Letter pubs.acs.org/NanoLett
Bandlike Transport in Strongly Coupled and Doped Quantum Dot Solids: A Route to High-Performance Thin-Film Electronics Ji-Hyuk Choi,‡,∥ Aaron T. Fafarman,† Soong Ju Oh,‡ Dong-Kyun Ko,‡ David K. Kim,‡ Benjamin T. Diroll,§ Shin Muramoto,⊥ J. Greg Gillen,⊥ Christopher B. Murray,†,§ and Cherie R. Kagan*,†,‡,§ †
Department of Electrical and Systems Engineering, ‡Department of Materials Science and Engineering, and §Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ∥ Complex Assemblies of Soft Matter, CNRS-Rhodia-UPenn UMI 3254, Bristol, Pennsylvania 19007, United States ⊥ National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States S Supporting Information *
ABSTRACT: We report bandlike transport in solutiondeposited, CdSe QD thin-films with room temperature fieldeffect mobilities for electrons of 27 cm2/(V s). A concomitant shift and broadening in the QD solid optical absorption compared to that of dispersed samples is consistent with electron delocalization and measured electron mobilities. Annealing indium contacts allows for thermal diffusion and doping of the QD thin-films, shifting the Fermi energy, filling traps, and providing access to the bands. Temperaturedependent measurements show bandlike transport to 220 K on a SiO2 gate insulator that is extended to 140 K by reducing the interface trap density using an Al2O3/SiO2 gate insulator. The use of compact ligands and doping provides a pathway to high performance, solution-deposited QD electronics and optoelectronics. KEYWORDS: Quantum dots, band-transport, field-effect transistor, cadmium-selenide, doping, thermal diffusion, thiocyanate
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QDs for strong coupling,13,14 and (iii) control of the free carrier concentration through surface trap passivation and doping.15,16 (i) Continued advancements in colloidal QD synthesis provide a wide range of semiconductor QDs that are monodisperse to within atomic roughness (σ < 5%). (ii) A number of compact molecules have been investigated as ligands to reduce interdot spacing.9 However, carrier mobilities in QD thin-films have been typified by values of 10−4 − 1 cm2/(V s),17,18 which are well below the mobility limit of extended state transport19 and indicative of thermally activated hopping between individual QDs with Coulombic charging energies.17 Only very recently has this hurdle been overcome as we, and others, have debuted new, compact ligands with which carriers in QD solids transport with mobilities >1 cm2/(V s).20−23 Such high mobilities alone strongly suggest the presence of extended states involving multiple QDs. (iii) Because of the high QD surface-to-volume ratio, large concentrations of surface states pose a significant problem especially in wide bandgap QDs, for example, CdX (X = S, Se,
he size dependence of the electronic and optical properties of nanocrystalline materials or quantum dots (QDs) offers new opportunities to control materials beyond just that of chemical composition.1,2 Over a decade ago, it was demonstrated that as semiconductor QDs are brought into close proximity, their quantum-confined electron and hole wave functions overlap and hybridize to form a delocalized, extended state.3,4 If strong interdot coupling could be combined with self-organization of colloidal QDs into arrays,5−8 it would enable a new era of electronic materials by design, “artificial solids” that retain the unique size tunable properties of the QD building blocks while exhibiting the high carrier mobilities and conductivities characteristic of bulk semiconductors. The quest to realize these artificial solids is motivated by the drive to understand the fundamental physics of coupling between QD building blocks and the tremendous technological applications for the low-cost, large-area solution-based fabrication of electronic materials possible using colloidal QDs. To realize strongly coupled, high mobility, and conductivity QD materials requires9,10 (i) colloidal synthesis of monodisperse QD building blocks11,12 to minimize the inhomogeneous distribution in QD energy levels or site-to-site dispersion, (ii) replacement of the bulky, insulating hydrocarbons used as ligands during synthesis with compact ligands to allow proximal © XXXX American Chemical Society
Received: March 21, 2012 Revised: April 11, 2012
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Figure 1. CdSe QD thin-film transistor with In/Au top contact. (a) Photograph of thiocyanate-exchanged CdSe QD films spin-cast onto 250 nm SiO2/n + Si substrates. The film thickness was varied from 25 (left) to 130 nm (right). (b) Output characteristics and (c) transfer characteristics with different VDS (0.1 to 50 V) of a 24 ± 4 nm thick CdSe QD transistor annealed at 250 °C for 10 min with channel length L = 180 μm, width W = 1800 μm. The inset shows a schematic of the device structure. The errors are based on the standard deviation of multiple measurements.
