Ultrafast photoresponse and enhanced photoresponsivity of Indium Nitride based broad band photodetector

Solar Energy Materials and Solar Cells 172 (2017) 376–383

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Ultrafast photoresponse and enhanced photoresponsivity of Indium Nitride based broad band photodetector Shibin Krishnaa,c, Alka Sharmab,c, Neha Aggarwala,c, Sudhir Husaleb, Govind Guptaa,c, a b c

MARK

⁎

Advanced Materials & Devices, CSIR-National Physical Laboratory, Dr K S Krishnan Road, New Delhi 110012, India Quantum Phenomena and Applications, CSIR-National Physical Laboratory (CSIR-NPL), Dr. K.S. Krishnan Road, New Delhi 110012, India Academy of Scientific & Innovative Research (AcSIR), CSIR-NPL Campus, Dr. K.S. Krishnan Road, New Delhi 110012, India

A R T I C L E I N F O

A B S T R A C T

Keywords: InN PAMBE NIR-VIS Photodetectors Fast response time

InN direct band gap semiconductor is a promising material in the nitride family for high power and high frequency optoelectronic devices. However, the reports on photo-sensing ability of the material are limited with photoresponsivity < 1 A/W only. Here, we report fast photoresponse from high quality molecular beam epitaxy grown InN islands delivering photoresponsivity of 13.5 A/W, about 30 times higher than the recently reported InN based photodetector operating in the near infrared spectral range. The ultra-broadband response with high photoresponsivity from visible to near infrared spectral range is experimentally demonstrated. To the best of our knowledge, this study presents the working of a photodetector having fast response (38 μs) and high photo detectivity (5.5 × 1010 W-1 Hz1/2 cm) operating at room temperature. The device yields a quiet prompt saturation and decay under periodic illumination which demonstrate excellent stability and reliability of the device with switching time. The sub-linear dependent photocurrent on the bias voltage and incident power offers good tunability for multipurpose applications. This is the first report on ultra-broadband spectral range of InN based photodetectors that open up opportunities for developing the next generation high efficiency photodetectors.

1. Introduction The ability to detect light over a broad spectral range in ultrafast detection time is vital to numerous technological applications such as imaging, sensing and communication [1,2]. III-Nitride semiconductors have become potential materials for fabrication of Photodetectors (PDs) since they have the asset of covering a wide range of spectrum varying from 0.7 to 6.2 eV (InN to AlN) along with the ability to tune the cut-off frequency of photodetectors. Moreover, these semiconducting materials hold better essential parameters required for an efficient ultra-violet (UV) photodetector such as high thermal stability, high electron saturation velocity, small dielectric constant & high breakdown field compared to the existing Si based technology for photodetectors [3]. Among III- nitride direct band gap semiconductors, Indium nitride (InN) has attracted much attention due to its unique potential characteristics such as high electron mobility, relatively high absorption coefficient, narrow band gap energy and peak drift velocity at room temperature, etc [4,5]. Up to now, most research efforts on development of III-Nitride based photodetector has been focused on UV operation [6,7]. Though, the developments in telecommunication demand for the extension of the photodetection range towards the infrared (IR) ⁎

spectral region apart from UV detection. To date, nano-materials including metallic nano-particles [8] and colloidal quantum dots have been used in IR photodetection [9]. IR photodetection generally relies on small band gap semiconductor compounds such as HgCdTe, PbS etc [10]. However during earlier portion of 2000s, preliminary interest was on InGaAs, InGaP and InP materials. The performance of these devices was generally not promising and often requires operating temperature below liquid nitrogen levels. Moreover, these device performances were still lackluster, with responsivity regularly below 1 A/W [10]. In contrast to these materials, graphene is also believed to have great potential in broadband photodetection [11], due to its unique gapless electronic structure [12]. However, its atomic thickness induced limited optical absorption (~ 2.3%) of a monolayer of carbon atoms along with the ultrafast recombination of photo-generated carriers results in suffering the poor responsivity (photo-generated current per incident optical power) [13–16]. Konstantatos et. al. demonstrates integration of colloidal quantum dots in the light absorption layer to improve the responsivity of graphene photodetectors to 107 A/W [17]. Owing to an interesting nature of InN semiconductor, low effective mass of electron (m*0.06me) [18] enables it to be a reliable candidate for high-speed electronics and its high internal gain which can lead to

