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在光电探测领域中,探测器的响应速度非常重要,为增加光电二极管的灵敏度,其耗尽区的宽度应做的尽可能大,PIN光电二极管符合这一要求,并且已经得到广泛应用[16-18]。PIN光电二极管的剖面图如图1所示,本征区宽度w比普通PN结的空间电荷区大得多。如果加反向偏置电压,则空间电荷区会延伸整个本征区。
光电二极管的响应速度主要受三大因素影响[19]:(1)光电二极管以及与其相关电路的RC时间常数;(2)耗尽区的光载流子的渡越时间;(3)耗尽区外产生的光载流子的扩散时间导致响应的延迟。
在有外加电压的情况下,在半导体材料的表面某一点产生一个脉冲的过剩载流子,也就是激光辐照结束后的状态,这些带电载流子会产生内建电场,会使过剩载流子拥有共同的迁移率和扩散系数,称为双极输运过程。其双极输运中两个重要的参数双极扩散系数D‘和双极迁移率μ' 分别为:
$${D^{'}} = \frac{{{D_n}{D_p}(n + p)}}{{{D_n}n + {D_p}p}}$$ (1) $${\mu ^{'}} = \frac{{{\mu _n}{\mu _p}\left( {p - n} \right)}}{{{\mu _n}n + {\mu _p}p}}$$ (2) 式中:Dn为电子扩散系数;Dp为空穴迁移率;n为电子浓度;p为空穴浓度;μn为电子迁移率;μp为空穴迁移率。由公式可见双极扩散系数D’和双极迁移率μ'均为载流子浓度的函数,并且载流子浓度n、p包括过剩载流子浓度бn、бp,因此,Dn和Dp都不是一个常数,双极输运方程是一个非线性的微分方程。
当激光辐照结束时,会有光子注入半导体的表面,当光子满足小注入也就是非平衡少数载流子远小于非平衡多数载流子,双极扩散系数可以表示为[20]:
$${D^{'}} = {D_n} = {D_p}$$ (3) 双极迁移率可以简化为:
$${\mu ^{'}} = {\mu _n} = - {\mu _p}$$ (4) 在大注入的条件下,也就是非平衡少数载流子大于等于非平衡多数载流子。此时双极扩散系数[21]:
$${D^{'}} = \frac{{{D_n}{D_p}(n + p)}}{{{D_n}n + {D_p}p}}\xrightarrow[{\rm high}]{}\frac{{2{D_n}{D_p}}}{{{D_n} + {D_p}}}$$ (5) 双极迁移率为:
$${\mu ^{'}} = \frac{{{\mu _n}{\mu _p}\left( {p - n} \right)}}{{{\mu _n}n + {\mu _p}p}}\xrightarrow[{\rm high}]{}0$$ (6) 在上述理论基础上,当激光辐照器件,能量密度较小时,满足小注入条件也就是器件饱和前的状态;随着注入光子的增加,内建电场增加会导致空间电荷区宽度增加,因此,会出现器件响应时间的展宽,单次辐照结束后,漂移速度会随着非平衡载流子的浓度减小而减小。满足大注入条件也就是器件饱和后的状态,随着注入光子的增加,内建电场增加会导致空间电荷区宽度增加,光生载流子的渡越时间增加,从而导致器件响应时间的展宽,同时双极迁移率趋近于0,所以信号下降沿初始会发生速度衰减的情况,随着时间推移恢复载流子小注入状态,载流子恢复之前的速度。同时部分载流子还没渡越完成,这样会使PIN结两端的电压信号接近饱和,导致PIN光电二极管内部的等效电容增大,RC时间常数会增大,使得整个单脉冲信号出现展宽现象,器件在不同激光能量密度辐照下饱和前后出现不同的瞬态响应性能退化的情况,影响高速光电探测器的响应速度。
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不同激光能量密度辐照二极管的峰值电压如图3所示,激光能量密度的范围为0.071~3.85 μJ/cm2,偏置电压为5 V时,当激光能量密度小于0.819 μJ/cm2时,对数据进行拟合,发现峰值电压随着激光能量密度的增加呈对数线性增长,属于器件的对数线性响应区[15],大于0.819 μJ/cm2时,峰值电压已经增长平缓,出现饱和现象。
图 3 不同激光能量密度辐照二极管的峰值电压
Figure 3. Peak voltage of irradiated diodes with different laser energy densities
不同激光能量密度下辐照硅基PIN光电二极管的脉冲响应信号波形如图4所示。随着激光能量密度的增大,整个脉冲信号响应时间宽度也随之增大,出现了展宽的现象,意味着器件发生了响应退化现象。信号在饱和后对比饱和前展宽明显增大,在不同的阶段有着不同的展宽形式,文中对于激光辐照器件饱和前后的上升沿、半高宽以及底宽分别进行详细地比较分析,分析激光辐照器件后的展宽规律。
图 4 不同激光能量密度下单脉冲信号响应波形
Figure 4. Single pulse signal response waveform under different laser energy densities
图5中给出饱和前后脉冲响应信号上升沿时间随着激光能量密度的变化情况,整体时间范围变化为69.7~17 ns,饱和前绝对上升和相对上升分别为50.3 ns,72.17%,饱和后的绝对下降和相对下降为4 ns,19%。因为外接电路的RC常数较小以及整个器件吸收层较薄,整体的上升沿变化时间对信号展宽作用较小。
图 5 不同激光能量密度辐照二极管的上升沿时间
Figure 5. Rising edge time of the diode irradiated by different laser energy densities
图6给出的是代表响应信号特征量的信号半高宽(Full Width at Half Maximum, FWHM)随着激光能量密度的变化情况,由图中可以看出变化范围为37.2~113 μs,随着激光能量密度的增加,信号半高宽增大,饱和前的绝对增幅和相对增幅分别为14.3 μs,38.44%,饱和后的绝对增幅和相对增幅别为61.5 μs,119.42%。图7给出的是代表信号特征量的底宽(Bottom Width, BW)随着激光能量密度的增加而增大,变化范围为181~322 μs,饱和前的相对增幅和绝对增幅为38 μs,21%,饱和后的绝对增幅和相对增幅分别为103 μs,47.03%,在外加电场存在的情况下,产生出一个脉冲过剩载流子,产生的内建电场、外部电路与设备一起存在RC时间常数和瞬态电容共同引起饱和前后展宽现象。
图 6 不同激光能量密度辐照二极管的半高宽
Figure 6. Half-height width of diodes irradiated by different laser energy densities
图 7 不同激光能量密度辐照二极管的底宽
