Growth model for plasma-assisted molecular beam epitaxy of N-polarand Ga-polar InxGa1−xN

Digbijoy N. Natha�

Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43210

Emre GürDepartment of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43210and Department of Physics, Faculty of Science, Atatürk University, 25240 Erzurum, Turkey

Steven A. Ringel and Siddharth RajanDepartment of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43210

�Received 27 August 2010; accepted 12 February 2011; published 10 March 2011�

The authors have developed a comprehensive model for the growth of N-polar and Ga-polarInxGa1−xN by N2 plasma-assisted molecular beam epitaxy. GaN films of both polarities werecoloaded and InxGa1−xN was grown in the composition range of 0.14�x�0.59 at different growthtemperatures keeping all other conditions identical. The compositions were estimated by triple-axis�-2� x-ray diffraction scans as well as by room temperature photoluminescence measurements. Thedependence of the In composition x in InxGa1−xN on growth temperature and the flux of incomingatomic species is explained using a comprehensive growth model which incorporates desorption ofatomic fluxes as well as decomposition of InN component of InxGa1−xN. The model was found tobe in good agreement with the experimental data for InxGa1−xN of both polarities. A N-polarIn0.31Ga0.69N / In0.05Ga0.95N multi-quantum-well structure grown with conditions predicted by ourgrowth model was found to match the compositions of the active layers well besides achieving asmooth surface morphology at the quantum-well/barrier interface. The understanding of growthkinetics presented here will guide the growth of InxGa1−xN for various device applications in a wide

range of growth conditions. © 2011 American Vacuum Society. �DOI: 10.1116/1.3562277�

I. INTRODUCTION

The ternary InxGa1−xN alloy system has been receivingwide attention especially for optoelectronic device applica-tions such as blue/green lasers and light emitting diodes�LEDs�. With its band gap �0.7–3.4 eV� spanning the entirevisible and near infrared spectrum, InxGa1−xN has also at-tracted interest for possible multijunction solar cells1 since itencompasses almost the entire solar spectrum. AlthoughInxGa1−xN based blue diodes and white light sources arenow commercially available, achieving emission in the green��520 nm� and longer wavelength range with high effi-ciency and acceptable output power still poses severe chal-lenges. It is difficult to grow high quality InxGa1−xN withhigh In-mole fractions in the range of 0.25�x to achieveemissions in the longer wavelengths because the optimalgrowth conditions for the alloy components InN and GaN aresignificantly different. While a steady-state Ga metal bilayercoverage at optimal growth temperatures ��710 °C� en-hances adatom diffusion leading to superior surface and ma-terial qualities for plasma-assisted molecular beam epitaxy�PAMBE� growth of GaN,2 InN growth by MBE is limited to�500 °C because of its inherent thermal instability and de-composition temperatures lower than that of desorption ofmetallic In.3 This is significantly lower than optimal growthtemperatures needed for the growth of GaN ��710 °C�

a�Author to whom correspondence should be addressed; electronic mail:

nathd@ece.osu.edu

021206-1 J. Vac. Sci. Technol. B 29„2…, Mar/Apr 2011 1071-1023/2011/2

which therefore deteriorates the quality of InxGa1−xN filmsgrown. More recently however, PAMBE grown Ga-polar In-GaN based LEDs have been shown to exhibit output powersof 9 mW at 385–400 nm �Ref. 4� and 3 mW at 450 nm �Ref.5� at injection currents of 1 and 0.1 A, respectively, which,however, are still far from achieving green or longer wave-length ranges.

Recently, the reversed direction of polarization of GaN,i.e., N-polar orientation, was explored to exploit the advan-tages for high-speed performance of highly scaledtransistors.6,7 It was also demonstrated that PAMBE growthof N-polar InN can be done at approximately 100 °C higherthan the thermal dissociation limit of In-polar InN.8–11 Thisimplies that N-polar InGaN with higher In compositions canbe grown at higher growth temperatures than In�Ga�-polarInGaN, and films with lower point defect incorporation andbetter optical properties may be expected. In addition, thereversed direction of polarization may have other advantagesfrom the device design perspective. However, N-polar In-GaN growth reports have been few so far12,13 due to thechallenges in the growth of high composition InGaN. Thispaper aims to provide a comprehensive understanding of Inincorporation in N-polar InGaN as compared to Ga polarwith a quantitative growth model that explains the variationof In-mole fraction in InxGa1−xN with respect to the changein the growth temperatures. There have been reports pub-lished earlier modeling the MBE growth of InxGa1−xN,14–16

but they were reported before the correct estimation of InN

021206-19„2…/021206/7/$30.00 ©2011 American Vacuum Society

Nath et al.: Growth model for plasma-assisted molecular beam epitaxy021206-2 021206-2

band gap as �0.7 eV. Thus, the model proposed here ismore accurate in light of a revised InN band gap.

