Isothermal Transient Spectroscopy Research Articles

Deep Level Transient Fourier Spectroscopy (DLTFS) and Isothermal Transient Spectroscopy (ITS) in vertical GaN-on-GaN Schottky barrier diodes

The paper explores the Deep Level Transient Fourier Spectroscopy (DLTFS) capabilities in characterizing electrically active traps in vertical GaN-on-GaN Schottky barrier diodes (SBDs). The capacitance-DLTFS (C-DLTFS) experiments reveal a prominent electron trap T2 at EC – 0.56 eV with a density (NT2) of 8 × 1014 cm−3 and a weak presence of another trap at EC – 0.18 eV (T1) with NT1 = 3.8 × 1013 cm−3 in the SBDs. The C-DLTFS acquired with two emission transients (TE = 20.48 ms and 2.048 s) has resulted in identical trap signatures for T1 and T2. Due to the high NT, the trap T2 at EC - 0.56 eV is identified from the current-DLTFS (I-DLTFS) and thermally stimulated capacitance (TSCAP) measurements. Especially the TSCAP results indicate the carrier freeze-out temperature (T < 75 K) in the GaN material. Furthermore, the carrier emission kinetics of the trap T2 is evaluated by performing isothermal transient spectroscopy at a stabilized temperature. The capture time constant of T2 is estimated from the isothermal transients attained with shorter trap-filling pulse widths (<100 ns).

Impact of thermal annealing on deep levels in nitrogen-implanted β-Ga2O3 Schottky barrier diodes

Understanding the properties of N-implanted β-Ga2O3 is fundamental for the optimization of doping and isolation structures based on gallium oxide. This paper reports an extensive analysis of the impact of thermal annealing on the concentration and properties of deep levels in N-implanted β-Ga2O3 Schottky barrier diodes by means of capacitance isothermal transient spectroscopy. Samples with annealing temperatures from 800 to 1200 °C were considered. The original results presented in this paper demonstrate the following: (a) The instability of current–voltage characteristics detected for all the samples under test can be attributed to the presence of three electron traps with activation energies of 0.6, 0.7, and 1 eV, consistent with previous reports in β-Ga2O3. (b) The detected traps are not the nitrogen level but intrinsic defects whose concentration is increased by the implantation process. (c) The concentration of deep levels decreases as the annealing temperature increases, demonstrating that the annealing process can effectively restore the quality of the material while keeping the conductivity decrease related to the presence of the nitrogen. Finally, (d) we demonstrate that the residual leakage and the turn-on voltage shift are correlated with the Arrhenius signature of the detected deep levels. An interpretation is proposed to explain the measurement results.

Defect properties of Sb2Se3 thin film solar cells and bulk crystals

As an absorber in photovoltaic devices, Sb2Se3 has rapidly achieved impressive power conversion efficiencies despite the lack of fundamental knowledge about its electronic defects. Here, we present a deep level transient spectroscopy (DLTS) study of deep level defects in both bulk crystal and thin film device material. DLTS study of Bridgman-grown n-type bulk crystals revealed traps at 358, 447, 505, and 685 meV below the conduction band edge. Of these, the energetically close pair at 447 and 505 meV could only be resolved using the isothermal transient spectroscopy (rate window variation) method. A completed Sb2Se3 thin film solar cell displayed similar trap spectra with traps identified at 378, 460, and 690 meV. The comparable nature of defects in thin film and bulk crystal material implies that there is minimal impact of polycrystallinity in Sb2Se3 supporting the concept of benign grain boundaries.

Electrical characteristics of low‐Mg‐doped p‐AlGaN and p‐InGaN Schottky contacts

Electrical characteristics of low‐Mg‐doped p‐Al0.02–0.07Ga0.98–0.93N and p‐In0.02Ga0.98N Schottky contacts were investigated. A memory effect was revealed in the current–voltage (I–V) characteristics, which is found also in p‐GaN contacts and may be due to charge and discharge of holes to acceptor‐type interfacial defects. High‐temperature isothermal transient spectroscopy revealed that the defect densities in the Al0.02Ga0.98N and In0.02Ga0.98N samples were nearly half that in the GaN samples. A large reverse current with a memory effect was observed in the In0.02Ga0.98N contacts. It was suggested that current can flow via relatively shallow acceptor‐type interfacial defects. For the AlxGa1−xN with x = 0.04 and 0.07, I–V characteristics became leaky. The Schottky barrier heights estimated from the photoresponse experiment were higher than the values estimated from the bandgap by 0.19, 0.21, 0.20, and 0.21 eV for Al0.02Ga0.98N, Al0.04Ga0.96N, Al0.07Ga0.93N, and In0.02Ga0.98N samples, respectively. A probable explanation for such discrepancies may be due to the total charge amount of the interfacial states, which varied the electric field and the barrier lowering. It was found that even though the Al or In content was only as small as 2%, the electrical characteristics of the contacts changed significantly.

