Shallow Donors In GaN Research Articles

Pervasive Shallow Donor Impurities in GaN

Understanding the nature of defects and their role on fundamental physical properties of semiconductors is essential to obtain the necessary material control to reach the full realization of their technological potential. High-resolution infrared measurements in combination with high-sensitive SIMS of unintentionally doped GaN films unambiguously identified Si and O shallow donors and yielded their ground state binding energies. High-resolution, variable-temperature photoluminescence studies of recombination processes associated with excitons bound to donors leaving the donors in the ground states and excited states after exciton annihilation are in excellent agreement with the energies of donor intra-impurity transitions measured by infrared absorption. Photoluminescence studies of unintentionally doped and doped GaN films clarified conflicting aspects of donor identification and binding energies of shallow pervasive Si and O donors. These results provided the experimental foundation for the detection and identification of pervasive shallow donors in GaN by optical methods.

Hybrid functional calculations ofDXcenters in AlN and GaN

Using hybrid functional calculations, we investigate the formation of $DX$ centers in GaN and AlN. We find that O, Si, and Ge are shallow donors in GaN, but form stable $DX$ centers in AlN for Fermi-level positions near the conduction band. Using a linear interpolation, we estimate the composition at which the onset of $DX$ behavior will occur in AlGaN alloys. Based on these predictions, we identify Si as the most effective donor for high Al-content AlGaN, and explain a number of seemingly conflicting experimental results for Si-doped AlGaN. Although based on size matching Ge was expected to be superior to Si as a shallow donor, it actually turns out to be more prone to $DX$-center formation.

Silicon and Oxygen in High-Al-Content AlGaN: Incorporation Kinetics and Electron Paramagnetic Resonance Study

The high-Al-content AlxGa1-xN alloys, x>0.70, and AlN is the fundamental wide-band-gap material system associated with the technology development of solid-state LEDs operating at the short wavelengths in the deep-UV (λ < 280 nm). Yet, their properties are insufficiently understood. The present study is intended to bring elucidation on the long-time debated and much speculated Si transition from shallow donor in GaN to a localized deep DX defect in AlxGa1-xN alloys with increasing Al content. For that purpose electron paramagnetic resonance is performed on a particular selection of high-Al-content epitaxial layers of Al0.77Ga0.23N, alternatively Al0.72Ga0.28N, alloy composition.

The effects of oxygen co‐doping on structural, magnetic and optical properties of Mn‐doped GaN

ABSTRACTThe GaMnN and GaMnN: O films were obtained by annealing at different NH3 flow rates. When the NH3 flow rate was decreased to 20 sccm, the oxygen was co‐doped into GaMnN film and substituted the few host atoms, which were confirmed by the X‐ray Photoelectron Spectra. Moreover, the oxygen co‐doping in GaMnN film drastically enhanced the gain of the ferromagnetic (FM) state but oxygen as shallow donor in GaN offers little carriers. Hence, the enhanced FM state is not due to increase of carrier concentration. On optical properties, the oxygen co‐doping in GaMnN film will suppress the blue luminescence centered near 2.8 eV and make a blue shift of the photoluminescence spectra. Copyright © 2013 John Wiley & Sons, Ltd.

Theoretical studies of low strain n-type GaN co-doped by Si and Sn

Electronic properties of Si- and Sn-doped GaN were studied by the first-principles calculations. According to the calculated density of states (DOS and PDOS), it is found that both the substitutional Si and Sn act as shallow donors in GaN. Results from charge density difference contour maps and Milliken population analysis are in good agreement with the results from DOS. Formation energy, ionization energy, and bond length results show that co-doping of Si and Sn in GaN can reduce the strain, and thus increases the carrier mobility due to smaller distortion of the lattice. Finally, the optimum Si/Sn ratio in GaN is proposed.

