Surface-Induced Carrier Localization and Recombination

Subscriber access provided by Macquarie University

C: Physical Processes in Nanomaterials and Nanostructures

Surface-Induced Carrier Localization and Recombination Characteristics in InGaN/GaN Quantum Dots in Nanopillars Zilan Wang, Zhibiao Hao, Jiadong Yu, Lai Wang, Jian Wang, Changzheng Sun, Bing Xiong, Yanjun Han, Hongtao Li, and Yi Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11830 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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 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 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.

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 16 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

The Journal of Physical Chemistry

Surface-induced carrier localization and recombination characteristics in InGaN/GaN quantum dots in nanopillars Z. L. Wang†, Z. B. Hao*, J. D. Yu, L. Wang, J. Wang, C. Z. Sun, B. Xiong, Y. J. Han, H. T. Li and Y. Luo* Beijing National Research Center for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China Key words: surface modification; carrier localization; temperature dependent PL; InGaN/GaN quantum dots in nanopillars; Abstract In quantum structures and nanomaterials, surface conditions have significant influence on the material’s optical and electrical properties, hence surface modification is an inevitable and critical step during the fabrication process of optoelectronic devices. A comprehensive understanding on how surface conditions impact on the performance is a key issue, however, it is difficult to apply experimental techniques to study the surface physics in atomic scale. In this paper, the photoluminescence properties of InGaN/GaN quantum dots in nanopillar samples were carefully investigated and compared after applying various surface manipulation techniques. Spectroscopic results show that the localization features including recombination energy level and peak shift with temperature are extremely sensitive to surface treatment. Based on localized state ensemble (LSE) model, a quantitative analysis is performed taking into account of thermal activation and DOS distribution function of localization. The correlation between optical properties and the physical mechanism of surface modification is established. This spectroscopic technique along with the analytical method provide physical insights and reliable basis for nanoscale device evaluation and optimization. * [email protected] * [email protected] † Z. L. Wang is currently working at the College of Science, Civil Aviation University of China.

Introduction Nitride based nanostructures exhibit excellent photonic properties as their bulk counterparts along with extra properties owing to the nanoscale geometry. In particular, InGaN quantum dots (QD) in nanostructures have promising potential in optoelectronic applications such as nano LEDs [1-3], solar cells [4-6], laser diodes [7, 8] and single photon emitters [9-11]. However, there is still a big gap in the performance of InGaN QD nanostructures to that of the mature quantum well devices. For example, the GaN-based QD have achieved single-photon emission at room temperature [11], while the quality of the emitted photons, as characterized by the second-order correlation, still needs to be improved. And the degradation of intensity and broadening of line width for single photon emitters with increasing temperature can be associated to the existence of defect states [12]. Meanwhile, due to the large surface-to-volume ratio, it is inevitable that the

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

nanostructure has a large number of surface states and dangling bonds, and the rate of surface recombination is high [13, 14], so that the generated carriers can be easily trapped causing current leakage and luminescence quenching [15, 16]. Therefore, in the preparation of nano-devices, optimizing surface condition is a crucial step to ensure high performance. At present, the modification on the surface of nanostructures mainly focuses on the removal of surface states, including acid/alkali solution etching [17, 18], sulfides [19, 20] and dielectric films coating [21-25]. However, the detailed mechanisms regarding various surface manipulation techniques are difficult to quantify from the micro-perspective and it is mainly limited by the experimental characterization methods. Hence a comprehensive study of the passivation mechanism with respect to surface modification is lacking. In the InGaN materials, carrier localization and the related phenomena have significant effect on electrical and optical properties [26, 27]. As temperature increases, the carriers are thermally activated to form S-shaped or V-shaped peak shift reflected from the temperature dependent PL spectrum [28]. And the carrier lifetime is affected by localization expressed as tail state [29, 30] and Bi-exponential characteristics [31]. These localization effects can be quantitatively analyzed by the localized state ensemble (LSE) model [32-34] which offers numerical solution to the distribution of density of states (DOS). As a type of structural imperfection, the surface states could also be seen as localization centers that take part in the carrier recombination of the InGaN QD nanostructures. The localization effects under the influence of surface states could be significantly altered by different surface treatment that corresponds to diverse spectroscopic features. In this paper, the influence of surface modifications on the distribution of localized carriers in InGaN/GaN QD in nanopillar were systematically investigated. The sample under different surface conditions exhibit distinguished peak shift in photoluminescence (PL) with temperature. The distribution function of the carriers in localized states were obtained and compared through the LSE model. This non-destructive and convenient experimental characterization method provides a quantitative analysis of the recombination mechanism of QD in nanopillar and the lucid physical interpretation of surface modification. Experiment

