- Email: [email protected]
UV-Visible optical photo-detection from porous silicon (PS) MSM device
Accepted Manuscript UV-Visible optical photo-detection from porous silicon (PS) MSM device M. Das, S. Sarmah, D. Sarkar
PII:
S0749-6036(16)30897-7
DOI:
10.1016/j.spmi.2016.11.052
Reference:
YSPMI 4687
To appear in:
Superlattices and Microstructures
Received Date: 8 September 2016 Revised Date:
21 November 2016
Accepted Date: 22 November 2016
Please cite this article as: M. Das, S. Sarmah, D. Sarkar, UV-Visible optical photo-detection from porous silicon (PS) MSM device, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.11.052. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
UV-Visible optical photo-detection from porous silicon (PS) MSM device
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M Das, S Sarmah and D Sarkar1
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Department of Physics, Gauhati University, Guwahati- 781014, Assam, India 1
Email: [email protected] Phone: +919435049434
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Si photodiodes have been in use as UV detectors and some compound semiconductors as
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visible detectors. However their implementation to the optoelectronic field is limited due to
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high fabrication cost and/or sophisticated prerequisites. The present article aims at fabricating
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porous silicon Metal-Semiconductor-Metal structure and its photodetection property for the
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UV wavelength range from 250-390 nm along with a portion of visible spectrum. PS
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thickness attained is ~ 2µm with uniform distribution of pores. It shows characteristic visible
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yellow/green luminescence under UV-Visible irradiation. The responsivities, obtained
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through photoconductivity measurement of the device, are obtained as ̴ 1.42 and 2.00 AW-1
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for UV and visible ranges respectively, whereas the response times in corresponding ranges
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as 0.70 and 1.00 s. These results suggest superiority of the device as a UV-Visible detector
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compared to silicon or other semiconductor detectors. However, the device shows ageing
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effect due to slow oxidation of the PS layer.
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Keywords: Porous Silicon; MSM device; Responsivity; Photodetector
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Introduction
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Low cost and high performance materials are need of the days for basic photonic application.
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On such a scenario Porous Silicon (PS) based photonic device is a good candidate since its
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fabrication requirement is simple at the same time highly cost effective [1]. Photodetectors
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(PDs) are ones among such photonic devices which attract great deal of interest among the
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researchers particularly for ultra violet (UV) detection in various fields such as chemical,
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ACCEPTED MANUSCRIPT biological, flame analysis and radiation detection, and in optical communication [2-5]. PS has
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gained much attention over silicon on development of optoelectronic devices [6, 7]. As the
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band gap of PS is within the visible region, there’s ample scope to employ PS as UV to
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visible optical based PDs. Zheng et al [8] have reported metal PS visible light PD with
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response time of 2ns for applied voltage of -9V. Balagurav L A et al [9] reported visible to
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NIR photodetector based on diode structure of Al/PS/c-Si and observed strong effect of
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annealing temperature on its photosensitivity property. These authors [10] also reported the
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applicable potential of PS based PDs for detection of visible to UV. However its
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implementation as a UV based PD are still limited. Although some recent research works
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suggest PS based hybrid device for efficient UV-Visible PDs [11, 12], still there is ample
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scope in the direction of improvement and standardization.
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fabrication of metal-PS-metal MSM structure and to efficiently modulate it for detecting UV-
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Visible radiation through measurement of photoconductivity. Finally, there’s a brief
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discussion on the ageing effect on the device performance.
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Experimental
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Fabrication of PS
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PS sample is prepared by photo-assisted electrochemical etching of (100) oriented
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phosphorous doped n-type crystalline silicon (c-Si) wafer of resistivity~ 1-100 Ωcm and
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thickness of 500 µm. Prior to etching, the wafer is subjected to RCA cleaning to remove the
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native oxide layer present on the wafer surface. To facilitate anodization process back side of
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the said wafer is coated with Al by vacuum coating followed by annealing at 150oC under
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vacuum of ~10-5 torr to achieve proper electrical contact for electrolysis with back side Al
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plate [13]. Configuration of the electrochemical cell and electrolyte concentration are kept
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same as that have been described by our recently reported work [14]. But in this case there is
The present work aims at
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ACCEPTED MANUSCRIPT little difference, the etching current density is not constant in the present case but varied
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cyclically in the current density limits of 22.1-13.2 mAcm-2 for total etching time of 54 sec.
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Etching time and etching current density for the anodization process are controlled manually
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using dc power supply Agilent-6634B with front side illumination with a 200 W Mercury
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light source (Newport-66901) placed at a distance of 50 cm from the electrochemical cell.
