Deep Level Transient Spectroscopy Research Articles

Perspective on electrically active defects in β-Ga2O3 from deep-level transient spectroscopy and first-principles calculations

The ultra-wide bandgap of gallium oxide provides a rich plethora of electrically active defects. Understanding and controlling such defects is of crucial importance in mature device processing. Deep-level transient spectroscopy is one of the most sensitive techniques for measuring electrically active defects in semiconductors and, hence, a key technique for progress toward gallium oxide-based components, including Schottky barrier diodes and field-effect transistors. However, deep-level transient spectroscopy does not provide chemical or configurational information about the defect signature and must, therefore, be combined with other experimental techniques or theoretical modeling to gain a deeper understanding of the defect physics. Here, we discuss the current status regarding the identification of electrically active defects in beta-phase gallium oxide, as observed by deep-level transient spectroscopy and supported by first-principles defect calculations based on the density functional theory. We also discuss the coordinated use of the experiment and theory as a powerful approach for studying electrically active defects and highlight some of the interesting but challenging issues related to the characterization and control of defects in this fascinating material.

Electrically active defects induced by thermal oxidation and post-oxidation annealing of n-type 4H-SiC

In this work, we have performed a detailed study of the defects created in the bulk of 4H-SiC after thermal oxidation and post oxidation annealing using deep level transient spectroscopy and minority carrier transient spectroscopy (MCTS). The study reveals the formation of several shallow and deep level majority carrier traps in the bandgap. The ON1 (EC−0.85 eV), ON2a (EC−1.05 eV), and ON2b (Ec−1.17 eV) levels are the most dominant and are observed across all the samples (EC denotes the conduction band edge). Three shallow levels Ti(k) (EC−0.17 eV), E0.23 (EC−0.23 eV), and C1/2 (EC−0.36/0.39 eV) are observed in the samples. For most of the majority carrier defects, the highest concentration is observed after an NO anneal at 1300°C. This behavior is sustained in the depth profile measurements where the defect concentration after the NO anneal at 1300°C is significantly higher than for the rest of the samples. The origin of most of the majority carrier defects has been attributed to C interstitial injection from the interface during thermal oxidation and annealing. MCTS measurements reveal two prominent minority carrier traps, labeled O0.17 (EV+0.17 eV) and B (EV+0.28 eV), where the concentration of O0.17 is independent of annealing parameters while the concentration of the B level increases after the NO anneal (EV denotes the valence band edge). Furthermore, the depth profiles of the defects are used to evaluate their diffusion parameters by solving the diffusion equation to fit the experimental profiles. The defect concentrations decay exponentially with depth, which evidences that the defects were created at or near the SiO2–SiC interface and migrate toward the bulk during oxidation and post-oxidation annealing.

On-state electrical stress-induced degradation of NiO/β-Ga2O3 heterojunction pn diodes

Degradation and trap evolution in NiO/β-Ga2O3 heterojunction pn diodes under on-state electrical stress were investigated in this work using deep-level transient spectroscopy measurements and density functional theory (DFT) calculations. The decrease in turn-on voltage and forward current appears to correlate with an increase in the concentration of the compensating acceptor-like traps. From the energy level of EV + 1.3 eV, the corresponding acceptor-like traps can be attributed to the Ga vacancy complex with hydrogen (VGa-H). Interestingly, accompanied by the increase in VGa-H concentration, the self-trapped holes (STH) originally passivated by H may gradually recover. DFT calculations show a monotonous decrease in energy, suggesting the spontaneous diffusion of hydrogen from STH passivated sites to the adjacent VGa, which are generated under stress. This phenomenon leads to the recovery of STH and the generation of VGa-H. This investigation offers new insights into the degradation mechanisms of β-Ga2O3-based devices under electrical stress.

