Defect Characterization of 6H-SiC Studied by Slow Positron Beam

利用单能慢正电子束流,对原生的和经过电子辐照的6H-SiC内的缺陷形成及其退火行为进行研究．发现在n型6H-SiC中,经过退火后缺陷浓度降低．这主要是因为在退火过程中缺陷和间隙子的相互作用所引起．n型6H-Si经过1400 oC、30 min真空退火后,在SiC表面形成一个大约20 nm的Si层,这是在高温退火过程中Si原子向表面逸出的有力证明．在高温退火中,在样品的近表面区域有一个明显的表面效应,既在这些区域的S参数整体较大,这种现象与高温退火中Si不断向表面逸出有关．经过10 MeV的电子辐照,在n型6H-SiC中,正电子有效扩散长度从86.2 nm减少至39.1 nm,说明在样品中由于电子辐照产生大量缺陷．但是对p型6H-SiC,经过10 MeV电子辐照后有效扩散长度变化不大,这与其中缺陷的正电性有关．同时还对n型6H-SiC进行了1.8 MeV电子辐照后的300 oC退火实验,发现退火后缺陷浓度不减反增,这主要是因为在退火过程中,一些双空位缺陷和Si间隙子互相作用从而产生了VC缺陷的缘故．     

Surface redox induced bipolar switching of transition metal oxide films examined by scanning probe microscopy

The bipolar resistive switching mechanisms of a p-type NiO film and n-type TiO₂ film were examined using local probe-based measurements. Scanning probe-based current–voltage (I–V) sweeps and surface potential/current maps obtained after the application of dc bias suggested that resistive switching is caused mainly by the surface redox reactions involving oxygen ions at the tip/oxide interface. This explanation can be applied generally to both p-type and n-type conducting resistive switching films. The contribution of oxygen migration to resistive switching was also observed indirectly, but only in the cases where the tip was in (quasi-) Ohmic contact with the oxide.     

EL2 deep level in sub-bandgap surface photovoltage spectra in GaAs bulk crystals

Experimentally observed surface-photovoltage-method (SPV) spectra in the subbandgap energy range are presented for a real (100)GaAs surface, treated with preepitaxial procedures. Conductive, n-type GaAs and semi-insulating GaAs are studied. It is shown that SPV spectra are formed as a result of the simultaneous action of both surface states and deep bulk levels. The spectral shape of the surface-state photoionization cross-section is qualitatively determined. The influence of the deep bulk levels on the SPV spectra is explained, and the photoionization cross-section for both Cr and EL2 levels is qualitatively determined.     

Optoelectronic characteristics of CuO nanorods

Optoelectronic characteristics of p-type CuO nanorods, synthesized by a simple hydrothermal method, were investigated at different atmospheres and oxygen pressures. The CuO nanorods have lower resistance in air than in a vacuum, unlike the n-type semiconductors. This is explained in terms of the surface accumulation conduction. Measurements at different oxygen pressures indicate that oxygen has an important effect on the optoelectronic properties of p-type nanomaterials.     

Alternating current surface photovoltage in thermally oxidized chromium-contaminated n-type silicon wafers

We investigated a variation of frequency-dependent alternating current (AC) surface photovoltages (SPVs) in thermally oxidized, chromium-contaminated, n-type silicon (Si) wafers. As previously reported, immediately after rinsing in Cr-contaminated solution, a Cr(OH)₃–Si contact causes a Schottky-barrier-type AC SPV on n-type Si. Upon oxidation at 373 K for 10 min, the Schottky barrier collapses and, with further oxidation, a metal-induced negative oxide charge, due to atomic bridging of (CrOSi)⁻ and/or CrO₂^-\mathrm{CrO}_{2}^{-} networks, definitely grows over time in SiO₂. For samples oxidized at temperatures between 823 and 1023 K for 30 min, the observed AC SPV gives evidence that the metal-induced negative oxide charge causes a strongly inverted state of the Si surface. At oxidation temperatures higher than 1023 K and /or for an oxidation time longer than 60 min, the level height of the AC SPV is reduced, implying that the strongly inverted state changes into a less depleted state, whilst, finally, the AC SPV disappears. In this case, the collapse of the (CrOSi)⁻ and/or CrO₂^-\mathrm{CrO}_{2}^{-} networks is anticipated, with a possible change into Cr₂O₃. The existence of the (CrOSi)⁻ and/or CrO₂^-\mathrm{CrO}_{2}^{-} networks has also been confirmed in p-type Si wafers.     

