Structure and Optoelectronic Properties of Perovskite-like (PEA)₂PbBr₃Cl on AlN/Sapphire Substrate Heterostructure

Abstract

This study presents the structure and optoelectronic properties of a perovskite-like (PEA)₂PbBr₃Cl material on an AlN/sapphire substrate heterostructure prepared using spin coating. The AlN/sapphire substrate comprised a 2 μm thick AlN epilayer on a sapphire wafer deposited via metal–organic chemical vapor deposition (MOCVD). The peak position of (PEA)₂PbBr₃Cl photoluminescence (PL) on the AlN/sapphire substrate heterostructure was 372 nm. The emission wavelength ranges of traditional lead halide perovskite light-emitting diodes are typically 410 to 780 nm, corresponding to the range of purple to deep red as the ratio of halide in the perovskite material changes. This indicates the potential for application as a UV perovskite light-emitting diode. In this study, we investigated the contact characteristics between Ag metal and the (PEA)₂PbBr₃Cl layer on an AlN/sapphire substrate heterostructure, which improved after annealing in an air environment due to the tunneling effect of the thermionic-field emission (TFE) mechanism.

1. Introduction

Semiconducting lead halide perovskites (LHPs) with an ABX₃ structure are potential semiconductor materials for use in high-performance electronic, photonic, and biosensing devices because of their outstanding physic characteristics, ease of processing, and low cost. Additionally, by adjusting the proportion of all elements of the A-site, B-site, and X-site in perovskite materials, their emission peaks can be tuned. They can exist in the form of perovskite or as perovskite-like crystals. For example, the emission wavelength range of MA (methylammonium)-based lead halide perovskite light-emitting diodes is 410 to 780 nm, achieved by changing the ratio of halide (Cl, Br, and I) in perovskite’s active layer [1,2,3,4,5,6,7,8,9,10]. Therefore, the “perovskite red wall” issue was addressed in 2017 [7,8,9]. Similarly, the “perovskite ultraviolet wall” problem also exists.

To solve the “perovskite red wall” issue, the mixed-cation compound FAPbI₃-CsPbI₃ (FA⁺ being too large and Cs⁺ being too small) material system emerged. Initially, the two-dimensional (2D), phenylethylamine (PEA)-based perovskite received more attention due to its better material stability in ambient atmosphere environments than 3D perovskite materials, owing to van der Waals forces in the 2D Ruddlesden–Popper (RP)-phase perovskite of the A-site phenylethylamine cation (PEA⁺) [11,12,13]. In the “perovskite ultraviolet wall” case, the bandgap widened when the PEA-based halide was introduced into the perovskite material due to the quantum size effect and formed the quasi-2D perovskite material. Therefore, in this study, we employed (PEA)₂PbBr₃Cl perovskite to grow on c-face sapphire (0001) substrates with a 2 μm thick AlN epilayer prepared via MOCVD. A peak shift in emission wavelength from 400 nm to 372 nm was observed due to the strain force caused by the lattice mismatch [14]. Additionally, AlN and (PEA)₂PbBr₃Cl perovskite are wide bandgap materials. On the other hand, AlN can be applied to UV devices for disinfection and high-power components for automobiles. During component operation, the remaining power is passed to the surrounding environment and semiconductor material itself in the form of heat. This causes the temperature of the component to increase locally, which worsens electronic characteristics such as mobility, saturation velocity, and conductivity. Because the thermal conductivity of AlN is very good, it is very suitable as a substrate material [15]. Thus, the relationships between contact behaviors and device applications are important. By utilizing the tunable bandgap of perovskite materials, those with emissions in the near-UV region and AlN exhibit similar optical and electronic characteristics. Therefore, we also investigated the contact mechanism between Ag and (PEA)₂PbBr₃Cl on an AlN/sapphire substrate heterostructure.

