The low dislocation gallium nitride layer by AP-MOCVD Fu-Chuan Chu, Sheng-Fu Yu, Chao-Hung Chen, Chou-Shuang Huang, Ray-Ming Lin* Dept. of Electronics Engineering, Chang Gung University, Taoyuan, Taiwan, R.O.C TEL:+886-3-2118800 FAX:+886-3-2118507 Email: rmlin@mail.cgu.edu.tw Abstract Using an atmospheric pressure metal-organic chemical vapor deposition system, we investigate the Gallium nitride (GaN) epitaxial layer on sapphire, the GaN layer were grown with different V/III ratios by varying the ammonia (NH3) flow rate, keeping the trimethylgallium (TMG) flow rate. The GaN crystal quality was enhanced, as evidenced using X-ray rocking curve widths a (102) reflection decreased from 421.2 arcsec for the 3083 V/III ratio to 241.2 arcsec and (002) reflection decreased from 205.2 arcsec to 183.6 arcsec. To evaluate etch pits density (EPD) of GaN epilayers by wet-chemical etching and scanning electron microscope (SEM), samples were etched in molten H 3 PO 4 at 200 C for 15min. The etch pit densities calculated by counting the number of etch pits in the images of the samples grown with low and high V/III ratios are on the order of 2.4E5/cm-2 and 8.4E6/cm-2. Index Terms: GaN, EPD, V/III Ratio. 1
Introduction GaN-base semiconductors have wide bandgap, which is known for its excellent light emitting properties and high-power electronic device [1-2]. Although the quality of GaN films grown on sapphire substrate has improved with the development of the growth method, there still exist some basic problem. GaN layer grown on (0001) sapphire contain high densities of threading dislocation (TDs), Due to the large lattice mismatch(~16%) and the different thermal expansion coefficients of the sapphire substrate and GaN. The threading dislocation density(tdd) ca 10 17-10 19 cm -2 in a grown GaN film, GaN LED usually exhibit greatly reduced internal quantum efficiencies(iqes) and causing an increased leakage current for GaN-HEMT device[3-4]. We focused to investigate the dislocation density of GaN epilayer by wet chemical etching and crystal quality has enhance as evidenced using HR-XRD techniques. Experimental details The GaN films were grown on a (0001) sapphire substrates by metallorganic vapor phase epitaxy was performed in Taiyo Nippon Sanso SR2000 atmospheric pressure system. Using trimethylgallium (TMG) and ammonia(nh 3 ) as precursors. The TMG flow rate was kept at 80μmol/s for all the growth runs and the ammonia flow in the reactor was varied from 1 to 5 SLM. The sapphire sample was heated at 1180 in a hydrogen ambient to remove surface contamination. Then a 25nm thick GaN nucleation layer grown at 500 and a 4um think unintentional GaN buffer layer grown at 1180. X-ray rocking curves were recorded using a PANalytical X Pert MRD system. GaN sample were etched using phosphoric acid (H 3 PO 4 ) at 200 for 20min to form etch pits, the morphology of etched surface and the etch pits were examined by an FE-SEM (Hitachi S-5000). 2
Measurement Results and Discussion Fig.1 shows the X-ray rocking curve full-widths at half-maxima (FWHM) of the (102) and (002) reflections for samples grown at different V/III ratios. FWHM of our GaN epilayers lie in the range 420 580 arcsec, which is comparable with the quality of the metal-organic chemical vapor deposition (MOCVD) grown gallium nitride samples reported in Ref. [5]. A nonmonotonic decrease in the rocking curve FWHMs of the (002) reflection is observed with decreasing V/III ratio. Heying et al. [6-7] reported that the (102) reflections are sensitive to pure threading edge dislocations. Broadening of the asymmetric (102) rocking curve with increasing V/III ratio evidences an increase in the density of pure threading edge dislocations in the crystal lattices as consequence of the rapid island coalescence. From the Fig.2. the density of screw dislocations has been found to decrease from 8.4 10 6 to 2.4x10 5 with decrease in V/III ratio from 3683 to 616. To confirm the effect of ammonia flow rate on the