n-ZnO Nanotube Heterojunctions with

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A Photodetector Based on p-Si/n-ZnO Nanotube Heterojunctions with High Ultraviolet Responsivity Tahani Hassan Flemban, Md Azimul Haque, Idris A. Ajia, Norah Alwadai, Somak Mitra, Tom Wu, and Iman S Roqan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09645 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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ACS Applied Materials & Interfaces

A Photodetector Based on p-Si/n-ZnO Nanotube Heterojunctions with High Ultraviolet Responsivity Tahani H. Flemban, Md Azimul Haque, Idris Ajia, Norah Alwadai, Somak Mitra, Tom Wu, and Iman S. Roqan* King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering Division, Thuwal 23955-6900, Saudi Arabia KEYWORDS: (photodetectors, oxides, time-resolved measurements, nanotubes, heterojunctions, ultraviolet, pulsed laser depositions) ABSTRACT Enhanced ultraviolet (UV) photodetectors (PDs) with high responsivity comparable to that of visible and infrared photodetectors are needed for commercial applications. N-type ZnO nanotubes (NTs) with high-quality optical, structural and electrical properties on a p-type Si (100) substrate are successfully fabricated by pulsed laser deposition (PLD) to produce a UV PD with high responsivity, for the first time. We measure the current−voltage characteristics of the device under dark and illuminated conditions and demonstrated the high stability and responsivity (that reaches ~ 101.2 AW-1) of the fabricated UV PD. Time-resolved spectroscopy, is employed to identify exciton confinement, indicating that the high PD performance is due to optical confinement, the high surface-to-volume ratio, the high structural quality of the NTs, and the high photo-induced carrier density. The superior detectivity and responsivity of our NTbased PD clearly demonstrate that fabrication of high-performance UV detection devices for commercial applications is possible. 1 ACS Paragon Plus Environment


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INTRODUCTION Room-temperature (RT) ultraviolet (UV) photodetectors (PDs) have applications in astronomy, chemical and biological detection, optical communications, flame sensing, and national defense.1 However, obtaining high responsivity and stability in the UV range of the spectrum that is comparable to that in the visible and infrared ranges remains a challenge. Commercially available Si-based PDs have low UV responsivity due to several limitations, such as the indirect and small bandgap energy of Si (1.1 eV), the effect of temperature variations on PD characteristics, and the need for high-pass optical filters and phosphors to stop low-energy photons.2,3 Therefore, the performance of UV PDs based on Si can be reduced due to the low efficiency and greater dark currents of Si. Consequently, the development of UV PDs using wide-bandgap semiconductors (ZnO, for example) would overcome these limitations. Wurtzite ZnO is an ideal RT UV PD

4-6

due to its wide and direct bandgap (3.3 eV at RT), High exciton

binding energy (60 meV) compared to the RT thermal energy (25 meV) provides stability against thermal dissociation of excitons.2 environmentally friendly properties, and chemical and thermal stability.7,8 Furthermore, ZnO has unique electric and piezophototronic properties that can be exploited in PD technologies.5,6, 9,10 The high stability of ZnO’s polar surface has been used to induce the formation of many nanostructures using different techniques.11 Therefore, significant efforts have been devoted to the development of ZnO-based UV PDs.12-15 ZnO-based p-n heterojunctions have the advantage of low applied fields and no oxygen dependency compared to ZnO-based Schottky contact UV detectors, which require a high applied field and rely on the oxygen adsorption/desorption process.16 This adsorption/desorption process can be explained as: free electrons from the surface can be trapped by oxygen molecules in the dark, to form a chemically adsorbed surface state, [ܱଶ + ݁ ି → ܱଶି ]. Under illumination at a photon energy

