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R. Singh et al.
electron gas (2DEG) was estimated at a 2DEG density of ~10
12 cm
−3 , by screening
the free carriers [55]. The development and RF performance of β-Ga 2 O 3 based
heterostructures are discussed next.
4 β-Ga 2 O 3 Based Heterostructure Devices
For a possible β-Ga 2 O 3 heterostructure, several early efforts were made like solubility
limit of Al 2 O 3 and In 2 O 3 in β-Ga 2 O 3 [58, 59], growing epitaxial β-(Al x Ga 1−x ) 2 O 3
thin films on (010) β-Ga 2 O 3 (native) substrate and on sapphire (foreign) substrates
using plasma-assisted molecular beam epitaxy (PA-MBE) and pulsed laser deposition (PLD), respectively [60, 61], and verification of carrier confinement at the
hetero-interface [62, 63]. Furthermore, the bandgap of β-(AlGa) 2 O 3 alloy can be
tuned in the range of 4.9–6.8 eV through the introduction of Al content between
0.22 and 0.98 in β-Ga 2 O 3 [60, 61]. This enabled realization of several ternary
β-(Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 heterostructure designs [64–68] like HFETs, delta-doped
FET, and MODFETs. Such device designs using conventional III-V nitrides have
already shown high-electron mobility in 2DEG channel by the screening of impurities and also allowed substantial scaling. These developments encouraged the
possible application of AGO/GO heterostructures to high-power HEMTs in which
the barrier layer β-(Al x Ga 1−x ) 2 O 3 is modulation-doped. It is worth noting that high
sheet charge density below the hetero-interface and large conduction band offset
(CBO) to accommodate 2DEG charges are some of the key desirable features of
HEMTs for high-power and high-frequency applications.
4.1 β-(Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 MODFETs
Although the first experimental β-(Al 0.15 Ga 0.85 ) 2 O 3 /Ga 2 O 3 (AGO/GO) heterostructure was demonstrated by Kaun et al. [64], surface roughness at the interface prevented carrier accumulation at the hetero-interface. Earlier reported β(Al x Ga 1−x ) 2 O 3 :Si/Ga 2 O 3 heterojunction [63] made use of unintentional Si contaminants, accumulated at the interface during the growth of β-Ga 2 O 3 substrate, for
doping the β-(Al x Ga 1−x ) 2 O 3 layer. This modulation doping of Si was analyzed using
secondary-ion mass spectroscopy (SIMS) profile as shown in Fig. 7, furthermore
sheet charge density of ~10
12 cm
−2 was estimated by capacitance–voltage (C–V)
analysis.
It is worth noting that auto-doping of Si contaminants lacks control on doping
profile as well as concentration, and also forms a parasitic channel in the AGO
layer which is detrimental for device performance, as mentioned in [62–68]. To
address the issue, Ahmadi et al. [65] used intentional doping of Ge in the AGO
barrier and employed Ga-polishing to reduce surface morphology and control the
auto-doping of Si impurity. Furthermore, the same authors were first to demonstrate
R. Singh et al.
electron gas (2DEG) was estimated at a 2DEG density of ~10
12 cm
−3 , by screening
the free carriers [55]. The development and RF performance of β-Ga 2 O 3 based
heterostructures are discussed next.
4 β-Ga 2 O 3 Based Heterostructure Devices
For a possible β-Ga 2 O 3 heterostructure, several early efforts were made like solubility
limit of Al 2 O 3 and In 2 O 3 in β-Ga 2 O 3 [58, 59], growing epitaxial β-(Al x Ga 1−x ) 2 O 3
thin films on (010) β-Ga 2 O 3 (native) substrate and on sapphire (foreign) substrates
using plasma-assisted molecular beam epitaxy (PA-MBE) and pulsed laser deposition (PLD), respectively [60, 61], and verification of carrier confinement at the
hetero-interface [62, 63]. Furthermore, the bandgap of β-(AlGa) 2 O 3 alloy can be
tuned in the range of 4.9–6.8 eV through the introduction of Al content between
0.22 and 0.98 in β-Ga 2 O 3 [60, 61]. This enabled realization of several ternary
β-(Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 heterostructure designs [64–68] like HFETs, delta-doped
FET, and MODFETs. Such device designs using conventional III-V nitrides have
already shown high-electron mobility in 2DEG channel by the screening of impurities and also allowed substantial scaling. These developments encouraged the
possible application of AGO/GO heterostructures to high-power HEMTs in which
the barrier layer β-(Al x Ga 1−x ) 2 O 3 is modulation-doped. It is worth noting that high
sheet charge density below the hetero-interface and large conduction band offset
(CBO) to accommodate 2DEG charges are some of the key desirable features of
HEMTs for high-power and high-frequency applications.
4.1 β-(Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 MODFETs
Although the first experimental β-(Al 0.15 Ga 0.85 ) 2 O 3 /Ga 2 O 3 (AGO/GO) heterostructure was demonstrated by Kaun et al. [64], surface roughness at the interface prevented carrier accumulation at the hetero-interface. Earlier reported β(Al x Ga 1−x ) 2 O 3 :Si/Ga 2 O 3 heterojunction [63] made use of unintentional Si contaminants, accumulated at the interface during the growth of β-Ga 2 O 3 substrate, for
doping the β-(Al x Ga 1−x ) 2 O 3 layer. This modulation doping of Si was analyzed using
secondary-ion mass spectroscopy (SIMS) profile as shown in Fig. 7, furthermore
sheet charge density of ~10
12 cm
−2 was estimated by capacitance–voltage (C–V)
analysis.
It is worth noting that auto-doping of Si contaminants lacks control on doping
profile as well as concentration, and also forms a parasitic channel in the AGO
layer which is detrimental for device performance, as mentioned in [62–68]. To
address the issue, Ahmadi et al. [65] used intentional doping of Ge in the AGO
barrier and employed Ga-polishing to reduce surface morphology and control the
auto-doping of Si impurity. Furthermore, the same authors were first to demonstrate