D.A. Redfern

Improved quantitative mobility spectrum analysis for Hall characterization

We present an improved quantitative mobility spectrum analysis (i-QMSA) procedure for determining free electron and hole densities and mobilities from magnetic-field-dependent Hall and resistivity measurements on bulk or layered semiconductor samples. The i-QMSA technique is based on a fundamentally new approach, which optimizes the fit to the conductivity tensor components and their slopes by making those adjustments in the mobility spectra that result in the greatest error reduction. Empirical procedures for manipulating the mobility spectra are also introduced, with the dual purpose of reducing the error of the fit and simplifying the shape of the spectra to minimize the presence of unphysical artifacts. A fully automated computer implementation of the improved QMSA is applied to representative synthetic and real data sets involving various semiconductor material systems. These results show that, as compared with previous approaches, the presented algorithm maximizes the information that may be extract...

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.

Diffusion length measurements in p-HgCdTe using laser beam induced current

The minority carrier diffusion length in p-HgCdTe is a key indicator of material quality and gives an indication of n-on-p diode performance when the zero bias resistance is diffusion limited. We present results of a temperature dependent study of diffusion length in p-HgCdTe using laser beam induced current (LBIC). Carriers are collected by a p-n junction formed using standard diode junction formation conditions, and thus not necessarily extending to the substrate. Two-dimensional modeling is used to examine the validity of results obtained using this geometry, as compared to the more standard diffusion length test structure geometries, which are harder to fabricate. The temperature dependence of the diffusion length can be compared with theoretical models to determine the dominant recombination mechanisms.

Characterization of electrically active defects in photovoltaic detector arrays using laser beam-induced current

One of the main limitations in the operability of modern infrared focal plane arrays of p-n junction diodes formed on molecular-beam epitaxy (MBE)-grown HgCdTe is the effect of localized defects. Such defects, including voids, triangles and microvoids, are a feature of the MBE growth regime and can compromise the performance of devices fabricated within the vicinity of electrically active defects. While such defects can often be identified visually, not all defects are electrically active such that they provide a current leakage path shunting the p-n junction of the individual photodiode. In this paper, the use of laser beam-induced current is proposed as a nondestructive characterization technique, and quantitative aspects of its use in the study of electrically active defects in photodiode arrays are examined.

PARAMETER IDENTIFICATION FOR SEMICONDUCTOR DIODES BY LBIC IMAGING

Laser-beam-induced-current (LBIC) imaging is a nondestructive technique used for the characterization of the electrical structure within a semiconductor. In this paper a model is formulated for this technique using the standard drift-diffusion model, and, subsequently, an approximate version and its dual are derived for the study of the inverse problem. The formulation is then applied to a cross-sectional model for n-on-p devices of finite depth to study in detail the relation between the LBIC images and the device parameters. Numerical methods are developed for the simulation of the LBIC image of a diode as well as for the identification of parameters from the LBIC image by least-squares formulation. Numerical examples are presented to illustrate the success of identifying parameters such as junction depth, diffusion length, and equilibrium potential of an abrupt p-n junction diode from its LBIC image. The differentiability of the image with respect to the parameters also is established.

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 (200 °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.

Investigation of laser beam-induced current techniques for heterojunction photodiode characterization

A reduced model is developed that has significant advantages over the full drift-diffusion model for the simulation of laser beam-induced current (LBIC) signals in the presence of heterojunctions. The model determines the contribution to the LBIC signal that would occur from photogeneration at any position within the semiconductor, and is particularly useful for heterostructures where judicious choice of illumination wavelength can result in photogeneration at different depths within the device structure. The reduced model is used to examine the basic features of LBIC as applied to two types of planar P‐n HgCdTe heterojunction photodiode structures. In particular, the question of correctly identifying erroneous device structures formed during the fabrication process is addressed, and experimental measurements are presented to support the simulation results.

LBIC imaging of semiconductor arrays: the cross-sectional model

Laser beam induced current (LBIC)/nondestructive technique that has been used for a number of years to qualitatively examine large arrays of p-n junctions, especially in HgCdTe infrared focal plane arrays. In this paper, we quantitatively study the application of the LBIC imaging technique to semiconductor arrays Based on a previous mathematical model for LBIC applied to individual devices, we employ the homogenization method to derive approximations of the LBIC images of large arrays. Such approximations reduce the computational burden in simulations of these LBIC bouges. We then illustrate the application of our approximations for the purpose of recovering array parameters from the LBIC images.

Laser-beam-induced current technique as a quantitative tool for HgCdTe photodiode characterization

A non-destructive optical characterization technique is used for the investigation of HgCdTe photovoltaic devices. The technique uses a scanning laser microscope to obtain Laser Beam Induced Current (LBIC) data from which it may be possible to extract information such as junction depth, array uniformity, and other material and device parameters. LBIC has been previously used only as a qualitative technique, but in this work the procedure is being developed into a quantitative tool. At present the only junction depth profiling techniques are destructive, while array uniformity can only be examined after bonding to readout circuits. In this paper we present both theoretical and experimental; results which show that LBIC can be employed as a quantitative tool for device characterization. The primary measure of performance of IR detectors is the zero bias dynamic resistance junction area product, R0A. LBIC measurements indicate that the peak LBIC signal varies by a factor of approximately 2 for long wavelength RI photodiodes for which the R0A varies between 70 (Omega) cm2 and 8 (Omega) cm2.

Non-contact evaluation of photodiode performance by laser beam induced current imaging

This work reports on the application of laser beam induced current (LBIC) to the determination of the product of zero-bias dynamic resistance and area, R/sub 0/A, of homojunction photodiodes. The technique involves using LBIC to measure temperature dependent values of the photocarrier spreading length of a particular device, and then fitting the theoretical temperature dependence. The photocarrier spreading length is a measure of the rate of decay of photoinduced forward bias of the p-n junction with distance from the location of electron-hole pair generation and is partly responsible for the shape of the LBIC profile. The LBIC magnitude is not required to be measured in any quantitative sense, although there is a requirement that the photocarrier spreading length be shorter than the device length in order for it to be measurable. The technique provides a non-contact method of determining the performance of individual devices within large two-dimensional focal plane arrays of photodiodes. Experimental results from Hg/sub 0.77/Cd/sub 0.23/Te infrared photodiodes are presented to demonstrate the procedure.