We have made AlGaN∕GaN high electron mobility transistors with a Si3N4 passivation layer that was deposited in situ in our metal-organic chemical-vapor deposition reactor in the same growth sequence as the rest of the layer stack. The Si3N4 is shown to be of high quality and stoichiometric in composition. It reduces the relaxation, cracking, and surface roughness of the AlGaN layer. It also neutralizes the charges at the top AlGaN interface, which leads to a higher two-dimensional electron-gas density. Moreover, it protects the surface during processing and improves the Ohmic source and drain contacts. This leads to devices with greatly improved characteristics.

In this work, we report on the growth by metalorganic vapor phase epitaxy (MOVPE) of GaN layers on AlN/Si(111) templates with step-graded AlGaN intermediate layers. First, we will discuss the optimization of the AlN/Si(111) templates and then we will discuss the incorporation of step-graded AlGaN intermediate layers. It is found that the growth stress in GaN on high-temperature (HT) AlN/Si(111) templates is compressive, although, due to relaxation, the stress we have measured is much lower than the theoretical value. In order to prevent the stress relaxation, step-graded AlGaN layers are introduced and a crack-free GaN epitaxial layer of thickness .1 mm is demonstrated. Under optimized growth conditions, the total layer stack, exceeding 2 mm in total, is kept under compressive stress, and the radius of the convex wafer bowing is as large as 119 m. The crystalline quality of the GaN layers is examined by highresolution x-ray diffraction (HR-XRD), and the full-width-at-half maximums (FWHMs) of the x-ray rocking curve (0002) v-scan and (ÿ1015) v-scan are 790 arc sec and 730 arc sec, respectively. It is found by cross-sectional transmission electron microscopy (TEM) that the step-graded AlGaN layers terminate or bend the dislocations at the interfaces.

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In this work, we demonstrate, for the first time, Al0.35GaN/GaN/Al0.25GaN double heterostructure field effect transistors on 200 mm Si(111) substrates. Thick crack-free Al0.25GaN buffer layers are achieved by optimizing Al0.75GaN/Al0.5GaN intermediate layers and AlN nucleation layers. The highest buffer breakdown voltage reaches 1380 V on a sample with a total buffer thickness of 4.6 µm. According to Van der Pauw Hall measurements, the electron mobility is 1766 cm2 V-1 s-1 and the electron density is 1.16×1013 cm-2, which results in a very low sheet resistance of 306±8 Ω/square.

High quality GaN layers with dislocation density of (3.0±0.5)×108∕cm2 have been grown on silicon(111) substrates using a combination of AlGaN intermediate layers and a SixNy interlayer. A smooth and fully coalesced layer was obtained by virtue of a high temperature growth process which accelerates coalescence and improves at the same time the crystalline quality. This was confirmed by high resolution x-ray diffraction showing a full width of half maximum of 415arcsec for the asymmetric (−2201) rocking curve.

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AlGaN/GaN/AlGaN double heterostructure field-effect transistors (DHFET) with high breakdown voltage and low on-resistance were fabricated on silicon substrates. A linear dependency of the breakdown voltage on the buffer thickness and on the buffer Aluminium concentration was found. A breakdown voltage as high as 830 V and an on-resistance as low as 6.2 Ω·mm were obtained in devices processed on 3.7 µm buffer thickness. The gate–drain spacing was 8 µm and the devices did not have any field plates.