E.P.G. Smith

Interpretation of current flow in photodiode structures using laser beam-induced current for characterization and diagnostics

This paper presents an interpretation of the physical mechanisms involved in the generation of laser beam-induced current (LBIC) in semiconductor p-n junction diodes. LBIC is a nondestructive semiconductor characterization technique that has been used in a qualitative manner for a number of years and is especially useful for examining individual photodiodes within large two-dimensional arrays of devices. The main thrust of this work is the analysis of LBIC in terms of nonzero steady-state circulatory current flow within the device and, hence, the interpretation of LBIC line profiles to diagnose the patterns of current flow within the structure. This provides an important basis for future studies seeking to relate LBIC to indicators of p-n junction performance and integrity such as dark current components and reverse bias saturation current. In particular, this paper examines the ideal cases of a single isolated p-n junction diode structure, and also considers an array of such devices in close proximity to each other. Modifications to the idealized theory that are required to account for localized junction leakage and surface recombination are presented, and the effect of Schottky contacts is discussed. Numerical simulations based on the HgCdTe family of semiconductors are presented to support the theory.

Mercury annealing of reactive ion etching induced p- to n-type conversion in extrinsically doped p-type HgCdTe

Mercury annealing of reactive ion etching (RIE) induced p- to n-type conversion in extrinsically doped p-type epitaxial layers of HgCdTe (x=0.31) has been used to reconvert n-type regions created during RIE processing. For the RIE processing conditions used (400 mT, CH4/H2, 90 W), p- to n-type conversion was observed using laser beam induced current (LBIC) measurements. After a sealed tube mercury anneal at 200u2009°C for 17 h, LBIC measurements clearly indicated that no n-type converted region remained. Subsequent Hall measurements confirmed that the material consisted of a uniform p-type layer, with electrical properties equivalent to that of the initial as-grown wafer (NA−ND=2×1016u2009cm−3, μ=350u2009cm2u2009V−1u2009s−1).

Estimation of doping density in HgCdTe p-n junctions using scanning laser microscopy

Quantitative assessment of p- to n- type conversion due to reactive ion etching (RIE) of p-type Hg0.71Cd0.29Te is presented using laser-beam-induced-current (LBIC) measurements. For the RIE processing conditions used (390 mT, CH4/H2, 0.4 W/cm2), n-type conversion was observed in extrinsic arsenic-doped p-type Hg0.71Cd0.29Te which had previously undergone a Hg anneal to eliminate Hg vacancies. Effective doping density of the n-type converted region is determined by fitting a theoretically determined LBIC signature to the measured LBIC signal over a temperature range 80–300 K. Effective n-type doping density is the only fitting parameter used in the simulation, which was carried out using a commercial semiconductor device modeling package (SEMICAD™ DEVICE). This noncontact experimental technique promises to be a useful tool in the characterization of p-n junction diodes in HgCdTe, and for studying the precise nature of p to n conversion in p-type HgCdTe.

Reactive ion etching for mesa structuring in HgCdTe

Both wet chemical and dry plasma etching techniques have been investigated for mesa structuring in n- and p-type HgCdTe. Scanning electron microscopy (SEM) confirms the isotropic nature of a bromine-based wet chemical etching solution, and the anisotropic profile that results from reactive ion etching. Laser-beam-induced-current (LBIC) measurements reveal no significant modifications to the electrical properties for chemically etched HgCdTe material, but clearly demonstrate a p- to n-type conversion in p-type samples and n+ doping in n-type samples for reactive ion etching (RIE) (processing conditions: 400 mTorr, CH4/5H2, 0.4 W/cm2). LBIC measurements following low-temperature (200u2009°C) mercury annealing of RIE-processed samples indicate the full restoration of electrical properties to that of the initial as-grown wafers, thus preserving the beneficial aspects of RIE for anisotropic mesa structuring in HgCdTe.

Two-dimensional modelling of HgCdTe photoconductive detectors

Abstract The performance of n-type HgCdTe mid-wavelength infrared (MWIR) photoconductors has been investigated using two-dimensional (2-D) device modelling. A comparison has been made between a practical detector structure with planar contacts on the upper HgCdTe surface, and a structure commonly used in one-dimensional (1-D) device modelling with end contacts to the photo-absorbing slab of semiconductor. This comparison highlights differences in detector responsivity, and spatial distribution of both the applied electric field and photogenerated minority carriers. The practical device structure, where 2-D effects are most evident, provided a good fit to experimental results for frontside-illuminated n-type HgCdTe photoconductive detectors with n + /n blocking contacts without needing to include S , the contact recombination velocity, which is commonly employed in 1-D models as a fitting parameter. Instead, only the n + doping density (1xa0×xa010 16 cm –3 ) and n + doping region (depth of 3 μm), were used to account for the partial blocking of minority carriers by the contact region. In addition, the 2-D model was used to examine the influence of n + blocking contact geometry and doping density on n-type HgCdTe photoconductor responsivity performance.

