Ultrathin GaN nanowires: Electronic, thermal, and thermoelectric properties

Abstract

We present a comprehensive computational study of the electronic, thermal, and thermoelectric (TE) properties of gallium nitride nanowires (NWs) over a wide range of thicknesses (3-9 nm), doping densities (10 18 -10 20 cm −3 ), and temperatures (300-1000 K). We calculate the low-field electron mobility based on ensemble Monte Carlo transport simulation coupled with a self-consistent solution of the Poisson and Schrödinger equations. We use the relaxation-time approximation and a Poisson-Schrodinger solver to calculate the electron Seebeck coefficient and thermal conductivity. Lattice thermal conductivity is calculated using a phonon ensemble Monte Carlo simulation, with a real-space rough surface described by a Gaussian autocorrelation function. Throughout the temperature range, the Seebeck coefficient increases while the lattice thermal conductivity decreases with decreasing wire cross section, both boding well for TE applications of thin GaN NWs. However, at room temperature these benefits are eventually overcome by the detrimental effect of surface roughness scattering on the electron mobility in very thin NWs. The highest room-temperature ZT of 0.2 is achieved for 4-nm-thick NWs, while further downscaling degrades it. In contrast, at 1000 K, the electron mobility varies weakly with the NW thickness owing to the dominance of polar optical phonon scattering and multiple subbands contributing to transport, so ZT increases with increasing confinement, reaching 0.8 for optimally doped 3-nm-thick NWs. The ZT of GaN NWs increases with increasing temperature beyond 1000 K, which further emphasizes their suitability for high-temperature TE applications.

Investigation and Mathematical Modelling for Different Scattering Mechanisms in AlGaN/GaN HEMT

“…In polar semiconductors the electron-longitudinal optical (LO) phonon scattering is the dominant inter subband scattering mechanism for separations of the sub-bands less than the LO phonon energy [6]. Polar optical phonon (POP) energy for the wurtzite GaN crystal is higher (ħω op = 92meV) [1] than other III-V semiconductors.…”

Significant Phonon Drag Enables High Power Factor in the AlGaN/GaN Two-Dimensional Electron Gas

“…25 This deviation is due to POP scattering, which forbids electrons with energies smaller than ℏ OP from emitting optical phonons. 29 The slight difference in the calculated values of d for the thick and thin GaN sample is found to arise from the difference in the roughness scattering component of ( ). We observe that the Seebeck coefficient for the thin GaN sample agrees well with the calculated d , however this model cannot describe the thick GaN sample ( Figure 2d).…”

Thermoelectric transport in ZnO and GaN nanowires

“…Although NWs can have different cross sections, we consider for simplicity a square crosssectional morphology with the understanding that the wire thickness or width stands in for a generic characteristic crosssectional feature size [17]. In Section 2, expressions for piezoelectric momentum relaxation rates are obtained for both ZB and WZ phases.…”

“…Electron mobility, one of the important electronic properties determining the performance of many semiconductor devices, has been investigated in GaN as well as ZnO NW systems [2,4,16,17]. Goldberger et al [4] have reported an average value of m ¼0.137 0.05 Â 10 À 2 m 2 V À 1 s À 1 , in ZnO NWs of diameter 101 nm and relatively low carrier concentrations of 5.2 72.5 Â 10 23 m À 3 in the temperature range 25 oT o300 K. On the other hand, a range of values of the electron mobility in GaN NWs, grown by different growth methods and having different cross sections, diameters and carrier density, are reported [2,16].…”