Deep Level Transient Spectroscopy Signal Research Articles

Electric‐Field Enhancement of Electron Emission Rates for Deep‐Level Traps in n‐type GaN

Results of a deep‐level transient spectroscopy (DLTS) study of trapping states in Ni/Au Schottky diodes made on Si‐doped GaN layers grown by metal–organic vapor‐phase epitaxy (MOVPE) on highly conductive n‐type Ammono‐GaN substrates are reported. In all as‐grown samples, the DLTS signals due to traps with the activation energies for electron emission (Eem) of about 0.26 and 0.60 eV (referred to as E1 and E3 traps, respectively) have been detected. It is found that the electric field (E) significantly enhances the electron emission rate (eem) for the E1 trap, while for the E3 trap the eem(E) dependence is not very strong. The eem(E) dependence for the E1 trap is found to be characteristic of attractive traps with spherically symmetric square well potential with a radius of about 2.9 ± 0.25 nm. The eem(E) dependence for the E3 trap is accounted for by a phonon‐assisted tunneling mechanism. The zero‐E Eem values for both traps are determined. Correlations between the concentrations of the E1 and E3 traps and concentrations of impurities (H, C, Si, O, Mg, and Fe) determined by secondary ion mass spectrometry in the samples are investigated. Charge states and origins of the E1 and E3 traps are discussed.

To define nonradiative defects in semiconductors: An accurate DLTS simulation based on first-principle

Deep level transient spectroscopy (DLTS) is a crucial technique to characterize the defects in semiconductor devices. Within the framework of the state-of-the-art density functional theory (DFT), the simulation for experimental DLTS spectra is realized successfully with high accuracy and intrinsic physics in this work, of which the reliability is confirmed by nonlocal hybrid functional. The electron–phonon coupling is included to describe the thermal-activated behavior of the hot carrier nonradiative capture reflected by DLTS signal. In the case study, the divacancy in silicon is employed for the flow of DLTS calculation as a benchmark, of which the results are verified by our 40 MeV Si irradiation experiments on the NPN and PNP bipolar junction transistors and previous reports. The problem we can address is the identification of defect characterized in DLTS experiments. The realization of DLTS simulation would have a profound significance for understanding the underlying physics in experimental observations and be a powerful leverage to define defect nature in semiconductors.

Inspection of the Defect State Using the Mobility Spectrum Analysis Method

Mobility spectrum analysis (MSA) is a method that enables the carrier density (and mobility) separation of the majority and minority carriers in multicarrier semiconductors, respectively. In this paper, we use the p-GaAs layer in order to demonstrate that the MSA can perform unique facilities for the defect analysis by using its resolvable features for the carriers. Using two proven methods, we reveal that the defect state can be anticipated at the characteristic temperature in which the ratio () that is associated with the density of the minority carrier , to the density of the majority carrier , exceeds 50%. (1) Using a p-GaAs Schottky diode in a reverse bias regime, the position of the deep level transient spectroscopy (DLTS) peak is shown directly as the defect signal. (2) Furthermore, by examining the current–voltage–temperature (I–V–T) characteristics in the forward bias regime, this peak position has been indirectly revealed as the generation–recombination center. The DLTS signals are dominant around the , according to the window rate, and it has been shown that the peak variation range is consistent with the temperature range of the temperature-dependent generation–recombination peak. The is also consistent with the temperature-dependent thermionic emission peak position. By having only through the MSA, it is possible to intuitively determine the existence and the peak position of the DLTS signal, and the majority carrier’s density enables a more accurate extraction of the deep trap density in the DLTS analysis.

Diffusion transport over grain-boundary barriers as the origin of N1 deep level transient spectroscopy signal in [formula omitted] solar cells

In this work, we explore the hypothesis that the N1 peak in Cu(In,Ga)Se2 (CIGS) solar cells is caused by secondary barriers. We indicate, through numerical modelling of the corresponding Deep Level Transient Spectroscopy (DLTS) signal, that these barriers may be located at grain boundaries. Moreover, the proper reconstruction of the N1 signal using the double-barrier model requires an assumption that corresponding capacitance transients are due to the diffusion limited transport of holes over grain boundaries. Only then we were able to achieve sufficiently small values of the saturation current for realistic values of material parameters. We show that the position of the N1 DLTS peak and its activation energy depends on the effective density and the effective mobility of holes which can be calculated based on grain size and the properties of grain boundaries. This interpretation opens a room for the explanation of the unusual properties of the N1 signal in CIGS solar cells.

