Concentration Of Deep Acceptors Research Articles

Transport and trap states in proton irradiated ultra-thick κ-Ga2O3

Changes induced by irradiation with 1.1 MeV protons in the transport properties and deep trap spectra of thick (>80 μm) undoped κ-Ga2O3 layers grown on sapphire are reported. Prior to irradiation, the films had a donor concentration of ∼1015 cm−3, with the two dominant donors having ionization energies of 0.25 and 0.15 eV, respectively. The main electron traps were located at Ec−0.7 eV. Deep acceptor spectra measured by capacitance-voltage profiling under illumination showed optical ionization thresholds near 2, 2.8, and 3.4 eV. The diffusion length of nonequilibrium charge carriers for ɛ-Ga2O3 was 70 ± 5 nm prior to irradiation. After irradiation with 1.1 MeV protons to a fluence of 1014 cm−2, there was total depletion of mobile charge carriers in the top 4.5 μm of the film, close to the estimated proton range. The carrier removal rate was 10–20 cm−1, a factor of 5–10 lower than in β-Ga2O3, while the concentration of deep acceptors in the lower half of the bandgap and the diffusion length showed no significant change.

On the nature of photosensitivity gain in Ga2O3 Schottky diode detectors: Effects of hole trapping by deep acceptors

Lightly n-type doped Ga2O3 layers grown by Halide Vapor Phase Epitaxy (HVPE) on bulk n+-Ga2O3 substrates were subjected to irradiation with fast reactor neutrons, 20 MeV protons, or treatment in high ion density Ar plasma. These treatments lead to a marked increase in the concentration of deep acceptors in the lower half of the bandgap. These acceptors have optical ionization thresholds near 2.3 eV and 3.1 eV. There is a simultaneous strong enhancement of the photocurrent of Schottky diodes fabricated on these layers in the UV spectral range, and a large increase in the Electron Beam Induced Current (EBIC) collection efficiency. The gain in photocurrent at −10 V reached 18 times for neutron and proton irradiated samples, and 104 times for the plasma treated samples. Similar increases in gain were observed in the EBIC current collection efficiency for beam energy 4 keV. With such beam energy, the electron–hole pairs are generated well within the space charge region. The results are explained by assuming that the capture of photoinduced or electron-beam-induced holes by the deep acceptors gives rise to a decrease in the effective Schottky barrier height and an increase of the electron current flow that is responsible for the observed high gain. The reported observation could form a basis for radical improvement of photosensitivity of Ga2O3-based solar-blind photodetectors. However, the photocurrent build-up and decay times in this mechanism are inherently long, on the order of some seconds.

On the threshold voltage of normally-OFF AlGaN/GaN heterostructure field effect transistors (HFETs) with p-(Al)GaN gate

A physics-based analytical model has been developed to correctly estimate the threshold voltage of normally-OFF AlGaN/GaN heterostructure field effect transistors (HFETs) with p-(Al)GaN gate. This analysis considers the effect of all polarization charges at different interfaces/surface, the layer structure, doping concentration of deep acceptors in p-(Al)GaN cap layer, incomplete ionization of deep Mg acceptors, out-diffusion of Mg to AlGaN barrier layer as well as the effect of compensated buffer/back barrier. Threshold voltage extracted from this analytical model is rigorously validated with experimental results. This analysis also provides a physical insight of charge distribution, gate voltage division in the heterostructure and the effect of device parameters on the threshold voltage. Besides, this model can also be extended to estimate the threshold voltage of normally-OFF MIS-HFETs with p-GaN gate.

Defects at the surface of β-Ga2O3 produced by Ar plasma exposure

Films of β-Ga2O3 grown by halide vapor phase epitaxy on native substrates were subjected to Ar inductively coupled plasma treatment. As a result, the built-in voltage of Ni Schottky diodes deposited on the plasma treated surfaces decreased from 1 V to −0.02 V due to the buildup of deep trap concentration in the near surface region. Deep level spectra measurements indicate a strong increase in the top ∼200 nm of the plasma treated layer of the concentration of E2* (Ec − 0.8 eV) and especially E3 (Ec − 1.05 eV) deep electron traps. Capacitance-voltage profiling with monochromatic illumination also indicated a large increase in the upper ∼100 nm of the film in the concentration of deep acceptors with optical threshold for an ionization of ∼2.3 eV and 3.1 eV. Such defects at the surface led to a significant increase in reverse current, an increase in the ideality factor in forward current, and a dramatic decrease in the diffusion length of nonequilibrium charge carriers from 450 nm to 150 nm.