Role of defects in tailoring the structural, electrical and optical properties of Schottky diodes based on GaAsBi alloy through gamma radiation

This work provides an extensive study of the influence of γ-rays on structural, electrical, and optical characteristics of GaAsBi Schottky diodes fabricated through Molecular Beam Epitaxy (MBE) on GaAs substrates. An important finding is the direct correlation observed between the radiation dose and the consequent increase in the turn-on voltage of the devices. X-ray diffraction (XRD) showed that only phases related to the GaAs and GaAs1-xBix layers were observed. Raman spectroscopy proved to be a powerful tool to elucidate the effects of ionizing radiation on GaAsBi samples. From the results of electrical measurements, such as current density-voltage, the characteristic parameters of the devices improved as a function of the radiation dose. Conversely, the Capacitance-Voltage (C–V) characteristics indicated a rise in the concentration of free carriers in all irradiated samples, suggesting an enhancement in the performance of the diodes. Assessment through Deep Level Transient Spectroscopy (DLTS) further revealed a reduction in the number of electrically active traps following irradiation. Consequently, the findings presented here highlight the potential improvements in diode performance resulting from sample irradiation. Upon subjecting the samples to irradiation doses of 50 kGys and 100 kGys, we identified the presence of three and two electron traps, respectively. This differs from the unirradiated diodes, which displayed four electron traps. Interestingly, the photoluminescence intensity of the main peak exhibited an increment with increasing irradiation dose. This observation implies an enhancement in the optical characteristic, as well as annihilation/contribution of Bi-related traps. These findings corroborate the electrical results obtained in our study. Finally, the Raman results showed that the decrease in the lifetime of the photoexcited carriers leads to a higher recombination rate which in turn leads to a higher photoluminescence (PL) intensity after radiation.

Theoretical modeling of defect diffusion in wide bandgap semiconductors

Since the 1940s, it has been known that diffusion in crystalline solids occurs due to lattice defects. The diffusion of defects can have a great impact on the processing and heat treatment of materials as the microstructural changes caused by diffusion can influence the material qualities and properties. It is, therefore, vital to be able to control the diffusion. This implies that we need a deep understanding of the interactions between impurities, matrix atoms, and intrinsic defects. The role of density functional theory (DFT) calculations in solid-state diffusion studies has become considerable. The main parameters to obtain in defect diffusion studies with DFT are formation energies, binding energies, and migration barriers. In particular, the utilization of the nudged elastic band and the dimer methods has improved the accuracy of these parameters. In systematic diffusion studies, the combination of experimentally obtained results and theoretical predictions can reveal information about the atomic diffusion processes. The combination of the theoretical predictions and the experimental results gives a unique opportunity to compare parameters found from the different methods and gain knowledge about atomic migration. In this Perspective paper, we present case studies on defect diffusion in wide bandgap semiconductors. The case studies cover examples from the three diffusion models: free diffusion, trap-limited diffusion, and reaction diffusion. We focus on the role of DFT in these studies combined with results obtained with the experimental techniques secondary ion mass spectrometry and deep-level transient spectroscopy combined with diffusion simulations.

The effect of FeGa (0/–) level presence on material properties in dilute AlxGa1−xN layers

AlxGa1−xN epilayers are used as the basis of ultraviolet LEDs and detectors. The trap states produced by defects and impurities can play a key role in the device performance. In this work, conventional deep-level transient spectroscopy, photoluminescence (PL), and secondary ion mass spectrometry have been used to characterize a deep-level trap termed as E3 in dilute AlxGa1−xN (x < 0.063) epilayers grown by metal-organic vapor phase epitaxy (MOVPE) on highly conductive ammono-GaN substrates. The AlxGa1−xN epilayers were doped with silicon to about 3 × 1016 cm−3. The electrical and the optical measurements were conducted on Ni/Au Schottky barrier diodes and virgin samples, respectively. First, we observed a general trend that the E3 (FeGa) electron trap concentration significantly changes along the wafers in AlxGa1−xN layers that is fully consistent with previously reported results for GaN materials grown by the MOVPE technique. Second, we report that the activation energies for electron emission for the E1 and E3 traps in dilute AlxGa1−xN exhibit linear variations with Al content. Moreover, low-temperature PL results show a proportional relation between the intensity of the line with its maximum at 1.299 eV and concentration of residual Fe impurity. Finally, we discuss how the presence of defects resulting from Fe contamination may result in degradation of AlxGa1−xN-based devices.

