Ni Acceptor Research Articles

Realization of High‐Resistive Ni‐Doped GaN Crystal by Hydride Vapor‐Phase Epitaxy

Herein, high‐resistivity GaN is studied for use as an epitaxial substrate in lateral power devices. Fe‐, C‐, Mn‐, and Zn‐doped GaN monocrystals have high resistivity at a doping concentration of ≈1 × 1018 cm−3. However, a low doping concentration is preferred for growing GaN monocrystals; therefore, other dopants for GaN that yield high resistivity at a doping concentration less than 1 × 1018 cm−3 must be identified. Herein, NiCl2 is used as a precursor to grow Ni‐doped GaN monocrystals on GaN substrates via hydride vapor‐phase epitaxy. Two Ni‐doped GaN substrates with Ni concentrations corresponding to 2.7 × 1017 and 2.9 × 1018 cm−3 are obtained by varying the partial pressure of NiCl2. The resistivity of Ni‐doped GaN monocrystals is measured as a function of temperature using Hall effect measurements. The GaN monocrystals doped with 2.7 × 1017 cm−3 of Ni have a higher resistivity than those doped with 2.9 × 1018 cm−3 of Ni at 600–900 K. Charge‐neutrality calculations have shown that the depth of the Ni acceptor level in GaN is 1.4–1.5 eV, indicating that Ni‐doped GaN monocrystals have high resistivity owing to the deep acceptor level of Ni.

Charge state transition levels of Ni in β-Ga2O3 crystals from experiment and theory: An attractive candidate for compensation doping

Nickel-doped β-Ga2O3 crystals were investigated by optical absorption and photoconductivity, revealing Ni-related deep levels. The photoconductivity spectra were fitted using the phenomenological Kopylov and Pikhtin model to identify the energy of the zero-phonon transition (thermal ionization), Franck–Condon shift, and effective phonon energy. The resulting values are compared with the predicted ones by first-principle calculations based on the density functional theory (DFT). An acceptor level (0/−) of 1.9 eV and a donor level (+/0) of 1.1 eV above the valence band minimum are consistently determined for NiGa, which preferentially incorporates on the octahedrally coordinated Ga site. Temperature-dependent resistivity measurements yield a thermal activation energy of ∼2.0 eV that agrees well with the determined Ni acceptor level. Conclusively, Ni is an eminently suitable candidate for compensation doping for producing semi-insulating β-Ga2O3 substrates due to the position of the acceptor level (below and close to the mid-bandgap).

Transition-metal ions in β-Ga2O3 crystals: Identification of Ni acceptors

Transition-metal ions (Ni, Cu, and Zn) in β-Ga2O3 crystals form deep acceptor levels in the lower half of the bandgap. In the present study, we characterize the Ni acceptors in a Czochralski-grown crystal and find that their (0/−) level is approximately 1.40 eV above the maximum of the valence band. Both Ni2+ (3d8) and Ni3+ (3d7) acceptors are present in the as-grown crystal. Also present are unintentional Ir3+ (5d6) and Ir4+ (5d5) donors. The neutral Ni3+ acceptors have a low-spin S = 1/2 ground state and are easily monitored with electron paramagnetic resonance (EPR). Principal values of the g matrix for these acceptors are 2.131, 2.138, and 2.233. Although paramagnetic, the singly ionized Ni2+ acceptors are not seen with EPR at X band (9.4 GHz). The Ir4+ donors are monitored with EPR and with infrared absorption spectroscopy. Exposing the Ni-doped β-Ga2O3 crystal to 275 nm light at room temperature increases the concentration of Ni3+ ions and reduces the concentration of Ir4+ ions as electrons move from the acceptors to the donors. After illumination, heating the crystal above 375 °C restores the initial concentrations of the Ni3+ and Ir4+ ions. Broad optical absorption bands peaking near 303 and 442 nm are attributed to the Ni3+ acceptors.

Photoinduced Intramolecular Electron Transfer in Phenylene Ethynylene Naphthalimide Oligomers.

