Fabrication of ZnO Nanorods p-n Homojunction Light-Emitting Diodes

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Fabrication of ZnO Nanorods p-n Homojunction Light-Emitting Diodes Using Ag Film as Self-Doping Source for p-Type ZnO Nanorods Do Kyun Kwon, Yoann Porte, and Jae-Min Myoung J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02330 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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The Journal of Physical Chemistry

Fabrication of ZnO Nanorods p-n Homojunction Light-Emitting Diodes using Ag Film as Self-Doping Source for p-type ZnO Nanorods

Do-Kyun Kwon, Yoann Porte, and Jae-Min Myoung* Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea

*E-mail) [email protected] 1

ACS Paragon Plus Environment

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ABSTRACT

In this work, homojunction light-emitting diodes (LEDs) based on n-type zinc oxide (ZnO) nanorods (NRs)/silver (Ag)-doped p-type ZnO NRs were successfully fabricated by low-temperature solution process. Here, the Ag thin film used as a template and dopant source for the growth of p-type ZnO NRs as well as a bottom electrode. The Agdoped p-type ZnO NRs were synthesized using an ammonium hydroxide solution, which possesses a high pH value of 11.6 in order to dissolve the Ag film and form Ag+ ions in the solution. Using these Ag-doped p-type ZnO NRs as a template, n-type ZnO NRs were epitaxially grown on top of them at 90 °C to form ZnO NRs p-n homojunction. These ZnO NRs p-n homojunction LEDs showed a typical rectifying behavior with a turn-on voltage of 3.5 V and a high rectifying ratio of 1.5 × 105 at 5 V. Furthermore, under a forward bias of 9 V, the LED exhibited a wide yellow EL emission centered at 645 nm, which was attributed to the various emission sites of ZnO deeplevel defects. This study suggests a facile fabrication method for ZnO NRs p-n homojunction LEDs by using a simple p-type doping approach with Ag during the lowtemperature solution process.

KEYWORDS: Ag doping, p-type ZnO NRs, Ag thin film, Homoepitaxial p-n junction, Solution process, Light-emitting diode.

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INTRODUCTION

The development of light-emitting diodes (LEDs) allowed to overcome the limitations of conventional incandescent light bulbs with their small size, high performance, and low power consumption [1,2]. However, there is still some room for improvement, especially regarding the materials used in the fabrication of LEDs. As a consequence, the focus has been brought onto materials that are abundant, environmentally friendly, and easy to process. The use of high vacuum along with high temperature in conventional GaN and GaAs-based LEDs to maintain a high purity makes it an expensive fabrication process [3]. In contrast, metal oxides are abundant, low cost, easy to process, and chemically stable, which makes them ideal candidates as alternative materials [4]. Amongst metal oxides, ZnO has attracted significant interests as a potential material for LED applications owing to its versatile properties such as environmental friendliness, wide direct bandgap (3.37 eV), and high exciton binding energy (~60 meV) at room temperature [5-8]. Generally, oxide-based films are polycrystalline which results in lower efficiency in LEDs due to an interruption of electron transfer [4]. However, ZnO can be synthesized with single crystal nanostructures for high efficiency LEDs such as nanorods (NRs), nanoparticles (NPs) using a simple low-cost solution process, i.e. hydrothermal growth [9,10]. Especially, vertical ZnO NRs can act as a waveguide, so help the light extraction to the top of the device [11]. But, despite these advantages, ZnO NRs are not used in commercial LEDs due to several issues, such as the difficulty to induce p-type doping in ZnO and the necessity to use a seed layer to induce a vertical growth of the NRs.

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One of the main issues lies in the nature of the conductivity in ZnO. Since ZnO is intrinsically n-type due to the favored formation of donor type defects in the lattice, the achievement of p-type conductivity remains challenging [12]. As a results, several studies using ZnO NRs in LEDs as n-type material have been conducted with alternative p-type materials for the fabrication of heterojunction [13-18]. However, the heterojunction LEDs usually show a poor luminance efficiency due to the large lattice mismatch at the interface of the p-n junction. Thus, to achieve high performance LEDs, the homojunction structure is always preferred. Recently, several studies reported successful p-type conductivity in doped ZnO NRs [19,20]. To synthesize p-type ZnO, vacuum-based processes such as molecular-beam epitaxy or chemical vapor deposition are usually adopted [21-24]. However, these synthesis methods generally require high temperature for dopant activation with high system cost [25-28]. In contrast with vacuum processes, solution-based p-type doping processes generally enable the lowtemperature and low-cost synthesis of p-type ZnO NRs [29-34]. Here, issues such as the stability of the p-type conductivity or the initial growth control for vertical ZnO NRs on the substrates represents additional difficulties in the LED fabrication process. Most of the p-type ZnO NRs reported require the use of a seed layer due to the large lattice mismatch or different crystal structure between the ZnO NRs and the substrate. Due to these difficulties, very few studies have been performed on the ZnO homojunction LED fabrication. Fang et al. reported the fabrication of phosphorous (P)-doped p-ZnO NRs/n-ZnO NRs homojunction LEDs [29]. However, their process required a high annealing temperature of 800 ° C and an additional n-type ZnO seed film. In addition, Hsu et al's undoped p-type ZnO NRs and Baek et al's Sb-doped p-type ZnO NR were

