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Manipulating the Band Bending of InGaN/GaN Quantum Dots in Nanowires by Surface Passivation Zilan Wang, Zhibiao Hao, Jiadong Yu, Chao Wu, 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.7b00578 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017

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The Journal of Physical Chemistry

Manipulating the Band Bending of InGaN/GaN Quantum Dots in Nanowires by Surface Passivation Z. L. Wang, Z. B. Hao*, J. D. Yu, C. Wu, L. Wang, J. Wang, C. Z. Sun, B. Xiong, Y. J. Han, H. T. Li and Y. Luo* Tsinghua National Laboratory for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China Abstract InGaN/GaN quantum dots (QD) in nanowires exhibit excellent optical properties and are promising candidates for nano-scale optoelectronic devices. However, large amount of surface states would cause low quantum efficiency more severely than bulk materials, through not only nonradiative recombination centers but also upward band bending. Therefore it is necessary to control the band bending effect in order to improve the quantum efficiency of QDs. In this work, quantitative measurements are carried out by ultraviolet photoelectron spectroscopy (UPS) to describe the band bending effect in InGaN/GaN QD in nanowires coated with three different dielectric layers including SiNx, Al2O3 and SiO2. Furthermore, their passivation mechanisms are investigated by photoluminescence (PL), time resolved PL. Contrary to SiO2 passivation, the SiNx and Al2O3 passivation nanowires demonstrate notable improvements in emission intensity. Most essentially, all experimental findings are consilience with the physical model that the deposition of dielectric layers effectively alter the surface states of nanowires resulting a weakening or strengthen in band bending near the surface. Our systematic studies on passivation of nanowires can provide strategies for optimizing the performance of nanowire-based optoelectronic applications.

* [email protected] * [email protected]

Introduction 1

ACS Paragon Plus Environment


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The tremendous success in gallium nitride (GaN) based optoelectronic devices has pushed forward the studies on its low dimensional counterparts such as nanowires to explore new physics and potential applications.1 GaN based nanowires grown on silicon demonstrate high level of crystal quality due to effective strain relaxation and free of extended defects.2, 3 Furthermore, InGaN disks-like quantum dot (QD) in GaN nanowire exhibits excellent optical properties such

as

strong

three-dimensional

quantum

confinement,

suppressed

quantum-confined Stark effect (QCSE),4, 5 shorter carrier lifetime6 and the Auger coefficient which is 2-3 orders of magnitude smaller than those in heteroepitaxial bulk materials.7 Meanwhile, the emission wavelength can be tuned from UV to visible region by adjusting the indium content or the dimension of nanowire.8 These excellent properties ensure promising applications in nano-scale optoelectronic devices such as single photon emitters9 and nanolasers,10 also prospect in interdisciplinary fields such as photocatalysis,11optochemical sensing12 and cellular level biomedicine.13

However, due to large surface-to-volume ratio, the rate of surface recombination is high, and the injected carriers would encounter large area of nanowire surface which significantly restricts the dynamics of carriers. These localized defects and charged-acceptor-like surface states (dangling bonds, adsorbates, etc.), act as traps of free carriers and nonradiative recombination centers causing harmful effect on the radiative efficiency, eventually uncontrolled and degraded properties.14 Therefore, it is inevitable to make the surface passive, i.e. virtually inert before fabricating nanowire-based device. The surface passivation on GaN based thin film and devices have been widely studied with approaches such as dielectric deposition,15 chemisorption,16, aqueous of NH4OH,16 HCL:HNO3,19,

20

17

annealing,18 surface cleaning in

KOH,19 sulfides.21-24 Among these

methods, dielectric deposition has the advantages of being reproducible, feasible to single nanowire, and without much changes in morphology. SiNx, Al2O3 and SiO2 are common dielectric materials to passivate surface states and enhance the performance of GaN-based thin film devices.25,

26

Although passivation

2


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technology has been studied for many years, the analysis on its physical mechanism and the effect of band manipulation are not comprehensive. Limited by the characterization techniques, it is difficult to analyze the nanoscale sample, especially on the surface. For low dimensional quantum devices, which were severely affected by surface states, very few quantitative measurements have been reported on the band bending. Hence, it is crucial to examine the outcome and understand the physical mechanism for different passivation materials.