Electrical characterization of GaN Schottky barrier diode at cryogenic temperatures

In this report, electrical characteristics of the Ni/GaN Schottky barrier diode grown on sapphire have been investigated in the range of 20 K–300 K, using current–voltage, capacitance–voltage, and deep level transient spectroscopy (DLTS). A unified forward current model, namely a modified thermionic emission diffusion model, has been developed to explain the forward characteristics, especially in the regime with a large ideality factor. Three leakage current mechanisms and their applicability boundaries have been identified for various bias conditions and temperature ranges: Frenkel–Poole emission for temperatures above 110 K; variable range hopping (VRH) for 20 K–110 K, but with a reverse bias less than 20 V; high-field VRH, in a similar form of Fowler–Nordheim tunneling, for cryogenic temperatures below 110 K, and relatively large bias (>25 V). Four trap levels with their energy separations from the conduction band edge of 0.100 ± 0.030 eV, 0.300 eV, 0.311 eV, and 0.362 eV have been tagged together with their capture cross sections and trap concentrations. The significantly reduced DLTS signal at 100 K suggested that traps practically became inactive at cryogenic temperatures, thus greatly suppressing the trap-assisted carrier hopping effects.

(Keynote) Border-Trap Characterization for Ge Gate Stacks Using Deep-Level Transient Spectroscopy

1) Introduction To fabricate high-performance Ge MOSFETs, a high-quality gate-stack of Ge is essential. Although a GeO2 interlayer is very effective to improve the interface quality between Ge and gate-stack, the existence of large number of border-traps (BTs) in GeO2 is still a serious issue that degrades the performance of Ge MOSFETs. Therefore, the BT characterization for GeO2/Ge structure is desired. In this work, we intentionally used relatively strong injection to carry out deep-level transient spectroscopy (DLTS) measurements. By separating out interface traps (IT) contribution from DLTS signals, we extracted the BT signals and calculated BT concentrations (N bt). BTs also exist in the SiO2/Si structure, and DLTS has been applied to evaluate N bt for Si MOSCAPs [1]. However, owing to the relatively low N bt in SiO2 and the large band-offsets at the SiO2/Si interface, BTs do not cause serious problems in Si MOSFETs. Therefore, an N bt evaluation has not been studied intensely. As for the GeO2/Ge structure, the BT problem becomes serious because the N bt in GeO2 is relatively high and the band offsets at the GeO2/Ge interface are lower than those of SiO2/Si. Actually, when we measured the IT density (D it) for a GeO2/Ge structure, we found that the DLTS signal intensity clearly increased with increasing intensity of the injection pulse [2], which must have resulted from the BTs in GeO2. We believe that DLTS is a promising method to evaluate BTs for Ge MOSCAPs. In this presentation, BT evaluation using the DLTS method for Ge MOSCAP is introduced. Both D it and N bt are evaluated for Ge MOSCAPs fabricated by post-passivation thermal oxidation (PTO) and electron cyclotron resonance (ECR) plasma oxidation. In addition, the effects of Al post metallization annealing (Al-PMA) are also investigated. 2) Experimental Both p- and n-type (100) Ge substrates with respective doping concentrations of 2.3x1016 and 9.3x1015 cm-3 were used. After substrate cleaning, SiO2/GeO2 bilayer passivation was performed [3]. Here, the thicknesses of SiO2 and GeO2 layers were ~1 nm each. Next, PTO was performed at 550°C for 15 min or 425°C for 9 h in O2 ambient. After 14-nm-thick SiO2 was deposited on the both samples, the annealing at 400°C for 30 min in N2 was performed. Then, the Al gate film was deposited on the SiO2 surface by thermal evaporation. Before the electrode patterning, an optional PMA was carried out at 300°C for 30 min in N2, which is the Al-PMA. The EOTs of the MOSCAPs with PTO at 550 and 425°C were 19.5 and 16.8 nm, corresponding to GeO2 thicknesses of 6.0 and 3.3 nm, respectively. DLTS measurements were performed using a lock-in integrator. The MOSCAP was reverse-biased at V R and was applied the pulse bias of V P from V R. V AP (=|V P-V FB|, V FB: flat band voltage) is the accumulation pulse voltage, which is an important parameter for the BT analysis, because the injection pulse intensity (E AP) at the GeO2/Ge interface is given by E AP =V AP/EOT. Frequency (f ) and pulse width (t w) are also important parameters for selecting the observed BT position (z 0) and for filling with carriers into BTs, respectively. 3) Summary of the r esults By using p-type MOSCAPs, BTs at the position of 0.4 nm from the GeO2/Ge interface were measured. The energy of these BTs was centralized at the position near to the valence band edge of Ge, and their N bt was in the range of 1017–1018 cm-3. By using n-type MOSCAPs, BTs at the position range of 2.8–3.4 nm from the GeO2/Ge interface were measured, of which Nbt varied little in the depth direction. The energy of these BTs was distributed in a relatively wide range near to the conduction band edge of Ge, and their N bt was approximately one order of magnitude higher than those measured by p-MOSCAPs. This high N bt value might originate from the states of the valence alternation pair with energy close to 1 eV above the conduction band edge of Ge. We also found that Al-PMA can passivate both ITs and BTs near to the valence band edge of Ge but not those near to the conduction band edge. Acknowledgement This work was supported by (JSPS) KAKENHI (grant No. 17H03237).

