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Generation and Enhancement of Surface Acoustic Waves on Highly Doped P-type GaAs and its Photoelectric Applications Open Access

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In the last decades, the application of dynamic strain and electric fields of surface acoustic waves (SAWs) has been used as successful tool for industrial applications such as filters, resonators and sensors due to its unique advantage to couple mechanical, electrical, and optical characteristics of piezoelectric materials. It is also reported that SAWs have advantages in other research fields such as being able to manipulate optically generated electronic excitations in semiconductors and to improve the quantum efficiency (QE) of photoelectric devices. Gallium Arsenide (GaAs) has both semiconductor photoelectric properties and piezoelectric effect, thus has promising potential for being utilized to build hybrid photoelectric devices with SAW-enhanced design. However, so far it has not been successful to achieve a wide range of practical SAW-enhanced GaAs photoelectric devices because of the relatively weak piezoelectricity of GaAs and the difficulty in generating SAWs on doped GaAs layer.To resolve this problem, we designed a unique structure that uses piezoelectric material Zinc Oxide (ZnO) to enhance the generation and propagation of SAWs on surface of highly doped p-type GaAs substrate, which is more extensively used in optoelectronic devices than intrinsic GaAs structure. To maximize the piezoelectricity and successfully generate the SAW, high quality c-axis orientation of ZnO film is needed, thus we experimented and developed optimized recipes of RF magnetron sputtering system to deposit ZnO on GaAs substrate. To further optimize the SAW performance, an intermediate Silicon Oxide (SiO2) layer was added between ZnO film and GaAs substrate. Additionally, we tested samples with varied thickness of ZnO films and dimension of interdigital transducer (IDT) fingers to figure out their individual effect on SAW properties. In this work, we used not only the method of modeling and simulation, but also experiments of fabrication and measurement, successfully verifying the feasibility and capability of our design. This novel technique provides the foundation for using SAWs to improve a variety of GaAs-based photoelectric devices.Next, based on this structure, we designed and studied two types of SAW-enhanced photocathode (bulk GaAs structure & thin film GaAs structure). We demonstrated that photoemission properties of GaAs photocathodes can be altered by SAWs generated on the sample’s surface due to dynamical piezoelectric fields of SAWs. Simulations with COMSOL indicate that electron’s effective lifetimes in p-doped GaAs may increase by a factor of 10x to 20x. It implies a significant, by a factor of 2x to 3x, increase of QE for GaAs photocathodes. Essential steps in device fabrication are demonstrated, including deposition of an additional layer of ZnO for piezoelectric effect enhancement, measurements of I-V characteristic of the SAW device, and high-temperature resistance of device after annealing.Additionally, we proposed the SAW-enhanced GaAs/InGaAs photodetection as an idea for future work. The SAW technique is utilized to reduce the recombination in GaAs by spatially separating the carriers, and to transport the photo-excited electrons and holes from where they were originally generated to the electrodes in the center. According to the simulations, the two advantages of using SAWs resulted in more photo-generated carriers being collected and more than 20 times higher photocurrent being generated at both visible light (505 nm – 875 nm) and near infrared spectrum (900 nm – 1700 nm). The results predicted by simulation provide good foundation for further study on using the SAW to improve the GaAs photodetection.Results and techniques demonstrated in this dissertation will provide guidance for further studies on enhancing the SAWs propagating along many other doped semiconductor materials. This combination of acoustics and optoelectronics in doped semiconductors is a promising start in building and inspiring more SAW-enhanced hybrid devices in a variety of applications across different fields.

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