antimonide semiconductors have potential application in a wide range of electronic and opto - electronic devices due to their unique band - structure alignments , and small effective mass as well as high mobility for electrons [ 1 - 4 ] .
while recent technical advancements have enabled high quality lattice matched gasb epitaxy on native substrates , for many applications gaas substrates are desirable .
this is because of the following reasons : gaas is inexpensive , has favorable thermal properties , transparent to more ( long wave length ) active regions , forms excellent n and p ohmic contacts , and can be semi - insulating compared to gasb .
however , the high ( 7.8% ) lattice mismatch between the gasb epilayer and the gaas substrate complicates the growth of sophisticated device structures .
currently , this mismatch is accommodated via metamorphic buffer layers   and strain - relief superlattices . in metamorphic buffer layer approach , initially the strain within the critical thickness is accommodated by tetragonal distortion followed by defect formation and filtering . while this approach has enabled a number of device demonstrations , it exhibits several deficiencies such as the necessity to grow thick buffer layers ( often > 1 m ) , poor thermal and electrical conductivity , and has resulted in significant material degradation through the presence of threading dislocations ( tds ) . recently , a fundamentally different growth mode , interfacial misfit dislocation ( imf ) growth mode , has been developed by our group . in this growth
, the strain is relieved instantaneously at the mismatched heterointerface unlike the traditional tetragonal distortion approach that relieves the strain after reaching a critical thickness .
buffer - free  approach to realize monolithic high quality gasb deposited on gaas substrate with exceptionally low threading dislocation ( td ) densities ( ~10 cm ) , despite the high lattice mismatch .
the strain created due to the 7.8% lattice mismatch is relieved at the gasb / gaas interface by the formation of a two - dimensional ( 2d ) , periodic imf arrays comprised of pure - edge ( 90 ) dislocations along both   and [ 1 - 10 ] . to facilitate the growth of  buffer - free  deposition of gasb on gaas substrate with low td densities , in complex device structures
, it is essential to understand the structural properties of imf - grown gasb epitaxial layers .
an attempt was made previously to show the proof of existence of the imf array at the gasb / gaas interface along [ 1 - 10 ] using cross - sectional transmission electron micrograph ( xtem ) and to calculate the td density using koh etching as shown in ref . .
the xtem images look only at one - dimensional sections and hence are not representative of the 2d interface .
also , the quantitative analyses like strain relaxation of bulk gasb deposited on gaas substrates , long - range uniformity of the imf array in 2d , and accurate td density calculation for gasb that was not presented earlier , are very important in realizing high quality gasb bulk layers on gaas substrate . in this study
, all the issues addressed earlier , namely the material quality of the gasb epitaxial layer is quantified using various analyses like xtem , selective area electron diffraction ( saed ) double spot pattern , moir fringe patterns , x - ray diffraction ( xrd ) , and plan - view tem .
the samples are grown on gaas substrates in a vg v80h molecular beam epitaxy ( mbe ) reactor equipped with valved crackers for as and sb , and an optical pyrometer for monitoring the substrate temperature .
various samples comprising gasb bulk layers are grown on gaas substrates , using imf growth .
the thickness of the imf - grown gasb epitaxial layers used for various analyses range from 15 nm to 5 m .
for example , thick samples like 5 , 0.5 m are used for xrd analyses , and samples with medium thickness , like 120 nm , are used for xtem and saed analyses , respectively . for td density analysis using plan - view tems ,
very thin 15-nm sample is grown separately for moir fringe analysis to facilitate the transmission of electrons through both the epitaxial layer and the underlying substrate . the sample required for moir fringe analysis
is prepared as follows , the substrate is lapped down to ~10 m and ion milled to 30 nm , resulting in a net thickness of 45 nm that includes the 15-nm imf - grown gasb epitaxial layer .
another set of gasb bulk samples , which are similar to those of the imf samples are deposited using non - imf growth on gaas substrate for comparison with the former in various analyses as mentioned earlier . if the interface is as - rich instead of ga - rich prior to the deposition of gasb , no imf
is observed at the heterointerface and this growth mode is called non - imf growth mode .
non - imf growth is also similar to that of the imf growth up to the deposition of gaas smoothing layer . after the smoothing layer ,
ga source is turned off and the as - overpressure is on while bringing the temperature down to 510 c from 560 c .
when the substrate temperature is 510 c , the resulting surface is as - rich . at this point ,
, imf is not formed at the interface as is explained in the following paragraphs .
figure 1shows the high - resolution tem ( hr - tem ) image of the gasb / gaas interface .
the burgers circuit completed around each misfit indicates a pure - edge dislocation along [ 1 - 10 ] .
one of such misfit dislocations are shown in fig.1as a bright spot representing the imf dislocation .
hence the dislocation network associated with the imf array formation along both   and [ 1 - 10 ] is characterized as a 2d lomer dislocation network . in general ,
relaxation kinetics favors the formation of 60 dislocations over 90 dislocations as the former dislocation can glide to the surface from the interface .
however , the latter is more preferable as it is more efficient in relieving the strain compared to the 60 dislocations and can be formed under favorable conditions as shown in fig .
