SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION Supplementary Information for Mid-infrared HgTe colloidal quantum dot photodetectors Sean Keuleyan, Emmanuel Lhuillier, Vuk Brajuskovic and Philippe Guyot-Sionnest* Optical absorption spectra were collected in a CaF2 cell on a Thermo-Nicolet NEXUS 670 FT-IR. TEM images were measured on a FEI Tecnai F30 electron microscope under 300 kv operation. DLS measurements were done on a Malvern Zetasizer. Noise spectral densities were recorded on a HP 3561a spectrum analyzer. 1. Dark current at low temperature Figure S1: Dark current as a function of the inverse of the temperature with a fit to a sum of two Arrhenius exponentials. The plot of Figure 2c from the main text is extended to lower temperature in Figure S1. This plot exhibits two slopes. At high temperature, the activation energy is ~ half of the gap energy, while a smaller slope is observed below 100K. We attribute this smaller activation energy to extrinsic carriers, with significant contribution to the current only at low temperature where the thermally excited intrinsic population is sufficiently low. nature photonics 1

2 supplementary information 2. Transient Photocurrent following 10ps 1064nm pulse Figure S2: Transient photocurrent for a 3 µm sample at room temperature, for three different biases. The transient photoresponse was measured by illuminating the sample with a 10 ps 1064 nm Nd:Yag pulsed laser and is given in Figure S2. There are fast and slow decay components. As stated in the text, the transients become faster with thicker films, higher temperature and larger bias, following the trend of smaller resistance - faster response. The slightly slower response of the 3 µm sample shown in Figure 2d is also consistent with this dependence. We tentatively suggest that this trend is due to capacitive effects, possibly associated with space-charges at the electrodes. 2 nature photonics

3 supplementary information 3. Accuracy of the external quantum efficiency and responsivity. The EQE is evaluated through the expression (1) Thus we can write the uncertainty on the EQE value as follow (2) Here is the current uncertainty and, are respectively the uncertainty on the flux coming from geometrical error and due to the evaluation of the blackbody source temperature. is less than 0.1 and comes from the possible change of the detector temperature which changes the dark current. The geometric uncertainty can be split into three terms With (3) which can be evaluated as follows (4) In our case: A d = 4.9mm!2mm (we assume no uncertainty on this parameter) nature photonics 3

4 supplementary information We thus obtained. Finally the uncertainty on the flux resulting from a change of the blackbody temperature may be written as in equation (5), (5) with. From the pyrometric measurement, we estimate a 50K temperature uncertainty, which provides value of 10 to 15% for. We conclude that our measurements of the EQE, responsivity, and detectivity are correct within a factor of two. The main uncertainty arises from the geometric value of the source and future measurements will be performed with large area and calibrated blackbody sources. 4 nature photonics

5 supplementary information 4. Corrected photocurrent spectrum Figure S3: Relative spectral response of the two samples at room temperature normalized by the MCT spectral response at Liquid Nitrogen. The short wavelength cut-off is due to the Ge filter. By normalizing the measured photoresponse such as those in Figure 3, with that of a liquid nitrogen cooled commercial MCT detector (Kolmar Technology, 1 mm 2 area) and using its specified spectral responsivity, we obtain a relative spectral responsivity shown in Figure S3. The validity of this procedure is limited due to the different detector areas. nature photonics 5

6 supplementary information Supplementary Table 1: Responsivity of some MWIR detectors. Technology Responsivity Wavelength Temperature Bias or electric References field QWIP 0.5 A/W 4.1 µm kV.cm -1 1 InSb 2.5 A/W µm 77K 2 InSb 3 A/W µm 77K 3 SLS 0.35 A/W 4.3 µm 77K 0V 4 PbSe 2500/ µm 295K 5 V/W MCT 10 5 V/W 4µm 80K 10V.cm -1 6 MCT 200 V/W 1-4.5µm 295K 1000 V/W 1-4.8µm 270K DTGS 30/2440 V/W µm 298K - 8 3µm Sample 0.12 A/W 1.7-3µm 290K 10V This work 750 V/W 5µm Sample 15 ma/w 10 V/W 1.7-5µm 290K 0.5V This work 7 1. Goldberg, A.C., Little, J.W., Kennerly, S.W., Beekman, D.W. & Leavitt, R.P. Temperature Dependence of the Responsivity of QWIPS. Long wavelength infrared detectors and arrays: physics and applications VI (1999) accessed on 01/08/ on the 01/08/ Plis, E. et al. Mid-infrared InAs/GaSb strained layer superlattice detectors with nbn design grown on a GaAs substrate. Semicond. Sci. Technol. 25, (2010) accessed on 03/08/ Smith, E.P.G., Winchester, K.J., Musca, C.A., Dell, J.M. & Faraone, L. A simplified fabrication process for HgCdTe photoconductive detectors using CH 4 /H 2 reactive-ion-etching-induced blocking contacts. Semicond. Sci. Technol. 16, (2001) on the 03/08/ nature photonics

7 supplementary information 8. Abedin, M. N., Mlynczak, M.G., Refaat, T.F. Infrared detectors overview in the short-wave infrared to far-infrared for the CLARREO mission, SPIE Proceedings Volume 7808, nature photonics 7