Supplementary Information for:
Enhanced NIR response of nano- and microstructured silicon/organic
hybrid photodetectors
Vedran Đerek, Eric Daniel Głowacki, Mykhailo Sytnyk, Wolfgang Heiss, Marijan
Marciuš, Mira Ristić, Mile Ivanda, and Niyazi Serdar Sariciftci
Materials
Boron-doped p-type silicon wafers with thickness of 500 µm and <100> orientation were
obtained from Topsil; R0= 5-10 Ω·cm. The wafers were one-side polished, with an
alkaline-etched backside.
A. General sample preparation
Silicon wafers were cut into 25×25 mm2 substrates. Substrates were pre-cleaned by
sonication in acetone, 2-propanol, 2% solution of chemical detergent Hellmanex III, and
DI water, followed by the RCA standard cleaning steps.1 Immediately prior to chemical
surface structuring, the chemical oxide formed by the RCA clean was removed from the
substrate by a 30 second dip in 5% HF solution. Next, the substrates were surface
structured by a number of well-established silicon structuring methods which will be
presented separately. Hierarchical Silicon surface structuring was accomplished by
combining the different structuring steps ordered by the dominant structure size. After the
structuring steps the prepared substrates were cut into 15×15 mm2 pieces and RCA
cleaned in order to remove the introduced organic impurities and metal contamination.1
Chemical oxide originating from the RCA cleaning process was removed from the
samples by a 30 second 5% HF dip. Immediately following the HF treatment, samples
were transferred to a vacuum chamber where an aluminum back contact was evaporated
through a shadow mask onto the alkaline-etched side of the wafer. After deposition of the
Al, the samples were annealed in N2 atmosphere at 450 C for 20 minutes to ensure an
Ohmic Al/p-Si contact. Next, samples were dipped into 5% HF for 30s, with Al contact
being protected from etching by adhesive polyimide tape. Then the sample was dipped
for 20s in RCA SC-2 solution heated at 70C to re-grow the oxide and remove residual
metal impurities. Next, a final 30s dip in 5% HF was conducted to generate a clean
hydride-terminated surface. The protective polyimide tape was removed and the samples
were immediately loaded into a vacuum chamber for evaporation of the Tyrian Purple
(TyP) layer. TyP is a natural indigoid pigment that has been reported to be an ambipolar
organic semiconductor with µh=µe=0.4 cm2/Vs. 2 , 3 TyP was synthesized according to
known literature methods4 and purified thrice by temperature gradient sublimation using
a source temperature of 270C at a pressure of ~110-6 mbar. The vacuum evaporation
technique used was hot-wall epitaxy, allowing independent temperature control of the
substrates.5 At a pressure of ~110-7 mbar, the substrates were heated to 580 C for 10
minutes. The substrates were then cooled over 30 minutes to ~70C and then TyP was
evaporated at a rate of 0.15 Å/s, to a thickness of ~40 nm. Samples were then removed
from vacuum and a ~2500 nm-thick poly(methyl methacrylate) (PMMA) buffer layer
was coated from chlorobenzene solution along the edge of the sample to prevent later
shunting through the TyP layer to the p-Si beneath during application of measurement
contact pins to the top Al contact. After drying the PMMA at 70C for 10 minutes, the
samples were transferred to a vacuum chamber for thermal evaporation of 100 nm of Al
for top contacts. Top contacts were defined by a shadow mask to give an active device
area of 0.07 cm2.
B. Electrochemically-grown porous silicon (PS)
A 100 nm-thick layer of Aluminum was deposited on the alkaline etched side of the
substrates by thermal evaporation for back contact during the anodization. Substrates
were annealed at 450 °C in N2 atmosphere for 20 minutes in order to establish Ohmic
contacts between pSi/Al. The polished surface of the substrate was micro-structured by
anodization in the Standard Etch Cell as defined by Sailor.6 Back contact to the silicon
substrate was established by directly contacting aluminum foil with the aluminized back
of the substrate, while the counter electrode was a 15 cm long spirally bent platinum
wire. Anodization was conducted in galvanostatic mode using a Keithley 2401
Sourcemeter with a current density of 50 mA/cm2 for the duration of 5 minutes. The
electrolyte consisted of a mixture of 2 M HF + 0.25 M of tetrabutylammonium
perchlorate + 2.4 M H2O in acetonitrile. 7,8 After the anodization, the electrolyte was
removed with a pasteur micropipette and the sample was rinsed twice with ethanol. Pores
in the freshly prepared porous silicon were chemically widened by direct application of
an oxidizing solution (4:1:1 DMSO:Ethanol:48% HF) into the anodization cell for 10
minutes.6 After the pore widening, the solution was emptied, and the substrate was rinsed
twice with ethanol and removed from the anodization cell. The aluminum back contact
was etched away in an aluminum etchant solution (H3PO4:HNO3:CH3COOH:H2O = 73%
: 3.1% : 3:1% : 20.8%), after which the substrates were thoroughly rinsed with DI water
and RCA SC-2 cleaned to remove the residual metal contamination introduced by the
aluminum etching.
