Showing papers by "Oliver Ambacher published in 2005"

Effects of solvent and annealing on the improved performance of solar cells based on poly(3-hexylthiophene): Fullerene

Abstract: Polymer solar cells based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) were fabricated using two different solvents. P3HT:PCBM films casted from chlorobenzene solution absorb more red light than the films casted from chloroform solution. After thermal annealing, the films casted from chloroform show higher absorption than the films casted from chlorobenzene. Solar cells made from P3HT:PCBM chlorobenzene solution show no change in the white light power conversion efficiency (2.2%) after annealing. Solar cells processed from P3HT:PCBM chloroform solution show a white light power conversion efficiency of 1.5% without thermal annealing and 3.4% after the thermal annealing. The stated efficiencies are not corrected for the spectral mismatch.

The role of Si as surfactant and donor in molecular-beam epitaxy of AlN

Abstract: The growth of Si-doped AlN(0001) thin films on Al2O3(0001) substrates by plasma-induced molecular-beam epitaxy is reported. We have found that Si positively affects the epitaxy being an effective surfactant for AlN growth with a remarkable impact on the crystal quality. It was proven that the characteristic surface reconstruction sequences frequently related to the Al adatoms are obviously Si induced on AlN(0001) surfaces. It was also observed that heavy doping conditions result in volume segregation of Si on the threading dislocation network and in the formation of an amorphous (AlO)(SiO)N cap layer caused by surface oxidation of the accumulated Al and segregated Si. The electron affinity was measured to be smaller than 0.5eV on the clean AlN surface after removing of the cap layer using Ar+ sputtering.

Raman studies of Ge-promoted stress modulation in 3C-SiC grown on Si(111)

Abstract: We present a study of the stress state in cubic silicon carbide (3C–SiC) thin films (120 and 300 nm) grown by solid-source molecular-beam epitaxy (SSMBE) on Si(111) substrates modified by the deposition of germanium prior to the carbonization of Si. μ-Raman measurements were used to determine the residual stress existing in the 3C–SiC layers. The stress is found to decrease linearly with increasing Ge quantity but with different strength depending on the 3C–SiC thickness deposited after the introduction of Ge. Based on secondary ions mass spectroscopy (SIMS) and transmission electron microscopy (TEM) analyses it is suggested that the Ge introduced prior to the carbonization step remains in the near-interface region and reduces the Si outdiffusion, which further reduces the stress state of the 3C–SiC layers.

Low Temperature Chemical Vapor Deposition of 3C-SiC on Si Substrates

Abstract: In the present work an UHVCVD method was developed which allows the epitaxial growth of 3C-SiC on Si substrates at temperatures below 1000°C. The developed method enable the growth of low stress or nearly stress free single crystalline 3C-SiC layers on Si. The influence of hydrogen on the growth process are be discussed. The structural properties of the 3C-SiC(100) layers were studied with reflection high-energy diffraction, atomic force microscopy, X-ray diffraction and the layer thickness were measured by reflectometry as well as visible ellipsometry. The tensile strain reduction at optimized growth temperature, Si/C ratio in the gas phase and deposition rate are demonstrated by the observation of freestanding SiC cantilevers.

Effect of nanoscale surface morphology on the phase stability of 3C-AlN films on Si(111)

Abstract: In this work, we report on the stabilization of 3C-AlN polytype by molecular-beam epitaxy (MBE) on 3C-SiC∕Si(111) pseudosubstrates. The main purpose of the present studies is to analyze the mechanisms forcing the epitaxy of the 3C-AlN at typical MBE conditions. The forces driving the cubic polytype formation have been considered including supersaturation, macroscopic stress, interfacial energy, and interface morphology. We conclude that the growth of 3C-AlN phase can be stabilized on the “rough” 3C-SiC(111) surface by the polytype replication due to the lateral character of the nucleation and the reduction in the total energy of the film provided by periodical undulations of the template surface.

Processing of novel SiC and group III-nitride based micro- and nanomechanical devices

Abstract: Micromechanical 3C-SiC- and AlN-resonator beams with resonant frequencies between 20 kHz and 5 MHz have been realized The 200 nm thin epitaxial SiC and group III-nitrides layers were grown on silicon (100) and (111) oriented substrates, respectively Subsequent the beams were dry-etched by an electron cyclotron resonance (ECR) plasma technique This new developed plasma based process allows anisotropic as well as isotropic etching for all materials involved The created freestanding resonator bars have dimensions in the μm to sub-μm range The operation principle based on a magneto motive actuation and a thin conductive metal layer on top of the resonator realizes the detection We present a measurement setup for the electrical characterization of the bars and show coupled beams, to isolate the sensor signal from the excitation signal Furthermore, we realized beams with a separate read-out unit (capacitive) as well as resonator structures with separate actuation (piezoelectric) to solve the problems of the superimposition of actuation and detection

