Properties of the oxygen vacancy in ZnO

As-grown ZnO bulk crystals and crystals annealed in vacuum, oxygen, or zinc vapour were characterized by electrical, optical and magnetic resonance spectroscopy. The experiments show that the residual carrier concentration is caused by residual H, Al, Ga and oxygen vacancies (V_O) in the material. Annealing the samples in O₂ at about 1000 °C (2 atm, 20 h) reduces the H and V_O donor concentration by typically one order of magnitude. The photoluminescence and deep level transient spectroscopy (DLTS) results suggest a correlation between the broad unstructured emission at 2.45 eV (“green band”) and a donor level 530 meV below the conduction band, it is attributed to the V_O ^0/++transition. By using DLTS experiments with optical excitation it is possible to observe a metastable level 140 meV below the conduction band which is assigned to the V_O ^2+/+ recharging. The results give evidence for the “negative-U” properties of the oxygen vacancy defects predicted by recent theoretical calculations. PACS 71.55.Gs; 72.20.Jv; 76.30.Mi; 76.70.Hb; 78.55.Et     

Hydrogen passivation effect on the yellow-green emission band and bound exciton in n - ZnO

Photoluminescence (PL) measurements performed on as-grown, hydrogenated, and annealed n-type ZnO bulk samples investigated the origins of their yellow (2.10 eV) and green (2.43 eV) emission bands. After hydrogenation, the defect-related peak at 2.10 eV was no longer present in the room temperature PL spectrum, the peak intensity at 2.43 eV was unchanged, and the intensity of the emission peak at 3.27 eV increased significantly. These results indicate that yellow band emission is due to oxygen vacancies, as the emission peak at 2.10 eV disappears when hydrogen atoms passivate these vacancies. The emission peak at 2.43 eV originates from complexes between oxygen vacancies and other crystal defects. We discuss the shallow donor impurities arising due to these hydrogen atoms in the ZnO bulk sample.     

Competition of N-passivation and Te-passivation in hydrogenation of Te-doped (Ga,In)(N,As)

(Ga,In)(N,As) lattice matched to GaAs with a band gap of 1 eV is employed as active material in high-efficiency III–V solar cells. Te-doped Ga_0.934In_0.066N_0.023As_0.977 layers were grown by metal-organic vapor-phase epitaxy on (1 0 0) GaAs. The samples were highly doped n-type with carrier concentrations ranging from about 10¹⁷–10¹⁹ cm⁻³. Pieces of the samples were hydrogenated with H-doses of 10¹⁸ ion/cm². The optical and electrical properties of the samples before and after hydrogenation were studied by low-temperature photoluminescence and magnetotransport. In undoped samples hydrogen is known to form N–H complexes which strongly reduce the local perturbation of the lattice due to nitrogen and thus reverse the N-induced global changes of the band structure. Combined analysis of photoluminescence and transport measurements on Te-doped samples, however, indicates a competition between N–H formation and passivation of the Te donor favoring the latter. Hardly any band structure changes due to hydrogenation are observed in these Te-doped samples, instead a strong reduction of the free-carrier concentration is observed after hydrogenation.     

An alternative approach to investigate the origin of p-type conductivity in arsenic doped ZnO

P-type conductivity in MOCVD grown ZnO was obtained by directional thermal diffusion of arsenic from semi-insulating GaAs substrate. The films were single crystalline in nature and oriented along (002) direction. Ab initio calculations in the framework of density functional theory have been carried out with different chemical states of arsenic in ZnO. Present calculations suggested As_Zn–2V_Zn defect is a shallow acceptor and results in ferromagnetism in ZnO. The magnetic measurements of the samples indeed showed ferromagnetic ordering at room temperature. X-ray photoelectron spectra confirmed the presence of As_Zn and V_Zn. The core level chemical shift in binding energy of As_Zn indicated the formation of As_Zn–2V_Zn. Diffused arsenic substitutes zinc atom and creates additional zinc vacancies. The zinc vacancies, surrounding the oxygen atoms, result in unpaired O 2p electrons which in turn induce ferromagnetism in the samples.     

