Centers In GaN Research Articles

Ultrafast broadband carrier and exciton dynamics of Fe-related centers in GaN

The effect of Fe defects on carrier recombination and two-photon induced ultrafast exciton dynamics in GaN crystals were investigated using femtosecond transient absorption spectroscopy. The absorption kinetics exhibited completely different characteristics under different nonequilibrium carrier concentrations and distributions. The carrier recombination mechanisms under different excitation conditions can be interpreted by the model of Fe defect-related energy level. The absorption under one-photon excitation originates from the intraband transition of free holes, and the capture of holes by Fe2+ leads to the decay of the absorption. The ultrafast excitonic state [Fe2+,hVB] formation time under two-photon excitation results in a rapid absorption decay (<20 ps), while the long tail of the absorption response could be attributed to long-lived excitonic state (>100 ns). These findings are crucial to the applications of GaN for ultrafast optoelectronics and integrated nonlinear optics.

Effects of proton irradiation-induced point defects on Shockley–Read–Hall recombination lifetimes in homoepitaxial GaN p–n junctions

This work examined the intentional generation of recombination centers in GaN p–n junctions on freestanding GaN substrates. Irradiation with a 4.2 MeV proton beam was used to create a uniform distribution of vacancies and interstitials across GaN p+/n− and p−/n+ junctions through anode electrodes. With increasing proton dose, the effective doping concentrations were found to be reduced. Because the reduction in the doping concentration was much higher than the hydrogen atom concentration, this decrease could not be attributed solely to carrier compensation resulting from interstitial hydrogen atoms. In fact, more than half of the electron and hole compensation was caused by the presence of point defects. These defects evidently served as Shockley–Read–Hall (SRH) recombination centers such that the SRH lifetimes were reduced to several picoseconds from several hundred picoseconds prior to irradiation. The compensation for holes in the p−/n+ junctions was almost double that for electrons in the p+/n− junctions. Furthermore, the SRH lifetimes associated with p−/n+ junctions were shorter than those for p+/n− junctions for a given proton dose. These differences can be explained by variations in the charge state and/or the formation energy of intrinsic point defects in the p-type and n-type GaN layers. The results of the present work indicate the asymmetry of defect formation in GaN based on the fact that intrinsic point defects in p-type GaN readily compensate for holes.

Space-Charge-Limited Efficiency of Electrically-Injected Carriers Localization

The mechanism for the influence of defects in theGaN on the efficiency of localization of electrically-injected charge carriers in the InGaN|GaN quantum well is studied using tunnel spectroscopy of deep centers. It is found that the curves of the quantum efficiency and the spectral efficiency of radiative recombination dependencies on forward bias have the shape with maximum and humps at the biases corresponding to the impurity bands of color centers in GaN. The quantum efficiency droop with increasing bias is accompanied by a blue shift of the emission spectrum. We explain these effects based on the model of carrier localization in the quantum well, which takes into account the significant contribution to the tunneling transparency of the potential walls of the quantum well from ionized deep centers and their recharging with increasing forward bias. Keywords: gallium nitride, quantum well, charge carrier localization, tunneling, quantum effeciency, color centers.

Local strain-dependent Zeeman splitting in GaN:Eu

The electronic spins of rare-earth materials are attractive candidates for spin qubits and quantum memories. To access individual spins, tuning of the g-factor is desirable. Here, we report on local strain-dependent g-factors of the 5D0–7F2 transitions of Eu3+ centers in GaN:Eu thin films. We have found a clear correlation between the effective g-factor and the emission energy shift induced by the local strain. The combination of micro-photoluminescence and scanning electron microscope/electron backscattering diffraction measurements has revealed that the compressive strain of 0.2%–0.4%, relative to a surrounding reference point, induces an energy shift of about 3 meV. The strain decreases the g-factor of the emission at 1.991 eV from 2.5 to 1.5, while the strain increases the g-factor of the emission at 1.994 eV from 1.1 to 1.7. The result suggests that the g-factor can be tuned by the local strain. On the basis of the strain-induced energy shift and the g-factor, we have identified the optical sites. The 5D0–7F2 transitions observed in this study consist of three optical sites with C3v symmetry and one site with C1h symmetry.

