Virtual slides in peer reviewed, open access medical publication

Klaus Kayser
Stephan Borkenfeld
Torsten Goldmann
Gian Kayser
Virtual slides in peer reviewed, open access medical publication
E D I T O R I A L
Background: Application of virtual slides (VS), the digitalization of complete glass slides, is in its infancy to be implemented in routine diagnostic surgical pathology and to issues that are related to tissue-based diagnosis, such as education and scientific publication.Approach: Electronic publication in Pathology offers new features of scientific communication in pathology that cannot be obtained by conventional paper based journals. Most of these features are based upon completely open or partly directed interaction between the reader and the system that distributes the article. One of these interactions can be applied to microscopic images allowing the reader to navigate and magnify the presented images. VS and interactive Virtual Microscopy (VM) are a tool to increase the scientific value of microscopic images.Technology and Performance:The open access journal Diagnostic Pathology http://www.diagnosticpathology.org has existed for about five years. It is a peer reviewed journal that publishes all types of scientific contributions, including original scientific work, case reports and review articles. In addition to digitized still images the authors of appropriate articles are requested to submit the underlying glass slides to an institution (DiagnomX.eu, and Leica. com) for digitalization and documentation. The images are stored in a separate image data bank which is adequately linked to the article. The normal review process is not involved. Both processes (peer review and VS acquisition) are performed contemporaneously in order to minimize a potential publication delay. VS are not provided with a DOI index (digital object identifier). The first articles that include VS were published in March 2011. Results and Perspectives: Several logistic constraints had to be overcome until the first articles including VS could be published.Step by step an automated acquisition and distribution system had to be implemented to the corresponding article. The acceptance of VS by the reader is high as well as by the authors. Of specific value are the increased confidence to and reputation of authors as well as the presented information to the reader. Additional associated functions such as access to author-owned related image collections, reader-controlled automated image measurements and image transformations are in preparation. Virtual Slides: The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/ vs/1232133347629819.

Introduction

Tissue based diagnosis or diagnostic surgical pathology is about to significantly change in its clinical importance and technology [1]. The times of autopsy and simple microscopic cellular morphology (H & E stained diagnosis) are still the gold standard for therapeutic decision making but are nowadays to a growing extent replaced by live imaging such as Computed Tomography (CT), nuclear resonance imaging (NRI), and nourished by molecular genetics and biology technology methods [2]. The combined application of these methods permits a so -called individualized diagnosis and treatment, especially for cancer patients [3,4]. The surgical pathologist acts as the clinician's guide, and the whole action is called predictive diagnosis [4,5].

In addition, modern communication tools and modules have been implemented in medicine, too [2,[6][7][8][9]. These include fast communication lines (fiber optics), standardized communication tools such as digital imaging and communication in medicine (DICOM), picture archiving and communication system (PACS), or the internet. Hospital information systems (HIS) and laboratory information systems (LIS) have been implemented in nearly all larger hospitals and pathology institutes [10][11][12].

Digitalization of complete microscopic glass slides is an additional tool that fits into the digitized environment of an institute of pathology. Consecutively, approaches are ongoing to implement virtual microscopy (VM) into the pathologist's routine diagnostics [2,6,[13][14][15].

Additional tools in this world of diagnosis include items that are mandatory for correct diagnostics, i.e., access to reference books, to experts who are specialized for certain diseases, or measurements of certain image features [16][17][18][19]. The performance of these aims is enhanced in electronic communication to a high degree.

In this article we describe a specialized tool that corresponds to VM and has been implemented in a specialized manner of electronic scientific communication, the so -called open access journals. The logistics and necessary implementation tools are described as well as the performance and acceptance of this innovative publication technology in medical sciences. In addition, the promising perspectives are briefly outlined.

Journal data

Open access publication has been developed after successful implementation of scientific electronic publication. In pathology, the Electronic Journal of Pathology and Histology (Elec J Pathol Histol) was the first scientific peer reviewed journal in pathology that has been solely electronically distributed to our knowledge [20]. It has been implemented in 1995 and was distributed via floppy-discs worldwide. Internet access and other electronic distribution media had not been available at that time. The main characteristics of Elec J Pathol Histol are displayed in (Figure 1). Each floppy-disc was distributed with specific "reader" software, which also allowed the compilation and execution of programs. Tests of so -called interactive publication were also performed. These test allowed the reader to add own data to an already existing article, and to publish the "extended" version by including the authors of the previous article, after they had agreed. All images were displayed in a . pcx format. Although the peer reviewed and published articles had been cited worldwide, the application for inclusion into Reuter's citation index failed several times because a solely electronic distribution of scientific articles was not appreciated in the 1990s [21].

The Elec J Pathol Histol was redesigned in HTLM format and distributed on a CD in 2000. However, several advantages of the DOS environment had to be given up (executable programs, interactive publication), and the production of the Elec J Pathol Histol was terminated in July 2000. The front page of last edition is depicted in (Figure 2).

The Elec J Pathol Histol has been brought back to live in 2006 once the internet had been established and acknowledged as a useful and flexible communication medium. The new journal was named Journal of Diagnostic Pathology (Diagn Pathol), <http://www.diagnosticpathology.org>. The potential benefits of the internet embedding were analyzed, and corresponding mandatory formal changes were undertaken. These included:

Open access publication. The authors have to pay for the publication of their article. In compensation, they still inhabit the publication rights etc. of their article, and the readers have access to all published articles for free.

Authors who are working in developing countries can apply for waivers to publish their scientific results for free or at a remarkably reduced price.

The principle formal organization remains related to the conventional style of publication; i.e., Diagn Pathol still possesses its own "front page" and is not just a domain which might be embedded in another front page such as "medicine".

Access to and reading of the published articles is documented as well as the country of the reader, and statistically analyzed.

Reviewers are chosen from the editorial board and contemporary from authors who have published articles that are related to the content under request. These authors are chosen from articles cited in the National Institute of Health (NIH) library (PubMed).

The number of included color images, tables, or drawing etc. is not limited and free of additional charges.

All published articles are provided with an ID number (DOI) and indexed for interactive search of readers etc.

Including Virtual Slides in Open Access Scientific Publication

Virtual slides (VS) are the digital representation of completely digitized glass slides [2,13]. VS have been developed at the beginning of this century. Image acquisition machines (scanners) are commercially available from about 10 different companies (e.g. 3DHistech, Aperio, General Electric, Leica, Olympus, Phillips, and others). They are now in daily use for interdisciplinary conferences, teaching of medical students, and postgraduate education in most of the Medical Universities located in Western Europe and the United States. Some of the larger institutes of pathology are equipped with scanners that are embedded in the daily routine workflow of diagnosis, and several other institutes of pathology are investigating to replace the conventional workflow by virtual microscopy (VM) [2,6,10].

