Giant UV Photoresponse of GaN-based photodetectors by surface

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Giant UV Photoresponse of GaN-based photodetectors by surface modification using Phenol functionalized Porphyrin organic molecules Manjari Garg, Bhera Ram Tak, V. Ramgopal Rao, and Rajendra Singh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20694 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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ACS Applied Materials & Interfaces

Type of Manuscript: Article Manuscript Title:

Giant UV Photoresponse of GaN-based photodetectors by surface modification using Phenol functionalized Porphyrin organic molecules

Corresponding Author: Manjari Garg Department of Physics, Indian Institute of Technology Delhi, Huaz Khas, New Delhi-110016, India Email: [email protected] Authors: Manjari Garg Department of Physics, Indian Institute of Technology Delhi, Huaz Khas, New Delhi-110016, India Email: [email protected] Bhera Ram Tak Department of Physics, Indian Institute of Technology Delhi, Huaz Khas, New Delhi-110016, India Email: [email protected] V. Ramgopal Rao Department of Electrical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110019, India Email: [email protected] Rajendra Singh Department of Physics, Indian Institute of Technology Delhi, Huaz Khas, New Delhi-110016, India Email: [email protected] ABSTRACT Organic molecular monolayers (MoLs) have been used for improving the performance of various electronic device structures. In this work, the concept of organic molecular surface modification is applied for improving the performance of GaN-based Metal-SemiconductorMetal (MSM) Ultraviolet (UV) Photodetectors (PDs). Organic molecules of phenol1 ACS Paragon Plus Environment

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functionalized-metallated-Porphyrin (Zn-TPPOH) were adsorbed on GaN and Ni/ZnTPPOH/GaN/Zn-TPPOH/Ni PD structures were fabricated. This process was beneficial in two ways, firstly the reverse bias dark current was reduced by ~1000 times and secondly, the photocurrent was enhanced by ~100 times, in comparison to the dark and photo current values obtained for Ni/GaN/Ni MSM PDs, at high voltages of 10V. The Responsivity of the devices was increased from 0.22 kA/W to 4.14 kA/W at 5 µW/cm2 optical power density at -10 V bias and at other voltages also. In addition to this, other PD parameters such as Photo-to-dark current ratio and UV-to-Visible rejection ratio were also enhanced. The spectral selectivity of the PDs was improved, which means that the molecularly modified devices became more responsive to UV spectral region and lesser responsive to visible spectral region, in comparison to bare-GaN based

devices.

Photoluminescence

measurements,

Power-dependent

photocurrent

characteristics and Time-resolved photocurrent measurements revealed that the MoL was passivating the defect-related states on GaN. In addition, Kelvin Probe Force Microscopy showed that the MoL was also playing with the surface charge (due to surface states) on GaN, leading to increased Schottky barrier height in dark conditions. Resultant to both these phenomena, the reverse bias dark current was reduced for metal/MoL/GaN/MoL/metal PD structures. Further, the unusual photoconductive gain in the molecularly modified devices has been attributed to Schottky-barrier lowering for UV-illuminated conditions, leading to enhanced photocurrent. KEYWORDS: MSM UV PDs, Photoluminescence, KPFM, Responsivity, Self-trapped holes I.

INTRODUCTION Ultraviolet (UV) Photodetectors (PDs) are becoming a part of the next-generation opto-

electronic device technologies owing to their successful usage in advanced space communications, flame detection, biological process sensing, air purification, ozone sensing

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and leak detection.1 Gallium Nitride (GaN) wide-bandgap semiconductor material remains one of the most promising candidates to contribute significantly to this endeavour.2,3 This is because GaN not only offers high spectral selectivity, high sensitivity and efficiency, due to its direct and wide bandgap, but also works without degradation in harsh conditions, such as high temperature, high power, or space environments.4 To date, various architectures of GaN-based PDs have been reported such as p-n junction diodes, p-i-n diodes, Schottky barrier diodes and metal-semiconductor-metal

(MSM)

structures.5-10

Among

these,

MSM

structured

photodetectors have attracted much attention owing to their fabrication simplicity, low intrinsic capacitance, high operation speed and low noise. The MSM PD geometry is basically two Schottky diodes (rectifying metal-semiconductor (MS) contacts) connected back to back.11 Different high work function Schottky metals such as Ni12,13 and Pt14 have been used in the form of interdigitated electrode (IDE) geometry to collect maximum photo-generated carriers. In order to increase the working area of the device, usage of transparent electrodes such as Titanium Tungsten (TiW),15 Tungsten (W) 15 and Graphene16 have been demonstrated. However, despite the benefits, the photodetection by these MSM UV PDs is not much efficient. Either the reverse bias dark current is not low (𝐼𝑑~10 ―6 ― 10 ―7𝐴),13-15 or the photocurrent is not much enhanced (as compared to dark current)12 or the photocurrent does not follow the power law (𝐼𝑝ℎ ∝ 𝑃0.62)16. Such observations have been attributed to certain imperfections in GaN such as dislocations and point defects. These limitations cause Schottky barrier inhomogeneity,17,18 allow tunneling of carriers during reverse bias conditions (in dark)13-15,17,18 and trap or recombine the photo-generated carriers12,16. In addition to this, lowering of Schottky barrier height, due to surface electronic states on GaN,19-25 is also responsible for allowing high reverse bias current to flow in dark conditions,26,27 thereby degrading the performance of the UV detector. Research has sought to alleviate these adverse effects by insertion of various oxide layers such as Ta2O5, SiO2, ZrO2, Ga2O3 and HfO2 at the 3 ACS Paragon Plus Environment


