256 × 256 Focal Plane Array Midwavelength Infrared Camera Based on InAs/GaSb Short-Period Superlattices

Journal of Electronic Materials, Jun 2005 by Walther, M, Rehm, R, Fuchs, F, Schmitz, J, Et al

An infrared camera based on a 256 × 256 focal plane array (FPA) for the second atmospheric window (3-5 µm) has been realized for the first time with InAs/GaSb short period superlattices (SLs). The SL detector structure with a broken gap type-II band alignment was grown by molecular beam epitaxy on GaSb substrates. Effective bandgap and strain in the superlattice were adjusted by varying the thickness of the InAs and GaSb layers and the controlled formation of InSb-like bonds at the interfaces. The FPAs were processed in a full wafer process using optical lithography, chemical-assisted ion beam etching, and conventional metallization technology. The FPAs were flip-chip bonded using indium solder bumps with a read-out integrated circuit and mounted into an integrated detector cooler assembly. The FPAs with a cutoff wavelength of 5.4 µm exhibit quantum efficiencies of 30% and detectivity values exceeding 10^sup 13^ Jones at T = 77 K. A noise equivalent temperature difference (NETD) of 11.1 mK was measured for an integration time of 5 ms using f/2 optics. The NETD scales inversely proportional to the square root of the integration time between 5 ms and 1 ms, revealing background limited performance. Excellent thermal images with low NETD values and a very good modulation transfer function demonstrate the high potential of this material system for the fabrication of future thermal imaging systems.

Key words: InAs/GaSb SLs, focal plane array, infrared camera

INTRODUCTION

InAs/GaSb short period superlattices (SLs) have been attracting increasing interest for the fabrication of mid- and long-wavelength infrared detectors. As proposed by Smith and Mailhiot1 in 1987, several groups have shown single detector elements with excellent electro-optical properties, similar to established mercury-cadmium-telluride (HgCdTe) detectors.2-4 The broken gap type-II band alignment of InAs and GaSb results in an overlap between the GaSb valence band and the InAs conduction band of about 140 meV.5 For InAs/GaSb SLs with confinement energies exceeding this band overlap, a spatially indirect bandgap opens, which is smaller than the bandgap of its constituents. InAs/GaSb SLs for mid-IR detection typically consist of InAs and GaSb layers with a thickness between five and fifteen monolayers. Heavy holes are largely confined in the GaSb layers, while electron wave functions overlap considerably from one InAs layer to adjacent InAs layers. The overlap of the electron wave functions results in the formation of an electron miniband in the conduction band. Spatially indirect transitions between the localized holes in the GaSb layers and the electron miniband are employed for the detection of mid-IR photons in the InAs/GaSb SL. A scheme of the band structure of an InAs/GaSb SL with an artificial band gap due to carrier confinement of electrons and holes is shown in Fig. 1. A detailed investigation of the dependence of bandgap and optical properties on layer thickness was recently reported by Haugan.6 For the fabrication of staring IR imaging systems based on InAs/GaSb SLs, stable and reproducible growth conditions as well as a reliable processing technology have to be developed. In this paper, we report on epitaxy, processing technology, diode characterization, and camera performance of a fully integrated InAs/GaSb SL focal plane array (FPA) mid-IR camera system with 256 × 256 detector elements.

FABRICATION TECHNOLOGY

InAs/GaSb SLs with a bandgap in the mid-IR wavelength region were grown by molecular beam epitaxy (MBE) in an Intevac Gen-II system on undoped (100)-GaSb substrates with 2-in. diameter using valved cracker cells for arsenic and antimony. The pinphotodetector structure consists of a latticematched Al^sub 0.5^Ga^sub 0.5^As^sub 0.04^Sb^sub 0.96^ buffer layer, a 500-nmthick p-type GaSb contact layer, 190 periods of an InAs/GaSb SL build with 9.5-monolayer-thick InAs and 12-monolayer-thick GaSb layers, and a 20-nmthick InAs cap layer. The lower part of the detector is p-type doped (1 × 10^sup 17^cm^sup -3^ with Be in GaSb), followed by a nominally undoped region and an n-type doped stack (5 × 10^sup 17^cm^sup -3^ with Si in InAs) on top of the structure. The carrier concentration in the undoped region is estimated to be p-type in the range of 10^sup 16^ cm^sup -3^ and results from the residual doping of the GaSb layers (p-type) and the InAs layers (η-type). To compensate for the tensile strain caused by the lattice mismatch between InAs and GaSb, the formation of InSb-like bonds was forced at the interfaces in the SL by an appropriate shutter sequence during growth.7 Growth conditions, i.e., growth temperature, V/III beam equivalent pressure ratios, and shutter sequences, have been optimized in order to improve materials quality and to establish a manufacturable growth process. Details of the growth conditions and materials characterization are reported elsewhere.8

Four FPA detector arrays with 256 × 256 elements and 40-µm pitch are processed on 2-in. epiwafers with a full wafer process using standard optical lithography. Processing starts with the formation of ohmic contacts on top of the mesa, followed by a chemical-assisted ion beam etching process using a chlorine-based chemistry for mesa definition. Sidewall damage, caused by the dry etch process on mesa sidewalls, is removed by wet chemical etching afterward. Then, the bottom p-contact is evaporated. The array is passivated with a silicon oxide layer and subsequently etched with a fluorine-based reactive ion etching process prior to the deposition of the reflector metallization. Then, a second silicon oxide passivation layer is deposited, followed by a reactive ion etching process to provide electrical contacts to each pixel. Finally, the bond metallization was evaporated on top of each detector element. A crosssectional scheme of a single pixel in the FPA arrangement is depicted in Fig. 2. After dicing the wafers, the detector chips were flip-chip bonded onto a read-out integrated circuit (ROIC) using indium solder bump technology. Following the hybridization with the ROIC, the GaSb substrate was removed to a thickness of about 20 µm to prevent thermally induced stress and to decrease absorption losses due to free carrier absorption in the GaSb substrate. Finally, the detector chip was mounted into an integrated detector cooler assembly.