Au-Nanoplasmonics-Mediated Surface Plasmon-Enhanced GaN Nanostructured UV Photodetectors.

The nanoplasmonic impact of chemically synthesized Au nanoparticles (Au NPs) on the performance of GaN nanostructure-based ultraviolet (UV) photodetectors is analyzed. The devices with uniformly distributed Au NPs on GaN nanostructures (nanoislands and nanoflowers) prominently respond toward UV illumination (325 nm) in both self-powered as well as photoconductive modes of operation and have shown fast and stable time-correlated response with significant enhancement in the performance parameters. A comprehensive analysis of the device design, laser power, and bias-dependent responsivity and response time is presented. The fabricated Au NP/GaN nanoflower-based device yields the highest photoresponsivity of ∼ 380 mA/W, detectivity of ∼ 1010 jones, reduced noise equivalent power of ∼ 5.5 × 10-13 W Hz-1/2, quantum efficiency of ∼ 145%, and fast response/recovery time of ∼40 ms. The report illustrates the mechanism where light interacts with the chemically synthesized nanoparticles guided by the surface plasmon to effectively enhance the device performance. It is observed that the Au NP-stimulated local surface plasmon resonance effect and reduced channel resistance contribute to the augmented performance of the devices. Further, the decoration of low-dimensional Au NPs on GaN nanostructures acts as a detection enhancer with a fast recovery time and paves the way toward the realization of energy-efficient optoelectronic device applications.

Introduction

Ultraviolet photodetector (UV PD) fabrication and development has been an area of intense research in the last few decades, and the ongoing research for finding new ways to enhance the UV detection capability of the detectors is very encouraging. The importance of UV PDs is universally recognized because of their diverse range of applications, such as in antimissile technology by plume detection, flame detection, calibration and monitoring of UV radiations, chemical as well as biological analyses, secure optical communications, environmental monitoring, and astronomical research.[1] For this, GaN semiconductors have demonstrated to be a potential backbone of durable device applications because of the numerous advantages offered by its significant inherent properties such as the wide and direct band gap, high thermal coefficient, high carrier mobility, higher resistance toward electric breakdown field, and high chemical stability.[2] The advancement in photodetection can be realized by using nanostructures (NSs), which offer additional advantages in detection applications by offering a large surface-to-volume ratio that can significantly increase the UV photon absorption rate. These advancements in UV PDs can lead to good photoconductivity and enhanced electrical conductivity.[3−5]

Recently, much attention has been paid for enhancing the performance of optoelectronic devices such as solar cells, light-emitting diodes (LEDs), and UV PDs via nanoplasmonic behavior of metal nanoparticles (NPs).[6−8] Some previous reports articulate that the use of nanoplasmonic behavior of metal NPs such as silver (Ag) NPs,[9] aluminum (Al) NPs,[6,10] platinum (Pt) NPs,[7] and gold (Au) NPs[8] has been realized as a remedy for the carrier enhancement seeker, GaN-based UV PDs. Thereby, metal NP-stimulated hot carriers are guided by local surface plasmon resonance (LSPR) for enhancing optical detector performance. Thus, LSPR-induced enhanced UV emission has been reported in Au NP-decorated ZnO NSs.[10,11] The Au NPs nowadays have splendidly established their identity in LSPR because of numerous inherent advanced physical properties such as nonlinear optics, magneto-plasmonics, optical trapping and optical activity,[12] high chemical and physical stabilities, ease of surface functionalization with biomolecules, and plasmon-oriented multitude of optical properties.[13,14] Besides, Au NPs are also known for access to easily polarizable hot carriers (hot resonant electrons). These polarizable conduction electrons interact with the electromagnetic field of the corresponding incident electromagnetic light wave and generate nonlinear optical phenomena.[15] Therefore, this is the first report where chemically synthesized monodispersed (∼10 nm) Au NPs’ nanoplasmonic impact has been realized on GaN-NS-based metal–semiconductor–metal (MSM) UV PDs as an UV detection enhancer. The study elaborates the nanoplasmonic effect, which leads to trapping of incident photons by the process of localized scattering and absorption as a consequence of incident photons and metal NP interaction, which increases the light and the material interaction time. This scattering stimulated an enhanced interaction to allow high photon absorptions, which generate more electron–hole pairs.[16]

