Deep Donor Level Research Articles

I–V characteristics in Nb2VSbO10-ceramics

Thermoelectric power and electrical resistivity as well as magnetic measurements carried out on Nb2VSbO10 showed a diamagnetic behavior, an insulating state up to 360K and above this temperature n-type semiconducting properties with the activation energy of 0.55eV. The I–V characteristics both at 300 and 400K are symmetric and non-linear (back to back varistor-like). A computer-assisted curve fitting procedure of the conductance G vs. the applied voltage V showed the relation G~V3/4, which is interpreted in a framework of deep donor and shallow trap levels as well as grain boundaries.

Nitrogen incorporation into GaAsN and InGaAsN layers grown by liquid‐phase epitaxy

AbstractThis paper presents the comparison of nitrogen incorporation in GaAsN and InGaAsN layers grown on GaAs substrate from Ga‐ and In‐rich solution, respectively, by liquid‐phase epitaxy. Polycrystalline GaN has been used as a source of nitrogen in two cases. The initial epitaxy temperature has been varied in the temperature range 600‐550 °C. Nitrogen content in Ga1‐xAsNx grown layers has been determined to be in the range 0.1‐0.5%. Higher nitrogen incorporation efficiency has been found for quaternary InGaAsN layers grown under carefully chosen lattice matched conditions. The incorporation of nitrogen into GaAsN and InGaAsN layers has been study by vibrational mode absorption spectroscopy. Nitrogen‐induced vibration mode near 472 cm‐1 has been registered in GaAsN samples. Preferential In‐N bonds and the formation of N‐centred In3Ga1 clusters have been identified for lattice matched to GaAs epitaxial InGaAsN layers. Electrical properties of the samples have been characterized by temperature‐dependent Hall effect measurements. Nominally undoped GaAsN and InGaAsN grown layers are n‐type with Hall concentration about one order of magnitude higher in comparison to layers not containing nitrogen. Thermally activated increase in the free carrier concentration at temperatures higher than 150 K is observed which indicates the presence of N‐related deep donor levels below dilute nitride conduction band edge. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Te vapor annealing of indium-doped CdMnTe crystals

Thermal annealing in Te vapor atmosphere was adopted to improve the properties of indium-doped Cd1−xMnxTe (x = 0.2, CdMnTe) wafers grown by the vertical Bridgman method. The wafers before and after annealing were characterized by measuring the Te inclusions, etch pit density (EPD), Mn composition, resistivity, and impurity. IR transmission microscopy and EPD measurements revealed that the densities of Te inclusions reduced from (5–9) × 104 cm−3 to (2–4) × 104 cm−3 and EPD from 105 cm−2 to 104 cm−2 after annealing. NIR transmission spectroscopy showed that the Mn composition increased by 0.002–0.005 mole fractions during the annealing. The resistivity of the wafers improved from (2.0–4.5) × 108 Ω·cm to (1.7–3.8) × 109 Ω·cm, which suggested that the deep-level donor of Te antisites was successfully introduced after annealing. Inductively coupled plasma-mass spectrometry (ICP-MS) revealed that the concentrations of impurities in the wafer decreased, which indicated the purifying effects of Te vapor annealing on the wafers. All the results demonstrate that the Te vapor annealing of the indium-doped CdMnTe crystal has positive effects on the crystallinity, resistivity and purity of CdMnTe wafers.

Excitation intensity- and temperature-dependent photoluminescence in layered structured Tl2GaInSe2S2 crystals

Photoluminescence spectra of Tl2GaInSe2S2 layered crystals have been registered in the temperature range of 17–62 K and in the wavelength region of 525–690 nm. A broad visible photoluminescence band centered at 590 nm (2.10 eV) was observed at T = 17 K. Variation of emission band has been studied as a function of laser excitation intensity in the 0.1–55.7 mW cm−2 range. The analysis of the spectra reveals that the peak energy position changes with excitation intensity (blue shift). The radiative transitions from moderately deep donor level to shallow acceptor level were suggested to be responsible for the observed band. From X-ray powder diffraction study, the parameters of monoclinic unit cell of Tl2GaInSe2S2 were determined.

