Positron lifetime measurements on InAs wafers have shown that the positron bulk lifetime in InAs is 246±2 ps. Most samples exhibit a defect lifetime of 287±6 ps, which is attributable to monovacancy-impurity complexes with a concentration of 7±2×10 16 cm-3. Very heavily doped n-type samples exhibit a defect lifetime of 332–340 ps, characteristic of divacancies. The concentration of these defects is also close to 1017 cm−3. Both types of defects are stable for rapid thermal annealing up to 850 °C, and both defects are neutral. The formation of the divacancytype defects may be correlated with a discrepancy between the carrier concentration and the total

Photoluminescence (PL) and electron paramagnetic resonance (EPR) studies were performed on a series of ZnSe samples grown by molecular beam epitaxy. The PL has been studied as a function of excitation wavelength, power, temperature, and time. The PL data indicates that the broad emission from a heavily nitrogen-doped ZnSe film is composed of three distinct recombination processes. The EPR spectra taken at 8 K and 9.45 GHz show an isotropic signal at g = 2.0027(3) which we attribute to singly ionized selenium vacancies (Vse+). The PL and EPR data help to clarify the role of defects in the compensation of heavily nitrogen-doped ZnSe thin films.

Concentration and origin of defect states in p-type nitrogen-doped ZnSe (p-ZnSe:N) grown by metalorganic vapor-phase epitaxy (MOVPE) are discussed by means of timeresolved photoluminescence and deep level transient spectroscopy. Thermal annealing, which is a useful tool for realizing p-type conductivity, results in deep defect states which seem to be associated with Zn vacancies and with nitrogen acceptors. By lowering the annealing temperature, the trap concentrations can be successfully reduced without seriously sacrificing the acceptor activation efficiency, although further reduction of Zn vacancies is pointed out as a remaining requirement for the improvement of quality of MOVPE-grown p-type layers.

We show how positron annihilation can distinguish vacancies in the different sublattices of a compound semiconductor by performing experiments in ZnSxSe1−x and Mgy Zn1−ySxSe1−x layers. We identify the Se vacancies (Vse) in N-doped and the Zn vacancies (Vzn) in Cl-doped material by the shape of the core electron momentum distribution. The charge of the defect involving VSe is neutral or negative in the p-type alloys, suggesting that the Se vacancy is complexed with an acceptor. The concentration of the VSe complexes is high (≥ 1018 cm−3 ), indicating that their role is important in the electrical compensation of p-type ZnSxSe1−x and MgyZn1−y SxSe1−x.

Using current-voltage measurements, deep-level transient spectroscopy and admittance spectroscopy we investigated nitrogen doped ZnSe grown on p-GaAs substrates by molecular beam epitaxy. Three major hole traps were observed with energy levels at 0. 11, 0.46, and 0.56 eV from the valence band. We attribute the level at 0.11 eV to a nitrogen acceptor. No other direct observations of this important acceptor level in p-ZnSe have been reported in the literature so far. The two remaining levels may originate from the nitrogen doping process. In addition, reverse current-voltage characteristics of the ZnSe/GaAs heterojunction show a hysteresis at low temperature and a soft saturation. At a constant reverse bias the current increases slowly until it reaches a steady state value. This behavior is attributed to a slow voltage-induced barrier lowering due to the presence of mismatch interface states. Therefore, these analyses are of a major interest for applications of ZnSe/GaAs based devices and illustrates the necessity of improving the growth conditions of such structures.

The photocurrent peak after preliminary optical excitation with specific wavelength in high resistivity (ρ≍1012 Ω∗cm), unintentionally doped ZnSe monocrystals was observed. To study photocurrent “flash” phenomenon, the photocurrent relaxation spectra, photon-capture crosssection (PCCS) spectra and photo-electron paramagnetic resonance (photo-EPR) were investigated. Two-levels model involving the recombination of an S-center and an acceptor A has been developed to obtain the relation between recombination probability and instantaneous photocurrent value. The S-centers ionization energies (1.06eV and 1.14eV from the bottom of the conduction band) and charge carriers life times were determined. The acceptor's positions in ZnSe band gap are equal to 2.05, 2.11, 2.16 and 2.21eV from the bottom of the conduction band. The photocurrent and PCCS “flash” nature as well as complicated photo-EPR are associated with the relaxation of S-centers attributed to Fe ion in the band gap of semi-insulated ZnSe.

Photoluminescence and conductivity of doped with chlorine and copper ZnTe, Zn(SeTe) and CdTe are investigated. We suppose that interstitial tellurium and complex defects of Tei with a donor dopant Cl can significantly affect the luminescence spectra and conductivity value of ZnTe and CdTe. This assumption is confirmed experimentally. We found that copper dopant in CdTe: Cl can increase the resistivity value of p-type CdTe.

