Optical Deep Level Transient Spectroscopy Research Articles

Resolving the Trap Distribution of Chalcogenide-Based Heterojunctions via Optical Injection Deep Level Transient Spectroscopy.

Chalcogenide semiconductors have emerged as promising candidates for next-generation optoelectronics, mainly benefiting from their cost-effective processability, chemical versatility, and excellent stability. However, the device performance is limited by the complex defect characteristics, necessitating a detailed understanding of the defect density, types, and distribution. This study employs optical injection deep level transient spectroscopy to characterize the trap features of antimony bismuth sulfide-based heterojunctions. Photoexcitation offers the possibility to determine the spatial distribution of traps. It reveals distinct distributions of hole and electron traps. Short-wavelength light (570 nm) detects hole traps near the hole transport layer, while long-wavelength light (830 nm) identifies electron traps near the electron transport layer. Additional intermediate-wavelength tests (670 and 755 nm) and light field distribution simulation further elucidate the trap distribution. Transient absorption and transient photocurrent also support the non-uniform trap distribution within the chalcogenide-based photodiodes.

Emission and capture characteristics of deep hole trap in n-GaN by optical deep level transient spectroscopy

Emission and capture characteristics of a deep hole trap (H1) in n-GaN Schottky barrier diodes (SBDs) have been investigated by optical deep level transient spectroscopy (ODLTS). Activation energy (E emi) and capture cross-section (σ p) of H1 are determined to be 0.75 eV and 4.67 × 10−15 cm2, respectively. Distribution of apparent trap concentration in space charge region is demonstrated. Temperature-enhanced emission process is revealed by decrease of emission time constant. Electric-field-boosted trap emission kinetics are analyzed by the Poole−Frenkel emission (PFE) model. In addition, H1 shows point defect capture properties and temperature-enhanced capture kinetics. Taking both hole capture and emission processes into account during laser beam incidence, H1 features a trap concentration of 2.67 × 1015 cm−3. The method and obtained results may facilitate understanding of minority carrier trap properties in wide bandgap semiconductor material and can be applied for device reliability assessment.

Study of minority carrier traps in p-GaN gate HEMT by optical deep level transient spectroscopy

Properties of minority carrier (electron) traps in Schottky type p-GaN gate high electron mobility transistors were explicitly investigated by optical deep level transient spectroscopy (ODLTS). By temperature-scanning ODLTS, three electron traps, namely, E1, E2, and E3, were revealed, together with activation energy, capture cross section, and trap concentration. A thermally accelerated electron-releasing process of traps was quantitatively studied by Laplace ODLTS with individual emission time constant disclosed. At 300 K, the emission time constant was determined to be 0.21 and 1.40 s for E2 and E3, respectively, which adjacently existed in the bandgap and held activation energies of over 0.6 eV. As varying the optical injection pulse duration, a three-dimensional mapping of capacitance transient was obtained for each trap, attesting to the electron capture capability of each trap. By varying the reverse bias, the analysis of the ODLTS signal amplitude indicates that all three electron traps are located inside the p-GaN layer rather than the surface defect related.

Revealing composition and structure dependent deep-level defect in antimony trisulfide photovoltaics

Antimony trisulfide (Sb2S3) is a kind of emerging light-harvesting material with excellent stability and abundant elemental storage. Due to the quasi-one-dimensional symmetry, theoretical investigations have pointed out that there exist complicated defect properties. However, there is no experimental verification on the defect property. Here, we conduct optical deep-level transient spectroscopy to investigate defect properties in Sb2S3 and show that there are maximum three kinds of deep-level defects observed, depending on the composition of Sb2S3. We also find that the Sb-interstitial (Sbi) defect does not show critical influence on the carrier lifetime, indicating the high tolerance of the one-dimensional crystal structure where the space of (Sb4S6)n ribbons is able to accommodate impurities to certain extent. This study provides basic understanding on the defect properties of quasi-one-dimensional materials and a guidance for the efficiency improvement of Sb2S3 solar cells.

Probing the trap states in N-i-P Sb2(S,Se)3 solar cells by deep-level transient spectroscopy.

