Deep Acceptor States Research Articles

Magnetic resonance and the structure of magnesium acceptors in gallium nitride

In recent years there have been several theoretical and experimental investigations concerned with the detailed structure of the acceptor states in Mg-doped GaN. Thus, the shallow effective-mass-like acceptor has been attributed to simple substitutional Mg at Ga sites in unstrained regions of the material, while earlier suggestions that the Mg is associated with H appear to have been discounted by more recent studies. Deeper acceptor states have also been attributed to simple substitutional Mg, but in strained regions. The present paper makes use of the extensive data available from electron spin resonance and optically detected magnetic resonance to confirm these assignments and to highlight further the crucial role played by strain in influencing the detailed nature of the acceptor states. In particular, the neutral deep acceptor states are found to be formed by localization of the holes in $p$-like orbits on N atoms that lie in the basal plane, rather than along the $c$ axis, relative to the Mg dopant.

Model for determination of mid-gap states in amorphous metal oxides from thin film transistors

The electronic density of states in metal oxide semiconductors like amorphous zinc oxide (a-ZnO) and its ternary and quaternary oxide alloys with indium, gallium, tin, or aluminum are different from amorphous silicon, or disordered materials such as pentacene, or P3HT. Many ZnO based semiconductors exhibit a steep decaying density of acceptor tail states (trap DOS) and a Fermi level (EF) close to the conduction band energy (EC). Considering thin film transistor (TFT) operation in accumulation mode, the quasi Fermi level for electrons (Eq) moves even closer to EC. Classic analytic TFT simulations use the simplification EC−EF> ‘several’kT and cannot reproduce exponential tail states with a characteristic energy smaller than 1/2 kT. We demonstrate an analytic model for tail and deep acceptor states, valid for all amorphous metal oxides and include the effect of trap assisted hopping instead of simpler percolation or mobility edge models, to account for the observed field dependent mobility.

Theoretical studies of single magnetic impurities on the surface of semiconductors and topological insulators

ABSTRACTWe present results of theoretical studies of transition metal dopants in GaAs, based on microscopic tight-binding model and ab-initio calculations. We focus in particular on how the vicinity of surface affects the properties of the hole-acceptor state, its magnetic anisotropy and its magnetic coupling to the magnetic dopant. In agreement with STM experiments, Mn substitutional dopants on the (110) GaAs surface give rise to a deep acceptor state, whose wavefunction is localized around the Mn center. We discuss a refinement of the theory that introduces explicitly the d-levels for the TM dopant. The explicit inclusion of d-levels is particularly important for addressing recent STM experiments on substitutional Fe in GaAs. In the second part of the paper we discuss an analogous investigation of single dopants in Bi2Se3 three-dimensional topological insulators, focusing in particular on how substitutional impurities positioned on the surface affect the electronic structure in the gap. We present explicit results for BiSe antisite defects and compare with STM experiments.

Piezo-Phototronic Effect on Electroluminescence Properties of p-Type GaN Thin Films

We present that the electroluminescence (EL) properties of Mg-doped p-type GaN thin films can be tuned by the piezo-phototronic effect via adjusting the minority carrier injection efficiency at the metal-semiconductor (M-S) interface by strain induced polarization charges. The device is a metal-semiconductor-metal structure of indium tin oxide (ITO)-GaN-ITO. Under different straining conditions, the changing trend of the transport properties of GaN films can be divided into two types, corresponding to the different c-axis orientations of the films. An extreme value was observed for the integral EL intensity under certain applied strain due to the adjusted minority carrier injection efficiency by piezoelectric charges introduced at the M-S interface. The external quantum efficiency of the blue EL at 430 nm was changed by 5.84% under different straining conditions, which is 1 order of magnitude larger than the change of the green peak at 540 nm. The results indicate that the piezo-phototronic effect has a larger impact on the shallow acceptor states related EL process than on the one related to the deep acceptor states in p-type GaN films. This study has great significance on the practical applications of GaN in optoelectronic devices under a working environment where mechanical deformation is unavoidable such as for flexible/printable light emitting diodes.

