Deep Level Transient Spectroscopy Study Research Articles

Investigation of Traps in HVPE Grown β-Ga2O3 Epilayers/n+-Ga2O3 using Deep Level Transient Spectroscopy

Abstract Detailed investigation of deep traps in HVPE-grown β-Ga2O3 epilayers has been done by performing deep-level transient spectroscopy (DLTS) from 200 K to 500 K on Pt/β-Ga2O3 and Ni/β-Ga2O3 Schottky diodes. Similar results were obtained with a fill pulse width of 100 milliseconds irrespective of different Schottky metal contacts and epilayers. Two electron traps at E2 (EC–ET =0.65 eV) and E3 (EC–ET =0.68 – 0.70 eV) with effective capture cross-sections of 4.10 × 10-14 cm2 and 5.75 × 10-15 cm2 above 300 K have been observed. Below 300 K, a deep trap with a negative DLTS signal peak was also observed at E1 (EC–ET = 0.34 – 0.35 eV) with very low capture cross-section of 3.28 × 10-17 cm2. For a short pulse width of 100 μs, only two electron traps E2 and E3 at energies of 0.72 eV, and = 0.73 eV were observed, and one order of higher corresponding effective capture cross-sections. All traps are found to be unaffected by the electric field during the field-dependence DLTS study. From the filling-pulse width dependence DLTS study, the decrease in capacitance transient amplitude with increasing pulse width has been observed opposite to the capture barrier kinetics of the traps and attributed to the emission of carriers during the capture process. Trap concentrations were found high at the interface from depth profiling DLTS. Based on the available literature, it is suggested that these traps are related to FeGa, Fe-related centers, and complexes with hydrogen or shallow donors and might be affected or generated during the metallization by the E-beam evaporator and the CMP polishing.

Hydrothermally deposited Sb2S3 absorber, and a Sb2S3/CdS solar cell with VOC approaching 800 mV

We report the hydrothermal deposition of Sb2S3 thin film on top of CdS buffer layer, and the fabrication of prototype photovoltaic devices utilizing spiro-OMeTAD as the hole transport layer. The as-deposited films were amorphous, which transformed to polycrystalline after thermal processing. The pristine films were annealed at different temperatures and showed effective recrystallization at 350 °C which resulted in larger grains, intense XRD patterns, and significantly improved device parameters. The obtained VOC of 795 mV is among the highest reported for a Sb2S3 based solar cell. Deep level transient spectroscopy studies detected an electron trap with activation energy 0.61 eV in the pristine annealed absorber, which became deeper (0.66 eV) upon Na incorporation. However, the capture cross-section decreased by an order of magnitude, and the trap density halved. The reduction in the capture cross-section and trap density for the Na-incorporated device coincides with the improved EQE response in the mid- and long-wavelength regions and a 9 % increase in device efficiency. The light intensity dependence of VOC clearly demonstrated that Na incorporation reduced the trap-assisted recombination and facilitated efficient charge transport in the device.

One-Step Passivation of Both Sulfur Vacancies and SiO2 Interface Traps of MoS2 Device.

Transition metal dichalcogenides (TMDs) benefit electrical devices with spin-orbit coupling and valley- and topology-related properties. However, TMD-based devices suffer from traps arising from defect sites inside the channel and the gate oxide interface. Deactivating them requires independent treatments, because the origins are dissimilar. This study introduces a single treatment to passivate defects in a multilayer MoS2 FET. By applying back-gate bias, protons from an H-TFSI droplet are injected into the MoS2, penetrating deeply enough to reach the SiO2 gate oxide. The characterizations employing low-temperature transport and deep-level transient spectroscopy (DLTS) studies reveal that the trap density of S vacancies in MoS2 drops to the lowest detection level. The temperature-dependent mobility plot on the SiO2 substrate resembles that of the h-BN substrate, implying that dangling bonds in SiO2 are passivated. The carrier mobility on the SiO2 substrate is enhanced by approximately 2200% after the injection.

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.

