The past decade witnessed the rapid development of III-nitride light emitters, such as light-emitting diodes (LEDs) [1,2] and laser diodes (LDs) [3,4], and their applications in general illumination [5], data storage [6], displays [7], and projectors [8]. In particular, blue InGaN/GaN quantum well (QW)-based LEDs have been the primary component for solid-state lighting (SSL) and more recently transceivers for visible light communication (VLC) [9]. However, efficiency droop [10] and relatively small modulation bandwidth ($<100\mathrm{MHz}$) in InGaN/GaN LEDs limit the performance of phosphor-coated-LED-based SSL-VLC systems. The limitation in bandwidth is primarily due to the carrier relaxation time of spontaneous photons in LEDs, and the YAG:Ce phosphor. The blue-to-yellow Stokes shift in YAG:Ce phosphor required for white light generation introduces $\sim 100\mathrm{ns}$ of carriers relaxation time. Group-III-nitride LDs do not suffer from efficiency droop due to the clamping of the carrier density at threshold [11]. This enabled the demonstration of high-power white light source, benefiting from the high wall-plug efficiency from GaN-based LDs [5]. Furthermore, the LDs exhibit much higher modulation bandwidths compared to LEDs [12,13]. Yet, the LD-based SSL-VLC system suffers from speckle noise and safety [11,14]. As a consequence, optoelectronic devices that combine the advantages of both LEDs and LDs for SSL-VLC functions are of great interest.

The superluminescent diode (SLD) is one of these devices. It works in the amplified spontaneous emission (ASE) regime, serving as a droop-free, speckle-free light emitter [15–21]. Continuous-wave (CW) blue-emitting (440–450 nm) SLDs have been reported on c-plane GaN substrates using tilted ridges and anti-reflection (AR) coating [18,19]. However, their limited optical power (40–100 mW), spectral linewidth (2.5–5 nm), and relatively large electroluminescence (EL) peak wavelength shift ($\sim 10\mathrm{nm}$) limit their effectiveness for high-brightness SSL applications. One issue on c-plane-oriented III-nitride SLDs is the separation of the electron-hole wavefunctions in the QWs, resulting from the strong piezoelectric field. This reduces material gain in c-plane InGaN/GaN QWs [22].

In this Letter, we demonstrated the first high optical power ($>250\mathrm{mW}$) blue-emitting SLD grown on semipolar $(20\overline{21})$ bulk GaN substrate. The device, which combines an integrated passive absorber with a gain section, shows large spectral linewidth ($>6\mathrm{nm}$ at $>100\mathrm{mW}$) and small EL peak shift (5.7 nm). Our SLD can generate high-quality white light when combined with conventional YAG:Ce phosphor, showing a color-rendering index (CRI) of 68.9 and a correlated color temperature (CCT) of 4340 K with chromaticity coordinates (0.37, 0.38) close to the Planckian locus. Furthermore, a 560-MHz modulation bandwidth was measured in our SLD; it is significantly higher than that of LEDs, suggesting that the semipolar SLD is a strong candidate for enabling SSL-VLC dual functionalities. Among other applications, the high-brightness blue-green SLDs could also have a major impact on florescence and biomedical imaging applications [16,23].

The structure, as featured in Fig. 1, was grown via metal-organic chemical vapor deposition (MOCVD), consisting of a four-period multiple quantum well (QW) active region with 3.6-nm-wide ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$ QWs. The 7.5-μm-wide ridge waveguide multi-section SLD consists of a 1000 μm long gain region and a 790 μm long integrated absorber, which was defined using UV photolithography and inductively coupled plasma (ICP) etching. The isolation trench between the two regions is 10 μm wide. Here, the etching process is carefully controlled, where the metal contact and highly doped GaN layer were removed, providing good electrical isolation while maintaining optical coupling between the gain section and absorber section. Pd/Au and Ti/Al/Ti/Au metallization layers were deposited as p- and n-electrodes, respectively.

