Complexes In GaN Research Articles

Sub-bandgap excited photoluminescence probing of deep defect complexes in GaN doped by Si, Ge and C impurities

Abstract With the sub-bandgap optical excitation, thermal dynamics of holes among multiple levels in n-type GaN epilayers with different dopants of Si, Ge and C are investigated via measuring and modelling variable-temperature yellow luminescence (YL) band of the samples. In sharp contrast to the case of above-bandgap optical excitation, the variable-temperature YL band of all the studied GaN samples including unintentionally-doped sample exhibit unusual negative thermal quenching (NTQ) behavior, suggesting a possible physical mechanism, namely thermally induced migration of holes from shallower levels to the luminescent deep level. By considering the possible presence of multiple hole levels in the doped GaN samples, a phenomenological model is developed for the thermal transfer of holes among the multi-levels and the interpretation of the observed NTQ phenomenon of the YL band. Different activation energies of 347.9, 520.8 and 348.5 meV are obtained for the Ge-doped, high C-containing, and Si-doped GaN samples, respectively. The results reveal the existence of multiple hole defect levels in the n-type GaN. Possible microstructural origins causing these different hole levels are further argued. The study may shed some light on the nature of various defect complexes in the technologically important GaN epilayers. Combined microstructural and optical investigations need to be further done for elucidating various optically- and electrically-active defect complexes in GaN.

Analysis of radiation defects in gallium nitride using deep level transient spectra and first principles methods

Radiation defects in gallium nitride devices were analyzed by combining experiments with simulation methods. Various forms of native defects, impurities, and these hydrogenated defect complexes in GaN were studied with first-principles calculations employing hybrid functionals which exhibit a reference data base for experimental analysis. We consider hydrogen complexes made of the combinations of single point defects (VGa, VN), di-vacancies (VGa-VGa, VN-VN, VGa-VN) and impurities (C, O, Si). Formation energies were computed for configurations with different charge states after full geometry optimizations. From the calculated formation energies, thermodynamic transition levels were evaluated, which are related to the thermal activation energies observed in experimental techniques such as deep level transient spectroscopy (DLTS). Hydrogen interstitial atoms were identified by the combination of DLTS and first-principles calculations. The transition level measured by DLTS was EC-0.5 eV, which is consistent with the calculation value. The signal of EC-0.9 eV peak in DLTS may be consisting of [VGa]-3/-2 and [VN-VN]+1/+2. According to the degradation of the electrical performance of the AlGaN/GaN high electron mobility transistors and the calculation results, the hydrogen passivation may be caused by [VGa]-3, [VGa-VGa]-2 and [VGa-VN]-1.

Comparative studies of interatomic potentials for modeling point defects in wurtzite GaN

In this paper, a new version of the Stillinger–Weber (SW) potential for wurtzite GaN is presented, by which we systematically explore the structural and thermodynamical properties of native point defects and their complexes. In parallel, the semi-empirical Modified Embedded-Atom Method (MEAM) potential is selected for comparison. The SW and MEAM potentials are assessed by the reproduction of the fundamental properties of wurtzite GaN and by the ability to describe the inversion domain boundaries and the wurtzite–rocksalt phase transition. Then the structural search of native point defects and their complexes in GaN is implemented using both SW and MEAM potentials with the benchmark of Density Functional Theory (DFT) calculations. Besides vacancies and antisites, four N and five Ga interstitials are confirmed by refining the DFT calculations, among which two N split interstitials N+−N⟨21̄1̄0⟩ and N+−Ga⟨011̄0⟩, and two Ga split interstitials, Ga+−Ga⟨011̄0⟩−g and Ga+−N⟨011̄0⟩, are observed for the first time. The SW potential correctly predicts the octahedral occupation GaOct to be the most stable Ga interstitial, while the MEAM potential predicts the ground state of the N+−N⟨011̄0⟩ split interstitial (N+−N⟨011̄0⟩−g) as the most stable N interstitial. However, neither of the two potentials could simultaneously generate the most stable configurations of N and Ga interstitials. The investigations of point defect complexes reveal that N octahedral Frenkel [FrenkelOct(N)] and paired antisite (NGaGaN) defects are unstable and get converted into VN⊕N+−N⟨011̄0⟩−g configurations with different separations between VN and N+−N⟨011̄0⟩−g point defects based on the DFT calculations. The formation energies calculated by the DFT and SW potential demonstrate that Schottky, Ga octahedral Frenkel [FrenkelOct(Ga)], and VN⊕N+−N⟨011̄0⟩−g point defect complexes are energetically feasible and that they should not dissociate into two isolated point defects. In contrast, the MEAM potential predicts the dissociation to be exothermic for Schottky and VN⊕N+−N⟨011̄0⟩−g. Overall, the structural features concerned with N–N or Ga–Ga bonds relaxed by the SW potential are more consistent with DFT calculations than the MEAM counterpart.

