Bidirectional Control of the Band Bending at the (2̅01) and (010) Surfaces of β-Ga₂O₃ Using Aryldiazonium Ion and Phosphonic Acid Grafting

Abstract

Beta gallium oxide (β-Ga 2 O 3 ) is an ultrawidebandgap semiconductor with one of the highest-known breakdown strengths (∼8 MV/cm) making it a strong candidate for highefficiency power electronics, deep-ultraviolet photodetectors, and transparent electronic devices. Bare β-Ga 2 O 3 surfaces exhibit a strong upward band bending and electron depletion that is reduced by the formation of a hydroxyl termination on exposure to the atmosphere, although this effect varies with exposure conditions and the processing history of different samples. This work investigates the covalent modification of (2̅ 01) and ( 010) β-Ga 2 O 3 surfaces with organic layers, as a mechanism for more effectively controlling their surface band bending. We compare the different electronic effects of grafted layers formed by the electrochemical reduction of 4-nitrobenzenediazonium ions and the spontaneous grafting of octadecylphosphonic acid (ODPA). Atomic force microscopy, synchrotron X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure spectroscopy confirmed the presence of few-nanometer layers of covalently attached nitrophenyl (NP) and ODPA molecules. Valence band XPS showed that NP modification produced upward band bending shifts of +0.51 and +0.53 eV on (2̅ 01) and (010) β-Ga 2 O 3 , respectively, with further increases of +0.35 and +0.20 eV after X-ray-induced reduction of the nitro substituent to aminolike moieties. In contrast, ODPA modification produced downward shifts in surface band bending of −0.36 and −0.07 eV on (2̅ 01) and (010) β-Ga 2 O 3 , respectively. Our study demonstrates that covalently bound NP and ODPA molecules can be used to significantly modify the electronic properties of β-Ga 2 O 3 surfaces, a finding that should prove useful for electronic applications of this material.

Modifying the Barrier Height of β-Ga2O3 Schottky Contacts Using Covalently Grafted Organic Layers

“…These voltammograms are similar in shape to those previously reported for NP electrografting on other transparent conducting oxides (TSOs) including β-Ga 2 O 3 , ZnO, SnO 2 , and ITO . The NP radicals are assumed to bind to the hydroxyl-terminated β-Ga 2 O 3 surface via the cleavage of the terminating O–H bonds with the formation of Ga–O–C bonds following a 2-radical hydrogen abstraction pathway (see Scheme a,b) similar to that proposed for other TSOs. − …”

“…Surprisingly, the measured thicknesses of the ODPA films were less than half of those previously reported on ( 2̅ 01 ) and (010) β-Ga 2 O 3 substrates, despite the same modification method and conditions being used. The length of an ODPA molecule is ∼2.5 nm (Avogadro).…”

“…The length of an ODPA molecule is ∼2.5 nm (Avogadro). Using the previously reported tilt angle of ∼31° for ODPA molecules on β-Ga 2 O 3 , the expected height of a close-packed ODPA monolayer is ∼2 nm, which is significantly higher than the measured ODPA film thicknesses in this work.…”

“…Recently, we have shown that the covalent binding of organic molecules can be used to bidirectionally tune the band bending at the (001) and ( 2̅ 01 ) surfaces of β-Ga 2 O 3 , with the electrografting of electron-accepting 4-nitrophenyl (NP) molecules producing upward shifts in band bending of up to ∼0.53 eV and the spontaneous grafting of electron-donating octadecylphosphonic acid (ODPA) molecules producing downward shifts of up to 0.36 eV . However, to date, it has not been established whether the band bending changes introduced by these modifiers will persist during the fabrication of β-Ga 2 O 3 electronic devices and, thereby, influence their performance.…”

“…In this work, we compare the barrier heights of Pd SCs fabricated on the (001), (010), and ( 2̅ 01 ) surfaces of β-Ga 2 O 3 following the electrografting of NP layers and the spontaneous grafting of ODPA molecules . SCs are the key component of Schottky rectifying diodes, photodiodes, metal–semiconductor–metal photodetectors, and metal–semiconductor field effect transistors, and their main figure of merit is the Schottky barrier height, which can be easily determined from their current–voltage and capacitance–voltage characteristics.…”