The photoluminescence measurements of alkaline metal doped zincselenide (ZnSe) single crystals were carried out to investigate how the impurity atoms substitute the intrinsic vacancy. Sample are grown by the sublimation method and the Li and Na with the concentrations of 0.1, 0.25 and 0.5 mol% were incorporated simultaneously. In the PL spectra with lower impurity concentrations, bound exciton lines of deep acceptor and donor-acceptor pair (DAP) bands were clearly observed accompanied by a weak free exciton line. Although the bound exciton lines becomes smaller with increasing the impurity concentration, the DAP band remains strong and its peak energy shift to lower energy side with the concentration. The decrease of the bound exciton lines is drastic for Li-doped sample than Na-doped sample. This indicates that Li atoms more easily occupy the Zn vacancies than Na.

The surface of Li-doped ZnSe grown on misoriented GaAs(001) substrates by molecular beam epitaxy is studied by means of reflection high energy electron diffraction. Sharp and curved streaky RHEED patterns are observed for all the layers grown on the misoriented substrate towards [110], irrespective of off-angles. No curved pattern, however, is observed on ZnSe layers grown on misoriented GaAs(001) towards [110] with off-angle larger than 5°. We confirmed that the Li-array is surely formed along [110], and that the length of the Li-array is longer than 32Å to be observable as curved streaks. It is also confirmed that the growth rate of Li-doped ZnSe is proportional to the step density, and that the growth rate on the misoriented substrate towards [110] is higher than that on the misoriented substrate towards [110]. The photoluminescence spectra of Li-doped ZnSe layers grown on misoriented GaAs are reported.

Deep levels in II-VI compounds have been investigated to understand their role in the compensation mechanism and their influence on the material electrical and optical properties. The electrical properties have been studied by current and capacitance transient spectroscopy, while the optical properties have been studied by cathodoluminescence. We have focused our attention on the traps involved in the compensation process, such as centre A and the deep levels located near midgap.

Variable angle spectroscopic ellipsometry (VASE) and atomic force microscopy (AFM) measurements have been employed to characterize the surface aging of HgI2 crystals. A surface model including top surface roughness and subsurface defects was established and studied by VASE analysis, as a function of real time, after the 10% KI chemical etching. In this model, the surface defects associated with the surface aging were modeled by the Bruggeman effective-medium approximation (EMA) as a HgI2/voids mixed layer. The relative 2-dimensional (2D) surface-defectdensities were monitored as the surface aging proceeds. The VASE measurements indicated that high surface aging rates were related to high initial effective 2D surface-defect densities. The AFM profile revealed increasing physical surface roughness as surface aging took place. The cleaved HgI2 crystal surface presented a smooth surface and the lowest surface aging rate, while the as-grown HgI2 surface also presented a very low surface aging. The HgI2 surfaces baked at elevated temperatures presented accelerated surface aging phenomena after a 30 min. baking.

The electronic structure of nitrogen donors in 6H-, 4H- and 3C-SiC is investigated by measuring the nitrogen hyperfine (hf) interactions with electron nuclear double resonance (ENDOR) and the temperature dependence of the hf split electron paramagnetic resonance (EPR) spectra. Superhyperfine (shf) interactions with many shells of 13C and 29Si were measured in 6H-SiC. The hf and shf interactions are discussed in the framework of effective mass theory. The temperature dependence is explained with the thermal occupation of the lowest valley-orbit split A1 and E states. It is proposed that the EPR spectra of P donors observed previously in neutron transmuted 6H-SiC at low temperature (<10K) and high temperature (>60K) are all due to substitutional P donors on the two quasi-cubic and hexagonal Si sites, whereby at low temperature the E state is occupied and at high temperature the A1 state. The low temperature spectra are thus thought not to be due to P-vacancy pair defects as proposed previously.

Annealing of electron irradiated bulk n-type 6H-SiC has shown that neutral carbon vacancies and neutral silicon vacancies undergo a major reduction in concentration in the 20–200 °C temperature interval after which only slight changes occur up to 1200 °C. The experiments suggest that the positively charged carbon vacancy, detected by electron paramagnetic resonance, constitutes only a small fraction of all carbon vacancies.

The characteristic defects of 4H-SiC homoepitaxial thin films grown on bulk substrates using chemical vapor deposition (CVD) are described based on transmission electron microscopy (TEM), atomic force microscopy (AFM) and surface decoration studies. Emphasis is placed on understanding the formation mechanism of surface triangular defects. Cross-sectional TEM observations revealed the existence of two variants of 3C-SiC inclusions in 4H epitaxial films. In the plan-view orientation, g4H = 3304 type reflections were found useful for distinguishing the two variants of 3C-SiC platelets that are present in the 4H epilayer. A decoration technique was employed to reveal the relationship between the 3C platelets and surface features, e.g., surface steps. A formation mechanism for surface triangular defects is proposed, which is partially confirmed by the etch pit patterns obtained on the epilayer surfaces after a molten KOH etch.

