Optical Study of Sub-10 nm In0.3Ga0.7N Quantum Nanodisks in GaN

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Optical Study of Sub-10nm In GaN Quantum Nanodisks in GaN Nanopillars Akio Higo, Takayuki Kiba, Shula Chen, Yafeng Chen, Tomoyuki Tanikawa, Cedric Thomas, Chang Yong Lee, Yi-Chun Lai, Takuya Ozaki, Junichi Takayama, Ichiro Yamashita, Akihiro Murayama, and Seiji Samukawa ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 07 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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Optical Study of Sub-10nm In0.3GaN Quantum Nanodisks in GaN Nanopillars Akio Higo,1,*,a) Takayuki Kiba,2 Shula Chen,3 Yafeng Chen, 3 Tomoyuki Tanikawa,4 Cedric Thomas,5 Chang Yong Lee,5 Yi-Chun Lai,1 Takuya Ozaki,5 Junichi Takayama,3 Ichiro Yamashita,6 Akihiro Murayama,3 and Seiji Samukawa1,*,b)

1

WPI-Advanced Institute for Material Research, Tohoku University, Sendai 980-8577, Japan 2

Department of Materials Science and Engineering, Kitami Institute of Technology, Kitami, Japan 3

Graduate School of Information Science and Technology, Hokkaido University, Sapporo 060-0814, Japan 4

IMR, Tohoku University, Sendai 980-8577, Japan

5

Institute of Fluid Science, Tohoku University, Sendai 980-8577, Japan

6

Nara Institute of Science and Technology, Ikoma 630-0101, Japan

* Corresponding authors Contact addresses: a) [email protected] b) [email protected]

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Abstract

We have demonstrated the fabrication of homogeneously distributed In0.3Ga0.7N/GaN quantum nanodisks (QNDs) with a high density and average diameter of 10 nm or less in 30-nmhigh nanopillars. The scalable top-down nanofabrication process used bio-templates that were spin coated on an In0.3Ga0.7N/GaN single quantum well (SQW) followed by low-damage dry etching on ferritins with 7-nm-diameter iron cores. The photoluminescence measurements at 70 K showed a blue shift of quantum energy of 42 meV from the InGaN/GaN SQW to the QND. The inertial quantum efficiency of the InGaN/GaN QND was 100 times that of SQW. A significant reduction in the quantum-confined Stark effect in the QND structure was observed, which concurred with the numerical simulation using a 3D Schrödinger equation. These results pave the way for the fabrication of large-scale III-N quantum devices using nanoprocessing, which is vital for optoelectronic communication devices.

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Keywords: III-N compound semiconductor, quantum nanodisk, photoluminescence

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III-nitride compound semiconductors such as GaN based materials have attracted much attention for their applications in photonic devices [1,2]. Optical devices made of InGaN/GaN multiple quantum wells (MQWs) such as light-emitting diodes (LEDs) and laser diodes [3-6] have been developed and widely used. However, it is not easy to overcome the green gap due to very low internal quantum efficiency (IQE) of the active layers in high-indium-content InGaN in the longer wavelength region because of high strain inside the materials and the quantum confined stark effect (QCSE). InGaN/GaN nanostructures have been investigated to realize high inertial quantum efficiency (IQE) optical devices because of the strain relaxation, weak QCSE, and quantum confinement effect achieved by the enhancement of photoluminescence (PL) intensity. Several GaN-based nanostructures have been manufactured using various fabrication methods such as inductively coupled plasma (ICP) reactive-ion etching [7] with nanoparticle lithography [8,9] and the growth of single-crystal GaN nanopillars by hydride vapor-phase epitaxy [10]. However, these reported methods have nanopillars larger than 10 nm in diameter. Our previous work on the combination of ferritin bio-template [11] and NBE was established with silicon [12] at first, and the process damage was investigated. Then, with III-V materials, and especially GaAs-based nanostructure fabrication, we realized a nanoscale structure by a dry process [13]. Subsequently, we investigated the activation energy of the neutral beam [15] and influence of surface GaAs-oxide during NBE. However, these techniques could not be directly transferred to the InGaN/GaN etching system because InGaN/GaN materials require high temperature during etching. Therefore, we investigated and achieved new etching conditions. Furthermore, using GaAs/AlGaAs system, we achieved an LED structure [16] and investigated the asymmetric barrier LED for room-temperature operation [17]. These two studies gave useful insights for InGaN/GaN based LED fabrication. The optical 4


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properties of InGaN/GaN QNDs have not been fully studied yet because the higher indium content in the c-plane single quantum well (SQW) could result in lower emission efficiency due to the strong QCSE.

