boosting responsivity of organic metal oxynitride hybrid … · 2016. 6. 2. · oxynitride (znon),...

Boosting Responsivity of Organic−Metal Oxynitride HybridHeterointerface PhototransistorYou Seung Rim,†,‡,# Kyung-Chul Ok,§,# Yang Michael Yang,†,‡,# Huajun Chen,†,‡ Sang-Hoon Bae,†,‡

Chen Wang,† Yu Huang,† Jin-Seong Park,*,§ and Yang Yang*,†,‡

†Department of Materials Science and Engineering, ‡California NanoSystems Institute, University of California, Los Angeles, LosAngeles, California 90095, United States§The Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Korea

*S Supporting Information

ABSTRACT: Amorphous metal oxides are attractive materials forvarious sensor applications, because of high electrical performanceand easy processing. However, low absorption coefficient, slowphotoresponse, and persistent photoconductivity of amorphousmetal oxide films from the origin of deep-level defects areobstacles to their use as photonic applications. Here, wedemonstrate ultrahigh photoresponsivity of organic−inorganichybrid phototransistors featuring bulk heterojunction polymersand low-bandgap zinc oxynitride. Spontaneous formation ofultrathin zinc oxide on the surface of zinc oxynitride films couldmake an effective band-alignment for electron transfer from thedissociation of excitons in the bulk heterojunction, while holes were blocked by the deep highest occupied molecular orbital levelof zinc oxide. These hybrid structure-based phototransistors are ultrasensitive to broad-bandwidth photons in ultraviolet to near-infrared regions. The detectivity and a linear dynamic range exceeded 1012 Jones and 122.3 dB, respectively.

KEYWORDS: phototransistor, bulk heterojunction, zinc oxynitride, oxide semiconductor, heterointerface, thin-film transistor

■ INTRODUCTIONAmorphous oxide semiconductors (AOSs) have been widelyresearched for their high electron mobility, transparency in thevisible range, and easy deposition for optoelectronic and sensorapplications.1−4 Their unique advantages, when used as channelmaterials in thin-film transistors (TFTs), are their high mobilityand stability in large-scale deposition, and low-temperatureprocessing.5,6 Representative candidates, such as InGaZnO,InZnSnO, and ZnO, have focused on improving mobility andstability.3 However, the elimination of oxygen vacancies (VO) indeep donor levels to improve the stability measured inillumination stress test remains a challenge. The photo-transition from deep-donor-like states (VO) near the valence-band maximum (VBM) to shallow-donor states (VO

+ or VO2+

formed by the photonic ionization of VO) near the conduction-band minimum (CBM) produces an unexpected instability inilluminated devices.7,8 To reduce this photon-acceleratedinstability, high-pressure annealing,7,9 oxygen plasma treat-ment,10 and addition of dopants (i.e., Hf or Zr)11,12 werepreviously proposed. More recently, following the generalstrategy of deactivating oxygen vacancies via the positioning ofdeep level VO region upper the oxygen 2p orbital, the formationof new orbital state by a nonoxide anion yielded a narrowbandgap (∼1.6 eV).13 The proposed TFTs, based on zincoxynitride (ZnON), not only have a high mobility (>50 cm2

V−1 s−1) but also suppress the high persistent photocurrents

(PPC).13−15 In particular, photocarrier relaxation time inZnON is as short as 10 ps, which is due to negligible Augerrecombination, and nanosecond-time- scale relaxation.16 Thesefeatures produce a high photoresponsivity and negligible PPC,as desired for optoelectronic applications.The photosensitivity of large-bandgap AOSs to short-

wavelength photons can be beneficial, e.g., for ultraviolet(UV) detectors. However, large PPC and the slow response ofthe deep oxygen-vacancy defects are also critical problems. Onthe other hand, ZnON has a narrow bandgap and can absorbphotons from the UV to near-infrared (NIR) regions, making itpotentially applicable to phototransistors and image sensors.Here, we report bulk heterojunction (BHJ) polymers and

