Scanning probe microscopy based electrical characterization of thin dielectric and organic semiconductor films

Scanning probe microscopy based electrical characterization of thin dielectric and organic semiconductor films

Microelectronics Reliability 53 (2013) 1430–1433 Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevi...

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Microelectronics Reliability 53 (2013) 1430–1433

Contents lists available at ScienceDirect

Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel

Scanning probe microscopy based electrical characterization of thin dielectric and organic semiconductor films Alexander Hofer a,⇑, Roland Biberger a, Günther Benstetter a, Björn Wilke b, Holger Göbel b a b

University of Applied Sciences Deggendorf, Edlmairstr. 6+8, 94469 Deggendorf, Germany Helmut-Schmidt-University, University of the Federal Armed Forces Hamburg, Holstenhofweg 85, 22043 Hamburg, Germany

a r t i c l e

i n f o

Article history: Received 24 May 2013 Received in revised form 3 July 2013 Accepted 20 July 2013

a b s t r a c t Scanning probe microscopy (SPM) techniques offer various characterization methods for thin organic films. However, the majority of the electrical SPM measurements is currently performed in contact mode operation and may lead to severe damage at the surface of soft organic materials. This work shows the electrical characterization of organic insulator and semiconductor films by use of two SPM techniques operating with reduced lateral forces between SPM tip and sample. The first one is intermittent-contact scanning-capacitance-microscopy (IC-SCM) which is used for the detection of the local surface capacitance. The second one is torsional resonance tunneling-atomic-force-microscopy (TR-TUNA) which shows the local conductivity respectively relative film thickness of the sample. It is found that the tunneling current distribution across 50 nm thick organic insulating films is very homogeneous and that inhomogeneities in P3HT and Pentacene films can be pinpointed even if no topographical variations are observable. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction

2. Experimental

Organic dielectric and semiconducting materials have gained in importance in recent years. Primarily they form the basis of organic field effect transistors (OFET), solar cells and light emitting devices. To ensure the correct and reliable operation of these new devices, material properties such as conductivity, capacitance and homogeneity of the organic semiconductor and insulation layers need to be tested. Typical methods to investigate these materials are Raman spectroscopy, X-ray diffraction or atomic force microscopy (AFM) [1–4]. Scanning probe microcopy (SPM) methods are capable to perform local conductivity measurements or to show the capacitance distribution among the surface. Conventional contact mode SPM techniques such as Tunneling-AFM (TUNA) or scanning capacitance microscopy (SCM) are not suitable for soft organic materials. Measurement artefacts like plowing or piling up of material can be observed because of high lateral forces resulting from contact mode operation. The key to analyse these organic materials is using non-contact electrical methods like torsional resonance TUNA (TR-TUNA) and intermittent contact SCM (ICSCM).

Three different types of samples have been used for the measurements as can be seen in Fig. 1. Firstly, samples with a thin layer of the organic insulator Bectron DP 8101 VP with a thickness of 50 nm have been deposited onto simple glass carriers coated with a thin indium tin oxide (ITO) film (Fig. 1a). This thickness was achieved by increasing the solvent content to 95%. The ITO layer serves as electric back contact which is needed for the IC-SCM measurements. Secondly, for the TR-TUNA measurements we used two different samples. One of the samples consists of a 10 nm thick P3HT organic semiconductor film which was deposited on the ITOcoated glass carriers (Fig. 1b). The third sample is a composite of a 30 nm thick Pentacene top layer and a 100 nm thick gold interface layer on top of a SiO2 layer based on a Si wafer (Fig. 1c). During the production process and afterwards the samples were kept in a nitrogen environment. A Bruker Dimension ICON AFM with Nanosensors PPP-EFM probes was used. All measurements were performed in standard environment. The samples were measured directly after leaving the nitrogen environment. Except the low conductive connection to the ITO or Au layer an additional sample preparation was not needed. Due to the very soft surface all contact mode based electrical AFM techniques turned out to fail because they lead to severe surface damage. The tip scrubs the surface during its movement. For this reason advanced electrical AFM techniques are necessary for the characterization of soft organic materials.

⇑ Corresponding author. Tel.: +49 (0) 991 3615 513; fax: +49 (0) 991 3615 562. E-mail addresses: [email protected] (A. Hofer), guenther. [email protected] (G. Benstetter). 0026-2714/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2013.07.086

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grain boundaries of 30 nm thick Pentacene layers can be imaged simultaneously with topography. 3. Results

Fig. 1. Sample layout (a) Insulator on ITO, (b) P3HT on ITO and (c) Pentacene on Au/ SiO2.

