Centers In GaN Research Articles

Photoluminescence studies of rare earth (Er, Eu, Tm) in situ doped GaN

The emission properties of rare earth (RE)-doped GaN are of significant current interest for applications in full color displays, white lighting technology, and optical communications. We are currently investigating the photoluminescence (PL) properties of RE (Er, Eu, Tm)-doped GaN thin-films prepared by solid-source molecular beam epitaxy. The most intense visible PL under above-gap excitation is observed from GaN:Eu (red: 622 nm) followed by GaN:Er (green: 537 nm, 558 nm), and then GaN:Tm (blue: 479 nm). In this paper, we present spectroscopic results on the Ga-flux dependence of the Er 3+ PL properties from GaN:Er and we report on the identification of different Eu 3+ centers in GaN:Eu through high-resolution PL excitation (PLE) studies. In addition, we observed an enhancement of the blue Tm 3+ PL from AlGaN:Tm compared to GaN:Tm. Intense blue PL from Tm 3+ ions was also obtained from AlN:Tm under below-gap pumping.

Er doped GaN by Gas Source Molecular Beam Epitaxy on GaN Templates

AbstractSince the work of Favennec et al.[1] it is well known that the quenching of luminescence from rare earth ions decreases with the host band gap. This has led to a large activity with silicon implanted or doped with RE, and then GaAs was used, in hope to realize simple, cheap light emitters. With a band gap of 3.4 eV at room temperature, GaN is even better suited to such applications. As a matter of fact, Steckl et al.[2] have demonstrated a green light emitting device based on Er doped GaN. This resulted in a renewed effort in this direction, but the crystal quality still have to be mastered and the physical phenomenon involved to be understood. In this work, GaN and Er-doped GaN with various Er concentrations were grown by gas source molecular beam epitaxy on high quality GaN templates grown by metalorganic chemical vapour deposition. In order to understand the influence of the Er incorporation on the crystal quality of GaN, Er-doped GaN were grown with a concentration between 0.1% and 5%. High quality undoped GaN were also grown, as a reference material, to show how the smallest amount of Er may affect drastically the structural and optical properties. All the samples were characterized by scanning electron microscopy, atomic force microscopy and X-ray diffraction. With these measurements, we demonstrate a strong correlation between the Er concentration and the surface roughness and the crystalline quality. This study shows that the activation of the Erbium luminescence is not improved with improving crystal quality. This assumption supports the idea that Er luminescence should be related to defect center in GaN.

Silicon behavior in GaN grown by radiofrequency plasma molecular beam epitaxy

The behavior of Si in GaN has been investigated in the wide doping range from 1×10 17 to 9×10 19 cm −3 by photoluminescence (PL). PL studies at 77 K for doped specimens show that the intensity of the near-band-edge emission increased monotonically as the Si concentration increases. The intensity of the yellow luminescence (YL), however, remains almost constant, which indicates that Si does not strongly influence the density of deep centers in GaN. The red-shift of the near-band-edge emission due to increase the Si concentration shows that bandgap renormalization (BGR) effect is much more important than the Burstein–Moss (B-M) effect in Si-doped GaN. With Si doping as high as 9×10 19 cm −3, a new peak due to donor-to-acceptor recombination appears in the PL spectrum. This donor-to-acceptor emission peak can also be observed in the PL spectrum of Si-doped GaN after thermal annealing. We attributed the appearance of the new acceptor level to the formation of Si N. A model has been proposed to illustrate the Si behavior in GaN in a wide range of Si concentrations.

Threading Dislocations and Optical Properties of GaN and GaInN

We categorized threading dislocations in GaN and GaInN multiple quantum wells and epitaxially lateral overgrown GaN into three types of line defects (edge, screw and mixed dislocations), and investigated the optical properties. It was confirmed by cathodoluminescence measurements that not only screw and mixed dislocations but also edge dislocations act as non-radiative centers in GaN. Epitaxial lateral overgrowth (ELO) technique can reduce the densities of all line-defects in a several μm wide wing region. Growth steps in the wing region were disturbed by the defects which were left in a seed region, and a complicated structure was formed at the surface of GaN and GaInN layers grown on ELO-GaN at low temperature. We believe that this surface structure formed by high supersaturation is a cause of In compositional spatial fluctuation and phase separation of GaInN alloy.

