1. Introduction

Decreasing the volume of light-matter interaction is the main issue in most photodetection and photovoltaic developments. The use of nanoscale absorbing region is a key for many applications as low-noise mid-infrared detectors [1, 2], high-speed telecommunication photodetectors [3], or next-generation solar cells [4, 5]. Nanowire-based devices are intensively studied, due to their unique properties for carrier and photon confinement [6–8]. However, fabrication difficulties and the extremely small photon absorption cross-section still limit the efficiency of nanowire photodetectors. Absorption cross-sections can be strongly increased with surface-plasmon resonances induced by metallic nanostructures [9, 10]. Their ability to concentrate light into deep-subwavelength volumes has been demonstrated in many configurations [11, 12]. Plasmonics provide new rules for the conception of photodetectors and solar cells [5,9,13,14], but these devices have to face up to metal absorption and the role of defects at the metal-semiconductor interfaces.

In this letter, we demonstrate high external quantum efficiencies (above 40 %) in nanoscopic metal-GaAs wire arrays. This is achieved in the nanoscale metal-semiconductor-metal (nano-MSM) photodetector [15] depicted in Fig. 1(b). It is shown that a metal nanowire array allows efficient light confinement in GaAs wires of tiny section (100 nm × 40 nm) near the electrodes, leading to highly efficient light trapping and photocarrier collection.

 

Fig. 1 (a) SEM photograph of a 5 × 5 μm² nano-MSM photodetector. (b) Schematic drawing of the nano-MSM photodetector, and cross-section of the calculated spatial distribution of the electric field intensity in the GaAs absorbing layer in TE polarization (λ = 790 nm). White regions show high absorption. (c) Schematic drawing of the active part of the photodetector showing the GaAs nanowire array where most of the absorption occurs, and carrier collection mechanism.

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2. Nearly-perfect resonant optical absorption in 40 nm-thick GaAs layer

Light trapping in the nano-MSM photodetector is based on a Fabry-Perot-type resonance [15, 16]: the lower mirror of the cavity is a multilayered Bragg reflector made of 24-period of quarter waves AlAs/Al_0.2Ga_0.8As layers. The upper mirror is a metal nanowire array with a period much smaller than the wavelength. It acts as a metamaterial whose optical properties (specular reflection and transmission) depend on the geometrical parameters (wire width, period and height) [15]. They are determined by electromagnetic computation. The metallic grating is made of a 2 nm-thick titanium adhesion layer and a 30 nm-thick gold layer with 100 nm finger spacings and finger widths. The resonant cavity is made of a Al_0.35Ga_0.65As non absorbing spacing layer (thickness: 30 nm) and a thin GaAs absorbing layer (thickness: 40 nm). The metallic wires are deposited on the absorbing layer, and they are connected to form interdigitated Schottky contacts (Fig. 1(a)). Nano-MSM devices were fabricated by molecular beam epitaxy and electron beam lithography, as described previously (see Refs. [15, 16] for details). Devices with 5 × 5 μm² (sample A, see Fig. 1(a)) and 10 × 10 μm² (sample B) surface area are analyzed experimentally when illuminated by TE-polarized incident light (electric field parallel to metallic wires). The results are compared with electromagnetic calculations obtained with an exact modal method and optical constants taken from [17] (dashed curves in Fig. 2).

 

Fig. 2 (a) Experimental and numerical reflection spectra in TE polarization (black curves), and numerical results of absorption (red curve) in the 40 nm-thick GaAs layer, at normal incidence. (b) Angular dependence of the absorption efficiency in the GaAs layer at resonance wavelength λ = 790 nm (numerical results).

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Reflection measurements are shown in Fig. 2(a) for normal incidence (solid curve, NA=0.3). Experimental results exhibit a strong resonance in the center of the stop-band of the Bragg mirror. The reflection dip and resonance width are in excellent agreement with theoretical predictions (dashed curves), and demonstrate nearly perfect absorption, with reflectivity as low as 5 % at 790 nm wavelength. The theoretical absorption in the GaAs layer is also plotted in Fig. 2(a) (red dashed curve). At 790 nm wavelength, 50 % absorption efficiency is predicted in the 40 nm-thick GaAs layer. The other 50 % of the incident power are divided as follows: reflection (2.5 %), transmission through the Bragg reflector (2.5 %), absorption in the gold grating (16 %) and absorption in the titanium adhesion layer (29 %). Low angular dependence is demonstrated in the ±15° angle range (numerical calculations, see Fig. 2(b)), allowing strong optical focusing on 5 × 5 μm² nano-MSM photodetectors (sample A) without significant effect on the device efficiency.

