Metalorganic chemical vapor deposition growth of InAs/GaSb type II superlattices with controllable As x Sb1-xinterfaces
© Li et al; licensee Springer. 2012
Received: 16 October 2011
Accepted: 28 February 2012
Published: 28 February 2012
InAs/GaSb type II superlattices were grown on (100) GaSb substrates by metalorganic chemical vapor deposition (MOCVD). A plane of mixed As and Sb atoms connecting the InAs and GaSb layers was introduced to compensate the tensile strain created by the InAs layer in the SL. Characterizations of the samples by atomic force microscopy and high-resolution X-ray diffraction demonstrate flat surface morphology and good crystalline quality. The lattice mismatch of approximately 0.18% between the SL and GaSb substrate is small compared to the MOCVD-grown supperlattice samples reported to date in the literature. Considerable optical absorption in 2- to 8-μm infrared region has been realized.
PACS: 78.67.Pt; 81.15.Gh; 63.22.Np; 81.05.Ea
InAs/GaSb superlattices (SLs) are important for long-wavelength infrared (IR) applications because of their broken gap type II band alignment with the conduction band minimum of InAs lying below the valence band maximum of GaSb. Such type II SL material has been investigated widely as a viable alternative to conventional HgCdTe IR detecting materials due to the unique capability for band structure engineering which results in great flexibility in controlling the detection wavelength (from 3 to 30 μm) , low Auger recombination rates [2, 3] and small tunneling current . Recently, quantum well IR photodetectors (QWIPs) [5–7] have also exhibited a number of potential advantages including highly uniform and well-controlled epitaxy growth, but their quatum efficiency cannot compete with HgCdTe photodiode due to the fact that optical transition is forbidden for normal incidence of light. More recently, new nanostructured IR photodectors based on quantum dots have been investigated intensively to outperform QWIPs since they are intrinsic sensitive to normal incidence light [8–10]. However, the absorption efficiency is still low due to the limited density of dots and inhomogeneous dot size. In the case of InAs/GaSb SL structures, the absorption is strong for normal incidence of light. Consequently, the SL structures provide the possibility to have both high absorption efficiency as reached with HgCdTe and high uniform as reached with QWIPs. So far, high-quality InAs/GaSb materials have been grown by molecular beam epitaxy (MBE) [11–13]. However, the capability to grow device-quality material and structure by metalorganic chemical vapor deposition (MOCVD) has not been fully achieved, which is the preferred growth technique in manufacturing due to the higher growth rate and multi-wafer capability. Reports on the MOCVD growth of InAs/GaSb are scarce in the literature [14, 15].
One of the key issues during the growth of InAs/GaSb SLs is to introduce suitable interfacial layers between InAs and GaSb to offset their lattice mismatch of 0.59%, since misfit dislocations will be generated as the number of layers in the SL increases. The widely investigated MBE growth of such SLs usually introduces InSb-like interface (IF) to make the SL match the GaSb substrate as much as possible [11, 12, 16]. However, we have shown that good-quality InSb cannot be obtained since InSb is unstable at the optimal temperature for the MOCVD growth of InAs/GaSb SLs . Even though GaAs-like IFs should be stable during MOCVD growth, SLs with GaAs-like IFs have poor crystalline quality since GaAs has a lattice 8% smaller than GaSb substrate, leading to a large lattice mismatch between the SL and GaSb substrate [18, 19]. Therefore, new types of IFs more stable than InSb with larger lattice constants than GaAs are necessary for improvement of MOCVD grown InAs/GaSb SL materials. In our recent work, ternary alloy IFs with mixed As and Sb were introduced in our MOCVD growth and have been demonstrated to improve material quality, significantly compared to InSb-like and GaAs-like IFs. Here, we report the growth and characterization of InAs/GaSb SLs on (100) GaSb substrates with an As x Sb1-xmixing plane that connects the InAs and GaSb layers as the interfacial layers.
InAs/GaSb SLs were grown on (100) GaSb substrates in an Aixtron (AIXTRON Ltd. Nanoinstruments, Swavesey, Cambridge, UK) MOCVD reactor system equipped with a close-coupled showerhead growth chamber and an EpiTT optical in situ sensor. The chamber pressure during growth was 100 Torr. Epi-pure™ (Epichem Inc., Haverhill, MA, USA) trimethylindium and triethylgallium were used as column III precursors and trimethylantimony (TMSb) and arsine (AsH3) were used as column V precursors. Prior to the growth, the substrates were cleaned in HCl to remove native surface oxide and then rinsed in isopropyl alcohol followed by N2 blow-drying. In a typical growth, 100-nm GaSb buffer was firstly deposited at 580°C, then the temperature was ramped down to 520°C for the growth of InAs/GaSb SL. Here, we grew 100-period SLs with nominal structures of 4.5-nm InAs and 3-nm GaSb with a plane of mixed As and Sb atoms connecting the GaSb and InAs as the IF layer. The growth of the IFs was controlled by atomic layer epitaxy-like switching sequence . The nominal x value of As x Sb1-xis 0.18. To obtain the AsSb mixing planes, a 3-s growth interruption with AsH3 flowing was introduced after the growth of InAs layer, then the AsH3 and TMSb switching valves were simultaneously closed and opened, respectively. The TMSb switching valve was left open for 10 s to form the nominal As0.18Sb0.82 plane.
The growth was resumed with the deposition of GaSb. The IF, thus, formed is a GaSb-on-InAs IF. For an InAs-on-GaSb IF, the GaSb surface was firstly smoothed by TMSb flowing for 0.5 s, then As was introduced for 5 s to exchange desired amount of Sb atoms on the GaSb surface followed by InAs deposition. The growth rate was 0.7 Å/s for InAs and 1 Å/s for GaSb layers, respectively. The surface morphologies of InAs/GaSb SLs were characterized by atomic force microscopy (AFM, Solver P47) operating in air at room temperature. High-resolution X-ray diffraction (HRXRD, Bede D1) was carried out with Cu Kα1 radiation as the source. Polarized Raman scattering measurements were performed in the backscattering geometry at room temperature with a Jobin Yvon (HORIBA Jobin Yvon Inc., Edison, NJ, USA) HR800 confocal micro-Raman spectrometer. Three scattering configurations, , and , were employed to assist in identifying the nature of the IF modes. X, Y' and Z' are defined along the ,  and crystallographic directions, respectively. The sample was excited by the 514.5-nm line of an Ar-ion laser to a 1-μm spot on the surface. IR transmission spectra were measured at room temperature using an IFS120HR Fourier-transform infrared (FTIR) spectrometer.
Results and discussion
We have achieved 100-period InAs/GaSb SLs on (100) GaSb substrates by MOCVD. An As0.1Sb0.9 plane connecting InAs and GaSb was introduced as the IF layer to compensate the strain of the SL to the GaSb substrate. The validity of the strain compensation was confirmed by the results of the in situ reflectance, AFM, HRXRD and Raman scattering measurements. The RMS roughness of the surface is 0.7 nm, and the lattice mismatch between the SL and the substrate is 0.18%. Absorption coefficient of approximately 2,000 cm-1 is realized in mid-IR region.
atomic force microscopy
first electron sub-band
first heavy hole sub-band
high-resolution X-ray diffraction
first electron sub-band
molecular beam epitaxy
metalorganic chemical vapor deposition
quantum well IR photodetectors
root mean square
This work was supported by the National Natural Science Foundation of China (grant nos. 50990064 and 60990315).
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