Evidence for causality between GaN RF HEMT degradation and the EC-0.57 eV trap in GaN

The degradation of industry-supplied GaN high electron mobility transistors (HEMTs) subjected to accelerated life testing (ALT) is directly related to increases in concentrations of two defects with trap energies of E_C-0.57 and E_C-0.75 eV. Pulsed I-V measurements and constant drain current deep level transient spectroscopy were employed to evaluate the quantitative impact of each trap. The trap concentration increases were only observed in devices that showed a 1 dB drop in output power and not the result of the ALT itself indicating that these traps and primarily the E_C-0.57 eV trap are responsible for the output power degradation. Increases from the E_C-0.57 eV level were responsible for 80% of the increased knee walkout while the E_C-0.75 eV contributed only 20%. These traps are located in the drain access region, likely in the GaN buffer, and cause increased knee walkout after the application of drain voltage.     

The effects of gate length variation and trapping effects on the transient response of AlGaN/GaN HEMT’s on SiC substrates

In AlGaN/GaN heterostructure field-effect transistors, the surface defects and dislocations may serve as trapping centers and affect the device performance via leakage current. In this paper we report results of our investigation of the trapping characteristics of Al_0.25Ga_0.75N/GaN HEMT using the Conductance Deep Level Transient Spectroscopy (CDLTS). Two deep level electronic defects were observed labeled E₁ and HL₁, with activation energies E_a1 = 1.36 eV and E_a2 = 0.63 eV. The hole-trap HL₁ is characterized for the first time in our studies. We identified the characteristics of the traps at the AlGaN/GaN interface adjoining the channel and the surface along the ungated region between the gate and the drain, as well as the effects of the surface traps.     

Evidence of surface states for AlGaN/GaN/SiC HEMTs passivated Si<Subscript>3</Subscript>N<Subscript>4</Subscript> by CDLTS

In AlGaN/GaN heterostructure field-effect transistors (HEMTs) structures, the surface defects and dislocations may serve as trapping centers and affect the device performance via leakage current and low frequency noise. This work demonstrates the effect of surface passivation on the current-voltage characteristics and we report results of our investigation of the trapping characteristics of Si₃N₄-passivated AlGaN/GaN HEMTs on SiC substrates using the conductance deep levels transient spectroscopy (CDLTS) technique. From the measured of CDLTS we identified one electron trap had an activation energy of 0.31 eV it has been located in the AlGaN layer and two hole-likes traps H ₁, H ₂. It has been pointed out that the two hole-likes traps signals did not originate from changes in hole trap population in the channel, but reflected the changes in the electron population in the surface states of the HEMT access regions.     

Traps centers impact on Silicon nanocrystal memories given by Random Telegraph Signal and low frequency noise

This work reports on a comprehensive process of trapping centers in Silicon nanocrystal (nc-Si) memories devices. The trap centers have been studied using Random Telegraph Signal (RTS) and Low Frequency (LF) techniques. The study of the traps which are responsible for RTS noise in non-volatile memories (NVM) devices as a function of gate voltage and temperature, offers the opportunity of studying the trapping/detrapping behaviour of a single interface trap center. The RTS parameters of the devices having random discrete fluctuations in the drain current get more information about trap energy level and spatial localization from the SiO₂/Si interface. The impact of trap centers has been also investigated showing the significant noise between memories and references devices. Furthermore, it has convincingly been shown that this discrete switching of the drain current between a high and a low state is the basic feature responsible for l/f^γ flicker noise in MOSFETs transistors.     

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.     

