Efficient radiative recombination from ⟨112¯2⟩ -oriented InxGa1−xN multiple quantum wells fabricated by the regrowth technique

K. Nishizuka, M. Funato, Y. Kawakami, Sg. Fujita, Y. Narukawa, T. Mukai
2004 Applied Physics Letters  
In x Ga 1−x N multiple quantum wells (QWs) with ͓0001͔, ͗1122͘, and ͗1120͘ orientations have been fabricated by means of the regrowth technique on patterned GaN template with striped geometry, normal planes of which are ͑0001͒ and ͕1120͖, on sapphire substrates. It was found that photoluminescence intensity of the ͕1122͖ QW is the strongest among the three QWs, and the internal quantum efficiency of the ͕1122͖ QW was estimated to be as large as about 40% at room temperature. The radiative
more » ... ination lifetime of the ͕1122͖ QW was about 0.38 ns at low temperature, which was 3.8 times shorter than that of conventional ͓0001͔-oriented In x Ga 1−x N QWs emitting at a similar wavelength of about 400 nm. These findings strongly suggest the achievement of stronger oscillator strength owing to the suppression of piezoelectric fields. Nitride-based light-emitting diodes (LEDs) have already been commercialized for the violet to green spectral range. 1,2 It is often reported that there are two competitive factors determining the internal quantum efficiency of the present LEDs; one is carrier/exciton localization 3,4 and the other is the quantum confinement Stark effect (QCSE) 5-7 which was caused by strong piezoelectric polarization in strained In x Ga 1−x N / GaN quantum wells (QWs). 8, 9 The former suppresses nonradiative processes to improve internal quantum efficiency, and the latter prevents the radiative recombination to degrade the efficiency. Furthermore, the QCSE becomes remarkable with increasing In composition in In x Ga 1−x N / GaN QWs and, so, can be a major drawback for realizing LEDs operating at longer wavelengths. Therefore, it is crucially important for LEDs with higher performances to avoid the QCSE. For this purpose, the use of nonpolar planes such as M-plane ͑1010͒ or A-plane ͑1120͒ has been proposed. In fact, a ͑Al, Ga͒N / GaN multiple quantum well (MQW) with a nonpolar ͓1010͔ orientation was grown on ␥ -LiAlO 2 ͑100͒ substrate. 10 However, it is generally difficult to grow polar materials such as GaN on nonpolar substrates. A clue to another approach can be found in Refs. 11 and 12, in which the magnitude of piezoelectric fields (PFs) was calculated as a function of the tilt angle of the c-axis from the surface normal, and it was shown that PFs could be zero at 90°and 39°. The plane inclined 90°corresponds to nonpolar planes such as M-or A-plane, while 39°is very close to the angle formed by ͑1124͒ and ͑1012͒ planes. In these papers, QWs were grown on the inclined ͑1011͒ facets but the QWs unexpectedly showed very weak luminescence due to the presence of a number of stacking faults. From this background, in this study, we propose the regrowth technique, in which well-established and high quality C-oriented GaN on sapphire ͑0001͒ substrate is used as a seed. We demonstrate that the multiple quantum wells (MQWs) grown on C-oriented GaN patterned with a stripe along the ͓1100͔ direction have the facets of ͑0001͒, ͕1122͖, and ͕1120͖, and that the ͕1122͖ QWs involve weaker PFs and drastically improve the luminescence efficiency compared with conventional C-oriented In x Ga 1−x N QWs. The GaN template with a thickness of 4 m was firstly grown on a ͑0001͒ sapphire substrate by metalorganic vaporphase epitaxy under atmosphere pressure. A stripe pattern with a repetition of 20 m was then formed along the ͓1100͔ direction by a reactive ion etching (RIE) technique. The GaN template was deeply etched to the sapphire substrate. The stripe width was 4 m and the sidewall is nonpolar ͕1120͖ planes. A 1-m-thick GaN layer was then grown on the patterned GaN to recover damage induced by the RIE process, and followed by the growth of three-period In x Ga 1−x N / GaN MQWs. The well thickness and In composition of the QWs were analyzed by the cross-sectional scanning transmission electron microscopy (STEM) equipped with energy dispersive x-ray spectroscopy (EDS) ͑JEOL: JEM-2100F͒ and x-ray diffraction (XRD) measurements. The specimen for STEM was prepared by a conventional Ar ion milling technique, the thickness of which was about 100 nm or less. The acceleration voltage for STEM was 200 kV. Microscopic photoluminescence (PL) was measured at room temperature (RT) using a confocal microscope, of which the spatial resolution was less than 500 nm. Both excitation and collection of PL were through a microscope objective lens, and therefore, the PL property of each growth facet could be assessed separately. The excitation pulses were from a frequency-doubled Ti:sapphire laser (Spectra Physics: TSUNAMI), of which the wavelength and excitation power density were 353 nm and 45.8 J/cm 2 , respectively. The PL was detected by a cooled head CCD camera. In order to estimate the internal quantum efficiency and lifetimes of both radiative and nonradiative processes, temperature dependence of PL and time-resolved (TR) PL was a)
doi:10.1063/1.1806266 fatcat:s3yx2wuw3fhj3chi2m32saic6i