Effects of Dielectric Material, HMDS Layer, and Channel Length on the Performance of the Perylenediimide-Based Organic Field-Effect Transistors.

It is well-known that the improvement in the performance of organic field-effect transistors (OFETs) relies primarily on growth properties of organic molecules on gate dielectrics, their interface behavior, and on understanding the physical processes occurring during device operation. In this work, the relation of varying the dielectric materials in an n-type OFET device based on 1,7-dibromo-N,N'-dioctadecyl-3,4,9,10-perylenetetracarboxylic diimide (Br2PTCDI-C18) molecule on a low-cost glass substrate at different channel lengths is reported, which is conceptually very important and fundamental in the context of device performance. Anodized alumina (Al2O3) along with dielectric films of polyvinyl alcohol (PVA) or polymethylmethacrylate (PMMA) was used to fabricate the devices and study their influence on various transistor properties. In addition, the effects of a thin hexamethyldisilazane (HMDS) layer on the performance of OFETs including their contact resistances were studied with the channel length variations. The devices with PVA dielectric material exhibited the maximum mobility values of 0.012-0.025 cm2 V-1 s-1 irrespective of varying channel lengths from 25 to 190 μm. The bias-stress measurements were recorded to realize the effects of the channel length and HMDS layer on the stability of the devices. The on/off ratios and electrical stabilities of these devices were enhanced significantly by modifying the surface of the PVA dielectric layer using a thin layer of HMDS. Similarly, in the case of PMMA dielectric layer, a drastic enhancement in the on/off ratio and bias-stress stability was observed. Characterization of all devices at different channel lengths using different dielectric materials permitted us to identify the effects of contact resistance on OFET devices. The stability of the devices in relation to the bias-stress measurements of devices by varying channel lengths and surface modification was systematically investigated. A careful analysis of oxide gate dielectrics modified with polymer-based dielectric materials, contact resistance, influence of thin HMDS layer on the electrical properties, and other parameters on top-contact bottom-gated configured n-type OFET devices is presented herein.

Introduction

In recent years, electronic devices based on organic materials have been receiving steady and increasing attention because of their versatile applications in optoelectronics, biomedical electronics, and digital electronics. Organic semiconducting materials are the active components in electronic devices, including solar cells,[1,2] organic light-emitting diodes (OLEDs),[3] and organic field-effect transistors (OFETs).[4,5] For a wide variety of low-cost, very large-area, flexible electronic applications, OFETs are the fundamental components.[6−8] Their relevance primarily includes active-matrix displays,[9] radio-frequency identification tags,[10] smart price and inventory tags, smart cards, and large sensor arrays.[11] OFETs offer two main advantages over inorganic semiconductor field-effect transistors (FETs). The former can be fabricated at lower temperatures, thereby lowering the cost significantly. Low-temperature processing of OFETs allows them to be fabricated on low-cost plastic/polymer substrates but not on glass and SiO2. Thus, the development of unbreakable, flexible, very light-weight flat-panel displays has found considerable commercial attention.

Despite the major improvements accomplished in the performance of OFETs over the past few years, several of these designs, materials, and experimental parameters, which affect the OFET operation, are yet to be understood clearly. Contact resistance remains as one such parameter.[12] Subsequently, the performance of OFET typically experiences large contact resistances, which may be limiting the speed of organic integrated circuits. In contrast to the FETs developed by utilizing the polycrystalline, single-crystalline, or hydrogenated amorphous silicon, the source and drain contacts of OFETs rather may not be optimized easily by conventional methods, such as doping the semiconductor or metal alloying. In OFETs, difference in the energy between Fermi levels of the metal contacts and the HOMO (highest occupied molecular orbital) or LUMO (lowest unoccupied molecular orbital) levels of the organic semiconductor (OSC) and the semiconductor morphology decides the contact resistance value. The gate modulation of the bulk resistivity in an OSC explains the obtained gate voltage dependence of the contact resistance for various thicknesses of the semiconducting thin films, and the contact resistance was Ohmic. However, for thicker semiconducting films, significant asymmetry between the source and drain contact resistances was observed with the drain resistances increasing more rapidly with the thickness as compared with the source resistance, revealing the vital role of diffusion at the drain contact.

