Deep Acceptor Level Research Articles

Comprehensive characterization of nitrogen-related defect states in β-Ga2O3 using quantitative optical and thermal defect spectroscopy methods

This study provides a comprehensive analysis of the dominant deep acceptor level in nitrogen-doped beta-phase gallium oxide (β-Ga2O3), elucidating and reconciling the hole emission features observed in deep-level optical spectroscopy (DLOS). The unique behavior of this defect, coupled with its small optical cross section, complicates trap concentration analysis using DLOS, which is essential for defect characterization in β-Ga2O3. A complex feature arises in DLOS results due to simultaneous electron emission to the conduction band and hole emission to the valence band from the same defect state, indicating the formation of two distinct atomic configurations and suggesting metastable defect characteristics. This study discusses the implications of this behavior on DLOS analysis and employs advanced spectroscopy techniques such as double-beam DLOS and optical isothermal measurements to address these complications. The double-beam DLOS method reveals a distinct hole emission process at EV+1.3 eV previously obscured in conventional DLOS. Optical isothermal measurements further characterize this energy level, appearing only in N-doped β-Ga2O3. This enables an estimate of the β-Ga2O3 hole effective mass by analyzing temperature-dependent carrier emission rates. This work highlights the impact of partial trap-filling behavior on DLOS analysis and identifies the presence of hole trapping and emission in β-Ga2O3. Although N-doping is ideal for creating semi-insulating material through the efficient compensation of free electrons, this study also reveals a significant hole emission and migration process within the weak electric fields of the Schottky diode depletion region.

Investigations on the Recovery of the Electrical Properties of Smart Cut™-Transferred SiC Thin Film Using SiC-on-Insulator Structures

SiC-on-Insulator (SiCOI) structures fabricated using the Smart Cut™ technique can be of great interest in order to probe the properties of a silicon carbide (SiC) transferred layer, by electrically insulating it from the receiver substrate. In this study, we report the fabrication of such a SiCOI structure using a SiC receiver, as well as its electrical and TEM characterization after high temperature annealing. We highlight a decrease of the transferred layer electrical resistivity with increasing annealing temperature, due to doping reactivation and electron mobility enhancement. After low temperature annealing (1200°C to 1400°C), deep acceptor levels, possibly located in a damaged region near the substrate’s surface, might be responsible of a non negligible electrical compensation. Beyond 1400°C however, the transferred SiC crystal is healed and electron transport is only subjected to shallow nitrogen ionization.

Investigation of carbon-related complexes in highly C-doped GaN grown by metalorganic vapor phase epitaxy

We investigated the C-related complexes in highly C-doped GaN by electron spin resonance (ESR) spectroscopy, Fourier transform IR spectroscopy (FTIR), and minority carrier transient spectroscopy (MCTS) measurements. In the ESR spectra, two resonances with g values of 2.02 and 2.04 were found to be assigned by (0/−) deep acceptor and (+/0) charge transition levels of carbon substituting for nitrogen site (CN). In the FTIR spectra, two local vibrational modes positioned at 1679 and 1718 cm−1 were confirmed to be associated with tri-carbon complexes of CN–CGa–CN (basal) and CN–CGa–CN (axial), respectively. In the MCTS spectra, we observed the hole trap level of E v + 0.25 ± 0.1 eV associated with the tri-carbon complexes, which are the dominant C-related defects, suggesting that these complexes affect the electronic properties in the highly C-doped GaN.

Effect of Fe Doping Profile on Current Collapse in GaN-based RF HEMTs.

Using computer-aided design (TCAD) simulation, the impact of the Fe doping profile, including concentration, decay rate, and depth of the doping region on current-collapse magnitude (▵CC) in 0.5-μm gated GaN-based high electron mobility transistors (HEMTs) is systematically investigated. Accurate simulation models are established and developed to facilitate the fabrication of electronics. It is elucidated that the intricate interplay between trapping and de-trapping of Fe-related traps at the gate-drain edge is responsible for current collapse. The concentration and decay rate of the doping region have a more significant impact on current collapse than the depth. Increased trap state density near two-dimensional electron gas (2DEG) channel caused by deep-level acceptors would boost ▵CC. However, a minor dynamic reduction in 2DEG density (nT) induces a relatively small ▵CC. By adjusting the concentration, decay rate, and depth of the doping region, ▵CC of GaN-based Radio Frequency (RF) HEMTs can be reduced by approximately 50.3 %. The optimized distribution of Fe doping discussed in this work helps to prepare GaN-based RF HEMTs with a limited current collapse effect.

