Centers In GaAs Research Articles

High-frequency EPR spectroscopy of paramagnetic manganese centers in GaAs : Mn crystals

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs : Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+-SH complex, which is a manganese ion with spin S=5/2, which replaces gallium (MnGa2+) and an ionized acceptor (A-) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J=3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2- replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the <111> axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+-SH complex. Keywords: high-frequency EPR, GaAs crystal, Mn acceptor, shallow hole, exchange interaction.

High-frequency EPR spectroscopy of paramagnetic manganese centers in GaAs : Mn crystals

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs : Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+-SH complex, which is a manganese ion with spin S=5/2, which replaces gallium (MnGa2+) and an ionized acceptor (A-) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J=3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2- replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the <111> axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+-SH complex. Keywords: high-frequency EPR, GaAs crystal, Mn acceptor, shallow hole, exchange interaction.

Remotely Pumped GHz Antibunched Emission from Single Exciton Centers in GaAs

Quantum communication networks require on-chip transfer and manipulation of single particles as well as their interconversion to single photons for long-range information exchange. Flying excitons propelled by GHz surface acoustic waves (SAWs) are outstanding messengers to fulfill these requirements. Here, we demonstrate the acoustic manipulation of single exciton centers consisting of individual excitons bound to shallow impurities centers embedded in a semiconductor quantum well. Time-resolved photoluminescence studies show that the emission intensity and energy from these centers oscillate at the SAW frequency of 3.5 GHz. Furthermore, these centers can be remotely pumped via acoustic transport of flying excitons along a quantum well channel over several microns. Time correlation studies reveal that the centers emit anti-bunched light, thus acting as single-photon sources operating at GHz frequencies. Our results pave the way for the exciton-based on-demand manipulation and on-chip transfer of single excitons at microwave frequencies with a natural photonic interface.

Enhanced Spontaneous Emission Rates for Single Isoelectronic Luminescence Centers in Photonic Crystal Cavities

Purcell effect enhancement of spontaneous emission rates is demonstrated for isoelectronic trap single-photon emitters. Two-dimensional photonic crystal slabs with L3 defects were fabricated in nitrogen delta-doped GaAs. Photoluminescence spectra of each photonic crystal cavity had a series of sharp and bright lines arising from individual nitrogen luminescence centers, which was confirmed by Hanbury-Brown and Twiss measurements. The emission rates of these lines depended on cavity detuning, indicating a resonant character of the enhancement. The observed emission lifetime in the cavity was 400 ps, which would be the shortest lifetime reported so far for luminescence centers in GaAs.

Anharmonic lattice relaxation during nonradiative carrier capture

Lattice vibrations of point defects are essential for understanding non-radiative electron and hole capture in semiconductors as they govern properties including persistent photoconductivity and Shockley-Read-Hall recombination rate. Although the harmonic approximation is sufficient to describe a defect with small lattice relaxation, for cases of large lattice relaxation it is likely to break down. We describe a first-principles procedure to account for anharmonic carrier capture and apply it to the important case of the \textit{DX} center in GaAs. This is a system where the harmonic approximation grossly fails. Our treatment of the anharmonic Morse-like potentials accurately describes the observed electron capture barrier, predicting the absence of quantum tunnelling at low temperature, and a high hole capture rate that is independent of temperature. The model also explains the origin of the composition-invariant electron emission barrier. These results highlight an important shortcoming of the standard approach for describing point defect ionization that is accompanied by large lattice relaxation, where charge transfer occurs far from the equilibrium configuration.

Photoluminescence characterization of nonradiative recombination centers in MOVPE grown GaAs:N δ-doped superlattice structure

Nonradiative recombination (NRR) centers in GaAs:N δ-doped superlattice (SL) structure grown by metal organic vapor phase epitaxy (MOVPE) are studied by two-wavelength-excited photoluminescence (PL) method. The PL intensity quenching due to the superposition of below-gap excitation (BGE) light at energies of 0.75, 0.80, 0.92, and 0.95 eV and an above-gap excitation (AGE) light either for the GaAs conduction band (1.69 eV) or E− band excitation (1.45 eV), respectively indicates the existence of NRR centers inside the GaAs:N δ-doped SL and GaAs layers. The AGE density and BGE density dependence of the PL intensity have been studied for both excitation schemes. Recombination model based on two levels has been proposed to interpret the experimental results and energy distribution of NRR centers inside the GaAs:N δ-doped SLs and GaAs layers. The defect-related parameters are estimated systematically by solving the rate equations and fitting the results with the experimental data that give a phenomenological insight into the sample.