II. EXPERIMENTAL DETAILS AND UNDERLYINGPHYSICS

Samples used in this study were grown by PAMBE in aVeeco Gen 930 system equipped with standard effusion cellsfor Ga and In. Active nitrogen was supplied using a Veeco rfplasma source. The substrates used were N-polar freestand-ing LED quality GaN templates �dislocation density of�108 cm−2� obtained from Lumilog and Ga-polar GaN onsapphire template �dislocation density of �109 cm−2� fromKyma. Both N-polar and Ga-polar templates were cleavedinto �1�1 cm2 sized pieces and coloaded on a single sili-con wafer using indium bonding to ensure identical growthconditions. This was to make sure that the difference in Inincorporation for InxGa1−xN of both polarities at a particulargrowth temperature could be studied. Since sapphire has alower thermal conductivity than GaN, the surface tempera-ture �using optical pyrometer� of the Ga-polar sample wasfound to be 3–5 °C higher than that of the N-polar sample.All InGaN layers discussed in this work were grown for 1 hwith a standard GaN growth rate of 5 nm/min. However, dueto decomposition of InGaN, the actual thickness of the filmsis expected to be less than expected from the nominal growthrate �i.e., 300 nm�.

Three sets of growths were done at growth temperaturesof 500, 550, and 600 °C in an In-rich regime, the tempera-tures being stabilized prior to growth. The growth tempera-ture was monitored by an optical pyrometer with readingscalibrated against the melting point of Al. The In incorpora-tion depends on the growth temperature and on the Ga/N-flux ratio;7 the Ga flux was kept constant at 9.5�10−8 Torr which corresponds to �0.39 times the stoichi-ometric GaN growth rate. A rf power of 350 W and N2 flowrate equivalent to 1.9�10−5 Torr of beam flux monitor pres-sure corresponding to a stoichiometric growth rate of 5 nm/min were used throughout. All growths were done in In-richregime with an In flux of 5�10−7 Torr which is more thantwice the stoichiometric Ga flux. We have assumed a “strongstoichiometric condition”15 which refers to the assumptionthat all impinging Ga atoms would be incorporated into thelattice and only those sites which have not been Ga occupiedwould be available for In incorporation. This translates intothe fact that a variation in In incorporation is simply depen-dent on growth temperature and Ga flux in a N-limitedgrowth regime.

Light emission from the N-polar and Ga-polar InGaNsamples was collected by 1-m-long monochromator with aruled grating of 1200 g/mm placed at surface normal and thesignals were detected by Hamamatsu R2658 photomultipliertube mounted exit slit of the monochromator. A StanfordResearch System SR830 DSP lock-in amplifier was used toanalyze the data. Ar ion laser with a wavelength of 488 nmwas used as an excitation light source. The photolumines-cence �PL� excitation density is 276 mW cm−2. A control

sample was used to confirm that the measurements were per-

J. Vac. Sci. Technol. B, Vol. 29, No. 2, Mar/Apr 2011

formed in identical conditions so that PL intensity valuesbetween each measurement performed can be comparable.All measurements were done at room temperature. Tripleaxis �-2� x-ray diffraction �XRD� scans of the samples were

performed symmetric around on-axis �0002� and �0002̄� forGa polar and N polar, respectively, using a Bede high reso-lution XRD system with Cu K�1 radiation �k=1.540 56 Å�and a Ge hybrid monochromator. The atomic force micro-scope �AFM� scans were performed using a Veeco DI 3000AFM equipment in tapping mode configuration.

III. RESULTS

Figure 1 shows the �-2� triple-axis scans of the N-polarfilms. All N-polar samples showed a single InGaN peak in-dicating the absence of phase segregation or compositionalnonuniformity in the range of compositions �0%–58%� ex-plored here. Using the growth model �derived later in thiswork�, the thicknesses of all N-polar and Ga-polar InxGa1−xNfilms are at least 150 nm. Since this is much higher than thecritical thickness17,18 corresponding to the relaxation ofInxGa1−xN on GaN for compositions obtained in this study,we can safely assume all the films to be fully relaxed. The Incompositions for the N-polar InxGa1−xN films as extractedfrom XRD data assuming complete relaxation are indicatedbeside each plot �Fig. 1�.

