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ZnO Nanobridge Array UV Photodetectors Shi-Ming Peng,† Yan-Kuin Su,*,†,‡,§ Liang-Wen Ji,*,‡,| Cheng-Zhi Wu,| Wei-Bin Cheng,| and Wan-Chun Chao| Institute of Microelectronics & Department of Electrical Engineering, National Cheng Kung UniVersity, Tainan 701, Taiwan, AdVanced Optoelectronic Technology Center, National Cheng Kung UniVersity, Tainan 701, Taiwan, Department of Electrical Engineering, Kun Shan UniVersity, Tainan 701, Taiwan, and Institute of Electro-Optical and Materials Science, National Formosa UniVersity, Yunlin 632, Taiwan ReceiVed: September 28, 2009; ReVised Manuscript ReceiVed: December 28, 2009
This study describes the fabrication of ultraviolet photodetectors with laterally aligned ZnO nanobridge arrays. These nanobridge arrays grow upward in the face-to-face direction, thereby forming biaxial compressive stress where nanobridges intersect. Compared with conventional thin-film photodetectors, the nanobridge devices markedly enhance photosensitivity and blue shift (30 nm) of the spectral response. These phenomena are caused by surface effects of the ZnO nanobridge and strain-induced polarization effects, leading to band structure change. Nanobridge devices are a promising alternative for transforming advanced optoelectronic integration circuits with a 1D structure into miniaturized devices. I. Introduction Advances in nanoscale fabrication strategies, innovative nanodevice architectures, and optoelectronic integrated circuit architectures have attracted considerable attention in recent years. Semiconductor nanowires (NWs) have the potential to stretch beyond the limits of conventional techniques. Notably, ZnO-based semiconductors are attractive for NW-based optoelectronics applications due to their beneficial electronic, optical, and piezoelectric properties. Vertical alignment, selective growth, and controlled morphology of NWs have recently stimulated interest in 1D nanostructure applications.1-6 Some technologies have been developed for orientation control, including electron-beam lithography, dispersal of isolated NWs on prefabricated electrodes,7 electricalfield-assisted assembly,8 fluid flow,9 controlled gas flow,10 and the Langmuir-Blodgett technique.11 Although these technologies facilitate the study of nanodevice characteristics, these methods cannot be applied as a general approach for industrialscale applications. By performing conventional photolithography, this study demonstrates the feasibility of fabricating laterally aligned ZnO NW (i.e., ZnO nanobridge) photodetectors (PDs) on a glass substrate using a hydrothermal solution. Conventional ZnO film PDs were constructed for comparison with laterally aligned ZnO nanobridge PDs. The laterally aligned ZnO nanobridge UV PDs performed better than the conventional ZnO film PDs.
nucleus layer covered with Ti/Au metal on the top was defined by photolithography. A 200 nm thick undoped ZnO seed layer was then deposited by a radio frequency (rf) magnetron sputter system. Ti/Au (100 nm/50 nm) contact pads were subsequently deposited on the ZnO film by electron-beam evaporation. Finally, ZnO patterns with Ti/Au metal on top were achieved after lifting-off processes. The active area of the entire device was 80 × 8 µm2. Finally, the patterned substrate was then immersed in a Zn(NO3)2/NH4OH hydrothermal solution for 4 h at 90 °C. The device was finally rinsed with acetone and ethanol and dried. Figure 1b schematically depicts the ZnO nanobridge PD. The conventional film PD with a 200 nm thick ZnO film was fabricated for comparison (Figure 1a). Surface morphologies of the ZnO PDs were characterized by a field emission scanning electron microscope (FE-SEM) (JEOL JSM-6700F (Figure 1c) and HITACHI-S4700 (Figure 1d)). The high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100), X-ray diffraction (XRD), and micro-Raman were then utilized to characterize the optical and crystallographic properties of the ZnO. An HP-4156C semiconductor parameter analyzer was then employed to measure current-voltage (I-V) characteristics of the proposed ZnO PDs. Spectral responsivity measurements were obtained using the TRAIX 180 system with a 300 W xenon arc lamp light source and standard synchronous detection scheme. III. Results and Discussion
II. Experimental Section The process for fabricating ZnO nanobridge UV PDs was as follows. First, the region that we would like to deposit a ZnO * To whom correspondence should be addressed. Tel: +886-6-2351864 (Y.-K.S.), +886-5-631-5679 (L.-W.J.). Fax: +886-6-2356226 (Y.-K.S.), +886-5-632-9257 (L.-W.J.). E-mail:
[email protected] (Y.-K.S.),
[email protected] (L.-W.J.). † Institute of Microelectronics & Department of Electrical Engineering, National Cheng Kung University. ‡ Advanced Optoelectronic Technology Center, National Cheng Kung University. § Kun Shan University. | National Formosa University.
