The first 30-mW-class semipolar blue light-emitting diode (LED) on a free-standing (10 1 1) GaN substrate has been demonstrated by using microscale periodic backside structures. The light extraction efficiency and corresponding output power were greatly enhanced, by up to 2.8-fold (bare chip) compare with conventional devices. At a driving current of 20 mA, the LED showed an output power of 31.1 mW and an external quantum efficiency of 54.7%. Semipolar GaN LED technology is now comparable to commercial c-plane blue LED technology, not only in terms of internal material properties but also in terms of chip processing techniques. # W urtzite (Al,Ga,In)N-based light-emitting diodes (LEDs) have attracted considerable attention since their first demonstration in the early to mid-1990s 1) for their wide applications including traffic signals, full-color displays, backlighting sources for liquidcrystal displays, and general lighting. Although device performance has improved steadily, current commercially available c-plane optoelectronic devices suffer from internal electric fields due to discontinuities in both spontaneous and piezoelectric polarization at the heterointerface. This leads to the separation of electrons and holes in the quantum wells and thus limits the radiative recombination rate. 2-5) On the other hand, devices grown on nonpolar planes such as the (1 100) m-plane and (11 20) a-plane, as well as devices grown on (11 22) and (10 1 1) semipolar planes have been demonstrated with eliminated or reduced polarization fields. [6][7][8][9][10][11][12][13] and are theoretically predicted to have a higher optical gain than c-plane devices, due to their anisotropic band structure. 14) Despite those advantages, however, the output power and efficiency of current semipolar and nonpolar LEDs are still lower than those of the best reported c-plane devices, mainly due to the poor light extraction efficiency ( extr ) compared with their polar counterparts. [15][16][17][18][19] Light extraction efficiency has become the most important limiting factor for the efficiency of LEDs, since the internal quantum efficiency (IQE) of nitride-based LEDs has been greatly improved (more than 80%) 20) by the availability of low-dislocation GaN substrates and advances in metal organic chemical vapor deposition (MOCVD) techniques. The low extr is primarily caused by the low critical angle (23 ) of the light escape cone, due to the large differences between the refractive indices of GaN (n $ 2:5) and air (n ¼ 1). 21) For c-plane devices, both extr and the corresponding output power have been greatly improved by surface roughening methods such as patterned sapphire substrate (PSS) and photoelectrochemical (PEC) etching techniques. [21][22][23][24][25] Semipolar and nonpolar devices, however, still suffer from low light extraction efficiency due to the lack of proper roughening techniques, which have hindered their performance. Recently, Zhong et al. have demonstrated that surface patterning with conical features by inductively coupled pla...
The quantum-confined Stark effect (QCSE) on InGaN-based light-emitting diodes (LEDs) was investigated as a part of the continuing study of exploring differences between photoluminescence (PL) and electroluminescence (EL) characteristics. The luminescence characteristics were related to electrical characteristics of green and amber LEDs by employing the electrical-bias-applied PL technique. By inspecting the band diagram, it has been found that the separation of quasi-Fermi levels, which strongly affects the QCSE, can be quantified and related to the luminescence. In order to compare PL and EL characteristics, attention was paid to the QCSE during the PL and EL measurements. Despite the control of the QCSE, differences were still confirmed between PL and EL characteristics, which have led us to the conclusion to that there are other unrevealed origins for the differences.
Long wavelength (525–575 nm) (112¯2) light emitting diodes were grown pseudomorphically on stress relaxed InGaN buffer layers. Basal plane dislocation glide led to the formation of misfit dislocations confined to the bottom of the InGaN buffer layer. This provided one-dimensional plastic relaxation in the film interior, including the device active region. The change of the stress state of the quantum well due to one-dimensional plastic relaxation altered the valence band structure, which produced a significant shift in polarization of emitted light. Devices grown on relaxed buffers demonstrated equivalent output power compared to those for control samples without relaxation.
We demonstrate an electrically injected semipolar (112¯2) laser diode (LD) grown on an intentionally stress relaxed n-In0.09Ga0.91N waveguiding layer. Detrimental effects of misfit dislocations (MDs) in the proximity of the active region were effectively suppressed by utilizing a p/n-Al0.2Ga0.8N electron/hole blocking layer between the dislocated heterointerfaces and the active region. The threshold current density of the LD was ∼20.3 kA/cm2 with a lasing wavelength of 444.9 nm. This LD demonstrates an alternative approach in semipolar AlInGaN LD waveguide design where the thickness and composition of the waveguiding and/or cladding layers are not limited by the critical thickness for MD formation.
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