Group-III-nitride laser diode (LD)-based solid-state lighting device has been demonstrated to be droop-free compared to light-emitting diodes (LEDs), and highly energy-efficient compared to that of the traditional incandescent and fluorescent white light systems. The YAG:Ce3+ phosphor used in LD-based solid-state lighting, however, is associated with rapid degradation issue. An alternate phosphor/LD architecture, which is capable of sustaining high temperature, high power density, while still intensity- and bandwidth-tunable for high color-quality remained unexplored. In this paper, we present for the first time, the proof-of-concept of the generation of high-quality white light using an InGaN-based orange nanowires (NWs) LED grown on silicon, in conjunction with a blue LD, and in place of the compound-phosphor. By changing the relative intensities of the ultrabroad linewidth orange and narrow-linewidth blue components, our LED/LD device architecture achieved correlated color temperature (CCT) ranging from 3000 K to above 6000K with color rendering index (CRI) values reaching 83.1, a value unsurpassed by the YAG-phosphor/blue-LD counterpart. The white-light wireless communications was implemented using the blue LD through on-off keying (OOK) modulation to obtain a data rate of 1.06 Gbps. We therefore achieved the best of both worlds when orange-emitting NWs LED are utilized as “active-phosphor”, while blue LD is used for both color mixing and optical wireless communications.
© 2016 Optical Society of America
Visible lighting and image projection  based on solid state devices have recently attracted considerable attention because of their small foot-print, long lifetime, stable light-output, low power consumption and heat generation, mercury-free, and high-speed modulation capability, which uncovers new areas such as optical wireless communications (OWC) [1–5]. This is because radio frequency (RF), as the primary source of communications, has been experiencing bandwidth-crunch due to the recent unprecedented increase in demand for higher data rates transmissions. Research into alternatives frequency bands , especially using the unregulated optical frequency in the visible region is most promising in off-loading the overburdened RF spectrum. Switching to visible frequencies will also reduce reliance on expensive hardware while providing diverse option of high speed and large bandwidths .
The research community has been extensively focusing on OWC mainly for indoor applications. To achieve white light, most conventional techniques utilize blue LED to excite yellow phosphor with highest reported luminous efficacy of 265 lm/watt , or combining red, green and blue (RGB) LEDs to achieve a broad white light spectrum [9, 10]. Phosphor based technique suffers from limited controllability of the yellow phosphor component in producing the desired white light characteristics. Also, a longer carrier relaxation lifetime in YAG:Ce3+ phosphor inhibits GHz communications, unless spectral-efficient modulation technique is utilized. For example, advanced communications schemes such as wavelength division multiplexing (WDM) and multiple-input and multiple-output (MIMO) have been adopted with the RGB LEDs triplet setup to achieve date throughput beyond 3 Gbps [10, 11].
Taking the technology further will require an optical device with much higher efficiency and larger bandwidth. This gap can be fulfilled by adopting laser diodes (LDs) which exhibit efficient electrical to optical efficiency, narrower linewidth compared to LED and support considerably higher parallel data channels with significantly lower interference. In this respect, D. Tsonev et.al theoretically argued that beyond-100-Gbps data rate can be achieved with optimized orthogonal frequency-division multiplexing (OFDM) encoding technique . By mixing three primary colors (red, green, and blue, i.e. RGB), one can produce white light with varying color temperatures . However, RGB triplets suffer from an inherent drawback of narrow linewidth which does not fill the visible spectrum, resulting in the poor rendering of colors of the illuminated object. Tsao et.al utilized the RYGB configuration with very promising results but the yellow optical source utilized was based on sum frequency generation which significantly reduces the cost effectiveness of the system .
It is apparent that a new white-lamp architecture for simultaneous lighting and communications should comprise coherent, small linewidth LD-spectrum, and the broad linewidth LED-spectrum. Although, this LED/LD combination is an attractive solution for high efficiency white lighting and high bandwidth visible light communications (VLC), the development is impeded by the lack of high-quality material and reduced efficiency in the “green gap” [14, 15]. Also, efficiency droop, a decrease in device efficiency with increase in operating bias current, has been a bottleneck in planar nitride devices . LEDs emitting in green-yellow regime have been demonstrated but the inherent problems of smaller linewidth (< 60 nm), efficiency droop, and lower internal quantum efficiency of the quantum well based planar structures, limit their deployment as an efficient yellow-orange-red wavelength source for white light generation [17–21].
