1. Introduction

Functioning as low threshold, low-power dissipation, miniaturized and compact coherent light sources, low-dimensional semiconductor lasers have gained extensive attentions as building blocks for the integrated optoelectronic devices, which can enable applications including photonic and optoelectronic circuits, high-speed communications, quantum information encryption technology, optical probes, and so on [1–6]. Over the years, significant progress has been developed in the fabrication of low-dimensional microlasers by noticing that miniaturizing the device size, lowering the pump threshold, and reducing the manufacturing cost of device architectures [7–11]. As a typical third generation semiconductor, ZnO (direct wide bandgap $\sim$ 3.3 eV, large exciton binding energy $\sim$ 60 meV) can provide a promising candidate to fabricate ultraviolet optoelectronic devices, especially for micro- and nanolasers [12–15]. Due to appealing optical gain characteristics and naturally formed microresonantors, electrically and optically pumped lasing actions containing random, Fabry-Perot (FP), and whispering-gallery-mode (WGM) modes have been achieved on account of ZnO micro/nanostructures with well-defined geometry [16–21].

By contrast, the WGM lasing exhibits a much higher quality factor and lower lasing threshold, but suffers from much more multimodes and relative poorer output directionality; while the greatest dilemma of random lasers is the lack of stabilized directionality and output modes [16,22–25]. Differ from the WGM and random microlasers, the output directionality, one of the most important merits of a laser can be fullfilled perfectly by a FP microlaser [26–28]. Until now, it remains a difficult challenge to achieve electrically pumped FP microlasers with low-threshold and desirable output directions. Due to the lack of stable and reproducible p-type ZnO template, the fabrication of efficient ZnO-based homojunction optoelectronic devices is still highly challenging [14]. As an alternative, p-GaN semiconductor materials can effectively be utilized to construct ZnO-based low-dimensional optoelectronic devices by combining ZnO micro/nanostructures because of their identical crystalline structure of hexagonal wurtzite, similar electronic properties and their relatively small lattice mismatch [19,20,29]. Unfortunately, the realization of electrically pumped lasers with n-ZnO/p-GaN heterojunctions has primarily been impeded by the great difficulties of p-GaN substrate involving efficiency droop and much lower hole concentration ($\sim$ 10$^{18}$ cm$^{-3}$ for commercially available p-GaN substrates) [20,30–32]. Therefore, increasingly aggressive efforts should be evolved to develop microsized light-emitting diodes (LEDs) and laser diodes (LDs) due to either novel operating principle or new materials systems that are of scientific basic for practical applications [31,33,34].

In this study, a bright, blue-violet microsized emission device made of a Ga-doped ZnO (ZnO:Ga) microribbon and p-type GaAs is realized experimentally, which operated under reverse-biasing condition. When the input current below a certain value, the device exhibits a broadband blue-violet emissions peaking around 410 nm, and a spectral linewidth of about 60 nm. A series of resonance peaks were further resolved from the emission spectra, and the waveguided output spectra can be ascribed to the exciton-polariton behaviors, with corresponding coupling strengths characterized by a Rabi splitting energy of about 500 meV. Further, using the similar device structure, we have succeeded in the fabrication of continuous wave (CW) operation of electrically pumped FP microlaser by exploiting the single microribbon with high-quality and well-defined geometry as the optical microresonator. The laser diode operates in the blue-violet spectrum and exhibits a threshold reverse-biased current of 32.5 mA capable of operating at room temperature. Employing the single microribbon as optical resonators, the lasing oscillations can be assigned to the FP resonant cavity. The FP lasing action and the resonant mechanism were further investigated in detail via experimental evidence and theoretical analysis. Our experiment results open a new paradigm in the design and development of low-dimensional FP microlasers, and can also facilitate the advancements in electrically pumped high-efficiency polaritonic coherent light sources and nonlinear devices.

