基于poly-Si键合层的SACM型Ge/Si APD的优化设计研究
Ge/Si avalanche photodiodes (APDs) are widely used in near-infrared detection; however, obtaining high-performance Ge/Si APD is challenging due to the 4.2% lattice mismatch between Ge and Si. Therefore, this study proposes introducing a polycrystalline silicon (poly-Si) bonding intermediate layer at the Ge/Si bonding interface to mitigate the effects of the Ge/Si lattice mismatch on APD device performance. With the introduction of poly-Si, the electric field at the bonding interface changes, causing a redistribution of the electric field inside the APD, which significantly impacts device performance. Consequently, this study focuses on regulating the doping concentrations of the Ge absorption layer and Si multiplication layer. It explores the effects of doping concentration on the electric field, recombination rate, carrier concentration, impact ionization, and other properties of Ge/Si APD. Ultimately, the aim is to design high-performance bonded Ge/Si APD. This study offers theoretical guidance for future research on Ge/Si APD with low noise and high gain.
In this study, a 2-nm thick layer of poly-Si material is introduced at the Ge/Si bonding interface, and the influence of the doping concentrations of the Ge and Si layers on the APD properties is investigated. Initially, changes in the APD optical and dark currents with doping concentration are simulated. The changes in the recombination rate and carrier concentration are then simulated to explore the reasons for the changes in the optical current. Next, to further understand the reasons for the change in electron concentration, changes in the energy band of the APD are simulated. Following this, changes in the charge concentration, impact ionization rate, electric field, and other parameters with the doping concentration are simulated. Finally, the gain, bandwidth, and gain-bandwidth product of the APD are simulated and compared with previous studies. The optimal doping concentration for APD devices is identified to improve device performance.
After introducing the polycrystalline silicon bonding layer, the dark current reaches 1×10-10 A, which is five orders of magnitude lower than that of the currently reported Ge/Si APD (Fig. 3). As the doping concentrations of the Ge and Si layers increase, the conduction band in the Ge layer gradually flattens. When the doping concentration is high, the conduction band bends upward at the bonding interface, gradually forming a barrier at the bonding interface that obstructs the transport of charge carriers, resulting in challenges in transporting electrons in the Ge layer to the multiplication layer. As the doping concentration increases, the valence band becomes steeper, which facilitates the migration of holes. The holes in the multiplication layer can reach the absorption layer smoothly under the influence of a higher potential energy difference (Fig. 6). The electron and hole ionization coefficients at the p-Ge/i-Ge interface rise sharply with increasing doping concentration of the Ge layer, primarily due to the significant increase in the electric field with rising doping concentration (Fig. 8).
In this study, a poly-Si material is introduced at the bonding interface of Ge/Si, and the influence of the doping concentrations of Ge and Si layers on the performance of Ge/Si APD is theoretically examined. After the poly-Si layer is introduced, the dark current is found to reach an order of 1×10-10 A. Furthermore, the gain of 12.21 is realized when the Ge layer doping concentration is set at 1×1012 cm-3 and the reverse bias is 28.0 V. The maximum gain of 12.14 is noted when the doping concentration of the Si layer is 1×1015 cm-3 and the reverse bias is 28.2 V. As the doping concentrations of the Ge and Si layers are increased from 1×1012 cm-3 to 1×1016 cm-3, under the same bias voltage, an overall upward trend in the 3-dB bandwidth is observed. However, a sharp drop in the bandwidth is observed when the Ge layer doping concentration exceeds 1×1016 cm-3. The gain bandwidth product is found to reach its maximum value of 225.76 GHz when the Ge layer doping concentration is 1×1012 cm-3. A peak value of 215.15 GHz for the gain bandwidth product is achieved when the doping concentration of the Si layer is 1×1012 cm-3, and the bias is 29.5 V. Thus, an optimal gain and gain-bandwidth product in a Ge/Si APD can be obtained when lower doping concentrations of the Ge absorption layer and Si multiplication layer are chosen, ensuring that no electric field or tunneling phenomenon is encountered.
