Letter pubs.acs.org/NanoLett
Imaging Schottky Barriers and Ohmic Contacts in PbS Quantum Dot Devices David B. Strasfeld,† August Dorn,‡ Darcy D. Wanger,† and Moungi G. Bawendi*,† †
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139 Institute for Applied Physics, University of Hamburg, Hamburg, Germany 20355
‡
S Supporting Information *
ABSTRACT: We fabricated planar PbS quantum dot devices with ohmic and Schottky type electrodes and characterized them using scanning photocurrent and photovoltage microscopies. The microscopy techniques used in this investigation allow for interrogation of the lateral depletion width and related photovoltaic properties in the planar Schottky type contacts. Titanium/QD contacts exhibited depletion widths that varied over a wide range as a function of bias voltage, while the gold/QD contacts showed ohmic behavior over the same voltage range. KEYWORDS: Quantum dots, Schottky, ohmic, scanning photocurrent microscopy
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by employing scanning photocurrent microscopy (SPCM), which measures photocurrent as a function of focal point position. SPCM has been used to elucidate the optoelectronic properties of carbon nanotubes,14,15 silicon nanowires,16−18 organic thin films,19 and PbS nanowires.20 We extend this technique to QD solids in order to explore the nature of the QD/metal contacts. In this study we investigate the vast differences between the local photocurrents that arise due to ohmic and Schottky contacts. This method provides a means for imaging changes in the depleted region as a function of source−drain voltage (VSD). Our device architecture results in an equilibrium depletion width that is far larger than what is observed in stacked structures. The setup, as diagrammed in Figure 1a, consists of a 514 nm cw Ar/Ion laser as the excitation source for a confocal microscope system. Experiments were performed with an excitation intensity of 1.5 kW/cm2. A 0.6 NA air objective with a correction collar was used to focus the excitation beam at the sample. This objective gives a calculated diffraction-limited xy diameter of 430 nm and a calculated diffraction-limited depth of field of 1430 nm. For the scanning photocurrent measurements, the source is connected to a voltage supply and the drain is attached to an I−V converter, which is monitored by a digital voltmeter. The laser is rasterscanned over the device with a 200 ms wait time, and the current between the source (ground) and drain (voltage) electrodes is monitored as a function of laser position. By measuring the photoinduced electronic signal and the reflection intensity simultaneously, we obtain ohmic and Schottky type
ead chalcogenide (PbX, X = S, Se) quantum dot (QD) thin films are finding their niche as the active material in photovoltaics1−3 and photodetectors.4−6 Their small bandgaps, which confer absorptivity into the near-infrared region of the spectrum, and their solution processability, which allows for potentially low production costs, make them desirable for device applications. By adjusting the synthesis procedure, the size of the particles can be controlled from approximately 2 to 12 nm, and the bandgap can, therefore, be easily tuned from 0.5 to 1.7 eV. However, before these optoelectronic properties can be fully utilized, device incorporation requires contact with metal electrodes, whether ohmic- or Schottky-type. Such contacts have been studied extensively for traditional semiconductors7 due to their importance in numerous device modalities. More recent studies are aiming to understand how these contacts form in devices containing granular semiconductors, such as quantum dots.3,8,9 The built-in field that accompanies a Schottky type contact can be used as the charge extraction mechanism in converting excitons into electric energy, a process that serves as the basis for Schottky solar cells2,3 and some photodetectors.10 Schottky barriers arise for p-type semiconductors when the work function of the metal relative to vacuum, ϕw is less than the semiconductor Fermi level, EF. The built-in electric field generated with Fermi level equilibration has provided the functionality for numerous stacked PbS QD devices.2,3,11,12 These devices are generally formed by depositing a QD film onto patterned ITO, followed by evaporation of the low workfunction metal electrode. We study planar Schottky and ohmic PbS QD devices with channel lengths two orders of magnitude larger than the depletion widths that have been reported in stacked device studies.3,12,13 Such dimensions allow for direct visualization of the photocurrent. We achieve such visualization © 2012 American Chemical Society
