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Nano Energy (2014) 5, 1–8

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nCorresponding auE-mail address: c

RAPID COMMUNICATION

Photoluminescence of GaInN/GaN multiplequantum well heterostructures on amorphoussurface through biaxial metal buffer layers

Liang Chena,n, Theeradetch Detchprohma, Christian Wetzela,b,Gwo-Ching Wanga, Toh-Ming Lua

aDepartment of Physics, Applied Physics and Astronomy, and Center for Integrated Electronics, RensselaerPolytechnic Institute, Troy, NY 12180, USAbFuture Chips Constellation, Rensselaer, Troy, NY 12180, USA

Received 16 October 2013; received in revised form 17 January 2014; accepted 26 January 2014Available online 2 February 2014

KEYWORDSEpitaxial GaN;Biaxial texture;Amorphous substrate;MQWs;Photoluminescence

0.1016/j.nanoen.2lsevier Ltd. All rig

thor. Tel.: +1 518henl9@rpi.edu (L.

AbstractWe report the successful epitaxial growth of biaxial GaN films using metal-organic vapor phaseepitaxy on amorphous surfaces (native oxide covered Si wafer) through biaxial metal bufferlayers of Mo and W. The biaxial metal films were grown using a dynamic oblique angledeposition technique. An epitaxial relationship of GaN(0002)//Mo(110) in the out-of-planeorientation and GaN[1010]//Mo[001] in the in-plane orientation was found by a detailed X-raydiffraction analysis. Similar epitaxial results have been found for GaN/W. These biaxial GaNfilms on biaxial Mo and W buffer layers were used as seeds to grow GaInN/GaN multiplequantum well structures (MQWs). A high intensity, narrow peak, and uniform luminance spacedistribution of photoluminescence signal was observed from these MQWs. This work opens up analternative strategy for the growth of GaN-based films on amorphous substrates using biaxialmetal buffer layers.& 2014 Elsevier Ltd. All rights reserved.

Introduction

Gallium nitride (GaN), one of the most important wide bandgap semiconductors, has been intensely studied due to its

014.01.007hts reserved.

2699756.Chen).

various applications in light-emitting diodes (LEDs), [1–6]photocatalysts, [7–9] field effect transistors, [10,11] andfield electron emission devices [12–14]. In order to achieve ahigh efficiency in light emission, researchers usually usesingle-crystal GaN films epitaxially grown on single crystalsubstrates. Various techniques have been developed toachieve decent GaN crystal quality with fewer dislocations,e.g., on sapphire [15–17] and Si [18–20] substrates.

L. Chen et al.2

GaN based nanostructures [21,22] were also used toincrease the light emission efficiency. Recently, heteroepi-taxial growth of GaN directly on single crystal metalsubstrates, e.g., Fe [23] and Cu, [24] has also been studied.Heteroepitaxial growth of GaN on a metallic surface has anadvantage in that the metallic substrate can be used as thebottom electrode for device applications. However, due tothe high cost of single crystal metal substrates, realistic andscale-up applications are limited. Amorphous substrates, e.g., glass and stainless steel, on the other hand, are idealdue to their possible large-scale manufacturing. Choi [25]demonstrated the possibility of forming GaN-based LEDs onglass with a Ti buffer layer and a SiO2 mask to confine theindividual GaN nucleation. The resulting GaN film actuallyhad a fiber texture.

