Recent advances in mid-ultraviolet light-emitting diodes grown pseudomorphically on bulk AlN substrates have led to improved efficiencies and lifetimes. For a 266 nm device an output power of 66 mW at 300 mA has been achieved with an external quantum efficiency of 4.5%. More importantly, the lifetimes of these devices have been increased substantially. Testing of LEDs in both surface mount design (SMD) and TO-39 packages show L<sub>50</sub> lifetimes well in excess of 1,000 hours under a variety of case temperatures and currents. Package-related catastrophic failures are eliminated through encapsulation and hermetic sealing, further reducing failure rates and extending the lifetime.
Native Aluminum Nitride (AlN) single-crystal substrates with ultra-low dislocation density are very promising for use
in III-nitride epitaxial growth required for ultraviolet (UV) electro-optical applications and high power radio frequency
(RF) devices. They offer a better lattice and thermal expansion match to AlGaN alloys, especially those with high Al
content, than foreign substrates such as SiC or sapphire. An additional advantage of bulk substrates is the possibility of
slicing and preparing surfaces with the desired orientation, such as non-polar and pre-determined, specific
misorientations, which will permit the fabrication of devices with specific, special properties. In this paper we present
chemical and electrical characterization of the AlN material. Secondary Ion Mass Spectroscopy (SIMS) measurements
show that oxygen is the main impurity, with concentrations in the order of mid 10<sup>18</sup> cm<sup>-3</sup>. The electrical resistivity of the
AlN was measured, giving a lower limit of 10<sup>12</sup>Ω-cm at room temperature. The prepared surface of substrates with
different orientations, as well as of homo-epitaxial and hetero-epitaxial layers of AlGaN with different Al:Ga ratios
were measured by Atomic Force Microscopy. The observation of atomic steps in the bare substrates and step flow in
the epilayers are an indication of the good surface preparation. The crystalline quality of the epilayers was assessed by
measuring the full width at half maximum (FWHM) of both symmetric and asymmetric X-ray rocking curves.
Relatively intense deep-green/yellow photoluminescence emission at ~600 nm is observed for InGaN/GaN multi quantum well (MQW) structures grown on bulk AlN substrates, demonstrating the potential to extend commercial III-Nitride LED technology to longer wavelengths. Optical spectroscopy has been performed on InGaN MQWs with an estimated In concentration of greater than 50% grown by metalorganic chemical vapor phase epitaxy at 750<sup>o</sup>C. Temperature- and power-dependence, time-resolved photoluminescence as well as spatially resolved cathodoluminescence measurements and transmission electron microscopy have been applied to understand and elucidate the nature of the mechanism responsible for radiative recombination at 600nm as well as higher energy emission band observed in the samples. A comparison between samples grown on bulk AlN and sapphire substrates indicate a lower degree of compositional and/or thickness fluctuation in the latter case. Our results indicate the presence of alloy compositional fluctuation in the active region despite the lower strain expected in the structure contrary to that of low In composition active regions deposited on bulk GaN substrates. Transient photoluminescence measurements signify a stretched exponential followed by a power decay to best fit the luminescence decay indicative of carrier hopping in the active region. Our results point to the fact that at such high In composition (>30%) InGaN compositional fluctuation is still a dominant effect despite lower strain at the substrate-epi interface.
Here, we report on our studies of ballistic electron transport across metal layers and metal/semiconductor interfaces using ballistic-electron-emission microscopy (BEEM). This new technique, which uses a scanning tunneling microscope to inject electrons with a controlled energy into a thin metal film, allows measurements (with spatial resolution approaching 1 nm) of (1) the local Schottky barrier height, (2) ballistic mean free paths of energetic electrons (or holes), and (3) transmission probability of hot carriers across the metal/semiconductor interface. We have measured the attenuation length of hot electrons (1.5 eV above the Fermi level) in PtSi to be approximately 4 nm. This should be compared with an attenuation length of 13 nm for similar energy electrons in Au layers. BEEM images of the Au/Si interface show features on a very small length scale suggesting that the inelastic mean free path of electrons in Au is close to the attenuation length. The SB height (as determined by BEEM) is 0.87 eV in good agreement with optical measurements. We have also used BEEM to observe the sharp onset of inelastic scattering mechanisms in Au/Si and in PtSi/Si. It is our belief that these studies of ballistic carrier transport will allow a fundamental determination of how to achieve higher quantum efficiencies in SBIRDs.
Conference Committee Involvement (1)
GaN Materials and Devices
23 January 2006 | San Jose, California, United States