Hangfeng Ji

Integrated micro-Raman/infrared thermography probe for monitoring of self-heating in AlGaN/GaN transistor structures

Self-heating in AlGaN/GaN device structures was probed using integrated micro-Raman/Infrared (IR) thermography. IR imaging provided large-area-overview temperature maps of powered devices. Micro-Raman spectroscopy was used to obtain high-spatial-resolution temperature profiles over the active area of the devices. Depth scans were performed to obtain temperature in the heat-sinking SiC substrate. Limitations in temperature and spatial resolution, and relative advantages of both techniques are discussed. Results are compared to three-dimensional finite-difference simulations

Thermal Boundary Resistance Between GaN and Substrate in AlGaN/GaN Electronic Devices

The influence of a thermal boundary resistance (TBR) on temperature distribution in ungated AlGaN/GaN field-effect devices was investigated using 3-D micro-Raman thermography. The temperature distribution in operating AlGaN/GaN devices on SiC, sapphire, and Si substrates was used to determine values for the TBR by comparing experimental results to finite-difference thermal simulations. While the measured TBR of about 3.3 x 10^-8 W^-1 ldr m² ldr K for devices on SiC and Si substrates has a sizeable effect on the self-heating in devices, the TBR of up to 1.2 x 10^-8 W^-1 ldr m² ldr K plays an insignificant role in devices on sapphire substrates due to the low thermal conductivity of the substrate. The determined effective TBR was found to increase with temperature at the GaN/SiC interface from 3.3 x 10^-8 W^-1 ldr m² ldr K at 150degC to 6.5 x 3.3 x 10^-8 W^-1 ldr m² ldr K at 275degC, respectively. The contribution of a low-thermal-conductivity GaN layer at the GaN/substrate interface toward the effective TBR in devices and its temperature dependence are also discussed.

Piezoelectric strain in AlGaN/GaN heterostructure field-effect transistors under bias

Micro-Raman spectroscopy was used to study piezoelectric strain in AlGaN∕GaN heterostructure field-effect transistors under bias. The measurements were made through the transparent SiC substrate. Strain in the GaN layer varied over the device area and was dependent on bias voltage, and affected, in particular, the gate-drain gap and area underneath the drain contact. The observed strain in GaN was shown to be related to the electric field component normal to the surface. Finite element simulations of electric field distribution show good qualitative agreement with the experimental data. Effects of strain on Raman temperature measurements in transistors are also discussed.

Improved Thermal Performance of AlGaN/GaN HEMTs by an Optimized Flip-Chip Design

AlGaN/GaN high electron mobility transistors (HEMT) on sapphire substrates have been studied for their potential application in RF power applications; however, the low thermal conductivity of the sapphire substrate is a major drawback. Aiming at RF system-in-a-package, the authors propose a flip-chip-integration approach, where the generated heat is dissipated to an AlN carrier substrate. Different flip-chip-bump designs are compared, using thermal simulations, electrical measurements, micro-Raman spectroscopy, and infrared thermography. The authors show that a novel bump design, where bumps are placed directly onto both source and drain ohmic contacts, improves the thermal performance of the HEMT

Vibrational and optical properties of GaN nanowires synthesized by Ni-assisted catalytic growth

GaN nanowires synthesized by Ni-assisted catalytic vapour?liquid?solid growth at different temperatures were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro-Raman spectroscopy, and photoluminescence spectroscopy. The nanowires exhibit low defect density. The growth direction of the nanowires is []. For nanowires grown at 800??C, Raman scattering is consistent with the presence of point defects and subsequently yellow luminescence dominating their photoluminescence properties. A low free carrier concentration of less than 1017?cm?3 is present in the nanowires. In contrast, for nanowires grown at 900??C, strong phonon?plasmon coupling was evidenced, suggesting a free carrier concentration in excess of the mid-1018?cm?3 region. Photoluminescence spectra show strong near-band-edge luminescence and negligible yellow luminescence. A Ni-related luminescence peak was observed at 3.436?eV at 80?K. Raman and photoluminescence results obtained from individual nanowires demonstrate that the nanowire crystalline quality improves not only with increasing growth temperature, but also along the nanowire growth direction.