This paper reports a photophysical investigation of a series of phenylene ethynylene oligomers (OPE) that are end-substituted with a 1,8-naphthalene imide (NI) acceptor. The NI acceptor is attached to the terminus of the OPEs via an ethynylene (-C≡C-) unit that is linked at the 4-position of the NI unit. A series of three oligomers is investigated, OPE1-NI, OPE3-NI, and OPE5-NI, which contain 1, 3, and 5 phenylene ethynylene repeat units, respectively. The properties of the OPEn-NI series are compared to a corresponding set of unsubstituted OPEs, OPE3 and OPE5, which contain 3 and 5 phenylene ethynylene repeats, respectively. The photophysics of all the compounds are interrogated using a variety of techniques including steady-state absorption, steady-state fluorescence, two-photon absorption, time-resolved fluorescence, and transient absorption spectroscopy on femtosecond-to-microsecond time scales. The effect of solvent polarity on the properties of the oligomers is examined. The results show that the NI-substituted oligomers feature a lowest charge transfer (CT) excited state, where the OPE segment acts as the donor and the NI moiety is the acceptor (OPEn•+-NI•-). The absorption spectra in one-photon and two-photon exhibit a clear manifold of absorption features that can be attributed to direct CT absorption. In moderately polar solvents, the emission is dominated by a broad, solvatochromic band that is due to radiative decay from the CT excited state. Ultrafast transient absorption provides evidence for initial population of a locally excited state (LE) which in moderately polar solvents rapidly (∼1 ps) evolves into the CT excited state. The structure, spectroscopy, and dynamics of the CT state are qualitatively similar for OPE3-NI and OPE5-NI, suggesting that delocalization in the OPE segment does not have much effect on the structure or energetics of the CT excited state.

Persistent Room-Temperature Radicals from Anionic Naphthalimides: Spin Pairing and Supramolecular Chemistry.

N-Substituted naphthalimides (NNIs) have been shown to exhibit highly efficient and persistent room-temperature phosphorescence from an NNI-localized triplet excited state, when the N-substitution is a sufficiently strong donor and mediates an intramolecular charge-transfer (ICT) state upon photo-excitation. This work shows that, when the electron-donating ability of the N-substitution is further increased in the presence of a carbanion or phenoxide, spontaneous electron transfer (ET) occurs and results in radical anions, verified with electron-paramagnetic resonance (EPR) spectroscopy. However, the EPR-active anion is surprisingly persistent and impervious to nucleophilic and radical reactions under anionic conditions. The stability is thought to originate from an intramolecular spin pairing between the N-donor and the NI acceptor post ET, which is demonstrated in supramolecular chemistry.

Structural and electron transport properties of single-crystalline In2O3 films compensated by Ni acceptors

For device applications, the ability to grow semi-insulating or p-type indium oxide (In2O3) is highly desirable. With this in focus, high quality single-crystalline Ni-doped In2O3 films have been grown by plasma-assisted molecular beam epitaxy and structurally and electrically characterized. Within a concentration range of approximately 1017–1019 cm−3, Ni is fully incorporated in the In2O3 lattice without the formation of secondary phases. At doping higher than roughly 1020 cm−3, secondary phases seem to start forming. No film exhibits p-type conductivity at room temperature. Instead, Ni is shown to be a deep compensating acceptor—confirming theoretical calculations, the effect of which only becomes apparent after annealing in oxygen. Combined Hall and Seebeck measurements reveal the compensation of bulk donors already at low Ni concentrations (∼1018 cm−3) and a residual film conductance due to mainly the interface region to the substrate. This residual conductance is gradually pinched off with increasing Ni doping, eventually resulting in semi-insulating films at excessive Ni concentrations (∼1021 cm−3).