Momentum matrix element and conduction band nonparabolicity in wurtzite GaN

Abstract: Comparing the imaginary part of the dielectric function of wurtzite GaN measured by spectroscopic ellipsometry at room temperature with calculations, we found the values of Ep=(19.8±0.5)eV, m0*=(0.201±0.005)m0, and F=−0.89±0.05 for the momentum matrix element, electron effective mass at the conduction band edge, and parameter F describing the influence of remote bands, respectively. We also observed a remarkable nonparabolicity of the conduction band. The data obtained are consistent with A sets of valence band parameters reported earlier providing a parameterization of band structure of wurtzite GaN in the vicinity of the Γ point.

Surface conductivity of epitaxial InN

Abstract: The electron accumulation at the surface of oxidised InN layers has been investigated by resistance measurements and simultaneous sputtering by Ar+ ions under gracing incidence. The removal of InN decreases continuously the cross section of the conductive InN layer on the insulating substrate. From the measurements the depth profile of the free electron concentration was derived and an excess sheet carrier density at the surface of 2.2 × 1013 cm–2 was estimated. The agreement of the Auger oxygen depth profiles and the free electron concentration profile motivates the conclusion that oxygen is responsible for the high surface conductivity of InN layers exposed to air. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Resonant localized donor state above the conduction band minimum in InN

Abstract: We have studied the pressure dependence of Hall electron concentration and mobility in degenerated, not intentionally doped InN samples. The results obtained for a whole set of samples with electron concentrations from 5.4×1017cm−3 to 3.3×1018cm−3 consistently reveal the existence of a localized donor type state, resonant with the conduction band. Its energy position is estimated to be about 80–90 meV above the bottom of the conduction band. This donor state is not the only source of electrons in these not intentionally doped samples and can be entirely populated and hidden in samples with very high electron concentrations.

Polytype control and properties of AlN on silicon

Abstract: In this paper we report on the optical and structural characterisation of cubic polytype AlN thin films on 3C-SiC/Si(111) pseudo-substrates prepared by carbonisation. We have found that 3C-AlN phase can be stabilised on the “waved” 3C-SiC(111) surface by polytype replication. On the other hand, 2H-AlN was grown on atomically smooth surfaces. X-ray diffraction and transmission electron microscopy imply that the cubic AlN was grown along with hexagonal inclusions up to 20%. Spectroscopic ellipsometry in the infrared region show the typical phonon lines for the two polytypes. The appearance of the TO phonon at 646 cm–1 confirms the existence of 3C-AlN. In the ultraviolet region the critical points in the band structure were identified. For 3C- and 2H-AlN the first direct transition at the Γ-point was determined to be at 5.74 eV and 6.16 eV, respectively. No indirect transition at lower energies was detected. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Defect related absorption and emission in AlGaN solar-blind UV photodetectors

Abstract: Room temperature electro-optical and structural analysis have been applied to study the origin of the defect-related absorption and emission in solar-blind AlGaN UV-photodetectors grown by molecular beam epitaxy on c-plane sapphire and Si(111) substrates. Two resonance absorption peaks have been observed in the photocurrent spectra for detectors grown on sapphire and silicon substrates. Correlation of the peaks in the photocurrent and cathodo-luminescence spectra, as well as their resonance character indicates that the resonance features relate to the transitions “deep level band” – “very shallow donor” with a depth of the order of the thermal energy (25–80 meV). (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

α-SiC–β-SiC heteropolytype structures on Si (111)

Abstract: Nanoscale α-SiC(0001) on β-SiC(111) heterostructures on Si (111) substrates fabricated by ultralow-pressure chemical vapor deposition are demonstrated. The intentional formation of the α-β structure was achieved by adjusting the SiH4 to C2H4 flux ratios to carbon rich conditions and by controlling the diffusion length. The developed method allows the formation of heteropolytypic structures with a controlled thickness of the intermediate cubic β-SiC layer.

Effect of Ge incorporation on stoichiometric composition of 3C‐SiC thin films grown on Si(111) substrates

Abstract: We present a study on the effect of Ge, deposited on Si(111) substrates prior to the 3C-SiC growth, on the stoichiometric composition of 3C-SiC thin layers grown by SSMBE on Si (111) substrates with Ge amount range from 0 to 1 ML. Secondary Ion Mass Spectroscopy (SIMS) depth profile measurements were used to determine the Si/C intensity ratio present in the 3C-SiC layers and for comparison the Si and C ratios measured in the 3C-SiC layers were compared with 6H-SiC and 4H-SiC respectively. SIMS investigations revealed that the Si/C intensity ratio in the epitaxial layers is different from the Si/C ratio of the single crystals. The intensity ratio of the 4H-SiC and 6H-SiC samples is found to be 2.01, thus the ratio for the sample without Ge is found to be 2.33 and decreases slightly with the introduction of Ge to 2.1 for the sample with 1 ML of Ge. It is suggested that this variation is related to a change in the defect densities caused by the Ge incorporation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Infrared Gratings Based on SiC/Si-Heterostructures

