Ga Vacancies Research Articles

High‐Frequency Electron Paramagnetic Resonance and Electron‐Nuclear Double Resonance Spectroscopy Study of the Ga Vacancy in β‐Ga2O3

The Ga vacancy (VGa) defect in β‐Ga2O3, generated by proton irradiation, is studied using high‐frequency electron paramagnetic resonance (EPR) and electron‐nuclear double resonance spectroscopy. The previous X‐band EPR studies of this defect, attributed to VGa2−, are extended to higher frequencies (240 GHz) and lower temperatures (T = 6 K). The spin Hamiltonian parameters of the VGa2− center are determined: electron spin S = 1/2, g‐tensor: gb = 2.0313, gc = 2.0079, and ga = 2.0025; the hyperfine interaction parameters with 2 equivalent Ga neighbors: Ab = 14.0 G, Ac = 14.6 G, and Aa* = 12.8 G for 69Ga; the superhyperfine interaction with distant Ga neighbors ASHF(69Ga) = 11 MHz; and the quadrupole interaction Qb(69Ga) = 0.32 MHz and Qb(71Ga) = 0.22 MHz. These results shall allow to refine the assignment of this center to a split vacancy or an unrelaxed VGa2− defect.

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.

On the nature of as-grown and irradiation-induced Ga vacancy defects in β-Ga2O3

On the nature of as-grown and irradiation-induced Ga vacancy defects in β-Ga2O3

Quasi-Van der Waals Epitaxial Growth of γ'-GaSe Nanometer-Thick Films on GaAs(111)B Substrates.

GaSe is an important member of the post-transition-metal chalcogenide family and is an emerging two-dimensional (2D) semiconductor material. Because it is a van der Waals material, it can be fabricated into atomic-scale ultrathin films, making it suitable for the preparation of compact, heterostructure devices. In addition, GaSe possesses unusual optical and electronic properties, such as a shift from an indirect-bandgap single-layer film to a direct-bandgap bulk material, rare intrinsic p-type conduction, and nonlinear optical behaviors. These properties make GaSe an appealing candidate for the fabrication of field-effect transistors, photodetectors, and photovoltaics. However, the wafer-scale production of pure GaSe single-crystal thin films remains challenging. This study develops an approach for the direct growth of nanometer-thick GaSe films on GaAs substrates by using molecular beam epitaxy. It yields smooth thin GaSe films with a rare γ'-polymorph. We analyze the formation mechanism of γ'-GaSe using density-functional theory and speculate that it is stabilized by Ga vacancies since the formation enthalpy of γ'-GaSe tends to become lower than that of other polymorphs when the Ga vacancy concentration increases. Finally, we investigate the growth conditions of GaSe, providing valuable insights for exploring 2D/three-dimensional (3D) quasi-van der Waals epitaxial growth.

Effect of gas pre-decomposition device on the growth of GaN epitaxial layer

In previous studies, the influence of gas phase and surface reactions on the growth of GaN was mainly calculated through simulations. In this study, a novel gas pre-decomposition device (GPDD) was designed to experimentally investigate the effects of gas phase and surface reactions on GaN growth by changing the length and height of the isolation plates (IPs). By varying the structure of the GPDD, the effects on the growth rate and thickness uniformity of the GaN films were studied. The growth rate of the GaN sample slowed with the extension of the IPs because the longer partition plates led to insufficient gas mixing and premature consumption of the precursor trimethylgallium (TMG). The use of GPDD simultaneously achieves high crystal quality and smooth surface morphology of the GaN film. Owing to the use of GPDD, the decomposition of TMG in the pyrolysis pathway was promoted, which suppressed Ga vacancies and C impurities, resulting in weak yellow luminescence bands in the photoluminescence. This study provides a comprehensive understanding of the chemical reaction mechanism of GaN and plays an important role in promoting the development of metal-organic chemical vapor deposition equipment.

