Measurement of induced surface charges, contact potentials, and surface states in GaN by electric force microscopy

We have studied molecular beam epitaxy grown GaN films of both polarities using electric force microscopy to detect sub 1 µm regions of charge density variations associated with GaN extended defects. The large piezoelectric coefficients of GaN together with strain introduced by crystalline imperfections produce variations in piezoelectrically induced electric fields around these defects. The consequent spatial rearrangement of charges can be detected by electrostatic force microscopy and was found to be on the order of the characteristic Debye length for GaN at our dopant concentration. The electric force microscope signal was also found to be a linear function of the contact potential between the metal coating on the tip and GaN. Electrostatic analysis yielded a surface state density of 9.4 ± 0.5 × 10^10 cm – 2 at an energy of 30 mV above the valence band indicating that the GaN surface is unpinned in this case

Electric force microscopy of induced charges and surface potentials in GaN modified by light and strain

We have studied molecular beam epitaxy grown GaN films using electric force microscopy to detect sub-1 µm regions of electric field gradient and surface potential variations associated with GaN extended defects. The large piezoelectric coefficients of GaN together with strain introduced by crystalline imperfections produce variation in piezoelectrically induced electric fields around these defects. The consequent spatial rearrangement of charges can be detected by electrostatic force microscopy, and can be additionally modified by externally applied strain and illumination. The electron force microscopy signal was found to be a function of the applied tip bias, showed reversal under externally applied strain, and was sensitive to above band gap illumination

Effect of Buffer Layer and III/V Ratio on the Surface Morphology of GaN Grown by MBE

The surface morphology of GaN is observed by atomic force microscopy for growth on GaN and AlN buffer layers and as a function of III/V flux ratio. Films are grown on sapphire substrates by molecular beam epitaxy using a radio frequency nitrogen plasma source. Growth using GaN buffer layers leads to N-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Ga-rich as possible, although Ga droplets tend to form. Ga-polar films can be grown on AlN buffer layers, with the surface morphology determined by the conditions of buffer layer deposition as well as the III/V ratio for growth of the GaN layer. Near-stoichiometric buffer layer growth conditions appear to support the flattest surfaces in this case. Three defect types are typically observed in GaN films on AlN buffers, including large and small pits and "loop" defects. It is possible to produce surfaces free from large pit defects by growing thicker films under more Ga-rich conditions. In such cases the surface roughness can be reduced to less than 1 nm RMS

Electron diffusion length and lifetime in p-type GaN

We report on electron beam induced current and current–voltage (I–V) measurements on Schottky diodes on p-type doped GaN layers grown by metal organic chemical vapor deposition. A Schottky barrier height of 0.9 eV was measured for the Ti/Au Schottky contact from the I–V data. A minority carrier diffusion length for electrons of (0.2 ± 0.05) µm was measured for the first time in GaN. This diffusion length corresponds to an electron lifetime of approximately 0.1 ns. We attempted to correlate the measured electron diffusion length and lifetime with several possible recombination mechanisms in GaN and establish connection with electronic and structural properties of GaN

Mechanisms of reduced maternal and fetal lopinavir exposure in a rat model of gestational diabetes

Abstract word count: 243 Introduction word count: 79

Patient Care in High-Level Containment Care Units: In a Resourced Setting

Vasa, A., Boulter, K., Horihan, Cates, D., Piquette, C., Sullivan, J., Johnson, D, & Hewlett, A. (2019). Patient Care in High-Level Containment Care Units. In T. Cieslak, M. Kortepeter, C. Kratochvil, & J. Lawler (Eds.), Nebraska Isolation and Quarantine Manual (pp. 87-101). Lincoln, NE: University of Nebraska Press.https://digitalcommons.unmc.edu/nm_books/1000/thumbnail.jp