Catalyst and processing effects on metal-assisted chemical etching for the production of highly porous GaN

Abstract

Metal-assisted chemical etching is a facile method to produce micro-/nanostructures in the near-surface region of gallium nitride (GaN) and other semiconductors. Detailed studies of the production of porous GaN (PGaN) using different metal catalysts and GaN doping conditions have been performed in order to understand the mechanism by which metal-assisted chemical etching is accomplished in GaN. Patterned catalysts show increasing metal-assisted chemical etching activity to n-GaN in the order Ag < Au < Ir < Pt. In addition, the catalytic behavior of continuous films is compared to discontinuous island films. Continuous metal films strongly shield the surface, hindering metal-assisted chemical etching, an effect which can be overcome by using discontinuous films or increasing the irradiance of the light source. With increasing etch time or irradiance, PGaN morphologies change from uniform porous structures to ridge and valley structures. The doping type plays an important role, with metal-assisted chemical etching activity increasing in the order p-GaN < intrinsic GaN < n-GaN. Both the catalyst identity and the doping type effects are explained by the work functions and the related band offsets that affect the metal-assisted chemical etching process through a combination of different barriers to hole injection and the formation of hole accumulation/depletion layers at the metal-semiconductor interface.

Evidences for redox reaction driven charge transfer and mass transport in metal-assisted chemical etching of silicon

“…Indeed, it has been suggested that the type of transport barrier at the metal/semiconductor interface (i.e., Schottky or Ohmic) will control the charge transfer and hence the MacEtch process3436373839. On perusal, this appears to be true.…”

GaAs pillar array-based light emitting diodes fabricated by metal-assisted chemical etching

“…The fact that doping type changes the etch rate with identical Au pattern feature sizes indicates that charge transport, instead of mass transport, is the rate determining factor. It has been reported that the Schottky barrier height, which can be affected by the metal work function, thickness, and size, as well as the semiconductor doping type and level, can affect the etch rate, with faster etch rates for ntype GaN 40 and thicker Au for p-type Si. 41 The Schottky barrier height analysis used in the case of GaN 40 cannot explain the doping type dependence observed in the current study, possibly because the etching condition that is low in oxidant concentration makes the majority carrier transport in the semiconductor play a dominant role in determining the etch rate.…”

In(Ga)N Nanostructures and Devices Grown by Molecular Beam Epitaxy and Metal-Assisted Photochemical Etching

“…Very long NWs (>10 μm) with a wurtzite structure were obtained. Geng et al [ 38 ] investigated the growth of porous GaN in various metal catalysts and GaN doping environments and demonstrated an improvement in the MacEtch technique and the catalytic scheme in the order of Ag < Au < Ir < Pt. The authors also observed that the catalytic action of continuous films contrasted with that of films with discontinuous islands.…”

“…MacEtch is the most popular technique because it produces a homogeneous porous morphology over a wide region and is simpler than other surface-roughening techniques. [ 38 , 39 ]. MacEtch prevents the surface damage caused by dry etching procedures, which is desired for potential optoelectronic applications.…”

“…MacEtch prevents the surface damage caused by dry etching procedures, which is desired for potential optoelectronic applications. MacEtch reactions can be broadly categorized into two types: single-step reactions with HAuCl 4 /HF [ 37 ], AgNO 3 /HF [ 40 ] or solutions containing AgNO 3 /HF/CH 4 O [ 38 ]; and two-step reactions involving predeposition of metal nanoparticles or patterned thin-film metals in the presence of HF/H 2 O 2 [ 41 ].…”