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GaN/AlGaN distributed Bragg reflectors have been grown by low-pressure metal organic chemical vapor deposition. Those structures were fabricated on the atmospheric pressure grown GaN layer on sapphire substrate. The aluminum content in low-pressure grown AlGaN layers was estimated to be 0.60 by X-ray diffraction. The GaN layers grown under the low-pressure condition in GaN/Al 0.60 Ga 0.40 N multilayer were compressively strained. The flat surfaces without cracks were successfully obtained for the growth of GaN/Al 0.60 Ga 0.40 N distributed Bragg reflector. For the 45.5 pairs, a peak reflectivity of over 98% was obtained at a wavelength of 421 nm.

We investigated the relationship between the degree of spatial memory impairment in an 8-arm radial maze and the changes in the contents of acetylcholine (ACh) and noradrenaline (NA) in the dorsal and ventral hippocampus and the frontal cortex, along with histological changes in kaolin-induced hydrocephalic rats. Kaolin-induced hydrocephalic rats were divided into three groups (non-impaired, impaired and severely impaired) according to the degree of impairment in a radial maze. Thirty percent of the hydrocephalic rats could not solve a radial maze (severely impaired group), while the remaining hydrocephalic rats could (non-impaired rats in the standard task). Forty percent of the non-impaired rats in the standard task failed to solve the delayed-response task (impaired group), whereas the remaining rats were able to solve it (non-impaired group). A positive correlation was observed between the impairment of spatial memory and ventricular dilatation. The ACh content in the dorsal and ventral hippocampus, and the NA content in the ventral hippocampus were decreased in the severely impaired group. Moreover, the NA content in the ventral hippocampus was decreased in the impaired group. These results suggest that the impairment of spatial memory in kaolin-induced hydrocephalic rats is associated with dysfunction of the hippocampal cholinergic and noradrenergic systems.

We demonstrate high-performance ultraviolet photodetectors (UV-PDs) based on lattice-matched (LM) InAlN/AlGaN heterostructure field-effect transistors (HFETs) gated by transparent ITO films. Low dark currents of 6.8 × 10−8 and 6.1 × 10−7 A/mm and high photocurrent gains over four and three orders of magnitude were obtained for the LM In0.12Al0.88N/Al0.21Ga0.79N and In0.10Al0.90N/Al0.34Ga0.66N HFETs, respectively. The negative threshold voltage shifts under illumination indicate that most of the photo-generated carriers are transported in the two-dimensional gas (2DEG) region around the InAlN/AlGaN interface. High peak responsivities of 2.2 × 104 and 5.4 × 104 A/W and large UV-to-visible rejection ratios greater than 104 and 103 were achieved for the LM In0.12Al0.88N/Al0.21Ga0.79N and In0.10Al0.90N/Al0.34Ga0.66N HFET-type UV-PDs, respectively. These improved performances with respect to other AlGaN UV-PDs around the same wavelength detection range may possibly be attributed to the greater contribution of t...

We conducted the study on the growth of rough-surface p-GaN layers on InGaN/GaN multiple-quantum-well (MQW) structures by metalorganic chemical vapor deposition (MOCVD). It was found that the sum of InGaN well thickness t well_total was a predominant factor to form the rough surface, in addition to the growth temperature as low as 800 °C for the p-GaN layers. Microstructure analyses revealed that the rough surfaces consisted of a certain number of hexagonal V-shaped pits starting from dislocations propagated through an under layer and they increased with the increased t well_total. It was confirmed that the light absorption was enlarged for MQW structure samples with rough-surface p-GaN layers on the top, owing to not only the thickness effect in MQWs but also their reduced light reflection on the surfaces. It was also confirmed that these optical properties contributed to the performance improvement in InGaN/GaN MQW solar cells.

The growth conditions of the AlN nucleation layer (AlN NL) by focusing on the partial pressure of trimethyl aluminum (TMA) with a constant V/III ratio are optimized. The relationship between the size and the density of the surface pits of the AlN NL and the variation in the vertical-direction breakdown voltage (VDBV) in the AlGaN/GaN high-electron-mobility transistor (HEMT) structure is determined. The maximum size of the pits in the surface of the AlN NL decreases as the partial pressure of TMA decreases. The density and the maximum size of the pits on the surface of the AlN NL decreased as the growth rate of AlN NL decreased. The pit density of the AlN NL decreases as the thickness of the AlN NL increased from 157 to 207 nm. Furthermore, the pit density is saturated when the thickness of the AlN NL was 207 nm. The optimum value of the thickness of AlN NL was about 200 nm. The variation in the VDBV of the HEMT structure decreases as the full width at half maximum (FWHM) of the rocking curve of the AlN (002) and AlN (102) planes, the pit density, and the maximum size of the pits in the AlN NL decrease. Thus, the variation in the VDBV of the HEMT structure can be related with the process of regrowth over the AlN NL.

