Donor-Acceptor Pair Recombination in Size-Purified Silicon Quantum

Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES

Communication

Donor-Acceptor Pair Recombination in Size-Purified Silicon Quantum Dots Hiroshi Sugimoto, Masataka Yamamura, Riku Fujii, and Minoru Fujii Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03489 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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 24 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 Letters

Donor-Acceptor Pair Recombination in SizePurified Silicon Quantum Dots Hiroshi Sugimoto*, Masataka Yamamura, Riku Fujii and Minoru Fujii* Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

Abstract

Shallow impurity doping is an efficient route to tailor optical and electronic features of semiconductor quantum dots (QDs). However, the effect of doping is often smeared by the size, shape and composition inhomogeneities. In this paper, we study optical properties of almost monodispersed spherical silicon (Si) QDs that are heavily doped with boron (B) and phosphorus (P). The narrow size distribution achieved by a size-separation process enables us to extract doping-induced phenomena clearly. The degree of doping-induced shrinkage of the optical band gap is obtained in a wide size range. Comparison of the optical band gap with theoretical calculations allow us to estimate the number of active donor-acceptor pairs in a QD. Furthermore, we found that the size and detection energy dependence of the luminescence decay rate is significantly modified below a critical diameter, that is ~5.5 nm. In the diameter range above 5.5 nm, the luminescence decay rate is distributed in a wide range depending on the detection energy even in size purified Si QDs. The distribution may arise from that of donor-

ACS Paragon Plus Environment

1

Nano Letters 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

Page 2 of 24

acceptor distances. On the other hand, in the diameter range below 5.5 nm, the detection energy dependence of the decay rate almost disappears. In this size range, which is smaller than twice of the effective Bohr radius of B and P in bulk Si crystal, the donor-acceptor distance is not a crucial factor to determine the recombination rate.

Keywords: silicon quantum dots, nanocrystals, doping, colloid, donor-acceptor pair

Defining the optoelectronic features of semiconductors by substitutional doping is a basis of modern silicon (Si) microelectronics technology. In the past decade, ever-increasing demands of device miniaturization have stimulated intensive research on impurity doping in semiconductor nanostructures.1,2 The primary role of impurity doping in bulk semiconductor crystal is providing charge carriers for the control of the electrical conductivity. The ionization energies of donors and acceptors are determined by the combination of host crystal and impurity elements, and the density of charge carriers can be accurately controlled by the dopant densities. This concept is not sustained in nanoscale semiconductors such as semiconductor quantum dots (QDs). Due to the reduction of the dielectric screening effect of the Coulomb potential, the ionization energy and the effective Bohr radius of an impurity becomes strongly size-dependent.3,4 The discrete energy state structure induced by the quantum confinement effects also affects the ionization energy significantly.5 Furthermore, due to very small number of impurity atoms in a QD, the effect of doping becomes stochastic. For example, the property of a QD is expected to be different depending on the position of an impurity atom even if the size, shape and the surface termination are the same. Although the stochastic behavior sometimes causes inhomogeneous


2

Page 3 of 24 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 Letters

broadening of impurity-induced physical phenomena in the ensemble of QDs, development of technology to accurately control the density and position of dopants may lead to the exploration of new functional QDs. Attempts to exploit optical and electrical properties of QDs by impurity doping have initiated for compound semiconductor QDs.1,2,6 Mocatta et al. studied the energy level structures of copper- or silver-doped indium arsenide QDs using scanning tunneling and optical spectroscopy in combination with theoretical calculation2 and demonstrated the emergence of a confined impurity band and band-tailing. Sahu et al. developed thin film transistors using silver-doped cadmium selenide QDs and studied the doping type and its effect on the device performance.6 Single dot photoluminescence (PL)7 spectroscopy and transient absorption spectroscopy8 identified electronic energy levels of various types of doped QDs. Doped QDs are considered to be a new functional material used as emission tunable probes9 and stokes-shift engineered phosphors for solar luminescent concentrator.10 In contrast to rapid exploration of applications in doped compound semiconductor QDs, impurity-doping in Si QDs is still at the stage of fundamental research despite its importance in microelectronics. Doped Si QDs have been studied by electron paramagnetic resonance (EPR) spectroscopy, and size dependence of the doping efficiency has been discussed.3,5 The donor and acceptor energy levels were determined by the combination of photoemission yield spectroscopy and photoluminescence spectroscopy11 and by scanning tunneling spectroscopy.12,13 Dopinginduced carriers in Si QDs have been characterized by infrared absorption spectroscopy14,15 and by ultrafast induced-absorption (IA) spectroscopy.16,17 In previous attempts for the exploration of noble electrical and optical properties of Si QDs by doping, the largest obstacle is the small solid solubility of impurities in Si crystal lattice. For