In this study, we provide fundamental understanding on defect properties of the Sb2(S,Se)3 absorber film and the impact on transmission of photo-excited carriers in N-i-P architecture solar cells by both deep level transient spectroscopy (DLTS) and optical deep level transient spectroscopy (ODLTS) characterizations. Through conductance-voltage and temperature-dependent current-voltage characterization under a dark condition, we find that the Sb2(S,Se)3 solar cell demonstrates good rectification and high temperature tolerance. The DLTS results indicates that there are two types of deep level hole traps H1 and H2 with active energy of 0.52 eV and 0.76 eV in the Sb2(S,Se)3 film, and this defect property is further verified by ODLTS. The two traps hinder the transmission of minority carrier (hole) and pinning the Fermi level, which plays a negative role in the improvement of open-circuit voltage for Sb2(S,Se)3 solar cells. This research suggests a critical direction toward the efficiency improvement of Sb2(S,Se)3 solar cells.

Deep traps and persistent photocapacitance in β-(Al0.14 Ga0.86)2O3/Ga2O3 heterojunctions

Persistent photocapacitance (PPC) was observed in β-(Al0.14Ga0.84)2O3/n+Ga2O3 heterojunctions at low temperatures. The effect is seen in capacitance-frequency measurements under illumination and in admittance spectra after illumination. In the latter case, the capacitance versus frequency curve after illumination returned to its dark values only at temperatures between 225 and 275 K. Prominent hole-trap-like peaks in optical deep level transient spectroscopy (ODLTS) near 130 K were attributed to quenching with temperature of the PPC effect. The phenomena were assigned to electrons being excited from deep traps in the (Al0.14Ga0.86)2O3 barrier into the two-dimensional electron gas (2DEG) in the Ga2O3 substrate. The reverse process then involves excitation of non-equilibrium electrons in the 2DEG back into the ternary barrier layer and subsequent re-capture by the host deep traps. The effective barrier height for this recovery process was ∼0.2 eV, as estimated from ODLTS measurements. The spectral dependence of the effect suggests the presence of deep traps with optical ionization thresholds near 1 eV and ∼2.3 eV. The shift of the threshold voltage necessary to deplete the 2DEG as a function of the photon energy during illumination at room temperature indicates that the most prominent centers in the barrier have an optical ionization energy near 2.3 eV and a sheet density of ∼1012 cm−2. In microcathodoluminescence (MCL) spectra of the heterojunction, a blue shift by 0.2 eV of the MCL band near 3.1 eV dominant in the substrate and the emergence of a weak MCL peak near 4.8 eV were observed. Electron beam induced current measurements point to the presence of small local areas where the 2DEG formation is handicapped due to Al composition variations or to defects.

Luminescence of undoped commercial ZnS crystals: A critical review and new evidence on the role of impurities using photoluminescence and electrical transient spectroscopy

Bulk undoped ZnS materials exhibit relatively bright yet diverse luminescence behavior, which has, in recent years, been attributed to intrinsic defects. However, the luminescence also resembles that of doped materials, implying a role of impurities. Luminescence features have also been attributed to oxygen impurities causing defect clusters or energy band anti-crossing. Thus, this study couples optical and electrical techniques, such as band edge transmission, photoluminescence (PL), PL excitation, radioluminescence, thermoluminescence, optical deep level transient spectroscopy, and photoinduced current transient spectroscopy, to explore the identity of defect levels. ZnS materials studied are commercial single crystals made by physical vapor transport, high-pressure Bridgman, and powder processing. These undoped bulk ZnS exhibit luminescence behavior similar to that of reported doped ZnS powders (10−4 to 10−2 mol. % doping for luminescent materials). Dopants (such as Al, Cl, Cu, and Ag) are also commonly found impurities in ZnS; hence, it is reasonable to believe they have a role in the luminescence of nominally undoped ZnS. By comparing the variation in optical and electrical properties between samples to the processing method and the rich literature on intentionally doped ZnS, this study shows a possible dominant contribution of impurities and impurity-containing defects on the luminescence of bulk “undoped” ZnS. Hence, there is no need to resort to complex mechanisms to explain the luminescence, but rather the metal and halide impurities and their defect complexes determine the main characteristics of luminescence in this wide-bandgap semiconductor.