Corresponding author at: Advanced Materials & Devices, CSIR-National Physical Laboratory, Dr K S Krishnan Road, New Delhi 110012, India. E-mail address: [email protected] (G. Gupta).

http://dx.doi.org/10.1016/j.solmat.2017.08.017 Received 2 April 2017; Received in revised form 10 August 2017; Accepted 11 August 2017 Available online 30 August 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.

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pronounced diffraction peak from InN (10−11) plane and AlN (0002) plane at 32.85° and 36° respectively, suggesting that the sample crystallizes in the wurtzite hexagonal structure. The visible presence of sharp and narrow x-ray diffractions of InN in the 2 theta-omega scan illustrates the highly crystalline InN film grown epitaxially along (10−11) direction (s-plane) on Si (111) substrate. The physical explanation could be the high growth temperature (540 °C) leads to the formation of InN islands epitaxially growing along (10−11) orientation [25]. Moreover, 2theta-omega scan shows a tiny peak at 39° (not visible in the spectra) which shows the presence of metallic In on the film. Anyebe et al. [25] also proven that metallic In adlayers will be present along with InN when the film are grown at extremely high growth temperature. This is due to increased InN dissociation rate due to high growth temperature which leads to an In rich condition, thereby a preferential adsorption of In metal on the surface (Influence of metallic In on the photoresponse behavior has included in the Supplementary information, S1). The d-spacing in the growth direction (along (10−11) plane) for InN is calculated directly by using Bragg’s law,

high photoresponsivity. Chen et al. [19] have presented temperature sensitive photosensitivity and long carrier life time induced high photocurrent gain (~ 107) in the InN nanowires. Although, having the superior properties for photodetection [20], the reported InN based photodetectors are very few [21–23]. Winden et al. reported a vertical nano-pyramid InN photodetector with spectral responsivity of 0.2 A/W [21]. So far, the highest photoresponsivity reported on InN based photodetector is 0.44 A/W [22]. It is because of challenges in synthesizing high-quality InN due to its low dissociation energy and the lack of appropriate substrates [24]. Moreover, the growth of high quality epitaxial InN on GaN, sapphire and silicon leads to the generation of high dislocation density which further guide to higher leakage current at the interface [25,26]. The main challenge is to develop and assess photodetectors having high photoresponsivity and simultaneously possesses a large active area, high internal efficiency and fast response time. In the present work, we demonstrated InN based ultra-broadband visible- near infrared (VIS-NIR) photodetector fabricated on Si (111) substrate using plasma assisted – molecular beam epitaxy (PAMBE) delivering the enhanced photo responsivity of 13.5 A/W and 2 A/W in the NIR and visible region respectively. In addition, the investigated switching time reveals a sharp response time in the range of microsecond for NIR detection. Moreover, the photoresponse characteristics observed from the fabricated InN based PD are comparable with the potential graphene based broadband PDs. This is the first report on ultra-broadband (VIS-NIR) and high responsive photodetector based on InN semiconductor operating at room temperature. Therefore, the results open up new opportunities for developing the next generation high efficient photodetectors with broad spectral range.

2dhkl sin θhkl = n. λ

(1)

where, d is the lattice spacing, which is given by dhkl = 1/ √{4/3(h + k2 + l2)/a2+ l2/c2} for hexagonal symmetry system, θ is the measured angle of diffraction, hkl are the miller indices, λ is the wavelength of xray source (CuKα1= 1.5406 Ǻ) and ‘a’, ‘c’ are the lattice parameters of the grown InN. The value of d-spacing along InN (10−11) plane of diffraction was calculated to be 0.2725 nm. Further, HRXRD analysis has been employed to procure the information of crystalline quality as well as dislocation density of InN epitaxial layer. Inset of Fig. 1(a) shows the ω-scan in the symmetric plane of diffraction of InN film having full width half maximum (fwhm) of 373″. The fwhm value has been used to calculate threading dislocation (TD) density by using the following equations, 2