ZnON hybrid phototransistors in a broad wavelength range(380−940 nm). BHJ structures involving a narrow-bandgappolymer and PC71BM effectively achieve the charge separationof excitons, allowing electrons to transfer to the spontaneousZnO and ZnON semiconductor surface. As a result, thephotosensitivity in hybrid structures is dramatically improved,relative to ZnON phototransistors.Phototransistors have been widely used in interactive

interfaces, control systems, and biological health systems.17−28

Received: March 8, 2016Accepted: May 19, 2016

Research Article

www.acsami.org

© XXXX American Chemical Society A DOI: 10.1021/acsami.6b02814ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

www.acsami.orghttp://dx.doi.org/10.1021/acsami.6b02814

In this perspective, photosensitivity and detectivity are criticalfigures of merit for evaluating photonic devices.29,30 Chargeseparation of electrons and holes in BHJ is accompanied bydissociating excitons.31 Proposed polymers are described inFigure 1a. The BHJ consists of the narrow-bandgap conjugatedpolymer poly[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1d,2-b;3,4-

b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione] (PBDTT-DPP) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). HomogeneousPBDTT-DPP:PC71BM has high absorption spectra in theultraviolet (UV) to near-infrared (NIR) regions and displaysstrong absorption between 650 nm and 850 nm, as shown in

Figure 1. Hybrid BHJ polymers−ZnON phototransistors: (a) chemical structures of PBDTT-DPP and PC71BM and schematic of the PBDTT-DPP:PC71BM/ZnON phototransistors with the bottom-gate and the top contact structure; (b, c) absorption spectra of ZnON (Eg = 1.50 eV) andPBDTT-DPP:PC71BM/ZnON films; (d) Auger depth profiling of a ZnON film for the tracing in Zn, O, and N; (e) energy-band alignment of aspontaneous ZnO buffer layer formed at the PC71BM and ZnON interface; and (f) photoluminescence (PL) spectra of ZnON and PC71BM/ZnONfilms. Photoexcited carriers were quenched at the interface between PC71BM and ZnON.

Figure 2. Photoresponse of different structures of ZnON phototransistors: (a) ZnON only, (b) PBDTT-DPP/ZnON, (c) PC71BM/ZnON, and (d)PBDTT-DPP:PC71BM/ZnON under illumination by wavelengths in the range of 380−940 nm.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b02814ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1c.32 ZnON also absorbs over a broad range from 400nm to 900 nm. The ZnON bandgap was previously measuredas ∼1.3 eV,14,15 less than our present measurement (∼1.5 eV)(Figure 1b). To find evidence of the extended bandgap inZnON, we investigated the ion distribution in ZnON films byAuger depth profiling (Figure 1d). Interestingly, the region nearthe surface (5−10 nm) was oxygen-rich and nitrogen-deficient,compared to the bulk region. That is, the surface of ZnON filmspontaneously forms a ZnO layer, thereby contributing to thebandgap widening. It could be attributed to the formation of aspontaneous passivation layer on metastable ZnON surface.However, the exact mechanism is still uncertain and requiresfurther study. The spontaneous band alignment in the ZnONsignificantly matched the BHJ polymers. This facilitates thetransfer of electrons and the blocking of holes by forming ZnOlayer, as shown in Figure 1e; the narrow bandgap of ZnONwithout ZnO buffer layer could not block the holes, so thatboth electron and hole generated inside the BHJ can transfer toZnON and recombine. On the other hand, the spontaneousformation of the ZnO layer can easily transfer electron fromBHJ to the ZnON, while the hole is blocked. Figure 1f showsthe photoluminescence (PL) intensity of the PC71BM andPC71BM/ZnON. The PL intensity of PC71BM/ZnON wasconsiderably quenched at the interface, compared to onlyPC71BM, which could be evidence of the electron transfer fromPC71BM to ZnO/ZnON. Thus, the electrons transfer fromPC71BM to ZnO/ZnON could be expected to change thepotential of ZnON and ultimately leads to the negative shift ofthe turn-on voltage of the device.Figure 2 shows the photosensitivity characteristics of the

ZnON phototransistors with the different structures. All devicesshowed similar field-effect mobilities (48−51 cm2 V−1 s−1),regardless of polymer types. The electrical variations in thedevices was examined for illumination wavelengths λ = 380−940 nm (power density = 1 mW cm−2). The threshold voltage(VTH) of the ZnON phototransistors shifted negatively withdecreasing wavelength (Figure 2a). The narrow bandgap ofZnON varied with photonic excitation and VTH changedaccordingly. (Detailed variations of the devices are listed inTable S1 in the Supporting Information.) To confirm the effectof organic materials on top of the ZnON phototransistors, weexamined PBDTT-DPP, PC71BM, and BHJ PBDTT-DPP:PC71BM. We confirmed that only the PBDTT-DPP:PC71BM/ZnON phototransistors were more photores-ponsive than the ZnON phototransistors. The PBDTT-DPP-and PC71BM-based ZnON phototransistors were less photo-

