PLASMONIC COMPONENT AND PLASMONIC PHOTODETECTOR AND METHOD FOR PRODUCING SAME

Abstract

The present invention relates to plasmonic components, more particularly plasmonic waveguides, and to plasmonic photodetectors that can be used in the field of microoptics and nanooptics, more particularly in highly integrated optical communications systems in the infrared range (IR range) as well as in power engineering, e.g. photovoltaics in the visible range. The present invention also specifies a method for producing a plasmonic component, more particularly for photodetection on the basis of internal photoemission.

Claims

1. A method for producing a plasmonic component comprising a metal-semiconductor-metal contact in which a first metal layer is separated from a second metal layer, which is different than the first metal layer, by a semiconductor layer having a thickness of 100 nm or less, the method comprising the following steps: providing a semiconductor layer on a substrate; providing an oxide layer at least partly on one of the surfaces of the semiconductor layer; applying a structured resist layer at least partly on the opposite surface of the oxide layer relative to the semiconductor layer in such a way that regions with the resist layer and regions without resist layer arise and regions with the resist layer correspond to a basic form of the metal-semiconductor-metal contact; removing the oxide layer and semiconductor layer in regions without resist layer to give rise to at least two surfaces of the semiconductor layer; and applying at least two layers each composed of different metals by angled vapor deposition on the surfaces of the semiconductor layer to form the metal-semiconductor-metal contact, wherein the first metal layer is not in direct contact with the second metal layer.

2. The method as claimed in claim 1, wherein the plasmonic component is present in the form of a waveguide.

3. The method as claimed in claim 1, wherein the structured resist layer is applied by means of electron or ion beam or laser lithography.

4. The method as claimed in claim 1, wherein the first metal layer is applied at least on a first surface of the semiconductor layer, and the second metal layer is applied on the surface of the semiconductor layer situated opposite the first surface.

5. The method as claimed in claim 1, further comprising the step of undercutting the oxide layer after the step of removing the oxide layer and semiconductor layer in regions without resist layer and before the step of applying the at least two layers each composed of different metals.

6. A method for producing a plasmonic photodetector comprising providing a plasmonic component prepared by using the method of claim 1; and coupling the plasmonic component to a voltage to be applied.

7. The method as claimed in claim 6, wherein the plasmonic component is coupled to a photonic silicon waveguide.

8. A plasmonic component comprising a metal-semiconductor-metal contact in which a first metal layer is separated from a second metal layer, which is different than the first metal layer, by a semiconductor layer having a thickness of less than 100 nm, wherein the first metal layer is not in direct contact with the second metal layer.

9. The plasmonic component as claimed in claim 8, in the form of a waveguide in which a first electrode in the form of a first metal layer is separated from a second electrode in the form of a metal layer different than the first metal layer by a semiconductor layer having a thickness of less than 100 nm, wherein the first metal layer is not in direct contact with the second metal layer.

10. The plasmonic component as claimed in claim 8, wherein an oxide layer separates the first metal layer from the second metal layer.

11. The plasmonic component as claimed in claim 10, wherein the oxide layer is an undercut oxide layer.

12. The plasmonic component as claimed in claim 8, wherein the two metal layers each comprise different metals.

13. The plasmonic component as claimed in claim 8, wherein the first metal layer consists of a metal selected from the group consisting of gold, silver and aluminum and alloys thereof.

14. The plasmonic component as claimed in claim 8, wherein the second metal layer consists of a metal selected from the group consisting of titanium, chromium, copper, zirconium, nickel, palladium, platinum, tin, lead and bismuth and alloys thereof.

15. The plasmonic component as claimed in claim 8, wherein the metal layers are selected in such a way that electromagnetic waves are predominantly absorbed in the second metal layer and the metal of the second metal layer, a metal layer exhibiting high absorption, has a low potential barrier with respect to the semiconductor layer.

16. The plasmonic component as claimed in claim 8, wherein the length of the metal-semiconductor-metal contact is 0.5 m to 100 m.

17. A plasmonic photodetector comprising the plasmonic component as claimed in claim 8, wherein the first and second metal layers constitute electrodes at which the photodetection takes place.

18. The plasmonic photodetector as claimed in claim 17, wherein the electrodes are a photoactive electrode and a counterelectrode.

19. The plasmonic photodetector as claimed in claim 17, wherein the plasmonic component is coupled directly to a photonic silicon waveguide.

20. An amplifier, electro-optical transducer or phase shifter comprising the plasmonic component of claim 8.

21. An optical chip-chip connection, an on chip connection, or free space optics comprising the plasmonic photodetector as claimed in claim 17 for the photodetection in the infrared range and visible range of the light spectrum.