Voltage-Induced Charge Redistribution in Cu(In,Ga)Se2 Devices Studied With High-Speed Capacitance–Voltage Profiling

Devices made from Cu(In,Ga)Se2 (CIGS) solar cell material have been evaluated with high-speed capacitance–voltage profiling after stepwise voltage changes. The changes primarily affect near-interface charge at deep acceptors within the CIGS absorber layer and generate temperature-dependent capacitance changes observed in deep-level transient spectroscopy measurements. SCAPS device modeling indicates that the deep acceptor concentration is up to the two orders of magnitude higher than the shallow doping level. High deep acceptor concentrations are found in all materials studied here. The large deep defect levels are high enough to limit minority carrier lifetime and cell efficiency.

Anneal induced transforms of radiation defects in heavily electron irradiated Si diodes

In this work, the spectra of deep traps attributed to radiation defects in n-type Czochralski (CZ) grown Si with dopant concentrations of 1015 cm−3 have been studied. Samples were irradiated with 6.6 MeV electrons with fluences in the range of (1–5) × 1016 e/cm2. Irradiated samples were annealed using isochronal 24 hour heat treatments in nitrogen (N2) gas at ambient temperature and by discretely varying the temperature from 80 °C to 280 °C. The as-irradiated and annealed samples were studied by combining capacitance-voltage (C-V), deep level transient spectroscopy (DLTS) and temperature dependent carrier trapping lifetime (TDTL) measurements. Schottky barrier contacts and ohmic contacts were deposited on the samples by Au magnetron sputtering and metal sintering, respectively, for C-V and DLTS measurements after each heat treatment procedure. In order to reduce the uncertainties due to contact quality, TDTL contact-less measurements were performed using microwave probed photoconductivity transient (MW-PC) technique. Using these techniques, we observe suppression of the rather deep carrier emission centres after heat treatment at elevated temperatures. However, the reduction of deep acceptor concentrations after anneals at elevated temperatures was observed to be accompanied by considerable increase of shallow acceptor concentrations which effectively compensate the dopants. The activation energies of the predominant peaks within the spectrum, displayed as temperature dependent carrier trapping lifetime variations, were measured and are shown to be in good agreement with those obtained from the DLTS spectra.

Numerical analysis of short‐gate GaN HEMTs with Fe‐doped buffer layers

AbstractThe influence of Fe‐doped GaN buffer layers on microwave GaN HEMT performance was analyzed using a two‐dimensional numerical model. The device structure model was based on a fabricated Ka‐band AlGaN/GaN HEMT with a 0.2‐µm gate. Deep acceptors brought about by Fe doping were introduced in the GaN buffer layer model, and the concentration profile was based on the Fe concentration profile measured by SIMS. The slow turn‐off of Fe doping was taken into account. The simulated drain current showed a sharp current‐off in the 0.2‐µm gate device, and it was found that this characteristic was influenced by the slow turn‐off Fe‐doping profile. To discuss the effect on drain current collapse in high‐frequency large‐signal operation, ionized deep acceptor concentrations were simulated and compared between the on‐state and off‐state. The results showed that the ionized concentration increases in the off‐state in the area of slow turn‐off Fe‐doping. It is expected that the ionization is one of the causes of drain current collapse in the 0.2‐µm gate device. The simulated results also indicated enhanced collapse in the 0.2‐µm gate device in comparison with the results for a 0.6‐µm gate device (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Converse piezoelectric strain in undoped and Fe-doped AlGaN/GaN heterostructure field effect transistors studied by Raman scattering

Converse piezoelectric strain in undoped and Fe-doped AlGaN/GaN heterostructure field effect transistors (HFETs), i.e. the strain induced by applying bias to a transistor, was studied using micro-Raman scattering spectroscopy as a function of applied source–drain voltage for different GaN buffer doping levels and substrate types. By monitoring the phonon frequency shifts and line width of the E2 and A1(LO) phonon modes of GaN, a considerable piezoelectric strain/stress was found in undoped devices, which exhibited a saturation above 40 V bias. This saturation voltage was used to quantify the deep acceptor concentration in the GaN buffer layer. Using experimental Raman data and numerical modelling of the electric field distribution in the device, it was furthermore established that Fe doping causes confinement of the strain/stress to the vicinity of the AlGaN/GaN interface, i.e. near the electron channel, with potential implications for device reliability. It was concluded that varying the structure and doping in the buffer layer has the potential to modify the converse piezoelectric strain and hence affect reliability issues in AlGaN/GaN HFETs.

