The n-type Ge photodetectors with gold nanoparticles deposited to enhance the responsivity
© Hsiao et al.; licensee Springer. 2014
Received: 30 June 2014
Accepted: 12 November 2014
Published: 27 November 2014
Gold nanoparticles (AuNPs) have been deposited on n-type Ge photodetectors to improve the responsivity. Two different coverage ratios, including 10.5 and 30.3% of AuNPs have been prepared, and the fabricated photodetectors are compared with the control sample. The 1,310-nm responsivities at -2 V of the control, 10.5% AuNPs, and 30.3% AuNPs samples are 465, 556, and 623 mA/W, respectively. The AuNPs could increase the responsivities due to the plasmon resonance. The reflectance spectra of these samples have been measured to verify that plasmon resonance contributes to the forward scattering of incident light. The reflectance decreases with AuNP deposition, and a denser coverage results in a smaller reflectance. The smaller reflectance indicates more light could penetrate into the Ge active layer, and it results in a larger responsivity.
Photodetectors are widely used in various fields in our daily life[1, 2]. The detection wavelength of a photodetector depends on the choice of the active semiconductor material. Si is a common semiconductor material for use in photodetectors due to its abundant existence on Earth. The bandgap of Si is 1.12 eV, which corresponds to a cutoff photodetection wavelength of 1,100 nm. Visible light and one of the common bands of near infrared, 850 nm, can be detected well by Si. However, other important near-infrared bands, including 1,310 and 1,550 nm, cannot be absorbed by Si. The low-loss and low-dispersion characteristics of infrared light in fiber at these wavelengths make these wavelengths necessary in optical communication[4, 5]. Ge, an element in the same group IV as Si, has a smaller bandgap of 0.66 eV, and it can be used for light detection at these wavelengths[6, 7]. Various approaches have been tried to boost the photoresponses (responsivities) of Si photodetectors[8–11]. Among them, the investigation of the use of metallic nanoparticles has grown rapidly in the last decade[11–14]. However, few studies have touched on the topic of photoresponse enhancement of Ge based on metallic-nanoparticle incorporation. Metallic nanoparticles on a semiconductor could contribute to the plasmonic interaction, which could result in forward scattering of incident light into the semiconductor to enhance the photoresponse[15, 16]. Such plasmon resonance can be used in solar cells, although it may upgrade the (quantum) efficiency at certain wavelengths while degrading at other wavelengths. As compared to applications on solar cells, where broadband utilization is needed, photodetectors could benefit more from plasmonic interaction, since only improvement of a certain wavelength is needed. In this manuscript, gold nanoparticles (AuNPs) have been deposited on the Ge photodetector. The reflectance and responsivities of n-type Ge photodetectors with and without AuNPs covering will be compared to show the enhancement contributed by AuNPs.
Circular Al gates with areas of 5 × 10-4 cm2 were then deposited on top of Ge substrates to form the photodetectors. Large-area C-shaped Al was also deposited to act as surrounding ohmic contacts at the same time. In order to identify the effect contributed by the AuNPs, the control sample (control) without AuNPs was also prepared, for comparison.
Results and discussion
To find the maximum response, we continuously moved the tip of the fiber output inward into the Al gate and measured the respective photocurrents. The alternate photocurrents and dark currents were measured until the representative current levels were obtained. It was found that the photocurrents gradually increased to the saturated maximum as the tip of the fiber output was moved inward into the gate and then decreased due to the blockage of light by the electrode. Hereafter, we only show the representative maximum photocurrent and the representative dark current.
The n-Ge photodetectors with and without AuNPs covering them are compared. The deposition of AuNPs can decrease the reflectance of near-infrared via the plasmon resonant nanoparticles. With a coating ratio of 10.5% AuNPs, the reflectance can decrease 4.4%, and the responsivity increases 20% as compared to the control sample. If the coverage ratio of AuNPs increases to 30.3%, the reflectance can decrease 10%, while the responsivity increases 34%. AuNPs is obviously beneficial for Ge near-infrared photodetection.
The authors are grateful to the National Nano Device Laboratories of the R.O.C. for the facility access. This work is supported by National Science Council of R.O.C. under contract no. NSC 101-2221-E-259 -023 -MY3.
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