We analyzed carrier trapping in AlGaN/GaN HEMT occurring between gate and drain with a bias-controllable Field Plate (CFP) on a Si3N4 passivation, which is structurally and electrically independent from other electrodes. We observed the recovery of a transient drain current which was associated with carrier emission after the momentary pulse bias of CFP. From the temperature dependence of pulsed I–V measurements, an activation energy of the trap state was 0.083 eV and capture cross section was 2.0 × 10−24 cm−2. This value is the same to the activation energy of the surface leakage current in AlGaN/GaN HEMT with Si3N4 passivation. The trap state with the activation energy of 0.083 eV has important role in pulse operation of AlGaN/GaN FP-HEMT with Si3N4 passivation.

The effects of post-deposition annealing (PDA) on Al2O3/AlGaN interfaces fabricated by atomic layer deposition (ALD) are studied by x-ray photoelectron spectroscopy (XPS) using synchrotron radiation. Current–voltage (I–V) measurements of annealed Al2O3/AlGaN/GaN metal–insulator–semiconductor (MIS)-type high electron mobility transistors (HEMTs) on Si substrates are also performed to investigate the effects of PDA. XPS results show that the amount of Al-O bonding in the Al2O3 layer and Ga-O bonding in the AlGaN layer changes considerably at a PDA temperature of 600 °C. The results of the I–V measurements show that the electrical characteristics of the MIS-HEMTs change substantially at a PDA temperature of 700 °C. After PDA at 700 °C, the dynamic threshold voltage shift (ΔV th) was reduced from 5 to 0.5 V with an increase of the gate leakage current (I g) from 1 × 10−7 to 7 × 10−5 mA mm−1. These results indicate that the deep electron traps, such as oxide traps near the Al2O3/AlGaN interfaces, almost disappear at approximately 700 °C with the crystallization of the Al2O3 layers.

In this paper, an AlGaN/GaN Field Effect Diode (FED) with high turn-on voltage (VON) controllability is proposed. The structural feature of a δ-doped GaN cap, an AlGaN barrier, and a GaN channel (GaN/AlGaN/GaN), and selective dry-etching of the GaN cap, ensure the precise control of VON which can be modulated by AlGaN barrier layer thickness. The VON as low as 0.3 V is obtained at the remained AlGaN thickness of 4-nm with an extremely low estimated VON deviation.

We investigated the effect of the Al content in the AlGaN buffer layer on the initial AlN nucleation layer, for AlGaN/GaN high-electron-mobility transistors (HEMT), on a Si substrate. Reducing the Al content in the AlGaN layer decreased the surface pit density of the AlGaN layer, and increased the leakage current of the AlGaN/AlN/Si structures, but the crystal quality of the AlGaN layer was not changed by the Al content. The dislocation density of the GaN layer and the two-dimensional electron gas characteristics of the HEMT structures, were almost the same for the AlGaN buffer layer with differing Al content. However, the vertical-direction breakdown voltage (VDBV) was decreased at an Al content of 0.760 compared with other HEMT structures. In addition, the fluctuations of SLS layer were observed at an Al content of 0.760. The warpage and cracking of the HEMT structures were minimized at an Al content of 0.558. Based on these results, the VDBV of HEMT structures are not correlated with the VDBV of the AlGaN buffer layer. However, the VDBV of HEMT structures is affected by the surface pit density of the AlGaN buffer layer. In addition, the warpage of HEMTs on a Si substrate, can be controlled by the Al content of the AlGaN buffer layer.

We fabricated graphene/Ni patterns directly on sapphire substrates through a self-forming process utilizing the pattern-controlled catalyst metal agglomeration technique, which was accomplished via a thermal annealing process of rectangular Ni patterns preformed on thin amorphous carbon films on sapphire. It was confirmed that graphene films were synthesized along with the preformed Ni patterns as a result of the progress of Ni agglomeration. Notably, a few-layer graphene film was observed in specific areas along the periphery of the preformed Ni patterns. The self-forming graphene/Ni patterns showed ohmic conductivity with a contact resistance ranging from 4 × 104 to 7 × 104 Ω μm.

Vertical breakdown studies on AlGaN/GaN high-electron-mobility transistors (HEMTs) grown by metal-organic chemical vapor deposition (MOCVD) on a silicon substrate are studied to analyze the breakdown dependence with regard to i-GaN thickness (TGaN) and buffer thickness (TBuf). A high breakdown field (Ec) of 2.3 MV/cm was observed for MOCVD grown epilayers of total thickness of 5.5 μm on Si. Increasing TBuf is more significant than TGaN toward controlling the vertical leak age and demonstrates a high breakdown. For transistor operation at high voltages, GaN layers grown on thick buffers are highly resistive to the flow of leakage currents. A high figure of merit (BV2/Rd.ON) of 5.4 × 108 V2 · Ω-1· cm-2 was observed for an AlGaN/GaN HEMT grown on Si using a thick buffer.

