Can photoemission measure valence-band discontinuities?

Abstract

The progress in understanding heterojunction band lineups has increasingly become dependent upon the availability of accurate and reliable measurements. The reported values of the band offset for the Si/GaAs system measured by photoemission range from 0.05 to 0.70 eV. Such a large scatter in experimentally determined discontinuities is common for other systems as well and not only makes the evaluation of different theories difficult, but raises the question as to whether such variations are real and representative of technological variations in the offset. Clearly the understanding and control of these variations is essential before any device applications can be realized. We attempted to study the source of these variations in the Si/GaAs(110) heterojunction. We altered the growth conditions to effect modifications in the overlayer crystalline order, strain, and chemical reactivity. While some of these studies have been previously reported, their collective consideration gives us a much more coherent overview of the problems involved in discontinuity measurements. In our studies we found the offset to be 0.23±0.05 eV for both amorphous and crystalline overlayers, either strained or unstrained. We did, however, observe discontinuities as large as 0.7 eV for room temperature growths. We show that such large discontinuities are due to dangling bond type states which extend into the band gap and change the apparent valence band maximum energy. These states can be removed by annealing the sample without crystallization. Offsets as small as 0.05 eV may be inferred from our data if we fail to account for the contributions of rather subtle chemical effects at the interface. Overall, we found the discontinuity to be more dependent on the interpretation of the data than on technological variations. If we discern and properly account for extrinsic effects and unambiguously define the discontinuity, photoemission can yield accurate and single valued measurements of the valence band discontinuity.