Masato Kudo

Electron emission from conduction band of diamond with negative electron affinity

Experimental evidence explaining the extremely low-threshold electron emission from diamond reported in 1996 has been obtained [K. Okano et al., Nature (London) 381, 140 (1996)]. Direct observation using combined ultraviolet photoelectron spectroscopy/field-emission spectroscopy proved that the origin of field-induced electron emission from heavily nitrogen (N)-doped chemical-vapor deposited (CVD) diamond was at conduction-band minimum utilizing negative-electron affinity (NEA). The significance of the result is that not only does it prove the utilization of NEA as the dominant factor for the extremely low-threshold electron emission from heavily N-doped CVD diamond but also strongly implies that such low-threshold emission is possible from other types of diamond and even other materials having NEA surface. The low-threshold voltage, along with the stable intensity and remarkably narrow energy width, suggests that this type of electron emission can be applied to develop a next generation vacuum nanoelectronic devices with long lifetime and high-energy resolution.

Dependencies of secondary electron yields on work function for metals by electron and ion bombardment

Secondary electron yields depending on work function were measured for 30 species of metal in ultrahigh vacuum by electron and ion bombardment. Secondary electron yields induced by electrons at 10 keV increase with work function, while those by Ar+ ions at 3 keV decrease with increasing work function. The opposite dependencies of secondary electron yields on work function between electron and ion bombardment are discussed on the basis of the different mechanisms of secondary electron emission, i.e., kinetic and potential emission for electron and ion bombardment, respectively.

Electron emission mechanism of diamond characterized using combined x-ray photoelectron spectroscopy/ultraviolet photoelectron spectroscopy/field emission spectroscopy system

Clarification on electron emission mechanism of diamond is one essential approach to realize the clear vision of vacuum nanoelectronics. Electric field of less than 5V∕μm is enough to extract electrons from diamond, whereas field of one to two orders of magnitude higher is needed to extract electrons from conventional metal emitter tips. Diamond has various advantages as an electron emitter in addition to the low-threshold voltage, such as negative electron affinity and high thermal conductivity. The difficulty in clarification of electron emission mechanism is the factor preventing diamond from being used in a practical way. In this study, combined spectroscopy of x-ray photoelectron spectroscopy/ultraviolet photoelectron spectroscopy/field emission spectroscopy was performed to characterize the electron emission mechanism of diamond. The results indicated the first successful observation of applied voltage dependence on the origin of field-emitted electrons.

Combined x-ray photoelectron spectroscopy/ultraviolet photoelectron spectroscopy/field emission spectroscopy for characterization of electron-emission mechanism of diamond

A possible mechanism for the field emission spectroscopy (FES) peak energy shift observed for lightly nitrogen (N)-doped chemical vapor deposition (CVD) diamond was investigated using natural IIb diamond as a reference. Combined ultraviolet photoelectron spectroscopy/FES spectra of natural IIb diamond indicated that the origin of field-emitted electrons is at the valence-band maximum and does not shift depending on the applied voltages. To further investigate the mechanism, FES peak energy was plotted versus emission current and the plot was best fitted to a straight line. The resistance of the diamond obtained from the slope was 109Ω and almost 0 for natural IIb diamond and lightly N-doped CVD diamond, respectively. The result was confirmed to be consistent with the resistivity of lightly N-doped CVD diamond and natural IIb diamond. Therefore, the result strongly implies that the observed energy shift is due to the voltage drop at the field emission site due to the resistance of the diamond bulk. Details...

Development of high spatial resolution Auger microscope as applied to semiconductor analysis

Abstract An Auger microscope equipped with aZrO/WSchottky field emitter was developed. For Auger analysis, spatial resolution under a probe current of more than 1 nA is important. Under such conditions, the aberration by the condenser lens is not negligible. To reduce it, the distance from the emitter to the principal plane of the first condenser lens is reduced by forming its magnetic field overlapping with the accelerating field of the electron. As a result, a spherical aberration coefficient of 10 mm (accelerating voltage 30 kV) was achieved and the spatial resolution was about 10 nm with a probe current of 10 nA and an accelerating voltage of 25 kV. The newly developed Auger microscope combined with the probe tracking system has enabled an Auger image to be obtained, in which a 40 nm gate oxide layer in a cross-sectioned integrated circuit device is clearly observed.

