Structural properties of ZnSe layers grown on (001) GaAs substrates tilted toward [110]and [010]Jin-Sang Kim, Sang-Hee Suh, Chang-Hoon Kim, and Su-Jin Chung Citation: Journal of Applied Physics 81, 6107 (1997); doi: 10.1063/1.364372 View online: http://dx.doi.org/10.1063/1.364372 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/81/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Monitoring of ZnCdSe layer properties by in situ x-ray diffraction during heteroepitaxy on (001)GaAs substrates Appl. Phys. Lett. 90, 162105 (2007); 10.1063/1.2724892 Optical and structural characterization of ZnSe films grown by molecular beam epitaxy on GaAs substrates withand without GaAs buffer layers J. Appl. Phys. 84, 1551 (1998); 10.1063/1.368222 ZnSe epitaxy on a GaAs(110) surface Appl. Phys. Lett. 71, 1192 (1997); 10.1063/1.119622 Structural and optical properties of lattice-matched ZnBeSe layers grown by molecular-beam epitaxy onto GaAssubstrates Appl. Phys. Lett. 70, 3564 (1997); 10.1063/1.119234 (001) GaAs substrate preparation for direct ZnSe heteroepitaxy J. Appl. Phys. 81, 7012 (1997); 10.1063/1.365266

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Structural properties of ZnSe layers grown on (001) GaAs substrates tiltedtoward [110] and [010]

Jin-Sang Kima) and Sang-Hee SuhDivision of Electronics and Information Technology, Korea Institute of Science and Technology,Cheongryang, Seoul 130-650, Korea

Chang-Hoon Kim and Su-Jin ChungDepartment of Inorganic Material Engineering, Seoul National University, Seoul 151-742, Korea

~Received 30 September 1996; accepted for publication 20 January 1997!

We have investigated the structural properties of ZnSe epilayers that were molecular beamepitaxially grown on~001! GaAs substrates with different tilt angles and tilt directions. Wemeasured the properties of the epilayers by x-ray diffraction, transmission electron microscopy, andetch pit density analysis. Tilting the~001! GaAs substrate toward@010# was very effective inreducing the surface defect density of the ZnSe layers, while tilting toward the@110# direction wasof no use. We could observe the increasingly two-dimensional nature of the initial growth mode inthe ~001! GaAs substrate tilted toward@010#. Growth of a 1.8-mm-thick ZnSe layer on~001! GaAstilted 4° toward@010# resulted in a very low surface defect density of 13104 cm22. Such a lowdefect density has seldom been obtained in ZnSe, without growing a GaAs buffer layer below theZnSe layer. ©1997 American Institute of Physics.@S0021-8979~97!03509-3#

I. INTRODUCTION

Degradation of laser diodes or light emitting diodes thatare made of ZnSe and related alloys such as ZnSSe andZnMgSSe is a major problem in developing room tempera-ture blue light emitting devices.1 The degradation of ZnSebased devices has been ascribed to propagation of crystaldefects generated during growth.2,3 Complexes of defectsconsisting of stacking faults which are nucleated at or nearthe II–VI /GaAs interfaces and associated threading disloca-tions have been the most commonly observed defects in thedevice structures.4 So the suppression of crystalline defectsin ZnSe related alloys is still a very important issue in therealization of blue laser diodes.

GaAs buffer layer growth, Zn beam irradiation beforeZnSe growth, and migration enhanced epitaxial~MEE!growth techniques have recently been tried to reduce the de-fect density of ZnSe related alloys.5–7 If growth begins in thethree-dimensional~3D! mode, defects are likely to nucleateat the boundaries between growth islands.8 Thus, a two-dimensional~2D! growth mode must be realized at the initialgrowth stage in order to reduce the defect density. Growth ofZnSe on misoriented substrates can be another approach forthis purpose. In the present work, ZnSe layers were grownon ~001! GaAs substrates tilted toward@010# and @110# bymolecular beam epitaxy~MBE!. The effect of a tilted sub-strate on crystallographic properties was investigated. Trans-mission electron microscope~TEM! and etch pit density~EPD! measurements were used to investigate the defects.Improvement of the epitaxial layer quality by using properlytilted substrates will be discussed on the basis of both theinitial growth and the lattice relaxation mechanism.

