FIG 1: SKPM surface potential traces next to a Schottky contact with an 8 V reverse bias of duration (b) 0.5 s, (c) 1 s, (d) 2 s, and (e) >5 s. Curve (a) is the steady state profile with no bias. Our objective is to improve GaN-based devices by investigating deleterious effects such as surface charging. We use scanning probe microscopy techniques such as scanning Kelvin probe microscopy (SKPM) that detect surface charge at the nanometer-scale. First, we examine surface band bending in a Schottky contact on GaN (see Fig. 1). After application of a reverse bias, the surface potential near the Schottky contact decreases, indicating an increase of band bending. Both a higher reverse bias and longer bias duration result in a larger increase of band bending. We believe this effect arises from electron tunneling at the edge of the reverse biased Schottky contact. Capture of these tunneled electrons by surface states causes the observed increase of band bending. To delineate any role played by polarization on surface band bending, we have examined both polar c-plane and non-polar a- plane GaN films. SKPM measures an upward surface band bending of 1.1 ± 0.1 V for a-plane films (see Fig. 2), which is comparable to that also observed for c-plane films. Because polarization effects are not present on a-plane GaN, we must attribute such band bending to the presence of charged surface states. Also, because band bending is comparable for both the a-plane and c-plane films, charged surface states must dominate the band bending in both cases, and thus current lag in FETs. Now that the effect of polarization is eliminated, our next step is to investigate the source and nature of these surface states and explore ways to eliminate and/or passivate them effectively. Charge and Current Imaging of GaN Devices Hadis Morkoç and Alison Baski, Virginia Commonwealth University, DMR FIG 2: (a,c) AFM topography images and (b,d) surface potential images of two different a-plane GaN films. Image sizes are 5 × 2.5 μm 2 and height variations are: (a)  z = 60 nm, (b)  V = 110 mV, (c)  z = 200 nm and (d)  V = 150 mV.

Education This grant supports one component of a comprehensive research program that involves the growth, characterization, and device fabrication of III-nitride films. This component concerns local surface potential and current imaging of GaN films and has involved the efforts of three graduate students (Sergei Chevtchenko, Shahriar Sabuktagin, Katherine Cooper), and three undergraduates (Lindsay Hussey, Kabongo Ngandu, Matt Sievert). Both Ms. Hussey and Mr. Ngandu have decided to pursue graduate degrees at VCU as a result of their research experiences. Outreach The undergraduates in this grant help Dr. Baski teach physical science lessons each summer. This summer we taught electrical circuits to 200 students participating in the National Youth Sports Program (NYSP). These lessons were held in the engineering building and have been very popular with the NYSP students. Dr. Baski teaches one of the NYSP classes about series and parallel circuits. She “pretends” to be a battery with her arms acting as wires connecting to two pink lightbulbs. Charge and Current Imaging of GaN Devices Hadis Morkoç and Alison Baski, Virginia Commonwealth University, DMR Two NYSP students build a series circuit and explore what happens when a lightbulb is unscrewed. Lindsay Hussey shows two students how to use a voltmeter. An NYSP student makes a lightbulb work with a battery.