Figure 1. (a) SEM image of QD in nanopillars with acquisition angle of 45 degrees. (b) Schematic diagram of sample preparation and passivation process including the top-down fabrication of QD


Page 2 of 16

Page 3 of 16 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


and the two-step passivation. The InGaN/GaN QD in nanopillar samples were fabricated by the top-down method which can be found in our previous work [35]. The as-grown single quantum well sample consists of 2-nm thickness well with 20% Indium composition and a 6-μm thickness GaN buffer layer. To start, the cylindrical nanopillar was obtained by ICP etching using a SiO2-nanosphere mask. Figure 1(a) shows the tilted SEM image of the nanocylinder having a height of 120 nm and diameter of 30 nm. In order to remove a large number of surface defects introduced during the preparation process, two-step surface passivation method is adopted, as schematically illustrated in Fig. 1(b). First, a 1:20 (mass ratio) diluted KOH solution is used for wet etching, while the sample is immersed for 10 s to reduce surface defects and then rinsed in deionized water. The etching duration is kept short as to avoid the corrosion of the active area as well as to maintain the nanopillar’s morphology. For comparison, some of the wet etched nanopillar samples were further coated with dielectric layer (Al2O3) for surface passivation. The Al2O3 film is deposited by atomic layer deposition (ALD) with substrate temperature of 200 oC using trimethyl aluminum (TMA) and H2O as precursors. The ALD cycle during growth is 100 based on previous experimental conditions as to keep the axial cladding thickness less than 5 nm. The Al2O3 layer introduces negative fixed charge forming an electric field to offset the effect of band bending [24, 36]. For PL characterization, the nanopillar samples were placed on the cold finger inside the Janis closed cycle cryostat with temperature ranging from 10 to 300 K, the excitation source came from a Kimmon 325 nm He-Cd laser with an output power of 28 mW. The laser beam was guided by a set of mirrors and focused to a 1 mm spot with power approximately 5 mW, and the photon density of laser excitation is around 1025 cm-3∙s-1. The PL signal was dispersed by a Jobin Yvon 550 monochromator with 0.5 nm resolution and then detected by a PMT. Discussion

Figure 2. PL plots at 10K, containing three samples, the pristine quantum dots sample (QD), the etched sample (EQD) and passivated sample (PQD). The interference fringe (IF) is fitted from PQD sample. The PL spectra of 3 sets of nanopillar samples were measured and compared, which are plotted in Fig. 2. The spectra at 10 K mainly consists of two emission bands, one is the near-band-edge



emission of intrinsic GaN in the ultraviolet region, the peak is located at 3.263 eV along with its phonon replica (PR) lines [37]; the other is the blue emission band located around 2.8-2.9 eV that originated from the InGaN QD, which is the focus of this study. For pristine nanopillar sample (without further surface treatment and denoted as QD for simplification), its emission peak (black curve) is located at 2.901 eV which yields a blue-shift comparing with the as-grown single quantum well at 2.737 eV. This shift in peak energy is a result of the increase in quantum confinement effect and a release of strain induced by the top-down etching [35]. After further wet etching in the KOH solution, the PL spectrum of etched QD samples (denoted as EQD, red curve) yields little difference in the emission intensity while its peak position slightly blue shifted which is consistent with the further increase of quantum confinement effect due to etching. On the other hand, the spectrum of sample after Al2O3 passivation (denoted as PQD, blue curve) is significantly different from the other two, with 5 times increase in emission intensity due to the enhancement in radiative recombination after passivation, while the red shift in peak position can be tentatively attributed to the added strain due to coating. In addition to enhancing the PL intensity, surface modification also alters the recombination mechanism of the carrier, which is manifested by variations in peak position and shape. The detailed physical mechanism of surface treatment will be discussed next. Meanwhile, the fine structure appearing in the emission peak shown in Fig. 2 is attributed to interference fringes (IF) from PL emission and substrate reflection [38, 39]. The magenta dashed line represents the interference distribution calculated from the thickness of buffer layer as well as the profile of PQD peak. In order to accurately identify the peak position from the PL band, the interference component of each spectra line is fitted and subtracted.

