Deposited CdTe Films Research Articles

Strain relaxation of CdTe films growing on lattice-mismatched substrates

We have deposited CdTe films by laser-assisted epitaxy approach and investigated the influence of substrate and film thickness on the film properties. Grown on Si(001), GaAs(001), and quartz substrates; the CdTe films exhibit preferential orientation along the cubic CdTe(111) direction. When the films are thin (<500 nm), a blueshift of the band gap and splitting of valence bands were observed. These results are attributed to the existence of residual strains induced by mismatch of the film lattice constant with that of the substrate, and by their difference in thermal expansion coefficients. The bulk band-gap energy of 1.5 eV was achieved on the surface of thick CdTe films grown on Si(001) substrate, indicating that strain was almost completely relaxed in this case. Our results demonstrate that by a proper selection of substrate and film thickness it is possible to grow film semiconductors with band gap approaching those of bulk crystals.

Atypical grain growth for (2 1 1) CdTe films deposited on surface reconstructed (1 0 0) SrTiO 3 substrates

The (1 0 0) SrTiO 3 substrate has emerged as the oxide substrate of choice for the deposition of a wide variety of materials. The substrate's unavoidable miscut leads to a step-terrace morphology when heated to high temperatures. This morphological transition is accompanied by an atomic scale repositioning of the uppermost terrace atoms, the nature of which is strongly dependent on the substrate temperature and ambient atmosphere used. Here, we report the deposition of CdTe films on the as-received and reconstructed surfaces of (1 0 0) SrTiO 3. The as-received substrate gives rise to a [1 1 1] CdTe film with four equally distributed in-plane grain orientations. The surface reconstruction, on the other hand, gives rise to an unprecedented reorientation of the film's grain structure. For this case, a [2 1 1] CdTe film emerges having twelve unevenly distributed in-plane orientations. We attribute the film's grain structure to an atomic scale surface reconstruction, with the anisotropic distribution of grain-types arising from a preferential formation due to the step edges.

Stimulation of photovoltaic properties of obliquely deposited CdTe films by doping with indium

The problem of doping of the CdTe films with anomalous photovoltaic property with the Ag impurity to obtain photoelectrets without external polarization field has already been analyzed in [1], where anomalous photovoltage was first observed in the region of impurity photoexcitation. In the present work, the anomalous photovoltaic effect (APV) in thin polycrystalline CdTe layers doped with In or Cl impurity is investigated to increase the working power of films as sources of photoelectromotive force and to elucidate a role of photosensitive centers in generation of anomalous photovoltage. Here, in particular, it is demonstrated that, unlike Ag, thermal field-induced migration of In and Cl ions and Cd j V − cadmium

Influence of Deposition Parameters on the Morphology of CdTe Films and on the Performance of CdTe Solar Cells

AbstractFor industrial CdTe solar cell production, thin, dense and pinhole-free films have to be deposited at high rates. Using a close-to-industrial close space sublimation (CSS) process, we deposited CdTe films at different temperatures and rates. The morphology of these films was analyzed by scanning electron microscopy (SEM) and x-ray diffraction (XRD). The measurements show the size, size distribution and orientation of the grains to strongly depend on deposition temperature and rate. A suitable combination of these parameters yields large grains of about 3 microns diameter even for deposition temperatures below 750 K and at high deposition rates of 3 microns per minute. The interplay of CdTe layer morphology and solar cell performance is elucidated employing a TCO-CdS-CdTe-Mo film sequence for complete solar cells.

Determination of the interdiffusion coefficient for the CdS/CdTe heteroestructure by AES sputter depth profiling

Abstract Double heterostructures were prepared by depositing CdTe films on stainless steel (ss) substrates by the close spaced sublimation (CSS) method. The CdTe films were treated with a saturated solution of CdCl 2 in methanol, dried in air and annealed at 400 °C. CdS layer of 0.2 μm was deposited on the CdTe film by the chemical bath method. The CdS/CdTe system was treated with saturated solution of CdCl 2 in methanol and annealed for 30 min in air at different temperatures from 300 to 400 °C. The samples were characterized by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The main effect of the temperature is to change the surface morphology of the CdS film from polycrystalline to an amorphous texture. By AES depth profiling the diffusion processes of the constituent element of the film was studied.

