Beckham et al., of Emcore, show a sheet resistivity map (Fig. 5) for a double planar doped p-HEMT wafer with a thin, lightly doped cap, showing a uniformity of better than 1.6% (1 sigma) [8].

In the MIMIC programme, Westinghouse, Avantek, Rockwell, and TRW have used sheet resistance and conductance diameter scans (Fig. 6),

Fig. 5. Sheet resistivity map for a double planar doped p-HEMT wafer

correlated with photon backscatter (PBS) red and blue wafers, in order to minimize subsurface damage caused by sawing and polishing, as well as to control the etching used to remove damage [9]. Fig. 7 shows the effect of pre-implantation surface treatment on the sheet conductance of Si implanted, 3 inch diameter GaAs wafers after PSG capping and 830oC/30 s furnace annealing.

Sheet resistance measurements and maps have been used by Aixtron, Emcore, and Riber to ensure that the epitaxial reactors are performing uniformly before sale and shipping. Tompa et al., of Emcore, show that, after growth parameter optimization, commercial III-V MOCVD production scale growth systems are capable of providing cross-sectional uniformity equivalent to that achieved by MBE [10]. CÐV, Hall effect, PL, X-ray diffraction and contactless sheet resistance probe measurements were used to evaluate the doping and compositional uniformity of a variety of structures. Fig. 8 shows a sheet resistivity map