2. Materials and Methods

The AlN/sapphire substrate was washed separately with HCl and ethanol solutions in a beaker using a supersonic oscillator for 5 min. Next, the AlN/sapphire substrate was rinsed using deionized water (DI water) for 5 min. The AlN/sapphire substrate comprised a 2 μm thick AlN epilayer on a c-axis (0002)-oriented sapphire wafer with a thickness of 450 nm deposited via metal–organic chemical vapor deposition (MOCVD). Then, a spinner was used to coat the (PEA)₂PbBr₃Cl precursor solution on the AlN/sapphire substrate at 3000 rpm for 60 s, and the sample was put into an oven to anneal at 110 °C for 10 min. The (PEA)₂PbBr₃Cl precursor solution was synthesized using a PEACl of 47.3 mg, PEABr of 60.6 mg, and PbBr₂ of 110.1 mg in a blended solvent of DMF and DMSO (0.8 mL:0.2 mL). Then, the mixture was stirred at 500 rpm at 25 °C overnight to form a two-dimensional perovskite (PEA)₂PbBr₃Cl layer with a thickness of 200 nm. Finally, 150 nm thick silver (Ag) metal was evaporated onto the (PEA)₂PbBr₃Cl film using a thermal coater under a vacuum pressure of around 10⁻⁶ torr, after which, annealing occurred at 150 and 300 °C for 10 min to form contacts, respectively. Figure 1 illustrates the whole structure. The spacing of the Ag contacts was 2 mm and the size of every Ag contact was 2 × 3 mm².

The crystallinities of (PEA)₂PbBr₃Cl perovskite on the AlN/sapphire substrates were examined using an X-ray diffractometer (Almelo, The Netherlands) with a Cu-target (λ = 1.5418 Å) source. Scanning electron microscopy (SEM, JSM-7610F; JEOL) was used to observe the surface morphology of the perovskite on the AlN wafers. Photoluminescence (PL) spectra were obtained using a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan). Their electronic characteristics were obtained using a Keithley 2420 source meter.

3. Results and Discussion

Figure 2a,b plot the XRD pattern of the (PEA)₂PbBr₃Cl perovskite on the glass substrate and AlN/sapphire substrate, respectively. Figure 2c shows the XRD pattern of Figure 2b with 7 times magnification. Six XRD spectra peaks of perovskite-like (PEA)₂PbBr₃Cl on the AlN/sapphire substrate and glass substrate were observed at around 10.83°, 16.15°, 21.50°, 26.90°, 32.38°, and 37.93°, corresponding to the (004), (006), (008), (0010), (0012), and (0014) phases of the quasi-2D (PEA)₂PbBr₃Cl structure, respectively [16,17]. The domain sizes D of the (PEA)₂PbBr₃Cl perovskite on the glass substrate and AlN/sapphire substrate were 50.7 and 17.1 nm, respectively, and were calculated using the Scherrer equation [18]. The crystallite size D of the (PEA)₂PbBr₃Cl film was

$$\mathrm{D}=\frac{k\lambda }{\beta cos\theta }$$

(1)

where k is the Scherrer constant, λ is the wavelength of the radiation (1.54 Å for the Cu target), and β and θ are the FWHM and Bragg angle of the (0010) phase, respectively. The degradation in crystal quality may have contributed to the preferred orientation of the (PEA)₂PbBr₃Cl perovskite, which was destroyed by the lattice of the AlN/sapphire substrate due to strained force.

Figure 3a,b show the top-view SEM image of the (PEA)₂PbBr₃Cl perovskite on AlN/sapphire without and with 300 °C annealing, respectively. The typical structure of (PEA)₂PbBr₃Cl perovskite is hexagonal [19]. For the pristine sample, oval- or round-shaped particles were observed on the surface of the (PEA)₂PbBr₃Cl perovskite film on the AlN/sapphire substrate. This may have been caused by the lattice mismatch during (PEA)₂PbBr₃Cl perovskite crystal growth (aggregation), thereby causing changes in the crystal shape. However, after annealing at 300 °C for 10 min, the structure of the (PEA)₂PbBr₃Cl perovskite crystal was destroyed, as shown in Figure 3b.

Figure 4a shows the photoluminescence (PL) spectra of the perovskite-like material (PEA)₂PbBr₃Cl on the glass substrate and AlN/sapphire substrate heterostructure, respectively. Figure 4b shows the normalized PL spectra of Figure 4a. The peak positions of PL spectra were 400 and 372 nm, with a full width at half maximum (FWHM) of 15 and 85 nm, respectively. It is well known that perovskite materials have excellent preferred-orientation crystallinity and defect allowance [20,21]. The PL spectrum of the (PEA)₂PbBr₃Cl showed a very narrow FWHM on the glass substrate and a broad FWHM on the AlN/sapphire substrate heterostructure, with the latter indicating degradation of the (PEA)₂PbBr₃Cl film crystal quality. A blue shift in the peak position of the (PEA)₂PbBr₃Cl film from 400 to 372 nm was observed, possibly due to the emissions of electron–hole recombination from the conduction band and valence band. The preferred-orientation growth was destroyed when the (PEA)₂PbBr₃Cl film was spin-coated onto the AlN/sapphire single-crystal substrate because the atoms of the crystal needed to fit the crystal lattice of the AlN epilayer, but lattice strain caused lattice mismatch, quality degradation, and bandgap widening [14].

