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Abstract
Far-UVC light with emission wavelengths between 207 nm and 222 nm has shown significant potential for killing pathogens without damaging exposed human tissues and can be an alternative for safe sterilization. This work first reports on different compressively strained (AlN)8/(GaN)2 nanorods constructing by strain engineering digitally alloyed GaN embedded in an AlN barrier. By controlling the atomically thin GaN well under increasing compressive stress, we use the top-down etching method to realize regular nanorod arrays based on (AlN)8/(GaN)2 with different compressive strains in the GaN well. The emission wavelength is as short as 220 nm in the far-UVC, as expected by the theoretical calculations. We believe that this study will play an essential role in the design and fabrication of short-wavelength and high-efficiency LED structures with far-UVC emissions and potential use in effective, reliable, and safe UV disinfection systems.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
UVC radiation between 280 nm and 200 nm has become indispensable for sterilization and disinfection, with unique capabilities in deactivating bacteria, viruses, and other pathogens by attacking their DNA or RNA [1,2]. However, because of the strong UVC attenuation by the Earth’s atmosphere, it must be artificially generated by manufactured UVC light sources. The most commonly used UVC sources are conventional low- and medium-pressure mercury lamps, which are bulky, toxic, and fragile, requiring long warm-up times and emitting at only a few specific wavelengths [3,4]. Since 2020, in response to the global health-threatening crisis from the COVID-19 pandemic, there have been tremendous concerns with more efficient, cost-effective, and environmentally friendly UVC sources, causing UVC market demand to grow explosively [1]. Therefore, the emergence of semiconductor-based UVC LEDs has great potential for sterilization and disinfection applications due to their advantages of design flexibility, low power consumption, environmentally friendly construction, and so on [5,6].
Unlike the commonly effective wavelength used in UV mercury lamps peaking at approximately 254 nm, semiconductor-based UVC LEDs can be tailored freely with controllable wavelengths based on elemental composition. By alloying GaN with an AlN material and varying the bandgap from 3.4 eV to 6.2 eV, the emission wavelength of AlGaN-based LEDs can cover almost the entire UVC range [7,8]. Thus, AlGaN materials have attracted considerable attention for the realization of UVC LEDs. Recent work on AlGaN-based LEDs has demonstrated UVC emission wavelengths of 250∼280 nm [9–11]. By implementing available techniques during growth and designing several new structures, the improved performance of the UVC LEDs peaking at approximately 254 nm has enabled the possibility for surface sterilization applications [12]. Unfortunately, exposure to UVC light with a wavelength approximately 254 nm can be a health hazard to human skin and eyes. More recently, researchers have shown that far-UVC light with emission wavelengths between 207 nm and 222 nm potentially kills pathogens without damaging exposed human tissues [13,14]. Additionally, 222-nm UVC has been reported to exert a sterilization effect comparable to that of 254 nm UVC light and can be a safe alternative for sterilizing human skin [15].
However, the realization of high-efficiency far-UVC LEDs based on AlGaN materials remains a challenge. As the aluminium content in AlGaN increases, the efficiency becomes extremely competitive due to the large dislocation density of AlGaN materials and the optical polarization crossover from conventional AlGaN-based quantum wells (QWs). To the limit of far-UVC LEDs, previous research has reported band-edge emission from an AlN emitting layer at the shortest wavelength of 210 nm [16]. However, the 210 nm LED efficiency is extremely low because of the dominant emission of transverse-magnetic (TM)-polarized light parallel to the LED interfaces. Thus, in monolayer-scaled GaN well separated with the AlN barrier layer, i.e., a digitally alloyed (AlN)m/(GaN)n structure, the confinement preserves the transverse-electric (TE) polarized emission of GaN. The switch from TM-polarized to TE-polarized emission will facilitate highly efficient surface emission by overcoming the strong edge emission in high-aluminium-content AlGaN [17]. Additionally, the bandgaps of the digitally alloyed (AlN)m/(GaN)n structure increase to values higher than that of bulk GaN and alloyed AlN; both theoretical and experimental results are revealed in Refs. [18,19].
