AlGaN Nanorods Grown by MOCVD

Subscriber access provided by UNIV OF ESSEX

Article

Ultraviolet-C Photodetector Fabricated using Sidoped n-AlGaN Nanorods Grown by MOCVD San Kang, Uddipta Chatterjee, Dae-Young Um, Yeontae Yu, Inseok Seo, and Cheul-Ro Lee ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01047 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Photonics is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Ultraviolet-C Photodetector Fabricated using Si-doped nAlGaN Nanorods Grown by MOCVD San Kang†, Uddipta Chatterjee†, Dae-Young Um, Yeon-Tae Yu, In-Seok Seo and CheulRo Lee* Semiconductor Materials Process Laboratory, School of Advanced Materials Engineering, Engineering College, Research Center for Advanced Materials Development (RCAMD), Chonbuk National University, Baekje-daero 567, Jeonju 54896, Republic of Korea. *

Corresponding Author

†

Equal Contributor

Abstract: Aluminum gallium nitride (AlxGa1-xN) alloy films and nanostructures have attracted extensive research attention for ultraviolet (UV) and deep ultraviolet optoelectronic applications. However, the morphology controlled growth of high quality AlxGa1-xN quasi one-dimensional nanostructures has been limited by the complex multi-component phase diagram and inhomogeneous composition distribution. Here, we demonstrated the growth of Si-doped n-type compositionally uniform Al0.45Ga0.55N nanorods employing a metal organic chemical vapor deposition (MOCVD) technique for the application in UV-C photodetectors. A 2-step growth process, namely, growth of undoped GaN seeds and subsequent growth of n-AlGaN nanorods over GaN seeds has been developed. Various characterization techniques have been used to study the crystalline quality, orientation and optical properties of the realized nanorods. Field emission scanning electron microscopy revealed a uniform distribution of vertically aligned n-AlGaN nanorods over the GaN seeds. X-ray diffraction studies showed that the grown nanorods are preferentially (0002) oriented with hexagonal crystal structure. High resolution transmission electron microscopy images indicated the nanorods are single crystalline in nature, without any significant crystalline defects and dislocations. Cathodoluminescence spectra of AlGaN nanorods displayed strong band edge excitonic emission peak at 276 nm at 77 K and shifted to lower energy as the temperature increased to 300 K. The photocurrent current (Ip) of the fabricated photoconductive device was significantly higher in the UV region (250-276 nm) compared to the corresponding dark current. The photocurrent displayed a non-linear power density (P) dependent characteristics (Ip∝P0.64). The photoresponsivity and sensitivity of the fabricated photodetector were estimated to be ~ 115 mA/W and ~ 64 %, respectively in the UV-C region.

Keywords: AlGaN, nanorods, MOCVD, ultraviolet-c, cathodoluminescence, photoconductive device

III-nitride materials namely, AlN, GaN, InN and their alloys have attracted enormous research attention in the past decades owing to their superior properties, such as direct wide band gap, high electron mobility and thermal/chemical resistance.1–6 Various optoelectronic devices, such as light emitting diodes, solar cells, field effect transistor, nanogenerators and photodetectors have already been demonstrated using III-nitride materials.7–11 Recently, 1dimensional (1-D) nanostructures, particularly, vertically aligned nanorods/nanowires are widely studied for the application in versatile optoelectronic devices due to their high surface

ACS Paragon Plus Environment

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

to volume ratio, high crystalline quality and quantum confinement effect.12–15 Solar-blind photodetectors detect lights in the ultraviolet-c (UV-C) region (wavelengths shorter than 280 nm), have important applications in flame detection, missile plume sensing, ozone layer monitoring, UV astronomy, water purification, chemical–biological agent sensing, submarine and space communication, and medical applications.16,17 In view of simple operation, smaller and lightweight systems, and low cost, semiconductor-based UV-C detectors are considered as a potential alternative to the photomultiplier tubes. For many years, diamond, β-Ga2O3, ZnMgO and AlGaN have been proposed as possible candidates for the application in UV-C photodetectors.18–22 Among these, the AlGaN material system is very promising for UV detection, because of the adjustable cut-off wavelength by varying the Al contents between 365 nm and 210 nm and, therefore, the realization of solar-blind detectors with tailored spectral properties is possible. In the last few years, epitaxial AlGaN thin film based UV-C photodetector was investigated by a number of groups. For example, Tut et al. reported on the high performance solar-blind Al0.4Ga0.6N p-i-n photodetectors displaying photoresponsivity of 0.093 A/W at reverse bias of 40 V.22 In this context, Albrecht et al. have shown the UV-C photodetectors fabricated with similar AlGaN p-i-n structure with an external quantum efficiency of ~ 55 %.16 Knigge et al. demonstrated a simple Schottky type metal-semiconductor-metal Al0.4Ga0.6N photodetectors which exhibited external quantum efficiency of ~ 77 % and photoresponsivity of ~ 100 mA/W.23 Monroy et al. reported ultraviolet photoconductive detectors based on Si-doped Al0.35Ga0.65N epitaxial layers grown on sapphire displaying highest photoresponsivity of ~ 100 A/W.24Another innovative approach relies on the use of AlGaN nanostructures and it has drawn significant research attention as potential candidate for high performance UV-C photodetectors. Furthermore, 1-D or quasi 1-D AlGaN nanostructures offer advantages of drastically reduced dislocation densities, increased surface area for enhanced light absorption leads to enhance electron-hole pair generation, direct path for carrier transport and thus high performance photodetectors can be achieved. However, the growth of 1-D nanostructures of ternary compound semiconductor AlxGa1-xN system with tunable band gap at 3.4 - 6.2 eV, has been limited by the complex multicomponent phase diagram and inhomogeneous composition distribution. A few 1-D AlxGa1xN nanostructures have been synthesized using chemical vapor deposition (CVD), metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE).25–31 Hong et al. synthesized AlxGa1−xN nanowires over the whole compositional range by the CVD technique using Al/Ga/NH3 as source materials.27 He et al. have reported the growth of AlGaN nanocones across the complete composition range by CVD using different source materials, namely, AlCl3/GaCl3/NH3.32 Chen et al. have investigated the evolution of various AlxGa1−xN nanostructures depending on the CVD growth parameters.33 In addition, optoelectronic devices have also been demonstrated utilizing MBE grown AlGaN nanowires operating in UV-C region.34–36 Despite of these intensive studies, the controlled synthesis of vertically aligned compositionally uniform 1-D AlxGa1-xN nanorods (NRs) is still remains challenging for extensive optoelectronic applications, particularly in ultraviolet light emitters and photodetectors. Further experimental study is required to realize the controlled


