Deep Level Spectra Research Articles

Trap states and carrier diffusion lengths in NiO/β-Ga2O3 heterojunctions

We report the electrical properties, deep trap spectra, and diffusion lengths of non-equilibrium carriers in Ni Schottky diodes and NiO/Ga2O3 heterojunctions (HJs) prepared on the same n−/n+ β-Ga2O3 epi structures. The heterojunctions decrease the reverse current of Ga2O3 high-power rectifiers. In HJs, in contrast to Schottky diodes, the capacitance and AC conductance show a prominent frequency and temperature dependence, suggesting the presence of two temperature activation processes with activation energies of 0.17 and 0.1 eV. The deep trap spectra of the Schottky diodes and HJs differ by the absence in the HJ of deep electron traps E2* with level near Ec − 0.7 eV considered to be an important center of non-radiative recombination. This correlates with the observed increase in the diffusion length of non-equilibrium charge carriers in the HJs to 370 nm compared to 240 nm in the Schottky diodes. The diffusion length of charge carriers in p-NiO was found to be quite short, 30 nm. Possible reasons for the observed differences and possible origin of the minority-trap-like feature commonly reported to be present in the deep level spectra of HJs and also observed in our experiments are discussed.

Influence of radiation defects on the photoelectric properties of polycrystalline CDTE films with deep impurity levels

In this paper, the effect of boron implantation at doses of 9⋅1014 cm-2 on the deep-level spectra of polycrystalline CdTe films doped with silver is considered. It was found that boron implantation results in the formation of radiation defects in the region close to the surface of 0.2 μm. It was determined that the location of deep local levels, the activation energy of which is equal to Ec-1.15 eV, is located in the intercrystal barriers. The implanted CdTe: Ag layers are thermally treated for 30 min at a temperature of 100⁰C and the restoration of the barriers in these layers is shown: the interlayer grows and the concentration of levels participating in the photovoltage increases. Heating of non-implanted layers under the same conditions does not affect the shape of the primacy absorption spectra, but only increases its value in its own region by 1.4 times, i.e. heating improves the barriers only in the implanted layers.

Defect analysis of 1-MeV electron irradiated flexible InGaAs solar cells by deep-level transient spectroscopy and photoluminescence

1-MeV electron irradiated InGaAs flexible solar cells were investigated based on solar cell I–V characteristics, external quantum efficiency, photoluminescence, and deep-level transient spectra (DLTS) mesurements. The electrical parameters of the solar cells, including Rs and Rsh, were extracted from the I–V curves. With an increase in irradiation fluence, the electrical and optical characteristics of the solar cells continuously degraded. The Voc, Isc, and Pmax values decreased to 83.16%, 81.96%, and 68.88% of their initial values, respectively, when the irradiation fluence was increased to 2 × 1015 e/cm2. Photoluminescence (PL) emission peak intensity decreased to 0.54% of its original value, and the minority carrier lifetimes derived from fitting PL emission spectra were reduced to 1.25%. DLTS technique was used to analyse the irradiation-induced defect properties. Similar fitting values for the minority carrier lifetimes were obtained from the PL and DLTS measurements.

Capacitive Spectroscopy of Deep Levels in Silicon with Samarium Impurity

The effect of thermal treatment on the behavior of samarium atoms introduced into silicon during the growth process was studied using the method of transient capacitive deep-level spectroscopy (DLTS). It has been shown that various high-temperature treatments lead to the activation of samarium atoms in the bulk of n-Si and the formation of deep levels. The energy spectrum of deep levels arising during heat treatments has been determined. The dependence of the efficiency of formation of these levels in n‑Si<Sm> on the processing temperature has been studied. It was found that the higher the content of samarium atoms in the bulk of silicon at the same high-temperature treatment temperature, the higher the concentration of the deep level EC–0.39 eV. From this, we can conclude that the EC–0.39 eV level is associated with the activation of samarium atoms in the n-Si<Sm> volume.

