Electron mobility in monoclinic β-Ga₂O₃—Effect of plasmon-phonon coupling, anisotropy, and confinement

Abstract

This work reports an investigation of electron transport in monoclinic β-Ga2O3 based on a combination of density functional perturbation theory based lattice dynamical computations, coupling calculation of lattice modes with collective plasmon oscillations and Boltzmann theory based transport calculations. The strong entanglement of the plasmon with the different longitudinal optical (LO) modes make the role LO-plasmon coupling crucial for transport. The electron density dependence of the electron mobility in β-Ga2O3 is studied in bulk material form and also in the form of two-dimensional electron gas. Under high electron density a bulk mobility of 182 cm 2 / V.s is predicted while in 2DEG form the corresponding mobility is about 418 cm 2 /V.s when remote impurities are present at the interface and improves further as the remote impurity center moves away from the interface. The trend of the electron mobility shows promise for realizing high electron mobility in dopant isolated electron channels. The experimentally observed small anisotropy in mobility is traced through a transient Monte Carlo simulation. It is found that the anisotropy of the IR active phonon modes is responsible for giving rise to the anisotropy in low-field electron mobility.

Electrical conductivity tensor of β−Ga2O3 analyzed by van der Pauw measurements: Inherent anisotropy, off-diagonal element, and the impact of grain boundaries

“…[23] but significantly contrasts the predicted stronger anisotropy of σ c * c * /σ bb ≈ 0.64 of Ref. [24] and σ aa /σ bb ≈ 0.84 of Ref. [23] for comparable electron concentrations.…”

“…Theory suggests a dependence of transport anisotropy on electron concentration or prevalent scattering mechanism (see Refs. [23,24] and Tab. I).…”

“…[23] In that work, the PLOPS-limited mobility (neglecting IIS) in the c * -direction has been predicted to be up to 9 % and 40 % higher than in the aand bdirection, respectively, to become more isotropic with decreasing electron concentrations, and anisotropies of up to 1.4 for the IIS-limited mobility were estimated. Another first principles study, [24] predicts for the PLOPS-limited mobility opposite trends with a 36 % lower mobility in the c *than in the b-direction, which becomes more isotropic with increasing electron concentration, and isotropic IIS.…”

“…The scattering time is related to the dominant scattering mechanism, polar longitudinal optical-phonon scattering (PLOPS) and ionized impurity scattering (IIS). [23,24,28] At and above room temperature, the mobility is limited by PLOPS [23,28], whereas IIS is dominating at high impurityor point-defect densities (including compensating impurities/point defects) or lower temperatures. Depending on the scattering mechanism the monoclinic symmetry of β-Ga 2 O 3 suggests more or less anisotropic scattering times.…”

Impact ionization in β-Ga2O3

“…Based on the hypothetical electric field profile, the critical electric fields are evaluated under the limit αn≈αp and is also listed on Table-I. As revealed in a previous work [45] by the current authors and also from experimental observations [46] the electron mobility is slightly lower in the x direction compared to that in y. But considering the joint effect of on-resistance and breakdown field, transport along x direction is supposed to provide a higher Baliga's figure of merit [47].…”