Vapor-Phase Incommensurate Heteroepitaxy of Oriented Single

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Vapor-Phase Incommensurate Heteroepitaxy of Oriented Single-Crystal CsPbBr on GaN: Towards Integrated Optoelectronic Applications 3

Liyun Zhao, Yan Gao, Man Su, Qiuyu Shang, Zhen Liu, Qi Li, Qi Wei, Meili Li, Lei Fu, Yangguang Zhong, Jia Shi, Jie Chen, Yue Zhao, Xiaohui Qiu, Xinfeng Liu, Ning Tang, Guichuan Xing, Xina Wang, Bo Shen, and Qing Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b02885 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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Vapor-Phase Incommensurate Heteroepitaxy of Oriented Single-Crystal CsPbBr3 on GaN: Towards Integrated Optoelectronic Applications Liyun Zhao,†, # Yan Gao,†, ‡, # Man Su,†, # Qiuyu Shang,† Zhen Liu,† Qi Li,†, ‡ Qi Wei,⊥ Meili Li,† Lei Fu,§ Yangguang Zhong,£ Jia Shi,£ Jie Chen,£ Yue Zhao, Xiaohui Qiu,£ Xinfeng Liu,£ Ning Tang,§ Guichuan Xing,⊥ Xina Wang,‡ Bo Shen,§ and Qing Zhang†,* †

Department of Materials Science and Engineering, College of Engineering, Peking University,

Beijing 100871, P. R. China ‡

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key

Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics and Electronic Science, Hubei University, Wuhan 430062, P. R. China ⊥

Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials

Engineering, University of Macau, Macao SAR 999078, P. R. China §

State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics,

Peking University, Beijing 100871, P. R. China £

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center of

Excellence for Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China Institute

for Quantum Science and Engineering and Department of Physics, Southern University

of Science and Technology, Shenzhen 518055, P. R. China *E-mail: [email protected].


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ABSTRACT: Integrating metallic halide perovskites with established modern semiconductor technology is significant for promoting the development of application-level optoelectronic devices. To realize such devices, exploring the growth dynamics and interfacial carrier dynamics of perovskites deposited on the core materials of semiconductor technology is essential. Herein, we report the incommensurate heteroepitaxy of highly oriented single-crystal cesium lead bromide (CsPbBr3) on c-wurtzite GaN/sapphire substrates with atomically smooth surface and uniform rectangular shape by chemical vapor deposition. The CsPbBr3 microplatelet crystal exhibits greencolored lasing under room temperature and has a structural stability comparable with that grown on van der Waals mica substrates. Time-resolved photoluminescence spectroscopy studies show that the type-II CsPbBr3-GaN heterojunction effectively enhances the separation and extraction of free carriers inside CsPbBr3. These findings provide insights into the fabrication and applicationlevel integrated optoelectronic devices of CsPbBr3 perovskites. KEYWORDS: cesium lead bromide; perovskite; incommensurate epitaxy; vapor-phase deposition; lasing; microplatelet


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Metal halide perovskites are a family of direct band-gap semiconductors with the chemical formula of ABX3 and the perovskites crystal structure (A sites for organic or inorganic cations, such as Cs+, and CH3NH3+; B sites for metal cations, such as Pb2+, Ge2+, Sn2+; X sites for halide anions, such as Cl–, Br–, I–). The metal halide perovskites exhibit desirable properties, including high defect tolerance, high emission quantum efficiency, easily tuned electronic band gaps and exciton binding energies, bipolar transport and long carrier diffusion distance.1-4 Owing to these advantages, the perovskites have been widely identified as promising materials for future electronic and optoelectronic devices, including solar cells, light-emitting diodes (LEDs), photodetectors, and lasers.2,

5-13

For example, an external quantum efficiency of ~20% has been

demonstrated for perovskite based green and red LEDs.2, 14-17 With the use of additives in the perovskite precursor, the power conversion efficiency (PCE) of perovskite/silicon monolithic tandem solar cells has reached ~25.4%.18 Very recently, the continuous-wave (CW) operation of perovskite micro-scale lasers has been realized at room temperature (T).19-22 So far, high-quality all-inorganic and inorganic-organic perovskites nanostructures have been fabricated by solutionprocessed and vapor phase deposition methods.5, 13, 23-24 Compared to hybrid inorganic-organic perovskites, all-inorganic perovskites, especially the CsPbX3, exhibit better structure stability and narrower photoluminescence (PL) because of the rigid lattice, high tolerance factor and weak bonds to out-coming polar molecules, holding great potentials for next-generation optoelectronic devices.24-26 Integrating the perovskites with established semiconductor technologies, typically based on Si, GaAs, and GaN, provide opportunities to develop application-level optoelectronic devices by combining the advantages of perovskites with these IV/III-V inorganic semiconductors. The first and key step to realize this is to fabricate large-area single-crystal perovskites on these


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semiconductors. For example, the growth of perovskites on the narrowband semiconductor Si has been widely studied, and a diversity of morphologies have been demonstrated, such as 1D nanowires, 2D microplatelets, and 3D micropyramids.27-29 GaN is considered as one of the core materials for next-generation semiconductor technology for high power optoelectronic and electronic devices. It has superior thermal conductivity (220 W/mK), high electron mobility ( 103 cm2/Vs), strong breakdown field strength (~3 MV/cm), and a high melting point (~3487 K).30-34 Meanwhile, GaN is wide band-gap semiconductor (3.4 eV), and it provides an ideal platform for optoelectronic applications fully covering the optical spectral range from the ultra-violet to the infrared via the pre-coating of blue, green, or red phosphors, or the formation of photovoltaic devices of ternary alloy systems.35-37 From the perspective of crystal growth, the wafer-scale fabrication technology of high quality GaN on sapphire has been established, and the atomically smooth surface of GaN wafers have plenty of atomically sharp dangling bonds, as well as dislocations, providing sites for uniform nucleation and heteroepitaxial deposition of perovskite.5, 38-40

