C-plane Light-emitting Diodes Research Articles

Raman and emission characteristics of a-plane InGaN/GaN blue-green light emitting diodes on r-sapphire substrates

Raman and emission properties of a nonpolar a-plane InGaN/GaN blue-green light emitting diode (LED) on an r-sapphire substrate are investigated and compared with a conventional c-plane blue-green LED. The output power of the a-plane LED was 1.4 mW at 20 mA. The c-plane LED has higher EQE, but it reaches the maximum at a lower forward current and the droop is faster than the a-plane counterpart. As the reverse bias increased, a blueshift in the PL spectra was not observed in the a-plane structure, which is indicative of an absence of quantum confined Stark effects. However, a strong blueshift in the electroluminescence spectra was still present, which means the In localization effects are relevant in nonpolar InGaN/GaN quantum wells. In the Raman spectra, a strong anisotropy of E2(high) phonon modes was observed. By comparing the frequency of the E2(high) modes, we demonstrate that the residual compressive strain in an a-plane LED is significantly smaller than in the polar counterpart.

Wet etching of non-polar gallium nitride light-emitting diode structure for enhanced light extraction

The surface of a non-polar a-plane GaN light-emitting diode (LED) was intentionally damaged with a KOH wet etch to enhance the extraction of light. This roughening technique has been commonly applied to c-plane polar GaN LEDs to extract photons that would otherwise suffer from total internal refraction. We show that wet etching of the non-polar LED does create a textured surface that increases the light extraction efficiency; however, the mechanism of the etch is quite dissimilar to the etch mechanism observed for c-plane LEDs. In fact, the etch proceeds perpendicular to the a-plane surface along unstable N-face (0 0 0 −1) plane with the Ga-face plane resistant to the etch. The photoluminescence intensity from a-plane non-polar LED after KOH-based wet etching was increased by 83% in our experiments. Therefore, surface roughening by KOH-based wet etch was found to be very effective to extract photons from a-plane non-polar GaN-based LEDs.

Highly Integrated InGaN/GaN Semipolar Micro-Pyramid Light-Emitting Diode Arrays by Confined Selective Area Growth

We report on the fabrication of highly integrated semipolar {101} GaN micro-pyramid light-emitting diode (LED) arrays on a c-plane sapphire substrate obtained via confined selective area growth (SAG). The cathodoluminescence (CL) revealed that the micro-pyramid arrays were under a severe local strain and potential fluctuations were observed depending on the location of the pyramid facets. The emissive region in the micro-pyramid facets was also found to be different in the monochromatically captured CL images. Furthermore, the electroluminescence (EL) of the micro-pyramid LED arrays had a palpable emissive spectra for high indium composition compared to c-plane LEDs, and optical polarization switching was also observed.

Ballistic transport in InGaN-based LEDs: impact on efficiency

Heterojunction light-emitting diodes (LEDs) based on the InGaN/GaN system have improved considerably but still suffer from efficiency degradation at high injection levels which unless overcome would aggravate LED lighting. Although Auger recombination has been proposed as the genesis of the efficiency degradation, it appears that the premise of electron overflow and non-uniform distribution of carriers in the active region being the immediate impediment is gaining popularity. The lack of temperature sensitivity and sizeable impact of the barrier height provided by an electron blocking layer and the electron cooling layer prior to electron injection into the active region suggest that the new concept of hot electrons and ballistic/quasi-ballistic transport be invoked to account for the electron overflow. The electron overflow siphons off the electrons before they can participate in the recombination process. If the electrons are made to remain in the active region e.g. by cooling them prior to injection and/or blocking the overflow by an electron blocking layer, they would have to either recombine, radiatively or nonradiatively (e.g. Shockley–Read–Hall and Auger), or accumulate in the active region. The essence of the proposed overflow model is in good agreement with the experimental electroluminescence data obtained for m-plane and c-plane LEDs with/without electron blocking layers and with/without staircase electron injectors.

High Power and High Efficiency Blue InGaN Light Emitting Diodes on Free-Standing Semipolar (3031) Bulk GaN Substrate

High power and high efficiency semipolar (3031) nitride light emitting diodes (LEDs), fabricated on low extended defect bulk GaN substrates, are reported for the first time. The LEDs were grown by metal organic chemical vapor deposition (MOCVD) at atmospheric pressure. The peak wavelength was 452 nm, and a minimal redshift of <1 nm was observed between 5–100 mA, in comparison to large blueshifts in c-plane LEDs. The output power and external quantum efficiency (EQE) of the packaged 200 ×500 µm2 was 14.48 mW and 26.5%, respectively, at 20 mA.

Low efficiency droop in blue-green m-plane InGaN/GaN light emitting diodes

We investigated the electroluminescence and relatively external quantum efficiency (EQE) of m-plane InGaN/GaN light emitting diodes (LEDs) emitting at 480 nm to elucidate the droop behaviors in nitride-based LEDs. With increasing the injection current density to 100 A/cm2, the m-plane LEDs exhibit only 13% efficiency droop, whereas conventional c-plane LEDs suffer from efficiency droop at very low injection current density and the EQE of c-plane LEDs decrease to as little as 50% of its maximum value. Our simulation models show that in m-plane LEDs the absence of polarization fields manifest not only the hole distribution more uniform among the wells but also the reduction in electron overflow out of electron blocking layer. These results suggest that the nonuniform distribution of holes and electron leakage current due to strong polarization fields are responsible for the relatively significant efficiency droop of conventional c-plane LEDs.

