gan - based 
 light emitting diodes ( leds ) have attracted a significant 
 amount of attention over the past decade , owing to their high optical 
 efficiency in the blue to uv spectral range . most research and commercial 
 products utilize the conventional polar c - plane orientation . however , due to the large discontinuities in the spontaneous and piezoelectric 
 polarization at the ingan / gan interface , large internal electric fields 
 along the c - axis separate the electron and hole wave 
 functions and result in a lower radiative recombination efficiency 
 and longer carrier lifetimes , and may contribute to efficiency droop 
 in leds . growth of non- and semipolar 
 gan - based structures is of interest due to the fact that it offers 
 a crystallographic solution to reduce the internal electric fields , 
 potentially allowing significant improvements in light emission efficiencies . semipolar ( 1122 ) gan - based 
 structures have been shown to have great potential for achieving efficient 
 green and yellow leds . gan is hindered by the lack of high quality gan pseudosubstrates with 
 low defect densities . non- and semipolar bulk gan substrates remain 
 expensive and small in size ( typically 1  0.5 cm ) , limiting their relevance to commercial consumer products . on the 
 other hand , heteroepitaxial growth of non- and semipolar gan is very 
 challenging and results in typical densities of basal plane stacking 
 faults ( bsfs ) and dislocations in excess of 10 cm and 10 cm , respectively , whereas c - plane gan templates 
 typically have a dislocation density of 10 cm and no bsfs . numerous efforts have been made to reduce 
 the defect densities in heteroepitaxial non- and semipolar gan . thus 
 far , in situ defect reduction schemes , such as the 
 application of low - temperature gan nucleation layers ( nls ) or aln nls , the use 
 of silicon nitride or scandium nitride 
 interlayers , followed by three - dimensional 
 ( 3d ) growth and coalescence , direct growth without a low - temperature 
 nl , and very high silicon doping can only reduce the dislocation density down 
 to the 10 cm regime , while the bsf 
 density remains well above 10 cm . ex situ methods , such as epitaxial lateral overgrowth ( elog ) 
 or the use of patterned substrates , appear more promising , as they 
 can more effectively reduce both dislocation and bsf densities . however , given the complex processing efforts involved , the fairly 
 limited size of the low defect density areas ( typically a few microns 
 wide in nonpolar elog ) , and/or the requirement to grow very thick 
 layers with associated wafer bowing problems , this may not be a very cost - effective way of achieving larger scale 
 non- and semipolar gan templates with very low defect densities . recently , 
 the emission efficiency of a - plane gan leds was improved 
 by incorporating silica nanospheres in between 3d gan islands , although only a modest reduction in dislocation 
 density was demonstrated and there was no observable impact on bsfs , 
 possibly due to the fact that the authors did not cover the entire 
 surface with nanospheres , thus limiting the dislocation filtering 
 efficiency . the defect reduction mechanisms of all the methods mentioned 
 above either rely on first bending and then annihilating the dislocations 
 during the subsequent regrowth process or blocking the defects with 
 an amorphous mask layer . given that the bsfs are confined on ( 0001 ) 
 planes and can not be bent easily , bsfs tend to merge and exhibit an 
 increase in the total length , so that their overall density is only 
 slightly reduced when employing dislocation filtering methods that 
 do not use physical masks . therefore , reducing the bsf densities especially 
 in nonpolar orientations remains very challenging , unless reorientation 
 of the bsfs is enabled . here we 
 demonstrate the dramatic reduction of defects in nonpolar 
 ( 1120 ) gan epilayers on r - plane sapphire 
 substrates using a porous mask formed using self - assembled multilayers 
 of silica nanospheres . during the metal  organic chemical vapor 
 deposition ( mocvd ) studies , we observed the following three phenomena : 
 ( 1 ) the crystal epitaxy is preserved during selective area deposition 
 through the nanogaps present in the multilayered silica nanospheres ; 
 ( 2 ) the formation of voids during the initial regrowth process and 
 the exposure of less defective regions in the gan seed layer below 
 the nanospheres ; and ( 3 ) lateral overgrowth and coalescence above 
 the silica nanospheres . it is shown that ingan / gan multiple quantum 
 well ( mqw ) structures grown on msn masked gan templates show significantly 
