Deep Level Transient Spectroscopy Method Research Articles

Correlation between deep level traps and reverse recovery of GaAs p–i–n diodes before and after neutron irradiation

Using the deep-level transient spectroscopy and reverse recovery method, the minority carrier lifetime in the base n 0-layers of high-voltage GaAs p +–p 0–i–n 0–n +-diodes grown by the liquid phase epitaxy in argon atmosphere has been estimated before and after irradiation by neutrons with the energy of 1 MeV and the fluence of 1.6 × 1014 cm–2. Correlation between the values of the minority carrier lifetime determined by both methods was found. EL2 defects where shown to govern the dynamic switching characteristics of non-irradiated GaAs diodes, while the switching process in the irradiated diodes is determined by the acceptor-like states of defect bands located above the midgap, which are D– states of a three-charged donor. Having been irradiated with neutrons, the diodes revealed a significant decrease in the time of their reverse recovery upon changing the character of the diode switching from the “hard” mode to the “soft” one.

Thermal DLTS study of the defect states of PELLE polymer

The thermal deep-level transient spectroscopy (DLTS) technique was applied to study the defect states and their activation energies of a polyurethane-based polymer network (PELLE) insulator. It was demonstrated that decomposition analysis of exponential transients allows a more accurate determination of the activation energy of the defective states. Two groups of activation energies for the PELLE polymer were observed, ranging from 0.49 eV to 1.9 eV and − 7.84 eV to − 2.53 eV, which were assigned to changes in the bond properties in the hard and soft segments of the studied polymer, respectively. The applicability of the DLTS method for the characterization of polymers was demonstrated. It was shown that the DLTS technique could contribute to a more comprehensive picture of the properties of polymers.

Influence of the type of interlayer on current transport mechanisms and defects in n-ZnO/ZnCdO/p-Si and n-ZnCdO/ZnO/p-Si heterojunctions grown by molecular beam epitaxy

A pair of heterojunctions with different type of interlayer: n-ZnO/ZnCdO and n-ZnCdO/ZnO were grown by molecular beam epitaxy on p-Si substrate and investigated in the present work. At first, their electrical properties and parameters were discussed following current-voltage (I-V) characteristics measured over a wide range of temperatures from 40 K to 300 K. The analysis of I-V plots in log-log scale pointed out the presence of different carrier transport mechanisms: tunneling, multi-tunneling capture-emission and space-charge limited currents. Next, the capacitance-voltage characteristics were studied to determine the dopant concentration of the investigated junctions, along with the net doping profiles and their temperature dependence in the range of 40 K to 300 K. Finally, a deep level transient spectroscopy method was applied to characterise defects in the studied junctions. Donor-like traps were identified and their parameters, such as activation energy and capture cross section, were calculated. Furthermore, the influence of cadmium ions in ZnCdO interlayer of n-ZnO/ZnCdO/p-Si diode on the energy position of trap levels was noticed. Moreover, it was demonstrated that the interlayer type affects junction parameters, especially its built-in potential, as well as it modifies the current transport mechanisms in the intermediate range of voltage.

Analysis of recombination centers near an interface of a metal–SiO2–Si structure by double carrier pulse deep-level transient spectroscopy

This paper proposes a new double carrier pulse deep-level transient spectroscopy (DC-DLTS) method that is applicable for evaluating metal–insulator–semiconductor (MIS) structures and the recombination centers in carrier-selective contact solar cells. Specifically, this study evaluated recombination characteristics of defects induced in bulk Si near SiO2/Si interfaces by reactive plasma deposition (RPD). In this method, a pulse voltage was first applied to inject majority carriers. Subsequently, a second pulse voltage was applied, which allowed minority carriers to be injected into the MIS structure. With these two types of carrier injections, carriers were recombined in recombination-active defects, and the DC-DLTS spectrum changed. During the injection of minority carriers, some majority carriers were thermally emitted from the defects, resulting in a decrease in the signal intensity. The recombination activity was analyzed by considering the effect of thermal emission on the change in signal intensity. The number of induced defect types and defect properties were estimated using Bayesian optimization. According to the results, three types of electron traps were generated using the RPD process. Based on the DC-DLTS results, defects with energy level 0.57 eV below the conduction band and capture cross section of ∼10−15 cm2 act as recombination centers.