2 ACS Paragon Plus Environment

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above the bandgap, electron-hole pairs are photogenerated, holes are captured in the surface and the negatively charged adsorbed oxygen ions are discharged [ℎା + ܱଶି → ܱଶ ]. The free electrons produced during the photodesorption process will increase the photoconductivity.2, 4 Wang and coworkers achieved ultrafast UV photoresponse in p-Si/n-ZnO, which is compatible to Si-based PDs.15 However, the UV responsivity of presently available ZnO-based p-n devices currently remains low.16-23 Enhancing the responsivity of UV PDs to the level comparable to that of visible-light PDs is thus needed before inexpensive and commercially viable UV PD devices can be fabricated. Here, we report on optimized ZnO nanotubes (NTs) grown on p-Si by pulsed laser deposition (PLD) and demonstrate their high-quality optical, structural and electrical properties suitable for UV photodetection. This is the first work reporting on a ZnO NTs-based PD fabricated by direct growth without seed layer or a need for costly and complex fabrication processing), leading to superior performance UV PDs. We also show a high-performance p-Si/nZnO NT heterojunction device with UV responsivity. We grew one-dimensional (1D) ZnO NTs on p-type silicon (p-Si) to fabricate a p-n heterojunction with superior responsivity and high quantum efficiency. PDs based on 1D nanostructured materials usually exhibit high responsivity and quantum efficiency due to their large surface-to-volume ratio.24-26 In addition, these 1D devices showed significant carrier transport owing to discontinuous interfaces, low grain boundary densities, and limited two-dimensional (2D) charge transport.4, 27,28 EXPEREMINTAL SECTION Sample preparation. The ZnO target for PLD was conventionally prepared from ZnO (99.999% pure) powder (Sigma-Aldrich). A pellet of 2.5 cm diameter was pressed under 100 bar pressure and sintered at ~ 1100 °C. N-type ZnO NTs were grown on p-Si without seeding or



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catalysts by a one-step PLD method. We used a krypton fluoride (krF) excimer laser with 248 nm wavelength. The laser frequency, target-substrate vertical distance and partial oxygen pressure were fixed at 10 Hz, ~ 9 cm, and 100 mTorr, respectively, at a deposition temperature of 650 °C. We used a p-type Si (100) wafer doped on a Boron (p-type) substrate with very low resistivity (0.001−0.005 Ω.cm). Structural characterizations. We used scanning electron microscopy (SEM) (FEI Nova Nano 630), High-resolution transmission electron microscopy (HR-TEM) (Titan), and x-ray diffraction (XRD) (Bruker D8 with CuKα and λ = 1.5406 Å) for structural characterizations. Optical characterization and carrier dynamics. We investigated the optical properties of ZnO NT through photoluminescence (PL) measurements conducted using a 325-nm He-Cd laser with a beam power of 8 mW. The samples were mounted in a He closed-cycle cryostat system for low-temperature PL measurements (6 K). The spectra were detected by an Andor spectrograph with a UV-visible charge coupled device (CCD) camera, allowing the PL spectra to be detected and analyzed. We performed temperature-dependent of time-resolved PL (TRPL) experiments by third harmonic UV (λ = 266 nm) pulses of a mode-locked Ti:sapphire femtosecond pulsed laser with a pulse width of ~150 fs and an average pulse power intensity of ~2.1 kW cm-2 (with a 76 MHz repetition rate). We used a coherent Verdi-V18 diode-pumped solid-state continuous wave laser to pump the Ti:sapphire laser and detected the emission emanating from the sample with a Princeton Instruments spectrograph attached to a UV-sensitive Hamamatsu C6860 streak camera with a temporal resolution of ~2 ps, using synchroscan mode. We mounted the sample in a variable-temperature closed-helium cryostat for measurements in the 4−300 K temperature range.


