Journal Search Engine
Search Advanced Search Adode Reader(link)
Download PDF Export Citaion korean bibliography PMC previewer
ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.29 No.10 pp.579-585
DOI : https://doi.org/10.3740/MRSK.2019.29.10.579

Effects of Growth Rate and III/V Ratio on Properties of AlN Films Grown on c-Plane Sapphire Substrates by Plasma-Assisted Molecular Beam Epitaxy

Se Hwan Lim1, Eun-Jung Shin1, Hyo Sung Lee2, Seok Kyu Han2, Duc Duy Le2, Soon-Ku Hong2
1Graduate School of Energy Science and Technology, Chungnam National University, Daejeon 34134, Republic of Korea
2Department of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Republic of Korea
Corresponding author E-Mail : soonku@cnu.ac.kr (S.-K. Hong, Chungnam Nat’l Univ.)
June 10, 2019 September 10, 2019 September 10, 2019

Abstract


In this study, we investigate the effect of Al/N source ratios and growth rates on the growth and structural properties of AlN films on c-plane sapphires by plasma-assisted molecular beam epitaxy. Both growth rates and Al/N ratios affect crystal qualities of AlN films. The full width at half maximum (FWHM) values of (1015) X-ray rocking curves (XRCs) change from 0.22 to 0.31° with changing of the Al/N ratios, but the curves of (0002) XRCs change from 0.04 to 0.45° with changing of the Al/N ratios. This means that structural deformation due to dislocations is slightly affected by the Al/N ratio in the (1015) XRCs but affected strongly for the (0002) XRCs. From the viewpoint of growth rate, the AlN films with high growth rate (HGR) show better crystal quality than the low growth rate (LGR) films overall, as shown by the FWHM values of the (0002) and (1015) XRCs. Based on cross-sectional transmission electron microscope observation, the HGR sample with an Al/N ratio of 3.1 shows more edge dislocations than there are screw and mixed dislocations in the LGR sample with Al/N ratio of 3.5.



초록


    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    III-nitride materials have been used for blue light emitting diode (LED) after development by S. Nakamura et al..1) Wurtzite-structure materials InN, GaN and AlN have bandgap energies of 3.4, 0.7, and 6.3 eV at room temperature.2-4) Therefore, ternary compound alloys of InGaN and AlGaN can have bandgap energies of 0.7 ~ 3.4 eV and 3.4 ~ 6.3 eV, respectively, depending on In and Al compositions. Generally, ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, that is, in the range 400 nm to 10 nm, corresponding to photon energies from 3.10 eV to 124 eV. 5) Recently, deep UV (DUV)- LEDs have received great attentions because of their potential applications in the field of environmental, biological and medical engineering.6–8) Therefore, among the III-nitride materials, AlN and high-Al composition AlGaN alloys (> ~ 45 at% Al) are hot material systems because their wavelengths belong to DUV. Especially, AlN is very attractive because it is feasible to emit DUV of 210inm.4, 9-11)

    AlN and AlGaN-based DUV LEDs have been reported but its external quantum efficiency (EQE) was reported to only a few percentage when the wavelength is below 275 nm,12-14) which is extremely low compared with visible light region of EQE reported to 70%.15) Liao et al., explained the poor EQE of UV LEDs by several reasons.16) They mentioned that atomic nitrogen is chemically very active and reacts instantly with arriving Al atoms and limits the diffusivity of the adatoms during heteroepitaxial growth, which leads to microstructures with small domains and thus high dislocation density.16) Also, they mentioned incorporation of oxygen in AlGaN due to high chemical affinity of aluminum for oxygen, which introduce states in the center of the energy gap of AlGaN alloys.16) Both the high dislocation density and the incorporation of oxygen impurities limit the internal quantum efficiency (IQE) of the AlGaN based DUV-LEDs.16)

    In order to grow high quality AlN and AlGaN films applicable to high EQE DUV-LEDs, several studies have been reported, mostly grown by metal organic chemical vapor deposition (MOCVD).17-20) Hirayama et al. reported multiple AlN buffer layers with total thickness more than 2 μm.14) However, there are very few reports on AlN and AlGaN growth by molecular beam epitaxy (MBE) for DUV LEDs. Recently, Liao reported high IQE from MBE grown materials for DUV LED emitting at 273 nm.16)