In this work, ultraviolet photoemission spectroscopy (UPS), as a nondestructive and most superficial inspection technique,27-29 was employed to quantitatively determine the shift of surface binding energy for different passivation. Samples of disk-like InGaN QDs in GaN nanowires grown by plasma-assisted molecular beam epitaxy (PA-MBE) are coated by three different dielectric passivation layers including SiNx, Al2O3 and SiO2. Temperature-dependent photoluminescence (PL) and time-resolved photoluminescence (TRPL) were carried out to investigate the connection between different passivation approaches to that of carrier recombination processes. The UPS results combined with optical characterization offer full perspective in explaining passivation mechanism for these dielectric layers based on a band bending manipulation model.

Experimental Section The two-dimensional (2D) disk-like InGaN quantum dots in GaN nanowires have been grown by Frank-van der Merwe (F-M) mode on the Si (111) substrate with temperature of 580 °C using PA-MBE. The nanowires have an average diameter of 30 nm and with density of 1010 cm2 shown in the SEM image of Figure 1(a). Each nanowire contains of 2 nm In0.19Ga0.81N disk with wurtzite structure along the c-axis.

Dielectric layers were employed to passivate the surface of nanowires. Al2O3 film was deposited by atomic layer deposition (ALD) with 200 °C substrate temperature using trimethyl aluminum (TMA) and H2O as precursors (200 cycles). 3



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SiNx and SiO2 were deposited at 300 °C by plasma enhanced chemical vapor deposition (PECVD). Figure 1(d) is the SEM image of InGaN/GaN QD in nanowires after Al2O3 deposition. A comparison of single nanowire before and after deposition can be found in Figure 1(b) and (e). The axial thickness of the coating was approximately 10 nm which is further confirmed by backscatter images (Figure 1(c) and (f)). Notice that SiNx and SiO2 coatings have similar geometrical features as Al2O3.

Figure 1 (a) and (b) are SEM images for group and single nanowire, respectively. (c) is the backscatter image of (b). Correspondingly, (d) (e) and (f) are images representing nanowires in the upper figures after Al2O3 layer coating. The 4 figures, (b), (c), (e) and (f) share the same scale bar. In (f) the dark shell is attributed to Al2O3 layer as the elements are lighter than GaN. To inspect the surface band bending of nanowires, the band diagrams were analyzed by UPS which is integrated inside the Kratos Axis Ultra DLD with He I (hυ=21.22eV) as excitation source under a base pressure at 10-10 Torr. The Fermi level of the system was measured on a clean Au substrate. Since UPS is extremely sensitive to the surface, the thickness of passivation layer for UPS measurement is ultra-thin, at 1-3 nm.

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The signal for temperature dependent (10-300 K) PL spectroscopy was collected by the Jobin Yvon 550 monochromator with a 0.5 nm resolution in wavelength measurement. The excitation for TRPL measurement was provided by a Ti:Sapphire laser (Spectra Physics) with 405 nm wavelength, pulse width of 170 fs and repetition rate of 80 MHz. The excitation wavelength was chosen to guarantee only the InGaN disks is excited while the GaN nanowire does not produce any carriers. The detector was a Picoquant single photon avalanche diode with a resolution time of 50 ps.

Results and Discussion

Figure 2 The UPS spectra of as-grown and passivated samples. The intensities of left part have been enlarged 5 times to show the onset of spectra. The spectra have been vertically shifted for clarity.