By reducing the thickness of the GaN well to correspond to a 3∼1 monolayer (ML), researchers have recently observed controllable UVC emission from 274 nm∼231 nm (4.5∼5.4 eV) for digitally alloyed (AlN)m/(GaN)n structures [17,20]. Even in theoretical calculations, the emission wavelength can be as short as 227 nm with 1 ML GaN embedded in 9 ML AlN, limiting the UVC emission and indicating that further blueshifting may be performed based on other technology [21]. Recent reports have shown that engineering AlGaN-based QWs under compressive strain contributes to the ordering of the valence bands (including the energy level position) and hence the improvement of the TE-polarized emission in UVC LEDs [22]. A promising way of controlling the strain status in QWs is proposed to fabricate AlGaN nanostructures, and recent work has revealed high structural quality and enhanced emission performance in AlGaN nanorod arrays [23,24]. Numerous methods for fabricating nanostructures have been investigated, such as electron beam lithography [25], nanoimprint [26], and self-assembly of polystyrene (PS) nanosphere template [27]. Generally, the nanosphere template is a simple technique that enables low cost and high throughput to produce structural controllable nanostructures. Therefore, an alternative approach by nanosphere template to utilize atomic-scaled GaN strain engineering is necessary to further realize shorter emission deeper into the far-UVC region with potentially improved light emission.
In this work, we employ first-principles calculations to control the atomically thin GaN well under different compressive in-plane strains of digitally alloyed (AlN)8/(GaN)2. Based on the predicted electronic band structures and radiative (optical) properties, far-UVC emission even shorter than 220 nm was realized by periodic nanorod arrays under different GaN well layer compressive stresses. Both theoretical and experimental results demonstrated that extreme quantum confinement shifts the emission wavelength into the far-UVC region and achieves TE-polarized emission in atomically thin GaN.
2. Theory and experimental details
2.1 Simulation details
We carried out first-principles calculations based on density functional theory (DFT) using the Vienna ab initio simulation package (VASP). The direction of the construction of the digitally alloyed (AlN)8/(GaN)2 structure was 80 atoms along the [0001], generated by 2a×2a×5c primitive cells, and the thicknesses of the GaN well and AlN barrier were 2 ML and 8 ML, respectively. The electron exchange and correlation pseudopotentials were described by the Perdew-Burke-Ernzerhof (PBE) method with the generalized gradient approximation (GGA) and Ga 3d electrons treated as a part of the valence electrons. For sampling the Brillouin zone of the (AlN)8/(GaN)2 structure, we used an 8×8×6 Monkhorst-Pack grid of k-points mesh and with a cut-off energy of 520 eV to expand the electronic wavefunctions. The geometry optimizations were performed by relaxing all degrees of freedom using the conjugate gradient algorithm with convergence energies of 1×10−3 eV and 1×10−4 eV for ions and electrons, respectively. The supercell’s volume and the atoms’ internal positions were initially determined by the AlN and GaN parameters, allowing them to be simultaneously relaxed to equilibrium. To further exert different compressive in-plane stresses on the atomically thin GaN well of the digitally alloyed (AlN)8/(GaN)2 structure, compressive strains of −1%, −1.5%, and −2% were employed by changing the in-plane lattice parameter. The crystal structure was allowed to relax along the c-plane direction. Finally, we used scissors to correct the values of the underestimated electronic bandgaps.