To fill the current technology and research gap, we utilize nanowires (NWs) based device, which has reduced defect density, improved light-extraction with a larger surface to volume ratio, and increased internal quantum efficiency due to a reduced lattice-strain, thus considerably mitigating efficiency droop . This paper focuses on utilizing an orange emitting LED based on a new platform of InGaN/GaN NWs grown on titanium-coated silicon substrate, as well as a narrow linewidth laser for simultaneous color mixing for solid state lighting (SSL) and data communications. We demonstrated the feasibility of achieving white light with CRI beyond 80, surpassing that of the phosphor/blue-LD combination with color rendering index (CRI) of less than 70, and large tunability of correlated color temperature (CCT), using the mixed NWs-LED/LD device combination. By utilizing the blue LD in conjunction with OOK modulation technique, we achieved 1.06 Gbps data rate hence bringing the design under the umbrella of OWC.
2. Experimental description
The orange NWs LED was grown using GEN 930 plasma-assisted molecular beam epitaxy (PA-MBE) system. Native oxide was removed from Si using HF-H2O solution followed by deposition of 100 nm of titanium (Ti). The silicon (Si) doped gallium nitride (GaN) was first nucleated at a lower substrate temperature of 500 °C followed by growth at a higher temperature of 600 °C for crystal quality improvement. Nitrogen (N2) flow was maintained at 1 sccm with RF power set to 350 W. Active region was grown using seven stacks of GaN quantum barrier (8 nm) and InGaN quantum disk (4 nm). The quantum disks were grown at a lower temperature of 515 °C followed by capping of 2 nm of GaN, to avoid dissociation when ramping up for quantum barrier growth. Indium (In) beam equivalent pressure (BEP) was set at 5×10−8 Torr while for Gallium (Ga) it was varied between 3×10−8 – 6×10−8 Torr. A 60 nm thick magnesium-doped GaN was then grown. Titanium nitride (TiN) has been seen to form at NWs base at the nucleation site, as confirmed by TEM and XRD, which considerably improves current injection. TiN in conjunction with underlying Ti layer reflects longer wavelength photons which also considerably increases light extraction efficiency of the device .
The orange NWs LED was fabricated using standard UV contact lithography process. The NWs were first planarized with parylene, etched back to reveal the p-GaN contact layers, and then deposited with Ni (5nm) / Au (5 nm), which forms an ohmic contact with p-GaN upon annealing. The LED mesa was then etched and the Ti buffer layer supporting the NWs was revealed as the n-contact metal. Then 500 nm of Au pad was sputtered to complete the top contact pads.
The designed experimental setup is shown in Fig. 1 where a blue LD was used in conjunction with orange NWs LED to generate white light by passing the laser beam through a commercially available diffuser. The blue LD (LP450-SF-15) from Thorlabs exhibits a nominal spectral linewidth of around 1 nm centered at 447 nm. A commercial blue LED with peak emission at ~460 nm was used to compare the white light characteristics. Plano-convex lens (LA1951-A) was used in front of the LD to collimate the laser beam. To obtain white light, the beam was passed through the diffuser (ED1-C50-MD) and mixed with orange NWs LED. The mixed white light is then measured using the GL Opti-probe attachment, fiber-coupled into the GL Spectis 5.0 Touch spectrometer.
