Hiromitsu Kudo

High Output Power InGaN Ultraviolet Light‐Emitting Diodes Fabricated on Patterned Substrates Using Metalorganic Vapor Phase Epitaxy

Ultraviolet (UV) light-emitting diodes (LEDs) with an InGaN multi-quantum-well (MQW) structure were fabricated on a patterned sapphire substrate (PSS) using a single growth process of metalorganic vapor phase epitaxy. The GaN layer grown by lateral epitaxy on a patterned substrate (LEPS) has a dislocation density of 1.5 x 10 8 cm -2 . The LEPS-UV-LED chips were mounted on the Si bases in a flip-chip bonding arrangement. When the UV-LED was operated at a forward-biased current of 20 mA at room temperature, the emission wavelength, the output power and the external quantum efficiency were estimated to be 382 nm, 15.6 mW and 24%, respectively. With increasing forward-biased current, the output power increased linearly and was estimated to be approximately 38 mW at 50 mA.

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.

Temperature dependence of Stokes shift in InxGa1−xN epitaxial layers

Optical properties of InxGa1−xN epitaxial layers with various indium compositions (x=0.02, 0.03, 0.05, 0.06, and 0.09) have been studied by means of temperature-dependent optical absorption and photoluminescence spectroscopy. A clear peak due to the absorption of InxGa1−xN ternary alloys was observed up to 300 K, which enabled us to investigate the temperature dependence of the Stokes shift. The Stokes shift at 4 K increased with an increase in the indium composition, and was estimated to be 22 and 45 meV for the samples with x=0.02 and 0.09, respectively. With an increase in temperature up to about 50 K, the Stokes shift increased slightly. With a further increase in temperature from 50 to 100 K, the Stokes shift decreased. Above 100 K, the Stokes shift was independent of the temperature and showed an almost constant value up to 300 K. The Stokes shift at 300 K was estimated to be 19 and 34 meV for the samples with x=0.02 and 0.09, respectively. This temperature dependence of the Stokes shift was characteristically common to all of the samples used in the present work, and was observed to be more prominent for the samples with higher indium compositions.Optical properties of InxGa1−xN epitaxial layers with various indium compositions (x=0.02, 0.03, 0.05, 0.06, and 0.09) have been studied by means of temperature-dependent optical absorption and photoluminescence spectroscopy. A clear peak due to the absorption of InxGa1−xN ternary alloys was observed up to 300 K, which enabled us to investigate the temperature dependence of the Stokes shift. The Stokes shift at 4 K increased with an increase in the indium composition, and was estimated to be 22 and 45 meV for the samples with x=0.02 and 0.09, respectively. With an increase in temperature up to about 50 K, the Stokes shift increased slightly. With a further increase in temperature from 50 to 100 K, the Stokes shift decreased. Above 100 K, the Stokes shift was independent of the temperature and showed an almost constant value up to 300 K. The Stokes shift at 300 K was estimated to be 19 and 34 meV for the samples with x=0.02 and 0.09, respectively. This temperature dependence of the Stokes shift was charact...

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.

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.

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.

Effects of electric field on photoluminescence spectra in InGaN ultraviolet light-emitting diodes

Abstract The effect of an external electric field on photoluminescence (PL) spectra at 77 K has been investigated in a reverse-biased InGaN/AlGaN double heterojunction ultraviolet light-emitting diode. There appear two emission components separated by about 20 meV in the vicinity of about 3.4 eV when reverse-bias voltages exceed about 8 V. The lower-energy component is dramatically decreased in PL intensity and its PL peak energies shift slightly towards lower-energy side with reverse-bias voltage. On the other hand, the PL peak energies of the higher-energy component are independent of reverse-bias voltage. These luminescence properties cannot be interpreted by the usual localized excitons and by a cancellation of the piezoelectric field. The obtained results support the free-carrier recombination model of the 3.4 eV band, which has previously been identified by magnetoluminescence studies.

Radiative Recombination Dynamics of Carriers in InxGa1-xN Epitaxial Layers Revealed by Temperature Dependence of Time-Resolved Photoluminescence Spectra

The radiative recombination process of InxGa1–xN epitaxial layers was investigated by means of the temperature dependence of time-resolved photoluminescence spectra. Two emission components separated by about 40 meV were clearly observed. At 6 K, the higher- and lower-energy components had decay-time constants of 30 and 540 ps, respectively. The energy transfer of carriers between the two levels was observed. With increasing temperature, the decay-time constant of the higher-energy component was almost constant of about 30 ps, whilst that of the lower-energy component decreased from 540 to 20 ps. On the basis of a strong electron–phonon interaction the contribution of a polaron state of electrons to the recombination process of the higher-energy component is proposed.

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.

EFFECT OF HIGH CURRENT INJECTION ON THE BLUE RADIATIVE RECOMBINATION IN INGAN SINGLE QUANTUM WELL LIGHT EMITTING DIODES

Radiative recombination properties of blue electroluminescence (EL) in the vicinity of 460 nm (about 2.7 eV) in an InGaN-based single quantum well light emitting diode (LED) have been investigated at 77 K under forward-bias pulsed high-current injection condition. The 460 nm blue emission band accompanying the LO-phonon sidebands was clearly observed at forward-biased steady-state currents, while its LO-phonon replicas were smeared out with increasing pulsed current. It is revealed that the forward-biased injection EL spectrum reflects the distribution function of hot electrons injected into the active layer. Emission spectral characteristics can be quantitatively explained by the drifted Maxwellian distribution function of electrons. The temperature of hot electrons was estimated to be about 400 K at current density of about 4500 A/cm2, which is much higher than the lattice temperature.