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MSM-PDs是在半导体表面制作金属电极形成金属−半导体接触的器件。器件基本结构分为两种。第一种器件结构为共面器件[17],电极通常由分立的两组金属条构成(即叉指电极),两组电极处于同一平面,即位于半导体的一侧,结构如图1(a)所示。第二种器件具有垂直形的结构[26],半导体层夹在上下两个金属电极中间,结构如图1(b)所示。在此,笔者将两种器件分别简称为共面MSM-PD和垂直MSM-PD。在共面MSM-PD中,半导体层直接暴露在光照下,而在垂直MSM-PD中,顶部金属电极必须做到足够薄以利于光吸收或载流子传输。与共面MSM-PDs相比,垂直MSM-PDs的两电极之间的距离更容易控制地比较小,这有利于降低器件寄生电容,从而获得更高的响应速度。
图 1 (a) 和 (b)分别为平面型MSM和垂直型MSM结构示意图;(c) 半导体材料本征和非本征光激发过程;(d) 肖特基MSM-PDs在VFB < V < VB 下的工作原理示意图;(e) 热载流子发射MSM-PDs的工作原理示意图,图中仅展示了左侧金属吸光发射热电子产生电流的过程
Figure 1. (a) and (b) Schematic diagrams of the planar and vertical MSMs, respectively; (c) Intrinsic and extrinsic light excitation processes of semiconductor materials; (d) The diagram illustrating the principle of Schottky MSM-PDs at VFB < V < VB; (e) The diagram illustrating the principle of hot carrier Schottky MSM-PDs with the hot electrons and the corresponding current produced due to the absorption of light by the left metal film left metal film
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根据金属电极与半导体材料接触形成的异质结类型不同,可将MSM-PDs分为光电导型和肖特基型。虽然通常所说的MSM-PDs具有肖特基型异质结接触,而近年来一些具有光电导特性的MSM-PDs也表现出优异的特性[15, 27],因此文中也涵盖了对光电导型MSM-PD的进展介绍。
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对于光电导型MSM-PDs,半导体材料与两端金属电极形成的是两个背靠背的欧姆接触,当光入射到半导体区域时,由于本征吸收和杂质吸收(见图1(c)),产生光生载流子(电子、空穴),引起半导体的电导率发生变化,光生载流子在外加电场的作用下漂移,被两端电极收集,从而在输出回路中产生光电流。
光电导探测器的主要优点是内部的光电子增益较高,光电流增益(G)代表光生载流子对光电流的贡献情况,定义为自由载流子寿命与渡越时间之比,计算方法见公式(1):
$$G = \dfrac{{{I_l}}}{{{I_p}}} = \dfrac{{({\mu _n} + {\mu _p})\tau \varepsilon }}{L} = \tau (\dfrac{1}{{{t_m}}} + \dfrac{1}{{{t_{rp}}}})$$ (1) 式中:Il 为两电极之间流过的光电流;Ip初始光电流;L电极间距;μn电子迁移率;μp空穴迁移率;τ载流子寿命;ε为光电导的内部电场;tm和trp分别为电子和空穴通过两个电极的渡越时间。当载流子寿命长,而多子渡越时间快时,光电流增益可以大于1。
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第一个肖特基型MSM-PDs由Sugeta等人于1979年报道,是由两个背靠背串联的金属−半导体肖特基二极管组成的[28]。早在1971年,Sze等人就给出了肖特基型MSM 器件在无光照情形下的基本工作原理[14]。例如,对于一个以N型硅为肖特基型MSM-PD,可以等效为如图2(a)所示的两个肖特基二极管。该器件的半导体区可以分为三部分,除了构成两个肖特基结的耗尽区以外,位于中间的区域在图2(a)中用一段平的能带来表示,这部分区域载流子浓度相对较高。在此,假设两端的肖特基结具有不同的电子势垒高度,分别为ϕ1和ϕ2,它们形成了方向相反的电场强度。左端的肖特基结形成的电场强度由右指向左,定义为反偏,而右边的肖特基结形成的电场强度方向指向右,定义为正偏,它们的电场强度分布如图2(f)所示,图中还标出了两个肖特基结相应的耗尽区宽度,分别为W1和W2。
图 2 MSM结构在不同偏压下的能带图(a-e)及相应的电场分布图(f-j)。(a)(f)无电压;(b)(g)为V小于穿通电压VRT;(c)和(h)为V等于穿通电压VRT;(d)和(i)为V等于平带电压VFB;(e)和(j)为V大于平带电压VFB但小于雪崩电压VB
