Advancing Ga₂O₃ doping

Nitrous oxide, ammonia, and several carbon-containing molecules are strong contenders for doping gallium oxide. But which one’s the best?

BY FIKADU ALEMA, AARON FINE, WILLIAM BRAND AND ANDREI OSINSKY FROM AGNITRON.

There’s a lot to like about Ga₂O₃. Thanks to a bandgap of around 4.8 eV, this semiconductor promises to provide more efficient switching than today’s rising duo, SiC and GaN. What’s more, crystals of Ga₂O₃ can be formed from the melt, indicating the potential for low-cost production of ultra-wide bandgap devices.

However, the picture is not all rosy. In addition to concerns over a limited thermal conductivity for Ga₂O₃, there are significant issues associated with doping [1].

Today it seems that the chances of effective p-type doping in Ga₂O₃ are rather bleak, due to the small energy dispersion of the valance band and the large effective masses in valance band states. These inherent weaknesses are limiting Ga₂O₃ device architectures to those that are unipolar. Although researchers have investigated whether impurities such as magnesium, iron, and nitrogen could unlock the door to p-type conductivity, all attempts resulted in deep acceptors.

Such efforts are still valuable, though, because deep acceptors are beneficial for engineering Ga₂O₃ power devices. When Ga₂O₃ substrates are not intentionally doped, they still exhibit unintentional n-type conductivity, associated with significant levels of silicon, a background impurity. One can compensate for this by adding acceptor impurities, such as magnesium or iron, to the Ga₂O₃ melt – this enables the production of semi-insulating or highly resistive β-Ga₂O₃ substrates. The addition of deep acceptor impurities causes the equilibrium Fermi level to move away from the conduction band edge and closer to acceptor dopant states. This shift in the Fermi level allows a junction to form with adjacent n-type material, thus opening the door to the realization of potential barriers for voltage blocking. Ultimately, this could enable enhanced-mode Ga₂O₃ MOSFETs with an accepter-doped channel.