Page 2 of 17

Page 3 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

fabrication of vertically aligned 1-D AlxGa1−xN nanostructures. Moreover, photoconductive AlGaN nanorods based UV-C photodetector with simple device structure has not been reported. In this article, we report the growth of Si-doped n-type compositionally uniform Al0.45Ga0.55N nanorods on Si (111) substrate using a horizontal MOCVD system. A detailed and systematic investigation of morphological, structural and optical properties of the grown AlGaN nanorods has been carried out. Additionally, a planner photoconductive device has been fabricated with the realized AlGaN nanorods. These results show the realized nanorods are high quality single crystals and display strong excitonic band edge emission in the UV-C region. Fabricated photoconductive device displays photoresponsivity of ~ 115 mA/W together with a sensitivity of ~ 64 % in the wavelength range of 250-276 nm.

Experimental AlxGa1-xN nanorods were grown on Si (111) substrate employing a home designed horizontal MOCVD system. Trimethylaluminium (TMAl), Trimethygallium (TMGa) and ammonia (NH3) were used as source materials for aluminium (Al), gallium (Ga) and nitrogen (N2), respectively. For n-type doping Silane (SiH4) was used as source and Hydrogen (H2) was supplied as carrier gas. All the chemicals were used without any further purification. We adapted 2-step pulsed growth process to grow undoped GaN (u-GaN) nucleation seeds on Si (111) for growing n-AlGaN nanorods. Prior to the deposition, Si (111) substrate was cleaned ultrasonically using acetone (CH3COCH4) and methanol (CH3OH). Then the substrate was subjected to a solution containing DI water:H2O2:H2SO4 (1:1:3), and subsequently to a mixture of H2O:HF (50:1) to remove the inorganic impurity and native oxide layer. Finally, the substrate was dried in N2 flow and used for the growth of nanorods. After cleaning, the Si substrate was subjected to a DC magnetron sputtering system to deposit Au nanoparticles for 25 seconds followed by the deposition of trimethylgallium (TMGa) for 30 seconds at 600°C. Then the Si (111) substrate was annealed at 650°C for 10 min with a reactor pressure of 600 Torr to form AuxGay nanodroplets.37 The size of growing nanorods and their density is conveniently controlled by the size and density of AuxGay droplets. Secondly, we deposited u-GaN seed layers on AuxGay nanodroplets for a short time (3/3 min, 15 pairs) by introducing TMGa and NH3 at the temperature of 730°C via pulsed precursor flow method. Once the AuxGay nanodroplets activated by the u-GaN seed layers via VLS growth mechanism, 37,38 we proceeded to deposit Si doped n-AlGaN nanorods on the u-GaN seeds using pulsed growth mode. During the pulsed mode, growth temperature and reactor pressure were kept constant at 900°C and 600 Torr, respectively. TMGa, TMAl and SiH4 respectively were supplied at flow rates of 0.3 sccm, 0.5 sccm, and 10 sccm during the first injection step. Then NH3 injection step proceeded at a flow rate of 1 SLM. Each precursor was flown for 3 minute, 15 pairs sequentially. The entire growth process and device fabrication is schematically represented in Figure 1.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Schematic representation of the n-AlGaN nanorods growth and fabrication process of photoconductive device.