Characterization of nonradiative recombination centers in proton-irradiated InAs/GaAs quantum dots by two-wavelength-excited photoluminescence

Nonradiative recombination (NRR) centers in as-grown and proton-irradiated InAs/GaAs quantum dot (QD) structures have been studied by two-wavelength-excited photoluminescence (PL). The PL intensity quenching of GaAs and QD emissions due to the addition of a below-gap excitation light of 0.80 eV energy indicates the presence of defect levels acting as NRR centers. The method enables us to discuss the distribution of NRR centers in GaAs and/or InAs QD regions by selecting either conduction band excitation (2.33 eV) or intermediate band excitation (1.27 eV). We have found that the densities of NRR centers in GaAs layers and the effect of quenching on GaAs emissions increase monotonically with increasing proton irradiation fluence. The QD emission intensity, however, increases at a moderate fluence of 7 × 1011 protons/cm2 owing to the defect-assisted trapping of electrons into QDs. Further incorporation of NRR centers after 4 × 1012 protons/cm2 fluence quenches the QD-PL intensity below that of an unirradiated sample.

Ferromagnetic ordering in doped by Mn GaAs quantum wells supported by the upper Hubbard band of Mn centers

We have studied experimentally ferromagnetic properties of GaAs-AlGaAs quantum wells where the center of GaAs well was doped by Mn (x<0.1%) while the neighboring barrier was doped by shallow acceptor Be with an aim to occupy (at least, partially) the states of the upper Hubbard band of Mn acceptors in the well (AMn+). It was found that while the measurements of the anomalous Hall effect unambiguously demonstrated a presence of ferromagnetic ordering up to rather high temperatures (of 250 K), the conductivity demonstrated activated behavior with high activation temperatures (of the order of 100 meV). In contrast to our previous studies of the structures with undoped barriers, the Curie temperatures were significantly higher (150 K instead of 100 K), but conductivities of barrier-doped structures were order of magnitudes lower. We suggest a theoretical model explaining this behavior. It takes into account, first, the electrostatic potential of charged acceptors in the barrier (Be-) forming a potential bend in the near-barrier region which accepts some of the holes supplied by the barrier acceptors. Second, we take into account the fact that at least part of the supplied holes occupies the upper Hubbard band mentioned above. The latter fact can explain observed increase in the Curie temperature with respect to the structures with undoped barriers. The suppression of conductivity is explained by a presence of long-range potential produced by the randomly distributed charged centers within the barrier. Thus we deals with droplets of delocalized holes within the potential bend and partially occupied upper Hubbard band which, with a help of the indirect exchange, form the islands of ferromagnetically ordered Mn ions. The detailed discussion of further experimental studies necessary to confirm our model is also presented.

Nonradiative recombination centers in GaAs:N δ-doped superlattice revealed by two-wavelength-excited photoluminescence

We use two-wavelength-excited photoluminescence (PL) to investigate nonradiative recombination (NRR) centers in GaAs:N δ-doped superlattice (SL) structures grown by molecular beam epitaxy. The change in photoluminescence (PL) intensity due to the superposition of below-gap excitation at energies of 0.75, 0.80, 0.92, and 0.95 eV and above-gap excitation at energies of 1.69 or 1.45 eV into the GaAs conduction band and the E− band implies the presence of NRR centers inside the GaAs:N δ-doped SL and/or GaAs layers. The change in PL intensity as a function of the photon number density of below-gap excitation is examined for both bands, which enables us to determine the distribution of NRR centers inside the GaAs:N δ-doped SL and GaAs layers. We propose recombination models to explain the experimental results. Defect-related parameters that give a qualitative insight into the samples are investigated systematically by fitting the rate equations to the experimental data.

Piezospectroscopy and first-principles calculations of the nitrogen-vacancy center in gallium arsenide

The nitrogen-vacancy (NV) center occurs in GaAs bulk crystals doped or implanted with nitrogen. The local vibration of nitrogen gives rise to a sharp infrared absorption band at 638 cm–1, exhibiting a fine structure due to the different masses of neighboring 69Ga and 71Ga host isotopes. Piezospectroscopic investigations in the crystallographic ⟨100⟩ direction prove that the center has C3v point symmetry, which is weakly perturbed by the isotope effect. The stress-induced shifts of some band components show an unusual non-linear behavior that can be explained by coupling between the isotope and the stress splitting. First-principles density-functional theory calculations are in full accordance with the experiments and confirm the C3v symmetry, caused by relaxation of the nitrogen atom from the anion lattice site towards the nearest-neighbor Ga plane. Furthermore, the calculations indicate the –3 charge state of the center as the most stable one for nearly all Fermi level positions. The NV center in GaAs is structurally analogous to the same center in diamond.

