Browsing by Author "Helberg, Hans Wilhelm"

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Open Access
AC conductivity of deformed germanium single crystals at T = 4.2 K
(1986) Dressel, Martin; Helberg, Hans Wilhelm
Dislocations introduced in semiconductor single crystals generate deep electronic states in the gap which form one-dimensional energy bands along the dislocation lines. Because dangling bonds can be charged by trapping electrons or holes, quasi-metallic conduction along the dislocations is expected. Plastic deformation produces a network of dislocations with only small unconnected segments of ideal behaviour. Therefore the conductivity of the dislocation core can only be obtained by special dc measurements or from high frequency conductivity.
Open Access
Microwave conductivity investigations in plastically deformed silicon
(1990) Brohl, Michael; Dressel, Martin; Helberg, Hans Wilhelm; Alexander, Helmut
We present new results on microwave continuous-wave (c.w.) conductivity investigations of plastically deformed floating-zone (FZ) silicon single crystals. N-type samples with various doping and deformation levels were investigated by the cavity perturbation technique. The anisotropy and absolute values turn out to be very sensitive to experimental conditions but on the whole the one-dimensional nature of dislocation conduction is confirmed. The C.W. anistropy follows the respective dislocation structure anisotropy closely. For clear results, care has to be taken of the number of deep point-defect centres also introduced by deformation. Furthermore the existence of a low-temperature conducting state is shown for a Czchrochalski (CZ) sample annealed at 650°C containing rod-like defects.
Open Access
Microwave dielectric properties of disubstituted diacetylene DNP single crystals
(1991) Dressel, Martin; Helberg, Hans Wilhelm
The dielectric properties of dlsubstltuted diacetylene DNP single crystals were measured by microwaves In order to study the ferroelectric phase transition as a function of solid state polymerization. The high frequency properties of the permittivity were studied In DNP monomer and also in DNP crystals that were partially polymerized at a temperature of 130°C for different periods of time. At Tc = 46.5 K the dielectric constant of the monomer shows a sharp peak. No change of this behaviour was found by cooling down for several times. The peak of ε' Is getting flat and shifted to lower temperatures with advanced duration of thermal polymerization. The monomer crystals show a small peak of dielectric losses ε"≈ 1at Tc. At the beginning of polymerization the peak height Is getting larger and dielectric losses were measured down to 4 K. For polymerizing more than 3 h the maximum is reduced and flat; no losses at all are found after full polymerization.
Open Access
Microwave hall mobility and conductivity in crystals of various (BEDT-TTF) radical salts
(1991) Dressel, Martin; Helberg, Hans Wilhelm; Schweitzer, Dieter
We report investigations of the microwave transport properties carried out on the organic conductors and superconductors of (BEDT-TTF)-salts: α-, αt- and β-(BEDT-TTF)2I3, α-(BEDT-TTF)3(NO3)2 and x-(BEDT-TTF)2Cu(NCS)2. The anisotropy and temperature dependence (300 K to 4 K) of the high-frequency conductivity was measured in a microwave cavity at 10.3 GHz. There is low anisotropy in the high conducting plane of the quasi two-dimensional crystals of the (BEDT-TTF)-family, e.g. the (a,b)-plane in β-(BEDT-TTF)2I3; perpendicular to this the microwave conductivity is one order of magnitude lower. At room temperature the microwave Hall mobility at 9.5 GHz of the different phases of (BEDT-TTF)2I3 and (BEDT-TTF)3(NO3)2 is 100 to 200 cm2/Vs in the high conducting planes of the crystals.
Open Access
Temperature and frequency dependence of microwave conductivity of isotropic reticulate doped polymers
(1990) Ulanski, Jacek; Lüpke, Gunter; Dressel, Martin; Helberg, Hans Wilhelm
Frequency dependence of conductivity for reticulate doped systems is observed at room temperature only around 1 GHz and even so it is relatively weak. The temperature dependence of conductivity is characteristic of the CT complex used and not of the polymer matrix. For the system containing TTF-TCNQ, for which the d.c. conductivity has a maximum at c. 230 K (i.e. metal-like behaviour at higher temperatures), this maximum becomes more pronounced and shifts towards lower temperatures with increasing frequency in the GHz range. The temperature dependence of the microwave conductivity is weaker than that of the d.c. conductivity. Such behaviour can be described by a modified Maxwell-Wagner model if an appropriate shape factor for the conducting inclusions is introduced, and if a relatively high conductivity of the continuous phase is assumed. We conclude that charge-carrier transport in reticulate doped polymers is not controlled by insulating barriers. The disorder within microcrystals plays a fundamental role, while the CT complex crystalline network is continuous in spite of very low concentration.