Observation of macro-terraces in cadmium telluride thin films

CdTe thin films (prepared on a Corning glass substrate by a modification of the close-space vapour transport technique) were studied by X-ray diffraction, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The conclusion of the study is that the films are formed by large microcrystals with very flat surfaces. The roughness measurements show that the surfaces of the microcrystals are flatter than CdTe layers grown by molecular beam epitaxy (MBE) on CdTe substrates. The terraces were almost atomically flat with sizes up to 0.2 μm2. This revised version was published online in July 2006 with corrections to the Cover Date.     

Space-charge-limited conductivity in evaporated cadmium selenide thin films

We have studied the basic d.c. electrical properties of evaporated CdSe films in the context of the development of inexpensive solar cells. Earlier measurements have shown significant variations, which depend on the preparation and previous treatment of the films. In the present work Al---CdSe---Al sandwich structures were studied, with the CdSe thickness in the range 0.1–1.0 (μm. D.c. capacitance varied inversely with film thickness, yielding a permittivity of 7.82 × 10⁻¹¹ F m⁻¹ (relative permittivity 8.83). Current density was proportional to applied voltage at low voltage levels, but followed a power law at higher voltages with exponent typically 2.5; the transitional voltage was directly proportional to the square of the film thickness. The results were interpreted as ohmic conduction at low voltages and space-charge-limited conductivity (SCLC) dominated by an exponential trap distribution, at higher voltage levels. Measurements of current density as a function of inverse temperature for different applied voltages in the SCLC region enabled the derivation of typical mobilities in the range of ( 7.65–10.15) × 10⁻⁵ m² V⁻¹ s⁻¹ using the results of our existing theory. This value of mobility and the derived trapping parameters were in general agreement with some earlier measurements.     

The structure and semiconducting properties of cadmium selenide films

CdSe films about 1 μm thick were vacuum deposited on unheated glass substrates. Reflection electron diffraction studies showed the growth of a one-dimensional {00.1} texture orientation of the hexagonal phase. Films of lower resistivity were characterized by a larger grain size and better ordering, whilst films of higher resistivity contained amorphous regions and were less ordered. A study of the carrier type, the concentration, the Hall mobility and the variation in the bond length u along the hexagonal c axis of “as-deposited” and heat-treated films showed that the lower resistivity films incorporated a small cadmium excess, which increased with heat treatment, resulting in higher carrier concentrations and a further lowering of the resistivity. Large increases in the resistivities of films deposited at high rates were attributed to the depletion of the small individual grains.     

Successively pulse plated cadmium selenide films and their photoelectrochemical behaviour

In successive pulse plating, the pulse deposition is repeated after a definite duration of plating preceded by a mild cleaning of the electrode and with a fresh deposition bath for the same duration. In this study, CdSe films were deposited on Ti substrates by successive pulse plating from a bath containing CdSO₄ and SeO₂ at a current density of 80 mA cm⁻² and a duty cycle of 3·3% for a duration of 30 min. The films heat-treated to 550°C for 20 min in argon atmosphere, were polycrystalline with a hexagonal structure. At an illumination of 60 mW cm⁻², a conversion efficiency of 4·5% for the photoetched film and 1·7% for the chemically etched one were determined.     

Structural and optical characterizations of chemically deposited cadmium selenide thin films

Synthesis of cadmium selenide thin films by CBD method has been presented. The deposited film samples were subjected to XRD, SEM, UV-vis-NIR and TEP characterization. X-ray diffraction analysis showed that CdSe film sample crystallized in zinc blende or cubic phase structure. SEM studies reveal that the grains are spherical in shape and uniformly distributed all over the surface of the substrates. The optical band gap energy of as deposited film sample was found to be in the order of 1.8 eV. The electrical conductivity of the film sample was found to be 10⁻⁶ (Ω cm)⁻¹ with n-type of conduction mechanism.     

