Acceptors In GaN Research Articles

Photoluminescence from CdGa and HgGa acceptors in GaN

Photoluminescence from GaN implanted with Cd or Hg ions was studied and compared with first-principles calculations. In Cd-implanted GaN, the blue band (BLCd) with a maximum at 2.7 eV is attributed to the CdGa acceptor with an ionization energy of 0.55 eV. In Hg-implanted GaN, the green band (GLHg) with a maximum at 2.44 eV is attributed to the HgGa acceptor with an ionization energy of 0.77 eV. The shapes of the BLCd and GLHg bands are asymmetric, with a similar Franck–Condon shift of about 0.28 eV. The electron- and hole-capture coefficients for the CdGa and HgGa acceptors are found. The experimentally found parameters agree reasonably well with first-principles calculations using HSE hybrid functional satisfying the generalized Koopmans' theorem.

Overcoming the compensation of acceptors in GaN:Mg by defect complex formation

In GaN:Mg, the MgGa acceptor is compensated extensively by the formation of nitrogen vacancies (VN) and Mg interstitials (Mgi). However, we show that such compensation can be overcome by forming two kinds of Mg-rich complexes: one that contains VN and the other that contains only MgGa and Mgi. Such complexing not only neutralizes VN and Mgi but also forms better complex acceptors that have lower formation energies and smaller hole localization energies than isolated MgGa. Our results help explain the different doping behaviors in samples grown by different methods.

Photoluminescence related to Ca in GaN

The ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor in GaN was studied using photoluminescence (PL) experiments and first-principles calculations. The experimentally found $\text{--}/0$ transition level of the ${\mathrm{Ca}}_{\mathrm{Ga}}$ at 0.50 \ifmmode\pm\else\textpm\fi{} 0.02 eV above the valence band reasonably agrees with the value calculated using the Heyd-Scuseria-Ernzerhof functional tuned to fulfill the generalized Koopmans condition (0.70 eV). The ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor is responsible for the green luminescence band (labeled ${\mathrm{GL}}_{\mathrm{Ca}}$) with a maximum at 2.5 eV. The electron- and hole-capture coefficients for the ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor are obtained from PL experiments: ${C}_{n}=9\ifmmode\times\else\texttimes\fi{}{10}^{\text{--}14}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}/\mathrm{s}$ and ${C}_{p}=6\ifmmode\times\else\texttimes\fi{}{10}^{\text{--}7}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}/\mathrm{s}$. The ${\mathrm{GL}}_{\mathrm{Ca}}$ band is an efficient radiative recombination channel in GaN samples implanted with Ca and in unintentionally doped GaN grown by molecular-beam epitaxy at relatively low temperatures, where Ca is a common contaminant.

Determination of the types of optical transitions and concentrations of donors and acceptors in GaN by the dependence of photoluminescence intensity on the excitation power

The results of the calculated and experimental depen-dence of the photoluminescence intensity on the excitation power density for silicon-doped GaN layers grown by molecular beam epitaxy are presented. A model was constructed for transitions in a compensated semiconductor upon interband generation of electron-hole pairs. It is shown that the dependence of the photoluminescence intensity on the excitation power density can be used to determine the recombination mechanism and concentrations of donors and acceptors in semiconductor. Keywords: GaN, ammonia-MBE, photoluminescence, heterostructures, point defects.

Stoichiometric imbalances in Mg-implanted GaN

Using numerical integration of the Boltzmann transport equation, depth distributions of recoil-implanted Ga and N atoms in Mg-implanted GaN are studied. Mg implantation into GaN is found to produce significant nonuniform stoichiometric distribution as a consequence of the recoil process, as compared to other III–V materials of GaP, GaAs and GaSb. Our calculation also indicates that the stoichiometric imbalance introduced by Mg implantation in GaN is insensitive to the changes in the implanted ion energy and the atom density of the target material. These results suggest that stoichiometric imbalance may be one of the principal reasons why the electrical activation of implanted Mg acceptors in GaN is rather difficult as compared to that in other III–V semiconductors.

Passivation of the beryllium acceptor in GaN and a possible route for p-type doping

Using theory and experiment, we address a possible p-type doping route of GaN using the Be acceptor. Hybrid functional calculations suggest that straightforward incorporation of Be in GaN will likely lead to compensated high resistivity samples. In this case, beryllium interstitials and nitrogen vacancies are the most likely compensating donors. However, calculations also show that simultaneous incorporation of beryllium and hydrogen in the nitrogen-rich growth regime is likely to lead to the formation of neutral BeGa-Hi complexes. These complexes are energetically favorable and do not exhibit any transition levels in the bandgap. Subsequent annealing in hydrogen-free or nitrogen ambient will dissociate these complexes and activate shallow BeGa acceptors in GaN. Experimental measurements, where an ultraviolet photoluminescence band related to the shallow BeGa acceptor emerges after annealing the Be-doped GaN, confirm these findings.

