INTRINSIC MECHANISM OF SMOOTH AND ROUGH MORPHOLOGY IN ETCHING OF INP BY CL2 DETERMINED BY INFRARED-LASER INTERFEROMETRY

The effect of Cl2 pressure and substrate temperature (T(s)) on the absolute, steady-state etch rates of InP (100) has been measured with approximately 1 % accuracy from approximately 0.02 to 1000 angstrom/sec by infrared laser interferometry. An unexpected finding was that the etch rate could exceed the calculated evaporation rate of InCl3 by a factor of 50, indicating a weaker binding energy of InCl3 to InP relative to itself. At sufficiently low T(s) the measured slow etch rate is only a factor of approximately 3.5 above that predicted from the evaporation rate of InCl3. The transition between the fast and slow etch rates occurs as a first-order phase transition at a substrate temperature, T(s)*, which depends on the Cl2 pressure. If InP is etched at T(s)*, the surface roughens while at other T(s) the surface remains smooth. The explanation for the surface roughening is that at T(s)*, InCl3 nucleates to form InCl3 islands. At T(s)* the etch rate is approximately fifteen times slower where the InP surface is covered by InCl3 compared to the bare InP surface. The large differential etch rates for bare and InCl3 covered InP produces a rough surface morphology. Below T(s)*, the surface is completely covered by an InCl3 multilayer and etches uniformly although slowly at the nominal InCl3 vaporization rate. This mechanism of surface roughening is intrinsic to the Cl2 + InP reaction.