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This illumination is essential for PS formation in n-type Si to facilitate creation of extra holes
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which is required for pore formation in n-Si [15], as n-type Si possesses very few numbers of
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holes as minority carriers.
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Fabrication of PS MSM structure
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The MSM finger pattern semi-transparent electrode of spacing and width of 2 mm are
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deposited on top of the PS surface using a shadow mask by thermal evaporation of high
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purity (99.99%) aluminium (Al) under vacuum of 10-5 torr in a 12A4D Hind High Vacuum
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coating unit. The thickness of the electrode is maintained at 50 nm by using digital thickness
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monitor (DTM) attached to the unit. Fig. 1(a) shows the schematic of the electrode
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configuration. The dark and photo current – voltage (I-V) characteristics of the fabricated
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MSM device is recorded using computer interfaced Keithley 2400 source meter at room
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temperature for an applied voltage range of -5 to +5V. The ON-OFF time response
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characteristics for different light intensities are also measured, whereas spectral responsivity
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is measured for bias voltage of -2V for different wavelengths of irradiation (250-600 nm).
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The schematic of the measurement setup is represented in fig. 1(b).
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Results and Discussion
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Figure 2 reveals surface morphology of the prepared PS with the visible appearance of the PS
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sample under UV-Visible irradiation (Fig. 2(a)), planar view of FESEM image (2(b)) and
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cross sectional view (2(c)). Fig. 2(a) shows yellowish green colour, the basic confirming
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feature of PS formation, 2(b) ascertains formation of uniform porous layers along with
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randomly distributed Si nano particle clusters and 2(c) gives the thickness of PS layer, which
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is found to be 2.3 µm.
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75 Suitability of PS-MSM photo-detector
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For a material to be used as an efficient photodetector, three key parameters needed to be
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checked, viz., spectral responsivity (Rλ), external quantum efficiency (EQE) and specific
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detectivity (D*). These are obtainable from the photoconductivity measurement of the
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sample. Rλ and The External Quantum Efficiency (EQE) are calculated using the relations
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[16]
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Rλ=
∆J P
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EQE=
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1.24 Rλ hc Rλ ≈ eλ λ ( µm)
(1)
Where ∆J photocurrent in Acm-2, P is the power of the irradiated light in Wcm-2, h is
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Planck’s constant, c is velocity of light, e is electronic charge and λ is the wavelength of the
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incident radiation.
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To observe the spectral photo-response for a particular wavelength (λ), Rλ and EQE vs λ are
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plotted. This plot is shown in fig. 3. From the plots it is evident that the device has better
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response in UV range (250-390 nm), though not that good in entire visible range. However,
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for a particular visible wavelength of ~ 450 nm the response is quite high, in fact much higher
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than that of the UV range. The calculated Rλ values are ~1.4 AW-1 in the UV range and ~2.0
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good agreement with the theoretically obtained ones [12] for PS based hybrid structured UV
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detectors, but are higher compared to some earlier reported PS based PDs [11, 17, 18] which
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reported 0.067-0.200, 0.027 and 0.100AW-1for UV and near UV (visible) band respectively.
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These values are much higher as obtained from the present work. Calculated EQE is found to
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be 708-590% at bias -2V for UV band to near UV (visible) band. This indicates wavelength
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dependent gain of the present photo-detector is high for UV band compared to that of the
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visible band. EQE values are high compared to those of earlier reported silicon based UV
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detectors [19-21]. Such high value of EQE (above 100%) might have originated due to
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multiple exciton generation (MEG) in the Si nano-crystal [22] region of the PS layer on
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photon absorption. The possible explanation for the process lies in the fact that when
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radiation of excitation energy equal or greater than twice the band gap energy ( E ex ≥ 2 E g ) is
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incident on the semiconductor structure the electron-hole pair is generated and the remaining
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excess energy (Eexcess= Eex - Eg) is employed to generate another electron hole pair via
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process called impact ionization in the nanostructure as described in the inset of fig. 3
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Specific detectivity (D*) of the structure for particular wavelength at applied bias of -2V is
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calculated using the equation [23]
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D* =
Rλ 2eJ d
(3)
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Where Jd is dark current density, the value of specific detectivity is found to be in the range
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of (6.24 − 9.4) × 1012 cm Hz 1 / 2W −1 for exciting wavelength of 250-450 nm which are much
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higher compared to the values reported for some earlier Si based detectors [24] and the wide
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direct band alloy based UV detector [25]. The calculated D* values are also listed in Table 1.
