Abstract
•Germanium targets were exposed under vacuum and H2 environment by nanosecond laser pulses.•The effect of laser fluence and ambient environment has been investigated.•The surface morphology is investigated by SEM analysis.•Raman and FTIR Spectroscopy are performed to reveal structural modification.•Electrical conductivity is probed by four probe method.
Laser fluence and ambient environment play a significant role for the formation and development of the micro/nano-structures on the laser irradiated targets. Single crystal (100) Germanium (Ge) has been ablated under two environments of vacuum (10−3Torr) and hydrogen (100Torr) at various fluences ranging from 4.5Jcm−2 to 6Jcm−2. For this purpose KrF Excimer laser with wavelength of 248nm, pulse duration of 18ns and repetition rate of 20Hz has been employed. Surface morphology has been observed by Scanning Electron Microscope (SEM). Whereas, structural modification of irradiated targets was explored by Fourier Transform Infrared Spectroscopy (FTIR) and Raman spectroscopy. Electrical conductivity of the irradiated Ge is measured by four probe method. SEM analysis exhibits the formation of laser-induced periodic surface structures (LIPSS), cones and micro-bumps in both ambient environments (vacuum and hydrogen). The formation as well as development of these structures is strongly dependent upon the laser fluence and environmental conditions. The periodicity of LIPSS or ripples varies from 38μm to 60μm in case of vacuum whereas in case of hydrogen environment, the periodicity varies from 20μm to 45μm. The difference in number of ripples and periodicity as well as in shape and size of cones and bumps in vacuum and hydrogen is explained on the basis of confinement and shielding effect of plasma. FTIR spectroscopy reveals that no new bands are formed for laser ablated Ge under vacuum, whereas CH stretching vibration band is formed for two moderate fluences (5Jcm−2 and 5.5Jcm−2) in case of ablation in hydrogen. Raman spectroscopy shows that no new bands are formed in case of ablation in both environments; however a slight Raman shift is observed which is attributed to laser-induced stresses. The electrical conductivity of the irradiated Ge increases with increasing fluence and is also dependent upon the environment as well as grown structures.