Thin CdTe Layers Research Articles

Numerical study of PbTe-Pb hybrid nanowires for engineering Majorana zero modes

Epitaxial semiconductor-superconductor (SM-SC) hybrid nanowires are potential candidates for implementing Majorana qubits. Recent experimental and theoretical works show that charged impurities in SM remain a major problem in all existing hybrid nanowires, in which the SM is either InAs or InSb while the SC is mainly Al. Here, we theoretically validate the recently proposed PbTe-Pb hybrid nanowire as a potential candidate for Majorana devices. By studying the electrostatic and electronic properties of PbTe nanowires, we demonstrate that the huge dielectric constant of PbTe endows itself a high tolerance of charged impurity, which is a potential advantage over InAs and InSb nanowires. Moreover, we find that the effective axial Land\'e $g$ factor and Rashba spin-orbit coupling strength of PbTe nanowires are comparable to those of InAs nanowires. The conceivable merits of using Pb as the SC are (i) Pb has a larger superconducting gap, higher critical temperature, and higher parallel critical magnetic field than those of Al; (ii) a superconducting gap comparable with those of InAs-Al and InSb-Al can be induced in PbTe-Pb even by a weak coupling between Pb and PbTe, which simultaneously relieves the adverse renormalization and induced disorder effects on SM from SC; and (iii) Pb film can be grown on PbTe with a thin buffer CdTe layer in between, solving the lattice mismatch problem as an important source of disorder. In the presence of a parallel magnetic field, we show that the typical BdG energy spectrum and tunneling spectroscopy of PbTe-Pb resemble those of InAs and InSb based hybrid nanowires exposed to a tilting magnetic field, as a result of the highly anisotropic Land\'e $g$ factors of PbTe nanowires. The calculated topological phase diagrams of PbTe-Pb indicate that the multivalley character of PbTe makes it easier than InAs and InSb to access topological superconducting phases. Our results could facilitate the experimental realization of PbTe-Pb hybrid nanowires and inspire further theoretical works.

Low-temperature growth of CdTe thin films as passivation layers for IR and THz functional elements

Low-temperature modes of growing thin (dCdTe = 80–360 nm) CdTe layers by the “hot-wall” method with a high degree of adhesion to CdHgTe epitaxial layers were implemented. Growth parameters (source and substrate temperatures 650 K and 373 K, respectively, deposition time 60 min), at which the film is optimal when used as a protective passivation coating for the functional elements of the IR and THz photoelectronics were selected. The study of morphology, chemical composition and electrical properties of CdTe thin films depending on temperature and growth time were carried out. It is shown that the CdTe semiconductor film on the CdHgTe surface grows in columns that aggregate with increasing deposition time. The study of capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics of CdTe/p-Si structures demonstrated the controllability of the space charge region in silicon by the voltage applied to CdTe, which indicates good dielectric properties of CdTe with low surface density at the CdTe/p-Si boundary (4.5 × 1010 сm−2) and small leakage currents through the dielectric.

Effects of CdTe selenization on the electrical properties of the absorber for the fabrication of CdSexTe1-x/CdTe based solar cells

CdTe thin film solar cells are the most successful thin film photovoltaic devices in terms of production yield. In the last few years, by changing the original solar cell structure, the efficiency was increased from around 17 % up to 22 %. In particular two main innovations have been introduced: a CdSexTe1-x layer, that narrows the band gap of the absorber near the junction, and a large band gap buffer layer (i.e. MgxZn1-xO). These allow to improve the absorption in the long and in the short wavelength region, respectively. Generally, CdSexTe1-x is formed either by the deposition of a CdSe layer at the junction subsequently mixed with CdTe or by directly depositing a CdSexTe1-x compound, we have instead introduced a different method where CdSexTe1-x is formed by treating a thin CdTe layer at high temperature in selenium atmosphere. The finished device has been fabricated by substituting CdS with SnO2 and efficiencies exceeding 14 %, with current densities higher than 26 mA/cm2 , are obtained by a low substrate temperature process. In this work we analyze and compare our older standard FTO/SnO2/CdS/CdTe with the novel FTO/SnO2/CdSexTe1-x/CdTe devices by capacitance voltage, drive level capacitance profiling, and admittance spectroscopy. The results highlight the effect of the selenization process in terms of carrier concentration, carrier profile, and formation of deep and shallow defects. The overall performance of the completed devices is also presented and compared.

