Current Transient Spectroscopy Research Articles

Radiation Hardness and Defects Activity in PEA2PbBr4 Single Crystals

AbstractMetal halide perovskites (MHPs) are low‐temperature processable hybrid semiconductor materials with exceptional performances that are revolutionizing the field of optoelectronic devices. Despite their great potential, commercial deployment is hindered by MHPs lack of stability and durability, mainly attributed to ion migration and chemical interactions with the electrodes. To address these issues, 2D layered MHPs are investigated as possible device interlayers or active material substitutes. Here, the 2D perovskite (PEA)2PbBr4 is considered that is recently discussed as promising candidate for X‐ray direct detection. While the increased resilience of (PEA)2PbBr4 radiation detectors has already been reported, the physical mechanisms responsible for such improvement compared to 3D perovskites are not still fully understood. To unravel the charge transport process in (PEA)2PbBr4 crystals thought to underly the device better performance, an investigation technique is adapted previously used on highly resistive inorganic semiconductors, called photo induced current transient spectroscopy (PICTS). It is demonstrated that PICTS can reliably detect three trap states (T1, T2, and T3), and that their evolution upon X‐ray exposure can explain (PEA)2PbBr4 superior radiation tolerance and reduced aging effects. Overall, the results provide essential insights into the electrical characteristics of 2D perovskites and their potential application as reliable direct X‐ray detectors.

Al–O–Al defect complexes as possible candidates for channel electron mobility reducing trapping centers in 4H-SiC metal–oxide–semiconductor field-effect transistors

The deep-level drain current transient spectroscopy (Id-DLTS) measurements of Al-doped SiC metal–oxide–semiconductor field-effect transistors (MOSFETs) strongly suggest that the reduction in the channel mobility at low temperatures is related to a shallow trap detectable at 70 K. Using the Shockley–Reed–Hall (SRH) theory, the level of this trap has been extracted to be around 0.15 eV below the conduction band minimum of SiC. Density functional theory (DFT) calculations of AlSiNCAlSi and AlSiOCAlSi defect complexes have found one configuration of the AlSiOCAlSi complex, which has a charge transition level within the SRH extracted trap level range. Therefore, we suggest that these AlSiOCAlSi defects are likely candidates for traps responsible for the channel mobility reduction.

Electrical characteristics and trap signatures for Schottky barrier diodes on 4H-SiC, GaN-on-GaN, AlGaN/GaN epitaxial substrates

The forward and reverse current transport mechanisms, temperature dependence of Schottky barrier height (SBH) and ideality factor, barrier inhomogeneity analysis, and trap parameters for Schottky barrier diodes (SBDs) fabricated on 4H-SiC, GaN-on-GaN and AlGaN/GaN epitaxial substrates are reported. High SBH is identified for Ni/4H-SiC (1.31 eV) and Ti/4H-SiC (1.18 eV) SBDs with a low leakage current density of <10−8 A cm−2 at −200 V. Thermally stimulated capacitance detects the well-known Z1/2 electron trap at E C—0.65 eV in both 4H-SiC SBDs, while an additional deep-level trap at E C—1.13 eV is found only in Ni/4H-SiC SBDs. The vertical Ni/GaN SBD exhibits a promising SBH of 0.83 eV, and two electron traps at E C—0.18 eV and E C—0.56 eV are identified from deep-level transient Fourier spectroscopy. A peculiar two-diode model behavior is detected at metal/GaN/AlGaN/GaN interface of high-electron mobility transistor (HEMT); the first diode (SBH-1 of 1.15 eV) exists at the standard Metal/GaN Schottky junction, whereas the second diode (SBH-2 of 0.72 eV) forms due to the energy difference between the AlGaN conduction band and the heterojunction Fermi level. The compensational Fe-doping-related buffer traps at E C—0.5 eV and E C—0.6 eV are determined in the AlGaN/GaN HEMT, through the drain current transient spectroscopy experiments.

