1. Introduction

The finding of random laser actions from polycrystalline ZnO films and powders [1,2] has spurred great efforts expending in the research on random lasing from ZnO-based semiconductors [3–7]. As a ZnO-based alloy semiconductor, Mg_xZn_1-xO has received intensive research interests. Because of little mismatch of ionic radius between Mg²⁺ (0.57 Å) and Zn²⁺ (0.60 Å), the bandgap energies of Mg_xZn_1-xO alloys can be successively tuned from 3.3 to 3.99eV by doping appropriate contents of Mg into ZnO without significant change in lattice constants [8]. With respect to ZnO, Mg_xZn_1-xO alloys can find applications in the optoelectronic devices operating in much deeper ultraviolet (UV) region. In recent years, the optically pumped UV random lasing from Mg_xZn_1-xO films has been reported [9–11]. Such achievements encourage us to develop the electrically pumped random lasers based on the Mg_xZn_1-xO films. In general, current injection via a p-n junction is preferred for the electrical pumping of lasers. Unfortunately, mature p-type doping strategies for the Mg_xZn_1-xO alloys especially those of high Mg contents have not been established to date. In this context, we have to find a way, in which the difficulty in p-type doping of Mg_xZn_1-xO can be circumvented, to realize the electrically pumped random lasing from the Mg_xZn_1-xO films. Recently, we have reported the electrically pumped random lasing from ZnO films or nanorods by means of metal-insulator-semiconductor (MIS) structures [12–14] fabricated on Si substrates. Moreover, we have also realized electroluminescence (EL) from the Mg_xZn_1-xO film-based MIS structure [15].

In this Express, we have further taken advantage of the MIS structures to achieve the electrically pumped random lasing from the Mg_xZn_1-xO films of different Mg contents. Overall, the central wavelength of random lasing spectrum is blue-shifted with the increase of Mg content in Mg_xZn_1-xO. It is believed that the results presented herein shed light on the development of Mg_xZn_1-xO-based lasers.

2. Experimental details

The procedures involved in the fabrication of the MIS devices based on the Mg_xZn_1-xO films are described as follows: (1) cleaning heavily arsenic-doped (n⁺), <100>-oriented Si wafers with a resistivity of ~5 × 10⁻³ Ω.cm using the standard RCA solution, (2) preparing different precursor sols in which Zn (CH₃COO)₂, Mg (CH₃COO)₂. CH₃OCH₂CH₂OH were used as the solvents and NH₂CH₂CH₂OH was used as a sol stabilizer. The molar ratio of Mg/(Zn + Mg), i.e. the nominal x value in Mg_xZn_1-xO, was adjusted to be 0, 0.05, 0.15, 0.20, 0.25 and 0.35, respectively, (3) after each aforementioned precursor sol was stirred for 12 h, a small amount of sol was dripped and then spin-coated on a n⁺-Si substrate. Through a soft-bake at 300 °C for 10 min, a ~60 nm thick Mg_xZn_1-xO precursor gel film was formed. Two cycles of the spin-coating and soft-bake as mentioned above were performed. Subsequently, the resulting gel film was annealed at 800 °C for 1 h in O₂ ambient, (4) depositing a ~60 nm thick SiO₂ film onto the Mg_xZn_1-xO film by a sol-gel process, which can be referred to our previous report [15], (5) sputtering a ~20 nm thick Au film onto the SiO₂ film and a ~100 nm thick Au film on the back-side of Si substrate. Both Au films were used as the electrodes of the Mg_xZn_1-xO film-based MIS devices.

The crystal structures of the Mg_xZn_1-xO films were characterized by x-ray diffraction (XRD) performed on a Japan Rigaku D/max-ga x-ray diffractometer with graphite momochromatized Cu K_α radiation (λ = 1.54178 Å). The morphologies of the Mg_xZn_1-xO films were observed with a FEI SIRION field emission scanning electron microscope (FESEM). Moreover, the composition of each film was determined by the energy dispersive x-ray spectroscope (EDX) attached to the FESEM. The photoluminescence (PL) spectra were measured at room temperature (RT) using an F-4500 fluorescence spectrophotometer. The EL spectra of the devices under different D. C. voltages were recorded at RT using an Acton spectraPro 2500i spectrometer with a lowest spectrum resolution of 0.5 Å and an accuracy of ± 2 Å. For the acquisition of spectra, the scanning step size was 1 Å. Moreover, the output power was measured using a Newport 1931-C power meter with an 818-UV/DB detector (~1cm in diameter). For the measurements, the devices were brought face to face with the detector. The devices were ~2 cm apart from the detector. For such a measurement configuration, it is roughly calculated that only ~2% of the output power of a device is detected by the above-mentioned power meter.