2. Experimental section

2.1 Sample preparation

Individual ZnO:Ga microstructures (such as microribbons, microwires with rectangular-shaped cross section) were prepared via a simple chemical vapor deposition method [7,18,19]. In the preparation procedure, high temperature tube furnace with double temperature zones is used as the crystal growing apparatus. A mixture of high-purity powders with the weight ratio of ZnO : Ga$_2$O$_3$ : graphite (C) = 9 : 1 : 10 serving as the precursors, was placed in a corundum boat. After being cleaned thoroughly, Si substrates (without any catalyst coating) were placed on the corundum boat to collect the products. During the synthesis process, a constant flow of argon ($Ar$) (99.99$\%$) (150 sccm) was introduced into the tube furnace as the protecting gas. Owing to the lower growth temperature of ZnO (950 $^\circ$C) than that of Ga$_2$O$_3$ (1100 $^\circ$C) in a vapor-solid progress, the furnace temperature was firstly increased to 1150 $^\circ$C in advance; then, the precursor mixtures were placed at the hottest zone. After maintaining about 5 mins, Ga and Zn vapors could be mixed thoroughly. The high-temperature growth environment could make Ga substitute Zn efficaciously. Continue to keeping the hypoxia synthesized condition $\sim$ 20 min, 8$\%$ oxygen ($O_2$) was introduced into the furnace chamber as the growth gas.

Afterwards, the furnace continued to maintain about 1 hour, and then naturally cooled down to room temperature. The as-synthesized microribbons can be collected on the Si-substrate. The width of the microribbons varies in the range of 5–100 $\mu$m, and the length can be up to 1 cm. Meanwhile, to fabricate ZnO:Ga microribbons with higher crystallization quality and controlled surface morphologies, the growth temperature and the reaction times should be increased and extended accordingly.

2.2 Device fabrication

Heterostructured light-emitting devices involving n-ZnO:Ga microribbon/p-GaAs were fabricated as follows: (1) the p-GaAs template was cleaned thoroughly; (2) MgO layer then deposited using Molecular-Beam-Epitaxy, served as an insulating layer; (3) Ni/Au electrode was deposited on GaAs template using the electron-beam evaporation system, and the Ohmic contacts were activated by a rapid thermal annealing; (4) finally, a single ZnO:Ga microribbon was selected and transferred across the boundary between MgO and GaAs template, and In particles were fixed the microribbon on the insulating layer while serving as the electrode. Schematic diagram of electrically driven light-emitting device involving single ZnO:Ga microribbon and p-GaAs heterojunction is schemically illustrated in Fig. 2(a). Therefore, n-ZnO:Ga microribbon/p-GaAs heterojuction light-emitting devices were fabricated [19,35,36].

2.3 Characterization and analysis instruments

The as-synthesized ZnO:Ga microribbons were characterized using a scanning electron microscopy (SEM). Optical properties of individual ZnO:Ga microribbons were performed by using a He-Cd laser as excitation source (the pumping wavelength at 325 nm), and the corresponding photoluminescence (PL) spectra were collected by using a Horiba Jobin-Yvon iHR500 spectrometer via an Andor Newton electron multiplying CCD camera. The electrical properties with current-voltage ($I$-$V$) characteristics curves of a single ZnO:Ga microribbon, p-GaAs template with Ni/Au electrodes, and n-ZnO:Ga microribbon/p-GaAs heterojunction devices were performed by using Keysight semiconductor device analyzer (B1500A). The working characteristics of electroluminescence (EL) emissions were measured under CW biasing conditions by using PIXIS 1024BR CCD detection system. Besides, the optical microscopic EL images of light emission from electrically illuminated n-ZnO:Ga microribbon/p-GaAs LEDs were recorded by utilizing a high numerical aperture microscope objective via a CCD camera (Olympus). And all the measurements were performed at room-temperature.

