ZnTe Layers Research Articles

Growth and characterisation of n- and p-type ZnTe thin films for applications in electronic devices

The growth of n- and p-type ZnTe thin films have been achieved intrinsically by potentiostatic electrodeposition method using a 2-electrode system. Cyclic voltammogram have been used to obtain range of growth voltages required to form stoichiometric thin films of ZnTe. The ZnTe thin films have been electrodeposited (ED) on glass/fluorine-doped tin oxide (FTO) conducting substrates in aqueous solutions of ZnSO4·7H2O and TeO2. The films have been characterised for their structural, electrical, morphological, compositional and optical properties by using X-ray diffraction (XRD), Raman spectroscopy, Photoelectrochemical (PEC) cell measurements, DC conductivity measurements, Scanning electron microscopy (SEM), Atomic force microscopy (AFM), energy-dispersive X-ray analysis (EDX) and Optical absorption techniques. The XRD results reveal that the electroplated films are polycrystalline and have hexagonal crystal structure with the preferred orientation along (002) plane. UV–Visible spectrophotometer has been used for the bandgap determination of as-deposited and heat-treated ZnTe layers. The bandgap of the heat-treated ZnTe films are in the range (1.90–2.60) eV depending on the deposition potential. PEC cell measurements show that the ED-ZnTe films have both n- and p-type electrical conductivity. The DC conductivity measurements revealed that the average resistivity of n-ZnTe and p-ZnTe layers of equal thickness is of the order of 104 Ωcm; the magnitude of the electrical resistivity of p-ZnTe is almost five times greater than that of the n-ZnTe layer. Using the n- and p-type ZnTe layers, p-n homo-junction diodes with device structure of glass/FTO/n-ZnTe/p-ZnTe/Au were fabricated. The fabricated diodes showed rectification factor of 102, reverse saturation current of ∼10.0 nA and potential barrier height greater than 0.77 eV indicating electronic device quality of these layers.

Density of dislocations in CdHgTe heteroepitaxial structures on GaAs(013) and Si(013) substrates

Epitaxial layers of Cd x Hg1–x Te (MCT) on GaAs(013) and Si(013) substrates were grown by molecular beam epitaxy. The introduction of ZnTe and CdTe intermediate layers into the structures made it possible to retain the orientation close to that of the substrate in MCT epitaxial layers despite the large mismatch between the lattice parameters. The structures were investigated using X-ray diffraction and transmission electron microscopy. The dislocation families predominantly removing the mismatch between the lattice parameters were found. Transmission electron microscopy revealed Γ-shaped misfit dislocations (MDs), which facilitated the annihilation of threading dislocations. The angles of rotation of the lattice due to the formation of networks of misfit dislocations were measured. It was shown that the density of threading dislocations in the active region of photodiodes is primarily determined by the network of misfit dislocations formed in the MCT/CdTe heterojunction. A decrease in the density of threading dislocations in the MCT film was achieved by cyclic annealing under conditions of the maximally facilitated nonconservative motion of dislocations. The dislocation density was determined from the etch pits.

Growth of ZnTe epilayers on r- and n-plane sapphire substrates

ZnTe epilayers were grown on r-plane () and n-plane () sapphire substrates by molecular beam epitaxy. The ZnTe domain distribution in the layer and the influence of the substrates’ c-plane location on the orientation of the epilayer were studied by means of X-ray diffraction pole figure measurements. Computer simulation was used to analyze the diffraction patterns. The (100)-plane ZnTe was formed on the r-plane substrate whereas two different {111}-plane mixed with the (511) domain were formed on n-plane substrate, respectively. The orientation of ZnTe layers on r-plane sapphire substrates could not change even by varying the buffer layer growth condition. From these results, it was revealed that the relationship of the sapphire c-plane and the (111) ZnTe strongly affected the orientation of the film. ZnTe layers grown on n-plane sapphire substrates, on the other hand, exhibited different relationships and (111) of ZnTe was not aligned to the sapphire c-plane.

Cr impurity-induced electronic states in ZnTe(110) surface

The impurity states of Cr atoms, which substituted Zn sites in the topmost layer of a p-type ZnTe(110) surface, were investigated by scanning tunneling microscopy/spectroscopy (STM/STS) and we firstly observed Cr-induced impurity states in the energy gap region of the host ZnTe including the unoccupied states by STS. Furthermore, we compared the observed energy levels and spatial distributions of the local density of states with those in the previous theoretical study [Katayama-Yoshida et al., Phys. Status Solidi A 204, 15 (2007)] and successfully identified the impurity states as the respective spin-polarized impurity states predicted by the theoretical study.

