Damage to Silicon Produced by Bombardment with Helium Ions

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Abstract
Some of the effects of bombarding a silicon single crystal surface with 3×104 electron volt helium ions have been examined experimentally. The surface recombination velocity was found to be increased by more than two orders of magnitude, but could be restored to its original value by the removal of approximately 1 μ of surface material. This is believed to correspond to the total depth of material affected by the bombardment, and is in fair agreement with a theoretical estimate of the range of the helium ions. Electron diffraction data shows that the effect of bombardment is to convert the surface layer to a quasi-stable amorphous form, which may be partially restored to the crystalline state upon annealing. The amorphous material is soluble in aqueous hydrofluoric acid, and its thickness has been measured by an interferometric technique. It is found to be approximately one tenth of the total thickness of silicon affected, depending on the bombardment dosage. The effect of annealing upon the diode characteristics of point contact diodes made with bombarded silicon have been studied. The general result is that the characteristics tend to revert to those of an unbombarded diode, as expected from the electron diffraction data. These experiments are discussed in terms of radiation damage theory.