1. Introduction

In the past decade, there has been an outburst of the research on the AlGaN-based ultraviolet light emitting diodes (UV-LEDs) with great potential in many application areas, such as sterilization, air and water purification, and resin/ink hardening [1–3]. However, the conventional polar AlGaN-based UV-LEDs fabricated along [0001] c-direction exhibit strong spontaneous and piezoelectric polarization fields with the order of magnitude up to several MV/cm. These polarization fields will lead to the serious reduction in the overlapping of the electron and hole wave functions and thus a serious droop in the internal quantum efficiency (IQE) due to the so-called quantum confinement Stark effect (QCSE) [4,5]. In theory, the polarization effects can be significantly suppressed by growing the AlGaN-based UV-LEDs with non-polar and/or semi-polar planes like $(11\overline{2}2)$ plane. As a result, superior performance is expected for the semi-polar $(11\overline{2}2)$ plane AlGaN-based UV-LEDs to their counterparts grown along the polar c-direction [6].

In order to achieve the semi-polar $\left(11\overline{2}2\right)$ plane AlGaN-based UV-LEDs with high emission efficiency, it is vital to grow the semi-polar $(11\overline{2}2)$ plane AlGaN-based multiple quantum wells (MQWs) with high crystalline quality and good interface smoothness. Up to date, there have been few reports on the epitaxial growth and optical characteristics of semi-polar $(11\overline{2}2)$ plane AlGaN-based MQWs. Balakrishnan et.al [7] adopted superlattice structure to improve the crystalline quality and reduce the stacking faults in the $(11\overline{2}2)$ plane active region. Wang et.al [8] and Ichikawa et.al [9] studied the optical anisotropy and the carrier radiative recombination lifetimes of the $(11\overline{2}2)$ plane AlGaN-based MQWs, respectively. However, the reported crystalline quality for the $(11\overline{2}2)$ plane AlGaN-based MQWs was still poor due to the difficulty in the epitaxial growth. Meanwhile, although the introduction of In-surfactant during the epitaxial growth process has been proven to be an effective technique to improve the optical and structural properties of the polar c-plane AlN/GaN MQWs [10,11], the research about the effects of In-surfactant on the growth process and the characteristics of the $(11\overline{2}2)$ plane AlGaN-based MQWs is still lack.

In this paper, the epitaxial growth and characterization of the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQWs with In as surfactant are reported. Particular attention was paid on the effects of In-surfactant on the epitaxial growth process as well as the structural and optical properties of the $(11\overline{2}2)$ plane AlGaN-based MQWs. It was revealed that the surface morphology and the MQWs-related photoluminescence (PL) intensity for the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQWs could be improved significantly with the introduction of In-surfactant.

2. Experimental

The semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples used in this study were grown on the $\left(10\overline{1}0\right)$ m-plane sapphire substrates in a low pressure (40 Torr) metal organic chemical vapor deposition (MOCVD) system. Trimethyl-aluminum (TMAl), trimethyl-gallium (TMGa), trimethyl-indium (TMIn), and ammonia (NH3) were used as the precursors for Al, Ga, In, and N, respectively. During the epitaxial growth process of the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW sample A without the introduction of In-surfactant, the sapphire substrate was heated up to 1,060 °C in H₂ ambient and was nitridized with NH₃ for 900 sec. Then, a ~20-nm thick low-temperature AlN nucleation layer was deposited on the substrate at 650 °C. Subsequently, a ~200-nm thick high-temperature AlN buffer layer was deposited on the nucleation layer at 1,110 °C. Afterwards, a Al_0.55Ga_0.45N interlayer was grown at 1,080 °C. Then, 5 periods of Al_0.39Ga_0.61N/Al_0.55Ga_0.45N MQWs were grown on the Al_0.55Ga_0.45N interlayer and finally covered with an Al_0.55Ga_0.45N cap layer. The epitaxial growth process and the layer structure for sample B with In as surfactant are basically the same as those for sample A with the exception that TMIn flow was kept injecting into the MOCVD reactor when the MQWs and the cap layer were been growing. The schematic layer structure for the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples A and B is shown in Fig. 1.

 

Fig. 1 The schematic layer structure for the two semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples A and B.

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The surface morphology of the two semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples was characterized with a Bruker BioScope Resolve atomic force microscopy (AFM) in tapping mode. And the structural properties of the two samples were evaluated by means of high resolution X-ray diffraction (HR-XRD) and scanning electron microscope (SEM), respectively. Furthermore, in order to characterize the optical properties and to determine the IQE for the two semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples, the room temperature photoluminescence (RT-PL) spectra were measured by using a 266 nm pulse laser as the excitation source, and the temperature-dependent PL spectra (TD-PL) were measured by using a 261 nm pulse laser as the excitation source at a repetition rate of 2.5 KHz, a pulse width of 4 ns, and an average excitation power of 10 mW.

3. Results and discussions

The effect of In-surfactant on the surface morphology for the two semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples A and B can be investigated in terms of the two-dimensional (2D) view AFM images, as shown in Fig. 2. These pictures actually demonstrate the surface morphology of the Al_0.55Ga_0.45N cap layers deposited on the MQWs. The evident triangle-like undulant structures observed along the $[1\overline{1}00]$ direction on the surface of sample A could be attributed to the anisotropy in the diffusion length for the group III-atoms on the $(11\overline{2}2)$ plane surface [12]. However, the formation of the undulation structures was significantly suppressed for sample B as shown in Fig. 2(b), implying a reduced anisotropy in the diffusion length on the surface. Furthermore, the root mean square (RMS) values for samples A and B were measured to be 1.77 and 1.02 nm, respectively. In other words, the RMS value for sample B were 42.3% less than that for sample A, implying a significant improvement in the surface morphology by employing In as surfactant during the epitaxial growth process. This phenomenon could be ascribed to the enhancement in surface adatom diffusion length induced by the In-surfactant which could saturate the free bonds of N atoms, allowing Ga and/or Al adatoms to diffuse freely and thus to form a relatively flat surface [13].