Moreover, the GaN and perovskite can absorb light in different spectra range as well as build

type-II semiconductor heterojunction that are promising for photonic and optoelectronic devices.41-43 For example, the GaN has been used as carrier transfer layer to drive the carrier dissociation and separation of perovskite based solar cell.41 Owing to these advantages, perovskiteGaN heterojunctions may provide great opportunities for developing higher-power, higher-speed, broader-band light emission and sensor devices. In this work, high-quality cesium halide bromide (CsPbBr3) microplatelet single-crystals (MPCs) are grown epitaxially on c-wurtzite GaN substrates with a uniform rectangular shape and an atomically sharp surface via chemical vapor deposition (CVD). The growth mechanism, PL properties as well as the interfacial carrier dynamics of the CsPbBr3-GaN heterojunctions are


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explored by time-resolved and temperature-dependent PL spectroscopy. The CsPbBr3 crystals are well aligned along the [-12-10], [11-20], and [2-1-10] directions of the (0001) face of the GaN substrate driven by the incommensurate lattice match. The as-fabricated CsPbBr3 crystals exhibit spectrally narrower PL emission and comparable structural stability under photo-excitation with those grown on mica substrates supporting van der Waals heteroepitaxy. Green-colored lasing is achieved in individual MPCs at room temperature. In addition, the CsPbBr3 and GaN can form a type-II heterojunction that effectively promotes the carrier dissociation of CsPbBr3, and is highly promising for electrically driven optoelectronic devices. These results not only suggest the considerable potential of CsPbBr3-GaN heterojunctions in integrated optoelectronic applications, but also provides insights into the fabrications of CsPbBr3. RESULTS AND DISCUSSION Figure 1a shows a schematic of a CsPbBr3-GaN heterojunction. The bottom of the conduction band of GaN is located at –3.4 eV, and the top of the valence band is located at –6.8 eV, while the conduction and valence bands of CsPbBr3 are located at –3.2 and –5.6 eV, respectively, which results in a type-II band alignment heterojunction and drives the separation as well as the extraction of electrons and holes.44-46 The n-doped GaN with a dopant concentration of 5×1017 cm–3 is adopted as an epitaxial layer owing to the mature fabrication technology, high carrier density and high electron mobility.47 The electrons migrate from CsPbBr3 to GaN and the holes migrate in the opposite direction. The CsPbBr3 crystals were grown on exposed c-wurtzite GaN/sapphire substrate by CVD; the CVD reactor is equipped with a single-zone quartz furnace (Supporting Information, SI, Figure S1).48 The powdered precursors of CsBr and PbBr2 with a molar ratio 1:1 were placed up-steam and GaN/sapphire substrates were placed down-steam of the furnace, respectively. The growth process was 15 minutes at ~575oC. Three types of microstructures


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including microwires, microplatelets, and micropyramids (Figure S2) can be obtained through controlling the pressure of the Ar carrier gas (~1.6×104 – 3.3×104 Pa). When the pressure of the carrier gas (p) is quite high (2.7×104  6.7×103 Pa), the mean-free-path of atomic diffusion is short, thus the growth rate along the out-of-plane direction is higher than the in-plane direction, leading to a three-dimensional growth mode and the formation of micropyramids and microwires.49-51 As p is reduced to ~1.6×104 – 1.7×104 Pa, the vapor concentration is reduced and the diffusion meanfree-path is increased, thus a two-dimensional growth mode is preferred, leading to the formation of MPCs.

a

bb

a

CsPbBr3 on GaN (0001)

CsPbBr CsPbBr3 3

Cs

c

Pb

CsPbBr3

h h

7

0.0

Equation 21

Sapphire

Plot y0

−6 −8

0

GaN

0.8 1.6 2.4 Height (m)

w

GaN 3.2

A

Model

y=y0 + (A/(w*sqrt(pi/2) ))*exp(-2*((x-xc)/w)^2)

Equation

Count

Plot

2.25899 ± 1.20986

14

Orthorhomic 1.08375 ± 0.06746

xc

h+ CsPbBr3

*

Gauss

ICSD#97851y0

0.7195 ± 0.13523

7

25.56671 ± 3.57465 4.17637

Reduced ChiR-Square(CO 0

xc

20

0.0 Adj. R-Square

0.9844 30 0.80.961 1.6 2.4 2q (degree) Height (m)

40 3.2

Gauss

y=y0 + (A/(w*sqrt(pi/2) ))*exp(-2*((x-xc)/w)^2) Count

*

2.25899 ± 1.20986

Orthorhombic 1.08375 ±ICSD#97851 0.06746

w

0.7195 ± 0.13523

A

25.56671 ± 3.57465

Reduced Chi-

f Absorbance PL

PL intensity (a.u.)

14

hv −4

Model

Absorbance * GaN PL

Sapphire

PL intensity (a.u.) Absorbance (a.u.)

Energy (eV)

21

28

CsPbBr e- 3

Intensity (a.u.) Count (002)

−2

CsPbBr3/GaNe * GaN CsPbBr3/GaN *

(002)/(110)

d

28

(224) (400) Intensity (a.u.) Absorbance (a.u.)

d

e (004) (220)

c

20 m

[-12-10]

(112) (200)

e e

GaN

e e GaN e e GaN

(004)/(220)

h h e e

ee

Br

[10-10]

h

(110)

h hh

Count

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

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4.17637

468 R-Square(CO

520 624 0.9844 572 15 20 250.96130 (nm) 35 40 45 Adj. R-Square Wavelength 2q (degree)

468

520 572 624 Wavelength (nm)

