Recombination property of nitrogen-acceptor-bound states in ZnO

The recombination property of nitrogen (N)-related acceptor-bound states in ZnO has been investigated by photoluminescence (PL), time-resolved PL, and selective PL. Several possible recombination processes were discussed by analyzing the relaxation and recombination properties under large Coulomb interaction. It is strongly suggested that bound exciton emission dominates the recombination process related to the N acceptor. The recombination lifetime is 750 ps and the binding energy is 67 meV for N-acceptor-bound exciton at low temperature. (c) 2006 American Institute of Physics.

Alloy states in dilute GaAs1-xNx alloys (x < 1%)

A set of GaAs1-xNx samples with small nitrogen composition (x<1%) were investigated by continuous-wave photoluminescence (PL), pulse-wave excitation PL, and time-resolved PL. In the PL spectra, an extra transition located at the higher-energy side of the commonly reported N-related emissions was observed. By measuring the PL dependence on temperature and excitation power, the PL peak was identified as a transition of alloy band edge-related recombination in GaAsN. The PL dynamics further confirms its intrinsic nature as being associated with the band edge rather than N-related bound states. (C) 2003 American Institute of Physics.

The second dissociation threshold bound levels of hydrogen molecule

The near-threshold highly bound states of all three stable isotopic variants of molecular hydrogen have been studied. Numerous perturbations and unexpected transitions are observed as far as 1cm(-1) just below the second dissociation threshold. This complex structure may arise from a combination of nonadiabatic coupling between B, B', C electronic states, perturbations due to. ne and hyperfine interactions, and strong shape resonances. The perturbed near-threshold states and vibrational continuum exhibit finegrained structure, differing greatly between isotopes because of varying nonadiabatic coupling.

Optical study of electronic states in GaAsN

GaAs1-xNx alloys with small N composition (x<1%) and GaAsN/GaAs quantum wells (QWs) were studied by continuous wave photoluminescence (PL), pulse wave excitaiton PL and time-resolved PL. In the PL spectra an extra transition located at the higher energy side of the commonly reported N-related emissions was observed. By measuring the PL dependence on temperature and excitation power, the new PL peak was identified as a transition of alloy band edge-related recombination in GaAsN and delocalized transition in QWs. The PL dynamics further confirms its intrinsic nature of band edge states rather than N-related bound states.

Electron tunneling through a planar single barrier in HgTe quantum wells with inverted band structures

We present a theoretical study on the electron tunneling through a single barrier created in a two-dimensional electron gas (2DEG) and quantum spin Hall (QSH) bar in a HgTe/CdTe quantum well with inverted band structures. For the 2DEG, the transmission shows the Fabry-Perot resonances for the interband tunneling process and is blocked when the incident energy lies in the bulk gap of the barrier region. For the QSH bar, the transmission gap is reduced to the edge gap caused by the finite size effect. Instead, transmission dips appear due to the interference between the edge states and the bound states originated from the bulk states. Such a Fano-like resonance leads to a sharp dip in the transmission which can be observed experimentally.

Resonant tunneling through S- and U-shaped graphene nanoribbons

We theoretically investigate resonant tunneling through S- and U-shaped nanostructured graphene nanoribbons. A rich structure of resonant tunneling peaks is found emanating from different quasi-bound states in the middle region. The tunneling current can be turned on and off by varying the Fermi energy. Tunability of resonant tunneling is realized by changing the width of the left and/or right leads and without the use of any external gates.

