Surface Morphology Evolution Mechanisms of InGaN/GaN Multiple Quantum Wells with Mixture N2/H2-Grown GaN Barrier

Zhou et al. Nanoscale Research Letters (2017) 12:354 DOI 10.1186/s11671-017-2115-8 NANO EXPRESS Open Access Surface Morphology Evolution Mechanisms of InGaN/GaN Multiple Quantum Wells with Mixture N2/H2-Grown GaN Barrier Xiaorun Zhou1,2, Taiping Lu1,2* , Yadan Zhu1,2, Guangzhou Zhao1,2, Hailiang Dong1, Zhigang Jia1,2, Yongzhen Yang1,2*, Yongkang Chen1 and Bingshe Xu1,2 Abstract Surface morphology evolution mechanisms of InGaN/GaN multiple quantum wells (MQWs) during GaN barrier growth with different hydrogen (H2) percentages have been systematically studied. Ga surface-diffusion rate, stress relaxation, and H2 etching effect are found to be the main affecting factors of the surface evolution. As the percentage of H2 increases from 0 to 6.25%, Ga surface-diffusion rate and the etch effect are gradually enhanced, which is beneficial to obtaining a smooth surface with low pits density. As the H2 proportion further increases, stress relaxation and H2 overetching effect begin to be the dominant factors, which degrade surface quality. Furthermore, the effects of surface evolution on the interface and optical properties of InGaN/GaN MQWs are also profoundly discussed. The comprehensive study on the surface evolution mechanisms herein provides both technical and theoretical support for the fabrication of high-quality InGaN/GaN heterostructures. Keywords: GaN barrier, Hydrogen, Surface, Interface Background InGaN/GaN-based high-brightness light-emitting diodes (LEDs) and laser diodes, as the representative devices of III-nitrides, have attracted much attention owing to their important role in digital signage, high-density optical storage, and general illumination [1–10]. Generally speaking, fabrication of blue or green LEDs requires relatively high indium composition of InGaN layer [11, 12]. Although the reduction of growth temperature and the increase of growth rate of the quantum well (QW) can alleviate indium atom desorption to obtain high indium content, these methods also deteriorate the optical performance of InGaN/GaN multiple quantum wells (MQWs) by worsening interface abruptness and introducing more defects [13, 14]. Moreover, these defects usually act as nonradiative recombination centers, thus weakening the internal quantum efficiency of the device [15–19]. Therefore, achieving required indium * Correspondence: ; 1 Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China Full list of author information is available at the end of the article content while maintaining high material quality is still a big challenge. In order to settle the problems mentioned above, various growth techniques have been employed in striving for smooth morphology and sharp interfaces within the InGaN/GaN stack. Quantum barriers (QBs) grown at elevated temperature [20, 21] and growth interruption after QWs [12, 22] are widely used to improve the morphology of InGaN/GaN heterostructures. However, they all have their own limitations. For instance, barriers grown at high temperature may lead to severe In loss [14, 23]. Although growth interruption can improve morphology as well as reduce inclusions, it is at the expense of the optical quality of the QWs [21]. Recently, it is reported that introducing a small amount of hydrogen during the growth of GaN barriers can improve both optical and interface properties [24–28]. However, the effect mechanism of H2 on surface evolution of InGaN/ GaN MQWs has not been fully understood yet. In this paper, the effects of H2 proportion, defined as H2 flow divided by total carrier gas flow, during GaN barrier deposition, on surface morphology evolution are systematically investigated. Ga surface-diffusion rate, © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Zhou et al. Nanoscale Research Letters (2017) 12:354 stress relaxation, and H2 etching effect are suggested to be the three main factors, affecting surface evolution. The dominant factors and their influences on the surface evolution are comprehensively discussed, which provides a technical guideline to obtain high-quality InGaN/GaN heterostructures. Methods The InGaN/GaN MQW structures were grown on cplane sapphire substrate by Aixtron TS300 metal organic chemical vapor deposition system. Trimethylgallium (TMG), triethylgallium (TEG), trimethylindium (TMI), and ammonia (NH3) were used as precursors. Silane (SiH4) was used as the n-type dopant source. The structure was composed of 3.2-μm-thick undoped GaN layer and nominally six-period 2.4-nm-thick InGaN QWs separated by 11-nm-thick lightly Si-doped (n-doping = 3×1017cm−3) GaN barriers. A 1.0-nm-thick low temperature GaN cap layer (LT-GaN) was deposited immediately after the growth of QW layer. InGaN wells and GaN barriers were grown at 730 and 850 °C, respectively. A conventional InGaN/GaN MQWs sample, labeled as S1, was grown in nitrogen atmosphere. Four other samples, denoted as S2, S3, S4, and S5, were grown with different proportion of H2 flow to total carrier gas (N2 + H2) during barriers deposition, with the other growth parameters the same with S1. The percentage of H2 was 2.5% (S2), 6.25% (S3), 10% (S4), and 50% (S5), respectively. The structures of InGaN/GaN MQWs were characterized by PANalytical Empyrean high resolution x-ray diffraction (HRXRD) system. Surface morphology was obtained by atomic force microscopy (AFM) (SPA300HV) using tapping model. Room temperature (RT) photoluminescence (PL) properties of the samples were studied by 226-nm Nd-YAG laser with an excitation power density of 1.36 W/cm2. Page 2 of 8 Results and Discussion The HRXRD ω-2θ scanning results of S1–S5 are illustrated in Fig. 1a. The strongest peak located at the center belongs to the underlying GaN template, and the satellite peaks correspond to the periodicity of the MQWs. It is found that the full-width at half-maximum (FWHM) of the strongest peaks in all samples is almost the same, indicating the similar crystal quality of GaN buffer layers for all samples. The presence of clearly distinguished “ + 4th” diffraction peak in samples S2–S4 manifest the improvement of crystal quality under low H2 percentage. The appearance of the “ + 5th” diffraction peak (represented by the rectangle in Fig. 1a) and the minimum FWHM value of InGaN “−1st” diffraction peak indicate the best interface quality in sample S3. The structure parameters determined by fitting the measured XRD curves are summarized in Table 1. The period thicknesses of the five samples are almost the same, and the values keep around 14. (...truncated)