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Functional nanocrystalline TiO2 thin films for UV enhanced highly responsive silicon photodetectors
Journal of Alloys and Compounds 792 (2019) 968e975
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Functional nanocrystalline TiO2 thin films for UV enhanced highly responsive silicon photodetectors Khushbu R. Chauhan, Dipal B. Patel* Department of Physics, Lovely Professional University, Phagwara, Punjab, 144411, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 February 2019 Received in revised form 8 April 2019 Accepted 10 April 2019 Available online 15 April 2019
In this paper, we have studied the effect of nanocrystalline TiO2 functional layer on the performance of Si photodetectors. It was found that the electron affinity of the deposited TiO2 films is much lower (3.2 eV) than the conventional films of the value 3.9 eV. In addition, the band gap of these newly developed films was found to be 3.6 eV which makes the films transparent for band to band optical absorption. However, a very high absorption co efficient of the value 4.6 105 cm1 and excitonic dissociation facilitate charge generation and transfer mechanisms in such films. It was found that the photodetector parameters like internal gain, response time, responsivity, detectivity, sensitivity and noise can be improved by inserting functional TiO2 layer to make either anisotype or isotype heterojunction with Si photodetectors. The major phenomena for such enhancements were found to be decrease in tunneling length and increased charge generation/transfer due to TiO2 nanocrystals. By this modified design, TiO2/p-Si hybrid device offer high responsivity of the value 23.07 A/W and detectivity of 4.5 1012 Jones for red light (620 nm) at an incident power of 1 mW/cm2. Furthermore, it was confirmed that faster devices with rise time of only 30 ms in broadband region from 425 to 620 nm can be made out of TiO2/n-Si configuration which allows slower diffusion of charge carriers. This study will provide a pathway to design ultramodern, highly responsive/sensitive and broadband Si photodetectors. © 2019 Elsevier B.V. All rights reserved.
Keywords: Photodetectors Tunneling Electron affinity TiO2 thin films
1. Introduction Photodetectors (PDs) are the major building blocks of digital electronics, optical fiber telecommunication, infrared astronomy and spectroscopy. Over the years, evolution of new materials and composites allowed the slow and steady improvisations of such devices. However, a single material is debilitated to operate the device for broadband applications and hence researchers always try to incorporate different materials for either stretched frequency response or superior photodetection properties. Till date, variety of materials has been explored to design an “All-in-One” PD which can deliver an exemplary performance. However, there has been always a trade-off between the responsivity (R) and speed/ response time of the PDs. To start with, Silicon (Si) PDs have reached up to extremely high R of the value 92 A W1 by using microstructured Si [1] and faster response up to picosecond (ps) by using interdigitated trenches [2]. On the other hand, novel materials like graphene [3], composites of graphene and perovskite [4],
* Corresponding author. E-mail address: [email protected] (D.B. Patel). https://doi.org/10.1016/j.jallcom.2019.04.111 0925-8388/© 2019 Elsevier B.V. All rights reserved.
MoS2 [5,6], TiO2 nanostructures [7,8], nonstoichiometric oxides [9e11] have been used to develop highly efficient PDs. One of the key approaches to develop broadband PDs is to employ heterojunctions. Researchers have achieved the broadband detections from UV to IR regions by using such junctions [12,13]. At the same time, high values of R can be obtained by using efficient charge collection grid and photoactive materials having very high absorption coefficient. Such practices have also been adopted especially for high purity single crystal materials like GaN or Si [14]. Another approach to achieve an efficient PD is to use surface plasmonic effect by metal nanoparticles or quantum dots [15,16]. In addition surface texturing [17] also leads to an improved light absorption in PDs and hence increases the overall performance. Amongst these various ways to improve the performance of conventionally available PDs, hybrid or heterojunctions have proved to be the potent solution. Recently, a research on hybrid structure of graphene and other 2D materials showed superior properties of PD [18]. Moreover, other heterojunctions like graphene/Si [19,20], ZnO/GaN [21], MoS2/Si [22], Bi2Te3/Si [23], Bi/ WS2/Si [24] and ZnO/CuO [25] have also proved to be of great importance when it comes to high speed and broadband
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applications. However, till date TiO2/Si hybrid structures have been very limitedly studied in-spite of having all the prerequisites to form a stable junction. This might be due to a band gap limited absorption properties of available TiO2 thin films which absorbs mostly in the UV region. The study of heterojunction PD with p-Si (111)/TiO2 nanorod array configuration showed that the UV sensitivity and response time of PDs can be improved by interfacial charge properties and oxygen vacancies [26]. In a similar study by using n-Si (111)/TiO2 nanorod array claimed the bias dependent detection properties of such interface [27]. However, in both these studies a complete set of PD properties (response time, linear dynamic range, sensitivity, and detectivity) was not provided which is necessary to show an overall highly efficient PD. Moreover, these studies are based on TiO2 nanorods which are highly sensitive to oxygen defects and the overall device performance may also be governed by such vacancies. Similar studies with either Si or TiO2 configurations led to the PDs with very slow response time (in ms) and only UV/visible light detection [28e30]. Hence, the fabrication of broadband PD with superior photodetection properties and characteristics in all aspects is still a challenging task. Recently we have developed nanocrystalline TiO2 thin films with excitonic properties and remould electron affinity which offer a unique absorption spectrum [10]. In addition, its low electron affinity facilitates extraordinary photodetection properties while interfaced with various metals [9]. In this work, we have made efforts to see the effect of very thin nanocrystalline TiO2 layer on the performance of Si PDs. Such a thin layer ensures the functionality of nanojunction/s which can greatly enhance the overall performance of the PDs. In particular, two different devices have been studied i.e. n-TiO2/p-Si and n-TiO2/n-Si heterojunction PDs. It will be shown that the unique properties of newly developed TiO2 films colossally improved the performance of Si PDs. 2. Experimental Mirror-like monocrystalline Si wafers with 450 mm thickness, 1e5 U cm electrical resistivity and (100) orientations were used as substrates. Metallization of p-Si and n-Si wafers was realized by sputtering thin layers of aluminum (Al) and silver (Ag), respectively. DC sputtering of high purity Ti target (99.995% pure, Sigma Aldrich) was done at 5 mT working pressure with continuous flow of Ar (40 sccm) and by applying a constant power of 150 W at room temperature for 900 s. The substrate stage was kept at 5 rotations per minute to ensure the uniform deposition of Ti film. Ti coated Silicon films were then thermally treated in vacuum furnace at 700 C for 600 s to convert Ti thin films into titanium dioxide (TiO2). The front metallization was done in the form of fingers and busbar of Ag metal by using DC sputtering. 3. Characterizations The phase identification of TiO2 thin film was carried out by XRay diffractometer (X'Pert Pro, PANalytical) using characteristic wavelength 1.45 Å of Cu-ka radiation source. The optical measurements were done by using UVeVis spectrophotometer (Shimadzu-2600) between wavelength range 220e1400 nm. Morphological images of n-TiO2 and a cross section analysis of the PD were obtained by using field emission scanning electron microscope (FE-SEM) (Ultra-55, Zeiss). Photoresponse measurements under chopping (100 Hz) light conditions were carried out by using Potentiostat/Galvanostat (PGSTAT302 N, Autolab). UV light illumination system consisted of the monochromatic UV LED (365 nm) and a function generator. In addition, visible light LEDs of 425 nm, 515 nm and 620 nm with an accuracy ±5 nm were used to record
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the photoresponse curves. Quantum efficiency (QE) measurements were done by using Optosolar instrument to calculate the responsivity (R) and specific detectivity (D) of the PDs. 4. Results and discussion 4.1. Material characterization Fig. 1 shows (A) XRD spectrum (B) Variation in absorption coefficient (a) with energy (C) FE-SEM surface morphology and (D) cross-sectional view of TiO2/Si interface. The XRD pattern of TiO2 thin film showed intense peaks at 27.83 , 36.27 and 54.92 , which belong to (110), (101) and (211) planes of tetragonal crystal structure of rutile TiO2. The other low intensity peaks were also found to be of only rutile TiO2 crystal. The high purity of deposited TiO2 thin film can be verified from the XRD spectrum as no impurity or elemental Ti peaks were found. According to Fig. 1(B), the maximum value of a (4.6 105 cm1) was found to be at 4 eV of photon energy and band gap of TiO2 was found to be 3.6 eV which is relatively high compared to rutile TiO2 films reported in the literature [31]. This higher value of band gap suggests that no band-toband transitions will be allowed for the energies of light used in this work. It can be seen from Fig. 1(C) that uniform TiO2 nanocrystals of the sizes 10e20 nm uniformly cover the Si substrate. The thickness of TiO2 film was found to be 142 nm by cross-sectional FE-SEM image shown in Fig. 1(D). 4.2. Device performance: quantitative analysis In this study, we have employed three different configurations of silicon photodetectors as (1) Anisotype heterojunction: Ag/nTiO2/p-Si/Al, (2) Isotype heterojunction: Ag/n-TiO2/n-Si/Ag and (3) Metal-semiconductor-metal (M-S-M): Ag/p-Si/Al. For convenience, Ag/n-TiO2/p-Si/Al PD is designated as D1, Ag/n-TiO2/n-Si/Ag as D2 and Ag/p-Si/Al as D3. The device structures are presented in Fig. 2 (D1-D3), in which D3 can be considered as a reference device for the comparison. It can be inferred that D1 works on the principle of heterojunctions whereas D2 and D3 will behave as M S junction. All these structures found to be functional broadband photodetectors under reverse biased conditions. Photoresponse curves of the devices under chopped light illumination condition are shown in Fig. 3(A-D) at 4 V. It can be seen from Fig. 3 that all the devices offer excellent dark current characteristics. When the devices were illuminated with different wavelengths of the solar spectrum, distinct nature of photocurrent was observed from all the devices. It was interesting to notice that the shape of photocurrent transient for D2 comprises of an initial overshoot which rapidly decreases to a stable plateau region. A detailed analysis on the shape of the transients can be found in the literature which critically analyzes each and every possible mechanism of different configurations [32]. The observed difference in the transient curve can be due to variety of phenomena including (i) excitonic dissociation [33] (ii) abundant generation of charge carriers at the interface (iii) unequal or trap assisted generation of minority and majority charge carriers in the device upon illumination [32]. It can be seen that such a feature was only observed for device D2 and hence the most probable reasons for this can be either (ii) or (iii). It can be also noticed that the lowest rise time of device D2 supports the possibility of abundant generation of carriers in the device. However, detailed analysis of the shape of transient is out of the scope of this study and we can only attribute this nature to the unique interfacial properties. The calculated values of photodetector parameters are listed in Table 1. We will first discuss the observations from Table 1 and then will try to explain the effect of functional TiO2 layer on the performance of Si PDs. It can be directly inferred from the Table that
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Fig. 1. (a) XRD spectrum of the nanocrystalline TiO2 thin film deposited over p-Si wafer (b) Variation in absorption coefficient of nanocrystalline TiO2 thin film with energy of incoming photons (c) Surface morphology of nanocrystalline TiO2 thin film deposited over p-Si wafer (d) Cross-sectional FE-SEM image of the TiO2/p-Si interface.
