The morphology evolution of high-temperature grown AlN nucleation layers on (111) silicon has been studied using atomic force microscopy (AFM). The structure and morphology of subsequently grown GaN film were characterized by optical microscopy, scanning electron microscopy, x-ray diffraction, and photoluminescence measurement. It was found that a thicker AlN buffer layer resulted in a higher crystalline quality of subsequently grown GaN films. The GaN with a thicker buffer layer has a narrower PL peak. Cracks were found in the GaN film which might be due to the formation of amorphous SiNx at the AlN and Si interface.Singapore-MIT Alliance (SMA

Chua, Soo-Jin

Thompson, Carl V.

Wang, Lianshan

Zang, Keyan

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Abstract— The morphology evolution of high-temperaturegrown AlN nucleation layers on (111) silicon has been studiedusing atomic force microscopy (AFM). The structure andmorphology of subsequently grown GaN film werecharacterized by optical microscopy, scanning electronmicroscopy, x-ray diffraction, and photoluminescencemeasurement.  It was found that a thicker AlN buffer layerresulted in a higher crystalline quality of subsequently grownGaN films. The GaN with a thicker buffer layer has anarrower PL peak.  Cracks were found in the GaN film whichmight be due to the formation of amorphous SiNx at the AlNand Si interface.   Index Terms— Metal-organic Chemical Vapour Deposition,III-V NitridesI. INTRODUCTIONallium-nitride-semiconductor offers good potential forapplication in a wide range of electro-optical [1, 2] andhigh-power microelectronic devices [3, 4] due to itswide band gap characteristics.  However it is very difficultto grow sufficiently large bulk single GaN substrates andGaN films are usually grown hetero-epitaxially on a varietyof alternative substrates, such as silicon, silicon carbide andsapphire. Growth of GaN onto silicon substrate offers very attractivepotential for incorporating GaN devices onto silicon-basedvery large-scale integrated circuits (Table 1). However, dueto the large difference in lattice constant, crystal structure,and thermal expansion coefficient, it is rather difficult toepitaxially grow GaN on Si substrate. This problem isapproached by the use of buffer layers which are meant tominimize the considerable mismatch between film andsubstrate so as to allow growth of GaN films of equalquality. Various types of buffer layer were attempted togrow GaN on Si until now, such as SiC [5], AlN [6, 7],GaAs [8] and Si3N4 [9]. AlN buffer layer is one of the mostpromising ways to achieve high quality GaN on Si [10].Nikiskin [11] reported high quality GaN on Si (111) withAlN buffer layer in gas-source molecular beam epitaxy(GSMBE). The crack-free thin GaN film with goodstructural characteristics can be achieved. However, suchhigh quality GaN film on Si was mostly grown by MBE.The GaN on Si with AlN buffer layer, the evolution of AlNnucleation layer and the effect on the subsequent GaNgrowth by metalorganic chemical vapor deposition(MOCVD) were seldom reported.In this paper, we studied the evolution of AlN buffer layeron Si (111) substrates and the effect on the subsequentexpitaxial GaN film using MOCVD. II. EXPERIMENTAL DETAILSBoron doped Si (111) wafer with resistivity of 1-4 Ω·cmwere used as the substrates. The substrates were cleanedusing the RCA recipe followed by dipping into HF solutionto form a hydrogen-terminated surface. The growth wascarried out in a vertical rotating disk low pressure(200Torr)-MOCVD system. Hydrogen was used as thecarrier gas. Trimethylaluminum (TMA), trimethylgallium(TMG), and ammonia (NH3) were used as the precursorsfor Al, Ga, and N sources respectively. Before growingAlN, the substrate was baked under H2 atmosphere at1000ºC for 15min to remove the native oxide. AlN bufferlayers were grown at 1030ºC for 10min, 46min and 80minrespectively. The flow rates of TMA and NH3 for AlNgrowth were 200µmol/min and 0.45mol/min, respectively.The reactor pressure is 50Torr After AlN growth, thetemperature was increased to 1020ºC. The flow rate ofTMG and NH3 for the GaN growth was 335µmol/min and0.40mol/min, respectively. The reactor pressure