Study of electrically active defects in n-GaN layer

Deep level defects in both p⁺/n junctions and n-type Schottky GaN diodes are studied using the Fourier transform deep level transient spectroscopy. An electron trap level was detected in the range of energies at E_c−E_t=0.23–0.27 eV with a capture cross-section of the order of 10⁻¹⁹–10⁻¹⁶ cm² for both the p⁺/n and n-type Schottky GaN diodes. For one set of p⁺/n diodes with a structure of Au/Pt/p⁺–GaN/n–GaN/n⁺–GaN/Ti/Al/Pd/Au and the n-type Schottky diodes, two other common electron traps are found at energy positions, E_c−E_t=0.53–0.56 eV and 0.79–0.82 eV. In addition, an electron trap level with energy position at E_c−E_t=1.07 eV and a capture cross-section of σ_n=1.6×10⁻¹³ cm² are detected for the n-type Schottky diodes. This trap level has not been previously reported in the literature. For the other set of p⁺/n diodes with a structure of Au/Ni/p⁺–GaN/n–GaN/n⁺–GaN/Ti/Al/Pd/Au, a prominent minority carrier (hole) trap level was also identified with an energy position at E_t−E_v=0.85 eV and a capture cross-section of σ_n=8.1×10⁻¹⁴ cm². The 0.56 eV electron trap level observed in n-type Schottky diode and the 0.23 eV electron trap level detected in the p⁺/n diode with Ni/Au contact are attributed to the extended defects based on the observation of logarithmic capture kinetics.     

Study of grown-in defects and effect of thermal annealing in Al0.3Ga0.7As and GaAs LPE layers

Studies of the grown-in deep-level defects in the undoped n-Al_xGa_1-xAs (x = 0.3) and GaAs epitaxial layers prepared by the liquid phase epitaxy (LPE) techniques have been made, using DLTS, I-V and C-V measurements. The effect of 300 °C thermal annealing on the grown-in defects was investigated as a function of annealing time. The results showed that significant reduction in these grown-in defects can be achieved via low temperature thermal annealing process. The main electron and hole traps observed in the Al_0.3Ga_0.7As LPE layer were due to the E_c-0.31 eV and E_v+0.18 eV level, respectively, while for the GaAs LPE layer, the electron traps were due to the E_c-0.42 and 0.60 eV levels, and the hole traps were due to E_v+0.40 and 0.71 eV levels. Research supported in part by the Air Force Wright Aeronautical Laboratories, Aeropropulsion Lab., Wright Patterson Air Force Base, Ohio, subcontract through SCEEE, contract F33615-81-C-2011, task-4, and in part by AFOSR grant no. 81-0187.     

A DLTS analysis of electron and hole traps in VPE grown n-GaAs using schottky barrier diodes

Capacitance lock-in amplifier deep level transient spectroscopy (DLTS) using Schottky barrier diodes (SBD’s) was used to characterize the electron and hole traps in VPEn-GaAs (N_D - N_A = 1 - 2 x 10¹⁵/cm³) layers grown on n⁺ (10¹⁸/cm³) GaAs substrates. The main electron traps observed were the EL2 atE _c - 0.81 eV and a level atE _c - 0.48 eV. The use of large forward bias electrical injection pulses (and no optical excitation) facilitated the detection of hole traps, of which the defect with an energy level atE _v + 0.42 eV, speculated to be Cu-related, was present in the highest concentration.     

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.     

Effective recombination levels in n- and p-type silicon irradiated by 4·5 MeV electrons