First observation of electronic trap levels in freestanding GaN crystals extracted from Si substrates by hydride vapour phase epitaxy

The electronic deep level states of defects embedded in freestanding GaN crystals exfoliated from Si substrates by hydride vapour phase epitaxy (HVPE) is investigated for the first time, using deep level transient spectroscopy (DLTS). The electron traps are positioned 0.24 eV (E1) and 1.06 eV (E2) below the conduction band edge, respectively. The capture cross sections of E1 and E2 are evaluated to be 1.65 × 10−17 cm2 and 1.76 × 10−14 cm2 and the corresponding trap densities are 1.07 × 1014 cm−3 and 2.19 × 1015 cm−3, respectively. The DLTS signal and concentration of the electronic deep levels are independent of the filling pulse width, and the depth toward the bottom of the sample, evidenced by the fact that they are correlated to noninteracting point defects. Furthermore, Photoluminescence (PL) measurement shows green luminescence, suggesting that unidentified point defects or complex, which affect the optical characterisitics, exhibit. Despite the Si-based materials, the freestanding GaN exhibits deep level characteristics comparable to those of conventional freestanding GaN, suggesting that it is a desirable material for use in the next generation optoelectronic devices with the large-scalibilityand low production costs.

Peak shape analysis of deep level transient spectra: An alternative to the Arrhenius plot

Abstract

Exploration of the two-diode model of deep level transient spectroscopy signal originating from secondary barriers

Deep level transient spectroscopy (DLTS) measurements on multilayer devices can be influenced by secondary barriers. Our starting point is a simple model used in the literature for simulations of DLTS signals induced by such barriers: the structure is represented by two diodes connected back-to-back and separated by a neutral region. Although the calculations confirm that capacitance transients can occur in multilayer structures without defects, we find that the quantitative modeling of corresponding DLTS peaks resembling the N1 signal in Cu(In,Ga)Se2 solar cells requires unphysical input parameters. Therefore, we carefully analyze the assumptions of the model and investigate the limits of its applicability. We look into details of the formation of a DLTS signal and unveil the connection between observed kinetics, capacitance-voltage profiles, and the shape of DLTS peaks. We show that the signals originating from secondary barriers exhibit a logarithmic dependence on the pulse duration which can mimic a response from extended defects. We also verify the validity of the criterion existing in the literature allowing to distinguish signals coming from a back contact and deep defects and show that it can be used only in very specific situations.

