J. Giess

Recent developments in the MOVPE of II–VI compounds

This review is concerned with developments that have taken place over the last year or so in the field of metalorganic vapour phase epitaxy of II–VI compounds. In this rapidly developing field the compounds attracting most study are CdTe, CdxHg1−xTe, HgT e, ZnSe, ZnSxSe1−x, ZnS, CdS and ZnO. These developments, which are considered within the framework of existing knowledge, cover organic reagents or precursors, alternative substrates, reaction mechanisms and developments in growth technique. Prominence is given to Photo-MOVPE and low temperature growth. Improvements in material quality and recent device advances using MOVPE are also reported.

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.

Recent developments in the pyrolytic and photolytic deposition of (Cd,Hg)Te and related II–VI materials

Abstract Recent results for the pyrolytic MOVPE of cadmium mercury telluride (CMT) and related compounds are reported. In pyrolytic MOVPE the role of “new” tellurium precursors and their potential for developing low temperature epitaxy is reviewed. Very high purity CMT layers which show classic p-type behaviour are discussed. The limitations of the cool down process in giving rise to complex (“anomalous”) Hall measurements are analysed. The interdiffused multilayer process (IMP) has been used to grow very uniform CMT ( Δx = 0.004 over 2.5 cm) using iPr 2 Te at 350°C. Growth on GaAs substrates has been shown to give some of the best crystallographic quality CMT layers yet reported. MOVPE CMT has been fabricated into infrared detectors showing state-of-the-art performance in the 8–14 μm band. Developments in photolytic growth of CMT and particularly CdTe are reported, in particular the use of laser photo-induced epitaxy, to achieve local area patterning is demonstrated.

MOVPE of narrow and wide gap II-VI compounds

Abstract The development of high quality II–VI compounds is dependent on advances made in certain key research activities. This review is concerned with these activities which includes precursors such as i Pr 2 Te, t Bu 2 Te, Me(allylTe and (allyl) 2 Te for low temperature growth, substrates including CdZnTe, CdTe, CdTeSe, GaAs on Si for epitaxy, doping for p-n junction formation and photo-assisted growth processes for low temperature growth, photo-patterning and selected area deposition.

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.

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).

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.

On the electrical properties and Hall effect behaviour of MOVPE CdxHg1−xTe

Abstract The electrical properties of as-grown MOVPE layers of Cd x Hg 1− x Te are discussed in the context of the fundamental and the preparation-dependent parameters which determine their characteristics. A simple model is used to predict the equilibrium vacancy concentration as a function of temperature and Hg pressure in the range 250–430°C. The preparation-dependent parameters associated with substrates, growth and post growth processes are considered with references to experimental examples. The results of both two layer and single layer Hall models are used to illustrate the role surfaces and interfaces and their significance on the measured Hall results.

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.

A combination of high-resolution X-ray diffractometry and diffraction imaging techniques applied to the study of MOVPE-grown CdxHg1−xTe/CdTe on GaAs

Abstract We have applied a range of high-resolution X-ray diffractometry and diffraction imaging techniques to study the structural properties of CdxHg1-xTe (CMT) grown epitaxially on GaAs by MOVPE. In this paper we specifically describe three such techniques and evaluate and compare the results from each. Automated double crystal diffractometry with a mapping facility provides information on the quality and uniformity of the layers. 004 rocking curve widths vary from A number of layers with varying degrees of structural quality have been examined using a combination of the above techniques. Recent results are reported illustrating the value of each technique and we demonstrate how the application of a combination of X-ray diffraction techniques can be a powerful tool for investigating the nature of structural defects in this highly mismatched heteroepitaxial system.