Journal of the Korean Physical Society, Vol. 50, No. 6, June 2007, pp. 1929∼1932

Strucrural Properties of GaSb Layers Grown on InAs, AlSb,and GaSb Buffer Layers on GaAs (001) Substrates

Y. K. Noh, Y. J. Hwang and M. D. Kim∗

Department of Physics, Chungnam National University, Daejeon 305-764

Y. J. Kwon and J. E. Oh

Division of Electrical and Computer Engineering, Hanyang University, Ansan 426-791

Y. H. Kim and J. Y. Lee

Department of Materials Science and Engineering,Korea Advanced Institute of Science and Technology, Daejeon 305-701

(Received 1 September 2006)

The strain-relief and the structural properties of GaSb films with thin InAs, AlSb, and GaSb bufferlayers grown on GaAs (001) substrates at low temperature (LT) by molecular beam epitaxy wereinvestigated using atomic force microscopy, transmission election microscopy, and X-ray diffraction.The strain arising from depositing the thin buffer layer onto the GaAs substrate was relieved by aperiodic array of 90◦ misfit dislocations with a Burgers vector of 1/2a<110> for the AlSb/GaAsand the GaSb/GaAs systems, but by both 60◦ and 90◦ misfit dislocations for the InAs/GaAssystem. The 90◦ misfit dislocation arrays at the AlSb/GaAs and GaSb/GaAs interface had averagespacing of 4.80 nm and 5.59 nm, respectively. The mean roughnesses and the full widths at halfmaximum of the rocking curves of the GaSb films on the thin AlSb and GaSb buffer layers werefound, respectively, to be less than 1 nm and about three times lower than the corresponding valuesfor the system with an InAs buffer layer. These results clearly demonstrate that the presence of athin, low-temperature AlSb or GaSb buffer layer is very useful for improving the quality of GaSbcrystals grown on GaAs substrates.

PACS numbers: 78.66.Fd, 78.70.Ck, 68.55.Jk, 68.35.Ct, 68.45.WsKeywords: Interfaces, Molecular beam epitaxy, AlSb, GaSb, InAs, Misfit dislocations

I. INTRODUCTION

Antimony-based compounds offer a wide range of elec-tronic band gaps, band-gap offsets, and electronic barri-ers along with extremely high electron mobility, therebyenabling a variety of devices that are much faster andhave lower power consumption than equivalent InP- andGaAs-based devices [1] and infrared light sources [2,3]. Lattice-mismatched epitaxy of Sb-based materials onGaAs and Si substrates has attracted considerable at-tention due to the numerous advances in optoelectronicdevices, such as laser diodes [4–6], detectors [7], and tran-sistors [8]. Therefore, much effort has been devoted togrowing high-quality Sb-based layers on GaAs and Sisubstrates.

In spite of the performance advantages offered by Sb-based layers on GaAs and Si substrates, growing III-Sb-

∗E-mail: mdkim@cnu.ac.kr;Tel: +82-42-821-5452; Fax: +82-42-822-8011

based compound semiconductors on GaAs and Si sub-strates is problematic due to the large difference in latticeconstants between the epitaxial layer and the substrate,which leads to stress and dislocations in the epitaxialfilm. Thus, epilayers of these types are limited to a crit-ical thickness, beyond which the material relieves thestrain energy by forming misfit and threading disloca-tions [9]. To overcome this problem, various buffer layers,such as As- and Sb-terminated layers, have been used,and growth techniques with low-temperature (LT), andcompositionally graded buffers have been implementedto reduce the density of threading dislocations [10–13].However, although arsenic and phosphorus material sys-tems have been widely studied, relatively little works hasbeen done on the interface between antimony-based ma-terials and GaAs substrates. In the development of high-quality device structures, it is important to understandthe source of confinement of misfit dislocations at hetero-interfaces.

In the present work, we compared the strain reliefin thin InAs, AlSb, and GaSb buffer layers grown on

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Fig. 1. AFM images of 500-nm-thick GaSb layers grownon 5-nm-thick buffer layers of (a) InAs, (b) AlSb, and (c)GaSb on GaAs substrates.

GaAs (001) substrates at LT. We investigated the de-pendence of the microstructural properties of uninten-tionally doped p-GaSb films grown on thin InAs, AlSb,and GaSb buffer layers. The effects of the buffer ma-terials were investigated using atomic force microscopy(AFM), high-resolution transmission election microscopy(TEM), and X-ray diffraction (XRD) measurements. Inthese studies, the use of high-resolution TEM and XRDproved useful in characterizing the interface.

