G. Y. Zhao

Characterizations of recessed gate AlGaN/GaN HEMTs on sapphire

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).

High-Quality GaN on Si Substrate Using AlGaN/AlN Intermediate Layer

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.

Determination of exciton transition energy and bowing parameter of AlGaN alloys in AlGaN/GaN heterostructure by means of reflectance measurement

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 ...

Electrical Characteristics of Schottky Contacts on GaN and Al0.11Ga0.89N

The electrical characteristics of Ag, Ti, Au, Pd and Ni Schottky contacts on GaN and Al0.11Ga0.89N grown by metalorganic chemical vapour deposition (MOCVD) on sapphire substrates have been investigated. Al0.11Ga0.89N Schottky barrier height values are bit higher than the values of GaN contacts except Ti Schottky contacts. Fermi-level pinning has been observed for both GaN and Al0.11Ga0.89N Schottky contacts. The pinning degree of GaN and Al0.11Ga0.89N are much less than GaAs, Si and GaP, but both of them may be similar to CdS.

Schottky diodes of Ni/Au on n-GaN grown on sapphire and SiC substrates

GaN films were grown on sapphire and SiC substrates. The crystal qualities of GaN films were investigated by photoluminescence, atomic force microscopy, and electron-beam-induced current measurements, etc. It was found that the crystal quality of GaN on SiC is better than the one on sapphire. Ni/Au Schottky contacts were formed on the both samples. The electronic characteristics were obtained by current–voltage and capacitance–voltage measurements. Schottky diodes on sapphire substrate show breakdown voltage of −80 V. While for SiC substrate, the strong breakdown was not observed even at −100 V. The reverse leakage current of diodes based on SiC is over three orders of magnitude lower than that of sapphire substrate when the reverse voltage is above 50 V, which is due to the presence of low dislocation density and high thermal conductivity.

Growth of Si delta-doped GaN by metalorganic chemical-vapor deposition

Si delta-doped GaN has been grown by metalorganic chemical-vapor deposition. A very high peak density and narrow full width at half maximum (FWHM) of the carrier profile are obtained. It is found that the peak carrier density of Si delta doping increases with the doping time and SiH4 flow rate, while the FWHM of the carrier profile decreases with both increasing doping time and SiH4 flow rate. Some saturation in the carrier density has also been observed for relatively longer doping time. Except for a broadened carrier distribution in GaN induced by Si diffusion due to high growth temperature, the Si delta-doping properties in GaN are found to be similar to those of GaAs.

Electron mobility on AlGaN/GaN heterostructure interface

Abstract High-quality AlGaN/GaN heterostructure with different Al compositions have been grown by metalorganic chemical vapor deposition (MOCVD) on sapphire. Photoluminescence spectra exhibit very strong recombination related to the two-dimensional electron gas (2DEG), which is dominated at higher Al composition. The unintentionally doped Al 0.11 Ga 0.89 N/GaN heterostructure shows the highest Hall mobility of 11 823 cm 2 / Vs at 5 K. In addition, we realized that the quality of interface can be improved with increasing AlGaN layer thickness, and Hall mobility will be enhanced.

Energy-gap narrowing in a current injected InGaN/AlGaN surface light emitting diode

The emission spectrum of a current injected InGaN/AlGaN surface emitting diode has been investigated. A clear redshift of the low energy edge with increasing injected current has been observed, and is attributed to the many body effects. The carrier density and band gap narrowing are obtained by fitting the line shape of the emission spectrum, using Landsberg model which includes many body effects. A redshift of around 92 meV of the low energy edge is obtained as injected current increases from 400 to 4000 mA. The band gap change can be described well in proportion to the 1/3 power of the carrier density, which is just suggested by the exchange energy of electron–electron, and hole–hole interactions.

Thermo-optical nonlinearity of GaN grown by metalorganic chemical- vapor deposition

A study of thermo-optical coefficient (dn/dT) of GaN using spectroscopic ellipsometry is made, and a large thermo-optical nonlinearity near band edge, which increases with increasing temperature, has been observed. Kramers–Kronig transformation has been used to verify our results and a qualitative consistency has been obtained.

Passivation of Bulk and Surface Defects in GaAs Grown on Si Substrate by Radio Frequency Phosphine/Hydrogen Plasma Exposure

The effects of PH3/H2 plasma exposure on GaAs grown on Si substrate (GaAs/Si) were investigated. It was found that incorporation of P atoms in H2 plasma not only hydrogenated the defect-related recombination centers of GaAs/Si epilayer, but also phosphidized the surface region of GaAs/Si epilayer by forming a phosphidized layer. Electron beam-induced current measurement directly proved that the defect-related dark spot density was effectively reduced by adding P atoms into the pure H2 plasma. In addition, PH3/H2 plasma exposure greatly increased the minority carrier lifetime properties and decreased the saturation current of the GaAs p+–n junction structure grown on Si substrate.