Electrical characterization and temperature reliability of 4H-SiC Schottky barrier diodes after Electron radiation

In this study, the operation of commercial SBD-based on 4H-SiC after radiation was assessed. The devices were subjected to electron radiation with an energy of 1.7 MeV at different fluences. Then the electrical characteristics of the devices were measured, and related parameters were calculated. The results show that the devices display good radiation resistance at the high fluence of 1015 e/cm2. However, the performance of the devices degrade when the total fluence is 1 × 1016 e/cm2. The variable temperature test and deep-level transient spectrum (DLTS) tests reveal the cause of performance degradation under high fluence. The variable temperature test result shows that the ideality factor of the device at 1 × 1016 e/cm2 increases compared with that devices, indicating more recombination centers are introduced. The donor concentration (ND) significantly decreases at 1 × 1016 e/cm2, and increases with increasing temperature in the range of 350–450 K. The DLTS results show that the change of ND with temperature is related to the decrease of Ec-0.91 defect concentration. During the variable temperature test, certain changes are found in the defects and the defects are due to the repair of the ionization damage during the heating process, the degeneration of the electrical characteristics is mainly caused by the ionization damage rather than the displacement damage. Finally, the radiation-induced displacement damage is mainly located in the region near the metal-semiconductor interface.

Permittivity modulation in Si-based PIN diode by electron irradiation

In this paper, we investigate the influence of defects induced by different doses of electron irradiation on the real part of permittivity of Si-based PIN diodes with a deep-level transient spectrum test. For a more precise result, the relationship between the real part of the permittivity of the sample and the irradiation dose, temperature and annealing is studied. The results show that the real part of the permittivity changes significantly with the irradiation dose and temperature. The absolute value of the real part of the permittivity decreases with the decrease in temperature, and the changing trend decreases after annealing. The deep-level transient spectroscopy results illustrate that different states of defects caused by radiation and annealing have changed the free carrier mobility, free carrier density and charge balance in the material, which means they have a significant impact on the electrical characteristics of the samples. The I–V and C–V curves are obtained through experiments to extract the leakage current and carrier concentration. The I–V characteristic and the permittivity are subjected to temperature-variation analysis.

Electrical spectroscopy methods for the characterization of defects in thin-film compound solar cells

The electronic activity of defects and their impact on the efficiency of Cu(In,Ga)Se2 and CdTe solar cells is a subject of continuing interest and dispute in the photovoltaic community. However, after many years of research, the conclusions are far from satisfying yet. Here, the electrical defect spectroscopy results for Cu(In,Ga)Se2 and CdTe absorbers and devices are discussed with focus on findings that have been confirmed on many samples but still do not have a well-grounded interpretation. Charged grain boundaries are proposed as a possible source of some signatures observed in deep level spectra in both materials. Electrical nano-characterization methods combined with standard defect spectroscopy are suggested as a promising solution for unraveling the role and origin of dominating defects for solar cells efficiency.

Synergistic Effect of Electrical Stress and Neutron Irradiation on Silicon Carbide Power MOSFETs

The synergistic effect of radiation and bias voltage for the commercial silicon carbide (SiC) power MOSFETs was investigated by 14-MeV neutron irradiation in this article. The types of C3M0065090D and C2M0080120D commercial SiC power MOSFETs from CREE studied in this article show consistent changes after the 14-MeV neutron irradiation. The results show that the devices biased during irradiation experienced more degradation than that only irradiated and biased devices. Compared with the devices only irradiated and biased, the devices biased during irradiation experienced an additional decrease in saturation current and a further decrease in peak transconductance. The defects in the devices were characterized by the low-frequency noise (LFN) and deep-level transient spectrum (DLTS) methods. The results show that the synergistic effect increased the defect concentration of the device, resulting in further degradation of the electrical performance of the device compared to the control group without a bias voltage. The defect characterization shows that the drain–source voltage leads to the accumulation of carbon and silicon vacancies induced by irradiation at the interface, which results in intensified radiation damage and significant changes in the performance of the device at low neutron fluence.

