Spectroscopic Characterization of Complex, Nanometer-scale, Semiconductor Heterostructures

Multi-layer, multi-semiconductor band gap engineered electronic devices made a revolution in modern electronics. High electron mobility transistors (HEMT), heterojunction bipolar transistors (HBT), and high brightness light emitting diodes (LED), are typically made of a sequence of nanometer-scale III-V semiconductor layers. These nanoscale heterostructures pose a challenge to silicon era characterization techniques that were developed for single material devices. As a result, band gap engineers developing such devices, rely mostly on simulations. In this work, we present a new spectroscopic tool capable of measuring built-in electric fields, band-gap energies, and band offsets, in band gap engineered devices. All these are measured in all the layers simultaneously. To demonstrate the technique, we chose a typical AlGaN/GaN HEMT. However, it can be used on any other heterostructure device. The method is based on sub-band gap electro-optical absorption due to the Franz-Keldysh effect. In addition to the measured parameters, simple mathematical model allows us to extract charge carrier concentration and electron mobility in the two-dimensional electron gas (2-DEG) and construct a band diagram of the device. Varying the gate voltage allows us to study the device not only at equilibrium but also over the entire range of its operating modes. The method presented in this study provides a reliable tool for band gap engineering of nanometer scale, multi-layer devices.