Asawin Sinsarp

Electrical Spin Injection from Out-of-Plane Magnetized FePt/MgO Tunneling Junction into GaAs at Room Temperature

We have succeeded in demonstrating zero-magnetic-field spin injection in metal–insulator–semiconductor (MIS) structure at room temperature using FePt/MgO/GaAs-based light-emitting diode heterojunction with out-of-plane magnetization. The spin polarization was investigated by spin-polarized electroluminescence (EL). The lower estimate injected at remanence was 1.5% and that injected at 1 T was reached up to 11.5%. The spin injection efficiency was estimated at least 29%. The bias dependence of the EL circular polarization showed that it decreases with increasing bias voltage for both at 1 T and at remanence.

Moiré-like Distribution of Local Tunneling Barrier Height of the Monolayer Graphite Adsorbed on Pt(111) Surface

The Moire-like distribution of the local tunneling barrier height (LBH) in the surface of monolayer graphite (MG) adsorbed on Pt(111) is discussed on the basis of atom-resolved LBH imaging. The Moire-like LBH distribution is explained by the local work function distribution induced by the lateral variations of inter-layer interaction between C and Pt atoms, which results from the lattice mismatch between MG and Pt(111). Further, the tip-sample distance dependence of the LBH image is discussed. When the tip-sample separation is improperly small, we find that the apparent LBH is influenced by the surface deformation induced by the interaction between the tip and the sample, resulting in the reversal of the Moire-like contrast. This is also explained in the framework of the inhomogeneous inter-layer interaction.

Microscopic Study on the Work Function Reduction Induced by Cs-Adsorption

The effects of Cs adsorption on Pt(111) and Si(111)-(7×7) surfaces have been investigated using the local tunneling barrier height (LBH) measurement with scanning tunneling microscope (STM). LBH has been evaluated from the tunneling current versus tip-sample gap distance (I-z) characteristic measurement. The histograms of LBHs obtained from the surfaces with different Cs coverages imply that the Cs adsorption effect on reduction in microscopic work function extends over a long distance beyond the classical picture. We conclude that the long-range modification in electronic structure of the substrate due to the presence of Cs atoms may be responsible for this long-range effect.

Growth condition dependence of spin-polarized electroluminescence in Fe∕MgO∕light-emitting diodes

We compared the electroluminescence (EL) polarization of two Fe∕MgO∕light-emitting-diode (LED) structures grown at different substrate temperatures for MgO growth: room temperature and 400°C. Two spin-LED wafers were prepared on molecular beam epitaxy grown LEDs by e-beam evaporation: one was LED∕MgO (RT)∕Fe (RT)∕Au cap (RT), and the other was LED∕MgO (400°C)∕Fe (150°C)∕Au cap (90°C). Spin-polarized EL was clearly observed in the latter sample, while the EL polarization was hardly observed in the former sample. The reasons for the near absence of EL polarization in the former sample are considered to be the degradation of the tunneling junction resulting from the crystallinity and the As-rich surface of the LED.

Local Tunneling Barrier Height Measurement on Au(111)

The systematic local tunneling barrier height (LBH) measurement on a Au(111) surface has been carried out in order to clarify the origin of the previously-reported peculiar behaviors; extremely small LBH values and its strong gap-distance dependence. In our study where carefully chosen tunneling tips are used, the LBH of Au(111) shows no such anomalies, indicating that these peculiar behaviors are artifacts due to tip-sample interaction. Furthermore, the bias and distance dependence of the LBH of Au(111) obtained here can be explained well by the simplified one-dimensional rectangular potential model for tunneling, which is similar to the case of Pt(111).

Scanning Tunneling Microscopy (STM)/Local Tunneling Barrier Height (LBH) Studies on Cs Adsorption on a Pt(111) Surface

The initial stage of Cs adsorption and the formation process of a Cs monolayer on a Pt(111) surface, as well as the variations of the local work function, are investigated by scanning tunneling microscopy (STM) and local tunneling barrier height (LBH) imaging. The STM/LBH images of the surface with a small amount of Cs adsorbate indicate increases in local density of states (LDOS) and LBH near the Cs adsorption sites, which are considered to provide a long-range effect of the Cs adsorption. At the Cs coverage of about 0.1, small (2×2) patches are seen throughout the surface, but they are so unstable that the Cs atoms in the patches diffuse to form stabilized patches at room temperature. With increasing Cs coverage, the area covered by the stabilized (2×2) patches increases. The stabilized (2×2) structure gives low LBH values and large atomic corrugations. The charge density in this structure is considered to be strongly localized due to the bond formation between Cs and Pt. This induces a strong surface dipole, resulting in a large work function reduction.

Microscopic Study of Work Function of Ag-Deposited Pt(111) and Ag/Pt Surface Alloy

The microscopic work function distributions of Ag-deposited Pt(111) surfaces and Ag/Pt surface alloy have been studied through local tunneling barrier height (LBH) imaging by means of scanning tunneling microscopy (STM). The difference in microscopic work function between Ag and Pt clusters in the Ag/Pt surface alloy is much smaller than that between Ag large islands and Pt terraces on a Ag-deposited Pt(111) surface although their Ag coverages as well as macroscopic work functions are the same. This features which occurrs when the dimensions of the clusters are a few nanometers is due to the long-range effect of work function reduction which is observed for the Cs-adsorption surfaces.

Local Tunneling Barrier Height Studies of the Initial Stage of Cs Adsorption on a Si(111) 7×7 Surface

The initial stage of Cs adsorption on a Si(111)7×7 surface is studied by means of scanning tunneling microscope (STM) and local tunneling barrier height (LBH) imaging which provides the microscopic work function distribution. Utilizing the combination of STM and LBH imaging, the Cs adsorption features can be classified into three types; clustering, adsorption on an adatom site, and adsorption on a rest atom site. For all these adsorption types, LBH is reduced locally just at the Cs adsorption sites. On the other hand, the increase in the local density of states at the adatom next to the adsorbed Cs is observed. This is considered as a long-ranged effect of Cs adsorption, which has been pointed out in theoretical works.

Magnetotransport Properties of MnAs Film on GaAs(001) Substrate

The magnetotransport properties at room temperature of the 250-㎚-thick MnAs(-1100) film grown on GaAs(001) substrate by molecular beam epitaxy was investigated. The results measured with various magnetic field directions were reported. They show the negative magnetoresistive effect for all field directions. The difference in the magnetoresistance curves for different field directions is in agreement with the magnetic anisotropy of the film.

Ordering of C60 on One-Dimensional Template of Single-Domain Ge(110)-16×2 and Si(110)-16×2 Surfaces

We demonstrated the ordering of C60 in one dimension by using Ge(110)-16×2 and Si(110)-16×2 single-domain surfaces as one-dimensional templates. We achieved the formation of one-dimensional order of C60 molecules along the atomic rows of the Ge(110)-16×2 surface where surface diffusion of C60 is activated at room temperature. In contrast, the substantial reactivity of the Si(110)-16×2 surface limited the diffusion of adsorbed C60, resulting in random adsorption even at elevated temperatures.