Table of contents

High-quality semiconductor material is a prerequisite to the production of high-performance, affordable semiconductor components. With infrared (IR) components, the material quality has an even greater significance, affecting not only the device characteristics, but also the system performance and cost. The quality of the infrared material is a major factor in the system design trade-offs. Sensitivity, spectral band, noise spectrum, spatial uniformity, input power and reliability are system parameters with a direct relationship to the infrared material. Each of these system requirements has an immediate impact on the material specifications, which then define the appropriate growth technique, wafer size, requirements for on-chip signal processing and relevant material figures-of-merit. To be successful in IR material development, a concurrent engineering approach is necessary, taking into account both the material limitations and the demands of the infrared imaging system for the proposed application. In addition to the material's performance figures-of-merit, the impact of the material on device fabrication and system cost must be taken into account.

The progress of HgCdTe MBE technology has been very significant over the last two to three years. In this overview paper, the authors first discuss the state-of-the- art HgCdTe MBE systems, then review the structural, optical and electrical properties of MBE-grown HgCdTe layers that are available today. In addition, various kinds of devices that have been fabricated with MBE-grown HgCdTe are described. Finally they discuss future directions and challenges to the perfection of HgCdTe MBE technology for focal plane applications.

The capability of chemical beam epitaxy to address the critical material issues of the growth and doping of Hg_1-xCd_xTe is discussed in conjunction with its advantages for enhanced compositional and growth rate control. The potential of this technique is demonstrated from a review of the electronic and photoconductive properties of narrow-bandgap alloys (x<0.3) and preliminary studies of the n-type iodine doping of CdTe.

The critical steps in the development of MOVPE for the fabrication of MCT focal plane arrays (FPAs) are outlined. Improved purity of the source organometallics has enabled background donor concentrations in the mid 10¹⁴ cm^-3 to be achieved, together with controlled back doping with donors or acceptors. Large area alloy uniformity is possible by the interdiffused multilayer process (IMP). However, large FPAs will need to be grown onto Si substrates to avoid thermal stress on the hybridized structure. Preliminary results are presented on a 256*256 element MWIR array. Results are also presented on optical in situ monitoring that will form the basis of improved epitaxial control in the future.

The present status of the interdiffused multilayer process (IMP) for the growth of (Hg,Cd)Te layers is presented, with reference to electronic, structural and device properties of the layers, and is compared with the low-temperature growth of mercury cadmium telluride (MCT) layers. The use of (211) GaAs substrates for the growth by IMP of MCT layers allows us to get rid of the hillocks. A growth temperature of about 280 degrees C has been found to be optimum for the growth of layers of high electronic and structural quality using precursors with a low decomposition temperature.

This paper describes work carried out towards achieving extrinsically doped Cd_xHg_1-xTe (CMT) heterostructures grown with stable dopants and sharp junctions. Both acceptor and donor doping of CMT has been achieved in our MOVPE growth reactor using the interdiffused multilayer process at approximately 400 degrees C with diethyltellurium (DET) as the tellurium alkyl source. The two dopants used were arsenic, introduced as AsH₃, and iodine, as vapour from the solid element. The donor doping range has been extended by the use of a concentrically arranged double-injection-tube system which reduced the pre-reaction between the cadmium alkyl and the iodine vapour. Investigations have been carried out into the growth of various double-layer structures which incorporate either a single dopant transition or a double transition, as in a fully doped structure.

A study has been made of the electrical behaviour of In-doped metal organic vapour phase epitaxy (MOVPE) layers of Hg_1-xCd_xTe grown by the interdiffused multilayer process (IMP) in order to establish the factors needed to obtain uniform extrinsic n-type layers with high electrical activity and mobility. Layers with x between 0.2 and 0.3 were grown on (100) CdTe and CdTeSe at 360 degrees C using di-isopropyltelluride (DIPT) and doped with trimethylindium (TMIn) in two ways, depending on In concentration, (In). For (In)>1*10¹⁷ cm^-3 layers were doped with TMIn on the CdTe part of the IMP cycle while for (In) in the range 1*10¹⁵ to 1*10¹⁷ cm^-3 the TMIn memory of the growth system was used, resulting in doping on the HgTe part of the IMP cycle because of a greater efficiency of incorporation. In profiles, which were determined by secondary-ion mass spectrometry (SIMS), are shown to depend on doping process due to TMIn partial pressure variations from adsorption effects.