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Device fabrication. To fabricate the device, we deposited silver as a bottom electrode. 150 nm indium-tin oxide (ITO) contact serving as a top electrode was deposited using a shadow mask (Figure S1 in supporting information) by radio frequency magnetron sputtering during insitu thermal annealing at 150 ˚C (To increase the ITO transparency and avoid high resistivity), using argon plasma at constant current of 0.15 A for 85 min, and a working pressure of 4 mTorr. We measured all device photoresponse characteristics using a probe station connected to a Keithley 4200 semiconductor analyzer under dark and UV-illuminated conditions (λ = 365 nm, whereby the light intensity was tuned using neutral density filters and was calibrated using an optical power meter). The responsivity (R) was calculated as R = ∆I/PS, where ∆I is the difference between the photocurrent and the dark current, P is the incident power density, and S is the effective area illuminated by the UV lamp (λ = 365 nm). External quantum efficiency (EQE) was calculated from the responsivity using the expression EQE = Rhc/eλ, where h is Plank’s constant, c is the speed of light, e is the electronic charge and λ is the incident light wavelength.29 Specific detectivity (D*) was calculated using the equation D* = R/(2eJdark)1/2, where Jdark is the dark current density. We assumed that the noise was dominated by shot noise.29 RESULTS AND DISCUSSION Nanorod (NR) and NT formation occurs initially though material nucleation as a result of the initial textured layer that is formed on the Si substrates during deposition due to the high lattice mismatch between Si and ZnO.30 Depending on the surface energy of the layers, this gives rise to Stranski-Krastanov (SK) nucleation, followed by NR and NT growth.31,32 In our work, this textured layer was formed during ZnO NT growth on p-Si using one-step method. We found that the laser fluence can affect the nanostructure type (such as NR, NT or quantum dots) formed on a p-Si substrate using PLD due to the changes in the kinetic energy E of the charged species



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upon arrival at the substrate surface, leading to NT formation. This relationship is described by the following equation:33  d E = Eο exp −  ;  λ

(1)

where Eο is the initial energy of the charged species escaping from the target and d is the distance between the target and the substrate, while λ is the mean free path of the ablated species (the plasma) traveling towards the substrate in the PLD chamber. We studied the effect of the growth temperature, oxygen partially pressure (P(O2)), laser fluence, and the number of laser pulses to optimize the growth. We found that 650 °C and 100 mTorr were the optimal growth temperature and P(O2), respectively, for forming ZnO NTs. Then, we investigated the effect of laser fluence by depositing samples by laser fluence ranging from 1.6 to 6.2 J cm-2 and 30,000 pulses. Figure 1 shows SEM images of samples deposited at different fluence conditions. Samples deposited using high laser fluence (in the 4−6.2 J cm-2 range) exhibit dense, film-like ZnO NRs, the size of which decreases with the decrease in laser fluence (Figure 1a-c). When the laser fluence decreases to 3.6 J cm-2 (Figure 1d), NT formation commences. Figure 1e shows that 2.8 J cm-2 is the optimal fluence for obtaining high NT density, as further decreasing the fluence to 1.6 J cm-2 results in a decline in ZnO NT density (Figure 1f). We observed that NT formation requires slightly lower energy, as shown in Figure 1. We also studied the effect of number of pulses by increasing the number of pulses from 30,000 to 40,000 at the laser fluence of 2.8 J cm-2 to optimize the NT quality (Figure 2) (Figure S2 in supporting information). We found that the optimal NT structure was achieved at 2.8 J cm-2 laser fluence and 40,000 pulses at a growth temperature of 650 °C and 100 mTorr pressure. Figure 2 shows optimized ZnO NTs with well-shaped hollowing that is on average ~ 650 nm long with a wall thickness (d) of ~10−25 nm and an external radius (r) of ∼90−110 nm 6 ACS Paragon Plus Environment

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(including the hollowing). In this case, the ratio of the wall thickness to the outer radius (d/r) is smaller than (3/8)1/2. At this ratio, light is confined via total internal reflection at the interface between the external and internal surface of the tube.34,35 This internal reflection can improve PD responsivity.36

Figure 1. ZnO NTs grown on p-Si (100) under different laser fluences at P(O2) = 100 mTorr. (a) 6.2 J cm-2; (b) 4.4 J cm-2; (c) 4.0 J cm-2; (d) 3.6 J cm-2; (e) 2.8 J cm-2; (f) 1.6 J cm-2.