We applied two new techniques; the first one is called IC-SCM. It was originally developed to analyze dopant concentrations on Sibased semiconductors [5,6]. The surface is scanned in IC-mode while the local capacitance C(x,y) is detected due to the amplitude change of the cantilever oscillation. Traps, voids or any foreign materials in the insulator give rise to a capacitance change and hence can be identified by IC-SCM measurements. An important requirement is the existence of a metal–oxide-semiconductor structure which is formed by the construct of doped tip, organic insulator and the ITO layer, thus IC-SCM can be used to monitor the dielectric film homogeneity during OFET manufacturing processes. In our case IC-SCM was used to study local capacitance variations of the P3HT layer. Another electrical characterization method preserving the surface from wear while scanning is called TR-TUNA [7,8]. This method generates a two dimensional distribution of the electric conductivity of the scanned area. Different to the IC-mode, where the cantilever oscillates at its vertical oscillation frequency the cantilever is excited by two separated piezoelectric actuators to oscillate at its lateral oscillation frequency [9,10]. Because of the torsional oscillation of the AFM cantilever, the tip-surface distance varies between 0 and 2 nm giving rise to a continuous tunneling current. Like in IC-mode operation, the force between tip and sample is very low. By applying TR-TUNA, organic semiconductors can be scanned and local defect paths can be pinpointed by locally increased or reduced current flows. This method was applied to a 50 nm thick organic insulator exhibiting the current distribution across the sample. TR-TUNA was also utilized to scan on the Pentacene sample (Fig. 1c) with high spatial resolution for areas with increased conductivity. Consequently the current distribution at

For the 50 nm thick organic insulator measurable tunneling currents could be obtained at a voltage of 10 V applied between sample and probe during the TR-TUNA measurements. As can be seen in Fig. 2a the topography is quite homogenous except hillocks with an average height of 20 nm. The sample shows a median tunneling current of 1 pA. At the hillocks the current increases to maximum of 2 pA (Fig. 2b). To enhance the contrast in Fig. 2b the background current of 1 pA was deducted from all current values. On the other side the current distribution was rather uniform even on topography changes caused by the rough surface of the glass carrier (Fig. 2a). An accumulation of the insulator at the hillocks is unlikely since the current increases. Overall the current value of about 1 pA is very low for the given film thickness and applied voltage hence indicating a low conductivity and high breakdown fields beyond 2  106 V/cm. The TR-TUNA measurements at sample 3 with the Pentacene covered SiO2 to gold transition (Fig. 1c) reveal increased current at the Pentacene grain boundaries at areas where the Pentacen was deposited on gold. Due to the low transversal conductivity the current is relatively low within the Pentacene grains. An additional sample with an ozone preprocessing prior to the deposition of the Pentacene layer showed fewer areas with high currents on the boundaries (Fig. 3d). This is in good correlation when comparing the topography of the two samples (Fig. 3a and c). Noteworthy is also that the roughness values Ra and Rq of both samples are in the same range. The simultaneous topographical and IC-SCM measurements of the 10 nm thick P3HT layer show roughness values Ra and Rq below 3 nm. In general the capacitance distribution (Fig. 4b) depends on the topography (Fig. 4a). Local Hillocks with a diameter of 100 nm lead to an enhanced capacitance in the same lateral dimension. However, several areas over the sample can be observed (encircled in Fig. 4a and b) where a significant capacitance change occurs without a clear correlation in topography, indicating a local material inhomogeneity. Topography influences on the capacitance signal determined with IC-SCM have been discussed by Biberger [11] and show very low impact for low doped semiconductors and topographic step sizes under 100 nm. 4. Conclusion AFM techniques offer unique possibilities to electrically characterize thin organic films or devices with satisfactory lateral

Fig. 2. Topography image (a) 5  5 lm2 of a 50 nm thick insulation layer and current mapping and (b) measured with TR-TUNA.

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Fig. 3. Topography images (a and c) 5  5 lm2 and current mappings (b and d) of Pentacene at the transition (marked with red line) from SiO2 to gold. The sample (c and d) with an ozone preprocessing shows a better coverage of the Pentacene layer resulting in lower current between the grains. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Topography image (a) 2  2 lm2 of 10 nm thick P3HT layer and capacitance distribution and (b) measured with IC-SCM. Topography changes with a resulting capacitance change are indicated with an arrow. The circles tag areas where a capacitance change is not correlated to a significant topography feature.

resolution. However, these methods must operate in a mode supporting low lateral forces between tip and sample to avoid surface damage and artefacts. The quality and repeatability of the topography images is also very high because of the low wear of tip and sample. IC-SCM and TR-TUNA are suitable methods, one for obtaining 2D capacitance images and the other for providing current mappings. Both methods can be used to optimize organic film deposition in respect to film homogeneity and dielectric performance. The results indicate that these methods can be applied to a wide range of organic semiconductor.

Acknowledgement This work has been supported by the BMBF, Bundesministerium für Bildung und Forschung, Germany.

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