Asian Journal of Nanoscience and Materials, 2018, 1(3), 122-134.

FULL PAPER Simulation and Characterization of PIN for Photonic Applications Waqas Ahmada,b,*, Muhammad Umair Alic, Vijay Laxmid, Ahmed Shuja Syed b aSZU-NUS Collaborative Innovation Centre for Optoelectronic Science & Technology, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, P.R. China. bCentre for Advanced Electronics & Photovoltaic Engineering, International Islamic University, Islamabad 44000, Pakistan. cDepartment of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P.R. China. dTHz Technical Research Center of Shenzhen University, College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, P.R. China.

Received: 28 March 2018, Revised: 05 May 2018 and Accepted: 09 May 2018. ABSTRACT: Research conducted on based technology has recently shown rapidly growing momentum to develop the robust silicon based detectors for photonic applications. The thrust is to manufacture low cost and high efficiency detectors with CMOS process compatibility. In this study, a new design and characterization of PIN photodiode is envisaged. The simulation tool, Silvaco TCAD (and its variants), was used to design and simulate the processes of the device. Electrical and optical measurements such as I-V characteristics (dark current), and internal/external quantum efficiencies were analysed to evaluate the designed and processed device structure for its potential applications in photonics and other detection mechanisms.

KEYWORDS: PIN Photodiode, CMOS, I-V Characteristics, Quantum Efficiency. GRAPHICAL ABSTRACT:

1. Introduction Over the last five decades, have applications including commercial use, and been used for extensive range of military purposes [1]. are

*Corresponding author: Waqas Ahmad, Email: [email protected], Tel: 0086-15602998010

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Asian Journal of Nanoscience and Materials optoelectronic devices which convert light . By increasing the bias energy into electrical energy and are voltage, time performance of photodetectors generally used in can also be increased [2, 3]. systems. Typically, there are two types of PIN photodiode is based on the P-N photodetectors which are commonly used, junction which has highly doped regions. P, named as p-i-n photodiodes and avalanche I and N photodiode can be structured in photodiodes (APDs). These devices have a different ways as demonstrated in Fig. 1. wide range of applications in various fields The planar PIN photodiode has lower cost of science and technology. For instance, and better efficiency as compared to the they are used in light sensors, diagnostic vertical PIN photodiodes [4]. In a PIN tools, digital radiography, and nuclear photodiode, intrinsic region is increased to medicines etc. The basic operation of a gain higher efficiency and photons produce silicon photodetector is to convert light more electron-hole pairs. PIN photodiode energy into electrical energy in which has the capability to increase transient time electron-hole pairs are generated via reverse and decrease the junction capacitance. The bias voltage to increase the depletion region. speed of a photodiode depends upon three

Due to carrier drift in opposite directions, a factors including diffusion, drift, and signal is generated towards the collecting junction capacitance. In diffusion, the electrodes. No signal will be generated even charge carriers are produced to the outer if there is presence of electric field in the sides of the depletion region which are close depletion region but the electron-hole pairs enough to diffuse them. The drift time, are produced outside. Photodetector which is in the diffusion region, can be performance depends upon the interaction reduced by increasing the intrinsic width of of photons and charge carriers in the the junction, while, the junction capacitance

124 Ahmad et al. Asian Journal of Nanoscience and Materials can be reduced by strong reverse bias [5].

Fig. 1. Basic structures of PIN Photodiodes.

Intrinsic silicon is used to fabricate the PIN the holes [6]. Compared to other optical photodiode. In this structure, heavily doped detectors, silicon PIN photodiode has

P and N regions are near the intrinsic various advantages such as lower cost, region, therefore it is referred as the P, I and higher efficiency, smaller size, better

N . In a PIN photodiode, junction resolution, as well as room temperature capacitance is inversely proportional to the operation capability. It is used to measure depletion region, therefore, depletion region the position of light and its pixels are used absorbs more photons. To achieve high for image sensing. This type of device can frequency response of the PIN photodiode, also be used to measure the short distance in mobility of electrons should be greater than optical communications as well as optical

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Asian Journal of Nanoscience and Materials storage. By reducing the surface reflectance, multipliers and resonators [9]. W. M. Jubadi the performance of a PIN photodiode can be and co-workers used the Sentaurus TCAD enhanced [7]. N. I. Shuhaimi et al. used the technology to simulate the four different

Sentaurus TCAD tools to design the silicon intrinsic layers of PIN diode with various

PIN diode. They investigated the I-V thickness (5 µm, 20 µm, 30 µm, and 50 µm) characteristics as well as the effects in and investigated the I-V characteristics of variations of un-doped regions on the device PIN diode. They explored the “I” layer’s performance. They varied the width of the thickness and its effect on PIN diode’s

PIN photodiode while the thickness was performance. The results obtained from kept constant at 40 µm. I-V characteristics simulation showed that the thinner is the “I” and device performance at different widths layer, the more forward current flows in the

