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.     

Flexible and lithography-compatible copper foil substrates for ferroelectric thin films

A process has been developed for preparing a low surface roughness copper foil by evaporation and subsequent peel-off of copper metal layers on glass slides. These 15 micron thick substrates exhibited roughness values between 1 and 2 nm root-mean-square (RMS) and 9 nm RMS over 25 μm² and 100 μm² analysis areas, respectively. The deposition and crystallization of barium strontium titanate layers were demonstrated on these smoother variant foils. The fully processed dielectric layers exhibited field tunability greater than 5:1, and could withstand fields in excess of 750 kV/cm. High field loss tangents below 0.007 were observed, making these materials excellent candidates for microwave devices. Finally, a process of lamination and contact lithography was used to demonstrate patterning of micron-scale features suitable for microwave circuit element designs.     

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.     

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.     

Photosensitivity and spectra of deep local states in sintered films of cadmium selenide and cadmium telluride alloys

The photoconductive and luminescence properties of non-intentionally doped and copper-doped CdSe_xTe_1−x (x=0−1) films were studied. The films were prepared by spraying an aqueous suspension of mixtures with varying composition onto ceramic substrate. Both the energy gaps determined using photoconductivity spectra and the maxima of the luminescence bands as functions of the film composition x were obtained. The correlation of the luminescence intensity and the photoconductivity with x was established. Using the decay curves of photocurrent excited under “light impact” conditions the electron capture cross-sections of the recombination centres in the films under investigation were estimated. The nature of the recombination centres in CdSe_xTe_1−x (x=0–1) films both copper free and copper doped are discussed.     

Evaporated aluminium nitride encapsulating films

AIN films were deposited onto GaAs and vitreous carbon substrates held at room temperature, by reactive evaporation of aluminium in the presence of nitrogen and/or NH₃ gas mixture. These films and their combination with very thin layers of Si₃N₄ were successfully used as encapsulants for GaAs and were found to withstand annealing temperatures of up to 1100°C. Films grown by this novel method were analysed by Rutherford backscattering spectrometry and reflection high energy electron diffraction. Oxygen, nitrogen and aluminium were the only elements detected in the encapsulants. However, the best encapsulants were found to have the lowest oxygen content. The deposition conditions were found to be very important in preventing the reaction of the films with the surface of GaAs during heat treatment.     

Solution growth of cadmium telluride

Cadmium telluride crystal has been grown by solution method from Te-rich (Cd_0·3Te_0·7) melt. Ingots having 9 mm diameter and length up to 30 mm were grown by cooling the melt slowly (1°C/h) under a vertical temperature gradient of about 30°C/cm. As-grown ingots were characterized for optical transmission and resistivity. The middle portion of the ingots exhibited better optical transmission properties. Resistivity (p-type) was found increasing, towards the last-to-grow end, from 10³ to 10⁶ Θ-cm. Surface barrier type of detectors, made from low resistivity (≅ 10⁴Θ-cm) materials, were found suitable for detection ofX- and low energy gamma radiations. In case of high resistivity (≅10⁶Θ-cm) detectors, the performance was seen to be affected by polarization. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Residual compressive stress in sputter-deposited TiC films on steel substrates

Polycrystalline TiC films with thickness between 0.1 and 2.8 microm were deposited by r.f. sputtering onto 1010 steel and borosilicate glass substrates at 200°C. All films were found to be in a state of compression. For a film grown under a given set of deposition conditions, the incremental compressive stress, i.e. the average stress in the uppermost deposited layer, was generally found to be largest near the film-substrate interface and to become constant with film thickness t_f for t_f ? 0.3 microm. However, for a given t_f the incremental stress increased with a decrease in the argon sputtering pressure P_Ar. Experimental results showed that the incremental compressive stress in bulk films could be directly related to the trapped argon concentration. Argon incorporation is due to the burial of energetic species incident on the growing film surface from two primary sources: energetic neutrals produced by Ar⁺ ions scattered off the target in binary collisions and Ar⁺ ions accelerated to the substrate owing to its induced negative potential with respect to the positive space charge region in the r.f. discharge. The trapped argon concentration from both contributions increased with decreasing P_Ar. All films grown on steel substrates exhibited good adhesion as indicated by indentation and diamond stylus scratch tests. The residual compressive stress in the films was found to be beneficial for wear-related applications in which the film was subjected to a large tensile stress.     

