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Hgcdte Infrared Detector Material: History, Status and Outlook

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Hgcdte Infrared Detector Material: History, Status and Outlook INSTITUTE OF PHYSICS PUBLISHING REPORTS ON PROGRESS IN PHYSICS Rep. Prog. Phys. 68 (2005) 2267–2336 doi:10.1088/0034-4885/68/10/R01 HgCdTe infrared detector material: history, status and outlook A Rogalski Institute of Applied Physics, Military University of Technology, 2 Kaliskiego St., 00-908 Warsaw, Poland Received 10 November 2004, in final form 13 April 2005 Published 22 August 2005 Online at stacks.iop.org/RoPP/68/2267 Abstract This article reviews the history, the present status and possible future developments of HgCdTe ternary alloy for infrared (IR) detector applications. HgCdTe IR detectors have been intensively developed since the first synthesis of this material in 1958. This article summarizes the fundamental properties of this versatile narrow gap semiconductor, and relates the material properties to its successful applications as an IR photoconductive and photovoltaic detector material. An emphasis is put on key developments in the crystal growth and their influence on device evolution. Competitive technologies to HgCdTe ternary alloy are also presented. Recent advances of backside illuminated HgCdTe heterojunction photodiodes have enabled a third generation of multispectral instruments for remote sensing applications and have led to the practicality of multiband IR focal plane array technology. Finally, evaluation of HgCdTe for room temperature long wavelength IR applications is presented. (Some figures in this article are in colour only in the electronic version) 0034-4885/05/102267+70$90.00 © 2005 IOP Publishing Ltd Printed in the UK 2267 2268 A Rogalski Contents Page 1. Introduction 2269 2. Fundamental HgCdTe properties 2269 2.1. Energy band gap 2271 2.2. Mobilities 2272 2.3. Optical properties 2272 2.4. Thermal generation–recombination processes 2274 2.4.1. Shockley–Read processes 2274 2.4.2. Radiative processes 2275 2.4.3. Auger processes 2275 2.5. α/G ratio and detectivity 2276 3. Photodiodes—principle of operation and figure of merit 2279 4. Historical perspective 2284 5. Impact of epitaxial growth on development of HgCdTe detectors 2291 6. HgCdTe photodiodes 2296 7. Third generation detectors 2306 7.1. Noise equivalent difference temperature 2306 7.2. Pixel and chip sizes 2307 7.3. Uniformity 2308 7.4. Identification and detection ranges 2309 7.5. Two-colour HgCdTe detectors 2311 8. Alternative material systems 2315 8.1. Lead salt ternary alloys 2315 8.2. InSb and InGaAs 2316 8.3. GaAs/AlGaAs QWIPs 2319 8.4. InAs/GaInSb strained layer superlattices 2320 8.5. Hg-based alternatives to HgCdTe 2322 9. HgCdTe versus thermal detectors 2322 10. Summary 2330 References 2331 HgCdTe infrared detector material 2269 1. Introduction In 1959, a publication by Lawson et al [1] triggered the development of variable band gap Hg1−x Cdx Te (HgCdTe) alloys providing an unprecedented degree of freedom in infrared (IR) detector design. HgCdTe is a pseudo-binary alloy semiconductor that crystallizes in a zinc blende structure. Because of its band gap tunability with x,Hg1−x Cdx Te has evolved to become the most important/versatile material for detector applications over the entire IR range. As the Cd composition increases, the energy gap for Hg1−x Cdx Te gradually increases from a negative value for HgTe to a positive value for CdTe. The band gap energy tunability results in IR detector applications that span the short wavelength IR (SWIR: 1–3 µm), middle wavelength (MWIR: 3–5 µm), long wavelength (LWIR: 8–14 µm) and very long wavelength (VLWIR: 14–30 µm) ranges. HgCdTe technology development was and continues to be primarily for military applications. A negative aspect of support of defence agencies has been the associated secrecy requirements that inhibit meaningful collaborations among research teams on a national and especially on an international level. In addition, the primary focus has been on focal plane array (FPA) demonstration and much less on establishing the knowledge base. Nevertheless, significant progress has been made over four decades. At present HgCdTe is the most widely used variable gap semiconductor for IR photodetectors. The specific advantages of HgCdTe are the direct energy gap, ability to obtain both low and high