burgers circuit completed around one misfit dislocation of the imf array at the gasb / gaas interface shown with the help of hr - tem image , where the dislocation is shown as abright spot figure 2a shows the bright - field xtem image of a 120-nm td free imf - grown gasb epitaxial layer on a gaas substrate along zone axis .
the imf is seen as dark spots in this figure with a periodicity of 5.6 nm .
this periodicity corresponds to exactly one misfit dislocation for every 14 lattice sites of gaas or 13 lattice sites of gasb .
this value is in good agreement with the theoretical periodicity for a relaxed gasb deposited on gaas .
the strain created by the lattice mismatch is relieved spontaneously by the formation of the imf at the gasb / gaas interface .
further proof of spontaneous relaxation of imf - based samples is provided via the saed double spot pattern as shown in fig .
the highly resolved diffraction spots in saed demonstrate two separate lattice constants associated with gaas ( as = 5.65  ) and gasb ( af = 6.09  ) , respectively .
the alignment of the 000 diffraction spot with , for instance , the two 220 spots indicates that there is no lattice rotation . in the imf growth ,
a sheet of sb atoms are deposited on ga - rich gaas surface before starting the growth of bulk gasb epitaxial layer .
if sb is deposited on as - rich gaas surface instead of ga - rich gaas surface , the resulting epitaxial layer will have high defect density as shown in the bright - field xtem of fig .
axtem showing a periodic imf array with a periodicity of 5.6 nm , asdark spots , at the gasb / gaas interfacebsaed double diffraction pattern of imf growth mode , andcxtem of non - imf growth mode with high threading dislocation density compared to the imf growth mode the -2 scan of symmetric ( 004 ) xrd spectra for a 0.5- m thick gasb epitaxial layers deposited using imf and non - imf growths , and 5- m thick sample deposited using imf growth are shown in fig .
in addition to the broad full width at half maximum ( fwhm ) , the non - imf spectrum differs to the imf spectrum due to the presence of additional peak near the gaas substrate as shown in fig .
this additional peak in the non - imf sample is attributed to the tetragonally distorted gasb .
this means that initially the in - plane lattice constant of the epitaxial layer and of the substrate are equal up to critical thickness , after which the epitaxial layer slowly relaxes to the original lattice constant of gasb by relieving the strain via the formation of misfit and often threading dislocations . in non - imf spectrum , this transition of lattice constant is represented by a negative slope via the transition from additional peak to the epi - peak .
similar type of behavior was not observed in the imf samples , and hence no tetragonal distortion is attributed to the imf - grown gasb epitaxial layers .
the relaxation of the imf - grown gasb epitaxial layer is determined from the analysis of xrd .
the calculation based on the symmetric ( 004 ) and asymmetric ( 115 ) xrd measurements show approximately 98.5% ( complete ) relaxation of the gasb epitaxial layer , and similar type of relaxation is observed in gasb grown on gaas with alsb nucleation layer .
we believe that the broad fwhm ( 194 arcsecs ) of gasb layers , thinner than 1 m , as shown in fig .
3a is due to the small amount of residual strain ( < 2% ) in the epitaxial layers after the creation of the imf array . as per our observations , with thicker layers ( 5 m )
the fwhm decreases considerably to ~20 arcsecs in imf - grown gasb epitaxial layers as shown in fig .
3b . xrd ( 004 ) scan ofa0.5 m gasb on gaas substrate grown using imf and non - imf growth mode , illustrating highly relaxed gasb for the imf growth , andb5 m gasb on gaas substrate showing a narrow fwhm of ~20 arcsecs for the gasb epitaxial layer figure 4a , b show the bright - field plan - view tems imaged along zone axis   for the center and edge of the imf sample , respectively .
the average td density was calculated to be 10 cmfrom the plan - view tems . even though , no tds are observed at the center , very few tds are observed at the edge of the imf sample and are attributed to the un - optimized imf growth at sample edges .
using the plan - view tem images , the dislocation density has been calculated based on the number of dislocations within the unit area from several wafer surfaces . in the non - imf
grown gasb layers , td density is measured to be ~10cmas shown in bright - field plan - view tem shown in fig .
this confirms the fact that the td density is reduced in the imf growth compared to the non - imf growth due to spontaneous strain relaxation .
also no 60 dislocations were observed in imf - grown gasb , which indicates that the imf dislocations are non - interacting and pure - edge ( 90 ) 2d arrays .
since the 90 dislocations can relieve strain almost completely at the interface , high quality  buffer - free  gasb epilayers can be deposited monolithically on gaas substrates in the imf growth .
plan - view tem showing tds fromacenter , bedge of the imf sample , andccenter of the non - imf sample for a 5 m gasb epilayer on a gaas substrate figure 5a , b shows the two - beam bright - field plan - view temg.3g[g = ( 220 ) and ( 2 - 20 ) ] obtained from gasb epitaxial layers deposited on gaas substrates using the imf growth .
these tems show moir fringe patterns , which are the interference patterns that are formed when two crystals with different orientations or lattice constants overlap , thus providing an excellent indication of whether the epitaxial layer is strained .
the moir fringes shown here are translational moir fringes as the planes and therebygvectors are parallel to each other .
moir fringe spacing , which is defined as the spacing between two consecutive white or dark lines is measured to be 2.8 nm from fig .
