Temperature dependence of electron redistribution in modulation-doped InAs/GaAs quantum dots

In this work we report the photoluminescence (PL) and interband absorption study of Si-modulation-doped multilayer InAs/GaAs quantum dots grown by molecular beam epitaxy (MBE) on (100) oriented GaAs substrates. Low-temperature PL shows a distinctive double-peak feature. Power-dependent PL and transmission electron microscopy (TEM) confirm that they stem from the ground states emission of islands of bimodal size distribution. Temperature-dependent PL study indicates that the family of small dots is ensemble effect dominated while the family of large dots is likely to be dominated by the intrinsic property of single quantum dots (QDs). The temperature-dependent PL and interband absorption measurements are discussed in terms of thermalized redistribution of the carriers among groups of QDs of different sizes in the ensemble. (C) 2000 Elsevier Science B.V. All rights reserved.

Photoluminescence of Eu2+ doped ZnS nanocrystals

Eu2+ doped ZnS nanocrystals exhibit new luminescence properties because of the enlarged energy gap of nanocrystalline ZnS host due to quantum confinement effects. Photoluminescence emission at about 520 nm from Eu2+ doped ZnS nanocrystals at room temperature is investigated by using photoluminescence emission and excitation spectroscopy. Such green emission with long lifetime (ms) is proposed to be a result of excitation, ionization, carriers recapture and recombination via Eu2+ centers in nanocrystalline ZnS host.

Surface states induced photoluminescence from Mn2+ doped CdS nanoparticles

Manganese doped CdS nanoparticles were synthesized in basic aqueous solution by using mercapto acetate as capping reagents. The nanoparticles were characterized by HRTEM, EPR, photoluminescence and optical absorption measurements. Out of our expectation, doping of Mn2+ ions altered the recombination paths in CdS nanoparticles markedly. The surface stares facilitated PL from Mn2+ doped CdS nanoparticles is reported. A complete suppression of the emission from surface states at room temperature when doping with Mn2+ ions has been observed for the first time. (C) 2000 Elsevier Science Ltd. All rights reserved.

Static and dynamic electric field domain formation in a doped GaAs/AlAs superlattice

We have observed the transition from static to dynamic electric field domain formation induced by a transverse magnetic field and the sample temperature in a doped GaAs/AlAs superlattice. The observations can be very well explained by a general analysis of instabilities and oscillations of the sequential tunnelling current in superlattices based solely on the magnitude of the negative differential resistance region in the tunnelling characteristic of a single barrier. Both increasing magnetic field and sample temperature change the negative differential resistance and cause the transition between static and dynamic electric field domain formation. (C) 2000 Elsevier Science B.V. All rights reserved.

DX-like centers in n-type Al-doped ZnS1-xTex grown by molecular-beam epitaxy

Al-related DX-like centers were observed in n-type Al-doped ZnS1-xTex epilayers grown by molecular-beam epitaxy on GaAs substrates. The capacitance-voltage measurement, deep-level transient spectroscopy, and photoconductivity spectroscopy revealed that the behaviors of Al donors in ZnS1-xTex were similar to the so-called DX centers in AlxGa1-xAs. The optical ionization energies (E-i) and emission barriers (E-e) for the observed two Al-related DX-like centers were determined as E-i similar to 1.0 and 2.0cV and E-e similar to 0.21 and 0.39 eV, respectively. It was also shown that the formation of Al-related DX-like centers resulted in a significantly large lattice relaxation in ZnS1-xTex. (C) 2000 Elsevier Science B.V. All rights reserved.

Correlated structural and optical investigation of terbium-doped zinc oxide nanocrystals

Tb3+-doped zinc oxide nanocrystals with a hexagonal wurzite structure were successfully prepared by reaction between Zn-O-Tb precursors and LiOH in ethanol. Good incorporation of Tb3+ in ZnO nanocrystals is proved by XRD, FTIR, PL and PLE measurements. The presence of acetate complexes to zinc atoms on particle surfaces is disclosed by FTIR results. Emission from both Tb3+ ions and surface states in ZnO matrix, as well as their correlation were observed. The luminescence mechanism is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.

High power 980 nm Al-free strained quantum-well lasers for erbium-doped fiber amplifiers

In this paper, we reported on the fabrication of 980 nm InGaAs/InGaAsP strained quantum-well (QW) lasers with broad waveguide. The laser structure was grown by low-pressure metalorganic chemical vapor deposition on a n(+)- GaAs substrate. For 3 mu m stripe ridge waveguide lasers, the threshold current is 30 mA and the maximum output power and the output power operating in fundamental mode are 350 mW and 200 mW, respectively. The output power from the single mode fiber is up to 100 mW, the coupling efficiency is 50%. We also fabricated 100 mu m broad stripe coated lasers with cavity length of 800 mu m, a threshold current density of 170 A/cm(2), a high slope efficiency of 1.03 W/A and a far-field pattern of 40 x 6 degrees are obtained. The maximum output power of 3.5 W is also obtained for 100 mu m wide coated lasers. (C) 2000 Elsevier Science B.V. All rights reserved.

Photoluminescence in Si and Be directly doped self-organized InAs/GaAs quantum dots

We report on the photoluminescence in directly Si- and Be-doped self-organized InAs/GaAs quantum dots (QDs). When the doping level is low, a decrease in linewidth is observed. However, it will decrease the uniformity and photoluminescence peak intensity of QDs when the doping level is high. We relate this phenomenon to a model that takes the Si or Be atoms as the nucleation centers for the formation of QDs. (C) 2000 Elsevier Science B.V. All rights reserved.