Intersubband optical absorption in quantum dots-in-a-well heterostructures

The theoretical analysis of intersubband optical transitions for InAs/ InGaAs quantum dots-in-a-well ( DWELL ) detectors are performed in the framework of effective-mass envelope- function theory. In contrast to InAs/ GaAs quantum dot (QD) structures, the calculated band structure of DWELL quantitatively confirms that an additional InGaAs quantum well effectively lowers the ground state of InAs QDs relative to the conduction-band edge of GaAs and enhances the confinement of electrons. By changing the doping level, the dominant optical transition can occur either between the bound states in the dots or from the ground state in the dots to bound states in the well, which corresponds to the far-infrared and long-wave infrared (LWIR ) peaks in the absorption spectra, respectively. Our calculated results also show that it is convenient to tailor the operating wavelength in the LWIR atmospheric window ( 8 - 12 mu m ) by adjusting the thickness of the InGaAs layer while keeping the size of the quantum dots fixed. Theoretical predictions agree well with the available experimental data. (c) 2005 American Institute of Physics.

A new type of photoelectric response in a double barrier structure with a wide quantum well

We have calculated the photoelectric response in a specially designed double barrier structure. It has been verilied that a transfer of the internal photovoltaic effect in the quantum well to the tunnelling transport through above-barrier quasibound states of the emitter barrier may give rise to a remarkable photocurrent.

Recombination kinetics of Te isoelectronic centers in ZnSTe

The recombination kinetics of Te isoelectronic centers in ZnS1-xTex (0.0065 less than or equal to x less than or equal to 0.85) alloys is studied by time-resolved photoluminescence (TRPL) at low temperature. The measured radiative recombination lifetimes of different Te bound exciton states are quite different, varying from a few nanoseconds to tens of nanosecond. As the bound exciton state evolves from a single Te impurity (Te-1) to larger Te clusters (Te-n, n=2,3,4), the recombination lifetime increases. It reaches maximum (similar to40 ns) for the Te-4 bound states at x=0.155. The increase of the exciton lifetime is attributed to the increasing exciton localization effect caused by larger localization potential. In the large Te composition range (x > 0.155), the exciton recombination lifetime decreases monotonically with Te composition. It is mainly due to the hybridization between the Te localized states and the host valence band states. The composition dependences of the exciton binding energy and the photoluminescence (PL) line width show the similar tendency that further support the localization picture obtained from the TRPL measurement. (C) 2005 American Institute of Physics.

Pressure behavior of the alloy band edge and nitrogen-related centers in GaAs0.999N0.001

The photoluminescence of a GaAsN alloy with 0.1% nitrogen has been studied under pressures up to 8.5 GPa at 33, 70, and 130 K. At ambient pressure, emissions from both the GaAsN alloy conduction band edge and discrete nitrogen-related bound states are observed. Under applied pressure, these two types of emissions shift with rather different pressure coefficients: about 40 meV/GPa for the nitrogen-related features, and about 80 meV/GPa for the alloy band-edge emission. Beyond 1 GPa, these discrete nitrogen-related peaks broaden and evolve into a broad band. Three new photoluminescence bands emerge on the high-energy side of the broad band, when the pressure is above 2.5, 4.5, and 5.25 GPa, respectively, at 33 K. In view of their relative energy positions and pressure behavior, we have attributed these new emissions to the nitrogen-pair states NN3 and NN4, and the isolated nitrogen state N-x. In addition, we have attributed the high-energy component of the broad band formed above 1 GPa to resonant or near-resonant NN1 and NN2, and its main body to deeper cluster centers involving more than two nitrogen atoms. This study reveals the persistence of all the paired and isolated nitrogen-related impurity states, previously observed only in the dilute doping limit, into a rather high doping level. Additionally, we find that the responses of different N-related states to varying N-doping levels differ significantly and in a nontrivial manner.

Resonant tunneling theory of planar quantum dot structures

The ballistic transport in the semiconductor, planar, circular quantum dot structures is studied theoretically. The transmission probabilities show apparent resonant tunneling peaks, which correspond to energies of bound states in the dot. By use of structures with different angles between the inject and exit channels, the resonant peaks can be identified very effectively. The perpendicular magnetic field has obvious effect on the energies of bound states in the quantum dot, and thus the resonant peaks. The treatment of the boundary conditions simplifies the problem to the solution of a set of linear algebraic equations. The theoretical results in this paper can be used to design planar resonant tunneling devices, whose resonant peaks are adjustable by the angle between the inject and exit channels and the applied magnetic field. The resonant tunneling in the circular dot structures can also be used to study the bound states in the absence and presence of magnetic field.