3

Nano Letters 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

Page 4 of 24

example, even in the case of P, which has the largest solubility in Si crystal (~1×1021 cm-3), the calculated maximum dopant number in a 2 nm Si QD is 4. Therefore, in these small Si QDs, impurities are easily pushed out due to a so-called self-purification effect.18 A strategy to overcome this effect and to modify the energy state structure of small Si QDs by doping is codoping n- and p-type impurities simultaneously.19,20 Ossicini et al. demonstrated by ab-initio calculations that codoping significantly reduces the formation energy of Si QDs.21 In fact, efficient incorporation of B and P in Si QDs as small as ~2 nm has been demonstrated by atom probe tomography.22 B and P codoped Si QDs are known to exhibit efficient luminescence due to the donoracceptor (D-A) pair recombination in a 0.85-1.8 eV range.19 In principle, the degree of band gap shrinkage by doping can be determined simply by comparing luminescence energies of undoped and codoped Si QDs with the same size. However, the luminescence band of codoped Si QDs is inhomogeneously broadened due to the distributions of several structural parameters, i.e., the size, the numbers of B and P atoms per a QD and their sites, and thus quantitative discussion on the doping-induced shrinkage of the band gap is not straightforward. In order to extract the information of impurity-induced effects, lifting the size inhomogeneity is crucial. Therefore, in this work, we develop a process to significantly reduce the size distribution of B and P codoped Si QDs. We have succeeded in preparing a colloidal solution of almost monodispersed codoped Si QDs. In the size-selected Si QDs solution, we determine the degree of doping-induced shrinkage of the optical band gap over a wide size range. From the comparison of the experimental data with recent theoretical calculations, we estimate the number of D-A pairs in a QD in a wide size range. Furthermore, we show that the luminescence decay rates depend strongly on the detection energy even in almost monodispersed codoped Si QDs when the


4

Page 5 of 24 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 Letters

diameter is larger than around 5.5 nm, while it is almost independent of the detection energy below the size. The results indicate that around 5.5 nm is a critical dimension, where the behavior of D-A pairs in a Si QD changes qualitatively.

Preparation of codoped monodispersed Si QDs by size-selective precipitation B and P codoped Si QDs were prepared by a cosputtering method.19,20 Briefly, a thick Si-rich borophosphosilicate glass (BPSG) film was deposited on a stainless steel plate by simultaneously sputtering Si, SiO2, B2O3 and phosphosilicate glass (PSG) (SiO2:P2O5=95:5 wt.%). After the deposition, the film was peeled off from a stainless steel plate and annealed in a N2 atmosphere for 30 min at 1050, 1100, 1150, and 1200°C. This results in the growth of B and P codoped Si QDs of different sizes in BPSG matrices. Hereafter, we distinguish samples by the growth temperature, e.g., T1050 for the sample grown at 1050°C. Finally, BPSG matrices were removed by hydrofluoric acid (HF) etching (48 wt.%, 30-60 min). Liberated Si QDs were transferred to methanol and stored for 10 days. In this work, we grew Si QDs from two Si-rich BPSG film samples with different B and P concentrations, i.e., 0.8 at.% B and 0.3 at.% P (refer to LBP), and 0.9 at.% B and 0.6 at.% P (refer to HBP). We mainly discuss the data obtained for HBP samples unless otherwise specified. Figure 1a and b show the transmission electron microscope (TEM) (JEOL, JEM-2100F) images of codoped colloidal Si QDs grown at 1150 and 1200oC. The average diameters (Dave) and the standard deviations (σ) are shown in the figures. In both samples, the polydispersity, defined by σ/Dave, is around 20%. A high-resolution image in Figure 1c demonstrates the high crystallinity of a codoped Si QD. Figure 1d shows the PL spectra of codoped Si QDs grown at


5

Nano Letters 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

Page 6 of 24

different temperatures. By changing the growth temperature from 1050 to 1200oC, the PL peak moves from 1.6 to 0.9 eV.19 We refer these samples as as-produced Si QDs. The multistep size-selective precipitation process was performed for as-produced Si QDs grown at different temperatures. The process is schematically shown in Figure 1e. By adding toluene to a methanol dispersion of Si QDs, the solution became cloudy and then was centrifuged for 10 min at 21,500 g. The supernatant solution was decanted and the precipitate was immediately redispersed in methanol and stored. The process was repeated until the supernatant solution became transparent. The obtained fractions formed clear dispersions as shown in the photos in Figure 1e.

