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IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 12, NO. 2, MARCH 2013
Electrical Measurement Under Atmospheric Conditions of PbSe Nanocrystal Thin Films Passivated by Remote Plasma Atomic Layer Deposition of Al2O3 Woojun Yoon, Member, IEEE, Anthony R. Smith, Edward E. Foos, Janice E. Boercker, William B. Heuer, and Joseph G. Tischler
Abstract—PbSe nanocrystal thin-film transistors (TFTs) were passivated using remote plasma atomic layer deposition (ALD) of a ∼10 nm thick Al2 O3 film at 150 ◦ C. By using a highly reactive remote oxygen plasma source, the time for one complete ˚ ALD cycle was about 15 s with growth rates of ∼1.1 A/cycle. The effective mobilities measured under atmospheric condition from Al2 O3 -passivated PbSe nanocrystal TFTs were comparable to the values reported previously for air-free PbSe nanocrystal TFTs, demonstrating that ALD Al2 O3 layers prevent oxidation and degradation of nanocrystal films from air exposure. The variation in the effective mobility of passivated devices was also found to be negligible under ambient conditions over a period of 30 days. The results show that remote plasma ALD processing of Al2 O3 is capable of producing an effective passivation layer on air-sensitive nanocrystals with high deposition rates at reduced temperature. Index Terms—Carrier transport, nanocrystal, PbSe, remote plasma atomic layer deposition (ALD), thin-film transistor (TFT).
I. INTRODUCTION
L
EAD chalcogenide nanostructures, encompassing nanocrystals, nanorods, and nanowires have been of particular interest for their potential applications in electronic and optoelectronic devices. These applications include field-effect transistors [1], [2], photovoltaic devices [3]–[5], photodetectors [6], lasers [7], and thermoelectrics [8]. Although this material system has desirable properties such as optical tunability through the near infrared [9], efficient multiexcition generation [10]–[13], and high carrier mobility relative to other nanocrystal systems [2], [14], carrier mobility and environmental stability still need further improvement.
Manuscript received July 13, 2012; revised October 29, 2012; accepted December 11, 2012. Date of publication December 21, 2012; date of current version March 6, 2013. The work of W. Yoon and A. R. Smith was supported by a National Research Council Research Associateship Award at the U.S. Naval Research Laboratory. The review of this paper was arranged by Associate Editor J. Li. W. Yoon, A. R. Smith, E. E. Foos, J. E. Boercker, and J. G. Tischler are with the Naval Research Laboratory, Washington, DC 20375 USA (e-mail: woojun.
[email protected]). W. B. Heuer is with the U.S. Naval Academy, Annapolis, MD 21401 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2012.2234761
Major improvements to the charge mobility of PbSe nanocrystal films have been reported using either short-chain dithiols or hydrazine for ligand exchange [2], [15]. However, these molecules introduce undesirable instability against oxidation upon exposure to air, degrading the electrical and optical properties of the films [16], [17]. Specifically, PbSe nanocrystal films capped with carboxylic acid functional groups have been shown to improve oxidation resistance while maintaining favorable mobility and conductivity, but the stability against oxidation is still limited only to short-term exposure to air [18], [19]. An alternative approach involving deposition of a thin Al2 O3 film by atomic layer deposition (ALD) has been shown to be effective at preventing degradation of nanostructured materials [20], [21]. Recently Liu et al. utilized low-temperature ALD (below 80 ◦ C) to deposit a thin film of Al2 O3 on PbSe nanocrystal films. They found that the Al2 O3 film prevents oxidation and photothermal damage without significant degradation of carrier mobility [21]. These studies have employed thermal ALD using trimethylaluminum (TMA) as the precursor and water as the oxidant. However, thermal ALD at low temperatures often requires an extensive purge time at each deposition step (typically 120–180 s/cycle) following the water reactant, leading to long overall cycle times [22]. This presents a significant limitation for practical applications. In these studies, the samples were not exposed to air between the film formation and ALD passivation [21], [22], also presenting a significant limitation. In contrast, we propose that the high effective growth rate of remote plasma ALD at low deposition temperatures is a superior choice for nanocrystal passivation. These high growth rates are achieved through the use of a highly reactive remote oxygen plasma as the oxidant, with a low ion energy usually below the threshold for potential plasma damage [23], [24]. In this study, we present electrical measurement in atmospheric conditions of PbSe nanocrystal thin-film transistors (TFTs) passivated using remote plasma ALD of a ∼10 nm thick film of Al2 O3 . PbSe nanocrystal films treated with 1, 2-ethanedithiol (EDT) were used due to high carrier mobility [25], while oxalic acid modified samples were used because of increased stability against oxidation, despite relatively low hole mobility [18], [19]. We found that the effective mobility of the passivated films was usually two orders of magnitude higher than that of nonpassivated films in air. Furthermore,
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YOON et al.: ELECTRICAL MEASUREMENT UNDER ATMOSPHERIC CONDITIONS OF PbSe NANOCRYSTAL THIN FILMS PASSIVATED
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Fig. 1. (a) Schematic of a PbSe nanocrystal TFT encapsulated with an ∼10-nm-thick Al2 O3 film deposited by remote plasma ALD at 150 ◦ C. (b) and (c) Plan view SEM images of the channel and the drain of a PbSe nanocrystal TFT treated with EDT and oxalic acid in acetonitrile, respectively. The top surfaces of both devices are overcoated with a thin Al2 O3 layer.
