Over the last few years, visible light communications (VLC) based on white light-emitting diodes (LEDs) have received significant attention due to advantages such as fast reaction speed, low cost, freedom from electromagnetic interference, and high reliability [1–4]. The widespread use of a VLC system based on LEDs requires a simultaneous increase in the modulation bandwidth and emission region. Nevertheless, commercially available broad-area LEDs developed for lighting can only support modulation bandwidths within tens of MHz, which can barely meet today’s rapidly increasing requirement for high-speed data transmission [5,6]. By shrinking the LED dimensions to microscale and growing these microLEDs on a semi/nonpolar GaN substrate, the modulation bandwidth can be significantly improved [7]. Unfortunately, semi/nonpolar wafers are complicated and expensive for commercial LEDs. As an alternative approach, the selective area growth (SAG) method was employed, in which micropyramid structures containing a semipolar facet can be easily obtained [8]. These microsized pyramids showed remarkably faster radiative recombination rates than the typical c-plane LEDs [7,9], since the internal polarization fields can be partially eliminated. However, these micropyramids structures lack a large enough color emission region, which is inapplicable for the integration of lighting and communications. Additionally, this monochromatic source cannot support the employment of wavelength division multiplexing (WDM) technology, which can significantly increase the total modulation bandwidth through the combination of individual subcarriers [10–12].

To produce broadband emission, the widely adopted approach is to use blue LEDs to excite some rare-earth phosphors. Although highly used, this approach is not suitable for a high-speed VLC system because the phosphors show a large fluorescent lifetime (∼μs to ms) and low intrinsic bandwidth [13,14]. These phosphor-related problems also can be overcomefv through the SAG method, by which phosphor-free microLEDs can be obtained on specially designed two-dimensional (2D) patterned substrates. These microstructures, including the microstripes, microtrapezoidal structures, and a hexagonal annular structure, contain various crystal planes, on which different multiple quantum well (MQW) thicknesses and indium compositions are formed [15–17]. However, the employment of these multifacets, including polar, semipolar, and nonpolar facets, such as (0001), {11–22}, {10–12}, {10–1–x}, results in a variation of the carrier lifetime from several tens of ps to several hundred of ns [18,19]. It causes a nonuniform modulation bandwidth for different wavelengths, which reduces the data transfer rate. Even though a high modulation bandwidth can be guaranteed without multifacets such as pyramid structures, a large enough emission region can not be reached. In other words, there are contradictory requirements to achieve broadband emission and a high modulation bandwidth, which are both extremely important for VLC systems.

To solve this contradiction, a new strategy based on the SAG method was proposed to achieve phosphor-free microstructures. A closely connected deep-hole patterned substrate was employed in this strategy to replace the individual 2D pattern in the traditional SAG method. Lateral overgrowth appeared during the microstructure growth because of this double-hole pattern, contributing to the growth rate distinction and broadband feature of these microstructures. Instead of multifacets, this multicolor asymmetric pyramid structure only contains (0001) and {1–101} facets, for which the variation of the carrier lifetime is effectively narrowed by reducing the number of the facets. Moreover, the spontaneous emission rate can be further improved by this deep-hole pattern and lateral overgrowth, due to the reduced residual stress and dislocation density. As a result, a high modulation bandwidth over broadband can be easily achieved by using our strategy. The total modulation bandwidth of multiple subcarriers will be beyond GHz by using WDM technology afterward. These ultrafast and multicolor 3D microLEDs are suitable for application in a high-speed VLC system. This is also the first attempt, to the best of our knowledge, to achieve high-speed microLEDs based on the SAG method, which offers an excellent opportunity to enable many new applications through the integration of lighting, display, and communications [20].

The morphology of the asymmetric pyramid was examined using the scanning electron microscope (GeminiSEM 500, Carl Zeiss Microscopy GmbH, Jena, Germany). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (JEM-F200, JEOL Ltd., Tokyo) was used to inspect the cross-section of the MQWs. We performed a room temperature photoluminescence (PL) and micro-PL experiment combined with SEM to characterize the broadband emission, and used Raman spectroscopy and a low-temperature PL to examine the material quality and residual stress. To inspect the recombination rate, the time-resolved PL (TRPL) spectra (FLS980, Edinburgh Instruments, Livingston, UK) were measured with a picosecond pulsed diode laser (EPL Series, Edinburgh Instruments). The wavelength, linewidth, and repetition rate of the pulsed laser were 375 nm, 1.5 nm, and 20 MHz, respectively.

The distance between the adjacent drilling holes must be carefully controlled to obtain completely merged asymmetric pyramids and broadband emission. Therefore, substrate patterns with hole distances L of 10 μm, 17 μm, 20 μm, and 25 μm were prepared, respectively, to get the optimal selection. Then, 3D microstructures were obtained with different substrate patterns under the same growth conditions.

The results demonstrate that the hole distance is not only associated with the structure morphology but, more importantly, it also affects the emission region. To obtain a high data transfer rate and broadband emission, sample 2 with the largest emission region is the best choice. Therefore, sample 2 was selected to further develop the deep reason for broadband emission and the impact of lateral growth on the modulation bandwidth.