MBE growth of epitaxial IV-VI lead salt layers on Si (111) substrates and fabrication of photovoltaic infrared devices in the layers is reviewed. IV-VI on Si IR sensors have potential as a low-cost technique of fabrication of large IR focal plane arrays for both the 3-5 mu m and 8-12 mu m ranges because of the easy fabrication procedure and because uniformity problems are much less severe in IV-VIs due to the weaker composition dependence of the bandgap compared with Cd_1-xHg_xTe. Sensor arrays are fabricated in 2-4 mu m thick PbTe, PbS_1-xSe_x and Pb_1-xEu_xSe for 3-5 mu m and in Pb_1-xSn_xSe for 8-12 mu m cut-off. An intermediate epitaxial stacked 0.2 mu m thick CaF₂-BaF₂ bilayer serves for compatibility and helps to overcome the large lattice and thermal expansion mismatch between the Si substrate and the IV-VI layer. Perfectly smooth surfaces with surface defect concentrations down to 10³ cm^-2, and X-ray rocking-curve linewidth of approximately=150 arcsec are obtained. Sensor arrays with 66 and 256 elements are described, the latter having been grown on standard Si chips with Al metallization.

The growth of InSb-based compounds on GaAs, Si and InSb has received increased interest over the past few years due to the potential use for this material in integrated infrared imaging arrays. In this paper a new technique for the epitaxial growth of InSb is reported for the first time. The technique, based on the sputter deposition of InSb from solid InSb and Sb targets, magnetron sputter epitaxy (MSE), has been used to produce layers of relatively high structural and electrical quality and with excellent thickness uniformity and surface morphology. The electrical properties of p⁺/n diode test structures fabricated using these epilayers is presented.

Superlattices consisting of combinations of III-V semiconductors with type II band alignments are of interest for infrared applications because their energy gaps can be made smaller than those of any 'natural' III-V compounds. Specifically, it has been demonstrated that both InSb/InAs_xSb_1-x superlattices and Ga_1-xIn_xSb/InAs superlattices can possess energy gaps in the 8-14 mu m range. The efforts have focused on the Ga_1-xIn_xSb/InAs system because of its extreme broken gap band alignment, which results in narrow energy gaps for very short superlattice periods. The authors report the use of in situ chemical doping of Ga_1-xIn_xSb/InAs superlattices to fabricate p-n photodiodes. These diodes display a clear photovoltaic response with a threshold near 12 mu m. They have also attained outstanding structural quality in Ga_1-xIn_xSb/InAs superlattices grown on radiatively heated GaSb substrates. Cross-sectional transmission electron microscope images of these superlattices display no dislocations, while high resolution X-ray diffraction scans reveal sharp high-order superlattice satellites and strong Pendellosung fringes.

The development of new epitaxial techniques has given rise to a variety of novel material combinations. Pseudomorphic combinations where the partners have lattice constants which differ by more than 1% are currently being extensively studied. The built-in strain can alter the symmetry and magnitudes of the bandgaps concerned. The growth and properties of narrow-gap semiconductor systems are reviewed together with their use as components for strained-layer structures. The materials discussed are InSb, InAs and the alloys of these two compounds. The alloy system InAs_1-xSb_x is prone to metallurgical problems such as ordering and phase separation in the mid-alloy range but high-mobility samples have been grown. Other alloy systems are also affected by similar problems. Spike-doped and n-i-p-i structures are studied in InSb and InAs. Minimal dopant diffusion is found. The control of the material properties achieved has enabled the fabrication of a number of prototype device structures including n-i-p-i photodetectors and resonant interband tunnelling structures with inversion barriers.