Figure 1. (a, b) TEM images of codoped Si QDs grown at 1150 and 1200oC. (c) High resolution TEM image of a codoped Si QD grown at 1150oC. (d) PL spectra of as-produced Si QDs grown


6

Page 7 of 24 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 Letters

at different temperatures excited at 405 nm. (e) Schematic of size-selective precipitation method and photographs of obtained fractions.

TEM analysis of size-separated codoped Si QDs We first perform TEM observations for the fractions (F2-F6). A Si QD solution is dropped on a copper TEM mesh covered by graphene oxide monolayers, which provides better contrast than a conventional amorphous carbon support film.23,24 Figure 2a-e shows the TEM images of F2-F6, respectively, of Si QDs grown at 1150oC (T1150). We can see that the QD size gradually decreases with increasing the fraction number. Moreover, we can recognize significant improvement of the size distribution compared to the as-produced sample in Figure 1a. Similar improvement of the size distribution is observed in TEM images of F2-F6 of Si QDs grown at 1200oC (T1200) in Figure 2f-j, respectively. Figure 2k shows Dave obtained from the TEM images as a function of the fraction number for T1150 and T1200. The error bars correspond to σ. As the fraction number increases, the Dave decreases from 7.6 to 3.5 nm for T1150 and from 10.8 to 5.0 nm for T1200. Figure 2l shows the polydispersity (σ/Dave) of as-produced and size-separated samples. The polydispersity of the separated fractions are much smaller than that of as-produced samples (AP). In T1200, the polydispersity of the fractions are in the range from 11% to 14%. In T1150, they are in the range from 8% to 10%. The polydispersity of 10% corresponds to the polydispersity index of 1.008.25 This value is comparable or better than the reported values of size-separated ligand stabilized Si QDs.26–28 It is known that nanoparticles with σ/Dave below 12% show the partial alignment during the self-assembly.29 Figure 2m displays a high-resolution image of Figure 2i. We can see that the


7

Nano Letters 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

Page 8 of 24

crystalline Si QDs are partially hexagonally packed. In Figure 2m, it is important to note that the distance between QDs is very small compared to ligand capped Si QD arrays, which always have the distance defined by the length of long alkyl chains (C8-12).24,30 The capability of the formation of a densely-packed QD layer with a small inter-dot distance is an advantage of codoped Si QDs for the optoelectronic applications. Indeed, the absence of organic ligands in codoped QD film leads to the high electrical conductivity without any thermal treatments.31

Figure 2. TEM images of size-separated samples (F2-F6) grown at (a-e) 1150 (T1150) and (f-j) 1200oC (T1200). (k) Average diameters and (l) polydispersity of as-produced and size-separated samples as a function of fraction number (F2-F6). (m) Higher magnification image of (i).

Photoluminescence spectra of size-separated Si QDs


8

Page 9 of 24 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 Letters

PL spectra of size-separated Si QDs were measured using a single spectrometer equipped with a liquid-N2 cooled InGaAs diode array (OMA-V-SE, Roper Scientific) and a CCD (Roper Scientific). The excitation source was 405 nm light from a semiconductor laser. Figure 3a-d show the normalized PL spectra of size-separated fractions of as-produced T1050, T1100, T1150 and T1200, respectively. In all the size-separated samples (F2-7), the peak blue-shifts monotonically with increasing the fraction numbers. In Figure 3e, we plot the PL peak energy of F2-F6 of T1150 (HBP and LBP) and T1200 (HBP) as a function of a Si QD diameter obtained from TEM images in Figure 2. The error bars in the abscissa are the same as those in Figure 2k. For comparison, the data of undoped Si QDs with similar size-distributions taken from a literature27 are shown. As has been studied in detail for many years,26,32,33 the PL peak energy of undoped Si QDs can be controlled from bulk Si bandgap to ~1.8 eV depending on the diameter. In codoped Si QDs, the PL energy is much lower than those of undoped Si QDs due to the optical transitions between donor and acceptor states as confirmed by STS.12 The PL energies of LBP samples are higher than those of HBP samples, but are still lower than those of undoped Si QDs. In Figure 3f, we plot the degree of the optical band gap shrinkage (∆E) of HBP and LBP samples as a function of diameter. ∆E of HBP samples (red spheres) is around 250-300 meV, while that of LBP samples (orange stars) is around 70-100 meV. In degenerately n- or p-type doped bulk Si crystal, the optical bandgap is reduced by up to 150 meV due to the formation of impurity bands and band-tailings.34–36 Similarly, the observed large ∆E values in codoped Si QDs may be explained by the interaction between impurity states. In addition, in Si QDs, the reduction of the dielectric screening increases ∆E.3,37 When the size of codoped Si QDs becomes below ~10 nm, the conduction and valence band edges start to shift by the quantum confinement