To investigate the reason for the broadband emission of the completely merged asymmetric pyramid, micro-PL excitation was carried out to characterize the wavelength variation of sample 2, which showed the largest emission region. Light spectra of 8 points from the edge to the middle of sample 2 were measured by the micro-PL, as shown in Fig. 2(c). It can be found that the peak wavelengths of the first four points, located on the {10–11} facets, are all focused at a position of 440 nm, with 1 nm variation. The constant emission peak of semipolar facets is highly consistent with the previous study [22,23]. A large red shift appears from point 4 to point 5 due to the crystal plane switch. Compared to point 5, the last three points show a significantly large red shift, with a several nanometers blue shift from point 6 to point 8.

The wavelength, QW thickness, and In composition of point 4 and point 6 are 439 nm, 4.7±0.2nm, and 15.2%, and 531 nm, 16.4±0.2nm, and 23.5%, respectively. Point 6, located on the c plane, shows a relatively higher growth rate compared to the structure grown on the {10–11} facets, resulting in a larger width of MQWs. Additionally, the In composition on InGaN MQWs is much higher along the c axis. The band tilt of the MQWs and the charge separation in QWs on the {10–11} facets are also alleviated because of the weakened polarization on the semipolar facets [24]. All these factors lead to a wavelength distinction between point 4 and point 6. A slight decrease of the InGaN well thickness and In contents appears on the points from the edge to the middle of (0001) facet, indicating that the growth rate is not uniform along the c plane. The c-plane growth rate distinction derives from the species migrating, triggering a reduction of the thickness-enhancement ratio and In contents [25]. This growth rate distinction results in a blue shift of several nanometers from point 6 to point 8. Figure 3(b) also indicates that the crystal quality is poor on the edge of the cross-section with a lot of V-shaped defects. It is originated from the spatial discontinuity and tremendous growth rate difference at the junction of the semipolar and polar facets [16,26], leading to a wavelength separation between point 5 and other points on the c plane. These results demonstrate that the broadband emission is mainly due to the growth rate distinction on the (0001) facet, which is totally different from the previous study of phosphor-free microLEDs [15–17].

To further investigate the lifetime and recombination rate of the carrier at different positions, the TRPL spectra of individual wavelengths are extracted from the TDMs and the normalized PL intensities are drawn on a natural logarithmic scale. Figure 4(d) shows the TRPL spectra of the planar LED, the single pyramid, and the asymmetric pyramid at the main emission wavelengths. Figure 4(e) is the TPRL spectra of the asymmetric pyramid with several characteristic peaks, corresponding to points 5, 6, 7, and 8 in Fig. 3.

The carrier lifetime τ, radiative-recombination carrier lifetime τr, and nonradiative recombination carrier lifetime τnr, can be calculated using [27] τnr=2τfinal,τr=2τinitial.τfinalτfinal−τinitial,(1)where τinitial and τfinal represent the lifetime at the initial and the final stage, respectively, which can be obtained from the slope on the curve of ln(PTRPL) and t. The overall time constant τ is equal to τ=11τr+1τnr.(2)

The maximum 3 dB modulation bandwidth f3dB can be written as f3dB=32πτeff,(3)where τeff=τ/2.5 is the differential carrier lifetime [28,29].

Table 1 lists the calculated results of τintal, τfinal, τr, τnr, τ, and f3dB for these characteristic peaks. For the different carrier frequency/characteristic peak, the corresponding carrier lifetime is different, resulting in a different modulation bandwidth. We define the modulation bandwidth for a specific point of the asymmetric pyramid as f3dBpoint_i. The modulation bandwidth and data transfer rate obtained based on WDM technology can be quantitatively calculated. According to Shannon’s theorems [30], the conventional channel capacity or the maximum data transfer rate is D=∑f3dBpoint_i·log2(1+Spoint_i/Npoint_i),(4)where S and N are the signal variance and the noise variance.Table 1.

Decay Parameters for Single Pyramid, Planar LEDs, and Asymmetric Pyramid

The difference in noise variance is negligible for different emission wavelengths, while the difference in signal variance is nonignorable, but within an order of magnitude. To simplify this calculation, we just assume that either S or N is the same for each characteristic peak. Thus, Eq. (4) can be simplified as D=log2(1+S/N)·∑f3dBpoint_i.(5)

This formula means that the value of the total channel capacity is proportional to total modulation bandwidth ∑f3dBpoint_i, which is the sum of the modulation bandwidth of these characteristic peaks.

