Selective area epitaxy of AlGaN nanowire arrays across nearly the entire compositional range for deep ultraviolet photonics

1. Introduction

Light sources that emit in the ultraviolet (UV)-C band (< 280 nm) are in demand for a broad range of applications, including water purification, disinfection, surface treatment, and medical diagnostics [1–7]. AlGaN, with tunable energy bandgap from 6.2 eV to 3.4 eV, has emerged as the material of choice for realizing all semiconductor based deep UV (DUV) light sources, which can potentially replace conventional mercury and xenon lamps. While high performance GaN-based devices have been demonstrated in the blue and near-UV wavelength range, the performance of AlGaN light-emitting diodes (LEDs) degrades considerably with increasing Al composition [8–13]. For instance, the external quantum efficiency (EQE) of AlGaN quantum well LEDs is well below 10% in a large part of the UV-C spectrum [10,12,14]. One of the primary challenges for the extremely low efficiency is associated with the dominance of transverse magnetic (TM) polarized optical emission, with the electric field direction parallel to the c-axis, due to the negative crystal field splitting energy of Al-rich AlGaN [15]. The TM polarized emission prevents the extraction of photons from the top c-plane surface. The resulting light reabsorption not only leads to extremely low efficiency but further causes severe heating effect and device instability.

Recent studies suggested that the efficiency bottleneck of DUV light sources could be addressed by using nanowire photonic crystals: Light extraction efficiency exceeding 70% has been predicted by exploiting the light transmission regime of AlGaN nanowire photonic crystal structures [16]. Moreover, AlGaN nanowires offer several additional advantages compared to conventional quantum well structures, including significantly reduced defect formation and much more efficient p-type current conduction, due to the reduced Al-substitutional Mg-dopant formation energy [17–21]. The achievement of such unprecedentedly high efficiency photonic crystal light emitters, however, requires a precise control of the nanowire size, spacing, and spatial arrangement. The currently reported AlGaN nanowires have been largely formed spontaneously [22–25], with random variations in size, spacing, and geometry, which leads to strong light trapping effect and extremely low light extraction efficiency [25–27]. Such issues can be potentially addressed by using the technique of selective area epitaxy [28–36]. In this process, the nanowire formation and nucleation only takes place in the nanoscale apertures created on a substrate using a top-down approach, due to the growth selectivity in the opening apertures vs. on the mask [32,35,36]. To date, however, selective area epitaxy has not been successfully applied to Al-rich AlGaN nanowires [30,37–41], and there has been no demonstration of AlGaN nanowires emitting in the UV-C band by selective area epitaxy [40], which is partly limited by the high temperature (often >1,000 °C) required for AlN epitaxy and the lack of a suitable growth mask.

In this context, we report on the site-controlled epitaxy and structural and optical characteristics of AlGaN nanowire arrays with Al incorporation controllably varied across nearly the entire compositional range. Selective area epitaxy of AlGaN nanowires with controlled size and spacing is achieved by growing AlGaN on a GaN nanowire template using plasma-assisted molecular beam epitaxy (MBE). It is also observed that an Al-rich AlGaN shell structure is spontaneously formed, which can significantly suppress nonradiative surface recombination based on previous studies [19,22,24,25,42]. An internal quantum efficiency up to 45% was measured at room-temperature. We have further demonstrated large area AlGaN nanowire LEDs operating in the UV-C band on sapphire substrate, which exhibit excellent optical and electrical performance, including a small turn-on voltage of ~4.4 V and an output power of ~0.93 W/cm² at a current density of 252 A/cm².