Effects of Ga Compositional Grading on CIGS Electronic Properties Relevant to Solar Cell Performance

AbstractTransient photocurrent (TPI) and photocapacitance (TPC) spectroscopy have been applied to a set of compositional graded CuIn1-xGaxSe2 (CIGS) solar cell devices deposited by the vacuum co-evaporation method at the National Renewable Energy Laboratory. These measurements provide a spectral map of the optically induced release of carriers for photon energies from below 1 eV to 2 eV. By comparing the two types of spectra one can distinguish majority from minority carrier processes and they clearly reveal a higher degree of minority carrier collection for devices in which the Ga fraction increased monotonically with distance from the junction. This agrees with notions of how compositional grading improves overall cell performance. Minority carrier collection was even more strongly enhanced in sample devices incorporating v-shaped Ga-grading. Spatial profiles of the free hole carrier densities and deep acceptor concentrations were examined using drive-level capacitance profiling (DLCP). In the compositionally graded sample devices we found that the free carrier density decreased and that defect density increased with increasing Ga fraction toward back contact.

Epitaxial silicon detectors for particle tracking—Radiation tolerance at extreme hadron fluences

Diodes processed on n-type epitaxial silicon with a thickness of 25, 50 and 75 μm had been irradiated with reactor neutrons and high-energy protons (24 GeV/ c) up to integrated fluences of Φ eq=10 16 cm −2. Systematic experiments on radiation-induced damage effects revealed the following results: in contrast to standard and oxygen-enriched float zone (FZ) silicon devices no space charge sign inversion was observed after irradiation. It is shown that the radiation-generated concentration of deep acceptors, dominating the behavior of n-type FZ diodes, is compensated by creation of shallow donors. Thus a positive space charge is maintained throughout the irradiation up to the highest fluence and even during prolonged elevated-temperature annealing cycles. Defect analysis studies using thermally stimulated current measurements attribute the effect to a damage-induced shallow donor at E C−0.23 eV. It is argued that, as in the case of thermal donors, oxygen dimers, out diffusing from the Cz substrate during the diode processing, are responsible precursers. Results from extensive annealing experiments at elevated temperatures are verified by comparison with prolonged room-temperature annealing. These results showed that in contrast to FZ detectors, which always have to be cooled, room-temperature storage during beam off periods of future elementary particle physics experiments would even be beneficial for n-type epi-silicon detectors. A dedicated experiment at CERN-PS had successfully proven this expectation. It was verified, that in such a scenario the depletion voltage for the epi-detector could always be kept at a moderate level throughout the full S-LHC operation (foreseen upgrade of the large hadron collider). Practically no difference with respect to FZ-silicon devices was found in the damage-induced bulk generation current. The charge trapping measured with 90Sr electrons (mip's) is also almost identical to what was expected. A charge collection efficiency of 60% (2500 e) in 50 μm n-type epi-diodes after 24 GeV/ c proton irradiation with Φ eq=6.2×10 15 cm −2 was reported recently, independent of the operating temperature down to −20 °C.

Resistivity analysis on n-semi-insulating-n and p-semi-insulating-p structures exemplified with semi-insulating InP

Current–voltage characteristic in semi-insulating (SI) InP layers sandwiched between n- or p-type layers have been simulated. Deep acceptor and deep donor levels were considered. A one-dimensional two-carrier numerical model was used, which takes into account carrier diffusion and Shockley–Read–Hall recombination through the deep traps. The current–voltage behavior is studied in the linear ohmic regime in order to deduce the resistivity, which is analyzed and compared to values obtained from equilibrium calculations. The simulations predict an increased resistivity for n-SI-n and a decreased resistivity for p-SI-p structures, while increasing the concentration of deep acceptors. Furthermore, a maximum in the resistivity as a function of the trap concentration is found for an n-SI-n structure, which accommodates electron injection. In addition, the influence of the position of the deep acceptor in the band gap on the resistivity is analyzed, and it is shown that it significantly affects the behavior when comparing n-SI-n and p-SI-p structures. Incorporating a deep donor level along with a deep acceptor level will improve the SI behavior of the p-SI-p structure, whereas a slight drop in resistivity occurs for the n-SI-n structure. Our predicted electrical behavior from simulation is found to be in accordance with the previously published experimental results in InP:Fe and InP:Fe,Ti.