VS are comparable with conventional glass slides in image quality that has been demonstrated by several investigations [2,6,10]. The specific features of VM are summarized in (Figure 3). In addition to the features of conventional microscopy they include contemporary viewing of different regions of interest (ROI) or stains, interactive labeling, automated scoring, and other electronic assistance.

Including VS in an open access peer reviewed scientific journal includes several advantages for the authors, readers, and the publisher (Figure 4).

The authors can demonstrate the morphological findings of whole microscopic image. The ROI can be analyzed independent from the authors' view.

The readers are assured for the originality of the findings, and can proof their own strategy for ROI selection.

The publisher increases the attraction of the included articles, and opens the journal for additional perspectives such as implementation of a repository or specific case collection, etc.

Constraints can be seen in the mandatory logistics:

The authors have to submit the original glass slides to a selected institution that scans the glass slides, because an official VS standard does not exist at present. Most of the companies have developed own specific viewers. A conversation of their own image format into a more general and open standard such as jp2000 is difficult if not impossible without knowing the individual image structure [22][23][24][25][26][27].

VS are large images of usually 2 -3 GB in size. They usually require a specific viewer and a related image data bank that can handle these images. Thus, VS have to be organized in an electronic system that has to be separated from that of the published articles. In addition, the publication of still images should not be affected by VS.

After several trials, we decided to provide each article with dummy links that are kept empty if VS are not  The chosen solution possesses logistic and content related advantages: The publication procedure of an article is not affected or delayed by VS as it is completely disconnected from the VS production, and VS do not require a DOI number (Digital Object Identifyer for article identification). Furthermore, VS can be included into each article at any time; even years after its publication by replacing the dummy link with an active one, and the reader has access to VS separately from included still images. The logistic frame is depicted in (Figure 5). The reviewers are not informed about potential inclusion of VS in order to completely separate this issue from scientific considerations. Thus, VS do not involve, delay or promote the articles' publication.

Experiences and Results

The preparations of the described procedure lasted for about 6 months. The authors of newly submitted articles were informed via email about the VS technology and asked to submit the corresponding glass slides for digitalization. A summary of the authors' responses, number of included VS, and published articles is shown in (Figure 6). About 50% of requested authors submitted glass slides for VS publication, most of them are working in Asia. There was no time delay in peer reviewing and production process in comparison to articles that do not contain VS. VS seem to promote the interest of readers and the journals reputation as the number of submissions increased remarkably after publication of VS. However, also other reasons might play a significant role in this manner, such as fast review process, the citation index, or increased number of subscribers. The quality of all published VS was judged good or even very good. The access to and display of VS naturally depends on the network and included servers of the readers internet. The navigation and VM control is fast and reliable as only the viewing partitions of VS have to be downloaded.

Discussion and Perspectives

This is the first report on successful VS publication in a peer reviewed scientific journal to our knowledge. The acceptance of this new technology by authors and readers can be judged good to very good. The reviewers are not informed about potential VS publication in the corresponding article. Thus, the reviewing process is completely separated from the newly introduced VS publication.

This might be a reason of debate. Reviewers are part of the scientific judgment of an article, and VS contribute to its scientific level too.

Why to completely exclude reviewers from VS?

There are practical and theoretical reasons: One aim in electronic publication is to maintain a short review/production time which could be delayed in combining VS From the theoretical point of view, the diagnostic judgment in tissue based diagnosis can be distinguished in a) selection of the ROI, and b) the diagnostic statement [12,[28][29][30][31]. In surgical pathology both algorithms are usually performed in only one combined step. Young colleagues are trained to perform the final diagnosis by viewing pre-selected areas (ROI) which are displayed in textbooks or atlases [2]. Publishing VS and  still images independently from the review procedure separates these two steps, which is of additional educational significance.

The main constraint of the described method still remains in logistic "hardware" problems, especially in submission of requested glass slides. The mailing costs have to be covered by the authors. The glass slides have to be sent to an "image acquisition center" which controls the image quality. It is responsible to accurately document and handle VS. After several different trials to accurately link included still images with VS we decided to separately publish still images and VS. The separation of the "conventional" publication procedure from VS inclusion was favored by the production team. It could maintain the production speed. VS can be included at any time after publication. No intervention is needed. The provided links can be transferred to different servers at any time without involving the publication procedure of text, included images, and references. Thus, the journal diagnostic pathology has been "opened" to a new innovative distribution of image content information that offers new regions for additional applications.

Most of published VS are derived from case reports which usually describe rare diseases displaying educational significance. They are arranged in an appropriate data bank which will be transformed in an adequate repository in the next step. Certainly, the authors have to agree if the images will be included in such an electronic image retrieval system. Such a system will offer images of rare and educational significance combined with extensive clinical data such as the patient's history and development of the disease.

An additional potential application is related to quantitative object related measurements of VS [7,15]. They can be performed on a broad variety of stains including conventional and fluorescent immunohistochemistry using open access systems such as EAMUS™ or equivalent systems [7,15]. The reader can use these systems to select ROI from VS and to perform content related measurement.

In summary, the publication of VS in an open access peer reviewed scientific journal has been accepted by a broad community of pathologists and colleagues working in related medical and scientific fields shortly after its implementation. It significantly increases the reader's tools that can only be provided in an electronic communication environment such as individual selection of areas of interest, interactive measurements of specific objects, or image content information related selection of case reports for tissue based diagnosis and training.

introduction and establishment of Diagn Pathol lasted for about six months. The total number of published articles rose from 46 in 2006 to 83 in 2010, and will probably extent the level of 120 in 2011. The rejection rate increased from 30% in 2006 to > 50% in 2011. The journal possesses about 1,200 registered readers in 2011. Most articles are assessed by more than 500 readers within their first year of publication.

Figure 2
2View of the front page of the Electronic Journal of Pathology and Histology. Diagn Pathol was included in the Citation Index (Reuter's) in 2010. It was calculated an impact factor of 1.39 for the year 2010.

Figure 3
3Specific features of Virtual Microscopy (compared to conventional microscopy).

the article. These links are activated and connected to the corresponding images if VS are included.

Figure 4
4Specific features of virtual slides focusing on open access publication. review. In addition, the selection of published still images depends upon the author. Any errors in selecting additional correct ROI can be judged by the reader if VS are published too.

