Transition from electron accumulation to depletion at β-Ga2O3 surfaces: The role of hydrogen and the charge neutrality level

Abstract

The surface electronic properties of bulk-grown β-Ga2O3 (2¯01) single crystals are investigated. The band gap is found using optical transmission to be 4.68 eV. High-resolution x-ray photoemission coupled with hybrid density functional theory calculation of the valence band density of states provides insights into the surface band bending. Importantly, the standard linear extrapolation method for determining the surface valence band maximum (VBM) binding energy is found to underestimate the separation from the Fermi level by ∼0.5 eV. According to our interpretation, most reports of surface electron depletion and upward band bending based on photoemission spectroscopy actually provide evidence of surface electron accumulation. For uncleaned surfaces, the surface VBM to Fermi level separation is found to be 4.95 ± 0.10 eV, corresponding to downward band bending of ∼0.24 eV and an electron accumulation layer with a sheet density of ∼5 × 1012 cm−2. Uncleaned surfaces possess hydrogen termination which acts as surface donors, creating electron accumulation and downward band bending at the surface. In situ cleaning by thermal annealing removes H from the surface, resulting in a ∼0.5 eV shift of the surface VBM and formation of a surface electron depletion layer with upward band bending of ∼0.26 eV due to native acceptor surface states. These results are discussed in the context of the charge neutrality level, calculated bulk interstitial hydrogen transition levels, and related previous experimental findings.

Enhancing the intrinsic p-type conductivity of the ultra-wide bandgap Ga2O3 semiconductor

“…While high n-type conductivity in β-Ga 2 O 3 can be efficiently achieved by impurity doping with Sn, Si, Ge, F or Cl [31,32] (and even metallic conductivity due to charge accumulation on the surface in undoped β-Ga 2 O 3 . [3,33] ), p-type conductivity is still controversial.…”

Hydrogen plasma treatment of β -Ga2O3: Changes in electrical properties and deep trap spectra

“…The proposed hydrogen passivation of donors would be in contrast to theoretical predictions and to some experimental results obtained for H introduction at high temperatures during growth. [6][7][8] Note that the Fermi level in the H treated sample is pinned near E c -1.05 eV, suggesting this trap to be the main uncompensated deep center. This is opposite to that observed in DLTS on reference samples and 550 C annealed samples where the E2 Ã (E c -0.8 eV) trap is the major deep center.…”

“…Hydrogen accumulation at the surface causes upwards band bending. 8 A question remains as to whether hydrogen behaves in Ga 2 O 3 as it often does in other semiconductors, i.e., forms neutral complexes with shallow donors or with deep level defects, thus passivating them. 11 One approach to check this is by introducing hydrogen (or deuterium) from plasmas.…”

Wide Bandgap Oxide Semiconductors: from Materials Physics to Optoelectronic Devices

“…Recently, 2DEG has been found generally at the surfaces of oxide semiconductors, including CdO, [46] In 2 O 3 , [29a,d,47] SnO 2 , [29c,48] ZnO, [46b,49] IGZO, [29b] and possibly Ga 2 O 3 . [50] The 2DEG is confined at a few nanometers of near surface region, exhibiting properties such as increased electron mobility and reduced bandgap. Furthermore, 2DEG can also be created by modulating charge transfer at the interfaces of Mg x Zn 1-x O/ZnO, [51] LaAlO 3 /SrTiO 3 , [52] In 2 O 3 / ZnO, [53] and (Al x Ga 1-x ) 2 O 3 /Ga 2 O 3 .…”

“…[58] However, during the past decade, a series of studies performed on high-quality, low-defect single crystals or epitaxial thin films suggested the opposite situation where a surface electron accumulation layer (SEAL) confined within a few nm thick near the surface region with a concomitant downward band bending at the surfaces of CdO, [46a-e] In 2 O 3 , [29a,d,47] SnO 2 , [29c,48] ZnO, [46b,49] IGZO, [29b] and Ga 2 O 3 . [50] Furthermore, the downward band bending creates a confining potential well, leading to quantization of 2DEG. It should be mentioned that, although the SEAL is only a few nm thick, such layer can have a pronounced effect on the electronic properties of oxide films and the subsequent device performance.…”