Thermal NIL-based graphene patterningfor permeable base heterojunction transistors
L.S.D. Antony1, C. Strobel1, R. Kirchner1, C.A. Chavarin2, Ch. Wenger2,3,
M. Albert1, J. W. Bartha1
1

2

TU Dresden, Institute of Semiconductors and Microsystems, 01062 Dresden, Germany
IHP-Leibniz-Institut für innovative Mikroelektronik, Im Technologiepark 25, 15236 Frankfurt(Oder), Germany
3
Brandenburg Medical School Theodor Fontane, 16816 Neuruppin, Germany
E-mail: robert.kirchner@tu-dresden.de, carsten.strobel@tu-dresden.deHigh-speed data communication and RF electronics demand fast switching transistors. A newclass of graphene-based transistors was proposed for this purpose [1]. Their structure leans on
a classical npn silicon bipolar transistor but with the p-type base being replaced by graphene(Fig. 1). Monolayer graphene films in such graphene-base heterojunction transistors (GBHT)
allow for short transit times due to the minimal base length. However, the gain of this GBHT
is expected to be low due to physical transport limitations across the graphene monolayer [2].
A patterned graphene base leads to a permeable base transistor design [3] potentially allowing
for much higher gain. To be able to fabricate such a permeable base GBHT (p-GBHT), a robust
technology for patterning graphene has to be developed. This work aims at laterally
nanopatterned graphene layers to control the transit across the base in a p-GBHT.
The developed process presents a potentially low-cost method using thermal
nanoimprint lithography (T-NIL) to pattern graphene films with periodic holes. The proposed
method allows for graphene patterning on an industrial scale. Our low-cost patterning approach
exploits the PMMA protective layer being present on top of the CVD-grown graphene on
copper substrates, eliminating the need for additional resists. This is a direct imprinting processwith subsequent transfer of graphene to the functional substrate (Fig. 2a). An alternative
process patterns graphene via being transferred on a substrate first and then imprinted (Fig. 2b).
Substrates were either 65 nm silicon nitride (SiN) films on silicon, for process optimization, or
n-doped silicon, for device fabrication.
The T-NIL process involves embossing of flexible PET molds (FleFimo, Soken
Chemical and Engineering Ltd.) with pillars into mr-I PMMA 35k or 75k (micro resist
technology GmbH) (Fig. 3). A sub-10 nm adhesion layer of AR-BR 5480 (Allresist GmbH) isused. PMMA spin-coating solutions were prepared by dilution in anisole (post-apply bake:
140°C, 2 mins). The embossing temperature was set at moderate 120-130°C to protect the PET
mold. The relatively low temperature was compensated by a long holding time of 10-20 min[4]. Subsequently, the sub-10 nm residual PMMA layer was removed along with the adhesion
layer and pattern-transferred into graphene using oxygen reactive ion etching. Subsequently,
PMMA was stripped in acetone and isopropanol (Fig. 4). High-temperature annealing at 450°C
in a nitrogen atmosphere for up to 5h was used to eliminate PMMA residues from the graphene
if needed. The patterned graphene was inspected by optical, scanning electron and atomic force
microscopy as well as Raman spectroscopy.
The p-GBHT devices were fabricated on n-doped Si wafers acting as the collector. An
insulating 50 nm aluminum oxide spacer was deposited by atomic layer deposition. The CVDgrown graphene was wet-transferred on to the substrate, and the T-NIL imprint was performed.
The emitter of amorphous n-doped Si (100nm) was deposited on top of the graphene by aplasma enhanced CVD process [5]. Finally, metal contacts were established, and the device
was electrically characterized. A permeable base with 1.5 µm holes showed significantly
reduced resistance (Fig. 5b and 6b) compared to the non-patterned graphene (Fig. 5a and 6a).
However, switching could not be achieved due to the large diameter. Ongoing work with sub200 nm holes aims at switching the base transit using a more effective Schottky barrier control.

References:[1] Di Lecce, V; Grassi, R; Gnudi, A; Gnani, E; Reggiani, S; and Baccarani, G; “Graphenebase heterojunction transistor: An attractive device for terahertz operation,” Electron
Devices IEEE Trans. on 2013 60 (12), 4263-4268.
[2] C. Strobel, C.A. Chavarin, B. Leszczynska, S. Leszczynski, F. Winkler, S. Killge, S.
Völkel, K. Richter, A. Hiess, M. Knaut, J. Reif, M. Albert, Ch. Wenger, J.W. Bartha,
Demonstration of a graphene-base heterojunction transistor with saturated output current,
J. Appl. Phys., accepted for publication[3] Bozler, C. O.; “Current prospects for the permeable base transistor,” Surf. Sci. 1986 174,487.
[4] Chidambaram, N.; Kirchner, R.; Altana, M.; Schift, H.; “High fidelity 3D thermal
nanoimprint with UV curable polydimethyl siloxane stamps,” J. Vac. Sci. Technol. B 201634, 06K401.
[5] Strobel, C.; Chavarin, C.A.; Kitzmann, J.; Lupina, G.; Wenger, Ch.; Albert, M.; andBartha, J. W.; “Towards high-frequency heterojunction transistors: Electricalcharacterization of N-doped amorphous silicon-graphene diodes,” J. Appl. Phys. 2017 121,
245302.
a)Figure 1. Device concept for a flexible
GBHT device with the potential for thecurrent modulation through a thinpatterned and permeable graphene layer
by controlling the Schottky barrier height.

b)

Figure 2. Methods for monolayer graphene patterning:a) direct imprinting into PMMA coated graphene (Gr)on Cu foils with subsequent wet-transfer, b) wet-transfer
of Gr and subsequent imprint on an additionally coated
PMMA film (preferred method due to Cu roughness).

Figure 3. 200 nm hole array in 160 nm Figure 4. SEM and AFM images of holes transferredthick PMMA 35k layer after T-NIL.
from PMMA into the graphene layer via oxygen plasma.
a)

b)

a)

b)
2.0E-03
Device 1

8.0V Ube

Emitter a-Si:H

7.0V Ube

Ic (A)

1.5E-03

6.0V Ube
5.0V Ube

1.0E-03

4.0V Ube
3.0V Ube
2.0V Ube

5.0E-04

1.0V Ube
0.0V Ube

0.0E+00
0.0 1.0 2.0 3.0 4.0 5.0 6.0
Uce (V)

Figure 5. Gain characteristic for a) Figure 6. I-V characteristic comparison of a) nonnon-patterned GBHT and b) p-GBHT patterned GBHT and b) p-GBHT devices after Tdevice after T-NIL with 1.5 µm holes. NIL with 1.5 µm holes.