Figure 5
5Logistic frame of virtual slides to be published in open access journals.

Figure 6
6Survey of published articles including virtual slides (Journal Diagnostic Pathology, September 2011).
AcknowledgementsThe financial support of the Verein zur Förderung des biologischtechnologischen Fortschritts in der Medizin e.V. is gratefully acknowledged.Submit your next manuscript to BioMed Central and take full advantage of:
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Criteria for the successful completion of medical dissertations -A multicenter study Kriterien für den erfolgreichen Abschluss medizinischer Dissertationen -eine multizentrische Studie

Olaf Kuhnigk
Volker Reissner

Prodekanat für Lehre
HamburgDeutschland

Medizinische Klinik III
HamburgDeutschland

Criteria for the successful completion of medical dissertations -A multicenter study Kriterien für den erfolgreichen Abschluss medizinischer Dissertationen -eine multizentrische Studie

Objective: In order to acquire the academic title "doctor" in Germany, it is essential to complete a dissertation. A high number of publications

methodology could also be a helpful tool toward successfully accomplishing a doctoral thesis or scientific projects in general.

Medizinische Klinik III, Hamburg, Deutschland

Introduction

In most countries physicians are called "doctor". However, in Germany a physician is only allowed to put "Dr." in front of his name if he has actually received a doctorate. In contrast to the situation in many European and overseas countries, medical students in Germany must conduct a research project and write a dissertation reviewed and graded by scientific experts in the respective field in order to obtain the title "doctor". Contrary to other academic faculties in Germany, it is possible to start the doctoral dissertation during undergraduate medical training. After successful graduation from medical school, evaluation of the thesis, and an oral examination, the "Dr. med." degree is awarded by the university. Even though a dissertation is a prerequisite for acquiring the academic title "doctor", it is not required to graduate, enter a residency, or practice medicine. The reasons why medical students want to obtain the degree "Dr. med." are not known. However, if graduates want to pursue an academic career at a university hospital, usually only the successful accomplishment of a doctoral thesis will open up this career path. The German Council of Science and Humanities, the governmental agency responsible for nationwide research evaluation and scientific research activity, considers most medical dissertations to be merely on the level of final-degree papers in terms of complexity and scientific quality [1]. Critics of the current dissertation method call it "pseudo-scientific medical dissertation" and "publication without any effective scientific relevance" [2]. Yet studies have revealed that more than one-third of all publications from medical schools are based on dissertations [3], [4]. Two-thirds of medical faculty members consider medical research related to dissertations to be important for sustaining a qualitatively and quantitatively high output of publications. Therefore, medical students writing dissertations significantly contribute to Germany's role as an important site within the global scientific landscape [5]. Successfully accomplished scientific research by students in other national or interna-tional programs (doctorate or post-doctorate) have a similar impact. In the winter semester 2007/2008, 78,000 students were registered at 36 German medical schools [6]. In the same year (2007), 6108 medical students completed their dissertation successfully, which is a rate of 47% [7]. Studies performed in the 1990s revealed that 17% of medical students had no intention of conducting a dissertation project during their undergraduate training [8], [9], and studies from three individual universities showed that 10-21% of all students abandoned at least one research project [4], [10], [11]. The reasons why some dissertations are successfully completed and why some are abandoned, and why there is a specific format for a structured scientific research project in general, are varied and have not been thoroughly analyzed thus far. In cases where a dissertation was accomplished successfully, the candidates had a positive view of the process in retrospect [12]. The new German Federal Medical Licensing Regulation from 2002 [13] does not require any courses for preparing a dissertation during medical undergraduate training. One study showed that 76% of the students assessed the preparation for scientific research at medical school as poor or nonexistent, while only 3% evaluated the preparation as good [11]. The students also mentioned poor supervision, methodological problems, difficulties with the topic, personal problems, or a lack of time and finances as reasons for abandoning a dissertation. 86% of doctoral candidates held poor supervision responsible for giving up on their dissertation project [11]. At the University of Würzburg, Germany, students still working on their dissertation assessed the supervision as hardly satisfying [14]. Regarding these data, further research seems to be necessary to investigate in more detail the reasons why students abandon their medical dissertation and to develop recommendations for promoting the successful completion of scientific research projects. On the one hand, this would enable students to achieve a relevant educational goal, i.e., an advanced degree; on the other hand, scientific research activity and its quality would improve, which is one of the declared aims of the German Council of Science and Humanities. So far, data are only available from studies performed at individual universities as well as from students who are working on or have completed their dissertation [14], [15]. Whether these data are applicable to all German universities or to research projects in general is currently unknown. The purpose of our study was to collect reliable data that would be helpful for drafting recommendations concerning good practice in the field of medical dissertations as an example for medical research projects. We included students from five representative German universitiesthose who abandoned their dissertation and those who were still continuing with their research project -and collected data on structure, content, and field of the respective dissertations. These data seem to be suitable for general recommendations.

Methods

We developed a questionnaire comprising one coversheet with general information about the study and 16 pages of questions from the following categories:

1. General questions about the students' attitudes toward scientific work, about the dissertation in general, and about the relevant personality traits of the supervisor and the doctoral candidate (8 questions) 2. Questions about the current or finished dissertation (Group A), e.g., type of project, reasons for choosing this project, accomplishment of individual project steps, expenditure of time for the project and influence on undergraduate studies, problems with the project, questions about supervision, and publication of the dissertations (36 questions) 3. This part asked about the first discontinued dissertation (Group B), the content of the questions being similar to Group A (37 questions) 4. Questions addressed to students without a dissertation project (Group C), inquiring about the reasons for not starting (2 questions) 5. General social background (6 questions)

We based the conceptual design of the questionnaire on research data regarding medical dissertations in Germany and on existing theoretical literature [12], [15], [16], [17]. The response options in this questionnaire included dichotomous answers, such as yes/no answers (e.g., did you start a dissertation during your undergraduate studies?), numeric answers (e.g., number of semesters), individual text (e.g., "reasons were: please specify"), and approval or refusal of a statement on a 6-point Likert scale (1 = I strongly agree, 2 = I agree, 3 = I somewhat agree, 4 = I somewhat disagree, 5 = I disagree, 6 = I strongly disagree).