Localized surface plasmon and transferred electron enhanced UV emission of ZnO by periodical aluminum nanoparticle arrays

Periodical Al nanoparticle (NP) arrays were applied to enhance the ultraviolet (UV) light emission of ZnO by localized surface plasmon resonance (LSPR) effect and electron transfer mechanism. By applying corresponding anodic aluminum oxide (AAO) templates, periodical Al NP arrays with different diameters (D) and spaces (S) were manufactured by magnetron sputtering. Theoretical simulation by FDTD-solution indicates that local electric field around Al NPs is significantly enhanced, and the surface plasmon resonance wavelength is determined by D and S. Al NP arrays with D of 70 nm and S of 125 nm enhanced near band-edge emission of ZnO by 2.4 times. By adding Al NP arrays, the photonic lifetime of ZnO decreased by 0.09 ns according to time-resolved photoluminescence spectra, indicating that the enhancement in UV luminescence intensity of ZnO is ascribed to the coupling between surface plasmons of Al NPs and ZnO excitons. Deep-level emission spectra analysis indicates that there lies another mechanism of electron transfer from the Al NPs to surface plasmon resonance (SPR) level and then further to ZnO conduction band.

Effect of neutron irradiation on the spectrum of deep-level defects in GaAs grown by liquid-phase epitaxy in hydrogen or argon ambient

We present results of experimental studies of capacitance--voltage characteristics, spectra of deep-level transient spectroscopy of graded high-voltage GaAs p+-p0-i-n0 diodes fabricated by liquid-phase epitaxy at a crystallization temperature of 900oC from one solution--melt due to autodoping with background impurities, in a hydrogen or argon ambient, before and after irradiation with neutrons are presented. After neutron irradiation, deep-level transient spectroscopy spectra revealed wide zones of defect clusters with acceptor-like negatively charged traps in the n0-layer, which arise as a result of electron emission from states located above the middle of the band gap. It was found that the differences in capacitance--voltage characteristics of the structures grown in hydrogen or argon ambient after irradiation are due to different doses of irradiation of GaAs p+-p0-i-n0 structures and different degrees of compensation of shallow donor impurities by deep traps in the layers. Keywords: GaAs, neutron irradiation, capacitance spectroscopy, p0-i-n0-junction, liquid-phase epitaxy, hydrogen, argon.

Влияние нейтронного облучения на спектр дефектов с глубокими уровнями в GaAs, изготовленном методом жидкофазной эпитаксии в атмосфере водорода и аргона

The results of experimental studies of capacitance– voltage characteristics, spectra of deep-level transient spectroscopy of graded high-voltage GaAs p+−p0−i−n0 diodes fabricated by liquid-phase epitaxy at a crystallization temperature of 900C from one solution–melt due to autodoping with background impurities, in a hydrogen or argon ambient, before and after irradiation with neutrons. After neutron irradiation, deep-level transient spectroscopy spectra revealed wide zones of defect clusters with acceptor-like negatively charged traps in the n0-layer, which arise as a result of electron emission from states located above the middle of the band gap. It was found that the differences in capacitance–voltage characteristics of the structures grown in hydrogen or argon ambient after irradiation are due to different doses of irradiation of GaAs p+−p0−i−n0 structures and different degrees of compensation of shallow donor impurities by deep traps in the layers.

Crystal orientation dependence of deep level spectra in proton irradiated bulk β-Ga2O3

The effects of 20 MeV proton irradiation with fluences of 5 × 1014 and 1015 p/cm2 on electrical properties of lightly Sn doped n-type (net donor concentration 3 × 1017 cm−3) bulk β-Ga2O3 samples with (010) and (−201) orientation were studied. Proton irradiation decreases the net donor density with a removal rate close to 200 cm−1 for both orientations and similar to the electron removal rates in lightly Si doped β-Ga2O3 epilayers. The main deep electron traps introduced in the β-Ga2O3 crystals of both orientations are near Ec−0.45 eV, while in Si doped films, the dominant centers were the so-called E2* (Ec−0.75 eV) and E3 (Ec−0.1 eV) traps. Deep acceptor spectra in our bulk –Ga2O3(Sn) crystals were dominated by the well-known centers with an optical ionization energy of near 2.3 eV, often attributed to split Ga vacancies. These deep acceptors are present in a higher concentration and are introduced by protons at a higher rate for the (010) orientation. Another important difference between the two orientations is the introduction in the surface region (∼0.1 μm from the surface) of the (010) of a very high density of deep acceptors with a level near Ec−0.27 eV, not observed in high densities in the (−201) orientation or in Si doped epitaxial layers. The presence of these traps gives rise to a very pronounced hysteresis in the low temperature forward current–voltage characteristics of the (010) samples. These results are yet another indication of a significant impact of the orientation of the β-Ga2O3 crystals on their properties, in this case, after proton irradiation.

