K. M. Chen

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.

HETEROJUNCTIONS OF SOLID C70 AND CRYSTALLINE SILICON : RECTIFYING PROPERTIES AND BARRIER HEIGHTS

Heterojunctions of solid C70 and n‐ or p‐type crystalline Si have been made. Current–voltage measurements show that both C70/n‐Si and C70/p‐Si contacts are rectifying but their directions of rectification are opposite to each other. 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 0.23 eV for C70/n‐Si and 0.27 eV for C70/p‐Si. Relative dielectric constant of solid C70 was determined to be 4.96 through the study of high‐frequency capacitance–voltage characteristics for Ti/C70/p‐Si structures.

Formation and dissolution of microcrystalline graphite in carbon-implanted GaN

Two sharp bands at ∼1350 and ∼1600 cm−1 were observed in the Raman spectra of carbon-implanted GaN after postimplantation annealing treatments. The intensities of these two bands increased while their full widths at half maximum decreased with increasing annealing temperature. The observation of these two bands indicates the formation of microcrystalline graphite in C-implanted GaN. Hall measurements demonstrated that some dispersed C in GaN acted as acceptors and played a role in reducing electron concentration and Hall mobility. The facts that in 1100 °C furnace annealing the intensities of these two Raman peaks decreased rapidly to zero and the resistivity increased by 3 orders of magnitude indicate the dissolution of microcrystalline graphite at this temperature.

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.

Using Fourier transform infrared grazing incidence reflectivity to study local vibrational modes in GaN

Both Fourier transform infrared (FTIR) grazing incidence reflectivity and FTIR transmission methods have been used to study GaN films grown on alpha-Al2O3 (0001) substrates by atmospheric pressure metal-organic chemical vapor deposition and low pressure metal-organic chemical vapor deposition. The results show that in the frequency range from 400 to 3500 cm(-1) the signal-to-noise ratio of the FTIR grazing incidence measurement is far higher than that of the FTIR transmission measurement. Some new vibrational structures appearing in the former measurement have been discussed. The features around 1460 and 1300 cm(-1) are tentatively assigned to scissoring and wagging local vibrational modes of CH2 in GaN, respectively

The bias-temperature effect in a rectifying Nb/C60/p-Si structure: evidence for mobile negative charges in the solid C60 film

Solid C60 film was grown on a p-type Si substrate and a rectifying Nb/C60/p-Si structure was prepared. Capacitance-voltage (C-V) measurements showed that for temperatures above 260 K the C-V curve of the Nb/Co60/p-Si structure shifted along the voltage axis depending on biasing conditions. We analysed this effect to reveal the existence of mobile negative charges in the C60 layer and determine the density of the mobile charges.

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.

Gamma-ray irradiation effects on Fourier transform infrared grazing incidence reflection-absorption spectra of GaN films

We have studied the effects of gamma-ray irradiation on the Fourier transform infrared grazing incidence reflection-absorption spectra of GaN films grown on alpha -Al2O3 (0001) substrates using metal-organic chemical vapour deposition at atmospheric pressure. After irradiation, the spectral intensity increased markedly and a series of new absorption features appeared, among which those at 3440, 3355, 3174, 3088, 2819, 2712, 2100, 1426 and 894 cm(-1) are discussed and identified. The reason why gamma-ray irradiation can enhance the intensity and give rise to these new features is also discussed.

Electron states at a solid interface

Electron states at a solid /Si(111) interface have been studied by the deep-level transient spectroscopy (DLTS) technique. An electron trap, (0.31), and three hole traps, (0.27), (0.36) and (0.47), exist at the solid interface. and are the dominant deep levels with densities of the order of magnitude , and both of them probably originate from the dangling bonds on silicon(111) surfaces. The fact that the density of the interface states at the interface is low indicates that passivates the Si surface.

Probing highly deep levels in solid C70 with isothermal capacitance transience-time spectroscopy

A technique, referred to as the isothermal capacitance transience-time spectroscopy, has been developed to probe the deep levels at insulator/semiconductor interfaces or in semiconductor materials with fairly wide band gaps held at moderately elevated temperatures. The highly deep levels in the solid C70/p-GaAs heterojunction were studied with this technique. Two discrete hole traps, H1 and H2, were found to exist in the solid C70 layer at the positions of Ev+0.856 eV and Ev+1.037 eV, respectively.