Analysis of aluminum nano-gratings assisted light reflection reduction

Transcription

1 Analysis of aluminum nano-gratings assisted light reflection reduction in GaAs metal-semiconductor-metal photodetectors Zhenzhu Fan a, Yahui Su *ab, Huayong Zhang c, Xiaohu Han a, Feifei Ren a a School of Electrical Engineering and Automation, Anhui University, Hefei , China; b Key Laboratory of Intelligent Computing and Signal Processing, Ministry of Education, Anhui University, Hefei , China; c School of Electrical and Information Engineering, Anhui University, Hefei , China ABSTRACT Plasmonics-based GaAs metal-semiconductor-metal photodetector (MSM-PD) with aluminum nano-gratings was proposed. A detailed numerical study of subwavelength nanogratings behavior to reduce the light reflection is performed by finite-difference time domain (FDTD) algorithm. The geometric parameters of nano-gratings, such as aperture width, the nano-gratings height, the duty cycles are optimized for subwavelength metal nanogratings on GaAs substrate and their impact on light reflection below the conventional MSM-PD is confirmed. Simulation results show that a light reflection factor around 15% can be obtained near the wavelength of 900 nm with optimized MSM-PDs, and in visible light spectrum, the Al nano-gratings show better performance than Au nano-gratings. Keywords: Metal-semiconductor-metal photodetector (MSM-PD); Surface plasmons; Finite-difference time-domain (FDTD) method; Nano-gratings; Sub-wavelength aperture; Light reflection factor (LRF) 1. INTRODUCTION Recent progress in optimization and miniaturization of optical and electrical components up to nanoscale dimensions has led to faster and more efficient performance, which has incorporated new capabilities in various aspects including high-speed telecommunication systems, solar cells, photodetectors, optical microscopy, etc. Metal-semiconductor-metal photodetectors are very attractive devices for the development of optoelectronic integrated circuits (OEIC) due to their high operating speed, ultra-low intrinsic capacitance and compatibility with high performance field effect transistor technologies. The contacts of conventional MSM-PD form two interdigitated, fork shaped electrodes, which are equivalent to two back-to-back connected Schottky diodes on a semiconductor substrate [1], such as GaAs which as a direct band gap semiconductor collects and emits photos more efficiently than indirect semiconductors such as Si and Ge [2]. Compared with standard PIN photodiodes with similar active areas, the interdigitated electrodes in MSM-PDs have to the benefits of significant band width increase and dark current reduction [3, 4]. Moreover, MSM-PDs have a much smaller capacitance per unit area due to their lateral geometry. Their response times are in the range of a few tens of picoseconds because of the nano-scale space between the electrode fingers, which are limited by the transit time of photogenerated carriers to the metallic contact pads. However, degraded responsivity by a decreased active area which will lead to the decrease of the electrode space should be considered. Reliability of Photovoltaic Cells, Modules, Components, and Systems VIII, edited by Neelkanth G. Dhere, John H. Wohlgemuth, Rebecca Jones-Albertus, Proc. of SPIE Vol. 9563, 95630T 2015 SPIE CCC code: X/15/$18 doi: / Proc. of SPIE Vol T-1

2 Since the extraordinary optical transmission (EOT) of periodic metal nanohole arrays through surface plasmons, which is reported in [5], much effort has been devoted to exploring EOT through metallic gratings for various sub-wavelength structures, for instance, periodic slit arrays, hole arrays and corrugated metal films [6-8]. It has also been validated that the transmission of light through a hole or sub-wavelength aperture in a metal film can be enhanced if the metal are patterned as nano-gratings. These nano-gratings couple the incident light with the surface plasmon polaritons (SPPs), which is guided into the sub-wavelength aperture. Several different implementations of SPP-enhanced MSM-PDs have been reported confirming that the confinement of light in subwavelength metal-semiconductor nano-gratings can be achieved through the Fabry-Perot resonances of the transverse magnetic and electric guided waves, resulting in the increase of the quantum efficiency [9, 10]. Previous studies have also shown that the Au nano-gratings have a significantly impact on the enhancement of light absorption [11, 12]. In this paper, the reduction of light reflection of Al nano-gratings is studied. Finite-difference time-domain (FDTD) simulation results have demonstrated significant reduction of light reflection for the design of ultrafast MSM-PDs and an LRF of 15% can be obtained. Compared with the work of Au nano-gratings [12, 13], it can be concluded that Al nano-gratings assisted MSM-PDs function well in the visible light wavelength and all values of LRF are smaller than one. However, in the NIR, there is no obvious advantage of the light reflection reduction of Al nano-gratings assisted MSM-PD in comparison with Au nano-gratings. 2. MSM-PDs DESIGN AND FDTD SIMULATION SETUP The FDTD algorithm was first proposed by Yee in 1966 [14]. This modeling technique is the diffraction of Maxwell equation in the field of time and space using the leap frog algorithm-- alternate calculation of the electric field and magnetic field in space. The effective response of metallic structures can be extracted using the tailored FDTD method. Thereafter, it has been used in different applications and many extensions of the basic algorithm have been developed. Nowadays, the FDTD method is widely applied in electromagnetic computations, such as fields, reflection and resonant modes. In this paper, the FDTD method is employed to analyze the light reflection when a plane wave perpendicularly incident through the metallic nanostructured grating. The geometry properties of the structure are changed and the simulation results are analyzed in detail to obtain the minimum light reflection of the device. MSM-PD structure with rectangular nano-gratings around the sub-wavelength aperture is used in our model. These nano-gratings are capable of capturing the light into the semiconductor region under the aperture and thus reducing the reflection. However, a substantial reflection loss at the sub-wavelength aperture region exits although the reflection loss at the upper surface is reduced by the aperture in the metallic film. Reflection power spectra of nano-gratings assisted MSM-PD are compared with reflection spectra obtained from a conventional device (i.e., without the nano-gratings). In our FDTD models, the normally incident light passes through the sub-wavelength aperture and reaches the semiconductor substrate--gaas, thus the electron-hole pairs are generated and the light absorption enhancement is appeared by the generation of SPPs excited near the metal-semiconductor interface. The phenomenon is described by: ω πl ε ' m ε d ksp = sinθ ± 2 = (1) c Λ ε ' +ε m d Proc. of SPIE Vol T-2