22. (canceled)

Description

[0095] The present invention is explained in greater detail below on the basis of non-limiting examples with reference to the accompanying figures, in which:

[0096] FIG. 1: shows a schematic illustration of an exemplary contour of the metal-semiconductor-metal contact to be produced with a substrate, a semiconductor layer and an oxide layer;

[0097] FIG. 2: shows a schematic illustration in cross section of a metal-semiconductor-metal contact in accordance with one preferred embodiment of the present invention;

[0098] FIG. 3: shows a simplified schematic illustration (view from above) of a metal-semiconductor-metal contact according to the invention, wherein the amplitude profile shows the profile of the magnetic field of the guided mode;

[0099] FIG. 4: shows a simplified schematic cross-sectional illustration of an asymmetrical metal-semiconductor-metal contact with light-induced charge separation for clarifying internal photoemission, which leads to a measurable current when voltage is applied;

[0100] FIG. 5: shows a schematic illustration in cross section of a metal-semiconductor-metal contact in accordance with one particularly preferred embodiment of the present invention;

[0101] FIG. 6: shows simulated standards of the electric field depending on the thickness d;

[0102] FIG. 7A: shows the ratio of the power losses in titanium and gold electrodes;

[0103] FIG. 7B: shows the penetration depth, i.e. the absorption length, i.e. the length after which the intensity of the optical signal has fallen to 1/e on account of absorption;

[0104] FIG. 8: shows schematic method steps of the method according to the invention;

[0105] FIG. 9: shows scanning electron microscope micrographs of a waveguide structure in accordance with one particularly preferred embodiment of the present invention;

[0106] FIG. 10: shows an optical setup for the measurement of photoinduced currents;

[0107] FIG. 11: shows the dependence of the photocurrent without applied voltage;

[0108] FIG. 12: shows measured photocurrents as a function of the incident light energy, wherein each line corresponds to a different applied voltage;

[0109] FIG. 13: shows the dependence of the photocurrents on the applied voltage with subtracted dark currents;

[0110] FIG. 14: shows an optical micrograph of a PMMA resist which defines electrodes of a plasmonic waveguide;

[0111] FIG. 15: shows a scanning electron microscope micrograph of a metalized photodetector in accordance with one preferred embodiment of the present invention; and

[0112] FIG. 16: shows dark currents of plasmonic photodetectors having a length of 10 m and 20 m, wherein the dark current is close to the maximum resolution of the instrument used.

SIMULATION OF OPTICAL PROPERTIES OF METAL-SEMICONDUCTOR-METAL CONTACTS

[0113] Reference is made below to the metal-semiconductor-metal contact in the form of a waveguide as illustrated in FIG. 5. This consists of a silicon nanowire having elliptical sidewalls, wherein its base surface is wider in cross section than at the upper end. A gold layer is deposited on the left surface of the waveguide, and a titanium layer on the right surface of the waveguide. The waveguide geometry corresponds to waveguides actually produced, as will be described below.

[0114] The optical properties of the waveguide were simulated with the aid of COMSOL (Comsol 4.3a Simulation Module: Electro-magnetic waves, frequency domain), wherein the electric field distribution (fundamental guided mode) and the absorption characteristics were examined in each case depending on the thickness d of the waveguide. These simulations were carried out at an optical wavelength of 1270 nm.

[0115] It is evident with reference to FIG. 6 that the electric field of the respective waveguide is all the more constricted, the narrower the waveguide. Furthermore, it is found that the penetration depth of the field toward the right-hand side with the titanium layer increases as the thickness d decreases. Since titanium effects much greater absorption in comparison with gold at the wavelength chosen, the proportion of light absorbed in the titanium also increases as the thickness d decreases.

[0116] In order to determine the density of the power losses in the electrodes which generate the photoinduced charge carriers, the power losses in each electrode were numerically integrated. FIG. 7(A) shows the ratio of the power losses in titanium and gold. As d decreases, the proportion of the power loss in titanium increases nonlinearly. For a waveguide having a width of 100 nm, the photogeneration in titanium exceeds that in gold by a factor of 10. The increased loss in the titanium electrode is accompanied by a reduced penetration depth into the waveguide, as can be seen in FIG. 7(B).

[0117] For waveguides having a width of less than 100 nm, a large portion of the power loss occurs solely at the titanium electrode, while a short penetration depth is made possible by small component lengths of less than 5 m.

Production of Photoactive Plasmonic Waveguides

[0118] Photoactive plasmonic waveguides on SOI (silicon on insulator) wafers with gold and titanium as electrode materials and an electrode spacing of less than 100 nm were produced as follows (also cf. FIG. 8).

[0119] SOI wafers having component thicknesses of 340 nm were thermally oxidized until an 80 nm thick oxide layer formed on the surface of the silicon. This was carried out at a temperature of 1040 C. in a dry oxygen atmosphere (>99%) at 1 bar. Silicon was consumed during this process, leaving behind an approximately 300 nm thick silicon layer (cf. FIG. 8A).