Figure 1. Fabrication and structure characterizations of CsPbBr3-GaN heterojunction by chemical vapor phase deposition. (a) The schematic and energy diagram of CsPbBr3-GaN heterojunction (upper panel), the lower panel shows a type-II band alignment between GaN and CsPbBr3, which illustrates the transfer of electron and hole that occurs after photo-excitation. (b) SEM image of oriented CsPbBr3 MPCs with uniform rectangular shape epitaxial on c-wurtzite GaN/sapphire substrate. Right: EDS images of the as-gown CsPbBr3 MPC. Scale bar: 20 m. (c) Cross-section SEM image of the CsPbBr3-GaN heterojunction. The cross-section of the CsPbBr3 MPC is uniform rectangular. Scale bar: 2 m. (d) Statistics on the height of as-grown CsPbBr3 MPCs in a selected area of ~200×100 μm2. The thicknesses are mainly in the range of 0.7 – 2 μm. (e) XRD pattern of the as-gown CsPbBr3 MPCs. Red: XRD pattern of CsPbBr3 MPCs on GaN substrate; green: XRD pattern of orthorhombic-phase CsPbBr3 extracted from ICSD 97851 card. (f) Optical absorption


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(green) and PL (red) spectra of the CsPbBr3 MPCs. The absorption spectroscopy exhibits an edge near 525 nm and the PL spectroscopy exhibits a single symmetric peak centered at 529 nm. Since the two-dimensional structure is of particular importance for integrated optoelectronic applications, herein we focus on the growth mechanism and spectroscopic studies of the CsPbBr3 MPCs. As revealed by the scanning electron microscope (SEM) micrograph (Figure 1b; area: 130180 m2), the as-fabricated CsPbBr3 MPCs exhibit uniformly rectangular shapes, which coincides with the orthorhombic or cubic phase. The lengths (L) of the CsPbBr3 MPCs are several to tens of micrometers. Although there is a large lattice mismatch between CsPbBr3 and GaN, all the CsPbBr3 MPCs are well ordered along the [-12-10] direction on the (0001) GaN substrate, suggesting an epitaxial deposition mechanism. Mapping by energy dispersive spectroscopy (EDS) of individual CsPbBr3 MPCs reveals that the distributions of Cs, Pb, and Br are homogeneous over the whole MPC (Figure 1b, inset). The atomic ratio of Cs, Pb, and Br is 0.8:1.0:2.8 (Figure S3), which is close to the stoichiometric ratio of 1:1:3. The surplus of Pb may be due to preferential absorption of metal ions by surface dangling bonds or sample degradation during measurement.5253

Figure 1c shows a SEM micrograph of the cross-section of the CsPbBr3-GaN heterostructure.

The CsPbBr3 MPC closely contact with the GaN and exhibit regular rectangular cross-section. The surface profiler height distribution of the MPCs (Figure 1d; area: 200100 m2) shows that the thicknesses of the CsPbBr3 MPCs are mainly 0.7 – 2.0 m. Generally, there are four crystalline phases for CsPbBr3: cubic (T > 130oC), tetragonal (130 − 88oC), orthorhombic (T < 88oC), and monoclinic (T ~25oC).54-58 At room temperature, the crystal phase of CsPbBr3 is expected to be monoclinic or orthorhombic.54-55 However, growth of cubic, orthorhombic, and monoclinic phases of perovskite nanostructures have been reported on substrates of mica, SrTiO3, and SiO2/Si by the CVD method, typically at a vapor-phase deposition


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T > 130oC.29, 48-49, 59 In these works, the high-T phases (cubic and tetragonal) may persist at ~297 K after the cooling process since the large kinetic barrier hinders the structural phase transition. X-ray diffraction (XRD) is conducted to probe the crystal phase of the CsPbBr3 MPCs. A careful analysis of the position, relative ratios, and the splitting of the XRD patterns is performed with the help of Inorganic Crystal Structure Database (ICSD) and Powder Diffraction File (PDF) cards. Four peaks are resolved at ~15.0o, 15.2o, 30.3o, and 30.7o which index to (002), (110), (004), and (220) planes of the orthorhombic phase of CsPbBr3 (ICSD 97851), respectively (Figure 1e, Figure S4).48, 60-62 It should be noted that the splitting at the (112)/(200) peaks and (004)/(220) peaks are direct evidences that exclude the cubic phase of CsPbBr3. Since the spot size of X-ray (~5×10 mm2) is larger than covering area of MPCs (0.5×0.5 mm2), the diffraction signals from the microwires and micropyramids are also be recorded, but the peaks are much weaker than that of MPCs (Figure S5). Furthermore, Raman spectroscopy shows three peaks of 74, 128, and 310 cm–1 attributed to Pb-Br stretching modes of the PbBr6 octahedron, head-to-head Cs motion coupled with proximal Br face expansion, and a second-order longitudinal optical (LO) mode associated with the octahedron’s vibration, respectively (Figure S6).63-65 The 128 cm−1 and 310 cm−1 peaks are not expected for PbBr2 and support the composition of CsPbBr3. A clear edge around 525 nm is resolved in absorption spectroscopy of an individual CsPbBr3 MPC (Figure 1f). The periodic peaks of absorption spectroscopy are attributed to the oscillatory behavior of light travelling between the top and bottom surfaces of GaN.66 In additional, single peak is observed at 529 nm in PL spectroscopy, which coincides with the absorption spectroscopy, confirming the formation of CsPbBr3.


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a

1 [-12-10] Cs

Pb

Br

Ga

2 [11-20]

N

3 2 1

c 2

1

[2-1-10]

3

b

2.9 nm

%

34.2 24.8

9.9

CsPbBr3 [-12-10]

[11-20]