structural, spectroscopic, and electronic measurements support the conclusion that carrier transport in these films occurs through extended electronic states, that is, states not localized on a single QD but extended over multiple QDs. In this respect, these glassy quantum dot films resemble the thoroughly studied example of noncrystalline semiconductors, where so-called “bandlike” transport, leads to high mobilities.32,33 Thin-films of CdSe QDs were prepared by spin-casting monodisperse, thiocyanate-exchanged CdSe QDs dispersed in dimethylformamide (DMF) on SiO2/Si substrates, which serve as the gate dielectric and gate, respectively, in FETs or alternatively on Al2O3/SiO2/Si substrates where Al2O3/SiO2 is a gate dielectric stack. The thickness of the films is readily controlled by tailoring the QD concentration in the dispersion, as shown in photographs of spin-cast films in Figure 1a. The film thickness’ were varied from 25 nm (6 monolayers, rootmean-square roughness of 0.5 nm) to 130 nm (from left to right) as characterized by atomic force microscopy (Supporting Information Figure S1). In/Au electrodes were thermally deposited on the QD films to complete back-gate/top-contact FETs. Figure 1 shows representative (b) output (ID−VDS) and (c) transfer (ID−VG) characteristics of devices annealed at 250 °C for 10 min (annealing temperature optimization described in Supporting Information Figure S2). The drain current is enhanced by a positive gate field showing n-type modulation. The linear ID− VDS relationship in the low bias region and vanishing normalized contact resistances extracted using the transmission
Te), where low intrinsic carrier concentrations can be outnumbered by high trap densities. Many researchers have sought to increase the carrier concentration through doping. Approaches have included thermal evaporation of potassium metal,24,25 electrochemical doping,24 “remote doping” using hydrazine16 as a ligand, and substitutional doping with impurity atoms during synthesis.26−29 Nonetheless, doping remains a significant challenge, and the high-performance electronic QD solids expected in the presence of both high carrier concentration and strong coupling have remained elusive. Here, we demonstrate high-performance CdSe QD thin-film field-effect transistors (FETs). We synthesize highly monodisperse CdSe QD samples and replace the long-chain organic ligands on the surface of the QDs by treatment with ammonium thiocyanate. Thiocyanate is an abundant, noncorrosive, compact, inorganic ligand, capable of preserving colloidal dispersibility of semiconductor QDs.21 In a single-step, low-temperature, solution-based process, we form highly uniform, crack-free, randomly close-packed QD thin-films. We achieve doping of the QD film through a new method, never applied previously to QDs, of facile thermal diffusion of a dopant metal, in this case, indium (In). This approach enables the low-cost, wide-area fabrication of QD thin-films in a process compatible with plastic substrates30,31 and transferrable to large-scale industrial fabrication modalities, such as spraycasting and roll-to-roll printing. The devices fabricated through the synergy of strong coupling and doping show performance metrics better not only than the state-of-the-art for QD FETs, but exceed those of amorphous silicon FETs. A combination of B
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Figure 2. Spectroscopic and structural characterization of CdSe QD thin-films. (a) Absorption spectrum of thiocyanate-capped CdSe QDs dipsersed in DMF and diffuse reflectance spectra of spin-cast QD thin-films as-cast and annealed at temperatures between 100 to 300 °C. (inset) Broadening (black) and redshift (red) in the 1Shole − 1Selectron transition as a function of annealing temperature, relative to the QD dispersion in DMF. (b) Grazing-angle total internal reflectance infrared spectra of CdSe QD films as a function of annealing temperature between 100 to 300 °C. (c) Transmission small-angle X-ray scattering of CdSe QD thin-films pre-exchange with TOPO ligands, after exchange with thiocyanate, and subsequently annealed at temperatures from 100 to 350 °C. Inset SEM image of a CdSe QD film annealed at 250 °C.