Graded composition CdxZn1−xTe films grown by Isothermal Close Space Sublimation technique

Graded composition CdxZn1−xTe films were prepared by growing several alternate layers of CdTe and ZnTe by the Isothermal Close Space Sublimation technique. The thickness of both kinds of layers was modified along the structure to produce an increase of the average concentration of CdTe towards the surface of the films. Due to Zn/Cd inter-diffusion during the growth process the sequence of layers was converted into a relatively smooth graded composition film. According to X-ray diffraction characterization the layers grew coherently with the (100) oriented GaAs substrates although they showed a relatively high mosaicity. θ−2θ plots show very wide diffraction peaks as expected from variable composition samples. The band gap grading effect in light absorption was also verified through transmission measurements, using transparent substrates. Graded composition profiles of the thin films were confirmed by x-ray photoelectron and secondary ion mass spectroscopies. Moreover, quantitative Cd, Zn and Te profiles were obtained by the analysis of Rutherford backscattering spectra of the samples. This analysis showed a CdTe molar fraction profile ranging from approximately x=0.8 at the surface of the sample and x=0.35 at the interface with the substrate. The possibility of growing graded composition layers using a simple and cost-effective procedure was demonstrated. This could be interesting in technological applications like CdxZn1−xTe layers of variable composition in CdS/CdTe solar cells.

Investigation of p-side contact layers for II–VI compound semiconductor optical devices fabricated on InP substrates by MBE

N-doped p-type ZnTe and ZnSeTe contact layers were investigated to evaluate which is more suitable for use in II–VI compound semiconductor optical devices on InP substrates. Contact resistances (Rc) between the contact layers and several electrode materials (Pd/Pt/Au, Pd/Au, and Au) were measured by the circular transmission line model (c-TLM) method using p-n diode samples grown on InP substrates by molecular beam epitaxy (MBE). The lowest Rc (6.5×10−5Ωcm2) was obtained in the case of the ZnTe contact and Pd/Pt/Au electrode combination, which proves that the combination is suitable for obtaining low Rc. Yellow light-emitting diode devices with a ZnTe and ZnSeTe p-contact layer were fabricated by MBE to investigate the effect of different contact layers. The devices were characterized under direct current injections at room temperature. Yellow emission at around 600nm was observed for each device. Higher emission intensity and lower slope resistance were obtained for the device with the ZnTe contact layer and Pd/Pt/Au electrode compared with other devices. These device performances are ascribed to the low Rc of the ZnTe contact and Pd/Pt/Au electrode combination.

Effects of temperature in electrodeposition of ZnTe thin films

ZnTe thin films have been deposited at various temperatures about 110–160 °C onto FTO-coated glass substrates by low-cost, easy and simple non-aqueous electrodeposition method for using it in solar cell application. The optimized growth conditions are chosen for ZnTe layers by investigating the photoelectrochemical, optical, structural and morphological properties of the deposited films. Optical measurement of the p-type ZnTe films have higher transmittance in the UV–Visible region; moderate transmittance in the visible region and moderate to lower transmittance in the visible–infrared region and have higher absorbance in the middle of the visible region to near infrared region. Band gap values were estimated to vary from 2.23 to 1.8 eV for the ZnTe films deposited at different conditions. Band gap of ZnTe decreases when the films annealed at 300, 350 and 425 °C in nitrogen environment. From the XRD and SEM study the films deposited from 110 to 150 °C were elemental Te film but Zn was not deposited at this temperature. When the deposition temperature was increased to 160 °C and annealing at 425 °C, the films were found to be polycrystalline ZnTe films with (111) preferential orientation of the cubic structure and appeared dense crack-free surfaces with regular granular shaped grains without any cauliflower like appearance. After annealing and all characterization it was concluded that ZnTe thin film can be deposited at 160 °C for 40 min at −0.75 V.

