Formation of indium–tin oxide ohmic contacts for β-Ga₂O₃

Abstract

Sputter-deposited indium–tin oxide (ITO) electrodes became ohmic contacts for unintentionally doped β-Ga2O3(010) substrates with a carrier concentration of 2 × 1017 cm−3 after rapid thermal annealing in a wide range of annealing temperatures of 900–1150 °C. The formation of an ohmic contact is attributed to interdiffusion between ITO and β-Ga2O3, as evidenced by the results of transmission electron microscopy and energy-dispersive X-ray spectroscopy. The interdiffusion decreases the band gap and increases the donor concentration of β-Ga2O3 at the interface, and forms an intermediate semiconductor layer desirable for carrier transport. The ITO ohmic contact is particularly useful for future β-Ga2O3 devices operated at high temperatures.

Band offsets in ITO/Ga2O3 heterostructures

“…An issue with any new wide bandgap semiconductor technology is the need for low resistance improved Ohmic contacts that do not require excessive thermal treatments (16,17) . At this stage, only n-type Ga 2 O 3 is available and so majority-carrier devices dominate.…”

“…They found that Pt/ITO contacts on n-Ga 2 O 3 showed superior Ohmic contacts to Pt/Ti and attributed this to the formation of an interfacial layer with lower bandgap and higher doping concentration than the Ga 2 O 3 alone (17) . This interfacial layer promoted improved electron transport across the heterointerface This is a case of one of the common approaches to lowering Ohmic contact resistance on wide bandgap oxide or nitride semiconductors, which generally take the form of surface cleaning or doping to reduce barrier height or increase of carrier concentration of the surface through preferential loss of oxygen (nitrogen) (3,6,7,18,19) .…”

Ohmic contacts on n-type β-Ga2O3 using AZO/Ti/Au

“…Previous reports of sputtered ITO as an intermediary layer for forming Ohmic contacts on Ga 2 O 3 also needed high temperature annealing to produce good quality contacts. 25 A Pt/ITO bi-layer contact to β-Ga 2 O 3 showed superior Ohmic contact properties compared to Pt/Ti and this was ascribed to the formation of an interfacial layer with lower bandgap and higher doping concentration than the Ga 2 O 3 . 25 The band alignment at the heterointerface is critically important in determining the favorability of carrier transport.…”

“…25 A Pt/ITO bi-layer contact to β-Ga 2 O 3 showed superior Ohmic contact properties compared to Pt/Ti and this was ascribed to the formation of an interfacial layer with lower bandgap and higher doping concentration than the Ga 2 O 3 . 25 The band alignment at the heterointerface is critically important in determining the favorability of carrier transport. AZO is another alternative that is widely used as a TCO with lower cost relative to ITO.…”

Recent Advances in β-Ga2O3–Metal Contacts

“…Oshima et al achieved platinum/indium–tin oxide (Pt/ITO) Ohmic contacts to β-Ga 2 O 3 with a wide range of process temperature window [77]. The large process window of 900–1150  ° C enables the realization of high-temperature operation.…”