Fig. 2. Device schematics of (D1) Anisotype heterojunction: Ag/n-TiO2/p-Si/Al (D2) Isotype heterojunction: Ag/n-TiO2/n-Si/Ag and (D3) Metal-semiconductor-metal: Ag/p-Si/Al photodetectors used in this study.
device D1 outperformed the other PDs in terms of all PD parameters except rise and fall time. Starting with photocurrent gain, the photo to dark current ratio was found to be the highest for D1 for all the wavelengths. It can be also noticed that the overall gain of the devices with TiO2 layer was found to be superior compared to the device without it. This is due to the exponential dependence of photocurrent in Schottky PDs on the photon energy which was also seen in ZnO/Graphene PDs [34]. Other important parameter for an efficient design of PD is the response time which is either fast rise and/or fall time. The rise time (tr) is defined as the time taken for the photocurrent value to increase from 10% to 90% of its final value whereas the fall time (tf) is
the time taken by the PD to reduce its photocurrent value from 90% to 10% of its maximum value when the light is turned off. The measured values of rise time and fall time for all the devices have been presented in the form of bar chart with error bar in Fig. 4(A) and (B), respectively. All the PDs were found to be fast reactive for different incident wavelengths of light as the magnitude of time taken by any device for creation/annihilation of charge carriers was in ms. Moreover, devices with TiO2 layer offered fast response compared to the device without TiO2. The fastest detection was achieved from the device D2 having the rise time of ~31 ms for the visible lights. On the other hand, D3 was found to be fastest for the detection of UV light with the response time of only 40 ms. It can be
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Fig. 3. The photoresponse spectra of all the photodetectors under the illumination of (a) 365 nm (b) 425 nm (c) 515 nm and (d) 620 nm.
Table 1 Calculated photodetector parameters extracted at a power density of 1 mW/cm2 and applied bias of 4 V for all the devices. Parameter
Wavelength (nm)
D1
D2
D3
Gain
365 nm 425 nm 515 nm 620 nm 365 nm 425 nm 515 nm 620 nm 365 nm 425 nm 515 nm 620 nm 365 nm 425 nm 515 nm 620 nm 365 nm 425 nm 515 nm 620 nm
39.6 58.8 29.0 46.6 3.86 103 5.78 103 2.80 103 4.56 103 31.9 35.4 29.2 33.4 8.18 104 1.48 105 2.02 105 2.80 105 7.62 1013 4.30 1013 3.08 1013 2.22 1013
16.5 21.4 17.1 19.9 1.55 103 2.04 103 1.61 103 1.89 103 24.3 26.6 24.7 26.0 2.73 104 1.24 105 1.95 105 1.88 105 2.28 1012 5.25 1013 3.40 1013 3.53 1013
24.6 10.0 14.4 14.6 2.36 103 9.01 103 1.34 103 1.36 103 27.8 20.0 23.2 23.3 3.96 104 9.84 104 1.44 105 1.03 105 1.47 1012 6.58 1013 4.43 1013 6.20 1013
Sensitivity
LDR (dB)
NPDR (W1)
NEP (W)
also noted that this time was the shortest compared to the time taken for visible light detection by device D3. In addition, the rise and fall time of devices D1 and D3 are of the almost same values indicating full depletion in the diode. Whereas, D2 offers almost half a rise time than the fall time suggesting prevalence of slow carriers and diffusion process in semiconductors. This phenomenon can be attributed to the like junctions (n-n type) which possess same polarity of majority and minority carriers. Since very less no. of counter carriers for recombination may be available in the device, the carriers are slowly diffused in the semiconductor before
reaching at the contacts. However, upon illumination, generation of minority carriers is more and the time taken for the photocurrent value to reach 100% of its value is less than other devices. Another figures of merit are, responsivity (R) and specific detectivity (D) which can be derived from formulas (1) and (2), respectively,
Jph Pin
(1)
R D ¼ pffiffiffiffiffiffiffiffiffiffiffiffi 2 q Jd
(2)
R¼ and
The spectral variations in R and D for all the devices between the wavelengths 340e850 nm are shown in Fig. 5 (A, B). At the same time, measured values of R and D upon illumination of wavelengths under study have been presented in the form of bar charts in Fig. 6 (A, B). The device D1 possesses extremely higher values of responsivity for each wavelength compared to D2 and D3. The responsivity of 6.74 A/W was achieved with D1 in the UV region which is comparable with commercially available UV enhanced silicon avalanche photodetector [35,36]. The maximum responsivity of the value 26 A/W was achieved at 800 nm, which is much higher than the available standard silicon photodetectors [37]. The direct effect of TiO2 functional layer can be observed from a huge difference in the responsivities of devices D1 and D3 in the visible light region. It can be seen that for the wavelengths 600e800 nm, D1 offers at least 2.5 times enhanced values of responsivity compared to D3. It can be also seen from Fig. 5 (A) that except UV, device D2 offers higher responsivity for all the wavelengths compared to D3. The detectivity of D1 was also found to be much
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Fig. 4. Variation in (A) Rise time and (B) Fall time with error bar for the devices D1, D2 and D3 under the illumination of UV (365 nm), Blue (425 nm), Green (515 nm) and Red (620 nm) wavelengths. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. Variation in (A) Responsivity and (B) Detectivity for all the devices with incident photon wavelength.
Fig. 6. Variation in (A) Responsivity and (B) Detectivity for the devices D1, D2 and D3 under the illumination of UV (365 nm), Blue (425 nm), Green (515 nm) and Red (620 nm) wavelengths. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
higher (1.31 1012 Jones) in UV region compared to other structures. Furthermore, the impact of TiO2 layer can be seen from the higher detectivity values of D2 and D1. The other figures of merit for characterizing a photodetector are sensitivity, linear dynamic range
(LDR), normalized photo to dark current ratio (NPDR) and noise equivalent power (NEP). The sensitivity is defined as in equation (3),
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Table 2 Comparison between photodetector parameters presented in this work and reported literatures on TiO2/Si configurations. Bias (V)
l (nm)
Gain
Rise time
Fall time
R (A/W)
Detectivity (Jones)
Sensitivity (%)
LDR (dB)
NPDR (W1)
NEP (W)
Reference
4
365 425 515 620 400 415 475 600 405 350 350 350 365 325
39.6 58.8 29.0 46.6 >2 >3 >5 >2
127.6 ms 73.2 ms 98.4 ms 64.5 ms
120.3 ms 80.8 ms 77.4 ms 77.8 ms
6.74 11.7 23.07 16.66 >0.1 >0.3 >0.2 >0.2 377 0.7 0.053 0.045 69.7
1.31 1012 2.33 1012 3.25 1012 4.50 1012
3.86 103 5.78 103 2.80 103 4.56 103
31.9 35.4 29.2 33.4
8.18 104 1.48 105 2.02 105 2.80 105
7.62 1013 4.30 1013 3.08 1013 2.22 1013
This work (Device-D1)
4
5 4 3 3 5 5
10.4 ms
77.1 4.79
18.5 ms 50.8 ms
11 ms
19.1 ms 57.8 ms
[27]
232 9.38 (@780 nm) 2.08 1012 1.91 1012
[26,27,39e43] with our PD (device-D1) is shown in Table 2.