MS interface.28-32 These metal-insulator-semiconductor UV PDs have shown reduced dark current which is beneficial, to some extent. But there are a few constrains. These reports show that the thickness of these dielectric layers vary from 10-130 nm, which may be a hindrance for fabricating small sized devices (which will require thinner passivating layer). Besides, not much enhancement in the photo-current and therefore Responsivity of the PDs, has been observed. As a solution to these issues, different processes such as acidic surface treatment of GaN33 or thermal annealing of Ni Schottky diodes on n-GaN (at different temperatures in the range of 100-700℃)34 have been demonstrated in the literature. Fabrication of vertical GaN Schottky barrier diodes on free-standing GaN substrates has also been shown.35 All these strategies have led to enhancement in the barrier height, as calculated using both currentvoltage and capacitance-voltage measurements, and reduction in reverse bias leakage current in dark conditions. While searching for more plausible solutions, we came across a fascinating concept of surface modification by adsorption of organic molecules. These organic molecules come under the category of molecular semiconductors36 (which are different from inorganic semiconductors) and exhibit HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels. The organic molecules have a specific structure consisting of three parts, the head group that binds to the substrate, the tail group that is away from the substrate and the backbone that connects them.37,38 Experimental and theoretical investigations have shown that such approach reconstructs the surface (on to which the molecules have bonded) and affects the surface electrical properties without actual transfer of electrons between the molecule and the substrate. When the molecules are adsorbed on to the surface of the semiconductor, they have been reported to tune the surface band-bending (surface potential generated due to surface charge) of the semiconductor. For example, adsorption of single layer of differently functionalized quinolinium-derived chromophores;39 4 ACS Paragon Plus Environment

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and 4,4’-tribenzenedithiol, 1,8-octanedithiol and 1,16-hexadecanedithiol40 have been reported to tune the band-bending of Si and GaAs semiconductors, respectively. Insertion of organic molecules at the metal-semiconductor interfaces of Schottky contacts on some semiconductors has also been shown to modify the Schottky barrier height. For example, barrier height of Au diodes on GaAs41 and SiO2/Si42 have been tuned by insertion of differently functionalized (CF3, -CN, -CH3 and -OCH3) tartaric acid organic molecules. In one of our previous reports, we have applied this novel concept of organic molecular surface modification for improving the electrical characteristics of different Schottky diodes on n-GaN.43 We have reported a significant decrease in the reverse bias leakage current by ~103-104 times and increase in the Schottky barrier height. In this work, we have applied this concept to improve the performance of GaN-based MSM UV PDs. ʻMetal ― Organic molecular layer ― Semiconductor (GaN) ― Organic molecular layer ― Metalʼ PD structures have shown lower reverse bias dark current by ~103 times, increased photocurrent by ~100 times, increased photo-to-dark current ratio by ~105 times, enhanced Responsivity by ~19 times, improved spectral selectivity, increased UV-to-Visible rejection ratio by ~100 times and faster slow-response rise and decay speed as compared to ʻMetal ― Semiconductor (GaN) ― Metalʼ PD structures. The working principle of organic molecular surface alteration has been explained using energy band diagram. II. EXPERIMENTAL SECTION Unintentionally doped GaN sample used for this work was grown on 2″ diameter cplane sapphire substrate using Metal Organic Vapour Phase Epitaxy (MOVPE) technique. The thickness of the GaN layer was approximately 4 µm. The Hall electron mobility and carrier concentration of the epilayer were measured as 315 cm2/Vs and 1.3 × 1017 cm-3, respectively, at room temperature (RT) using Ecopia Hall effect measurement system (HMS 5000) at 0.57



