Gallium Sites Research Articles

Formation energies of metal impurities in GaN

We present ab initio local density pseudopotential calculations on neutral nickel, titanium and gold impurities in GaN. Formation energies are calculated for substitution on the gallium site M Ga, the nitrogen site M N and for incorporation in the octahedral interstitial site M i. These elements are commonly employed for metal contacts in GaN-based LED, LD and FET devices and may be present in significant concentrations within the GaN lattice due to the use of high anneal temperatures. The majority of defects studied were found to have a high formation energy in excess of 4 eV. Ti Ga is found to have a particularly low formation energy of 1.2 eV and can be expected to incorporate readily into GaN. Also, deep gap states are found to be introduced by Au i,Ti i,Ni i,Ti N,Au Ga and by Ni Ga. Ti Ga and Ni Ga are found to act as single donors whereas Ni N and Au N act as double donors.

Carrier relaxation dynamics for As defects in GaN

Long decay times in the 50–150 ns range have been measured for the characteristic blue photoluminescence that peaks at 2.6 eV in GaN:As. We interpret these long decay times according to the theoretical predictions that this blue photoluminescence is caused by the incorporation of arsenic on the gallium site. The long decay times are characteristics of the large lattice relaxation for such a deep donor with a negative-U center behavior.

Ab initio study of substitutional nitrogen in GaAs

We investigate the atomic geometry, formation energies, and electronic structure of nitrogen occupying both arsenic and gallium sites in GaAs (NAs and NGa) using first-principles total-energy calculations. We find that both neutral defects induce impurity-like empty levels in the band gap acting as acceptors. While NAs shows a s-like a1 level in the middle of the band gap, NGa shows a p-like t2 level close to the bottom of the conduction band. The gap level of NAs gives theoretical support for the experimentally observed band-edge redshift on the GaAsN alloy for a N concentration ∼3%. Strong inward relaxations preserving the Td symmetry characterize the NAs equilibrium geometry in all the charge states investigated. In contrast, NGa exhibits a structural metastability in neutral charge state and Jahn–Teller off-center distortions in negative charge states forming a negative-U center. Formation energies of competing NAs and NGa defects are also discussed.

Site-selective electron nuclear double resonance in the exchange-narrowed regime of the ESR in conducting solids.

Electron nuclear magnetic double resonance on conduction electrons reveals the hyperfine interaction hidden by the fast electron spin exchange. We used the Overhauser shift technique to investigate the electron spin density of the conduction band of gallium oxide, beta-Ga(2)O(3). Due to the monoclinic structure, the conduction band of beta-Ga(2)O(3) is anisotropic and it is dominated by contributions from the two nonequivalent Ga sites. The large quadrupole couplings of the two gallium isotopes (69)Ga and (71)Ga (both with I = 3/2) are completely resolved in our double-resonance experiments. This resolved quadrupole interaction allows the determination of the electric field gradients at both gallium sites with high precision and high sensitivity. The resolved quadrupole splitting is the key to the site-selected determination of the hyperfine interaction. The concepts behind these double-resonance techniques are rather general and should be applicable in similar semiconductor systems.

SITE-SPECIFIC SURFACE CHEMISTRY OF GaAs (001)

In this article, we summarize our studies of the surface chemistry of gallium arsenide as it pertains to the metal organic chemical vapor deposition of compound semiconductors. It has been found by scanning tunneling microscopy and vibrational spectroscopy that the adsorption of reactant molecules on reconstruted GaAs (001) surfaces is "site-specific." The adsorption sites on the semiconductor surface are revealed by the vibrational spectrum of adsorbed hydrogen. Studies of arsine adsorption have shown that it dissociatively adsorbs only on gallium sites and transfers hydrogen to the neighboring As atom. Studies of carbon doping with carbon tetrachloride have shown that adsorbed chlorine attacks the exposed gallium and generates volatile GaCl x species. The site-specific nature of this reaction leads to a dramatic change in the film morphology, with the formation of etch pits primarily distributed along the step edges.

