Characteristics of A Zn0.7Mg0.3O/ZnO Heterostructure Field-Effect Transistor Grown on Sapphire Substrate by Molecular-Beam Epitaxy

Characterization of a Zn0.7Mg0.3O/ZnO heterostructure field-effect transistor (HFET) is reported. The HFET was based on a Zn0.7Mg0.3O/ZnO/Zn0.7Mg0.3O single quantum well(SQW)grown on an a-plane sapphire substrate by molecular-beam epitaxy, and was fabricated by a conventional photolithography technique combined with dry etching. Room-temperature characteristic of the HFET was a n-channel depletion type with a transconductance of 0.70mS/mm and a field-effect mobility of 140cm2/Vs, in good agreement with the electron Hall mobility in SQW of 130cm2/Vs. The on∕off ratio at VDS=3V was ∼800, which was limited by an insufficiently suppressed leakage current through the bottom Zn0.7Mg0.3O barrier.

In this paper we describe recent experimental efforts to produce high quality thick (300 μm) GaN layers on sapphire, the removal of such a layer from the sapphire substrate, and the properties of the so obtained free-standing GaN material. The growth process is described in some detail in the vertical reactor geometry used in this work. Defects like dislocations, micro-cracks and pits produced during growth are discussed, along with procedures to minimize their concentration on the growing surface. The laser lift-off technique is shown to be a feasible technology, in particular if a powerful laser with a large spot size can be used. A major problem with the free-standing material is the typically large bowing of such a wafer, due to the built in defect concentrations near the former GaN-sapphire interface. This bowing typically causes a rather large width of the XRD rocking curve of the free-standing material, while optical data confirm virtually strain free material of excellent quality at the top surface.


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