Recently, we have achieved InGaN/GaN single quantum nanodisk blue emission by using fusion nanoprocessing of iron oxide core and ferritin protein particles [11], and damage-less plasma neutral beam etching [12-17]. The optical properties of InGaN/GaN QNDs have not been fully studied yet since the higher indium content in the c-plane single quantum well (SQW) could result in lower emission efficiency due to the strong QCSE. In this study, we present InGaN/GaN QNDs with enhanced emission intensity properties and analyze their characteristics using temperature-dependent PL, time resolved PL, and a theoretical simulation using 3-dimensional finite difference methods [18].

Methods Sample preparation

The samples were grown by metal-organic vapor phase epitaxy on a c-plane sapphire (0001) substrate. The QND profile structure consisted of low temperature GaN nucleation layer, a 2-µm-thick GaN buffer layer, three pairs of 2-nm-thick In0.3GaN MQWs/8-nm-thick GaN barrier layers, and 10-nmthick GaN capping layer. Additionally, 2-nm-thick In0.3GaN QW/8-nm-thick GaN barrier layers and a 10nm-thick GaN capping layer were used for optical measurement.

Fabrication process and results 5


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A schematic illustration of the fabricated In0.3GaN/GaN QND is shown in Figure 1. Ferritins with an iron-oxide core inset were modified using polyethylene glycol (PEG 2000, molecular weight: 2000 g/mol) and synthesized [19]. After cleansing with organic solutions and deionized water, we dropped the PEGferritin on the top of the samples. Following an incubation time of 4 min, the solution was spin-coated at 500 rpm for 2 s and at 3000 rpm for 30 s. Oxygen annealing in vacuum was performed at 100 °C at a flow rate of 100 sccm and a chamber pressure of 37 Pa. Following this process, the iron core remained on the surface and acted as an etching mask. Hydrogen radical treatment was performed to remove the surface oxide at chamber pressure of 51 Pa at 80 °C for 30 min. The microwave power was 155 W. After the hydrogen radical treatment, we used the same flow condition and decreased the process temperature to room temperature (RT) for 30 min for passivation. Subsequently, the QW was etched completely to obtain nanopillars using 40 sccm chlorine gas at a chamber pressure of 0.1 Pa at 100 °C with ICP power of 400 W and bottom bias power of 10 W. As a result, approximately 5-nm-diameter and 20-nm-high InGaN nanodisks could be fabricated by the damage-less dry etching process. The crosssection of the InGaN/GaN nanopillars recorded by scanning transmission electron microscope (TEM) with energy dispersive X-ray spectroscope (STEM-EDX, JSM-6500F, JEOL) is shown in Figure 2.

Figure 2 shows the top-view image of QND by scanning electron microscope (SEM), the side view image by TEM, and the STEM-EDX image of the In0.3GaN/GaN QNDs. A nanopillar density of 2.5 × 10 11 cm−2 was measured in Fig 2(a). The nanopillar diameter and etching depth profile were measured by TEM to be approximately 5 nm and approximately 30 nm, respectively. In Figure 2(b), two pairs of 2-nmthick In0.3GaN layers and 8-nm-thick GaN barrier layers can be clearly observed and sub-10-nm in diameter QDs. This is the minimum feature of NDs by a top-down dry process. We could not fabricate 6


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NDs with diameter less than 10 nm in the previous studies of GaAs/AlGaAs systems. We investigated the surface oxide [14] and activation energy of NBE [15] because they are key factors of the process. Indeed, in GaAs, Ga-Ox and As-Ox are easily generated and Ga-Ox cannot be etched at a low-temperature (−20 o

C) by NBE. Moreover, there was no other way to remove it without impacting device characteristics.