Modulation of NiGe/Ge Schottky barrier height by S and P co-introduction

The profiles and current-voltage characteristics of phosphorus (P)- and/or sulfur (S)-introduced Ge and NiGe/Ge have been investigated to clarify the mechanism of Schottky barrier height (SBH) modulation. For NiGe/nGe (NiGe/pGe), the introduction of P and the co-introduction of P and S effectively reduced (increased) SBH, which is explained by the properties of S acting as an n-type dopant with deep donor levels in Ge. In modulating the SBH, the co-introduction was the most effective. It is probably related to the fact that a higher increase in electron concentrations was observed in S and P co-introduced Ge than in P-introduced Ge.

Oxygen vacancy migration in compressively strained SrTiO3

The intrinsic properties of strontium titanate render it promising in applications such as gate dielectrics and capacitors. However, there is growing evidence that oxygen vacancies significantly impact upon its use, with the diffusion and deep donor level of the oxygen vacancy leading to electrical leakage. Where grown epitaxially on a lattice mismatched substrate, SrTiO3 undergoes bi-axial strain, altering its crystal structure and electronic properties. In this paper, we present the results of first-principles simulations to evaluate the impact of strain in a (001) plane upon the migration of oxygen vacancies. We show that in the range of strains consistent with common substrate materials, diffusion energies in different directions are significantly affected, and for high values of strain may be altered by as much as a factor of two. The resulting diffusion anisotropy is expected to impact upon the rate at which oxygen vacancies are injected into the films under bias, a critical factor in the leakage and resistive switching seen in this material.

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.

Post-Growth Annealing Effects on the Photoluminescence of ZnO Nanoparticle-Based Discs

The luminescence of ZnO exhibits a band-edge UV emission peak and a broad emission in the blue-green-red regions associated with the deep level defects as well as the oxygen vacancies and zinc interstitials in the discs. The mechanism of the defect is attributed to the electronic transitions from near conduction band-edge to the deep level acceptors and transitions from the deep donor levels to the valence band. In this study, post-growth annealing was conducted to investigate the mechanism involved in the visible luminescence of ZnO nanoparticle discs prepared. Post-growth thermal annealing can be applied as a new technique in controlling the optical properties of ZnO discs.

Abundance of CuZn + SnZn and 2CuZn + SnZn defect clusters in kesterite solar cells

Kesterite solar cells show the highest efficiency when the absorber layers (Cu2ZnSnS4 [CZTS], Cu2ZnSnSe4 [CZTSe] and their alloys) are non-stoichiometric with Cu/(Zn+Sn)≈0.8 and Zn/Sn≈1.2. The fundamental cause is so far not understood. Using a first-principles theory, we show that passivated defect clusters such as CuZn+SnZn and 2CuZn+SnZn have high concentrations even in stoichiometric samples with Cu/(Zn+Sn) and Zn/Sn ratios near 1. The partially passivated CuZn+SnZn cluster produces a deep donor level in the band gap of CZTS, and the fully passivated 2CuZn+SnZn cluster causes a significant band gap decrease. Both effects are detrimental to photovoltaic performance, so Zn-rich and Cu, Sn-poor conditions are required to prevent their formation and increase the efficiency. The donor level is relatively shallower in CZTSe than in CZTS, which gives an explanation to the higher efficiency obtained in Cu2ZnSn(S,Se)4 (CZTSSe) cells with high Se content.

Photoluminescence Study of Deep Levels in CuInS2 Thin Films Grown by Sulfurization Using Ditertiarybutylsulfide

Photoluminescence (PL) spectroscopy measurements are carried out to determine the deep defect levels of Cu-rich CuInS2 thin films prepared by sulfurization using ditertiarybutylsulfide [(t-C4H9)2S: DTBS]. Several PL emission peaks were detected at 1.07, 1.01, and 0.93 eV in Cu-rich CuInS2 thin films. These peaks are considered to be due to both donor–acceptor pair emission as well as transitions related to trap. On the basis of excitation power dependent and temperature dependent PL measurements, the defect levels are calculated. Copper interstitial (Cui) was determined to be the deep donor level, which is easier to create in samples prepared under Cu-rich conditions. A new trap level at 625 meV below the conduction band was found. Using these data and our previous study data, the intrinsic defects are easier to exist in what kind of CuInS2 thin films is analyzed, a complete defect levels diagram of CuInS2 thin films is also proposed.