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ACCEPTED MANUSCRIPT 116 Photo current-voltage (I-V) characteristics of PS-MSM structure
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UV irradiation impact on the I-V characteristics and kinetics of photoresponse are shown in
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fig 4, (a) shows I-V characteristics, (b) the ON-OFF kinetics, (c) response time and (d) the
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intensity dependence. I-V (4a) reveals typical Schottky diode nature. From the linear region
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(> ±1V) of this plot, the ideality factor (n) calculated and the values are found to be 3.34 to
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4.9 and 2.84 to 4.30 for forward and reverse bias in the wavelength range 250nm-390nm,
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such high value of n is attributed to the porous nature of PS confirming high density of
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surface states at Al:PS and PS:n-Si junction. However, on UV irradiation there is an
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enhancement of rectification in both forward and reverse regimes indicating a good photo
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enhancing property of the device. Calculated normalized photo to dark current ratio (NPDR)
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at applied bias voltage of -5V is obtained to be in the range (15.0-14.10) x 106 W-1 for
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incident power density of (28.2-41.4µWcm-2). These values are quite high compared to those
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of some recently reported UV detectors [26, 27]. These NPDR values are also listed in
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Table1 along with the other characteristic parameters. From the best of our knowledge the
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values are some of the highest reported NPDR values for any porous silicon based UV
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photodetectors. ON/OFF kinetic response (4(b)) ascertains the structure to exhibit fair
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photoresponse and also that the photocurrent increases with increasing wavelength in the UV
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range. The response times i.e., rise and decay times, as calculated from fig. 4(c), are obtained
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as 0.7 and 0.8 sec. respectively. However, these values are relatively large compared to some
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of the earlier reported UV PDs [28, 29], i.e., the device is slower in the present case. Slow
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response is due to the presence of inhomogenity and high surface density at the metal-PS and
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PS-n-Si heterostructure. Nonetheless the values less than 1sec suggest possible application of
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the device in the field of optoelectronics for UV detection. Plot of power density (µWcm-2)
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vs. photocurrent (Fig 4(d)) can be fitted by simple power law in the form
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I p = AP α
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(4)
Where Ip , A and P represent photocurrent, proportionality constant, and intensity of irradiated
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light respectively. The exponent α is a fitting parameter which depicts linearity or non-
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linearity of the plot. From the plots value of α is found to be 0.96, which is approaching
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unity, i.e., almost linear. One may infer that photocurrent increases linearly with increasing
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UV wavelength. Fig.5 (a-d) shows similar photoconductivity characteristics for the visible
148
wavelength of 450 nm. From these figures it can be seen that the structure is also sensitive to
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visible light with enhanced rectification in both forward and reverse direction proving
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excellent photoresponse characteristics with NPDR of 2.14 X 107 W-1 for incident power
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density of 259µWcm-2 at applied reverse bias of -5V. From the slope of the linear region (low
152
voltage region) of fig 5(a), the values of n are found to be 9.2-6.3 and 7.5-6.52 for both
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forward and reverse directions respectively. This increase in n values for both UV and visible
154
irradiation suggests that the current transport mechanism in PS-MSM structure are dominated
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by recombination and tunnelling of the charge carriers at the interface layer [30] i.e. Al:PS
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and PS:n-Si junctions. Fig 5(b) describes the on/off kinetic response for light of different
157
intensity. From these curves it is evident that with increasing light intensity photocurrent
158
increases with an on/off ratio 2.95, 6.98, 7.5, 9.3 respectively. The response time (rise and
159
decay time) as calculated from fig.5(c) is found to be 1s, i.e., device is slower compared to
160
UV response. However for both UV and visible ranges the structure shows steep rise and
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decay when light is on and off periodically which also indicates that there is ample
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contribution from the slow diffusion (electron diffusion) of the charge carriers while the
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whole photoconduction process is dominated by fast drift (electron drift) [31]. Fig 5(d) shows
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the plot of photocurrent vs. irradiated power density at a bias voltage of -5V. The plots show
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strong dependence of photocurrent on incident light power density with ‘α’ value of 1.47.
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Since ‘α’ is directly linked to trap level density of the semiconductor [32, 33] the value more
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ACCEPTED MANUSCRIPT than 1 suggest nonlinear behaviour revealing high trap density in the PS MSM
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heterostructure which is also supported by the n value showing deviation from the ideal diode
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value.