Figure 3. Contour lines of the PL intensity of quantum well (QW) and the other three QD samples. The emission features of InGaN materials is governed by the localization effect which is sensitive to temperature as the localized carriers can be de-localize due to the increase of thermal energy. Thus, temperature dependent PL measurements, from 10-300 K, were performed as to further


Page 4 of 16

Page 5 of 16 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


investigate the localization emission among these nanopillar samples. Figure 3 shows the contour plot of the PL intensity as a function of temperature for various samples. The vertical axis represents photon energy around the emission peak and the PL intensity is normalized and represented by different colors bands: the normalized intensity below 0.2 is colored in black while violet is used to represent the peak intensity (normalized intensity > 0.98). The violet color band visualizes the shift of PL peak position as a function of temperature. The four sets of data exhibit different temperature dependency which all deviate from the standard Varshni’s red shift, indicating the dominance of localized emission in these samples. First, consistent with previous reports [28], the emission peak of quantum well (QW) sample shows an S-shaped shift with temperature (first red-shifted and then blue-shifted) which is a typical result of the localization effect. The QD sample also shows S-shape shift with slight difference to that of QW indicating the carrier recombination process for QD is still driven by localization, although the localized center is different. The peak shift of EQD sample does not follow the S-shape trend as that of QD although they have similar PL spectra at low temperature (Fig. 2). The peak of EQD sample exhibits further red-shift in the temperature range of 100-200 K, meaning that the EQD sample is affected by different types of localized state. Finally, after passivation with Al2O3, the PQD sample shows another type of peak shift which is again different from EQD sample. The emission peak shifts to higher energy at initial temperature stage, and then shifts to lower energy till room temperature. In order to investigate these temperature dependent spectral features along with different surface modification techniques, quantitative analysis is carried out to elucidate detailed physics behind localization effect. In the past decades, various models, such as the band tail model [29], Eliseev model [40] and the newly developed localized state ensemble (LSE) model [32-34], were proposed to quantitively study carrier localization in nitride-based materials. Among them, the LSE model, proposed by Q. Li and S. J. Xu, successfully elucidated the localized emission for InGaN material regarding spectral features such as line shape, emission intensity and carrier lifetime. In this study, LSE model is adopted to analyze the surface induced localization features in InGaN QD. Assuming that the localized-state ensemble exhibits a Gaussian-type density of state, the LSE model describes the physical process of radiative recombination, non-radiative recombination, thermal escape and redistribution of carriers, and the peak position as a function of temperature is expressed as [32]:

E (T )  Eg 

T 2  x  k BT  T

Eq. (1)

where Eg is the bandgap at T=0. The second term is attributed to bandgap shrinkage,  is the Varshni parameter and  is the Debye temperature of the material. The third term represents carriers’ thermal redistribution within the localized states. kB is the Boltzmann constant, and x parameter characterizes the level of redistribution which can be solved by the following equation [32]

  2     ( E  E )/ k T xe   x   r e 0 a B   k BT     tr  x

Eq. (2)

where σ is the standard deviation of localized distribution, τr and τtr are the time constants characterizing the radiative recombination process and the thermal activation of carriers, respectively. τr/τtr represents the escape rate of carrier from a localization center. E0 is the mean



value of the DOS distribution while Ea is the energy level of which delocalized carriers have to overcome, and it is related to properties such as Indium composite, carrier lifetime, strain condition and etc.

