Abstract-The optical saturation of nonresonant photoionization of shallow acceptors m p-Ge was investigated at low temperatures using a high-power FIR molecular laser. The experimental results were analyzed in terms of a rate equation model. Very small saturation intensities in the order of 10 mW,icm-' and long free hole recombination times in the range of 10 ps were observed.

The photothermal spectra of the shallow acceptor boron in high purity p-Ge has been investigated for various magnetic field strengths at low temperatures. A large number of lines observed at frequencies above the binding energy of boron are attributed to optical excitation of quasi-bound Coulomb states, which arise in a magnetic field below each Landau level. By comparison with calculated Landau level energies the final states of the observed transitions can be unambigously identified.Shallow impurities in semiconductors represent hydrogen-like systems for which the high magnetic field case can be exprimentally realized by standard laboratory magnets due to the large effective Bohr radius. Therefore the optical excitation spectra of shallow impurities in a uniform magnetic field have been extensively studied by far-infrared (FIR) photothermal ionization and photoionization spectroscopy [1]. The hydrogen atom in a magnetic field is a fundamental problem of quantum theory, which cannot be solved in closed form because of different symmetries of the Coulomb potential and the uniform magnetic field. The energy spectrum of this system consists besides of continuum states, corresponding to Landau levels with quantum number n, of bound (stable) or quasi-bound (metastable) Coulomb states depending on their angular momentum quantum number m [2,3]. Due to the relation m < n, the Coulomb states connected with the higher Landau levels n > 1 and m < n -1 are degenerate with the continuum of lower Landau levels with the same m quantum number, to which they are coupled by the Coulomb potential. Thus autoionization transitions may occur and, therefore, these states are called quasi-bound or metastable. In contrast, the Coulomb states connected with the lowest Landau level n = 0 and m < 0 and those with n = m, which are not degenerate with a continuum, are bound or stable.In a previous investigation of shallow donors in nGaAs several structures in FIR photoconductivity spectra could be attributed to optical transitions from the donor ground state to those metastable states [4]. The identification of the final states was based on the fact that pairs of states with m = 4-1 separated by the cyclotron resonance energy hwc are optically accessible in Faraday configuration from the donor ground state using unpolarized light. In the fourfold degenerate valence band of tetrahedrally coordinated semiconductors the situation is more complex because the Landau levels are irregularly spaced [5]. Zverev et al. analyzed p-Ge in detail by investigating magneto-photoconductivity upon monochromatic irradiation [6] and optical phonon emission induced resonant recombination of holes excited by broad band infrared illumination [7]. For each degenerate Landau level they observed a single resonance attributed to quasi-bound acceptor states.In the present paper we report on FIR photoconductivity measurements on boron doped p-Ge carried out by Fourier-spectroscopy. In high purity samples a whole series of spectral lines has been observed and identified...

Abstract. We present experimental investigations on the spatio-temporal nonlinear current flow in the post-breakdown regime of p-germanium at liquid-helium temperatures. The basic nonlinear effects are characterized in terms of the underlying semiconductor physics, taking into account the influence of different experimental parameters. 05.45. + b, 72.20.Ht, 72.70. + m It is well known that a large number of physical and nonphysical systems show spontaneous formation of spatial or temporal structures as a result of instability. Close to such instability points the dynamics of the system and its emerging structures are determined by a set of, in general, a few collective variables, often called order parameters. The underlying synergetic approach introduced by Haken [1] can explain the unexpected order and coherence arising on the macroscopic scale, regardless of the large number of competing physical forces interacting on the microscopic scale. Motivation for the intensive study of cooperative dynamics and pattern formation phenomena during the past few years derives from an increasing appreciation of the remarkable diversity of behavior encountered in nonlinear systems and of universal features shared by entire classes of similar nonlinear dynamic processes. PACS:So far, it appears that the subject of such complex nonlinear behavior is dominated by theoretical investigations and computer studies, whereas experimental measurements on real physical systems represent the minority. Among the various objects which can be studied experimentally, solid-state turbulence in semiconductors appears particularly interesting [2]. Nonlinear current transport behavior during lowtemperature avalanche breakdown of extrinsic germanium comprises the self-sustained development of spatio-temporal dissipative structures in the formerly homogeneous semiconductor [3]. This kind of nonequilibrium phase transition between different conducting states results from the autocatalytic nature of impurity impact ionization generating mobile charge carriers [4]. The simple and direct experimental accessibility via advanced measurement techniques favors semiconductors as a nearly ideal study object for complex nonlinear dynamics compared to other physical systems. Further representing a convenient model reaction-diffusion system that exhibits distinct universal features, the present semiconductor system may acquire general significance for many synergetic systems in nature. Finally, in view of the rapidly growing application of semiconductor technologies, the understanding, control, and possible exploitation of sources of instability in these systems have considerable practical importance. This paper gives a classification of our experimental investigations on the spatio-temporal nonlinear current flow in the post-breakdown regime of p-germanium at liquid-helium temperatures. Section 1 briefly outlines an example of a set of nonlinear current instabilities obtained from our semiconductor system. Section 2 reports the characterization of the basic...

The far-infrared magneto-photoconductivity due to optical transitions from the acceptor ground state into quasi-bound Coulomb states in p-Ge has been investigated at low temperatures as a function of intensity applying a high-power cw molecular laser. For intensities above about 1 mW/cm 2 the photoconductive signal shows a square root dependence on intensity, which is attributed to nonlinear free carrier capture in the low compensated material. The experimental results are analyzed in terms of a rate equation model yielding the kinetic parameters of the carrier generation-and recombination process involved.

The photothermal spectrum of shallow acceptors in p-Ge has been investigated at various magnetic field strengths up to 5.6 T at a temperature of 7.5 K by FIR-Fourier-spectroscopy. From the observed Zeeman splittings of the excited states of the boron acceptor the coefficients of the linear and quadratic field dependence have been evaluated and g-factors of the D-, C-and the G-transitions have been determined based on a standard group theoretical approach.