Photoelastic strain measurement in GaP (100) wafers under external stresses

The sign of the absolute value of residual strain measured in LEC-grown GaP wafers with scanning infrared polariscope (SIRP) has been determined by measuring the residual strain in the round-shape wafer with and without compressive load to the diametric edges. Since the external stress induced by the compressive load is additive to the residual strain in wafer, the residual strain measured in the wafer with the compressive load is increased if the sign is negative while it is decreased if the sign is positive. The measurement of residual strain was successively performed in various LEC-grown GaP (100) wafers clamped their diametric edges in a specially-made vise, in which a movable jaw is pulled by a coil spring to slide parallel toward a fixed jaw with a compressive load. It is found from their results that the residual strain component of (S-r - S-t), where S-r is the radial strain component and S-t is the tangential strain component defined in the cylindrical coordinate system matching to the round-shape wafer, is negative; that is, radially compressive in most wafers.