Study of selective gate recess etching of InGaAs/InAlAs/InGaAs metamorphic HEMT structures using succinic acid based etchant

Metamorphic HEMTs on GaAs substrates are promising devices of today as they are operated at even higher frequencies for microwave applications compared to pseudomorphic HEMTs. The selective removal of n⁺ InGaAs ohmic contact layer from the top of the device structure poses a major challenge during fabrication. We have studied the influence of temperature on the selectivity of etch rate between the n⁺ InGaAs and underlying InAlAs layers using succinic acid based etchant. The etchant was composed of succinic acid solution mixed with hydrogen peroxide and deionized water. The pH of the solution was adjusted to 5 by adding NH₄OH. The etch rates at different temperatures between 14degC to 30degC were estimated by profiling the etched pattern using atomic force microscopy (AFM). Surface roughness of the etched area also was studied using AFM. It was found that the selectivity has improved with temperature. This is possibly due to simultaneous occurrence of low etch rates of InAlAs due to presence of aluminum oxide and high etch rates of InGaAs due to increased temperature. It was also found that the surface roughness was higher at lower temperatures contrary to the observations made in the case of pseudomorphic HEMTs.