Therefore, the remaining Ga-Ox layer worked also as an etching mask, enlarging GaAs QDs diameter over 10 nm. However, in InGaN/GaN QDs fabrication process, high temperature is required because of the vapor pressure of the etched products. After thorough scanning of etching parameters for InGaN/GaN systems, we found etching conditions at 100 oC for which ferritin nanoparticles remained. Using these optimized etching parameters, we succeeded in fabricating the InGaN/GaN nanodisks with a diameter of 10 nm or less, without being impacted by the remaining oxidized layer as in the GaAs ND case.

Next, the optical properties at various temperatures were studied using photoluminescence (PL) spectroscopy. The single QW (SQW) and QND samples were placed in a vacuum cryostat with quartz windows and cooled on a cold finger by a closed Helium compressor. The PL spectra were observed from 6 to 300 K with excitation wavelengths of 385 nm for SQW and 267 nm for QND, respectively. The excitation light sources were the outputs of second or third harmonics of a mode-locked Ti-sapphire laser with a pulse width of 150 fs and repetition rate of 76 MHz. The time-resolved PL spectra were detected by a synchroscan streak camera (Hamamatsu Photonics, Hamamatsu, Japan). The spot diameter of the laser light focused on the sample surface was 100 μm. The excitation power density was 0.34 µJcm−2. The Band structures including the internal electrical fields and energy levels of the electronhole transitions in the SQW and QND structures were simulated by applying three-dimensional 7


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Schrodinger equation using the finite difference method, nextnano3, with global strain minimization [20-22]. The SQW and QND structures consisted of 10-nm-thick GaN bottom layer, 2-nm-thick In0.3GaN middle layer, and a 10-nm-thick GaN top layer. We set the diameter of the cylindrical structure in the simulation as 5 nm based on the estimation from the TEM image.

Temperature-dependent PL and TRPL results and discussion

Temperature-dependent PL spectra were obtained as shown in Figure 3. The PL peak positions in the initial SQW sample and QND sample were located at 491 nm (2.53 eV) and 420 nm (2.95 eV), respectively, at 6 K. The peak energy of the QND PL spectrum shifted approximately 420 meV compared to that of the initial QW. A significant blue shift of the emission peaks from the QND was observed in comparison to QW. The PL peak positions in the QW were plotted as a function of temperature in Figure 3(c). The small energy shift of 20 meV in SQW probably originated due to exciton localization. At lowtemperature limits, the excitons are localized at various potential minima induced by the inhomogeneity of the indium content because of which the transition energy had energy potential fluctuation due to the indium concentration dispersion in QW. With slight increase in temperature, the excitons trapped at shallow potential minima can escape towards the in-plane direction. However, these movable excitons are easily trapped by the non-radiative center resulting in PL quenching. Nevertheless, the excitons in the deep potential minima are still alive and they can contribute to the red side of the PL spectrum. In contrast, no peak shift was observed in the QND with change in temperature. The effect of localized potential in the QND can be less than that in the SQW because of the spatially limited area of each QND.

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The discrete energy levels due to quantum confinement and the relatively flat band line-up without QCSE (shown in Figure 4(b)) also result in a temperature independent PL peak energy in QND.

Because the 5 nm diameter of QND is close to the Bohr radius, the blue-shift of the emission peak in the QNDs could correspond to the reduction in the piezoelectric field caused by partial strain relaxation. Figure 4(a) shows the transition energy, EQW (e1-hh1), between the ground states of the electron e1 and the heavy hole hh1 in the fully strained In0.3GaN QW. This is calculated to be 2.48 eV in the simulation test. The wave function overlap between the electrons and the holes is small in the QW region because of the strong internal electric field of the high Indium content in InGaN. On the other hand, the transition energy EQND (e1-hh1) in the strained InGaN QND is calculated to be 2.9 eV as shown in Figure 4 (b). These values show good agreement with the measurement results. Strain relaxation occurs in the band line-ups of InGaN QND in the band as shown in Figure 4. Therefore, the measured blue shift of the QND PL peak is explained as having been caused by partial strain relaxation and quantum confinement because the band slope of QND was released in both bands.

