Nanoscale visualization of hot carrier generation and transfer at non-noble metal and oxide interface

doi:10.1016/j.jmst.2021.04.064

Abstract

Abstract:

The conversion efficiency of energy-harvesting devices can be increased by utilizing hot-carriers (HCs). However, due to ultrafast carrier-carrier scattering and the lack of carrier injection dynamics, HC-based devices have low efficiencies. In the present work, we report the effective utilization of HCs at the nanoscale and their transfer dynamics from a non-noble metal to a metal oxide interface by means of real-space photocurrent mapping by using local probe techniques and conducting femtosecond transient absorption (TA) measurements. The photocurrent maps obtained under white light unambiguously show that the HCs are injected into the metal oxide layer from the TiN layer, as also confirmed by conductive atomic force microscopy. In addition, the increased photocurrent in the bilayer structure indicates the injection of HCs from both layers due to the broadband absorption efficiency of TiN layer, passivation of the surface states by the top TiN layer, and smaller barrier height of the interfaces. Furthermore, electrostatic force microscopy and Kelvin probe force microscopy provide direct evidence of charge injection from TiN to the MoO_x film at the nanoscale. The TA absorption spectra show a strong photo-bleaching signal over wide spectral range and ultrafast decaying behavior at the picosecond time scale, which indicate efficient electron transfer from TiN to MoO_x. Thus, our simple and effective approach can facilitate HC collection under white light, thereby achieving high conversion efficiency for optoelectronic devices.

Key words: Non-noble metal, Hot carrier, Real-space photocurrent mapping, Local-probe force microscopy

Ranveer Singh, QadeerAkbar Sial, Seung-ik Han, Sanghee Nah, Ji-Yong Park, Hyungtak Seo. Nanoscale visualization of hot carrier generation and transfer at non-noble metal and oxide interface[J]. J. Mater. Sci. Technol., 2022, 98: 151-159.

Figures/Tables 5

Fig. 1. Carrier generation and transport at metal-semiconductor interface. (a) Schematic diagram of the device. (b) Schematic representation of scattering and injection of hot carriers. Hot carriers with sufficient momentum and kinetic energy can cross the Schottky barrier. (c) Schematic of nanoscale electrical measurements conducted using atomic force microscopy..

Fig. 2. Surface morphology and compositional characterization. (a) AFM and (b) planar-view SEM images of TiN/MoOx/TiN thin films show granular nanostructures. The observed nanostructures are marked by blue circles. (c) Cross-sectional SEM image of TiN/MoOx/TiN films grown on Si substrate. (d) EDS spectra of the full device, confirming the presence of titanium (Ti), nitrogen (N), molybdenum (Mo), oxygen (O), and silicon (Si). XPS spectra corresponding to (e) Ti, (f) N, (g) Mo, and (h) O. The inset in (f) shows the XPS spectra of O in the TiN film..

Fig. 3. Local probe EFM and KPFM measurements. EFM maps of MoOx/TiN corresponding to different Vtip: (a) +5.0 V, (b) 0.0 V, and (c) -5.0 V. EFM maps of TiN/MoOx/TiN corresponding to different Vtip: (d) +5.0 V, (e) 0.0 V, and (f) -5.0 V. The bright contrast represents the presence of negative charges at the surface corresponding to negative Vtip. (g) and (h) Phase change as a function of applied bias on Vtip under dark and white light illumination conditions for MoOx/TiN and TiN/MoOx/TiN, respectively. Shifting of the parabola under white light illumination indicates improved charge transfer. (i) Change in surface potential (ΔVCPD) of MoOx/TiN and TiN/MoOx/TiN as a function of light intensity. The scale bar is 400 nm for all EFM phase images.

Fig. 4. Nanoscale electrical current measurements. Current map of the TiN/MoOx/TiN device under 0 bias at the nanoscale (a) under dark and (b) under white light illumination, confirming the generation and injection of charge carriers. (c) Change in current without any bias with increasing intensity of light illumination. The current reaches the initial value after the light is switched off. (d) Local current-voltage measurements of the TiN/MoOx/TiN device.

Fig. 5. Femtosecond TA dynamics. Spectrally resolved TA (ΔT/T) surface maps of (a) TiN, (b) MoOx/TiN, and (c) TiN/ MoOx/TiN as a function of the pump-probe time delay (t). (d) TA spectra of TiN (dark blue), MoOx/TiN (light blue), and TiN/ MoOx/TiN (red) films at t = 0.5 ps as a function of probe wavelength. TA intensity changes of three films as a function of the pump-probe time delay (e) at 650 nm and (f) at 770 nm.

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