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Abstract
In emerging miniaturized applications, semiconductor micro/nanostructures laser devices have drawn great public attentions of late years. The device performances of micro/nanostructured microlasers are highly restricted to the different reflective conditions at various side surfaces of microresonators and junction interface quality. In this study, an electrically driven whispering-gallery-mode (WGM) microlaser composed of a Ga-doped ZnO microwire covered by a MgO layer (MgO@ZnO:Ga MW) and a p-type GaN substrate is illustrated experimentally. Incorporating a MgO layer on the side surfaces of ZnO:Ga MWs can be used to reduce light leakage along the sharp edges and the ZnO:Ga/GaN interface. This buffer layer incorporation also enables engineering the energy band alignment of n-ZnO:Ga/p-GaN heterojunction and manipulating the current transport properties. The as-constructed n-MgO@ZnO:Ga MW/p-GaN heterojunction device can emit at an ultraviolet wavelength of 375.5 nm and a linewidth of about 25.5 nm, achieving the excitonic-related recombination in the ZnO:Ga MW. The broadband spectrum collapsed into a series of sharp peaks upon continuous-wave (CW) operation of electrical pumping, especially for operating current above 15.2 mA. The dominant emission line was centered at 378.5 nm, and the line width narrowed to approximately 0.95 nm. These sharp peaks emerged from the spontaneous emission spectrum and had an average spacing of approximately 5.5 nm, following the WGM cavity modes. The results highlight the significance of interfacial engineering for optimizing the performance of low-dimensional heterostructured devices and shed light on developing future miniaturized microlasers.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultraviolet (UV) light sources, especially light-emitting diodes (LEDs) and lasers have emerged as a viable technological platform for medical diagnostics, optical communications, environmental sterilization, and material processing [1–5]. In recent decades, UV LEDs and lasers have been developed for wide-bandgap semiconductors such as GaN, diamond, and ZnO [6–9]. Especially for the epitaxial heterostructures of III-V broadband inorganic semiconductors, such as AlGaN, has made great progress for designing and developing ultraviolet optoelectronic devices [10,11]. In emerging miniaturized applications, the fabrication of low-dimensional UV emission devices on account of broad-bandgap semiconductor micro/nanostructures has undergone significant progress [12,13]. However, the devices require expensive epitaxial growth equipment and complicated preparation processes. In addition, the fabrication of low-dimensional microlaser devices requires post-fabrication substrate replacement with highly reflective mirror structures to increase their poor power output, which originates from the mismatch of the refractive index between these materials and common substrates [12,14–16].
ZnO is a direct wide-bandgap semiconductor (bandgap $\sim$ 3.34 eV, exciton binding energy $\sim$ 60 meV) with high carrier mobility and high thermal-chemical stability [6,17,18]. It has been widely employed to construct optoelectronic devices operating in the UV regime [2,4]. While possessing numerous micro- and nanostructures, the ZnO has been used to construct low-dimensional optoelectronic devices at UV wavelengths [17,19]. Electrical-pumping UV ZnO-based nano/microlasers have been reported [6,20–22]. Due to the lack of stable and reproducible p-type ZnO materials, these published devices are mainly based on heterojunctions involving n-type ZnO micro/nanostructures and p-type dissimilar materials, such as GaN. The advantages provided by materials would make the as-constructed UV light emitter competitive for use in the field [23–27]. The device performances are significantly constrained by the optical loss at the planar-contact interface of the p-type substrates and the energy barrier created at the ZnO/GaN heterostructural interface [20,24,28]. Tremendous efforts, especially for the insertion of low-dielectric insulating layers, such as MgO, AlN, and SiO$_2$, have been utilized as electron-blocking layers to engineer the energy band alignment of n-ZnO/p-GaN heterojunctions [29–33]. Thus, this contributes to achieving high-efficiency micro/nanostructure-based electrically driven LEDs and lasers. However, the as-constructed n-ZnO/MgO/p-GaN heterojunction microlasers still suffer from a much stronger spontaneous emission background, lower $Q$-factor, and the serious light leakage at the corners of the ZnO micro/nanostructures [17,33–36].
We design an electrically driven whispering-gallery-mode (WGM) microlaser made of a Ga-doped ZnO microwire covered by a MgO dielectric layer (MgO@ZnO:Ga MW) and p-type GaN substrate in this study. The electroluminescence (EL) spectra were dominated by a series of sharp peaks in the UV region when the operating current increased above the threshold. The electrically driven WGM lasing characteristics and resonant mechanisms were studied in detail. A dielectric MgO interlayer working as an electron-blocking layer can be used to engineer the band alignment of the n-ZnO:Ga/p-GaN heterojunction in the as-constructed device. Thus, the leakage current is lowered, and the current transport path is manipulated to yield efficient carrier radiative recombination in the wire gain medium. In addition, the deposition of the MgO layer on the ZnO:Ga wire enhances the MW cavity performances, and reduces the optical loss at the ZnO:Ga/GaN interface. Therefore, we propose a promising candidate for the construction of single-MW electrically driven WGM microlasers.
