Epitaxial growth of thin films

based on the lecture of Prof. Gabriel Ferro
Université Claude Bernard Lyon, France

                                                                                                         Epitaxial growth of thin films
Abstract
The term 'epitaxial' is applied to a film grown on top of the crystalline substrate in ordered fashion that atomic arrangement of the film accepts crystallographic structure of the substrate. Epitaxial growth is one of the most important techniques to fabricate various 'state of the art' electronic and optical devices.

Modern devices require very sophisticated structure, which are composed of thin layers with various compositions. Quality, performance and lifetime of these devices are determined by the purity, structural perfection and homogeneity of the epitaxial layers. Epitaxial crystal growth resulting in epitaxial layer perfection, surface flatness and interface abruptness depend on number of factors like: the epitaxial layer growth method, the interfacial energy between substrate and epitaxial film, as well as the growth parameters – thermodynamic driving force, substrate and layer misfit, substrate misorientation, growth temperature, etc…

Recently epitaxial growth is also used for fabrication of semiconductor quantum structures like quantum dots giving highly perfect structures with high density. In this report the aspect determining the epitaxial growth mode, epitaxial layer growth techniques and additional focusing on SiC epitaxial growth is discussed.

 

1. Epitaxial growth modes
The occurrence of the epitaxial growth modes depends on various parameters of which the most important are the thermodynamic driving force and the misfit between substrate and layer. The growth mode characterizes the nucleation and growth process. There is a direct correspondence between the growth mode and the film morphology, which gives the structural properties such as perfection, flatness and interface abruptness of the layers. It is determined by the kinetics of the transport and diffusion processes on the surface. Different atomistic processes may occur on the surface during film growth: deposition, diffusion on terraces, nucleation on islands, nucleation on second-layer island, diffusion to a lower terrace, attachment to an island, diffusion along a step edge, detachment from an island, diffusion of dimmer (see Figure 1).

Experimentally, the distinction between three classical growth modes is well known: Frank-van der Merwe (FV), Volmer-Weber (VW) and Stranski-Krastonov (SK). In addition to the three well-known epitaxial growth modes mentioned above there are four distinct growth modes: step flow mode, columnar growth, step bunching, screw-island growth.

Volmer-Weber (VW) growth mode
A VW growth mode consists in first phase of large number of surface nuclei and in
second phase of their spreading. Thus VW growth often results in a high mosaicity of the material inside the layer. Usually continues growth of the layer, after initial VW growth, occurs by columnar growth, but in the case of 3C-SiC on Si a VW growth mode results in growth of layers that are not columnar using right conditions. Stranski-Krastonov (SK) growth mode SK mode is considered as intermediate between the FV and VW growth modes, and it is caused by significant lattice misfit from film and substrate. The lattice mismatch between the substrate and the film creates a build-in strain as a consequence of the increasing elastic energy with increasing layer thickness. The first deposited layer is atomically smooth (FV growth mode), compressively strained layer up to a certain thickness called critical thickness. When the deposition time is enough exceeding the critical thickness – phase transition to islands rapidly takes place (VW growth mode), because the nonuniform strain field can reduce the strain energy by an island array, compared with a uniform flat film, resulting in the SK growth mechanism. Step flow growth mode Step flow mode is clearly distinct from layer-by layer growth in FV mode. Unidirectional step flow is induced by substrate missorientation (off cut angle). This trick is often used to avoid island formation, their coalescence and following columnar growth in epitaxy from the vapor phase. Step bunching growth mode Step bunching is observed when a high density of steps moves whit large step velocities over the growth surface. By fluctuations, higher steps catch up with lower steps and then move together as double, triple…. Or in general as macro steps that can exceed thickness of thousands of monosteps. The microsteps cause different incorporation rates of impurities and dopands due to locally varying growth rate Spiral-island growth mode Coalescence of larger number of initial growth islands may lead to screw dislocations due to the layer structure resulting in spiral-island growth mode.

