crystals Article The Effect of Trimethylaluminum Flow Rate on the Structure and Optical Properties of AlInGaN Quaternary Epilayers Dongbo Wang 1, Gang Liu 2,*, Shujie Jiao 1,*, Lingping Kong 2, Teren Liu 1, Tong Liu 3, Jinzhong Wang 1, Fengyun Guo 1, Chunyang Luan 1 and Zhenghao Li 1 1 Department of Opto-Electronic Information Science, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China; [email protected] (D.W.); [email protected] (T.L.); [email protected] (J.W.); [email protected] (F.G.); [email protected] (C.L.); [email protected] (Z.L.) 2 Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China; [email protected] 3 Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS), Suzhou 215123, China; [email protected] * Correspondence: [email protected] (G.L.); [email protected] (S.J.); Tel./Fax: +86-451-8641-8745 (S.J.) Academic Editors: Alain Largeteau, Mythili Prakasam and Helmut Cölfen Received: 27 November 2016; Accepted: 22 February 2017; Published: 6 March 2017 Abstract: In this work, a series of quaternary AlxInyGa1−x−yN thin films have been successfully achieved using metal organic chemical vapor deposition (MOCVD) method with adjustable trimethylaluminum (TMA) flows. Surface morphology and optical properties of AlxInyGa1−x−yN films have been evaluated. The indium segregation effect on the enhancement of UV luminescence emission in AlxInyGa1-x-yN films with increasing TMA flows was investigated. Our results shed some lights on future optical materials design and LED/LD applications. Keywords: trimethylaluminum flows; AlInGaN alloys; photoluminescence 1. Introduction In the past decades, GaN-based ternary alloys have made significant progress in blue, green, and ultraviolet (UV) light emitting diodes (LEDs) and laser diodes (LDs) [1–3]. Despite the great progress that has been made, the commercial InGaN/GaN blue LEDs still suffered from some fundamental problems related to the basic material properties. Even though InGaN-based blue LEDs are less sensitive to crystalline quality because of the carrier localization effect induced by alloy composition fluctuation [4], the piezoelectric polarization effect induced by the strain between barrier and well has been considered as one of the reasons of efficiency droop effect [5,6]. Meanwhile, AlGaN-based UV LEDs combined with RGB phosphors have also been proposed to avoid the color rendering problems in white LED made by blue LED with YAG phosphors. However, because of low doping efficiency and poor crystal quality, highly efficient AlGaN-based UV LEDs are still hard to achieve. In order to solve these problems in ternary InGaN- and AlGaN-based devices, several researchers have recently grown quaternary AlInGaN films [7,8]. Quaternary AlInGaN films are considered to have the potential to overcome the above shortcomings of ternary alloys, since AlInGaN films can be grown by independently controlling the lattice constant and the band gap by adjusting the composition [9]. Therefore, AlInGaN attracts much attention as a candidate material for high efficiency LEDs or LDs. The growth of AlInGaN has been proven to be extremely challenging, attributed to the large difference in the growth condition between InN, GaN, and AlN [10,11]. Crystals 2017, 7, 69; doi:10.3390/cryst7030069 www.mdpi.com/journal/crystals Crystals 2017, 7, x FOR PEER REVIEW 2 o f 8 Crystalschallenging2017, 7, 69attributed to the large difference in the growth condition between InN, GaN, and 2AlN of 8 [10,11]. Recently, Yu et al. [12] reported the optical and electrical properties of AlInGaN with various Recently, Yu et al. [12] reported the optical and electrical properties of AlInGaN with various trimethylindium (TMIn) molar rates. Fu et al. [13] reported the effect of trimethylgallium (TMGa) flow trimethylindium (TMIn) molar rates. Fu et al. [13] reported the effect of trimethylgallium (TMGa) rate on the optoelectrical characteristics of AlInGaN. However, there are few reports available about flow rate on the optoelectrical characteristics of AlInGaN. However, there are few reports available AlInGaN structure and optical properties with varied trimethylaluminum (TMA) flow rate. Because about AlInGaN structure and optical properties with varied trimethylaluminum (TMA) flow rate. of the large lattice mismatch between the InN and AlN, there is a great increasing trend in Because of the large lattice mismatch between the InN and AlN, there is a great increasing trend in compositional disorders with increasing Al/In content, and their influence on the optical properties compositional disorders with increasing Al/In content, and their influence on the optical properties of of AlxInyGa1−x−yN quaternary alloys arise as a result [14,15]. It is important to know the influence of AlxInyGa1−x−yN quaternary alloys arise as a result [14,15]. It is important to know the influence of Al Al content on the properties of AlInGaN quaternary alloys