2. Materials and methods The InN photosensing film was grown by PAMBE system (Riber Compact 21) equipped with a radio frequency plasma source (Addon) for supplying active nitrogen species and effusion cells to evaporate Indium and Aluminum on the substrate. The Si (111) (Boron doped (p type), 5–15 Ω-cm, 325 ± 25 µm) has been used as substrate for the growth of InN film, which was chemically pre-cleaned by employing the standard RCA cleaning process and immediately loaded into MBE system to avoid re-oxidation followed by out-gassing in the buffer chamber at 600 °C. The nitrogen flow of 1.5 sccm and RF plasma power of 500 W was kept constant throughout the growth process. Initially, an AlN buffer layer was grown on an atomically clean Si (111) 7×7 reconstructed surface at 850 °C, followed by the epitaxial InN film which was grown at 540 °C with In beam equivalent flux of 2 × 10-7 Torr. The epitaxial growth of InN film was analyzed in-situ by Reflection High Energy Electron Diffraction (RHEED) using STAIB electron gun operating at 12 keV. The structural quality of InN film was examined by High Resolution X-Ray Diffraction (HRXRD, Panalytical X’Pert PRO MRD System) instrument. Field Emission Secondary Electron Microscopy (FESEM, ZEISS AURIGA) was employed to analyze the morphology of the grown InN film. Crystallinity and orientation of the as-grown InN film was further explored by using High Resolution Transmission Electron Microscopy (HRTEM, FEI, Tecnai F30 G2 STWIN). A schematic diagram of the fabricated device is illustrated in the result part. Two electrodes of Ag metal contact were deposited on the InN film to collect the photo-generated current. Optoelectrical measurements were carried out using probe station setup (Cascade Microtech EPS150TRIAX) which has shield enclosure (EPS-ACC-SE750) for low signal measurements. The spectrometer is equipped with halogen source (power density, Pd = 22 mW/cm2) for VIS detection and focused laser (λ = 1064 nm, Pd = 6–29 mW/cm2) for NIR detection.

TD =

β2 9b2

(2)

where β is the fwhm value measured for (10−11) by HRXRD ω-scan and b is the Burgers vector length. The direction of burger vector is along 1/6 < 20–23 > [27] where the magnitude of b taken here for TD calculation is aInN = 3.533 Å. The calculated TD (screw) density was found to be 2.8 × 108 cm-2 which is one order magnitude lower than the conventional InN epilayer. Recently, Wang et al. [28] reported Npolar InN with significantly reduced TD density (screw TD of 6 × 108 cm-2) grown on sapphire substrate. The detailed structural characterization of the InN islands grown on Si (111) was performed by HRTEM. The cross sectional TEM image (Fig. 1(b)) shows the confirmation of the InN islands grown on AlN buffer layer on Si (111) substrate. The thickness of the AlN buffer layer was found to be 70 nm whereas InN islands are between 150 and 200 nm. The lattice spacing of 0.27 nm (inset Fig. 1(b)) corresponds to d-spacing of InN (10−11) crystal plane indicating the growth of crystalline islands along s-plane direction. The observed d-spacing value from HRTEM image is in good accordance with the calculated d-spacing value from HRXRD by Bragg’s law. Furthermore, morphological analysis by FESEM reveals that InN growth on Si (111) surface at 540 °C possesses a continuous surface morphology in large scale with uniformly distributed InN island like structure on the surface (Fig. 1(c)). The growth kinetic plays a dominant role in the observed island formation where surface energy anisotropy has been considered as the main reason to form nano sized structures. It can be explained using Wulff construction [29] where the surface Gibbs free energy is minimized by assuming a shape of low surface energy. Therefore, a nanostructure/island like structure growth will be preferable at high temperature. Anyebe et al. [25] studied the evolution of InN nanorods to microstructures on Si (111) by MBE where they have observed the similar structure of InN islands grown epitaxially along (10−11) direction at varied substrate temperature of