3 where ω is the angular frequency of the incident light wave; ϲ is the speed of light in vacuum; θ is the angle of light with respect to the device normal; Λ is the metal nano-gratings period and l is an integer i.e., l=1, 2, 3,..., N. In the analysis, the complex dielectric permittivity of the aluminum is ε + ε ' i ε" m m m =, from which the real part has been used, the dielectric permittivity of air is denoted as ε d. (a) L.1 Cb} Incident light nano -gratings H r 1 1h=30{un xhsemiconductor substrate Fig.1 Schematic diagram of MSM-PD structure with integrated electrodes (metal fingers) and semiconductor substrate (GaAs) (a) conventional structure and (b) the metal nano-gratings having a rectangular-shaped cross-sectional profile. Fig.1(a) shows the schematic diagram of a conventional MSM-PD and Fig.1(b) is the schematic diagram of an MSM-PD with plasmatic rectangular profile nano-gratings etched inside the top part of the Al layer. The thickness of the unperturbed Al layer containing sub-wavelength aperture is hs (here it is 30 nm). The height of metallic nano-gratings hg (varying from 40 nm to 200 nm) is a crucial parameter which significantly affects the reflection and absorption of the device. Another important parameter--the aperture width, which is denoted as Xw, varies between 50 nm and 250 nm in our modeling. A single grating period Λ of 800 nm is fixed in our design and modeling, which contains a protrusion and a groove with the duty cycles varying in the range of 0.3 to RESULTS AND DISCUSSION In this section, the LRF is used to demonstrate the proficiency of the plasmonic-based MSM-PDs. LRF is a dimensionless quantity, which is defined as the ratio of nano-gratings assisted device reflected power spectrum normalized to the power reflection of an identical structure without the nano-gratings at the sub-wavelength aperture area. 3.1 Impact of sub-wavelength aperture width on the LRF In this subsection, the impact of sub-wavelength aperture width on the light reflection for MSM-PDs is discussed. As the aperture width is very small compared with the incident light wavelength (λ 0 ), only symmetric and fundamental SP modes will propagate into the aperture. Fig.2 shows the simulation results of the reflection spectrum for different Proc. of SPIE Vol T-3

4 sub-wavelength aperture width varying from 50 nm to 250 nm, the duty cycle is 0.5 while the period is kept constant at 800 nm, and the nano-gratings height is also kept constant at 120 nm. When the sub-wavelength aperture width is much smaller than the incident wavelength (λ 0 ), the light reflection decreased in the GaAs semiconductor substrate in addition to the light transmission and absorption caused by the metal nano-gratings. The simulated results show that the LRF increases with the increase of the sub-wavelength aperture width. It also shows that the LRF is less than 20% with 0.5 DC for a 50 nm and 100 nm sub-wavelength aperture width. Even for a 250 nm sub-wavelength aperture width, which is the widest aperture width in this simulation, the LRF is about 30%. The effective refractive index is a function of aperture width for symmetrical SP modes when the aperture experiences the TM incident wave. Therefore, with the decrease of the sub-wavelength aperture width, the refractive index increases [12] and leads to a light reflection decrease feature. It should be pointed out that decreasing the sub-wavelength aperture width also increases the speed of the MSM-PD. Previous work has shown that the LRF of MSM-PDs with gold nano-gratings [13] in the wavelength range from 600 nm to 750 nm is above one, which means the quality of the MSM-PDs is decreased. However, using Al as the nano-grating, the LRF spectra of MSM-PD are smaller than one in a wide wavelength range between 500 nm and 1000 nm. For the wavelength from 750 nm to 950 nm, similar characteristics can be obtained from Au nano-gratings and Al nano-gratings, although the minimum LRF of Al nano-gratings is a little higher than Au nano-gratings Xw=50nm Xw=100nm Xw=150nm Xw=200nm i wavelength (nm) Fig.2 LRF spectra for plasmon-assisted MSM-PD with different sub-wavelength aperture width. 3.2 Influence of duty cycles on the LRF This subsection discussed the effect of duty cycles on the LRF of the nanostructured MSM-PDs. The reflection of the MSM-PDs can be changed because of the aluminum nano-gratings profile on the GaAs substrate. The LRF for rectangular-shaped nano-gratings structures for different duty cycles are calculated, as shown in Fig.3 where the duty cycles varies from 0.3 to 0.7, while the sub-wavelength aperture width has been kept constant at 50 nm, the nano-gratings height and the underlayer thickness are 120 nm and 30 nm, respectively. Proc. of SPIE Vol T-4

