Probing Device Mechanism
using Operando Scanning Probe

Material properties determine device behavior. However, it is often not very straightforward to correlate the nanometer-scale material properties with macroscopic device behavior. Scanning probe microscopy (SPM) is a microscopy technique that is capable of probing materials' surface properties in μm scale with sub-nm resolution. Numerous variations of SPM techniques can probe electronic, mechanical, and chemical properties.

Kelvin probe force microscopy (KPFM; also called Scanning Kelvin probe microscopy, SKPM) is an SPM that can probe the surface potential variation of the sample surface. KPFM is widely adopted to characterize the electronic properties of semiconductor junctions, due to its fine energetic resolution (~meV). KPFM probes the surface potential by detecting the Coulomb force applied between the surface and the biased scanning probe tip: Coulomb force is minimized once the potentials of the biased probe tip and the sample surface match. Knowing the work function of the probe tip and the applied bias (contact potential difference; CPD), the surface potential of the sample is determined.

Surface potential is determined by two contributing factors: the samples' intrinsic work function, and the bias/field applied to the sample. Two samples with different work functions will have different surface potentials; on the other hand, two identical samples, one of which is applied with bias, will also have different surface potentials. Based on this principle, the operando-KPFM probes the surface potential profiles of a device that is operating under bias. The profiles include information on the work functions of consisting materials (Φ), the Fermi level change due to the field-effect (ΔΦ), and the applied potentials (V). Compared with the reference profile, quantitative potential drop profiles can be achieved (ΔV) at each bias condition. 

Operando KPFM characterization technique was able to reveal the operating principle of solution-processed In2O3 TFT. Doping- and contact-engineered solution-processed In2O3 TFTs performed strong output saturation, which allows for <1V of operating voltage and ≈25 μW of operating power. 

Operando KPFM revealed that the exceptional output saturation is due to the Schottky barrier at the source contact, i.e., the device was operating in a source-gated transistor (SGT) regime. This work will provide a novel approach for low-cost power-efficient TFTs. 

In contrast to the excellent electronic transport performance of single-sheet MoS2 FETs, MoS2 (or any other 2D semiconductor)-based printed TFTs suffer from poor electrostatic gating, low mobility, and/or high off-current. 

This paper, led by Zhehao Zhu, investigates the mechanism of such performance degradation of printed films, via a combination of scanning probe microscopy characterization and resistor network modeling. The charged edges of MoS2 flakes induce an electrostatic gating effect on adjacent flakes, causing performance degradation.