Wavelength-Tunable Ultraviolet Electroluminescence from Ga-Doped

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Wavelength-tunable Ultraviolet Electroluminescence from Ga-doped ZnO Microwires Yang Liu, Mingming Jiang, Gaohang He, Shunfang Li, Zhenzhong Zhang, Binghui Li, Haifeng Zhao, Chongxin Shan, and De-Zhen Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14084 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 3, 2017

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ACS Applied Materials & Interfaces

Wavelength-tunable Ultraviolet Electroluminescence from Ga-doped ZnO Microwires Yang Liu,†,‡ Mingming Jiang,*,† Gaohang He,†,‡ Shunfang Li,†† Zhenzhong Zhang,† Binghui Li,† Haifeng Zhao,† Chongxin Shan,*,†,†† and Dezhen Shen*,† † State Key Laboratory of Luminescence and Applications, Changchun Institute of

Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, No.3888 Dongnanhu Road, Changchun, 130033, China. ‡ University of the Chinese Academy of Sciences, Beijing 100049, China. †† School of Physics and Engineering, Zhengzhou University, Zhengzhou 450001, China.

Abstract The usage of ZnO as active layers to fabricate hybrid heterojunction light-emitting diodes is expected to be an effective approach for ultraviolet light sources. Individual ZnO microwires with controlled Gallium (Ga) incorporation (ZnO:Ga MWs) have been fabricated via chemical vapor deposition method. It is found that with the increasing

Ga-incorporated

concentration,

the

near-band-edge

(NBE)

photoluminescence of the ZnO MWs blueshifted gradually from 390 nm to 370 nm. Heterojunction diodes comprising single ZnO:Ga MWs and p-GaN have been fabricated. Increasing injection currents, the interfacial emissions can be suppressed effectively, and the typical NBE emission dominates the electroluminescence (EL). In particular, with increasing Ga-doping concentration, the dominant EL emission wavelengths of the ZnO:Ga MWs based heterojunction diodes blueshifted from 384 nm to 372 nm, and the blueshift can be ascribed to the Burstein-Moss effect induced by the Ga incorporation. The presented work demonstrates the feasibility of optical bandgap

engineering

of

ZnO

MWs

and

the

potential

application

for

wavelength-tuning ultraviolet light sources. Keywords:

heterojunction

diode,

ZnO:Ga,

microwire,

Burstein-Moss effect

1

ACS Paragon Plus Environment

electroluminescence,


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1. Introduction Ultraviolet light sources, such as light-emitting diodes (LEDs) and laser diodes (LDs), have drawn widespread attention for their versatile potential applications, such as advanced information sciences, water and air purification, biomedical sterilization, photolithography, et al.1-9 Compared with toxic, bloated, inefficient gas lasers and mercury lamps, semiconducting light sources with compact structure and flexible fabrication, have broadened the application in military affair and civil area.10-15 Wide-bandgap semiconductors, such as diamond, III-V nitride semiconductors (GaN, AlGaN and AlN), and II-VI oxide semiconductors (ZnO, Ga2O3 and MgO) are potential candidates for ultraviolet optoelectronic devices.16-18 ZnO has been widely acknowledged as a competent candidate for short wavelength optoelectronic devices, such as ultraviolet photodetectors, LEDs and LDs (Wide direct bandgap of 3.37 eV and large exciton binding energy of 60 meV). The development of ZnO based optoelectronic devices has never been satisfactory, which suffering from reproducible, stable and reliable p-type ZnO materials.19-24 While possessing the same wurtzite crystal structure and similar lattice parameters, p-type GaN have been widely used as strategic alternative to design and fabricate ZnO-based heterojunctioned optoelectronic devices.25-31 Owing to high crystalline quality, superior optical gain property, and excellent electrical transport properties, individual ZnO micro/nanowires have become attractive candidates to fabricate optoelectronic devices, such as photodetector, LEDs, LDs, et al.32-37 Notwithstanding, great progress has been made on one-dimensional (1D) heterojunctioned ultraviolet light sources, difficult challenges remain in many aspects. Firstly, compared with ZnO films and nanowire arrays, the research performed on the wavelength-tuning of single ZnO micro/nanostructures based light-emitting devices is still at the fledgling stage and limited in scope, especially, in the ultraviolet region. Secondly, compared with photoluminescence

