X.H. Zhou

RECTIFICATION PROPERTIES AND INTERFACE STATES OF HETEROJUNCTIONS BETWEEN SOLID C60 AND N-TYPE GAAS

Solid C60/n‐GaAs heterojunctions have been fabricated by deposition of solid C60 film on the (100)‐oriented epitaxial n‐type GaAs substrates and their electrical characteristics have been measured. The rectification ratio is greater than 106 at a bias of ±1 V. The current for a fixed forward bias is an exponential function of reciprocal temperature, from which the effective potential barrier height of the heterojunction is determined to be 0.58 eV. A trap with an energy level 0.35 eV below the conduction band of GaAs at the C60/GaAs interface has been observed by the deep level transient spectroscopy technique.

Growth and structure of C60 thin films on NaCl, glass and mica substrates

Abstract We studied the growth and structure of C60 thin film condensed on NaCl, glass and mica substrates by transmission electron microscopy. Highly ordered, (111) textured and epitaxial thin films were obtained on (001) NaCl and mica respectively. Various deposition parameters including different substrate temperatures, deposition rates and film thicknesses were experimented with. The orientational order and nature of defects present in the films were assessed by electron diffraction, bright and dark field images. We ascribe the abnormal reflections in the electron diffraction patterns which had been thought by some workers to belong to the diffraction of an h.c.p. structure phase to the existence of stacking disorder in the f.c.c. structure.

Heterojunctions of solid C60 and crystalline silicon: rectifying properties and energy-band models

Heterojunctions of undoped solid C60 and n- or p-type-doped crystalline Si have been obtained. Current-voltage measurements show that both C60/n-Si and C60/p-Si contacts are rectifying but their directions of rectification are opposite. Thermal activation measurements at a fixed forward bias show an exponential dependence of current on the reciprocal of temperature, from which we determine the effective barrier height as 0.30 eV for C60/n-Si and 0.48 eV for C60/p-Si. Using energy-band models for heterojunctions we assign values to the positions of the conduction and valence bands of the solid C60 relative to those of crystalline Si and derive the electron affinity and band gap of solid C60 film as 3.92 eV and <1.72 eV, respectively.

Study of microstructure and anomalous variation of resistivity in metal-C60 multilayer thin films

Abstract The structure and electrical transport properties of C60 thin films and some metal-C60 multilayer thin films were studied. We find the resistance decreases sharply when adding C60 on some specific metal layers. There are two possible explainations: one is the formation of a conduction layer of metal doped C60, the other is the bonding interfacial interaction between the metal and C60 layers, which may result a better continuous metal layer.

Study of microstructure of epitaxial fullerenes films

Abstract A comprehensive study was made on the structure of epitaxial thin films of C 60 and C 70 by means of transmission electron microscopy. Both the films show similar face-centered cubic structure and are epitaxial on (001) mica with close-packed plane parallel to the substrate surface. Two main kinds of defects-stacking faults and twins-were observed and are discussed. The effect of the remaining C 70 impurity on the crystal orientation of C 60 films was studied by comparing different samples made from high-purity fullerene and C 60 /C 70 mixtures. The results show that there is a higher density of planar defects in the films containing larger amounts of impurities: moreover, some faint anomalous reflections located at so-called 2a 0 fcc reciprocal lattice points were also detected, probably as a result of C 70 contamination. Finally, it is found that stacking disorders can be easily increased by keeping the high-quality pure C 60 film in air at room temperature for a few weeks, implying the instability of the crystal orientation of the epitaxial fullerene films.

Surface enhanced electronic transport. A new method to probe the possible interactions between C60 and non-alkali metals

Abstract By measuring the resistance in situ during the deposition of C 60 on ultrathin metal layers in a UHV system, we find the resistance decreases sharply in most cases. This enhanced electronic transport in metal—C 60 multilayers (bilayers), may be related to charge transfer from the metal to C 60 and surface bonding effects, and hence can be used as a new possible approach for probing the interfacial interactions between C 60 and metals.

Epitaxial thin films of C70: Growth and structure characterization

Abstract Epitaxial thin films of C 70 have been grown on (001) mica substrate by resistive evaporation at a vacuum pressure of about 10 -3 Pa. The orientational ordering and the nature of the defects presented in the films were assessed by transmission electron diffraction and electron microscopy. The fundamental structure of the C 70 crystals is face-centered cubic with the lattice parameter a 0 = 1.50 nm, but forbidden reflections resulted from hcp stacking were also detected which are usually appeared in the prior study of alloys with low stacking fault energy and the solid C 60 . Furthermore, we also studied the films deposited on (001) NaCl and Si single crystals and the results show these substrates promoted polycrystal growth under the same evaporation conditions where epitaxy was observed on mica.

A possible interaction between nonalkali metals and C60 thin films

By measuring the resistance in situ during the deposition of metals-C60 multilayer thin films on sapphire substrates at room temperature, the authors find that the resistance decreases sharply when adding C60 on some metal layers. There are two possible explanations: one is the formation of a conduction layer of metal- (Sn-, Ba-, Ga-) doped C60; the other is a bonding interfacial interaction between the metal and C60 layers, which may result in a better continuous metal layer.

Electrical characteristics for solid C60/GaN heterojunctions

Solid C60/n-GaN heterojunctions have been fabricated and their electrical properties have been studied. It has been found that the heterojunction is a strongly rectifying contact with a rectification ratio greater than 106 and with an ideality factor close to 1. The current–temperature measurement shows an exponential decrease of current with increasing reciprocal temperature, from which the effective barrier height is determined to be 0.535 eV. The series resistance measured decreases with increasing forward voltage and finally tends to be constant.

Synthesis of K3C60 single crystal thin films with high critical currents

Abstract High quality K3C60 single crystal thin films have been synthesized on (001) mica substrates. The normal state electrical resistivity shows classic metallic temperature dependence and can be fit to a functional form of ϱ(T)=a+bT2. The film has a sharp transition to superconductivity near 22K, with a 10% to 90% width less than 700 mK. The experimental data near Tc have poor agreements with the Aslmozov-Larkin type fluctuation. Furthermore, critical current density up to 50 000 A/cm2 has been obtained at 4.7 K by continuous DC measurements, the preliminary analysis indicate that Jc is proportional to 1-(T/Tc)3.