Wafer scale heteroepitaxy of silicon carbon and silicon carbide thin films and their material properties

For years now, many have believed the solution to reducing the cost of the wide bandgap compound semiconductor silicon carbide (SiC) is to grow its cubic form (3C-SiC) heteroepitaxially on silicon (Si). This has the potential to reduce cost, increase wafer size and integrate SiC with Si technology. After decades of research, 3C-SiC grown on Si is still yet to penetrate the commercial market as the process is plagued with various issues such as very high growth temperatures, thermal stresses, high cost, poor epitaxial material quality and poor scalability to wafer sizes beyond 100 mm diameter. The first section of this thesis starts with a focus on the traditional, high temperature growth of 3C-SiC carried out in the first industrial type SiC based reduced pressure chemical vapour deposition (RP-CVD) reactor installed in a UK University. After the process demonstrated little promise for mass scale implementation into the semiconductor industry, a radical change in strategy was made. The research pivoted away from SiC and instead focussed on silicon carbon alloys (Si1-yCy) with carbon (C) contents in the range of 1-3%. Si1-yCy has a range of applications in strain engineering and reducing contact resistance, differing from 3C- SiC quite significantly. Crystalline alloys with C contents around 1.5% were achieved using an industry standard Si based RP-CVD growth system. Analysis was carried out on the defects that form due to the saturation of C in higher content alloys. The high temperature annealing of Si1-yCy resulted in out diffusion of C and traces of 3C-SiC growth which presented itself as a potential buffer layer for 3C-SiC epitaxy. Through the careful selection of growth precursors and process optimisation, high crystalline quality 3C-SiC was grown heteroepitaxially on Si within the industry standard Si based RP-CVD and in-depth material characterisation has been carried out using a vast range of techniques. High levels of electrically active dopants were incorporated into the 3C-SiC and its electrical properties were investigated. Various investigations were carried out on suspended 3C-SiC and Si1-yCy films including strain and tilt measurements through micro X-ray diffraction and the effect of thickness and doping on their optical properties. The results led to a greater understanding of suspended films and provide a foundation for a number of applications in microelectromechanical systems (MEMS) and optical devices. Further material growth research was carried out on Si1-yCy multilayers, selective epitaxy of 3C-SiC and the growth of 3C-SiC on suspended growth platforms. Each topic presents an interesting area for further research. The research presented demonstrates new, state of the art 3C-SiC heteroepitaxial material and its basic structural, electrical and optical properties. A new low-cost and scalable process has been developed for the heteroepitaxial growth of 3C-SiC on Si substrates up to 100 mm with a clear path to scaling the technology up to 200 mm and beyond. Not only does the developed technology have a high commercial impact, it also paves the way for many interesting future research topics, some of which have been briefly investigated as part of this work

Ultrasonic inspection and self-healing of Ge and 3C-SiC semiconductor membranes

Knowledge of the mechanical properties and stability of thin film structures is important for device operation. Potential failures related to crack initiation and growth must be identified early, to enable healing through e.g. annealing. Here, three square suspended membranes, formed from a thin layer of cubic silicon carbide (3C-SiC) or germanium (Ge) on a silicon substrate, were characterised by their response to ultrasonic excitation. The resonant frequencies and mode shapes were measured during thermal cycling over a temperature range of 20--100~$^\circ$C. The influence of temperature on the stress was explored by comparison with predictions from a model of thermal expansion of the combined membrane and substrate. For an ideal, non-cracked sample the stress and Q-factor behaved as predicted. In contrast, for a 3C-SiC and a Ge membrane that had undergone vibration and thermal cycling to simulate extended use, measurements of the stress and Q-factor showed the presence of damage, with the 3C-SiC membrane subsequently breaking. However, the damaged Ge sample showed an improvement to the resonant behaviour on subsequent heating. Scanning electron microscopy showed that this was due to a self-healing of sub-micrometer cracks, caused by expansion of the germanium layer to form bridges over the cracked regions, with the effect also observable in the ultrasonic inspection

