ZHANG Han, PEI Lei-lei, SU Shi-chen

(Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, Institute of Opto-electronic Materials and Technology, South China Normal University, Guangzhou 510631, China)

Realization of UVB Lasing in High Quality Cubic ZnMgO Films

ZHANG Han, PEI Lei-lei, SU Shi-chen*

(Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, Institute of Opto-electronic Materials and Technology, South China Normal University, Guangzhou 510631, China)

The optical and structural properties of high-quality epitaxial Zn1-xMgxO films deposited by pulsed-laser deposition (PLD) were studied. Zn1-xMgxO films with ～45% Mg incorporation were measured by EDS (Energy dispersive spectroscopy). XRD (X-ray diffraction) measurement results show that Zn0.55Mg0.45O films have a cubic phase structure without phase separation and are epitaxial grown along thec-axis of Al2O3substrate. In the films, intense UVB optical pumped stimulated emission of this pure cubic-phase ZnMgO can be observed. The lasing threshold is about 22 kW/cm2. Lasing occurs at UVB wavelength of ～310 nm under optical pumping.

ZnMgO; PLD; threshold

1 Introduction

Semiconductor oxide materials based on energy-gap engineering have garnered widespread interest in many aspects, for instance, in catalysts, sensors, electronic devices, UV detectors and solar cells, among others[1-11].Among these semiconductor oxide materials, ZnO as a direct wide band gap semiconductor material, because of its wide band gap energy (3.37 eV at room temperature), large exciton binding energy (60 meV), much higher than the room temperature heat ionization energy 26 meV, and high optical gain (300 cm-1). So that exciton-stimulated ultraviolet radiation at room temperature or higher can be achieved, which makes ZnO has attracted extensive studies in the preparation of UV light-emitting diodes and laser devices.

However, the P-type doping of ZnO is still a significant difficulty[12]. In addition, another key problem in the application of ZnO materials is the regulation of the band. To utilize the optical and electrical properties of ZnO sufficiently, an excellent method is to dope proper transition elements, such as Co, Mn, Fe, Ni,etc.[13-14]. Since the radius of Mg2+(0.057 nm) is very close to the radius of Zn2+(0.060 nm), the lattice mismatch of Mg in the position of substitution of Zn is very small (only 0.1%), therefore, Mg is an appropriate element. By varying the Mg composition, the band gap can be tuned from 3.37 to 7.8 eV for wurtzite and cubic-structured MgxZn1-xO, extending the cutoff wavelength from UV-A (320-400 nm) to UV-B (280-320 nm) and UV-C (200-280 nm) regions[15-17]. Thus, ZnMgO has become a very suitable material for the preparation of ZnO/ZnMgO superlattices, quantum wells and UVB optoelectronic devices.

Due to the high energy and short wavelength of the photon in the UVB band, the UVB laser source based on wide bandgap semiconductor has a broad prospect in high density data storage, laser precision machining, large screen display, biomedicine, food sterilization, water purification and so on, while the resulting economic and environmental benefits will be immeasurable.

Because the lattice structure of MgO and ZnO is cubic rock salt and hexagonal wurtzite structure,when the molar fraction of Mg is in the range of 0.4-0.6, the phase separation phenomenon exists in the ZnMgO material. Despite the existence of phase separation, pulsed laser deposition(PLD)[18-20], molecular beam epitaxy(MBE)[21-23], RF reactive magnetron sputtering[24-25]and metal organic chemical vapor deposition(MOCVD)[26-27], such continuous improvement in epitaxial film deposition techniques has contributed the successful growth of wurtzite ZnMgO with a Mg content up to 0.37(4.28 eV) and cubic ZnMgO for more than 0.62(5.40 eV) Mg content.

Band gap engineering and stimulated emissions of nanostructures with different Mg doping concentrations were demonstrated[28-30], however, the threshold for the ZnMgO nanowire is about 200 kW/cm2. It is noteworthy that few reports have been previously reported on the use of cubic ZnMgO thin films (low thresholds) in UVB lasers. It is widely accepted that high-quality film is fundamentally important for high performance optoelectronic devices. In this letter, we report the demonstration of ultraviolet (UV) laser action in Zn0.55Mg0.45O high quality films grown on sapphire substrates by PLD.

