O. de Melo

Low resistivity cubic phase CdS films by chemical bath deposition technique

In this letter we report on cubic CdS thin films with low resistivity by chemical bath deposition (CBD) technique and subsequent annealings in S2 and H2+In. Low temperature photoluminescence, x rays, and transmission spectra support the assumption that S2 annealings contribute to fill the vacancies in the as‐deposited films leading to an enlargement of the CdS cubic cell. This fact is revealed by an increase in interplanar distances, evanescence of the PL red broad band, and decrease in band‐gap energies. Cubic phase remains after H2+In annealing at higher temperatures. A resistivity as low as 11 Ω cm was obtained at an optimum annealing temperature of 350 °C.

Strain relaxation during the layer by layer growth of cubic CdSe onto ZnSe

A detailed reflection high-energy electron diffraction analysis shows relevant features of the lattice parameter relaxation of CdSe thin films grown in a layer-by-layer mode onto ZnSe. In situ investigations of different azimuths show a clear lattice parameter oscillation in the [110] azimuth. The lattice parameter has a minimum value (similar to that of ZnSe) during Se exposure steps, and a higher and increasing lattice parameter during Cd exposure steps. The behavior is ascribed to the formation of CdSe islands during Cd exposure steps. The cumulative effect in CdSe exposure steps is considered to be a consequence of a decrease in the island size with the number of cycles. Actual plastic deformation does occur after 5 ML.

Thickness measurement and optical properties of very thin ZnxCd(1−x)Te layers

A theoretical procedure based on the model dielectric function is used to fit the room temperature reflectivity spectra of ZnxCd1−xTe alloys in the range 1.5–4.5 eV. This procedure allows calculation of the thickness of very thin films as well as determination of the composition dependence of the critical points. A parabolic dependence on Zn molar fraction is reported. Calculated optical data, such as refractive index and extinction coefficient, are also presented.

Investigation of deep levels in ZnSe:Cl films grown by molecular beam epitaxy

Using ZnCl2 as a doping source, we have grown epitaxial layers of Cl‐doped ZnSe onto GaAs(100) substrates at a temperature of 285 °C by molecular beam epitaxy. The carrier concentration was controlled by the ZnCl2 source temperature. The maximum carrier concentration obtained was 1.2×1019 cm−3 with a resistivity of 2.7×10−3 Ω cm. Higher doses of Cl atoms tend to decrease free carrier concentration and introduce additional defects; this effect was consistent with an increase of deep‐level photoluminescence emission. From deep‐level transient spectroscopy measurements we determined the presence of four traps with energy levels Ec around 0.50, 0.68, 0.74, and 1.20 eV. The last three traps have not been previously reported. One of them, Ec∼0.74 eV, presents a clear dependence with Cl concentration and thus can affect the performance of ZnSe:Cl‐based devices. The microscopic nature of these traps is discussed in terms of Zn vacancies (VZn) and VZn–Cl complexes.

Photoluminescence Study of Ultra-Thin CdSe Quantum Wells

CdSe ultra-thin quantum wells (UTQWs) with thickness in the 0.5 to 4 monolayers (ML) range were grown by pulsed beam epitaxy (PBE) between barriers of ZnSe at substrate temperatures of 260 and 290 °C. In each sample, five similar QWs separated by barriers around 200 A thick were deposited onto a 5000 A buffer layer of ZnSe. During the growth process, the surface was monitored in-situ by reflection high energy electron diffraction (RHEED); a 2D growth mode was observed in all the UTQWs. The analysis of the streak positions in the RHEED patterns indicated a critical thickness around 1.5 ML. The photoluminescence (PL) experiments exhibited intense and narrow peaks in the blue-green region of the spectra. The energy of the PL peaks presented a clear dependence on growth temperature and QW thickness. The emission of the UTQWs grown at 290 °C was blue-shifted in comparison to those grown at 260 °C. The PL peaks of the samples show the presence of different transitions, which cannot be explained in terms of thickness fluctuations.