Native point defects in HgCdTe infrared detector material: Identifying deep centers from first principles

We investigate the native point defects in the long-wavelength infrared (LWIR) detector material Hg0.75Cd0.25Te using a dielectric-dependent hybrid density functional combined with spin–orbit coupling. Characterizing these point defects is essential as they are responsible for intrinsic doping and nonradiative recombination centers in the detector material. The dielectric-dependent hybrid functional allows for an accurate description of the bandgap (Eg) for Hg1−xCdxTe (MCT) over the entire compositional range, a level of accuracy challenging with standard hybrid functionals. Our comprehensive examination of the native point defects confirms that cation vacancies VHg(Cd) are the primary sources of p-type conductivity in the LWIR material given their low defect formation energies and the presence of a shallow acceptor level (−/0) near the valence-band maximum. In addition to the shallow acceptor level, the cation vacancies exhibit a deep charge transition level (2−/−) situated near the midgap, characteristic of nonradiative recombination centers. Our results indicate that Hg interstitial could also be a deep center in the LWIR MCT through a metastable configuration under the Hg-rich growth conditions. While an isolated Te antisite does not show deep levels, the formation of VHg–TeHg defect complex introduces a deep acceptor level within the bandgap.

Realization of High‐Resistive Ni‐Doped GaN Crystal by Hydride Vapor‐Phase Epitaxy

Herein, high‐resistivity GaN is studied for use as an epitaxial substrate in lateral power devices. Fe‐, C‐, Mn‐, and Zn‐doped GaN monocrystals have high resistivity at a doping concentration of ≈1 × 1018 cm−3. However, a low doping concentration is preferred for growing GaN monocrystals; therefore, other dopants for GaN that yield high resistivity at a doping concentration less than 1 × 1018 cm−3 must be identified. Herein, NiCl2 is used as a precursor to grow Ni‐doped GaN monocrystals on GaN substrates via hydride vapor‐phase epitaxy. Two Ni‐doped GaN substrates with Ni concentrations corresponding to 2.7 × 1017 and 2.9 × 1018 cm−3 are obtained by varying the partial pressure of NiCl2. The resistivity of Ni‐doped GaN monocrystals is measured as a function of temperature using Hall effect measurements. The GaN monocrystals doped with 2.7 × 1017 cm−3 of Ni have a higher resistivity than those doped with 2.9 × 1018 cm−3 of Ni at 600–900 K. Charge‐neutrality calculations have shown that the depth of the Ni acceptor level in GaN is 1.4–1.5 eV, indicating that Ni‐doped GaN monocrystals have high resistivity owing to the deep acceptor level of Ni.

Electron-Phonon Coupling-Mediated Ultralong Carrier Lifetime in an All-Inorganic Two-Dimensional Cs2PbI2Cl2 Perovskite: Explanation for the High Antisite Defect Tolerance.

Two-dimensional (2D) halide perovskite are appealing candidates for applications in optoelectronics and photovoltaics, but their energy conversion efficiency is severely limited by nonradiative electron-hole recombination. In most investigations, point defects with deep defect levels and deep charge-state transition levels in the band gap are treated as the carrier recombination centers. For the all-inorganic 2D Css 2PbI2Cl2, the IPb antisite defect is the most likely to form and cause nonradiative electron-hole recombination. By using density functional theory and ab initio nonradiative molecular dynamics calculations, we found that the IPb defect can introduce the deep acceptor and donor levels into the band gap. Because electron-phonon coupling gives rise to weak nonadiabatic coupling and rapid loss of electronic coherence, those levels lead to a reduction of the carrier loss and the prolongation of the excited-state carrier lifetime, thereby enhancing the photoelectric and defect tolerance properties of the Cs2PbI2Cl2 material. These results could deepen the understanding of the chemistry of defects and carrier dynamics in perovskite materials.

