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本文利用飞秒瞬态吸收光谱技术, 在近红外波段对Ge掺杂GaN(GaN: Ge)晶体进行了超快载流子动力学研究. 在双光子激发下, 瞬态吸收动力学呈现出双指数衰减, 其中慢过程寿命随着泵浦光强增加而增加. 瞬态吸收响应随着探测波长而单调增强, 并在约1050 nm处由空穴吸收占据主导. 利用简化模型模拟载流子动力学发现, GaN: Ge中碳杂质形成的深受主能级对空穴有很强的俘获能力, 并且引起了缺陷发光. 在较适中的载流子注入下, n型GaN中的载流子寿命可以通过控制缺陷浓度和载流子浓度来共同调控, 使其可应用于发光二极管和光通信等不同的领域.Gallium nitride (GaN) is a key material in blue light-emitting devices and is recognized as one of the most important semiconductors after Si. Its outstanding thermal conductivity, high saturation velocity, and high breakdown electric field have enabled the use of GaN for high-power and high-frequency devices. Although lots of researches have been done on the optical and optoelectrical properties of GaN, the defect-related ultrafast dynamics of the photo-excitation and the relaxation mechanism are still completely unclear at present, especially when the photo-generated carrier concentration is close to the defect density in n-type GaN. The transient absorption spectroscopy has become a powerful spectroscopic method, and the advantages of this method are contact-free, highly sensitive to free carriers, and femtosecond time resolved. In this article, by employing optical pump and infrared probe spectroscopy, we investigate the ultrafast photo-generated carriers dynamics in representative high-purity n-type and Ge-doped GaN (GaN:Ge) crystal. The transient absorption response increased as probe wavelengths increased, and hole-related absorption was superior to electron-related absorption, especially at 1050 nm. The transient absorption kinetics in GaN:Ge appeared to be double exponential decay under two-photon excitation. By modelling the carrier population dynamics in energy levels, which contained both radiative and non-radiative defect states, the carrier dynamics and carrier capture coefficients in GaN: Ge can be interpreted and determined unambiguously. The faster component (30–60 ps) of absorption decay kinetics corresponded to the capturing process of holes by negatively charged acceptor CN. However, the capturing process was limited by the recombination of electron and trapped holes under higher excitation after the saturation of deep acceptors. As a result, the slower component decayed slower as the excitation fluence increased. Moreover, the experimental and theoretical results found that, the carrier lifetime in n-GaN can be modulated by controlling the defect density and carrier concentration under a moderate carrier injection, making GaN applicable in different fields such as LED and optical communication.
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Keywords:
- carrier dynamics /
- transient absorption spectroscopy /
- two-photon absorption /
- GaN
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图 1 (a) GaN: Ge晶体的线性吸收谱, 内插图为2PE下的发光图片; (b)不同脉冲能量激发下GaN: Ge的开孔Z扫描曲线, 实线为理论拟合曲线
Fig. 1. (a) Linear absorption spectrum of GaN: Ge crystal. The inset shows the two-photon excited photoluminescence photograph of sample; (b) open-aperture Z-scan data of GaN: Ge at several input pulse energies, the solid lines are theoretical fitting curves.
图 2 (a) 2PE下GaN: Ge的超快瞬态吸收光谱, 激发能流为0.8 mJ/cm2; (b) 1PE下GaN: Ge的超快瞬态吸收光谱, 激发能流为0.5 mJ/cm2. 内插图均为可见光探测下的结果
Fig. 2. (a) Ultrafast TAS in GaN: Ge using 2PE under the excitation fluence of 0.8 mJ/cm2; (b) ultrafast TAS in GaN: Ge using 1PE under the excitation fluence of 0.5 mJ/cm2. The insets show the TAS probed at visible wavelengths.
图 3 (a)不同激发能流下GaN: Ge的瞬态吸收动力学, 探测波长为1050 nm, 实线为双指数拟合曲线, 内插图为较短时间尺度下(7 ps)的数据; (b)不同激发能流下瞬态吸收衰减曲线拟合得到的快速和慢速弛豫寿命(分别为τ1和τ2)
Fig. 3. (a) The transient absorption kinetics in GaN: Ge under various excitation fluence probed at 1050 nm, the solid lines denote the theoretical curves using bi-exponential decay, and the inset illustrates the transient absorption kinetics in a 7 ps time window; (b) the fast and slow relaxation time (τ1 and τ2, respectively) extracted from transient absorption kinetics under various excitation fluence.
