Control of Ge Homojunction Band Offsets Via Ultrathin Ga-as Dipole Layers

We have created 0.35-0.45 eV band offsets at Ge homojunctions using Ga-As dipole intralayers, with the Ge valence band edge on the As side of the junction at lower energy. This is, to our knowledge, the first time that intralayer control of band discontinuities is extended to homojunctions, thereby expanding the potential domain of band gap engineering. Because these offsets occur over just a few atomic spacings, they rival heterojunction band edge discontinuities in breadth. The offsets were measured with synchrotron-radiation photoemission spectroscopy. Similar band offset magnitudes occur for both "Ga-first" and "As-first" growth sequences, consistent with a truly dipolar effect. Both cleaved Ge(111) and thick almost-equal-to 50 angstrom Ge films deposited on cleaved Ge(111) were used as substrates, obtaining consistent results. The sign and magnitude of the effect is in agreement with a "theoretical alchemy" model. Experimental evidence of microdiffusion is discussed.