Effect of iodine impurity on relaxation of photoexcited silver chloride

The time and temperature dependences of relaxation of excited AgCl and AgCl:I crystals is studied by the method of photostimulated flash of luminescence. The presence of iodine impurity in silver chloride gives rise to hole recombination (luminescence) centers and hole traps in the band gap. It is shown that the main contribution to the decrease in the concentration of electrons localized at deep traps is made by the recombination of electrons with holes released thermally from shallow localization levels (iodine-related centers). Estimation of activation energy for the relaxation process showed that these energies for the AgCl and AgCl:I samples under study are the same within the experimental error and are equal to E _{rel 1} = 0.01 ± 0.0005 eV for the initial stage of relaxation and E _{rel 2} = 0.09 ± 0.005 eV for the final state. This fact indicates that the majority of hole traps involved in the relaxation process in AgCl are related to iodine impurity. In the course of thermal relaxation in AgCl, relocalization of nonequilibrium charge carriers from shallow levels to deep levels is observed. The depth of the corresponding trap is E _arl = 0.174 ± 0.03 eV.     

Analysis of current collapse effect in AlGaN/GaN HEMT: Experiments and numerical simulations

In this work, current collapse effects in AlGaN/GaN HEMTs are investigated by means of measurements and two-dimensional physical simulations. According to pulsed measurements, the used devices exhibit a significant gate-lag and a less pronounced drain-lag ascribed to the presence of surface/barrier and buffer traps, respectively. As a matter of fact, two trap levels (0.45 eV and 0.78 eV) were extracted by trapping analysis based on isothermal current transient. On the other hand, 2D physical simulations suggest that the kink effect can be explained by electron trapping into barrier traps and a consequent electron emission after a certain electric-field is reached.     

Effect of composition on deep levels in heteroepitaxial GexSi1−x layers and evidence for dominant intrinsic recombination-Generation in relaxed ge layers on Si

Electron traps, hole traps, and the dominant recombination-generation (R-G) centers have been investigated with deep level transient spectroscopy and current-voltage/temperature measurements in heteroepitaxial Ge_xSi_1-x alloys with x ranging from 0.15 to 1, grown on graded Ge_y.Si_1−y/Si substrates. For all samples with compositions x < 0.85, which retain the Si-like conduction band structure, we detect a dominant electron trap and R-G center whose activation energy is ΔE = 0.5 eV, independent of composition. This energy agrees with that of electron traps previously reported for plastically deformed (PD) Si, suggesting a connection to the Si-like band structure. This 0.5 eV level dominates the reverse leakage current over a wide range of growth and annealing conditions for the 30% Ge samples, indicating that the electronic state at ΔE = 0.5 eV is a very efficient R-G center, as would be expected from its midgap position. Alternatively, for strain relaxed, pure Ge (< 1), we detect electron traps at E_c − 0.42 eV and E_c − 0.28 eV, in agreement with the literature on PD Ge and Ge bicrystals. These energies are significantly different from those observed for x < 0.85, and we conclude that these changes in activation energy are due to changes in the conduction band structure for high Ge content. Moreover, in contrast with the Si-like samples (x < 0.85), the reverse leakage current in the relaxed Ge cap layer is not controlled by deep levels, but is rather dictated by intrinsic, band-to-band generation due to the reduced bandgap of Ge as compared to Si-like alloys. Only for reverse bias magnitudes which incorporate a significant portion of the graded buffer within the depletion region do R-G centers dominate the reverse leakage current. These results confirm the high quality of the strain-relaxed, pure Ge cap region which was grown on a Ge_ySi_1−y/Si step graded heterostructure (where y was increased from 0 to 1) by ultra high vacuum chemical vapor deposition. Finally, we report for the first time, what is apparently the dislocation kink site state at E_c − 0.37 eV, in a Ge_xSi_1−x alloy.     

