Raoul Schlesser

The growth and optical properties of large, high-quality AlN single crystals

The effect of impurities and defects on the optical properties of AlN was investigated. High-quality AlN single crystals of more than 20mm2 size were examined. Different crucible materials and growth procedures were applied to the growth of bulk AlN by physical vapor transport method to vary the defect and the impurity concentrations. The crystalline orientation was investigated by Raman spectroscopy. Glow discharge mass spectrometry was used to determine the trace concentration of the incorporated impurities such as oxygen and carbon. The photoluminescence emission and absorption properties of the crystals revealed bands around 3.5 and 4.3eV at room temperature. Absorption edges ranging between 4.1 and 5.95eV were observed. Since no straight correlation of the oxygen concentration was obtained, a major contribution of oxygen or oxygen-related impurities was ruled out to generate the observed emission and absorption bands in the Ultraviolet spectral range. The carbon-related impurities and intrinsic defec...

Deep-Ultraviolet Light-Emitting Diodes Fabricated on AlN Substrates Prepared by Hydride Vapor Phase Epitaxy

AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) were fabricated on AlN substrates. The AlN substrates were prepared by growing thick hydride vapor phase epitaxy (HVPE)-AlN layers on bulk AlN substrates prepared by physical vapor transport (PVT). After growing an LED structure, the PVT-AlN substrates were removed by mechanical polishing. This process allowed the fabrication of DUV-LEDs on HVPE-AlN substrates with high crystalline quality and DUV optical transparency. The DUV-LEDs exhibited a single emission peaking at 268 nm through the HVPE-AlN substrates. The output power as high as 28 mW was obtained at an injection current of 250 mA.

Growth and Characterization of AlN and AlGaN Epitaxial Films on AlN Single Crystal Substrates

AlN and AlGaN epitaxial films were deposited by metal organic chemical vapor deposition on single crystal AlN substrates processed from AlN boules grown by physical vapor transport. Structural, chemical, and optical characterization demonstrated the high crystalline quality of the films and interfaces.

Lasing and longitudinal cavity modes in photo-pumped deep ultraviolet AlGaN heterostructures

To unambiguously distinguish lasing from super luminescence, key elements of lasing such as longitudinal cavity modes with narrow line-width, polarized emission, and elliptically shaped far-field pattern, need to be demonstrated at the same time. Here, we show transverse electric polarized lasing at 280.8 nm and 263.9 nm for AlGaN based multi-quantum-wells and double heterojunction structures fabricated on single crystalline AlN substrates. An elliptically shaped far-field pattern was recorded when pumped above threshold. With cavities shorter than 200 μm, well-defined, equally spaced longitudinal modes with line widths as narrow as 0.014 nm were observed. The low threshold pumping density of 84 kW/cm2 suggests that the electrically pumped sub-300 nm ultraviolet laser diodes are imminent.

Preparation of a Freestanding AlN Substrate from a Thick AlN Layer Grown by Hydride Vapor Phase Epitaxy on a Bulk AlN Substrate Prepared by Physical Vapor Transport

The structural and optical quality of a freestanding AlN substrate prepared from a thick AlN layer grown by hydride vapor phase epitaxy (HVPE) on a bulk (0001)AlN substrate prepared by physical vapor transport (PVT) were investigated. The prepared HVPE-AlN substrate was crack- and stress-free. High-resolution X-ray diffraction ω-rocking curves of symmetric (0002) and skew-symmetric (1011) reflections had small full widths at half maximum (FWHMs) of 31 and 32 arcsec, respectively. Deep-ultraviolet optical transparency of the HVPE-AlN substrate was higher than that of the PVT-AlN substrate, which was related to lower concentrations of C, O impurities, and Al vacancy.

Bias voltage dependent field-emission energy distribution analysis of wide band-gap field emitters

We have studied the origin of field emission from wide band-gap semiconductors by a combination of voltage dependent field-emission energy distribution and I–V measurements. For this purpose, tip-shaped molybdenum emitters were coated with 100–1000 nm thick layers of nominally undoped diamond and cubic boron nitride (c-BN) powders. Electron energy spectra revealed that significant band bending occurred due to field penetration into wide band-gap materials. Voltage drops on the order of several volts were measured across the coatings, for applied voltages on the order of 1 kV, and a cathode–gate distance of 500 μm. These voltage drops showed a linear dependence with the applied bias voltage for well-annealed diamond coatings and a strongly nonlinear behavior for unannealed diamond and c-BN coatings. In general, annealing of diamond coated Mo tips led to improved emission current stability and lower “turn-on” voltages due to the removal of oxide and the formation of conductive carbide layers between the met...

Band-edge exciton states in AlN single crystals and epitaxial layers

The band-edge excitonic properties of AlN are investigated using low-temperature (1.7K) optical reflectance and transmission measurements of samples with various crystal orientations. The A, B, and C excitons are found to have energies of 6.025, 6.243, and 6.257eV in unstrained material, which shift with strain. The results are compared to a calculation of exciton energies and oscillator strengths to yield a crystal-field splitting of −230meV in unstrained AlN, in good agreement with previous ab initio calculations.

Synchrotron white beam topography characterization of physical vapor transport grown AlN and ammonothermal GaN

Structural defects in AlN single crystals grown by the sublimation method and GaN single crystals grown by the ammonothermal method are characterized by synchrotron white-beam X-ray topography in conjunction with optical microscopy. AlN platelets are either of (1120) or (0001) type depending on the growth conditions. Dislocation densities of the order of 10 3 cm -2 or lower are observed in some crystals. X-ray topographs reveal the presence of growth sector boundaries, inclusions, and growth dislocations that indicate slight impurity contamination. The 2H crystal structure of GaN single crystals obtained by the ammonothermal method was verified by Laue X-ray pattern analysis. GaN crystals grown are of the order of 1 mm in size and are either (0001) platelets or [0001] prismatic needles. Generally, prismatic needles are characterized by lower degree of mosaicity than (0001) platelets.

Two field-emission states of single-walled carbon nanotubes

Two field-emission states of single-walled carbon nanotubes have been identified according to their respective emission current levels. The state yielding increased emission current has been attributed to the presence of adsorbates on the nanotubes. It was realized that, by application of high electric fields inducing large emission currents, a transition between the two states could be induced. For the high current state, field-emitted electrons originated from states located 1 eV below the Fermi level, as was determined by field-emission energy distribution measurements. This suggested that adsorbates introduced a resonant state on the surface that enhanced the tunneling probability of the electrons. These states are removed when the nanotubes are cleaned by application of a large electric field, thus, decreasing the field-emitted current.

Role of adsorbates in field emission from nanotubes

Two field-emission states of single-walled carbon nanotubes were identified according to their respective emission current levels. The state yielding increased emission current was attributed to the presence of adsorbates on the nanotubes as confirmed by electron emission measurements at different background pressures. Application of high electric fields induced large emission currents and a transition between the two states. During this transition, a current drop to 1/10 of the original value was observed. For the high current state, field-emitted electrons originated from states located below the Fermi level, as was determined by field-emission energy distribution measurements. This suggested that adsorbates introduced a resonant state on the surface that enhanced the tunneling probability of electrons. The adsorbed states are removed at high applied electric fields, presumably due to thermal effects caused by large emission currents. This adsorption/desorption process is completely reversible.