Using first-principles calculations, we explored all the 21 defect-pairs in GaN and considered 6 configurations with different defect-defect distances for each defect-pair. 15 defect-pairs with short defect-defect distances are found to be stable during structural relaxation, so they can exist in the GaN lattice once formed during the irradiation of high-energy particles. 9 defect-pairs have formation energies lower than 10 eV in the neutral state. The vacancy-pair VN-VN is found to have very low formation energies, as low as 0 eV in p-type and Ga-rich GaN, and act as efficient donors producing two deep donor levels, which can limit the p-type doping and minority carrier lifetime in GaN. VN-VN has been overlooked in the previous study of defects in GaN. Most of these defect-pairs act as donors and produce a large number of defect levels in the band gap. Their formation energies and concentrations are sensitive to the chemical potentials of Ga and N, so their influences on the electrical and optical properties of Ga-rich and N-rich GaN after irradiation should differ significantly. These results about the defect-pairs provide fundamental data for understanding the radiation damage mechanism in GaN and simulating the defect formation and diffusion behavior under irradiation.
Magnetic field dependent polarization selective photoluminescence(PL) study has been carried out at 1.5~K on Gd-doped GaN epitaxial layers grown on c-SiC substrates by molecular beam epitaxy technique. It has been found that the incorporation of Gd in GaN leads to the generation of three types of donor like defects that result in neutral donor bound excitonic features in low temperature PL. The study reveals that the rate of spin-flip scattering for all the three excitonic features becomes almost $B$-independent suggesting that these signals must be stemming from defects which are ferromagneti…
The integration of advanced optoelectronic properties in nanoscale devices of group III nitride can be realized by understanding the coupling of charge carriers with optical excitations in these nanostructures. The native defect induced electron-phonon coupling in GaN nanowires are reported using various spectroscopic studies. The GaN nanowires having different native defects are grown in atmospheric pressure chemical vapor deposition technique. X-ray photoelectron spectroscopic analysis revealed the variation of Ga/N ratios in nanowires having possible native defects, with respect to their gr…
Oxygen is the most common unintentional impurity found in GaN. We study the interaction between substitutional oxygen (ON) and the gallium vacancy (VGa) to form a point defect complex in GaN. The formation energy of the gallium vacancy is largely reduced in n-type GaN by complexing with oxygen, while thermodynamic and optical transition levels remain within the band gap. We study the spectroscopy of this complex using a hybrid quantum-mechanical molecular-mechanical (QM/MM) embedded-crystal approach. We reveal how a single defect center can be responsible for multiband luminescence, including …
Erbium (Er) doped GaN has been studied extensively for optoelectronic applications, yet its defect physics is still not well understood. In this work, we report a first-principles hybrid density functional study of the structure, energetics, and thermodynamic transition levels of Er-related defect complexes in GaN. We discover for the first time that Er$_{\rm Ga}$-C$_{\rm N}$-$V_{\rm N}$, a defect complex of Er, a C impurity, and an N vacancy, and Er$_{\rm Ga}$-O$_{\rm N}$-$V_{\rm N}$, a complex of Er, an O impurity, and an N vacancy, form defect levels at 0.18 and 0.46 eV below the conduction…
We study the formation energies of native point defects in GaN through density-functional theory. In our first-principles scheme, the band edges are positioned in accord with hybrid density functional calculations, thus yielding a band-gap in agreement with experiment. With respect to previous semilocal calculations, the calculated formation energies and charge transition levels are found to be significantly different in quantitative terms, while the overall qualitative trend remains similar. In Ga-rich conditions, the nitrogen vacancy corresponds to the most stable defect for all Fermi energi…
The semiconducting behaviour and optoelectronic response of gallium nitride is governed by point defect processes, which, despite many years of research, remain poorly understood. The key difficulty in the description of the dominant charged defects is determining a consistent position of the corresponding defect levels, which is difficult to derive using standard supercell calculations. In a complementary approach, we take advantage of the embedded cluster methodology that provides direct access to a common zero of the electrostatic potential for all point defects in all charge states. Charge…