(PL),

the full

width at

half

maximum

(FWHM)

of

electroluminescence (EL) spectra is broadening, still suffering from severely 2


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interfacial emission; thus, typical near-band-edge (NBE) EL emission cannot dominate the EL emissions.25-31 Thirdly, active tuning of tunable electronics characteristics remain extremely challenging. In this study, individual ZnO microwires (MWs) with controlled Ga-incorporation (ZnO:Ga) were fabricated.38,39 To assess the optical characteristics and crystalline qualities of ZnO:Ga MWs, PL measurements demonstrates that exclusive NBE emission with wavelength-tuning in the ultraviolet spectral regions can be achieved by means of increasing Ga2O3 weight ratios in the precursor mixtures; meanwhile, whisper-gallery-mode (WGM) lasing were also observed from single ZnO:Ga MWs with hexagonal cross section. Meanwhile, heterostructured light-emitting devices comprising single ZnO:Ga MWs and p-GaN substrate were fabricated, suggesting excellent diode behavior. With increasing the injection currents, the dominant EL emission turned from interfacial emission into typical NBE emission. In particular, with increasing the Ga-doping concentration in the ZnO MWs, wavelength-tunable NBE emission can be realized from the heterojunction diodes in the ultraviolet region.

2. Experimental Section Synthesis of ZnO:Ga MWs: The ZnO:Ga MWs with hexagonal cross section were synthesized via chemical vapor deposition (CVD) method. Mixtures of ZnO, Ga2O3 and graphite (C) powders with a weight ratio of 10:1:11 was placed in a corundum boat and served as the precursors. Cleaned Si wafers were placed on the corundum boat to collect the products. The growth temperature was kept at 1150〬C. During the synthesis process, a constant flow of argon (Ar2) (99.99%) (120 standard cubic centimeters per minute) mixed with 10% oxygen (O2) was introduced into the tube furnace as the carrier gas. Meanwhile, in order to increase Ga-doping concentration in ZnO MWs, corresponding weight ratios of Ga2O3 in the mixtures should be increased. Four different weight ratios of the precursor mixtures of ZnO : Ga2O3 : C, such as 10:1:11 (Sample-1), 9:1:10 (Sample-2), 8:1:9 (Sample-3) and 7:1:8 (Sample-4) were fabricated. Owing to the lower growth temperature of ZnO (950 °C) than Ga2O3 3



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(1100 °C) in a vapor−solid progress, the furnace temperature should raise to 1150 °C in advance, then the precursor mixtures were placed into the hottest area. Consequently, it could lead to intensive mixing of Ga and Zn vapors. Aided by carriers, Ga-incorporated ZnO MWs can be synthesized. Therefore, higher temperature growth environment could make Ga substitute Zn efficaciously in ZnO. The width of the MWs ranged from 5 ~ 30 μm and the length can be up to 2 cm. Meanwhile, to fabricate higher crystallization quality, and controlled Ga-incorporation, reaction temperature and the growth times should be increased and extended accordingly. Device Fabrication: Cleaned quartz plates with the size of 3 cm × 2 cm were used as the substrates. Single ZnO:Ga MWs were selected and transferred onto the quartz substrate. Two indium (In) particles was then applied to fix both ends of MWs, serving as the both electrodes to form a metal-semiconductor-metal (MSM) structure. After annealing for 5 minutes at 200 ℃, the ZnO:Ga MW based MSM structure were fabricated. In addition, heterojunctioned light-emitting devices were fabricated as following: firstly, the p-GaN substrate was cleaned; secondly, the PMMA solution (5 mg∙ml-1) was then spin-coated on GaN substrate (The speed of 2500 rotations per second), and an insulating layer of PMMA film was formed; thirdly, a single ZnO:Ga MW was transferred to across the boundary between the PMMA and GaN films, accompanied with In particle fixed the MWs on the insulating layer serving as the electrode; fourthly, the Au electrode and Ni/Au electrode were deposited on the GaN using the electron-beam evaporation system, the electrode size was about 0.5 mm in diameter. Therefore, heterojunction light-emitting devices were built up. Analysis Instruments: The morphologies of the undoped ZnO and ZnO:Ga MWs were characterized using a Hitachi S-3000N/S-4800 scanning electron microscope (SEM). In order to determine the internal structure and composition of ZnO:Ga MWs, elemental mapping using energy dispersive X-ray spectroscopy (EDX) were performed. The current-voltage (I-V) characteristics of the devices were measured using a Keithley 2611 system. The photoluminescence (PL) measurements were 4


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performed using the 325 nm line of a He-Cd laser in a LABRAM-UV Jobin Yvon spectrometer. The optically pumped lasing of the ZnO:Ga MWs was excited by a femtosecond pulsed laser at 325 nm. The EL spectra of the device were obtained using a Hitachi F4500 spectrometer. The EL emission videos and photos were taken using an optical microscope.

