Strukturierung von Zinkoxid und Galliumnitrid mit Femtosekundenlaserpulsen : Ablation, Oberflächenstrukturen und optische Eigenschaften

Abstract

Structuring the surface of semiconductor devices with femtosecond laser pulses is a promising method for enhancing the device performance while keeping thermal damage as small as possible. The surface processing with femtosecond pulses provides two main opportunities that are hard to realize with other processing techniques. First femtosecond laser pulses can generate a large number of different self-organized surface morphologies with characteristic sizes ranging from nanometers to micrometers. They make it possible to create subwavelength structures not available by laser with longer pulses. Thus they offer the possibility to roughen the surface of semiconductors at different spatial scales and therefore allow one to adapt the laser process to the desired functionality of the device surface. Second femtosecond pulses of sufficient intensity allow to trap a very large amount of dopants within a small depth from the surface. In this way it is possible to create highly doped surfaces that can be used to increase the efficiency of solar cells or photodetectors. In this thesis the author examines the femtosecond laser pulse structuring of gallium nitride and zinc oxide. Both are technologically important wide band gap semiconductors. The first part of the thesis explores the ablation and surface modification of both semiconductors. For zinc oxide it is found that surface is ablated at a laser fluence of 0.5 J/cm^2. For gallium nitride the measured single laser pulse threshold lies at 0.6 J/cm^2. The single pulse threshold values agree with what is expected for an electrostatic ablation process. The ablation threshold depends strongly on the number of structuring laser pulses. Crossectional energy dispersive x-ray measurements show that it is possible to incorporate antimony into the zinc oxide surface layer at level of 1-2 atomic percent. In the second part of the thesis laser-induced self-organized surface structures are analyzed. The presented studies focus on so called LIPSS (laser induced periodic surface structures) on the c-plane zinc oxide and gallium nitride. By varying laser parameters such as laser fluence and the angle of incidence the formation mechanism is studied. It is observed that the periodicity of the surface structures increases for larger incidence angles in a way that can be explained by surface scattering of the incident laser light. When lowering the applied laser fluence a transition from wavelength-sized LIPSS (650 nm) to subwavelength-sized LIPSS (200 nm) is observed indicating a pronounced transient change of the optical properties of the surface layer. This alteration allows the excitation of high frequency surface plasmons that might well explain the observed LIPSS. The LIPSS behave similar on both semiconductor surfaces. In the third part of the thesis the optical properties of femtosecond laser processed zinc oxide are studied by Raman spectroscopy, photoluminescence spectroscopy and absorption spectroscopy. High above the ablation threshold of zinc oxide a low number of laser pulses will cause large stress on the surface layer that leads to surface cracking and a delamination of crack tiles. This cracking does not fully relax the surface layer and a residual strain of up to 1.8 % is detected. The observed strain pattern supports an origin by thermal stress generated by the cooling following the laser-matter-interaction. For samples structured with a larger number of laser pulses, a phonon confinement induced broadening and shift of the E2(high) mode is observed. Simultaneously the intensity of the A1(LO) mode is enhanced and polar surface phonons are observed pointing to a nanocrystalline surface layer rich in defects. Photoluminescence measurements confirm this and provide evidence for an increase in the zinc interstitial density. As a consequence the light absorption increases drastically from the near-UV to near-IR spectral region. The laser processing allows to control the optical absorption of zinc oxide over a wide range not known from other processing techniques.