1. Introduction

Energy-efficient lighting based on semiconductor light-emitting diodes (LEDs) has been extensively used in a wide variety of applications because such devices provide both improved efficiency and pollution free light generations [1]. In particular, tuning the indium content of InGaN/GaN multiple quantum wells (MQWs) used as the active mediums of the LEDs allows for the corresponding output light to be easily tuned to emit colors ranging from ultraviolet to green [2]. In addition to being used in general lighting, another potential application for these visible LEDs is to establish an indoor optical wireless link, in which the modulated light signal is directly generated from the LEDs [3]. Unlike radio-frequency (RF) links, LED-based visible light communications offer several advantages including low system cost, license-free operation, immunity to electromagnetic interference, network security, and high and unregulated bandwidth. O’Brien et al. have reported constructing a line-of-sight optical link using a flip-chip packaged 968-nm resonant cavity light-emitting diode (RCLED) array along with an imaging diversity receiver, allowing for the successful 100 Mbit/s (Manchester) transmission of coded data [4]. Tsonev et al. reported that optical wireless link consists of GaN μLEDs together with orthogonal frequency division multiplexing (OFDM) modulation have the potential to transmit at a data rate in excess of 3 Gb/s [5]. Wang et al. reported that the transmission data rate in LED-based wavelength division multiplexing communication systems can be boosted to 4.5 Gb/s with the introduction of carrier-less amplitude, phase modulation and recursive least square-based adaptive equalization [6]. Recently, a unified performance analysis of free-space optical transmission systems over the Málaga atmospheric turbulence channel is conducted to account for pointing errors under the indirect modulation/direct detection or heterodyne detection techniques [7]. With this approach, several performance metrics such as the outage probability, the ergodic capacity, and the average error rate for binary and M-ary modulation schemes can be given numerically. This will help to build a free-space optical link suitable for long-distance data transmission over various ambient conditions.

In general illumination applications, InGaN LEDs exhibit an efficiency “droop” at high currents, obstructing the route to use LED luminaires instead of conventional fluorescent lamps [2,8]. The physical origin of this phenomenon is still unconfirmed but it is widely attributed to the presence of nonuniform carrier distribution within the InGaN MQWs. In InGaN LEDs, injected carriers tend to recombine radiatively at the quantum wells closest to the p-GaN because the energetic electrons from the n-GaN can easily cross the MQW active regions while not for the holes having a large effective mass and a low mobility [9]. This will result in an increased probability of carrier leakage in LEDs operating at elevated current levels. This efficiency droop can be improved upon using advanced design strategies for the epitaxial growth of InGaN MQWs, e.g., the use of a wide well to reduce carrier density or cooling the hot electrons before their injection into the InGaN MQWs using a staircase electron injector [10,11]. In addition to stronger carrier localization in LEDs grown with asymmetric barrier layer (ABL) to help the effective recombination, LEDs operating at high currents also feature improved roll-off behavior [12]. However, the use of such LEDs as an optical transmitter in a directed line-of-sight optical link has not yet been investigated. In this work, according to the results from the dynamic response analysis of LEDs with ABL, these LEDs cannot only have enhanced 3 dB modulation bandwidth but enable data to be transmitted at a higher rate than their counterparts without it. In addition, real-time transmissions of digital TV signals over a moderate distance (~100 cm) in free space is shown to be available using white InGaN LEDs grown with ABL.

2. Experiments

The used epi-wafer of normal LEDs grown on c-plane sapphire substrate consisted of a GaN nucleation layer, a GaN buffer layer, a Si-doped GaN, a five-pair In_0.135Ga_0.865N (2 nm)/GaN (10 nm) MQW, a p-Al_0.15Ga_0.85N electron blocking layer, and the Mg-doped GaN layer. In contrast, the proposed LEDs have a modified InGaN MQW, shown in the inset of Fig. 1, in which three GaN barriers adjacent to the n-GaN have a reduced layer thickness (~4.8 nm) while the remaining structures are identical to those of the conventional LED. As evaluated from the asymmetric reciprocal space mapping (RSM) for the $(10\overline{1}5)$ reflections of the LED with ABL, shown in Fig. 1, the position of the GaN peak with respect to the InGaN/GaN MQW-related diffraction peak is located at the same Q_x axis (a similar result is presented in conventional LEDs), implying that both LEDs were pseudomorphically grown on the GaN buffer layers. Therefore, the probability of material defects forming due to strain relaxation in LEDs grown with ABL should be trivially small. This is supported by the similar etch-pit density of 8.24 × 10⁸ cm⁻² and 8.40 × 10⁸ cm⁻² found in the LEDs with and without ABL using atomic force microscopy (AFM) (not shown here). The fabrication process of the LEDs involves the formation of a 300 × 300 μm² indium tin oxide (ITO)-coated mesa, followed by the formation of n contacts on the exposed n-GaN layer and p-contacts on part of the ITO. In on-wafer measurements, the completed LEDs were driven by a Keithley Model 2400 source meter and the corresponding light output power was measured by a calibrated integrating optical sphere sensor (Newport Corp.).