3. Results and discussion 3.1. Optical characteristics of individual ZnO:Ga MWs SEM images of undoped ZnO and ZnO:Ga MWs (Sample-1, Sample-2, Sample-3 and Sample-4, respectively) are demonstrated in Fig.1. Compared with undoped ZnO, it could be found out that the Ga-dopant has little influence on the morphologies of the ZnO MWs. The SEM images of the ZnO:Ga MWs were demonstrated in Fig. 1(b) for Sample-1, Figure 1(c) for Sample-2, Figure 1(d) for Sample-3, and Figure 1(e) for Sample-4, respectively. Figure 1(f) displays the magnified hexagonal cross section of the ZnO:Ga MW of Sample-1 (The cross-sectional images of ZnO:Ga MWs for Sample-2, Sample-3 and Sample-4 were displayed in Figure S1). The elemental distributions of Zn, O and Ga were characterized using EDX mapping, as shown in Fig. 1(g). It could be found out that Ga-dopant distributed uniformly throughout the MWs. Taken Sample-1 and Sample-3 for comparison, Ga-doping concentration in ZnO:Ga MWs with the weight ratios of 8:1:9 in mixtures is higher than ZnO:Ga MWs with Ga2O3 weight ratios of 10:1:11 in the mixtures intuitively (Figure S2). Further to illustrate the Ga-incorporation influence on the optical properties of the ZnO MWs, room temperature PL measurements were carried out, as shown in Fig. 2(a). It could be found out that the dominant PL emission wavelengths of undoped ZnO and ZnO:Ga MWs centered in the ultraviolet region, accompanied with negligible visible emission, thus the as-synthesized MWs possessed excellent crystallization quality.21,22,44–47 Specifically, with increasing Ga-incorporation concentration, it can be confirmed that the NBE emission wavelengths is blue-shifted 5



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from 390 nm for un-doped ZnO MWs, to 370 nm for Sample-4. Therefore, one can conclude that optical bandgap of ZnO became widening, which caused by Ga-incorporation. Besides, with increasing Ga-doping concentration, the full width at half-maximum (FWHM) of the NBE peak narrowed down from 14 nm (un-doped ZnO MWs) to 12 nm (Sample-1), and finally drops to 8 nm (Sample-4). The above apparent band gap broadening upon Ga-incorporation can be ascribed to Burstein-Moss (BM) effect,8,49 as supported by the following theoretical analysis. In Fig. 2(b), the schematic diagram of the electronic energy band structures of the ZnO:Ga were constructed. Relative to the ZnO:Ga MWs samples with slightly Ga-doping concentration (Sample-1), increasing Ga2O3 weight ratios in the precursor mixtures can drive the doped electrons to occupy the conduction band (Sample-4) of the ZnO. The magnitude of the BM shift (∆BM) from free-electron theory is proportional to ne2/3, where ne is the electron carrier concentration.48,49 Here, first-principles total energy calculations within density functional theory (DFT) were performed to further convincingly support our statement on the experimental observations. As shown in Fig. 2(c), the local-projected density of states (LPDOS) of un-doped ZnO crystal simulated by a hexagonal supercell consisting of 150 Zn atoms (black solid lines), a supercell containing 150 Zn atoms with one substitution of Ga for Zn adopted for ZnO:Ga crystal (Sample-1) (red solid lines), and a supercell containing 150 Zn atoms with two substitutions of Ga for Zn adopted for ZnO:Ga crystal (Sample-4) (green solid lines) are presented, respectively. Note that in our energy band calculations, both the valence band maximum and the conduction band minimum located at the same G point indicated that the pure ZnO is a direct band gap semiconductor. The calculated band gap is 0.78 eV, as also indicated by the LPDOS in Fig. 2(c), by using the projector augmented wave (PAW) method and selecting the generalized gradient approximation of Perdew, Burke, and Ernzerh form as the exchange-correlation functional. The calculated band gap of 0.78 eV is seriously underestimated than the experimental value of 3.37 eV, due to the well-known intrinsic feature of first-principles calculations based on DFT.37,48 However, such an inaccuracy can be systematically eliminated when comparatively considering the 6