 enhanced optical performance . in addition , the msn masking technique 
 is also applied to the epitaxial growth of semipolar ( 1122 ) 
 gan , and , here too , significant defect reduction is achieved . the integration of self - assembled multilayers 
 of silica nanospheres and the subsequent gan regrowth via selective 
 area deposition are shown schematically in figure 1a . after an 1 m thick gan 
 seed layer with a smooth surface was grown on r - plane 
 sapphire by mocvd , an oxygen plasma treatment 
 was carried out for 5 min to make the gan surface hydrophilic . silica 
 nanospheres ( d = 180 nm ) dispersed in ethanol were subsequently 
 coated onto the gan seed layer using convective assembly with a varying 
 deposition rate ( mm / min ) , which can result in the controlled formation 
 of a monolayer ( ml ) or multilayers of silica nanospheres . after the silica nanosphere deposition , the 
 samples were reintroduced into the mocvd reactor for the gan regrowth . the cross - sectional sem image of a fully coalesced multilayer sample 
 prepared with a focused ion beam ( fib ) is shown in figure 1b . it is seen that there are 
 vertically elongated voids below the multilayers of nanospheres in 
 the gan seed layer , which were not present in the original seed layer 
 growth , nor in the monolayer samples , 
 and are not related to the cross - sectional sample preparation ( since 
 cleaved multilayer samples also show voids ) ( see supporting information and figures s1 ) , suggesting that these 
 voids were formed during the initial regrowth process at high temperature . ( a ) schematic 
 illustrating the regrowth process on a gan seed layer 
 covered with multilayers of silica nanospheres and the dislocation 
 reduction process ( dark lines represent the dislocations ) , viewed 
 along the c - axis . . low temperature ( 15 
 k ) ( c ) sem and cl images of a multilayer sample stopped during the 
 initial stages of growth , taken at ( d ) gan nbe emission at 3.46 ev 
 and ( e ) bsf emission energy at 3.40 ev . white dotted circles in each 
 image encompass an elongated island emitting at both gan and bsf energies . red dotted circle indicates an area of only gan emission but no bsf 
 emission originating from the gan filling the nanogaps between the 
 silica nanospheres . in conventional selective 
 area deposition methods , the gan regrowth 
 commences on the unmasked seed regions and proceeds by laterally overgrowing 
 the masked areas , and polycrystalline nucleation on the mask does 
 not occur . in this novel approach , the ensemble 
 of nanospheres does not act as a conventional mask like the lithographically 
 defined oxide stripes or patterns in an elog process , but rather as 
 a porous masking material covering the entire surface . in the example 
 of the multilayer specimen , the cross - sectional sem image in figure 1b reveals that the 
 msn is not perfectly closely packed . it shows that the regrowth was 
 initiated from the gan seed layer , filled the nanogaps between the 
 silica nanospheres , and continued beyond as a fully coalesced layer , 
 which has been first demonstrated in gaas selective area deposition . the lack of polycrystalline nucleation has been 
 confirmed by studying a multilayer sample for which the regrowth was 
 stopped at an early stage . figure 1c shows the sem of this sample and reveals that the 
 gan regrowth takes place rather homogeneously across the surface through 
 the nanogaps in between silica nanospheres but also causes the formation 
 of a number of large crystalline islands ( elongated along , 
 with { 1011 } and ( 0001 ) facets ) . low temperature ( 15 k ) cl image taken at the gan near band - edge 
 ( nbe ) emission energy of 3.46 ev ( figure 1d ) shows that there is gan nbe emission from 
 the 3d islands ( as indicated by the white circles ) as well as originating 
 from between the nanogaps ( red circles ) . furthermore , since a bsf 
 is often regarded as a type ii zincblende qw layer embedded in a wurtzite 
 matrix , these planar defects strongly luminesce 
 at an energy of 3.40 ev . the cl image taken at 3.40 ev suggests that 
 the bsfs are only present in the 3d islands ( white circles ) but could 
 not propagate through the nanogaps ( red circles ) as shown in figure 1e . by varying 
 the silica nanosphere deposition rate between 0.15 and 
 0.6 mm / min , a series of samples were prepared with multilayers and 
 ml with different silica nanosphere coverage , namely densely packed 
 ml and loosely packed ml , as shown in figure 2a  c . figure 2d  i show the room temperature sem 
 and cathodoluminescence ( cl ) data of the fully coalesced samples grown 
 on these nanosphere masked layers . for deposition rates faster than 
