Te-rich Conditions Research Articles

Self-compensation in phosphorus-doped CdTe

We investigate the self-compensation mechanism in phosphorus-doped CdTe. The formation energies, charge transition levels, and defects states of several P-related point defects susceptible to cause self-compensation are addressed by first-principles calculations. Moreover, we assess the in uence of the spin-orbit coupling and supercell-size effects on the stability of AX centers donors, which are believed to be responsible for most of the self-compensation. We report an improved result for the lowest-energy configuration of the P interstitial (P$_\text{i}$) and find that the self-compensation mechanism is not due to the formation of AX centers. Under Te-rich growth conditions, (P$_\text{i}$) exhibits a formation energy lower than the substitutional acceptor (P$_\text{Te}$) when the Fermi level is near the valence band, acting as compensating donor. While, for Cd-rich growth conditions, our results suggest that p-type doping is limited by the formation of (P$_\text{Te}$-V$_\text{Te}$) complexes.

Effect of Stoichiometry on the Lifetime and Doping Concentration of Polycrystalline CdTe

The effect of gas phase Cd/Te deposition ratio on the minority carrier lifetime and doping concentration of polycrystalline cadmium telluride (CdTe) was investigated. CdTe thin films were deposited by the elemental vapor transport (EVT) with various Cd/Te gas phase ratios. The films were fabricated into solar cells, and characterized by current–voltage ( JV ) and spectral response measurements. The minority carrier lifetime was measured using time-resolved photoluminescence measurements. Capacitance–voltage ( CV ) measurements indicated a change in doping concentration as a function of gas phase Cd/Te ratio. Films deposited under Te-rich deposition conditions exhibited superior electrical characteristics. Minority carrier lifetimes up to 7 ns and doping concentrations on the order of 1015 cm–3 were measured for CdCl2 heat-treated Te-rich deposited films. The addition of controlled amounts of Cu increased the doping concentration up to 1016 cm–3, however lowered the carrier lifetimes to less than 1 ns. EVT deposited CdTe solar cells exhibited $V_{{\rm{OC}}}$ of 800 mV without Cu and up to 850 mV with Cu doping.

First-Principles Calculation of Defect Formation and Positron Annihilation States in Bi<sub>2</sub>Te<sub>3</sub>

Defect formation energy in Bi2Te3 thermoelectric material was calculated using a first principles approach based on the Density Functional Theory (DFT). For vacancy-type defect, the Te1 vacancy (VTe1) is the most stable defect with low formation energy in both Bi-rich and Te-rich conditions, which indicates that the Te1 vacancies have higher probability to be formed. For antisite defects, the formation energy of BiTe1 is much lower than that of BiTe2 in Bi-rich condition, while in Te-rich condition it is beneficial for TeBi with lower formation energy. Positron wave function distribution and positron lifetimes of different annihilation states in Bi2Te3 were also calculated using the atomic superposition (ATSUP) method. The positron bulk lifetime in Bi2Te3 is about 231 ps, and for the neutral vacancy-type defects without relaxation, the positron lifetimes of VBi, VTe1 and VTe2 are 275 ps, 295 ps and 269 ps, respectively.

Improvement of the THz response of Zn1−xMnxTe bulk crystals grown by a temperature gradient solution method

As a type of AII1−xMnxBVI alloy, Zn1−xMnxTe ingots with a diameter of 30 mm are grown as large single crystals by a temperature gradient solvent method under Te-rich conditions. The orange-red Zn1−xMnxTe crystals are cut and processed into size-appropriate wafers for fundamental studies, as well as for THz spectroscopy and magnetization response analyses. Mn segregation associated with the growth conditions is identified in the as-grown crystals, and an enriched higher concentration of Mn is observed inside the Te inclusion. To evaluate the effect of Mn on the Zn1−xMnxTe crystal, the site occupation is calculated via an ab initio study, and is further confirmed by electrical and optical property measurements. Mn tends to substitute Zn to form MnZn in Zn1−xMnxTe, which results in a low density of free charge carriers, through which the THz detection sensitivity is enhanced by 15–25% compared to the intrinsic ZnTe. Moreover, evident paramagnetic magnetization behavior is observed at variable temperatures due to the random distribution of an isolated Mn (Mnioct,Te6) spin with S = 5/2. We note that further optimization of the THz performance can be achieved by optimizing the growth process and tailoring the Mn content.