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doping effect of the Ga atoms with different concentrations. To consider the doping effect, in the simulation supercell, Zn atoms are randomly replaced by Ga atoms. As shown in Fig. 2(c), relative to the un-doped case (black line), when the doping concentration x = 0.67 at.%, the LPDOS (red lines) of both Zn and O are significantly downward shifted, by about 0.8 eV, indicating that the Fermi level of now is already upward shifted to the conduction band upon doping.

When further increasing the

substitution value x, up to 1.33 at.%, the Fermi level is further upward shifted, as confirmed by the continually downward shifted LPDOS (green lines). Thus, the interband transition of an electron from the valence band to the unoccupied states of the conduction band will need more energy in ZnO:Ga MWs, and the optical absorption edge of ZnO:Ga will be blue-shifted to the high-energy regime relative to the undoped ZnO. Therefore, the BM effect could be served as a supportive tool to explain the observed band gap widening associated with Ga-incorporation in ZnO MWs. Furthermore, to assess the crystallization qualities of the ZnO:Ga MWs, optical pumped lasing characteristics from individual ZnO:Ga MWs (Taken Sample-3 for example, D ~ 10 µm) were carried out, as shown in Fig. 3. When the excitation power being 4.5 mW/µm2, some sharp peaks with FWHM δλ ~ 0.3 nm can emerge. Meanwhile, lasing modes with mode spacing ∆λ ~ 0.68 nm can be distinguished in the emission spectrum, with the peak centered at 393 nm. The Q factor could be calculated to be 1310 according to the definition Q = λ/δλ, where λ is the PL emission wavelength and ∆λ is the FWHM. Figure 3(b) demonstrated the nonlinear relationship between input excitation power and the output EL intensities, suggesting the transition from spontaneous emission into stimulated emission. To demonstrate the WGM-mode lasing in more details, bright field optical images of the PL emission from the ZnO:Ga MW was shown in Fig. 3(c), as well as optical image of dark field (Figure 3(d)). In addition, a model with suitable parameters, such as nZnO= 2.5, incident wavelength λ=390 nm, ne ~1020 cm-3, was used to calculate and simulate light field resonant modes, which distributed in the hexagonal cross section of ZnO:Ga MWs.41,52,53 7



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During the simulation, two-dimensional (2D) time domain and frequency domain finite element method (FDTD) have been carried out with the aid of eigenfrequency analysis. Three kinds of standing-wave field distributions can be obtained, and denoted as the fundamental mode (Figure 3(e)), nonradiative mode (Figure 3(f)), and radiative mode (Figure 3(g)) respectively. Therefore, ZnO:Ga MWs with hexagonal cross section can be served as a WGM-mode optical cavities. Thus the confined photons could propagate inside the structure through consecutive total internal reflection at the inner walls. Therefore, all the above results indicate the high optical quality of the synthesized ZnO:Ga MWs.

3.2. EL emission characteristics of single ZnO:Ga MWs based Ultraviolet heterojunction diodes To broaden the application of ZnO:Ga MWs, heterojunction light-emitting devices were fabricated. p-GaN (Detailed parameters of p-type GaN layer can be referred to Table-S1), being considered as a strategic alternative p-type layer, can be used to construct heterojunction diodes owing to similar lattice structure (wurtzite) and electronic properties with ZnO.25–29,54 Schematic diagram of heterojunctioned device structure based on single ZnO:Ga MW is shown in Figure S3. Figure 4(a) demonstrated the schematic diagram of EL emission characteristics from single ZnO:Ga MWs based heterojunction diodes. The preparation process of the device is presented as following:25-30 Firstly, the Mg-doped GaN layer is used as p-type substrate. The GaN substrate is ultrasonically cleaned by means of acetone, ethanol, and deionized (DI) water for 5 min successively. Then, the GaN substrate is rinsed using DI water and dried in N2. Ni (20 nm)/Au (50 nm) was deposited onto the GaN as the anode. Secondly, a thin PMMA layer of 20 nm thickness was partly spin-coated on the GaN substrate, served as an insulating layer. After annealing process (keeping the temperature 50 ℃ for 20 min), the quality of insulating layer can be improved. Then, ZnO:Ga MW was selected and transformed to cross the boundary between the p-GaN and the insulating 8