Surface 210 nm light emission from an AlN p–n junction light-emitting diode enhanced by A-plane growth orientation

( 11 2 ¯ 0 ) A-plane AlN p–n junction light-emitting diode (LED) with a wavelength of 210 nm is demonstrated. The electroluminescence from the A-plane LED is inherently polarized for the electric field parallel to the [0001] c-axis due to a negative crystal-field splitting energy. The polarization ratio (electric-field component ratio of parallel and perpendicular to c-axis) is as high as 0.9. The radiation pattern of the A-plane LED shows higher emission intensity along the surface normal, while that of a conventional (0001) C-plane LED shows lower emission intensity along the surface normal. The different radiation patterns can be explained by the polarization property.

On carrier spillover in c- and m-plane InGaN light emitting diodes

The internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) in InGaN light-emitting diodes (LEDs) emitting at 400 nm with and without electron blocking layers (EBLs) on c-plane GaN and m-plane GaN were investigated in order to shed some light on any effect of polarization charge induced field on efficiency killer carrier spillover. Without an EBL the EQE values suffered considerably (by 80%) for both orientations, which is clearly attributable to carrier spillover. Substantial carrier spillover in both polarities, therefore, suggests that the polarization charge is not the major factor in efficiency degradation observed, particularly at high injection levels. Furthermore, the m-plane variety with EBL did not show any discernable efficiency degradation up to a maximum current density of 2250 A cm−2 employed while that on c-plane showed a reduction by ∼40%. In addition, IQE of m-plane LED structure determined from excitation power dependent photoluminescence was ∼80% compared to 50% in c-plane LEDs under resonant and moderate excitation condition. This too is indicative of the superiority of m-plane LED structures, most probably due to relatively larger optical matrix elements for m-plane orientation.

Efficiency retention at high current injection levels in m-plane InGaN light emitting diodes

We investigated the internal quantum efficiency (IQE) and the relative external quantum efficiency (EQE) of m-plane InGaN light emitting diodes (LEDs) grown on m-plane freestanding GaN emitting at ∼400 nm for current densities up to 2500 A/cm2. IQE values extracted from intensity and temperature dependent photoluminescence measurements were consistently higher, by some 30%, for the m-plane LEDs than for reference c-plane LEDs having the same structure, e.g., 80% versus 60% at an injected steady-state carrier concentration of 1.2×1018 cm−3. With increasing current injection up to 2500 A/cm2, the maximum EQE is nearly retained in m-plane LEDs, whereas c-plane LEDs exhibit approximately 25% droop. The negligible droop in m-plane LEDs is consistent with the reported enhanced hole carrier concentration and light holes in m-plane orientation, thereby enhanced hole transport throughout the active region, and lack of polarization induced field. A high quantum efficiency and in particular its retention at high injection levels bode well for m-plane LEDs as candidates for general lighting applications.

Bright semipolar GaInN∕GaN blue light emitting diode on side facets of selectively grown GaN stripes

The authors demonstrate the fabrication and evaluation of bright semipolar GaInN∕GaN blue light emitting diodes (LEDs). The structures are realized by growing five GaInN∕GaN quantum wells on the {11¯01} side facets of selectively grown n-GaN stripes with triangular shape running along the ⟨112¯0⟩ direction covered with a Mg-doped GaN top layer. The growth was done by metal organic vapor phase epitaxy using a conventional [0001] sapphire substrate. The devices have circular mesa structures with diameters between 70 and 140μm. Continuous wave on-wafer optical output powers as high as 700μW and 3mW could be achieved under dc conditions for 20 and 110mA, respectively. The current dependent blueshift of the peak emission wavelength caused by screening effects of the piezoelectric field was only 1.5nm for currents between 1 and 50mA. This is less than half the value measured on c-plane LEDs and confirms the reduced piezoelectric field in our LED structures.

Demonstration of a semipolar (101¯3¯) InGaN∕GaN green light emitting diode

We demonstrate the growth and fabrication of a semipolar (101¯3¯) InGaN∕GaN green (∼525nm) light emitting diode (LED). The fabricated devices demonstrated a low turn-on voltage of 3.2V and a series resistance of 14.3Ω. Electroluminescence measurements on the semipolar LED yielded a reduced blueshifting of the peak emission wavelength with increasing drive current, compared to a reference commercial c-plane LED. On-wafer measurements yielded an approximately linear increase in output power with drive current, with measured values of 19.3 and 264μW at drive currents of 20 and 250mA, respectively. The external quantum efficiency did not decrease appreciably at high currents. Polarization anisotropy was also observed in the electroluminescence from the semipolar green LED, with the strongest emission intensity parallel to the [12¯10] direction. A polarization ratio of 0.32 was obtained at a drive current of 20mA.