Controlling threshold voltage and leakage currents in vertical organic field effect transistors by inversion mode operation

Controlling threshold voltage and leakage currents in vertical organic field effect transistors by inversion mode operation Controlling threshold voltage and leakage currents in vertical organic field[.]

Controlling threshold voltage and leakage currents in vertical organic field-effect transistors by inversion mode operation

Alrun A Günther, Christoph Hossbach, Michael Sawatzki, Daniel Kasemann, Johann W Bartha, and Karl Leo

Citation: Appl Phys Lett. 107, 233302 (2015); doi: 10.1063/1.4937439

View online: http://dx.doi.org/10.1063/1.4937439

View Table of Contents: http://aip.scitation.org/toc/apl/107/23

Published by the American Institute of Physics

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Controlling threshold voltage and leakage currents in vertical organic

field-effect transistors by inversion mode operation

Alrun A.G€unther,1,a)ChristophHossbach,2MichaelSawatzki,1DanielKasemann,3

Johann W.Bartha,2and KarlLeo1,4

1

Institut f€ ur Angewandte Photophysik, Technische Universit€ at Dresden, 01062 Dresden, Germany

2

Institut f€ ur Halbleiter- und Mikrosystemtechnik, Technische Universit€ at Dresden, 01062 Dresden, Germany

3

CreaPhys GmbH, Niedersedlitzer Straße 75, 01257 Dresden, Germany

4

Canadian Institute for Advanced Research (CIFAR), 180 Dundas Street West, Suite 1400, Toronto,

Ontario M5G 1Z8, Canada

(Received 26 August 2015; accepted 26 November 2015; published online 10 December 2015)

The interest in vertical organic transistors as a means to overcome the limitations of conventional

organic field-effect transistors (OFETs) has been growing steadily in recent years Current vertical

architectures, however, often suffer from a lack of parameter control, as they are limited to certain

materials and processing techniques, making a controlled shift of, e.g., the transistor threshold

voltage difficult In this contribution, we present a vertical OFET (VOFET) operating in the

inversion regime By varying the thickness or doping concentration of a p-doped layer in an

otherwise n-type VOFET, we are able to shift the threshold voltage in a controlled manner from

1.61 V (for a normal n-type VOFET) to 4.83 V (for the highest doping concentration of 50 mol %)

Furthermore, it is found that low doping concentrations of 20 mol % can improve the Off state of

the VOFET through reduction of the source-drain leakage current.V C 2015 Author(s) All article

content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0

Unported License [http://dx.doi.org/10.1063/1.4937439]

Organic field-effect transistors (OFETs) are the key

devi-ces for many future flexible electronics applications.13They

should be easy and cheap to fabricate, yet show high

perform-ance and adaptability to the specific needs of the application

A simple way to meet these requirements is the vertical

stacking of the transistor electrodes, resulting in a so-called

vertical organic transistor.4 Many such devices have been

reported recently,5 9 showing promising performance and

easy fabrication procedures In many cases, however, a

change in material system or geometry seems necessary in

order to tune certain parameters, such as the threshold voltage

or On/Off ratio, to the needs of a specific application In this

letter, we report on a vertical OFET (VOFET)10,11operating

ininversion mode.12–15Variation of the doping concentration

or thickness of the inversion layer allows to control the

threshold voltage and Off state current of the VOFET without

altering the geometry or material system as a whole

Previous efforts in this area have shown the good

per-formance of VOFETs using C60as the organic

semiconduc-tor.11In order to test whether this VOFET geometry allows

for inclusion of the inversion operation principle, it is

imple-mented into VOFETs of the same type, with molybdenum

trioxide (MoO3) as dopant16,17 for the p-doped inversion

channel (see Fig.1) The VOFET geometry works much like

a conventional OFET in the sense that charge carriers

emit-ted from the source electrode first accumulate at the gate

dielectric interface before entering the vertical channel to the

drain electrode For an n-type VOFET, it is therefore

expected that a p-doped layer, placed into the accumulation

region, will lead to an inversion operation in the same way

as demonstrated previously for conventional p-type OFETs.13,14 To check for the formation of this inversion regime, capacitance-voltage (CV) measurements are per-formed on a series of metal-insulator-semiconductor (MIS) capacitors, resembling a material stack identical to the chan-nel region underneath the source electrode of the VOFET (see Fig.1for the VOFET geometry)