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layer of PMMA. Finally, ZnO:Ga MWs were fixed on the insulating layer by In particles. When the devices was running under forward biased currents, bright light-emitting could be seen clearly from the ZnO:Ga MWs through naked eyes, as shown in Fig. 4(b) (Magnifying high-resolution images of EL emissions can be referred to Figure S4). The emitting regions presented interrupted, and uneven brightness along the MWs, corresponding dark regions may be caused by local poor-contact due to uneven pressure exerted between MWs and GaN substrates. The I-V curve shown in Fig. 4(c), demonstrated typical rectification characteristics with the turn-on voltage to be 3.4 V.25,28,38,39 Under lower injection currents (0.05 mA ~ 1.0 mA), the dominant EL emission centered at 412 nm can be derived from the interfacial emissions between ZnO:Ga MW and p-GaN layer, as shown in Fig. 4(d).26,27,29,30 With increasing the injection currents, such as 8.0 mA, EL emissions centered at 367 and 412 nm can be observed. Continue to increase the injection currents, there is one dominant EL emission centered at 370 nm. More importantly, typical NBE EL emission of the ZnO:Ga MW increases dramatically and becomes the prominent gradually. Meanwhile, the interfacial EL emissions centered at 412 nm are effectively suppressed, accompanied with the FWHM being 10 nm. By contrast, heterojunction LEDs comprising a single undoped ZnO MW and p-GaN substrate have been constructed. Detailed information of I-V characteristics and EL emission has been demonstrated in Figure S5. EL emission characteristics, such as the dominant EL emission wavelengths centered at 395 nm, accompanied with the FWHM being 40 nm, suggested that EL emission of the heterojunction diodes could be evolved into three distinct emissions, such as the NBE emission of undoped ZnO MWs, p-type GaN layer and the interfacial radiative recombination between ZnO MWs and p-GaN.25-31 Therefore, blue shifted of the EL emission wavelengths, the narrowing of the FWHM, and effectively suppression of interfacial EL emission from single ZnO:Ga MWs based heterojunction diodes can be attributed to Ga incorporation (The EL emission characteristics comparison between single undoped ZnO and ZnO:Ga MWs based heterojunction diodes can be referred to the Table-2 in the supporting information). 9



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Further to demonstrate the EL emission characteristics of the heterojunction diodes, the relationship between the integrated intensities of the EL emissions and the injection currents (L-I) curve were calculated, as shown in Fig. 5(a). The results could be fitted using exponential curve of L ~ Im, where m could be used to define the nonradiative and radiative modes of the EL emissions.54 The L-I curve displays a superlinear dependence at lower currents (I < 1.0 mA) with m ~ 1.27; then becomes sublinear (m ~ 0.9) at high current (I > 1.0 mA). It can be found that nonradiative recombination, dependent strongly on the injection carrier densities, is responsible for the output saturation. The turning point in our experiments may correspond to the current value at which nonradiative recombination is saturated; thus, the energy conversion efficiency reaches the maximum simultaneously. Figure 5(b) demonstrates the tendency of the dominant EL emission wavelengths with increasing the injection currents. Meanwhile, Figure 5(c) displays the FWHM narrowing tendency of EL emission spectra with increasing the injection currents. For example, when the injection current changed from 0.05 mA to 8.0 mA, the EL emission wavelengths blue shifted from 420 nm to 412 nm, with the FWHM narrowing from 60 nm to 50 nm. When the injection currents ranged from 8.0 mA to 30.0 mA, the EL emission wavelengths blue shifted further from 412 nm to 375 nm, with the FWHM narrowing from 50 nm to 10 nm. By means of EL spectra being fitted into several sub-bands using Gaussian deconvolution (Figure S5), the interfacial emission played the dominant role in the EL emissions under lower injection currents. When the injection current greater than 8.0 mA, the output intensities of the typical NBE emission from ZnO:Ga MWs increased much faster than others, and become the dominant gradually. In addition, a little red shifted of the dominant EL emission wavelengths from 365 nm to 375 nm were also observed, with the injection currents ranged from 8.0 mA to 30 mA. Presumably, under lower injection current, light-emitting maybe collected from the radiative recombination between the electrons of ZnO:Ga MWs and holes of p-GaN layer. With increasing the injection currents, radiative recombination can occur in both places: interfacial emission between ZnO:Ga MWs and p-GaN, and NBE-type emission from ZnO:Ga MWs. Once the applied currents exceeds a certain value, for 10