. Id S ¼ Iph Id
(3) 4.3. The role of functional TiO2 layer: qualitative discussion
The LDR for any PD can be calculated by using equation (4),
. LDR ¼ 20 log Iph Id
(4)
The values of NPDR can be derived from equation (5),
NPDR ¼ R=Id
(5)
The noise equivalent power (NEP) can be found from the formula given in (6),
NEP ¼
1 NPDR
76.06 102 3.79 102
[26] [39] [40] [41] [42] [43]
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2q=I d
(6)
The values of sensitivity for all the PDs were found to be of the order of 103 which make them highly suitable for industry applications. The LDR for all the devices was found to be in the range of 20e35 dB, which is typically observed for Si-PDs. However, the devices with TiO2 layer possessed relatively broad LDR. The values of NPDR were found to be as high as 2.8 105 W1 which is much higher than the available silicon photodetectors [38]. Similarly, NEP values of all the devices in the broadband spectrum range were found to be of the order of ~1013 W, which makes them suitable for picowatt power detection. The comparison between the parameters of other TiO2/Si based PDs available in the literature
To understand the role of functional TiO2 layer, we calculated the band edges of nanocrystalline TiO2 film by using Mott-Schottky analysis. It was found that the TiO2 films fabricated for this work possessed relatively lower electron affinity compared to the films deposited via any route till date. These lower values of affinity and higher band-gap make these semi-transparent films unique for their utilization as functional transport layer in Si PDs. The band diagrams of each device with actual values of band edges under equilibrium condition have been presented in Fig. 7 to thoroughly understand the current mechanisms in PDs. It is also known that in reverse bias condition, injection of minority carriers control the overall current of the device. The nanocrystalline TiO2 film with lower electron affinity provides a very steep bending of the bands at the TiO2/Si interface as shown in Fig. 7 (D1 and D2). As the applied reverse bias increases such steepness increases and provides minimum tunneling distance for the minority carriers compared to devices D2 and D3. When the applied bias reaches at its optimum value of 4 V, maximum tunneling probability is achieved and hence maximum photocurrent is recorded. This claim can be further confirmed by the variation in responsivity curve with different bias for the devices D1 and D3 as shown in Fig. 8 (A, B). We have shown in our earlier reports [10] that the photocurrent in this nanocrystalline TiO2 films is via excitonic dissociation and trap
Fig. 7. Band diagrams of (D1) Anisotype heterojunction: Ag/n-TiO2/p-Si/Al (D2) Isotype heterojunction: Ag/n-TiO2/n-Si/Ag and (D3) Metal-semiconductor-metal: Ag/p-Si/Al photodetectors under equilibrium condition.
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Fig. 8. Variation in responsivity curves with different bias for the devices (A) D1 and (B) D3 with incident photon wavelength.
states. Under illumination condition at -4 V, the photogenerated electrons in TiO2 can also tunnel through the barrier and reach to the Si bulk. Now this extra current due to photogeneration in TiO2 layer also increases the overall gain of the devices D1 and D2. In addition, the trap assisted tunneling or thermally assisted multistep tunneling is likely to be prominent through TiO2 layer which enhances the responsivity of the PDs. However, a detailed temperature dependent study needs to be done for the quantitative analysis of such processes. Now when the isojunction is created (as in D2), the improved photogeneration due to TiO2 still remains the same as in D1. However, less band bending (more tunneling distance) and weak built in field at the interface result into relatively lower photocurrent values than device D1. The effect of inserted TiO2 layer can be prominently seen when we compare the operation of device D2 and D3. It can be seen that the device can be fast operative in absence of TiO2 layer which can be seen from the fast rise and fall time of D3. In other words, higher fall time of D1 and D2 suggest that the recombination process is slower than in device D3. This in turn suggests that the higher gain or responsivity can be achieved by inserting TiO2 transport layer which also minimizes the recombination current due to efficient charge transfer. This can be confirmed from a higher dark current density in device D3 which was 6.62 104 A cm2. Such a huge dark current is a typical characteristic of Schottky devices with relatively lower barrier height and has been explained very well in literature [44]. In-spite of possessing good responsivity, device D3 offers very low detectivity at each wavelength. This is due to a very high dark current of the device which results from inefficient charge transfer and recombination in the device. 5. Conclusion In conclusion, we have shown that very unique properties of nanocrystalline TiO2 films greatly improve the performance of Si photodetectors. We have thoroughly examined the effect of TiO2 layer on the charge transfer processes in hybrid Si photodetectors. In addition, each photodetector parameter was critically analyzed and the improvements were explained in detail. It was found that TiO2/p-Si device can be utilized as highly responsive and sensitive photodetector whereas TiO2/n-Si can be used for quick operation/ fast detection applications. The availability of excitonic transitions and intermediate states in TiO2 nanocrystals was found to be a major source of better functionality of the Si photodetectors. Nonetheless, an exploitation of slow diffusion process and enhanced generation rate in isotype Si devices can be availed by
using n-TiO2 functional layer. We have shown that the broadband photodetectors with very high responsivity and sensitivity can be made by using TiO2/Si configuration. In an all, for the visible light (425 nm), gain of Si device can be increased to ~6 times and noise level can be minimized to 4.30 1013 W by incorporation a functional film of TiO2 nanocrystals. This study opens up a possibility for the use of functional layers with modified electron affinity, band alignment and tunneling length for efficient designs of photoelectric devices. Acknowledgements Authors acknowledge technical support from Pandit Deendayal Petroleum University, Gandhinagar, Gujarat and administrative support from their current organization, Lovely Professional University, Phagwara, Punjab. Both the authors have contributed equally in this work. References [1] Z. Huang, J.E. Carey, M. Liu, X. Guo, E. Mazur, J.C. Campbell, Microstructured silicon photodetector, Appl. Phys. Lett. 89 (3) (2006), 033506. [2] J.Y. Ho, K.S. Wong, Bandwidth enhancement in silicon metal-semiconductormetal photodetector by trench formation, IEEE Photonics Technol. Lett. 8 (8) (1996) 1064e1066. [3] A. Pospischil, M. Humer, M.M. Furchi, D. Bachmann, R. Guider, T. Fromherz, T. Mueller, CMOS-compatible graphene photodetector covering all optical communication bands, Nat. Photon. 7 (11) (2013) 892. [4] Y. Lee, J. Kwon, E. Hwang, C.H. Ra, W.J. Yoo, J.H. Ahn, J.H. Park, J.H. Cho, Highperformance perovskiteegraphene hybrid photodetector, Adv. Mater. 27 (1) (2015) 41e46. [5] D. Kufer, G. Konstantatos, Highly sensitive, encapsulated MoS2 photodetector with gate controllable gain and speed, Nano Lett. 15 (11) (2015) 7307e7313. [6] Y. Xie, B. Zhang, S. Wang, D. Wang, A. Wang, Z. Wang, H. Yu, H. Zhang, Y. Chen, M. Zhao, B. Huang, Ultrabroadband MoS2 photodetector with spectral response from 445 to 2717 nm, Adv. Mater. 29 (17) (2017) 1605972. [7] J. Zou, Q. Zhang, K. Huang, N. Marzari, Ultraviolet photodetectors based on anodic TiO2 nanotube arrays, J. Phys. Chem. C 114 (24) (2010) 10725e10729. [8] X. Li, C. Gao, H. Duan, B. Lu, X. Pan, E. Xie, Nanocrystalline TiO2 film based photoelectrochemical cell as self-powered UV-photodetector, Nano Energy 1 (4) (2012) 640e645. [9] D.B. Patel, K.R. Chauhan, S.H. Park, J. Kim, High-performing transparent photodetectors based on Schottky contacts, Mater. Sci. Semicond. Process. 64 (2017) 137e142. [10] D.B. Patel, K.R. Chauhan, W.H. Park, H.S. Kim, J. Kim, J.H. Yun, Tunable TiO2 films for high-performing transparent Schottky photodetector, Mater. Sci. Semicond. Process. 61 (2017) 45e49. [11] H.S. Kim, K.R. Chauhan, J. Kim, E.H. Choi, Flexible vanadium oxide film for broadband transparent photodetector, Appl. Phys. Lett. 110 (10) (2017) 101907. [12] K.K. Manga, J. Wang, M. Lin, J. Zhang, M. Nesladek, V. Nalla, W. Ji, K.P. Loh, High-performance broadband photodetector using solution-processible