Electrically-active defects in reduced and hydrogenated rutile

We report on electrically-active defects located between 0.054 and 0.69 eV below the conduction band edge in rutile single crystals subjected to reducing and hydrogenating heat treatments. Deep-level transient spectroscopy measurements recorded on samples subjected to different heat treatments are compared. In samples annealed in gas, three defect levels are commonly observed. One of these levels, , located 0.43 eV below the conduction band edge is tentatively assigned to a hydrogen-impurity complex. Two levels at 0.054 and 0.087 eV below the conduction band edge, which were present after all different heat treatments, are tentatively assigned as being related to O vacancies or Ti self-interstitials. Deep-level transient spectroscopy spectra of samples heat-treated in display a larger number of defect levels and larger concentrations compared to samples heat-treated in gas. treatments are performed at considerably higher temperatures. Four energy levels located between 0.28 and 0.69 eV, induced by annealing in , are tentatively attributed to O vacancy- or Ti interstitial-related complexes with impurities.

Characterization of nano-bio silicon carbide

Plasma-enhanced chemical vapor deposition, reactive magnetron sputtering, hot-wire chemical vapor deposition and radio frequency plasma-enhanced chemical vapor deposition were used to develop technology for preparation of nano-bio silicon carbide coating of ceramic materials for dental applications. The effect of the bias voltage applied to the ceramic prostheses and dental crowns on the crystallization processes have been recognized. The optimal bias voltage applied to conductive substrate was –200 V, whereas for dielectric substrate the bias voltage Vbias did not affect the properties of SiC coating. The analysis of CVCs and spectroscopic diagnostics as the methods for studying the mechanism of interfacial rearrangements to investigate SiC phase transition in nano silicon carbide coatings were used. The conductivity of the SiC coating coincided with the conductivity on the dielectric (µn0 = 1012…1013 сm–1·s–1·V–1). The conductive substrate had a significant effect on the properties of the coating and thus depended on the bias voltage Vbias. The conductivity increased by three-four orders of magnitude (µn0 = 3·1017 сm–1·s–1·V–1), if the bias voltage Vbias = –200 V. The increase of the bias voltage (Vbias = –600 V) led to a decrease in the conductivity (µn0 = 1011…1012 сm–1·s–1·V–1). It was found that there was the double injection regime with bimolecular recombination in this structure with the dependence I = V3/2 for CVCs of SiC. The luminescence spectrum of SiC coating on non-dielectric ceramics (if Vbias = – 200 V during deposition) was significantly different from the luminescence spectrum of SiC coating on dielectric ceramics. Increasing the applied voltage to the substrate Vbias during deposition led to increasing the fraction of hexagonal polytypes. Directions in the crystal lattice according to the photoluminescence spectra were identified from the comparing the values of the width of the non-phonon parts of stacking faults and deep level spectra in the low-temperature photoluminescence with arrangements of atoms in the SiC lattice structure. The displacement of each atom participating in photoluminescence allowed to find the correlation with technology of SiC deposition and to develop technology of SiC coating on the dental materials.

Capacitance Spectroscopy of Heteroepitaxial AlGaAs/GaAs p–i–n Structures

The temperature dependences of the capacitance-voltage (C–V) characteristics and deep-level spectra of a graded high-voltage AlxGa1 – xAs p0–i–n0 junction grown by liquid-phase epitaxy via autodoping with background impurities are studied. Changes in the C–V characteristics at varied measurement temperature and optical illumination demonstrate that the p0-, i-, and n0-type layers in AlxGa1 – xAs under study contain bistable DX centers. Spectra furnished by deep-level transient spectroscopy (DLTS), measured at various bias voltages Vr and filling-pulse voltages Vf, show a positive DLTS peak for the n0-type layer, with a thermal activation energy of Et = 280 meV and electron-capture cross section of σn = 3.17 × 10–14 cm2, which is unusual for a majority-charge-carrier trap. This peak is attributed to the negatively charged state of the Se/Te donor impurity, which is a bistable DX center with negative correlation energy U.