3. Results and discussion The structural quality of the grown InN islands was inspected using HRXRD measurement (Fig. 1(a)). The HRXRD result shows a 377

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b

Intensity (a.u)

Intensity (a.u)

a

InN (10-11)

Si (111)

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AlN (0002)

15

26

28

30

32

34

36

16

17

18

Omega (degree)

38

40

42

2 theta- Omega (degree)

d

c

Fig. 1 Structural Analysis of PAMBE grown InN.. a, HRXRD 2θ-ω scan of InN/AlN heterostructure grown on Si (111) substrate, Inset shows the symmetric ω-scan of InN film b, The cross sectional TEM image of the InN island growth on AlN buffered Si (111) substrate c, Top-view FESEM image of the grown InN heterostructure, d, EDX spectra of the spherical shaped InN structures (marked with red colour square, Fig. 1c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

of the photodetector at different voltage bias and zero gate voltage with illumination on the whole device by halogen source for VIS detection. Fig. 3(a) shows the rise in photocurrent upon turning on the light and its decay after turning off the incident light under different applied bias (10–200 mV). The device yields a quiet prompt saturation and decay under periodic illumination which demonstrates good stability and reliability with switching time (supplementary information, S4). The dependence of photocurrent with bias voltage has been examined where maximum value of photocurrent is found to be ~ 78 µA for 200 mV bias and observed a sub-linear increase in photocurrent with applied bias (Supplementary information, S5). The higher bias voltage leads to increase in drift velocity and reduce the transit time of the carrier. Therefore, the recombination of photo-generated carriers reduced efficiently, thus resulting in easier carrier separation and rise in photocurrent. The highest value of photoresponsivity in the VIS range is calculated to be 2.0 A/W (λ = 400–700 nm, P = 22 mW/cm2). The reported value is competitive to the recently published pure graphene monolayer based PD which shows broadband photoresponse from visible to mid IR range and the photoresponsivity of 1.25 A/W (λ = 532 nm) in the VIS range [31]. Moreover, theoretically calculated photoresponsivity [32] value of the device was found to be 0.322–0.560 A/W(for VIS range, 400–700 nm). The high value of experimental responsivity (2.0 A/W) in the present study could be due to the application of applied bias/illumination power which has not been included in theoretical limit calculations. For better understanding, we have re-plotted the bias voltage vs responsivity graph of VIS PD where the spectra had been linearly extrapolated to 0 V bias (Supplementary information, S6). The observed responsivity value (0.2032 A/W) is below the limit of theoretical value (0.322–0.560 A/W) as we have not considered the bias voltage. Katz et al. [33] reported that the

520–560 °C. Fig. 1(d) shows the Energy Dispersive X-Ray (EDX) spectra of the spherical shaped InN structures (marked with red colour square, Fig. 1(c)). The atomic percentage of Indium, Nitrogen, Aluminum and Silicon are found to be 65%, 23%, 6% and 6% respectively which shows the presence of InN film grown on the silicon substrate with AlN buffer layer. The EDX map of the entire FESEM image shows more information about the underlying film and substrate with atomic percentage of 6%, 39%, 21% and 34% for Indium, Nitrogen, Aluminum and Silicon, respectively (Supplementary information, S2). A schematic diagram of the InN photodetector is represented in Fig. 2(a). The device consists of PAMBE grown InN islands on continuous InN film grown on Si (111) substrate with AlN (70 nm) buffer layer and two Ag contact electrodes. Fig. 2(b) shows the current-voltage (I-V) characteristics of InN photodetector without as well as with illumination having optical power densities between 6 and 29 mW/cm2 at 1064 nm. The value of current shows a linear increase with bias voltage and higher current value is observed for high power density (29 mW/ cm2). The linear and symmetric behavior in both forward and reverse bias indicates the Ohmic like contact which ensures the intrinsic photoresponse gain from the InN film instead of the interface between InN and the electrodes. The presence of high dark current could be due to the existence of strong surface charge accumulation layer on InN [30]. Inset of Fig. 2(b) shows the schematic cross sectional view of the grown structure together with electrical connections used to characterize the device. One of the silver electrode act as drain while the other is grounded (source electrode). The optical microscopic image of the measured photodetector is shown in the Supplementary information, S3, where the illumination area (A) is calculated to be 294 × 592 µm2. Fig. 3 summarizes the VIS photoresponse of the fabricated InN photodetection device. We measured the photoresponse characteristics 378