Tesla magnetic field. The wafer was diced into small pieces having dimension of 5 mm × 5 mm. The sample pieces were cleaned using the standard cleaning procedure.37 HydroxylPhenyl-Zinc-Tetra-Phenyl-Porphyrin (Zn-TPPOH) organic molecules, whose chemical structure has been illustrated in Figure 1(a), were adsorbed onto the surface of cleaned GaN sample via the solution phase. In this process, as the cleaned GaN sample was placed in the pre-processed molecular solution,43-45 for an optimized time43 in air tight conditions and clean room environment, the head group of the molecules were bonded with the surface of GaN, as illustrated in Figure 1(a). This was followed by slow organization of the backbone and the tail end. This way the organic molecules formed an infinite 2-dimensional periodic array on the surface of the whole sample, leading to the formation of a self-terminating single layer of hydrocarbon chains.37 The thickness of the molecular layer was estimated to be ~ 8-9 Å using different characterization techniques such as Spectroscopic ellipsometry and Cross-sectional Transmission Electron Microscopy, as described in our previous works.43,46 Thereafter, the samples were gently cleaned using iso-propanol and rinsed-off using DI-water. The samples were dried, gently, using dry nitrogen jet. Further, to remove the moisture, the samples were placed in hot air oven at 120° C for 10-15 minutes. Schottky contacts were deposited on the bare cleaned GaN samples and molecularcoated GaN samples in the form of two inter digitated electrode (IDE) structures (MetalSemiconductor-Metal, MSM geometry) by using standard photolithography (by Intelligent Micro Patterning, SF-100 Xpress), thermal evaporation (by Advanced Process Technology) and lift-off process. Shipley 1828 positive photoresist was spin coated over the samples and baked. The samples were exposed to UV light of wavelength 423 nm for 4 sec and developed using MF-319 developer for 1 min to form the IDE structures. After post-baking, the patterned samples were loaded in thermal evaporation system at a base pressure of 10-6 Torr. Schottky metal contact Nickel (Ni)/Gold (Au) (20 nm/ 50 nm) were deposited at deposition rate of about 6 ACS Paragon Plus Environment

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1Å/s. Metal film was finally lifted-off by dipping the samples in hot acetone (100° C) for 5-10 min leaving the Schottky IDE on the sample surface. The fingers of the interdigitated contact electrodes were 50 µm wide and 800 µm long with spacing of 50 µm, as shown in the form of optical image (of the actual device) and schematic illustration mentioning the device dimensions in Figure 1(b).

Figure 1. (a) Schematic illustration of Schottky metal/Zn-TPPOH/GaN structure showing the cross-sectional view and chemical structure of Zn-TPPOH organic molecule bonded to GaN (b) Optical image and schematic representation of MSM IDE structure fabricated for UV PD, mentioning the device dimensions The Photoluminescence (PL) measurements and Kelvin Probe Force Microscopy (KPFM) were performed on the non-contacted samples using PL: Horiba: LabRAM HR Evolution instrument and Dimension Icon microscope (Bruker Corporation, Billerica, MA, USA), respectively. The current-voltage (I-V) and current-time (I-t) characteristics of IDE structures on bare GaN samples and molecular layer coated GaN samples were measured using Keithley Semiconductor Characterization System (SCS-4200) and EverBeing DC Probe station (EB 6). The UV light was produced with 75-W Xenon arc lamp and transmitted through 7 ACS Paragon Plus Environment


bandpass filters (monochromator: Bentham TMC 300) and an optical fibre to attain spectral selectivity. The power to the light source was controlled by current stabled power supply from Bentham Instruments Ltd (Model No. 306) and was measured using a Silicon detector and a power meter by Thorlabs (model S130VC). III. RESULTS AND DISCUSSION Photolumiscence (PL) measurements and Kelvin Probe Force Microscopy (KPFM) were performed on both bare GaN samples and molecular layer (MoL)-coated GaN samples before metallization. The PL spectra obtained from both the samples have been compared in Figure 2(a). It can be seen that the PL intensity for MoL/GaN samples is higher as compared to the intensity obtained from bare GaN samples. Specifically, a 6-fold enhancement in the band-edge emission intensity was observed. Such enhancement in the PL intensity is indicative of decrease in surface recombination velocity (SRV)28,47 due to the molecular surface modification. In addition to the band-edge emissions, Figure 2(a) shows the emergence of few extra PL peaks at ~ 370 nm wavelength for both bare GaN samples and MoL/GaN samples. The specific energies of two prominent peaks lie in the range of 3.35-3.36 eV and 3.34 eV which correspond to 𝑌4 and 𝑌5 lines in GaN, respectively. These unusual 𝑌-lines are assigned to recombination with excitons, which are bound to the structural defects on the surface.48 PL measurements convey that the organic MoL was acting as a surface passivating layer on GaN. KPFM images were collected from different locations on the surfaces of both the samples at a scan size of 1 × 1 µm2 in tapping mode using a caliberated Pt-coated Si tip. The contact potential difference (CPD) was then calculated by using Nanoscope Analysis software. KPFM potential images of GaN and Zn-TPPOH/GaN samples have been shown in Figure 2(b) and 2(c), respectively. The value of CPD was found to be 901±11 mV for bare cleaned GaN sample surface. It was observed that CPD was decreased to 592±8 mV when MoL of Zn-TPPOH organic molecules was chemisorbed on GaN. These measurements reveal that the adsorption 8 ACS Paragon Plus Environment

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of organic molecular monolayer was responsible for considerable reduction in surface potential of GaN by ~309 meV (since the CPD measured by KPFM is related to the surface potential of a semiconductor surface, due to the space charge layer)49-51.

