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Enhanced photocarrier generation in large-scale photonic nanostructures fabricated from vertically aligned quantum dots

Takeshi Tayagaki, Yusuke Hoshi, Yuko Kishimoto, and Noritaka Usami

Open Access Open Access

Abstract

We demonstrate enhanced photocarrier generation using photonic nanostructures fabricated by a wet etching technique with vertically aligned quantum dots (QDs). Using photoluminescence excitation spectroscopy, we found that the photocarrier generation in Ge/Si QDs placed close to the surface is enhanced below the band gap energy of crystalline silicon. The enhancement is explained by light trapping owing to the photonic nanostructures. Electromagnetic wave simulations indicate that the photonic nanostructure with a subwavelength size will be available to light trapping for efficient photocarrier generation by increasing their dip depth.

© 2014 Optical Society of America

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Figures (4)
Fig. 1
Fig. 1 (a) Schematic illustration of the side view of the surface photonic structures formed by selective wet etching using HF/HNO3 and KOH. (b) Atomic force microscope images of the photonic structures formed by HF/HNO3 (left) and KOH (right) etching. Images are ~1 × 1 μm in size. (c) Distribution of the width (left) and depth (right) of the dip formed by HF/HNO3 etching. (d) Distribution of the width (left) and depth (right) of the convex formed by KOH etching. Error bars show the FWHM of the distributions.

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Fig. 2
Fig. 2 Transmission, reflection, and extinction spectra of photonic structures formed by (a) HF/HNO3 and (b) KOH etching. Inset: Extinction at 1100 nm for different etching times.

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Fig. 3
Fig. 3 (a) PL and PLE spectra of the Ge/Si QDs without etching measured at 20 K. Inset: Schematic of the PLE measurements. Normalized PL intensity of Ge/Si QDs in the samples etched for different etching times with (b) HF/HNO3 and (c) KOH. Excitation wavelength dependence of the normalized PL intensity of Ge/Si QDs in the sample etched with (d) HNO3 and (e) KOH. Curves show the results for different optical thickness d = 20 (solid), 40 (dotted), and 60 nm (broken), calculated using Eq. (1).

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Fig. 4
Fig. 4 (a) Schematic illustration of the photonic nanostructure used in the simulation. (b) Dip depth dependence of the electric-field density at a 1000 nm wavelength (TE and TM polarization). (c) Typical electric-field distribution for the photonic structures with a 200 nm depth. The calculation was performed for the sample without PEC reflector at the rear surface.

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Equations (1)

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M= I ′ I 0 = 1−exp(−αxd) 1−exp(−αd) ,
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