Yoshifumi Takanashi

The thermoelectric properties of bulk crystalline n- and p-type Mg2Si prepared by the vertical Bridgman method

Bulk Mg2Si crystals were grown using the vertical Bridgman melt growth method. The n-type and p-type dopants, bismuth (Bi) and silver (Ag), respectively, were incorporated during the growth. X-ray powder diffraction analysis revealed clear peaks of Mg2Si with no peaks associated with the metallic Mg and Si phases. Residual impurities and process induced contaminants were investigated by using glow discharge mass spectrometry (GDMS). A comparison between the results of GDMS and Hall effect measurements indicated that electrical activation of the Bi doping in the Mg2Si was sufficient, while activation of the Ag doping was relatively smaller. It was shown that an undoped n-type specimen contained a certain amount of aluminum (Al), which was due either to residual impurities in the Mg source or the incorporation of process-induced impurities. Thermoelectric properties such as the Seebeck coefficient and the electrical and thermal conductivities were measured as a function of temperature up to 850 K. The dimen...

Heteroepitaxial growth and characterization of InP on Si substrates

Heteroepitaxy of a highly mismatched system (∼8%), InP/Si, has been studied using low‐pressure organometallic vapor phase epitaxy. GaAs buffer layer effects on residual stress and defect density in InP/Si have been clarified. Using a 1‐μm‐thick GaAs buffer layer, residual stress in the InP layer has been reduced to as low as 2×108 dyn/cm2 compared to ∼4×108 dyn/cm2 for InP directly grown on Si. Moreover, the GaAs buffer layer has also been confirmed to be effective for improving InP/Si quality by evaluation of etch‐pit density, x‐ray diffraction measurement, and cross‐sectional transmission electron microscopy. Electrical properties of InP layers on GaAs/Si were evaluated with the van der Pauw and deep level transient spectroscopy (DLTS) methods. The heteroepitaxial layer’s own electron trap has also been observed by DLTS measurements. For an InP/GaAs/Si structure, InP growth temperature effect on surface morphology and etch‐pit density is also shown. High quality InP films with an etch‐pit density of 8×1...

Required Donor Concentration of Epitaxial Layers for Efficient InGaAsP Avalanche Photodiodes

Distinction between an avalanche and a zener breakdown has been made theoretically for the In1-xGaxAs1-yPy alloy lattice-matched to InP substrate. Experimental results of temperature dependence and donor concentration dependence of the breakdown voltages for In0.89Ga0.11As0.26P0.74 and In0.53Ga0.47As APDs are in good agreement with the theoretical result. Based on both theory and experiment, the critical donor concentration ND+, in which an avalanche and a zener mechanism exchange places, has been established for the In1-xGaxAs1-yPy alloys. It has been found that ND+s lie near 6×1014 cm-3 for In0.53Ga0.47As, 9×1015 cm-3 for In0.72Ga0.28As0.62P0.38 and 2×1017 cm-3 for InP, respectively.

Thermoelectric properties of semiconductorlike intermetallic compounds TMGa3 (TM=Fe, Ru, and Os)

Transport properties of polycrystalline TMGa3 (TM=Fe, Ru, and Os) compounds are reported in the temperature range 313K<T<973K. These compounds exhibit semiconductorlike behavior with relatively high Seebeck coefficient and electrical resistivity. Hall carrier concentrations at room temperature are reported in the range of 1017–1018cm−3. Seebeck coefficient measurements indicate that FeGa3 and OsGa3 are n-type material at the operating temperature; on the other hand, the Seebeck coefficient of RuGa3 changes its sign rapidly from large positive value to large negative value around 450K. The thermal conductivity of these compounds is estimated to be 3.5W∕mK at room temperature and decreases with increasing temperature. The absolute value of the lattice thermal conductivity for FeGa3, RuGa3, and OsGa3 is 3.5W∕mK at room temperature, and decreases down to 2W∕mK at high temperature. The resulting thermoelectric figure of merit ZT at 945K for RuGa3 reaches 0.18.

