Control of Photoelectrochemical (PEC) Etching by Modification of the Local Electrochemical Potential of the Semiconductor Structure

Brief Description

A method for locally controlling an electrical potential of a semiconductor structure or device, and thus locally controlling lateral and/or vertical photoelectrochemical (PEC) etch rates.

Background

PEC etching uses above-bandgap illumination to generate carriers (specifically holes) needed to etch III-nitrides. The electrochemical potential of the semiconductor material surface relative to the electrolyte causes holes to be drawn toward the semiconductor-electrolyte interface in n-type (unintentionally-doped or doped) material, allowing them to participate in the electrochemical reactions necessary for material removal. Because the etching mechanism relies heavily on the absorption of incident light and the electrochemical potential of the semiconductor material relative to the electrolyte, PEC etching can be defect-selective, dopant-selective, and bandgap-selective. Most applications of PEC etching have pertained to vertical etching of the material, either through direct illumination of the material surface, or by illumination through a masking layer.  However, specific descriptions of local control of the etch process through modifications of the electrochemical component of etching have not been presented. Previously published and patented techniques that apply to III-nitride PEC etching  suffer from certain limitations.

Description

This invention describes a scheme for fabricating III-nitride semiconductor structures wherein a highly selective photo-induced etch is achieved through strategic modification of the local electrochemical potential of the semiconductor structure relative to the electrolyte. This is accomplished through:

1. The suitable placement of electrically resistive (unintentionally-doped, doped, alloyed) or electron-blocking layers in the semiconductor structure.
2. The selective placement of the cathode in PEC etching, wherein the cathode acts as a "channel" for the controlled collection of photo-generated electrons from the semiconductor layers with which it is in contact.
3. The use of a suitable light source during PEC etching, which enables the photo-generation of electrons and holes in layers with bandgap energies lower than the energy of the incident light. The etch will not proceed without photo-induced carriers. The light source may be a laser or a broad-spectrum source with/without a filter.
4. The use of a suitable electrolyte solution during PEC etching, wherein the concentration and type of electrolyte determines the etch rate and etch selectivity.

Advantages

PEC etching is a viable method for producing specific geometries in the III-nitride material system, forming three-dimensional structures that would be extremely challenging to produce with gas-phase etching processes or more standard wet chemical etching. The existence of a controlled three-dimensional etch process can give rise to numerous useful device geometries. Specifically, an undercut geometry is desirable in several applications including but not limited to microdisk resonators, air-gap DBRs, semiconductor membranes and cantilevers, electrical and optical apertures, and in substrate removal.

Applications

• Bandgap-selective lateral etching

Patent Status