Waveguide-integrated black phosphorus photodetector with high responsivity and low dark current

Transcription

1 SUPPLEMENTARY INFORMATION DOI: /NPHOTON Waveguide-integrated black phosphorus photodetector with high responsivity and low dark current Nathan Youngblood 1, Che Chen 1, Steven J. Koester 1, and Mo Li 1* 1 Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, USA S1. Black phosphorus transfer Black phosphorus was transferred using the wet transfer method described in [1]. The silicon transfer substrates were prepared by spinning 15 nm PVA dissolved in water and baking at 150 C for 1 minute. 300 nm PMMA was then spun on top of the PVA layer and baked at 150 C for 5 minutes. Black phosphorus was then exfoliated onto the transfer substrates using typical scotch tape mechanical exfoliation. It was found that Nitto wafer dicing tape left much less residue and a higher yield could be achieved by gently pressing the tape onto the transfer substrate, heating the substrate between 50 and 60 C for 5 minutes, and then peeling the tape from the substrate very slowly. Once black phosphorus pieces were identified and the height measured with AFM, the transfer substrate was cut to the appropriate size and placed in water until the PMMA membrane separated from the silicon substrate. Care was taken to prevent water from coming Figure S1: Wet transfer process using a PMMA membrane and PVA sacrificial layer. * Corresponding author: moli@umn.edu NATURE PHOTONICS 1

2 into contact with the black phosphorus. Once the membrane was free to float on the water, a 7 mm diameter TEM loop was used to lift the membrane out of the water and was placed on a hot plate at 90 C for 20 minutes to dry. A micromanipulator and microscope were used to align the black phosphorus flakes to the waveguides on the target substrate. Once in contact, heat was applied to the target substrate (120 C for 10 minutes) to melt the PMMA from the TEM loop and ensure good adhesion. The PMMA was removed with an overnight soak in NMP. The target substrate with the transferred black phosphorus flakes was heated in vacuum to 120 C for 5 hours after the removal of the PMMA membrane before subsequent processing. This helped to prevent potential degradation due to water. Figure S1 illustrates this wet transfer process.

3 S2. Raman characterization Raman spectra was taken to determine the orientation of transferred black phosphorus. Figure S2 shows the polarization dependent Raman spectra we observed for the devices presented. The spectra are normalized to the out-of-plane A 1 g peaks since they should be independent of in-plane polarization. We can determine the crystal orientation based on the relative ratio of the B2g peak (zigzag direction) to the A 2 g peak (armchair direction). The polarization angle is relative to the flow of carriers in the channel. Therefore, we can see that the BP for the 100 nm device is oriented with the armchair axis parallel to the flow of carriers in the channel, while for the 11.5 nm device, the armchair axis is perpendicular to the direction of carrier flow. According to [8] and [2], the armchair direction has the greatest optical absorption and highest carrier mobility. Therefore, the crystal orientation of the 100 nm device is optimally aligned for maximal mobility and absorption while the 11.5 nm device is not. Figure S2: Raman spectrum of BP devices. The 11.5 nm and 100 nm devices are discussed in the main text, while data from the 10 nm device is included in section S4.

4 S3. Effective electron mobility calculation The effective mobility of the carriers in our black phosphorus FETs can be calculated using the linear long-channel MOSFET equation: This can be rewritten as: I D W = μ (V GS V T ) effc ox V L DS μ eff = ΔI D/W ΔV GS L = Δσ sheet 1 C ox V DS ΔV GS C ox Figure S3 shows the linear fit to the σ sheet versus V GS plots at various bias voltages. Using a relative permittivity of εr = 8 for Al2O3 and L = 1.5 μm, the carrier mobility of the n-doped side was estimated to be 21.9 cm 2 /V*s. This effective mobility is lower than other reports [3] and likely underestimates the intrinsic carrier mobility. The lower value of the effective mobility could be caused by non-ideal effects due to surface passivation such as a high density of oxide traps in the dielectric layer [4]. Figure S3: Fit to linear region of sheet conductivity. Mobility was estimated to be 21.9 cm 2 /V*s.

5 S4. Other top gated BP devices An additional black phosphorus device with comparable thickness was fabricated with a top gate to verify the doping abnormality induced by the top gate and dielectric layer. This device showed similar transport properties, but was not as heavily n-doped as our original device which made seeing the heavily doped photocurrent sign change impossible. Figure S4 shows the photocurrent at various biases and gate conditions on the left while on the right, the high conduction is again seen on the n-doped side. This is consistent with Figure 2a of the main text. Photocurrent (µa) vs Bias and Gate Voltage Figure S4: Optical (top left) and AFM (top right) images of a 10 nm gated BP photodetector similar to the one presented in the main text. Photocurrent (µa) as a function of gate and bias voltage (bottom left). Source-drain current as a function of gate voltage for various biasing conditions (bottom right). Inset is the same data shown in linear scale.