In the last decade, AlGaN/GaN based Enhancement-mode (E-mode) devices on Si substrate have been studied extensively [1-3]. In spite of improvements in the threshold voltage (Vth), the AlGaN/GaN E-mode devices could not show high drain current density (Idsmax), which is highly desirable for automotive applications [4, 5]. Alternatively, InAlN/GaN based E-mode devices are preferred for high-temperature and high-speed applications. A few research groups have demonstrated InAlN/GaN based E-mode devices mainly on SiC and sapphire substrates [69]. The main focus in this work is to demonstrate high performance InAlN/GaN based E-Mode metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMT) on low cost Si. The In0.15Al0.85N/GaN heterostructure on Si was grown by MOCVD with 3 m thick strained layer super lattice (SLS) structures. The Hall effect measurements showed a high sheet carrier density of 3.25 x 10 cm and mobility of 1020 cmVs respectively. Device isolation was carried out by reactive ion etching using BCl3 gas. Alloyed ohmic contacts were formed and gate recess was made upto the AlN layer by using a gate mask. A 20 nm thick Al2O3 layer was deposited using atomic layer deposition and subsequent annealing at 600 C was done. Finally, gate and contact metals were deposited as shown in Fig. 1. TLM measurements showed a low sheet resistance (Rsh) of 157 / . Figure 2 shows the Ids-Vds characteristics of InAlN/GaN E-mode operating at a gate bias (Vgs) of +8 V with a high Idsmax of 0.84 A/mm and low on-resistance (Ron) of 5.9 mm. The semi-log and linear scale transfer characteristics are shown in Fig. 3 (a) and (b) respectively. The InAlN/GaN E-mode MOS-HEMT device showed a high Ion/Ioff ratio of 7 10 and a steep subtreshold swing slope of 74 mV/dec. The gate leakage was low (~10A/mm), even at a high Vgs of +8 V. The Vth extracted was +1.9 V from the linear extrapolation of Ids at peak transconductance (157 mS/mm). The isolation current measured between two ohmic mesas separated by a 10 m gap, showed two orders of lower current after Al2O3 layer passivation as shown in Fig. 4 (a). Figure 4 (b) shows the gate-lag measurements performed with a pulse width of 500 s and a base of 0 V. Negligible dispersions were observed in pulsed Ids-Vds curves due to adequate passivation by Al2O3. The Rsh in the recessed gate region can be extracted from the plot of Ron versus recessed gate lengths [3]. As shown in 5 (a), the Rsh in the recessed region was 1580 / , which is one order higher than the Rsh in the non-recessed region. Nevertheless, the Rsh of recessed InAlN/GaN E-mode device is one order lower when compared to gate recessed AlGaN/GaN E-mode devices [3]. Figure 5 (b) shows the benchmarking plot of the product of Idsmax and gate length (Lg) of existing E-mode InAlN/GaN and AlGaN/GaN E-mode devices on various substrates. In summary, we have demonstrated low leakage InAlN/GaN E-mode MOSHEMTs with high drain current density and low Ron on low cost Si substrate. References: [1] T. Mizutani et. al., IEEE Electron Device Lett. 28, 549, 2007, [2] F. Medjoub et. al., IEEE Electron Device Lett. 31, 948, 2010; [3] B. Lu et. al., IEEE Electron Device Lett. 31, 990, 2010; [4] N. Maeda et. al., Apex, 5, 084201, 2012; [5] Z. Tang, IEEE Electron Device Lett. 34, 1373, 2013;[6] C. Ostermaier, IEEE Electron Device Lett. 30, 1030, 2009; [7] R. Wang et. al.., IEEE Electron Device Lett. 32, 309, 2011 [8] R. Wang et. al., IEEE Electron Device Lett. 31, 1383, 2010 [9] D. Morgan et. al., Apex, 4, 114101, 2011; [10] J.J. Freedsman et. al., Apex, 7, 041003 (2014).

A recessed gate AlGaN/GaN high-electron mobility transistor (HEMT) on sapphire (0 0 0 1), a GaN metal-semiconductor field-effect transistor (MESFET) and an InGaN multiple-quantum well green light-emitting diode (LED) on Si (1 1 1) substrates have been grown by metalorganic chemical vapor deposition. The AlGaN/GaN intermediate layers have been used for the growth of GaN MESFET and LED on Si substrates. A two-dimensional electron gas mobility as high as 9260 cm 2 /V s with a sheet carrier density of 4.8×10 12 cm −2 was measured at 4.6 K for the AlGaN/GaN heterostructure on the sapphire substrate. The recessed gate device on sapphire showed a maximum extrinsic transconductance of 146 mS/mm and a drain–source current of 900 mA/mm for the AlGaN/GaN HEMT with a gate length of 2.1 μm at 25°C. The GaN MESFET on Si showed a maximum extrinsic transconductance of 25 mS/mm and a drain–source current of 169 mA/mm with a complete pinch-off for the 2.5-μm-gate length. The LED on Si exhibited an operating voltage of 18 V, a series resistance of 300 Ω, an optical output power of 10 μW and a peak emission wavelength of 505 nm with a full-width at half-maximum of 33 nm at 20 mA drive current.

