Ulrich Goesele

Diffusion Engineering by Carbon in Silicon

The possibility to suppress undesirable diffusion of the base dopant boron in siliconbased bipolar transistor structures by the incorporation of a high concentration of carbon has lead to renewed interest in the behavior of carbon in crystalline silicon. The present paper will review essential features of carbon in silicon including solubility, diffusion mechanisms and precipitation behavior. Based on this information the possibilities to use carbon to influence diffusion of dopants in silicon by the introduction of non-equilibrium concentrations of intrinsic point defects will be discussed as well as the reason for the relatively high resilience against carbon precipitation. Interactions between carbon and oxygen will be mentioned, especially in the context of an as yet unexplained fast out-diffusion of carbon close to the surface.

Controlled in situ boron doping of short silicon nanowires grown by molecular beam epitaxy

Epitaxial silicon nanowires (NWs) of short heights (∼280nm) on Si ⟨111⟩ substrate were grown and doped in situ with boron on a concentration range of 1015–1019cm−3 by coevaporation of atomic Si and B by molecular beam epitaxy. Transmission electron microscopy revealed a single-crystalline structure of the NWs. Electrical measurements of the individual NWs confirmed the doping. However, the low doped (1015cm−3) and medium doped (3×1016 and 1×1017cm−3) NWs were heavily depleted by the surface states while the high doped (1018 and 1019cm−3) ones showed volume conductivities expected for the corresponding intended doping levels.

Three-dimensional structuring of silicon for photonic crystals with complete photonic bandgaps

The fabrication of three-dimensional photonic bandgap materials and the controlled incorporation of point, linear and planar defects into these crystals is a major challenge in materials research today. We show in this report that these purposes can be achieved by photoelectrochemical etching of lithographically prestructured silicon. Our advanced etching method allows the fabrication of three-dimensional photonic crystals with simple cubic symmetry. The performed calculations suggest complete bandgaps of 5% for the realized bulk structures. By lithographic prestructuring vertical line and planar defects can be induced, whereas horizontal planar defects can be created during the etching step. By combining both structuring techniques point defects can be fabricated.

Characterization of simple cubic 3D photonic crystals with complete photonic bandgaps

The major challenge in todays photonic crystal fabrication is the experimental realization of perfect, disorder-free structures. Macroporous silicon etching is a versatile technique for the manufacturing of large-scale well-ordered porous materials and three-dimensional photonic crystals. We investigate the degree of local disorder by scanning electron microscopy and a subsequent image processing, as well as the homogeneity of our large area crystals by an optical two-dimensional mapping. The observed disorder is related to the applied fabrication parameters. The deduced dependencies help to avoid disorder and to optimize our structures.

Transfer and handling of thin semiconductor materials by a combination of wafer bonding and controlled crack propagation

Direct waferbonding is an appropriate technology to join two or more wafers of the same or of different materials. Waferbonding can be used to stiffen thin wafers during fabrication. However, conventional fabrication processes lead to an increase of the bond strength, which inhibits the required de-bonding. The propagation of cracks, which is based on a subcritical crack growth in the bonded interface, was used to cleave the bonded wafers. The subcritical crack growth is limited to the bonded interface, since the adjacent bulk semiconductor materials are inherently resistant to subcritical crack growth. The process allows the separation of Si-Si and Si-GaAs wafers after annealing. Wafer-bonded SOI wafers can also be separated with this technology even if they were annealed at 1100oC. The first examples for wafer stiffening during fabrication and wafer transfer using the developed approach will be presented.

Investigation of Blistering Phenomena in Hydrogen-Implanted GaN and AlN for Thin Film Layer Transfer Applications

High dose hydrogen implantation-induced blistering phenomena in GaN and AlN have been investigated for potential thin film layer transfer applications. GaN and AlN were implanted with 100 keV H 2 + ions with various ion doses in the range of 5´10 16 to 2.5´10 17 cm −2 . After implantation the samples were annealed at higher temperatures up to 800°C in order to observe the formation of surface blisters. In the case of GaN only those samples that were implanted with a dose of 1.3´10 17 cm −2 or higher showed surface blistering after post-implantation annealing. For AlN the samples those were implanted with a dose of 1.0´10 17 or 1.5´10 17 cm −2 displayed surface blistering after post-implantation annealing. Cross-sectional transmission electron microscopy was utilized to observe the microscopic defects that eventually cause surface blistering. Large area microcracks, as revealed in the XTEM images, were clearly observed in the case of both GaN and AlN after post-implantation annealing. A comparison of the hydrogen implantation-induced blistering in GaN and AlN has also been presented.

