Johannes Konle

Self-assembled Ge-islands for photovoltaic applications

Abstract We have fabricated silicon solar cells with embedded germanium layers to form three-dimensional islands in the Stranski–Krastanov growth mode. The additional Ge-layers increase the infrared absorption in the base of the cell to achieve higher overall photocurrent and overcome the loss in open circuit voltage of the heterostructure. In an UHV-MBE chamber up to 75 layers of germanium, each about 8 monolayers thick, separated by Si-spacer layers (9– 16 nm ) were grown on each other using standard 10 Ω cm p-type Si-substrates. The density of islands in the layers was increased by the use of antimony as surfactant, thus densities >10 11 cm −2 were realized. The islands were covered by a 200 nm thick Si-layer (n-type) on top which is used as emitter of the cell. Photoluminescence measurements, AFM and TEM-microscopy were used to characterize the growth of Ge-islands under various growth conditions and post-thermal treatment. Photocurrent measurements exhibit a higher response of the fabricated solar cells in the infrared regime compared to standard Si-cells.

Nano-sized pore formation in p-type silicon for automotive applications

Self-organized electrochemical etching of p-type silicon (Si) has been used to study random micropore formation which produces porous Si structures with nanometer well thickness. The density of micropores, i.e. the porosity, can be varied in a wide range by choice of the substrate doping level. Surface enlargement up to a factor of 10000 and more can be easily achieved by choice of appropriate conditions in the anodic etch process. In addition, we demonstrate deep anodic etching (DAE) of a pinhole array in Si by lithographic pre-patterning and subsequent etch using potassium hydroxide (KOH). The Si wafer is then anodically etched which produces deep channels, thus creating porous structures with enlarged surface. Such channels have large application potential as a carrier structure for the catalyst in micro-steam fuel reformers in compact fuel cells used as auxiliary power units for the on-board electronics in vehicles or can be used for fuel injection or fuel heating systems.

Mid-infrared silicon/germanium focal plane detector arrays

Highly p-doped silicon/silicon-germanium (Si/SiGe) quantum well (QW) structures have been grown by molecular beam epitaxy (MBE) on <100> Si substrates for mid-infrared (3(mu) - 5(mu) ; 8(mu) - 12(mu) ) detection. These detectors operate at 77 K and are based on hetero-internal photoemission (HIP) of holes from a highly p-doped SiGe quantum well into a undoped silicon layer. They are grown on weakly-doped (50(Omega) cm), double-sided polished Si substrate on which the highly p-doped Si1-xGex wells (p++ approximately 5 1020cm-3) are deposited. The Ge content, doping level and the well width determine the operating wavelength. The samples have been characterized by secondary ion mass spectroscopy (SIMS), X-ray diffraction, and absorption measurements. Single mesa detectors as well as large area focal plane arrays (FPA) with 256 X 256 pixels have been fabricated from these samples using standard Si integrated processing techniques. Dark- and photocurrents have been measured at 77 K and up to 225 K. Broad photoresponse curves with peak external quantum efficiencies up to 0.5% have been measured at 77 K and 4

Structure and doping optimization of SiGe heterojunction internal photoemission detectors for mid-infrared applications

u. Detectivities in excess of 2 1010 cm(root)Hz/W have been obtained. We demonstrate that by varying the well design of the SiGe HIP detectors by means of Ge-content, doping and Ge gradients in double and stacked multi-wells the photoresponse peak and the shape of the spectrum can be tuned over a wide wavelength range. The fabricated 256 X 256 Si/SiGe FPA array has a pixel size of 21 X 21(mu) 2 and a pitch of 24(mu) . The mesa detectors diameters range from 1 mm down to 125(mu) . The epitaxial versatility and the compatibility of the Si/SiGe array fabrication compared to the existing silicide fabrication process demonstrates the advantages of the SiGe system in comparison over commercially used silicide detectors.

Fabrication of Si trench solar cells with improved radiation hardness for space applications

We present full-scale calculations of highly p-type doped siGe heterojunction internal photoemission (Hip) detectors that operate in the mid-infrared (3- to 5-?m) range of wavelength. We explore the effects of including undoped spacer layers within the highly doped siGe wells, for which a systematic body of experimental data is available to us, and which demonstrates a substantial reduction in the dark current produced by these devices. We model the doping explicitly by means of a screened Coulomb potential, leading to a full description of the resultant impurity band, and compare our calculated optical line shapes with recent state-of-the-art molecular-beam epitaxy experiments. The variation of the optical response with well width, germanium concentration, spacer width and position, and doping concentration are all considered.

Freely suspended actin cortex models on arrays of microfabricated pillars

Mono-crystalline silicon (Si) space solar cells were fabricated with deep holes (0.5-0.7 /spl times/ substrate thickness) etched into a standard p-doped Si substrate by a high etch rate plasma apparatus. After the etch an n/sup +/ emitter diffusion is done inside and outside of the holes followed by cleaning and thermal passivation. The cells have been electron irradiated by a 1 MeV radiation with different doses up to 3 10/sup 15/ cm/sup -2/ and the cell efficiency /spl eta/, the short circuit current I/sub sc/ and the open circuit voltage V/sub oc/ have been measured before and after radiation and compared to a Si reference cell. The fabrication process of the trench cell has been kept fully compatible with standard Si cell processing.