From crystalline to glassy gallium fluoride materials: an NMR study of [formula omitted] and [formula omitted] quadrupolar nuclei

Owing to the implementation of acquisition techniques specific for nuclei with very large quadrupolar interaction (full shifted echo and variable offset cumulative spectra (VOCS)), NMR spectra of 69 Ga and 71 Ga are obtained in crystallised (PbGaF 5, Pb 3Ga 2F 12, Pb 9Ga 2F 24 and CsZnGaF 6) and glassy (PbF 2–ZnF 2–GaF 3) gallium fluorides. Simulations of both static (full echo or VOCS) and 15 kHz MAS spectra allow to obtain consistent determinations of isotropic chemical shifts and very large quadrupolar parameters ( ν Q up to 14 MHz). In the crystalline compounds whose structures are unknown, the number and the local symmetry of the different gallium sites are tentatively worked out. For the glassy systems, a continuous Czjzek's distribution of the NMR quadrupolar parameters accounts for the particular shape of the NMR spectrum.

Site-specific chemistry of gallium arsenide metalorganic chemical vapor deposition

Herein, we summarize our studies of the surface chemistry of gallium arsenide as it pertains to the metalorganic chemical-vapor deposition of compound semiconductors. It has been found by scanning tunneling microscopy and vibrational spectroscopy that the adsorption of reactant molecules on reconstructed GaAs (001) surfaces is site-specific. Trimethylgallium dissociatively adsorbs only on arsenic sites, whereas arsine dissociatively adsorbs only on gallium sites. The decomposition of one precursor molecule (Ga(CH 3 ) 3 or AsH 3 ) creates the site for the decomposition of the other molecule (AsH 3 or Ga(CH 3 ) 3 , respectively). In this fashion, the crystal grows one atomic layer at a time. Studies of carbon doping with carbon tetrachloride have shown that adsorbed chlorine attacks the exposed gallium and generates volatile GaCl x , species. The site-specific nature of this reaction leads to a dramatic change in the film morphology, with the formation of 30-nm etch pits randomly distributed over the surface.

Electrical and Optical Properties of Mg Doped MBE Grown Cubic GaN Epilayers

Mg doping of cubic GaN epilayers grown by rf plasma-assisted molecular beam epitaxy on (100) GaAs is investigated by secondary ion mass spectroscopy (SIMS), low temperature photoluminescence (PL) and Hall-effect measurements. SIMS measurements show that the amount of incorporation of Mg saturates and is limited to a value of about 5 × 1018 cm−3 at substrate temperatures of 720 °C. Besides a shallow acceptor level at EA ≅ 0.230 eV PL spectra reveal an additional Mg-related deeper defect band indicating an incorporation of Mg at off gallium sites or as complexes. The thermal activation energy of the shallow Mg acceptor is measured by temperature dependent Hall effect to be 0.110 eV.

Crystal Structure Studies by Single-Crystal NMR Spectroscopy. 71Ga and 69Ga Single-Crystal NMR of β-Ga2O3 Twins

71Ga and 69Ga single-crystal NMR spectra of a β-Ga2O3 twin have been analyzed in terms of the 71Ga/69Ga quadrupole coupling and chemical shielding anisotropy (CSA) interactions. Analyses of the magnitudes and orientations of the interaction tensors for the two distinct crystallographic gallium sites in the two twin crystals resulted in accurate values for the parameters describing the quadrupole coupling and CSA tensors. Moreover, it has been possible to determine the actual twin-law of our sample crystal. Along with a recent 71Ga single-crystal NMR investigation of YGG, this work shows that gallium CSA generally has to be taken into account in addition to the dominant quadrupole coupling interaction.