The performance of n-type OSCs has been lagging behind the more intensively examined p-type organic materials over the years.[13,14] However, both kinds of semiconductors are vital to fabricate complementary integrated circuits,[15−17] bipolar transistors, or organic p/n junctions. Over the past years, organic semiconducting materials appeared with high electron mobilities compared with those of the amorphous silicon devices.[18−20] In general, electron-deficient organic semiconducting materials have been developed by introducing electron-withdrawing functionalities at different substituent positions over the extended aromatic π-systems with promising p-channel performance.[21,22] Molecular derivatives of perylenetetracarboxylic diimides (PTCDIs) are an exceptional class of n-type OSC materials.[23,24] PTCDIs have been studied extensively because of their high molar absorptivity and higher quantum yields with good photochemical and thermal stability. Additionally, the self-assembly of π-conjugated materials directly from solution provides a practical and potent approach to generate supramolecular nanomorphological structures with long-range ordering, which remain immensely important for the progress of organic optoelectronic devices. PTCDIs or PDIs (perylene diimides) are, in general, very attractive building blocks to self-assemble because the chemical functionalization of the PDI core at their imide,[25,26] ortho,[27,28] or bay[29] positions modifies their molecular packing in films/solid state, resulting in exciting electronic and optical properties. The solid-state molecular packing significantly effects both the exciton migration and carrier mobility, which remain central for optoelectronic devices, namely, OLEDs,[30,31] FETs,[32] and photovoltaics.[33]

To enhance the carrier injection into OSC films, it is essential to apply a higher voltage to the gate electrode or the gate insulator of higher dielectric constant value.[34] OFETs with heavily doped conductive Si substrates were also reported by using thermally oxidized SiO2 dielectrics.[35] Even though SiO2 is confirmed to be a good insulator with higher dielectric strength (∼10 MV/cm), SiO2 has a relatively lower dielectric constant (∼4). Therefore, attempts to construct an OFET with a high-k and high-dielectric-constant gate insulator to inject additional number of carriers have gained momentum.[36,37] An additional advantage of high-k dielectric materials is their ability to reduce the operating voltage.[38] In the present study, polyvinyl alcohol (PVA) and poly methyl methacrylate (PMMA) were considered to be polymer dielectric materials. The dielectric constants of PVA and PMMA are ∼8 and 3, respectively. Therefore, injecting thrice the amount of charge carriers into organic dielectric PVA than in PMMA would be possible at the same thickness, and yet the best dielectric breakdown behavior is actually realized. Hence, it remains crucial to obtain a good polymer film with higher dielectric property and smoother surface on the top of which a high-quality morphology OSC thin film could be deposited.[39]

In common practice, dielectric thin films are deposited by chemical vapor deposition, sputtering, e-beam evaporation, and so forth. These methods required costly equipment and are too sophisticated techniques to grow a low-defect smooth dielectric thin film. In this work, we attempted to fabricate an Al2O3 thin film by anodization technique that seldom needs vacuum or elevated temperatures.[40] Herein, uniform Al2O3 films are grown on the Al layer. In this anodization process, the Al2O3 film growth takes place under “negative-feedback” state, indicating higher electrochemical current flowing at defective or thinner parts in the developing oxide film, in a way that these defects get automatically restored. Herein, an attempt to develop an Al2O3 surface on a thermally deposited Al was discussed. Over the anodized Al2O3/Al gate substrates, bottom gate-type OFETs using n-type N,N′-dioctadecyl-1,7-dibromo-3,4,9,10-perylenetetracarboxylic diimide (Br2PTCDI-C18) were fabricated with varying channel lengths. It was clearly observed that these new OFETs worked exceedingly well and showed consistent results, even on varying the length of the channel. In this paper, the overall focus was to develop an economical fabrication process and consistent device performance with the variation in the channel length and the surface modification of the dielectric layer, which has not been attempted previously.

