A description of incorporation and solubility limit of substitutional impurities is made using the alloy CdxHg1−xTe as a model of analysis and a source of experimental data. Then non-equilibrium incorporation of impurities under light excitation is considered. A model of photo assisted doping is presented which accounts for the high doping efficiency reported for the donor indium in photoassisted grown CdTe.

The formation energies of defects in ZnO taken from recently published first-principles calculations are used to simulate the incorporation and compensation of group-V impurities. A detailed analysis is carried out for the specific case of nitrogen-doped ZnO as a function of doping temperature, O/Zn stoichiometric ratio, and dopant activity. This study shows that net p-type doping of ZnO:N in thermodynamic equilibrium conditions is rather limited (∼1016 cm−3) and involves a delicate balance between the achievement of high dopant solubility (on the Zn-rich side) and low compensation by donor defects (on the O-rich side). Efficient doping appears possible in various alternative ways: simultaneous incorporation of hydrogen as an interim compensating donor to be subsequently eliminated; use of a high activity dopant in the external phase (atomic N, NO) while preventing the formation of (N2)O compensating donors; planar doping for separate optimization of nitrogen incorporation and a defect-free crystal. The other group-V impurities (P, As) introduce deep acceptor levels when substituted for O, but have been predicted to form a single acceptor complex in the antisite position, like [ AsZn–2V Zn], with both low formation and ionization energies. This leads to a moderate p-type behaviour. Comparison of the present theoretical modelling with experiments shows that accounting for the wide range of reported hole concentrations (1016–1019 cm−3) necessarily implies some departure from equilibrium.

Vickers microindentation of bulk (0001¯) ZnO single crystal has been studied by means of a scanning microscope using cathodoluminescence (CL) monochromatic imaging and spectral modes. A main feature of the sample's deformation is the propagation of the latter away from the indenter contact diameter, in well-defined directions, characterizing the hexagonal crystal systems. An attempt is made to understand plastic deformation mechanism, which is responsible for the observed deformation behavior. The deformation-produced dislocations act as nonradiative recombination centers, which is confirmed by the absence of any additional emission band and the diminution of the band edge luminescence intensity in the Cl spectra after micro-indentation.

The growth of ZnO films deposited by Closed Space Vapor Transport (CSVT) on sapphire substrates has been investigated. Deposition on R oriented sapphire substrates gives rise to a-(11-20) oriented ZnO films. Under optimised conditions, flat surfaces can be achieved and rocking curves with full half width below 500 arcsec are observed. The electrical properties of the films were studied. Hall measurements reveal that the measured n-carrier concentration decreases linearly upon the thickness of the sample. This is interpreted as interface conduction probably related to diffusion of aluminium from the substrate. On thinnest films, the n-carrier concentration can be dramatically decreased with thermal annealing under oxygen. Furthermore, the effect of this annealing under oxygen is found to be completely reversible after a further thermal annealing under oxygen free atmosphere.

The incorporation and electrical activity of nitrogen as an acceptor in ZnO has been investigated. Low temperature Metalorganic Vapor Phase Epitaxy (MOVPE) growth, using diallylamine as nitrogen precursor, yields to incorporation of nitrogen in the range 10(16)-10(21) cm(-3). The electrical activity of nitrogen is demonstrated through the increased compensation of the natural donors with doping level. Close Space Vapor transport (CSVT) and Chemical Vapor Tranport (CVT) are found to be less efficient for nitrogen incorporation. This suggests that the use of high temperature growth is a limiting factor for nitrogen incorporation in ZnO. Ex-situ techniques have been tried for both electrical activation and nitrogen incorporation in ZnO. High pressure annealing under oxygen pressure shows a conversion to p-type on nitrogen doped samples grown by MOVPE. Finally, diffusion of nitrogen was carried out on undoped MOVPE layers under high pressure conditions stemming from the decomposition of NH4NO3. Conversion to p-type conductivity was observed in a systematic way with measured hole concentrations up to 6.5. 10(17) cm(-3). These results suggest that ex-situ treatment can be a practical way to realize p-type ZnO layers.