The temperature dependency of the PL intensity was investigated to determine the IQE of the QW and QND as shown in Figure 5. The IQE of QW and QND were calculated to be 0.31% and 11.9% at 200 K, respectively, under the assumption that the non-radiative relaxation process centers are completely frozen, i.e., IQE at 6 K is 100%. The significant increase in the IQE of QND is attributed to the improvement in the spatial overlap between the electron and hole wave functions due to the relaxation of QCSE. The efficient coupling of electron and hole wave functions enhances the oscillator strength of the e1-hh1 transition in the QND. Compared to this, the localized positions of the electrons and holes in 9


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the QW are spatially different due to the band-slope caused by the intrinsic piezo-electric fields. Therefore, a high IQE of QND was achieved, which was 38 times that of the SQW.

To gain better insight into the temperature-dependent PL phenomena, we estimated the activation energies for PL quenching based on the Arrhenius plots of the integrated PL intensity obtained from the In0.3GaN/GaN SQW and the QND samples with various temperature inversions as shown in Figure 5(b). The activation energies of the SQW and QND were stimulated according to the relationships between the integrated PL intensity and various temperatures as expressed by the following equation, where I0 is the PL intensity at 0 K and Ea and A are the activation energy and the coefficient related to the number of non-radiative centers, respectively [23-25].

It =

∑

(1)

The activation energies Ea1 of the SQW and QND were 20 and 120 meV, respectively. The activation energy of the SQW was significantly smaller than that of QND. This value is attributed to the fluctuation of energy potential in the high indium content of the QW region. At a low temperature such as 6 K, the observed emission in the SQW could have originated from the electron-hole recombination at such energy minima within the fluctuated potential. However, with increase in temperature, the thermally activated carriers can escape beyond the potential barrier of 20 meV and can freely move towards the in-plane direction of the SQW. In the InGaN QW, particularly those with high indium content, a nonradiative recombination center is observed due to the existence of defects. Therefore, the movable carriers can be easily trapped in such a non-radiative center, resulting in the rapid PL quenching that is 10


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observed from 50 K onwards. In contrast, the PL quenching for QND is moderate up to 150 K after which it shows a rapid decrease in the PL intensity with further increase in temperature. The activation energy for this PL quenching is estimated to be 120 meV and can be assigned to the band-offset for the valence band, which was shown in Figure 4(b). The band offset for the valence band was shallow and relatively closer to the observed Ea value compared to the conduction band (more than 500 meV offset) in the nextnano3 simulation although its band line-up had a complicated structure due to its strain. The bandoffset value from the eigenstate of hh1 to the peak of the sharp GaN barrier (z = 12 nm in Figure 5(b)) was 160 meV. The experimentally observed activation energy of 120 meV shows reasonable agreement with the simulated offset value when considering the effective barrier height to be much lower because the calculated GaN barrier shape is like a narrow peak and carriers can tunnel towards the bending GaN barrier. At the low temperature range below 150 K, the carriers are confined by the disk structure and their motion was limited within the QND. Therefore, the carrier generated in the QND has a lesser chance to be captured by the non-radiative center unlike in QW, resulting in a moderate decrease in PL intensity below 150 K.

In order to investigate the carrier dynamics in QW and QND, the time-resolved PL (TRPL) measurements [26] were performed and results were fitted with the stretched exponential equation

It = exp − , τ

(2)

where I(t), I0 are PL intensity as a function of time and initial PL intensity, respectively, τ is the PL lifetime, and parameter β is a distribution of the lifetime with values between 0 (broad distribution) and 1 (narrow distribution = single exponential function) [27]. The observed PL decay curves with 11


