We fabricated InGaN LEDs prepared on β-Ga<sub>2</sub>O<sub>3</sub> (201) single-crystal substrates. The substrates were produced by
using the edge-defined film-fed growth (EFG) method. A Si-doped GaN epitaxial layer was grown on an electrically
conductive β-Ga<sub>2</sub>O<sub>3</sub> (201) substrate by metal organic chemical vapor deposition (MOCVD). The full-width at half
maximum (FWHM) of (0002) and (101 1) X-ray rocking curves (XRCs) of the Si-doped GaN layer were 220 arcsec and
223 arcsec, respectively. The dark spot density measured by cathode luminescence (CL) was approximately 1.5×10<sup>8</sup> cm<sup>-2</sup>.
The crystalline quality was equal to that of GaN layer on sapphire. We fabricated a vertical LED in the p-side down
configuration. The peak wavelength was approximately 450 nm. The p-contact metal area was 300 ×300 μm<sup>2</sup>. The light
output power did not saturate at 1000 A/cm<sup>2</sup>. This device characteristic indicates the great potential of Ga<sub>2</sub>O<sub>3</sub> for use in
We report a highly efficient GaN-based blue light-emitting diodes (LEDs) structure with an emitting wavelength of 450nm on flat sapphire substrate by utilizing a nano-porous (NP) GaN insertion layer. Unlike the LED on patterned sapphire substrates (PSS), the presented substrate has a new morphology which not only can generate an embedded nano-dimensional void structure as a mirror layer to reflect the light from active layers for enhancing the light extraction, but can also easily enlarge the wafer size to a large scale, such as wafer diameter larger than 6 inches. With a chip size of 45 mil × 45 mil under a driving current of 350 mA, the light output powers of the NP GaN LEDs without and with encapsulation are 455 and 554 mW respectively. The light output power is improved about 2 -fold comparing to the LED on a flat sapphire substrate, and even comparable to the LED on PSS which all of them have a flat p-type GaN surface. The characterization and performance of this newly NP LED structure will be discussed in detail.
The effect of active layer design on the efficiency of InGaN light emitting diodes (LEDs) with the light emission in blue
(~420 nm) has been studied. Correlation between the internal quantum efficiency (IQE) and relative external quantum
efficiency (EQE) and salient features of structures on c-plane InGaN LEDs which contain multiple quantum wells
(MQWs) of different barrier height (either In<sub>0.01</sub>Ga<sub>0.99</sub>N or In<sub>0.06</sub>Ga<sub>0.94</sub>N barriers) and thickness (3 nm and 12 nm) as well
as different double heterostructure (DH) designs (3 nm, dual 3 nm, 6 nm, dual 6 nm, 9 nm and 11 nm) with inserted 3
nm In<sub>0.06</sub>Ga<sub>0.94</sub>N barrier. Pulsed electroluminescence (EL) and optical excitation power-dependent photoluminescence
(PL) measurements indicated that the thinner and lower In<sub>0.06</sub>Ga<sub>0.94</sub>N barriers bode well for high EQE and IQE.
Furthermore, increase of the effective active region thickness by multiple InGaN DH structures (dual, quad and hex)
separated by 3 nm In<sub>0.06</sub>Ga<sub>0.94</sub>N barriers is promising at high injection levels. Although increasing the single DH
thickness from 3 to 6 nm improves the peak relative EQE by nearly 3.6 times due to increased density of states and
increased emitting volume, the IQE suffers a nearly 30% loss. Further increase in the DH thickness to 9 and 11 nm
results in a significantly slower rate of increase of EQE with current injection and lower peak EQE values presumably
due to degradation of the InGaN layer. Increasing the number of 3 nm DH active regions with 3 nm In<sub>0.06</sub>Ga<sub>0.94</sub>N
barriers improves EQE, while still maintaining high IQE (above 95% at a carrier concentration of 10<sup>18</sup> cm<sup>-3</sup>) and
showing negligible EQE degradation up to 550 A/cm<sup>2</sup> due to increased emitting volume and high radiative
recombination coefficients and high IQE.
We report the efficiency droop behaviors of InGaN/GaN blue LEDs with different thickness of GaN quantum barriers
(QBs). The droop percentage from efficiency peak to 70 A/cm<sup>2</sup> is only about 10% as reducing the thickness of GaN QBs
from 104 Å to 33 Å. A less carrier localization has been observed from wavelength dependent time resoled
photoluminescence measurement as reducing the thickness of GaN QBs. The alleviation of droop percentage may due to
more uniform distribution of electron and hole carrier in the active region, which resulted from super-lattice (SL) like
active structure. The crystalline quality does not become worse from the results of v-pits density even thickness of GaN
QBs is as low as 33 Å. The SL like active structure could be a potential structure to alleviate the efficiency droop for the
application of solid state general lighting.
Light extraction efficiency is important to the brightness of LEDs. In this study, various texturing and roughing
schemes were formed on the surface or interface of InGaN-based LED structure grown on sapphire substrate to
investigate their effects. Throughout the research, temperature-dependent PL measurement was used to calculate the
internal quantum efficiency so as to derive the light extraction efficiency. The light extraction efficiency is around 60
to 65% while the epitaxy and substrate are flat. On the other hand, the light extraction efficiency reaches an optimal
value of around 85% while the p-GaN surface is textured and the substrate is patterned. However, for LED having
only one-side surface texturing structure optimized on either p- or n-side, the light extraction efficiency can be
already as high as 75 to 80%. Methods for further enhancement, such as use of ZnO nanorod on chip surface, were