David W. Greve

Large grain polycrystalline silicon by low-temperature annealing of low-pressure chemical vapor deposited amorphous silicon films

The crystallization of undoped amorphous silicon films deposited by low‐pressure chemical vapor deposition in the temperature range 580–530 °C and annealed from 550 to 950 °C has been studied by transmission electron microscopy. The average grain size of the crystallized films depends on the annealing temperature and the deposition conditions. The nucleation rate of new grains during annealing decreases as the deposition temperature decreases from 580 to 545 °C and/or when the deposition rate increases. The final grain size is also influenced by the annealing temperature with the largest grain size obtained at low annealing temperatures. A simple model is described which explains the dependence of grain size on the annealing temperature. An average grain size of 500 nm has been obtained in a 200‐nm film deposited at 545 °C and annealed at 550 °C.

Determination of wurtzite GaN lattice polarity based on surface reconstruction

We identify two categories of reconstructions occurring on wurtzite GaN surfaces, the first associated with the N face, (0001), and the second associated with the Ga face, (0001). Not only do these two categories of reconstructions have completely different symmetries, but they also have different temperature dependence. It is thus demonstrated that surface reconstructions can be used to identify lattice polarity. Confirmation of the polarity assignment is provided by polarity-selective wet chemical etching of these surfaces.

Reconstructions of GaN(0001) and (0001̄) surfaces: Ga-rich metallic structures

Reconstructions of GaN(0001) and (0001) surfaces are studied by scanning tunneling microscopy and spectroscopy, by electron diffraction, by Auger electron spectroscopy, and using first-principles theory. Attention is focused on Ga-rich reconstructions for each surface, which are found to have a metallic character involving significant overlap between Ga valence electrons. The electron counting rule is thus violated for these surfaces, but they nonetheless form minimum energy structures.

Inversion of wurtzite GaN(0001) by exposure to magnesium

Magnesium incorporation during the molecular-beam epitaxy growth of wurtzite GaN is found to invert the Ga-polar (0001) face to the N-polar face. The polarity is identified based on the two different sets of reconstructions seen on the film prior to and after about 1 monolayer Mg exposure. The inversion boundary is seen to lie on the (0001) plane from transmission electron microscopy images, and a structural model is presented for the inversion. On the Ga-polar face, Mg is also seen to stabilize growth in the N-rich regime.

Single-chip computers with microelectromechanical systems-based magnetic memory (invited)

This article describes an approach for implementing a complete computer system (CPU, RAM, I/O, and nonvolatile mass memory) on a single integrated-circuit substrate (a chip)—hence, the name “single-chip computer.” The approach presented combines advances in the field of microelectromechanical systems (MEMS) and micromagnetics with traditional low-cost very-large-scale integrated circuit style parallel lithographic manufacturing. The primary barrier to the creation of a computer on a chip is the incorporation of a high-capacity [many gigabytes (GB)] re-writable nonvolatile memory (in today’s terminology, a disk drive) into an integrated circuit (IC) manufacturing process. This article presents the following design example: a MEMS-based magnetic memory that can store over 2 GB of data in 2 cm2 of die area and whose fabrication is compatible with a standard IC manufacturing process.

Deep centers and their spatial distribution in undoped GaN films grown by organometallic vapor phase epitaxy

Deep traps in undoped n-GaN layers grown by organometallic vapor phase epitaxy on sapphire substrates were studied by temperature dependent conductivity, photoinduced current transient spectroscopy (PICTS), thermally stimulated current, electron beam induced current (EBIC), and band edge cathodoluminescence (CL) methods. Presence of electron traps with energy levels 0.1–0.2 eV below the conduction band and hole traps with energy levels of about 0.25, 0.5, and 0.85 eV above the valence band edge was detected. CL and EBIC measurements show that the deep recombination centers in GaN are distributed inhomogeneously with well defined cellular pattern. Both carrier lifetime and luminescence intensity are enhanced at cell walls indicating lower density of recombination centers. However, the density of main hole trap (0.85 eV) is enhanced in these regions as determined by local PICTS measurements. Photoconductivity in many GaN samples exhibits very long decay times at temperatures between 100 and 300 K. The effec...

Simulation of microelectrode impedance changes due to cell growth

Observation of impedance changes caused by cell growth on microelectrodes provides information about cell coverage and other important quantities, such as the cell-electrode gap. In this paper, we use finite-element simulations to calculate the impedance changes caused by cell growth on electrodes comparable in size to the cell. Parameters describing the impedance of the electrode without cells are derived from measurement. The results show that the impedance magnitude is expected to increase by at least 100% for partial cell coverage. We also report the frequency dependence of the magnitude and phase of the impedance and compare the simulation results with a lumped-element model. The simulation results are important for the design and modeling of arrays of cell-sized electrodes and also for the interpretation of experiments.

GaN(0001) surface structures studied using scanning tunneling microscopy and first-principles total energy calculations

Abstract Surface reconstructions occurring on the (0001) surface of wurtzite GaN are studied using scanning tunneling microscopy, electron diffraction, and Auger electron spectroscopy. The family of reconstructions found on this face includes 2×2, 5×5, 6×4, and ‘1×1’, in order of increasing surface Ga/N ratio. Detailed experimental results are presented for each of these reconstructions. First-principles total energy calculations are employed to identify possible model structures. An adatom model, with N-adatoms occupying H3 sites, is proposed for the 2×2 reconstruction. A model composed of N adatoms, Ga adatoms, and Ga vacancies is proposed for the 5×5 reconstruction.

On the origin of electrically active defects in AlGaN alloys grown by organometallic vapor phase epitaxy

Shallow and deep centers were studied by means of temperature dependent Hall effect and photoluminescence (PL) measurements in two sets of undoped n‐AlGaN samples grown by organometallic vapor phase epitaxy. The samples of these two series were grown under different conditions and had, as a result, electron concentrations differing by several orders of magnitude. The composition dependence of ionization energies of dominant donors in these two sets of samples is very different indicating that different types of centers are involved, but in both cases they are most probably related to some native defects. These defects behave as hydrogen‐like donors for low Al compositions and become increasingly deeper with increasing Al content. The shallow‐deep transition occurs at about x=0.2 in the low conductivity AlxGa1−xN series and at about x=0.5 for the high conductivity series. Several PL bands were detected in AlGaN and it is shown that the band at 3.05 eV is due to a radiative transition between deep donors in...

Properties of Si donors and persistent photoconductivity in AlGaN

The behavior of Si donors was studied in AlxGa1−xN films with composition 0<x<0.6. It is shown that the Si donors ionization energy increases from 18 meV for 0<x<0.1 to about 50 meV for x=0.4 and does not exceed 90 meV for x=0.6. Increase in Al composition is also accompanied by the growth of the density of defects with energy levels deeper than Si. Combined action of the two above effects leads to increased difficulty in n-type doping of AlGaN films with higher Al mole fractions. Persistent photoconductivity (PPC) is shown to be a characteristic feature of AlGaN samples of all compositions. It is shown that PPC in AlGaN is related to the presence of centers with a relatively high (0.1–0.2 eV) barrier for capture of electrons. In GaN and AlGaN with x<0.1 such centers are not associated with Si donors per se.