Effects of Illumination on Ar<Superscript>+</Superscript>-Implanted <Emphasis Type="Italic">n</Emphasis>-Type 6H-SiC Epitaxial Layers

Argon ions were implanted into n-type 6H-SiC epitaxial layers at 600°C. Postimplantation annealing was carried out at 1,600°C for 5 min in an Ar ambient. Four implantation-induced defect levels were observed at E_C-0.28 eV, E_C-0.34 eV, E_C-0.46 eV, and E_C-0.62 eV by deep level transient spectroscopy. The defect center at E_C-0.28 eV is correlated with ED₁/ED₂ and with ID₅. The defect at E_C-0.46 eV with a capture cross section of 7.8 × 10⁻¹⁶ cm² is correlated with E1/E2, while the defect at E_C-0.62 eV with a capture cross section of 2.6 × 10⁻¹⁴ cm² is correlated with Z1/Z2. Photo deep level transient spectroscopy was also used to study these defects. Upon illumination, the amplitudes of the deep level transient spectroscopy (DLTS) peaks increased considerably. Two emission components of Z1/Z2 were revealed: one fast and the other slow. The fast component could only be observed with a narrow rate window. In addition, a new defect was observed on the low-temperature side of the defect at E_C-0.28 eV when the sample was illuminated. [rl](Received ...; accepted ...)     

Impurity states in cobalt-doped silicon

Cobalt was diffused into p⁺ pn⁺ silicon structures at 900° and 1150°C for 2−4 hours followed by various quenching conditions. Four primary hole traps and two electron traps associated with cobalt in these devices were observed. The hole traps are labeled H1(E_v + 0.22 eV), H2(E_v + 0.29 eV), H3 (E_v + 0.40 eV) and H4(E_v + 0.45 eV) while the electron traps labeled E1 and E2 are located at E_c − 0.36 eV and E_c − 0.44 eV, respectively. The concentrations, thermal emission rates, and the capture cross sections for the majority carriers at these defects are reported. The behavior of these defects under heat treatment and the emergence of secondary defects, H5(E_v +0.22 eV) and H6 (E_v +0.34 eV), will be discussed.     

High and low-energy proton radiation damage in p/n InP MOCVD solar cells

Indium phosphide p⁺/n/n⁺ solar cells, fabricated by metal organic chemical vapor deposition, were irradiated with 0.2-MeV and 10-MeV protons to a fluence of 10¹³ cm⁻². The power output degradation, 1-V behavior, carrier concentration and defect concentration were observed at intermediate points throughout the irradiations. The 0.2-MeV proton-irradiated solar cells suffered much greater and more rapid degradation in power output than those irradiated wit h 10 MeV protons. The efficiency losses were accompanied by larger increases in the recombination currents in the 0.2-MeV proton-irradiated solar cells. The low-energy proton irradiations also had a larger impact on the series resistance of the solar cell s. Despite the radiation-induced damage, the carrier concentration in the base of the solar cells showed no reduction after 10-MeV or 0.2-MeV proton irradiations and even increased during irradiation with 0.2-MeV protons. In a deep-level transient spectro scopy study of the irradiated samples, the minority carrier defects H4 and H5 at E_v + 0.33 and E_v + 0.52 eV and the majority carrier defects E7 and E10 at E_c − 0.39 and E_c − 0.74 eV were observed. Th e defect introduction rates for the 0.2-MeV proton irradiations were about 20 times higher than for the 10-MeV proton irradiations. The defect E10, observed here after irradiation, has been shown to act as a donor in irradiated n-type InP and may be responsible for obscuring carrier removal. The results of this study are consistent with the much greater damage produced by low-energy protons whose limited range causes them to stop in the active region of the solar cell. © This article is a US Government work and, as such, is in the public domain in the United States of America     