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grown by solution process and applied to ZnO homojunction LEDs, but additional seed layer and dopant were needed.[31,35] In this paper, an Ag thin film was employed as a multiple role implementing layer, which acted as a dopant source for p-type ZnO NRs, a growth template for ZnO NRs, and a bottom electrode. During the growth of ZnO NRs by hydrothermal synthesis, Ag+ ions are dissolved from the Ag film and, subsequently incorporated into the ZnO lattice. Due to the substitution of Zn2+ ions with dissolved Ag+ ions, the p-type ZnO NRs were synthesized [36]. Then, the n-type ZnO NRs were homoepitaxially grown on these Agdoped p-type ZnO NRs to fabricate an efficient p-n homojunction LED with minimum interface defects. The ZnO NRs p-n homojunction LEDs fabricated through this method exhibited a bright yellow light with a large rectifying ratio.

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EXPERIMENTAL

Synthesis of Ag-doped p-type ZnO NRs. An Ag (150 nm)/Ti (4 nm) layer was deposited on a glass substrate by using an e-beam evaporator. A part of the Ag thin film was covered with self-adhesive PI tape which was used as mask to keep a contact area for the bottom electrode. Then, the ZnO NRs were grown on the Ag thin film by a lowtemperature hydrothermal process using an aqueous solution (25 mL) of zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma-Aldrich, 0.2 g) and an ammonium hydroxide solution (NH4OH, OCI, 1 mL) in an oven at 90 °C. During the growth, Ag+ ions are dissolved from the film and incorporated into the ZnO lattice. The Ag doping concentration was adjusted by changing the growth time of the ZnO NRs in the aqueous solution. After the growth process, the ZnO NRs were rinsed with DI water and dried at room temperature for 30 min in a vacuum chamber. Synthesis of n-type ZnO NRs. The space between the Ag-doped p-type ZnO NRs was filled by an AZ 5214E photoresist (PR) deposited by spin coating. Then the PR layer covered on the p-type ZnO NRs was etched by reactive ion etching (RIE) using oxygen plasma of 150 W until the top surface of NRs are exposed. The n-type ZnO NRs were grown on the Ag-doped p-type ZnO NRs by a similar aqueous hydrothermal process. Zinc

nitrate

hexahydrate

(Zn(NO3)2·6H2O,

Sigma-Aldrich,

0.52

g)

and

hexamethylenetetramine (HMTA, C6H12N4, Sigma-Aldrich, 0.24 g) were dissolved in 50 mL of DI water with a solution concentration of 40 mM. The growth temperature of the solution was maintained in a heating block at 90 °C for 2 h. After the reaction, the samples were washed with DI water and dried at room temperature for 30 min in a vacuum chamber.