Al2 O3 -passivated PbSe nanocrystal films showed negligible degradation in the effective mobility under ambient conditions over a period of 30 days. II. EXPERIMENTAL DETAILS PbSe nanocrystal TFTs were fabricated and characterized in air. The devices were built as shown in Fig. 1(a) and consisted of a heavily doped n-type silicon substrate as the gate electrode, a gate dielectric, and source/drain electrodes. For the gate dielectric, HfO2 films were deposited by remote plasma ALD at 250 ◦ C on silicon substrates. The high-frequency capacitancevoltage response clearly showed accumulation at the gate voltage VG of −5 V and depletion regions at VG = +5 V (not shown). The source and drain contacts were formed by standard photolithography and liftoff of Au (100 nm) with a 15-nm Ti adhesive layer atop of the HfO2 layer. The channel length L of the devices was 10 µm, while the channel width W was 1000 µm. After photolithography and metal liftoff, the patterned substrates were cleaned with oxygen plasma for 1 min. Thin films of PbSe nanocrystals (∼6.5-nm diameter) synthesized by solution methods [26] were then deposited using a layer-by-layer dip coating method [15], [19]. Inert atmosphere was maintained except for a brief exposure to air during isolation and reprecipitation of the oleic acid-capped nanocrystals. The patterned substrates were dipped in a 5 mg/mL solution of PbSe nanocrystals in chloroform, followed by immersion in either a 0.1 mM EDT in acetonitrile solution or a 0.5 mM oxalic acid in acetonitrile solution to replace the insulating oleic acid. In the case of the oxalic acid, pure acetonitrile and chloroform were used to rinse the films after each cycle to remove any oxalic acid residue across the substrates. This dipping sequence was repeated a total of ten times per device to yield ∼35-nm-thick PbSe films as measured by profilometry and scanning electron microscope (SEM). All layer-by-layer dip-coating procedures were performed in a nitrogen-filled glove bag to prevent oxidation. After fabrication, the devices were held in sealed contain-
Fig. 2. Output characteristics (ID − V D ) in air for (a) EDT- and (b) oxalicacid-treated PbSe nanocrystal TFTs with Al2 O3 and without Al2 O3 film immediately after exposure to air. At low drain voltages of ID − V D , the drain conductance gd was estimated from the output curves of (c) EDT- and (d) oxalic-acid-treated PbSe nanocrystal devices with Al2 O3 and without Al2 O3 film, respectively. For these devices, the channel length L and width W were 10 and 1000 µm, respectively.
ers under N2 atmosphere until transferred to the ALD chamber which resulted in a brief exposure to air. Finally, a thin Al2 O3 film (∼10 nm) was added atop the PbSe nanocrystal TFTs by sequential exposure to the TMA precursor and oxygen plasma using remote plasma ALD (FlexAL, Oxford Instruments Inc.). During the ALD process at 150 ◦ C, a constant 60 sccm oxygen flow and pressure of 15 mTorr were maintained in the chamber. The TMA dose and purge time were 30 ms and 6 s, respectively. The inductively coupled remote oxygen plasma was used at an rf power of 400 W for 2 s with a plasma purge time of 3 s. The time for one complete cycle was ∼15 s at 150 ◦ C. Electrical characterization of the devices was performed with a semiconductor parameter analyzer (HP 4145B) at room temperature under darkness in air. III. RESULTS AND DISCUSSION Fig. 1(b) and (c) shows plan view SEM images of the channel and the drain overcoated with a thin Al2 O3 films in a PbSe nanocrystal TFT treated with EDT and oxalic acid in acetonitrile, respectively. The deposited Al2 O3 films by 90 cycles of ALD process were found to be smooth, pin-hole free, and conformal. The thickness and the refractive index determined using ˚ and ∼1.63 (at 633 nm), spectroscopic ellisometry were ∼100 A ˚ respectively. ALD growth rate of Al2 O3 was ∼1.1 A/cycle. Fig. 2(a) and (b) shows the output characteristics for PbSe nanocrystal TFTs both with and without Al2 O3 passivation for an applied drain voltage VD ranging from 0 to −5 V, and a VG varying from 0 to −5 V in steps of −1 V. As the gate
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bias was increased to −5 V, the drain current at a fixed VD increased, showing a typical behavior for a p-channel transistor operating in the accumulation region. All devices exhibited relatively large off-state drain currents ID (i.e., ID at VG = 0 V) and no saturation in ID was observed at an elevated VG . However, devices with and without the Al2 O3 passivation layer showed significantly different modulation in ID on initial exposure to air as can be seen in Fig. 2(a) and (b). At positive gate biases zero to +5 V (not shown), the ID at a fixed VD only decreased. For a typical n-channel transistor, an increase in ID should occur under this operating condition [2]. Previously, it has been reported that air-free EDT-treated PbSe nanocrystal TFTs exhibit both n-and p-channel transistor behavior in inert atmosphere but even after brief exposure to air (