2. Experimental section
2.1 Growth of individual ZnO:Ga MWs
Samples of individual ZnO micro/nanowires via Ga incorporation were prepared by chemical vapor deposition method [17,37]. The uniform source materials were highly purified ZnO, Ga$_2$O$_3$, and graphite (C) powders. After mixing thoroughly, the precursor materials were placed in a corundum boat with the size of 10.0 cm (length) $\times$ 2.0 cm (width) $\times$ 1.5 cm (depth)). A clean Si wafer without any catalyst was placed on the corundum boat, where a vertical distance of about 0.2 cm from the mixture was maintained. The corundum boat was positioned in the hottest area of a quartz tube in a horizontal tube electric furnace. A mixture of $Ar$ and $O_2$ being used as the carrier gas and a protecting gas at a total flow rate of 130 sccm was introduced into the tube furnace. Before naturally cooling to room temperature, the furnace was maintained at a temperature of 1100 $^{\circ }$C for approximately one hour. Finally, individual ZnO:Ga samples were obtained on Si substrates. A photograph of the sample is presented in Fig. 1(a). The size of the as-prepared ZnO:Ga wires can be tuned by manipulating the growth conditions, such as the mass of the precursor mixtures, growth time, growth temperature, and carrier gas. An ultralong length of approximately 2.0 cm was achieved in the as-grown ZnO:Ga products, and the diameter of the samples varied from 1 to 50 $\mu$m.
Fig. 1. (a) Optical image of as-synthesized ZnO:Ga samples. (b) SEM image of a ZnO:Ga MW. (c) XRD result of the as-grown ZnO:Ga samples. (d) EDS elemental mapping of the ZnO:Ga MW, illustrating the uniform composition of Zn, Ga, and O species. (e) TEM image of ZnO:Ga wire with a diameter of approximately 200 nm. (f) High-magnification TEM image of the as-grown ZnO:Ga wire showing well-resolved lattice fringes with a lattice spacing of approximately 0.289 nm.
2.2 Device preparation
An ultrathin MgO layer with a thickness of approximately 10 nm was deposited on the ZnO:Ga MWs using electron beam evaporation. A relatively pure UV light-emitting device composed of a ZnO:Ga MW covered by a MgO layer and p-type GaN template was fabricated. A commercially available p-type GaN template was used as the hole-injection source in the device structure. The fabrication procedure for the single-MW heterojunction emission device is summarized as follows: (1) A Ni/Au ohmic contact (30/30 nm) was first deposited on an activated p-type GaN film via electron beam-heating evaporation. (2) A MgO film with a thickness of approximately 150 nm was deposited on one side of the GaN film using electron beam evaporation. (3) An individual MgO@ZnO:Ga MW was subsequently placed across the slit of the p-GaN film and the deposited MgO film. (4) Finally, an Indium (In) particle was fixed onto the MW with the end not covered by the MgO layer on the MgO insulating layer. In the as-constructed LED device, the In and Ni/Au electrodes were responsible for current injection. The typical device configuration of the designed n-MgO@ZnO:Ga MW/p-GaN heterojunction is shown in Fig. 5(a).
2.3 Sample characterization
The lasing characteristics of the ZnO:Ga samples uncovered and covered by the MgO layer were measured using a home-built micro-photoluminescence (PL) system. The system comprised an optical parametric amplifier (OPERA SOLO) with a Ti:sapphire laser and a confocal micro-PL system (Olympus BX53). The emitted photons were collected using a spectrometer (SpectraPro-2500i, Acton Research Corporation). The PL properties of the p-GaN template, a ZnO:Ga MW uncovered or covered by MgO layers, were investigated using a He-Cd laser (excitation wavelength of 325 nm) via a LabRAM-UV Jobin-Yvon spectrometer. A Keysight B1500A sourcemeter system was used to measure the electrical characteristics in terms of the current-voltage ($I$-$V$) characteristics of the as-constructed emission devices. EL characterization of the as-fabricated heterojunction device was performed using a microspectral detection system made of an ANDOR detector (CCD-13448) and Omni-$\lambda$ 500 spectrograph. An optical microscope was used to collect the EL images of the fabricated heterojunction light sources.
