Letter pubs.acs.org/NanoLett
High Quantum Efficiency Nanopillar Photodiodes Overcoming the Diffraction Limit of Light Wook-Jae Lee,*,† Pradeep Senanayake,† Alan C. Farrell,† Andrew Lin,† Chung-Hong Hung,‡ and Diana L. Huffaker†,§ †
Department of Electrical Engineering and §California Nano-Systems Institute, University of California - Los Angeles, Los Angeles, California 90095, United States ‡ Nanopixel Technologies LLC, Los Angeles, California 90095, United States S Supporting Information *
ABSTRACT: InAs1−xSbx nanowires have recently attracted interest for infrared sensing applications due to the small bandgap and high thermal conductivity. However, previous reports on nanowire-based infrared sensors required low operating temperatures in order to mitigate the high dark current and have shown poor sensitivities resulting from reduced light coupling efficiency beyond the diffraction limit. Here, InAsSb nanopillar photodiodes with high quantum efficiency are achieved by partially coating the nanopillar with metal that excites localized surface plasmon resonances, leading to quantum efficiencies of ∼29% at 2390 nm. These high quantum efficiency nanopillar photodiodes, with 180 nm diameters and 1000 nm heights, allow operation at temperatures as high as 220 K and exhibit a detection wavelength up to 3000 nm, well beyond the diffraction limit. The InAsSb nanopillars are grown on low cost GaAs (111) B substrates using an InAs buffer layer, making our device architecture a promising path toward low-cost infrared focal plane arrays with high operating temperature. KEYWORDS: Nanowires, nanopillar, InAsSb, nanophotodiode, diffraction limit, surface plasmons In this Letter, we for the first time demonstrate vertically grown InAsSb NP photodiodes (PDs) overcoming the diffraction limit of the light owing to LSPs by partially covering the NPs with gold, as shown in Figure 1b. LSPs allow extreme light confinement in NPs with a diameter of 180 nm and a height of 1000 nm allowing detection up to 3000 nm. Moreover, we show that our InAsSb NP PDs can be operated not only above 220 K, which eliminates the cryogenic coolers and allows the use of inexpensive thermoelectric coolers, but also operate at zero-bias voltage. Figure 1c shows the calculated absorption in square lattice NPs as a function of the diameter and the wavelength of the incident light taken under x-polarized and normal incident illumination. The calculations were carried out using the finitedifference time-domain (FDTD) method. We considered hexagonal-shaped InSb NPs on InAs substrate with 1000 nm height and 1000 nm pitch surrounded by SiO2 (see Figure 1a). It is clear that the absorption peak location due to HE11 leaky mode is red-shifted with increasing diameter19,21 and the maximum absorption peak is limited to 1200 nm at 200 nm diameter according to the inherent diffraction limit as shown in
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emiconductor nanopillars (NPs) or nanowires with dia meters from 2−200 nm1 have been implemented as nanoscale photovoltaic cells, including photodetectors2−7 and solar cells,8−12 to take advantage of reduced dark current and strong light trapping resulting from their small size and threedimensional geometry. Vertically oriented NP arrays, in particular, have attracted great interest for more efficient photodetection than single and horizontal NPs due to their enhanced light absorption by localized surface plasmon (LSP) resonances,5,13 increased the path length of incident light14,15 and diffusion length,16 and reduced reflection.17,18 A typical NP device can only absorb in a specific spectral range limited by the diameter11,14,19−21 which gives rise to very low responsivity beyond the diffraction limit of the light.22 Although vertical NPs, as shown Figure 1a, support hybrid (HE) modes for normal-incidence illumination,19 NPs lying on a substrate can absorb longer wavelengths via the excitation of transverseelectric and transverse-magnetic leaky modes. Both horizontal and vertical NPs, however, are still dependent on NP diameter; this holds true for vertical NP arrays as well. Unfortunately, this means that the performance of semiconductor NPs are ultimately limited by geometry, not material bandgap. Previous studies on semiconductor NP photodetectors, therefore, have focused primarily on the visible4 and near-infrared (IR) spectral region6,7,19,23,24 with a few works carried out in mid-IR range25,26 through large-diameter NPs and agglomerated wires. © XXXX American Chemical Society
Received: August 29, 2015 Revised: December 2, 2015
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DOI: 10.1021/acs.nanolett.5b03485 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. (a, b) Schematic illustration of hexagonal-shaped NP array and proposed NPs structure with partially covered metal (Au 150 nm thick) used for absorption calculations, respectively. InSb NPs on InAs substrate are considered, and they have the same height (1000 nm), diameter (200 nm), and periodicity (1000 nm). (c, d) Calculated absorption for (a) and (b) structures, respectively. (e) Electric field profiles in the xz plane for NP arrays and NP with partially covered metal at the peak wavelengths (λ). The white lines indicate the interfaces of materials.