Controlled vapor-pressure heat-treatment effect on deep levels in liquid-encapsulated czochralski-grown GaP crystals

Photocapacitance (PHCAP) measurements have been carried out on GaP crystals grown by the liquid-encapsulated Czochralski (LEC) method with heat treatment under various phosphorus-vapor pressures at different temperatures. Electron traps of E_C−1.1 eV, E_C−1.6 eV, E_C−1.9 eV, and a hole trap of E_V+2.26 eV are mainly detected. The phosphorus-vapor pressure dependence of the E_C−1.9 eV trap density and their diffusion behavior indicate that they are interstitial phosphorus atoms. The densities of both E_C−1.1 eV and E_C−1.6 eV traps are strongly dependent on the shallow impurity concentrations. Moreover, the density of E_C−1.1 eV traps increases with increasing phosphorus-vapor pressure. From these results, we suggest that E_C−1.1 eV traps are the complexes of shallow donors and antisite phosphorus atoms. Deep-level densities in GaP crystals after annealing at 860°C or 960°C for 60 min are decreased almost one order of magnitude lower than those in untreated substrate crystals, which should have occurred via out-diffusion of interstitial phosphorus atoms. However, such an effect is not prominent for 800°C treatment for 60 min.     

Electron and hole traps in GaN p-i-n photodetectors grown by reactive molecular beam epitaxy

GaN p-i-n photodetectors grown on sapphire by reactive molecular beam epitaxy have been characterized by measurements of room-temperature current-voltage (I-V), temperature-dependent capacitance (C-V-T), and deep level transient spectroscopy (DLTS) under both majority and minority carrier injection. Due to what we believe to be threading dislocations, the reverse I-V curves of p-i-n photodetectors show typical electric-field enhanced soft breakdown characteristics. A carrier freeze-out due to the de-ionization of Mg-related deep acceptors has been found by C-V-T measurements. Three electron traps, B (0.61 eV), D (0.23 eV), and E₁ (0.25 eV) and one hole trap, H₃ (0.79 eV) have been revealed by DLTS measurements. The photodetectors with lower leakage currents usually show higher responsivity and lower trap densities of D and E₁.     

Identification of an RF degradation mechanism in GaN based HEMTs triggered by midgap traps

Quantitative defect spectroscopy was performed on low gate leakage operational S-band GaN HEMTs before and after RF accelerated life testing (ALT) to investigate and quantify potential connections between the evolution of observed traps and RF output power loss in these HEMTs after stressing. Constant drain current deep level transient spectroscopy and deep level optical spectroscopy (CI_D-DLTS and CI_D-DLOS, respectively) were used to interrogate thermally-emitting traps (CI_D-DLTS) and deeper optically-stimulated traps (CI_D-DLOS) so that the entire bandgap can be probed systematically before and after ALT. Using drain-controlled CI_D-DLTS/DLOS, with which traps in the drain access region are resolved, it is found that an increase in the concentration of a broad range of deep states between E_C–1.6 to 3.0 eV, detected by CI_D-DLOS, causes a persistent increase in on-resistance of ~ 0.22 Ω-mm, which is a likely source for the 1.2 dB reduction in RF output power that was observed after stressing. In contrast, the combined effect of the upper bandgap states at E_C–0.57 and E_C–0.72 eV, observed by CI_D-DLTS, is responsible for only ~ 10% of the on-resistance increase. These results demonstrate the importance of discriminating between traps throughout the entire bandgap with regard to the relative roles of individual traps on degradation of GaN HEMTs after ALT.     

Evidence of interaction between two DX centers in N-Type AlGaAs from RDLTS and temperature dependent pulse-width DLTS measurements

Two well-separated electron traps with activation energies: E_{t 1} ≊ 0.286 eV and E_{t 2} ≊ 0.433 eV have been consistently detected in the n-type Al_0.6Ga_0.4As confinement layer of AlGaAs/GaAs single quantum well laser diodes. The physical characteristic parameters for these two traps, including capture cross section, emission time constant, and capture time constant, have been calculated. Reverse-bias pulsed deep level transient spectroscopy (RDLTS) results provide the evidence for the first time that these two traps have strong interaction during emission processes. This allows us to conclude that E_{t 1} and E_{t 2} are indeed both donor-unknown centers. Furthermore, using a temperature-dependent pulse-width method, DLTS signals from E_{t 1} alone can be obtained. The corrected activation energy appears to be a little shallower at E_{t 1} ≊ 0.265 eV.     