the films as oscillator strength is concentrated in QDs compared to bulk,40 and the continued evolution of the absorption features at the higher annealing temperature of 300 °C (magenta). To understand the chemical and structural effects of annealing on the QD films, we monitored the infrared absorption spectra and the transmission small-angle X-ray scattering (tSAXS) and wide-angle X-ray scattering (WAXS) spectra. Figure 2b shows the signature absorption band of the CN stretch from the thiocyanate ligand (full mid-infrared spectra in Supporting Information Figure S5a). After annealing, there is a reduction in CN stretch intensity, consistent with the well-documented decomposition of metal-SCN bonds around 200 °C, leaving a metal sulfide.41 Figure 2c, shows tSAXS for CdSe QD films, spin-cast from the QD dispersion assynthesized, and after exchange with the thiocyanate ligand. A strong reflection due to nearest-neighbor interactions in the randomly close-packed films is seen at 0.117 Å−1 in the preexchanged sample and 0.134 Å−1 after exchange, corresponding to a reduction in interdot spacing from 1.5 to 0.5 nm. With increasing annealing temperature, a steady decrease is observed in the intensity of this reflection, without any change in peak position. A strong reflection is still seen (blue) after 250 °C annealing. To test whether Ostwald ripening of the QDs was responsible for any of the broadening of the optical bands (Figure 2a), we acquired WAXS data and saw no evidence for changes in grain size with annealing by 250 °C, which is in contrast to the changes seen upon annealing to 400 °C (Supporting Information Figure S5b). The structural and chemical probes are consistent with a reduction in thiocyanate coverage at elevated temperature without a change in QD size, leading to a polydispersity in interparticle spacing, ranging from ∼0.5 nm down to touching or near-touching for a subset of particles under the conditions used to fabricate FETs. SEM images of annealed films (inset in Figure 2c) strongly support the evidence from UV−vis, tSAXS, and WAXS that these films consist of randomly close-packed, individual particles. We note
line method34,35 (Supporting Information Figure S3) confirms that Ohmic contacts are formed. The field-effect mobility for electrons extracted from ID−VG curves in the linear regime (VDS = 0.1 V) is 19 cm2/(V s) and in the saturation regime is similarly 20 cm2/(V s), implying negligible mobility dependence on the drain-source electric field. Field-dependent mobility is characteristic of hopping transfer between localized sites of different energies in disordered semiconductors.36−38The high carrier mobilities, larger than the Mott− Ioffe−Regel limit,19,33 and negligible VDS dependence of QD devices, summarized in Supporting Information Figure S4, are a strong indication of transport through extended states where the carrier mean free path is larger than the interdot distance. For 20 devices measured, the electron mobility ranged from 13 to 22 cm2/(V s) which is orders of magnitude higher than what has typified QD FETs. Mobility is only one metric in which these devices excel: the devices exhibit a high ION/IOFF >106 and a subthreshold swing (S−1) of ∼3 V/dec at VDS = 5 V and very low hysteresis (threshold voltage shift ΔVth of ∼7 V) compared to previous QD FETs (Supporting Information Figure S4). Additional support that the exemplary performance observed in FETs is due to extended states in the QD solid, comes from optical absorption spectra (Figure 2a). The absorption of thiocyanate-exchanged CdSe QDs dispersed in DMF (black) shows discrete resonances characteristic of excitations between quantum-confined hole and electron states. After spin-casting the solution-dispersed QDs into a thin-film, the diffuse reflectance spectrum (red) was collected. Upon film formation, the 1Shole − 1Selectron excitation peak downshifts and broadens in the film by 15 and 6 meV (from a peak max at 2149 meV and fwhm of 80 meV, respectively, in DMF)39 and much more dramatically upon annealing at 250 °C (blue), by 120 and 180 meV, respectively (Figure 2a, inset). Quantum confinement is still evident upon annealing to 250 °C (blue) as demonstrated by the residual excitonic structure in absorption, the band gap being larger than the bulk value, the high per-cm absorptivity of C
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Figure 3. Characterization of In thermal diffusion by SIMS and electrical measurements. (a) Lateral distribution of 113In+ as a function of depth (left) preannealed and (right) postannealed sample. The inset shows a schematic of the measured 100 μm × 100 μm sample area. (b) Depth profile of the 113 + In distribution for preannealed and postannealed samples. (c) Conductivity of spin-coated 40 nm thick CdSe films on quartz with In (20 nm)/Au (30 nm) and Al electrodes as a function of annealing temperature. Inset shows the conductivity of CdSe QD films annealed at 250 °C for 10 min as function of indium thickness. (d) Capacitance of CdSe QD films as function of gate-voltage for (In/Au)/CdSe QD film/SiO2 (250 nm)/ n+ Si structure at a temperature from 200 to 250 °C.
quantum confined wave functions of two near-neighbors. For packing densities ranging from 6 to 12 near-neighbors, β = We/ 12 to We/16 respectively,49 yielding β = 6−8 meV under the conditions of FET fabrication. Using β, we calculate a minimum rate of carrier transfer from one QD to another of equal energy Γ: β = hΓ/4. And with the Einstein−Smoluchowski relationship, we calculate the mobility, μ = Γea2/kBT, where e is the elementary charge, kB is the Boltzmann constant, a is the center-to-center distance between QDs, and T is the temperature. We calculate a mobility as high as 50 cm2/(V s), which is in qualitative agreement with our high FET mobilities, providing support for the use of the spectroscopically approximated bandwidth. The existence of a band does not guarantee that carriers will have access to the band at room temperature. CdSe QD FETs fabricated with similar workfunction Al electrodes and processed under the same film and annealing conditions show orders of magnitude lower electron mobilities of 10−2 to 10−1 cm2/(V s), higher Vth and larger hysteresis (Supporting Information Figure S6). Dangling Cd and Se bonds at the QD surface are known to give rise to shallow and midgap trap states50 that are expected to limit carrier mobility and give rise to device hysteresis, especially as the passivating thiocyanate ligands decompose. We show that access to the QD solid bands is achieved in our FETs through doping the film by thermal diffusion of a low temperature, high diffusivity metal such as In.