0.9 Roughness (nm)

31.1

d

3

~1.16 nm Couts

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

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[2-1-10]

others

RMS = 0.37 nm Ra = 0.23 nm

0.6

0.3

200 nm –4.0 nm

Growth direction

0

3

6 9 Number

12

Figure 2. Heteroepitaxy mechanism of CsPbBr3 on c-wurtzite GaN/sapphire substrate. (a) Incommensurate lattice match of CsPbBr3 on c-wurtzite GaN/sapphire substrate. The three mostlikely growth directions, [-12-10], [11-20], [2-1-10], are indicated by red arrows, respectively. (b) Statistics on the growth directions of CsPbBr3 MPCs. Inset: the optical images of CsPbBr3 MPCs grown along the [-12-10], [11-20], [2-1-10] directions of GaN from left to right, respectively. The ratio of MPCs grown along the three directions is 31.1%, 34.2% and 24.8%, respectively. (c) Atomic force microscopy image of a typical CsPbBr3 MPC. Inset: the height of measured terrace indicated by white dot line. (d) Surface roughness average (Ra) and root mean square (RMS) roughness of twelve CsPbBr3 MPCs measured by atomic force microscopy. The Ra is ~0.23 nm and the RMS is ~0.37 nm, respectively. Next, the heteroepitaxial mechanism of the oriented CsPbBr3 MPCs is explored, as shown in Figure 2. The GaN-(0001) face is Ga-atom-terminated and the Ga and N atoms are arranged in the form of hexagonal lattice with a P63mc space group (Figure 2a).67 As indicated by the red arrows, the CsPbBr3 MPCs are primarily aligned to the three equivalent directions of the c-GaN/sapphire substrate of [-12-10], [11-20], and [2-1-10], respectively. As shown in Figure 2b, the distribution ratios of the MPCs aligned to the three directions are almost the same and of approximately 31.1%, 34.2%, 24.8%, respectively. Next, we discuss the lattice match model of CsPbBr3. Since the deposition temperature for CsPbBr3 is 380 − 450oC (Figure S1b), the cubic phase CsPbBr3 is


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anticipated to form at the initial growth stage and before the cooling process.29 For twodimensional growth mode, a growth face of CsPbBr3 (001) is preferred. The c-GaN has a large lattice mismatch with CsPbBr3 (ac-GaN = 3.189 Å and aCsPbBr3 = 5.830 Å), thus an incommensurate epitaxial growth mode is presented.59, 68 Two types of lattice match models of CsPbBr3 (001) and GaN (0001) are considered, (1) CsPbBr3[010]‖GaN[10-10] and CsPbBr3[100]‖GaN[-12-10]; (2) CsPbBr3[010]‖GaN[2 1/2 -5/2 0] and CsPbBr3[100]‖GaN[3 -9/2 3/2 0].69 The lattice constant mismatch f is (1-doverlayer/dsubstrate)100%, where d is the lattice spacing. Corresponding calculated results are shown in Table 1. The lattice match values of the two types, −5.5% and −0.28% for type (1) and 3.3%, 0.27% for type (2), are almost the same. While, type (1) is energetically preferred, which is possibly due to a stronger chemical bond between the substrate and asdeposited atoms. Figure 2c shows atomic-force microscope (AFM) micrographs of the CsPbBr3 MPC. The atomic terraces and helical dislocation can be seen. The height of the measured terrace is 1.16 nm (indicated by white dot line), close to the (001)-plane interplanar spacing (1.17 nm) of the orthorhombic phase of CsPbBr3 (Table S1). The root-mean-square (RMS) value of surface roughness is ~0.37 nm, summarized by AFM micrographs of twelve MPCs (Figure 2d and Table S2), further suggesting an atomically sharp surface of the CsPbBr3 MPCs. The observations of atomically sharp helical dislocation and island morphology indicate that the heteroepitaxy of CsPbBr3 on GaN/sapphire substrate is a hybrid layer-by-layer and Volmer-Weber island formation mode.38, 49 Increasing reaction temperature may enhance the homogeneous nucleation and the diffusion of adatoms, overcome the Volmer-Weber growth mode, and realize the thin-film-growth essential to massive industry production.49 However, possibly owing to surface dangling bonds and discontinuity, an amount of pinholes and crystal boundaries are still observed even at a growth


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temperature of 650℃ (Figures S7 and S8). Surface engineering such as thermal treatment or introducing wetting layers may be necessary to improve the crystallinity of the film. Table 1. Lattice match models of the exposure of (001)-plane CsPbBr3 grown on c-GaN (0001)/sapphire. Corresponding lattice parameters: CsPbBr3, JCPDS 54-0752, dCsPbBr3(100) = 5.830 Å, dCsPbBr3(010) = 5.830 Å; GaN, JCPDS 50-0792, dGaN(2 1/2 -5/2 0) = 8.437 Å, dGaN(3 9/2 3/2 0) = 14.614 Å, dGaN(10-10) = 2.762 Å, dGaN(-12-10) = 1.5945 Å. lattice match models

f1

f2

Type 1

1dCsPbBr3(010) ≈ 2dGaN(10-10)

-5.5 %

3dCsPbBr3(100) ≈ 11dGaN(-12-10)

-0.28 %

Type 2

7dCsPbBr3(010) ≈ 5dGaN(2 1/2 -5/2 0)

3.3 %

5dCsPbBr3(100) ≈ 2dGaN(3 -9/2 3/2 0)

0.27 %

Layered mica is a well-established substrate for the incommensurate van der Waals heteroepitaxy of CsPbBr3 and the other semiconductors.68, 70 We compare emission spectroscopy of CsPbBr3 grown on GaN and mica substrates, respectively. As shown in Figure 3a, the steadystate PL spectra of CsPbBr3 MPC on GaN shows single Gaussian peak (excitation laser: CW 405 nm, power density: 1.3 kW cm−2). Owing to the type-II band alignment of CsPbBr3-GaN heterojunction, PL intensity of the MPC on GaN substrate (red line) is ~7.2% of that on mica with comparable dimension (blue line). Meanwhile, after transferred onto 300 nm SiO2/Si substrate, the PL intensity of MPC increases more than seven-fold (Figure S9), suggesting that the PL quenching is not due to the difference in sample crystallinity but the interface structure. The PL spectroscopy could also be tuned by the dielectric screening of substrate, but because the MPCs is quite thick (~1 m), the effect could be ignored.71 Figure 3b compares the full width at half maximum (FWHM, upper panel) and center wavelength (low panel) of the MPC grown on GaN (red) and mica (blue), respectively. The PL FWHM of the CsPbBr3 MPC on GaN is ~19.6  0.5 nm which is of ~1.5  0.8 nm narrower than that of MPC on mica (FWHM, ~21.1  0.3 nm), and its center wavelength is ~531.5  0.2 nm which is of ~5.0  0.3 nm shorter than ~536.5  0.1 nm of MPC