2 Experiments

Using pulsed laser deposition (PLD-450) to grow thec-plane sapphire ZnMgO films with thickness of 300 nm. In order to conduct systematic studies and obtain conclusive results, films are grown with substrate temperatures (Tsub=600 ℃) and oxygen pressures (P(O2)=0.5 Pa). The substrate was continuously rinsed in acetone, ethanol and distilled water and dried by nitrogen gas and then loaded into a growth chamber. In order to ensure uniform ablation without damage, the target is continuously rotated throughout the deposition process.The target used in the present study was made of Mg-doped high purity ZnO powder(99.999%, Kurt J. Lesker Co.) to achieve Mg incorporation of 45%(mole fraction) in ZnO. The background pressure for the growth is 10-4Pa.The 248 nm laser pulse from the Coherent COMPexPro 102 excimer laser with a pulse energy of 300 mJ and a repetition rate of 2 Hz was used for the PLD growth. After the end of the growth and natural cooling to room temperature, remove the sample.

3 Results and Discussion

Fig.1 shows XRD patterns of Zn0.55Mg0.45O film. As shown inθ-2θangular scan in the XRD patterns, only two strong peaks at 36.67° and 41.68° can be observed. The peak at 41.68° is from thec-plane sapphire substrate(006), the appearance of (111) peak at 36.67° indicates that Zn0.55Mg0.45O film is highlyc-axis preferred orientation with pure cubic phase. Obviously, the XRD of MgO (111) peak and ZnO (002) peak are at 2θ=37°and 34.4°, respectively. High-quality cubic ZnMgO(111) without phase separation was prepared in this figure and the diffraction angle is 36.67°. It is clear that the optical band gap increases with increasing Mg doping concentration. In addition, the lattice constantsaandcaxis of ZnMgO films decrease slightly with the increase of Mg content, which is indicated by the lattice geometric equation:

(1)

As a result, XRD results showed that (111)c-axis-preferred orientation peak has a slight shift toward the direction of angle increases with increasing Mg concentration. The above results are consistent with the reasoning by the Bragg’s law:

2dsinθ=nλ.

(2)

Thefullwidthathalfmaximum(FWHM)ofthe(111)diffractionpeakofZnMgOthinfilmasshownininsertofFig.1.TheFWHMisabout0.16°.TheaboveresultsalsoprovethattheZnMgOfilmsarehigh-qualitycubicstructure.

Fig.1XRDθ-2θangular scan of the Zn0.55Mg0.45O films deposited onc-plane sapphire substrates, the insert is theω-rocking curve of the (111) of Zn0.55Mg0.45O film.

A typical transmission spectrum of the ZnMgO films is shown in Fig.2. The films show a high transmission of over 80% in the ultraviolet spectrum region, while they have a very sharp absorption edge at around 310 nm (4 eV). Therefore, ZnMgO thin film material for the preparation of UVB optoelectronic devices has important significance. The typical feature of phase separation is the appearance of multiphase absorption edges. For this sample, no multiphase absorption edge was observed, which confirms that in our case does not occur in phase separation which is often observed in ZnMgO alloys with high Mg content. Thus, the absence of phase separation largely improves the performance of the devices can be obtained on our films. Similarly, this reveals that this cubic ZnMgO film has a very high crystalline quality.

Fig.2 Transmission spectrum of the cubic ZnMgO films in the UV spectrum

The bright-field transmission electron microscope(TEM)micrograph in cross section from a Zn0.55Mg0.45O sample is shown in Fig.3, with the corresponding diffraction pattern from the ZnMgO film. As shown in Fig.3(a), the Zn0.55Mg0.45O/Al2O3structure clearly appears smooth cross-section, which is a typical image of a single crystalline material. In addition, the Zn0.55Mg0.45O films were found to cohere well to the sapphire substrate and displayed a uniform thickness on a microscopic scale. The diffraction crystal lattice pattern in Fig.3(b) shows the films are crack free, homogeneous, well covered with granular deposits and without any evidence of lattice defects. As a consequence, the micrograph and diffraction patterns clearly show the structure of single cubic phase ZnMgO (111), there is no evidence of any phase separation, which is consistent with previous results.

Fig.3 Cross-sectional TEM and transmission lattice micrographs of a Zn0.55Mg0.45O film

Lasing characteristics of ZnMgO film were investigated by aQ-switch Nd∶YAG laser (266 nm) under pulsed operation (6 ns, 30 Hz). Fig.4 shows the evolution of the emission spectrum as the average pump power (hereinafter referred to as pump power) increases. Comparative analysis of the three color lines in the figure, the central wavelength of the stimulated emission does not change with the pump power, which is always in the UVB about 310 nm. In the pump power from low to high changes in the process, showing a different stimulated emission spectrum, at low excitation intensity, the spectrum consisted of a single wide spontaneous emission peak. When the pump power is increased, the emission peak became narrower due to the preferential amplification at the frequency close to the maximum value of the gain spectrum. When the pump power increases further, more sharp peaks appear, and the line-width of peak drops sharply. When the pump power density is increased above the threshold, the emission intensity of the narrower feature becomes dominant. The narrow and strong emission exhibits a superlinear increase, accompanied by a slight redshift, Which indicates the appearance of stimulated emission. The stimulated emission may be attributed to the stimulated recombination of exciton-exciton scattering[26-27].