Figure 2. (a) Room temperature PL spectra for GaN and Zn-TPPOH MoL/GaN samples, and KPFM images of (b) Bare GaN and (c) Zn-TPPOH/GaN samples, showing the measured values of surface potential Current-Voltage (I-V) measurements were performed on two PD device structures, Ni/GaN/Ni and Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni. I-V was measured using tungsten (W) needles with tip diameter of 20 µm connected to micromanipulators in DC Probe station, and semiconductor characterization system in voltage sweep mode. One probe was mechanically contacted to one interdigitated electrode (IDE) structure and the other was connected to the



other IDE, as represented in Figure 3(c). The measurements were performed in two conditions, one in dark condition and the other upon illumination by the Ultraviolet light (UV) (Figure 3(c)). The UV light shone on the samples was having wavelength of 365 nm and high power density of ~ 2 mW/cm2. I-V characteristics of Ni/GaN/Ni and Ni/Zn-TPPOH/GaN/ZnTPPOH/Ni device structures have been plotted in Figure 3(a) and (b), respectively. It is evident from the figures that insertion of organic molecules at the metal-semiconductor interface has benefitted in two ways. Firstly, the dark current (𝐼𝑑) has reduced significantly by ~3-4 orders of magnitude. 𝐼𝑑 for Ni/GaN/Ni was measured as 1.7 µA and 4.3 µA at -10 V and +10 V, respectively. 𝐼𝑑 got reduced to 2.4 nA and 1.73 nA at -10 V and +10 V, respectively for Ni/ZnTPPOH/GaN/Zn-TPPOH/Ni structures. Secondly, the enhancement in photocurrent (𝐼𝑖𝑙𝑙𝑢𝑚), which occurs due to UV light-matter interaction and electron-hole pair generation, is more for molecularly modified PD structures, as compared to PDs fabricated on bare GaN samples. 𝐼𝑖𝑙𝑙𝑢𝑚 for Ni/GaN/Ni MSM structures was measured as 18.4 µA and 20.3 µA at -10 V and +10 V, correspondingly. 𝐼𝑖𝑙𝑙𝑢𝑚 for Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni PD structures was measured as 1.1 mA and 1.2 mA at -10 V and +10 V, correspondingly.


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Figure 3. I-V characteristics of (a) Ni/GaN/Ni and (b) Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni UV PD structures in dark and upon illumination with UV light having wavelength of 365 nm and power density of 2 mW/cm2, (c) Schematic illustration of electrical characterization of IDEs on the samples in UV light (d) PDCR Vs Voltage plot for Ni/GaN/Ni and Ni/ZnTPPOH/GaN/Zn-TPPOH/Ni PD structures The effect of organic molecular adsorption can be also assessed by evaluating the photo-to-dark current ratio (PDCR) which is mathematically expressed as5

𝑃𝐷𝐶𝑅 =

𝐼𝑖𝑙𝑙𝑢𝑚 ― 𝐼𝑑

(1)

𝐼𝑑

PDCR was calculated for both the samples at different voltages. The variation of PDCR with applied voltage was been plotted and has been shown in Figure 3(d). It can be seen that PDCR 11 ACS Paragon Plus Environment


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for Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni PD structures is more than the values obtained for Ni/GaN/Ni MSM structures at all voltages. The values of PDCR at -10 V and +10 V for Ni/GaN/Ni PD structures were 9.58 and 3.77, respectively. The values were increased to 3.8×105 and 6.6×105 for -10 V and +10 V, correspondingly, for Ni/Zn-TPPOH/GaN/ZnTPPOH/Ni samples. Conclusively, it can be seen that by adsorption of organic molecules, the metal-semiconductor interface was improved in such a way that the reverse bias dark current was reduced by ~1000 times, photocurrent was enhanced by ~100 times (as compared to bare GaN case) and PDCR was increased by 105 times. The barrier height of Ni Schottky diodes on both bare and molecular-coated GaN samples in both dark and UV-illuminated conditions were calculated using the equation52 𝑘𝑇 𝐴𝐴 ∗ 𝑇2 𝜙𝐵0 = 𝑙𝑛 𝑞 𝐼𝑜

(

)

(2𝑎)

where, 𝑘 is the Boltzmann constant, 𝑇 is the temperature, 𝑞 is the electronic charge, 𝐴 is the area of the device and 𝐴 ∗ is the Richardson constant. The term 𝐼𝑜 is the saturation current which is evaluated using the mathematical expression52

[ ( )]

𝑙𝑛 𝐼𝑒𝑥𝑝

𝑒𝑉 𝑒𝑉 = 𝑙𝑛(𝐼𝑜) + 𝑘𝑇 𝜂𝑘𝑇

(2𝑏)

where, 𝑉 is the applied voltage and 𝜂 is the ideality factor. The intercept of the plot between 𝑙𝑛

[𝐼𝑒𝑥𝑝(𝑒𝑉 𝑘𝑇)] and 𝑉 gives the value of 𝐼𝑜. Plots between 𝑙𝑛[𝐼𝑒𝑥𝑝(𝑒𝑉 𝑘𝑇)] (calculated from I-V plots in Figure 3(a) and 3(b)) and 𝑉 (one side of the applied bias voltage from 0V to -10V) for Ni/GaN/Ni and Ni/MoL/GaN/MoL/Ni Schottky structures in both dark and UV illuminated conditions have been shown in Figure 4. A zoomed view of the graph restricting to lesser voltage range of -4V to -5V has been shown in the inset of Figure 4. From these graphs, the intercepts was calculated and fitted in equation 2(a). The value of barrier height for Ni Schottky 12 ACS Paragon Plus Environment