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various temperatures for QW and QND are shown in Figures 6(a) and (b), respectively. From these analyses, the temperature dependence on the lifetime τ for both QW and QND were plotted as filled circles in Figures 7(a) and (b), respectively. The PL lifetime of QW was more than 100 ns at 6 K. At lower temperatures, the small oscillator strength, which was caused by the small overlap of the electron and hole wave functions due to QCSE, results in a longer lifetime in QW. With the increase in temperature, the PL lifetime of the QW rapidly decreased even at the low temperature regime, and this observed trend was similar to that of the PL intensity. There are two possible reason for the shortened lifetime— one is the rapid trapping of the excitons by the non-radiative center; another is the increase in the oscillator strength leading to the recombination of thermally excited electrons and holes in the QCSEaffected band structure as mentioned while explaining the temperature-dependent red shift of the PL peak energy of the QW. Assuming that the IQE is 100% at 6 K with maximum PL intensity (I(6K)), the PL

lifetime can be separated into a radiative (τr) and non-radiative (τnr) lifetimes (

1

τ PL

=

and τnr were calculated by using following equations:

ηIQE (T ) =

τ r (T ) =

I (T ) 1 τ r (T ) 1 τ r (T ) = = , I (6K ) 1 τ r (T ) + 1 τ nr (T ) 1 τ PL (T )

(3)

τ PL (T ) I (6 K ) = τ PL (T ) , I (T ) η IQE (T )

τ nr (T ) =

(4)

τ PL (T ) I (6K ) = τ PL (T ) , 1 − ηIQE I (6K ) − I (T )

(5)

12


1

τr

+

1

τ nr

). The τr

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where τPL is the measured value of the PL decay time and I(T) is the PL intensity at a certain temperature T. The individual radiative (τr) and non-radiative (τnr) lifetimes of the QW and QND are plotted in Figures 7(a) and (b), respectively. The initial slight decrease in τr below 50 K in the QW can be attributed to the increase in the oscillator strength due to the thermal excitation induced moderation of the QCSE as mentioned above. Except for the low temperature region below 50 K, the PL lifetime was governed by τnr in the QW. This is probably due to the fast trapping of mobile excitons or electron sand holes by the non-radiative center in the InGaN QW. This explanation is supported by the temperature dependence of the PL intensity and the interpretation of the activation energy in the QW. Therefore, the reason for the lower IQE at RT in the QW is mainly due to the free movement of carriers towards the inplane direction in this InGaN QW. The excitons or carriers dynamics in the QW estimated from the temperature dependency of the PL intensity and lifetimes show good correspondence to that of the stretching parameter β for fitting of the PL decay curves. Figure 7(c) shows the stretching parameter β as a function of temperature. When the potential fluctuation due to the inhomogeneity of the indium content is significant in the InGaN region, the localization in the different potential minima with different energy can cause the distribution of lifetimes for radiative recombination, which results in a smaller value of the parameter β = 0.5−0.6 as observed at low temperatures. However, as the temperature increases, the excitons thermally escape from the shallow potential minima. Thus, the recombination of excitons takes place at the deep localized minima in the QW. Therefore, the distribution of the lifetimes should be much narrower, resulting in the increase in parameter β by 0.8 at temperatures above 100 K. These observations correspond well with the small red shift in PL peak energy in the QW. 13


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In contrast, the PL lifetime of QND at 6 K was approximately 100 ps and it was significantly short compared to that of QW. There are three possible reasons for explaining this drastic decrease in PL lifetime in the QND: (i). the enhancement of radiative lifetime due to the increase in oscillator strength because of QCSE relaxation. (ii). Non-radiative recombination occurs at the surface defects. If we assume that the IQE at low-temperature limit is 100%, the IQE was greatly improved at middle temperature range (150–200 K) compared to that in the QW. Under the same assumption, the PL lifetime was almost governed by the radiative contribution below 150 K in the QND. These features can be partly attributed to the QCSE relaxation. Finally, the temperature dependence of parameter β in the QND was plotted in Figure 7(c). The QND showed smaller values of β compared to the QW for the entire temperature range. This is attributed to the small effects of the localized potential induced by the high indium content, which was moderated from the spatial confinement of excitons within the QND. The increase in β was observed at the low temperature regime. This was probably due to the small localization within the QND, which causes a small distribution of radiative lifetime at low temperatures, and the thermal escape from potential minima within the QND gives a true PL emission from the QND, resulting in a single-exponential-like (β ≈ 1) decay profile.

