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AlGaN/GaN heterostructures were grown on Si (1 1 1) substrates with different AlN nucleation layers (NL) by metal–organic chemical vapor deposition (MOCVD). The results indicate that the growth temperature of AlN NL has a noticeable influence on the structural, electronic and optical properties of the AlGaN/GaN heterostructures. Optimizing the growth temperature to 1040 °C led to quasi-2D smooth surface of the AlN NL with providing sufficient compressive stress to suppress cracking of the subsequent GaN layer during the cooling process, resulting in improved crystalline quality of GaN layer and superior two-dimensional electron gas (2DEG) performance of the AlGaN/GaN heterostructure.

AlGaN/GaN heterostructures with multiple AlN interlayers deposited at a high-temperature (HT) of 900 °C were grown by metalorganic chemical vapor deposition. Flat surface with few dark pits was obtained by using multiple HT-AlN interlayers. Hall effect measurements and non-contact sheet resistance mappings demonstrated the improvement of the electrical properties of AlGaN/GaN heterostructure. The sheet carrier density and Hall mobility continuously increased with the increase of the AlN interlayer number. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The electrical properties of two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures using high temperature (HT) AlN interlayers (ITs) grown on c-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD) have been investigated. It is found that the electrical properties (electron mobility and sheet carrier density) are improved compared with those in the conventional AlGaN/GaN heterostructures without HT AlN ITs, and the improved 2DEG properties result in the reduction of the sheet resistance. The results from high resolution X-ray diffraction (HRXRD) and Raman spectroscopy measurements show that HT AlN ITs increase the in-plane compressive strain in the upper GaN layer, which enhances the piezoelectric polarization in it and consequently causes increasing of 2DEG density at the AlGaN/GaN interface. Meanwhile, the compressive strain induced by HT AlN ITs leads to a less tensile strain in AlGaN barrier layer and causes positive and negative effects on the sheet carrier density of 2DEG, which counteract each other. The HT AlN ITs reduce the lattice mismatch between the GaN and AlGaN layers and smooth the interface between them, thus increasing the electric mobility of 2DEG by weakening the alloy-related interface roughness and scattering. In addition, the surface morphology of AlGaN/GaN heterostructures is improved by the insertion of HT AlN ITs. The reason for the improved properties is discussed in this paper.

A multi-step rapid thermal annealing process of Ti/Al/Ni/Au was investigated for ohmic contact of AlGaN/GaN high electron mobility transistor (HEMT). The samples were studied by Transmission Line Model (TLM), Scanning Electron Microscopy (SEM), Auger electron spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) measurements. By the multi-step annealing process, the specific contact resistance was decreased from 10−5 Ω cm2 level to 4–3 × 10−6 Ω cm2 and the surface morphology was improved. The AES measurements showed that the limitation indiffusion of Au and outdiffusion of Al were account for the surface morphology improvement and the surface Fermi level towards the conduction-band edge resulted in a lower specific contact resistance.

The effect of reactor pressure on the growth rate, surface morphology and crystalline quality of GaN films grown on sapphire by metalorganic chemical vapor deposition is studied. The results show that as the reactor pressure increases from 2500 to 20000 Pa, the GaN surface becomes rough and the growth rate of GaN films decreases. The rough surface morphology is associated with the initial high temperature GaN islands, which are large with low density due to low adatom surface diffusion under high reactor pressure. These islands prolong the occurrence of 2D growth mode and decrease the growth rate of GaN film. Meanwhile, the large GaN islands with low density lead to the reduction of threading dislocation density during subsequent island growth and coalescence, and consequently decrease the full width at half maximum of X-ray rocking curve of the GaN film.

We report on a GaN metal-oxide-semiconductor high electron mobility transistor (MOS-HEMT) using atomic-layer deposited (ALD) Al 2 O 3 as the gate dielectric. Through further decreasing the thickness of the gate oxide to 3.5 nm and optimizing the device fabrication process, a device with maximum transconductance of 150 mS/mm was produced. The drain current of this 0.8 μm gate-length MOS-HEMT could reach 800 mA/mm at +3.0 V gate bias. Compared to a conventional AlGaN/GaN HEMT of similar design, better interface property, lower leakage current, and smaller capacitance-voltage (C-V) hysteresis were obtained, and the superiority of this MOS-HEMT device structure with ALD Al 2 O 3 gate dielectric was exhibited.