Figure 2. (a-e) Energy band and (f-j) electric field distribution diagrams of a typical MSM structure. (a) (f) without applied voltage; (b)(g) V is lower than VRT; (c)(h) V is equal to VRT; (d)(i) V is equal to VFB; (e)(j) V is greater than VFB but smaller than VB
该器件在施加不同偏压时,器件的工作状态不同。当施加一个很小的反向偏压时,器件的能带结构和电场分布如图2(b)和2(g)所示,此时,反偏的肖特基结耗尽区略有展宽,而正偏的肖特基结耗尽区稍稍变窄,但只要两个耗尽区的宽度之和还小于两金属电极之间的指间距,回路中的暗电流就非常微弱,器件性能与不加偏压时接近。
继续增加反偏电压,处于反偏状态的耗尽区宽度持续增大,而正偏下的耗尽区宽度不断减小,外加电压大部分降在反偏置下的肖特基结上。当外加偏压达到某一临界值,即拉通电压(reach through voltage, VRT) 时,正向偏置的耗尽区与反向偏置的结耗尽区相遇,如图2(h)所示,此时,接触点处电场强度为0,相应的能带结构如图2(c)所示。
随着外加偏压的进一步增加,空穴要从右端电极注入回路的势垒不断下降,回路中的暗电流明显上升,直到右端处于正偏状态的耗尽区完全消失,半导体最右端处能带变平,如图2(d)所示,此时所加的偏压定义为平带电压(flat band voltage, VFB)。从图中可以看出,在平带电压下,整个器件工作在反偏状态,耗尽区的宽度与两金属电极之间的指间距相等,电场由右指向左,且其强度由右向左线性增加,如图2(i)所示。此时,空穴势垒达到高度接近其极限值,进一步增大电压,空穴势垒只会由于镜像力[29]缓慢降低,因而,电流增速较VRT < V < VFB区间明显变缓。
当电压继续增加,会引起器件击穿,该电压定义为击穿电压(break down voltage, VB)。击穿是器件中的载流子在强电场作用下与晶体原子发生连锁碰撞,产生的电流雪崩式倍增效应。击穿电压不依赖于耗尽区宽度的变化而变化[30]。在VFB < V < VB区间,器件的能带结构与电场强度如图2(e)及2(j)所示。值得一提的是,有机半导体中由于激子束缚能比较高,只能通过陷阱载流子在界面处积累引发的载流子隧穿行为实现光电倍增,该效应也被用来制备高性能光电探测器[31-33]。
在肖特基型MSM-PDs中,常见的器件是利用半导体材料进行感光的。在图2(a)~(c)中所示正偏肖特基结没有消失的情形下,位于两个肖特基结区的半导体层均吸收光,产生与肖特基内建电势相反的光生电势,在回路中产生两个相反方向的光电流。当器件处于图2(d)~(e)的情形下时,即VFB < V < VB时,器件只包含一个反偏肖特基结,此时回路中只有单方向的光生电流,如图1(d)所示。通常,用于光电探测时,所加偏压位于这一区间,但是此时器件的暗电流较高。为了降低暗电流,可以在金属和半导体之间引入一个薄势垒增强层[34-35]。相比于光电导型PDs,由于肖特基结增强了半导体区的电场强度,所以载流子传输性能被进一步改善,器件的响应速度得以提升[36]。
在肖特基型MSM-PDs中,也可利用金属吸收光产生光电流[37-38],这样便可在半导体不吸光的波段实现光电探测。例如,笔者可以使用ZnO等宽带隙半导体实现对可见光的探测,使用Si半导体实现1.30 μm和1.55 μm两个光纤通信波段的光信号检测[39]。这类型器件的工作原理如图1(e)所示。当光照射到器件上时,两端的金属吸收光子后会激发热载流子[40],部分热载流子传输到金属与半导体之间的界面处,具有足够动能的热载流子可以隧穿通过顶部金属与半导体界面之间形成的肖特基势垒,进入半导体层,随后这些热载流子传输到达半导体与对向金属电极构成的界面时,会再一次隧穿进入对向金属电极,产生光电流。由于两端金属吸光所产生的两个光电流方向相反,这两个电流分别记为I1和I2,当操控这两个光电流的大小不等时,就会在外电路中产生净电流实现光电探测[41-43]。当一侧金属半导体界面是欧姆接触时,就构成了通常所说的金属−半导体二极管型光电探测器[44]。因为这种器件是利用了金属中的热载流子来实现光电探测的,所以又被叫做热载流子光电探测器,包括了热电子和热空穴两种类型[45-46],其中以热电子型光电探测器更为常见。
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在此罗列了半导体光电探测器的一些主要性能参数,随后将着重探讨这些相关性能指标。
外量子效率(EQE):指光入射到器件中每一个光子所产生的电子−空穴对数目。计算公式如下:
$$EQE = \dfrac{{{{{I_{ph}}} / e}}}{{{{{P_{in}}} / {h\nu }}}}$$ (2) 式中:Iph为光生电流,实际应用中为光照下所测电流(Il)减去暗电流(Id);e为单位电子所带电荷量的绝对值;h为普朗克常量;ν为入射光频率;hν为光子能量;Pin为入射光功率。