2.2 Fabrication and characterization details
The digitally alloyed (AlN)8/(GaN)2 structures were grown by metal-organic vapor phase epitaxy (MOVPE, 3×2-inch CCS Aixtron) on c-plane sapphire substrates. Following an approximately 1-µm-thick AlN buffer layer, (AlN)8/(GaN)2 structures were generated with integral monolayers and atomically sharp interfaces. We have described the growth and structural details in our previous work. Notably, the digitally alloyed AlN/GaN structures featuring atomically sharp interfaces were prepared with the hierarchical growth method. The AlN barriers and GaN wells are coherently grown through accurately stacking AlN and GaN monolayers in an integral scale [17]. Based on digitally alloyed (AlN)8/(GaN)2 structures, we employed self-assembled PS technology to change the GaN well strain status in the AlN barriers by forming nanorod arrays of different diameters. The process was initiated with a 200 nm-thick silica (SiO2) film deposited onto the (AlN)8/(GaN)2 epilayer by plasma-enhanced chemical vapor deposition (PECVD). PS nanospheres with diameters of 720 nm, 540 nm, and 360 nm were dip-coated on the above SiO2 surface. Inductively coupled plasma (ICP) was subsequently carried out to shrink the diameter of the nanospheres and etch down to (AlN)8/(GaN)2 by controlling the gas, pressure and time in the reactor. Through wet etching in the mixed solution (HF: NH4F=1:6) for 3 minutes, the residual layer of SiO2 was removed. Furthermore, to remove the surface states on the sidewall of the nanorods that inevitably lower sample quality during dry etching, the chemical treatment was carried out for all the nanorod samples by using KOH and dilute acid solution at 90 °C in a water bath. Finally, nanorod arrays based on (AlN)8/(GaN)2 with different nanorod diameters and periods were achieved by the top-down approach.
A series of characterizations were carried out to examine the properties of the nanorods fabricated on (AlN)8/(GaN)2, including a comparison to the properties of the planar (AlN)8/(GaN)2 structure. The nanorod morphologies were observed through high-resolution field-emission scanning electron microscopy (FESEM, ZEISS SIGMA). Using a confocal Raman spectroscopy imaging system (WITec alpha 300RA), the Raman scattering spectra were recorded in z(x, x)$\; \bar{z}$ backscattering geometry. The wavelength of the excitation was 488 nm, with an excitation power of 30∼40 mW. Cathodoluminescence (CL) spectra were collected by a Horiba Jobin Yvon model iHR320 spectrometer system at a spectral resolution of 0.06 nm with an electron gun (Orsay Physics “Eclipse” FEB Column) operating at 10 kV and 0.5 nA. All of the measurements were performed at room temperature (RT).
3. Results and discussion
To understand the underlying mechanism theoretically determining the radiative properties, we calculated the full band structures for the 2 ML-thick GaN well under different compressive in-plane strains. Stress-free (AlN)8/(GaN)2 was also considered for comparison to (AlN)8/(GaN)2 under controllable compressive stress. As shown in Fig. 1(a), energy quantization occurred in the stress-free GaN well. Along the high-symmetry lines from Γ to A in the Brillouin zone, i.e., the [0001] direction, the appearance of discrete energy levels was due to the extremely confined atomically thin GaN well. The top valence bands are identified by the heavy- and light-hole (HH-LH) bands, while the crystal-field (CH) band shifts to lower energy. This valence-band ordering ensures that the TE-polarized emission dominates in the stress-free (AlN)8/(GaN)2 digitally alloyed structure.
Fig. 1. Electronic band structures of atomically thin GaN wells in AlN barriers by first-principles calculations for (a) stress-free and (b) compressively strained (AlN)8/(GaN)2 with stress of −1%, (c) −1.5%, and (d) −2%.
When exerting compressive in-plane strains of −1%, −1.5%, and −2% on the atomically thin GaN well, the bandgaps increased to higher values, as shown in Fig. 1(b-d); thus, a higher transition energy provides the possibility of shorter emission. The CH band is shifted downward as the compressive in-plane stress increases, resulting in the crystal-field splitting energy increasing to as high as 218 meV more than that of the unstrained state. Furthermore, towards a deeper understanding of the optical polarization of compressively strained structures, the transition matrix elements (shown in Fig. 2) were calculated, with values of 3.42, 3.37, and 3.30 for the HH-LH band compressively strained at −1%, −1.5%, and −2%, respectively. However, the CH band values were determined to be 2.19, 2.25, and 2.32 at −1%, −1.5%, and −2%, respectively. Although the TM-polarized proportion increased as the strain in the atomically thin GaN well increased, the results show a higher transition probability from the HH-LH bands than from the CH band. Thus, the TE-polarized emission is expected to govern the efficient emission, even shortening to the far-UVC region.