sensitive than ZnON (Figures 2b and 2c). PBDTT-DPP orPC71BM shows strong exciton binding energy, so thatphotogenerated excitons does not separate into holes andelectrons.33 It is also difficult to separate the electron and thehole near the ZnON/ZnO surface, because of the limitedexciton diffusion length (∼10 nm) of the polymer.34 On theother hand, BHJ PBDTT-DPP:PC71BM can generate electronsand holes through efficient exciton dissociation and increaseddiffusion lengths. In other words, photoinduced carriers inPBDTT-DPP:PC71BM contribute to the change in VTH and toan off-current (Figure 2d).To confirm the phototransistor performance, the responsivity

(R), detectivity (D*), linear dynamic range (LDR), and noiseequivalent power (NEP) were investigated. R represents theratio of the photocurrent (Iph) to the photopower density(Llight), and reflects the detector efficiency:

35,36

=RI

Lph

light (1)

D* is expressed by

* =DAfi R

( )( / )

1/2

n (2)

where A is the active area of the phototransistor, f is theelectrical bandwidth, and in is the noise current. It wasmeasured from the noise current using a lock-in amplifier,giving 0.045 pA Hz−1/2 at 1 Hz (Figure S1 in the SupportingInformation). The calculated detectivity, for various wave-lengths and gate voltages (VGS), is shown in Figures 3a and 3b.Values in ZnON phototransistors had over 1011 Jones withinthe broad range of 380−940 nm. Unlike conventional ZnO-based phototransistors (Eg > 3.0 eV), used as UV detectors, thenarrow bandgap of ZnON favors a high photosensitivity over abroad wavelength range (Figure 3a). On the other hand, ZnONphototransistors with embedded PBDTT-DPP were lessphotosensitive than ZnON, because photoexcited electron/hole pairs cannot be generated, because of the strong excitonbinding energy of a single polymer (>0.3 eV). Thus, mostexcitons do not dissociate at only one interface. ZnONphototransistors with embedded PC71BM showed higherdetectivity, compared with ZnON phototransistors, becauseof well-matched CB and electron transfer into the ZnONsurface. In the case of BHJ PBDTT-DPP:PC71BM, electronsand holes can be efficiently separated at the donor−acceptorinterface, which contributes to a large increase in free electrons.

Figure 3. Detectivity, responsivity, and effective quantum efficiency (EQE) of phototransistors. Detectivity of ZnON (black, □), PC71BM/ZnON(red, ○), PBDTT-DPP/ZnON (green, △), and PBDTT:PC71BM/ZnON (blue, ▽) phototransistors (a) under illumination by wavelengths in therange of 380−940 nm and (b) for varying VGS. (c) Responsivity and EQE of ZnON and PBDTT-DPP:PC71BM/ZnON phototransistors underillumination by wavelengths in the range of 380−940 nm.



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In particular, a spontaneously formed ZnO layer transportselectrons and blocks holes, thereby helping to reduce electron−hole recombination within the ZnON. Significantly, thedetectivity of BHJ PBDTT-DPP:PC71BM-embedded ZnONphototransistors attained 1.00 × 1012 Jones, compared withZnON (D* = 1.82 × 1011) at 630 nm (VGS = −10 V). Inaddition, we investigated detectivity dependence with VGSvariations, as shown in Figure 3b. PBDTT-DPP:PC71BM-embedded ZnON phototransistors showed a higher detectivityfor VGS values ranging from −40 V to 0 V, compared with otherstructures. At VGS = −40 V, the detectivity of PBDTT-DPP:PC71BM-embedded ZnON phototransistors was morethan 103 times higher than that of ZnON only. The effectivequantum efficiency (EQE) is given by R × E × 100%, where Eis the incident photon energy.37 The photoresponsivities ofZnON and PBDTT-DPP:PC71BM/ZnON photodetectors at630 nm were 0.031 A W−1 and 0.16 A W−1, respectively (Figure3c). EQE values for PBDTT-DPP:PC71BM/ZnON photo-detectors at the same wavelength were ∼5 times higher thanthose of ZnON photodetectors.The NEP is the performance of weak light detection for the

photonic device. In order to obtain exact NEP value, wecalculated it using the measured noise current, which is givenby29

