Andrei Sarua

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

Measurement of temperature distribution in multifinger AlGaN/GaN heterostructure field-effect transistors using micro-Raman spectroscopy

The temperature distribution in multifinger high-power AlGaN/GaN heterostructure field-effect transistors grown on SiC substrates was studied. Micro-Raman spectroscopy was used to measure channel temperature with 1 μm spatial resolution, not possible using infrared techniques. Thermal resistance values were determined for four different device layouts with varying number of fingers, finger width, and spacing. The experimental thermal resistance was in fair agreement to that predicted by three-dimensional finite difference heat dissipation simulations. Uncertainties in thermal properties of this device system made simulation less reliable than experiment.

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.

Self-heating effects at high pump currents in deep ultraviolet light-emitting diodes at 324 nm

We present a detailed high-pump-current study of self-heating effects in ultraviolet light-emitting diodes (LEDs) grown on sapphire. For deep ultraviolet LEDs on sapphire, our results establish self-heating to be a primary cause of premature power saturation under dc pumping. Even the flip-chip packaged devices undergo a steady-state temperature rise to about 70 °C at a dc pump current of only 50 mA (at 8 V) resulting in a significant decrease in LED output. Temperature rise values estimated from peak emission wavelength shifts and from micro-Raman mapping of the active devices were in good agreement.

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.

Time-Resolved Temperature Measurement of AlGaN/GaN Electronic Devices Using Micro-Raman Spectroscopy

We report on the development of time-resolved Raman thermography to measure transient temperatures in semiconductor devices with submicrometer spatial resolution. This new technique is illustrated for AlGaN/GaN HFETs and ungated devices grown on SiC and sapphire substrates. A temporal resolution of 200 ns is demonstrated. Temperature changes rapidly within sub-200 ns after switching the devices on or off, followed by a slower change in device temperature with a time constant of ~10 and ~140 mus for AlGaN/GaN devices grown on SiC and sapphire substrates, respectively. Heat diffusion into the device substrate is also demonstrated

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

Deformation potentials of the E2(high) phonon mode of AlN

AlN layers grown on (111)-oriented silicon substrates were studied by Raman spectroscopy. The deformation potentials of the nonpolar E2(high) phonon mode of hexagonal AlN were derived from phonon frequency shifts under biaxial stress applied to the layer. Stress was applied by mechanical bending of the wafer with resulting in-plane biaxial stress in AlN. The technique allows one to avoid the uncertainty of x-ray diffraction strain determination inherent to experimental methods commonly used for deformation potentials determination in III–V nitrides. The obtained values for the phonon deformation potentials are in reasonably good agreement with previous theoretical calculations. For pure biaxial stress, we determine a phonon frequency shift of 3 cm−1/GPa.

Bioengineered magnetic crystals

In this paper we report on the successful application of a protein crystallization technique to fabricate a three-dimensionally ordered array of magnetic nanoparticles, i.e. a novel type of metamaterial with unique magnetic properties. We utilize ferritin protein cages for the template-constrained growth of superparamagnetic nanoparticles of magnetite/maghemite Fe3O4-γ-Fe2O3 (magnetoferritin), followed by thorough nanoparticle bioprocessing and purification, and finally by protein crystallization. Protein crystallization is driven by the natural response of proteins to the supersaturation of the electrolyte, which leads to spontaneous nucleation and 3D crystal growth. Within a short period of time (hours to days) we were able to grow functional crystals on the meso-scale, with sizes of the order of tens, up to a few hundred micrometres. We present initial magnetic and Raman spectroscopy characterization results for the obtained 3D arrays of magnetic nanoparticles.

Raman spectral investigation of thiourea complexes

Spectroscopic properties of metal complexes of thiourea single crystals (tris thiourea zinc acetate, bis thiourea cadmium zinc acetate and bis thiourea ammonium chloride) which are non-linear optic materials were investigated by Raman scattering spectroscopy. The vibrational frequencies of the functional groups are identified and assigned. Effects due to the coordination of thiourea with metal ions are analyzed. Hydrogen bonding interactions involved in the metal complexes are observed in the Raman spectra.