Properties of r.f. sputtered cadmium telluride thin films

CdTe thin films were prepared using r.f. magnetron sputtering in an Ar atmosphere. Substrate temperatures in the range 100–320 °C were used. XRD results showed that the films are amorphous below 200 °C while above 200 °C the firms were polycrystalline with cubic structure and grains preferentially oriented along the [1 1 1] crystallographic direction. SEM measurements showed significant enhancement of crystallite size with increase of T _s or with post-preparation annealing above 400 °C. The 5 K photoluminescence spectrum showed a broad (FWHM=80 meV) band with a maximum at 1.538 eV. This band showed significant narrowing after annealing above 400 °C suggesting that it originates from transitions involving grain boundary defects. The refractive index n was determined from the interference pattern of the optical transmission. The results agree with the values of n calculated using the Jensen theory. The absorption coefficient was determined for photon energies hE _g (the energy bandgap) from the optical transmission spectra in the absorption region using the Swanepoel theory. Several direct and indirect allowed optical transitions were identified. It was found that the transitions can be grouped into four main allowed transitions (two direct; E _o, E ₃ and two indirect; E ₁, E ₂) whose energy values vary from one sample to another due the quantum size effect associated with small grain size. The main transitions are: E _o (1.50–1.77 eV) assigned to ₈ valence band (VB)₆ conduction band (CB) transition, E ₁ (1.84–2.05 eV) assigned to L_4,5(VB) _i transition where _i is an impurity level at 1.2 eV above the ₈ (VB), E ₂ (2.37–2.49 eV) assigned to L_4,5 (VB)₆ (CB) transition and E ₃ (2.25–2.55 eV) assigned to ₇ (VB) _i transition. The impurity is attributed to native centers or grain-boundary-related defects.     

Mechanisms of formation and charge states of intrinsic defects in lead telluride films

A crystallochemical model of the vapor-phase epitaxy of lead telluride films is suggested. The model is based on the simultaneous formation of singly-and doubly-charged Frenkel defects in the cationic sublattice. The results of numerical calculations based on this model are in good agreement with the known experimental data.     

Effects of substrate temperature and post-deposition anneal on properties of evaporated cadmium telluride films

The effects of substrate temperature and post-deposition heat treatment steps on the morphology, structural, optical and electrical properties of thin film CdTe layers grown by vacuum evaporation were investigated. Scanning electron microscopy and X-ray diffraction (XRD) techniques were employed to study the structural changes. It was observed that the grain sizes and morphologies of as-deposited layers were similar for substrate temperatures of − 173 °C and − 73 °C. However, CdTe films produced at a substrate temperature of 27 °C had substantially larger grain size and clearly facetted morphology. Annealing at 200-400 °C in air did not cause any appreciable grain growth in any of the films irrespective of their growth temperature. However, annealing at 400 °C reduced faceting in all cases and initiated fusing between grains. XRD studies showed that this behavior after annealing at 400 °C coincided with an onset of a degree of randomization in the originally strong (111) texture of the as-grown layers. Optical band gap measurements showed sharpening of the band-edge upon annealing at 400 °C and a band gap value in the range of 1.46-1.49 eV. Resistivity measurements indicated that annealing at 400 °C in air forms a highly resistive compensated CdTe film. All results point to 400 °C to be a critical annealing temperature at which optical, structural and electrical properties of CdTe layers start to change.     

Proper point defects in cadmium telluride with excess of cadmium

Calculation of concentrations of free charge carriers and point defects in monocrystals of cadmium telluride is carried out at the boundary of the region of existence of the compound. The type of dominating proper point defects determining the electrical properties of the material at an excess of cadmium is identified. The maximum annealing temperatures and partial vapor pressures of two-temperature annealing at which a material with electron conductivity is obtained are determined.     

Synthesis and characterization of cadmium telluride nanowire

CdTe nanowires with controlled composition were cathodically electrodeposited using track-etched polycarbonate membrane as scaffolds and their material and electrical properties were systematically investigated. As-deposited CdTe nanowires show nanocrystalline cubic phase structures with grain sizes of up to 60 nm. The dark-field images of nanowires reveal that the crystallinity of nanowires was greatly improved from nanocrystalline to a few single crystals within nanowires upon annealing at 200?°C for 6?h in a reducing environment (5%?H(2)+95%?N(2)). For electrical characterization, a single CdTe nanowire was assembled across microfabricated gold electrodes using the drop-casting method. In addition to an increase in grain size, the electrical resistivity of an annealed single nanowire (a few 10(5)?Ω?cm) was one order of magnitude greater than in an as-deposited nanowire, indicating that crystallinity of nanowires improved and defects within nanowires were reduced during annealing. By controlling the dopants levels (e.g.?Te content of nanowires), the resistivity of nanowires was varied from 10(4) to 10(0)?Ω?cm. Current-voltage (I-V) characteristics of nanowires indicated the presence of Schottky barriers at both ends of the Au/CdTe interface. Temperature-dependent I-V measurements show that the electron transport mode was determined by a thermally activated component at T>-50?°C and a temperature-independent component below -50?°C. Under optical illumination, the single CdTe nanowire exhibited enhanced conductance.     