Two-electron photo-ionization from deep states in semiconductors

Photo-ionization (P-I) from deep states in semiconductors can be used to explore the band structure of the crystal as well as to give detailed information about the symmetry and other properties of the bound states themselves. When 2 electrons are bound to a defect the P-I spectrum at high photon energy reflects the initial and final state properties of both electrons. Only at low energies, where one electron is transferred into its 1-electron ground state does the P-I process of a 2-electron state resemble that of a 1-electron state. This fact offers a resolution of the two different interpretations of P-I from the 0^- state in GaP as reported by Henry et al. from photo-capacitance studies and by Grimmeiss et al. from photo-conductivity. In the latter the 4 lowest enercry thresholds (E_i = 0.65 to 1.19 eV) correspond to “1-electron” P-I with the 0 center left in its neutral ground state. Only above about 1.4 eV do the “2-electron” processes appear. In photo-capacitance the “2-electron” processes dominate (the “1-electron” thresholds being too weak or too diffuse to be identified), and the spectrum is strongly modified by relaxation of the lattice around the 0⁺ ion.     

Spectroscopic differentiation between O-atom vacancy and divacancy defects,respectively, in TiO2 and HfO2 by X-ray absorption spectroscopy

Defect state features have been detected in second derivative O K edge spectra for thin films of nano-crystalline TiO₂ and HfO₂. Based on soft X-ray photoelectron band edge spectra, and the occurrence of occupied band edge 4f states in Gd(Sc,Ti)O₃, complementary spectroscopic features have been confirmed in the pre-edge (<530 eV) and vacuum continuum (>545 eV) regimes of O K edge spectra. Qualitatively similar spectral features have been obtained for thin films of HfO₂ and TiO₂, and these have been assigned to defect states associated with vacancies. The two electrons/removed O-atom are not distributed uniformly over the TM atoms defining the vacancy geometry, but instead are localized in equivalent d-states: a d² state for a Ti monovacancy and a d⁴ state for a Hf divacancy. This new model for electronic structure provides an unambiguous way to differentiate between monovacancy and divacancy arrangements, as well as immobile (or fixed) and mobile vacancies.     

Neutron-induced trapping levels in aluminum gallium arsenide

Deep level transient spectroscopy (DLTS) measurements have been performed on a variety of Al_xGa_1-xAs p-n junctions prior to and following a series of fast neutron irradiations at room temperature and subsequent isochronal anneals. In contrast with electron and proton irradiated GaAs, neutron irradiation produces a single, broad featureless DLTS band which is a majority carrier trap in both n and p type material. The characteristics of this neutron-induced trap are relatively independent of growth method, dopant type and concentration. In GaAs, the thermal emission energies of the trap are 0.58 to 0.68 eV depending on the particular junction. These energies increase with Al content to 0.94 eV at 20% Al. The trap introduction rate, which also increases with Al content, is 0.7 cm_-1 in GaAs. Isochronal annealing to temperatures as high as 400‡C results in a smaller FWHM of the DLTS band, a shift in the peak to higher temperatures, and a modest decrease in magnitude. Above 400‡C the magnitude decreases rapidly, suggesting a similarity with the antisite defect, As_Ga, which has been observed to anneal in this range.     

Transient current spectroscopy and frequency dispersion studies of low temperature GaAs and Al0.3Ga0.7As metal-insulator-semiconductor diodes

Low temperature (LT)-grown GaAs and Al_0.3Ga_0.7As metal-insulator-n⁺-GaAs (MIN) diodes have been fabricated and their electrical properties analyzed. Studies were carried out to evaluate the interfacial quality of the LT layer and the underlying n⁺-GaAs layer using transient current spectroscopy (TCS) and capacitance-frequency (C-f) characterization. TCS studies on LT-GaAs revealed a high concentration of a continuum of states anda dominant electron trap with an activation energy of 0.52eV. In LT-Al_0.3Ga_0.7As, a shallow trap at 0.36eV and two deep level traps at 0.85eV and 1.12eV were observed. Frequency dispersion was observed to be less for LT-GaAs samples with an AlAs barrier layer than without an AlAs barrier layer. However, LT-Al_0.3Ga_0.7As MIN diodes displayed a smaller frequency dispersion than LT-GaAs MIN diodes. Upon further investigation into MISFET devices, it was found that LT-Al_0.3Ga_0.7As MISFET devices had better transconductance frequency dispersion characteristics than LT-GaAs MISFET devices did.     

