Supplementary Information

Thin-layer black phosphorous/GaAs heterojunction p-n diodes

Pascal Gehring,1 Roberto Urcuyo,1 Dinh Loc Duong,1 Marko Burghard,1,a) and Klaus Kern1,2

1Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany

2Institut de Physique de la Matière Condensée, Ecole Polytechnique Féderale de Lausanne, CH-1015 Lausanne, Switzerland

Cleaning of GaAs and surface oxide removal

The GaAs wafers were first cleaned by 30 s of O2 plasma treatment (200 W, 0.3 Torr). The

surface native oxide was then removed by the following subsequent etching steps: (i) 2 min

Semiconclean, (ii) 5 s rinsing with DI water, (ii) 5 s rinse with aqueous HCl (30%), (iv) 2s

rinsing with DI water, and (v) blow drying under argon. Finally, the substrates were directly

provided with a spin-coated PMMA layer or loaded into the vacuum chamber for metal

evaporation.

Ohmic contacts to n+-GaAs

100 nm of AuGeNi alloy (54.6% Au, 26.4% Ge, 20% Ni) were thermally evaporated onto the

GaAs, and then diffused into the substrate by an annealing step.

Figure S1. Electrical characterization of bare black phosphorous sheets. (a) Reflection

image and (b) photocurrent map of black phosphorous on Si/SiO2. The photocurrent is only

generated close to the metallic contacts, indicating the formation of Schottky barriers. The value

of the photocurrent is on the order of several nA, about 1000 times smaller than the photocurrent

generated at the heterostructures discussed in the main text. (c) A transfer characteristic at a bias

of 3 mV of a typical black phosphorous device where an EMI-TFSA liquid electrolyte is used for

gating. It follows that the black phosphorous is a p-type semiconductor but can be changed to n-

type by applying high positive gate voltages.

Figure S2. Estimation of the depletion layer thickness. The thickness of the depletion layer

can be estimated by W =√ 2 ∙ εGaAs ε bP ∙ (N AbP+N D

GaAs )2 ( V bi−V )q N A

bP N DGaAs (N A

bP εbP+N DGaAs εGaAs)

(pink curve) if GaAs is considered,

or by W =√ 2 ∙ εbP ∙ (V bi−V )q N A

bP (blue curve) if it is assumed that N AbP ≪N D

GaAs and thus the depletion

layer only forms inside the black phosphorous sheet. In these equations N AbP , N D

GaAs and ε GaAs , εbP

are the carrier densities and dielectric constants of black phosphorous and GaAs, respectively,

and Vbi is the built in potential. For the estimation we used N AbP ≈ 1.2·1023 m-3, N D

GaAs ≈ 2.7·1024 m-

3, ε bP=10, ε GaAs=12 and Vbi = 0.44 eV. It can be seen, that the part of the depletion layer inside

GaAs can be neglected.

Figure S3. Estimation of the shunt and series resistance. (a) Full bias range of the I-V curves

shown in the main text. The linear parts around V = 0 were used to estimate the shunt resistance

Rsh, the linear parts around I = 0 could be used to estimate the series resistance Rs. (b) Equivalent

circuit for a real solar cell. The current IL is generated inside the cell. It can flow through the

diode (blue) or a parallel shunt Rsh resistance (pink). All resistances of the contacts/interfaces are

included in a series resistance Rs (orange). (c) Rsh (extracted from (a)) for different light powers.

The characteristic resistance Voc / Isc is given as a comparison. Since both values are on the same

order of magnitude the fill factor (see main text) is expected to be low. (d) Rs (extracted from (a))

for different light powers. The series resistance, which is most likely dominated by the resistance

between the black phosphorous sheet and the underlying GaAs and the metallic contacts to the

black phosphorous flakes is on the order of 1MΩ which further reduces the efficiency of the

devices.

Figure S4. Gate dependence of the photocurrent around the flat band condition. The

photocurrent of the device changes sign from negative (a) to positive (b) when the bias is larger

than the built-in voltage Vbi (see main text).

Figure S5. AFM height profile of the black phosphorous sheet in the device in the main

text. The cross-sectional profile reveals a sheet height of ~15 nm.