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J. Phys. Chem. C 2009, 113, 9480–9485
Surface-Induced Transients in Gallium Nitride Nanowires B. S. Simpkins* Chemistry DiVision, NaVal Research Laboratory, Washington, D.C. 20375
M. A. Mastro and C. R. Eddy, Jr. Electronics Science and Technology, NaVal Research Laboratory, Washington, D.C. 20375
P. E. Pehrsson Chemistry DiVision, NaVal Research Laboratory, Washington, D.C. 20375 ReceiVed: February 6, 2009; ReVised Manuscript ReceiVed: April 20, 2009
Traps responsible for gate-induced current transients and rate-dependent transport properties in gallium nitride nanowire field-effect transistors have been investigated. Our results suggest these traps are acceptor states located ∼0.85 eV above the valence band and are at or near the nanowire surface. Deposition of a 40 nm SiNx passivation layer reduces trap density by a factor of ∼4 resulting in a lower limit estimate of surface state density of 1.2 × 1011 cm-2/eV. Introduction Semiconducting nanowires (NWs) present a significant scientific and technological promise due to their potential to reduce optical and electronic device length scales,1 as well as providing a testbed for examining surface2,3 and quantum effects.4,5 Elemental and compound semiconducting crystals of extremely high aspect ratio (∼1000) have been produced and explored for electronic6 and optical7,8 device applications as well as for sensing9 in biomedical technologies. A heightened impact of surfaces and interfaces on material properties is inherent in high aspect ratio semiconducting crystals. Indeed, NW conductivity modulation has been observed due to just a single surfacebound virus9 and the extension of the evanescent optical field outside of the NW due to its small size has been exploited for optical interrogation of surface-bound molecules.10 The extreme sensitivity of NW opto-electronic properties to its surface and the lack of a complete understanding, however, can become a detriment to device performance and slow the technological impact of these materials. It has been shown that the enhanced influence of surface depletion modifies photoconductivity,11 results in diameter-dependent free carrier mobility,12,13 and can introduce measurement artifacts in basic parameter extraction in NW-based field effect transistors (NW-FETs).2 This last effect has been observed for InAs NWs14 and Ge NWs15 and in both cases is attributed to interfacial charge rearrangement due to an applied gate bias. For this work, such an effect will be referred to as a gate-induced transient. Gallium nitride (GaN), an appealing material for many opto-electronic applications, possesses surface state densities of 1011-1013 cm-2 and therefore may be susceptible to gate-induced transients.16,17 Charge movement into or out of interface states can be observed through modulation of device conductivity due to resulting bias screening. Promotion of charge from trap states under monochromatic illumination reveals subgap trap state energies. Therefore, monitoring of gate-induced current transients as a function of * To whom correspondence should be addressed. Naval Research Laboratory 4555 Overlook Ave. SW, Washington, DC 20375. Phone: 202404-1901. Fax 202-767-3321. E-mail:
[email protected].
10.1021/jp901122k
monochromatic illumination will be combined with photocurrent decay measurements to examine the impact of interfacial charging on transport measurements of GaN NW-FETs, characterize the states responsible, and assess the effectiveness of SiNx as a passivation layer for such states. SiNx passivation has been applied to planar GaN18 and AlGaN19 and is believed to reduce surface state density. Methods GaN NWs were produced in a vertical impinging-flow MOCVD system.20 A 50-Torr, N2/H2 mixed atmosphere was used during the ramp to growth temperature. Trimethylgallium (TMG) was flowed for 2 s prior to the onset of NH3 flow to prevent nitridation of the Ni seed. The NWs were grown on a nickel nitrate coated Si-(111) wafer in a H2 ambient at a temperature of 725 °C, a pressure of 50 Torr, and a V/III (NH3/ TMG) ratio of 50. These NWs have a hexagonal crystal structure and typically grow along either the a- or m-direction. The resulting triangular cross section is bounded by the GaN basal plane and two prismatic planes.21 NW-FETs were prepared by sonicating the as-grown samples in isopropyl alcohol to release the NWs, then drop drying the suspension of wires onto a 500 nm SiO2/n+ Si substrate.22 Electron beam lithography was used to deposit source and drain electrodes of Ti/Al/Ni/Au (20/100/ 40/50 nm) over the ends of the NW. The doped wafer served as a global back gate and was contacted by depositing Al through a window etched through the top oxide. All measurements were carried out under flowing N2 in a glovebag with
