Fabrication of semiconductor devices for microelectronics and optoelectronics typically requires a series of growth or deposition process steps and etching process steps. For example, in fabrication of buried heterostructure semiconductor lasers, after forming an active layer on a semiconductor substrate and etching a mesa ridge through the active layer, selective area epitaxial regrowth around the mesa ridge is required. In this type of structure, edges of the active layer are exposed after etching, and during the regrowth step. A significant challenge for metal-organic chemical vapour deposition (MOCVD) growth is removal of residual contaminants at the regrowth interface.
Thermal decomposition along the mesa sidewalls during a heat-up cycle before regrowth may create non-radiative recombination centres, which would degrade the reliability of the device. For example, failure of devices grown on InP has been attributed to defects originating at the mesa sidewall. Typical contaminants of InP surfaces are oxygen, and silicon, which is a n-type dopant in InP.
In fabrication of integrated circuits and optoelectronic devices, advanced growth and processing for device integration typically requires multiple in situ growth and etching steps, i.e. sequences of process steps carried out in the same reactor, to reduce opportunities for surface contamination between process steps. For example, where semiconductor layers are deposited by low pressure MOCVD, an compatible method of in situ vapour etching in the same MOCVD reactor is desirable.
Known methods of in-situ vapour etching of GaAs materials use HCl, or etchant precursors including AsCl.sub.3 or CH.sub.3 I. In use of the latter two etchants, the precursor pyrolizes in a hot zone of a reactor to provide reactive species, believed to be HCl or HI respectively. As described by Shimoyama et al., in J. Crystal Growth 107 (1991) 767, in situ etching of GaAs/AlGaAs hetero-strutures using HCl before crystal growth of subsequent layers was useful in preventing surface oxidation of etched AlGaAs and accumulation of impurities at the regrown interface.
Etching of InP in an MOCVD reactor using HCl gas is discussed, for example, by C. Caneau et al., in J. Crystal Growth 107 (1991) p.203 and by P. D. Agnello et al., in J. Crystal Growth 73 (1985) p.453. However, it was found that etching could not be done reproducibly on a susceptor (i.e. wafer support and heater) that was previously coated with deposits from previous growths. This is a disadvantage when a series of growth steps and etching steps must be carried out.
Etching in HCl alone may result in poor surface morphology with pits. Good etching morphology has been achieved at crystal growth temperatures by providing a phosphorous overpressure at the wafer surfaces as well as an etchant gas during etching of InP. For example, PH.sub.3 may be added as a phosphorus source during etching. The Caneau reference describes HCl etching in an MOCVD reactor where PH.sub.3 was added as a phosphorus source during etching. Low partial pressures of HCl and PH.sub.3 and an etching temperature of at least 625.degree. C. were necessary to avoid the formation of pits in the etched material. It was necessary to use a bare susceptor, free of deposits, to achieve reproducible etching rates.
Another known method of etching indium phosphide semiconductor materials for advanced integrated microelectronic and optoelectronic circuit applications is chemical beam etching with a combination etchant and surface preservative. For etching in a Chemical Beam Epitaxy (CBE) apparatus, typically PCl.sub.3 is used as a combined etchant and phosphorus source, as described by W. T. Tsang et al., in J. Crystal Growth 136 (1994), p. 42.