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structures on bare GaN samples were obtained as 0.67 eV and 0.58 eV for dark (𝜙𝐵𝑑) and UVilluminated (𝜙𝐵 ― 𝑈𝑉) conditions, respectively. It was observed that the barrier height for dark condition (𝜙𝑀𝑜𝐿 𝐵𝑑 ) was enhanced to 0.91 eV when the MoL was inserted between the metal and semiconductor. Also, Schottky barrier was lowered to a very small value of 0.48 eV (𝜙𝑀𝑜𝐿 𝐵 ― 𝑈𝑉) when the metal/MoL/GaN structures were illuminated with UV light.

Figure 4. Plot of 𝑙𝑛[𝐼𝑒𝑥𝑝(𝑒𝑉 𝑘𝑇)] Vs 𝑉 (Bias Voltage) for Ni/GaN/Ni and Ni/MoL/GaN/MoL/Ni Schottky structures in both dark and UV illuminated conditions for calculating the barrier height and inset figure: zoomed-in view of the graph in the voltage range of -4V to -5V In order to further assess the usefulness of the molecularly modified photodetectors, power dependency of photocurrent was investigated. For a photodetector, the variation of photocurrent with power density of the incident light follows the Power law, which is given by 𝐼𝑝ℎ = 𝐴𝑃𝑐 where 𝐼𝑝ℎ is the measured photocurrent, 𝐴 is the proportionality constant, 𝑃 is the power density and 𝑐 is the empirical coefficient.53,54 According to the law, the fractional power dependence is related to the density of carrier traps present at the metal-semiconductor 13 ACS Paragon Plus Environment


interface. Ideally, for a trap-free condition, the photocurrent varies linearly with the power density, which means, 𝑐 = 1. The condition of 𝑐 < 1 is indicative of presence of interface states. The low value of exponent c (of the photocurrent dependence on light intensity) has been attributed to the fact that the trap states become recombination centres under illumination, leading to the weak power dependence of photocurrent.55 Figure 5(a) and 5(c) shows the variation of the photocurrent, obtained at -10 V, with the power density of the incident light for Ni/GaN/Ni and Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni PD structures, respectively. The measurements were taken keeping the wavelength of light fixed at 365 nm and at room temperature. It was observed that for both the PD structures, the photocurrent increased as the power density was increased. The curves were fitted using the power law. The values of exponent c were obtained as 0.72 and 0.95 for PDs fabricated on bare GaN samples and MoL/GaN samples, respectively, as shown in the corresponding figures. This signifies that the quality of the UV photodetector was improved by adsorption of the MoL.


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Figure 5. Variation of (a) Photocurrent and (b) Responsivity with Power density of incident UV light for Ni/GaN/Ni MSM UV PDs and (c) Photocurrent Vs Power density and (d) Responsivity Vs Power density curves for Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni UV PDs, measurements taken at -10V and 365 nm wavelength After analyzing the UV photodetection capability and quality of both GaN-based and MoL/GaN-based PDs, the response of the optical detectors to the radiation of specific wavelength was investigated. Mathematically, the Responsivity of the PDs is expressed as56,57

𝑅𝜆 =

𝑅𝜆 =

(

𝐼𝑝 device current in ampere (due to illumination) = incident optical power in watt (required to generate that current) 𝑃𝜆

(

)

)

𝐼𝑖𝑙𝑙𝑢𝑚 ― 𝐼𝑑 𝐴𝑃𝜆

(3𝑎)

(3𝑏)

where, A is the area of device which is illuminated and 𝑃𝜆 is the incident optical power density



(in Watt/cm2) required to generate the photo-current. Higher be the spectral response, better the photodetector is. In order to find the maximum Responsivity of both the PDs, the power dependency of Responsivity was studied. The variation of Responsivity with power density of incident UV radiation at -10V and at 365 nm wavelength for Ni/GaN/Ni and Ni/MoL/GaN/MoL/Ni PDs have been plotted in Figure 5(b) and 5(d), respectively. It can be seen that for both the cases, the Responsivity first increased, attained a maximum value and then decreased as the power of the radiation was increased.58 The maximum responsivity for the GaN-based photodetectors was obtained as 0.22 kA/W at a very low power density of 5 µW/cm2. When Zn-TPPOH organic MoL was adsorbed on GaN, considerable enhancement in response of the device for UV radiation was observed, as shown in Figure 5(d). Maximum responsivity was increased to 4.14 kA/W at power density of 5 µW/cm2. Not only at higher voltages, but responsivity was enhanced for almost all the voltages in the range of -10 V to +10 V, as shown in Figure 6(a). This enhancement in Responsivity, by ~19 times (at 10V), is attributed to increment in the photocurrent of the device consequent to molecular surface modification.

