The H$\text{}_{2}$ Molecule in Semiconductors: An Angel in GaAs, a Devil in Si

The electrical and optical properties of semiconductors are largely determined by the defects and impurities they contain. Without a doubt, hydrogen is the impurity which exhibits the most varied and exotic properties. In most semiconductors, it is found in three charge states and four configurations. It forms (at least) two types of dimers as well as small and large precipitates such as platelets. H also interacts with impurities and defects. It removes or changes the electrical activity of many shallow and deep centers, and catalyzes the diffusion of interstitial oxygen (in Si). Sometimes, it exhibits quantum tunneling and is associated with unusual effects such as Fermi resonances. But one of the most exotic forms of hydrogen in GaAs and Si is the interstitial H$\text{}_{2}$ molecule, which appears to play a critical role in processes such as the "smart cut". It is the only interstitial molecule observed (so far) in semiconductors. In GaAs, it behaves like a nearly-free rotator, with properties very much as one would expect them to be. But in Si, the early experiments were puzzling. No ortho/para splitting was observed, the symmetry appeared to be C$\text{}_{1}$, the single HD line was at the wrong place and had the wrong amplitude, and other features seemed strange as well. Recent experimental studies have now resolved many issues. However, the behavior of the simplest molecule in the Universe proved to be a tough nut to crack, which goes to show that devils can be a lot more fun than angels after all.