Thermoreflectance Imaging of Superlattice Micro Refrigerators

High resolution thermal images of operating micro refrigerators are presented. Using the thermoreflectance method and a high dynamic range PIN array camera, thermal images with 50mK thermal resolution and high spatial resolution are presented. This general method can be applied to any operating semiconductors, and can be used as a tool for identifying fabrication failures. With further optimization of the experimental setup, we expect to achieve sub-micron spatial resolution thermal images. Introduction For various applications in optoelectronic or high power electronic devices, it is useful to control the temperature on a microscopic scale. For example, semiconductor lasers used in wavelength division multiplexed fiber optics communication systems require less than a degree centigrade variation in their operating temperature in order to have stable wavelength and output power. The traditional thermoelectric effect that can provide cooling at the interface between two materials can be enhanced using thermionic emission in superlattice barriers [1,2]. By integrating these heterostructure integrated thermionic(HIT) micro coolers with lasers, and other optoelectronic devices, we can have active temperature control on a small scale thus improving the reliability of thermally sensitive systems. Room temperature thermocouple measurements show 4 degrees centigrade of cooling on the surface of the cooler. However, since the size of the microcoolers can be smaller then the thermocouple, and the measurement is effected by the thermal mass of the thermocouple, non-contact high-resolution methods are preferred for obtaining device performance. Thermoreflectance techniques Many experiments have used the thermoreflectance method for thermal measurements on a microscopic scale. In particular experiments by Goodson[3], Quintard[4] and Claeys[5] have shown good results on metal trace experiments and also several experiments by Mansares[6] and Batista[7] have shown thermal imaging with a scanning method. The thermoreflectance technique exploits the change in the reflection coefficient of material with temperature. Using visible wavelength one can achieve submicron spatial resolution. It is known that the reflection coefficient has a small linear dependence on temperature. The normalized change in reflection per unit temperature is called the thermoreflectance constant and is denoted by Cth.