Design guidelines suppressing dynamic punch-through in GaN vertical MOSFETs by considering the Poole–Frenkel effect

Due to the large ionization energy of Mg acceptors in GaN, dynamic punch-through will occur in vertical GaN MOSFETs. To avoid this, higher doping and/or a thicker p-body region should be utilized. However, this increases the channel resistance. In this letter, we suggest that the Poole–Frenkel (P–F) effect has significant impact on dynamic punch-through because of the high electric field in the depletion region under a large bias voltage. Systematic TCAD simulations of simplified vertical GaN MOSFET structures were carried out. We show that the device design considering the P–F effect results in a reduction in the increase in channel resistance.

Shallow and Deep States of Beryllium Acceptor in GaN: Why Photoluminescence Experiments Do Not Reveal Small Polarons for Defects in Semiconductors.

Currently, only one shallow acceptor (Mg) has been discovered in GaN. Here, using photoluminescence (PL) measurements combined with hybrid density functional theory, we demonstrate that a shallow effective-mass state also exists for the Be_{Ga} acceptor. A PL band with a maximum at 3.38eV reveals a shallow Be_{Ga} acceptor level at 113±5 meV above the valence band, which is the lowest value among any dopants in GaN reported to date. Calculations suggest that the Be_{Ga} is a dual-nature acceptor with the "bright" shallow state responsible for the 3.38eV PL band, and the "dark," strongly localized small polaronic state with a significantly lower hole capture efficiency.

Electrical and Optical Properties of Beryllium Deep Acceptors in GaN

Beryllium (Be) may be a substitute for GaN p-type dopant magnesium due to its smaller atomic size and higher electronegativity. However, Be has been shown to be a deep acceptor in recent experiments. By using the hybrid functional based on density functional theory (DFT), we predict two acceptors with symmetric-broken configurations (SB1 and SB2) in neutral charge state of Be substitutional doping, and analyze their optical and electronic properties. The ionization energy and emission peak of the symmetric-broken configuration SB2 are almost the same as the ground-state (neutral charge state) configuration SB1. Our work provides a comprehensive understanding of the deep acceptor behavior of Be in wurtzite GaN.

Anisotropic optical phonons in MOCVD grown Si-doped GaN/Sapphire epilayers

Micro-Raman spectroscopy is employed to study the anisotropic optical phonons of Si-doped GaN/Sapphire epifilms grown by metal organic chemical vapor deposition method. In an undoped 3.6 μm thick sample – our polarized Raman measurements in the backscattering geometry revealed major first order modes of GaN and sapphire. Careful analyses of the second-order Raman spectra using critical-point-phonons from a rigid-ion-model fitted inelastic X-ray spectroscopy data with appropriate selection rules helped us attain expedient data for the lattice dynamics of GaN. In Si-doped films, a modified phonon confinement model is used for simulating Raman line shapes of E2high phonons to monitor crystalline quality. While the optical phonons in lightly doped samples are coupled to electron plasma– at higher carrier concentration the over-damped A1(LO) mode vanished in the background. For each sample we assessed the transport parameters by simulating Raman profiles of A1(LO) line shape with contributions from plasmon-LO-phonon and Lorentzian shaped Eg sapphire mode. A realistic Green’s function theory is adopted to study the vibrational modes of Si donors and Mg acceptors in GaN by including force constant changes estimated from lattice relaxations using first-principles bond-orbital model. Theoretical results of impurity-activated modes compared favorably well with the existing Raman scattering data.

Koopmans’ tuning of HSE hybrid density functional for calculations of defects in semiconductors: A case study of carbon acceptor in GaN

Hybrid density functional theory has become a standard method for calculations of defects in semiconductors. The majority of work in this field is done using hybrid functionals tuned to reproduce the experimental bandgap of the host material. This approach usually yields results in reasonable agreement with the experiment. Alternatively, hybrid functional can be tuned to fulfill the generalized Koopmans' condition for defect orbitals, which cancels self-interaction energy and restores the linear behavior of energy with respect to electron occupation. Here, we investigate the methods of hybrid functional tuning, which both satisfy the generalized Koopmans' condition and reproduce the experimental bandgap, using one of the most well-studied defects in GaN, carbon acceptor. We test different charged defect correction schemes, the influence of Ga3d-electrons, and compare the results with accurate photoluminescence measurements. We find that using different charged defect correction methods can lead to substantially different hybrid functional parametrizations. However, the calculated optical properties of the carbon acceptor are found to be weakly dependent on specific parameters.