K.R. Chauhan, D.B. Patel / Journal of Alloys and Compounds 792 (2019) 968e975 PbSeeTiO2egraphene hybrids, Adv. Mater. 24 (13) (2012) 1697e1702. [13] H. Yuan, X. Liu, F. Afshinmanesh, W. Li, G. Xu, J. Sun, B. Lian, A.G. Curto, G. Ye, Y. Hikita, Z. Shen, Polarization-sensitive broadband photodetector using a black phosphorus vertical pen junction, Nat. Nanotechnol. 10 (8) (2015) 707. [14] Van Hove, J.M., Kuznia, J.N., Olson, D.T., Kahn, M.A. and Blasingame, M.C., APA Optics Inc, 1994. High responsivity ultraviolet gallium nitride detector. U.S. Patent 5,278,435. [15] D.M. Schaadt, B. Feng, E.T. Yu, Enhanced semiconductor optical absorption via surface plasmon excitation in metal nanoparticles, Appl. Phys. Lett. 86 (6) (2005), 063106. [16] C.C. Chang, Y.D. Sharma, Y.S. Kim, J.A. Bur, R.V. Shenoi, S. Krishna, D. Huang, S.Y. Lin, A surface plasmon enhanced infrared photodetector based on InAs quantum dots, Nano Lett. 10 (5) (2010) 1704e1709. [17] Zaidi, S.H., Zaidi Saleem H, 2006. Method of making an enhanced optical absorption and radiation tolerance in thin-film solar cells and photodetectors. U.S. Patent 7,109,517. [18] F.H.L. Koppens, T. Mueller, P. Avouris, A.C. Ferrari, M.S. Vitiello, M. Polini, Photodetectors based on graphene, other two-dimensional materials and hybrid systems, Nat. Nanotechnol. 9 (10) (2014) 780. [19] X. Li, M. Zhu, M. Du, Z. Lv, L. Zhang, Y. Li, Y. Yang, T. Yang, X. Li, K. Wang, H. Zhu, High detectivity graphene-silicon heterojunction photodetector, Small 12 (5) (2016) 595e601. [20] Y. Cao, J. Zhu, J. Xu, J. He, J.L. Sun, Y. Wang, Z. Zhao, Ultra-broadband photodetector for the visible to terahertz range by self-assembling reduced graphene oxide-silicon nanowire array heterojunctions, Small 10 (12) (2014) 2345e2351. [21] C.H. Chen, S.J. Chang, S.P. Chang, M.J. Li, I.C. Chen, T.J. Hsueh, C.L. Hsu, Novel fabrication of UV photodetector based on ZnO nanowire/p-GaN heterojunction, Chem. Phys. Lett. 476 (1e3) (2009) 69e72. [22] M.R. Esmaeili-Rad, S. Salahuddin, High performance molybdenum disulfide amorphous silicon heterojunction photodetector, Sci. Rep. 3 (2013) 2345. [23] J. Yao, J. Shao, Y. Wang, Z. Zhao, G. Yang, Ultra-broadband and high response of the Bi2Te3/Si heterojunction and its application as a photodetector at room temperature in harsh working environments, Nanoscale 7 (29) (2015) 12535e12541. [24] J. Yao, Z. Zheng, J. Shao, G. Yang, Promoting photosensitivity and detectivity of the Bi/Si heterojunction photodetector by inserting a WS2 layer, ACS Appl. Mater. Interfaces 7 (48) (2015) 26701e26708. [25] R.R. Prabhu, A.C. Saritha, M.R. Shijeesh, M.K. Jayaraj, Fabrication of p-CuO/nZnO heterojunction diode via sol-gel spin coating technique, Mater. Sci. Eng., B 220 (2017) 82e90. [26] A.M. Selman, Z. Hassan, M. Husham, N.M. Ahmed, A high-sensitivity, fastresponse, rapid-recovery pen heterojunction photodiode based on rutile TiO2 nanorod array on p-Si (1 1 1), Appl. Surf. Sci. 305 (2014) 445e452. [27] T. Ji, Q. Liu, R. Zou, Y. Sun, K. Xu, L. Sang, M. Liao, Y. Koide, L. Yu, J. Hu, An interface engineered multicolor photodetector based on n-Si (111)/TiO2 nanorod array heterojunction, Adv. Funct. Mater. 26 (9) (2016) 1400e1410. [28] C. Gao, X. Li, X. Zhu, L. Chen, Y. Wang, F. Teng, Z. Zhang, H. Duan, E. Xie, High performance, self-powered UV-photodetector based on ultrathin, transparent, SnO2eTiO2 coreeshell electrodes, J. Alloys Compd. 616 (2014) 510e515.
975
[29] M.B. Sarkar, A. Mondal, B. Choudhuri, B.K. Mahajan, S. Chakrabartty, C. Ngangbam, Enlarged broad band photodetection using Indium doped TiO2 alloy thin film, J. Alloys Compd. 615 (2014) 440e445. [30] N.M. Khusayfan, A.A. Al-Ghamdi, F. Yakuphanoglu, Solar light photodetectors based on nanocrystalline copper indium oxide/p-Si heterojunctions, J. Alloys Compd. 663 (2016) 796e807. [31] M. Landmann, E. Rauls, W.G. Schmidt, The electronic structure and optical response of rutile, anatase and brookite TiO2, J. Phys. Condens. Matter 24 (19) (2012) 195503. [32] M. Mokhtarimehr, S.A. Tatarkova, Photocurrent transients of thin-film solar cells, JOSA B 34 (8) (2017) 1705e1712. [33] D.L. Li, W. Si, W.C. Yang, Y. Yao, X.Y. Hou, C.Q. Wu, Spike in transient photocurrent of organic solar cell: exciton dissociation at interface, Phys. Lett. 376 (4) (2012) 227e230. [34] H. Lee, N. An, S. Jeong, S. Kang, S. Kwon, J. Lee, Y. Lee, D.Y. Kim, S. Lee, Strong dependence of photocurrent on illumination-light colors for ZnO/graphene Schottky diode, Curr. Appl. Phys. 17 (4) (2017) 552e556. [35] A. Rochas, A.R. Pauchard, P.A. Besse, D. Pantic, Z. Prijic, R.S. Popovic, Low-noise silicon avalanche photodiodes fabricated in conventional CMOS technologies, IEEE Trans. Electron Devices 49 (3) (2002) 387e394. [36] M. Hohenbild, P. Seegebrecht, H. Pless, W. Einbrodt, November. High-speed photodiodes with reduced dark current and enhanced responsivity in the blue/uv spectra, The 11th IEEE International Symposium on, in: Electron Devices for Microwave and Optoelectronic Applications, 2003, vol. 2003EDMO, 2003, pp. 60e65 (IEEE). [37] X. Wang, Z. Cheng, K. Xu, H.K. Tsang, J.B. Xu, High-responsivity graphene/ silicon-heterostructure waveguide photodetectors, Nat. Photon. 7 (11) (2013) 888. [38] Y. An, A. Behnam, E. Pop, A. Ural, Metal-semiconductor-metal photodetectors based on graphene/p-type silicon Schottky junctions, Appl. Phys. Lett. 102 (1) (2013), 013110. [39] H.M. Ali, S.A. Makki, A.N. Abd, Enhanced photo-response of porous silicon photo-detectors by embedding Titanium-dioxide nano-particles, in: Journal of Physics: Conference Series, vol. 1003, IOP Publishing, 2018, May, 012073. No. 1. [40] U.M. Nayef, K.A. Hubeatir, Z.J. Abdulkareem, Characterisation of TiO2 nanoparticles on porous silicon for optoelectronics application, Mater. Technol. 31 (14) (2016) 884e889. [41] U.M. Nayef, K.A. Hubeatir, Z.J. Abdulkareem, Ultraviolet photodetector based on TiO2 nanoparticles/porous silicon hetrojunction, Optik-Int. J. Light Electron Optics 127 (5) (2016) 2806e2810. [42] A.M. Selman, Z. Hassan, Fabrication and characterization of metalesemiconductoremetal ultraviolet photodetector based on rutile TiO2 nanorod, Mater. Res. Bull. 73 (2016) 29e37. [43] A.M. Selman, Z. Hassan, Highly sensitive fast-response UV photodiode fabricated from rutile TiO2 nanorod array on silicon substrate, Sensor Actuator Phys. 221 (2015) 15e21. [44] K.R. Chauhan, I. Mukhopadhyay, On the electrical and interface properties of nanostructured CdTe Schottky diodes electrodeposited from an ionic liquid medium, J. Appl. Phys. 115 (22) (2014) 224506.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Functional nanocrystalline TiO2 thin films for UV enhanced highly responsive silicon photodetectors Khushbu R. Chauhan, Dipal B. Patel* Department of Physics, Lovely Professional University, Phagwara, Punjab, 144411, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 February 2019 Received in revised form 8 April 2019 Accepted 10 April 2019 Available online 15 April 2019
In this paper, we have studied the effect of nanocrystalline TiO2 functional layer on the performance of Si photodetectors. It was found that the electron affinity of the deposited TiO2 films is much lower (3.2 eV) than the conventional films of the value 3.9 eV. In addition, the band gap of these newly developed films was found to be 3.6 eV which makes the films transparent for band to band optical absorption. However, a very high absorption co efficient of the value 4.6 105 cm1 and excitonic dissociation facilitate charge generation and transfer mechanisms in such films. It was found that the photodetector parameters like internal gain, response time, responsivity, detectivity, sensitivity and noise can be improved by inserting functional TiO2 layer to make either anisotype or isotype heterojunction with Si photodetectors. The major phenomena for such enhancements were found to be decrease in tunneling length and increased charge generation/transfer due to TiO2 nanocrystals. By this modified design, TiO2/p-Si hybrid device offer high responsivity of the value 23.07 A/W and detectivity of 4.5 1012 Jones for red light (620 nm) at an incident power of 1 mW/cm2. Furthermore, it was confirmed that faster devices with rise time of only 30 ms in broadband region from 425 to 620 nm can be made out of TiO2/n-Si configuration which allows slower diffusion of charge carriers. This study will provide a pathway to design ultramodern, highly responsive/sensitive and broadband Si photodetectors. © 2019 Elsevier B.V. All rights reserved.