The effective recombination levels created at room temperature by 4·5 MeV electron irradiation are deduced from the variations in lifetime vs carrier injection rate, electron fluence, and temperature. This paper aims to compare the properties of the created recombination energy levels and defect centers in N- and P-type silicon single crystals. The characteristics of the samples used extend over a wide range of resistivities, doping impurities and crystal growth techniques. A pulsed neodymium laser has been employed to carry out these studies, and the carrier lifetime has been measured by the photoconductivity-decay method. Information on the specific centers is deduced from the comparison of the present macroscopic results on energy levels and annealing studies with the known properties of microscopic defects.From the results obtained, several types of recombination centers are simultaneously created in N- and P-type silicon, and crystal impurities other than oxygen and dopants may play a big part in the constitution of such centers. In the P-type silicon case, 3 types of recombination centers are clearly operative: (1) centers with a ～E_v+0·20 eV energy level, which could be divacancies, and which would cease to act as recombination centers by trapping irradiation induced interstitial carbon atoms, (2) centers with a ～E_v+0·24 eV level which may involve aluminium interstitial atoms, and finally (3) centers with a ～E_v+0·27 eV level, which are K centres. These recombination centers are more or less active, depending on the initial characteristics of the sample. In the N-type silicon case, only two groups of effective recombination levels, ～E_c?0·17 and E_v+0·3 eV, appear in the irradiated materials. However, the effects of centers possibly linked to the presence of contaminants, such as carbon and aluminium, must be added to the known effects of the divacancy, doping atom-vacancy and oxygen-vacancy complexes to explain the carrier lifetime degradation and recovery.     

Deep level studies in MBE GaAs grown at low temperature

The photo-induced current transient spectroscopy (PICTS), thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) spectroscopy have been used to characterize the deep levels in the GaAs materials grown at low temperature by molecular beam epitaxy. At least five hole traps and five electron traps have been identified by the TEES measurement employing a simplified sample arrangement. We have studied the behavior of various traps as a function of the growth temperature and the post-growth annealing temperature. Some of the shallower hole traps were annealed out above 650‡ C. Electron traps atE _c- 0.29 eV andE _c- 0.49 eV were present in the material, and have been identified as M3 and M4, respectively. The dominant electron trap, atE _c- 0.57 eV, is believed to be associated with the stoichiometric defect caused by the excess As in the material, and our data show evidence of forming a defect band by this trap. A possible model involving As precipitates is proposed for this trap atE _c-0.57 eV.     

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     

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.     

Electrical characterisation of Cu-Diffusedn-GaAs epitaxial layers using optical deep level transient spectroscopy

Undopedn ^--GaAs epitaxial layers were grown by OMVPE onn ⁺ (1 × 10¹⁸ cm³) and semi-insulating (SI) GaAs substrates. The as-grown epitaxial layers grown onn ⁺ substrates contained several deep level defects whereas those grown on SI substrates were, apart from the EL2, virtually “defect free”. Upon Cu diffusion, deep levels which may reduce hole and electron diffusion lengths and lifetimes, were formed. Optical deep level transient spectrocopy (ODLTS) has been used to identify such levels atE _v + 0,41 eV andE _c-0,31 eV respectively. The EO1 (EL2) trap concentration reduced after Cu had been diffused into the epitaxial layers. The magnitude of this reduction was approximately equal to the concentration of the trap found atE _c - 0,31 eV which suggests that the two may be related. Activation energies and capture cross-section values are presented for the deep levels detected in these epitaxial layers.     

Stoichiometry-dependent deep levels in undoped p-type Al0.38Ga0.62As grown by liquid phase epitaxy

Photocapacitance (PHCAP) and photoluminescence (PL) measurements were applied to unintentionally doped p-type Al_0.38Ga_0.62As grown by liquid phase epitaxy using the temperature difference method under controlled vapor pressure. PHCAP spectra revealed three dominant deep levels at E_v+0.9, E_v + 1.45, and E_v+1.96 eV, and a deep level at E_v+0.9−1.5 eV which was not neutralized by forward bias injection. These level densities increase with increasing arsenic vapor pressure and net shallow acceptor density. Furthermore, PL spectra reveal a deep level at 1.6–1.7 eV. The PL intensity of this deep level increases with increasing arsenic vapor pressure. These deep levels are thought to be associated with excess As.     

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 ...)     

