Further integration in semiconductor devices requires less-damaged wafers and damage-free fabrication processes, and sensitive and nondestructive evaluation for low-level damages is in high demand. A new measurement method for photothermal displacement has been developed and successfully applied to evaluation of lattice damages in semiconductor. When a semiconductor surface is illuminated by a focused and intensity-modulated laser pump beam, an absorbed laser power causes a small photothermal displacement of several tens of picometers on the surface depending upon thermal properties of the surface layer. The thermal properties, especially thermal conductivity, are drastically influenced by the concentration of lattice damages. A highly sensitive laser heterodyne-interferometric probe with a resolution of 0.1 picometers detects small displacement and quantifies lattice damages in the surface layer. This paper describes the principle of this method focusing upon how to obtain high sensitivity to photothermal displacement, in other words, to lattice damages. Demonstrations of photothermal displacements observed with polished wafers, low-dose ion implanted wafers and ECR etched wafers, and discussions on the limit of this photothermal displacement are also presented.
In order to lower the epitaxy temperature of GaAs on ZnSe, effect of photo-irradiation on the growth and/or reaction mechanism in the initial growth stage in MOMBE of GaAs has been studied by using the surface photo-absorption (SPA) technique. It has been found that excess carriers photoinduced in ZnSe can contribute to the decomposition of TEGa on ZnSe, but those photoinduced in GaAs cannot. This has been attributed to the photocatalytic effect on the decomposition of TEGa on the ZnSe surface. It has been confirmed that the photo-irradiation is effective to enhance the growth rate at low temperatures, resulting in decreased epitaxy temperature of GaAs on ZnSe.
Scanning tunneling microscopy (STM) is used to elucidate the atomic structure of a Si (111)√3×√3: Al surface and the solid phase epitaxy (SPE) processes of Ar-sputtered Si (100) surface. A √3×√3: Al structure is observed upon annealing at 580°C where Al adatoms occupy the T4 sites. At a domain boundary between 7×7 and √3×√3 phases, Al adatoms are located in the unfaulted half of the 7×7 unit cell. The phase transition from √3×√3 to 7×7 structure occurred at the annealing temperature of 680_??_700°C. On the other hand, the 2×2 and c(4×4) reconstructed surfaces are observed upon annealing of amorphodized Si by Ar sputtering at 587_??_621°C. The possible arrangements of the missing-dimer defect are proposed to explain corresponding 2×2 and c(4×4) reconstructed structures.
Microcharacterization of semiconductor devices is performed using scanning tunneling microscopy (STM). Novel impurity profiling method employing chemical etching and STM measurements is proposed. This method reveals localization of Zn segregation at the heterointerface of n-InP/InGaAsp and enables profiling a wide range of boron impurity concentration (1016_??_1020cm-3) with spatial resolution of 10nm. Spectroscopic technique is successfully applied to identify each layer of double heterostructure for laser diodes. This technique also clarified that spectra for ultra-thin (1.5nm) SiO2 on Si structure reflect the band structure of Si substrate together with SiO2/Si interface gap-states and spectra on depressed sites of STM images show negative differential resistance associated with oxide defects. Fowler-Nordheim tunneling characteristics is also found for the relatively thick (2.5nm) SiO2/Si structures.
The frequency stabilization of a semiconductor laser using an absorption line of atoms under direct frequency shift keying (FSK) is degraded because of inevitable sideband spectra. However, we devised the peak-hold method and obtained the better stability than in a traditional method under direct FSK. The stability hardly changed with the duty ratio of FSK signal in the peak-hold method. Two oscillation frequencies, i.e. mark and space frequencies of FSK signal, did not change with its duty ratio either, although they changed in the traditional method. In our modulation frequency, the oscillation frequency changed slowly and deviated from FSK in spite of rectangular wave modulation to the injection current because of the frequency chirping of the diode laser by a thermal effect. Therefore, we improved FSK signal using the modified modulation signal through the RC circuit, which improved the frequency stability and lessened the frequency deviation.
In order to realize a superconductor/semiconductor three terminal devices such as SUBSIT, it is necessary to make a superconductor/semiconductor interface without Schottky behavior. We selected BSCCO superconductor and Te semiconductor as materials of the junction and conducted a basic experimental study concerning, (i) the electrical property of BSCCO/Te junction and, (ii) the characteristics of three terminal devices with the BSCCO/Te junction. It is found that the BSCCO/Te junction reveals small Schottky-barrier with the barrier height is about 17.5 meV at 10 K. In the three terminal devices composed of BSCCO, BSCO, Te and Au layers, the modulation of current-voltage characteristics of the BSCCO/Te junction is observed by current injection in the BSCCO/BSCO/BSCCO junction. The current gain coefficient is about 7.2×10-3 at 4.2K.
A superconducting compact synchrotron light source, the NIJI-III has been completed. The development of the NIJI-III was entrusted to Sumitomo Electric Industries Ltd. by the Research Development Corporation of Japan and was deemed successful in October 1991 with the attainment of the design goal of the stored beam current of 200 mA at the final beam energy of 600 MeV. The lattice has a four-sector configuration; four superconducting bending magnets and eight quadrupole magnets, achieving the optimum beam characteristics for lithography. The supercondeucting bending magnet developed first time in the world was adapted, which was characterized by a cos θ type without an iron, a large bore of 200 mm, and a strong curvature of 0.5m. Futhermore, a newly developed vacuum system and an rf system were installed. The high-speed electron-beam wobbling method was investigated at a 20Hz triangular wave for the exposure of vertical field of the synchrotron light. We demonstrated that the expansion of the vertical exposure area agreed with the theoretical calculation, and the there was no effect on the beam lifetime. The achievement of the NIJI-III can be expected to contribute much to the development of various research applications of synchrotron light.
In this paper, We estimate the probability density function for the stochastic process whose probability density function belongs to the exponential family. First, we hypothesize the candidate probability density functions on basis of a priori information, and then estimate the statistics of the probability density function by the maximum likelihood method. Next, a posteriori probability of each candidate density function can be calculated by the Bayesian theorem. After enough observation data were obtained, we may select the probability density function whose probability is the highest among the candidate functions. Furthermore, we investigate the asymptotic property of a posteriori probability. A posteriori probability of the density function which is the most closed to the true probability function in the sense of Kullback's information, approaches to one as data increase enough.