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Ageing effect on PS-MSM performance
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Since with ageing PS surface layer is known to get oxidised [34], it would surely affect its
172
structural, optical and electrical properties. If kept in ambient, the device degrades by 30% or
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so in a month. However, we have stored the device in vacuum desiccators, which generally
174
have low vacuum ~10-1 Torr, where the efficiency can be retained to some extent up to 30
175
days or so, but there’s significant efficiency loss in few months. We have some results for the
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device stored for 6 months. Fig 6 (a-c) show plots of various parameters obtained from
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photoconductivity study. Fig 6 (a) shows plots for Rλ and EQE vs λ for the device. These
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values are shown to decrease with a blue shift of the peak compared to that of the freshly
179
prepared device (fig 3). Here the maximum Rλ value of 0.25AW-1 and EQE of 82.1% are
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observed at 390nm for the UV region and 0.31AW-1 with 90% EQE at 431nm for the visible
181
region respectively. Fig 6 (b) shows the reverse region I-V characteristics of the ageing
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device for the UV-Visible wavelength range of 250-450nm. It is seen that device dark current
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(Id) increases while photo current (Ip) decreases upon ageing compared to the fresh device
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(Fig 4(a) and 5(a)). This decrease is mainly due to the formation of thin oxide layer between
185
Al:PS interface which decreases the tunnelling probability of photocarriers through the oxide
186
layer towards the contact Al electrode [35]. The UV to Visible NPDR of the device after
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storage is calculated to be in the range (0.2-0.61) X 106 W-1 for incident power density (28.2-
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259 µWcm-2) at applied voltage of -5V. Fig 6(c) shows on/off kinetics of the device at
189
applied bias voltage of -5V. From the curve on/off ratio is calculated to be in the range 1.05-
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1.15 for incident power density of 29.2 and 31.3µWcm-2 with a response time of 0.8s. All
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these parameters are listed in parentheses of table 1 along with the data for fresh device for a
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ready comparison.
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Conclusion
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Freshly prepared porous silicon MSM (PS MSM) structure shows good to excellent
196
photoconduction response for near UV to UV range of the spectrum. Some important
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parameters, such as responsivity, external quantum efficiency (EQE), detectivity and
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normalized photo to dark current ratio (NPDR) of the structure exceeds the value of some
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earlier reported Si based photodetectors. Also, the response time of the device is less than 1s
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for the UV range and 1s for the visible. These results indicate possible application of the PS
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MSM structure as UV-Visible optical detector and switching device in optoelectronic
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application. However, on storing the device for 6 months in vacuum desiccator, due to slow
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oxidation of the PS surface the device performance degrades.
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Acknowledgement
205
Authors
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DST/TSG/PT/2009/96), University Grants Commission (Project grant no.-40-438/2011 SR)
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for providing instrumental facility, IASST, Guwahati for FESEM measurements, Dr M P C
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Kalita, Department of Physics, Gauhati University for PL measurement and UGC-BSR
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scheme for proving fellowship to Mr M. Das.
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Figure captions:
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Figure 1: (a) Schematic of the PS MSM structure (b) Schematic of experimental setup for
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photocurrent measurement
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Figure 2: (a) Visible appearance of the PS sample under UV (b) Planar view of FESEM
287
image and (b) cross-sectional view of FESEM image of the PS sample
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Figure 3: plot of Rλ and EQE vs. λ of PS MSM structure, mechanism of MEG (inset)
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Figure 4: (a) I-V plots for different UV wavelengths (b) ON/OFF kinetic response for
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wavelength range (200-390 nm), (c) rise and decay response for 350nm light (d) plot of
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intensity dependent photocurrent at - 5V bias of PS MSM
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Figure 5: (a) I-V plots at different intensity of 450nm light (b) ON/OFF kinetic response for
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different light intensity, (c) rise and decay response at power density of 259 µWcm-2 (d) plot
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of intensity dependent photocurrent of PS MSM
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Figure 6: (a) plot of Rλ and EQE vs. λ at -2V (b) Reverse region I-V plot for incident power
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density 28.2-259 µWcm-2 (c) ON/OFF kinetics at -5V of aged (oxidised) PS MSM structure
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ACCEPTED MANUSCRIPT Table 1: Responsivity (Rλ), Ideality factor (n), external quantum efficiency (EQE), specific detectivity (D*), NPDR and response time of PS MSM structure, data in parentheses are the values for aged sample (λ)
(nm)/intensity
D*
EQE (%)
Reverse
decay
Hz1/2W-1
(τ)
(σ)
0.7(0.8)
0.8(0.8)
1.42 (0.01)
708 (26)
6.24
300
3.54
3.7
1.46 (0.03)
606 (12)
6.4
350
4.20
3.8
1.42 [0.05]
502 (20)
6.18
390
4.9
4.3
1.34 (0.3)
426 (82)
5.8
450(116, 163, 188,
9.2, 6.9,
7.5, 8.4,
2.004 (0.1)
590 (25)
9.4
259µ Wcm-2)
6.7, 6.3
7.52,
15x106 (1.85x105)
15.3x106(1.31x105) 15x106(4.69x106)
14.1x106 (4.23x106)
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Response Time (Sec.) Rise
3.34
6.52
NPDR (W-1 )
1012 cm
250
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Porous silicon MSM device is fabricated from n-type Si The device shows good photoresponse in UV and visible range of incident radiation External quantum efficiency and detectivity of the device are quite high Much faster response to incident radiation
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PII:
S0749-6036(16)30897-7
DOI:
10.1016/j.spmi.2016.11.052
Reference:
YSPMI 4687
To appear in:
Superlattices and Microstructures
Received Date: 8 September 2016 Revised Date:
21 November 2016
Accepted Date: 22 November 2016
Please cite this article as: M. Das, S. Sarmah, D. Sarkar, UV-Visible optical photo-detection from porous silicon (PS) MSM device, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.11.052. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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UV-Visible optical photo-detection from porous silicon (PS) MSM device
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M Das, S Sarmah and D Sarkar1
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Department of Physics, Gauhati University, Guwahati- 781014, Assam, India 1
Email: [email protected] Phone: +919435049434
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Si photodiodes have been in use as UV detectors and some compound semiconductors as
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visible detectors. However their implementation to the optoelectronic field is limited due to
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high fabrication cost and/or sophisticated prerequisites. The present article aims at fabricating
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porous silicon Metal-Semiconductor-Metal structure and its photodetection property for the
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UV wavelength range from 250-390 nm along with a portion of visible spectrum. PS
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thickness attained is ~ 2µm with uniform distribution of pores. It shows characteristic visible
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yellow/green luminescence under UV-Visible irradiation. The responsivities, obtained
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through photoconductivity measurement of the device, are obtained as ̴ 1.42 and 2.00 AW-1
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for UV and visible ranges respectively, whereas the response times in corresponding ranges
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as 0.70 and 1.00 s. These results suggest superiority of the device as a UV-Visible detector
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compared to silicon or other semiconductor detectors. However, the device shows ageing
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effect due to slow oxidation of the PS layer.
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Keywords: Porous Silicon; MSM device; Responsivity; Photodetector
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Introduction
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Low cost and high performance materials are need of the days for basic photonic application.
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On such a scenario Porous Silicon (PS) based photonic device is a good candidate since its
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fabrication requirement is simple at the same time highly cost effective [1]. Photodetectors
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(PDs) are ones among such photonic devices which attract great deal of interest among the
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researchers particularly for ultra violet (UV) detection in various fields such as chemical,
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gained much attention over silicon on development of optoelectronic devices [6, 7]. As the
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band gap of PS is within the visible region, there’s ample scope to employ PS as UV to
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visible optical based PDs. Zheng et al [8] have reported metal PS visible light PD with
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response time of 2ns for applied voltage of -9V. Balagurav L A et al [9] reported visible to
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NIR photodetector based on diode structure of Al/PS/c-Si and observed strong effect of
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annealing temperature on its photosensitivity property. These authors [10] also reported the
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applicable potential of PS based PDs for detection of visible to UV. However its
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implementation as a UV based PD are still limited. Although some recent research works
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suggest PS based hybrid device for efficient UV-Visible PDs [11, 12], still there is ample
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scope in the direction of improvement and standardization.
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fabrication of metal-PS-metal MSM structure and to efficiently modulate it for detecting UV-
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Visible radiation through measurement of photoconductivity. Finally, there’s a brief
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discussion on the ageing effect on the device performance.
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Experimental
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Fabrication of PS
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PS sample is prepared by photo-assisted electrochemical etching of (100) oriented
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phosphorous doped n-type crystalline silicon (c-Si) wafer of resistivity~ 1-100 Ωcm and
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thickness of 500 µm. Prior to etching, the wafer is subjected to RCA cleaning to remove the
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native oxide layer present on the wafer surface. To facilitate anodization process back side of
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the said wafer is coated with Al by vacuum coating followed by annealing at 150oC under
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vacuum of ~10-5 torr to achieve proper electrical contact for electrolysis with back side Al
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plate [13]. Configuration of the electrochemical cell and electrolyte concentration are kept
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same as that have been described by our recently reported work [14]. But in this case there is
The present work aims at
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cyclically in the current density limits of 22.1-13.2 mAcm-2 for total etching time of 54 sec.
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Etching time and etching current density for the anodization process are controlled manually
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using dc power supply Agilent-6634B with front side illumination with a 200 W Mercury
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light source (Newport-66901) placed at a distance of 50 cm from the electrochemical cell.
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This illumination is essential for PS formation in n-type Si to facilitate creation of extra holes
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which is required for pore formation in n-Si [15], as n-type Si possesses very few numbers of
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holes as minority carriers.