Hole trapping in E-beam irradiated SiO2 films

Low energy (25 kV) electron beam irradiation of MOS capacitors is shown to produce neutral hole traps in thin ‘radiation hardened’ SiO₂ films. These traps are found in an uncharged state after irradiation and are populated by passing a small hole current, generated by avalanche breakdown of then-type silicon substrate, through the oxide. From the time dependence of the observed trapping, a capture cross-section between 1 × 10∼⁻¹³ and 1 × 10⁻¹⁴ cm² is deduced. The trap density is found to depend on the annealing conditions and incident electron beam dosage. The density of traps increases with incident electron beam exposure. Once introduced into the oxide by the radiation the traps can be removed by thermal anneals at temperatures above 500° C. Parallels between electron and hole trapping on these neutral centers are strong evidence for an amphoteric uncharged trap generated by ionizing radiation.     

Point Defects in Pb-, Bi-, and In-Doped CdZnTe Detectors: Deep-Level Transient Spectroscopy (DLTS) Measurements

We studied, by current deep-level transient spectroscopy (I-DLTS), point defects induced in CdZnTe detectors by three dopants: Pb, Bi, and In. Pb-doped CdZnTe detectors have a new acceptor trap at around 0.48?eV. The absence of a V_Cd trap suggests that all Cd vacancies are compensated by Pb interstitials after they form a deep-acceptor complex [[Pb_Cd]⁺-V _Cd ^2? ]^?. Bi-doped CdZnTe detectors had two distinct traps: a shallow trap at around 36?meV and a deep donor trap at around 0.82?eV. In detectors doped with In, we noted three well-known traps: two acceptor levels at around 0.18?eV (A-centers) and 0.31?eV (V_Cd), and a deep trap at around 1.1?eV.     

Defect structure and generation of interface states in MOS structures

The correlation between the defect structure of the SiO₂ and the generation of interface traps upon avalanche injection of electrons and holes in MOS capacitor was investigated. Using samples with widely varying densities of intrinsic and H₂O-related trapping centers, and with different oxide thicknesses and gate electrodes it was established from experiments around room temperature that a one to one correlation between the densities of captured carriers and generated interface states exists as long as only a single type of carrier is involved. If simultaneously the second type of carrier is injected the picture becomes more complex. We confirm reports by other authors that at reduced temperatures (typically 77 K) two steps may be distinguished in the generation process, one of which is thermally activated. In this case the “yield” of interface states drops to values distinctly below one, even after warm up. In many cases the energy distribution of the states shows characteristic features: at E = E_v + 0.45 eV for electron injection in samples with H₂O related traps, at E = E_v + 0.75 eV for hole injection. Samples exhibiting these features always show the occurrence of slow states.     

Interface states and deep-level centers in silicon-on-insulator structures

Deep-level centers in a split-off silicon layer and trap levels were studied by deep-level transient spectroscopy both at the Si/SiO₂ interface obtained by direct bonding and also at the Si(substrate)/〈thermal SiO₂〉 interface in the silicon-on-insulator structures formed by bonding the silicon wafers and delaminating one of the wafers using hydrogen implantation. It is shown that the Si/〈thermal SiO₂〉 interface in a silicon-on-insulator structure has a continuous spectrum of trap states, which is close to that for classical metal-insulator-semiconductor structures. The distribution of states in the upper half of the band gap for the bonded Si/SiO₂ interface is characterized by a relatively narrow band of states within the range from E _c −0.17 eV to E _c −0.36 eV. Furthermore, two centers with levels at E _c −0.39 eV and E _c −0.58 eV are observed in the split-off silicon layer; these centers are concentrated in a surface layer with the thickness of up to 0.21 μm and are supposedly related to residual postimplantation defects. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 8, 2001, pp. 948–953. Original Russian Text Copyright ? 2001 by Antonova, Stano, Nikolaev, Naumova, Popov, Skuratov.     