Hydrogen impurity in paratelluriteα-TeO2: Muon-spin rotation andab initiostudies

We present a systematic study of isolated hydrogen in $\ensuremath{\alpha}$-TeO${}_{2}$ (paratellurite) by means of muon-spin spectroscopy measurements complemented by ab initio calculations based on density-functional theory (DFT). The observable metastable states accessible by means of the muon implantation allowed us to probe both the donor and the acceptor configurations of hydrogen, as well as to follow their dynamics. A shallow donor state with an ionization energy of 6 meV as well as a deep acceptor state are proposed, together with their atomic-level configurations and associated formation energies obtained from the DFT calculations. The latter show a tendency of interstitial hydrogen to bind strongly to bridging oxygen ions but also to coexist at sites deeper in the interior of the Te--O--Te rings with a more atomiclike character and a defect level in the gap. Atomlike interstitial muonium was observed; it has a hyperfine interaction of about 3.5 GHz. Charge and site changes with temperature are discussed.

Anisotropies in the optical ac and dc conductivities in lightly dopedLa2 − xSrxCuO4: the role of deep and shallow acceptor states

We investigate the origin of the optical ac and dc conductivity anisotropiesobserved in the low temperature orthorhombic phase of lightly doped, untwinnedLa2 − xSrxNiO4 single crystals. We show that these anisotropies can be naturally ascribed to the emergenceof two odd parity, rotational-symmetry-broken, localized impurity acceptor states, onedeeper and one shallower, resulting from the trapping of doped holes by the Coulombpotential provided by the Sr ions. These two lowest-energy, p-wave-like states are split byorthorhombicity and are partially filled with holes. This leaves a unique imprint in theoptical ac conductivity, which shows two distinct far-infrared continuum absorptionenergies corresponding to the photoionization of the deep and shallow acceptor states.Furthermore, we argue that the existence of two independent and orthogonal channels forhopping conductivity, directly associated with the two orthorhombic directions, alsoquantitatively explains the observed low temperature anisotropies in the dc conductivity.

Photoluminescence Analysis of Deep Acceptor in CdTe Films on GaAs(100) Substrates

In this study, we used photoluminescence (PL) measurement for analysis of deep acceptor states in unintentionally doped CdTe films on GaAs substrates. We analyzed PL and time-resolved spectra (TRS) in the vicinity of the 1.42 eV band. By analyzing the peak shift of the 1.42 eV band as a function of time after pulsed excitation, the bound-to-bound reaction constant for the unintentionally doped CdTe film on a GaAs substrate is estimated to be W0=4.0×107 s-1. The impurity concentration in an unintentionally doped CdTe film is estimated to be N=2.1×1016 cm-3. By analyzing the TRS of the 1.42 eV band as a function of time after pulsed excitation, the acceptor binding energy is estimated to be 105 meV. The acceptor binding energy is reasonably comparable to that in the case of a VII-donor-doped CdTe. The acceptor bound exciton, which can probably be attributed to a recombination of excitons bound to a complex acceptor consisting of a Cd vacancy and two Cl donors, is observed. From these features, it appears that the origin of the deep acceptor in unintentionally doped CdTe films on GaAs substrates is a complex acceptor consisting of a Cd vacancy and a Cl donor.

Carrier Compensation by Excess Oxygen Atoms in Anatase Ti0.94Nb0.06O2+δ Epitaxial Thin Films

We report the effect of post-deposition annealing on the electrical transport properties of anatase Ti0.94Nb0.06O2 (TNO) epitaxial thin films. Annealing TNO films in pure oxygen drastically suppressed the carrier density (ne). A high ne of the order of 1021 cm-3 was recovered by successive annealing in pure hydrogen. Core-level X-ray photoemission spectroscopy revealed that Ti and Nb respectively exist as tetravalent and pentavalent ions in fully oxidized samples. The concentration of Nb5+ relative to that of Nb4+ tends to increase with O2 annealing, suggesting that carriers released by Nb donors are compensated by electron-killing impurity states created by O2 annealing. Based on these findings, we propose that excess oxygen atoms incorporated by O2 annealing occupy interstitial sites and behave as deep acceptor states, which compensate electron carriers generated by Nb doping. Resonant valence-band photoemission spectroscopy directly confirmed the formation of deep acceptor states associated with oxygen annealing.