3. Results and discussion
Individual ZnO:Ga wires with controlled sizes, high yields, and highly crystallized qualities were successfully synthesized using a carbothermal reduction reaction [17,37]. The optical photograph, as shown in Fig. 1(a), illustrates individual ZnO:Ga wires growing directly on the precursor Si substrate and the inner wall of the corundum boat. Figure 1(b) shows a scanning electron microscopy (SEM) image of a ZnO:Ga MW with a hexagon-shaped cross-sectional profile and smooth boundaries. The diameter of the wire is approximately 12 $\mu$m. The crystal phase of the as-prepared ZnO:Ga samples were examined by using an X-ray diffraction (XRD). Figure 1(c) shows that the XRD patterns peaked at $31.7^\circ$, $34.3^\circ$ and $36.2^\circ$, corresponding to the (100), (002), and (101) crystal planes, respectively. The XRD spectra suggest that the as-synthesized samples are wurtzite with a hexagonal structure (JCPDS No. 80-0075). Furthermore, it was observed that the full width at half maximum (fwhm) of the (100) peak was $0.06^\circ$, suggesting that the as-synthesized samples possessed relatively high crystalline quality [17,36–38].
Energy-dispersive X-ray spectroscopy (EDS) mapping shown in Fig. 1(d) depicts the uniform spatial distribution of the contained chemical elements (Zn, Ga, and O, respectively) with a clear boundary of the ZnO:Ga MW. The as-grown ZnO:Ga wires were investigated using transmission electron microscopy (TEM). Figure 1(e) shows a TEM image of a ZnO:Ga wire, illustrating straight boundaries. High-resolution TEM analysis was performed at the sharp edge of the wire. The high-resolution TEM image demonstrates a clear lattice fringe, as shown in Fig. 1(f). The measured spacing of the lattice fringes was evaluated to be approximately 0.289 nm. The interspacing observed in the ZnO:Ga wires is slightly larger than that in the intrinsic undoped ZnO products ($\sim$ 0.260 nm), suggesting lattice expansion due to the substitution of Ga for Zn [37,39]. Thus, we established that individual ZnO:Ga micro- and nanowires can be successfully grown on Si substrates.
Numerous investigations have illustrated that incorporating the insulator materials such as HfO$_2$, MgO, and AlN is advantageous in engineering the optical and electrical properties, surface defects, and interfacial modulation [29,31–33]. An ultrathin MgO film with a thickness of $\sim$ 10 nm was evaporated on a ZnO:Ga MW, as described in the Experimental Section. An SEM image of a ZnO:Ga MW covered with a MgO layer is shown in Fig. 2(a). The influence of the MgO insulating layer on the optical properties was studied using a He-Cd laser. The PL spectra obtained are shown in Fig. 2(b). The PL profile (the black solid line) of the bare ZnO:Ga MW shows that the main emission wavelength is at $\sim$ 370.0 nm and the line width of the peak is evaluated to be approximately 12.0 nm. This peak probably originates from a typical near-band-edge (NBE) emission [17,39]. In addition, we also observed a much weaker visible light emission around a wavelength of 500 nm. This emission may be a result of intrinsic defects (such as zinc interstitials ($Zn_{i}$), zinc anti-sites ($Zn_{O}$), and oxygen vacancies ($V_{O}$)), and Ga-incorporation induced impurity levels [17,37,38]. A slight enhancement of the UV emission was achieved by incorporating the MgO nanolayer. In contrast, the visible emission was evidently suppressed. The electrical properties of the ZnO:Ga MWs uncovered and covered by the MgO nanolayer were tested. The $I$-$V$ characteristic curve shown in Fig. 2(c) is close to linear, indicating that the electrode and wire formed a relatively good ohmic contact [25,33]. Thus, the MgO insulating layer had little effect on the electrical properties of the ZnO:Ga MWs.
Fig. 2. (a) SEM image of a ZnO:Ga MW covered by MgO layer. The inset shows an enlarged SEM image of the MgO layer deposited on the wire. (b) PL spectra of a ZnO:Ga MW uncovered and covered with MgO layer. (c) $I$-$V$ curves of a single ZnO:Ga MW uncovered and covered with a MgO layer. Numerically simulated electric field intensity distribution within the hexagon-shaped cross section of (d) a ZnO:Ga MW, and (e) the ZnO:Ga MW covered by MgO layer. The models were performed using the FDTD solution. (f) Energy distribution curves along the $y$-direction of the hexagonal cross-section of a ZnO:Ga MW uncovered and covered by the MgO layer.
