N. G. Chew

An experimental and theoretical study of the formation and microstructure of porous silicon

Abstract We report a systematic study of the formation and properties of porous silicon formed by anodising silicon under a wide range of conditions. The current-voltage characteristics of the silicon-hydrofluoric acid system are presented. The detailed microstructure of the two types of porous silicon that form depending on the dopant concentration in the silicon, were determined by cross-sectional transmission electron microscopy. A theory for the formation of porous silicon is proposed which accounts for the dependence of the microstructure on the anodising conditions. The formation of porous gallium arsenide, analogous to porous silicon, is reported for the first time.

Microstructure and formation mechanism of porous silicon

A systematic study is presented of the effects of silicon dopant type, resistivity, current density, and hydrofluoric acid concentration on the formation and properties of porous silicon. Cross‐section transmission electron microscopy revealed the presence of two distinct microstructures. The structure formed is determined by the doping level with the transition occurring near degeneracy. A model of the anodisation process is presented which is based on the semiconducting properties of the material and which explains the formation of the two different types of porous structure observed.

Effect of small changes in composition on the electrical and structural properties of YBa2Cu3O7 thin films

Epitaxial thin films of YBa2Cu3O7 have been grown in situ by evaporation onto (001) MgO substrates. The composition was varied systematically to investigate the effects of changes in Cu content and Ba/Y ratio on the film properties. The results demonstrate that deviations from stoichiometry at the limit of resolution of most analytic techniques can have a large effect on structural and transport properties, as well as causing marked changes in surface morphology. The best properties (Jc≳3×106 A/cm2 at 77 K) are only found for a narrow range of compositions, which can be readily identified from the surface morphology.

Computer simulation of high speed melting of amorphous silicon

The laser melting of amorphous Si is accurately modelled by computer calculations. It is found that the thermal conductivity of the amorphous phase must be set at approximately 10−2 W/cm K, a value much lower than that of crystalline material to obtain close agreement with experimental measurements. This low value is, however, consistent with the thermal conductivities of other amorphous materials. The results of the computations, when compared with experimental observations, confirm that the melting point of ion implanted amorphous Si is below that of crystalline Si, with a best estimate in the range 1185–1385 K.

Iodine ion milling of indium-containing compound semiconductors

The effects of reactive I+ ion beams, derived from a source of solid elemental I, on In‐containing compound semiconductors have been investigated using transmission electron microscopy. The results are compared with the effects produced by beams of Ar+ and Xe+ inert gas ions. It is shown that the surface accumulation of metallic In due to the disproportionation normally associated with ion milling of these materials can be eliminated by the use of I+ ion beams. Transmission electron microscope specimens in cross‐sectional configuration are used to demonstrate the excellent results which may be obtained by I+ ion milling of InP and InSb.

Growth interface breakdown during laser recrystallization from the melt

Morphological instability occurring during high‐velocity Si crystal growth from an impurity containing melt is examined in detail. The experimental conditions are achieved by annealing an ion‐implanted Si layer with pulsed laser radiation. Computer modeling is employed to understand the heat flow and impurity diffusion behavior that occurs. The stability and size of impurity segregation cells observed to occur under particular conditions are related to the predictions of morphological stability theory.

InGaAs‐InP multiple quantum wells grown by atmospheric pressure metalorganic chemical vapor deposition

The optical and structural properties of multiple quantum wells of InGaAs‐InP grown by atmospheric pressure metalorganic chemical vapor deposition are reported. Room‐temperature excitons are resolved for well widths from 50 to 200 A. Below 50 K, exciton linewidths, in absorption, of less than 10 meV are obtained. Absorbances for allowed valence to conduction subband transitions are found to be independent of well width, as expected in the two‐dimensional limit. A lower bound for the conduction‐band discontinuity of 235±20 meV is obtained.

Orientation dependence of high speed silicon crystal growth from the melt

Abstract The high speed melt growth behaviour of elemental Si crystals in (001), (011), (112) and (111) orientations is assessed in some detail. Rapid growth conditions are established by transient melting of Si samples using nanosecond laser radiation pulses. Very fast cooling after irradiation leads to the formation of highly undercooled melts and yields crystal growth rates of up to and beyond 10 m/s. Under the most extreme conditions the melt undercooling becomes so large that crystal growth breaks down and an amorphous final solid phase is produced. The dependence of the maximum crystal growth velocity upon the crystallographic orientation of the growth surface is determined. Discrete high velocity regimes of defective crystal growth are identified. Defect formation and propagation processes are characterized on the atomic scale woth the aid ofhigh resolution electron microscope images of growth breakdown interfaces.

Correlation of the structure and electrical properties of ion‐implanted and laser‐annealed silicon

Both transmission electron microscopy and four‐point probe measurements have been used to compare the structure and electrical properties of As+‐ and P+‐ion‐implanted and Q‐switched laser‐annealed Si. Use of a uniform laser beam permitted accurate correlations to be made. It was found that the thickness of the initial amorphous surface layer controlled the radiation energy densities required for annealing onset and completion. Annealing onset variations were probably due to changes in the coupling of the radiation with the ion‐damaged region. The variation in depth distribution of residual defects with changes in annealing conditions has been determined and shown often to produce a two‐stage annealing characteristic. The effect of such defects on annealed‐layer electrical properties has been determined. The presence of very‐small‐scale undulations on annealed surfaces is thought to be due to the propagation of shock waves through the annealed region.

The formation of porous silicon by chemical stain etches

Abstract We report an investigation into the nature of the stain films formed on single crystal silicon by hydrofluoric acid solutions containing small amounts of either sodium nitrite or chromium trioxide. Cross sectional transmission electron microscopy revealed that the stain films retain the crystallinity of the substrate but contain networks of pores on the scale of several hundred angstroms. From the microstructure and electrical properties of the stain films, we deduce that they are porous silicon, conventionally produced by the anodisation of silicon in hydrofluoric acid. We propose a mechanism for the formation process of the stain films and relate it to both the anodic formation of porous silicon and to the chemical polishing of silicon in a mixture of hydrofluoric and nitric acids.