N. Shaw

The sources and behaviour of impurities in LPE-grown (Cd,Hg)Te layers on CdTe(111) substrates

Abstract The sources of background impurities in LPE layers of (Cd,Hg)Te grown on CdTe(111) substrates are investigated using Spark-source mass spectrometry (SSMS) and secondary-ion mass spectrometry (SIMS). The principal sources of impurities were (i) the compounds HgTe and CdTe used in the LPE solution synthesis, (ii) the graphite sliding boat system and (iii) the CdTe substrate. The main impurities and their typical levels in the LPE layers were found to be Li (0.2 ppma), Na (0.3-1.0 ppma), Si (0.1-0.3 ppma), Cl (0.3-0.4 ppma) and K (0.03-0.06 ppma). To reduce the levels various changes in procedure were adopted such as cleaning of the graphite boat parts in aqua regia, the use of elemental starting materials (Hg, Cd, Te), substrate purification and the use of in-situ wash melts. The effect of Hg saturated isothermal annealing on the impurity concentrations and distribution has been studied, revealing rapid surface segregation of the Group IA impurities. The values of the distribution coefficients for several impurities have been calculated from the analytical results, these values being generally in good agreement with other published work.

Growth of CdxHg1-xTe by a pressurised cast-recrystallise-anneal technique

Growth of large-grained CdxHg1-xTe ingots with 0.15<x < 0.30 has been achieved using a casting and recrystallisation technique in which the sealed quartz growth ampoule is subjected to a high external pressure of helium gas. This opposes the internal vapour pressure, permitting the use of thin-walled silica tube and the exploration of a wide range of casting rates. This investigation indicates that the optimum casting rate is a balance between the formation of porosity at high rates and segregation of the constituents at low rates. The cast structure and recrystallisation temperature gradient play a major part in determining the rate and efficacy of the recrystallisation process and details are given of changes in the structure of CdxHg1-xTe which result from changes in recrystallisation procedures. Finally, we present data on the correlation between annealing conditions and the observed electrical carrier concentration, which indicate that, at low carrier concentrations, the electrical activity is dominated by the presence of a donor impurity.

Growth of CdTe by solvent evaporation

Recent improvements to the Solvent Evaporation (SE) process are described. It is shown that good crystallinity is achievable provided that the solid-liquid interface is convex with respect to the solid and that the pyrolysed carbon coating of the ampoule adheres firmly to the wall. Breakdown of the coating leads to carbon-rich inclusions in the material and to dissolved carbon in the crystal lattice. C-V measurements indicate that carbon-rich areas are higher n-type than carbon-free regions, suggesting that carbon is a residual donor impurity in CdTe.

Boron-segregation in Czochralski-grown CdTe

Abstract Chemical analysis of CdTe ingots grown by the liquid encapsulated Czochralski technique using B 2 O 3 encapsulant show that the material contains up to 90 ppma boron and that the boron distribution is non-uniform. High concentrations are found at twin and grain boundaries. Boron-rich precipitation is also observed. The observation of low carrier concentrations ( n 77 K = 8×10 14 cm -3 ) and high electrical mobility (μ 77 K = 4800 cm 2 V -1 s -1 ) in this material also supports the thesis that most of the boron is not electrically active.

Improved control of composition and electrical properties of liquid phase epitaxial (CdHg)Te layers

Abstract The use of powdered HgTe as a source of mercury in a graphite sliding boat used for liquid phase epitaxial (LPE) growth of CdxHg1−xTe alloys from Te-rich solutions at 460°C has been found to give excellent reproducibility of alloy composition, x and thickness due to the low Hg loss rates during growth (typically 0.3 mg min-1). Reproducibility of the wavelength corresponding to an absorption coefficient, α, of 500 cm-1 (λα = 500) is ± 0.15 μm, while the thickness reproducibility is typically ± 1 μm for 20μm thick layers. The variation of composition with depth in the layers is typically (2–4)x10-4 in x per μm. Background carrier concentrations in undoped layers after Hg-rich annealing to remove Hg vacancies are n = (6–8)x1013 cm-3. Using SIMS analysis we have shown that In is 100% electrically active after Hg-rich annealing and that the In dopant does not diffuse during the anneal treatment. Copper (Cu) doping has been studied for p-type material in the range 8×1015 to 2×1018 cm-3. The Cu appears to be nearly 100% electrically active in as-grown layers, but during Hg-rich isothermal annealing at 250° C, a certain proportion of the Cu diffuses to the surface leaving only about 30% of the Cu in the bulk of the layer still electrically active.

Studies on the diffusion of the halogens into CdTe

A comparison of the diffusion of the halogens into CdTe is given. Diffusion anneals were carried out at selected temperatures in the range between 20 and in evacuated silica ampoules using a diffusion source of , or under saturated vapour pressure conditions. The concentration profiles were measured using either a radiotracer sectioning (RTS) technique or secondary-ion mass spectrometry (SIMS). In most cases the profiles were found to be composed of four parts to which a computer package consisting of the sum of four complementary error functions (erfc) gave satisfactory fits to the data. This fitting method was purely empirical but it proved to be satisfactory to describe the diffusion profiles as well as to show how the diffusivities behaved with various parameters. Proposals explaining how this type of diffusion may occur are given.

Techniques for improving the control of properties of liquid phase epitaxial (CdHg)Te

Liquid phase epitaxy (LPE) is currently the most widely used thin-film technology for (CdHg)Te IR photodiode arrays. This paper discusses several key materials parameters and how they have been improved The main areas addressed are: (i) A reduction in background impurities following an extensive chemical analysis programme, leading to improved LPE charge preparation, LPE boat cleaning and epitaxial layer annealing. Typical n-type background electrical properties are n77=6*1013-1*1014 cm-3, mu 77=(2.2-2.4)*105 cm2 V-1 s-1 (x=0.21). (ii) Donor doping with In and Al and acceptor doping with Cu and Rb to provide n- and p-type carrier concentration control using impurities rather than native defects and residual impurities. In doping is shown to give good control in the range n=1014-1018 cm-3 with approximately 100% activity after annealing. Cu doping gives approximately 100% activity in the range mid-1017 to mid-1018 cm-3 in the as-grown state, but after annealing to remove Hg vacancies, most of the Cu diffuses to the surface. Initial work using Rb doping has shown that the Rb rapidly diffuses to the surface and the substrate-layer interface. (iii) A novel design of LPE graphite sliding boat featuring integral HgTe sources for Hg overpressure control and very low loss-rates of Hg, giving excellent compositional reproducibility ( Delta x+or-0.0025, Lambda lambda RT+or-0.2 mu m) at x=0.21.

Incorporation of iodine into CdTe by diffusion

Initial studies on the diffusion of iodine into CdTe are described. Diffusion anneals were carried out at selected temperatures in the range between 20 and 270°C in evacuated silica ampoules using a diffusion source of either elemental iodine or CdI2, both under saturated vapour pressure conditions. The concentration profiles were measured using either a radiotracer sectioning technique or secondary ion mass spectrometry (SIMS). The profiles were found to be composed of four parts to which a computer package consisting of the sum of four complementary error functions (erfc) gave satisfactory fits to the data. The fastest diffusing component gave values of the diffusivity, which agreed with previously published data. Proposals explaining how this type of diffusion may occur are given, but the results indicate that iodine diffused into CdTe from the vapour is not suitable as a long term stable dopant in devices where sharp junctions are required.

Control of the electrical properties of In-doped HgCdTe grown by MOVPE for IR detector applications

A study has been made of the electrical behaviour of In-doped metal organic vapour phase epitaxy (MOVPE) layers of Hg1-xCdxTe 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.