Development of efficient semipolar InGaN long wavelength light-emitting diodes and blue laser diodes grown on a high quality semipolar GaN/sapphire template

Semipolar/nonpolar GaN-based optoelectronic devices become attractive due to several advantages such as alleviation of quantum-confinement Stark effect, high polarization ratio and optical gain. High performance semipolar/nonpolar InGaN light-emitting diodes (LEDs) and laser diodes (LDs) grown on semipolar/nonpolar bulk GaN substrate have been demonstrated. Owing to the limited size of such costly substrate, hetero-epitaxial growth of semipolar/nonpolar LEDs and LDs on foreign substrate causes lots of attentions. However, it is very challenging to realize efficient semipolar/nonpolar optoelectronic devices on foreign substrate due to the high dislocation density and possibly high basal plane stacking fault density. In this article, we review two growth methods to obtain high crystal quality semipolar (11-22) and (20-21) GaN layers on specially patterned sapphire substrate. The use of these substrates leads to the realization of efficient long wavelength InGaN semipolar LEDs and the first demonstration of semipolar blue LDs grown on foreign substrate shown in our previous reports. These results demonstrate significant progress in exploring the semipolar GaN materials quality and the devices efficiency grown on foreign substrate.


Introduction
III-nitride optical devices like light emitting diodes (LEDs) and laser diodes (LDs) have been well developed due the wide application in general illumination, display backlighting, and automotive headlights [1][2][3]. The commercially available GaN-based LEDs with the wurtzite structure grown along the c-direction, however, suffer from the quantum-confined Stark effect (QCSE) due to the large polarization-related electric fields, leading to a reduction of electron-hole wave-function overlap in the quantum wells (QWs) [4,5]. The QCSE becomes more severe in long wavelength InGaN LEDs such as green and yellow LEDs [6].
To overcome the challenges of the QCSE in the InGaN QWs, semipolar and nonpolar orientations are proposed to grow GaN optical devices, which show reduced or eliminated polarization fields [7][8][9][10]. Semipolar and nonpolar GaN optical devices also offer other advantages such as a high polarization ratio and high optical gain [7][8][9][10]. The polarized emitting light from semipolar and nonpolar LEDs could be employed as the backlighting source for liquid crystal displays (LCDs). In LCDs, a polarizer is required since the emission light from commercial c-plane LEDs is unpolarized, which results in an energy conversion loss [11,12]. High efficiency semipolar/nonpolar LEDs and LDs can be only demonstrated on semipolar and nonpolar bulk GaN substrates, which are very costly and only available with a small area [13]. This limits the application of semipolar/nonpolar optical devices. Growing semipolar/nonpolar GaN devices on low cost and large size foreign substrate like sapphire and silicon is attractive [9,10]. However, semipolar and  [17], with the permission of ACS publishing. nonpolar GaN layers grown on a foreign substrate suffer from high defect densities like basal stacking faults (BSFs) and threading dislocations (TDs), resulting in a low quantum efficiency and poor device performance [14]. In our recent studies, we presented state-of-the-art efficient semipolar (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) and (20)(21) GaN LEDs grown on high crystal quality semipolar GaN templates on a patterned sapphire substrate [15][16][17][18]. Also, blue semipolar LDs have been firstly demonstrated on a high crystal quality (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) GaN/sapphire template [19]. In this paper, we discuss the progress of materials growth for a high crystal quality semipolar GaN template on a sapphire substrate, efficient semipolar GaN long wavelength LEDs, and semipolar blue LDs grown on a foreign substrate. Polarized phosphor-free white semipolar (20-21) LEDs grown on a patterned sapphire template have also been presented [20].

Growth of high crystal quality (20-21) GaN on a patterned sapphire substrate
An unintentionally doped semipolar (20-21) GaN layer was grown on a 4 inch (22-43) patterned sapphire substrate using MOCVD [16,23,24]. A schematic diagram of the growth process and sapphire orientations are shown in figure 2(a). Patterned trenches with a 6 µm period were formed on the sapphire substrate and  the inclined c-axis was exposed by dry etching. The semipolar (20-21) GaN layer was achieved by coalescing the adjacent nucleated crystals. A detailed description of the growth process of the semipolar (20-21) GaN template on a patterned sapphire substrate can be found in [23]. Figure 2(b) is an image of a 4 inch polished (20-21) GaN template on a patterned sapphire substrate. The roughness of the surface is measured by AFM to be 0.5 nm in an area of 10 × 10 µm 2 as shown in figure 2(c). The X-ray rocking curve width of the (20-21) peak in figure 2(d) was 192 and 217 arcsec parallel and perpendicular to the stripes, respectively, which suggests a high crystal quality of the semipolar (20-21) GaN template grown on a patterned sapphire substrate [24]. The density of the final TDs is around 2 × 10 8 cm −2 and the density of the BSF is low.