Structural and optical properties of CdS/Cu(In,Ga)Se<Subscript>2</Subscript> heterostructures irradiated by high-energy electrons*

Thin films of Cu(In, Ga)Se₂ (CIGS) with a Ga/(Ga + In) ratio of ~0.27 corresponding to the standard elemental composition for solar-energy transducers were grown on Mo-coated glass substrates by the Cu, In, Ga, and Se co-evaporation technique from different sources. Transmission (T), photoluminescence (PL), and photoluminescence excitation (PLE) spectra at 4.2 K were used to analyze electronic properties in the asgrown and electron-irradiated CIGS films. The band-gap energy (E_g) of the CIGS films measured using both transmission and PLE methods was found to be about 1.28 eV at 4.2 K. Two deep bands in the PL spectra of the irradiated CIGS films, P₁ at ~0.91 eV and P₂ at ~0.77 eV, have been detected. These bands are tentatively associated with copper atoms substituting indium (Cu_In) and indium vacancies V_In, respectively, as the simplest radiation-induced defects.     

On the validity of the amphoteric-defect model in gallium arsenide and a criterion for Fermi-level pinning by defects

Using the theoretically calculated point-defect total-energy values of Baraff and Schlüter in GaAs, anamphoteric-defect model has been proposed by Walukiewicz to explain a large number of experimental results. The suggested amphoteric-defect system consists of two point-defect species capable of transforming into each other: the doubly negatively charged Ga vacancyV _Ga ^2– and the triply positively charged defect complex (AS_Ga+V _As)³⁺, with As_Ga being the antisite defect of an As atom occupying a Ga site andV _As being an As vacancy. When present in sufficiently high concentrations, the amphoteric defect systemV _Ga ^2– /(As_Ga+V _As)³⁺ is supposed to be able to pin the GaAs Fermi level at approximately theE _v +0.6 eV level position, which requires that the net free energy of theV _Ga/(As_Ga+V _As) defect system to be minimum at the same Fermi-level position. We have carried out a quantitative study of the net energy of this defect system in accordance with the individual point-defect total-energy results of Baraff and Schlüter, and found that the minimum net defect-system-energy position is located at about theE _v +1.2 eV level position instead of the neededE _v +0.6 eV position. Therefore, the validity of the amphoteric-defect model is in doubt. We have proposed a simple criterion for determining the Fermi-level pinning position in the deeper part of the GaAs band gap due to two oppositely charged point-defect species, which should be useful in the future.     

Emission and excitation spectra of ZnO:Ga and ZnO:Ga,N ceramics

The spectral characteristics of ZnO:Ga and ZnO:Ga,N ceramics prepared by uniaxial hot pressing have been investigated. At room temperature, the edge (exciton) band at 3.12 eV dominates in the luminescence spectra of ZnO:Ga, while a wide luminescence band at 2.37 eV, which is likely to be due to zinc vacancies, is observed in the spectra of ZnO:Ga,N. Upon heating, the edge band maximum shifts to lower energies and the bandwidth increases. The extrapolated position of the edge-band maximum at zero temperature, E _m (0) = 3.367 ± 0.005 eV, is in agreement with the data for thin zinc oxide films. The luminescence excitation spectra in the range from 3 to 6.5 eV are reported and the mechanism of energy transfer to excitons and luminescence centers is considered.     

Thermal stability study of palladium and cobalt Schottky contacts on n-Ge (1 0 0) and defects introduced during contacts fabrication and annealing process

Palladium (Pd) and cobalt (Co) Schottky barrier diodes were fabricated on n-Ge (1 0 0). The Pd-Schottky contacts were deposited by resistive evaporation while the Co-contacts were deposited by resistive evaporation and electron beam deposition. Current-voltage (I-V), capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements were performed on as-deposited and annealed samples. Electrical properties of Pd and Co samples annealed between 30 and 600 °C indicate the formation of one phase of palladium germanide and two phases of cobalt germanide. No defects were observed for the resistively evaporated as-deposited Pd-and Co-Schottky contacts. A hole trap at 0.33 eV above the valence band was observed on the Pd-Schottky contacts after annealing at 300 °C. An electron trap at 0.37 eV below the conduction band and a hole trap at 0.29 eV above the valence band was observed on as-deposited Co-electron beam deposited Schottky contacts. Rutherford back scattering (RBS) technique was also used to characterise the Co-Ge, for as-deposited and annealed samples.     