图 4 用于模拟2PE下GaN载流子动力学的能带示意图. 直虚线箭头表示无辐射跃迁, 向下曲线箭头表示通过辐射复合产生的发光
Fig. 4. Energy band diagram used to model the carrier dynamics of GaN under 2PE. Straight broken arrows denote non-radiative transitions and curvy downwards arrows denote emissions via radiative recombination.
图 5 利用载流子复合模型拟合和模拟不同激发能流下GaN: Ge的超快载流子弛豫动力学 (a)实验结果拟合; (b)更大的激发能流和1PE情况
Fig. 5. Fitting and simulation of ultrafast carrier relaxation dynamics in GaN: Ge using carrier recombination model: (a) The fitting of experimental results; (b) under higher excitation fluence and 1PE.
图 6 1PE(0.8 mJ/cm2)和2PE(1.6 mJ/cm2)下GaN:Ge在通讯波段1310 nm下的超快瞬态吸收动力学
Fig. 6. Ultrafast transient absorption kinetics in GaN:Ge probed at communication band 1310 nm under both 1PE (0.8 mJ/cm2) and 2PE (1.6 mJ/cm2).
表 1 用于模拟实验结果使用和确定的参数. Ni和τnRad的数值为预估值, BRad数值来自参考文献[18], Cni, Cpi和S数值为拟合实验数据确定的参数
Table 1. Parameters used/determined to model the experimental results. The values of Ni and τnRad were estimated. The value of BRad was extracted from Ref. [18]. The values of Cni, Cpi and S were determined by fitting the data.
参数 数值 Ni 1 × 1016 cm–3 Cni (2.7 ± 0.8) × 10–9 cm3·s–1 Cpi (5.9 ± 0.7) × 10–7 cm3·s–1 τnRad 40 ns BRad 3 × 10–11 cm3·s–1 S 7 ± 1 
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[1] Nakamura S, Pearton S, Fasol G 2013 The Blue Laser Diode: the Complete Sstory (2nd Ed.) (Berlin: Springer-Verlag) pp3,4
[2] Pearton S J, Ren F 2000 Adv. Mater. 12 1571
Google Scholar
[3] Xiong C, Pernice W, Ryu K K, Schuck C, Fong K Y, Palacios T, Tang H X 2011 Opt. Express 19 10462
Google Scholar
[4] Bruch A W, Xiong C, Leung B, Poot M, Han J, Tang H X 2015 Appl. Phys. Lett. 107 141113
Google Scholar
[5] Monteagudo-Lerma L, Naranjo F B, Valdueza-Felip S, Jiménez-Rodríguez M, Monroy E, Postigo P A, Corredera P, González-Herráez M 2016 Phys. Status Solidi A 213 1269
Google Scholar
[6] van de Walle C G, Neugebauer J 2004 J. Appl. Phys. 95 3851
Google Scholar
[7] Reshchikov M A, Morkoc H 2005 J. Appl. Phys. 97 061301
Google Scholar
[8] Chichibu S F, Uedono A, Kojima K, Ikeda H, Fujito K, Takashima S, Edo M, Ueno K, Ishibashi S 2018 J. Appl. Phys. 123 161413
Google Scholar
[9] Jarašiūnas K, Malinauskas T, Nargelas S, Gudelis V, Vaitkus J V, Soukhoveev V, Usikov A 2010 Phys. Status Solidi B 247 1703
Google Scholar
[10] Iwinska M, Takekawa N, Ivanov V Y, Amilusik M, Kruszewski P, Piotrzkowski R, Litwin-Staszewska E, Lucznik B, Fijalkowski M, Sochacki T, Teisseyre H, Murakami H, Bockowski M 2017 J. Cryst. Growth 480 102
Google Scholar
[11] Ueno K, Arakawa Y, Kobayashi A, Ohta J, Fujioka H 2017 Appl. Phys. Express 10 101002
Google Scholar
[12] Götz W, Johnson N, Chen C, Liu H, Kuo C, Imler W 1996 Appl. Phys. Lett. 68 3144
Google Scholar
[13] Götz W, Kern R S, Chen C H, Liu H, Steigerwald D A, Fletcher R M 1999 Mater. Sci. Eng. B 59 211
Google Scholar
[14] Nenstiel C, Bügler M, Callsen G, Nippert F, Kure T, Fritze S, Dadgar A, Witte H, Bläsing J, Krost A, Hoffmann A 2015 Phys. Status SolidiRRL 9 716
Google Scholar