Comparison of SiGe Virtual Substrates for the Fabrication of Strained Silicon-On- Insulator (sSOI) Using Wafer Bonding and Layer Transfer

Different methods of preparing sSOI wafers have been analyzed. The initial virtual substrate wafers are characterized by a 17 20 nm thick strained silicon layer grown either on a thick relaxed SiGe layer on a graded buffer or on a thin SiGe buffer relaxed by He implantation. Bonding and layer transfer experiments using different oxide layers proved that strained silicon layers are completely transferred if designed PE-CVD oxide layers were used. For both types of virtual substrates the oxide layers are deposited on top of the strained silicon and bonded to non-oxidized (blank) silicon wafers. A perfect layers transfer is obtained for virtual substrates having thick SiGe buffer layers (type A) even at 350°C, while annealing at 450 °C is required for substrates with thin SiGe buffer layers (type B). The lower annealing temperature for substrates of type A is caused by the lower activation energy for blistering. The hydrogen implantation is here into the SiGe. For type B substrates the hydrogen implantation is into the underlying Si requiring a higher temperature for layer splitting (higher activation energy for Si).

Director fields of nematic liquid crystals in tunable photonic crystals

Variations of the refractive index can be utilized in order to shift the stop band in photonic crystals. Here, two- and three-dimensional structures made of macroporous silicon were filled with liquid crystals. Optical investigations in the infrared wavelength range indicate temperature-induced spectral shifts of the edges of stopbands. In addition, the defect modes corresponding to microcavities within the periodic structure can be thermally controlled. Investigations of the director field within the pores by means of 2H-NMR and confocal microscopy indicate that both parallel and escaped radial director fields can appear, depending on the surface treatment of the substrates. In cylindrical pores with a periodic modulation of the pore diameter, the escaped radial director field is modified thereby showing a regular array of disclination rings.

Highly luminescent Si quantum dots: new ways for size, position, and density control

Phase separation and thermal crystallization of SiO/SiO2 superlattices result in ordered arranged silicon nanocrystals. The preparation method enables independent control of particle size as well as of particle density and spatial position by using a constant stoichiometry of the layers. Infrared absorption and photoluminescence spectra are measured as a function of annealing temperature to study the phase separation process. Three photoluminescence emission bands are observed. A band centered at 560 nm is found in as-prepared samples and vanishes for annealing above 700oC. A second band around 760 nm to 890 nm is detected for annealing temperatures above 500oC. The superlattices show a strong luminescence and a size dependent blue shift in the visible and near-infrared region after crystallization for temperatures above 900oC. The origin of the different photoluminescence bands at different phase separation stages of ultra thin SiOx layers are discussed based on transmission electron microscopy investigations and on correlations seen in photoluminescence spectra and infrared absorption. In addition, we report the PECVD preparation of amorphous SiO/SiO2 superlattices which show a similar size dependent luminescence after crystallization.

Homoepitaxial Growth of Vertical Si Nanowires on Si(100) Substrate using Anodic Aluminum Oxide Template

Homo-epitaxial growth of Si nanowires on Si (100) substrate was accomplished using a combination of anodic aluminum oxide (AAO) template and Vapor-Liquid-Solid (VLS) growth. We prepared two types of AAO template for epitaxial growth of Si nanowires. We observed vertically grown epitaxial Si (100) nanowires in the AAO template. In addition, after leaving filled pores, Si nanowires changed their growth direction from [100] to . This result shows that the walls of the pores forced the growth direction of Si nanowires parallel to the direction of the pores, and after complete filling, the growth direction changes to that of the Si nanowires on a bare Si substrate.