Crystal orientation mechanism of ZnTe epilayers formed on different orientations of sapphire substrates by molecular beam epitaxy

The electrooptic effect in ZnTe has recently attracted research attention, and various device structures using ZnTe have been explored. For application to practical terahertz wave detector devices based on ZnTe thin films, sapphire substrates are preferred because they enable the optical path alignment to be simplified. ZnTe/sapphire heterostructures were focused upon, and ZnTe epilayers were prepared on highly mismatched sapphire substrates by molecular beam epitaxy. Epitaxial relationships between the ZnTe thin films and the sapphire substrates with their various orientations were investigated using an X-ray diffraction pole figure method. (0001) c-plane, (1-102) r-plane, (1-100) m-plane, and (11-20) a-plane oriented sapphire substrates were used in this study. The epitaxial relationship between ZnTe and c-plane sapphire was found to be (111) ZnTe//(0001) sapphire with an in-plane orientation relationship of [−211] ZnTe//[1-100] sapphire. It was found that the (211)-plane ZnTe layer was grown on the m-plane of the sapphire substrates, and the (100)-plane ZnTe layer was grown on the r-plane sapphire. When the sapphire substrates were inclined from the c-plane towards the m-axis direction, the orientation of the ZnTe thin films was then tilted from the (111)-plane to the (211)-plane. The c-plane of the sapphire substrates governs the formation of the (111) ZnTe domain and the ZnTe epilayer orientation. These crystallographic features were also related to the atom arrangements of ZnTe and sapphire.

OPTIMIZATION OF OPTICAL CHARACTERISTICS OF CdMgTe/Cu(In,Ga)Se2 TANDEM SOLAR CELL

Thin-film tandem structure with CuInSe2 and Cd1-xMgx Te solar cells and thin conducting transparent ZnTe layer between them are investigated. Cd1-xMgx Te (x < 0,1) in the top solar cell is used to expand the band gap of CdTe. From the measured transmission spectra of the top solar cell on the portion of the spectrum with low transmittance when periodic changes in transmittance (oscillations) significantly reduced, the absorption curves α(λ) and band gap of Cd1-xMgx Te (1,54 and 1,6 eV) have been found. Since the reflection coefficient and the oscillations in the measured transmission spectra significantly attenuated in the case of optical contact of Cd1-xMgx Te with ZnTe, transmission of the top solar cell of the tandem can be calculated with a small error ignoring the oscillations. It has been shown that the short-circuit currents of the top and bottom solar cells become equal with the band gap of Cd1‑x Mgx Te ~ 1,65 еВ.

Investigation of multilayered quantum dot-sensitized solar cells with different Zn chalcogenide passivation layers

In the case of cadmium sulfide (CdS) and cadmium selenide (CdSe)-based quantum dot-sensitized solar cells (QDSSCs), the addition of a zinc sulfide (ZnS) passivation layer improves the solar cell performance. In this study, multilayered QDSSCs were fabricated using CdS and CdSe quantum dots prepared by successive ionic layer adsorption and reaction (SILAR) method. The optimized QDSSCs were used to study the passivation effect of zinc chalcogenide layers: ZnS, zinc selenide (ZnSe), and zinc telluride (ZnTe). The best performing solar cell prepared from four SILAR cycles of CdS followed by six SILAR cycles of CdSe were used for subsequent deposition of Zn chalcogenide layers. It was observed that capping with ZnSe or ZnTe layer on the multilayered Cd chalcogenide QDs did not improve the solar cell performance. Only the addition of ZnS layer contributed to the better performance of the solar cell. The efficiency obtained in the optimized multilayered CdS/CdSe QDSSC with ZnS layer was 1.37 %, while the QDSSC with ZnSe or ZnTe capping showed lower performance. The behavior of the solar cells is explained with electrochemical impedance spectroscopy study.

Intermediate Band to Conduction Band Optical Absorption in ZnTeO

ZnTe doped with high concentrations of oxygen has been proposed in previous works as an intermediate band (IB) material for photovoltaic applications. The existence of extra optical transitions related to the presence of an IB has already been demonstrated in this material and it has been possible to measure the absorption coefficient of the transitions from the valence band (VB) to the IB. In this study, we present the first measurement of the absorption coefficient associated with transitions from the IB to the conduction band (CB) in ZnTeO. The samples used are 4-μm-thick ZnTe layers with or without O in a concentration ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , which have been grown on semiinsulating GaAs substrates by molecular beam epitaxy (MBE). The IB-CB absorption coefficient peaks for photon energies ~0.4 eV. It is extracted from reflectance and transmittance spectra measured using Fourier transform infrared (FTIR) spectroscopy. Under typical FTIR measurement conditions (low light intensity, broadband spectrum), the absorption coefficient in IB-to-CB transitions reaches 700 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . This is much weaker than the one observed for VB-IB absorption. This result is consistent with the fact that the IB is expected to be nearly empty of electrons under equilibrium conditions in ZnTe(O).

Design of a high efficiency ultrathin CdS/CdTe solar cell using back surface field and backside distributed Bragg reflector.