Sample and Design

We handed out the questionnaires in 2003 to 467 students at the medical schools of five German universities (Berlin, Bochum, Düsseldorf, Essen, and Hamburg). The participating students were in their ninth semester (the total number for undergraduate training in Germany is twelve semesters) with one exception: in Bochum the students who participated were in their eighth semester. The testing took place during mandatory courses. Participation was anonymous and optional. The collected data were protected by the Data Protection Act. The vice deans of education and the directors of the departments agreed to this study. Return rates of questionnaires were between 85% and 99%. The data from all universities were combined, as a comparison of universities was not a focus of this study.

Statistical Analysis

For data evaluation we divided the students into three groups: Group A: students with an ongoing or completed dissertation, regardless of the fact that they may have abandoned a previous project. Group B: students with abandoned dissertation projects, regardless of the fact that they might have started a new one. Group C: students who had never started a dissertation. Data are presented as arithmetic means and frequency distributions. We used the chi-square-distribution test and the t-test as statistical tests for independent samples. We applied the non-parametric Mann-Whitney-U-Test to confirm the t-test results. We combined all discontinued dissertations, even if a student started a new dissertation in the meantime. In this case, the students were part of both groups and were asked the respective questions. Nevertheless, we chose the t-test for independent samples because it reacts robustly to violations of its assumptions and is more conservative than the t-test for related samples. As we designed our study to collect data to generate hypotheses for further research in this field, we applied no Bonferoni correction to avoid an alpha error. We calculated the effect size when significant results appeared: 0.2 indicates small, 0.5 medium, and 0.8 large effects [18]. We used SPSS for all statistical analyses.

Results

The return rate of the questionnaires from the 467 students at the five universities was 93.5% (n = 437). 71% had started at least one dissertation and were still working on it (Group A), 15% had abandoned at least one dissertation (Group B), and 14% had never started a comparable project (Group C). We found no significant group differences in how students searched for a dissertation project. In Group A, 51% of the projects included basic research (e.g., laboratory work), followed by clinical research (36%), and empirical studies (11%); 2% were literature studies only. The two main reasons why students in Group A started a dissertation were "interest in the topic" (M = 2.18, SD = 1.17) and "advantage for job applications" (M = 2.48, SD = 1.65), followed by "scientific methods" (M = 2.71, SD = 1.33), "learning how to do scientific work independently" (M = 2.88, SD = 1.40), "gain insight into research" (M = 3.01, SD = 1.35), "prestige" (M = 3.04, SD = 1.50), and "patients' prejudice against physicians with no doctorate" (M = 3.32, SD = 1.65), data not shown. Irrespective of group status, the three main reasons for choosing a specific dissertation topic were: "interest in the specialty", "interest in the topic", and "better occupational opportunities" (all means < 3.0). The only significant difference between the groups was that students in Group A mentioned "the personality of the supervisor" as the reason for the choice of their project much more frequently (Group A: M = 2.8, Group B: M = 3.8; p < 0.001, ε = 0.67) (see Figure 1). Doctoral colloquia, i.e., academic meetings at which specialists deliver addresses on a particular topic and then answer questions relating to them or give technical advice, were offered neither to Group A (57.9%) nor to Group B (67.2%). 64.9% in Group A and 70.0% in Group B who had not had such colloquia stated that they would have wanted to participate in a colloquium had they been given the opportunity. Students in Group A (46.6%) affirmed the question "do you work with other doctoral candidates in study groups in which you give each other methodological support?" significantly more often than students in Group B (24.3%; p < 0.01). Irrespective of group status, 57.5% of all dissertations were never discussed with a statistician (see Figure 2). 18.6% of the students did not know whether the project had been discussed with a statistician. 20.6% of Group A discussed their work with a statistician before starting the practical phase of the project, while in Group B only 6.6% did so. In total, significantly more dissertations from Group A were discussed with a statistician (45.4%) as compared to Group B (27.9%) (p < 0.05) (see Figure 2). ; P < 0.05) significantly more than students in Group A. We found no significant group differences for the answers "too few patients" and "lack of material" (see Figure 3). The opposite was the case in only 14.0%. Another 35.1% stated that both parties were responsible, and 5.3% saw the responsibility neither in their supervisor nor in themselves. "Deficient tutoring by the supervisor" (M = 2.9, SD = 1.95) and an "oversized expenditure of time" (M = 3.7, SD = 2.01) were given as the main reasons for abandoning their dissertations. Only 14.4% (n = 63) of all participating students had not started a dissertation at the time of this study. 53% of all students in Group C stated they had not yet started a dissertation because of not having found an appropriate topic thus far. 37% wanted to start a dissertation after completing their undergraduate training, and 10% stated no interest at all to do so. The latter group mostly claimed "adverse effects on their studies while working on a dissertation" (M = 2.8, SD = 1.34) and "no time" (M = 2.8, SD = 1.60) as reasons. The statement "a dissertation is not an additional qualification for medical work" also received some agreement (M = 3.1, SD = 1.75).

