I. Physika1isches lnstitut, Universitiit Stuttgart. W·7rxxl Stuttgart SO, Fed. Rep. of Gennany In oxygen doped germanium we find by phonon spectroscopy with superconducting tunnelling junctions [II (Fig.I) a series of states from 0.18 meV up to 4.08 meV above the ground state (Fig.3). The sequence can be approximated by a free rotation of the interstitial oxygen atom (

One-phonon ionization of donors in germanium via intervalley scattering by slow TA phonons at the X point of the Brillouin zone is demonstrated by the stress dependence of phonon-induced conductivity. Superconducting aluminum junctions are used as tunable quasimonochromatic phonon sources up to these extreme frequencies. The ionization probability is found to be increased when donor levels associated with stress-shifted higher valleys of the conduction band cross the lowest valley(s). PACS numbers: 63.20.Kr, 63.20.Mt, 71.55.-iThe fast trapping of free carriers into shallow states at low temperatures in lightly doped semiconductors is an old 1,2 but as yet open problem. The capture into the impurity ground state (GS) by one-phonon emission is possible in principle for binding energies below the Debye limit, although the probability is in general small because of the difficulty to conserve momentum. So, other processes have been invoked to explain the short carrier lifetimes: the cascading down of the carrier through excited states by low-frequency phonon emission, 2 " 4 a onestep decay by two-phonon emission, 5 and trapping by neutral donors or acceptors in D ~,A + intermediate states. 6 ' 7The process inverse to relaxation, the excitation of carriers from bound states by nonthermal phonons, has become accessible by phonon-induced electrical conductivity (PIC), a new spectroscopic tool, 8 using superconducting Al junctions as tunable phonon sources. At bias U the phonon spectrum emitted by the junction has a sharp limit at e[/~2AAi (2 AAI = superconductor gap) which by modulation (differentiation) techniques becomes effectively a quasimonochromatic line. Here we show that, in the special case of donors in Ge, one-phonon excitation is possible by the large wave-vector transfer via intervalley scattering of the carriers in the excitation process.The experimental technique has been described earlier. 9,10 A thin Al junction is evaporated onto one of the 15x5-mm 2 faces of the 2-mm-thick samples doped with = (2-5)x 10 14 cm ~3 of Sb, P, or As. The face opposite to the junction is irradiated with visible light to create free carriers and thus a finite sample conductivity.The PIC signals obtained vary with stress. This can be understood in terms of the theoretical stress dependence of the shallow donor states and the corresponding transition probabilities. The eigenvalues and eigenfunctions are given by several authors. 11 " 14 The unstressed GS consists of a singlet [\s(A { ), n =0] and a triplet [\s(T\),/i -1,2,3] separated by the valley-orbit splitting 4A (Fig. 1). With increasing stress the ionization energy initially falls and then remains constant. The eigenfunctions V n are represented as a superposition of Bloch functions ^' } (r) of the four conduction-band (CB) minima at k (/) : **(r) = £ ¥" G) -X a ( n i) Table I). (p)Fi i) (rM i) (r). Here /=l-4 corresponds to [111], [TlT], [lTT], and [III], respectively, F" (T) are envelope functions and aJt'Hp) are stress-dependent coefficients (see Phonon-induced transit...

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...

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I. Physika1isches lnstitut, Universitiit Stuttgart. W·7rxxl Stuttgart SO, Fed. Rep. of Gennany In oxygen doped germanium we find by phonon spectroscopy with superconducting tunnelling junctions [II (Fig.I) a series of states from 0.18 meV up to 4.08 meV above the ground state (Fig.3). The sequence can be approximated by a free rotation of the interstitial oxygen atom (