Investigation of the yellow light center in GaN p-i-n structure grown by MOCVD with electro-optical measurements

Bu çalışmada, metal organik buhar fazlı epitaksi ile üretilmiş GaN p-i-n yapı, elektrolüminesans (EL) ve sıcaklığa akım-gerilim ölçümleriyle incelenmiştir. GaN p-i-n yapının, düz besleme geriliminde uzay yükü sınırlamalı bölgeye karşılık gelen 50 mA enjeksiyon akımı altında, tepe noktası 2.2 eV enerjisinde olan sarı ışık yaydığı gözlenmiştir. Ayrıca, uzay yükü sınırlamalı bölgeden türetilen mobilitenin, elektrik alan bağlılığının Poole-Frenkel türünde olduğu saptanmış ve termal enerji aralığı 0.299 eV olarak hesaplanmıştır. Hesaplanan bu enerji aralığı, EL ölçümünden elde edilen sonuçlarla desteklenmiştir.

Tuning the valence and concentration of europium and luminescence centers in GaN through co-doping and defect association

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 region. The Eu$^{2+}$/Eu$^{3+}$ ratio can be tuned by tuning the position of Fermi level and through defect association. We find co-doping with oxygen can facilitate the incorporation of Eu into the lattice. The unassociated Eu$_{\rm Ga}$ is an electrically and optically active defect center and its behavior is profoundly impacted by local defect--defect interaction. Defect complexes such as Eu$_{\rm Ga}$-O$_{\rm N}$, Eu$_{\rm Ga}$-Si$_{\rm Ga}$, Eu$_{\rm Ga}$-H$_i$, Eu$_{\rm Ga}$-Mg$_{\rm Ga}$, and Eu$_{\rm Ga}$-O$_{\rm N}$-Mg$_{\rm Ga}$ can efficiently act as deep carrier traps and mediate energy transfer from the host into the Eu$^{3+}$ $4f$-electron core which then leads to sharp red intra-$f$ luminescence. Eu-related defects can also give rise to defect-to-band luminescence. The unassociated Eu$_{\rm Ga}$, for example, is identified as a possible source of the broad blue emission observed in n-type, Eu$^{2+}$-containing GaN. This work calls for a re-assessment of certain assumptions regarding specific defect configurations previously made for Eu-doped GaN and further investigation into the origin of the photoluminescence hysteresis observed in (Eu,Mg)-doped samples.

Observation of sharp emission lines from Zn-doped GaN

We report the observation and characterization of sharp emission lines from Zn-related emission centers in GaN. Initial studies on the emission lines show that they appear only at low temperatures (T < 50 K), are energetically stable, and exhibit linewidths of a few meV. A brief discussion on their possible origins is given.

Photo-EPR study of compensated defects in Be-doped GaN substrates

Time-dependent photo-induced electron paramagnetic resonance measurements were performed on GaN crystals grown by the high nitrogen pressure solution method. A resonance was induced in Be-doped samples by illuminating with photon energies greater than 2.7 eV. The EPR was thermally stable at T=4K and was quenched with photon energies between 0.5 and 2.7 eV. Optical cross sections for EPR excitation and quenching processes were extracted from time-dependent measurements using a model for defect charge state populations. A fit to the cross sections was performed using a lineshape for photon absorption by deep centers in GaN. The threshold for photo-excitation of a compensated acceptor (A−+hν→A0+eCB−) was 2.5 eV followed by a lattice relaxation of 0.3 eV. The threshold for the complimentary process (A0+hν→A−+hVB+) was 0.75 eV with a relaxation energy of 0.3 eV. These values are consistent with CN being the EPR-active acceptor. Thermal quenching is attributed to delocalization of carriers from a beryllium-related electron trap, which are then captured by neutral CN.