C. Metal-assisted chemical etching of silicon (Si MACE)
A roughened silicon surface consisting of conical pores of size 10-100 nm was prepared
by metal-assisted chemical etching of silicon.9 Silver nanoparticles were deposited on the
substrate surface by a 5 minute dip in a mixture of NH4F:AgNO3:H2O = 2% : 0.2% :
97.8%.10 Silicon surface was etched by placing a silver nanoparticle coated substrate in a
mixture of 1:1 = 31% H2O2: 5% HF for 15 minutes. Silver nanoparticles were dissolved
from the substrate by a 5 minute dip in 35% nitric acid, followed by a thorough DI water
rinse and a RCA SC-2 cleaning step in order to eliminate residual silver ion
contamination from the substrate.
D. Silicon micropyramids (µ-pyramid Si)
Micropyramids were formed on <100> silicon substrates by anisotropic etching in a
water solution of 2% KOH and 10% 2-propanol.11,12 Etchant was heated to 90 °C in a
beaker topped by a reflux condenser on a stirrer/hot plate. A PTFE coated stir bar was
placed in the beaker, and the etchant was stirred at 1000 rpm. Substrates were positioned
vertically in a PTFE sample holder and placed in the beaker for 30 minutes. After the µpyramids were formed the substrates were thoroughly rinsed with DI water and RCA SC2 cleaned to remove the residual metal ion contamination introduced by the KOH.
E. Characterization and measurement techniques
Diode devices were contacted using either pogo pin contacts or wires bonded to the
contacts with silver paste. Electrical IV measurements were conducted using a computer
controlled Keithley 2401 Sourcementer. IV measurements were taken in dark or under
the diode laser illumination, while JSC values were measured by keeping the device at
short circuit during illumination in order to measure the steady-state current. Photocurrent
output was measured for 2-3 minutes for each sample. Two IR sources were used: For IV
measurements a diode laser emitting at 1.48 µm, giving an irradiance of ~200 mW/cm2;
for spectral responsivity a broadband light source coupled through a grating
monochromator was used. Temporal response was not measured for these devices due to
the large device area (~ 0.1 cm2), which from previous studies is known to give a
bandwidth of about 5 MHz due to the large RC time-constant. 13 SEM images of
structured silicon and final devices were taken by FE-SEM Jeol 7000 and Zeiss 1540XB
CrossBeam FE-SEM/FIB.
FIG. S1. J-V characteristics of Al/p-Si/TyP/Al in dark and under illumination with a 1.48
µm laser diode (200 mW/cm2) for hierarchically structured substrates. Dark J-V curves
are shown with dotted lines, while solid lines represent J-V characteristics under
illumination. a) Comparison of PS/µ-pyramid Si with planar devices; b) µ-pyramid
Si/PS/Si MACE versus planar; c) µ-pyramid Si/Si MACE diodes versus planar.
FIG. S2. Comparison of spectral responsivity of Al/µ-pyramid Si/TyP/Al diodes biased
at -1V (purple line) with literature responsivity data for black silicon and InGaAs
FIG. S3. (a),(b) Reflectance values for the silicon samples without and with TyP,
respectively; (c),(d) Transmittance of Si samples, without and with TyP.
FIG. S4. (a) SEM of a silicon µ-pyramid sample (b) 3D model of the µ-pyramid
structured surface based on the same SEM. Surface area of the micro-pyramid covered
sample was calculated to be 1.55 times that of the planar sample of the same size.
1
W. Kern and J.E. Soc, J. Electrochem. Soc. 137, 1887 (1990).
E.D. Głowacki, L. Leonat, G. Voss, M.-A. Bodea, Z. Bozkurt, A.M. Ramil, M. Irimia-Vladu, S. Bauer,
and N.S. Sariciftci, AIP Adv. 1, 042132 (2011).
2
Y. Kanbur, M. Irimia-Vladu, E.D. Głowacki, G. Voss, M. Baumgartner, G. Schwabegger, L. Leonat, M.
Ullah, H. Sarica, S. Erten-Ela, R. Schwödiauer, H. Sitter, Z. Küçükyavuz, S. Bauer, and N.S. Sariciftci,
Org. Electron. 13, 919 (2012).
3
4
G. Voss and H. Gerlach, Chem. Ber. 122, 1199 (1989).
5
H. Sitter, a. Andreev, G. Matt, and N.S. Sariciftci, Synth. Met. 138, 9 (2003).
6
M.J. Sailor, Porous Silicon in Practice (Wiley-VCH, Weinheim, 2011).
7
E.A. Ponomarev, Electrochem. Solid-State Lett. 1, 42 (1998).
8
J.P. Zheng, Electrochem. Solid-State Lett. 3, 338 (1999).
9
Z. Huang, N. Geyer, P. Werner, J. De Boor, and U. Gösele, Adv. Mater. 23, 285 (2011).
10
P.R. Brejna and P.R. Griffiths, Appl. Spectrosc. 64, 493 (2010).
11
H. Seidel, L. Csepregi, A. Heuberger, and H. Baumgärtel, J. Electrochem. Soc. 137, 3612 (1990).
12
H. Angermann, Appl. Surf. Sci. 254, 8067 (2008).
M. Bednorz, G.J. Matt, E.D. Głowacki, T. Fromherz, C.J. Brabec, M.C. Scharber, H. Sitter, and N.S.
Sariciftci, Org. Electron. 14, 1344 (2013).
13