Deep Level Transient Spectroscopy Research Articles

Defect suppression and optoelectronic property investigation of a self-powered β-Ga2O3/p-Si photodetector grown by pulsed laser deposition

Abstract The pulsed laser deposition method is recognized for its ability to minimize defects in semiconductors by carefully controlling gas pressure and temperature during deposition. This study discusses the presence of defect states in β-Ga2O3/p-Si heterojunction created using pulsed laser deposition at different oxygen pressures through deep-level transient spectroscopy (DLTS). When the oxygen pressure rises from 0 to 25 mTorr, the electron trap at EC −1.21 eV associated with an oxygen vacancy vanished, resulting in a 1.6 fold decrease in the total defect density. A gallium vacancy hole trap located at EV + 0.57 eV was identified when the oxygen pressure reached 55 mTorr. The presence of this hole trap suggested that β-Ga2O3 was grown in an oxygen-rich environment. The photoluminescence findings are outstanding and in agreement with DLTS data. The β-Ga2O3/p-Si device showed its peak photoresponsivity at 25 mTorr (330 mA W−1). This shows that carrier transport is improved because defects are reduced. Our discovery provides a possible path for enhancing the high optoelectronic properties of β-Ga2O3/p-Si devices.

Solvent Engineering-Enabled Surface Defect Passivation in Cu2ZnSn(S,Se)4 Solar Cells with Low Open-Circuit Voltage Losses and Improved Carrier Lifetime.

The efficiency of earth-abundant kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells has been lagging behind the Shockley-Queisser limit primarily due to the presence of deep-level defects. These deep-level defects cause critical issues such as short carrier diffusion length, significant band tailing, and a large open-circuit voltage (Voc) deficit, ultimately leading to low device efficiency. To address these issues, we propose a post-fabrication defect healing strategy by dip-coating the CZTSSe film in dimethylformamide (DMF) solvent. Immersing the absorber layer in DMF (a polar solvent), neutralizes CuSn antisite defects through chemical bonding and facilitates the formation of a dense, smooth CZTSSe film with larger grain size. Deep-level transient spectroscopy revealed a remarkable increase in carrier diffusion length from 93 nm (control device) to 142 nm (champion device), confirming the beneficial effect of solvent-assisted post-treatment on mitigating CuSn antisite defects. The reduction in defect densities led to a decrease in Voc deficit by up to 289 mV, accompanied by an increased champion device efficiency of 11.4 %. This work highlights the huge potential of the DMF post-treatment strategy for defect healing in CZTSSe solar cells.

DLTS characterisation of 107 MeV krypton ion-irradiated nitrogen-doped 4H-silicon carbide

Swift heavy ions, such as krypton ions, play a significant role in developing and enhancing the performance of various devices. In this study, the influence of Kr2+ on nitrogen-doped 4H-silicon carbide has been investigated using deep level transient spectroscopy (DLTS). Krypton ions, with an energy of 107 MeV, were used to irradiate the Au/Ni/4H-SiC Schottky barrier diodes (SBDs) at a fluence of 1 × 1010 cm–2 at room temperature (300 K). Before the irradiation of the samples, the electrical measurements revealed good rectifying behaviour. However, rectification properties of the Au/Ni/4H-SiC SBDs were completely lost after irradiation at a fluence of 1 × 1010 cm–2. Annealing was performed at 300 °C in flowing argon, and the current–voltage (I–V) and capacitance–voltage (C–V) revealed partial rectification. DLTS of the as-grown devices analyses revealed the presence of four deep level defects. After annealing the irradiated device, the DLTS spectra showed a reduction in the intensity of the E0.10 and the disappearance of the E0.12 as well as the E0.16 defects compared to that as-grown. Two defects with energies of 280 and 410 meV showed inverted peaks, as would have been expected from minority carriers trap instead of majority carriers, which led to confusion as the peaks were inverted. It was concluded that the peculiar characteristics of DLTS measurements on SBDs may be due to the extremely high value of the series resistance as well as the low capacitance. The results of this study provide insight into the behaviour of SBDs under extreme irradiation and can be used to improve the radiation tolerance of electronic devices made from SiC.