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Conclusions We have successfully fabricated a damage-less, uniform, 10-nm-diameter or smaller InGaN QND by using a top-down technique that combines a bio-template and neutral beam etching. The timedependent PL measurements revealed that the optical properties of the QND exhibited significant differences from the SQW including differences in the PL spectra at various temperatures. Partially relaxed and weak QCSE were observed in the PL measurements at various temperatures and in the numerical simulation. Therefore, we achieved high IQE improvement of the QND. As a result, this QND structure has great potential for high quantum efficiency light emitting devices.

Acknowledgements The authors would like to thank Professor Takashi Matsuoka, IMR, Tohoku University for the collaborative research extended by him. This work was supported by Japan Society for Promotion of Science, Grant-in-Aid for Scientific Research (S) No.16H06359.

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Author Contributions A. H. conceived of the study, participated in its design and coordination. A. H. and T. K. performed experiments, analysis and interpretation of data, simulations and drafted the manuscript. S. C. and C. T. performed experiments, analysis and interpretation of data, and helped to draft the manuscript. Y. C. and J. T. participated in the acquisition of data and its analysis in optical measurements. Y. L., C. L., T. O. and T. T. performed experiments in sample fabrication. I. Y., A. M. and S. S. edited the manuscript. All authors read and approved the final manuscript.

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References [1] Nakamura, S. The Roles of Structural Imperfections in InGaN-Based Blue Light Emitting Diodes and Laser Diodes. Science 1998, 281, 956-961.

[2] Holmes, M.; Kako, S.; Choi, K.; Arita M.; Arakawa, Y. Spectral diffusion and its influence on the emission linewidths of site-controlled GaN nanowire quantum dots. Phys. Rev. B 2015, 92, 115447.

[3] Someya, T.; Werner, R.; Forchel, A.; Catalano, M.; Cingolani, R.; Arakawa, Y. Room temperature lasing at blue wavelengths in gallium nitride microcavities. Science 1999, 285, 1905-1906.

[4] Butté, R.; Feltin, E.; Dorsaz, J.; Chirstmann, G.; Carlin, J.-F.; Grandjean, N.; Ilegems, M. Recent progress in the growth of highly reflective nitride-based distributed Bragg reflectors and their use in microcavities. Jpn. J. Appl. Phys. 2005 44, 7207-7216.

[5] Pattison, P. M.; David, A; Sharma, R.; Weisbuch, C.; DenBaars, S.; Nakamura, S. Gallium nitride based microcavity light emitting diodes with 2λ effective cavity thickness. Appl. Phys. Lett. 2007, 90, 031111.

[6] Liu, W. J.; Hu, X. L.; Ying, L. Y.; Chen, S. Q.; Zhang, J. Y.; Akiyama, H.; Cai, Z. P.;Zhang, B. P. On the importance of cavity-length and heat dissipation in GaN-based vertical-cavity surface-emitting lasers. Sci. Rep. 2015, 5, 9600.

[7] Lee, L. K.; Ku, P. C. Fabrcation of site-controlled InGaN quantum dots using reactive-ion etching. Phys. Status Solidi (c) 2012, 9, 609-612.. 17


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[8] Hu, Y.; Hao, Z; Lai, W.; Geng, C.; Luo, Y.; Yan, Q. Nano-fabrication and related optical properties of InGaN/GaN nanopillars. Nanotechnology 2015, 26, 075302.

[9] Lai, Y. C.; Higo, A.; Kiba, T.; Thomas, C.; Chen, S.; Lee, C. Y.; Tanikawa, T.; Kuboya, S.; Katayama, R.; Shojiki, K.; Takayama, J.; Yamashita, I.; Murayama, A.; Chi, G. C., Yu, P., Samukawa, S. Nanometer scale fabrication and optical response of InGaN/GaN quantum disks. Nanotechnology 2016, 27, 1-5.

[10] Holmes, M. J.; Park, Y. S.; Wang, X.; Chan, C. C. S.; Reid, B. P. L.; Kim, H. D.; Luo, J.; Warner, J. H.; Taylor, R. A. Optical studies of GaN nanocolumns containing InGaN quantum disks and the effect of strain relaxation on the carrier distribution. Phys. Status Solidi (c) 2012, 9, 712-714.