Recently there has been intense interest in growingsemiconductors on low cost, biaxially textured substrates[26–33]. Biaxial substrates are not single crystal, but theyhave strong preferred out-of-plane and in-plane orienta-tions. The as grown semiconductor films on the biaxialsubstrates can be considered as near-single crystal filmsand they possess small angle grain boundaries. It was shownthat these films with small angle grain boundaries can havedramatically higher mobility and other electrical propertiescompared to that of the polycrystalline films with randomgrain orientations [26,34]. First, a biaxial buffer layer canbe created on non-single crystal surfaces such as glass orstainless steel using either oblique angle deposition [35–39]or ion beam assisted deposition [27,40–42]. Then, functionalsemiconductors such as Si, [26–28,30,43] Ge, [33,44] andCdTe, [32,45] can be grown epitaxially on top of the biaxialbuffer layers. Most of the buffer layer materials areinsulators, e.g., CaF2 or MgO. A challenge associated withthe insulating buffer layers is that one needs to create a

Figure 1 (a) SEM cross section view and (b) top view images of theview images of the HT-GaN/biaxial Mo film. The dashed lines in (bdeposition of the Mo buffer layer.

conductive bottom electrode as a contact in realisticapplications. Therefore, it is very desirable to grow semi-conductors directly on biaxial metal surfaces.

The successful growth of nano-structured, biaxial refrac-tory metal buffer layers such as W(110) and Mo(110) by adynamic oblique angle flipping rotation technique [46,47]opens up an opportunity to grow epitaxial functionalsemiconductors on amorphous substrates through the biax-ial metal buffer layers. The nano-structured metal bufferlayers can serve not only as a biaxial seed layer but also as ametal back contact. Due to its high melting temperature,the refractory metals can survive in the harsh growthconditions of wide band-gap semiconductors such as thenitrides. Here we report the epitaxial growth of GaN usingmetal-organic vapor phase epitaxy (MOVPE) directly onnano-structured, biaxial Mo(110) and W(110) buffer layers.These biaxial GaN films on biaxial Mo and W buffer layerswere used as seeds to grow GaInN/GaN multiple quantumwell structures (MQWs). A strong photoluminescence signalwas observed from these MQWs.

Material and methods

Substrate preparation

Biaxial Mo and W films were grown by DC magnetron sputterdeposition with the flipping rotation [46,47] mode onamorphous surfaces (Si wafer with native oxide). In theflipping rotation deposition, the rotation axis is lying withinthe substrate and perpendicular to the incident flux direc-tion. Rotation speeds were optimized to achieve biaxialfilms with minimum dispersions. 1 RPM and 0.3 RPM wereused for Mo and W film growth, respectively.

LT-GaN/biaxial Mo film. (c) SEM cross section view and (d) top) and (d) indicate the rotation axis during the flipping rotation

3GaN grown on amorphous surfaces

GaN seed layers and GaInN/GaN MQWs preparation

Before low temperature (LT)-GaN growth, the biaxial metalbuffer layers were treated in H2 environment at 750 1C for5 min to remove the surface metal oxide. The LT-GaN seedlayers (250–300 nm) were grown at 530 1C by metal-organicvapor phase epitaxy (MOVPE) using Trimethylgallium, NH3,and H2 as precursors on Mo and W substrates. High tem-perature (HT)-GaN films (�800 nm thick) were grow at1030 1C on the LT-GaN seeds layers (�20 nm thick). For

Figure 2 (a) XRD theta-2 theta scans of the LT-GaN film(black) and HT-GaN film (red) deposited on biaxial Mo bufferlayers. (b) GaN{1011} x-ray pole figures of the LT-GaN and(c) HT-GaN films, which indicate the epitaxial nature of thefilm. Pole indices of LT-GaN film are labeled in (b). Note thecenter pole in (b) is an artifact due to the high x-ray back-ground intensity near the center. The dashed lines in (b) and(c) indicate the rotation axis during the flipping rotationdeposition of the Mo buffer layer.

MQWs films, five repeats of GaInN (3 nm)/GaN (15 nm)MQWs were grown directly on the LT-GaN seed layer.Trimethylindium was used as In precursor. The growthtemperatures for GaInN and GaN in MQWs were 750 and800 1C, respectively.