Thin CdTe Layers Deposited by a Chamberless Inline Process using MOCVD, Simulation and Experiment

The deposition of thin Cadmium Telluride (CdTe) layers was performed by a chamberless metalorganic chemical vapour deposition process, and trends in growth rates were compared with computational fluid dynamics numerical modelling. Dimethylcadmium and diisopropyltelluride were used as the reactants, released from a recently developed coating head orientated above the glass substrate (of area 15 × 15 cm2). Depositions were performed in static mode and dynamic mode (i.e., over a moving substrate). The deposited CdTe film weights were compared against the calculated theoretical value of the molar supply of the precursors, in order to estimate material utilisation. The numerical simulation gave insight into the effect that the exhaust’s restricted flow orifice configuration had on the deposition uniformity observed in the static experiments. It was shown that > 59% of material utilisation could be achieved under favourable deposition conditions. The activation energy determined from the Arrhenius plot of growth rate was ~ 60 kJ/mol and was in good agreement with previously reported CdTe growth using metalorganic chemical vapour deposition (MOCVD). Process requirements for using a chamberless environment for the inline deposition of compound semiconductor layers were presented.

Optical properties of CdTe/ZnTe structures with thin CdTe layers

The optical properties of series of CdTe/ZnTe heterostructures with two built-in planar CdTe inclusions of thickness 4.0 and 1.5 monolayers are investigated. The temperature behavior of the exciton luminescence intensity at above-barrier and under-barrier excitation depends on the ZnTe barrier thickness which varies from 15 to 55 monolayers. It occurs that the shape of exciton luminescence bands changes strongly under the increasing optical excitation. The excitation luminescence spectra are studied in the temperature range 5 – 80 K.

Exciton Light Emission of CdTe/ZnTe Heterostructures with Double Ultrathin Narrow-Gap Layers

The exciton luminescence spectra of CdTe/ZnTe heterostructures containing two thin CdTe layers separated by barriers of different thickness were studied. The complex temperature dependence of the intensity of luminescence from these layers in its underbarrier and abovebarrier excitation led to the conclusions about the effect of the thickness of the barriers on the energy transfer between the CdTe layers and gave the possibility to observe, at specific temperatures, the implementation of a resonant-type excitation. The dependence of the shape of the exciton emission contours on the excitation level gives information about the real structure of CdTe layers with the monolayer thickness of 1.5 and 4.0 monolayers.

Экситонное излучение гетероструктур CdTe/ZnTe с двойными ультратонкими узкозонными слоями

AbstractThe exciton luminescence spectra of CdTe/ZnTe heterostructures containing two thin CdTe layers separated by barriers of different thickness were studied. The complex temperature dependence of the intensity of luminescence from these layers in its underbarrier and abovebarrier excitation led to the conclusions about the effect of the thickness of the barriers on the energy transfer between the CdTe layers and gave the possibility to observe, at specific temperatures, the implementation of a resonant-type excitation. The dependence of the shape of the exciton emission contours on the excitation level gives information about the real structure of CdTe layers with the monolayer thickness of 1.5 and 4.0 monolayers.

CdHgTe heterostructures for new-generation IR photodetectors operating at elevated temperatures

The parameters of multilayer CdxHg1–xTe heterostructures for photodetectors operating at wavelengths of up to 5 μm, grown by molecular-beam epitaxy (MBE) on silicon substrates, are studied. The passivating properties of thin CdTe layers on the surface of these structures are analyzed by measuring the C–V characteristics. The temperature dependences of the minority carrier lifetime in the photoabsorption layer after growth and thermal annealing are investigated. Samples of p+–n-type photodiodes are fabricated by the implantation of arsenic ions into n-type layers, doped with In to a concentration of (1–5) × 1015 cm–3. The temperature dependences of the reverse currents are measured at several bias voltages; these currents turn out to be almost two orders of magnitude lower than those for n+–p-type diodes.