Rare-earth (RE) doped GaN is of interest for optoelectronics and spintronics and potentially for quantum applications. A fundamental understanding of the interaction between RE dopants and the semiconductor host is key to realizing the material's full potential. This work reports an investigation of lanthanide ($Ln$) defects in GaN using hybrid density-functional defect calculations. We find that all the $Ln$ dopants incorporated at the Ga lattice site, $Ln_{\rm Ga}$ ($Ln$ = La--Lu), are stable as trivalent ions, but Eu and Yb can also be stabilized as divalent and Ce, Pr, and Tb as tetravalen…
Present work investigates the structural, electronic and magnetic properties of wurtzite (0001) GaN nanowires (NWs) doped with Gd and point defects by employing the GGA+U approximation. We find that Ga vacancies (VGa) exhibit lower formation energy compared to N vacancies (VN). Further stabilization of point defects occurs due to the presence of Gd and ambient ferromagnetism (FM) can be stabilized in the NW by the additional positive charge induced by the VGa. Electronic structure analysis shows that VGa introduces additional levels in the band gap leading to ferromagnetic coupling due to the …
Defect physics of europium (Eu) doped GaN is investigated using first-principles hybrid density-functional defect calculations. This includes the interaction between the rare-earth dopant and native point defects (Ga and N vacancies) and other impurities (O, Si, C, H, and Mg) unintentionally present or intentionally incorporated into the host material. While the trivalent Eu$^{3+}$ ion is often found to be predominant when Eu is incorporated at the Ga site in wurtzite GaN, the divalent Eu$^{2+}$ is also stable and found to be predominant in a small range of Fermi-level values in the band-gap r…
GaN films with thicknesses up to 3 mm were grown in two custom-made halide vapor phase epitaxy (HVPE) reactors. V-shaped defects (pits) with densities from 1 cm$^{-2}$ to 100 cm$^{-2}$ were found on the surfaces of the films. Origins of pit formation and the process of pit overgrowth were studied by analyzing the kinematics of pit evolution. Two mechanisms of pit overgrowth were observed. Pits can be overgrown intentionally by varying growth parameters to increase the growth rate of pit facets. Pits can overgrow spontaneously if a fast-growing facet nucleates at their bottom under constant gro…
In this work, doping-defect interactions relevant to self-compensation in p-type GaN were investigated using atom probe tomography. The 3D visualization of ion distribution revealed the formation of spherical Mg-rich clusters and the segregation of Mg dopant towards dislocations in MOCVD-grown GaN:Mg. Impurities related to self-compensation, such as oxygen and hydrogen, were identified and detected adjacent to Mg-rich dislocations. Crystal stoichiometry around the defect regions was investigated to understand how the defects can serve as traps and influence dopant diffusion. Non-stoichiometric…
Investigation of charged defects is necessary to understand the properties of semiconductors. While density functional theory calculations can accurately describe the relevant physical quantities, these calculations increase the computational loads substantially, which often limits the application of this method to large-scale systems. In this study, we propose a new scheme of neural network potential (NNP) to analyze the point defect behavior in multiple charge states. The proposed scheme necessitates only minimal modifications to the conventional scheme. We demonstrated the prediction perfor…
Determining the types and concentrations of vacancies present in intentionally doped GaN is a notoriously difficult and long-debated problem. Here we use an unconventional approach, based on thermal transport modeling, to determine the prevalence of vacancies in previous measurements. This allows us to provide conclusive evidence of the recent hypothesis that gallium vacancies in ammonothermally grown samples can be complexed with hydrogen. Our calculations for O-doped and Mg-O co-doped samples yield a consistent picture interlinking dopant and vacancy concentration, carrier density, and therm…
GaN(0001) surfaces with Ga- and H-adsorbates are fundamental stages for epitaxial growth of semiconductor thin films. We explore stable surface structures with nanometer scale by the density-functional calculations combined with Bayesian optimization, and succeed to reach a single structure with satisfactorily low mixing enthalpy among hundreds of thousand possible candidate structures. We find that the obtained structure is free from any postulated high symmetry previously introduced by human intuition, satisfies electron counting rule locally, and shows new adsorbate arrangement, reflecting …
