1. Introduction

Aluminum-Gallium-Nitride (AlGaN)-based semiconductor ultraviolet (UV) lasers are of tremendous importance for a wide range of applications in lighting, display, water purification, disinfection, chemical and biochemical sensors, and medical diagnostics [1–3]. To date, however, the performance of such devices degrades considerably with increasing Al content, i.e. decreasing operation wavelength [3–5]. For example, the shortest wavelength of electrically injected AlGaN quantum well lasers reported to date is ~336 nm, with a large threshold current density on the order of ~10 kA/cm², or higher [6–8]. The poor performance of such devices is fundamentally limited by the presence of large densities of defects and dislocations, the strong polarization fields, and the inefficient p-doping in AlGaN materials [9–17]. Recently, with the use of nanowire structures, high-efficiency light emitting diodes (LEDs) and lasers have been demonstrated in the visible range [18–27]. Nanowire structures can offer several fundamental advantages, including low defect densities, reduced strain-induced polarization fields, enhanced dopant incorporation and compatibility with foreign substrates [28,29]. However, the realization of efficient nanowire lasers in the UV wavelength range has been limited [30–32]. For practical applications, it is of significant interest in developing large-area electrically injected photonic crystal surface by using such dislocation-free nanowire structures, wherein the optical mode can be tailored to achieve high performance lasing in the UV spectrum. To realize nanowire photonic crystal lasers, any optical scattering loss related to variations of nanowire size, density, and spacing should be substantially reduced. Such issues can be addressed by using the special technique of selective area growth (SAG), wherein the nanowire formation are directly controlled by the nanoscale apertures created on a substrate using the well-defined top-down process [33–35].

Recent theoretical studies have suggested that defect-free nanowire photonic crystal structures, with arbitrary geometry or shape, can behave as mirrorless resonant cavities with high quality factor and fineness for surface emitting lasers [36]. This unique design concept does not require the use of any perfect mirrors or precisely fabricated point or line defects [37–39], which have been extensively used in conventional photonic crystal structures incorporated in optical filters [40], surface emitting lasers [41], and light emitting diodes [42]. The defect-free photonic crystal structures can sustain surface avoiding modes, which have been observed experimentally at acoustic frequency in semiconductor superlattices [43,44]. To date, however, there have been no reports on such photonic crystal lasers operating in the UV wavelength range. In this context, we have studied the design, fabrication, and characterization of defect-free AlGaN nanowire photonic crystal lasers. The AlGaN nanowire arrays with controlled size and spacing are selectively grown on GaN-on-sapphire template using plasma-assisted molecular beam epitaxy. Such nanowire structures exhibit nearly identical size distribution and strong photoluminescence emission in the wavelength range of 370 nm. By exploiting the presence of topological high-Q resonance of defect-free nanowire photonic crystals, we have achieved an electrically pumped semiconductor laser that can operate at 369.5 nm. The lasers exhibit a low threshold current density (~2.1 kA/cm²) under continuous wave operation at room-temperature, with a very narrow spectral linewidth (~0.2 nm).

2. Photonic crystal laser structure and design

Using the finite-element method, we have studied the simulation of defect-free AlGaN nanowire photonic crystal structures as a topological high-Q resonator. The simulation is performed using the RF module of Comsol Multiphysics. A topological resonator is expected to operate near the band edge of a photonic crystal structure with an arbitrary shape [36]. In this study, a photonic crystal with band edge around 370 nm is designed as an example. The lattice constant a of the nanowire photonic crystal is 400 nm, and the spacing between nanowires is ~36 nm. The effective refractive index is ~2.49 for GaN/Al_xGa_1-xN nanowires. The corresponding band structure for transverse magnetic polarization (E in parallel with growth direction) is shown in Fig. 1(a). The normalized frequency a/λ for the operation wavelength 370 nm is ~1.08, which is found to be near the Γ point of the band shown in red color. It is therefore expected such a photonic crystal structure with an arbitrary shape to behave like a topological high-Q factor cavity near this wavelength [36]. To further confirm the presence of a high-Q resonance at this wavelength, a nanowire photonic crystal with an arbitrary irregular shape is generated and the mode is simulated, as shown in Fig. 1(b). The hexagons represent nanowires. The profile shown in Fig. 1(b) extends through the entire structure, with very weak field intensity along the edges, which is the unambiguous surface-avoiding feature of a topological resonator as discussed in Ref. [36] and can further lead to dominant surface emission by optimizing the nanowire size and spacing. The Q factor of 11,460 is derived from the simulation using the equation $Q=0.5 \times Real (f) /Imag(f)$, wherein f, $Real (f)$and $Imag(f)$ are eigenfrequency, the real part of eigenfrequency representing the optical frequency, and the imaginary part of eigenfrequency representing the decay of energy in this mode, respectively.

 

Fig. 1 (a) Band structure of the designed photonic crystal structure targeted to operate at a/λ = 1.08 (red). (b) Schematic of the topological nanowire photonic crystal high-Q resonator represented by hexagons and the intensity profile of one of the calculated modes.