2. Selective area epitaxy of AlGaN nanowire arrays over the entire compositional range

Illustrated in Fig. 1(a) is the selective area epitaxy of Al_xGa_1-xN nanowire arrays on a GaN nanowire template formed on c-plane GaN-on-sapphire substrate. A thin Ti layer (~10 nm) was first employed as the growth mask for the selective area epitaxy of GaN nanowires [30,43]. Nanoscale hexagonal apertures, with a lateral size d arranged in a triangle lattice of a lattice constant a were fabricated using e-beam lithography and reactive ion etching. Prior to the growth of nanowires, nitridation of the Ti mask was performed at 400°C to prevent crack and degradation during the growth process [30,32,35,37,38,44]. GaN nanowire templates were then grown with a substrate temperature of 955°C, nitrogen flow rate of 0.55 sccm, and Ga flux of ~3.7 × 10⁻⁷ Torr using a Veeco GENxlor MBE system [36]. Subsequently, Al_xGa_1-xN nanowires were grown with substrate temperatures and nitrogen flow rates in the range of 935°C to 1025°C and 0.3 sccm to 0.55 sccm, respectively. Ga and Al beam fluxes were varied in the range of ~10⁻⁷ to ~4.2 × 10⁻⁷ Torr and ~10⁻⁸ to ~7.1 × 10⁻⁸ Torr, respectively to tune the alloy composition and emission wavelength. The substrate temperature mentioned here refers to the thermocouple reading on the backside of the substrate. The real substrate surface temperature is estimated to be ~100–150°C lower, depending on the substrate and sample size. Because of the shadowing effect of GaN nanowires, direct epitaxy of AlGaN on the substrate is largely eliminated. Shown in Fig. 1(b) is a scanning electron microscope (SEM) image of GaN/Al_xGa_1-xN nanowire arrays, which exhibit controlled size and spacing and well-defined hexagonal morphology, with a very high degree of uniformity. Detailed scanning transmission electron microscopy studies (see Appendix) further confirm that an Al-rich AlGaN shell structure is spontaneously formed surrounding the nanowires, which can suppress nonradiative surface recombination [19,25,42].

 

Fig. 1 (a) Left: Nanoscale aperture arrays defined by e-beam lithography on a 10 nm thick Ti mask on a c-plane GaN-on-sapphire substrate. Right: Schematic of the selective area epitaxy of GaN/Al_xGa_1-xN nanowires on the patterned substrate. (b) An SEM image of GaN/Al_xGa_1-xN nanowire arrays grown by selective area epitaxy.

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Photoluminescence (PL) spectra of these Al_xGa_1-xN nanowire arrays were measured at room-temperature. The nanowire sample was excited using a 193 nm ArF excimer laser, and the PL emission was collected and spectrally resolved by a high-resolution spectrometer and detected by a liquid nitrogen cooled charge coupled device detector. Illustrated in Fig. 2(a), strong PL emission from 210 nm to 327 nm was measured from Al_xGa_1-xN nanowires grown with different Al compositions. The correlation between the energy bandgap of Al_xGa_1-xN and Al composition x can be approximately derived from the equation below [40].

 

Fig. 2 (a) Normalized room-temperature PL spectra of Al_xGa_1-xN nanowire arrays with Al compositions tuned from ~20% to 100%. (b) Plot of emission wavelength vs. Al composition for AlGaN nanowires demonstrated in this work (blue diamond) and reported previously (red circle) by selective area epitaxy.

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Illustrated in Fig. 2(b) are variations of the PL emission wavelength vs. Al composition. It is seen that Al_xGa_1-xN nanowires with Al composition varying across nearly the entire compositional range, i.e. from x = 0 to 1 can be readily achieved by selective area epitaxy. For comparison, previous studies on the selective area epitaxy of Al_xGa_1-xN nanowires were largely limited to Al composition below 40% [40].

3. Selective area epitaxy and characterization of AlGaN nanowire LEDs

The realization of high quality Al-rich Al_xGa_1-xN nanowire arrays by selective area epitaxy provides a distinct opportunity to demonstrate high efficiency nanowire light emitters operating in UV-C band [16,43]. In this work, we first investigated the epitaxy and electrical and optical performance of AlGaN nanowire LEDs operating at ~280 nm. Illustrated in Fig. 3(a) is the schematic diagram of Al_xGa_1-xN nanowire LEDs grown on c-plane sapphire substrate, which consists of a 300-nm Si-doped GaN, 80-nm Si-doped Al_0.64Ga_0.36N, 30-nm undoped Al_0.48Ga_0.52N active region, and 60-nm Mg-doped Al_0.64Ga_0.36N segment. The growth for Al_xGa_1-xN nanowire LED structures included the following steps. The n- and p-type Al_xGa_1-xN cladding layers were grown with the substrate temperature of 1025°C, nitrogen flow rate of 0.55 sccm, and Ga and Al beam fluxes of ~3.7 × 10⁻⁷ Torr and ~3.7 × 10⁻⁸ Torr, respectively. The AlGaN active layer sandwiched between the n- and p-type cladding layers was grown with Ga and Al beam fluxes of ~3.7 × 10⁻⁷ Torr and ~3.0 × 10⁻⁸ Torr, respectively. Mg beam flux was ~3.2 × 10⁻⁹ Torr for the Mg-doped AlGaN layer. During the growth of AlGaN nanowire segments, the lateral growth is enhanced, due to the small diffusion length of Al atoms, which leads to increased nanowire diameter and reduced air gap between adjacent nanowires. The approximate axial and lateral growth rates are ~150 nm/hr and ~20 nm/hr, respectively.

