(Invited) Ballistic Hot Electron Effects in Nanosilicon Dots and Their Photonic Applications
Authors: Koshida Nobuyoshi , Ikegami Naokatsu , Kojima Akira , Mentek Romain , Suda Ryutaro , Yagi Mamiko , Shirakashi Junichi , Gelloz Bernard , Mori Nobuya 
Published: February 2020

The efficient generation of ballistic hot electrons in nanosilicon dots under an electric field is based on the intrinsic carrier transport therein. Due to suppressed phonon scattering losses followed by enhanced tunneling rate, electrons become hot at the early stage of injection [1]. Generated ballistic hot electrons can be utilized either inside or outside the nanosilicon dot layer. The topics on this effect are discussed here toward possible photonic applications.
The nanosilicon device is a kind of MIS diode consisting of a thin film surface electrode, a nanosilicon layer (~1 μm thick), a crystalline silicon wafer substrate, and a back contact. The nanosilicon layer includes parallel chains of quantum-sized nanosilicon dots (~3 nm in mean diameter) interconnected with tunnel oxides. When a positive voltage is applied to the surface electrode, electrons injected from the substrate into the nanosilicon layer are drifted toward the outer surface, and then some of them reach the surface electrode as ballistic hot electrons.
One possible application of this effect is the avalanche photoconduction by sequential impact ionization in nanosilicon dots. Actually, the photo-carriers multiplication effect was observed in thin oxidized nanosilicon diodes under the reverse bias condition [2]. For a monochromatic near-uv light incidence at low temperatures (e.g., 77 K), the photoconduction quantum efficiency was increased up to 2500% at an external electric field of 9×105 V/cm. The experimental data on the temperature, electric field, and sample thickness dependencies of the photocurrent suggest that the field-induced avalanche multiplication of photo-excited carriers occurs inside the nanosilicon dot layer. From theoretical analyses of carrier dynamics [3], it has been clarified that the impact ionization rate is significantly enhanced in nanosilicon dots compared with in bulk silicon owing to the efficient transition between quantized discrete energy states and the enhanced Coulomb interaction in nanosilicon dots. The threshold electron energy of impact ionization is decreased in realistic nanosilicon dots with disordered surface structures [4]. These facts provide useful information for the development of efficient photo-sensors and photovoltaic cells.
Ballistic hot electrons generated in the nanosilicon layer uniformly ejects to outside by tunneling through the thin surface electrode [5]. The most characteristic feature of this emission phenomenon is the high kinetic energy of electrons and its controllability. At applied voltages of 15-20 V, for instance, the mean energy of output electrons varies from 5 to 7 eV. No conventional cold cathodes can emit such energetic electrons. Being a supplier of electrons with an extremely high physical and electrochemical activity, this emitter is available for the operation in every media: vacuum, atmospheric gases, and solutions.
In vacuum, this device is applicable to the exposure source for parallel EB lithography [6]. By integrating the emitter array with active-matrix CMOS driver circuit and combining with focusing electron optics system, alternative advanced nanofabrication technology would be developed. Another promising application in vacuum is a probing source for an extremely high-sensitivity image pick-up.
The usefulness of the nanosilicon ballistic emitter in atmospheric gases has been demonstrated in air and in Xe gases as a negative ion source and a vacuum-ultraviolet (VUV) light generator, respectively. The former relates to the dissociative hot electron attachment on O2 molecules. In the latter, the output electron energy match well with the internal excitation of Xe gas molecules followed by VUV emission without any impact ionizations.
This emitter acts even in solutions as an active electrode which supplies highly reducing electrons. Based on the experimental confirmation of the usefulness in aqueous solutions for H2 generation and pH control, it has been demonstrated that the operation of the emitter in metal-salt solutions is available for depositing thin metal (Cu, Ni, Co, Zn, and so on) films without using any counter electrodes.
In contrast to the exchange of thermalized electrons, the ballistic electro-reduction scheme is effective in SiCl4 and GeCl4 solutions. When driven in these solutions, thin Si and Ge films are deposited uniformly on the emitting surface [7]. As in the case in metal-salt solutions, injected energetic electrons can directly reduce Si4+ and Ge4+ ions at the interface leading to the nuclei formation and the subsequent film growth with neither contaminations nor byproducts. Because of a large electrochemical window, it is difficult to get uniform thin Si and Ge films by electroplating. The deposition rate in this unilateral electro-reduction process is comparable to that in the conventional dry process. The structural analyses indicate that the deposited thin Si and Ge films are of amorphous. Visible photoluminescence was observed in ultra-thin Si films. In addition, it is possible to deposit thin SiGe films by the operation in mixture (SiCl4+GeCl4) solutions. Since the ballistic electro-reduction is an alternative clean approach, this is attractive for the low-temperature fabrication of thin-film and multilayered photoelectronic devices.
This work was partially supported by the JSPS through a Grant-in-Aid for Scientific Research and the FIRST Program and by the NEDO through the R&D Project.
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