II. EXPERIMENTAL PROCEDURE

ZnSe layers were grown by MBE on GaAs substrateswhose surface orientations are several degrees off~001!:nominal~001!, tilted 2°, 4°, and 10° toward@010#. For com-parison,~001! GaAs substrates tilted 2° and 4° toward@110#were also used. ZnSe epilayers were grown using elementalsources with a Zn to Se beam pressure ratio of about 1/2. Allsamples were grown at a 280 °C substrate temperature with agrowth rate of about 0.6mm/h. Several GaAs substratestilted towards the same direction but with different degreesof orientation were mounted side by side on the same mo-lybdenum block. Just before growth, the substrates werechemically etched in H2SO4:H2O2:H2O ~5:1:1! and thenthermally cleaned at 580 °C for 10 min in the growth cham-ber. A GaAs buffer layer was not grown in this study. Areflection high energy electron diffraction~RHEED! patternwas observed during growth.

The crystalline qualities of the ZnSe layers were evalu-ated by using a double-crystal x-ray diffractometer. The EPDmeasurements by Hanet al.7 was also tried using a 2%bromine–methanol solution at room temperature. Specimensfor TEM were prepared by mechanical polishing followed byAr ion milling. TEM studies were performed using a JEOL2000 FX-II transmission electron microscope.

III. RESULTS AND DISCUSSION

A. Growth rate

Figure 1 shows the dependence of the relative verticalgrowth rate on the tile direction and the tilt angle of the~001!GaAs substrate. The vertical growth rate was determined bymeasuring the thickness of each layer. The relative growthrate is the growth rate divided by that on the nominal~001!GaAs substrate. For the@010# tilt direction, the growth ratea!Electronic mail: jskim@kistmail.kist.re.kr

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increases with increasing tilt angle. For the@110# tilt direc-tion, however, the growth rate does not depend on the tiltangle.

In order to investigate the dependence of the lateralgrowth rate on the crystallographic direction, the morpho-logical change of a negative circular mesa~2 mm in diameterand 500 Å in depth! on the~001! GaAs substrate surface wasexamined. Typical surface morphologies before and after 1.5h growth on the 2-mm-diam circular mesa are shown in Fig.2. The original circular mesa was transformed during growthinto a hexagonal shape. Note that the@110# distance was the

greatest of all the directions on the~001! surface~2.9mm forthe @110# and 2.6mm for the @110#, @010#, and @100# direc-tions, respectively!. This result indicates that the lateralgrowth rate has anisotropy with respect to crystallographicdirections on the~001! surface. The growth rate in the@110#direction is smaller than the ones in any other directions atthis growth condition.

These experimental results could be discussed qualita-tively from the view point of the atomic-step structure of thesubstrate surface. Asai9 has reported that the difference oflateral growth rate between@110# and@110# directions is dueto the different atomic structures with directions in GaAs. InGaAs, steps are likely to form along the^110& directions.10

Thus the~001! GaAs substrate tilted toward@110# wouldhave periodic steps with one@110# step edge and several~001! terrace atoms, as shown in Fig. 3~a!. The absorbedatoms on this surface would create many kink sites becauseof the higher growth rate in the@110# direction than in the@110# direction. Thus the front of the steps’ line might bedisturbed. Further growth on this surface would not haveinitial @110# atomic steps on the surface. On this surface, thetime to grow a monolayer of ZnSe will depend on the@110#lateral growth rate. Therefore, the growth rate would notchange discernibly with degrees of tilt.

