Hands-On Physical Science for In-Service Teachers

The initiation of the Virginia Commonwealth University B.S. in Science program was reported in this journal Fall 1999 as a program designed to meet the academic content major of a teacher preparation program for elementary and middle school mathematics and science teachers [1]. This paper reports the current status of the interdisciplinary B.S. in Science degree program including program enrollment data and trends. Also described are refinements in the required curriculum, which include a newly developed geometry, a mathematical computing course, and an emerging teaching technology course featuring graphing calculators, CBLs, and computer software applications

Surface photovoltage in undoped n-type GaN

Steady-state and transient surface photovoltage (SPV) in undoped GaN is studied in vacuum and air ambient at room temperature and 400 K with a Kelvin probe. The results are explained within a phenomenological model accounting for the accumulation of photogenerated holes at the surface, capture of free electrons from the bulk over the near-surface potential barrier, and emission of electrons from surface states into the bulk. Simple analytical expressions are obtained and compared with experimental results. In particular, the proposed model explains the logarithmic decay of the SPV after stopping illumination. Internal and external mechanisms of the SPV are discussed in detail. It is shown that an internal mechanism dominates at low illumination intensity and/or small photon energies, while external mechanisms such as charging of a surface oxide layer and photoinduced processes play a significant role for above-bandgap illumination with sufficient intensity

Structure and Stability of Si(114)-(2x1)

We describe a recently discovered stable planar surface of silicon, Si(114). This high-index surface, oriented 19.5 degrees away from (001) toward (111), undergoes a 2x1 reconstruction. We propose a complete model for the reconstructed surface based on scanning tunneling microscopy images and first-principles total-energy calculations. The structure and stability of Si(114)-(2x1) arises from a balance between surface dangling bond reduction and surface stress relief, and provides a key to understanding the morphology of a family of surfaces oriented between (001) and (114).Comment: REVTeX, 4 pages + 3 figures. A preprint with high-resolution figures is available at http://cst-www.nrl.navy.mil/papers/si114.ps . To be published in Phys. Rev. Let

Photoadsorption and photodesorption for GaN

The effect of an ambient environment on the surface photovoltage and photoluminescence observed for GaN is studied. In air ambient the upward band bending gradually increases under UVillumination and is explained by the photoinduced chemisorption of surface adsorbates. Specifically, the increase in negative surface charge is consistent with the transfer of electrons from surface states or bulk to oxygen species physisorbed at the GaNsurface. In contrast, the upward band bending gradually decreases in vacuum under UVillumination and can be explained by the photoinduced desorption of these species. The photoadsorption and photodesorption of negatively charged species cause the surface depletion region to increase and decrease, respectively. This change in depletion region width is consistent with the observed decrease in photoluminescence intensity in air ambient and its significant increase in vacuum for a sample with low free electron concentration

Surface charging and current collapse in an AlGaN∕GaN heterostructure field effect transistor

This work investigates the correlation between surfacecharging and current collapse in an AlGaN∕GaNheterostructurefield effect transistor.Surfacecharging due to applied biases was sensed by mapping the surface potential between the gate and drain using scanning Kelvin probe microscopy. Due to the bias, the surface band bending near the gate edge was observed to increase by as much as 1 eV. This increase of band bending is caused by an accumulation of excess charge near the surface during the applied bias. By varying the duration of the applied bias, we find that this accumulation of excess charge near the gate takes about 20 s to saturate. Continuous monitoring of the surface potential after switching off the bias shows that a complete relaxation of the excess band bending requires about 800 s. Drain current transient measurements show that the collapse and recovery of the drain current also occur on similar time scales. This correlation between time scales indicates that the accumulation of excess charge near the gate edge causes current collapse by depletion of the channel

Temperature-dependent Kelvin probe measurements of band bending in p-type GaN

The band bending in a Mg-doped, p-type GaN film grown by hydride vapor phase epitaxy was studied at various temperatures. At 295 K, the band bending in dark was calculated to be approximately −1.5 eV. However, when the sample was heated to 600 K for 1 h in dark before performing a measurement at 295 K, the calculated value of band bending in dark became about −2.0 eV. These results are explained by the fact that increasing the sample temperature exponentially increases the rate at which the band bending restores and allows for a more accurate value of band bending to be measured

Surface band bending of a-plane GaN studied by scanning Kelvin probe microscopy

We report the value of surface band bending for undoped, a-plane GaN layers grown on r-plane sapphire by metalorganic vapor phase epitaxy. The surface potential was measured directly by ambient scanning Kelvin probe microscopy. The upward surface band bending of GaN films grown in the [112¯0] direction was found to be 1.1±0.1V. 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. It should be noted that the band bending observed for a-plane layers is comparable to that obtained on polar c-plane layers, and both a-plane and c-plane GaN films with similar surface treatments demonstrate comparable band bending behavior, indicating that charged surface states dominate band banding in both cases

Investigation of inversion domains in GaN by electric-force microscopy

Inversion domains in III-nitride semiconductors degrade the performance of devicesfabricated in them. Consequently, it is imperative that we understand their electrostatic manifestation, the growth conditions under which such domains form, and an effective means of their identification. In what is nominally referred to as Ga-polarity samples, N-polarity domains have a polarization that is reversed with respect to the remainder of the surface, and therefore, have a different potential under strain. We have used surface-potential electric-force microscopy (SP-EFM) to image the electrostaticsurface potential of GaNgrown on sapphire, which is strained due to the thermal mismatch between the substrate and GaN. Employing a control sample with side-by-side Ga- and N-polarity regions, we have established the EFM mode necessary to identify inversion domains on GaN samples grown by molecular-beam epitaxy. This method is not sensitive to topology and has a spatial resolution of under 100 nm. The measured surface potentials for Ga-face and N-face regions are +25±10 and −30±10 mV, respectively, with respect to the sapphire substrate, where the sign is consistent with Ga- and N-polarity GaN under compressive strain due to thermal mismatch with the sapphire substrate

Investigation of forward and reverse current conduction in GaN films by conductive atomic force microscopy

We have used conductive atomic force microscopy (C–AFM) to investigate the forward and reverse bias current conduction of homo- and heteroepitaxial GaN-based films grown by molecular beam epitaxy. In the case of homoepitaxy, C–AFM shows enhanced current conduction at the centers of ∼30% of spiral hillocks, which are associated with screw dislocations. Local current–voltage spectra taken by C–AFM on and off such hillocks indicate Frenkel–Poole and field emission mechanisms, respectively, for low current levels in forward conduction. In the case of heteroepitaxialGaN films grown on sapphire, the correlation between conduction pathways and topography is more complex. We do observe, however, that films with more rectifying nominal Schottky behavior (less reverse leakage current) produce forward and reverse bias C–AFM images with strong asymmetry

Current mapping of GaN films by conductive atomic force microscopy

Conductive atomic force microscopy has been used to investigate the local conductivity in hydride vapor-phase epitaxy and molecular-beam epitaxyGaN films, focusing on the effect of off-axis facet planes. We investigated two different types of samples, in which the facet planes were either present on the perimeters of as-grown islands, or on the edges of etch pits created by post-growth chemical etching. The results show that crystallographic planes tilted with respect to the c-plane growth direction show a significantly higher conductivity than surrounding areas. The n-type (or p-type) samples required a negative (or positive) sample bias for current conduction, consistent with the formation of a Schottky barrier between the metallized atomic force microscope tip and sample. The time dependence of this enhanced conductivity was different for the two types of samples, possibly indicating different conduction mechanisms