    In this report, we investigated plasma-assisted MBE (PAMBE) grown AlN films on c-plane sapphire substrates. We compared growth behavior and structural properties of AlN films in which difference in growth rate is focused. Additionally effects of III/V ratio are addressed. In order to change growth rate, growth temperature or source flux in MBE growth have to be changed. However, these two important factors can affect properties of grown films by themselves in addition to the growth rate. Therefore, careful and systematic experiments are needed to investigate the growth rate effect exclusively as much as possible. We have grown two sets of samples. In each set, the III/V ratio is varied. However, the III/V ratio is same in set to set by changing the Al and N fluxes simultaneously.

    2. Experimental

    Commercial c-plane sapphire substrates were used for the film growth. The substrates were cleaned by ultrasonic agitation with a sequence of in acetone, methanol and deionized water. Then the substrates were chemically cleaned in a chemical solution of H2SO4:H3PO4 = 3:1 (vol%) at 160 °C for 15 min followed by rinsing in deionized water and drying by nitrogen blowing. Chemically cleaned substrates were installed in the loadlock chamber and transferrred to the growth chamber for the thermal cleaning. Thermal cleaning was performed at 900 °C for 20 min in the growth chamber. During the thermal cleaning pressure of the growth chamber was manintained under 5 × 10-8 torr. The thermally cleaned substrates were nitrided by radio frequency (RF) nitrogen plasma at 860 °C for 15 min. We have grown two sets of AlN film growth. In each set, the III/V ratio is varied. However, the III/V ratio is same in set to set by changing the Al and N fluxes simultaneously. By doing that we have grown two set of samples with largely different growth rate but have the same III/V ratios. The AlN films were grown at 860 °C for 13 and 30 min for sets 1 and 2, respectively. The reason for different growth time is to maintain similar thickness for all the samples because the film thickness also affect properties like surface morphology and crystal quality. In both set of samples, the III/V ratio was set to 3.1, 3.3, and 3.5 by changing the Al and N fluxes simultaneously. N flux was supplied by RF nitrogen plasma cell and the plasma power was set to 150 W. Fluxes of Al were measurred by ion gauge at the substrate position and it is normally called as a beam equivalent pressure. Detailed conditions are given in Table 1. Base pressure of the growth chamber with liquid nitrogen supply into the shraud was about 2.0 × 10-9 torr and working pressure during the growth by nitrgen gas supply to the rf-plasma cell was about 6.3 × 10−6 ~ 1.7 × 10−5 depending on the flow rates. Growth process was monitored by in-situ reflection high energy electron diffraction (RHEED). Crystallinity and structural quality of the films were addressed by x-ray diffraction (XRD) using θ-2θ and ω rocking curve measurements for (0002) and (1015) reflections. To investigate threading dislocation configuration in the film cross-sectional transmission electron microscopy (TEM) was performed. TEM specimens were prepared by mechanical polishing with a tripod polisher followed by Ar+ ion milling for the electron transparency. TEM observations were pergprmed with the AlN <1-100> zone axis.

    3. Results and Discussion

    Figure 1 shows in-situ RHEED patterns in the progress of nitridation. In the beginning, only the diffraction pattern of sapphire was seen but as nitridation progressed, the diffraction pattern of AlN was appeared. A thin AlN layer was formed through nitridation and worked as a buffer layer to decreases lattice misfit between sapphire substrate and AIN film The epitaxy relationships between AlN and c-plane sapphire substrate are [1120]AlN//[1010] Al2O3 and [1010]AlN//[1120]Al2O3. Figure 2 shows in-situ RHEED patterns for AlN films grown with independent Al/N ratio and growth rates, which were observed from the azimuth [1120]AlN and [1010]AlN. Figures 2(a-d) are RHEED patterns for the samples with AI/N ratio of 3.1 and 3.3 grown in high growth rate (HGR) of 730 nm/ hr. The spotty RHEED patterns show the characteristics of three-dimensional growth but RHEED patterns in Figs. 2(e, f) with AI/N ratio of 3.5 show the streaky patterns, which implies two-dimensional growth. Figures 2(g-j) are RHEED patterns for the samples grown in low growth rate (LGR) of 315 nm/hr. The RHEED characteristics for HGR and LGR were similar but the RHEED patterns for LGR growth were a little bit streaky than ones for HGR growth. In each growth rate scheme, higher Al/N ratio resulted in streaky RHEED patterns.