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Device fabrication and characteristization. The PR molding and etching processes were repeated to avoid Schottky contact between the Ag-doped p-type ZnO NRs and the top electrode (Al). An 80 nm-thick Al electrode was deposited on the n-type ZnO NRs by using an e-beam evaporator. In order to confirm the reliability of the device fabrication process and device characteristics, more than 20 devices were fabricated by the same process. The morphology of the Ag-doped p-type ZnO NRs and the n-type ZnO NRs were observed by field-emission scanning electron microscopy (FESEM, JSM-6701F, JEOL). The crystalline structures of the Ag-doped p-type ZnO NRs and the n-type ZnO NRs were analyzed by high-resolution transmission electron microscopy (HRTEM, JEM-ARM 200F, JEOL) with a focused ion beam system (JIB-4601F, JEOL) and X-ray diffraction measurement (XRD, Ultima IV, Rigaku). To investigate the chemical composition of Ag-doped p-type ZnO NRs, X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo VG, U.K. Inc.) was carried out with a monochromatic Al-Kα line (1486.6 eV). In order to confirm the optical properties of the Ag-doped p-type ZnO NRs and the n-type ZnO NRs/Ag-doped p-type ZnO NRs homojunction, roomtemperature photoluminescence (PL, Dongwoo Optron) measurements were conducted by using a He‒Cd laser (λ = 325 nm) as an excitation source. The electrical properties of the ZnO NRs p-n homojunction were measured by using a semiconductor parameter analyzer (Agilent B1500A, Agilent Technologies) in ambient atmosphere at room temperature. To prevent any damages to the ZnO NRs p-n homojunction and top Al electrode, the physical contact was made between a gold (Au) wire probe (0.25 mm in diameter) and the surface of the Al electrode. Finally, the electroluminescence (EL) spectroscopy measurements of the ZnO NRs p-n homojunction were performed by using Andor SOLIS simulation software connected with a charge-coupled device (CCD) 7



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camera (DV401A-BV).

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RESULTS AND DISCUSSION

The fabrication steps of homoepitaxial p-n junction LED are schematically illustrated in Figure. 1. First, a 150 nm-thick Ag film on 4 nm-thick Ti thin film was deposited on a glass substrate as a bottom electrode. The Ag thin film acts as a template for the growth of ZnO NRs since the (111) plane of Ag (2.89 Å) has a lattice spacing similar to that of the a-axis lattice constant of ZnO (3.25 Å) [37]. This allows the ZnO NRs to grow along the c-axis on the Ag thin film without an additional seed layer. The growth of Ag-doped p-type ZnO NRs was processed with an ammonium hydroxide solution, which is a widely used agent for the dense growth of vertical ZnO NRs (step 1) [38]. The ammonium hydroxide solution was selected because of its high pH value of 11.6. It allows to dissolve a part of the Ag thin film to form Ag+ ions in the solution, which act as a p-type dopant source for ZnO [39]. Within the short duration of the growth of the p-type ZnO NRs, the Ag layer cannot be completely dissolved. Furthermore, when the p-type ZnO NRs are uniformly grown and fully cover the Ag film, they act as a capping layer and prevent the Ag layer from being dissolved. As a consequence, the Ag thin film can still be used as a bottom electrode, even after the p-type ZnO NR growth. After preparing the Ag-doped p-type ZnO NRs, the exposed areas of Ag thin film between the NRs were passivated with PR in order to prevent any electrical short between the Ag bottom electrode and upper p-type ZnO NRs layer (step 2). In order to achieve an epitaxial growth of the n-type ZnO NRs on top of the p-type ZnO NRs, the PR was etched by RIE to expose the surface of the p-type ZnO NRs. Then to create the ZnO NRs p-n homojunction, the n-type ZnO NRs were grown by HMTA solution (step 3). Ag+ ions used as a dopant source of p-type ZnO NRs cannot be produced because of the

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passivation by PR layer and the relatively low pH value of the HMTA solution (6.7). Therefore the ZnO NRs grow with intrinsic n-type characteristics. Figures. S1a−c shows the SEM image of the surface of the Ag layer as deposited, after bathing in ammonium hydroxide solution, and after bathing in HMTA solution, respectively. It can be clearly observed that only the ammonium hydroxide solution dissolves the surface of the Ag film, owing to its high pH value. Although the reactant solutions for p-type ZnO NRs and n-type ZnO NRs were different, the synthesized ZnO NRs formed a homojunction with minimized lattice strain due to matching lattice parameters. The n-type ZnO NRs were passivated by PR and etched by RIE to expose their surface (step 4). Finally, the homoepitaxial p-n junction ZnO NRs LEDs were fabricated by depositing an Al thin film as a top electrode (step 5).

Figure. 1. Schematic illustration of fabrication step for the ZnO NRs p-n homojunction LED.