1 引言
在光探测系统的实际应用中,大多是对微弱光信号进行探测[1],雪崩光电二极管(APD)由于其内部倍增效应可以获得较高的响应度,适用于远距离通信系统中微弱信号的探测[2-3]。APD具有量子效率高、内部增益高、灵敏度高、频率响应快、尺寸小、电压低等优点[4-5],广泛应用于光纤通信、激光测距、量子密钥分配、量子成像、生物检测以及光纤传感等领域[6-8],因此具备雪崩倍增效应的APD作为一种高速、灵敏、可靠的固态光电探测器件,逐渐受到研究者的青睐。
APD的器件性能主要受器件结构和使用材料的影响,用于制备APD的材料主要有锗(Ge)、硅(Si)和III-V族化合物[9-10]。III-V族半导体材料(如InGaAs/InP和InGaAs/InAlAs)制备的APD具有量子效率高、灵敏度高、暗电流小等优点[11-14],但其电子空穴电离系数比(K值)较高,价格昂贵,导热性能、力学性能及其与硅基互补金属氧化物半导体(CMOS)工艺兼容性较差等,这些缺点限制了器件的进一步发展[15]。相较于传统的III-V族化合物[16],Si材料的成本较低,与CMOS工艺兼容性好,禁带宽度为1.12 eV,光吸收截止波长仅为1100 nm,且电子空穴的电离系数比K值小,是良好的雪崩倍增材料。此外,同为IV族元素的Ge材料,其禁带宽度相对较小,为0.66 eV,光吸收截止波长可扩展到1700 nm。因此,结合Ge材料的高吸收以及Si材料的高倍增优势制备Ge/Si APD是理想选择。
Ge/Si APD凭借其在近红外波段吸收系数高、与传统Si基CMOS工艺兼容[17-18]、载流子迁移速率高、电子空穴电离系数比小等特点,被广泛应用于近红外探测领域[19-20]。然而,目前Ge/Si APD大部分采用外延方式制备,外延的优点是可以实现Si基Ge薄膜的大面积制备,但是由于Ge和Si之间存在4.2%的晶格失配[21-23],Ge薄膜在直接异质外延过程中,会出现大量穿透位错成核现象[24-26],这些实空间的穿透位错会在Ge材料的禁带中引入一系列缺陷能级[27-29],形成有源区的非故意掺杂,导致Ge/Si APD的漏电流较大以及增益带宽积较小[30],从而导致器件性能下降。因此,制备高质量的Ge/Si异质结材料是实现高性能Ge/Si APD制备的关键。
近年来,有研究报道低温Ge/Si异质键合技术可以用来实现高质量的无位错的Si基Ge薄膜材料的制备[31-34]。在Ge/Si异质键合界面处插入一层非晶或者多晶半导体中间层可以阻断失配晶格[35],使器件结构能带保持连续,异质结导热性较好。虽然目前关于采用键合技术制备Ge/Si APD的研究较少,但是键合的优势在于Ge薄膜的晶体质量可以达到接近体Ge的晶体质量,可以将Ge薄膜中的穿透位错密度降到最低。因此本文提出采用异质键合技术在Ge/Si键合界面处引入一层多晶Si(下文称poly-Si)键合中间层,阻断Ge和Si之间的失配晶格,在理论上分别研究了不同吸收层掺杂浓度和不同倍增层掺杂浓度对Ge/Si APD性能参数的影响,并分析了各性能参数之间的联系,为后续研究超低噪声的Ge/Si APD指明了方向。
2 结构模型与模拟软件
键合Ge/Si APD器件结构如
表 1. Ge/Si APD材料参数
Table 1. Material parameters of Ge/Si APD
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Ge/Si APD涉及到载流子的输运,因此需要泊松方程、电流连续方程以及依赖平行电场的载流子迁移模型。APD涉及到载流子的产生与复合,因此需要引入的复合模型有俄歇复合模型、浓度决定的Shockley-Read-Hall(SRH)复合模型、光学辐射复合模型、缺陷辅助俄歇复合模型。APD需要工作在高电压下,电场会发生倾斜,导致载流子容易从价带隧穿到导带或从导带隧穿到价带,故需要引入标准能带跃迁模型和Trap-Assisted跃迁模型,使载流子在能带之间更好地跃迁。APD在高电场下的势垒可能会降低,因此需要利用库伦势阱Poole-Frenkel势垒降低模型对Ge/Si APD器件性能进行理论计算。以下是各方程和模型的表达式。
泊松方程表示为
式中:
电流连续方程表示为
式中:t为时间;
依赖平行电场的载流子迁移模型表示为
式中:
俄歇复合模型表示为
式中:
浓度决定的SRH复合模型表示为
式中:
光学辐射复合模型表示为
式中:
缺陷辅助俄歇复合模型表示为
式中:
标准能带跃迁模型表示为
式中:
Trap-Assisted跃迁模型表示为
式中:
库伦势阱Poole-Frenkel势垒降低模型表示为
式中:
3 结果与讨论
理论上Ge/Si APD可以实现宽光谱的光信号检测,因此,我们模拟了Ge/Si APD对紫外-可见-近红外光(375、532、808、1550 nm)的响应性能,如
图 2. 当T=300 K和P=-20 dBm时不同波长下的电流
Fig. 2. Currents at different wavelengths when T=300 K and P=-20 dBm