Divacancy complexes induced by Cu diffusion in Zn-doped GaAs

Positron annihilation spectroscopy was applied to investigate the nature and thermal behavior of defects induced by Cu diffusion in Zn-doped p-type GaAs crystals. Cu atoms were intentionally introduced in the GaAs lattice through thermally activated diffusion from a thin Cu capping layer at 1100 °C under defined arsenic vapor pressure. During isochronal annealing of the obtained Cu-diffused GaAs in the temperature range of 450?850 K, vacancy clusters were found to form, grow and finally disappear. We found that annealing at 650 K triggers the formation of divacancies, whereas further increasing in the annealing temperature up to 750 K leads to the formation of divacancy-copper complexes. The observations suggest that the formation of these vacancy-like defects in GaAs is related to the out-diffusion of Cu. Two kinds of acceptors are detected with a concentration of about 10¹⁶ ? 10¹⁷ cm^?3, negative ions and arsenic vacancy copper complexes. Transmission electron microscopy showed the presence of voids and Cu precipitates which are not observed by positron measurements. The positron binding energy to shallow traps is estimated using the positron trapping model. Coincidence Doppler broadening spectroscopy showed the presence of Cu in the immediate vicinity of the detected vacancies. Theoretical calculations suggested that the detected defect is V _Ga V _As-2Cu_Ga.     

The annealing of damage in ion implanted gallium arsenide

Photoluminescence measurements at 77°K and Rutherford scattering of 450 keV protons were used to study radiation damage and annealing in ion implanted GaAs. The characteristic band edge luminescence (8225 Å) in GaAs is completely quenched by ion implantation. Photoluminescence measurements on samples which were isochronally annealed show a single annealing stage at 600°C. A luminescence peak at 9140 Å is introduced into the spectra of all implanted and annealed samples. This peak is attributed to an acceptor level created by As vacancies. The intensity of the peak is greatly reduced by protecting the surface of implanted layers with SiO₂ during annealing. Rutherford scattering measurements on isochronally annealed samples reveal two annealing stages. A 300°C annealing stage is observed on samples which have an initial aligned yield less than random while a 650°C stage is observed on samples which have an initial aligned yield equal to random.     

Defect chemistry of CuInS2, investigated by electrical measurements and Mössbauer spectroscopy

Electrical measurements are performed on CuInS₂ single crystals (grown via chemical vapour transport), which were annealed under well defined conditions of molecularity and stoichiometry on four different positions of the CuInS₂ phase boundary. In this way two acceptor levels are detected, viz. at 0.10 eV in In-rich material and at 0.15 eV in Cu-rich material. The first is ascribed to V_cu, the latter to V_In or Cu_In. Also a donor level at 0.035 eV is observed, which is assigned to V_s or In_cu as most likely defects. The incorporation of iron, which is by far the most important impurity as revealed by spectrochemical analysis (concentration about 5 × 10¹⁸ cm^?3), was studied by means of Mössbauer spectroscopy. Irrespective of the molecularity and the stoichiometry iron appeared to be incorporated as Fe²⁺ on both Cu and In sites in a concentration ratio of roughly 3:1.     

Study of n type porous GaAs by photoluminescence spectroscopy: Effect of the etching time on the deep levels

Photoluminescence (PL) analysis is used to study porous layers elaborated by electrochemical etching of n⁺ Si-doped GaAs substrate with different etching times. Temperature and power dependence photoluminescence (PL) studies were achieved to characterize the effect of the etching time on the deep levels of the n⁺ Si-doped GaAs. The energy emission at about 1.46 eV is attributed to the band-to-band (B-B) (e-h) recombination of a hole gas with electrons in the conduction band. The emission band is composed of four deep levels due to the complex of (V_AsSi_GaV_Ga), a complex of a (Ga vacancy - donor - As vacancy), a (Si_Ga-V_Ga³⁻) defect or Si clustering, and a (gallium antisite double acceptor-effective mass donor pair complex) and which peaked, respectively, at about (0.94, 1, 1.14, and 1.32 eV). The PL intensity behavior as function of the temperature is investigated, and the experimental results are fitted with a rate equation model with double thermal activation energies.     