Keywords: Photodetectors Tunneling Electron affinity TiO2 thin films
1. Introduction Photodetectors (PDs) are the major building blocks of digital electronics, optical fiber telecommunication, infrared astronomy and spectroscopy. Over the years, evolution of new materials and composites allowed the slow and steady improvisations of such devices. However, a single material is debilitated to operate the device for broadband applications and hence researchers always try to incorporate different materials for either stretched frequency response or superior photodetection properties. Till date, variety of materials has been explored to design an “All-in-One” PD which can deliver an exemplary performance. However, there has been always a trade-off between the responsivity (R) and speed/ response time of the PDs. To start with, Silicon (Si) PDs have reached up to extremely high R of the value 92 A W1 by using microstructured Si [1] and faster response up to picosecond (ps) by using interdigitated trenches [2]. On the other hand, novel materials like graphene [3], composites of graphene and perovskite [4],
* Corresponding author. E-mail address: [email protected] (D.B. Patel). https://doi.org/10.1016/j.jallcom.2019.04.111 0925-8388/© 2019 Elsevier B.V. All rights reserved.
MoS2 [5,6], TiO2 nanostructures [7,8], nonstoichiometric oxides [9e11] have been used to develop highly efficient PDs. One of the key approaches to develop broadband PDs is to employ heterojunctions. Researchers have achieved the broadband detections from UV to IR regions by using such junctions [12,13]. At the same time, high values of R can be obtained by using efficient charge collection grid and photoactive materials having very high absorption coefficient. Such practices have also been adopted especially for high purity single crystal materials like GaN or Si [14]. Another approach to achieve an efficient PD is to use surface plasmonic effect by metal nanoparticles or quantum dots [15,16]. In addition surface texturing [17] also leads to an improved light absorption in PDs and hence increases the overall performance. Amongst these various ways to improve the performance of conventionally available PDs, hybrid or heterojunctions have proved to be the potent solution. Recently, a research on hybrid structure of graphene and other 2D materials showed superior properties of PD [18]. Moreover, other heterojunctions like graphene/Si [19,20], ZnO/GaN [21], MoS2/Si [22], Bi2Te3/Si [23], Bi/ WS2/Si [24] and ZnO/CuO [25] have also proved to be of great importance when it comes to high speed and broadband
K.R. Chauhan, D.B. Patel / Journal of Alloys and Compounds 792 (2019) 968e975
applications. However, till date TiO2/Si hybrid structures have been very limitedly studied in-spite of having all the prerequisites to form a stable junction. This might be due to a band gap limited absorption properties of available TiO2 thin films which absorbs mostly in the UV region. The study of heterojunction PD with p-Si (111)/TiO2 nanorod array configuration showed that the UV sensitivity and response time of PDs can be improved by interfacial charge properties and oxygen vacancies [26]. In a similar study by using n-Si (111)/TiO2 nanorod array claimed the bias dependent detection properties of such interface [27]. However, in both these studies a complete set of PD properties (response time, linear dynamic range, sensitivity, and detectivity) was not provided which is necessary to show an overall highly efficient PD. Moreover, these studies are based on TiO2 nanorods which are highly sensitive to oxygen defects and the overall device performance may also be governed by such vacancies. Similar studies with either Si or TiO2 configurations led to the PDs with very slow response time (in ms) and only UV/visible light detection [28e30]. Hence, the fabrication of broadband PD with superior photodetection properties and characteristics in all aspects is still a challenging task. Recently we have developed nanocrystalline TiO2 thin films with excitonic properties and remould electron affinity which offer a unique absorption spectrum [10]. In addition, its low electron affinity facilitates extraordinary photodetection properties while interfaced with various metals [9]. In this work, we have made efforts to see the effect of very thin nanocrystalline TiO2 layer on the performance of Si PDs. Such a thin layer ensures the functionality of nanojunction/s which can greatly enhance the overall performance of the PDs. In particular, two different devices have been studied i.e. n-TiO2/p-Si and n-TiO2/n-Si heterojunction PDs. It will be shown that the unique properties of newly developed TiO2 films colossally improved the performance of Si PDs. 2. Experimental Mirror-like monocrystalline Si wafers with 450 mm thickness, 1e5 U cm electrical resistivity and (100) orientations were used as substrates. Metallization of p-Si and n-Si wafers was realized by sputtering thin layers of aluminum (Al) and silver (Ag), respectively. DC sputtering of high purity Ti target (99.995% pure, Sigma Aldrich) was done at 5 mT working pressure with continuous flow of Ar (40 sccm) and by applying a constant power of 150 W at room temperature for 900 s. The substrate stage was kept at 5 rotations per minute to ensure the uniform deposition of Ti film. Ti coated Silicon films were then thermally treated in vacuum furnace at 700 C for 600 s to convert Ti thin films into titanium dioxide (TiO2). The front metallization was done in the form of fingers and busbar of Ag metal by using DC sputtering. 3. Characterizations The phase identification of TiO2 thin film was carried out by XRay diffractometer (X'Pert Pro, PANalytical) using characteristic wavelength 1.45 Å of Cu-ka radiation source. The optical measurements were done by using UVeVis spectrophotometer (Shimadzu-2600) between wavelength range 220e1400 nm. Morphological images of n-TiO2 and a cross section analysis of the PD were obtained by using field emission scanning electron microscope (FE-SEM) (Ultra-55, Zeiss). Photoresponse measurements under chopping (100 Hz) light conditions were carried out by using Potentiostat/Galvanostat (PGSTAT302 N, Autolab). UV light illumination system consisted of the monochromatic UV LED (365 nm) and a function generator. In addition, visible light LEDs of 425 nm, 515 nm and 620 nm with an accuracy ±5 nm were used to record
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the photoresponse curves. Quantum efficiency (QE) measurements were done by using Optosolar instrument to calculate the responsivity (R) and specific detectivity (D) of the PDs. 4. Results and discussion 4.1. Material characterization Fig. 1 shows (A) XRD spectrum (B) Variation in absorption coefficient (a) with energy (C) FE-SEM surface morphology and (D) cross-sectional view of TiO2/Si interface. The XRD pattern of TiO2 thin film showed intense peaks at 27.83 , 36.27 and 54.92 , which belong to (110), (101) and (211) planes of tetragonal crystal structure of rutile TiO2. The other low intensity peaks were also found to be of only rutile TiO2 crystal. The high purity of deposited TiO2 thin film can be verified from the XRD spectrum as no impurity or elemental Ti peaks were found. According to Fig. 1(B), the maximum value of a (4.6 105 cm1) was found to be at 4 eV of photon energy and band gap of TiO2 was found to be 3.6 eV which is relatively high compared to rutile TiO2 films reported in the literature [31]. This higher value of band gap suggests that no band-toband transitions will be allowed for the energies of light used in this work. It can be seen from Fig. 1(C) that uniform TiO2 nanocrystals of the sizes 10e20 nm uniformly cover the Si substrate. The thickness of TiO2 film was found to be 142 nm by cross-sectional FE-SEM image shown in Fig. 1(D). 4.2. Device performance: quantitative analysis In this study, we have employed three different configurations of silicon photodetectors as (1) Anisotype heterojunction: Ag/nTiO2/p-Si/Al, (2) Isotype heterojunction: Ag/n-TiO2/n-Si/Ag and (3) Metal-semiconductor-metal (M-S-M): Ag/p-Si/Al. For convenience, Ag/n-TiO2/p-Si/Al PD is designated as D1, Ag/n-TiO2/n-Si/Ag as D2 and Ag/p-Si/Al as D3. The device structures are presented in Fig. 2 (D1-D3), in which D3 can be considered as a reference device for the comparison. It can be inferred that D1 works on the principle of heterojunctions whereas D2 and D3 will behave as M S junction. All these structures found to be functional broadband photodetectors under reverse biased conditions. Photoresponse curves of the devices under chopped light illumination condition are shown in Fig. 3(A-D) at 4 V. It can be seen from Fig. 3 that all the devices offer excellent dark current characteristics. When the devices were illuminated with different wavelengths of the solar spectrum, distinct nature of photocurrent was observed from all the devices. It was interesting to notice that the shape of photocurrent transient for D2 comprises of an initial overshoot which rapidly decreases to a stable plateau region. A detailed analysis on the shape of the transients can be found in the literature which critically analyzes each and every possible mechanism of different configurations [32]. The observed difference in the transient curve can be due to variety of phenomena including (i) excitonic dissociation [33] (ii) abundant generation of charge carriers at the interface (iii) unequal or trap assisted generation of minority and majority charge carriers in the device upon illumination [32]. It can be seen that such a feature was only observed for device D2 and hence the most probable reasons for this can be either (ii) or (iii). It can be also noticed that the lowest rise time of device D2 supports the possibility of abundant generation of carriers in the device. However, detailed analysis of the shape of transient is out of the scope of this study and we can only attribute this nature to the unique interfacial properties. The calculated values of photodetector parameters are listed in Table 1. We will first discuss the observations from Table 1 and then will try to explain the effect of functional TiO2 layer on the performance of Si PDs. It can be directly inferred from the Table that
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Fig. 1. (a) XRD spectrum of the nanocrystalline TiO2 thin film deposited over p-Si wafer (b) Variation in absorption coefficient of nanocrystalline TiO2 thin film with energy of incoming photons (c) Surface morphology of nanocrystalline TiO2 thin film deposited over p-Si wafer (d) Cross-sectional FE-SEM image of the TiO2/p-Si interface.