The physicochemical processes at the surfaces of semiconductor nanostructures involved in electrochemical and sensing devices are strongly influenced by the presence of intrinsic or extrinsic defects. To reveal the surface controlled sensing mechanism, intentional lattice oxygen defects are created on the surfaces of GaN nanowires for the elucidation of charge transfer process in methane (CH4) sensing. Experimental and simulation results of electron energy loss spectroscopy (EELS) studies on oxygen rich GaN nanowires confirmed the possible presence of 2(ON) and VGa-3ON defect complexes. A glob…
We determine atomic and electronic structure, formation energy, stability and magnetic properties of native point defects, such as Gallium (Ga) and Nitrogen (N) vacancies in bulk and at non-polar (10$\overline{1}$0) surface of wurtzite Gallium Nitride (\textit w-GaN) using, first-principles calculations based on Density Functional Theory (DFT). Under both Ga-rich and N-rich conditions, formation energy of N-vacancies is significantly lower than that of Ga-vacancies in bulk and at (10$\overline{1}$0) surface. Experimental evidence of the presence of N-vacancies was noted from electron energy lo…
Carbon doping is used to obtain semi-insulating GaN crystals. If the carbon doping concentration exceeds $5*10^{17}$ $cm^{-3}$, the carbon atoms increasingly form triatomic clusters. The tri-carbon defect structure is unambiguously proven by the isotope effect on the defects' local vibrational modes (LVMs) originally found in samples containing carbon of natural isotopic composition $(~99 $%$ ^{12}C, ~1$%$ ^{13}C)$ at $1679$ $cm^{-1}$ and $1718$ $cm^{-1}$. Number, spectral positions, and intensities of the LVMs for samples enriched with the $^{13}C$ isotope (~99 % and ~50 %) are consistently i…
Annealing Mg-implanted homoepitaxial GaN at temperatures at or above 1400 °C eliminates the formation of inversion domains and leads to improved dopant activation efficiency. Extended defects in the form of inversion domains contain electrically inactive Mg after post-implantation annealing at temperatures as high as 1300 °C (one GPa N2 overpressure), which results in a low dopant activation efficiency. Triple axis X-ray data show that the implant-induced strain is fully relieved after annealing at 1300 °C for 10 min, indicating that the strain-inducing point defects formed during implantation…
We report a correlative microscopy study of a sample containing three stacks of InGaN/GaN quantum dots (QDs) grown at different substrate temperature, each stack consisting of 3 layers of QDs. Decreasing the substrate temperature along the growth axis leads to the proliferation of structural defects. However, the luminescence intensity increases towards the surface, in spite of the higher density of threading dislocations, revealing that the QD layers closer to the substrate behave as traps for non-radiative point defects. During atom probe tomography experiments combined with in-situ micro-ph…
Using hybrid density functional theory, we address point defects susceptible to cause charge compensation upon Mg doping of GaN. We determine the free energy of formation of the nitrogen vacancy and of several Mg-related defects. The entropic contribution as a function of temperature is determined within the quasiharmonic approximation. We find that the Mg interstitial shows a noticeably lower free energy of formation than the Mg substitutional to Ga in p-type conditions. Therefore, the Mg impurity is amphoteric behaving like an acceptor when substitutional to Ga and like a double donor when a…
GaN nanowires grown by molecular beam epitaxy generally suffer from dominant nonradiative recombination, which is believed to originate from point defects. To suppress the formation of these defects, we explore the synthesis of GaN nanowires at temperatures up to 915 $°C$ enabled by the use of thermally stable TiN$_x$/Al$_2$O$_3$ substrates. These samples exhibit indeed bound exciton decay times approaching those measured for state-of-the-art bulk GaN. However, the decay time is not correlated with the growth temperature, but rather with the nanowire diameter. The inverse dependence of the dec…
Leibniz-Institut für Kristallzüchtung (IKZ)（莱布尼茨晶体生长研究所）；Center for Computational Materials Science, United States Naval Research Laboratory（美国海军研究实验室计算材料科学中心）；Institute of Experimental Physics, TU Bergakademie Freiberg（弗莱贝格矿业技术大学实验物理研究所）；Ferdinand-Braun-Institut (FBH)（费迪南德·布劳恩研究所）；Institute of Solid State Physics, Technische Universität Berlin（柏林工业大学固体物理研究所）；Technische Physik, University of Würzburg（维尔茨堡大学技术物理系）