[0120] Negative resist maN-2401 (from Micro Resist Technology GmbH, Berlin) was spin-coated onto the surface of the oxidized wafer. Using electron beam lithography, the contour of the waveguides, of markers and coupling regions was written into the resist, the resist protecting the oxide layer in the subsequent etching steps (cf. FIG. 8B).

[0121] CHF.sub.3 was used to remove the unprotected regions of the oxide layer. By means of the cryo process using SF.sub.6 (temperature: 115 C., pressure: 5 mtorr, gas flow rates: 36 sccm SF.sub.6, 18 sccm O.sub.2, 10 sccm Ar, RF power: 20 W, ICP power: 700 W), the waveguides were anisotropically etched into the silicon. In this case, the oxide layer on the surface functions as a mask during etching with SF.sub.6 (cf. FIG. 8C).

[0122] A second electron beam lithography method was carried out in order to protect the photonic parts in the next etching step and in order to define the electrode form. Afterward, the silicon waveguide was laterally undercut in a further isotropic etching step at room temperature using SF.sub.6 (cf. FIG. 8D).

[0123] It is important here that in the processing also firstly the undercut regions and the electrodes can be produced and afterward the feeding silicon waveguides can be produced anisotropically using SF.sub.6 or HBr. The order of the structuring of the plasmonic and purely photonic components can thus be interchanged.

[0124] An asymmetrical plasmonic waveguide having different metal layers, including gold electrodes, on each side was produced in a total number of five metallization steps at five angles. In this case, the oxide mask (oxide layer) prevents the two sides from short-circuiting one another during deposition. In the first two vapor depositions (metallization steps), titanium and gold were respectively applied on the sidewalls of the undercut silicon at an angle of from 70 to 85 with respect to the surface normal. That part of the detector which interacts with light is thus defined (cf. FIG. 8E).

[0125] The last vapor deposition at 0 defines the electrodes at which the external electronics tap off the photocurrent. Said electrodes are usually composed of gold or aluminum or an alloy of titanium and aluminum, in order to ensure the best possible electrical contact between the conductors on the chip and the electronics, wherein the deposited metal layer may be 100 nm, for example.

[0126] The vapor deposition steps in between at an angle of approximately 45 provide for a good electrical contact of the plasmonic region at the waveguide and the electrodes. The material used for these steps can be the same as that also used for depositing the electrodes. The first two steps with different metals are thus crucial for photodetection. All further steps provide for a good electrical contact toward the outside and can be realized with arbitrary conductive materials.

[0127] Plasmonic waveguides having lengths of more than 10 mm were produced on the substrate. The substrate was then scribed and broken using a diamond cutter, thus giving rise to a fracture edge perpendicular to the longitudinal direction of the waveguides. Said fracture edge uncovers the facet and thus the cross section of the waveguide. FIG. 9 shows scanning electron microscope micrographs of the waveguide thus produced. The front part of the oxide layer was removed by the scribing, as a result of which the underlying metal-semiconductor-metal contact was revealed. The micrographs were taken at an angle of 45. The micrograph illustrated in FIG. 9(B) shows the side view of the detector likewise at an angle of 45. The smallest distance between the two metal electrodes is less than 100 nm.

Characterization of Plasmonic Photodetectors

[0128] The uncovered facet of the plasmonic waveguide was illuminated by a tapering fiber with a focal spot at the focus of 5 m. The optical setup is illustrated in FIG. 10. An adjustable laser source ranging from 1270 nm to 1350 nm was used. After careful alignment of the polarization and the fiber position, the photocurrent was measured as a function of the photon energy used, the incident laser energy and the voltage applied to the photodiode externally.

[0129] It is assumed that the photocurrent has a power law dependence in relation to the photon energy in accordance with the formula for the photoelectric effect that was presented by Fowler in 1931. This formula was adapted to the case of internal photoemission (IPE) by using the height of the potential step, seen by an electron at the Fermi level (potential of the Fermi level subtracted from the potential of the barrier) between gold and silicon .sub.Au-Si. This formula applies to the present structure if no external voltage is applied, and reads as follows:

[00001]$I_{\mathrm{photo}}.Math..Math.\frac{{\left(-{}_{\mathrm{Au}-\mathrm{Si}}\right)}^2}{\left({}_0-\right)},$

wherein is the photon energy and .sub.0 is the total height of the potential step given by the interface.

[0130] FIG. 11 shows the measured currents in relation to the photon energy. By adapting the formula to these data using the method of weighted least squares, the barrier height between gold and silicon was able to be obtained as a parameter. The barrier height obtained is .sub.Au-Si=0.82 eV, which corresponds well to literature indications for said barrier height (cf. Chen et al., Current Transport and its Effect on the Schottky-Barrier Height in a typical System: Gold on Silicon, Solid-State Electronics, vol. 36, no. 7, pp. 949-954, 1993). This correspondence constitutes first evidence of internal photoemission (IPE).