HgCdTe p+‐n structures grown by MBE on Si (013) substrates for high operating temperature SWIR detectors

AbstractElectrophysical properties of multilayered heteroepitaxial structures of Hg1‐xCdxTe with x=0.3‐0.4 grown by molecular beam epitaxy on silicon substrates are presented. The passivating effect of thin CdTe layers grown on top of the structures in single process is demonstrated. Comparison between experimental and theoretical temperature dependencies of reverse currents in n‐on‐p and p‐on‐n diodes fabricated by boron and arsenic ion implantation in vacancy‐doped p‐type and In‐doped n‐type Hg1‐xCdxTe films, respectively, are presented. The influence of p‐n junction position in double‐layer heterostructure on temperature dependencies of reverse currents is examined. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

MBE-Grown CdTe Layers on GaAs with In-assisted Thermal Deoxidation

Molecular beam epitaxy (MBE) growth of thin (∼2 μm) CdTe layers characterized by high crystal quality and low defect density on lattice mismatched substrates, such as GaAs and Si, has thus far been difficult to achieve. In this work, we report the effects of in situ thermal deoxidation under In and As4 overpressure prior to the CdTe growth on epiready GaAs(211)B wafers, aiming to enhance CdTe crystal quality. Thermally deoxidized GaAs samples were analyzed using in situ reflection high energy electron diffraction, along with ex situ x-ray photo-electron spectroscopy (XPS) and atomic force microscopy. MBE-grown CdTe layers were characterized using x-ray diffraction (XRD) and Everson-type wet chemical defect decoration etching. We found that In-assisted desorption allowed for easier surface preparation and resulted in a smoother surface compared to As-assisted surface preparation. By applying In-assisted thermal deoxidation to GaAs substrates prior to the CdTe growth, we have obtained single crystal CdTe films with a CdTe(422) XRD rocking curve with a full-width half-maximum value of 130.8 arc-s and etch pit density of 4 × 106 cm−2 for 2.54 μm thickness. We confirmed, by XPS analysis, no In contamination on the thermally deoxidized surface.

A possible way to reduce absorber layer thickness in thin film CdTe solar cells

Reducing the thin film module production cost is a strong focus in the current thin film solar cell research. One possible way to reduce the cost is minimising material consumption by reducing film thickness. Ultra thin CdTe layer will also help to design p–i–n structure solar cells. The standard CdTe layer thickness is normally in the range of 4–5μm. Experimental trials were carried out in order to reduce the CdTe layer thickness below 1μm using close spaced sublimation technique. Simply reducing the CdTe layer thickness induces pinholes in the CdTe layer which results in poor device performance. The film thickness reduction below 1.5μm was achieved by employing a double layer structure. In the process of making CdTe double layer the first layer was deposited at higher substrate temperature (~520°C) and the second layer was deposited at low substrate temperature (~350°C). The maximum cell efficiency of 12.5% was obtained for ~3μm CdTe layer and comparable device performance was obtained for the 1.5μm CdTe layer (11.2%). Further reducing the film thickness below 1.5μm reduces the device performance. Solar cell efficiencies for ~0.8μm and 0.5μm CdTe layers were 9.5% and 5.2% respectively. The Capacitance–Voltage measurements show that the CdTe layer is fully depleted when the thickness is reduced below 1μm. Further, the possible cause for the decrease in efficiency with decrease in CdTe film thickness is discussed in this article.

MOCVD of Cd(1−x)Zn(x)S/CdTe PV cells using an ultra-thin absorber layer

Ultra-thin Cd(1−x)Zn(x)S/CdTe devices were produced by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD) with varying CdTe absorber thicknesses ranging from 1.0 to 0.2μm and compared to baseline cells with total CdTe thickness of 2.25μm. The ultra-thin CdTe layers (≤1μm) were intentionally doped with As to induce p-type conductivity in the absorber. Cell performance reduced with CdTe thickness, with the magnitude of photo-current generation loss becoming more significant for the very thin CdTe layers. The decline in cell performance was lower than the optically limited performance relating to a decrease in shunt resistance, Rsh, especially for the thinnest cells due to areas of incomplete CdTe coverage and large presence of pin-holes leading to micro-shorts. Incorporation of Zn into the CdS window layer improved cell performance for all devices except when 0.2μm thick CdTe was used. This improvement was markedly in the blue region owing to enhanced optical transparency of the window layer. External quantum efficiency (EQE) measurements showed a red-shift of the window layer absorption edge due to leaching out of Zn during the CdCl2 treatment. Reduction of the CdCl2 deposition time was demonstrated to recover the blue response of the ultra-thin cells.

The role of copper in bifacial CdTe based solar cells

We present an innovative back contact for CdTe solar cell by the application of a transparent conducting oxide, typically ITO, as a back electrical contact on CdTe/CdS photovoltaic devices that acts as a free-Cu stable back contact and at the same time allows to realize bifacial CdTe solar cells, which can be illuminated from either or both sides. The controlled insertion of a very limited amount of copper into the ITO back contact permits to have reproducible devices with high efficiencies still keeping the bifacial configuration. Thin CdTe layer solar cells with ITO back contact have been realized with efficiencies exceeding 10%, the reduced thickness of CdTe allows to have a better performance on the back-side illumination and reduces the amount of CdTe material.

