Junkichi Nakai

Photoacoustic Spectroscopy Theory for Multi-Layered Samples and Interference Effect

The Rosencwaig-Gersho theory of photoacoustic spectroscopy has been expanded into the two-layer model and the photoacoustic signal in the multi-layer model has been deduced from its approximations in special cases. Furthermore, the photoacoustic signal is discussed considering multi-reflection of the light in a sample. This calculation accounts for the interference effect on photoacoustic signals. As an example, the interference theory was applied to a sample of silicon on sapphire (SOS), which showed considerable effect of interference.

Some Optical Properties of Layer-Type Semiconductor GaTe

Some optical properties of GaTe single crystals, such as absorption coefficient, photoconductivity and reflectivity, have been measured. The crystal is usually p -type with carrier density 10 16 cm -3 , mobility 15 cm 2 /volt·sec and resistivity 20\(\varOmega\)·cm at room temperature. In the measurement of the photoconductivity, a minimum of the photoresponse corresponding to the strong line structure of absorption is observed and therefore the line structure is interpreted to be due to the formation of excitons. The energy gap and exciton binding energy are deduced from the shape of the absorption curve near the edge. The energy gap is 1.797 eV at 77°K and 1.700 eV at 300°K and the temperature coefficient ∂ E g /∂ T is found to be -4.35×10 -4 eV/°K. The exciton binding energy and the reduced effective mass associated with conduction and valence band are 0.025 eV and 0.089 m 0 , respectively. The reflectivity is found to change rapidly at the photon energy corresponding to the exciton absorption.

Electroabsorption of GaS around the Indirect Edge

The electroabsorption spectra in the indirect transition region are measured on layer compound GaS with experimental condition of incident light normal to the c -axis. Six phonon-assisted transitions are found at 4.2K and eight at 77K. The energy value of the indirect edge was found to be 2591±1 meV at 77 K and 2596±1 meV at 4.2 K. The line shape of the electroabsorption spectra are discussed in connection with the highly anisotropic crystal structure of GaS.

Electroreflectance of GaSe. I. Around 3.4 eV

Electroreflectance of layer compound e-GaSe is measured around 3.4 eV at 77 K and around 3.2 eV at 300 K with the Schottky-barrier technique for the modulation field F parallel to the c -axis and with the transverse electroreflectance for F ⊥ c , where the light is incident nearly normal to the layers. It is found from the experimental results that the structure is due to an excitonic transition at M 0 critical point. The transition energy is estimated the to be 3382 meV at 77 K and 3225 meV at 300 K, and binding energy of the exciton 9 meV.

Magnetophonon Resonance of Hot Electrons in n -InSb at 77 K

Magnetophonon resonances, which were first predicted by Gurevich and Firsov, have been observed in the hot electron mobility in pure n -InSb. The resonance experiment was carried out at 77 K in transverse magnetic field up to 22 kG with application of sufficiently small electric field to produce a slight change in the electron mobility. The quantity β in the formula µ=µ 0 (1-β E 2 ) was measured with extremely good accuracy ( Δ µ/µ 0 ∼10 -7 ) by adopting a bridge circuit. The minima in the quantity |β| were observed at magnetic fields, B =17.5, 11.3, 8.53, 6.89, 5.72, 4.83, 4.12, 3.68, 3.72, and 2.93 kG. These resonance extrema are attributed to resonant cooling of hot electrons due to optical phonon induced transitions from the high energy tail of the electron distribution near k z =0 at higher Landau levels to lower Landau levels. The extremal positions are well explained by taking the non-parabolicity of the conduction band of InSb into account.

Electrical and optical properties of ZnTe-GaSb heterojunctions☆

Heterojunctions between ZnTe and GaSb is produced by the interface-alloy technique with the use of movable quartz tube loaded in a furnace instead of a strip-heater. X-ray measurement shows that the recrystallized GaSb is monocrystalline and oriented parallel to the ZnTe substrate with (111) as the growth direction. Photoresponse for p-p ZnTe-GaSb heterojunctions decreases exponentially with decreasing incident photon energy for hν < 2·18 eV at room temperature. This behavior is explained by a model of linearly-graded bandgap region of the order of 500 A long, which was first proposed for GaAs-InSb heterojunction by Hinkley and Rediker. The photocurrent is produced by hot holes generated by absorbed photons in the graded-gap region which traverse this region with a mean free path of about 40 A. Space-charge-limited current observed in I–V characteristics of p-p ZnTe-GaSb heterojunctions is explained by Lampert theory. The thickness of semi-insulating region is estimated at about 1 to 3 μm from the capacitance measurements without d.c. bias. This thickness is much larger than the graded bandgap region. Capacitance depends strongly on both forward d.c. bias voltage and a.c. frequency, but is almost independent of reverse bias voltage. The results of capacitance measurements for forward bias are qualitatively interpreted in terms of a change in the trapped charge density in the semi-insulating region. A significant discrepancy between the theory and experimental results exists in the region of trap-free limit.

New series of the magnetophonon resonance under hot electron conditions in n-InSb

Abstract Measurements of the magnetophonon resonance in n -InSb under hot electron conditions are performed for the longitudinal configuration over a wide range of temperatures. Besides the ordinary one-phonon series, new minima are observed at magnetic field where two-phonon process is expected.

On the Tunneling Current through Thin Aluminum-Oxide Films

The mechanism of the electrical conduction in a metal-insulator-metal thin film diode is discussed by the tunneling process. Experimental current-voltage characteristic and its temperature dependence of an Al-Al oxide-Al diode agree well with a theoretical calculation when the potential barrier is assumed to have a geometrically simple shape. Practical barrier height of the diode is estimated at 2.2 eV from a photosensitive measurement. Electrode effects are also discussed from the viewpoint of rectification. The surface potential of the insulator on Al film is measured to be 3.6 eV, which is consistent with a simple potential diagram of the metal and insulator surfaces.

Electroreflectance of GaSe. II. 3.5-4.1 eV Region

Electroreflectance spectra of e-GaSe in the photon energy region 3.5-4.1 eV have been investigated at 77 K and at 300 K for F // c with the Schottky-barrier technique and for F ⊥ c with the transverse electroreflectance, where the light was incident nearly normal to the layers. It was found from the Kramers-Kronig analysis of the electroreflectance spectra at 77 K that the following five transitions exist whose transition energies, broadening energies in meV, and assignments, respectively, are those in the following parentheses: (3600±10, 40±5, M 1 ), (3700±5, ≃20, excitonic M 0 ), (3860±10, 100±10, M 1 2 D ), (3895±10, 100±10, M 1 2 D ), and (4040±10, 50±10, M 1 ). The anisotropy of the reduced masses at the M 0 and M 1 critical points are found to be not so large as expected from the highly anisotropic crystal structure.

Electroreflectance of Si-MOS

This paper describes the applicability of low-field electroreflectance (ER) to MOS capacitors to determine the surface properties of thermally oxidized silicon. The ER signal amplitude versus the dc gate voltage is calculated, including interface states. The flat-band voltage of the MOS structure is estimated from the ER method. This value is almost the same as that obtained from the conductance method within the experimental errors. It is shown that the relative change in reflectivity in the Si-MOS system includes not only the ER signal but also a signal possibly due to piezoreflectance ascribed to piezoelectrically active regions of the thermally grown Sio2. layer. This explanation is based on our observations of piezoelectric phenomena in Si MOS capacitors fabricated by thermal oxidation technique.