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example 10.0 mA, the interfacial emission would no longer increase, and this declining tendency maybe attributed to the limited holes concentration in the p-GaN. Finally, typical NBE emission from ZnO:Ga MW increases rapidly, and being the predominant. Consequently, increasing injection currents, the dominant EL emission wavelengths centered at 375 nm can be observed, with FWHM being 10 nm, which could be attributed to typical NBE emission from single ZnO:Ga MWs. PL measurements of p-GaN and ZnO:Ga MWs have been carried out and shown in Figure S6. The p-GaN film exhibits the NBE-related emission, which centered at 362 nm, accompanied with weak blue emission due to the radiative recombination between the conduction band and Mg-related deep acceptor level. The dominant PL emission of the ZnO:Ga MWs centered at 375 nm can be attributed to the radiative recombination of NBE emission, corresponding FWHM being 10 nm. Therefore, the FWHM of EL spectral is almost equivalent to PL spectral FWHM of single ZnO:Ga MW. Therefore, the EL spectra present a structure similar to the PL emission of ZnO:Ga MWs. Taken the EL emissions and the PL emissions together for comparison, EL emissions from heterojunction diodes can be ascribed to the NBE-type radiative recombination from ZnO:Ga MWs. In addition, the wavelengths centered at 412 nm could be derived from the radiative recombination between the electrons from the conduction band of ZnO:Ga MWs and Mg-related deep acceptor levels. The junction barrier heights of 0.38 eV for the holes, and 0.36 eV for the electrons, respectively, could be speculated in view of the energy band structure and EL emission wavelengths, thus the band diagram considering the interfacial barriers was depicted, and shown in Fig. 5(d).25-31 In addition, electrostatic potential field distribution was carried out on account of numerical simulation by means of computational fluid dynamics approach, as shown in Figs. 5(e) and (f). Therefore, under low bias voltage, such 5 V, the active region is mainly distributed in the interface between ZnO:Ga MW and p-GaN. With increasing the bias voltage exceed to a value, such as 20 V, corresponding active region mainly distributed in the ZnO:Ga MWs, as indicated in Figure S7. 11



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3.3. Wavelength-tunable ultraviolet EL emissions from single ZnO:Ga MWs based heterojunction diodes As described previously, PL emissions indicated that the NBE emission wavelengths of Ga-doped ZnO MWs were near linearly blue shifted from 385 nm to 370 nm with Ga2O3 weight ratios in the mixtures increased from 10 : 1 : 11 to 7 : 1 : 8, indicating that the optical band-gap engineering of ZnO could be successfully tailored by Ga-incorporation. In addition, to illustrate electronic transport properties of individual ZnO MWs with controlled Ga-incorporation, Figure 6(a) shows the typical I-V characteristics of single ZnO:Ga MWs, which selected from the four samples. It is well-known that, n-type conduction of undoped ZnO could be ascribe to intrinsic donor defects, such as Zni and VO.23 Meanwhile, due to the negligible contact resistance between MWs and In electrodes, the resistivity of undoped ZnO MWs was calculated to be about 4 × 10-1 Ω·cm.38 While, the resistivity of ZnO:Ga MW (Sample-4) could be calculated to be 5 × 10-4 Ω·cm.38,39 Increasing Ga2O3 weight ratio in the precursor mixtures, can lead to lower resistivity reduced by three orders of magnitude. In addition, individual ZnO:Ga MWs are quasi-1D semiconductor microstructures, with the cross-sectional dimensions of MWs being on the scale of micrometers, thus, there is no effective technical means to measure the electrical characteristic parameters, such as carrier concentration, mobility and et al. Therefore, the Ga concentration could be estimated to be ∼1at.% using XPS measurement, with the carrier concentration ne on the order of 1020 cm-3 being roughly estimated from the resistivity values.24,50,51,55 Therefore, tunable n-type conduction can be achieved from single ZnO MWs with controlled Ga-incorporation, with resistivity decreasing from 4 × 10-1 Ω·cm to 3 × 10-4 Ω·cm.38,39,57 In particular, LEDs were fabricated from single ZnO:Ga MWs, which have been selected from different samples. Accordingly, the plotted I-V curves, shown in Fig. 6(b), demonstrated that all the heterojunction diodes indicated p-n junction rectifying characteristics (The rectification ratios of single ZnO:Ga microwires of different 12