Deep level transient spectroscopy: Tracing interface and bulk trap‐induced degradation in AlGaN/GaN‐heterostructure based devices

AbstractThe exceptional physical properties of gallium nitride (GaN) position GaN‐based power devices as leading candidates for next‐generation high‐efficiency smart power conversion systems. However, GaN's multi‐component nature results in a high density of epitaxial defects, whereas the introduction of dielectric layers further contributes to severe interface states and dielectric traps. These factors collectively impair reliability, manifesting as threshold voltage instability and current collapse, which pose significant barriers to the advancement of GaN‐based electronics. Establishing the intrinsic relationship between device reliability and defects is crucial for understanding and addressing reliability degradation issue. Deep level transient spectroscopy (DLTS) offers valuable insights by revealing defect‐induced changes in electrical parameters during the capture and emission processes under varying biases, thereby elucidating the influence of defects from GaN buffer layers, AlGaN barriers, dielectric layer, and even at dielectric/(Al)GaN interfaces. This research aims to provide a foundational understanding of reliability degradation whereas further enabling enhancements in device performance from the perspectives of epitaxial growth and process preparation, ultimately striving to improve the reliability of GaN‐based devices and unlock their full potential for practical applications.

Probing n-ZnMgO/p-Si nanowire junctions: Insights into composition, strain, and defects via Raman spectroscopy and electrical measurements

This article focuses on investigating physical properties of n-ZnMgO/p-Si heterojunctions containing ZnMgO nanowires (NWs) with ZnMgO/ZnO/ZnMgO quantum wells (QWs) grown by molecular beam epitaxy. Detailed analysis of chemical composition, crystal lattice strain, and defects in the structures was conducted via Raman spectroscopy, deep level transient spectroscopy (DLTS), and capacitance-versus-temperature characteristics, C(T). Additionally, current-voltage (I-V) curves were included to demonstrate the rectifying properties of the studied n-ZnMgO/p-Si NW junctions. The samples varied in the thicknesses of the ZnO QWs and the ZnMgO barriers. Raman spectra provided information on strain within the crystal lattice of the nanowires. The values of the biaxial in-plane strain were calculated for the selected samples, and the origin of this strain was discussed in detail. A broadened part of spectrum in the range of 480–600 cm−1 was ascribed to the contribution of A1LO mode, which is associated with lattice defects such as oxygen vacancies, zinc interstitials, or their complexes. The presence of deep traps in the junctions was revealed by the DLTS method. Measurements of C(T) characteristics of the p-n junctions disclosed the presence of deep-level traps in the structures as well.

Low‐frequency noise and deep‐level transient spectroscopy in LWIR Auger‐suppressed Hg1-xCdxTe heterostructure detector

Low-frequency noise spectroscopy (LFNS) along with deep-level transient spectroscopy (DLTS) are complementary and effective tools to study and characterize the carrier traps in semiconductors. These traps caused, e.g., by contamination by foreign atoms or various types of dislocations, can significantly affect quantum efficiency, dark current, responsivity, and noise generated by devices especially when operating under bias. Since DLTS is difficult to apply in high leakage current devices, LFNS can be used to overcome this limitation, so the use of both methods gives very effective and reliable results during research on various devices. In this paper, we reported a study of defects activation energies in HgCdTe Auger-suppressed long-wavelength infrared (LWIR) heterostructure-based detector using these two experimental methods. By proper structure design, the examined detector was optimized for high operating temperature (HOT) conditions ≥ 200 K. The results obtained showed that in such detectors, grown by the metal organic chemical vapor deposition (MOCVD) technique, a few traps can be extracted. The found trap levels and activation energies were located below and above the absorber bandgap, so they can be identified in both absorber and other heterostructure layers. Due to specific multilayer architecture, a precise interpretation of the results is difficult. Nevertheless, the most probable trap locations based on the current state of knowledge were discussed and proposed.

Effect of 5 MeV proton irradiation on electrical and trap characteristics of β-Ga2O3 power diode

In this study, impact of 5 MeV proton irradiation with radiation fluence of 1013 cm−2 on β-Ga2O3 power diode is investigated by a β-Ga2O3 Schottky barrier diode (SBD). Via temperature-dependent measurements, carrier removal rate RC is determined to be 7.26 × 102 cm−1 at 300 K. Meanwhile, the threshold voltage (Von) and ideality factor (n) almost remain stable after proton irradiation. A close-to-unity n was observed for a wide temperature range indicating near-ideal Schottky characteristics. Dynamic degradation was observed at 300K, but was greatly suppressed at a low temperature of 100K. Meanwhile, two more bulk traps are discovered in proton irradiated β-Ga2O3 SBD by deep-level transient spectroscopy (DLTS). The larger corrected trap concentration (NTa) in proton irradiated β-Ga2O3 SBD was regarded as the reason behind slightly worsened dynamic on-resistance instability at 300 K. Furthermore, lower low frequency noise is revealed for proton irradiated device at room temperature and cryogenic temperature. The study demonstrates the competitive irradiation hardness of β-Ga2O3 power diodes and paves a solid path for the deployment of β-Ga2O3 in space.

