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Electrically Driven Single Microwirebased Heterojuction Light-emitting Devices Anqi Chen, Hai Zhu, Yanyan Wu, Guanlin Lou, Yunfeng Liang, Jinyu Li, Zhiyang Chen, Yuhao Ren, Xuchun Gui, Shuang peng Wang, and Zikang Tang ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 21 Apr 2017 Downloaded from http://pubs.acs.org on April 24, 2017
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Electrically
Driven
Single
Microwire-based
Heterojuction Light-emitting Devices Anqi Chen,* Hai Zhu,* Yanyan Wu,* Guanlin Lou,* Yunfeng Liang,* Jinyu Li,* Zhiyang Chen,* Yuhao Ren,* Xuchun Gui,‡ Shuangpeng Wang† and Zikang Tang† *State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun Yat-Sen University, Guangzhou 510275, China ‡
State Key Laboratory of Optoelectronic Materials and Technologies, School of
Electronics and Information Technology, Sun Yat-Sen University, Guangzhou 510275, China †
The Institute of Applied Physics and Materials Engineering, University of Macau,
Avenida da Universidade, Taipa, Macau, China E-mail:
[email protected] (Hai Zhu),
[email protected] (Zikang Tang) ABSTRACT: Semiconductor micro/nanowire is an attractive candidate for light-emitting devices (LED) especially laser diodes, due to its ideal geometric shape ， excellent optical performance and electrical transport properties. However, the realization of single micro/nano-structure semiconductor LED or lasers is still a challenge topic. In this letter, we demonstrated a feasible route to fabricate electrically injection single microwire (MW) light-emitting devices. Firstly, the excellent optical properties of single MW were investigated comprehensively, especially for the self-formed high-Q whisper gallery mode lasing. By properly engineering the band alignment of n-ZnO MW/p-GaN heterojunction using a dielectric MgO interlayer, the effective carrier injection and excitonic-type recombination electroluminescence was realized in the single MW active media. Our results present a significant step towards future fabrication of single micro/nanowire light-emitting diode and laser diode. KEYWORDS: semiconductor, heterojunction, electrically injection, microwire
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Semiconductor micro/nanowire has received much attention over the past decade due to their high crystalline quality, superior optical gain property and excellent electrical transport properties. Meanwhile, the single micro/nanowire with the uniformity and well-defined geometric structure can be acted as an excellent optical resonance cavity.1-3 In addition, the low-cost fabrication technique and controllable growth method of micro/nanowires could reduce the processing complexity of the nanoscale optoelectronic device significantly.4 Therefore, individual semiconductor micro/nanowire has become an attractive candidate media for light-emitting device (LED) especially for laser diode (LD). Actually, most of the reported micro/nanowire lasers are obtained by optically pumping, for example, Fabry–Pérot and whisper gallery mode (WGM).5-7 However, the electrical injected LED and LD is indispensable for the practical application. To date, only a few research groups have realized electrically injected single micro/nanowire homojuncton LED through artificial doping, to the best of our knowledge, although those are highly desired.8-10 In addition, it is widely accepted that intentional doping of micro/nanowire is still a major challenge and the elaborate device processing for such pn junction will also fade the unique advantages of micro/nanowire. Therefore, the effective carrier injection for micro/nanowires in a feasible route is the key issue to achieve the single micro/nanowires-based electrical injection luminescence devices. In order to realize carrier injection, the heterojunction structure has been proposed to provide the hole carrier for micro/nanowires from p-Si or p-GaN.11-12 Nevertheless, the carrier leakage deteriorates the injection efficiency in the case of simple directly contact. Significantly, it was demonstrated that the high concentration carrier can be captured and confined in active region through the modulation of carrier transport in n-ZnO/p-GaN film heterojunction.13-15 Hence, the engineering of energy band alignment of the heterojunction may be a practicable technique to realize high efficiency micro/nanowires-based electrically driven light-emission devices. 2
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In this letter, we testified a feasible approach to realize electrically injected single microwire (MW) light-emitting device. By properly engineer the band alignment of n-ZnO MW/p-GaN heterojunction using a MgO layer, the electrons and holes are confined and accumulated in the ZnO MW active medium simultaneously. In this way, the excitonic radiative recombination was realized in the MW. The results about electrically injected single MW LED have important implication value for other semiconductor micro/nano structures (CdSe, CdS, and ZnS). The single MW LED or LD may be realized in these MWs based on the carrier transport modulation technique.
