J. Storasta

Interaction of deep levels and potential fluctuations in scattering and recombination phenomena in semi‐insulating GaAs

The complex influence of recombination centers and potential fluctuations of the band gap on the scattering and recombination phenomena in n‐type semiinsulating liquid‐ encapsulated‐Czochralski‐grown GaAs were investigated by using the transient photoconductivity and photo‐Hall effects. The inhomogeneities cause a hyperbolic decrease of nonequilibrium carrier concentration and the saturation of Hall mobility, while the exponential parts of the decay appear due to the recharge of deep levels. The mean recombination barrier heights of potential fluctuations were evaluated. We propose a complex ‘‘island’’ model of scattering and recombination centers, consisting of defect clusters and their associations around dislocations, surrounded by potential barriers. At low light intensities and at the temperatures below 330 K they are insulating for majority charge carriers, thus reducing an effective crystal volume and causing percolation transport effects. At the temperature higher than 330–360 K the main barrier o...

Traps in GaAs detectors (before and after irradiation) and electric field redistribution in excited SI-GaAs

Abstract We report here on investigations of non-equilibrium conductivity and different types of defects in SI-GaAs, analyzing the effects of different growth techniques and thermal treatments. The influence of local inhomogeneities on the determination of electrical properties and parameters of local levels are discussed. The interaction of dislocation related defects and their charge have been recognized as the main factors influencing non-equilibrium carrier behaviour. The GaAs irradiation effects could be analyzed in terms of formation of microinhomogeneities, with different non-equilibrium carrier drift and capture in the inhomogeneity volume and the surrounding defect-free region. The trap spectra were studied using thermally stimulated current and polarization measurements. EL2 centre modification was used to separate electron and hole traps. Data from measurements of Hall and photo-Hall effects and light-induced non-linear optical effect measurements have also been used in the analysis.

GaAs peculiarities related with inhomogeneities and the methods for reveal of their properties

Abstract The inhomogeneities in as-grown and irradiated GaAs crystals and structures, their properties and role have been analysed. The influence of inhomogeneities on the properties of crystal and Schottky diode structure are presented. The possibilities of different methods for material and structure characterisation (Hall effect and magnetoresistance, noise spectra, thermally stimulated current and polarisation, photoconductivity kinetics and spectrum, light-induced diffraction) are discussed. Existence of the inhomogeneity of the crystal structure has been revealed by diffusion of copper through the GaAs wafer, and a percolation of it through doped crystal has been determined by means of the Hall effect. Modification of the recombination-induced defect system has been analysed to get deeper insight into the crystal and device degradation under illumination by ionising radiation. Role of free carriers in local electric field generation has been analysed via modification of the EL2 centre in GaAs. Complicated behaviour of non-equilibrium conductivity during quenching and activation of EL2 centres has been determined by precise measurement of the photoconductivity spectra in different crystals. Influence of the EL2 centre on the non-linear polarisation of sample and on the additional non-active light absorption has been analysed.

Impact of extended defects on Hall and magnetoresistivity effects in cubic silicon carbide

From magnetoresistivity effect measurements the carrier mobility at room- temperature is 200 cm2/Vs in heteroepitaxially grown 3C-SiC on 6H-SiC by sublimation epitaxy. The main scattering mechanism ...

Electrical Parameters of Bulk 3C-SiC Crystals Determined by Hall Effect, Magnetoresistivity, and Contactless Time-Resolved Optical Techniques

The electrical and optical techniques have been applied for investigation of carrier transport and recombination features in thick free-standing 3C-SiC layers. Temperature dependencies of Hall mobility, magneto-resistivity, and conductivity indicated presence of high potential barriers, up to 0.4 eV. The carrier mobilities and equilibrium densities were calculated in the barrier and inter-barrier regions. Contactless measurements of the excess carrier ambipolar mobility and lifetime at 1016-18 cm-3 injection levels revealed carrier scattering solely by phonons in 80 – 800 K range. A correlation between the temperature dependencies of carrier lifetime and ambipolar mobility pointed out that diffusion-limited surface recombination at extended defects contributes significantly to the carrier lifetime.

Impact of nonintentional Al impurity to carrier lifetime and diffusion in sublimation grown 3C heterostructures

Using optical techniques, we analyzed an impact of non‐intentional Al mpurity and twin boundaries to photoelectrical properties of sublimation‐grown 3C heterostructures. Differential transmission techniques revealed Al related contribution to probe beam absorption with cross‐section σAl = (1.8±0.5)×10−17 cm2 at 1064 nm, being four times stronger that the free‐carrier absorption cross‐section at given wavelength. Temperature dependent carrier recombination rates provided trap activation energy of 170 and 210 meV in two samples with different Al concentration. Saturation of probe beam absorption with excitation allowed determination of electrically active Al concentration, not gettered at grain boundaries. Increase of room‐temperature mobility with injection in the highly defective layer and the corresponding lifetime decrease pointed out contribution of point and structural defects to carrier scattering.

Transient electron transport in indium-doped semiinsulating GaAs

We investigated the effect of In-doping up to a concentration of 2×1020 cm−3 on transient transport phenomena in semiinsulating liquid-encapsulated-Czochralski grown GaAs. The changes in time after a strong laser excitation of the photoconductivity and nonequilibrium Hall mobility were analyzed. We did not find extra energy levels caused by indium. Nevertheless, In-doping caused significant changes in the behavior of the nonequilibrium mobility in the temperature range of 300–420 K, which were not observed in other crystals, undoped or doped by other dopants. They could not be explained merely by the reduction of dislocation density caused by In. Besides, the In-doping was demonstrated to cause the rearrangement of defect inhomogeneities. It is considered that lattice defects become distributed more homogeneously and appear more probably as short-range inhomogeneities instead of accumulations around dislocations. This diminishes the role of percolation phenomena and intensifies the effect of smaller defec...

Analysis of trap spectra in LEC and epitaxial GaAs

Abstract Different methods of trap parameter measurement are analysed. Transient photoconductivity and thermally stimulated effects were used to investigate the influence of traps in LEC SI-GaAs and high-resistivity epitaxial GaAs. The peculiarities of the TSC were analysed and shown to be related to the influence of crystal micro-inhomogeneities.

Role of growth defects on carrier dynamics: semi-insulating GaAs

Using transient light-induced grating experiments, we demonstrate important consequences of interaction between photoexcited electrons and EL2 centers in semiinsulating GaAs at room temperature. Carrier lifetime is found to depend on the local density and ionization ratio of the EL2 centers. A substantial slow down of diffusive grating decay due to the interaction between electrons and photoionized EL2 donors is observed.

Polytype Inclusions in Cubic Silicon Carbide

The 3C-SiC layers on nominally on-axis 6H-SiC substrates were grown using sublimation epitaxy. More than 90% coverage by 3C-SiC is typically achieved at growth temperature of 1775°C. The main reason for the polytype inclusions to appear is local supersaturation non-uniformities over the sample surface which appear due to the temperature gradient and spiral growth nature of 6H-SiC. On the 6H-SiC spirals with small steps supersaturation is smaller and 3C-SiC nucleation and growth is diminished. Due to surface free energy and surface diffusion differences, polytype inclusions appear differently when 3C-SiC is grown on the Si- and C-faces. The 6H-SiC inclusions as well as twin boundaries act as neutral scattering centers and lower charge carrier mobility.