Experimental Section

The Au NPs were synthesized in the presence of trisodium citrate via the Turkevich method[17] using gold(III) chloride hydrate (HAuCl4·3H2O) precursor solution with trisodium citrate (C6H5Na3O7·2H2O) [for details, see Supporting Information (Figure S1)]. The as-synthesized Au NPs were characterized by ultraviolet–visible (UV–vis) spectroscopy (Agilent, Cary 5000i) and high-resolution transmission electron microscopy (HRTEM) (FEI, Tecnai F30 G2 S-TWIN). The HRTEM images of bare Au NPs elucidate spherelike shape with an average size of ∼ 10 nm, and the UV–vis absorption spectrum shows the LSPR peak position at ∼520 nm, which also confirms the size of Au NPs to be ∼10 nm[18] (Figure ). The UV–vis absorption data have also been used to calculate the number density of Au NPs to be 3.5 × 1012 using the calculation technique mentioned by Haiss et al.[19] The broadening of the UV–vis absorption spectrum is found directly in correlation with the concentration of Au NPs;[18] lower broadening and higher absorption postulates higher concentration of Au NPs.

Figure 1

(a,b) HRTEM micrographs of NPs with lower and higher magnifications. (c) UV–vis absorbance spectra of Au NPs. (d) RT-PL spectra of Au NPs.

Thus, it contributes more LSPR-generated hot carriers into the devices as attributed by room-temperature (RT) photoluminescence (PL) measurements of Au NPs (Figure d), which were carried out using a FLS980 D2D2 (Edinburg) system with a 325 nm excitation source (Hd-Cd Kimmon laser). The spectra exhibit a prominent emission peak at 363 nm and a secondary peak at ∼430 nm. The secondary blue emission peak at 430 nm corresponding to the Au 6S-to-6P intraband transition is also attributed to the presence of isolated spherelike Au NPs of size < 20 nm.[20] These chemically synthesized Au NPs were spin-coated uniformly on GaN nanostructured surfaces, that is, on GaN nanoislands (GaN-NIs) and GaN nanoflowers (GaN-NFs) at a constant speed of 3000 rpm. Both the GaN nanostructured surfaces (GaN-NIs and GaN-NFs) were grown by using plasma-assisted molecular beam epitaxy (Riber Compact 21). The high-density GaN-NIs have been grown at the substrate temperature of 770 °C in the presence of N2 plasma (1.5 sccm, 400 W) and Ga beam equivalent (BEP) of 6.5 × 10–7 Torr for 120 min.[21] These high density and vertically aligned GaN-NIs were grown on the Si(111) substrate with a distribution density of ∼ 2 × 1010 cm–2, average width of 50 ± 5 nm, and aspect ratio of 1.92 (width-to-height). For the growth of GaN-NFs, the buffer layer of AlN (thinness 30 nm) was deposited at a substrate temperature of 825 °C under stoichiometric conditions on Si(111). Further, the growth of the GaN film (80 nm) followed by GaN-NFs was performed at 730 °C at rf plasma power of 500 W with Ga BEP of 1 × 10–6 Torr.[22] Thus, the uniformly distributed GaN-NFs with the bottom tapered portion of ∼200 nm to top higher opening part of ∼300 ± 50 nm was performed on Si(111) with the AlN buffer layer. The high rms roughness of these nanostructured surfaces (5.8 nm for GaN-NIs and 9.8 nm for GaN-NFs) has been utilized to settle low-dimensional Au NPs. A schematic diagram elaborating the complete sample preparation process is demonstrated in Figure S1. The morphology and elemental mapping of uniformly distributed Au NPs decorated on GaN-NS surfaces were carried out using field-emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDS) techniques (Zeiss, Auriga) as shown in Figures S2 and S3. For the fabrication of UV PDs, Au-metal electrodes (200 nm thick) were deposited as a MSM structure by the thermal evaporation technique on bare and Au NP-coated GaN-NS samples, and the active area of the devices was fixed as 3 × 10–3 cm2. The impact of Au NPs on the opto-electrical transport is analyzed by RT current–voltage and photoresponse measurements, which were carried out using a Cascade Microtech instrument (EPS150TRIAX with shield enclosure EPS-ACC-SE750).