Figure 7. Bottom width of diodes irradiated by different laser energy densities
通过上述的数据对比分析可知,首先从上升沿可以看出,饱和前,漂移速度随着电场增加而增大,饱和后,漂移速度达到最大,上升沿时间明显下降且变化较小,同时上升沿范围在69.7~17 ns之间,对信号展宽影响较小,说明响应信号的下降沿的宽度可以反应激光辐照结束后光生载流子的输运情况。由表1和表2可以看出,对于FWHM和BW来说,无论是绝对增幅还是相对增幅,都是饱和后的增幅要高,并且饱和后下降沿速度呈两相变化,该现象与参考文献[13]中研究的飞秒激光辐照PIN光电二极管的饱和特性,下降沿速度呈三相变化有着明显的不同,在皮秒激光高注入时还未出现空间电荷的屏蔽效应。由此可以看出,饱和前后信号宽度都有增幅,饱和前满足小注入条件,双极扩散速度等于器件自身的漂移速度,由于空间电荷区随着内建电场增加增宽,信号出现展宽;饱和后满足光生载流子高注入条件,双极扩散速度趋近于0,导致饱和后初始下降沿出现的速度衰减,当载流子浓度降低时又恢复饱和前的输运方式,因此饱和后有更加明显的增幅。
表 1 器件脉冲信号FWHM变化
Table 1. Device pulse signal FWHM changes
Item Absolute increase/μs Relative increase Before saturation 14.3 38.44% After saturation 61.5 119.42% 表 2 器件脉冲信号BW变化
Table 2. Device pulse signal BW changes
Project Absolute increase/μs Relative increase Before saturation 38 21% After saturation 103 47.03%
Analysis of the transient response signal of a silicon-based PIN photodiode irradiated by picosecond laser
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摘要: 实验研究了超短脉冲皮秒激光辐照硅基PIN光电二极管的瞬态响应的规律特性,测量了在不同激光能量密度辐照下的脉冲响应信号。分析实验结果表明,随着激光能量密度的增大,器件出现了非线性饱和状态,半高宽从37.2 μs到113 μs,底宽从181 μs到322 μs,脉冲响应信号出现展宽现象,信号的展宽意味着器件的瞬态响应发生了退化,同时对于饱和前后的信号特征量的半高宽和底宽进行了分析,发现无论是绝对增幅还是相对增幅,可以看出饱和后有更为显著的展宽现象,并且主要由于器件饱和后的下降沿出现速度的衰减所致。通过理论分析发现,由于注入的光生载流子浓度变化对双极输运过程造成影响,从而改变其载流子输运过程中的速度,导致器件响应出现退化现象。Abstract: The regular characteristics of the transient response of a silicon-based PIN photodiode were stuidied experimentally, which irradiated by an ultrashort pulse picosecond laser, and the pulse response signals under different laser energy densities were measured. The analysis and experiment results show that with the increase of laser energy density, the device appeares a non-linear saturation state. The FWHM is from 37.2 μs to 113 μs, and the bottom width is from 181 μs to 322 μs. The impulse response signal has a broadening phenomenon, and the signal broadening means that the transient response of the device is degraded. At the same time, the analysis of the half-height width and the bottom width of the signal characteristics before and after saturation shows that whether it is an absolute increase or a relative increase, it can be seen that there is a more significant broadening phenomenon after saturation. It is caused by the attenuation of the speed on the falling edge after the device is saturated. Through theoretical analysis, the change in the concentration of injected photogenerated carriers affects the bipolar transport process, thereby changing the speed of the carrier transport process, resulting in degradation of the device response.
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表 1 器件脉冲信号FWHM变化
Table 1. Device pulse signal FWHM changes
Item Absolute increase/μs Relative increase Before saturation 14.3 38.44% After saturation 61.5 119.42% 表 2 器件脉冲信号BW变化
Table 2. Device pulse signal BW changes
Project Absolute increase/μs Relative increase Before saturation 38 21% After saturation 103 47.03% -
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