Figure 5. a) Fully suspended platform to measure local temperature rise using the bulk GaN heterostructure. The six labeled AlGaN-GaN Seebeck 2DEG mesa regions (e.g., 2-2†, shaded white for clarity) can be used to estimate temperature rise near the heater line, which mimics a power device. Note that the temperatures are measured at the “hot” ends of the mesa regions, marked by a † symbol. b) Finite element model of temperature rise with a heater power of 25 mW using the extracted thermal properties of the bulk GaN sample. The model is for a quarter symmetry region of (a). Three locations for temperature rise measurement (2†, 3†, and 6†) are also shown. The Si substrate is assumed to be at 25 °C in this model. c) Simulated and experimentally determined temperature rise at locations. The measured temperatures are noted to be accurate within ≈15%. We used the measured Seebeck coefficient of the bulk GaN sample to extract the temperature rise. d) Measured thermoelectric figure of merit (zT) for thin and bulk GaN samples, showing steady rise with temperature. An improvement of ≈22–51% is noted for the thin GaN sample over the bulk GaN sample across the temperature range. Scale bars of (a) and (b), 500 and 100 µm.
energy harvesting devices using III-V heterostructures. As an example, we demonstrate how the measured electrical and thermal properties could be used to detect local temperatures from on-chip heat loads using the fully suspended bulk GaN platform. Figure 5a shows an on-chip circuit with four resistive heat sources (similar to the heater and sensor metal electrodes) with six 2DEG mesa regions (spanning the suspension and substrate regions) across which the Seebeck voltage can be measured. The substrate is held at 25 °C using a temperature-controlled probe station. We then apply a heating power (25 mW) to induce a temperature gradient relative to the substrate in the suspended heterostructures. The simulated temperature profile for a quarter region using the measured thermal properties and external losses (Section S3, Supporting Information) is shown in Figure 5b. Three sample locations of the “hot” ends of the 2DEG mesa where temperature is measured are illustrated in Figure 5b. Finally, with the heater power held constant, we measured the differential Seebeck voltage for
the six 2DEG mesa regions. Using the measured temperature-dependent Seebeck voltage for the bulk GaN film, we extracted the temperatures at the hot ends of the 2DEG mesa regions and compared them with the simulated local temperatures from the finite element model. Excellent agreement is observed (within a maximum of ≈15% error), as seen in Figure 5c, which demonstrates how the measured properties could be used for local on-chip monitoring of thermal loads in addition to validating our measurements.
Finally, Figure 5d shows the temperature-dependent thermoelectric figure of merit ($zT$) up to ≈300 °C using the measurements of 2DEG electrical conductivity, Seebeck coefficient, and in-plane heterostructure thermal conductivity. These $zT$ values are 3 to 4× higher than in bulk doped GaN films,[10] owing to large 2DEG conductivities and thermal conductivity reduction from size effect. Further, we notice that the thin GaN film displays almost ≈22–51% higher $zT$ values in comparison to the bulk GaN film due to the large, ≈2.5× decrease of the in-plane