This work examines the carbon defects associated with recently reported and novel peaks of infrared (IR) absorption and Raman scattering appearing in GaN crystals at carbon ($^{12}C$) doping in the range of concentrations from $3.2*10^{17}$ to $3.5*10^{19} cm^{-3}$. 14 unique vibrational modes of defects are observed in GaN samples grown by hydride vapor phase epitaxy (HVPE) and then compared with defect properties predicted from first-principles calculations. The vibrational frequency shift in two $^{13}C$ enriched samples related to the effect of the isotope mass indicates six distinct confi…
We report accurate energetics of defects introduced in GaN on doping with divalent metals, focussing on the technologically important case of Mg doping, using a model which takes into consideration both the effect of hole localisation and dipolar polarisation of the host material, and includes a well-defined reference level. Defect formation and ionisation energies show that divalent dopants are counterbalanced in GaN by nitrogen vacancies and not by holes, which explains both the difficulty in achieving p-type conductivity in GaN and the associated major spectroscopic features, including the …
Gas sensors at low operating temperature with high sensitivity are the demand for the group III nitrides owing to their high chemical and thermal stability. The CH4 sensing is realized for the Al0.07Ga0.93N nanowires (NWs) with an improved response over the GaN NWs at a low operating temperature of 50 °C, for the first time. Al0.07Ga0.93N NWs were synthesized via ion beam mixing process using inert gas ion irradiation on the bilayer of Al/GaN NWs. The sensing mechanism is explained with the help of native defects present in the system. The number of shallow acceptors created by Ga vacancy (V_G…
We uncover the origin of blue luminescence (BL) peak in Mg doped GaN thin film using a combination of experimental X-ray absorption near edge spectroscopy (XANES), first-principles calculations based on density functional theory and full multiple scattering theoretical analysis of various possible defect complexes and their XANES signatures. We demonstrate that a defect complex composed of Mg substituted at Ga site (MgGa) and Mg at interstitial site (Mgi) is primarily responsible for the observed BL by Donor-Acceptor Pair transition (DAP) associated with a deep donor state in the gap. It corre…
Here we report on the effect of rare earth Gd-doping on the magnetic properties and magnetotransport of GaN two-dimensional electron gasses (2DEGs). Samples are grown by plasma-assisted molecular beam epitaxy and consist of AlN/GaN heterostructures where Gd is delta-doped within a polarization-induced 2DEG. Ferromagnetism is observed in these Gd-doped 2DEGs with a Curie temperature above room temperature and an anisotropic spontaneous magnetization preferring an out-of-plane (c-axis) orientation. At magnetic fields up to 50 kOe, the magnetization remains smaller for in-plane configuration than…
This work presents an in-depth investigation of the properties of complexes composed of hydrogen, silicon or oxygen with carbon, which are major unintentional impurities in undoped GaN. This manuscript is a complement to our previous work on carbon--carbon and carbon-vacancy complexes. We have employed a first-principles method using Heyd-Scuseria-Ernzerhof hybrid functionals within the framework of generalized Kohn-Sham density functional theory. Two H--C, four Si--C and five O--C complexes in different charge states have been considered. After full geometry relaxations, formation energies, b…
Defect engineering of ultrathin gallium nitride via electric fields for advanced electronic, magnetic, and gas sensing applications
Nanyang Technological University（南洋理工大学）；Infineon Technologies Asia Pacific Pte. Ltd.（英飞凌亚太私人有限公司）；China-Singapore International Joint Research Institute（中新国际联合研究院）
Spins bound to point defects are increasingly viewed as an important resource for solid-state implementations of quantum information technologies. In particular, there is a growing interest in the identification of new classes of defect spin that can be controlled optically. Here we demonstrate ensemble optical spin polarization and optically detected magnetic resonance (ODMR) of the S = 1 electronic ground state of chromium (Cr4+) impurities in silicon carbide (SiC) and gallium nitride (GaN). Spin polarization is made possible by the narrow optical linewidths of these ensembles (< 8.5 GHz), w…
Single photon emitters play a central role in many photonic quantum technologies. A promising class of single photon emitters consists of atomic color centers in wide-bandgap crystals, such as diamond silicon carbide and hexagonal boron nitride. However, it is currently not possible to grow these materials as sub-micron thick films on low-refractive index substrates, which is necessary for mature photonic integrated circuit technologies. Hence, there is great interest in identifying quantum emitters in technologically mature semiconductors that are compatible with suitable heteroepitaxies. Her…