Figure 5 plots the current–voltage (I-V) characteristics of Ag/(PEA)₂PbBr₃Cl on the AlN/sapphire substrate heterostructure under different annealing temperatures. AlN with a 6.2 eV wide bandgap is a III-nitride semiconductor. Therefore, its conductivity is very poor, and it is close to being an insulator. The ohmic contact on AlN presents a serious issue for future optoelectronic device applications [22,23]. Similarly, perovskite-like (PEA)₂PbBr₃Cl is a wide-bandgap material. According to the PL spectrum, the bandgap of the (PEA)₂PbBr₃Cl on the AlN/sapphire substrate should be 3.33 eV. Hence, in this study, we used the perovskite-like (PEA)₂PbBr₃Cl material as a buffer layer between the ohmic contact metal and AlN layer to reduce the metal contact resistance. As shown in Figure 5, the Ag/(PEA)₂PbBr₃Cl on the AlN layer was almost non-conductive. After annealing treatment at 150 and 300 °C for 10 min, the conductivity of Ag/(PEA)₂PbBr₃Cl on the AlN layer improved. The resistance of the applied voltage at 9.9 V decreased to 1.7 and 1.1 × 10¹⁰ Ω from 2.7 × 10¹⁰ Ω of the as-deposited film, respectively. The mechanism of conductivity increase is shown in Figure 6. The work function of the Ag metal and electron affinity of the (PEA)₂PbBr₃Cl perovskite layer were 4.7 and 3.65 eV, respectively. The work functions of Ag and Ag₂O were 4.7 and 5.7 eV, respectively, as shown in Figure 6a. The transportation of an electron from Ag to the (PEA)₂PbBr₃Cl layer was difficult due to the 1.05 eV barrier, as shown in Figure 6b. Part of the Ag transformed into Ag₂O after annealing due to processes occurring in the air environment and formed the Ag and Ag₂O complex layer. This caused the Fermi level of the (PEA)₂PbBr₃Cl layer to lower because of the thermal equilibrium [24]. Therefore, the electron in the Ag contact layer could easily transfer to the (PEA)₂PbBr₃Cl layer via the thermionic-field emission (TFE) mechanism. The electrons in the Ag contact could acquire a small amount of energy, enabling them to penetrate the top of the potential barrier and reach the (PEA)₂PbBr₃Cl layer via the tunneling effect, such that the conductivity between the Ag contact and (PEA)₂PbBr₃Cl semiconductor layer improved, as shown in Figure 6c. Hence, the contact between the Ag and (PEA)₂PbBr₃Cl was not an ohmic contact but a non-blocking contact due to the narrow potential barrier for electrons and the possibility of tunneling.

4. Conclusions

In this study, we demonstrated the characteristics of (PEA)₂PbBr₃Cl on an AlN/sapphire substrate heterostructure. A quasi-2D structure was observed. Six XRD spectra peaks of perovskite-like (PEA)₂PbBr₃Cl on both the AlN/sapphire substrate and glass substrate were observed at around 10.83°, 16.15°, 21.50°, 26.90°, 32.38°, and 37.93°, corresponding to the (004), (006), (008), (0010), (0012), and (0014) phases of the quasi-2D (PEA)₂PbBr₃Cl structure, respectively. The peak position of photoluminescence (PL) of (PEA)₂PbBr₃Cl on the AlN/sapphire substrate heterostructure was 372 nm. The emission wavelength was lower than that of the traditional lead halide perovskite light-emitting diodes (typically 410 to 780 nm). This indicates the potential for application in UV perovskite light-emitting diodes. The conductivity between the Ag contact area and as-deposited (PEA)₂PbBr₃Cl on the AlN was almost non-conductive. However, after annealing treatment at 150 and 300 °C for 10 min, the conductivity of the Ag/(PEA)₂PbBr₃Cl structure on the AlN/sapphire substrate improved. The work function mechanism of Ag₂O with 5.7 eV caused the Fermi level of the (PEA)₂PbBr₃Cl layer to lower because of the thermal equilibrium with Ag and Ag₂O contacts after the annealing process, such that the electron in the Ag contact could easily transfer to the (PEA)₂PbBr₃Cl layer via the thermionic field emission (TFE) mechanism. Due to having higher stability than that of 3D perovskite materials, the two-dimensional, perovskite-like (PEA)₂PbBr₃Cl layer could be a candidate material for UV LEDs and AlN-based devices for ohmic contact and high-power devices.