Attaining quantitatively fewer defects in close-packed InGaZnO synthesized using atomic layer deposition

Owing to their uniformity across large areas, low-temperature processability, and low off-current characteristics, oxide semiconductors demonstrate significant potential for integration into displays, memories, and logic devices. In the hyper scaling era, atomic layer deposition (ALD) is well suited for downsizing device fabrication, leveraging self-limiting chemical reactions to provide nanoscale control, and conformal coating on substrates with high aspect ratios. Hence, in this study, we fabricated highly oriented c-axis-aligned crystalline (CAAC) indium-gallium-zinc oxide (IGZO) thin films using plasma-enhanced ALD (PEALD), introducing an additional thermodynamic driving force for crystallization. A comparative study was conducted on the properties of CAAC-IGZO in correspondence to conventional sputter-based IGZO. In particular, photo-induced current transient spectroscopy (PICTS) is employed to map the electronic structure (density of states (DOS)) of the films, which quantitatively confirmed fewer defects in CAAC-IGZO. Fewer defects result in highly stable CAAC-IGZO transistors with positive bias stress threshold (PBTS) at 0.03 V and negative bias stress threshold (NBTS) at - 0.06 V. Thus, this research on thermally and electrically stable CAAC-IGZO synthesized at low temperatures through PEALD offers insights into the processing and material perspectives for the application of oxide semiconductors in 3D integration.

Deep-Level Structure of the Spin-Active Recombination Center in Dilute Nitrides.

A gallium interstitial defect is thought to be responsible for the spectacular spin-dependent recombination in GaAs_{1-x}N_{x} dilute nitrides. Current understanding associates this defect with at least two in-gap levels corresponding to the (+/0) and (++/+) charge-state transitions. Using a spin-sensitive photoinduced current transient spectroscopy, the in-gap electronic structure of a x=0.021 alloy is revealed. The (+/0) state lies ≈0.27 eV below the conduction band edge, and an anomalous, negative activation energy reveals the presence of not one but two other in-gap states. The observations are consistent with a (++/+) state ≈0.19 eV above the valence band edge, and a (+++/++) state ≈25 meV above the valence band edge.

Tracking deep-level defects in degrading perovskite solar cells with a multifactorial approach

In this work, we provide a mechanistic understanding of the degradation of perovskite solar cells in operation by focusing on methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) and tracking the evolution of electronic defects via photo-induced current transient spectroscopy (PICTS). Moreover, we also record the degradation of its photovaltaic characteristics over time under various electric load and temperature conditions. Using PICTS, we found that bands of trap states, initially highly localized deep within the band gap of the perovskite, widened over the exposure period. This effect was exacerbated with increasing temperature. Further, using the design of experiment methodology for this multifactorial study, we found that two interaction factors (temperature× load & temperature× time) were significant in the degradation of the perovskite cells, validating the importance of our holistic approach. Through these observations, we establish a mechanistic link between deep-level traps and photovoltaic characteristics.

Suppressed current collapse and improved threshold voltage stability of AlGaN/GaN HEMT via O2 plasma treatment

A comprehensive study about the effects of O2 plasma treatment on the dynamic performance of AlGaN/GaN high electron mobility transistors (HEMTs) is presented. The drain current transient spectroscopy indicated a much decelerated and mitigated current degradation process for the HEMT with O2 plasma treatment. Upon negative gate bias stressing, current collapse of 10.7 % and minor threshold voltage shift of 0.16 V were achieved by O2 plasma treatment. In addition, current collapse ratio of the HEMTs as a function of stress/recovery time showed that the HEMT with O2 plasma treatment had a superior performance in various on-off conditions. Particularly in high-frequency switching events, current collapse ratio was reduced from about 50 % to 0.2 %. At last, the quality of AlGaN/metal interface with O2 plasma treatment is demonstrated by capacitance-frequency measurements, with the interface trap density Dit is estimated to be 1.39 × 1012 cm−2 eV−1. These results indicate that GaN HEMT with O2 plasma treatment is a promising technology in power switching applications.

Investigation of high resistivity [formula omitted]-type FZ silicon diodes after 60Co [formula omitted]-irradiation

In this work, the effects of 60Co γ-ray irradiation on high resistivity p-type diodes have been investigated. The diodes were exposed to dose values of 0.1, 0.2, 1, and 2MGy. Both macroscopic (I–V, C–V) and microscopic investigations, by means of Thermally Stimulated Current (TSC) and Deep Level Transient Spectroscopy (DLTS) techniques, were conducted to characterize the radiation-induced changes. The investigated diodes were manufactured on high resistivity p-type Float Zone (FZ) silicon and were further classified into two types based on the isolation technique between the pad and guard ring: p-stop and p-spray. After irradiation, the macroscopic results of current–voltage and capacitance–voltage measurements were obtained and compared with existing literature data. Additionally, the microscopic measurements focused on the development of the concentration of different radiation-induced defects, including the Boron interstitial-Oxygen interstitial (BiOi) complex, the Carbon interstitial-Oxygen interstitial (CiOi) defect, the H40K, and the so-called IP∗.To investigate the thermal stability of induced defects in the bulk, isochronal annealing studies were performed in the temperature range of 100°C to 300°C. These annealing processes were carried out on diodes irradiated with doses of 1 and 2MGy. Furthermore, in order to investigate the unexpected results observed in the C–V measurements after irradiation with high dose values, the surface conductance between the pad and guard ring was measured as a function of both dose and annealing temperature.

