Semiconductor Polytype Heterostructures: A Pathway to Superior Power Electronics

Overview: In this project, we will develop a new building block for electronics, semiconductor polytype heterostructures, which consists of adjacent layers of lattice-matched materials differing only in their atomic stacking sequences. Semiconductor polytype heterostructures are expected to result in the formation of a polarization-doped 2-dimensional electron gas (2DEG), with both high carrier concentration and high carrier mobility, resulting in ultra-high conductivity; therefore, they are expected to offer a timely solution to overcome a near-decade plateau in transistor speed. Building upon our expertise developed for polar-charge-induced growth phenomena, we will examine the effects of electrostatic phenomena on polytype selection in thin film heterostructures, towards the development of semiconductor polytype heterostructures for superior power electronics.

Intellectual Merit: In this project, we seek new understanding of the mechanisms for ZB vs. WZ polytype selection, in order to examine the electronic states and transport properties of ZB/WZ polytype heterostructures in both ZB-preferring GaAs and WZ-preferring GaN, thereby informing strategies for fabrication of polytype heterostructures for superior power electronics. We will tailor the polytype selection during epitaxy of GaN and GaAs using electrostatic phenomena such as surface charging and surface/layer polarization discontinuities. During epitaxy, we will monitor the interplay between surface reconstruction, polytype, and local electronic states in real-time using in-situ reflection high-energy electron diffraction (RHEED), multi-beam optical stress sensing (MOSS), and scanning-tunneling microscopy (STM). In addition to examining growth kinetics, we will explore the influence of electrostatic phenomena, including both thermal and electron-induced charging, on WZ vs. ZB polytype selection. We will develop a machine-learning approach using convolutional neural networks (CNN) to quantify and classify RHEED patterns, thereby accelerating the process of identifying appropriate growth kinetics and induced surface charging to select WZ vs. ZB polytypes. Following epitaxy, we will examine the interface structure and polarity using a combination of high-resolution and scanning transmission electron microscopy (HREM/STEM), selected-area and convergent-beam electron diffraction (SAED/CBED), and x-ray diffraction. We will also examine the electronic states using scanning tunneling spectroscopy and optical spectroscopic tools based upon the Franz-Keldysh effect. Upon identification of the key growth kinetics and/or electrostatic phenomena to tailor polytype selection, we will prepare and measure the transport properties of ZB/WZ polytype heterostructures for HEMTs. Expected outcomes of this work include the identification of strategies for polytype selection during epitaxy of thin films that prefer the ZB or WZ polytype, as well as the design and fabrication of ZB/WZ polytype HEMT structures that will facilitate the discovery of new strategies for transistors, with the potential for integration of logic and memory beyond Moore's Law.

Broader Impacts: The purpose of this work is to develop the knowledge to controllably synthesize both stable and metastable polytypes, using a combination of epitaxy with intentional electrostatic disturbances, in-situ and ex-situ atomic-scale characterization, and optical spectroscopy of films, heterostructures, and fabricated heterostructure devices. Although we focus on GaAs and GaN as model systems, these results will be widely applicable to systems that have multiple metastable polytypes (such as InN, AlN, and ZnO). The new knowledge generated will significantly broaden our capabilities to produce layers of complex materials composed of multiple polytypes, with properties of interest for various functional devices. For example, the WZ polytype exhibits a spontaneous polarization, which in principle could be useful for separating charge in photodetectors and photovoltaics, while the ZB polytype lacks a spontaneous polarization and in GaN exhibits enhanced p-type doping, both of which facilitate efficient carrier recombination for light-emitters. The polarization discontinuities at polar/non-polar WZ/ZB polytype junctions are expected to lead to the accumulation of free carriers in two-dimensional electron gases. Two-dimensional electron gases are the cornerstone for electronic power switching – an enabling technology for electric vehicles, which is based on high electron mobility transistors (HEMTs) of GaAs and GaN. WZ/ZB polytype junctions are expected to advance this technology by enabling greater electron mobilities. Furthermore, WZ/ZB polytype superlattices are expected to yield unique optoelectronic, electromechanical, and thermoelectric properties, with implications for the telecommunications, radar, and automotive industries.