Quantum dots in glass spherical microcavity

Glass spherical microcavities containing CdSexS1-x semiconductor quantum dots (QDs) are fabricated. The coupling between the optical emission of embedded CdSexS1-x QDs and spherical cavity modes is realized. When the luminescence of QDs is excited by a laser beam, the strong whispering gallery mode resonance with high Q factors is achieved in the photoluminescence spectra. (C) 2001 American Institute of Physics.

Optical transitions and type-II band lineup of MBE-grown GaNAs/GaAs single-quantum-well structures

We have investigated transitions above and below band edge of GaNAs/GaAs and InGaNAs/GaAs single quantum wells (QWs) by photoluminescence (PL) as well as by absorption spectra via photovoltaic effects. The interband PL peak is observed to be dominant under high excitation intensity and at low temperature. The broad luminescence band below band edge due to the nitrogen-related potential fluctuations can be effectively suppressed by increasing indium incorporation into InGaNAs. In contrast to InGaNAs/GaAs QWs, the measured interband transition energy of GaNAs/GaAs QWs can be well fitted to the theoretical calculations if a type-II band lineup is assumed. (C) 2001 Elsevier Science B.V. All rights reserved.

Analysis of the temperature-induced transition to current self-oscillations in doped GaAs/AlAs superlattices

We observed the decrease of the hysteresis effect and the transition from the stable to the dynamic domain regime in doped superlattices with increasing temperature. The current-voltage characteristics and the behaviours of the domain boundary are dominated by the temperature-dependent lineshape of the electric field dependence of the drift velocity (V(F)), As the peak-valley ratio in the V(F) curve decreases with increasing temperature, the hysteresis will diminish and temporal current self-oscillations will occur. The simulated calculation, which takes the difference in V(F) curves into consideration, gives a good agreement with the experimental results.

Conduction band offset and electron effective mass in GaInNAs/GaAs quantum-well structures with low nitrogen concentration

We have investigated the optical transitions in Ga1-yInyNxAs1-x/GaAs single and multiple quantum wells using photovoltaic measurements at room temperature. From a theoretical fit to the experimental data, the conduction band offset Q(c), electron effective mass m(e)*, and band gap energy E-g were estimated. It was found that the Q(c) is dependent on the indium concentration, but independent on the nitrogen concentration over the range x=(0-1)%. The m(e)* of GaInNAs is much greater than that of InGaAs with the same concentration of indium, and increases as the nitrogen concentration increases up to 1%. Our experimental results for the m(e)* and E-g of GaInNAs are quantitatively explained by the two-band model based on the strong interaction of the conduction band minimum with the localized N states. (C) 2001 American Institute of Physics.

Phonon-induced Raman scattering in GaNAs

The Raman scattering spectra of MBE-grown GaNAs epilayers were investigated. The resonant enhancement of Raman scattering due to the E+ states in the conduction band was observed and the Raman peaks related to the phonons at non-Gamma points of the Brillouin Zone were detected. II was clearly seen that the local vibrational mode induced by nitrogen impurities evolves to the GaN-like lattice phonon mode when the nitrogen content increases. By comparing the Raman spectra measured before and after 850 degreesC rapid thermal annealing, it was tentatively suggested that two weak peaks were induced by the pairing or clustering effect of nitrogen.

Strain-compensated quantum cascade lasers operating at room temperature

Quantum cascade (QC) lasers based on strain-compensated InxGa(1-x)As/InyAl(1-y)As grown on InP substrate using molecular beam epitaxy is reported. The epitaxial quality is demonstrated by the abundant narrow satellite peaks of double-crystal X-ray diffraction and cross-section transmission electron microscopy of the QC laser wafer. Laser action in quasi-continuous wave operation is achieved at lambda approximate to 3.6-3.7 mum at room temperature (34 degreesC) for 20 mum x 1.6 mm devices, with peak output powers of similar to 10.6mW and threshold current density of 2.7kA/cm(2) at this temperature. (C) 2000 Published by Elsevier Science B.V.

