Hybrid Dielectrics Composed of Al2o3 and Phosphonic Acid Self

Jang et al. Nano Convergence (2018) 5:20 https://doi.org/10.1186/s40580-018-0152-3 LETTERS Open Access Hybrid dielectrics composed of Al2O3 and phosphonic acid self‑assembled monolayers for performance improvement in low voltage organic feld efect transistors Sukjae Jang, Dabin Son, Sunbin Hwang, Minji Kang, Seoung‑Ki Lee, Dae‑Young Jeon, Sukang Bae, Sang Hyun Lee, Dong Su Lee and Tae‑Wook Kim* Abstract Low voltage operational organic transistors (< 4 V) based on pentacene were successfully fabricated with hybrid dielectric flms composed of aluminum oxide using atomic layer deposition and various phosphonic acid-based self-assembled monolayers as the gate dielectrics. High capacitances up to 279 nF/cm2, low leakage current densities 8 2 of 10− A/cm at 6 V, and high breakdown felds up to 7.5 MV/cm were obtained. The transistors with the octade‑ cylphosphonic acid hybrid dielectric exhibited an improved saturation mobility of 0.58 cm2/Vs, a subthreshold slope of 151 mV/decade, a threshold voltage of 1.84 V and an on–of current ratio of 106. The low surface energies of the self-assembled monolayers having non-polar− terminal groups, such as methyl and pentafuorophenoxy, improved the carrier conduction of the transistors due to the pentacene growth with an edge-on orientation for low voltage opera‑ tion. The pentafuorophenoxy end-group showed an accumulation of holes at the semiconductor-dielectric interface. Keywords: Hybrid dielectrics, ALD Al2O3, PA-SAM, Phosphonic acid, Water contact angle, Organic transistor Organic feld-efect transistors (OFETs) have great poten- than those of low-k gate dielectrics [7]. High-k dielectric tial for a wide variety of applications, such as fexible dis- flms, such as metal oxides, were typically deposited using plays, electronic paper, radiofrequency identifcation tags atomic layer deposition (ALD) based on the sequen- and sensors, due to their low cost, low temperature fab- tial use of a self-limited surface reaction [8, 9]. Te ALD rication, solution process compatibility, light weight and technique has various advantages, such as precise thick- mechanical fexibility compared with conventional inor- ness control, high quality flms based on layer-by-layer ganic materials [1–4]. Te relative high operating volt- growth, a lower growth temperature (< 350 °C) than that ages of OFETs have hindered their development in the of classical chemical vapor deposition methods, a high commercial market. To implement low voltage operating uniformity, an excellent conformality over high aspect OFETs, signifcant eforts have been devoted to increas- ratio structures, and dense and pinhole free flms [8, 10]. ing the capacitive density of gate dielectrics by decreas- Among the various high-k dielectrics, aluminum oxide ing the thickness of the gate dielectrics or increasing the (Al2O3) is considered as a potential gate dielectric flm of dielectric constant (k) of the dielectric flms [5, 6]. feld efect transistors due to its thermodynamic stabil- High-k dielectric flms can induce greater surface ity on Si up to high temperatures, mechanical robustness charge densities at the semiconductor-dielectric interface and highly insulating properties that are due to its high band gap (8.7 eV) and medium-k value (8–9) [7]. Another strategy for low voltage OFETs is improving *Correspondence: [email protected] Functional Composite Materials Research Center, Institute of Advanced the charge transport in organic semiconductors. Charge Composite Materials, Korea Institute of Science and Technology, transport occurs in the few monolayers near the inter- Wanju‑gun, Jeollabuk‑do 55324, Republic of Korea face between the organic semiconductor/dielectric and © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Jang et al. Nano Convergence (2018) 5:20 Page 2 of 8 is limited by the thermally activated hopping process Terefore, hybrid dielectrics composed of PA-SAMs between the molecules in the disordered regions [11, on a high-k dielectric are excellent candidates for OFET 12]. To improve the device performance, the disordered applications [15]. Tey are suitable for gate dielectrics in region of the semiconductor should be suppressed, OFETs due to their low voltage operation, low gate leak- and additional interface treatments are required for age currents and interface modifcation by the functional optimal charge transport. For example, the molecular end-groups of SAMs. In this letter, we introduce various parameters of organic semiconductors were modifed hybrid dielectrics comprised of PA-SAMs on ALD Al 2O3 by controlling their regioregularity, molecular weight, and further investigate the relationship between various side chain length, doping level and end-group. Inter- functional end-groups of PA-SAMs and the device per- face engineering has been introduced to improve the formances of