Defects in as-grown and annealed AlxGa1−xAs single crystals

Photoluminescence studies were made on Sn-, Te- and Ge-doped Al_xGa_1-xAs single crystals before and after annealing at 800°C for 21 h in evacuated and sealed quartz ampoules. The thin p-type surface layer, which was present on all nondegenerate n-Al_xGa_1-x As crystals after annealing, is attributed to the introduction of Si from the inner wall of the ampoule to form shallow Si_As and V_As-Si_As acceptor defects. Sn_As, V_Ga-Te (or) V_Al-Te), Ge_As and V_As-Ge_As defects were observed in Sn-, Te- and Ge-doped Al_x Ga_1-xAs after annealing. The defects were identified by plotting their characteristic recombination radiation energies versus Al composition x and comparing with previously established assignments in GaAs (x = 0). Two prominent bands, located at 1.53 and 1.46 eV independent of Al composition x, were detected in annealed (p) Al_xGa_1-xAs: Ge. The exact nature of the defects giving rise to these bands is not known.     

Size effects on the magnetic and optical properties of CuO nanoparticles

Optical and magnetic studies on CuO nanoparticles prepared by a chemical route are reported and the effect of size variation on these properties is discussed. SEM images show that the nanoparticles are interlinked into microspheres with the cages containing visible nanoscale holes. Diffuse reflectance spectroscopy indicates a consistent red shift in the fundamental band gap (indirect band gap) from 1.23 to 1 eV as the size decreases from 29 to 11 nm. This observed red shift is attributed to the presence of defect states within the band gap. A clear blue shift is observed in the direct band gap of these nanoparticles presumably due to the quantum confinement effects. Air-annealed samples show a paramagnetic response whereas particles annealed in a reducing atmosphere show additionally a weak ferromagnetic component at room temperature. For both types of particles, the paramagnetic and ferromagnetic moments, respectively, increase with decreasing size. The role of oxygen vacancies is understood to relate to the generation of free carriers mediating ferromagnetism between Cu spins. AC susceptibility measurements show both the antiferromagnetic transitions of CuO including the one at 231 K which is associated with the onset of the spiral antiferromagnetic phase transition.     

Effect of annealing temperature on the electrical properties of Au/Ta2O5/n-GaN metal–insulator–semiconductor (MIS) structure

We report on the effect of an annealing temperature on the electrical properties of Au/Ta₂O₅/n-GaN metal–insulator–semiconductor (MIS) structure by current–voltage (I–V) and capacitance–voltage (C–V) measurements. The measured Schottky barrier height (Φ _bo) and ideality factor n values of the as-deposited Au/Ta₂O₅/n-GaN MIS structure are 0.93 eV (I–V) and 1.19. The barrier height (BH) increases to 1.03 eV and ideality factor decreases to 1.13 upon annealing at 500 ^°C for 1 min under nitrogen ambient. When the contact is annealed at 600 ^°C, the barrier height decreases and the ideality factor increases to 0.99 eV and 1.15. The barrier heights obtained from the C–V measurements are higher than those obtained from I–V measurements, and this indicates the existence of spatial inhomogeneity at the interface. Cheung’s functions are also used to calculate the barrier height (Φ _bo), ideality factor (n), and series resistance (R _s ) of the Au/Ta₂O₅/n-GaN MIS structure. Investigations reveal that the Schottky emission is the dominant mechanism and the Poole–Frenkel emission occurs only in the high voltage region. The energy distribution of interface states is determined from the forward bias I–V characteristics by taking into account the bias dependence of the effective barrier height. It is observed that the density value of interface states for the annealed samples with interfacial layer is lower than that of the density value of interface states of the as-deposited sample.     

Luminescent properties of the oxygen impurity centres in A1N

Emission luminescence spectra of A1N with oxygen impurity are composed of several overlapping bands ranging from 2.37 eV to 3.51 eV. The intensity of these bands depends on the excitation conditions. The Stokes shift of corresponding excitation bands is ～0.93 eV; this shift is compatible with the experimental half-width of individual bandsΘ ～ 0.46 eV. The observed complex spectra are ascribed to the excitation and recombination processes in D-A complexes formed of substitutional oxygen donors O_N and aluminum vacancies V _Al . Tentative models of these centers are proposed.     

