Mohamed Lachab

Investigation on the P-Type Activation Mechanism in Mg-doped GaN Films Grown by Metalorganic Chemical Vapor Deposition

An investigation on the p-type activation in Mg-doped GaN epilayers has been carried out in relation to the defect structure. The samples were grown by the metalorganic chemical vapor deposition method. Sapphire with (0001) orientation (C-face) was used as the substrate. After growth, the samples were heat-treated under flowing N2, at temperatures ranging from 600 to 850°C. The p-type activation arises from the dissociation of electrically inactive Mg–H complexes and the neutralization of the dissociated H+ during the annealing process. The annealing temperature dependence of hole concentration and hole mobility was studied. The p-type activation process resulted in a different maximum hole concentration and an optimum annealing temperature. Subsequent microstructural characterization of our samples revealed that the dislocations play a key role in p-type conductivity and may explain the difference observed in the electrical properties. Indeed, the analyses of transmission electron microscopy (TEM) images and X-ray diffraction (XRD) data show that Mg-doped GaN exhibits a different X-ray rocking curve full width at half maximum (FWHM) and dislocation density. Furthermore, it was found that the higher the dislocation density, the higher the hole concentration. Therefore, we suggest that dislocations could act as a migration path or a neutralizing source for dissociated hydrogen impurities.

276 nm Substrate-Free Flip-Chip AlGaN Light-Emitting Diodes

Lateral-conduction, substrate-free flip-chip (SFFC) light-emitting diodes (LEDs) with peak emission at 276 nm are demonstrated for the first time. The AlGaN multiple quantum well LED structures were grown by metal?organic chemical vapor deposition (MOCVD) on thick-AlN laterally overgrown on sapphire substrates. To fabricate the SFFC LEDs, a newly-developed laser-assisted ablation process was employed to separate the substrate from the LED chips. The chips had physical dimensions of 1100?900 ?m2, and were comprised of four devices each with a 100?100 ?m2 junction area. Electrical and optical characterization of the devices revealed no noticeable degradation to their performance due to the laser-lift-off process.

First Demonstration of Semipolar Deep Ultraviolet Light Emitting Diode on m-Plane Sapphire with AlGaN Multiple Quantum Wells

We report on the first demonstration of a semipolar AlGaN based deep ultraviolet (UV) light emitting diode (LED), with a peak emission wavelength of 307 nm. The LED structure was grown on an m-plane sapphire substrate using metal organic chemical vapor deposition (MOCVD). A combination of pulsed MOCVD (PMOCVD) grown AlN and a short period AlN/AlGaN superlattice structure was found to be instrumental in achieving singular semipolar structural phase of (1122) with a defect density low enough to fabricate the light emitting device. The on-wafer optical output power of the fabricated LED was ~20 µW at a dc pump current of 20 mA.

A Hybrid Micro-Pixel Based Deep Ultraviolet Light-Emitting Diode Lamp

We report on the room temperature electrical and optical characterization of a multichip light-emitting diode (LED) lamp with peak emission at 281 nm. Four pairs of micro-pixel LED arrays were connected in series to fabricate the lamp, which delivered a pulsed output power of 235 mW at 1.18 A (duty cycle ~0.5%), and attained a high external quantum efficiency of 4.63%. Under dc operation, the maximum power achieved by this lamp was ~20 mW at a drive current of 220 mA. The peak output power improved 1.62-fold after a thermoelectric cooler was added to the device packaging assembly.

Enhancement-Mode Insulating-Gate AlInN/AlN/GaN Heterostructure Field-Effect Transistors with Threshold Voltage in Excess of +1.5 V

This letter presents the dc characteristics of normally Off AlInN/AlN/GaN metal–oxide–semiconductor heterostructure field-effect transistors (MOS-HFETs). The devices were fabricated using a recessed gate and SiON dielectric layers for gate isolation. For a device with a 1.5 µm gate length and an 8-µm-long channel, the threshold voltage was above +1.5 V and a maximum drain current density of 0.7 A/mm was reached under 6 V gate bias. These enhancement-mode MOS-HFETs have an excellent potential for power electronics applications.

Effect of Oxygen on the Activation of Mg Acceptor in GaN Epilayers Grown by Metalorganic Chemical Vapor Deposition

The effect of oxygen mixed in nitrogen on p-type activation in Mg-doped GaN epilayers grown by metalorganic chemical vapor deposition (MOCVD) was investigated. The samples annealed in N2/O2 (1%) ambient exhibited the best electrical properties with respect to hole concentration. SIMS data suggested that oxygen reacted with hydrogen present in the Mg-doped GaN samples during the thermal annealing process, thereby enhancing the activation of Mg acceptors.

Deep ultraviolet photopumped stimulated emission from partially relaxed AlGaN multiple quantum well heterostructures grown on sapphire substrates

A low threshold optical power density of 240 kW/cm2 is achieved for room temperature stimulated emission at 276 nm in AlGaN/AlGaN multiple quantum well (MQW) structures over AlN/sapphire templates. The heterostructures were grown by low-pressure metalorganic chemical vapor deposition whereas a pulsed ArF excimer laser (λexc = 193 nm) was used as the pumping source for photoluminescence measurements in edge configuration. The light emitted from the MQWs above threshold exhibits a minimum linewidth of 1.2 nm and is dominated by transverse electric polarization above threshold. The optical confinement factor in the active region was calculated to be about 2%.

Ultraviolet Stimulated Emission From Optically Pumped AlGaN/AlGaN Multiple Quantum Wells on Pulsed Laterally Overgrown

We report the room temperature photoluminescence properties of AlGaN/AlGaN multiple quantum well (MQW) structures grown on high-quality a-plane GaN pillars oriented along the m-axis. The non-coalesced thick GaN films were laterally overgrown on r-plane sapphire at 900 °C using pulsed metalorganic chemical vapor deposition. This growth approach relies on the modulation of the nitrogen source supply while keeping the Ga precursor flow constant. Optically pumped stimulated emission (SE) from the AlGaN MQWs was demonstrated at ~355.5 nm in the edge-emitting geometry. Unlike their c-plane counterparts, these nonpolar quantum structures exhibited SE that was predominantly polarized in the transverse magnetic mode.

a

We demonstrate the capability to control the optical polarization of room-temperature stimulated emissions (SEs) at 238–239 nm from optically pumped AlGaN multiple-quantum-well (MQW) heterostructures on bulk AlN. The results of structural and optical characterizations provided evidence that altering the strain state in the pseudomorphically grown MQW laser structures enabled the switching of the polarization direction of the SE from predominantly transverse electric (TE) at 238 nm to predominantly transverse magnetic (TM) at 239 nm. The SE observed at 238 nm represents the shortest peak wavelength with TE polarization yet reported for AlGaN materials grown on any type of substrate.

-Plane GaN

285 nm stimulated emission (SE) is demonstrated at room temperature from optically-pumped AlGaN multiple quantum wells (MQWs) deposited between a Si-doped AlGaN current spreading layer and a 25 nm Mg-doped p-type AlGaN clad layer. The epitaxial structures were grown by low pressure metalorganic chemical vapor deposition (MOCVD) on high-quality AlN/sapphire templates and a pulsed ArF excimer laser was used as the excitation source for lasing experiments. The threshold power density (Pth) was measured to be 970 kW/cm2 and the SE was strongly polarized in transverse electric (TE) mode. The minimum emission linewidth was ~2 nm. The high value of Pth is primarily attributed to optical losses due to pump laser absorption in the top p-AlGaN. Our results show the viability of the MOCVD growth technology in conjunction with c-plane sapphire substrates to develop electrically-pumped deep-UV laser diodes.