Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
The Simulation of Ge-on-Si APD Xiaohui Yi Abstract-Ge/Si avalanche photodiode(APD) become a promising candidiate operating at 1.31-1.55um due to complementary metal-oxide-semiconductor compatibility, high-absorption coefficient of Ge, low impact ionization rate of silicon,and high thermal conductivity. To the next research, we designed a Ge/Si APD and as follow simulated a series of performances by using commecial software Silvaco, then we obtain the dark current level, breakdown voltage, dc gain, and electric field profile, as well as frequency response and gain-bandwidth product. Index Terms-Avalanche photodiode, germanium, silicon, simulation. 1. INTRUDUCTION In optical fiber communication systems avalanche photodiodes can be applied to receive photo information and improve the detection sensitivity due to its internal current multiplication mechanism.As we see, in the past several years, standard InP-base APDs instead of silicon-base APD have been produced successfully in a large scale. But It looks less promising than Si-base devices. Because there is a lower multiplication noise and also complete integration for APDs produced by silicon. As we all know, silicon material is extremely important for integrate circuit, but it is difficult to produce active devices because of the intrinsic properties-indirect gap. We should find a new materia which can be easily combine with Si. Ge and Si both belongs to elements. they have a common crystal structure-diamond structure. Germanium is one of the most attractive material due to the strong absorption of 1.55um,and the Ge absorption range can be extended up to 1.6um by introduce bandgap shrinkage to tensile strain. Consequently, it is a promising candidate for high quality material applied in a large scale. Ge/Si APD with a separated absorption-charge-multiplication(SACM) structure decouple the absorption region and avalanche region, has a extremely high performance. For instance, high internal gain, high gain-bandwidth product. Silicon has a low k(defined as the ratio of the ionization rate of electron and hole), which has been proved play a important role in multiplication coefficient and also excess noise. In silicon, the multiplication happen but photo-generated carriers are produced in Ge. There is a 4.2% mismation between Si and Ge, which is considered as the most annoying problem. Epitaxial growth of high quality thick germanium becomes very difficult for the reason of high concentration of threading dislocation in epitaxial layer and dismatch dislocation in the interface. Fortunately, a great many approach have been demonstrated. High-quality epitaxial Ge-on-Si growth has also been achieved through suitable direct Ge growth without using SiGe buffers layers. In the case, a two-temperature Ge growth technique is used to prevent islanding during the ultrahigh vacuum chemical vapour deposition(UHV-CVD) process system, with subsequent annealing to significantly decrease the threading dislocation density. In the first growth temperature, a thin epitaxial Ge buffer layer of 30-60 nm is directly grown on Si at 320-360℃. At such low temperatures, the low surface diffusivity of Ge kinetically suppresses the islanding of Ge. The main growth temperature is>600℃, which is purpose on achieving higher growth rates and better crystal quality. the equivalent circuit models of APDs have been reviewed in the past years. Some passive devices like resistance, capacitance and conductivity and a few active devices like current source have been introduced. The process of designing such complicated modern devices as APDs requires deep understanding of semiconductor physic. How ever, even in such case, it is extremely difficult to predict overall properties of structure. Modern technology can be helpful in designing photodiodes via more and more sophisticated simulation software. This type of software package belongs to a category defined as technology computer-aided design(TCAD). We use commercial software Silvaco to simulate my devices. Silvaco is a famous company software, which is devote to process simulation and device simulation of semiconductor. It have been used to electrical circle automation. In 2006, silvaco entered into Chinese market, and it has develop the basic theory research for semiconductor physic. 