 0.6 mm / min , a very loosely packed ml is formed and the cl image of 
 the related overgrown sample is mostly dominated by a very large number 
 of dislocation related dark spots with less than 10% of the surface 
 appearing bright , which is as expected since most of the dislocations 
 from the seed layer can propagate freely up into the film due to lack 
 of silica nanosphere coverage and thus less blocking by the nanospheres 
 ( sample not shown here : see supporting information figure s2 ) . for the loosely packed ml ( 0.6 mm / min ) , the silica 
 nanospheres appear in bands of small clusters , and from the overgrown 
 sample , bright regions in the cl map are emerging and have been found 
 to cover 35% of the sample area ( figure 2i ) , implying that some dislocations were 
 blocked during the coalescence process . the location of threading 
 dislocations are visible as small surface depressions in the sem image 
 in figure 2f and as 
 dark spots in the cl image since threading dislocation cores are known 
 to be nonradiative recombination centers . the cl image of the densely packed ml sample ( 0.3 mm / min ) shows 
 a large bright region covering 52% of the area with a few 
 narrow bands of dark spots in figure 2h . the data show some similarities to samples grown 
 by the conventional elog process , a periodic alternation of low defect 
 density and highly defective regions is observed . in particular , there are large areas with a number of dark spots 
 ( similar to the -c wing in elog ) , and small areas 
 with almost no dark spots ( like the + c wing in elog ) 
 adjacent to the dislocation clusters ( the elog window ) . in the multilayer sample , however , the bright 
 region has been found to cover over 85% of the sample area with only 
 a small number of dark spots , indicating a significant reduction of 
 the dislocation density ( figure 2 g ) . a dislocation density of 2  10 cm is estimated for the multilayer sample by 
 counting the dark spots in the cl images ( although this may be an 
 underestimation of the dislocation density as two or more dark spots 
 may merge in cl ) , which represents 4 orders of magnitude reduction 
 from the 1  10 cm dislocations 
 originally in the seed layer . the optimal number of layers has been 
 found to be 5 , where not only can the entire surface be covered , 
 but also the nanogaps in between the silica nanospheres will be sufficiently 
 large to allow the regrowth to initiate from the underlying gan seed 
 layer . however , the silica nanosphere stack will crack upon drying 
 if the number of layers exceed 10 , in which case the regrowth 
 only occurs through the cracks ( see supporting information figure s3 ) . sem images of silica nanospheres deposited 
 on a gan / sapphire template 
 with different deposition rates : ( a ) 0.15 mm / min , ( b ) 0.3 mm / min , 
 ( c ) 0.6 mm / min . plan - view sem and room - temperature panchromatic cl 
 images of fully coalesced samples with ( d , g ) multilayer , ( e , h ) densely 
 packed ml , and ( f , i ) loosely packed ml of silica nanospheres . detailed cl analysis of the densely 
 packed ml and the multilayer 
 samples were carried out at 77 k. the cl spectrum taken from the densely 
 packed ml sample shows emission peaks at 3.46 ev , which can 
 be assigned to the donor bound exciton ( dx ) in gan , and 
 at 3.40 ev , which is bsf related emission . on the other hand , the multilayer sample shows a very strong dx emission 
 at 3.46 ev and only a weak bsf emission at 3.40 ev 
 ( figure 3a ) . additionally , 
 the free exciton peak ( fx ) observed on the high energy 
 side of the dx peak at 3.51 ev indicates a high 
 optical quality and thus low defect density of the multilayer sample . note that the small shift ( 7 mev ) in the dx peak 
 between the two samples can be explained by the presence of different 
 strain states in the samples . assessment 
 of the defect reduction efficiency and improvements in the material 
 quality is obtained by evaluating the cl intensity ratio of the gan 
 bound exciton to the bsf emission . this ratio is found to be 5.8 for 
 the multilayer sample , which is 2.5 times larger than that 
 of the densely packed ml sample and 5 times larger than that 
 of an elog sample ( 1.1 ) measured 
 under the same conditions in cl ( averaged across the elog sample over 
 the same area size including both the wing and window regions ) . in 
 addition , cl images taken at different emission energies show that 
 the gan dx emission is only visible in the bright bands 
 in the densely packed ml sample ( figure 3b ) , as the large number of dislocations may 
 have quenched the emission from the other regions . also , a large number 
 of bright linear features along the [ 1100 ] direction are seen 
