1.Jiangsu Key Laboratory of Micro and Nano Heat Fluid Flow Technology and Energy Application, School of Physical Science and Technology, Suzhou University of Science and Technology, Suzhou 215009, China 2.School of Physical Science and Technology, Soochow University, Suzhou 215006, China
Abstract: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. Keywords:carrier dynamics/ transient absorption spectroscopy/ two-photon absorption/ GaN
图1(a)显示了GaN: Ge晶体的线性吸收光谱, 当波长大于带隙对应的波长时(365 nm, 3.4 eV)时, 样品不存在任何光吸收, 证明了样品具有很高的纯度及极低的缺陷密度. 利用650 nm下的飞秒Z扫描测量技术来验证GaN: Ge晶体的双光子吸收特性. 光源与瞬态吸收光谱所用的光源一致, 在焦点处光斑的半径约为45 μm. 在不同入射脉冲能量下的开孔Z扫描曲线如图1(b)所示. 样品表现出明显的反饱和吸收, 样品的吸收系数变化Δα可表示为 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 Figure1. (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.
$\Delta \alpha = \beta I,$
其中β为双光子吸收系数, I为样品处的峰值光强. 利用Z扫描理论[27]拟合得到β ≈ 5.1 cm/GW, 其值不随入射能量变化, 证明了反饱和吸收来自于双光子吸收. 图1(a)的内插图还显示了在2PE (650 nm)下GaN: Ge晶体的照片, 整个晶体发出了明亮的黄光. GaN的黄带发光是GaN中最常见的由点缺陷引起的发光[7]. HVPE生长的低位错GaN晶体在室温下的光致发光(photoluminescence)光谱可参考文献[28,29], 除了很强的带边发射(band-edge emission, BE)以外, 依然可以观测到中心约为2.2 eV的黄色发光(yellow luminescence, YL)带. 说明即使在较低的位错密度和杂质浓度下, GaN中的缺陷依然会对其光学性质产生严重的影响. 图2显示了2PE(650 nm)和1PE(325 nm)下GaN:Ge的TAS响应. 在不同的激发波长下, 整个吸收光谱(1.1—2.6 eV)都随着探测波长的增加单调增强, 这是自由载流子吸收才具备的特点[30,31]. 此外, 没有任何吸收峰的存在, 也证明可以忽略带内缺陷引起的光吸收. 据此, 瞬态吸收可认为由导带和价带内的电子和空穴间接吸收引起, 瞬态吸收响应的衰减对应着导带电子和价带空穴的复合. 但是与2PE下TAS响应不同的是, 1PE下的TAS响应几乎不随延迟时间衰减, 即使在特别低的激发能流下(0.1 mJ/cm2). 这是由于1PE下非平衡载流子浓度(约1018 cm–3)远大于样品中的缺陷浓度(约1016 cm–3), 缺陷对载流子弛豫的影响很小. 而2PE下的非平衡载流子浓度与缺陷浓度相近, 因此, 本文重点分析2PE下的TAS响应来研究缺陷对GaN: Ge超快载流子动力学的影响及机制. 利用关系${\sigma _{{\rm{eh}}}} \propto \lambda _{\rm{p}}^b$拟合2PE下的吸收光谱(σeh为载流子吸收截面), 可见光光谱下b = 2.5, 这满足极性光学声子辅助的载流子吸收[32]. 但是, 对于近红外光谱, b值从刚激发时(3 ps)的6.5逐渐降低到2.5(500 ps). 根据文献[31]的报道: 在1050 nm附近由于GaN的能带结构, 空穴引起的吸收将占主导; 而在较短的波长至可见光波段, 电子吸收的比重逐渐增大; 此外, 能带的非抛物性也会导致b > 2.5. 对瞬态吸收光谱机制的进一步探究将在3.3节中进行. 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 Figure2. (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.
23.2.超快载流子动力学 -->
3.2.超快载流子动力学
图3(a)显示了2PE不同激发能流下从TAS响应中提取的GaN: Ge在探测波长1050 nm下的瞬态吸收衰减响应. 为了看到更快的超快过程, 图3(a)中的内插图展示了短时间尺度下的瞬态吸收响应(0.8 mJ/cm2). 在零延迟附近超快的尖峰响应(0—0.4 ps)来源于泵浦光和探测光重合时产生的相干散射, 而在之后观察到吸收信号约1 ps的上升时间, 这可认为是非平衡载流子产生与弛豫到导带底部的过程(带内载流子弛豫过程). 在2—3 ps后吸收响应逐渐衰减, 利用双指数衰减方程可以很好地拟合实验结果: 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) Figure3. (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.
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 Table1.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.
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 Figure5. 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.
载流子寿命是光子器件的关键, 而根据(5)式以及载流子动力学实验和模拟结果可知: 1)在高载流子注入下(>1018 cm–3), n型GaN的载流子寿命主要由位错密度和辐射复合决定; 2)在适中的载流子注入下(1016—1017 cm–3), 辐射缺陷、非辐射复合缺陷以及固有载流子浓度将会共同决定载流子的寿命. 图6显示了在1PE和2PE下GaN: Ge在通讯波段1310 nm探测下的超快瞬态吸收响应. 和我们分析的一致, 1PE下的载流子寿命(约10 ns)远远大于2PE下的载流子寿命. 根据参考文献[41], 可估算出1PE下GaN:Ge的发光内量子效率约50%. 2PE下内量子效率虽然严重下降, 但更快的载流子寿命反而有利于其应用于超快全光开关器件. 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 Figure6. 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).