Boosting the open-circuit voltage of Cu2ZnSn(S,Se)4 solar cell by Cd/NH3 soaking

In this paper, the open-circuit voltages (V OC) of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells were improved by soaking the CZTSSe films in Cd/NH3 solutions before the deposition of buffer layers. The effects of Cd/NH3 soaking on the properties of CZTSSe films and solar cells were investigated in depth. We found Cd/NH3 soaking can promote downward band bending of the CZTSSe layer at the surface region. In addition, the Cd/NH3 soaking can eliminate the deep level acceptor in CZTSSe and decrease the density of the defects at the absorber/buffer interface. These effects can significantly reduce the carrier recombination in the depletion region of CZTSSe solar cells. As a result, the V OC of CZTSSe solar cells were increased from ∼460 mV to ∼480 mV, the efficiency of the best CZTSSe solar cell was increased from 10.2% to 11.6% by the Cd/NH3 soaking process.

Probing Boron Vacancy Complexes in h-BN Semi-Bulk Crystals Synthesized by Hydride Vapor Phase Epitaxy

Hexagonal BN (h-BN) has emerged as an important ultrawide bandgap (UWBG) semiconductor (Eg~6 eV). The crystal growth technologies for producing semi-bulk crystals/epilayers in large wafer sizes and understanding of defect properties lag decades behind conventional III-nitride wide bandgap (WBG) semiconductors. Here we report probing of boron vacancy (VB)-related defects in freestanding h-BN semi-bulk wafers synthesized by hydride vapor phase epitaxy (HVPE). A photocurrent excitation spectroscopy (PES) was designed to monitor the transport of photoexcited holes from deep-level acceptors. A dominant transition line at 1.66 eV with a side band near 1.62 eV has been directly observed, which matches well with the calculated energy levels of 1.65 for the VB-H deep acceptor in h-BN. The identification of VB complexes via PES measurement was further corroborated by the temperature-dependent dark resistivity and secondary ion mass spectrometry measurements. The results presented here suggested that it is necessary to focus on the optimization of V/III ratio during HVPE growth to minimize the generation of VB-related defects and to improve the overall material quality of h-BN semi-bulk crystals. The work also provided a better understanding of how VB complexes behave and affect the electronic and optical properties of h-BN.

Effect of induced defects on conduction mechanisms of noble-gas-implanted ScN thin films

Noble-gas implantation was used to introduce defects in n-type degenerate ScN thin films to tailor their transport properties. The electrical resistivity increased significantly with the damage levels created, while the electron mobility decreased regardless of the nature of the ion implanted and their doses. However, the transport property characterizations showed that two types of defects were formed during implantation, named point-like and complex-like defects depending on their temperature stability. The point-like defects changed the electrical conduction mode from metallic-like to semiconducting behavior. In the low temperature range, where both groups of defects were present, the dominant operative conduction mechanism was the variable range hopping conduction mode. Beyond a temperature of about 400 K, the point-like defects started to recover with an activation energy of 90 meV resulting in a decrease in resistivity, independent of the incident ion. The complex-like defects were, therefore, the only remaining group of defects after annealing above 700 K. These latter, thermally stable at least up to 750 K, introduced deep acceptor levels in the bandgap resulting in an increase in the electrical resistivity with higher carrier scattering while keeping the metallic-like behavior of the sample. The generation of both types of defects, as determined by resistivity measurements, appeared to occur through a similar mechanism within a single collision cascade.

A Method to Simulate Extrinsic Light Excitation of Vanadium-Compensated 6H-SiC

Extrinsic light excitation has much lower absorption coefficient compared to intrinsic light excitation, which can better utilize the “bulk” of semiconductor rather than a thin surface as the depth of light absorption is much larger, making it suitable for higher power applications. However, commercial technology computer aided design (TCAD) software has not developed a model for extrinsic light excitation. Therefore, we construct a model of Vanadium-compensated semi-insulating (VCSI) 6H-SiC photoconductive semiconductor switch (PCSS) illuminated with sub-bandgap light, and realize the process of light absorption at V deep acceptor level in Silvaco TCAD simulation by modifying the electron emission rate. Then, we simulate the transient response of 6H-SiC triggered by a nanosecond light pulse and discuss the feasibility of this method.