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on mica substrate (Figure S10).66, 72-73 Since the refractive index between the CsPbBr3 (2.2) and GaN (2.5) is quite low, the reflectively of CsPbBr3/GaN interface is only 0.4%, thus interference effects and self-absorption effects are substantially suppressed, leading to a spectrally purer emission. Also, similarly to the mica case, the PL intensity of CsPbBr3 MPC on GaN exhibits a small decrease after one-hour exposure to laser irradiation with a power density of 2 and 4 kW cm−2 (Figure S11).

b

500


d

1.0

4E15

GaN

25

mica

4.8

8

1.5 ns 6.0

2.9

6.1 ns

1.0

1.0 6.7 ns

10 12 14 Decay time (ns)

16

I~L2.4

IPL (t=0) (a.u.)

539

GaN I~L2.2

532 0

0.3

1 2 3 4 5 Power density (kW/cm2)

e Carrier density 2.0 Mica ( 1016 cm-3) t1 13.5 5.6 ns

Carrier density (cm-3) 1E16 3E16 8E16

Mica

20

0.6 1 2 5 Pump fluence (J cm-2)

f

Normalized intensity (a.u.)

1.5

c

Carrier density GaN t2 ( 1016 cm-3) t1 13.5 7.3 ns 0.3 ns 4.8 2.9

8.2 ns 0.9 ns 9.2 ns 1.6 ns

1.0 10.7 ns 4.8 ns

8

10 12 14 Decay time (ns)

16

10 8 Time (ns)

Intensity (a.u.)

GaN Mica

Wavelength (nm) FWHM (nm)

a

Normalized intensity (a.u.)

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

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t1 on GaN

6 4 2 0 1E16

t1 on mica

t2 on GaN 5E16 9E16 1.3E17 Carrier density (cm-3)

Figure 3. Interfacial carrier dynamics of the CsPbBr3-GaN and CsPbBr3-mica heterostructure. (a) PL spectroscopy of CsPbBr3 MPC on GaN (red) /mica (blue) at room temperature. Excitation laser: CW, 405 nm; the power density: 1.3 kW cm−2. (b) The FWHM (upper, panel) and center wavelength (lower, panel) of PL spectroscopy of the CsPbBr3 MPC on GaN (red) and mica (blue) as a function of power density. The FWHM of the CsPbBr3-GaN is smaller and the center wavelength is shorter than that of the CsPbBr3-mica. The FWHM and intensity of the MPCs on the two substrates show little fluctuation. (c) The PL intensity at t = 0, IPL(0) of CsPbBr3 MPC on mica (blue) and GaN (red) versus pump fluence (carrier density) under excitation by a femtosecond pulsed laser (400 nm, 50 fs, 1 kHz), respectively. Scatters: date points; lines: fitting curves. (d-e) Power-dependent TRPL spectroscopy of CsPbBr3 MPC on mica (d) and GaN (e) pumped by a


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femtosecond pulsed laser (400 nm, 120 fs, 76 MHz), respectively. The TRPL curve of CsPbBr3−mica is fitted with single exponential decay. The TRPL of CsPbBr3 on GaN is fitted by bi-exponential decay (slower recombination channel t1, faster recombination channel t2). (f) The decay times of CsPbBr3 MPCs on mica (blue scatters) and GaN (red and olive scatters) as function of carrier density. Scatters: date points; lines: fitting curves using a function of τ = 1/An, where A is a constant. Carrier dynamics of the CsPbBr3-GaN heterojunction are further studied by excitation carrier density (n) dependent time-resolved photoluminescence (TRPL) spectroscopy of CsPbBr3-mica as a comparison. The absorption coefficient of CsPbBr3 is 9.27×104 cm−1 at 400 nm, the light penetration length is ~300 nm, which is much smaller than the thickness of MPCs. Therefore, the influence of dielectric reflection at the interface of MPC-substrate on the carrier density could be ignored, and the carrier/exciton densities are the same for MPCs on the two substrates.74 To probe the nature of photo-generated carriers, the carrier density dependence of PL intensity at initial time, IPL(0), is measured by a streak camera (time response: 10 ps) when the CsPbBr3 MPC is pumped by femtosecond laser excitation (wavelength: 400 nm; pulse duration: 50 fs; repetition rate: 1 kHz, Methods).75 The pump fluence can be converted to carrier density based on 1 J cm−2 = 1.65×1016 cm−3 (SI Note). IPL(0) is mainly linear with the carrier density n for the exciton recombination, and proportional to n2 for free carrier recombination, respectively.76-79 As shown in Figure 3c, as n is 5.25×1015 ‒ 1.05×1017 cm−3, the n-IPL(0) relation for the MPCs on GaN and mica substrates mainly contain a quadratic term, suggesting that the excitons are dissociated into free carriers inside the MPCs, which are in good agreement with the previous literatures.77-79 The power orders of n-IPL(0) reduce as n is > 3.15×1016 cm−3 possibly due to the arising of saturated absorption and/or nonradiative recombination processes. To determine which of these recombination processes occurs, TRPL spectroscopy with broad time window are conducted by high-efficiency time correlated single photon counting (TCSPC) technique with a femtosecond pulsed laser excitation source (wavelength: 400 nm; repetition rate: 76 MHz; pulse duration: 120 fs; Method). The TRPL