In Fig. 2, the variation of the In compositions of theInxGa1−xN films of both polarities as obtained from roomtemperature PL peaks and XRD scans as a function ofgrowth temperature is shown. The energy band-gap valuesextracted from the room temperature PL peaks were used todetermine the In compositions in the films using bowing pa-rameter of b=1.8 eV from Ref. 19 with a GaN band gap=3.4 eV. The wavelength corresponding to the lowest com-position Ga-polar InGaN ��14% from XRD� grown at600 °C was outside the range of the PL setup since its emis-sion wavelength is expected to be lower than the excitationlight source wavelength �488 nm, �2.541 eV�. The compo-sitions determined from PL measurements are close to thoseobtained from XRD scans using linear interpolation of lattice

FIG. 1. �Color online� High resolution XRD �-2� triple-axis scans of theN-polar InGaN films showing no phase segregation or compositionalnonuniformity.

constants �Vegard’s law�. The small mismatch between the

Nath et al.: Growth model for plasma-assisted molecular beam epitaxy021206-3 021206-3

two ways of composition estimation can be attributed to un-certainty in bowing parameter as well as to the nonideality ofVegard’s law. Also for PL, the energy band gaps have beenextracted from room temperature PL peak positions and re-lated these to the In content. The PL peak position, however,cannot be related precisely to the band gap. Because of thetypical higher energy of the PL peak positions with respect tothe actual band gap, this may cause decreased In contents asderived from PL. From Fig. 2, it is clear that In incorporationdrops as growth temperature increases due to higher decom-position of InN at higher temperatures irrespective of thepolarity. Besides, for a given growth temperature, In incor-poration is higher for N-polar InxGa1−xN than for Ga-polarInxGa1−xN. This implies that N-polar InxGa1−xN can begrown at a higher temperature for a specified In-mole frac-tion than Ga-polar InxGa1−xN.

IV. DISCUSSION

We developed20 a growth model for N-polar InxGa1−xNbased on InN decomposition to explain the growth tempera-ture and Ga-flux dependent compositional variation of In-mole fraction and later extend it for Ga-polar InxGa1−xN aswell. The model has been developed for the metal richN-limited growth regime where the active N flux availabledetermines the nominal growth rate with an excess of Incoverage on the surface. Thus, the composition of In inInxGa1−xN samples is independent of In atoms impinging on

11

FIG. 2. �Color online� Indium mole fraction as a function of growth tem-perature for both Ga and N-polar InGaN films as extracted from room tem-perature PL peaks and XRD scans.

or desorbing from the surface. It has been shown that

JVST B - Microelectronics and Nanometer Structures

N-polar InN decomposition in the metal rich growth regimecan be considered to be N atoms leaving the surface �inset ofFig. 3�. If FN is the nitrogen-stoichiometric flux defining thegrowth rate in the absence of decomposition, then the actualstoichiometric flux FN�

defining the true growth rate as InNdecomposes is less than FN by a decomposition rate FD,

FN�= FN − FD �1�

The In composition, x, in an InxGa1−xN film is related to theeffective �reduced� growth rate FN� and the incident Ga flux,FGa, by the equation �assuming strong stoichiometriccondition15�

x = 1 −FGa

FN� . �2�

We assume that the decomposition rate FD is proportionalto the In-mole fraction x in the InxGa1−xN film �so that for apure GaN film, there will be no decomposition as expected�.The decomposition rate FD is also shown to have an Arrhen-ius dependence on substrate temperature Tsub,

11

FD�x exp�−Ea

kT� , �3�

where Ea is the activation energy of decomposition ofN-polar InN which has been calculated earlier to be 1.2 eV.11

Using Eqs. �1� and �4� in Eq. �3�, the In-composition x in theInxGa1−xN film can be expressed as a function of

FIG. 3. �Color online� Indium mole fraction as a function of growth tem-perature for various Ga/N-flux ratios as predicted by our growth model forN-polar InxGa1−xN.

temperature,

x =�1 + �e�−Ea/kT�� − ��1 + �e�−Ea/kT��2 − 4�e�−Ea/kT��1 − FGa/FN�

2�e�−Ea/kT� , �4�

where � is the proportionality factor of Eq. �3� normalized tothe value of nitrogen flux.