Results and Discussion

To analyze the enhancement in photon-generated hot carriers from Au NP-decorated GaN-NS devices, the RT-PL measurements were carried out. RT-PL spectra demonstrate UV emission at 364 nm and an additional peak of violet-blue emission for all samples (bare GaN-NIs, Au NPs/GaN-NIs, bare GaN-NFs, and Au NPs/GaN-NFs) when excited by the 325 nm UV laser as shown in Figure . The UV emission at 364 nm resembles the near-band edge emission (GaN band gap of 3.406 eV) and recognized the radiative recombination of excitons, although the presence of violet-blue emission (380–440 nm) is due to trapping states related to the optical states existing in between the band gap regime.[23,24] The spectra validate significantly enhanced UV emission for Au NP-decorated GaN-NIs (×1.4) and GaN-NFs (×3) as compared to their bare counterparts, as shown in Figure a,b, respectively. The coupling of Au NPs’ localized surface plasmons at the interface with GaN-NSs postulates resonance interaction between the electromagnetic field of the incident UV light and the electron charge near the surface of Au NPs (emission at 363 nm as shown in Figure d) and contributes more electrons in-flow with the existing GaN channel.

Figure 2

(a) RT-PL spectra of (a) bare GaN-NIs and Au NPs/GaN-NIs and (b) bare GaN-NFs and Au NPs/GaN-NFs.

The effect of nanoplasmonics on the opto-electrical behavior of GaN-NS-based devices can be realized because of the Au NP-stimulated LSPR effect, which can significantly enhance photocurrent in the fabricated device because of enhanced light absorption in the near-surface region.[25,26] As the synthesized metallic NPs having size dimensionality to the metal skin depth, thereby, the electric field of incident electromagnetic radiation of light having ease of penetration of Au NPs’ surface. Thus, this surface penetration by UV radiation allow the polarization of Au NPs’ conduction electrons.[27,28] The LSPR with uniformly distributed plasmon oscillation over the entire NP volume where the size of NPs is much smaller (∼10 nm) than the incident photon wavelength (325 nm) has nonpropagating excitations. The generated plasmons over the NPs have two possibilities, that is, either they got absorbed or scattered. The probability of absorption or scattering can be determined by σAbs and σSca, respectively, which depend on the size, shape, and structure of the NPs.[29,30] The Au NPs of size < 25 nm have pure plasmon absorber properties due to the negligible scattering coefficient, that is, σAbs ≫> σSca.[31] This absorbed plasmon-originated resonance or electron clouds are damped (through Landau damping) by these low-dimensional NPs in two ways: nonradiatively and radiatively through hot electron–hole pairs generation and re-emission of photons, respectively.[32] The distribution of these generated carriers depends upon factors such as particle size, plasmon energy, plasmon mode symmetry, state density of the material, and electronic structure.[33] The plasmon decay-generated carriers release their energy (via carrier relaxation) among many lower-energy electrons through their interscattering process, for example, Auger transition.[34]Figure A–C displays a complete schematic diagram (including the energy band diagram) of NP-decorated GaN-NS-based UV PDs having nanoplasmonics-generated LSPR excitation of Au NPs and their damping around the medium (GaN-NSs), carrier generation, and their scattering, relaxation, and distribution. These Au NPs’ LSPR-injected extra carriers (resonant hot electrons) in addition to existing UV photon-generated carriers of GaN-NSs enhanced the cumulative photocurrent.[35]

Figure 3

Schematic illustration of (A,B) both NP-decorated UV PDs and (C) their nanoplasmonic effect. (a) Local surface plasmons are excited by the electromagnetic wave of UV light propagating in free space. (b) Process of Landau damping. (c) Scattering and relaxation of LSPR-generated hot carriers. (d) Energy dissipation. (e) Energy band diagram of GaN-NSs and Au NP system showing local surface plasmon coupling (325 nm laser source excites GaN-NSs as well as Au NPs). The schematic demonstrates that GaN-generated carriers are getting merged with Au NPs’ local surface plasmon-originated resonant hot electrons in the same band regime of GaN, i.e., ∼364 nm.