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curves of CsPbBr3-mica are well fitted by a single exponential decay function. When the carrier density n increases from 1.01016 to 1.351017 cm−3, τ1 decreases from 6.7 to 5.6 ns that agrees with the bimolecular recombination of free carriers (τ  1/n) (Figure 3d).80 By contrast, the TRPL of CsPbBr3-GaN contains two decay channels with time constants of τ1 and τ2, respectively (Figure 3e). The slow decay channel, with a longer lifetime τ1 corresponds to the radiative recombination of free carriers that are not influenced by the CsPbBr3/GaN interface. The time constant is slightly larger than that on mica substrate, indicating the low density of defects. The fast initial decay channel, with a shorter lifetime τ2 attributes to the loss of carriers at the interface due to charge extraction and/or interface recombination, which has been widely observed in the semiconductor heterojunction.81-82 On increasing the injected charge carrier density from 1.0 1016 cm−3 to 1.351017 cm−3 (Figure 3f), the long lifetime τ1 on GaN decreases from 10.7 to 7.3 ns, and the fast lifetime τ2 reduces from 4.8 to 0.3 ns. Both of the two decay channels described more appropriately by the tendency expressed by τ  1/n.75,

83

The appearance of interface

recombination/extraction decay channel suggests a sharp interface between as-grown CsPbBr3 and GaN, which would provide a good platform to develop hybrid functional optoelectronic devices. Temperature and power-dependent PL spectroscopy are conducted to explore the emission properties of the CsPbBr3 MPCs. Figure 4a shows the PL spectroscopy of a typical CsPbBr3 MPC with L of ~2.5 m at T = 77 − 296 K. The PL mapping of CsPbBr3 MPCs under the excitation of the 405 nm CW laser is uniform across the whole surface of the MPC. In PL mapping, light is mainly emitted from the CsPbBr3 MPC edges away from the excitation point, suggesting a low density of scattering centers inside the CsPbBr3 MPC. At 296 K, PL spectroscopy exhibits a single peak with center wavelength and FWHM of ~527.6 and 16.5 nm (Figure S12), respectively. With increasing T from 77 to 176 K, the PL peak presents a blue shift from 530.1 to 527.7 nm (~2.4 nm)


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owing to the increased band gap as a result of thermal expansion. With further increase of T from 176 to 296 K, electron-phonon scattering prevails, lowering the blue shift rate of the PL peak (~0.1 nm, from 527.7 to 527.6 nm at 176 K and 296 K, respectively).84 With decreasing the temperature, a low energy peak with center wavelength of 535 nm emerges possibly due to the bound exciton recombination related to Br vacancy (Figure S13).76, 85 The T-dependent PL intensity can be expressed by the function I(T) = I0/(1+Ae−Ea/kBT) with activity energy Ea of 52.2 meV, where I0, A and kB represent the PL intensity at 0 K, a fitting constant, and the Boltzmann constant, respectively (Figure 4b, red). The amplitude of Ea is consistent with the exciton binding energy reported in the literature, which is higher than the thermal energy (25 meV).86 The reason on how these excitons dissociate is unclear and explored in the future works. The T-dependent FWHM was fitted by a Boson model: Γ(T) = Γ0+T+Γop/(eEop/kBT-1) (Figure 4b, blue), where the first, second, and third term represent the contributions of inhomogeneous broadening, acoustic phonons, and optical phonons, respectively (Table S3).87 The average optical photon energy Eop is extracted as 21.9  1.2 meV, smaller than the LO energy (38 meV), which may be due to some of the lower-energy phonons contributing to the scattering process. The exciton-acoustic phonon interaction coefficient

 is 7.210−3  1.610−3 meV/K and the exciton-optical phonon coupling coefficient Γop is ~78.27.0 meV. Based on the fitting, PL broadening contributed to by the acoustic phonon (blue dash dot line) and exciton-optical phonon interaction (blue dashed line), are determined, and the optical phonon contribution prevails particularly at high temperatures, which is in good agreement with the previous studies.88


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b

c

96 K 116 K 136 K

176 K

80

G0 + GOC G0 + GAC

60

40

120 180 240 Temperature (K)

276 K 296 K

d 4E18


12

6

0.4 0.8 1.5 Pump fluence (mJ cm-2)


e

Carrier density (cm-3) 1.5E19 4E19

18

0.2

0.3 510

0

520

0.9 0.7

300

Intergrated intensity (a.u.)

256 K

2.4 1.5

20 60

FWHM (nm)

236 K

mJ cm-2

Ea = 52.2 meV

196 K 216 K

FWHM (meV) Intensity (a.u.)

77 K

156 K

G0+GAC+GOC

Intergrated intensity (a.u.)

L = 2.5 m

3

F-P：ng = 6.8 8 WGM：ng = 4.8 D (nm)

a

Normalized Intensity (a.u.)

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F-P

6 4

WGM 2 0.1 0.2 0.3 0.4 0.5 Reciprocal edge length (1/m)

Figure 4. The low temperature PL spectroscopy and lasing of CsPbBr3 MPC epitaxial on cwurtzite GaN/sapphire substrate. (a) Temperature-dependent PL spectra of a typical CsPbBr3 MPC with L ~2.5 m. The PL peak blue shifts from ~530.1 nm to 527.7 nm as temperature increases from 77 K to 176 K. With further increasing temperature from 527.7 nm to 527.6 nm, the rate of blue shift decreases, e. g. from 176 K to 296 K. Inset PL mapping (upper panel) and PL photograph presents (lower panel). Excitation: CW 405 nm laser with a power density of 0.3 kW cm−2. (b) The temperature-dependence of the integrated PL intensity (red) and FWHM (blue) extracted from (a). The temperature-dependence FWHM broaden of MPC is mainly from the contribution of inhomogeneous broadening (Γ0) and optical phonon (ΓOC), while the contribution of acoustic phonon (ΓAC) is much less than the ΓOC. Scatters: experimental data; solid line: fitting curve including the contributions of Γ0, ΓAC, ΓOC. (c) Lasing from a CsPbBr3 MPC with L ~ 2.5 m at room temperature. The excitation source is femtosecond pulsed laser with wavelength of 400 nm, pulse duration of 80 fs and repetition rate of 1 kHz, respectively. The excitation fluence is indicated along with the spectra, and a peak appears at ~ 547 nm. (d) Integrated emission intensity and FWHM as a function of excitation fluence, the data is extracted out from (c). The superlinear response and linewidth narrowing of the lasing mode is observed as the pump fluence exceeds the threshold (0.7 mJ cm−2), which indicate the occurrence of lasing action in CsPbBr3 MPC. The red and blue solid lines are guide to the eye. (e) The free spectra range ∆ as a function of the edge length L, showing ∆λ is linear to the inverse of L. Scatter: experimental data; solid line: fitting curve. Two types of microcavities, Fabry-Pérot and whispering-gallery-mode, are possible (inset: the diagram).