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Figure. 2 shows the structural and optical properties of Ag-doped p-type ZnO NRs synthesized by hydrothermal growth at low-temperature. Figure. 2a reveals a crosssectional SEM image of the Ag-doped p-type ZnO NRs indicating individual tapered shape of the NRs which directly grown on the Ag thin film. Also, the inset of Figure. 2a shows the top SEM image of the densely grown Ag-doped p-type ZnO NRs. This tapered shape of the NRs is due to the high pH of ammonium hydroxide solution [40]. A high-resolution TEM image of the Ag-doped p-type ZnO NRs reveals a clear lattice fringe with a lattice spacing of 0.52 nm corresponding to (0001) planes of the single crystalline p-type ZnO NRs, as shown in Figure. 2b. Also, the selected-area electron diffraction (SAED) pattern in the inset of Figure. 2b indicates a single crystallinity of the Ag-doped p-type ZnO NRs. The XRD spectra of the Ag-doped p-type ZnO NRs in Figure. 2c show that the NRs have a wurtzite crystal structure with diffraction along of the (1010) and (0002) planes. As previously confirmed in Figures. 2a and b, strong (0002) peak signifies that the Ag-doped p-type ZnO NRs were grown along c-axis predominantly. In order to confirm the incorporation of Ag atoms in the ZnO NRs, XPS analyses were performed, as shown in Figure. 2d. The O 1s narrow-scan spectrum is composed of four sub-peaks which correspond to adsorbed H2O (532.08 eV), Zn−OH bonds (531.36 eV), Zn−O (530.19 eV), and Ag−O bonds (529.01 eV), respectively [36]. The bonding peak of Ag−O bond indicates that the Ag+ ions are doped into the lattice of ZnO NRs. The XPS spectra of Ag 3d, in the inset of Figure. 2d, show the Ag 3d5/2 and 3d3/2 peaks centered at 367.4 eV and 373.4 eV, respectively, which means that the doped Ag+ ions are bonded with O atoms [41]. Furthermore, no secondary phases were observed in the XRD spectra, confirming the successful incorporation of Ag in the ZnO lattice. The optical properties of the Ag-doped p-type ZnO NRs were measured by 11



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room-temperature PL as shown in Figure. 2e. The room-temperature PL spectra of the Ag-doped p-type ZnO NRs exhibit a very weak near band-edge (NBE) emission and a broad deep-level emission (DLE), centered at 368.6 nm and 550 nm, respectively. It is believed that due to the solution growth process, the Ag-doped p-type ZnO NRs contain a large concentration of deep-level defects, accounting for the large DLE measured [42].

Figure. 2. (a) Cross-sectional SEM image of Ag-doped p-type ZnO NRs and corresponding top-view SEM image (inset). (b) HRTEM image showing a clear lattice fringe with an interplanar spacing of 0.52 nm and corresponding SAED pattern revealing a single crystallinity of the Ag-doped p-type ZnO NRs (inset). (c) XRD spectra of the Ag-doped p-type ZnO NRs. (d) XPS spectra for the O 1s of Ag-doped ptype ZnO NRs and for the Ag 3d (inset). (e) Room-temperature PL spectra of the Agdoped p-type ZnO NRs with four prominent deconvoluted peaks.

In a previous study, we focused on the properties of Ag-doped p-type ZnO NRs rather than their application in LEDs [36]. In that case, the PL peak shift of the NBE region

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was observed intensively by increasing the NBE peak intensity through the high temperature annealing. However, in this study, since the annealing process was not performed to study the application of ZnO NWs-based visible light LEDs synthesized by the low temperature solution process, strong DLE emission peak was observed. This DLE peak was deconvoluted into four emission sub-peaks at 495, 541, 612, and 700 nm corresponding to photon energies of 2.51, 2.29, 2.02, and 1.77 eV, respectively. The green emissions corresponding to the 495 and 541 nm sub-peaks are attributed to the transitions of electrons from the conduction-band minimum (CBM) and a zinc interstitial (Zni) to a zinc vacancy (VZn), respectively [43,44]. Also, the orange emission of the 612 nm sub-peak is related to a transition of electron from the Zni to an oxygen interstitial (Oi) [45]. Finally, the red emission with a 700 nm sub-peak occurs with the electron transition from an oxygen vacancy (VO) to the valence-band maximum (VBM) [44]. The n-type ZnO NRs with an average length of 2.5 μm were epitaxially grown on the Ag-doped p-type ZnO NRs, as shown in the tilted-view SEM image of Figure. 3a. The corresponding tilted SEM image in the inset of Figure. 3a presents the uniformity of the grown ZnO NRs at a larger scale. The homojunction interface between the n-type ZnO NR and the Ag-doped p-type ZnO NR, shown by the low-magnification TEM in Figure. 3b, confirms both NRs have been grown along the [0001] direction. The HRTEM analysis of the interface, Figure. 3c, reveals a lattice spacing of 0.52 nm for the n-type NRs, corresponding to the (0001) plane, which is the same value as for the p-type NRs. This indicates the n-type NRs were grown homo-epitaxially on the p-type NRs in the [0001] direction with minimized interface defects, owing to the matching lattices. In the inset of the Figure. 3c, the SAED pattern at the interface region of the n-type ZnO 13