CdTe/CdS Solar cells on flexible molybdenum substrates

Development of CdTe/CdS solar cells on flexible metallic substrates is highly interesting due to the light weight and flexible nature of the solar modules. We have deposited CdTe films onto flexible molybdenum substrates using close-spaced sublimation technique and the CdTe/CdS junction was developed by depositing a thin layer of CdS onto the CdTe substrate from a chemical bath. The devices were characterized by Current–voltage (I–V) and photocurrent spectroscopy techniques. Prior to the deposition of the transparent conducting layer, the devices were annealed in air at different temperatures and found that the devices annealed at 400°C have better photovoltaic parameters. The efficiency of a typical device under 60 mW cm−2 illumination was estimated as 3.5%.

Structural characterization of CdTe thin films developed on metallic substrates by close spaced sublimation

Cadmium telluride (CdTe) is one of the most investigated semiconductors for photovoltaic applications in the last four decades and is one of the most promising photovoltaic materials due to low-cost, high-efficiency, nearly optimum band gap (1.5 eV) for efficient photo conversion, high optical absorption coefficient (>104 cm−1) at band edge and the variety of deposition techniques available for its deposition as thin films. The close spaced sublimation (CSS) is one of the most promising methods to develop good quality CdTe films on large area substrates. There are various reports of the CdTe/CdS solar cells with efficiencies exceeding 16%, all prepared by CSS [1, 2]. In conventional solar cells, the CdTe is developed on the CdS window layer deposited on conducting glass substrates. But devices on glass substrates have the disadvantage of weight, inability to withstand high processing temperatures and are prone to damages. On the other hand devices on flexible metallic substrates can overcome all the above mentioned disadvantages of the glass based devices. Hence the development and characterization of CdTe on thin flexible metallic substrates are interesting. There are a number of reports regarding the characterization of CdTe thin films developed on CdS substrates by CSS, but practically no reports regarding the characterization of CdTe films developed on metallic substrates by CSS technique. We have deposited CdTe films on stainless steel by CSS at various source and substrate temperatures and the films were characterized using SEM and XRD. In this paper we are reporting the influence of the temperatures of substrate and source on the various structural parameters such as lattice parameter, grain size and stress. The investigations regarding the development of an interlayer between CdTe and the substrate are in progress and will be published in subsequent papers. The CSS system consists of a graphite reactor enclosed in a quartz tube. The tube was evacuated and

Effect of in-situ CdCl2 treatmenton spray deposited CdTe films: Photoluminescence study

In-situ ${CdCl}_2$ treatment for spray deposited CdTe films has been done by spraying a 0.01 M ${CdCl}_2$ solution on CdS substrates. Scanning electron micrographs show the improvement in grain growth with the ${CdCl}_2$ spray. Photoluminescence spectra for films with the in-situ treatment show a shift in the bandgap by 0.06 eV and a reduction in the defect band intensity as compared to those for the films without the treatment. The shift is due to conversion of CdTe into CdTeS by S diffusion from the CdS substrate. Our studies show that an in-situ ${CdCl}_2$ treatment is effective in improving the CdTe thin film properties.

Electrodeposited Films of CdxHg1?xTe with Bandgap Energy in the Range 1.2-1.5 eV

Electrodeposition of CdxHg1—xTe films with a thickness of about 1 μm and a bandgap energy in the range 1.2–1.5 eV (0.87 < x < 1) is investigated. Deposition is performed on stainless steel substrates at 80–90 °C from a bath containing CdSO4, TeO2 and HgCl2. Film composition and bandgap energy are evaluated, respectively, from the variation of the deposition current with time and from the spectral response of photocurrent in Schottky devices formed on these films. In addition, other characterization techniques such as XRD, SEM and EDAX are also used. Films grow with a graded composition unless HgCl2 is added periodically into the solution during deposition. Films consisting of several layers with different thickness and composition can also be deposited. Films consist of grains with an average size of about 100 nm and are less oriented than grains in identically deposited CdTe films.

