Terahertz MicroNano Technology blog -BIMETALLIC GRATING STRUCTURES – A new concept for large-scale bias-free terahertz emitters

http://amoprotemics.wordpress.com/2014/01/28/bimetallic-grating-structures-a-new-concept-for-large-scale-bias-free-terahertz-emitters/

Fig. 1: (top) Top-view showing both investigated lateral schemes of the fabricated radial mode Terahertz emitter structures featuring complementary bimetal/semiconductor sequences. (bottom) Cross-section view of device including the principle THz field distribution after optical excitation.

A new concept for the optical generation of THz radiation has been introduced by AMO GmbH, Germany. The approach – filed for patent application [1] by the company – uses grating structures made of two different metal materials which are configured on a semiconducting substrate. THz pulse generation is triggered by optical excitation of this structure through femtosecond near-infrared pulses and subsequent acceleration of the photo-induced charge-carriers. Scaling of the actively emitting areas into a range of square-mm sizes (helpful to avoid conversion efficiency degradation through pump saturation effects) is straightforward.
Earlier large-scale THz emitter concepts used voltage-biased metal-semiconductor-metal (MSM)-structures [2,3]. For these structures increasing emitter area means an increasing probability for device fade-out through a single short-cut defect. Now, the emitter is operated bias-free because inherent Schottky-fields present at the bimetallic/semiconductor interfaces are used for charge-carrier acceleration. As a result, the emitter stays functionally unimpaired even in case of short-cut defects.

Bias-free Terahertz emission – Schottky-field vs. photo-Dember

The introduced emitter is sharing this robustness with an earlier bias-free THz emitter concept based on lateral photo-Dember field induction [4], but it features the important further advantages of simple monolithic fabrication and higher efficiency. In order to generate lateral photo-Dember fields the fabrication of (mono-) metallic gratings with three-dimensional wedged profiles is required. These profiles are difficult to realize for arbitrary shaped gratings like radial or curved ones instead of shown linear gratings. More important, lateral charge carrier acceleration through Schottky-fields is not limited to semiconductor materials with pronounces differences in electron and hole mobilities and diffusion processes (as used for photo-Dember field induction). Consequently, the new approach enables bias-free Terahertz emitters with improved efficiency. A demonstration device in terms of a radial mode THz emitter has now been presented using the new bimetal grating concept.

No bimetal, no Terahertz emission

The key feature enabling Terahertz generation within the novel large-scale grating structure is based on the application of two different metal materials: While the lateral Schottky-fields in a monometallic grating always cancel out over a full grating area, there is a net lateral field at bimetallic gratings resulting from the Schottky-field difference between both metals applied. Fig. 1 (top) is showing the principle configuration of the investigated radial grating emitter. By choosing the medial sequential order of the applied metals (e.g. metal 1/metal 2/semiconductor instead of metal 2/metal 1/semiconductor) it is possible to flip the direction of the generated THz field by 180° as shown in the cross-section view at the bottom of Fig. 1. A representative surface-profile is shown in Fig. 2. Three height levels visible in this plot correspond to plateau areas representing the bare semiconductor surface (dark green), metal 1 or metal 2 (light green) and metal 1/metal 2 one above the other (yellow). No wedged structures have been formed in this case.

Fig. 2: Surface-profile measurement of the center region of a radial bimetal grating emitter.

Advanced monitoring of THz near-field emission

The THz emission process has been measured using photoconductive microprobes (from the TeraSpike TD-800 series) developed in-house through AMO. The probes allow the selective time-domain sampling of every THz vector-field component in x-, y- and z-direction in terms of amplitude and phase. Fig. 3 is showing a single snap-shot of the field amplitude distribution in z-direction measured shortly after optical excitation at a pair of radial emitters. The measurement plane is on the emitter backside as sketched by the dashed red line in Fig. 1. As expected for the z-component of a radial mode, the largest field magnitudes are observed at the center of each emitter. Both emitters have been fabricated using the converse bimetal sequences also illustrated in Fig. 1. As a consequence, the field lines on both radial emitters are pointing in opposite direction which confirms that the Schottky-field induced THz generation is working as expected. A movie showing the time evolution of the excitation process can be watched by following the link in the caption of Fig. 3.

Fig. 3: Measurement of the z-component of the Terahertz near-field distribution shortly after optical excitation. To watch a movie showing the full time-domain excitation process click here.