 

Fig. 3 (a) Schematic of AlGaN nanowire LEDs grown by selective area epitaxy. (b) PL spectra of AlGaN nanowire LED heterostructures measured at 300 K under different excitation powers. Each spectrum was normalized by its individual peak intensity and shifted vertically for display purpose. (c) Variations of the PL spectral linewidth and peak energy as a function of excitation power. (d) Arrhenius plots of the integrated PL intensity measured from 14 K to 300 K for the active region (E₁) emission and the whole spectra. The inset shows PL spectra measured between 300 K and 20 K under an excitation power of 50 mW. E₁, E₂ and E₃ correspond to peak emissions from Al_0.48Ga_0.52N active region, Al_0.64Ga_0.36N cladding layers, and GaN, respectively.

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Optical properties of Al_xGa_1-xN nanowire LED heterostructures were studied using temperature-dependent PL spectroscopy. Illustrated in Fig. 3(b) are the normalized PL spectra measured at room temperature under different excitation powers. At a relatively low excitation power of 4 mW, PL emission (~283 nm) from the Al_0.48Ga_0.52N active region dominates, confirming the excellent carrier confinement provided by the core-shell structure. The spectral linewidth is ~11 nm, which is narrower compared to that of previously reported spontaneous AlGaN nanowires [19,22,24,42], due to the improved size uniformity. With increasing excitation power, a shorter wavelength emission peak E₂ ~255 nm emerges, which corresponds to the emission from Al_0.64Ga_0.36N cladding layers. It is also seen that, with increasing excitation power, the emission peak E₁ (~283 nm) remains highly stable, shown in Fig. 3(c). The spectral linewidth exhibits a small broadening with increasing power. Illustrated in the inset of Fig. 3(d) are the PL spectra measured between 20 K and room-temperature under an excitation power of 50 mW. The emission peaks E₁ and E₂ (~255 nm) correspond to the transitions in the Al_0.48Ga_0.52N active region and Al_0.64Ga_0.36N cladding layers, respectively. The GaN-related emission (E₃) at ~355 nm is also identified at low temperature. S-shaped behavior was not observed for the peak position of E₁. Figure 3(d) shows the integrated PL intensities for emissions from both the whole spectra and active region as a function of temperature under excitation of 50 mW. The internal quantum efficiency can be approximately estimated by I_PL(RT)/I_PL(LT), wherein I_PL(RT) and I_PL(LT) are the integrated PL intensity measured at room temperature and low temperature, respectively. By assuming a near-unity quantum efficiency at 14 K, The internal quantum efficiency of the active region emission (E₁) is derived to be ~45% at room temperature, which is comparable with previously reported high quality Al_xGa_1-xN quantum well structures [45–47]. However, it is worthwhile mentioning that the measurement of internal quantum efficiency may depend on the excitation power [47,48]. In our studies, the internal quantum efficiency was measured to be ~49% under excitation power ~100 mW. For nanowire devices, the internal quantum efficiency may also be affected by the presence of surface states/defects. In this regard, the incorporation of a large bandgap AlGaN shell structure can significantly reduce the effect of surface recombination on the quantum efficiency. The internal quantum efficiency of the presented AlGaN nanowires can be further improved by optimizing the growth conditions and the large bandgap AlGaN shell coverage.

AlGaN nanowire LEDs were subsequently fabricated and characterized. First, polyimide (PI2610 from HD MicroSystems) was spin-coated to serve as a surface planarization and passivation layer, followed by dry etching to reveal the nanowire top surface. Prior to contact metallization, the surface was treated with hydrochloric acid to remove any oxides. Ni (20 nm)/Au (10 nm) and Ti (20 nm)/Au (100 nm) contact layers were subsequently deposited on the device top surface and n-GaN template to serve as p- and n-metal contacts, respectively. An annealing at 550 °C was performed in N₂ gas ambient for 1 min. Subsequently, metal grid patterns were deposited on the device surface to facilitate current injection. Although the device fabrication process is more involved compared to the previously reported controlled coalescence of AlGaN nanowire LEDs [41], the controlled synthesis of AlGaN subwavelength nanostructures with well-defined size and spacing opens up new opportunities for developing high efficiency LEDs and lasers. The devices exhibit excellent current-voltage (I-V) characteristics. Shown in Fig. 4(a) is the I-V curve for a device with a size of 50 × 50 μm², which exhibits a turn-on voltage of ~4.4 V. A current density of 100 A/cm² was measured for a bias voltage of ~5.0 V, which is better than previously reported Al_xGa_1-xN quantum well LED devices operating at similar wavelengths [49–51]. The room-temperature electroluminescence (EL) spectra are shown in Fig. 4(b). The devices exhibit strong emission at 279 nm. With increasing injection current, the peak emission (E₁) exhibits a small blue-shift as shown in Fig. 4(c), i.e. from 279.6 nm at 50 A/cm² to 278.9 nm at 252 A/cm². The weaker emission peak at 260 nm is due to electron overflow and the resulting emission from the p-Al_0.64Ga_0.36N layer. The output power density as a function of injection current density is also shown in Fig. 4(c). The measurements were performed for current density up to 252 A/cm² under pulsed mode (1% duty cycle) to reduce any heating effect. The output power density increases near-linearly with increasing injection current. The output power at current density of 252 A/cm² is estimated to be ~0.93 W/cm². The output power can be significantly increased by optimizing the nanowire size and spacing to achieve enhanced light extraction for TM polarized emission [16]. Moreover, the device performance can be further improved by utilizing the scheme of tunnel junction to significantly enhance the hole injection and transport [52,53].

