1. Introduction

InGaN-based violet-blue light-emitting diodes (LEDs) has been widely used as the fundamental component for solid-state lighting (SSL) due to its advantages such as high efficiency, long lifetime, reduced heat generation, and fast turn-on [1–4]. In addition to the illumination application, the LEDs were recently demonstrated as transmitters in visible-light communication (VLC) system, advantages of which include license-free spectrum range, high security and free-from electromagnetic interference [5–7]. The modulation bandwidth of conventional III-nitride LEDs, however, is limited to 10 ~100 MHz owing to a relative large RC delay and a relative long carrier lifetime, which is originating from the spontaneous emission (SE) process [8]. Recently, micrometer- sized LED pixels were demonstrated to achieve a high modulation bandwidth of 225 MHz [9] and 462 MHz [6], albeit with a relatively low optical power of 1 ~2 mW. Although GaN-based laser diode (LD) has shown a modulation bandwidth of ~2.6 GHz [10] and a data rate of ~2 Gbps for white light communication [11], further development is required to address the limited etendue, speckle noise, and eye-safety related concerns [12, 13]. The superluminescent diode (SLD) has lately been studied as a high-brightness, speckle-free light source for SSL, combining the advantages of both LEDs and LDs [14–17]. However, there is a limited investigation on the high-speed operation of SLDs for VLC application. As white light with high color rendering index (CRI) can be achieved using violet LEDs pumped phosphors [2], the investigation into violet-emitting SLD constitute an important research topic. In this letter, we demonstrated a high-speed, absorber-free InGaN-based SLD on semipolar $\left(20\overline{2}\overline{1}\right)$ GaN substrate, emitting at 405 nm. The SLD exhibits > 800 MHz modulation bandwidth with an optical power of > 20 mW, benefiting from the small RC delay, the short lifetime associated with the amplified spontaneous emission (ASE) process, and the reduced polarization field in semipolar quantum-wells (QWs) [18]. A data rate of 1.3 Gbps was achieved for SLD transmitter with bit-error rate (BER) of 2.9 × 10⁻³. Our work suggests that SLD is promising for SSL-VLC dual-functionalities light source.

2. Experiment

The SLD was grown on a semipolar $\left(20\overline{2}\overline{1}\right)$ bulk GaN substrate using metal-organic chemical vapor deposition (MOCVD). The crystal orientation is chosen as the LEDs grown on semipolar GaN substrate was reported to have better performance [19–22]. It consists of 4 periods of In_0.1Ga_0.9N/GaN multi-quantum-wells (MQWs) sandwiched between 60-mn InGaN separate confinement heterostructure (SCH) waveguiding layers, low doped (~10¹⁸ cm⁻³) GaN cladding layers, highly doped GaN contact layers [Fig. 1(a)]. The In composition in the graded InGaN SCH layer is ~0.025. An Al_0.18Ga_0.82N electron blocking layer (EBL) was also included in the epitaxial structure. The 4-µm ridge waveguide SLD has 45ᵒ tilted facet on one-side to suppress feedback oscillation, without the need of using an integrated absorber [Fig. 1(b)]. There is no anti-reflection (AR) coating on the facets. The Pd/Au and Ti/Al/Ni/Au stacks are deposited as the p- and n-contacts, respectively, for ground-signal (GS) probing. The ridge waveguide structure and device mesa were defined using UV photolithography and plasma etching. The self-aligned SiO₂ layer was sputtered for passivation of ridge sidewalls.

 

Fig. 1 (a) Layer structure, and (b) 3D illustration of the 405-nm emitting superluminescent diode on semipolar GaN substrate with tilted facet configuration. The thickness for P⁺-GaN, p-GaN, P-InGaN SCH, AlGaN EBL, N-InGaN SCH, N-GaN layer are 10 nm, 600 nm, 60 nm, 16 nm, 60 nm, 350 nm, respectively. The length of the device is 590 µm. The GS contact pitch size is 50 µm. The device has InGaN/GaN multi-quantum-wells (MQWs) as the active region.

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The fabricated SLD was tested using a radio frequency (RF) prober with a Picoprobe Model-10 microwave probe. For emission spectra measurement, the device was driven using Keithley 2400 source meter and the spectra were collected using Ocean Optics HR4000 spectrometer. The optical power–current (L-I) measurement was performed using Keithley 2520 diode-laser test system with calibrated integrating sphere from Labsphere. The current–voltage (I-V) and capacitance–voltage (C-V) measurements were carried out using Keithley 4200 semiconductor characterization system with 4225-RPM remote amplifier/switch module. All the measurements were performed under continuous wave (CW) operation. The frequency response measurement was carried out on an RF prober and the setup involves an Agilent E8361C PNA network analyzer, a Picosecond 5543 bias tee, and a Menlo Systems APD 210 Si avalanche photodetector. The system was calibrated using Agilent 85093-60010 RF electronic calibration module (E-cal). The same setup was used for on-off keying (OOK) data transmission, involving an Agilent N4903B J-BERT and an Agilent DCA-86100C digital communication analyzer.