Radiation-induced defects in the InGaN/GaN superlattice structure

With the molecular dynamics method, this paper investigates radiation-induced defects in the In0.16Ga0.84N/GaN superlattice structure (SLS) and the In0.04Ga0.96N/GaN SLS. In the temporal evolution of cascades, most of vacancies recombine with interstitials. The Monte Carlo simulations about the proportions of PKAs induced by 3 MeV protons were also considered in this work for calculating the weighted averages of surviving defects. For the In0.16Ga0.84N/GaN SLS irradiated by protons, around 82.6 percent of surviving vacancies are Ga vacancies while around 88.9 percent of surviving interstitials are Ga interstitials. For the In0.04Ga0.96N/GaN SLS irradiated by protons, around 87.3 percent of surviving vacancies are Ga vacancies while around 88.6 percent of surviving interstitials are Ga interstitials. N vacancies, N interstitials, and In vacancies also exist in irradiated InGaN/GaN SLS. Details about different types of defects are presented in this paper, which helps explain the microscopic mechanism of irradiated InGaN/GaN SLS. Since different types of defects have different influences on electronic and optical properties, simulations about the proportions of various defects in irradiated InGaN/GaN SLS help experimentalists find the effective factors of radiation-related changes in electronic and optical properties.

Impacts of vacancy complexes on the room-temperature photoluminescence lifetimes of state-of-the-art GaN substrates, epitaxial layers, and Mg-implanted layers

For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission (τPLRT) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τPLRT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3–4, in ammonothermal GaN. The values of τPLRT in n-GaN samples are compared with those of p-GaN, in which τPLRT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively.

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.

Improved midgap recombination lifetimes in GaN crystals grown by the low-pressure acidic ammonothermal method

To investigate the carrier recombination processes in GaN crystals grown by the low-pressure acidic ammonothermal (LPAAT) method, the photoluminescence (PL) spectra and PL lifetimes of LPAAT GaN crystals grown on acidic ammonothermal (AAT) GaN seed crystals were correlated with the growth polarity and species/concentration of point defects. The PL spectra of LPAAT GaN grown toward the (0001¯) direction (−c region), which provided the highest growth rate, exhibited a predominant near-band edge (NBE) emission. Neither bandgap narrowing nor Burstein–Moss shifts due to high concentration residual impurities were observed in the NBE emissions, indicating higher purity than the previously reported AAT GaN crystals. In addition, strain-induced energy shift or energy broadening of excitonic emission peaks was not observed, indicating excellent crystal coherency. Because of the reduced concentration of midgap recombination centers, a record-long room-temperature PL lifetime for the NBE emission of ammonothermal GaN (40 ps) was obtained from the −c region. Meanwhile, the PL spectra also exhibited the yellow and blue luminescence bands originating from particular deep-state radiative recombination centers. The major vacancy-type defects acting as midgap recombination centers are identified as vacancy complexes comprising a Ga vacancy (VGa) and a few N vacancies (VN), namely, VGa(VN)n buried by H and/or O, where n is an integer. Further reduction of such defect complexes will allow less compensated stable carrier concentration in the LPAAT GaN crystals.

Heavy Ge Doping in GaN via Pulsed Sputtering to Tailor the Optical Bandgap Energy

The epitaxial growth of heavily Ge‐doped GaN films using pulsed sputtering deposition (PSD) on AlN (0001)/sapphire substrates is presented and the correlations among their structural, electrical, and optical properties are investigated. High‐quality Ge‐doped PSD‐GaN films are grown with electron concentrations of 0.33–4.4 × 1020 cm−3. Notably, cathode luminescence spectra reveal the absence of defect‐induced emissions associated with Ga vacancies (2.3–2.4 eV), consistent with the high electron mobilities of the films. As the electron concentration increases, the optical bandgap also increases. This correlation can be explained by combining the theoretical formulas for the Burstein–Moss shift and the bandgap renormalization effects. Remarkably, the optical bandgap reaches 3.84 eV at 4.4 × 1020 cm−3 carrier concentration, marking the highest value reported for the optical bandgap of GaN. These results indicate the potential of heavy Ge doping in GaN via PSD for applications in highly transparent conductive layers and tunneling junctions within GaN‐based optoelectronic devices.