The effect of low energy high-dose X-ray irradiation on the performance of CdZnTe detector

The paper investigated the irradiation damage of CZT detectors caused by high flux an X-ray from X-ray tube operated at a tube voltage of 140 kV and an accumulated dose of 130 kGy. Deep-level transient spectroscopy (i-DLTS) was employed to characterize the defects. Four types of irradiation-induced defects were identified, corresponding to the energy levels at Ec-0.1eV, Ev+0.17eV, Ev+0.27eV and Ec-0.55eV, which are associated with the point defects, InCd+/0, VCd-/0, VCd2−/− and TeCd2+/+ respectively. After the irradiation, the concentration of VCd-/0 changed from 5.63 × 1012 cm−3 to 3.30 × 1013 cm−3 and VCd2−/− increased from 1.13 × 1012 cm−3 to 1.88 × 1013 cm−3. The resistivity of all samples showed an upward trend after the irradiation, with the most significant change observed in sample 1, where it doubled from 1.09 × 1011 Ω cm to 1.94 × 1011 Ω cm. However, the energy resolution for the spectrum peak of 241Am (59.5 keV) decreased from 6.2% to 10.7% and the signal noise to ratio decreased from 42.24 to 27.33 when irradiated from anode. The counting performance of the photon counting module decreased by approximately 1.81% after irradiation on the cathode side.

Investigation of traps in halide vapor-phase epitaxy-grown β-Ga2O3 epilayers/n+-Ga2O3 using deep-level transient spectroscopy

Abstract A detailed investigation of deep traps in halide vapor-phase epitaxy (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 ms irrespective of the different Schottky metal contacts and epilayers. Two electron traps at E2 (E C–E T = 0.65 eV) and E3 (E C–E T = 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 were observed. Below 300 K, a deep trap with a negative DLTS signal peak was also observed at E1 (E C–E T = 0.34–0.35 eV) with a 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 were found to be unaffected by the electric field during the field-dependent DLTS study. From the filling pulse width dependence DLTS study, a decrease in the capacitance transient amplitude with the increasing pulse width was 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 to be high at the interface using 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 metallization by the electron beam evaporator and chemical mechanical polishing.

Interface properties study of black silicon solar cells with front surface a-Si:H/c-Si heterojunction

Abstract The exceptionally low reflectance of black silicon (Si) across a broad wavelength range makes it an intriguing surface texture for solar cell applications. Si heterojunction (SHJ) solar cells fabricated on black Si formed by dry reactive ion etching using inductive coupled plasma on n-type Si are explored. The study is focused on the properties of the a-Si:H/ crystalline silicon interface, being a key issue for the photovoltaic performance of SHJ. Deep-level transient spectroscopy detected no radiation defect in Si after the etching. The surface of black Si was passivated with an intrinsic a-Si:H layer, followed by the deposition of p-type and n-type a-Si:H on the front and back sides, respectively. The SHJ solar cell photovoltaic performance based on black Si is strongly influenced by defect density at the a-Si:H/Si interface. Admittance spectroscopy and effective charge carrier lifetime measurements demonstrate that interface defect density decreases with the increase of (i) a-Si:H thickness. The value of 0.75 ms effective charge carrier lifetime was reached for single (i) a-Si:H layer passivation and 0.25 ms when (p)a-Si:H was deposited over the intrinsic a-Si:H layer. The measured open circuit voltage values for the SHJ solar cells increase with the (i) a-Si:H layer thickness reaching 658 mV. However, the fill factor decreases with increasing (i) a-Si:H layer thickness, limiting the efficiency at the maximum value below 14% due to the thickness uniformity of the a-Si:H layer. The development of conformal growth of a-Si:H is a key issue for further improvement of black Si heterojunction solar cell photovoltaic performance.