Preparation and Crystal Structure Analysis of Ba2BiGa11O20

X-ray structure investigations of ternary BaO–Ga2O3crystals grown in a Bi2O3flux led to discovery of a quaternary compound of ideal composition Ba2BiGa11O20. Single-crystal X-ray diffraction indicates a monoclinic unit cell with parametersa=14.9283(14) Å,b=11.7046(8) Å,c=5.1170(5) Å, andβ=91.137(7)°. The structure is similar to that of Ba3TiAl10O20and related compounds. Three different refinement trials were completed, involving different treatments of positioning of bismuth or gallium atoms shifted from normal barium and gallium sites. The refinement selected as representing the best model has only one bismuth atom positioned near a barium site at a displacement of 0.495 Å. A crystal chemical analysis based on the X-ray data suggests that the displacement of bismuth atoms is due to the stereochemical activity of the 6s2lone pair. X-ray refinement was carried out by full-matrix least squares onF2on all data, to giveR1=0.0689 andwR2=0.1802 for 104 parameters and 1361 independent reflections. The final position assignments were analyzed via bond valence sum calculations. Other crystal data:M=1570.58, space groupI2/m(No. 12),V=893.92(14) Å3,Z=2,Dc=5.835 g cm−3, MoKα=0.71073 Å,μ=30.506 mm−1, 2θmax=59.98°.

Degradation of commercial high-brightness GaP:N green light emitting diodes

Deep level transient spectra, electroluminescence spectra, and light output versus current were measured on diodes degraded by a current stress. It has been shown that the drop in electroluminescence efficiency can be related to a new deep level formed by recombination enhanced reaction from the electron trap attributed to the nitrogen pair in the phosphorus site with silicon in the nearest gallium site.

Theoretical study of antisite arsenic incorporation in the low temperature molecular beam epitaxy of gallium arsenide

A stochastic model for simulating the surface growth processes in the low temperature molecular beam epitaxy of gallium arsenide is developed, including the presence and dynamics of a weakly bound physisorbed state for arsenic. The physisorbed arsenic is allowed to incorporate into the arsenic site or gallium site (antisite) and evaporate. Additionally, the antisite As is allowed to evaporate from the surface of the crystal. The arsenic flux, temperature and growth rate dependences of antisite arsenic (AsGa) concentration and the resultant % lattice mismatch obtained from our simulation are in excellent agreement with the experimental results. The activation energy of 1.16 eV for the evaporation of antisite arsenic from the crystal obtained from our model is in good agreement with theoretical estimates. At a constant substrate temperature and growth rate (Ga flux rate), the antisite arsenic concentration and hence, the % lattice mismatch increase with arsenic flux in the low flux regime and saturate for high flux regime. The critical arsenic flux at which the AsGa concentration and the % lattice mismatch saturate, increases with temperature. The AsGa concentration and % lattice mismatch saturate at lower values for higher temperatures. As the arsenic flux increases, the coverage of the physisorbed layer increases and at a critical flux dictated by the fixed temperature and growth rate, the coverage saturates at its maximum value of unity (a complete monolayer) and hence, the concentration of AsGa and % lattice mismatch saturate. Lower AsGa concentration and % lattice mismatch result at higher temperature due to more evaporation of AsGa from the surface of the growing crystal. Additionally, an analytical model is developed to predict the AsGa concentration and % lattice mismatch for various growth conditions.

27Aland69Gaimpurity nuclear magnetic resonance in ZnO:Al and ZnO:Ga

We report ${}^{27}\mathrm{Al}$ and ${}^{69}\mathrm{Ga}$ impurity and ${}^{67}\mathrm{Zn}$ host species NMR experiments on ZnO powders containing Al or Ga dopants in the range 0.03--3 at. %. Impurity resonance shift and nuclear spin-lattice relaxation data are analyzed to yield values of the impurity nucleus chemical shift, Knight shift, and hyperfine coupling. The electric quadrupole coupling of dilute ${}^{69}\mathrm{Ga}$ in ZnO is determined to be ${(e}^{2}{qQ/h)}_{69}=3.9\ifmmode\pm\else\textpm\fi{}0.3$ MHz. The concentration dependence of the Knight shifts and relaxation rates indicate rapid development of an impurity band as the dopant concentration falls below about 0.5 at. %. At all concentrations investigated, Knight shifts and relaxation rates satisfy the Korringa relation; electron correlation and localization effects are essentially absent. The shapes of ${}^{69}\mathrm{Ga}$ spin echoes reveal that a second, more distorted gallium site is present with a relative concentration that increases with the ratio of oxygen to total metal content. Magnetic hyperfine coupling of carriers to the distorted sites is very weak and the apparent carrier density at substitutional sites decreases with the fraction of observed substitutional sites. These results imply that the distorted sites are part of a diamagnetic carrier trap involving gallium complexed with a defect. We suggest that the complex consists of two gallium ions and an oxygen interstitial, i.e. ${\mathrm{Ga}}_{2}^{3+}{\mathrm{O}}_{i}^{2\ensuremath{-}}.$