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Optically pumped lasing with green colored emission is achieved in the CsPbBr3-GaN heterojunction at room temperature. The rectangular CsPbBr3 MPCs can form active optical microcavities for lasing devices, with the crystal facets as the optical reflectors and perovskite as the gain media, respectively.12 Figure 4c shows the power-dependent PL spectra of a CsPbBr3 MPC (L = 2.5 m) when it is pumped by femtosecond laser excitation at 297 K (pulse duration: 80 fs; repetition rate: 1 kHz, Methods). For a pump fluence of 0.3 mJ cm−2, a spontaneous emission (SE) peak is observed at 533.0 nm with a FWHM of ~19.2 nm; for a pump fluence > 0.7 mJ cm−2 (threshold, Pth), a sharp peak arises on the low energy side of the SE peak with a FWHM of ~0.7 nm. The intensity and FWHMs of the emission versus the excitation fluence are extracted and plotted in Figure 4d, in which an “S”-shaped curve presents a linear to superlinear transition at Pth ~0.7 mJ cm−2, manifesting the onset of lasing. The lasing threshold of CsPbBr3 MPCs on GaN is higher than that of MPCs on mica (Figure S14), for two possible reasons: 1) the much lower refractive index contrast of CsPbBr3 and GaN (Figure S15), and 2) the effective carrier separation in the CsPbBr3-GaN heterojunction. Nevertheless, the lasing has still been achieved at room temperature, which is possibly enabled by the long lifetime, uniform morphology and good thermal conductivity of GaN substrate. The carrier density threshold (1.2×1019 cm−3) is larger than Mott density (4.8×1018 cm−3), suggesting that the lasing originates from stimulated emission of an electron-hole plasma (Figure S16), which are in good agreement with the recent literature by Zhu and co-workers.89 L-dependent lasing spectra are performed and show that the free spectral range ∆λ at a typical lasing wavelength (λ) is inversely proportional to L (Figure 4e and Figure S14c-d). Further, more lasing modes emerge with increasing L. Three types of optical microcavities are possible: Fabry-Pérot (F-P) cavities consisting of top- and bottom-facets that confine the light, F-


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P cavities consisting of two side-facets, and whispering-gallery-mode cavities (WGM) consisting of four side-facets, respectively. The quality factor of the top-bottom F-P cavity is very low because of the low reflectivity of the CsPbBr3/GaN interface, prohibiting the occurrence of lasing. Both of the remaining two cavities types (Figure 4e, inset), the WGM and F-P cavity confined by the side-facets of MPC, the group refractive indices ng are extracted as approximately 4.8 and 6.8 by the ∆λ-L relation of ∆λ=λ2/(2√2ngL) and ∆λ=λ2/(2ngL), respectively. The values are in good agreement with previous works.48, 90-91 Further investigation will be conducted to distinguish the cavity type of lasing in the MPCs. CONCLUSIONS In summary, we demonstrated the heteroepitaxial growth of large area, highly quality CsPbBr3 single crystals on GaN/sapphire substrate via the CVD method. CsPbBr3 exhibits an atomically sharp surface, uniform rectangular shape, and well-defined growth direction along the [-12-10], [11-20] and [2-1-10] directions of GaN. The crystalline and optical quality of CsPbBr3 is comparable to that grown on mica substrate, leading to the occurrence of green colored laser emission under ambient conditions. Moreover, CsPbBr3-GaN forms a type-II semiconductor heterojunction, enhancing the carrier separation and extraction in CsPbBr3. This work demonstrates the potential of the CsPbBr3-GaN heterostructure in optoelectronics and providing great opportunities for the development of application-level, broadband perovskite micro/nanosized optoelectronic and photonic devices. METHODS CsPbBr3 Growth on GaN Substrate. Commercial grade c-plane GaN, grown on sapphire by metal organic CVD (MOCVD) method was adopted as an epitaxial substrate for CsPbBr3 MPCs through a single-temperature quartz furnace by CVD (Figure S1a). The substrate was cleaned by


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acetone, ethanol and deionized water for 15 minutes, respectively, and placed at the downstream of furnace. The mixed powders, CsBr and PbBr2, 99.99%, Sigma Aldrich with molar ratio of CsBr and PbBr2, 1:1, were located at the center of furnace. The growth temperature was set as ~ 575℃. The flow rate of carrier gas Ar was ~ 30 sccm. Rectangular CsPbBr3 MPC were obtained under the pressure of ~1.6×104 – 1.7×104 Pa, and micropyramids and microwires formed when the pressure is ~2.7×104  6.7×103 Pa. Optical Characterizations. The steady-state and temperature-dependent PL spectroscopy were performed by a home-built confocal microscopy system. For steady-state PL spectroscopy, a 405 nm CW laser was focused onto individual CsPbBr3 MPCs by a 100× (number aperture, NA = 0.9) or a 50×(NA = 0.5) objective lens. The PL signals are collected by the same objective using a reflective configuration and analyzed by a monochromator equipped with a liquid nitrogencooled device detector. The initial time after photo-excitation, IPL(0), was excitation by a femtosecond pulsed laser (400 nm) frequency doubled by a BBO crystal from a Coherent Libra amplifier laser (800 nm, 50 fs, 1 kHz). The signals were collected by an Optronis Optoscope streak camera system (resolution: ~10 ps). For TRPL spectroscopy to probe recombination channel and lifetime, laser pluses generated by frequency-doubled pulse (Coherent Mira 900, 120 fs, 800 nm, 76 MHz) was focused onto individual CsPbBr3 MPCs and the signal was analyzed by timecorrelated single photon counting (TCSPC) technique. For the lasing characterization, laser pulses, which was frequency doubled by barium boron oxide (BBO) doubling crystal from a Coherent Astrella regenerative amplifier (80 fs, 1 kHz, 2.5 mJ per pulse), were used as excitation sources and focused onto individual MPCs with a 50×objective lens (NA = 0.5). The emission signal was collected by the same objective and analyzed by a monochromator with liquid nitrogen-cooled device detector.