The electronic properties of 3d transition metal (TM) defects located on one of the four different tetrahedral positions in 3C SiC are shown to be quite site-dependent. We explain the differences for the 3d TMs on the two substitutional sites within the vacancy model: the difference of the electronic structure between the carbon vacancy VC and the silicon vacancy VSi is responsible for the differences of the 3d TMs. The electronic properties of 3d TMs on the two tetrahedral interstitial sites differ even more: the TMs surrounded tetrahedrally by four Si atoms experience a large crystal field splitting while the tetrahedral C environment does not give rise to a significant crystal field splitting at all. It is only in the latter case that high-spin configurations are predicted.

A new technique for investigation of the electrical effects of micropipes in single-crystal 6H-SiC is presented. The setup allows the application of a parallel or normal electric field to MSM (metal-semiconductor-metal) test structures and the visualization of light emission sites in the test gap, including light activity underneath the metal contact. A special transparent metal [indium-tin-oxide(ITO)] was chosen for the metallic contacts. A map of micropipe locations was initially obtained at zero applied field using a laser scattering method. The initial map is compared with that of light emissions at different applied fields.

Several tests on undoped and vanadium-doped (compensated) SiC, using NiCr/Au or ITO contacts, indicated the rapid activation of micropipes at relatively low fields in vertical MSM devices. A good match between the laser imaging map of micropipes at zero field and the map of field-induced light emission sites indicates that micropipes are the main current paths in vertical devices, carrying a large current density, and leading to light emissions and partial or total bulk breakdown of the test device.

Deep levels in 6H-SiC wafers implanted with Al+ ions at high-temperature were studied using current deep level transient spectroscopy (iDLTS). Aluminum was implanted at a temperature of 1800 °C with an energy of 40 keV and a dose of 2 × 1016 cm−2 into n-type epitaxial layers with different carrier concentration. Four levels were found, at Ec−0.12, Ec−0.13, Ec−1.06 and Ev+0.35 eV. It was established that modification of the carrier concentration in original ntype 6H-SiC epitaxial layers affects the deep levels concentration. The relationship between the thickness of the space charge region and the relative deep level concentration was considered.

The interface between 6H and 15R polytypes of silicon carbide, grown by Physical Vapor Transport, was studied by high-resolution transmission electron microscopy. The sample was investigated in cross-section cut perpendicular to the [11.0] irection. The atomic stacking sequence at the interface of the polytypes was determined. Polytype boundaries with orientations parallel and perpendicular to the 6H c-axis were investigated. Stacking faults associated with low angle grain boundaries in both 6H and 15R regions were observed and the 15R regions systematically showed a higher fault density than the 6H regions.

Micropipes, the hollow cores of superscrew dislocations that lie along the c-direction in SiC single crystals, are quite deleterious to the performance of semiconductor devices. In the x-ray topography of longitudinal-cut samples of these crystals, topographs in the reflection (006) show dislocation image contrast associated with the superscrew dislocations lying along the c-axis of the crystal, which is also the direction of the dislocations' Burgers vectors. In the (110) reflection, whose g-vector is perpendicular to the c-axis and the dislocations' Burgers vector, there is also an image of the superscrew dislocation formed, albeit weaker than the corresponding image in the (006) reflection. This dislocation image is thought to represent stress components of the superscrew dislocation in directions perpendicular to the c-axis.

In order to investigate the origin of these stress components, steps have been carried out to determine whether the dislocation image is anisotropic in all possible reflections where g={110}. To achieve this we have excised a hexagonal prism-shaped sample from a 6H-SiC wafer, 100μm wide, polished along the six {100} crystallographic faces, such that a single micropipe ran along the axis of the sample. This enabled x-ray topographs to be taken through each of these {100} faces

Changes in the defect structure in carbon rich a-SiC:H films deposited on various substrates using ditertiary butyl silane were investigated as a function of thermal treatment. Films grown at high deposition rates exhibit hydrogen trapped in voids. The incorporation of CHn, groups is thought to be the origin for these microvoids. With increasing annealing temperature the effusion of CHn, groups as determined by thermal desorption experiments promotes void growth which was studied using a variable energy positron beam. At annealing temperatures above 600 °C the films densify due to the breaking of C-H bonds and the formation of additional C-C bonds when the voids anneal out.