Figure 4. The left side (a) - (d) show the peak shift with temperature of four samples fitted by LSE model. The black squares denote the peak energy at each temperature and red lines represent the fitting results. The dashed line in (c) indicates the peak movement obtained from the Varshni equation. The right side (e) – (h) show the Gaussian distribution of density of states (DOS) and carriers filling states affected by localization. Fig. 4(a)-(d) show the fitting curve of the PL peak position as a function of temperature using the LSE model where Table 1 listed the fitting parameters. The curves fitted according to Eq. (1) are in good agreement with PL data for the four set of samples. Fig. 4(e)-(h) are DOS distribution functions drawn according to the obtained fitting parameters, i. e. localization center E0 and the deviation σ from Eq. (2), as listed in Table 1. The red shaded region below the energy level Ea represents the filling states of localized carriers. As shown from the distribution figures (Fig. 4(e)(h)), the discrepancy between the value of filling energy level (Ea) to the center of DOS distribution (E0) in all samples is the key reason of the unusual peak shift with temperature which can be explained as follows: the configuration of peak shift could be divided into two situations, one is the case of Ea < E0, such as Sample QW and QD (Fig. 4(a), (b)). At the initial stage when temperature T < 80 K, localized carriers exhibit red-shift due to band gap shrinkage. As the temperature further increases from 80 K to 180 K, carriers are delocalized by thermal activation. The escaped carriers are re-distributed to higher energy state according to the DOS of localization, and hence resulting the blue shift in PL spectrum. At the stage approaching room temperature, the peak shift is again dominated by Varshni movement. These three stages consist of an S-shaped shift of peak positions in QW and QD samples since they have similar distribution of localized states. The other is the case of Ea > E0, shown in sample of EQD. The main difference is that, for EQD sample, carriers thermally activated from the localized center are re-distributed to the lower energy state according to the DOS of localization. Therefore, the peak position will be further red-shifted in addition to the


Page 6 of 16

Page 7 of 16 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


influence of Varshni shift (represented by the dashed line in Fig. 4(c) for comparison). For PQD sample, although the fitting results shows consistency with the case of Ea < E0 (Fig. 4(h)), the thermal escape of carriers is dominated at low temperature. Thus, the whole process contains only two stages, which is different from the conventional S-shaped movement. Table I The fitting parameter in LSE model for four samples. QW QD EQD PQD