Results and Discussion

The schematic diagram (Figure a) shows the OFETs prepared over anodized Al2O3, employed in this study with different dielectric materials and hexamethyldisilazane (HMDS) surface-modifying layer. The OFETs were fabricated using 13 nm thick anodized Al2O3/100 nm PVA, Al2O3/100 nm PVA/HMDS, Al2O3/100 nm PMMA, and Al2O3/100 nm PMMA/HMDS on a glass substrate. The structures of the semiconducting material Br2PTCDI-C18 and HMDS are shown in Figure b,c. All four devices were tested to identify the effect of the channel length on the performance and stability of the OFET devices. A thin HMDS layer was also grown to identify the effect of the HMDS layer on the growth of PVA or PMMA surfaces. In general, anodized aluminum films have extremely high rough surfaces (∼28 nm). To lessen this surface roughness, a 100 nm PVA or PMMA film was spin-coated on these anodized surfaces. The semiconductor films deposited on different dielectric materials were characterized using atomic force microscopy (AFM) (Figure ). The temperature and pressure parameters were 50 °C and ∼10–6 mbar, respectively, during deposition. On to these films, OSC films were thermally deposited and characterized by AFM and X-ray diffraction (XRD).

Figure 1

(a) Schematic diagram of the device structure, (b) structure of Br2PTCDI-C18 molecule, and (c) structure of the surface-modifying agent HMDS.

Figure 2

(a) AFM topography images of Br2PTCDI-C18 n-type semiconductor molecule deposited on (a) PVA, (b) PMMA, (c) PVA/HMDS, and (d) PMMA/HMDS surfaces.

AFM and FESEM Analyses

The surface features of Br2PTCDI-C18 thin films on diverse substrates were characterized using AFM (Figure ). The growth of Br2PTCDI-C18 films on every HMDS-treated substrate exhibited improved structural ordering and displayed grain formation than the untreated films. However, the grain size and surface roughness varied greatly at different surface energies. Figure a presents the topography image of Br2PTCDI-C18 deposited over the PVA surface having a root mean square (rms) roughness (σ) of 2.82 nm; whereas on PMMA, the σ was 3.02 nm (Figure b) within the 1.5 × 1.5 μm2 scan scale. On the HMDS-modified surface of PVA and PMMA, the σ was found to be 3.47 and 3.22 nm, respectively (Figure c,d). It was observed that the grain size increased when the dielectric surface was modified with HMDS because of the increased hydrophobicity, which made the surface more favorable for the organic molecule growth. In addition to the rms roughness of all films interpreted from AFM (see Figure ), the enhanced growth on HMDS-treated substrates was further confirmed by XRD analysis and surface energy measurements.

Br2PTCDI-C18 OFETs were fabricated over different dielectric materials with varying channel lengths and then characterized carefully in their saturation regime, defined by the standard MOSFET models. To further investigate the influence of different dielectric materials (13 nm Al2O3/100 nm PVA and 13 nm Al2O3/100 nm PMMA) and HMDS treatment on the device performance, the transistors having channel length L = 25, 50, 100, and 190 μm with a channel width of W = 750 μm were evaluated. The FESEM images of the different channel lengths are shown in Figure a–d. Copper wires with different diameters were used as masks to define the varying channel lengths.

Figure 3

Field emission scanning electron microscopy (FESEM) images of different channel lengths on the fabricated devices with the same channel width of 750 μm and with channel lengths of (a) 25, (b) 50, (c) 100, and (d) 190 μm.

XRD Analysis

XRD analysis was performed with Cu Kα1 (λ = 1.542 Å) to further confirm the morphological changes identified in the AFM images and the structural changes. The substrates for XRD analysis were prepared in the same manner as done for the OFET device fabrication. In all four cases, a primary diffraction peak at around 2θ = 2.62°–2.74° was observed. In the case of PVA, the peak intensity is low and the fwhm (full width half-maxima) is high among all, which indicated the formation of smaller grain size, as observed in AFM. After modifying the surface of the dielectric materials PVA and PMMA with HMDS, there was an increase in the intensity and decrease in the fwhm (shown in Figure S1) of the low-angle diffraction peak observed in the XRD spectra, as shown in Figure . The fwhm values extracted from the peaks and fitted with Lorentzian function are tabulated in Table .

Figure 4

XRD X-ray diffraction patterns of Br2PTCDI-C18 deposited on different dielectric materials and after the surface modification with HMDS.