Figure 2. ZnO NTs grown by increasing the number of pulses (up to 40,000) with laser fluence of 2.8 J cm-2 (a) SEM top view; the inset is a zoomed image of a single NT; (b) cross sectional SEM image.



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HR-TEM images of a cross-section taken parallel to the [0001] c-axis near the interface between a ZnO NT and the p-Si substrate shown in Figure 3 confirm that the hollowing starts from the bottom of the NT near the interface, resulting in enhancing light absorption of ZnO NT in comparison to ZnO NR due to the high aspect ratio

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. The SEM and HR-TEM results also

reveal a significant surface-to-volume ratio, which ensures that a highly active area is exposed to light to produce sufficient photo-induced carrier density.

Figure 3. HR-TEM of a cross section of ZnO NTs near the interface, indicating that the hollowing emerged from the interface.

Figure 4. (Log. scale) XRD pattern of ZnO NTs grown on p-Si substrate. Figure 4 shows XRD measurements that reveal the high-quality crystalline structure of the n-ZnO NTs. A sharp and intense peak (at 2θ = 32.94° with a small full width at half 8 ACS Paragon Plus Environment

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maximum [FWHM] of 0.011°) and a minor peak (at 2θ = 34.42° with a FWHM of 0.213°) correspond to the (100) and (002) ZnO planes, respectively. The ZnO NT lattice constants were calculated using Bragg’s law, whereby the spacing between the planes in hexagonal structures is given by Miller indices h, k and l.38 The a and c lattice parameters are calculated as a = 3.14 Å and c = 5.20 Å. To investigate the material quality, the optical properties of ZnO NTs were studied. Lowtemperature (6 K) and RTPL spectra reveal strong and sharp bandgap peaks at 3.36 and 3.27 eV, respectively, with a very weak defect band in the visible region (Figure 5a), confirming the high quality of the ZnO NT structures.39 The weak defect band indicates that the density of the trap states in the bandgap introduced by defects is low, resulting in a significant improvement in the photocurrent that rapidly reaches a steady state, as will be shown later. This phenomenon leads to substantial PD UV responsivity.26, 40,41 As 1D confinement leads to high PD responsivity, we studied the temperature-dependent PL and TRPL to investigate the confinement behavior of the NTs. Figure 5b shows the temperature-dependent PL of ZnO NTs. As the temperature increases, the near-band-edge (NBE) emission shows a redshift. The dependence of the NBE exciton peak energy on temperature is determined by Varshni’s formula42 and indicates that NBE is dominated by free excitons; similar behavior was observed in 1D confinement in ZnO quantum wells.43 In order to elucidate the confinement behavior of the ZnO NTs, we studied temperature-dependent TRPL to investigate the carrier dynamics (Figure S3 in supporting information). Figure 5c shows the TRPL spectra of ZnO NT samples at 4 K and RT. The TRPL decay lifetime of the sample followed a nonexponential trend in all temperature ranges. The TRPL spectra were fitted, with excellent convergence, using the following bi-exponential decay model:44



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‫ܫ‬௧ = ‫ܣ‬ଵ ݁

ି

೟ ഓభ

+ ‫ܣ‬ଶ ݁

ି

೟ ഓమ

;

