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Abstract
The optical and structural properties of non-polar a-plane Al0.4Ga0.6N epi-layers were improved significantly with the assistance of a bicyclopentadienyl-magnesium (Cp2Mg) and ammonia (NH3) (BA) treatment. The defects-related emission was remarkably suppressed with the introduced BA treatment. Moreover, the crystalline quality could be improved significantly using a BA treatment at a relatively low-temperature (LT), whereas the anisotropy in crystalline quality could be suppressed remarkably using a BA treatment at a relatively high-temperature (HT). In fact, the full width at half maximum values in the X-ray rocking curves measured along c- and m-directions were found to be decreased by approximately 15.5% and 56.0%, respectively, with the introduction of both LT-BA and HT-BA treatments. Meanwhile, the root-mean-square value measured with an atomic force microscope was decreased from 7.1 to 3.3 nm due to the LT-BA and HT-BA treatments.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The nonpolar (110)-oriented a-plane AlGaN-based optoelectronic devices are promising deep ultraviolet light sources due to the elimination of quantum confined Stark effect [1–4]. However, the surface morphology and crystalline quality of the nonpolar a-plane AlGaN film are much poorer than that for the (0001)-oriented polar c-plane AlGaN film. In fact, dense superficial pyramidal defects and intense anisotropy in crystalline quality generated in the epitaxial growth process are the typical structural features for the a-plane AlGaN epi-layers [5, 6]. Therefore, it is still a big challenge to achieve nonpolar a-plane AlGaN epi-layers with high crystalline quality and smooth surface morphology [7, 8]. Although the usage of intermediate layers was reported to be effective to some extent for improving the crystalline quality of the nonpolar a-plane GaN epi-layers [9], and the Mg surface treatment of p-type GaN was reported to be highly effective in enabling Ohmic contact to p-type c-plane GaN grown with ammonia molecular beam epitaxy technology [10], the effects of bicyclopentadienyl-magnesium (Cp2Mg) and ammonia (NH3) (BA) treatment in the epitaxial growth process on the structural and optical properties of the nonpolar a-plane AlGaN epi-layers are not yet investigated.
In this work, the structural and optical properties of nonpolar a-plane AlGaN epi-layers grown with the BA treatments were studied in detail with high-resolution X-ray diffraction (HR-XRD), atomic force microscope (AFM), and photoluminescence (PL) spectroscopy. It was revealed that the crystalline quality and surface morphology for the a-plane AlGaN epi-layer could be significantly improved with the assistance of appropriate BA treatment.
2. Experimental
The nonpolar a-plane AlGaN epi-layers were grown with a low pressure (40 Torr) two-way pulsed-flow metal organic chemical vapor deposition (MOCVD) technology [5, 11]. As schematically shown in Figs. 1(a)-1(d), four 400 nm-thick a-plane AlGaN epi-layer samples named as samples A1-A4, respectively, were deposited at exactly the same growth conditions except for the BA treatments. In specific, sample A1 was grown without any BA treatment. Sample A2 was grown with a BA treatment at a low-temperature (LT, 600 °C) before the deposition of LT-grown AlN nucleation layer (LT-AlN NL). Sample A3 was grown with a BA treatment at a high-temperature (HT, 1000 °C) performed in between the growth processes for the top AlGaN epi-layer and the HT-grown AlN buffer layer. Sample A4 was grown with both the LT-BA and HT-BA treatments conducted at the identical conditions described above. Both the LT-BA and HT-BA treatments were carried out with a Cp2Mg flow of 0.52 μmol/min and a NH3 flow of 52 mmol/min for 300 sec. More detailed growth conditions on the MOCVD growth process for the a-plane AlGaN epi-layers can be found in our previously published paper [5, 11].
Fig. 1 Schematic layer structures of the nonpolar a-plane AlGaN epi-layer samples A1 (a), A2 (b), A3 (c), and A4 (d).
The structural properties of the nonpolar a-plane AlGaN epi-layers were characterized with the HR-XRD and AFM measurements. To evaluate the optical properties of the a-plane AlGaN epi-layers, the PL spectra were measured at room temperature (RT) by using a 261 nm laser as the excitation source.
3. Results and discussions
The HR-XRD 2θ-ω scanning curve for sample A4 is shown in Fig. 2(a). It was identified that the peaks located at 52.6, 58.4, and 59.3° were the XRDs from the r-plane sapphire substrate, a-plane AlGaN, and a-plane AlN epi-layers, respectively. Highly similar curves were obtained the rest three samples A1-A3 (not shown), and the Al composition was calculated to be 0.40 for the four samples A1-A4 [12], which is consistent very well with the result obtained from the PL spectra to be described below. On the other hand, the AlGaN-related X-ray rocking curves (XRCs) for samples A1-A4 were measured along c-direction (phi = 0°) and m-direction (phi = 90°), respectively, and demonstrated in Figs. 2(b) and 2(c). Here, the anisotropy in crystalline quality (ACQ) is defined as:
where CFWHM and MFWHM are the full width at half maximum (FWHM) values of the XRCs measured along c- and m-directions, respectively. The XRC FWHM and ACQ values for samples A1-A4 are summarized in Table 1. It is demonstrated that both the CFWHM and MFWHM decreased remarkably while ACQ decreased slightly for sample A2 which was grown only with LT-BA treatment. However, for sample A3 which was grown only with HT-BA treatment, both ACQ and MFWHM decreased evidently whereas CFWHM increased slightly. These facts indicate that the crystalline quality could be improved significantly by using the LT-BA treatment and the anisotropy in crystalline quality could be suppressed remarkably by using the HT-BA treatment. The LT-BA treatment could result in the formation of amorphous MgNx islands on the surface of sapphire substrate, that will work like the nano-patterned sapphire substrates, and thus should be responsible for the significant improvement in crystalline quality [13, 14]. On the other hand, the HT-BA treatment could induce the formation of ultra-thin MgNx layer which played a similar role to that of our previously reported SiNx interlayer [9, 15], leading to the suppression of the anisotropy in crystalline quality in the nonpolar a-plane AlGaN epi-layers. Furthermore, CFWHM, MFWHM, and ACQ values were reduced from 1726, 2598 arcsec, and 47.4% for sample A1 grown without any BA treatment to 1526, 1665 arcsec, and 9.1% for sample A4 grown with both the LT-BA and HT-BA treatments, respectively. In other words, CFWHM and MFWHM were decreased by approximately 15.5% and 56%, respectively, after both the LT-BA and HT-BA treatments.Fig. 2 HR-XRD 2θ-ω scanning curve for sample A4 (a); the AlGaN-related XRCs for samples A1-A4 measured at φ = 0° (b) and φ = 90° (c), respectively.
