Nanoscale electronic properties of semiconductor surfaces and interfaces
Several issues involving semiconductor materials have been studied using the techniques of scanning probe microscopy and molecular beam epitaxy.
A method for performing scanning tunneling potentiometry of semiconductor heterojunctions is developed. The method yields a direct measure of the electrostatic potential distribution across the interface, with microscopic resolution. The measurement is accomplished by scanning the probe tip at constant sample-tip separation across the junction, and adjusting the sample-tip voltage to maintain a constant tunnel current. An example is given of potentiometry across a GaAs pn-junction. Work on more complex systems is discussed.
Compositionally abrupt InGaP/GaAs heterojunctions grown by gas-source molecular beam epitaxy have been investigated by cross-sectional scanning tunneling microscopy and spectroscopy. Images inside the InGaP layer show non-uniform In and Ga distributions. About 1.5 nm of transition region at the interfaces is observed, with indium carryover identified at the GaAs-on-InGaP interface. InGaAs-like interfaces are identified. Spatially resolved tunneling spectra with nanometer spacing across the interface were acquired. Possible systematic errors in the measurement due to tip-induced band bending are considered, using 3-dimensional electrostatic potential simulations. Band offsets of 0.13 ± 0.01 eV for the conduction band and 0.35 ± 0.01 eV and for the valence band (type I) are deduced.
We study the effect of introducing hydrogen gas through the RF-plasma source during plasma-assisted molecular-beam epitaxy of GaN(0001). The well-known smooth-to-rough transition that occurs for this surface as a function of decreasing Ga flux in the absence of H is found to persist even with H present, although the critical Ga flux for this transition increases. Under Ga-rich conditions, the presence of hydrogen is found to induce step bunching (facetting) on the surface. Conductive atomic force microscopy reveals that leakage current through dislocation cores is significantly reduced when hydrogen is present during the growth.
Oxidation of Ga-polar GaN surfaces was performed in-situ. Oxidized GaN surfaces (either oxidized at room temperature or elevated temperature) are studied by STM, STS, and separation-dependent tunneling current measurements. For elevated temperature oxidization, oxidized GaN surfaces are found to contain two phases: a relatively smooth phase, showing [special characters omitted] - R30° periodicity, and a relatively rough phase displaying disordered, 2x periodicity. For the GaN surface oxidized at elevated temperature, nearly flat band conditions are revealed by scanning tunneling spectroscopy and the separation dependence of the tunneling current, indicative of a very low density of oxide-derived surface or interfaces states.