Enhanced in-plane thermoelectric figure of merit in p-type SiGe thin films by nanograin boundaries

Abstract

Boron-doped polycrystalline silicon-germanium (SiGe) thin films are grown by low-pressure chemical vapor deposition (LPCVD) and their thermoelectric properties are characterized from 120 K to 300 K for the potential applications in integrated microscale cooling. The naturally formed grain boundaries are found to play a crucial role in determining both the charge and thermal transport properties of the films.Particularly, the grain boundaries create energy barriers for charge transport which lead to abnormal dependences of charge mobility on doping concentration and temperature. Meanwhile, the unique columnar grain structures result in remarkable thermal conductivity anisotropy with the in-plane thermal conductivities of SiGe films about 50% lower than the cross-plane values. By optimizing the growth conditions and doping level, a high figure of merit (ZT) of 0.2 for SiGe films is achieved at 300 K, which is about 100% higher than the previous record for p-type SiGe alloys, mainly due to the significant reduction in the in-plane thermal conductivity caused by nanograin boundaries. The low cost and excellent scalability of LPCVD render these high-performance SiGe films ideal candidates for thin-film thermoelectric applications.

Realizing High Thermoelectric Performance at Ambient Temperature by Ternary Alloying in Polycrystalline Si1-x-yGexSny Thin Films with Boron Ion Implantation

“…The conductivity of Si 1-x-y Ge x Sn y samples increases faster than that of Si 0.889 Ge 0.111 samples and when the annealing temperature over 1100 °C the mobility of Si 1-x-y Ge x Sn y samples are twice as large as corresponding Si 0.889 Ge 0.111 samples. Moreover, the improved annealing temperature under conditions of carrier concentration of 10 20 cm −3 order of magnitude which is contrary to the rule that the mobility decreases with the increase of carrier concentration 29 as shown in Fig. 6(a,b), which attributing to suitable grain sizes and boundaries increase influence with heavy B doping 30,31 .…”

“…6(a,b), which attributing to suitable grain sizes and boundaries increase influence with heavy B doping 30,31 . After checking the carrier concentration data, one can find at least 900 °C-annealing is necessary for activation the implanted B ions 29,32 for heavily doped samples of more than 10 15 cm −2 dosage, furthermore annealing of more than 1000 °C, even if only for 15 seconds, can activate most of implanted B atoms, and even at the temperature of 1150 °C, almost all the implanted B atoms are activated which leading the measured Hall carrier concentration value is larger than the designed value 1.8 × 10 20 cm −3 .

Figure 6Hall measurement results (mobility, carrier concentration, and conductivity) of ( a ) Si 1-x-y Ge x Sn y and ( b ) Si 0.889 Ge 0.111 samples after 15 seconds-RTA as a function of the RTA temperature.

…”

Exceptional thermoelectric power factors in hyperdoped, fully dehydrogenated nanocrystalline silicon thin films

“…SiGe alloys were also considered. Although their electronic structure and physical chemistry obviously differ from that of silicon, a SiGe system quite similar to the one we have investigated was discussed by Li et al [54]. Thin films of Si 74 Ge 26 alloys were grown by LPCVD on a Si (100) wafer that was previously coated with a SiO x film and an amorphous Si thin layer.…”

Recent Advances on Thermoelectric Silicon for Low-Temperature Applications

“…SiGe alloys were also considered [ 62 ]. Thin films of Si Ge alloys were grown by LPCVD on a Si (100) wafer that was previously coated with a SiO film and an amorphous Si thin layer.…”