Trap states in multication mesoscopic perovskite solar cells: A deep levels transient spectroscopy investigation

This work presents a study of trap levels in a mesoscopic multication lead halide perovskite solar cell structure. The investigation is performed by combining capacitance measurements, admittance measurements, Deep Level Transient Spectroscopy (DLTS), and Optical DLTS. We found a donor level with an energy of 0.2 eV below the conduction band of perovskite. The donor density reaches a concentration of 1018 cm−3 in the accumulation region present at the interface between the perovskite and transporting layers. Other two deep trap levels are found with energies of 0.57 eV and 0.74 eV. The first level is related to a hole trap while the second one to an electron trap.

Impact of deep level defects induced by high energy neutron radiation in β-Ga2O3

The effects of high energy neutron irradiation on the deep level defect concentration profile throughout the bandgap of β-Ga2O3 were investigated by a combination of deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS). For the unintentionally doped edge-defined film-fed growth-grown (010) β-Ga2O3 substrates investigated here, it was found that the dominant effects of neutron irradiation were to produce defects detected by DLOS having energy levels of EC −1.29 eV and EC −2.00 eV, with no discernable impact on traps within ∼1 eV of the conduction band edge. Commensurate with the introduction of these states was a significant amount of net doping reduction, for which lighted capacitance-voltage studies revealed that both of these irradiation-induced deep states are responsible, likely through a compensation mechanism. The sensitivity of the EC −1.29 eV and EC −2.00 eV states on irradiation suggests an intrinsic source, and whereas the EC −2.00 eV state was already present in the as-grown material, the EC −1.29 eV state was not detected prior to irradiation. DLOS and DLTS revealed other defect states at EC −0.63 eV, EC −0.81 eV, and EC −4.48 eV, but none of these responded to neutron irradiation for two different 1 MeV equivalent fluences 8.5 × 1014 cm−2 and 1.7 × 1015 cm−2, which is consistent with the behavior expected for defect states having an extrinsic source.

Electrical characterization of HVPE GaN containing different concentrations of carbon dopants

A comprehensive study of the electrical characteristics in Schottky diodes made of GaN:C grown by hydride vapour phase epitaxy (HVPE) technology has been reported. The Schottky junctions were made of an Ni/Au (25/200 nm) metal stack and ohmic contacts were fabricated by Ti/Al/Ni/Au (30/90/20/100 nm) e-beam deposition of the metal thin-films followed by rapid thermal annealing. A good quality of the fabricated Schottky diodes has been proved by considering the transient shape using a pulsed technique of barrier evaluation under linearly increasing voltage (BELIV). The concentrations of equilibrium carriers of 1 × 1011 cm−3 and of 2 × 1010 cm−3 have been evaluated for relatively low and high carbon density doped samples, respectively, using photo-capacitance characteristics dependent on excitation intensity. The effective mobility of carriers of μeff = 610 cm2/Vs has been estimated by considering the serial resistance variations dependent on excitation density, through analysis of delay times that appeared in the formation of peaks within BELIV transients. Optical deep level transient spectroscopy has shown that thermal emission from the rather shallow centres prevails. The centres, ascribed to vacancies as well as to carbon on Ga sites, have been identified. It has been demonstrated that Schottky junctions made of heavy carbon doped (NC ≥ 1018 cm−3) HVPE GaN material are able to withstand voltages of ≥300 V.