Determination of trap levels in CZT:In by thermally stimulated current spectroscopy

Many defects in semi-insulating (SI) cadmium zinc telluride (Cd1-xZnxTe or CZT) ingots grown by the melt methods act as trapping centers to introduce deep levels in the band gap, which has strong effects on CZT detection properties. The thermally stimulated current (TSC) spectroscopy was used to measure these traps, and the initial rise method and the simultaneous multiple peaks analysis (SIMPA) method were introduced to characterize trap levels in SI-CZT:In. The results show that there is a larger error in the determination for the trap peaks with the initial rise method due to the interference of overlapping peaks, while the SIMPA method demonstrates a better performance in resolving these overlapping peaks simultaneously for a full characterization of trap levels. On this basis, a theoretical SIMPA fitting, which is composed of ten trap levels and a deep donor level EDD dominating the dark current in SI-CZT:In, is achieved. Furthermore, the reason of high resistivity in CZT:In was explained by the relationship between EDD level and Fermi level.

Thermally stimulated current spectroscopy applied on defect characterization in semi-insulating Cd0.9Zn0.1Te

Defects exhibiting trapping behaviors in semi-insulating (SI) materials were investigated by thermally stimulated current (TSC) spectroscopy. The variation of measurement conditions during the initial photoexcitation and thermal emission, such as the heating rate, bias voltage, illumination time and delay time, may bring some significant effects on TSC spectra, leading to incomplete characterization of trap levels. In this work, defects with deep levels in the band gap of SI-Cd0.9Zn0.1Te crystal, grown by the modified vertical Bridgman (MVB) method, were studied via TSC measurements. TSC measurement of the SI-CZT sample was performed with the optimized measurement conditions. Ten different traps and a deep donor (EDD) level were characterized from the as-obtained spectrum in the temperature range from 25 to 310K with the aid of simultaneous multiple peak analysis (SIMPA). The origins of these traps were identified in detail as well.

Irradiation-Induced Defects in Cd0.9Zn0.1Te:Al

The effects of gamma-irradiation, produced by a 60Co source with a dose of 2.7 kGy, on defect levels in a Cd0.9Zn0.1Te:Al (CZT:Al) crystal were investigated by thermally stimulated current spectroscopy. Nine observed defect levels were identified, and the irradiation-induced variations of trap signatures for these levels, i.e., the thermal activation energy, concentration, and capture cross-section, have been evaluated using simultaneous multiple peak analysis. In addition, the dark-current spectra dominated by the deep donor (EDD) level have been certified. By fitting plots of the logarithm of dark current ln(IDC) versus 1/kT, the EDD level is determined to be 0.554 ± 0.001 eV and 0.792 ± 0.004 eV before and after gamma-irradiation, respectively. One month after irradiation, the EDD level moved slightly to 0.782 ± 0.003 eV. This result indicates that the EDD level is closer to the middle of the bandgap of CZT:Al after irradiation, which should be responsible for Fermi-level pinning near the middle of the bandgap, leading to high resistivity. This is consistent with the resistivity variation of CZT:Al at room temperature from the original value of 7.5 × 109 Ω cm before irradiation to the irradiated value of 5.6 × 1010 Ω cm as determined by current–voltage measurements.

Unique Deep Level in Spectroscopic CdZnTe: Compensation, Trapping, and Polarization

As yet, the role of the main native defects in the compensation, trapping, and polarization of x-ray and gamma-ray room-temperature detectors based on semi-insulated cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) is indeterminate. To better quantify it, we assessed the ionization energy, i.e., the binding energy for the hole of the second (2-/1-) acceptor level of Cd vacancies in Cd 1−x Zn x Te (x ≈ 0.1), and that of the deep donor levels located at E C -0.4 eV and E C -0.7 eV. We characterized the defects in several ways, including measuring the photoconductivity at below-bandgap excitation, and photoconductivity quenching by comparing their positions in the bandgap with that of the native energy-levels in CdTe quantum dots (QDs) and other II-VI semiconductors. In this way, we determined unambiguously that a deep acceptor, Cd vacancy, behaves as a doubly charged acceptor, and the second ionization level is located at ∼ EV+(0.5±0.05) eV, i.e., far from the mid-gap. This configuration may determine the lifetime of holes, but it does not stabilize precisely the compensation condition, and it is not responsible for electron trapping and polarization. We demonstrated that a self-consistent model of compensation, electron trapping, and polarization should be based on a doubly charged donor (D) with two electrical states D(2+/1+) and D(1+/0), one of which is located close to the mid-gap and is separated from the second by a potential barrier that prevents fast trapping of the photoelectrons from the conduction band, but can be responsible for polarization.

