Table of contents

An extensive industrial survey of the importance and use of characterization measurements for HgCdTe materials, processes and devices has been completed. Seventy-two characterization/measurement techniques were considered and thirty-five responses were received. This information was sought for a study on materials characterization and measurement techniques of parameters and properties necessary to improve the manufacturing capabilities of HgCdTe infrared detectors. The nature of materials characterization is defined, and an overview is given of how it is related to improving IR detector manufacturing. Finally, the authors present a description of the characterization survey and a summary of the survey results. Major aspects of the results include: (1) ranking the 72 techniques by their importance and frequency of use, (2) listing the parameters or properties determined by each technique, (3) enumerating the most important properties that need to be measured, (4) indicating the key measurement techniques that most need to be developed, enhanced or improved, and (5) giving key overall comments.

The growth of the silicon device and integrated circuit industry has been closely coupled with the development of materials characterization technology. The author traces this development from the beginning, when the industry was young and each manufacturer had to grow its own materials, develop its processes, assemble its measurement systems from component instruments, and fabricate its processing equipment, to the present, when a complex infrastructure supports the industry. He also describes examples of both successful and unsuccessful developments in connection with other electronic materials. The critical factors in the widespread standardization and application of silicon characterization technology are: shared results and common coals among industry, government and universities; an efficient mechanism for development of consensus standards with adequate expertise; standardization of parameters needed for the orderly flow of materials in manufacturing together with standardized terminology, test methods and formats to support detailed purchase specifications; and refinement of the measurement technology as the industry develops and becomes more sophisticated. Finally, a good methodology for process diagnosis and control is an essential feature for applying test procedures to the manufacturing environment. The successes and failures which occurred during the course of developing metrology tools for silicon can provide some insights into similar developments for the special needs of other electronic materials. In addition, many of the test procedures and standards developed for silicon materials and supporting technologies can be applied directly.

The authors review those characterization techniques that have played significant roles in the development of HgCdTe infrared detector technology. They focus on the two specific HgCdTe devices that have achieved widespread application for infrared detection in the LWIR (8-12 mu m) and VLWIR (12-20 mu m) spectral regions: the simple n-type photoconductor and the P-on-n LPE heterojunction photodiode. They review the device physics of these two detectors, relate device performance to starting material properties and processing parameters, and describe the most important characterization techniques that have had a role in their development.

Advanced techniques for the magnetotransport characterization of semiconductor IR detector materials are reviewed. Both conventional mixed-conduction and 'mobility spectrum' analyses of the resistivity tensor as a function of magnetic field and temperature are discussed, as well as a hybrid approach which exploits the advantages of both methods. Assuming that the data are sufficiently sensitive, one obtains concentrations and mobilities for each electron and hole species present in a given sample. This is particularly valuable for narrow-gap infrared detector materials such as Hg_1-xCd_xTe, since the coexistence of multiple species tends to be the rule rather than the exception, and 'anomalous' Hall data are easily misinterpreted if inferences are drawn from measurements at only a single magnetic field. In addition to bulk electron and hole densities and mobilities, one can determine inversion and accumulation layer properties from the anomalous Hall effect, acceptor binding energies and compensation ratios from the low-temperature freeze-out of free holes, and energy gaps from the temperature dependence of the intrinsic carrier concentration. Shubnikov-de Haas and quantum Hall measurements provide additional information about the spatial distribution of the carriers. Electron and hole mobilities in Hg_1-xCd_xTe detector materials will be briefly reviewed, and theoretical and experimental results compared.

The authors review the current status of minority carrier lifetime in n-type and p-type (Hg, Cd)Te. This review includes a discussion of the relevant (Hg, Cd)Te recombination mechanisms and measurement techniques. The reported experimentally determined lifetimes were related to (Hg, Cd)Te material properties of carrier concentration, Shockley-Read-Hall centres, non-uniformities and dislocation densities.

Three techniques for determining the composition (x identical to mole fraction of CdTe) of Hg_1-xCd_xTe are reviewed. These three techniques are infrared transmission (often called FTIR, for Fourier transform infrared) spectroscopy, energy dispersive X-ray analysis (EDX) and optical reflectance (OR). A brief summary of several methods for determining composition in Hg_1-xCd_xTe is included.

In situ diagnostic techniques are broadly divided into two categories, those involving an electron probe or reflected beam and those relying just on photon beams. The former, such as RHEED, XPS, AES and UPS, can be used in MBE monitoring but are not suitable for the high-pressure environment of MOVPE. Optical diagnostic techniques such as ellipsometry, laser reflectometry, quasielectric light scattering, reflection difference spectroscopy (RDS) and infrared reflection-absorption spectroscopy (IRRAS) can be used as an in situ monitor in both UHV and high-pressure growth chambers. These are compared with the electron beam techniques and some analogues are established in terms of the properties of the growing films. The in situ monitoring of MCT growth is helping to improve layer quality and reproducibility. Some specific examples are discussed, including the potential for in situ feedback control.