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samples based heterojunction diodes can be referred to the Table-3 in the supporting information). Increasing Ga-doping concentration, can bring reducing the threshold voltages, which agrees with previous reports.38,50 In particular, under forward injection currents, EL emission wavelengths can be tuned by increasing the Ga2O3 weight ratios in the precursor mixtures, with the EL emission wavelengths centered at 384 nm (Figure 6(c)) for Sample-1, 382 nm (Figure 6(d)) for Sample-2, 374 nm (Figure 6(e)) for Sample-3, and 372 nm (Figure 6(f)) for Sample-4, respectively. In particular, compared with undoped ZnO MWs based LEDs (Figure S8 and Figure S9), EL emission characteristics, such as the dominant EL emission wavelengths in the ultraviolet regions, efficiently suppression of the interfacial emission, and the FWHM being 10 nm can also be achieved from single ZnO MWs with controlled Ga-incorporation based heterojunction diodes. Therefore, heterojunction diodes composed of p-GaN layers and ZnO MWs with controlled Ga-incorporation behave perfect p-n diodes. In particular, under forward bias, heterojunction diodes yield intense, wavelength-tuning, and efficient light-emitting behaviors.

4. Conclusions In summary, individual ZnO MWs with controlled Ga-incorporation were synthesized via CVD methods. With increasing the Ga2O3 weight ratio in the precursor mixtures, the blueshift of PL emission ranged from 390 nm to 370 nm can be attributed to the filling of the conduction band induced by the Ga incorporation. heterojunction diodes comprising single ZnO:Ga MWs and p-GaN have been fabricated, and with the increase of the Ga-doping concentration, wavelength-tunable EL emission has been achieved in the ultraviolet region. EL emission from heterojunction diodes can be attributed to typical NBE radiative recombination from ZnO:Ga MWs. These wavelength-tuning ultraviolet EL emissions from heterojunction diodes based on single

ZnO:Ga

MWs

offer

practicable

platform

to

fabricate

ultraviolet

wavelength-tuning, and reliable building blocks for short wavelengths light sources.

■ ASSOCIATED CONTENT 13



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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. The cross section morphologies of ZnO:Ga MWs, EL spectra decomposed with Gaussian functions, EL emission characteristics from single undoped ZnO MWs based heterojunction diodes, parameters of p-GaN film, and rectification ratios of single ZnO:Ga microwires based heterojunction diodes.

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (M. M. Jiang). *E-mail: [email protected] (C. X. Shan). *E-mail: [email protected] (D. Z. Shen).

Notes The authors declare no competing financial interest.

■ Acknowledgements The authors are grateful to Prof. Shunfang Li of Zhengzhou University for his theoretical analysis and numerical calculation, and valuable discussions. This work was supported by the National Natural Science Foundation of China (Grant Nos. 11404328, 11574307, 61505033, 51471051, 11374296, and 61376054), the National Science Fund for Distinguished Young Scholars (Grant Nos. 61425021 and 61525404), and the 100 Talents Program of the Chinese Academy of Sciences.

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Figure 1. (a) SEM image of undoped ZnO MWs with hexagonal cross section, the inset shows perfect flat hexagonal cross section. SEM images of ZnO:Ga MWs corresponding to (b) Sample-1, (c) Sample-2, (d) Sample-2 and (e) Sample-4. (f) SEM image of an individual ZnO:Ga MW, with perfect hexagonal cross section, corresponding to Sample-1 (Hexagonal cross section images of ZnO:Ga MWs of Sample-2, Sample-3 and Sample-4 respectively, can be referred to Figure S1 in the Supporting Information). (g) The distributions of the different elements for Zn, O and Ga detected by EDS mapping according to the four samples denoted as Sample-1, Sample-2, Sample-3 and Sample-4, respectively.

