Takuya Hoshi

Relation between Al vacancies and deep emission bands in AlN epitaxial films grown by NH3-source molecular beam epitaxy

Intensity ratios of characteristic deep cathodoluminescence (CL) bands at 4.6, 3.8, and 3.1eV to the near-band-edge emissions at 11K of AlN epilayers grown by NH3-source molecular beam epitaxy were correlated with the change in the S parameter of positron annihilation measurement, which represents the concentration or size of Al vacancies (VAl). Since the relative intensities of 3.1 and 3.8eV bands increased remarkably with lowering supply ratio of NH3 to Al (V/III ratio) and growth temperature (Tg), they were assigned to originate from VAl-O complexes. The VAl concentration could be decreased by adjusting V/III ratio and Tg, resulting in observation of fine excitonic features in the CL spectra. From the energy separation between the ground and first excited states, the binding energy of A exciton was determined to be 48meV.

Impact of strain on free-exciton resonance energies in wurtzite AlN

The strain dependence of the free-exciton resonance energies in AlN epilayers is presented and the values are analyzed using an appropriate Hamiltonian assuming equibiaxial stress for the wurtzite crystal structure in order to obtain valence band parameters. Based on the results, we study the strain dependence of the valence band ordering, optical transition probability, and free-exciton binding energy. As a result of these calculations, the following strain-free values are obtained for the energy gap, averaged dielectric constants, and ordinary and extraordinary dielectric constants: Eg=6.095 eV at T=11 K, ϵ=7.87, ϵ⊥=7.33, and ϵ∥=8.45, respectively. A brief discussion of the valence band ordering in bulk AlxGa1−xN is also presented.

Light polarization characteristics of m-plane AlxGa1−xN films suffering from in-plane anisotropic tensile stresses

Polarization characteristics of the near-band-edge optical transitions in m-plane AlxGa1−xN epilayers suffering from anisotropic stresses were quantified. The epilayers were grown by both ammonia-source molecular beam epitaxy and metalorganic vapor phase epitaxy methods on an m-plane freestanding GaN substrate. The light polarization direction altered from E⊥c to E∥c at the AlN molar fraction, x, between 0.25 and 0.32, where E is the electric field component of the light and ⊥ and ∥ represent perpendicular and parallel, respectively. To give a quantitative explanation for the result, energies and oscillator strengths of the exciton transitions involving three separate valence bands were calculated as functions of strains using the Bir–Pikus Hamiltonian. The calculation predicted that the lowest energy transition (E1) is polarized to the m-axis normal to the surface (X3) for 0<x≤1, meaning that E1 emission is principally undetectable from the surface normal for any in-plane tensile strained AlxGa1−xN. The ...

Improvement of High-Frequency Characteristics of InGaAsSb-Base Double Heterojunction Bipolar Transistors by Inserting a Highly Doped GaAsSb Base Contact Layer

This letter presents InP/GaAsSb/InGaAsSb/InP double heterojunction bipolar transistors (DHBTs) with a highly doped base contact layer. In order to reduce the base contact resistivity, a 3-nm-thick highly doped GaAsSb contact layer is inserted between the InP emitter and 17-nm-thick composition- and doping-graded InGaAsSb base. Fabricated DHBTs with a 0.25-μm emitter show a current gain of 32 and a high open-base breakdown voltage BVCEO of 5.2 V. The DHBTs also exhibit fT/fmax = 513/637 GHz at a collector current density of 9.5 mA/μm2 and VCE = 1 V. The fmax is higher by 124 GHz than that for InP/InGaAsSb DHBTs without the GaAsSb contact layer. These results indicate that the use of the GaAsSb/InGaAsSb base structure is very effective in improving fmax.

Composition- and Doping-Graded-Base InP/InGaAsSb Double Heterojunction Bipolar Transistors Exhibiting Simultaneous \(f_{t}\) and \(f_{\textrm {max}}\) of Over 500 GHz

We demonstrate composition- and doping-graded-base InP/InGaAsSb double heterojunction bipolar transistors (DHBTs) with a passivation ledge fabricated in a self-aligned process with i-line lithography. We obtained a high current gain of 52 and high breakdown voltage of 5 V for 0.2-μm-emitter DHBTs featuring 30-nm-thick composition- and doping-graded InGaAsSb base and 100-nm-thick InP collector. The HBTs exhibit an ft of 501 GHz and an fmax of 503 GHz at a collector current density of 10.6 mA/μm2 .

