Matt D. Brubaker

Effect of AlN buffer layer properties on the morphology and polarity of GaN nanowires grown by molecular beam epitaxy

Low-temperature AlN buffer layers grown via plasma-assisted molecular beam epitaxy on Si (111) were found to significantly affect the subsequent growth morphology of GaN nanowires. The AlN buffer layers exhibited nanowire-like columnar protrusions, with their size, shape, and tilt determined by the AlN V/III flux ratio. GaN nanowires were frequently observed to adopt the structural characteristics of the underlying AlN columns, including the size and the degree of tilt. Piezoresponse force microscopy and polarity-sensitive etching indicate that the AlN films and the protruding columns have a mixed crystallographic polarity. Convergent beam electron diffraction indicates that GaN nanowires are Ga-polar, suggesting that Al-polar columns are nanowire nucleation sites for Ga-polar nanowires. GaN nanowires of low density could be grown on AlN buffers that were predominantly N-polar with isolated Al-polar columns, indicating a high growth rate for Ga-polar nanowires and suppressed growth of N-polar nanowires under typical growth conditions. AlN buffer layers grown under slightly N-rich conditions (V/III flux ratio = 1.0 to 1.3) were found to provide a favorable growth surface for low-density, coalescence-free nanowires.

On-chip optical interconnects made with gallium nitride nanowires.

In this Letter we report on the fabrication, device characteristics, and optical coupling of a two-nanowire device comprising GaN nanowires with light-emitting and photoconductive capabilities. Axial p-n junction GaN nanowires were grown by molecular beam epitaxy, transferred to a non-native substrate, and selectively contacted to form discrete optical source or detector nanowire components. The optical coupling demonstrated for this device may provide new opportunities for integration of optical interconnects between on-chip electrical subsystems.

Atom probe tomography evaporation behavior of C-axis GaN nanowires: Crystallographic, stoichiometric, and detection efficiency aspects

The field evaporation behavior of c-axis GaN nanowires was explored in two different laser-pulsed atom probe tomography (APT) instruments. Transmission electron microscopy imaging before and after atom probe tomography analysis was used to assist in reconstructing the data and assess the observed evaporation behavior. It was found that the ionic species exhibited preferential locations for evaporation related to the underlying crystal structure of the GaN and that the species which evaporated from these locations was dependent on the pulsed laser energy. Additionally, the overall stoichiometry measured by APT was significantly correlated with the energy of the laser pulses. At the lowest laser energies, the apparent composition was nitrogen-rich, while higher laser energies resulted in measurements of predominantly gallium compositions. The percent of ions detected (detection efficiency) for these specimens was found to be considerably below that shown for other materials, even for laser energies which pr...

Toward Discrete Axial p–n Junction Nanowire Light-Emitting Diodes Grown by Plasma-Assisted Molecular Beam Epitaxy

In this paper we investigate axial p–n junction GaN nanowires grown by plasma-assisted molecular beam epitaxy (MBE), with particular attention to the effect of Mg doping on the device characteristics of individual nanowire light-emitting diodes (LEDs). We observe that a significant fraction of single-nanowire LEDs produce measurable band-gap electroluminescence when a thin AlGaN electron blocking layer (EBL) is incorporated into the device structure near the junction. Similar devices with no EBL typically yield below-detection-limit electroluminescence, despite diode-like I–V characteristics and optically measured internal quantum efficiencies (IQEs) of ∼1%. I–V measurements of the p-regions in p–n junction nanowires, as well as nanowires doped with Mg only, indicate low p-type conductivity and asymmetric Schottky-like p-contacts. These observations suggest that imbalanced carrier injection from the junction and p-contact can produce significant nonradiative losses.

Imaging the p-n junction in a gallium nitride nanowire with a scanning microwave microscope

We used a broadband, atomic-force-microscope-based, scanning microwave microscope (SMM) to probe the axial dependence of the charge depletion in a p-n junction within a gallium nitride nanowire (NW). SMM enables the visualization of the p-n junction location without the need to make patterned electrical contacts to the NW. Spatially resolved measurements of S11′, which is the derivative of the RF reflection coefficient S11 with respect to voltage, varied strongly when probing axially along the NW and across the p-n junction. The axial variation in S11′u2009 effectively mapped the asymmetric depletion arising from the doping concentrations on either side of the junction. Furthermore, variation of the probe tip voltage altered the apparent extent of features associated with the p-n junction in S11′ images.

