Supporting Information for

Colossal Ultraviolet Photoresponsivity of Few-Layer Black Phosphorus

Jing Wu ‡ 1,2,3, Gavin Kok Wai Koon ‡1,2,3, Du Xiang‡1,2, Cheng Han1,2, Chee Tat Toh1,2, Eeshan S. Kulkarni1,2, Ivan Verzhbitskiy1,2,4, Alexandra Carvalho1,2, Aleksandr S. Rodin2, Steven P. Koenig1,2, Goki Eda1,2,4, Wei Chen1,2,4,5, A. H. Castro Neto1,2, and Barbaros Özyilmaz1,2,3*

1

Department of Physics, National University of Singapore, 117542, Singapore

2

Centre for Advanced 2D Materials and Graphene Research Centre, National University of

Singapore, 117542, Singapore 3

NanoCore, National University of Singapore, 117576, Singapore

4

Department of Chemistry, National University of Singapore, 117542, Singapore

5

National University of Singapore (Suzhou) Research Institute, 377 Lin Quan Street, Suzhou

Industrial Park, Jiang Su 215123, China

Additional results:

Figure S1 | Photo current response at low enery range (VL-NIR). Time dependence photo current response excited by different wavelength sources, 400, 650 and 900 nm measured at VBG=-80 V and VSD=0.1 V. The response time shown is in the order of ~1 ms. First of all, transport measurements were done in these devices by measuring the conductance as a function of back gate voltage. From the transport measurements, we can

calculate the field effect mobility and the current modulation of the device. I-V measurements were done on the junctions used to measure the photo current response later on. After the initial electrical characterizations, we measure time dependence photo response measurements. Shown above, are the measurements done in the low energy excitation range (VL-NIR) with source drain bias of VSD=0.1 V and external back gate of VBG=-80 V.

Figure S2 | Photo current response at low enery range with miliseconds time scale. Time dependence photo current response excited by wavelength source of 500 nm measured at VBG=80 V and VSD=0.1 V used to extract the photo response time. The rise response time is τ0~1 ms and fall response time is τ1~4 ms.

Figure S3 | Photo current response at high enery range (near UV). Time dependence of the photo current response excited by different wavelength sources, 310 and 350 nm measured at VBG=-80 V and VSD=0.1 V. At the high energy range, the response time increases to ~200 s. Figure S3 shows the similar time dependence photo-response measurements but in the high energy range (near UV). In the high energy range, the photo current is much higher and the response time increase to ~200 s. After shining the light, the current increases suddenly and then slowly to a saturated value. Calculation of External Photogain The external photogain is defined as the ratio of photocurrent (in electrons per second) to incident photons; ∆


  = ×∆× , 

where ∆ is the number of excited electron hole pairs and  is the electron charge. To estimate ∆ we can employ the following relation;

∆ =  ×

∆ , 

where  is the capacitance of a dielectric film of 300 nm SiO2, 1.15×10-8 F/cm2 and ∆ is the shift of threshold voltage. By using ∆ ~ 10  ,  ~10 !" and ∆#~ 10 , the external gain calculated can be as high as 106.

Figure S4 | Energy dependence of absorbance of black phosphorus. Absorbance spectrum measurements of few-layer black phosphorus showing high absorption at high excitation energy. The absorption spectrum of black phosphorus can be directly obtained from measuring the differential reflectance. The sample absorbance, A is related to differential reflectance, dR via the following equation12;

 =

" $ 1 &', 4

where n is the refractive index of black phosphorus. The fact that we do see the rapid increase of differential reflectance/absorbance in the near-UV range is a strong argument for the high activity of black phosphorus at larger energies.

Figure S5 | Band structures. Band structures of black phosphorus and SiO2 with the grey shaded area corresponding to interface charge traps such as Si dangling bonds.

Figure S6 | Backgate dependence of photo-responsivity. Photo-responsivity calculated for excitation source of 400 nm with varying applied back gate VBG and fixed source drain bias VSD=0.1 V. Here we see for a fixed excitation source of wavelength 400 nm, the photo-responsivity increases as we tune the back gate voltage from VBG=0 to VBG=-80 V. When we change the back gate voltage, we are effectively moving the Fermi level of the system towards the valence band. This compresses the Schottky barrier region and thus reduces the effective barrier height as well as the contact resistance. This significantly facilitates the electrical transport of photo induced carriers, thereby leading to the apparent photocurrent enhancement.

Comparison of our UV photo-detector with previous results: In order to assess the performance of few-layer black phosphorus UV photo-detector, we summarize in Table 1 the relevant figures-of-merit of other reported UV photodetectors.

Materials

Measurement

Responsivity

Detectivity

Condition

(A/W)

(Jones)

ZnO

Vsd = 5V, Vg =

nanowire

0V

WO3

Vsd = 1V, Vg =

nanowire

0V

SnO2

Vsd = 1V, Vg =

nanowire

0V

In2O3

Vsd = 0.05V, Vg

nanowire

= 0V

GaN

Vsd = 5V, Vg =

Photo gain

Response

Spectra

time

Region

Reference

̶

̶

～108

～30s

UV

Ref[1]

̶

̶

～4.3× ×103

～300s

UV

Ref[2]

̶

̶

～1.32× ×107

～50s

UV

Ref[3]

～104

～100s

UV

Ref[4]

UV

Ref[5]

UV

Ref[6]

UV

Ref[7]

̶

̶

0.15

~1010

4.4× 4.4×103

2.6× ×1011

̶

̶

0V InAs

Vsd = 2V, Vg =

nanowire

0V

AIGaN

Vsd = 0V, Vg =

̶

̶

̶

~1014

～3.9× ×105 ̶

̶

～20s

UV

Ref[8]

～9× ×104

～3× ×1013

～106

～200s

UV

Our

̶

̶

0V In2Ge2O7

Vsd = 5V, Vg =

nanobelt

0V

BP flake

Vsd = 3V, Vg = 80V

results

Table 1 | Comparison of figures-of –merit for reported UV photo-detectors.

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