Hitoshi Yasunaga

Electromigration on semiconductor surfaces

Abstract Electromigration on semiconductor surfaces is a preferential mass transport towards either the cathode or the anode over a clean semiconductor substrate heated by a DC current. Since the first observation of this electrically driven movement for an In overlayer on Si(111) in 1982, a systematic investigation of electromigration has been made for a variety of elements on several semiconductor substrates with scanning Auger electron spectroscopy and UHV reflection high energy electron microscopy. As a result, it has been widely observed and has proved to be a novel type of mass transport entirely different from conventional electromigration, which manifests itself in the presence of an extremely high density of DC current in bulk and thin-film materials. The novel electromigration exhibits a mass flux of deposited material in a direction mostly contrary to the conventional one of the same material. Moreover, during the novel electromigration, a homogeneous overlayer with a characteristic coverage and structure grows up laterally in the preferential direction over a clean bare surface of semiconductor substrate. This also contrasts with the fact that damages such as hillocks, cracks and voids are formed in current-carrying media during the conventional electromigration. The scope of the present review article includes an introductory description of the novel electromigration and the conventional one with a comparison of them, a brief explanation of related mass transports and related aspects of the surface diffusion of hetero-adatoms on single crystals, the experimental method of electromigration on semiconductor surfaces, detailed data of the electromigration for In, Ag and Au on Si(111), and Ag on Ge(111) along with summarized data of other elements on Si(111), effects of surface steps on the lateral migration of adatoms, the driving force of the electromigration, and a computer simulation of the electromigrative growth of the overlayer based on the lattice gas model with three types of nearest-neighbor atomic interactions and the Schwoebel factor. The macroscopic behaviors observed by the experimental studies are qualitatively well explained in terms of the basic processes of adatoms introduced into the computer simulation.

Surface electromigration of metal atoms on Si(111) surfaces studied by UHV reflection electron microscopy

Abstract Surface electromigration of metal atoms on Si(111) surfaces, reported previously by reflection high energy electron diffraction and Auger electron microscopy, is studied by ultra-high-vacuum reflection electron microscopy (REM). Au and In are deposited in the form of a stripe on clean Si(111)7 × 7 surfaces, and movement of the stripe edges and changes of surface structures due to surface electromigration are in-situ observed when a current is fed through the Si crystal perpendicularly to the stripe. In the case of Au deposit, migration to the anode is observed and structure changes from the 7 × 7 to Si(111)5 × 1-Au structures are noted on the surface between the stripe and the anode. The shapes of the surface atomic steps change during the transition. The cathode side edge of the stripe is noted to move slowly to the anode. The migration not only on adsorbed layers but also on clean Si(111) surfaces is noted. In the case of In, electromigration to the cathode is observed. Changes of the structure from the 7 × 7 to Si(111)4 × 1-In structures are noted in the area between the stripe and the cathode. The step shapes do not change during the transition.

Hetero-electromigration on semiconductor surfaces

Abstract A preferential movement of hetero-adatoms on a clean surface of a semiconductor substrate heated by a DC current (referred to as the hetero-electromigration on semiconductor surfaces to distinguish it from the conventional electromigration) has been investigated for a variety of systems with scanning Auger microscopy (SAM). Several SAM observations for Ag/Si(111) are described as the most evident demonstration of the general features of the mass transport. Unpublished data for Al and Ge on Si(111) and Ag on Ge(111) are also demonstrated briefly with a summary of all the systems investigated. The atomic process of the mass transport is explained in terms of single adatoms with a great mobility exclusively on the intermediate adlayer. The observed dependence of the adatom flux on the doping level of the semiconductor substrate favors rather the electric field acting on adatom ions as a driving force than the wind force predominant in the conventional electromigration.

Surface electromigration and diffusion of Au on Si(111)7 × 7

Abstract The movement of a patch of Au film on clean Si(111) when a DC current was fed through the Si substrate was investigated by scanning Auger microscopy. The gold overlayer exhibited a directional movement towards the anode in addition to isotropic spreading due to surface thermal diffusion. The movement of the overlayer exhibited successively three distinct processes with time: (I) fast isotropic spreading supplemented by the directional movement over the intermediate layer of 1 monolayer (ML) in coverage, (II) the advance of the foremost overlayer on the anode side at the expense of the coverage from 1 to 0.7 ML on the cathode side and (III) a slow directional movement of the overlayer as a whole causing an oscillatory profile of the Auger peak height on the cathode side. The lateral flux of atoms at the leading edge was 8.1 × 10 8 and 2.0 × 10 8 atoms/cm · s for a DC current of 352 mA (596°C) in processes II and III, respectively and the corresponding activation energy was 0.77 and 1.2 eV. We discuss the atomic process during the movement and the relation between the present mass transport and the conventional electromigration.

