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ZnO: Ultrafast photodoping: Ultrashort and metastable photoinduced metallization of the ZnO(10-10) surface


Gierster,  Lukas
Physical Chemistry, Fritz Haber Institute, Max Planck Society;

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Gierster, L. (2021). ZnO: Ultrafast photodoping: Ultrashort and metastable photoinduced metallization of the ZnO(10-10) surface. PhD Thesis, Technische Universität, Berlin.

Cite as: http://hdl.handle.net/21.11116/0000-0009-2F17-A
In recent years, band bending at oxide semiconductor surfaces induced by chemical doping or electric fields has gained considerable attention, because it leads to metallic surfaces with interesting properties not found in the bulk semiconductor. These properties include high electron mobility, magnetism and superconductivity. Optical generation of such surface metals on ultrafast timescales would pave the way for novel high-speed electronics. In this thesis, I investigate the photoresponse of the (10-10) surface of zinc oxide (ZnO) to femtosecond laser pulses using time- and angle-resolved photoelectron spectroscopy. The semiconductor ZnO is widely used in optoelectronics due to its transparency for visible light and its ease of nanostructuring. I show that photoexcitation acts like chemical doping of the ZnO surface and induces band bending on ultrafast time scales. The 'ultrafast photodoping' effect is reached by positive surface charging due to the photoexcitation of deep donor-type defects at the surface. For low photoexcitation densities, surface-confined excitons are formed. At a critical density, an exciton Mott transition occurs, and within only 20 femtoseconds the surface becomes metallic. The surface metal decays on a timescale of few hundred picoseconds, due to reduction of the defect exciton density upon electron-hole recombination. After the initial decay, a fraction of the photoexcited defect excitons remains and exhibits a lifetime exceeding the inverse laser repetition rate (5 microseconds). In the course of investigating this long-lived defect exciton population, I develop a new mathematical model, which allows for lifetime determination by tuning of the laser repetition rate and facilitates to analyze pump-probe scans in the presence of photostationary states. The long-lived defect exciton lead to metastable doping of the ZnO surface. Complementary to the ultrafast metallization due to short-lived defect excitons described above, a photostationary semiconductor-to-metal transition can be reached upon increasing the density of deep defects at the sample surface by sustained UV illumination. These observations are consistent with the persistent photoconductivity of ZnO reported in literature and provide a microscopic explanation for this phenomenon. In summary, a novel ultrafast photodoping mechanism is reported that leads to surface metallization of ZnO. Compared to hitherto known photoinduced phase transitions, the ultrafast generation and decay of the surface metal occurs with three- to four orders of magnitude lower laser fluences. Beyond implementations of ZnO as a transparent photoconductive switch, this work is the starting point for studies on optically generated surface metals with emerging properties beyond metallicity in the ultrafast time domain.