Spatially resolved characterization of HgCdTe materials and p-n junction diodes using scanning laser microscopy is reviewed. Several techniques that yield spatial maps of electrical inhomogeneities in HgCdTe material and non-uniformities in various performance parameters of p-n junctions fabricated using these materials have been developed. Many of the techniques are non-destructive, or can be made such with minor changes in sample preparation, and are scalable to large full wafers. A high-resolution and non-destructive technique called 'laser beam induced current (LBIC)' has been developed for spatial mapping of electrically active regions in HgCdTe. When applied to unprocessed HgCdTe material, LBIC images represent spatial distributions of electrically active defects including inclusions, strain, damage, precipitates, stacking faults, twin boundaries, dislocation clusters, bandgap and doping variations. Device structures such as p-n junctions are special cases of electrically active regions, therefore the LBIC technique lends itself to a non-destructive study of p-n junction arrays without requiring any direct electrical contact to the individual elements of the array. The remote contacting, especially using pressure contacts, makes the application of the LBIC technique non-destructive, allowing testing at various stages of device processing to identify particular processing procedures that need optimization. LBIC has also been used to spatially map electrical non-uniformities at the HgCdTe surface near its interface with an insulating passivation layer. Besides LBIC imaging, scanning laser microscopy has been used for several other applications. For HgCdTe p-n junctions, applications include photoresponse mapping (uniformity, active area and diffusion length determination, contact bonding effects), spatially resolved light-induced degradation, and avalanche photodiode properties (ionization coefficients, localized breakdown). A key technique currently in development is the non-contact diode R₀A determination based on trapping mode photoconductivity. Examples of scanning laser microscopy on HgCdTe materials are infrared transmission mapping (thickness and bandgap variations), photoluminescence mapping, and minority carrier lifetime mapping (distribution of recombination centres).

The impact of microelectronic test structures, as they have been applied to silicon integrated circuits (ICS) and gallium arsenide monolithic microwave integrated circuits (MMICS), is reviewed. General principles for the use of test structures with possible applications to infrared (IR) detector technology based on HgCdTe and other materials are emphasized. The uses of test structures for Si and GaAs, test chip design methodology and some examples of how test structures have been applied for process control and to increase yield are discussed. Specific test structures and techniques that have been applied to IR detectors are also reviewed. The basic design considerations and measurements possible with each class of test structure are discussed. The important experience of the Si and GaAs industries, applicable to IR detectors, is that significant yield improvement is possible with improved process control using test structures. Increased research efforts to expand the applications of test structures to IR detector manufacture are indicated.

The authors have developed detailed non-destructive mapping and spatial imaging techniques for comparing boule and wafer properties with analytical predictions from high-fidelity process models for seeded vertical Bridgman-Stockbarger growth of CdZnTe crystals. They have emphasized the prediction of the magnitude and distribution of residual stress and strain, as well as longitudinal and radial segregation within the boules and across wafers cut from the boules. Boule and wafer compositional distributions were mapped using photoreflectance, precision lattice parameter measurements and FTIR spectroscopy. Defect and strain distributions within the boules and wafers were imaged using synchrotron topography, synchrotron strain contour mapping, and double-crystal rocking-curve mapping. Thermomechanical and thermosolutal models specifically addressing the seeded vertical Bridgman-Stockbarger growth of CdZnTe crystals were developed and empiricized. These models addressed solute redistribution and stress generation as a result of the interface shape, aspect ratio and growth parameters during the seeding, initial transient (including the shoulder), steady-state and terminal transient regions of the boule. Finally, the stress and strain distributions on specific wafers 'cut' from the processed (modelled) boules were predicted and X-ray synchrotron strain contour, double-crystal rocking-curve, FTIR, and photoreflectance maps were generated on the real wafers for comparison. Implications of these results with respect to substrate quality, screening, performance and producibility, will be discussed.

The sessile drop technique is a method of evaluating and comparing surfaces by measuring the contact angle between a liquid drop and the surface on which it is placed. The technique is extremely sensitive to the outermost surface layer and is thus a potentially useful tool for the evaluation, comparison and monitoring of (Hg, Cd)Te surfaces subjected to various chemical treatments. Drops of deionized water were used to show that many chemicals commonly used in (Hg, Cd)Te processing create surfaces with characteristic contact angles: e.g. ozone <10 degrees , concentrated hydrogen peroxide 45 degrees , 0.5 vol.% Br/methanol solution 73 degrees . Sulphuric and nitric acids gave values that were dependent on chemical concentration. The kinetics of oxidation of a chemically treated (Hg, Cd)Te surface were monitored by X-ray photoelectron spectroscopy and the sessile drop technique, with good agreement. Ellipsometry and the sessile drop method were in good agreement during the monitoring of the initial stages of surface film growth.

