H. Sitter

Molecular Beam Epitaxy

MBE is a versatile technique for growing thin epitaxial structures made of semiconductors, metals or insulators [7.1] (see also the definition given in Sect. 6.1.3). What distinguishes MBE from previous vacuum deposition techniques is its significantly more precise control of the beam fluxes and growth conditions. Because of vacuum deposition, MBE is carried out under conditions far from thermodynamic equilibrium and is governed mainly by the kinetics of the surface processes occurring when the impinging beams react with the outermost atomic layers of the substrate crystal. This is in contrast to other epitaxial growth techniques, such as LPE or atmospheric pressure VPE, which proceed at conditions near thermodynamic equilibrium and are most frequently controlled by diffusion processes occurring in the crystallizing phase surrounding the substrate crystal.

High performance n-channel organic field-effect transistors and ring oscillators based on C60 fullerene films

We report on organic n-channel field-effect transistors and circuits based on C60 films grown by hot wall epitaxy. Electron mobility is found to be dependent strongly on the substrate temperature during film growth and on the type of the gate dielectric employed. Top-contact transistors employing LiF∕Al electrodes and a polymer dielectric exhibit maximum electron mobility of 6cm2∕Vs. When the same films are employed in bottom-contact transistors, using SiO2 as gate dielectric, mobility is reduced to 0.2cm2∕Vs. By integrating several transistors we are able to fabricate high performance unipolar (n-channel) ring oscillators with stage delay of 2.3μs.

Random laser action in self-organized para-sexiphenyl nanofibers grown by hot-wall epitaxy

We report on the observation of amplified spontaneous emission and random lasing in self-organized crystalline para-sexiphenyl nanofibers. Using subpicosecond excitation, a lasing threshold is observed on the 0–1 emission band near 425 nm at excitation fluences as low as 0.5 μJ/cm2 (6×1016 cm−3 equivalent density), near the onset of density-dependent recombination processes. The dependence of the nonlinear emission spectrum on both the pump intensity and position of the excitation area are attributed to the interplay between random lasing and amplified spontaneous emission occurring along the nanofibers.

Correlation of crystalline and structural properties of C60 thin films grown at various temperature with charge carrier mobility

Transistors fabricated from C60 films grown by hot wall epitaxy at higher substrate temperature, showed an order of magnitude increased charge carrier mobility up to 6cm2∕Vs. In this letter, the authors present an extensive study of morphology and crystallinity of the fullerene films using atomic force microscopy and grazing-incidence x-ray diffraction. A clear correlation of crystalline quality of the C60 film and charge carrier mobility was found. A higher substrate temperature leads to a single crystal-like faceted fullerene crystals. The high crystalline quality solely brings a drastic improvement in the charge carrier mobility. A gate voltage independent mobility is also observed in these devices which can be attributed to the highly conjugated nature of the C60 thin film.

Doping of zinc‐selenide‐telluride

We investigate the doping behavior of ZnSe/ZnTe short period superlattices. p‐type doping is achieved with a dc nitrogen plasma source, n‐type doping with chlorine from a ZnCl2 Knudsen source. Even a small Te content has a strong positive effect on p doping: Doping levels in the upper 1019 cm−3 range are achieved, and ohmic contacts can be obtained even for low carrier concentrations. The data are in excellent agreement with a theory based on the amphoteric native defect model. The opposite is valid for n doping: At Te concentrations above 20% electron concentrations are below 1016 cm−3. As a possible way to get both good n‐ and p‐type doping at the same lattice constant we propose the use of the quaternary compound Zn(1−y)Mg(y)Se(1−x)Te(x).

Type conversion of p-(HgCd)Te using and Ar reactive ion etching

Hydrogen/methane gas mixtures and pure argon were used for reactive ion etching (RIE) of ( and 0.28). The effect of the ratio on the depth of the etched surface and the depth of the p - n junction created under the etched surface were studied for the RIE process. It was found that the etch depth reaches a maximum at an ratio and the depth of the p - n junction decreases with increasing fraction in the mixture. The roughness of the etched surface is smallest using a gas mixture with a small amount of (20 - 30%). For the pure Ar RIE process the etch and p - n junction depths were studied as functions of etch time, Ar pressure and rf power. Clear evidence for the creation of p - n junctions using various kinds of Ar RIE processes is found.

High quality CdTe epilayers on GaAs grown by hot-wall epitaxy

Hot‐wall epitaxy is used for the first time for the growth of high quality CdTe epilayers on semi‐insulating GaAs. The quality of the films is investigated by photoluminescence at 10 K, 78 K, and room temperature. The photoluminescence spectra at 78 K show a strong bound exciton recombination at about 1.58 eV with a linewidth of 11 meV. The linewidth of the bound exciton emission at 10 K is 1 meV. The electrical properties of the as‐grown layers are measured by Hall effect yielding a hole concentration of p=3×1012 cm−3 and a mobility of 100 cm2/V s at 270 K.

Epitaxy of Rodlike Organic Molecules on Sheet Silicates—A Growth Model Based on Experiments and Simulations

During the last years, self-assembled organic nanostructures have been recognized as a proper fundament for several electrical and optical applications. In particular, phenylenes deposited on muscovite mica have turned out to be an outstanding material combination. They tend to align parallel to each other forming needlelike structures. In that way, they provide the key for macroscopic highly polarized emission, waveguiding, and lasing. The resulting anisotropy has been interpreted so far by an induced dipole originating from the muscovite mica substrate. Based on a combined experimental and theoretical approach, we present an alternative growth model being able to explain molecular adsorption on sheet silicates in terms of molecule−surface interactions only. By a comprehensive comparison between experiments and simulations, we demonstrate that geometrical changes in the substrate surface or molecule lead to different molecular adsorption geometries and needle directions which can be predicted by our growth model.

Intermolecular hydrogen-bonded organic semiconductors—Quinacridone versus pentacene

Quinacridone is a five-ring hydrogen-bonded molecule analogous in structure and size to the well-known organic semiconductor pentacene. Unlike pentacene, quinacridone has limited intramolecular π-conjugation and becomes highly colored in the solid state due to strong intermolecular electronic coupling. We found that quinacridone shows a field-effect mobility of 0.1 cm2/V·s, comparable to mobilities of pentacene in similarly prepared devices. Photoinduced charge generation in single-layer quinacridone metal-insulator-metal diodes is more than a hundred times more efficient than in pentacene devices. Photoinduced charge transfer from quinacridone to C60 is not effective, as evidenced by measurements in heterojunctions with C60. Hydrogen-bonded organic solids may provide new avenues for organic semiconductor design.

Chemical reaction at the ZnSe/GaAs interface detected by Raman spectroscopy

When ZnSe is deposited at temperatures commonly used for epitaxy onto GaAs, the possibility arises that selenium or zinc reacts with the substrate and thin interfacial layers consisting of a gallium selenide or a zinc arsenide are formed. In particular, Ga2Se3, which is thermodynamically the most stable, has been suggested as a likely candidate. In this study we present evidence for the formation of Ga2Se3 using Raman spectroscopy as a fingerprint technique. Ga2Se3 layers were grown on GaAs and the Raman spectra thereof were compared with those of ZnSe/GaAs heterostructures.