Magnetopolaron effect on shallow donors in GaN

Resonant interaction between longitudinal-optic (LO) phonons and electrons bound on shallow donors in GaN is studied using magnetoluminescence of neutral-donor bound excitons $({D}^{0}X)$. The experiments were performed on high-quality freestanding GaN material and heteroepitaxial GaN layers grown on sapphire. In addition to the principal recombination channel of ${D}^{0}X$, in which donors are left in their ground states, two-electron satellites (TES) involving different excited donor excitations, as well as replicas of the principal ${D}^{0}X$ transition due to LO phonons, were observed for the oxygen and silicon donors. In order to separate transitions involving ground and excited ${D}^{0}X$ states, variable-temperature experiments were performed. The application of high magnetic fields allows tuning of the donor excitations into resonance with the LO phonon energy. This results in a strong enhancement of TES intensity and the appearance of several anticrossing processes in the vicinity of a LO phonon replica of the principal ${D}^{0}X$ transition. The observed behavior is explained in terms of a resonant interaction between LO phonons and donor-bound electrons (magnetopolaron effect). The experimental data are described using a phenomenological model that combines the theory of a hydrogen atom in a magnetic field with a model which includes the effects of nonparabolicity, nonresonant polaron corrections, and the resonant magnetopolaron effect on electrons bound to donors. It was found that the interaction in the resonant magnetopolaron regime is stronger for the oxygen than for the silicon donor.

A hydrogen-related shallow donor in GaN?

We present photoluminescence (PL) data for deliberately O-doped, high-resistive GaN samples where a new shallow donor-bound exciton (DBE) peak at about 3.4746 eV (corrected for strain shift) at 2 K appears. This DBE is strongly enhanced upon annealing in the entire range 450–900 °C. The possible relation of this DBE to a metastable H donor state is discussed.

Defects in wide band gap semiconductors: magneto-optical double resonance studies

Abstract Recent results obtained in the investigation of shallow donors in GaN with optical detection of electron paramagnetic resonance via the so-called yellow luminescence are reviewed as well as recent results on the silicon vacancy in SiC using optical detection of electron paramagnetic resonance via the magnetic circular dichroism of the absorption. The power of these double resonance methods is illustrated. The possibility of electrical detection of electron paramagnetic resonance in the electrical conductivity is also briefly discussed as future development.

Shallow donors in GaN

AbstractHigh‐resolution, variable temperature PL experiments were performed in the spectral region associated with recombination processes involving the ground and excited states of the neutral donor bound excitons. High‐resolution infrared measurements in combination with high‐sensitive SIMS unambiguously identified Si and O shallow donors and yield their ground state binding energies. These binding energies are in excellent agreement with values obtained by the analysis of the two‐electron‐satellite PL spectra considering the participation of ground and excited state donor bound excitons. This work clarifies conflicting aspects existing in donor identification and the binding energies of the impurities and excitons. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Donors in hydride-vapor-phase epitaxial GaN

Although considerable progress has been made in reducing background impurities and controlling optical and electronic properties of heteroepitaxial GaN layers over the last three decades, strong experimental evidence that clarifies the chemical identity of the shallow donors has been presented only recently. Variable temperature photoluminescence studies of recombination processes associated with excitons bound to donors and Fourier transform infrared (FTIR) measurements of donor absorption in hydride-vapor-phase epitaxial GaN are presented. PL studies of undoped and Si-doped homoepitaxial layers are consistent with and support the identification of Si and O as the dominants pervasive shallow donors in GaN, as measured by FTIR and secondary ion mass spectroscopy. Analyses of the two-electron satellite (2ES) transitions reveal the observation of a number of transitions associated with ground and excited states of both the donors and the donor-bound exciton complexes. All the 2ES lines observed in this work can be accounted for by recombination of excitons bound to Si and O donors and line positions are in excellent agreement with the energies of donor intraimpurity transitions measured previously by infrared absorption.

Point defect characterization of GaN and ZnO

Point defects are created in bulk ZnO and epitaxial GaN by 1–2 MeV electron irradiation at 300 K, and are studied by temperature-dependent Hall effect, photoluminescence, and deep level transient spectroscopy measurements. The N vacancy is identified as a fairly shallow donor in GaN, whereas defect identifications in ZnO are uncertain at this time. Both materials, but especially ZnO, are quite resistant to displacement damage.

Zeeman splitting of shallow donors in GaN

The Zeeman splitting of the donor spectra in cubic and hexagonal GaN is studied using an effective mass theory approach. Soft-core pseudopotentials were used to describe the chemical shift of the different substitutional dopants. The donor ground states calculated range from 29.5 to 33.7 meV, with typically 1 meV higher binding in the hexagonal phase. Carbon is found to produce the largest donor binding energy. The ionization levels and excited states are in excellent agreement with Hall and optical measurements, and suggest the presence of residual C in recent experiments.