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Fabrication of PS MSM structure
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The MSM finger pattern semi-transparent electrode of spacing and width of 2 mm are
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deposited on top of the PS surface using a shadow mask by thermal evaporation of high
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purity (99.99%) aluminium (Al) under vacuum of 10-5 torr in a 12A4D Hind High Vacuum
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coating unit. The thickness of the electrode is maintained at 50 nm by using digital thickness
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monitor (DTM) attached to the unit. Fig. 1(a) shows the schematic of the electrode
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configuration. The dark and photo current – voltage (I-V) characteristics of the fabricated
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MSM device is recorded using computer interfaced Keithley 2400 source meter at room
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temperature for an applied voltage range of -5 to +5V. The ON-OFF time response
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characteristics for different light intensities are also measured, whereas spectral responsivity
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is measured for bias voltage of -2V for different wavelengths of irradiation (250-600 nm).
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The schematic of the measurement setup is represented in fig. 1(b).
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Results and Discussion
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Figure 2 reveals surface morphology of the prepared PS with the visible appearance of the PS
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sample under UV-Visible irradiation (Fig. 2(a)), planar view of FESEM image (2(b)) and
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cross sectional view (2(c)). Fig. 2(a) shows yellowish green colour, the basic confirming
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feature of PS formation, 2(b) ascertains formation of uniform porous layers along with
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randomly distributed Si nano particle clusters and 2(c) gives the thickness of PS layer, which
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is found to be 2.3 µm.
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75 Suitability of PS-MSM photo-detector
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For a material to be used as an efficient photodetector, three key parameters needed to be
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checked, viz., spectral responsivity (Rλ), external quantum efficiency (EQE) and specific
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detectivity (D*). These are obtainable from the photoconductivity measurement of the
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sample. Rλ and The External Quantum Efficiency (EQE) are calculated using the relations
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[16]
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Rλ=
∆J P
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EQE=
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1.24 Rλ hc Rλ ≈ eλ λ ( µm)
(1)
Where ∆J photocurrent in Acm-2, P is the power of the irradiated light in Wcm-2, h is
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Planck’s constant, c is velocity of light, e is electronic charge and λ is the wavelength of the
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incident radiation.
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To observe the spectral photo-response for a particular wavelength (λ), Rλ and EQE vs λ are
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plotted. This plot is shown in fig. 3. From the plots it is evident that the device has better
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response in UV range (250-390 nm), though not that good in entire visible range. However,
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for a particular visible wavelength of ~ 450 nm the response is quite high, in fact much higher
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than that of the UV range. The calculated Rλ values are ~1.4 AW-1 in the UV range and ~2.0
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good agreement with the theoretically obtained ones [12] for PS based hybrid structured UV
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detectors, but are higher compared to some earlier reported PS based PDs [11, 17, 18] which
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reported 0.067-0.200, 0.027 and 0.100AW-1for UV and near UV (visible) band respectively.
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These values are much higher as obtained from the present work. Calculated EQE is found to
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be 708-590% at bias -2V for UV band to near UV (visible) band. This indicates wavelength
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dependent gain of the present photo-detector is high for UV band compared to that of the
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visible band. EQE values are high compared to those of earlier reported silicon based UV
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detectors [19-21]. Such high value of EQE (above 100%) might have originated due to
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multiple exciton generation (MEG) in the Si nano-crystal [22] region of the PS layer on
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photon absorption. The possible explanation for the process lies in the fact that when
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radiation of excitation energy equal or greater than twice the band gap energy ( E ex ≥ 2 E g ) is
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incident on the semiconductor structure the electron-hole pair is generated and the remaining
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excess energy (Eexcess= Eex - Eg) is employed to generate another electron hole pair via
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process called impact ionization in the nanostructure as described in the inset of fig. 3
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Specific detectivity (D*) of the structure for particular wavelength at applied bias of -2V is
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calculated using the equation [23]
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D* =
Rλ 2eJ d
(3)
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Where Jd is dark current density, the value of specific detectivity is found to be in the range
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of (6.24 − 9.4) × 1012 cm Hz 1 / 2W −1 for exciting wavelength of 250-450 nm which are much
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higher compared to the values reported for some earlier Si based detectors [24] and the wide
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direct band alloy based UV detector [25]. The calculated D* values are also listed in Table 1.