[15] Ajay A, Lim C B, Browne D A, Polaczyński J, Bellet-Amalric E, Bleuse J, den Hertog M I, Monroy E 2017 Nanotechnology 28 405204
Google Scholar
[16] Zhong Y, Wong K S, Zhang W, Look D C 2006 Appl. Phys. Lett. 89 022108
Google Scholar
[17] Williams K W, Monahan N R, Koleske D D, Crawford M H, Zhu X Y 2016 Appl. Phys. Lett. 108 141105
Google Scholar
[18] Ščajev P, Jarašiūnas K, Okur S, Özgür Ü, Morkoç H 2012 J. Appl. Phys. 111 023702
Google Scholar
[19] Ohashi Y, Katayama K, Shen Q, Toyoda T 2009 J. Appl. Phys. 106 063515
Google Scholar
[20] Upadhya P C, Martinez J A, Li Q, Wang G T, Swartzentruber B S, Taylor A J, Prasankumar R P 2015 Appl. Phys. Lett. 106 263103
Google Scholar
[21] Chen Y T, Yang C Y, Chen P C, Sheu J K, Lin K H 2017 Sci. Rep. 7 5788
Google Scholar
[22] Dugar P, Kumar M, T. C S K, Aggarwal N, Gupta G 2015 RSC Adv. 5 83969
Google Scholar
[23] Marcinkevičius S, Uždavinys T K, Foronda H M, Cohen D A, Weisbuch C, Speck J S 2016 Phys. Rev. B 94 235205
Google Scholar
[24] Fang Y, Yang J, Yong Y, Wu X, Xiao Z, Zhou F, Song Y 2016 J. Phys. D: Appl. Phys. 49 045105
Google Scholar
[25] 方宇 2016 博士学位论文 (苏州: 苏州大学)
Fang Y 2016 Ph. D. Dissertation (Suzhou: Soochow University) (in Chinese)
[26] 聂媱, 王友云, 吴雪琴, 方宇 2019 激光与光电子学进展 56 063201
Google Scholar
Nie Y, Wang Y, Wu X, Fang Y 2019 Laser & Optoelectronics Progress 56 063201
Google Scholar
[27] Zhao W, Palffy-Muhoray P 1993 Appl. Phys. Lett. 63 1613
Google Scholar
[28] Gu H, Ren G, Zhou T, Tian F, Xu Y, Zhang Y, Wang M, Zhang Z, Cai D, Wang J 2016 J. Alloys Compd. 674 218
Google Scholar
[29] Zhang Y M, Wang J F, Cai D M, Ren G Q, Xu Y, Wang M Y, Hu X J, Xu K 2020 Chin. Phys. B 29 026104
Google Scholar
[30] Kioupakis E, Rinke P, Schleife A, Bechstedt F, van de Walle C G 2010 Phys. Rev. B 81 241201
Google Scholar
[31] Ščajev P, Jarašiūnas K, Özgür Ü, Morkoç H, Leach J, Paskova T 2012 Appl. Phys. Lett. 100 022112
Google Scholar
[32] Ridley B K 2013 Quantum Processes in Semiconductors (5th Ed.) (Oxford: Oxford University Press) pp194–195
[33] Lyons J, Janotti A, van de Walle C G 2010 Appl. Phys. Lett. 97 152108
Google Scholar
[34] Demchenko D O, Diallo I C, Reshchikov M A 2013 Phys. Rev. Lett. 110 087404
Google Scholar
[35] Zhang H S, Shi L, Yang X B, Zhao Y J, Xu K, Wang L W 2017 Adv. Opt. Mater. 5 1700404
Google Scholar
[36] Christenson S G, Xie W, Sun Y, Zhang S 2015 J. Appl. Phys. 118 135708
Google Scholar
[37] Wu S, Yang X, Zhang H, Shi L, Zhang Q, Shang Q, Qi Z, Xu Y, Zhang J, Tang N 2018 Phys. Rev. Lett. 121 145505
Google Scholar
[38] Fang Y, Zhou F, Yang J, Wu X, Xiao Z, Li Z, Song Y 2015 Appl. Phys. Lett. 106 131903
Google Scholar
[39] Reshchikov M A, Albarakati N M, Monavarian M, Avrutin V, Morkoç H 2018 J. Appl. Phys. 123 161520
Google Scholar
[40] Reshchikov M A, Korotkov R Y 2001 Phys. Rev. B 64 115205
Google Scholar
[41] Dreyer C E, Alkauskas A, Lyons J L, Speck J S, Van de Walle C G 2016 Appl. Phys. Lett. 108 141101
Google Scholar
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