Figure 1c. However, NPs partially covered with metal (see Figure 1b) have a number of distinct peaks in longer wavelength ranges at 1550 and 2600 nm within the extreme subwavelength-scale diameter of NP, which is well beyond the diffraction limit, as shown in Figure 1d. The physical origin of these peaks can be understood by analyzing the electric field distributions at the peak wavelengths. Figure 1e shows the electric field distributions of absorption maxima of the bare NP and the metal covered NP. The bare NP shows the HE11 leaky mode at 1200 nm, while the metal covered NP shows enhanced fields at the edges of the metal and absorbed fields in the NP at longer wavelengths supported by the 3D LSP resonances. We propose that 3D LSP resonances allow absorption beyond the diffraction limit and can be used to realize ultracompact absorbers for infrared photodetector applications. To demonstrate the use of our NP structure for overcoming the diffraction limit of light, we have fabricated NP PDs that consist of arrays of InAsSb NPs with a diameter of 180 nm and
a height of 1000 nm grown on p-doped InAs buffer layer with doping concentration of 5 × 1018 cm−3 on GaAs (111)B substrate. Subsequently, a SiO2 growth mask is patterned by ebeam lithography for selective area NP growth. A pn heterojunction is formed between the p-doped InAs buffer layer and an n-doped InAsSb NP array with doping concentration of 3 × 1017 cm−3 to generate a built in field for extracting photogenerated carriers. The gas-phase Sb composition, Sb/[Sb + As], during the growth was 40% which is corresponding to InAs0.93Sb0.07.27 The detector area of 50 μm × 50 μm, corresponding to 3086 NPs for a pitch of 900 nm, was defined by photolithography followed by the tilted metal deposition. This also functions as a top electrode which is electrically isolated by the benzocyclobutene (BCB) planarization layer. The bottom and top metals are Cr/Au (10 nm/150 nm). Figure 2a shows the scanning electron microscope (SEM) images of the fabricated NP PDs. The NP PDs were wirebonded to a 68 pin leadless chip carrier (LCC) as depicted in B
DOI: 10.1021/acs.nanolett.5b03485 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 2. (a) SEMs of the fabricated NP PDs. Inset shows the diameter of NP. The scale bar indicates 100 nm in the inset. (b) Photograph of the wire-bonded device. (c) Schematic of proposed NP PD structure.
nation current. The formation of a nanowire pn heterojunction allows dark current reduction and higher temperature operation compared to a previous report,25 which showed a relatively high dark current at 5 K because photoconductive NPs with high Sb composition (InAs0.24Sb0.76) were measured. In order to evaluate the electro-optic properties of the NP PD, the LCC is mounted in a liquid nitrogen flow cryostat integrated with a Nicolet 8700 Fourier-transform infrared (FTIR) spectrometer. The NP PDs were illuminated by IR source passing through the FTIR and then focused by means of a reflective objective. The photocurrent spectra was recorded in the rapid scan mode with a spectral resolution of 4 cm−1. To account for the spectral response of the IR source and system optics, the photocurrent spectra were normalized to the flat response of a deuterated triglycine sulfate (DTGS) detector. The frequency response of the DTGS detector was also compensated by taking advantage of the variable mirror velocity of the FTIR, allowing the true spectral shape of the photocurrent spectrum to be recorded. The arbitrary units of the photocurrent spectra were converted to units of amperes per watt using a calibrated commercial InGaAs detector (see Supporting Information for a detailed explanation). Figure 4 shows the measured spectral responsivity taken under unpolarized IR light at 77 K (blue, left axis) and 227 K (red, right axis). The cutoff wavelength (λ) of our NP PDs is around 3000 nm, which is strong evidence for plasmonically enhanced absorption overcoming the diffraction limit of the light because NPs partially covered with metal absorb longer wavelengths within the extreme subwavelength-scale diameter of NP (below λ/16) through the LSP resonances, whereas the bare NPs show a near zero absorption beyond 1500 nm wavelength (see Figure 5). Further evidence for plasmonically enhanced absorption is the occurrence of the enhanced
Figure 2b. The schematic of our NP PD with p-type InAs buffer layer grown by molecular beam epitaxy is also shown in Figure 2c (see Supporting Information for a detailed fabrication process). The wire-bonded NP PDs were mounted in a lakeshore TTPX cryogenic probe station for dark current measurements. Figure 3a shows temperature-dependent dark current characteristics of the InAs/InAsSb heterojunction PD. At 77 K, dark currents as low as 40 pA were achieved at a reverse bias of 0.1 V, corresponding to a dark current density of 200 mA/cm2 which is normalized to the NP junction area. We note that the diode behavior and the kinks in the low temperature IV curves under forward bias are a common feature of InAs photodiodes (in our case, InAsSb with a small amount of Sb) and has been attributed to tunneling current.28,29 The temperature dependence of the dark current of a PD is described by eq 1 below, I = AT 3/2e−Ea / kT (eqVbias/2kT − 1)
(1)
where A is proportionality constant, Ea is the activation energy, q is the elementary charge, k is Boltzmann constant, Vbias is the bias voltage, and T is the temperature. Ea can be extracted by fitting the slope of the semilog plot of I/V3/2 with 1/kT. Figure 3b shows the extracted activation energies at reverse biases of 0.1 and 0.3 V. An activation energy of 0.17 eV is measured, which is approximately half the bandgap of our InAs0.93Sb0.07 nanopillar. Dark current analysis of bulk InAs diodes has shown that, while the bulk material has an activation energy of 0.36 eV, the activation energy of the surface current is 0.18 eV and is the dominant component of the leakage current for small area diodes (diameter