5 The dependence of light transmission properties on nano-gratings numbers is reported in [13], the calculated maximum light absorption enhancement factor increases (that means the LRF decreases) with the increase of the nano-gratings numbers, and saturates when the number is four or higher. The results shown in Fig.3 indicate that the LRF is a function of duty cycle which drops down rapidly towards the 0.4 DC and grows gradually to 0.7 DC. In this case, it can be seen that the minimum LRF is obtained around 15% for 0.4 DC, and for 0.5 DC, the LRF is a little higher. It is also shown that the peak wavelength is different for each specific duty cycle and the peak wavelength are red-shifted with the increase of duty cycles. Obviously, the duty cycle not only plays an important part in the peak wavelength, but also has an effect on the amount of light flux reflected from the active area of the MSM-PDs. Compared with Au nano-gratings assisted MSM-PDs, the minimum LRF of Al nano-gratings assisted MSM-PDs appears when the duty cycle is 0.4 or 0.5, while 0.6 or 0.7 for Au. It can be seen in [12] that the LRF spectra are almost all under the line of value 0.5 in the wavelength range between 800 nm to 100 nm. But in the wavelength smaller than 800 nm, the LRF spectra are significantly increased. Fig 4 has shown that although the reflection reduction of MSM-PD with Al nano-gratings in NIR cannot be comparable with Au, the value of LRF in visible spectrum is much lower DC=0.3 DC=0.4 DC=0.5 DC=0.6 DC= wavelength (nm) Fig.3 LRF spectra for plasmon-assisted MSM-PD with different duty cycles. 3.3 Effect of nano-gratings heights on the LRF In this subsection, the amount of light flux reflected from the aperture for five different rectangular nano-gratings heights have been calculated and the LRF spectra is presented in Fig.4. Here, the sub wavelength aperture width is 50 nm for constant; the duty cycle for 800 nm period nano-gratings is 0.5 in the simulation. The light was perpendicularly incident on groove profiles. The height of nano-gratings is an effective parameter as different sets of results show significant changes in the amount of light reflected from the active area of MSM-PD with the variation of nano-gratings height. It can be seen from the Fig.4 that LRF spectra are red-shifted and as the nano-grating s height increases. The minimum reflection of MSM-PDs of 40 nm nano-grating s height is higher than 70% compared with MSM-PDs without nano-gratings. However,when the height of nano-gratings reaches 120 nm, LRF has a minimum value of about 15%. Proc. of SPIE Vol T-5

6 Interpretations for the curves are that the SPPs coupling process and the expected reflection can easily occur for higher gratings and increases after certain heights because the SPs rather than SPPs are coupled to radiative modes. Studies have shown that the LRF increases with increasing the nano-gratings heights in visible wavelength of light and an LRF of about four can be reached of a 200 nm Au nano-gratings assisted MSM-PD while the value of zero almost can be reached with higher nano-gratings heights in the NIR area [13]. FDTD simulation results implies that the reflection of MSM-PD with Al nano-gratings decreases with higher nano-gratings in visible wavelength of light, and in NIR, the LRF decreases to a certain value and then increases with nano-gratings height increasing * NGH=40nm NGH=80nm NGH=120nm x NGH=160nm + NGH=200nm wavelength (nm) Fig.4 LRF spectra for plasmon-assisted MSM-PD with different nano-gratings heights. 4. CONCLUSION The interaction of incident light as electromagnetic waves through the central sub-wavelength aperture surrounded by periodic nano-gratings is discussed in this work. The concept of SPPs is introduced and the excited SPPs are generated at metal-dielectric interface which are used for plasmonic-based applications. Plasmonics offers the ability to concentrate light into the sub-wavelength aperture and make impression in nano-scale PD development. The performance of a low reflection plasmonics-msm-pd employing metal gratings etched onto the metal fingers has been investigated. The sub-wavelength aperture width, the nano-gratings height, the duty cycles of metallic nano-gratings have been optimized to minimum the optical reflection of the MSM-PD, by minimizing the reflection from the sub-wavelength aperture. The FDTD simulation tool has been used and the results have shown that the nano-gratings assisted MSM-PD structures can theoretically attain a minimum light reflection near 900 nm of around 15% of the conventionally-designed MSM-PDs. The results provide useful information for the design and fabrication of high absorption, nano-scale optoelectronic devices. ACKNOWLEDGMENTS This work is supported by National Science Foundation of China (No and ), Anhui College of Natural Science Foundation of China (KJ2011A014) and Provincial Natural Science Foundation of China ( ME76). Proc. of SPIE Vol T-6

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