Identification of Iron Transition Group Trace Impurities in GaN Bulk Crystals by Electron Paramagnetic Resonance

Abstract We report on the observation of electron paramagnetic resonance of iron, manganese and nickel trace impurities in bulk GaN crystals grown by the sublimation sandwich method. The resolved hyperfine structure due to interaction with 55Mn ( I = 5 2 ) nuclei has been observed in GaN, allowing unambiguous identification of the impurity. Manganese and nickel exist in Mn2+ (3d5) and Ni3+ (3d7) charge states with electron spin S = 5 2 and S = 3 2 , respectively, and occupy gallium sites in the GaN lattice. For Mn2+ we found g = 1.999, hyperfine structure constant A = 70.10−4 cm−1 and fine structure parameter /D/ = 240.10−4 cm−1. The EPR spectrum of Ni3+ in GaN had the characteristic anisotropy of an S = 3 2 system in a strong axial crystalline field. The effective g-factor values were found to be g′∥ = 2.10 and g′⊥ ≅ 4.20 for a system with an effective spin S′ = 1 2 . An analogy was revealed between the parameters of Mn2+ and Ni3+ in GaN and ZnO crystals. The zero-phonon line at 1.047 eV seems to belong to transition 4T2(F)-4A2(F) within 3d levels of Ni3+ ion with a 3d7 electronic configuration.

Synthesis and Characterization of a Chiral 3D-Framework Material: d-Co(en)3[H3Ga2P4O16

A chiral material, d-Co(en)3[H3Ga2P4O16] (en = 1,2-diaminoethane), with a three-dimensionally connected framework has been prepared hydrothermally, using optically pure d-Co(en)33+ as a structure directing agent, and the structure characterized by X-ray diffraction. Aluminum-containing analogues of this material have also been synthesized. The material can be thought of as a diamondlike network, consisting of tetrahedrally coordinated gallium sites linked by doubly bridging phosphates, with the metal complex sitting in the interstices. Co(C2N2H8)3[H3Ga2P4O16], Mr = 761.56, monoclinic, I2, a = 9.580(2)Å, b = 12.6789(5)Å, c = 9.9631(6)Å, β = 97.85(2)°, Z = 2, ρcalc = 2.109 g cm-3, ρobs = 2.11 g cm-3, Cu Kα, RF = 0.032. Co(C2N2H8)3[H3Ga1.25Al0.75P4O16], Mr = 729.51, monoclinic, I2, a = 9.567(1)Å, b = 12.6923(5)Å, c = 9.9241(5)Å, β = 97.601(8)o, Z = 2, ρcalc = 2.028 g cm-3, ρobs = 2.08 g cm-3, Cu Kα, RF = 0.026.

Identification of iron transition group trace impurities in GaN bulk crystals by electron paramagnetic resonance

We report on the observation of electron paramagnetic resonance of iron, manganese and nickel trace impurities in bulk GaN crystals grown by the sublimation sandwich method. The resolved hyperfine structure due to interaction with 55Mn ( I = 5 2 ) nuclei has been observed in GaN, allowing unambiguous identification of the impurity. Manganese and nickel exist in Mn 2+ (3 d 5) and Ni 3+ (3 d 7) charge states with electron spin S = 5 2 and S = 3 2 , respectively, and occupy gallium sites in the GaN lattice. For Mn 2+ we found g = 1.999, hyperfine structure constant A = 70.10 −4 cm −1 and fine structure parameter / D/ = 240.10 −4 cm −1. The EPR spectrum of Ni 3+ in GaN had the characteristic anisotropy of an S = 3 2 system in a strong axial crystalline field. The effective g-factor values were found to be g′ ∥ = 2.10 and g′ ⊥ ≅ 4.20 for a system with an effective spin S′ = 1 2 . An analogy was revealed between the parameters of Mn 2+ and Ni 3+ in GaN and ZnO crystals. The zero-phonon line at 1.047 eV seems to belong to transition 4 T 2( F)- 4 A 2( F) within 3 d levels of Ni 3+ ion with a 3 d 7 electronic configuration.

