Band Gap Of ZnTe Research Articles

Carrier-mediated ferromagnetism in Mn(II)-doped ZnTe thin films and their optical properties: A first-principles study

We have investigated the opto-electronic and magnetic properties of Mn(II)-doped ZnTe thin films by employing Density Functional Theory (DFT). In the absence of additional carriers, spin-up Mn-ta levels are fully occupied, leading to a super-exchange mechanism. The effect of additional p-type doping on the ferromagnetism is studied by considering the addition of hole carriers, Zn vacancies, and C co-doping, and we found that in all cases, coupling of hole carriers with the spin-up Mn-states causes the stability of the ferromagnetism in the Mn(II)-doped ZnTe thin films. We discovered through the optical study that Mn(II) doping at the Zn site widens ZnTe's bandgap and produces spin-forbidden d-d transition peaks on the low energy side of the bandgap. The p-type defects in Mn(II)-doped films produce absorption peaks in the infrared region and improve the absorption efficiency. In addition, the optical bandgap and spin-forbidden d-d transition of Mn(II) to different modes of spin-spin interactions were correlated, and we found a red shift of d-d intra-band transition peaks as well as an optical bandgap in the FM-coupled Mn(II) ions system and a blue shift in the AFM coupled ions system, supporting the experimental observations.

Effect of thermal annealing on physical properties of Cu-doped ZnTe thin films: Functionality as interface layer

Achieving a high quality interface is of great importance in CdTe based solar cells, as it appreciably inhibits interfacial recombination and consequently improves the photovoltaic performance. The low carrier density of CdTe absorber contributes to formation of back contact Schottky barrier which acts as diode setup in opposition to the CdS/CdTe junction. To overcome this problem, the Copper doped Zinc Telluride (ZnTe:Cu) material is estimated to be a good nominee for such interface layer due to its high chemical immovability and appropriate band gap alignment with CdTe absorber. Hence, the present work demonstrates thermal annealing evolution to 200 nm thin resistive heating grown ZnTe:Cu 1 % films to find their feasibility as back/rear contact material in the CdTe based superstrate solar cell devices. The structural analysis revealed that as deposited and annealed ZnTe:Cu 1 % films have (103) preferred reflection with cubic phase and crystallite size is enhanced from 22 nm to 42 nm with annealing. The optical transmittance is varied with annealing and the direct optical energy band gap is found within the range of 2.10–2.74 eV. The pristine and annealed films showed ohmic behavior and the AFM images displayed hill like topographies. Smaller to larger size spherical shaped grains and non-uniform cylindrical shaped grains morphology is obtained in FESEM images whereas the EDS patterns confirmed successful deposition of ZnTe:Cu 1 % thin films. The obtained results signify that the 300 °C annealed ZnTe:Cu 1 % films might be considered as interface layer material for the fabrication of CdTe based solar cell devices. • Annealing induced properties of ZnTe:Cu interface layer are investigated for solar cells. • The ZnTe:Cu films showed crystal growth along (103) preferred orientation. • Optical band gap of ZnTe:Cu films is found in 2.10–2.74 eV range with annealing. • AFM images showed hill-like topographies and EDS patterns confirmed films deposition. • SEM images revealed spherical grains and I-V curves ensured ohmic nature.

Computational insights into electronic, magnetic and optical properties of Mn(II)-doped ZnTe with and without vacancy defects

First principles calculations have been performed to investigate the optoelectronic and magnetic properties of Mn(II); ZnTe DMS with and without native defects using first principle calculations. Our results find that anti-ferromagnetic coupling dominates in the pure Mn(II)-doped ZnTe system due to super-exchange (SE) mechanism. The impact of p-type and n-type defects on magnetic coupling in Mn(II)-doped ZnTe were discussed and we found that p-type defect such as Zn vacancy defects play important role in stabilization of FM state due to the formation of bound magnetic polaron (BMP) while n-type defect such as Te vacancy defect does not affect the ground state of Mn(II); ZnTe. The magnetic coupling of Mn(II) spins in the absence and presence of Zn vacancy were explained on the basis of phenomenological band structure model. The optical absorption spectra of pure ZnTe and Mn(II); ZnTe with and without vacancies have been investigated and we found that single Mn(II)-doping in the lattice of ZnTe generates an absorption band at energy around 2.02 eV, which is assigned to intra-band d-d transitions of Mn(II) dopant. The band at energy around 0.44 eV in the absorption spectrum of Mn(II)-doped ZnTe with Zn-vacancy may be due to the acceptor states produced by Zn vacancy at Fermi level and band around 1.6 eV in the spectrum of Te vacancy defect system are related to the donor states produced by Te vacancy defect. Moreover, the correlation of magnetic coupling with intra-band d-d transition bands and fundamental bandgaps of ZnTe were also discussed and it was found that intra-band transition peak of Mn(II) ions and optical band gap of ZnTe are blue-shift in AFM coupled Mn(II) ions and are red-shift in FM coupled ions system. Our present findings highlight the worthwhile half-metallic properties of Mn(II)-doped ZnTe, which can be obtained via Zn vacancy defect engineering; this can find broad applications for the fabrication of optoelectronic and spintronic devices.