is 200Torr.The AlN layer was characterized by atomic forcemicroscopy (AFM). X-ray diffraction (XRD), Micro PL,and scanning electron microscopy (SEM) were used tocharacterize the GaN film. III. RESULTS AND DISCUSSIONA. Morphology and structure of AlN buffer layerThe morphological evolution of AlN for 10min, 46min and80min is illustrated in the AFM images of Fig. 1 a, b and c.During this nucleation/growth step, the grains underwentsubstantial growth, resulting in an increase of the averagegrain size, from 20-30nm to 80-160nm. Furthermore, theRMS surface roughness increased from 0.749nm to3.923nm. Fig. 1a shows high density of AlN grains on the surface ofSi as 3-dimensional islands, and the average size of thegrains is about 20-30nm.  In general, GaN was grown onEvolution of AlN buffer layers on Silicon andthe effect on the property of the expitaxial GaNfilmZang Keyan, Wang Lianshan, Chua Soo Jin, Carl V. ThompsonGsapphire using 2-step growth procedure [12, 13]. The firststep consists of AlN or GaN growth at lower temperatures(~600ºC) in order to achieve a smooth, fine-grained film;the second step consists of GaN growth at highertemperature (~1100ºC). However, studies found that theSiNx tended to form below 900ºC on Si substrate due to thestrong bonding of Si-N [14]. But even at high temperatures,SiNx tends to form when TMA and ammonia wasintroduced into the chamber. This will be explained later. Inour growth, high temperature (1100ºC) was used for theAlN layer. A high density of AlN nuclei (2.2×1011/cm2 )was observed and these nuclei grew in size and at 10minthe average height of these grains was about 2-4nm asshown in Fig. 1a. Fig. 1b shows the morphology of AlN at 46min growth.The grain size has increased to 80-160nm, which shows therapid and continuous growth of the grains. From Fig. 1b,we notice the grains form triangular islands on the surfaceof Si substrate. This might be explained by the competitionof surface energy vs. the strain energy. At shorter time,surface energy minimization dominates, in which certaincrystallographic planes confined the shape of the islands. Comparing Fig. 1b and 1c, we found that grain size did notchange much, but the RMS surface roughness increasedfrom 3.923nm to 8.069nm, and the average height of theAlN grains increased to 10-30nm at 80min. With longertime evolution of AlN buffer layer, the film grew thicker,but is confined by the shape of the grains as deposited bystrain energy minimization. The top of the islands wasrelatively flat as seen from the AFM figure. In the growth condition used, conversion from 3D to 2Dgrowth was observed which was proposed by other paper[11, 15]. The possible reason might be the SiNx form at theinterface of AlN and Si even at the high temperatures. Dueto the strength of Si-N bonds, SiNx islands would form onthe surface of Si when TMA and ammonia were introducedinto the chamber. Suppressing the formation of SiNx wouldbe an effective way to acquire high quality of GaN film. B. Characteristics of GaN epilayerFig. 2 shows the optical microscope pictures of GaN grownfor 60min on 46min-AlN buffer layer (Fig. 2a) and on80min-AlN buffer layer (Fig. 2b). Fig. 3 shows the SEMdiagrams of GaN films on 46min-AlN buffer layer (Fig. 3a)and on 80min AlN buffer layer (Fig. 3b). From Fig. 2a and3a, the surface of GaN film was rough and obvious islandswere observed on the surface. Fig. 2b and 3b shows that thesurface is smooth, but cracks were observed in the GaNfilm. Despite the rough morphology of the buffer layer(80min-AlN), the epitaxial GaN above showed a smoothsurface. The cracks were produced due to the difference inthermal expansion coefficient during cooling from thegrowth. Comparing these two samples, we found that thethicker buffer layer gave better surface morphology of thesubsequently grown epitaxial GaN film. This might due tothe relatively flat islands, which offer better templates forthe subsequent growth of GaN.XRD was performed to investigate the crystal phase of theGaN films. Fig 4a and 4b show the XRD pattern of GaNfilm deposited on 46min-AlN and 80min-AlN buffer layer,respectively. From Fig. 4b, besides a (111) peak of Si(2θ=28.5º), and (0002) peak of AlN (2θ=36.1º), there is apronounced (0002) peak of the GaN with the c-planespacing for the (0002) at 0.5167nm, which is consistentwith the hexagonal lattice. Compared with that of Fig. 4b,the (0002) peak of GaN in Fig. 4a has a slightly different 2θ=34.6º, which has different c-plane spacing of 0.5186nm.In Fig. 4a, AlN is thinner than that of Fig. 4b resulting in aweaker AlN peak. The double-crystal X-ray rocking curve (DCXRD) of the(0002) diffraction of GaN film deposited on 46min-AlNand 80min-AlN respectively (shown as Fig 5 a, b), showedfull width at half maximum (FWHM) of 1.8213º and0.4551º respectively. GaN film deposited on the thickerAlN buffer layer shows higher crystallity, which meanshigher quality, compared to that of the thinner buffer layer. The optical properties of the GaN samples werecharacterized by photoluminescence measurement (PL).The PL spectrum of the GaN samples is shown in Fig. 6aand 6b. From Fig. 6, we found that GaN grew on the thickerbuffer layer has a narrower peak, but weaker in intensity. C. Figures(b)(a)Fig. 1 The surface of AlN buffer layers on Si (111)substrate with different duration time (a) 10min, (b) 46min,(c) 80minFig 2 Optical Microscope imagegrown on 46min AlN buffer lay80min AlN buffer layer   Fig 3 SEM photography of GaN film a) GaN film grown on46min AlN buffer layer b) GaN film grown on 80min AlNbuffer layer10000c=0.51671 nm28.47o2θ=34.7215o#1293( 0002)GaN(111)Si )1000010000028.47o#1323 (111)Si .u)(c)(a)(b)(a)25 30 351101001000c=0.49742θ=36.1150o(0002)AlN XRD Intensity(a.u2θ/ω(degrees)25 30 351101001000c=0.51866 nmAlN (?)2θ=34.5866o(0002)GaN XRD Intensity (a2θ/ω(degrees)(a)(b) of GaN film a) GaN filmer b) GaN film grown on    (b)Fig. 4 XRD Curves of samples a) GaN grown on 46minAlN b) GaN grown on 80min AlNFig. 5 Rocking curve of (0002) diffraction of GaN film a)on 46min-AlN b) on 80min-AlN        Fig .6 PL spectrum of GaN film a) grown on 46min-AlN b)grown on 80min-AlN  Table 1 Different substrate for GaN growthC. CONCLUSIONSIn summary, we studied the evolution of AlN buffer layergrown at a high temperature using MOCVD, and examinedthe quality of the subsequent epitaxial GaN film. Theproperties were characterized using AFM, XRD, SEM,optical microscope, and PL. We found that a thicker bufferlayer resulted in a better quality of the GaN film. But therewere still cracks in the GaN film, which might be due to theformation of SiNx at the AlN and Si interface. REFERENCES1. S. Nakamura, M. Senoh and T. Mukai, Candela-class high-brightness InGaN/AlGaN double-thterostructure bule-light-emitting diodes, Appl. Phys. Lett., 1994 (64): 16872. S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada,T. Matushita, H. Kiyoku, Y. Sugimoto, InGaN-Based Multi-Quantum-Well-Structure Laser Diodes, Jpn. J. Appl. Phys., Part2 1996 (35): L743. S. N. Mohammad, A. A. Salvador and H. Morkoc, Proc. IEEE,1995 (83): 1306Substrate Sapphire SiC/ZnO SiDisadvantageNot conductive;electrodes fordevices are notreadily fabricateddifficult toobtainLattice mismatch (17%);difference in thermalexpansion coefficientAdvantageHas been widelystudied, lesslattice mismatch Conductive;relativelygood latticematch withGaNAvailable in large size; highquality at low cost;integration with siliconmicroelectronics andoptoelectronics2.50 2.75 3.00 3.25 3.50 3.75010002000300040005000∆Ε=53.8meV3.391eV  MicroPL Intensity(a.u)Photon Energy(eV)#1293 on flat region2.50 2.75 3.00 3.25 3.50 3.7501000020000300004000050000∆Ε=89.6meV3.414eV  MicoPL Intensity (a.u)Photon Energy (a .u)#132316.5 17.0 17.5 18.0 18.50200040006000800010000120001400016000(0002)GaN∆ω=0.4551o  Intensity (counts/s)ω(degrees)#1293 rocking curve14 16 18 200100200300400500600(0002)GaN∆ω=1.8213o  XRD Intensity (a.u)ω(degrees)#1323 rocking curve(a)(b)(a)(b)4. S. Strite and H. Morkoc, GaN, AlN and InN: A review, J. Vac.Sci. Technol. 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Evolution of AlN buffer layers on Silicon and the effect on the property of the expitaxial GaN film

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Evolution of AlN buffer layers on Silicon and the effect on the property of the expitaxial GaN film

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