Identification of manganese trace impurity in GaN crystals by electron paramagnetic resonance

We report on the first observation of electron paramagnetic resonance of manganese trace impurity in GaN crystals. The resolved hyperfine structure due to interaction with nuclei has been observed in GaN for the first time, allowing unambiguous identification of the impurity. Manganese exists in the charge state with electron spin S = 5/2 and occupies gallium sites in the GaN lattice. We found g = 1.999, and the fine structure parameter .

Dissociative and selective adsorption of H2S on the GaAs(001)-(4×2) surface

The adsorption of H2S on the GaAs(001)-(4×2) surface has been studied using high-resolution electron energy loss spectroscopy and temperature-programmed desorption. The H2S adsorption is predominantly dissociative at 100 K, producing mercaptan (HS-) and H- species, the latter of which are bonded only to As. After annealing to 700 K, vacant gallium sites, which are available for further sulfur chemisorption, are monitored by post-exposure of atomic hydrogen. Several cycles of H2S exposure and thermal annealing build up more S-Ga bonding on the surface and yield a completely sulfur-saturated layer on which only sulfur is available for post-hydrogenation.

Properties of carbon-doped InGaAs grown by atmospheric pressure metalorganic chemical vapor deposition using CCl 4

This paper shows the results of a van der Pauw-Hall analysis of carbon-doped InGaAs epilayers grown by atmospheric pressure metalorganic chemical vapor deposition using CCl 4 gas. Heavily C-doped In x Ga 1 − x As ( x = 0 to 0.52) epilayers were obtained by varying the V III ratio from 1.3 to 32 and at a low growth temperature of 550°C, while keeping the flow rate of CCl 4 constant. For up to an InAs mole fraction of 0.43, a hole concentration of about 1.1 × 10 19 cm −3 was obtained. In the case of as-grown samples, a type conversion from p-type to n-type to n-type occurred at an InAs mole fraction of 0.48. This work also describes the effects of a rapid thermal annealing (RTA) on the electrical properties, particularly on a type conversion of InGaAs. After the RTA process at an annealing temperature of 750°C for 5 s, as-grown In 0.44Ga 0.56As with the p-type of 5.6 × 10 17 cm −3 was converted into the n-type of 3.9 × 10 17 cm −3. This can be attributed to the breaking of the CH or CH x bonds after the RTA process. The broken carbon atoms could enter into arsenic sites or indium (or gallium) sites in the epilayers. With increasing InAs mole fraction, more carbon atoms were supposed to enter into indium sites rather than arsenic sites, due to a lower binding energy of InC than that of AsC.

Photoluminescence excitation spectroscopy of GaAs:Er,O in the near-band-edge region

We report a spectroscopic study of GaAs:Er,O samples grown by metalorganic chemical vapor deposition, with excitation in the near-band-edge region, 1.22–1.62 eV. Photoluminescence under host-excitation is dominated by luminescence due to the Er-2O center (an erbium atom at a gallium site coupled with two adjacent oxygen atoms) in these samples. The characterization of different Er centers is demonstrated by the observation of 4I15/2→4I11/2 intra-4f-shell transitions by photoluminescence excitation (PLE) spectroscopy. The transitions between these crystal-field-split 4f-shell levels are observed at ∼1.26 eV (∼980 nm) which is at an energy attainable by PLE spectroscopy with a Ti:Sapphire laser. Two lines are assigned to the Er-2O center. This allows a semi-quantitative measurement of the relative concentrations of different centers to be made. PLE spectroscopy was also employed to study the trap levels related to the Er-2O center. However, we find no evidence for such a trap level in this energy range. The expected position of this trap is discussed in the light of these results and recent calculations.