The electric field intensity (||E||²) along a device cross-section is represented in Fig. 1(b) for λ = 790 nm (resonance wavelength). The local absorption is proportional to the electric field intensity. The optical interaction with the metallic grating induces a specific shape for the absorption into the GaAs layer. The absorption enhancement (up to 4-fold below the GaAs-air interface) is localized in an array of GaAs nanowires, spatially shifted by half a period compared to the metal nanowire array. This is illustrated schematically in Fig. 1(c). As expected in TE polarization, no plasmonic effect is involved in the optical resonance, as confirmed in Fig. 1(b). This configuration is well adapted to achieve a very efficient collection of photogenerated electrons and holes: the mean collection path is strongly reduced (50–100 nm) compared to standard photodetectors, and the absorbing region is separated from the GaAs-Ti/Au interfaces. Hence, we expect a low level of electron-hole recombinations induced by surface defects at the GaAs-metal interface. In addition, it has to be noted that transport and carrier collection occur on the sides of the nanowires, unlike most semiconductor nanowire photodetectors where the carriers have first to reach the wire extremity to be collected [6].

3. Evidence of 40 % external quantum efficiency

Electro-optical measurements are shown in Fig. 3 (λ = 790 nm, normal incidence). Very low dark current (500 pA) and high bias voltage without breakdown (up to 2 V) are obtained with nanoscale (100 nm) finger spacing and finger width. Low level of dark current density (< 5 pA/μm²) allows to realize high sensitive detectors of optical radiation on these structures. The Schottky barrier height is evaluated to 0.55 eV and its ideality factor is n=1.05. These results demonstrate the high quality of the metal-GaAs interfaces. The photocurrent-voltage characteristics do not show saturation up to 2 V. At low voltage, the photocurrent can be limited by surface recombinations [18]. At higher voltage, the very high electric field close to the metal wires can induce gain effects due to barrier lowering and internal emission of photoexcited electrons into the metal, close to the GaAs interface [19].

 

Fig. 3 IV characteristics of nano-MSM photodetector (sample B) in dark (black dots) and illuminated (color dots) conditions, for different light powers and for λ = 790 nm wavelength.

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Figure 4 shows the external quantum efficiency η as a function of wavelength and incident light power. This efficiency can be expressed as the product of the absorption efficiency in the GaAs layer A(λ) and the internal quantum efficiency η_i (η = η_i × A(λ)). The contributions of carrier recombinations and gain effects mentioned above depend on the bias voltage and incident light power, and can lead to internal quantum efficiencies η_i smaller or greater than 1. Therefore, the exact absorption efficiency cannot be deduced from experimental data. However, external quantum efficiency measurements confirm the resonant behavior of the nano-MSM photodetectors, and the high absorption efficiency predicted in the 40 nm-thick GaAs layer. For low incident power, external quantum efficiencies above 40 % are obtained at 790 nm wavelength, close to the maximum theoretical absorption efficiency.

 

Fig. 4 External quantum efficiency η (color dots: measurements, lines: Lorentzian fit) as a function of the wavelength and incident light power on sample A (Bias voltage: 2 V). The theoretical absorption A_th(λ) calculated in the GaAs layer is also shown (black curve). Inset: External quantum efficiency as a function of the incident light power (measurements under 2 V bias voltage).

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4. Conclusion

In conclusion, nearly perfect absorption (95 %) is demonstrated in a nanoscale MSM device. Light trapping in a 40 nm-thick GaAs layer is achieved with a Fabry-Perot-type resonator between a gold nanowire array and a multilayered Bragg reflector. External quantum efficiency reaches 40 %. This is an efficiency improvement by orders of magnitude in comparison with conventional MSM photodetector with a similar geometry of 100 nm finger spacing [18]. Light trapping leads to a twenty-fold decrease of the absorption layer thickness compared to bulk GaAs. The efficient collection of photocarriers is achieved through a high and homogeneous electric field with mean collection path of the carriers below 100 nm. The diode capacitance of 5 × 5 μm² nano-MSM devices is 8 fF. As a result, the transit time and RC time are below 1ps, leading to potential cutoff frequencies above 300–400 GHz. This nano-MSM structure allows to overcome the usual trade-off between speed and efficiency in MSM photodetectors [15].

These results demonstrate the achievement of efficient light absorption in nanoscale semiconductor layers by resonant mechanisms, together with efficient collection of photogenerated carriers. They show that optical confinement can be achieved in nanowire arrays without the help of plasmonic effects, and could lead to novel architectures for nanowire devices. They pave the way to the realization of photodetectors and photovoltaic devices with nanoscale absorption layer. Broadband operation by resonance broadening or multi-resonant light trapping is under study. This is the next step toward the development of original designs required for next-generation solar cells using ultra-thin absorbers.