SEM and XRD analysis

The top and cross section view micrographs of all the sampleswere imaged using ZEISS SUPRA 55 FE scanning electronmicroscope (SEM). The crystalline structures of the sampleswere analyzed using a PanAlytical X0Pert Pro X-ray Diffract-ometer (XRD). Theta-2 theta scans were performed with adivergent beam and a line detector. Pole figures wereperformed with a parallel beam and a point detector. Thestep sizes in χ and ϕ angles are 31 and 51, respectively.

Figure 3 (a) Mo {110} x-ray pole figure of the LT-GaN/biaxialMo film. The dashed line in (a) represents the flipping rotationaxis during the deposition of biaxial Mo film. Indices of the {110}poles are labeled in (a). (b) Schematic plots of superlatticematching of GaN(0002) on Mo(110), which indicate the in planeepitaxial relationship of GaN on Mo. Black dots stand for Mo(110) lattices, and green dots stand for GaN(0002) lattices. Theblack dashed rectangle and green dashed rectangle are thematched superlattices corresponding to Mo lattice and GaNlattice, respectively. The in-plane and out-of-plane orientationsare labeled for Mo and GaN films in (b).

L. Chen et al.4

Optical measurement

Photoluminescence spectra for the MQW samples wereperformed at room temperature. The excitation sourcewas a He–Cd gas laser with a selective excitation wave-length of 325 nm at about 0.7 mW (intensity at the samplesurface). The incident laser beam was introduced at anangle of 30–451 off the sample surface normal. The detec-tion unit was a combination of optical lens, optical fiber,and charge-coupled-device (CCD) spectrometer with aspectral resolution of 1 nm.

Results and discussion

Epitaxial growth of biaxial GaN seed layer

The nano-structured buffer layers were produced by DCmagnetron sputter deposition with a flipping rotation tech-nique at near room temperature [46,47] on Si substratescovered with amorphous native oxide layers. The flippingrotation technique is one of the oblique angle depositionmethods where the flux incident direction is varied dyna-mically to achieve a biaxial film texture with verticallyaligned nanocolumns. Figure 1(a–d) shows the SEM images ofGaN layer grown at different temperatures on the biaxial Mo(110) buffer layer. Figure 1(a and b) is for the sample grownat low temperature of 530 1C and Figure 1(c and d) is for thesample grown at high temperature of 1030 1C. The HT-GaNsample was grown on very thin LT-GaN seed layer which wasfirst grown on Mo. From the cross section view image of theLT-GaN sample, individual Mo nanocolumns are visible andeach of those nanocolumns is a single crystal. The LT-GaNpreserves the columnar structure through the epitaxialgrowth, and has a sharp tip for each column. The top viewimage shows that the LT-GaN has a pyramidal structure,similar to that reported by Choi et al. on a glass substratewhere a LT-GaN nucleation layer was grown on a Ti pre-orienting layer [25]. However, there are morphologicalchanges in the Mo layer for the HT-GaN sample. The HT-GaN has a much larger grain size and a flat top surface. Acloser examination of Figure 1(b and d) reveals thatdifferent pyramids actually orient almost in the same

Figure 4 Ten smallest mismatched areas calculated from superlattfilm on Mo film. The superlattice area is within 200 Å2, and the supeand Mo. The first matching (first row) with the smallest mismatche

direction. This means that they have similar in-planeorientations. The GaN grown on biaxial W buffer layer alsoshows a similar morphology (Supplement S1).