3. Results and discussion

Figure 2 presents the spectral characteristics of the SLD and its comparison with that of a LED and an LD, to confirm the appearance of superluminescence. The three kinds of devices were based on the same epitaxial structure and were fabricated simultaneously. The electroluminescence (EL) emission spectra with injection current from 50 mA to 400 mA at room temperature, collected from the tilted facet, is shown in Fig. 2(a). At 400 mA, the emission spectrum from the SLD is compared with that of a LED and an LD, as shown in Fig. 2(b). The full-width at half-maximum (FWHM), and the peak wavelength of SLD as a function of injecting current are summarized in Fig. 2(c). The SLD has a peak wavelength of ~405 nm. A slight red shift is observed with increasing injection current, likely due to junction heating induced thermal expansion, which is similar to those observed in GaN-based LEDs and LDs [23]. The SLD has a peak FWHM of ~16 nm at 50 mA and 100 mA injection current, which is comparable to that of the spontaneous emission from conventional violet LEDs. With increasing injection current of >100 mA, the peak FWHM of the device reduces from ~16 nm (at 100 mA) to ~9 nm (at 400 mA), indicating the onset of amplified spontaneous emission at ~100 mA. For the LED, SLD and LD with a similar epitaxial design operating, the emissions at 400 mA exhibit an FWHM of 16.3 nm, 9 nm, and 2 nm, respectively. This further confirms the achievement of ASE mode in our 405-nm SLD.

 

Fig. 2 Plot of: (a) electroluminescence (EL) spectra of the SLD under current injection of 50 mA - 400 mA; (b) Comparison of EL spectra from an LD, SLD and LED at 400 mA; (c) FWHM and peak wavelength of the SLD as a function of injection current at room temperature.

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The optical power versus current (L-I) relation of the SLD is shown in Fig. 3(a). The photodetector was placed either at the edge, or on top of the SLD device to measure the coupled ASE-SE components, or SE component alone, respectively. As expected, the superluminescence is observed at ~100 mA and beyond, which is in consistent with the spectral characteristics discussed previously. The SLD has an optical power of 20.5 mW at 400 mA. Figure 3(b) shows the measured current versus voltage (I-V) in linear and log scale (inset). To derive the series resistance of the SLD in superluminescence mode, linear fitting of the measured I-V data was performed between 150 mA and 400 mA and the slope of the fitted curve can then be calculated. Our device exhibits a turn-on voltage of ~3 V, a series resistance of ~5.9 Ω. Figure 3(c) shows the capacitance versus voltage (C-V) characteristics of the SLD and Fig. 3(d) illustrates the 1/C² versus voltage relation showing a slope of −4.2 × 10¹⁸. The C-V measurement was carried out at 1 MHz. The measured device shows a capacitance of ~35 pF at −4V. As a result, the RC time-constant for the SLD is ~0.2 ns, corresponding to the RC-limited bandwidth of ~800 MHz.

 

Fig. 3 Plot of: (a) Optical power vs. injection current (L-I) from the top emission (SE) and edge emission (ASE + SE) of the SLD. The current density is labeled as well. Inset: Photo of the edge-emitting SLD operating at 300 mA. (b) Current vs. voltage (I-V) relation of the SLD. Inset: I-V curve in log scale. (c) Capacitance vs. voltage (C-V) characteristics of the SLD. (d). 1/C² vs. voltage relation with a slope of −4.2 × 10¹⁸.

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The frequency response of the SLD operating at different current is presented in Fig. 4(a). The 3-dB bandwidth is measured to be 333 MHz, 616 MHz, 784 MHz, and 807 MHz at an injection current of 100 mA, 200 mA, 300 mA, and 400 mA, respectively. There is no pre-/post-equalization processing unit used in obtaining the small signal response curve. It should also be noted that the rapid drop of the response signal beyond 1 GHz was originating from the 1 GHz bandwidth limit of the APD used in the measurement setup. The 3-dB bandwidth as a function of the injection current is plotted in Fig. 4(b). It suggests that the bandwidth of SLD is limited by the RC delay, rather than the carrier lifetime, at the driving current above 300 mA. The SLD shows a considerably high bandwidth compared to conventional LEDs, which is attributed to the rapid recombination of carriers during the ASE process.

 

Fig. 4 (a) Modulation response at different current. (b) −3 dB bandwidth vs. current of the SLD.

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Therefore, it is expected that a reduced effective carrier lifetime, τ_e, would be observed in SLDs, leading to a higher modulation bandwidth, when compared to that of the LEDs. The 3 dB optical bandwidth can be expressed using the power transfer function as [24]:

The effective carrier lifetime (τ_e) at 300 mA for the SLD demonstrated is extracted to be ~0.35 ns, and would be even smaller at a higher current. This value is shorter than the minority carrier lifetime of 0.6 ~6 ns in micro-LED pixels [6, 25].

To demonstrate the data transmission applications, the SLD was utilized as a transmitter for optical communication using on-off keying modulation scheme. With the SLD operating at 400 mA, a BER of 4 × 10⁻⁴, 8.4 × 10⁻⁴, and 2.1 × 10⁻³ was measured at data rates of 622 Mbps, 1.06 Gbps, and 1.3 Gbps, respectively (Fig. 5). Clear open-eye is achieved at data rates up to 1.3 Gbps with the BER below the forward-error correction (FEC) limit of 3.8 × 10⁻³, thus demonstrating the significant advantage of high-speed SLD for VLC applications.

 

Fig. 5 Plot of bit-error rate (BER) vs. data rate for on-off keying modulation of SLD. The forward error correction (FEC) limit BER of 3.8 × 10⁻³ is indicated. Insets: the eye diagrams for data rates of 622 Mbps, 1.06 Gbps, and 1.3 Gbps.

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4. Conclusions

We demonstrated a high-speed InGaN/GaN QW based superluminescent diode emitting at 405 nm. Benefiting from the amplified spontaneous emission process induced short carrier lifetime and a low RC delay, the SLD exhibits 807 MHz 3-dB bandwidth, enabling a high data rate of 1.3 Gbps with BER of 2.9 × 10⁻³ using OOK modulation scheme. Such SLD, in standalone or array form, can be applied as the light-emitter in SSL-VLC systems.