The activation energy Ea=1.2 eV obtained in Ref. 11 isin the temperature range of 590–635 °C and so using this

Nath et al.: Growth model for plasma-assisted molecular beam epitaxy021206-4 021206-4

value of Ea might not be accurate for a temperature range of450–650 °C modeled in our study. It has been pointed outthat16 In loss cannot be accounted for simply by a singlethermally activated process since the surface stoichiometricmodifies the process behind it, implying a temperature de-pendence of Ea. However, the exact dependence of Ea forInN decomposition on growth temperature is not yet wellunderstood or derived and hence we shall use the value re-ported in Ref. 11 for our work here with a room for betteraccuracy provided we know Ea as a function of temperature.It is also noteworthy that in Ref. 14, the activation energy forInxGa1−xN decomposition is reported to be 3.5 eV as ex-tracted from the plot of the ratio of InN loss to In composi-tion vs 1 /kT. However, a value of activation energy ofInxGa1−xN decomposition reported in Ref. 14 would requirea revision now in the light of a revised and correct InN bandgap of �0.7 eV. From the data points obtained in this study,we plotted the ratio of FD /xIn vs 1 /kT �not shown here� andfrom the slope, derived Ea�1.5 eV which is close to 1.2 eVreported in Ref. 11. The difference springs from various fac-tors such as dependence of Ea on growth temperature, valueof 1.2 eV �Ref. 11� reported for 590–635 °C range, anduncertainty in precisely determining the In composition�from both PL and XRD�.

Using Eq. �4� and the data points of In composition cor-responding to the three N-polar InxGa1−xN samples, a curvefitting was done to evaluate �. The resulting variation in x asa function of substrate temperature as obtained from ourmodel is plotted in Fig. 3. The different curves correspond todifferent Ga/N-flux ratios. As can be observed, In incorpora-tion x decreases as Ga/N-flux ratio increases for a givengrowth temperature. Thus, using the curves as guidelines, arequired In composition x can be achieved either by choosingthe growth temperature and determining the Ga/N-flux ratiofrom the plots or vice versa. This growth model thus pro-vides comprehensive flexibility in obtaining higher In incor-poration even at higher growth temperatures by choosing therequired Ga/N-flux ratio.

To validate our growth model, we plotted the experimen-tally obtained data points �i.e., In composition x� for N-polarInxGa1−xN for a Ga/N-flux ratio of 0.39 used in this work. Asshown in Fig. 4, the data points are found to be in goodagreement with the curve predicted by our growth model.Besides, using the same approach as used for N-polarInxGa1−xN, we use the same set of equations for modelingthe growth of Ga-polar InxGa1−xN. The Ea for decompositionof In-polar InN, however, has been investigated earlier forlower growth temperatures only ��500 °C� and found to be1.92 eV.21 The prefactor relating the decomposition rate togrowth temperature and In-mole fraction �Eq. �3�� is ex-pected to be the same for growth kinetics of both Ga-polarand N-polar InxGa1−xN since it accounts for any factor whichis not temperature dependent. More precisely, the prefactor isrelated to the number of planar sites containing N atoms onthe surface because that is what is assumed to define thedecomposition of InN apart from temperature and In-mole

fraction. In simple words, with a different prefactor for Ga-

J. Vac. Sci. Technol. B, Vol. 29, No. 2, Mar/Apr 2011

polar polarity, the higher incorporation of In in N-polarInxGa1−xN at the same growth temperature can no longer beattributed to a difference in Ea only. Borrowing the value ofprefactor from the growth model developed for N-polar po-larity of InxGa1−xN, we use a Ga/N-flux ratio of 0.39 used inthis work with Ea as the unknown while fitting the datapoints for Ga-polar InxGa1−xN. The activation energy for Ga-polar polarity is thus found to be 1.12 eV against 1.2 eV forN-polar polarity. Again, the data points are found to be innice fit with the model as shown in Fig. 4. Several datapoints showing In compositions and growth temperatures forGa-polar InxGa1−xN by PAMBE from the literature4,22–24

have also been included in Fig. 4 as a comparison. Thehigher In incorporation for N-polar polarity is obvious fromFig. 4.