In order to clarify the mechanism by which pGlu-Asn-Cys(Cys)-Pro-Arg-Gly-NH 2 (vasopressin-(4–9)), a major metabolite C-terminal fragment of [Arg 8 ]-vasopressin (vasopressin-(1–9)), improves learning and memory, we used several different drugs such as an acetylcholine receptor antagonist, a Ca 2+ /calmodulin-dependent protein kinase II inhibitor, vasopressin receptor antagonists and L-type Ca 2+ channel blocker to disrupt spatial memory in rats. Moreover, we examined the effect of vasopressin-(4–9) on acetylcholine release in the ventral hippocampus using microdialysis. Vasopressin-(4–9) (10 fg/brain, i.c.v.) improved the impairment of spatial memory in the eight-arm radial maze induced by scopolamine, pirenzepine and Ca 2+ /calmodulin -dependent protein kinase II inhibitor. Pirenzepine, a vasopressin V 1A receptor antagonist, and L-type Ca 2+ channel blocker, but not a vasopressin V 2 receptor antagonist, suppressed the effects of vasopressin-(4–9) on scopolamine-induced impairment of spatial memory. Moreover, vasopressin-(4–9) did not affect acetylcholine release in the ventral hippocampus of intact rats or of scopolamine-treated rats as assessed by microdialysis. These results suggest that vasopressin-(4–9) activates vasopressin V 1A receptors on the postsynaptic membrane of cholinergic neurons, and induces a transient influx of intracellular Ca 2+ through L-type Ca 2+ channels to interact with muscarinic M 1 receptors. The activation of these processes by vasopressin-(4–9) is critically involved in the positive effect of vasopressin-(4–9) on scopolamine-induced impairment of spatial memory.

A recessed gate high electron mobility transistor (HEMT) has been fabricated with AlGaN/GaN heterostructure on a sapphire substrate using metalorganic chemical vapor deposition. Capacitance-voltage (C-V) and Shubnikov-de Haas measurements have shown the formation of two-dimensional (2-D) electron gas (2DEG) at Al/sub 0.11/Ga/sub 0.89/N/GaN heterointerface. A 2DEG mobility 12000 cm/sup 2//V-s with a sheet carrier density 2.8/spl times/10/sup 12/ cm/sup -2/ was measured on Al/sub 0.11/Ga/sub 0.89/N/GaN heterostructure at 8.9 K. The recessed gate Al/sub 0.26/Ga/sub 0.74/N/GaN HEMT structure showed maximum extrinsic transconductance 181 mS/mm and drain-source current 1120 mA/mm for a gate length 1.5 /spl mu/m at 25/spl deg/C. The device exhibited stable operation characteristics at 350/spl deg/C for long time (500 h). No interfacial change has been observed at metal/AlGaN interface even after 350/spl deg/C for 500 h treatment. The threshold voltage of device does not depend very much on operating temperature (25 to 350/spl deg/C).

The normal-incidence reflectance measurement was employed to obtain the free exciton transition energy (EFX) of AlGaN alloys in AlxGa1-xN/GaN/sapphire heterostructure grown by metalorganic chemical vapor deposition. It was found that the thickness variation of the AlGaN layer may cause a noticeable change in the line shape of reflectance spectrum and impede the identification of the desired excitonic position. By using a reflection model of two absorbing layers with a transparent substrate, the experimental reflectance spectra were theoretically simulated and utilized to explain the reflection mechanism in AlxGa1-xN/GaN heterostructures. On the basis of the above analysis, the feasibility of the reflectance measurement for such heterostructures is confirmed. At room temperature, the EFXs obtained from the fitting showed an excellent agreement with the corresponding peak energies in the photoluminescence spectra. Furthermore, at the optical energy position about 100 meV above the EFX, the spectral feature of exciton-LO phonon interaction was observed in the reflectance spectrum record for low Al composition (x?0.16). Using the Al mole fraction derived from x-ray diffraction measurement, the bowing parameter of the epitaxial AlGaN layer was determined. In the range of 0?x

We report the growth of self-formed InGaAs quantum dot (QD) and the fabrication of self-formed In0.2Ga0.8As QD-like laser on Si substrate by metal organic chemical vapor deposition (MOCVD). The dots are grown by Stransky–Krastanov (S–K) growth mode. The laser shows room temperature continuous wave (CW) light output with a threshold current density of 1.32 kA/cm2 and a lasing wavelength of 854 nm.

Investigations were carried out on metalorganic-chemical-vapor-deposition (MOCVD)-grown strained AlGaN/ GaN/sapphire structures using X-ray diffratometry. While AlGaN with lower AlN molar fraction (< 0.1) is under the in-plane compressive stress, it is under the in-plane tensile stress with high AlN molar fraction (> 0.1). Though tensile stress caused the cracks in AlGaN layer with high AlN molar fraction, we found that the cracks dramatically reduced when the GaN layer quality was not good. Using this technique, we fabricated a GaInN multi-quantum-well (MQW) surface emitting diodes were fabricated on 15 pairs of AlGaN/GaN distributed Bragg reflector (DBR) structures. The reflectivity of 15 pairs of AlGaN/GaN DBR structure has been shown as 75% at 435 nm. Considerably higher output power (1.5 times) has been observed for DBR based GaInN MQW LED when compared with non-DBR based MQW structures.