Depth profiles of hole traps in the tail region of Al ion implantation into p-type 4H-SiC

Abstract Hole traps generated in the tail region of Al ion implantation in p-type 4H-SiC were characterized by deep-level transient spectroscopy measurements. Hole traps energetically located at E v + 0.51 eV, E v + 0.72 eV, E v + 0.77 eV, and E v + 1.40 eV (E v: energy of the valence band top) were detected. The hole trap densities were roughly 7–40 times smaller than the implanted Al atom density in the tail region, and the densities of the observed traps exponentially decreased with the decay lengths of 84–150 nm.

Investigation of the leakage mechanism in solar-blind AlGaN p-i-n photodetector at high reverse bias

The leakage mechanism of solar-blind AlGaN p-i-n photodetectors has been investigated. By studying the temperature-dependent I–V curves, we determined that Poole–Frenkel emission is the main source of the leakage current, and the corresponding trap energy level is 0.61 eV. Cathodoluminescence measurement demonstrates that the leakage current of the device may be related to the deep-level traps in the p-AlGaN layer. An analysis of deep-level transient spectroscopy testing results for p-AlGaN suggests that nitrogen vacancies formed during the epitaxial growth of high Al-content p-AlGaN act as electron traps. The trap potential energy decreases at high reverse bias voltage, making trapped electrons easier to escape, which increases the leakage current.

The Influence of Etching Method on the Occurrence of Defect Levels in III-V and II-VI Materials.

The influence of the etching method on the occurrence of defect levels in InAs/InAsSb type-II superlattice (T2SLs) and MCT photodiode is presented. For both analyzed detectors, the etching process was performed by two methods: wet chemical etching and dry etching using an ion beam (RIE-reactive ion etching). The deep-level transient spectroscopy (DLTS) method was used to determine the defect levels occurring in the analyzed structures. The obtained results indicate that the choice of etching method affects the occurrence of additional defect levels in the MCT material, but it has no significance for InAs/InAsSb T2SLs.

Characterization Study of Deep-Level Defect Spatial Distribution, Emission Mechanisms, and Structural Identification

Abstract The characterization of defects in semiconductor materials and devices is crucial for enhancing the performance and reliability of semiconductor products. This tutorial review focuses on Deep Level Transient Spectroscopy (DLTS) as the primary analytical tool, thoroughly discussing its distinct advantages in deep-level defect characterization. However, it is unable to reveal the concentration-depth distribution of deep-level defects, neglects the dependency of carrier emission rates on the electric field, and fails to accurately identify defect structures.
To overcome these limitations, two enhanced DLTS techniques have been developed to extend the capabilities of DLTS. These enhancements include the utilization of graded fill pulse technology to accurately map defect distributions at various depths within devices, facilitating individual defect characterization across different layers of multilayered structures; the application of varying electric field strengths to samples to delve into the intricate physical mechanisms of defects during carrier emission processes; and the adjustment of the duration of electric pulse injection to monitor signal growth trends, deducing the microstructure of defects. The paper integrates research findings from a wide array of field experts, meticulously outlines a description of how to obtain the depth distribution of defect concentration in devices, furnishes quantitative criteria for both the Poole-Frenkel effect and phonon-assisted tunneling mechanisms of carrier emission, and provides specific examples for distinguishing between interface states/bulk defects and point defects/extended defects. This enhances both the theoretical and practical knowledge in this field. The advanced DLTS techniques outlined provide crucial guidance for defect characterization and performance optimization in semiconductor devices with new structures and materials.

Multifaceted Characterization Methodology for Understanding Nonidealities in Perovskite Solar Cells: A Passivation Case Study

A multifaceted characterization approach is proposed, aiming to establish a link between nanoscale electrical properties and macroscale device characteristics. Current–voltage (I–V) measurements are combined with admittance spectroscopy (AS) and deep‐level transient spectroscopy (DLTS) for the analysis of charge‐related performance losses with time‐of‐flight secondary‐ion mass spectrometry to complete the understanding of ionic motion in the device. This is applied to the study of surface treatment in perovskite solar cells, which implements several strategies to improve band alignment, perovskite grain growth, and chemical passivation. An increase of both open‐circuit voltage (Voc) and fill factor of respectively 90 mV and 11% is shown after surface treatment, with an absolute efficiency increase of 4%. AS measurements, coupled with a lumped elements model, rule out the impact of transport layers as the origin of the performance improvement, rather pointing toward a reduction in ionic resistance in the perovskite bulk. Analysis of the DLTS response yields an activation energy of 0.41 eV, which is likely related to the same ionic mechanism discovered with AS. Finally, both of these techniques enable to show that the surface treatment main contribution is to reduce ion‐related recombination of charge carriers.