Investigating the impact of physical vapour deposition of palladium on electron-irradiated silicon substrates

This study reports on the effect of resistive physical vapour deposition of Pd Schottky contacts on the defects observed in an irradiated n-type Si substrate. Deep-level transient spectroscopy (DLTS) was used in order to investigate the possible defects in Schottky diodes on the n-type Si. In this study the defects present where the diode was deposited first and then irradiated (“post irradiated”) were compared to those present where the substrate was irradiated first, and the Pd deposited after irradiation (“pre-irradiated”). In the post-irradiated material, the familiar radiation-induced defects were observed. However, in the pre-irradiated material, fourteen new defects were observed, with DLTS signatures differing from those of the defects in the post-irradiated diodes. The newly identified defects seemed to be specifically caused by the deposition of Pd contacts after irradiation of the substrate, as these defects were not observed when other metals such as Au, Ni, Al and Ag were deposited after irradiation. In this paper, we will refer to the observed effect that Pd deposition replaced the familiar radiation-induced defects in Si with the new set of defects as the “Pd effect”. Careful experiments ruled out annealing due to inadvertent heating of the sample during deposition as a possible cause. Care was taken to avoid all sources of contamination: The effect was observed for different sources of Pd and crucibles. This effect was inhibited by the presence of a thin intermediate layer of Pd or Au deposited before irradiation. We therefore conclude that the effect is only observed when Pd is deposited directly onto the irradiated Si surface. Out of the fourteen newly observed defects, four defects, with activation energy of 182, 220, 360 and 607 meV, had DLTS signatures corresponding to those of defects previously observed in Pd-containing Si. For the rest of the defects, no defects with similar DLTS signatures were found in literature. We therefore believe that the defects observed are produced by defect enhanced diffusion of Pd. Overall, the study enhances our understanding of defect behaviour in silicon-based devices, particularly under irradiation and metal deposition conditions, and reveals the unique properties and effects of Pd.

Reassessing iron–gallium recombination activity in silicon

In this work, we present a comprehensive re-evaluation of the iron–gallium (FeGa) recombination parameters in silicon using injection-dependent lifetime spectroscopy (IDLS). Ga-doped silicon wafers (of varying resistivities) with precise concentrations of intentional iron contamination in the silicon wafer bulk, through ion implantation and distribution, were used. The presence of interstitial Fei and FeGa, and their lifetime-limiting effects in these silicon wafers, were confirmed through measuring the effective minority carrier lifetime changes during the conditions that are known to cause FeGa dissociation and association. The presence of Fe was also confirmed by deep-level transient spectroscopy. To ensure accurate IDLS analysis of the FeGa defect in silicon, a lifetime linearization scheme was employed to effectively filter out interference by other defects. Error analysis was employed to find the combination of defect parameters that best fit the experimental data and to ascertain the range of uncertainty associated with the IDLS best-fit results. The optimal fitting of the experimental IDLS by Shockley–Read–Hall statistics produced an electron capture cross section σn=2.3×10−14cm2, hole capture cross section σp=1.1×10−14cm2, and a trap energy level Et=EV+0.2−0.01+0.02eV for the FeGa defect in silicon. The extracted defect parameters are also verified by experimentally measuring the crossover point of Fei and FeGa lifetime curves.

Generation of deep levels near the 4H-SiC surface by thermal oxidation

Abstract Deep levels near the surface of 4H-SiC after dry oxidation were investigated. A large and broad peak appeared in the low-temperature range of deep level transient spectroscopy (DLTS) spectra after oxidation of SiC at 1300 °C, indicating multiple deep levels energetically located near the conduction band edge are generated inside SiC by thermal oxidation. Analyses of the DLTS spectra acquired with changing the bias voltage revealed that the majority of deep levels is located very near the SiC surface, within about 6 nm deep region from the surface. The area density of the observed deep levels is higher than 3 × 1012 cm−2.