Received: August 23, 2011 Revised: January 6, 2012 Published: January 17, 2012 569
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Ti device, and 75 nm for the Au/Ti device), and to ensure comparable deposition and morphology. The lateral dimensions are 20 times larger than the focal spot size, while the vertical dimensions are 10 times smaller than the depth of field and are therefore probed equally during lateral scanning. The AFM images along with more details about the PbS QD synthesis, device preparation, and experimental setup are available in the Supporting Information. In our devices, Au serves as a large-work-function metal (ϕw,Au = −5.2 eV) and we expect ohmic behavior when it is in contact with a PbS QD thin film. Titanium serves as a small work-function metal (ϕw,Ti = −4.3 eV), and we anticipate Schottky behavior due to the inherent p-type nature of the PbS QDs, which will shift the Fermi level toward the valence band and away from ϕw,Ti. Titanium was chosen due to the conductivity and low work function of its native oxide.21−23 The Ti, PbS QD, and Au energy bands are plotted relative to vacuum in Figure 2a. The valence and conduction band energy levels relative to vacuum are from Hyun et al.24 for 4.4 nm diameter PbS QDs. The band bending that generates a Schottky barrier when Ti and the PbS QD thin film come into contact is depicted in Figure 2b. The barrier height for an ideal p-type semiconductor in contact with a metal is ϕB ≈ EVB − ϕW, where ϕW is the metal work function and EVB is the valence band energy relative to vacuum (ϕB = 0.7 eV). Minimal bending is expected for the Au/PbS QD contact7 (Figure 2b), and the current will, therefore, be determined by the bulk resistance of the QD film. Holes, the dominant free carrier in the p-type PbS QD solid, migrate into the Ti as electrons migrate out to achieve Fermi level equilibration. With equilibration, an interfacial region depleted of mobile carriers and an attending built-in potential, Vbi, are formed. The depleted region is eliminated at a source-drain bias equal to Vbi. The built-in potential dictates the directionality of photogenerated current when our devices are operated as a photovoltaic (VSD = 0 V), with electrons flowing toward and holes flowing away from the Ti electrode. A simulation of the electric field strength25 is shown in the inset of Figure 2c for a Au/Au device with a 100 nm thick PbS QD film. The PbS QD film was parametrized using values from the literature (conductivity = 4 × 10−7 S cm−1 and εr = 17).11,26 This simulation demonstrates the well-known effect of electric field bunching at the electrode edges.27 This field strength allows for efficient exciton dissociation with applied bias even when the incident laser pulse is focused in the center of the channel. The SPCM signal then arises due to absorption and ease of carrier extraction as a function of focal point position. Reflectance and photocurrent images for each of the devices are featured in Figure 3. In the case of the Au/Au electrode device (Figure 2a), the symmetry of the photocurrent map at VSD = 1 V and VSD = −1 V are nearly identical, and there is a global maximum centered in between the two electrodes. The position of the global maximum can be attributed to carrier extraction efficiency, as a finite mean free path for both holes and electrons leads to the shortest transit distance yielding the highest likelihood of extraction. The SPCM images of the Ti/Ti device at VSD = 1 V and −1 V exhibit photocurrent intensity localized at the source electrode for positive bias and at the drain electrode for negative bias. This pronounced bias dependence, the presence of sharp features with dimensionalities comparable to the diffraction limited spot size, and the lack of excitation-intensity-dependence on the signal profile (see Supporting Information) precludes significant contribution
Figure 1. Scanning photocurrent microscopy of PbS QD thin films. (a) A schematic of the microscope setup. A 0.60 NA objective focuses 10 μW of 514 nm light to subdevice dimensions. Each device consists of two metal electrodes and a PbS QD thin film with lateral dimensions defined by photolithography techniques. (b) An absorption spectrum of the n-butylamine capped PbS QDs reveals Eg = 1.0 eV. A representative TEM image is shown in the inset.