E0 (eV) 2.752 2.944 2.901 2.852

σ (meV) 15.7 26.3 27.1 8.6

Ea(eV) 2.733 2.904 2.934 2.816

E0-Ea (meV) 19.2 39.8 -33.2 37.0

τr/τtr 100 735 332 42

The physical meaning of the obtained parameters and their relationship to the surface condition among the four samples is discussed as follows: For QD sample, the increase of Ea and E0 comparing with that of QW is attributed to the quantum confinement effect and the effect due to surface states. A large amount surface states were induced during the top-down fabrication process of the QD sample. These surface states act as the ‘capture centers’ of carriers and can be considered as newly formed localization centers with different energy levels. Thus, for QD sample, the DOS distribution tends to yield larger deviation (σ, 26.3 meV) due to the additional localization centers. Under the influence of surface states, the band structure near the surface is bended upwards [4143]. Therefore, the distribution range of carriers is extended to a higher energy level, corresponding to the rise of the mean value E0 of the distribution function. For EQD sample, surface damage and defects introduced by top-down fabrication were removed by KOH etching. Thus, the band structure that originally bending upward for QD sample is partly recovered [35] and the center of DOS distribution (E0) is shifted down from 2.944 eV to 2.901 eV. On the other hand, and the strain is further released as the size is slightly reduced. According to the QCSE effect, the weakened strain of the EQD sample corresponds to higher recombination energy. Hence Ea for EQD sample slightly rises compared to that of QD since that the position of Ea should locate above the recombination energy according to LSE model. Consequently, due to the divergent changes (decrease and increase) of E0 and Ea in EQD sample, the energy level of Ea is eventually higher than E0, which is the major difference from other three samples. The passivation effect of Al2O3 further reduces the surface state and the band bending. The carriers that are originally distributed at higher energy level due to surface states can return to lower energy levels. Accordingly, the deviation of the carrier distribution function σ decreases (27.1 meV to 8.6 meV) and the center E0 value decreases (from 2.901 eV to 2.852 eV). Based on the results of the spectrum, the coating of Al2O3 increases the strain of the nanopillar, and Ea moves to the lower energy level, so the relationship between E0 and Ea returns back to the case of Ea< E0. The last item in Table I is the time constant related to the recombination efficiency. The internal quantum efficiency of the sample can be written as 𝜂 = 1/(1 + 𝜏𝑟/𝜏𝑛𝑟). In LSE model, 𝜏𝑟 represents the lifetime of radiative recombination, and 𝜏𝑡𝑟 represents the lifetime of carrier thermal activation which is proportional to the 𝜏𝑛𝑟, the lifetime of non-radiative recombination [28]. Thus, the term of 𝜏𝑟/𝜏𝑡𝑟 is proportional to 𝜏𝑟/𝜏𝑛𝑟. According to the fitting result in Table I, the smallest 𝜏𝑟/𝜏𝑡𝑟 indicates the highest 𝜂 for PQD sample, which is in good agreement with the highest PL intensity shown in Fig. 2. The physical reason is that the weakened band bending



increases the electron hole overlap integral, which is favorable for radiation recombination and 𝜏𝑟 becomes shorter. Meanwhile, the reduced surface states decrease the non-radiative recombination center, and 𝜏𝑛𝑟 becomes longer. In addition, the trend of FWHM and integrated intensity with temperature can be quantitatively described by LSE model [44]. The energy separation of Ea and E0 is a decisive parameter indicating the deviation of localized carriers from non-localized carriers during temperature dependent luminescence. And both types of emission yield a decay trend with rising temperature, however, the localized emission has a slow decay process comparing with non-localized emission. Therefore, the luminescence properties of carrier localization and recombination can be analyzed by LSE fitting, and surface modifications have significant impact on the performance of nano-scale QD devices. Conclusion In this paper, InGaN/GaN QDs in nanopillar and their passivated samples including alkaline solution etched and Al2O3 coated samples were studied. These surface modification methods have slight effect on the size and morphology of the samples, but have an important influence on their localization characteristics. The surface conditions, i.e. surface states, are altered by the passivation process which give rises to different localization levels and the corresponding DOS distribution. Through spectroscopic techniques, the difference in carrier recombination among these samples with different DOS distribution can be illustrated by characterizing the emission energy, peak shifts with temperature and internal quantum efficiency. These physical parameters are further analyzed by LSE model which quantitatively describe the anomalous luminescence features caused by localization. And the results show that surface modifications have a significant impact on localization characteristics in InGaN nanostructures which provide physical insight and technical guidance on the fabrication and device optimization for optoelectronics application. Acknowledgement This work was supported by the National Key Research and Development Program (Grant No. 2018YFB0406601), the Science Challenge Project (Grant No. TZ2016003), the National Natural Science Foundation of China (Grant Nos.51561165012, 51561145005, 61574082, 61621064, 61822404, 61875014), the Tsinghua University Initiative Scientific Research Program (Grant Nos.20161080068, 20161080062, 2015THZ02-3), and the Collaborative Innovation Centre of SolidState Lighting and Energy-Saving Electronics. Reference [1] Tchernycheva, M., Messanvi, A., de Luna Bugallo, A., Jacopin, G., Lavenus, P., Rigutti, L., ... & Durand, C. (2014). Integrated photonic platform based on InGaN/GaN nanowire emitters and detectors. Nano letters, 14(6), 3515-3520. [2] Jang, E., Jun, S., Jang, H., Lim, J., Kim, B., & Kim, Y. (2010). White‐light‐emitting diodes with quantum dot color converters for display backlights. Advanced materials, 22(28), 3076-3080. [3] Zhang, M., Bhattacharya, P., & Guo, W. (2010). InGaN/GaN self-organized quantum dot green light emitting diodes with reduced efficiency droop. Applied Physics Letters, 97(1), 011103. [4] Jiang, C., Jing, L., Huang, X., Liu, M., Du, C., Liu, T., ... & Wang, Z. L. (2017). Enhanced Solar Cell Conversion Efficiency of InGaN/GaN Multiple Quantum Wells by Piezo-Phototronic Effect. ACS