Емкостная спектроскопия гетероэпитакиальных AlGaAs/GaAs p-i-n-структур

Temperature dependences of capacitance-voltage (C−V) characteristics and deep-level spectra of the graded highvoltage AlxGa1−xAs p0 −i−n0 junction grown by liquid-phase epitaxy via autodoping with background impurities were investigated. The changes of the C−V characteristics at varied measurement temperature and optical illumination demonstrated that the p0-, i-,n0-type layers in the AlxGa1−xAs under study contain bistable DX centers. In spectra of deep-level transient spectroscopy (DLTS), measured at various bias voltages Vr and filling pulses Vf , a positive DLTS peak is observed for the n 0 -type layer with thermal activation energy Et = 280 meV and electron-capture cross-section σn = 3.17 · 10−14 cm2, which is unusual for a majority-carrier trap. This peak is related to the negatively charged state of the Se/Te donor impurity, which is a bistable DX center with negative correlation energy U.

Impact of the Percolation Effect on the Temperature Dependences of the Capacitance–Voltage Characteristics of Heterostructures Based on Composite Layers of Silicon and Gold Nanoparticles

The temperature dependences of the capacitance–voltage characteristics and deep-level spectra of a Au–n-Si:Au–Si–p-Si heterostructure based on a composite layer of Au and Si nanoparticles are investigated. The structure manifests the properties of a transistor connected to a circuit with a common emitter with a disconnected base and an emitter Schottky barrier between the Au point contact and the n–(Si:Au) layer. Nanoparticles in this layer form finite clusters with hopping conductivity; herewith, charge accumulation is observed in the region of the Au point contact. The system at a measurement temperature below 180 K transitions from the finite-cluster phase to the infinite-cluster phase due to the percolation effect. This phase manifests metallic properties in the lateral plane of the heterostructure, which transforms into a p–n diode.

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.

Carbon in GaN: Calculations with an optimized hybrid functional

GaN is of great importance for optoelectronic and power electronic devices, and carbon is one of its most common impurities, used also actively for preparing semi-insulating buffer layers. Experimental investigations reveal at least five carbon-related centers, however, theory could provide definite assignment so far for only two of those: in terms of simple carbon substitutionals on nitrogen $({\mathrm{C}}_{\mathrm{N}})$ and gallium $({\mathrm{C}}_{\mathrm{Ga}})$ sites. We apply here an optimized hybrid functional (which reproduces the band gap and is Koopmans' compliant) to investigate small carbon complexes. We find that an increasing carbon concentration, beyond the compensation of the $n$-type doping by the acceptor ${\mathrm{C}}_{\mathrm{N}}$, makes its complexes with ${\mathrm{C}}_{\mathrm{Ga}}$ and ${(\mathrm{C}\ensuremath{-}\mathrm{N})}_{\mathrm{N}}$ split interstitials competitive in formation energy. Except for $p$-doped samples, these complexes are electrically passive, so the Fermi level is pinned a little below midgap, ensuring the semi-isolating behavior even for high carbon concentrations. The charge transition levels, calculated by the optimized functional, allow the interpretation of all carbon-related deep-level spectra as well as of their relation to the carbon-to-donor ratio.

Influence of deep defects on electrical properties of Ni/4H-SiC Schottky diode**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400402).

In this paper, we investigate the influence of deep level defects on the electrical properties of Ni/4H–SiC Schottky diodes by analyzing device current–voltage (I–V) characteristics and deep-level transient spectra (DLTS). Two Schottky barrier heights (SBHs) with different temperature dependences are found in Ni/4H–SiC Schottky diode above room temperature. DLTS measurements further reveal that two kinds of defects Z1/2 and Ti(c)a are located near the interface between Ni and SiC with the energy levels of EC–0.67 eV and EC–0.16 eV respectively. The latter one as the ionized titanium acceptor residing at cubic Si lattice site is thought to be responsible for the low SBH in the localized region of the diode, and therefore inducing the high reverse leakage current of the diode. The experimental results indicate that the Ti(c)a defect has a strong influence on the electrical and thermal properties of the 4H–SiC Schottky diode.