Probing dynamic behavior of electric fields and band diagrams in complex semiconductor heterostructures

TL;DR

This paper introduces an experimental method using optical photocurrent spectroscopy to evaluate bandgap, band offsets, and electric fields in complex multi-layer semiconductor heterostructures, enabling detailed band diagram construction under various conditions.

Contribution

The method allows simultaneous, experimental characterization of all layers in complex heterostructures, bridging the gap between simulation and real device conditions.

Findings

Successfully applied to GaN HEMT structures

Constructed band diagrams under different electric fields

Determined charge density and mobility variations

Abstract

Modern bandgap engineered electronic devices are typically made of multi-semiconductor multi-layer heterostructures that pose a major challenge to silicon-era characterization methods. As a result, contemporary bandgap engineering relies mostly on simulated band structures that are hardly ever verified experimentally. Here, we present a method that experimentally evaluates bandgap, band offsets, and electric fields, in complex multi-semiconductor layered structures and it does so simultaneously in all the layers. The method uses a modest optical photocurrent spectroscopy setup at ambient conditions. The results are analyzed using a simple model for electro-absorption. As an example, we apply the method to a typical GaN high electron mobility transistor structure. Measurements under various external electric fields allow us to experimentally construct band diagrams, not only at equilibrium, but also under any other working conditions of the device. The electric fields are then used to obtain the charge carrier density and mobility in the quantum well as a function of the gate voltage over the entire range of operating conditions of the device. The principles exemplified here may serve as guidelines for the development of methods for simultaneous characterization of all the layers in complex, multi-semiconductor structures.