(70 µm, 80 µm and 90 µm) were analysed PIN diode, improving the overall device and it was observed that as the width is performance [10]. A. W. Vergilio proposed increased, the current level is also increased, three terminal PIN diode structure which suggesting the importance of width works at forward conductive state, optimization to realize high performance demonstrating the accuracy in simulation photodiodes [9]. E Abiri et al. described [11]. Z. Zainudin et al designed silicon on various applications of PIN diode by insulator (SOI) PIN diode at Athena and demonstrating a device which is very analysed its electrical characteristics using similar to the conventional PIN diode with Atlas Silvaco software. They investigated an extra layer at the centre of the layers in the performance of SOI PIN diode and

PIN diode, realizing a device which possess simple PIN diode and realized that former diverse applications such as modulators and produces lower leakage current as compared demodulators, , high frequency to the simple PIN diode. However, SOI

126 Ahmad et al. Asian Journal of Nanoscience and Materials photodiode shows poor performance in the absorption coefficient and “w” is the temperature variations [12]. Here, in this width of the absorption layer. The paper, we developed a simulation based PIN photodetection performance can be photodiode using the commercial software determined if quantum efficiency of the

TCAD Silvaco i.e. Athena is employed for device is known. The sensitivity of a designing the structure of the photodiode photodetector, its noise equivalent power and Atlas is used for the evaluation of (NEP) and responsivity depend upon the electrical and optical characteristics of the quantum efficiency [4]. Surface reflectivity proposed device. is a unique property of a photodetector

2. Theory which is defined as output current divided

Photodetection is one of the most emerging by the incident light i.e. research fields in optoelectronics. Silicon R = (2) photodiodes are low cost devices which where “ ” is the output current and have the ability to detect light in different “ ” is the optical power. It is linearly ranges of wavelength. Silicon photodetector independent of the quantum efficiency [5]. has unique features such as 90 % surface A relatively small electric current is present reflection, response time approaching to 30 in the absence of light which flows through ps and wavelength approaching to >850 nm the photo-sensitive devices. Due to dark [13]. The quantum efficiency of a current, photodetector can operate under an photodetector is defined as the number of applied bias and generates electrical signals electron-hole pairs generated per incident in the absence of light. The performance of photon and is given by: the dark current will be increased if a bias η = (1-R) [1-exp (-αw)] (1) voltage is applied across the photodetector. where, “R” is the surface reflectivity, “α” is

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It has the capability to detect small signals upon the electrical frequency. It is used to from the photodetector [5, 15]. In a photo- find the magnitude of the noise if the detecting device, the noise power is defined electrical frequency of a photodetector is as the collection of light resulting in signal known. To find the noise power of the to noise ratio of 1 or in other words, root system, a term, power spectral density is mean square (RMS) noise current divided used which is described as the noise content by the responsivity is known as noise power as a function of frequency. Mathematically and its unit is . it depends on the it is expressed as: frequency of light and normally (4) photodetectors are characterized by the where, PN is the noise power over a single value of NEP. It is used to evaluate bandwidth range, S (f) is the power spectral photodetector’s performance as well as to density, and Δf is the bandwidth [5]. compare the performance of various 3. Experiment Details photodetectors [15]. Detectivity is another We employed Silvaco TCAD tool to design

Fig. of merit for a photodetector to evaluate the PIN photodiode and investigated its its performance which is reciprocal of NEP electrical and optical characteristics. Silvaco and is given by the following formula: is a simulator tool of TCAD process for

D = (3) device simulation and consists of Athena and Atlas [16]. Athena is used for the design Detectivity is also used for comparing the of physical structure and Atlas is used for its performance of photodetectors, however, it electrical and optical characterizations [17]. is not suitable to find the response of a The dimension of the designed two- photodetector to an intensity spectrum [15]. dimensional device in this study is 2500×22 Noise spectrum of a photodiode depends µm, having silicon with phosphorous

128 Ahmad et al. Asian Journal of Nanoscience and Materials concentration of 1000 ohm/cm2. 0.6 µm silicon wafer was diffused with phosphorus oxide layer was deposited on the silicon with a concentration of the 8.02 × 1018 at wafer for the masking purpose as well as a 1150 °C for 70 min. Aluminium is mostly passivation layer. For making the P well and used for the purpose of metallization to

N well, it is necessary to make the make electric contacts between circuits of geometrical etching at silicon based wafer the semiconductor materials. In the next to form the P well and N-well. First, we step, aluminium was annealed at 500 °C for fabricated P-well for which geometrical 30 min. The final simulated structure of the etching was done by diffusing Boron atoms PIN photodiode is presented in Fig. 2. All with a concentration of 8.09 ×1016 on the the simulation codes used for the device left side of the silicon wafer at 1200 °C for design and characterization in this work are

120 min. For N- Well, the right side of the provided in Supplementary File.

Fig. 2. Simulated structure of PIN photodiode.

4. Electrical and Optical Atlas is the module of the Silvaco simulator Characterizations. which is used to obtain the electrical

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Asian Journal of Nanoscience and Materials characteristics of the device. It is a two or thermal behaviour of semiconductors with a three dimensional simulator, used to get special reference to the output efficiency of electrical characterization of the device. the fabricated device [18].