 in the bsf emission image ( figure 3c ) . the multilayer sample shows a uniform gan dx emission across the cl image shown in figure 3e , and a much - reduced number of bsf related 
 bright linear features in figure 3f . an average bsf density of 3200 cm has been measured for the multilayer sample by measuring the bright 
 linear features . although this is likely to be an underestimation 
 of the bsf density , as these bright linear features may involve a 
 bundle of bsfs , this provides a lower limit for the bsf density present 
 in the multilayer sample . ( a ) low temperature ( 77 k ) cl spectra and monochromatic 
 cl images 
 of ( b and c ) densely packed ml and ( d  f ) multilayer samples 
 taken at various emission energies corresponding to fxgan at 3.51 ev , dxgan at 3.46 
 ev , bsf emission at 3.40 ev . cross - sectional transmission electron microscopy ( tem ) was 
 used 
 to further study the microstructure of the multilayer sample and gain 
 insights into the regrowth and defect reduction mechanisms . figure 4a shows a high - angle 
 annular dark field scanning tem ( haadf - stem ) image taken at the conditions 
 where the direction of the electron beam was parallel to the 0001 
 zone axis . it reveals the microstructural transitions from the sapphire 
 substrate to the seed gan layer , presence of voids in the seed layer , 
 to the incorporated multilayers of nanospheres and regrown gan layer , 
 where a dramatic reduction of defects in the regrown gan layer can 
 be seen due to the introduction of the msn . a few dislocations are 
 visible in the regrown gan layer and are labeled with blue arrows . 
 in order to further analyze the defects , weak beam dark field ( wbdf ) 
 imaging figure 4b is a wbdf image acquired at g - g ( g = 1120 ) conditions close to the c zone axis from 
 the same sample as in figure 4a . under these diffraction conditions , a - type 
 and ( a+c)-type dislocations and some partial 
 dislocations in addition , several prismatic stacking faults ( psfs ) are also visible , 
 as marked by the yellow arrows . this image confirms that the dislocation 
 density in the gan above the msn is much lower than that in the gan 
 seed layer , verifying that the dislocations are efficiently blocked 
 by the msn . there are several underlying dislocation reduction mechanisms 
 that might simultaneously play important roles for achieving such 
 a low dislocation density in the multilayer sample . first , the formation 
 of voids in the seed layer suggests that materials have evaporated 
 through the nanogaps in the msn during the initial regrowth process , 
 with the exposed surfaces acting to terminate some defects , similar 
 to the scn interlayer case . second , the 
 dislocations are blocked at the gan / nanosphere interface , similar 
 to dislocation blocking at an elog mask . a third mechanism is the 
 nucleation and epitaxial growth of gan through the channels in the 
 msn ( as shown earlier in figure 1c ) and the subsequent lateral coalescence process . dislocation annihilation resulting from island coalescence is evidenced 
 by the formation of half - loops between bent - over dislocations during 
 lateral growth , as marked by the blue arrow with an asterisk in figure 4a and 4b . ( a ) haadf taken at the c - zone 0001 
 axis and ( b ) wbdf image taken using g = 1120 
 at close to the c - zone 0001 axis , 
 and ( c ) bf image viewed close to the a - zone 1120 
 axis , such that the bsfs are visible . ( a ) and ( b ) are from the same 
 tem sample , where the blue arrows point out several dislocations and 
 the yellow ones mark the psfs . the white arrow marks the position 
 of a coalescence boundary , and the pink arrows indicate the bsfs in 
 ( c ) . figure 4c shows 
 a bright field ( bf ) tem image taken from another sample , in which 
 the sample was oriented at close to the < 1120 > zone 
 axis 
 so that the bsfs are in contrast . the bsfs in the regrown gan layer 
 are observed as narrow dark bands extending toward the sample surface 
 some of which are labeled with pink arrows . in contrast , in the gan 
 seed layer , it is almost impossible to distinguish individual bsfs 
 in the gan seed layer since their density is so high . a closer look 
 at the gan / silica nanosphere interface reveals that the majority of 
 the bsfs have been blocked by the msn , but new bsfs appear to have 
 been initiated at the interfaces between the silica nanosphere and 
 the regrown gan layers , possibly being formed during island coalescence . the bsf density above the silica nanospheres 
 can be estimated using these tem images , giving a value of 3 
  10 cm , which is a higher value 
 than the bsf densities measured using the cl data . it should be noted 