The surface morphology of the grown n-AlGaN nanorods was analyzed by using fieldemission scanning electron microscopy (FE-SEM, Hitachi S-7400, Hitachi, Japan) with an operating voltage of 15 kV and 13° tilt-view. Single-crystal X-ray diffraction (XRD) measurements were performed using a Rigaku diffractometer equipped with a Cu-Kα radiation source. The crystallinity was examined by a JEM-ARM-200F Cs-corrected-field emission scanning transmission electron microscopy (Cs-corrected-FE-STEM) instrument equipped with X-ray dispersive spectroscopy (EDX) facility operating at an accelerating voltage of 200 kV. The FE-STEM samples of the n-AlGaN nanorods structure were prepared by being coated with platinum using a dual-beam focused ion beam (FIB, Quanta 3D FEG) technique with a beam current of 65 nA and a resolution of 7 nm at 30 kV. The optical properties of the n-AlGaN nanorods were investigated by cathodeluminescence (CL) spectroscopy at low temperature using the FE-SEM system equipped with backscattered/CL detector (spectral range : 200 – 1800 nm). Finally, a planner photoconductive device has been fabricated with grown n-AlGaN nanorods by depositing two Ti/Au metal electrodes. The current-voltage (I - V) characteristics of the photoconductive device were measured in dark and under illumination of solar simulator (McScience Lab 100, maximum power density 100 mW/cm2). A monochromator (Oriel Cornerstone 130) was used to provide the monochromatic light incident on the channel.

Results and Discussion The surface morphology of the MOCVD grown Si-doped n-AlGaN nanorods was characterized by FE-SEM. Figures 2 (a) shows the FE-SEM image of u-GaN seeds grown on Si substrate. Uniformly distributed u-GaN seeds are consist of smaller sized nanorods and the


Page 4 of 17

Page 5 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

average length of the u-GaN seed is ~ 0.22 µm, as shown in the inset of Figure 2 (a). The FESEM image of Si-doped n-AlGaN nanorods grown on u-GaN seeds is shown in Figure 2(b). FE-SEM image clearly reveals the growth of nearly vertically aligned hexagonal shaped nAlGaN nanorods over the u-GaN seeds. Although, the lengths of the nanorods are found to be slightly non-uniform, however most of the nanorods are micron in lengths. The average length and diameter of the nanorods are measured to be ~ 2.0 µm and ~ 0.2 µm, respectively, corresponding to an aspect ratio of ~ 10. In addition, the hexagonal nanorods are terminated with pyramidal r-plane tip, usually observed in MOCVD grown GaN nanorods.37,39

Figure 2. FE-SEM images of the MOCVD grown (a) u-GaN seeds and (b) n-AlGaN nanorods (13.0° tilt-view). Insets show the magnified view of a single nanorod.

The crystal structure and orientation of n-AlGaN nanorods have been characterized by X-ray diffraction (XRD). Figure 3 (a) shows the full scan XRD pattern of n-AlGaN nanorods in the 2θ range of 30-80°, in which the XRD pattern of u-GaN seed is also included. Both the samples exhibit a strong reflection from (0002) plane, indicating their preferred c-axis orientation. However, the presence of reflection from asymmetric 1013 plane in the diffraction patterns is an indication of vertical misalignment of nanorods, as observed in FESEM images. The slow scan XRD patterns of n-AlGaN nanorods and u-GaN in the 2θ ranges of 34-37° are shown in Figure 3 (b). The XRD pattern of u-GaN displays a single peak at 2θ value of 34.65°, corresponding to (0002) reflection of wurtzite GaN. Interestingly, the XRD pattern of n-AlGaN nanorods exhibits a strong GaN (0002) peak at 2θ = 34.6°, along with additional peaks at 2θ = 35.3° and 36.1°. The peak at 2θ = 36.1° is identified as the reflection from (0002) plane of hexagonal AlN. Thus, it can be established that the diffraction peak at 2θ = 35.3° is associated with the reflection from (0002) crystal plane of hexagonal n-AlGaN. The (0002) peak positions for both GaN and AlN are closed to the respective JCPDS values of bulk materials. Therefore, it is also confirmed that the realized AlGaN nanorods are nearly relaxed, without significant strain. The lattice parameter c was calculated from the 2θ peak position of (0002) plane and the estimated value is ~ 5.095 Å, which is nearly between those of pure AlN (c=4.997 Å) and GaN (c=5.189 Å) as mentioned in JCPDS file number 79-2497 for AlN and JCPDS file number 76-0703 for GaN. Using Vegard’s law, the Al-mole fraction


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of the AlxGa1-xN nanorods is estimated to be ~ 0.45.40

Figure 3. (a) Full scan XRD patterns of u-GaN seeds and n-AlGaN nanorods; (b) Slow scan XRD patterns of uGaN seeds and n-AlGaN nanorods in the 2θ ranges of 34 - 37°, showing reflection from (0002) crystal plane.

Figure 4. (a) FE-SEM image of single n-AlGaN nanorod; (b) Monochromatic CL mapping of the grown nAlGaN nanorod; (c) Blue mapping of the grown n-AlGaN nanorod.