First-Principles Study of Intrinsic Point Defects of Monolayer GeS

The properties of six kinds of intrinsic point defects in monolayer GeS are systematically investigated using the“transfer to real state”model,based on density functional theory.We find that Ge vacancy is the dominant intrinsic acceptor defect,due to its shallow acceptor transition energy level and lowest formation energy,which is primarily responsible for the intrinsic p-type conductivity of monolayer GeS,and effectively explains the native p-type conductivity of GeS observed in experiment.The shallow acceptor transition level derives from the local structural distortion induced by Coulomb repulsion between the charged vacancy center and its surrounding anions.Furthermore,with respect to growth conditions,Ge vacancies will be compensated by fewer n-type intrinsic defects under Ge-poor growth conditions.Our results have established the physical origin of the intrinsic p-type conductivity in monolayer GeS,as well as expanding the understanding of defect properties in lowdimensional semiconductor materials.     

Evidence for p-type doping of InN

The first evidence of successful p-type doping of InN is presented. It is shown that InN:Mg films consist of a p-type bulk region with a thin n-type inversion layer at the surface that prevents electrical contact to the bulk. Capacitance-voltage measurements indicate a net concentration of ionized acceptors below the -type surface. Irradiation with 2 MeV He+ ions is used to convert the bulk of InN:Mg from p to n-type, at which point photoluminescence is recovered. The conversion is well explained by a model assuming two parallel conducting layers (the surface and the bulk) in the films.     

Wet-chemical treatment and electronic interface properties of silicon solar cell substrates

On textured n-type silicon substrates for solar cell manufacturing, the relation between light trapping behavior, structural imperfections, energetic distribution of interface state densities and interface recombination losses were investigated by applying surface sensitive techniques. The field-modulated surface photovoltage (SPV), in-situ photoluminescence (PL) measurements, total hemispherical UV-NIR-reflectance measurements and electron microscopy (SEM) were employed to yield detailed information on the influence of wet-chemical treatments on preparation induced micro-roughness and electronic properties of polished and textured silicon substrates. It was shown that isotropic as well as anisotropic etching of light trapping structures result in high surface micro-roughness and density of interface states. Removing damaged surface layers in the nm range by wet-chemical treatments, the density of these states and the related interface recombination loss can be reduced. In-situ PL measurements were applied to optimise HF-treatment times aimed at undamaged, oxide-free and hydrogen-terminated substrate surfaces as starting material for subsequent solar cell preparations.      

Electronic surface properties of uhv-cleaved III–V compounds

Combined CPD and photoemission measurements were performed on uhv-cleaved surfaces of the III–V compounds InAs, GaSb, GaAs and GaP with moderate p-type and n-type dopings. Except for n-type GaP these materials show practically no band bending. N-type GaP exhibits surface Fermi level stabilization at 0.55 eV below the conduction band edge. This is ascribed to an intrinsic empty surface state band in the forbidden zone. On the basis of our experiments together with available data from literature we propose an empirical model for the (110) plane of III–V compounds containing In or Ga as metal and Sb, As or P as non-metal atoms from which the lower edge of the empty surface state band can be predicted. The model indicates that for any of these compounds except for GaP no empty surface state band exist in the band gap on the (110) surface.     

Surface photovoltage for real n- and p-type in the visible and near spectroscopy (110) CdTe surfaces infra-red spectral range

Surface photovoltage spectroscopy has been carried out on real n- and p-type (110) CdTe surfaces in the wavelength range 0.36-1 μm at room temperature (300 K), and at atmospheric pressure. The measurements show the existence of surface states at 1.3; 1.48, and 1.2; 1.46 eV within the energy gap of n- and p-type CdTe, respectively. Surface states greater than the energy gap at 2.24, 2.38, 2.68, and 3.1 eV have also been detected in n-type samples and at 1.66, 2.12, 2.69 eV in p-type samples.     

Analysis of thin thermal silicon nitride films on silicon

We have investigated the chemical and electrical properties of very thin (<32 Å thick) silicon nitride films grown by rapid thermal nitridation of silicon. These films were of interest as a possible means of tailoring the barrier heights of silicon Schottky barrier diodes. Auger and XPS analysis showed that the level of oxygen contamination in the films was very low ([N]/[N]+[O]) =0.85 to 0.95). The oxygen is located primarily at the surface and interface of the films. Metal-nitride-silicon devices were characterized by I-V and C-V techniques. These measurements indicated an increase in barrier heights to p-type substrates and a decrease in barrier heights to n-type substrates compared to values measured in the absence of the nitride layers. The magnitude of the change in barrier height increases with increasing nitride thickness. The barrier height can be varied reproducibly over a wide range. For molybdenum on p-type, this range is greater than half the bandgap. For titanium and molybdenum on p-type diodes, barrier heights higher than 1.0 V can be achieved. These measurements could be explained by a reduction in the density of silicon interface states with increasing nitride thickness or by the presence of positive fixed charge in the nitride layer.     