Fig. 2. The interband transition matrix element of the (AlN)8/(GaN)2 structure under stress-free and increasing compressive strain values of −1%, −1.5%, and −2%.
Based on first-principles predictions, an (AlN)8/(GaN)2 structure with a digital well and barrier thickness was first grown. The schematic in Fig. 3(a) shows the fabrication process of the periodic nanorod arrays. Utilizing a self-assembled top-down approach based on (AlN)8/(GaN)2, the fabricated nanorod samples used PS nanosphere periods of 720 nm, 540 nm, and 360 nm, defined as samples N1, N2, and N3, respectively. Figure 3(b-i) shows typical top-view and cross-sectional SEM images of the periodic nanorod arrays. Nanorod arrays are observed in highly ordered hexagonal arrays, with controllable diameters over a large uniform area. From Fig. 3(b), (c), (f), and g, the nanorod diameters were determined to be approximately 550 nm, 400 nm, and 280 nm for samples N1, N2, and N3, respectively. Therefore, the diameters and periods of all samples equalled those for hexagonal close-packed nanospheres as the mask, indicating successful transfer and etching-down. Furthermore, the slanted sidewalls had an angle of 107.8° for samples N1, N2, and N3 in Fig. 3(e), (h), and (i). All samples appeared with fewer residuals covering them after eliminating the damage induced during nanorod formation.
Fig. 3. (a) Schematic of the nanorod fabrication by top-down technology consisting of SiO2 film deposition, self-assembly of PS nanospheres dip-coated onto the SiO2 surface, ICP etching down to the SiO2 layer, and removal of both nanospheres and residual SiO2. (b-i) Top-view and cross-sectional SEM images of the periodic nanorod arrays for samples N1 (b, c, e), N2 (f, h), and N3 (g, i) and a tilted-view image of (d) sample N1, with magnified morphology insets in (f-i).
To examine the stress variation in the periodic nanorod arrays compared to that in the planar-structured (AlN)8/(GaN)2, an evaluation of the Raman shift concentrating in the E2H mode was necessary. For the planar-structured (AlN)8/(GaN)2, the E2H modes located at 614.3 cm−1 and 654.5 cm−1 are assigned as GaN-like and AlN-like, respectively, as shown in Fig. 4(a). The latter phonon mode appears to have a higher intensity with a narrower full-width at half maximum (FWHM), which is attributed to the presence of the AlN buffer layer and the relatively thicker AlN barrier. When the planar structure was fabricated into nanorod arrays with different diameters and periods, the E2H (GaN- and AlN-like) modes showed redshifts towards high phonon frequencies. Notably, E2H (AlN-like) was shifted to 656.6 cm−1 in all the nanorod samples, shown in Fig. 4(b-d). Considering a phonon frequency of 657.4 cm−1 for stress-free AlN as a reference, the E2H (AlN-like) mode shift towards high frequency indicates that more stress had been released in the AlN layers. On the other hand, the E2H mode has shown broader FWHM for all the nanorods than the planar (AlN)8/(GaN)2 structure. This can be attributed to the non-uniform strain distribution along the direction perpendicular to the well plane (z-axis) induced by the inclined sidewall of nanorods.
Fig. 4. Raman spectra of (a) the planar-structured (AlN)8/(GaN)2 and (b) nanorod samples N1, (c) N2, and (d) N3. The red dashed lines represent the measured E2H mode.