Page 6 of 17

Page 7 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

The optical properties of n-AlGaN nanorods were assessed by low temperature (77 K) and temperature dependent CL spectroscopy. Figure 4 (a) shows the FE-SEM image of an isolated single n-AlGaN nanorod. The presence of u-GaN seeds at the bottom of n-AlGaN nanorod is a clear evidence of the growth of n-AlGaN nanorods over the u-GaN seeds. The monochromatic CL mapping and blue mapping of n-AlGaN nanorod are shown in Figures 4 (b) and (c), respectively. Taking the advantages of the high spatial resolution of CL technique, the variation of the emission spectra along the length of a single nanorod is analyzed. Figure 5 (a) shows the spatial positions that corresponds to the spot mode CL spectra presented in Figure 5 (b). The CL spectra of the n-AlGaN nanorod recorded at various spots display an UV emission peak at 276 nm. The UV emission peak is attributed to the excitonic band edge emission, corresponding to the optical band gap of ~ 4.49 eV at 77 K for the AlGaN nanorod with Al content of ~ 0.45.41–43 However, a variation in the intensity of the band edge emission can be observed in the CL spectra recorded at different spots. This variation may be related to the non-uniform diameter and inhomogeneous distribution crystalline defects along the length of the nanorod. The low temperature CL (77 K) spectra of the n-AlGaN nanorod at different incident current is shown in Figure 5 (c). As expected, the band edge emission intensity monotonously increases with increase of the incident current.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. (a) FE-SEM image of single n-AlGaN nanorod captured during low temperature CL measurement at various spots (as indicated); (b) Low temperature CL spectra recorded at 77 K at different selected spots; (c) Low temperature CL spectra recorded at different incident currents; (d) Temperature dependent CL spectra of nAlGaN nanorod in the temperature range from 77 to 300 K.

The temperature dependent CL spectra of n-AlGaN nanorod measured in the temperature range from 77 to 300 K is shown in Figure 5 (d). A strong variation in the excitonic band edge emission peak can be seen with increasing the temperature. The intensity of the emission peak is gradually decreases with increasing the temperature, due to the thermal activation of localized excitons.41,42 Concurrently, a significant redshift (~ 12 nm) in the UV band edge emission peak is observed as the temperatures increases from 77 K to 300 K. Further, the width of the band edge emission is also increases with temperature. The temperature dependent behavior of band gap of a semiconductor, which typically decreases with increase of temperature and is often described by the Varshni formula. The shift in the relative position of the conduction and valence bands is attributed to the temperature


Page 8 of 17

Page 9 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

dependent dilatation of the lattice and electron lattice interaction.41,44 Using the Varshni formula, the energy versus temperature plot can be fitted by44

T = 0 −

(1)

where 0 describes the energy gap at zero temperature, B = (3.7 ± 0.2)×10-4 eV/K and ΘD = 400 ± 40 K.41,45 The value of 0 is estimated to be ~ 4.61 eV for the grown Al0.45Ga0.55N nanorods. The Al content in the grown n-AlGaN nanorods can also be estimated from bowing equation using the optical band gap obtained from the RT-CL spectra. The bowing equation is given by

= 1 − + − 1 −

(2)

where x is the Al content and bowing parameter b is about 1.3.46 The Al content is found to be Al0.45Ga0.55N using equation (2). The obtained value of Al content (x) is similar to the value calculated from XRD results, indicating the band gap is nearly a linear function of the lattice parameter. Cs-corrected-FE-STEM technique was employed to further study the crystallinity of realized n-AlGaN nanorods and these results are presented in Figure 6. Cs-corrected-FE-STEM image of an isolated single n-AlGaN nanorod is shown in Figure 6 (a). The morphological features of the nanorods are well matched with the corresponding FE-SEM images. No extended defects, such as misfit dislocations and stacking faults are observed in the n-AlGaN nanorod. High resolution lattice image shows that the n-AlGaN nanorod grows along directions with an interplanner spacing of ~ 0.255 nm. A representative selected area electron diffraction (SAED) pattern of a single nanorod is shown in Figure 6 (c). The indexing of the spots in the electron diffraction pattern along with the zone axis < 1120 > is also shown in the figure. The spot pattern in the electron diffraction confirms that the nanorods are single crystalline in nature without any as-grown defects or secondary phases. Figure 7 (a-d) displays the elemental composition of the n-AlGaN nanorods analyzed by energy dispersive X-ray spectroscopy (EDX). All the desired elements, namely Al, Ga and N, are seen to be present in the nanorod. More importantly, EDX mapping of Al, Ga and N along the radial and growth directions showed that, all the elements are uniformly distributed in n-AlGaN nanorods. The Al content obtained from the EDX elemental count data (Figure 7 (b) and (d)) along the growth and radial direction is similar to the estimation from XRD and CL spectra.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. (a) Cs-corrected-FE-STEM image of an isolated n-AlGaN nanorod; (b) High magnification TEM image of the realized n-AlGaN nanorod showing no defects and dislocations; (c) SAED pattern of the isolated single n-AlGaN nanorod; (d) Inverse fast Fourier transform (IFFT) lattice image for measuring the interplanar spacing for the (0002) crystal plane.


Page 10 of 17

Page 11 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 7. (a) EDX elemental mapping along the radial direction of n-AlGaN nanorod (false color); (b) EDX elemental count data along the radial direction of n-AlGaN nanorod; (c) EDX elemental mapping along growth direction of n-AlGaN nanorod (false color); (d) EDX elemental count data along growth direction of n-AlGaN nanorod.

