Recently, the demands for the high performance Short-wave infrared (SWIR) image sensors in the defense, the industrial or the biomedical applications have increased due to its advantages of the day-to-night imaging, the atmospheric transmission and the visible-like imaging characteristics. For these applications, a dark current, the fundamental leakage current of the focal plane arrays (FPAs), needs to be reduced to obtain the sufficient sensitivity or detectivity. In order to improve the dark current characteristics of SWIR image sensors, the planar- and the mesa-type device structures are commonly used in InGaAs/InP PIN photodetectors. Among them, the planar-type PIN photodetectors are widely utilized due to the low dark current characteristics which is the key parameter required in the high performance SWIR PIN photodetectors.
In this thesis, the InGaAs/InP planar-type PIN photodiodes with guard-ring structures and ALD-$Al_2O_3$ passivation techniques have been developed for the low dark current characteristics. At the first, the devices with the guard-ring structures are fabricated using the conventional passivation layer of a PECVD-$SiO_2$ for the reduction of the bulk dark current densities. The fabrication steps for the planar-type PIN photodiodes are as follows. Firstly, the active pixel areas and guard-ring regions are diffused by the Zn-doped spin-on-glass (SOG) diffusion technique to form p-doped areas. After the diffusion process, the $SiO_2$ passivation film is deposited using a PECVD technique. Then, on the p-doped areas of the InGaAs photodiode, a p-contact is formed by evaporation of Zn/Au/Cr/Au and lift-off followed by a 400 ˚C annealing process. The fabricated p-ohmic contact shows a specific contact resistivity of 5.96x10-4 $\Omega$·$cm^2$. Subsequently, the top contact metal evaporation of Ti/Pt/Au and lift-off is performed and followed by a post-metallization annealing process. Finally, the fabricated InGaAs/InP planar-type PIN photodiode shows almost 1.3 times higher R0A product of 7.4x104 $WOmega$·$cm^2$ than the R0A product of the non-guarded device.
Next, the InGaAs/InP planar-type PIN photodiodes with guard-ring structures are optimized with the ALD-$Al_2O_3$ passivation layer. Because of its low damage process, the thermally grown ALD-$Al_2O_3$ passivation layer is used for reducing the surface dark current densities. The device with guard-ring structures passivated by the ALD-$Al_2O_3$ shows the R0A product of 1.4x105 $\Omega$·$cm^2$ which is almost 1.9 times higher than the R0A product of the PEVCD-$SiO_2$ passivated device. In sequence, for the further improvement of the surface dark current characteristics, the effects of the post-deposition annealing with different temperatures are investigated and analyzed. In order to obtain the optimal temperature of the post-deposition annealing process, the experiments in various temperature conditions are performed. As a result, the optimized InGaAs/InP planar-type PIN photodiode with the post-deposition annealing process of 500 ˚C shows the highest R0A product of 2.3x105 $\Omega$·$cm^2$ in this work. The quantum efficiency of the fabricated device is 65 % and the detectivity is calculated to be 3.0x$10^{12}$ cmHz$^{1/2}$/W.
Consequently, the results show that the fabricated InGaAs/InP planar-type PIN photodiodes using guard-ring structures and ALD-$Al_2O_3$ passivation techniques are very promising for the SWIR imaging system applications.
높은 detectivity 를 요구하는 SWIR Imaging system 으로써의 응용을 위하여 낮은 dark current 특성을 갖도록 guard-ring 및 ALD-$Al_2O_3$ passivation 을 InGaAs/InP planar-type PIN photodiode 에 적용하여보고 그 특성을 분석하였다.
흡수층의 electron-hole pair 로 인한 dark current 감소를 위하여 guard-ring 이 적용된 구조를 설계하고, PECVD-$SiO_2$ passivation 을 이용하여 소자를 제작하였다. Active pixel 과 Guard-ring 을 형성하는 확산공정 시간을 줄이고, reliability 및 uniformity 를 개선하기 위하여 기존의 Sealed ampoule 방식이 아닌 Zn-doped SOG 방식을 적용하였고, 높은 양자효율을 얻기 위하여 1.0 $\mu$m 로 diffusion depth 를 설정하고, 공정조건을 확립하였다. 이 후, Zn/Au/Cr/Au metal layer 의 annealing 을 통해 p-type InP 와 metal layer 간에 ohmic contact 을 형성하였고, 5.9x10-4 $\Omega$·$cm^2$ 의 specific contact resistivity 를 얻을 수 있었다. 이를 기반으로 InGaAs/InP planar-type PIN photodiode 의 제작 및 측정을 진행하여, guard-ring 이 적용된 소자의 경우 7.4x104 $\Omega$·$cm^2$, guard-ring 이 적용되지 않은 소자의 경우 5.5x10 $\Omega$·$cm^2$ 의 R0A product 를 얻을 수 있었으며, guard-ring 을 통해 R0A product 가 약 1.3배 증가할 수 있는 것을 확인하였다.
다음으로, 표면의 trap 으로 인한 dark current 를 감소하기 위하여 ALD-$Al_2O_3$ passivation 을 적용하고, dark current density 의 개선을 최적화할 수 있도록 annealing 을 진행하였다. Guard-ring 구조가 적용된 소자에 ALD-$Al_2O_3$ passivation 을 적용하여 본 후, R0A product 를 측정해본 결과 1.4x105 $\Omega$·$cm^2$ 로 PECVD-$SiO_2$ passivation 이 적용되었던 소자 대비 1.9 배 정도 커진 값을 보여주었다. 이 후, dark current density 의 더 나은 성능을 위해 post-deposition annealing 을 진행하였고, 500 ˚C 로 annealing 하는 조건에서 R0A 가 2.3x105 $\Omega$·$cm^2$으로 가장 좋은 특성을 나타내었다. Detectivity 의 산출을 위하여 양자효율 측정을 진행하였고, 65 % 의 평균값을 보여 양자효율 목표에 만족하였다. 최종적으로 R0A 및 양자효율로부터 3.0x$10^{12}$ cmHz$^{1/2}$/W 의 Detectivity 를 산출하였으며, 이로부터 SWIR Imaging system 으로의 응용에 적합한 InGaAs/InP planar-type PIN photodiode 가 제작된 것을 알 수 있었다.