The authors review the current status of knowledge of fundamental properties of the alloy Hg_1-xCd_xTe. The most vexing questions are about its correlation state. Several different experiments now suggest it is highly correlated, but no theory predicts this result. They also discuss other properties, including dislocations at interfaces, the residual donor, worms, surface segregation and its impact on passivation, and concentration fluctuations. The forces driving these phenomena, where they are known, will be presented. Most of the paper focuses on the following: correlations; native defects, formation enthalpies and entropies; native defect equilibria with mercury gas and with tellurium inclusions; and self-diffusion coefficient activation energies including its contribution from migration energies. They will take advantage of new first-principles, high-accuracy calculations to help explain the experimental situation. The calculations predict that the main native defects found in alloys equilibrated at low Hg pressures are Hg vacancies, while at high Hg pressures they are Hg interstitials, and, surprisingly, Hg antisites.

The authors report on Fourier transform photoluminescence studies, carried out on bulk Hg_1-xCd_xTe. Using a double-modulation technique, luminescence in the 10 mu m range has been measured. The dependence of the luminescence spectra on temperature and excitation density has been studied. A single line with a FWHM of 130 cm^-1 is observed, which exhibits an anomalous increase of the intensity with rising temperature. In addition, a change of the lineshape and a large blue shift of the spectra of 6k_B (0.52 meV K^-1) occurs. These observations are explained on the basis of alloy disorder. Assuming exponential band tails and considering the effect of carrier localization on the minority carrier lifetime, the experimental results are consistently explained.

The authors have investigated the valence band offset ( Delta E_V) of the CdTe-HgTe heterojunction for three orientations, (100), (110) and (111)B, using in situ X-ray photoelectron spectroscopy. The difference in energy between the Cd 4d and Hg 5d_5/2 core levels, Delta E_CL, and consequently Delta E_V was found to be independent of surface orientation and the surface structure immediately prior to growth of the uppermost layer. Delta E_V was found to be 0.37+or-0.07 eV.

The authors report here new transport and far-infrared magneto-optical experiments performed in n-type HgZnTe-CdTe superlattices over a large temperature range (1.5-300 K). They show that these superlattices are semi-metallic at low temperature and are degenerate intrinsic semiconductors for T>100 K. Theoretical calculations are presented which account very well for this behaviour.

A novel semiphenomenological kinetic electron-affected model for diffusion in Hg_1-xCd_xTe systems is proposed. The model suggests electron-affected mechanisms for an explanation of the observed large variations in Arrhenius activation energies Delta E and pre-factors D₀, the linear correlation in D₀ versus Delta E (the compensation effect), the differences between diffusions at low and high temperatures and the exponential dependence of diffusivity D on x.

This paper gives a description of the infrared photovoltaic detector technology developed by LETI and now industrialized by SOFRADIR. Results obtained on several kinds of arrays both in the 3-5 mu m and 8-12 mu m wavelength ranges are presented. The technological choices made by the laboratory to achieve components with increased complexities are also given.

A new economic route for IR photoconductor processing is presented with wavelengths ranging from 3-12 mu m and higher. n-type epitaxial layers, grown either by liquid phase epitaxy (LPE) or metalorganic chemical vapour deposition (MOCVD), are demonstrated to be quite suitable for the photoconductive mode. Background-limited infrared performances have bean achieved at 200 K, 30 degrees FOV (field of view), 3-5 mu m spectral range with LPE layers. MOCVD layers grown on GaAs have been successfully used for photoconductors operating at room temperature in the 3-5 mu m region.

With the exception of PtSi, responsivity uniformity of present IR detector arrays, for example in HgCdTe, is limited by control of the geometry of the optically sensitive area and/or material uniformity. PtSi, in the best examples, appears to be limited by the state of the art of photolithography. This paper discusses the issues involved in understanding and improving upon the current uniformity limits in HgCdTe and other detector materials.

Narrow-gap semiconductors have many unique properties which are advantageous for optical and electronic applications. These properties and the principles of the non-equilibrium techniques which are required to access them at or near ambient temperature are reviewed. The application of these techniques to raise the operating temperature of cadmium mercury telluride infrared detectors is discussed. The growth of InSb and In_1-xAl_xSb to demonstrate the operation of other electronic devices in a narrow-gap semiconductor is described. Heterostructure diodes incorporating a thin In_1-xAl_xSb layer have values of R₀A several orders of magnitude greater than conventional devices at ambient temperature. Negative differential resistance predicted to accompany Auger suppression has been observed. Enhancement-mode MISFETS have near-classical output characteristics at 294 K. with a typical dynamic range of 23dB and transconductance of 34 mS mm^-1.