that the resolution of each of these techniques is insufficient to rule out the possibility of interparticle necking. In light of this structural data, the majority of the increase in fwhm, (Figure 2a, inset), is attributed to enhanced interparticle coupling, with at most a small contribution from possible necking. Broadening and redshifting of a similar magnitude is also observed when very small and thus highly quantum confined InP, CdS, or CdSe QDs are condensed into thin films.42−45 Whether for the literature example of very small, highly confined QDs, or the present example of strongly coupled QDs, these changes are readily understood by analogy to the formation of delocalized states from atomic orbitals in the traditional tight-binding model of semiconductor solids: the coupling between local wave functions on single QDs gives rise to new delocalized states of the QD solid, that is, a band.46,47 The measured optical transition is between bands formed from 1Shole and 1Selectron states, and therefore the transition line width depends on the bandwidths, Wh and We, respectively. In addition the width depends on the homogeneous and inhomogeneous broadening in the QD ensemble, described by W0, and modeled using the 80 meV fwhm of the dispersion of isolated QDs. For 250 °C annealing, where the fwhm is 260 meV, We is therefore estimated by We = (fwhm − W0)/2 = 90 meV.48 Under these assumptions, the tight-binding model relates We to the interparticle coupling energy, β = ⟨φa|H|φb⟩, where φi corresponds to the isolated electron- (or hole-) D
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Figure 4. Temperature-dependent characteristic of devices with SiO2 and Al2O3/SiO2 gate insulators and band diagram depicting charge transport. (a) Transfer characteristics of CdSe QD transistors with channel length L = 180 μm, width W = 1800 μm, and Al2O3(20 nm)/SiO2(250 nm) gate dielectric stack at drain-voltages, VDS of 0.1 V. The inset shows the output characteristics of the CdSe QD transistor. (b) Temperature-dependent mobility of devices with SiO2 and Al2O3/SiO2 gate insulators. (c) Electronic structure depicting isolated, annealed, and In-doped CdSe QD films. The width of the 1Selectron state (red) and the bandwidth (purple) are evaluated as a function of annealing temperature from optical absorption measurements and (inset) schematically shown to give rise to wave functions extending over multiple QDs with energies that fall within the developed band. Trap states (green) are drawn schematically to approximate unpassivated QD surface states and QD/dielectric interface states that tail into the energy gap. Increasing In doping by thermal diffusion raises the Fermi energy (EF) and fills traps within the band gap.
Beyond the stark difference in performance between In- versus Al-based electrodes, intentional changes in device performance can be achieved with subtle variation in the In electrode, further pointing to its role in doping. For example, devices with thicker In electrodes or longer annealing times showed higher off currents and a negative Vth shift (Supporting Information
The incorporation of metals (indium, tin, gallium, etc.) as ntype dopants in CdSe has been studied in thin-films51,52 and nanowires.53 In polycrystalline CdSe thin-films, thermal diffusion of In metal occurs primarily through grain boundaries, neutralizing trap states associated with dangling bonds.51,54,55 This is also expected to occur in CdSe QDs where In may saturate surface states and facilitate a shift in EF through doping. E
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with SiO2 (1.36 × 1013 cm−2) consistent with the higher mobility, lower hysteresis, and low S−1. While mobility values above the Mott−Ioffe−Regel limit and negligible VDS dependence as well as the dramatic redshifts and broadening observed spectroscopically all support the existence of extended or bandlike states, we further test this model by acquiring the temperature dependence of mobility for CdSe QD FETs (Figure 4b). With a SiO2 gate dielectric layer, we observe increases in both linear and saturation mobility with decreasing temperature for 220 K < T < 300 K, which is the hallmark of band transport, and directly contravenes a thermally activated hopping mechanism. In region II (77 K < T < 220 K), carrier transport is governed by shallow traps with a small activation energy of 7.5 meV. For devices with an Al2O3/SiO2 dielectric stack, the region of negative slope extends further from room temperature to 140 K with lower activation energy of 6.2 meV (Region II). This suggests that devices with SiO2 introduce a larger density of interfacial trap states. Thus, traps at the QD/dielectric interface play a crucial role in QD FETs, governing the transition between bandlike and thermally activated transport. In Figure 4c, we present a model for the electronic structure in CdSe QD thin-films giving rise to bandlike transport. Colloidal synthesis provides a narrow inhomogeneous distribution in size (σ < 5%) and site-energy. The compact thiocyanate ligands and mild annealing allows for decreased interdot distance (