We investigated the relationship between the induction of spatial cognition impairment in the 8-arm radial maze task and regional changes (ventral hippocampus (VH), dorsal hippocampus, frontal cortex, and basolateral amygdala nucleus) in brain acetylcholine (ACh) release using microdialysis in rats treated with muscarinic (M) receptor antagonists. In a behavioral study, two M1 antagonists, scopolamine (0.5 mg/kg, i.p. and 20 microg, i.c.v.) and pirenzepine (80 microg, i.c.v.), but not an M2 antagonist, AF-DX116 (40-80 microg, i.c.v.), disrupted spatial cognition in the 8-arm radial maze task. In brain microdialysis with Ringer's solution containing 0.1 mM eserine sulfate, scopolamine and AF-DX116, but not pirenzepine, increased ACh release in the VH. Moreover, in the bilateral injection of scopolamine (2 microg/side), the VH and dorsomedial thalamus nucleus were important regions for scopolamine-induced impairment of spatial cognition. A simultaneous determination of the behavioral changes revealed that scopolamine (0.5 mg/kg, i.p.) markedly decreased the ACh contents and also increased the ACh release in all regions tested. Especially, the changes in the ACh release of the VH closely paralleled the induction of the scopolamine-induced impairment of spatial cognition. These results suggest that the blocking balance between M1 and M2 muscarinic receptor in the VH therefore plays a major role in the spatial cognition impairment induced by scopolamine in the 8-arm radial maze task.

A single crystal GaN thin film was successfully grown on a Si(111) substrate by means of atmospheric-pressure metalorganic chemical vapor deposition. An intermediate layer consisting of AlN and AlGaN improved the quality of GaN on Si with a mirror-like surface and reduced the pits and cracks over the surface. The full width at half maximum (FWHM) of the double-crystal X-ray rocking curve for GaN(0004) was 600 arcsec. Photoluminescence measurement at 4.2 K for a nondoped film revealed a sharp band-edge emission with a FWHM of 8.8 meV, which is the narrowest value reported to date. GaInN multi-quantum-well structure was grown on this structure and showed a strong blue emission peaking at 470 nm. The results suggest GaN on Si with an AlGaN/AlN intermediate layer provides reliable light emitting devices on Si substrate.

An AlGaAs/GaAs multi-quantum well vertical-cavity surface-emitting laser diode (VCSELD) has been grown on a Si substrate using metalorganic chemical vapour deposition (MOCVD). The VCSELD with a 23-pair of AlAs/Al0·1Ga0·9As distributed Bragg reflector on a Si substrate exhibited a threshold current of 223 mA under continuous-wave condition at 220 K. Electroluminescence observation showed that an optical degradation was caused by generation and growth of dark-line defects. An MOCVD-grown InGaN/AlGaN double-heterostructure light-emitting diode on a sapphire substrate exhibited an optical output power of 0·17 mW, an external quantum efficiency of 0·2%, a peak emission wavelength at 440 nm with a full width at half-maximum of 63 nm and a stable operation up to 3000 h under 30 mA DC operation at 30°C. A high current level of 281 mA/mm and a large transconductance (gm) of 33 mS/mm have been achieved for a GaN metal semiconductor field-effect transistor (MESFET) with a gate length of 2 µm and a width of 200 µm at 25°C. The GaN MESFET at 400°C showed degraded characteristics: a lowgm of 13·4 mS/mm, a gate leakage and a poor pinch-off.

AlN/AlGaN/GaN MIS-HEMTs were grown on 4 inch silicon to suppress the gate leakage and achieve high breakdown. However the drain current density maximum obtained is low (361 mA/mm) for 15 μm MIS-HEMTs. Since the contact resistance of the Ohmic contacts on the insulating AlN is high (5.1 Ωmm), the thin AlN layer was etched prior to the evaporation of the source-drain Ohmic contact. Though the recessed Ohmic contact reduced the contact resistance to 2.5 Ωmm there is no improvement in the drain current density. The poor quality of the 2 nm AlN growth on silicon could be the possible reason behind the low drain current density. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the fabrication and characterization of high-performance AlGaN solar-blind p–i–n photodiodes grown on AlN/sapphire templates by metal organic chemical vapor deposition. The realized devices demonstrate a sharp spectral cutoff with a drop of four orders of magnitude from 260 to 310 nm under the 1 µW/cm2 illumination. Dark current density as low as 2.6×10-11 A/cm2 at -5 V bias has been measured. The zero bias external quantum efficiency peaks at 258 nm with a value of 30%. The upper-bound detectivity limited by Johnson noise is calculated to be 7.1×1014 cmHz1/2/W at 300 K.