响应率(R):定义为光照下所产生的平均光电流与入射光功率的比值,单位为A/W。计算公式如下:
$$R = \dfrac{{{I_{ph}}}}{{{P_{in}}}}$$ (3) 响应时间:探测器对入射光的响应速度快慢,一般定义为在脉冲光照射下,信号由最高值的10%上升至90%所需要的时间,或者从信号最高值下降到最高值的10%所经过的时间。对于肖特基型共面MSM-PD而言,其时间常数包含了光生载流子渡越时间常数和电路RC时间常数两个方面。其中,光生载流子渡越时间与叉指电极的间距成正比,间距越大,响应时间越长。而电路RC时间常数与叉指电极的间距及电极的厚度成反比,与指宽成正比。电极指间距越大或电极厚度越厚,电路RC时间常数越小;指宽越小,电路时间常数越小[47]。要获得尽可能低的响应时间,在设计时如何选取合适的电极指间距需要综合考虑。相应地,器件的频率响应带宽与其响应时间常数成反比。
明暗电流比:光电流Il与暗电流Id的比值。只有当比值大于1的时候,光信号才能被探测到。在实际中,我们希望器件具有强的信号电流及弱的暗电流。光电流相同的情况下,暗电流越低,器件的信噪比越高,器件对弱光的检测能力也越高。对于光电导型MSM-PDs,暗电流主要来源于热噪声和散粒噪声,其中热噪声与半导体材料的电导率有关[48]。而肖特基型MSM-PDs的暗电流包含电子暗电流和空穴暗电流两部分。电子暗电流主要是金属电极热电子发射并越过肖特基势垒所形成的电流。空穴电流在低电压下主要来源于空穴扩散,这是因为空穴要穿越的势垒高度远大于其平均自由程,空穴在穿越势垒途中被散射,造成大量积累,这样半导体内空穴的浓度呈现不均匀,而出现扩散。当V > VFB时,空穴越过势垒的几率大大增加,空穴电流由扩散电流为主转变为热发射电流为主。具体地,MSM-PDs的暗电流与偏压、掺杂浓度及电极间距有关,详细的公式参见参考文献[49]。实际中,为了获得尽可能低的暗电流,测试时控制偏压接近平带电压。在固定偏压的条件下,降低掺杂浓度,选择较大的电极间距,均有利于降低暗电流[50]。
探测率(D*)通常用以下公式表示:
$$D* =\dfrac{{\sqrt {A \cdot \Delta f} }}{{NEP}}$$ (4) 式中:A为探测器器件的面积;
$\Delta f$ 为测量带宽;NEP为噪声等效功率;D*的单位是cm·Hz1/2/W。
Research progress in metal-inorganic semiconductor-metal photodetectors
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摘要: 金属−半导体−金属光电探测器(MSM-PDs)本身固有的高速、高响应率、易集成等特性使其在光纤通信、传感、制导等多个领域受到广泛关注。文中围绕金属−无机半导体−金属光电探测器展开综述。首先介绍了MSM-PDs的基本结构,包含共面和垂直两种类型。紧接着,介绍了MSM-PDs具体的工作原理,除了常见的光电导型及肖特基型工作原理,还介绍了以金属作为吸光层的热载流子光电探测器的工作原理。随后,详细介绍了以GaAs、InGaAs、Si/Ge等无机材料作为半导体层的MSM-PDs在过去所取得的研究进展。此外,还介绍了利用金属微纳结构拓展较宽带隙半导体材料MSM-PDs在红外波段响应特性的研究进展。最后,总结全文并对MSM-PDs未来的发展做出了展望。Abstract: The metal-semiconductor-metal photodetectors (MSM-PDs) have received great attention in areas of optical fiber communication, sensing, missile guidance, etc., due to their inherent merits of high speed, high responsitivity, and easy integration. This review focused on MSM PDs with the semiconductor layer made of inorganic materials. Firstly, the basic structures of MSM-PDs was introduced, including the planar and vertical configurations. Then, the working principles of MSM-PDs were introduced. In addition to the common photoconductive and Schottky principles, the principle of hot carrier photodetectors with the metal layer as the light absorbing part was also introduced. Subsequently, the