Subthreshold Electron Damage inn-Type Germanium

The minimum energy at which intrinsic defects resulting from electron bombardment have been detected in $n$-type Ge at 78°K is 360 keV. Defects referred to as subthreshold defects are produced at lower energies, but only in crystals grown in hydrogen. Heat treatment of such a crystal in either vacuum or hydrogen atmosphere at 450-500°C for 24 h renders the crystal immune to subthreshold damage. This treatment does not change the electrical conductivity. Subthreshold defects have hole-trapping properties which render the measured carrier concentration highly sensitive to the intensity of the electron beam used for irradiation. The traps can be emptied completely by heating the samples at 140-280°K without modifying the defects. They can be refilled by a short irradiation. There is no dependence of the subthreshold-damage production rate on crystal orientation. The defects have different electrical properties and annealing behavior from those of defects produced by higher-energy electrons, suggesting that they may not involve the vacancy or interstitial. The production of subthreshold damage appears to be zero below about 40 keV. Damage rates at 78°K are larger than those at 273°K. Crystals grown in deuterium exhibit little or no damage. It is concluded that subthreshold damage is caused by a relocation of hydrogen impurity in the lattice. The surprising isotopic effect is suggestive of an ionization mechanism for relocation. The dependence of damage production rate on bombardment energy is consistent with an ionization-induced process, except that it should have a "zero-energy" threshold. Possible explanations are given to account for the apparent 40-keV threshold.