AND ITS APPLICATION FOR PHONON SPECTROSCOPY

Thermal emptying of traps is mostly a cooperative action of many phonons. A new technique makes it possible to study trap depths down to 0.01 eV. Such traps can be emptied by one phonon and are able to act as detectors for single phonons. If a limited number of single phonons is generated in a He-cooled crystal and its temperature remains unchanged, the phonons do not interact with each other and have long free paths being able to reach the traps distributed in the crystal and to release charge carriers. Single phonons can be generated e.g. by neutron collisions, by bombardment of the crystal surface with hot gas atoms, etc. The range of phonons can be determined. The use of traps with different depths enables a phonon spectroscopy and a detection of the phonon range. Experiments confirmed this idea. Bombardment of a He-cooled phosphor crystal with room temperature Ne- or He-atoms produces a bright light spike although the temperature rise is less than 0.001 K! In ZnO the 0.035 eV LA-phonons have at 5 K a penetration depth about 0.65 mm emptying inside this distance traps with ET<0.035 eV and leaving unaffected the deeper traps. Further applications and improve­ ments of the method are in progress.