Figure 1c shows the FE-SEM image of the conventional film PDs with 200 nm thick ZnO films. The inset of Figure 1c shows an optical micrograph of the patterned PDs. Figure 1d shows FE-SEM images at a 45° tilt angle of the laterally aligned ZnO nanobridge PDs. The laterally aligned ZnO nanobridge arrays were successfully grown across the 8 µm gap between the two Ti/Au electrodes. The average diameter of these ZnO nanobridges was around 300 nm. The observed growth direction of the ZnO nanobridges was parallel to the glass substrate due to the pre-existing lateral seed layer that functioned as a nucleation site. Besides, the laterally grown ZnO NWs were not only in contact with each other but also overlapped in the middle and
10.1021/jp909299y 2010 American Chemical Society Published on Web 01/28/2010
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Figure 1. Schematic for the fabricated UV photodetector and the FE-SEM image with a 45° tilt angle for the devices. Panels (a) and (c) show the conventional ZnO film device. Panels (b) and (d) show the nanobridge device. The inset of (c) shows an optical micrograph of the patterned photodetectors.
Figure 2. Typical TEM image analysis results. (a) Low-magnification TEM image; the yellow dotted line indicates an elemental line scan across the nanowire. (b) High-resolution TEM of ZnO nanowire with (002) growth direction; the inset is the corresponding SAED image. Panels (c) and (d) show the line scan of the EDS analysis for an individual ZnO nanowire.
thus led the neighbor hexagonal ZnO NWs to coalesce during the growth process. These NW networks provide electrical conducting paths for electrons.12,13 However, the adjacent pattern and parameters of the growth conditions were both important, which can significantly influence the electrical transport behavior of ZnO NWs at NW/NW junction or metal/NW junction. Furthermore, compared with vertically aligned ZnO NW arrays, the laterally aligned ZnO NW structures can offer better carrier confinement in one dimension. The carrier transport path in vertically aligned ZnO NW devices15 (i.e., through ZnO seed layer to electrode) is much longer than that in laterally aligned ZnO NW devices, which implies that the laterally aligned ZnO NWs in application to PDs have the faster transmission time than that of vertically aligned ZnO NWs. Figure 2a,b shows TEM images of NWs from a nanobridge device under low and high magnification, respectively. The ZnO NWs were preferentially oriented in the c-axis direction. The selective area electron diffraction (SAED) pattern of the NW
Figure 3. (a) XRD result of the ZnO film and ZnO nanobridge device. (b) Raman scattering spectra of the vertical ZnO nanowire material and nanobridge device.
