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.

A standard metallization scheme for the formation of Ohmic contacts on n-type GaN does exist. It has the following multilayer structure: Ti∕Al∕metal∕Au. Ti is known to extract N out of the GaN. This leaves a high density of N vacancies (donors) near the interface pinning the Fermi level. The created tunnel junction is responsible for an Ohmic contact behavior. Au is deposited as the final metal layer to exclude oxidation of the contact and the metal should limit the diffusion of Au into the layers below and vice versa. Al in the metallization scheme is known to improve the contact resistance, but the reason why has not been reported yet. We studied Ti and Ti∕Al contacts on GaN and AlGaN∕GaN as a function of annealing temperature by transmission electron microscopy. The role of Al in the metal multilayer, and of Al in the AlGaN on the Ohmic contact formation, has been determined. The latter result indicates that the standard metallization scheme for GaN cannot be simply transferred to AlGaN∕GaN structures.

The semi-insulating character of GaN epitaxial layers can be achieved by the control of the early stages of growth on the substrate. Adding two low temperature (LT) AlN interlayers is a technique enough powerful to reduce threading dislocation densities by up to one order of magnitude. A compressive strain as high as 2.8 × 10 −3 is induced in the uppermost GaN epilayer. The global structure is kept semi-insulating so that it is a perfect template for undoped AlGaN − GaN HEMTs (High Electron Mobility Transistors). HEMTs with interlayers present better two dimensional electron gas (2DEG) properties: up to 20% higher carrier density (n S ) and 40% higher mobility. Typically n S is as high as 1.7 × 10 13 cm −2 for a record mobility of 1200 cm 2 /Vs. The improvement of the mobility can be correlated to the reduction of nano-scale V-shaped defects in the AlGaN (less morphological-relaxation). The improvement of n S could be explained by the higher piezo-doping resulting from GaN extra-compression and AlGaN weaker relaxation. As a consequence, the DC transistors characteristics are improved: in 2 µm gate transistors, the maximum current and transconductance are increased by up to 80% and 20%, respectively, and could be extrapolated to values as high as 1500 mA/mm and 250 mS/mm for 0.2 µm gate devices.

In x Ga 1 − x N ∕ Ga N light-emitting diode structures with a high In concentration may lose all optical output after capping the active region with a p-type GaN layer. Transmission electron microscopy has been applied to determine the microstructural changes that occur in the quantum-well (QW) region during this capping process. The loss of the optical output is related to a clustering of In into metallic In platelets in the QW region. The properties of these In platelets are described and a formation model is proposed.

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