Page 8 of 16

Page 9 of 16 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


nano, 11(9), 9405-9412. [5] Sang, L., Liao, M., Liang, Q., Takeguchi, M., Dierre, B., Shen, B., ... & Sumiya, M. (2014). A Multilevel Intermediate‐Band Solar Cell by InGaN/GaN Quantum Dots with a Strain‐Modulated Structure. Advanced Materials, 26(9), 1414-1420. [6] Bhuiyan, A. G., Sugita, K., Hashimoto, A., & Yamamoto, A. (2012). InGaN solar cells: present state of the art and important challenges. IEEE Journal of photovoltaics, 2(3), 276-293. [7] Li, C., Wright, J. B., Liu, S., Lu, P., Figiel, J. J., Leung, B., ... & Feezell, D. F. (2017). Nonpolar InGaN/GaN core–shell single nanowire lasers. Nano letters, 17(2), 1049-1055. [8] Zhang, M., Banerjee, A., Lee, C. S., Hinckley, J. M., & Bhattacharya, P. (2011). A InGaN/GaN quantum dot green (λ= 524 nm) laser. Applied Physics Letters, 98(22), 221104. [9] Arita, M., Le Roux, F., Holmes, M. J., Kako, S., & Arakawa, Y. (2017). Ultraclean single photon emission from a GaN quantum dot. Nano letters, 17(5), 2902-2907. [10] Deshpande, S., Heo, J., Das, A., & Bhattacharya, P. (2013). Electrically driven polarized singlephoton emission from an InGaN quantum dot in a GaN nanowire. Nature communications, 4, 1675. [11] Holmes, M. J., Choi, K., Kako, S., Arita, M., & Arakawa, Y. (2014). Room-temperature triggered single photon emission from a III-nitride site-controlled nanowire quantum dot. Nano letters, 14(2), 982-986. [12] Deshpande, S., Das, A., & Bhattacharya, P. (2013). Blue single photon emission up to 200 K from an InGaN quantum dot in AlGaN nanowire. Applied Physics Letters, 102(16), 161114. [13] Guo, W., Banerjee, A., Bhattacharya, P., & Ooi, B. S. (2011). InGaN/GaN disk-in-nanowire white light emitting diodes on (001) silicon. Applied Physics Letters, 98(19), 193102. [14] Nguyen, H. P. T., Cui, K., Zhang, S., Djavid, M., Korinek, A., Botton, G. A., & Mi, Z. (2012). Controlling electron overflow in phosphor-free InGaN/GaN nanowire white light-emitting diodes. Nano letters, 12(3), 1317-1323. [15] Saxena, D., Mokkapati, S., Parkinson, P., Jiang, N., Gao, Q., Tan, H. H., & Jagadish, C. (2013). Optically pumped room-temperature GaAs nanowire lasers. Nature Photonics, 7(12), 963. [16] Hong, Y. J., Lee, C. H., Yoon, A., Kim, M., Seong, H. K., Chung, H. J., ... & Yi, G. C. (2011). Visible‐ color‐tunable light‐emitting diodes. Advanced Materials, 23(29), 3284-3288. [17] Spradlin, J., Dogan, S., Mikkelson, M., Huang, D., He, L., Johnstone, D., ... & Molnar, R. J. (2003). Improvement of n-GaN Schottky diode rectifying characteristics using KOH etching. Applied physics letters, 82(20), 3556-3558. [18] Wang, X. H., Ning, J. Q., Xu, S. J., & Choi, H. W. (2011). Raman and photoluminescence characterization of focused ion beam patterned InGaN/GaN multi-quantum-wells nanopillar array. Journal of Applied Physics, 110(9), 093111. [19] Zhao, C., Ng, T. K., Prabaswara, A., Conroy, M., Jahangir, S., Frost, T., ... & Ooi, B. S. (2015). An enhanced surface passivation effect in InGaN/GaN disk-in-nanowire light emitting diodes for mitigating Shockley–Read–Hall recombination. Nanoscale, 7(40), 16658-16665. [20] Sun, M. H., Joyce, H. J., Gao, Q., Tan, H. H., Jagadish, C., & Ning, C. Z. (2012). Removal of surface states and recovery of band-edge emission in InAs nanowires through surface passivation. Nano letters, 12(7), 3378-3384. [21] Jahangir, S., Banerjee, A., & Bhattacharya, P. (2013). Carrier lifetimes in green emitting InGaN/GaN disks‐in‐nanowire and characteristics of green light emitting diodes. physica status solidi c, 10(5), 812-815. [22] Chevtchenko, S. A., Reshchikov, M. A., Fan, Q., Ni, X., Moon, Y. T., Baski, A. A., & Morkoc, H.