Compensation and persistent photocapacitance in homoepitaxial Sn-doped β-Ga2O3

The electrical properties of epitaxial β-Ga2O3 doped with Sn (1016–9 × 1018 cm−3) and grown by metalorganic chemical vapor deposition on semi-insulating β-Ga2O3 substrates are reported. Shallow donors attributable to Sn were observed only in a narrow region near the film/substrate interface and with a much lower concentration than the total Sn density. For heavily Sn doped films (Sn concentration, 9 × 1018 cm−3), the electrical properties in the top portion of the layer were determined by deep centers with a level at Ec-0.21 eV not described previously. In more lightly doped layers, the Ec-0.21 eV centers and deeper traps at Ec-0.8 eV were present, with the latter pinning the Fermi level. Low temperature photocapacitance and capacitance voltage measurements of illuminated samples indicated the presence of high densities (1017–1018 cm−3) of deep acceptors with an optical ionization threshold of 2.3 eV. Optical deep level transient spectroscopy (ODLTS) and photoinduced current transient spectroscopy (PICTS) detected electron traps at Ec-0.8 eV and Ec-1.1 eV. For lightly doped layers, the compensation of film conductivity was mostly provided by the Ec-2.3 eV acceptors. For heavily Sn doped films, deep acceptor centers possibly related to Ga vacancies were significant. The photocapacitance and the photocurrent caused by illumination at low temperatures were persistent, with an optical threshold of 1.9 eV and vanished only at temperatures of ∼400 K. The capture barrier for electrons causing the persistent photocapacitance effect was estimated from ODLTS and PICTS to be 0.25–0.35 eV.

Deep level defects in Ge-doped (010) β-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy

Deep level defects were characterized in Ge-doped (010) β-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy (PAMBE) using deep level optical spectroscopy (DLOS) and deep level transient (thermal) spectroscopy (DLTS) applied to Ni/β-Ga2O3:Ge (010) Schottky diodes that displayed Schottky barrier heights of 1.50 eV. DLOS revealed states at EC − 2.00 eV, EC − 3.25 eV, and EC − 4.37 eV with concentrations on the order of 1016 cm−3, and a lower concentration level at EC − 1.27 eV. In contrast to these states within the middle and lower parts of the bandgap probed by DLOS, DLTS measurements revealed much lower concentrations of states within the upper bandgap region at EC − 0.1 – 0.2 eV and EC − 0.98 eV. There was no evidence of the commonly observed trap state at ∼EC − 0.82 eV that has been reported to dominate the DLTS spectrum in substrate materials synthesized by melt-based growth methods such as edge defined film fed growth (EFG) and Czochralski methods [Zhang et al., Appl. Phys. Lett. 108, 052105 (2016) and Irmscher et al., J. Appl. Phys. 110, 063720 (2011)]. This strong sensitivity of defect incorporation on crystal growth method and conditions is unsurprising, which for PAMBE-grown β-Ga2O3:Ge manifests as a relatively “clean” upper part of the bandgap. However, the states at ∼EC − 0.98 eV, EC − 2.00 eV, and EC − 4.37 eV are reminiscent of similar findings from these earlier results on EFG-grown materials, suggesting that possible common sources might also be present irrespective of growth method.

Tutorial: Junction spectroscopy techniques and deep-level defects in semiconductors

The term junction spectroscopy embraces a wide range of techniques used to explore the properties of semiconductor materials and semiconductor devices. In this tutorial review, we describe the most widely used junction spectroscopy approaches for characterizing deep-level defects in semiconductors and present some of the early work on which the principles of today's methodology are based. We outline ab-initio calculations of defect properties and give examples of how density functional theory in conjunction with formation energy and marker methods can be used to guide the interpretation of experimental results. We review recombination, generation, and trapping of charge carriers associated with defects. We consider thermally driven emission and capture and describe the techniques of Deep Level Transient Spectroscopy (DLTS), high resolution Laplace DLTS, admittance spectroscopy, and scanning DLTS. For the study of minority carrier related processes and wide gap materials, we consider Minority Carrier Transient Spectroscopy (MCTS), Optical DLTS, and deep level optical transient spectroscopy together with some of their many variants. Capacitance, current, and conductance measurements enable carrier exchange processes associated with the defects to be detected. We explain how these methods are used in order to understand the behaviour of point defects and the determination of charge states and negative-U (Hubbard correlation energy) behaviour. We provide, or reference, examples from a wide range of materials including Si, SiGe, GaAs, GaP, GaN, InGaN, InAlN, and ZnO.