Phosphorus implanted cadmium telluride solar cells

Polycrystalline cadmium telluride layers were implanted with phosphorus (P) in order to obtain an enhanced p-type doping close to the back contact of CdTe solar cells. The implantation parameters were adjusted based on computer simulations using SRIM. While the implantation profile was kept constant, different CdTe layer thicknesses were investigated. Furthermore, different annealing and activation processes and their influence on the P distribution in the device as well as ion beam induced damage were investigated. The P level was identified by photoluminescence measurements, the effective doping was investigated by means of capacitance-voltage measurements, and the effect on the solar cell properties was analyzed by current voltage characteristics. The results show the P distribution in the CdTe layer to depend strongly on the thermal and chemical post-implantation treatment. The effect of the P-doping on the solar cell properties becomes obvious by an increase of the open-circuit voltage due to the implantation.     

Heavy element diffusion in cadmium telluride

The diffusion behaviour near the surface for gold and bismuth in cadmium telluride has been investigated by the charged particle Rutherford back-scattering and ion microprobe analysis techniques in the temperature range 200°–450°C.For all temperatures above 370°C penetration saturation effects are observed. Preliminary results for the activation energy (1.7 eV for gold, 0.3-0.4 eV for bismuth) and diffusion constants D₀ are presented. For comparable temperatures, a much more rapid diffusion of gold occurs in evaporated layers of cadmium telluride than in the crystalline case. It further appears that for near-surface diffusion (<1000 Å) the back-scattering technique is more useful, with its higher resolution and its ability to give information on atom location, than the ion microprobe. However, the latter technique shows advantages when information from greater depths is required.     

Electro-optic coefficient spectrum of cadmium telluride

A spectrum for the electro-optic coefficient of cadmium telluride measured from 3 to 14 μm is reported. The spectrum shows that the quantity n(3)r(41) has a nearly constant value of 1.09 × 10(-10) m/V over this spectral band, with a slight (5%) dip at the weak absorption band centered at 6 μm. Measurements were performed with an infrared Mueller matrix spectropolarimeter. Transmission spectra of the Mueller matrix were acquired at a set of applied voltages. Retardance spectra were calculated from Mueller matrix spectra, and then the electro-optic coefficient was calculated at each wavelength by a least-squares fit to the resulting retardance as a function of voltage.     

Ellipsometric studies on cupric telluride thin films

The optical properties of cupric telluride (CuTe) thin films have been studied in the wavelength range 310-800 nm using spectroscopic ellipsometry (SE). Thin films of thickness between 30 and 150 nm were prepared by thermal evaporation at the rate of 15.6 Å/s on well cleaned glass substrates kept at 300 K under the vacuum better than 2×10⁻⁵ mbar. It has been found that the optical band gap increases with the thickness of the films. The refractive index of the films increases with the energy but the extinction coefficient first increases and then decreases gradually with energy. The analysis of the absorption coefficient determined from the extinction coefficient reveals that there is allowed direct transition with a band gap of about 1.5 eV. The increase in the band gap with the increase in the film thickness has been ascribed to defect levels in the band gap formed by defects in the films.     

Some investigations on offstoichiometric zinc telluride films

Stoichiometric and offstoichiometric zinc telluride thin films are prepared by flash evaporation onto glass substrate at 50° C. The films are investigated by transmission electron microscopy, electron diffraction (for selected area diffraction and micro area diffraction) and IR transmittance. The electrical resistivity and its temperature dependence were studied by sandwiching the zinc telluride film between silver electrodes making ohmic contacts. The degradation in structure has been observed on disturbing the stoichiometry of zinc telluride films. Interestingly, the composition dependence of electrical resistivity of the films agrees well with the IR transmission spectra.     

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.     

Flexible cadmium telluride/cadmium sulphide thin film solar cells on mica substrate

Polycrystalline thin film II–VI compound semiconductors of cadmium sulfide (CdS) and cadmium telluride (CdTe) are the leading materials for the development of cost effective and reliable photovoltaic systems. The two important properties of these materials are its nearness to the ideal band gap for photovoltaic conversion efficiency and they have high optical absorption coefficients. Usually thin film solar cells are made by hetero-junction of p-type CdTe with n-type CdS partner window layer. In this article, we have deposited CdTe films on mica substrates using thermal evaporation technique and CdTe/CdS junction were developed by depositing a thin layer of CdS on to the CdTe substrate from chemical bath deposition method. The device was characterized by current voltage and photocurrent spectroscopy technique prior to the deposition of the transparent conducting layer. The devices were annealed in air at different temperatures and found that the device annealed at 673?K had better photovoltaic parameters. The efficiency of a typical device under 50?mW?cm^?2 illumination was estimated as 4%.     

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.