Discussion

Our study, conducted at five German universities, revealed that the most important reasons for choosing a dissertation research project are a personal interest in both the specialty related to the dissertation and the topic itself. Students with an ongoing dissertation (Group A) mentioned the supervisor significantly more frequently as a cause for not having discontinued their project. Asked for the reasons for abandoning their dissertation, 45% of the students in Group B held their supervisor responsible for the discontinuation, compared to only 14% who blamed themselves. Inadequate tutoring was also a major reason for giving up on a dissertation. Our results coincide with those of studies at individual German universities, which also found that students considered inadequate tutoring as the main reason for not finishing a dissertation [10], [11], even though some of these studies relied only on a small number of participants [14]. Furthermore, in one of these studies, doctoral candidates who had successfully completed a dissertation assessed their supervisors considerably better regarding "time" and "support" [11]. Since the scientific research topic is usually something in which the supervisor specializes or has an active interest, and as adequate tutoring seems to have an important influence on the success or failure of a research project, universities should consider -especially with regard to publication output [5] -offering specific training to support supervisors in their tutoring function. In this context, attention should be drawn to the fact that the actual supervision has to be performed by the students' academic supervisors and not only by technical assistants [14]. Successful faculty development courses for university teachers have already been implemented at some German medical schools. Instruction courses for research supervisors of scientific projects have not been established in any German medical school thus far. Besides courses, structured dissertation contracts might also be helpful to ensure the success of doctoral candidates, since clear work schedules and timeframes were present in Group A significantly more often than in Group B. In another study, about 50% of doctoral candidates expressed a desire to have a dissertation contract [15]. More than 85% of the doctoral candidates worked in dissertation projects involving basic sciences or clinical studies. Both require in-depth knowledge of biostatistics, which is usually not acquired during undergraduate medical training [19]. Our study shows that 45.5% of students in Group A and only 27.9% in Group B discussed their project with a statistician. In total, only 57.5% of dissertations were supported by a statistician. While one reason for the group difference might be related to greater personal initiative of successful candidates, these data nevertheless seem to support the need for courses on how to properly perform scientific research [19]. Considering that biometric courses are usually very unpopular among medical students [20], medical schools need to find new means to support their students in doing scientific research if this is an educational goal of their undergraduate curriculum or other scientific research programs. It is unlikely that several weeks of problem-based EBM courses [21] or peer education workshops for preparation of medical dissertations [19] are sufficient to close this gap. Regular working groups for the discussion of methodological questions also seem to support the successful completion of dissertations. Students in Group A participated significantly more frequently in such groups compared to students in Group B. Student feedback on a PhD program showed very good ratings for structured courses and supervision [22]. Both results suggest that method- ological courses and embedding a dissertation into a structural framework support successful completion, which is relevant for any kind of scientific research program. Such courses could be integrated into the regular medical curriculum as mandatory electives for students working on a dissertation. Several German universities (e.g., Johannes-Gutenberg-University Mainz, University of Cologne) have meanwhile taken this approach in the form of voluntary courses. A strength of our study lies in its large sample size and its design as a multicenter study including five representative German medical schools. The questionnaire, based on the literature, covers all relevant fields of influence regarding the realization of medical dissertations as an example for scientific research projects. However, some limitations have to be taken into account. The influence of specific personal traits (e.g., motivation, IQ, etc.) on the successful completion of a dissertation was not examined with our questionnaire. Furthermore, the perspective of supervisors on the successful realization of dissertation projects was not considered. In summary, our data suggest that the handling of statistics and colloquia for doctoral candidates provide successful support for finishing a dissertation project. To improve supervisory support and structural quality for successful dissertations at German medical schools, we suggest implementing the following aspects, which are based on both research and the literature: Structural recommendations for dissertations at medical schools:

1. Central announcement of dissertation projects, structured application procedure, acceptance of the project by the faculty before work is started 2. Contract between doctoral candidate and supervisor, including a rough time schedule and structural framework and covering rights and obligations of doctoral candidate and supervisor as part of the dissertation regulations of the faculty Advanced training programs for doctoral candidates and supervisors:

1. Mandatory preparatory courses for doctoral candidates in basic knowledge of scientific research methods, statistics, and literature search 2. Continuous colloquia for doctoral candidates including scientific exchange and follow-up on project's progress 3. Advanced training for supervisors of the doctoral candidates including teaching techniques for scientific research and soft skills for mentoring

Figure 1 :
1Students´reasons for starting their dissertation

Figure 2 :
2Doctoral candidates' participation in study groups and/or colloquia; support by a statistician More than 50% of students from both groups (A = 49.7%; B = 56.5%; n.s.) had no clear work or time schedule for their project. If a schedule did exist, it was mainly the result of the supervisor's planning for the doctoral project (A = 69.2%, Group B = 77.7%). Only in a few cases did a schedule exist due to participation in a research group (A = 10.0%, B = 7.4%) or because the students were members of a singular research project (A = 20.7%, B=14.8%). In cases where a schedule existed, it was followed by 47.2% of Group A but only by 14.7% of Group B. With reference to self-management, only 23.7% of the students in Group B stated that they were able to manage time themselves ("could you manage the time you had for the experimental/empirical work of your dissertation by yourself?"). Significantly more students in Group A (44.9%) answered this question positively (p < 0.001). 54.3% of Group A and 41.0% of Group B (n.s.) agreed to the question "have there been any delays that were not caused by your studies?". In case of delays not related to their studies, students in Group B stressed "missing support"(Group A: M = 4.05, SD = 1.97, Group B: M = 2.59, SD = 1.97; p < 0.001), "flaws in the concept of the project" (Group A: M = 4.16, SD = 1.90, Group B: M = 2.95, SD = 2.05; p < 0.001) , and "non-availability of the supervisor" (Group A: M = 4.06, SD = 1.98, Group B: M = 3.21, SD = 2.78

Figure 3 :
3Reasons for difficulties or delays during the doctoral project 45.6% of the students in Group B held their supervisors responsible for their project's failure (M = 2.5, SD = 1.59) rather than blaming themselves (M = 3.5, SD = 1.80).

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/GMS Zeitschrift für Medizinische Ausbildung 2010, Vol. 27(3), ISSN 1860-3572
Universitätsklinikum Hamburg-Eppendorf, Klinik Results: Students in Group A referred significantly more often to their supervisor as being essential for choosing the topic of their dissertation und Poliklinik für Psychiatrie project. Furthermore, students in Group A worked together with other und Psychotherapie, Hamburg, Deutschland students in self-organized support groups to debate methodological questions. They also discussed their project more frequently with a 2 Universitätsklinikum Hamburg-Eppendorf, statistician. Students in Group C gave "interference with undergraduate studies" and "no time" as the most common reasons for not starting a dissertation. Prodekanat für Lehre, Hamburg, Deutschland Conclusions: As the supervisor seems to play an important role for the successful completion of dissertations, universities should think about 3 Rheinische Kliniken Essen, Klinik für Psychiatrie und professional programs that would prepare supervisors for this educa-Psychotherapie des Kindestional task. Courses for doctoral candidates in scientific research und Jugendalters, Essen, Deutschland
AcknowledgmentsThe authors wish to thank the deans of education and the directors of the participating departments of the five universities for allowing us to distribute the questionnaires during students' courses. We wish to thank all medical students who participated in our study.FundingThe study was not funded.DisclaimerThere is no conflict of interest.Ethical approvalThe ethics committee of the University Essen-Duisburg was asked for permission. As students participated in this study voluntarily and anonymously, the ethics committee stated that no special approval needed to be given.The authors
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Phosphorus diffusion gettering process of multicrystalline silicon using a sacrificial porous silicon layer