Current Noise and Efficiency Droop of Light-Emitting Diodes in Defect-Assisted Carrier Tunneling from an InGaN/GaN Quantum Well

The current dependences of the spectral noise density and quantum efficiency in green and blue light-emitting diodes with InGaN/GaN quantum wells (QWs) are measured. It is shown that the noise level greatly increases at high currents at which there is a quantum efficiency droop. The mechanism by which the current noise is formed is associated with hopping transport via the deep states of color centers in GaN across the n barrier of an InGaN/GaN QW. The source of the noise is the hopping resistance of the space-charge region, which limits the current of thermally activated electrons into the QW. The efficiency droop and the increase in noise level are attributed to a change in the electric-field direction near the QW at high injection levels and to an increase in the tunneling leakage of holes from the QW. It is shown that the experimental frequency-related noise spectra having the shape of a Lorentzian spectrum at the working currents are related to the frequency of hopping between deep centers near the InGaN/GaN QW and to Maxwell relaxation in the space-charge region.

Токовый шум и падение эффективности светодиодов при туннелировании носителей из квантовой ямы InGaN/GaN с участием дефектов

AbstractThe current dependences of the spectral noise density and quantum efficiency in green and blue light-emitting diodes with InGaN/GaN quantum wells (QWs) are measured. It is shown that the noise level greatly increases at high currents at which there is a quantum efficiency droop. The mechanism by which the current noise is formed is associated with hopping transport via the deep states of color centers in GaN across the n barrier of an InGaN/GaN QW. The source of the noise is the hopping resistance of the space-charge region, which limits the current of thermally activated electrons into the QW. The efficiency droop and the increase in noise level are attributed to a change in the electric-field direction near the QW at high injection levels and to an increase in the tunneling leakage of holes from the QW. It is shown that the experimental frequency-related noise spectra having the shape of a Lorentzian spectrum at the working currents are related to the frequency of hopping between deep centers near the InGaN/GaN QW and to Maxwell relaxation in the space-charge region.

Self-Organized Eu-Doped GaN Nanocolumn Light-Emitting Diode Grown by RF-Molecular-Beam Epitaxy

High-concentration doping of Eu ions working as luminescent centers in GaN is required to improve light output of optical devices. GaN nanocolumns can be used to achieve high crystallinity in high-concentration Eu-doped GaN (GaN:Eu). In this study, optical properties of a GaN:Eu nanocolumn grown by RF-plasma-assisted molecular-beam epitaxy (RF-MBE) are investigated and GaN:Eu nanocolumn light-emitting diodes (LEDs) are demonstrated. Suppression of yellow luminescence and concentration quenching of PL intensity are observed at RT for GaN:Eu nanocolumns. When the effect of Eu concentration on optical properties is analyzed using rate equation, an increase in the Eu concentration is observed to affect 5D0 lifetime corresponding to back transfer, and not the effective cross-section corresponding to transfer efficiency. Based on these results, GaN:Eu nanocolumn LEDs are fabricated on an n-type (111) Si substrate. Clear rectification characteristics and sharp red luminescence are observed. The light output increases with increasing Eu concentration up to a concentration of 6 × 1020 cm−3. These results suggested that GaN:Eu nanocolumns provide feasible crystal to realize novel optical devices.

Excited states of neutral donor bound excitons in GaN

We investigate the excited states of a neutral donor bound exciton (D0X) in bulk GaN by means of high-resolution, polychromatic photoluminescence excitation (PLE) spectroscopy. The optically most prominent donor in our sample is silicon accompanied by only a minor contribution of oxygen—the key for an unambiguous assignment of excited states. Consequently, we can observe a multitude of Si0X-related excitation channels with linewidths down to 200 μeV. Two groups of excitation channels are identified, belonging either to rotational-vibrational or electronic excited states of the hole in the Si0X complex. Such identification is achieved by modeling the excited states based on the equations of motion for a Kratzer potential, taking into account the particularly large anisotropy of effective hole masses in GaN. Furthermore, several ground- and excited states of the exciton-polaritons and the dominant bound exciton are observed in the photoluminescence (PL) and PLE spectra, facilitating an estimate of the associated complex binding energies. Our data clearly show that great care must be taken if only PL spectra of D0X centers in GaN are analyzed. Every PL feature we observe at higher emission energies with regard to the Si0X ground state corresponds to an excited state. Hence, any unambiguous peak identification renders PLE spectra highly valuable, as important spectral features are obscured in common PL spectra. Here, GaN represents a particular case among the wide-bandgap, wurtzite semiconductors, as comparably low localization energies for common D0X centers are usually paired with large emission linewidths and the prominent optical signature of exciton-polaritons, making the sole analysis of PL spectra a challenging task.