Optical and electrophysical properties of defects in high-purity CdTe

The electronic spectrum of defects formed in low-temperature synthesis and growth of high-purity CdTe[111]from the vapor phase of the starting components has been studied by the photoluminescence and photoconductivity methods, as well as using the analysis of the behavior with temperature of the conductivity. The studies have revealed in the crystals, in addition to the comparatively shallow centers involving primarily donors and acceptors contained in residual substitutional impurities, deep acceptor states with activation energies of 0.25, 0.60, and 0.86 eV, which differ in the character and magnitude of the localizing potential. While the deep centers at 0.60 and 0.86 eV display strong localization of electronic states, the center with the 0.25-eV activation energy is associated with defects for which the major part of the localizing potential extends uniformly in space over several unit cells. Such centers are assumed to originate from twinning-induced extended defects.

Point Defects in γ-Irradiated Germanium: High- and Low- Momentum Positron Annihilation Study Before and After n-p-Conversion

The electron momentum distribution and microstructure of centers incorporating a vacancy (vacancies) and a group-V-impurity atom (P, As, Sb, or Bi) in oxygen-lean n-Ge crystals have been investigated by means of the angular correlation of the annihilation radiation (ACAR). The vacancy-group-V-impurity atom complexes have been induced by irradiation with 60Co γ – rays at Tirr. ≈ 280K. A split between the intensities of the high-momentum emission of the annihilation radiation measured before and after n-p-conversion has been revealed for the complexes containing smaller ion cores (P, As) and the larger ones (Sb, Bi), respectively. After n-p-conversion the electron density decreases slightly (but markedly) around the positron localized at the vacancy complexes incorporating P, Sb, and Bi impurity atom. This decrease is accompanied by a lessening of intensity of the high-momentum emission of the annihilation radiation thus bringing in a direct evidence of a multi-vacancy structure of the vacancy-group-V-impurity atom complexes after n-p-conversion; the electron density was found to be affected by the localized deep acceptor states related to these centers. The relaxation inward open volume is a common feature which is pronounced for As-containing complexes. Subvalent band states are suggested to contribute the high-momentum annihilation most markedly. The electron momentum density around the positron is due to rather by the elemental specificity of the surrounding atoms than by changes of the electron-positron many-body interaction in the vacancy-group-V-impurity atom complexes.

Positron probing of gamma-irradiated Ge doped with P, As, Sb, and Bi: Changes in atomic structures of defects due to n→p conversion

Abstract The emission of the high-momentum annihilation radiation from the subvalent ion core shells and electron density around a positron localized at a vacancy-group-V-impurity atom complexes produced in oxygen-lean Ge doped with P, As, Sb, and Bi by irradiation with 60Co gamma-rays at room temperature have been investigated with the help of the angular correlation of annihilation radiation (ACAR) before and after n→p conversion. The probability of positron annihilation in the subvalent shells of atoms incorporated in dominant radiation centers was found to be dependent on the ratio of the ion core radii ri(P5+, As5+)/ri(Ge4+) 1, respectively. In passing from P to As impurity atoms the activation energy ΔEe of electron emission to be detected by DLTS measurements is increased by ~(+0.017 eV) vs. the increase of the electron density parameter to be reconstructed by ACAR data, Δr′s=r′s(As)−r′s(P)≈0.029 a.u. On the contrary, in passing from Sb to Bi impurity atoms, ΔEe value is decreased by ~(−0.028 eV) whereas the electron density parameter rises by Δr′s=r′s(Bi)−r′s(Sb)≈0.04 a.u. After n→p conversion a marked decrease in both the electron density and the number of ion cores around the positron points to the fact that the radiation-produced complexes with group-V-impurity atoms (P, As, Sb, Bi) are of a multi-vacancy character. The deep acceptor states in the forbidden gap (Ev+0.1), (Ev+0.12), (Ev+0.16) eV to be attributed to the P-, As-, Sb-, and Bi-containing multi-vacancy centers, respectively, were found to contribute to lessening the electron density around the trapped positron. It is argued that a close similarity of the As5+ and Ge4+ ion cores results in a small (but marked) augmentation in the electron density around the positron in As-containing multi-vacancy centers after n→p conversion. A trend for inward relaxation of the ion cores is observed in all radiation-produced centers studied.