 that the estimation of the bsf density in this case assumes that all 
 bsfs run across the entire thickness of the tem foil , so this value 
 represents an upper bound on the bsf density . the 
 differences in the tem and cl data can be further explained by the 
 limited size of the tem specimen . overall , these data show that the 
 use of a self - assembled msn can dramatically reduce the dislocation 
 density in nonpolar a - plane gan with a simple one - step 
 regrowth process and can also reduce the bsf density by one or 2 orders 
 of magnitude compared to the seed layer which has a bsf density 1 
  10 cm . to assess the 
 impact of the change in defect densities on the optical 
 efficiency of qw overgrown layers , 5 periods of ingan / gan mqws were 
 grown with an in fraction ( x ) of 0.15 in inxga1xn , and qw and barrier 
 thicknesses of 4  0.2 nm and 7  0.2 nm on samples with 
 and without the msn ( essentially a gan seed layer ) . temperature dependent 
 pl spectra of the samples with msn and without msn are shown in figure 5 , where the qw emission 
 peak at 7 k is centered at 2.8 ev and 2.6 ev , respectively . it is known that the use of silica nanospheres can result in less 
 strained gan layers . although the two 
 samples were grown at the same time , the sample without the msn may 
 exhibit much greater wafer bowing and inhomogeneity in the ingan growth 
 temperature and thus give rise to a different qw emission energy . in addition , the differences in the nanoscale surface morphology and 
 indeed local differences in indium incorporation at dislocation and 
 stacking faults will have a greater effect on the qw emission energy 
 in the sample without the msn . in both samples , a weaker and broader 
 emission band at 2.2 ev is present and is attributed to impurity - related 
 recombination ( on the basis of the very long pl decay times ( > 10 
 s ) . 
 at 7 k , the pl decay time of the qw emission is around 400 
 ps ( see inset of figure 5a ) , consistent with the absence of built - in electric fields across 
 the qw . the monotonic temperature dependence 
 of the qw pl peak energy shift and its magnitude ( 160 mev ) 
 is also consistent with other reports on the optical properties of 
 nonpolar ingan qws . due to the complex interplay between the bsf emission 
 and the true qw emission in the temperature dependent pl data , it is not possible to analyze the data to extract 
 meaningful internal quantum efficiencies . nonetheless , it is notable 
 that the ratio of the integrated qw pl intensity of the msn sample 
 between 7 k and room temperature is more than 33 times larger than 
 that of the sample without the msn . we attribute this large enhancement 
 to the greatly reduced defect densities in the gan template prepared 
 by the msn method . to our knowledge , this marks by far the largest 
 enhancement in the emission efficiency of nonpolar a - plane ingan / gan mqws when compared to elog or other patterning techniques . furthermore , even at 7 k , the pl intensity of the qw emission ( and 
 impurity emission ) in the sample with msns is markedly greater ( by 
 a factor of 5 ) than it is for the sample with no msns . this 
 is likely to be due to enhanced light extraction efficiency related 
 to the refractive index contract between the gan and the silica nanospheres , 
 i.e. the msn acts as a reflector . temperature 
 dependent photoluminescence ( pl ) spectra taken from 
 samples ( a ) with and ( b ) without the integration of msn in the gan 
 template ( essentially a gan seed layer ) . inset shows a pl decay spectrum 
 taken at 7 k , revealing a short exciton lifetime 400 ps . to demonstrate 
 the applicability of the msn technique to gan grown on other orientations , 
 it has also been applied to reduce the defect density in semipolar a cross - sectional sem image of an uncoalesced semipolar multilayer 
 sample and a schematic representation of the regrowth process are 
 shown in figure 6a 
 and 6b , respectively . the growth conditions 
 were chosen to achieve an enhanced growth rate along the   direction . gan grows through the gaps between the silica nanospheres , and then 
 islands grow along the inclined c - axis ( 32 
 to the film surface ) and coalesce . at the resulting voids or coalescence 
 boundaries , both dislocations and bsfs therefore , in the case of incorporating 
 multilayers of silica nanospheres in the growth of semipolar ( 1122 ) 
 gan , the growth and defect reduction mechanisms appear to be similar 
 to the asymmetric elog process.figure 6c shows a cross - sectional sem image of the 
 fully coalesced multilayer sample , which reveals the presence of a 
 coalescence void above the silica nanospheres ( red dotted circle ) . 