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ASSOCIATED CONTENT Supporting Information. Supporting Information Available: Schematic illustration of CsPbBr3 MPCs grown on GaN/sapphire substrates and the temperature along the quartz furnace. SEM and EDS images of CsPbBr3 micropyramids and microwires grown on c-GaN/sapphire substrate. XRD pattern of CsPbBr3 MPCs on GaN substrate compared to the ICSD/PDF cards of cubic, tetragonal, orthorhombic, and monoclinic phases of CsPbBr3 perovskites. XRD spectra of CsPbBr3 MPCs compared with micropyramids and microwires. Raman spectra of GaN/sapphire and CsPbBr3 MPC on GaN/sapphire substrates at room temperature, respectively. Optical images of CsPbBr3 grown on GaN through one-step CVD and two-step physical vapor deposition methods at different growth temperature. PL spectroscopy of CsPbBr3 MPC grown on GaN before and after transferred onto SiO2 substrate. PL spectroscopy of CsPbBr3 MPC on c-GaN/sapphire and mica fitted by Gaussian functions. Normalized PL intensity of CsPbBr3 MPC on GaN and mica under CW laser pump with power density of ~2 and 4 kW cm−2. Power-dependent PL spectra and intergrated intensity of CsPbBr3 MPC on GaN from 0.02 to 8.67 kW cm−2 at 77 K. Lasing spectra and threshold of CsPbBr3 MPC on GaN with different edge length. Angle-resolved PL mapping and power-dependent lasing spectra of CsPbBr3 MPC. The photo-generated carrier density and interface recombination/extraction velocity are calculated. This material is available free of charge via the Internet at http://pubs.acs.org. The authors declare no competing financial interest. AUTHOR INFORMATION Author Contributions #

L.Z, Y.G. and M.S. contributed equally to this work.

ACKNOWLEDGMENT The work is supported by National Natural Science Foundation of China (Nos. 61774003, 61521004, 21673054, 51472080, 11874130, 61574006 and 61605073), National Key Research and

Development

Program

of

China

(Nos.

2017YFA0205700,

2017YFA0304600,

2016YFA0200700 and 2017YFA0205004), Open Research Fund Program of the State Key Laboratory of Low-dimensional Quantum Physics (Nos. KF201706 and KF201907), Excellent


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Youth Foundation of Hubei Province (No. 2017CFA038), Macau Science and Technology Development Fund (Nos. FDCT-116/2016/A3 and FDCT-091/2017/A2), Research Grant (Nos. SRG2016-00087-FST and MYRG2018-00148-IAPME) from the University of Macau, Young 1000 Talents Global Recruitment Program of China (No. 51235086), and China Postdoctoral Science Foundation (No. 2019M653721). REFERENCES 1. Akkerman, Q. A.; Rainò, G.; Kovalenko, M. V.; Manna, L. Genesis, Challenges and Opportunities for Colloidal Lead Halide Perovskite Nanocrystals. Nat. Mat. 2018, 17, 394– 405. 2. Song, J.; Li, J.; Li, X.; Xu, L.; Dong, Y.; Zeng, H. Quantum Dot Light-Emitting Diodes Based on Inorganic Perovskite Cesium Lead Halides (CsPbX3). Adv. Mater. 2015, 27, 7162–7167. 3. Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341–344. 4. Siegler, T. D.; Houck, D. W.; Cho, S. H.; Milliron, D. J.; Korgel, B. A. Bismuth Enhances the Stability of CH3NH3PbI3 (MAPI) Perovskite under High Humidity. J. Phys. Chem. C 2019, 123, 963–970. 5. Dou, L.; Yang, Y.; You, J.; Hong, Z.; Chang, W.-H.; Li, G.; Yang, Y. Solution-Processed Hybrid Perovskite Photodetectors with High Detectivity. Nat. Commun. 2014, 5, 5404. 6. Saidaminov, M. I.; Adinolfi, V.; Comin, R.; Abdelhady, A. L.; Peng, W.; Dursun, I.; Yuan, M.; Hoogland, S.; Sargent, E. H.; Bakr, O. M. Planar-Integrated Single-Crystalline Perovskite Photodetectors. Nat. Commun. 2015, 6, 8724. 7. Kim, Y.-H.; Cho, H.; Heo, J. H.; Kim, T.-S.; Myoung, N.; Lee, C.-L.; Im, S. H.; Lee, T.-W. Multicolored Organic/Inorganic Hybrid Perovskite Light-Emitting Diodes. Adv. Mater. 2015, 27, 1248–1254. 8. Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright Light-Emitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687–692. 9. Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I. Iodide Management in Formamidinium-Lead-Halide-Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376–1379. 10. Wu, Y.; Shen, H.; Walter, D.; Jacobs, D.; Duong, T.; Peng, J.; Jiang, L.; Cheng, Y.-B.; Weber, K. On the Origin of Hysteresis in Perovskite Solar Cells. Adv. Funct. Mater. 2016, 26, 6807– 6813. 11. Liu, B.; Wang, L.; Gu, H.; Sun, H.; Demir, H. V. Highly Efficient Green Light-Emitting Diodes from All-Inorganic Perovskite Nanocrystals Enabled by a New Electron Transport Layer. Adv. Opt. Mater. 2018, 6, 1800220. 12. Zhang, Q.; Ha, S. T.; Liu, X.; Sum, T. C.; Xiong, Q. Room-Temperature Near-Infrared HighQ Perovskite Whispering-Gallery Planar Nanolasers. Nano Lett. 2014, 14, 5995–6001.


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