Anomaly of the current self-oscillation frequency in the sequential tunneling of a doped GaAs/AlAs superlattice

An anomalous behavior of the current self-oscillation frequency is observed in the dynamic de voltage bands, emerging from each sawtoothlike branch of the current-voltage characteristic of a doped GaAs/A1As superlattice in the transition process from static to dynamic electric field domain formations. Varying the applied de voltage at a fixed temperature, we find that the frequency increases while the averaged current decreases. Inside each voltage band, the frequency has a strong voltage dependence in the temperature range where the averaged current changes with the applied de voltage. This dependence can be understood in terms of motion of the system along a limit cycle.

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation-delta-doped GaAs/Al0.3Ga0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E-LO at B > 27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E-LO + (E-2 - E-1), where E-2, and E-1 are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm(-1) around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LI, of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence: of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Room-temperature optical transitions in Mg-doped cubic GaN/GaAs(100) grown by metalorganic chemical vapor deposition

The mechanism of room-temperature optical transitions in a Mg-doped cubic GaN epilayer grown on GaAs(100) by metalorganic chemical vapor deposition has been investigated. By examining the dependence of photoluminescence on the excitation intensity (which varied over four orders) at room temperature, four different emissions with different origins were identified. A blue emission at similar to 3.037 eV was associated with a shallow Mg acceptor, while three different lower-energy emissions at similar to 2.895, similar to 2.716, and similar to 2.639 eV were associated with a deep Mg complex. In addition to a shallow acceptor at E congruent to 0.213 eV, three Mg-related deep defect levels were also found at around 215, 374, and 570 meV (from the conduction band). (C) 2000 American Institute of Physics. [S0021-8979(00)01904-6].

Effect of thermal annealing on hole trap levels in Mg-doped GaN grown by metalorganic vapor phase epitaxy

Deep trap levels in a Mg-doped GaN grown by metalorganic vapor phase epitaxy are studied with deep level transient spectroscopy (DLTS). The Mg concentration of the sample was 4.8 x 10(19) cm(-3), but the hole concentration was as low as 1.3x10(17) cm-3 at room temperature. The DLTS spectrum has a dominant peak D-1 with an activation energy of 0.41+/-0.05 eV, accompanied by two additional peaks with activation energies of 0.49+/-0.09 eV (D-2) and 0.59+/-0.05 eV (D-3). It was found that the dominant peak D-1 consists of five peaks, each of which has different activation energy and capture cross section. In order to investigate these deep levels further, we performed heat treatment on the same samples to observe the variations of activation energy, capture cross section, and amplitude of DLTS signals. It was found that the longer the heat treatment duration is, the lower the amplitude of DLTS peaks become. This suggests that the decrease of the DLTS signal originates from hydrogen atom outgoing from the film during the annealing process. The possible originality of multiple trap levels was discussed in terms of the Mg-N-H complex. (C) 2000 American Vacuum Society. [S0734-2101(00)01701-2].

Pulsed excimer laser annealing of Mg-doped cubic GaN

A KrF (248 nm) excimer laser with a 38 ns pulse width was used to study pulsed laser annealing (PLA) on Mg-doped cubic GaN alms. The laser-induced changes were monitored by photoluminescence (PL) measurement. It indicated that deep levels in as-grown cubic GaN : Mg films were neutralized by H and PLA treatment could break Mg-H-N complex. The evolution of emissions around 426 and 468 nm with different PLA conditions reflected the different activation of the involved deep levels. Rapid thermal annealing (RTA) in N-2 atmosphere reverts the luminescence of laser annealed samples to that of the pre-annealing state. The reason is that most H atoms still remained in the epilayers after PLA due to the short duration of the pulses and reoccupied the original locations during RTA. (C) 2000 Elsevier Science B.V. All rights reserved. PACS: 61.72.Vv; 61.72.Cc; 18.55. -m.

Dynamic dc voltage band observed within each current branch in the transition from static to dynamic electric-field domain formation in a doped GaAs/AlAs superlattice

A dynamic dc voltage band was found emerging from each sawtooth-like branch of the current-voltage characteristics of a doped GaAs/AlAs superlattice in the transition process from static to dynamic electric-field domain formation caused by increasing the sample temperature. As the temperature increases, these dynamic dc voltage bands expand within each sawtooth-like branch, squeeze out the static regions, and join up together to turn the whole plateau into dynamic electric-field domain formation. These results are well explained by a general analysis of stability of the sequential tunneling current in superlattices. (C) 1999 American Institute of Physics. [S0003-6951(99)04443-5].

Current self-oscillation induced by a transverse magnetic field in a doped GaAs/AlAs superlattice

We investigate the influence of a transverse magnetic field on the current-voltage characteristics of a doped GaAs/AlAs superlattice at 1.6 K. The current transport regimes-stable electric field domain formation and current selfoscillation-are observed with increasing transverse magnetic field up to 13 T. Magnetic-field-induced redistribution of electron momentum and energy is identified as the mechanism triggering the switching over of one process to another lending to a change in the dependence of the effective electron drift velocity on electric field. Simulation yields excellent agreement with observed results.