2. Control of growth modes
There are two main types of epitaxy – homoepitaxy and heteroepitaxy. Homoepitaxy is when the same material (or polytype) as the substrate is grown for example: Si on Si, 4H-SiC on 4H-SiC. Heteroepitaxy is when a different material (or polytype) from the substrate is grown for example: GaN on sapphire, 3C-SiC on 6H-SiC. In heteroepitaxy the lattice mismatch between substrate and film and the supersaturation, plays a key role on growth mode and this is demonstrated. This layer-by-layer growth mode FV requires the zero misfit as indicated Large lattice misfit normally induces VW mode except for large interface energies between substrate and film, which will cause SK mode. If structural perfect layer are required, either homoepitaxy or substrate with zero misfit are needed. On another hand the misorientation of the substrate provides steps on surface depending on the angle and direction of misorientation. The density of the steps can be made so high and interstep distance so small that VW or SK modes can be suppressed. The layers growth in the step flow mode have relatively high crystal perfection because defects due to coalescence are prevented.

3. SiC Polytype control
The best eptaxial layer qualities are achieved by using homoepitaxy, because of the compatibility of grown material with substrate. However not all materials substrates are commercially available. For example the only commercially available SiC substrates are of the 4H and 6H polytypes. So if one wants to grow 4H or 6H-SiC, there are no big problems to do homoepitaxy, however to grow 3CSiC heteroepitaxy has to be applied.
Using heteroepitaxy one has to care about lattice misfit, temperature expansion
coefficient and etc. - not all substrates can be used.
It is known that SiC exists in different polytypes, there are more then 200 of them. Most stable are 4H-, 6H- and 3C-SiC (Figure 4). When growing homoepitaxy or heteroepitaxy one should care about polytype inclusions, which are very common. One could think about temperature dependence for polytype stability, it is clear, that two or more polytypes can grow at the same temperature, so other polytype control methods has to be applied. For homoepitaxy of hexagonal polytypes, one solution is to increase surface mobility of the adatoms by higher temperature, but usually this is not preferable, because of technical problems. The most popular thing in SiC homoepitaxy is to cut substrate off-axis to create steps.

Double positioning boundaries (DBPs) of 3C-SiC
DBPs is a special defect for 3C-SiC growth on hexagonal substrate which comes from the two possible orientations of the cubic 3C-SiC axis on the hexagonal α- SiC basis [8]. Because 3C-SiC has two different orders in stacking sequence either ABCABC… or ACBACB… Normally in on-axis substrate, these two stacking sequences can be formed on the substrate in alter positions (see Figure 7). When the nucleated domains expand via 2D-nucleation mechanism, these two domains cannot blend together according to a different stacking order. The boundaries of these two domains are so-called "Double Positioning Boundaries". DBPs can be observed as 60 degrees angle difference of triangular defects according to the crystal structure. These defects will limit the expanding of domains and the size of single crystal growth.

4. TECHNIQUES FOR EPITAXY
The techniques of epitaxy can be classified according to the phase (till ex: liquid
(solution), or vapor) of material use to form the epitaxial layer. Growth techniques: liquid phase epitaxy (LPE), physical vapor deposition (PVD) and molecular beam epitxay (MBE).

4.1 Molecular beam epitaxy (MBE)
Molecular beam epitaxy is a technique for epitaxial growth via the interaction of one or several molecular or atomic beams that occurs on a surface of a heated crystalline substrate. In Figure 8 scheme of a typical MBE system is shown. The substrate, on witch the heterostructure to be grown, is placed on a sample holder which is heated to the necessary temperature and, when needed, continuously rotated to improve the growth homogeneity. The growth in the MBE requires ultra high-vacuum (UHV), typically 10-6 – 10-4 mbar during growth. After outgasing under such a high vacuum, O2, CO2, H2O, and N2 contamination on the growing surface can be neglected. The typical growth conditions make possible to reduce the rate down to nm/sec, so that precise control of the growth thickness is possible – this is a great advantage.