Figure 6. (a) Plot of Responsivity Vs Voltage for Ni/GaN/Ni and Ni/Zn-TPPOH/GaN/ZnTPPOH/Ni PD structures at 365 nm wavelength and at -10V (b) Spectral Responsivity for Ni


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MSM UV PD structures fabricated on GaN and Zn-TPPOH/GaN samples at different wavelengths at -10V In the next step, the spectral selectivity of both the detectors was examined. This implies that maximum Responsivity of both the PD structures was calculated for different wavelengths in the spectral range of 250-590 nm (UV to Visible spectral region). For high spectral selectivity, the UV photodetector should have higher responsivity for UV spectral region and lower responsivity for visible wavelengths.59,60 The variation of responsivity with wavelength at power density of 5 µW/cm2 and at -10 V for both Ni/GaN/Ni and Ni/Zn-TPPOH/GaN/ZnTPPOH/Ni PDs have been plotted in Figure 6(b). It was observed that for PDs fabricated on bare GaN samples, as the wavelength was increased from 260 nm to 360 nm, the responsivity was increased. The maximum responsivity was obtained at 365 nm. After that, as the visible region was approached, a sharp dip in responsivity was observed. For higher wavelengths ranging from 380–550 nm, the response of the device was lesser. PDs fabricated on MoL/GaN samples showed similar spectral response curve. But the spectral selectivity of the devices was enhanced noticeably. As is evident from Figure 6(b), the response of the devices for visibleblind spectral region was increased (by ~19 times for 365 nm). Not only was this, but the responsivity of the devices towards visible spectral region got reduced. An extra dip in responsivity at the wavelength of ~430 nm was observed in the spectral curve. This was attributed to the fact that the porphyrin based organic molecules absorb light of wavelength ~430 nm.61 The UV-to-visible rejection ratio, which we define as the responsivity at 340 nm divided by responsivity at 450 nm, was found to increase from 16.59 (-10V) for Ni/GaN/Ni PDs to 1867.7 (-10V) for Ni/Zn-TPPOH/GaN/ Zn-TPPOH /Ni MSM PDs (increment by ~110 times). Lastly, the effect of the molecular surface alteration was tested on the temporal response of the PD devices. The measurements were performed at 365 nm UV light illumination (2 17 ACS Paragon Plus Environment


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mW/cm2) and at bias of -10 V. The time resolved photocurrent curves (photocurrent Vs time (I−t) plots) for Ni/GaN/Ni and for Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni PD structures have been shown in Figure 7(a) and Figure 7(b), respectively. The figures show the rise curves when UV light was switched on and the photocurrent was enhanced, and the decay curves when UV light was switched off and the photocurrent was reduced to the original level. It can be seen that the response (rising) edges and the recovery (decaying) edges of both the samples consist of two parts each, a fast-response part and a slow-response part. For a detailed comparison of the response times, the rise and decay curves for both the device structures were fitted by two equations, 4 and 5,62 respectively: 𝑡 𝑡 + 𝐵 𝑒𝑥𝑝 𝜏𝑟1 𝜏𝑟2

[ ( )] [ ( )]

(4)

[ ( )] [ ( )]

(5)

𝐼𝑖𝑙𝑙𝑢𝑚 = 𝐼𝑑 + 𝐴 𝑒𝑥𝑝

𝐼𝑖𝑙𝑙𝑢𝑚 = 𝐼𝑑 + 𝐴 𝑒𝑥𝑝 ―

𝑡 𝑡 + 𝐵 𝑒𝑥𝑝 ― 𝜏𝑑1 𝜏𝑑2

where A and B are the scaling constants, t is the time when the light was turned on or off, 𝜏𝑟1 is the fast-rise time constant, 𝜏𝑟2 is the slow-rise time constant, 𝜏𝑑1 is the fast-decay time constant and 𝜏𝑑2 is the slow-decay time constant. The exponential curve fitting of the rise time for Ni/GaN/Ni MSM structures has been presented on left side of Figure 7(a). Two different rise times, 𝜏𝑟1 and 𝜏𝑟2, were obtained as 0.53 s and 2.32 s, respectively. Usually, the fastresponse component is attributed to the rapid change of the carrier concentration as soon as the light is turned on (or off for the decay curve).63 The appearance of slow-response component is likely due to carrier trapping/releasing owing to the existence defects at the metalsemiconductor interface.63 The exponential curve fitting of the rise time for Ni/MoL/GaN/MoL/Ni PD structures has been presented on left side of Figure 7(b). For this case, the rise times 𝜏𝑟1 and 𝜏𝑟2 were obtained as 0.70 s and 1.28 s, respectively. An increase in