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NR/Ag-doped p-type ZnO NR homojunction proves the mono-crystallinity of the interface. The XRD spectra of the n-type NRs grown on the p-type NRs, Figure. 3d, confirm the NRs adopted the wurtzite crystal structure with growth direction similar to the Ag-doped p-type ZnO NRs with the (1010) and (0002) planes (JCPDS #36-1451). Especially, the vertical alignment of the n-type ZnO NRs along the c-axis was confirmed by the strong intensity of the (0002) peak. The position of the (0002) peak for the n-type and p-type NRs are compared in the inset of Figure. 3d. In the case of the Agdoped p-type NRs, the (0002) peak is shifted toward lower 2θ angle from 34.02° to 33.84°compared to the n-type NRs. According to Bragg’s equation, nλ=2dsin(θ), a lower diffraction angle θ results in a larger lattice d-spacing. Therefore, this peak shift is attributed to the incorporation of Ag+ in the ZnO lattice. When substituted on Zn2+ sites, the Ag+ ions induce an expansion of the lattice size owing to the large difference in ionic radius between Ag+ (1.26 Å) and Zn2+ (0.74 Å) [46]. This information confirms that the Ag+ ions dissolved from the Ag bottom electrode substitute the Zn2+ ions in the ZnO lattice during the growth of ZnO NRs, and it induces the expansion of the lattice resulting in a peak shift towards lower diffraction angle in the XRD spectra. The optical properties of the n-type ZnO NRs were measured by room-temperature PL, as shown in Figure. 3e. The n-type ZnO NRs show a weak NBE peak centered at 378.6 nm and a broad visible-range DLE peak ranging from 500 to 800 nm. It is considered that electron transitions through the defect levels are favored instead of the direct band-to-band transition due to a large concentration of deep-level defects existing in the n-type ZnO NRs. Through deconvolution of the PL spectra of the n-type ZnO NRs at DLE, four sub-peaks were obtained which were centered at 541, 612, 700, and 742 nm, representing photon energies of 2.29, 2.02, 1.77, and 1.67 eV, respectively. 14


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Compared with the Ag-doped p-type ZnO NRs, the sub-peak positioned at 612 nm in the PL spectra of the n-type ZnO NRs shows the strongest intensity, while in the p-type NRs the sub-peak at 541 nm was the most intense. Regarding the 495 nm sub-peak, nothing was measured, as opposed to the Ag-doped p-type ZnO NRs. The intensity of the 541 nm sub-peak was decreased, while the intensity of 700 nm sub-peak was increased. Moreover, an additional 742 nm sub-peak corresponding to deep red emission was detected due to the electron transitions from CBM to VO [44]. In order to insist on the differences in optical properties between the n-type and p-type NRs, the PL spectra of the n-type ZnO NRs was compared with that of the Ag-doped p-type ZnO NRs, as shown in Figure. S2 with a focus on the NBE. The NBE peak intensity of the ntype ZnO NRs was observed to be more intense than the one of the Ag-doped p-type ZnO NRs, whereas the NBE peak position (378.6 nm) of the n-type ZnO NRs was blueshifted to 368.6 nm for the Ag-doped p-type ZnO NRs. In general, Ag doping in ZnO is reported to induce an increase in the NBE peak intensity [47]. However, in this study, Ag-doped p-type ZnO NRs and n-type ZnO NRs were synthesized under different conditions using ammonium hydroxide solution and HMTA solution, respectively. During the synthesis of ZnO NRs, the pH value of the solution not only affects the shape of the NRs but also the nature and concentration of surface defects [48, 49]. Therefore, the Ag-doped p-type ZnO NRs synthesized with ammonium hydroxide solution, which possesses a high pH value, show a weaker NBE peak than the n-type ZnO NRs synthesized using the HMTA solution. However, this NBE peak shift is a direct consequence of the Ag-doping. Several studies reported an increase of the bandgap when Ag is doped in ZnO [47, 50], owing to a decreased bond length, which leads to the blue shift of NBE peak position. 15



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Figure. 3. (a) 40º-tilted SEM of the vertically grown n-type ZnO NRs on the Ag-doped p-type ZnO NRs and corresponding top-view of the n-type ZnO NRs (inset). (b) Lowmagnification TEM image of the interface between Ag-doped p-type ZnO NR and ntype ZnO NR. (c) HRTEM image of the interface indicating the epitaxial growth of the n-type ZnO NR on the Ag-doped p-type ZnO NR with a prominent lattice fringe and corresponding SAED pattern revealing a single crystallinity of the interface (inset). (d) XRD spectra of the as-grown vertical n-type ZnO NRs confirming a predominant growth along the c-axis. Comparison of (0002) peaks from the XRD spectra of the Agdoped p-type ZnO NRs and the n-type ZnO NRs (inset). (e) Room-temperature PL spectra of the n-type ZnO NRs with four prominent deconvoluted peaks.