=iR

NEP n(3)

LDR represents the linearity of the photosensitivity at variousincident photopower intensities and is given by38

= ⎜ ⎟⎛⎝

⎞⎠

PLDR 10 log

NEPsat

(4)

where Psat is the saturation power density, measured at thephotopower intensity of the maximum photocurrent linearity.The LDR of the PBDTT-DPP:PC71BM/ZnON phototransis-tors, under a photopower density, exhibited a value of 1 ×10−4−5 mW cm−2 (for a calculated photon flux density rangefrom 3.2 × 1011 numbers−1 cm−2 to 1.6 × 1016 numbers−1

cm−2) at 630 nm (Figure 4a). The calculated LDR for thedevice was 122.3 dB, which is much higher than forconventional photodetectors made from, e.g., Si (120 dB) orInGaAs (66 dB).36 Figure 4b shows the photoresponsivity andEQE of PBDTT-DPP:PC71BM/ZnON phototransistors, asfunctions of the photopower density. These values aresignificantly improved, compared to those of ZnON photo-transistors and attained, respectively, 168 A W−1 and 3.3 ×104% (at VG = −10 V; light intensity = 1 × 10−4 mW cm−2).Recently, some hybrid-structure-based photodetectors showedvery high photoresponsivities (∼107 A W−1) under very lowincident light intensities (∼1 pW cm−2).39 To compare thesevalues, we plotted the photoresponsivity with different lightintensities (Figure S2 in the Supporting Information).37

Although we could measure the photocurrent up to 100 nWcm−2, the photoresponsivity showed linearity. High-gain hybridphotodetectors are expected to be realized. Figure 4c depictsthe photoswitching of PBDTT-DPP:PC71BM/ZnON photo-transistors at various frequencies from 1 to 100 Hz and awavelength of 630 nm. Devices displayed fast photoswitching.For drain voltages of 2−10 V, the photogain changedsignificantly and the photocurrent showed fast saturation/recovery (Figure 4d). Larger drain voltages effectively amplifyelectron transfer throughout suppress recombination at thePBDTT-DPP:PC71BM and ZnON interface.

Figure 4. LDR and photoresponse of phototransistors: (a) linear dynamic range of PBDTT-DPP:PC71BM/ZnON phototransistors under aphotopower density ranging from 1 × 10−4 mW cm−2 to 5 mW cm−2. The linear dynamic range of the devices exceeded 120 dB. (b) Responsivityand EQE of PBDTT-DPP:PC71BM/ZnON phototransistors at 630 nm wavelength under a photopower density ranging from 1 × 10

−4 mW cm−2 to5 mW cm−2. (c) Transient photocurrent response of PBDTT-DPP:PC71BM/ZnON phototransistors at a pulse frequency ranging from 1 Hz to 100Hz. (d) Amplified photocurrent response of PBDTT-DPP:PC71BM/ZnON phototransistors with VDS set to 2, 5, or 10 V, at a pulse frequency of 1Hz.



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■ CONCLUSIONIn summary, we developed narrow-bandgap ZnON semi-conductor-based phototransistors involving bulk heterojunction(BHJ) polymers for boosting photosensitivity, for use as high-performance photodetectors. The spontaneous band alignmentof ZnON with an oxygen-rich surface was well-matched to theBHJ polymers to facilitate electron transfer electrons and holeblocking. This can improve the electron charge transport overthe conduction band under photon illumination. BHJ PBDTT-DPP:PC71BM contributed to improving the photodetectivityover the infrared (IR) and near-infrared (NIR) regions, and thephotodetectivity and LDR of hybrid structures increasedsignificantly to over 1012 Jones and 122.3 dB, respectively.The calculated photoresponsivity reached values of 1.7 × 102 AW−1 at 100 nW cm−2. We found that hybrid ZnONphototransistors not only maintain high field-effect mobilitybut also improve the photosensitivity. These approaches arepromising avenues for further developing simple, affordable,and high-performance photosensing technologies.