Defect states of chemical vapor deposition grown GaN nanowires: Effects and mechanisms in the relaxation of carriers

Carrier relaxation in GaN nanowires, grown by atmospheric pressure chemical vapor deposition, via direct nitridation of Ga with NH3 at 950 °C has been investigated in detail. Differential absorption measurements reveal a large number of defect states located within the band gap. The relaxation dynamics of the photogenerated carriers suggest three distinct regions of energy states below the band edge identified as shallow donor states, midgap states, and deep acceptor states. Measurements suggest that Auger recombination is not a contributing factor in carrier relaxation even at the highest fluence (∼1 mJ/cm2) used in this work for carriers located within the conduction band. On the contrary, Auger recombination has been observed when probing the shallow donor states for fluences above 40 μJ/cm2. Measurements at the lowest fluence reveal a biexponential relaxation for the donor states with the fast component (∼50 ps) corresponding to the relaxation of carriers into the midgap states and the slow component of 0.65 ns associated with the relaxation into the deep acceptor states. Measurements reveal free-carrier absorption contribution from the deep acceptor states to the U-valley with an observed threshold limit of 3.5 eV suggesting the U-valley is located approximately 4.7 eV from the valence band.

Photoconductivity and photoluminescence in chemically deposited films of CdSSe:CdCl2,Ho

AbstractResults of the scanning electron microscopy (SEM), X-ray diffraction (XRD), optical absorption, photoconductivity (PC), and photoluminescence (PL) studies for the CdSSe:CdCl2,Ho films are presented in this paper. The SEM studies of different CdSSe films show a layered growth structure. A crystalline nature of the films is observed in the XRD studies. The regions with stacking fault were also observed in the X-ray diffractograms. The optical absorption spectra of these films show variations corresponding to the band gaps and the grain-sizes obtained under various deposition conditions and also with annealing. The effect of flux, impurities and annealing on the saturated photo to dark current ratio Ipc/Idc is observed in the PC rise and decay studies. The maximum value of Ipc/Idc ∼107 is obtained for the impurity doped annealed films. The PL emission spectra of CdSSe films show two emission peaks associated with the annihilation of free excitons and the transitions between shallow donor and deep acceptor states. In CdSSe:CdCl2,Ho films, two PL emission peaks are observed at 495 nm and 545 nm corresponding to the transitions 5S2→5I8 and 5F3→5I8, respectively, in Ho. The effect of pH on PL and grain size is also included in the present studies.

Mott−Schottky Analysis and Impedance Spectroscopy of TiO2/6T and ZnO/6T devices

Schottky junctions have been realized by evaporating gold spots on top of sexithiophen (6T), which is deposited on TiO 2 or ZnO with e-beam and spray pyrolysis. Using Mott-Schottky analysis of 6T/TiO2 and 6T/ZnO devices acceptor densities of 4.5x10(16) and 3.7x10(16) cm(-3) are obtained, respectively. For 6T/TiO2 deposited with the e-beam evaporation a conductivity of 9x10(-8) S cm(-1) and a charge carrier mobility of 1.2x10(-5) cm2/V s is found. Impedance spectroscopy is used to model the sample response in detail in terms of resistances and capacitances. An equivalent circuit is derived from the impedance measurements. The high-frequency data are analyzed in terms of the space-charge capacitance. In these frequencies shallow acceptor states dominate the heterojunction time constant. The high-frequency RC time constant is 8 micros. Deep acceptor states are represented by a resistance and a CPE connected in series. The equivalent circuit is validated in the potential range (from -1.2 to 0.8 V) for 6T/ZnO obtained with spray pyrolysis.

Nano-star formation in Al-doped ZnO thin film deposited by dip-dry method and its characterization using atomic force microscopy, electron probe microscopy, photoluminescence and laser Raman spectroscopy

Zinc oxide doped with Al (AZO) thin films were prepared on borosilicate glass substrates by dip and dry technique using sodium zincate bath. Effects of doping on the structural and optical properties of ZnO film were investigated by XRD, EPMA, AFM, optical transmittance, PL and Raman spectroscopy. The band gap for ZnO:Al (5.0 at. wt.%) film was found to be 3.29 eV compared with 3.25 eV band gap for pure ZnO film. Doping with Al introduces aggregation of crystallites to form micro-size clusters affecting the smoothness of the film surface. Al 3+ ion was found to promote chemisorption of oxygen into the film, which in turn affects the roughness of the sample. Six photoluminescence bands were observed at 390, 419, 449, 480, 525 and 574 nm in the emission spectra. Excitation spectra of ZnO film showed bands at 200, 217, 232 and 328 nm, whereas bands at 200, 235, 257 and 267 nm were observed for ZnO:Al film. On the basis of transitions from conduction band or deep donors (CB, Zn i or V OZn i ) to valence band and/or deep acceptor states (VB, V Zn or O i or O Zn), a tentative model has been proposed to explain the PL spectra. Doping with Al 3+ ions reduced the polar character of the film. This has been confirmed from laser Raman studies.