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b

a LASER Beam

InN film

InN Islands

Fig. 2. InN photodetector layout. a, Schematic diagram of the InN photodetector under a focused laser beam b, I-V characteristics of InN photodetector illuminated for optical power densities between 6 and 29 mW/cm2 at 1064 nm. Increasing illumination level results in an enhanced current due to electron-hole pair generation. Inset shows the schematic crosssectional view of the fabricated device.

To gain further insight into the characteristics of this photodetector, the response time constants (rise time constant, τr and decay time constant, τd ) is a key figure of merit and is also relevant in revealing the physical mechanism of the device operation. The response time is

photoresponse increases with bias voltage due to lowering of barrier height which is responsible for the gain. Hence, the bias voltage can also play a dominant role for high photoresponsivity value of the device.

Fig. 3. VIS photoresponse of the InN photodetector. a, Time dependent photocurrent measurement by illuminating halogen source under various bias voltages (10–200 mV). b, A single cycle fitted curve for estimating the response time and decay time constants in the VIS region. c, The plot of wavelength dependent photocurrent measured at different illumination power density. d, Time dependent photocurrent measurement at varied applied bias by illuminating with a focused laser source having emission wavelength of 532 nm.

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calculated using the fitting equations, for τr , I = Io (1−e τr ) and for τd ,

comparative studies uncovering the state of art of NIR photodetectors are summarized in Table 1. The photo-responsivity, 13.5 A/W (illumination power density of 6 mW/cm2) of our device shows obvious advantage over other reported NIR photodetectors and can further enhance the device performance by additional photo-absorption boosting such as surface plasmon [4], metal nano-particle decoration etc [36]. Further, we have theoretically calculated the responsivity [32] value of the device and it was found to be 1.5 A/W. In the present study, the high value of experimental responsivity (13.5 A/W) is because of the high internal gain in the device which has not been considered in the theoretical calculations. After carefully considering the current gain in the fabricated device by using equation [37], G = EI ph/eɳP (Iph is photocurrent, e is the electron charge and E is the photon energy, P is the absorbed optical power and ɳ is the efficiency of photon absorption to create electron-hole pairs), it was found that high internal gain is present in the device. Chen et al. [20] reported that high internal gain (which could be high as 107) persist in InN based photodetector which is higher than those in different types of high-efficiency photodetectors. This high internal gain leads to high photoresponsivity value of the InN based photodetectors. Fig. 5(a) demonstrates that photocurrent of the device increases gradually with increasing light intensity. The dependence of the photocurrent on illumination power can be expressed by power law, Iph α Pθ, where Iph is the photocurrent, P is the power of illumination, θ explains the response of photocurrent to power density [38]. By fitting this equation, the value of θ is calculated to be 0.78. The non-unity exponential are the results of complex processes of electron-hole generation, trapping and recombination etc [39]. while the value of exponent close to 1 reveals that the number of trapping states are less in energy gap between Fermi level and conduction band [40]. In the present study, the lower value of θ reveals presence of some trap/defect states in mid-gap of the InN semiconductor. Moreover, the sub-linear dependence of photocurrent on light intensity at higher illumination power can be explained by reduced number of available hole-traps present at the surface which is leading to the saturation of photoresponse. A detailed study on the high internal gain by hole-trapping effect has been reported by Soci et al. [41]. Fig. 4(b) plots the power density as a function of responsivity, clearly revealing remarkable responsivity at low excitation power under 200 mV bias. The device exhibits a photoresponsivity of 13.5 A/W for an illumination power density of 6 mW/cm2 and shows a monotonous decrease with increasing power density. Moreover, the responsivity does not show any sign of saturation and a higher photoresponse is expected at lower excitation power and higher applied bias [9]. The plots unveiling relationships of response time constants with power density and bias voltage are represented in Fig. 4(c) and (d) respectively. The

