Gallium arsenide quantum well modulators grown on silicon substrates

This thesis describes an investigation into GaAs/GaAlAs multiple quantum well (MQW) modulators grown on silicon substrates. The basic device consists of a pin diode containing quantum wells within the intrinsic region. Application of a reverse bias voltage to the device causes a change in the absorption of the MQWs. This change may be used to modulate a beam of light incident upon the modulator. By enclosing the device within a Fabry- Perot cavity, resonance effects can be used to enhance the degree of modulation. Such a device is known as an asymmetric Fabry-Perot modulator (AFPM). The motivations for the integration of GaAs and Si are derived from an appreciation of the merits of the two different materials. For purely electronic systems there may be advantages, but most exciting are the possibilities for new systems using optics to enhance the capabilities of electronics. Chapter 1 introduces these issues and presents a discussion on possible future optoelectronic systems. Given that a high enough level of integration can be achieved, the AFPM is shown to be a promising device for use in such systems. The properties of MQW devices epitaxially grown on silicon have therefore been investigated. A problem of GaAs on Si growth is the material mismatch, which results in a degree of strain in the epitaxial layer. The effect of this on MQW devices is discussed in Chapter 2. The material mismatch also creates a number of difficulties in the growth process and has an impact on material quality. These issues are discussed in Chapter 3, together with an outline of methods by which material quality could be improved. A study of the material quality of devices grown by both MOCVD and MBE is presented. Substrates used included plain, patterned, and GaAs pre-coated silicon. Material analysis was performed by electron microscopy, optical microscopy, photoluminescence, and optical beam induced current (OBIC) scanning optical microscopy. Photocurrent and absorption measurements were made on a number of GaAs on Si MQW pin diode test structures and the results described in Chapter 4. Quarter-wave reflector stacks were also grown on silicon and their performance compared to similar devices grown on GaAs. The results of both these studies were used to model the performance of a complete AFPM structure. Experimental results for these modulators are presented in Chapter 5. These include both photocurrent and reflection spectra