Ruisheng Zheng

Intense Ultraviolet Electroluminescence Properties of the High-Power InGaN-Based Light-Emitting Diodes Fabricated on Patterned Sapphire Substrates.

The electroluminescence and photoluminescence characteristics of high-efficient InGaN multi-quantum-well ultraviolet light-emitting diodes have been investigated. There appeared a single emission band in the electroluminescence spectra at about 3.235 eV with a band width of 90 meV at room temperature under direct current. With increasing forward current, the luminescence intensity was not saturated, and increased linearly with increasing injection current up to 50 mA. Under pulsed current conditions at room temperature, the luminescence intensity increased linearly with increasing injection current up to 1000 mA, and a shift of the electroluminescence peak position was not observed. These results indicated that the injected carriers were confined efficiently in the active layer, and also suggested the possibility of realizing ultraviolet laser diodes. It was revealed that the forward-biased electroluminescence spectrum at 4 K reflected the distribution of hot electrons injected into the active layer. The maximum temperature of hot electrons was estimated to be about 350 K under a forward-biased pulsed current of about 500 mA, which was much higher than the lattice temperature.

Properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells

The fundamental properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells are investigated theoretically. The mode number and the energies of optical phonons, the high frequency and static dielectric constants of Ga1−xInxN crystals, are obtained by using an improved mixed crystal theory. The carrier-longitudinal optical (LO)-phonon interaction in Ga1−xInxN mixed crystals is studied by a perturbational polaron theory. The feature of optical phonons in Ga1−xInxN/GaN quantum wells is studied based on the dielectric continuum approach. The frequencies and the dispersion relation of the interface phonons are obtained. The 1s and 2s exciton binding energies and the exciton–LO-phonon interaction energies in Ga1−xInxN/GaN quantum wells are calculated numerically. Based on the present theoretical results, the general properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells are discussed.

Optical design of large-area GaN-based LEDs

The light extraction process in GaN-based light emitting diodes (LEDs) is studied in this paper. In order to increase the light extraction efficiency of large area LEDs, several novel LED geometries are discussed. The light propagation in the LEDs is simulated numerically by using the finite-difference time-domain (FDTD) method. It is shown that the following improvements in the GaN-based LEDs are very effective for increasing the light extraction: (1) To fabricate GaN micro-pyramid array on the surface of the LED, which guides the generated light to the surface; (2) To make inverted V-shaped groove formation on the GaN layer, which restricts the average length of ray path in the LEDs and refracts the waveguide-mode light to the surface; (3) To separate the LED epilayer from its substrate and then mount it on a metal mirror base, which is used to reflect the backside light to the LED surface. The FDTD simulation results show clearly that these improved geometries guide most of the internal luminescence to escape from the LED, and increase greatly the external light-extraction efficiency of GaN-based LEDs.

Recombination dynamics of carriers in an InGaN/AlGaN single-quantum-well light-emitting diode under reverse-bias voltages

The radiative recombination process of the blue emission band in an InGaN single-quantum-well light-emitting diode has extensively been investigated by means of the dependence of an external electric field on photoluminescence and time-resolved photoluminescence spectra. Two emission (higher and lower) components separated by about 40 meV are found in the emission band on the condition of reverse bias at 77 K. It is also found that the luminescence intensity decreases dramatically with increasing reverse-bias voltage at room temperature. The model based on field ionization of excitons cannot explain the present experimental phenomena. It is, therefore, suggested that the free-carrier recombination process is dominant at room temperature.

Stokes shift in InGaN epitaxial layers

By analyzing the features of quasi-low-dimensional structures, we recognize that the Stokes shift is a characteristic of quantum-wire and quantum-disk systems. Including the smearing effect of the broad distribution of sizes of the nanostructures into consideration, we found that the Stokes shift is proportional to the light-emission peak energy of the systems. We propose that the Stokes shift observed in the optical spectra of InGaN epitaxial layers might originate from the self-formed quantum-wire and/or quantum-disk structures in the epitaxial layers.

Radiative recombination process in InGaN active layers of GaN-based light emitting diodes

We present a theoretical analysis of radiative recombination process in active layers of blue/green InGaN-based light emitting diodes (LEDs) in the framework of a quantum disk model. Taking the structural and compositional inhomogeneity and the finite subband-states effects into account we modify the optical absorption and energy relaxation equations for quantum-disk systems. The carrier relaxation dynamic process and related time-dependent photoluminescence spectra are calculated numerically. Our results show that the quantum-disk model can interpret the main optical properties of InGaN-based LEDs reasonably.

Temperature-independent Stokes shift in an In0.08Ga0.92N epitaxial layer revealed by photoluminescence excitation spectroscopy

Optical properties of an In 0.08 Ga 0.92 N epitaxial layer have been studied by means of photoluminescence excitation spectroscopy. The photoluminescence spectrum of the In 0.08 Ga 0.92 N epitaxial layer was composed of two emission components with an energy separation of 40 meV. Photoluminescence excitation measurements allowed us to observe a clear peak due to the absorption of InGaN and to investigate the temperature dependence of the Stokes shift. At 100 K, the Stokes shifts of the higher and lower energy components were estimated to be 44 and 79 meV, respectively. The Stokes shifts were well consistent with the energy shifts expected from the polaron interaction. The absorption peaks for both the higher and lower energy components were located at the same energy position. Furthermore, the Stokes shift of the higher energy component was not dependent on temperature and indicated a constant value up to room temperature.

Composition dependence of optical phonon properties and dielectric functions of group-III arsenide ternary and quaternary mixed crystals

Optical properties of group-III arsenide ternary and quaternary alloys are investigated theoretically by a generalized mixed crystal theory. On the basis of a modified random element isodisplacement approach, long-wavelength optical phonon energies, phonon mode strengths and dielectric functions of AlxGa1−xAs, AlxIn1−xAs, and GaxIn1−xAs ternary alloys, and also of AlxGayIn1−x−yAs quaternary alloys are calculated numerically in the whole compositional range. The results are discussed and compared with experimental results. It is confirmed that the optical phonon spectra of AlxGayIn1−x−yAs alloys will exhibit three optical phonon modes in most of the compositional range.

Temperature dependence of electric-field induced photoluminescence from an InGaN-based light-emitting diode

Temperature dependence of reverse-biased photoluminescence has been investigated for understanding the radiative recombination mechanism in an InGaN single-quantum-well light-emitting diode. It is found that the applied-voltage dependence of luminescence intensities is strongly affected by temperature from 17 to 100 K, and a dramatic decrease in the luminescence intensity is observed over 100 K. The model of a field ionization of excitons cannot explain this dramatic decrease in the luminescence intensity. It is therefore suggested that the free-carrier recombination process becomes dominant over 100 K. Two emission components are found on the condition of reverse bias. The lower-energy component becomes strongly dependent on reverse-bias voltage with increasing temperature, and fully disappears under the applied voltage of only −2 V at 100 K.

Ultraviolet emission properties in InxGa1−xN epitaxial layer revealed by magnetoluminescence and time-resolved luminescence studies

Abstract Two recombination channels (higher- and lower-energy states) have been found in the efficient ultraviolet (UV) emission of an In0.08Ga0.92N epitaxial layer. The time-resolved luminescence spectra and the temperature dependence of decay time have shown that an energy-transfer process of carriers between two states has significantly taken place. The magnetoluminescence studies have revealed from a large Landau energy shift that the higher-energy state is not related to the localized excitons, but is due to an electronic transition.