A simplified fabrication process for HgCdTe photoconductive detectors using CH4/H2 reactive-ion-etching-induced blocking contacts

A fabrication procedure using dry plasma process technology has been developed for HgCdTe photoconductive detectors. Dry plasma, in the form of CH4/H2 reactive ion etching, was used in a multi-functional process capacity after the mesa delineation of individual detector elements and prior to metal contact deposition to (i)xa0etch the passivation layer to open contact areas for metal deposition and (ii)xa0take advantage of the increase in effective doping in HgCdTe material when it is exposed to the CH4/H2 plasma and increase the n-type doping in the opened contact areas. The limited undercut that results from etching the passivation layer with dry plasma, as opposed to traditional wet-chemical etching techniques, eliminates the need for a photolithographic masking step to align the metal contacts over the contact areas after they have been opened in the passivation layer. Increasing the n-type doping in the contact areas has the purpose of providing both low-resistance ohmic contacts and a potential barrier to minimize photogenerated carrier recombination at the semiconductor/metal interface and thereby improve detector responsivity performance. The process improvements outlined in this paper make it possible to fabricate high-performance infrared photoconductive devices using two photolithographic masking steps. Mid-wavelength infrared HgCdTe photoconductive detectors fabricated using this procedure demonstrate a background-limited Dλ* of 2.0×1011xa0cmxa0Hz1/2xa0W-1 at an operating temperature of 80xa0K, illustrating the application of dry plasma technology to HgCdTe infrared detector fabrication for both process and performance optimization.

On the transient photoconductive decay technique for lifetime extraction in HgCdTe

The minority carrier lifetime of HgCdTe is studied using the photoconductive decay technique. The focus of this paper is to examine the various methods of extracting minority carrier lifetime and/or surface recombination parameters from photoconductive decay data, with particular reference to HgCdTe. It is shown that none of the current models unambiguously explain experimental results and that detailed lifetime extraction by photoconductive decay is still not a quantitative technique.

Performance and stability of HgCdTe photoconductive devices: a study of passivation and contact technology

HgCdTe is the material of choice for high performance infrared (IR) detectors operating in the long-wavelength and mid-wavelength (LWIR and MWIR, respectively) regions of the electromagnetic spectrum. Photoconductive HgCdTe IR detectors are commonly used for single element and linear arrays due to their higher yield when compared to photovoltaic devices. In this work characterisation of fabricated n-type photoconductors with blocking contacts and heterostructure passivation are presented. The two-layer heterostructure photoconductors exhibit up to a 100% increase in responsivity over the equivalent single-layer photoconductors at an applied electric field of 10 V/cm. The two-layer and single-layer photoconductors have been subjected to bake tests at 85/spl deg/C for 20 hours with the single-layer photoconductors showing a large degradation in performance compared to the two-layer photoconductors. A suitably modified commercial device simulation package has been used to model blocking contacts in two dimensions.

Minority carrier sweepout effects in HgCdTe infrared photoconductors at temperatures above 80K

The effect of drift and diffusion of minority carriers into regions of high recombination which occur at the metal/semiconductor interface of contacts, has the effect of reducing the density of photogenerated excess carriers in photoconductive devices. This loss of photogenerated carriers, which is enhanced at higher applied electric fields, is known as minority carrier sweepout, and is an important mechanism that limits the performance of HgCdTe photoconductive devices operating at high bias fields. In this study, experimentally determined contact recombination velocities range from 25 cm/s at 80 K, to 600 cm/s at 200 K for x=0.31 Hg/sub 1-x/Cd/sub x/Te. Hence, it is concluded that contact recombination is a dominant mechanism at higher temperatures even though it may not be significant at 80 K.

Temperature dependent noise performance of Hg/sub 1-x/Cd/sub x/Te photoconductive detectors

Background limited infrared photodetector (BLIP) performance, attained at cryogenic temperatures, is the maximum performance achievable by a photoconductive detector. By increasing the operating temperature, low cost detectors can be realised using thermoelectric cooling (>200 K). Investigation of temperature dependent performance of Hg/sub 1-x/Cd/sub x/Te infrared detectors is essential for developing device structures and fabrication technologies capable of producing BLIP performance. In this paper the performance of n-type x=0.31 Hg/sub 1-x/Cd/sub x/Te photoconductors under reduced cooling is examined experimentally and theoretically. Measurements gave a background limited D/sub /spl lambda//* of 1.9/spl times/10/sup 11/ cm Hz/sup 1/2/W/sup -1/ up to 170 K and D/sub /spl lambda//* values of 7.3/spl times/10/sup 10/ and 6.4/spl times/10/sup 9/ cm Hz/sup 1/2/W/sup -1/ were measured at 200 K and 300 K, respectively.