research progresses of MSM-PDs made of inorganic materials such as GaAs, InGaAs, Si/Ge was described in detail. Additionally, the research progress of using metallic micro/nano structures to extend the response of wide energy band semiconductor based MSM-PDs in infrared wavelength range was presented. Finally, the full text was summarized and the future development of MSM-PDs was prospected.
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Key words:
- metal-semiconductor-metal /
- photodetectors /
- Schottky /
- infrared /
- metallic micro/nano structures
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图 1 (a) 和 (b)分别为平面型MSM和垂直型MSM结构示意图;(c) 半导体材料本征和非本征光激发过程;(d) 肖特基MSM-PDs在VFB < V < VB 下的工作原理示意图;(e) 热载流子发射MSM-PDs的工作原理示意图,图中仅展示了左侧金属吸光发射热电子产生电流的过程
Figure 1. (a) and (b) Schematic diagrams of the planar and vertical MSMs, respectively; (c) Intrinsic and extrinsic light excitation processes of semiconductor materials; (d) The diagram illustrating the principle of Schottky MSM-PDs at VFB < V < VB; (e) The diagram illustrating the principle of hot carrier Schottky MSM-PDs with the hot electrons and the corresponding current produced due to the absorption of light by the left metal film left metal film
图 2 MSM结构在不同偏压下的能带图(a-e)及相应的电场分布图(f-j)。(a)(f)无电压;(b)(g)为V小于穿通电压VRT;(c)和(h)为V等于穿通电压VRT;(d)和(i)为V等于平带电压VFB;(e)和(j)为V大于平带电压VFB但小于雪崩电压VB
Figure 2. (a-e) Energy band and (f-j) electric field distribution diagrams of a typical MSM structure. (a) (f) without applied voltage; (b)(g) V is lower than VRT; (c)(h) V is equal to VRT; (d)(i) V is equal to VFB; (e)(j) V is greater than VFB but smaller than VB
图 3 (a) GaAs MSM-PD的脉冲响应实验与数值结果对比图[52];(b)指宽和指间距均为300 nm的GaAs MSM-PDs 的脉冲响应图[53];(c)谐振腔增强型 GaAs MSM-PD 结构示意图[54];(d)具有凹陷阳极和阴极的GaAs MSM-PD 的结构示意图[18];(e)表面等离激元增强的MSM-PD 的结构示意图[55];(f)基于二维电子气和空穴气的GaAs MSM-PD的结构示意图[56]
Figure 3. (a) Experimental and calculated impulse response of a GaAs MSM-PD[52]; (b) Impulse response of a GaAs MSM-PD with both finger spacing and width of 300 nm[53]; (c) Schematic diagram of a resonant cavity enhanced GaAs MSM-PD[54]; (d) Schematic diagram of GaAs MSM-PD with recessed anode and cathode[18]; (e) Schematic diagram of the plasmonic MSM-PD structure[55]; (f) Structure diagram of a GaAs MSM-PD based on 2D electron gas and 2D hole gas[56]