Influence of intrinsic or extrinsic doping on charge state of carbon and its interaction with hydrogen in GaN

It has been established that the formation of point defects and their behaviors could be regulated by growth details such as growth techniques and growth conditions. In this work, we prove that C doping approaches have great influence on the charge state of CN, thus the interaction between H and C in GaN. For GaN with intrinsic C doping, which is realized by reducing the V/III ratio, CN mainly exists in the form of CN− charged from the higher concentration of VN and, thus, may attract H+ by coulomb interaction. Whereas for the extrinsically C doped GaN with propane as the doping source, the concentration of VN is reduced, and CN mainly exists in neutral charge state and, thus, nearly does not attract H ions. Therefore, we demonstrate that the interplay between H and C atoms is weaker for the extrinsically C doped GaN compared to the intrinsically doped GaN, thus gives a clear picture about the different charge states of CN and the formation of C–H complexes in GaN with different C doping approaches.

(Invited) Defect Microstructural Evolution of Co-Implanted and Gyrotron Microwave-Annealed GaN

The success of the next generation of (Al)GaN-based power devices depends on the ability to form selective-area p-type regions, which can be accomplished by ion implantation of the prevailing acceptor dopant, Mg. Ion implantation induces lattice damage and creates point defects within the material, which evolve in response to activation annealing. Lattice dilation can be reduced and point defects removed or agglomerated into clusters by annealing [1], however sufficiently extreme annealing conditions degrade or decompose GaN [2]. The p-type activation of Mg-implanted GaN can be further aided by co-implantation with N, which is believed to enhance the removal of implant-induced nitrogen vacancies (VN) in the material that act as donors and compensate free holes [3].To restore the bulk crystalline quality, lower the concentration of defects, and activate the implanted dopants without degrading the material, specialized annealing techniques are required either to heat very rapidly [4] and/or use ultra-high pressure conditions [5] in order to obtain high-conductivity p-type material, and activation of Mg-implanted material remains an area of active research.In this study, we report on the micro-structural changes in p-type implanted GaN material after activation-annealing by pulsed gyrotron microwave. MOCVD-grown unintentionally doped (UID) GaN is grown on n+-HVPE GaN, co-implanted with Mg/N to a depth of 250 nm, capped with AlN, and annealed using a gyrotron under an N2 overpressure of 3 MPa. Gyrotron microwave annealing is performed in pulses of 5–8 sec with maximum temperatures between 1350–1450 °C. Detailed optical, structural, and electrical characterization are conducted to better understand the impact on defect microstructure, and to verify p-type conductivity. Mitigation of implant-induced damage to the lattice is further characterized by transmission electron microscopy (TEM) and x-ray diffraction (XRD). We correlate damage removal (by this annealing methodology) with a trend in optical signature through spatially-resolved low-temperature PL. At the optimized anneal condition of 5 pulses at 1450 °C maximum temperature, and 8 s pulse length, the average intensity ratio of the MgGa acceptor-related UVL to the implant-induced donor VN-related GL2 is 6.6 with a standard deviation 66% of the mean across the material, suggesting high and uniform acceptor activation relative to unoptimized beam conditions. We also characterize regions of the sample in which the GL2 luminescence is suppressed and a dominant YL emerges in the 2.2–2.5 eV range, which is attributed to unintentionally introduced CN-ON complexes in GaN grown by MOCVD [5]. For the optimized annealing condition, diode I-V measurements indicate clear p-n diode behavior with threshold voltages between 3.1–3.5 V and Ion/Ioff ratio of up to 106.This work is funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy under the PNDIODES programs directed by Dr. Isik Kizilyalli.[1] Y. Wang, et al., physica status solidi (b), 257.4, 1900705 (2020)[2] G. Alfieri, et al. J. Appl. Phys., 123, 205303 (2018)[3] R. Tanaka et al. Japan. J. Appl. Phys., 59 , SGGD02 (2020)[4] J. D. Greenlee, et al, ECS J. Solid State Sci Technol., 4(9) (2015)[5] K. Iwata et al, J. Appl. Phys., 127(10) (2020)[6] V. Meyers et al. in review, J. Appl. Phys. (2021) Figure 1