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(2)

where A1 and A2 are the relative initial intensities of the fast and slow decay components, respectively, τ1 and τ2 are the fast and slow lifetimes, respectively, and It is the time-dependent intensity of the photoemission from the sample. As τ1 and τ2 at 4 K (RT) were 10.7 ps and 35.7 ps (7.9 ps and 25.2 ps), respectively, our results are in good agreement with the previously reported values for ZnO nanostructures.45,46 The non-exponential decay of nanostructures has been extensively discussed in pertinent literature.39, 46,47 Fast decay, τ1, has been attributed to the effects of surface recombination states when the surface-to-volume ratio is high, whereas slow decay, τ2, has been ascribed to the bulk-like states in the thicknesses of NT walls,46 leading to multiexcitation centers. Figure 5d shows PL lifetimes as a function of temperature and reveals the carrier confinement behavior of the NTs. We extracted the radiative (τrad) and nonradiative (τnr) lifetimes using the following expressions:48 ߬௥௔ௗ (ܶ) = ߬௉௅ (ܶ)/ߟ௧௛ (ܶ);

(3)

and ߬௡௥ (ܶ) = ߬௉௅ (ܶ)/(1 − ߟ௧௛ (ܶ));

(4)

where τPL(T) is the PL lifetime of the NTs at any given temperature and ηth(T) is the ratio between the PL intensity at any given temperature and at 4 K. The validity of this method is based on the assumption that internal quantum efficiency is at its maximum at 4 K.48 We observed that the radiative lifetime seems to increase in an approximately linear fashion as the temperature increases; that is, τrad α T, which is a signature of 1D-like confinement of carriers.49,50 The three-dimensional (3D) behavior exhibited by bulk materials (represented by the dotted line in Figure 5d) diverges significantly from the experimentally observed dependence of the radiative lifetime on temperature. As the NT wall thickness (10–25 nm) is greater than the 10 ACS Paragon Plus Environment

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Bohr diameter (2aB), 1D-like confinement can be ascribed to the quantum-confined Stark effect (QCSE) induced by both spontaneous and piezoelectric polarizations in wurtzite ZnO.51 In addition, the reduction in the dimensionality of the thickness of NT walls decreases the photoinduced carrier transit time, which can result in improving responsivity.4,41

Figure 5. (a) PL of ZnO NTs at 6 K and RT; (b) temperature-dependent PL. The inset shows photon energy as a function of temperature; (c) (log. scale) TRPL of ZnO NTs at RT and 4 K with a double exponential decay fit (blue and magenta lines); (d) temperature dependence of radiative and non-radiative recombination lifetimes. The dotted line represents the bulk behavior. To fabricate the PD device, we deposited ITO as an n-contact on NTs and Ag as a pcontact on p-Si. Figure 6a presents a schematic diagram of the vertically injected p-Si/n-ZnO 11 ACS Paragon Plus Environment


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NT-based PD. Relevant energy levels of the materials used in this device structure are shown in Figure 6b. The valence band (VB) and conduction band (CB) of Si are −5.17 and −4.05 eV, respectively, while the VB and CB of ZnO are −7.71 and −4.35 eV, respectively.15,52 Upon 365nm UV illumination, photo-induced carriers are generated in ZnO NTs. The energy band alignment of the device ensures that electrons are injected from the photosensitive ZnO NTs and are collected by the ITO electrode, while the VB holes reach the Ag electrode through the p-Si substrate.

Figure 6. (a) A schematic illustration of the structure of our p-Si/ n-ZnO NTs UV PD; (b) an energy band diagram of our PD. We measured the I–V characteristics of our p-Si/n-ZnO NT-based PD under 365-nm illumination at different power densities (1−100 mW cm-2) as shown in Figure 7a, revealing high UV photoresponse at the reverse-bias voltage of 2 V. At −2 V bias voltage, the output current increases from 7.19 (dark) to 176.06 µA at a power density of 1 mW cm-2. We observe rectifying behavior in the dark, which is a p-n junction characteristic. However, under illumination,


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electrons and holes generated in the depletion region of the p-Si/n-ZnO NT heterojunctions drift in opposite directions in the presence of an electric field operating under a reverse bias, leading to the collection of more photocurrent than dark current and the disappearance of the rectification characteristics of the device. Such behavior has been observed in p-n PDs.19,20,22,40,53,54 A reverse bias is thus advantageous for PDs because it increases the depletion width, while decreasing the photo-induced carrier transit time and the carrier loss.19 Under a forward bias, photoresponse to UV light is also possible.