Si-doped GaN layers were grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD) and characterized by using electron beam-induced current (EBIC) and molten KOH etching. Dark spot densities in EBIC image are be 1.0 and 1.2×10 8 cm −2 depending on the accelerating voltage. The dark spots in EBIC image correspond to recombination centers. When we compare dark spot density with etch pit density revealed molten KOH, this is the same order as the EPD revealed by molten KOH. We confirm there are many dislocations which influence recombination of minority carriers in the layers.

We studied thermal annealing effects of low-temperature-grown GaN (LT-GaN) on Si substrate by use of the two step growth technique. Difficulties were encountered when LT-GaN films were annealed up to the temperature of 1050°C on Si substrate. However, the LT-GaN on sapphire was stable after the same annealing condition. The decomposition of LT-GaN and the wavy surface were observed. Thus, thermal stability of LT-GaN on Si was lower than that on sapphire. To improve this low thermal stability, LT-GaN on Si was coated by middle-temperature-grown GaN (MT-GaN). However, swellings were observed on the surface after annealing to the temperature of 1050°C and there were hollows, which indicated that the corrosion of Si occurred below the temperature of the melting point of Si, under swellings. Energy dispersive X-ray (EDX) analysis indicated that a large amount of Ga was detected in swellings. Ga is responsible for the corrosion of Si substrate and the possible reason for the formation of hollows was the meltback etching of Si by Ga.

We report characteristics and degradation of an InGaN/AlGaN double-heterostructure light-emitting diode (LED) grown by metalorganic chemical vapor deposition on a sapphire substrate. The InGaN/ AlGaN LED exhibited an optical output power of 0.17 mW, an external quantum efficiency of 0.2%, a peak emission wavelength at 440 nm with a full width at half-maximum of 63 nm, and a stable operation up to 3000 h under 30 mA dc operation at 30 °C. However, the InGaN/AlGaN LED showed electrical and optical degradations under high injected current density and high ambient temperature. Electroluminescence, electron-beam-induced current and cathodoluminescence observations showed that the degraded InGaN/AlGaN LED exhibited formation and propagation of dark regions, which act as nonradiative recombination centers. The values of the degradation rate were determined to be 1.1×10-3, 1.9×10-3, and 3.9×10-3 h-1 under the injected current density of 100 A/cm2, and 1.6×10-2, 3.6×10-2, and 8×10-2h-1 under 200 A/cm2 at ambient temperatures of 30, 50, and 80 °C, respectively. The activation energy of degradation was also determined to be 0.23-0.25 eV. The degradation of electrical and optical characteristics was caused by the growth of dark regions. It was also observed that GaN-based LEDs on sapphire substrates have longer lifetime than the ZnSe-based LED, but shorter than the AlGaAs and InGaAsP LEDs.

Ionic conduction of trivalent aluminum in solid aluminum tungstate, Al2(WO4)3, has been directly and quantitatively demonstrated. Trivalent ions, especially for aluminum where the polarizability is considerably low, are strongly bonded in a crystal lattice and have been believed to be unlikely to exhibit migration in solids because of high electrostatic interaction with the surrounding skeleton structure. The material, Al2(WO4)3, which has the Sc2(WO4)3 structure, was selected in order to reduce the interaction between the framework and the mobile species, Al3+, as much as possible. The ionic conduction characteristics of Al2(WO4)3 were investigated by means of electrolysis and electromotive force measurement by constructing an aluminum concentration cell. The typical electrical conductivity of Al2(WO4)3 was, approximately 2 × 10-5 S cm-1 at 800 °C. The dc electrolysis data strongly support the conclusion that ionic Al3+ is the mobile species in Al2(WO4)3. By further electrochemical measurements, it was c...

An AlGaAs/GaAs multi-quantum well vertical-cavity surface-emitting laser diode (VCSELD) has been grown on a Si substrate using metalorganic chemical vapor deposition. The VCSELD structure grown on a Si substrate consists of ten quantum well active layers and a 23-pairs of AlAs/Al0.1Ga0.9As distributed Bragg reflector (DBR). The VCSELD on a Si substrate exhibited a threshold current of 82 mA and a threshold current density of 4.2 kA/cm2 under continuous-wave (cw) condition at 150 K. Cross-sectional scanning electron microscopy observation showed quasi-periodic zigzag roughness and nonuniformity in the DBR structure. Auger-electron spectroscopy also showed compositional transitions at the heterointerfaces of DBR. A low reflectivity of the DBR on Si substrate is caused by the degraded heterointerfaces, which prevent 300 K cw operation for the VCSELD grown on Si.

Single crystalline epitaxial layers of Al0.22Ga0.78As have been grown on Si substrates. Silicon diffusion in the heteroepitaxial layers has been explained by a pipe diffusion model and a gas phase reaction model. The dislocation bending at the AlGaAs Si interfaces has been explained in terms of electrical potential difference, thermal stress and point defect complexes. The Schottky diode characteristics of AlGaAs Si have been studied and the ideality factor (n) and Schottky barrier height (φb) estimated in the present study were 1.2 and 0.89 eV, respectively. Capture cross-sections, activation energies and concentrations of the deep levels have been calculated by deep level transient spectroscopy. Two deep levels with thermal emission energies of 0.43 and 0.64 eV have been observed in the 900°C thermal cycle annealed samples. Influence of silicon complex defects on the nature of the deep levels has been addressed.

