Negative Electron Affinity Property Research Articles

Theoretical prediction of hafnium-terminated diamond (100) surface with promising negative electron affinity for electron emission applications

Controlling the chemical termination of diamond surface shows great promise in achieving negative electron affinity (NEA). In this work, we explore the feasibility of obtaining stable NEA properties by hafnium (Hf) termination on bare and oxidized diamond (100) surfaces based on swarm-intelligence structural search and first-principles calculations. The negative adsorption energies demonstrate that Hf-terminated diamond surfaces are energetically favorable. Remarkably, the lowest-energy structure at 0.25 monolayer Hf coverage on an oxidized surface has an extremely large adsorption energy of −11.15 eV, which is consistent with the largest bonding orbital population of 7.28 eV. The most negative NEA values of bare and oxidized surfaces are −3.50 and −3.19 eV at 0.25 monolayer Hf coverage, respectively. Both structures have dynamic stability, and the oxidized surface exhibits excellent thermal stability even up to 1200 K. Owing to their super stability and outstanding NEA properties, Hf-terminated diamond surfaces are suggested as promising candidate materials for electron emission applications.

Study of Negative Electron Affinity Property for Varied Doping GaN Photocathode

Negative electron affinity (NEA) property means that the vacuum level is lower than the conduction band minimum in the bulk. For GaN photocathode, the effective electron affinity of the material is smaller than zero. According to Spicer “3-step model”, the photoemission mechanism for varied GaN photocathode was studied. For the varied doping GaN photocathode, the photoelectrons can be transported to the surface by diffusing and drifting in the directional inside electric field, and the higher photoemission performance can be obtained. By covering with low escape power materials such as Cs or O on an atomically clean surface, Negative electron affinity property can be achieved for the varied doping GaN photocathode. Using the activation and evaluation system for NEA photocathode, the varied doping GaN photocathode was activated with Cs and O, and the photocurrent curve for varied doping GaN photocathode was gotten.

ダイヤモンド半導体の特異な物性とデバイス展開

Diamond is a new semiconductor which has been developed in the last around 15 years. It has unique properties and the potential of unique electronic devices. The typical example is negative electron affinity (NEA) for hydrogen terminated diamond surface. The nature of diamond electron affinity was investigated using by photo-yield spectroscopy (TPYS). Using the NEA property, electron emission p-n (p-i-n) diodes were fabricated, and this diode can be used as a new principle power device; a vacuum micro switch operated by a p-n diode.

Photoemission mechanism of GaN vacuum surface electron source

GaN photocathode is fully activated by employing a continuous Cs source and an alternate O source. The quantum efficiency curve of transmission-mode photocathode is tested in situ. The quantum efficiency reaches up to 13% in transmission-mode. According to the one-dimensional Schrdinger equation, the electron transmission coefficient formula of GaN vacuum electron source material is deduced. For a certain profile of photocathode surface potential barrier, the electron transmission coefficient relates to the incident electron energy, the height and the width of the surface potential. The energy band of transmission-mode negative electron affinity (NEA) GaN photocathode and the change of surface barrier in the deposit course of Cs,O are given. Using the double dipole layer surface model [CaN(Mg):Cs]:O-Cs, the NEA property formation of GaN vacuum electron source material is analyzed. The results show that the double dipole layer formed in the activation course of Cs,O is conducible to the escape of electrons, and it is the formation of double dipole layer that causes the drop of vacuum energy level of the material surface.

Activation mechanism of negative electron affinity GaN photocathode

The photocurrent curves during either Cs or Cs/O activation of GaN photocathode were tested by using dedicated experimental system for activation and evaluation of negative electron affinity （NEA） photocathode. Aiming at explaining the formation of NEA property of GaN photocathode and according to the rule of photocurrent change during activation period and the surface model of a fully activated photocathode，the activation mechanism for NEA GaN photocathode was studied. The experiment results show that：the obvious NEA property is be induced in GaN photocathode mainly due to the activation by Cs. The increase extent of photocurrent is not large after introducing O during Cs/O activation process for GaN photocathode. The NEA property formation reasons of GaN photocathode after being activated successfully can be well explained using the double dipole layer model ［GaN（Mg）：Cs］：O—Cs.

XPS study of diamond surface after mass-separated low-energy phosphorus ion irradiation

Phosphorus-ion irradiation of synthetic type II a diamond crystals with a hydrogen-terminated (100) surface was carried out by using a 40-eV mass-separated 31P + ion beam at ion doses from 4.4×10 11 to 3.4×10 14 ions/cm 2. The effects of the dose strength on the electrical state of the diamond surface were investigated. The diamond surface was characterized by using X-ray photoelectron spectroscopy (XPS). Work functions were determined from the XPS photoemission cutoff and valence band spectrum. The hydrogen-terminated II a (100) surface demonstrated the negative electron affinity (NEA) property, and the work function was found to be 3.9 eV. The NEA property disappeared after phosphorus-ion irradiation higher than 2.3×10 12 ions/cm 2, and the work function increased to 4.7 eV at 1.3×10 13 ions/cm 2. We believe that a new energy level is formed due to the displacement of phosphorus. Defects were introduced at ion irradiation doses greater than 3.0×10 13 ions/cm 2, and a defect-related energy state was formed.

Atomic-scale desorption of hydrogen from hydrogenated diamond surfaces using the STM

Diamond has a number of unique chemical and physical properties. In particular, when covered with hydrogen, diamond surfaces acquire a negative electron affinity (NEA). This NEA property has already been used to fabricate high-efficiency diamond-based light detectors and/or electron emitters. We have used the scanning tunnelling microscope for (i) atomic-scale visualisation of the hydrogenated diamond surface, (ii) probing the surface electronic structure and (iii) atomic-scale desorption of hydrogen atoms. Desorption of individual hydrogen atoms has been used to pattern pre-selected areas on the hydrogenated diamond surface. This is considered to be a promising way to fabricate atomic-scale photon detectors and/or electron emitters. The feasibility of the tip-induced atomic-scale desorption of hydrogen from the diamond surface is discussed in comparison with the similar studies on hydrogenated silicon and germanium surfaces performed previously.

Photoemission from the negative electron affinity (100) natural hydrogen terminated diamond surface

We reported recently about the cleaning and polishing of natural doped (100) and (111) diamond surfaces in a hydrogen plasma at 870°C and 40 mbar [Küttel et al., Surf. Sci. 337 (1995) L812]. This smooth (100) surface shows a sharp negative electron affinity (NEA) peak for the 2 × 1 monohydride terminated surface upon annealing to 300°C, experimentally observed by ultraviolet photoemission spectroscopy (UPS). The effect of annealing the crystal up to 1000°C in an UHV environment ( p < 3 × 10 −9 mbar) results in a positive electron affinity (PEA) whereas a subsequent atomic hydrogen adsorption from a heated filament leads to an 1 × 1 reconstruction and to the reappearance of the NEA peak with a lower intensity than the plasma exposed surface. Upon annealing the surface up to 1000°C at a pressure of 5 × 10 −8 mbar, a NEA is observed which probably is caused by remaining hydrogen at the surface. The hydride terminated surface seems to be responsible for the NEA property. The link between the NEA property and the band bending of the (100) surface are elucidated by photoemission spectroscopy of the core level and the valence band and we measured the spatial distribution of the NEA peak for the plasma exposed surface by angle resolved UPS. Finally, we show photoelectron current measurements and the influence of the bias applied to the surface.