Bulk Aluminum Nitride Substrates Research Articles

Prismatic Slip in AlN Crystals Grown By PVT

Aluminum nitride (AlN) single crystals are desired as substrates for nitride based electronic and optoelectronic applications[1]. A low density of dislocations and other structural defects in bulk AlN substrates is in demand to grow high-quality heteroepitaxial epilayers. Physical vapor transport (PVT) grown AlN crystals possess low density dislocations and other structural defects[2]. In AlN crystals grown by PVT, basal plane slip is the most frequently observed deformation mechanism. However, prismatic slip takes place as well in such crystals [3, 4].When the diameter of AlN wafers expands to 50 mm and larger, managing the thermal gradients in the PVT growth chamber becomes critical for reducing thermal stresses that cause deformation. Recent synchrotron X-ray topography [5] studies of 50 mm diameter AlN wafers showed that while most wafers contained few to no basal plane dislocations (BPDs), some wafers possessed a 6-fold BPD dislocation pattern [2] which was aligned along the <11-20> directions. This configuration indicated that prismatic slip had likely occurred. Screw typed prismatic dislocations cross slipped from the prismatic plane onto the basal plane, and underwent basal plane slip, leading to dislocation multiplication. To investigate the origins of prismatic slip, a radial thermal gradient model was established showing[6] the resolved shear stress across the entire area of the crystal boule during growth. The results from the model were compared with experimental observations to investigate the role of radial thermal gradients in the nucleation of prismatic slip in AlN during bulk growth. This insight will help guide efforts to minimize structural defects in PVT AlN.

Manufacturability of high power ultraviolet-C light emitting diodes on bulk aluminum nitride substrates

As the technological challenges in obtaining high power ultraviolet-C light emitting diodes (LEDs) on bulk AlN substrates are being overcome, the next challenge faced is the manufacturability. One of these challenges consists of introducing a 10×10mm substrate into a fabrication facility that is processing 2” and larger diameter substrates. This has been successfully carried out using a standard visible LED production facility that is involved in high volume manufacturing of blue LEDs. By introducing specifications on the substrates, the epitaxial wafers, and the fabrication process, the establishment of a pilot production process with pathways to high volume manufacturing has been established.

Reliability and performance of pseudomorphic ultraviolet light emitting diodes on bulk aluminum nitride substrates

AbstractHigh quality, bulk aluminum nitride substrates were used to obtain pseudomorphic AlxGa1‐xN layers with low dislocation density, smooth surfaces, and high conductivity. These layers were fabricated into mid‐ultraviolet light emitting diodes with peak wavelengths in the range of 240‐260 nm. From transmission electron microscope images, it was confirmed that the dislocation density in the n‐type Al0.7Ga0.3N layer is low with no dislocations in the field of view. The low dislocation density continued through the quantum wells and electron blocking layer. However, the lattice mismatch between the pseudomorphic AlxGa1‐xN layers and the p‐type GaN contact layer is high (approximately 2.4%) and the pseudomorphic growth could not be achieved for thick layers. Instead, a network of misfit dislocations was formed at the GaN/AlxGa1–xN interface, which relieved strain in the GaN layer. The threading dislocations did not appear to propagate into the active region and thus enable the achievement of high internal (IQE) and external quantum efficiencies (EQE) from the devices fabricated on low dislocation density pseudomorphic active layers. Values of EQE as high as 1.44% for a device emitting at 248 nm operated in pulsed mode were achieved. Devices were packaged in lead frame packages and achieved output powers of 45 mW at 1.5 A for a 257 nm device with a peak EQE of 0.8% at 200 mA when operated in pulsed mode. The thermal properties of the packaged devices limited high current performance in CW mode. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

UV transparent single‐crystalline bulk AlN substrates

AbstractBulk aluminum nitride (AlN) is a very promising substrate material for UV optoelectronics, and its UV transparency is of high interest for UV devices designed to emit through the substrate. We report on 500 μm thick bulk AlN substrates with plain UV transmittance exceeding 50% (i.e., absorption coefficients below 14 cm–1) for wavelengths from 220 nm to 380 nm in the main wafer area. Comparing the spectra of different AlN bulk samples, four major below band‐gap absorption bands are identified and interpreted based on a point defect model for AlN crystals. We conclude that a further reduction of oxygen contamination of the source material led to a significant decrease of UV absorption in both colorless and yellowish areas of those substrates. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Reliability and Performance of Pseudomorphic Ultraviolet Light Emitting Diodes on Bulk Aluminum Nitride Substrates

AbstractLow dislocation density epitaxial layers of AlxGa1-xN can be grown pseudomorphically on c-face AlN substrates prepared from high quality, bulk crystals. Here, we will report on initial characterization results from deep ultraviolet (UV) light emitting diodes (LEDs) which have been fabricated and packaged from these structures. As reported previously, pseudomorphic growth and atomically smooth surfaces can be achieved for a full LED device structure with an emission wavelength between 250 nm and 280 nm.A benefit of pseudomorphic growth is the ability to run the devices at high input powers and current densities. The high aluminum content AlxGa1-xN (x∼70%) epitaxial layer can be doped n-type to obtain sheet resistances &lt; 200 Ohms/sq/μm due to the low dislocation density. Bulk crystal growth allows for the ability to fabricate substrates of both polar and non-polar orientations. Non-polar substrates are of particular interest for nitride growth because they eliminate electric field due to spontaneous polarization and piezoelectric effects which limit device performance. Initial studies of epitaxial growth on non-polar substrates will also be presented.

Reliability and Performance of Pseudomorphic Ultraviolet Light Emitting Diodes on Bulk Aluminum Nitride Substrates

AbstractReliability and performance of ultraviolet light emitting diodes have suffered due to the high dislocation density of the AlN and high Al-content AlxGa1-xN layers when grown on foreign substrates such as sapphire. The development of pseudomorphic layers on low dislocation density AlN substrates is leading to improvements in reliability and performance of devices operating in the ultraviolet-C (UVC) range. One major improvement is the ability to operate devices at much higher current densities and input powers than devices on sapphire substrates. This is due to the better thermal properties and lower dislocation density of devices on AlN substrates. Devices with active area of 0.001 cm2 emitting at ∼265 nm have been measured for their reliability and change in power output over time at input currents of 20 mA (20 A/cm2), 100 mA (100A/cm2) and 150 mA (150 A/cm2). When operating at currents of 20 mA over 3500 hours of consecutive operation has been demonstrated with typical decay of ∼27% over the 3500 hours. Extrapolating the decay with a linear fit gives a L50 (time to 50% of initial power) of ∼5000 hrs. However it is desirable to be able to model the decay to better understand the kinetics and better understand the mechanisms. In order to do this, the lifetime at 20 mA and 100 mA were modeled using an exponential decay function, square root transformation and a log transformation to both be able to fit the experimental data and predict future performance.

Elastic constants of aluminum nitride

AbstractWe report on the application of Brillouin spectroscopy as an approach to non‐destructive optical characterization of the elastic constants of semiconductors with the wurtzite symmetry. Three different configurations were used to achieve a complete determination of the elastic stiffness constants of bulk AlN substrates grown by the Physical Vapor Transport (PVT) method. The scattering diagrams of these three configurations are presented showing the geometrical arrangements necessary to observe all the elastic stiffness constants for the partially nontransparent wurtzite type of the crystal structure. Because aluminum nitride (AlN) is a suitable material for the fabrication of light emitting devices, the characterization of its elastic constants was carried out very precisely to provide a reliable data which can be used for the determination of residual stress arising during the growth of AlN thin films or wide band gap semiconductor thin films on substrates of AlN. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)