G.W. Blackmore

The growth of high quality CdxHg1−xTe by MOVPE onto GaAs substrates

Abstract The growth of Cd x Hg 1− x Te layers by metalorganic vapour phase epitaxy (MOVPE) onto (100) 2°→(110) GaAs substrates is reported. The mirror smooth epitaxial layers have been grown with a reproducible structural quality that is comparable to layers grown onto the best CdTe substrates. By growing a sufficiently thick buffer layer to isolate the active layer from the substrate, the potential problem of Ga diffusion out of the substrate into the layer has been successfully controlled. As a consequence the Ga concentration in the active layer has been reduced to a level well below that for normal background contamination.

The potential for abrupt interfaces in CdxHg1−xTe using thermal and photo-MOVPE

This paper reviews the current progress in the growth of abrupt structures in the infrared detector alloy CdxHg1−xTe, with special reference to metalorganic vapour phase epitaxy (MOVPE). Recent results on the growth of heterostructures using the interdiffused multilayer process (IMP) are described for epitaxy onto GaAs as well as CdTe substrates. It is envisaged that useful heterostructure devices can be grown where the interface widths are of the order of 0.3 μm. For more abrupt structures, lower growth temperatures are needed and this has been demonstrated using the new photolytic MOVPE process. Epitaxial growth at temperatures as low as 200°C has enabled measured interface widths of approximately 100 A to be realised for a HgTe/CdTe structure. Lower growth temperatures also reduce the rates of diffusion of dopants such as Ga from a GaAs substrate. Ga concentrations of just 0.05 ppma have been measured 500 A from a CdTe/GaAs interface. Detailed Hall measurements on photo-MOVPE HgTe and HgTe/CdTe structures have shown that high quality epitaxial layers can be grown. A study of the limitations on the electrical quality has shown some influence of impurity diffusion in very thin layers but for thicker layers ( > 1 μm) this has little effect. The possibility of using photo-MOVPE for the growth of CdTe/HgTe superlattices has been explored by predicting the extent of interdiffusion at 200°C and 150°C. Even at 150°C, the predicted interdiffusion in just 10 min is significant. However, interdiffusion may depend critically on the dislocation structure and strain at the interface. Structural studies on thin epitaxial layers shows the critical dependence of structure, and strain, on the substrate orientation and layer thickness.

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.

Epitaxial growth of CdxHg1−xTe by photo-MOVPE

Abstract We report on the first growth of epitaxial films of cadmium mercury telluride (CMT) using photolytic metal-organic vapour-phase epitaxy (photo-MOVPE).

Growth of CdTe‐InSb multilayer structures on (100) InSb substrates using molecular beam epitaxy

Molecular beam epitaxy has been used to grow multilayer structures of CdTe/InSb on (100)InSb substrates. To minimize interdiffusion effects, a particularly low growth temperature was used. This study presents the first transmission electron microscope and secondary‐ion mass spectrometer investigations of this multilayer system. The work clearly demonstrates that the multilayers have high structural quality and that there is no large scale interdiffusion of the matrix elements. Roughening is observed at the ‘‘InSb grown on CdTe’’ interface, although only relatively minor undulations are present at the inverted interface. A possible explanation for this effect is described.

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.

Interdiffusion characteristics of constituent atoms during the MOCVD growth of wide band gap II–VI multilayer structures

Abstract Data are presented on the interdiffusion between constituent atoms in multiple layer structures of wide band gap materials grown by MOCVD on to GaAs substrates. The systems ZnS/ZnSe, CdS/ZnS and CdSe/ZnSe have been used as the vehicles for this study. The results obtained suggest that with a common group II element, interdiffusion between the group VI atoms is negligible but that with a common group VI element some evidence exists for interdiffusion on the group II site. The implications of these findings upon device structures are discussed.

The use of in-situ wash melts in the LPE growth of (CdHg)Te

The use of Bi solutions as in-situ wash melts in the liquid phase epitaxial (LPE) growth of (CdHg)Te from Te-rich solutions onto CdTe substrates is described. Bismuth has several advantages for this purpose: (i) the solubility of CdTe in it at the LPE growth temperature (≌460°C) is such that a reasonable rate of removal (≌10 μm min-1) is achieved; (ii) the removal of the wash melt is complete, hence avoiding contamination of the on coming growth solution; (iii) it has a very low vapour pressure; (iv) it has a very low segregation coefficient so that any small amounts of Bi which were picked up by the Te growth solution would only be incorporated into the growing (CdHg)Te layer at very low levels. The epitaxial layers grown with a wash melt treatment of the substrate have been assessed by Secondary Ion Mass Spectrometry (SIMS) and Hall measurements.

The use of alternative solvents for the low-temperature LPE growth of CdTe films

Abstract The use of alternative solvents, in particular Bi and In, has been investigated for the LPE growth of CdTe films at temperatures down to 250°C. In comparison with earlier work using Te as a solvent, this approach has several benefits: (i) improved surface topography; (ii) better solution removal; (iii) more abrupt interfaces; (iv) improved electrical properties. SIMS measurements showed that the layers grown from Bi solutions contained ∽0.1 ppma Bi, while those from In solutions contained ∽100 ppma In. Doping experiments with In, Sb and As were performed using Bi as the solvent. Estimated values for the segregation coefficients for growth at 500-400°C were k In ∽6×10 -4 , k Sb ∽10 -4 and k As ∽2×10 -3 . Layers grown from In solutions showed long minority carrier diffusion lengths ( L h ≳20 μm ) and very high luminescence efficiency, indicating extremely high quality CdTe.

SIMS analysis of epilayers

Abstract Secondary Ion Mass Spectrometry (SIMS) is an invaluable tool in support of semiconductor development, especially in the chemical characterisation of epitaxial layers. This review considers the quantitative aspects of depth profiling and is principally concerned with the practical considerations necessary to ensure meaningful results. Topics discussed included instrumental features, operational conditions, sample quality and inherent limitations of the technique. Emphasis is given to factors which the prospective user should bear in mind when planning, carrying out and interpreting SIMS analyses of epitaxial layers.