Engineering the half-metallic and magnetic semiconductor natures in gallium phosphide monolayer towards spintronic applications

In this work, magnetism engineering in the Gallium phosphide (GaP) monolayer is explored in order to create new two-dimensional (2D) spintronic materials. Pristine monolayer is a non-magnetic indirect gap semiconductor with band gap of 1.61(2.49) eV obtained from the PBE(HSE06)-based calculations. Both ionic and covalent characters form the Ga-P chemical bond, which are generated by the charge transfer and electronic hybridization, respectively. It is found that creating a single Ga vacancy magnetizes significantly GaP monolayer (VGa), where a large total magnetic moment of 2.88 μB is obtained and magnetic properties are produced mainly by P atoms around the defect site. Meanwhile, GaP monolayer is metallized under effects of single P vacancy, where the non-magnetic nature is preserved. Significant magnetization is also achieved by doping with alkali (Li and Na - LiGa and NaGa) and alkaline earth (Be and Mg - BeGa and MgGa) metals. In these cases, total magnetic moments of 2.00 and 1.00 μB are obtained, respectively. Besides, doping with S (SP) and Se (SeP) atoms induces weaker magnetization, which is reflected in smaller total magnetic moments of 0.60 and 0.96 μB, respectively. Interestingly, the half-metallicity is observed in VGa, BeGa, and MgGa systems; while LiGa, NaGa, and SeP systems exhibit the diluted magnetic semiconductor behavior. In contrast, Cl (ClP) and Br (BrP) impurities induces no magnetism in GaP monolayer. However both ClP and BrP systems have energy gap of 0.76 eV, which corresponds to a reduction of the order of 52.80% from that of pristine monolayer. Results presented herein may introduce efficient methods to modify the GaP monolayer electronic and magnetic properties, such that new multifunctional 2D materials can be created for spintronic and optoelectronic applications.

Electrical conductivity improvement of charged Ga vacancies in wurtzite GaN

This investigation explores the complex role of Ga vacancy charged defect in the crystalline structure of wurtzite Gallium Nitride (GaN), highlighting a significant increase in electrical conductivity—contrary to conventional beliefs. Through advanced Density Functional Theory (DFT) calculations, the study reveals that specific configurations of Ga vacancy charged defect contribute to a sixfold increase in electrical conductivity in wurtzite GaN. However, it is crucial to note that N vacancy charged defects negatively impact these electrical properties. This detailed theoretical analysis sheds light on how these vacancy charged defects can positively affect conductivity and provides a novel perspective on charged defects in GaN, paving the way for the optimized design of GaN in the future.

Origin of Black Color in Heavily Doped n‐Type GaN Crystal

In semiconductor materials, doping is used mainly for controlling the electrical properties. There have been attempts to grow low‐resistivity n‐type gallium nitride (GaN) crystals by doping oxygen, germanium, and silicon, because a low‐resistivity GaN substrate is required to reduce the power losses of optical and electrical devices. However, in those efforts, the crystal color turns black with the increase in the concentration of the n‐type additives, even though they are shallow donors. Herein, it is explained why heavily doped n‐type GaN crystals exhibit low transparency. From optical absorption profiles, the appearance of a band tail from the band edge to 1.5 eV is observed. Considering the band tail theory and our observations, it is concluded that Ga vacancy or Ga vacancy complexes behaving as acceptors induce the band tail and the black color. It is proposed that neutralizing the high charge of defects ensures that low‐colored GaN crystals with low resistivity can be obtained. Moreover, the fabrication of low‐resistivity wafers sliced from a large crystal with a laser produces inexpensive wafers and allows the spread of high‐efficiency GaN devices fabricated on low‐resistivity substrates for saving electric power.