(2007). Study of SiNx and SiO2 passivation of GaN surfaces. Journal of applied physics, 101(11), 113709. [23] Hashizume, T., Ootomo, S., & Hasegawa, H. (2003). Suppression of current collapse in insulated gate AlGaN/GaN heterostructure field-effect transistors using ultrathin Al 2 O 3 dielectric. Applied physics letters, 83(14), 2952-2954. [24] Lee, L. K., & Ku, P. C. (2012). Fabrication of site ‐ controlled InGaN quantum dots using reactive‐ion etching. physica status solidi c, 9(3‐4), 609-612. [25] Sanford, N. A., Blanchard, P. T., Bertness, K. A., Mansfield, L., Schlager, J. B., Sanders, A. W., ... & George, S. M. (2010). Steady-state and transient photoconductivity in c-axis GaN nanowires grown by nitrogen-plasma-assisted molecular beam epitaxy. Journal of Applied Physics, 107(3), 034318. [26] Ivanov, R., Marcinkevičius, S., Zhao, Y., Becerra, D. L., Nakamura, S., DenBaars, S. P., & Speck, J. S. (2015). Impact of carrier localization on radiative recombination times in semipolar (20 2¯ 1) plane InGaN/GaN quantum wells. Applied Physics Letters, 107(21), 211109. [27] Wang, J., Wang, L., Zhao, W., Hao, Z., & Luo, Y. (2010). Understanding efficiency droop effect in InGaN/GaN multiple-quantum-well blue light-emitting diodes with different degree of carrier localization. Applied Physics Letters, 97(20), 201112. [28]Su, Z., & Xu, S. (2017). A generalized model for time-resolved luminescence of localized carriers and applications: Dispersive thermodynamics of localized carriers. Scientific Reports, 7(1), 13. [29]Narukawa, Y., Kawakami, Y., Fujita, S., Fujita, S., & Nakamura, S. (1997). Recombination dynamics of localized excitons in In 0.20 Ga 0.80 N-In 0.05 Ga 0.95 N multiple quantum wells. Physical Review B, 55(4), R1938. [30] Nakamura, S.; Chichibu, S. F. Introduction to Nitride Semiconductor Blue Lasers and Light Emitting Diodes (Taylor & Francis, London and New York, 2000), p. 153–257. [31] Wang, Z., Wang, L., Xing, Y., Yang, D., Yu, J., Hao, Z., ... & Li, H. (2017). Consistency on two kinds of localized centers examined from temperature-dependent and time-resolved photoluminescence in InGaN/GaN multiple quantum wells. ACS Photonics, 4(8), 2078-2084. [32] Li, Q., Xu, S. J., Cheng, W. C., Xie, M. H., Tong, S. Y., Che, C. M., & Yang, H. (2001). Thermal redistribution of localized excitons and its effect on the luminescence band in InGaN ternary alloys. Applied Physics Letters, 79(12), 1810-1812. [33] Li, Q., Xu, S. J., Xie, M. H., & Tong, S. Y. (2005). Origin of the ‘S-shaped’ temperature dependence of luminescent peaks from semiconductors. Journal of Physics: Condensed Matter, 17(30), 4853. [34] Li, Q., Xu, S. J., Xie, M. H., & Tong, S. Y. (2005). A model for steady-state luminescence of localized-state ensemble. EPL (Europhysics Letters), 71(6), 994. [35] Hu, Y., Hao, Z., Lai, W., Geng, C., Luo, Y., & Yan, Q. (2015). Nano-fabrication and related optical properties of InGaN/GaN nanopillars. Nanotechnology, 26(7), 075302. [36] Wang, Z. L., Hao, Z. B., Yu, J. D., Wu, C., Wang, L., Wang, J., ... & Luo, Y. (2017). Manipulating the Band Bending of InGaN/GaN Quantum Dots in Nanowires by Surface Passivation. The Journal of Physical Chemistry C, 121(11), 6380-6385. [37] H. Morkoç, Handbook of GaN Materials and Devices (Springer, Berlin, 2007). [38] Holm, R. T., McKnight, S. W., Palik, E. D., & Lukosz, W. (1982). Interference effects in luminescence studies of thin films. Applied optics, 21(14), 2512-2519. [39] Namvar, E., & Fattahi, M. (2008). Interference effects on the photoluminescence spectrum of