In order to investigate the applicability towards the ultraviolet-c (UV-C) optoelectronic devices, a planner photoconductive device has been fabricated with MOCVD grown nAlGaN nanorods. The schematic diagram of the fabricated device is shown in Figure 1 (f). IV characteristics of the photoconductive device were measured in dark and under illumination of solar simulator. Figure 8 (a) shows the I-V characteristics of the photoconductive device measured in dark (black line) and under illumination of UV light (red line). After illumination of UV light an enhancement in the current is observed and the corresponding photocurrent (IP = Ilight - Idark) is ~ 8.74 mA at bias voltage of 3 V. Figure 8 (b) represents the I-V characteristics of the device measured at different incident light power densities. The photocurrent increases with increase in the incident light power density, as expected. The dependence relation is typically expressed by an power law (Ip∝Pθopt), where θ is an empirical value (usually smaller than unity).47,48 The θ value can be obtained by fitting the measured data (shown in the inset of Figure 8(b)) and estimated to be ~ 0.64. It is noted that the photocurrent displays a non-linear light intensity dependent characteristics. The nonunity exponentials have also been observed previously in many nanostructure-based photodetectors, which are originated from the complex processes of electron–hole generation,


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

trapping and recombination in the semiconductor.49–51 The photoresponsivity (IP/(Illuminated area × Power density of light)) of the photoconductive device was calculated at different bias voltage and plotted against the bias voltage, as shown in Figure 8 (c). The photoresponsivity displays nearly a linear dependency on the bias voltage and the photoresponsivity is ~ 115 mA/W at 3 V bias. The spectral sensitivity (IP/Idark) was also studied in order to evaluate the wavelength selectivity of the fabricated photoconductive device. Figure 8 (d) shows the spectral sensitivity curves of the photoconductive device measured at 3 V bias. The sensitivity of the device is nearly constant in the UV region (250-276 nm) and maximum sensitivity is ~ 64%. From Figure 8 (b) it is also seen that the sensitivity of the device significantly decreases in the higher wavelength region (> 276 nm), indicating higher degree of wavelength selectivity of the photoconductive device. Although the photoresponsivity of the device remains below the epitaxial film based photoconductors7,52, the realized AlGaN nanorods promise to be beneficial for the fabrication of low-dimensional optoelectronic devices operating in UV-C region.

Figure 8. Photoresponse of photoconductive device fabricated with MOCVD grown n-AlGaN nanorods; (a) The I-V characteristics photoconductive device measured in dark (dark line) and under UV illumination (red line); (b) Photocurrent Vs. bias voltage at different light power densities (inset shows the photocurrent as a function of illumination light intensity at 3 V bias); (c) Photoresponsivity Vs. bias voltage plot under UV illumination; (d) Wavelength dependent sensitivity of photoconductive device measured at 3.0 V.


Page 12 of 17

Page 13 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Conclusion In summary, uniformly distributed and nearly vertically aligned Si-doped n-type compositionally homogeneous Al0.45Ga0.55N nanorods have been successfully grown on uGaN seeded Si substrate by MOCVD technique. The surface morphology, crystal structure and optical properties of n-AlGaN nanorods were studied by FE-SEM, XRD, FE-STEM and CL measurements. I-V measurements in dark and under illumination were employed to characterize the fabricated planner photoconductive device. The MOCVD grown n-AlGaN nanorods are of ~ 2.0 µm in length and ~ 0.2 µm in diameter leading to an aspect ratio of ~10. XRD results show that the nanorods are hexagonal crystal structure and preferentially c-axis oriented. FE-STEM studies reveal the high single crystalline nanorods are grown along (0002) direction, without any as-grown defects, dislocations or secondary phases. The results of low temperature and temperature dependent CL measurements evidenced the grown n-AlGaN nanorods are of good optical quality with lower crystalline defects and the optical band gap is ~ 4.31 eV at room temperature. In order to study photoresponse characteristics a planner photoconductive device has been fabricated utilizing the n-AlGaN nanorods. The photoresponsivity of the planer photoconductive device is ~ 115 mA/W at a bias voltage of 3 V. The sensitivity of the device is ~ 64% in the UV region (250-276 nm), which decreases substantially in the higher wavelength region, owing to the higher degree of wavelength selectivity. However, further optimization of growth process and response characteristics of photoconductor utilizing n-AlGaN nanorods may explore the plausible implication in UV-C photodetectors.

Competing Interests The authors declare no competing financial interest. Conflicts of interest There are no conflicts of interest to declare. Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A2A01002877) and (No. 2015R1A4A1042417). References (1)

Calarco, R. InN Nanowires: Growth and Optoelectronic Properties. Materials (Basel). 2012, 5, 2137– 2150.

(2)

Bugallo, A. D. L.; Rigutti, L.; Jacopin, G.; Julien, F. H.; Durand, C.; Chen, X. J.; Salomon, D.; Eymery, J.; Tchernycheva, M. Single-Wire Photodetectors Based on InGaN/GaN Radial Quantum Wells in GaN Wires Grown by Catalyst-Free Metal-Organic Vapor Phase Epitaxy. Appl. Phys. Lett. 2011, 98, 233107.