Recombination processes in erbium-doped MBE silicon

The incorporation of erbium from a solid source into molecular beam epitaxy (MBE) Si and Si/Ge alloys grown at substrate temperatures of 500 degrees C and 700 degrees C has been studied by photoluminescence, electrical measurements, secondary-ion mass spectrometry (SIMS), Rutherford backscattering (RBS) and transmission electron microscopy (TEM). Erbium concentrations between 1018 and 1022 cm-3 were obtained but the maximum photoluminescence intensity was from samples with an erbium concentration of 2*1018 cm-3. Above this concentration the onset of erbium precipitation could just be observed by TEM. The authors found no shallow donors or acceptors attributable to erbium but they observed a high concentration of deep acceptors with an activation energy of 360 meV; these may be due to impurities in the erbium source rather than being directly related to the rare earth. Implantation with oxygen is found to enhance the Er3+-related photoluminescence signal when measured at temperatures greater than 77 K but to have little effect on the low-temperature luminescence. A detailed study of the temperature dependence of the luminescence reveals tree quenching mechanisms with average activation energies of approximately 5, 20 and 130 meV. The authors attribute the first two to de-excitation effects in the matrix, and the last to processes competing with the internal 4f transition.

Electron accumulation in AlGaSb/InAs/AlGaSb quantum well system

The mechanism of the electron accumulation in the AlGaSb/InAs/AlGaSb QW system with Al composition higher than 0.5 is discussed. In this QW system, it is found that the dominant electron source is donor levels in AlGaSb rather than interface levels which have, so far, been proposed as the electron source. We have found two contradictory results, that undoped AlGaSb shows p-type conduction and Be-doping is compensated electrically, are satisfactorily explained by the model of coexistence of donors and a larger concentration of deep acceptors. Electron accumulation in InAs QW is shown to be controllable by shifting the quantum level with respect to the deep acceptor level.

Optical characterisation of acceptors in doped and undoped VPE InP

The compensation ratios (= N A/ N D) of a large number of InP layers, undoped and doped with S and Si donors, have been determined by analysis of photoluminescence spectra. Good agreement with electrical estimates of θ was obtained for uniform samples with θ⪅0.4, while non-uniform samples and samples where the electrical data indicated high compensation showed large discrepancies. A good correlation was obtained with deep acceptor concentrations from DLTS measurements, indicating that the compensation is dominated by deep acceptors except at low carrier densities where the shallow (Zn) acceptors are important. N A was found to increase with carrier density, independent of dopant, and the increase was dominated by the deep acceptors.

Electrical and optical properties of deep levels in MOVPE grown GaAs

In this paper we present a characterization and suggest an identification of deep levels that are present in our MOVPE grown GaAs. The undoped material is n-type with carrier concentrations of 3 − 7 × 10 15 cm -3. From low temperature photoluminescence we identify carbon as the dominant shallow acceptor impurity. Deep Level Transient Spectroscopy (DLTS) investigations show a dominant electron trap ≈0.74 eV below the conduction band edge. This level is known in the literature as the “O”-level, EL2 etc. The concentration of this defect is in our material in the range 5 × 10 13 − 3 × 10 14 cm -3, while other deep electron traps are present in concentrations ≲3 × 10 12 cm -3. In n-doped layers we also observe the 1.2 eV luminescence band corresponding to a level positioned ≈0.2 eV from the valence band edge. This band is not detectable in undoped material. The n-doping incorporation and the deep acceptor concentration in n-type material are found to be in agreement with recent calculations by Hurle. For the “O”-level concentration we have, for the first time, found a unique dependence on the AsH 3 partial pressure, N T ∝ P 1 /2 AsH 3 . Comparison with theoretical calculations suggests that the “O”-level is directly related to gallium vacancies.