    The structural properties of grown AlN films were investigated by XRD 2θ-ω measurements as shown in Fig. 3. The (0002) AlN peak and (0006) Al2O3 peak appear in both HGR and LGR samples with Al/N ratios of 3.1 and 3.3. It means that most part of the AlN thin film grows parallel to the c-plane sapphire. However, Al/N ratio of 3.5 samples in both HGR and LGR showed (111) Al and (200) Al peaks, which implied the existence of cubic phase Al.20) It was concluded that AlN single crystal is hardly achieved as the Al/N ratio over 3.5 in our experiments independent of the growth rates. Therefore, we thought that the epitaxial growth is more dependent on the Al/N ratio than the growth rate.

    XRD ω-rocking curves (XRCs) measurements of (0002) and (1015) AlN were carried out to investigate crystalline quality of the AlN films. Figures 4(a, c) shows XRCs of (0002) reflection which can be a criteria for the deformation of tilt by screw and mixed dislocations that exist in the thin film.21,22) The lowest (0002) full width at half maximum (FWHM) value was obtained from HGR sample with Al/ N ratio 3.1 and it was 0.04°, as shown in Fig. 5. On the other hand, LGR sample with Al/N ratio 3.5 has highest FWHM value of 0.45°. The HGR (1015) XRCs reflect mostly the twist of columnar AlN films (Fig. 4(b, d)). The FWHM value of (1015) XRCs is mostly affected by edge and mixed dislocations.21) The lowest FWHM value for (1015) XRC was 0.22° from HGR sample with Al/N ratio 3.1. On the other hand, the highest FWHM value for (1015) XRC was 0.31° from LGR sample with Al/N ratio 3.1, as shown in Fig. 5. In case of (0002) XRCs, the lowest FWHM value for (0002) XRC was 0.04° from HGR sample with Al/N ratio 3.1. On the other hand, the highest FWHM value for (0002) XRC was 0.45° from LGR sample with Al/N ratio 3.5, as shown in Fig 5. The FWHM values of (1015) XRCs were changed from 0.22 to 0.31° by changing the Al/N ratios but the ones of (0002) XRCs were changed from 0.04 to 0.45° by changing the Al/N ratios. It means that the structural deformation due to the dislocations was affected a little by Al/N ratio in (1015) XRCs but affected by much for (0002) XRCs.

    That is, generation of edge dislocation was little bit dependent on the Al/N ratio but generation of screw/ mixed dislocations was strongly dependent on the Al/N ratio. We think that the different flux ratio results in changes in migration of atoms on the growth front, which affected the microstructure and crystallinity of the grown films. In the view point of growth rate the AlN films with HGR showed better crystal quality than LGR films in overall as addressed by FWHM values of (0002) and (1015) XRCs.

    Figures 6(a) and 7(a) are bright field (BF) cross-sectional TEM micrographs of the AlN films with Al/N ratio of 3.1 in HGR and Al/N ratio of 3.3 and LGR. Figures 6(b, c) and 7(b, c) show TEM micrographs under two-beam diffraction conditions with g = [0002] and [1010]. Figures 6(d) and 7(d) are selected area electron diffraction (SAED) patterns at AlN/Al2O3 taken along <1120> AlN zone axis. The AlN/Al2O3 epitaxial relationship can be confirmed again as (0001) AlN//(0001) Al2O3 and [1100]AlN//[1210] Al2O. We can determine the dislocation types based on the visible and invisible criteria of bg=0. Where b is the burgers vector of the dislocation and g is the diffraction vector. By comparing images of Figs. 6(b, c) and Figs. 7(b, c) with different diffraction vectors, the dislocation types were defined. Screw type threading dislocation with a b of <0001> and a mixed type threading dislocation with a b of 1/3<1123> can be observed with g = [0002], while edge type threading dislocation with a b of 1/ 3<1120> and mixed type threading dislocations with a b of 1/3<1123> can be observed with g = [1010], as the dislocation types were marked to e (edge), m (mixed), s (screw) on the figures. Base on the cross-sectional TEM observation, we noticed that sample of Al/N ratio 3.1 in HGR has less dislocation than the sample of Al/N ratio 3.3 in LGR and it is agreeable to prior XRCs results. From the XRCs, FWHM values for HGR sample with AlN ratio of 3.1 showed larger FWHM for of (1015) than that of (0002) and this is agreed with the more edge dislocations in Fig. 6 than more screw and mixed dislocations in LGR sample with AlN ratio of 3.5 in Fig. 7, which showed larger FWHM for of (0002) that that of (1015).