3. Results and discussion

3.1 Optically pumped FP lasing action of a single ZnO:Ga microribbon

Figure 1(a) demonstrates a typical SEM image of a ZnO:Ga microribbon. From the figure, the ribbon demonstrates rectangular-shaped cross section and smooth sidewall surfaces. The width of microribbon is about 70 $\mu$m. PL emission measurement of a single ZnO:Ga microribbon was performed using the 325-nm line of a He-Cd laser as an excitation source; and the recorded PL spectrum is shown in Fig. 1(b). It is clearly illustrated that the near-band-edge (NBE) emissions of ZnO:Ga dominated the PL spectrum [18,29]; While, it was also accompanied by an ignorable weaker broadband defect-related emission at about 500 nm, which may be assigned to the intrinsic defect, or the Ga-doping related impurity levels [13,19,37].

 

Fig. 1. (a) SEM image of the as-prepared single ZnO:Ga microribbon. (b) PL spectrum of the ZnO:Ga microribbon. (c) Under optical characterization via a fs pulse laser, PL spectra as a function of the pumping fluence varying in the range of 46.6–103.6 kW/cm$^2$. (d) PL spectrum of the ZnO:Ga microribbon, with the pumping fluence of 90.0 kW/cm$^2$; the peak spacing $\Delta \lambda$ $\sim$ 0.40 nm, and corresponding linewidth $\delta \lambda$ $\sim$ 0.20 nm. (e) Integrated PL intensity as a function of the pumping fluence varying in the range of 46.6–103.6 kW/cm$^2$.

Download Full Size | PDF

A ZnO:Ga microribbon with smooth sidewalls, may potentially serve the reflecting mirrors for the emitted photons propagating back and forth in the rectangular-shaped cross section, leading to the cavity construction of FP microresonator [24,38]. Meanwhile, serving as the superior optical gain medium, it is highly possible to realize FP lasing in the ultraviolet spectrum, with lateral cavity enabled by the bilateral sides of ZnO:Ga microribbons [19,22,39,40]. In this regard, we use a standard microphotoluminescence ($\mu$-PL) measurement with a fs pulsed laser (the excitation wavelength of 355 nm) via a 50 $\times$ microscope objective to both pump the microribbon and record its emitted photons from the top surface [7,16,18]. Figure 1(c) exhibits the pumping energy-dependent luminescence spectra of a single ZnO:Ga microribbon. Varying the pumping power density $I_{exc}$ in the range of 46.6 to 71.5 kW/cm$^2$, only a broadband PL peak is collected peaking at 387.0 nm and a spectral linewidth of about 12 nm. And the corresponding PL intensity increases slowly. When the pump intensity $I_{exc}$ reaches 71.5 kW/cm$^2$, a series of narrow peaks emerge and the full width at half maximum (FWHM) value rapidly reduces to approximately 0.20 nm. It indicates that an optical microresonator with specific modes is formed in the rectangular-shaped cross section of single ZnO:Ga microribbon [18,24]. As the pumping energy $I_{exc}$ increases more than 71.5 kW/cm$^2$, the PL spectra were dominated by a series of sharp emission lines, accompanying by the PL intensity increases sharply (See Fig. 1(c)).

Further to exploit the PL characteristics, the spectral peak spacing ($\Delta \lambda$), a critical parameter for a typical microresonator, was taken into account. As shown in Fig. 1(d), regardless of the pumping power, the peak spacing between two adjacent oscillation modes, keeps an approximately constant. Specifically, uniformly spaced oscillation peaks ($\Delta \lambda$ $\sim$ 0.40 nm) and FWHM ($\delta \lambda$ $\sim$ 0.20 nm) can be resolved in the PL spectrum, indicating the same waveguide origin of the optical modes. The $Q$-factor of the single microribbon based FP microcavity was determined as 1900 according to the formula $Q$=$\lambda$/$\delta \lambda$, where $\lambda$ is the peak wavelength. To analyze the working characteristics of PL emission in more details, a formation of the optical microcavity was expressed. The volume of the lateral microresonator is determined by the width of the microribbons. For an optical FP-mode microcavity, the formula of the mode spacing is described as follows [16,22,39],