Derbali Lotfi
Ezzaouia Hatem
Phosphorus diffusion gettering process of multicrystalline silicon using a sacrificial porous silicon layer
N A N O E X P R E S SMulticrystalline siliconPorous siliconDefect densityGrain boundariesGetteringWTC-120 lifetime tester
The aims of this work are to getter undesirable impurities from low-cost multicrystalline silicon (mc-Si) wafers and then enhance their electronic properties. We used an efficient process which consists of applying phosphorus diffusion into a sacrificial porous silicon (PS) layer in which the gettered impurities have been trapped after the heat treatment. As we have expected, after removing the phosphorus-rich PS layer, the electrical properties of the mc-Si wafers were significantly improved. The PS layers, realized on both sides of the mc-Si substrates, were formed by the stain-etching technique. The phosphorus treatment was achieved using a liquid POCl 3 -based source on both sides of the mc-Si wafers. The realized phosphorus/PS/Si/PS/phosphorus structures were annealed at a temperature ranging between 700°C and 950°C under a controlled O 2 atmosphere, which allows phosphorus to diffuse throughout the PS layers and to getter eventual metal impurities towards the phosphorus-doped PS layer. The effect of this gettering procedure was investigated by means of internal quantum efficiency and the dark current-voltage (I-V) characteristics. The minority carrier lifetime measurements were made using a WTC-120 photoconductance lifetime tester. The serial resistance and the shunt resistance carried out from the dark I-V curves confirm this gettering-related solar cell improvement. It has been shown that the photovoltaic parameters of the gettered silicon solar cells were improved with regard to the ungettered one, which proves the beneficial effect of this gettering process on the conversion efficiency of the multicrystalline silicon solar cells.

Background

Due to the high price of fossil energy source and huge pollution of environmental issue, in the recent years, the development of renewable energy has regained our attention. The development of the solar energy industry is one of the most popular technologies in renewable energy. The majority of solar cells are made from crystalline silicon; more than half of the crystalline silicon solar cell production is based on multicrystalline silicon (mc-Si). The quality of multicrystalline silicon (mc-Si) wafers may become worse in the future for several reasons; the main reason is that lower-quality feedstock will probably be used for cost reduction and availability reasons [1]. Improving conversion efficiency using a low-cost material development is the main research activity in the photovoltaic field [2,3]. Several experiments demonstrated that the electrical properties of crystalline silicon solar cells can be improved by coating the n + emitter region with a porous silicon layer [2][3][4][5], and many technologies have been used for this purpose [6][7][8], such as using vanadium oxide as an antireflection coating [9]. The possibility of improving the electrical properties of silicon wafers, by extracting impurities from them, using thermal treatment under oxygen atmosphere, or phosphorus diffusion, is well known [10][11][12][13]. Besides, the porous silicon (PS) layer may be used as an efficient sacrificial layer for gettering metallic impurities [11,14]. There are two general classifications of gettering, namely extrinsic and intrinsic. Extrinsic gettering refers to gettering that employs external means to create the damage or stress in the silicon lattice in such a way that extended defects needed for trapping impurities are formed. These chemically reactive trapping sites are usually located at the wafer surfaces, away from the bulk. Intrinsic gettering is using oxygen to enforce precipitation of metallic impurities, such as copper, not at the wafer surface but in its bulk. A preparatory step in this case is a thermal treatment causing precipitation of the excess oxygen which, for a variety of reasons related to the specific properties of oxygen in silicon, forms oxygen precipitates in the bulk of the wafer rather than at its surface. Those oxygen precipitates then act as gettering sites for metallic impurities. PS may also be used as an excellent antireflective coating or as a passivating layer on the emitter of crystalline silicon [15]. Most often, the formation of PS can create defects in the surface of the Si substrate that may enhance the gettering effect as extrinsic gettering effect. A phosphorus gettering model was proposed [16], which stated that the gettering speed is controlled by two steps. The first step is to limit the gettering temperature by releasing diffusion of metal impurities, and the second step is to control the best gettering temperature by the segregation function. Other authors [17] proposed that silicon self-interstitial current generated during phosphorus (P) diffusion is an essential factor of the gettering mechanism, and also found that Fermi level-enhanced solubility in the P diffused layer contributes to the gettering effect. The use of an infrared (IR) furnace for the fabrication of p-n junction [18,19] and gettering impurity through the porous silicon layer [20] is one of the processes that may be used to reach this goal. In this work, we will investigate the increase of carrier lifetime and the conversion efficiency of multicrystalline silicon solar cells gettered by a phosphorous gettering process using a sacrificial porous silicon layer in which the impurities will be trapped.

Methods

The starting material was a p-type (boron-doped) multicrystalline silicon substrate, 400 μm thick with a resistivity of 1 to 2 Ω cm. To avoid changes in grains and grain barriers (GBs) from wafer to wafer, samples were selected from consecutive mc-Si wafers sharpened successively in the same ingot. Similar wafers, which were vertically adjacent to each other in the ingot, were used to compare and confirm the gettering effect. Therefore, differences between them, after different processing, can be interpreted as being due to variations in the process parameters rather than to material variations. PS layers were formed on both sides by the stain-etching technique using HF/HNO 3 /H 2 O solution with a 1:3:5 volume composition [3]. A POCl 3 /acetone liquid source was used for the diffusion process [20]. We optimized the POCl 3 / acetone ratio to 1:5. The POCl 3 spreading out was realized by the spinning technique onto p-type multicrystalline silicon wafers. After drying at 200°C for solvent evaporation, the realized P/PS/Si/PS/P structure undergoes a heat treatment in an IR furnace under an O 2 atmosphere. The temperature annealing was varied in the range of 700°C to 950°C for 60 min. This heat treatment has been applied in order to allow P diffusion throughout the PS layer. After annealing the samples, the phosphorous-doped region (the phosphorous-doped PS layer) was removed from both sides using a chemical etching (HF 16%: HNO 3 64%: CH 3 COOH 20%) solution. In order to perform the solar cell fabrication process, the n+/p junction was achieved using a simple phosphorus diffusion technique. The back aluminum (Ag/Al) and the front Ag contacts were screen printed using AMI Presco CP885 screen printer (Affiliated Manufacturers, Inc., North Branch, NJ, USA) and fired at 850°C and 620°C, respectively. The gettering effect has been evaluated by measuring the minority carrier lifetime, the dark and illuminated I-V characteristics, and the defect density at the GBs of the mc-Si substrates. The I-V measurements under illumination have been performed using PASAN cell tester CT 801 (PASAN Measurement Systems, Neuchâtel, Switzerland). The effective minority carrier lifetime (τ eff ) in the mc-Si substrates was measured using a WTC-120 photoconductance lifetime tester (Sinton Instruments, Boulder, CO, USA) under the quasi-steadystate lifetime measurement using the generalized analysis condition.

Results and discussion

The porous silicon layer created onto the front and back surfaces of the multicrystalline silicon wafers has a great importance in this work because it has the role of an external trapping site of impurities extracted from the samples. Figure 1a,b presents an atomic force microscopy (AFM) topography of the porous silicon layer formed by the stain-etching technique.