 

Fig. 4 (a) Current-voltage characteristics of AlGaN nanowire LEDs with an area of 50 × 50 µm². Inset: I-V characteristics of device under forward and reverse bias displayed in semi-log scale; (b) Electroluminescence spectra of AlGaN nanowire LEDs measured under different injection currents. (c) Power density and peak position as a function of current density measured at room-temperature under pulsed biasing condition.

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4. Summary

In summary, with the use of a GaN nanowire template, we have successfully demonstrated the selective area epitaxy of Al_xGa_1-xN nanowire arrays with Al composition varying across nearly the entire compositional range. The nanowires exhibit spontaneously formed core-shell structure and relatively high internal quantum efficiency at room temperature. With the use of selective area epitaxy, we have further demonstrated functional AlGaN nanowire LEDs operating in the UV-C band, which exhibit excellent electrical and optical performance. The controlled synthesis of AlGaN subwavelength nanostructures with well-defined size, spacing, and spatial arrangement and tunable emission opens up new opportunities for developing high efficiency LEDs and lasers and promises to break the efficiency bottleneck of deep UV photonics.

Appendix

Detailed structural characterization of Al_xGa_1-xN nanowire arrays was performed by an aberration-corrected FEI Titan Cubed 80-300 scanning transmission electron microscope (STEM) operated at 200 kV on a cross-sectional TEM specimen of the nanowire arrays prepared by focused ion beam milling using a Zeiss NVision 40 dual-beam system with deposited Pt and C as protection layers. Atomic-number sensitive (Z-contrast) high-angle annular dark-field (HAADF) image of an array of Al_xGa_1-xN nanowire LED heterostructure observed in cross-section along the a-plane orientation is shown in Fig. 5(a). A higher magnification image is shown in Fig. 5(b). The brighter intensity regions correspond to the n-GaN nanowire template and the darker intensity regions correspond to AlGaN LED heterostructure. Electron energy-loss spectroscopy (EELS) was also carried out for elemental mapping of the AlGaN heterostructure and shown in Figs. 5 (c)-(e). An Al-rich core-shell configuration was observed. The underlying growth mechanism is directly related to the incorporation of Al and Ga adatoms and growth conditions. At elevated temperatures, such as the growth temperatures used in this study, the Ga adatom diffusion length is significantly larger than that of Al adatom, forming Ga-rich core of the nanowire. Simultaneously, a large number of Al adatoms get incorporated directly on the sidewalls, resulting in the formation of an Al-rich shell on the nanowire sidewalls. It is worthwhile mentioning that in these studies, due to the instability of the Al effusion cell, the top AlGaN segment has higher Al composition that the lower AlGaN cladding layer.

 

Fig. 5 STEM studies of AlGaN nanowire heterostructures. (a) Low magnification STEM- HAADF image of multiple AlGaN nanowire devices in cross-section and oriented along the $[11\overline{2}0]$ axis. (b) High magnification STEM-HAADF image of one nanowire (c) PCA-treated EELS elemental maps representing respectively, the distribution of Ga and Al in pseudo-color overlay (green for Al and red for Ga). (d) and (e) the distribution of Ga using its L_2,3-edge and the distribution of Al using its K-edge in greyscale.

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Funding

US Army Research Office (W911NF-17-1-0109).

Acknowledgment

Aberration-corrected STEM was performed in the Canadian Centre for Electron Microscopy, a national facility supported by NSERC, the Canada Foundation for Innovation, and McMaster University. S.Y.W. and G.A.B. thank T. Casagrande for the sample preparation using FIB.

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