图 4 背照射式 InGaAs MSM-PD的 结构示意图(a)及光响应和暗电流曲线(b)[70];具有透明肖特基接触的 InGaAs MSM-PD 结构示意图(c)及不同金属电极厚度下器件响应率随入射光功率变化的关系曲线图(d)[71];BCB侧壁钝化InGaAs MSM-PD 的结构示意图(e)及钝化前后器件的亮暗电流电压曲线(f)[76]
Figure 4. Schematic diagram of a back-illuminated InGaAs MSM-PD (a) and its photo response and dark current characteristics (b)[70]; Schematic diagram of an InGaAs MSM-PD with semi-transparent Schottky contacts (c) and curves of responsivity versus optical power for the devices with different contact thicknesses[71]. The structural diagram of the BCB passivated InGaAs MSM-PD (e) and the I–V characteristics in dark and under illumination for devices before and after passivation (f)[76]
图 5 (a) 基于有纹理的硅薄膜上制作的 MSM-PD 的截面图[79];(b) 具有脊形硅结构的 MSM-PD 的结构示意图[80];(c)具有对称电极和不对称电极的 Ge MSM-PDs 的光响应和暗电流曲线[81];(d) Si MSM-PD的非对称电极SEM图[82]
Figure 5. (a) Cross-sectional diagram of an MSM detector based on a textured silicon membrane[79]; (b) Schematic diagram of an MSM-PD based on silicon trenches[80]; (c) Photo response and dark currents measured for both symmetric and asymmetric MSM-PDs on Ge substrate[81]; (d) SEM image of an asymmetric contact for a Si MSM PD[82]
图 6 (a)~(b) 叉指电极的一组引入了金属纳米天线阵列的MoS2 MSM-PD 结构示意图及电场分布图与能带示意图[112];(c)具有一维保形光栅的垂直型 ZnO MSM-PD结构示意图[26];(d)具有二维保形光栅结构的Au/TiO2/ITO光电探测器结构及原理示意图[115];(e) 含金属纳米颗粒的MoS2 MSM-PD结构示意图[116];(f) Tamm表面等离激元热电子ZnO MSM-PDs 结构示意图[119];(g) 基于慢波吸光原理的宽谱高效多MSM 光电探测器结构示意图[121]
Figure 6. (a)~(b) Schematic diagram of structure, electric field distribution and energy band of the MoS2 MSM-PD with one of the finger electrode including an optical antenna array[112]; (c) Schematic diagram of the vertical type ZnO MSM-PD with the conformal grating structure[26]; (d) Schematic of structure and principle of Au/TiO2/ITO photodetector with two dimensional conformal grating[115]; (e) Schematic diagram of the MoS2 MSM-PD dressed with metallic nanoparticles[116]; (f) Schematic diagram of the Tamm plasmon based hot electron ZnO MSM-PD[119]; (g) Structural diagram of broadband and efficient photodetectors composed of multiple MSMs based on the slow light absorption principle[121]
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