Synergistic effect of electrical bias and proton irradiation on the electrical performance of β-Ga2O3 p–n diode

The synergistic impact of reverse bias stress and 3 MeV proton irradiation on the β-Ga2O3 p–n diode has been studied from the perspective of the defect in this work. The forward current density (JF) is significantly decreased with the increase in proton irradiation fluence. According to the deep-level transient spectroscopy results, the increase in the acceptor-like trap with an energy level of EC-0.75 eV within the β-Ga2O3 drift layer, which is most likely to be Ga vacancy-related defects, can be the key origin of the device degradation. The increase in these acceptor-like traps results in the carrier concentration reduction, which in turn leads to a decrease in JF. Furthermore, compared with the case of proton irradiation with no bias, the introduction of −100 V electrical stress induced a nearly double decrease in JF. Based on the capacitance–voltage (C–V) measurement, with the support of the electric field, the carrier removal rate increased from 335 to 600 cm−1. Similar to the above-mentioned phenomenon, the trap concentration also increased significantly. We propose a hypothesis elucidating the synergistic effect of electrical stress and proton irradiation through the behavior of recoil nuclei under the electric field.

Improvement in stability and exploring the photovoltaic properties of CsPbI2Br thin films for perovskite solar cells

The phase stability and quality of CsPbI2Br perovskite film directly determines the performance of perovskite solar cells (PSCs). However, there is a lack of research on phase stability of perovskite films of PSCs. Here, by simply controlling precursors with specified ratios and solvents, the phase stability, charge transfer resistance and electron lifetime improve significantly which is useful for high performance in perovskite solar cells. Besides this, the information about trap centres by a novel technique Charge Deep Level Transient Spectroscopy (Q-DLTS) also helps to improve efficiency of perovskite solar cells. In this work, CsPbI2Br perovskite thin films have been synthesized using simple solution processing method, by varying CsBr and PbI2 precursors with specified ratios of 1.05 M:1 M and 1.10 M:1 M in Dimethysulfoxide (DMSO) and Dimethyl formamide (DMF). The CsPbI2Br film with ratio of 1.05:1 has more phase stability in ambient air at room temperature than that of 1.10:1. Drop casting method was adopted for thin films preparation and structurally cubic phase of perovskite films obtained by X-ray diffraction (XRD). Scanning electron microscope (SEM) showed more homogeneity on the surface of the sample with ratio 1.05:1 and energy dispersive x-ray spectroscopy (EDX) confirmed the presence of same amounts in chemical composition. Fourier transform infrared spectroscopy (FTIR) transmittance spectra showed that main peak is generally observed because of stretching vibrations of lead-iodide ions. In UV–visible (UV-Vis) little effect was observed on the energy band gaps i.e. 1.93 eV for 1.05 M:1 M and 1.96 eV for 1.10 M:1 M. The electrochemical impedance spectroscopy (EIS) measurements were performed for electronic charges accumulation on the surface of the films which showed semi-circles Nyquist plots and frequency dependent Bode plots were used to find electron lifetime. The charge transfer resistance (Rct) decreased from 52Ω to 45Ω and electron life time were approximately equal to 0.10 s and 0.09 s for the ratio of 1.10:1 and 1.05:1. I-V measurements showed an Ohmic contact formation providing insights into defect concentrations with a linear response to applied bias. Recombination lifetimes were investigated using transient photovoltage (TPV) at various biases, no correlation between the applied bias and recombination lifetime was found. Q-DLTS indicated presence of 3 trap centers for the charge carriers which were actively responsible for degradation of the perovskite material, these traps serve as recombination centres that reduce the overall power conversion efficiency (PCE) of solar cells. Photoluminescence spectroscopy (PL) analysis was done to verify the energy bandgap value which was found similar in comparison to UV–VIS.