9

Nano Letters 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

Page 10 of 24

effect, which further increases ∆E. When the size becomes smaller than ~5 nm, quantum confinement of donor and acceptor starts,3,38 which further increases ∆E. On the other hand, the reduction of the number of impurities per a QD with decreasing the size decreases ∆E due to the decrease of the number of impurity states interacting each other. The observed size dependence of ∆E is considered to be the interplay of these effects. To quantitatively discuss the size dependent shrinkage of the optical band gap and to extract the information on the number of B and P pairs in a QD, we compare the data of size dependence of the PL energy with those obtained by the atomistic tight-binding calculations.39 In Figure 3e, we plot calculated size-dependence of the PL energy of Si QDs doped with different numbers of B and P pairs (2 (�▽), 5 (◊) and 10 (◁ ) pairs). The calculations are performed for 40 random configurations of impurity positions and the mean values are shown. Thanks to the comprehensive calculation data with different number of B-P pairs and sizes, we can roughly estimate the number of B-P pairs in a QD. In HBP samples, when the QD diameter is < 6 nm, the PL energy almost follows the curve of calculated results for 5 B-P pairs. Above the size, it deviates from the curve and approaches the values of 10 B-P pairs. Therefore, the number of BP pairs in a QD changes depending on the size. In LBP, the PL energy is close to the calculated values for QDs with 2 B-P pairs. Although the size range of the LBP sample is very narrow, we can see a similar trend, that is the number of B-P pairs changes from 6 nm) is in general ascribed to a fast nonradiative Auger recombination of a photoexcited e−h pair with the interaction of excess carriers in not-perfectly-compensated QDs.11,19,41 Recently, Limpens et al.16 demonstrated by ultrafast IA spectroscopy that interaction between excess carriers and a photoexcited e−h pair becomes small below 6 nm, and they do not interfere the recombination process anymore. It is very plausible that this effect increases the QY below the critical size in Figure 3h. In the energy range larger than the bulk Si band gap, the QY is nearly constant till ~1.5 eV and then rises steeply. It reaches maximum around 1.6 eV and then decreases. The maximum QY is 30%, which is the highest in all-inorganic Si QDs in polar solvents reported so far. Very similar detection energy dependence of the QY has been reported in undoped QDs by several groups,26,28,42 although the mechanism is not fully elucidated. A general explanation is that the QY is determined by the competition between enhanced radiative recombination rate by the quantum confinement effect and increased surface defect density in smaller QDs. The very similar trend of the QY between undoped and codoped Si QDs in the high energy range suggests that doping does not strongly influence the PL property of very small Si QDs.


12

Page 13 of 24 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 Letters

Figure 3. (a-d) PL spectra of as-produced and size-separated codoped Si QDs grown at different temperatures (HBP samples). (e) PL peak energies of codoped (HBP (red and blue spheres) and LBP (orange stars) samples) and undoped Si QDs (Ref.[27]) (gray squares) as a function of diameter. Calculated PL energies of Si QDs codoped with different numbers of B-P pairs (2 (�▽), 5 (◊) and 10 (◁ ) pairs) taken from Ref.[39] are also shown. (f) Energy difference of the PL peak energies (∆E) between undoped and codoped QDs (HBP and LBP) obtained from (e). (g) PL FWHM of as-produced (open circle) and size-separated (spheres) codoped QDs. The data of single codoped QDs (Ref.[40]) (magenta square) and size-separated undoped QDs (Ref.[27]) (gray square) are also shown. (h) PL QY of size-separated codoped QDs prepared from Si QDs grown at different temperatures and different B and P concentrations (HBP and LBP). Inset is the expansion of the low energy range.