 

Fig. 1 Reciprocal space mapping around the asymmetric $(10\overline{1}5)$ reflection for the LEDs with ABL. The inset shows the high-resolution transmission electron microscopy (HRTEM) image of the InGaN MQWs with ABL.

Download Full Size | PDF

3. Results and discussion

Figure 2 displays the light output power–current–voltage characteristics (L–I–V) of the LEDs with and without ABL at 25 °C. Data taken from ten LEDs is shown to consider the statistical deviation in on-wafer testing. In the experiment, a similar forward voltage of 3.1 V at 20 mA and an equivalent series resistance of 8 Ω were found for all fabricated LEDs. In addition, the mean value of the light output power at 250 and 350 mA is respectively evaluated as 5.0 and 5.7 mW, and 3.9 and 4.3 mW for the LEDs with and without ABL. It is worth noting that the output performance of the LED with ABL is superior to that of the conventional LED over the entire range of current injections. Our previous work [12] found this is due to the increase in carrier localization at the potential minima caused by phase separation or spinodal decomposition in the ABL-containing InGaN MQWs so that the output performance of the fabricated LEDs can be improved at low currents. Owing to band filling of the localized states in indium-rich areas increasing with current, the subsequent carrier injection will be released to the conduction band of the quantum wells [13]. Moreover, the use of three thin quantum barriers located near the n-GaN improves the uniformity of carrier distribution in InGaN MQWs. Both effects are considered to be responsible for the improved roll-off behavior in the L–I characteristics of proposed LEDs operating at high current levels.

 

Fig. 2 Light output power–current–voltage characteristics (L–I–V) of the LEDs without (a) and with (b) ABL at 25 °C. Data taken from ten LEDs is shown to consider the statistical deviation in on-wafer testing.

Download Full Size | PDF

On the other hand, small- and large-signal measurements were performed to clarify their capability of the developed LEDs to serve as optical transmitters in directed line-of-sight optical links. The detailed experimental setup for small-signal measurements is similar to Ref [14]. including the use of a Rohde & Schwarz ZVL network analyzer, a bias tee-driven LED with Transistor Outline (TO)-can package, a light coupling lens (two plano-convex lenses), and an optical receiver (Pacific AD500-9-400M-TO5). As for the large-signal response analysis of a directly modulated LED, the bias current and nonreturn-to-zero (NRZ) pseudorandom bit sequence (PRBS) generated from an Agilent 81160A pulse function arbitrary noise generator were combined in a bias tee and fed to the TO-packaged LEDs. The collimated light propagation through a given distance in free space is collected by the optical receiver using the light coupling lens and then analyzed by a wide-bandwidth sampling oscilloscope (Agilent 86100A). Figure 3 shows the frequency response of the LEDs with and without ABL at 200 mA. As a result of poor heat dissipation from the TO-packaged LED to ambient air, significant thermal heating occurred at higher currents, causing the limited 3 dB modulation bandwidth of 79.2 and 71.7 MHz at 200 mA, respectively, for LEDs with and without ABL. Taking the parasitic elements such as the depletion capacitance (C_P) into account, the RC_P-limited bandwidth is calculated to be ~199 MHz for both LEDs provided the zero-bias capacitance of ~100 pF (given from the capacitance–voltage analysis) and the series resistance of 8 Ω are used. Because the measured bandwidth is significantly lower than the value given from the parasitic RC_P elements and the LED under testing was biased at a high injection current (under such situation C_P has been partly filled with injected carriers [15]), the modulation bandwidth of the fabricated LEDs should be dominated by the carrier recombination time [16]. The differential carrier lifetime (τ_c) of the LEDs can be described by [17]

 

Fig. 3 Frequency response of the LEDs with and without ABL at 200 mA. The inset shows the corresponding eye diagram for data transmission over a distance of 100 cm in free space for the ABL-containing LED operated at a data rate of 280 Mbit/s (I_Bias = 170 mA, PRBS = 2⁷-1 and V_PP = 5 V).

Download Full Size | PDF

The blue LEDs packaged with a phosphor layer were also used as transmitters with a white light emission through the combination of two complementary colors (i.e., blue and yellow). Figure 4 show the eye diagrams measured at 180 Mbit/s with I_Bias = 170 mA, V_PP = 5 V and PRBS = 2⁷-1, respectively, for phosphorescent white InGaN LEDs grown with and without ABL. The distance between the transmitter and the receiver is set at 100 cm. Experimentally, no well-resolved eye patterns could be obtained from these white LEDs operating at the same modulation parameters. However, as shown in Figs. 4(a) and 4(b), placing an additional optical filter (Semrock FF01-450/70) in front of the receiver significantly improves the corresponding eye-pattern quality. This result indicates that the slow phosphorescent components emitted from the phosphor layer restrict the available transmission data rate of free-space optical communication with phosphor-converted white LEDs. Otherwise, the rise time, fall time and peak-to-peak jitter are respectively found to be 2.48 ns, 2.0 ns and 1.33 ns for transmitters made from white LEDs grown with ABL, and 3.28 ns, 3.76 ns and 1.92 ns for those without ABL. It is obvious that the improvement of the eye-pattern quality can be achieved as white light was made from the LEDs having a higher modulation bandwidth. Taking propagation loss into account, the use of the LEDs with larger emission intensity is also essential to increasing the transmission distance in visible light communications.

