Blanchard, Paul T.
Bertness, Kris A.
Brubaker, Matthew D.
Harvey, Todd E.
Sanders, Aric W.
Sanford, Norman A.
We demonstrate the effect that the different morphologies of molecular beam epitaxy-grown GaN nanowires (NWs) can have upon current-voltage (I-V) behavior. Two aspects of NW morphology were investigated. The first aspect was the NW diameter, d(NW). For single-crystal Si-doped GaN NW devices with d(NW) < 120 nm, I-V curves were nonlinear. In contrast, single-crystal Si-doped NWs from the same growth run with d(NW) > 120 nm consistently showed ohmic I-V behavior. This discrepancy is likely the result of the comparatively larger surface depletion in thin NWs, which contributes to 1) an increased contact barrier, and 2) a barrier resulting from an axial band offset between the portion of the NW directly beneath the contact and the portion extending from the contact. The second aspect of NW morphology that we investigated was NW coalescence, which occurs when neighboring NWs fuse together during growth. I-V measurements of undoped coalesced NWs showed that these structures can have a free carrier concentration that is significantly higher than the background carrier concentration that is present in single-crystal (noncoalesced), undoped NWs.
Herrero, Andrew M.
Blanchard, Paul T.
Bertness, Kris A.
Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO2 interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices. This procedure determines the degree of adhesion of the contact films to the substrate and NWs and allows for the characterization of the morphology and composition of the contact interface with the substrate and nanowires. This technique is also useful for assessing the amount of residual contamination that remains from the NW suspension and from photolithographic processes on the NW-SiO2 surface prior to metal deposition. The detailed steps of this procedure are presented for the removal of annealed Ni/Au contacts to Mg-doped GaN NWs on a SiO2 substrate.
Blanchard, Paul T.
Bertness, Kris A.
Harvey, Todd E.
Sanders, Aric W.
Sanford, Norman A.
George, Steven M.
Seghete, Dragos
We report novel metal-oxide-semiconductor field effect transistors (MOSFETs) based on individual gallium nitride (GaN) nanowires with fully conformal cylindrical gates. The W/Al2O3 gates were deposited by atomic layer deposition. Reversebias breakdown voltages exceeded the largest gate voltage tested (-35 V). The nanowire MOSFETs showed complete pinchoff, with threshold voltages between -4 and -12V. Maximum transcon-ductances exceeded 10 mu S, and ON/OFF current ratios higher than 10(8) were measured. Significant gating hysteresis and memory effects were also present, indicative of charge traps. Although further optimization is needed, these results represent a promising step forward in the development of efficient GaN nanowire-based FETs.
Blanchard, Paul T.
Bertness, Kris A.
Harvey, Todd E.
Sanders, Aric W.
Sanford, Norman A.
George, Steven M.
Seghete, Dragos
We report novel metal-oxide-semiconductor field effect transistors (MOSFETs) based on individual gallium nitride (GaN) nanowires with fully conformal cylindrical gates. The W/Al 2O 3 gates were deposited by atomic layer deposition. Reverse-bias breakdown voltages exceeded the largest gate voltage tested (-35 V). The nanowire MOSFETs showed complete pinchoff, with threshold voltages between -4 and -12 V. Maximum transconductances exceeded 10 muS, and ON/OFF current ratios higher than 10 8 were measured. Significant gating hysteresis and memory effects were also present, indicative of charge traps. Although further optimization is needed, these results represent a promising step forward in the development of efficient GaN nanowire-based FETs.
Blanchard, Paul T.
Bertness, Kris A.
Harvey, Todd E.
Mansfield, Lorelle M.
Sanders, Aric W.
Sanford, Norman A.
In this paper, we demonstrate novel MESFETs based on individual GaN nanowires. The Pt/Au Schottky gates exhibited excellent two-terminal Schottky diode rectification behavior. The average effective Schottky barrier height was 0.87 eV, with an average ideality factor of 1.6. In addition, the Schottky gates efficiently modulated the conduction of the nanowires. The threshold gate voltages required for complete pinch off were as small as -2.6 V, and transconductances exceeded 1.4 mu S. Subthreshold swings approaching 60 mV/decade and ON/OFF current ratios of up to 5 x 10(8) were achieved. These results show that the Schottky gate has the potential to significantly improve the performance of GaN nanowire field-effect devices.
Sahu, Abhishek
Aaen, Peter H.
Lewandowski, Arkadiusz
Shkunov, Maxim
Rigas, Grigorios
Blanchard, Paul T.
Wallis, Thomas Mitch
Devabhaktuni, Vijay K.
In this paper, we propose a robust algorithm to automate a microwave characterization process, which simultaneously extracts the electrical parameters (conductivity and dielectric constant) of inkjet-printed components on flexible substrates. Initially, the algorithm extracts the propagation constant of printed coplanar waveguide (CPW) standards by the use of a multiline thru-reflect-line calibration. Then, the proposed algorithm utilizes finite-element modeling to dynamically create an interpolated search space by automatic simulation. The algorithm utilizes a least-square optimization routine to minimize the deviation between the model and the measurements. Our technique significantly reduces the computing resources and is able to extract the material parameters using even a nominal ink profile. Characteristic impedances for CPWs are extracted using series resistor measurements from 10 MHz to 20 GHz. It is shown that the proposed characterization methodology is able to detect small changes in material properties induced by changes in fabrication parameters, such as sintering temperature. Ink conductivities of approximately 2.973 x 10(7) S/m and a supporting spacer dielectric constant of 1.78 were obtained for the inkjet-printed CPWs on polyethylene terephthalate. In addition, the inkjet-printed CPWs sintered at 170 degrees C and 220 degrees C on Kapton had conductivities of 0.187 x 10(7) and 0.201 x 10(7) S/m, respectively. We demonstrate the advantages of our technique by measuring the material parameters with the conventional approach.