Furthermore, σxx was adopted to evaluate the in-plane compressive or tensile stress quantitatively, which can be described by the formula below [28]:
Table 1. E2H phonon frequencies (cm−1) measured at RT and the calculated stress and strain for both planar-structured (AlN)8/(GaN)2 and nanorod samples
In addition, the effect on electronic bandgaps induced by the structural change from planar to nanorods is explored through the normalized CL spectra, as illustrated in Fig. 5(a-b). All the samples exhibited a single emission from the (AlN)8/(GaN)2, in good agreement with our previous work that the transition originated from 1h–1e energy levels [2,17]. The energy peaked at approximately 5.36 eV for the planar structure, with an emission wavelength of approximately 231 nm in the UVC region. Once the design changed from planar to periodic nanorod arrays, a blueshift was observed as the nanorod diameter decreased, i.e., the compressive in-plane strain increased in the atomically thin GaN well. Recently, S. Park et al. reported that the interface effect caused by the crystalline GaN nanotube sidewall surface could contribute to the redshift [30]. However, the emission wavelength was significantly dominated by extreme quantum confinement in this work. As a result of the atomically thin GaN well and the confinement from residual stress distributed around the nanorod’s edge, the biaxial stress is likely to change in the (AlN)8/(GaN)2 nanorods in comparison to the Al0.8Ga0.2N ternary alloy with the same composition and even the planar (AlN)8/(GaN)2 structure. In contrast to the near-band-edge transition of Al0.8Ga0.2N at approximately 235 nm observed by D. G. Ebling et al. from the CL spectrum [31], the (AlN)8/(GaN)2 planar structure shifts the emission wavelength to 231 nm. After controlling the compressive strain status of the well-fabricated (AlN)8/(GaN)2 nanorod arrays, the emission wavelength was as short as 220 nm for sample N3 in the far-UVC region, in excellent agreement with the theoretical predictions.
Fig. 5. (a) Normalized CL spectra of the planar- and nanorod samples based on the (AlN)8/(GaN)2 structure excited by an electron beam operating at 10 kV and 0.5 nA, respectively. (b) The emission wavelengths for the Al0.8Ga0.2N ternary alloy [31], (AlN)8/(GaN)2 planar structure, and compressively strained (AlN)8/(GaN)2 nanorods in this work by CL measurements.
4. Conclusion
In summary, we investigated the electronic and radiative properties of strain engineered digitally alloyed (AlN)8/(GaN)2 from first-principles calculations. The bandgap of the (AlN)8/(GaN)2 structure increased due to the increase in compressive strain in the GaN well. The TE-polarized emission is expected to dominate due to the shortened emission wavelength in the UVC region. Through a top-down-etching approach, we fabricated regular AlGaN nanorods with varying diameters to achieve increasing compressive strain in the GaN layer of the (AlN)8/(GaN)2 nanorods. The room-temperature CL results further demonstrate that such nanostructures based on digitally alloyed (AlN)8/(GaN)2 enable emission wavelengths as short as 220 nm. Due to the extreme quantum confinement, the transition energy is higher than those for Al0.8Ga0.2N ternary alloys with the same composition and the planar (AlN)8/(GaN)2 structure. Thus, it is beneficial to generate a shorter wavelength emission in the far-UVC region. Digital alloying of (AlN)8/(GaN)2 by controllable strain engineering shows potential for generating emissions from the active layer in far-UVC emitters and enhancing high-efficiency emission.
Funding
National Key Research and Development Program of China (2016YFB0400903); National Natural Science Foundation of China (61874090, 61874091, 62074133); Natural Science Foundation of Fujian Province (2021H0001).
Acknowledgements
All authors participated in the conception of the project and took part in the discussion of results. NG, JXC, XF, and HYC fabricated the samples and made characterizations. SQL, WL, and JCL performed the first-principles calculations. NG, KH, and JYK wrote the draft and revised the final manuscript. All authors have given approval to the final version of the manuscript.
Disclosures
The authors declare no competing financial interest.
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