Effect of gallium doping and ball milling process on the thermoelectric performance of n-type ZnO

Abstract

First-principles study of the effect of BiGa heteroantisites in GaAs:Bi alloy

First-principles calculations have been carried out to study effects of BiGa heteroantisite defects in GaAs:Bi on the related electronic structures. Our results show that the heteroantisite defect BiGa can be formed after the isovalent impurity BiAs reaches the solubility limit. This BiGa defect is a deep level donor, and the related defect band is composed of 6s state of BiGa hybridized with 4p states of the nearest As atoms. It is also found that the bandgap of GaAs:Bi alloy varies slightly with the increase of Bi compositions, as well as the relative position of the defect level. The co-existing, especially when BiGa and BiAs stay in the first-nearest neighbor, counteract the reduction of the bandgap caused by isovalent impurities BiAs in GaAs:Bi alloy. These results suggest that heteroantisite defect BiGa does not contribute to the reduction of the bandgap of GaAs:Bi alloy, and might be not an ideal candidate for the bandgap engineering of GaAs:Bi alloy.

Metal impurities in crystallographic voids of beta-rhombohedral boron lattice: Binding energies and electron levels

Applying quasi-classical approach, the binding energies and electron levels of metal impurities (Li, Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Hf, Ta, and Re) introduced into crystallographic voids of types A, D and E in the beta-rhombohedral boron lattice are calculated. Binding energies are estimated as ∼1 eV–60 eV per bond. The most of the obtained metal–boron bond lengths are very close to the mean radii of voids in the undoped crystal. Relatively light impurities (from Li to Cu) are found to form donor electron states directly inside the conduction band, i.e., they cause metallization of the material being introduced at sufficiently high concentrations. Heavy impurities (from Zr to Re) form shallow or deep donor levels inside the band gap or even valence band depending on dopants and voids of accommodation.

Electronic structure and impurity states of S-doped cBN: A first-principle study

The electronic structure and impurity states of S-doped cBN were investigated by first-principle approaches. Our calculation shows that S substituted for an N atom creates shallow donor levels merged to the states at the conduction band edge, S substituted for a B atom creates deep donor levels within the band gap, both forming n-type cBN, and band gap of S-doped cBN decreases with the increase of dopant concentration. Moreover, from the view of energy, S is more likely to be substituted for a B atom than an N atom.

Effect of Cu ions on the morphology, structure and luminescence properties of ZnO nanorod arrays prepared by hydrothermal method

Well-aligned ZnO nanorod arrays with different average diameters were grown by hydrothermal method in the presence of Cu ions in the deposition solution. Cu ions significantly inhibit diameter growth of ZnO nanorods and influence the luminescence properties. Quantitative analyses reveal that Cu ions were not incorporated into the synthesized nanorods. Photoluminescence spectra show that a weak ultraviolet emission and a strongly broad visible emission band in yellow region under ultraviolet light excitation for as-grown ZnO nanorods. The intensity of yellow emission bands decreases with Cu ions concentration increasing and the peak position has a blue-shift after low temperature thermal treatment. Photoluminescence spectra under different excitation energies and photoluminescence excitation spectra reveal that the yellow emission bands and green (blue) emission bands are associated with zinc interstitial related shallow donor level and oxygen vacancies related deep donor level as initial states, respectively. A plausible growth mechanism has been proposed to explain the narrowed diameter for ZnO nanorods in the presence of Cu ions based on crystallographic habits of wurtzite hexagonal.

Role of deep and shallow donor levels on n-type conductivity of hydrothermal ZnO

The residual n-type conductivity of O-polar hydrothermally grown ZnO single crystals and the role of annealing on the transport properties are assessed by temperature dependent Hall measurements on a wide 20–800 K temperature range. A deep level lying 250 meV below the conduction band is responsible for the residual n-type conductivity of unannealed samples. After annealing, a shallow donor level with 25 meV ionization energy becomes responsible for the sample conductivity in the room temperature range. Thanks to high temperature Hall measurement, the coexistence of the deep and the shallow level has been demonstrated in the case of annealed sample.