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The band structure at the surface has been adjusted after coating with different dielectric layers. A change in surface potential can be measured quantitatively using UPS through measuring a parallel shift of all binding energies. Figure 2 shows the energy distribution of emitted electrons under irradiated He I excitation. The scanning range is -2 ~ 23 eV with the resolution of 0.025 eV. The left side of the spectra provides the energy position of the valence band maximum (VBM) relative to Fermi level (EF), and the peak on the high binding energy side represents the deepest energy level where the electrons can be extracted in the valence band.30

For as-grown sample, EV is located at 2.099 ± 0.05 eV below EF by linearly extrapolating the leading edge of the valence band spectrum to the baseline, whereas EV of the passivated samples have different degree of energy shift comparing with the as-grown ones. In Figure 2, the values of EF - EV for SiNx and Al2O3 coated samples shift to larger than as-grown sample, whereas that of SiO2 coated sample becomes smaller. These changes are attributed to the downward and upward bending of the valence band minimum near the surface, respectively.

Figure 3 Schematics of the band bending for (a) a wire with large diameter and (b) and (c) wires with small diameter, along with the distribution of electrons and

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holes. Arrows represent the corresponding transitions. The relative energetic locations of EC, EV, and EF are not on scale. Figure 3 shows a schematic drawing of equilibrium energy band diagram illustrating the physical band bending under different circumstances. The large concentration of surface states lead to various physical outcomes that substantially affects the surface electronic properties, such as space-charge related surface depletion,31 upward band bending and Fermi level pinning at the surface of GaN nanowire.32 For nanowire with diameter exceeding 80 nm, as in Figure 3(a), only the surface region gets depleted while the physical properties near center resemble to that of the bulk GaN.33 For example, Bao et al. studied the emission mechanism of InGaN nano-discs in GaN nanowires with diameters of 160 nm and 100 nm which were obtained by etching planar multiple quantum wells.34 In that case, the radiative recombination mainly comes from the interior of the nanowire while surface state acts as nonradiative recombination center, which is similar to that presented in Figure 3(a). However, the surface band bending in our samples becomes more significant as the whole nanowire is almost completely depleted due to the small diameter of 30 nm. The effect of band bending causes electrons prefer the inner part of the column, whereas holes tend to accumulate on the surface in Figure 3(b). This leads to the separation of the wave function of electron and hole, the recombination of non-equilibrium carriers is thus reduced. After successful reduction of surface state, carriers are redistributed and the separation of electrons and holes has been weakened, and thus the band structure becomes flatten (Figure 3(c)).

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Figure 4 Low temperature (LT, blue line) and room temperature (RT, red line) PL spectra of InGaN/GaN quantum dots in nanowire passivated by different dielectric layers. Comparing with the as-grown sample (dash line), these spectra show shifts of emission peak. Table 1 PL peak position at RT and 10K, as well as the IQE of as-grown and passivated samples. Peak Position (RT)

Peak Position (10K)

IQE

As-grown

457.5 nm

447.5 nm

10.23%

SiNx passivation

445.0 nm

439.0 nm

19.23%

Al2O3 passivation

449.0 nm

440.0 nm

14.88%

SiO2 passivation

458.0 nm

447.0 nm

7.39%

To further illustrate the band bending manipulation, optical properties have been characterized systematically. Figure 4 shows the PL spectra at 10 K and 300 K of samples passivated by different dielectric layers comparing with the as-grown sample. The intensities and peak positions at 300 K have been changed significantly before and after passivations, where SiNx and Al2O3 passivated samples show higher emission intensity and blue shifts in peak position comparing with the as-grown and SiO2 passivated sample. Despite the light extraction efficiency affected by different refractive indices of the passivation material, the differences of transmittance are less than 1% using the simulation of

8


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transfer matrix method. These spectral features have been verified through repeated experiments and the results of PL measurements were summarized in Table I for comparison. The internal quantum efficiency (IQE) is used to quantify the emission property of QD in nanowire, which is defined as the integrated intensity ratio between room temperature and 10 K assuming that the nonradiative centers are frozen at 10 K.35,

