M.G. Astles

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.

High resolution X-ray diffraction studies of CdxHg1-xTe/CdTe epitaxial layers grown by MOVPE on GaAs substrates

Abstract We have applied high resolution X-ray diffractometry and topography techniques to investigate both the lateral uniformity and structural properties of Cd x Hg 1− x Te layers grown by MOVPE onto CdTe buffer layers on GaAs. On samples ∼1–2 cm square, maps of rocking curve width (β) have shown values varying from 60 arc sec (comparable to the best reported) to over 1000 arc sec on the same slice, indicating the superior value of mapping over single point measurements on this material. A good correlation has been observed between rocking curve widths, lattice tilts and the density of pyramid-like surface defects, the last of which are also associated with an increased twin density. However, on rotating the sample about its surface normal, the 400 surface symmetric β-value varies by up to an order of magnitude, indicating that lattice tilts play an important role in broadening the rocking curve. X-ray topography reveals large tilt boundaries in the CMT epilayer which correlate with the dislocation structure in the GaAs substrate.

Minority carrier diffusion length in CdTe

Abstract Measurements of the minority carrier diffusion length (L) in CdTe have been performed at 300 K using the “water drop” technique. In this reliminary study, a variety of single crystal materials have been compared, including bulk grown (solvent exaporation technique), molecular beam, metal organic, and liquid phase epitaxial samples and typical values for L were found to be in the micrometer range. In order to obtain accurate values of L by this method, it was necessary to measure the absorption coefficient (α) at the band edge in the range 10-105 cm-1. Cathodoluminescence intensity and decay time measurements have been used to seek confirmation of the diffusion length values and to determine whether they are limited by fundamental radiative processes.

The influence of substrate surface preparation on the microstructure of CdTe grown on (001) GaAs by metalorganic chemical vapour deposition

Abstract A cross-sectional transmission electron microscopy study has been performed on the microstructure of CdTe epitaxial buffer layers, which were grown upon 2°-off (001) GaAs substrates by metalorganic chemical vapour deposition (MOCVD). The substrates were prebaked at various temperatures instead of the usual acid etching treatment. Ut was shown that single crystal films were only obtained for baking temperatures of 550 and 590°C. These single crystal deposits exhibited a distinctive anisotropic defect structure with subgrain boundaries and 90°-type misfit dislocations perpendicular to the off-cut axis and stacking faults and a mixture of 60°-type and 90°-type misfit dislocations parallel to the off-cut axis. All of the stacking faults and 60°-type dislocations lie on the plane (111) producing a 6° rotation between the substrate and deposit lattices about the offcut axis. The development of this microstructure has been ascribed to the formation of high aspect ratio island nuclei and differences in Schmid factors due to the offcut. This microstructure was compared to that produced on an etched substrate and it appears that fine scale substrate surface roughness eliminates both of these influences.

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.

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.

Transmission electron microscopy of LPE grown CdHgTe

The defect structure in CdxHg1-xTe layers grown by LPE on {111} B oriented CdTe and CdZnTe substrates has been studied by transmission electron microscopy. The interface between the epilayer and the substrate is characterized by a dislocation network and a variation in Hg/Cd concentration both of which have a finite thickness. The geometry and nature of the interfacial dislocations have been determined by examination of a combination of transverse and planar bevelled TEM specimens. One source for the interface dislocations has been shown to be the dislocations in the substrate. Layers grown on CdZnTe show a much smaller density of interfacial dislocations. These are in the epilayer rather than at the chemical interface.

Surface damage of CdTe by mechanical polishing investigated by cross-sectional TEM

Abstract The damage caused to CdTe substrates by five mechanical polishing treatments has been assessed and compared by cross-sectional transmission electron microscopy. The best polishes were 0.05 μm Al 2 o 3 in ethanediol and Syton/H 2 O 2 slurry.

Characterisation of epitaxial CdxHg1−xTe using electrolyte electroreflectance with in-situ electrochemical etching

Abstract Electrolyte electroreflectance with in-situ electrochemical etching are successfully employed to reveal the compositional variations and lattice strain that occurs in CMT epilayer structures grown on GaAs 〈100〉 by the IMP-MOVPE method, with a CdTe cap on the surface. There is excellent agreement between these results and those obtained using Rutherford Backscattering Spectrometry.