demonstrates that the NW has a single crystalline phase (Figure 2b). Figure 2c,d shows the elemental line scanning on the ZnO NW (Figure 2a), indicating that the NW is composed of Zn and O, respectively. Figure 3a shows the XRD results for the pure ZnO film and nanobridge PDs. The measured (100), (002), (101), and (103) diffraction peaks are indexed to confirm the ZnO wurtzite hexagonal structure.14 The Au(111) peak was expected as the nanobridge PDs were prepared with an electrode contact pattern. The (002) peak of the nanobridge shifted slightly toward a higher
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Peng et al. Figure 5 shows schematic energy band diagrams of piezoelectricity (φPZ) across the intersection of nanobridges. The most recent research determined that piezoelectric potential induced asymmetrical changes in barrier height (BH) when a ZnO NW was bent.6,21,22 The abundant research on bending ZnO NWs indicates that electrical transport of ZnO NWs significantly influences energy band structure (∆bs)23-26 and piezoelectric polarization effects (∆PZ) under compressive or tensile strain.6,21,26 In this study, the laterally aligned ZnO nanobridges grow upward in the face-to-face direction, thereby forming biaxial compressive stress where nanobridges intersect (Figure 1d). If the effect of the metal-semiconductor interface can be neglected, we assume unstrained nanobidges exist where nanobridges intersect; thus, BHs at two contacts are regarded as identical φdb ) φsb(Figure 5a). Additionally, charge-trapping effects by impurity and vacancy states in ZnO, which exhibit metastable charge states located between shallow and deep energy levels. It is well-known that the defect states in the surface layer of ZnO readily act as adsorption sites by combining ambient negative charges such as oxygen molecules.20,23 Oxygen molecules act as acceptors that deplete the surface electron states and capture free carrier density [O2(g) + e- f O2-(ad)], which can influence the effect of the surface state on surface band bending (φB).4,20,23,24 The surface band bending (φB) caused by adsorption is given by23
φB )
Figure 4. I-V characteristics of the two fabricated ZnO photodetectors measured in darkness (a) and under 340 nm illumination (b).
diffraction angle (∼∆2θ ) 0.68°) compared with that of ZnO film. Furthermore, the (002) peak shifted about ∆2θ ) 0.26° compared with that of the vertically aligned ZnO NWs in our previous study.15 The shift of XRD peaks toward higher diffraction angles is indicative of compressive stresses in the inward radial direction.16 Hexagonal wurtzite ZnO (W-ZnO) belongs to the C6V4/ P63mc space group. In the Raman spectrum of vertically aligned ZnO NWs and nanobridges (Figure 3b), E2 (high) modes only exist at 438 and 447 cm-1. The E2 (high) mode in the nanobridge shifted to 9 cm-1 higher than that in ZnO NWs. Decremps et al.17 and Chen et al.18 reported that the E2 (high) of ZnO crystal shifting toward a high frequency can be attributed to biaxial compressive stress in the inward radial direction. Figure 4a shows the current-voltage (I-V) characteristics of the fabricated conventional ZnO film and ZnO nanobridge PDs with the same Ti/Au; the electrodes were measured in darkness. With a 5 V applied bias, the dark current was 29.5 pA and 7.9 nA for the conventional ZnO film and ZnO nanobidge PDs, respectively. The I-V curves of the nanobidge PDs exhibited asymmetrical nonlinear behaviors, indicating the existence of barriers between the ZnO NWs and Ti/Au electrodes, such as conduction via defective sites, space-chargelimited currents (SCLCs) through ZnO NWs, and piezoelectricdependent optoelectric transport.19-21 The measured photocurrent of nanobridge devices increased to about 7.77 × 103 (photo/ dark current ratio) under UV illumination (λ ) 340 nm, 5 V applied bias, and an incident optical power of 57.46 mW/cm2) (Figure 4b). Thus, the nanobridge device has higher sensitivity (Iph-nanobridge/Iph-traditional ) 1.02 × 103) to UV illumination than the conventional ZnO film device; this difference in sensitivity can be attributed to the surface effects of the ZnO nanostructure.
