Room temperature InGaAs camera for NIR imaging

DIODE WAVELENGTH (g.m)
Due both to the smaller baudgap and to interface defects resulting from the lattice mismatch, "long wavelength" InGaAs photodiodes have considerably higher dark currents than those fabricated from the lattice-matched alloy. For a variety of reasons, likely dominated by greater alloy stability under processing conditions, lnGaAs photodiode arrays tend to have lower (Jalt currents than HgCdTe arrays with similar long wavelength cutoffs.
436 /SPIE Vol. 1946 Infrared Detectors and Instrumentation (1993)
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2. InGaAs FPAS 2.1 Why InGaAs?
Figure 1 shows the bandgap vs. lattice constants of alloys of some common ffl-V semiconductors.3 Of interest here is the InAs/GaAs ternary system whose bandgaps span 035 eV (3.5 jim) for InAs to 1.43 eV (0.87p.m) for GaAs. The alloy In33Ga47As has a bandgap of 0.73 eV (1.7 tim) and has the same room temperature lattice constant as In?. This lattice match minimizes the defect density at the Ln33Ga47ASfInP interface and lends itself to devices with room temperature dark current densities of order 100 nA/cm2.
The 1-3 jnn band (SWIR) is ofrecent interest for many commercial and industrial as well as military applications. Of particular interest has been fiberoptic communications. Optical fibers exhibit a minimum in their absorption at 1.55 jim and indium gallium arsenide technology has dramatically advanced to service this need. In particular, In53Ga.47As is sensitive out to 1.7 j.un, is lattice-matched to indium phosphide substrates, and has particulanly high performance for optoelectronic applications.1'2 In this paper, we describe a camera based on a 128x128 pixel Inj3Ga47As focal plane aiTay. The camera
has very high sensitivity (D 1013 4H/W - or "Jones") with the focal plane array held at room temperature. It
is an attractive choice for applications requiring low cost ruggedness, and a lack of laboratory services such as cooling water or cryogenic liquids.
0-8194-1 182-5/93/$6.00
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When the indium content of the alloy is increased to 80%, the long wavelength cutoff extends to 2.5 im allowing photodiodes to cover the entire traditional near infrared band.4 The lattice constant of In.8Ga2As is about 2% larger than that of the hip substrate and a graded layer siructure is used to accommodate the
A'co(im): R0A(W-cm2):
1.4
InGaAs
HgCdTe
4x1O'(292K)
7x106 (230K)
2.5x1O(3OOK)
1.3xห้องสมุดไป่ตู้O (220K)
1.7 2.1
12.5x105(300K)
1.3x108 (220K)
1 2x102(300K)
the mean detectivity of the FPA, D*pk, exceeded 1013 4HfW and increased to 3.5x1014 cm4HzfW when cooled to 230K. Potential applications for this wavelength band are discussed.
1. INTRODUCTION
During the past 20 years, there has been dramatic progress in staring focal plane arrays used in a wide vanety of consumer, commercia] industrial, and militaiy applications. Applications tend to be divided by wavelength band which is often dictated by the availability of sensor material technologies. Silicon CCDs, for example, cover the visible band (and the shortest wavelengths of the near infrared) and are used extensively in applications ranging from consumer video camcorders through medical and astronomical imaging and optical speciroscopy.
At longer wavelengths, most activity has centered on the 3-5 jim (MWIR) and the 8-12 jim (LWIR) bands. The application is primarily thermal imaging. Development has been driven by the militaly, and these bands have been identified because they represent minima in atmospheric absorption.
Room temperature InGaAs camera for N imaging
Marshall J. Cohen and Gregory H. Olsen Sensors Unlimited, Inc. 3490 U.S. Route 1, Princeton, NJ 08691
ABSTRACT
A useful parameter is "RØA", the product of the zero-bias resistance and the Fig. 1 . Cutoff wavelengths and lattice constants for ffl-V compound photodiode active area.5 Table 1 compares semiconductors. this parameter for InGaAs and HgCdTe two dimensional photodiode arrays for a variety of cutoff wavelengths and temperatures. For 1.7 jim material, RØA of InGaAs is 3 orders of magnitude higher than that of HgCdTe. This is a direct result of its lattice-matched structure which does not occur with HgCdTe. For longer wavelengths, InGaAs is no longer lattice matched to its substrate, yet its RØA continues to be an order of magnitude higher than that of HgCdTe.