The porous silicon structure has been treated with phosphorus, followed by heat treatment at various temperatures ranging between 700°C and 950°C. Then, the treated PS layers were removed, and as a result, purified mc-Si substrates have been obtained. To confirm this gettering effect on the electrical properties of the samples, we have investigated the variation of the effective minority carrier lifetime.

The effective minority carrier lifetime of the gettered mc-Si wafers has been measured under the quasi-steadystate lifetime measurement using the generalized analysis condition. The measured area always includes some grain boundaries and thus provides an overall picture of both the intra-grain material and the extra recombination at grain boundaries. It is a combination of mechanisms that determines the performance of practical solar cells. The surfaces of the wafer are always present; we have reduced their possible contribution, but especially for the highest lifetimes measured, they are still likely to have a significant effect. The lifetimes reported here should be considered as effective lifetimes, which include both surface and volume recombination components. In many practical cases, there may be several sources of recombination in a sample, such as recombination through impurities in the wafer bulk or recombination at the surfaces. The effective lifetime represents the combined impact of all of these competing recombination channels. The effective lifetime τ eff measured under low-level injection at Δn = 1 × 10 14 cm -3 was taken for the calculations under generalized conditions. For 2 Ω cm (the resistivity of the wafers) and N D = 5 × 10 14 cm -3 , the wafers satisfy the lowlevel injection condition (Δn < <N D ).

The effectiveness of this gettering process is confirmed by the increase of the minority carrier lifetime, measured at the same carrier density, before and after treatment. Table 1 summarizes the effect of this treatment process on the effective minority carrier lifetime (τ eff ). We have obtained a very high lifetime value of the minority carrier after a thermal treatment at 900°C. We notice that τ eff increases when the annealing temperature increases, as shown in Table 1.

Multicrystalline silicon incorporates many impurities and defects that limit the minority carrier lifetime and, thus, the solar cell performance [21]. This significant variation of the minority carrier lifetime would indicate that a non-negligible quantity of unwanted impurities has been gettered and removed from the Si material and a decrease of the grain and GBs carrier recombination activities. Considering that the measurement area includes several grain boundaries, the results presented here indicate a very low recombination activity at the grain boundaries when comparing treated wafers to untreated one (reference).

It is important to note that the surface and volume recombination mechanisms coexist. We considered that both front and back surfaces of the wafer are identical since both of them have been subjected to the same treatment. The equation that takes into account all the recombination mechanisms that are present in the wafer is given by [22]:
1 τ eff ¼ 1 τ bulk þ 2S W :
Supposing that both front and back surfaces of all samples were subjected to the same passivation effect and have the same surface state, we can neglect the effect of the surfaces [22,23]. Consequently, (τ bulk ) can be deduced:
1 τ eff ∝ 1 τ bulk :
Therefore, we can determine the bulk recombination properties of the wafer from the effective minority carrier lifetime measurements. As a result, the obtained  improvement indicates that impurities in the bulk of the treated wafers have been removed, and the recombination activities have been decreased noticeably.

It is now interesting to investigate the variation of the defect density at the GBs while changing the treatment conditions, which is a one of the important parameters to evaluate the effect of our gettering process. We have performed four aluminum ohmic contacts on two adjacent grains (as shown in Figure 2). The aluminum was deposited by screen printing using Al/Ag containing aluminum and a small amount of silver in order to ensure the welding of the contact's edges. A DC current passes through the selected GBs between two adjacent grains using two external contacts. Using the two inner contacts, we have measured the voltage U. Figure 2 shows a schematic illustration of the dark I-V measurements across four selected GBs to evaluate the defect density (N B ) [24].

The zero-bias conductance for grain boundary is defined by [24,25]:
σ B ¼ J DC U
where U is the applied voltage. It can be written as [26]:
σ B ¼ 1 R S ¼ e 2 KT N a ν t ð1 À CÞ expðÀ V B KT Þ
The GB potential barrier can be written as following:
V B ¼ KT Log eAN a S N v σ B KT
where K is the Boltzmann constant, T is the ambient temperature, e is the elementary charge, A is the Richardson constant, and S is the grain boundary surface crossed by the current flow. N v and N a are the effective densities of the valence states and the doping concentration, respectively. The defect density N B (cm -2 ) was determined as a function of the GB potential barrier V B [27]:
N B ¼ Q B e ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 32ε γ ε 0 N a V B p e :
Identical wafers have been treated and identical GBs in the gettered samples show that defect density decreases when samples were subjected to thermal treatment, as shown in Figure 3. This significant variation of the defect density at the GBs would indicate that a non-negligible quantity of undesirable impurities have been gettered and removed from the grain boundaries. Thus, the recombination activities at the GBs have been reduced noticeably. These improvements could be attributed to the removal of eventual bulk metal impurities (Fe, Cu, etc.) and their trapping into the n + /PS layer. We notice that hightemperature annealing in infrared furnace has enhanced the impurity diffusion into the sacrificial porous silicon layer. Thus, the phosphorus-rich PS acts as an efficient external gettering site in which the impurities are captured due to the high thermal treatment. The phosphorus diffusion into grain boundaries, at least in the region near the front and back surfaces, can explain the considerable decrease of the defect density and the recombination activities into the GBs, as shown in Figure 3.

To understand the impact of this gettering process on cell performance, the internal quantum efficiency (IQE) measurements were evaluated at various annealing temperatures. All annealing temperatures exhibit an increased blue response in short wavelengths (400 to 750 nm), which is due to an important decrease of the surface recombination velocity and an improvement of the carrier collection at the emitter region, as shown in Figure 4.

A significant increase of the IQE in the long-wavelength range 700 to 1,000 nm (red response) is observed. This improvement can be explained by the important reduction of the carrier recombination activities in the bulk of the treated wafers, which is proven by the obtained minority carrier lifetime values in Table 1, and the significant reduction of the defect density at the GBs, at least in the region near the front and back surfaces. The observed behavior of the spectral response indicates that our gettering process leads to an efficient surface and bulk passivation and indicates an extended effect deep into the bulk of the substrate, which we suggest to be considered especially at the grain boundaries. The gettered solar cell at 900°C shows the highest IQE, which is not surprising because the effective lifetime of the minority carrier in the wafer treated at 900°C proved to be the highest.