Spatially resolved electronic structure of an isovalent nitrogen center in GaAs

Small numbers of nitrogen dopants dramatically modify the electronic properties of GaAs, generating very large shifts in the conduction-band energies with nonlinear concentration dependence, and impurity-associated spatially-localized resonant states within the conduction band. Cross-sectional scanning tunneling microscopy provides the local electronic structure of single nitrogen dopants at the (110) GaAs surface, yielding highly anisotropic spatial shapes when the empty states are imaged. Measurements of the resonant states relative to the GaAs surface states and their spatial extent allow an unambiguous assignment of specific features to nitrogen atoms at different depths below the cleaved (110) surface. Multiband tight binding calculations around the resonance energy of nitrogen in the conduction band match the imaged features. The spatial anisotropy is attributed to the tetrahedral symmetry of the bulk lattice. Additionally, the voltage dependence of the electronic contrast for two features in the filled state imaging suggest these features could be related to a locally modified surface state.

Quantum interference of two photons emitted from a luminescence center in GaAs:N

The indistinguishability of photons emitted from a nitrogen luminescence center in GaAs is investigated by two-photon interference under nonresonant optical excitation. A clear dip is observed in a parallel polarization configuration for consecutively emitted two photons with a 2-ns time interval. The indistinguishability is approximately 0.24, and is found to be independent of the time interval between 2 ns and 4 ns. These results suggest the existence of a very fast dephasing mechanism within 2 ns.

Scattering of an exciton polariton by impurity centers in GaAs

Scattering of an exciton polariton by impurity centers at low temperatures has not been investigated comprehensively in spite of its significant role in processes accompanying Bose–Einstein condensation of an exciton polariton. For studying the peculiarities of the interaction of an exciton polariton with impurity centers, we have studied the integrated absorption of the ground state (n 0 = 1) of the exciton in GaAs in thin (micrometer-thick) wafers with an appreciable optical transmission. Comparative analysis of the transmission in the vicinity of the exciton resonance performed on 15 samples of crystalline GaAs wafers with different concentrations N of impurity has revealed an unexpected regularity. The value of N increases by almost five decimal orders of magnitude, while the normalized spectrally integrated absorption of light exhibits a slight increase, following the power dependence N m on the concentration, where m = 1/6. It has been shown that this dependence indicates the diffusion mechanism of propagation of the exciton polaritons through the bulk of the semiconductor, which is present along with the ballistic propagation of light through the sample.

Arsenic antisite and oxygen incorporation trends in GaAs grown by water-mediated close-spaced vapor transport

Close-spaced vapor transport (CSVT) provides a plausible path to lower the costs of GaAs deposition as it uses only solid precursors and provides precursor utilization in principle approaching 100%. However, the use of H2O as a transport agent causes O to be incorporated in CSVT films, and O has been associated with a number of electrically active defect centers in GaAs, which decrease minority carrier lifetimes. Using deep-level transient spectroscopy, we study the effect of H2O concentration and substrate temperature on electron trap concentrations in n-type GaAs. We find that the most-prominent O-related center (ELO) typically has a much higher concentration than the center usually associated with As antisites (EL2), but that overall defect concentrations can be as low as those in films deposited by common vapor phase techniques. The trends with increasing H2O concentration suggest that ELO is most likely a defect complex with two As antisites. We also consider the optimal conditions for achieving high growth rates and low defect concentrations using CSVT. The results of this study have implications for the future CSVT growth using halide transport agents, where the ELO defect would be eliminated but EL2 might have a higher concentration.

The impact of electron and hole trapping coefficient on nonlinear phenomena in photorefractive multiple quantum well structures

