Kyoungnae Lee

Molecular beam epitaxy of YMnO3 on c-plane GaN

Epitaxial YMnO3 films were grown on (0001) GaN-on-sapphire templates using molecular beam epitaxy. The YMnO3 maintained the (0001) orientation with an in-plane YMnO3∕GaN epitaxial relationship of (0001)‖(0001); [11¯00]‖[112¯0]. The YMnO3 was ferroelectric at room temperature with a remanent polarization of ∼3.2μC∕cm2 and a saturation polarization of ∼12μC∕cm2. This heterostructure is a promising candidate for multifunctional structures that integrate ferroelectrics with GaN-based high-power and short-wavelength light-emitting devices.

High temperature limitations for GaN growth by rf-plasma assisted molecular beam epitaxy: Effects of active nitrogen species, surface polarity, hydrogen, and excess Ga-overpressure

The temperature used for growth of GaN by molecular beam epitaxy is ultimately limited by the greatly reduced growth rate related to thermal decomposition. This limiting temperature apparently varies from group to group. Factors influencing thermal decomposition are growth species (atomic versus metastable molecular nitrogen), surface polarity (N- versus Ga-polar), the presence of atomic hydrogen, and varying Ga-overpressure. Surface polarity and growth species are the predominant influence determining the onset of thermal decomposition. There are indications that the use of a significant Ga-overpressure can suppress decomposition allowing for an increase in obtainable growth temperatures for a given polarity. Electrical properties are shown to be strongly influenced by Ga-overpressure and thermal decomposition.

Harmonic surface acoustic waves on gallium nitride thin films

SAW devices operating at the fundamental frequency and the 5th, 7th, 9th, and 11th harmonics have been designed, fabricated, and measured. Devices were fabricated on GaN thin films on sapphire substrates, which were grown via metal organic vapor phase epitaxy (MOVPE). Operating frequencies of 230, 962, 1338, 1720, and 2100 MHz were achieved with devices that had a fundamental wavelength, /spl lambda/0 = 20 /spl mu/m. Gigahertz operation is realized with relatively large interdigital transducers that do not require complicated submicrometer fabrication techniques. SAW devices fabricated on the GaN/sapphire bilayer have an anisotropic propagation when the wavelength is longer than the GaN film thickness. It is shown that for GaN thin films, where khGaN > 10 (k = 2/spl pi///spl lambda/ and hGaN = GaN film thickness), effects of the substrate on the SAW propagation are eliminated. Bulk mode suppression at harmonic operation is also demonstrated.

Improvement in the Light Extraction of Blue InGaN/GaN-Based LEDs Using Patterned Metal Contacts

We demonstrate a method to improve the light extraction from an LED using photonic crystal (PhC)-like structures in metal contacts. A patterned metal contact with an array of Silicon Oxide (SiOx) pillars (440 nm in size) on an InGaN/GaN-based MQW LED has shown to increase output illumination uniformity through experimental characterization. Structural methods of improving light extraction using transparent contacts or dielectric photonic crystals typically require a tradeoff between improving light extraction and optimal electrical characteristics. The method presented here provides an alternate solution to provide a 15% directional improvement (surface normal) in the radiation profile and ~ 30% increase in the respective intensity profile without affecting the electrical characteristics of the device. Electron beam patterning of hydrogen silesquioxane (HSQ), a novel electron beam resist is used in patterning these metal contacts. After patterning, thermal curing of the patterned resist is done to form SiOx pillars. These SiOx pillars aid as a mask for transferring the pattern to the p-metal contact. Electrical and optical characterization results of LEDs fabricated with and without patterned contacts are presented. We present the radiation and intensity profiles of the planar and patterned devices extracted using Matlab-based image analysis technique from 200 μm (diameter) circular unpackaged LEDs.

The Use of Cathodoluminescence in Gallium Nitride During Growth to Determine Substrate Temperature

The Use of Cathodoluminescence in Gallium Nitride During Growth to Determine Substrate Temperature Accurate measurement of the substrate temperature during growth of gallium nitride by molecular beam epitaxy is crucial. Typically, thermocouples are usually used to measure the temperature of the back side of block which is holding the substrate. Alternatively, pyrometers are often used. However, there is a big range of an error. In-situ cathodoluminescence (CL) occurring during reflection high energy electron diffraction is a strong candidate to determine the growth temperature. The electron beam supplied by our RHEED gun has an energy of 13keV which was used for each measurement. CL was easily detected up to and beyond typical growth temperatures. The CL was directed into a monochromator using fiber optics. The final signal was detected with a photomultiplier tube. This technique appears quite useful to accurately and reproducably determine substrate temperature during growth. The CL could also be observed using a ccd camera. Thus, we investigated using the CL to image the sample during growth. This could be used to see temperature inhomogenaities, and potentially to map alloy composition fluctuations. We calibrated the wavelength vs. growth temperature by using narrow band-pass interference filters. Background subtraction with blanking the e-beam could be used to remove black body radiation and other undesired sources of light. For gallium nitride, the photon energy at the growth temperature of 750°C is about 3.0eV. Using different filters, we can take a picture of e-beam on the surface of substrate for each filter and analyze the peak intensity using the line profile. We will present CL images of various samples at differing temperatures. This work was supported by the AFOSR MURI F49620-03-1-0330 monitored by Todd Steiner and Gerald Witt and by ONR Grants N00014-02-1-0974 and N00014-01-1-0571, both monitored by Colin E. C. Wood.