Far-Infrared Magnetospectroscopy of Shallow Donors in GaN

We report a complete far-infrared investigation of shallow level centres in GaN. Transitions between the ground and excited states of shallow donors have been observed by means of transmission and far-infrared photoconductivity measurements — the latter for two configurations of the magnetic field relative to the crystal axis. The hitherto unobtainable high quality of the spectra allows, for the first time, a comparison with a theoretical model of shallow donors in a crystal with a wurtzite structure.

Dry Etching Damage Effect on Ohmic Characteristics of GaN

ABSTRACTWe report on the dry etching damage effects on the Ohmic contact characteristics of Sidoped n-type GaN. The 2-µm thick n-type GaN samples whose carrier concentrations are ∼7 × 1017/cm3 are grown on c-plane sapphire substrates by metalorganic chemical vapor deposition(MOCVD). The samples are processed to measure the transmission line method(TLM). We evaporated Ti/Al/Ni/Au=150/2700/400/4000 Å on the samples using an ebeam evaporator under 5 × 10−7 torr and annealed at 700 °C for lminute using a rapid thermal annealer(RTA). AES and XPS data show that the surface stoichiometry of the sample etched with H2 gas is gallium- richer than that of the one etched with Cl2 gas. According to the previous report[l], N-vacancy acts as shallow donor in GaN. However, specific contact resistivity of the sample etched with H2 gas is 3.03 × 10−6 Ω cm 2, which is much higher than that of the sample etched with C12 gas, 7.9 × 10 −7 Ω cm2. The difference of the N- vacancy formed during the etching process cannot completely explain the result. We suggest that formation of GaxOy during etching process, and the weakening of the bonding by ion bombardments might be responsible for the difference in the ohmic characteristics.

Zeeman spectroscopy of shallow donors in GaN

Gallium nitride grown by hydride vapor phase epitaxy has been studied by infrared spectroscopy in the region of donor intrasite electronic excitations as a function of magnetic field and temperature. Infrared-absorption features which are present only when the sample temperature is cooled to near 4 K have been observed. These features are identified as due to ${1s\ensuremath{-}2p}_{\ifmmode\pm\else\textpm\fi{}}$ transitions which split in a magnetic field, and a ${1s\ensuremath{-}2p}_{0}$ transition which does not split. A donor effective mass of ${0.22m}_{0}$ is calculated from the rate of splitting with field. We assume that the lower-energy transition occurs at an effective-mass donor and calculate donor binding energies of 31.1 and 33.8 meV for the two donors observed, and a static dielectric constant of 9.8.

Doping properties of C, Si, and Ge impurities in GaN and AlN

Doping properties of substitutional C, Si, and Ge impurities in wurtzite GaN and AlN were studied by quantum molecular dynamics. We considered incorporation of impurities on both cation and anion sublattices. When substituting for cations, Si and Ge are shallow donors in GaN, while Ge becomes a deep donor in AlN. Both impurities are deep acceptors on the N site. Substitutional C${}_{\mathrm{cation}}$ is a shallow donor in GaN, but a deep one in AlN; C${}_{\mathrm{N}}$ is a relatively shallow acceptor in both materials. Two effects that potentially quench doping efficiency were investigated. The first one is the transition of a donor from a substitutional position to a $\mathrm{DX}$-like configuration. In crystals with a wurtzite symmetry, there are two possible variants of a $\mathrm{DX}$-like state, and they have substantially different properties. In GaN, ${\mathrm{DX}}^{\ensuremath{-}}$ states of both Si and Ge are unstable, or metastable, and thus they do not affect doping efficiency. In contrast, they are stable in AlN, and therefore neither Si nor Ge is a dopant in this material. Estimates obtained for Al${}_{x}$Ga${}_{1\ensuremath{-}x}$N alloys show that the crossover composition for $\mathrm{DX}$ stability is much lower for Ge $(x\ensuremath{\simeq}0.3)$ than for Si $(x\ensuremath{\simeq}0.6)$. The second effect quenching the doping efficiency is self-compensation, i.e., simultaneous incorporation of impurity atoms on both cation and anion sublattice. This effect may be enhanced by the formation of nearest-neighbor donor-acceptor pairs. The calculated binding energies of such pairs are large, about 1 eV, influencing self-compensation in some cases. Finally, the computed formation energies are used to identify growth conditions under which all these impurities may be efficient dopants in wide-band-gap nitrides.

