K. Jacobs

Origin and evolution of background impurity content of materials used in the preparation of (Hg, Cd) Te LPE layers on CdTe substrates

Abstract The presence of unintentional background impurities found in LPE-grown Hg 1- x Cd x Te layers has been traced back to the starting materials and different technological steps in the course of preparation of the layers. The purified elements Cd and Te, the binary compounds HgTe and CdTe synthesized from them, VB-grown CdTe monocrystals, LPE source solutions and the final LPE (Hg 0.78 Cd 0.22 Te/CdTe) layer/substrate structures have been analysed with regard to their impurity content. Spark source mass spectrometry, atomic absorption spectrophotometry and secondary ion mass spectrometry were the analytical techniques employed. Generally, any high-temperature and handling procedures cause an increase in the concentration of most of the impurities. For CdTe Bridgman ingots, a non-uniform distribution with enrichment in the last-to-freeze part of the as-grown crystal is observed. Furthermore, it was found that the carrier concentration and conductivity type of annealed LPE layers are influenced by the varying impurity levels of substrates from different axial positions within the CdTe ingot. The impurity depth profiles of LPE layers show a gettering effect of the layer surface and the layer/substrate interface resulting in a reduced impurity level in the central part of the layers.

Molecular beam epitaxial growth and characterization of ZnSe on GaAs

Abstract Systematic investigations have been performed concerning the effect of growth temperature and beam pressure ratio on the structural perfection and optical properties of molecular beam epitaxy (MBE) grown ZnSe on GaAs substrates. A surface phase diagram of ZnSe has been derived showing the different surface reconstructions as a function of these growth conditions. RHEED reflex profiling reveals more details about the surface morphology occurring in different stages of the growth process. The epitaxial layers are further characterized by means of X-ray diffraction and various optical techniques.

Systematic steps towards exactly stoichiometric and uncompensated CdTe Bridgman crystals

A modified vertical Bridgman arrangement with Cd extra source and variable CdTe melt surface temperature is used to find out conditions for the growth of near stoichiometric CdTe crystals. Growth experiments were carried out with different temperature courses according to predictions for the optimum temperature program obtained from thermodynamic calculations. The transition point from p- to n-type conductivity for inclusion-free crystals was observed at a CdTe melt surface temperature of 1118°C and a Cd source temperature of 850°C. The incorporation of shallow acceptors (Ag, Cu) as a function of the deviation from stoichiometry during the growth was analysed by photoluminescence, mass spectroscopy and atomic absorption spectrophotometry. The incorporation coefficients of atoms substituting Cd were deduced in dependence on their total concentration in the melt and the Te excess. The maintenance of nearly stoichiometric growth conditions drastically reduces the substitutional impurity fraction, acting as shallow acceptors, and therefore the carrier concentration.

Investigations on the correlation between growth rate and gate oxide integrity of Czochralski-grown silicon

Abstract Czochralski-grown silicon crystals show two concentric regions providing gate oxide layers of markedly different breakdown stability. The two regions are separated by a “wreath”, several millimeters in width, that is built-up from microdefects which act as nucleation centres for the formation of oxidation induced stacking faults. The core region of inferior gate oxide integrity (GOI) becomes smaller in diameter with decreasing pulling speed. The three regions have been extensively characterized by numerous analytical techniques. The most striking difference between the inner and outer regions, besides the GOI, is the enhanced oxygen precipitation in the core region occurring after annealing. An attempt is undertaken to analyze the defect structure in the different regions, its origin, correlation with growth and annealing processes, and its effect on the GOI.

Surface engineering during molecular beam epitaxial growth of wide-gap II-VI structures

Abstract Systematic investigations have been carried out in order to control the surface morphology of ZnSe and related compounds. Rough surfaces can be smoothed by means of growth interruptions. The smoothing of the surface is reflected in the halfwidth of the intensity distribution along the 00 rod of the reflection high-energy electron diffraction (RHEED) pattern. Reduction of the halfwidth is observed to occur within times of less than a minute; a fast process removes short range disturbances of the surface morphology. A more detailed analysis of the intensity distribution reveals a fine structure. From this, it can be derived that the distance of surface terraces is made uniform during the growth interruption, but this takes several minutes; a slow process removes long range disturbances. Degree and time dependence of surface smoothing are influenced by the beam equivalent pressure ration during layer growth.