5a , b. the theoretical spacing for translational moir fringes is given by : , wheredis the inter - planar spacing assuming thatdgasb = 2.155 nm anddgaas = 0.1999 nm for { 220 } reflections and is calculated to be 2.75 nm .
the measured value of 2.8 nm is in good agreement with the theoretical spacing , which again indicates that the film is fully relaxed .
plan - view tems showing moir fringes of 2d imf arrays alongab[1 - 10 ] , andc2d lomer dislocation network along both   and [ 1 - 10 ] measured using diffraction vectors ( 220 ) , ( 2 - 20 ) , and both ( 220 ) and ( 2 - 20 ) , respectively .
consecutivewhiteanddark linesrepresent moir fringes , and thewhite circlesrepresent the edge dislocations moir fringes are often used to identify dislocations in semiconductors [ 12 - 14 ] as well as metals .
5a , b , indicated by white circles illustrate the projection of pure - edge dislocations and are similar to the observations made by other groups in various material systems .
the pure - edge dislocation density from various areas of the moir fringes averages to 6.62  10 cm .
the thls in the moir fringes might also represent tds as shown in ref . .
the tds revealed in this way are attributed to the half - period shifts in the moir fringes , which are produced as a result of the interaction between 60 and 90 dislocations .
however , no half - period shifts are observed in the moir fringes of imf - grown gasb samples as shown in fig .
5a , b. moreover , no 60 dislocations are observed in the imf sample , which are considered to be the main source for the formation of td when the former interacts with the 90 dislocations .
generally , distortions local to the interface , such as stacking faults are revealed as displacements in moir fringes . in this study ,
displacement of the moir fringes is not observed in the imf samples , hence stacking faults or partial dislocations are not ascribed to the imf growth .
the moir fringes are imaged along both   and [ 1 - 10 ] using ( 220 ) and ( 2 - 20 ) g vectors as shown in fig .
the projection of 2d lomer dislocation network is observed to be uniform over a large area that was imaged ( 0.72 m ) .
in conclusion , high quality  buffer - free  gasb is grown on gaas substrates with very low td densities ( ~10 cm ) despite the high ( 7.8% ) lattice mismatch .
the strain due to lattice mismatch is relieved immediately at the gasb / gaas heterointerface with the help of periodic , pure - edge misfit ( imf ) arrays of dislocations along both   and [ 1 - 10 ] in the imf - grown gasb .
instead , if the gasb is deposited using a non - imf growth , the resulting epitaxial layer has very high td density ( 10 cm ) due to buildup of strain in tetragonal distortion . comparing the imf and non - imf samples using xrd and xtem analyses
have shown that the strain is completely ( 98.5% ) relieved in imf sample , whereas it is not the case for non - imf sample .
the plan - view tem analysis for both samples also confirmed similar results , where the td density is very low for imf sample ( ~10 cm ) compared to non - imf sample ( ~10 cm ) .
the long - range uniformity and the strain relief of the imf - grown gasb epitaxial layer measured using the moir fringe patterns have shown a uniform 2d lomer dislocation network over the entire scan area .
the moir fringe spacing of 2.8 nm agrees well with the theoretical spacing of 2.75 nm , which proves that the gasb layer is completely relaxed .
further proof of strain is also achieved from saed measurements , which shows that gasb and gaas has lattice constants almost similar to the expected lattice constants of the corresponding relaxed materials .
we believe that this approach is useful for the deposition of  buffer - free  high quality gasb on well - studied gaas substrates in complex device structures .
the authors gratefully acknowledge the financial support of afosr through fa 9550 - 08 - 1 - 0198 .
 we report structural analysis of completely relaxed gasb epitaxial layers deposited monolithically on gaas substrates using interfacial misfit ( imf ) array growth mode . unlike the traditional tetragonal distortion approach , strain due to the lattice mismatch is spontaneously relieved at the heterointerface in this growth . 
 the complete and instantaneous strain relief at the gasb / gaas interface is achieved by the formation of a two - dimensional lomer dislocation network comprising of pure - edge ( 90 ) dislocations along both [ 110 ] and [ 1 - 10 ] . in the present analysis , 
 structural properties of gasb deposited using both imf and non - imf growths are compared . 
 moir fringe patterns along with x - ray diffraction measure the long - range uniformity and strain relaxation of the imf samples . 
 the proof for the existence of the imf array and low threading dislocation density is provided with the help of transmission electron micrographs for the gasb epitaxial layer . 
 our results indicate that the imf - grown gasb is completely ( 98.5% ) relaxed with very low density of threading dislocations ( 105 cm2 ) , while gasb deposited using non - imf growth is compressively strained and has a higher average density of threading dislocations ( > 109 cm2 ) . 
Introduction
Experiments
Results and Discussion
Conclusions
Acknowledgments