Conclusion

Bimetallic grating structures are highly attractive for the production of bias-free large-scale THz emitters of arbitrary shape and size. The given example of a radial-mode emitter demonstrates the flexibility of this approach very nicely. Further important attributes are robustness and efficiency. In addition to pulsed generation the concept should also be attractive for continuous wave (cw) Terahertz signal generation [5] using semiconducting materials with sufficiently short carrier lifetimes.

References:

[1] M. Nagel, German patent application, DE 102012010926 A1

[2] A. Dreyhaupt et al., Appl. Phys. Lett. 86, 121114 (2005),http://dx.doi.org/10.1063/1.1891304

[3] M. Awad et al., Appl. Phys. Lett. 91, 181124 (2007); http://dx.doi.org/10.1063/1.2800885

[4] G. Klatt et al., Optics Express, Vol. 18, Issue 5, pp. 4939-4947 (2010),http://dx.doi.org/10.1364/OE.18.004939

Repost from "Terahertz MicroNano Technology blog" TERAHERTZ PROBING – From sub-nanometers to micrometers

Contactless Terahertz microprobes can measure the sheet resistivity and thickness of large-area conductor films at unprecedented speed and resolution.
http://amoprotemics.wordpress.com/2013/11/05/terahertz-probing-from-sub-nanometers-to-micrometers/
By: Michael Nagel

Emerging terahertz technology is on course to define the next state-of-the-art for thin-film conductor inspection [1]. A recent example is given by a novel instrument employing miniaturized terahertz near-field detectors [2]. Non-destructive high-resolution inspection of various conduction layers as used in touch-screens, electronic paper, displays, solar cells or OLED devices is efficiently accomplished by this new technology.

Background

While THz radiation can virtually not be transmitted through highly conducting bulk materials it penetrates fairly well through thin conductor layers with a thickness below skin-depth [3]. This property can be used to measure the absolute sheet-resistance and (indirectly) also the thickness of many technically relevant conductor layers under the usually satisfied assumption of constant bulk conductivity. THz radiation is sharing this property with microwave or even longer wavelength radiation. However, high-resolution measurements are much more difficult in these low-frequency regimes and thus have only been available for small measurement areas on the order of 100 µm x 100 µm using atomic-force-microscope-type equipment. Other methods which can be used for full-wafer mapping (like Eddy-current measurements) suffer from very low mm-scale spatial resolution. The THz microprobe-based technology is now enabling micron-scale resolution and high-speed full wafer mapping which has not been possible up to now. This increased performance is supplemented by capabilities to measure layers buried under isolating capping layers and generally contact-free probing.

Measurement results

In Fig. 1 the dependency of THz transmission against layer thickness is shown for some selected conductor materials. The accessible thickness range extends from sub-nanometers for a single layer of graphene to the micrometer range for indium-tin-oxide (ITO). The corresponding sheet resistance range is from sub-Ohm to some k-Ohm per square. For optical measurement systems such a large range of thickness values is usually inaccessible. In case of ellipsometric measurements a pre-knowledge of the approximate thickness value may allow the execution of measurements, but only in a small few-nm-range of thickness deviation.

 

In Fig. 2 an exemplary sheet resistance plot measured with a THz microprobe is shown. The investigated sample is a glass wafer covered with TiN and Ti layers of various thicknesses. The measurement speed is up to 5 ms/data-point which is sufficiently high to enable high-resolution full wafer mappings in a few minutes and up to three orders of magnitude higher compared to standard methods like four-point-probing or spectroscopic ellipsometry. The observed sheet resistance values range from 6 Ohm to 400 Ohm per square corresponding to 7 nm – 230 nm of TiN. The well visible radial increase of sheet resistance refers to a thickness decrease of up to 20% caused by a sputtering process inhomogeneity.

Fig. 2: (Right) wafer-scale mapping of sheet resistance values measured at a glass wafer covered with differently thick TiN and Ti layers. The black area refers to uncovered glass. (Left) Colour-scale plot high-lighting the sheet resistance inhomogeneity within the marked wafer area.

 

References:

[1] M. Nagel,
A. Safiei, S. Sawallich, C. Matheisen, T.-M. Pletzer, A.A. Mewe, N.J.C.M. van der Borg, I. Cesar, H. Kurz, “THz Microprobe System for Contact-Free High-Resolution Sheet-Resistance Imaging,” 28th EU PVSEC conference, 30 September 2013 – 4 October 2013, Paris.

[2] http://www.amo.de/?id=798

3] M. Walther, D. G. Cooke, C. Sherstan, M. Hajar, M. R. Freeman, and F. A. Hegmann, “Terahertz conductivity of thin gold films at the metal-insulator percolation transition,” Phys. Rev. B 76, 125408 (2007).