A high efficiency CdS/CdTe solar cell was designed with a reduced CdTe absorber-layer thickness and a distributed Bragg reflector (DBR) as an optical reflector and a ZnTe layer as back surface field (BSF) layer. Simulation results showed that with combination of DBR and BSF layers and 0.3 µm thick CdTe, the conversion efficiency was increased about 3.2% as compared with a reference cell (with 4 µm thick CdTe layer). It was also shown that the efficiency can be increased up to 6.02% by using a long carrier lifetime in the absorber layer. Under global AM 1.5G conditions, the proposed cell structure had an open-circuit voltage of 1.062 V, a short-circuit current density of 24.64 mA/cm², and a fill factor of 81.3%, corresponding to a total area conversion efficiency of 21.02%.

Deposition of polycrystalline Cd1‐xZnx Te films on ZnTe/graphite and graphite substrates by close‐spaced sublimation

AbstractThe effects of a ZnTe layer on the deposition of a Cd1‐xZnxTe (CZT) layer in the initial stage of the close‐spaced sublimation (CSS) deposition were investigated. The deposition rate was almost constant in the initial stage of the CdTe deposition on the ZnTe/graphite substrates. However, the deposition rate within 1 minute was lower than that after 1 minute in the CdTe deposition on graphite substrates. This result suggests that nucleation of CdTe directly deposited on graphite substrate is difficult when compared to that with a ZnTe layer. Furthermore, the effects of CdCl2 and ZnTe additions to the CdTe sources in the CSS deposition were also investigated. Both the grain size and the intensity of donor‐acceptor pair (DAP) emission in photoluminescence (PL) spectra were decreased by the effect of CdCl2 addition. Zn content in CZT films was controlled by the ZnTe ratio in the CdTe/ZnTe powder sources. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Design of Very thin CdTe Solar Cells with High Efficiency

After several years without change, First Solar announced in 2011 an efficiency record for CdS/CdTe solar cells set at 17.3%, as confirmed by NREL [1]. This is a significant milestone that demonstrates the ongoing potential of CdTe thin-film solar cells. Then, a question arises on the new limits likely to be achieved for the efficiency of this kind of solar cells. The author [2] had previously predicted that with current technology, the practical limit was around 17.5%, but below 18%. In this work, a systematic study of the expected efficiency evolution as a function of fabrication parameters is made with the help of a new solar cell simulator (wxAMPS) [3]. We start from known parameters, so that the simulated results coincide with a previous reported record efficiency (16.5% under AM1.5 G radiation). Then, we show that as the thickness of the CdTe layer is reduced the efficiency will be increased mostly due to a reduced series resistance. It will be shown that there is an optimum thickness for which the highest efficiency becomes 17.7%, confirming the preceding author's prediction. In addition, it will be shown that the introduction of a ZnTe layer at the back of the solar cell (i. e. with a new device structure: CdS/CdTe/ZnTe) will help reaching even higher efficiencies, for very thin solar cells, above 19.5%. These results should be useful for guiding experimentalists to make more efficient CdTe solar cells.

HgCdTe structures for dual-band photodetectors operating in the 3–5 and 8–12 µm spectral ranges

A bilayer cadmium-mercury-tellurium (CMT) heterostructure was designed consisting of photosensitive layers of compositions xCdTe = 0.29–0.32 and xCdTe = 0.220–0.230, sensitive in the spectral ranges of 3–5 and 8–12 µm, a barrier layer between them, and wide-band variable-gap layers on the heterojunction and the surface grown on a GaAs substrate with ZnTe and CdTe buffer layers. The molecular beam epitaxial (MBE) growth of the heteroepitaxial structure (HES) was controlled by real time ellipsometry. After the growth, the composition distribution throughout the thickness was measured by reflection spectra with layer-by-layer chemical etching. There is good agreement between the results of composition measurements using ellipsometry and reflection spectra. P-type conductivity of bilayer MBE CMT HESs was obtained after thermal annealing at 220–240 °C in an inert gas (helium) for 24 h. The concentration of holes in the photosensitive layers is (4–10) ·1015 cm−3 and (8–20) · 1015 cm−3 at 78 K.

