Karim S. Boutros

Capacitance–voltage characterization of AlN/GaN metal–insulator–semiconductor structures grown on sapphire substrate by metalorganic chemical vapor deposition

Electrical characterization of AlN/GaN interfaces was carried out by the capacitance–voltage (C–V) technique in materials grown by metalorganic chemical vapor deposition. The high-frequency C–V characteristics showed clear deep-depletion behavior at room temperature, and the doping density derived from the slope of 1/C2 plots under the deep depletion condition agreed well with the growth design parameters. A low value of interface state density Dit of 1×1011u200acm−2u200aeV−1 or less around the energy position of Ec−0.8u200aeV was demonstrated, in agreement with an average Dit value estimated from photoassisted C–V characteristics.

Current limitation after pinch-off in AlGaN/GaN FETs

Piezoelectric AlGaN/GaN FETs on SiC with high carrier mobility have been fabricated yielding I DS =450 mA/mm and g m =200 mS/mm. In the on-state, under UV-illumination, the devices sustain a drain voltage of V DS =49 V, corresponding to a power dissipation of 26.5 W/mm. On turn-on of the device from the pinch-off state, a significant delay in the drain current build-up is observed. This effect depends on the pinch-off time and the pinch-off voltage and can be removed by either a brief UV-illumination or a V DS >25 V applied in the on-state. The drain current transients are characterized by a relaxation time τ, which is in the order of several hundred seconds. From the temperature dependence of τ, an activation energy of about 280 meV and a capture cross section of 4.4·10 −18 cm 2 were determined. The devices show pronounced persistent photoconductivity (PPC) and the drain current I D is sensitive to illumination.

Long Time-Constant Trap Effects in Nitride Heterostructure Field Effect Transistors

Current collapse effects in an Al 0.25Ga0.75N/GaN HFET have been investigated under pulsed bias conditions, and a detailed investigation of current responses to changes in drain or gate bias voltage (drain-lag and gate-lag, respectively) has been performed. Three components of transient current response to changes in drain and gate bias voltages are distinguished. Surface treatment using KOH etching and the influence of pulsed bias conditions on threshold voltage are investigated to explore the origins of traps associated with each current transient component.

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.

Yellow photoluminescence in MOCVD-grown n-type GaN

The near-band gap and the yellow optical transitions in n- type GaN grown by MOCVD have been studied by photoluminescence experiments. The excitation density ranged from 5 X 10-6 W/cm-2 to 50 W/cm-2. The UV PL intensity increases linearly in the entire range of excitation density. The yellow PL intensity exhibits a linear dependence oat low excitation densities and a square-root dependence at high excitation densities. A theoretical model is developed describing the intensity of the two radiative transition between continuum states and one defect level deep in the band gap as a function of the excitation density, free carrier and defect concentrations. The calculated dependences of the two luminescence channels follow power laws with exponents of 1/2 and 1 depending on excitation density. These dependences are in very good agreement with experimental results. The measured intensity of the yellow luminescence does not saturate at high excitation densities. This rules out the possibility that the yellow PL could arise from a sequential transition via two deep levels in the gap. It is shown that the intensity modulation that frequently appears in the PL spectra is caused by a micro-cavity which is formed by the semiconductor-substrate and semiconductor-air interfaces. Finally, the dependence of the yellow luminescence intensity on n-type doping concentration indicates that the deep center causing the yellow luminescence is an acceptor-like level.

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.