3. Results and discussion

Figure 1a shows the XRD patterns of the Mg_xZn_1-xO films with different nominal compositions. In each XRD pattern, there are (100), (002), (101) and (102) peaks ascribed to hexagonal phase, indicating that the sol-gel derived films are not crystallized in a preferential orientation. For the nominal Mg_0.35Zn_0.65O film, in its XRD spectrum there is an obvious peak belonging to cubic MgO phase besides the peaks of hexagonal Mg_xZn_1-xO phase. Actually, for the nominal Mg_0.25Zn_0.75O and Mg_0.20Zn_0.80O films, although without discernable peaks of cubic MgO phase in their XRD patterns, there is a small quantity of MgO crystallites incorporating into the two films, which will be indicated later.

 

Fig. 1 (a). XRD patterns of the Mg_xZn_1-xO films with different nominal compositions. (b). Plan-view FESEM images of the Mg_xZn_1-xO films with different nominal compositions: (i) x = 0, (ii) x = 0.05, (iii) x = 0.15, (iv) x = 0.2, (v) x = 0.25, (vi) x = 0.35.

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Figure 1b shows the typical plan-view FESEM images of the Mg_xZn_1-xO films of different nominal compositions. It can be seen that in the films with x = 0.05 and 0.15 there are no distinct secondary phase crystallites. While, in the films with x = 0.2, 0.25 and 0.35, there are secondary phase crystallites which are manifested themselves as white objects in the FESEM images. Moreover, the number of the secondary phase crystallites increases with the Mg content. The EDX analysis (not shown herein) indicates that the secondary phase crystallites contain a considerably high content of Mg. In association with the XRD results as given above, we believe that the secondary phase crystallites are of MgO phase. The sol-gel derived Mg_xZn_1-xO films, unlike those prepared by the sputtering or pulse laser deposition which is far from equilibrium states, cannot sustain to be of single hexagonal phase at a high Mg content.

Figure 2a shows the PL spectrum of Mg_0.15Zn_0.85O film. The PL spectrum exhibits a UV band related to the near-band-edge (NBE) emission. Moreover, a broad emission band in the visible region is revealed, which is presumably due to the intrinsic defects such as interstitial Zn atoms and O vacancies [16,17]. Other Mg_xZn_1-xO films prepared in this work also exhibit emissions in both the UV and visible regions. Figure 2b shows the decrease of the UV peak wavelength from 377 to 352 nm with the increase of Mg content. Obviously, the blueshift of the UV peak along with the increase of Mg content is ascribed to the widening of Mg_xZn_1-xO bandgap as a result of the enhanced Mg incorporation into ZnO lattice. Note that the UV peak wavelength is nearly unchanged as x > 0.25. As revealed above, in the case of the sol-gel derived Mg_xZn_1-xO films, as x exceeds a critical value, the further incorporation of Mg atoms into ZnO lattice becomes essentially limited because the segregation of MgO is more energetically favorable than the substitution of Mg into ZnO lattice.

 

Fig. 2 (a). PL spectrum of Mg_0.15Zn_0.85O film. (b). Decrease of UV peak wavelength with the increase of Mg content in the Mg_xZn_1-xO films.

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Figure 3a shows the evolution of EL spectra for the Mg_0.15Zn_0.85O film-based MIS device with the increasing forward bias voltages. Herein, the forward bias means that the Au gate electrode of the MIS device is connected to the positive voltage. It can be seen that the EL spectra feature a number of discrete sharp peaks with an extremely narrow linewidth less than 5 Å. Moreover, in each EL spectrum the wavelength spacing between the neighboring two sharp peaks is not a constant. Figure 3b shows the dependence of the detected output power on the injection current for the Mg_0.15Zn_0.85O film-based MIS device. It is obvious that above a threshold current the output power increases much more rapidly with the injection current, exhibiting nearly a linear dependence due to the gain saturation that forms an intrinsic aspect of an amplifying system above the threshold current [18]. The above-mentioned facts indicate that the MIS device exhibits random lasing under sufficiently high forward bias voltages. It should be mentioned that the MIS devices based on the Mg_xZn_1-xO films of other Mg contents also exhibit random laser actions with the above-mentioned features.

 

Fig. 3 (a). EL spectra of the Mg_0.15Zn_0.85O film-based MIS device under different forward bias voltages/currents. (b). Detected output power as a function of injection current for the Mg_0.15Zn_0.85O film-based MIS device.