Raman spectroscopy based measurements of carrier concentration in n-type GaN nanowires grown by plasma-assisted molecular beam epitaxy

The carrier concentration in as-grown ensembles of n-type GaN nanowires was determined by Raman spectroscopy of the coupled longitudinal phonon–plasmon (LPP+) mode and modeling of the carrier concentration dependence of the LPP+ frequency. The Raman measurements and analyses enabled estimation of the carrier concentration in single-nanowire devices fabricated from the as-grown ensembles. The nanowires were grown by plasma-assisted molecular beam epitaxy in either of the two growth systems. Twelve samples were examined, of which 11 samples were Si-doped and one was undoped. The Raman-measured carrier concentrations in the Si-doped samples ranged from (5.28u2009±u20091.19) × 1016u2009cm−3 to (6.16u2009±u20090.35)u2009×u20091017u2009cm−3. For a subset of samples grown with varying Si cell temperature, from 1125u2009°C to 1175u2009°C, the carrier concentration was found to be an Arrhenius function of Si cell temperature, with activation energy of 6.281±0.011u2009eV. Co-illumination by an above band gap UV laser (325u2009nm, excitation intensityu2009=u20090.7u2009W/cm2...

Localized phonon-polariton modes in periodic GaN nanowire arrays grown by selective area epitaxy (Conference Presentation)

Localized surface phonon-polariton (SPhP) resonances in polar semiconductor nanostructures can provide highly sub-diffractional electromagnetic fields. Furthermore, SPhP resonances offer enhanced resonant quality factors when compared to plasmon-polariton based systems. The various material platforms and nanostructure geometries achievable in polar semiconductors suggest they would be ideal platforms for tunable, long-wavelength photonics applications. Moreover, the constituent atomic basis defines the operating frequency regime for SPhP resonances; tunable from the mid-infrared to THz. Here, we investigate Raman active aspects of SPhP modes in GaN nanowire arrays that are grown via selective area molecular beam epitaxy. We detect strong Raman peaks within the Reststrahlen band of GaN that are not found in the bulk GaN Raman spectrum. These SPhP modes occur around 700 cm^-1 (~ 14.3 microns), offering a spectral region for device applications which is currently not accessible by plasmonic based systems or other SPhP enabled materials. Utilizing selective area epitaxy, we created GaN nanowire arrays with various diameters and pitches, from which the Raman spectra showed tuning of the apparent SPhP resonances. Infrared reflectance measurements were also performed with an FTIR microscope to further establish the physical properties of the resonances. Finally, computational studies of the structures’ reflectance were used to solidify our understanding of the geometry/SPhP-resonance-tuning relationship.

Core-shell p-i-n GaN nanowire LEDs by N-polar selective area growth

GaN nanowire LEDs with radial p-i-n junctions were grown by molecular beam epitaxy using N-polar selective area growth on Si(111) substrates. The N-polar selective area growth process facilitated the growth of isolated and highaspect-ratio n-type NW cores that were not subject to self-shadowing effects during the subsequent growth of a conformal low-temperature Mg:GaN shell. LED devices were fabricated from single-NW and multiple-NW arrays in their as-grown configuration by contacting the n-type core through an underlying conductive GaN layer and the p-type NW shell via a metallization layer. The NW LEDs exhibited rectifying I-V characteristics with a sharp turn-on voltage near the GaN bandgap and low reverse bias leakage current. Under forward bias, the NW LEDs produced electroluminescence with a peak emission wavelength near 380 nm and exhibited a small spectral blueshift with increasing current injection, both of which are consistent with electron recombination in the p-type shell layer through donor-acceptor-pair recombination. These core-shell NW devices demonstrate N-polar selective area growth as an effective technique for producing on-chip nanoscale light sources.