Naoteru Shigekawa

Optical study of high-biased AlGaN/GaN high-electron-mobility transistors

Microscopic electroluminescence (EL) and photoluminescence (PL) measurements of high-biased AlGaN/GaN high-electron-mobility transistors are reported. We observed that the EL intensity reveals peaks around the edge of the channel and the electron temperature there is higher than the electron temperature at the center of the channel. These EL features were found to be consistent with the change in the junction temperature, which we locally estimated by comparing the PL data with measurements in raised ambient temperatures.

Electroluminescence of InAlAs/InGaAs HEMTs lattice-matched to InP substrates

Electroluminescence (EL) due to the electron-hole recombination in the channel of InAlAs/InGaAs HEMTs lattice-matched to InP substrates has been measured at room temperature. The carrier temperature extracted from the obtained spectra has been found to be approximately 300 K. It has also been found that the EL comes from a region between the source and the gate by measuring its spatial distribution. These two features imply that holes generated at the drain edge in the channel due to the impact ionization pile up and recombine with the majority electrons between the source and the gate, and agree with results of theoretical analysis.<<ETX>>

Ultrahigh-speed InP/InGaAs DHPTs for OEMMICs

This paper presents an ultrahigh-speed InP/InGaAs double-heterostructure phototransistor (DHPT) with a record optical gain cutoff frequency of 82 GHz. This excellent performance originates from the double-heterostructures compatibility with high-performance double-heterostructure bipolar transistor (DHBT) and a new self-aligned process. To demonstrate the excellent performance of the DHPT, two kinds of optoelectronic MMICs (OEMMICs) were designed and fabricated. One is a 40-GHz-band DHPT/DHBT photoreceiver that shows the DHPTs ability to be simultaneously integrated with a high-performance DHBT. The 40 GHz operation frequency is also the highest reported for monolithically integrated HPT/HBT photoreceivers. The other is a direct optical injection-locked oscillator that can extract an electrical clock signal from optical data streams. The OEMMICs are promising for compact and low-power-consumption optical receivers on an InP platform for millimeter-wave photonics and ultrahigh-speed optical communication systems.

Electroluminescence characterization of AlGaN/GaN high-electron-mobility transistors

Spectral analysis of the electroluminesence (EL) of AlGaN/GaN high-electron-mobility transistors is reported. The shape of the EL spectra is completely different from the shape of the photoluminescence spectrum. The wavelength for the peak of the EL spectrum gets shorter when the gate–bias voltage is decreased. Its intensity shows a bell shape when the gate-bias voltage is swept. These features suggest that the EL signal is due to the intraband transition of the channel electrons in the high-field region at the drain edge.

Measurement of valence-band offsets of InAlN/GaN heterostructures grown by metal-organic vapor phase epitaxy

The valence band offsets, ΔEV, of In0.17Al0.83N/GaN, In0.25Al0.75N/GaN, and In0.30Al0.70N/GaN heterostructures grown by metal-organic vapor phase epitaxy were evaluated by using x-ray photoelectron spectroscopy (XPS). The dependence of the energy position and the full width at half maximum of the Al 2p spectrum on the exit angle indicated that there was sharp band bending caused by the polarization-induced electric field combined with surface Fermi-level pinning in each ultrathin InAlN layer. The ΔEV values evaluated without taking into account band bending indicated large discrepancies from the theoretical estimates for all samples. Erroneous results due to band bending were corrected by applying numerical calculations, which led to acceptable results. The evaluated ΔEV values were 0.2±0.2 eV for In0.17Al0.83N/GaN, 0.1±0.2 eV for In0.25Al0.75N/GaN, and 0.0±0.2 eV for In0.30Al0.70N/GaN. Despite the large decrease of around 1.0 eV in the band gap of InAlN layers according to the increase in the In molar fr...

Small valence-band offset of In0.17Al0.83N/GaN heterostructure grown by metal-organic vapor phase epitaxy

The valence-band offset of a lattice-matched In0.17Al0.83N/GaN heterostructure grown by metal-organic vapor phase epitaxy (MOVPE) was investigated by x-ray photoelectron spectroscopy (XPS). Atomic force microscopy and angle-resolved XPS indicated that a thin In0.17Al0.83N (2.5 nm) layer was successfully grown by MOVPE on GaN. The XPS result showed that the valence band offset was 0.2±0.3 eV. This result indicates that the conduction-band offset at the In0.17Al0.83N/GaN interface is large, i.e., 0.9 to 1.0 eV, and occupies a large part of the entire band discontinuity.

Time‐of‐flight measurement of electron velocity in an In0.52Al0.48As/In0.53Ga0.47As /In0.52Al0.48As double heterostructure

The electron velocity versus electric field (v‐E) relationship was measured between 0 and 12 kV/cm at room temperature for a selectively Be‐doped In0.52Al0.48As/In0.53Ga0.47As /In0.52Al0.48As double heterostructure. It was found that the observed electron velocity is greater than that previously measured for an AlGaAs/GaAs/AlGaAs double heterostructure over the enitre range of field investigated. This indicates the superiority of In0.53Ga0.47As as a material for high‐speed semiconductor devices. The experimental results were also compared with those of the Monte Carlo calculation, and a remarkable discrepancy between the experiment and the calculation was found above the threshold field.

Electrical properties of Si/Si interfaces by using surface-activated bonding

Electrical properties of n-Si/n-Si, p-Si/n-Si, and p−-Si/n+-Si junctions fabricated by using surface-activated-bonding are investigated. The transmission electron microscopy/energy dispersive X-ray spectroscopy of the n-Si/n-Si interfaces reveals no evidence of oxide layers at the interfaces. From the current-voltage (I-V) and the capacitance-voltage (C-V) characteristics of the p-Si/n-Si and p−-Si/n+-Si junctions, it is found that the interface states, likely to have formed due to the surface activation process using Ar plasma, have a more marked impact on the electrical properties of the p-Si/n-Si junctions. An analysis of the temperature dependence of the I-V characteristics indicates that the properties of carrier transport across the bonding interfaces for reverse-bias voltages in the p-Si/n-Si and p−-Si/n+-Si junctions can be explained using the trap-assisted-tunneling and Frenkel-Poole models, respectively.

Side-gate effects on transfer characteristics in GaN-based transversal filters

We examine the transfer characteristics of transversal filters with side gates fabricated on unintentionally doped GaN layers grown on (0001) sapphire substrates. By positively biasing the side gates, the transfer characteristics of the filters are efficiently improved, which means that GaN-based transversal filters with side gates are potentially applicable for filtering and modulating rf signals. We further examine the capacitance in an interdigital transducer, which decreases when the side-gate bias voltage increases. This indicates that the variation in the transfer characteristics is likely to be attributable to a change in the depletion-layer thickness in the GaN layers.

Thermal stability of electrical properties in AlGaN/GaN heterostructures

An annealing study of an AlGaN/GaN two-dimensional electron gas structure was conducted in combination with measurements of precise epitaxial layer thickness and AlGaN crystal quality. We found that the sheet resistance (Rsheet) increases significantly for samples with a less-than-180-A-thick AlGaN layer when annealing is performed below the growth temperature. The Rsheet increase also depends on the GaN thickness, which determines AlGaN crystal quality. Hall measurements revealed that the decrease in sheet carrier density is responsible for the Rsheet increase. One possible explanation for the Rsheet increase is that Si donors in poor-surface-morphology AlGaN layers are passivated or compensated from the top surfaces upon annealing. These results have a great impact on wafer selection, device process, design, and performance for short-gate AlGaN/GaN high-electron-mobility transistors.