Figure. 4 shows the structural and electrical properties of the completed ZnO NRs p-n homojunction LED. In this experiment, the growth time of the Ag-doped p-type ZnO NRs was optimized to 30 min leading to an average length of 0.8 μm. The ZnO NRs not only grow vertically along the c-axis, but also grow in lateral direction at the same time. 16


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As the growth time increases, the ZnO NRs cover a larger surface of the Ag thin film. This leads to the gradual decrease of the exposed Ag area until a complete covering of the film, which slows down the dissolution of Ag in the ammonium hydroxide solution. This results in a falling gradient of Ag-doping concentration along the perpendicular direction of the Ag-doped p-type ZnO NRs, and the loss of p-type ZnO properties [36].

Figure. 4. Cross-sectional SEM images of ZnO NRs p-n homojucntions after PR molding and RIE process. The ZnO NRs p-n homojucntions were prepared with different growth times for the Ag-doped p-type ZnO NRs; (a) 30 min, (b) 60 min, and (c) 90 min, with each inset showing the 40º-tilted SEM image of the respective growth times. J−V characteristics of the ZnO NRs p-n homojunction LEDs with (d) 30 min, (e) 60 min, and (f) 90 min-grown Ag-doped p-type ZnO NRs.

Figures. 4a−c show the cross-sectional SEM images of the ZnO NRs p-n homojunction after PR passivation and a RIE process with different growth times for the Ag-doped p-type ZnO NRs, 30, 60, and 90 min, respectively. With increasing

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growth time of the Ag-doped p-type ZnO NRs from 30, to 60 and to 90 min, the length of the p-type NRs was observed to be increased from 0.8, to 1.5 and to 1.9 μm, respectively (inset of Figures. 4a−c). In addition, the SEM image of Ag-doped ZnO NRs grown for 15 min is shown in Figure. S3. The ZnO NRs were sparse and short of length with only 200 nm. So, it was not suitable to apply them for the device. Figures. 4d−f show the current density-voltage (J−V) characteristics of the devices with different growth times for the Ag-doped p-type ZnO NRs. The J−V characteristics of the ZnO NRs p-n homojunction fabricated with the 30 min-grown Ag-doped p-type ZnO NRs show a superior diode behavior compared with other growth time conditions. A high rectifying ratio of 1.5 105 at 5 V was measured with a turn-on voltage of 3.5 V, as shown in Figure. 4d. To verify the origin of the superior rectifying property of this optimized ZnO NRs homojunction, Al/n-type ZnO NRs/Ga-doped zinc oxide (GZO) bottom electrode structure, Ag/Ag-doped p-type ZnO NRs/Ag structure, and Al/Agdoped p-type ZnO NRs/Ag structure were fabricated, respectively, as shown in Figure. S4. The J−V curve in Figure. S4a shows Ohmic contact was obtained from the Al/ntype ZnO NRs/GZO structure, implying that the n-type ZnO NRs have n-type semiconducting properties. On the other hand, the Ag/Ag-doped p-type ZnO NRs/Ag structure in Figure. S4b shows resistive Ohmic characteristic. The reason for this resistance is that the carrier density in Ag-doped p-type ZnO NRs is low due to low Ag doping concentration with self-doping. However, compared to the Al/Ag-doped p-type ZnO NRs/Ag structure showing Schottky junction properties in Figure. S4c, the Ag/Agdoped p-type ZnO NRs/Ag structure clearly exhibits an Ohmic characteristic, implying Ag-doped p-type ZnO NRs have p-type properties. From these, it can be confirmed that the superior rectifying property in Figure 4d originates from a p-n junction composed of 18