All devices are fabricated on doped silicon wafers serving both as substrate and as the bottom contact for MIS samples/gate electrode of the transistors An aluminum oxide (Al2O3, e¼ 7.8) dielectric of only 33 nm thickness is depos-ited on top of the wafers via thermal atomic layer deposition (ALD) (at IHM, for CV samples) or plasma-assisted ALD (at IAPP, for VOFET samples) The substrates are then immersed in acetone and isopropanol and cleaned in an

FIG 1 Device schematic of the VOFET with a p-doped layer for inversion operation in the Off and On state.

a)

alrun.guenther@iapp.de

ultrasonic bath for 5 min each, followed by exposure to

oxygen plasma for 10 min In order to reduce interface traps,

the cleaned substrates are dipped into hexamethyldisilazane

(HMDS, Merck) for 30 min and residuals of HMDS are

removed by spin-rinsing with isopropanol All MIS capacitor

samples forCV measurements are prepared in a single run

by thermal deposition under high-vacuum conditions, using

a single-chamber UHV tool (Kurt J Lesker Company) with

a base pressure of 107mbar Thin layers of C60 (Sensient,

sublimated twice before use, ¼ 1.54 g/cm3, and thickness

0–10 nm), doped with varying concentrations of

molybde-num oxide (MoO3, Sigma Aldrich, used as received,

.¼ 4.50 g/cm3, and concentrations 0–50 mol %), are

depos-ited on the Al2O3-coated substrates The layer thickness is

monitored using individual quartz crystal micro-balances

(QCMs) for both materials Intrinsic C60is deposited by the

same process to achieve a total thickness of 30 nm of the two

layers 10 nm-thick Au contact pads are deposited through a

shadow mask, followed by 40 nm of Al, which are deposited

through the same shadow mask Layer thickness and doping

concentration variations are achieved in this single run by

use of the UHV system’s wedging tool, so as to provide the

best comparability between samples Inversion VOFETs are

prepared on the same type of substrates, but deposition is

done in a multi-chamber UHV tool (base pressure also 107

mbar) in separate runs, so that the reference samples are

never exposed to the MoO3dopant Just as for MIS

capaci-tors, thin layers of C60doped with varying concentrations of

MoO3are deposited first, followed by intrinsic C60to give a

total layer thickness of 30 nm The source contacts of the

VOFET are made of 50 nm Au and patterned using a bi-layer

orthogonal photolithography process.18 120 nm of SiO2 is

also deposited through this lithography mask via magnetron

sputtering in order to reduce leakage currents between the

source and drain electrodes The photolithography mask is

subsequently lifted off, and the VOFETs are completed by

depositing 30 nm of intrinsic C60and a drain electrode, made

of 40 nm Au, through a second photolithography mask,

which is lifted off prior to electrical characterization All

devices are annealed for 2 h at 60C under nitrogen

atmos-phere to recover the p-doping effect of MoO3in C60 This

step is necessary as the photolithography is performed under

ambient conditions and the energy levels of MoO3shift upon

contact with air.17All VOFETs have the same dimensions of

60 nm vertical channel length (L) and 600 lm channel width

(W) Contact doping19–22for either n- or p-type injection is

not employed for the transistors as a sufficient injection of

both carriers from the Au contacts into the C60 LUMO/

HOMO is expected without the aid of injection barrier

low-ering Electrical characterization (using an Autolab

PGSTAT302N galvanostat for CV and a HP 4145B

semicon-ductor parameter analyzer for VOFET transfer curves) is

performed in a nitrogen glovebox to prevent additional

p-doping by oxygen and the aforementioned energy level shift

of MoO3

CV curves are obtained at a frequency of 100 kHz and

an amplitude of 50 mV The DC voltage is applied to the

injecting top contact, while the substrate, i.e., the back

con-tact, is kept at ground potential The total capacitance per

unit area of the MIS stack can be approximated as

1

C¼ 1

Cox

þ 1

Cinv

þ 1

Csemi

¼ dox

e0eox

þVSG

qd þdtot d

e0eC60

; (1)

whereCox,Cinv, andCsemiare the capacitances per unit area

of the gate oxide, inversion layer, and intrinsic semiconduc-tor layer q is the charge carrier density inside the inversion layer, which depends on the doping concentration, dox, d, anddtotare the oxide, inversion layer, and total semiconduc-tor thicknesses and all other variables have their usual mean-ings As can be observed in Fig 2, the total capacitance varies noticeably between the individual samples We attrib-ute this to a convolution of the doping effect and variations

indtotdue to dopant addition, as well as small variations in device area due to processing conditions

The application of a positive voltage to the injecting contact results in the accumulation of holes at the interface between the p-doped C60and the underlying Al2O3 It may

be noted that this hole accumulation effect is observed not only for the samples with a p-doped layer, but, to a certain extent, also for the undoped reference sample, shown in black We attribute this to the processing conditions in the chamber: All samples were prepared in a single run, and even though the wedging tool employed during deposition covered the reference samples during MoO3 deposition, a slight MoO3contamination of the reference samples is possi-ble Applying a negative voltage to the injection contact instead accumulates electrons at the dielectric interface, i.e.,

FIG 2 CV characteristics of (a) samples with a fixed doping layer thickness

of 5 nm and varying doping concentrations of MoO 3 and (b) samples with a fixed MoO 3 concentration of 30 mol % and varying doping layer thick-nesses The turn-over point between majority carrier accumulation and depletion is marked as a dashed line, and a reference device without a p-doped C layer is shown as a black line.