Computational materials design for efficient red luminescence: InGaN codoped with Eu and the donor–acceptor pair of Mg and O

We propose that InGaN is superior to GaN as a host material for GaN-based red-light-emitting diodes (LEDs). In our previous paper, we proposed that codoping of Eu and a Mg and O pair generates an efficiently luminescent center in GaN. This is caused by the quantum confinement of the quantum dot constructions generated by the codoping method. The present report illustrates that InGaN allows the expansion of such electronic structures throughout the crystal owing to spontaneous phase decomposition. This can be used for self-organized fabrication and self-regenerated products.

Photoexcited carrier trapping and recombination at Fe centers in GaN

Fe doped GaN was studied by time-resolved photoluminescence (PL) spectroscopy. The shape of PL transients at different temperatures and excitation powers allowed discrimination between electron and hole capture to Fe3+ and Fe2+ centers, respectively. Analysis of the internal structure of Fe ions and intra-ion relaxation rates suggests that for high repetition rates of photoexciting laser pulses the electron and hole trapping takes place in the excited state rather than the ground state of Fe ions. Hence, the estimated electron and hole capture coefficients of 5.5 × 10−8 cm3/s and 1.8 × 10−8 cm3/s should be attributed to excited Fe3+ and Fe2+ states. The difference in electron capture rates determined for high (MHz) and low (Hz) (Fang et al., Appl. Phys. Lett. 107, 051901 (2015)) pulse repetition rates may be assigned to the different Fe states participating in the carrier capture. A weak temperature dependence of the electron trapping rate shows that the potential barrier for the multiphonon electron capture is small. A spectral feature observed at ∼420 nm is assigned to the radiative recombination of an electron in the ground Fe2+ state and a bound hole.

Electronic and optical characteristics of an m-plane GaN single crystal grown by hydride vapor phase epitaxy on a GaN seed synthesized by the ammonothermal method using an acidic mineralizer

Fundamental electronic and optical properties of a low-resistivity m-plane GaN single crystal, which was grown by hydride vapor phase epitaxy on a bulk GaN seed crystal synthesized by the ammonothermal method in supercritical ammonia using an acidic mineralizer, were investigated. The threading dislocation and basal-plane staking-fault densities of the crystal were around 104 cm−2 and less than 100 cm−1, respectively. Oxygen doping achieved a high electron concentration of 4 × 1018 cm−3 at room temperature. Accordingly, a photoluminescence (PL) band originating from the recombination of hot carriers was observed at low temperatures, even under weak excitation conditions. The simultaneous realization of low-level incorporation of Ga vacancies (VGa) less than 1016 cm−3 was confirmed by using the positron annihilation technique. Consistent with our long-standing claim that VGa complexes are the major nonradiative recombination centers in GaN, the fast-component PL lifetime of the near-band-edge emission at room temperature longer than 2 ns was achieved.

Hybrid density functional study of optically active Er3+ centers in GaN

Understanding the luminescence of GaN doped with erbium (Er) requires a detailed knowledge of the interaction between the rare-earth dopant and the nitride host, including intrinsic defects and other impurities that may be present in the host material. We address this problem through a first-principles hybrid density functional study of the structure, energetics, and transition levels of the Er impurity and its complexes with N and Ga vacancies, substitutional C and O impurities, and H interstitials in wurtzite GaN. We find that, in the interior of the material, Er$_{\rm Ga}$ is the dominant Er$^{3+}$ center with a formation energy of 1.55 eV; Er$_{\rm Ga}$-$V_{\rm N}$ possesses a deep donor level at 0.61 eV which can assist in the transfer of energy to the 4$f$-electron core. Multiple optically active Er$^{3+}$ centers are possible in Er-doped GaN.

