High hole concentrations in AlxGa1−xN become increasingly difficult to obtain as the Al mole fraction increases. The problem is believed to be related to compensation, extended defects, and the band gap of the alloy. Whereas electrical measurements are commonly used to measure hole density, in this work we used electron paramagnetic resonance (EPR) spectroscopy to investigate a defect related to the neutral Mg acceptor. The amount and symmetry of neutral Mg in MOCVD-grown AlxGa1−xN with x = 0 to 0.28 was monitored for films with different dislocation densities and surface conditions. EPR measurements indicated that the amount of neutral Mg decreased by 60% in 900°C-annealed AlxGa1−xN films for x = 0.18 and 0.28 as compared with x = 0.00 and 0.08. A decrease in the angular dependence of the EPR signal accompanied the increased x, suggesting a change in the local environment of the Mg. Neither dislocation density nor annealing conditions contribute to the reduced amount of neutral Mg in samples with the higher Al concentration. Rather, compensation is the simplest explanation of the observations, because a donor could both reduce the number of neutral acceptors and cause the variation in the angular dependence.
We fabricated optically pumped and electrically injected ultraviolet (UV) lasers on reduced-threading-dislocation-density (reduced-TDD) AlGaN templates. The overgrowth of sub-micron-wide mesas in the Al0.32Ga0.68N templates enabled a tenfold reduction in TDD, to (2-3) × 108cm%2. Optical pumping of AlGaN hetero-structures grown on the reduced-TDD templates yielded a low lasing threshold of 34kW/cm2 at 346 nm. Roomtemperature pulsed operation of laser diodes at 353nm was demonstrated, with a threshold of 22.5 kA/cm2. Reduced-TDD templates have been developed across the entire range of AlGaN compositions, presenting a promising approach for extending laser diodes into the deep UV.
We use planar metamaterial resonators to enhance by more than two orders of magnitude the near infrared second harmonic generation obtained from intersubband transitions in III-Nitride heterostructures. The improvement arises from two factors: employing an asymmetric double quantum well design and aligning the resonators' cross-polarized resonances with the intersubband transition energies. The resulting nonlinear metamaterial operates at wavelengths where single photon detection is available, and represents a different class of sources for quantum photonics related phenomena.
Realization of efficient laser diodes with ultra-violet (UV) emission from ∼260-360 nm would enable many applications including fluorescence-based biological agent detection, sterilization, and portable water purification. While InGaN-based laser diodes are well developed down to ∼370 nm, achieving shorter UV wavelengths requires higher Al-content AlGaN alloys with increasing challenges in achieving p-type doping, strain-management, and low threading-dislocation-density (TDD) AlGaN templates. Given these challenges, few groups have reported AlGaN-based edge-emitting laser diodes (LDs) with emission < 355 nm.[1, 2] Most recently, random lasing via Anderson localization in AlGaN nanowire structures has demonstrated a novel approach to realizing deep-UV laser diodes.[3]
Epitaxial (111) MgO films were prepared on (0001) AlxGa1-xN via molecular-beam epitaxy for x=0 to x=0.67. Valence band offsets of MgO to AlxGa1-xN were measured using X-ray photoelectron spectroscopy as 1.65±0.07eV, 1.36±0.05eV, and 1.05±0.09eV for x=0, 0.28, and 0.67, respectively. This yielded conduction band offsets of 2.75eV, 2.39eV, and 1.63eV for x=0, 0.28, and 0.67, respectively. All band offsets measured between MgO and AlxGa1-xN provide a>1eV barrier height to the semiconductor.
Selective layer disordering in an intersubband Al0.028Ga0.972N/AlN superlattice using a silicon nitride (SiNx) capping layer is demonstrated. The SiNx capped superlattice exhibits suppressed layer disordering under high-temperature annealing. Additionally, the rate of layer disordering is reduced with increased SiNx thickness. The layer disordering is caused by Si diffusion, and the SiNx layer inhibits vacancy formation at the crystal surface and ultimately, the movement of Al and Ga atoms across the heterointerfaces. Patterning of the SiNx layer results in selective layer disordering, an attractive method to integrate active and passive III-nitride-based intersubband devices.
The growth temperature dependence of Si doping efficiency and deep level defect formation was investigated for n-type Al0.7Ga0.3N. It was observed that dopant compensation was greatly reduced with reduced growth temperature. Deep level optical spectroscopy and lighted capacitance-voltage were used to understand the role of acceptor-like deep level defects on doping efficiency. Deep level defects were observed at 2.34 eV, 3.56 eV, and 4.74 eV below the conduction band minimum. The latter two deep levels were identified as the major compensators because the reduction in their concentrations at reduced growth temperature correlated closely with the concomitant increase in free electron concentration. Possible mechanisms for the strong growth temperature dependence of deep level formation are considered, including thermodynamically driven compensating defect formation that can arise for a semiconductor with very large band gap energy, such as Al0.7Ga0.3N.