A new lock-in amplifier-based deep-level transient spectroscopy test and measurement system for solar cells

A new, widely-used, lock-in amplifier-based deep-level transient spectroscopy (DLTS) test and measurement system has been developed that efficiently digitizes and analyzes through conventional DLTS methods. The fundamental principle of DLTS, using the n+p diode, is introduced first. Part of the DLTS system setup is discussed in terms of the accuracy, optimization, and processing of measurement results. The established system is subsequently applied to study deep-level traps in the antimony sulfide (Sb2S3) and the Copper Indium Gallium Selenium (CIGS) thin-film solar cell. One electron trap and one hole trap were found in each of the two solar cells, and the calculated results are within a reasonable range compared with previous reports. It is shown experimentally that the DLTS system can be employed in a variety of test scenarios and the results are fundamentally in agreement with the mainstream system performance.

Quantum Sensor for Nanoscale Defect Characterization

The optical and electronic properties of semiconductors are strongly affected by structural and stoichiometric defects. The precise incorporation of dopants and the control of impurities are essentially what makes semiconductors useful materials for a broad range of devices. The standard defect and impurity characterization methods are sensitive only on a macroscopic scale, like the most widely used method of deep-level transient spectroscopy (DLTS). We perform time-resolved measurements of the resonance fluorescence of a single self-assembled $(\mathrm{In},\mathrm{Ga})$$\mathrm{As}$ quantum dot (QD) at low temperatures ($4.2\phantom{\rule{0.2em}{0ex}}\mathrm{K}$). By pulsing the applied gate voltage, we are able to selectively occupy and unoccupy individual defects in the vicinity of the dot. We address the exciton transition of the QD with a tunable diode laser. Our time-resolved measurements exhibit a shift of the resonance energy of the optical transition. We attribute this to a change of the electric field in the dot's vicinity, caused by electrons tunneling from a reservoir to the defect sites. Furthermore, we are able to characterize the defects concerning their position and activation energy by modeling our experimental data. Our results thus demonstrate how a quantum dot can be used as a quantum sensor to characterize the position and activation energy of individual shallow defects on the nanoscale.

Formation and annealing of radiation defects in silicon, implanted with hydrogen ions

Energy spectrum of radiation defect levels in n-type epitaxial silicon irradiated with 300 keV hydrogen ions was studied by DLTS (deep level transient spectroscopy) method. The increase in the amplitude of DLTS peak with the increase in the temperature of its registration was found. This indicates the formation of areas of defects accumulation with displacement density lower initial level of doping. After exposure of irradiated samples at room temperature for several months, these areas decay with isolated point A-, E-centers and hydrogen defects with an Ec – 0.31 eV level formation. It is shown that complexes with an Ec – 0.31 eV level are formed by attaching hydrogen atoms to A-center. At Т > 150 °С, this defect begins to anneal, and at the same time A-center concentration is increased.

(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).

Direct Nanoscale Characterization of Deep Levels in AgCuInGaSe2 Using Electron Energy‐Loss Spectroscopy in the Scanning Transmission Electron Microscope

AbstractA new experimental framework for the characterization of defects in semiconductors is demonstrated. Through the direct, energy‐resolved correlation of three analytical techniques spanning six orders of magnitude in spatial resolution, a critical mid‐bandgap electronic trap level (EV + 0.56 eV) within Ag0.2Cu0.8In1−xGaxSe2 is traced to its nanoscale physical location and chemical source. This is achieved through a stepwise, site‐specific correlated characterization workflow consisting of device‐scale (≈1 mm2) deep level transient spectroscopy (DLTS) to survey the traps present, scanning probe–based DLTS (scanning‐DLTS) for mesoscale‐resolved (hundreds of nanometers) mapping of the target trap state's spatial distribution, and scanning transmission electron microscope based electron energy‐loss spectroscopy (STEM‐EELS) and X‐ray energy‐dispersive spectroscopy for nanoscale energy‐, structure, and chemical‐resolved investigation of the defect source. This first demonstration of the direct observation of sub‐bandgap defect levels via STEM‐EELS, combined with the DLTS methods, provides strong evidence that the long‐suspected CuIn/Ga substitutional defects are indeed the most likely source of the EV + 0.56 eV trap state and serves as a key example of this approach for the fundamental identification of defects within semiconductors, in general.