Deep Levels Induced by CdTe/ZnTe Quantum Dots

The electrical properties of the CdTe/ZnTe quantum dot system have been analyzed to identify deep-level defects related with the presence of quantum dots. The capacitance‐voltage (C‐V ) and deep level transient spectroscopy measurements were used to investigate the samples. A reference ZnTe sample (without dots) was also studied for comparison. Both samples were grown by molecular beam epitaxy technique on the n-type GaAs substrate. The quantum dots were formed by a Zn-induced reorganization of a thin CdTe layer. The presence of quantum dot formation was confirmed by micro-photoluminescence measurements. The deep level transient spectra for both samples are complex. In order to characterize individual contributions to the deep level transient spectra the latter have been simulated by separated Gaussian components [1]. The results of the deep level transient spectroscopy measurements yield the conclusion that the same defects are present in both materials but there is an increased concentration of the defects in the quantum dot structures. No deep level associated directly with the quantum dot confinement has been identified.

Features of the light current-voltage characteristics of bifacial solar cells based on thin CdTe layers

It is shown that the features of the illuminated current-voltage characteristics of bifacial solar cells glass/SnO2:F/CdS/CdTe/Cu/ITO with a thin base layer are associated with the photovoltaic effect at the back contact. An equivalent circuit of the device structure under study, which takes into account the existence of two illuminated diodes—a frontal diode (main separating barrier) and a diode at a back contact—is suggested.

Dependence of efficiency of thin-film CdS/CdTe solar cell on parameters of absorber layer and barrier structure

Dependences of the open-circuit voltage, short-circuit current, fill factor, and efficiency of a CdS/CdTe solar cell on the resistivity and thickness of the p-CdTe absorber layer, the noncompensated acceptor concentration N a– N d, and carrier lifetime τ in CdTe, are investigated, and optimization of these parameters in order to improve the solar cell efficiency is performed. It has been shown that the observed low efficiency of CdS/CdTe solar cells is caused by the too short electron lifetime in the range of 10 − 10 –10 − 9 s and too thin (3–5 µm) CdTe layer currently used for fabrication of CdTe/CdS solar cells. To achieve an efficiency of 28–30%, the resistivity and thickness of the CdTe absorber layer, the noncompensated acceptor concentration, and carrier lifetime should be ∼ 0.1 Ω·cm, ≥ 20–30 µm, ≥ 10 16 cm − 3 , and ≥ 10 − 6 s, respectively.

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

Bifacial configurations for CdTe solar cells

We present a different back contact for CdTe solar cell by the application of only a transparent conducting oxide (TCO), typically ITO, as a back electrical contact on all-PVD CdTe/CdS photovoltaic devices that acts as a free-Cu stable back contact and at the same time allows to realize bifacial CdTe solar cells, which can be illuminated from either or both sides. Also devices with thin CdTe layers (from ∼2 μm down to 1 μm) have been prepared to improve the conversion efficiency on the back side illumination, which is limited by the collection of carriers far away from the junction and to reduce the amount of material in the CdTe device. Reproducible solar cells exceeding 10% efficiency on the front side illumination and exceeding 3% on the back side illumination are reported.

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.

A Si/CdTe Compton Camera for gamma-ray lens experiment

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and γ-ray detection. However, a considerable amount of charge loss in these detectors results in a reduced energy resolution. We have achieved a significant improvement in the spectral properties by forming the Schottky junction on the Te side of the CdTe wafer. With the further reduction of leakage current by an adoption of guard ring structure, we have demonstrated a CdTe pixel detector with high energy resolution and full charge collection capabilty. The detector has a pixel size of a few mm and a thickness of 0.5 – 1 mm. We apply this high resolution detector to a new silicon and CdTe Compton Camera which features high angular resolution. We also describe a concept of the stack detector which consists of many thin CdTe layers and provides sufficient efficiency for hard X-rays and gamma-rays up to several hundred keV maintaining good energy resolution. A narrow-FOV Compton telescope can be realized by installing a Si/CdTe Compton Camera inside the deep well of an active shield. This configuration is very suitable as focal plane detector for future focusing gamma-ray missions.