Compensation processes in high-resistivity CdZnTe crystals doped with In/Al

The high resistivity performances for aluminum/indium doped Cd0.9Zn0.1Te crystals (CZT:Al/CZT:In), grown via the modified vertical Bridgman method and under excess Cd/Te conditions, were investigated by the relationship between Al/In dopant behaviors and defects induced by crystal growth. Subsequently, their defects compensation processes responsible for high resistivity were proposed by thermally stimulated current spectroscopy. It was revealed that the donor levels from impurity Al/In are too shallow to account for the Fermi level pinned near the mid-gap of CZT materials. Considering the growth of CZT:Al crystal under Cd-rich condition, the doubly ionized Cd interstitial (Cdi2+) as a deep donor was formed with an activation energy of 0.554eV. Correspondingly, a deep donor level of doubly ionized Te antisite (TeCd2+) located at 0.704eV was observed in CZT:In crystal grown under Te-rich condition. In addition, the Fermi level dominated by deep level defects were evaluated at 0.716eV for CZT:Al and 0.740eV for CZT:In by the temperature-dependent resistivity using current-voltage measurements, which almost approach the mid-gap. We therefore suggest that these deep donor defects (TeCd2+/Cdi2+) can stabilize the Fermi level deep near the mid-gap and thus result in high resistivity.

Research into the electrical property variation of undoped CdTe and ZnTe crystals grown under Te-rich conditions

Both undoped ZnTe and CdTe bulk single crystals are grown under Te-saturated conditions from the solution and melt, respectively. To give an insight into the variation of the electrical properties, the defects structures in both tellurides are discussed. According to the actual growth velocities and the entire cooling history, tellurium interstitials (Tei) and Zinc vacancies (VZn) are proposed as the dominant grown-in defects, account for the low resistivity of p-type ZnTe. However, relatively high pulling rates and slow cooling-down processes result in tellurium anti-sites (TeCd) as the principle grown-in defects, leading to the high resistivity of light n-type CdTe. Further low-temperature (8.6K) photoluminescence spectra of both tellurides are obtained. The donor–acceptor pair (DAP) and recombination of free electron to neutral acceptor (eA) dominate the luminescence, however, with their intensities are anti-correlated. eA is superior to DAP in undoped Te-rich ZnTe, suggests a high concentration of Tei or VZn. On the contrary, DAP is the principal emission for undoped Te-rich CdTe. In addition, V-line is clearly identified in undoped Te-rich ZnTe, which possibly associated with VZn or close Frenkel pair VZn–Zni.

Structure and Morphology in Diffusion-Driven Growth of Nanowires: The Case of ZnTe

Gold-catalyzed ZnTe nanowires were grown at low temperature by molecular beam epitaxy on a ZnTe(111) B buffer layer, under different II/VI flux ratios, including with CdTe insertions. High-resolution electron microscopy and energy-dispersive X-ray spectroscopy (EDX) gave information about the crystal structure, polarity, and growth mechanisms. We observe, under stoichiometric conditions, the simultaneous presence of zinc-blende and wurtzite nanowires spread homogeneously on the same sample. Wurtzite nanowires are cylinder-shaped with a pyramidal-structured base. Zinc-blende nanowires are cone-shaped with a crater at their base. Both nanowires and substrate show a Te-ended polarity. Te-rich conditions favor zinc-blende nanowires, while Zn-rich suppress nanowire growth. Using a diffusion-driven growth model, we present a criterion for the existence of a crater or a pyramid at the base of the nanowires. The difference in nanowire morphology indicates lateral growth only for zinc-blende nanowires. The role of the direct impinging flux on the nanowire's sidewall is discussed.

Optical properties of single ZnTe nanowires grown at low temperature

Optically active gold-catalyzed ZnTe nanowires have been grown by molecular beam epitaxy, on a ZnTe(111) buffer layer, at low temperature (350 °C) under Te rich conditions, and at ultra-low density (from 1 to 5 nanowires per μm2). The crystalline structure is zinc blende as identified by transmission electron microscopy. All nanowires are tapered and the majority of them are ⟨111⟩ oriented. Low temperature micro-photoluminescence and cathodoluminescence experiments have been performed on single nanowires. We observe a narrow emission line with a blue-shift of 2 or 3 meV with respect to the exciton energy in bulk ZnTe. This shift is attributed to the strain induced by a 5 nm-thick oxide layer covering the nanowires, and this assumption is supported by a quantitative estimation of the strain in the nanowires.

