Low-Temperature Deposition of Thin Si, Ge, and SiGe Films Using Reducing Activity of Ballistic Hot Electrons

Quantum-sized nanocrystalline Si (nc-Si) dots behave as a drift layer which efficiently generates ballistic hot electrons. This is due to little phonon scattering losses in nc-Si at the early stage of injection and subsequent multiple-tunneling cascade through nc-Si dots chain interconnected with thin oxides [1]. At a sufficient electric field, generated ballistic hot electrons are ejected through the surface electrode. In solutions, the nc-Si emitter operates as an active electrode supplying highly reducing electrons. This is applicable to deposition of thin films of metals [2], Si [3], and Ge [4]. Details of this thin film deposition scheme based on the ballistic reduction effects is discussed here including the availability for the deposition of thin Ge and SiGe films.

The nc-Si emitter 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 nc-Si 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 nc-Si layer are accelerated toward the outer surface, and then some of them are emitted from the surface electrode as ballistic hot electrons [5]. This planar cold cathode emits energetic and directional electrons. The kinetic energy of electrons can be controlled by the applied voltage. At applied voltages of 15-20 V, for instance, the mean energy of output electrons varies from 5 to 7 eV.

Besides in vacuum, the nc-Si emitter operates in atmospheric-pressure gases and even in solutions. Since electrons with an extremely high and electrochemical activity are supplied at relatively low applied voltage, positive ion species are directly reduced. Following the experimental confirmation of the usefulness in aqueous solutions as means for H₂ generation and pH control, it has been demonstrated that the ballistic hot electron injection into metal-salt solutions leads to the deposition of thin metal (Cu, Ni, Co, Zn, and so on) films without using any counter electrodes.

After the device is mounted in a N₂ gas filled glove box, a small amount (5~10 μl) of SiCl₄, GeCl₄, or their mixture (1:1 in volume) was dripped onto the emission surface, and then the emitter was driven at V_b=10-15 V. After the emitter operation for several min without using any counter electrodes, thin films were uniformly deposited on the emitting area in the same way as the previous dipping method. The structure and composition of the deposited films were investigated by transmission electron microscope (TEM), scanning electron microscope (SEM), atomic force microscope (AFM), energy dispersive X-ray (EDX) spectra, X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectroscopy (SIMS).

In accordance with TEM analyses, the deposited films are amorphous. The observed EDX spectra indicate that the respective deposited films are composed of Si, Ge, and SiGe. The contamination signals of Cl were below the detection limit of XPS measurements. The result of SIMS of thin SiGe films, the compositional ratio of Si to Ge was about 0.9:0.1. This can be controlled by changing the mixture ratio of SiCl₄+GeCl₄ solution.

In contrast to the electro-plating based on exchange of thermalized electrons, the ballistic electro-reduction proceeds with neither Cl₂ evolution nor byproducts generation at the counter electrode. Despite a significantly large electrochemical window between the reduction (Si/Si⁴⁺) and oxidation (Cl₂/Cl^-) potentials, injected electrons have energies sufficient enough for unilateral reduction. Being a low-temperature and clean process, this ballistic electro-reduction mode is attractive as an alternative approach for fabrication of group-IV thin-films, multilayered nanostructures, and 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.

 References

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