Hydrogen is frequently found as an impurity in semiconducting materials used in the electronics industry. It becomes incorporated during growth or deposition, either deliberately or unintentionally, and can profoundly alter electronic properties even in trace quantities.
In silicon devices hydrogen can mitigate the effects of defects and improve performance but can also act against deliberate dopants if concentrations are too high.
A very different behaviour has recently been found in certain compound semiconductors, where hydrogen itself acts as a dopant, causing electrical conductivity rather than opposing it. Examples include ZnO and InN, both used in opto-electronic applications.
Roger Lichti at Texas Tech University and collaborators used ISIS to model hydrogen-atom behaviour and predict how hydrogen will behave in different materials [Lichti et al., Phys. Rev. Lett. (2008) 101 136403].
Hydrogen is often difficult to study directly in semiconductors as it is highly mobile and reactive. Instead, information can now be obtained by using the hydrogen analogue ‘muonium.’
Muonium is formed when positive muons are implanted into a material. Positive muons can act like light protons (muons have a mass of about one ninth that of the proton) when implanted and it is possible to follow the behaviour of muons to find out more about hydrogen behaviour – the lattice sites, charge states and energy levels that hydrogen is likely to form in a semiconductor.
The team's detailed research has demonstrated the underlying principle that enables prediction of hydrogen behaviour in materials where it has not been studied directly.
Their research also shows how muon results are related to their hydrogen atom counterparts.
As an increasing variety of semiconductors are used in electronic devices, this work is important to enable the effects of hydrogen impurity to be properly taken into account and used for the benefit of applications.