The decomposition rate can be quantitatively estimated asFD�x exp�−�Ea /kT��, and therefore the reduced growth rateFN�

=FN−FD as a function of growth temperature. Figure 5shows the overall growth diagram for N-polar InxGa1−xNgrowth by MBE calculated using experimental compositionand temperature values from this study. The stoichiometricgrowth rate in the absence of decomposition is 5 nm/min; to

FIG. 4. �Color online� Experimental data points obtained in this work fittedto curves predicted by our growth model for both N-polar and Ga-polarpolarity InxGa1−xN; a few data points from reports published elsewhere arealso shown to stress the overall higher achievability of indium incorporationin N-polar polarity. Inset: schematic of InN decomposition modeled as ni-trogen atoms leaving the surface.

FIG. 5. �Color online� Growth diagram for the growth of N-polar InxGa1−xN:the difference between reduced nitrogen stoichiometric and Ga flux defines

indium incorporation assuming strong stoichiometric condition.

Nath et al.: Growth model for plasma-assisted molecular beam epitaxy021206-5 021206-5

ensure excess In coverage, an In flux of approximately twicethe stoichiometric ��10 nm /min� is used while the Ga fluxused is �2 nm /min which is 0.4 times the usual growth rateof 5 nm/min. As seen, in the presence of decomposition, thegrowth rate no longer stays constant at 5 nm/min but dropssignificantly with increasing growth temperature. The differ-ence between the reduced nitrogen stoichiometric �or re-duced growth rate� and the Ga flux, i.e., FN� −FGa, in fact,corresponds to the amount of In incorporated. Thus, fromFig. 5, the decreasing In incorporation for a given Ga/N-fluxratio with increasing growth temperature becomes clear.

The model developed above is valid in temperature andIn-flux regimes where a stable excess indium coverage ismaintained. It has been established11 that in PAMBE growthof N-polar InN, metallic In accumulation �adlayer and drop-lets� on the surface due to In flux from InN decompositionand impinging In atoms from the source is limited by maxi-mum In-desorption rate Fdes up to a growth temperature of�610 °C.11 However, at higher growth temperatures above�610 °C, desorption rate of In exceeds decomposition rateof InN. In Fig. 5, the shaded region above �630 °C hasbeen indicated to have “no indium incorporation” as excessIn can remain on the surface in the form of droplets only ifthe impinging In flux is greater than Fdes−FD. If the surfaceis devoid of excess In but there is an excess of N, the growthwill shift to In-limited regime instead of being N limited thusinvalidating our growth model. Since all our growths havebeen performed in an In-rich regime at temperatures below�610 °C, the In incorporation will depend only on the re-duced N-flux FN�

and the Ga-flux FGa which remains un-changed.

To assess the optical quality of the N-polar InGaN films,we plot the PL intensity for the various N-polar and Ga-polarInxGa1−xN samples in a linear scale �inset of Fig. 6� with the

FIG. 6. �Color online� Peak PL intensity �normalized with respect to thehighest intensity obtained� and full width at half maximum of InGaNsamples of both polarities as a function of wavelengths �corresponding toband gaps extracted from room temperature PL measurements�. The linesare a guide for the eyes. Inset: PL intensity �normalized with respect to thehighest intensity obtained� of Ga-polar and N-polar InGaN on a linear scale;the magnification factor, polarity �G for Ga polar and N for N polar�, andgrowth temperature are indicated against each peak.

intensities of Ga-polar samples multiplied 100 times to bring

JVST B - Microelectronics and Nanometer Structures

them to a comparable level to the peak intensity of theN-polar sample grown at 600 °C corresponding to 600 nmemission. The intensities of other N-polar InGaN samples aremultiplied ten times as well. Figure 6 shows on its left axisthe peak intensity corresponding to the PL measurements ofeach sample against the corresponding wavelength. The peakintensity for N-polar InxGa1−xN samples shows more thantwo orders of magnitude increase as wavelength decreasesfrom �900 to �600 nm, corresponding to a 100 °C in-crease in growth temperature. Besides, the N-polar sampleshows much higher intensity than the Ga-polar sample espe-cially at shorter wavelengths. However, the surface rough-ness or morphological difference plays a major role in PLintensity. Figure 7 shows the �5�5 m2� AFM scans ofboth Ga-polar and N-polar samples grown at 550 and600 °C. As can be seen, the N-polar samples have relativelyhigher rms roughness than that of the Ga-polar samples. Par-ticularly, the N-polar sample grown at 600 °C has a veryhigh rms roughness of �15 nm. The surface morphologiesof a particular sample were found to be similar in variousregions of the sample concerned. This might explain the rela-tively higher PL intensity of N-polar �bulk� InGaN samples.Besides, the composition of In in the Ga-polar and N-polarsamples grown at a particular temperature is different, andhence it would not be justified to comment on the opticalqualities of the samples containing different In compositions.