The electrical activity of nitrogen as an acceptor in ZnO has been investigated in two ways. First, nitrogen was introduced by means of diallylamine during metalorganic vapor phase epitaxy (MOVPE) yielding incorporation of nitrogen in the range 1016–1021 cm−3. This led to significant compensation of the natural donors with a minimum electron concentration of 5×1014 cm−3. Second, diffusion of nitrogen was carried out on undoped MOVPE layers under high pressure conditions stemming from the decomposition of NH4NO3. Conversion to p-type conductivity was observed in a systematic way with measured hole concentrations up to 6.5×1017 cm−3.

The effect of hydrogenation on defects associated with dislocations has been studied in n-type CdTe crystals with the help of cathodoluminescence imaging and photoluminescence spectra. Dislocations were introduced by performing indentations on the Cd(111) and Te(1̄1̄1̄) crystal faces using a Vickers microhardness apparatus. Hydrogenation was achieved by annealing the samples in closed ampoules at 500°C for 70h under deuterium atmosphere at a pressure of 700mbar. Cathodoluminescence images of hydrogenated samples show a removal of the long arms of the indentation rosette on Te face and short arms on Cd face. This is accounted for in terms of the formation of TeH2-like volatile entities as Te atoms are involved in both cases. Photoluminescence spectra indicate that the general effect of plastic deformation is to create acceptor centres related to mobile impurities (Cu) and to Cd vacancies. Those acceptors may be passivated by formation of neutral complexes with deuterium.

The effect of codoping in semiconductors takes on the appearance of reduced impurity ionisation and enhanced solubility of the majority impurity in presence of the compensating one. These two issues were addressed by considering the changes in the impurity ionisation process due to a high concentration of donors and acceptors. Specifically potential fluctuations associated with random distributions of impurities are dealt with in a simple but tractable model which allows one to define the optimum conditions of codoping. The required compensation ratio turns out to be around 0.5. Low activation energy is also predicted. Potential fluctuations are shown to affect as well impurity incorporation from an external phase in codoping experiments. A pending question is the actual carrier mobility in such disordered material.

A short theoretical analysis of the main mechanisms governing the efficiency of impurity doping in CdTe is first presented. Then doping experiments with donor (Al, In, Cl, Br, I) and acceptor (N, As) impurities are reviewed. It has been shown that uncompensated n-type and p-type doping up to a limit in the range 1017–1019 cm−3, depending on the element, can be achieved under proper conditions of incorporation. The implication of impurities in the fabrication steps of CdS/CdTe solar cells, including the CdCl2 treatment, is briefly discussed. Finally, the present state of knowledge about particular native point defects in tellurium-saturated CdTe (VCd, TeCd) is examined.

Polycrystalline layers of the alloy CdSe0.9Te0.1 were prepared from compacted plates of CdSe-CdTe-Cr2O3 powders. By sintering these plates in a temperature gradient, sublimation of CdSe and CdTe occurs which finally leads to crystallization of a CdSeTe alloy layer on the remaining Cr2O3 particles. Extensive characterization of the layers has been performed using X-ray diffraction, SEM observations, photoconductivity and photoluminescence measurements. A grain size up to 40 μm has been obtained with some preferential micro-crystalline orientation along the c-axis of the wurtzite structure. Relative photoconductivity approaching 106 under 500 Lx illumination has been measured, which demonstrates the high photosensitivity of this material grown in the presence of α chromium tri-oxide.

CdS layers were grown by MOVPE on (100) GaAs and (211)B GaAs substrates. The last substrate orientation yields homogeneous wurtzite CdS layers with sharply defined free excitonic structures seen in optical reflectivity and photoluminescence. Free exciton luminescence dominates the spectrum at T > 50 K. Structural studies indicate that the layers grown on (100) GaAs substrates are a mixture of hexagonal and cubic CdS phases spatially arranged in a quasi-periodic fashion. The luminescence spectrum reflects this situation.