The performance of infrared detector arrays at thermoelectric temperatures can be greatly improved by a new scheme in which the detector array is optically coupled to an array of individual field lenses. The advantages and limitations of the scheme are discussed and it is concluded that current designs are limited by total internal reflection in the glue layer at the lens-detector interface. By considering a specific example, it is shown that a higher performance can be achieved by taking advantage of frustrated internal reflection across the glue layer.

Optical concentration is a means of improving the performance of CdHgTe photodiodes so that they can be used at longer wavelengths and higher temperatures. The authors describe the fabrication of silicon microlens arrays the fabrication of 128*1 arrays of miniature CdHgTe diodes, the mating of these two components, and the assessment of the resulting devices. For a device operating at 77 K and having a cut-off wavelength of 12 mu m the R₀I₀ product is improved by an order of magnitude over a conventional detector. At 192 K and a wavelength of 9 mu m useful peak detectivities of 4*10⁹ cm Hz^1/2 W^-1 are obtained. The lens/diode combination has a field of view of 30 degrees . The photocurrent gain on immersion is 10 to 12 times.

The authors discuss the device physics and detector performance of quantum well infrared photodetectors (QWIPS). On the basis of these results, large two-dimensional arrays of 128*128 pixels were fabricated and measured. The excellent long-wavelength imaging results (99% of the pixels working and a noise-equivalent temperature difference NE Delta T=10 mK) demonstrate the potential of this new GaAs QWIP technology.

The authors report on a new type of GaAs quantum well intersubband photodetector operating at wavelengths of 3-5 mu m. In these structures, the intersubband spacing is considerably enlarged by using ultrathin AlAs barriers on either side of the GaAs quantum wells followed by a thicker Al_0.3Ga_0.7As layer. These AlAs layers act as tunnel barriers allowing both photoconductive and photovoltaic detection of the intersubband excitation. Photovoltaic detector operation is possible because of an asymmetry between the confining AlAs layers, which is attributed to the epitaxial growth process. They also discuss the low-signal detector noise which is determined from the dark-current behaviour. The noise behaviour strongly favours photovoltaic detectors over photoconductive ones.

The authors investigate the possibility of long-wavelength infrared (LWIR) photodetection using a single quantum well in an asymmetrical double-barrier resonant tunnelling structure. The goal is to produce very fast (>10 GHz) LWIR detectors. GaA-AlGaAs structures with both Si-doped and undoped quantum wells designed for the 8-12 mu m wavelength region have been studied. Electrons responsible for the absorption are dynamically stored in the well for the undoped case under a finite bias voltage. A circuit-limited temporal response risetime of about 800 ps was observed. The device physics can be understood using a simple resonant tunnelling model. To compare with the experiments quantitatively, self-consistent numerical calculations have been carried out.

As a first step to the realization of an integrated solid state infrared spectrometer the authors have fabricated integrated GaAs/AlGaAs infrared detectors with double wavelength selectivity. An efficient coupling mechanism based upon the excitation and emission of surface plasmons has been applied to achieve high responsivity.

The authors report on electron interference effects in n-Hg_0.8Cd_0.2Te (CMT). This narrow-gap semiconductor has a very light effective electron mass and a comparatively long electron wave coherence length. Due to these properties a non-classical conductivity was observed in bulk samples at elevated temperatures up to 35 K. Dimensional crossovers were discovered in samples as large as 1*2.5* 35 mu m³: with decreasing temperature size-quantization from three- via two- to one-dimensional behaviour takes place. The evidence of pronounced electron wave interference effects favours CMT as a candidate for novel quantum size devices, as cutoff wavelength transistors or quantum-valve infrared detectors.

The authors consider the resonant absorption of infrared radiation by a layer of dopant atoms. It is shown that using such delta layers a detector can be achieved that operates like devices based on quantum well heterostructures. For p-type layers they demonstrate that detection of normally incident light, which is polarized parallel to the surface, can be achieved.