The calculated modulation bandwidth of single pyramid and asymmetric pyramid MQWs on the semipolar facet (point 4) increased dozens of times compared to planar LED, due to the suppression of the quantum confinement stark effect (QCSE) of the semipolar facet. More importantly, the decay time of the asymmetric pyramid MQWs on a polar facet (points 6–8) also showed the similar or even better performance compared to that grown on the semipolar facets. Even point 5, at the junction of the polar facet and semipolar facet with plenty of V-shaped defects, still shows an enhancement of several times. The total modulation bandwidth of multiple subcarriers of the asymmetric pyramid is beyond GHz, which is almost a 100-times improvement compared to the planar LED. The results indicate that the asymmetric pyramid structure can provide a high recombination rate and modulation bandwidth over a broad emission region, contributing to a superior data transfer rate using WDM technology.

The E2 (high) peak also was observed to examine the effectiveness of the strain relief for recombination rate enhancement. Referring back to the intrinsic value of 567.2cm−1 [33] for stress-free GaN, the frequency shifts ‘Δω’ were 0.22, 0.77, 0.33, and 3.09cm−1. The corresponding strain also can be estimated by [34]ε=Δω/[2(a−bC13C33)],(6)where a and b are phonon deformation potentials, and C13 and C33 are elastic constants, respectively. The calculated values of stress and strain for different samples are given in Table 2. These data imply that the stress of the asymmetric pyramid has been significantly reduced in contrast to the planar LEDs, with a frequency shift range from 567.4 to 568cm−1. Even compared to the previous broadband emission microstructures (569.2 to 570cm−1) [19], which were grown on 2D patterned substrates by the traditional SAG method, our asymmetric pyramid growth on 3D patterned substrates still appears to be superior. It demonstrates that the deep-hole pattern is an effective method to reduce the residual stress. Additionally, we found that the lateral overgrowth region, including the LO wing and edge, has lower stress than the LO window. It shows that the lateral overgrowth can also contribute to stress relaxation.Table 2.

Calculated Residual Stress, Polarization Field, and Built-in Electric Field Based on Raman Results

To further investigate the influence mechanism of the residual stress on the radiative recombination rate, a theoretical calculation and a numerical analysis were performed based on the Raman test results. Since the spot size of the Raman test is far less than the dimension of each facet, the test results can be used to represent the residual stress of the specific plane. According to Fiorentini et al. [35], the spontaneous polarization (in C/m2) of InGaN can be expressed to the second order in the composition parameter x as PInxG1−xNsp=−0.042x−0.034(1−x)+0.038x(1−x).(7)

For nitride nanostructures grown along the c axis and the strain imposed onto the epitaxial layer in the basal plane, the piezoelectric polarization can be expressed as a Vegard interpolation, PInxG1−xNpz=xPInNpz[ε(x)]+(1−x)PGaNpz[ε(x)],(8)where the bulk piezoelectric polarization PInNpz and PGaNpz can be expressed accurately and compactly (in C/m2) as PInNpz=−1.373ε+7.559ε2,(9)PGaNpz=−0.918ε+9.541ε2,(10)as a function of the basal strain ε(x)=αbasal−α(x)α(x),(11)where αbasal is the basal GaN lattice constant and can be obtained using the residual stress ε calculated from the Raman result by αbasal=(ε+1)·αGaN and α(x) is the lattice constant of the unstrained alloy at composition x.

The built-in electric field in different strain conditions uses [36]FInGaN=|−PInGaNsp+PInGaNpz−PGaNspεeInGaNε0|,(12)where FInGaN denotes the built-in electric field of InGaN quantum well and εeInGaN is the electronic dielectric constant of InGaN quantum well. The calculation results, which are listed in Table 2, demonstrate that the piezoelectric contribution dominates the polarization inside the QW because the spontaneous polarizations of GaN and InN do not noticeably differ. The residual stress strongly influences the piezoelectric polarization of QW. Thus, stress relief of the asymmetric (0001) facet due to the employment of the 3D substrate pattern and lateral overgrowth can effectively reduce the piezoelectric polarization of the MQWs, resulting in an effectively decreased built-in electric field.

Experimental and theoretical analyses demonstrate that stress relief due to the lateral overgrowth of the asymmetric pyramid can efficiently reduce the piezoelectric polarization and built-in electric filed, contributing to an increased possibility for a radiative recombination rate. This increased radiative recombination rate or reduced carrier lifetime is responsible for the enhancement of the modulation bandwidth.

In summary, we presented the fabrication of asymmetric pyramid microLEDs directly on the double-hole patterned sapphire substrate using laser drilling and SAG. Coalescence appears by reducing the hole distance and completely merged asymmetric pyramids are obtained by lateral overgrowth. Large-scale broadband emission appears for samples that have gone through the whole coalescence process, originating from the In segregation and QWs thickness variation at different locations via lateral overgrowth. The 3D substrate pattern and the lateral overgrowth also help reduce the strain and the dislocation density of the structure, resulting in an enhancement of the radiative recombination rate over a large emission range. As a result, the modulation bandwidth is significantly improved. This broadband high-speed emission makes an asymmetric pyramid viable for practical VLC applications.

Acknowledgment. The SEM work was done at International Center for Dielectric Research (ICDR), Xi’an Jiaotong University. Dr. Zhenhuan Tian also thanks Dr. Jian Zhu for his help in life.