212
R. Singh et al.
2 Properties of β-Ga 2 O 3
Apart from large bandgap of 4.9 eV, what makes β-Ga 2 O 3 most competitive and
compelling for power device applications is availability of inexpensive, large-size,
and high-quality single-crystal substrates. Also, latest advances in β-Ga 2 O 3 bulk
crystal growth bring to the table renewed interest in Ga 2 O 3 , and remain as a key
driver for its potential applications in power electronics. These bulk substrates can
be grown using economical melt-growth techniques: Czochralski (CZ), floating-zone
(FZ) and edge-defined film-fed growth (EFG) [23–26]. In addition, experimental, βGa 2 O 3 devices have shown high breakdown field of 3.8 MV/cm [16], saturation
velocity v sat ~1.1 × 10
7 cm/s [27], and reasonable carrier mobility ~180 cm
2 /Vs
[20]. Owing to these properties, β-Ga 2 O 3 based power devices can offer reasonable
advantages in terms of size, weight, and output delivery power.
Concerning β-Ga 2 O 3 electronic and structural properties, most of the earlier
studies [28–31] used density functional theory (DFT), largely due to difficulty in
isolation of different Ga 2 O 3 polymorphs in crystalline form, to calculate its electron effective mass, lattice, and structural parameters, and reported β-Ga 2 O 3 band
structure. The β-Ga 2 O 3 band structure has conduction band minima (CBM) at the
center and almost flat balance band which suggests large effective mass for holes
[28, 29]. The effective mass of electron, largely responsible for transport properties,
was reported ~0.342m 0 , where m 0 is electron rest mass [28]. Anionic (O-2p with
Ga-3d and -4s states) and cationic (Ga-4s states) constitute the valence band top
and conduction band bottom, respectively. As valence band states are made up of
O-2p states and are slightly dispersed, resulting in large effective mass for holes and
high valence band densities of states. Therefore, p-type conduction either intrinsic
or extrinsic does not look feasible due to the tendency of holes to form localized
polarons alias self-trapped holes (STHs) [32], and due to deep acceptor dopants with
ionization energy levels higher than 1 eV. However, some groups demonstrated the
p-type conductivity in Ga 2 O 3 : p-type Ga 2 O 3 based nanowires using nitrogen and
zinc as acceptor dopants [33, 34]; by manipulating hydrogen incorporation in the
lattice [35], and by finding Ga-vacancy as possible acceptor [36]. On the other hand,
wide variety of intentional n-type dopants like Si, Ge, and Sn for both β-Ga 2 O 3 bulk
crystals and epitaxial films are available with very controllable carrier concentrations
ranging from 10
16 to 10
20 cm
−3 [10, 11, 37, 38]. To date, EFG has been proved as
most successful technique for growing β-Ga 2 O 3 bulk crystals, and conductive as well
as semi-insulating (S.I.) wafer sizes up to 4 inch diameter are commercially available
from novel crystal technologies [1]. EFG grown bulk β-Ga 2 O 3 crystals have major
impurities like Si, Ir, and Fe originated from the crucible and primary source material. This unintentional doping causes background n-type conductivity which can be
compensated by adding deep acceptors like Fe and Mg to obtain semi-insulating
β-Ga 2 O 3 bulk [39].
In the crystal structure of β-Ga 2 O 3 , each unit cell contains two inequivalent
gallium and three inequivalent oxygen ions commonly mentioned as Ga
(I) , Ga
(II) ,
and O
(I) , O
(II) , O
(III) , respectively. Due to this low symmetry of β-Ga 2 O 3 crystal,
R. Singh et al.
2 Properties of β-Ga 2 O 3
Apart from large bandgap of 4.9 eV, what makes β-Ga 2 O 3 most competitive and
compelling for power device applications is availability of inexpensive, large-size,
and high-quality single-crystal substrates. Also, latest advances in β-Ga 2 O 3 bulk
crystal growth bring to the table renewed interest in Ga 2 O 3 , and remain as a key
driver for its potential applications in power electronics. These bulk substrates can
be grown using economical melt-growth techniques: Czochralski (CZ), floating-zone
(FZ) and edge-defined film-fed growth (EFG) [23–26]. In addition, experimental, βGa 2 O 3 devices have shown high breakdown field of 3.8 MV/cm [16], saturation
velocity v sat ~1.1 × 10
7 cm/s [27], and reasonable carrier mobility ~180 cm
2 /Vs
[20]. Owing to these properties, β-Ga 2 O 3 based power devices can offer reasonable
advantages in terms of size, weight, and output delivery power.
Concerning β-Ga 2 O 3 electronic and structural properties, most of the earlier
studies [28–31] used density functional theory (DFT), largely due to difficulty in
isolation of different Ga 2 O 3 polymorphs in crystalline form, to calculate its electron effective mass, lattice, and structural parameters, and reported β-Ga 2 O 3 band
structure. The β-Ga 2 O 3 band structure has conduction band minima (CBM) at the
center and almost flat balance band which suggests large effective mass for holes
[28, 29]. The effective mass of electron, largely responsible for transport properties,
was reported ~0.342m 0 , where m 0 is electron rest mass [28]. Anionic (O-2p with
Ga-3d and -4s states) and cationic (Ga-4s states) constitute the valence band top
and conduction band bottom, respectively. As valence band states are made up of
O-2p states and are slightly dispersed, resulting in large effective mass for holes and
high valence band densities of states. Therefore, p-type conduction either intrinsic
or extrinsic does not look feasible due to the tendency of holes to form localized
polarons alias self-trapped holes (STHs) [32], and due to deep acceptor dopants with
ionization energy levels higher than 1 eV. However, some groups demonstrated the
p-type conductivity in Ga 2 O 3 : p-type Ga 2 O 3 based nanowires using nitrogen and
zinc as acceptor dopants [33, 34]; by manipulating hydrogen incorporation in the
lattice [35], and by finding Ga-vacancy as possible acceptor [36]. On the other hand,
wide variety of intentional n-type dopants like Si, Ge, and Sn for both β-Ga 2 O 3 bulk
crystals and epitaxial films are available with very controllable carrier concentrations
ranging from 10
16 to 10
20 cm
−3 [10, 11, 37, 38]. To date, EFG has been proved as
most successful technique for growing β-Ga 2 O 3 bulk crystals, and conductive as well
as semi-insulating (S.I.) wafer sizes up to 4 inch diameter are commercially available
from novel crystal technologies [1]. EFG grown bulk β-Ga 2 O 3 crystals have major
impurities like Si, Ir, and Fe originated from the crucible and primary source material. This unintentional doping causes background n-type conductivity which can be
compensated by adding deep acceptors like Fe and Mg to obtain semi-insulating
β-Ga 2 O 3 bulk [39].
In the crystal structure of β-Ga 2 O 3 , each unit cell contains two inequivalent
gallium and three inequivalent oxygen ions commonly mentioned as Ga
(I) , Ga
(II) ,
and O
(I) , O
(II) , O
(III) , respectively. Due to this low symmetry of β-Ga 2 O 3 crystal,