[0131] Using the mode overlap integral, it was possible to estimate the minimum internal quantum efficiency in this structure, which is between 5 and 10% for an optical wavelength of 1270 nm and increases exponentially for shorter wavelengths. In this structure, the presence of IPE is not restricted to photons below the band gap energy of silicon. For photons of higher energy, i.e. in the visible range, the absorption in silicon is smaller by orders of magnitude in comparison with the metals.

[0132] Further series of measurements were carried out in order to determine the dependence of the photocurrent on the incident laser energy. In accordance with Fowler's theory, even if an external voltage is applied, a linear rise in the photocurrent with energy was expected. The results obtained are plotted in FIG. 12. Each line of the graph corresponds to a different applied voltage, wherein the dark currents of the diode were subtracted. The lowest curve was measured at 0.5 V, and the highest curve at 0.7 V. The difference between each curve is 50 mV.

[0133] As shown in FIG. 12, the photocurrent increases with higher voltages. Positive currents correspond to the carrier injection of titanium; negative currents are transferred from gold. Each measured curve shows a linear dependence on energy. As was also discussed in the simulation above, it is found that the titanium electrode makes a greater contribution.

[0134] FIG. 13 shows the measured photocurrent dependence on the applied voltage. It is found that a limit voltage exists which separates the charge carrier injection of titanium from that of gold. The charge carriers are transferred from titanium for voltages greater than the limit voltage, and from gold for lower voltages. This voltage corresponds to the diffusion potential of the diode, which can be calculated by V.sub.th=0.256 V. Consequently, the difference should result from the measured barrier height at the gold-silicon interface and the diffusion potential in the barrier height at the titanium-silicon interface, i.e. .sub.Ti-Si=.sub.Au-SiqV.sub.th=0.564 eV. This value is in the range of previous literature values (.sub.Ti-Si=0.5 eV to 0.6 eV) (cf. A. M. Cowley, Titanium-Silicon Schottky Barrier Diodes, Solid-State Electronics, Pergamon Press 1970, vol. 12, pp. 403-414).

[0135] Production of Ultracompact Plasmonic Photowaveguides

[0136] The integration of active plasmonic waveguides into a photonic silicon platform requires the coupling of photonic and plasmonic regions. This was accomplished by firstly producing a photonic silicon waveguide on an SOI substrate. Afterward, the lengths of the plasmonic region and of the connected electrodes were defined by means of electron beam lithography (cf. FIG. 14), wherein the silicon waveguide and the oxide substrate were accessible for etching and metal deposition. The production method described above was subsequently carried out on the accessible parts of the waveguide, as a result of which a component as illustrated in FIG. 15 was obtained.

[0137] The dark currents of the plasmonic photodetector having lengths of 10 m and 20 m were measured, which are depicted in FIG. 16 (instruments: Agilent B2900A series, electrodes contacted on-chip with DC probes from Cascade Microtech Inc.). On account of the low signal strength, the series of measurements were carried out close to the resolution limit with a different time constant and voltage resolution, i.e. short averaging time and low voltage resolution (coarse). The procedure for fine was implemented correspondingly. In comparison with known plasmonic photodetectors, as described in M. Casalino et al., Optics Express, Vol. 21 (23), pp. 28072-28082 (2013), the dark current is smaller by an order of magnitude.

[0138] On account of the small cross section, the RC time constant is the crucial speed-determining variable. With an assumed capacitance of the order of magnitude of 1 fF/m component length and a 50 connection to the external drive electronics, it was possible to estimate an RC time constant of approximately 0.25 ps and a resultant limiting frequency (maximum frequency) of 4 THz for a component having a length of 5 m.

[0139] In view of the above results it was possible to show that on account of the linear rise in the photocurrent with the incident optical energy and the exponential rise with photonic energy, the photodetection in the described component takes place on the basis of internal photoemission. In particular, two-photon absorption, which would have shown a parabolic rise in the photocurrent with the incident optical energy, was able to be ruled out on the basis of the measurement data.

LIST OF REFERENCE SIGNS

[0140] 1 Oxide layer [0141] 2 Semiconductor layer [0142] 3 Substrate [0143] 4 First metal layer [0144] 5 Second metal layer [0145] 6 Resist [0146] d Thickness of the semiconductor layer [0147] d.sub.B Maximum thickness of the semiconductor layer [0148] 10 3-axis piezo-stage [0149] 11 Si chip with metal-semiconductor-metal waveguides and connected measuring electronics [0150] 12 Optical waveguide with lens, focal spot 5 m [0151] 13 NIR laser source [0152] 14 Single-mode optical fiber [0153] 15 Polarization control