[11] Yamashita, I. Fabrication of a two-dimensional array of nano-particles using ferritin molecule. Thin Solid Films 2001, 393, 12.

[12] Samukawa, S. Ultimate top-down etching processes for future nanoscale devices: advanced neutral-beam etching. Jpn. J. Appl. Phys. 2006, 45, 2395.

[13] Tamura, Y.; Kaizu, T.; Kiba, T.; Igarashi, M.; Tsukamoto, R.; Higo, A.; Hu, W.; Thomas, C.; Fauzi, M E.; Hoshii, T.; Yamashita, I.; Okada, Y.; Murayama, A.; Samukawa, S. Quantum size effects in GaAs nanodisks fabricated using a combination of the bio-template technique and neutral beam etching. Nanotechnology 2013, 24, 285301.

[14] Thomas, C.; Tamura, Y.; Syazwan, M. E.; Higo, A.; Samukawa, S. Oxidation states of GaAs surface and their effects on neutral beam etching during nanopillar fabrication. J. Phys. D: Appl. Phys. 2014, 47,

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215203.

[15] Thomas, C.; Tamura, Y.; Okada, T.; Higo, A.; Samukawa, S. Estimation of activation energy and surface reaction mechanism of chlorine neutral beam etching of GaAs for nanostructure fabrication. J. Phys. D: Appl. Phys. 2014, 47, 275201.

[16] Higo, A.; Kiba, T.; Tamura, Y.; Thomas, C.; Takayama, J.; Wang, Y.; Sodabanlu, H.; Sugiyama, M.; Nakano, Y.; Yamashita, I.; Murayama, A.; Samukawa, S. Light-Emitting Devices Based on Top-down Fabricated GaAs Quantum Nanodisks. Sci. Rep. 2015, 5, 9371.

[17] Tamura, Y.; Higo, A.; Kiba, T.; Thomas, C.; Takayama, J.; Yamashita, I.; Murayama, A.; Samukawa, S. Temperature-Dependent Operation of GaAs Quantum Nanodisk LEDs with Asymmetric AlGaAs Barriers. IEEE Trans. Nanotechnol. 2016, 15, 557-562.

[18] nextnano software available online: http://www.nextnano.com/

[19] Tsukamoto, R.; Godonoga, M.; Matsuyama, R.; Igarashi, M.; Heddle, J. G.; Samukawa, S.; Yamashita, I. Effect of PEGylation on controllably spaced adsorption of ferritin molecules. Langmuir 2013, 29, 12737-12743.

[20] Birner, S.; Zibold, T.; Andlauer, T.; Kubis, T.; Sabathil, M.; Trellakis, A.; Vogl, P. Nextnano: general purpose 3-D simulations. IEEE Trans. Electron Devices 2007, 54, 2137-2142.

[21] Fonoberov, V. A; Balandin, A. A. Excitonic properties of strained wurtzite and zinc-blende GaN/AlxGa1−xN quantum dots. J. Appl. Phys. 2003, 94, 7178. 19


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[22] Povolotskyi, M.; Muar, A. D. M.; Carlo, D. Strain effects in freestanding three-dimensional nitride nanostructures. Phys. Status Solidi (c) 2005, 2, 3891.

[23] Heitz, R.; Mukhametzhanov, I.; Madhukar, A.; Hoffmann, A.; Bimberg, D. J. Temperature dependent optical properties of self-organized InAs/GaAs quantum dots. Electron. Mater. 1999, 28, 520527.

[24] Gelinas, G.; Lanacer, A.; Leonelli, R.; Masut, R. A.; Poole, P. J. Carrier thermal escape in families of InAs/InP self-assembled quantum dots. Phys. Rev. B. 2010, 81, 235426.

[25] Jahan, N. A.; Hermanstadter, C.; Huh, J. H.; Sasakura, H.; Rotter, T. J.; Ahirwar, P.; Balakrshnan, G.; Akahane, K.; Sasaki, M.; Kumano, H.; Suemune, I. Temperature dependent carrier dynamics in telecommunication band InAs quantum dots and dashes grown on InP substrates. J. Appl. Phys. 2013, 113, 033506.