Data processing methods for deep level transients measurement

The year 2019 marks 45 years in the development of Deep Level Transient Spectroscopy (DLTS) - the signal processing method for determination of overalpping deep levels in semiconductors. From its introduction in 1974 by David Lang (D.V. Lang, J. Appl. Phys. 45, 1974, p.3023) to this date the DLTS method has undergone many changes and modifications: some were purely theoretical speculations, some were to also include new experimental arrangement and technique. This paper provides almost complete review on DLTS, focusing on the main three approaches widely used today. We also summarize the development of this method in the Faculty of Physics, VNU-Hanoi University of Sciences.

Study of Deep Levels in a HIT Solar Cell

The results of studying a HIT (heterojunction with an intrinsic thin layer) Ag/ITO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n+)/ITO/Ag solar cell by the capacitance–voltage characteristic and current deep-level relaxation transient spectroscopy methods are presented. The temperature dependence of the capacitance–voltage characteristics of the HIT structure and deep-energy-level parameters are studied. The results of comprehensive studies by the above methods are used to determine the features of the energy-band diagram of actual HIT structures.

Исследование глубоких энергетических уровней в солнечном элементе типа HIT

AbstractThe results of studying a HIT (heterojunction with an intrinsic thin layer) Ag/ITO/ a -Si:H( p )/ a -Si:H( i )/ c -Si( n )/ a -Si:H( i )/ a -Si:H( n ^+)/ITO/Ag solar cell by the capacitance–voltage characteristic and current deep-level relaxation transient spectroscopy methods are presented. The temperature dependence of the capacitance–voltage characteristics of the HIT structure and deep-energy-level parameters are studied. The results of comprehensive studies by the above methods are used to determine the features of the energy-band diagram of actual HIT structures.

Evaluation of the concentration of point defects in GaN

Photoluminescence (PL) was used to estimate the concentration of point defects in GaN. The results are compared with data from positron annihilation spectroscopy (PAS), secondary ion mass spectrometry (SIMS), and deep level transient spectroscopy (DLTS). Defect-related PL intensity in undoped GaN grown by hydride vapor phase epitaxy increases linearly with the concentration of related defects only up to 1016 cm−3. At higher concentrations, the PL intensity associated with individual defects tends to saturate, and accordingly, does not directly correlate with the concentration of defects. For this reason, SIMS analysis, with relatively high detection limits, may not be helpful for classifying unidentified point defects in GaN. Additionally, we highlight challenges in correlating defects identified by PL with those by PAS and DLTS methods.

Mechanism of mobility enhancement in Ge p-channel metal-oxide-semiconductor field-effect transistor due to introduction of Al atoms into SiO2/GeO2 gate stack

In this paper, we present comprehensive results on Al-postmetallization annealing (Al-PMA) effect for the SiO2/GeO2 gate stack on a Ge substrate, which were fabricated by a physical vapor deposition method. The effective oxide thickness of metal-oxide-semiconductor (MOS) capacitor (CAP) was ~7nm, and the Al-PMA was performed at a temperature in the range of 300–400°C. The flat band voltage (VFB), the hysteresis (HT), the interfacial states density (Dit), and the border traps density (Dbt) for MOSCAPs were characterized by a capacitance–voltage method and a constant-temperature deep-level transient spectroscopy method. The MOSCAP without Al-PMA had an electrical dipole of ~−0.8eV at a SiO2/GeO2 interface, which was disappeared after Al-PMA at 300°C. The HT, Dit, and Dbt were decreased after Al-PMA at 300°C and were maintained in the temperature range of 300–400°C. On the other hand, the VFB was monotonically shifted in the positive direction with an increase in PMA temperature, suggesting the generation of negatively charged atoms. Structural analyses for MOSCAPs without and with Al-PMA were performed by a time-of-flight secondary ion mass spectroscopy method and an X-ray photoelectron spectroscopy method. It was confirmed that Al atoms diffused from an Al electrode to a SiO2 film and reacted with GeO2. The dipole disappearance after Al-PMA at 300°C is likely to be associated with the structural change at the SiO2/GeO2 interface. We also present the device performances of Al-gated p-channel MOS field-effect transistors (FET) with PMA treatments, which were fabricated using PtGe/Ge contacts as source/drain. The peak field-effect mobility (μh) of the p-MOSFET was reached a value of 468cm2/Vs after Al-PMA at 325°C. The μh enhancement was explained by a decrease in the total charge densities at/near the GeO2/Ge interface.

