INTERNALLY DETECTED ELECTRON PHOTOEXCITATION SPECTROSCOPY ON HETEROSTRUCTURES

We applied for the first time the internal photoemission technique (IPE) to the direct observation of deep levels together with barrier heights and band discontinuities at different semiconductor interfaces. Its performances and capabilities are superior to those of established techniques: the energy resolution is excellent and it can be applied to "real" device interfaces under bias. The technique was successfully tested for the Pd/n-GaAs Schottky barrier and for the a-Ge/n-GaAs heterojunction. We found a Schottky barrier height of 0.78 eV and an a-Ge/n-GaAs valence band discontinuity of 0.42 eV; in both cases the Fermi level was "pinned" at an average value of 0.76 +/- 0.03 eV above the top of the GaAs valence band. Complementary optical absorption and photoconductivity spectra revealed clean-GaAs features at 1.05 and 1.27 eV. A third feature at 1.09 eV, due to Pd-induced empty states approximately 1.1 eV above the valence band, is present only for the heterostructures. The Schottky barrier also exhibits a feature at 1.36 eV, explained by transitions from the valence band to a localized state, made possible by the band bending.