Table 1

Summary of fwhm Extracted from XRD Diffraction Peaks of Br2PTCDI-C18 Molecule Deposited on Different Dielectric Materials and after the Surface Modification with HMDS Fitted with Lorentzian Function

dielectric	2θ	fwhm
PVA/HMDS	2.718	0.3001
PMMA/HMDS	2.736	0.31903
PMMA	2.730	0.5312
PVA	2.622	0.6784

The decrease in the fwhm of the diffraction peak after the modification of the surface of PVA and PMMA is a clear signature of increase in the grain size of the Br2PTCDI-C18, which is in well-agreement with the results of the AFM analysis. Thus, by performing surface modification of the dielectrics with HMDS, the film morphology is improved, which enhanced the IDS correspondingly.

Surface Energy Measurements

To examine the surface properties of dielectrics, the PVA and PMMA films were carefully characterized by calculating the contact angle with deionized water and hexane having varying polarities. Both contact angles and surface free energies were recorded using a contact angle measurement system [KRUSS drop shape analyzer (DSA 25E)] at 30 °C and ∼60% humidity. The contact angle values and surface free energy for both PVA and PMMA after various surface treatments are listed in Table . HMDS-treated surfaces (Table ) showed smaller surface free energy than unmodified surfaces. This decrease in the surface energies after HMDS treatment indicated the improved morphology of the Br2PTCDI-C18 grown thermally over the dielectric surfaces. HMDS can form self-assembled layers on PVA and PMMA surfaces. Therefore, HMDS treatments could exert good influence on increasing the crystallinity, which results in the increase in the drain currents as well.

Table 2

Summary of the Contact Angles and Surface Energies of PVA and PMMA Dielectric Materials before and after the Surface Modification with HMDS

	contact angle
dielectric	water	hexane	surface energy (mN/m)
PVA/HMDS	74.14 (±0.65)°	13.27 (±0.12)°	31.77 ± 0.42
PMMA/HMDS	64.90 (±0.10)°	12.86 (±0.13)°	38.10 ± 0.08
PMMA	58.09 (±0.56)°	13.62 (±0.42)°	43.15 ± 0.46
PVA	56.84 (±0.87)°	12.14 (±0.11)°	44.12 ± 0.67

Devices Fabricated with Al2O3/PVA

The output and transfer characteristic curves of the OFET devices fabricated using the Al2O3/PVA dielectric material with different channel lengths are depicted in Figure . It was confirmed that the drain current (IDS) decreased when the channel length increases. At 25 μm channel length, the IDS at VDS = 10 V was 500 nA, which further decreased to 380 nA at a channel length of 50 μm. It further decreased up to 250 and 120 nA for 100 and 190 μm channel lengths, respectively. The data were analyzed after plotting the square root of IDS as a function of the gate voltage (VGS). The slope of a fit to the obtained linear portion of the obtained plot, above the threshold value, provides the field-effect electron mobility (μ), whereas intercept of the fit line on the x-axis provides the threshold voltage (VTh) value. The subthreshold slope is established by a linear fit to the log(IDS) just as the current begins to rise. The on/off ratios were also calculated using the minimum current value observed in the off region and the maximum current value observed in the on region. The field-effect mobilities (μ) were obtained from the slope of a plot of (IDS)1/2 against VGS, using the equationwhere VTh and Cdiel are the threshold voltage and the capacitance per unit area, respectively. The OFETs using Al2O3/PVA gate dielectrics exhibited a mobility of 0.012–0.025 cm2 V–1 s–1 for different channel lengths. The threshold voltages were in the range of 0.23 to −3.12 V. The averaged transistor parameters of OFETs for different channel lengths are summarized in Table . The μ value of the 25 μm channel length device was 2 times lower than that of OFET with μ of the 190 μm channel length prepared using the same dielectric material.

Figure 5

Output and transfer characteristic curves of the OFET devices fabricated over Al2O3/PVA dielectric material with varying channel lengths (a) L = 25 μm, (b) L = 50 μm, (c) L = 100 μm, and (d) L = 190 μm.