(3)

Nguyen, H. P. T.; Zhang, S.; Cui, K.; Han, X.; Fathololoumi, S.; Couillard, M.; Botton, G. A.; Mi, Z. P-


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Type Modulation Doped InGaN/GaN Dot-in-a-Wire White-Light-Emitting Diodes Monolithically Grown on Si(111). Nano Lett. 2011, 11, 1919–1924. (4)

Taniyasu, Y.; Kasu, M.; Makimoto, T. An Aluminium Nitride Light-Emitting Diode with a Wavelength of 210 Nanometres. Nature 2006, 441, 325–328.

(5)

Meneghesso, G.; Verzellesi, G.; Danesin, F.; Rampazzo, F.; Zanon, F.; Tazzoli, A.; Meneghini, M.; Zanoni, E. Reliability of GaN High-Electron-Mobility Transistors: State of the Art and Perspectives. IEEE Trans. Device Mater. Reliab. 2008, 8, 332–343.

(6)

Guo, W.; Zhang, M.; Banerjee, A.; Bhattacharya, P. Catalyst-Free InGaN/GaN Nanowire Light Emitting Diodes Grown on (001) Silicon by Molecular Beam Epitaxy. Nano Lett. 2010, 10, 3356–3359.

(7)

Sang, L.; Liao, M.; Sumiya, M. A Comprehensive Review of Semiconductor Ultraviolet Photodetectors: From Thin Film to One-Dimensional Nanostructures. Sensors 2013, 13, 10482–10518.

(8)

Shur, M. S.; Davis, R. F. GaN-Based Materials and Devices, World Scientific Publishing Co. Pte. Ltd. 2004.

(9)

Jones, E. A.; Member, S.; Wang, F. F.; Costinett, D. Review of Commercial GaN Power Devices and GaN-Based Converter Design Challenges. IEEE Journal of Emerging and Selected Topics in Power Electronics 2016, 4, 707–719.

(10)

Mokkapati, S.; Jagadish, C. III-V Compound SC for Optoelectronic Devices. Mater. Today 2009, 12, 22–32.

(11)

Kuech, T. F. III-V Compound Semiconductors : Growth and Structures. Prog. Cryst. Growth Charact. Mater. 2016, 62, 352–370.

(12)

Huang, Y.; Duan, X.; Cui, Y.; Lieber, C. M. Gallium Nitride Nanowire Nanodevices. Nano Lett. 2002, 2, 101–104.

(13)

Huang, C. Te; Song, J.; Lee, W. F.; Ding, Y.; Gao, Z.; Hao, Y.; Chen, L. J.; Wang, Z. L. GaN Nanowire Arrays for High-Output Nanogenerators. J. Am. Chem. Soc. 2010, 132, 4766–4771.

(14)

Chatterjee, U.; Park, J. H.; Um, D. Y.; Lee, C. R. III-Nitride Nanowires for Solar Light Harvesting: A Review. Renew. Sustain. Energy Rev. 2017, 79, 1002–1015.

(15)

Ra, Y.; Navamathavan, R.; Park, J.; Lee, C. Coaxial InxGa1–xN/GaN Multiple Quantum Well Nanowire Arrays on Si(111) Substrate for High-Performance Light-Emitting Diodes. Nano Lett. 2013, 13, 3506– 3516.

(16)

Albrecht, B.; Kopta, S.; John, O.; Kirste, L.; Driad, R.; Köhler, K.; Walther, M.; Ambacher, O. AlGaN Ultraviolet A and Ultraviolet C Photodetectors with Very High Specific Detectivity D*. Jpn. J. Appl. Phys. 2013, 52, 08JB28.

(17)

Biyikli, N.; Kimukin, I.; Aytur, O.; Ozbay, E.; We, A. Solar-blind AlGaN-based p-i-n photodiodes with low dark current and high detectivity. IEEE Photon. Technol. Lett. 2004, 16, 1718–1720.

(18)

Knigge, A.; Brendel, M.; Brunner, F.; Einfeldt, S.; Knauer, A.; Kueller, V.; Weyers, M. AlGaN photodetectors for the UV-C spectral region on planar and epitaxial laterally overgrown AlN/sapphire templates. Phys. Status Solidi C 2013, 10, 294–297.

(19)

Oshima, T.; Okuno, T.; Arai, N.; Suzuki, N.; Hino, H.; Fujita, S. Flame Detection by a β-Ga2O3-Based Sensor. Jpn. J. Appl. Phys. 2009, 48, 011605.

(20)

Fan, M.; Liu, K.; Chen, X.; Zhang, Z.; Li, B. Realization of Cubic ZnMgO Photodetectors for UVB Applications. J. Mater. Chem. C 2014, 3, 313–317.

(21)

Mendoza, F.; Makarov, V.; Weiner, B. R.; Morell, G. Solar-Blind Field-Emission Diamond Ultraviolet Detector. Appl. Phys. Lett. 2015, 107, 201605.