Combined XBIC/μ‐XRF/μ‐XAS/DLTS investigation of chemical character and electrical properties of Cu and Ni precipitates in silicon

AbstractCombination of DLTS method and synchrotron‐based analytical microprobe techniques was used to study the precipitation of Cu and Ni atoms at two kinds of structural defects in silicon lattice: dislocation network created by direct wafer bonding and oxygen‐induced microdefects. Results of our measurements revealed the difference in the preferred precipitation places: Ni particles in form of NiSi2 were found only at the dislocation network, while Cu particles in form of Cu3Si were found both at the dislocation network and at the oxygen‐induced microdefects. DLTS measurements showed Ni acceptor levels in Ni contaminated sample and a broad band related to Cu precipitates in Cu contaminated one. In case of simultaneous Cu and Ni contamination NiSi2 and Cu3Si precipitates were found definitely at the same places indicating therefore that the metals interact during precipitation. DLTS showed the superposition of spectra for only Ni and for only Cu contaminated samples. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Preparation of Multilayer Semiconducting BaTiO3 Ceramics Co-Fired with Ni Inner Electrodes

A multilayer semiconducting BaTiO3 ceramics having positive temperature coefficient of resistivity (PTCR) characteristics is investigated with the aim of lowering its resistance. The multilayer BaTiO3 ceramics was fired in reducing atmosphere and reoxidized at 700 °C in air. The result reveals that the multilayer semiconducting BaTiO3 ceramics with Ni inner electrodes shows high resistance even when the thickness is lowered; however, excessive donor doping markedly decreases the resistance of BaTiO3 ceramics to a very low value. The donor in excessive amounts is considered to cancel the Ni acceptors occupying the Ti sites; these acceptors diffuse from the Ni inner electrode and increase resistivity. As a result, a multilayer BaTiO3 ceramics element (3.2×2.5 mm2) with a low resistance of 0.05 Ω is realized by using excess donor doping (Sm=0.35 mol %).

Exaggerated grain growth in Ni-doped BaTiO 3 ceramics

With approximately 0.7 mol% Ni doping, the formation of hexagonal phase in BaTiO 3 after sintering at a temperature above 1385 °C in air has been observed. The shape of the h-BaTiO 3 grains is plate-like, and the length of these h-BaTiO 3 plates can be longer than 15 mm. As the amount of Ni doping increases above or decreases below 0.7 mol%, the hexagonal phase disappears and the resulting microstructure is dominated by small BaTiO 3 grains. This critical amount of Ni (i.e. 0.7 mol%) corresponds to the solubility of Ni in BaTiO 3. The exaggerated growth of the plate-like grains in BaTiO 3 relates strongly to the formation of oxygen vacancies due to the presence of Ni acceptors.

Process Window of BaTiO3–Ni Ferroelectric–Ferromagnetic Composites

Experimental study and thermodynamic analysis are conducted to identify a process window for the preparation of BaTiO3–Ni ferroelectric–ferromagnetic composites. A tiny amount of Ni solutes (<0.5 mol%) can play a key role in the preparation of the composites. The Ni solute acts as an acceptor that enhances the reduction resistance of BaTiO3. Owing to the presence of Ni acceptors, dense BaTiO3–Ni composites can be prepared by sintering above 1330°C in an atmosphere of 1 Pa oxygen partial pressure.

Electron Transfer Involving Nonbonded Superexchange Interactions in Rigid Donor−Acceptor Arrays

Rigid intramolecular trichromophoric arrays based on a 4,5-diaminoxanthene bridge B were prepared in which the xanthene bridge orients A2 relative to D, so that nonbonded interactions between 1*D and A2 can be studied. The electron donor, D, is 4-(N-piperidinyl)naphthalene-1,8-dicarboximide (ANI), and the acceptors A1 and A2 are pyromellitimide (PI) and naphthalene-1,8:4,5-bis(dicarboximide) (NI), respectively, and X is either n-C8H17 or H. The torsional angles of the xanthene−imide single bonds orient the NI and PI acceptors approximately cofacial to one another. Femtosecond transient absorption measurements show that electron transfer from 1*ANI to NI occurs by nonbonded superexchange interactions that include contributions from both the n-C8H17 substituent on the NI acceptor and nearby solvent molecules. The structural rigidity of these compounds allows evaluation of the distance dependence of both charge separation and recombination. For charge separation, β = 1.1−1.3 A-1, and evidence exists for solv...