Semipolar (20-21) InGaN yellow-green LEDs on a patterned sapphire substrate
Semipolar (20-21) 550 nm yellow-green LEDs with a single QW were grown on a (20-21) GaN template on a patterned sapphire substrate using MOCVD. The epitaxial wafer was fabricated into micro-size devices [28]. Figure 4(a) shows the electrical luminous (EL) spectrum of the LEDs at 20 A cm −2 , which exhibits an emission peak wavelength of 550 nm and a FWHM of 37 nm. As shown in figure 4(b), the packaged semipolar (20-21) yellow-green LEDs show a state-of-the-art EQE of 2.3%. A detailed study of materials growth and characterizations and the electrical and optical properties of yellow-green LEDs is published elsewhere [28].

Polarized phosphor-free white semipolar (20-21) LEDs on a patterned sapphire substrate
Polarized monolithic white semipolar LEDs were realized by integrating blue and yellow QWs directly on a 4 inch patterned sapphire substrate, based on the high crystal quality semipolar InGaN QWs described in 3.2 [20]. In this design, the emission spectrum and color temperature can be precisely controlled by tuning the number of QWs and the In content in the QWs. Figure 5(a) shows a side view of the distribution of In atoms in the active region using atom probe tomography (APT). The top blue QW and the bottom yellow QW can be clearly observed. Figure 5(b) presents the LIV characteristic of standard LEDs with a size of 0.1 mm 2 . The output power was measured to be 3.9 mW at 100 mA, which is the highest output power among white semipolar InGaN LEDs on foreign substrates [29,30]. The forward voltage was 3.3 V at 20 mA. The polarization ratio is defined by ρ = (I x ′ − I y ′ )/(I x ′ + I y ′ ), where I x ′ and I y ′ are the maximum and minimum integrated intensities of the emission spectra that pass through the polarizer when the polarizer is aligned along the x ′ -direction and y ′ -direction. In the (20)(21) orientation, the x ′ -direction and y ′ -direction are along (1-210) and (10-1-4), respectively. Figure 5(c) presents the emission spectra of the semipolar (20)(21) LEDs with the polarizer aligned along (1-210) and (10-1-4), respectively. Two emission peaks of 445 and 565 nm can be seen, which originate from the blue and yellow QWs, respectively. The polarization ratio was calculated to be 0.30. The phosphor-free white semipolar LEDs can be employed in visible light communication (VLC) owing to a larger electron-hole wave-function overlap and a shorter carrier lifetime on semipolar orientation [31]. The conventional yellow phosphor converted white LEDs show a limited 3 dB modulation bandwidth (MB) of only 30 MHz due to the low frequency response of yellow phosphor [32]. The measurement results of 3 dB MBs of the monolithic white semipolar µLEDs are plotted in figure 5(d). It is found that the highest MB reaches 660 MHz in the 20 × 20 µm 2 size µLEDs, which shows a large potential application in VLC.

First demonstration of blue semipolar LDs grown on a high crystal quality (11-22)/sapphire template
The first semipolar blue LDs grown on a foreign sapphire substrate were recently successfully demonstrated by our group by optimizing the structure and growth condition on high crystal quality semipolar (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) GaN layers on a patterned sapphire substrate described in 2.1 [19]. The far field pattern of the semipolar LDs grown on a sapphire template is shown in figure 6(a), followed by a lasing peak wavelength of 439 nm as shown in figure 6(b). The LIV characteristic of a semipolar LD with a length of 1800 µm and width of 2.5 µm is presented in figure 6(c). The semipolar blue LD shows a threshold current density of ∼20 kA cm −2 and an output power of 38 mW at 800 mA under a pulse condition. These results represent significant progress in semipolar optical devices grown on sapphire substrates, which could overcome the limitation of costly and small size semipolar bulk GaN substrates. We believe the performance of the devices could be dramatically improved by reducing the BSFs and TDs in the semipolar GaN layers on sapphire substrates and the misfit dislocations in the devices [33].