A fast approach to measuring the thickness uniformity of a homoepilayer grown on to any type of silicon wafer

Optical reflection spectroscopy techniques offer a non-destructive and fast method of measuring the thickness of silicon (Si) epilayers, enabling very fast thickness uniformity mapping across the full surface of epiwafers up to 450 mm in diameter. However, their use for undoped or low doped epilayers has traditionally been constrained by a dependence on high levels of substitutional doping in the Si wafer, at values of approximately 5 × 1019 cm−3. Whilst the high dopant concentration of this wafer creates the necessary reflectance boundary for optical reflection, their commercial availability is mainly limited to the (001) surface orientation only. Optical reflectance techniques are therefore also limited in use to this orientation. In this article, an approach to measure the thickness of a Si epilayer on any Si wafer, independent of its crystallographic orientation, doping type and value, using Fourier transform infrared reflection spectroscopy is proposed and demonstrated. Because the use of non-destructive optical reflection spectroscopy is already common and well-understood within both industry and academia, this technique could easily be implemented within existing industrial and research fabrication facilities. Furthermore, this approach could be adapted, with further work, to suit other semiconductor materials and other optical reflection techniques

Strain mapping of silicon carbon suspended membranes

The alloy silicon carbon (Si1-yCy) has various strain engineering applications. It is often implemented as a dopant diffusion barrier and has been identified as a potential buffer layer for cubic silicon carbide (3C-SiC) heteroepitaxy. While suspended membranes formed from thin films of semiconductor (Ge and 3C-SiC) and dielectric (Si3N4) materials have been well studied, pseudomorphic, defect-free epilayers under high levels of tensile strain have received little attention. Often, tensile strain is a desired quality of semiconductors and enhancing this property can lead to various benefits of subsequent device applications. The strain state and crystalline tilt of suspended Si1-yCy epilayers have been investigated through micro-X-ray diffraction techniques. The in-plane tensile strain of the alloy was found to increase from 0.67% to 0.82%. This strain increase could reduce the C content required to achieve suitable levels of strain in such alloys and further strain enhancement could be externally induced. The source of this strain increase was found to stem from slight tilts at the edges of the membranes, however, the bulk of the suspended films remained flat. The novel process utilised to fabricate suspended Si1-yCy thin-films is applicable to many other materials that are typically not resistant to anisotropic Si wet etchants

Controlling the optical properties of monocrystalline 3C-SiC heteroepitaxially grown on silicon at low temperatures

Cubic silicon carbide (3C-SiC) offers an alternative wide bandgap semiconductor to conventional materials such as hexagonal silicon carbide (4H-SiC) or gallium nitride (GaN) for the detection of UV light and can offer a closely lattice matched virtual substrate for subsequent GaN heteroepitaxy. As 3C-SiC can be heteroepitaxially grown on silicon (Si) substrates its optical properties can be manipulated by controlling the thickness and doping concentrations. The optical properties of 3C-SiC epilayers have been characterized by measuring the transmission of light through suspended membranes. Decreasing the thickness of the 3C-SiC epilayers is shown to shift the absorbance edge to lower wavelengths, a result of the indirect bandgap nature of silicon carbide. This property, among others, can be exploited to fabricate very low-cost, tuneable 3C-SiC based UV photodetectors. This study investigates the effect of thickness and doping concentration on the optical properties of 3C-SiC epilayers grown at low temperatures by a standard Si based growth process. The results demonstrate the potential photonic applications of 3C-SiC and its heterogeneous integration into the Si industry

Electrical properties of n-type 3C-SiC epilayers in situ doped with extremely high levels of phosphorus