Fig. 2. Device schematics of (D1) Anisotype heterojunction: Ag/n-TiO2/p-Si/Al (D2) Isotype heterojunction: Ag/n-TiO2/n-Si/Ag and (D3) Metal-semiconductor-metal: Ag/p-Si/Al photodetectors used in this study.
device D1 outperformed the other PDs in terms of all PD parameters except rise and fall time. Starting with photocurrent gain, the photo to dark current ratio was found to be the highest for D1 for all the wavelengths. It can be also noticed that the overall gain of the devices with TiO2 layer was found to be superior compared to the device without it. This is due to the exponential dependence of photocurrent in Schottky PDs on the photon energy which was also seen in ZnO/Graphene PDs [34]. Other important parameter for an efficient design of PD is the response time which is either fast rise and/or fall time. The rise time (tr) is defined as the time taken for the photocurrent value to increase from 10% to 90% of its final value whereas the fall time (tf) is
the time taken by the PD to reduce its photocurrent value from 90% to 10% of its maximum value when the light is turned off. The measured values of rise time and fall time for all the devices have been presented in the form of bar chart with error bar in Fig. 4(A) and (B), respectively. All the PDs were found to be fast reactive for different incident wavelengths of light as the magnitude of time taken by any device for creation/annihilation of charge carriers was in ms. Moreover, devices with TiO2 layer offered fast response compared to the device without TiO2. The fastest detection was achieved from the device D2 having the rise time of ~31 ms for the visible lights. On the other hand, D3 was found to be fastest for the detection of UV light with the response time of only 40 ms. It can be
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Fig. 3. The photoresponse spectra of all the photodetectors under the illumination of (a) 365 nm (b) 425 nm (c) 515 nm and (d) 620 nm.
Table 1 Calculated photodetector parameters extracted at a power density of 1 mW/cm2 and applied bias of 4 V for all the devices. Parameter
Wavelength (nm)
D1
D2
D3
Gain
365 nm 425 nm 515 nm 620 nm 365 nm 425 nm 515 nm 620 nm 365 nm 425 nm 515 nm 620 nm 365 nm 425 nm 515 nm 620 nm 365 nm 425 nm 515 nm 620 nm
39.6 58.8 29.0 46.6 3.86 103 5.78 103 2.80 103 4.56 103 31.9 35.4 29.2 33.4 8.18 104 1.48 105 2.02 105 2.80 105 7.62 1013 4.30 1013 3.08 1013 2.22 1013
16.5 21.4 17.1 19.9 1.55 103 2.04 103 1.61 103 1.89 103 24.3 26.6 24.7 26.0 2.73 104 1.24 105 1.95 105 1.88 105 2.28 1012 5.25 1013 3.40 1013 3.53 1013
24.6 10.0 14.4 14.6 2.36 103 9.01 103 1.34 103 1.36 103 27.8 20.0 23.2 23.3 3.96 104 9.84 104 1.44 105 1.03 105 1.47 1012 6.58 1013 4.43 1013 6.20 1013
Sensitivity
LDR (dB)
NPDR (W1)
NEP (W)
also noted that this time was the shortest compared to the time taken for visible light detection by device D3. In addition, the rise and fall time of devices D1 and D3 are of the almost same values indicating full depletion in the diode. Whereas, D2 offers almost half a rise time than the fall time suggesting prevalence of slow carriers and diffusion process in semiconductors. This phenomenon can be attributed to the like junctions (n-n type) which possess same polarity of majority and minority carriers. Since very less no. of counter carriers for recombination may be available in the device, the carriers are slowly diffused in the semiconductor before
reaching at the contacts. However, upon illumination, generation of minority carriers is more and the time taken for the photocurrent value to reach 100% of its value is less than other devices. Another figures of merit are, responsivity (R) and specific detectivity (D) which can be derived from formulas (1) and (2), respectively,
Jph Pin
(1)
R D ¼ pffiffiffiffiffiffiffiffiffiffiffiffi 2 q Jd
(2)
R¼ and
The spectral variations in R and D for all the devices between the wavelengths 340e850 nm are shown in Fig. 5 (A, B). At the same time, measured values of R and D upon illumination of wavelengths under study have been presented in the form of bar charts in Fig. 6 (A, B). The device D1 possesses extremely higher values of responsivity for each wavelength compared to D2 and D3. The responsivity of 6.74 A/W was achieved with D1 in the UV region which is comparable with commercially available UV enhanced silicon avalanche photodetector [35,36]. The maximum responsivity of the value 26 A/W was achieved at 800 nm, which is much higher than the available standard silicon photodetectors [37]. The direct effect of TiO2 functional layer can be observed from a huge difference in the responsivities of devices D1 and D3 in the visible light region. It can be seen that for the wavelengths 600e800 nm, D1 offers at least 2.5 times enhanced values of responsivity compared to D3. It can be also seen from Fig. 5 (A) that except UV, device D2 offers higher responsivity for all the wavelengths compared to D3. The detectivity of D1 was also found to be much
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Fig. 4. Variation in (A) Rise time and (B) Fall time with error bar for the devices D1, D2 and D3 under the illumination of UV (365 nm), Blue (425 nm), Green (515 nm) and Red (620 nm) wavelengths. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. Variation in (A) Responsivity and (B) Detectivity for all the devices with incident photon wavelength.
Fig. 6. Variation in (A) Responsivity and (B) Detectivity for the devices D1, D2 and D3 under the illumination of UV (365 nm), Blue (425 nm), Green (515 nm) and Red (620 nm) wavelengths. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
higher (1.31 1012 Jones) in UV region compared to other structures. Furthermore, the impact of TiO2 layer can be seen from the higher detectivity values of D2 and D1. The other figures of merit for characterizing a photodetector are sensitivity, linear dynamic range
(LDR), normalized photo to dark current ratio (NPDR) and noise equivalent power (NEP). The sensitivity is defined as in equation (3),
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Table 2 Comparison between photodetector parameters presented in this work and reported literatures on TiO2/Si configurations. Bias (V)
l (nm)
Gain
Rise time
Fall time
R (A/W)
Detectivity (Jones)
Sensitivity (%)
LDR (dB)
NPDR (W1)
NEP (W)
Reference
4
365 425 515 620 400 415 475 600 405 350 350 350 365 325
39.6 58.8 29.0 46.6 >2 >3 >5 >2
127.6 ms 73.2 ms 98.4 ms 64.5 ms
120.3 ms 80.8 ms 77.4 ms 77.8 ms
6.74 11.7 23.07 16.66 >0.1 >0.3 >0.2 >0.2 377 0.7 0.053 0.045 69.7
1.31 1012 2.33 1012 3.25 1012 4.50 1012
3.86 103 5.78 103 2.80 103 4.56 103
31.9 35.4 29.2 33.4
8.18 104 1.48 105 2.02 105 2.80 105
7.62 1013 4.30 1013 3.08 1013 2.22 1013
This work (Device-D1)
4
5 4 3 3 5 5
10.4 ms
77.1 4.79
18.5 ms 50.8 ms
11 ms
19.1 ms 57.8 ms
[27]
232 9.38 (@780 nm) 2.08 1012 1.91 1012
[26,27,39e43] with our PD (device-D1) is shown in Table 2.