the inversion regime is reached The turn-over point between

these two regimes is marked in Fig.2 as a dashed line for

each sample As the doping concentration or doping layer

thickness at the dielectric interface is increased in a

con-trolled manner, we see a shift of the turn-over points from

accumulation into depletion mode and then further into

inversion, as typically observed for MOSFETs with a

high-frequency test signal.15We can only partly resolve the

inver-sion regime in highly doped samples, as we are limited by

sample processing and experimental set-up The

characteri-zation tool used for the CV measurements is limited to the

voltage regime investigated here and low frequency

meas-urements (which typically show a clear accumulation of

minority carriers in inversion MOSFETs) could not produce

reliable results due to the noise level in and around the

glovebox and measurement set-up N-type contact doping

would normally be employed in such a case to enhance the

injection of minority charge carriers and thus make the

inver-sion regime more visible This would necessitate an

air-stable n-dopant for C60, since we aim for the same material

system in the MIS structures and VOFET samples and part

of the fabrication process for VOFETs is performed in air

(see above and Ref.18) As air-stable n-dopants are not

read-ily available, we choose gold for the electrodes of both

de-vice types The work function of Au is approximately

half-way between the LUMO and HOMO of C60 (Au¼ 5.1 eV,

C60HOMO¼ 6.4 eV, and C60LUMO¼ 4 eV (Refs.23–25)),

and the injection barriers for both carrier types are thus

roughly comparable

To investigate the effects of the inversion layer on the

working VOFET, the transfer characteristic of each VOFET is

measured in the saturation regime The transistor threshold

voltageVth, which can be extracted from these transfer

charac-teristics, is expected to depend on the density of activated

dop-antsNA(c) and on the doping layer thickness d according to

Vth¼ VFBþeNAð Þdc

Cox

whereVFBis the transistor flatband voltage,c is the doping

concentration, ande is the elementary charge.14 To obtain

transfer characteristics, a constant potential is applied to the

drain electrode, while keeping the source electrode at ground

potential and sweeping the gate in the same way as for the

CV measurement

Figures 3(a) and 4(a) show a controlled threshold

voltage shift with the increasing doping concentration in the

inversion channel This same effect was previously

demon-strated by L€ussem and coworkers for a standard

lateral-channel OFET.14 It may be noted that the Vth determined

from the transfer characteristics is not identical to the

turn-over point marked in the CV curves This is attributed to

differences in processing conditions and substrate quality

(see methods section) It can be shown further that the

threshold voltage is shifted by an increased thickness of the

doped layer, as suggested in Eq (2) As can be observed

from Figs.3 and4(b), a change in layer thickness does in

fact produce a more systematic threshold voltage shift than a

change in doping concentration, as layer thickness is more

easily controlled in our setup than doping concentration

