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Tuesday, 24 November 2015 15:38

Localized Charge Transfer Process

Localized Charge Transfer Process and Surface Band

Bending in Methane Sensing by GaN Nanowires

Avinash Patsha, Prasana Kumar Sahoo, Sankarakumar Amirthapandian, Arun K.

Prasad, Arindam Das, Ashok Kumar Tyagi, Monica Alonso Cotta, and Sandip Dhara

J. Phys. Chem. C,27 Aug 2015

Unique optical, electrical and mechanical properties of semiconductor nanowires make them very attractive as building blocks for novel LEDs, transistors, chemical and biological sensors. Recently was found that GaN nanowires are sensitive to methane CH4 gas and can be used for fabrication of the advanced gas sensor. The detection of potential greenhouse gas methane CH4   is extremely demanding in environmental safety.

Study of the sensing mechanism in the GaN nanowires is very important for improvement of the gas detectors performances. Recent studies were dedicated to the global characterization of the sensing processes in nanowires ensemble based gas detectors. The underlying physiochemical mechanism of gas detector operation is absorption of the gaseous molecules and subsequent transfer and transport of charge on semiconductor surface. However, detectors based on the nanostructures show more complex behavior due to the high surface- to-volume ratio.  Thermally stable and chemically inert semiconducting surfaces such as GaN and AlGaN utilize the Schottky junction with metal or heterojunctions with other semiconducting surfaces as active sites in charge transfer and thus in sensing process. However, the sensitive properties of these semiconductors nanowires in III-nitrides group are strongly influenced by the present of intrinsic or extrinsic defects such as oxygen impurities.

Most studies of the GaN nanowire based detectors characterize them globally.  This paper for the first time reports about the study of the localized charge transfer and thus sensing mechanism in single GaN nanowire. Role of the surface defects formed by the oxygen impurities in the single nanowire was also investigated.  This localized characterization of the single nanowire was possible due to Nanonics Multiprobe Scanning Microscope. This multiprobe SPM enables to use two independent cantilevered probes (up to four probes) for different SPM methods. Study of the localized charge transfer in the single nanowire in result of gaseous molecular adsorption requires application of two probes: one for local gas delivery and the second one to probe the contact potential difference (CPD).  During the sensing process, the chemisorbed molecules on the semiconductor surface induce bending of the energy band near the semiconductor surface, which cause changes in CPD. This surface band bending (SBB) can be studied as a function of CPD distribution, which is probed by scanning Kelvin probe microscopy. SBB is also strongly influenced by intrinsic and extrinsic defects, surface states, adsorbed gas or chemical molecules, temperature.

Experimental setup: Four GaN nanowires samples with different oxygen concentrations: 105 ppm, 103 ppm, 102 ppm and <2pmm were synthesized. Presence of N and O in the samples was identified with EELS method.

 Multiprobe system with cantilevered fountain nano-pipette (opening diameter 200 nm) for localized gas delivery and conductive probe for Kelvin probe microscopy were used. The probes were moved close to each other and approached to the single nanowire with AFM feedback. Thus GaN single nanowire was exposed locally to the methane while the CPD and topography were acquired with conductive Kelvin probe.   CPD was studied as a function of the nanowire diameters and oxygen impurity concentration.

The CPD showed a deceasing in the SBB value with an increasing in diameter. The increasing of the oxygen concentration in the nanowire causes the decreasing of the SBB value as well.

The observed variations in SBB value, depletion width, and surface charge density during nanowires exposed to methane confirmed the occurrence of gas adsorption and change transfer process in GaN nanowires. A localized charge transfer process, involving defect complex in nanowires is attributed in controlling the global sensing behavior of the oxygen rich GaN nanowire ensemble.

Wednesday, 11 November 2015 15:34

Nanonics at MRS 2015 Fall Meeting & Exhibit

Tuesday, 27 October 2015 01:14

October 2015 newsletter

Monday, 19 October 2015 16:24


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