4H-SiC Schottky barrier diodes as radiation detectors: A role of Schottky contact area

Large-area SiC Schottky barrier diodes can significantly improve the sensitivity in radiation detection due to the increased interacting SiC volume. In this work, we tested a large SiC detector with an area of 1 × 1 cm2. Extensive electrical characterization was performed at temperatures ranging from 150 K to 390 K, demonstrating the impact of barrier inhomogeneities on the electrical performance of the diode. Forward current-voltage (I-V) measurements of the diodes revealed two distinct regions caused by Schottky barrier inhomogeneity present throughout the entire temperature range. The barrier heights in the low- and high-current forward voltage regions were extracted from Richardson plots corrected for the Gaussian distribution of barrier heights, yielding values of 1.52 eV and 1.79 eV, respectively. Deep-level transient spectroscopy (DLTS) revealed only one deep-level defect, Z1/2, with an activation energy for electron emission of 0.67 eV, which was assigned to the known carbon vacancy. The DLTS study showed no correlation between electrically active defects and barrier inhomogeneity. An excellent energy resolution of 3.2 % was measured using a large area 241Am radiation source, consistent with values for small area SiC detectors that exhibited no barrier height inhomogeneities. The impact of temperature on the alpha particle radiation response was determined within a temperature range of 200 K to 390 K.

Electrical characterization of Au/Ni Schottky contacts on GaN synthesized using electrodeposition

Gallium nitride thin films have attracted attention due to their prospects in semiconductor devices and technology. In this study, we investigate the electrical properties and perform deep level transient spectroscopy (DLTS) on Au/Ni Schottky diodes fabricated on gallium nitride thin films that were synthesized by electrodeposition on a Si(111) substrate from a solution containing gallium nitrate (Ga(NO3)3) and ammonium nitrate (NH4(NO3)) using current densities of 1 and 3 mA cm−2. The thin films were found to crystallize in the wurtzite hexagonal structure with crystallite sizes of approximately 20 nm. Scanning electron microscopy and atomic force microscopy were used to characterize the microstructure of the GaN thin films. The Schottky diodes had good rectifying properties, corresponding to n-type material. The diodes had a IV barrier heights of 0.76 eV and 0.60 eV; a CV barrier heights of 0.92 eV and 0.71 eV; and carrier densities of 1.2 × 1016 cm−3, and 1.7 × 1016 cm−3, for material deposited under 1 mA cm−2 and 3 mA cm−2, respectively. By fitting a model taking both thermionic emission as well as conduction through the polycrystalline bulk into account, it was found that the grain boundary potential of the GaN crystallites was 0.29 V. The DLTS study revealed different dominant DLTS peaks in each as-grown sample with activation energies of 0.49 and 0.48 eV with capture cross-sections of 9 × 10−15 and 4 × 10−16 cm2, in the material grown under 1 mA cm−2 and 3 mA cm−2 current density, respectively. The defects seem to be similar to defects found in heavily damaged GaN and GaN grown on highly mismatched or cracked substrates, and are probably not simple defects.

Laplace DLTS study of defects introduced in GaAs during sputter deposition of Au Schottky contacts

The RF sputter deposition was used to fabricate Au Schottky barrier diodes on Si-doped n-type GaAs with two different carrier densities (1 × 1015 cm−3, 1 × 1016 cm−3 and partially 8 × 1016 cm−3) at a power of 150 W. Deep-level transient spectroscopy (DLTS) and Laplace DLTS were used to measure the electrical properties of the sputter-induced defects near the surface of the GaAs. By using DLTS, we found that the samples contained six defects (S1 (0.046 eV), S2 (0.22 eV), S3 (0.30 eV), S4 (0.55 eV), S5 (0.56 eV) and S6 of which three (S1, S3 and S5) were metastable and one (S4) was highly dopant concentration dependent. S6 was observed close to where the EL2 was expected, but had different properties and two components which could be resolved by Laplace DLTS: S6a (0.83 eV) and S6b (0.84 eV). The depth profile showed a difference in depth distribution of the defects in the samples which shows by increasing depth, a rapid decay of defect concentration. The electric field dependence of the S3 defect could be explained by a combination of Poole-Frenkel and phonon-assisted tunnelling mechanisms while for the S4 and S5 defects the emission of carriers indicate the phonon-assisted tunnelling. True capture cross-section measurements were done, and the S3 defect had a much lower capture cross-section than the S4 and S5 defects and the real capture cross-section of S3 and S4 defects is in the range of Auger mechanism while for the S5 defect indicates multiphonon emission. Finally, measurements as to the effect of annealing on I–V characteristics were carried out on the sputtered samples as well.

