Humboldt-Universität zu Berlin, Institut für Physik


Mathematisch-Naturwissenschaftliche Fakultät

Institut für Physik


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Motivation Equipment Findings Investigators


Semiconductor plasmonics

ZnO is known as a semiconductor tolerant to extremely high doping levels. So far, most of the efforts have focused on the development of transparent conducting oxides for flat panel or photovoltaic industry. Only very recently, it has been realized that this feature might be exploited for plasmonics in the infrared spectral range. By using advanced growth techniques, semiconductors might outperform traditional metals in terms of tuneability and flexibility in device fabrication and could provide better plasmonic performance.

Using Molecular Beam Epitaxy, we are able to fabricate n-type ZnO:Ga films with high structural quality up to Ga mole fractions of about 7.0 %, reaching free-carrier concentrations of more than 8 x 1020 cm-3. Controlling the doping levels allows for tuning of the positive-to-negative crossover wavelength of the real part of the resultant metallic dielectric function (λC) from the mid infrared up to about 1.2 Ám.

(a) Reflectivity spectra of ZnO:Ga layers and an undoped reference layer. (b) Measured (solid) and by transfer matrix calculated (dotted lines) relfectivity (R), transmission (T) and absorbance (A) of the sample with the highest carrier concentration (n = 8.1 x 1020 cm-3). (c) Zero-crossover wavelength (empty squares) and plasmonic damping (filled dots) as a function of the carrier concentration.

The surface plasmon polariton (SPP) dispersion at the air/ZnO:Ga interface can be tuned via the doping level in a wide spectral range from the midinfrared up to the telecommunication wavelength band at about 1.55 Ám. This allows us to adjust the SPP frequency resonantly to molecular vibrations, band edge related transitions of quantum dots (e.g., PbS) or transitions of other infrared emitters like Er+3 ions.

(a) ATR spectra in TM-polarization of a layer with n = 3.2 x 1020 cm-3 at various angle of incidence. The inset schematizes the ATR configuration where a silicon hemisphere is used to excite evanescent waves along the surface plane. (b) ATR spectra of three samples with different free-electron concentrations at three selected angle of incidence. (c) Dispersion relations deduced for the free-electron concentrations of (b). Circles: experimental ATR minima. Curves: calculated dispersion for the air/ZnO:Ga interface. Note that the exact position of the ATR minima depends on the thickness of the air gap between sample surface and silicon hemisphere.

Furthermore, we fabricate ZnO:Ga multilayer structures with well-defined doping levels as well as smooth interfaces and investigate their plasmonic properties. In particular, we focus on engineering of the dispersion relation of SPPs at metal/metal-interfaces and of coupled SPPs in multi-layer structures for nanophotonic manipulation of infrared light, even at telecommunication wavelengths.

letzte Änderung: 21.09.2015 id