Dependence of the Minority-Carrier Lifetime on the Stoichiometry of CdTe Using Time-Resolved Photoluminescence and First-Principles Calculations

CdTe is one of the most promising materials for thin-film solar cells. However, further improvement of its performance is hindered by its relatively short minority-carrier lifetime. Combining theoretical calculations and experimental measurements, we find that for both intrinsic CdTe and CdTe solar cell devices, longer minority-carrier lifetimes can be achieved under Cd-rich conditions, in contrast to the previous belief that Te-rich conditions are more beneficial. First-principles calculations suggest that the dominant recombination centers limiting the minority-carrier lifetime are the Te antisite and Te interstitial. Therefore, we propose that to optimize the solar cell performance, extrinsic p-type doping (e.g., N, P, or As substitution on Te sites) in CdTe under Cd-rich conditions should be a good approach to simultaneously increase both the minority-carrier lifetime and hole concentration.

Relationship between high resistivity and the deep level defects in CZT:In

The high resistivity performance for an indium doped Cd0.9Zn0.1Te crystal (CZT:In), grown by the modified vertical Bridgman (MVB) method and under Te-rich conditions, was investigated by the relationship between the deep donor level EDD and the Fermi level. Using the temperature-dependent resistivity measurement, the energy value of Fermi level was evaluated to be 0.740eV, which almost approaches the mid-gap of CZT:In. Further, the carrier transport behaviors were characterized by gamma-ray energy spectroscopy response with various bias voltages. The determination of the carrier mobility–lifetime products and the broadening in the photo-peak resolution implied incomplete charge collection which can be attributed to the deep level defects in the band gap. Therefore, the deep level defects were identified by thermally stimulated current (TSC) spectroscopy in the temperature range of 25–310K. Fitted by the plots of the natural logarithm of current intensity ln(IDC) versus 1/(kT), the EDD level dominated by dark current was characterized to be 0.704eV near the mid-gap. As a deep donor level of doubly ionized Te antisite (TeCd2+) existed in the CZT:In crystal grown under excess tellurium conditions, the origin of EDD level was ascribed to TeCd2+ below the conduction band. As a result, the EDD level can stabilize the Fermi level deep near the mid-gap of CZT:In and the resulting high resistivity.

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.

Thermodynamic and stoichiometric stability of the Cd-terminated CdTe (111) surface

Employing ab initio thermodynamics calculations, we elucidate for the first time the effects of growth conditions on the reconstruction phase diagram of the polar Cd-terminated CdTe (111) surface and resolve an existing experimental controversy of the low-temperature reconstruction under Te-rich conditions. We demonstrate that the Cd-vacancy $(2\ifmmode\times\else\texttimes\fi{}2)$ reconstruction is the most stable configuration in the allowed range of the Te chemical potential. Under Te-rich conditions, however, the calculations reveal a transition from a high-temperature semiconducting ($2\ifmmode\times\else\texttimes\fi{}2$) to a low-temperature metallic ($1\ifmmode\times\else\texttimes\fi{}1$)-Te${}_{n}$ reconstruction consisting of $n$ Te adlayers forming a spiral trigonal structure. We also predict the temperature and pressure dependence of the Te thickness adlayer, which has remained experimentally unresolved.

Characterization of Indium-Doped CdZnTe Crystal Grown under Te-Rich Conditions

CdZnTe (CZT) crystals with a diameter of 52 mm was grown using low pressure Bridgman method and were indium-doped under Te-rich conditions. Results showed the Te concentration was almost the same in different part of the as-grown ingot. Through IR transmission, Te inclusion with regular triangular or circular shapes could be observed, and the size of Te inclusion was around 7 µm and the concentration was 1.5  105 cm-3 for the CZT in the middle part of the ingot. I-V characteristic showed a good linear behavior with a resistivity of ~1010 Ω•cm.

First-principles density functional theory study of native point defects in Bi2Te3

We present a first-principles study of the native point defects in the thermoelectric material Bi${}_{2}$Te${}_{3}$. Calculated formation energies of defects and electronic densities of states were analyzed in detail. The most prominent native point defects considered are vacancies and antisite defects on the Bi, Te1, and Te2 sublattices of the Bi${}_{2}$Te${}_{3}$ structure. Vacancies on all three sublattices are found to have much higher formation energies than antisite defects. The most dominant antisite defects are found to be Bi${}_{\mathrm{Te}1}$ at Bi-rich conditions, and Te${}_{\mathrm{Bi}}$ at Te-rich conditions. These lead to the formation of resonant defect states at the top of the valence band and bottom of the conduction band, respectively. Hence they are expected to impact charge and energy transport in a profound way. Furthermore antisite defect pairs tend to form at nearest-neighbor distances, and lead to substantial changes in the electronic structure and hence in the thermoelectric properties of Bi${}_{2}$Te${}_{3}$.