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UV irradiation impact on the I-V characteristics and kinetics of photoresponse are shown in
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fig 4, (a) shows I-V characteristics, (b) the ON-OFF kinetics, (c) response time and (d) the
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intensity dependence. I-V (4a) reveals typical Schottky diode nature. From the linear region
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(> ±1V) of this plot, the ideality factor (n) calculated and the values are found to be 3.34 to
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4.9 and 2.84 to 4.30 for forward and reverse bias in the wavelength range 250nm-390nm,
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such high value of n is attributed to the porous nature of PS confirming high density of
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surface states at Al:PS and PS:n-Si junction. However, on UV irradiation there is an
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enhancement of rectification in both forward and reverse regimes indicating a good photo
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enhancing property of the device. Calculated normalized photo to dark current ratio (NPDR)
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at applied bias voltage of -5V is obtained to be in the range (15.0-14.10) x 106 W-1 for
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incident power density of (28.2-41.4µWcm-2). These values are quite high compared to those
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of some recently reported UV detectors [26, 27]. These NPDR values are also listed in
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Table1 along with the other characteristic parameters. From the best of our knowledge the
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values are some of the highest reported NPDR values for any porous silicon based UV
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photodetectors. ON/OFF kinetic response (4(b)) ascertains the structure to exhibit fair
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photoresponse and also that the photocurrent increases with increasing wavelength in the UV
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range. The response times i.e., rise and decay times, as calculated from fig. 4(c), are obtained
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as 0.7 and 0.8 sec. respectively. However, these values are relatively large compared to some
136
of the earlier reported UV PDs [28, 29], i.e., the device is slower in the present case. Slow
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response is due to the presence of inhomogenity and high surface density at the metal-PS and
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PS-n-Si heterostructure. Nonetheless the values less than 1sec suggest possible application of
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the device in the field of optoelectronics for UV detection. Plot of power density (µWcm-2)
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vs. photocurrent (Fig 4(d)) can be fitted by simple power law in the form
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I p = AP α
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(4)
Where Ip , A and P represent photocurrent, proportionality constant, and intensity of irradiated
144
light respectively. The exponent α is a fitting parameter which depicts linearity or non-
145
linearity of the plot. From the plots value of α is found to be 0.96, which is approaching
146
unity, i.e., almost linear. One may infer that photocurrent increases linearly with increasing
147
UV wavelength. Fig.5 (a-d) shows similar photoconductivity characteristics for the visible
148
wavelength of 450 nm. From these figures it can be seen that the structure is also sensitive to
149
visible light with enhanced rectification in both forward and reverse direction proving
150
excellent photoresponse characteristics with NPDR of 2.14 X 107 W-1 for incident power
151
density of 259µWcm-2 at applied reverse bias of -5V. From the slope of the linear region (low
152
voltage region) of fig 5(a), the values of n are found to be 9.2-6.3 and 7.5-6.52 for both
153
forward and reverse directions respectively. This increase in n values for both UV and visible
154
irradiation suggests that the current transport mechanism in PS-MSM structure are dominated
155
by recombination and tunnelling of the charge carriers at the interface layer [30] i.e. Al:PS
156
and PS:n-Si junctions. Fig 5(b) describes the on/off kinetic response for light of different
157
intensity. From these curves it is evident that with increasing light intensity photocurrent
158
increases with an on/off ratio 2.95, 6.98, 7.5, 9.3 respectively. The response time (rise and
159
decay time) as calculated from fig.5(c) is found to be 1s, i.e., device is slower compared to
160
UV response. However for both UV and visible ranges the structure shows steep rise and
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decay when light is on and off periodically which also indicates that there is ample
162
contribution from the slow diffusion (electron diffusion) of the charge carriers while the
163
whole photoconduction process is dominated by fast drift (electron drift) [31]. Fig 5(d) shows
164
the plot of photocurrent vs. irradiated power density at a bias voltage of -5V. The plots show
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strong dependence of photocurrent on incident light power density with ‘α’ value of 1.47.
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Since ‘α’ is directly linked to trap level density of the semiconductor [32, 33] the value more
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heterostructure which is also supported by the n value showing deviation from the ideal diode
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value.