For a~001! GaAs substrate tilted toward@010#, the@100#steps’ front would consist of many atomic kink sites, asshown in Fig. 3~b!.9–11 Since the probability of adatoms’incorporation into the kink sites is higher than for flat sur-faces, Zn or Se atoms are easily caught in@100# steps accord-ing to a classical growth model. In GaAs metalorganic

FIG. 1. Relative vertical growth rates as a function of substrate tile direc-tions and degrees of tilt. Filled circles and open squares are for ZnSe layersgrown on the GaAs surface tilted from~001! toward @110# and @010#, re-spectively.

FIG. 2. Surface photographs of~a! the original circular mesa and~b! thedeformed mesa after 1.5 h growth.

FIG. 3. Schematic representation of~a! the ~001! surface tilted toward@110#and ~b! the ~001! surface tilted toward@010#.

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chemical vapor deposition~MOCVD! growth, the lateralgrowth velocities in the@110# and@110# directions are influ-enced by the AsH3 pressure.

9,12 On the other hand, in@100#steps’ front as in this experiment, the lateral growth rate willnot be influenced much by the flux ratio of the source beam.This is because steps’ front consists of kinks with both@110#and@110# steps. This would result in the high lateral growthrate and would increase the vertical growth rate with increas-ing degree of tilt.

B. X-ray diffraction

Double-crystal x-ray rocking curves of the~004! reflec-tion are shown in Fig. 4 for 0.15-mm-thick ZnSe epilayers onthe ~001! GaAs substrates with 0°, 4°, and 10° tilts toward@010#. The full width at half-maximum~FWHM! value of thex-ray diffraction peak decreases with increasing tilt. For theZnSe layers on~001! GaAs tilted toward@110#, there is al-most no difference in FWHM values with the degree of tilt.As thickness increases, the FWHM increases until it reachesa maximum of about 0.3–0.5mm. The thickness of theFWHM maximum increased with the increasing degree oftilt.

Nucleation of ZnSe on~001! GaAs without a MBEGaAs buffer layer typically gives a spotty RHEED patternfor the first 200–300 Å, indicative of three-dimensionalnucleation, and then the spots are elongated into streaks.13,14

In this study, use of a~001! GaAs substrate with 4° tilt to-ward @010# gave a streaklike RHEED pattern in much lesstime than ~001! GaAs substrates with 0° or 4° tilt toward@110#. Early observation~after about 10 s for off 4° toward@010#! of a streaked RHEED pattern suggests a more two-

dimensional character of the nucleation. Two-dimensionalnucleation would result in a narrow FWHM of x-ray diffrac-tion as shown in Fig. 4. This increasingly two-dimensionalnature of the initial growth mode for the substrate tilted to-ward @010# is closely related to the surface geometry. Asshown in Fig. 3~b!, the kink sites’ density increases as thedegree of tilt toward@010# increases. Thus, a large number ofkink sites would offer stable nucleation sites and preventisland nucleation for the~100! substrate tilted toward the@010# direction.

The diffraction peak positions varied when the configu-rations measured was changed because the~001! plane of theZnSe layer was inclined toward that of the GaAs substrate.In order to calculate the tilt angle between the~004! planesof the epilayer and the substrate, we obtained diffraction pat-terns for two arrangements. One is with a larger x-ray inci-dent angle to the film surface than the Bragg angle of GaAs~004! and the other with a smaller incident angle. Tilt angleswere measured as a function of ZnSe layer thickness and Fig.5 shows the results obtained for films grown on~001! GaAstilted toward @010#. As can be seen from Fig. 5, the tiltchanges of sign from negative to positive value as the layerthickness increases. Here, the negative sign means that thedirection of tilt is away from the surface. For ZnSe layersless than 0.2mm thick on a GaAs substrate tilted toward@010#, the observed tilt increased from 20 arcsec for 2° tilt to200 arcsec for 10° tilt. At the middle stage of growth~0.2–1mm thick!, the tilt sign changes from negative to positive andmonotonically increases. Such a feature was also observedby Ohki et al.15 in metalorganic vapor-phase epitaxialgrowth of ZnSe layers on~001! GaAs tilted toward@110#.

