Surface roughness contribution to MHEMT mobility characteristics

Characterizations on the surface roughness of metamorphic high electron mobility transistor structure (MHEMT) denoted as 2211_#1 (step-graded metamorphic buffer) and 2212_#1 (linearly-graded metamorphic buffer) have been carried out using Atomic Force Microscopy (AFM). Surface roughness effect of linearly and step graded metamorphic buffer on sheet resistance and mobility at room temperature have been studied using a four-point probe and Hall effect techniques, respectively. From the AFM measurement, it was found that surface roughness of linearly graded buffer (2.84 nm) is lower compare to the step graded buffer (3.07 nm). The lower surface roughness sample which is sample 2212_#1 gave lower sheet resistance value and higher mobility value which is 7280 cm²/V-sec compare to the mobility of sample 2211_#1 which is 5850 cm²/V-sec. It is believed that surface roughness of MHEMT structures reflects the growth quality of the structures and smooth surface roughness introduces to a higher mobility value. It is assumed that rough surface introduces higher sheet resistance due to higher scattering effect and this in turns to the lower mobility value.