( )
q2ND W 2εZnO
where q is the electronic charge, εZnO is the dielectric constant of ZnO, ND is donor density, and W is depletion width. Figure 5b shows the piezoelectric-dependent charge-transport process and strain-induced charge influencing the BH in darkness. The outer and inner bending surfaces can induce positively charged (stretching) and negatively charged (compression) surfaces and form a polarization field, which is denoted by b E ) ε/d, where d is the piezoelectric coefficient;6 thus, a piezoelectric potential drop was produced across the width of the ZnO nanobridge, which can be partially neutralized by changing the electrical charges at the interface states between thestretchingandcompressionsurfaceandthemetal-semiconductor interface along the length of the nanobridge, leading to piezoelectric polarization (∆PZ) of the BH.21,25 The change in BH by piezoelectric polarization can be expressed as25
∆φPZ )
(
σPZ 1 1+ D 2qsW
)
-1
where σpz is the area density of piezoelectric polarization changes (in units of electron charge) and D is a two-dimensional density of interface states at the Fermi level in the semiconductor band gap at the BH. A two-dimensional screening parameter [qs ) (2πq2/ εZnO)D] associated with the states in the band gap at the interface.25 He et al.22 investigated rectifiable I-V behavior using a Au/Ticoated tungsten probe to bend a ZnO NW. Additionally, Zhou et al.21 examined the asymmetrical change in Schottky-barrier height at both the source and the drain electrodes under different straining conditions, including stretching and compression along the piezoelectric fine wire. In this study, the I-V curves of nanobidge PDs (Figure 4a) exhibit asymmetrical nonlinear behaviors due to different BHs at the drain and source (φd > φs), which can be attributed to unbalanced biaxial compressive stress where nano-
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Figure 5. Schematic energy band diagram illustrations for barrier height at the metal electrode-semiconductor interface and the interface barrier where nanobridges intersect. (a) The unstrained nanobidge photodetectors where nanobridges intersect. (b) The piezoelectric-dependent chargetransport process and stain-induced charge influence the barrier height in the darkness. (c) Under the UV-illuminated state.
nanobridge PDs, respectively. Therefore, this study defines the rejection ratio of UV to visible (UV/vis) as the responsivity of 360 nm/450 nm and 340 nm/430 nm in conventional ZnO film and nanobridge PDs, respectively. Using this definition and a 5 V applied bias, the UV/vis rejection ratios were 1.83 × 102 and 1.35 × 103 for conventional ZnO film and nanobridge PDs, respectively. At a 5 V applied bias, the measured responsivities were 0.11 (370 nm) and 166.84 (340 nm) A/W (ampere/watt) for the conventional ZnO film and nanobridge PDs, respectively. The responsivity of a detector R is defined as30
Figure 6. Measured spectral reponsivities of the conventional ZnO film and nanobridge photodetectors at 5 V applied bias.
bridges intersect, leading to piezoelectric polarization effects (∆PZ) and induced charge influencing BHs. Therefore, the changes in BH can be attributed to the strain-induced band structure change (∆bs), piezoelectric effect (∆PZ), and charge-trapping effect by surface/vacancy states in ZnO; thus, the total change in BH of ZnO nanobridge PDs can be expressed as20,21
φd ) φdb + ∆φd-bs + ∆φd-PZ The unbalanced biaxial compressive stress where nanobridges intersect markedly modifies the potential barrier at both the source and the drain electrodes (φd > φs) and is caused by strain-induced piezoelectricity across the intersection of the nanobridges. The electron-hole pairs were photogenerated (hυ f e- + h+). The photon energy of illumination exceeded the energy band gap (hυ > Eg) (Figure 5c). The photogenerated holes react with adsorbed oxygen ions [h+ + O2-(ad) f O2(g)], thereby increasing the free carrier concentration and persistent photoconductive gain.4,23,27,28 Therefore, electrical transport of ZnO nanobridges markedly reduces the BHs at the metal electrodesemiconductor interface (φ′d < φd) and the interface barrier where nanobridges intersect.20,29 Figure 6 shows spectral responsivity R (λ) characteristics of the fabricated conventional ZnO film and nanobridge PDs with an applied bias of 5 V. A sharp cutoff occurred at around 360 and 340 nm for the fabricated conventional ZnO film and
R (A/W) )
Ip qλ )η G Popt hc
( )
