Properties of InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) grown by MOCVD on sapphire substrates are investigated over a temperature range from 290 to 340 K. Two types of wafers are used to fabricate the devices: one with Mg dopants in <i>p</i>-type epilayers pre-activated in N<sub>2</sub> ambient for 4 min at 800 °C, and the other as-grown, without any pre-activation of Mg acceptors. Measured specific resistances of <i>p</i>-side contacts are 1.49x10<sup>-4</sup> Ωcm<sup>2</sup> for contacts on pre-activated samples annealed at 650°C for 4 min, and 1.55x10<sup>-5</sup> Ωcm<sup>2</sup> for contacts on as-grown samples annealed at 600 °C for 30 min. Based on the specific contact resistance experiments, interdigitated LEDs are fabricated using either the standard annealing procedures (separate annealings for <i>p</i>-type conduction activation and for ohmic contact formation) or a single-step annealing process (simultaneous annealing for activation of <i>p</i>-type conduction and for ohmic contact formation). In devices fabricated using the standard annealing procedures, the electroluminescence (EL) peak position at 300 K is at 2.379 eV (~521.3 nm) and the full width at half maximum (FWHM) is ~132 meV, while in devices fabricated using a single-step annealing, the EL peak position shows a red shift by ~10 meV without affecting the FWHM. Over the entire voltage range up to 4 V, tunneling is the dominant carrier transport mechanism. The operating voltage is comparable in both types of LEDs, and the output power of LEDs fabricated using the single-step annealing process is somewhat improved.
Properties of InGaN/AlInGaN/AlGaN single- and multiple-quantum-well (MQW) light-emitting diodes grown by MOCVD on sapphire substrates are investigated over a wide temperature range from 12 to 298 K. The room-temperature (RT) UV emission band, observed in both single-quantum-well (SQW) and MQW samples, is at 3.307 eV (375 nm) and its full width at half maximum is ~82 meV. In addition to the UV band, a blue emission band at 2.96 eV (419 nm) is observed in SQW samples. The relative intensities of these UV and blue emission bands depend on the injection current. We attribute the blue emission to the carrier overflow over the quantum well (QW) and subsequent radiative recombination involving a Mg-related-level in p-GaN. In MQW LEDs, we observe an anomalous temperature-induced "blue jump" between 170-190 K, with the main emission peak switching from blue to UV. The blue band emission dominates below 170 K, and is practically absent at RT. Thus, we demonstrate a significant advantage in utilizing MQW structures that provide a more effective capture of injected carriers into the QWs.
High quality InAlGaN alloys, quantum wells and associated light emitting diodes have been grown by metalorganic chemical vapor deposition for ultraviolet (UV) emitters. In-situ reflection and ex-situ atomic force microscopy measurements show that InAlGaN epilayers and structures have good surface morphology. InAlGaN epilayers have also a narrow (0006) reflection X-ray diffraction rocking curve linewidth of ~ 340 arcsec and a strong band edge photoluminescence (PL) emission peak from 320 nm to 355 nm at room temperature. Several X-ray satellite peaks were observed from InAlGaN based quantum well structures, revealing that they were periodic with good interfaces. PL mapping measurements of the quantum well structures show excellent wavelength uniformity over a 2" wafer with a standard deviation of ~ 0.4% for structures emitting from 351-372 nm. Ultraviolet light emitting diodes (UV LEDs) based on the same InAlGaN quantum well structures have an electroluminescence (EL) emission at ~ 375 nm with a linewidth of ~10 nm and an excellent wavelength uniformity of less than 1 nm across a 2" wafer. Temperature dependent study of EL spectrum from an UV LED shows "blue jump" from a broad blue emission at <170 K to a narrow UV emission at higher temperatures.
Laser annealing, laser surface processing and laser lift-off procedure are reviewed as applied to semiconductor nitride- based structures (GaN films and InGaN/GaN optoelectronic device structures grown on sapphire substrates). Data on laser ablation of composite GaN/sapphire material are reviewed with more detailed consideration of the ablation rate under subpicosecond laser pulses and under long-pulse irradiation in the IR range (wavelengths of 5.0-5.8 and 9.6 micrometers ).
In order to achieve controlled degree of intermixing in selected areas (CISA), SiO<SUB>2</SUB> gratings are checked first to be able to influence the degree of intermixing during high-temperature rapid thermal annealing of InGaAs/GaAs quantum wells. Subsequently, SiO<SUB>2</SUB>/MgF<SUB>2</SUB> gratings with different periods are used to cover different parts of MWQ sample and found to be suitable for achieving CISA after only a single annealing procedure.
Experimental data on photoluminescence of various bulk and quantum-well epitaxial InGaN/GaN structures grown by MOCVD are interpreted in terms of a band-tail model of inhomogeneously broadened radiative recombination. The anomalous temperature-induced blue spectral is shown to result from band-tail recombination under non-degenerate conditions. Significant differences are observed between epilayers grown on sapphire substrates and on GaN substrates prepared by the sublimination method, with no apparent evidence of band tails in homoepitaxial structures, indicating their higher crystalline quality.
A brief review and discussion of ongoing research on radiation effects in key optoelectronic-photonic waveguide and periodic structures is presented. The review focuses on optoelectronic components composed of III-V and II-VI materials that are currently of interest in space applications. Reported and predicted radiation-induced responses in acousto-optic modulators, optical waveguide couplers, distributed Bragg reflectors, and organic materials are discussed.