Localized growth of heteromaterials by VLS transport for power electronic applications – V-LOC

The diamond substrates used are commercial substrates produced either High Pressure High Temperature, or by Chemical Vapour Deposition (CVD). Before the SiC epitaxial growth, an important work has been dedicated to the diamond surface preparation by a polishing seeing to create a smooth and misoriented surface, or by the epitaxial deposition of a intrinsic or boron doped diamond thin layer by plasma assisted CVD.
To deposit SiC, we used the growth by Vapour-Liquid-Solid. To grow this layer, we prepared the deposit of a Si-Al bilayer on the diamond substrate. The propane introduced in the reactor is cracked at high temperature and thus allows the carbon dissolution that, by reacting with the Si-Al liquid produced 3C-SiC
The deposits were characterized by X ray diffraction, optical microscope, scanning electron microscope, transmission electron microscope to identify the film nature and morphology. For the doping, we made Raman spectroscopy. Electrical characterisations were done I-V measurements.

The localized heteroepitaxial growth of 3C-SiC on diamond was obtained by Vapour-Liquid-Solid transport producing previously a SiC buffer layer by solid state reaction between a Si layer thermally deposited and the diamond substrate. The deposit isn’t fully covering and exhibits facetted islands (Fig. 1). We showed the fragility of the buffer layer that its stability must be preserved with the limitation of the liquid amount. A carbon out-diffusion of the substrate towards the liquid is observed through the buffer layer. The growths realized without the use of the buffer layer led to the production of a dense polycristalline deposit of 3C-SiC by a dissolution-precipitation mechanism. The aluminium in the liquid phase brought a strong p-doping of the SiC deposit which could be considered as a first step in the production of localized ohmic contacts on diamond by this technique.
By inspiring this process of the SiC buffer layer on diamond, we got very promising results using a classical deposition technique, the chemical vapour deposition (CVD) that not belong to the project. By this way, it is possible to get epitaxial undoped 3C-SiC thin films on diamond (Fig. 2). This constitutes a major advance because heteroepitaxial thin films weren’t obtained on diamond

The growths made by Vapour-Liquid-Solid transport allow to get a localized cubic silicon carbide deposit on the diamond surface. To obtain a heteroepitaxial growth we formed a buff relayer of SiC (solid state reaction between Si layer and the diamond) before to realize the VLS growth. Its deposit is p-doped and formed a ohmic contact on diamond. Promising results were obtained for heteroepitaxial SiC growth on diamond using CVD technique, by a total covering of the substrate with a undoped film. This technique should allow to study the heterojunction SiC(n)/diamond? and bypass the n doping problem of diamond for pn junction based on this material.

• SiC growth on diamond
A. Vo-Ha et al, Mater. Sci. Forum, Vols. 740-742 (2013) pp 331-334
A. Vo-Ha et al, Diam. & Relat. Mater., 35, (2013) pp 24-28
• Diamond etching of diamond by metal catalyst
H. Mehedi et al., Carbon, 59, (2013) 448-456
H-A. Mehedi, Nanotechnology, 23, 455302
• Influence of the misorientation of diamond substrate on the morphology and the defects propagation
M, Naamoun et al., Phys. Status Solidi A, 1-6 (2013)

VLS transport is a crystal growth process which could be described as an interaction between three phases. A solid phase (the substrate) in contact with a liquid phase (the catalyst) who reacts with a gaseous phase (the reactant) in order to form a deposition at the solid-liquid interface. This process is highly used since several years for fabrication of wire-type nano-objects. However, it is rarely used for thin layers epitaxial growth due to numerous technical difficulties associated. Thanks to the project partners competences, this process is well-controlled since it has been studied by one of them since 10 years for SiC thin layers epitaxial growth. In this case, the substrate is a seed of SiC, the liquid is an alloy Metal-Si and the gaseous phase is the carbon source, C3H8 for instance. The carbon generated by cracking of propane is dissolved in the liquid alloy, diffuse until the interface solid-liquid ad reacts with silicon of the liquid to form an epitaxial thin layer of silicon carbide. Among the advantages of this process, we can quote:
- a stable crystal growth (regulated by gas flow)
- a high crystalline quality because of the liquid phase
- the use of lower temperatures than those used with conventional techniques like CVD.
- the possibility to obtain high doping by choosing the right liquid phase.
- the possibility to develop a selective (or localized) growth.
The last point is the most interesting because its high potential, and will be develop in this V-LOC project. The VLS localized transport is based on the fact that it cannot have a growth only if there is a liquid, and only under this liquid. Thus, by using liquid pins containing Si on the surface, SiC could be selectively obtained in the desired areas.
In this project, the objective is to apply the VLS transport to the heteroepitaxial growth of wide band gap semiconductors.
For instance, the localized deposition of (p+)SiC on diamond substrate appears to be a very promising way to improve the ohmic contacts. The VSL transport with a liquid alloy (Al-Si) should warranty a p+ doping a strong adherence of the deposition, which could result in good electrical characteristics.
Concerning the Schottky contacts, we will study the possibility to use localized VLS transport on boron doped diamond (p). As observed for ohmic contacts, we will try to succeed in the deposition of tungsten carbide which allow to obtain Schottky contacs of good quality.
At last, the localized growth by VLS transport ill be used for the following heterostructures: SiC(n) on Diamond(p), AlN(n) on Diamond(p) and AlN on SiC substrate. The n-doped SiC growth will be realized using Ge-Si alloys fed by nitrogen. The AlN growth will be done using an Al-based alloy with ammoniac NH3.
The study of these different systems will allow us to put forward the different assets of the VLS transport: the high quality of crystal growth obtained at lower temperatures than those of usual techniques as CVD. An important point also concerning the investigated system is the possibility to obtain a doping of the material by a smooth way using the liquid phase during the growth.
The partners of this project own well-complementary competences and knowledge that are indispensable for the achievement of the project. The writing of patterns will be considered if the results reach our expectations.