Abstract: The fabrication process and the spectral properties of gratings for the infrared wavelength region on the basis of 3C-SiC layers grown by CVD on (100) oriented Si substrates are demonstrated. The formed 3C-SiC gratings on Si support two phonon polaritons as a function of the geometrical properties excited between 10.3 and 11.4 µm. They appear as a dip in the transmission spectrum. A third minimum in the transmission spectrum is caused by the substrate – grating interaction. The obtained resonances were polarization sensitive, i.e. they appeared only under TMpolarized illumination.

Ion beam synthesis of 3C–(Si1–xC1–y)Gex+y solid solutions

Abstract: In the present study cubic 3C-(Si 1-x C 1-y )Ge x+y solid solutions were created by using ion beam synthesis. 3C-SiC thin layers grown on on-axis Si (111) substrates by Molecular Beam Epitaxy were implanted with Ge in order to incorporate Ge atoms in the Silicon Carbide lattice. Two series of experiments were carried out. The implantation energy was chosen to be 140 keV and 200 keV and the implantation dose 1 × 10 17 cm -2 and 4.7 × 10 16 cm -2 respectively. The samples were annealed under rapid thermal annealing conditions in the temperature range between 800 °C and 1300 °C. X-ray diffraction measurements indicate an enlargement of the lattice constant. The observed higher absorption in the implanted layers could be a sign of a band gap reduction as a consequence of Ge incorporation.

Method for transferring a semiconductor body from a growth substrate to a support material

Abstract: A method for transferring a semiconductor body selected from the group consisting of a semiconductor layer, a semiconductor layer sequence or a semiconductor layer structure from a growth substrate to a support material. An interface between the growth substrate and the semiconductor body or a region in the vicinity of the interface is exposed to electromagnetic radiation through one of the semiconductor body and the growth substrate. A material at or in proximity to the interface is decomposed by absorption of the electromagnetic radiation in proximity to or at the interface so that the semiconductor body can be separated from the growth substrate. The semiconductor body is connected to the support material.

Computer Simulation of the Early Stages of Nano Scale SiC Growth on Si

Abstract: Solid source molecular beam epitaxy was applied to create silicon carbide nanoclusters on silicon. The island size distribution can be controlled by an appropriate substrate temperature, carbon fluxes and process times. Rate equation computer simulation was applied to simulate the experimental obtained nano scale nuclei properties.

Growth of 3C-(Si1-xC1-y)Gex+y Layers on 4H-SiC by Molecular Beam Epitaxy

Abstract: In the present work cubic 3C-(Si1-xC1-y)Gex+y solid solutions were grown at different^temperatures by molecular beam epitaxy on on-axis 4H-SiC (0001) substrates. Two different growth methods are compared in order to explore the optimal growth conditions for the incorporation of Ge into the SiC lattice during the low temperature epitaxy. For this reason simultaneous growth and migration enhanced epitaxy were used. The chemical composition of the grown layers were analyzed by energy dispersive x-ray methods during transmission electron microscopy investigations. It was found that the migration enhanced epitaxy is a more suitable technique for the formation of high quality (Si1-xC1-y)Gex+y solid solutions. Additionally, polytypes transition from 4H-SiC to 3C-SiC occurs during the growth independent of the applied growth technique.

Strain relaxation and void reduction in SiC on Si by Ge predeposition

Abstract: In this work, 120 nm cubic SiC layers have been grown on Si (111) by SSMBE, depositing 1ML of Ge at different temperatures before carbonization. In every case, SiC was epitaxially grown on Si (111) showing characteristic defects and more relaxation than a reference sample where Ge was not employed. Depending on the temperature of Ge predeposition, a reduction of voids or stacking faults was achieved. The residual strain depended on this temperature, as was confirmed by electron diffraction and infrared ellipsometry measurements.

Beta to alpha transition and defects on SiC on Si grown by CVD

Abstract: The features of α-SiC (0001) epitaxially grown on top of β-SiC(l11)/Si(111) is reported by means of transmission electron microscopy (TEM). Hexagonal and rhombohedral polytype nuclei, mainly 4H-SiC, appear after the growth of a fixed cubic SiC thickness which is related to the selected growth conditions: Si/C ratio and growth temperature. The defect structure of these multilayer systems (voids, planar defects, facets and polycrystalline top clusters) and the hexagonality of the SiC surface are determined and described.