Centers In GaN Research Articles

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.

The nature of Cr center in GaN: Magnetic anisotropy of GaN:Cr single crystals

Magnetization measurements of the strain-free bulk GaN:Cr single crystals of wurtzite structure are reported. Strong magnetic anisotropy at low temperatures (2–10 K) was observed. The data were analyzed assuming Cr2+(d4) configuration. The crystal field model taking into account cubic field of tetrahedral symmetry, trigonal field along the c-axis simulating hexagonal structure, tetragonal static Jahn-Teller distortion, and the spin-orbit interaction provide a good description of the experimental magnetization data.

Formation energy of optically active Er3+ centers in Er doped GaN

Erbium doped GaN (GaN:Er) and low In-content InxGa1−xN (x∼0.05) epilayers were synthesized by metal organic chemical deposition. The 1.54 μm PL emission intensity was monitored for GaN:Er epilayers grown at different growth temperatures and utilized to establish a value of 1.8 ± 0.2 eV for the formation energy (EF) of the optically active Er3+ centers in GaN. The optically active Er+ centers are presumably Er and nitrogen vacancy (Er-VN) complexes. The experimentally measured value of the EF of the optically active Er3+ centers is about 0.98 eV larger than the calculated formation energy of Er ions at Ga sites; however, it is 1.1–2.2 eV lower than the formation energy of VN in GaN. Due to the large EF values, relatively high growth temperatures are required to improve the 1.54 μm emission efficiency in GaN:Er.

Spin-polarization of VGaON center in GaN and its application in spin qubit

VGaON center in cubic gallium nitride is a defect complex composing of a substitutional oxygen atom at nitrogen site (ON) and an adjacent gallium vacancy (VGa). Based on first-principles calculations, we predicted that this VGaON center has much in common with the famous nitrogen-vacancy center in diamond, but the excitation energy is very low. The electron spin-polarization of the centers can be tuned by changing the charge states. The neutral ONVGa center has the v↓ and exy↓ states being well isolated from the bulk bands with appropriate spacing which are suitable for achieving spin qubit operation with low excitation energy.

Lattice site location of optical centers in GaN:Eu light emitting diode material grown by organometallic vapor phase epitaxy

Eu-doped GaN was grown by organometallic vapor phase epitaxy at temperatures from 900 to 1100 °C. Eu incorporation is influenced by temperature with the highest concentration found for growth at 1000 °C. In all samples, Eu is incorporated entirely on substitutional Ga sites with a slight displacement which is highest (∼0.2 Å) in the sample grown at 900 °C and mainly directed along the c-axis. The major optical Eu3+ centers are identical for in situ doped and ion-implanted samples after high temperature and pressure annealing. The dominant Eu3+ luminescence lines are attributed to isolated, substitutional Eu.

Photoionization study of deep centers in GaN∕AlGaN multiple quantum wells

Transient photocapacitance (TPC) measurements were performed to investigate deep centers in GaN∕AlGaN multiple quantum wells. The influence of the persistent photovoltaic effect was successfully separated during the TPC experiments. The resolution obtained by the TPC measurements is much better than that of steady-state photocapacitance. The spectral dependence of photoionization cross section of deep centers in GaN is quantitatively determined in the energy range from 1.68to3.30eV. The absolute values of photoionization cross sections of these centers are found to be of the order of 10−15–10−14cm2.

V N-Mg defect complexes as compensating centers in GaN:Mg

Defect equilibrium diagrams have been constructed for intrinsic and extrinsic defects in GaN:Mg crystals using the quasi-chemical formalism, and the formation of (VN-MgGa)× and (VN-MgGa••) defect complexes has been analyzed. The results indicate that, under equilibrium conditions, VN vacancies in the charge state 3+ and Mg′Ga impurities form (VN-MgGa)•• defect complexes, which compensate acceptors.

Spectroscopic and energy transfer studies of Eu3+ centers in GaN

Photoluminescence (PL), photoluminescence excitation (PLE), and time-resolved PL spectroscopies have been carried out at room temperature and 86K on transitions from D25, D15, and D05 excited states to numerous FJ7 ground states of Eu-doped GaN films grown by conventional solid-source molecular beam epitaxy (MBE) and interrupted growth epitaxy MBE. Within the visible spectral range of 1.8–2.7eV, 42 spectral features were observed and assignments were attempted for each transition. PL and PLE indicate that four Eu3+ centers exist in the GaN lattice whose relative concentration can be controlled by the duration of growth interruption. The energy levels for these four sites are self-consistently obtained, and time-resolved photoluminescence measurements reveal details about the radiative and nonradiative relaxations of excitation among these levels. The data indicate a near-resonant cross relaxation among these sites. The D25 and D15 states are observed to decay nonradiatively by filling the D05 state with characteristic times of 2.4 and 2.8μs, respectively. The D05 state is found to relax in a manner that depends slightly on the final state and dopant site.

