nanostructures for microelectronic applications need to be well controlled in terms of the shape , ordering and composition . since ge islands tend to nucleate randomly on flat si substrates , substrate patterning can be used to achieve controlled positioning by creating material traps . besides positioning the sige structures on the substrate , an appropriate pattern can also control their size . using electron beam lithography ( ebl ) , it is possible to vary width , depth and spacing of seed holes in order to trap the deposited material in a controlled way . similarly , it is also necessary to control the composition , since this parameter influences a number of electronic and optical properties [ 4 - 6 ] . in this paper , we use ebl and reactive ion etching ( rie ) to realize periodical seed holes in si substrates . the deposition of a few layer of ge directly on si - patterned substrates by lepecvd results in the nucleation of sige islands at the preferential sites . we investigate the impact of the lepecvd technique on composition and strain relaxation of sige islands using afm and -raman spectroscopy . pit - patterned si substrates were prepared by ebl upon a surface cleaned by an ultrasonic treatment with organic solvents . then , two layers of poly(methyl methacrylate ) were deposited by spinning ( 3,000 rpm ) and baking for 1 h at 160c . during ebl , different doses were used ( 250 , 300 , 350 and 400 c cm ) to create pits with final diameter between 80 and 300 nm . for every exposure , the patterned area was 25  25 m with pits at a regular spacing of 1 m , and 7075 nm of si was etched by rie . after rie , all samples were analyzed by scanning electron microscope and afm to check the depth and quality of etching ( fig . 1 ) . to obtain a clearer visualization of the surface in all the images shown in this work , we have used a grayscale corresponding to the surface gradient . scanning electron micrograph ( a ) and afm gradient image ( b ) of patterned region on ( 001 ) si substrate . the pit diameter was 100 nm while the depth ~70 nm . we choose the coordinate axes x and y to be parallel to the images edges , which are aligned to the   and   directions , respectively pure ge was deposited by lepecvd   directly on the cleaned patterned si ( 001 ) substrates at different temperatures between 600 and 750c . the thickness of deposited ge ranged from 0.8 to 3.2 nm , while the growth rate was either 0.015 or 0.1 nms . the distribution and size of the islands were characterized by afm , while the ge content was measured using -raman spectroscopy . all afm analyses were carried out in tapping mode using an innova  veeco instrument with ultra sharp tip ( typical curvature radius 2 nm ) . the -raman spectra were acquired in back - scattering configuration with a 532-nm excitation wavelength in order to obtain a high signal - to - noise ratio with ge - rich structures . due to self - absorption , the investigated depth corresponds to about one half of the penetration depth dp , ranging from about 10 to 40 nm in ge - rich si1xgex alloys . the spectral positions of the sige alloy vibrational bands were used to measure both the strain and ge content . 2a , the mobility of ge atoms was too low , and we obtained small islands between patterned areas . increasing the temperature ( 750c ) , the ge mobility increases , and we obtained 3d islands positioned correctly ( fig . 2c ) with only very few islands between the pits . at this temperature , we varied the amount of deposited ge to investigate the trapping effect of the holes . in the range of amount of deposited ge ( 0.93.2 nm ) following models reported in literature , this means that the migration length of ge adatoms ( with d the diffusion coefficient ) is greater than the distance between the holes . for a fixed patterning dimension , the island volume increases proportionally to the thickness of the deposited ge . from high - resolution afm scans , we can identify { 105 } , { 113 } and { 15 3 23 } facets at the sides of the islands and { 1 1 10 } facets near their tops , while the tops themselves are parallel to ( 001 ) . afm gradient images of a patterned area following the deposition of 2.8 nm of ge at 600c ( a ) , 700c ( b ) and 750c ( c ) . the growth rate was 0.1 nm s. at low temperature ( a ) , we have formation of small sige 3d structures around pits . they are also observed at intermediate temperature ( b ) , although in smaller quantity . at high temperature ( c ) , only very few islands are formed which are not on the top of pits , and some pits appear to be uncapped with islands we evaluated the effect of growth rate on the positioning and composition of the islands . the growth rates were 0.1 or 0.015 n ms . at these growth rates , keeping the other growth parameters fixed , we did not observe island formation between pits . this result has been expected , because by low growth rate we reduce the ge adatom concentration , and consequently we decrease the possibility of nucleating new islands between pits . the volume of the islands changed : the structures grown at high growth rates have a lower volume and the aspect ratio ( ar , which is defined as the height h divided by the square root of the base area a ) ; those obtained at low rates have larger volume , and the ar is comparable to islands grown by mbe at the same temperature and growth rates ( fig . the effect of ge thickness and growth rates described above has been characterized also by -raman spectroscopy ( fig . 4 ) . in table 1 , we report results on the ge content for different ge thickness deposited and growth rates . -raman analyses show that if we increase the ge thickness , the ge content in the island increases . the residual strain measured by -raman spectroscopy is always very small and of tensile nature , which probably indicates plastic relaxation at the growth temperature followed by thermal stress on cooling to room temperature . the ge content of islands within the pattern does not change significantly when the growth rate is reduced from 0.1 to 0.015 nm s despite the increase in volume . afm gradient images of a patterned region following the deposition of 2.8 nm of ge at 750c . the growth rate was a 0.015 nm s and b 0.1 nm s. in ( a ) the ar is 0.2 with a volume mean value of 7.118  10 m , while in ( b ) ar is ~0.11 with a volume of 1.380  10 m examples of -raman spectroscopy results obtained from single island inside the pattern ( black data ) and island outside the pattern ( red data ) . the data refer to islands grown with a total amount of 2.8 nm of ge with a 0.1 nm s rate with a substrate temperature of 750c . a semilogarithmic plot of the raman spectra with the indication of the three bands related to the islands ( marked with si si , si - ge and ge  ge ) and the single band related to the si bulk substrate . b composition - strain plane showing the calibration curves extracted from ref .   corresponding to the peaks shown in panel ( a ) . the intersection of the curves is highlighted with the dot and marks the value of composition and strain for the islands . ge content x on patterned and unpatterned regions for a range of deposition thicknesses and growth rates the error bars refer to the single measurement . we have demonstrated regular sige islands on prepatterned substrate formed after the deposition of ge by lepecvd . in particular , we have analyzed by afm and -raman , the effects of temperature , thickness and growth rate on the shape , volume and composition of the sige islands . optimal growth conditions for ordered ge - rich island formation were found for the chosen geometry of the pattern . the cariplo foundation is gratefully acknowledged for partially financing this research through the project mandis . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
sige islands have been proposed for applications in the fields of microelectronics , optoelectronics and thermoelectrics . 
 although most of the works in literature are based on mbe , one of the possible advantages of low - energy plasma - enhanced chemical vapor deposition ( lepecvd ) is a wider range of deposition rates , which in turn results in the possibility of growing islands with a high ge concentration . 
 we will show that lepecvd can be effectively used for the controlled growth of ordered arrays of sige islands . in order to control the nucleation of the islands , patterned si ( 001 ) 
 substrates were obtained by e - beam lithography ( ebl ) and dry etching . 
 we realized periodic circular pits with diameters ranging from 80 to 300 nm and depths from 65 to 75 nm . subsequently , thin films ( 0.83.2 nm ) of pure ge were deposited by lepecvd , resulting in regular and uniform arrays of ge - rich islands . 
 lepecvd allowed the use of a wide range of growth rates ( 0.010.1 nm s1 ) and substrates temperatures ( 600750c ) , so that the ge content of the islands could be varied . 
 island morphology was characterized by afm , while -raman was used to analyze the ge content inside the islands and the composition differences between islands on patterned and unpatterned areas of the substrate .