Table 1. Summary of the FWHM and ACQ values for the a-plane AlGaN-related XRCs for samples A1-A4.
The two-dimensional (2D) view AFM images for the four samples detected within an area of 5 × 5 µm2 are shown in Fig. 3. It was demonstrated clearly from Fig. 3(a) that dense pyramidal defects are distributed along c-direction on the surface of sample A1. The existence of such kind of pyramidal defect was known to be one of the structural features for the nonpolar AlGaN epi-layers [5, 16]. When the LT-BA or HT-BA treatment or the combination of LT-BA and HT-BA treatments was introduced into the epitaxial growth process for samples A2-A4, the surface morphology was improved remarkably as shown clearly in Fig. 3. The effect of LT-BA treatment on the reduction in surface roughness for sample A2 was identified to be more evident than that of HT-BA treatment for sample A3 since the RMS value for sample A2, 3.8 nm is smaller than that of 4.2 nm for sample A3 as shown in Figs. 3(b) and 3(c). This result implies that the LT-BA treatment was more effective than the HT-BA treatment for reducing the pyramidal defects that usually appear on the surface of a-plane AlGaN epi-layers [17]. Moreover, it was found that the RMS value decreased further from 3.8 nm for sample A2 grown only with the LT-BA treatment to 3.3 nm for sample A4 grown with both the LT-BA and HT-BA treatments. This fact indicates that the combination of LT-BA and HT-BA is the most effective to improve the surface morphology for the nonpolar a-plane AlGaN epi-layers.
The PL spectra measured at RT for samples A1-A4 are shown in Fig. 4. The primary PL emission peak located at 287 nm for each sample is assigned to the near band edge (NBE) emission from a-plane AlGaN epi-layer with an Al composition of 0.40. The line shape of the major emission peaks in the PL spectra for samples A1 and A3 is obviously asymmetrical. In fact, there is a “shoulder” lying on the right (lower energy) side of the major NBE emission peak. By fitting the PL spectra with Gauss line-shape, the “shoulder” could be split and identified to be originated from the structural defects-related (SDR) emission, especially the pyramidal defects-related emission [18, 19]. Due to the remarkable decrease in the density of pyramidal defects as shown clearly in Fig. 3, the SDR emission intensity for samples A2 and A4 was evidently reduced. In fact, the ratio of the PL intensity for NBE emission to that for SDR emission, INBE/ISDR increased from 5.1 for sample A1 grown without any BA treatment to 25 for sample A2 grown only with LT-BA treatment, and further increased to 58 for sample A4 grown with both LT-BA and HT-BA treatments. On the other hand, the ratio of the integrated PL intensity with respect to that of sample A1 for the four samples was calculated to be A1:A2:A3:A4 = 1.0:9.5:3.7:12.5. These facts demonstrated once again that the combination of LT-BA and HT-BA treatments is the most effective to decrease the superficial pyramidal defects and to improve the optical properties, such as the PL emission intensity for the nonpolar AlGaN films.
Fig. 4 The measured (black circle) and fitted (blue line) PL spectra that are consisted of a major NBE peak (red line) and a SDR emission peak (green line) appearing as a “shoulder” next to the major NBE peak of samples A1-A4. The inserted figures in Figs. 4(b) and 4(d) demonstrate the enlarged PL spectra near SDR emission peaks for samples A2 and A4, respectively.
4. Conclusions
The nonpolar a-plane Al0.4Ga0.6N epi-layers with improved crystalline quality and reduced anisotropy in crystalline quality were successfully grown on r-plane sapphire substrates. The significantly decreased FWHM values of the Al0.4Ga0.6N -related XRCs demonstrated that the crystalline quality could be improved effectively by using the LT-BA treatment whereas the anisotropy in crystalline quality could be suppressed remarkably by using HT-BA treatment. Both the AFM images and the PL spectra indicate that the combination of LT-BA and HT-BA treatments is the most effective for decreasing the superficial pyramidal defects and improving the surface morphology of the nonpolar a-plane Al0.4Ga0.6N epi-layer. In fact, the RMS value was reduced from 7.1 nm for sample A1 grown without any BA treatment to 3.3 nm for sample A4 grown with both LT-BA and HT-BA treatments.
Funding
Key Research and Development Project of Science and Technology Department of Jiangsu Province, People’s Republic of China (Grant No. BE2015159); the Scientific Research Foundation of Graduate School of Southeast University (Grant No. YBJJ1763); the Fundamental Research Funds for the Central Universities, Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. KYCX17_0094).
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