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p-type and n-type semiconductors. However, when increasing the growth time of Agdoped p-type ZnO NRs to 60 and 90 min, the rectifying ratio was found to decrease to 32.2 and 5.8, respectively, as shown in Figures. 4e and f. Moreover, the degradation of the electrical properties was also observed through an increase of the leakage current at reverse bias, while the turn-on voltage was decreased under 1 V. This is a direct consequence of the degradation in the quality of the p-n homojunction. As the growth time is increased above 30 min, Ag-doped p-type ZnO NRs lose their p-type property due to the decreasing Ag concentration. These results lead to undefined p-n junction interfaces and to poor rectification. Furthermore, if the Ag doping concentration in the ZnO NRs is too low, the Ag-doped ZnO NRs will no longer exhibit p-type characteristics but n-type instead, and the rectification does not occur even when n-type ZnO is epitaxially grown. Therefore, as the growth time increases beyond a certain threshold, i.e. when Ag doping concentration is too low, more Schottky contact characteristics are exhibited instead of those of a p-n junction. As a consequence, the turn-on voltage is lowered due to the lower forward voltage drop present in the Schottky contact as shown in Figures. 4d-f [51]. Figure. 5a shows the EL spectra of the ZnO NRs p-n homojunction LED under forward biases. As the forward bias is increased from 6 to 9 V with steps of 1 V, the EL intensity increases gradually since an increased charge flow induces more electron-hole pair recombinations. Figure. S5 shows the stable operation of the ZnO NRs p-n homojunction LED under various forward biases from 6 to 9 V for 50 s. This shows that the currents are stable during the whole measurement of the device without electrical breakdown or current drop. As shown in Figure. 5b, the broad EL emission of the LED centered at 645 nm was deconvoluted into five sub-peaks centered at 495, 541, 612, 700, 19



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and 742 nm.

Figure. 5. (a) EL spectra of the ZnO NRs p-n homojunction LED under various dc forward biases indicating broad-visible-range photon emissions. (b) Deconvoluted EL spectra of the LED at a forward bias of 9 V revealing five prominent peaks. (c) The optical microscopy (OM) images of the light-emission under dark and inset shows OM image of the top Al electrode with a diameter of 250 µm.

The sub-peaks of the EL spectra correspond to the sub-peaks from the deconvoluted PL spectra extracted from the Ag-doped p-type and n-type ZnO NRs. This is clear evidence that the recombination between electrons and holes occur at the interface of 20


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the p-n junction. The schematic energy band diagrams between the n-type ZnO NRs and the Ag-doped p-type ZnO NRs, as shown in Figures. S6a and b, show the recombination sites when the p-type and n-type ZnO NRs are separated, and when the p-n homojunction is formed, respectively. It reveals that the bluish green, green, orange, red, and deep red light emissions are caused by the different radiative recombinations possible, owing to the various defect-related energy levels at the ZnO NRs homojunction. The mixing of the various emitted wavelength results in an intense emission wavelength for the ZnO NRs p-n homojunction LED, centered at 645 nm, and corresponding to a bright yellow emission. The bright yellow light emitted from the ZnO NRs p-n homojunction LED can be observed by optical microscopic image, as shown in Figure. 5c. The upper inset image in the figure shows the Al top electrode with a diameter of 250 μm. As the forward bias increased from 6 to 9 V, the intensity of the light emitted increased gradually and the yellow color emission was observable on the entire electrode area after the forward bias reached 7 V.

CONCLUSIONS

In summary, we have fabricated n-type ZnO NRs/Ag-doped p-type ZnO NRs homojunction LEDs by using a low-temperature aqueous solution process. The Agdoped p-type ZnO NRs were vertically grown on the Ag thin film which was used as a bottom electrode, an epitaxial template, and a dopant source for p-type ZnO doping. The n-type ZnO NRs were epitaxially grown on the Ag-doped p-type ZnO NRs, resulting in the ZnO NRs p-n homojunction. The Ag+ ions doping concentration of the Ag-doped ptype ZnO NRs decreased gradually with increasing NRs growth time. The fabricated

21



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homojunction LEDs with the optimized growth time of Ag-doped p-type ZnO NRs showed a high rectifying ratio of 1.5

105 at 5 V with a turn-on voltage of 3.5 V.