Fig.4 Stimulated emission spectra of the Zn0.55Mg0.45O films at different average pump powers ranging from 0.18 mW (a), 1.5 mW (b), 1.8 mW (c), respectively.

Discrete red dots in Fig.5 show FWHM as the non-linearity reduction function of the excitation density, the FWHM narrowed much more rapidly with the excitation density at the threshold. Another set of blue dots in the figure indicates the excitation density as a non-linear increasing function of the emission intensity. On the blue dot-fitted line, it is observed that the emission intensity increased much more rapidly with the excitation density above the excitation density of multiple sharp peaks emerged in the emission spectrum, which is the threshold behavior. This increased process is superlinear, which the threshold value is 22 kW/cm2. This proves that spontaneous radiation is transformed into stimulated radiation.

Fig.5 FWHM and emission intensity of the Zn0.55Mg0.45O films as a non-linear function of the excitation density

These results indicate that laser action has occurred in the Zn0.55Mg0.45O films. Compared with other literatures, this result of threshold power was lower than several orders of magnitude. This is a momentous progress to improve the work performance of laser devices.

The optical cavity formed by repeated scattering has a different loss. When the pump power increases, the gain first achieves low loss cavity loss. After that, laser oscillation occurs in these cavities, and the laser frequency is determined by the cavity resonance. Laser emission from these resonators results in a small amount of discrete emission spectra narrow peaks. When the pump power is further increased, the gain increases and exceeds the loss in the lossier cavities. Due to the laser oscillations in these cavities, the emission spectrum adds more discrete peaks. These theories are consistent with the experimental data in this paper.

4 Conclusion

In conclusion, pure cubic MgxZn1-xO thin films with Mg content of 45% have been prepared by PLD. High crystalline quality non-phase separation of the Zn0.55Mg0.45O films was observed in the results of XRD and TEM. Under optical pumping, excitation occurs in the UVB wavelength of ～310 nm. As the excitation density increased, the peak intensity increased superlinear, and the linewidth of peak decreased dramatically. The high-quality Zn0.55Mg0.45O alloys films with a thresholds of 22 kW/cm2that we prepared have potential applications in various optoelectronic devices.

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張晗(1991-)，女，山東濟寧人，碩士研究生，2015年于曲阜師范大學獲得學士學位，主要從事半導體光電子材料與器件的研究。

E-mail: 15625138017@163.com

宿世臣(1979-)， 男， 黑龍江佳木斯人，博士，副研究員，2009年于中科院長春光機所獲得博士學位，主要從事寬禁帶半導體的研究。

E-mail: shichensu@126.com

2017-03-07；

2017-04-17

國家自然科學基金(61574063)； 廣東省科技計劃(2016A040403106)； 廣州市科技計劃(2016201604030047)資助項目 Supported by National Natural Science Foundation of China(61574063); Science and Technology Program of Guangdong Province(2016A040403106); Science and Technology Project of Guangzhou City(2016201604030047)

高質量立方相ZnMgO的制備與紫外受激發(fā)射特性研究

張 晗， 裴磊磊， 宿世臣*

(廣東省光電功能材料與器件重點實驗室， 華南師范大學 光電材料與技術研究所， 廣東 廣州 510631)

利用脈沖激光沉積(PLD)設備在藍寶石襯底上制備了高質量Zn1-xMgxO單晶薄膜，并對其結構和光學特性進行了深入細致的研究。通過能量衍射譜(EDS)確認Zn1-xMgxO薄膜的Mg組分為45%。在Zn0.55Mg0.45O薄膜的X射線衍射譜(XRD)中觀測到了明顯的位于36.67°的衍射峰，對應的是(111)晶向的立方相ZnMgO。從透射光譜中可以看出，Zn0.55Mg0.45O具有陡峭的吸收邊，沒有發(fā)生相分離，在透射電鏡圖譜中也得到了證實。該ZnMgO薄膜還表現(xiàn)出了優(yōu)異的光學特性，在Zn0.55Mg0.45O材料體系中實現(xiàn)了峰位位于310 nm的紫外光泵浦受激發(fā)射，其激光發(fā)射的閾值僅為22 kW/cm2。

氧化鋅鎂； 脈沖激光沉積； 閾值

1000-7032(2017)07-0905-06

O484.4 Document code： A

10.3788/fgxb20173807.0905

*Corresponding Author, E-mail: shichensu@126.com