Previously published literature suggests that semiconductor micro/nanostructures with hexagon-shaped cross-sectional profiles can be used to construct WGM microcavities because of the total internal reflection at the boundaries [17,36,40,41]. The effect of the MgO layer on the cavity performances of hexagon-shaped ZnO:Ga MWs was studied. The optical field distributions of ZnO:Ga MWs with hexagon-shaped cross-sections uncovered and covered by a MgO layer were simulated using the finite-difference time-domain(FDTD) method [36,42]. A Gaussian beam with a line width of 10 nm centered at 375 nm was set as the light source in the simulation. This matches the near-band emission of the ZnO:Ga MWs. The refractive indices of the ZnO:Ga MWs, air, SiO$_2$ substrate, and MgO layer were regarded as 2.35, 1.0, 1.35, and 1.5, respectively. The diameter of the ZnO:Ga MW was 10 $\mu$m, and the thickness of the MgO layer was 10 nm. Figure 2(d) shows the simulated electric field intensity $\sim$ $|E(x,y)|^2$-distribution inside the ZnO:Ga MW cavity under an optical resonant mode, suggesting that a WGM can be supported by an MW with a hexagon-shaped cross-section. An intuitive enhancement of the electric field pattern can be observed by incorporating the MgO dielectric layer, as shown in Fig. 2(e). The electric field intensities along the $y$-direction are plotted quantificationally in Fig. 2(f). The simulated results confirm that light can be more efficiently confined in the hexagonal ZnO:Ga MW microcavity when all six side facets are surrounded by a low reflective index of the MgO layer. The results indicate a well-faceted morphology, hexagon-shaped structure, highly crystallized quality, and optical features of an individual ZnO:Ga MW covered by a MgO layer, making these one-dimensional structures promising for integration in light-emitting devices and laser devices [25,36,43].
We studied the lasing characteristics inspired by the superior PL properties and the hexagon-shaped ZnO:Ga MW covered by the MgO layer, which may naturally serve as the cavity resonator. A femtosecond-amplified laser was used as the excitation source. Bright light emission from the sharp edges can be seen when pumped optically, especially from the corners via a single ZnO:Ga MW. Thus, this suggested that light can be emitted from the six edges and corners of the hexagon-shaped wire [17,21,40]. The emitted photons were collected for a bare ZnO:Ga MW being pumped optically, and the corresponding PL spectra are shown in Fig. 3(a). The obtained PL spectrum was governed by spontaneous broadband emission at a low pumping fluence. The prominent PL wavelength peaks at approximately 391.0 nm and has a spectral linewidth of about 12.5 nm. A group of sharp spikes with evenly spaced modes was observed when the excitation fluence reached 130.5 $\mu$J/cm$^2$, An increase in the pump intensity caused sharp emission lines and a dramatic increase in the PL intensity. Thus, the lasing action was achieved in the hexagon-shaped ZnO:Ga MW microcavity.
Fig. 3. WGM lasing characteristics of single ZnO:Ga MW uncovered and covered by MgO layer. (a) Pump fluence-dependent PL spectra of a ZnO:Ga MW, with the pump intensity varying from 103.2–180.3 $\mu$J/cm$^2$. (b) Pump fluence-dependent PL spectra of the ZnO:Ga MW covered by a MgO dielectric layer with a thickness of 10 nm; the pump intensity varies from 78.8 to 153.6 $\mu$J/cm$^2$. (c) Lasing spectra of ZnO:Ga MW uncovered and covered by the MgO layer, plotted at the same excitation intensity of $\sim$ 153.6 $\mu$J/cm$^2$. (d) Integrated PL intensity as a function of pump density. The laser threshold of the ZnO:Ga MW was approximately $\sim$ 112.8 $\mu$J/cm$^2$. By incorporating MgO layer, the laser threshold was reduced to approximately 92.3 $\mu$J/cm$^2$.
The same MW was further optically pumped using an fs laser beam by incorporating a MgO dielectric layer (10 nm thick). The output signal was also collected. PL spectra depicted in Fig. 3(b) shows that the obtained light signals display a transition from spontaneous emission to stimulated emission with an increase in pumping intensity above 95.6 $\mu$J/cm$^2$. Generally, the lasing action from optically pumped hexagonal ZnO:Ga MWs covered by a MgO layer is dominantly assigned to WGM [36,42]. Studying the influence of the MgO layer on the lasing features was enabled by observing the PL spectrum of the bare hexagonal ZnO:Ga MW at a pumping fluence of 153.6 $\mu$J/cm$^2$, as shown in Fig. 3(c) (solid black lines). The PL spectrum obtained by incorporating the MgO layer deposited on the wire is plotted in Fig. 3(c) (solid red line) at an identical pumping fluence($\sim$ 153.6 $\mu$J/cm$^2$). Clearly, in addition to an observable enhancement in the PL intensity, the entire spectrum profile exhibited an observable broadening by comparing to that of the bare wire. And this broadening is assigned to the optical field confinement and enhancement, which is induced by cladding MgO nanolayer. Moreover, the integrated PL intensity of the ZnO:Ga MW uncovered and covered by the MgO layer was derived by varying the pump-power intensity. The laser threshold of the bare ZnO:Ga MW was determined to be 112.8 $\mu$J/cm$^2$, as illustrated in Fig. 3(d). This is much higher than that of the same wire covered by the MgO layer ($\sim$ 92.3 $\mu$J/cm$^2$).