2.EXPERIMENT In general, silvaco is consist of a large number of simulationmodule,for instance, Athena, Atlas , Devedit . Atlas is the module of semiconductor device simulation, which include several sub-module. It can simulate but not limit the performance of electricity, phonics, thermotics, direct current, and alternating current. Atlas is a powerful device simulation module connected with a series of sub-module. We can define easy devices directly by atlas, a module of silvaco, which can be invoked by deck-build. But for a complicated structure, it will employ more convenient to describe the device by the DevEdit, in which the device is constructed by a series of node. In the experiment, we use DevEdit to get structure of device. First of all, we will define a mesh in both x direction and y direcion. Then, the region wich a unique number will be introduced in the mesh. Next, doping concentration and type(acceptor or donnor) will be specified. Then, at least there is one electrode in the surface of the device. Fig. 1 shows the schematic configuration of the Ge/Si SACM APD. There is a thin charge layer with 0.1um between the multiplication layer and the absorption layer. This designment can obtains sufficient gain via a high electric field in the multiplication layer while the electric field in the absorber is low enough to ensure carrier drift without multiplication. Fig. 1. Schematic of the Ge/Si SACM-APD structure modeled in this paper. The Ge/Si APD is modeled by solving the Poisson’s equation coupled with the charge continuity equations. In this model the total charge caused by the presence of traps is subtracted from the right hand side of the Poisson’s equation as follows: E q p n N D N A x (1) In (1), n and p are the electron and hole carrier densities, respectively, which are calculated by solving the carrier continuity equations ((2) and (3)) coupled with current density equations ((4) and (5)), self consistently: n 1 J n Gn Rn t q x (2) p 1 J p G p Rp t q x (3) n x p J p qpv p ( E ) qD p x J n qnvn ( E ) qDn (4) (5) where Jn and Jp are the electron and hole current densities, Respectively. Gn,p and Rn,p in (2) and (3) represent generation and recombination processes, such as photoabsorption and impact ionization as well as trap-assisted (Shockley, Read, Hall) recombination (RSRH). To describe the impact ionization process the Selberherr model, which shows a strong dependence of the impact ionization coefficients on the electric field, is used. Phonon transitions occur in the presence of a trap (or defect) within the forbidden gap of the semiconductor. Therefore, carrier recombination processes such as SRH, given by (6) are also included in the model: R SRH np ni2 E Et E Ei n p ni exp i p n ni exp t kT kT (6) 3. RESULT AND ANALYSIS Fig. 2 show the impuriy concentration distribution of devices in y direction. The concentration of each layers all obtain by the analysis and caculation. Its purpose is deserve appropriate field distribution, as we can see from Fig. 3. Fig. 2. impuriy concentration distribution of devices in y direction. The electric field distribution across the device is illustrated in Fig. 3 for an ideal APD, The electric field inside the silicon multiplication region must be above the critical value of 4105 V/cm to initiate an avalanche, but in gemanium layer, the electric field must be lower than 1105 V/cm to ensure there is no any impact ionization and also be higer than 1 10 4 V/cm to make the velocity of carriers saturated. Fig. 3. Field distribution in y directuon at 30V bias. As it is shown in Fig. 4, the most of impact ionization happen in Silicon mutiplication layer, although there is a little impact ionization process in Ge layer.We can see from Fig 5. Almost light is absorbed in Ge layers and the absorption rate exponential decay since the depth increase. Fig. 4. generation rate caused by impact ionization Fig. 5. light absorption rate in material Fig. 6. spectral response of Ge/Si APD Fig. 7. shows the simulated dark current (DC) and total current(TC) of a circular 30 μm-diameter APD versus bias voltage under an input optical power of 5 w / cm 2 at 1310 nm. As the reverse bias applied to the device increases, the depletion region expands into the Ge region. The APD breakdown voltage is defined as the reverse bias voltage at which the dark current is 100 μA. The dark current for the APD device is 0.3 nA at a bias equal to 90% of the breakdown voltage (Vbd = −34.24 V). Fig. 7. I-V characteristic of Ge/Si APD Fig. 7. Gain vs bias of Ge/Si APD Fig. 7. Frequency response of Ge/Si APD 4.CONCLUSION 5. In this papar we simulated the Ge/Si SACM APD by Silvaco. The result shows that Ge/Si APD has a high performance for the future photoelectricity. REFENRENCES [1] Yimin Kang, Han-Din Liu, et al. Monolithic germanium/silicon avalanche photodiodes with 340GHz gain-bandwidth product. Nature photonics, 2008, 247:10.1038. [2] K.E. Ponomarev, A.A. Shklyaev, et al. Shape of epitaxial Ge islands on Si(100) surfaces, 2013, Electron Devices IEEE Transactions 2013, 978-1-4799-0762-5 [3] Zhang Q, Wu N, Osipowicz T, et al. Formation and thermal stability of nickel germanide on germanium substrate[J]. Japanese journal of applied physics, 2005, 44(10L): L1389.