INJECTION ANNEALING OF THE SELF DI-INTERSTITIAL – OXYGEN COMPLEX IN p-TYPE SILICON

With the use of deep level transient spectroscopy (DLTS) the effect of injection of minority charge carriers (electrons) on an annealing rate of self di-interstitial – oxygen (I2O) complex in silicon has been studied. The complex has been formed by irradiation of epitaxial boron-doped n+–p diode structures with alpha-particles at room temperature. It has been shown that the disappearance of this complex at room temperature begins at a direct current density of ~1.5 A/cm2. This characteristic current density has been found for 10 W·cm p-type silicon when the total radiation defect density was less than 15 % of the initial boron concentration, a divalent hole trap with energy levels of Ev + 0.43 eV and Ev + 0.54 eV has been found to appear as a result of recombination-enhanced annealing of the I2O. When the I2O complex is annealed thermally, the concurrent appearance of an electron trap with an energy level of Ec – 0.35 eV has been observed. It has been shown that the divalent hole trap represents a metastable configuration (BH-configuration) of the bistable defect, whereas the electron trap is stab le in the p-Si configuration (ME-configuration). From the comparison of DLTS signals related to different defect configurations it is found that the ME-configuration of this bistable defect can be characterized as a center with negative correlation energy. It has been shown that the injection-stimulated processes make it very difficult to obtain reliable data on the formation kinetics of the bistable defect in the BH-configuration when studying the thermal annealing of the I2O complex.

Properties of a previously unobserved donor-related electrically active defect in Ge induced by alpha particle irradiation

Alpha particle irradiation was used to study the radiation-induced defects in n-type germanium (Ge). Investigation of the well-known antimony (Sb)-vacancy complex (commonly known as the E-center) in Ge, with an activation energy of 0.37eV (E0.37), has led to the discovery of another defect with a DLTS signature virtually indistinguishable from the E-center, but with different annealing characteristics. We shall refer to this new defect as the E-prime. Although the two defects are easily distinguishable by annealing, the DLTS signal produced by the E-center and E-prime were not distinguishable through conventional deep level transient spectroscopy (DLTS). Separation of the two peaks was only possible through the use of low noise equipment in conjunction with high resolution Laplace-DLTS. The activation energy of the Sb-vacancy and the E-prime was determined to be 0.370±0.005eV and 0.375±0.005eV. Depth profiles showed uniform distributions of both defects below the Schottky junction.

Two-Dimensional Imaging of Trap Distribution in SiO<sub>2</sub>/SiC Interface Using Local Deep Level Transient Spectroscopy Based on Super-Higher-Order Scanning Nonlinear Dielectric Microscopy

A new technique for local deep level transient spectroscopy (DLTS) imaging using super-higher-order scanning nonlinear dielectric microscopy is proposed. Using this technique, SiO2/SiC structure samples with different post oxidation annealing (POA) conditions were measured. We observed that the local DLTS signal decreases with POA levels, which agrees with the well-known phenomena that POA reduces trap density. Furthermore, obtained local DLTS images had dark and bright areas, which is considered to show the trap distribution at/near SiO2/SiC interface.

On Deep Level Transient Spectroscopy of Extended Defects in n-Type 4H-SiC

We have performed capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements on Schottky contacts fabricated on triangular defects in 4H-SiC epitaxial layers. These measurements are a case study on the effect of a specific extended defect on the DLTS spectrum in order to contribute to the physical understanding of curious features occasionally observed in DLTS spectra. Our measurements reveal an inversion of the DLTS signal depending on applied voltages and filling pulse lengths, and a step in the C-V characteristic of the Schottky diode. We present a model that qualitatively describes the experimentally obtained data. In this model, we assume that stacking faults within a triangular defect form quantum wells, which can capture electrons from other defects during the DLTS measurement leading to the inversion of the DLTS spectrum. Moreover, by calculating the differential capacitance using a self-consistent Schrödinger-Poisson-Solver, the step in the C-V measurements is reproduced by our model.

