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The thermal conductivity is a fundamental material property what is particular important in high-power/high-frequency microelectronic devices since the ability to dissipate heat is often the limiting factor that determines device performance. GaN-based field effect transistors suffer from surface related problems including collapse in drain current and excess leakage current through the gate (Schottky barrier). This current becomes an event larger problem as the device operating temperature increases because of enhanced thermionic emission over Schottky barrier. The efficient way to improve Schottky contact properties is to use an ultra thin insulating layer prior to the deposition of gate electrode. In this regard, controlling thermal properties of thin insulating films are of utmost importance. In the case of thin films the transport properties can be considerably different from equivalent bulk materials and show large scatter depending on technique of the film growth. Therefore, there is a need to know exact data on thermal conductivity of deposited layers, since the available literature data present large discrepancy.
We have examined several insulating films used as gate dielectrics or passivating layers, namely PECVD-deposited SiNO and sputter-deposited ZnO, Al2O3 and AlN. Sputter-deposited layers with thickness of 100 nm were formed by reactive rf process from metallic targets. The deposition parameters were adjusted so as to obtain the maximum breakdown voltage and thermal conductivity.
In this work we present the application of photothermal technique to determine thermal conductivity of thin films deposited on wide band gap substrates (GaN, SiC). The applied method is based on generation of temperature field disturbance in the sample by modulated light beam. Due to the mirage effect the probing beam (parallel to the surface) is deflected and this deflection, is related to the thermal properties of the sample. The deflection of the probing beam is determined experimentally in two geometries: for the sample illuminated from the coated and uncoated surfaces. Comparison of obtained dependencies allows determination of the thermal conductivity of the film.
Part of the research was supported by the grant from EC HYPHEN Contract Number: FP6-027455.
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