To analyze the Au NPs nanoplasmonics-stimulated enhancement in the opto-electrical transport, self-powered (without bias) as well as photoconductive modes of operation were performed where the I–V measurement for bare GaN-NIs, GaN-NFs, Au NPs/GaNIs, and Au NPs/GaN-NFs based MSM UV PD devices was performed under dark and UV illumination (325 nm) at RT, as shown in Figure . The data revealed that Au NPs/GaN-NS-based UV PDs yield enhanced photocurrent (Iph) characteristics as compared to bare GaN-NS UV PD devices in the self-powered mode (0 V bias) as well as under variable bias conditions (from −1 to +1 V). At zero volt bias, the Au NPs/GaN-NIs UV PDs display very low dark current (Id) ∼ 22 nA and very high light current (IL) ∼ 25 μA; however, in the case of bare GaN-NIs UV PDs, these values are found to be ∼12 nA (Id) and ∼2.1 μA (IL), which elucidates ∼12-fold enhancement in photocurrent (Iph = IL – Id), as shown in the inset of Figure a. The analogous impact was observed in the case of the Au NP/GaN-NF-based device, where Id and IL ∼ are found to be 97.4 nA and 60.3 μA, respectively, leading to Iph ∼ 60 μA, which is significantly higher than ∼24 μA in the case of bare GaN-NF (Id: 47.7 nA, IL: 24.3 μA)-based UV PDs, as shown in the inset of Figure b. The self-powered behavior originated from the Au/GaN Schottky junction, and asymmetric Au/GaN interfaces triggered the inbuilt potential gradient as explained elsewhere.[36] The analysis reveals that, due to the presence of Au NPs, the fabricated Au NP/GaN-NI and Au NP/GaN-NF devices display enhanced self-powered characteristics as compared to their respective bare GaN-NS-based devices. Further, a significant variation in the photocurrent has also been observed for these devices (bare and Au NPs/GaN-NSs) under applied bias (−1 to +1 V). The Schottky barrier height (SBH) (φb) and the ideality factor (n) of the Au NPs/GaN-NSs as compared to their respective bare counterparts have been evaluated for forward as well as reverse bias by using standard thermionic emission theory,[37,38] as shown in Table .

Figure 4

I–V characteristics of (a) bare and Au NPs/GaN-NI- and (b) bare and Au NPs/GaN-NF-based devices under dark and UV illumination.

Table 1

Ideality Factors and SBH Variations at Two Au/GaN Interfaces on Each of Four Fabricated UV PDs

	GaN-NI (A)		Au NP/GaN-NI (B)		GaN-NF (C)		Au NP/GaN-NF (D)
Au/GaN	1	2	1	2	1	2	1	2
n	nA1	nA2	nB1	nB2	nC1	nC2	nD1	nD2
	1.8	1.92	1.2	1.5	1.7	1.53	1.1	1.2
φb	φbA1	φbA2	φbB1	φbB2	φbC1	φbC2	φbD1	φbD2
	0.75	0.85	0.703	0.8	0.97	0.88	0.7	0.73

The performance parameters such as detectivity (D), responsivity (R), noise equivalent power (NEP), and quantum efficiency (η) of the fabricated UV PDs were calculated by using the following relations.[39]where Pinc is the incident optical power illuminated on the active area of the device to generate corresponding carriers out of absorbed incident UV photons, A0 is the active area of the fabricated device, h is the Planck’s constant, c is the speed of light, e is the elementary charge, and λ is the incident photon wavelength of the illuminating source. The responsivity, detectivity, and NEP of all fabricated devices are calculated for zero and applied biases between −3 to +3 V. It was observed that the responsivity of the fabricated detectors in the self-powered mode is enhanced by ∼12× and ∼2.5× for Au NP-decorated GaN-NI and GaN-NF detectors as compared to their bare devices, respectively. The responsivity of these devices was analyzed at an applied bias of −3 to +3 V, where all fabricated detectors show a gradual increment in the value of “R” in either direction of bias polarity (Figure a). A significant enhancement in “R” from 151.28 mA/W (bare GaN-NIs) to 215.67 mA/W (Au NPs/GaN-NIs) and 199.36 mA/W (bare GaN-NFs) to 380.6 mA/W (Au NPs/GaN-NFs) at −3 V is determined (Table ).

Figure 5

Detector performance parameters variation with respect to (a–d) applied bias voltages.