The integrated PL intensity dependent on pumping fluence is plotted in Fig. 1(e). From the figure, the relationship of the integrated PL intensity versus the pumping fluence exhibits a joint between two linear regions. The linear region at low pumping fluence is ascribed to spontaneous radiation, and the other linear increase region at higher pumping fluence indicates a full lasing action, with the corresponding lasing threshold extracted to be about 71.5 kW/cm$^2$. In addition, once the pumping fluence increasing beyond 71.5 kW/cm$^2$, the spectral linewidth declines sharply from 13.5 to 0.2 nm, further substantiating the lasing behavior. All the results indicate that FP lasing action can be obtained on account of the single ZnO:Ga microribbon with rectangular-shaped cross section. Therefore, the relatively uniform and smooth side facets of the as-synthesized ZnO:Ga microribbons can support a competitive platform to construct FP-mode microlasers, with rectangular-shaped cross section serving as the optical microresonantor [19,22,38].

3.2 Current-driven exciton-polariton emission device of a n-ZnO:Ga microribbon/p-GaAs heterojunction

As everyone knows, typical semiconductor materials with the high refractive index can provide a promising platform to construct low-dimensional optoelectronic devices with the physical size scaled down to only a few hundred nanometres. Especially, micro/nanostructures can facilitate low-loss optical waveguiding and optical recirculation in the active region due to their unique well-defined geometries [16,26,41,42]. GaAs, a type III/V semiconductor possessing high electron mobility and a high saturated electron velocity, has enabled intense research efforts for optoelectronic devices including integrated circuits, LEDs, LDs, solar cells, etc [2,4,9,43]. Suffering from large surface recombination velocity and auger recombination, GaAs micro/nanostructures employing as a gain medium has limited progress on the low-dimensional optoelectronic devices in the technologically important near-infrared spectral range [4,9,44]. Since GaAs wafer is still one of the modern electronic industry and has asserted its dominance in large-scale device integration. By a carefully desigened heterostructure composed of a single ZnO:Ga microribbon and p-GaAs template, the association of the superiorities of both the semiconductors would make as-fabricated devices applicative value and can be applied widely [20,45,46].

As depicted schematically in Fig. 2(a), heterostructured light-emitting device involving a single ZnO:Ga microribbon and p-GaAs template was fabricated [20,31,47]. A single ZnO:Ga microribbon was used to construct metal-semiconductor-metal structure, with In particles serving as electrodes. Electrical property of the single ZnO:Ga microribbon was performed. As shown in Fig. 2(b), $I$-$V$ curve exhibits a linear and symmetric character, suggesting the ohmic contacts between the In electrodes and ZnO:Ga microribbon. Owing to Ga-incorporation, individual ZnO microribbons reveal excellent n-type conduction [13,48,49]. Afterwards, the electrical properties of n-ZnO:Ga microribbon/p-GaAs heterojunction devices were carried out. As illustrated in the inset of Fig. 2(c), the $I$-$V$ curve demonstrates typical rectification characteristics with a turn-on voltage of about 2.8 V [19,29,36,50]. And a much lower leakage current of about 10$^{-2}$ mA at a reverse bias up to 5 V was also obtained. Besides, varying the reverse bias above 12 V, the leakage current illustrates a dramatically increase (See Fig. 2(c)). Because of the high doping concentration in p-GaAs, the space charge layer is primarily distributed in the ZnO:Ga microribbon. When operated at high reverse bias, electrons may tunnel from valence band of p-GaAs to the conduction band of ZnO:Ga, yielding tunneling breakdown. Thus, the large leakage current can be assigned to the tunneling effect [51,52].