Bistability and electrical activity of the vacancy-dioxygen complex in silicon

The methods of infrared absorption. Hall effect, and deep-level transient spectroscopy are used to study the complexes that consist of a vacancy and two oxygen atoms (the vacancy-dioxygen complexes, VO₂) in irradiated n-Si crystals with various levels of doping. The previously observed bistability of VO₂ is confirmed and evidence is provided for electrical activity of this defect in the metastable configuration VO*₂. It is established that the defect with this configuration features an acceptor level located at E _C ? 0.06 eV. It is shown that the absorption bands at 967 and 1023 cm^?1 are caused by the negatively charged VO*₂ state, while the bands peaking at 928 and 1004 cm^?1 correspond to the neutral charge state of the defect.     

Improved Interface and Electrical Properties by Inserting an Ultrathin SiO2 Buffer Layer in the Al2O3/Si Heterojunction

An ultrathin SiO₂ interfacial buffer layer is formed using the nitric acid oxidation of Si (NAOS) method to improve the interface and electrical properties of Al₂O₃/Si, and its effect on the leakage current and interfacial states is analyzed. The leakage current density of the Al₂O₃/Si sample (8.1 × 10^?9 A cm^?2) due to the formation of low‐density SiO_x layer during the atomic layer deposition (ALD) process, decreases by approximately two orders of magnitude when SiO₂ buffer layer is inserted using the NAOS method (1.1 × 10^?11 A cm^?2), and further decreases after post‐metallization annealing (PMA) (1.4 × 10^?12 A cm^?2). Based on these results, the influence of interfacial defect states is analyzed. The equilibrium density of defect sites (N_d) and fixed charge density (N_f) are both reduced after NAOS and then further decreased by PMA treatment. The interface state density (D_it) at 0.11 eV decreases about one order of magnitude from 2.5 × 10¹² to 7.3 × 10¹¹ atoms eV^?1 cm^?2 after NAOS, and to 3.0 × 10¹⁰ atoms eV^?1 cm^?2 after PMA. Consequently, it is demonstrated that the high defect density of the Al₂O₃/Si interface is drastically reduced by fabricating ultrathin high density SiO₂ buffer layer, and the insulating properties are improved.     

Electrical properties of Si:H/<Emphasis Type="Italic">p</Emphasis>-Si structures fabricated by hydrogen implantation

Hydrogenated silicon (Si:H) layers and Si:H/p-Si heterostructures were produced by multiple-energy (3–24 keV) high-dose (5×10¹⁶–3×10¹⁷ cm^?2) hydrogen implantation into p-Si wafers. After implantation, current transport across the structures is controlled by the Poole-Frenkel mechanism, with the energy of the dominating emission center equal to E _c ?0.89 eV. The maximum photosensitivity is observed for structures implanted with 3.2×10¹⁷ cm^?2 of hydrogen and annealed in the temperature range of 250–300°C. The band gap of the Si:H layer E _g ≈2.4 eV, and the dielectric constant ?≈3.2. The density of states near the Fermi level is (1–2)×10¹⁷ cm^?3 eV^?1.     

Gamma-induced trapping levels in si with and without gold doping

Because gold doping can be used to suppress ionization-induced photocurrents in Si devices exposed to radiation environments, it is of interest to know whether or not radiation-induced mobile defects interact with Au to form deep level defect complexes. Trapping level studies have been undertaken using the technique of deep level transient spectroscopy (DLTS) prior to and following Co-60 gamma irradiation and thermal annealing of p⁺/n Si junctions doped with 3-6 × 10¹⁴ Au/cm³ and 1-3 × 10¹³ Au/cm³ , and of non-Au doped p⁺ /n and n⁺ /p junctions. Prior to irradiation, the Au acceptor level at E_c - 0.57 eV is observed in the gold doped junctions. During irradiation, five additional levels at E_c - 0.18 eV, E_c - 0.20 eV, E_c - 0.46 eV, E_v +0.17 eV, and E_v + 0.39 eV grow in linearly with gamma dose. None of these levels are associated with Au because they are observed at equal concentrations in the non-Au doped and Audoped junctions, and because the Au concentration as monitored by the E_c - 0.57 eV level does not change even when defect concentrations are of the same order. Isochronal annealing studies also indicate that the Au acceptors are inert to defect re-ordering effects which occur during annealing. This work was supported by the United States Department of Energy (DOE) under contract number DE-AC04-76-DP00789. A United States Department of Energy Facility.     