Electrical properties and spectra of deep centers in GaN p-i-n rectifier structures

GaN p-i-n rectifiers with 4 μm thick i-layers show typical reverse breakdown voltages of 100–600 V. We have studied the temperature dependence of current-voltage characteristics in these diodes, along with hole diffusion lengths and the deep level defects present. Generally we find that i-layer background doping varies significantly (from <1014 cm−3 to 2–3×1016), which influences the current conduction mechanisms. The hole diffusion lengths were in the range 0.6–0.8 μm, while deep level concentrations were ∼1016 cm−3.

Evolution of deep centers in GaN grown by hydride vapor phase epitaxy

Deep centers and dislocation densities in undoped n GaN, grown by hydride vapor phase epitaxy (HVPE), were characterized as a function of the layer thickness by deep level transient spectroscopy and transmission electron microscopy, respectively. As the layer thickness decreases, the variety and concentration of deep centers increase, in conjunction with the increase of dislocation density. Based on comparison with electron-irradiation induced centers, some dominant centers in HVPE GaN are identified as possible point defects.

Synthesis and Luminescent Properties of GaN and GaN-Mn Blue Nanocrystalline Thin-Film Phosphor for FED

AbstractThe technologies of fabrication of thin film phosphors based on gallium nitride using rf-magnetron sputtering are developed and the structural properties of these films are studied. Luminescence and electron spin resonance (ESR) spectra of GaN and GaN-Mn thin films are obtained. A correlation between cathodo-luminescence intensity and conductivity of GaN films is found. The nature of emission centers in GaN and GaN-Mn thin films is discussed and a mechanism of luminescence in these films is proposed.

Sites and motion of Mu − defect centers in n-type gallium nitride

We have investigated the sites and motional properties of Mu − defect centers in GaN using muon nuclear-quadrupole level-crossing resonances (QLCR) and zero-field muon-spin depolarization measurements. The QLCR spectra imply that Mu − occupies interstitial locations next to Ga atoms, consistent with the two inequivalent Ga anti-bonding sites for the Wurtzite structure. The Mu − related depolarization shows an increase in average dipolar fields near 200 K and indicates motion above 500 K. We discuss the properties of the two separate Mu − states obtained from a model-dependent constrained fit to the zero-field data.

Correlation of vibrational modes and DX-like centers in GaN[formula omitted

Vibrational modes in O-doped GaN have been observed at 544 cm−1 in Raman spectroscopy. Under perturbation of large hydrostatic pressure the mode appears as a set of three different lines Q1⋯3 whose relative intensities change by pressure. A switching between the modes occurs near 10 and 20 GPa and is found to correlate with the electron capture process to the DX-like state of O. We employ a simple oscillator model to predict the vibrational frequencies of ON. A localization energy of 23 cm−1 with respect to the optical phonon band is predicted. This is in reasonable agreement with the observed vibrational frequencies. Therefore, we assign the Q modes to the local vibration of O on N site in GaN. Modes Q1⋯3 are tentatively assigned to three different charge states of the O defect center.

DX CENTERS IN AlGaN

In this letter, recent theoretical and experimental investigations of DX centers in Al x Ga 1-x N are reviewed. Due to the technological importance of III–V nitride semiconductors, studies of deep-level defects in AlGaN have attracted a great deal of interest. Oxygen impurities form DX centers in GaN under hydrostatic pressure and in Al x Ga 1-x N alloys. For GaN under pressures greater than 20 GPa, the DX level emerges from the conduction band, leading to a decrease in the free-electron concentration. The localization of free carriers leads to a decrease in the far-infrared absorption and an increase in the LO Raman peak intensity. In Al x Ga 1-x N alloys, Hall effect and persistent photoconductivity measurements indicate that the DX state is energetically favorable for x&gt;0.3. The experimental data for oxygen DX centers are in excellent agreement with first-principles calculations. Experiments have shown that silicon remains a shallow donor up to at least x=0.5 and theory indicates that it may remain shallow up to x=1.