II. EXPERIMENT

The samples used in this work were grown on semi-insulating (001)-oriented GaAs substrates by using con-ventional solid source molecular beam epitaxy (MBE).First, a 200-nm GaAs buffer layer was grown on theGaAs substrate at 530 ◦C. The substrate temperaturewas then lowered to 440 ◦C for the growth of the LTInAs and GaSb buffer layers or to 460 ◦C for the growthof the AlSb buffer layer. The As/In, Sb/Ga, and Sb/Alflux ratios during the growth were 20, 8, and 10, re-spectively. The InAs, AlSb, and GaSb growth rates persecond were set to 0.08, 0.3, and 0.7 monolayers (ML),respectively, and the growth process was monitored insitu by using reflection high energy electron diffraction(RHEED). The RHEED patterns of the InAs, AlSb, andGaSb buffer layers became slightly streaky during theinitial stage of growth, but subsequently underwent anabrupt change to a spotty pattern. These series of pat-terns can be attributed to the formation of islands duringthe initial stage, followed by coalescence of these islandsinto a rough-and-flat surface. The substrate temperaturewas then increased to 500 ◦C for the deposition of theGaSb layer with a Sb/Ga flux ratio of 8 and a growthrate of 0.7 ML/s. The thicknesses of the unintentionallydoped GaSb layers and the buffer layers were fixed at500 and 5 nm, respectively. AFM observations were per-formed to measure the roughness of each surface. Thestructural and the interfacial properties of the grown un-doped GaSb and buffer layers on the GaAs substrateswere measured by TEM with a JEM 2000 EX transmis-sion electron microscope operating at 200 kV.

Fig. 2. Cross-sectional bright-field TEM images of GaSblayers on (a) InAs, (b) AlSb, and (c) GaSb buffer layers.

III. RESULTS AND DISCUSSION

The surface morphologies of the 500 nm GaSb filmson the InAs, AlSb, and GaSb buffer layers grown at LTare shown in Figures 1(a), (b), and (c), respectively. AsFigure 1(b) and (c) show, the GaSb filmsgrown on theAlSb and the GaSb buffer layers have smooth surfaces.These films had a mean surface roughness of less than 1nm, and no cracks or aggregation related to dislocationswere observed in the films. By contrast, the GaSb filmdeposited on the thin InAs buffer exhibited a roughnessgreater than 10 nm, and hatch lines related to disloca-tions in the [110] and the [110] directions, as well as pitsin the GaSb layer, were observed. Although the source ofthis behavior is currently unclear, it can be explained bytaking into account the critical thickness dictated by thestrain and the effect of dislocation confinement at the in-terface. Given that it is possible to obtain a smooth sur-face by controlling the confinement of misfit dislocationsat the interface, it is important to clarify the microscopicmechanism by which dislocations are confined.

Cross-sectional and high-resolution TEM (HR-TEM)images were obtained in order to investigate the struc-tural properties and the dislocation confinement charac-teristics of the buffer and the GaSb layers grown on theGaAs substrate. Figures 2(a) - (c) show cross-sectionalimages of the GaSb layers on thin InAs, AlSb, and GaSbbuffers grown on GaAs substrates, respectively. Thethicknesses of the GaSb and the buffer layers were about500 nm and 5 nm, respectively. The coherently strainedlines of the interface are indicated by characteristic darkand bright contrasts and are related to inhomogeneouslattice deformation. The density of dislocations at theinterface in the system with an InAs buffer (Figure 2(a))is higher than that of the system with an AlSb (Fig-ure 2(b)) or a GaSb (Figure 2(c)) buffer. Moreover, inthe InAs buffer system, threading dislocations are prop-

Strucrural Properties of GaSb Layers Grown on InAs,· · · – Y. K. Noh et al. -1931-

Fig. 3. Cross-sectional high-resolution TEM images alongthe [110] zone axis of thin (a) InAs, (b) AlSb, and (c) GaSbbuffer layers grown on GaAs. Arrows indicate 60◦ and 90◦

misfit dislocations at the interface between the substrate andbuffer layer. The lattice spacing of GaAs is 3.998 A.

agated to the GaSb layer.Figure 3 shows HR-TEM micrographs of the interfaces

between the buffer layers and the GaAs (001) substrates.A regular array of misfit dislocations is observed at theinterface between each buffer layer and the GaAs sub-strate. Each of these dislocations, indicated by arrowsin the figure, coincides with a {111} plane of GaAs. Therelative orientations of the InAs, the AlSb, and the GaSbbuffer layers with respect to the GaAs substrates are[110] buffer layer // [110]GaAs substrate and (001)buffer layer

// (001)GaAs substrate, respectively. Figure 3(a) showsan HR-TEM image of the interface between the InAsbuffer layer and the GaAs substrate. A spaced arrayof 60◦ and 90◦ misfit dislocations, which have Burgersvectors of 1/2 a[011] (or 1/2 a[101]) and 1/2 a[110], isobserved along the [110] directions. he driving force forthe generation of these 60◦ and 90◦ dislocations is thelattice mismatch between the buffer layer and the GaAssubstrate. At present, the precise source of the 60◦ mis-fits is unclear, although they presumably arise due tofactors such as the degree of mismatch, the growth pa-rameters, and island coalescence [14]. The lattice mis-match between the buffer layer and the GaAs substrateis also known to cause threading dislocations in the GaSblayer. The 90◦ misfits appear every 13 lattice spacingsin the GaAs substrate. The average distance betweenthe misfit dislocations in the InAs buffer (∼5.20 nm) is

Table 1. The lattice constants, strains, and FWHM valuesfor the GaSb films with InAs, AlSb and GaSb buffer layerson GaAs substrates.