Electric-field-induced simultaneous diffusion of Mg and H in Mg-doped GaN prepared using ultra-high-pressure annealing

To investigate Mg diffusion during ultra-high-pressure annealing, which activates Mg acceptors in GaN, GaN samples with p–n junctions prepared via epitaxial growth were annealed at 1573 K under 1 GPa. The profiles of Mg diffusion toward the underlying n-type layer cannot be explained by a simple diffusion model. We found that H atoms diffused along with Mg atoms. By considering the suppressed diffusion of positively charged interstitial H atoms due to the electric field in the depletion layer, we could better reproduce the Mg–H diffusion profiles, suggesting that H atoms play a key role in the Mg diffusion process.

Hysteretic Photochromic Switching (HPS) in Doubly Doped GaN(Mg):Eu-A Summary of Recent Results.

Europium is the most-studied and least-well-understood rare earth ion (REI) dopant in GaN. While attempting to increase the efficiency of red GaN light-emitting diodes (LEDs) by implanting Eu+ into p-type GaN templates, the Strathclyde University group, in collaboration with IST Lisbon and Unipress Warsaw, discovered hysteretic photochromic switching (HPS) in the photoluminescence spectrum of doubly doped GaN(Mg):Eu. Our recent work, summarised in this contribution, has used time-, temperature- and light-induced changes in the Eu intra-4f shell emission spectrum to deduce the microscopic nature of the Mg-Eu defects that form in this material. As well as shedding light on the Mg acceptor in GaN, we propose a possible role for these emission centres in quantum information and computing.

Eu–Mg defects and donor–acceptor pairs in GaN: photodissociation and the excitation transfer problem

We have investigated the temperature-dependent photoluminescence (TDPL) profiles of Eu3+ ions implanted in an HVPE-grown bulk GaN sample doped with Mg and of donor–acceptor pairs (DAP) involving the shallow Mg acceptor in GaN(Mg) (unimplanted) and GaN(Mg):Eu samples. Below 125 K, the TDPL of Eu3+ in GaN(Mg):Eu correlates with that of the DAP. Below 75 K, the intensity of Eu3+ emission saturates, indicating a limitation to the numbers of Eu–Mg defects available to receive excitation transferred from the host, while the DAP continues to increase, albeit more slowly in the implanted than the unimplanted sample. Prolonged exposure to UV light at low temperature results in the photodissociation of Eu–Mg defects in their Eu1(Mg) configuration, with a corresponding increase in shallow DAP emission and the emergence of emission from unassociated EuGa (Eu2) defects.

Identification of yellow luminescence centers in Be-doped GaN through pressure-dependent studies

Effective acceptor doping of wide-band-gap semiconductors is still an outstanding problem. Beryllium has been suggested as a shallow acceptor in GaN, but despite sporadic announcements, Be-induced p-type doping has never been practically realized. Be-doped GaN possesses two luminescence bands; one at 3.38 eV and a second near 2.2 eV at an energy close to that of the parasitic yellow luminescence often found in undoped GaN crystals. We have performed high hydrostatic pressure studies of bulk, Be-doped gallium nitride crystals using the diamond anvil cell technique. We observed a splitting of the yellow luminescence line under hydrostatic pressure into two components, one which is strongly dependent on applied pressure and another whose pressure dependence is more modest. Together with hybrid functional calculations, we attribute the strongly-varying component to the beryllium–oxygen complex. The second component of the yellow luminescence possesses very similar pressure behavior to the yellow luminescence observed in undoped samples grown by the same method, behavior which we find consistent with the CN acceptor. At higher pressure, we observe the vanishing of yellow luminescence and a rapid increase in luminescence intensity of the UV line. We explain this as the pressure-induced transformation of the Be–O complex from a highly localized state with large lattice relaxation to a delocalized state with limited lattice relaxation.