High-performance strain-compensated InGaAs/InAlAs quantum cascade lasers

We report on the realization of quantum cascade (QC) lasers based on strain-compensated InxGa(1-x)As/In(y)A((1-y))As grown on InP substrates using molecular beam epitaxy. X-ray diffraction and cross section transmission electron microscopy have been used to ascertain the quality of the QC laser materials. Quasi-continuous wave lasing at lambda approximate to 3.54-3.7 mum at room temperature was achieved. For a laser with 1.6 mm cavity length and 20 mum ridge-waveguide width,quasi-continuous wave lasing at 34 degreesC persists for more than 30 min, with a maximum power of 11.4 mW and threshold current density of 1.2 kA cm(-2), both record values for QC lasers of comparable wavelength.

Influence of dual incorporation of In and N on the luminescence of GaInNAs/GaAs single quantum wells

The optical properties of above- and below-band-edge transitions have been investigated by incorporating In atoms into GaNAs/GaAs single quantum wells. The experimental results show that with increasing In concentration the interband luminescence is improved and the luminescence intensity below the band edge in GaInNAs/GaAs decreases significantly. An interpretation is given that N atoms are preferable to form a covalent bond with In than with Ga atoms in a GaInNAs alloy, due to the compensation of the atomic-size difference between In and N atoms on the GaAs substrate. The photoreflectance spectra of the GaInNAs/GaAs single quantum well support the assignment of an intrinsic mechanism to the high-energy luminescence peak. (C) 2000 American Institute of Physics. [S0003- 6951(00)01752-6].

Novel coupled multi-active region high power semiconductor lasers cascaded via tunnel junction

A novel semiconductor laser structure is put forward to resolve the major difficulties of high power laser diodes. In this structure, several active regions are cascaded by tunnel junctions to form a large optical cavity and to achieve super high efficiency. This structure can solve the problems of catastrophic optical damage of facet, thermal damage and poor light beam quality effectively. Low-pressure metalorganic chemical vapor deposition method is adopted to grow the novel semiconductor laser structures, which are composed of Si:GaAs/C:GaAs tunnel junctions, GaAs/InGaAs strain quantum well active regions. External differential quantum efficiency as high as 2.2 and light power output of 2.5 W per facet (under 2A drive current) are achieved from an uncoated novel laser device with three active regions.

Room temperature (34 degrees C) operation of strain-compensated quantum cascade lasers

A short wavelength (lambda similar or equal to 3.5 mu m) strain-compensated InxGa(1-x)As/InyAl(1-y)As quantum cascade laser is reported. Quasi-continuous wave operation of this device at 34 degrees C with an output power of 11.4mW persisted for more than 30 minutes without obvious degradation. A very low threshold current density of 1.2KA/cm(2) at this temperature was observed.

Excitation transfer in vertically self-organized pairs of unequal-sized InAs/GaAs quantum dots

The excitation transfer processes in vertically self organized pairs of unequal-sized quantum dots (QD's), which are created in InAs/GaAs bilayers with different InAs deposition amounts in the first and second layers, have been investigated experimentally by photoluminescence technique. The distance between the two dot layers is varied from 3 to 12 nm. The optical properties of the formed pairs of unequal-sized QD's with clearly discernible ground-state transition energy depend on the spacer thickness. When the spacer layer of GaAs is thin enough, only one photoluminescence peak related to the large QD ensemble has been observed as a result of strong electronic coupling in the InAs QD pairs. The results provide evidence for nonresonant energy transfer from the smaller QDs in the second layer to the larger QD's in the first layer in such an asymmetric QD pair.

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.

Interband luminescence and absorption of GaNAs/GaAs single-quantum-well structures

We have investigated the interband electron transitions in a GaNAs/GaAs single quantum well (QW) by photoluminescence and absorption spectra. The experimental results show that the dominant photoluminescence at low temperature and high excitation intensity originates from transitions within the GaNAs layer. The interband transition energy for QWs with different well widths can be well fitted if a type-II band line up of GaNAs/GaAs QWs is assumed. (C) 2000 American Institute of Physics. [S0003-6951(00)03220-4].

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.

Effective method for evaluation of semiconductor laser quality

The measurements of one hundred 1.3 mu m planar buried crescent (PBC) structure InGaAsP/InP lasers demonstrate that parameters given by the electrical derivative of varied temperature and the variation of the parameters with temperature can be used to appraise the quality and reliability of semiconductor lasers effectual. By measurement of electrical derivative curves one can evaluate the quality of epitaxial wafer and chip, find the problems in the material and the technology, offer the useful information on increasing the quality and improving the technology of devices. (C) 2000 Elsevier Science Ltd. All rights reserved.