photocurrent signatures that are correlated to the electrode positions. The PbS QDs used in this study were prepared as previously reported.1 Briefly, a Pb precursor was degassed and heated before the swift introduction of a sulfur precursor. The resulting PbS QDs were purified by exposure to a mixture of methanol and butanol followed by centrifugation. Ligand exchange took place after dissolving the purified QDs in n-butylamine and stirring for three days. Exchanging the native oleic acid ligands for n-butylamine significantly improves film conductivity.4 The PbS QDs used in this study exhibit a band-edge absorption of 1 eV and a diameter of 4.4 nm (see Figure 1b). Devices were generated by spin-coating a solution of PbS QDs in octane onto 50-nm thick Au/Au, Ti/Ti, or Au/Ti electrode pairs. In order to generate adequate contacts in the channel, a well-defined region of PbS QD thin film was deposited both on top of the electrodes and in the electrode gap. Standard optical lithography techniques, and acetone lift-off following the spin-coating procedure were performed. Atomic force microscopy (AFM) was used to determine the thickness of our PbS QD thin films (90 nm for the Au/Au device, 50 nm for the Ti/ 570
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the focal point is localized in the depleted region, photogenerated free carriers improve the conductivity of the depleted region, leading to the large relative photocurrent enhancement observed in our devices. For both ohmic- and Schottky-type devices, photocurrent extraction relies on the presence of background conductivity in the film, such that the PbS thin film regions on either side of the photoexcitation act as electrodes that complete the circuit. For a film of CdSe/CdS core(shell) QDs, which did not exhibit a background conductivity, no photocurrent was observed.28 The SPCM slices for VSD = −1.6 to 1.6 V through the dotted white lines of the reflectance images in Figure 3 are included in the left column of Figure 4. The images featured in Figure 3 were taken by stepping the piezo-mounted objective in 500 nm increments, while the slices in Figure 4 were generated using a 100 nm step size, which is well below the diffraction limited focal point. The position of the SPCM intensity relative to the electrode edges is monitored with reflectance. The global maxima (or minima for negative photocurrent) are indicated by black circles. The positions of these extrema were determined by finding the y-intercept of the first derivative in the region of the maximum. The extrema are useful metrics for demonstrating the Schottky or ohmic behavior of our devices: in the case of a Schottky contact, the extrema are correlated with the depletion region and should shift as a function of VSD; in the case of an ohmic contact, the lack of a VSD-dependent depletion width should render the symmetry of the photocurrent intensity unchanged as a function of VSD. For the Ti electrodes, which make Schottky-like contacts, the depletion width will increase as VSD increases in magnitude for reverse bias (negative VSD), while in forward bias (positive VSD), the free carriers are forced to the electrode edge and the depleted region is eliminated when the built-in potential is overcome. In the case of the Au/Au device (Figure 4a), the locations of the global extrema remain unchanged as a function of VSD. The lack of a change in symmetry of the SPCM trace with small variations in bias (below those featured in Figure 3) further indicates that the Au/PbS QD contacts are ohmic in nature. The Ti/Ti device (Figure 4b) demonstrates large changes in symmetry at small values of VSD, which is in accordance with the Schottky nature of the Ti/PbS QD contacts. For a negative bias, the position of the maximum shifts toward the drain electrode by 1 μm when going from −1.6 to 0 V. This shift arises again due to changes in the depletion width. At 0 V, the localization of the SPCM extremum shifts from the drain electrode to the source electrode. For positive biases, a shift of 1.4 μm is observed, along with the emergence of a secondary maximum when going from 0 to 1.6 V. In the Ti/Ti device, there are two Schottky contacts with opposing polarity in series. Hence, in order to simplify our analysis of the Schottky-type contact, we fabricated a Au/Ti device. In this case the Au contact is ohmic and we can directly monitor the isolated Schottky-type Ti contact. In the case of a Au/Ti device (Figure 4c), the ridge observed in reverse bias shifts away from the Ti (drain) electrode by 0.7 μm as the magnitude increases from 0.2 to −1.6 V. When going from VSD = 0.2 V to VSD = 0.4 V, the ridge disappears. This indicates that 0.4 V > Vbi > 0.2 V, and that for biases larger than Vbi, the depletion region no longer exists. This value for Vbi agrees with those previously reported for PbS QD thin films.3,11,12 The static position of the extrema in the middle of the channel for forward bias greater than 0.2 V is similar to the Au/Au signal profile, which is indicative of the ohmic contact expected in the absence of a depleted region.