Electrophysical Properties of GaAs P–I–N Structures for Concentrator Solar Cell Applications

Abstract This paper is dedicated to electro-physical characterisation of a GaAs p-i-n structure grown for solar cell applications, which was carried out by light and dark current-voltage (I–V) and Deep Level Transient Fourier Spectroscopy (DLTFS) methods. The conversion efficiency and open-circuit voltage were determined from I–V measurement at 1 and 20× sun light concentrations. Three electron like defects TAn1, TAn2, TDn and one hole like defect TBp obtained by DLTFS measurements were confirmed. The origin of these defect states was stated as native GaAs impurities.

(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.

A charge-based deep level transient spectroscopy measurement system and characterization of a ZnO-based varistor and a Fe-doped SrTiO3 dielectric

A charge-based deep level transient spectroscopy (Q-DLTS) method is applied to provide insights into the electronic behavior near grain boundaries and may provide new insights into mechanisms such as fatigue, degradation, dielectric aging, and dielectric breakdown. Here, we tested the Q-DLTS in both a ZnO varistor material and Fe-doped SrTiO3 materials. Comparisons are made to other data on ZnO varistors, and we obtain very good agreement for the energy levels. The status of deep traps in Fe-doped SrTiO3 dielectrics has been investigated where the relaxation was contrasted in a single crystal and polycrystalline ceramic materials. The relaxation is only observable in the polycrystalline materials, and was absent in single crystal Fe-doped crystals indicating that the deep traps originating from the Schottky barriers at the grain boundaries provide the DLTS signals. The energy associated with this grain boundary trap was found to be 1.26 eV.

Structures and electronic properties of defects on the borders of silicon bonded wafers

The complex research of the structure and electronic properties of defects occurring on the bonding border of twist misfit Si(001) wafers of n-type conductivity was carried out by the methods of transmission electron microscopy, deep level transient spectroscopy (DLTS), and photoluminescence. The revealed defects main are two types of dislocation structure: the orthogonal dislocation network composed of two screw dislocation families and zigzag mixed dislocations. The dislocation structures observed are the sources of intense luminescence, whose spectrum differ considerably from the standard dislocation luminescence spectrum at all investigated misfit angles of bonded Si wafers. It is shown that an increase of the misfit angle results in a strong transformation of the dislocation luminescence spectra consisting in the changes of the spectra form and a decrease in the integral luminescence intensity. In the studied samples, by means of the deep level transient spectroscopy method, the presence of deep centers whose concentration increases with an increase of the misfit angle of the bonded wafers is revealed. It is established that the deep centers are related to the dislocation structures observed by means of transmission electron microscopy.

Structures and Electronic Properties of Defects on the Borders of Silicon Bonded Wafers

Comprehensive studies of the structure and electronic properties of defects occurring on the connection boundary of disarranged n − type Si(001) wafers have been made by the methods of transmission electron microscopy, deep level transient spectroscopy (DLTS) and photoluminescence. The main revealed defects are two types of dislocation structure: orthogonal dislocation network composed of two screw dislocation families and zigzag mixed dislocations. The dislocation structures observed are sources of intense luminescence whose spectra are appreciably different from the standard dislocation luminescence spectra at all the investigated misfit angles of the Si bonded wafers. We show that an increase of the misfit angle results in a strong transformation of the dislocation luminescence spectra consisting in changes of the form of the spectra and a decrease in the integral luminescence intensity. In the samples in question the DLTS method revealed the presence of deep centers the concentration of which increased with increasing of twist misorientation of bonded wafers. It has been established that the deep centers are related to the dislocation structures observed by means of transmission electron microscopy.

Low-temperature fabrication of Y2O3/Ge gate stacks with ultrathin GeOx interlayer and low interface states density characterized by a reliable deep-level transient spectroscopy method

Y2O3/Ge gate stacks with ultrathin GeOx interlayer were fabricated by two-step rf sputtering using a Y2O3 target followed by a vacuum-annealing, which were carried out in the same chamber without vacuum breaking. TiN-gate Ge metal–insulator–semiconductor (MIS) capacitors were fabricated with equivalent oxide thicknesses in the range of 2.1–2.3nm. The highest temperature was 400°C for the entire fabrication process. Interface states density (Dit) was characterized using a deep-level transient spectroscopy method with optimized injection pulse and quiescent reverse-bias voltages at each temperature. Dit values were approximately 4×1013, 5×1011, and 3×1012cm−2eV−1 at energy positions around valence band, mid-gap, and conduction band, respectively. The slow trap contribution was also small in the upper half of the band-gap, implying a potential application of the Y2O3/Ge gate stack to the fabrication of high-performance Ge-n-MIS field effect transistors.