Study of SEM images obtained with an electron energy and take-off angle (E-θ) selective detector

Scanning electron microscopes (SEMs) are usually equipped with different types of electron detectors. This allows acquisition of images showing diverse contrast, which is caused by the range of detectable energy and take-off angle, respectively. However, the phenomenon is not fully understood yet. We have manufactured an E-θ detector, which can detect electrons emitted from a sample with a selectable range of both energy and take-off angle [1].

SEM Image Observation Using an Electron Energy and Electron Take-off Angle Filtered Detector

Scanning electron microscopes (SEMs) are usually equipped with two types of detectors: secondary and backscattered electron detectors. The former produces secondary electron images (SEI) rich in topographic information[1, 2], whereas the latter produces backscattered electron images (BEI) rich in composition information[3]. Recently, however, a few other detectors have been installed in addition to these two types of conventional detectors. For example, four different detectors can be installed in a field emission SEM JSM-7800F: an upper electron detector (UED), an upper secondary electron detector (USD), a backscattered electron detector (BED), and a lower electron detector (LED). They allow to separate electron energy and take-off angle. Therefore, they help to observe specimen shape and structure on nano-scale[4]. On the other hand, they detect electrons having passed through the electromagnetic field of the objective lens, which changes actual energy and take-off angle ranges. Consequently, it is difficult to distinguish the image contrasts in accordance with physical properties. In this study, an electron detector was designed and experimentally manufactured to detect electrons emitted in a definite, variable range of energy and take-off angle.

Remaining problems in the combined XPS/UPS/FES system

An electric field of less than 5 V/um is enough to extract electrons from diamond, whereas field of one to two orders of magnitude higher is needed to extract electrons from metal emitter tips. Diamond has various advantages as an electron emitter in addition to the low-threshold field, such as negative electron affinity (NEA), high thermal conductivity, high mechanical hardness, and high chemical stability. Despite advantages of the diamond as an electron emitter, however, the difficulty in clarification of electron emission mechanism is the factor preventing diamond from being used in a practical application. Although quite a few numbers of possible mechanisms were proposed to better understand the origin and properties of the observed emission, most of these mechanisms were based on the conventional emission current — anode voltage (I–V) characteristics. Energy distribution of the field-emitted electrons is essential in direct clarification of the mechanism. It enables one to draw the energy band diagram of emitting diamond. Our ultimate goal is to draw the energy band diagram of emitting diamond using the electron energy distribution obtained from combined X-ray Photoelectron Spectroscopy/Ultraviolet Photoelectron Spectroscopy/Field Emission Spectroscopy (XPS/UPS/FES).

Combined XPS/UPS/FES for characterisation of electron emission mechanism from diamond

In this study, natural type lib diamond was used to characterise the origin of emitting electrons by the means of combined XPS/UPS/FES. Natural type lib diamond is one of the most intensively studied diamond due to its semiconductor properties and negative electron affinity (NEA) on hydrogenated (111) surface. The origin of emitting electrons from hydrogenated natural type lib diamond (111) surface, thus, can be a one of the most appropriate reference data for the future studies for diamond with various dopants and various surface terminations.

Electron Emission Mechanism of Doped CVD Diamond Characterised by Combined XPS/UPS/FES System

In this study, combined X-ray photoelectron spectroscopy (XPS)/ultraviolet photoelectron spectroscopy (UPS)/field emission spectroscopy (FES) system was used to characterize the electron emission mechanism of doped CVD diamond. The energy band diagram of emitting diamond is drawn using the electron energy distribution obtained from the system