Construction of a natural gas pipeline from Niigata to Sendai was completed in March 1996. Pipeline construction consists of engineering works such as civil engineering, material technology, welding technology and so on. This lecture presents the following items regarding the engineering works to be overcome and some technical skills associated with the project are introduced.i) Seismic designii) Material technology, welding technology, field bendingiii) Bridge design, tunnel designiv) Leak detection

We investigated the effects of (+/-)-rolipram, a phosphodiesterase (PDE) 4 inhibitor, and its isomers on scopolamine-induced deficits of learning and memory in rats using an 8-arm radial maze task and a passive avoidance task. 1) In the 8-arm radial maze task, (+/-)-rolipram (0.02-0.2 mg/kg, p.o.), (-)-rolipram (0.01-0.02 and 0.2-0.5 mg/kg, p.o.) and (+)-rolipram (20-50 mg/kg, p.o.) attenuated the scopolamine-induced deficits of spatial cognition. As for the minimum effective dose of each drug, (-)-rolipram was 2 and 2000 times as potent as (+/-)-rolipram and (+)-rolipram, respectively. (-)-Rolipram produced a biphasic dose-response and (+/-)-rolipram produced a broad dose-response. 2) (+/-)-Rolipram and its isomers also attenuated the scopolamine-induced deficits in the passive avoidance response. Also for the minimum effective dose, (-)-rolipram (0.01-0.02 mg/kg) was 2 and 200 times as potent as (+/-)-rolipram (0.02-0.1 mg/kg) and (+)-rolipram (2mg/kg). 3) The behaviorally effective doses of (+/-)-rolipram and its isomers also enhanced the oxotremorine-induced tremors in mice. Comparing these racemic isomers, (-)- and (+/-)-rolipram have more potent effects than (+)-rolipram on scopolamine-induced deficits in the 8-arm radial maze task and passive avoidance task. Especially (+/-)-rolipram has a wide dose range in these behavioral study. These results suggest that the ameliorating effects of rolipram might result from the indirect potentiation of various transmitters including cholinergic and noradrenergic systems by an increase in cAMP with the inhibition of PDE4.

We report on the fabrication of AlGaN/GaN double-insulator MOSHEMTs using SiO2 and SiN as the prime gate insulators. Both the dielectrics showed enhanced saturation current density and pinch-off voltage compared to conventional HEMTs. The SiO2/SiN/AlGaN/GaN MOSHEMTs exhibited a low current collapse and low leakage current compared to SiN MISHEMTs and unpassivated HEMTs. However, the SiN/SiO2/AlGaN/GaN MOSHEMTs exhibited a lower current collapse coupled with moderate leakage current compared to SiO2 MOSHEMTs and unpassivated HEMTs. The comparison of device dc characteristics of double-insulator MOSHEMTs with single-insulator MOSHEMTs revealed its superiority in reducing the problem of current collapse and leakage current simultaneously. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Metal–oxide–semiconductor high-electron-mobility transistors were demonstrated on AlGaN/GaN with electron beam (EB) evaporated ZrO2. The composition of the EB deposited ZrO2 thin films was confirmed using X-ray photoelectron spectroscopy (XPS). The fabricated ZrO2-based MOSHEMTs exhibited high positive gate voltage of operation up to +7 V with low gate leakage current. For a comparison, conventional high-electron-mobility transistors (HEMTs) were also fabricated with identical device dimensions. The maximum drain current densities of 1168 and 538 mA/mm were observed on MOSHEMTs and HEMTs, respectively. Low gate leakage current density of four orders of magnitude was observed on ZrO2-based MOSHEMTs when compared with the conventional HEMTs. The observation of high forward on-voltage with low gate leakage current density and high positive operational voltage reveals the importance of ZrO2 dielectric films for MOSHEMT devices. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We firstly demonstrate dynamic change in a drain conductance of an AlGaN/GaN HEMT from DC to high frequency using sinusoidal wave input signal from a network analyzer which can sweep from hertz to giga hertz. Prior to measurements, a bias-T which is adaptable at a frequency of hertz to mega hertz has been developed. S-parameter measurements sweeping from 5 Hz to 3 GHz reveals that the magnitude of S22 significantly decreases at a hertz to mega hertz frequency range although the phase of S22 is negligibly stable. Also, it is found that the drain conductance evaluated from drain I-V characteristics and that extracted from S- parameters at 100 Hz are comparable and that a drain conductance from mega hertz to hundreds mega hertz is stable, indicating that some trapping or de-trapping effects occur at this frequency range.

Developments of heteroepitaxial growth and characteristics of MOCVD-grown AlGaN/GaN HEMTs on Si substrates are presented. High-temperature-grown AlGaN/AlN intermediate layers and GaN/AlN strained layer superlattice are effective in improving the crystal quality of a following GaN layer and in growing thick device structure on Si. The AlGaN/GaN HEMT has been successfully grown on 8-inch Si, which indicates that the gas phase reaction is minimized in this growth. The device exhibited the excellent uniformity, the 2DEG properties and the device characteristics.