Effect of Carrier Gas on Spray Pyrolysis Deposited SnO2 Thin Films

Nanostructured SnO2 thin films are synthesized by the chemical Spray pyrolysis method in presence of air and nitrogen as carrier gas. The study has emphasized on the impact of carrier gas on the underlying, electrical, and optical properties of the deposited thin films. XRD studies uncover that there is no variation in the crystalline structure of the film. The SnO2 thin films prepared using nitrogen as carrier gas is found more conductive as compared to the film prepared using air as a carrier gas. The change in conductivity is due to the increased oxygen deficiency in the nitrogen atmosphere. The HBM and EMA models are used to calculate particle radius by optical absorption data, showing the particle diameter is low for nitrogen as compared to air as carrier gas. The activation energy is also found to decrease in presence of nitrogen as carrier gas. This may be due to shallow donor and deep acceptor levels of semiconductors. The decrease in particle size increases the number of surface atoms, responsible for a decrease in activation energy of films.

Wide-Bandgap Perovskite Solar Cell Using a Fluoride-Assisted Surface Gradient Passivation Strategy.

Wide-bandgap (1.68 eV) perovskite solar cells (PSCs) are important components of perovskite/Si tandem devices. However, the efficiency of wide bandgap PSCs has been limited by their huge open-circuit voltage (VOC) deficit due to non-radiative recombination. Deep-level acceptor defects are identified as the major killers of VOC, and they can be effectively improved by passivation with ammonium salts. Theoretical calculation predicts that increasing the distance between F and -NH3+ of fluorinated ammonium can dramatically enhance the electropositivity of -NH3+ terminals, thus providing strong adsorption onto the negatively charged IA and IPb anti-site defects. Characterizations further confirm that surface gradient passivation employing p-FPEAI demonstrates the most efficient passivation effect. Consequently, a record-efficiency of 21.63% with the smallest VOC deficit of 441 mV is achieved for 1.68 eV-bandgap inverted PSCs. Additionally, a flexible PSC and 1 cm2 opaque device also deliver the highest PCEs of 21.02% and 19.31%, respectively.

Характеризация широкозонных слоев в лазерных структурах на основе CdHgTe

The results of a study of the optical and structural properties of wide-bandgap (x~0.7) layers in laser heterostructures based on CdxHg1-xTe solid solutions grown by molecular beam epitaxy on (013)GaAs substrates, as well as epitaxial films similar to these layers in chemical composition, are presented. It is shown that the position of the maximum of the photoluminescence spectrum and the nature of its temperature shift are related to the disordering of the composition of the solid solution. Shallow and deep acceptor levels were found in the bandgap. The possible influence of disordering and acceptor levels in laser structures on the energy spectrum of carriers is discussed.

Characterization of wide-bandgap layers in laser structures based on CdHgTe

The results of a study of the optical and structural properties of wide-bandgap (x~0.7) layers in laser heterostructures based on Cd x Hg 1-x Te solid solutions grown by molecular beam epitaxy on (013)GaAs substrates, as well as epitaxial films similar to these layers in chemical composition, are presented. It is shown that the position of the maximum of the photoluminescence spectrum and the nature of its temperature shift are related to the disordering of the composition of the solid solution. Shallow and deep acceptor levels were found in the bandgap. The possible influence of disordering and acceptor levels in laser structures on the energy spectrum of carriers is discussed. Keywords: CdHgTe, laser structures, photoluminescence, defects, structural properties.

Transition-metal ions in β-Ga2O3 crystals: Identification of Ni acceptors

Transition-metal ions (Ni, Cu, and Zn) in β-Ga2O3 crystals form deep acceptor levels in the lower half of the bandgap. In the present study, we characterize the Ni acceptors in a Czochralski-grown crystal and find that their (0/−) level is approximately 1.40 eV above the maximum of the valence band. Both Ni2+ (3d8) and Ni3+ (3d7) acceptors are present in the as-grown crystal. Also present are unintentional Ir3+ (5d6) and Ir4+ (5d5) donors. The neutral Ni3+ acceptors have a low-spin S = 1/2 ground state and are easily monitored with electron paramagnetic resonance (EPR). Principal values of the g matrix for these acceptors are 2.131, 2.138, and 2.233. Although paramagnetic, the singly ionized Ni2+ acceptors are not seen with EPR at X band (9.4 GHz). The Ir4+ donors are monitored with EPR and with infrared absorption spectroscopy. Exposing the Ni-doped β-Ga2O3 crystal to 275 nm light at room temperature increases the concentration of Ni3+ ions and reduces the concentration of Ir4+ ions as electrons move from the acceptors to the donors. After illumination, heating the crystal above 375 °C restores the initial concentrations of the Ni3+ and Ir4+ ions. Broad optical absorption bands peaking near 303 and 442 nm are attributed to the Ni3+ acceptors.