where Ip is photocurrent, Popt is incident optical power, η is quantum efficiency, h is Planck’s constant, c is the speed of light, λ is the incident light wavelength, and G is the photoconductive gain. Additionally, the gain is defined as the ratio between the number of electrons collected per unit time (Nel) and the number of absorbed photons per unit time (Nph) or the ratio of carrier lifetime (τ) to carrier transit time (τtr); the photoconductive gain can be expressed as10,30
G)
Nel Iph 1 τ ) = 2 τµeV ) Nph qF τtr L
where L is interelectrode spacing, µe is electron mobility, and V is applied bias. As discussed, this study applies 166.84 A/W and 340 nm to the expression; thus, the gain is estimated at 6.08 × 102 by assuming η ) 1 for simplicity. Such a computational result indicates that internal photoconductive gain exists in the nanobridge devices. Soci et al.4 examined that the substantial photoconductive gain is attributed to the presence of deep level surface trap states in NWs that greatly prolong the photocarrier lifetime and prevent electron-hole recombination. Additionally, the spectral responsivity sharp cutoff occurred at around 340 nm in the nanobridge devices. This refers to devices that have a clear 30 nm blue shift compared with that of conventional ZnO film. This study proposed that the blue shift was related to the biaxial compressive stress at the interface
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Peng et al. nanobridge devices are a promising alternative for fabricating future miniaturized low-cost UV and high-resolution “visibleblind” UV detectors. Acknowledgment. Funding from the Advanced Optoelectronic Technology Center, National Cheng Kung University, under projects from the Ministry of Education and the National Science Council (NSC 96-2221-E-006-079-MY3) of Taiwan is gratefully acknowledged. This work was partially supported by the TDPA “Lamp Development of White Light-Emitting Diode for Local Lighting” program and, in part, by the National Science Council of the Republic of China (R.O.C.) in Taiwan under Contract Nos. TDPA 97-EC-17-A-07-S1-105, NSC 972623-E-168-001-IT, and NSC-98-2221-E-150-005-MY3.
Figure 7. Photoresponse of the conventional ZnO film and nanobridge photodetectors with turning off the UV light at 5 V applied bias.
barrier where nanobridges intersect. When the biaxial compressive stress-induced polarization field appears across where the nanobridges intersect, some free electrons in the n-type ZnO NW may be trapped and accumulate at the stretching surface (positive side surface) and become nonmobile charges, which will shift the local Fermi level and modify the local conduction band.21,25,31 Figure 7 shows time-resolved measurements of photoresponse by turning the UV on (λ ) 340 nm and 5 V applied bias) and off for conventional ZnO film and ZnO nanobridge PDs, respectively. The time decay constants follow a second-order exponential relaxation function. The conventional ZnO film PDs show the stability decay component (τd1-F = τd2-F ) 0.07 s) after turning the UV excitation off. Conversely, the ZnO nanobridge PDs have slower (τd1-NB ) 2.54 s, τd2-NB ) 0.32 s) components of carrier relaxation dynamics than conventional ZnO film PDs. As a result, the carrier relaxation dynamics can be attributed to the following two factors. (i) The high surface trap states in the nanobridge arrays significantly prolong carrier lifetime and strongly depend on the oxygen adsorption/desorption process, which has a significant influence on photoresponse time (τd1-NB) of the ZnO nanobridge.4,27,28 (ii) The strain-induced charge influences the BHs at the metal electrode-semiconductor interface and the interface barrier where nanobridges intersect, which are caused by a piezoelectric effect.6 The free electrons are repulsed by the piezo-induced electrical field, leaving a charge depletion region around the compressed side and reducing the width of the conducting channel; thus, the carrier relaxation dynamics (τd2-NB) becomes rapid.29,31 IV. Conclusion In summary, this study performed conventional photolithography to fabricate laterally aligned ZnO nanobridge PDs on a glass substrate. Compared with conventional ZnO film PDs, the fabricated nanobridge PDs have a higher photoresponse and deeper response near 340 nm. The compressive stress in the intersection of nanobridges markedly influences the BHs at the interface of the metal electrode and semiconductor and interface barrier where nanobridges intersect. Furthermore, time-resolved measurements demonstrate that the carrier relaxation dynamics becomes shorter due to the piezo-induced electrical field reducing the width of the conducting channel. However,
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