Heat capacity and thermodynamic properties of lead selenide and lead telluride

Given that the heat capacities of PbSe and PbTe had been previously measured only below 240 and 260 K, respectively, we evaluated unreported heat capacity values up to 298.15 K. The low- and high-temperature heat capacity data were matched, and the thermodynamic functions of PbSe and PbTe were calculated from 298.15 K to temperatures close to their melting points.     

Electronic and adsorption properties of the real surface of cadmium selenide films containing indium whiskers

An investigation is made to determine how surface doping of thin semiconducting cadmium selenide films with heavy metal atoms influences their binding energy and their absorption sensitivity to oxygen. Under certain doping conditions, indium atom whiskers are formed on the surface of the films. These samples show an appreciable reduction in the oxygen binding energy with the surface and a reversible response in the interaction with oxygen at 353 K. Pis’ma Zh. Tekh. Fiz. 23, 13–17 (August 12, 1997)     

Infrared reflectance spectra of oxide films on aluminum-magnesium alloys

Infrared reflectance spectra of oxide films on hot rolled 3004 and 5182 aluminum alloy surfaces indicate the presence of two types of magnesium-doped aluminum oxide. Heating the samples to 350°C or more causes shifting and broadening of the 900 cm^?1 aluminum-oxygen absorption band for 3004 metal, and the appearance of a magnesium-oxygen absorption band at 670 cm^?1 in the reflectance spectra of both 3004 and 5182 metal.Reflectance spectra of the surfaces on cold rolled 3004 and 5182 metal show no absorption bands on unheated samples. However, results from time-temperature studies indicate that further oxidation of these samples is more rapid than in hot rolled samples. The oxides formed on cold and hot rolled 5182 metal are similar, while cold rolled 3004 metal initially forms an oxide which differs from that on hot rolled 3004 metal, exhibiting a broader absorption band at approximately 850 cm^?1.     

Laser implantation of impurities in cadmium telluride crystals

Processes of laser implantation of shallow donors (aluminum and indium) and an acceptor (antimony) in CdTe crystals (n,p∼10¹⁵ cm⁻³) are investigated. Thin dopant films vacuum deposited on the etched surface of the crystals are irradiated by ruby (λ=0.694 μm) and Nd:YAG (λ=1.06 μm) laser pulses (pulse duration 20 ns) over a wide energy interval (0.1–1.8 J/cm²). The irradiated surfaces are studied by x-ray microanalysis, Auger spectroscopy, and the thermopower method. It is it is shown that irradiation by a Nd:YAG laser produces a uniform doping of a subsurface layer of the crystal by aluminum. The implantation of indium leads to the formation of a precipitate. The concentration of implanted impurities reaches 10¹⁹–10²¹ cm⁻³. Pis’ma Zh. Tekh. Fiz. 24, 1–6 (June 12, 1998)     

Epitaxy of cadmium telluride in a quasi-closed cell

The structure and morphology of epitaxial CdTe films obtained by the method of vacuum condensation in a quasi-closed cell on mica (muscovite) with substrate temperatures ranging from 50° to 650°C and evaporation temperatures of 400°–700°C and with various degrees of supersaturation and different gas phase compositions have been investigated. It has been shown that this method allows epitaxial films to be synthesized even at relatively low substrate temperatures (～50°C), and under conditions close to equilibrium ones it is possible to obtain epitaxial layers with a low density of figures of growth. The effect of an excess pressure of one component on the morphology and phase composition of the films has been studied. An excess pressure of cadmium over the substrate increases the relative rate of growth of crystallites in the [111] direction and promotes the growth of films of cubic modification with a more perfect structure. An excess pressure of tellurium has the reverse effect. Under conditions close to equilibrium ones (α≈1.05) we have observed the growth of CdTe films of cubic modification only, irrespective of the composition of the gas phase.     

The avalanche multiplication of electrons and holes in cadmium telluride

It is experimentally demonstrated that the process of impact ionization in cadmium telluride crystals is stimulated by holes. The ratio of the impact ionization coefficients of holes (α_p) and electrons (α_n) amounts to α_p/α_n ≈ 30–40.