Time Domain Thermoreflectance

Time Domain Thermoreflectance (TDTR) is a photothermal technique capable of measuring thermal conductivity of thin films and bulk substrates as well as thermal boundary resistance. This technique employs an ultrafast (pulse duration < 1 ps) laser in a pump (heating) probe (temperature sensing) configuration. The pump pulse is modulated between 1 and 12 MHz to control the thermal penetration depth and to use a lock-in amplifier to extract the signal. The probe pulse is sent through a mechanical stage to delay its arrival between 0.1 to 7 ns compared with the pump pulse to allow the temperature decay of the surface to be measured through this time frame. The temperature decay of the sample surface is related to a radial 2D multi-layer heat conduction model that allows the determination of various thermophysical properties. The temperature is measured through changes in reflectivity of a thin metal film (50-100 nm) deposited on top of the sample. Because the reflectivity of the surface is measured the sample must be smooth enough for a specular reflection. The system is capable of measuring thermal conductivity between 0.1 and 1000 W/m-K and thermal boundary resistance between 2 and 500 m2-K/GW.

Renishaw InVia Micro-Raman/Photoluminescence System

The Renishaw Raman Spectrometer is an invaluable tool in modern day physics. Allowing for analysis of specific vibrational modes of materials, Raman spectroscopy has been widely used for material characterization. In our lab, we primarily look at how phonon modes change in relation to temperature and stress. This allows for unique insight into material properties. Additionally, incorporation of an extended stage allows for probing of electronic devices and in-situ measurements. Raman thermometry is a powerful technique for measuring the temperature of the semiconductor material within an electronic device in-operando.  The system uses an incident monochromatic laser beam to excite specific vibrational modes within the material.  The activation of these modes results in backscattered radiation where the photon energy is slightly shifted relative to the incident energies.  These shifts can be correlated to a change in the temperature of the material, providing point-based thermometry of the semiconductor material within the device.  Scanning Raman thermometry can also be performed via the integration of a moving sample stage to generate line-scans and 2D maps of device temperatures in-operando.

Horiba Jobin Yvon HR800 Micro-Raman/Photoluminescence System

The Horiba HR800 Spectrometer is equiped with a 325nm He-Cd laser capable of probing wide band gap materials such as Gallium Nitride and Silicon Carbide. We are able to use both photoluminesence and and Raman spectroscopy to monitor changes in a materials band gap and phonon modes as a result of changes in temperature and stress. An additional 532nm laser excitation is incorporated which allows for further material characterization.

Nanosecond Transient Themoreflectance
Nanosecond transient thermoreflectance is an important technique for measuring the thermal transport properties across deeply buried interfaces in semiconductor material system, such as those often encountered for epitaxially-grown films deposited on bulk substrates.  The thicknesses of these epi-films can often reach the 2 – 10 μm range or even thicker, depending on the eventual application.  In situations where the epi-layer itself presents a large thermal resistance due to factors such as dislocations or other crystal defects, degenerate-doping levels of impurity atoms or exotic nanostructuring in the form of superlatticies, the thermal interface resistance where the epi-layer meets the bulk crystal can be difficult, if not impossible to measure with TDTR.  The nanosecond transient system uses nanosecond-width pulses and much longer time-delay windows to interrogate transport processes at longer diffusion times than the 6 ns window offered by TDTR.  This system fills an important niche regarding the measurement of these buried interfaces which can often exhibit significant thermal resistances.

Glove Box

Keithley 4200A-SCS Parameter Analyzer

Thermal Storage Benchtop Test Facility 

Transient Thermoreflectance Imaging

Transient thermoreflectance imaging is a full-field imaging technique, which takes advantage of the temperature dependent reflectivity of light on various metal and semiconductor surfaces to measure transient temperatures in devices under pulsed operation.  The system provides 2D thermal images with micron spatial resolution and can be used to provide temporal resolution of transient surface temperatures down to 10 ns under pulsed operation of electrical devices.

Transient Electroluminescence Imaging
Transient electroluminescence imaging is an extremely powerful technique for mapping the location and relative intensity of electron-trap and electron-defect/impurity interactions within the channel region of actual device structures in-operando.  Electroluminescence arises from the radiative recombination of electron-hole pairs following their excitation when a device is subjected to high electric fields.  Under such large fields, the electrons are accelerated to energies where upon encountering a defect such as a trap state or an impurity atom, photoemission may result as a consequence of carrier recombination at the defect site.  Through the use of an extremely sensitive imaging system and pulsed-bias operation, the transient evolution of the electroluminescence signal emanating from the device can be captured, providing valuable information about carrier transport/scattering within the device while under applied bias.

CVD Growth Furnaces (graphene and carbon nanotubes)



George Woodruff School of Mechanical Engineering