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Notes: The interfacial reaction in Ni and amorphous Si (a-Si) multilayers (Ni/a-Si) has been studied. Transmission electron microscope observation was used to monitor the progress of the solid-state reaction. It was found that amorphous Ni-silicide phase [a-(Ni,Si)] is the first phase formed in the Ni and a-Si interfacial reaction. A relatively large composition range for the amorphous phase exists in these Ni/a-Si multilayers. In the as-deposited Ni/a-Si multilayers with shorter modulation period, the uniform a-(Ni,Si) phase forms at least in the composition range of 25–62 at. % Ni. These results are consistent with predictions from the calculated Gibbs free-energy diagram. The δ-Ni2Si phase is the preferred phase in the crystallization process of a-(Ni,Si) even for the equiatomic Ni/a-Si multilayers. The mechanism that controls phase selection in the Ni/a-Si interfacial reaction is discussed using nucleation theory. A nucleation control model for phase selection is proposed.

Notes: Interdiffusion-induced solid-state amorphization reaction (SSAR) in polycrystalline Ni/amorphous Si multilayers has been studied using an in situ x-ray diffraction technique together with transmission electron microscope observations. The amorphization reaction was found to occur both on the Ni/Si interfaces in terms of a planar-layer growth model and along the grain boundaries in the Ni sublayers. Thermodynamic and kinetic interpretations for the SSAR at grain boundaries are presented and an amorphous growth model is also suggested for elucidating the SSAR in polycrystalline Ni/amorphous Si multilayers.

Notes: We studied interfacial reactions in Co/amorphous Si(a-Si) multilayers by transmission electron microscopy. We found that an intermixed layer of amorphous cobalt silicide formed in the as-deposited state. To explain the solid-state amorphization reaction, two parameters were used. They were the thermodynamic driving force (heat of formation) and the interfacial energy. The initial amorphization reaction in Co/a-Si multilayers was thermodynamically and kinetically favored. However, the formed amorphous interlayer remained about 1 nm thick and did not grow thicker with increasing modulation period and annealing temperature. The reason for this phenomenon was that the amorphous interlayer acted as a diffusion barrier to impede the amorphization reaction in Co/a-Si multilayers. Co2Si phase was always the preferred phase in the crystallization process for different average compositions of the multilayers. The mechanism that controlled the phase selection in Co/a-Si interfacial reaction was interpreted by using the model of modified heat of formation.

Notes: Little is known about the interdiffusion in the amorphous Ni–Si multilayer due to the lack of suitable experimental method. In this paper, the interdiffusion phenomena in the amorphous Ni–Si multilayer are investigated by an in situ x-ray diffraction technique. The temperature-dependent interdiffusivity obtained by monitoring the decay of the first-order modulation peak as a function of annealing time can be described in terms of the Arrhenius relation. The effective interdiffusivities can be expressed as De(T)=2.13 ×10−17 exp[−(0.61±0.02)/kBT] m2/s (423–613 K). A retarded interstitial diffusion mechanism is suggested to explain the diffusion process in the amorphous multilayer films.

Notes: We present a theoretical investigation of the growth orientation dependence of valence-subband structures in ZnSxSe1−x/ZnyMg1−ySzSe1−z quantum wells grown in the [001], [115], [113], [112], and [111] directions. The results indicate that the in-plane effective mass of the heavy-hole subband in the [111]-oriented structure is substantially smaller than that in the [001] quantum wells. For applications to quantum-well lasers, the lighter effective mass will lead to a smaller threshold current density, and therefore a better laser performance. Our investigations should provide useful guidelines for the design of II-VI quantum-well blue lasers.