Download Full Size | PDF

3. Selective area epitaxial growth of AlGaN nanowire photonic crystal lasers and performance characterization

Extensive growth optimization was performed to achieve GaN/Al_xGa_1-xN nanowire photonic crystals. In this work, the selective area epitaxial growth takes place on n-GaN templates on c-plane sapphire substrates. A thin (~10 nm) Ti layer was employed as the growth mask. Subsequently, periodically nanoscale patterns with a hole diameter of 300 nm and a center-to-center spacing of 400 nm arranged in a triangle lattice were created using standard e-beam lithography and reactive ion etching processes [34,35]. Sizes of the nanohole arrays were varied in the range of tens to hundreds of µm for the fabrication of large-area nanowire photonic crystal lasers. Prior to loading into the growth chamber, the nanohole patterned substrates were carefully cleaned using standard solvents and chemically treated using hydrochloric acid to remove any oxide layer on GaN surface. The Ti masking layer was then nitridized in the reaction chamber at ~400 °C to prevent the crack formation and degradation at elevated temperature [34,45–48]. Subsequently, vertically aligned GaN/Al_xGa_1-xN nanowire arrays, schematically illustrated in Fig. 2(a), were selectively grown in the opening apertures using a Veeco Gen II plasma-assisted molecular beam epitaxial system. The epitaxy of GaN/Al_xGa_1-xN nanowires included the following steps. An n-type GaN contact layer was first grown with a substrate temperature of 920 °C, a nitrogen flow rate of 0.33 sccm, and a Ga flux of ~4 × 10⁻⁷ Torr. Subsequently, the n- and p-Al_0.2Ga_0.8N cladding layers were grown at a substrate temperature of 940 °C, a nitrogen flow rate of 0.33 sccm, Ga and Al beam flux of ~4 × 10⁻⁷ Torr and ~2.2 × 10⁻⁸ Torr, respectively. The epitaxial growth conditions of Al_0.09Ga_0.91N waveguide layers included a substrate temperature of 940 °C, a nitrogen flow rate of 0.33 sccm, Ga and Al beam flux of ~4 × 10⁻⁷ torr and ~1.1 × 10⁻⁸ Torr, respectively. Five GaN/Al_0.09Ga_0.91N quantum disks were incorporated as the gain media. The mentioned substrate temperature refers to the thermocouple reading on the backside of the substrate, which is ~100–150 °C higher than the actual sample surface temperature, depending on the substrate and sample size. Mg beam equivalent pressure was ∼1.0⋅10⁻⁹ Torr and ~8.0⋅10⁻¹⁰ for the Mg-doped Al_0.2Ga_0.8N cladding and p-GaN contact layers, respectively. During the growth of AlGaN nanowire segments, the lateral growth is significantly enhanced, leading to increased nanowire diameter and smaller air gap amongst nanowires. The incorporation of Al during Al_xGa_1-xN growth leads to vertical and lateral growth rates of ~1 nm/min and ~0.2 nm/min, respectively.

 

Fig. 2 (a) Schematic illustration of GaN/Al_xGa_1-xN multiple quantum disk nanowire structure. (b) Titled view SEM image of GaN/Al_xGa_1-xN nanowire arrays grown by selective area epitaxy. (c) Room-temperature photoluminescence spectrum of GaN/Al_xGa_1-xN nanowire arrays.

Download Full Size | PDF

Figure 2(b) shows a scanning electron microscope (SEM) image of the GaN/Al_xGa_1-xN nanowire arrays taken at a tilted angle. It is seen that the nanowires exhibit a high level of size uniformity. Photoluminescence (PL) spectra of these Al_xGa_1-xN nanowire arrays were measured at room-temperature using a 193 ArF excimer laser as an excitation source. The PL emission was collected and spectrally resolved by a high-resolution spectrometer equipped with a liquid nitrogen cooled charge coupled device detector (CCD). Illustrated in Fig. 2(c), strong emission of GaN/Al_xGa_1-xN nanowire arrays at ~370 nm can be clearly measured. The PL peak emission at ~355 nm originates from the Al_xGa_1-xN guide layer.

Subsequently, the nanowire arrays were fabricated into photonic crystal lasers using standard photolithography, e-beam lithography, dry etching and contact-metallization techniques. The devices were chemically treated by hydrochloric acid for 30 s prior to metal evaporation. Ni/Au and Ti/Au metal layers were deposited on the nanowire top surfaces and n-GaN template to serve as p-and n-metal contacts, respectively. Subsequently, an annealing process at 550 °C was performed in N₂ ambient for 1 min for Ohmic contact formation. Taking advantages of well-controlled and small air gaps amongst nanowires in SAG process, no surface passivation or planarization layer was used during the device fabrication. This promotes the use of nanowires for large-area UV emitting devices, which has been often limited by the lack of UV-transparent polymer for surface passivation. The characterization of electrical and optical properties of photonic crystal lasers was performed under a direct current bias condition at room-temperature. The operation of the GaN/Al_xGa_1-xN nanowire photonic crystal lasers is schematically illustrated in Fig. 3(a)_, wherein the laser emission is expected to be around the entire photonic crystal surface area. The device area is ~500 µm². The current-voltage (I-V) characteristics of GaN/Al_xGa_1-xN photonic crystal lasers are shown in Fig. 3(b). The fabricated nanowire photonic crystal lasers exhibit good current-voltage characteristics. The device shows a relatively low turn on voltage of ~4V and exhibits a low leakage current of ~69 µA at −9.5V, shown in the inset, which is significantly smaller than that of previously reported nanowire devices [49].