Theoretical calculations were performed using a simple plane-wave model for resonant WGMs in hexagonal cavities to analyze further the influence of the MgO nanolayer on the lasing characteristics of a ZnO:Ga MW [36,42]. The peak positions obtained from the lasing spectrum of the bare ZnO:Ga MW (bottom) fit the predicted results, which were simulated theoretically, as shown in Fig. 4(a). The solid black line represents the predicted energetic positions with suggested interference orders $N$ for an MW diameter of $\sim$ 10 $\mu$m (middle). The distance between the resonant energies decreases toward the higher-energy shoulder of the PL spectrum. The refractive index acquired from the fitting procedure is shown in the figure as a solid black line (top). From the figure, the lasing mode numbers vary from 188 to 207. The simulated results of the ZnO:Ga MW covered by the MgO nanolayer were also achieved by introducing an MgO nanolayer. Figure 4(b) (the middle) reveals the lasing modes with mode numbers ranging from 187 to 211, which are larger than those of the bare ZnO:Ga MW. Therefore, the incorporation of a low-dielectric nanolayer on ZnO:Ga MWs can lead to optical field confinement and enhancement of the light emission from the WGM microcavity [17,31].
Fig. 4. WGM lasing features of a ZnO:Ga MW uncovered and covered by MgO layer. (a) Simulated lasing modes labeled with the mode numbers and refractive index in the dispersion curve, and lasing spectrum of the bare ZnO:Ga MW. (b) Simulated lasing modes labeled with the mode numbers and refractive index in the dispersion curve, and lasing spectrum of the same ZnO:Ga MW covered by the MgO layer. (c) Gaussian-fitted fwhm line of the lasing peak of a ZnO:Ga MW. (d) Gaussian-fitted fwhm line of the lasing peak of the same ZnO:Ga MW covered by the MgO layer.
Figure 4(c) illustrates the Gaussian fitted spectrum of the bare ZnO:Ga MW at a pumping density of $\sim$ 153.6 $\mu$J/cm$^2$. From the spectrum profile, Uniformly spaced oscillation peaks ($\Delta \lambda$ $\sim$ 0.34 nm), and the fwhm ($\delta \lambda$ $\sim$ 0.12 nm) can be obtained from the spectrum profile. Thus, it is suggested that the same waveguide origin of the optical modes exists. The average ${Q}$-factor was estimated as 3270 according to the formula Q=$\lambda$/$\delta \lambda$, where $\lambda$ is the lasing wavelength [17]. According to the hexagonal WGM resonance equation, the obtained PL emission can be assigned to WGM lasing behavior. The Gaussian fitted spectrum of the same wire incorporating MgO nanolayer is depicted in Fig. 4(d). The figure reveals that the lasing mode spacing observed in the ZnO:Ga MW covered by the MgO layer is about 0.30 nm. The fwhm of the resonant peak is determined to be 0.07 nm. The $Q$-factor of about 5600 is derived. Therefore, the evaporation of MgO nanolayer on ZnO:Ga micro/nanostructures is highly sensitive to manipulations from the PL performances via enhanced WGM lasing characteristics. These characteristics include lowering the threshold, increasing quality (Q) factor, and increasing lasing output [36,42].
A type of UV emission device composed of a MgO@ZnO:Ga MW and p-type GaN film was designed, as described in the Experimental Section. Figure 5(a) shows a schematic of the n-MgO@ZnO:Ga MW/p-GaN heterojunction light-emitting device. The device architecture used a p-type GaN film as the hole supplier. The $I$-$V$ curve of the as-constructed heterojunction device is shown in Fig. 5(b). The contacts between Ni/Au-GaN and In-MgO@ZnO:Ga MW were proven to have Ohmic properties (Shown in the inset of Fig. 5(b)). The typical rectifying diode-like behavior should be assigned to the high-quality heterojunction formed between p-type GaN and n-type MgO@ZnO:Ga MW [25,29,32,33]. The turn-on voltage of the n-MgO@ZnO:Ga MW/p-GaN was determined to be about 5.8 V. The single-MW heterojunction device was measured with a forward current of 0.6 mA at 15 V and a leakage current of approximately 0.01 mA at a reverse bias of 15 V. The good rectification feature of the n-MgO@ZnO:Ga/p-GaN heterojunction device suggests that current transport can be manipulated by introducing a MgO insulating layer [25,29,32,33].