    4. Conclusions

    In this study, we investigated the effect of Al/N source ratios and growth rates on the growth and structural properties of AlN films on c-plane sapphires. Both growth rates and Al/N ratios affected crystal qualities of AlN films grown by PAMBE. The FWHM values of (1015) XRCs were changed from 0.22 to 0.31° by changing the Al/N ratios but the ones of (0002) XRCs were changed from 0.04 to 0.45° by changing the Al/N ratios. It means that the structural deformation due to the dislocations was affected a little by Al/N ratio in (1015) XRCs but affected by much for (0002) XRCs. In the view point of growth rate the AlN films with HGR showed better crystal quality than LGR films in overall as addressed by FWHM values of (0002) and (1015) XRCs. We observed that the main defect dislocation was edge type threading dislocations in the HGR with Al/N ratio of 3.1 sample. On the other hand, in the LGR with Al/N ratio of 3.3 sample, there are much more of mixed and screw type threading dislocations. The lowest (0002) XRC FWHM value was obtained from HGR sample with Al/N ratio 3.1 and it was 0.04° and the lowest FWHM value for (1015) XRC was 0.22° from HGR sample with Al/N ratio 3.1.

    Acknowledgment

    This research was supported by the research fund of Chungnam National University (Grant No. 2017-1675-01).

    Figure

    MRSK-29-10-579_F1.gif

    In-situ RHEED patterns during the nitridation of sapphire substrate.

    MRSK-29-10-579_F2.gif

    RHEED patterns of AlN films grown with different Al/N ratio and growth rate (a, b) HGR Al/N ratio of 3.1, (c, d) HGR Al/ N ratio of 3.3, (e, f) HGR Al/N ratio of 3.5, (g,h) LGR Al/N ratio of 3.1, (i,j) LGR Al/N ratio of 3.3, (k,l) LGR Al/N ratio of 3.5. All RHEED patterns are obtained along with [1120] AlN and [1010] AlN directions.

    MRSK-29-10-579_F3.gif

    XRD 2θ-ω spectra for the AlN films grown with different Al/N ratios and growth rates. (a) HGR and (b) LGR.

    MRSK-29-10-579_F4.gif

    XRD ω rocking curves of (a) HGR (0002) AlN, (b) LGR(0002) AlN (c) HGR (1015) AlN, (d) LGR(1015) AlN.

    MRSK-29-10-579_F5.gif

    Plot of the FWHM values for (0002) and (1015) XRCs from the AlN films with different growth rates and Al/N ratios.

    MRSK-29-10-579_F6.gif

    (a) Bright field TEM micrograph of the AlN film grown with the Al/N ratio 3.1 in HGR. (b) Two-beam bright field TEM micrograph with g = [0002] and (c) with g = [1010]. (d) Corresponding selected area electron diffraction patterns of AlN/Al2O3 taken along <1120> AlN zone axis.

    MRSK-29-10-579_F7.gif

    (a) Bright field TEM micrograph of the AlN film grown with the Al/N ratio 3.3 in LGR. (b) Two-beam bright field TEM micrograph with g = [0002] and (c) with g=[1010]. (d) Corresponding selected area electron diffraction patterns of AlN/Al2O3 taken along <1120> AlN zone axis.

    Table

    Summary of detailed growth conditions for AlN films grown with different growth rates and Al/N ratios.

