Structural Analysis of Highly Relaxed GaSb Grown on GaAs Substrates with Periodic Interfacial Array of 90° Misfit Dislocations
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  and [1-10]. In the present analysis, structural properties of GaSb deposited using both IMF and non-IMF growths are compared. Moiré 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 cm−2), while GaSb deposited using non-IMF growth is compressively strained and has a higher average density of threading dislocations (>109 cm−2).
KeywordsSemiconductor GaSb/GaAs Molecular beam epitaxy Interfacial misfit dislocations (IMF) or Lomer dislocations Strain relief Structural properties Moiré fringes
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 μm), 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 [8, 9]. 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. The IMF growth offers a “buffer-free” approach to realize monolithic high quality GaSb deposited on GaAs substrate with exceptionally low threading dislocation (TD) densities (~105 cm−2), 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. . However, 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, moiré 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 details of the IMF growth are presented elsewhere . The thickness of the IMF-grown GaSb epitaxial layers used for various analyses range from 15 nm to 5 μm. For example, thick samples like 5, 0.5 μm 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, the sample is lapped down from 5-μm GaSb epitaxial layer to 45 nm. Very thin 15-nm sample is grown separately for moiré fringe analysis to facilitate the transmission of electrons through both the epitaxial layer and the underlying substrate. The sample required for moiré fringe analysis is prepared as follows, the substrate is lapped down to ~10 μm 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, both Ga and Sb sources are turned on. In this case, IMF is not formed at the interface as is explained in the following paragraphs.
Results and Discussion
Moiré fringes are often used to identify dislocations in semiconductors [12–14] as well as metals . The terminating half lines (THLs) shown in Fig. 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 [13, 15]. The pure-edge dislocation density from various areas of the moiré fringes averages to 6.62 × 1010 cm−2. The THLs in the moiré fringes might also represent TDs as shown in Ref. . The TDs revealed in this way are attributed to the half-period shifts in the moiré fringes, which are produced as a result of the interaction between 60° and 90° dislocations. However, no half-period shifts are observed in the moiré 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 moiré fringes. In this study, displacement of the moiré fringes is not observed in the IMF samples, hence stacking faults or partial dislocations are not ascribed to the IMF growth. The moiré fringes are imaged along both  and [1-10] using (220) and (2-20) g vectors as shown in Fig. 5c. The projection of 2D Lomer dislocation network is observed to be uniform over a large area that was imaged (0.72 μm2).
In conclusion, high quality “buffer-free” GaSb is grown on GaAs substrates with very low TD densities (~105 cm−2) 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 (109 cm−2) 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 (~105 cm−2) compared to non-IMF sample (~109 cm−2). The long-range uniformity and the strain relief of the IMF-grown GaSb epitaxial layer measured using the moiré fringe patterns have shown a uniform 2D Lomer dislocation network over the entire scan area. The moiré 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.