−t Io (e τd )

I= where Io is the maximum current value at a particular time t. We performed the fitting of rising and falling curves of the photoresponse spectra (Fig. 3(b)) taken at an applied bias of 200 mV and calculated the rise time constant of 352 ms and decay time constant of 188 ms. Till date, this is the first report on faster response times in VISInN photodetector. The τr and τd reported in this study are comparable to corresponding values observed in graphene based photodetectors [34]. Additionally, an attention should be given on back contact gate voltage and high bias which can further enhance the device performance [35]. In order to prove the different wavelength absorption in the visible spectra region, the fabricated InN photodetector has been illuminated by using halogen source with various filters: 400 nm, 480 nm, 550 nm, 600 nm, 660 nm and 700 nm. As each optical filter have different illumination power density, wavelength vs responsivity curve will not give the comparable information. So, the time dependent photocurrent for different wavelengths are plotted together as shown in Fig. 3(c) which manifests a strong dependency of illumination power with photocurrent. Further, a continuous laser source of emission wavelength 532 nm has been used to illuminate the fabricated device to confirm the visible detection. Fig. 3(d) shows the time correlated photocurrent measurement at various applied bias (10–200 mV) upon illuminating with a visible range laser source (λ = 532 nm). The device yields a quiet prompt saturation and decay under periodic illumination which demonstrates good detectivity under visible spectral range. We next turned our interest to the photoresponse of the InN photodetector in NIR region. Fig. 4(a) shows the time dependent photoresponse using a focused laser beam (λ = 1064 nm) under on-off light modulation for different power densities (6–29 mW/cm2). The device shows a maximum photocurrent at high power density (29 mW/cm2) and as the power density is increased from 6 mW/cm2 to 29 mW/cm2, the photocurrent demonstrate a linear dependence on it (Fig. 4(a)). The time-dependent photocurrent measurement over a 10-periods on–off operation under NIR light (1064 nm) illumination manifests the good reliability and stability of the photodetector (Supplementary information, S4). Fig. 4(b) illustrates the fitted rise and falling curves of the photodetector at an incident power of 29 mW/cm2 (λ = 1064 nm, VDS = 200 mV). A sharp response has been ascertained with very fast switching time (performed at 110 µs step size) from which we evaluated the rise and decay time constants to be 38 µs and 1 ms respectively (200 µs, 400 µs and 50 ms step size measurements were included in the Supplementary information, S7 where the fast detection has been confirmed). Inset of Fig. 4(b) shows the magnified plot of rise time fitted curve where the ultra-fast detection in microseconds is clearly visible. The NIR operation speed of PD device is much faster than the earlier reported values of NIR photodetector [34]. The detailed

Fig. 4. NIR photoresponse of the InN photodetector. a, Time dependent photocurrent measurement by illuminating focused laser source having different power density (6–29 mW/ cm2). b, A single cycle fitted curve for estimating the rise time and decay time constants in the NIR region. Inset show the magnified plot of rise time fitted curve.

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Table 1 Summary of the device performance parameters of present NIR photodetectors. Device structure

Responsivity (A/W)

Rise/decay time constants

Detectivity (W-1 Hz1/2 cm)

Ref

InN/AlN/Si (111) InN/GaN/Al2O3 InNnano-pyramid/GaN/Al2O3 Graphene/Si Graphene/ Ge wafer