    Reference

    1. S. Nakamura, M. Senoh and T. Mukai, Jpn. J. Appl. Phys., 32 L8 (1993).
    2. J. Wu, W. Walukiewicz, K. M. Yu, J.W. Ager, E. E. Haller, H. Lu, W. J. Schaff, Y. Saito and Y. Nanishi, Appl. Phys. Lett., 80, 3967 (2002).
    3. ISO 21348 Definitions of Solar Irradiance Spectral Categories. Space Environment Technologies Home Page. Retrieved June 6, 2019 from http://www.spacewx.com/pdf/SET_21348_2004.pdf.
    4. J.-Y. Shin, S.-J. Kim, D.-K. Kim and D.-H. Kang, Appl. Environ. Microbiol., 82, 2 (2016).
    5. Y. Nagasawa and A. Hirano, Appl. Sci., 8, 1264 (2018).
    6. M. Kneissl, F. Mehnke, C. Kuhn, C. Reich, M. Guttmann, J. Enslin, T. Wernicke, A. Knauer, V. Kueller, U. Zeimer, M. Lapeyrade, J. Raß, N. Lobo-Ploch, T. Kolbe, J. Glaab, S. Einfeldt and M. Weyers, 2015 IEEE Summer Top. Meet. Ser., p.9 (2015).
    7. P. B. Perry and R. F. Rutz, Appl. Phys. Lett., 33, 319 (1978).
    8. T. Onuma, S. F. Chichibu, T. Sota, K. Asai, S. Sumiya, T. Shibata and M. Tanaka, Appl. Phys. Lett., 81, 652 (2002).
    9. W. M. Yim, E. J. Stofko, P. J. Zanzucchi, J. I. Pankove, M. Ettenberg and S. L. Gilbert, J. Appl. Phys., 44, 292 (1973).
    10. L. Chen, B. J. Skromme, R. F. Dalmau, R. Schlesser, Z. Sitar, C. Chen, W. Sun, J. Yang, M. A. Khan, M. L. Nakarmi, J. Y. Lin and H.-X. Jiang, Appl. Phys. Lett., 85, 4334 (2004).
    11. A. Khan, K. Balakrishnan and T. Katona, Nat. Photonics., 2, 77 (2008).
    12. V. Adivarahan, A. Heidari, B. Zhang, Q. Fareed, M. Islam, S. Hwang, K. Balakrishnan and A. Khan, Appl. Phys. Express, 2, 092102 (2009).
    13. H. Hirayama, Y. Tsukada, T. Maeda, N. Kamata, Appl. Phys. Express., 3, 031002 (2010).
    14. B. P. Yonkee, E. C. Young, S. P. DenBaars, S. Nakamura and J. S. Speck, Appl. Phys. Lett., 109 , 191104 (2016).
    15. Y. Liao, C. Thomidis, C. Kao and T. D. Moustakas, Appl. Phys. Lett., 98, 081110 (2011).
    16. H. Hirayama, S. Fujikawa, N. Noguchi, J. Norimatsu, T. Takano, K. Tsubaki and N. Kamata, Phys. Status Solidi A, 206, 1176 (2009).
    17. M. Kneissl, T. Kolbe, C. Chua, V. Kueller, N. Lobo, J. Stellmach, A. Knauer, H. Rodriguez, S. Einfeldt, Z. Yang, N. M. Johnson and M. Weyers, Semicond. Sci. Technol., 26, 014036 (2010).
    18. T. M. Altahtamouni, J. Y. Lin and H. X. Jiang, AIP Adv., 4, 047122 (2014).
    19. M. A. Khan, N. Maeda, M. Jo, Y. Akamatsu, R. Tanabe, Y. Yamada and H. Hirayama, J. Mater. Chem. C., 7, 143 (2019).
    20. Q. X. Guo, K. Yahata, T. Tanaka, M. Nishio and H. Ogawa, J. Cryst. Growth., 257, 123 (2003).
    21. J. Bai, T. Wang, P. J. Parbrook, K. B. Lee and A. G. Cullis, J. Cryst. Growth., 282, 290 (2005).
    22. L. Lu, B. Shen, F. J. Xu, J. Xu, B. Gao, Z. J. Yang, G. Y. Zhang, X. P. Zhang, J. Xu and D. P. Yu, J. Appl. Phys., 102, 033510 (2007).