Integrated Raman - IR Thermography on AlGaN/GaN Transistors

We report on the integrated Raman-IR thermography analysis of AlGaN/GaN heterostructure field effect transistors (HFETs). IR thermography provides fast temperature overviews while micro-Raman enables accurate peak temperature determination with high spatial resolution in selected device areas. Excellent agreement between Raman determined device temperatures and finite difference simulations was achieved in the active device region. IR provided temperature determination over large device areas, and complemented the Raman thermal data. In IR thermography, lateral averaging of temperature in the small active device region resulted in an underestimation of device peak temperature and an overestimation of temperature profile linewidth. Dependent on device layout, depth averaging of device temperature occurs in IR thermography. The developed novel approach opens unique possibilities for the thermal screening of MMICs and reliability optimization

Three-dimensional thermal analysis of a flip-chip mounted AlGaN/GaN HFET using confocal micro-Raman spectroscopy

The authors demonstrate the potential of confocal micro-Raman spectroscopy to enable three-dimensional (3-D) thermal analysis of solid state devices. This is illustrated on a flip-chip mounted AlGaN/GaN heterostructure field-effect transistor. To better understand its heat dissipation and for device optimization, it is desirable to know temperature distribution not only in the active device area, but also in the bulk substrate. This cannot be achieved using traditional thermal imaging techniques. 3-D thermal imaging was demonstrated by probing the temperature dependent Raman shift of phonons at different depths within the bulk substrate using confocal micro-Raman spectroscopy. The heatsinking through the metal bumps connecting the active device area to the flip-chip carrier is illustrated. Experimental temperature results are in reasonably good agreement with 3-D finite difference simulations

Converse piezoelectric strain in undoped and Fe-doped AlGaN/GaN heterostructure field effect transistors studied by Raman scattering

Converse piezoelectric strain in undoped and Fe-doped AlGaN/GaN heterostructure field effect transistors (HFETs), i.e. the strain induced by applying bias to a transistor, was studied using micro-Raman scattering spectroscopy as a function of applied source–drain voltage for different GaN buffer doping levels and substrate types. By monitoring the phonon frequency shifts and line width of the E2 and A1(LO) phonon modes of GaN, a considerable piezoelectric strain/stress was found in undoped devices, which exhibited a saturation above 40 V bias. This saturation voltage was used to quantify the deep acceptor concentration in the GaN buffer layer. Using experimental Raman data and numerical modelling of the electric field distribution in the device, it was furthermore established that Fe doping causes confinement of the strain/stress to the vicinity of the AlGaN/GaN interface, i.e. near the electron channel, with potential implications for device reliability. It was concluded that varying the structure and doping in the buffer layer has the potential to modify the converse piezoelectric strain and hence affect reliability issues in AlGaN/GaN HFETs.

Thermal Properties and Reliability of GaN Microelectronics: Sub-Micron Spatial and Nanosecond Time Resolution Thermography

We review our latest developments in the field of Raman thermography and its application to GaN microelectronics. Device self-heating, the temperature rise in a device generated by electrical power dissipation, plays an important role for device performance and reliability, however, is difficult to assess as it occurs on sub-micrometer length scales in most devices, not observable using traditional thermography techniques. The new technique of Raman thermography enables to gain unprecedented insight into device self-heating with sub-micron spatial and with nanosecond time resolution. Thermal resistance of GaN electronic devices on different substrates and with different layouts are compared, interface thermal resistance between the GaN and the substrate was determined. Temperature measurements in the device plane and three dimensionally from the device into the substrate are discussed. Temperature in devices operated in pulsed mode as function of time, dependent on duty cycle and pulse length was studied. A comparison to temperature measurements performed using electrical methods illustrates that care must be taken when identifying junction temperatures using electrical methods.