Impact of Electric Field on Thermoemission of Carriers from Shallow Dislocation-Related Electronic States

Shallow dislocation-related electronic states near the bottom of the conduction band in n-type Si bonded sample have been investigated with deep-level transient spectroscopy (DLTS), isothermal transient spectroscopy (ITS) and energy-resolved DLTS. The effect of thermoemission (TE) enhancement in external electric field was found and the dependence of the TE activation energy reduction as a function of the filling grade was obtained for these states. A new model of dislocation-strain-related Poole-Frenkel effect that accounts for the own electric field of internal charge of dislocation line is suggested and compared with the experimental data.

Deep hole traps in boron-doped diamond

Deep hole traps in boron-doped diamond epitaxial layers are studied by means of several types of deep-level transient spectroscopy and density-functional theory calculations. Standard deep-level transient spectroscopy and high-resolution isothermal transient spectroscopy permit to identify nine deep hole traps. Their capture cross-sections and ionization energies are systematically determined. In parallel, the ionization energies of donor and acceptor levels related to boron- and/or hydrogen-related complexes in diamond are assessed by ab initio calculations in this work and summarized with others from the literature including native defects. Tentative assignments of the measured deep hole traps to the calculated ones are proposed.

Electrically active defects in boron doped diamond homoepitaxial layers studied from deep level transient spectroscopies and other techniques

AbstractHomoepitaxial thin films of boron doped diamond are investigated with the help of Fourier transform deep level transient spectroscopy (FT‐DLTS) and high resolution isothermal transient spectroscopy (HR‐ITS) in order to determine the properties of hole traps. Conductivity studies and other characterisation techniques of defects are also used to bring complementary information. The main conclusions are as follows: (i) many hole traps can be found with activation energies in the range 0.9–1.6 eV; (ii) most of them show inwards decreasing concentrations, which passes below 1015/cm3 at a depth close to a few hundred nanometers from the surface; (iii) some of them are metastable, in the sense the emission rate distribution undergoes changes after thermal treatments performed at temperatures well below those used for growth and previous annealing. These two last facts suggest that hydrogen atoms may be involved in charged defects. In other samples, an irreversible decrease of the conductivity correlated with the increase of the cathodoluminescence (CL) A band demonstrates that a deep donor is connected with dislocations and acts as a very efficient compensating centre in the first 400 nm just below the surface but with a decreasing concentration deeper inside the homoepitaxial layer. The physical origin of these deep levels is discussed and believed to be likely connected with hydrogen atoms.

Investigation of both interface states spectrum and deep oxide states from differential isothermal transient spectroscopy

Metal/insulator/Si (MIS) structures using a high-permittivity (high- k) dielectric layer, here the perovskite compound SrTiO 3, are investigated by differential isothermal transient spectroscopy in order to study electronic states at interface and inside the oxide. The isothermal transient capacitance responses of these MIS capacitors, obtained at varying depletion bias voltages and recording times, are analysed by fast Fourier transform (FFT). Different values of the accumulation pulse duration may induce changes in the trapped charge, allowing identification of the various mechanisms for restoring thermodynamical equilibrium in the interface states. Thus, discrimination between the proper contribution of interfaces states and that of oxide deep states is evidenced. The effects of several post-annealing of the oxide layer onto the energy spectra are also described.

