Y. Fujikawa

Thin bismuth film as a template for pentacene growth

Thin Bi(001) films grown by ultrahigh vacuum deposition on Si(111)‐7×7 surfaces at room temperature, were annealed at ∼400K in order to improve their morphology by reducing the step density on the surface. Annealed, well-ordered Bi(001) films have been subsequently used as substrates for growth of pentacene (Pn). It has been determined using low-energy electron microscope that Pn nucleates on Bi(001) into a highly ordered, crystalline layer, with Pn molecules “standing up” on the Bi surface, and the (001) plane on the growth front. Moreover, the Pn layer is aligned with the Bi(001) surface having a “point-on-line” commensurate relationship with the substrate. The Pn∕Bi(001) film crystallizes in a bulk-like structure directly from the first Pn layer. Formation of the thin film phase reported for the Pn growth on SiO2 and other inert substrates was not observed in our experiments.

Photoinduced inverse spin-Hall effect: Conversion of light-polarization information into electric voltage

The photoinduced inverse spin-Hall effect was observed in a Pt/GaAs hybrid structure. In the GaAs layer, circularly polarized light generates spin-polarized carriers, inducing a pure spin current into the Pt layer through the interface. This pure spin current is, by the inverse spin-Hall effect in the Pt layer, converted into electric voltage. By changing the direction and ellipticity of the circularly polarized light, the electromotive force varies systematically, consistent with the prediction of the photoinduced inverse spin-Hall effect. The observed phenomenon allows the direct conversion of circular-polarization information into electric voltage; this phenomenon can be used as a spin photodetector.

Development of a metal–tip cantilever for noncontact atomic force microscopy

We report on a focused-ion-beam fabrication of a metal–tip cantilever for noncontact atomic force microscopy (AFM) and demonstrate its superior performance by observing atomically resolved AFM images of the Si(111)7×7 surface. Characterization of the tip apex by transmission electron microscope revealed that the tip radius is less than 5nm. Detrimental changes in the resonance frequency and the Q factor of the cantilever due to the attachment of the metal tip are small and do not affect the performance of the AFM imaging. Since the fabrication technique is applicable to any materials, various functional probes can be developed with this method.

Spontaneous aggregation of pentacene molecules and its influence on field effect mobility

The authors have found that pentacene molecules deposited on SiO2 substrates treated with self-assembled monolayers, such as hexamethyldisilazane or octadecyltrichlorosilane, aggregate spontaneously. In situ, real-time low-energy electron microscopy investigation of the morpho-logical changes in pentacene films deposited under ultrahigh vacuum reveals that the balance between pentacene film and substrate surface energies is an origin of this aggregation. With in situ atomic force microscopy - field effect transistor measurements, the authors demonstrate that field effect mobility is critically affected by this aggregation.

Heteroepitaxial Growth of Layered Semiconductor GaSe on a Hydrogen-Terminated Si(111) Surface*

Layered III-VI semiconductor GaSe has been heteroepitaxially grown on HF-treated Si(111) surfaces. The HF-treated Si surface is chemically inactive because of the hydrogen termination of active dangling bonds. GaSe can be grown on such a surface through weak van der Waals interaction, which relaxes the lattice matching requirement. With careful control of the substrate temperature, thermal desorption of surface hydrogen atoms was prevented so that a single-domain film of GaSe could be grown with good crystallinity. Auger electron spectra and high-resolution electron energy loss spectra revealed high quality of the grown GaSe film.

Stability of the quasicubic phase in the initial stage of the growth of bismuth films on Si(111)-7×7

We discuss the results of the scanning tunneling microscopy (STM) investigations and ab initio calculations of the structure and stability of the quasicubic Bi{012} film formed in the initial stage of the bismuth deposition on the Si(111)-7×7 surface at room temperature. Results of our STM experiments show that paired-layer Bi{012} film grows on top of the initially formed wetting layer, with the Si 7×7 lattice preserved underneath. The pairing of the layers in the {012} film leads to the substantial stabilization of the film when it consists of an even number of layers and only even-number layered Bi{012} islands are observed to be stable. The buckling of the atoms in the topmost paired layer induced by the relaxation of the film is evidenced by the high-resolution STM images.

Real-Time Observation and Control of Pentacene Film Growth on an Artificially Structured Substrate

Suppression of nucleation around a gold electrode during pentacene growth on a SiO2 channel is found by photoemission electron microscopy. Mass flow is driven by the difference between the molecular orientations on SiO2 and gold. The poor connectivity at the channel/electrode boundary causes degradation in the performance of a field-effect transistor, which is found to be improved by self-assembled monolayer treatment on the electrode (see figure; thickness in monolayers (ML)).

Atomistic study of GaN surface grown on Si"111…

GaN is directly grown on Si(111) by radio-frequency plasma-assisted molecular-beam epitaxy, and the surface is studied using in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). By optimizing the growth condition, well-defined surface reconstructions are observed in atomically-resolved STM images after the additional Ga deposition, indicating the uniform N-polarity of the grown film. We show that N-rich condition in the initial GaN growth and slightly Ga-rich condition in the subsequent growth are critical in order to achieve monopolar uniform GaN films.

Direct conversion of light-polarization information into electric voltage using photoinduced inverse spin-Hall effect in Pt/GaAs hybrid structure: Spin photodetector

The direct conversion of light-polarization information into electric voltage has been demonstrated using the photoinduced inverse spin-Hall effect in a Pt/GaAs hybrid structure. In the GaAs layer, spin-polarized carriers are generated by the illumination of circularly polarized light, which induces a pure-spin current in the Pt layer through the interface. The pure-spin current is converted into an electromotive force using the inverse spin-Hall effect (ISHE) in the Pt layer. The electromotive force due to the photoinduced ISHE was found to be proportional to the degree of circular polarization of the illuminated light outside the sample in spite of the presence of the Pt top layer, which is consistent with a calculation based on the analysis for light propagation in multilayer structures. This conversion of light-polarization information into electric voltage works at room temperature without bias voltage and magnetic fields, and thus can be used as a spin photodetector.

Effect of nitridation on the growth of GaN on ZrB2(0001)/Si(111) by molecular-beam epitaxy

The effect of nitridation on the epitaxial growth of GaN on lattice-matched ZrB2(0001) films prepared ex situ and in situ was studied using an ultrahigh-vacuum molecular-beam epitaxy (MBE)-scanning probe microscopy system. The growth of GaN was carried out by rf-plasma-assisted MBE, and epitaxy of wurtzite GaN was observed on both ex situ and in situ prepared ZrB2 samples. The polarity was found to be consistently N-polar regardless of the samples, based on the observation of a series of N-polar Ga-rich reconstructions: (3×3), (6×6), and c(6×12). The nitridation of ZrB2 film was conducted by exposing it to active nitrogen and well-ordered hexagonal-BN (h-BN) formation was observed when the annealing temperature was above 900°C. The partially formed BN layer affected neither the epitaxy nor the polarity of GaN, but when the surface was fully covered with well-ordered h-BN, GaN growth did not occur.