FIG. 4. Double-crystal x-ray rocking curves of the~004! reflection from0.15-mm-thick ZnSe epilayers grown on the~001! GaAs substrate, tilted 4°and 10° toward@010#, respectively.

FIG. 5. Tilted angle of the ZnSe~001! plane to the GaAs~001! planes as afunction of ZnSe thickness. Filled circles, open squares, and filled diamondsare for layers grown on the GaAs surface tilted from~001! toward@010# by2°, 4°, and 10°, respectively.

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But for ZnSe layers.2 mm thick on GaAs tilted toward@010#, the observed tilt gradually decreases with layer thick-ness. This result is significantly different from the one ofOhki et al.15 For ZnSe layers.2 mm thick on GaAs tiltedtoward@110#, they obtained a slow increase in tilt with layerthickness.

For ZnSe films of less than 0.15mm thickness~experi-mentally measured critical thickness!,16 the direction andmagnitude of tilt are well explained by the theoretical modelproposed by Nagai.17 When the stress builds up so that thelattice can hardly bear the stress, misfit dislocations are gen-erated to relax the mismatch stress. The experimental resultindicates that a part of the mismatch stress is relaxed bypositive tilt. Ayerset al.18 proposed that this positive tilt canbe explained by the preferential glide of certain types ofdislocations in the asymmetry of the slip system due to sub-strate tilt. But for the layer thicker than 2mm, the Ayersmodel cannot explain our results where tilt direction changedfrom positive to negative for ZnSe layers on the GaAs sub-strate tilted toward@010#. The layer thickness dependence oftilting may be due to the stress that might have developed inthe epilayer at different stages of growth and cooling.

In the ZnSe/GaAs system, the difference in thermal ex-pansion coefficients between the epilayer and the substratewould induce a tensile stress in the epilayer near the interfaceregion.19 But the lattice parameter difference between theepilayer and the substrate would induce compressive stressin the interface region. The tensile stress due to the thermalexpansion coefficient difference would depend on the layerthickness. Thus, the increased tilting of the layer with in-creasing layer thickness can be explained by a gradual reliefof elastic strain due to misfit between the lattices. For thickerlayers, however, when the effect of thermal stress becomesdominant, the layer tilting will start to change its directionbecause the stress changes its direction.

For the 2.5–3-mm-thick layers grown on~001! GaAstilted toward @010#, the narrowest range of the FWHM islocated in the vicinity of the 4° tilt of the substrate. ZnSefilms grown on the 4° tilt substrate have better quality thanthose grown on exact~001! or 10° tilt substrates, eventhough ZnSe film on the 10° tilt substrate has smallerFWHM values initially than the others. But, for layers grownto 2.5–3mm on ~001! GaAs tilted toward@110#, it was re-ported that the narrowest range of the FWHM is located inthe vicinity of the 2° tilt of the substrate.15 In the case of2–2.4-mm-thick ZnSe, the FWHM value shows 90 arcsec for4° tilt toward @010#, whereas it is 150 arcsec for the layerwith 4° tilt toward @110#.

C. Characterization of defects structures

The substrate tilt direction has a dramatic effect on thearrangement and density of misfit dislocations in MBEgrown ZnSe. The bright field TEM images of Figs. 6~a! and6~b! illustrate this effect. These images are plan views of thesubstrate–epitaxial layer interfaces of undoped ZnSe layerson ~001! GaAs with 4° tilt toward@010# @Fig. 6~a!# and@110#@Fig. 6~b!# substrates. Both of the layers in Fig. 6 are about1.8 mm thick. Ar ion milling of ZnSe may result in genera-tion of small dislocation loops,19 so we ignored these small

defects as ion milling artifacts. The image of Fig. 6~a! showsa regular array of 60° misfit dislocations lying along the@110# and@110# directions. This feature is different from thetypical interfacial structure of the undoped ZnSe layer thathas a random and irregular array of dislocations.8,19,20In ad-dition to dislocations with line directions along the^110&,some misfit dislocations running approximately parallel tothe ^100& directions ~marked by arrow! are also observed.These features were also observed by Guhaet al. in theirTEM studies on the initial two-dimensional growth mode ofZnSe on As stabilized GaAs substrates.21 A misfit disloca-tion with ^100& line direction is formed by cross slippingthreading segments between different$111% planes.21 For theZnSe layer on GaAs tilted 4° toward@110#, the misfit dislo-cation array becomes somewhat random and irregular, asshown in Fig. 6~b!. In this sample a high density of misfitdislocations deviates from the110& direction.

