Microfacet QWs

The width of the striped GaN seed region ranged from 4 to 15 (Am with a period of 10 to 100 )im. The shape of the microfacet structure varied with the mask geometry multifaceted structures tended to appear on masks with a large occupancy of the seed region within a period, while (1122) facets were dominant on masks with a small occupancy. On the contrary. 

---

This chapter is structured as follows The crystalline structure of the 1122 plane is briefly described in Section 14.2 and the results for the microfacet QWs are described in Section 14.3. Finally, Section 14.4 reviews planar 1122 ... 

Figure 14.3 Schematic of the two different GaN template —> (e) Si02 mask patterning formation processes of microfacet QWs. (a) by photolithography (f) fabrication of GaN template -> (b) patterning by RIE -> QWs by regrowth.

Typically, when the well width is thin, the internal electric field in the well does not affect the PL peak positions. In fact, our blue-emitting 1122 microfacet QW with a well width as thin as 2.8 nm and the reference (0001)... 

Figure 14.7 Temperature dependence of the integrated PL intensities of a 1122 microfacet QW and a conventional (0001) InGaN QW that emit at 400 nm. Intensities are normalized by those at 13 K so that they are proportional to the IQE.

Figure 14.8 Temperature dependence of Trl, Trad, and Tnon-rad of a (1122) microfacet QW that emits at 400 nm. Inset shows PL decay at 14 K acquired at the emission peaks of the 1122 microfacet QW as well as a reference planar (0001) QW for comparison.

When the intra-facet variation of the In composition is emphasized, a broadband emission should be realized. Herein, a multiwavelength (rainbow color) luminescence from a 1122 microfacet QW is demonstrated. The sample was a microfacet single quantum well (SQW). A STEM observation confirmed that an InGaN well with a thickness of 2 0.2 nm was successfully and uniformly formed within the (1122) facet. On the contrary, the In composition estimated by EDS equipped with the STEM system monotonously increased from 25% on the (1120) side to 40% on the (0001) side. Considering a 2 nm uniform well width and an internal electric field due to the polarization effects, the estimated QW transition energy ranged from 2.43 (510 nm) to 2.79 eV (444 nm). 

The influence of this surprisingly large In spatial distribution in the 1122 microfacet QW on the optical properties was microscopically investigated by CL at RT. The lower inset of Figure 14.9 shows representative monochromatic CL images acquired at wavelengths of 420 nm (blue), 500 nm (green), and... 

Figure 14.9 Position dependence of the CL 420 nm (blue), 500 nm (green), and 580 nm peak wavelength of a (1122) microfacet QW. (amber), while the upper inset shows a Vertical bars represent the emission line schematic sample structure where the arrow...

The macroscopic optical properties of the 1122 microfacet QW were assessed by selectively exciting the InGaN wells with a frequency-doubled Ti sapphire laser with a wavelength of400 nm. The repetition rate, pulse width, and excitation density were 80 MHz, 1.5 ps, and 1.31 p, cm , respectively. Figure 14.10 shows a PL spectrum acquired at RT. The PL peaked at 535 nm (2.32 eV) and encompassed wavelengths from 450 to 650 nm, which correspond to almost the entire visible range. The estimated FWHM was... 

To investigate the reason for this wavelength-independent high IQE in (1122 microfacet QWs, TRPL measurements were performed at 13 K using the same excitation conditions as above. Figure 14.11 shows the PL decay curves at 460 nm (blue), 530 nm (green), and 575 nm (amber) within the hroad (1122) microfacet QW emission. Each curve is composed of fast and slow decays. 

Figure 14.11 PL decay curves ofa 1122 microfacet QW at 13 K. Decay curves are monitored at 460 nm (blue), 530 nm (green), and 575 nm (amber).

Two microfacet QWs that emitted different colors were designed. One emitted 500 nm from the (0001) facet and 400 nm from the 1122 facet (sample A), whereas the other emitted 580 nm from the (0001) facet and 430 nm from the 1122 facet (sample B). The In composition in the 1120 facet was insufficient to emit visible hght. Hence, the contribution from this facet was ignored. The emission color from each facet of the fabricated microfacet QWs was microscopically confirmed by CL at RT and was consistent with the design. 

The macroscopic optical properties of the microfacet QWs were assessed by PL measurements at RT. Figure 14.12a and b show the PL spectra of the samples. The excitation source was a He-Cd laser with an excitation density of 10 W cm and an excited spot of 300 pm, which included many (0001) and 1122 facets. The origins of the emissions, which were indicated in Figure 14.12a and b, were identified using the CL spectra. The PL spectra were... 

The superior optical properties of our microfacet QWs and reports on planar heteroepitaxial films lead us to heheve that if planar structures are grown on high-quality GaN substrates, then device performances could he drastically improved. In this section, we describe the MOVPE of GaN and InGaN/GaN QWs on semipolar (1122) GaN substrates, their fundamental optical properties, and the fabrication of LEDs. 

A five-period InGaN/GaN multiple quantum well (MQW) was fabricated on this high-quality GaN hornoepitaxial layer. The XRD profile of the MQW consisted of satellite peaks, where the evaluated well and barrier layer thicknesses were 3.4 and 11.3 nm, respectively. It is noteworthy that the growth rates of InGaN and GaN derived from the well and barrier thicknesses are comparable with those on the (0001) plane. As for the optical properties, we were interested in if the short radiative lifetime observed for the microfacet QWs was preserved. Therefore, TRPL measurements were conducted at 10 K. The excitation pulses were from a frequency-doubled Ti sapphire laser with a wavelength of 380 nm to selectively excite InGaN wells and a power density as low as 470 nj cm. The PL was detected by a streak camera. Figure 14.15... 

In summary, we have demonstrated recent progress on the microfacet QWs and planar, semipolar QWs and LEDs. The 1122 plane plays a key role in both structures. The major achievements are as follows (i) the 1122 microfacet QWs with a high IQE due to weak internal electric fields, (ii) white... 

---