Recombination and trapping centers in pure and doped TlBr crystals

TlBr is a promising wide-gap semiconductor for developing γ-radiation detectors. One of the limiting factors in developing the technology of detectors is the lack of experimentally determined trapping and recombination centers. In this paper, a generalized model of the formation and behavior of intrinsic defects in pure and doped TlBr single crystals is presented. The relation of intrinsic defects to growth conditions and electrical properties is determined. The previously obtained temperature dependences of the photoconductivity, the data of current deep level transient spectroscopy and microcathodoluminescence, and the kinetic characteristics of the photoconductivity are used as objects of analysis. It is shown that the compensation of charged centers control the transport properties of charge carriers. In compensated doped TlBr crystals, the product of the mobility and lifetime can reach μτ = 5 × 10^?4 cm² V^?1. The energy-level diagram of local levels in pure and doped TlBr crystals is proposed. The ionization energies of major structural and impurity defects in TlBr, i.e., the anion vacancy V _a ⁺ , cation vacancy V _c ^? , and Pb²⁺, O^2?, S^2? ions, are determined. The energy position of a single anion vacancy V _a ⁺ is E _c ? 0.22 eV. The energy level of the cation vacancy is E _v + 0.85 eV for a single cation vacancy and E _v + 0.58 eV for a vacancy incorporated into the {Pb²⁺ V _c ^? }⁰ complex. The ionization energy of the Pb²⁺ Coulomb trap is E _c ? 0.08 eV in doped TlBr crystals.     

Modification of the spectrum of electronic states in polycrystalline p-CdTe as a result of annealing in Cd vapors or natural aging

Electrical properties, steady-state photoconductivity, and kinetics of photoconductivity have been studied in the samples of polycrystalline p-CdTe (as-grown by the method of low-temperature synthesis from thoroughly purified components; subjected to subsequent annealing in Cd vapors; and aged at room temperature). It is shown that electrical properties of as-grown p-CdTe are controlled by a complex defect with the level at E _v + 0.16 eV; the latter transforms into the level E _v + 0.25 eV as a result of annealing in Cd vapors and aging of the samples. In addition, the deep centers with the levels E _v + 0.6 eV and E _v + 0.86 eV were detected; these centers govern the decay of the photocurrent.     

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.     

Au and non-Au based rare earth metal-silicide ohmic contacts to p-InGaAs

The aim of this study is to improve the electrical properties of ohmic contacts that plays crucial role on the performance of optoelectronic devices such as laser diodes (LDs), light emitting diodes (LEDs) and photodetectors (PDs). The conventional (Pd/Ir/Au, Ti/Pt/Au and Pt/Ti/Pt/Au), Au and non-Au based rare earth metal-silicide ohmic contacts (Gd/Si/Ti/Au, Gd/Si/Pt/Au and Gd/Si/Pt) to p-InGaAs were investigated and compared each other. To calculate the specific contact resistivities the Transmission Line Model (TLM) was used. Minimum specific contact resistivity of the conventional contacts was found as 0.111 × 10⁻⁶ Ω cm² for Pt/Ti/Pt/Au contact at 400 °C annealing temperature. For the rare earth metal-silicide ohmic contacts, the non-Au based Gd/Si/Pt has the minimum value of 4.410 × 10⁻⁶ Ω cm² at 300 °C annealing temperature. As a result, non-Au based Gd/Si/Pt contact shows the best ohmic contact behavior at a relatively low annealing temperature among the rare earth metal-silicide ohmic contacts. Although the Au based conventional ohmic contacts are thermally stable and have lower noise in electronic circuits, by using the non-Au based rare earth metal-silicide ohmic contacts may overcome the problems of Au-based ohmic contacts such as higher cost, poorer reliability, weaker thermal stability, and the device degradation due to relatively higher alloying temperatures. To the best of our knowledge, the Au and non-Au based rare earth metal-silicide (GdSi_x) ohmic contacts to p-InGaAs have been proposed for the first time.     

Drift mobility and photoluminescence measurements on high resistivity Cd1−xZnxTe crystals grown from Te-Rich solution

The drift mobilities of chlorine doped high resistivity Cd_0.8Zn_0.2 Te have been investigated by using the time-of-flight technique. Electron as well as hole mobility in the as-grown crystals is limited by trap-controlled carrier transport. The energy locations of the defects responsible for carrier trapping are determined to be E_c- 0.03 and E_v+ 0.14 eV for electrons and holes, respectively. After annealing at 400°C for 80 h, no evidence of trap-controlled mobility was recognized for electrons. On the other hand, no significant change before and after the annealing was observed for hole transport. Those results and the change in the photoluminescence spectra before and after the annealing are explained by the complex defect model composed of the Cd vacancy and chlorine donor. Further, the alloy scattering potentials of ΔU_e and ΔU_h were estimated by employing the theoretical calculation method recently reported by D. Chattopadhyay [Solid State Commun. 91, 149 (1994)].     

