Research On Selective Liquid Phase Epitaxial Growth Of GaAs Microtips

Scanning near-field optical microscopy (SNOM) is a novel photoelectrical tool for the research down to the nanometer scale. It uses the evanescent field confined at the tiny aperture to provide images of rough surfaces with a resolution beyond the classical optical diffraction limit. One of the most important potential application of SNOM may be found in ultra-high density optical recording. Generally speaking, SNOM sensor is composed of three main parts, semiconductor laser, photoelectronic detector and the microtip. Since microtip is one essential part of SNOM, the fabrication of high quality microtips has become a key issue.Up to now, many microtip fabricating techniques have been explored. As for fiber microtips, the main fabrication techniques are thermal stretching and chemical etching. As for semiconductor microtips, the main fabrication techniques are wet etching, MOCVD, mixed etching, self-assembled liquid phase epitaxy and so on. According to the conditions of our lab, we present a new simple selective liquid phase epitaxial growth of GaAs pyramidal microtips. The main steps of the technology can be divided into three parts. First, a 20-30nm thick thin film is deposited on the treated GaAs substrate. Second, the periodic windows with regular sizes are created in the mask using standard photolithography and wet etching. Third, the GaAs substrate with the mask openings is loaded into a conventional LPE system to carry out the selective liquid phase epitaxial growth of microtips in an atmosphere of Pd-purified hydrogen. This method not only can grow GaAs microtips directly on the VCSEL wafer and avoid transferring microtips but also settle the problems of aligning the microtips with light-emiting windows of lasers. It also provides a technology basis for batch production and parallel scanning with several microtips.The (001) GaAs substrates are used instead of VCSEL wafers in our preliminary experiments, Scanning electron microscopy (SEM) is used to characterize the morphology of GaAs microtips. The results indicate that in appropriate conditions the microtips are pyramid-like and distribute uniformly on the wafers.This technology of fabricating microtips is introduced in great detail in this thesis, including mask preparation, photolithography, wet etching and liquid phase epitaxial growth. The effects of different masks and different mask openings on the liquid phase epitaxial growth of GaAs microtips are investigated. The morphological evolution of GaAs microtips in different mask openings are also illustrated and the LPE experimental conditions are optimized to improve the quality of tips. Besides, we also provide two methods to transferring GaAs microtips, selective wet etching Al_0.7Ga_0.3As buffer layer using concentrated HCl solution and selective wet etching GaAs substrate by diluent ammonia solution. Both of the two methods are compatible with selective liquid phase epitaxy. Furthermore, we realize the integration of microtips and VCSEL, which expands the application of the selective liquid phase epitaxial GaAs microtips for other scanning probe microscopy.In the end, Bravais law, BFDH theory and SHAPE software are employed to preview and simulate the morphology of GaAs microtips grown in free growth system. And the results agree with the experimental results well.This work is financially supported by two National Nature Science Foundations of China under project No.60377005 and No.60777009, Special Fund for Preliminary Research of Key Basic Research Project from Ministry of Science and Technology of China under project No.2004CCA03700 and Special Research Fund for Doctoral Program of Higher Education under project No.20060141026. One national patent has been authorized and the Patent Number is ZL 03 1 33404.0.