Unusual combination repetitions of the zero phonon line of Ni acceptor excitons in ZnSe : Ni and ZnO : Ni due to photoinduced lattice vibrations

Multiphonon vibration repetitions of the zero phonon line of a Ni impurity [d 9h] acceptor exciton for ZnSe : Ni are analyzed. The first observed unusual structure of combination repetitions is explained by a significant anharmonism of a dominant mode which is formed by a moving of ions in the photoinduced lattice distortion area.

The pressure dependence of the intraimpurity absorption and the charge transfer process of ZnS:Ni and ZnSe:Ni

The absorption spectra of ZnS:Ni and ZnSe:Ni were measured at 100 K under hydrostatic pressure up to 10 GPa. Two kinds of optical transition were investigated: (i) the d-d transitions between crystal field levels of Ni2+ in the tetrahedral field of the host lattice and (ii) a charge transfer band that occurs when an electron is transferred from the valence band to the Ni2+ ion. The pressure dependence of the crystal field parameter Dq and of the Racah parameter B were determined and a revised assignment of the d-d transitions was obtained. The charge transfer can be considered as a deep-acceptor transition. In this case the Ni acceptor level may be used as an energy reference, which enabled us to determine the hydrostatic deformation potential of the valence band.

Persistent electron redistribution among deep levels in CdTe

Photo electron paramagnetic resonance measurements in CdTe show that sub-band-gap illumination causes a transfer of electrons from deep Ge donors to deep Ni acceptor states at a higher energy. This metastable state persists in the dark and can be quenched by illumination with photon energies in the midgap range. The metastability is explained in terms of the strong localization of both the Ni and the Ge states.

Identification of the isolated deep Ni acceptor in CdTe and ZnTe: comparison with isomorphous systems

Single crystals of CdTe and ZnTe doped with nickel have been investigated by electron spin resonance. Among other signals they exhibit an isotropic line near mod g mod =1.4 which is identified as being due to the isolated, ionised Ni acceptor state, NiCd+(3d9) and NiZn+(3d9). Central hyperfine interaction with 61Ni and ligand hyperfine interaction with 125Te could be resolved for CdTe:Ni. Intra d-shell optical transitions of Ni+ in CdTe were also observed. The general properties of the Ni(d8/d9) acceptor in cubic IIB-VI and III-V compounds are pointed out and are contrasted with those of the copper acceptor in IIB-VI compounds. A tentative prediction for the Ni acceptor level in CdTe is made.

Compensation mechanism in Ni-diffused n-type GaP

Optical investigation of the diffusion profile of Ni acceptor in n-type Gap is presented. It is found that a single Ni impurity atom is responsible for compensation of 4 to 5 electrons from shallow donors. Generation of point defects, such as vacancies, by a dislocation climb mechanism is responsible for this large compensation.

Negative magnetoresistance in GaAs containing Mn or Ni acceptors

Transverse magnetoresistance was measured, from 20 to 30 K and for fields up to 1 T, in GaAs crystals doped with enough Mn or Ni acceptors to result in activationless impurity conduction. Anomalies in the weak-field magnetoresistance were seen in the impurity conduction range. Nickel-doped samples showed consistently zero or positive Δρ/ρo, though the near-zero magnetoresistance below 0.5 T for the temperature range 40–100 K suggested cancellation of conventional monotonically increasing positive magnetoresistance by a negative component that saturated for large fields. With manganese-doped crystals, a small negative total magnetoresistance (−Δρ/ρo<4×10−5) was seen for fields up to 0.6 T, but only within the limited temperature range 40–75 K. The field dependence again suggested that total magnetoresistance should be regarded as the algebraic sum of an increasing positive term and a saturating-type negative component. The magnitude and field dependence of the negative magnetoresistance of GaAs : Mn was compared with models such as those based on the magnetic ordering of localized spins, and yielded a localized impurity magnetic moment of 2.2μB when compared with the magnetic ordering model of Pöder.