Reverse characteristics of rectifying TeSeCd structures

Current density j, measured as a function of reverse voltage V R in rectifying TeSeCd structures, is found to be much lower under pulsed than steady voltage conditions. For pulsed voltages, it is observed that ln j is proportional to (V 0 + V R) l n , with n between 2 and 3 and V 0 = 0.5 V. This form of dependence is consistent with reduction of the barrier potential by the electric field in the depletion layer. As the applied frequency is increased from 20 to 10 6 Hz, the measured parallel junction capacitance is found to show an approximate 4 to 1 decrease, with the frequency-dependent portion varying as ( frequency) −1 2 at higher frequencies. This is in accordance with theory in the literature for deep acceptors, which are thus considered to be dominant in the selenium of the structures. Trap release times, determined within the depletion layer, are found to decrease with increase in the total concentration of deep acceptors.

Electric field and impurity concentration effects on the ionization energy of impurities. Application to acceptors in ZnTe

The ionization energies of donor and acceptor centers measured by thermal stimulation methods are systematically lower than the energies deduced from optical measurements. This result can be explained by the perturbations of the emission probability due to tunneling, Poole-Frenkel effect and coulombic interaction between impurities. The effect of the last two terms is calculated in realistic situations corresponding to DLTS or admittance spectroscopy experiments and compared with experimental results obtained for three impurities (Au, Cu, Ag) in ZnTe. After correction the thermal ionization energies are well identified with the optical ones if the deep acceptor concentration does not exceed too much that of the shallow ones.

Optical and electrical properties of CdGeAs2

Large-grained ingots of CdGeAs2 have been grown from near-stoichiometric melts. Resistivity and Hall coefficient (RH) measurements were made on a large number of samples at 77 and 300 K and in some cases up to 450 K. Good fits to the log RH vs 1/T plots are obtained by using a model that assumes three kinds of electronic levels within the energy gap: donors, shallow acceptors, and acceptors with an ionization energy of 0. 30 eV. The deep acceptors are probably native defects, since their concentration varies by nearly four orders of magnitude from ingot to ingot with little change in impurity concentration. Between the intrinsic absorption edge at about 2. 5 Μm and the two-phonon absorption band at 18 Μm, the optical absorption increases with increasing deep acceptor concentration. By using oriented single-crystal samples ∼ 1 cm on a side, conversion efficiencies as high as 27% have been achieved for second-harmonic generation with single-mode pulses from a CO2 TEA laser.

Reverse bias capacity effects and defect energy levels in electron irradiated gaasp+-njunctions

Abstract Results are presented of capacity as a function of reverse bias for GaAs p + n solar cells before and after irradiation by fast electrons. The effect on capacity measurements of the presence of additional donor or acceptor defects on the n-type side is disccssed theoretically. It is shown that a variety of characteristic effects can occur depending on the position of the defect energy level in the band gap and the associated time constant for electron emission and capture. It is deduced that the defects introduced by irradiation with a predominant influence on the results were a uniform concentration of deep acceptors with levels in the upper half of the gap. Other defects with deep levels in the top half of the gap were present before irradiation. The time constants were greater than 3 × 10−3 secs. It is inferred that the levels of other acceptors known to be introduced by electron irradiation are shallow and in the top half of the gap, and those of donors in the lower half. More tentatively it ...

Anomalous Capacitance in Gallium Phosphide Electroluminescent p-n Junctions

Anomalous capacitance-voltage and current-voltage characteristics have been observed in GaP p-n junctions. Capacitance in these diodes abruptly decreases in the forward direction when the applied voltage exceeds 1.5 V, and is largely frequency-dependent above 100 kHz. Moreover, both capacitance and parallel conductance are temperature-dependent above 100 kHz. The observed anomalies have quantitatively been explained by a model in which deep centers take part in space charges in the depletion layer. The deep centers are considered to be acceptors due to Ga vacancies produced by zinc diffusion which occurs during junction formation by liquid phase epitaxy or during heat treatment, and to be distributed in the p-side. Analyses of experimental data have revealed that the junction has a p-π-i-n structure and that, in the π-layer, the deep acceptor concentration is predominantly high as compared with the net shallow acceptor concentration. Voltage dependence of diode current and red electroluminescence has also been interpreted in terms of this junction structure.