RESULTS AND DISCUSSION The maximum length of the as-grown ZnO MWs can be up to 10 mm with the diameter of about 10-70 μm. The X-ray diffraction (XRD) pattern indicates the MWs are wurtzite structure with high crystalline quality (Figure 1a). In order to investigate the optical property of the bare MW, the single MW with diameter of 26 μm was selected individually, as shown in the inset of Figure 1a. This MW exhibits a perfect hexagonal cross-section and smooth side surfaces. Figure 1b presents the room temperature PL of the single MW. A strong near bandedge emission (NBE) located at 390 nm dominates in the spectrum, meanwhile, the defect related emission band was hardly detected. The strong NBE band was constituted by a typical exciton emission peak and a series of resonance peaks at its low energy shoulder. The fine structure of these resonance peaks at difference measured temperature was given in Figure 1c. These resonance fringes are known as the whispering-gallery mode (WGM) interference peaks2, which come from the light interference around the hexagonal cross section through total internal reflection at the ZnO/air interfaces. The detailed evolution of NBE as a function of temperature was presented in the Figure 1d. Noticeably, the NBE peak exhibits a redshift obviously with 3
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increasing of temperature, and the WGM interference fringes can also be observed even at the temperature as high as 560 K. To further explore the MW luminescence properties in-depth, the hyperfine structure of the NBE from the MWs was measured at 15 K (Figure 1e). Two peaks at the high energy shoulder of donor bound exciton (DBX) can be attributed to free-exciton (FX) A and B, respectively. The energy spacing between FX A and B is 8 meV, which is consistent with reported value (7.5 meV).16 At low energy shoulder of DBX, a two-electron satellite (TES) emission line of the neutral donor bound excitons was indicated.17 Moreover, the equal spacing longitudinal optical (LO) phonon replicas of DBX and FX can be seen obviously.18 The appearance of four-order LO-phonon replicas confirms the high crystalline quality and excellent optical properties of the MW. In addition, the typical Raman scattering spectra measurement of the single MW was also performed (Figure S1c), the sharp five Raman active modes also indicate that the ZnO microwire has a high crystalline quality. For an electroluminescence device, the optical amplification ability of active layer is a critical factor for its application. Here, the lasing action of single MW was investigated using a 266 nm nanosecond pulsed laser (10 Hz repetition rate, and 30 ns temporal duration). To carry out the lasing experiment, the laser beam was focused into a strip and perpendicular to the MW by a cylindrical lens (Figure 2a). Figure 2b gives the emission spectra of the single MW (26 μm diameter) at different excitation power. At low excitation density (41.9 kW cm-2), a broad spontaneous emission (SPE) band centred at 390 nm with a full-width at half-maximum (FWHM) of Δλ = 8 nm can be seen in the spectrum, which is the typical NBE of ZnO.19 As the excitation density increases to 76.0 kW cm-2, several sharp peaks emerge and its intensity grows rapidly with increasing of pumping density. Under higher excitation power (137.6 kW cm-2), the number and intensity of these sharp peaks were enhanced dramatically, and the FWHM of the peak is just about 0.06 nm (inset of Figure 2b). The above emission features display the transition procedure from SPE into well-defined lasing mode. 4
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The light-output versus light-input (L-L) exhibits a superlinear behavior obviously, and the threshold (Pth) was extracted to be about 68 kW cm-2 from the L-L curve (Figure 2c). Moreover, the FWHM plot of peak shows a larger value (8 nm) below Pth, then a sudden drop was occurred by nearly two orders of magnitude smaller (
