Noble M. Johnson

Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off

Indium–gallium nitride (InGaN) multiple-quantum-well (MQW) light-emitting diode (LED) membranes, prefabricated on sapphire growth substrates, were created using pulsed-excimer laser processing. The thin-film InGaN MQW LED structures, grown on sapphire substrates, were first bonded onto a Si support substrate with an ethyl cyanoacrylate-based adhesive. A single 600 mJ/cm2, 38 ns KrF (248 nm) excimer laser pulse was directed through the transparent sapphire, followed by a low-temperature heat treatment to remove the substrate. Free-standing InGaN LED membranes were then fabricated by immersing the InGaN LED/adhesive/Si structure in acetone to release the device from the supporting Si substrate. The current–voltage characteristics and room-temperature emission spectrum of the LEDs before and after laser lift-off were unchanged.

Interface traps and Pb centers in oxidized (100) silicon wafers

The band‐gap energy distribution of Pb centers on oxidized (100) Si wafers has been determined and compared with interface electrical trap density Dit. Two different Pb centers are observed on (100) Si: Pb0, which has the structure ⋅Si≡Si3, and is essentially identical to the sole Pb center observed on (111) Si; and Pb1, of presently uncertain identity, but clearly different in nature from Pb0. By electric field‐controlled electron paramagnetic resonance (EPR) and capacitance‐voltage (C‐V) measurements, it is found that Pb0 has its (0↔1) electron transition at Ev+0.3 eV and its (1↔2) transition at Ev+0.85 eV. Similarly, Pb1 has its (0↔1) transition at Ev+0.45 eV and its (1↔2) transition at Ev+0.8 eV. The Pb band‐gap density correlates qualitatively and quantitatively with the electrical trap density Dit from C‐V analysis; nonbonded Pb orbitals are found to be the source of about 50% of the characteristic traps in dry‐oxidized, unannealed (100) Si wafers.

Deuterium passivation of grain‐boundary dangling bonds in silicon thin films

Hydrogen passivation of silicon grain boundaries has been investigated by using deuterium as a readily identifiable isotope which duplicates hydrogen chemistry. Deuterium detection with high sensitivity was achieved with secondary‐ion mass spectrometry. Diffusion of deuterium in single‐ crystal silicon and polycrystalline silicon thin films at low temperatures (e.g., 350 °C) clearly demonstrates enhanced diffusion along grain boundaries. Defects at grain boundaries were detected by electron‐spin resonance and identified as silicon‐dangling bonds. Deuterium passivation of grain boundaries is revealed by correlated deuterium diffusion and dangling‐bond annihilation in polycrystalline silicon films.

Unidirectional lasing from InGaN multiple-quantum-well spiral-shaped micropillars

We report unidirectional emission from lasing in In0.09Ga0.91N/In0.01Ga0.99N multiple-quantum-well spiral micropillars. Our imaging technique shows that the maximum emission comes from the notch of the spiral microcavities at an angle about 40° from the normal of the notch. At room temperature, the spiral microcavity lases near 400 nm when optically pumped with 266 or 355 nm light. A reduction in the lasing threshold and an improvement in unidirectionality occurs when the microcavity is selectively pumped near its boundary.

InxGa1−xN light emitting diodes on Si substrates fabricated by Pd–In metal bonding and laser lift-off

Indium–gallium nitride (InxGa1−xN) single-quantum-well (SQW) light emitting diodes (LEDs), grown by metalorganic chemical vapor deposition on sapphire, were transferred onto Si substrates. The thin-film InxGa1−xN SQW LED structures were first bonded onto a n+-Si substrate using a transient-liquid-phase Pd–In wafer-bonding process followed by a laser lift-off technique to remove the sapphire growth substrate. Individual, 250×250 μm2, LEDs with a backside contact through the n+-Si substrate were then fabricated. The LEDs had a typical turn-on voltage of 2.5 V and a forward current of 100 mA at 5.4 V. The room-temperature emission peak for the InxGa1−xN SQW LEDs was centered at 455 nm with a full width at half maximum of 19 nm.

Deep level defects in n‐type GaN

In n‐type GaN grown by metalorganic chemical vapor deposition two new electronic defects were detected and characterized by deep level transient spectroscopy (DLTS). Schottky‐barrier diodes with Ohmic back contacts and low series resistance were fabricated in GaN layers grown on sapphire. The diodes display well behaved current‐voltage and capacitance‐voltage characteristics and permit unambiguous DLTS evaluation. The new deep levels display thermal activation energies for electron emission of 0.49 and 0.18 eV.

PHASE SEPARATION IN InGaN/GaN MULTIPLE QUANTUM WELLS

Evidence is presented for phase separation in In0.27Ga0.73N/GaN multiple quantum wells. After annealing for 40 h at a temperature of 950 °C, the absorption threshold at 2.95 eV is replaced by a broad peak at 2.65 eV. This peak is attributed to the formation of In-rich InGaN phases in the active region. X-ray diffraction measurements show a shift in the diffraction peaks toward GaN, consistent with the formation of an In-poor phase. A diffraction peak corresponding to an In-rich phase is also present in the annealed material. Nanoscale In-rich InGaN precipitates are observed by transmission electron microscopy and energy dispersive x-ray chemical analysis.

Self‐limiting oxidation of Si nanowires

Achieving tolerances of the order of 1 nm for sub‐10 nm structures is both challenging and necessary for controlled experiments on such structures. Here the use of a self‐limiting oxidation reaction to yield silicon (Si) wires of less than 10 nm diam with a tolerance of ±1 nm over 0.5 μm. The final self‐limiting diameters were found to be controlled by oxidation temperature. For 30 nm initial Si column diameters, the asymptotic diameters were found to be 11 and 6 nm for dry oxidation at 800 and 850 °C, respectively. The mechanism of the self‐limiting reaction is not yet fully understood but the tiny radius of curvature is obviously a factor. In addition, there appears to be an anomalous loss of Si; this may be due to sublimation of SiO.

Hydrogenation of p‐type gallium nitride

Hole concentrations of up to 1019 cm−3 have been reported for GaN:Mg films grown by molecular beam epitaxy without any post‐growth treatment. Comparing results from Hall measurements and secondary ion mass spectrometry, we observe doping efficiencies of up to 10% at room temperatures in such p‐type material. By hydrogenating at temperatures above 500 °C, the hole concentration can be reduced by an order of magnitude. A new photoluminescence line at 3.35 eV is observed after this treatment, both in p‐type and unintentionally doped n‐type material, which suggests the introduction of a hydrogen‐related donor level in GaN.

Hydrogen passivation of Mg acceptors in GaN grown by metalorganic chemical vapor deposition

The effects of the deliberate hydrogenation of GaN were investigated for heteroepitaxial layers grown by metalorganic chemical vapor deposition. The GaN layers were either Mg‐doped, p‐type after thermal activation, or Si‐doped, n type. Elemental depth profiles from secondary ion mass spectroscopy reveal a striking contrast after a deuteration at 600 °C: the deuterium concentration in Mg‐doped GaN is ∼1019 cm−3 while there is no detectable deuterium incorporation in the n‐type material. Variable temperature Hall effect measurements provide the most direct evidence to date for Mg–H complex formation with the decrease in the hole concentration upon hydrogenation accompanied by an increase in the hole Hall mobility.