Terahertz generation from ZnTe optically pumped above and below the bandgap.

We report on the generation of THz waves through optical rectification in ZnTe of femtosecond laser pulses whose photon energy is tuned from below to above the ZnTe bandgap energy. The THz signal exhibits a pronounced peak at the bandgap energy, at THz frequencies for which losses in ZnTe remain small. This peak is likely due to the resonance of the ZnTe nonlinear susceptibility in the vicinity of the bandgap.

Development of highly luminescent and color-tunable Zn1-xPmxTe QDs synthesized using 4,4-bis-N-1-naphyl-N-phenylaminobiphenyl assisted microwave technique for solar cell applications

A new family based on the synthesis of highly luminescent and color-tunable Zn1-xPmxTe QDs using 4,4-bis-N-1-naphyl-N-phenylaminobiphenyl assisted microwave technique. This new series of Zn1-xPmxTe QDs showed remarkable optoelectronic properties. The promethium ions exhibited a modulating of the bandgap of ZnTe Qs, where the bandgap reduced from 3.06 eV to 2.8 eV and the particle size changed from 1.4 nm to 4.3 nm. The promethium ions created trap levels within the ZnTe bandgap which leads to highly luminescent emission spectra exceeded the undoped ZnTe QD by 5 times. The inclusion of promethium ions in ZnTe QDs resulted in color tunable QDs in the visible region. The quantum yield of the Zn1-xPmxTe QDs increased 7%–82%, according to the amounts of promethium ions incorporate ZnTe QDs. Therefore. The developed Zn1-xPmxTe QDs are promising candidate for photovoltaic devices.

Hot exciton relaxation in coupled ultra-thin CdTe/ZnTe quantum well structures

The photoluminescence (PL) and PL excitation (PLE) spectra of CdTe/ZnTe asymmetric double quantum well (QW) structures are studied on a series of samples containing two CdTe layers with nominal thicknesses of 2 and 4 monolayers (ML) in the ZnTe matrix. The samples differ in the thickness of the ZnTe spacer between CdTe QWs which is 45, 65 and 75 ML. It has been found that at above-barrier excitation the PL from a shallow QW at sufficiently weak excitation intensities is determined by recombination of hot excitons. It is shown that under these conditions, when PL is excited by lasers with different wavelengths, the ratio of the PL intensities from shallow and deep QWs decreases exponentially with an increase of the initial kinetic energy of hot excitons. It is found that energy relaxation of hot excitons with LO phonon emission determine the shape of the PLE spectrum of shallow QW in the range of exciton kinetic energies up to more than 20 LO phonons above ZnTe bandgap. We have shown that the results obtained are well described by the model of charge and energy transfer between QWs.

Review on optical, structural and electrical properties of ZnTe thin films: effect of deposition techniques, annealing and doping

Zinc Telluride (ZnTe) has become a very fascinating research material for the scientists over past many years. This p type semiconducting material has a wide band gap, which makes it useful in many optoelectronic applications such as solar cells, light emitting diodes, laser screens etc. With the desire for eco-friendly alternative energy resources, it excites to investigate potential semiconducting materials at nano scale for solar cell applications. Due to direct, wide and controllable optical band gap of ZnTe with easy doping makes it a potential material for photoelectrochemical applications. In the present work, an attempt has been made to compile the work done by various researchers on optical properties (band gap, refractive index and absorption/transmission spectra, etc.), structural properties (crystallite/grain size, lattice constant and Zn:Te, etc.) and electrical properties (resistivity, activation energy, carrier concentration and hall mobility, etc.) of ZnTe thin films with main emphasis on the effect of deposition techniques, doping and annealing on these properties. Most of the thin films reported in literature shown polycrystalline cubic structure. It is found that annealed ZnTe thin films show enhanced optical, structural and electrical properties at different annealing temperatures. Doping methods used by various researchers have been discussed in detail and results show good doping impact on these properties as well.