Transformation of interface states in silicon-on-insulator structures under annealing in hydrogen atmosphere

Changes induced by annealing the spectrum of states on a Si/SiO₂ interface obtained by direct bonding and on a Si(substrate)/〈thermal SiO₂〉 interface in silicon-on-insulator (SOI) structures were investigated by charge-related deep-level transient spectroscopy. The structures were formed by bonding silicon wafers and slicing one of the wafers along a plane weakened by hydrogen implantation. The SOI structures were annealed at 430°C for 15 min in hydrogen, which corresponded to the conventional mode of passivation of the Si/SiO₂-interface states. The passivation of interface states by hydrogen was shown to take place for the Si/〈thermal SiO₂〉 interface, as a result of which the density of traps substantially decreased, and the continuous spectrum of states was replaced by a band of states in the energy range E _c=0.1–0.35 eV within the entire band. For the traps on the bonded Si/SiO₂ interface, the transformation of the centers occurs; namely, a shift of the energy-state band is observed from E _c=0.17–0.36 to 0.08–0.22 eV. The trapping cross section decreases by about an order of magnitude, and the density of traps observed increases slightly.     

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.     

Germanium as a deep level in alxga1− xas grown by organometallic vapor phase epitaxy

Optoelectronic devices require materials which exhibit extremely low trap concentrations. The Al_xGa_1−xAs system has been used extensively for optoelectronic applications despite trap concentrations in the Al_xGa_1−xAs which limit the efficiency of the resulting devices. Deep level transient spectroscopy (DLTS) performed on Al_0.2Ga_0.8As layers grown by organometallic vapor phase epitaxy (OMVPE) has revealed three traps with concentrations >10¹³ cm⁻³ -E _c-E_t = 0.3, 0.5 and 0.7 eV. The dominant source of the 0.3 eV trap has proven to be a Ge impurity in arsine. SIMS analysis of Al_0.2Ga_0.8As samples show Ge as the only candidate for the impurity responsible for the 0.3 eV trap. DLTS and SIMS analysis performed on Al_0.2Ga_0.8As samples intentionally doped with Ge displayed a proportional increase in the 0.3 eV trap concentration with the Ge concentration and establishes that Ge is indeed the source of the 0.3 eV trap in Al_xGa_1−xAs. Comparison of C-V, SIMS and DLTS measurements performed on Al_xGa_1-xAs:Ge indicate that approximately 30% of elemental Ge incorporated created the 0.3 eV trap, DX_Ge.     

Effects of Electron Irradiation on Deep Centers in High-Purity Semi-Insulating 6H-SiC

Many point-defect–related centers have been investigated in electron-irradiated 6H-SiC by deep-level transient spectroscopy (DLTS). Most of them are believed to be related to vacancies. Our DLTS studies on deep centers produced by electron-irradiation (EI) in conductive epi-6H-SiC are in agreement with the literature data. However, for semi-insulating SiC, DLTS cannot be used for trap studies so we have applied thermally stimulated current (TSC) spectroscopy. At least nine TSC traps have been observed in high-purity/semi-insulating (HPSI) 6H-SiC. To understand the nature of these centers, 1-MeV EI and postirradiation annealing at 600°C were applied to the sample. The TSC spectroscopy and 4.2 K photoluminescence (PL) have been used to study the effects of EI and annealing on the centers in HPSI 6H-SiC. It was found that (1) some of the major EI-induced DLTS centers in conductive 6H-SiC, such as ED1, E₁/E₂, E_i, Z₁/Z₂, and L₉, have TSC counterparts even in as-grown HPSI 6H-SiC; (2) EI-induced TSC centers in HPSI 6H-SiC are due to point defects, which have been confirmed by typical PL lines (such as S, L, and V lines); and (3) the concentration of the 1.1-eV center, which controls material conductivity, can be increased by 1-MeV EI and decreased by 600°C annealing.