To investigate the surface morphology of InGaN multiplequantum well �MQW� grown under these conditions, a testsample was grown at �625 °C in identical conditions as forthe growth of the multiple quantum-well structure �explainedlater� with a single �3-nm-thick In0.31Ga0.69N layer coveredby �2 nm of In0.05Ga0.95N barrier to prevent the decompo-sition of the active layer as the sample was cooled down. Thesurface was found to be smooth with rms roughness of�0.5 nm for a 2�2 m2 AFM scan �inset of Fig. 8�.

Based on our growth model, a Ga/N-flux ratio was chosento achieve 29% In-mole fraction in an InxGa1−xN /GaNMQW structure corresponding to green ��520 nm� emis-sion. Figure 8 shows the epitaxial structure grown. Thegrowth temperature for the growth of active regions was cho-sen to be �625–630 °C which is fairly high for PAMBEgrowth of InxGa1−xN in general. The growth of InxGa1−xNand GaN successively for MQW without growth interruptionwith only one Ga source poses severe challenges. Both theoptimal growth temperature and the Ga flux for the growthof InxGa1−xN and GaN are significantly different. A pulsingscheme was employed to grow the GaN barrier: the In andGa shutters were open throughout the barrier growth whilethe nitrogen shutter was pulsed with a duty cycle equal to theGa/N-flux ratio, the Ga-flux being the same as used in thepreceding InxGa1−xN quantum-well layer. Using our growthmodel, the decomposition rate was calculated for the givengrowth conditions to determine the actual growth time toachieve 3-nm-thick quantum wells. A fitting of the dynamicsimulation of triple-axis �-2� HRXRD scan with the actualscan data �inset of Fig. 8� indicated a close match of the In

composition and thickness to that originally aimed for thus

Nath et al.: Growth model for plasma-assisted molecular beam epitaxy021206-6 021206-6

once again validating our growth model. Postgrowth 3�3reconstructions were observed while sample was cooled in-dicating N-polar polarity; the top p+ GaN layer was found tohave smooth surface morphology �AFMs not shown�

In conclusion, a comprehensive growth model for thegrowth of N-polar InxGa1−xN by PAMBE has been proposedand validated which explains quantitatively the variation ofIn incorporation as a function of growth temperature andGa/N-flux ratio. It has been extended for Ga-polar InxGa1−xNas well and found to fit experimental data points excellently.The model has been found to predict In incorporation x inN-polar InxGa1−xN grown under different growth �i.e., vari-ous growth temperatures and Ga fluxes� conditions fairly ac-curately. Finally, an In0.31Ga0.69N / In0.05Ga0.95N MQW struc-ture was grown with conditions predicted by our growthmodel. Our growth model is hence expected to guide thegrowth of N-polar InxGa1−xN by PAMBE in a wide range of

FIG. 7. AFM scans of �5�5 m2�InxGa1−xN films: �a� Ga polar, 550 °C,rms�2.8 nm; �b� Ga polar, 600 °C,rms�2.3 nm; �c� N polar, 550 °C,rms�5.7 nm; and �d� N polar,600 °C, rms�14.1 nm.

growth conditions for optoelectronic device applications.

J. Vac. Sci. Technol. B, Vol. 29, No. 2, Mar/Apr 2011

FIG. 8. �Color online� �a� Epitaxial structure used for InGaN/GaN MQW; �b�XRD scan data and simulation confirming the composition and thicknessesof epilayers; Inset: AFM of the quantum well �with two barriers� showing

smooth surface.

Nath et al.: Growth model for plasma-assisted molecular beam epitaxy021206-7 021206-7

ACKNOWLEDGMENTS

The authors would like to acknowledge funding fromONR �Program manager: Paul Maki� and OSU Institute forMaterials Research �IMR�. E.G. would like to thank TheScientific and Technological Research Council of Turkey�TUBITAK� Grant No. 2219 project program for the support.

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