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Determination of the photocarrier diffusion length in intrinsic Ge nanowires

Yun-Sok Shin, Donghun Lee, Hyun-Seung Lee, Yong-Jun Cho, Cheol-Joo Kim, and Moon-Ho Jo

Open Access Open Access

Abstract

We quantitatively determined the photocarrier diffusion length in intrinsic Ge nanowires (NWs) using scanning photocurrent microscopy. Specifically, the spatial mapping of one-dimensional decay in the photocurrent along the Ge NWs under the scanning laser beam (λ= 532 nm) was analyzed in a one-dimensional diffusion rate equation to extract the diffusion length of ~4-5 μm. We further attempt to determine the photocarrier lifetime under a finite bias across the Ge NWs, and discuss the role of surface scattering.

©2011 Optical Society of America

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Figures (3)
Fig. 1
Fig. 1 (Color online) (a) Schematics of scanning photocurrent microscopy (SPM) on a Ge nanowire (NW) on a SiO2/p+-Si substrate, where a laser beam (λ = 532 nm) was used with a chopper at a frequency of 10 kHz. The lock-in amplifier and photodiode were used for the photocurrent and reflectance measurements, respectively. (b) Dark conductance G as a function of gate voltages VG , showing p-type characteristics. The inset shows ohmic I-V characteristics for VSD = −5 V (red), 0 V (black), and 5 V (blue), respectively.

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Fig. 2
Fig. 2 (Color online) (a) SPM images of the photocurrent (I ph) at VSD = 35 mV, 15 mV, 0 V, −15 mV, and −35 mV, respectively, where the gray and dashed lines indicate the reflectance image and Ge NW, respectively. The S (D) stands for the drain (source) at x1(2) of the NW axial coordinate x. Here, the scale bar indicates 3 μm. (b) 3-dimensional Iph profile at VSD = 0 V. (c) The NW axial I ph profile for VSD = 35 mV (red), 0 mV (black), and −35 mV (blue), respectively.

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Fig. 3
Fig. 3 (online color) (a) Photocurrent, I ph profile along the NW at VS = 0 V with the fitting curve, where the circles (black) and line (red) indicate the experiment data and the fitting, respectively. See the details in the text. (b) The characteristic parameter, λh/μhτh for various VSD with error bars.

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Equations (5)

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∂ δ n h ( e ) ∂ t = D h(e) ∂ 2 δ n h ( e ) ∂ x 2 + μ h(e) E ∂ δ n h ( e ) ∂ x + g - δ n h ( e ) τ h ( e ) ,
I ph = I S - I D = (- I h2 + I e2 ) - (- I h1 + I e1 ),
I ph = I h1 - I h2 = I h0 {exp(- |x-x 1 | λ h ) - exp(- |x-x 2 | λ h ) } .
I ph = I h0 {exp(- |x-x 1 | λ h ) - exp(- |x-x 2 | λ h )} - I e0 {exp(- |x-x 1 | λ e ) - exp(- |x-x 2 | λ e )},
d(ln( I ph )) dx ∝ d(ln( δ n h )) dx ∝ -E/2 + (-E/2) 2 + ( λ h / ( μ h τ h ) ) 2 kT/e .
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