Determination of Critical Thickness for Epitaxial ZnTe Layers Grown by Molecular Beam Epitaxy on (211)B and (100) GaSb Substrates

Cross-section electron micrographs, cathodoluminescence images, and confocal photoluminescence (cPL) images have been acquired for ZnTe layers deposited to various thicknesses on GaSb substrates with (211)B and (100) orientations. The critical thickness of ZnTe on GaSb is predicted to range between 115 nm and 329 nm, depending on the theoretical approach chosen. For ZnTe layers grown on (211)B GaSb with thickness exceeding 150 nm, dark spots and lines are present in all images. We associate these with dislocations generated at the ZnTe/GaSb interface. The discrepancy between this thickness value and a critical thickness value (350 nm to 375 nm) obtained for the (211)B orientation in a previous study is related to the distinction between the onset of misfit dislocations and the onset of significant plastic deformation. The former requires a direct imaging technique, as strain-related measurements such as x-ray diffraction do not have the resolution to detect the effects of small numbers of dislocations. For ZnTe layers on (100) GaSb, x-ray diffraction measurements indicate an abrupt change characteristic of dislocation multiplication at a thickness value in the range from 250 nm to 275 nm. High-resolution electron micrographs of the ZnTe/GaSb interface indicate that deoxidation using atomic hydrogen produces GaSb surfaces suitable for ZnTe epitaxy. cPL images of a 1.2-μm-thick lattice-matched ZnTe0.99Se 0.01 layer grown on a 150-nm-thick ZnTe buffer layer on a (211)B GaSb substrate yield a threading dislocation density of ∼7 × 104 cm−2.

Development of ZnTe layers using an electrochemical technique for applications in thin-film solar cells

Zinc telluride layers were grown by an electrochemical technique using acidic and aqueous solutions containing ZnCl2 and TeO2. The layers were deposited on glass/fluorine-doped tin oxide substrates using a two-electrode system. The deposited ZnTe layers were characterized using x-ray diffraction, x-ray fluorescence, Raman spectroscopy, optical absorption, photoelectrochemical cell measurements, scanning electron microscopy and 3D-atomic force microscopy (3D-AFM) for the structural, optical, electrical and morphological properties. The electrodeposited ZnTe layers grow as columns, and have cubic crystal structure, the band gap in the range of (2.00–2.20) eV and p-type electrical conductivity. Surface morphology studies using SEM indicate the presence of two types of material clusters varying in size up to ∼125 nm. 3D-AFM studies with higher magnification show that the material tends to grow as columns with different sizes leaving gaps in between in some areas.

Effect of Annealing Temperature on Electrical Properties of ZnTe Layers Grown by Thermal Evaporation

The electrical properties of evaporated ZnTe films were investigated with emphasis on the effects of an annealing temperature from 600oC to 800oC by RTA technique. Crystallinity, carrier concentration, sheet resistance, and mobility are shown to be dependent on the annealing temperature. The highest carrier concentration and lowest sheet resistance are 7.9×1015cm-3and 9300 Ω/□ at an annealing temperature of 700oC, respectively. The mobility was found to vary from 20 to 50 cm2V-1S-1. The ZnTe thin films using thermal evaporation can find applications in solar cell or light emitting diodes

Effects of annealing treatment upon electrical and photoluminescence properties of phosphorus-doped ZnMgTe epilayers grown by metalorganic vapor phase epitaxy

Post-annealing treatment in nitrogen gas flow has been carried out for P-doped Zn1−xMgxTe layer grown under a Te-rich or Te-poor condition by metalorganic vapor phase epitaxy. The electrical and photoluminescence properties of P-doped Zn1−xMgxTe layers are altered by annealing treatment. The post-annealing is very effective in obtaining p-type conductive Zn1−xMgxTe for the layer grown under a Te-poor condition. On the other hand, Zn1−xMgxTe layer is characterized by a high compensation ratio and DAP luminescence for the layer grown under a Te-rich condition, even after annealing treatment. Similar tendencies are also found in P-doped ZnTe layers.

Surface morphologies and photoluminescence properties of undoped and P-doped ZnTe layers grown by metalorganic vapor phase epitaxy

ZnTe homoepitaxial growth has been carried out by varying source transport rate or substrate temperature to cover from mass transport limited region to the kinetically controlled region. The correlation among the growth rate, surface morphology, roughness and photoluminescence (PL) property of undoped ZnTe on (100) substrate has been clarified as a function of source transport rate. The effect of substrate temperature upon the surface roughness and PL property of P-doped ZnTe layer has been investigated using (111)Te substrate and compared with the case of (100) substrate. The growth conditions close to the mass to surface-kinetics transition region provide an optimum growth not only for obtaining undoped ZnTe layer with smooth surface but also for achieving a better PL property. The surface roughness of P-doped ZnTe layer grown on (111)Te or (100) substrate is also improved under above conditions. The surface roughness of the layer is not so sensitive to substrate temperature in the case of (111)Te substrate, which is different significantly from the case of (100) substrate.