Characterization of electrically active defects at III-N/dielectric interfaces

1.3 Goal of this thesis

In this thesis we address the problem of defects at the AlGaN/dielectric interface. We illustrate here the contents, giving an overview of its structure and a short summary of each chapter.

The present Chapter 1 introduces the subject, gives the motivation for the work, an overview on the state–of–the–art of the research in this field and summarizes the fundamental facts that will be used in the following chapters.

We present some experimental details in Chapter 2, including a description of the typical sample preparation and the setup used for the measurements. Two more detailed sections regard the introduction of a lock–in amplifier for fast impedance measurements, and the integration of the electrical probe station system with an optical setup for opto–electrical characterization.

In Chapter 3 we discuss the role of the AlGaN barrier in GaN/AlGaN MIS–HEMTs and the challenges that it represents in impedance measurements. We develop an alternative methodology for interface trap characterization based on stress and recovery transients, proposing two techniques which focus on the device response during stress (the on–the–fly technique) and during recovery (the measurement–stress–measurement technique).

Chapter 4 summarizes the theoretical considerations which allow to extract the interesting defect properties from the experimental data. We focus especially on the concept of activation energy, and we perform some simulations to explore the effect of different physical mechanisms on the final, measurable .

In Chapter 5 we apply the electrical characterization methods presented in Chapter 3 and we analyze the data according to the theory introduced in Chapter 4. A physical interpretation of the trapping mechanisms in standard GaN/AlGaN HEMTs is discussed, in comparison with that of fluorinated devices with modified native surface donor states.

Finally, in Chapter 6 we present the results obtained with the optical setup. We discuss the challenges of this characterization method applied to GaN/AlGaN MIS–HEMTs, we give an overview of the basic theoretical considerations in order to analyze the data, and we perform DLOS measurements to study the trap levels with very large activation energy present in our devices.