Figure 7. (a) I–V characteristics of our p-Si/n-ZnO PD under dark and 365-nm illumination conditions with different power densities; (b) photoresponse under 365-nm UV illumination of 1 mW cm-2 and bias voltage of ±2 V; (c) Rise and fall photoresponse time of PD fit by doubleexponential functions under 365 nm illumination with intensity of 1 mW cm-2 and a bias of -2 V; (d) The responsivity and detectivity under different bias voltage for a fixed illumination power of 365 nm with 1 mW cm-2; (e) Responsivity and detectivity under 365 nm illumination with different light intensities and a bias of -2 V.



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The responsivity of our device was examined using the 1 mW cm-2 and 365 nm UV source under ±2V bias. Figure 7b shows the device photoresponse response obtained by switching the light source on and off six times in order to test its repeatability in the UV region under a forward- and reverse-bias of 2 V. As shown in Figure 7b, the photocurrent was consistent and repeatable under both reverse (red curve) and forward (black curve) bias. The curve in Figure 7c shows the photoresponse time pertaining to reverse bias of 2 V when the PD was operated under UV illumination (a power density of 1 mW cm-2). Here, to determine the changes in the photoresponse time during the rise and decay, we fitted the data using various functions, whereby the best fit was achieved with a double-exponential function.4,55 (Figure S4 in supporting information) This behavior is prevalent and characterizes many materials, including ZnO NR/nanowire (NW)-based UV photodetectors 4, 55,56 due to the presence of the defect traps, which delays charge carrier evacuation. 57,58 Based on the fitting functions and parameters (blue curve), the time constants for the photocurrent rise τr1 and τr2 and decay τd1 and τd2 are estimated at 0.44 s, 3.19 s, 0.559 s, and 7.18 s, respectively. This indicates a rapid rise, followed by a slow rise and a rapid decay, followed by a slow decay. We derived the weighted-average rise and fall times of our device at 0.44 s and 0.599 s, respectively. A typical photoresponse and recovery time is observed compared to high performance PDs based on ZnO reported in previous work, as shown in Table 1. Constant photocurrent over 3600 s demonstrates the stability of the operational device after a storage time of 5 months (Figure S5 in supporting information). Visible blindness characteristics are crucial in UV PDs. Therefore, we demonstrate that the I−V measurements of our PD under white light show a very week response (Figure S6 in the supporting information), confirming with visible blindness characteristics of our PD. 20, 59,60


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Table 1. Progress in the ZnO NTs-based PD and other high performance ZnO-based PDs.

Structure

Bias

Power of

Voltage

illumination

Electrode

Responsivity

Rise time

Fall time

Ref

0.01 A/W

-

-

17

0.07 A/W

-

-

19

0.34 A/W

-

-

22

0.38 A/W

11 s

14 s

20

24 A/W

1s

45 s

23

0.67 s

1.02 s

18

0.1 s

1s

21

0.44 s

0.599 s

xenon lamp At p-Si/n-ZnO NWs

Au/Ti/ITO

-1 V 480 nm 25 µW

p-Si/n-ZnO NWs

Ti

-20 V at 365 nm 650 µW

p-Si/n-ZnO

Ti/Al

-2 V at 365 nm 150 W xenon

p-Si/n-ZnO NRs

Ag

-4 V lamp at 360 nm 15 mW/cm2

ZnO film (MSM)

Au/Pt

3V at 325 nm

ZnO NWs

10 V

0.5 mW/cm2

40 A/W

Au (MSM)

(5 V)

at 370 nm

(1.2 A/W)

ZnO 1.06 mW/cm2 nanoparticles

Au

120 V

61 A/W at 370 nm.

film (MSM) -5 V

101.2 A/W 1 mW/cm2

p-Si/n-ZnO NTs

ITO

This

-3 V

40 A/W at 365 nm

work

-2 V

21.5 A/W

R and D* are the most critical parameters determining PD performance. Previous reports indicated that R and specific D* are calculated based on the assumption that noise in the current