Detection of3T1−3T2 and3T1−3A2 transitions of Ni by conventional Fourier transform infrared FT-IR spectroscopy

A FT-IR spectroscopic study was carried out in the region 4000–400 cm^?1 for ZnSe∶Ni and ZnS∶Ni at room temperature. The data obtained were examined on the basis of the energy states calculations of the (3d)ⁿ configuration, based on the defect molecule approach. The present investigation reveals the transition from the ground state to the first two excited states namely³T₁?³A₂ and³T₁?³T₂ of Ni⁺² (d⁸).     

Studies of defect states of ZnO thin films under different annealing conditions

ZnO thin films were grown by the pulsed laser deposition technique on c-plane sapphire substrates at a substrate temperature of 500 °C with 1×10⁻⁴ Torr ambient gas. After the deposition process, ZnO thin films were annealed at 1000 °C for 5 min under N₂ or O₂ ambient gas, respectively. In the X-ray patterns, the (0 0 2) peak of the annealed sample was shifted from that of the as-grown sample, which indicates a reduced lattice constant of about 1%. Even though the X-ray diffraction patterns in the samples annealed under O₂ and N₂ annealing gases were almost the same, photoluminescence spectra showed the generation of a shallow level with a few meV, and deep-level states were generated at E_v+0.594 eV. In addition, a defect state appeared at E_c−0.607 eV, which originated from hydrogen plasma irradiation on the ZnO sample.     

Characterization of the iron and nickel impurities in gaas0.6p0.4

Characterization of nickel and iron impurity centers in GaAs_0.6P_0.4 has been made using the technique of capacitance transients on reverse biased zinc-diffused p⁺n diodes. Both the nickel and iron levels have been identified by thermal hole emission having activation energies of 0.3eV and 0.58eV, respectively. Relative photoresponse measurements resulted in a threshold for optical hole emission of 0.3eV and 0.58eV, therefore, confirming the thermal hole emission measurements. Capture studies for nickel and iron centers indicated a relatively large capture cross-section and a strong electric field dependence. This work is supported by the National Science Foundation, Grant ENG76-80128.     

Importance of the choice of the profile model for ap-n junction in the location of deep levels

The energy levels introduced by Pt in silicon have been measured in a non-abruptp ⁺-n junction using constant-capacitance thermal-emission rate measurements and a numerical simulation of high frequency-capacitance. Two levels have been detected with activation energies of:E _c -E _T = 0.22 eV with acceptor character andE _T -E _v = 0.34 eV with donor character. The sample preparation and diffusion of Pt is similar to previous works in which an acceptor levelE _c -E _T = 0.34 eV was found instead of or besides a donorlike levelE _T -E _v = 0.34 eV. Our numerical calculation of the shallow-impurity profile points to the existence of a gradual transition near the metallurgical junction for these samples. We have demonstrated that the well-known model of an abrupt junction is not appropriate for these types of junctions, and could lead to errors in the location attributed to the detected levels. Simulation of the electrical behavior leads to the non-existence of the acceptor levelE _c −E _T = 0.34 eV located in then-side of the junction.     

Defect studies in Cu2ZnSnSe4 and Cu2ZnSn(Se0.75S0.25)4 by admittance and photoluminescence spectroscopy