 

Fig. 2. Fabrication and characterization of n-ZnO:Ga microribbon/p-GaAs heterojunction LED: (a) Schematic illustration of the fabricated single ZnO:Ga microribbon/p-GaAs heterojunction device. (b) Employing In particles as electrodes, $I$-$V$ characteristics curve of a single ZnO:Ga microribbon suggestes a well-defined Ohmic behavior. (c) $I$-$V$ characteristics curve of n-ZnO:Ga microribbon/p-GaAs heterojunction emission device. Inset: $I$-$V$ characteristics curve of the heterojunction device in the range of -4–5 V. (d) EL spectra of a single ZnO:Ga microribbon-based heterojunction LED under various reverse-biased currents at room temperature. (e) The main peak wavelengths and integrated EL intensity as functions of reversed input current. (f) Emission spectrum acquired from the single microribbon based LED, and corresponding energy-wavevector dispersion curves.

Download Full Size | PDF

Due to the larger leakage current operated under large reverse bias, the working characteristics of EL emission were performed at room temperature. Increasing the reverse bias more than a certain value, bright and blue/violet illuminating can be clearly observed, with the emission regions distributed along both the bilateral sides of the ribbon. We collected the emitted light from the top surface of the single ZnO:Ga microribbon, as plotted in Fig. 2(d). From the figure, the emission bands covered in the range of 375 to 550 nm. Furthermore, the integrated EL intensity and the main peak wavelengths versus the reverse currents were performed, and depicted in Fig. 2(e). The intergrated EL intensity increased under various increasing injection current following an approximately linear relationship. Additionally, a slight blueshift of the main wavelengths of EL peaks was acquired, with a spectral linewidth of approximately 60 nm.

It is distinctly noted that the EL spectra can be spatially resolved into a series of sharp peaks over a broadband spectral range in the blue-ultraviolet spectrum [19,36,46]. The sharp peaks with average mode spacing of about 8.0 nm can be extracted on spontaneous emission background. The average linewidth of each multipeak is evaluated to be 3.0 nm, and the $Q$-factor is calculated to be about 120. It is much smaller than that obtained in the single ZnO:Ga microribbon by optical excitation [18,19]. The much lower $Q$-factor of the electrically biased waveguide emission could be caused by the light leakage at the ZnO:Ga/GaAs interface, as well as the propagation loss in the rectangular-shaped cross section of microribbon [19,20,30]. Besides, the EL spectra illustrate that with reducing the photon energy in the lower energy side of the EL spectrum, the peak spacing of the resonance energies exhibits a significant broadening, which could not be explained by pure photonic modes. As the input current increasing, the main EL peak positions illustrate significant blueshift, agreeing well with the typical characteristics of localized excitons in the as-fabricated single microribbon based LED. Thus, strong exciton-photon coupling modified the EL spectra, covering a blue-violet emission band in the range of 375 to 500 nm, can be achieved [19,36,46].

Further to analyze the multipeak behavior of the single microribbon based LED, one of the unique characteristics of conventional photonic model is the identical mode spacing due to the same group index for different photon-energy [16,53,54]. Taken a reverse current of 9.0 mA for instance, the EL spectrum was resolved into a series of narrow peaks. As the peak spacing changed from 6.0 nm to 14.0 nm toward the longer wavelength shoulder, thus, the oscillation modes suggest that strong light-matter interaction can occur in the as-fabricated LED, leading to the formation of exciton-polariton quasi-particles [53–55]. Accordingly, the energy-wavevector dispersion curves is further demonstrated in Fig. 2(f). From the picture, the coupling strength of exciton-polariton defined as Rabi splitting energy was extracted to be about 500 meV. Therefore, the observed series of resonance peaks on the EL spectra can be assigned to the exciton-polariton illuminating features [19,36,46].

When lit up electrically, the as-fabricated LED can emit blue-violet light. The luminous photos of microscopy EL images were captured via a microscope CCD camera, as seen in Fig. 3. As seen in the photos, the corresponding birghtest lighting regions distributed along both the bilateral sides of the microribbon. With a gradual increase of the reverse current, the EL lighting became brighter and brighter. Therefore, the homogeneity of the light-emitting distributed along the bilateral sides of the LED can be caused by the uniform distribution of recombination centers as well as uniform current spreading across the microribbon [19,39,40].