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is primarily dominated by shot noise.29,61,62 The photocurrent response of our PD leads very high R (in the AW-1 range), D* and external quantum efficiency (EQE). Specifically, under 365 nm UV illumination of 1 mW cm-2 and -2 V, R = 21.51 AW−1 is obtained in the UV region, corresponding to D* = 1.26×1012 cm Hz1/2 W-1 and EQE = 73.1×102 % with an effective area of ~0.79 mm2. When, the voltage bias increases to -5 V, the responsivity increases to 101.2 AW-1 as shown in Figure 7d. Thus, we achieved an R value that was greater than that of commercial UV Si-based PDs (mostly in the 0.1–0.2 AW−1 range, with effective areas of 0.8 mm2).1,63 Table 1 provides a comparison of the responsivity, bias voltage, power density of illumination, and photoresponse time of ZnO NWs-based photoconductors reported in extant publications with those obtained in our work, indicating superiority of the responsivity of our ZnO/p-Si NTs. To confirm that this high responsivity is attributed to the NTs and not the nanostructures between the NTs, the I−V measurement of a ZnO layer without NTs grown on p-Si was carried out (Figure S7, supporting information). No p-n characteristic is observed, and very low responsivity (0.48 AW-1) is obtained, indicating that the high responsivity of our PD is due to the NTs. Figure 7e shows that R decreased as the UV intensity increased, which is consistent with the observations made in previous studies.16, 24,64,65 At relatively high intensities, the decrease in R is attributed to a manifestation of hole-trap saturation

4

and the shrinking of the depletion

region.66 Therefore, R is defined as the ratio of the photocurrent (IPh) and the incident power received over the device effective area, given by the relation R = IPh/P.64 It can be clearly seen that, at a fixed bias voltage, the responsivity decreases with the increasing illumination power. In addition, for a fixed illumination power, the detector exhibits a higher responsivity at a greater bias voltage.


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We demonstrate that the photodetecting performance was significantly increased in ZnO NT-based PDs. We ascribe this superior performance compared to those obtained in previous studies to several factors. First, the superior crystal quality of ZnO NTs with low defect densities, as shown by PL, leads to a remarkable reduction in the density of traps induced by defects in the bandgap and allows sufficient photo-induced carrier transport. Second, the large surface-tovolume ratio of ZnO NTs (as the hollowing emerges from the ZnO-Si interface) boosts the responsivity due to the surface states. This, in turn, enhances the density of trapped carriers (mainly holes), leading to an increase in the carrier lifetime accompanied by a reduction in the carrier transient time due to low dimensionality of the NTs compared to NWs.4 As most photons are absorbed at the surface before arriving at the internal surface of the NT,40,67 the abovebandgap excitation (using UV light) leads to induced the surface states. In addition, the oxygen adsorption/desorption process may occur in the surface state, leading to more generated carriers.2,

4, 68

.Third, the nano-dimensions of ZnO NTs allow 1D-like confinement, which is

essential for enhancing absorption during illumination and reducing the temperature dependence of the devices, while also diminishing carrier scattering, thereby enhancing carrier transport. Fourth, as the d/r ratio is smaller than (3/8)1/2, it allows internal reflection 34,35 light to be trapped inside the active layer, resulting in a reduction in the transit time of the photo-induced carriers and consequently an increase in the responsivity.36 Therefore, the high-quality structure, with a large surface-to-volume ratio leads to a strong carrier transport process, which results in the superior responsivity of the NTs. In sum, the high responsivity and stability of the ZnO NT structures suggest that are impressive candidates for use in UV PD devices compared to nanowires. CONCLUSIONS



