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近几年, 由于用分子束外延法在SrTiO3衬底表面制备的单层FeSe具有很高的超导转变温度而引发了极大的研究热潮, 随之而来的是对多层FeSe薄膜日益增长的研究兴趣. 但目前还没有对成功生长高质量多层FeSe薄膜的详细报道. 本文利用高能电子衍射仪(RHEED) 实时监控在不同生长条件下制备的多层FeSe薄膜, 发现在FeSe薄膜的生长初期, RHEED图像的强度演化基本符合台阶密度模型的描述特征, 即台阶密度 正相关于衍射条纹强度. FeSe(02)衍射条纹的强度在第一生长周期内呈现稳定而明显的峰型振荡, 而且不受高能电子掠射角的影响, 最适合用来标定FeSe薄膜的厚度. 结合扫描隧道显微镜对FeSe薄膜质量的原位观察, 确定了制备多层FeSe薄膜的最佳生长条件, 为FeSe薄膜的物性研究提供了重要的材料基础.Single-layer FeSe film grown on SrTiO3(001) surface (STO surface) by molecular beam epitaxy has aroused a great research boom ever since the discovery of its huge superconductive energy gap which indicates a possible critical temperature (Tc) higher than the liquid nitrogen temperature. The interface enhanced superconductivity with a Tc above 100 K is revealed in an in situ electrical transport measurement by using a four-point probe installed in a scanning tunneling microscope (STM). Consequent research interest in multi-layer FeSe films grown on STO surface is also increasing. The quality of thick FeSe film, however, has not been well studied yet in previous studies, although it is related to the sample properties including superconductivity. Here, reflection high-energy electron diffraction (RHEED) is used to monitor the growths of multi-layer FeSe thin films on STO surface under different growth conditions. Combing the RHEED results with STM observations taken at various FeSe coverages, we find that the intensity evolution of the RHEED pattern in the early growth stage can be well explained by the step density model but not by the widely known facet model. The intensity evolution of the FeSe(02) diffraction streak exhibits a single-peak oscillation in the growing of the first layer of FeSe. As the oscillation does not depend on the grazing angle of the high-energy electron beam, the FeSe(02) diffraction streak is very suitable for calibrating the FeSe growth rate. In contrast, the intensity of the specular spot exhibits different evolution pattern when the grazing angle of electron beam is changed. It is found in STM observations that only at an appropriate substrate temperature and a growth rate can the high-quality multi-layer FeSe films be grown on STO substrates. If the growth temperature is too high, the FeSe molecules nucleate into islands so that FeSe films with various thickness values eventually come into being on the STO surface. If the growth temperature is too low, a different phase of FeSe film is formed. The optimal growth temperature is in a range from 400 ℃ to 430 ℃, within which a two-layer FeSe film grown at a low rate (0.15 layer/min) coveres the whole STO surface with a negligible number of small FeSe islands. In contrast, a larger growth rate is necessary for growing thicker FeSe film. This is because FeSe islands tend to come into form at steps when the growth rate is too low, which is more distinct in a thicker FeSe film. An STM image of 80-layer FeSe film grown under an optimal condition, i.e., the substrate temperature of 420 ℃ and the growth rate of 2.3 layer/min, shows that it is in a perfect layer-by-layer growth mode. These experimental results are useful for growing high-quality multi-layer FeSe films on STO substrates, which could be critical for studying their physical properties and relevant physical phenomena.