Defect properties of Sn- and Ge-doped ZnTe: suitability for intermediate-band solar cells

We investigate the electronic structure and defect properties of Sn- and Ge- doped ZnTe by first-principles calculations within the DFT+GW formalism. We find that and introduce isolated energy levels deep in the band gap of ZnTe, derived from Sn- and Ge- states, respectively. Moreover, the incorporation of Sn and Ge on the Zn site is favored in p-type ZnTe, in both Zn-rich and Te-rich environments. The optical absorption spectra obtained by solving the Bethe–Salpeter equation reveals that sub-bandgap absorptance is greatly enhanced due to the formation of the intermediate band. Our results suggest that Sn- and Ge-doped ZnTe would be a suitable material for the development of intermediate-band solar cells, which have the potential to achieve efficiencies beyond the single-junction limit.

Time-Resolved Spectroscopy of ZnTe Photocathodes for Solar Fuel Production

The negative conduction band potential and small bandgap of ZnTe make the material a promising photoelectrode for solar fuels production, photocatalyst, and solar cell component. However, the factors controlling the underlying efficiencies of the light-driven processes on ZnTe are not well understood. Here we report a combined spectroelectrochemical and transient absorption (TA) spectroscopic investigation of ZnTe photoelectrodes for CO2 reduction. In the visible region TA spectra are dominated by a broad positive photoinduced absorption at 540 nm following initial charge carrier relaxation (<540 nm). The 540 nm spectral feature is shown to be related to deeply trapped photoelectrons with charge carrier recombination occurring via a trapping–detrapping model on the microsecond time scale. Significantly these deeply trapped electrons are insensitive to the presence of electron acceptors and to the applied potential of the ZnTe electrode. Trapping at such states is proposed to be a significant factor limiti...

High power conversion efficiency of intermediate band photovoltaic solar cell based on Cr-doped ZnTe

We report on a high-performance intermediate band solar cell (IBSC) based on Cr-doped ZnTe (ZnTe:Cr) fabricated using a pulsed laser deposition (PLD) method. Chromium (Cr) was uniformly distributed in the ZnTe:Cr thin film with an atomic concentration of about 3.5%, and the ZnTe:Cr thin film showed p-type electrical conductivity. The ZnTe:Cr thin film had higher absorption coefficients than those of undoped ZnTe films in the photon energy range below band gap of ZnTe (2.2eV). The enhanced absorption coefficients of the ZnTe:Cr thin film were attributed to the photoionization energy between Cr2+ and Cr+ (Cr2+⇌ Cr+), which acted as the IB to absorb photons below the bandgap of ZnTe (2.2eV). Illumination with an AM 1.5G solar spectrum on the ZnTe:Cr IBSC generated a large short circuit current of 21.18mA/cm2, an open circuit voltage of 0.48V, and a fill factor of 0.58, yielding a power conversion efficiency (PCE) of 5.9%, the highest reported PCE in an IBSC based on impurity-doped ZnTe.

Investigation on the effect of Cu-doping to ZnTe layers by low-cost electrochemical approach

We have reported the electrochemical synthesis zinc telluride (ZnTe) and copper-doped ZnTe (Cu–ZnTe) thin films by electrodeposition technique from non-aqueous electrolyte. Voltammetric investigations under dark and illumination on the deposition were performed to optimize the growth potentials for ZnTe and Cu–ZnTe. The effect of Cu-doping on the structural, morphological, compositional and optical properties has been studied by means of X-ray diffraction, scanning electron microscopy, atomic absorption spectroscopy and UV–Vis spectroscopy. The growth of semiconducting phase was revealed by photovoltammetry. The direction of photocurrent remains unchanged upon the doping of CuCl2 in ZnTe revealed no change in majority charge carriers. The values of the energy band gap of ZnTe and Cu–ZnTe, 10−4 and 2 × 10−4 Cu contents were respectively determined as 2.26, 2.23 and 2.21 eV with sharp absorption edge and enhancement in absorption for the sample grown with 1 × 10−4 M CuCl2. The structure and surface morphology of ZnTe is sensitive to the choice of the deposition potential. The un-doped film deposited at −0.75 and −0.95 V yielded a mixed cubic as well as hexagonal phase of ZnTe along with metallic tellurium. The surface morphology changes from dendritic to globular. Presence of Cu2+ ions in the ZnTe bath not only helped in preventing the formation of tellurium dendrites, it also yielded single hexagonal phase of ZnTe. In case of Cu-doped layer, the surface morphology changes from flake like structure to globular cluster upon changing the growth potential. The un-doped ZnTe samples were Te-rich, whereas Zn/Te ratio for Cu–ZnTe with 10−4 M CuCl2 was found to be nearly unity. The direction of thermoemf was indicative of a p-type response. The increased electrical conductivity in Cu–ZnTe film is associated to increase in the hole density as Cu plays a role of acceptor in ZnTe.

