Event

Ph.D. Dissertation Defense: Lisa Krayer

Friday, November 15, 2019
10:00 a.m.-12:00 p.m.
AVW 2460
Maria Hoo
301 405 3681
mch@umd.edu

ANNOUNCEMENT: Ph.D. Dissertation Defense


NAME: Lisa Krayer

Committee:
Professor Jeremy N. Munday, Chair/Advisor
Professor Kevin Daniels
Professor Neil Goldsman
Professor Thomas E. Murphy
Professor Lourdes G. Salamanca-Riba, Dean's Representative

Date/Time: Friday, November 15th, 2019 at 10 AM - 12 PM

Place: AVW 2460

Title: Photodetection using ultrathin metal films


Abstract:
Silicon is the most widely used material for visible photodetection, with extensive applications in both consumer and industrial products. Further, its excellent optoelectronic properties and natural abundance have made it nearly ideal for microelectronic devices and solar cells. However, silicon's lack of absorption in the infrared limits its use in infrared detectors and imaging sensors, severely constraining its implementation in telecommunications for low-cost integrated optical circuitry.  We will show that this limitation can be overcome by exploiting resonant absorption in ultrathin metal fi lms (< 20 nm). Our approach paves the way to implement
scalable, lithography-free, and low-cost silicon-based optoelectronics beyond the material bandgap.

Light absorption in metal films can excite hot carriers, which are useful for photodetection, solar energy conversion, and many other applications. However, metals are highly reflective, and therefore, careful optical design is required to achieve high absorption in these fi lms. Through appropriate optical design, we achieved a Fabry-P erot-like resonance in ultrathin metal fi lms deposited on a semiconductor enabling > 70% light absorption below the bandgap of the semiconductor. We experimentally demonstrate this phenomenon with four ultrathin planar metal fi lms: Pt, Fe, Cr, and Ti. These metals were chosen to satisfy the resonant condition for high absorption over a wide range of wavelengths, and with these designs we realize a near-infrared imaging detector.

In addition, we utilize an index-near-zero (INZ) substrate to further improve the absorption to near-unity. By employing aluminum-doped zinc oxide (AZO) as the INZ medium in the near-infrared range, we enhance the metal fi lm absorption by nearly a factor of 2. To exploit this absorption enhancement in an optoelectronic device, we fabricate a Schottky photodiode with a Pt film on Si and find that the photocurrent generated in the photodiode is enhanced by > 80% with the INZ substrate. The enhancement arises from a combination of improved carrier generation and carrier transport resulting from the addition of the AZO film.

Finally, we explore the possibility of tuning material properties through alloying metals. We explore AgAu alloys for controlling the optical and electrical responses to achieve improved functionality as hot carrier photodetectors. An ideal metal-semiconductor photodetector requires not only high absorption, but also long hot carrier attenuation lengths in order to efficiently collect excited carriers. While pure Ag and Au do not have high absorption, they have long hot carrier attenuation lengths >20 nm. We find that alloying Ag and Au enhances the absorption by ~ 50% while maintaining attenuation lengths >15 nm, although pure Au remains the best material for maximizing the hot carrier attenuation length because the alloys are limited by high grain boundary scattering.


 
 

Audience: Graduate  Faculty 

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