Noboru Negoro

Al2O3 insulated-gate structure for AlGaN/GaN heterostructure field effect transistors having thin AlGaN barrier layers

An Al2O3 insulated-gate (IG) structure was utilized for controlling the surface potential and suppressing the gate leakage in Al0.2Ga0.8N/GaN heterostructure field effect transistors (HFETs) having thin AlGaN barrier layers (less than 10 nm). In comparison with the Schottky-gate devices, the Al2O3 IG device showed successful gate control of drain current up to VGS = +4 V without leakage problems. The threshold voltage in the Al2O3 IG HFET was about -0.3 V, resulting in the quasi-normally-off mode operation.

Effects of gap states on scanning tunneling spectra observed on (110)- and (001)-oriented clean surfaces and ultrathin Si layer covered surfaces of GaAs prepared by molecular beam epitaxy

In an attempt to understand and control Fermi level pinning on GaAs surfaces, an ultrahigh vacuum (UHV) scanning tunneling spectroscopy (STS) study was made on (110) and (001) clean surfaces and Si covered (001) surfaces of n-type GaAs prepared by molecular beam epitaxy. Normal STS spectra showing conductance gaps corresponding to GaAs energy gap and anomalous spectra showing much larger gaps coexisted on all samples. The rate of finding normal spectra was very low on the initial surfaces, but it greatly increased after Si deposition particularly on the c(4×4) surface. A previous explanation of the gap anomaly by tip-induced electrostatic bend bending change is invalid. A new model based on a band bending change due to tip-induced local charging of surface states is presented where tunneling proximity makes occupancy of surface states in equilibrium with the scanning tunneling microscopy (STM) tip. Spots with anomalous spectra correspond to Fermi level pinning centers where the tip directly “writes” or “e...

Effects of Si deposition on the properties of Ga-rich (4×6) GaAs (001) surfaces

In order to achieve better surface passivation on technologically important (001) GaAs surfaces, microscopic and macroscopic properties of the clean (001) surface with Ga-rich (4×6) reconstruction, and effects of monolayer level Si deposition on this surface, were investigated. According to scanning tunneling spectroscopy and contactless capacitance–voltage (C–V) measurements, the Ga-rich genuine (G-)(4×6) surface had high-density acceptor-type states above EC−0.5 eV. However, the surface exhibited a much lower and wider surface state density (NSS) distribution as well as a much stronger band-edge photoluminescence (PL) intensity than the conventional As-rich surfaces. Irradiation of Si molecular beam on the (4×6) surface resulted in epitaxial growth of Si. This completely removed the acceptor-type surface states from the energy gap, and further enhanced the PL intensity. The result indicates that the Ga-rich G-(4×6) surface is more ordered and more stable against processing than conventional As-rich surf...

Strong Photoluminescence and Low Surface State Densities on Clean and Silicon Deposited (001) Surfaces of GaAs with (4×6) Reconstruction

In an attempt to determine an optimum initial GaAs surface for application of the silicon interface control layer (Si ICL)-based surface passivation method, properties of the (4×6) reconstructed Ga-rich (001) surface of GaAs were investigated before and after Si deposition. An ultrahigh vacuum (UHV)-based multi-chamber system was used where samples were prepared by molecular beam epitaxy (MBE), and characterized by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), UHV photoluminescence (PL), and contactless capacitance–voltage (C–V) methods. For comparison, (2×4) and c(4×4) As-rich surfaces were also prepared. STM observation clarified the microscopic structure of the (4×6) surface. XPS measurements confirmed the Ga-rich nature of the initial surface and detected the reduction of band bending on both initial and Si-deposited (4×6) surfaces. A surprisingly large band-edge PL intensity was obtained on the (4×6) surface, and it further increased after Si deposition, reaching as high as 8.9 times that of the (2×4) surface. UHV contactless C–V measurements indicated low and wide Nss distributions on the initial and Si-deposited (4×6) surfaces. All the results consistently indicate that the (4×6) surface is the most promising surface for application of the Si ICL-based surface passivation process for GaAs.

Scanned-probe topological and spectroscopic study of surface states on clean and Si-deposited GaAs (0 0 1)-c(4×4) surfaces

Abstract Microscopic topological and spectroscopic properties of MBE-grown GaAs c(4×4) surfaces without and with monolayer Si deposition were investigated by the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Empty state STM images on as-grown surface showed bright and dark cells, and they exhibited strong correlation with the spatial distribution of normal and anomalous conductance gaps of the STS spectra. Bias dependent STM images indicated presence of pinning areas with continuous space and energy distribution of surface gap states. By deposition of monolayer Si, dark areas reduced a great deal and the rate of finding normal STS spectra increased, indicating large reduction of surface states.

Scanning tunneling microscopy and spectroscopy study of ultrathin Si interface control layers grown on (001) GaAs for surface passivation

Abstract Microscopic topological and spectroscopic properties of the ultrathin silicon interface control layer (Si-ICL) grown by molecular beam epitaxy on the (001) GaAs surface were investigated by the ultrahigh vacuum (UHV), scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) techniques, and the results were correlated with the macroscopic electronic properties measured by in situ X-ray photoelectron spectroscopy (XPS) and UHV photoluminescence (PL) techniques. Growth of the Si-ICL on c(4×4) GaAs surface produced much more ordered STM topology, leading to better electronic properties as observed by XPS, PL and STS methods than the growth on (2×4) surface. In addition to normal STS spectra showing GaAs energy gap, many anomalous spectra showing much wider apparent gaps were observed on all the surfaces, and they were interpreted to correspond to the pinning centers for the Fermi level where surface states exchange electronic charge with the STM tip and modify the band bending.

Improved hysteresis in a normally-off AlGaN/GaN MOS heterojunction field-effect transistor with a recessed gate structure formed by selective regrowth

A normally-off AlGaN/GaN MOS heterojunction field-effect transistor (MOS-HFET) with a recessed gate structure formed by selective area regrowth is demonstrated. The fabricated MOS-HFET exhibits a threshold voltage of 1.7 V with an improved hysteresis of 0.5 V as compared with a device fabricated by a conventional dry etching process. An analysis of capacitance–voltage (C–V) characteristics reveals that the dry etching process increases interface state density and introduces an additional discrete trap. The use of the selective area regrowth technique effectively suppresses such degradation, avoiding the MOS interface from being exposed to dry etching. The results presented in this paper indicate that the selective area regrowth technique is promising for the fabrication of normally-off AlGaN/GaN MOS-HFETs.

A 26 GHz Transceiver Chipset for Short Range Radar Using Post-Passivation Interconnection

We demonstrate a 26 GHz SiGe bipolar complementary metal oxide semiconductor (BiCMOS) transceiver chipset for short-range spread-spectrum (SS) radar system. The integrated frequency triplers lower the local oscillation frequency down to 8.8 GHz, and eliminate the carrier leak in the transmitting signal which enables high sensitivity. The on-chip balun connected to the mixer demodulating by the pseudo noise (PN) code in the receiver increases the dynamic range of the receiving signal. Low-loss transmission lines on Si substrate are fabricated by post-passivation interconnection process using thick benzocyclobutene (BCB). We have confirmed the transmitting signal having spread-spectrum with suppressing the carrier leak, the improvement of the dynamic range of the receiver, and the reduction of the transmission loss on Si substrate. We have demonstrated the operation of the 26 GHz ultra wide band (UWB) SS radar, corresponding to detecting a human located at 7 m away from the system.