Page 14 of 17

Page 15 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

(22)

Tut, T.; Yelboga, T.; Ulker, E.; Ozbay, E.; Tut, T.; Yelboga, T.; Ulker, E.; Ozbay, E. Solar-blind AlGaN-based p-i-np-i-n photodetectors with high breakdown voltage and detectivity. Appl. Phys. Lett.. 2008, 92, 103502.

(23)

Knigge, A.; Brendel, M.; Brunner, F.; Einfeldt, S.; Knauer, A.; Kueller, V.; Weyers, M. AlGaN Photodetectors for the UV-C Spectral Region on Planar and Epitaxial Laterally Overgrown AlN/sapphire Templates. Phys. Status Solidi Curr. Top. Solid State Phys. 2013, 10, 294–297.

(24)

Monroy, E.; Calle, F.; Garrido, J. A.; Youinou, P.; Muñoz, E.; Omnès, F.; Beaumont, B.; Gibart, P. Si-Doped AlxGa1-xN Photoconductive Detectors. Semicond. Sci. Technol. 1999, 14, 685–689.

(25)

Ambacher, O. Growth and Applications of Group III-Nitrides. J. Phys. D. Appl. Phys. 1998, 31, 2653.

(26)

Ristic, J.; Sanchez-Garcia, M. A.; Calleja, E.; Sanchez-Paramo, J.; Calleja, J. M.; Jahn, U.; Ploog K. H. AlGaN Nanocolumns Grown by Molecular Beam Epitaxy : Optical and Structural Characterization. Phys. Status Solidi 2002, 66, 60–66.

(27)

Hong, L.; Liu, Z.; Zhang, X. T.; Hark, S. K. Self-Catalytic Growth of Single-Phase AlGaN Alloy Nanowires by Chemical Vapor Deposition. Appl. Phys. Lett. 2006, 89, 193105.

(28)

Su, J.; Cui, G.; Gherasimova, M.; Tsukamoto, H.; Han, J.; Ciuparu, D.; Lim, S.; Pfefferle, L.; He, Y.; Nurmikko, A. V.; et al. Catalytic Growth of Group III-Nitride Nanowires and Nanostructures by Metalorganic Chemical Vapor Deposition. Appl. Phys. Lett. 2005, 86, 013105.

(29)

Su, J.; Gherasimova, M.; Cui, G.; Tsukamoto, H.; Han, J.; Onuma, T.; Kurimoto, M.; Chichibu, S. F.; Broadbridge, C.; He, Y.; et al. Growth of AlGaN Nanowires by Metalorganic Chemical Vapor Deposition. Appl. Phys. Lett. 2005, 87, 183108.

(30)

Choi, H.-J.; Johnson, J. C.; He, R.; Lee, S.-K.; Kim, F.; Pauzauskie, P.; Goldberger, J.; Saykally, R. J.; Yang, P. Self-Organized GaN Quantum Wire UV Lasers. J. Phys. Chem. B 2003, 107, 8721–8725.

(31)

Wang, X.; Song, J.; Zhang, F.; He, C.; Hu, Z.; Wang, Z. Electricity Generation Based on OneDimensional Group-III Nitride Nanomaterials. Adv. Mater. 2010, 22, 2155–2158.

(32)

He, C.; Wu, Q.; Wang, X.; Zhang, Y.; Yang, L.; Liu, N.; Zhao, Y.; Lu, Y.; Hu, Z. Growth and Characterization of Ternary AlGaN Alloy Nanocones across the Entire Composition Range. ACS Nano 2011, 5, 1291–1296.

(33)

Chen, F.; Ji, X.; Zhang, Q. Morphology-Controlled Synthesis and Structural Characterization of Ternary AlxGa1-xN Nanostructures by Chemical Vapor Deposition. CrystEngComm 2015, 17, 1249–1257.

(34)

Sadaf, S. M.; Zhao, S.; Wu, Y.; Ra, Y. H.; Liu, X.; Vanka, S.; Mi, Z. An AlGaN Core-Shell Tunnel Junction Nanowire Light-Emitting Diode Operating in the Ultraviolet-C Band. Nano Lett. 2017, 17, 1212–1218.

(35)

Zhao, S.; Liu, X.; Woo, S. Y.; Kang, J.; Botton, G. A.; Mi, Z. An Electrically Injected AlGaN Nanowire Laser Operating in the Ultraviolet-C Band. Appl. Phys. Lett. 2015, 107, 043101.

(36)

Li, K. H.; Liu, X.; Wang, Q.; Zhao, S.; Mi, Z. Ultralow-Threshold Electrically Injected AlGaN Nanowire Ultraviolet Lasers on Si Operating at Low Temperature. Nat. Nanotechnol. 2015, 10, 140– 144.

(37)

Ra, Y. H.; Navamathavan, R.; Park, J. H.; Song, K. Y.; Lee, Y. M.; Kim, D. W.; Jun, B. B.; Lee, C. R. Highly Uniform Characteristics of GaN Nanorods Grown on Si(111) by Metalorganic Chemical Vapor Deposition. Jpn. J. Appl. Phys. 2010, 49, 091003.

