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This thesis is concerned with a detailed study of the performance of superlattice avalanche photodiodes (SAPDs) and the implications for high bit rate direct-detection optical fibre communication systems. In these advanced detectors the electron to hole ionisation rate ratio is artificially enhanced through selective heating of the electron distribution to reduce the excess noise associated with the randomness of the avalanche multiplication and to ensure high gain-bandwidth product. Thus SAPDs are suitable for long wavelength applications (1.3-1.6 pm) where most compound semiconductor materials otherwise have comparable electron and hole ionisation rates. A comprehensive discrete ionisation model is developed to assess the performance of SAPDs; emphasis being placed on the gain, excess noise factor, gain moment generating function (MGF), and gain-bandwidth product. The model is quite flexible and it is found that other device impairments such as dark current and the number of ionisations per stage caused by the injected carrier can be readily incorporated into the formulation. The performance of optical receivers employing SAPDs is examined using a Gaussian approximation (GA) and taking into account the influence of various device impairments. To assess the accuracy of GA a rigorous statistical analysis is developed using a MGF formulation. New signal designs for optical communications devised specifically for APD receivers are described. These signals achieve simultaneously both zero intersymbol interference and zero telegraph distortion with respect to a depressed optimum threshold and are thus well suited to untimed transmission. Importantly, they also offer improved tolerance to alignment jitter when they used in conventional fully retimed receivers.

We use high rate magnetically-confined-plasma-triode sputtering guns in a diffusion pumped vacuum chamber to fabricate metal-metal superlattices. By feedback control of the sputtering rates and microprocessor control of the substrate rotation the individual layer thicknesses were kept constant to within ±0.3% over the entire sample thickness ∼0.5 μm. We describe in detail the results of a number of structural characterization techniques applied to these materials, including Bragg Θ-2Θ x-ray diffraction, transmission and reflection Laue diffraction, wide film Debye-Scherrer diffraction, and Rutherford Backscattering Spectrometry (RBS). By depositing Ta onto freshly deposited Mo surfaces and using RBS to measure the resultant Ta coverage we determined the dependence of the Ta sticking coefficient on coverage. The same was done for Mo deposited on Ta surfaces. We grew a series of Mo/Ta superlattices with superlattice wavelengths covering the range from 10 to 120 Å. A number of four-wire resistance measuring patterns were etched in each superlattice using standard photolithographic techniques. Resistivities of the superlattice films of various layer thicknesses were then measured to a relative precision of 0.01% and an absolute accuracy of 1%. The layer thickness dependence and temperature coefficient of the resistivity of these superlattices was analyzed using grain and size effect theories.

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No description available.