Investigation of the structural properties of MBE grown ZnSe/GaAs heterostructures

The results of photoluminescence and Raman spectroscopies, high resolution X-ray diffraction, and Auger electron spectroscopy are analyzed in terms of the structural properties of the ZnSe/GaAs(l 0 0) system as a function of film thickness and substrate temperature. The results of Raman spectroscopy and X-ray diffraction clearly show that the strain in the film is inhomogeneous and depends only on film thickness and not on growth temperature in the 285-325°C range. From these experiments a value of N 0.17 pm is inferred for the critical thickness of ZnSe on GaAs. Photoluminescence experiments sensitive to the ZnSe/GaAs interface reveal the presence of strain in the GaAs substrate. Analysis of the intensities of the LMM Auger transitions of Zn and Se indicate the formation of an interfacial layer with excess of Se, suggesting the formation of a pseudomorphic Ga-Se compound mixed with ZnSe at the interfacial region.

Crystallographic properties of some ternary and multinary Te-based semiconductors

Crystallographic properties of several single crystals of ternary ABC2 (A = Cu, Ag; B = In, Ga; C = Te, Se) compounds, of Cd1−xMnxTe (0.5 ≤ x ≤ 0.66), of quaternary and pentenary (CuIn1−xGaxSe2(1−yTe2y with x = 0, 0.7, 1 and y = 0.135, 0.65, 0.4) solid solutions have been determined by single crystal X-ray diffraction. Many chalcopyrite compounds and alloys show thermal polymorphism involving order-disorder phase transitions, and the structure changes are characterized by a first order transition from ordered tetragonal chalcopyrite to the completely disordered zinc-blende and/or wurtzite structures. All the single crystals were grown by chemical vapour phase transport in a close tube using iodine as the transport agent. The phases observed in growing ternary compounds depend on the temperature used during the growth process. For the multinary solid solutions the situation appears to be more complex: in fact, together with the expected phases some altogether unexpected ones were detected, confirming the idea that the growth process of these compounds is dependent not only on temperature but also on parameters involving the composition and growth kinetics.

Optical and structural properties of ZnSe/GaAs interfaces

Abstract The results of high resolution X-ray diffraction clearly show that the strain in the ZnSe films is inhomogeneous and depends only on the film thickness and not on the growth temperature in the range of 285–325°C; a value of ∼0.17 μ m is inferred for the critical thickness of ZnSe on GaAs. The measurements of the spectral response of the heterostructures give a strong photocurrent due to the absorption edges of both GaAs and ZnSe. However, in the case of the GaAs signal a shift towards lower energies is observed with the increase of substrate temperature, indicating local differences in the structure of the interfacial region. The analysis of the intensities of the LMM Auger transitions of Zn and Se indicates the formation of an interfacial layer with excess of Se, suggesting the formation of a pseudomorphic Ga 2 Se 3 compound mixed with ZnSe at the interfacial region.

CVT growth, and magnetic and electronic properties of NiGa2O4 single crystals

Abstract Growth of NiGa 2 O 4 spinel single crystals by the chemical vapor transport method is carried out together with a detailed thermodynamical investigation of the NiGa 2 O 4 -Cl 2 system. Moreover, structural, magnetic and electronic properties by using single crystal X-ray diffraction, AC susceptibility measurements and X-ray photoelectron spectroscopy are reported.

CVT growth, thermodynamic and magneto-structural study of Cd1-xMnxTe single crystals

The growth of Cd1-xMnxTe (with 0.5 ≤ x ≤ 0.66) single crystals by the chemical vapour transport technique in a closed tube using iodine as transport agent is reported. As regards the growth process, a qualitative thermodynamic picture of the CdTe-MnTe-I2 system is given with the aim of avoiding the presence of liquid or solid spurious species (i.e., MnI2 liquid, CdI2 liquid and MnTe2 solid. The grown single crystals showed the cubic zincblende structure; the hexagonal symmetry, verified in various cases, was found to be apparent and due to a particular type of twinning consisting in a syntaxic coalescence of cubic domains. The magnetic characterization by susceptibility measurements is reported. XPS and Auger electron spectroscopy are used for studying the electronic structure of the crystals, and for monitoring possible modifications in the chemical composition of their surface.