. Id S ¼ Iph Id
(3) 4.3. The role of functional TiO2 layer: qualitative discussion
The LDR for any PD can be calculated by using equation (4),
. LDR ¼ 20 log Iph Id
(4)
The values of NPDR can be derived from equation (5),
NPDR ¼ R=Id
(5)
The noise equivalent power (NEP) can be found from the formula given in (6),
NEP ¼
1 NPDR
76.06 102 3.79 102
[26] [39] [40] [41] [42] [43]
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2q=I d
(6)
The values of sensitivity for all the PDs were found to be of the order of 103 which make them highly suitable for industry applications. The LDR for all the devices was found to be in the range of 20e35 dB, which is typically observed for Si-PDs. However, the devices with TiO2 layer possessed relatively broad LDR. The values of NPDR were found to be as high as 2.8 105 W1 which is much higher than the available silicon photodetectors [38]. Similarly, NEP values of all the devices in the broadband spectrum range were found to be of the order of ~1013 W, which makes them suitable for picowatt power detection. The comparison between the parameters of other TiO2/Si based PDs available in the literature
To understand the role of functional TiO2 layer, we calculated the band edges of nanocrystalline TiO2 film by using Mott-Schottky analysis. It was found that the TiO2 films fabricated for this work possessed relatively lower electron affinity compared to the films deposited via any route till date. These lower values of affinity and higher band-gap make these semi-transparent films unique for their utilization as functional transport layer in Si PDs. The band diagrams of each device with actual values of band edges under equilibrium condition have been presented in Fig. 7 to thoroughly understand the current mechanisms in PDs. It is also known that in reverse bias condition, injection of minority carriers control the overall current of the device. The nanocrystalline TiO2 film with lower electron affinity provides a very steep bending of the bands at the TiO2/Si interface as shown in Fig. 7 (D1 and D2). As the applied reverse bias increases such steepness increases and provides minimum tunneling distance for the minority carriers compared to devices D2 and D3. When the applied bias reaches at its optimum value of 4 V, maximum tunneling probability is achieved and hence maximum photocurrent is recorded. This claim can be further confirmed by the variation in responsivity curve with different bias for the devices D1 and D3 as shown in Fig. 8 (A, B). We have shown in our earlier reports [10] that the photocurrent in this nanocrystalline TiO2 films is via excitonic dissociation and trap
Fig. 7. Band diagrams of (D1) Anisotype heterojunction: Ag/n-TiO2/p-Si/Al (D2) Isotype heterojunction: Ag/n-TiO2/n-Si/Ag and (D3) Metal-semiconductor-metal: Ag/p-Si/Al photodetectors under equilibrium condition.
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Fig. 8. Variation in responsivity curves with different bias for the devices (A) D1 and (B) D3 with incident photon wavelength.
states. Under illumination condition at -4 V, the photogenerated electrons in TiO2 can also tunnel through the barrier and reach to the Si bulk. Now this extra current due to photogeneration in TiO2 layer also increases the overall gain of the devices D1 and D2. In addition, the trap assisted tunneling or thermally assisted multistep tunneling is likely to be prominent through TiO2 layer which enhances the responsivity of the PDs. However, a detailed temperature dependent study needs to be done for the quantitative analysis of such processes. Now when the isojunction is created (as in D2), the improved photogeneration due to TiO2 still remains the same as in D1. However, less band bending (more tunneling distance) and weak built in field at the interface result into relatively lower photocurrent values than device D1. The effect of inserted TiO2 layer can be prominently seen when we compare the operation of device D2 and D3. It can be seen that the device can be fast operative in absence of TiO2 layer which can be seen from the fast rise and fall time of D3. In other words, higher fall time of D1 and D2 suggest that the recombination process is slower than in device D3. This in turn suggests that the higher gain or responsivity can be achieved by inserting TiO2 transport layer which also minimizes the recombination current due to efficient charge transfer. This can be confirmed from a higher dark current density in device D3 which was 6.62 104 A cm2. Such a huge dark current is a typical characteristic of Schottky devices with relatively lower barrier height and has been explained very well in literature [44]. In-spite of possessing good responsivity, device D3 offers very low detectivity at each wavelength. This is due to a very high dark current of the device which results from inefficient charge transfer and recombination in the device. 5. Conclusion In conclusion, we have shown that very unique properties of nanocrystalline TiO2 films greatly improve the performance of Si photodetectors. We have thoroughly examined the effect of TiO2 layer on the charge transfer processes in hybrid Si photodetectors. In addition, each photodetector parameter was critically analyzed and the improvements were explained in detail. It was found that TiO2/p-Si device can be utilized as highly responsive and sensitive photodetector whereas TiO2/n-Si can be used for quick operation/ fast detection applications. The availability of excitonic transitions and intermediate states in TiO2 nanocrystals was found to be a major source of better functionality of the Si photodetectors. Nonetheless, an exploitation of slow diffusion process and enhanced generation rate in isotype Si devices can be availed by
using n-TiO2 functional layer. We have shown that the broadband photodetectors with very high responsivity and sensitivity can be made by using TiO2/Si configuration. In an all, for the visible light (425 nm), gain of Si device can be increased to ~6 times and noise level can be minimized to 4.30 1013 W by incorporation a functional film of TiO2 nanocrystals. This study opens up a possibility for the use of functional layers with modified electron affinity, band alignment and tunneling length for efficient designs of photoelectric devices. Acknowledgements Authors acknowledge technical support from Pandit Deendayal Petroleum University, Gandhinagar, Gujarat and administrative support from their current organization, Lovely Professional University, Phagwara, Punjab. Both the authors have contributed equally in this work. References [1] Z. Huang, J.E. Carey, M. Liu, X. Guo, E. Mazur, J.C. Campbell, Microstructured silicon photodetector, Appl. Phys. Lett. 89 (3) (2006), 033506. [2] J.Y. Ho, K.S. Wong, Bandwidth enhancement in silicon metal-semiconductormetal photodetector by trench formation, IEEE Photonics Technol. Lett. 8 (8) (1996) 1064e1066. [3] A. Pospischil, M. Humer, M.M. Furchi, D. Bachmann, R. Guider, T. Fromherz, T. Mueller, CMOS-compatible graphene photodetector covering all optical communication bands, Nat. Photon. 7 (11) (2013) 892. [4] Y. Lee, J. Kwon, E. Hwang, C.H. Ra, W.J. Yoo, J.H. Ahn, J.H. Park, J.H. Cho, Highperformance perovskiteegraphene hybrid photodetector, Adv. Mater. 27 (1) (2015) 41e46. [5] D. Kufer, G. Konstantatos, Highly sensitive, encapsulated MoS2 photodetector with gate controllable gain and speed, Nano Lett. 15 (11) (2015) 7307e7313. [6] Y. Xie, B. Zhang, S. Wang, D. Wang, A. Wang, Z. Wang, H. Yu, H. Zhang, Y. Chen, M. Zhao, B. Huang, Ultrabroadband MoS2 photodetector with spectral response from 445 to 2717 nm, Adv. Mater. 29 (17) (2017) 1605972. [7] J. Zou, Q. Zhang, K. Huang, N. Marzari, Ultraviolet photodetectors based on anodic TiO2 nanotube arrays, J. Phys. Chem. C 114 (24) (2010) 10725e10729. [8] X. Li, C. Gao, H. Duan, B. Lu, X. Pan, E. Xie, Nanocrystalline TiO2 film based photoelectrochemical cell as self-powered UV-photodetector, Nano Energy 1 (4) (2012) 640e645. [9] D.B. Patel, K.R. Chauhan, S.H. Park, J. Kim, High-performing transparent photodetectors based on Schottky contacts, Mater. Sci. Semicond. Process. 64 (2017) 137e142. [10] D.B. Patel, K.R. Chauhan, W.H. Park, H.S. Kim, J. Kim, J.H. Yun, Tunable TiO2 films for high-performing transparent Schottky photodetector, Mater. Sci. Semicond. Process. 61 (2017) 45e49. [11] H.S. Kim, K.R. Chauhan, J. Kim, E.H. Choi, Flexible vanadium oxide film for broadband transparent photodetector, Appl. Phys. Lett. 110 (10) (2017) 101907. [12] K.K. Manga, J. Wang, M. Lin, J. Zhang, M. Nesladek, V. Nalla, W. Ji, K.P. Loh, High-performance broadband photodetector using solution-processible