Contribution of Surface Defects to the Interface Conductivity of SrTiO_3/LaAlO_3

Based on the first-principles method,the structural stability and the contribution of point defects such as O,Sr or Ti vacancies on two-dimensional electron gas of n-and p-type LaAlO_3/SrTiO_3 interfaces are investigated.The results show that O vacancies at p-type interfaces have much lower formation energies,and Sr or Ti vacancies at n-type interfaces are more stable than the ones at p-type interfaces under O-rich conditions.The calculated densities of states indicate that O vacancies act as donors and give a significant compensation to hole carriers,resulting in insulating behavior at p-type interfaces.In contrast,Sr or Ti vacancies tend to trap electrons and behave as acceptors.Sr vacancies are the most stable defects at high oxygen partial pressures,and the Sr vacancies rather than Ti vacancies are responsible for the insulator-metal transition of n-type interface.The calculated results can be helpful to understand the tuned electronic properties of LaAlO_2/SrTiO_3 heterointerfaces.     

Electrical conductivity of Co doped cuprous oxide

The solubility limit of metals in cuprous oxide is very low and it is therefore difficult to form solid solutions of metal oxides with cuprous oxide. In an on going research looking for such solid solutions and their properties we have prepared Co doped Cu₂O. We report here on measurements of the electrical conductivity of Co doped Cu₂O as a function of the oxygen partial pressure. It is found that Co doped material is an n-type semiconductor in the low oxygen partial pressure regime and p-type at higher oxygen pressures (while undoped Cu₂O is a p-type material throughout the whole existence regime). A point defect model is discussed. The ionic transference number is also measured and is found to be less than 2⋅10⁻⁴. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Positron lifetime spectroscopic studies of as-grown 6H-silicon carbide

Positron lifetime spectroscopy has been employed to study the as-grown n-type 1.2᎒¹⁸ cm^-3 N-doped and p-type 1.8᎒¹⁸ cm^-3 Al-doped 6H-silicon carbide in the temperature range 10 K-300 K. For the p-type material, a positron trapping site, which has a lifetime of 225ᆟ ps, was found and is attributed to positron annihilating from the V_SiV_C divacancy-related defect having a neutral charge. For the case of the n-type material, a positron trapping centre having a lifetime of 200Nj ps, attributed to a V_Si-related defect, and a shallow trap were observed. The shallow trap, having binding energy of l8 meV was attributed to Rydberg-like states resulting from positron binding with a negative ion.     

Surface photovoltage,band-bending and surface states on a-Si : H

We have measured the surface photovoltage (SPV) of intrinsic (i.e., undoped) and phosphorus-doped amorphous Si : H between ?168 and 25°C in the spectral range from 0.5 to 2.5 eV. The a-Si : H was grown in a silane glow discharge. Vibrating Kelvin probe techniques were used for the SPV measurements; Auger spectroscopy was used for monitoring surface cleanliness and chemistry. At all temperatures and for both materials, (1) the SPV was invariably negative, (2) there was no correlation between the spectral, thermal and response-time properties of the SPV and the bulk photoconductivity, and (3) surface treatments such as sputtering and oxygen physisorption strongly affected the SPV but not the photoconductivity. These facts indicated that the SPV was due to the emptying of surface-states via surface transitions, and corresponded to the flattening of bands which, when unilluminated, were bent upwards. Intrinsic material showed a maximum SPV of about 0.2 V. The SPV was characterized at ?168°C by strong electronic isolation between surface-states and valence band (i.e., once light was removed, there was no surface-state refilling or decay of the SPV), slow rise times (～min), saturation at photon fluxes of about 10¹¹/cm² · s, and a SPV spectral threshold occurring at 0.7 eV. At 25°C, all SPV responses were much faster (<0.5 s) and the optical threshold was 0.9 eV. The thermal activation energies associated with the SPV were 0.11 eV for surface-state emptying and 0.22 eV for surface-state refilling. For P-doped material the maximum SPV at ?168°C was 0.3 V and its properties indicated less electronic isolation between surface-states and valence band. There was no SPV at room temperature. Our results are discussed in terms of an energy level scheme which contains a distribution of filled surface states isolated from both conduction and valence bands. The surface-state density is estimated to be about (1?2) × 10¹¹/ cm², a relatively low value which is consistent with the observed lack of Fermi level pinning. In both materials there is a very fast component of the SPV which suggests the presence of additional surface states below the valence band edge.     

