Dependence of the Phonoionization of A⁺-States in Si on Uniaxial Pressure

Abstract

By the new technique of phonon induced conductance we have investigated the dependence Ort pressure of the phonoionization response of shallow A+ -states in Si with superconducting AI-junctions as monochromatic phonon generators. In the case of B+ and AI+ we obtain a much more complicated behaviour than previously found for 8+ with FIR-photoconductivity which may be connected with differences in coupling for short wavelength phonons. In the case of In+ on the other hand a shift to lower energies is observed for uniaxial pressure in [100]-direction whereas for pressure In [111]-direction only the signal intensity varies but not the position of the threshold.It is well established from thresholds in FIR photoconductivity response that shallow impurities in semiconductors at low enough temperatures can bind a second carrier to form a metastable state analogous to the H--Ion. It has been shown recently[1] that phonon spectroscopy with superconducting AI-junctions gives the same threshold energies of phonoionlzation e.g. In the case of B+ or P-In SI. however. with much steeper and narrower signal forms. This is due to the fact that the wavelength of the Interacting phonons is comparable to the extent of the A+-state. Increasing phonon energy means shortening of the wavelength and thus a reduction of the interaction. Phonon spectroscopy turned out to be a rather sensitive and relatively simple means to determine binding energies of A+-states. In the case of acceptors in Si it has been found by this technique [2], that there Is a shallowto-deep instability of the binding energies. with about 1.8 meV for 8+ • AI+. and Ga+ and 5.9 meV for In+ . No indication for a split multiplet character from hole-hole-coupling has been found in these experiments as might be inferred from the interpretation of the luminescence multiplet of acceptor-bound excitons [3.4]. However. since the working temperature with superconducting AI-junctions. namely 1 K. Is rather low it is only for favourable values of the splitting to be registered In such phonoionlzatlon experiments. In principle information on zerofleld splitting 8 might be .expected from the nonlinear dependence of threshold energy on uniaxial pressure. since the levels split by strain e as A somewhat different interpretation for large pressure has been given qualitatively in [6], where the shift of the threshold to smaller energies under uniaxial pressure has been measured with FIR-photoconductivity: beyond a critical value of pressure the threshold does not move any more since It is pinned to the heavy hole valence band together with the AO -binding energy. No analysis of the change of threshold In the low pressure regime has been given.Here we present analogous phonoconductivlty measurements for 8+ and also for the deeper acceptors AI+ and In+. Fig. 1 shows the complicated stress dependence for 8+ : Instead of a gradual decrease of the threshold energy as found with photoconductivity [6] there is first a flattening of the threshold (Fig. 1a) and then a sharp line emerging at lo...