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Figure 2. (a) Room temperature PL spectra for undoped ZnO and ZnO:Ga MWs, NBE PL emission wavelengths centered at 389 nm (undoped ZnO), 380 nm (ZnO:Ga for Sample-1), 378 nm (ZnO:Ga for Sample-2), 375 nm (ZnO:Ga for Sample-3), and 370 nm (ZnO:Ga for Sample-4); thus increasing Ga-doping concentration can lead to blueshift of NBE PL emissions. (b) Schematic diagram of the structural energy-level configuration, with increasing Ga-doping concentration. (c) LPDOS of (i) O atom, (ii) Zn atom and (iii) Ga atom in Ga-incorporated ZnO crystals, respectively. The black solid lines demonstrate undoped ZnO perfect crystals; red solid line demonstrate the supercell of the ZnO incorporated with Ga concentration being 0.67 at.%; the green solid lines present the supercell of the ZnO incorporated with Ga concentration being 1.33 at.%.

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Figure 3. (a) PL emission spectrum of the ZnO:Ga MW (Sample-3) (D ∼ 10 µm) excited by a 325 nm lasing with excitation power 4.5 mW/µm2. (b) The relationship between output lasing intensities and excitation power density. (c) Bright field optical images of PL emission from an individual ZnO:Ga MW. (d) Dark field optical images of PL emission from an individual ZnO:Ga MW. To demonstrate the optical pumped WGM lasing behaviors from individual ZnO:Ga MWs, numerical simulations on the standing wave field distributions, which confined in the hexagonal cross section, were performed (Considering the electrons concentration in ZnO:Ga MWs being n ~ 1020 cm-3) along the x-y plane (e) fundamental mode, (f) non-radiative mode and (g) radiative mode. Corresponding parameters nZnO= 2.5, nquartz= 1.5, nair= 1, and the diameter of ZnO MWs D = 10 µm, with corresponding calculated wavelength λ = 390 nm. The center of the MW defines the origin (x = y = 0).

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Figure 4. (a) A schematic diagram of Ultraviolet heterostructure light emitting diodes composed of single ZnO:Ga MW and p-GaN layer. (b) EL lighting from heterojunction diode, with corresponding injection currents, such as 0.5 mA, 12 mA, and 20 mA respectively (Scale bar: 200 µm). (c) Room temperature I-V characteristics of heterojunction diodes. (d) The EL spectra of the diode under different injected currents changed from 0.1 mA to 1.0 mA, the dominant EL emission wavelengths centered at 410 nm. (e) The EL spectra of the diode under different injection currents changed from 0.2 mA to 30 mA, the dominant EL emission wavelengths blueshifted from 410 nm to 375 nm.

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Figure 5. (a) Integrated EL intensities as a function of the injection current. (b) EL emission wavelengths as a function of the injection currents. (c) FWHM of EL emission spectra as a function of the injection currents. (d) Schematic diagram and characterization of the energy band structure of ZnO:Ga MWs/GaN heterojunction devices and corresponding recombination. Electric potential field distribution along the cross section of heterojunction diodes with different forward bias: (e) 5 V and (f) 20 V. Considering that electron concentration of ZnO:Ga MWs 1021cm−3, hole concentration of p-GaN substrate 1017cm−3.

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Figure 6. (a) Room temperature I-V characteristics of ZnO:Ga MWs corresponding to different samples with increasing Ga-doping concentration. (b) I-V rectification characteristics of LEDs consisting of p-GaN substrate and individual ZnO:Ga MWs, which in accord with different samples denoted as Sample-1, Sample-2, Sample-3, and Sample-4. EL emission characteristics of ZnO:Ga MWs based LEDs: (c) ZnO:Ga MWs of Sample-1, with EL emission wavelength centered at 384 nm; (d) ZnO:Ga MWs of Sample-2, with EL emission wavelength centered at 382 nm; (e) ZnO:Ga MWs of Sample-3, with EL emission wavelength centered at 374 nm; (f) ZnO:Ga MWs of Sample-4, with EL emission wavelength centered at 372 nm.

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