For modulating blue LD, DC biasing current (Ibias) and peak-to-peak modulation current (Ipp) are adjusted to optimize the signal-to-noise ratio (SNR) of the transmission. The 1.06 Gbps on-off keying non-return-to-zero (NRZ-OOK) laser modulation is realized with a pseudorandom binary sequence (PRBS) generator (Agilent Technologies’s J-BERT) having a 210-1 long words. The 210-1 long PRBS pattern is consistent with data pattern length found in applications such as Gigabit Ethernet, and SATA 1 that use 8b/10b, as well as other related encodings. The NRZ-OOK data was electrically pre-amplified with an ultra broadband amplifier (Picosecond Pulse Labs, 5868) of 28.5 dB gain to increase the RF signal power and improve the extinction ratio (ER). The transmitted NRZ-OOK optical signal is sent to the APD receiver for optical-to-electrical conversion and error detection measurements using an Agilent Technologies’s Digital Communication Analyzer (DCA-J 86100C).
3. Results and discussion
The transmission electron microscopy (TEM) image of the NWs sample in Fig. 2(a) clearly show the well-defined InGaN quantum disks (white horizontal lines) embedded in NWs, separated by GaN quantum barriers. The tapered NWs nucleate with a small base and the lateral size gradually increases as growth progresses. This is attributed to the decrease in temperature with NWs height thus favoring lateral growth. A top view scanning electron microscope (SEM) image of the InGaN NWs (see Fig. 2(b)) shows the NWs mean height of 800 nm and lateral size of 175 nm, respectively. Figure 2(c) shows that the NWs were mostly vertically aligned and disjointed with an areal density of 6.8 × 109 cm−2.
The optical properties the NWs were investigated using temperature dependent photoluminescence (PL) as shown in Fig. 3(a). A broad PL linewidth of 184 nm was obtained at room temperature, which is attributed to the presence of compositional inhomogeneity and alloy disordering among the nanowires and across individual quantum disks [23, 24]. Insets show weak S-shape evolution of the peak wavelengths versus temperature for the two deconvoluted-peaks, which are in the orange-red and far-red color regime, respectively. The ultrabroad linewidth is favorable for our application herewith for generating high CRI white light as evident in the room temperature electroluminescence measurement of an orange LED in Fig. 3(c). With further band-filling effect in the electronic transitions of the clusters of NWs, the emission peak wavelength and linewidth were found to be 614 nm (orange color) and above 120 nm, respectively. In particular, the peak exhibit weak shift with the increase in current injection which is usually the case for quantum well based InGaN planar devices in the presence of high polarization fields [24, 25]. This provides evidence of minimized polarization fields due to smaller crystal volume and inherent lower strain in the nanowires, which nucleate via strain relaxation . As the EL peak correlates well with that of PL, we thus achieve the required broad linewidth emission by design.
The L-I-V characteristics of the orange NWs LED was characterized at different biases is shown in Fig. 4(a). A turn-on voltage of 4.2 V was obtained which is relatively larger than its planar counterpart. This can be due to the presence of higher contact resistance at the top p-GaN layer and insertion of AlGaN carrier blocking layers. An important feature to be noted is that no saturation in power is observed up to 70 mA of current injection. Figure 4(b)-(c) shows the L-I-V characteristics of the blue LD having a threshold current of 34.0 mA, 22.2% differential efficiency with peak wavelength at 447 nm and linewidth of 1 nm, while the blue LED having a turn on voltage of ~2.6 V with peak wavelength of 459 nm and linewidth of 16 nm, respectively. Due to optical coupling and optical fiber losses, the LD shows a flat power response up to injection current of 25 mA.
For the white light experiment, the bias current for the blue LD was kept at the optimum operating condition of 39 mA. The intensity level of the ultrabroad linewidth orange LED was varied to improve white light characteristics by changing the bias current from 50 mA to 200 mA. In parallel, the intensity of blue LD was also adjusted using a variable attenuator keeping the bias at 39 mA. It was seen that the white light color temperature drastically changed with the blue light intensity. For the blue LED, the intensity was adjusted by varying the voltage. In Fig. 5(b)-(c), the diffused white light spectral characteristics, which correspond to the integrated intensity ratio between the blue light and orange light, along with the color rendering index (CRI) and the correlated color temperature (CCT) based on CIE 1931 standard were measured with a GL Opti-probe connected to GL Spectis 5.0 Touch spectrometer. When the orange LED was operated at a bias current of 140 mA, a CCT value of 4138 K and a CRI of 83.1 were obtained as shown in Fig. 5(b). As compared to the case when using blue LD to excite single-crystal YAG phosphor with the resultant linewidth of ~100 nm, a CRI of mere 58 was reported . The blue laser based white lighting using orange NWs LED is thus a better candidate for achieving a prominent CRI.