Fig. 5. EL characterization of the as-constructed n-MgO@ZnO:Ga MW/p-GaN heterojunction LED. (a) Schematic of the n-MgO@ZnO:Ga MW/p-GaN heterojunction device structure. The In particles and Ni/Au work as electrodes for current injection in the device structure. (b) The characteristic curve of the constructed n-MgO@ZnO:Ga MW/p-GaN heterojunction device. The insets illustrate the $I$-$V$ curves of the MgO@ZnO:Ga MW and p-GaN substrates. (c) EL spectra of the as-constructed LED by varying the operating current in the range 0.3–6.6 mA. (d) Integrated EL intensity of the device as a function of the injection current. (e) The normalized PL spectrum of p-GaN and EL spectrum of the as-constructed LED are provided for comparison. (f) Diagrammatic drawing of the energy-band structure for the as-designed n-MgO@ZnO:Ga MW/p-GaN heterojunction LED under forward bias. The injected electrons are confined and accumulated in the ZnO:Ga MW active medium, whereas the holes can be injected into the ZnO:Ga MW through the barrier.
The fabricated n-MgO@ZnO:Ga MW/p-GaN heterojunction device was operated at room temperature under forward-bias conditions. Significant UV light emission was observed when the forward bias increased above the turn-on voltage. Light signals were recorded, and a charge-coupled device (CCD) spectrometer (PIXIS) was used to collect the emitted photons. Illustration in Fig. 5(c) shows the EL spectra of the heterojunction LED, which were measured by varying the injection current in the range of 0.3–6.6 mA. A relatively narrow EL light emission peak was observed at approximately 375.5 nm under various forward injection currents. The spectral linewidth was measured to be about 25.5 nm. The peak position of the UV emission band remained unchanged throughout the applied voltage range. The light-current features shown in Fig. 5(d) shows that the integrated EL intensity increases almost linearly with an increase in the input current.
We can study the EL emission mechanism through the normalized intensities of the PL spectra of the p-GaN layer and MgO@ZnO:Ga MW and the EL spectrum of the as-fabricated heterojunction LED, which are shown in Fig. 5(e). The figure (the solid red line) shows that the PL spectrum of the p-GaN film is characterized by a broad blue emission peaking at 435.5 nm with a line width of about 50 nm, which was usually attributed to the transition from the conduction band to the Mg acceptor level in the Mg-doped p-GaN film. The PL spectrum of the MgO@ZnO:Ga MW (solid violet line) exhibits an NBE recombination emission band at 375.0 nm with an fwhm of about 12.5 nm. The EL spectrum (the navy-blue solid line) illustrates a typical ultraviolet emission peaking at around 376.8 nm, which can be attributed to the NBE emission in MgO@ZnO:Ga MW. Nevertheless, the EL band extends unsymmetrically to 450 nm. The line width of the EL spectra was derived to be approximately 25.5 nm, which is much broader than that of the PL profile of the MgO@ZnO:Ga MW. Therefore, an energy barrier may form at the MgO@ZnO:Ga/GaN heterostructural interface, which is generally produced during the preparation process [20,24,25,39].
The working principles of current transport and EL emission were investigated. The energy-band structure diagram of the n-MgO@ZnO:Ga/p-GaN heterojunction is shown in Fig. 5(f). The electron affinities $\psi$ of ZnO:Ga and GaN are $\psi _{ZnO:Ga}$ $\sim$ 4.5 and $\psi _{GaN}$ $\sim$ 4.2 eV, and the energy bandgaps of ZnO:Ga ($E_{ZnO:Ga}$) and GaN ($E_{GaN}$) are $\sim$ 3.34 and $\sim$ 3.4 eV, respectively [29,32]. The conduction-band offset between ZnO:Ga and MgO was evaluated to be about 3.55 eV. The valence-band offset $\sim$ $\Delta E_v$ was estimated to be approximately 0.90 eV. The bias is mainly applied on the MgO insulating layer due to its dielectric nature under the operation of the forward biasing condition. The energy bands of MgO were also bent. Meanwhile, the effective barrier of the valence-band offset in the vicinity of MgO/GaN was dramatically reduced. Consequently, the injected electrons will be restrained and accumulated in the ZnO:Ga MW because of the large conduction band offset between MgO and ZnO:Ga, whereas the holes in the GaN layer can tunnel through the barrier and be injected into the MW simultaneously. Therefore, the highly efficient electron-hole recombination region is principally confined in the ZnO:Ga MW active region, yielding the interband transition emission in ZnO:Ga MW [29,31,44].