The VUV ring at the National Synchrotron Light Source (NSLS, Brookhaven national Laboratory) is a source of nanosecond-duration, high-brightness, broadband IR pulses. The authors are developing several measurement techniques for characterizing Hg_1-xCd_xTe and other IR detector materials using this source. For example, the broadband IR pulses can be used to study transient photoconductive decay at various photon energies near the bandgap. A particularly novel technique they have developed is far-infrared photoinduced nanosecond spectroscopy (FIR-PINS). In this all-optical (contactless) measurement, a short laser pulse generates photocarriers which are subsequently sensed by a far-infrared pulse from the synchrotron. Spectroscopic analysis of the far-infrared yields the free carrier plasma frequency, providing information on the photocarrier density. By varying the delay time of the far-infrared pulse (relative to the laser pulse), the photocarrier relaxation is determined with nanosecond resolution. In addition to being contactless, the technique offers other potential advantages over electrical measurements. Results for MBE-grown Hg_1-xCd_xTe are presented.

A non-contact lifetime screening technique for measuring excess carrier lifetime in HgCdTe (MCT) using transient millimetre-wave reflectance (TMR) is presented. The TMR system described is capable of measuring lifetimes ranging from <or approximately=10 ns found in high-doped (>or approximately=1*10¹⁶ cm^-3) p-type material to >1 mu s found in low-doped (<or approximately=1*10¹⁵ cm^-3) n-type material. In TMR the reflected 90 GHz millimetre-wave (mm-wave) signal is proportional to a transient change in photoconductivity induced by a short (1.06 mu m) YAG laser pulse. The details of the TMR test system including the mm-wave circuit design are described. The system uses a computer controller for 80-300 K temperature control, x-y wafer translation and data acquisition and analysis. The TMR technique is shown to be equivalent to standard photoconductive decay. Applications of the technique, including MCT epilayer characterization by both frontside and backside illumination and in-process wafer characterization, are discussed. Spatial mapping of lifetime over an MCT wafer is demonstrated.

The authors present the details of a photoinduced microwave reflectance set-up operating at 35 GHz for measuring the excess carrier lifetime and surface recombination velocity of semiconductor epitaxial layers or bulk wafers. The principles upon which the operation of the set-up is based are given in detail, pointing out those situations in which the measurements can give erroneous results. Evaluation of HgCdTe epitaxial layers grown in the laboratory by MOCVD on semi-insulating CdTe substrates have shown excess carrier lifetimes of 1.1 mu s and a surface recombination velocity of 1400 cm s^-1 at 77 K.

Using a double-modulation, infrared photoluminescence technique, the authors have examined a variety of long-wavelength, HgCdTe materials under development for focal-plane array applications. Near-bandgap photoluminescence was easily observed in 'good quality' HgCdTe at 77 K, and they demonstrate the feasibility of using this technique for routine characterization of HgCdTe prior to device processing. Information regarding minority carrier lifetime, compositional uniformity, and native defects is obtained from photoluminescence data.

The authors investigate the separate dark current components which are dominant in the diffusion-limited regime in MWIR n/p HgCdTe/CdTe/sapphire photodetectors. Both mesa and planar configurations of variable-area diodes were fabricated and evaluated over the temperature range from 78 to 250 K. Simple analytical expressions are used to calculate the contributions of bulk, lateral and surface effects from the perimeter/area dependence of R₀A and measurement of the minority carrier diffusion length. The analysis indicates that at 180 K the mesa diode results can be accounted for by bulk and lateral currents, but that the planar diodes are limited by surface currents. The 180 K median R₀A for the mesa diodes ranges from 63 Omega cm² for 500*500 mu m² diode areas to 14 Omega cm² for 30*30 mu m² diodes at a cut-off wavelength of 4.64 mu m. Scanning laser microscope measurements determine the 180 K electron minority carrier diffusion length to be 17-18 mu m.

Faraday rotation was measured in liquid-phase epitaxial (LPE) Hg_1-xCd_xTe (x approximately=0.22) at CO₂ laser photon energies over the temperature range of 77 K to 300 K for magnetic fields up to 16 kG. The authors find that the Faraday rotation is composed of free-carrier, interband and spin-induced contributions. At 77 K, the interband and spin contributions dominate, resulting in large observed Faraday rotation, i.e. of the order of 10² deg cm^-1 kG^-1. All three contributions are needed to interpret the temperature and magnetic field dependence of the Faraday rotation in the samples. Resonant behaviour observed in the rotation angle is successfully described by impurity-to-band magneto-optical absorption. They conclude that Faraday rotation could serve as a viable technique for the non-contact measurement of carrier concentration in Hg_1-xCd_xTe.