13.5 0.443 0.2 0.435 0.05

38 µs/1 ms – – 1.2 ms/3 ms 23 µs/108 µs

5.5 × 1010 – – ~ 108 ~ 1010

Present work [22] [21] [34] [43]

diagrams shown in Fig. 6. The direct band gap of InN semiconductor is 0.7 eV and the work function of Ag metal contact is 4.7 eV. On the basis of these parameters, the energy band diagram of the Ag-InN (metalsemiconductor) junction is depicted in Fig. 6(a). The presence of electrons in the conduction band could be the existence of strong surface charge accumulation layer on InN [30] which leads to high dark current in the photodetectors. Under the illumination of laser light (λ = 1064 nm), electrons from highly dense valance band (EV) is excited to conduction band (EC) as shown in Fig. 6(b). As the lifetime of these excitons is very less, an external bias voltage (Vds) is applied to block the recombination of generated photo carriers by drifting them to the corresponding electrodes. The photocurrent increases with applied voltage due to the increase in carrier drift velocity and related reduction of the carrier transit time [34]. In order to clearly see performance of the fabricated device with existing counterparts, the key parameters

dependence of time constants of the device on power density is studied by applying a constant bias (200 mV). The τr of 9 ms is observed at 6 mW/cm2 power density whereas a significant reduction in rise time constant (38 µs) has been witnessed at 29 mW/cm2. The reduction in time constants with higher power density has been attributed to the generation of large number density photons which could lead to faster response time. The dependence of applied bias on rise/fall time reveals that large external voltage can block the recombination of photo-generated carriers more efficiently which results in easier carrier separation and faster response time. Moreover, at higher external voltage, the increased drift velocity and reduced transit time of the carrier (Tt = l2/ µVds, where l is the device length, µ is the carrier mobility and Vds is the bias voltage) leads to very fast rise/fall time. The carrier generation mechanism of the InN based metal-semiconductor-metal photodetector is explained through schematic band

Fig. 5. Dependence of photocurrent and rise/fall time constants on power density and bias voltage. a, The fitting of the relationship between photocurrent and power density by power law. b, Photoresponsivity of the InN photodetector in NIR region with power density. Rise and decay time constants dependence on c, power density and d, bias voltage.

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a

b

Vacuum level

hγ

VD EF

W (4.7 eV)

EC

EC

(0.7 eV)

EF

EV Ag

InN

EV Ag

InN

Fig. 6. Energy band diagram of a photodetector. a, initial band diagram. b, band diagram after excitation by a laser having wavelength of 1064 nm.

improvements of photodetector performance with respect to sensitivity, noise, speed, and especially tunability are necessary.

which are used to quantify the performance of the device like responsivity (R), detectivity (D), external quantum efficiency (EQE) and noise equivalent power (NEP) are calculated and compared. For the present NIR (VIS) photodetector, the R, D, EQE and NEP were calculated to be 13.5 (2) A/W, 5.5 × 1010 (8.1 × 109) W-1 Hz1/2 cm, 15.75 (5.84 for 600 nm wavelength emission) and 6.8 × 10-13(4.6 × 10-12) W Hz-1/ 2 respectively (the calculations for these parameters are described in the Supplementary information, S8). Table 1 summarizes the performance parameters of the developed device along with the comparison from some recently reported NIR photodetectors. As compared with low dimensional (monolayer, nano-wire, nano-tubes, etc.) material devices, the present photodetector shows an excellent responsivity and quick response time. This work demonstrates an easy and simple way to develop high performance photo-sensing devices for optoelectronic applications. In the present InN based VIS-NIR photodetector fabrication, all the measurements were performed at lower bias voltage (10–200 mV) for effectively eliminating the carrier generation (high dark current) occurred by applied bias. Even at very low bias voltage (10 mV), the device shows very good performance with high photoresponsivity of 0.4 A/W, which confirms the improvement in the efficiency of InN based photodetector. Previous reported InN based photodetectors are operated at high bias voltage (1 V) as compared to the present study (200 mV) and the observed photoresponsivity in mA/W range [42]. Recently reported ultrasensitive photodetector based on monolayer MoS2 [34] shows a maximum photoresponsivity of 880 A/W operated at bias voltage of 8 V and 70 V gate voltages. Hence, the InN photodetector fabricated in this study (photoresponsivity 30 times higher than the recently reported value [22]) without gate voltage and minimum bias voltage (in mV) shows an improved efficiency and encouraging results. Further enhancement could be realized by applying high bias voltage and gate voltage.

