A Novel Structure of a-SiN:H/a-Si:H Multilayer Avalanche Photodiode (MAPD)
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A NOVEL STRUCTURE OF a-SiN:H/a-Si:H MULTILAYER AVALANCHE PHOTODIODE (MAPD) JIAO LIHONG, MENG ZHIGUO, SUN ZHONGLIN NANKAI UNIVERSITY, INSTITUTE OF PHOTOELECTRONIC DEVICE AND TECHNOLOGY ,TIANJIN, 300071,P.R.OF CHINA ABSTRACT Because of the lower density of interface states in a-Si:H/a-SiN:H than that in a-Si:H/a-SiC:H, an a-Si:H/a-SiN multilayer reach-through avalanche photodiode is fabricated on an ITO/glass substrate by plasma-enhanced chemical vapor deposition(PECVD). In order to improve the performance of the a-Si:H/a-SiN:H APD'S, a novel structure is used. By controlling the deposition ratio of silicon and nitrogen of amorphous SiN, the valence band top of a-Si:H is deeper than that of a-SiN:H, that is, the a-Si:H/a-SiN:H system has the electron potential well in a-Si:H, while the hole well is in a-SiN:H, thus we can successfully suppress the hole impact ionization, correspondingly enhance the electron impact ionization effectively. The measurement of current versus voltage is employed to study the multiplication factors and the impact ionization coefficients. The characteristics of a-Si:H/a-SiN:H APD's,such as I-V curves, optical gains, impact ionization rates, excess noise factors, the relative response and the relationship between the breakdown voltage and wavelength, are studied. The electron multiplication factor is M,=4.5 at reverse bias V=12v. An optical gain of 3.7 at reverse bias VR=1 2 v and an incident light power Pm=3iw is obtained.Homojunction a-Si:H reach-through APD's and homojunction a-Si:H APD's are also fabricated for comparison.The results show that the novel a-Si:H/a-SiN:H APD's is promising in high-gain, low-noise photodetectors. 1.
Introduction
Recently,different kinds of MAPD, especiallyn -V compound MAPD have been studied carefully'. Amorphous Silicon MAPD's study 2 also has been conducted . But few studies on a-SiN:H/a - Si:H MAPD has been done 3 . Compared to a-SiC:H/a-Si:H, a-SiN:H/a-Si:H has many advantages, such as lower interface states. But a-SiN:H has higher resistance. If it was used to the conventional structure of MAPD, high speed, high gain and low noise can not be obtained easily. We propose a new structure:Valley Reach-Through: when the ratio of N/Si in the films is properly controlled, the avalanche region of electrons and holes will separate spatially as shown in fig.l. That is, the relative position of a-SiN:H and a-Si:H will shift compared to the conventional structure, the impact ionization of holes is suppressed effectively. Thus the contradiction between high resistance and high gain disappears. 2.Device structure and fabrication: When the ratio of N/Si in a-SiN:H material achieves a proper value x=0.38, l-x=0.62,a different structure will appear 4 , as shown in fig.2. Egs 8 N =2.2ev, AEc =0.55ev,
AEv
=0.15ev.
Mat. Res. Soc. Symp. Proc. Vol. 297. ©1993Materials Research Society
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For electrons, a-SiN:H is a barrier layer, electrons will accelerate on it, a-Si:H is a well layer, electrons will impact in it. For holes, a-SiN:H becomes a well layer and a-Si:H
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