Semiconductor photorefractive quantum wells belong to materials with strong optical nonlinearity. One of the parameters that may affect the course of nonlinear phenomena in these materials is the electron and hole trapping coefficient. We present the results of a numerical analysis aimed to find out, how electric field-dependent trapping coefficients affect the process of space-charge field formation in multiple quantum wells in the phenomenon of photorefractive two-wave mixing. Full Text: PDF ReferencesQ. Wang, R. M. Brubaker, D. D. Nolte and M. R. Melloch, "Photorefractive quantum wells: transverse Franz-Keldysh geometry," J. Opt. Soc. Am. B 9, 1626 (1992) CrossRef D.D. Nolte and M.R. Melloch, in: Photorefractive effects and Materials, Chap.6, ed. by D. D. Nolte (Kluwer Academic, Boston, 1995) CrossRef D.D. Nolte, "Semi-insulating semiconductor heterostructures: Optoelectronic properties and applications"", J. Appl. Phys. 85, 6259 (1999) CrossRef Q.Wang, R. M. Brubaker and D. D. Nolte, "Photorefractive phase shift induced by hot-electron transport: Multiple-quantum-well structures", J. Opt. Soc. Am. B 9 (1994) 1773. CrossRef V. Ya. Prinz, S. N. Rechkunov, "Influence of a Strong Electric Field on the Carrier Capture by nonradiative Deep-Level Centers in GaAs", Phys. Stat. Sol. (b) 118, 159 (1983) CrossRef S.M. Sze, Physics of Semiconductors Devices, second ed., Wiley, New York, 1981 (Chapter 10) DirectLink B. Jablonski, "Impact of donor compensation ratio on photorefractive two-wave mixing dynamics in multiple quantum wells structures", JNOPM 23, 1450029 (2014) CrossRef

Vacancy and Doping States in Monolayer and bulk Black Phosphorus.

The atomic geometries and transition levels of point defects and substitutional dopants in few-layer and bulk black phosphorus are calculated. The vacancy is found to reconstruct in monolayer P to leave a single dangling bond, giving a negative U defect with a +/− transition level at 0.24 eV above the valence band edge. The V− state forms an unusual 4-fold coordinated site. In few-layer and bulk black P, the defect becomes a positive U site. The divacancy is much more stable than the monovacancy, and it reconstructs to give no deep gap states. Substitutional dopants such as C, Si, O or S do not give rise to shallow donor or acceptor states but instead reconstruct to form non-doping sites analogous to DX or AX centers in GaAs. Impurities on black P adopt the 8-N rule of bonding, as in amorphous semiconductors, rather than simple substitutional geometries seen in tetrahedral semiconductors.

Dynamics of excitons bound to nitrogen isoelectronic centers in GaAs

The first comprehensive study is made of the exciton dynamics and spin-relaxation processes in isoelectronic centers in GaAs, single excitons bound to two nitrogen atoms. Such atomic defects may offer advantageous properties for quantum information applications over the more established quantum dots and NV centers in diamond. Time-resolved spectroscopy is coupled with a unique analytical model.

Complete quantum control of exciton qubits bound to isoelectronic centres.

In recent years, impressive demonstrations related to quantum information processing have been realized. The scalability of quantum interactions between arbitrary qubits within an array remains however a significant hurdle to the practical realization of a quantum computer. Among the proposed ideas to achieve fully scalable quantum processing, the use of photons is appealing because they can mediate long-range quantum interactions and could serve as buses to build quantum networks. Quantum dots or nitrogen-vacancy centres in diamond can be coupled to light, but the former system lacks optical homogeneity while the latter suffers from a low dipole moment, rendering their large-scale interconnection challenging. Here, through the complete quantum control of exciton qubits, we demonstrate that nitrogen isoelectronic centres in GaAs combine both the uniformity and predictability of atomic defects and the dipole moment of semiconductor quantum dots. This establishes isoelectronic centres as a promising platform for quantum information processing.

Single Photon Generation from an Impurity Center with Well-Defined Emission Energy in GaAs

We studied optical properties of isoelectronic traps in nitrogen δ-doped GaAs by micro-photoluminescence (µ-PL) spectroscopy. We found that these nitrogen impurity centers (NNA) emit photons with nearly identical emission energy at 1475 meV and polarization direction. Furthermore, single photon generation from a single impurity center was confirmed by a strong photon antibunching under the continuous optical excitation at 5 K. Our results suggest that the nitrogen impurity center in GaAs might be well suited for the energetically-defined single photon source for the quantum information application.

Compensation effects at electron traps in semiconductors

The basic qualities for fulfilling the Meyer-Neldel rule (MNR) for thermal electron emission from semiconductor traps are investigated. A trap model including vibronic properties is used with varying entropy arising from the change in elasticity of the ionic part of the trap potential when an electron transition takes place. This gives rise to a system where the compensation effect originates from the increasing entropy change as a function of the enthalpy supply needed for the transition process in concord with Yelon-Movaghar theory. The entropy increase connects to a decrease in the activation energy for electron capture, which amplifies the compensation effect for MNR manifestation. By comparing with experimental data, the result achieved from the model clarifies the experimental observation of class partitioning for centers in GaAs, obeying the MNR. Furthermore, it is demonstrated that traps at metal-oxide-silicon interfaces, with the same properties as bulk traps following the MNR, give rise to capture cross-sections steeply increasing with the Gibbs free energy involved in carrier emission, as found by experiment.