 as in the nonpolar multilayer sample , there are also voids formed 
 in the seed layer during the initial regrowth process , which were 
 not observed for the ml samples ( see supporting information figure s4 ) . coalescence boundaries can be seen 
 in the cl image in figure 6d , which have been marked by the white dashed line . ( a ) sem image 
 of the semipolar ( 1122 ) gan regrowth at an 
 early stage and ( b ) schematic illustration of the crystal growth and 
 defect reduction process ( dark lines represents the defects , and dotted 
 lines indicate the position of the coalescence boundaries ) . room temperature 
 cross - sectional ( c ) sem and ( d ) cl images of the fully coalesced multilayer 
 sample , where a coalescence void ( red dotted circle ) and the coalescence 
 boundaries ( white dashed lines ) can be seen . similar to the nonpolar gan , the room temperature cl images 
 of 
 the ml samples are mostly dominated by a very large number of dislocation - associated 
 dark spots ( see supporting information figure s5 ) . figure 7a and b shows the room temperature sem and cl images of the multilayer 
 sample . the multilayer sample exhibits very large bright areas with 
 many fewer dark spots , suggesting that the msn has reduced the dislocation 
 density significantly . the dislocation density in the multilayer sample 
 has been measured to be 3  10 cm by counting the dark spots observed in the cl images . note that 
 there are some chevron - shaped surface features pointing toward [ 1123 ] 
 present in all samples as shown in figure 7a , which are also evident in the corresponding 
 cl image in figure 7b . these are commonly observed in ( 1122 ) gan and have been 
 attributed to asymmetric growth fronts and the subsequent coalescence 
 process . it is worth noting that the presence of 
 dislocation clusters might imply that additional dislocations were 
 generated to compensate the misorientation between growth facets during 
 the coalescence process . plan - view room temperature 
 ( a ) sem and ( b ) cl images of fully coalesced 
 multilayer sample . ( c ) low temperature cl spectra taken from both 
 multilayer ( red ) and densely packed ml samples ( black ) . ( d ) cl image 
 taken at the bsf emission energy at 3.37 ev from the multilayer sample . low temperature ( 77 k ) cl spectra 
 of the multilayer and the densely 
 packed ml samples are shown in figure 7c . both samples exhibit very strong luminescence from 
 the gan dx recombination at 3.46 ev , relatively 
 weak emission from the bsfs at 3.37 ev , and negligible psf 
 emission at around 3.28 ev . bsf emission features can be seen as bright 
 linear features along the [ 1100 ] direction in the monochromatic 
 cl image ( figure 7d ) . a bsf density of 2400 cm has been estimated 
 and determined by the cl images . and the cl intensity ratio between 
 the dx and the bsf emission has been found to be 9.6 
 for the multilayer sample , which is more than two times larger than 
 that of an asymmetric semipolar elog sample ( 4.3 ) measured under the same conditions in cl ( averaged 
 across the elog sample over the same area size including both the 
 wing and window regions ) . other samples with mls of silica nanospheres 
 have also been explored ( as in the former nonpolar case ) , but the 
 msn gives the best results for both dislocation and bsf reduction . given that defect reduction is achieved via island nucleation through 
 the nanogaps between the silica nanospheres in a self - assembled fashion , 
 a small number of bsfs can still propagate up to the top surface due 
 to local nonuniformity in the initial island growth and the subsequent 
 coalescence and defect blocking process . further optimization of the 
 msn deposition and growth conditions is thus needed to achieve bsf - free 
 ( 1122 ) semipolar gan . epitaxial integration of self - assembled multilayers of silica nanospheres 
 in heteroepitaxial growth of non- and semipolar gan is demonstrated 
 as a new defect reduction technique , which not only reduces the dislocations 
 down to 10 cm and 3  
 10 cm , respectively , but also allows 
 us to significantly reduce the bsf density by more than an order of 
 magnitude in both cases . the strong luminescence enhancement in nonpolar 
 ingan / gan mqws grown on the template prepared by the msn technique 
 is believed to relate to both improved internal quantum efficiency 
 and enhanced light extraction , demonstrating that this is a successful 
 multifunctional approach to improving optical performance . the msn 
 technique could be beneficial for heteroepitaxial growth of other 