Deep levels in GaInAs grown by molecular beam epitaxy and their concentration reduction with annealing treatment

X-ray diffraction (XRD), current–voltage ( IV), capacitance–voltage ( CV), deep-level transient Fourier spectroscopy (DLTFS) and isothermal transient spectroscopy (ITS) techniques are used to investigate the thermal annealing behaviour of three deep levels in Ga 0.986In 0.014As heavily doped with Si (6.8 × 10 17 cm −3) grown by molecular beam epitaxy (MBE). The thermal annealing was performed at 625 °C, 650 °C, 675 °C, 700 °C and 750 °C for 5 min. XRD study shows good structural quality of the samples and yields an In composition of 1.4%. Two main electron traps are detected by DLTFS and ITS around 280 K, with activation energies of 0.58 eV and 0.57 eV, capture cross sections of 9 × 10 −15 cm 2 and 8.6 × 10 −14 cm 2 and densities of 2.8 × 10 16 cm −3 and 9.6 × 10 15 cm −3, respectively. They appear overlapped and as a single peak, which divides into two smaller peaks after annealing at 625 °C for 5 min. Annealing at higher temperatures further reduces the trap concentrations. A secondary electron trap is found at 150 K with an activation energy of 0.274 eV, a capture cross section of 8.64 × 10 −15 cm 2 and a density of 1.38 × 10 15 cm −3. The concentration of this trap level is also decreased by thermal annealing.

Deep centres in bulk MOCVD n‐type hexagonal GaN thin films and near their interface

Deep level transient spectroscopy (DLTS) is performed in metal organic chemical vapour deposition (MOCVD) hexagonal GaN doped with silicon either intentionally or not. Capacitance transients are measured in junctions and analysed with the help of several techniques. In these samples, the main contribution due to deep levels appears as a prominent peak in the Fourier transform DLTS (FTDLTS) spectrum with an apparent ionisation energy of 0.95 eV and a capture cross section close to 5 × 10–15 cm2. However, these values must be considered as apparent ones. Isothermal transient spectroscopy (ITS) and a high resolution method based on the analysis of the transients recorded over five decades of time show that several sub-levels exist with ionisation energies between 0.40 and 0.76 eV, and capture cross sections in the range 5 × 10–20– 5 × 10–17 cm2. Capture kinetics confirms this fact, indicating that at least three charge states (either +, 0, – or 0, –, 2–) are involved. The possible origins of long-term instabilities and reversible evolutions of the current–voltage characteristics of the diodes which are also detected, are discussed in relation with these deep states.

Electrical Signature of Ion-Implantation Induced Defects in n-Silicon in the Defect Cluster Regime Studied using DLTS and Isothermal Transient Spectroscopies

AbstractWe study electrical signature of defect clusters in KeV Ar ion-implanted n-silicon using Deep Level Transient Spectroscopy (DLTS) and isothermal capacitance spectroscopies such as time analyzed transient spectroscopy (TATS) and high resolution Laplace-DLTS. The samples are annealed at relatively low temperatures of 350 °C - 600 °C at which defect clusters are known to form and evolve. Contrary to the view that few dominant point-defect like traps are associated with defect clusters, our results show that the band gap may be replete with bands of multiple trap states; however their occupation and hence observation depends on experimental conditions dictated by dynamics of carrier capture and emission at these traps. Charge redistribution among multiple states and deepening of effective emission energy with capture are shown to be commonly occurring at these defects. Isothermal transient spectroscopy is shown to be appropriate tool for recognition of some of these features.

Anomalous optical and electrical recovery processes of the photoquenched EL2 defect produced by oxygen and boron ion implantation in gallium arsenide

The optical and electrical recovery processes of the metastable state of the EL2 defect artificially created in n-type GaAs by boron or oxygen implantation are analyzed at 80 K using optical isothermal transient spectroscopy. In both cases, we have found an inhibition of the electrical recovery and the existence of an optical recovery in the range 1.1–1.4 eV, competing with the photoquenching effect. The similar results obtained with both elements and the different behavior observed in comparison with the native EL2 defect has been related to the network damage produced by the implantation process. From the different behavior with the technological process, it can be deduced that the electrical and optical anomalies have a different origin. The electrical inhibition is due to the existence of an interaction between the EL2 defect and other implantation-created defects. However, the optical recovery seems to be related to a change in the microscopic metastable state configuration involving the presence of vacancies.

Characterization of EL2 in GaAs wafers by scanning isothermal transient spectroscopy

The authors have developed a novel scanning deep level transient spectroscopy system based on laser-excited junction capacitance or current transient measurements under isothermal conditions (scanning isothermal transient spectroscopy or SICTS). By applying the system to the measurement of deep levels in GaAs, they have successfully obtained precise information on the spatial distribution of EL2 in n-type GaAs wafers.