carrier concentrations, high mobility of electrons and low dielectric constant. The extremely small change of lattice constant with composition makes it possible to grow high quality layers and heterostructures. HgCdTe can be used for detectors operated at various modes, and can be optimized for operation at the extremely wide range of the IR spectrum (1–30 µm) and at temperatures ranging from that of liquid helium to room temperature. The main motivations for replacing HgCdTe are the technological disadvantages of this material. One of them is a weak Hg–Te bond, which results in bulk, surface and interface instabilities. Uniformity and yield are still issues especially in the LWIR spectral range. Nevertheless, HgCdTe remains the leading semiconductor for IR detectors. This article summarizes the fundamental properties of HgCdTe versatile alloy semiconductor, and relates these material properties, which have successful applications as an IR detector material. The intent is to concentrate on device approaches and technologies that are having the most impact today mainly in photovoltaic detectors. A secondary aim is to outline the historical evolution of HgCdTe detector technology showing why certain device designs and architecture have emerged as more successful and more useful today, especially in FPA technology and development of third generation IR detectors. Also alternative technologies competitive to HgCdTe, technology are considered. 2. Fundamental HgCdTe properties HgCdTe ternary alloy is a nearly ideal IR detector material system. Its position is conditioned by three key features [2]: • tailorable energy band gap over the 1–30 µm range, • large optical coefficients that enable high quantum efficiency and • favourable inherent recombination mechanisms that lead to high operating temperature. These properties are a direct consequence of the energy band structure of the zinc blende semiconductor. Moreover, the specific advantages of HgCdTe are the ability to obtain both low and high carrier concentrations, high mobility of electrons and low dielectric 2270 A Rogalski Table 1. Summary of the material properties for the Hg1−x Cdx Te ternary alloy, listed for the binary components HgTe and CdTe, and for several technologically important alloy compositions (after [2]). Property HgTe Hg1−x Cdx Te CdTe x 0 0.194 0.205 0.225 0.31 0.44 0.62 1.0 a (Å) 6.461 6.464 6.464 6.464 6.465 6.468 6.472 6.481 77 K 77 K 77 K 77 K 140 K 200 K 250 K 300 K Eg (eV) −0.261 0.073 0.091 0.123 0.272 0.474 0.749 1.490 λc (µm) — 16.9 13.6 10.1 4.6 2.6 1.7 0.8 −3 14 13 12 12 11 10 5 ni (cm )—1.9 × 10 5.8 × 10 6.3 × 10 3.7 × 10 7.1 × 10 3.1 × 10 4.1 × 10 mc/m0 — 0.006 0.007 0.010 0.021 0.035 0.053 0.102 gc — −150 −118 −84 −33 −15 −7 −1.2 εs/ε0 20.0 18.2 18.1 17.9 17.1 15.9 14.2 10.6 ε∞/ε0 14.4 12.8 12.7 12.5 11.9 10.8 9.3 6.2 nr 3.79 3.58 3.57 3.54 3.44 3.29 3.06 2.50 2 −1 −1 5 5 5 µe (cm V s )— 4.5 × 10 3.0 × 10 1.0 × 10 ———— 2 −1 −1 µhh (cm V s ) — 450 450 450 — — — — b = µe/µη — 1000 667 222 — — — — τR (µs) — 16.5 13.9 10.4 11.3 11.2 10.6 2 3 τA1 (µs) — 0.45 0.85 1.8 39.6 453 4.75 × 10 τtypical (µs) — 0.4 0.8 1 7 — — — Ep (eV) 19 (eV) 0.93 mhh/m0 0.40–0.53 Ev (eV) 0.35–0.55 15 −3 τR and τA1 calculated for n-type HgCdTe with Nd = 1×10 cm . The last four material properties are independent of or relatively insensitive to alloy composition. Table 2. Some physical properties of narrow gap semiconductors. −3 4 2 −1 −1 4 2 −1 −1 Eg (eV) ni (cm ) µe (10 cm V s ) µh (10 cm V s ) Material 77 K 300 K 77 K 300 K ε 77 K 300 K 77 K 300 K InAs 0.414 0.359 6.5 × 103 9.3 × 1014 14.5 8 3 0.07 0.02 InSb 0.228 0.18 2.6 × 109 1.9 × 1016 17.9 100 8 1 0.08 11 In0.53Ga0.47As 0.66 0.75 — 5.4 × 10 14.6 7 1.38 — 0.05 PbS 0.31 0.42 3 × 107 1.0 × 1015 172 1.5 0.05 1.5 0.06 PbSe 0.17 0.28 6 × 1011 2.0 × 1016 227 3 0.10 3 0.10 PbTe 0.22 0.31 1.5 × 1010 1.5 × 1016 428 3 0.17 2 0.08 13 16 Pb1−x Snx Te 0.1 0.1 3.0 × 10 2.0 × 10 400 3 0.12 2 0.08 13 16 Hg1−x Cdx Te 0.1 0.1 3.2 × 10 2.3 × 10 18.0 20 1 0.044 0.01 8 15 Hg1−x Cdx Te 0.25 0.25 7.2 × 10 2.3 × 10 16.7 8 0.6 0.044 0.01 constant. The extremely small change of lattice constant with composition makes it possible to grow high quality layered and graded gap structures. As a result, HgCdTe can be used for detectors operated at various modes (photoconductor, photodiode or metal–insulator– semiconductor (MIS) detector).
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