Evidence of minority carrier traps contribution in deep level transient spectroscopy measurement in n–GaN Schottky diode

It is shown that deep level transient spectroscopy can be carried out on Schottky diodes to investigate, in addition to majority carrier traps, minority carrier traps. This is possible thanks to the application of a large reverse bias to the device which allows minority carrier injection by lowering their corresponding effective Schottky barrier height. Indeed, when increasing the reverse bias voltage, the deep level transient spectroscopy signal, initially negative and thus showing only majority carrier traps signature, becomes positive, revealing minority carrier traps involvement. A careful analysis of the recorded spectra leads to the identification of four minority carrier traps which have been so far only evidenced using dedicated technique such as minority carrier transient spectroscopy.

Detection of electron emission as DLTS signal in CdTe solar cells

This work identifies an incongruity in the detection of the minority carrier signal in CdTe solar cells during the deep level transient spectroscopy (DLTS) measurement. Use of quasi-Fermi level instead of Fermi level of majority carriers to estimate the probability of emitting carriers seems to correct the ambiguity. During the experiment, signals from minority carrier traps (electron traps) were detected by using a long filling pulse time instead of an electron injection pulse. The DLTS measurements of CdTe solar cells observed a single electron trap with energy level EE1 = 0.47 eV, and two hole traps with energy levels, EH1 = 0.17 eV and EH2 = 0.27 eV. The possibility of any impact from the back contact was excluded, and the phenomenon was clarified by the simulation. It was further observed that when the condition of quasi-Fermi level is considered, the results of calculated probability were significantly different from that of the results that used only Fermi level of majority carriers. The simulations further aided the explanation of the defect behavior in DLTS measurements and the overlapping phenomenon of the capacitance spectrum of hole and electron traps.

Current transport and deep levels in p-Si/n-Zn0.9Mg0.1O/n-ZnO heterojunction

The paper describes the deep level transient spectroscopy (DLTS) measurements that were performed in order to investigate defects in p-Si/n-Zn0.9Mg0.1O/n-ZnO heterojunction (HJ). The HJ structure was grown by molecular beam epitaxy (MBE) technique on a p-type Si(111) substrate at 500 °C. The DLTS studies had been preceded by the current-voltage (I-V) and capacitance-voltage (C-V) measurements. The I-V measurements show the rectifying behavior of the HJ and indicate that the low forward bias (less than 0.6 V) mechanism of current transport is dominated by recombination-tunneling whereas at higher voltage space charge limited current (SCLC) governs the current transport. The I-V and C-V measurements yield donor concentration of ND = 5·1015 cm−3 in the Zn0.9Mg0.1O layer. This value is two orders of magnitude less than the acceptor concentration, NA = 2·1017 cm−3 in the Si substrate. Therefore it has been assumed that the depletion region extends mainly on the Zn0.9Mg0.1O side of the HJ and the DLTS signal has its origin in this layer and at the HJ interface. The DLTS measurements performed within the temperature range of 20–400 K revealed the presence of three electron traps, with activation energies of 0.06 eV, 0.38 eV, and 0.39 eV respectively. The latter two are ascribed to the defects in the Zn0.9Mg0.1O layer, while the trap with the activation energy of 0.06 eV is located either in the Zn0.9Mg0.1O layer or at the interface with Si.

Influence of surface states on deep level transient spectroscopy in AlGaN/GaN heterostructure**Project supported by the National Key Basic Research Program of China (Grant No. 2011CBA00606), the Program for New Century Excellent Talents in University, China (Grant No. NCET-12-0915), and the National Natural Science Foundation of China (Grant Nos. 61334002 and 61404097).

Deep level transient spectroscopy (DLTS) as a method to investigate deep traps in AlGaN/GaN heterostructure or high electron mobility transistors (HEMTs) has been widely utilized. The DLTS measurements under different bias conditions are carried out in this paper. Two hole-like traps with active energies of Ev + 0.47 eV, and Ev + 0.10 eV are observed, which are related to surface states. The electron traps with active energies of Ec − 0.56 eV are located in the channel, those with Ec − 0.33 eV and Ec − 0.88 eV are located in the AlGaN layer. The presence of surface states has a strong influence on the detection of electron traps, especially when the electron traps are low in density. The DLTS signal peak height of the electron trap is reduced and even disappears due to the presence of plentiful surface state.