Effect of VI/II ratio upon photoluminescence and electrical properties of phosphorus-doped ZnTe films grown by metalorganic vapor phase epitaxy

We have investigated photoluminescence and electrical properties of P−doped ZnTe layers grown by metalorganic vapor phase epitaxy as a function of VI/II ratio. Near band-edge emissions are strongly influenced by VI/II ratio. The low VI/II ratio corresponding to Zn-rich condition brings about heavy p-type doping with P as compared to Te-rich condition. The best VI/II ratio for obtaining high quality layer is one. A maximum carrier concentration of 5.4 × 10 18 cm −3 is attained even for as-grown ZnTe layer. In this sample, the carrier concentration is almost independent of the measurement temperature, indicating conduction in an impurity band formed by shallow acceptor.

Cation-site intrinsic defects in Zn-doped CdTe

The properties of the cation vacancy and the Te antisite, two dominant defects in CdTe and Cd1-xZnxTe alloys grown in Te-rich conditions, are examined using first-principles calculations. First, the structure, electronic levels, and migration paths of V-Cd and Te-Cd in CdTe are studied in detail. Additionally, we analyze the evolution of the stability and electronic properties in Cd1-xZnxTe alloys, taking into account both the role of alloying in the position of the ionization levels and its effects on the equilibrium concentration of those two defects. It is shown that the formation of cation vacancies becomes progressively more favorable as x increases, whereas Te antisites become less stable, backing the trend towards p-type conductivity in dilute Cd1-xZnxTe.

Vacuum evaporation of CdTe thin films on Ni, Mo, Ti and TiN-deposited sapphire single-crystal substrates

Cadmium telluride (CdTe) thin films were grown on sapphire c-face single-crystal substrates by vacuum evaporation. Prior to growing the CdTe thin films, conducting thin films of Ni, Mo, Ti and titanium nitride (TiN) were grown on the sapphire substrates. The crystallinities and photoluminescence (PL) properties of the CdTe thin films grown on the various conducting thin films were investigated and they were compared with those of CdTe thin films grown directly on sapphire substrates without a conducting thin film. CdTe thin films with relatively high crystallinities were obtained when Ti and TiN were used as the conducting thin film. Therefore, the PL properties of the CdTe thin films on such conducting thin films were not inferior to those of the CdTe thin film grown directly on the sapphire substrate. The effect of the Cd and Te supply ratio on the properties of the CdTe thin films was also investigated. The CdTe thin films had much higher crystallinities when they were grown in Te-rich conditions than in Cd-rich conditions.

First-principles study of gold p-type doping in Hg1－xCdxTe

Using plane-wave pseudopotential methods based on the density functional theory, we have studied the structural and the electronic properties of gold doped Hg1-xCdxTe （MCT）. The results indicate that the in situ gold impurity maintains stabile bonds with the host atoms. Moreover, the in situ gold impurity creates a shallow acceptor level behaving as an efficient p-type dopant. For all mole fractions under molecular beam epitaxy （MBE） cation-rich growth condition, for 0.75＜x≤1 under MBE Te-rich growth condition, and for 0.75≤x≤1 under liquid phase epitaxy cation rich growth condition, the self-compensating effect occurs and the gold impurity cannot behave as an efficient p-type dopant in MCT.

Intrinsic defects in CdTe and CdZnTe alloys

The Cd vacancy (VCd) and Te anti-site (TeCd) are two dominant defects in CdTe and CdZnTe alloys grown in Te-rich conditions. We examine the properties of these two intrinsic defects in Cd1-xZnxTe alloys with x<0.5 using first-principles calculations. It is shown that Cd vacancies become progressively more favourable with increasing Zn content, in contrast with Te anti-sites, which show the opposite behaviour, explaining the trend towards p-type conductivity in Cd1-xZnxTe.

Electronic properties of the Au impurity in : First-principles study

The structural and electronic properties of the gold in situ impurity (AuHg) and gold interstitial defect (Aui) in Hg0.75Cd0.25Te (MCT) have been studied by plane-wave pseudopotential methods based on the density functional theory. Valence charge density, electron localization function (ELF), density of states and formation energy of gold impurity and defect were calculated to reveal the dopant stability and doping efficiency. The results indicate that the in situ gold impurity maintains a relative strong bonding with the host atom. The impurity AuHg creates a shallow acceptor level that is 23 meV above VBM. However, the interstitial gold defect is dominant donor. Under Hg-rich molecular beam epitaxy (MBE) growth condition, due to the self-compensating effect between AuHg and Aui, the Fermi level is pinned close to CBM by which the gold impurity cannot behave as an efficient acceptor. Whereas under Te-rich MBE growth condition and liquid phase epitaxy (LPE) growth condition, the self-compensating effect is overcame and the in situ gold impurity acts as an efficient p-type dopant.