2, Cornell University, Ithaca, New York, United States
3, UCLA, Los Angeles, California, United States
Extreme bandgap semiconductors (xBGS) such as AlN and GaN have attracted much interest due to their potential applications in power electronics, and deep UV photonics. Recent advances in molecular beam epitaxy (MBE) technology has enabled the growth of ultra-thin AlN/GaN quantum heterostructures for deep-UV emission [1,2], which is suitable for diagnostic and therapeutic application, where thermal conductivity plays an important role in heat dissipation of operating devices.
Here, we use the 3-omega technique  to study thermal conductivity of AlN single crystals and ultrathin AlN/GaN superlattices over a wide temperature range (100 to 400 K) for the first time. For the 3-omega technique, four-probe metal lines are patterned on the sample surface as both heater and thermometer. A current with frequency ω causes a second harmonic temperature rise in the sample under the heater. The metal heater resistance varies linearly with temperature, thus a voltage is measured at the 3ω frequency using a lock-in amplifier, and this voltage is correlated to the thermal conductivity of the substrate.
First, we measured the thermal conductivity of AlN bulk, finding it ranges from ~680 W/m/K at 100 K to ~200 W/m/K at 400 K. Comparison with simulations based on the Boltzmann Transport equation (BTE) shows this behavior is consistent with phonon Umklapp scattering, and a contribution from atomic impurity defects, leading to roughly 1/T dependence of thermal conductivity over this temperature range. (The contribution of electrons to thermal transport is negligible in these xBGS materials.)
Next, AlN/GaN superlattices (SL) of thickness ~300 nm are grown on 1 µm AlN template on sapphire substrates by MBE, with (repeating) SL layer thickness from 0.25 nm to 2.25 nm. After subtracting the contribution of the interfaces and substrate, we obtain the SL thermal conductivity, which ranges from ~47 W/m/K at 100 K to ~33 W/m/K at 400 K. Due to strong interface scattering, the SL has much lower thermal conductivity than AlN bulk, which is an important finding for thermally-limited devices based on SLs .
In summary, we report the thermal conductivity of AlN and AlN/GaN SLs over a wide temperature range, uncovering the role of interfaces, defects, and their temperature dependence from 100 to 400 K. AlN/GaN SLs in deep-UV photonic devices are commonly used as light emitting regions and buffers layers, thus knowledge of their thermal conductivity is valuable in device design and heat dissipation considerations. This work is supported in part by the NSF DMREF program.
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