The dark I-V characteristics have been investigated in Figure 5. After PS treatment, a significant improvement in the typical dark I-V characteristic was observed: the leakage current has considerably decreased, which is due to the reduction of the impurity concentration by gettering undesirable metallic impurities and defects that diffuse at getter sites present at the pore walls of the phosphorous-rich PS layer. The I-V characteristics clearly show a significant improvement of the rectifying behavior and a noticeable decrease of the reverse current after treatment, indicating an efficient passivation of grain and grain boundaries of the mc-Si wafers.

In multicrystalline silicon solar cells, the saturation current is essentially due to short circuiting via GBs. The current-voltage characteristic of the gettered samples shows an enhancement of the saturation current, which is a sign of the passivation effect of this gettering process at GBs. From these dark I-V curves, we determined the series resistance (R s ) [28] and the shunt resistance (R sh ) of the gettered and untreated cells. Figure 6 depicts the variation of the series and shunt resistances. We notice a significant increase of the shunt resistance (R sh ) and a diminution of the series resistance (R s ). The improvement of R sh can be attributed to an efficient passivation of the grain and GBs of the mc-Si wafers and the gettering of unwanted impurities.

However, the enhancement of R s could be due to the gettering effect in adjacent grains [29]. The deep recombination centers present at the GBs and bulk defects have been reduced, and an important decrease of the surface recombination velocity has been obtained. Gettering at 950°C seems to have a bad effect on the electrical properties of the cells, which could be due to the deep diffusion of phosphorus into the mc-Si substrate when annealing the sample at 950°C.

In Figure 7, we carried out the I-V characteristic measurement of obtained solar cells under AM1.5 illumination (100 mW/cm 2 ). Four parameters of solar cells were used to define illuminated solar cells: the short-circuit current (I sc ), the open-circuit voltage (V oc ), the fill factor (FF), and the efficiency (η).

IQE (%)

Wavelength (nm) Figure 4 The internal quantum efficiency dependence with the thermal treatment.

The optimal temperature is about 900°C. This optimum could be the result of the competition between the release of impurities from the bulk and a capture of impurities in the gettering layer, which has been removed after the thermal treatment. Below the optimum temperature, gettering process is limited by the release or the diffusion [28,30] of metallic impurities towards the gettering layer. This behavior is confirmed by the degradation of the I-V characteristics for the samples treated at temperatures exceeding the optimum one. Table 2 shows the variation of the I sc and the V oc after phosphorous gettering at different temperatures. Our gettering process improves the I sc from 74 to 109 mA (sample gettered at 900°C) and the V oc from 0.48 to 0.55 mV (sample gettered at 900°C). The effect of gettering mc-Si using the combination of phosphorous diffusion into a sacrificial PS layer and a thermal treatment becomes apparent when looking at the different cell parameters. The untreated solar cell (reference sample) shows a drastically lower short-circuit current density (J sc ) compared to the cells subjected to the above treatment, especially when annealed at 900°C. The same tendency can be stated for the open-circuit voltage (V oc ), which is 70 mV higher in the case of the gettered solar cell at 900°C, compared to the reference wafer (untreated). We conclude that phosphorous gettering process under O 2 atmosphere, using a sacrificial PS layer on both sides, improves the electrical parameters and the performances of the cells via the reduction of the carrier recombination activities. This leads to an enhancement of the short-circuit current and the open-circuit voltage and an increase of the conversion efficiency of the mc-Si solar cells.

Conclusions

The application of a sacrificial porous silicon layer, followed by a deposition of thin POCl 3 liquid film and then subjecting to a thermal treatment, has been proven to be able to getter undesirable impurities from the mc-Si substrate. The best results were achieved after gettering at 900°C for 60 min. This gettering process has led to a significant increase of the minority carrier lifetime and a noticeable decrease of the defect density at grain boundaries. The improvement of the internal quantum efficiency confirms the beneficial effect of the above treatment.   Impurities and defects at the surfaces and at the GBs have been trapped inside the P-doped PS layer after the heat treatment. Obtained results show a significant improvement of the electrical performances of the mc-Si solar cells, and the conversion efficiency has been increased from 5.9% to 11.6% in the sample gettered at 900°C.

Figure 1
1AFM topography of the porous silicon layer formed by the stain-etching technique. (a) Porous silicon formed on both sides of the samples. (b) 3D AFM topography before treatment with phosphorous.

Figure 3 Figure 2
32Effect of annealing the PS/phosphorus structure on the defect density at four selected GBs. Schematic illustration of the dark I-V measurements at a selected grain boundary.

Figure 5
5REFERENCE P-gettering at 700°C P-gettering at 800°C P-gettering at 850°C P-gettering at 900°C P-gettering at 950°C Voltage (V) Dark I-V characteristics of the untreated and the PSgettered samples at 700°C, 800°C, and 900°C.

Figure 7
7I-V characteristics under AM1.5 illumination of the gettered multicrystalline solar cells.

Figure 6
6Series (R s ) (a) and shunt (R sh ) (b) resistances of the gettered multicrystalline solar cells.

Table 1
1Measured effective lifetime τ eff under low-level injectionTemperature (°C)
τ eff (μsec)

Reference
2.92

700
37.17

800
59.76

850
83.16

900
90.88

950
13.07

τ eff , effective minority carrier lifetime.

Table 2
2Comparison of the electrical parameters of phosphorous gettered and ungettered multicrystalline silicon solar cells FF, fill factor; J sc , short-circuit current density; V oc , open-circuit voltage; η, efficiency.Temperature (°C)
J sc (mA/cm²)
V oc (V)
FF
η

Reference
18.5
0.48
67
5.9

700
21.3
0.50
69.4
7.4

800
24.4
0.52
70.5
8.9

850
26.3
0.53
74.8
10.4

900
27.4
0.55
77.2
11.6

http://www.nanoscalereslett.com/content/7/1/338
Competing interestsThe authors declare that they have no competing interests.Authors' contributions DL carried out the synthesis and analysis of the experiment, fabricated the porous silicon layers, measured the minority carrier lifetime and performed the AFM imaging and IQE and I-V measurements of the gettered solar cells. DL and HE contributed to the conception and design of the experiments, data interpretation and writing of the manuscript, discussed the results, contributed to the manuscript text, commented on the manuscript and approved its final version. Both authors read and approved the final manuscript. Submit your manuscript to a journal and benefi t from:7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the fi eld 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com
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Cite this article as: Lotfi and Hatem: Phosphorus diffusion gettering process of multicrystalline silicon using a sacrificial porous silicon layer. Nanoscale Research Letters. 338Cite this article as: Lotfi and Hatem: Phosphorus diffusion gettering process of multicrystalline silicon using a sacrificial porous silicon layer. Nanoscale Research Letters 2012 7:338.