Low temperature heteroepitaxy of cubic silicon carbide (3C-SiC) on silicon substrates is key to the low-cost and mass scale hetergeneous integration of 3C-SiC into the semiconductor market. Low temperature growth also opens up the opportunity to dope 3C-SiC in situ during the epitaxial growth with standard Si based n-type and p-type dopants. In situ doping offers many advantages over ion implantation, such as complex doping profiles, more abrupt interfaces and minimal crystal damage. In this study, 3C-SiC thin films have been doped with phosphorus to a range of concentrations during epitaxial growth on standard silicon (Si) substrates. Both the material and electrical properties of the films have been investigated. Hall effect measurements and secondary ion mass spectroscopy profiling confirm 100% electrically active n-type dopants up to 2 × 1020 cm−3. The process offers extreme control over the 3C-SiC electrical properties without relying on post-growth ion implantation and high temperature activation annealing, enabling the formation of more complex 3C-SiC based devices and low resistance contacts

RP-CVD growth of high carbon content Si1−xCx epilayers using disilane and trimethylsilane precursors

We have demonstrated Si1-xCx epilayers growth by RP-CVD and using Disilane and Trimethylsilane precursors. Very high Carbon content, up to 2.4%, has been obtained. It is close to the best results obtained using more expensive precursors. Use of the RP-CVD is vital from an industrial standpoint as, although MBE offers greater control over growth parameters, the RP-CVD is one of the only viable methods of mass epitaxial growth

Analysis of surface defects in Si1−yCyepilayers formed by the oversaturation of carbon

Strained Si1−y C y epilayers have been grown on Si (001) by reduced pressure chemical vapor deposition, using low-cost precursors disilane and trimethylsilane. Substitutional C incorporation has been achieved in strained epilayers up to y = 1.5%, while higher C content of at least 2.4% is observed in relaxed layers. These results are comparable to the highest concentrations achieved using more highly reactive, but expensive, precursors. These relatively high C content epilayers were found to form defects throughout growth attributed to the clustering of C adatoms, which result in localized accelerated amorphous growth and, consequently, hillocks forming on the epilayer surface. The formation, size and distribution of these surface defects has been analyzed through the use of various microscopic techniques. The size and density of these structural defects increases with both C content and epilayer thickness. In our layers of fixed growth time, substitutional C compositions above 1.5% causes hillocks to fuse on the surface; subsequently amorphous growth occurs, which forms an amorphous layer over the crystalline Si1−y C y epilayer and hence prevents further epitaxy or reliable device fabrication. The results of this investigation suggest that substitutional C composition of below 1.5% could be achieved without the need for expensive and volatile precursors or complex growth processes, assuming sufficiently thin layers are grown

The effect of Ge precursor on the heteroepitaxy of Ge1−x Sn x epilayers on a Si (001) substrate

The heteroepitaxial growth of Ge1−x Sn x on a Si (001) substrate, via a relaxed Ge buffer, has been studied using two commonly available commercial Ge precursors, Germane (GeH4) and Digermane (Ge2H6), by means of chemical vapour deposition at reduced pressures (RP-CVD). Both precursors demonstrate growth of strained and relaxed Ge1−x Sn x epilayers, however Sn incorporation is significantly higher when using the more reactive Ge2H6 precursor. As Ge2H6 is significantly more expensive, difficult to handle or store than GeH4, developing high Sn content epilayers using the latter precursor is of great interest. This study demonstrates the key differences between the two precursors and offers routes to process optimisation which will enable high Sn content alloys at relatively low cost

Non-linear vibrational response of Ge and SiC membranes

Characterisation of membranes produced for use as micro-electro-mechanical systems using vibrational techniques can give a measure of their behaviour and suitability for operation in different environments. Two membranes are studied here: germanium (Ge) and cubic silicon carbide (3C-SiC) on a silicon (Si) substrate. When driven at higher displacements, the membranes exhibit self-protecting behaviour. The resonant vibration amplitude is limited to a maximum value of around 10 nm, through dissipation of energy via higher harmonic vibrations. This is observed for both materials, despite their different Young's moduli and defect densities