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the fast-time response of the rise curve (𝜏𝑟1) by insertion of the MoL supports the enhanced gain mechanism for Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni PD structures (as larger gain shows slower time response). Further, an improved slow-time response of the rise curves (𝜏𝑟2) for MoL/GaN case is indicative of lesser photo-generated carrier trapping and hence improved quality of the metal-semiconductor interface. In the similar manner, the decay times for both Ni/GaN/Ni and Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni PD structures have been plotted and exponentially fitted on the right side of Figure 7(a) and 7(b), respectively. Two different decay times, 𝜏𝑑1 and 𝜏𝑑2, were obtained as 0.10 s and 0.75 s, correspondingly, for the former case. The decay times were decreased to 0.06 s and 0.58 s, respectively, for the latter case. It may be noted that the actual fast-decay time constants (𝜏𝑑1) for both the device structures is likely to be lower than the values mentioned above, as the temporal resolution of the measuring system was in the range of 0.1 s. In addition, the plausible reason for the slow-decay time constant ( 𝜏𝑑2) to be in the range of seconds may be attributed to persistent photoconductivity (PPC) in GaN.64 However, PPC was getting reduced for the MoL/GaN case (as implied by the reduction in 𝜏𝑑2) as the defect/trap states were being passivated by the MoL, thereby improving the characteristics of the molecularly modified electrical devices.



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Figure 7. Time resolved photocurrent for (a) Ni/GaN/Ni PD structures and (b) Ni/ZnTPPOH/GaN/Zn-TPPOH/Ni PD structures measured at 365 nm UV light illumination and at bias of -10 V Such improved performance of MSM PD structures fabricated on MoL/GaN samples draws attention towards understanding the underlying phenomena. In this section, an explanation is provided for the same with the help of energy-band diagram as shown in Figure 8. It is known that GaN semiconductor faces the issues of electronic gap states, which are localized at the surface of the semiconductor, and defects (dislocations, point defects). Both these issues have adverse effects on the surface electrical properties of GaN and the electrical characteristics of the Schottky metal-semiconductor (MS) contacts thereon. A substantial density of electronic surface states yield a net charge on the surface of the semiconductor65 which leads to upward band-bending at the semiconductor surface66, as shown in Figure 8(a). The extent of the band-bending is described by the surface potential 𝑉𝑠. 𝑉𝑠 was measured by KPFM as 901 meV for our bare GaN samples. When the metal forms contact with the semiconductor (either singly or in MSM geometry), the charge on semiconductor surface forms image charge on the surface of metal and an interface potential 𝑉𝑖 is generated at the MS interface, as shown in Figure 8(b).67 The interface potential has a negative effect on the barrier height (𝜙𝐵𝑛) of the Schottky diode according to the equation 𝜙𝐵𝑛 = 𝜙𝑚 ― 𝜒𝑠 ― 𝑉𝑖

(6)

where, 𝜙𝑚 is the work function of the metal, 𝜒𝑠 is the electron affinity of the semiconductor and 𝑉𝑖 is the interface potential. We can see that higher be the value of 𝑉𝑖, lesser will be 𝜙𝐵𝑛 (lesser than the value it should have been for an ideal trap-free case), as shown in Figure 8(b). 𝜙𝐵𝑜 for Ni Schottky diodes on bare GaN sample was evaluated as 0.67 eV for dark conditions. Furthermore, the defects on the surface of GaN makes the Schottky MS contact spatially 20 ACS Paragon Plus Environment

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inhomogeneous. In dark conditions, when bias is applied to the MSM structure, one Schottky diode becomes reverse biased and the other diode becomes forward biased, as illustrated in the part (c) of Figure 8. The electrons tend to flow from the first metal contact to the second metal contact, as illustrated by red arrows. In this process, they find an open trap-state pathway due to the defects at the MS interface. These defect-states allow tunneling of the carriers through the interface, as shown in Figure 8(c). Further, as the Schottky barrier height gets lowered, due to surface states, the carriers also jump over the barrier while travelling from the metal to the semiconductor (Figure 8(c)). Thereby, for MSM structures on bare GaN samples, the experimentally observed reverse bias dark current was higher (reverse dark current for Ni Schottky diodes on GaN was of the order of µA). When UV light is shone on the MSM PD structure on GaN, electron-hole pairs are generated. The photo-generated electrons jump to the conduction band and flow towards the positive collecting electrode, as represented by maroon colored arrow in Figure 8(d). The photo-generated holes move towards the negative collecting electrode. However, during this process, few photo-generated holes get trapped. Such hole trapping may be either at the metalsemiconductor interface68,69 or beneath the surface of the semiconductor70,71. This leads to either Schottky barrier lowering68-70 or thinning of the space charge region at the MS interface71. During reverse bias, the former (barrier lowering) allows jumping of the carriers above the Schottky barrier (red colored arrow in Figure 8(d)) and the latter (barrier thinning) allows tunneling of the carriers across the MS interface. These carriers add to the photogenerated carriers and leads to enhanced photocurrent as compared to the dark current. For our case, Schottky barrier height (𝜙𝐵 ― 𝑈𝑉) was lowered (as illustrated in Figure 8(d)) to 0.58 eV for UV-illuminated conditions. Resultantly, the photocurrent was enhanced as compared to dark current, though slightly.