Amorphous Ga2O3 Schottky photodiodes with high-responsivity and photo-to-dark current ratio

Schottky photodiodes with sputtered amorphous Ga2O3 (a-Ga2O3) and asymmetric electrodes were fabricated for the first time and achieved excellent performances under 254-nm light illumination with the superhigh responsivity of 1021.8 A W−1, high photo-to-dark current ratio of 2.3 × 106, fast rise/decay response time of 144/208 ms, high detectivity of 1.66 × 1016 Jones, and high external quantum efficiency of 5.0 × 105 %. The excellent solar-blind detection performances are suggested to be attribute to the large concentration of oxygen vacancies, the possible photo released carriers from the deep-level acceptors, and the high film uniformity, while the former two contribute to the high photo carrier concentration, and the latter one contributes to high quality Schottky contact to achieve the low dark current. A 10 × 10 array based on these a-Ga2O3 Schottky photodiodes realized the imaging of a “E” character with high contrast. The results give a feasible way to achieve large area, low cost, high contrast, and high detection sensitivity solar-blind imaging.

Acceptor defects in polycrystalline Ge layers evaluated using linear regression analysis

Polycrystalline Ge thin films have recently attracted renewed attention as a material for various electronic and optical devices. However, the difficulty in the Fermi level control of polycrystalline Ge films owing to their high density of defect-induced acceptors has limited their application in the aforementioned devices. Here, we experimentally estimated the origin of acceptor defects by significantly modulating the crystallinity and electrical properties of polycrystalline Ge layers and investigating their correlation. Our proposed linear regression analysis method, which is based on deriving the acceptor levels and their densities from the temperature dependence of the hole concentration, revealed the presence of two different acceptor levels. A systematic analysis of the effects of grain size and post annealing on the hole concentration suggests that deep acceptor levels (53–103 meV) could be attributed to dangling bonds located at grain boundaries, whereas shallow acceptor levels (< 15 meV) could be attributed to vacancies in grains. Thus, this study proposed a machine learning-based simulation method that can be widely applied in the analysis of physical properties, and can provide insights into the understanding and control of acceptor defects in polycrystalline Ge thin films.

Role of Intrinsic Defects in Enhancing the Photoabsorption Capability of CuZn2AlSe4.

As a promising candidate for low-cost and eco-friendly thin-film photovoltaics, the emerging quaternary chalcogenide based solar cells have experienced rapid advances over the past decade. Here, we propose quaternary semiconducting chalcogenides CuZn2AlSe4 (CZASe) through cross-substitutions (cation mutations). The nonexistence of imaginary modes in the entire Brillouin zone of CZASe represents the inherent dynamic stability of the system. The electronic, optical, and defect properties of stannite CZASe quaternary semiconducting material was systematically investigated using density functional theory calculations. We have found that the chemical-potential control is very important for growing good-quality crystals and also to avoid secondary-phase formations such as ZnSe, Al2ZnSe4, and Cu3Se2. The observed p-type conductivity is mainly due to antisite defect CuZn, which has the lowest formation energy with a relatively deeper acceptor level than that of the Cu vacant site (VCu). The electronic band structures of vacancies and antisite defects by means of hybrid functional calculations show energy band shifting and energy band narrowing or broadening, which eventually tunes the optical band gap and improves the solar energy-conversion performance of semiconducting CZASe. Our results suggest that the stannite CZASe quaternary chalcogenides could be promising candidates for the efficient earth-abundant thin-film solar cells.

N2, NO, and O2 molecules in LiGaO2 in both Ga and Li sites and their relation to the vacancies

Doping of the ultrawide bandgap semiconductor LiGaO2 (Eg=5.6 eV) with N2, NO, and O2 molecules placed in either Ga or Li vacancies is studied using first-principles calculations. These molecular dopants are considered potential acceptors for p-type doping. Their optimal placement and orientation relative to the lattice is studied as well as their transition levels and energy of formation. Unfortunately, they are found to have deep acceptor level transition states. We discuss the relation of the transition levels to the one-electron levels, their spin state, and defect wave functions. They are found to be closely related to those of the corresponding vacancies.