Vacancy-related defects in n-type Si implanted with a rarefied microbeam of accelerated heavy ions in the MeV range

Deep level transient spectroscopy (DLTS) has been used to study vacancy-related defects formed in bulk n-type Czochralski-grown silicon after implantation of accelerated heavy ions: 6.5MeV O, 10.5MeV Si, 10.5MeV Ge, and 11MeV Er in the single ion regime with fluences from 109cm−2 to 1010cm−2 and a direct comparison made with defects formed in the same material irradiated with 0.7MeV fast neutron fluences up to 1012cm−2. A scanning ion microprobe was used as the ion implantation tool of n-Cz:Si samples prepared as Schottky diodes, while the ion beam induced current (IBIC) technique was utilized for direct ion counting. The single acceptor state of the divacancy V2(−/0) is the most prominent defect state observed in DLTS spectra of n-CZ:Si samples implanted by selected ions and the sample irradiated by neutrons. The complete suppression of the DLTS signal related to the double acceptor state of divacancy, V2(=/−) has been observed in all samples irradiated by ions and neutrons. Moreover, the DLTS peak associated with formation of the vacancy-oxygen complex VO in the neutron irradiated sample was also completely suppressed in DLTS spectra of samples implanted with the raster scanned ion microbeam. The reason for such behaviour is twofold, (i) the local depletion of the carrier concentration in the highly disordered regions, and (ii) the effect of the microprobe-assisted single ion implantation. The activation energy for electron emission for states assigned to the V2(−/0) defect formed in samples implanted by single ions follows the Meyer–Neldel rule. An increase of the activation energy is strongly correlated with increasing ion mass.

Determination of Majority Carrier Capture Rates via Deep Level Transient Spectroscopy

In Deep Level Transient Spectroscopy (DLTS) experiments the majority carrier capture rate is often determined by observing the growth of the signal amplitude as a function of filling pulse duration at constant temperature. The analysis of such experiments is complicated by the phenomenon of slow capture: carrier capture by defects in the Debye tail of the depletion layer at the pulse voltage. We review here three approaches for analyzing isothermal pulse duration variation DLTS experiments that have been described or at least have been frequently used in literature. These methods are compared for their ability to correctly extract capture rates from simulated data as well as from actual experimental data for the Fe− /2 − level in crystalline germanium. Finally, we tested the performance of the three methods for analysis of DLTS signals that experience a delayed growth, modeled by an additional time constant in the system.

Synergistic Effect of Ionization and Displacement Defects in NPN Transistors Induced by 40-MeV Si Ion Irradiation With Low Fluence

Based on 3DG110 transistors and special gated NPN (GNPN) transistors, the characteristic of the synergistic effect between ionization and displacement defects induced by 40-MeV silicon (Si) ions with low fluence was investigated in terms of deep-level transient spectroscopy (DLTS). Nonlinear relationship, produced by the synergistic effect, between low fluence and radiation response can be obviously observed in the bipolar junction transistors (BJTs) under the exposure of 40-MeV Si ions. The DLTS signals of displacement defect centers and oxide-trapped charges are detected in the base–collector junction of the 3DG110 transistors. Meanwhile, based on the GNPN transistors, the DLTS signals of interface traps and displacement defects are measured in the base–collector junction. By comparison of the change in electrical parameters and the DLTS signals, it is found that the interface traps induce an enhanced effect to displacement defects in the base–collector junction of NPN BJT, whereas the oxide-trapped charge can suppress the DLTS signals of deep-level centers to a certain extent. Compared with the suppression action, the enhanced effect rising from ionization damage gives more contribution to the displacement damage. Moreover, the bias used during DLTS measurements can influence the characteristics of DLTS signals caused by oxide-trapped charge and interface traps. With increasing bias, both the height and temperature of ionization defect peaks in the DLTS spectra will increase, illustrating that concentration and energy level of these defects are raised.