Ichitaro Saito

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.

A transparent ultraviolet triggered amorphous selenium p-n junction

This paper will introduce a semitransparent amorphous selenium (a-Se) film exhibiting photovoltaic effects under ultraviolet light created through a simple and inexpensive method. We found that chlorine can be doped into a-Se through electrolysis of saturated salt water, and converts the weak p-type material into an n-type material. Furthermore, we found that a p-n diode fabricated through this process has shown an open circuit voltage of 0.35 V toward ultraviolet illumination. Our results suggest the possibility of doping control depending on the electric current during electrolysis and the possibility of developing a simple doping method for amorphous photoconductors.

Field emission mechanism of oxidized highly phosphorus-doped homoepitaxial diamond (111)

Spatially resolved electron field emission experiments on oxidized highly phosphorus-doped homoepitaxial diamond (111) were applied at room temperature. The diamond layer shows hopping conductivity. Field emission properties have three distinct regions. We attribute the variation in emission currents to: (a) Electron emission from conduction-band minimum (Region I), (b) Depletion of conduction-band electrons at the surface (Region II), and (c) emission from the phosphorus level (Region III). From these data, we calculate an effective positive electron affinity for the oxidized surface of 1–1.5eV.

Conditions for a carrier multiplication in amorphous-selenium based photodetector

Amorphous selenium is a promising candidate for high sensitivity photodetector due to its unique carrier multiplication phenomenon. More than 10 carriers can be generated per incident photon, which leads to high photo-conversion efficiency of 1000% that allows real-time imaging in dark ambient. However, application of this effect has been limited to specific devices due to the lack in material characterization. In this article, mechanism of carrier multiplication has been clarified using time-of-flight secondary ion mass spectroscopy and Raman spectroscopy. A prototype photodetector achieved photo conversion efficiency of 4000%, which explains the signal enhancement mechanism in a-Se based photodetector.

Development of an Amorphous Selenium-Based Photodetector Driven by a Diamond Cold Cathode

Amorphous-selenium (a-Se) based photodetectors are promising candidates for imaging devices, due to their high spatial resolution and response speed, as well as extremely high sensitivity enhanced by an internal carrier multiplication. In addition, a-Se is reported to show sensitivity against wide variety of wavelengths, including visible, UV and X-ray, where a-Se based flat-panel X-ray detector was proposed. In order to develop an ultra high-sensitivity photodetector with a wide detectable wavelength range, a photodetector was fabricated using a-Se photoconductor and a nitrogen-doped diamond cold cathode. In the study, a prototype photodetector has been developed, and its response to visible and ultraviolet light are characterized.

Anneal-Induced Degradation of Amorphous Selenium Characterized by Photoconductivity Measurements

Photoconductivity measurements of amorphous selenium (a-Se) and arsenic selenide (a-As2Se3) were carried out. Samples where annealed from 333–358 K at 5 K step for 5 minutes and the photoconductivity was measured after each annealing step. It was found that the dark current increases permanently for a-Se and the photoconductivity ratio increased drastically after 338 K, but drops after further annealing. Amorphous-As2Se3 however, has no noticeable change in the dark current neither in the photoconductivity ratio. Arsenic is introduced into a-Se to prevent degradation, which causes loss of image quality, but the volume must be controlled in order to maintain high photoconductivity.

Field Emission from Modified P-Doped Diamond Surfaces with Different Barrier Heights

In this study, the field emission properties of P-doped diamond with various surface treatments were measured in order to understand the effects of surface treatments on the barrier height. The emission properties were acquired for diamond with C-reconstructed, oxidized, and H-plasma treated surfaces. The voltage drop across the vacuum was estimated for each surface, using threshold voltage–anode distance (V–d) measurement. The estimated electric fields near the diamond surface were 4.95, 26.6, and 54.1 V/µm for the C-reconstructed, oxidized, and H-plasma treated surfaces, respectively. The barrier height ratio of these surfaces derived from their electric fields was 1:3.1:4.9, which agrees with the result derived from Fowler–Nordheim (F–N) plots. Considering the electron affinities of all surfaces and the obtained results, positive electron affinities dominate the field emission properties of the C-reconstructed and oxidized surfaces. An internal barrier due to upward band bending on the H-plasma treated surface limits the field emission properties, even though it has a negative electron affinity. Our results suggest that the emission properties strongly depend on the barrier height, which is modified by surface treatment.

Amorphous selenium based photodetector driven by field emission current from N-doped diamond cold cathode

Operation of prototype photodetector using a-Se based target and diamond cold cathode is investigated. In our previous study, successful operation of the prototype photodetector as a unit pixel in future imaging device was reported. Clear photoresponse was obtained even at 10−5Torr, several orders of magnitude higher than the pressure used for conventional imaging tube devices. Electron emission characteristics of diamond are widely reported, but the precise fabrication process of a-Se based target film is covered with patents. In this study, we have fabricated following three types of target films: (A) a-Se film without incorporation, (B) a-Se film with incorporation of arsenic (As), and (C) a-Se film with incorporation of As and tellurium (Te). Successful operation of the photodetector driven by diamond cathode with target C was observed. The detector showed clear response to red, green, and blue light-emitting diodes in addition to white halogen light.

Characterization of a-Se p-n junction fabricated using electrolysis in NaCl aq

In this paper, we introduce an electro-chemical doping method of amorphous selenium (a-Se) using NaClaq. Recently, an a-Se photovoltaic device fabricated using this method [1], has been announced and opened up the potential of a new impurity doping method. This study will further explore its possibilities by doping chlorine (Cl) and sodium (Na) and aim to fabricate a p-n junction by reversing the applied voltage during the electrolysis. The device is characterized through photoelectric measurements. The I-V characteristics show rectification under light illumination.

Characterization of amorphous selenium based photoconductor for a high-sensitivity photodetector driven by diamond cold cathode

In this study, amorphous selenium based photoconductor with different film thicknesses were fabricated and applied to prototype photodetectors. The use of thick photoconductor resulted in higher sensitivity due to carrier multiplication. Advantages and disadvantages of the thick photoconductor were investigated.