How does hydrogen transform into shallow donors in silicon?

We conducted electron paramagnetic resonance (EPR) and cyclotron resonance studies on hydrogen-related donors (HDs) in silicon, whose origins have remained unclear for 50 years. In floating-zone silicon substrates irradiated with 2-MeV protons, followed by optimized annealing at 400 degrees C and 500 degrees C, we identified three original EPR spectral signals originating from HDs, labeled the MT1, MT2, and MT3 centers. The three centers exhibited a tetragonal symmetry around the (100) axis and were classified into two types of HDs: a HD with a lower thermal stability and a normal single-donor nature/positive-U nature (MT1), and a HD with a higher thermal stability and a double-donor nature/negative-U nature (MT2 and MT3). Most importantly, the MT2 center revealed 29Si hyperfine interactions closely resembling the B3 EPR center with a tetragonal symmetry, which has been identified as a tetrainterstitial cluster, I4. Contrarily, we could not resolve any 1H hyperfine interactions for the three MT centers. According to the experimental results, an atomistic model was proposed for the HDs based on the I4 center weakly coupled with hydrogen atom(s). We deduce that different numbers of incorporated hydrogen atoms and/or different types of hydrogen bonding to I4 generated the two types of HDs. These findings are the first step in understanding how hydrogen works as a donor in Si and offer important insights into the use of proton-irradiation doping processes in device applications.