Determination of the nitrogen vacancy as a shallow compensating center in GaN doped with divalent metals.

We report accurate energetics of defects introduced in GaN on doping with divalent metals, focusing on the technologically important case of Mg doping, using a model that takes into consideration both the effect of hole localization and dipolar polarization of the host material, and includes a well-defined reference level. Defect formation and ionization 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 ubiquitous 3.46eV photoluminescence line, a characteristic of all lightly divalent-metal-doped GaN materials that has also been shown to occur in pure GaN samples. Our results give a comprehensive explanation for the observed behavior of GaN doped with low concentrations of divalent metals in good agreement with relevant experiment.

First-principles theory of nonradiative carrier capture via multiphonon emission

We develop a practical first-principles methodology to determine nonradiative carrier capture coefficients at defects in semiconductors. We consider transitions that occur via multiphonon emission. Parameters in the theory, including electron-phonon coupling matrix elements, are computed consistently using state-of-the-art electronic structure techniques based on hybrid density functional theory. These provide a significantly improved description of bulk band structures, as well as defect geometries and wavefunctions. In order to properly describe carrier capture processes at charged centers, we put forward an approach to treat the effect of long-range Coulomb interactions on scattering states in the framework of supercell calculations. We also discuss the choice of initial conditions for a perturbative treatment of carrier capture. As a benchmark, we apply our theory to several hole-capturing centers in GaN and ZnO, materials of high technological importance in which the role of defects is being actively investigated. Calculated hole capture coefficients are in good agreement with experimental data. We discuss the insights gained into the physics of defects in wide-band-gap semiconductors, such as the strength of electron-phonon coupling and the role of different phonon modes.

Optical and magnetic resonance studies of Be-doped GaN bulk crystals

Low temperature photoluminescence (PL) and optically-detected magnetic resonance (ODMR) at 24GHz have been performed on a set of Be-doped GaN bulk crystals grown by the high pressure nitrogen solution method. Most notably, these crystals exhibit an intense 2.25eV “yellow” PL band that is currently of high interest for use as a converter in white light-emitting-diode applications. In addition, the samples investigated in this work are found to emit a broad “violet” PL band of varying intensity with peak energy near 3.16eV that is shown to be associated with a high concentration of residual Mg acceptor impurities as supported by secondary ion mass spectroscopy measurements. ODMR angular rotation studies obtained on the “yellow” and “violet” emission bands revealed two strong signals in each case and helped to provide information on the nature of these recombination processes. One feature, common to the ODMR of each PL band, has Zeeman splitting g-values of ~1.95 and is ascribed to residual Si and O shallow donor impurities. The second ODMR signal observed on the 2.25eV “yellow” PL exhibits nearly isotropic g-values of ~2.0 characteristic of deep centers in GaN and is tentatively associated with Be-related deep acceptors. In contrast, the second ODMR feature found on the 3.16eV “violet” PL has a larger degree of anisotropy with g║~2.1 and g⊥~2.0 and are similar to the g-values assigned previously to Mg shallow acceptors from ODMR studies of highly Mg-doped GaN heteroepitaxial layers with large degree (≥10%) of compensation.

Luminescent centers in GaN codoped with Eu and Mg: Calculation on density functional theory

We investigate GaN codoped with Eu and Mg from calculations on the density functional theory. As a result, we obtain the total energies that are almost independent of Mg positions relative to Eu positions. The result suggests the coexistence of the luminescent centers that are Eu–Mg complexes and typical Eu atoms at least. Eu ions are trivalent in both centers; hence, the magnetization in the ground state 7F0 is zero. Considering the low excited states via the Boltzmann distribution, we can expect the finite magnetization on each Eu atom, but the Curie temperature is unlikely to be room temperature.