Magnetoluminescence Studies of GaN:Fe

We report on magneto-optical studies on iron doped GaN crystals grown using hydride vapor phase epitaxy method on bulk GaN substrate. The investigated samples showed an intensive 1.3 eV luminescence band, characteristic of Fe(d) center in GaN. A high quality of the investigated samples allowed us to observe a well-resolved fine structure of intracenter transitions between T1(G) and A1(S) states, consisting of four sharp no-phonon lines. All the observed no-phonon lines showed pronounced splittings in magnetic field. From the analysis of the magneto-optical data, the structure of split T1(G) multiplet in the magnetic field applied along c-axis of GaN crystals was established.

Photoinduced transient spectroscopy of defect centers in GaN and SiC

The potentialities of photoinduced transient spectroscopy in terms of investigation of defect centers in wide-band-gap semiconductors are presented. An experimental system dedicated to measurements of the photocurrent transients at temperatures 20–800 K is described and a new approach to extraction of trap parameters from the photocurrent relaxation waveforms recorded in a selected temperature range is presented. The approach is based on the two-dimensional analysis of the waveforms as a function of time and temperature using the correlation procedure. As a result, three-dimensional images showing the temperature changes of the emission rate for detected defect centers are produced and a neural network method is applied to determine the parameters of defect centers. The new approach is exemplified by studies of defect centers in high-resistivity GaN: Mg and semi-insulating 6H-SiC: V.

Theoretical analysis of defect formation in GaN:Mg crystals

The method of quasi-chemical reactions was used to construct the diagrams of point-defect equilibrium in GaN and GaN:Mg at 1400 K. According to these diagrams, it is impossible to obtain GaN with hole conductivity in equilibrium conditions. The nitrogen vacancies are shown to be the main acceptor-compensating centers in GaN.

Mn- and Fe- doped GaN for spintronic applications

AbstractIn the context of ferromagnetic spin-coupling in dilute magnetic semiconductors, we present optical investigations on Mg co-doped GaMnN and Fe doped GaN. A complex luminescence feature occurring in Mg co-doped GaMnN around 1 eV was previously attributed to the internal 4T2(F)—4T1(F) transition of Mn4+ involved in different complexes. Selective excitation studies indicate the presence of at least three different complexes. Photoluminescence excitation spectra suggest that the internal Mn3+ transition may represent an excitation mechanism. Magneto photoluminescence spectra indicate equal g values for the ground and excited state. Low temperature infrared absorption spectra of Fe doped GaN allow to unambiguously establish the electronic structure of the Fe2+ center in GaN. Our results suggest that the Fe2+(5T2) state is stabilized against Jahn-Teller coupling by the reduced site-symmetry of the hexagonal lattice.

Investigation of molecular co‐doping for low ionization energy p‐type centers in GaN

This work initiates an investigation of molecular co-doping to produce p-type centers in (Ga,Al)N with ionization energies lower than Mg. Dopant complexes can be formed between a doubly ionized acceptor such as (Li or Ag) and a singly ionized donor (silicon). Ion implantation of Li and Ag into silicon doped GaN films grown by Metalorganic Chemical Vapor Deposition (MOCVD) has been performed. High resolution X-ray diffraction was used to characterize the crystalline damage and subsequent recovery upon anneal. A complete recovery was observed after annealing at 850 °C. Higher temperature anneal was required to activate the implanted Si dopants and was also performed with the co-doped samples. Conductivity measurements – Hall and Van der Pauw – were employed to analyze the conductivity type of these samples. None of the samples analyzed showed any indication of p-type conductivity. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High Resolution Site-Selective Studies of Erbium-Centers in GaN and GaN:Mg.

AbstractIn this work the high resolution optical spectroscopy is used in order to determine the positions of the Stark split 4I15/2, 4I11/2 and 4I9/2 energy levels of Er3+ in GaN and GaN:Mg. Photoluminescence and photoluminescence excitation spectra were measured at energies corresponding to the 4I15/2→4I9/2 and 4I15/2→4I11/2 absorption transitions. Low Er implant doses were applied to reduce the number of possible defects. In undoped GaN only a single Er-center was observed and no influence of Mg doping on the energy level splittings of this center in GaN:Mg was found. Numerical analysis based on point charge model was used to calculate parameters of the local crystal field (CF) acting on Er3+ ions. The splittings of the 4I9/2 and 4I11/2 energy levels were calculated in weak CF approach and good agreement with experimental results was obtained. The calculations confirmed that the symmetry of the erbium center in hexagonal GaN is C3v.