Study of in situ CdCl 2 treatment on CSS deposited CdTe films and CdS/CdTe solar cells

Effect of in situ CdCl 2 treatment on the morphological, structural and electrical properties of CdTe films as well as on solar cell characteristics of CdS/CdTe junction has been investigated. XRD measurements show that the presence of CdCl 2 vapours induces 〈111〉 oriented growth in the CdTe films. CdCl 2 concentration required for this oriented growth is found to be directly proportional to the substrate temperature. SEM measurements show enhanced grain growth in the presence of CdCl 2. Spectral response of the CdCl 2 treated CdS/CdTe solar cells shows an enhanced CdS diffusion in to the CdTe, which results in an improved spectral response in UV range and a consequent reduction in the interface states density. A drastic reduction in the deep levels due to the CdCl 2 treatment, as seen in the photo-capacitance studies, has results in CdS/CdTe solar cells having efficiency >8%.

Improvement of CdTe solar cell performance with discharge control during film deposition by magnetron sputtering

We have used a combination of rf and dc excitation to control the plasma density during planar magnetron sputter deposition of CdTe films and CdTe-based solar cells. While adding dc current we have adjusted the rf power to maintain a constant deposition rate. We find that the lower plasma density obtained with negative dc currents yields films with a more highly faceted surface, more columnar growth structure, and stronger photoluminescence. The solar cells prepared with rf sputtered CdS and with CdTe deposited at reduced rf power and −70 mA of dc current produce substantially higher performance. The results indicate advantages to reducing the ion bombardment energy and flux to the growing film by the addition of dc control during rf sputtering from highly insulating semiconductor targets.

The impact of MOCVD growth ambient on carrier transport, defects, and performance of CdTe/CdS heterojunction solar cells

CdTe solar cells were fabricated by depositing CdTe films on CdS/SnO2/glass substrates in various metalorganic chemical vapor deposition growth ambient with varying Te/Cd mole ratio in the range of 0.02 to 15. The short-circuit current density (Jsc) showed a minimum at a Te/Cd ratio of 0.1 and increased on both sides of this minimum. The open-circuit voltage (Voc) was found to be the highest for the Te-rich growth ambient (Te/Cd∼6)and was appreciably lower (600 mV as opposed to 720 mV) for the stoichiometric and the Cd-rich growth conditions. This pattern resulted in highest cell efficiency (12%) on Te-rich CdTe films. Auger electron spectroscopy revealed a high degree of atomic interdiffusion at the CdS/CdTe interface when the CdTe films were grown in the Te-rich conditions. It was found that the current transport in the cells grown in the Cd-rich ambient was controlled by the tunneling/interface recombination mechanism, but the depletion region recombination became dominant in the Te-rich cells. These observations suggest that the enhanced interdiffusion reduces interface states due to stress reduction or to the gradual transition from CdS to CdTe. The hypothesis of reduced defect density in the CdTe cells grown in the Te-rich conditions is further supported by the high effective lifetime, measured by time-resolved photoluminescence, and the reduced sensitivity of quantum efficiency to forward/light bias.

The Wet Etching of CdZnTe Substrates for OMVPE Growth

ABSTRACTThe etching of CdZnTe substrates by bromine-methanol solutions was studied by chemi-mechanical polishing. Over a bromine concentration of 0.5 to 3.5 volume percent, the etch rates varied linearly with bromine concentration, were independent of substrate orientation, and, within experimental uncertainties, were identical to those for the etching of CdTe. The etched samples were used as substrates for the organometallic vapor phase epitaxy (OMVPE) growth of CdTe in an impinging jet reactor. The bromine concentration used in etching did not affect the growth rate or the morphology of the deposited CdTe films, but a higher defect density was found for the film grown on a substrate with the orange-peel structure.

Effects of CdCl2 on the growth of CdTe on CdS films for solar cells by isothermal close-spaced vapour transport

CdS/CdTe solar cells were made by depositing CdTe films by an isothermal close-spaced vapour transport method on sintered CdS/glass substrates. The influence of amounts of CdCl2 ranging from 0 wt.% to 8 wt.% in the CdTe source on the solar cells performance was studied. Increasing the CdCl2 content enhances the CdTe grain size but degrades the spectral response and increases the reverse saturation current. An optimal CdCl2 concentration of 1 wt.% was found for a growth temperature of 620 degrees C.

