Shigeru Minomura

Nondestructive depth profiling using soft X-ray emission spectroscopy by incident angle variation method

We constructed a sample reclining stage for soft X-ray emission spectroscopy (SXES), where we intended to carry out nondestructive depth profiling using an incident angle variation (IAV) method. The penetration depth of an incident electron decreases with a reclining sample stage. One of the characteristics of the IAV method is that the incident electron beam energy is constant; thus, it is easy to obtain information related to a quantitative analysis of the integrated intensity of soft X-ray emission spectra. This method has been applied for a specimen with NiSi2/Si(111) structure. We have analyzed the change in shape of the Si L2,3 emission band spectrum in a region of the Si substrate to the NiSi2 layer, and have shown that we can determine the distribution of the Si element from the integrated intensity of Si L2,3 emission spectra.

The High Pressure Phase in the Chalcopyrite AgGaTe2

High pressure X-ray diffraction measurements were performed on AgGaTe2 chalcopyrite semiconductor up to 30 GPa; the space group and atomic positions under high pressure were refined by the Rietveld method. The first phase transition occurred at 3.1 GPa from the chalcopyrite phase to the coexisting phase of the P-4 which is disordered zinc blende (d-B3) and a second phase which is not inconsistent with a Cmcm structure but this is by no means proven. Between 3.1 and 5.4 GPa, all three phases were apparent in the X-ray spectra until the chalcopyrite phase disappeared at 5.4 GPa. Using the Birch equation, the bulk moduli of the chalcopyrite, d-B3, and the very tentative Cmcm phases were determined to be 71.5, 55.2, and 34.0. GPa, respectively. Moreover, two high-pressure phases appeared around 8.5 GPa and these phases existed up to 30 GPa. One phase was assigned as the disordered NaCl (d-B1) structure. The d-B3—d-B1 transition was accompanied by a volume reduction of 8% which is compatible with the phase transition from a fourfold-coordinated phase to a sixford-coordinated phase transition.

Photoluminescence and Raman spectra of Si/Si1−xGex strained superlattices under high pressure

Abstract We investigated photoluminescence spectra in strained Si/Si 1− x Ge x superlattices as a function of pressure. The double-crystal X-ray diffraction patterns prove that there is tetragonal lattice distortion in the alloy layer grown epitaxially on a Si substrate. The effects of lattice distortion and alloy composition are confirmed by Raman spectra. The excitonic photoluminescence spectra are assigned to the freeexcitonic no-phonon transitions and their phonon replicas. The photoluminescence peaks show negative pressure dependencies of −15.0 to −16.0meV/GPa. This result means that the conduction-band minimum is at the X point as in bulk Si.

Photoluminescence Spectra of (GaAs)12(AlAs)12 Superlattice under High Pressure

Photoluminescence spectra of MBE-grown (GaAs) 12 (AlAs) 12 superlattices under high pressure and low temperature are measured. Two main emission bands are observed. The higher emission band behaves like a \(\varGamma\)-edge under pressure; the lower emission band, on the other hand, behaves like an X -edge. This result shows that the \(\varGamma-X\) crossover occurs beyond the twelfth layers in (GaAs) m (AlAs) m superlattices.

Gamma -X crossover in GaAs/AlAs short period superlattice under pressure

Photoluminescence (PL) spectra under pressure have been measured at 80 K for MBE grown (GaAS)m(AlAs)m superlattices (SLS). Two main emission bands have been observed for the (GaAs)12(AlAs)12SL. The higher emission band (1.785 eV) behaves like a Gamma edge under pressure and the lower emission band (1.749 eV), on the other hand, behaves like an X edge. One emission band at 1.664 eV has been observed for (GaAs)18(AlAs)18SL at atmospheric pressure. This band moves to higher energy when pressure is applied, which shows the lowest lying conduction band is Gamma like. The PL emission intensity of this SL decreases abruptly to the order of 10-2 at 7.5 kbar, which is due to the pressure-induced Gamma -X crossover. The pressure-induced change-over from Gamma - to X-like emission has been clearly observed for (GaAs)15(AlAs)15SL. These results give conclusive evidence to verify the finding that, on increasing the number of monolayers, the lowest conduction band edge of (GaAs)m(AlAs)m switches from the X-like to Gamma -like state at a thickness of 14 monolayers.

Resonant tunneling in triple barrier diode under pressure

Pressure-induced changes in the resonant voltages(quantum bound state energies) and the resonant currents(wave functions) in Al0.4Ga0.6As/GaAs triple barrier diode reveal that both of the ΓbXb contact (the subscript b denotes the barrier potential) and the ΓbXw alignment (the w denotes the well potential) in energy induce new boundary conditions for the Kronig-Penny potential.

Electron emission spectra of DX center under pressure

Multiple emission‐rates of the DX center were investigated using isothermal capacitance transient spectroscopy. Four emission‐rates were identified for the DX center in Al0.3Ga0.7:Si (Si‐DX) and seven emission‐rates were identified for that in Al0.3Ga0.7As:Te (Te‐DX). The thermal emission energy of Si‐DX for each of the four emissions is the same energy of 0.41±0.02 meV. It means that there are at least five emissions from the DX center in AlGaAs:Si with 0.34 eV emission observed in GaAs and dilute AlGaAs. Pressure effects of Si‐DX were investigated. The emission energy and the isothermal emission‐rates are independent on pressures. The result is discussed based on the microscopic model for the DX center.

Resonant tunneling in triple barriers diode under pressure

Fine resonant tunneling was investigated under pressures up to 2 GPa at 77 K in Al0.4Ga0.6As/GaAs triple barrier diode. It was found that the resonant current decreased linearly with increasing pressure if the total current increased by the pressure‐induced band alignment of the X‐valley in the barrier with the resonant state in the well. This result indicates that the increase in the non‐resonant (ΓXX) current does not decrease the resonant (ΓΓΓ) current.

Multiplicity and lattice relaxation of the dx center studied by electron emission spectra under pressure

Abstract Multiple emission-rates of the DX center were investigated using isothermal capacitance transient spectroscopy (ICTS) under pressure. Four emission rates were resolved for the DX centers in Al 0.3 Ga 0.7 As:Si(Si-DX) and eight emission-rates were resolved for those in Al 0.3 Ga 0.7 As:Te(Te-DX). The thermal emission energies of Si-DX for each of the four emission rates are the same at 410 + 2 meV. All emission energies and isothermal emission rates of Si-DX are independent of pressure. The emission energies of Te-DX with four nearest neighbor (1 nn) Ga atoms and that with four 1 nn Al atoms are 238 ± 3 meV and 288 ± 6 meV, respectively.

Study of Sensitization Processes in Superionic Conductor α-AgI by Photoacoustic Spectroscopy (I)

The change in the photoacoustic (PA) spectrum of α-AgI has been measured by exposing an above-band-gap light as a function of time from 0 to 20 min at 443 K. It has been found that the PA signal below the band gap of α-AgI increases monotonically up to a factor of 4 with increasing lapse of exposure time. The behavior seems to be attributable to a photoinduced clustering of Ag+ ions.