13

Nano Letters 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

Page 14 of 24

Photoluminescence decay time of size-separated codoped Si QDs In order to reveal the mechanism of the PL broadening in size-purified codoped Si QDs, we study the PL decay rates at different emission energies. The time-resolved PL was excited by modulated 405 nm light and detected by a NIR photomultiplier (R5509-41, Hamamatsu Photonics) combined with a multichannel scaler. Figure 4a shows the decay curves of sizeseparated Si QDs (F2-F6) in T1150 detected at 1.1 eV (see the Supporting Information for the other detection energies). The decay curve of the as-produced sample is also shown as a reference. Since the decay curves are not a single-exponential function, a stretched exponential function, I = I0 exp (-t/τ)β, where τ is the apparent decay constant and β is the stretching parameter, is used for the analysis (see the Supporting Information for details of PL lifetime analysis and for the parameters extracted from the analysis (Table S1)). The stretching parameters are much smaller than unity and are in the range of 0.6-0.8 even for size-separated samples. The small stretching parameters mean that there is a broad distribution of the lifetime. The lifetime distribution may arise from different configurations of dopants in QDs. We will discuss this in detail later. The obtained lifetimes are in the range of 20 to 80 µsec which are much shorter than those of undoped Si QDs (200-500 µsec).27 The short lifetime suggests further localization of photoexcited e-h pairs at the impurity states in codoped QDs. A recent first principle calculation demonstrated that the lifetime of Si QDs doped with a single D-A pair with different configurations is in a 10 to 100 µsec range,43 which is in good agreement with our results. The agreement suggests that the observed shortening of the lifetime is predominantly due to that of


14

Page 15 of 24 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 Letters

the radiative lifetime. In the following discussion, we assume that a codoped Si QDs sample is composed of non-emitting “dark” QDs and “bright” QDs with a nearly 100% QY. The “dark” QDs do not contribute to the lifetime data, and thus the observed lifetime is the radiative lifetime of “bright” QDs. This assumption is appropriate considering generally observed experimental results that the internal QYs obtained by external modulation of a PL decay rate are much higher than the external QYs in Si QDs.44–46 Furthermore, in our codoped Si QDs, there is no correlation between the PL lifetime and the PL QY.19 This supports the existence of large number of “dark” QDs. In codoped Si QDs, the most plausible origin of “dark” QDs is the nonperfect compensation. If there are excess electrons or holes in a QD, photo-excited carriers recombine by the Auger process (~nsec), which is much faster than the radiative recombination,41,47 and the QY of that QD becomes close to 0%. In undoped Si QDs, the energy and the radiative decay rate of quantum confined excitons are determined solely by the size.44,45 Hence, the decay rate is in principle determined by the detection energy. This is not the case in codoped Si QDs as can be seen in Figure 4a. Despite the same detection energy, the decay rate is different between the fractions (i.e., QD diameter). To understand the implication of this phenomenon, we plot in Figure 4b the lifetimes obtained at different detection energies as a function of the diameter of a Si QD. Below ~5.5 nm, the lifetime depends only slightly on the diameter and the detection energy dependence is very small. This indicates that these fractions are a quite homogeneous system. On the other hand, when the diameter is larger than ~5.5 nm, the lifetime depends strongly on the detection energy. The system thus contains inhomogeneities other than the size. If we look at Figure 4b from other direction, we can say that the PL lifetime detected at 1.3 eV is almost independent of the diameter, while those at smaller energies, e.g., 1.1 eV, depend strongly on the diameter.


15

Nano Letters 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

Page 16 of 24

In D-A pair recombination in bulk semiconductor crystal, the emission energy and the decay rate are determined by the D-A distance.48 A distant D-A pair has a smaller transition energy due to the smaller Coulomb attraction and a longer lifetime due to the smaller overlap of bound electron and hole wavefunctions. It is very plausible that the detection energy dependence of the PL lifetime is the manifestation of the distribution of D-A distances in codoped Si QDs as discussed below. Although 5-10 pairs of B and P are doped in HBP samples as shown in Figure 3e, for simplicity, we illustrate the situation that only one B and one P atoms are doped in a QD. We then assume that the energy and the lifetime of PL are determined both by the physical size of a QD and the distance between B and P atoms. As schematically shown in Figure 4c, codoped Si QDs 3.5 nm in diameter has several emission bands depending on the B-P distance. The emission energy is the largest and the lifetime is the shortest when the distance is the smallest (“close”), and vice versa when the distance is the largest (“far”). However, since the physical size of a QD is very small and close to the impurity Bohr radius in bulk Si crystal, the distribution of the lifetime is expected to be small (i.e., τ3.5nm,close ≈ τ3.5nm,far). On the other hand, in a Si QD 5.8 nm in diameter, the largest B-P distance is much larger than the case of the 3.5 nm QD, while the smallest distance is the same. This results in the large distribution of the lifetime depending on the detection energy (i.e., τ5.8nm,close < τ5.8nm,far). It should be noted in Figure 4c that in the same emission energy, the D-A distance of a 5.8 nm QD is always larger than that of a 3.5 nm QD. Finally, it is important to mention the appropriateness of the very large D-A distancedependence of the emission energy. In bulk Si crystal, the energy difference between D-A pairs with the distances of 1.9 nm and 3.3 nm is around 0.025eV,49 which is one order of magnitude