Band alignment of type I at (100)ZnTe/PbSe interface

A junction of lattice-matched cubic semiconductors ZnTe and PbSe results in a band alignment of type I so that the narrow band gap of PbSe is completely within the wider band gap of ZnTe. The valence band offset of 0.27 eV was found, representing a minor barrier during injection of holes from PbSe into ZnTe. Simple linear extrapolation of the valence band edge results in a smaller calculated band offset, but a more elaborate square root approximation was used instead, which accounts for parabolic bands. PbSe was electrodeposited at room temperature with and without Cd2+ ions in the electrolyte. Although Cd adsorbs at the surface, the presence of Cd in the electrolyte does not influence the band offset.

The study of electronic structure and absorption coefficient of ZnTe:O alloys: A GGA+U method

In this study the electronic structures and absorption coefficient for highly mismatched ZnTe1−xOx alloys have been investigated on the basis of the GGA+U approximation. It is found that an isolated intermediate band (E−) is formed within the band gap of ZnTe by the incorporation of low content of O (x<0.0625) into ZnTe. In the range of the low O content, the variation of E− and E+ is in agreement with the theoretical prediction by the well-known band anti-crossing model. As the O content x exceeds 0.0625, the E+ energy position decreases and intermediate band (E−) cannot exist in isolation states, which is consistent with the experimental results but in contrast to the band anti-crossing model. The chemical bonding pictures show that the intermediate band states are mainly caused by the antibonding states that are formed through the hybridization of a localized O 2s state and some localized Zn 3d4s4p states. The calculated absorption coefficient exhibits an obvious enhancement within the ZnTe:O bandgap, which indicates the formation of intermediate states can effectively make use of low energy photons to improve the conversion efficiency of final solar cell based on ZnTe:O materials.

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.

Creating intermediate bands in ZnTe via co-alloying approach

We propose an effective co-alloying approach to creating an intermediate band (IB) in bulk ZnTe. First-principles calculations show that a donor–acceptor co-alloying scheme can produce an IB within the band gap of ZnTe and that the position of the IB can be tuned by choosing appropriate donor–acceptor pairs. We find that the position of the IB is governed by the atomic d-orbital energies of dopants, and also by the charge transfer between donor and acceptor atoms and their subsequent intra- and inter-Coulomb interactions. Therefore, the location of the IB can be manipulated by selecting donors and acceptors with desirable d-orbital energies and electronegativities.

Structural and Optical Characterization of Zinc Telluride Thin Films

Group II-VI compounds have been investigated largely in last two decades due to their interesting optoelectronic properties. ZnTe, a member of this family, possesses a bandgap around 2.26eV. This material is now a day investigated in thin film form due to its potential towards various viable applications. In this paper, the authors report their investigations on the preparation of ZnTe thin films using vacuum evaporation technique and their structural and optical characterizations. The structural characterization, carried out using an X-ray diffraction (XRD) technique shows that ZnTe used in present case possesses a cubic structure. Using the same data, the micro strain and dislocation density were evaluated and found to be around 1.465×10-3lines-m2and 1.639×1015lines/m2respecctively. The optical characterization carried out in UV-VIS-NIR region reveals the fact that band gap of ZnTe is around 2.2eV in present case. In addition to this, it was observed that the value of bandgap decreases as the thickness of films increases. The direct transitions of the carries are involved in ZnTe. Using the data of UV-VIS-NIR spectroscopy, the transmission coefficient and extinction coefficient were also calculated for ZnTe thin films. Besides, the variation of extinction coefficient with wavelength has also been discussed here.