 

Fig. 3 (a) Schematic of GaN/Al_xGa_1-xN nanowire photonic crystal laser grown by selective area epitaxy. (b) Current-voltage characteristics of GaN/Al_xGa_1-xN nanowire photonic crystal laser. Inset: I-V characteristics plotted in semi-log scale.

Download Full Size | PDF

The lasing characteristics of such devices were investigated under electrical injection under continuous-wave operation at room-temperature. Electroluminescence (EL) from the devices was collected using an UV-enhanced optical fiber and was spectrally analyzed by a high-resolution spectrometer (spectral resolution ~0.1 nm) equipped with a photomultiplier tube. Figure 4(a) shows the room-temperature EL spectra of GaN/Al_xGa_1-xN photonic crystal lasers at injection currents below and above threshold current density (J_th). It is observed that, with increasing injection current density, a sharp peak emerged and superimposed on the broad background emission. Above threshold, the EL spectrum shows a sharp peak centered at 369.5 nm with full-width-at-half-maximum (FWHM) of ~0.2 nm. The light-current density (L-J) characteristics of photonic crystal laser are illustrated in Fig. 4(b), clearly showing a threshold current density of ~2.1 kA/cm², which is much lower than that of previous reports of GaN-based lasers operating in similar wavelength range [6,50–52]. The relatively low J_th is directly related to the dislocation-free AlGaN nanowire heterostructures, the core-shell nanowire arrays with suppressed surface recombination, and the topological high-Q resonance of defect-free nanowire photonic crystals [34,36,49]. The variations of output power and linewidth of the lasing peak as a function of injection current in logarithmic scale are plotted in the inset of Figs. 4(b) and 4(c), respectively. The S-shaped L-J curve, together with the significant reduction of spectral linewidth near threshold confirm the evolution from spontaneous emission, amplified spontaneous emission to linear lasing emission with increasing injection current, providing an unambiguous evidence for the achievement of lasing. Shown in Fig. 4(d) are variations of the laser peak position with increasing injection current density. It is observed that the peak position exhibits a negligible change as injection current increases, suggesting an extremely stable lasing operation. Under injection current density of ~3.28 kA/cm², the estimated output power is in the range of ~10 µW. The output power is limited, to a certain extent, by the small collection angle (~25°) of the optical fiber. It is also important to notice that the emission of such a photonic crystal laser is not limited to one direction. The optical fiber, however, can only measure lasing emission from a single direction. Therefore, the measured power of the AlGaN nanowire photonic crystal lasers is significantly smaller than the actual output power. The slope efficiency η_s of AlGaN nanowire photonic crystal lasers derived from the L-I curve above threshold current is ~1.37 × 10⁻³ W/A, equivalent to an external differential quantum efficiency η_d of ~0.04%. Compared to other electrically pumped photonic crystal lasers operating from violet to infrared wavelength range, the η_s of defect-free AlGaN nanowire photonic crystals reported in this study is at least one order of magnitude higher [52–54]. It is expected that, with further improved design and fabrication process, the output power and slope efficiency can be significantly improved.

 

Fig. 4 (a) Room-temperature electroluminescence spectra of GaN/Al_xGa_1-xN photonic crystal laser below (red) and above (blue) threshold current density. The spectra are vertically shifted for display purpose. (b) Estimated output power of the photonic crystal laser as a function of current density. Inset: Variation of output power vs. current density in log scale. (c) EL spectral linewidth vs. current density. (d) Variation of lasing peak position vs. current density.

Download Full Size | PDF

4. Summary

In summary, we have demonstrated a low threshold electrically injected semiconductor UV laser by utilizing defect-free AlGaN nanowire photonic crystals. The photonic crystal lasers were designed to achieve surface-avoiding modes of a topological resonator. The photonic crystal lasers exhibit a peak emission at 369.5 nm with a low threshold current density. The spectral linewidth is as narrow as ~0.2 nm. The device performance can be further improved by optimizing the design of nanowire photonic crystal optical cavity to achieve redominantly surface or edge emission. Such nanowire photonic crystals provide a unique approach for achieving semiconductor laser diodes in the UV-B and UV-C bands. This work also bridges the gap between conventional single nanowire devices and large-area laser diodes that are often required for practical applications.