Influence of carrier trapping on radiation detection properties in CVD grown 4H-SiC epitaxial layers with varying thickness up to 250 µm

Low doped n-type 4H-SiC epitaxial layers of thickness 50, 150, and 250 µm grown by hot wall chemical vapor deposition were used to fabricate Ni/4H-SiC Schottky radiation detectors. The epitaxial layers were grown on the (0001) face of highly conductive bulk 4H-SiC substrates 8° offcut towards the 〈112¯0〉 direction. The 50, 150, and 250 µm thick epilayer detectors, under optimized settings, showed energy resolutions of 2.0%, 0.78%, and 0.63%, respectively for 5.48 MeV alpha particles. Deep level transient spectroscopy studies showed that the observed variation in the detector resolution is linked to the defect parameters in the devices. Least-squares fitting of the bias dependence of the charge collection efficiency according to a drift-diffusion model, revealed minority carrier diffusion lengths of 16, 10, and 9.2 µm, respectively, implying that the detectors are not limited by minority carrier trapping. The detector performance was observed to be primarily dependent on the concentration and capture cross-sections of the lifetime killing electron traps Z1/2 and EH6/7.

Механизмы токопереноса в полупроводниковой структуре с пленкой пористого кремния, сформированной металл-стимулированным травлением

It is shown that during a porous Si film formation by metal-stimulated etching a barrier layer is formed on a monocrystal p-Si substrate. The rectifying properties of the semiconductor structure can be explained by the fixation of the Fermi level in the near-surface layer of porous Si due to a high concentration of electrically active defects (deep centers or traps). It causes to energy bands bending and the appearance of a potential barrier. The study of Raman scattering showed the absence of size effects and a change in the band gap in the porous Si film. Activation energies of deep centers by the temperature dependence of the current-voltage characteristics and deep level transient spectroscopy study were determined.

Current Transfer in a Semiconductor Structure with a Porous Silicon Film formed by Metal-Stimulated Etching

It is shown that during a porous Si film formation by metal-stimulated etching a barrier layer is formed on a monocrystal p-Si substrate. The rectifying properties of the semiconductor structure can be explained by the fixation of the Fermi level in the near-surface layer of porous Si due to a high concentration of electrically active defects (deep centers or traps). It causes to energy bands bending and the appearance of a potential barrier. The study of Raman scattering showed the absence of size effects and a change in the band gap in the porous Si film. Activation energies of deep centers by the temperature dependence of the current-voltage characteristics and deep level transient spectroscopy study were determined. Keywords: porous silicon, deep level, Raman scattering, current-voltage characteristics, deep level transient spectroscopy.

(Invited) Defect Engineering for Monolithic Integration of III-V Semiconductors on Silicon Substrates

Implementation of high-mobility channel materials enables to increase the device performance due to the higher carrier mobility compared to silicon devices. III-V based materials such as GaAs and ternary systems (e.g. InxGa1-xAs alloys) are achieving high performing n-channel transistors. The strong progress in hetero-epitaxy using nano ridge engineering [1] leading to monolithic heterogeneous integration of III-V materials on silicon substrates resulted in state-of-the-art FinFETs, gate-all-around (GAA) and horizontal or vertical stacked nanowires (NW) on 300 mm Si wafers [2,3]. Very promising results have also been reported for GaN on Si triggering their use for high power and RF applications [4].One of the main challenges remains a good defect control in order not to degrade the electrical performance of devices and circuits [5]. The nucleation of extended defects is directly related to the hetero-epitaxial growth of layers with a lattice mismatch and their concentration depends on the composition of the layers such as e.g. the In content in InxGa1-xAs and the Al content in AlxGa1-xN. The defects have a direct impact on the generation and recombination lifetime and the leakage current [5], while also the low frequency noise (LFN) performance is influenced [6]. The identification of the defects (energy level ET, capture cross section and concentration) can be determined using Deep Level Transient Spectroscopy (DLTS) and low frequency noise spectroscopy by measuring LFN as a function of temperature.The goal of this work is to review the knowledge about defect engineering and to analyze the electrical activity of extended defects in III-V semiconductors on silicon substrates. Different case studies will be used to illustrate both the defect identification and the electrical activity of the defects. The impact of the used processing schemes to fabricate the devices is also discussed. It will e.g. be shown that defects in GaN/AlGaN MOSHEMTs increase the low frequency noise due to generation-recombination centers in the GaN depletion layer [7]. These traps can lead to a distortion of the current-voltage characteristics of short channel devices. DLTS studies are a powerful tool to identify traps and to investigate their impact on the leakage current of III-V on Si devices. Double correlation DLTS investigations on GaAs/InGaP/GaAs diodes pointed out that traps associated with sidewall interface states can dominate the peripheral diode leakage current. The focus of the presentation will be on the current understanding of defect engineering associated with the monolithic integration of III-V materials on Si and a discussion of recent results concerning the electrical activity of the defects and the defect modeling.[1] M. Baryshnikova et al., Crystals, 10, 330 (2020).[2] X. Sun et al., Digest VLSI Technology, T3-4 (2017).[3] A.P. Milemin et al., Microelectron. Eng., 192, 14 (2019).[4] L. Li et al., IEEE Electron Device Lett., 41, 689 (2020).[5] C. Claeys et al., ECS Solid State Electronics, 9, 03301 (2020).[6] C. Claeys et al., J. Phys. Cond. Ser., 1190, 012001 (2019).[7] K. Takakura et al., IEEE Trans. Electron Dev., 67, 3062 (2020).