The initial stages of deposition of Cu on ZnO(101&#x0304;0) and MgO(001) surfaces studied by electron energy loss spectroscopy

The copper deposition on single crystal ZnO(101&#x0304;0) and MgO(001) surfaces has been studied by electron energy loss spectroscopy (EELS) in UHV at room temperature. The initial deposited Cu (well below one monolayer) induces a loss peak at about 2 eV on both oxide surfaces and at 4.3 eV on the MgO(001) surface. Based upon heat treatment and oxidation experiments the 2 eV structure is assigned to the electronic resonance of Cu(I) from the Cu deposit on the oxide matrix substrates. On the basis of the experiments the colour-center-related loss peaks, at 2.6 eV for MgO(001) and at 1.9 eV for ZnO(101&#x0304;0), are believed to be due to electronic resonance of a V⁻_s center, and the metal ion vacancies are suggested to be active centers which interact with the submonolayer copper deposits. Finally, the electronic energy loss spectra from the Cu-covered oxide surfaces are discussed in the framework of electronic band structures.     

Prediction of enhanced ferromagnetism in (Ga,Mn)As by intrinsic defect manipulation

In the diluted magnetic semiconductor (Ga,Mn)As the excess of As incorporated as As antisites (As_Ga) is responsible for the hole compensation. The As_Ga defect can be transformed into a As interstitial–Ga vacancy pair (As_i–V_Ga) upon illumination. In this paper we study the effects of such a transition on the ferromagnetism of (Ga,Mn)As using density functional theory within the local spin density approximation. We find that the ferromagnetic order in (Ga,Mn)As is strongly enhanced if As_Ga are transformed into As_i–V_Ga pairs, since the hole compensation is reduced. This suggests a valuable way to tune the carrier concentration and hence the T_c in (Ga,Mn)As, without changing the Mn concentration nor the microscopic configuration of the Mn ions.     

Synthesis,characterization and radiation damage studies of high-k dielectric (HfO2) films for MOS device applications

The current trend in miniaturization of metal oxide semiconductor devices needs high-k dielectric materials as gate dielectrics. Among all the high-k dielectric materials, HfO₂ enticed the most attention, and it has already been introduced as a new gate dielectric by the semiconductor industry. High dielectric constant (HfO₂) films (10?nm) were deposited on Si substrates using the e-beam evaporation technique. These samples were characterized by various structural and electrical characterization techniques. Rutherford backscattering spectrometry, X-ray reflectivity, and energy-dispersive X-ray analysis measurements were performed to determine the thickness and stoichiometry of these films. The results obtained from various measurements are found to be consistent with each other. These samples were further characterized by I–V (leakage current) and C–V measurements after depositing suitable metal contacts. A significant decrease in the leakage current and the corresponding increase in device capacitance are observed when these samples were annealed in oxygen atmosphere. Furthermore, we have studied the influence of gamma irradiation on the electrical properties of these films as a function of the irradiation dose. The observed increase in the leakage current accompanied by changes in various other parameters, such as accumulation capacitance, inversion capacitance, flat band voltage, mid-gap voltage, etc., indicates the presence of various types of defects in irradiated samples.     

Influence of post-deposition annealing on the structural,optical and electrical properties of Li and Mg co-doped ZnO thin films deposited by sol–gel technique

Lithium (Li) and magnesium (Mg) co-doped zinc oxide (ZnO) thin films were deposited by sol–gel method using spin coating technique. The films were deposited on glass substrates and annealed at different temperatures. The effects of annealing temperature on the structural, optical and electrical properties of the deposited films were investigated using X-ray diffraction (XRD), Ultraviolet–Visible absorption spectra (UV–VIS), photoluminescence spectra (PL), X-ray photo electron spectroscopy (XPS) and Hall measurements. XRD patterns indicated that the deposited films had a polycrystalline hexagonal wurtzite structure with preferred (0 0 0 2) orientation. All films were found to exhibit a good transparency in the visible range. Analysis of the absorption edge revealed that the optical band gap energies of the films annealed at different temperatures varies between 3.49 eV and 3.69 eV. Room temperature PL spectra of the deposited films annealed at various temperatures consist of a near band edge emission and visible emission due to the electronic defects, which are related to deep level emissions, such as oxide antisite (O_Zn), interstitial zinc (Zn_i), interstitial oxygen (O_i) and zinc vacancy (V_Zn) which are generated during annealing process. The influence of annealing temperature on the chemical state of the dopants in the film was analysed by XPS spectra. Ion beam analysis (Rutherford back scattering) experiments were performed to evaluate the content of Li and Mg in the films. Hall measurements confirmed the p-type nature of the deposited films.