X-ray diffraction (XRD) and pole figure analysis confirmthe biaxial texture of the GaN films. Figure 2(a) shows XRDtheta-2 theta scans of the LT-GaN and HT-GaN films onbiaxial Mo substrates. Both the as grown biaxial Mo and Wmetal buffer layers have the (110) out-of-plane orientation[46,47]. For the LT-GaN film, only the GaN(0002) peak andMo(110) peak appear which reveal that the out-of-planeorientation is along the GaN c-axis with the epitaxialrelationship of GaN(0002)//Mo(110). For the HT-GaN film,we can also see a huge GaN(0002) peak which indicates thesame out-of-plane epitaxial relationship. However, we canalso see three small peaks corresponding to GaN(1010),GaN(1011), and GaN(1012), which indicate that the HT-GaN film has a small amount of highly-misoriented grains.Figure 2(b) shows the {1011} pole figure of the LT-GaN film.The six separate poles around χ�621 (polar angle from thesurface normal) with a six-fold symmetry reveal the biaxialtexture of the film. This is consistent with the (0002) out-of-plane orientation. The appearance of the center pole isdue to the relatively low intensity of the six peripheralpoles and the high background intensity at the center sincewe did not see any peak corresponding to the {1011} planeof the GaN film in the x-ray theta-2 theta scan. Figure 2(c) shows the {1011} pole figure of the HT-GaN film. We canalso see the six separate poles. Comparing both the GaNpole figures, we can see more localized and higherintensity poles for the HT-GaN film. This indicates a bettercrystal quality of the HT-GaN sample. Even though it haslarger grains and better crystal quality, the very roughisland-like surface of the as-deposited HT-GaN films maynot be ideal to serve as a seed layer for further MQWsgrowth in realistic applications.

Epitaxial growth relationship of LT-GaN on Mo

To study the in-plane epitaxial relationship between GaNand Mo after the LT-GaN film growth, a measured Mo{110}pole figure has been analyzed for the same sample afterLT-GaN growth. The center pole in the Mo{110} pole figure

ice matchings with different in-plane epitaxial relations of GaNrlattice length and angle are within 5% mismatch between GaNd area (1.44 Å2) is consistent with our experimental results.

5GaN grown on amorphous surfaces

shown in Figure 3(a) indicates the out-of-plane orientationof Mo is [110], consistent with the XRD theta-2theta scans.Comparing the pole figures shown in Figure 2(b) and Figure 3(a), one can conclude that the in-plane epitaxial relation-ship are GaN[1230�//Mo[110] and GaN[1010]//Mo[001].The selected epitaxial relationship between GaN and Mo isdue to their excellent superlattice matching in the GaN[1010] direction. The lattice constants for GaN and Mo are3.180 Å (a) and 3.147 Å (a0), respectively. The mismatch issmall, i.e., Δa/a=(a�a0)/a=(3.180–3.147)/3.181=1.05%. Ifwe consider the superlattice matching [48,49], in the GaN

Figure 5 (a) SEM cross section view and (b) top view images,and (c) GaN{1011} x-ray pole figure of the GaInN/GaN MQWs/LT-GaN/biaxial Mo sample, which indicate the epitaxial growthof the MQWs. The inset in (b) is the optical microscopy with PLexcitation. The PL microscopy shows the good uniformity of thefilm. The dashed lines in (b) and (c) indicate the rotation axisduring flipping rotation deposition of the Mo buffer layer. TheGaN pole indices are labeled in Figure 2.

[1230] direction, the mismatch is even smaller, i.e., Δb/b=(b�b0)/b=�0.99%, where bðGaNÞ ¼ 3:180�

ffiffiffi

3p

� 4, andb0ðMoÞ ¼ 3:147 �

ffiffiffi

2p

� 5. See Figure 3(b). In this case fourperiods of GaN match five periods of Mo. This matching also hasthe smallest mismatched area, which controls the heteroepi-taxial growth condition according to Yapsir et al. [49]. Themismatched area is defined by ΔA=A[(Δa/a)+(Δb/b)+Δα cotα], where A is the superlattice area, α is the angle between aand b. Figure 4 is a table of possible in-plane epitaxial relationsof the superlattice matchings for the ten smallest mismatchedareas given the constrains that the superlattice area is within200 Å2 and the superlattice length and angle mismatches arewithin 5% between GaN and Mo. The table shows the epitaxialrelationship of the matching where the case with the smallestmismatched area (1.44 Å2) is consistent with our experimentalresults. The GaN film on W substrate also shows the samebiaxial texture and epitaxial relationship. This epitaxial growthrelation has also been found for AlN/Mo(110) and AlN/W(110)systems [50,51]. Note that GaN and AlN have a small differencein their lattice constants, and so are Mo and W.