Role of the Substitutional Oxygen Donor in the Residual N-Type Conductivity in GaN

A detailed photoluminescence (PL) study reveals a striking similarity in local vibrational properties of a defect center in GaN as compared to that for the substitutional OP donor in GaP. This observation could be interpreted as if the center is in fact related to the substitutional oxygen donor in GaN. The deep-level nature experimentally determined for the defect center calls for caution of a commonly referred model that the substitutional oxygen donor is responsible for the residual n-type conductivity in GaN.

Deep centers and their spatial distribution in undoped GaN films grown by organometallic vapor phase epitaxy

Deep traps in undoped n-GaN layers grown by organometallic vapor phase epitaxy on sapphire substrates were studied by temperature dependent conductivity, photoinduced current transient spectroscopy (PICTS), thermally stimulated current, electron beam induced current (EBIC), and band edge cathodoluminescence (CL) methods. Presence of electron traps with energy levels 0.1–0.2 eV below the conduction band and hole traps with energy levels of about 0.25, 0.5, and 0.85 eV above the valence band edge was detected. CL and EBIC measurements show that the deep recombination centers in GaN are distributed inhomogeneously with well defined cellular pattern. Both carrier lifetime and luminescence intensity are enhanced at cell walls indicating lower density of recombination centers. However, the density of main hole trap (0.85 eV) is enhanced in these regions as determined by local PICTS measurements. Photoconductivity in many GaN samples exhibits very long decay times at temperatures between 100 and 300 K. The effect most probably is not related to shallow donors such as silicon, but rather is associated with unidentified deep centers with a 0.2 eV barrier for electron capture.

Role of the Substitutional Oxygen Donor in the Residual N-Type Conductivity in GaN

AbstractA detailed photoluminescence (PL) study reveals a striking similarity in local vibrational properties of a defect center in GaN as compared to that for the substitutional Op donor in GaP. This observation could be interpreted as if the center is in fact related to the substitutional oxygen donor in GaN. The deep-level nature experimentally determined for the defect center calls for caution of a commonly referred model that the substitutional oxygen donor is responsible for the residual n-type conductivity in GaN.

Stability of deep donor and acceptor centers in GaN, AlN, and BN

We examine the atomic and electronic structure of substitutional Be, Mg, and C acceptor impurities and of Si, Ge, S, and O donor impurities in GaN, AlN, and BN through first-principles calculations. The small bond lengths in III-V nitrides are found to inhibit large lattice relaxations around impurities and, with a few exceptions, this leads to a significant stabilization of effective-mass states over deep centers despite the large band gaps of these materials. In particular, we find that Ge and S impurities do not have stable or even metastable DX centers in GaN, AlN, or BN, independent of crystal structure. Similarly, the effective-mass states of Be, Mg, and C acceptor impurities in GaN are found to be more stable than the corresponding deep AX centers. We propose that persistent photoconductivity in Mg-doped GaN arises from the bistability of a N-site vacancy that is accompanied by a charge-state change from +3e to +1e.

Ultraviolet and violet light-emitting GaN diodes grown by low-pressure metalorganic chemical vapor deposition

Both metal-insulator-semiconductor and p-n junction electroluminescence have been observed in thin-film, metalorganic chemical vapor deposition-grown GaN diodes thermally annealed in N2. UV radiation, peaking near 380 nm, is emitted when electrons are injected from the undoped, n-type material into the Mg-doped, p-type GaN. Violet light, peaking near 430 nm, is obtained by injecting electrons into p-type material from either n-type material or non-ohmic metal contacts. The present results support and extend earlier interpretations of the nature of the recombination centers in GaN.