Photoinduced Current Transient Spectroscopy on Metal Halide Perovskites: Electron Trapping and Ion Drift.

Metal halide perovskites (MHPs) are disruptive materials for a vast class of optoelectronic devices. The presence of electronic trap states has been a tough challenge in terms of characterization and thus mitigation. Many attempts based on electronic spectroscopies have been tested, but due to the mixed electronic-ionic nature of MHP conductivity, many experimental results retain a large ambiguity in resolving electronic and ionic charge contributions. Here we adapt a method, previously used in highly resistive inorganic semiconductors, called photoinduced current transient spectroscopy (PICTS) on lead bromide 2D-like ((PEA)2PbBr4) and standard "3D" (MAPbBr3) MHP single crystals. We present two conceptually different outcomes of the PICTS measurements, distinguishing the different electronic and ionic contributions to the photocurrents based on the different ion drift of the two materials. Our experiments unveil deep level trap states on the 2D, "ion-frozen" (PEA)2PbBr4 and set new boundaries for the applicability of PICTS on 3D MHPs.

Quantitative analysis of defect states in InGaZnO within 2 eV below the conduction band via photo-induced current transient spectroscopy

This work investigates the function of the oxygen partial pressure in photo-induced current measurement of extended defect properties related to the distribution and quantity of defect states in electronic structures. The Fermi level was adjusted by applying a negative gate bias in the TFT structure, and the measurable range of activation energy was extended to < 2.0 eV. Calculations based on density functional theory are used to investigate the changes in defect characteristics and the role of defects at shallow and deep levels as a function of oxygen partial pressure. Device characteristics, such as mobility and threshold voltage shift under a negative gate bias, showed a linear correlation with the ratio of shallow level to deep level defect density. Shallow level and deep level defects are organically related, and both defects must be considered when understanding device characteristics.

Assessment of deep levels with selenium concentration in Cd1–xZnxTe1–ySey room temperature detector materials

Incorporation of Se into Cd1−xZnxTe (CZT) to form the quaternary compound semiconductor Cd1−xZnxTe1–ySey (CZTS) has proven to be an effective solution for compensating the major flaws associated with CZT, including poor homogeneity and high concentrations of electronically active deep levels that limit the performance of CZT detectors. In order to investigate how deep levels are affected by the Se concentration in CZTS, we performed photoinduced current transient spectroscopy (PICTS) measurements on CZTS crystals grown by the traveling heater method (THM) with 10% atomic Zn and varying atomic percentage of Se from 1.5% to 7.0%. The PICTS scans for up to 4% Se showed an exponential reduction in the capture cross section of deep levels associated with Te secondary phases in conjunction with an increase in a deep level positioned near the mid-gap, which initially increases the electron trapping time before degrading again at higher Se concentrations. The PICTS peaks present in 7% Se were anomalous relative to the other crystals and are expected to originate from transition metal impurities found in the lower-purity CdSe precursor material.

Current Transient Spectroscopic Study of Vacancy Complexes in Diamond Schottky p-i-n Diode

The present state-of-the-art of junction-based diamond electronic devices is limited by the challenges inherent to the synthesis of n-type layers, where the dopant itself forms defect complexes that are donor compensating centers. Recently, the fabrication of diamond Schottky p-i-n diodes (SPINDs) by chemical vapor deposition (CVD) of intrinsic diamond films followed by an n-type layer on p-type diamond substrates has been reported to demonstrate excellent diode-like behavior with high rectification factors and surface barrier height at elevated temperatures. However, due to the ultrawide bandgap of diamond and small contact area of the devices, it is challenging to characterize the defect centers that limit the performance of the SPINDs using conventional capacitance transient spectroscopy. In this article, we report the characterization of defect centers in the diamond SPINDs with a contact area of 280 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}\,\,\times \,\,280\,\,\mu \text{m}$ </tex-math></inline-formula> using current transient spectroscopy (CTS), which measures thermally induced emission rates from trap centers through change in device current instead of junction capacitance. The CTS scans in the temperature range 80–800 K revealed the presence of five trap-related peaks. Results from density functional theory (DFT) calculations were used to identify the nature of the traps. While most of the detected traps were identified to be vacancy complexes coupled to phosphorus and hydrogen impurities, one trap has been identified as single carbon vacancy. Most of the phosphorus-related defect complexes were observed to be in neutral and/or negative charge states and were identified as acceptor-like states.