Effect of hot-electron injection of high-frequency characteristics of abrupt In/sub 0.52/(Ga/sub 1-x/Al/sub x/)/sub 0.48/As/InGaAs HBT's

The effect of hot-electron injection energy (E/sub i/) into the base on the high-frequency characteristics of In/sub 0/52/(Ga/sub 1-x/Al/sub x/)/sub 0.48/As/InGaAs abrupt heterojunction bipolar transistors (HBTs) is investigated by changing the composition of the emitter. There exists an optimum E/sub i/ at which a maximum current gain cutoff frequency (ft) is obtained. Analysis of hot-electron transport in the base and collector by Monte Carlo simulation is carried out to understand the above phenomenon. The collector transit time ( tau /sub c/) increases with E/sub i/, because electrons with higher energy transfer from the Gamma valley into the upper L and X valleys. At first, the base transit time ( tau /sub b/) decreases with E/sub i/ at the low E/sub i/ region. However, tau /sub b/ does not decrease monotically with E/sub i/, because of the nonparabolicity in the energy-band structure of InGaAs. Consequently, there exists a minimum in the sum of tau /sub b/ and tau /sub c/, in other words a maximum f/sub t/, at an intermediate value of E/sub i/. >

Temperature Dependence of Ionization Coefficients for InP and 1.3 µm InGaAsP Avalanche Photodiodes

Investigations were made on the temperature dependence of excess noise characteristics for InP and 1.3 µm InGaAsP APDs for the case where electrons are injected into the high field region. The effective ratio of the ionization coefficients Keff(β/α) was found to decrease with decreasing temperature for both APDs. Theoretical investigations on hot carriers are combined with McIntyres theory for the excess noise characteristics to explain the experimental results. The dominant scattering process for hot electrons is the intervalley scattering; this leads to the fact that the temperature dependence of the mean free path is greater in the case of electrons than holes. The theoretical analysis provides the best fit to the experimental results. On the basis of the theoretical results, the compositonal dependence of the mean free path for hot electrons is discussed.

Low-frequency noise of InP/InGaAs heterojunction bipolar transistors

The equivalent base noise SI/sub b/ of InP/InGaAs heterojunction bipolar transistors (HBTs) with a circular pattern emitter is investigated experimentally at a low frequency ranging from 10-10/sup 5/ Hz. The measured SI/sub b/ exhibits the 1/f dependence in an overall frequency range without any accompanying burst noise. Furthermore, SI/sub b/ varies as I/sub b//sup /spl gamma// for the base current I/sub b/ and as d/sup -2/ for the emitter diameter d, where the value of /spl gamma/ ranges from 1.62-1.72 depending on d of HBTs used. The 1/f noise model, which rigorously deals with the recombination current at the base surface I/sub bs/ as a function of I/sub b/ as well as of d is proposed. Applying our noise model to the dependence of SI/sub b/ on I/sub b/, as well as on d, reveals that even though /spl gamma/ is less than two, the origin of SI/sub b/ is due to the recombination of electrons at the exposed base surface near the emitter edges. On the basis of theoretical considerations for the diffusion length of electrons and traps at the base surface, the Hooge parameter /spl alpha//sub H/ for the noise due to the base surface recombination is deduced to be in the order of 10/sup -2/ for the first time.

Observation of deep levels in undoped GaSb grown by molecular beam epitaxy

Deep levels in undoped GaSb grown by molecular beam epitaxy are investigated with the deep level transient spectroscopy method. The measurements are performed on diodes consisting of undoped and Te‐doped GaSb layers. Three hole traps are detected in the undoped GaSb, the activation energies of shallowest and deepest traps being 0.25 and 0.63 eV, respectively. The depth profile of trap concentration suggests that the origin of these two traps is related to native defects or interdiffusion of Te from n+ layer. The capture cross sections of these traps are also measured by using the filling pulse technique.

Surface currents in InP/InGaAs heterojunction bipolar transistors produced by passivation film formation

The correlation between the passivation film formation for InP/InGaAs heterojunction bipolar transistors and transistor current-voltage (I-V) characteristics is investigated. The I-V characteristics vary significantly depending on the kind of passivation films. The major change in I-V characteristics is the increase in base current (Ib),which implies the generation of an additional recombination process around the surface of the emitter-base junction. A change in collector current (Ic) is also observed, indicating that the electron injection mechanism changed near the passivated semiconductor surface. These surface currents are produced by the interaction of the semiconductor surface with the passivation films. It is shown that these surface currents are greatly suppressed using a buffered hydrofluoric acid solution before the passivation film formation.

Deep trap in InGaAs grown by gas source molecular beam epitaxy

The deep-level transient spectroscopy (DLTS) measurement method is used to investigate the deep traps in In0.53Ga0.47As grown by gas source molecular beam epitaxy. An electron trap with activation energy of 0.28 eV is detected and a capture cross section is estimated to be 8.03×10−22 cm2 by measuring the dependence of DLTS spectrum on the majority carrier pulse width. The concentration of this trap is determined from the peak value of the DLTS signal to be 3.7×1014 cm−3.