 

Fig. 1. Schematic of InGaN/GaN QWs SLD, consisting of a gain section and an absorber section.

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In SLDs, the spontaneously generated photons are amplified by stimulated process, but the oscillations of light in the resonating cavity are suppressed to avoid lasing. Therefore, the optical gain is less than the total loss (internal loss and mirror loss) in the structure, preventing the device from reaching the threshold modal gain condition for lasing. The emitted optical power can be expressed by [17]

Here, $P_{\text{sp}}$ is the power generated by spontaneous emission and $G(J,L)$ is the optical gain. The single-pass optical gain in such SLDs is given by [24]

The optical power–current ($L-I$) and voltage–current ($V-I$) characteristics for an SLD at room temperature are shown in Fig. 2(a). The $L-I$ curves were measured with a photodetector placed at the facet, in-plane to the waveguide or above the device, and normal to the waveguide to measure the optical power of edge emission and top emission, respectively. The former measured the amplified spontaneous emission together with the spontaneous emission ($\mathrm{ASE}+\mathrm{SE}$) of the SLD while the latter measured only the uncoupled, spontaneous emission (SE). All the measurements were carried out under CW current injection conditions. The onset of superluminescence can be observed at the threshold current of 400 mA, based on the divergence of the intensities of $\mathrm{ASE}+\mathrm{SE}$ and SE. Instead of a linear increase of optical power above the threshold in LDs, the SLD exhibits a superlinear increase in the intensity, as it follows the exponential trend in Eq. (2). The corresponding forward voltage at threshold is 6.8 V. The optical powers of the SLD are 123 mW at 600 mA and 256 mW at 700 mA, respectively. Our blue-emitting SLD shows improved optical power output over those devices grown on c-plane GaN substrates [16]. Though 200-mW violet SLD has been demonstrated [15], achieving high-power blue-emitting SLD is a challenge due to the increasing indium incorporation in the InGaN-QW. The high optical power is partially attributed to the improved optical gain of the SLD, resulting from the long cavity design used here, as well as the higher material gain in InGaN/GaN QWs grown on semipolar GaN substrate, corresponding to increased $L$ and $g_0$ in Eq. (2). The latter was also confirmed by an enhanced overlap of the electron-hole wavefunctions of $\sim 0.9$ in the SLD with 3 nm ${\mathrm{In}}_{0.23}{\mathrm{Ga}}_{0.77}\mathrm{N}$ QW grown on semipolar $(20\overline{21})$ GaN substrate, while the calculated overlap for blue-emitting QW was grown on c-plane GaN is $<0.4$ [25]. Of note is that the device is directly tested in the present form for proof-of-concept demonstration. Therefore, an even higher optical power is expected in devices fabricated in the form of tilted-angle and AR-coated configuration, with proper packaging. To investigate the efficiency droop, the calculated external quantum efficiency (EQE) as a function of injected current density is plotted in Fig. 2(b). As expected, the EQE of an LED quickly reaches its peak value at a relatively low-injection current density and reduces with increasing injection, i.e., suffering from efficiency droop, thereby limiting its performance. The origin of efficiency droop in LEDs has been widely studied and several mechanisms were reported, including Auger recombination [26] and carrier leakage [27]. This is, however, not observed in SLD. As Fig. 2(b) shows, the EQE of an SLD actually increases, rather than decreases, at relatively high current densities ($\mathrm{kA}/{\mathrm{cm}}^2$). We attributed this observation to the amplified spontaneous emission nature of SLDs, in which a high photon density in SLDs can cause carriers to burn out, and the ASE-related process consumes carriers at a far higher rate than the Auger process; for instance, the amplified spontaneous emission lifetime is shorter than the Auger recombination lifetime. Besides, there is no observable speckle noise from the optical image of SLD emission at 600 mA, as shown in the inset of Fig. 2(b). This is due to the temporal incoherence characteristic of SLDs. Therefore, the droop-free and speckle-free characteristics of SLDs make them promising candidates for solid-state lighting applications.