Point defect creation by proton and carbon irradiation of α-Ga2O3

Films of α-Ga2O3 grown by Halide Vapor Phase Epitaxy (HVPE) were irradiated with protons at energies of 330, 400, and 460 keV with fluences 6 × 1015 cm−2 and with 7 MeV C4+ ions with a fluence of 1.3 × 1013 cm−2 and characterized by a suite of measurements, including Photoinduced Transient Current Spectroscopy (PICTS), Thermally Stimulated Current (TSC), Microcathodoluminescence (MCL), Capacitance–frequency (C–f), photocapacitance and Admittance Spectroscopy (AS), as well as by Positron Annihilation Spectroscopy (PAS). Proton irradiation creates a conducting layer near the peak of the ion distribution and vacancy defects distribution and introduces deep traps at Ec-0.25, 0.8, and 1.4 eV associated with Ga interstitials, gallium–oxygen divacancies VGa–VO, and oxygen vacancies VO. Similar defects were observed in C implanted samples. The PAS results can also be interpreted by assuming that the observed changes are due to the introduction of VGa and VGa–VO.

Electrical properties of oxygen precipitate-related defects in Czochralski silicon

Oxygen precipitate-related defects in Czochralski (Cz) silicon have been studied by electron-beam-induced current (EBIC) and deep-level transient spectroscopy (DLTS). The EBIC results present that oxygen precipitates combined with dislocations can strengthen carrier recombination. The DLTS data reveals two levels of T 1 (E v + 0.43 eV) and T 2 (E v + 0.26 eV), which are related to the oxygen precipitates and dislocations, respectively. Hydrogen can partly passivate the recombination activity of oxygen precipitates and dislocations, as well as their induced electronic states. The results are of interest for deeply understanding the electronic properties of oxygen precipitates and their induced dislocations in Cz silicon used in the semiconductor industry.

Electrical properties of α-Ga2O3 films grown by halide vapor phase epitaxy on sapphire with α-Cr2O3 buffers

We report on growth and electrical properties of α-Ga2O3 films prepared by halide vapor phase epitaxy (HVPE) at 500 °C on α-Cr2O3 buffers predeposited on sapphire by magnetron sputtering. The α-Cr2O3 buffers showed a wide microcathodoluminescence (MCL) peak near 350 nm corresponding to the α-Cr2O3 bandgap and a sharp MCL line near 700 nm due to the Cr+ intracenter transition. Ohmic contacts to Cr2O3 were made with both Ti/Au or Ni, producing linear current–voltage (I–V) characteristics over a wide temperature range with an activation energy of conductivity of ∼75 meV. The sign of thermoelectric power indicated p-type conductivity of the buffers. Sn-doped, 2-μm-thick α-Ga2O3 films prepared on this buffer by HVPE showed donor ionization energies of 0.2–0.25 eV, while undoped films were resistive with the Fermi level pinned at EC of 0.3 eV. The I–V and capacitance–voltage (C–V) characteristics of Ni Schottky diodes on Sn-doped samples using a Cr2O3 buffer indicated the presence of two face-to-face junctions, one between n-Ga2O3 and p-Cr2O3, the other due to the Ni Schottky diode with n-Ga2O3. The spectral dependence of the photocurrent measured on the structure showed the presence of three major deep traps with optical ionization thresholds near 1.3, 2, and 2.8 eV. Photoinduced current transient spectroscopy spectra of the structures were dominated by deep traps with an ionization energy of 0.95 eV. These experiments suggest another pathway to obtain p–n heterojunctions in the α-Ga2O3 system.

Temperature-Dependent Dynamic Performance of p-GaN Gate HEMT on Si

Temperature-dependent dc and dynamic characteristics of p-GaN gate HEMT were thoroughly investigated from 300 to 140 K. At low temperature, in addition to barely shifted threshold voltage, substantial improvement in drain current was observed. At 300 K both positive and negative gate bias stressing were applied to the device in order to form a complete mapping of threshold voltage instability of p-GaN gate HEMT. Three mechanisms, namely hole trap emission, carrier out-spilling, and hole accumulation have been employed to elucidate the trends of threshold voltage shift at room temperature. Temperature-dependent dynamic performances, including threshold voltage instability and drain current degradation, were investigated. Via drain current transient spectroscopy, activation energy for hole emission and trapping process were extracted as 132 and 70 meV, respectively. Detailed pulsed output characteristics at various quiescent biases were explored. At 300 K, gate quiescent bias played a dominant role in determining current collapse compared with drain quiescent bias. Moreover, current collapse caused by gate pulse stressing was much mitigated with decrease of temperature. The detailed temperature-reliant dynamic performance provides valuable information for justifying feasibility of p-GaN gate HEMT for low-temperature applications and further device optimizations.