These differences in dislocation configuration can be ex-plained on the basis of different initial growth mode. WhenZnSe growth is initiated on GaAs tilted 4° toward@110#, thespotty RHEED pattern is observed for the first 2–3 min. Thismeans that the initial growth mode is three dimensional.Coalescence of three-dimensional islands may lead to theformation of defects,21 and this would lead to an irregulararrangement of misfit dislocations as shown in Fig. 6~b!. Inthe case of the ZnSe layer on GaAs tilted 4° toward@010#,

FIG. 6. Bright field plan view TEM images of the misfit dislocation arraysat the ZnSe–GaAs interface for the~001! GaAs substrate~a! off 4° toward@010# and ~b! off 4° toward @110#. Arrows indicate misfit dislocations run-ning approximately parallel to the100& directions.

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the early observation~after 10 s! of a streaked RHEED pat-tern suggests a more two-dimensional character to the nucle-ation. So the dislocation generation at the coalescing islandboundaries would become insignificant and other dislocationgeneration mechanism such as half-loop generation or fullloop generation would come into play.8

The difference in crystalline quality between thesesamples was observed by using a defect etching technique aswell. Figures 7~a! and 7~b! show etched surfaces of the ZnSelayers on~001! GaAs substrates tilted 4° toward@010# and@110#, respectively. The pits are elongated along the@110#direction. Both of the layers in Fig. 7 are about 1.8mm thick.A remarkable decrease in etch pit density was observed forthe sample on the substrate tilted 4° toward the@010# direc-tion. The EPD was 13104 cm22in ZnSe on the substratetilted 4° toward @010#, while it was higher than5 3 107 cm22 in the ZnSe on the substrate tilted 4° toward@110#. For the layer on the substrate tilted 10° toward@010#,however, the EPD was about 1–33 106/cm2.The lattice re-laxation mechanism responsible for higher EPD in ZnSe lay-ers on the substrate tilted 10° toward@010# is not clear. TheZnSe film grown on the substrate tilted 4° toward@010# hasthe best quality among all the films grown with differenttypes of substrates.

IV. CONCLUSIONS

It is obvious from growth rate, x-ray diffraction, andTEM observations that the initial growth and lattice relax-ation mechanism of the ZnSe on a tilted~001! GaAs sub-strate depend on the substrate tilted direction. For the ZnSelayers on~001! GaAs tilted toward@010#, the growth rateincreased with increasing substrate tilt. On the other hand,the growth rate did not change with tilt for layers on the~001! substrates tilted toward the@110# direction. The lateralgrowth rate has anisotropy with respect to crystallographicdirections on the~001! surface. This growth rate differenceswere explained by considering the surface atomic structuresat @010# and @110# step sites. At the initial growth stage thecrystalline quality was improved with degree of tilt for thelayers on~001! substrates misoriented toward the@010# di-rection. The increasingly two-dimensional nature at the ini-tial growth stage of the ZnSe layers on the~001! GaAs sub-strate tilted toward the@010# direction results in lower defectdensities and improved crystalline quality. But different fromthe initial growth stage, further growth results in a decreaseof crystalline quality for the ZnSe layer off 10° toward the@010# direction. This may be closely related to the latticerelaxation mechanism. Further investigation into the sub-strate orientation dependency of the lattice relaxation mecha-nism and other properties is needed.

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FIG. 7. Normarski micrographic images of the ZnSe surfaces etched with2% bromine–methanol solution. ZnSe layers on~001! GaAs substrates of~a! off 4° toward @010# and ~b! off 4° toward @110#, respectively.

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