when Al atoms are added in the film content on the properties of AlInGaN quaternary alloys when Al atoms are added in the film layer to layer to adjust the band gap of the quaternary. adjust the band gap of the quaternary. In this manuscript, we reported AlInGaN/GaN heterostructure grown by MOCV D with In this manuscript, we reported AlInGaN/GaN heterostructure grown by MOCVD with varying varying TMA flow rate. Structural and optical properties versus TMA flow rate in AlInGaN were TMA flow rate. Structural and optical properties versus TMA flow rate in AlInGaN were systematically systematically investigated. The purpose of this article is to experimentally study the influence of investigated. The purpose of this article is to experimentally study the influence of TMA flow rate TMA flow rate on the AlInGaN quaternary, which will be highly useful for optimizing growth of on the AlInGaN quaternary, which will be highly useful for optimizing growth of AlInGaN- and AlInGaN- and AlInGaN- based device application. AlInGaN- based device application. 2. Results Figure 11aa showsshows (0004)(0004) !ω--22θθ scans of AlInGaN/GaN samples samples 1 1to to 4. 4. The The diffraction diffraction peaks peaks at at 0 0arcsec arcsec originated originated from from the the GaN GaN buffer buffer layers. layers. For For all all the the sampl samples,es, the the diffraction peaks from thethe quaternary AlInGaNAlInGaN thin filmsfilms shiftedshifted to thethe rightright sideside withwith increasingincreasing TMATMA flowflow rate.rate. ThisThis waswas ascribed toto thethe increaseincrease of of the the lattice lattice constant constant due due to to the the increase increase in Alin Al incorporation. incorporation. For For sample sample 1, the 1, AlInGaNthe AlInGaN and and GaN GaN peaks pea were ks were overlapped, overlapped, which which means means that that the lattices the lattice matchs match along along the c-axes the c-axes [15]. For[15] sample. For sample 4, without 4, without any Al any incorporation, Al incorporation, the diffraction the diffraction peak peak originated originated from InGaN.from InGaN. Figure 1. XRDXRD spectraspectra of of Al AlxInxInyyGaGa11−−x−−yN/GaNyN/GaN laye layers rs with with diffe different re nt trime trimethylaluminum thylaluminum ( (TMA)TMA) flow flow rate:rate: ((aa)) (0004) ω!--22θθ scan; ((bb)) 22θθ scan.s c a n. Figure1 1bb showsshows 2 2θθ scans scans ofof AlInGaN/GaNAlInGaN/GaN samples. samples. The The peaks located at 34.5 ◦° originated from the GaNGaN buffer buffer layers. layers. The The shoulder shoulder peaks peaks at the at right the sideright of side GaN of peaks GaN originated peaks originated from the AlInGaNfrom the layers.AlInGaN For layers. samples For 1, sample 2, ands 3, 1, the 2, and peaks 3, the located peaks at located 33.8◦ on at the 33.8 left° on side the of left the side GaN of peaks the GaN belong peaks to InGaN,belong to which InGaN mean, which compositional mean compositional fluctuations fluctuations were formed were in formed samples. in samples. It is worth It is noting worth thatnoting in increasingthat in increasing the TMA the flow TMA rate flow from rate 1.15 tofrom 1.9 µ1.15mol/min, to 1.9 anotherμmol/min, peak another located pea atk 31.1 located◦ appeared at 31.1 in° samplesappeared 2 in and sample 3, correspondings 2 and 3, corresponding to InN. The appearance to InN. The of InNappear diffractionance of peaksInN diffraction in samples peaks 2 and in 3 indicatesamples that2 and phase 3 indicate separation that appears phase separation in the AlInGaN appears films in with the incrasingAlInGaN Alfilms content. with incrasing Al content.To further investigate the crystal quality of AlInGaN epilayer, the full width at half maximum (FWHM)To further of XRD investigate (0004) ! -2theθ scanscrystal of qual AlInGaNity of AlInGaN epilayers epilayer, versus TMAthe full flow width were at alsohalf maximum measured (listed(FWHM in) of Table XRD1). (0004) The FWHM ω-2θ scans of (0004) of AlInGaN!-2θ scan epilayers of AlInGaN versus TMA epilayers flow were were between also measured 195 to 252(listed arcsec, in Table corresponding 1). The FWHM withsamples of (0004) 1–4. ω-2θ When scan the of TMAAlInGaN flow epilayers rate was low,were the between lattice constant 195 to 252 of AlInGaNarcsec, corresponding epilayers was with closer sample to thats 1 of–4. GaN, When and the meanwhile TMA flow the rate composition was low, the of AlInGaN lattice constant epilayers of becameAlInGaN homogeneous epilayers was because closer to of that strain-free of GaN, growth;and meanwhile both of the these composition were expected of AlInGaN to narrow epilayers X-ray Crystals 2017, 7, 69 3 of 8 peaks. From Table2, the FWHM of AlInGaN diffraction peaks broadened as the Al content increased. Inhomogeneous strain, finite grain size, dislocations, and composition fluctuation are all responsible for broadening the diffraction peaks.