(Invited) DLTS Studies of Defects in n-GaN

Introduction There exist various defects in GaN depending on growth methods or growth conditions or substrates used for the epitaxial growth. There are point defects, such as vacancy-related defects, antisite defects, interstitials as well as line defects, i.e. dislocations. These affect the performance of GaN-based devices, since they create deep levels in the bandgap and act as traps and generation-recombination centers. Foreign atoms, such as carbon and iron, also create deep levels. It is important to characterize these defects and identify their origins to realize the high performance of GaN-based devices. Current collapse in AlGaN/GaN high electron mobility transistors is well known to be caused by trapping of carriers [1]. In this work, we present our results on the characterization of traps in Si-doped n-GaN grown by metal organic chemical vapor deposition (MOCVD) on different substrates, that is, sapphire, n+-GaN, SiC and Si [2]. Experimental Lateral and vertical Schottky diodes were fabricated using Pt/Au or Ni/Au as a Schottky contact and Ti/Al/Ni/Au as an ohmic contact for n-GaN on sapphire, Si and SiC substrates and for n-GaN on n+-GaN substrates, respectively. Carrier concentrations determined from capacitance-voltage measurements at room temperature are in the range from 1016 to 1017 cm-3. Deep level transient spectroscopy (DLTS) using bias pulses has been applied to estimate electron traps for fabricated Schottky diodes. Minority carrier transient spectroscopy (MCTS) using above-band-gap light pulses has been used to characterize hole traps. Results and Discussion Temperature-scan DLTS and MCTS have been performed in the temperature range from 90 to around 400 K. DLTS measurements detected four electron traps labeled E1 (Ec-0.24 eV), E3 (Ec-0.57 eV), E6 (Ec-0.40 eV) and E7 (Ec-0.73 eV). E1 and E3 might be ascribed to VGa-VN pairs and NGa antisite defects, respectively. E6 and E7 are observed for n-GaN on SiC and Si, suggesting their correlation to dislocation-related defects. MCTS measurements detected three hole traps labeled H1 (Ev+0.86 eV), H2 (Ev+0.25 eV) and H3 (Ev+0.25 eV). Trap E3 and H1 concentrations are as high as 1016 cm-3 and show a large variation of trap concentration on each wafer. Moreover, trap H1 concentration increases in n-GaN intentionally doped with carbon, suggesting its correlation to carbon-related defects, possibly in addition to VGa-related defects. We have studied a distribution of trap E3 and H1 concentration for a quarter of 2-inch n-GaN on n+-GaN using isothermal DLTS and MCTS measurements at room temperature, respectively. A variation of trap E3 concentration over four orders of magnitude is found in the range from ~1012 to ~1016 cm-3 with the average concentration of 4.6x1014 cm-3, while trap H1 shows a relatively small variation in the range from~1015 to ~1016 cm-3 with the average concentration of 5.9x1015 cm-3. Average trap concentration of trap H1 is higher in n-GaN doped with carbon of 6x1016 cm-3 and is found to be 4.2x1016 cm-3. It is thought that the increased concentration of trap H1 in carbon-doped GaN corresponds to the concentration of carbon-related defects. In this context, we will present a method to distinguish between VGa-related and carbon-related defects with similar thermal emission activation energies by a combination of MCTS and optical deep level transient spectroscopy (ODLTS) using below-band-gap light pulses. Further, we will discuss on the identification of these traps by varying MOCVD growth conditions, namely, V/III ratios, growth pressures and growth temperatures. Acknowledgments This work has been performed as an MEXT-Supported Program for the Strategic Research Foundation at Private Universities (2010-2014).