These findings, together with theCV measurements, are further proof that the threshold voltage shift is due to an increased number of holes near the gate dielectric interface, resulting from the p-doping effect of MoO3in C60 Once the holes are depleted from the p-doped layer, it is possible to accumulate electrons at the gate dielectric interface in the same way as in the reference VOFET The precise voltage at which this turn-over happens, i.e., the transistor threshold voltage, is determined by the amount of holes in the p-doped layer

The presented data suggest another benefit of the inver-sion operation, which has not been demonstrated before: The p-doping layer at the gate dielectric interface allows for con-trol of the Off state current As visible in Fig.3, a compara-bly small MoO3 concentration of 20 mol % does not yet affect the threshold voltage much but results in the presence

of small amounts of holes near the gate dielectric interface This leads to carrier recombination with the electrons leaking out of the source electrode in the transistor’s Off state This effect reduces the Off state current while having only a slight effect on the On state current If combined with an efficient n-dopant interlayer as injection booster at the source, this effect could provide another useful tool to enhance the On/Off ratio in n-type VOFETs As higher MoO3 concentra-tions introduce more holes into the p-doped layer, however, there are no longer sufficient amounts of electrons from the source-drain leakage current to recombine with; thus, the

FIG 3 (a) Transfer curves of VOFETs with a fixed doping layer thickness

of 5 nm and varying doping concentrations of MoO 3 The applied V D

is þ6 V (b) Transfer characteristics of VOFETs with a fixed MoO 3 concen-tration of 30 mol % and varying doping layer thicknesses The applied V D

is þ8 V Reference devices without a p-doped C 60 layer are shown as black lines in both cases.

holes themselves now form a considerable leakage current in

the Off state of the transistor, as can be seen particularly well

for an MoO3 concentration of 50 mol % or a doped layer

thickness of 10 nm

Our results show that vertical organic field-effect

tran-sistors can be improved by using the inversion operation

concept Variations in the doping concentration or layer

thickness of a p-doped layer in an n-type VOFET can control

the threshold voltage and Off state of the device, with the

layer thickness variation producing a particularly controlled

shift of these parameters Indeed, the reduction of leakage

currents, and thus the potential for improving the On/Off

ratio, is an especially interesting feature of this operation, since

leakage currents are an issue often faced in short-channel

devices Being able to control them with simple fabrication processes can give VOFETs the boost required to make them truly successful high-performance control devices for flexible electronics applications

This work has received funding from the Dr Isolde-Dietrich-Stiftung and the European Community’s 7th Framework Programme under Project NUDEV (FP7-267995)

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FIG 4 Threshold voltage (a) and On/Off ratio (b) as a function of doping

concentration and doping layer thickness for at least three devices per

sam-ple Box areas represent the interval of 25% to 75% of the data distribution,

and whiskers denote the maximum and minimum values of the distribution.

The horizontal black lines within the boxes represent the mean value of the

distribution.