 

Fig. 3. Upon illuminated electrically, optical microscopic EL images of blue/violet emitting from single n-ZnO:Ga microribbon/p-GaAs heterojunction emission device were captured via a CCD camera by increasing the reverse current in the range of 6.0-20.0 mA.

Download Full Size | PDF

As we described above, the main wavelength of PL emission positioned at 378.0 nm, corresponding to the NBE-type emission of ZnO:Ga (the wide bandgap of 3.37 eV); While the narrow bandgap of GaAs is about 1.43 eV. According to previous literature, the electron affinity $\psi$ of ZnO and GaAs are 4.5 eV and 4.07 eV, respectively. Further, heterojunction of n-ZnO:Ga microribbon/p-GaAs has been formed, thus, a band discontinuity would occur at the heterointerface due to the Anderson model [20,43]. The band structure of n-ZnO:Ga/p-GaAs heterojunction was depicted in Fig. 4(a). From the figure, the conduction band offset was extracted to be $\Delta E_c$ = 0.43 eV, and the valence band offset $\Delta E_v$ was evaluated to be 2.37 eV. Therefore, n-ZnO:Ga/p-GaAs heterostructure has an asymmetrically large band offset between two energy bandgaps, a considerably large valence band offset of $\Delta E_v$ and a relatively low conduction band offset of $\Delta E_c$ [20,34,45]. Since the large difference between $\Delta E_c$ and $\Delta E_v$, the energetic barrier was much lower for electrons than holes. Under such an energy band configuration of n-ZnO:Ga/p-GaAs heterojunction, the band offset between n-ZnO:Ga microribbon and p-GaAs template could be extracted to be around 1.0 eV in the near-infrared spectrum [33,34,45].

 

Fig. 4. (a) The energy band diagram of the n-ZnO:Ga microribbon/p-GaAs heterojunction under thermal equilibrium. (b) As the reverse bias increased beyond certain value, schematic energy band of the n-ZnO:Ga microribbon/p-GaAs heterojunction LED.

Download Full Size | PDF

When operating at proper reverse bias, the origin of blue-violet EL from n-ZnO:Ga microribbon/p-GaAs heterojunction LED was further investigated. As the narrow band of GaAs being much smaller than ZnO, the EL illuminating with the main peak wavelength of around 410 nm can be derived from the band-to-band transitions induced light-emission of ZnO:Ga, which is approximate to the EL peak wavelengths of n-ZnO/p-GaN heterojunction LEDs being reported [20,35,36,46]. Thereby, the radiative recombination is primarily distributed at the side of ZnO:Ga microribbon, rather than at the interface region in the as-fabricated LED. Figure 4(b) schematically describes band diagram of the n-ZnO:Ga microribbon/p-GaAs heterojunction under reverse-biasing condition. It clearly exhibits the blue-violet lighting mechanisms, as well the carrier travelling paths. Progressively increase the applied bias, the overall band structure of p-GaAs would move upward, causing the bending of the barrier zones. The increasing of reverse bias can result in more inclined of the barrier band, which further makes the maximum valance band of GaAs higher than the minimum conduction band of ZnO:Ga. The physical process will tend to thiner and thiner of the tunneling barrier, which is approximate to the broken-gap band at the pn-junction [19,34,46]. In turn, the built-in electric-field with much higher intensity can be established across the n-ZnO:Ga/p-GaAs heterojunction, and this intense electric-field will enable the electrons situating at the valance band to obtain adequate potential energies. Ultimately, the electrons could tunnel through the narrow barrier from valence band of p-GaAs to conduction band of ZnO:Ga. Meanwhile, large amounts of electrons can also flow into ZnO:Ga through the conduction band of p-GaAs by this large reverse electric-field. These electrons arriving at the conduction band of ZnO:Ga, can recombine with the holes in the valance band at the side of ZnO:Ga microribbon to produce blue-violet emissions peaking around 410 nm [19,36,50].

