The Influence of GaSb Layer Thickness on the Band Gap of InAs/GaSb Type-II Superlattices for Mid-Infrared Detection
- PDF / 164,943 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 81 Downloads / 154 Views
Z5.31.1
The Influence of GaSb Layer Thickness on the Band Gap of InAs/GaSb Type-II Superlattices for Mid-Infrared Detection Heather J. Haugan, Frank Szmulowicz, Gail J. Brown Air Force Research Laboratory, Materials & Manufacturing Directorate, Wright-Patterson AFB, OH 45433-7707, U.S.A. ABSTRACT The effect of small changes in GaSb layer thickness on the photoresponse spectrum of InAs/GaSb superlattices (SLs) designed for mid-infrared detection was systematically investigated. The samples were grown by molecular beam epitaxy with precisely calibrated growth rates. The basic SL used for this study consisted of 40 periods of InAs (20.5 Å)/GaSb (X Å), where the nominal value for X was adjusted from 18 to 27 Å in four different samples. An InSb-like interface (IF) was inserted between the layers to balance the SL strain. By decreasing the GaSb width, the photoresponse cut-off wavelength (λc) was adjusted from 4.03 µm to 4.55 µm, i.e., the SL energy band gap is being decreased. This decrease in the energy separation between the first heavy hole band (HH1) and the first conduction band (C1) as the GaSb layer is narrowed is counter intuitive. However, this experimental trend can be explained by a modified envelope function approximation (EFA) calculation that includes the effect of in-plane asymmetry at InAs/GaSb interfaces. As expected, the HH band is pushed away from the top of the GaSb valence band as the GaSb layer width becomes narrower. However, at the same time the C1 band is significantly broadened by the increased wave function overlap of the electron states in the InAs layer. The trend to smaller band gap with narrower GaSb layers and other effects of the design changes on the photoresponse spectrum are discussed.
INTRODUCTION In recent years, there has been an increasing demand for photodetectors that can sense infrared (IR) radiation in the mid–IR detection range (3-5 µm) and preferably operate at ambient temperatures with low power consumption. One new material system that shows promise for high-speed infrared detection at ambient temperature is a SL of InAs and GaSb [1]. In this hetero-structure, the bands have the type-II alignment, making possible intra-band optical transitions between the confined electron and hole states, the difference between which is the SL energy band gap (Eg). By adjusting the individual layer thicknesses, Eg can be tuned to the midIR. In addition, through careful design, SL energy bands can be structured to reduce Auger recombination noise and enhance carrier lifetimes [2,3], which can potentially lead to improved sensitivity at ambient temperatures. This material system can be used in the photovoltaic devices, which would reduce the power requirement for imaging array operation [4,5]. Based on these advantages, we undertook a study to exploit the binary SL design for mid-IR detector applications. It is of interest to predict the proper SL band structure for mid-IR detection and to optimize the SL design for optimum detector performance. However, the standard EFA model [6,7] that
Data Loading...