Figure 6 (a) XRD theta-2 theta scans of the GaInN/GaNMQWs/ LT-GaN/Mo sample. (b) XRD ϕ scan at χ=601 for theMo {110} poles after the MQWs growth on biaxial Mo surface,which confirm the same epitaxial relationship after MQWsgrowth. The arrow in (b) shows the position of the rotationaxis during the Mo film growth by flipping rotation. (c) Grainsizes and in-plane dispersions in multilayer of Mo buffer, LT-GaNand MQWs.

Figure 7 (a) Measured PL spectra of the MQWs on biaxial Mo(black), and biaxial W (red) substrates. (b) Tabulated PL peakposition, peak width (FWHM), and peak integral intensity fromthese two samples. The PL spectra intensity and FWHM indicategood optical properties of the MQWs.

L. Chen et al.6

Epitaxial growth of biaxial GaInN/GaN MQWs

GaInN/GaN MQWs were grown on the LT-GaN seeds. Thisspecific MQWs were chosen because the high tunability ofthe peak wavelength in the LED application through thecontrol of MQWs layer thickness or In concentration [52,53].The morphology and texture quality were studied after thegrowth using field emission scanning electron microscopy(FE-SEM) and XRD. Figure 5(a) indicates that the morphologyof Mo nanocolumn structure changes during the MQWgrowth. However, the LT-GaN seed layer was stable anddid not go through any obvious morphological change. Thusthe further epitaxial growth of MQWs on LT-GaN is possible.Figure 5(b) shows a SEM top view image of the as-grownMQW film on Mo substrate. It has the pyramidal structurewith the lateral size of �200 nm, almost doubled the LT-GaN pyramids size of �100 nm shown in Figure 1(b). Thismeans the MQWs had a lateral growth. Figure 5(c) shows the{1011} XRD pole figure of GaN after the MQW growth. Thereare six poles with high intensity concentrations at χ�601.The pole figure has the same symmetry in pole positions anda higher pole intensity as compared to that in Figure 2(b).These results imply that the MQWs grow epitaxially on theLT-GaN surface. To double check the epitaxial growthrelationship, XRD theta-2 theta scans of the GaInN/GaNMQWs/LT-GaN/Mo sample and an XRD ϕ scan at χ=601 forMo {110} has also been performed and shown in Figure 6(aand b), respectively. Figure 6(c) is a summary of the grainsize and in-plane dispersion in each layer of the multilayerstructure grown on the amorphous substrate. An increasinggrain size and a decreasing of the in-plane dispersion wereobserved. Both of these results indicate a progressiveimprovement of the texture quality from the Mo bufferlayer to GaN seed layer and then to the MQWs.

Photoluminescence of the biaxial GaInN/GaN MQWs

Optical properties of these MQWs on the biaxial substrateswere investigated using photoluminescence (PL) spectro-scopy. Figure 7(a) shows the PL spectra of the GaInN/GaNMQWs on LT-GaN on biaxial Mo and W substrates. Figure 7(b) lists the PL peak position, FWHM, and integral intensityfrom these two samples. Previous studies indicate thatMQWs grown on the nano-patterned substrate possess fewerdislocations [16]. The self-assembled column-like structureof Mo and W buffer layers grown by flipping rotation [46,47]may produce similar effects as the patterned substrates do.The PL optical spectroscopy image in the inset of Figure 5(b) and measured PL spectra in Figure 7(a) show that the PLimage from the GaInN/GaN MQWs is quite uniform through-out the entire film and the PL spectra are quite narrow eventhough the MQWs have pyramidal features. The FWHM ofthe PL spectrum of the sample grown on biaxial Mo metalbuffer layer is 29.3 nm. It has been reported that LEDdevices based on nanorods [54] or pyramidal structures[34,53] usually have non-uniform luminance and differentpeak positions at difference locations of the pyramids.A broadened spectrum affects the tunability of the outputwavelength [52] in certain specific applications. The uni-formity and the narrowness in the PL spectrum may indicatea more uniform In concentration in the MQWs. Similar

results have been found for the MQWs grown on biaxial Wbuffer layer (Supplement S2), except that the W bufferlayer did not show an obvious morphological change afterhigh temperature growth of MQWs.