36

The recombination losses in our

sample are dominated by point defect, surface recombination and surface state while dislocations, the common nonradiative centers for GaN based thin film samples, are absent due to the growth method.2, 3 Therefore, surface passivation can effectively alter the density and working mechanism of surface state resulting the variation of IQE. For SiNx passivated sample, its IQE almost doubled comparing with the as-grown sample indicating a successful manipulation to the surface states. This is because that band bending reduction leads to an increase of overlap in electron and hole wave function, and facilitates the radiative recombination (Figure 3(c)). Meanwhile, the peak position of Al2O3 and SiNx passivated samples shift to higher energies (the change up to 70meV), but the SiO2 passivated sample remains. The blue shift in peak position also benefits from band flattening, since the separation of electrons and holes has been weakened (Figure 3(b) and (c)).

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Figure 5 The TRPL results (symbol) of as-grown and passivated sample measured at 300 K with 405 nm excitation. The lines represent fitting result using stretched exponential model. The spectra have been vertically shifted for clarity.

Table 2 Variation of radiative, non-radiative, and total carrier lifetime in as-grown and passivated samples fitted with stretched exponential model.

τ

τr

τnr

As-grown

439 ps

4294 ps

489 ps

SiNx passivation

460 ps

2392 ps

570 ps

Al2O3 passivation

454 ps

3065 ps

533 ps

SiO2 passivation

341 ps

4618 ps

368 ps

To further analyze the influence of passivation on recombination mechanism, TRPL measurements were performed at the wavelength of PL peak position (Figure 5). The transient response of our QD sample is studied by the stretched exponential model,37

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I(t) = I 0 exp[−(t / τ )β ], where

I0

stands for the photon counts at

lifetime and β

t = 0, τ is the photo-excited carrier

is the stretching parameter which suggests the polarization field

in the disks.38 This is expected because of the radial relaxation of strain during the growth of nanowires.

The lifetime of carriers can be calculated by the following equations,

1 /τ = 1 / τr + 1 / τnr , ηin = τnr /(τr + τnr ) , where

τr

and

τnr

represent radiative recombination and non-radiative

recombination lifetime respectively and

ηin

is the internal quantum efficiency

described in Table I. The results of nonradiative and radiative lifetime are listed in Table II. For SiNx and Al2O3 passivated samples, the radiative recombination lifetime become shorter and the non-radiative recombination lifetime become longer, which are contrary to that of SiO2 passivated sample. This can be explained from the band bending model, the overlapping of wave functions is increased after band flattening causing the decrease in radiative recombination lifetime. Meanwhile, the reduction in nonradiative centers leads to an increase in nonradiative lifetime. Thus, the SiNx passivation is more effective in reducing the surface state concentration than Al2O3 as supported by the PL data. On the other hand, the SiO2 passivation may introduce extra surface states so that its radiative lifetime is comparable and even slightly longer than that of the as-grown while its nonradiative life time significantly decreases. According to previous studies on the surface passivation for bulk materials,15, 35, 39-41

we tentatively deduce the physical mechanism of these passivation methods

on the QD in nanowire samples. We have good reasons to believe that SiNx layer successfully eliminates the N deficiency defects while for Al2O3 introduces negative fixed charge forming an electric field to offset the effect of band bending.

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Conclusions To conclude, a systematic study of passivation mechanism has been done on InGaN/GaN QD in nanowire with three different dielectric layers. These passivation layers reduce the surface state and simultaneously adjust the band structure on the surface. Comparing with the as-grown nanowire, SiNx and Al2O3 passivated samples exhibit manifest improvement on their IQE by 88% and 45%, respectively. Based on the analysis of PL and TRPL, we are able to investigate the influence of band bending on the nanowires’ emission intensity, the shift of peak position, radiative and nonradiative lifetime. More importantly, these results can bond with data from UPS which quantitatively probe the changes in surface band bending effect, and thus forming a comprehensive picture on the physical nature of band bending manipulation. We believe our work will receive great attentions in further studies on optimizing the performance of nanowire-based optoelectronic applications.