K.R. Chauhan, D.B. Patel / Journal of Alloys and Compounds 792 (2019) 968e975 PbSeeTiO2egraphene hybrids, Adv. Mater. 24 (13) (2012) 1697e1702. [13] H. Yuan, X. Liu, F. Afshinmanesh, W. Li, G. Xu, J. Sun, B. Lian, A.G. Curto, G. Ye, Y. Hikita, Z. Shen, Polarization-sensitive broadband photodetector using a black phosphorus vertical pen junction, Nat. Nanotechnol. 10 (8) (2015) 707. [14] Van Hove, J.M., Kuznia, J.N., Olson, D.T., Kahn, M.A. and Blasingame, M.C., APA Optics Inc, 1994. High responsivity ultraviolet gallium nitride detector. U.S. Patent 5,278,435. [15] D.M. Schaadt, B. Feng, E.T. Yu, Enhanced semiconductor optical absorption via surface plasmon excitation in metal nanoparticles, Appl. Phys. Lett. 86 (6) (2005), 063106. [16] C.C. Chang, Y.D. Sharma, Y.S. Kim, J.A. Bur, R.V. Shenoi, S. Krishna, D. Huang, S.Y. Lin, A surface plasmon enhanced infrared photodetector based on InAs quantum dots, Nano Lett. 10 (5) (2010) 1704e1709. [17] Zaidi, S.H., Zaidi Saleem H, 2006. Method of making an enhanced optical absorption and radiation tolerance in thin-film solar cells and photodetectors. U.S. Patent 7,109,517. [18] F.H.L. Koppens, T. Mueller, P. Avouris, A.C. Ferrari, M.S. Vitiello, M. Polini, Photodetectors based on graphene, other two-dimensional materials and hybrid systems, Nat. Nanotechnol. 9 (10) (2014) 780. [19] X. Li, M. Zhu, M. Du, Z. Lv, L. Zhang, Y. Li, Y. Yang, T. Yang, X. Li, K. Wang, H. Zhu, High detectivity graphene-silicon heterojunction photodetector, Small 12 (5) (2016) 595e601. [20] Y. Cao, J. Zhu, J. Xu, J. He, J.L. Sun, Y. Wang, Z. Zhao, Ultra-broadband photodetector for the visible to terahertz range by self-assembling reduced graphene oxide-silicon nanowire array heterojunctions, Small 10 (12) (2014) 2345e2351. [21] C.H. Chen, S.J. Chang, S.P. Chang, M.J. Li, I.C. Chen, T.J. Hsueh, C.L. Hsu, Novel fabrication of UV photodetector based on ZnO nanowire/p-GaN heterojunction, Chem. Phys. Lett. 476 (1e3) (2009) 69e72. [22] M.R. Esmaeili-Rad, S. Salahuddin, High performance molybdenum disulfide amorphous silicon heterojunction photodetector, Sci. Rep. 3 (2013) 2345. [23] J. Yao, J. Shao, Y. Wang, Z. Zhao, G. Yang, Ultra-broadband and high response of the Bi2Te3/Si heterojunction and its application as a photodetector at room temperature in harsh working environments, Nanoscale 7 (29) (2015) 12535e12541. [24] J. Yao, Z. Zheng, J. Shao, G. Yang, Promoting photosensitivity and detectivity of the Bi/Si heterojunction photodetector by inserting a WS2 layer, ACS Appl. Mater. Interfaces 7 (48) (2015) 26701e26708. [25] R.R. Prabhu, A.C. Saritha, M.R. Shijeesh, M.K. Jayaraj, Fabrication of p-CuO/nZnO heterojunction diode via sol-gel spin coating technique, Mater. Sci. Eng., B 220 (2017) 82e90. [26] A.M. Selman, Z. Hassan, M. Husham, N.M. Ahmed, A high-sensitivity, fastresponse, rapid-recovery pen heterojunction photodiode based on rutile TiO2 nanorod array on p-Si (1 1 1), Appl. Surf. Sci. 305 (2014) 445e452. [27] T. Ji, Q. Liu, R. Zou, Y. Sun, K. Xu, L. Sang, M. Liao, Y. Koide, L. Yu, J. Hu, An interface engineered multicolor photodetector based on n-Si (111)/TiO2 nanorod array heterojunction, Adv. Funct. Mater. 26 (9) (2016) 1400e1410. [28] C. Gao, X. Li, X. Zhu, L. Chen, Y. Wang, F. Teng, Z. Zhang, H. Duan, E. Xie, High performance, self-powered UV-photodetector based on ultrathin, transparent, SnO2eTiO2 coreeshell electrodes, J. Alloys Compd. 616 (2014) 510e515.
975
[29] M.B. Sarkar, A. Mondal, B. Choudhuri, B.K. Mahajan, S. Chakrabartty, C. Ngangbam, Enlarged broad band photodetection using Indium doped TiO2 alloy thin film, J. Alloys Compd. 615 (2014) 440e445. [30] N.M. Khusayfan, A.A. Al-Ghamdi, F. Yakuphanoglu, Solar light photodetectors based on nanocrystalline copper indium oxide/p-Si heterojunctions, J. Alloys Compd. 663 (2016) 796e807. [31] M. Landmann, E. Rauls, W.G. Schmidt, The electronic structure and optical response of rutile, anatase and brookite TiO2, J. Phys. Condens. Matter 24 (19) (2012) 195503. [32] M. Mokhtarimehr, S.A. Tatarkova, Photocurrent transients of thin-film solar cells, JOSA B 34 (8) (2017) 1705e1712. [33] D.L. Li, W. Si, W.C. Yang, Y. Yao, X.Y. Hou, C.Q. Wu, Spike in transient photocurrent of organic solar cell: exciton dissociation at interface, Phys. Lett. 376 (4) (2012) 227e230. [34] H. Lee, N. An, S. Jeong, S. Kang, S. Kwon, J. Lee, Y. Lee, D.Y. Kim, S. Lee, Strong dependence of photocurrent on illumination-light colors for ZnO/graphene Schottky diode, Curr. Appl. Phys. 17 (4) (2017) 552e556. [35] A. Rochas, A.R. Pauchard, P.A. Besse, D. Pantic, Z. Prijic, R.S. Popovic, Low-noise silicon avalanche photodiodes fabricated in conventional CMOS technologies, IEEE Trans. Electron Devices 49 (3) (2002) 387e394. [36] M. Hohenbild, P. Seegebrecht, H. Pless, W. Einbrodt, November. High-speed photodiodes with reduced dark current and enhanced responsivity in the blue/uv spectra, The 11th IEEE International Symposium on, in: Electron Devices for Microwave and Optoelectronic Applications, 2003, vol. 2003EDMO, 2003, pp. 60e65 (IEEE). [37] X. Wang, Z. Cheng, K. Xu, H.K. Tsang, J.B. Xu, High-responsivity graphene/ silicon-heterostructure waveguide photodetectors, Nat. Photon. 7 (11) (2013) 888. [38] Y. An, A. Behnam, E. Pop, A. Ural, Metal-semiconductor-metal photodetectors based on graphene/p-type silicon Schottky junctions, Appl. Phys. Lett. 102 (1) (2013), 013110. [39] H.M. Ali, S.A. Makki, A.N. Abd, Enhanced photo-response of porous silicon photo-detectors by embedding Titanium-dioxide nano-particles, in: Journal of Physics: Conference Series, vol. 1003, IOP Publishing, 2018, May, 012073. No. 1. [40] U.M. Nayef, K.A. Hubeatir, Z.J. Abdulkareem, Characterisation of TiO2 nanoparticles on porous silicon for optoelectronics application, Mater. Technol. 31 (14) (2016) 884e889. [41] U.M. Nayef, K.A. Hubeatir, Z.J. Abdulkareem, Ultraviolet photodetector based on TiO2 nanoparticles/porous silicon hetrojunction, Optik-Int. J. Light Electron Optics 127 (5) (2016) 2806e2810. [42] A.M. Selman, Z. Hassan, Fabrication and characterization of metalesemiconductoremetal ultraviolet photodetector based on rutile TiO2 nanorod, Mater. Res. Bull. 73 (2016) 29e37. [43] A.M. Selman, Z. Hassan, Highly sensitive fast-response UV photodiode fabricated from rutile TiO2 nanorod array on silicon substrate, Sensor Actuator Phys. 221 (2015) 15e21. [44] K.R. Chauhan, I. Mukhopadhyay, On the electrical and interface properties of nanostructured CdTe Schottky diodes electrodeposited from an ionic liquid medium, J. Appl. Phys. 115 (22) (2014) 224506.