Measurement of active carbon on ruthenium (110): Relevance to catalytic methanation

Hall measurements of charge carrier mobility and concentration in the channels appearing at germanium surfaces cleaved in liquid nitrogen, have been carried out to define the surface state charge Q_ss. The n-type germanium samples with various Sb atom concentrations and p-type germanium samples with various Sb and Au atom concentrations have been investigated. It has been established that surface states lie below the valence band top, and ¦Q_ss¦increases with the growth of the bulk doping level. The results obtained imply that the surface state density depends on the bulk impurity concentration. Possible explanation of the found bulk impurity influence on the surface state characteristics is proposed.     

Assessing the effect of hydrogen on the electronic properties of 4H-SiC

As a common impurity in 4H silicon carbide (4H-SiC), hydrogen (H) may play a role in tuning the electronic properties of 4H-SiC. In this work, we systemically explore the effect of H on the electronic properties of both n-type and p-type 4H-SiC. The passivation of H on intrinsic defects such as carbon vacancies (V_C) and silicon vacancies (V_Si) in 4H-SiC is also evaluated. We find that interstitial H at the bonding center of the Si-C bond (H_i^bc) and interstitial H at the tetrahedral center of Si (H_i^Si-te) dominate the defect configurations of H in p-type and n-type 4H-SiC, respectively. In n-type 4H-SiC, the compensation of H_i^Si-te is found to pin the Fermi energy and hinder the increase of the electron concentration for highly N-doped 4H-SiC. The compensation of H_i^bc is negligible compared to that of V_C on the p-type doping of Al-doped 4H-SiC. We further examine whether H can passivate V_C and improve the carrier lifetime in 4H-SiC. It turns out that nonequilibrium passivation of V_C by H is effective to eliminate the defect states of V_C, which enhances the carrier lifetime of moderately doped 4H-SiC. Regarding the quantum-qubit applications of 4H-SiC, we find that H can readily passivate V_Si during the creation of V_Si centers. Thermal annealing is needed to decompose the resulting V_Si-nH (n=1-4) complexes and promote the uniformity of the photoluminescence of V_Si arrays in 4H-SiC. The current work may inspire the impurity engineering of H in 4H-SiC.     

Surface photovoltage spectroscopy of electronic surface states on cleaved germanium(111) surfaces

Surface photovoltage (SPV) measurements on UHV cleaved Ge(111) surfaces at 100 K are reported for photon energies 0.4 < ?ω < 1 eV. The SPV spectra are sensitive to surface treatment. Upon annealing to temperatures above 200°C, which is accompanied by a reconstruction change from the (2 × 1) to an (8) superstructure, the SPV spectrum shows 2 shoulders below band gap energy with threshold energies near 0.4 and 0.45 eV. These structures are interpreted in terms of electronic transitions from the valence band into empty surface state levels which are related to the (8) superstructure. Adsorbed oxygen and water vapor both cause new similar transitions from the valence band into empty surface states at 0.08 eV below the bottom of the conduction band.     

Electronic properties of cleaved CdTe(110) surfaces

Photoemission yield spectroscopy measurements were performed on a set of n- and p-doped CdTe single crystals. The surfaces were obtained by cleavage in ultrahigh vacuum and characterized by low energy electron diffraction and Auger electron spectroscopy. On clean and properly cleaved surfaces, no band bending was found, neither on n- nor on p-type samples, showing the absence of intrinsic surface states in the gap. The ionization energy is found at 5.80±0.05 eV. Oxygen adsorption removes defect-induced surface states on the valence band side of the gap and develops a band bending on n-type samples which indicates the presence of acceptor surface states in the gap down to 0.70 eV below the conduction band edge. The ionization energy remains constant.     

Electrical and Raman properties of p-type and n-type modified graphene by inorganic quantum dot and organic molecule modification

We obtained n-type and p-type modified graphene by mixing quantum dots and depositing electron-acceptor molecules on the surface of graphene, respectively. The electrical and optical properties of these two types of samples were measured. For n-type modified graphene, the electrons were transferred from quantum dots to graphene. The resistance of these quantum dots in modified n-type graphene is significantly smaller than that of pristine graphene. For p-type graphene, modified by electron-acceptor organic ...