Hole capture cross section of the Al acceptor level in 4H-SiC

We performed current deep level transient spectroscopy (I-DLTS) on Al-doped p-type 4H-SiC epilayers. The epilayers exhibited a peak at 100–120 K in the I-DLTS spectra. The correspondence of the deep-level concentrations with the net acceptor concentration implies that the peak originates from the Al acceptor level. The observed activation energy of the peak was low compared to the Al acceptor level reported by Hall measurements owing to the Poole-Frenkel effect. We estimated capture cross section σ for holes of the Al acceptor level at the peak temperature based on the I-DLTS peak height dependence on the injection pulse widths and analyzed the temperature dependence of σ for holes.

Identification of Semi-ON-State Current Collapse in AlGaN/GaN HEMTs by Drain Current Deep Level Transient Spectroscopy

A drain-controlled current-mode deep level transient spectroscopy (I-DLTS), was developed for investigation of Semi-ON-state current collapse in AlGaN/GaN high electron mobility transistors (HEMTs). By inserting a graded AlGaN back-barrier between the GaN channel and AlGaN buffer layer, hot-electron effect induced charging of buffer traps is effectively blocked, which contributes to a remarkable suppressed current collapse in AlGaN/GaN HEMTs under Semi-ON-state stress. A broad-distributed electron trap <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {T1}}$ </tex-math></inline-formula> , which features an energy level depth <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {C}}$ </tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {T}}$ </tex-math></inline-formula> and apparent capture cross section <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\sigma }_{\text {T}}$ </tex-math></inline-formula> of 0.25 ± 0.05 eV and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$9.0\,\, {\pm } 1.0 {\times } 10^{-18}$ </tex-math></inline-formula> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , was verified to locate in the AlGaN buffer layer by I-DLTS. A combination of pulsed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}$ </tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula> and I-DLTS measurements, could be an effective method for in-situ study of current collapse mechanism in AlGaN/GaN HEMTs.

Dynamic Characteristics of GaN MISHEMT With 5-nm In-Situ SiNx Dielectric Layer

A comprehensive study on dynamic characteristics of GaN MISHEMT with a 5nm-thick in-situ SiNx dielectric is presented. Effects of both negative and positive gate bias on threshold voltage instability were investigated and miniature threshold voltage shift was acquired. The slight shift was considered to be associated with the traps at the insulator/AlGaN interface and in the dielectric layer itself. Pulsed I-V measurements with various gate quiescent biases presented small current collapse (11%) and low enhancement of dynamic Ron for zero quiescent drain bias. When drain quiescent bias was strengthened to 20V, an increased dynamic Ron/static Ron ratio was identified but still limited to a low value of 1.24. The conduction reduction was in a good agreement with measurement results from drain current transient spectroscopy and possibly originates from trap states existed in the access region. Additional current collapse was observed in hard switching-on operation, resulted from energetic hot electrons accelerated by drain-source electrical field during the off-to-on step. The measurement results showed stabilized threshold voltage, a low dynamic Ron/static Ron ratio, and suppressed current collapse via employing a 5-nm thin in-situ SiNx layer in GaN MISHEMT, enabling it a promising solution for high-efficiency power switching applications.

Deep levels related to the carbon antisite–vacancy pair in 4H-SiC

Photo-induced current transient spectroscopy (PICTS) and electron paramagnetic resonance (EPR) are used to study irradiation-induced defects in high-purity semi-insulating (HPSI) 4H-SiC. Several deep levels with the ionization energy ranging from 0.1 to ∼1.1 eV have been observed in irradiated and annealed samples by PICTS. Among these, two deep levels, labeled E370 and E700 at ∼0.72 and ∼1.07 eV below the conduction band, respectively, are detected after high-temperature annealing. The appearance and disappearance of these two deep levels and the EPR signal of the positive C antisite–vacancy pair (CSiVC+) in the sample annealed at 1000 and 1200 °C, respectively, are well correlated. Based on data from PICTS and EPR and the energies predicted by previous calculations for different charge states of dominant intrinsic defects, the E370 and E700 levels are suggested to be related to the charge transition levels (0|–) and (+|0), respectively, of the C antisite–vacancy pair. The activation energy of Ea ∼ 1.1 eV in commercial HPSI 4H-SiC materials is, therefore, reassigned to be related to the single donor (+|0) level of CSiVC.