首先,本文模拟了Ge/Si APD光/暗电流随Ge吸收层掺杂浓度的变化,当Ge层掺杂浓度达到5×1016 cm-3和7×1016 cm-3时,器件的光电流急剧上升,如
图 3. 当λ=1310 nm,T=300 K,P=-20 dBm时,掺杂浓度对Ge/Si APD的影响
Fig. 3. Influence of doping concentration on Ge/Si APD when λ=1310 nm, T=300 K, and P=-20 dBm
为探究光电流随Ge层和Si层掺杂浓度变化的原因,本文模拟了APD复合率随掺杂浓度的变化,如
图 4. 当λ=1310 nm,T=300 K,P=-20 dBm,V=0.95Vbr时,复合率随掺杂浓度的变化(插图为键合层复合率)。(a) Ge-APD;(b) Si-APD
Fig. 4. Recombination rate versus doping concentration with recombination rate in bond layer shown in inset when λ=1310 nm,T=300 K, P=-20 dBm, and V=0.95Vbr. (a) Ge-APD; (b) Si-APD
接着,本文模拟了APD中载流子浓度随掺杂浓度的变化,如
图 5. 当λ=1310 nm,T=300 K,P=-20 dBm,V=0.95Vbr时,载流子浓度随掺杂浓度的变化
Fig. 5. Carrier concentration versus doping concentration when λ=1310 nm, T=300 K, P=-20 dBm, and V=0.95Vbr
然后,本文模拟了Ge/Si APD导带和价带随Ge层和Si层掺杂浓度的变化,如
图 6. 当λ=1310 nm,T=300 K,P=-20 dBm,V=0.95Vbr时,APD能带随掺杂浓度的变化
Fig. 6. APD energy band versus doping concentration when λ=1310 nm, T=300 K, P=-20 dBm, and V=0.95Vbr
图 7. 当λ=1310 nm,T=300 K,P=-20 dBm,V=0.95Vbr时,电荷浓度随掺杂浓度的变化。(a) Ge-APD;(b) Si-APD
Fig. 7. Charge concentration versus doping concentration when λ=1310 nm, T=300 K, P=-20 dBm, and V=0.95Vbr. (a) Ge-APD; (b) Si-APD
本文模拟了APD中碰撞电离率(Igr)随Ge层和Si层掺杂浓度的变化,如
图 8. 当λ=1310 nm,T=300 K,P=-20 dBm,V=0.95Vbr时,掺杂浓度对Ge/Si APD的性能影响。(a)碰撞电离率随Ge层掺杂浓度的变化;(b)碰撞电离率随Si层掺杂浓度的变化;(c)电子离化系数随Ge层掺杂浓度的变化;(d)空穴离化系数随Ge层掺杂浓度的变化;(e)电子离化系数随Si层掺杂浓度的变化;(f)空穴离化系数随Si层掺杂浓度的变化
Fig. 8. Effect of doping concentration on performance of Ge/Si APD when λ=1310 nm, T=300 K, P=-20 dBm, and V=0.95Vbr. (a) Impact ionization rate versus doping concentration of Ge layer; (b) impact ionization rate versus doping concentration of Si layer; (c) electron ionization coefficient versus doping concentration of Ge layer; (d) hole ionization coefficient versus doping concentration of Ge layer; (e) electron ionization coefficient versus doping concentration of Si layer; (f) hole ionization coefficient versus doping concentration of Si layer
离化系数(Ic)可以用来表示碰撞电离的难易程度,因此,我们模拟了电子和空穴的离化系数随Ge层和Si层掺杂浓度的变化,如
图 9. 当λ=1310 nm,T=300 K,P=-20 dBm,V=0.95Vbr时,电场强度随掺杂浓度的变化。(a) Ge-APD;(b) Si-APD