16

Page 17 of 24 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 Letters

smaller than our experimental results.49 However, in Si QDs, this value is expected to be largely enhanced. Ab-initio calculations of very small Si QDs (~2 nm) demonstrate that the HOMOLUMO gap changes about 0.4 eV when the distance is changed from 0.4 to 1.4 nm.21 Similarly, in atomistic tight-binding calculations, the energy gap variation of several hundreds of eV depending on the dopant configurations is shown for codoped Si QDs 2 to 10 nm in diameter.39

Figure 4. (a) PL decay curves of as-produced (T1150) and size-separated samples measured at 1.1 eV. (b) Average lifetimes measured at different photon energies as a function of QD diameter. (c) Schematic illustration and energy level structures of 3.5 and 5.8 nm QDs with “close” and “far” D-A pairs emitting at 1.1 (blue) and 1.3 eV (red).


17

Nano Letters 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

Page 18 of 24

In summary, we investigated the doping-induced bandgap shrinkage in a wide size range through PL spectroscopy of heavily B and P codoped Si QDs. To this end, we developed almost monodispersed codoped QDs with σ < 10% by the size-selective precipitation method. Despite the reduced size distribution, the emission spectra of codoped QDs were much broader than that of undoped QDs. The reduction of the size inhomogeneity enabled us to compare the PL energy with that of undoped Si QDs and to provide quantitative information on the size dependence of the doping-induced shrinkage of the optical gap. We also succeeded in estimating the number of D-A pairs in Si QDs over a wide size range by comparing the PL energy with tight-binding calculations. We showed that 2-10 D-A pairs are formed in our codoped Si QDs depending on the size and the B and P concentration in starting materials. From detailed analyses of the PL lifetimes of size-purified Si QDs, we found that the diameter of around 5.5 nm is a critical dimension. Above ~5.5 nm, the recombination rate depends on the D-A pair distance, while below ~5.5 nm, it is governed by the physical size of a QD rather than the D-A pair distance. Our findings provide quantitative data of photo-physical properties of heavily doped Si nanostructures useful in forthcoming nano-electronic devices.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Photoluminescence decay analyses and Supplementary Figures of additional experimental data (PDF) Conflict of Interest


18

Page 19 of 24 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 Letters

The authors declare no competing financial interests.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

ACKNOWLEDGMENT This work was partly supported by the 2015 JST Visegrad Group (V4)−Japan Joint Research Project on Advanced Materials, JSPS KAKENHI Grant Number 16H03828 and 18K14092, and JSPS 2018 Bilateral Joint Research Projects (Japan–Australia).

REFERENCES (1)

Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Nature 2005, 436 (7047), 91–94.

(2)

Mocatta, D.; Cohen, G.; Schattner, J.; Millo, O.; Rabani, E.; Banin, U. Science 2011, 332 (6025), 77–81.

(3)

Pereira, R. N.; Stegner, A. R.; Andlauer, T.; Klein, K.; Wiggers, H.; Brandt, M. S.; Stutzmann, M. Phys. Rev. B 2009, 79 (16).

(4)

Stegner, A. R.; Pereira, R. N.; Lechner, R.; Klein, K.; Wiggers, H.; Stutzmann, M.; Brandt, M. S. Phys. Rev. B 2009, 80 (16), 165326.


19

Nano Letters 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

(5)

Page 20 of 24

Fujii, M.; Mimura, A.; Hayashi, S.; Yamamoto, Y.; Murakami, K. Phys. Rev. Lett. 2002, 89 (20), 206805.