A first-principles study of carbon-related energy levels in GaN. II. Complexes formed by carbon and hydrogen, silicon or oxygen

This work presents an in-depth investigation of the properties of complexes composed of hydrogen, silicon, or oxygen with carbon, which are the 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, binding energies, and both thermal and optical transition levels were obtained. The calculated energy levels have been systematically compared with the experimentally observed carbon related trap levels. Furthermore, we computed vibrational frequencies for selected defect complexes and defect concentrations were estimated in the low, mid, and high carbon doping scenarios considering two different cases where electrically active defects: (a) only carbon and vacancies and (b) not only carbon and vacancies but also hydrogen, silicon, and oxygen. We confirmed that CN is a dominant acceptor in GaN. In addition to it, a substantial amount of SiGa–CN complex exists in a neutral form. This complex is a likely candidate for the unknown form of carbon observed in undoped n-type GaN.

Hysteretic photochromic switching of Eu-Mg defects in GaN links the shallow transient and deep ground states of the Mg acceptor

Although p-type activation of GaN by Mg underpins a mature commercial technology, the nature of the Mg acceptor in GaN is still controversial. Here, we use implanted Eu as a ‘spectator ion’ to probe the lattice location of Mg in doubly doped GaN(Mg):Eu. Photoluminescence spectroscopy of this material exemplifies hysteretic photochromic switching (HPS) between two configurations, Eu0 and Eu1(Mg), of the same Eu-Mg defect, with a hyperbolic time dependence on ‘switchdown’ from Eu0 to Eu1(Mg). The sample temperature and the incident light intensity at 355 nm tune the characteristic switching time over several orders of magnitude, from less than a second at 12.5 K, ~100 mW/cm2 to (an estimated) several hours at 50 K, 1 mW/cm2. Linking the distinct Eu-Mg defect configurations with the shallow transient and deep ground states of the Mg acceptor in the Lany-Zunger model, we determine the energy barrier between the states to be 27.7(4) meV, in good agreement with the predictions of theory. The experimental results further suggest that at low temperatures holes in deep ground states are localized on N atoms axially bonded to Mg acceptors.

C-Implanted N-Polar GaN Films Grown by Metal Organic Chemical Vapor Deposition**Supported by the National Natural Science Foundation of China under Grant Nos 61204006, 61574108, 61334002, 61474086 and 51302306.

C-implantation N-polar GaN films are grown on c-plane sapphire substrates by metal organic chemical vapor deposition. C-implantation induces a large number of defects and causes disorder of the lattice structure in the N-polar GaN film. Raman measurements performed on the N-polar GaN film before C-implantation after C-implantation and subsequent annealing at 1050°C for 5 min indicate that after annealing the disordered GaN lattice is almost recovered. High resolution x-ray diffraction shows that after implantation there is an obvious increase of screw-dislocation densities, and the densities of edge dislocation show slight change. Carbon implantation can induce deep acceptors in GaN, thus the background carriers induced by the high oxygen incorporation in the N-polar GaN film will be partially compensated for, resulting in 25 times the resistivity, which is demonstrated by the temperature-dependent Hall-effect measurement.

P-type doping of GaN by magnesium ion implantation

Magnesium ion implantation has been performed on a GaN substrate, whose surface has a high thermal stability, thus allowing postimplantation annealing without the use of a protective layer. The current–voltage characteristics of p–n diodes fabricated on GaN showed distinct rectification at a turn-on voltage of about 3 V, although the leakage current varied widely among the diodes. Coimplantation with magnesium and hydrogen ions effectively suppressed the leakage currents and device-to-device variations. In addition, an electroluminescence band was observed at wavelengths shorter than 450 nm for these diodes. These results provide strong evidence that implanted magnesium ions create acceptors in GaN.

Anisotropic polaron localization and spontaneous symmetry breaking: Comparison of cation-site acceptors in GaN and ZnO

The behavior of cation substitutional hole doping in GaN and ZnO is investigated using hybrid density functional calculations. Our results reveal that Mg substitution for Ga $({\mathrm{Mg}}_{\mathrm{Ga}})$ in GaN can assume three different configurations. Two of the configurations are characterized by the formation of defect-bound small polaron (i.e., a large structural distortion accompanied by hole localization on one of the neighboring N atoms). The third one has a relatively small but significant distortion that is characterized by highly anisotropic polaron localization. In this third configuration, ${\mathrm{Mg}}_{\mathrm{Ga}}$ exhibits both effective-mass-like and noneffective-mass-like characters. In contrast, a similar defect in ZnO, ${\mathrm{Li}}_{\mathrm{Zn}}$, cannot sustain the anisotropic polaron in the hybrid functional calculation, but undergoes spontaneous breaking of a mirror symmetry through a mechanism driven by the hole localization. Finally, using ${\mathrm{Na}}_{\mathrm{Zn}}$ in ZnO as an example, we show that the deep acceptor levels of the small-polaron defects could be made shallower by applying compressive strain to the material.