 

Fig. 2. (a) Optical power vs. current and voltage vs. current characteristics of the SLD. Optical powers were collected from the edge of the device (amplified spontaneous emission and spontaneous emission, $\mathrm{ASE}+\mathrm{SE}$), and the top surface of the device (spontaneous emission, SE), respectively; (b) plot of external quantum efficiency (EQE) vs. injected current density of LED and SLD. Inset shows an optical image of the speckle-free emission pattern of SLD operating at 600 mA.

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The spectral characteristics of an SLD and its comparison with that of an LD are presented in Fig. 3. The spectral analysis was performed using an Ocean Optics HR 4000 high-resolution spectrometer. The EL of the SLD, collected at the edge of the device, is shown in Fig. 3(a) for different injection currents. The spectrum at low current ($<400\mathrm{mA}$) corresponds to spontaneous emission with a relatively large full-width at half-maximum (FWHM) of $>12\mathrm{nm}$. With increasing current above the threshold ($>400\mathrm{mA}$), the light amplification narrows the emission peak FWHM to 8 nm at 500 mA, 6.3 nm at 600 mA, and 2 nm at 700 mA. The current-dependent changes in FWHM and peak wavelength are summarized in Fig. 3(b). A blueshift in the peak wavelength from 452.3 nm (100 mA) to 448.2 nm (400 mA), and to 446.6 nm (700 mA), was observed with increasing drive current. This is likely due to carrier induced screening of the built-in electric field. It is worth noting that a blueshift of 5.7 nm observed in this device is considerably smaller than the $\sim 10\mathrm{nm}$ shift reported in blue c-plane-oriented SLDs [19]. This is a result of the reduced piezoelectric field in semipolar $(20\overline{21})$-oriented devices.

 

Fig. 3. Spectral characteristics of the fabricated SLD: (a) spectra collected from the edge of the device with injection current increasing from 100 mA to 700 mA; (b) change in full-width at half-maximum (FWHM) and wavelength ($\lambda$) of the emission peak as a function of injection current; (c) comparison of spectra of top emission from the SLD (SE), edge emission from the SLD ($\mathrm{ASE}+\mathrm{SE}$), and stimulated emission from a laser diode with the same active region design.

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Comparison of EL spectra of SLD top emission (SE), SLD edge emission ($\mathrm{ASE}+\mathrm{SE}$), and stimulated emission from an LD at the current of 600 mA is presented in Fig. 3(c). The ASE emission exhibits a peak FWHM of 6.3 nm, which is narrower than that of SE (29 nm) but significantly larger than that of the stimulated emission (0.5 nm, which is close to the resolution limit of the spectrometer). The large spectral linewidth above the threshold current, together with the superlinear increase of the optical power as a function of the injection current, is a strong signature of the superluminescence behavior in the SLD. Our SLD shows a large spectral linewidth at 600 mA, with EL peak FWHM of $>6\mathrm{nm}$ and optical power $>100\mathrm{mW}$, compared to 2.5-nm FWHM at 105 mW [18] and 2.7-nm FWHM at 100 mW [15] in the reported blue SLDs’ work. It therefore benefits micro-projection and SSL-VLC applications. The ASE and stimulated emission peaks are observed in the long-wavelength (low-energy) end of the spectrum compared to in the spontaneous emission spectrum as a result of weaker absorption at longer wavelengths in the passive absorber [21].