(6)

Sahu, A.; Kang, M. S.; Kompch, A.; Notthoff, C.; Wills, A. W.; Deng, D.; Winterer, M.; Frisbie, C. D.; Norris, D. J. Nano Lett. 2012, 12 (5), 2587–2594.

(7)

Whitham, P. J.; Knowles, K. E.; Reid, P. J.; Gamelin, D. R. Nano Lett. 2015, 15 (6), 4045–4051.

(8)

Hughes, K. E.; Hartstein, K. H.; Gamelin, D. R. ACS Nano 2018, 12 (1), 718–728.

(9)

Pradhan, N.; Das Adhikari, S.; Nag, A.; Sarma, D. D. Angew. Chemie Int. Ed. 2017, 56 (25), 7038–7054.

(10)

Sharma, M.; Gungor, K.; Yeltik, A.; Olutas, M.; Guzelturk, B.; Kelestemur, Y.; Erdem, T.; Delikanli, S.; McBride, J. R.; Demir, H. V. Adv. Mater. 2017, 29 (30), 1700821.

(11)

Hori, Y.; Kano, S.; Sugimoto, H.; Imakita, K.; Fujii, M. Nano Lett. 2016, 16 (4), 2615– 2620.

(12)

Ashkenazi, O.; Azulay, D.; Balberg, I.; Kano, S.; Sugimoto, H.; Fujii, M.; Millo, O. Nanoscale 2017, 9 (45), 17884–17892.

(13)

Angı, A.; Sinelnikov, R.; Heenen, H. H.; Meldrum, A.; Veinot, J. G. C.; Scheurer, C.; Reuter, K.; Ashkenazy, O.; Azulay, D.; Balberg, I.; Millo, O.; Rieger, B. Nanotechnology 2018, 29 (35), 355705.

(14)

Rowe, D. J.; Jeong, J. S.; Mkhoyan, K. A.; Kortshagen, U. R. Nano Lett. 2013, 13 (3), 1317–1322.


20

Page 21 of 24 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 Letters

(15)

Zhou, S.; Pi, X.; Ni, Z.; Ding, Y.; Jiang, Y.; Jin, C.; Delerue, C.; Yang, D.; Nozaki, T. ACS Nano 2015, 9 (1), 378–386.

(16)

Limpens, R.; Fujii, M.; Neale, N. R.; Gregorkiewicz, T. J. Phys. Chem. C 2018, 122 (11), 6397–6404.

(17)

Limpens, R.; Sugimoto, H.; Neale, N. R.; Fujii, M. ACS Photonics 2018 DOI: 10.1021/acsphotonics.8b00671.

(18)

Dalpian, G. M.; Chelikowsky, J. R. Phys. Rev. Lett. 2006, 96 (June), 226802.

(19)

Sugimoto, H.; Fujii, M.; Imakita, K.; Hayashi, S.; Akamatsu, K. J. Phys. Chem. C 2013, 117 (22), 11850–11857.

(20)

Sugimoto, H.; Fujii, M.; Imakita, K.; Hayashi, S.; Akamatsu, K. J. Phys. Chem. C 2012, 116 (33), 17969–17974.

(21)

Iori, F.; Degoli, E.; Magri, R.; Marri, I.; Cantele, G.; Ninno, D.; Trani, F.; Pulci, O.; Ossicini, S. Phys. Rev. B - Condens. Matter Mater. Phys. 2007, 76 (8), 1–14.

(22)

Nomoto, K.; Sugimoto, H.; Breen, A.; Ceguerra, A. V.; Kanno, T.; Ringer, S. P.; Wurfl, I. P.; Conibeer, G.; Fujii, M. J. Phys. Chem. C 2016, 120 (31), 17845–17852.

(23)

Sugimoto, H.; Yamamura, M.; Sakiyama, M.; Fujii, M. Nanoscale 2018, 10 (16), 7357– 7362.

(24)

Panthani, M. G.; Hessel, C. M.; Reid, D.; Casillas, G.; José-Yacamán, M.; Korgel, B. A. J. Phys. Chem. C 2012, 116 (42), 22463–22468.


21

Nano Letters 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

(25)

Page 22 of 24

Miller, J. B.; Van Sickle, A. R.; Anthony, R. J.; Kroll, D. M.; Kortshagen, U. R.; Hobbie, E. K. ACS Nano 2012, 6 (8), 7389–7396.

(26)

Mastronardi, M. L.; Maier-Flaig, F.; Faulkner, D.; Henderson, E. J.; Kübel, C.; Lemmer, U.; Ozin, G. A. Nano Lett. 2012, 12 (1), 337–342.