Notes: This article reviews our recent works on frequency control of semiconductor lasers. The magnitudes of quantum noise limited frequency modulation (FM) noise, realized by the negative electrical feedback, are given for four methods of using an external Fabry–Perot cavity as a frequency demodulator. It is shown that the theoretical expression for the quantum noise-limited FM noise of the feedback laser contains a factor of 1/8 as compared with that of the free running laser, which is due to the different ways of injecting the vacuum fluctuations to the laser cavity and to the external Fabry–Perot cavity for negative electrical feedback. The FM sideband technique is shown to be an effective method to reject the contribution of laser power fluctuations to the FM noise detection for the negative electrical feedback system. As a candidate for a high reflectivity and frequency selective external reflector for the optical feedback, characteristics of the semiconductor laser as a phase conjugate mirror, i.e., the characteristics of the nearly degenerate four-wave mixing and the nondegenerate four-wave mixing in a semiconductor laser, are shown. Optical feedback by using a velocity selective optical pumping and polarization spectroscopy of an atomic vapor is proposed as an effective method to realize simultaneously the center frequency stabilization and linewidth reduction of the field spectrum of the laser, and also the fine detuning of the stabilized center frequency.For the heterodyne frequency locking between two lasers, a spectroscopic method of using a Doppler-free spectrum of the three-level atomic vapor, obtained by using the phenomenon of coherent population trapping, is shown. In order to realize a highly efficient nonlinear optical frequency conversion for wideband frequency sweep of semiconductor lasers, a method of adding the output powers of several lasers, i.e., the coherent addition, is presented. After emphasizing that the wideband frequency sweep (covering from the near-infrared to the visible region) can be realized by using the techniques of nonlinear optical frequency conversions and the optical phase locking, relevant experimental results of nonlinear optical frequency conversions are presented which are the second harmonics generation, sum and difference frequency conversions. A highly accurate optical frequency measurement system is proposed using an optical frequency comb generator with a modulation sidebands up to several THz. Performances of the optical frequency comb generator used for this system are presented. As a candidate for an ultrafast and wavelength insensitive photodetector for optical frequency counting, nonbolometric optical response characteristics of a high transition temperature (Tc) oxide superconducting film are demonstrated.

Notes: The band-to-band Auger recombination rate in bulk GaSb and in a GaSb quantum well is calculated. It turns out to be larger in the quantum well because the threshold of the Auger process is located at the band edge where the density of states is larger in the quantum well than in bulk. A simple picture is developed to illustrate the physics of the Auger processes in bulk and quantum-well direct-band-gap semiconductors. With this picture, we propose that the composition-disorder-induced band mixing should be considered in order to explain the unusual behavior of the Auger process in an InGaAsP quantum-well structure.

Notes: Theoretical investigations are presented of the electric-field dependence of normal-incidence interconduction subband absorption in Ga1−xAlxSb/AlSb L-valley quantum wells. Under an applied electric field of 50 kV/cm, a blue shift of the absorption peak from 4.94 to 4.82 μm was found in a Ga0.7Al0.3Sb/AlSb structure with well width of 25 A(ring). The ability to absorb normally incident light and to achieve significant Stark shifts with bias makes the Ga1−xAlxSb/AlSb L-valley system an attractive choice for the 3–5 μm vertical optical modulators.

Notes: In this article, we propose and demonstrate a novel punchthrough heterojunction phototransistor (HPT). The base of the transistor is lightly doped and completely depleted under the operating condition. The collector bias current can be applied without the base terminal. The transistors exhibit optical conversion gain as high as 1240 at an incident optical power as low as 0.5 μW, and the gain changes less than 15% over a 20 dB range of incident optical power. The transient measurements showed that the transistor has a high response speed than that of conventional two or three terminal HPTs. This represents the best performance of HPTs with similar dimensions. The results of simulation showed that the punchthrough HPTs have much lower noise characteristics than conventional HPTs. The principle reported here can be applied to HPTs made from other material systems, such as AlGaSb/GaSb and InP/InGaAs, for long wavelength optical communications.

Notes: We present the first systematic studies of infrared absorption from interconduction subband transitions for AlAs/Ga1−xAlxAs X-valley superlattices grown in the [001], [115], [113], [112], [111], and [110] directions. In the AlAs quantum well material, electrons occupy X valleys with ellipsoidal constant energy surfaces. Due to the effective mass anisotropy of electrons in the ellipsoidal valleys, these structures can absorb normally incident radiation when the superlattice growth direction is not collinear with the principal axes of at least one of the ellipsoids (i.e., not grown along the 〈001(approximately-greater-than) directions). For both parallel and normal incidence radiation at wavelengths of 12–20 μm, peak absorption coefficients of 3000–6000 cm−1 were obtained for the [113] and [112] superlattices with well widths in the range of 30–50 A(ring) and sheet doping concentrations of 1012 cm−2. Their ability to detect normally incident light and to obtain absorption comparable to that in the GaAs/Ga1−xAlxAs superlattice detectors makes these novel structures promising for use as normal incidence infrared photodetectors.