As Moore’s Law ends, the search for approaches enabling faster computing systems and diversification of semiconductor device applications has significantly intensified. In our group we work on challenges through the lens of two questions:
- Can materials and devices be grown at low-temperatures and non-epitaxial substrates to enable functional diversification of CMOS platforms?
- Can hot-electrons be used efficiently in electronic and optoelectronic devices?
In the first part of the talk, we discuss our work on growing III-V semiconductors on non-epitaxial substrates using a combination of templated liquid phase growth (TLP) and metal-organic chemical vapor deposition (MOCVD). Specifically, we will discuss the fundamentals of the TLP growth technique, which enables crystalline III-V mesas to be grown directly on oxide and metal substrates. This is achieved by creating micro-scale metal templates (e.g. indium), and then carrying out a phase transformation to change each template into a single crystalline III-V (e.g. InP). After discussing the basic growth processes and resulting material quality, we highlight some recent results with TLP growth at CMOS back-end compatible temperatures of <400 oC. Due to the low temperature used here, these material growth techniques are directly compatible with back-end CMOS integration. Uniquely, it is shown that InAs grown directly on oxides at 300 oC exhibits mobilities of ~5800 cm2/V-s. We then discuss electronic devices fabricated from TLP based materials, specifically III-V on oxide MOSFETs and synaptic devices.
Next, we show how hot-electron processes can dramatically reduce the optical power densities required for photoemission. Specifically, we show that a waveguide integrated graphene electron emitter excited with 3.06 eV photons from a continuous wave (CW) laser exhibits two hot-electron processes that drive photoemission at peak powers >5 orders of magnitude lower than previously reported multi-photon and strong-field metallic photoemitters. Optical power dependent studies combined with modeling illustrate that the observed behavior can be explained by considering direct emission of excited electrons. Theoretical and simulation exploration of the fundamental limits of this process highlights that a new class of ultra-fast electron emitters could be enabled using the hot-electron emission process in conjunction with integrated photonics.
For the second hot-electron device, we highlight that the onset of electrochemical reactions on a graphene or metal surface can be modified with a silicon-insulator-metal type structure, where injection of hot-electrons from silicon into metal or graphene modify the hydrogen evolution reaction (HER) rate on the metal/graphene surface. Uniquely, hot-electrons injected from silicon into a thin gold layers demonstrates quantum efficiencies of >50% when driving the hydrogen evolution reaction. As a model system, the hydrogen evolution reaction on graphene is shown to be modified n-Si/Al2O3/graphene electrochemical device, and a p-Si/Al2O3/graphene photoelectrochemical device. Uniquely, it is shown that the onset potential of the electrochemical reaction, normally considered a material property, can be modified through purely electronic approaches.