Dept. of Applied Physics
Oregon Graduate Center
Semi-insulating (SI) GaAs is currently the object of active investigation due to its applications in microwave and digital technology. The SI status of "undoped" GaAs derives from the electrical compensation of acceptors by an intrinsic donor labeled EL2. Thus the material uniformity is expected to depend, among other parameters, on the EL2 spatial fluctuations GaAs substrates used in the integrated circuit industry are usually grown by the liquid encapsulated Czochralski (LEC) method, and are heavily dislocated. Dislocations may affect the electrical uniformity of the substrates either directly or indirectly, acting as sinks and sources for EL2. Because of the high cost of SI GaAs, it is appropriate to develop nondestructive techniques to evaluate the uniformity of the material and the relations between different parameters such as EL2 and dislocations. High resolution transmittance measurements of GaAs wafers of customary thickness (0.5 mm) have been performed in this study to gain knowledge of the EL2 concentration and its spatial distribution in the material. A computer controlled system has been originally developed to map the EL2 distribution inferring its local concentration from transmittance data in the 0.95-1.1 Âµm range. The same experimental set-up has then been used to map nondestructively the residual stress in the wafers, as well as to get qualitative information about the dislocation pattern in chemically etched wafers. Stress in LEC grown GaAs crystals produce dislocations and may even lead to the cracking of the ingot. While the stress distribution is usually destructively deduced from the dislocation pattern, the measurements have provided the first experimental verification of theoretical thermoelastic models of the stress distribution in LEC ingots. More than a hundred GaAs wafers from various commercial suppliers have been mapped, and the results of this study can be summarized as follows: (a) in only about 50% of the samples the EL2 and stress/dislocation distributions are qualitatively similar in that they have the same four-fold crystallographic symmetry; (b) these extrinsic parameter distributions appear to be independent, since each can be individually detected in the absence of the other; (c) in the presence of linear clusters of dislocations, commonly known as lineages, enhanced EL2 can be found segregated about them. The spatial coincidence of EL2 and dislocations makes it difficult to differentiate the effect they individually have on devices built in GaAs substrates. A few wafers, whose EL2 and dislocation patterns were known to be different,have been processed to yield arrays of field effect transistors (FETs). The results suggest that EL2 fluctuations, rather than dislocation nonuniforrnities, are responsible for the electrical inhomogeneity of FETs implanted in SI GaAs substrates.
Dobrilla, Paolo, "Infrared transmittance study of GaAs: Mapping stress, dislocation, and EL2 distributions" (1986). Scholar Archive. 96.