As the organic molecular layer (MoL) is adsorbed on GaN, the surface defects are passivated. This was experimentally revealed by significant enhancement in the band-edge emission intensity of MoL/GaN samples using PL measurements, higher slope of PhotocurrentPower density curve and improved slow-response of the rising and decaying PhotocurrentTime curves. In addition to this, the surface band bending due to surface charge also gets reduced (Figure 8(e)), as indicated by decrease in the surface potential 𝑉𝑠, to 592 meV, using KPFM. When metal forms contact with MoL/GaN surface, the interface potential 𝑉𝑖 reduces (since 𝑉𝑠 has decreased), and the Schottky barrier height increases according to equation 6, as represented in Figure 8(f). 𝜙𝑀𝑜𝐿 𝐵𝑑 for Ni Schottky diodes on MoL/GaN sample was evaluated as 0.91 eV for dark conditions. During reverse bias in dark conditions, lesser electrons are allowed to pass from the metal to the semiconductor side. This is because firstly, the carriers find a blocked trap-state pathway for tunneling across the barrier. Secondly, due to the enhanced barrier height, lesser carriers are able to jump over the barrier from the metal to the semiconductor, as shown in Figure 8(g). Therefore, the experimentally observed reverse biased dark current was reduced by several orders of magnitude (~nA range). Upon illuminating the metal/Zn-TPPOH/GaN/Zn-TPPOH/metal PD structures with UV light, electron-hole pairs are generated. The photo-generated carriers flow towards their respective collecting electrodes and contribute to the photocurrent (maroon arrow in Figure 8(h)). However, enhanced Schottky barrier lowering occurs for the molecularly modified case as compared to the bare GaN case (represented in Figure 8(h)). The calculated value of Schottky barrier height (𝜙𝑀𝑜𝐿 𝐵 ― 𝑈𝑉) for Ni Schottky diodes on MoL/GaN sample was 0.48 eV for UV-illuminated conditions. Though, at this point, we are unable to explain the exact mechanism at the molecule-semiconductor interface which has caused such lowering. Hence during reverse bias in UV-illuminated conditions, more carriers jump over the Schottky barrier and contribute to the photocurrent (red arrow in Figure 8(h)). Therefore, the value of 22 ACS Paragon Plus Environment

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photocurrent or the photoconductive gain obtained for Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni PD structures was amplified as compared to the of photocurrent or the photoconductive gain obtained for Ni/GaN/Ni PD structures. Now, the Responsivity is directly proportional to the photo-current. Therefore, for the same device area and the same incident optical power, as the photocurrent is increased, the responsivity also enhances, proportionally.




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Figure 8. Energy band-diagrams for (a) bare GaN (b) MSM interface (c) reverse biasing and (d) UV photodetection for MSM structures; (e) MoL/GaN (f) metal/MoL/GaN/MoL/metal (g) reverse biasing and (h) UV photodetection for metal/MoL/semiconductor/MoL/metal structures IV. CONCLUSIONS In conclusion, organic molecules of Zn-TPPOH have been adsorbed on cleaned GaN samples and Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni UV photodetector structures have been fabricated. Enhancement in PL intensity for Zn-TPPOH/GaN samples by 6-fold of magnitude indicated that the molecular layer (MoL) was passivating the defect-related states on GaN. Decrease in surface potential of GaN by approx. 300 meV, due to the molecular surface modification, signified that the MoL was also playing with the surface charge on GaN which led to increase in Schottky barrier height from 0.67 eV (GaN) to 0.91 eV (MoL/GaN) in dark conditions. Consequent to both these mechanisms, the motion of charge carriers across the metal-semiconductor interface of Schottky contacts on GaN was reduced, leading to decrease in reverse bias dark current by 3 orders of magnitude at 10V. Upon UV illumination, current due to photo-generated carriers along with current due to Schottky barrier lowering formed the net photocurrent, which was slightly higher than the dark current for the bare GaN case. Enhanced Schottky barrier lowering, from 0.58 eV for UV-illuminated bare GaN case to 0.48 eV for UV-illuminated MoL/GaN case, was observed. This further led to increase in photocurrent by ~100 times for MoL/GaN samples, as compared to bare GaN case at 10V. UV photodetector parameters such as Responsivity, photo-to-dark current ratio and UV-tovisible rejection ratio were increased by ~19 times, ~105 times and ~110 times, respectively at 10V. The devices on MoL/GaN became more responsive to UV wavelengths and lesser responsive to visible wavelengths, as compared to those fabricated on bare GaN. The slowresponse rise and decay time constant of the devices was also improved. Overall, such metal25 ACS Paragon Plus Environment


MoL-semiconductor device structures can be useful for the fabrication of more efficient GaNbased Ultraviolet optical detectors, mitigating the adverse effects of electronic surface states in these materials. ACKNOWLEDGEMENTS Manjari Garg is grateful to Council of Scientific and Industrial Research (CSIR), India for providing research fellowship. The authors would like to acknowledge Prof. M. Ravikanth and Indian Nanoelectronics User Program (INUP) at Indian Institute of Technology Bombay for the molecular chemisorption work. The authors are also obliged to Nanoscale Research Facility (NRF) at Indian Institute of Technology Delhi for the PL, KPFM and UV photodetection measurements. REFERENCES (1)

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Giant UV Photoresponse of GaN-based photodetectors by surface

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