Study of GSMBE growth and characteristics of high-quality strained In0.63Ga0.37As/InP quantum wells

High-quality compressively strained In0.63Ga0.37As/InP quantum wells with different well widths (1-11 nm) have been grown coherently on InP substrates using a home-made gas source molecular beam epitaxy (GSMBE) system. The indium composition in the wells of the sample was determined by means of high-resolution X-ray diffraction and its computer simulation. it is found that the exciton transition energies determined by photoluminescence (PL) at 10 K are in good agreement with those calculated using a deformation potential model. Sharp and intense peaks for each well can be well resolved in the 10 K PL spectra. For wells narrower than 4 nm, the line width of the PL peaks are smaller than the theoretical values of the line-width broadening due to 1 hit interface fluctuation, showing that the interface fluctuation of our sample is within 1 ML. For wells of 7 and 9 nm, the PL peak widths are as low as 4.5 meV.

High-pressure study of optical transitions in strained In0.2Ga0.8As/GaAs multiple quantum wells

We have measured low-temperature photoluminescence (PL) and absorption spectra of In0.2Ga0.8As/GaAs multiple quantum wells (MQW's) under hydrostatic pressures up to 8 GPa. In PL, only a single peak is observed below 4.9 GPa corresponding to the n = 1 heavy-hole (HH) exciton in the InxGa1-xAs wells. Above 4.9 GPa, new PL lines related to X-like conduction band states appear. They are assigned to the type-II transition from the X(Z) states in GaAs to the HH subband of the InxGa1-xAs wells and to the zero-phonon line and LO-phonon replica of the type-I transition involving the X(XY) valleys of the wells. In addition to absorption peaks corresponding to direct exciton transitions in the wells, a new strong absorption feature is apparent in spectra for pressures between 4.5 and 5.5 GPa. This absorption is attributed to the pseudodirect transition between the HH subband and the X, state of the wells. This gives clear evidence for an enhanced strength of indirect optical transitions due to the breakdown of translational invariance in MQW structures. From experimental level splittings we determine the valence band offset and the shear deformation potential for X states in the In0.2Ga0.8As layer.

HIGH-PRESSURE STUDY OF GAMMA-X MIXING IN INAS/GAAS QUANTUM DOTS

We have measured low-temperature photoluminescence spectra of InAs quantum dots embedded in a GaAs crystalline matrix under hydrostatic pressures up to 7 GPa. Below 4.2 GPa the spectra are dominated by the Gamma-like electron-heavy hole (HH) exciton transition in the InAs dots. Above 4.2 GPa the spectra show two X-related luminescence bands which are attributed to the indirect type-I transition between X(Xy) and HH states of the dots and the type-II transition from X states in GaAs to InAs HH states, respectively. In the Gamma-X crossover regime we find evidence for a pronounced mixing interaction between InAs Gamma-like and GaAs X-like states. The corresponding interaction potential is estimated to be 9 meV.

PHOTOLUMINESCENCE STUDIES ON VERY HIGH-DENSITY 2-DIMENSIONAL ELECTRON GASES IN PSEUDOMORPHIC MODULATION-DOPED QUANTUM-WELLS

We present photoluminescence studies on highly dense two-dimensional electron gases in selectively Si delta-doped GaAs/In0.18Ga0.82As/Al0.25Ga0.75As quantum wells (N(s) = 4.24 x 10(12) cm-2). Five well-resolved photoluminescence lines centered at 1.4194, 1.4506, 1.4609, 1.4695 and 1.4808 eV were observed, which are attributed to the subband excition emission. The subband separations clearly exhibit the feature of a typical quantum well with triangle and square potential. These very intensive and sharp luminescence peaks with linewidths of 2.2 to 3.5 meV indicate the high quality of the structures. Their dependence on the excitation intensity and temperatures are also discussed.

Stress and resistivity controls on in situ boron doped LPCVD polysilicon films for high-Q MEMS applications

The simultaneous control of residual stress and resistivity of polysilicon thin films by adjusting the deposition parameters and annealing conditions is studied. In situ boron doped polysilicon thin films deposited at 520 ℃ by low pressure chemical vapor deposition (LPCVD) are amorphous with relatively large compressive residual stress and high resistivity. Annealing the amorphous films in a temperature range of 600-800 ℃ gives polysilicon films nearly zero-stress and relatively low resistivity. The low residual stress and low resistivity make the polysilicon films attractive for potential applications in micro-electro-mechanical-systems (MEMS) devices, especially in high resonance frequency (high-f) and high quality factor (high-Q MEMS resonators. In addition, polysilicon thin films deposited at 570 ℃ and those without the post annealing process have low resistivities of 2-5 mΩ·cm. These reported approaches avoid the high temperature annealing process (> 1000℃), and the promising properties of these films make them suitable for high-Q and high-f MEMS devices.

Potential light source for photonic integrated circuits based on the equilateral triangle resonator

The semiconductor microlasers based on the equilateral triangle resonator (ETR) can be fabricated from the edge-emitting laser wafer by dry-etching technique, and the directional emission can be obtained by connecting an output waveguide to one of the vertices of the ETR. We investigate the mode characteristics, especially the mode quality factor, for the ETR with imperfect vertices, which is inevitable in the real technique process. The numerical simulations show that the confined modes can still have a high quality factor in the ETR with imperfect vertices. We can expect that the microlasers is a suitable light source for photonic integrated circuits.

Growth and fabrication of high performance 980nm strained InGaAs quantum well lasers using novel hybrid material system of InGaAsP and AlGaAs

In this paper, we report on the design, growth and fabrication of 980nm strained InGaAs quantum well lasers employing novel material system of Al-free active region and AlGaAs cladding layers. The use of AlGaAs cladding instead of InGaP provides potential advantages in laser structure design, improvement of surface morphology and laser performance. We demonstrate an optimized broad-waveguide structure for obtaining high power 980nm quantum well lasers with low vertical beam divergence. The laser structure was grown by low-pressure metalorganic chemical vapor deposition, which exhibit a high internal quantum efficiency of similar to 90% and a low internal loss of 1.5-2.5 cm(-1). The broad-area and ridge-waveguide laser devices are both fabricated. For 100 mu m wide stripe lasers with cavity length of 800 mu m, a low threshold current of 170mA, a high slope efficiency of 1.0W/A and high output power of more than 3.5W are achieved. The temperature dependences of the threshold current and the emitting spectra demonstrate a very high characteristic temperature coefficient (T-o) of 200-250K and a wavelength shift coefficient of 0.34nm/degrees C. For 4 mu m-width ridge waveguide structure laser devices, a maximum output power of 340mW with GOD-free thermal roll-over characteristics is obtained.

Splitting the Fast and Slow Motions in Molecular Dynamics Simulations Based on the Change of Cold Potential Well Bottom

The atomic motion is coupled by the fast and slow components due to the high frequency vibration of atoms and the low frequency deformation of atomic lattice, respectively. A two-step approximate method was presented to determine the atomic slow motion. The first step is based on the change of the location of the cold potential well bottom and the second step is based on the average of the appropriate slow velocities of the surrounding atoms. The simple tensions of one-dimensional atoms and two-dimensional atoms were performed with the full molecular dynamics simulations. The conjugate gradient method was employed to determine the corresponding location of cold potential well bottom. Results show that our two-step approximate method is appropriate to determine the atomic slow motion under the low strain rate loading. This splitting method may be helpful to develop more efficient molecular modeling methods and simulations pertinent to realistic loading conditions of materials.