Comparing the effect between room temperature and low temperature heavy ion irradiation by deep level transient spectroscopy

To promote the application of SiC devices in extreme environments, the irradiation damages of 4H-SiC Schottky barrier diodes (SBDs) irradiated with 6 MeV Au ions under room temperature (RT) and low temperature (LT) were investigated in this work. Deep level transient spectroscopy (DLTS) was used to analyze the changes in primary defects. DLTS measurements indicated the presence of six deep levels labeled EN, E1, E2, E3, E4 and E5. Notably, the peak positions for the samples irradiated under RT shifted to the right compared to those irradiated at LT with increasing fluence. This means that the deeper energy defects are produced under RT irradiation. The experimental results also demonstrated that the HI irradiation-induced defects are mainly located at interface between metal and SiC, especially the metal–semiconductor (M−S) interface edge. Therefore, these results suggest that the evolution of defects in SiC devices under LT and RT irradiation,which is helpful to provide its application in power electronics and extreme environments.

Recovery at room temperature annealing on 4H–SiC SBDs by gamma irradiation

The impact of gamma irradiation and subsequent recovery at room temperature on the device performance of commercial 4H–SiC Schottky barrier diodes (SBDs) was investigated through the analysis of the electrical properties and the deep level transient spectroscopy (DLTS). The ideality factor (n) of the SBDs increased from 1.01 to 1.13 with an increasing irradiation dose, but recovered after 7 days of room temperature annealing. To determine Schottky barrier heights (ΦB), both current-voltage (I–V) and capacitance-voltage (C–V) measurements were performed. The results of the combined I–V and C–V analysis showed that a little variations in ΦB were consistent with the variations in irradiation dose, except for the ΦB calculated by C–V measurement at 30 kGy. Similarly, the ΦB recovered after 7 days at room temperature. DLTS analysis revealed the presence of Z1/Z2 traps in the samples after 7 days at room temperature annealing. These traps exhibited activation energies ranging from 0.46 eV to 0.55 eV. The trap concentration (NT) increased from 9.48 × 1012 cm−3 to 2.23 × 1013 cm−3 with the increasing irradiation dose. These findings indicate that gamma irradiation-induced point defects were the primary cause of the degradation of 4H–SiC SBDs.

Impact of carbon injection in 4H-SiC on defect formation and minority carrier lifetime

Point defects in silicon carbide (SiC) can act as charge carrier traps and have a pronounced impact on material properties such as the mobility and carrier lifetime. Prominent among these traps is the carbon vacancy (VC) with a demonstrated detrimental effect on the minority carrier lifetime in 4H-SiC epitaxial layers. Hence, a variety of methods for VC removal have been proposed. Common to all is that they involve some sort of C-injection into the epi-layer that leads to annihilation of VC with C interstitials (Ci) via the reaction Ci＋ VC→0̸. However, many studies on injection of Ci for removal of VC do not take into account the potential effect of any additional defects that are formed as a result of excess C in the material, including electrically active defects that introduce energy levels in the SiC band gap. Herein, we study the formation and impact of carbon related defects that are introduced in 4H-SiC epi-layers by injection of excess carbon. C-injection is achieved by annealing 4H-SiC epi-layers covered by a graphitized photoresist known as a C-cap at 1250°C for different durations. The resulting appearance of defects in the samples is monitored using deep level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS) measurements, which reveal the formation of both minority and majority carrier traps as a result of the C-injection. Intriguingly, the injected carbon is also found to interact with a perennially present impurity in 4H-SiC epi-layers and a potentially lifetime limiting defect, namely boron. Furthermore, we monitor the minority carrier lifetime as a function of C-injection time and depth from the surface using cross-sectional time-resolved photoluminescence (TRPL) measurements. Both the defect distributions and the minority carrier lifetime are found to depend strongly on the duration of the C-cap anneal, with a marked depth dependence being present for all samples studied. The moderate temperature C-cap annealing treatment is proposed as a method for enhancing the carrier lifetime in n-type 4H-SiC epi-layers for power device applications.