 lattice - mismatched materials , such as gan - on - silicon and cubic gan . the non- and semipolar gan samples were grown by mocvd in a 6  2 in . thomas swan close - coupled 
 showerhead reactor on r - plane ( 1102 ) and m - plane ( 1100 ) sapphire substrates , respectively . hydrogen and nitrogen were used as carrier gas for gan and ingan / gan 
 qws , respectively . nonpolar ( 1120 ) gan seed layer of 1 
 m was first grown with a v / iii ratio of 60 at 1050 
 c , following a 30 nm gan nucleation layer grown at 540 c 
 and 500 torr . this a - plane gan seed layer typically has a dislocation density of 2  
 10 cm , and a bsf density of 1  
 10 cm . semipolar ( 1122 ) gan 
 seed layer of 2 m was grown under similar growth conditions 
 as the nonpolar seed layer , but with a v / iii ratio of 800 
 and has a dislocation and bsf density of 3.0  10 cm and 3.2  10 cm , respectively . then the wafers 
 were followed by an oxygen plasma treatment for 
 5 min to make the gan surface hydrophilic . silica nanospheres ( d = 
 180 nm , homemade ) were dispersed 
 in ethanol and the nanosphere suspension contained 0.6% v / v of silica . coating on the gan seed layer was performed by injecting 0.2 ml of 
 suspension at the edge of an inclined , precleaned glass slide ( 25 
  75 mm ) and pulling the slide at a constant angle to the surface 
 away from the meniscus using a syringe pump as a linear motor . at 
 room temperature , the following speeds resulted in 0.15 mm / min  multilayer , 
 ( b ) 0.3 mm / min  dense , but less than close - packed monolayer 
 coverage , and ( c ) 0.6 mm / min  loose monolayer coverage . regrowth 
 was initially carried out with a v / iii ratio of 60 at 1050 c 
 and 100 torr to promote lateral growth and completed by the growth 
 of another 2 m of gan with a v / iii ratio of 740 to improve 
 luminescence properties . sem - cl 
 studies were performed on a liquid helium cooled stage in 
 a philips xl30s sem operating at 5 kv and equipped with a gatan monocl4 
 system . a dual beam focused ion beam ( fib ) microscope ( fei helios 
 nanolab ) was used to prepare tem lamellas whose surface normals are 
 close to the c - zone 0001 and m - zone 1100 axis respectively , according 
 to the standard in situ lift - out technique . figure 1b is a cross - sectional 
 image of the sample which was taking before being lift - out . two fei 
 tem microscopes ( fei tecnai osiris and f20 ) with field - emission guns 
 were used to investigate the samples at operating voltage 200 kv using 
 bright - field and dark - field imaging modes and haadf - stem mode where 
 applicable . in order to bring bsf in contrast , the tem lamella with 
 the m - zone 1100 normal was 
 tilted to be approximately adjacent to the a - zone 
 1120 axis . pl was excited using pulsed excitation from a 3.147 ev diode laser , 
 using an average excitation density of 100 w / cm . the emission was collected and focused into a spectrograph equipped 
 with a uv enhanced ccd array . the sample temperature was controlled 
 using a he - flow microstat . to record the pl transients , the spectrally 
 dispersed signal was detected using a photomultiplier tube and processed 
 using time - correlated single photon counting .
non- and semipolar 
 gan have great potential to improve the efficiency 
 of light emitting devices due to much reduced internal electric fields . 
 however , heteroepitaxial gan growth in these crystal orientations 
 suffers from very high dislocation and stacking faults densities . 
 here 
 , we report a facile method to obtain low defect density non- 
 and semipolar heteroepitaxial gan via selective area epitaxy using 
 self - assembled multilayers of silica nanospheres ( msn ) . 
 nonpolar ( 1120 ) 
 and semipolar ( 1122 ) gan layers with high crystal quality 
 have been achieved by epitaxial integration of the msn and a simple 
 one - step overgrowth process , by which both dislocation and basal plane 
 stacking fault densities can be significantly reduced . 
 the underlying 
 defect reduction mechanisms include epitaxial growth through the msn 
 covered template , island nucleation via nanogaps in the msn , and lateral 
 overgrowth and coalescence above the msn . 
 ingan / gan multiple quantum 
 wells structures grown on a nonpolar gan / msn template show more than 
 30-fold increase in the luminescence intensity compared to a control 
 sample without the msn . 
 this self - assembled msn technique provides 
 a new platform for epitaxial growth of nitride semiconductors and 
 offers unique opportunities for improving the material quality of 
 gan grown on other orientations and foreign substrates or heteroepitaxial 
 growth of other lattice - mismatched materials .