Competing financial interests The authors declare no competing financial interests. Acknowledgment This work is financially supported by CSIR-12th FYP network project PSC-0109 and NWP-55. The authors would like to thank Dr. K. K. Maurya for performing HRXRD experiments, Mr. Dinesh Singh for executing TEM measurements and Mrs. Mandeep Kaur for carrying out FE-SEM & EDX measurements. One of the authors (Shibin Krishna) acknowledges Department of Science & Technology (DST), Govt. of India and M/s Simco Global Technology & Systems Ltd. for financial support under the prestigious Prime Minister Doctoral Fellowship award. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2017.08.017. References [1] A. Rogalski, Infrared detectors: status and trends, Prog. Quant. Electron 27 (2003) 59–210. [2] J. Clark, G. Lanzani, Organic photonics for communications, Nat. Photonics 4 (2010) 438–446. [3] L. Liu, J.H. Edgar, Substrate for gallium nitride epitaxy, Mater. Sci. Eng. R 37 (2002) 61–127. [4] S.K. O’Leary, B.E. Foutz, M.S. Shur, L.F. Eastman, Steady-state and transient electron transport within bulk wurtzite indium nitride: an updated semi-classical threevalley Monte Carlo simulation analysis, Appl. Phys. Lett. 87 (222103) (2005) 1–3. [5] V.Y. Davydov, A.A. Klochikhin, V.V. Emtsev, S.V. Ivanov, V.V. Vekshin, F. Bechstedt, J. Furthmuller, H. Harima, A.V. Mudryi, A. Hashimoto, A. Yamamoto, J. Aderhold, J. Graul, E.E. Haller, Bandgap of InN and In rich InxGa1−xN alloys (0.36 < x < 1), Phys. Status Solidi B 230 (2002) R4–R6. [6] L. Sang, M. Liao, M.A. Sumiya, Comprehensive review of semiconductor ultraviolet photodetectors: from thin film to one-dimensional nanostructures, Sensors 13 (2013) 10482–10518. [7] J. Lia, Z.Y. Fan, R. Dahal, M.L. Nakarmi, J.Y. Lin, H.X. Jiang, 200 nm deep ultraviolet photodetectors based on AlN, Appl. Phys. Lett. 89 (213510) (2006) 1–3. [8] L.B. Luo, L.H. Zeng, C. Xie, Y.Q. Yu, F.X. Liang, C.Y. Wu, L. Wang, J.G. Hu, Light trapping and surface Plasmon enhanced high-performance NIR photodetector, Sci. Rep. 4 (3914) (2014) 1–8. [9] E. Lhuillier, M. Scarafagio, P. Hease, B. Nadal, H. Aubin, X. Xu, N. Lequeux, G. Patriarche, S. Ithurria, B. Dubertret, Infrared photo-detection based on colloidal quantum-dot films with high mobility and optical absorption up to the THz, Nano Lett. 16 (2) (2016) 1282–1286. [10] C. Downs, T.E. Vandervelde, Progress in infrared photodetectors since 2000, Sensors 13 (2013) 5054–5098. [11] C.H. Liu, Y.C. Chang, T.B. Norris, Z. Zhong, Graphene photodetectors with ultrabroadband and high responsivity at room temperature, Nat. Nanotechnol. 9 (2014) 273–278. [12] A.N. Grigorenko, M. Polini, K.S. Novoselov, Graphene plasmonics, Nat. Photonics 6 (2012) 749–758.

4. Summary An ultra-sensitive photodetector based on InN direct band gap semiconductor is demonstrated and its photo-sensing characteristics have been investigated. Ultra-broadband spectral response with high photoresponsivity from visible to near infrared spectral range is experimentally probed and a photoresponsivity of 13.5 A/W is observed in the NIR spectral region. More importantly, this is the best reported NIR photoresponsivity for a thin film photodetection device. The fabricated device shows very fast response time in microseconds which was not yet been reported in NIR-InN based photodetectors. Thus, we provide a simple NIR photodetector device architecture based on InN semiconductor which is a promising candidate for future high frequency optoelectronics. The potential contribution from bias and gate voltage in generation and separation of carrier would be definitely an interesting topic to be investigated upon. Beyond that, further 382

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