High hole mobility front-gate InAs/InGaSb-OI single structure CMOS on Si

We have demonstrated the front-gate (FG) III-V single structure CMOS using ultra-thin body (UTB) InAs/InGaSb on insulator (-OI) on Si substrates with high hole mobility (μ_eff) of 240 cm²V^-1s^-1. We have found that the μ_eff is enhanced by the buffered-HF (BHF)-cleaned InAs MOS interfaces, Ni alloy S/D, and the InAs/strained InGaSb-OI hetero-interface channel. The CMOS operation using FG InAs/InGaSb-OI n/p-MOSFETs has been realized.

Low-Turn-on-Voltage Heterojunction Bipolar Transistors with a C-Doped InGaAsSb Base Grown by Metalorganic Chemical Vapor Deposition

We demonstrate the low-turn-on-voltage InP-based heterojunction bipolar transistors (HBTs) with a C-doped InGaAsSb base grown by metalorganic chemical vapor deposition. As the solid In content of InGaAsSb increases, not only the near-band-edge emission peak energy in photoluminescence spectra of the InGaAsSb film but also the turn-on base–emitter voltage of the HBT with an InGaAsSb base decrease. These results are attributed to the reduction of the InGaAsSb band gap with the increase of solid In content. We obtain the turn-on voltage of 0.35 V at collector current density of 1 A/cm2 in the HBT with a In0.22Ga0.78As0.73Sb0.27 base, which is one of the lowest turn-on voltages ever reported.

Reduction of In Composition in Heavily Zn-Doped InAlGaAs Layers Grown at Low Temperature by Metalorganic Chemical Vapor Deposition

Growth of heavily Zn-doped InAlGaAs at low temperature (560 °C) by metalorganic chemical vapor deposition (MOCVD) is investigated. The lattice constant contracts and the growth rate decreases with increasing dithylzinc (DEZn) flow rate. To clarify the reason, the growth rates of InAs, AlAs, and GaAs components are examined. The growth rates of AlAs and GaAs components are almost constant; only that of InAs dramatically decreases when the DEZn flow increases. This indicates that the incorporation of In is suppressed by the DEZn supply. The doping behavior during the growth is well fit by the surface adsorption-trapping model, which suggests that excess Zn atoms on the growth surface induce the reduction of the InAs component.

Effects of buffered HF cleaning on metal–oxide–semiconductor interface properties of Al2O3/InAs/GaSb structures

We studied the impact of buffered HF (BHF) cleaning on the interface properties of Al2O3/InAs/GaSb metal–oxide–semiconductor (MOS) structures fabricated by the ex-situ surface cleaning process. The Al2O3/InAs/GaSb MOS structures fabricated with BHF cleaning exhibited lower Dit values than those fabricated with sulfur passivation. In addition, the Al2O3/InAs/GaSb MOS structures fabricated with BHF cleaning were robust with respect to the MOS field-effect transistor fabrication process by using W gate metal with PMA in the 250–300 °C range.

Operation of the GaSb p-channel metal-oxide-semiconductor field-effect transistors fabricated on (111)A surfaces

We demonstrate the operation of GaSb p-channel metal-oxide-semiconductor field-effect transistors (p-MOSFETs) on (111)A surfaces with Al2O3 gate dielectrics formed by atomic-layer deposition at 150 °C. The p-MOSFETs on (111)A surfaces exhibit higher drain current and lower subthreshold swing than those on (100) surfaces. We find that the interface-state density (Dit) values at the Al2O3/GaSb MOS interfaces on the (111)A surfaces are lower than those on the (100) surfaces, which can lead to performance enhancement of the GaSb p-MOSFETs on (111)A surfaces. The mobility of the GaSb p-MOSFETs on (111)A surfaces is 80% higher than that on (100) surfaces.