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We demonstrated that our 1D p-Si/ZnO NT-based UV PD optimized to obtain high UV responsivity (fabricated by direct growth without a need for costly and complex fabrication processing). SEM and TEM confirmed the high quality of the vertical n-ZnO NTs (with welldefined hollowing) grown on p-Si. TRPL revealed that the radiative carrier lifetime is due to 1Dlike carrier confinement. We fabricated p-Si/n-ZnO NT heterojunctions using a process that can potentially serve as a viable approach for enhancing the performance of UV PD devices due to high photo-induced carrier density, the high surface-to-volume ratio of the ZnO NTs and the extremely low density of defect trapping states. These characteristics result in superior responsivity in the hundreds ampere-per-watt range (101.2 AW−1), as well as high external efficiency. Using this process, PDs based on both Si and ZnO can be produced at low cost for biological, environmental and industrial applications. In addition, the junction devices based on Si are compatible with the complementary metal-oxide-semiconductor (CMOS) technology.

ASSOCIATED CONTENT Supporting Information Supporting Information is available from ACS website. The microscope image of ITO electrode; a titled SEM image of ZnO NTs; The TRPL spectra for different temperature; The method of the double-exponential function fitting of the response time; Photocurrent stability of the device over time; I–V characteristics of our p-Si/n-ZnO PD under visible light conditions; SEM images of the wetting layer without NTs and its I-V characteristics.

AUTHOR INFORMATION Corresponding Author


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Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Tahani Flemban is grateful for a scholarship from Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia. The authors wish to thank Ms. Ecaterina Ware from Imperial College London, UK for TEM sample preparation, and Dr M. A. Roldan from KAUST core lab for HRTEM measurements. The authors thank KAUST for the financial support.

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A Table of Contents graphic



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Figure 1. ZnO NTs grown on p-Si (100) under different laser fluences at P (O2) = 100 mTorr. (a) 6.2 J cm-2; (b) 4.4 J cm-2; (c) 4.0 J cm-2; (d) 3.6 J cm-2; (e) 2.8 J cm-2; (f) 1.6 J cm-2. 139x81mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Figure 2. ZnO NTs grown by increasing the number of pulses (up to 40,000) with laser fluence of 2.8 J cm-2 (a) SEM top view; the inset is a zoomed image of a single NT; (b) cross sectional SEM image. 83x38mm (300 x 300 DPI)


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Figure 3. HR-TEM of a cross section of ZnO NTs near the interface, indicating that the hollowing emerged from the interface. 83x41mm (300 x 300 DPI)



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Figure 4. (Log. scale) XRD pattern of ZnO NTs grown on p-Si substrate. 83x68mm (300 x 300 DPI)


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Figure 5. (a) PL of ZnO NTs at 6 K and RT; (b) temperature-dependent PL. The inset shows photon energy as a function of temperature; (c) (log. scale) TRPL of ZnO NTs at RT and 4 K with a double exponential decay fit (blue and magenta lines); (d) temperature dependence of radiative and non-radiative recombination lifetimes. The dotted line represents the bulk behavior. 177x135mm (300 x 300 DPI)



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Figure 6. (a) A schematic illustration of the structure of our p-Si/ n-ZnO NTs UV PD; (b) an energy band diagram of our PD. 177x91mm (300 x 300 DPI)


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Figure 7. (a) I–V characteristics of our p-Si/n-ZnO PD under dark and 365-nm illumination conditions with different power densities; (b) photoresponse under 365-nm UV illumination of 1 mW cm-2 and bias voltage of ±2 V; (c) Rise and fall photoresponse time of PD fit by double-exponential functions under 365 nm illumination with intensity of 1 mW cm-2 and a bias of -2 V; (d) The responsivity and detectivity under different bias voltage for a fixed illumination power of 365 nm with 1 mW cm-2; (e) Responsivity and detectivity under 365 nm illumination with different light intensities and a bias of -2 V. 177x90mm (300 x 300 DPI)



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TOC 83x33mm (300 x 300 DPI)


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n-ZnO Nanotube Heterojunctions with

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