Figure 2. (a) The energy level diagram of the metals and PbS QDs used in this investigation, referenced relative to vacuum. (b) The PbS QD thin film energy bands are shown at the Ti/PbS QD Schottky contact and the Au/PbS QD ohmic contact. In the case of the Ti/PbS QD interface, holes migrate from the PbS QDs into the Ti and electrons migrate in the opposite direction, resulting in bending of the vacuum energy level (Evacuum), the conduction band (ECB) and the valence band (EVB). and Fermi level equilibration shown in the figure. (c) A cross section schematic that includes a simulation of the electric field magnitude for a 100 nm PbS QD film deposited on a Au/Au device with VSD = 1 V.
from induced thermoelectric potential or wave-guiding in the silicon dioxide layer of our devices. Finally, for the Au/Ti device at 1 V forward bias, the photocurrent intensity is centered in between the electrodes, as would be expected for an ohmic contact, while at 1 V reverse bias, the photocurrent intensity shifts toward the Ti electrode. In reverse bias, a noticeable ridge in the photocurrent map (Figure 3c(iii)) appears along the edge of the Ti electrode. This ridge arises due to current enhancement with photoexcitation in the depleted region. The current in our devices can be thought to encounter two resistors in series, where the depleted region represents a highly insulating resistor and the remaining PbS QD thin film in the channel represents a highly conductive resistor. When the excitation beam is focused in the center of the channel for a Schottky type device, the current flow is limited by the highly resistive depleted region. However, when 571
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Figure 3. Reflectance and scanning photocurrent images of the Au/Au (a), Ti/Ti (b), and Au/Ti (c) devices. For each image the source electrode is on the left and the drain electrode is on the right. Reflectance images are featured in column (i), scanning photocurrent images taken at VSD = 1 V are featured in column (ii), and scanning photocurrent images taken at VSD = −1 V are featured in column (iii). Black dotted lines indicate the position of the electrodes for each of the scanning photocurrent images. The white dotted lines in the reflectance images indicate the positions of the slices in Figure 4. The insets in (ii) and (iii) are band diagrams indicating the shifts that arise due to the corresponding applied bias.
