Mohammad Nazari

Self-Heating Profile in an AlGaN/GaN Heterojunction Field-Effect Transistor Studied by Ultraviolet and Visible Micro-Raman Spectroscopy

Direct measurements of self-heating in gallium nitride (GaN) transistor using ultraviolet (UV) and visible micro-Raman spectroscopy are reported. The material stack was grown on silicon substrates and consists of an AlN nucleation, AlGaN transition, GaN buffer, and AlGaN barrier layers. Phonon shifts are used to estimate the temperature rise. UV measurements probe the current-carrying 2-D electron gas (2-DEG) in the GaN near the interface with the barrier region. Visible micro-Raman measurements provide an average temperature rise through GaN, AlN, and substrate near its interface with AlN. Together, these measurements provide a temperature depth profile. Under identical drive conditions, the increase in temperature from UV micro-Raman is approximately twice what is obtained from the visible measurements, reaching as high as 350°C above ambient temperature at input power of 7.8 W/mm. The temperature depth profile is simulated using finite-element analysis. We find that the temperature dependence of the thermal conductivity of GaN is important to incorporate in these simulations due to the large temperature rise in the 2-DEG region. A thermal boundary resistance of 1× 10-8~K·m2 /W is obtained from the combined simulation and experimental results.

Self-Heating in a GaN-Based Heterojunction Field-Effect Transistor Investigated by Ultraviolet and Visible Micro-Raman Spectroscopy

Direct measurements are reported of self-heating in an AlGaN/GaN transistor using ultraviolet and visible micro-Raman. Device stack is comprised of silicon substrate, AlN nucleation, AlGaN transition, GaN buffer, and AlGaN barrier layers. Phonon shifts are used to estimate temperature rise. Ultraviolet measurements probe the current- carrying GaN near the interface. Visible micro- Raman measurements provide an average temperature rise through GaN, AlN, and substrate near its interface with AlN. Under identical drive conditions, temperature rises from UV micro-Raman are approximately twice those from visible measurements. Correlating the data with a thermal simulation provides an estimated thermal boundary resistance 1×10.8 K.m2/W.

Implementations of CVD diamond on gan device layers: Optical characterization of wafer-scale stress uniformity

The use of CVD diamond films for electronic thermal management is quickly being realized as a viable application in high-power and high-temperature electronics, owing to the exceptional thermal properties of diamond and the recent advances in diamond CVD growth technology. In this study, a thick (~100 μm) CVD diamond film was grown on a 75-mm wafer consisting of AlGaN/GaN high electron mobility transistor (HEMT) device layers. By removing the original growth substrate and transition nitride layers, the CVD diamond can be deposited on the back side of the wafer and used as a heat-spreading substrate in close proximity to the active region of the device layers. Ultraviolet (UV) and visible micro-Raman spectroscopy, as well as UV photoluminescence (PL) were used to characterize the stress distributions within the (~1 μm-thick) GaN active layer across the entire wafer. The shallow optical penetration depth of the UV excitation allows for measurements to be accomplished within the first ~100 nm of the material; measuring from both sides of the wafer yields separate stress information for the top and bottom regions of the GaN. Examinations of the stress profile across the wafer reveal an average tensile stress of ~ 0.86 GPa from the top side of the GaN layer (near the AlGaN/GaN interface) and a lower tensile stress of ~0.23 GPa from the bottom side (near the GaN/Diamond interface), resulting in a significant stress gradient along the growth direction of the material which is otherwise unexpected for a uniform layer. Factors influencing the stress and stress gradient will be presented and discussed, supported by results from finite element simulations of thermal growth stresses and electron microscopy of the device layers and interfaces.

Investigation of Stresses in GaN HEMT Layers on a Diamond Substrate Using Micro-Raman Spectroscopy

Visible and ultraviolet (UV) micro-Raman spectroscopies are used to study the stress in GaN integrated with diamond grown by chemical vapor deposition. Mapping of stress is accomplished across a 75-mm GaN-on-diamond wafer. UV measurements from both sides of the wafer reveal an unexpected gradient between the tensile stress from the free GaN surface (~0.86 GPa) and the GaN/Diamond interface (~0.23 GPa). This gradient is understood through non-uniformities in the material along the growth direction of the layers, with relaxation attributed to threading dislocations. Simulations incorporating stress relaxation in the elastic modulus describe the observed dependence. Measurements from TEM support this conclusion.