# Presentation: 453364

## Charge and Current Imaging of GaN DevicesHadis Morkoç and Alison Baski, Virginia Commonwealth University, DMR 0309095

- 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 DevicesHadis Morkoç and Alison Baski, Virginia Commonwealth University, DMR 0309095

**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 μm2 and height variations are: (a) *z* = 60 nm, (b) *V* = 110 mV, (c) *z *= 200 nm  and (d) *V* = 150 mV.

**Notes:**
AIM OF THE PROJECT:  Currently, the high frequency power output of wide bandgap III-nitride devices is lower than expected due to effects such as “current collapse” and associated dispersion effects, in addition to high density of non-equilibrium (hot) phonons. Schottky contacts are pivotal components in a number of semiconductor devices, most notably field effect transistors (FETs). Excess leakage current and premature breakdown under reverse bias are the two main undesirable effects commonly observed in Schottky diodes. They may also be the source, though not the source of the problem, of carriers injected into the surface states which causes current lag also. Our research is in part aimed at determining the source of carriers as well. 
RESEARCH RESULTS:
Scanning Kelvin probe microscopy (SKPM) has been used to detect reverse-bias induced surface potential changes near the Schottky contact of a GaN Schottky diode in conjunction with the role of the surface states on current lag in GaN based FET devices. An n-type GaN sample was grown by metal organic chemical vapor deposition (MOCVD). The sample’s electron concentration as determined from Hall measurements was 3×1017 cm–3. Schottky contacts of 300 μm diameter were formed using Ni/Au (30 nm/ 75 nm) metallization and ohmic contacts using Ti/Al/Ti/Au (30 nm/ 100 nm/ 30 nm/ 15 nm) metallization. The spacing between the Schottky and ohmic contacts was 25 μm, and the diode’s leakage current was measured to be 1 mA at 10 V reverse bias. 
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 suggest that a reverse bias facilitates electron tunneling at the edge of the Schottky contact by decreasing the potential barrier width. Capture of these tunneled electrons by surface states causes the observed increase of band bending. This increase occurs within ~5 s of turning on a reverse bias, and relaxes within ~15 s of turning off the bias. We have also found that an 18 V reverse bias results in an excess band bending of 2.8 eV, which indicates a surface state density of at least 2.9×1012 cm–2. The injected electrons are captured by surface states which in turn cause the drain current lag. However, there are possibly two other mechanisms that may also be responsible for current lag in a FET such as hot electron injection from the channel and carrier trapping in the wide bandgap barrier layer. The latter issues can be addressed by insertion of a large bandgap AlN barrier at the heterointerface and by improving layer quality. 
It should be noted that the relaxation time of ~15 s is much shorter than the 800 s relaxation time observed for an AlGaN/GaN HFET (S. Sabuktagin, S. Dogan, A. A. Baski, and H. Morkoç, Appl. Phys. Lett. 86, 083506, 2005).  This shorter relaxation time is most likely due to the shallower location of surface states in the band gap of GaN as compared to AlGaN. Another notable difference is the roughly four times wider region of increased band bending in the case of the diode compared to the HFET. Apparently, electrons hop/transport more easily along the GaN surface as compared to the AlGaN surface. The band bending has also been attributed to polarization in the literature, as the c-plane of GaN is polar. To further gain insight as to the source of band bending, we have also examined comparatively 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, which is comparable to that also observed for c-plane films. Because polarization effects are not present on a-plane GaN, we attribute such band bending to the presence of charged surface states. We have modeled the surface band bending assuming a localized level of surface states in the band gap on the surface. The band bending is consistent with an acceptor like level at 1.2 eV having a surface state density of 2×1012 cm-2, or with a level at 1.6 eV having a density of only 3×1011 cm-2. Also, because band bending is comparable for both the a-plane and c-plane films, charged surface states must dominate band bending in both cases. The experiments on a-plane GaN give us confidence that the band bending is due to surface states and that current lag is associated with electrons captured, albeit temporarily but long enough to cause phase lag, at surface states. 
SIGNIFICANCE OF THIS WORK  
GaN heterojunction FETs are considered the device for high power amplifiers for communication and radar. In any device, particularly mobile varieties, the efficiency is a crucial parameter. The current lag robs the device from attaining its expected efficiency and power levels. Understanding of the cause(s) of this current lag is essential to its eventual solution. Now that the source of the problem is converging on surface states, appropriate surface passivation methods can be explored to treat the problem with precision as opposed to blanket experimental attempts which also included surface passivation. 

## 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 DevicesHadis Morkoç and Alison Baski, Virginia Commonwealth University, DMR 0309095

- 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.