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Ageing effect on PS-MSM performance
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Since with ageing PS surface layer is known to get oxidised [34], it would surely affect its
172
structural, optical and electrical properties. If kept in ambient, the device degrades by 30% or
173
so in a month. However, we have stored the device in vacuum desiccators, which generally
174
have low vacuum ~10-1 Torr, where the efficiency can be retained to some extent up to 30
175
days or so, but there’s significant efficiency loss in few months. We have some results for the
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device stored for 6 months. Fig 6 (a-c) show plots of various parameters obtained from
177
photoconductivity study. Fig 6 (a) shows plots for Rλ and EQE vs λ for the device. These
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values are shown to decrease with a blue shift of the peak compared to that of the freshly
179
prepared device (fig 3). Here the maximum Rλ value of 0.25AW-1 and EQE of 82.1% are
180
observed at 390nm for the UV region and 0.31AW-1 with 90% EQE at 431nm for the visible
181
region respectively. Fig 6 (b) shows the reverse region I-V characteristics of the ageing
182
device for the UV-Visible wavelength range of 250-450nm. It is seen that device dark current
183
(Id) increases while photo current (Ip) decreases upon ageing compared to the fresh device
184
(Fig 4(a) and 5(a)). This decrease is mainly due to the formation of thin oxide layer between
185
Al:PS interface which decreases the tunnelling probability of photocarriers through the oxide
186
layer towards the contact Al electrode [35]. The UV to Visible NPDR of the device after
187
storage is calculated to be in the range (0.2-0.61) X 106 W-1 for incident power density (28.2-
188
259 µWcm-2) at applied voltage of -5V. Fig 6(c) shows on/off kinetics of the device at
189
applied bias voltage of -5V. From the curve on/off ratio is calculated to be in the range 1.05-
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1.15 for incident power density of 29.2 and 31.3µWcm-2 with a response time of 0.8s. All
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these parameters are listed in parentheses of table 1 along with the data for fresh device for a
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ready comparison.
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Conclusion
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Freshly prepared porous silicon MSM (PS MSM) structure shows good to excellent
196
photoconduction response for near UV to UV range of the spectrum. Some important
197
parameters, such as responsivity, external quantum efficiency (EQE), detectivity and
198
normalized photo to dark current ratio (NPDR) of the structure exceeds the value of some
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earlier reported Si based photodetectors. Also, the response time of the device is less than 1s
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for the UV range and 1s for the visible. These results indicate possible application of the PS
201
MSM structure as UV-Visible optical detector and switching device in optoelectronic
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application. However, on storing the device for 6 months in vacuum desiccator, due to slow
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oxidation of the PS surface the device performance degrades.
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Acknowledgement
205
Authors
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DST/TSG/PT/2009/96), University Grants Commission (Project grant no.-40-438/2011 SR)
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for providing instrumental facility, IASST, Guwahati for FESEM measurements, Dr M P C
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Kalita, Department of Physics, Gauhati University for PL measurement and UGC-BSR
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scheme for proving fellowship to Mr M. Das.
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Figure captions:
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Figure 1: (a) Schematic of the PS MSM structure (b) Schematic of experimental setup for
285
photocurrent measurement
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Figure 2: (a) Visible appearance of the PS sample under UV (b) Planar view of FESEM
287
image and (b) cross-sectional view of FESEM image of the PS sample
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Figure 3: plot of Rλ and EQE vs. λ of PS MSM structure, mechanism of MEG (inset)
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Figure 4: (a) I-V plots for different UV wavelengths (b) ON/OFF kinetic response for
290
wavelength range (200-390 nm), (c) rise and decay response for 350nm light (d) plot of
291
intensity dependent photocurrent at - 5V bias of PS MSM
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Figure 5: (a) I-V plots at different intensity of 450nm light (b) ON/OFF kinetic response for
293
different light intensity, (c) rise and decay response at power density of 259 µWcm-2 (d) plot
294
of intensity dependent photocurrent of PS MSM
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Figure 6: (a) plot of Rλ and EQE vs. λ at -2V (b) Reverse region I-V plot for incident power
296
density 28.2-259 µWcm-2 (c) ON/OFF kinetics at -5V of aged (oxidised) PS MSM structure
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ACCEPTED MANUSCRIPT Table 1: Responsivity (Rλ), Ideality factor (n), external quantum efficiency (EQE), specific detectivity (D*), NPDR and response time of PS MSM structure, data in parentheses are the values for aged sample (λ)
(nm)/intensity
D*
EQE (%)
Reverse
decay
Hz1/2W-1
(τ)
(σ)
0.7(0.8)
0.8(0.8)
1.42 (0.01)
708 (26)
6.24
300
3.54
3.7
1.46 (0.03)
606 (12)
6.4
350
4.20
3.8
1.42 [0.05]
502 (20)
6.18
390
4.9
4.3
1.34 (0.3)
426 (82)
5.8
450(116, 163, 188,
9.2, 6.9,
7.5, 8.4,
2.004 (0.1)
590 (25)
9.4
259µ Wcm-2)
6.7, 6.3
7.52,
15x106 (1.85x105)
15.3x106(1.31x105) 15x106(4.69x106)
14.1x106 (4.23x106)
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Response Time (Sec.) Rise
3.34
6.52
NPDR (W-1 )
1012 cm
250
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Porous silicon MSM device is fabricated from n-type Si The device shows good photoresponse in UV and visible range of incident radiation External quantum efficiency and detectivity of the device are quite high Much faster response to incident radiation
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