ALBERT

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: Significant deformation of a grooved Si surface is discovered during low temperature processing (650–900 °C) in a ultra high vacuum. The lowest temperature at which deformation results is determined to be 650 °C for a (111)Si substrate. In addition, clear facets are formed after high temperature processing (850–900 °C). These new findings are tentatively considered to originate from the surface migration of Si atoms on atomically clean surfaces.

Notes: Limited-area molecular-beam epitaxial growth of Si1−xGex films, on a Si substrate with patterned SiO2, has been studied in order to eliminate misfit dislocations in Si1−xGex/Si heterostructures. This method is found to dramatically reduce misfit dislocations in Si1−xGex films. Hence, a thicker Si1−xGex film can be grown, without introducing misfit dislocations, on a Si substrate with patterned SiO2 than on a Si substrate without patterned SiO2. This phenomenon is attributed to the blocking of misfit dislocation extension and to a partial relaxation of the residual strain in the boundary between the crystal and polycrystalline Si1−xGex film.

Notes: Co/Pd(111) and Ni/Pd(111) bilayer films were grown epitaxially on single crystal MgO (111) substrates by ultrahigh vacuum deposition. In situ observation of the strain of Co and Ni deposited on Pd (111) plane was performed by reflection high energy electron diffraction (RHEED) during fabrication. From RHEED patterns it was confirmed that the lattice constant of a Co film of 2 A(ring) in thickness deposited on Pd was about 3.7 A(ring), which was different from that of Pd (3.89 A(ring)). The strain (εCo) of the Co layer was rapidly released during further Co deposition. The strain εCo of the third Co monatomic layer on Pd is nearly equal to zero. The large strain of the Co layer is localized at about one or two monatomic layers of the interface between Co and Pd. On the other hand, a Ni film deposited on Pd was not strained from the beginning of the deposition. The relationship between the perpendicular magnetic anisotropy of Co/Pd and Ni/Pd compositionally modulated multilayer films and the strain of Co and Ni is discussed.

Notes: A cell and gas-handling system of the supercritical material for x-ray absorption measurement are designed. The cell was made of stainless steel with a pair of Be windows of 3 mm thickness. The x-ray path can be controlled by changing the distance between two Be windows. Electric cartridge heaters are embedded in the cell to control the temperature. With this system we have studied the supercritical state of CF3Br, whose critical point is 67 °C and 39.1 atm. The results of x-ray absorption fine structure measurements for Br K edge for gaseous, liquid, and supercritical states of this material are presented together with the performance of the gas-handling system. © 1995 American Institute of Physics.

Notes: A new method to fabricate atomic layer doping structures in silicon using a combination of molecular-beam epitaxy and solid-phase epitaxy is developed. The antimony dopants are restricted to a thickness less than the resolution limit of secondary ion mass spectrometry, and exhibit extremely sharp C-V carrier profiles of less than 2 nm for a single Sb monolayer (ML). Hall and resistivity data show full activation for 0.1 ML and saturation at 8×1013 electrons cm−2. Measurements down to 4 K show metallic conduction for highly doped samples (above 0.05 ML) two-carrier conduction for intermediate levels, and strong freezeout for low amounts of Sb (below 0.01 ML). The two-dimensional electron gas is shown to exist by angular dependence of magnetoresistance.

Notes: Pinhole-free, pseudomorphic CoSi2 films with thicknesses from 1.3 to 16.4 nm have been grown epitaxially on Si(111) by sequentially depositing thin Co and Si films at room temperature, and subsequently annealing the resulting a-Si:Co2Si:Si(111) structure at 670 K. The film morphology is studied by high-resolution Rutherford backscattering, transmission electron microscopy, and reflection high-energy electron diffraction. The absence of pinholes in the as-grown layers is explained by a lowering of the barrier of CoSi2 nucleation owing to the presence of amorphous Si. Upon further heating the films remain uniform up to a temperature of 1000 K. Above that temperature the layers break up in islands, which corresponds with the thermodynamically most stable morphology. The lattice strain in pseudomorphic layers is found to persist even after islanding.

Notes: Electroluminescence (EL) was observed at temperatures up to 60 °C in p-type strained Si0.65Ge0.35/Si multiple-quantum-well (MQW) diode structures grown on Si(111) substrates by Si molecular-beam epitaxy. No-phonon line and its transverse optical phonon replica were well-resolved in the room-temperature EL spectrum for the first time. EL spectrum was dominated by a broad alloy band located below the band-edge state by ≈100 meV at lower temperatures, which was taken over by clear band-edge emissions at elevated temperatures. The emission intensity of the alloy band exhibited a saturation behavior with increasing injection current while the band-edge emission was found to develop with a power exponent of 1.4.

Notes: A novel method for modulating frequency-doubled blue radiation from an AlGaAs laser diode was developed utilizing the electro-optic effect in a KNbO3 nonlinear crystal. Stable blue radiation was obtained from an external ring resonant cavity containing a KNbO3 crystal. The weakly excited counterpropagating mode in the resonant cavity was re-injected into the laser diode to establish self-locking. Nearly 100% intensity modulation was observed when a 3 kV/cm ac electric field was applied parallel to the c axis of the KNbO3 crystal.

Notes: Nanometer-scale Si quantum dots have been spontaneously fabricated on SiO2 by controlling the early stages of low-pressure chemical vapor deposition from pure silane. The tunneling current through Au/1 nm-SiO2/a single Si quantum dot/1 nm-SiO2/n+-Si(100) double-barrier structures has exhibited the clear current bump or negative conductance at 300 K with a peak current to valley ratio as high as 10. © 1997 American Institute of Physics.