order to quantify the distance between the electrode edge and the electric field onset, we deconvolute both the reflection image and the JSC signal with a Gaussian point-spread function representing the focal point diameter. We subsequently fit the JSC signal in the region of the electrode edge to a line, and take the x-intercept as the signal turn-on (Figure 5b). This approach yields a value of 1.8 μm for the lateral depletion width, though SPCM signal that deviates from ideal Schottky behavior extends further into the channel. This value is independent of incident laser intensity and, therefore, cannot be attributed to plasmonic effects. A schematic of the cross-section of the devices used in this investigation is featured in Figure 5b. Since PbS QDs are deposited both on top of and in-between the metal electrodes, we expect both vertical depletion, DV, and lateral depletion, DL. Full vertical depletion should be achieved regardless of bias, because the PbS QD film thicknesses were kept below measured equilibrium depletion widths on top of the electrode. Full vertical depletion of the QD film above the electrode may enhance DL as the metal recruits free carriers from the channel for Fermi-level equilibration and decreases free carrier concentration (NA) at the electrode edge. The depletion width is proportional to NA−1/2 and therefore increases as NA is diminished. Additional depletion enhancement likely arises due to equilibration with surface state charges of the SiO2 layer in our device,30,31 which could lead to coupling between DL and a vertical depletion region in the channel. This offers an explanation for the sizable difference between the depletion widths observed for stacked devices3,12 and the depletion width observed in this investigation. Scanning open-circuit voltage (VOC) images for each device are featured in Figure 6. The absence of a built-in field in the
Photocurrent and dark-current traces further indicate the Schottky versus ohmic nature of our devices. The 10−15 μm channel lengths and high contact areas used in this experiment lead to I−V curves dominated by bulk conductivity. The large channel lengths increase series resistance, yielding more ohmiclike I−V curves.29 In Figure 4(b,d,f) dark I−V curves for each of the devices are plotted with photocurrent intensities at each of the electrode edges. For the Au/Au device, each trace is linear, as is observed for ohmic junctions. Each current trace is qualitatively similar to two Schottky diodes with opposing polarity in series. For the dark current trace, the point of inflection is centered at VSD = 0, while it is shifted for the SPCM traces depending on which electrode edge is being illuminated. The Au/Ti traces also exhibit a point of inflection near VSD = 0 V; however, in forward bias, the upward concavity is minimal and suggests an ohmic contact. Differences in intensity for reverse bias again indicate that the depleted region is localized at the drain electrode. Weak relative photocurrent intensity is observed for the Au/Ti device due to the ratio of channel length to width for this device (see Supporting Information). At VSD = 0 V, the SPCM signal can be exclusively attributed to carrier extraction induced by the built-in electric field. For a perfect Schottky contact, the built-in electric field decreases linearly to zero in going from the metal/semiconductor interface to the edge of the depletion region. Hence, the onset of the short-circuit current (JSC) signal indicates the location of the edge of the depleted region. In Figure 5a, we plot the JSC signal as a function of focal point position along with the corresponding reflectivity trace for two different Au/Ti devices. It is clear that the JSC signal onset occurs well before the electrode edge, as determined by the reflection image. In 572
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Figure 5. (a) Reflectance and JSC images for two different Au/Ti PbS QD devices. (b) Zoom in on the deconvoluted JSCA trace and linear fit to this trace at the electrode edge. (c) A diagram of the cross-section of the planar devices used in this investigation. I, indicates the thickness of the PbS QD thin film, DV is the vertical depletion width, DL is the lateral depletion width, and DSiO2 is a vertical depletion region that arises due to charging at the SiO2/QD interface.
Figure 4. Scanning photocurrent slices are shown as a function of VSD from +1.6 to −1.6 V for the Au/Au (a), Ti/Ti (c) and Au/Ti (e) devices. The local maximum for each slice is indicated by a circle. The source electrode is on the left and the drain electrode is on the right for panels a, c, and e. Additionally, dark I−V curves are plotted along with photocurrent intensities at the electrode edges as a function of VSD (b,d,f). In order to demonstrate the elimination of the ridge that indicates the depleted region we magnified VSD = 0.0−0.6 V and background subtracted to maximize contrast for the Au/Ti device.