[26] Kiba, T.; Mizushima, Y.; Igarashi, M.; Huang, C. H.; Samukawa, S.; Murayama, A. Temperature dependence of time-resolved photoluminescence in closely packed alignment of Si nanodisks with SiC barriers. Nanoscale Res. Lett. 2013, 8, 223.

[27] Pophristic, M., Long, F. H., Tran, C., Ferguson, I. T. & Karlicek Jr, R. F. Time-resolved photoluminescence measurements of quantum dots in InGaN multiple quantum wells and light-emitting diodes. J. Appl. Phys. 1999, 86, 1114.

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Figure Captions Figure 1. Schematic illustration of the fabrication process of InGaN QND

Figure 2. SEM and TEM images of a nanopillar

a. Top view of the etching surface

b. TEM image of a nanopillar

c. STEM-EDX profiles of InGaN/GaN in nanopillars with indium content distribution

Figure 3. Photoluminescence measurements of the fabricated InGaN/GaN nanopillar and initial InGaN/GaN SQW at various temperatures

a. PL measurements for SQW

b. PL measurements for QND

c. Peak energies for SQW

d. Peak energy for QND

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Figure 4. Energy band diagrams for SQW and QD: (a) Energy band diagram of SQW with strain; the bandgap energy e1-hh1 is 2.48eV, (b) Energy band diagram of QD with strain relaxation; the bandgap energy e1-hh1 is 2.90 eV.

Figure 5. Arrhenius plots and fitting of the integrated PL intensities for SQW and QND

a. IQE at various temperatures for SQW and QND

b. PL intensity of the In0.3GaN/GaN SQW and the QND sample at various temperature inversions

Figure 6. Time resolved PL spectra and estimated lifetime for SQW and QND

a. Temperature dependence of time resolved PL for QW

b. Temperature dependence of time resolved PL for QND

Figure 7. Temperature dependence of radiative and non-radiative recombination of SQW, QND and beta parameters

a. Temperature dependence of radiative and non-radiative recombination of QW 22


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b. Temperature dependence of radiative and non-radiative recombination of QND

c. Temperature dependence of beta parameters of QW and QND

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Figures

Figure 1. Schematic illustration of the fabrication process of InGaN QND

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Figure 2 a

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Figure 2 b

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Figure 2 c

Figure 2. SEM and TEM images of a nanopillar

a. Top view of the etching surface

b. TEM image of a nanopillar

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c. STEM-EDX profiles of InGaN/GaN in nanopillars with indium content distribution

Figure 3 a

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Figure 3 b

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Figure 3 c

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Figure 3 d

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Figure 3. Photoluminescence measurements of the fabricated InGaN/GaN nanopillar and initial InGaN/GaN SQW at various temperatures

a. PL measurements for SQW

b. PL measurements for QND

c. Peak energies for SQW

d. Peak energy for QND

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Figure 4

Figure 4. Energy band diagrams for SQW and QD: a. Energy band diagram of SQW with strain; the bandgap energy e1-hh1 is 2.48eV, b. Energy band diagram of QD with strain relaxation; the bandgap energy e1-hh1 is 2.90 eV.

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Figure 5 a

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Figure 5 b

Figure 5. Arrhenius plots and fitting of the integrated PL intensities for SQW and QND

a. IQE at various temperatures for SQW and QND

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b. PL intensity of the In0.3GaN/GaN SQW and the QND sample at various temperature inversions

Figure 6 a

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Figure 6 b

Figure 6. Time resolved PL spectra and estimated lifetime for SQW and QND

a. Temperature dependence of time resolved PL for QW

b. Temperature dependence of time resolved PL for QND

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Figure 7 a

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Figure 7 b

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Figure 7 c

Figure 7. Temperature dependence of radiative and non-radiative recombination of SQW, QND and beta parameters

a. Temperature dependence of radiative and non-radiative recombination of QW

b. Temperature dependence of radiative and non-radiative recombination of QND 40


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c. Temperature dependence of beta parameter of QW and QND

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Optical Study of Sub-10 nm In0.3Ga0.7N Quantum Nanodisks in GaN

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