Page 10 of 16

Page 11 of 16 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


GaN/InxGa1−xN single quantum well structures. Journal of Luminescence, 128(1), 155-160. [40] Eliseev, P. G. (2003). The red σ2/kT spectral shift in partially disordered semiconductors. Journal of applied physics, 93(9), 5404-5415. [41]Sabuktagin, S., Reshchikov, M. A., Johnstone, D. K., & Morkoç, H. (2003). Band bending near the surface in GaN as detected by a charge sensitive probe. MRS Online Proceedings Library Archive, 798. [42] Chevtchenko, S., Ni, X., Fan, Q., Baski, A. A., & Morkoç, H. (2006). Surface band bending of aplane GaN studied by scanning Kelvin probe microscopy. Applied physics letters, 88(12), 122104. [43] Peng, L. H., Shih, C. W., Lai, C. M., Chuo, C. C., & Chyi, J. I. (2003). Surface band-bending effects on the optical properties of indium gallium nitride multiple quantum wells. Applied physics letters, 82(24), 4268-4270. [44] Bao, W., Su, Z., Zheng, C., Ning, J., & Xu, S. (2016). Carrier localization effects in InGaN/GaN multiple-quantum-wells LED nanowires: luminescence quantum efficiency improvement and “negative” thermal activation energy. Scientific reports, 6, 34545.



Figure 1.The schematic diagram of the sample preparation and passivation process. (a) is the SEM image of surface morphology of QD in nanopillars, the acquisition angle is tilted by 45 degrees. (b) is the process of top-down fabrication QD and two step passivation. 204x159mm (300 x 300 DPI)


Page 12 of 16

Page 13 of 16 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 2. PL plots at 10K, containing three samples, the pristine quantum dots sample (QD), the etched sample (EQD) and passivated sample (PQD). The interference fringe (IF) is fitted from PQD sample. 178x145mm (300 x 300 DPI)



Figure 3. The contour line of the PL intensity of quantum well and three samples. 110x129mm (300 x 300 DPI)


Page 14 of 16

Page 15 of 16 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 4. (a) - (d) The left side show the peak shift with temperature of four samples fitted by LSE model. The black squares denote the peak energy at each temperature and red lines represent the fitting results. The dashed line in (c) indicates the peak movement obtained from the Varshni equation. (e) – (h) The right side show the Gaussian distribution of density of states (DOS) and carriers filling states affected by localization. 210x187mm (300 x 300 DPI)



50x50mm (300 x 300 DPI)


Page 16 of 16

Surface-Induced Carrier Localization and Recombination

Recommend Documents