Acknowledgements This work was supported by National Basic Research Program of China (Grant No. 2013CB632804，2015CB351900), National Key Research and Development Plan (Grant No. 2016YFB0400102), the National Natural Science Foundation of China

(Grant

Nos.

61574082,

61210014,

61321004,

61307024,

and

51561165012), the High Technology Research and Development Program of China (Grant No. 2015AA017101), Tsinghua University Initiative Scientific Research Program (Grant No. 2013023Z09N, 2015THZ02-3), the Open Fund of the

State

Key

Laboratory

on

Integrated

Optoelectronics

(Grant

No.

IOSKL2015KF10), the CAEP Microsystem and THz Science and Technology Foundation (Grant No. CAEPMT201505) and S&T Challenging Project.

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H.;

Yan,

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25th

anniversary

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36. Watanabe, S.; Yamada, N.; Nagashima, M.; Ueki, Y.; Sasaki, C.; Yamada, Y.; Taguchi, T.; Tadatomo, K.; Okagawa, H.; Kudo, H. Internal quantum efficiency of highly-efficient InxGa1-xN-based near-ultraviolet light-emitting diodes. Appl. Phys. Lett. 2003. 83, 4906-4908. 37. Johnston, D. C. Stretched exponential relaxation arising from a continuous sum of exponential decays. Phys. Rev. B 2006, 74, 184430. 38. Jahangir, S.; Mandl, M.; Strassburg, M.; Bhattacharya, P. Molecular beam epitaxial growth and optical properties of red-emitting (λ= 650 nm) InGaN/GaN disks-in-nanowires on silicon. Appl. Phys. Lett. 2013, 102, 071101. 39. Heinrich, J.; Huggenberger, A.; Heindel, T.; Reitzenstein, S.; Höfling, S.; Worschech, L.; Forchel, A. Single photon emission from positioned GaAs/AlGaAs photonic nanowires. Appl. Phys. Lett. 2010, 96, 211117. 40. Hashizume, T.; Ootomo, S.; Inagaki, T.; Hasegawa, H. Surface passivation of GaN and GaN/AlGaN heterostructures by dielectric films and its application to insulated-gate heterostructure transistors. J. Vac. Sci. Technol. B 2003, 21, 1828. 41. Hashizume, T.; Hasegawa, H. Effects of nitrogen deficiency on electronic properties of AlGaN surfaces subjected to thermal and plasma processes. Appl. Surf. Sci. 2004, 234, 387-394.

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Figure 1 (a) and (b) are SEM images for group and single nanowire, respectively. (c) is the backscatter image of (b). Correspondingly, (d) (e) and (f) are images representing nanowires in the upper figures after Al2O3 layer coating. The 4 figures, (b), (c), (e) and (f) share the same scale bar. In (f) the dark shell is attributed to Al2O3 layer as the elements are lighter than GaN. 101x71mm (300 x 300 DPI)



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Figure 2 The UPS spectra of as-grown and passivated samples. The intensities of left part have been enlarged 5 times to show the onset of spectra. The spectra have been vertically shifted for clarity. 180x184mm (300 x 300 DPI)


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Figure 3 Schematics of the band bending for (a) a wire with large diameter and (b) and (c) wires with small diameter, along with the distribution of electrons and holes. Arrows represent the corresponding transitions. The relative energetic locations of EC, EV, and EF are not on scale. 202x120mm (300 x 300 DPI)



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Figure 4 Low temperature (LT, blue line) and room temperature (RT, red line) PL spectra of InGaN/GaN quantum dots in nanowire passivated by different dielectric layers. Comparing with the as-grown sample (dash line), these spectra show shifts of emission peak. 251x130mm (300 x 300 DPI)


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Figure 5 The TRPL results (symbol) of as-grown and passivated sample measured at 300 K with 405 nm excitation. The lines represent fitting result using stretched exponential model. The spectra have been vertically shifted for clarity. 254x190mm (300 x 300 DPI)



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GaN Quantum Dots in

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