The invention relates to the field of semiconductor etching, and in particular to a process of crystallographic wet chemical etching of III-Nitride material, and in particular gallium nitride.
The III-Nitride materials system, gallium nitride (GaN) and its indium- and aluminum-containing alloys, shows great promise for producing high-speed electronic devices that operate efficiently at high temperatures. III-Nitrides are the material of choice for producing light-emitting devices including LEDs and lasers that operate at wavelengths throughout the visible and UV parts of the electromagnetic spectrum. These materials are extremely stable, allowing them to be used in high temperature and corrosive environments. Because of this stability, it is difficult to find methods for etching of III-Nitrides.
Most processing of the III-Nitrides is currently done by dry plasma etching. There are several disadvantages to dry etching, including rough surface profiles, ion-induced damage in the exposed surface areas and difficulty in obtaining smooth etched sidewalls, which are required for lasers, the expense of the equipment required.
Photoenhanced electro-chemical (PEC) wet etching has also been used for etching of GaN. PEC etching has the advantage of low surface damage and relatively low equipment cost, but there has not yet been found a method for producing smooth etched vertical sidewalls, which are required for lasers.
Even what seems to be a very small roughness can have a large effect on the reflectivity of a laser facet. FIG. 1 illustrates this point, showing a graph of the maximum reflectivity of a laser facet as a function of surface roughness in a typical InGaN/GaN laser structure. The best published roughness results from several different experimental groups are indicated by arrows in the figure. Note that most etching techniques produce a roughness of approximately 50 nm, allowing a maximum specular reflectivity of less than 1%. Production of laser facets using an etching technique is preferable to using cleaving or polishing because an entire wafer of several thousand lasers can be produced at one time by etching, but cleaving must be done on individual rows of devices, and polishing is an even more labor-intensive technique.
Recently, researchers at Xerox Coxp. fabricated a laser using chemically assisted ion beam etching (CAIBE), producing sidewalls with a roughness of 4-6 nm. This is the lowest sidewall roughness ever reported for etching in the III-Nitride material system, but even with this roughness the reflectivity is only 60-70% of the ideal reflectivity, and the roughness is enough to cause interference fringes in the emitted laser beam.
It would be very beneficial to take advantage of the crystal structure of the III-Nitride material itself to produce atomically smooth crystallographic planes without damaging the surface. Such crystallographic planes would have reflectivities greater than 99% of the ideal reflectivity, given by the well known Fresnel formula, as compared to the above values of 1% and 60-70% for plasma etching and CAIBE, respectively. Improvement in facet reflectivity corresponds to higher output power and a better defined laser spot. Wet chemical etching has been used for producing atomically smooth crystallographic planes in other semiconductor materials systems, such as silicon, gallium arsenide, and gallium phosphide. Previously, prior to the invention, this had never been achieved in the III-Nitride materials system.