Ga 2 O 3 Based Heterostructure FETs (HFETs) for Microwave …
215
As electron mobility in β-Ga 2 O 3 based experimental devices is comparatively
low and reported in the range of 60 to 200 cm
2 /Vs at room temperature. Earlier this
variation in electron mobility was interpreted as due to anisotropy in electron effective mass [10]. However, subsequent theoretical studies [26, 38, 46–49] suggested
almost isotropic conduction band effective mass. So different mechanism affecting
transport properties is of critical importance for relatively new and immature βGa 2 O 3 semiconductor. The first-principles study was reported to explain the scattering mechanism considering the role of polar and non-polar optical phonons, and
ionized impurities. Parisini et al. [50] suggested non-polar optical (NOP) phonons
scattering as the main mechanism in β-Ga 2 O 3 crystals and estimated lattice deformation potential of about 4 × 10
9 eV/cm. However, subsequent reports [51–53]
advocated and suggested polar optical (PO) phonon with energy between 21 and
48 meV scattering as most likely scattering phenomena in β-Ga 2 O 3 limiting room
temperature electron mobility of <200 cm
2 /Vs for doping concentration <10
18 cm
−3 .
The similar mechanism holds good in other polar semiconductors like GaN, and
GaAs; while in non-polar semiconductors (Si, Ge) lattice deformation enabled NOP
phonons scattering is significant. So it may be concluded that electron mobility in
β-Ga 2 O 3 is limited by two different scattering mechanisms—ionized impurity and
optical phonon at low and high temperature, respectively, as shown in Fig. 3.
Even though electron effective mass of GaN (~0.2m 0 ) and β-Ga 2 O 3 (0.28m 0 ) is
almost equal, PO phonon energy (E PO ) in β-Ga 2 O 3 is almost half that in GaN (E PO
~92 meV), Ma et al. [53] reported very strong Fröhlich coupling constant showing
enhanced electron—PO phonon interaction in β-Ga 2 O 3 , mainly due to its high bond
iconicity and a large difference in static- and high-frequency dielectric constant
values of 10.2 and 3.57, respectively.
Due to low PO phonon energy (èω 0 ) in β-Ga 2 O 3 , as mentioned above, and since
Fröhlich coupling constant α f ∝ 1
( 0 ), electron—PO phonon interaction was
Different Scattering
Mechanisms:
IO – Ionized impurity
NI – Neutral impurity
DP – Non-polar
acoustic phonon
NOP – Non-polar
optical phonon
PO – Polar optical
phonon
Fig. 3 Drift mobility curves as a function of temperature in β-Ga 2 O 3 , The solid bold line is total
drift mobility while solid thin lines are related to individual scattering mechanism, and dotted line
related to single NOP mobility, redrawn from Ref. [50]
215
As electron mobility in β-Ga 2 O 3 based experimental devices is comparatively
low and reported in the range of 60 to 200 cm
2 /Vs at room temperature. Earlier this
variation in electron mobility was interpreted as due to anisotropy in electron effective mass [10]. However, subsequent theoretical studies [26, 38, 46–49] suggested
almost isotropic conduction band effective mass. So different mechanism affecting
transport properties is of critical importance for relatively new and immature βGa 2 O 3 semiconductor. The first-principles study was reported to explain the scattering mechanism considering the role of polar and non-polar optical phonons, and
ionized impurities. Parisini et al. [50] suggested non-polar optical (NOP) phonons
scattering as the main mechanism in β-Ga 2 O 3 crystals and estimated lattice deformation potential of about 4 × 10
9 eV/cm. However, subsequent reports [51–53]
advocated and suggested polar optical (PO) phonon with energy between 21 and
48 meV scattering as most likely scattering phenomena in β-Ga 2 O 3 limiting room
temperature electron mobility of <200 cm
2 /Vs for doping concentration <10
18 cm
−3 .
The similar mechanism holds good in other polar semiconductors like GaN, and
GaAs; while in non-polar semiconductors (Si, Ge) lattice deformation enabled NOP
phonons scattering is significant. So it may be concluded that electron mobility in
β-Ga 2 O 3 is limited by two different scattering mechanisms—ionized impurity and
optical phonon at low and high temperature, respectively, as shown in Fig. 3.
Even though electron effective mass of GaN (~0.2m 0 ) and β-Ga 2 O 3 (0.28m 0 ) is
almost equal, PO phonon energy (E PO ) in β-Ga 2 O 3 is almost half that in GaN (E PO
~92 meV), Ma et al. [53] reported very strong Fröhlich coupling constant showing
enhanced electron—PO phonon interaction in β-Ga 2 O 3 , mainly due to its high bond
iconicity and a large difference in static- and high-frequency dielectric constant
values of 10.2 and 3.57, respectively.
Due to low PO phonon energy (èω 0 ) in β-Ga 2 O 3 , as mentioned above, and since
Fröhlich coupling constant α f ∝ 1
( 0 ), electron—PO phonon interaction was
Different Scattering
Mechanisms:
IO – Ionized impurity
NI – Neutral impurity
DP – Non-polar
acoustic phonon
NOP – Non-polar
optical phonon
PO – Polar optical
phonon
Fig. 3 Drift mobility curves as a function of temperature in β-Ga 2 O 3 , The solid bold line is total
drift mobility while solid thin lines are related to individual scattering mechanism, and dotted line
related to single NOP mobility, redrawn from Ref. [50]