InAs buffer AlSb buffer GaSb buffer

GaSb lattice

constant (A)6.0787 6.0953 6.0960

Strain (%) 7.572 7.820 7.832

FWHM (arcsec) 1296 517 468

less than the theoretical value of 5.83 nm derived usingthe formula S = b/f [15], where b is the Burgers vector(b = 4.17 A) and f is the lattice mismatch (f = 7.16× 10−2). Figures 3(b) and (c) show HR-TEM imagesof the interfaces between the AlSb and GaSb buffer lay-ers, respectively, and the GaAs substrates. The imagesshow small AlSb and GaSb distortions, manifesting ascharacteristic dark and bright contrasts, mainly boundin the interface. An array of 90◦ misfit dislocations witha spacing of 12 is observed at the AlSb/GaAs interface.The average distance between the misfit dislocations inthe AlSb buffer is about 4.80 nm, which is quite closeto the theoretical value of 4.95 nm. This result meansthat in the AlSb buffer layer, the mismatch strain be-tween the epilayer and the substrate should be almostcompletely compensated for by the misfit dislocations atroom temperature and that the confinement of the mis-fit dislocations at the heterointerface should enable theformation of a smooth growth surface free of distortions.In the case of GaSb/GaAs interface, the average spacingof 90◦ misfit dislocations is 14. The average distance be-tween the misfits dislocations in the GaSb buffer (∼5.59nm) is very close the theoretical value of 5.53 nm. The90◦ dislocations are the most efficient misfit dislocationsfor strain relaxation because the length of the Burgersvector edge component projected onto the interface, be,is a/

√2. Therefore, it is of great importance to clarify

the microscopic mechanism by which these dislocationsarise, although the growth mode is very sensitive to vari-ables such as the growth temperature, the growth rate,and the V/III flux ratio.

Figure 4 shows the high-resolution XRD rockingcurves of the 500-nm-thick GaSb layers grown on theInAs, AlSb, and GaSb buffer layers. The peaks cor-responding to the (004) planes are sharp for the GaSbbuffer layer (Figure 4(c)), but broad for the InAs bufferlayer (Figure 4(a)). The broadness of this peak whenthe GaSb layer is deposited on a InAs buffer layer isattributed to the generation of threading dislocations re-sulting from strain relaxation. These results are in agree-ment with the AFM and the TEM results. The full widthat half maximum (FWHM) of the XRD rocking curve forGaSb on a GaSb buffer layer (Figure 4(b)) is 468 arcsec,which is much smaller than the FWHM of 1296 arcsecobtained for the sample with an InAs buffer layer. Inaddition, the samples containing the AlSb and the GaSb

-1932- Journal of the Korean Physical Society, Vol. 50, No. 6, June 2007

Fig. 4. High-resolution double-crystal X-ray rockingcurves for GaSb layers grown on thin (a) InAs, (b) AlSb,and (c) GaSb buffer layers on GaAs substrates.

buffer layers were found to exhibit mirror-like surfacesafter GaSb growth. The strains and the FWHM valuesof the three samples, as determined from the XRD mea-surements, are summarized in Table 1. As Table I shown,the GaSb epilayers on the AlSb and the GaSb buffer lay-ers have the same characteristics as fully relaxed bulkGaSb. These results clearly demonstrate that the pres-ence of a thin, LT AlSb or GaSb buffer layer is veryuseful for improving the quality of GaSb crystals grownon GaAs substrates.

IV. CONCLUSION

The microstructural properties of GaSb films grown onthin InAs, AlSb, and GaSb buffer layers deposited at LTby using MBE were investigated using AFM, HR-TEM,and XRD measurements. HR-TEM of the GaSb layergrown on the thin LT AlSb and GaSb buffers revealed 90◦misfit dislocations with a Burgers vector of 1/2a<110>at the AlSb/GaAs and the GaSb/GaAs interfaces. Thesedefects represent the most efficient type of dislocation formisfit relaxation. By contrast, both 60◦ and 90◦ misfit

dislocations were observed at the interface between theInAs layer and the GaAs substrate. Taken together, thepresent findings for the GaSb layer grown on a thin LTAlSb or GaSb buffer - mean roughness less than 1 nm,90◦ misfit dislocations, and low FWHM value by XRD -confirm that the LT AlSb or GaSb buffer layer plays akey role in the growth of high quality GaSb layers. Theseobservations can be used to improve the crystal qualityof large mismatch heteroepitaxy structures.

ACKNOWLEDGMENTS

This work was supported by the Basic Research Pro-gram of the Korea Science & Engineering Founda-tion (Grant No. R01-2003-000-10268-0/ R01-2006-000-10874-0), the Terabit Nano Device (TND) of Frontier-21 program sponsored by Korea Ministry of Scienceand Technology, and by a Korea Research Foundationgrant funded by the Korean Government (KRF-2004-041-D00433)

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