Conclusion

In conclusion, we have demonstrated the successful growthof quality GaInN/GaN MQWs on amorphous substratesthrough LT-GaN seeds and biaxial Mo and W buffer layers.The GaN films grown on the biaxial metal buffer layers alsohave a biaxial texture. The PL measurements of the MQWson amorphous surfaces with biaxial buffer layers showedgood optical properties in terms of peak intensity, peakbroadening, and uniformity. This work opens up an alter-native strategy for the growth of GaN-based films onamorphous substrates using biaxial metal buffer layers.

Acknowledgments

We thank the support from NSF DMR 1104786, New YorkState Office of Science, Technology & Academic Research(NYSTAR) through Focus Center–New York, the Future ChipsConstellation Fund and the Wellfleet Endowment.

7GaN grown on amorphous surfaces

Appendix A. Supporting information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2014.01.007.

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Liang Chen received his B.S. degree inPhysics from Fudan University in ShanghaiChina in 2009. He is currently a Ph.D.student in applied physics at RensselaerPolytechnic Institute. His research interestsfocus on biaxial nano-structure thin films onamorphous substrate and low cost solidstate devices.

L. Chen et al.8

Theeradetch Detchprohm received his Ph.D. degree from Nagoya University, Nagoya,Japan in 1996. He joined RPI as a researchassociate in 2004. In 2008, he was promotedas a research associate professor at RPI. Heis currently a Research Engineer in GeorgiaInstitute of Technology since 2013. Hisprimary focus lays in the epitaxial technol-ogy, device fabrication and device physicsof group III-nitride semiconductor. At RPI,

he concentrates on improving performance of GaInN based light

emitters in the green (520-560 nm) for solid state lighting and laserapplications.

Christian Wetzel received his Dr. rer. nat.(PhD) in physics (1993) at Technical Univer-sity Munich, Germany. In 1994, he joinedUniversity of California Berkeley and Lawr-ence Berkeley National Laboratory. In 1997,he joined High Tech Research Center atMeijo University, Nagoya, Japan, and in2000 Uniroyal Optoelectronics, Tampa FL.In 2004 he joined the Department of Physicsat Rensselaer Polytechnic Institute, Troy NY,

as Associate Professor and Wellfleet Career Development Constella-tion Professor, Future Chips. Since 2011 he is Professor. His workconcerns energy efficiency by wide bandgap semiconductors,primarily group-III nitrides for solid state lighting and solar cells.

Gwo-Ching Wang received her Ph.D. inMaterials Physics from University of Wiscon-sin-Madison. She worked at NBS (NationalBureau of Standards, now NIST) in Washing-ton DC and Solid State Physics Division atOak Ridge National Lab, Oak Ridge, Tenn.before joining the Department of Physics atRensselaer Polytechnic Institute. She holdsthe title of Travelstead Institute Chair. Shepublished over 250 papers and three books.

Her recent research interests are growth of biaxial epitaxial thin

films and applications of RHEED (reflection high energy electrondiffraction) pole figure technique.

Toh-Ming Lu received the BS degree inphysics from National Cheng Kung Universityin Taiwan in 1969 and Ph.D. degree inphysics from the University of Wisconsin,Madison, in 1976. He is currently the RayPalmer Baker Distinguished Professor ofPhysics, Rensselaer Polytechnic Institute,Troy, NY. He has authored and co-authoredover 490 publications and 8 books in thearea of electronic materials. His current

interests include materials for gigascale interconnects, solar cells,and batteries.