The 3D-and 2D-behaviour of wide parabolic PbTe single quantum wells, which consist of PbTe p-n-p-structures, are studied theoretically and experimentally. A simple model combines the 2D-subband levels and the 3D-Landau levels in order to calculate the density of states in a magnetic field perpendicular to the 2D plane. It is shown that at a channel width of about 300nm on can expect to observe 3D-and 2D-behaviour at the same time. Magnetotransport experiments in selectively contacted Hall bar samples are performed at temperatures down to T = 50 mK and at magnetic fields up to B = 17 T.

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Single period nipis (i.e. pnp-structures) turn out to be interesting elements for the design of arbitrary nipi-potentials. In addition, the possibility of applying selective contacts to n-type layers allows the use of p-type buffer layers on either side of the pnp-structure in order to screen band bending effects both at the buffer interface and at the surface. From transient photoconductivity experiments typical lifetimes result up to the millisecond time regime (i.e. about iO to iO times the bulk lifetimes). The amplitude of the photoconductive response cannot be attributed to simply an equivalent change of the electron density in the selectively contacted n-layer. This fact is confirmed by Photo Hall experiments. The enhancement of the response can be explained in terms of the special characteristics of the narrow gap material PbTe. Additional tuning of the electron densities via background illumination is performed in order to control the lifetimes. The resulting trends are well understood if the modification of the nipi-potential caused by excess carriers is taken into account. In a simple model the role of the background radiation is explained..

The surface topography and structure of low-pressure chemical vapor deposited silicon films on thermal oxide grown on (100) silicon substrates have been investigated after different processing steps. Atomic force microscopy topographic measurements are performed on undoped as-grown samples and after ex situ phosphorous doping from a POCl3 source. In addition, topographies of the doped films are obtained using constant current scanning tunneling microscopy on hydrofluoric acid passivated surfaces under high vacuum. As a result we have found that surface topography and roughness are mainly determined by the deposition process. Roughness of films deposited at 620 °C is related to the grain structure represented by hillocks with typical lateral dimensions between 50 and 150 nm in the images. Doping by high temperature diffusion and subsequent annealing causes a complete recrystallisation of the film, leading to typical lateral grain sizes between 200 and 600 nm. However, the surface topography of the doped films still remains determined by the hillocks formed by the deposition process. Values of the surface roughness are between 7 and 14 nm depending on the process step.

The basic properties of nipi structures are discussed in terms of their potential interest for photon detection. A comparison of different semiconductors (GaAs, InSb, InAs, PbTe) for the realization of either short-period nipis with strong overlap of the e-h wavefunctions or long-period nipis without overlap is presented. The advantages of long-period nipis are demonstrated with PbTe p-n-p and p-i-n-i-p structures. Barrier leakage is shown to be of substantial importance for gaining the detector response in selectively contacted structures which are based on narrow-gap semiconductors.