An optical microscopic EL image of the device at an injection current of 6.0 mA was captured, as shown in Fig. 6(a). A bright light-emitting pattern along the MgO@ZnO:Ga MW profile was clearly observed. The EL image exhibits a distinct purple color compared with previously reported literature. In addition, the fine structure of the EL peak presents a series of resonance fringes, which is consistent with the WGM interference peaks in the MWs. Therefore, the effect of the MgO insulating layer on the EL characteristics of the n-ZnO:Ga MW/p-GaN heterojunction should be considered [21,23–25,28]. First, a bare ZnO:Ga MW with an identical size working as the active medium was employed to construct a p-n heterojunction LED by combining the p-GaN film. EL characterization of the constructed n-ZnO:Ga MW/p-GaN heterojunction emission device was performed. The EL spectrum recorded at a current of 7.8 mA is shown in Fig. 6(b) (solid black lines). We can observe from the spectrum that the as-fabricated n-ZnO:Ga MW/p-GaN heterojunction diode can emit at a near-ultraviolet wavelength of 411.5 nm with a linewidth of about 55 nm. In particular, the EL spectra can be resolved into a series of resonance fringes, in accordance with the WGM interference peaks in the MW. The $Q$-factor was evaluated as 120. The broader EL emission band can be attributed to the superposition of ZnO:Ga near bandgap recombination, strong n-ZnO:Ga/p-GaN interfacial radiation, and very weak emission from the p-GaN film [23,25,32,39].
Fig. 6. (a) Top-view optical microscope EL image of the as-fabricated n-MgO@ZnO:Ga MW/p-GaN heterojunction LED. (b) Normalized EL spectra of the as-fabricated single-MW heterojunction LED with the wire not covered and covered by the MgO layer. (c) EL spectra of n-MgO@ZnO:Ga MW/p-GaN heterojunction LED under CW operation, with injection current varying from 15.4 to 27.0 mA. Above 15.4 mA, the lasing characteristics were observably captured. The emission arrows in the 27.0 mA EL spectrum show quasi-equidistant peaks. (d) Variations of the integrated EL spectral intensity and spectral fwhm as functions of the operating current, yielding a microlaser threshold of approximately 15.2 mA. (e) Fitted EL spectrum obtained from the heterojunction LED at an injection current of 24.8 mA. The lasing arrows highlight WGM modes with an average mode spacing $\Delta \lambda$ $\sim$ 5.0 nm, the fwhm ($\delta \lambda$) is measured to about 0.95 nm, thus, the $Q$-factor is estimated to about 400.
As described above, electrons were confined and accumulated in the ZnO:Ga MW due to the MgO layer’s transport modulation. Meanwhile, hole carriers can be injected into the MW through the triangular energy barrier of the valence-band offset via quantum tunneling. As a result, excitonic ultraviolet EL was realized in the ZnO:Ga MWs [29,32,45]. The EL spectrum of the constructed n-MgO@ZnO:Ga MW/p-GaN heterojunction LED is shown in Fig. 6(b) (solid red line). The main EL wavelengths of the n-MgO@ZnO:Ga MW/p-GaN heterojunction LED exhibited a significant blue shift compared to that of the LED in the absence of the MgO layer. In addition, the line width displayed visible narrowing. The $Q$-factor is extracted as approximately 250, which is larger than that of the n-ZnO:Ga MW/p-GaN heterojunction LED. The demonstration of an n-MgO@ZnO:Ga MW/p-GaN heterojunction LED can avoid the limitation of inherently efficiency droop even at record high current densities. The advantages of the as-fabricated single MW heterojunction LED are summarized as follows. First, the highly-crystallized ZnO:Ga MWs are expected to have a lower potential drop and resistive loss in the fabricated LEDs. Second, the flow of electrons (electron leakage) to the p-GaN is not observed in MW LEDs. While, the p-GaN contact is also effective for the current spreading and for the effective hole injection into the ZnO:Ga MWs. The designed LED is conducive to limiting the droop effect and, thus helps in boosting internal quantum efficiency, which is a determining parameter in the overall performance of the LED. Third, at high injection regime, the EL results also reveal that, the generation still occurs in the ZnO:Ga MWs, and the EL intensity grows linearly with the injected current, suggesting that carrier loss due to Auger recombination is almost negligible [24,26,27].
Compared with the introduction of a low-dielectric layer deposited on the planar-contact interface, the use of a ZnO:Ga MW covered by an ultrathin MgO layer can engineer the band alignment of the n-ZnO:Ga/p-GaN heterojunction and the current transport path. It can also enhance the cavity performance due to the enhancement of the optical gain, which is related to the higher efficiency of the total internal reflection at the ZnO:Ga MW microcavity boundary [29,36,42]. Furthermore, the lasing action of the as-constructed n-MgO@ZnO:Ga MW/p-GaN heterojunction emission device was performed upon continuous wave (CW) operation of electrical excitation at room temperature [46–48]. Figure 6(c) shows representative EL spectra when the input current is varied from 15.4 to 27.0 mA. The broad spontaneous emission collapsed to several sharp emission modes featuring lasing characteristics when the injection current reaches up to 17.5 mA [21,24]. The sharp peak operates at 378.8 nm wavelength in the UV band, with the fwhm sharply narrowing to approximately 0.95 nm. We observed that the EL spectra are dominated by a series of sharp and narrow emission peaks when the input current was above 17.5 mA. Thus, the UV light-emitting is selectively amplified by the optical feedback in the MgO@ZnO:Ga MW [7,21,24].