Radiation damage effects on electronic and optical properties of β-Ga2O3 from first-principles

β-Ga2O3 with an ultra-wide bandgap demonstrates great promise in applications of space missions as power electronics and solar-blind photodetector. Unraveling the radiation damage effects on its material properties is of crucial importance, especially for improving the radiation tolerance of Ga2O3-based devices. Herein, we evaluate the formation energy of gallium and oxygen vacancy defects and comprehensively investigate their influence on the electronic and optical properties of β-Ga2O3 using first-principles calculations. Ga vacancies act as deep acceptors and produce p-type defects in β-Ga2O3, while the defective Ga2O3 with O vacancies exhibits the n-type characteristics. A semimetal characteristic is observed in the defective Ga2O3 with Ga vacancies, and an apparent optical absorption peak in the infrared spectral range emerges. Moreover, the self-compensation effect emerges when β-Ga2O3 contains both Ga vacancies and O vacancies, leading to the reduced absorption peak. The doping effect on the defect formation energy of β-Ga2O3 is also investigated, and Ga vacancies are found to be easily formed in the case of In doped β-Ga2O3 (InGa2O3) compared to the undoped β-Ga2O3, while O vacancies are much harder to form. This work provides insights into how gallium and oxygen vacancy defects alter electronic and optical properties of β-Ga2O3, seeking to strengthen its radiation tolerance.

The optimized size distribution and NIR persistent luminescence of Zn2Ga2.98-4x/3HfxGe0.75O8:Cr0.02 nanoparticles via high-valence ion doping

In this study, spinel-phase Zn2Ga4-4y/3GeyO8 (ZGO:Ge4+y) (y = 0–0.90) and Zn2Ga2.98-4x/3HfxGe0.75O8:Cr0.02 (ZGGO:Cr3+,Hf4+x) (x = 0–0.075) nanoparticles were prepared by a hydrothermal method combined with a subsequent vacuum annealing. For Cr3+doped ZGO:Ge4+y nanoparticles, it was found that the replacement of Ga3+ with high-valence Ge4+ can lead to more uniform morphology and enhanced afterglow. Meanwhile, for ZGGO:Cr3+,Hf4+x nanoparticles, the non-equivalent replacement of Ga3+ with high-valence Hf4+ can give rise to the decrease in standard deviation gradually from 12.3 to 7.0 nm and the afterglow intensity is increased to 3.4 times of the undoped ZGGO:Cr3+ nanoparticles. Based on the first principle calculations using an ideal model of Zn8Ga16O32 (ZGO) unit, it is found that the formation energy (11.32 eV) using a doped model of Zn8Ga12Ge3O32 unit (i.e. three Ge ions substitute for three Ga ions and one Ga vacancy (VGa′′′) is created in a unit due to the charge compensation effect) is lower than that (13.35 eV) using a doped model of Zn11Ga10Ge3O32 unit (i.e. three Ge ions and three Zn ions substitute for six Ga ions without the charge compensation effect). Combined with the spectral data, we can confirm that, in the case of the same Ge doping concentration (y = 0.75), the formula of ZGGO host is Zn2Ga4-4y/3GeyO8 rather than Zn2+yGa4-2yGeyO8. Based on the above ZGGO (Zn8Ga12Ge3O32) model, it is found that the smaller formation energies (−1.49 to −3.34 eV) of the Hf4+ doped systems further prove that high-valence Hf4+ ions prefer to substitute for octahedral and tetrahedral Ga3+ and Ge4+ sites and promote the creation of Zn’Ga–Ga°Zn and Ga°Zn defects and it leads to the improved afterglow. Furthermore, contrary to other isovalent ions (such as Nd3+) doped ZGGO nanoparticles, ZGGO:Cr3+,Hf4+x nanoparticles exhibit excellent dispersion stability in an acidic fluid. These findings suggest ZGGO:Cr3+,Hf4+x nanoparticles might be potential for bioimaging in an acidic body fluid.

Investigation of the surface band structure and the evolution of defects in β-(AlxGa1−x)2O3

This study examines the electronic and luminescent properties of β-(AlxGa1−x)2O3 (0 ≤ x ≤ 0.42) thin films grown on (0001) sapphire using laser-MBE, with a focus on the evolution of defect energy levels and their impact on surface Fermi level pinning and luminescence. X-ray photoelectron spectroscopy (XPS) and cathodoluminescence (CL) have been employed to analyze surface band bending and defect evolution as a function of aluminum content. The results have revealed a pinned Fermi level at 3.6 eV above the valence band maximum despite the increase in the bandgap. The consequent upward band bending has been confirmed by a peak shift in the core level XPS. The defects that lead to the Fermi level pinning effect are attributed to E2*, which is related to a Ga vacancy or Ga vacancy-O vacancy complex. In addition, CL spectroscopy and depth-resolved CL have demonstrated consistent blue and ultraviolet emissions across the Al content range and a similar suppression of electron concentration on blue and ultraviolet emissions in β-(AlxGa1−x)2O3 and β-Ga2O3. Based on the observed evolution of defects with Al content, the blue band emission is attributed to electron transition in the donor–accepter pair.