Moreover, these ZnO NRs p-n homojunction LEDs were found to be very stable under various forward biases from 6 to 9 V. The EL spectra of the ZnO NRs p-n homojunction LEDs showed 5 visible emission peaks at 495, 541, 612, 700, and 742 nm corresponding to bluish green, green, orange, red, and deep red emission, respectively. The comparative analyses of the energy level diagram and the deconvoluted EL peaks reveal that the largest orange emission is related to the transition of electron from Zni to Oi, the green emissions are assigned to the electron transition from CBM or Zni to VO, and the red emissions are attributed to the electronic transition from CBM to VO or VO to VBM. Due to the combination of these emission peaks, yellow light emission from the ZnO NRs p-n homojunction LEDs was observed through the naked eye and CCD camera in the entire electrode-deposition area. Therefore, it is believed that this research can provides novel method for applications in ZnO NRs p-n homojunction LEDs by using simple Ag thin film dissolving process without adopting any additional seed layer and a doping process.

ASSOCIATED CONTENT

Supporting Information Top-view SEM images of Ag thin film after being immersed in chemical solution while being in an oven (90 ℃, 0.5 h) with the corresponding insets of cross-sectional SEM images, Room-temperature PL spectra of the Ag-doped p-type ZnO NRs and the n-type ZnO NRs in the UV region, 40º-tilted SEM image of Ag-doped ZnO NRs grown 22


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for 15 min, J-V characteristics of (a) Al/n-ZnO NRs/GZO (Ohmic contact), (b) Ag/Agdoped ZnO NRs/Ag (Ohmic contact), (c) Al/Ag-doped ZnO NRs/Ag (Schottky contact) and each inset, Stability test results of the ZnO NRs p-n homojunction LEDs for 50 s at various dc forward biases, Schematic energy band diagrams of n-type ZnO NRs and Agdoped p-type ZnO NRs.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2017M3D1A1027831).

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Table of Contents

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Figure. 1. Schematic illustration of fabrication step for the ZnO NRs p-n homojunction LED. 41x20mm (300 x 300 DPI)



Figure. 2. (a) Cross-sectional SEM image of Ag-doped p-type ZnO NRs and corresponding top-view SEM image (inset). (b) HRTEM image showing a clear lattice fringe with an interplanar spacing of 0.52 nm and corresponding SAED pattern revealing a single crystallinity of the Ag-doped p-type ZnO NRs (inset). (c) XRD spectra of the Ag-doped p-type ZnO NRs. (d) XPS spectra for the O 1s of Ag-doped p-type ZnO NRs and for the Ag 3d (inset). (e) Room-temperature PL spectra of the Ag-doped p-type ZnO NRs with four prominent deconvoluted peaks. 86x42mm (300 x 300 DPI)


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Figure. 3. (a) 40º-tilted SEM of the vertically grown n-type ZnO NRs on the Ag-doped p-type ZnO NRs and corresponding top-view of the n-type ZnO NRs (inset). (b) Low-magnification TEM image of the interface between Ag-doped p-type ZnO NR and n-type ZnO NR. (c) HRTEM image of the interface indicating the epitaxial growth of the n-type ZnO NR on the Ag-doped p-type ZnO NR with a prominent lattice fringe and corresponding SAED pattern revealing a single crystallinity of the interface (inset). (d) XRD spectra of the as-grown vertical n-type ZnO NRs confirming a predominant growth along the c-axis. Comparison of (0002) peaks from the XRD spectra of the Ag-doped p-type ZnO NRs and the n-type ZnO NRs (inset). (e) Roomtemperature PL spectra of the n-type ZnO NRs with four prominent deconvoluted peaks. 83x39mm (300 x 300 DPI)



Figure. 4. Cross-sectional SEM images of ZnO NRs p-n homojucntions after PR molding and RIE process. The ZnO NRs p-n homojucntions were prepared with different growth times for the Ag-doped p-type ZnO NRs; (a) 30 min, (b) 60 min, and (c) 90 min, with each inset showing the 40º-tilted SEM image of the respective growth times. J−V characteristics of the ZnO NRs p-n homojunction LEDs with (d) 30 min, (e) 60 min, and (f) 90 min-grown Ag-doped p-type ZnO NRs. 89x44mm (300 x 300 DPI)


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Figure. 5. (a) EL spectra of the ZnO NRs p-n homojunction LED under various dc forward biases indicating broad-visible-range photon emissions. (b) Deconvoluted EL spectra of the LED at a forward bias of 9 V revealing five prominent peaks. (c) The optical microscopy (OM) images of the light-emission under dark and inset shows OM image of the top Al electrode with a diameter of 250 µm. 71x62mm (300 x 300 DPI)



By using Ag thin film as a multiple role implementing layer, ZnO NRs p-n homojunction LEDs can fabricated easily 25x11mm (300 x 300 DPI)


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Fabrication of ZnO Nanorods p-n Homojunction Light-Emitting Diodes

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