Fig. 9. Electrical field intensity versus doping concentration when λ=1310 nm, T=300 K, P=-20 dBm, and V=0.95Vbr. (a) Ge-APD; (b) Si-APD
我们分别研究了APD的增益随Ge层和Si层掺杂浓度的变化,如
图 10. 增益随掺杂浓度的变化。(a)(b) λ=1310 nm,T=300 K,P=-20 dBm;(c)(d) λ=1310 nm,T=300 K,P=-20 dBm,V=0.95Vbr
Fig. 10. Gain changes with doping concentration. (a)(b) λ=1310 nm, T=300 K, and P=-20 dBm; (c)(d) λ=1310 nm, T=300 K, P=-20 dBm, and V=0.95Vbr
同时本文还研究了Ge/Si APD的3 dB带宽随Ge层和Si层掺杂浓度的变化趋势,如
图 11. 当λ=1310 nm,T=300 K,P=-20 dBm 时,3 dB带宽和增益带宽积随掺杂浓度的变化
Fig. 11. 3 dB bandwidth and gain bandwidth product versus doping concentration when λ=1310 nm, T=300 K, and P=-20 dBm
此外,本文还模拟了掺杂浓度对APD增益带宽积的影响,如
为解释带宽变化的原因,我们模拟了电子/空穴速率随Ge层和Si层掺杂浓度的变化,如
图 12. 当λ=1310 nm,T=300 K,P=-20 dBm,V=0.95Vbr时,载流子速率随掺杂浓度的变化
Fig. 12. Carrier velocity versus doping concentration when λ=1310 nm, T=300 K, P=-20 dBm, and V=0.95Vbr
电子在电场作用下从Ge吸收层漂移到电荷层,由于导带没有带阶,故电子可以无阻碍通过电荷层到达倍增层。随着Ge吸收层掺杂浓度的增加,p型重掺杂(p+)-Ge/i-Ge界面电场逐渐增强,载流子在此处的碰撞电离也增强,载流子浓度增大,但Ge吸收层内部的电场却逐渐减弱,导致Ge吸收层内的电子在向电荷层漂移的过程中受到限制,载流子速率下降,因此电子浓度在Ge吸收层内部增大,致使到达倍增层的电子数量减少。随着Ge吸收层掺杂浓度的增加,Si倍增层内的电场减小,电子在倍增层内的碰撞电离效应减弱,导致Si倍增层内的电子浓度和空穴浓度同时减小,因此最后到达n型重掺杂(n+)-Si并被吸收的电子数量减少。另外,随着Si倍增层掺杂浓度的增加,Ge吸收层内的电场逐渐增大,碰撞电离系数逐渐增加,导致吸收层电子浓度增加,因此通过漂移穿过电荷层达到倍增层的电子数量增加。随着Si倍增层掺杂浓度的增加,p型轻掺杂(p-)-Si/i-Si界面的电场逐渐增强,电子/空穴离化系数变大,导致Si倍增层电子浓度增加,但Si内部电场却逐渐减弱,电子/空穴离化系数变小,导致空穴浓度在Si倍增层内呈现减小趋势。对于器件而言,键合界面的价带带阶对空穴有一定的限制作用,导致空穴在键合界面堆积,空穴浓度增大,不利于空穴的漂移。
4 结论
在Ge/Si键合界面处引入poly-Si材料,在理论上研究了Ge层和Si层掺杂浓度对Ge/Si APD性能的影响。研究表明,引入poly-Si层后,暗电流达到1×10-10 A量级,当Ge层掺杂浓度为1×1012 cm-3且反向偏压为28.0 V时,增益最大为12.21。当Si层掺杂浓度为1×1015 cm-3且反向偏压为28.2 V时,增益最大为12.14。随着Ge层和Si层掺杂浓度(1×1012~1×1016 cm-3)的增加,在相同偏压下,3 dB带宽整体呈现上升趋势,当Ge层掺杂浓度超过1×1016 cm-3时,带宽急剧下降。当Ge层掺杂浓度为1×1012 cm-3时,增益带宽积达到最大值225.76 GHz;当Si层掺杂浓度为1×1012 cm-3且偏压为29.5 V时,增益带宽积达到最大值215.15 GHz。因此,选择较低的Ge吸收层和Si倍增层掺杂浓度,能够获得增益和增益带宽积均较为理想的Ge/Si APD,同时也可避免无电场和隧穿现象的发生。
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Article Outline
张娟, 苏小萍, 李嘉辉, 王战仁, 柯少颖. 基于poly-Si键合层的SACM型Ge/Si APD的优化设计研究[J]. 中国激光, 2024, 51(8): 0803001. Juan Zhang, Xiaoping Su, Jiahui Li, Zhanren Wang, Shaoying Ke. Optimal Design of SACM Ge/Si APD Based on Poly-Si Bonding Layer[J]. Chinese Journal of Lasers, 2024, 51(8): 0803001.