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Figure 4 shows the EL spectra of the MIS devices based on the Mg_xZn_1-xO films of different nominal Mg contents, which operate at different bias voltages/currents. All the EL spectra feature a number of narrow spikes in the UV region, which are ascribed to the random lasing from the Mg_xZn_1-xO films. Overall, the central wavelength of the random lasing spectrum of the MIS device is blue-shifted with the increase of Mg content. This is obviously due to the increased bandgap energy of Mg_xZn_1-xO. It should be mentioned that the MIS devices have different threshold voltages/currents for random lasing due to the two main reasons as following. Firstly, the thickness of SiO₂ film is not essentially the same in each device. As shown in Fig. 2, the morphologies of Mg_xZn_1-xO films vary significantly with the composition. This will result in that the sol-gel derived SiO₂ films on the Mg_xZn_1-xO films possess different thicknesses, although they are deposited under nearly the same conditions. It should be pointed out that most of the forward bias voltage applied on the MIS device drops across the SiO₂ film. If roughly assuming that the critical electric field in the SiO₂ film is the same for the onset of random lasing from the different Mg_xZn_1-xO film-based MIS devices, it is understandable the threshold voltage for random lasing varies with the thickness of SiO₂ film. Secondly, the injection of electrons from the n⁺-Si substrate to the Mg_xZn_1-xO film depends on the conduction band offset between Si and Mg_xZn_1-xO (neglecting the possible existence of ultrathin SiO₂ film between Si and Mg_xZn_1-xO, herein), which changes with the content of Mg in Mg_xZn_1-xO. Therefore, in Fig. 4 the random lasing spectra for the different Mg_xZn_1-xO film-based MIS devices cannot be illustrated at the same voltage/current. Anyway, of significance herein is the demonstration of wavelength-tunable random lasing in the UV region. Moreover, the Mg_xZn_1-xO films have been definitely proved to be promising in the UV optoelectronic devices including the lasers. In the following, we will address the mechanism for the electrically pumped random lasing from the Mg_xZn_1-xO film-based MIS devices.

 

Fig. 4 EL spectra of different Mg_xZn_1-xO film-based MIS devices under certain forward bias voltages/currents.

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It is well known that optical gain and multiple optical scattering are crucial for the random lasing from semiconductors [18]. Herein, the Mg_xZn_1-xO films are polycrystalline in nature, featuring random distribution of crystal grains. Naturally, the light propagating within the Mg_xZn_1-xO films will be subjected to multiple scattering by the grains in disorder. Moreover, the existence of secondary phase crystallites in the high-Mg-content films actually leads to the spatially variation of refractive index within the films, which facilitates the optical scattering and therefore random lasing. For a semiconductor, the optical gain owing to stimulated emission is necessitated by the condition that the energy gap between the quasi-Fermi-level of electrons (E_fn) and that of holes (E_fp) is larger than bandgap energy (E_g), i.e. E_fn- E_fp > E_g.

In the case of Mg_xZn_1-xO film-based MIS device applied with a sufficiently high forward bias, the energy band of Mg_xZn_1-xO substantially bends downwards in the region adjacent to the Mg_xZn_1-xO/SiO₂ interface, as illustrated in Fig. 5a . In this region, the concentration of electrons in the conduction band is very high so that the E_fn is well above the conduction band edge (E_c). On the other hand, under forward bias a number of electrons in the valence band of Mg_xZn_1-xO are electrically driven into the defect states within the sol-gel derived SiO₂ film, which is much more defective than the thermally grown SiO₂ film. Therefore, an equivalent number of holes are generated in the valence band of Mg_xZn_1-xO. With a sufficiently high forward bias voltage, the concentration of holes in Mg_xZn_1-xO becomes considerably high. In this case, as illustrated in Fig. 5b, the E_fp can be close to or even lower than the valence band edge (E_v). Conclusively, we believe that in the region adjacent to the Mg_xZn_1-xO/SiO₂ interface, the condition for the substantial stimulated emission and therefore the optical gain, namely, E_fn- E_fp > E_g, can be satisfied when the Mg_xZn_1-xO film-based MIS device is applied with sufficiently high forward bias voltages. It should be stated that the optical gain changes with the photon energy (wavelength) in a specific range, as schematically shown in Fig. 5c. Therefore, as illustrated in Fig. 5, the random lasing occurs only in a range of wavelengths at which the photons can achieve optical gain. As for the laser spikes appearing at specific wavelengths, they are corresponding to a number of spontaneous emission events that propagate with random walks due to multiple inter-grain scattering in the plane of Mg_xZn_1-xO film and pick up optical gains through the stimulated emission [19], which is larger than the optical losses that are due to different reasons. As mentioned above, the optical gain varies with the photon wavelength. Moreover, the scattered light paths along which the photons of different wavelengths experience are quite different. Accordingly, the random lasing intensities at different wavelengths are not uniform.

 

Fig. 5 (a). Schematic energy band diagram of a sufficiently forward-biased MIS structure of Au/SiO₂/ Mg_xZn_1-xO. (b). The density of states and energy distribution of electrons and holes in the conduction and valence bands respectively in the band-downward region adjacent to SiO₂/Mg_xZn_1-xO interface under forward bias such that E_Fn − E_Fp > E_g. (c). Schematic diagram for the optical gain as a function of photon energy.

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4. Conclusions

In summary, we have demonstrated the electrically pumped wavelength-tunable random lasing from the Mg_xZn_1-xO films of different bandgap energies, which act as the optically active semiconductor components in the MIS structures. Overall, the central wavelength of random lasing spectrum is blueshifted with the increase of Mg content in the Mg_xZn_1-xO films. It is reasonably believed that the Mg_xZn_1-xO alloys are promising materials for the UV lasers.