Doping and impurity compensation by ion implantation in a-SiGe films

This paper discusses the electrical properties of a-SiGe films (N _Ge∼2.2 at. %) prepared by co-evaporation of Si and Ge from separate sources and doped by ion implantation of substitutional impurities (B⁺ and P⁺), as well as the results of controlled impurity compensation by ion-beam doping. It was found that B⁺ and P⁺ implantation into a-SiGe films in the dose range 1.3×10¹⁴–1.3×10¹⁷ cm⁻², followed by annealing at 350 °C, increased the conductivity of these films from 10⁻⁹ to 10⁻⁴ and to 10⁻⁵ S/cm for B⁺ and P⁺, respectively. The position of the Fermi level could be varied from (E _v+0.27) to (E _c−0.19) eV. These investigations indicate that compensation of pre-doped a-SiGe films by ion implantation is feasible and reproducible. It is also found that higher doping efficiency of a-SiGe films is obtained by using boron than by using phosphorus. Fiz. Tekh. Poluprovodn. 32, 1260–1262 (October 1998)     

Defect levels in chromium-doped silicon

The transient capacitance technique has been used to study the chromium-related levels in the silicon band gap. Chromium was diffused at temperature of 1100 and 1150°C for 0.5 and 3 hr. Five different levels at E_c?0.11 eV, E_c?0.21 eV, E_c?0.28 eV, E_c?0.36 eV and E_c?0.45 eV were obtained from the Arrheniu plots of the electron thermal-emission rates. The number of levels in the upper half of the band gap decreased from five to two with an increase of Cr-diffusion period. Two levels were located at E_c?0.20 eV (donor) and E_c?0.43 eV (acceptor). A donor level was also observed at E_v + 0.25 eV. The donor level was not affected by the diffusion condition. The majority carrier capture cross sections of the three dominant levels have been measured by the transient capacitance technique modified by the pulse transformer. The values were σ_n = 4.1 × 10^?15 cm² for the upper donor at E_c?0.20 eV, σ_n = 2.0 × 10^?16 cm² for the acceptor at E_c ?0.43 eV and σ_p = 9.1 × 10^?18 cm² for the lower donor at E_v + 0.25 eV, and were independent of temperature. The three dominant levels are due to distinct chromium centers.     

Detection of hydrogen impurity in silicon radiation detectors

The behavior of silicon particle detectors irradiated with 2.5-MeV electrons during subsequent annealing is studied. Annealing at 100–250°C was found to result in the formation of two types of traps with the levels E _c ?0.32 eV and E_v+0.29 eV. Increasing the annealing temperature to 300°C makes both traps disappear. On the basis of data obtained, it was concluded that these traps are related to hydrogen-containing complexes. The presence of hydrogen in a crystal results in a decrease in the annealing temperature for vacancy-oxygen (VO) complexes and complexes consisting of carbon and oxygen interstitials (C_iO_i). The reason for this phenomenon is the passivation of these complexes by hydrogen, which results in the formation of electrically active VOH centers {with the level E _c ?0.32 eV} in an intermediate stage of this process. It is assumed that hydrogen penetrates the structures under investigation in one of the stages of their fabrication.     

Isochronal annealing study of hydrogen interaction with implantation-induced point defects in p-type silicon

Isochronal annealing with zero and reverse bias applied to Schottky diodes was used to monitor the evolution of hydrogen interaction with point defects observed in hydrogen-implanted p-type silicon, i.e., divacancy (VV), carbon–oxygen interstitial pair (C_iO_i) and two levels at E_v+0.28 and E_v+0.50 eV. The VV and C_iO_i are passivated by hydrogen liberated from hydrogen-containing defects during annealing in the temperature range 90–150°C and reappear upon annealing above 180°C under reverse bias due to hydrogen liberation and its field drift. Two levels at E_v+0.50 and E_v+0.28 eV are ascribed to irradiation-induced and hydrogen-related defects, respectively.