Indentation plasticity and fracture in silicon

Measurements of the ductile-brittle transition temperature of heavily doped silicon were carried out using indentation techniques. Diamond pyramid hardness tests were performed on the (100) face of heavily doped N-type and P-type and intrinsic silicon single crystals. Tests were performed over the range 200 C to 850 C and loads of 100 to 500 g were used. Samples were subsequently etched to reveal dislocation rosettes produced by indentation. Intrinsic silicon underwent a ductile-brittle transition at 660 C, P-type at 645 C and N-type at 625 C. Hardness values varied from 1.1 GPa at 700 C to 11.7 GPa at 200 C. Significant effects of hardness on doping were present only at the highest temperatures. Lower loads generally produced higher hardness but load did not affect the Ductile-Brittle Transition Temperature (DBTT). Fracture toughness values ranged from 0.9 MPa m(1/2) at 200 C to 2.75 MPa m(1/2) near the DBTT. Doping did not affect the fracture toughness of silicon. P-type doping increased the size of dislocation rosettes observed after indentation, but N-type did not, in contradiction of the expected results. Results are discussed in terms of the effect of doping on the dislocation mobility in silicon

The effect of plastic deformation on the electrical properties of indium antimonide

Imperial Users onl

A critical examination of discrete lattice and dispersed barrier hardening

Critical assessment of discrete lattice and dispersed barrier hardening theories of thermally activated deformation of metal

Predictive Modeling for Ductile Machining of Brittle Materials

Brittle materials such as silicon, germanium, glass and ceramics are widely used in semiconductor, optical, micro-electronics and various other fields. Traditionally, grinding, polishing and lapping have been employed to achieve high tolerance in surface texture of silicon wafers in semiconductor applications, lenses for optical instruments etc. The conventional machining processes such as single point turning and milling are not conducive to brittle materials as they produce discontinuous chips owing to brittle failure at the shear plane before any tangible plastic flow occurs. In order to improve surface finish on machined brittle materials, ductile regime machining is being extensively studied lately. The process of machining brittle materials where the material is removed by plastic flow, thus leaving a crack free surface is known as ductile-regime machining. Ductile machining of brittle materials can produce surfaces of very high quality comparable with processes such as polishing, lapping etc. The objective of this project is to develop a comprehensive predictive model for ductile machining of brittle materials. The model would predict the critical undeformed chip thickness required to achieve ductile-regime machining. The input to the model includes tool geometry, workpiece material properties and machining process parameters. The fact that the scale of ductile regime machining is very small leads to a number of factors assuming significance which would otherwise be neglected. The effects of tool edge radius, grain size, grain boundaries, crystal orientation etc. are studied so as to make better predictions of forces and hence the critical undeformed chip thickness. The model is validated using a series of experiments with varying materials and cutting conditions. This research would aid in predicting forces and undeformed chip thickness values for micro-machining brittle materials given their material properties and process conditions. The output could be used to machine brittle materials without fracture and hence preserve their surface texture quality. The need for resorting to experimental trial and error is greatly reduced as the critical parameter, namely undeformed chip thickness, is predicted using this approach. This can in turn pave way for brittle materials to be utilized in a variety of applications.Ph.D.Committee Chair: Liang, Steven; Committee Co-Chair: Li, Xiaoping; Committee Member: Garmestani, Hamid; Committee Member: Griffin, Paul; Committee Member: Melkote, Shreyes; Committee Member: Neu, Richar

Influence of Arsenic Pressure on the Doping of Gallium Arsenide with Germanium

THE doping of III-V compounds with elements from the IVth column of the periodic table has been studied under standard conditions of preparation by several investigators. In most cases, the IV element was found to act as an n-type dopant of low doping efficiency, a result that is usually interpreted to mean that more of the impurity atoms are located on the III element sublattice than on the V element sublattice. Causes for the unequal distribution of impurity atoms between the two sublattices have been sought in the sizes of the atoms and in their binding energies. An additional influence on the impurity atom distribution, namely, the vapor pressure of the V element, is considered in this note. A simple estimate will be given of the magnitude expected for the pressure effect, followed by some qualitative results for Ge-doped GaAs

Master of Science

thesisIn recent years the demand for germanium has swiftly increased due to its use in Infrared (IR) optics, gamma-radiation detectors, and in large part to the importance as a substrate for concentrator multijunction celestial and terrestrial based solar cells. Because of the high cost of germanium, and the weight limits of space systems, germanium wafers used in multijunction space solar cells are ultra thin and therefore susceptible to failure due to defects laid in from Czochralski (CZ) crystal growth, and wafer processing. These defects can greatly alter or hinder the electrical properties of the device made from these germanium wafers because of stress, or affect the growth of any material such as gallium arsenide grown epitaxially on the germanium wafer. The ability to locate and measure these defects is critical in developing a growth and wafering process to produce dislocation free germanium crystals and ultrathin wafers cut from them. A chemical etching solution has been found to reveal pits that correspond to dislocations in p-type germanium wafers. The etching solutions, which includes Cu(NO3)2 dissolved in HF & HNO3 and H2O2 & HNO3, are shown to disclose defect points for germanium wafers that were grown off the [100] plane 4°-8° towards the [111] plane to provide multiple and random lattice sites for high quality epitaxial growth. Alterations of the etch solution were also examined in order to develop a chemical polishing technique, which aided the turnaround time of dislocation examination. The morphology of the etched surface was examined with varying etch times. The surface of the etched wafers was observed using a light microscope that possessed Nomarski Differential Interference Contrast (DIC) imaging capability

An EBSD study of the deformation of service-aged 316 austenitic steel

Electron backscatter diffraction (EBSD) has been used to examine the plastic deformation of an ex-service 316 austenitic stainless steel at 297K and 823K (24 °C and 550 °C)at strain rates 3.5x10-3 to 4 x 10-7 s-1. The distribution of local misorientations was found to depend on the imposed plastic strain following a lognormal distribution at true strains 0.1. At 823 K (550 °C), the distribution of misorientations depended on the applied strain rate. The evolution of lattice misorientations with increasing plastic strain up to 0.23 was quantified using the metrics kernel average misorientation, average intragrain misorientation, and low angle misorientation fraction. For strain rate down to 10-5 s-1 all metrics were insensitive to deformation temperature, mode (tension vs. compression) and orientation of the measurement plane. The strain sensitivity of the different metrics was found to depend on the misorientation ranges considered in their calculation. A simple new metric, proportion of undeformed grains, is proposed for assessing strain in both aged and unaged material. Lattice misorientations build up with strain faster in aged steel than in un-aged material and most of the metrics were sensitive to the effects of thermal aging. Ignoring aging effects leads to significant overestimation of the strains around welds. The EBSD results were compared with nanohardness measurements and good agreement established between the two techniques of assessing plastic strain in aged 316 steel