3.3 CW operation of an electrically pumped single microribbon based FP microlaser

Furthermore, the working characteristics of the single microribbon based LED was implemented under CW reverse-biasing conditions at room temperature [56–58]. Figure 5(a) demonstrates the EL spectra of the single microribbon based LED device under different injection currents. As plotted in the figure, two discrete sharp peaks positioned at 410.0 and 450.5 nm respectively, can be captured in the enlarged EL spectra. And the spectral positions of the dominating modes, remain nearly constant as a function of reversed driving-current. Two sharp peaks with unique resonance modes and spectrally distant spacing were observed. It is highly possible determined by relatively few FP modes falling within the gain spectrum of ZnO:Ga microribbon [19,56]. Thus, the increase of input current can lead to an explicit spectral transition in the presence of the as-fabricated single microribbon based LED. A group of sharp EL peaks arised on the broadband spectral response of the spontaneous radiation as stimulated illuminating begins to dominate the EL emissions.

 

Fig. 5. CW lasing characteristics of n-ZnO:Ga microribbon/p-GaAs heterojunction laser device. (a) EL spectra with increasing the input currents of the single microribbon based heterojunction device under CW operation at room temperature. (b) The $L$-$I$ characteristic curve of integrated EL intensity versus injection current measured under CW reverse-biasing condition. (c) Comparison of lasing spectrum and waveguide EL spectrum. (d) Lorentz fitting of a lasing oscillation mode (the lasing peak $\lambda$ centered at 450.5 nm), giving the FWHM $\delta \lambda$ of the lasing peak $\sim$ 1.15 nm at the input current of 39.3 mA under reverse-biasing condition. (e) Polarization-sensitive EL spectra of the single microribbon based microlaser device at the input current of 39.3 mA. Inset: In the EL measurement, the orthogonal polarization with blue-double arrow solid line was paralleled to the axial direction of ZnO:Ga microribbon; while the violet-double arrow solid line was perpendiculared to the axial direction of microribbon.

Download Full Size | PDF

Figure 5(b) exhibits the integrated EL intensity as a function of the reverse-biased current. A "kink" point appeared in the $L$–$I$ characteristic curve, which can also be evidenced for the lasing behavior [20,31,57,58]. And a lasing threshold of 32.5 mA represented from spontaneous emission to stable lasing action is deduced. In contrast, the distinguish of spontaneous radiation and CW-pumping lasing is also evidenced by the comparison of the lasing spectrum and waveguide EL spectrum. The spontaneous radiation exhibits a waveguide lighting feature, which being assigned to exciton-polariton [19,35,36,46]; At the lasing regime, the emission intensity is strongly enhanced by FP microresonator, which being formed by the rectangular-shaped cross section of the microribbon. And then, the device can emit light signals from bilateral facets of the microribbon, suggesting a FP microlaser [14,19]. Figure 5(c) illustrates exciton-polariton EL spectrum and the lasing spectrum above the threshold of the same single microribbon based emission device. From the figure, one lasing peak of 410.0 nm located at the middle of the corresponding waveguide mode spectrum; while the other lasing peak (450.5 nm) positioned at the low-energy side of the waveguide EL spectrum. Thus, the lasing peaks matched well with the multiple interference peaks [59,60].

General analyzing, the lasing modes are mostly occurred in longer wavelengths shoulder for strong coupling strength of exciton-polariton by comparing with corresponding waveguide EL spectra. By increasing the reverse current, the lasing peaks of the microlaser remain nearly the same without any shift above the threshold current. In the n-ZnO:Ga microribbon/p-GaAs heterojunction device, polaritons are decoherent before they could maintain the final state population via relaxation, leading to photonic lasing rather than polariton condensation. Therefore, the observed stimulated radiation in the as-fabricated n-ZnO:Ga microribbon/p-GaAs heterojunction emission device belongs to FP-mode polaritonic lasing [12,55,59,60]. The $Q$-factor of the single microribbon based microlaser device at the lasing wavelength of 450.5 nm is calculated with the formular $Q=\lambda /\delta \lambda$, where $\lambda$ is the peak wavelength of the resonant mode and $\delta \lambda$ is the line width of the corresponding lasing peak. The $Q$-factor is extracted to be about 400, as depicted in Fig. 5(d). The much lower $Q$-factor of the single microribbon based microlaser could be caused by the nonignorable optical loss: (i) due to the relatively lower refractive index of ZnO:Ga microribbon (2.35), the emitted light can refract into the GaAs template, leading to the serious refractive loss at the ZnO:Ga/GaAs interface; (ii) the emitted light can travel back and forth within the rectangular-shaped cross section, and can bump with the bilateral facets of the microribbon, resulting in the field leakage from the cavity facets [19,20,31].

