Simulation Analysis of Low-Noise InGaAs/InP Avalanche Photodiodes

Objective At present, the main research is focused on lower-noise and higher-gain-bandwidth product of an APD in order to adapt to the evolving optical fiber communication systems. Usually, lower noise is related to higher speed, because a high gain tail can result in higher noise and longer transmission time in the gain distribution. Therefore, a major issue is how to effectively reduce noises in the APD research. An effective way to reduce noises is using beneficial heterojunction structures to impact ionization engineering ((IE)-E-2) design. A heterojunction structure is used in this method to provide spaces with different ionization threshold energies. This structure is more localized than the impact ionization that can be achieved in a spatially uniform structure. Through impacting ionization engineering, it has been achieved in the GaAs/AlGaAs material system with ultra-low noise of k=0.1. Now we have known that the heterojunction structure can reduce the multiplication noise of an APD by controlling the spatial separation of electrons and holes. There is a specific design which is to place a thin narrow band gap layer with a lower threshold energy adjacent to a wider band gap layer with a higher threshold energy, so that more ionization events occur in the thin narrow band gap layer with a lower threshold energy. At present, the first choice is the use of InGaAs/InP APD detectors for optical fiber communication. Because InP material has the advantages of large electron-hole ionization ratio k and low excess noise. In addition, the lattice constants of InP and In0.53Ga0.47As are very small. High quality extension can be realized by molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD). Here, we design InP/InGaAsP SACM APD through impact ionization engineering. An InGaAsP multiplication layer is inserted into the multiplication layer of a traditional InP-based APD, and there are two regions with different ionization threshold energies in the multiplication layer to make ionization events occur more intensively. In addition, the ionization of electrons and holes is regulated spatially in order to reduce the noise. Methods A kind of separation of absorption region-charge region-multiplication region (SACM) InGaAs/InP avalanche photodiodes is designed. Through the impact ionization engineering, an InP APD with double charge layers and double multiplication layers is designed. In order to control the location of impact ionization and to reduce the noise, a classification of ionization threshold energy is set in the multiplication region of this structure. The device simulator Silvaco is used to model this device. The energy band structure, electric field distribution, dark current, 1.55 mu m pulse light response current, gain, etc. are simulated and calculated. At a 30 V voltage, a lower k value of 0.15 can be obtained for the new structure. And compared with the conventional SACM InP APD, the results show that the APD devices with new structures can obtain better noise characteristics. Results and Discussions The device simulator Silvaco is used to calculate and simulate the light and dark currents and gain of the device. The photocurrent of the device is obtained under the condition of the incident wavelength of 1310 nm, the dark current of the APD detector is less than 10 nA under the punch-through voltage, the punch-through voltage of the device is about 20 V, and the breakdown voltage is about 30 V. The device gain is set as 1.0 at the punch-through point. When the bias voltage rises by 30 V, the gain of the device increases from 1.0 to 8.0 (Fig. 2). The ionization coefficients alpha and beta of holes and electrons are simulated and calculated in the device simulator Silvaco. We extract the hole and electron ionization coefficient values of 20 points, integrate them to calculate the k value (alpha/beta) of each point and take the average value to obtain the noise coefficient k=0.15. The excess noise and gain simulation diagram are obtained, and we can see that (IE)-E-2 APD has a low excess noise (Figs. 3 and 4). The same treatment is also performed on the conventional InP APD. The ionization coefficient values of holes and electrons at 20 points are selected and integrated to obtain the excess noise coefficient k=0.35. The design of impact ionization engineering is to insert a thin layer with a relatively low threshold energy in the area with a higher ionization threshold energy, so that the most ionization events occur in the trap layer with a low threshold energy, which leads to more certainty in the location of impact ionization, and thus the noise is reduced. From the partial energy band structures of the two devices in the equilibrium state, we can see that the well layer structure is designed in the (IE)-E-2 APD energy band (Fig. 5). Conclusions The charge layer and multiplication layer of the conventional InP/InGaAs SACM APD are improved through impact ionization engineering, and an APD with double multiplication layers and double charge layers is designed. Two InGaAsP layers with lower ionization threshold energy are inserted between the InP multiplier layer and the InP charge layer, so that the most ionization events occur in the InGaAsP well layer. Through the structure designed by impact ionization engineering combined with the step distribution electric field in the multiplication region, the spatial modulation of the impact ionization events of electrons and holes leads to a lower excess noise with k=0.15, which has a lower excess noise compared with the conventional InP APD with the single multiplication layer. The design of this structure takes into account the positive influence of the dead zone on the excess noise and also takes into account the positive effects of energy band design and ionization threshold energy on the position of carrier multiplication. This will have a great guiding effect on device preparation in future.