Stability of the CNHi Complex and the Blue Luminescence Band in GaN

The dissociation of the CNHi complex in GaN is studied in detail using photoluminescence measurements and first‐principles calculations. The blue luminescence (BL2) band with a maximum at 3.0 eV is caused by electron transitions from an excited state located at 0.02 eV below the conduction band to the ground state of the CNHi donor with the 0/+ level 0.15 eV above the valence band. The dissociation releases hydrogen atom, and the remaining CN defect with the −/0 state at 0.92 eV above the valence band is responsible for the yellow band (YL1) with a maximum at about 2.2 eV. The dissociation of the CNHi complex can be caused by the photoinduced defect reaction mechanism under UV illumination at low temperature (≈20 K), leading to the bleaching of the BL2 band and simultaneous rise in the YL1 band. The bleaching is reversible. Alternatively, the complex dissociates after annealing at temperatures above 600 °C. The activation energy of this process (3–4 eV, depending on the annealing geometry) corresponds to the removal of hydrogen from the sample and not to the dissociation of the complex itself.

Prediction of multiband luminescence due to the gallium vacancy–oxygen defect complex in GaN

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 bandgap. We study the spectroscopy of this complex using a hybrid quantum-mechanical molecular-mechanical embedded-cluster approach. We reveal how a single defect center can be responsible for multiband luminescence, including possible contributions to the ubiquitous yellow luminescence signatures observed in n-type GaN, owing to the coexistence of diffuse (extended) and compact (localized) holes.

A first-principles study of carbon-related energy levels in GaN. I. Complexes formed by substitutional/interstitial carbons and gallium/nitrogen vacancies

Various forms of carbon based complexes in GaN are studied with first-principles calculations employing Heyd-Scuseria-Ernzerhof hybrid functionals within the framework of the density functional theory. We consider carbon complexes made of the combinations of single impurities, i.e., CN−CGa, CI−CN, and CI−CGa, where CN, CGa, and CI denote C substituting nitrogen, C substituting gallium, and interstitial C, respectively, and of neighboring gallium/nitrogen vacancies (VGa/VN), i.e., CN−VGa and CGa−VN. Formation energies are computed for all these configurations with different charge states after full geometry optimizations. From our calculated formation energies, thermodynamic transition levels are evaluated, which are related to the thermal activation energies observed in experimental techniques such as deep level transient spectroscopy. Furthermore, the lattice relaxation energies (Franck-Condon shift) are computed to obtain optical activation energies, which are observed in experimental techniques such as deep level optical spectroscopy. We compare our calculated values of activation energies with the energies of experimentally observed C-related trap levels and identify the physical origins of these traps, which were unknown before.