dislocation densities, the samples were etched with H 3 PO 4 at 200 C for 20 min. Fig. 3 displays the SEM images of as-grown and etched surfaces of the samples grown using the V/III ratios of 3683 and 616. To show the difference between etch pits formed by screw and edge dislocations, a relatively poor quality GaN sample grown with different parameters was etched under the same etching conditions and a well defined hexagonal with black core etch pit of a screw dislocation was acquired as shown in Fig.2(f) On comparing it with the etch pits on the samples grown with different V/III ratios,we may say that the pits observed in the latter samples correspond to edge and mixed type threading dislocations.the etch pit densities calculated by counting the number of etch pits in the images of the samples grown with low and high V/III ratios are on the order of 8.4 10 6 and 2.4x10 5. 3
Conclusions In summary, we reported the GaN epitaxial layers grown with various ammonia flow rates in AP-MOCVD have been investigated. As the V/III ratio decreases the densities of pure edge dislocations, The density of dislocations has been found to decrease from 8.4 10 6 to 2.4x10 5 with decrease in V/III ratio from 3683 to 616. Reference [1] Suthan Kissinger, Seong-Muk Jeong, Seok-Hyo Yun, Seung Jae Lee, Dong-Wook Kim,In-Hwan Lee, Cheul-Ro Lee, Enhancement in emission angle of the blue LED chip fabricated on lens patterned sapphire (0 0 0 1), Solid-State Electronics., 54 (2010) 509 515. [2] Z. H. Feng,, S. J. Cai, K. J. Chen, and Kei May Lau, Enhanced-Performance of AlGaN GaN HEMTs Grown on Grooved Sapphire Substrates, Appl. Phys. Lett., vol. 80, pp. 3216 3218, 2002. IEEE ELECTRON DEVICE LETTERS, VOL. 26, NO. 12, DECEMBER 2005. [3] Hae-Kon Oh, Jung-Gyu Kim, Hyun-Hee Hwang, Young Jun Choi, Hae-Yong Lee, Won-Jae Lee, Byoung-Chul Shin, and Jonghee Hwang, Characterization of etch pit density for gallium nitride layer grown by HVPE and MOCVD P hys. Status Solidi C., No. 7 8, 1794 1797 (2010) [4] Ray-Ming Lin,Yuan-Chieh Lu, Sheng-Fu Yu, YewChung Sermon Wu, Chung-Hao Chiang, Wen-Ching Hsu, and Shoou-Jinn Chang Enhanced Extraction and Efficiency of Blue Light-Emitting Diodes Prepared Using Two-Step-Etched Patterned Sapphire Substrates, Journal of The Electrochemical Society., 156.11 H874-H876.2009. 4
[5] X.H.Zheng,H.Chen,Z.B.Yan,Y.J.Han,H., Determination of twist angle of in-plane mosaic spread of GaN films by high-resolution X-ray diffraction J. Crystal Growth 255 (2003) 63. [6] B.Heying,X.H.Wu,S.Keller,Y.Li,D.Kapolnel,B.P.Keller,S.P.DenBaars, Role of threading dislocation structure on the x-ray diffraction peak widths in epitaxial GaN films Appl. Phys. Lett. 68 (1996) 643. [7] S. Suresh, S.Lourdudoss, G.Landgren, K.Baskar, Studies on the effect of ammonia flow rate induced defects in gallium nitride grown by MOCVD. Journal of Crystal Growth, 312-(2010)3151-3155. Figure captions Fig.1 Variations of X-ray rocking curve FWHMs of the (002) and (102) reflections in arcsec with V/III ratio. Fig.2.Etch pits density of different V/III ratio from 3683 to 616. Fig.3 Fig.3 SEM image of the GaN Layer surface grown different V/III ratio of (a) 3683 (b) 2460 and (c) 1850 (d) 1233 (e) 616.(f). SEM image of etch pit of a screw dislocation formed on a different GaN sample (provided for comparison). 5
(002) Omega Scan- (arcsec) 210 205 200 195 190 185 FWHM(002) FWHM(102) 180 500 1000 1500 2000 2500 3000 V/III Ratio 450 400 350 300 250 200 (102) Omega Scan- (arcsec) Fig.1.Variations of X-ray rocking curve FWHMs of the (002) and (102) reflections in arcsec with V/III ratio. 1.0x10 7 Etch Pit Density (cm -2 ) 8.0x10 6 6.0x10 6 4.0x10 6 2.0x10 6 0.0 500 1000 1500 2000 2500 3000 3500 V/III Ratio Fig.2.Etch pits density of different V/III ratio from 3683 to 616. 6
Fig.3 SEM image of the GaN Layer surface grown different V/III ratio of (a) 3683 (b) 2460 and (c) 1850 (d) 1233 (e) 616.(f) SEM image of etch pit of a screw dislocation formed on a different GaN sample (provided for comparison). 7