Table 3

Summary of the Electrical Parameters of the Al2O3/PVA Dielectric and Br2PTCDI-C18 Organic Semiconductor-Based OFETs Fabricated with Different Channel Lengths Measured under Vacuum Conditions Inside of the Probe Station

	Al2O3/PVA/Br2PTCDI-C18
device	L = 25 μm	L = 50 μm	L = 100 μm	L = 190 μm
μ (cm2 V–1 s–1)	0.012	0.014	0.024	0.025
VTh (V)	–2.15	–2.01	–3.12	0.23
S (V/dec)	3.95	4.02	4.15	4.36
Ion/off × 101	1.7	1.5	1.3	1.5

Devices Fabricated with Al2O3/PVA/PMMA

The output and transfer characteristic curves (Figure ) of the fabricated OFET devices were obtained by modifying the surface of Al2O3/PVA dielectric material with the HMDS layer and varying the channel lengths. It was observed that the drain current (IDS) decreased moderately compared with that of the unmodified layer, as shown in Figure . At 25 μm channel length, the IDS at VDS = 10 V was 400 nA and decreased up to 350 nA at a channel length of 50 μm. It further decreased to 320 and 80 nA for 100 and 190 μm channel lengths, respectively. The OFETs using Al2O3/PVA/HMDS gate dielectrics exhibited high performance and high mobility value in the range of 0.011–0.013 cm2 V–1 s–1 for different channel lengths. The threshold voltages were observed to be on the order 0.15–2.25 V. In addition, we observed an improvement in the threshold voltages and the subthreshold slopes compared with those of the unmodified device. The “on/off current ratios” also increased in these devices. The averaged transistor parameters of OFETs for different channel lengths are shown in Table .

Figure 6

Output and transfer characteristic curves of the OFET devices over the Al2O3/PVA/HMDS dielectrics with varying channel lengths (a) L = 25 μm, (b) L = 50 μm, (c) L = 100 μm, and (d) L = 190 μm.

Table 4

Summary of Electrical Parameters of the Al2O3/PVA/HMDS Dielectric and Br2PTCDI-C18 Organic Semiconductor-Based OFETs Fabricated with Different Channel Lengths Measured under Vacuum Conditions Inside of the Probe Station

	Al2O3/PVA/HMDS/Br2PTCDI-C18
device	L = 25 μm	L = 50 μm	L = 100 μm	L = 190 μm
μ (cm2 V–1 s–1)	0.011	0.012	0.012	0.013
VTh (V)	2.25	2.14	0.15	0.29
S (V/dec)	2.75	2.52	3.15	3.37
Ion/off × 103	3	2	2	1.6

Devices Fabricated with Al2O3/PMMA

Figure S2 presents the output and transfer characteristic curves of the OFETs having Al2O3/PMMA dielectric material and Br2PTCDI-C18 as the dielectric material at different channel lengths. Here also, it was observed that the drain current (IDS) decreased compared with that of the PVA dielectric-based devices; however, the threshold voltages and other properties Ion/off ratios improved significantly. At 25 μm channel length, the IDS at VDS = 10 V was 80 nA, which decreased to 70 nA when the channel length was 50 μm. It further decreased to 60 and 50 nA for 100 and 190 μm channel lengths, respectively. The field-effect mobility was extracted for all four transistors and was in the range of 0.0025–0.0045 cm2 V–1 s–1. All device parameters are tabulated in Table S1.

Devices Fabricated with Al2O3/PMMA/HMDS

Figure S3 shows the output and transfer curves of the OFETs having Al2O3/PMMA/HMDS dielectric material and Br2PTCDI-C18 as the dielectric material at different channel lengths. This resulted in an increase in the drain current (IDS) compared with that of the PMMA dielectric layer-based devices along with improvements in other properties such as threshold voltage and Ion/off ratios. At 25 μm channel length, the IDS at VDS = 10 V was 120 nA, and it decreased to 80 nA when the channel length was 50 μm. It further decreased up to 50 and 20 nA for 100 and 190 μm channel lengths, respectively. The field-effect mobility was extracted for all four transistors and was in the range of 0.0015–0.0025 cm2 V–1 s–1. All device parameters are tabulated in Table S2.

