Growth of InAs/GaSb short-period superlattices for high-resolution mid-wavelength infrared focal plane array detectors

doi:10.1016/j.jcrysgro.2004.12.044

Journal of Crystal Growth

Volume 278, Issues 1–4, 1 May 2005, Pages 156-161

https://doi.org/10.1016/j.jcrysgro.2004.12.044 Get rights and content

Abstract

InAs/GaSb short-period superlattices (SLs) with a broken gap type-II band alignment are investigated for the fabrication of photovoltaic pin-photodetectors on GaSb substrates. The structures were grown by molecular beam epitaxy using valved cracker cells for arsenic and antimony. Effective bandgap and strain in the SL were adjusted by varying the thickness of the InAs and GaSb layers in the SL and the controlled formation of InSb-like or GaAs-like bonds at the interfaces. MBE growth conditions were investigated and optimized in order to achieve good morphological, electrical and optical properties. IR-photodiodes with a cut-off wavelength of 5.4 μm reveal quantum efficiencies around 30% and detectivity values exceeding 10¹³ Jones at 77 K. A focal plane array camera with 256×256 detector elements and 40 μm pitch based on InAs/GaSb short-period SLs was fabricated for the first time. The camera system reveals an excellent thermal resolution with a noise equivalent temperature difference below 12 mK for an integration time of 5 ms using f/2 optics. The detector performance, comparable with state of the art mercury–cadmium–telluride IR detectors, makes this material system very interesting for the fabrication of advanced thermal imaging systems.

Introduction

InAs/GaSb superlattices (SLs) with a broken gap type-II band alignment [1] have attracted increasing interest over the years for the fabrication of infrared detectors for the mid- (3–5 μm) and long-wavelength infrared region (8–12 μm). The materials system, proposed for IR-detectors by Smith and Mailhiot [2] in 1987, is investigated by several groups. Since the early 1990s, very promising electro-optical properties of single detector elements have been reported with characteristics similar to established mercury–cadmium–telluride (HgCdTe) detectors [3], [4], [5].

Compared to camera systems based on quantum well infrared photodetectors (QWIPs) [6], much higher quantum efficiencies can be obtained with InAs/GaSb SL detectors, making this material very promising for camera systems where short integration times and high frame rates are required.

For the fabrication of staring IR imagers based on InAs/GaSb SLs, stable and reproducible growth conditions as well as a reliable device processing technology have to be developed. In this paper, we report on MBE growth and processing of InAs/GaSb SLs and the successful fabrication of a fully integrated focal plane array (FPA) mid-wavelength infrared camera system with 256×256 detector elements.

Section snippets

Design of InAs/GaSb superlattices for MWIR detectors

Binary InAs/GaSb SLs grown on GaSb substrates form a nearly ideal material system for the fabrication of short-period SLs with broken gap type-II band alignment. The overlap of the GaSb valence band with the InAs conduction band is in the range of 140 meV [7]. For short-period SLs, the confinement energies of electron and holes exceed the overlap between the GaSb valence band and the InAs conduction band. As a consequence, a spatially indirect bandgap opens in the SL, which is smaller than the

MBE growth of InAs/GaSb superlattices

Samples were grown by MBE in a Gen-II system on undoped (1 0 0)-GaSb substrates with 2″ diameter. The system is equipped with standard group-III effusion cells for gallium, aluminium and indium and valved cracker cells for arsenic and antimony. Cracker temperatures were held at 800 °C for both arsenic and antimony. Silicon and GaTe as well as beryllium can be used for n- and p-type doping.

The detector structure consists of an Al_0.5Ga_0.5As_0.04Sb_0.96 buffer layer, a p-doped GaSb contact layer, 190

Detector processing

FPAs with 256×256 detector elements and 40 μm pitch were fabricated on InAs/GaSb SLs in a full wafer process using standard optical lithography. Each wafer contains four FPA chips with a size of 11.2×11.2 mm² and various test diodes for materials characterization. The scheme of a processed pixel in the FPA is shown in Fig. 4. Processing starts with the formation of ohmic contacts on top of the mesa, followed by chemical-assisted ion beam etching for mesa definition. The sidewall damage, caused by

Diode characterization and infrared imaging

The I–V characteristics of the diodes were measured on test diodes with the same size and geometry (37×37 μm²) as the detector diodes in the FPA. The dark current vs. applied voltage for a detector diode with a cut-off wavelength of 5.4 μm is shown in Fig. 6. The measurements reveal dynamic impedance values of $R_{0} A = 4 \times 1 0^{5} Ω {cm}^{2}$ at 77 K and beyond 1×10⁶ Ω cm² at 67 K, respectively. These diodes are limited by generation–recombination currents and show background limited performance (BLIP).

Large area test

Summary

InAs/GaSb SLs with broken gap type-II band alignment have been grown by molecular beam epitaxy for the fabrication of mid-IR imaging systems. Materials properties, epitaxial growth conditions and processing of these structures have been reported. A fully integrated 256×256 FPA camera system based on InAs/GaSb detectors has been fabricated for the first time. The high-performance camera system shows a quantum efficiency of about 30% and reveals excellent NETD values below 12 mK comparable with

Acknowledgements

The authors are grateful to K. Schwarz, L. Kirste, H. Güllich and F. Windscheid for characterization of the detector structures and to D. Eich, M. Finck, W. Rode, J. Wendler and R. Wollrab for camera fabrication and characterization. We also express our thanks to G. Weimann for continuous support and fruitful discussions. Financial support by the Bundesministerium für Verteidigung is gratefully acknowledged.

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The detector wavelength is flexible to change from about 1 [4] to 30 μm by tuning the SL period and the thickness ratio of InAs to GaSb. To date, encouraging progress has been made for the short [5,6], mid [7,8], long [9,10], and very long wavelength [11,12] (SW, MW, LW, and VLW) ranges. Compared to the HgCdTe detectors, which have superior performance, T2SL detectors may have nearly comparable performance, but have the merits of easier focal plane array device fabrication with high operability, better uniformity and lower cost.
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