Fabrication of enhancement-mode high electron mobility transistors on AlGaN/GaN heterostructures grown on sapphire substrates is reported. These devices with 1 μm gate-length, 10 nm recessed-gate depth, 4 μm distance of source and drain ex-hibit a maximum drain current of 233 mA/mm at 1.5 V, a maximum transconduc-tance of 210 mS/mm, and a threshold voltage of 0.12 V. The threshold voltage of these devices increased to 0.53 V after 500℃ 5 min annealing in N2 ambient. The saturation drain current and transconductance of 15 nm recessed-gate depth re-duced compared to those of 10 nm recessed-gate depth, but the threshold voltage increased to 0.47 V. The relations between threshold voltage, controlling ability of gate and recess depth were validated by testing C-V structures on AlGaN/GaN het-erostructures with different etching depth.

Sapphire substrate was treated by SiH 4 under NH 3 atmosphere and nano-size islands SiN x mask was formed on the surface. GaN films were grown on the SiN x patterned sapphire substrates to form GaN/SiN x /Al 2 O 3 structure. The influence of SiH 4 treatment time on crystalline quality and luminance properities of GaN films were studied. The lowest density of screw and edge type dislocations of 1.3×108 cm−2 and 5.0×109cm−2 was got by SiH 4 treatment time of 120s, together with the lowest FWHM of GaN PL spectra of 5.4nm. The result indicated that SiH 4 treatment time could affect the size and density of SiN x islands. The SiN x mask with modified SiH 4 treatment time increased 3D to 2D growth time of high temperature GaN, which was the major factor improving the crystalline quality of GaN films.

We have developed a novel AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor using a stack gate HfO2/Al2O3 structure grown by atomic layer deposition. The stack gate consists of a thin HfO2 (30-A) gate dielectric and a thin Al2O3 (20- A) interfacial passivation layer (IPL). For the 50-A stack gate, no measurable C-V hysteresis and a smaller threshold voltage shift were observed, indicating that a high-quality interface can be achieved using a Al2O3 IPL on an AlGaN substrate. Good surface passivation effects of the Al2O3 IPL have also been confirmed by pulsed gate measurements. Devices with 1- mum gate lengths exhibit a cutoff frequency (fT) of 12 GHz and a maximum frequency of oscillation (f MAX) of 34 GHz, as well as a maximum drain current of 800 mA/mm and a peak transconductance of 150 mS/mm, whereas the gate leakage current is at least six orders of magnitude lower than that of the reference high-electron mobility transistors at a positive gate bias.

A novel structure of AlGaN/GaN heterostructure which has a high temperature AlN interlayer (HT-AlN) in GaN buffer grown by metal organic chemical vapor deposition(MOCVD) on c-plane sapphire substrate has been researched. It is found that both electron mobility and sheet carrier concentration are increased by the HT-AlN interlayer compared to AlGaN/GaN heterostructure without HT-AlN interlayer. The sheet carrier concentration and Hall mobility measured by Hall increased from 1.446×1013 cm−2 and 1019 cm2/v·s to 1.605×1013cm−2 and 1036 cm2/v·s, respectively, hence the sheet resistance decreased from 424Ω/□ to 376Ω/□.

A GaN HEMT with a polarization-graded AlGaN buffer is performed by two-dimensional analysis of drift-diffusion simulations. The bulk trap-induced current collapse of the proposed structure is effectively restrained in contrast to that of conventional HEMTs with a GaN or AlGaN buffer, resulting from the fact that the high and flat back-barrier altitude in the proposed structure prevents the two-dimensional electron gas (2DEG) from spilling over from the channel, with the reduction of hot carriers injecting into the buffer followed by trapping in deep acceptor-like levels. Simultaneously, the off-state breakdown voltage is remarkably enhanced, due to the strong electric breakdown field of the polarization-graded AlGaN buffer and the restraint of the buffer leakage current. In addition, the relationship between the off-state breakdown voltage and the thickness of the polarization-graded AlGaN buffer is analyzed.

Through a comparative study of the scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) results, polarity is found to play an important role in the GaN wet etching process. For the pure screw dislocation it is easy to be etched along the steps that the dislocation terminates. Consequently a small Ga-polar plane is formed to prevent further vertical etching, which results in an etch pit of inverted truncated hexagonal pyramid. However, for the pure edge dislocation it is easy to be etched along the dislocation line, inducing an etch pit of inverted pyramid.