GaN Photonic-Crystal Surface-Emitting Laser at Blue-Violet Wavelengths

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Science  25 Jan 2008:
Vol. 319, Issue 5862, pp. 445-447
DOI: 10.1126/science.1150413


Shorter-wavelength surface-emitting laser sources are important for a variety of fields, including photonics, information processing, and biology. We report on the creation of a current-driven blue-violet photonic-crystal surface-emitting laser. We have developed a fabrication method, named “air holes retained over growth,” in order to construct a two-dimensional gallium nitride (GaN)/air photonic-crystal structure. The resulting periodic structure has a photonic-crystal band-edge effect sufficient for the successful operation of a current-injection surface-emitting laser. This represents an important step in the development of laser sources that could be focused to a size much less than the wavelength and be integrated two-dimensionally at such short wavelengths.

The lasing principle of photonic-crystal surface-emitting lasers (PC-SELs) (15) is based on the band-edge effect in a two-dimensional (2D) PC, where the group velocity of light becomes zero and a 2D cavity mode is formed. The output power is coupled to the vertical direction by the PC itself, which gives rise to the surface-emitting function. PC-SELs have the following features: first, perfect, single longitudinal and lateral mode oscillation can be achieved even when the lasing area becomes very large (for example, devices >300 μm in diameter) (1, 3, 5); second, the polarization mode (3) and the beam pattern (5) can be controlled by appropriate design of the unit cell and/or lattice phase in the 2D PC. However, the shortest lasing wavelength achieved so far is 980 nm. A lasing wavelength in the blue-to-ultraviolet region would open the door to a much broader range of applications such as super–high-resolution laser sources, which can be focused to spot sizes smaller than blue-violet wavelengths by the use of doughnut beams (5, 6), and optical tweezers for ultrafine manipulation.

One issue in the creation of a gallium nitride (GaN)–based PC-SEL has been whether a 2D PC structure could be constructed with a sufficient band-edge effect. To do so requires the fabrication of a high-quality 2D GaN/air periodic structure with a lattice constant between 100 and 200 nm, close to an active layer with optical gain. Figure 1A shows the schematic structure of the target GaN-based PC-SEL. In order to obtain the required band-edge effect, the PC structure must be constructed within 300 nm of the active layer.

Fig. 1.

(A) Schematic structure of the target GaN-based PC-SEL. MQW, multi–quantum well. (B) SEM image of the triangular lattice of air holes with a period of 186 nm, a diameter of 85 nm, and a depth of 100 nm, formed in an epitaxial GaN/AlGaN layer above a GaN substrate. (C) Cross-sectional SEM image of the PC-SEL, showing a well-defined GaN/air periodic structure inside the GaN epitaxial layer.

Previously, 2D semiconductor/air PC structures in PC-SELs have generally been constructed using a wafer-fusion technique (1, 3, 5), in which one of the important processes is to remove the sacrificial substrate after the 2D semiconductor/air periodic structure has been formed. Although there are several reports on sacrificial etching techniques in GaN systems (7, 8), the application of these processes for the fabrication of PC-SELs has not yet been established. We therefore developed a new method, “air holes retained over growth” (AROG), for the construction of such a periodic structure. Our method is based on the particular characteristics of GaN growth; namely, that growth proceeds much faster in the lateral direction than it does vertically from the (0001) crystal plane (9). Initially, a triangular lattice of air holes (Fig. 1B) with a period of 186 nm, a diameter of 85 nm, and a depth of 100 nm was formed in an epitaxial GaN/aluminum gallium nitride (AlGaN) layer situated above a GaN substrate, and a silicon dioxide (SiO2) layer was then deposited at the bottom of each air hole. The GaN layer was then overgrown by means of a low-pressure metal-organic vapor-phase epitaxial method. Further fabrication details are described in fig. S1 (10). Figure 1C shows a cross-sectional scanning electron microscope (SEM) image of our fabricated device. The periodic arrangement of air holes was well defined inside the GaN epitaxial layer. The GaN overgrowth appeared to proceed laterally, capping the top of the air holes, whereas the SiO2 deposited at the bottom of the air holes had a growth-blocking action (10). The structures of the air holes were essentially uniform and had not been degraded by the overgrowth process. Transmission electron microscopy confirmed that no dislocations were generated at the overgrowth interface (fig. S2) (10). The establishment of this AROG method allowed the formation of a 2D GaN/air PC structure near the active layer in the GaN system.