6 S5. Responsivity and quantum efficiency of devices made with thicker BP flakes Figure S5: Optical (left) and AFM (right) images of 100 nm device shown in figure 3e of main text. Higher responsivity and internal quantum efficiency were found for devices which were thicker and therefore had higher absorption. The device with the highest internal quantum efficiency had a thickness of 100 nm and a channel width of 4.2 µm. The channel absorption measured from the transmission spectrum of the MZI was 3.34 db/µm. The I-V curves for various optical powers can be seen in Figure S6 along with the responsivity and internal quantum efficiency. Figure S6: Source-drain current for 100 nm thick device at various optical powers (left). Responsivity and internal quantum efficiency at 1.57 mw for various applied biases (right). At high applied source-drain bias, the photocurrent appears to begin to saturate. This is likely due to the transit time being shorter than the recombination time. As one can see from Figure

7 S6, there is a trade-off between higher responsivity and increased dark current when using a thicker flake. As the flake thickness surpasses the screening limit (about 10 nm), an added gate will do little to improve the dark current of the detector. Additionally, the asymmetry of the I-V plot could be due to non-uniformity in the source-drain contact and flake thickness (see Figure S5 AFM) and was not typical for other fabricated devices.

8 S6. FEM simulation of the absorption coefficient To determine the optical absorption of black phosphorus on the waveguide, we performed cross-section analysis using the FEM software COMSOL. The waveguide supports a single, quasi- TE mode as expected, with the maximum electric field in x-direction. According to [5], the optical conductance is anisotropic in multilayer black phosphorus, so the relative orientation between maximum absorption and the strongest electric field needs to be considered in this simulation. When simulating 11.5 nm thick black phosphorus, we use the complex refractive index (nxx = i, nyy = 2.83, nzz = i) which corresponds to the predicted optical conductivity of a 10 nm thick flake [5, 6] (σxx = 0.7σ0, σyy = 0, σzz = 1.7σ0, where σ0 is the universal conductivity of graphene). This orientation means we have the weakest absorption since the electric field maxima and the material absorption maxima are orthogonal. The mode profile is shown in Fig. S7(a) and the calculated absorption is db/µm which is quite close to our experimental result of db/µm. This configuration matches our predicted crystal orientation from the Raman data presented in section S2. As the thickness of black phosphorus approaches bulk, the optical conductivity changes slightly for 1.55 µm wavelengths. When simulating black phosphorus flakes with 20 nm to 100 nm thicknesses, we used a fixed complex refractive index (nxx = i, nyy = 2.83, nzz = i) which corresponds to optical conductivity of a 20 nm flake [5] (σxx = 4.1σ0, σyy = 0, σzz = 1.4σ0). We also rotated the crystal orientation by 90 degrees to match the results of section S2 for the 100 nm thick device. This orientation should produce the maximum absorption. Shown in Fig. S7(b), the absorption of a 100 nm flake was found to be 3.15 db/µm which is comparable to our experimental result of 3.34 db/µm. When solving for the waveguide mode, we see that the mode actually begins to be guided in the bulk black phosphorus layer when its thickness is comparable to 120 nm thick silicon waveguide. This leads to a significant enhancement of absorption. The absorption per unit length for the 100 nm device is double that which is achievable (around 1.7 db/µm) in a typical germanium waveguide photodetector [7]. To account for both orientations, we calculated the theoretical upper and lower limit of the optical absorption coefficient as a function of thickness as shown in Fig. S7(c). We also included the absorption data of 5 devices with various thicknesses. It is worth noting that on closer inspection the 22 nm device was found to have extra debris touching the waveguide which contributes additional absorption loss to the waveguide mode.

9 Figure S7: Waveguide cross-section of electric field intensity distribution and power flow represented by arrows (not to scale) for: (a) 11.5 nm thick BP and (b) 100 nm thick BP. Layers from top to bottom: ALD deposited Al 2O 3, black phosphorus, HfO 2, Silicon waveguide. (c) The calculated upper and lower limit of absorption in black phosphorus of different thickness in two different orientations (0 degrees = armchair axis along the x-direction) with data from five fabricated devices. Closer examination of 22 nm device revealed other BP debris touching the waveguide, thus contributing additional absorption loss.

