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Highly selective PEC etching of gallium nitride device structures


Photoelectrochemical (PEC) wet etching is an attractive wet etch approach for III-Nitride materials. Compared to dry etch techniques normally applied in prevalent GaN device fabrications, PEC wet etching can provide low damage, selective etching and understanding of material defects. This dissertation work has carried out an in-depth exploration of the dependence of PEC etching on both process variables and materials composition. In particular, a detailed study of bandgap-selective etching is carried out. To focus these studies, we describe the fabrication of a novel vertical electronic device, the  CAVET (C&barbelow;urrent A&barbelow;perture V&barbelow;ertical E&barbelow;lectron T&barbelow;ransistor). The key feature of this device is a current aperture that restricts the flow of current in a 2DEG to a direction perpendicular to the surface. By keeping current away from surface states, this device geometry provides low DC-RF dispersion compared to a more conventional AlGaN/GaN HEMT. The composition and dimensions of the aperture cannot otherwise interfere with the operation of the device, thus the fabrication process will involve highly selective etching of a very thin sacrificial layer. We utilized PEC bandgap selective etching of a 60nm InGaN sacrificial layer, and great effort was employed to optimize the etch process to obtain smooth, controllable lateral undercut etching. We describe two generations of device fabrication and the accompanying modifications in selective etch process that were required.

In developing a selective undercut etch process in our initialdevices, it becomes important to understand the differences in etch rate and mechanism for both the Ga-face and N-face (0001¯) crystallographic planes. Tremendous insights were provided by PEC etch studies on LEO (Lateral Epitaxial Overgrowth) a-plane GaN, where we could compare the effects of dislocation and crystallographic plane on etch rate and etch morphology. It is observed that dislocations retard the etch process. The N-face is far more chemically active than theGa-face, producing a crystallographic etching morphology of {101¯1¯} hexagonal pyramids. The N-face can even be etched without illumination, thus limiting the selectivity obtainable in a photo-enhanced etch process. This morphology can be used to increase the light extraction efficiency of GaN LEDs by a factor of 2--3.
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