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Keywords:
- FeSe /
- reflection high-energy electron diffraction /
- molecular beam epitaxy /
- step-density
[1] Hsu F C, Luo J Y, Yeh K W, Chen T K, Huang T W, Wu Phillip M, Lee Y C, Huang Y L, Chu Y Y, Yan D C, Wu M K 2008 Proc. Natl. Acad. Sci. USA 105 14262
[2] Wang Q Y, Li Z, Zhang W H, Zhang Z C, Zhang J S, Li W, Ding H, Ou Y B, Deng P, Chang K, Wen J, Song C L, He K, Jia J F, Ji S H, Wang Y Y, Wang L L, Chen X, Ma X C, Xue Q K 2012 Chin. Phys. Lett. 29 037402
[3] Ge J F, Liu Z L, Liu C H, Gao C L, Qian D, Xue Q K, Liu Y, Jia J F 2015 Nat. Mater. 14 285
[4] Miyata Y, Nakayama K, Sugawara K, Sato T, Takahashi T 2015 Nat. Mater. 14 775
[5] Wang L L, Ma X C, Chen X, Xue Q K 2013 Chin. Phys. B 22 086801
[6] Imai Y, Sawada Y, Nabeshima F, Maedaet A 2015 Proc. Natl. Acad. Sci. USA 112 1937
[7] Medvedev S, McQueen T M, Troyan I A, Palasyuk T, Eremets M I, Cava R J, Naghavi1 S, Casper1 F, Ksenofontov V, Wortmann G, Felseret C 2009 Nat. Mater. 8 630
[8] Margadonna S, Takabayashi Y, Ohishi Y, Mizuguchi Y, Takano Y, Kagayama T, Nakagawa T, Takata M, Prassides K 2009 Phys. Rev. B 80 064206
[9] Jung S G, Lee N H, Choi E M, Kang W N, Lee S I, Hwang T J, Kim D H 2010 Physica C 470 1977
[10] Chen L, Tsai C F, Zhu Y Y, Bi Z X, Wang H Y 2011 Physica C 471 515
[11] Li Z, Peng J P, Zhang H M, Zhang W H, Ding H, Deng P, Chang K, Song C L, Ji S H, Wang L L, He K, Chen X, Xue Q K, Ma X C 2014 J. Phys: Condens. Mat. 26 265002
[12] Wang M, Ou Y B, Li F S, Zhang W H, Tang C J, Wang L L, Xue Q K, Ma X C 2014 Acta Phys. Sin. 63 027401 (in Chinese) [王萌, 欧云波, 李坊森, 张文号, 汤辰佳, 王立莉, 薛其坤, 马旭村 2014 63 027401]
[13] Hanzawa K, Sato H, Hiramatsu H, Kamiya T, Hosono H 2015 arXiv: 1508 07689
[14] Resh J, Jamison K D, Strozier J, Bensaoula A, Ignatiev A 1989 Phys. Rev. B 40 11799
[15] Nemcsics 2002 Thin Solid Films 412 60
[16] Zhang J, Neave J H, Dobson P J, Joyce B A 1987 Appl. Phys. A 42 317
[17] Tan S, Zhang Y, Xia M, Ye Z R, Chen F, Xie X, Peng R, Xu D F, Fan Q, Xu H C, Jiang J, Zhang T, Lai X H, Xiang T, Hu J P, Xie B P, Feng D L 2013 Nat. Mater. 12 634
[18] Huang D, Song C L, Webb T A, Fang S A, Chang C Z, Moodera J S, Kaxiras E, Hoffman J E 2015 Phys. Rev. Lett. 115 017002
[19] Neave J H, Joyce B A, Dobson P J, Norton N 1983 Appl. Phys. A 31 1
[20] Blger B, Larsen P K 1986 Rev. Sci. Instrum. 57 1363
[21] Chen K M, Zhou T C, Fan Y L, Sheng C, Yu M R 1990 Acta Phys. Sin. 39 1937 (in Chinese) [陈可明, 周铁城, 樊永良, 盛篪, 俞鸣人 1990 39 1937]
[22] Wang Z L 1993 Rep. Prog. Phys. 56 997
[23] Shin B, Leonard J P, McCamy J W, Aziz Michael J 2007 J. Vac. Sci. Technol. A 25 221
[24] Okamoto H 1991 J. Phase. Equilib. Diff. 12 383
[25] Ohring M 2001 Materials Science of Thin Films 7.6.1 (San Diego: Academic press) pp340-345
[26] Clarke S, Vvedensky D D 1988 J. Appl. Phys. 63 2272
[27] Braun W, Trampert A, Dweritz L, Ploog K H 1997 Phys. Rev. B 55 1689
[28] Sudijono J, Johnson M D, Snyder C W, Elowitz M B, Orr B G 1992 Phys. Rev. Lett. 69 2811
[29] Korte U, Maksym P A 1997 Phys. Rev. Lett. 78 2381
[30] Zhang W, Li Z, Li F, Zhang H M, Peng J P, Tang C J, Wang Q Y, He K, Chen X, Wang L L, Ma X C, Xue Q K 2014 Phys. Rev. B 89 060506