Formation of Mn-Sin complexes in GaN: A first principles investigation

Using first principles calculations, we study the relative spatial distribution of Mn and Si impurities in the host matrix of GaN. It stems from our investigation that the co-doping of (Ga,Mn)N with Si will result in the formation of Mn-Si complexes with a large absolute pairing energy of 2.27eV. The formation of such complexes is energetically feasible already at the growth surface. According to our results, larger complexes involving two Si ions are also possible in bulk but exhibit a much smaller absolute pairing energy. Finally, the co-doping of (Ga,Mn)N with Si results in the reduction of the charge state of Mn from 3+ to 2+ regardless of the relative position of Si and Mn in the gallium sublattice.

Excitonic complexes in GaN/(Al,Ga)N quantum dots

Here we report a theoretical investigation of excitonic complexes in polar GaN/(Al,Ga)N quantum dots (QDs). A sum rule between the binding energies of charged excitons is used to calculate the biexciton binding energy. The binding energies of excitonic complexes in GaN/AlN are shown to be strongly correlated to the QD size. Due to the large hole localization, the positively charged exciton energy is found to be always blueshifted compared to the exciton one. The negatively charged exciton and the biexciton energy can be blueshifted or redshifted according to the QD size. Increasing the size of GaN/AlN QDs makes the identification of charged excitons difficult, and the use of an Al0.5Ga0.5N barrier can be advantageous for clear identification. Our theoretical results for the binding energy of exciton complexes are also confronted with values deduced experimentally for InAs/GaAs QDs, confirming our theoretical prediction for charged excitonic complexes in GaN/(Al,Ga)N QDs. Finally, we realize that the trends of excitonic complexes in QDs are significantly related to competition between the local charge separation (whatever its origin) and the correlation effect. Following our findings, entangled photons pairs can be produced in QDs with careful control of their size in order to obtain zero exciton–biexciton energy separation.

First‐principles identification of defect levels in Er‐doped GaN

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 ErGa–CN–VN, a defect complex of Er, a C impurity, and an N vacancy, and ErGa–ON–VN, a complex of Er, an O impurity, and an N vacancy, form defect levels at 0.18 eV and 0.46 eV below the conduction band, respectively. Together with ErGa–VN, a complex of Er and an N vacancy which has recently been found to produce a donor level at 0.61 eV, these defect complexes provide explanation for the Er‐related defect levels observed in experiments. The role of these defects in optical excitation of the luminescent Er center is also discussed.

TiGa–VN complexes in GaN: a new prospect of carrier mediated ferromagnetism

First principle investigations exploring the effects of nitrogen vacancies on ferromagnetism in Ti doped wurtzite GaN are reported.

Decrease in the binding energy of donors in heavily doped GaN:Si layers

The properties of Si-doped GaN layers grown by molecular-beam epitaxy from ammonia are studied by photoluminescence spectroscopy. It is shown that the low-temperature photoluminescence is due to the recombination of excitons bound to donors at Si-atom concentrations below 1019 cm−3. At a Si-atom concentration of 1.6 × 1019 cm−3, the band of free excitons is dominant in the photoluminescence spectrum; in more heavily doped layers, the interband recombination band is dominant. A reduction in the binding energy of exciton-donor complexes with increasing doping level is observed. With the use of Haynes rule, whereby the binding energy of the complex in GaN is 0.2 of the donor ionization energy ED, it is shown that ED decreases with increasing Si concentration. This effect is described by the dependence {ie1134-1}, where EDotp is the ionization energy of an individual Si atom in GaN. The coefficient that describes a decrease in the depth of the impurity-band edge with increasing Si concentration is found to be α = 8.4 × 10−6 meV cm−1.