(27)

Yu, Y.; Fan, G.; Fermi, A.; Mazzaro, R.; Morandi, V.; Ceroni, P.; Smilgies, D.-M.; Korgel, B. A. J. Phys. Chem. C 2017, 121 (41), 23240–23248.

(28)

Liu, X.; Zhang, Y.; Yu, T.; Qiao, X.; Gresback, R.; Pi, X.; Yang, D. Part. Part. Syst. Charact. 2016, 33 (1), 44–52.

(29)

Auer, S.; Frenkel, D. Nature 2001, 413 (6857), 711–713.

(30)

Yu, Y.; Lu, X.; Guillaussier, A.; Voggu, V. R.; Pineros, W.; de la Mata, M.; Arbiol, J.; Smilgies, D.-M.; Truskett, T. M.; Korgel, B. A. Nano Lett. 2016, 16 (12), 7814–7821.

(31)

Kano, S.; Tada, Y.; Matsuda, S.; Fujii, M. ACS Appl. Mater. Interfaces 2018, 10 (24), 20672–20678.

(32)

Kovalev, D.; Heckler, H.; Polisski, G.; Koch, F. Phys. status solidi 1999, 215 (2), 871– 932.

(33)

Hessel, C. M.; Reid, D.; Panthani, M. G.; Rasch, M. R.; Goodfellow, B. W.; Wei, J.; Fujii, H.; Akhavan, V.; Korgel, B. A. Chem. Mater. 2012, 24 (2), 393–401.

(34)

Wagner. Phys. Rev. B. Condens. Matter 1985, 32 (2), 1323–1325.

(35)

Schmid, P. E. Phys. Rev. B 1981, 23 (10), 5531–5536.


22

Page 23 of 24 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 Letters

(36)

Wagner, J. Phys. Rev. B 1984, 29 (4), 2002–2009.

(37)

Almeida, A. J.; Sugimoto, H.; Fujii, M.; Brandt, M. S.; Stutzmann, M.; Pereira, R. N. Phys. Rev. B 2016, 93 (11), 115425.

(38)

Melnikov, D. V; Chelikowsky, J. R. Phys. Rev. Lett. 2004, 92 (4), 046802.

(39)

Delerue, C. Phys. Rev. B 2018, 98 (4), 045434.

(40)

Kanno, T.; Sugimoto, H.; Fucikova, A.; Valenta, J.; Fujii, M. J. Appl. Phys. J. Appl. Phys. Appl. Phys. Lett 2016, 120 (94), 164307–183103.

(41)

Delerue, C.; Lannoo, M.; Allan, G.; Martin, E.; Mihalcescu, I.; Vial, J. C.; Romestain, R.; Muller, F.; Bsiesy, A. Phys. Rev. Lett. 1995, 75 (11), 2228–2231.

(42)

Sun, W.; Qian, C.; Wang, L.; Wei, M.; Mastronardi, M. L.; Casillas, G.; Breu, J.; Ozin, G. A. Adv. Mater. 2015, 27 (4), 746–749.

(43)

Somogyi, B.; Derian, R.; Štich, I.; Gali, A. J. Phys. Chem. C 2017, 121 (49), 27741– 27750.

(44)

Walters, R. J.; Kalkman, J.; Polman, A.; Atwater, H. A.; de Dood, M. J. A. Phys. Rev. B 2006, 73 (13), 132302.

(45)

Miura, S.; Nakamura, T.; Fujii, M.; Inui, M.; Hayashi, S. Phys. Rev. B 2006, 73 (24), 245333.

(46)

Sangghaleh, F.; Sychugov, I.; Yang, Z.; Veinot, J. G. C.; Linnros, J. ACS Nano 2015, 9 (7), 7097–7104.


23

Nano Letters 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

Page 24 of 24

(47)

Limpens, R.; Neale, N. R. Nanoscale 2018, 10 (25), 12068–12077.

(48)

Yu, P.; Cardona, M. Fundamentals of Semiconductors: Physics and Materials Properties; Graduate Texts in Physics; Springer Berlin Heidelberg, 2010.

(49)

Tajima, M.; Iwai, T.; Toyota, H.; Binetti, S.; Macdonald, D. J. Appl. Phys. 2011, 110 (4), 043506.

SYNOPSIS


24

Donor-Acceptor Pair Recombination in Size-Purified Silicon Quantum

Recommend Documents