We further investigated the potential of SLD for white light generation in SSL application by using the blue InGaN SLD to excite a $\mathrm{YAG}\text{:}{\mathrm{Ce}}^{3+}$ phosphor (LMY-4453-C, Dalian Luming Group). As a proof-of-concept, a phosphor plate was directly placed in front of the SLD chip. The spectrum and the chromaticity diagram (CIE 1931) coordinates of the generated white light are shown in Figs. 4(a) and 4(b), respectively. When operating at 500 mA, the SLD generates a white light with a CRI of 68.9 and a CCT of 4340 K, after its emission passes through the phosphor layer, which is suitable for indoor illumination. Owing to its relatively broad FWHM, the SLD-based white light provides better CRI than LD-based white light sources, which have a relatively low CRI of 58 [13]. The CIE coordinates of the SLD-base white light were (0.37, 0.38), which is closer to the Planckian locus compared to the reported values (0.36, 0.43) of LD-based white light [13]. Although the CRI is relatively lower than that of the LED-based white light, it can be improved by replacing the yellow phosphor with green and red phosphor mixture, or by designing a 405-nm violet-emitting SLD exciting a combination of red, green, and blue (RGB) phosphors. For studying the speckle noise in the phosphor-converted white light using SLDs and LDs, we measured the white light profile using an Ophir Spiricon SP503U beam-profiling camera placed in front of the phosphor plate. The 3D emission profiles of SLD- and LD-based white light were shown in Figs. 4(c) and 4(d), respectively, with the spectral profiles along both X and Y directions labeled. As expected, the significant spot-to-spot intensity fluctuation is observed in LD-based white light, which is undesired for SSL applications. The SLD-based white light, however, presents a speckle-free operation owing to its temporal incoherence. Therefore, SLD-based white lighting is a viable approach for solid-state lighting application. These devices not only overcome the efficiency droop-related issues typically observed in LED-based white light, but also solved the speckle-related issues observed in LD-based white light [17].

 

Fig. 4. Plot of (a) emission spectrum and (b) chromaticity coordinates at (0.37, 0.38) generated using blue SLD with YAG:Ce phosphor. Inset in (a) shows an optical image of blue-SLD generated white light, and the corresponding CRI and CCT were indicated. The chromaticity coordinates of LD based white light (0.36, 0.43) reported by C. Lee et al. [13] were also labeled. The measured 3D beam profile of white light based on blue SLD with YAG:Ce phosphor is shown in (c) and blue LD with YAG:Ce phosphor is shown in (d). The speckle noise is not present in SLD-based white light but is significant in LD-based white light.

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Finally, to study the frequency-response properties of the SLD, we measured the small signal modulation using the setup featured in Fig. 5(a). The $-10\mathrm{dBm}$ modulation signal was generated using an Agilent E8361C PNA network analyzer, and a Keithley 2400 source meter was used as the DC power supply. The setup also involves a Picosecond 5543 bias tee and a Menlo Systems APD 210 Si avalanche photodetector with a bandwidth of 1 GHz. The measured frequency response of the SLD is shown in Fig. 5(b). The SLD exhibits a $-3\mathrm{dB}$ bandwidth of 430 MHz, 490 MHz, and 560 MHz at a DC driving currents of 400 mA, 500 mA, and 600 mA, respectively, which are considerably higher than those of the LEDs. This is attributed to the fast stimulated recombination of carriers in the amplified spontaneous emission process [24]. Noted is that the SLD with 560-MHz modulation bandwidth will enable Gbit/s data communications by employing spectral-efficient modulation techniques, such as orthogonal-frequency divisional multiplexing (OFDM) [28].

 

Fig. 5. (a) Schematic setup of small signal modulation using SLD; (b) The $-3\mathrm{dB}$ frequency response of up to 560 MHz at 600 mA current.

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In conclusion, high-power, droop-free, and speckle-free blue-emitting SLDs have been demonstrated using a semipolar $(20\overline{21})$ GaN-based waveguide structure, with a record CW optical power of 256 mW, as well as a broadband emission of 6.3 nm at 100 mW optical power. The SLD generated speckle-free, eye-safe white light with CRI of 68.9 and CCT of 4340 K using a yellow-emitting YAG:Ce phosphor, while having a modulation bandwidth of 560 MHz, thus demonstrating the feasibility for simultaneous SSL-VLC implementations.