OFETs. molecular ordering, orientation, assembly, packing and Figure 1 illustrates the schematic structure of an flm morphology of organic semiconductors [13]. Self- OFET with a hybrid dielectric consisting of ALD Al 2O3 assembled monolayers (SAMs) are good candidates for and PA-SAMs and molecular structures of various PA- efective surface treatment due to their high-packing SAMs with diferent functional end-groups. We pre- density, being only a few nm thick and allowing inter- pared seven kinds of PA-SAMs to investigate the efects face control with end-functional groups [14]. Among of the alkyl chain length and functional end-groups on various SAMs with diferent binding groups, such as the OFET performance. Hexylphosphonic acid (HPA), carboxyl groups, thiols and silanes, phosphonic acid dodecylphosphonic acid (DDPA), octadecylphosphonic self-assembled monolayers (PA-SAMs) have attracted acid (ODPA), phosphonohexadecanoic acid (PHDA), attention due to readily assembling on activated metal 12-mercaptododecylphosphonic acid (MDPA), 12-pen- oxides because their reaction is not limited by the con- tafuorophenoxydodecylphosphonic acid (PFPA), tents of surface hydroxyl groups and supplies their 11-hydroxyundecylphosphonic acid (HUPA), penta- own hydroxyl moieties [15]. Tey also have the advan- cene (99.9% purity) and isopropanol (99.8% purity) tages of stability and prevention of homocondensation were purchased commercially from Sigma-Aldrich, between phosphonic acid molecules compared with Korea and used as received. HPA (C6), DDPA (C12) other binding groups, resulting in the highly ordered, and ODPA (C18) are methyl-terminated PA-SAMs with dense and robust monolayers of phosphonic acid mol- increasing alkyl chain lengths ranging from C6 to C18. ecules on metal oxide surfaces. PHDA, MDPA, PFPA and HUPA have diferent end Fig. 1 a Schematic structure of an OFET with a hybrid dielectric. b Molecular structures of the various PA‑SAMs used in this study. [1: Hexylphosphonic acid (HPA) (C6), 2: dodecylphosphonic acid (DDPA) (C12), 3: octadecylphosphonic acid (ODPA) (C18), 4: 16‑phosphonohexadecanoic acid (PHDA), 5: 12‑mercaptododecylphosphonic acid (MDPA), 6: 12‑pentafuorophenoxydodecylphosphonic acid (PFPA), 7: 11‑hydroxyundecylphosphonic acid (HUPA)] Jang et al. Nano Convergence (2018) 5:20 Page 3 of 8 functionalized terminals with the carboxyl group, thiol, Phosphonic acid solutions were prepared by dissolving (2,3,4,5,6-pentafuorophenoxy) group and hydroxyl 3 mM of each phosphonic acid in 1 ml of isopropanol. group, respectively (Fig. 1b). Te Al2O3 thin flms were exposed to UV-Ozone for Heavily n-doped bare Si (100) substrates were diced, 30 min to generate enough density of the hydroxyl groups cleaned using ultra-sonication in acetone and isopro- on the surface for the SAM treatment. Te prepared PA- panol for 10 min, and then dried with nitrogen gas. An SAM solutions after fltration with a 0.45 µm PTFE mem- n+-doped bare Si wafer acted as the back-gate electrode brane were immediately spin-coated on the UV-Ozone in the OFETs. Te cleaned substrates were annealed in treated Al2O3 thin flms at a spin rate of 3000 rpm for a convection oven for 1 h to remove residual solvent. 20 s and annealed at 140 °C for 10 min in a nitrogen-flled Al2O3 dielectric flms were deposited on the cleaned glovebox. Te annealed substrates were rinsed using substrates by 200 ALD cycles (Lucida D100 ALD, NCD) ultra-sonication in isopropanol for 10 min to remove any at a process temperature of 200 °C. Trimethylaluminum remaining phosphonic acids except for the self-assem- (TMA, Al(CH3)3) and water were used as the Al and bled monolayer on the surface. O precursors, respectively. Te growth rate per cycle Figure 2a displays images of water droplets on the (GPC) was calculated to be ~ 0.12 nm/cycle. hybrid dielectrics that were modifed with various PA- SAM molecules and the measured water contact angles. Fig. 2 a Water droplets on hybrid dielectrics and measured contact angles, b capacitive densities–frequency (Ci–F) and c leakage current densities– voltage (J–V) of hybrid dielectrics with various PA‑SAMs Jang et al. Nano Convergence (2018) 5:20 Page 4 of 8 Te contact angle of the reference Al 2O3 surface was k measured to be less than 10°, implying a hydrophilic Ci = ε0 ti (1) property. After the formation of PA-SAM molecules on the Al2O3 dielectric, the water contact angles were dra- matically changed depending on the kind of molecules where ε0 is the permittivity of free space and ti is the that were formed. Alkyl-phosphate SAMs and PFPA thickness of the insulator. Te thickness of Al2O3 pro- exhibited a low energy surface with a high contact angle duced by 200 ALD cycles was measured to be ~ 23 nm, (> 100°) due to their methyl and fuorinated functional and the calculated k of the reference Al2O3 was ~ 7.2, groups, respectively [15].

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