Anisotropic spatial structure of deep acceptor states in GaAs and GaP

Cross-sectional scanning tunneling microscopy is used to identify the origin of the anisotropic electronic structure of acceptor states in III--V semiconductors. The density of states introduced by a hole bound to an individual Cd acceptor in GaP is spatially mapped at room temperature. Similar to the Mn hole wave function in GaAs, we found a highly anisotropic, crosslike shape of the hole bound to Cd both at the GaP(110) and the $\mathrm{Ga}\mathrm{P}(1\overline{1}0)$ orthogonal cleavage planes. The experimentally observed similarity of the symmetry properties of Mn:GaAs to Cd:GaP shows that the anisotropic structure of acceptor states in zinc-blende III--V compounds is determined by the cubic symmetry of the host crystal. Nevertheless, the weak spin-orbit interaction in GaP leads to a slight modification of the Cd bound-hole wave function relative to that of Mn in GaAs. In addition to the anisotropic angular structure of the $d$-like spherical harmonic of the wave function, which dominates the appearance of the hole ground state far from the ionic core, the admixture of $g$-like and higher order spherical harmonics is identified at the sides of the Cd hole wave function. The experimentally obtained results agree with both atomistic tight-binding and envelope-function effective-mass theoretical models.

Carrier Compensation Mechanism of Highly Conductive Anatase Ti0.94Nb0.06O2 Epitaxial Thin Films

ABSTRACTWe investigated electrical conduction of anatase Ti0.94Nb0.06O2 (TNO) epitaxial thin films in relation to oxygen defects generated by post-annealing. Annealing of TNO in oxygen was found to cause dramatic decreases in ne. Resonant photoemission spectroscopy measurements revealed that a deep acceptor state just above the top of valence band evolves, synchronized with the decrease of ne. We proposed that the acceptor state originates from interstitial oxygen atoms combined with Nb dopants and compensates electron carriers.

Metal In-Diffusion during Fe and Co-Germanidation of Germanium

In this paper, the deep levels occurring in Fe- or Co-germanide Schottky barriers on ntype Ge have been studied by Deep Level Transient Spectroscopy (DLTS). As is shown, no traps have been found for germanidation temperatures up to 500 oC, suggesting that in both cases no marked metal in-diffusion takes place during the Rapid Thermal Annealing (RTA) step. Deep acceptor states in the upper half of the Ge band gap and belonging to substitutional Co and Fe can be detected by DLTS only at higher RTA temperatures (TRTA). For the highest TRTA, deep levels belonging to other metal contaminants (Cu) have been observed as well. Simultaneously, the reverse current of the Schottky barriers increases with TRTA, while the barrier height is also strongly affected.

Modification of the electronic properties of GaN nanowires by Mn doping

The electronic transport properties of individual GaN nanowire configured as field-effect transistors (FETs) show them to be highly conductive n-type semiconductors. Accordingly, the source-drain current (ISD) of these FETs cannot be varied significantly by the gate potential. When doped with Mn by thermal diffusion, the carrier density of resultant nanowire drops approximately two orders of magnitude likely due to the fact that Mn doping produces deep acceptor states that compensate for the native shallow donors of as-grown GaN nanowires. The ISD of the Mn-doped GaN nanowires could be modulated by a factor of 5 using the gate potential.

Modification of the properties of porous silicon on adsorption of iodine molecules

Infrared spectroscopy and electron spin resonance measurements are used to study the properties of porous silicon layers on adsorption of the I2 iodine molecules. The layers are formed on the p-an n-Si single-crystal wafers. It is established that, in the atmosphere of I2 molecules, the charge-carrier concentration in the layers produced on the p-type wafers can be noticeably increased: the concentration of holes can attain values on the order of ∼1018−1019 cm−3. In porous silicon layers formed on the n-type wafers, the adsorption-induced inversion of the type of charge carriers and the partial substitution of silicon-hydrogen bonds by silicon-iodine bonds are observed. A decrease in the concentration of surface paramagnetic defects, P b centers, is observed in the samples with adsorbed iodine. The experimental data are interpreted in the context of the model in which it is assumed that both deep and shallow acceptor states are formed at the surface of silicon nanocrystals upon the adsorption of I2 molecules.