Table 2

Comparative Evaluation of Various Performance Parameters of Bare and Au NP-Incorporated GaN-NS-Based MSM UV PDs

	R (mA/W)		D (109 Jones)		NEP (10–11 W Hz–1/2)		Q.E (%)
fabricated detectors	0 V	–3 V	0 V	–3 V	0 V	–3 V	0 V	–3 V
GaN-NIs	3.89	151.28	3.43	0.708	1.59	7.73	1.48	58.73
GaN-NFs	43.95	199.36	19.5	0.507	0.281	10.08	16.8	76.2
Au NPs/GaN-NIs	45.99	215.67	30.5	1.22	0.179	4.49	17.6	82.4
Au NPs/GaN-NFs	109.1	380.60	33.8	0.826	0.162	6.63	41.7	145.5

Furthermore, detectivity of the fabricated devices was also analyzed at self-powered as well as at variable bias conditions (±3 V). Au NPs’ highly conductive behavior accelerated the reduction in the dark current (Figure b) and enhanced the responsivity, significantly reciprocating increased D values (Figure c and Table ), that is, from 3.43 × 109 Jones (bare GaN-NIs) to 3.05 × 1010 Jones (Au NPs/GaN-NIs) and 1.95 × 1010 Jones (bare GaN-NFs) to 3.38 × 1010 Jones (Au NPs/GaN-NFs) in the self-powered mode. At higher applied bias, the increase in D values is limited due to the high dark current. The bias-dependent analysis also revealed the criticality of dark current, which can directly affect detectivity as well as the NEP of the detectors, as shown in Figure (c,d) and Table . The observation elucidates that unlike higher dark current values at higher bias voltage (−3 V), lower dark current at the self-powered mode allows the fabricated detectors to achieve higher D values and lower NEP, that is, 1.79 × 10–12 W Hz–1/2 (Au NPs/GaN-NIs) and 1.62 × 10–12 W Hz–1/2 (Au NPs/GaN-NFs) as compared to their bare counterparts (Table and Figure d). The study divulges that the GaN-NS-based UV PDs with Au NPs demonstrates comparatively superior performance. The capability of these detectors have also been investigated by applied bias voltages and realized as an increment in the quantum efficiency, where they achieved their maximum values comparatively, that is, 82.4 and 145.5% by Au NPs/GaN-NIs and Au NPs/GaN-NFs, respectively, at −3 V, as shown in Figure d and Table .

The performance of all fabricated UV PDs with varying optical power is tested, which shows promising linear inclination in photocurrent values with increasing incident optical power “Popt” (from 1 to 13 mW), as shown in Figure a,b for GaN-NI- and GaN-NF-based UV PDs, respectively. The results demonstrate increasing Iph values from 1.02 μA (1 mW) to 2.87 μA (13 mW) (∼2.8× increment) for bare-GaN-NIs UV PDs and −1.05 μA (1 mW) to 22.6 μA (13 mW) revealing ∼22× increment in Iph for bare-GaN-NF-based detectors. The Au NP-induced LSPR significantly enhances Iph values with increasing power such as 2.46–26.2 μA (13 mW) and shows ∼11-fold enhancement in Au NPs/GaN-NIs, whereas Iph displays ∼85-fold enhancement (from −0.68 μA at 1 mW to 58.5 μA at 13 mW) for Au NPs/GaN-NFs. Thus, Au NP hot carrier-induced enhancement is witnessed in the performance of the fabricated devices. Moreover, the NEP of Au NP/GaN-NF-based detectors has also been investigated by varying the applied optical power (1 to 13 mW), where NEP values start decreasing from 1.072 × 10–10 W Hz–1/2 (at 13 mW, −3 V) and reached its minimum value of 5.55 × 10–13 W Hz–1/2 (at 1 mW, 0 V) because of inverse effect of increasing the responsivity values 21.61 mA/W (1 mW, 0 V) to 109.1 mA/W (13 mW, 0 V), as shown in Figure S4.

Figure 6

Power-dependent transient response of (a) bare and Au NP/GaN-NI and (b) bare and Au NP/GaN-NF detectors.