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[1] Hsu F C, Luo J Y, Yeh K W, Chen T K, Huang T W, Wu Phillip M, Lee Y C, Huang Y L, Chu Y Y, Yan D C, Wu M K 2008 Proc. Natl. Acad. Sci. USA 105 14262
[2] Wang Q Y, Li Z, Zhang W H, Zhang Z C, Zhang J S, Li W, Ding H, Ou Y B, Deng P, Chang K, Wen J, Song C L, He K, Jia J F, Ji S H, Wang Y Y, Wang L L, Chen X, Ma X C, Xue Q K 2012 Chin. Phys. Lett. 29 037402
[3] Ge J F, Liu Z L, Liu C H, Gao C L, Qian D, Xue Q K, Liu Y, Jia J F 2015 Nat. Mater. 14 285
[4] Miyata Y, Nakayama K, Sugawara K, Sato T, Takahashi T 2015 Nat. Mater. 14 775
[5] Wang L L, Ma X C, Chen X, Xue Q K 2013 Chin. Phys. B 22 086801
[6] Imai Y, Sawada Y, Nabeshima F, Maedaet A 2015 Proc. Natl. Acad. Sci. USA 112 1937
[7] Medvedev S, McQueen T M, Troyan I A, Palasyuk T, Eremets M I, Cava R J, Naghavi1 S, Casper1 F, Ksenofontov V, Wortmann G, Felseret C 2009 Nat. Mater. 8 630
[8] Margadonna S, Takabayashi Y, Ohishi Y, Mizuguchi Y, Takano Y, Kagayama T, Nakagawa T, Takata M, Prassides K 2009 Phys. Rev. B 80 064206
[9] Jung S G, Lee N H, Choi E M, Kang W N, Lee S I, Hwang T J, Kim D H 2010 Physica C 470 1977
[10] Chen L, Tsai C F, Zhu Y Y, Bi Z X, Wang H Y 2011 Physica C 471 515
[11] Li Z, Peng J P, Zhang H M, Zhang W H, Ding H, Deng P, Chang K, Song C L, Ji S H, Wang L L, He K, Chen X, Xue Q K, Ma X C 2014 J. Phys: Condens. Mat. 26 265002
[12] Wang M, Ou Y B, Li F S, Zhang W H, Tang C J, Wang L L, Xue Q K, Ma X C 2014 Acta Phys. Sin. 63 027401 (in Chinese) [王萌, 欧云波, 李坊森, 张文号, 汤辰佳, 王立莉, 薛其坤, 马旭村 2014 63 027401]
[13] Hanzawa K, Sato H, Hiramatsu H, Kamiya T, Hosono H 2015 arXiv: 1508 07689
[14] Resh J, Jamison K D, Strozier J, Bensaoula A, Ignatiev A 1989 Phys. Rev. B 40 11799
[15] Nemcsics 2002 Thin Solid Films 412 60
[16] Zhang J, Neave J H, Dobson P J, Joyce B A 1987 Appl. Phys. A 42 317
[17] Tan S, Zhang Y, Xia M, Ye Z R, Chen F, Xie X, Peng R, Xu D F, Fan Q, Xu H C, Jiang J, Zhang T, Lai X H, Xiang T, Hu J P, Xie B P, Feng D L 2013 Nat. Mater. 12 634
[18] Huang D, Song C L, Webb T A, Fang S A, Chang C Z, Moodera J S, Kaxiras E, Hoffman J E 2015 Phys. Rev. Lett. 115 017002
[19] Neave J H, Joyce B A, Dobson P J, Norton N 1983 Appl. Phys. A 31 1
[20] Blger B, Larsen P K 1986 Rev. Sci. Instrum. 57 1363
[21] Chen K M, Zhou T C, Fan Y L, Sheng C, Yu M R 1990 Acta Phys. Sin. 39 1937 (in Chinese) [陈可明, 周铁城, 樊永良, 盛篪, 俞鸣人 1990 39 1937]
[22] Wang Z L 1993 Rep. Prog. Phys. 56 997
[23] Shin B, Leonard J P, McCamy J W, Aziz Michael J 2007 J. Vac. Sci. Technol. A 25 221
[24] Okamoto H 1991 J. Phase. Equilib. Diff. 12 383
[25] Ohring M 2001 Materials Science of Thin Films 7.6.1 (San Diego: Academic press) pp340-345
[26] Clarke S, Vvedensky D D 1988 J. Appl. Phys. 63 2272
[27] Braun W, Trampert A, Dweritz L, Ploog K H 1997 Phys. Rev. B 55 1689
[28] Sudijono J, Johnson M D, Snyder C W, Elowitz M B, Orr B G 1992 Phys. Rev. Lett. 69 2811
[29] Korte U, Maksym P A 1997 Phys. Rev. Lett. 78 2381
[30] Zhang W, Li Z, Li F, Zhang H M, Peng J P, Tang C J, Wang Q Y, He K, Chen X, Wang L L, Ma X C, Xue Q K 2014 Phys. Rev. B 89 060506
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