Defect studies on short-wave infrared photovoltaic devices based on HgTe nanocrystals/TiO2 heterojunction

Narrow bandgap (<0.5 eV) colloidal semiconductor nanocrystals (e.g. mercury chalcogenides) provide practical platforms for next generation short wave infrared, mid wave infrared and long wave infrared optoelectronic devices. Until now, most of the efforts in the field of infrared active nanocrystals have been taken on synthesizing nanocrystals, determining quantum states and building different geometries for optoelectronic devices. However, studies on interface trap states in the devices made from these narrow band gap nanocrystals are mostly unexplored. Herein, we investigate the defects or traps in these nanocrystals—embedded devices, which will be critical for improving their optoelectronic performance. In this article, we fabricate HgTe nanocrystals/TiO2 based photovoltaic devices and used capacitance-voltage (C–V) and deep level transient spectroscopy (DLTS) to investigate and obtain quantitative information on deep level trap states. Interestingly, frequency dependent C–V measurements show two peaks in the capacitance at lower frequency (<40 kHz), which is attributed to the presence of trap states. However, at high frequency the presence of a weak hump-like structure almost at the center of above two peaks validate the role of interface traps. DLTS studies show that traps at the interface of HgTe nanocrystals/TiO2 acts as recombination centers having activation energies of 0.27, 0.4 and 0.45 eV with corresponding trap densities of 1.4 , 1. and 1. and estimated capture cross-sections of 6.3 , 7.5 and 3.7 , respectively. In this work, DLTS has revealed the existence of interface trap states and the frequency dependent capacitance measurements corroborate the effect of charge storage on the heterostructures built from these nanocrystals that helps in the development of futuristic devices.

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.

Origin and anomalous behavior of dominant defects in 4H-SiC studied by conventional and Laplace deep level transient spectroscopy

Several deep level defects were observed by conventional deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS (LDLTS) in n-type 4H-SiC junction barrier Schottky diodes. We have shown that the broad DLTS peak labeled Z1/2 has, in fact, two components, Z1 and Z2, with activation enthalpies for electron emission of 0.63 eV and 0.68 eV, respectively. The reorientation process between these two components was observed. A combination of double-correlated DLTS and LDLTS demonstrated an anomalous reduction of the emission rate and an increase of the activation enthalpy of Z2 with an increase of the reverse bias applied to the diode. The possible explanation of this phenomenon could be correlated with a tensile stress in epitaxial SiC layers. The results observed are discussed in the frame of the model that correlates Z1 and Z2 with carbon vacancies (VC), located at hexagonal (h) and cubic (k) lattice sites, respectively. We also discussed the origin of other traps E0–E5 with particular emphasis on a N-related shallow donor level located at 0.04 eV below the conduction band, which has never been previously reported by DLTS studies.

Deep Level Transient Spectroscopy (DLTS) Study of 4H-SiC Schottky Diodes and PiN Diodes

We have studied capacitance mode Deep Level Transient Spectroscopy (DLTS) of five 4H-SiC Schottky diode and PiN diode designs. Comparing with previous DLTS studies, we have identified four traps levels, Z1/2, EH1, EH3 and EH5. Additionally, a new trap level, EH1, is prominent in blanket Al+ and B+ high-energy implanted samples but less so in mask-implanted samples. Al+ implantation increases EH3 (associated with silicon vacancy) and EH5, while B+ implantation significantly reduces EH3. The Z1/2 peak (associated with carbon vacancy) is reduced to very low levels after B+ and Al+ implantation.

