Puzzling robust 2D metallic conductivity in undoped β-Ga2O3 thin films

Abstract

Here we report the analogous of an extremely stable topological-like ultra-wide bandgap insulator, (a solid that is a pure insulator in its bulk but has a metallic conductive surface), presenting a two-dimensional conductive channel at its surface that challenges our current thinking about semiconductor conductivity engineering. Nominally undoped epitaxial β-Ga 2 O 3 thin-films without any detectable defect (after a range of state-of-the-art techniques) showed the unexpectedly low resistivity of (3×10 -2 Ωcm) which was found to be also resistant to high dose proton irradiation (2MeV, 5x10 15 cm -2 dose) and was largely invariant (metallic) over the phenomenal temperature range of 2K up to 850K. The unique resilience and stability of the electrical properties under thermal and highly ionising radiation stressing, combined with the extended transparency range (thanks to the ultra-wide bandgap) and the already known toughness under high electrical field could open up new perspectives for use as expanded spectral range transparent electrodes (e.g. for UV harvesting solar cells or UV LEDs/lasers) as well as robust Ohmic contacts for use in extreme environments/applications and for novel optoelectronic and power device concepts.

Functional Oxides for Photoneuromorphic Engineering: Toward a Solar Brain

“…The low resistivity in TCOs is generally achieved by increasing the oxide carrier concentration extrinsically (i.e. doping) while not degrading in excess its mobility (although there are intrinsic transparent conductors without extrinsic doping [129] , [130] ). While the largest impurity concentration achievable by doping is limited by the dopant solubility within the oxide lattice (typically ~10 21 cm -3 ), the free carrier mobility is degraded by its Coulomb scattering at the large number of ionized impurities (a common phenomenon taking place in any degenerately doped semiconductors [131] ).…”

“…Therefore, optically engineered oxides and their alloys can be tuned to respond to any wavelength in the germicidal UVC to the thermal imaging IRC range (~20 THz) which is a much broader range that the human visual perception range of 400-700 nm. At the time being Ga 2 O 3 (5 eV) is the widest band-gap transparent conductive oxide to be used as electrode [130] . Besides its tunable light harvesting features, some non-centrosymetric functional oxide thin-films are among the best(inverse) piezoelectric (i.e., material deformations after an electrical field) and photostrictive (light induced non-thermal deformations) materials and, therefore, very interesting candidates to implement motoractive inference in (photo)-neuromorphic systems [795] , [796] , [797] .…”

p-Type Ultrawide-Band-Gap Spinel ZnGa2O4: New Perspectives for Energy Electronics

“…Ultra-wide bandgap semiconductor oxides [1,2] are a class of materials that currently are attracting a lot of attention as an emerging multi-functional platform owing to their unusual material properties [3][4][5] , endurance (high critical fields, radiation hardess,etc) [6][7][8][9] and unique optoelectronic properties [10][11][12] . The emerging transparent semiconductor oxide (TSO) technology based on ultra-wide bandgap oxides holds the promise of extending many energy and optoelectronic applications further into the deep ultraviolet range, overpassing the conventional wide bandgap TSO's near ultraviolet limit (~3.5 eV) (e.g.…”

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

“…Recently, ultra-wide band gap gallium oxide (Ga2O3) is receiving a lot of renewed attention as a transparent semiconducting oxide (TSO) champion owing to its unusual material properties [1], [2], [3], large tuneable conductivity [4], [5] , extremely high breakdown field [6], [7], [8], unique optoelectronic properties [9], [10], [11] and low cost [12] . TSOs are a class of key enabling material in increasingly high demand because of the immediate applications they can find in a variety of new technologies, ranging from thin-film coatings and sensor devices, to transparent electronics and optoelectronics in telecommunications [13,14] .…”

“…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.…”