(38)

Ra, Y.-H.; Navamathavan, R.; Lee, C.-R. Growth Characteristics of Uniaxial InGaN/GaN MQW/n-GaN Nanowires on Si(111) Using MOCVD. CrystEngComm 2012, 14, 8208–8214.

(39)

Ra, Y.-H.; Navamathavan, R.; Kang, S.; Lee, C.-R. Different Characteristics of InGaN/GaN Multiple Quantum Well Heterostructures Grown on M- and R-Planes of a Single N-GaN Nanowire Using


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Metalorganic Chemical Vapor Deposition. J. Mater. Chem. C 2014, 2, 2692-2701. (40)

Görgens, L.; Ambacher, O.; Stutzmann, M.; Miskys, C.; Scholz, F.; Off, J. Characterization of InGaN Thin Films Using High-Resolution X-Ray Diffraction. Appl. Phys. Lett. 2000, 76, 577.

(41)

Bell, A.; Srinivasan, S.; Plumlee, C.; Omiya, H.; Ponce, F. A.; Christen, J.; Tanaka, S.; Fujioka, A.; Nakagawa, Y. Exciton Freeze-out and Thermally Activated Relaxation at Local Potential Fluctuations in Thick Al xGa 1-xN Layers. J. Appl. Phys. 2004, 95, 4670–4674.

(42)

Nepal, N.; Li, J.; Nakarmi, M. L.; Lin, J. Y.; Jiang, H. X. Exciton Localization in AlGaN Alloys. Appl. Phys. Lett. 2006, 88, 062103.

(43)

Coli, G.; Bajaj, K. K.; Li, J.; Lin, J. Y.; Jiang, H. X. Linewidths of Excitonic Luminescence Transitions in AlGaN Alloys. Appl. Phys. Lett. 2001, 78, 1829–1831.

(44)

Varshni, Y. P. Temperature Dependence of the Energy Gap in Semiconductors. Physica 1967, 34, 149– 154.

(45)

Najar, A.; Shafa, M.; Anjum, D. Synthesis, Optical Properties and Residual Strain Effect of GaN Nanowires Generated via Metal- Assisted Photochemical Electroless Etching. RSC Adv. 2017, 7, 21697–21702.

(46)

Stutzmann, M.; Ambacher, O.; Cros, a; Brandt, M. .; Angerer, H.; Dimitrov, R.; Reinacher, N.; Metzger, T.; Höpler, R.; Brunner, D.; et al. Properties and Applications of MBE Grown AlGaN. Mater. Sci. Eng. B 1997, 50, 212–218.

(47)

Wu, P.; Dai, Y.; Ye, Y.; Yin, Y.; Dai, L. Fast-Speed and High-Gain Photodetectors of Individual Single Crystalline Zn3P2 Nanowires. J. Mater. Chem. 2011, 21, 2563-2567 .

(48)

Prabhakar, R. R.; Mathews, N.; Jinesh, K. B.; Karthik, K. R. G.; Pramana, S. S.; Varghese, B.; Sow, C. H.; Mhaisalkar, S. Efficient Multispectral Photodetection Using Mn Doped ZnO Nanowires. J. Mater. Chem. 2012, 22, 9678-9683.

(49)

Lan, C.; Li, C.; Yin, Y.; Guo, H.; Wang, S. Single-Crystalline GeS Nanoribbons for High Sensitivity Visible-Light Photodetectors. J. Mater. Chem. C 2015, 3, 8074–8079.

(50)

Hansen, B. J.; Kouklin, N.; Lu, G.; Lin, I. K.; Chen, J.; Zhang, X. Transport, Analyte Detection, and Opto-Electronic Response of P-Type CuO Nanowires. J. Phys. Chem. C 2010, 114, 2440–2447.

(51)

Shaygan, M.; Davami, K.; Kheirabi, N.; Baek, C. K.; Cuniberti, G.; Meyyappan, M.; Lee, J.-S. SingleCrystalline CdTe Nanowire Field Effect Transistors as Nanowire-Based Photodetector. Phys. Chem. Chem. Phys. 2014, 16, 22687–22693.

(52)

Ozbay, E.; Biyikli, N.; Kimukin, I.; Kartaloglu, T.; Tut, T.; Aytür, O. High-Performance Solar-Blind Photodetectors Based on AlxGa1-xN Heterostructures. IEEE J. Sel. Top. Quantum Electron. 2004, 10, 742–751.


Page 16 of 17

Page 17 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

For Table of Contents Use Only

Manuscript title: Ultraviolet-C Photodetector Fabricated using Si-doped n-AlGaN Nanorods Grown by MOCVD Authors: San Kang, Uddipta Chatterjee, Dae-Young Um, Yeon-Tae Yu, In-Seok Seo and Cheul-Ro Lee

Compositionally uniform high crystalline quality Al0.45Ga0.55N nanorods have been synthesized by metal organic chemical vapor deposition (MOCVD) technique for the application in UV-C photodetectors. The photodetectors fabricated with nanorods displayed photoresponsivity of ~ 115 mA/W along with sensitivity of ~ 64 %, at 3 V bias in the UV-C region.


AlGaN Nanorods Grown by MOCVD

Recommend Documents