A DIRECT METHOD TO PRODUCE AND MEASURE COMPOSITIONAL GRADING IN ALXGA1-XAS ALLOYS

We present a method to calibrate the profile of Al mole fraction versus depth, deposited in growth of graded Al(x)Ga(1-x)As alloys in a molecular-beam epitaxy machine. A computer is used to either ramp the Al oven temperature (analog alloy), or pulse the Al beam (digital alloy), with a fractional monolayer depth resolution that permits averaged alloy profiles corresponding to a range of different design profiles to be obtained. The profiles are measured in calibration runs by using a fast picoammeter to track the ion-collector current of the beam flux monitor ion gauge (facing the ovens), and integrating the ion current with time. Parabolic quantum wells are grown by either technique and the corresponding measured profiles are compared to each other and to the design profile. The ability of the digital-alloy technique to obtain almost arbitrarily varying graded-alloy profiles is illustrated.