Gerard Colston

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 Youngs moduli and defect densities.

Mapping the Strain State of 3C-SiC/Si (001) Suspended Structures Using μ-XRD

The residual strain has been mapped across suspended 3C-SiC membranes and wires using synchrotron based micro X-ray diffraction (μ-XRD). Residual tensile strain is observed to relax slightly upon suspension in both sets of structures. Similar maps were acquired by calculating the residual strain from the shift in 3C-SiC Raman peaks. Comparable trends in strain relaxation are observed by both methods, although the sensitivity of μ-XRD is higher using our measurement conditions. While Raman shift provides a fast and convenient method for mapping strain variations, it cannot give direct measurements of the lattice parameters that can be achieved with μ-XRD, making these techniques excellent complimentary methods of mapping residual strain in 3C-SiC.

Integration of silicon carbide on silicon for its application in ultraviolet photodetectors

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. Novel low-temperature epitaxy technology, leading to the invention of state of the art wafer scale heteroepitaxy of 3C-SiC on standard, up to 150 mm diameter so far, Si(001) and Si(111) substrates is demonstrated. The initial experiments demonstrate that 3C-SiC offers a route to tuneable UV photodetectors or protective coatings on UV susceptible devices by simply controlling epilayer thickness and impurity doping concentrations. The obtained results open an opportunity for the integration of 3C-SiC with other semiconductor materials on Si substrates and for the creation of low-cost and high performance UV and visible spectral range photodetectors.

Impact of Sn segregation on Ge1−xSnx epi-layers growth by RP-CVD

This work investigates the impact of Sn segregation on the growth of Ge1−xSnx epi-layers using a reduced pressure chemical vapour deposition (RP-CVD) system with the common precursors Ge2H6 and SnCl4. The investigated samples were grown on top of a 1 µm thick relaxed Ge buffer layer with different amounts of Sn incorporation, achieved by increasing the SnCl4 partial pressure. The grown Ge1−xSnx epi-layers themselves are fully strained with respect to the Ge buffer underneath. A range of advanced analytical techniques have been used to characterize the material properties. The crystal structure, quality and thickness of the Ge1−xSnx epi-layers were analysed by using cross-sectional high resolution transmission electron microscopy, high resolution X-ray diffraction and fourier transform infrared spectrometry. Atomic force microscopy and Scanning electron microscopy in combination with energy dispersive X-ray spectroscopy are used for analysing the surface. It is shown that simply increasing the SnCl4 partial pressure is insufficient for achieving Sn contents beyond ∼8%. Above these concentrations the epitaxial growth breaks down due to the segregation of Sn resulting in the formation of dots on the epilayer surface, which consist of pure Sn.

Quality-factor and frequency shifts of suspended Ge membranes

Crack initiation and growth are key issues when it comes to the mechanical reliability of microelectronic devices and microelectromechanical systems, especially when a system is fabricated using a suspended membrane as a platform. In this study, two suspended Ge membranes on a silicon substrate were investigated by measuring their vibrational response over a frequency range of 50 kHz to 500 kHz. Key material properties were identified by measuring the frequencies and mode patterns for a set of vibrational resonant modes using a laser interferometer. The membranes were subject to thermal cycling over a temperature range of 25 °C to 97 °C. The temperature dependence of mechanical properties, including residual stress and the quality (Q-) factor of each membrane, were studied at low pressure (10−3 mbar)and at room pressure. Experimental results are compared with models. The change in biaxial stress with temperature agrees well for a perfect membrane. Unexpected behaviour was observed in one membrane which sh...

Laser ultrasonic characterization of membranes for use as micro-electronic mechanical systems (MEMS)

Germanium (Ge) on Silicon (Si) has the potential to produce a wide variety of devices, including sensors, solar cells and transistors. Modification of these materials so that a suspended membrane layer is formed, through removing regions of the Si substrate, offers the potential for sensors with a more rapid response and higher sensitivity. Such membranes are a very simple micro-electronic mechanical system (MEMS). It is essential to ensure that the membranes are robust against shock and vibration, with well-characterised resonant frequencies, prior to any practical application. We present work using laser interferometry to characterise the resonant modes of membranes produced from Ge or silicon carbide (SiC) on a Si substrate, with the membranes typically having around 1 mm lateral dimensions. Two dimensional scanning of the sample enables visualisation of each mode. The stress measured from the resonant frequencies agrees well with that calculated from the growth conditions. SiC provides a more robust p...

Disilane and trimethylsilane as precursors for RP-CVD growth of Si 1−y C y epilayers on Si(001)

Strained Si1-yCy epilayers with up to 1.48% C content have been grown on Si(001) by RP-CVD using the low-cost precursors disilane and trimethylsilane. These epilayers, with higher C content, were found to form point defects throughout growth resulting in amorphous hillocks forming on the epilayer surface. The size and density of these surface defects increases with C content and epilayer thickness. With substitutional C compositions above 1.48% the hillocks fuse on the surface and subsequent amorphous growth occurs.

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.

Si1-xCx/Si(001) Heterostructures for Use in Schottky Diode Rectifiers Demonstrating High Breakdown Voltage and Negligible Leakage Current

Vertical Schottky diodes have been fabricated on low C content Si1-xCx and 3C-SiC epilayers epitaxially grown on Si(001) substrates. Significant leakage current was observed in 3C-SiC diodes under reverse bias, masking any rectifying behavior. This issue is far less pronounced in Si1-xCx based Schottky diodes which demonstrate a clear critical breakdown. Leakage current is shown to be greater in relaxed Si1-xCx layers. While crystalline Si1-xCx is not currently a viable material for high power electronics it is useful for assessing the impact lattice mismatch and crystalline quality has on the behavior of rectifiers.

RP-CVD growth of high carbon content Si 1−x C x 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.