Growth of SiC by MBE
Although growth with MBE has a lot of advantages, like: growth of atomically abrupt interfaces, heteropolytype engineering (for example 4H/3C/4H heterostructure) and in-situ characterization, for growth of SiC it is almost not used. Firstly because of high costs. Secondly because of source material availability, Si source is not a problem, but C is not so easy, graphite can be used, but high temperature cells are needed, C60 doesn't require high temperature cells, but material is expensive. And for usage of gaseous precursors a lot of technical problems arises like: keeping ultra high vacuum and high temperatures are needed (>1200 0C), which is hard to implement in MBE. Also grown layers have high background doping levels and polytype inclusion is a serious.

4. 2 Liquid phase epitaxy (LPE)
The process known as LPE is a technique for the deposition of the epitaxial layers
from supersatured solution. The chosen solvent has generally low melting point and low vapor pressure. LPE method is mainly used for the growth of compound semiconductors. Very thin, uniform and high quality layers can be produced. Typical example of LPE method is given by the growth of III-V compounds. In this case, the process can be described as follows: a melt of pure gallium exposed to a GaAs wafer will dissolve some of the solid to produce a dilute solution of group V element. Cooling this solution to induce a slight supersaturation, and bringing a substrate into the contact with the melt surface, will result in the growth of a layer of GaAs all over the substrate surface. At conditions that are close to the equilibrium, deposition of the semiconductor crystal on the substrate is slowly and uniform. The equilibrium conditions depend very much on the temperature and on the concentration of the dissolved semiconductor in the melt. The thickness of the epitaxial layer is controlled by the contact time between substrate and solution, the cooling rate, rate of diffusion of the slowest component elements etc… The major advantage of the LPE is that the growth temperature can be well below the melting point of the compound semiconductor which is being decomposed. Furthermore, equipment is simple and inexpensive, also non-hazardous. Key problem in the production of the epilayer is that the composition of relatively small volumes of each melt will rapidly change as crystal growth proceeds. LPE is to simple to grow more complicated nanostructures, because of the difficult thickness and composition control, etc… Figure 9 shows sketches of three kinds of LPE growth process: a tipping arrangement, slightly more complicated sliding substrate holder and sandwich arrangement.

LPE of SiC
SiC does not form a stoichiometric liquid phase at normal conditions. The Si-C phase diagram is shown in Figure 10. Instead the material decomposes to vapor at 2830 °C. Silicon carbide can be grown from the liquid phase by using a nonstoichiometric melt. The natural choice as base for the solution would be Si since this is a constituent of SiC and high-purity Si is commercially available. The growth rates using silicon as a solvent are not high since the solubility of C in Si is very low at temperatures less than 2000 °C. By introducing a transition metal to the silicon melt the solubility of carbon is increased. An example is given by using Si-Sc melts for which the liquid phase epitaxy of SiC has successfully demonstrated good influence on the growth rate and on the structural properties (crystallinity and surface morphology) of the SiC epitaxial layers. The tipping and sliding arrangement, as shown in figure 9, are not used for SiC epitaxy by LPE because of the high reactivity of the melt with the crucible. Most of the time, a dipping or sandwich arrangement is preferred.

4.3 Vapor-liquid-solid (VLS) method
The vapor-liquid-solid (VLS) method has recently been re-examined to produce one dimensional structures (whiskers) for nano-physics technology or other applications [12]. The VLS mechanism has been also developed for growth of SiC epitaxial layers. Some of the basic mechanisms involved in the VLS method are similar to LPE. In case of LPE, carbon is supplied by the graphite container, a solid SiC source in direct contact with the solution or initial dissolution of the substrate while in the VLS method the carbon is provided through the reaction of a carbon containing gas phase with silicon containing liquid phase. The difference to "conventional" LPE growth conditions is that VLS growth may be performed even at a negative temperature gradient, i.e. the temperature is higher at the substrate than in the liquid or the top of the solution, and the requirements on the temperature gradient are not as strict.

Barbara Scarlett Betancourt Morales

CAF