First-principles calculation of positron states and annihilation parameters for group-III nitrides

We calculated theoretically positron states and annihilation parameters for VGa-C complexes in GaN. Positron lifetimes and Doppler-broadening S–W parameters were evaluated. The obtained results are compared other various types of defects in group-III nitrides. A random alloy In0.5Ga0.5N and its multilayer system together with GaN are investigated also. The results of the present study are expected to be useful for interpretation of experimental results on practical materials.

Functional Mn–Mgk cation complexes in GaN featured by Raman spectroscopy

The evolution of the optical branch in the Raman spectra of (Ga,Mn)N:Mg epitaxial layers as a function of the Mn and Mg concentrations, reveals the interplay between the two dopants. We demonstrate that the various Mn-Mg-induced vibrational modes can be understood in the picture of functional Mn–Mgk complexes formed when substitutional Mn cations are bound to k substitutional Mg through nitrogen atoms, the number of ligands k being driven by the ratio between the Mg and the Mn concentrations.

Ab initio study of metastability of Eu3+ defect complexes in GaN

Density functional theory (DFT) within the generalized gradient approximation (GGA) has been used to study the structural and electronic properties of Eu3+ defect complexes in GaN under Ga-rich conditions. Two distinct configurations of the EuGaVN defect complex, the axial and basal configuration, have been investigated. We report two forms of metastable defects namely; the Negative U defect in the lower half of the GaN band-gap and a metastable defect with two distinct configurations each with levels at EC −0.46eV and −0.56eV in the upper half of the GaN band-gap.

First principle study of Ce doping and related complexes in GaN

Using LDA+U method, we investigate the formation energy, electronic structure, and optical properties of rare earth Ce isolated substitutional and related complexes with neighboring N or Ga vacancies in wurtzite GaN. Our results reveal that CeGa introduces a localized defect level contributed by Ce-4f states in the band gap. The defect complexes CeGaVN and CeGaVGa introduce the shallow donor levels and acceptor levels, respectively. The doping of Ce in GaN gives rise to spin polarization. Both the substitutional Ce dopants and defect complexes can lead to new peaks from defect levels electronic transitions in the low energy regions of imaginary dielectric function and absorption spectrum.

Stable In-defect complexes in GaN and AlN

Abstract Perturbed angular correlation measurements in Indium implanted GaN and AlN have shown that after annealing up to 50% of the In probes occupy relatively undisturbed substitutional group III sites at 293 K while the remaining fraction traps a nearest neighbor point defect. Above 293 K this fraction decreases strongly until, at 1100 K, all In probes are found in undisturbed substitutional sites. The effect is completely reversible. A model involving an Indium–nitrogen vacancy complex is suggested to explain this behavior. Possibly this complex can act as radiative recombination center or as seed for phase segregation during growth.

Metal (Ni, Au)‐vacancy complexes in GaN

AbstractThe stability and electronic properties of metal (Ni, Au)‐vacancy complexes in GaN were studied using density functional calculations. Two complexes (NiGa‐VN and VGa‐Aui‐VN) were found to exhibit relatively low formation energies in their neutral charge state under p‐type conditions and are predicted to act as deep acceptors in GaN. As such they are unlikely to contribute to the hole carrier concentration in p‐GaN and are probably not responsible for the low electrical resistance of Au/Ni contacts. They may, however, act as compensation centres in n‐GaN. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Local vibrational modes of H complexes in Mg-doped GaN grown by molecular beam epitaxy

Local vibrational modes (LVM’s) in Mg-doped GaN grown by molecular beam epitaxy have been studied by Raman scattering. Besides Mg local vibrational modes, several local mode peaks associated with hydrogen complexes are observed in the region around 2200 cm−1 and peaks assigned to carbon-hydrogen complexes are also detected in the region around 2900 cm−1. These modes arise from the presence of unintentional carbon and hydrogen impurities in the sample, which was corroborated by secondary ion mass spectroscopy measurements. Raman scattering evidence of local vibrational modes of the C-H complex in GaN is reported. The behavior of the LVM’s associated with the hydrogen and carbon impurity complexes with annealing temperature is also reported.