I.G. Gale

Doping studies in MOVPE-grown CdxHg1-xTe

In two recent review papers on metal organic vapour phase epitaxy (MOVPE) of cadmium mercury telluride (CMT) particular emphasis was placed on the crucial importance of doping studies to the realization of future device structures Irvine et al. (1991) and Tribowet (1991). If the full potential of MOVPE growth of CMT is to be realized then extrinsic doping of heterostructures is required. If the doping and composition junctions can be grown with the correct degree of grading then this will create the potential for the production of device structures leading to either improved performance and/or increased operating temperatures. This papers reviews published doping studies and also presents some recent results on both acceptor and donor doping studies carried out by the authors. In the latter studies, interdiffused multilayer process (IMP) growth of CMT has been performed at approximately=360 degrees C using dimethyl cadmium (DMC) and di-isopropyl tellurium (DIPT) as the MO precursors while the Hg overpressure was provided by a heated elemental source. Alternative acceptor doping sources to arsine have been investigated including phosphine, triphenyl arsenic, and phenyl arsenic of which the latter appears to be most suitable. Iodine has continued to show the donor dopant potential in CMT that it exhibited with higher-temperature ( approximately=400 degrees C) MOVPE growth using di-ethyl tellurium (DET). Characterization of fully doped structures is described.

Impurity segregation in CdxHg1-xTe crystals grown using ACRT

Abstract The segregation behaviour of a number of important elements in Cd x Hg 1- x Te (CMT) has been established in crystals grown under conditions of accelerated crucible rotation (ACRT). Chemical analysis by atomic absorption spectrometry (AAS), laser scan mass spectrometry (LSMS) and spark source mass spectrometry (SSMS) has been carried out on both deliberately doped and undoped crystals. Estimates of segregation coefficients have been obtained and these are compared to those found in normal Bridgman grown crystals. In general the segregation coefficients are found to be lower in ACRT material and possible causes for this are suggested.

Matrix and impurity element distributions in CdHgTe (CMT) and (Cd,Zn)(Te,Se) compounds by chemical analysis

Abstract This review describes several of the main techniques used to determine matrix element distributions and those which can provide a survey of impurity levels and assess deliberate doping concentrations in Cd x Hg 1 − x Te and CdTe-based substrate materials. The most widely used method to non-destructively determine x is that of Fourier transform infrared (FTIR) spectrometry and lateral x variations in current bulk, LPE and MOVPE material measured by this technique will be presented. Auger electron spectrometry (AES) has been used on bevelled samples to assess variations in x with depth and interface widths in LPE, MOVPE and MBE layers and examples will be given. Near IR spectrometry is also now being used to monitor the variations in Zn and Se content, in CdZnTe and CdTeSe respectively, and results in this area will be described along with measurements of Zn on the micro-scale using AES. All of these techniques need to be calibrated against an absolute chemical analysis technique and we have used atomic absorption spectrometry (AAS). The latter technique also provides the accurate measure of dopant and impurity elements to standardise other techniques. Secondary ion mass spectrometry (SIMS) is mainly used for the determination of dopant depth distributions while laser scan mass spectrometry (LSMS) has the unique capability of providing a survey of low levels of impurities in thin epitaxial layers. Depth profiles of arsenic and iodine in MOVPE heterostructures, using SIMS, will be given. Impurity surveys, using LSMS, in bulk CMT and substrate materials and in CMT epitaxial layers grown by LPE, MOVPE and MBE will be described. Reported glow discharge mass spectrometry (GDMS) results on substrate materials will be compared to the present results.

Extrinsic doping at low concentrations for CDxHg1-xTe layers grown by MOVPE

The study of arsenic doping has resulted in the capability to control the acceptor level between 1×1016 cm-3 and 2×1017 cm-3 in CdxHg1-xTe grown by MOVPE. An understanding of the dopant incorporation mechanism and diffusion rate has enabled the growth parameters to be adjusted to ensure high (≈100%) electrical activity and homogeneous distribution of the dopant atoms. The acceptor ionization energy obtained from such layers was consistent with extrinsically doped material. Minority carrier lifetime data are also presented. Doped/undoped heterostructures have been produced which have demonstrated p-n junctions following Hg annealing. The high-x regions can behave as barriers to Hg in-diffusion if they cap the undoped region.

Analysis of cadmium mercury telluride and related materials

Abstract The roles of various analytical techniques for both matrix element composition and impurity content of cadmium mercury telluride and related materials are reviewed from the viewpoint of a user. The main techniques considered are infrared (IR) transmission, atomic absorption spectrometry (AAS), X-ray flourescence (XRF), electrolyte electroreflectance (EER), Rutherford backscattering (RBS), spark source mass spectrometry (SSMS), laser scan mass spectrometry (LSMS) and secondary ion mass spectrometry (SIMS). The role of these techniques in the analysis of bulk materials, epitaxial layers and device structures is considered. It is shown that whilst SIMS can uniquely provide analytical information with excellent depth and lateral resolution, some compositional and purity information can also be achieved more cost-effectively using developments of what may be considered more “traditional” techniques. Laser scan mass spectrometry is shown to be an interesting alternative to SIMS for high sensitivity general analyses when depth and spatial distributions are not required.

Electron beam induced current assessment of doped and diffused junctions in epitaxial CdxHg1−xTe

Electron beam induced current (EBIC) measurements have been applied to shallow-angle bevelled sections through Cd x Hg 1-x Te (CMT) epitaxial layers containing p-n junctions. Samples studied include (a) fully doped homo- and heterostructures grown by metalorganic vapour phase epitaxy (MOVPE) and (b) vacancy-doped CMT layers grown by MOVPE or liquid-phase epitaxy (LPE) where the junction has been formed by ion-beam milling or Hg in-diffusion from a surface oxide layer. The bevelled sections were formed by chemical etching and profiles of the bevel surfaces were measured to enable accurate junction positions to be obtained

Assessment of doped CdxHg1-xTe structures using bevelled sections

Doped homo- and heterostructures of CdxHg1-xTe (CMT) are required for some types of new-generation devices. These structures have been grown by metal organic vapour phase epitaxy (MOVPE) at approximately 360 degrees C onto both CdTe and GaAs (buffered with CdTe) substrates. The structures were produced using both acceptor and donor doping with arsenic and iodine respectively, and the junctions formed following annealing in mercury to eliminate metal vacancies. This paper summarizes progress in characterizing these junctions both in terms of x and electrical behaviour.

Impurities and metal organic chemical‐vapor deposition growth of mercury cadmium telluride

The doping behavior of indium and iodine have been investigated for mercury cadmium telluride (MCT, Hg1−xCdxTe) layers deposited by the interdiffused multilayer process procedure at 400 °C using diethyl tellurium and dimethyl cadmium. Trimethyl indium and solid iodine were used as dopant sources. Both elements exhibited donor behavior under the conditions employed. Secondary ion mass spectrometry profile analysis was used to demonstrate that indium required a relatively long period during growth to attain an equilibrium concentration in the layer; in addition a significant reactor system memory was observed allied to a relatively fast diffusion rate. Iodine showed encouraging dopant properties at low concentration levels, the chemical concentration of iodine was in good agreement with the free donor level from Hall measurements. The advantage and applicability of each of the three chemical analysis procedures used in this work are discussed together with components on the residual impurity content and ele...