We have also systematically compared the white light characteristics, based on CRI values, of the blue LD against that of a commercially available blue LED. CRI provides a quantitative measure of the degree of a light source revealing the color of an object under consideration, when compared to a Planckian light source having the same Kelvin temperature. As shown in Fig. 5(c), considerably good white light characteristics can be achieved with both blue LD and LED. Compared to a Planckian radiator emitting around 4000 K, the LD light, having smaller linewidth, reveal the color of an object more faithfully compared to that of a blue LED with a much wider linewidth. Thus blue LD, compared to LED exhibits higher CRI value. Color temperatures were seen to drastically change with blue light intensity as shown in Fig. 5(d)-(e). In another study we introduced RG LDs component at 532 nm and 640 nm and were able to achieve CRI value above 90 but at the cost of color temperature which went below 2000 K as shown in Fig. 6(a)-(b); thus elucidating the strength of our demonstrated orange NWs LED / blue LD device architecture.
As for high speed data communications, we first measure the maximum allowable modulation bandwidth (see Fig. 7(a)). The small signal response at 50 mA DC bias current, i.e. the −3dB bandwidth of the communications system (inclusive of the blue LD, the laser driver, and the APD) is 1.02 GHz. As compared to an LED, this is two orders of magnitude enhancement in modulation bandwidth, and therefore the use of a blue LD serves well in high data rate OWC .
It is noted that prior to OWC measurement, the modulation performance in terms of BER of the blue LD encoded signals was investigated under different bias currents and peak-to-peak voltage (Fig. 7(b), and the inset, respectively) in order to find an optimum operating point. The power reaching the PD was kept constant at 20 µW by using a variable attenuator for the above optimization. An optimized operating condition was obtained when the bias current of the blue LD was set to 39 mA and the amplitude of the modulating voltage signal to 125 mV as shown in Fig. 6(b). At a lower bias current, we observed clipping of the modulated signal, which degrades the BER of the encoded OOK data stream. An overly biased operation also declines the laser throughput response, and degrades the high-frequency response which increases the transmitted BER. By using the experimental setup depicted in Fig. 1 and the optimized operating condition described above, we have achieved a transmission rate of up to 1.06 Gbps. The eye diagrams at 0.622 Gbps and 1.06 Gbps measured using an Agilent digital communication analyzer (DCA) are shown in Figs. 7(c)-(d). At 0.622 Gbps and 1.06 Gbps, with the corresponding BER of 6.4x10−4 and 1.93×10−3 below the forward error correction (FEC) criterion of 3.8×10−3, error-free operation was realized.
We experimentally demonstrated a white light device architecture based on ultrabroad linewidth orange NWs LED and <1 nm linewidth blue LD to achieve both white light generation and optical wireless communication (OWC). The PAMBE grown NWs were observed to be vertically aligned with density, diameter and length of 6.8×109 cm−2, 175 nm and 800 nm, respectively. The emitted spectrum under 120 mA bias current had a peak wavelength of 614 nm with invariant shift as the bias current increases. A high data transmission rate of 1.06 Gbps was achieved without the need of an optical blue-filter based on NRZ-OOK modulation scheme. At 1.06 Gbps transmission, open eye diagrams and FEC compliant BER of 1.93×10−3 were successfully obtained. In addition, colorimetric properties of the white light source were characterized. At 140 mA injection current, we achieved white light with a CCT of 4138 K and a CRI of 83.1, a value unmatched by the blue LD – phosphor counterpart. Our demonstrated ultrabroad linewidth orange NWs LED in conjunction with narrow linewidth blue LD based white light source will be applicable for next-generation high-efficiency indoor illumination and optical wireless communications systems.
King Abdulaziz City for Science and Technology (KACST) (KACST TIC R2-FP-008); King Abdullah University of Science and Technology (KAUST) (BAS/1/1614-01-01).
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