Monolithic single gallium nitride nanowire laser on silicon

Semiconductor lasers have been a key technology for diverse applications that require coherent light. Nanoscale lasers, in particular, will enable the integration of optoelectronic components into electronic circuits for optical interconnects and medical diagnostics, to give just two examples. For hybrid integration, silicon (Si) appears to be a convenient substrate material on which to grow silica waveguides for integrated optics. However, Si is an indirect bandgap material that inherently does not emit light at useful wavelengths. Consequently, development of nanoscale lasers monolithically on Si is an area of great interest.

One viable technology is single nanowires, which have been investigated for use as Fabry-Pérot cavities and waveguides.^{1, 2} However, poor reflectivity (<0.4)³ between semiconductors and air requires the nanowire to be very long (∼40μm) to achieve stimulated emission,^{3, 4} which is not conducive to realizing nanoscale lasers. Other efforts have focused on combining semiconductor nanowires with lithographically defined cavities, such as a 1D photonic crystal microcavity,⁵ a microstadium cavity,⁶ and a silica microfiber knot cavity.⁷ While the modal gain of a nanowire Fabry-Pérot cavity depends strongly on the length of the nanowire,⁴ an external high-quality-factor microcavity could provide strong feedback regardless of the wire dimension. However, all these approaches expose the limitation of transferring the nanowires to a Si substrate before fabricating the cavity. We have demonstrated a monolithic laser by creating a 2D photonic crystal microcavity centered on a gallium nitride (GaN) nanowire directly grown on a Si substrate.

Figure 1. (a) Cross-sectional scanning electron microscopy (SEM) image of catalyst-free gallium nitride (GaN) nanowires grown by plasma-assisted molecular beam epitaxy with a density of ∼1×10⁸cm⁻² on (111) silicon (Si). (b) High-resolution transmission electron microscopy image of GaN nanowire that exhibits no observable defect. The inset shows the diffraction pattern indicating that the nanowire is a single-crystal wurtzite structure with the c-axis along the direction of growth.

Figure 2. (a) Schematic representation of a nanowire laser consisting of a single GaN nanowire and a 2D photonic crystal microcavity. (b) An oblique-view SEM image of the fabricated device. TiO₂: Titanium dioxide.

Figure 3. (a) Photoluminescence of the laser at pump power densities of 95kW/cm²(below threshold), 143kW/cm² (near threshold), and 477kW/cm²(above threshold). Spectra are offset for clarity. The inset shows the lasing spectrum (blue circles), which is matched to the sum of two Gaussian peaks (green and red solid lines). (b) Calculated resonant modes with the nanowire in the center (black) and off-center by 60nm (blue). Mode profiles of each case are shown on the right. a.u.: Arbitrary units. Γ, M, K: High symmetry points in the first Brillouin zone (i.e., the reciprocal space unit cell).

We grew GaN nanowires on Si in the absence of a foreign metal catalyst in a plasma-assisted molecular beam epitaxy system.⁸ By carefully controlling growth conditions, we were able to achieve an extremely low density of ∼10⁸/cm². Structural studies show that the nanowires are relatively defect-free because of a large surface-to-volume ratio and have a single-crystal wurtzite structure with the c-axis parallel to the growth direction (see Figure 1). The device heterostructure—see Figure 2(a)—consists of a 2D photonic crystal pattern in a titanium dioxide (TiO₂) layer and a single GaN nanowire at the center of an H2 point defect. To minimize optical loss from the TiO₂ layer to the Si substrate, we place a spin-on-glass layer beneath the TiO₂ layer as a low-refractive-index material. The fabricated device shows a single nanowire located at the center of the H2 defect of the photonic crystal microcavity: see Figure 2(b).

We optically characterized the device at room temperature. The photoluminescence spectrum below threshold shows a broad GaN band-edge emission with a full-width at half-maximum of ∼10nm. The output emission also exhibits a fairly narrow peak (∼4.5nm) at λ=370.4nm due to the Purcell effect, which enhances spontaneous emission. As the pump power density increases, the enhanced spontaneous emission peak becomes more pronounced and finally evolves into a coherent lasing peak above threshold: see Figure 3(a). It is worth noting that a narrow lasing peak above threshold consists of two lasing transitions—see Figure 3(a), inset—indicated by the two Gaussian fitting curves with a minimum linewidth of ∼0.5nm.

To understand the origin of the dual lasing peaks, we carried out theoretical studies of the resonant modes. When the nanowire is placed precisely in the center, there are two degenerate modes due to the symmetry of the H2 defect.⁹ However, imperfect alignment of the microcavity induces a small off-positioning of the nanowire within the defect, which breaks the symmetry, resulting in two individual modes (blue lines). Consequently, the dual lasing peaks result from a breaking of the degeneracy of the dominant H2 cavity modes because of the imperfection of fabrication.

In summary, we have demonstrated a Si-based monolithic single GaN nanowire laser with a 2D photonic crystal microcavity. Catalyst-free low-density nanowires grown on Si substrates by plasma-assisted molecular beam epitaxy show the superior material quality compared to bulk material. We characterized our device by a lasing transition at λ=371.3nm with a linewidth of 0.5nm. The lasing peak could be decomposed to two individual lasing transitions, which result from off-center positioning of the nanowire in the H2 defect microcavity due to misalignment. As a next step, we plan to further explore this technology as a viable light source for Si photonics, for example, electrically injected nanowire lasers and integration with silica waveguides.

This work was supported by the Air Force Office of Scientific Research and the Defense Advanced Research Projects Agency.