Electrochemical deposition and characterization of CdTe polycrystalline thin films

CdTe thin films have been prepared by potentiostatic electrodeposition from acidic solutions containing CdSO 4 and TeO 2, or CdCl 2 and TeO 2. The films were characterized by X-ray diffraction and scanning electron microscopy. The influence of the deposition potential and bath temperature on the presence of tellurium and crystallite size was studied. The effect of annealing treatment on the increase in grain size of the electrochemically deposited CdTe films has been investigated.

Effect of preferred orientation on photovoltage of CdTe thin films

Photovoltaic property of angularly deposited CdTe films is studied by the X-ray diffraction. The variation of photovoltage of the film deposited at different angles and different substrate temperatures are co-related with the preferential orientation of the crystallites. The magnetude of the photo-e.m.f. developed is dependent on the relative number of planes having {220} and {311} orientation and corresponding optical absorption.

Ion-assisted sputter deposition of CdTe layers

Epitaxial, single-phase CdTe layers have been deposited on a variety of single-crystal substrates, including freshly cleaved KBr and NaCl and polished BaF2, by ion-assisted, planar, rf magnetron sputtering. The substrate temperature and bias were used as independent, operator-controlled process parameters to greatly enhance the degree of orientation in the films and (or) modify the crystallographic form of the deposited layer. Thus, depending primarily on the substrate temperature and bias, the deposited CdTe films exhibited one of several forms: (i) single-phase, highly oriented cubic, (ii) single-phase, highly oriented hexagonal, (iii) two-phase, highly oriented, hexagonal plus cubic or hexagonal plus tetragonal, and (iv) single-phase, epitaxial tetragonal. Compositional analysis revealed no impurities. However, films of all crystallographic phases contained a large density of planar defects such as twins, stacking faults, and low-angle grain boundaries.

Resistivity, carrier concentration, and carrier mobility of electrochemically deposited CdTe films

The electrical type, resistivity, and donor or acceptor concentration of CdTe films deposited electrochemically at various potentials were measured. The carrier mobilities of the films were determined from these results. The deposition potential dependence of the mobility was small and the deposition potential dependence of the resistivity was mainly controlled by the deposition potential dependence of the donor or acceptor concentration. The carrier mobilities were very small compared with those in single crystals due to the scattering of the carriers at grain boundaries.

Evidence of VLS growth in CdTe film preparation by NH4Cl closed‐tube CVD

AbstractBy thermodynamical calculations of the vapour phase composition expected to occur in the closed — tube CVD of CdTe films on CdTe substrates, it is shown that a VLS growth mechanism should be active in the growth process when use is made of the thermal decomposition of NH4Cl for producing the proper amount of HCl as a transport agent. Responsible of this VLS mechanism is the liquid tellurium, which is always present when the NH4Cl concentration reaches values sufficiently high for the epitaxial deposition of CdTe films.

Effects of heat treatment on the composition and semiconductivity of electrochemically deposited CdTe films

Effects of heat treatments on crystalline diameter, composition, and semiconductivity of CdTe films deposited electrochemically at −0.35 V (vs Ag/AgCl) from an acidic solution containing 1-M CdSO4 and 1-mM TeO2 were studied. As-grown films contained free Te in addition to CdTe. The intensity of x-ray diffraction peaks due to CdTe became stronger by the heat treatment at higher temperatures but x-ray diffraction peak due to metallic Te disappeared at the films annealed above 350 °C in a He atmosphere. The intensity of Auger peaks due to Cd increased and that due to Te decreased by increasing the annealing temperature. The composition of the films annealed above 350 °C was close to that of pure CdTe. The similar effects were observed at the films annealed at 350 °C for various heat treatment periods in a He atmosphere. The change of semiconductivity of the films from p to n type was observed at the films annealed at enough high temperatures and for enough long time. The disappearance of free Te from the films was explained in terms of the temperature dependence of vapor pressure of Te2 from CdTe and metallic Te.