Excitation-Wavelength Dependent and Time-Resolved Photoluminescence Studies of Europium Doped GaN Grown by Interrupted Growth Epitaxy (IGE)

AbstractThe emission properties of Eu doped GaN thin films prepared by interrupted growth epitaxy (IGE) were investigated through excitation-wavelength dependent and time-resolved photoluminescence (PL) studies. Under above-gap excitation (333-363 nm) large differences were observed in the Eu3+ PL intensity and spectral features as a function of Ga shutter cycling time. The overall strongest red Eu3+ PL intensity was obtained from a sample grown with a Gashutter cycling time of 20 minutes. The main Eu3+ emission line originating from 5D0→ 7F2 transition was composed of two peaks located at 620 nm and 622 nm, which varied in relative intensity depending on the growth conditions. The room-temperature emission lifetimes of the samples were non-exponential and varied from ∼50 νs to ∼200 νs (1/e lifetimes). Under resonant excitation at 471 nm (7F0→5D2) all samples exhibited nearly identical PL spectra independent of Ga shutter cycling time. Moreover, the Eu3+ PL intensities and lifetimes varied significantly less compared to above-gap excitation. The excitation wavelengths dependent PL results indicate the existence of different Eu3+ centers in GaN: Eu, which can be controlled by the Ga shutter cycling time.

Elementary blue-emission bands in the luminescence spectrum of undoped gallium nitride films

Inhomogeneous broadening of the blue-emission band in the luminescence spectrum of nominally undoped gallium nitride films grown on substrates of sapphire with orientation (0001) and silicon with orientation (001) using chemical vapor deposition is observed. Studies of the emission spectra under different conditions of excitation of GaN films made it possible to detect three elementary bands with peaks at 2.65, 2.84, and 3.01 eV in the blue region of luminescence of these films at room temperature. Assumptions are made about the types of intrinsic and impurity defects involved in the formation of various centers in GaN as sources of blue emission.

Muonium as a shallow center in GaN.

A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter was observed for the first time in single-crystalline GaN below 25 K. It has a highly anisotropic hyperfine structure with axial symmetry along the <0001> direction, suggesting that it is located either at a nitrogen-antibonding or a bond-centered site oriented parallel to the c axis. Its small ionization energy (<or=14 meV) and small hyperfine parameter (approximately 10(-4) times the vacuum value) indicate that muonium in one of its possible sites produces a shallow state, raising the possibility that the analogous hydrogen center could be a source of n-type conductivity in as-grown GaN.

Site symmetry of erbium centers in GaN

In this work site selective excitation at an energy range corresponding to the 4 I 15/2 → 4 I 9/2 transition of Er 3+ was applied in order to determine the site symmetry of luminescent erbium centers in cubic and hexagonal GaN layers. From the analysis of the Stark splitting it was found that in zinc blende GaN Er occupies a nitrogen interstitial site. A small deviation from cubic symmetry, best described by a tetragonal distortion, was observed, possibly related to the layer strain. In wurtzite GaN Er was found to occupy substitutional Ga sites, with C 3v symmetry.

Fine Structure of Effective Mass Acceptors in Gallium Nitride

Magnetoluminescence of the exciton bound to a neutral acceptor was measured to investigate the electronic structure of a shallow acceptor center in GaN. The application of magnetic fields along different directions with respect to the crystal c axis allowed us to determine the symmetry of the ground (Gamma(9)) and the first excited state (Gamma(7)) of the acceptor. The observed Zeeman splitting pattern has axial symmetry but can be explained well only by assuming a significant reduction of the spin-orbit interaction for this acceptor state. Because of this reduction, the energy structure of the neutral acceptor is found to be very sensitive to any local, axial perturbation.

Photoluminescence in platinum doped GaN

An epitaxial GaN layer was doped with the radioactive isotope 191Pt by ion implantation at the online mass separator facility ISOLDE at CERN (energy 60keV, maximum dose of 3×1012ions/cm2). The isotope 191Pt decays with a half-life of 2.9d into stable 191Ir. After annealing at 1300K several Pt-related photoluminescence transitions in the near-infrared region are detected. Platinum is found to produce near-infrared luminescence consisting of two single transitions (Pt1) at 1.461 and 1.446eV followed by a set of transitions (Pt2) starting at 1.273eV each separated by 15meV in energy. The photoluminescence spectra recorded within 30 days after implantation will be presented. The half-lives resulting from exponential fits to the intensities of these optical transitions yield half-lives of t1/2(Pt1)=(2.6±0.6)d and t1/2(Pt2)=(3.1±0.3)d, respectively, in very good agreement with the nuclear half-life of the isotope 191Pt. Therefore, the recombination centers responsible for this near-infrared luminescence have to involve exactly one Pt atom. We do not observe any other changes in the whole spectral region between 0.9 up to 3.5eV, in particular no new lines turn up, so we conclude that iridium does not introduce any optically active recombination centers in GaN.