Effect of the Channel Length

Figure a shows the plots of the resistance versus the channel length of these devices. All devices exhibited a linear increase in the resistance with the channel length. The contact resistances are extracted using the linear fitting of these curves by transmission line method and are shown in the Table . The contact resistance changes with the morphology of the OSC at the source/drain electrode and with the nature of the dielectric material. As the number of charge traps in the OSC channel increased, the contact resistance also increased (Figure b). With respect to the PVA dielectric, the grain size of the deposited Br2PTCDI-C18 is less compared with that of the one treated with HMDS. This resulted in the higher density of grain boundaries at the electrode and OSC interface compared with HMDS-treated devices, which caused high contact resistance. After HMDS treatment, the contact resistance reduced because of the increase in the grain size. Because of the hydrophobic nature of the PMMA dielectric, the contact resistance is less than that in the case of PVA. After modification with HMDS, the contact resistance further decreased similar to that in the case of PVA dielectric. Under all above conditions, the PMMA/HMDS showed very low contact resistance, 1.38 × 108 Ω. The contact resistance also depends on the vertical overlap between gates with source–drain contacts. In this work, the overlaps of the contacts are kept the same for all devices.

Figure 7

(a) Resistance vs channel length graph for the devices having varying dielectrics and (b) IDS vs channel length graph for devices having various dielectrics of studied channel length (L) 25, 50, 100, and 190 μm.

Table 5

Summary of Contact Resistances Calculated for the OFETs Fabricated on Different Dielectrics Al2O3/PVA, Al2O3/PVA/HMDS, Al2O3/PMMA, and Al2O3/PMMA/HMDS by the Transmission Line Method

sl. no.	dielectric material used	contact resistance (RC/Ω)
1	Al/Al2O3/PVA/Br2PTCDI-C18	7.89 × 108
2	Al/Al2O3/PVA/HMDS/Br2PTCDI-C18	6.22 × 108
3	Al/Al2O3/PMMA/Br2PTCDI-C18	1.98 × 108
4	Al/Al2O3/PMMA/HMDS/Br2PTCDI-C18	1.38 × 108

Bias-Stress and Stability Measurements

To further explore and understand the electrical stability (under vacuum conditions) of these devices, we recorded the bias-stress measurements on these devices having different channel lengths. The devices were seen exhibiting a decay behavior of IDS for 1 h of bias stress under vacuum conditions, as shown in Figure a–d. As the channel length increases, the increase in the decay of current is due to the more number of charge traps present at the higher channel length. For 25 μm channel length, in the case of Al2O3/PVA, the decay in the current was about 45% (Figure a). When the surface of the dielectric is modified by HMDS, the decay reduced to 30% (Figure b) in the case of Al2O3/PVA/HMDS. The additional decay is due to the hydrophilic nature of PVA dielectric material. By contrast, for the devices fabricated with PMMA, the decay in the current significantly reduced to 25% (Figure c), and upon the surface modification of the dielectric with HMDS, it further decreased to below 20% (Figure d). The same trends were observed for all devices fabricated with different channel lengths. Thus, the devices fabricated on Al2O3/PMMA/HMDS with 25 μm channel length showed better stability among all devices reported here.

Figure 8

Bias-stress measurements of the devices fabricated with different dielectric materials: (a) Al2O3/PVA, (b) Al2O3/PVA/HMDS, (c) Al2O3/PMMA, and (d) Al2O3/PMMA/HMDS at different channel lengths of 25, 50, 100, and 190 μm.

Conclusions

In summary, it was observed that the OFETs obtained utilizing oxide gate dielectrics customized with the polymer dielectric materials and thin layer of HMDS significantly improved the electrical behavior as compared with the ones fabricated without the thin HMDS layer. We were successful in demonstrating low-voltage operating Br2PTCDI-C18-based n-type OFETs with improved performance on low-cost glass substrates and systematically studied the effect of the channel length, dielectric material, and HMDS thin layer on the electrical properties of the OFETs with enhanced stability at 25 up to 190 μm channel lengths. In the process, the vital role of HMDS in combination with gate dielectrics Al2O3/PVA and Al2O3/PMMA was successfully demonstrated. As the channel length of the devices decreased, the IDS showed an increase. The devices fabricated on Al2O3/PVA showed better mobility, and Ion/off ratios and the stability of the device significantly increased by HMDS modification even though there is a negligible change in the mobility. The HMDS layer significantly improved the Ion/off ratios and the electrical stability of the device. The electrical stabilities of the devices were tested by stressing the device for 1 h at 10 V. However, the devices fabricated on Al2O3/PMMA/HMDS showed improved stability because of the hydrophobic nature of PMMA.