Nano Particles and Transport
Metal-Semiconductor interfaces play a significant role in all solid state devices. As the demand for more and better functionality increases, an understanding of how electrical contacts scale down in the nanometer regime is crucial for further improvement. When a metal makes contact with a semiconductor, a Schottky barrier (SB) forms, inducing band-bending. At room temperature, the electron transport is mainly governed by Thermionic Emission (TE). With valid I-V curves, this theory can be used to determine the Schottky Barrier heights ΦB at the interface.
This research will focus on the Schottky Barrier formation using Au and Pt nanoparticles on various Nb-doped SrTiO3 (NSTO) substrate.
- Tapping mode
Samples are under AC scan, to obtain topographical images and search for a suitable particle for I-V test.
- Contact mode
In contact mode, I-V curves were obtained on a specific nanoparticle. Tip, sample and sample holder can form a perfect circuit as shown above.
I. Orientation relationship
AFM images imply two orientation relations of Au/STO interface systems: Au(100) //STO(100), Au //STO in a) and Au(111) //STO(100), Au //STO in b). The arrows indicate the  crystal orientation for STO substrates.
II. Interface Transport Properties
The Schottky barrier height ΦB and ideality factor n approach to theoretical values when size increases but exhibit variations for (100) junction. (111) system shows similar trend. (lines to guide the eye)
Ideality factor n is derived from the slope of ln I-V curve. Several factors lead to non-ideal transport at nanoscale interfaces. At dimensions smaller than the depletion width, the Schottky barrier is not well developed and has a smaller 'effective' value. The local electric fields at the edge of the interface lead to 'pinch off' of the barrier. The statistical variation of dopant concentration leads to local depletion width variations. These mechanisms are manifest in ideality factors >1.
Nano-sized Resistive Switching
Hysteretic behavior associated with resistive switching is observed for all interfaces with sizes from 200 nm to 20 nm.
- positive bias: high resistive state (HRS) → low resistive state (LRS)
- negative bias: LRS → HRS
- High cycle stability and reproducibility
Orientation dependence of resistive switching is obvious in the negative bias regime, corresponding to leakage current. Two examples are shown below. The resistance ratio differs by more than two orders of magnitude.
R-V curve derived from I-V data to demonstrate orientation dependence for different Au nanoparticle sizes: 40 nm in a) and 100 nm in b).
Nanoparticle Mechanical Properties
The effects of tip loading force on the contact quality and local current-voltage character between conductive AFM tips and individual noble metal nanoparticle-strontium titanate (NP-STO) interfaces can be demonstrated. The applied force of tip on the nanoparticle can be calculated from tip spring constant, system sensitivity and deflection. Therefore, force can be controlled by varying contact set point. The differences of nanoparticle height can be detected before and after AC scan.
Plots of size variations in Au nanoparticles for a) various diameter nanoparticles under an applied 48 nN load and b) applied tip loading forces ranging from 10-9 10-6 N; c) the current versus applied tip loading force for a Ti/Pt coated AFM tip in direct contact with a 0.02 at% Nb-doped STO substrate.