On the mechanism of reflection high-energy electron diffraction oscillations studied by phase-locked epitaxy of ZnSe

Abstract Molecular beam epitaxy (MBE) is one of the most promising methods for growth of sophisticated device structures. Starting the growth on a flat surface, reflection high-energy electron diffraction (RHEED) oscillations occur. The question which phase of the oscillations corresponds to lattice plane completion and the most flat surface morphology is not yet fully resolved. There is hardly a direct access to the answer. Phase-locked epitaxy (PLE), however, appears to be a tool for studying this phase problem. PLE permits the growth of layers without losing RHEED oscillations due to damping the thickness of which is great enough to become measurable by common techniques. This enables to compare the directly determined layer thickness with that obtained by counting the number of RHEED oscillation periods. Results are presented that show the phase relation between RHEED oscillations and lattice plane completion.

Magneto-optical study of ZnSe/(Zn,Mn)Se and ZnSe/(Zn,Cd,Mn)Se quantum well structures and superlattices

Abstract Quantum well structures with semimagnetic barriers, ZnSe /( Zn , Mn ) Se , or semimagnetic wells, ( Zn , Cd , Mn ) Se / ZnSe , are studied by luminescence and reflection spectroscopy in an external magnetic field up to 7.5 T.A. unique offset dependence on the manganese concentration and a type conversion of the ZnSe /( Zn , Mn ) Se heterostructures due to the giant Zeeman splitting of the barriers are found. In the ( Zn , Cd , Mn ) Se / ZnSe structures, a strong reduced energy transfer from the exciton states to the Mn internal transition was found, which is further suppressed by an increasing magnetic field. Therefore (Zn,Cd,Mn)Se quantum wells can be proposed to be suitable and effective for tuneable blue-green laser devices in an external magnetic field. The field dependent energy splitting both of well and barrier excitons are significantly anisotropic with respect to the orientation of the magnetic field B parallel or perpendicular to the growth axis. This magneto-optical anisotropy can be explained by the mixing of the heavy and light hole states in the framework of a multi-band envelope-function approximation.

MBE growth and characterization of (Zn, Mn)Se/ZnSe and MnSe/ZnSe structures on (001) GaAs

Abstract The MBE growth of ZnSe, (Zn,Mn)Se, MnSe, and related quantum well structures has been studied. Results are presented concerning the effect of growth interruptions and optical properties of SQW and MQW structures.

Thermodynamic investigations on the liquid phase epitaxy of Hg1-xCdxTe layers

Liquidus temperatures (450 <T l < 480°C) and the corresponding mixed crystal compositions (0.2 ⩽ x ⩽ 0.4) of Hg1-xCdxTe have been determined experimentally in the HgCdTe phase diagram as a function of various melt compositions. The data have been determined ex situ from special experiments as well as in situ from liquid phase epitaxy (LPE) experiments. At the same time, expressions for liquidus temperatures and materials constants have been derived from common equations describing the LPE growth kinetics. The materials constants appearing in these equations have been determined from LPE growth runs. LPE experiments have been performed in the step-cooling mode. Good lateral and vertical homogeneity in the composition as well as reproducibility of thickness, composition and good surface morphology can only be achieved after optimizing the homogenization procedure. The only reliable method is mechanical stirring of the melt solution. A criterion is given, characterizing the extent of source melt homogenization. The analysis of the experimentally determined correlations between layer thickness, growth times and deposition temperatures confirms the ex situ determined liquidus temperatures. Linear extrapolation of the x values to the liquidus temperatures allows the determination of the equilibrium mixed crystal composition.

Growth and optical properties of ZnSe/ZnMnSe quantum structures

ZnSe/ZnMnSe MQW structures are grown by MBE. In situ RHEED control allows one to lock the growth cycle on the phase of the RHEED oscillations so that lattice plane completion is achieved independent of beam flux fluctuations and other irregularities. The band-edge resonant optical properties of the structures are dominated by sharp and pronounced excitonic features. The influence of strain and confinement on these excitons, their localization and interaction with phonons are discussed.