Wallis, T. Mitch
Gu, Dazhen
Imtiaz, Atif
Smith, Christopher S.
Chiang, Chin-Jen
Kabos, Pavel
Blanchard, Paul T.
Sanford, Norman A.
Bertness, Kris A.
The electrical response of two-port photoconductive GaN nanowire devices was measured from 50 MHz to 33 GHz. The admittance of the nanowire devices showed an increase on the order of 10% throughout the measured frequency range after exposure to steady ultraviolet illumination. Two different two-port microwave network models were used to extract microwave circuit parameters in the photoconductive and dark states. After illumination, the GaN nanowire devices showed a measurable increase in shunt capacitance and decreases in both the contact and nanowire resistances.
Chiang, Chin-Jen
Wallis, T. Mitch
Gu, Dazhen
Imtiaz, Atif
Kabos, Pavel
Blanchard, Paul T.
Bertness, Kristine A.
Sanford, Norman A.
Kim, Kichul
Filipovic, Dejan
A two-port GaN nanowire (NW) device with one Schottky contact and one Ohmic contact was characterized up to 10 GHz using on-wafer microwave measurements. In addition to the measurement of the broadband response, two additional applications of microwave measurements are introduced: (1) the capability to distinguish a Schottky-type contact from an Ohmic contact based on the reflected broadband signals (S(11) and S(22)) and (2) the measurement of a capacitance voltage (CV) curve for a Schottky contact to a bundle of a few NWs. The junction capacitance of the Schottky contact is determined at various bias voltages by fitting the broadband response with a microwave circuit model. The carrier concentration is estimated from the resulting CV curve to be 5.3 X 10(18)/cm(3) and the Schottky barrier height is estimated to be 0.89 eV. (c) 2010 American Institute of Physics. [doi:10.1063/1.3428391]
Sprenger, Jaclyn K.
Sun, Huaxing
Cavanagh, Andrew S.
Roshko, Alexana
Blanchard, Paul T.
George, Steven M.
Electron-enhanced atomic layer deposition (EE-ALD) was used to deposit boron nitride (BN) thin films at room temperature and 100 degrees C using sequential exposures of borazine (B3N3H6) and electrons. Electron-stimulated desorption (ESD) of hydrogen surface species and the corresponding creation of reactive dangling bonds are believed to facilitate borazine adsorption and reduce the temperature required for BN film deposition. In situ ellipsometry measurements showed that the BN film thickness increased linearly versus the number of EE-ALD cycles at room temperature. Maximum growth rates of similar to 3.2 angstrom/cycle were measured at electron energies of 80-160 eV. BN film growth was self-limiting versus borazine and electron exposures, as expected for an ALD process. The calculated average hydrogen ESD cross section was sigma =3D 4.2 X 10(-17) cm(2). Ex situ spectroscopic ellipsometry measurements across the similar to 1 cm(2) area of the BN film defined by the electron beam displayed good uniformity in thickness. Ex situ X-ray photoelectron spectroscopy and in situ Auger spectroscopy revealed high purity, slightly boron-rich BN films with C and O impurity levels <3 at. %. High-resolution transmission electron microscopy (HR-TEM) imaging revealed polycrystalline hexagonal and turbostratic BN with the basal planes approximately parallel to the substrate surface. Ex situ grazing incidence X-ray diffraction measurements observed peaks consistent with hexagonal BN with domain sizes of 1-2 nm. The BN EE-ALD growth rate of similar to 3.2 angstrom/cycle is close to the distance of 3.3 angstrom between BN planes in hexagonal BN. The growth rate and HR-TEM images suggest that approximately one monolayer of BN is deposited for every BN EE-ALD cycle. TEM and scanning TEM/electron energy loss spectroscopy measurements of BN EE-ALD on trenched wafers also showed preferential BN EE-ALD on the horizontal surfaces. This selective deposition on the horizontal surfaces suggests that EE-ALD may enable bottom-up filling of vias and trenches.
Brubaker, Matt D.
Blanchard, Paul T.
Schlager, John B.
Sanders, Aric W.
Herrero, Andrew M.
Roshko, Alexana
Duff, Shannon M.
Harvey, Todd E.
Bright, Victor M.
Sanford, Norman A.
Bertness, Kris A.
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 similar to 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.
Roshko, Alexana
Brubaker, Matt D.
Blanchard, Paul T.
Bertness, Kris A.
Harvey, Todd E.
Geiss, Roy H.
Levin, Igor
A comparison of two electron microscopy techniques used to determine the polarity of GaN nanowires is presented. The techniques are convergent beam electron diffraction (CBED) in TEM mode and annular bright field (ABF) imaging in aberration corrected STEM mode. Both measurements were made at nominally the same locations on a variety of GaN nanowires. In all cases the two techniques gave the same polarity result. An important aspect of the study was the calibration of the CBED pattern rotation relative to the TEM image. Three different microscopes were used for CBED measurements. For all three instruments there was a substantial rotation of the diffraction pattern (120 or 180 degrees) relative to the image, which, if unaccounted for, would have resulted in incorrect polarity determination. The study also shows that structural defects such as inversion domains can be readily identified by ABF imaging, but may escape identification by CBED. The relative advantages of the two techniques are discussed.
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