■ METHODSMaterials. For the BHJ layer , poly[2,60 ′ -4,8-bis(5-

ethylhexylthienyl)benzo[1,2-b;3,4-b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]- pyrrole-1,4-dione](PBDTT-DPP) was used as a light-sensitive polymer, previouslydeveloped in our group.32 [6,6]-Phenyl C71-butyric acid methyl esterwas purchased from Nano-C (Westwood, MA, USA). We dissolved 16mg of PBDTT-DPP:PC71BM (1:2) in 2 mL of 1,2-dichlorobenzene(DCB, Aldrich, 99%).Device Fabrication. A ZnON channel layer was deposited onto

SiO2 (100 nm)/p++Si by direct current (DC) reactive sputtering witha metallic Zn target.15 An Ar/O2/N2 ratio of 5/1.2/40 was used for thedeposition, and the working pressure was fixed at 5 mTorr. Thechannel thickness of the ZnON film was 30 nm, and the channelregion was defined using a shadow mask. For the source/drain (S/D)electrodes, indium−tin oxide (ITO) was deposited by sputtering overa shadow mask. The channel region of ZnON TFTs was defined witha width (W) and a length (L) of 800 and 200 μm, respectively. Deviceswere annealed in air at 250 °C for 5 h. Subsequently, PBDTT-DPP,PC71BM, and PBDTT-DPP:PC71BM were spin-coated onto ZnONTFTs at 5000 rpm for 60 s in a nitrogen-filled glovebox. The overallpolymer thickness was ∼100 nm.Film and Device Characterization. The optical transmittance

was measured using a ultraviolet−visible (UV-vis) spectrophotometer(U-4100, Hitachi) and the optical bandgap was extrapolated from aTauc plot.40 The work function and Fermi level of the ZnON filmwere examined by X-ray photoelectron spectroscopy (XPS, Omicron;see Figure S8 in the Supporting Information) and ultravioletphotoelectron spectroscopy (UPS, Omicron). Auger depth profilingwas performed to measure the vertical element distribution of theZnON film. The microstructures and morphologies of the films wereinvestigated using atomic force microscopy (AFM, Dimension Iconsystem, Bruker) in tapping mode and a scanning electron microscopy(SEM) system. Microphotoluminescence (PL) was performed using aHoriba LabRAM HR Evolution confocal Raman system with an Ar ionlaser (488 nm) excitation (100× objective; 100 μW power). The PLintensity of the ZnON film is shown in Figure S3 in the SupportingInformation. The Agilent 4155C Semiconductor Parameter Analyzer

was used to measure the devices in darkness. The inducing VGS was setin the range of −40 V to 40 V, and VDS was set in the range of −40 Vto 10 V. Detailed device parameters of devices are listed in Table 1.Monochromatic light-emitting diodes (LEDs, 380−940 nm) wereused as the light source to measure the photoresponse. The lightintensity and frequency were controlled using a function generator(AFG3252, Techtronix). The actual light intensity was tuned using apower meter (Model 1830-C, Newport).

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b02814.

Dark current noise measurements through the lock-inamplifier, expected linear dynamic range results, and thePL spectra of the ZnON film (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected] (J. S. Park).*E-mail: [email protected] (Y. Yang).Author Contributions#Y.S.R., K.-C.O., and Y.(M.).Y. contributed equally to thiswork.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by a grant from theNational Science Foundation (Grant No. CHE-1230598,Program Manager Linda S. Sapochak), the Office of NavalResearch (Program Manager Dr. Paul Armistead; Grant No.N000141410648), and UCLA internal funds. This research wasalso supported by Global Frontier Program through the GlobalFrontier Hybrid Interface Materials (GFHIM) of the NationalResearch Foundation of Korea (NRF) funded by the Ministryof Science, ICT & Future Planning (No. 2013M3A6B1078870)and done by the MOTIE (Ministry of Trade, Industry &Energy (Grant No. 10051403) and KDRC (Korea DisplayResearch Corporation) support program for the developmentof future devices technology for display industry.

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Table 1. Device Performance of ZnON Phototransistors with the Different Structures in Darkness

structure mobility (cm2 V−1 s−1) VTH (V) S.S (V dec−1) on/off ratio

ZnON 51.94 ± 2.46 −6.19 ± 0.98 0.71 ± 0.08 108

PC71BM/ZnON 50.98 ± 3.11 −7.41 ± 1.87 1.12 ± 0.14 108

PBDTT-DPP/ZnON 48.49 ± 1.79 −4.26 ± 1.33 1.26 ± 0.19 108

PBDTT-DPP:PC71BM/ZnON 48.57 ± 3.35 −2.72 ± 1.66 1.27 ± 0.16 108



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