To achieve higher record efficiencies for solar cells containing Cu₂ZnSnSe₄ (CZTSe), Cu₂ZnSnS₄ (CZTS) or their solid solution Cu₂ZnSn(Se_xS_1?x)₄ (CZTSSe) as an absorber, it is necessary to obtain more knowledge about defect structure of these materials. In this work, admittance spectroscopy (AS) and low temperature photoluminescence spectroscopy (PL) were used for defect studies. Admittance spectroscopy in the frequency range from 20 Hz to 10 MHz was used for studies of CZTSe/CdS and CZTSSe/CdS monograin layer heterojunctions. The measurement temperature varied from 140 K to 245 K. Two defect states (labelled E_A1 and E_A2) were found in Cu₂ZnSnSe₄ and Cu₂ZnSn(Se_0.75S_0.25)₄. In different CZTSe/CdS heterojunctions the E_A2 state was present at 74 meV, but the second E_A1 defect state changed from 87 meV to 100 meV during time and had varying properties. In Cu₂ZnSn(Se_0.75S_0.25)₄ the E_A2 state was found at 25 meV. The E_A1 state at 154 meV showed the same properties as the two defect levels in CZTSe. In both cases the E_A2 defect state was attributed to an acceptor defect and the E_A1 state with changing properties to interface states. The detected PL bands were at 0.946 eV in CZTSe and at 1.028 eV in Cu₂ZnSn(Se_0.75S_0.25)₄. Obtained by PL measurements, defect states at 69 meV in CZTSe and at 39 meV in Cu₂ZnSn(Se_0.75S_0.25)₄ were attributed to the same acceptor defect that was found from the AS measurements.     

Capacitance voltage measurements on n-type InAs MOS diodes

The electrical properties of the interface between pyrolytically deposited SiO₂ and InAs have been investigated by measuring the admittance of Al-SiO₂-InAs MOS diodes at room temperature and 77°K. The room temperature measurements yield a rather high surface state density of 2·5 × 10¹² states/eV/cm². The surface state density undergoes an anomalous decrease to 2·8 × 10¹¹ states/eV/cm² at 77°K. A charge which is linearly dependent on the voltage across the oxide is trapped in the oxide. Conduction through the oxide causes the diodes to enter a deep depletion condition at 77°K.     

The influence of hydrogen gas on the characteristics of amorphous silicon deposited by RF sputtering

The electrical and optical properties of a-Si depend greatly on the fill gas and substrate temperature during RF sputtering. From the measured absorption coefficient, it was estimated that after the introduction of H₂ gas during sputtering the gap state density reduces from 3.2 × 10¹⁹ cm^?3 to 4.8 × 10¹⁷ cm^?3. As a consequence the optical band gap was found to increase from 1.74 to 1.82 eV. The d.c. conductivity measurement shows three distinct conducting mechanisms at different temperature regions. The SiH bonds in RF sputtered samples are persistant to higher temperature treatment than the CVD prepared ones.     

Optically active interface states in MOS structures

The results of studies of the interface states in clean n-type MOS capacitors by the low-temperature photocapacitance technique are reported. Energy distribution of the interface states density and photoionization cross-section are determined from the analysis of the photocapacitance kinetic. It is shown that the energy distribution of the interface state density can be described by the Gaussian distribution with maximum at about 0.7 eV below the conduction band edge and standard deviation of about 0.1 eV. The photoionization cross-section of these states can be described by the Lucovsky relationship. The maximum of the photoionization cross-section vs photon energy was equal to about 5 × 10^?20/(E_c ? E_ss) cm² for a given energy of the interface states E_ss.From the comparison of the photocapacitance and quasi-static C-V measurements the existence of two types of interface states is suggested: “optically active states” with density decreasing towards the band edges, and “optically inactive states” with density increasing towards the band edges. No photocapacitance associated with “optically inactive states” was observed.     

Processing and reconstruction effects on Al-GaAs(100) barrier heights

We have used low-energy electron diffraction, soft x-ray photoemission, and cathodoluminescence (CLS) spectroscopies to investigate the effects of the GaAs(100) surface geometry and composition on the formation of electrically active interface states at Al-GaAs(100) contacts. Clean GaAs(100) surfaces in the 350 to 620°C annealing temperature range undergo large compositional, structural, and electronic changes with temperature. Aluminum thin film deposition induces new discrete deep level CLS features between ∼0.80 and 1.20 eV photon energy, whose properties depend sensitively and systematically on surface annealing temperature and/or reconstruction. Fermi level (E_F) stabilization energies at these interfaces span the range from 0.58 eV above the valence band maximum (E_v) for an arsenic-rich starting surface to 0.46 eV above E_v for a gallium-rich starting surface. Correlation between the aluminum-induced deep level energies and the interface E_F position suggests an important role of localized bandgap states in determining the interface barrier height. The sensitivity of these states to starting surface composition and reconstruction may open new possibilities for tailoring Schottky barrier properties.