Exciton-phonon coupling in individual ZnTe nanorods studied by resonant Raman spectroscopy

The exciton-phonon coupling in high-quality cubic phase zinc telluride (ZnTe) nanorods (NRs) is investigated by resonant micro-Raman spectroscopy near the direct bandgap of ZnTe. The scattering cross section of longitudinal optical (LO) phonon is enhanced significantly in the resonant process, where the enhancement factor of LO modes is much higher than that of the transverse optical (TO) modes, indicating a dominant Fr\"ohlich electron-phonon interaction mechanism. Up to fifth-order LO phonons are observed by resonant Raman scattering at room temperature. The Huang-Rhys factor of individual NRs---and thus the exciton-LO coupling strengths---is evaluated, showing increasing with the NR diameter. Surface optical (SO) phonon and its high-order overtones are observed between $n$LO and ($n$ \ensuremath{-} 1)LO + TO for the first time, whose positions are consistent with a dielectric continuum model. Strong acoustic phonon-exciton coupling induces a high-frequency shoulder above each $n$LO peaks with two maxima located around 14 cm${}^{\ensuremath{-}1}$ and 32 cm${}^{\ensuremath{-}1}$, which are assigned to transverse acoustic and longitudinal acoustic phonons, respectively. The resonant multiphonon scattering process involving acoustic and LO phonons is discussed based on an exciton-intermediated cascade model, where a scattering sequence of acoustic phonon followed by LO phonons is favorable. These results advance the understanding of electron-phonon coupling and exciton scattering in quasi-one-dimensional systems, especially in the scarcely documented ZnTe compound, facilitating the development and optimization of NR-based optoelectronic devices.

Preparation of nanocomposite for optical application using ZnTe nanoparticles and a zero-birefringence polymer

Surface-modified ZnTe nanoparticles were mixed in a zero-birefringence polymer matrix. Transmission electron microscopy images revealed that aggregates of ZnTe nanoparticles with a diameter of ∼20 nm were uniformly dispersed in the polymer. The transmittance of ZnTe nanocomposites rapidly decreased at wavelengths shorter than the critical wavelength corresponding to the band gap of ZnTe nanoparticles, an effect which became significant as the volume fraction of particles increased. In this way, the optical characteristic of ZnTe nanoparticles was added to the polymer. The intrinsic zero-birefringence was confirmed in the heat-drawn ZnTe nanocomposites. As the ZnTe nanocomposites were left in air, a lowering of transmittance was observed. This was due to the oxidation of Zn and the resultant deposition of Te in the ZnTe nanocomposite, as the light absorption of Te is significant. The formation of oxygen non-permeable SiO2 films onto the ZnTe nanocomposite by the sol-gel method was useful in preventing oxidation so that the decrement of transmittance decreased from 47.2% to 14.9% at 530 nm near the ZnTe band gap.

Suppression of ferromagnetism due to hole doping in Zn1−xCrxTe grown by molecular-beam epitaxy

Electric and magnetic properties were investigated on p-type Zn1−xCrxTe doped with nitrogen (N) as an acceptor. Thin films of p-Zn1−xCrxTe(x≦0.09) were grown by molecular-beam epitaxy with the supply of N2 gas excited by rf plasma. With the increase of Cr composition x at an almost fixed N concentration of the order of 1020cm−3, the temperature dependence of resistivity changed from metallic behavior to an insulating one, accompanied with a significant decrease of the hole concentration. The magnetization measurements revealed that ferromagnetic behaviors observed in undoped Zn1−xCrxTe were suppressed due to the nitrogen doping; with N concentrations of the order of 1020cm−3, hysteresis loops in the magnetization curve disappeared, the magnitude of magnetization decreased, and the ferromagnetic transition were not observed down to 2 K according to the Arrott plot analysis. These experimental findings are discussed on the basis of the ferromagnetic double exchange interaction which is considered to work on the Cr 3d impurity level formed in the band gap of ZnTe.

Effects of wavelength upon photoluminescence properties of ZnTe layers grown by photo-assisted MOVPE

Effects of illumination wavelength upon the growth rate and photoluminescence properties of undoped and aluminum (Al)-doped ZnTe layers have been investigated under a Te-rich growth condition by xenon-lamp-assisted metalorganic vapor-phase epitaxy. Even illumination with wavelengths longer than 600 nm, which hardly enhance the growth rate, deteriorates the quality of undoped and Al-doped ZnTe epilayers. Illumination with wavelengths, which enhance the growth rate, i.e., illumination by photons having energies higher than the band gap of ZnTe, ensures the appearance of strong free exciton emission for undoped layers. This also brings about a strong donor–acceptor pair luminescence for Al-doped layers.