不同晶态Ge薄膜键合层对InGaAs/Si雪崩光电二极管性能的影响研究

Effect of Different Crystalline Ge Film Bonding Layers on Properties of InGaAs/Si Avalanche Photodiodes

由于InGaAs与Si之间存在7.7%的晶格失配,因此难以获得制备方式简单、性能良好的InGaAs/Si雪崩光电二极管(APD)。从理论上提出了一种从源头弱化InGaAs/Si晶格失配对APD性能影响的办法,即在InGaAs/Si键合界面引入一层a-Ge或poly-Ge键合层,模拟比较了InGaAs/Si APD性能随键合层厚度的变化情况。研究指出,a-Ge和poly-Ge材料作为键合层对载流子有阻挡或俘获作用,因此器件能够获得超低暗电流,且由于键合层导带势垒对载流子的阻挡作用,APD雪崩之后出现了光暗电流间隙,可以在较小暗电流情况下获得大的光电流。当a-Ge厚度为0.5 nm时,APD雪崩击穿前增益可达最大值451.3,而当poly-Ge厚度为0.5 nm时,雪崩击穿前增益仅为7.9。这种差异是由于poly-Ge键合层处价带出现了势阱,载流子浓度下降。该工作为超低噪声和高增益InGaAs/Si APD的研制提供了理论指导。

Objective InGaAs materials as absorption layers and Si materials as multiplication layers are potential alternatives for achieving high-performance avalanche photodiodes(APDs).However,simple and well-performing InGaAs/Si APDs are difficult to fabricate owing to the 7.7% lattice mismatch between InGaAs and Si.Investigators have recently reported that a-Si was introduced at the InGaAs/Si APD bonding interface to inhibit the nucleation of mismatch dislocations and realize an ultra-low dark current.However,owing to the large bandgap of a-Si,the bonding interface has a large conduction band and valence band offset.This causes the gain of the device to decrease.Ge and Si are both indirect band gap semiconductors,and Ge materials have the advantages of a small gap width and a long absorption cutoff wavelength in the infrared region.Hence,in this study,a method to mitigate the effect of the InGaAs/Si lattice mismatch on APD performance from the source side is theoretically proposed.Here,a-Ge or poly-Ge bond layers are introduced into the InGaAs/Si bond interface,and the variation in the InGaAs/Si APD performance with the bond layer thickness is simulated and compared.In this work,theoretical guidance for the development of ultralow-noise and high-gain InGaAs/Si APDs will emerge.Methods An a-Ge or poly-Ge bond layer is introduced into the InGaAs/Si bond interface,and variations in APD performance with bond layer thickness are simulated and compared.Initially,the optical and dark currents of the APD are simulated and compared considering the thickness of the bonding layer.Subsequently,the recombination rate and carrier concentration of the APD under light conditions are simulated to understand the cause of the change in the APD optical current.To further understand the cause of the change in the electron concentration of the APD,the changes in the APD energy band under light conditions are simulated.Then,the changes in charge concentration,impact ionization rate,electric field,and other parameters with the bond layer thickness are simulated and compared.Finally,the gain and gain bandwidth products of the APD are simulated and compared to further explore the performance improvement of the device.Results and Discussions After introducing a-Ge or poly-Ge bond layers,the dark current of the APD before avalanche can be as low as 10~(-11) A(Fig.2).Moreover,potential barriers or wells appear in the energy band of the bonding interface(Figs.8 and 9).Owing to the barrier effect and hole trapping effect,optical and dark current gaps appear in both(Fig.2),and this phenomenon is more obvious in the APD with the poly-Ge bond layer.These results indicate that both the a-Ge and poly-Ge bonding layers can reduce device noise.The gain and gain-bandwidth products of the APD are simulated and compared.The results show that when aGe is used as the bonding layer and the thickness of the bonding layer is 0.5 nm,the gain and gain bandwidth product can reach its maximum.The maximum gain of the APD can reach 451.3(Fig.15),and the maximum gain bandwidth product can reach 13.7 GHz(Fig.20).Theoretically,an InGaAs/Si APD with high gain and ultralow noise is obtained.Conclusions In this study,the effects of a-Ge and poly-Ge bonding layer thicknesses introduced at the InGaAs/Si bonding interface on the performance of InGaAs/Si APD are theoretically studied.The results show that the dark current before the avalanche can be as low as 10~(-11) A when the a-Ge and poly-Ge bond layers are introduced.Furthermore,the APD with a-Ge or poly-Ge bond layers introduced after the avalanche exhibits optical and dark current bandgaps.This will help the APD achieve ultralow noise.Similarly,the gain and gain bandwidth products of the APD with an a-Ge bonding layer are much larger than those with a poly-Ge binding layer.The gain and gain bandwidth products of the APD decrease with an increase in the bond layer thickness.The APD performance is optimum when a-Ge is used as the bonding layer and the bond layer thickness is 0.5 nm.At this time,the maximum gain can reach 451.3 and the maximum gain bandwidth product can reach 13.7 GHz.However,when poly-Ge is used as the bonding layer,the maximum gain of the APD is only 7.9 and the maximum gain bandwidth product is only 598 MHz.Therefore,the use of aGe as the bonding layer material and the selection of a thin bonding layer are considered ideal schemes for preparing InGaAs/Si APD with improved device performances.