Doru I. Florescu

Ultraviolet nitride LED fabrication for high-flux white LED

The requirements for maximizing the external quantum efficiency of UV nitride LEDs are discussed. It is shown that as the chip wavelength progressively decreases, nitride epi growth on a sapphire substrate becomes advantageous in terms of light extraction. The epilayer requirements for UV LEDs dictate the growth of n-AlGaN, with increasing Al contents, and the growth of UV-transparent p-GaN. It is shown that MOCVD growth in a Emcore D-180 or Ganzilla reactor is ideal for meeting the stringent epilayer requirements. Increasing light extraction efficiency and wall-plug efficiency also requires optimization of the reflecting P-contact. The relative merits of Al- and Ag-based reflecting contacts are discussed. Performance data for UV LEDs on sapphire, for drive currents up to 700 mA is shown. Finally, a practical high power UV-based white lamp is demonstrated.

Doping Dependence of the Thermal Conductivity of Hydride Vapor Phase Epitaxy Grown n-GaN/Sapphire (0001) Using a Scanning Thermal Microscope

The authors have measured the doping concentration dependence of the room temperature thermal conductivity ({kappa}) of two series of n-GaN/sapphire (0001) fabricated by hydride vapor phase epitaxy (HVPE). In both sets {kappa} decreased linearly with log n, the variation being about a factor two decrease in {kappa} for every decade increase in n. {kappa} {approx} 1.95 W/cm-K was obtained for one of the most lightly doped samples (n = 6.9 x 10{sup 16} cm{sup {minus}3}), higher than the previously reported {kappa} {approx} 1.7--1.8 W/cm-K on lateral epitaxial overgrown material [V.A. Asnin et al, Appl. Phys. Lett. 75, 1240 (1999)] and {kappa} {approx} 1.3 W/cm-K on a thick HVPE sample [E.K. Sichel and J.I. Pankove, J. Phys. Chemc. Solids 38, 330 (1977)]. The decrease in the lattice component of {kappa} due to increased phonon scattering from both the impurities and free electrons outweighs the increase in the electronic contribution to {kappa}.

Confinement Effects of Hard Disk Lubricant Investigation by Atomic Force Microscopy

As the critical design requirements decrease, we need to understand how confinement affects the performance of the materials we use. One example of this occurs with the lubricant used on magnetic recording disks to reduce wear between the disk and head reader during unexpected intermittent contact. Lubricants for use in hard drives need to have sufficient reflow and redistribution properties which are stable in a wide range of temperatures experienced in todays hard drive. Since the material properties of the confined system often differ from the bulk properties, the tools needed to properly investigate these systems need to be developed and understood. In this study, we utilized atomic force microscopy (AFM) techniques, to study the confinement effects on lubricant stability with temperature. Our studies have demonstrated distinct changes in adhesion force with changes in the thickness of the confining substrate layer. As adhesion is a component of friction, these changes in adhesion are directly related to lubricant performance and its ability to reduce wear between disk and reader.

Optical and electrical properties of interdigitated InGaN/GaN green light-emitting diodes

Properties of InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) grown by MOCVD on sapphire substrates are investigated over a temperature range from 290 to 340 K. Two types of wafers are used to fabricate the devices: one with Mg dopants in p-type epilayers pre-activated in N2 ambient for 4 min at 800 °C, and the other as-grown, without any pre-activation of Mg acceptors. Measured specific resistances of p-side contacts are 1.49x10-4 Ωcm2 for contacts on pre-activated samples annealed at 650°C for 4 min, and 1.55x10-5 Ωcm2 for contacts on as-grown samples annealed at 600 °C for 30 min. Based on the specific contact resistance experiments, interdigitated LEDs are fabricated using either the standard annealing procedures (separate annealings for p-type conduction activation and for ohmic contact formation) or a single-step annealing process (simultaneous annealing for activation of p-type conduction and for ohmic contact formation). In devices fabricated using the standard annealing procedures, the electroluminescence (EL) peak position at 300 K is at 2.379 eV (~521.3 nm) and the full width at half maximum (FWHM) is ~132 meV, while in devices fabricated using a single-step annealing, the EL peak position shows a red shift by ~10 meV without affecting the FWHM. Over the entire voltage range up to 4 V, tunneling is the dominant carrier transport mechanism. The operating voltage is comparable in both types of LEDs, and the output power of LEDs fabricated using the single-step annealing process is somewhat improved.

Direct AFM Observation of Strain Effects on MOCVD-Grown GaN Epilayer Surface Morphology

In this study, we investigate the dependence of GaN surface morphology on the absolute strain values for thin ( c -plane sapphire substrates of various miscut angles towards the m -plane. Results indicate an excellent correlation between the surface roughness observed employing an AFM tool and epilayer strain values. An overall increase of surface roughness (decrease of atomic terrace width) is found with decreasing compressive strain (epilayer vs. bulk value). In addition, sapphire substrates with increasing miscut angle (0.30 deg) appear to relax the inherent, built-in strain differently in the vertical (growth) direction when compared to just (0.00 deg) substrates. Strain relaxation by typical V-shaped, hexagonal pits is directly imaged through the comparison of surface features inside and outside of pits in the thin GaN epilayer films.

Correlation of Thermal with Structural and Optical Properties of High Quality GaN/Sapphire (0001) Grown by Metalorganic Chemical Vapor Deposition

In this study, we compare three unintentionally doped GaN samples, grown in an EMCORE commercial reactor by metalorganic chemical vapor deposition (MOCVD) on sapphire (0001), with the growth parameters optimized in order to improve the crystalline quality. A direct correlation between higher thermal conductivity values and increasing low temperature (77 K) ratio of the near band edge/deep level photoluminescence (PL) signals, decreasing surface roughness and dislocation density, and narrowing of the full-width at half maximum (FWHM) from X-ray diffraction (XRD) scans was established indicating the importance of growth optimization. The implications of these results for optoelectronic device applications will be discussed.