AlGaN based 229 nm light emitting diodes (LEDs), employing p-type Si to signifificantly increase hole injection, were fabricated on single crystal bulk aluminum nitride (AlN) substrates. Nitride heterostructures were epitaxially deposited by organometallic vapor phase epitaxy and inherit the low dislocation density of the native substrate. Following epitaxy, a p-Si layer is bonded to the heterostructure. LEDs were characterized both electrically and optically. Owing to the low defect density fifilms, large concentration of holes from p-Si, and effificient hole injection, no effificiency droop was observed up to a current density of 76 A/cm2 under continuous wave operation and without external thermal management. An optical output power of 160 lW was obtained with the corresponding external quantum effificiency of 0.03%. This study demonstrates that by adopting p-type Si nanomembrane contacts as a hole injector, practical levels of hole injection can be realized in UV light-emitting diodes with very high Al composition AlGaN quantum wells, enabling emission wavelengths and power levels that were previously inaccessible using traditional p-i-n structures with poor hole injection effificiency. Published by AIP Publishing.
Demand for ultraviolet (UV) light emitting diodes (LEDs) is increasing due to broad applications in biological and chemical detection, decontamination, medical treatment, high density optical recording, and lithography. The group III-nitride material system is the most attractive candidate for UV LEDs spanning UVA, UVB, and UVC owing to its wide bandgap range (GaN: 3.3 eV–AlN: ~6.2 eV). However, as the emission wavelength gets shorter, the external quantum effificiency (EQE) becomes signifificantly degraded. Along with challenges in growth of high Al composition AlxGa1-xN materials with low defect densities, the doping concentration limitations and high ionization energy of acceptors for wide gap AlGaN render the p-side of the diode structure quite resistive and the resulting hole injection effificiency is poor. As a result of the insuffificient hole injection, electron leakage ensues due to imbalance between electrons and holes, which is commonly addressed by the use of an electron blocking layer. However, the EBL approach based on AlN ceases to be effective as the Al composition in the active quantum epistructures is high, reaching nearly 100%. In addition, achieving an Ohmic metal contact to typical p-layers with low contact resistance remains a critical limitation to obtaining an electrically effificient UVC LED. The approach used in this work overcomes both limitations.
A variety of approaches have been deployed to circumvent the fundamental p-type doping challenges, such as polarization doping and tunnel junctions. Both methods require careful control of precursor flfluxes for grading the Al composition during the growth process, which complicates the epitaxy technique. We have reported a 237 nm UV LED using silicon as an effificient hole injector and postulated that shorter wavelength emission would be obtainable using this method. In this paper, a 229 nm wavelength LED operating under continuous wave (CW) drive current is reported using a higher Al composition for the multi-quantum wells (MQWs).
The UV LED structure in Fig. 1(a) was grown on a bulk AlN substrate by low pressure organometallic vapor phase epitaxy (LP-OMVPE) in a custom high-temperature reactor. As shown in Fig. 1(a), following an initial 400 nm AlN homoepitaxial layer on an AlN substrate, a Si doped (concentration: 1x1019 cm3 ) 600 nm n-Al0.7Ga0.3N contact and an electron injection layer were grown prior to the 3-period 3/6 nm Al0.77Ga0.23N/AlN MQW active region. The epitaxial growth was terminated with a 20 nm Mg doped (~4x1019 cm3 ) p-GaN layer to prevent rapid oxidation of the AlN surface, the challenges of which are explained elsewhere. The dopant atomic concentration and free carrier concentration were characterized by secondary ion mass spectrometry (SIMS) and Hall Effect measurements, respectively. Prior to transferring a 100 nm thick, heavily doped single-crystal p-type Si nanomembrane (NM) with a doping concentration of 5x1019 cm3 , a 0.5 nm thick Al2O3 layer, which acts as a quantum tunnel barrier and a passivation layer, was deposited by fifive cycles of an atomic layer deposition (ALD) process using an Ultratech/ Cambridge Nanotech Savannah S200 ALD system.
Fig1
In order to realize substantial light output from LEDs, epitaxial layers with low threading dislocation densities and an atomically flflat surface are highly desirable. Regarding these challenges, bulk AlN substrates were used in our work to grow Al-face high Al composition AlGaN epitaxial heterostructures, reducing the dislocation density by several orders of magnitude compared with layers on non-native substrates, as the epitaxial device layers inherit the low dislocation density (<104 cm3 ) of the single-crystalline bulk AlN.24,25 Reducing surface roughness of the epitaxial layers is especially critical in our scenario with a foreign membrane transfer incorporated during fabrication. Surface roughness above ~2 nm leads to non-uniform bonding between the Si-NM and GaN, which hinders carrier transport across the Si/GaN interface, often resulting in leakage paths and reduced effificiency of the devices.
Given the crucial role of the surface roughness, we characterized the surface of the as-grown epitaxy samples and the epitaxy samples with the Al2O3 layer coated and p-type Si NM bonded using both an optical microscope and a Bruker Catalyst atomic force microscope (AFM). Figure 3(a)(i) shows a fifiltered optical microscopic image of the surface of the AlGaN sample. Figure 3(a)(ii) shows an AFM image of the epitaxial sample surface. The extracted AFM root-mean-square (RMS) surface roughness results marked in Fig. 3(a)(ii) were taken from a 2x 2 lm2 scan area. An averaged RMS roughness of 0.547 nm was measured, and the images show a crack-free surface. The smooth surface allowed the high-yield (100%) transfer of the Si NM to the epitaxy surface and also enabled intimate contact between the Si NM and the top GaN layer.
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