Stability of ordered missing-dimer structures and the ordering dynamics on Si(001)

Abstract The stability of (2 × n )-ordered missing-dimer structures on Si(001) and the ordering dynamics of missing dimers are investigated. The surface free energy of (2 × n )-ordered missing-dimer structure was calculated under strains parallel or perpendicular to the dimer row using the Stillinger-Weber potential among Si atoms. The (2 × n )-ordered missing-dimer structure becomes more stable than the (2 × 1)-ordered surface without missing-dimers only if a compressive strain larger than 0.5% is applied parallel to the dimer row. The relaxation process towards the equilibrium configuration of excess missing dimers formed initially at random on the (001) surface was studied by a time-dependent Monte Carlo simulation based on the lattice gas model. The ordering of missing dimers towards the (2 × n ) structure evolves via a three-stage relaxation process at 300 K, while it evolves via a one-stage, fast relaxation process at 600 and 900 K. The degree of ordering at equilibrium is significantly improved as the annealing temperature decreases and the missing-dimer concentration increases. The effect of the missing-dimer hopping across the dimer row is also studied.

Quantum Computer Using Coupled-Quantum-Dot Molecules

We propose a method for implementing a quantum computer using artificial molecules. The artificial molecule consists of two coupled quantum dots stacked along the z direction and one single electron. One-qubit and two-qubit gates are constructed by one molecule and two coupled molecules, respectively. The ground state and the first excited state of the molecule are used to encode the |0> and |1> states of a qubit. The qubit is manipulated by a resonant electromagnetic wave that is applied directly to the qubit through a microstrip line. The coupling between two qubits in a quantum-controlled NOT gate is switched on (off) by floating (grounding) the metal film electrodes. We study the operations of the gates using a box-shaped quantum dot model and numerically solving a time-dependent Schrodinger equation, and demonstrate that the quantum gates can perform quantum computations. The operating speed of the gates is about one operation per 4 ps. The reading operation of the output of the quantum computer can be performed by detecting the polarization of the qubits.

Surface mass transport of Ag and In on vicinal Si(111)

Abstract Mass transport of Ag and In on vicinal Si(111) has been investigated by scanning Auger microscopy (SAM). Highly anisotropic surface diffusion and surface electromigration due to direct current were observed for Ag and In adatoms on 0°−, 0.5°−, 3°− and 6°−off vicinal Si(111) surfaces. The diffusion on the intermediate layer is strongly enhanced in the direction parallel to the step edge for Ag adatoms, while it is remarkably suppressed in the direction perpendicular to the step edge for In adatoms. The activation energy of the diffusion for the Ag adatoms ranged between 0.81 and 1.3 eV, while that for In adatoms increased from 0.31 to 0.66 eV with increasing the vicinal angle. The anisotropic diffusion transport is explained in terms of the step structure and the difference in the binding energy at the step site and the terrace site.

Mass transport of a Ag thin film on a stepped Si(111) surface

Abstract The mass transport of Ag on stepped Si(111) surfaces was investigated by a scanning Auger microscope (SAM). A highly anisotropic surface diffusion of Ag ultra-thin films was observed on 0°, 0.5°, 3° and 6° vicinal Si(111). The mass transport parallel to the step edge is overwhelmingly greater than that perpendicular to the edge. The preferential mass transport toward the cathode due to DC current heating was also observed. This increased with the resistivity of the Si(111) substrate. The anisotropic mass transport is correlated with the difference of the binding energies at the step edge sites and at the terrace sites.

Effect of Schottky barrier on alternating current response of lead phthalocyanine

Measurements were made of the capacitance (1 kHz) and the dc conductivity of degassed, oxygen‐doped, and iodine‐doped single crystals of lead phthalocyanine (PbPc) as a function of temperature between room temperature and 250 °C. As the temperature was increased, the capacitance initially remained constant as expected, followed by an exponential increase similar to that of the conductivity and then saturated with an extraordinarily large value. Frequency dependence below 1 MHz of the capacitance and the loss tangent of PbPc film with aluminium electrodes was also measured at degassed and oxygen‐doped states. An apparent Debye‐type dispersion was observed. All the results obtained are consistently understood in terms of the Schottky barrier capacitance associated with the PbPc‐electrode contact. The oxygen doping exhibited remarkable influence on the alternating current response and this effect is considered to be due to changes in the barrier capacitance and the dc conductivity with doping.

Structural change of a heterogeneous thin film patch by surface electro-migration

Abstract A new model is proposed for the mechanism of the structural change of a thin film patch by surface electro-migration. The evolution in time of the structure of a thin film patch is investigated under the influence of the uniform driving force. The computer simulation is executed, based on the lattice gas model with the nearest neighbour atomic interactions. The numerical results can reproduce well the diverse observed behaviours for In/Si(111), Ag/Si(111) and Sn/Si(111).