Recently, LEDs based on heteroepitaxial GaN on Si substrates (GaN/Si) are attracting a great attention and are industrially developed by many companies in order to compete with LEDs based on GaN/sappire and GaN/SiC which are commercially marketed advancing GaN/Si LEDs, though GaN/Si LEDs have been realized early in 2002. The recent attention on GaN/Si is due to the availability of low cost and large wafer size availability (up to 300 mm diameter) of Si substrates. In this article, we review the developments of the GaN epitaxial growth on Si and summarize the developments made in our laboratory including the device structures achieving the GaN/Si LEDs with higher emission efficiency. We describe on GaN/Si using (i) high temperature (HT) AlN/AlGaN intermediate layers, (ii) HT intermediate layers (ILs) and multilayers (MLs), and (iii) strained-layer superlattices (SLS) interlayers and their LED performances respectively. We believe that GaN/Si LEDs with low prices will become important LEDs for general lighting in the near future.

We have reported that the crystallinity of GaAs on Si (GaAs/Si) grown by chemical beam epitaxy (CBE) depends on the growth temperatures of the top GaAs and nucleation layers. The GaAs layer on Si, grown at 500°C for the top GaAs layer and 400°C for the nucleation layer, exhibited a high crystal quality even at a lower growth temperature of 500°C. By performing in-situ thermal cycle annealing for thisGaAs/Si, the values of etch pit density and the full width at half maximum of X-ray double-crystal measurement were2.3 × 107cm−2and 147 arcsec, respectively. These samples were superior to those grown by the conventional techniques such as metal organic chemical vapor deposition and molecular beam epitaxy.

Using droplet-epitaxy, self-formed GaAs islands were successfully grown onGaAs/Si substrates by metalorganic chemical vapor deposition (MOCVD). The shapes and sizes of the islands were observed by atomic force microscope (AFM). In addition, a GaAs-based light emitting diode (LED) on Si with self-formed GaAs islands active region was fabricated for the first time. This LED was operated up to 27μW at 190 mA under room-temperature direct current (dc) conditions. Compared with a GaAs-based LED on Si with a GaAs quantum well active region, the reliability of this LED was much superior because of low dislocation density due to reduction of the active region by use of a GaAs island structure.

A high efficiency AlGaAs/Si tandem solar cell has been fabricated by metal-organic chemical vapor deposition (MOCVD). It consists of a Al 0.15 Ga 0.85 As top cell and a Si bottom cell. The crystalline quality of the Al 0.15 Ga 0.85 As heteroepitaxial layer grown on Si has been improved using a high temperature growth process (800°C) and thermal cycle annealing (300 ∼ 900°C). The quantum efficiency of the Si bottom cell in the long wavelength region has been improved by back surface field. The conversion efficiencies of the tandem solar cell under AMO and 1 sun measurement conditions with 4-terminal and 2-terminal configuration are 20.0% and 19.0%, respectively. The conversion efficiencies of the tandem solar cell with graded band gap emitter Al x Ga 1− x As layer achieved 20.6% and 19.9% under the same condition with 4-terminal and 2-terminal configuration, respectively.

GaNAs/GaAs triple quantum wells (QWs) grown by chemical beam epitaxy are annealed by two methods, ex situ rapid thermal annealing (RTA) and in situ annealing in a growth chamber followed by ex situ RTA (NRTA). The effects of annealing method on the structural, morphological, and optical properties of the QWs are studied comparatively. The results show that although there is surface desorption for both RTA and NRTA, the holes on the sample surface after desorption are clearly different. RTA is better than NRTA from the viewpoints of both surface morphology and optical properties.

Developments of heteroepitaxial growth and characteristics of an AlGaN/GaN HEMT on a Si substrate are reported. High-temperature-grown AlGaN/AlN intermediate layers and GaN/AlN strained layer superlattice are effective in improving the crystallinity of a following GaN layer and for growing thick device structure on Si, which resulted in obtaining high-breakdown voltage. The AlGaN/GaN HEMT on Si exhibited the breakdown voltage as high as 1402 V with a state-of-the-art figure-of-merit (FOM = BV2/R on ) of 2.6×108 V2Ω−1cm−2.

We report novel GaN fully vertical p–n diode on Si grown by metalorganic chemical vapor deposition. The thick strained layer superlattice is effective in controlling a doping level of 1016 cm−3 in an n−-GaN drift layer. The GaN p–n diode exhibits a differential on-resistance R on of 7.4 mΩ cm2, a turn-on voltage of 3.4 V, and a breakdown voltage V B of 288 V. The corresponding Baliga’s figure of merit (FOM) is 11.2 MW/cm2. A good FOM value for the GaN-on-Si vertical p–n diode is realized for a drift layer thickness of 1.5 µm without using substrate removal technology.