10 S7. Other works on the origin of photocurrent in black phosphorus photodetectors Modeling of the photothermoelectric, photobolometric, and photovoltaic contributions to the net photocurrent in black phosphorus have been discussed in detail by Low et. al. (see [8]). The authors also included brief experimental verification of their model and concluded both from simulation and experiment that under a moderate bias (VDS > 50 mv), the bolometric effect dominates the net photocurrent. This bolometric photocurrent is opposite in sign relative to the dark current compared to the photovoltaic current, which has the same sign as the dark current (i.e. bias direction). This difference in sign provides a distinctive method for identifying whether the bolometric effect or the photovoltaic effect dominates the net photocurrent as stated on page 4 of their paper. The conclusion drawn from this paper, while correct for the case of bulk material, is not comprehensive for all black phosphorus photodetectors as we clearly demonstrate in Figure 3b and 3c of our manuscript. The authors of [8] use a 100 nm thick flake of black phosphorus and are not able to significantly modulate the source-drain current with a back gate due to a finite screening length (~10 nm). Therefore, when calculating the carrier density, the authors conclude that their device has a considerable hole carrier density of 2.5 x cm -2. This heavily doped device shows photocurrent dominated by the bolometric effect because their BP device has substantial p doping, and electron-electron scattering can significantly reduce n*, where n* is the number of photoexcited carriers. Our experimental results confirm the theory presented in [8], but because we are able to control the doping level and use a flake with a thickness (11.5 nm) comparable to the screening length, we are able to observe photocurrent in the intrinsically-doped regime. In this regime, the lack of carriers both suppress dark current and drastically reduce photobolometric current. Therefore, we observe a flipping of the sign of the photocurrent when the doping level passes from heavily doped to lightly doped (see Figures 3b and 3c) as predicted in [8]. Additionally, we see a drastic decrease in the bandwidth of our photodetector when we go from low doping to high doping (see Figure 4a). The lower bandwidth for higher doping is due to the fact that the bolometric effect is dependent on the in-plane lattice thermal conductivity of 12.1 W m -1 K -1 [9] which is much smaller than in graphene (>2000 W m -1 K -1 [10]). This low thermal conductivity limits the operation speed of our detector in the highly doped region (an effect not seen in graphene) and gives us yet another method for identifying the mechanism of photocurrent generation. As for the photothermoelectric current, since our contacts are symmetrically placed relative to the waveguide and have the same work function, we do not observe any contribution to the photocurrent at zero bias. This is in perfect agreement with Figure 2c of [8] where the thermoelectric photocurrent is zero when optical power is at the exact center of the channel, between two identical contacts.

11 Additional recent works on the mechanisms of photocurrent in black phosphorus have served to confirm our results. T. Hong et al. [11] demonstrated in thin black phosphorus flakes (8 nm) that photovoltaic current dominates in regions of low doping, while thermal effects dominate for heavily doped regions. Other works by Y. Deng et al. [12] and M. Buscema et al. [13] demonstrate a black phosphorus pn junctions biased at low doping levels where the photovoltaic effect dominates the photocurrent.

12 S8. Comparison with other black phosphorus photodetectors We have included the following table as a comparison of our black phosphorus photodetectors with current state-of-the-art black phosphorus photodetectors published to date. Our photodetectors show the highest responsivity for near-ir wavelengths and are six orders of magnitude faster than any black phosphorus photodetector yet demonstrated. Using our MZI devices, we are also able to determine the maxim achievable responsivity based on the optical power absorbed in the black phosphorus layer. Material Area (µm 2 ) Responsivity (ma/w) I dark (A) f 3dB (Hz) Spectral Range Ref nm BP 1.5 x (ext.) / 125 (int.) 2.2 x x µm this work 100 nm BP 1.5 x (ext.) / 657 (int.) 5.6 x x µm this work 8 nm BP x 10 3 Visible nm BP Visible nm BP 2.5 x / 5 (532 / 1550 nm) 0 bias >4 x & 1550 nm 3 11 nm BP / 1L MoS unknown 633 nm 4 1. Buscema M, Groenendijk DJ, Blanter SI, Steele GA, van der Zant HSJ, Castellanos-Gomez A. Fast and broadband photoresponse of few-layer black phosphorus field-effect transistors. Nano Lett 2014, 14(6): Buscema M, Groenendijk DJ, Steele GA, van der Zant HSJ, Castellanos-Gomez A. Photovoltaic effect in few-layer black phosphorus PN junctions defined by local electrostatic gating. Nature communications 2014, 5: Engel M, Steiner M, Avouris P. Black Phosphorus Photodetector for Multispectral, High- Resolution Imaging. Nano Letters (11), Deng Y, Luo Z, Conrad NJ, Liu H, Gong Y, Najmaei S, Ajayan PM, Lou J, Xu X, Ye PD. Black Phosphorus Monolayer MoS2 van der Waals Heterojunction p n Diode. ACS Nano, 2014, 8 (8), pp

13 References: [1] C.R. Dean et al. Nature Nanotechnology 5, (2010) [2] F. Xia et al., Nature Communications 5, Article number: 4458 (2014) [3] L. Li et al., Nature Nanotechnology 9, (2014) [4] H. Liu et al., arxiv: [cond-mat.mes-hall] (2014) [5] T. Low, et al., Phys. Rev. B 90, (2014). [6] H. Asahina & A. Morita, J. Phys. C Solid State Phys. 17, (1984). [7] D. Feng et al., Appl. Phys. Lett. 95, (2009) [8] T. Low et al., Phys. Rev. B 90, (R) (2014) [9] G.A. Slack, Phys. Rev. 139, A507 (1965) [10] A. A. Balandin, Nature Materials 10, 569 (2011) [11] T. Hong et al., Nanoscale 6, 8978 (2014) [12] Y. Deng et al., ACS Nano, 8(8), pp (2014) [13] M. Buscema et al., Nature Communications 5, Article number: 4651 (2014)