Electrical and optical properties of oxygen doped GaN grown by MOCVD using N2O

Oxygen doped GaN has been grown by metalorganic chemical vapor deposition using N2O as oxygen dopant source. The layers were deposited on 2″ sapphire substrates from trimethylgallium and especially dried ammonia using nitrogen (N2) as carrier gas. Prior to the growth of the films, an AIN nucleation layer with a thickness of about 300A was grown using trimethylaluminum. The films were deposited at 1085°C at a growth rate of 1.0 µm/h and showed a specular, mirrorlike surface. Not intentionally doped layers have high resistivity (>20 kW/square). The gas phase concentration of the N2O was varied between 25 and 400 ppm with respect to the total gas volume. The doped layers were n-type with carrier concentrations in the range of 4×1016 cm−3 to 4×1018 cm−3 as measured by Hall effect. The observed carrier concentration increased with increasing N2O concentration. Low temperature photoluminescence experiments performed on the doped layers revealed besides free A and B exciton emission an exciton bound to a shallow donor. With increasing N2O concentration in the gas phase, the intensity of the donor bound exciton increased relative to that of the free excitons. These observations indicate that oxygen behaves as a shallow donor in GaN. This interpretation is supported by covalent radius and electronegativity arguments.

Comment on “Shallow donors in GaN studied by electronic Raman scattering in resonance with yellow luminescence transitions” [Appl. Phys. Lett. 69, 1276 (1996)

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Response to “Comment on ‘Shallow donors in GaN studied by electronic Raman scattering in resonance with yellow luminescence transitions’ ” [Appl. Phys. Lett. 70, 909 (1997)

In their comment, Siegle et al. present Raman and photoluminescence ~PL! data from samples grown either on GaAs by molecular beam epitaxy ~MBE! or on sapphire by metal organic vapor phase epitaxy and hydride vapor phase epitaxy. They observe electronic Raman scattering ~ERS! peaks like those reported in Ref. 2 and claim, however, our assignments to be incorrect. Siegle et al. report peaks at 60 ~a!, 95 ~b!, 102.5 ~g!, 125 ~d!, and 250 cm ~e! in addition to the ERS peaks at 151 ~A*!, 189 (A), 218 ~B*!, and 237 cm (B) discussed in Ref. 2 and assert that all these peaks have the same origin. In fact, we also observe peaks b and d in the spectra of our GaN samples. According to our model, we expect a very similar Raman resonance behavior for different donor species with sufficiently low binding energy. Therefore, the argument based on the resonance behavior is invalid in attributing all the observed peaks to the same origin. If the same origin is assumed for all peaks, their relative intensities should be nearly constant for different samples. This is by far not the case as shown, e.g., in Fig. 1 of Ref. 2 for peaks A and B vs A* and B* as well as for peaks b vs d in Fig. 1 ~compare also Figs. 1 and 2 in Ref. 1!. In contrast to the ERS peaks discussed in Ref. 2, peak d has been observed also in p-type GaN and shows a weak temperature dependence. Therefore, it is not even clear whether peak d is due to ERS. Siegle et al. further claim as a main statement that the ERS peaks are only observable for GaN grown on GaAs. However, we observe the ERS peaks reported in Ref. 2 also in GaN layers after removal of the GaAs substrate @Fig. 1~c!# demonstrating that they do not originate from GaAs. It is furthermore known that N impurities in GaAs introduce a resonant energy level in the conduction band which is not occupied under normal conditions. Moreover, peak B* was observed previously by absorption spectroscopy and, more recently, peaks A* and B* by Raman scattering from hexagonal GaN grown on sapphire. These findings completely rule out any connection with GaAs and therefore refute the statement of Ref. 1. The observability of the ERS peaks rather depends on the donor concentration and the Fermi level in the GaN layer. For too small donor concentrations, the intensity of the corresponding Raman excitations vanishes below the detection limit. At very large concentrations, a pronounced formation of impurity level bands is expected to destroy the observability of individual electronic transitions. In addition, the resonance enhancement should be only observable for sufficiently high concentrations of the involved deep level defect. In this respect, arguments based

Shallow donors in GaN studied by electronic Raman scattering in resonance with yellow luminescence transitions

Electronic excitations in nominally undoped GaN have been investigated by Raman scattering. Peaks in the range between 18.5 and 30 meV have been assigned to internal shallow donor transitions in cubic and hexagonal GaN, respectively. The photon energy dependence of the scattering efficiencies in the cubic phase is explained by Raman scattering in resonance with the so-called ‘‘yellow’’ luminescence transitions. This interpretation supports models for the notorious yellow luminescence in which shallow donors are involved. These shallow donors most likely do not originate from native point defects.