We have further exploited the polarization features in the far-field light emission of the single microribbon based FP-microlaser device, with the input current at 39.3 mA. In the EL meaurement, the EL signals were collected by rotating the rotatory polarizer. EL intensity $I_{0^\circ }$ was corresponded to the electric-field perpendicularing to the axial direction of ZnO:Ga microribbon; while the EL intensity $I_{90^\circ }$ was corresponded to the electric-field parallelling to the axial direction of microribbon. And the corresponding EL spectra are shown in Fig. 5(e). From the figure, the maximum lasing emission was distributed along both the bilateral sides of the micribbon, which is much stronger than the parallel components. It suggests that the lasing emission intensity perpendicular to the axial direction of the microribbon dominates the output of the electrically pumped the single ZnO:Ga microribbon based microlaser device. Therefore, the polarization characteristic of lasing emission confirms that single microribbon can be utilized to construct FP-mode microlaser devices. The lasing ouput is polarized perpendicular to the axial direction of microribbon, and the corresponding FP-microresonator was perfectly matched with the illuminating polarization of the microribbon [9,19,26,59].

Additionally, in the presented device architecture, the carrier leakage deteriorates the injection efficiency in the case of simple directly contact between single microribbon and p-GaAs template [19,29]. Therefore, a workable approach of the effective hole-injection from p-GaAs template into ZnO:Ga microribbon should be developed to achieve high-performance of the n-ZnO:Ga microribbon/p-GaAs heterojunction FP microlaser. By properly designing novel n-ZnO:Ga microribbon/p-GaAs based heterojunction, such as the incorporating low-dielectric interlayers (for example MgO, HfO$_2$, Al$_2$O$_3$ nanofilms, etc) as blocking interlayer for the carrier transport, the modulation of the holes injection efficiency and reducing the optical loss may be further improved significantly through optimization of the heterojunction interface. And the influence of blocking interlayer on the band alignment of the ZnO/GaAs heterostructure should be further investigated in more details [31,46,50]. Continued investigation is under way and we have observed further optimization on the EL characteristics, especially for a hexagonal ZnO:Ga MW based heterojunction device by combining p-GaAs template working as their hole injectors, which will be accomplished and published separately.

4. Conclusions

In summary, a new generation of CW operation of electrically pumped FP-mode microlaser device made of single ZnO:Ga microribbon/p-GaAs heterojunction has been realized upon reverse-biased condition. Under lower reverse bias, the device can emit blue-violet light, and the multipeak features resolved from the EL spectra suggestes that strong light–matter interaction can occur in a single ZnO:Ga microribbon-based heterojunction LED, yielding the exciton–polariton illuminating and the corresponding coupling strength defined as Rabi splitting energy of about 500 meV. When operated under CW biasing conditions above certain value, electrically pumped FP lasing action was achieved, and the presence of a clear lasing threshold, and sharp reducing of the spectral linewidth, can be served as unambiguous evidences for the achievement of coherent lasing oscillation. As the main emission band located in the blue-violet spectral region, the luminescent layer is mainly distributed in the side of n-ZnO:Ga microribbon in the as-fabricated microlaser device. This work can enable a new paradigm in the design and development of low-dimensional, blue-violet microlasers, wherein the p-GaAs template was utilized an alternative hole supplier to construct on-chip light sources, which no longer suffer from the availability of additional optical microcavities, lattice mismatch, and substrate availability.