 

Fig. 2 The 2D-view AFM images for the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples A (a) and B (b) measured with a detection area of 5 × 5 µm².

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The HR-XRD ω-2θ scanning results for the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples A and B are shown in Fig. 3(a). The −5th to + 2nd satellite peaks could be clearly resolved for sample B while only the satellite peaks from −3rd to + 2nd order could be observed for sample A. This result indicates that the periodicity and abruptness of the hetero-interfaces in sample B are superior to those for sample A. Meanwhile, it is evident that all of the satellite peak locations for sample A were the same as those for sample B, implying that the layer thicknesses of the MQWs for samples A and B were identical. Meanwhile, from Fig. 3(a), the period thickness is determined to be 16 nm with a well width of 4.6 nm and a barrier layer thickness of 11.4 nm. The calculation was based on the average distance between the MQWs-related satellite peaks, which was measured to be around 1210 arcsec [10, 14].

 

Fig. 3 The HR-XRD 2θ–ω scanning curves for the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples A and B (a) and the cross-sectional SEM micrographs for sample B (b).

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The cross-sectional SEM micrograph for the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW sample B is shown in Fig. 3(b). Apparently, the hetero-interfaces between the barriers and the wells in the MQWs structure were evident and abrupt. From Fig. 3(b), the well width and the barrier layer thickness were estimated to be 5 and 11 nm, respectively, which are in good agreement with the values determined by HR-XRD measurement as described above. In addition, the cap layer thickness was measured to be approximately 30 nm.

Figure 4(a) shows the RT-PL spectra for the two semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples A and B measured under the same conditions. It is clear that the dominant MQWs-related exciton emission (EE) peaks for both samples are located at 279 nm, indicating that the influence of the introduction of In-surfactant on the EE energy could be ignored. Moreover, the full width at half maximum (FWHM) values of the EE peaks for samples A and B were measured to be 22 and 14 nm, respectively. This remarkable reduction in the FWHM of the EE peak observed for sample B could be attributed to the improvement in the homogeneity and the abruptness in the hetero-interface of the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQWs grown with In surfactant [10,15]. Meanwhile, a defect-related emission peak located at 476 nm was also detected for both samples [16]. Nevertheless, the PL intensity of the defect-related emission peak for sample B is much smaller than that for sample A, indicating that the defect-related emission could be suppressed effectively with the introduction of In-surfactant.

 

Fig. 4 The normalized RT-PL spectra (a), the TD-PL spectra measured within the temperature range of 10-300 K (b, c), and the integrated PL intensity as a function of temperature (d) for the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples A and B.

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The TD-PL spectra measured in the temperature range of 10–300 K for the two semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples A and B are illustrated in Figs. 4(b) and 4(c), respectively. Obviously, there is a shoulder-like emission peak on the high energy side of the major MQWs-related EE peak located at around 265 nm which was observed for both samples at the temperature lower than 150 K. This peak was identified to be originated from the emission of the Al_0.55Ga_0.45N cap layer [17,18]. Besides, an “S-shaped” variation for the dominant MQWs-related EE peak position can be seen in the TD-PL spectra when increasing the temperature from 10 to 300 K for both samples. This feature is a typical indication of the existence of the localized exciton states. In fact, the initial red-shift was attributed to the carrier redistribution to the deeper localized states. As the temperature was increased, however, the carriers were delocalized into the quasi-continuous states of the AlGaN wells, resulting in the blue-shift in the peak position. As the temperature was further increased, the red-shift in the peak position was due to the normal temperature-induced bandgap shrinkage [19,20]. There was another “shoulder” appearing on the low energy side of the major MQWs-related EE peak observed for both samples, which was assigned to be the basal stacking faults (BSFs)-related emission [21]. However, the PL intensity of the BSFs-related emission for sample B is weaker than that for sample A throughout the whole varied temperature range, implying a reduction in the density of BSFs.

The integrated MQWs-related EE peak intensity as a function of the temperature for the two semi-polar $(11\overline{2}2)$ plane AlGaN-based MQW samples A and B are depicted in Fig. 4(d). It was found that the integrated intensity of the EE peak for sample B was much stronger than that for sample A. In fact, the integrated EE PL intensity for sample B was found to be more than 2.5 times higher than that for sample A at 300 K, indicating a significant improvement in crystalline quality. Moreover, the thermal quenching effect of the MQWs-related EE for sample B was much weaker than that for sample A within the entire temperature range. Hence, it is reasonable to deduce that In-surfactant was effective to shorten the fast radiative recombination lifetime for the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQWs, leading to the intense EE and the greatly enhanced IQE [22].

4. Conclusion

In summary, the influence of the In-surfactant on the epitaxial growth and the characteristics of the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQWs have been investigated extensively. The AFM results revealed that a remarkable improvement in surface morphology could be realized by employing In as surfactant during the epitaxial growth process for the semi-polar $(11\overline{2}2)$ plane AlGaN-based MQWs. The PL measurement results indicate that the homogeneity and the abruptness as well as the crystalline quality for the MQWs could be improved remarkably with the introduction of In-surfactant. Furthermore, the integrated EE peak intensity and the radiative recombination probabilities in MQWs could be increased as well with the help of In-surfactant, resulting in an enhanced internal quantum efficiency eventually.