On the artificial creation of the EL2 center by means of boron implantation in gallium arsenide

In the present work, an analysis of the dark and optical capacitance transients obtained from Schottky Au:GaAs barriers implanted with boron has been carried out by means of the isothermal transient spectroscopy (ITS) and differential and optical ITS techniques. Unlike deep level transient spectroscopy, the use of these techniques allows one to easily distinguish contributions to the transients different from those of the usual deep trap emission kinetics. The results obtained show the artificial creation of the EL2, EL6, and EL5 defects by the boron implantation process. Moreover, the interaction mechanism between the EL2 and other defects, which gives rise to the U band, has been analyzed. The existence of a reorganization process of the defects involved has been observed, which prevents the interaction as the temperature increases. The activation energy of this process has been found to be dependent on the temperature of the annealing treatment after implantation, with values of 0.51 and 0.26 eV for the as-implanted and 400 °C annealed samples, respectively. The analysis of the optical data has corroborated the existence of such interactions involving all the observed defects that affect their optical parameters.

Poole Frenkel Effect on the DX Centers in III-V Ternary Alloys

AbstractThe dependence of the DX centers thermal emission rates on the electric field has been measured by Isothermal Transient Spectroscopy (ITS) and differential ITS (DITS) techniques. These techniques allow us to achieve a more accurate analysis of these defects than the DLTS, which ability for the study of DX centers is discussed. This dependence has been found to be mainly due to a Poole-Frenkel effect and is the cause of the non-exponentiality of the thermal emission transients.

Time-dependent response of interface states determined by using differential isothermal transient spectroscopy

A method based on the differential analysis of the isothermal transients is proposed to study the dynamical properties of the charging and discharging of interface states, and several possibilities of using this method are shown. The results obtained in SiO 2/Si and Si 3N 4/SiO 2/Si samples are in agreement with the existence of a spatial and energy distribution of interface states within the insulator. From the experimental data, the concentration of traps within the insulator at 35 Å is estimated to be 5×10 9 cm -2 eV -1, with a tunneling cross section ∼10 -19 cm 2, at E c- E ≈ 0.2 eV.

Optical Assessment of the Interaction Between EL2 and EL6 Levels in Boron Implanted GaAs

The majority optical cross section and the concentration of levels with optical response induced by boron implantation at 1010 ions/cm2 in GaAs, have been obtained using the technique labelled Optical Isothermal Transient Spectroscopy, OITS, in an energy range between 0.8 eV and 1.4 eV. The experimental optical data measured at different annealing temperatures, show only the presence of the EL2 and EL6 electron trap levels. The optical behaviour found is in complete agreement with the existence of an interaction between these two deep levels, EL2 and EL6. Besides, this interaction has been found to be the origin of the U-band in the DLTS spectra and of the anomalous dependence of the level concentration on the annealing temperature deduced from the OITS and DLTS spectra.

Analysis of the near-intrinsic and extrinsic photocapacitance due to the EL2 level in boron-implanted GaAs

A detailed analysis of the photocapacitance signal at the near-band and extrinsic energetic ranges in Schottky barriers obtained on horizontal Bridgman GaAs wafers, which were implanted with boron at different doses and annealed at several temperatures, has been carried out by using the optical isothermal transient spectroscopy, OITS. The optical cross sections have been determined as well as the quenching efficiency of the EL2 level which has been found to be independent of the annealing temperature. Moreover, the quenching relaxation presents two significant features: (i) a strong increase of the quenching efficiency from 1.35 eV on and (ii) a diminution of the quenching transient amplitude in relation with that shown by the fundamental EL2 level. In order to explain this behavior, different cases are discussed assuming the presence of several energy levels, the existence of an optical recuperation, or the association of the EL2 trap with two levels located, respectively, at Ev+0.45 eV and Ec−0.75 eV. The theoretical simulation, taking into account these two last cases, is in agreement with the experimental photocapacitance data at low temperature, as well as at room temperature where the EL2 filling phototransient shows an anomalous behavior. Moreover, unlike the previous data reported for the EL2 electron optical cross section, the values found using our experimental technique are in agreement with the behavior deduced from the theoretical calculation. The utilization of the OITS method has also allowed the determination of another level, whose faster optical contribution is often added to that of the EL2 level when the DLOS or standard photocapacitance is used.