(Invited) Toward an Understanding of Defects and Device Reliability in GaN Hemts

GaN device reliability has become a major factor in technology acceptance for various electronics applications including high radiation and space applications. GaN’s properties make it ideal for a variety of harsh conditions including high energy particle radiation, high power RF biasing, high temperature, and simultaneous combinations of these stressors. Understanding how trapping behavior impacts the electrical characteristics and how defects form or are activated with these various stressors is important to predicting lifetime and end of life performance and approaches to reduce the degradation rate, improve the radiation hardness, etc. This work explores different modes of GaN high electron mobility transistors (HEMT) degradation when subjected to proton irradiation and electrical stressing. Specific trap states associated with different stress conditions are revealed using conventional and HEMT-based deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) methods. These trap characterization techniques provide quantitative information about degradation-limiting traps that can exist anywhere within the device bandgap profile and can be distributed within specific regions of the device structure. In addition, using the newly developed scanning-DLTS, we will show the ability to quantitatively map specific defects on the nanometer scale to identify how defects are related to specific microstructure. Multiple types of AlGaN/GaN HEMTs were characterized in a systematic fashion using conventional, HEMT-based, and scanning-DLTS and DLOS before and after the application of RF switching, high voltage switching and irradiation. Overall, radiation is found to form defects that primarily impact the threshold voltage where shallower traps cause dynamic threshold voltage VT instabilities and deeper traps cause static VT shifts. In particular, proton irradiation is found to increase the concentration of traps at EC-1.3 eV and EC-3.28 eV in the GaN buffer, which appear to be responsible for large and persistent VT shifts in GaN HEMTs, and a state at EC-0.72 eV in the GaN buffer that separately correlates with an increase in dynamic VT instability. The RF accelerated life testing (ALT) has led to different degradation pathways in different generations of HEMTs and different vendors but has consistently been in the drain access region where the electric fields are maximized, which is quite different than the radiation-induced degradation. Early generations consistently showed correlation of the RF output power with the concentration of the EC-0.57 eV level, which we have recently mapped with scanning-DLTS to directly correlate the EC-0.57 eV level with edge dislocations and Fe doping. Newer generations have degraded due to deeper, near mid-gap defects indicating that several degradation mechanisms are possible in GaN HEMTs and they evolve as the materials and device designs are improved. Semi-on DC ALT exhibits degradation somewhat different to RF ALT where the degradation is still due to defects in the drain access region but the traps responsible are different indicating that different testing mechanisms may not be predictive of actual degradation in some cases. The use of DLOS and DLTS to track the activation of these defects will be shown as essential to accurately determine exact degradation mechanisms to establish accurate predictive reliability models and guidelines for improved end of life material and device designs.

Deep traps and instabilities in AlGaN/GaN high electron mobility transistors on Si substrates

Deep traps were studied in multiple-finger AlGaN/GaN transistors with broad periphery by means of current–voltage and capacitance–voltage characteristics, reverse deep level transient spectroscopy, deep level transient spectroscopy with electrical (DLTS) and optical DLTS injection, and current DLTS (CDLTS) with gate voltage and drain voltage pulsing. Deep electron traps with activation energies of 0.25, 0.36, 0.56, and 0.8–0.9 eV are found to be responsible for variations in threshold current with applied gate bias. These traps also give rise to current transients observed at different temperatures in CDLTS. The 0.25 and 0.56 eV centers are most likely located at the AlGaN/GaN interface. The 0.25 eV state is believed to be nitrogen vacancy-related, the 0.36 eV level is of unknown origin, the 0.56 eV level is likely due to point defects enhanced in the presence of dislocations, and responsible for low luminescence efficiency of nonpolar n-GaN and for degradation of GaN transistors and light-emitting diodes and light-emitting diodes, while the 0.8 eV-level has been attributed to either interstitial Ga or N. Hole-trap-like relaxations in CDLTS were observed at temperatures higher than ∼300 K and attributed to hole traps near Ev + 0.9 eV located in the buffer layer. These have previously been assigned to either gallium vacancy-oxygen acceptor complexes (VGa-O)2− or carbon on nitrogen site acceptor (CN−) complexes. These results show that the stability of GaN-based high electron mobility transistors on Si is still dominated by the high concentration of point defects present in the material.