Next, the switching speed of the detector which elucidates the quality of a detector to detect illuminating photons has been evaluated. The switching speed of the fabricated UV PDs in terms of time-correlated response and recovery was evaluated from its transient response by UV light illumination turning on and turning off. Figure shows the time-correlated photoresponse under self-powered mode (at 13 mW optical power) of all fabricated UV PDs, where the rise (response) time and decay (recovery) time curves are fitted with respect to the experimental data by the following equations[3]where I0 is the maximum saturation photocurrent value at a particular time t and tr and td stand for the rise and decay times, respectively. The rise time from 10 to 90% of the final value, that is, “tr” under UV illumination and decay time from 90 to 10% of the final value “td” after switching off UV illumination have been measured from time correlated transient response of the fabricated devices.

Figure 7

Time-correlated photoresponse rise time and decay time fitted curves. (a) Bare GaN-NIs, (b) Au NPs/GaN-NIs, (c) bare GaN-NFs, and (d) Au NPs/GaN-NFs MSM UV PDs under self-powered mode at 13 mW.

The observation reveals that the fabricated detectors with Au NPs are benefited by LSPR-generated high-density charge carriers, wherein, the highly conducting behavior of Au NPs plays a significant role to reduce the defect-originated obstructions in the flow of charge carriers. This reduction in offered resistance to the photocurrent allows quick retardation of the photocurrent after switching off the UV light. Figure shows the effect of quick retardation as the quick recovery time, that is, “td” of GaN-NI- (∼63.6 ms) and GaN-NF-based detectors (∼43.7 ms) is reduced to 40 ms of Au NP/GaN-NI- and Au NP/GaN-NF-based UV PDs. The rise and decay time measurement of the fabricated detectors is restricted to <40 ms because of system limitation.

Table shows, upon comparing with previous reports on nanoplasmonics, that the fabricated GaN-NS UV PDs with Au NPs, herein, exhibit fast rise and decay times. Such a high switching speed is contributed by an additional potential force because of variation in SBHs, reduced channel resistance due to additional effect of Au NPs’ higher conductivity, and a better crystalline quality, which were elaborately discussed elsewhere,[21] and revealed a negligible defect band in PL and low stress via micro-Raman spectroscopy.

Table 3

Switching Speed Comparison with Various Devices Incorporating Hot Carrier Generated by Nanoplasmonic Effects

	response time (switching speed)
devices	rise time (ms)	decay time (ms)	references
Au NPs/GaN-NF/Si(111)	40	40	this work
Au NPs/GaN-NS/Si(111)	40	40	this work
Au NP/Ga-polar GaN	2900	6200	(40)
Au NP/ZnO/PET	10,300	14,200	(41)
Ag-NP/ZnO-NW	80	3270	(42)
Pt-NP/GaN-NS/Al2O3	1100	650	(7)
ZnO-NP/quartz	600	8000	(43)
ZnO-NP/quartz	22,000	11,000	(44)
ZnO-NP/glass	2000	12,000	(45)
TiO2-NP/SiOx	1000	900	(46)
TiO2-nanofibers/SiO2/Si	1500	7800	(47)

Conclusions

In conclusion, the present analysis postulates a significant impact of chemically synthesized, monodispersed Au NPs (∼10 nm) on the performance of GaN-NS-based UV PDs, wherein fabricated GaN detectors with Au NPs prominently respond toward UV in both self-powered as well as the photoconductive modes of operation and display fast and stable time-correlated photoresponse with significant enhancement in the performance parameters. The enhanced performance of the fabricated devices is due to nanoplasmonics-induced LSPR-stimulated hot resonant carriers. Moreover, the electronic transport properties affirmed the influence of Au NPs in lowering the ideality factor and SBH at the Au/GaN interfaces because of which the channel resistance of the fabricated detector is reduced. These distinctive barrier height-stimulated self-powered UV PDs have shown fast switching speed (rapid rise and decay time of 40 ms), enhanced responsivity, detectivity, higher quantum efficiency, and very low NEP. The best performance of the fabricated device with Au NPs/GaN-NFs yields photo responsivity of ∼ 380 mA/W, detectivity of ∼ 3.38 × 1010 jones, reduced NEP of ∼ 5.5 × 10–13 W Hz–1/2, fast response/recovery time of ∼40 ms, and higher quantum efficiency of 145.5%. The decoration of low-dimensional Au NPs’ nanoplasmonic behavior on GaN-NSs acts as a detection enhancer with a fast recovery time and paves the way toward the realization of energy-efficient optoelectronic device applications.