The effects of rapid thermal annealing on the optical properties of Zn1-xMnxSe epilayer grown by MOCVD on GaAs substrate

Zn1-xMnxSe thin films with different Mn compositions are grown by metal-organic chemical vapor deposition on GaAs substrate. Good crystallinity of sample is evidenced by X-ray diffraction and rocking-curve measurements. Photoluminescence (PL) properties were carefully studied. A dominant PL peak close to the band edge is observed at low temperature for samples with higher Mn concentration. The temperature-dependent PL and time-resolved photoluminescence show that this emission peak is associated with the recombination of exciton bound to Mn-induced impurity bound states. It is found that rapid thermal annealing can induce reorganization of Mn composition in alloys and significantly reduce the density of impurity induced by Mn incorporation and improve the intrinsic interband transition. (C) 2002 Elsevier Science B.V. All rights reserved.

Anomalous temperature dependence of photoluminescence in GaInNAs/GaAs multiple quantum wells

Photoluminescence (PL) spectra of GaInNAs/GaAs multiple quantum wells grown on a GaAs substrate by molecular beam epitaxy are measured in a range of temperatures and excitation power densities. The energy position of the dominant PL peak shows an anomalous S-shape temperature dependence instead of the Varshni relation. By careful inspection, especially for the PL under lower excitation power density, two near bandedge peaks are well identified. These are assigned to carriers localized in nitrogen-induced bound states and interband excitonic recombinations, respectively. It is suggested that the temperature-induced switch of such two luminescence peaks in relative intensity causes a significant mechanism responsible for the S-shape shift observed in GaInNAs. A quantitative model based on the thermal depopulation of carriers is used to explain the temperature dependence of the PL peak related to N-induced bound states.

ENERGY-LEVEL CALCULATIONS OF THE RECONSTRUCTED 90-DEGREES PARTIAL DISLOCATION IN SILICON

An LCAO scheme taking into account 10 atomic orbitals (s-, p-, and d-type) applied to a supercell containing 256 atoms is used to calculate the bound states of the reconstructed 90-degrees partial dislocation in Si. The results differ significantly from our earlier calculations on the unreconstructed 90-degrees partial using the same method. We find two bands separate from each other in the entire Brillouin zone and the upper band penetrates deep into the indirect band gap which is in contradiction with the general opinion that core reconstruction clears the band gap of dislocation states.

Optical Pr operties of GaNAs and GaAsSb Semiconductors

Under short pulse laser excitation, it has been observed, for the first time, a new high-energy photoluminescence emission from GaNx As1- x/GaAs SQWs. This new emission has totally different optical properties compared with the localized exciton transition in GaNx As1-x, and is attributed to the recombination of delocalized excitons in QWs. At the same time, a competition process between localized and delocalized exciton emissions in GaNx As1-x/GaAs quantum wells is observed in the temperaturedependent PL spectra under the short pulse excitation. This competition process for the first time, reveals the physical origin of the temperature-induced S-shaped PL peak shift, which was often reported in the disordered alloy semiconductor system under continuous-wave excitation and puzzled people for a long time. We have also investigated a set of GaNx As1- x samples with small nitrogen composition( x ＜ 1% )by PL, and time-resolved PL. After the PL dependence on temperature and excitation power and PL dynamics were measured, the new PL peak was identified as an intrinsic transition of alloy, rather than N-related bound states. This is the first observation in PL, showing that alloy state exists in GaNx As1- x materials even when N composition is smaller than 0.1%. Finally by selective excitation,both type-Ⅰ and type-Ⅱ transitions were observed simultaneously in GaAs1-xSbx/GaAs SQWs for the first time.