Atlas simulates the electrical, optical and

Fig. 3. Graph between optical wavelength and photocurrent (internal quantum efficiency). The internal quantum efficiency of the PIN dynamics. When the light is incident on the photodiode is defined as the number of the device, every single photon is supposed to charge carriers composed by the device to yield electrons-hole pairs. These electron- the number of photons of a given energy hole pairs contribute to the photodetector’s incident on the device, which are then output current. It can be seen that after absorbed by the device [19, 20]. It is certain rise, it is independent of the energy influenced by the absorption coefficient as of the photons striking at the surface of the shown in Fig. 3. The internal quantum device [20]. High quantum efficiency can be efficiency varies with the wavelength of the seen in Fig. 4, which is ensured by the prop- incident light. osed device structure.

Fig. 4 reveals the relationship between the optical wavelength and the current

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Fig. 4. Graph between optical wavelength and normalized anode current.

Fig. 5. Graph between cathode current and cathode voltage.

The relationship between the cathode small reverse saturation current [21, 22]. current and the cathode voltage is The device current is also observed to be demonstrated in Fig. 5. It is referred as the decreased as the voltage increases on the dark current of photodetection process. The cathode terminal of the photodetector. reverse bias is applied to achieve a very

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Fig. 6. Graph between light intensity and cathode current.

Fig. 6 exhibits the relation between the light with an increase in the light intensity, intensity and the current on the cathode. cathode current will start rising, resulting in

When the light has no impact on the device, a linear relationship, as evident by the graph negligible current will be observed, while, [23].

Fig. 7. Graph between photocurrent and cathode current.

Relationship between the photocurrent and detect even at very narrow scales [24]. It is cathode current is shown in Fig. 7, which evident from the graph that when the reveals high detection capability of the photocurrent increases, the cathode current photodiode, when light is incident on it. The increases linearly. graph clearly presents the capability to

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Fig. 8. Graph between source photocurrent and cathode current.

The variation in cathode current as a photodetector compatible with concurrent function of source photocurrent is depicted CMOS technology. The trade-off designed in Fig. 8 for the case when light is incident parameters between the quadrant geometry on the PIN photodiode. The photocurrent and PIN photodiode processing yields a increases when the light is incident on the significantly cost effective and efficient detector, which in turn increases the cathode solution to manufacture such current in a linear fashion [24]. These photodetectors. Less number of processing optical and electrical characterizations steps with better uniformity was observed as confirm standard device performance of the an advantage of the proposed design simulated device. strategy. The applicability of the process

5. Conclusion and the device scheme was testified and

A rigorous design approach was maintained presented as a proficient design for the in this work to provide an industrial driven consequent fabrication. process recipe in order to effectively design Vertical geometry of a photodetector based a “planar” structure of quadrant on PIN diode structure may be investigated

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Asian Journal of Nanoscience and Materials in order to evaluate the impact of geometry 2. Bisogni MG, Morrocchi M (2016) Nucl. Instr. Meth. Phys. Res A 809: on the efficiency of detection and sensing. 140–148. Detailed analysis and possible tailoring of 3. Palai G (2017) Optik 133: 108–113. underlining physical model of the 4. Menon PS, Shaari S (2004) The 4th Annual Seminar of National Science simulation tool may yield a better Fellowship, Malaysia. understanding on the fine tuning of process 5. Yotter RA, Wilson DM (2003) IEEE and device parameters for photonic Sens. J 3: 288-303. 6. Alexandre A, Pinna A, Granado B, applications. One may look for other Garda P (2004) IEEE ICIT’04 1: competing choices of starting material 142-147. (substrate), for example SiGe or III-V 7. Andjelkov MS, Risti GS (2015) Radiat. Meas 75: 29-38. semiconductors, to design this device in 8. Shuhaimi NI, Mohamad M, Jubadi “beyond CMOS” approach for variability of WM, Tugiman R, Zinal N, Zin RM applications. (2010) ICSE 12-14. 9. Abiri E, Salehi MR, Kohan S, Acknowledgment Mirzazadeh M (2010) PACCS 1: 60-

The authors would like to appreciate M. Ali 62. 10. Jubudi WM, Norafzaniza S, Noor M from the Advanced Electronics (2010) SIEA 428-432. Laboratories, International Islamic 11. Vergilio AW, Pekarik JJ, Jain V

University Islamabad, Pakistan for his (2014) BCTM 207-210. 12. Zainudin Z, Ismail AF, Hasbullah precious support and guidance. NF (2014) Computer and Communications Engineering 268- References 268. 1. Bank SR, Campbell JC, Maddox SJ, 13. Mohammed WF, Tikriti MN (2014) Ren M, Rockwell A-K, Woodson SSD1-5. ME, March SD (2018) IEEE J. Sel. 14. Conradi J (1981) Proc. SPIE Int. Top. Quant. Electron 24: 1-7. Soc. Opt Eng 266: 49-55.

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How to cite this manuscript: Waqas Ahmad*, Muhammad Umair Ali, Vijay Laxmi, Shuja Ahmed Syed. Simulation and Characterization of PIN Photodiode for Photonic Applications. Asian Journal of Nanoscience and Materials, 2018, 1, 122-134.