Tutorial: Microscopic properties of O–H centers in β-Ga2O3 revealed by infrared spectroscopy and theory

β-Ga2O3 is an ultrawide bandgap semiconductor that is attracting much attention for applications in next-generation high-power, deep UV, and extreme-environment devices. Hydrogen impurities have been found to have a strong effect on the electrical properties of β-Ga2O3. This Tutorial is a survey of what has been learned about O–H centers in β-Ga2O3 from their vibrational properties. More than a dozen, O–H centers have been discovered by infrared absorption spectroscopy. Theory predicts defect structures with H trapped at split configurations of a Ga(1) vacancy that are consistent with the isotope and polarization dependence of the O–H vibrational spectra that have been measured by experiment. Furthermore, O–H centers in β-Ga2O3 have been found to evolve upon thermal annealing, giving defect reactions that modify conductivity. While much progress has been made toward understanding the microscopic properties and reactions of O–H centers in β-Ga2O3, many questions are discussed that remain unanswered. A goal of this Tutorial is to inspire future research that might solve these puzzles.

First-principles study on the effect of point defects on the magnetic new mechanism and optical properties of the GaN:Be/Mg/Ca system

The new magnetic mechanism and optical properties of Ga vacancies and Hi interstitial in the GaN: Be/Mg/Ca system have not been fully understood, and the use of first principles can solve this problem. The effect of point defects on the magnetic mechanism and optical properties of the GaN: Be/Mg/Ca system was investigated using the first nature principle of the hybridized generalized HSE06 method. Results show that all doped systems have N2− ions in addition to N3− ions, and N2− ions have the dual property of itinerant electrons in the off-domain (donor) and of local electrons (acceptor). The magnetism of magnetic doped systems is generated by the hybrid coupling of Ga4s and N2− 2p states. In comparison with the Ga34MN36 (M = Be/Ca) system, the magnetic moments of Ga34MHiN36 (M = Be/Ca) system are reduced after doping with Hi interstitial. The magnetic properties of the Ga34MgN36 system can be regulated by the presence or absence of Hi interstitial, which is advantageous as a magnetic switch. The absorption spectral distribution of the Ga34MgHiN36 system extends to the mid-infrared optical region. This material has some reference value as infrared thermophotovoltaic cells, infrared photodetectors, or infrared semiconductor lasers.

Low-temperature photoluminescence characteristic of Tm-doped Ga2O3 films for light emitting diodes application

Tm-doped Ga2O3 films hold significant promise for application in light-emitting diodes (LEDs). A comprehensive understanding of their photoluminescence properties is pivotal for unlocking their full potential. In this work, we present findings on the photoluminescence (PL) spectra of Tm-doped Ga2O3 films, elucidating variations at different measurement temperatures and excitation energies utilizing synchrotron radiation light. The intensity of ultraviolet light (UVL1 and UVL2) exhibits a temperature dependence and its activation energy illustrates an ionization energy of shallow donor and migration through tunneling of self-trapped holes to triply ionized Ga vacancy (VGa) centers forming doubly ionized VGa acceptors. Additionally, the excitation energy-dependent PL spectra provide insights into the excitation mechanism of Tm ions, indicating a most likely through defects in the Ga2O3 host.

Theoretical Insights into Band Gap Tuning Through Cu Doping and Ga Vacancy in GaSe Monolayer: A First-Principles Perspective

Theoretical Insights into Band Gap Tuning Through Cu Doping and Ga Vacancy in GaSe Monolayer: A First-Principles Perspective