To investigate whether our periodic structure possessed the characteristics of a 2D PC, we measured the photonic band structure of our device by observing the resonant coupling of light from the active layer to the bands of the 2D GaN/air periodic structure (11). For a given frequency, resonant coupling occurs when the in-plane wave vector of the light matches the wave vector of the photonic bands. In spontaneous emission spectra, this coupling can be observed as a sharp peak. The in-plane wave vector k is related to the polar angle θ (shown in Fig. 1A) by the relation k = (2π/λ0)sinθ, where λ0 is the wavelength of the light in free space. By varying the θ defined with respect to the direction normal to the plane, and shifting the in-plane direction from Γ–X to Γ–J, which are the two characteristic directions of the 2D triangular lattice (shown in Fig. 1B), the photonic bands can be mapped out around the Γ point (fig. S3) (10). Figure 2A shows the results of the measurements, for which the injection current to the device was set at 20 mA. Figure 2B shows the band structure of the device, which was calculated by the plane wave-expansion method (12). The two figures seem to be in good agreement, especially for the central mode splitting (Γ point). As the in-plane k vector was increased, four peaks (denoted by I, II, III, and IV, respectively, in the inset of Fig. 2A) could be distinguished for the normal direction (Γ point); thus, four dispersion bands, including degenerate (or closely placed) bands, were constructed. These results indicate that the 2D GaN/air periodic structure formed in the device has the characteristics of a 2D PC. Moreover, based on 2D-coupled wave theory (13, 14) for a triangular-lattice PC, the frequencies of the four peaks shown in the inset of Fig. 2A can be used to determine the 2D optical coupling coefficients κ1, κ2, and κ3, which quantify the in-plane optical coupling effects for light waves propagating along the six equivalent Γ–X directions at 60°, 120°, and 180° to each other. Coefficients of κ1 ∼ 830 cm–1, κ2 ∼ 510 cm–1, and κ3 ∼ 160 cm–1, respectively, were obtained (10). Because the 2D band-edge effect is determined by the product of the coupling coefficients and the light-propagation length, we fixed the active PC area, where the current was injected, to 100 × 100 μm2 in order to obtain sufficient band-edge effects.

Fig. 2.

(A) Measured 2D photonic band structure of the GaN/air periodic structure constructed with the AROG method. c/a, the unit of normalized frequency, where c is the velocity of light in vacuum and a is the lattice constant. The inset shows the spectrum observed at the Γ point, where four peaks (I, II, III, and IV) are resolved. (B) Band structure calculated by the plane wave-expansion method.

We then measured the current–light-output power characteristics (10) of our device under the pulsed condition (a pulse width of 500 ns and a repetition rate of 1 kHz) at room temperature (Fig. 3A). A clear threshold characteristic at 6.7 A (equivalent to a current density of 67 kA/cm2) was apparent. Figure 3B shows the emission spectra below and above the threshold. The spectra were measured by coupling the output light from the device to an optical fiber and then transferring it to a monochromator (10). Below the threshold current (6.5 A), the emission spectrum was broad, and four peaks were observed with a distribution similar to that shown in the inset of Fig. 2A. In contrast, the spectrum became sharp with a peak width of ∼0.15 nm above the threshold (6.9 and 7.4 A), which was close to the resolution limit of the measurement system. The peak wavelength was 406.5 nm (in the blue-violet region). The emission peak above the threshold was much stronger than that below the threshold, which is due to substantial improvement in the optical coupling between the output and the optical fiber (10). Finally, we measured the far-field pattern (FFP) below and above the threshold by placing a fluorescent substance at a distance of 10 cm above the device. Because of the need to insert a digital camera to record the FFP, it was not possible to align the fluorescent substance completely parallel to the device surface, which resulted in slightly asymmetric FFPs (Fig. 4). The FFP was broad below the threshold current (6.5 A) but was reduced to a small spot above the threshold (6.9 and 7.4 A). The beam divergence angle was as narrow as 1°, which indicated that large-area coherent lasing oscillation had been achieved, reflecting the characteristics of the PC-SEL.

Fig. 3.

(A) Current–light-output power characteristics of the device under the pulsed condition (a pulse width of 500 ns and a repetition rate of 1 kHz) at room temperature (RT). (B) Emission spectra above and below the current threshold. A.U., arbitrary units.

Fig. 4.

(A) FFP observed below the threshold current (6.7 A) by setting a fluorescent substance at a distance of 10 cm above the device. (B and C) FFPs observed above the threshold (6.9 and 7.4 A, respectively).

At present, the laser operates with a large threshold current; however, the performance could be substantially improved by the following methods: (i) Improvement of the crystalline quality of the multiple–quantum-well active layer. Currently, the growth condition of the active layer on the 2D GaN/air PC formed by AROG process has not yet been optimized. Modification of growth conditions such as growth pressure and III-V ratio would improve the quality of the active layer. (ii) Optimization of the distance between the active layer and the PC. Currently, the distance is ∼150 nm, and the degree of mode overlap with the air holes is limited to ∼3.5% (fig. S1) (10). If this distance were reduced to, for example, ∼60 nm, the band-edge effect could be increased, causing the threshold current to be substantially reduced. (iii) Use of a transparent electrode. Currently, the top electrode is not transparent and thus blocks much of the surface emission. If a transparent electrode (or ring-type electrode) were used, the output power and/or efficiency could be improved.

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Figs. S1 to S3


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