Synergistic Effects of Radiation and Stress Localization on Wide-Bandgap Semiconductor Devices
Open Access
- Author:
- Rasel, Md Abu
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 04, 2024
- Committee Members:
- Robert Kunz, Professor in Charge/Director of Graduate Studies
Sukwon Choi, Major Field Member
Jing Du, Major Field Member
Md Haque, Chair & Dissertation Advisor
Douglas Wolfe, Dissertation Co-Advisor
Mia Jin, Outside Unit & Field Member - Keywords:
- GaN HEMT
Heavy Ion Irradiation
Proton Irradiation
Gamma Irradiation
In situ Irradiation
In situ Heating
TEM
Electron Wind Force Annealing - Abstract:
- The importance of identifying radiation effects and radiation sensitive regions within electronic devices is ever-increasing as many crucial electronic devices operate in damaging radiation environments, such as space, aviation, defense, medicine, and nuclear power. Radiation-induced upsets and damage pose a significant threat to performance and reliability, ranging from temporary loss of data to the complete and permanent loss of functionality. The importance of radiation effects on wide bandgap semiconductor devices (such as Ga2O3 Schottky diode, GaN High Electron Mobility Transistor (HEMT) is growing rapidly due to their touted suitability for harsh radiation environments. Radiation degrades their electrical, thermal, and mechanical properties, resulting in lower output and larger stress and temperature gradients across the conducting channel. This research is motivated by the observation that radiation effects literature has only focused on the electrical domain. A new perspective is presented, where we argue that nanoscale confined mechanical stress fields act as pre-existing conditions – thereby significantly influencing radiation damage. Such stress confinements unavoidably develop in microelectronic devices due to material mismatches, structural design, and fabrication processing aspects. Their global average is insignificant; hence, they are typically ignored since the literature focuses on strain engineering (to increase carrier mobility). This dissertation is dedicated to the role of stress confinement and its effects on radiation damage. A model specimen is the AlGaN/GaN HEMT, where a large electric field appears under the gate edge across the barrier, adding to the already existing substantial intrinsic tensile strain at the interface due to lattice mismatch. The current trend is to study radiation effects with electrical characterization (as a function of radiation damage) followed by post-mortem microscopy. A major shortcoming of this trend is that it assumes the entire device (e.g., a transistor) or material exhibits uniform sensitivity to radiation. In contrast, we propose a hypothesis that radiation sensitivity is a function of the pre-existing defects, defects induced from stressors during irradiation, and/or mechanical residual stress. For example, the gate region in a transistor experiences larger mechanical stress in the OFF condition and larger electrical stress during the ON condition compared to the source and drain regions – hence, it should be more vulnerable to radiation. Similarly, interfaces in transistors are the regions to closely observe for nucleation of radiation damage. The deleterious nature of the combinatorial effect of superimposed environmental and operational stressors on devices also needs immediate focus and attention. The literature has not adequately addressed some fundamental questions from a mechanical point of view: How do the physical parameters, such as thermal and mechanical stress, progress after irradiation? Is the thermally/mechanically strained hotspot most sensitive to irradiation? Where do defects nucleate, and failure initiates during the ON state of the device? Can we mitigate defects in the post-irradiated device in an efficient way? The objectives of this dissertation are to conduct a detailed investigation of the above-mentioned issues by exploiting the in-situ/operando philosophy where electron transparent transistors and/or diodes will be operated to develop micro to atomic-scale insights on radiation sensitivity of wide bandgap GaN HEMT devices. This project aims to introduce the concept of highly localized stress (‘mechanical hotspot’) in radiation effects in electronics literature and elucidate the role of device-specific structure, materials heterogeneity and interfaces, and process-induced residual stress on radiation vulnerability. Our quantitative and qualitative can effectively indicate the correlation of the mechanical hotspots with higher post-radiation stress or defect generation. The insights of this study can be helpful in identifying radiation-sensitive areas in electronic devices, an area that has been historically reliant only on the effects of electrical fields. Finally, a novel approach will be introduced to mitigate irradiation-induced defects by generating electron wind force in the device to recover electrical and crystal quality in an energy-efficient way. We subjected the GaN HEMT devices to irradiation using various ion species, including heavy ions like gold (Au4+) and light ions such as protons (H+). Additionally, we exposed the devices to electromagnetic radiation, such as gamma rays, to assess their vulnerability in diverse irradiation environments. The level of damage observed in various irradiation scenarios is contingent upon the conditions of the devices, including whether they were in an ON/OFF/Pinched OFF state or subjected to accelerated prestressed conditions or external heating. Degradation patterns can exhibit a complete reversal depending on the ion species involved. For instance, with 2.8 MeV gold irradiation, ON devices may display greater radiation tolerance compared to OFF modes. Conversely, with 300 KV proton irradiation, ON mode devices may degrade more rapidly than those in the OFF condition, according to our findings. Prestressed devices experience more degradation than ON and OFF mode devices in the case of Gamma irradiation. Devices subjected to external heating and Pinched-off conditions during 2.8 MeV Au3+ show less/no degradation, whereas OFF devices degrade proportionally as the fluence increases. Following in situ biasing measurements, each device underwent additional analysis using in situ micro-Raman spectroscopy to examine the evolution of thermoelastic stress and channel temperature subsequent to irradiation. Devices after irradiation exhibit notable thermoelastic stress alongside a moderate increase in channel temperature. For defect characterization, we employed HRTEM (High-Resolution Transmission Electron Microscopy) images of irradiated samples and utilized them for strain mapping to discern dislocations. We present significant evidence regarding the interactions between radiation species and GaN HEMT devices under various operating conditions. The scientific contribution of the research is the experimental validation of the hypothesis that localized mechanical stress/strain (referred to as 'mechanical hotspots') when combined with environmental and operational stressors, can expedite radiation degradation in electronic devices. Growing diverse engineering applications now require electronic devices tested for reliability in a myriad of operating conditions. However, the current approach to designing experiments to meet these needs is based on applying a single stressor or post-analysis after the stressor is applied. The combinatorial effect of superimposed environmental and operational stressors, together with in situ philosophy, can significantly capture real situations and provide fundamental insight into the synergistic effect of various crucial physical phenomena. We expect that this dissertation will introduce a fresh perspective to the field of localized mechanical stress/strain, offering insights into how it contributes to radiation degradation in electronic devices. The core concept that localized stress enhances radiation vulnerability can be extended to non-electronic materials for far-reaching impact. As we unfold some crucial yet overlooked research questions, our unique experimental method and evidence allow us to elucidate the local, underlying physical mechanisms of radiation vulnerability that are not captured or obvious from the global response of devices.