the frequency dependence of this EQE measurement can be found in the Supporting Information. The spatial dependence of both the JSC (Figure 5a) and the VOC suggest that power density can be influenced by device geometry. Such signal depedence on position relative to the point of carrier collection is analogous to what is exploited in nanowire-based solar cells32−34 and bulk-heterojunction type devices35,36 to improve exciton collection efficiency. In conclusion, we have generated planar PbS QD devices and characterized the nature of their metal/semiconductor contacts using SPCM. This study has resulted in the direct visualization of the equilibrium depletion width and changes in that depletion width as a function of bias. The shape of the SPCM signal is independent of VSD for symmetric ohmic contacts, while the magnitude scales linearly with changes in VSD. In the case of Schottky contacts, the shape of the SPCM signal changes with applied bias, while the magnitude scales with Schottky-type behavior. The observed changes in signal profile arise due to the bias dependence of the depleted region. The depletion region vanishes for biases above 0.2 V and below 0.4 V, which indicates an equilibrium built in voltage that lies in that range. This value agrees with those previously reported for Vbi in stacked devices.3,12 The bandbending observed in our devices takes place on micrometer scales, despite the fact that the QDs are encapsulated by organic ligands and carrier equilibration must take place via a hopping or tunneling mechanism. These ligands and QD surface states can also lead
Au/Au device results in the absence of a photovoltage signal. This lack of voltage indicates that our signal arises due to photogenerated carriers and not a photothermoelectric effect. For the Ti containing devices, a photovoltage signal appears when the laser focus is centered above the Ti electrodes. In treating the PbS QD thin film as a homogeneous, p-type semiconductor with holes as the dominant carrier, we expect a negative voltage when the excitation source is localized above the voltage-monitored lead, and a positive voltage above the grounded electrode due to the change in polarity. This is due to the built-in-potential-mediated direction of carrier diffusion. In the case of the Ti/Ti device, a maximum VOC of 0.78 mV is observed when the focal point is localized near the grounded electrode edge and a minimum VOC of −1.23 mV is observed at the voltage-monitored electrode edge. In the Au/Ti device, a minimum VOC of −0.46 mV is observed near the edge of the voltage-monitored Ti electrode. The slices featured in the insets indicate a steeper rise in VOC when approaching the electrode edge for the Au/Ti device than for the Ti/Ti device. In Figure 5a, the unnormalized JSC signal can be scaled by absorption and photon flux to give external quantum efficiency (EQE). The EQE maximum occurs at the Ti electrode edge and decreases monotonically in going away from the edge. A full discussion of 573
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ASSOCIATED CONTENT
S Supporting Information *
Detailed information concerning PbS QD synthesis, device fabrication, and the experimental setup. Control experiments to characterize the effect of incident light intensity and frequency are discussed. Contribution to the SPCM signal due to gain is also discussed. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
ACKNOWLEDGMENTS The authors thank Vladimir Bulovic, Jennifer Scherer, Tim Osedach, and Jesus del Alamo for helpful discussions. D.B.S. was funded during this work as part of the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001088. D.D.W. gratefully acknowledges support from the Fannie and John Hertz Foundation Fellowship. This work made use of the NSF-funded Harvard NSEC (DMR-D213282) and MIT MRSEC (DMR-0819762) shared exeprimental facilities.
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REFERENCES
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Figure 6. Scanning photovoltage images are shown for the Au/Au (a), Ti/Ti (b), and Au/Ti (c) devices. Black dotted lines indicate the position of the electrodes for each image. Insets are slices taken through the white dotted lines in Figure 2 and are included to further illustrate photovoltage intensity relative to electrode position. The distance and voltage scales for the insets are the same as the images.
to photoconductive gain, especially in the case of an ohmic-type contact, for which photogenerated holes can pass through the circuit numerous times for each trapped photoelectron.37 A full discussion of photoconductive gain is included in the Supporting Information. Hysteresis that arises due to photoconductive gain does not affect our interpretation of the data. Our methods have also resulted in direct visualization of the spatially dependent EQE and VOC, offering a means for device optimization beyond the constraints of traditional stacked architectures. Additionally, planar geometries eliminate loss due to reflection and absorption from a top contact, as well as loss due to a reflection node in the depleted region. This study can also be extended to characterize the nature of QD/metal contacts in stacked devices. Heterogeneous defect regions of ohmic type contacts in such devices can be characterized using SPCM, as has been demonstrated previously.38 Finally, we have generated photocurrent images of Schottky and ohmic type contacts made between a metal and granular semiconductor media, directly imaging the spatial effects of Fermi level equilibration in the process. 574
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