Emission and capture characteristics of deep hole trap in n-GaN by optical deep level transient spectroscopy

Emission and capture characteristics of a deep hole trap (H1) in n-GaN Schottky barrier diodes (SBDs) have been investigated by optical deep level transient spectroscopy (ODLTS). Activation energy (E emi) and capture cross-section (σ p) of H1 are determined to be 0.75 eV and 4.67 × 10−15 cm2, respectively. Distribution of apparent trap concentration in space charge region is demonstrated. Temperature-enhanced emission process is revealed by decrease of emission time constant. Electric-field-boosted trap emission kinetics are analyzed by the Poole−Frenkel emission (PFE) model. In addition, H1 shows point defect capture properties and temperature-enhanced capture kinetics. Taking both hole capture and emission processes into account during laser beam incidence, H1 features a trap concentration of 2.67 × 1015 cm−3. The method and obtained results may facilitate understanding of minority carrier trap properties in wide bandgap semiconductor material and can be applied for device reliability assessment.

Constant Photocurrent Method to Probe the Sub‐Bandgap Absorption in Wide Bandgap Semiconductor Films: The Case of α‐Ga2O3

The optical absorption coefficient is one of the fundamental properties of semiconductors and is critical to the development of optical devices. Herein, a revival of the constant photocurrent method is presented to measure sub‐bandgap absorption in wide bandgap semiconductor films. The method involves maintaining a constant photocurrent by continually adjusting the impinging photon flux across the energy spectrum. Under such conditions, the reciprocal of the photon flux for uniformly absorbed light is proportional to the absorption coefficient. This method is applied to α‐Ga2O3 and reveals that it can access the absorption coefficient from 1 × 105 cm−1 at the band edge (5.3 eV) to 0.8 cm−1 close to mid‐bandgap (2.7 eV). Changes in the steepness of the absorption curve in the sub‐bandgap region are in excellent agreement with defect states of α‐Ga2O3 reported by deep level transient spectroscopy, indicating that the technique shows promise as a probe of energetically distributed defect states in thin film wide bandgap semiconductors.

Deep-Level Transient Spectroscopy Studies on Four Different Zinc Oxide Morphologies

In this work, we described the variations in the defect energy levels of four different ZnO morphologies, namely nanoribbons, nanorods, nanoparticles, and nanoshuttles. All the ZnO morphologies were grown on a seeded 4% Boron-doped p-type silicon (p-Si) wafer by using two different synthesis techniques, which are chemical bath deposition and microwave-assisted methods. The defect energy levels were analyzed by using the Deep-Level Transient Spectroscopy (DLTS) characterization method. The DLTS measurements were performed in the 123 K to 423 K temperature range. From the DLTS spectra, we found the presence of different trap-related defects in the synthesized ZnO nanostructures. We labeled all the traps related to the four different ZnO nanostructures as P1, P2, P3, P4, and P5. We discussed the presence of defects by measuring the activation energy (Ea) and capture cross-section (α). The lowest number of defect energy levels was exhibited by the ZnO nanorods at 0.27 eV, 0.18 eV, and 0.75 eV. Both the ZnO nanoribbons and nanoparticles show four traps, which have energies of 0.31 eV, 0.23 eV, 0.87 eV, and 0.44 eV and 0.27 eV, 0.22 eV, 0.88 eV, and 0.51 eV, respectively. From the DLTS spectrum of the nanoshuttles, we observe five traps with different activation energies of 0.13 eV, 0.28 eV, 0.25 eV, 0.94 eV, and 0.50 eV. The DLTS analysis revealed that the origin of the nanostructure defect energy levels can be attributed to Zinc vacancies (Vzn), Oxygen vacancies (Vo), Zinc interstitials (Zni), Oxygen interstitials (Oi), and Zinc antisites (Zno). Based on our analysis, the ZnO nanorods showed the lowest number of defect energy levels compared to the other ZnO morphologies.