Optical and electrical studies on the TS defect in 4H-SiC

Abstract When annealing a 4H silicon carbide (SiC) crystal, a sequence of optically active defect centers
occurs among which the TS center is a prominent example. Here, we present low-temperature photoluminescence
analyses on the single defect level. They reveal that the three occurring spectral
signatures TS1, TS2 and TS3 originate from one single defect. Their polarization dependences expose
three different crystallographic orientations in the basal plane, which relate to the projections of
the nearest neighbor directions. Accordingly, we find a three-fold level-splitting in ensemble studies,
when applying mechanical strain. This dependency is quantitatively calibrated. A complementary
electrical measurement, deep level transient spectroscopy, reveals a charge transition level of the TS
defect at 0.6 eV above the valence band. For a future identification, this accurate characterization
of its optical and electronic properties along with their response to mechanical strain is a milestone.

Study on the electrical performance degradation mechanism of β -Ga2O3 p-n diode under heavy ion radiation

The impact of heavy ion irradiation on the β-Ga2O3 p-n diode and its physical mechanism have been studied in this Letter. After the irradiation fluence of 1 × 108 cm−2, it is observed that the electrical performance of the device is significantly degraded. The forward current density (JF) is reduced by 49.4%, the reverse current density (JR) is increased by more than two orders of magnitude, and the breakdown voltage (VBR) is decreased by 30%. Based on the results of the deep-level transient spectroscopy measurement, it is concluded that acceptor-like traps generated with an energy level of EC-0.75 eV in the β-Ga2O3 drift layer dominate the JF degradation of the device, which are most likely related to Ga vacancies. These acceptor-like traps result in the reduction of change carrier concentration, which in turn leads to a decrease in JF. In addition, according to the conductive atomic force microscope measurements and theoretical calculation, it is clearly observed that the latent tracks induced by heavy ion irradiation can act as leakage paths, leading to a significant degradation of JR and VBR.

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

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

A DLTS analysis of alpha particle irradiated commercial 4H-SiC Schottky barrier diodes

4H-SiC Schottky barrier diodes (SBDs) were exposed to 5.4 MeV alpha particles with fluences of 2.55× 1011 cm−2, 5.11 × 1011 cm−2 and 7.67 × 1011 cm−2, respectively. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) was used to determine the structure and cross-sectional elemental composition of the device, while current–voltage and capacitance–voltage profiling were used to determine the primary electrical device-characteristics before and after irradiation. EDS revealed the presence of a <1 μm Ti layer, covered by 5 μm Al layer, in intimate contact with the SiC. Deep level transient spectroscopy (DLTS), performed in the temperature range 15–310 K, revealed one dominant peak around 50 K (Ec - 0.07 eV) in the unirradiated samples. This peak showed asymmetry suggesting that it may consist of more than one defect. Notably, Z1/2, the carbon vacancy-related (Vc) defect commonly observed in as-grown n-type 4H-SiC, was not detected in the unirradiated reference sample. After irradiation, a broad peak emerged around 280 K (at 80 Hz), most likely Z1/2, having a shoulder around 180 K, was detected. Increasing the fluence resulted in a corresponding decrease in the concentration of the electron trap observed around 50 K (Ec - 0.07 eV), while the concentration increases for the defect detected around 280 K. Notably, the concentration of Z1/2 was found to be strongly fluence dependent and linked to what we believe is a related to a silicon vacancy transition, labelled S1/2 in literature. Laplace DLTS confirmed that the peak observed around 50 K is composed of multiple defects.

Analysis of ionic defect distribution on perovskite photodetector by temperature-dependent deep level transient spectroscopy

The High-efficiency Organohalide perovskites solar cells are produced via a solution-based method. Thus, a lot of defects are inevitable generated during fabrication processes, which are the main factors that affect the efficiency and long-term stability. The defect has been linked to the observation of reduce device stability against degradation and it can also create mid-gap states in a band structure as well act as recombination centers. Although the efficiency of the perovskite solar cells (PSCs) has been improved by curing defects using various passivation methods, deeply trapped defects in the perovskite layer have not been systematically studied, and their function is still unclear. In this study, we perform temperature-dependent deep-level transient spectroscopy (T-DLTS) measurements on PSCs, ionic defect parameters (activation energy, trap density, diffusion constants) for each observed ionic species were compared in different PSCs.