Robert P. Vaudo

Large Free-Standing GaN Substrates by Hydride Vapor Phase Epitaxy and Laser-Induced Liftoff

Free-standing GaN, nearly equal in area to the original 2 inch wafer, was produced from 250–300 µm thick GaN films grown on sapphire by hydride vapor phase epitaxy (HVPE). The thick films were separated from the growth substrate by laser-induced liftoff, using a pulsed laser to thermally decompose a thin layer of GaN at the film-substrate interface. Sequentially scanned pulses were employed and the liftoff was performed at elevated temperature (>600°C) to relieve postgrowth bowing. After liftoff, the bow is only slight or absent in the resulting free GaN.

Characteristics of semi-insulating, Fe-doped GaN substrates

Semi-insulating freestanding GaN substrates were produced by hydride vapor phase epitaxy using intentionally introduced iron impurity atoms to compensate residual donors in GaN. Variable temperature resistivity measurements determined the resistivity of an iron-doped GaN sample to be ∼3 x 10 5 Ω cm at 250°C. The activation energy of the carrier was 0.51 eV and room temperature resistivity was determined to be ∼2 x 10 9 Ω cm at room temperature by linear fitting and extrapolation to room temperature. Near-infrared photoluminescence at 1.6K exhibited sharp emission at 1.3 eV, associated with the 4 T 1 (G) → 6 A 1 (S) internal transition of the Fe 3+ charge state.

Dislocation Distribution and Subgrain Structure of GaN Films Deposited on Sapphire by HVPE and MOVPE

Transmission electron microscopy (TEM) was used to characterize the microstructure in GaN films deposited by two different methods. An 11 μm thick film was deposited directly on a sapphire substrate by HVPE; an 8 μm thick film was deposited on a 15 nm buffer layer of AIN on sapphire by MOVPE. The dislocation densities in the top layer of the HVPE and MOVPE ilms were ∼10 9 cm -2 and ∼5 x 10 9 cm -2 respectively. In the HVPE film this was almost exclusively threading dislocations (TDs), ∼70% of which had edge character. In addition to the TDs, the MOVPE sample also contained an appreciable number of dislocations lying in the basal plane. The microstructure of each film was dominated by a subgrain structure of slightly misoriented cells. In the MOVPE specimen, approximately 90% of the TDs were associated with subgrain walls, whereas only approximately 75% of the dislocations in the HVPE specimen were associated with walls. Both the HVPE and MOVPE samples experienced 40% coarsening of the cells through the thickness of the film. The subgrains of the MOVPE sample were 75% smaller than those in the HVPE sample (350 and 1300 nm, respectively). The average dislocation spacing in the walls was 50% smaller in the MOVPE sample than in the HVPE sample (82 and 180 nm, respectively).

InGaN Double-Heterostructures and Dh-Leds on Hvpe Gan-on-Sapphire Substrates

We report on the growth of InGaN films, and the fabrication and characterization of GaN homojunction LEDs and InGaN double heterostructure (DH) LEDs on HVPE GaNon- sapphire substrates. The use of these substrates facilitates the III-nitrides growth process, as it avoids the use of complicated buffer layers. We have achieved InGaN films with strong and sharp band-to-band photoluminescence (PL) from 370 to 540 nm. Typical In 0.o9Ga0. g9N/GaN DH films had double-crystal XRD FWHM ∼ 300 arcsec, and 400 nm peak PL emission with FWHM ∼ 100 meV. DH-LEDs were fabricated with InGaN layers with various compositions, and produced strong electroluminescence (EL) in the blue and blue/green regions.

InGaN/GaN double heterostructure laser with cleaved facets

Laser action is demonstrated in InGaN/GaN double heterostructures with cleaved facets. Hydride vapor phase epitaxy is used to grow a 10-micrometer-thick buffer layer of GaN on (0001) sapphire, and metal-organic vapor phase epitaxy is used to subsequently grow a GaN/In0.09Ga0.91N/GaN double heterostructure. One-mm-long cavities are produced by cleaving the structure along the (1010) plane of the sapphire substrate. A pulsed Nitrogen laser is used for optical excitation. At room temperature, the laser threshold occurs at an incident power density of 1.3 MW/cm2. Above threshold, the differential quantum efficiency increases by a factor of 34, the emission linewidth decreases to 13.5 meV, and the output becomes highly TE polarized.

InGaN/GaN double-heterostructure LEDs on HVPE GaN-on-sapphire substrates

InGaN double heterostructure light emitting diodes (DH-LEDs) were fabricated on hydride vapor phase epitaxy (HVPE) GaN- on-sapphire substrates. These substrates consisted of a thick HVPE GaN layer grown directly on sapphire and eliminated the need for the growth of a low-temperature buffer layer for GaN epitaxy on sapphire. Homojunction and DH-LEDs have been fabricated with various composition InGaN active regions resulting in strong electroluminescence in the blue, green, and yellow portion of the visible spectra. These devices had turn-on voltages as low as 3.6 volts.