 

Fig. 4 Eye diagrams measured at 180 Mbit/s with I_Bias = 170 mA, V_PP = 5 V and PRBS = 2⁷-1 for white InGaN LEDs grown without (a) and with (b) ABL. The distance between the transmitter and the receiver is set at 100 cm. In addition, an optical filter is placed in front of the receiver to remove the slow phosphorescent components from the phosphor layer.

Download Full Size | PDF

Given that a universal platform suitable for ubiquitous wireless communications can be realized by radio over free-space optics (RoFSO) technology [18], the potential for transmitting RF signals (TV signals) in a directed line-of-sight optical link with white InGaN LEDs is further investigated experimentally. Figure 5(a) shows the proposed design for optical wireless links for transmitting digital video broadcasting-terrestrial (DVB-T) signals using white LEDs as transmitters. In the transmitting terminal, the broadcasting RF signals from the antenna first used a commercial Thomson DVB-T receiver to attain the demodulated TV signals. To simplify the associated circuitry, only video components from the DVB-T signals were used. Then, a 8-bit analog-to-digital (A/D) video converter chip (Texas Instruments TLC5510) along with a field programmable gate array (FPGA) implementation of the video encoder (Altera CPLD EPM570T144C5N) [19] were used to generate a serial video signal, which is fed to the phosphor-converted LEDs grown with ABL to generate the modulated white light. In the receiving end, the collected light is converted back to TV signals and then displayed in the monitor through the use of an optical detector (Pacific AD500-9-400M-TO5), a video decoder (Altera CPLD EPM570T144C5N) and a D/A converter (Texas Instruments TLC5602C) in sequence. As shown in Fig. 5(a), the distance between the transmitter and the receiver is set at 100 cm. In comparison with the blocking link (i.e., the display monitor shows a blank screen), real-time TV signals (video only) with reasonable quality are observed in the monitor, shown in Fig. 5(b), as the optical link is established between the transmitter and the receiver. The channel number, frequency, bandwidth, and other pertinent information for Formosa Television News Channel can also be found on the screen. In addition, the measured transmission rate at the receiving end is around 150 Mbit/s. Instead of using an analog pre-equalization technique to increase the LED bandwidth and to realize a 28.419 Mbit/s OFDM-based optical wireless link with analog front end [20], our work indicates that real-time optical wireless transmission of high-speed digital signals could be accomplished using a high-speed white LED transmitter even without an exceptional modulation scheme or equalization technique in optical links.

 

Fig. 5 Proposed design for optical wireless links for transmitting the digital video broadcasting-terrestrial (DVB-T) signals using white LEDs as transmitters. The distance between the transmitter and the receiver is set at 100 cm. In comparison with the blocking link (a), real-time TV signals (video only) with reasonable quality are observed in the monitor as the optical link is established between the transmitter and the receiver (b). The channel number, frequency, bandwidth, and other pertinent information for Formosa Television News Channel can also be found on the screen.

Download Full Size | PDF

7. Conclusion

In summary, we have demonstrated that an optical transmitter made of InGaN LEDs grown with ABL is suitable for use in visible light communications. For the LEDs with ABL, HRTEM, RSM and AFM revealed that the modified InGaN MQWs with three thin barriers near the n-GaN have a reasonable crystalline quality and no strain relaxation-induced defect formation in pseudomorphically grown epi-structures. In addition, to improve the light output power by 28.2% and 32.6% at 250 and 350 mA, a thermal heating-limited modulation bandwidth found in LEDs with ABL is increased to 10.5%. Compared with conventional LEDs, the improved device performance may be due to the presence of stronger carrier localization and more uniform carrier distribution in the ABL-containing InGaN MQWs so as to increase the spontaneous emission rate. In a directed line-of-sight optical link with white LEDs, the decrease in blue light intensity due to the removal of the slow phosphorescent components generated from the phosphor layer resulted in a maximum transmission rate of 280 Mbit/s from the blue LEDs, degrading to 180 Mbit/s. Finally, a 150 Mbit/s visible light communication system based upon an on-off keying modulation scheme and capable of transmitting real-time TV signals over a moderate distance (~100 cm) in free space is realized through white InGaN LEDs grown with ABL.