The integrated emission intensity and linewidth dependent on the forward driving currents are plotted in Fig. 6(d). The integrated EL intensity is the integral of all area under the EL profiles, which is identified with total output light-emission at all wavelengths. The injection current-dependent integrated EL intensity is plotted on a double logarithmic scale. The representative "$S$"-shaped profile can be well-fitted by a lasing rate formula with stimulated-radiation factor value ($\beta$) of approximatively 0.25, confirming the lasing action upon CW operation. As seen in the figure (the blue solid line), the light-emitting increases slowly when the operating current is lower than 15.2 mA, and then increases sharply as the operating current increases above the 15.2 mA. Additionally, at relatively low driving current, a broad spontaneous emission band centered at 378.5 nm and fwhm of 25.5 nm can be observed. As the operating current exceeds 15.6 mA, a series of narrow peaks arise over the spontaneous emission band and further dominates the EL spectrum. Especially, the spectral fwhm of EL emission drastically drop from 25.5 to 0.95 nm. The curve profiles suggest that an unambiguous transition from spontaneous radiation to stimulated radiation can occur at 15.2 mA, contributing to the lasing threshold. Thereby, the occurrence of lasing action was acknowledged through the depiction of integrated EL intensity and spectral fwhm versus injection current [21,24,27,33].
As previously reported, hexagon-shaped ZnO micro/nanostructured WGM cavities are very sensitive to boundary conditions [17,36,49]. The refractive index of GaN is close to that of ZnO in the as-constructed n-ZnO/p-GaN and n-ZnO/MgO/p-GaN heterojunction emission device. The light leaks from ZnO to the GaN substrate because the total internal reflection condition cannot be satisfied at the ZnO/GaN interface. Although the light can still be effectively confined by the rest of the side surfaces of ZnO/air, ZnO/MgO, and ZnO/electrode, achieving lasing emission, the optical gain should be sufficiently high to overcome the optical loss along the entire WGM optical path [20,21,24,25]. Incorporating a low-dielectric layer deposited on the external facets may have influenced the cavity performance and enhanced the optical confinement ability in the present study.
The lasing characteristics of the constructed n-MgO@ZnO:Ga MW/p-GaN heterojunction microlasers were investigated. The mode spacing of the WGM laser can be determined using the following equation [17,21]:
In the formula, $\lambda$ is the lasing wavelength of the sharp peaks, $n_{ZnO:Ga}$ is the refractive index of the ZnO:Ga MW ($\sim$ 2.35), and $L_{eff}$ is the cavity length. In our as-constructed single ZnO:Ga MW heterojunction microlaser, the efficient cavity length is assumed to be the maximum possible optical path traversed by light inside the hexagon-shaped cross-section of the wire. According to the EL spectrum plotted at a current of 24.8 mA (Fig. 6(e)), the calculated diameter was evaluated to be approximately 10 $\mu$m. It is almost identical to the size of the wire used in the device, which was experimentally observed (Fig. 6(a)). It is demonstrated that the as-constructed n-MgO@ZnO:Ga MW/p-GaN heterojunction devices presented in this work exhibit electrically driven highly purified UV emission of ZnO:Ga and have a much narrower line width in the EL spectra. The above EL characteristics suggest that individual ZnO:Ga MWs covered by a low dielectric layer are promising candidates for the development of ultraviolet light-emitting devices, especially ultraviolet microlasers [21,24,27,33].
4. Conclusions
We proposed a workable approach to fabricate electrically driven WGM microlasers by constructing an n-MgO@ZnO:Ga MW/p-GaN heterostructure. Using individual ZnO:Ga MW covered by a MgO dielectric layer can achieve modulation of the optical loss, carrier transport, energy band structure, and device performance-enhanced UV LEDs. The devices exhibited a highly stabilized and purified UV emission peak at $\sim$ 378.0 nm with a linewidth of approximately 25.5 nm, and the EL profiles illustrate a series of resonant modes. Many sharp lasing peaks dominated the broadband spontaneous emission spectra with an fwhm of about 0.95 nm when CW-operation of electrical pumping was done, especially for the injection current above 15.2 mA. Thus, this indicated that there is enough laser gain to sustain cavity modes in the hexagon-shaped MgO@ZnO:Ga MW. The origin of lasing is ascribed to excitonic recombination in the MgO@ZnO:Ga MW WGM cavity. Electrically driven WGM lasing with an unambiguous multiple-mode structure was realized based on the n-MgO@ZnO:Ga MW/p-GaN heterojunction. The lasing features exhibit a relatively wide emission band, multiple modes, and low $Q$-factor. Although, this investigation is essential step toward the realization of electrically pumped WGM microlasers.
Funding
National Natural Science Foundation of China (11874220, 11974182).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
The data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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