Deep level defects throughout the bandgap of (010) β-Ga2O3 detected by optically and thermally stimulated defect spectroscopy

Deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) measurements performed on Ni/β-Ga2O3 Schottky diodes fabricated on unintentionally doped (010) substrates prepared by edge-defined film-fed growth revealed a rich spectrum of defect states throughout the 4.84 eV bandgap of β-Ga2O3. Five distinct defect states were detected at EC − 0.62 eV, 0.82 eV, 1.00 eV, 2.16 eV, and 4.40 eV. The EC − 0.82 eV and 4.40 eV levels are dominant, with concentrations on the order of 1016 cm−3. The three DLTS-detected traps at EC − 0.62 eV, 0.82 eV, and 1.00 eV are similar to traps reported in Czochralski-grown β-Ga2O3, [K. Irmscher et al., J. Appl. Phys. 110, 063720 (2011)], suggesting possibly common sources. The DLOS-detected states at EC − 2.16 eV and 4.40 eV exhibit significant lattice relaxation effects in their optical transitions associated with strongly bound defects. As a consequence of this study, the Ni/β-Ga2O3 (010) Schottky barrier height was determined to be 1.55 eV, with good consistency achieved between different characterization techniques.

Comparative study of deep levels in HVPE and MOCVD GaN by combiningO-DLTS and pulsed photo-ionization spectroscopy

Operational characteristics of sensors made of GaN significantly depend on technologically introduced defects acting as rapid traps of excess carriers which reduce charge collection efficiency of detectors. In order to reveal the prevailing defects in HVPE and MOCVD grown GaN, the carrier lifetime and photo-ionization spectra have been simultaneously measured by using microwave probed photo-conductivity transient technique. Several traps ascribed to impurities as well as vacancy and anti-site type defects have been identified in HVPE GaN material samples by combining photo-ionization and electron spin resonance spectroscopy. The optical deep level transient spectroscopy technique has been applied for spectroscopy of the parameters of thermal emission from the traps ascribed to technological defects in the Schottky barrier terrace structures fabricated on MOCVD GaN.

Low rate deep level transient spectroscopy - a powerful tool for defect characterization in wide bandgap semiconductors

We present an overview of implementation and application of low rate Deep-Level Transient Spectroscopy (LR-DLTS). In conventional DLTS the sensitivity of the capacitance meter must be chosen so low that the whole capacitance drift range between lowest and highest temperature can be measured. In LR-DLTS the bridge is automatically balanced (capacitance and conductivity) after each measured transient. Thus, the highest available sensitivity still avoiding an overload can be used. With LR-DLTS it is now possible to extend the rate windows to the mHz range while preserving highest possible sensitivity. This allows the detection of energetically close levels and levels with large thermal activation energy. Also low emission rates in optical DLTS can be detected this way.

Determination of bandgap states in p-type In0.49Ga0.51P grown on SiGe/Si and GaAs by deep level optical spectroscopy and deep level transient spectroscopy

The presence and properties of traps in p-type In0.49Ga0.51P grown on low dislocation density, metamorphic Ge/SiGe/Si substrates and GaAs substrates were determined using deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) leading to the quantification of trap behavior throughout the entire 1.9 eV bandgap of the In0.49Ga0.51P material as a function of substrate. Thermal emission-based DLTS revealed a single hole trap at Ev + 0.71 eV for growth on both lattice matched and mismatched substrates with similar concentrations. Complementary, optical emission-based DLOS measurements detected bandgap states at Ev + 1.18 eV, Ev + 1.36 eV, and Ev + 1.78 eV for p-type In0.49Ga0.51P grown on both substrate types. The total concentration of the DLOS-detected states was found to comprise approximately 80% of the entire trap concentration in p-type In0.49Ga0.51P bandgap. This relatively high concentration of above midgap levels may be of great significance for minority carrier devices that utilize p-type In0.49Ga0.51P (such as high efficiency III–V multijunction solar cells) since their position in the bandgap and high concentrations suggest that strong minority carrier electron trapping behavior can be expected. The primary effect of substituting the GaAs substrate by Ge/SiGe/Si is to increase the concentration of these states by a factor of 2–3, with no additional levels detected due to the replacement by the Si-based substrates, indicating that all detected traps are native to the epitaxial In0.49Ga0.51P material (regardless of the substrate), but whose concentrations appear to be influenced by dislocation density.