THERMAL CONDUCTIVITY OF InAs/AlSb SUPERLATTICES

In this work, we present experimental studies on the cross-plane thermal conductivity of InAs/AlSb superlattices. The thermal conductivities of MBE grown InAs/AlSb superlattices are measured using the 3ω method from 80-300 K. The influence of the growth temperature and post annealing is investigated. Significant reductions in thermal conductivity are observed in these superlattices compared to the predictions of the Fourier heat conduction theory based on the bulk material properties. These results suggest that the interface conditions strongly influence the thermal conductivity. Proceedings of the Heat Transfer and Transport Phenomena in Microscale, pp. 369-372, Ed. G.P. Celeta, Banff, Canada, October 15-20, 2000. Revised for Journal Publication. 2. SAMPLES AND MEASUREMENT TECHNIQUE Table 1 summarizes the growth and annealing conditions of the samples studied in this work. The InAs/AlSb superlattices are grown by molecular beam epitaxy on GaSb substrates. The sample’s growth began with a 3000Å GaSb buffer layer deposited at 490oC followed by 162 periods of InAs(33.4Å)/AlSb(31.8Å). The growth of the superlattice layers is carried out for three different sets of samples at respectively 390oC, 425oC, and 460oC temperatures. Two of the samples grown at 390oC and 425oC are then post-annealed at 490oC for 5 minutes. A differential 3ω method is used in this work to measure the cross-plane thermal conductivity of the InAs/AlSb superlattices. In the 3ω method [35], a metal wire is deposited onto the film to act as both a heater and a temperature sensor. Since the superlattice film is semiconducting, the metallic wire must be insulated from the film to avoid current leakage. The electrical insulation is provided by a ~119 nm SiNx film deposited by plasma enhanced chemical vapor deposition (PECVD) at 150oC onto the samples. In order to determine the thermal conductivity of the InAs/AlSb superlattice films, the temperature drop across the superlattices is experimentally determined using a differential technique. Since the composition of the GaSb buffer layer is identical to the composition of the substrate, the differential 3ω thermal conductivity characterization of the superlattice films requires deposition of wires on two different samples, including a substrate reference and the superlattice. The insulation layer and the metallic heaters/temperature sensors with wire widths between 2 50 μm are processed for all the samples during the same process flow. For a given wire width, the temperature rise at a given frequency and identical power input is recorded for the substrate and superlattice samples and the measured temperature difference is used to determine the thermal conductivity of the superlattice. Both the in-plane and the cross-plane thermal conductivity of the superlattice film can be determined if the experiment is carried out for different heater width/film thickness aspect ratios and a twodimensional heat conduction model is used to fit the experimental temperature drop across the film [19, 36]. However, a simpler one-dimensional heat conduction model can be used to determine the cross-plane thermal conductivity of the film if the width of the heater is much larger than the film thickness, and if the film thermal conductivity is much smaller than the substrate thermal conductivity [36]. In this work, 30 μm wide heaters are used to measure the temperature drop across ~ 1.05 μm thick superlattice films. The heaterwidth/superlattice-thickness aspect ratio is ~28, and the ratio between the superlattice thermal conductivity and substrate thermal conductivity is at most ~0.1. Under the above conditions, one-dimensional heat conduction modeling for the heat transport across one anisotropic film yields an error of <~5% for its thermal conductivity, as long as the film anisotropy is smaller than eight [36]. The temperature dependent thermal conductivity measurements were carried out in a cryostat in the temperature range from 80K to room temperature. The temperature in the chamber was adjusted to the desired temperature by a temperature controller. During the 3ω measurements, the ambient temperature variations in the cryogenic chamber were within 0.1K. The calibration of the temperature coefficient of resistance (TCR) of the metallic wire was carried out during the slow warm up of the cryostat. The 3ω voltage was measured by a lock-in amplifier. A computer was used for automatic data acquisition and control. 3. EXPERIMENTAL RESULTS AND DISCUSSION The data points in Fig. 1 are examples of the frequency dependent experimental temperature amplitude measured at 80K and 300K for 30 μm wide heaters deposited on the reference and one of the superlattice samples (2801). The temperature difference measured between the superlattice sample (SL) and the substrate reference is constant over a wide frequency range and is used to determine the thermal Table 1. Growth and annealing conditions of the samples. Sample Growth temperature Annealing