Deep-level transient spectroscopy studies of electron and hole traps in n-type GaN homoepitaxial layers grown by quartz-free hydride-vapor-phase epitaxy

We studied deep levels in quartz-free hydride-vapor-phase epitaxy (QF-HVPE)-grown homoepitaxial n-type GaN layers within which three electron and eight hole traps were detected. The dominant electron and hole traps observed in the QF-HVPE-grown GaN layers were E3 (EC − 0.60 eV) and H1 (EV + 0.87 eV), respectively. We found that the E3 trap density of QF-HVPE-grown GaN (∼1014 cm−3) was comparable with that of MOVPE-grown GaN layers, whereas the H1 trap density of QF-HVPE-grown GaN (∼1014 cm−3) was much smaller than that of an MOVPE-grown GaN layer with a low-residual-carbon growth condition. A detailed analysis of the QF-HVPE-grown GaN layers revealed that the H1 trap density is almost equal to the carbon impurity concentration and other impurities that compensate the Si donors besides the carbon impurity were hardly detected in the QF-HVPE-grown GaN layers.

DLTS study of extended defects in HgCdTe photodiodes

Extended electrically active defects have been investigated in Short Wave InfraRed (SWIR) HgCdTe n on p photodiodes, using the Deep Level Transient Spectroscopy (DLTS) technique. Three localized defects have been found in the dislocations core or in their close environment. DLTS studies have also been performed before and after indentation, a technique which generates dislocations in the material. DLTS spectra are discussed and dislocations generated by indentation have been found to be electrically active only after annealing.

Correlated annealing and formation of vacancy-hydrogen related complexes in silicon

We report on a deep level transient spectroscopy study of annealing kinetics of a deep, vacancy-hydrogen related level, labeled E5*, at 0.42 eV below the conduction band in hydrogen-implanted n-type silicon. The E5* annealing correlates with the formation of another commonly observed vacancy-hydrogen related level, labeled E5, at 0.45 eV below the conduction band. The annealing of E5* and the formation of E5 exhibit first-order kinetics with an activation energy of 1.61 ± 0.07 eV and a pre-factor of ~1013–1014 s−1. The pre-factor indicates a dissociation or structural transformation mechanism. The analysis of electron capture cross-sections for E5* and E5 reveals considerable transition entropies for both states and a temperature dependent capture cross-section for E5*. Two possible identifications of E5* and E5 are put forward. Firstly, E5* can be attributed to V2H2(−/0) or V2H3(−/0), which dissociate with the emission of VH (E5). Secondly, E5* and E5 can be assigned to two different configurations of V3H.

Laplace DLTS study of the fine structure and metastability of the radiation-induced E3 defect level in GaAs

In this paper we used high-resolution Laplace deep-level transient spectroscopy (DLTS) to study the electrical properties of the E3 defect family introduced in GaAs by MeV electron irradiation. We found that the peak conventionally referred to as the E3 contained 3 components which we labeled E3a, E3b and E3c, with E3a being the most prominent component. The activation energy of E3a for different carrier densities varied between 0.36 and 0.375 eV. From dopant dependent introduction rate measurements, we found that the introduction rates of the E3a and E3b defect did not depend on doping density. However, the E3c concentration increased with increasing carrier density. Furthermore, the E3c was found to be metastable: it was reversibly removed during minority carrier injection at low temperatures and re-introduced by annealing above 160 K under zero bias. Interestingly, annealing measurements revealed that, of the three components, the E3b annealed first at around 500 K while both the other components annealed together at a higher temperature of 525 K. By comparing our results with previous studies, we concluded that the origin of E3a is VAs, E3b is Asi and E3c is VGa–SiGa.

Deep level transient spectroscopy study of heavy ion implantation induced defects in silicon

Defects introduced by low fluence arsenic, antimony, erbium, and bismuth ion implantation have been investigated as a function of annealing temperature using deep level transient spectroscopy (DLTS) and Laplace-DLTS. The defects produced by heavy ion implantation are stable up to higher temperatures than those introduced by electron irradiation and low mass ions. This result is attributed to the enhanced defect interactions that take place in the dense collision cascades created by heavy ion implantation. As a consequence, broadened DLTS features are apparent, especially after annealing. Using high energy resolution Laplace-DLTS, the well-known singly charged divacancy and vacancy-donor pair are accompanied by additional apparent defect signals. This shows that Laplace-DLTS is highly sensitive to the type of damage present, and extreme care must be exercised for reliable Arrhenius analysis.