ZnS layers were grown by metalorganic vapor phase epitaxy on GaAs substrates using diethylzinc, ditertiarybutyl sulphide, and triallylamine as organometallic sources. After postgrowth rapid thermal annealing, the ZnS layers showed p-type conductivity with hole concentrations up to 1018 cm−3. Photoluminescence measurements gave additional indications of the presence of electrically active nitrogen acceptors. In separate experiments, lithium was diffused from a LiH solid source into ZnS layers grown without the nitrogen precursor. High-conductivity p-type material was directly obtained with no need of thermal anneal.

Nitrogen doped ZnSe layers grown by molecular beam epitaxy have been exposed to a hydrogen or deuterium plasma. Deuterium diffusion profiles have been measured by secondary ion mass spectroscopy. The main feature of these profiles is the presence of a plateau on which the H concentration closely matches the total N content of the layers. Electrical and photoluminescence studies of the layers, before and after hydrogen plasma exposure, show that both acceptor and donor N-related centers are passivated by hydrogen. These results demonstrate that in ZnSe:N hydrogen passivates the nitrogen acceptors and the related nitrogen donors.

The photorefractive properties of CdTe and CdZnTe alloys are reviewed. Firstly, the energy level spectrum of the deep centre vanadium is analyzed from the data brought about by various spectroscopic techniques. Secondly, the photorefractivity parameters as deduced from two-wave-mixing experiments are presented and discussed. The insufficiency of the one-level model is pointed out. Thirdly, a few demonstrations of possible applications in the field of optical detection and processing are briefly reported. In conclusion, the specific features of photorefractivity in Cd(Zn)Te:V crystals are stressed, in comparison with Fe-doped InP, and possible ways of improvement are indicated. (C) 1999 Elsevier Science B.V. All rights reserved.

Deep levels in vanadium-doped CdTe and CdZnTe crystals grown by vertical Bridgman for photorefractive applications are reviewed. Based on photo-induced current transient spectroscopy, deep-level transient spectroscopy and deep-level optical spectroscopy, deep traps are identified and their electrical and optical properties are characterized. A discussion about their origin and a comparison with results obtained by other spectroscopy techniques are given. (C) 1999 Elsevier Science B.V. All rights reserved.

The effect of alternate n—p—n—p doping on the in-plane photoconductivity of Hg1−xCdxTe structures is investigated. A theoretical analysis leads to evaluation of the effective lifetime enhancement due to the spatial separation of photogenerated electrons and holes as a function of electronic and constitutive parameters. Enhancement factors greater than 20 are predicted for low band gap alloy (x = 0.22). Experimental realisations made use of layers grown by MOVPE, whose electronic properties were adjusted by thermal annealing or impurity (As) doping. Photoconductivity measurements performed on simple single-junction structures with x = 0.22 show a signal enhancement at low temperature, in agreement with theoretical simulation.

Substrate (GaAs) orientation dependence of arsenic incorporation in MOVPE-grown CdTe is reported. Arsine was used as the dopant carrier gas. The substrate orientations were (1 0 0), (5 1 1)A, B, (3 1 1)A, B and (2 1 1)A, B. The highest incorporation was obtained for (1 0 0) substrates and the lowest for (2 1 1)A orientation. As a general rule, incorporation was larger on B faces compared to A ones. A strong polarity dependence was observed between (2 1 1)B and (2 1 1)A surfaces with arsenic concentration differing by an order of magnitude. Thermal anneals led to acceptor activation with maximum hole conductivities related to the amount of incorporated arsenic. Analysis of these results was done by considering the bonding geometry of the different exposed surfaces and the fact that As is introduced through an intermediate species formed in the vapour phase, the adduct DimethylCdAsH3.