Relaxation kinetics of interface states and bulk traps in atomic layer deposited ZrO2/β-Ga2O3 metal-oxide-semiconductor capacitors

The study of interface states and bulk traps and their connection to device instability is highly demanded to achieve reliable β-Ga2O3 metal-oxide-semiconductor (MOS) devices. However, a comprehensive analysis of the capture/emission behavior of interface states and bulk traps can be challenging due to widespread time constant distribution. In this study, using capacitance transient measurement tools, trap states of the ZrO2/β-Ga2O3 MOS gate stack were explicitly investigated, particularly its bias- and temperature-dependent relaxation kinetics. As forward bias is enlarged, it is observed that the interface state density (Dit) increases by 12.6%. Two bulk traps with discrete levels identified as 0.43 eV (E1) and 0.74 eV (E2) below the conduction band minimum were extracted by deep-level transient spectroscopy. It is further revealed that the emission processes of E1 and E2 are thermally enhanced, while the capture processes remain insensitive to temperature. The electric-field dependence of E1 indicates that the dominant mechanism follows the rule of Poole–Frenkel emission. The capacitance–voltage (C–V) hysteresis deteriorated at a higher forward bias due to the higher trap density and increased population of trapped charges. These findings provide an important framework for future device optimization to improve the reliability and performance of β-Ga2O3 MOS devices.

Thermal behavior of irradiation-induced-deep level in bulk GaN used for fabricating blue light emitting diodes

Bulk GaN was irradiated by 2 MeV electron beam at a fluence of 5 × 1016 cm2 and studied by deep level transient spectroscopy (DLTS). After irradiation, a broad peak, including at least two traps, was detected. The trap D1 (EC – 0.16 eV) observed from a broad peak is induced during the annealing process below 550K and completely annealed out at 550K after ~ 10 hours. The annealing process at 550K also forms a new trap D2 (EC – 0.25 eV). From the isothermal study the activation energy of the trap D2 in the annihilation process is obtained and has a value of 1.3 ± 0.3 eV. The pre-factor of the annihilation process suggested this process to be related to the free-carrier capture by multi-phonon emission. From the thermal behavior, the trap D2 was suggested to be related to gallium vacancy.

Review on ultrahigh growth rate GaAs solar cells by metalorganic vapor-phase epitaxy

The aim of this review paper is to summarize a decade of research focused on enhancing metalorganic vapor-phase epitaxy (MOVPE) growth rates of GaAs, driven by the imperative for most cost-effective and energy-efficient III–V compounds’ production. While MOVPE is renowned for producing high-quality devices, it has been constrained by production cost. For example, MOVPE was traditionally thought to have moderate growth rates that limit the throughput of the cost-intensive reactors. Recent research endeavors, however, have demonstrated ultrafast growth rates, exceeding 280 μm/h, with a remarkable group III precursor utilization efficiency of over 50%. It is worth noting that even with increased growth rates, the surface quality remains unaffected in terms of roughness and morphology. Nonetheless, optoelectronic properties, such as minority carrier lifetime, deteriorate for both p- and n-doped materials under constant growth conditions. This is attributed to an increase in the defect density of arsenic antisites, particularly EL2 and HM1 defects, as revealed by deep-level transient spectroscopy investigations. Some of these losses can be mitigated by optimizing growth conditions, such as elevating the temperature and reducing the V/III ratio. The latter not only restores some of the material quality but also increases the growth rate and reduces precursor consumption. Still, fully recovering the original reference lifetimes remains a challenge. Solar cell results indicate that structures with predominantly n-type absorbers are less affected by reduced minority carrier lifetimes. A remarkable 24.5% efficiency was achieved in a GaAs single-junction solar cell grown at 120 μm/h, representing less than 1 min of growth time for the absorber layers.