Tae-Kyung Yoo

The effect of silicon doping in the selected barrier on the electroluminescence of InGaN/GaN multiquantum well light emitting diode

The effect of silicon doping in the selected barrier on the electroluminescence of InGaN∕GaN multiquantum well light emitting diode (LED) was studied using dual wavelength LEDs. The result verified that the hole carrier transport is easily blocked by the silicon doped barrier, and the dominant electron and hole recombination occurs at the wells between p-GaN and the silicon doped barrier. The electroluminescence spectrum and the wavelength blueshift of the silicon doped LEDs were compared with undoped LEDs. The numerical simulation was done to clearly explain the hole blocking effect by the silicon doped barrier.

InGaN-light emitting diode with high density truncated hexagonal pyramid shaped p-GaN hillocks on the emission surface

To increase the light extraction efficiency, high density truncated hexagonal pyramid shaped submicron p-GaN hillocks were formed on the emission surface of an InGaN∕GaN multiple quantum well light emitting dicode (LED) using an in situ silicon carbon nitride self-masking layer. The self-assembled hillock density was raised up to a low 109cm−2 using several nanometers of a Si0.4C0.6N1 self-masking layer. The self-assembled hillock LED resulted in the optical power improvement up to 80% with similar electrical properties as a normal LED. This device showed a higher electrostatic discharge pass yield at over 1000V reverse stress voltage.

Optical gain and luminescence of a ZnO-MgZnO quantum well

The optical gain and the luminescence of a ZnO quantum well with MgZnO barriers is studied theoretically. We calculated the non-Markovian optical gain and the luminescence for the strained-layer wurtzite quantum well taking into account the excitonic effects. It is predicted that both optical gain and luminescence are enhanced for the ZnO quantum well when compared with those of the InGaN-AlGaN quantum well structure due to the significant reduction of the piezoelectric effects in the ZnO-MgZnO systems.

Many-body optical gain and intraband relaxation time of wurtzite InGaN∕GaN quantum-well lasers and comparison with experiment

The optical gain and the intraband relaxation time of wurtzite InGaN∕GaN QW lasers are investigated theoretically considering the non-Markovian gain model with many-body effects. Gain spectra are compared with those obtained from the experiment. The calculated intraband relaxation time is about 24fs at the subband edge and gradually increases with the energy. The intraband relaxation time is shown to be nearly independent of the sheet carrier density in an investigated range. The calculated gain spectra is in good agreement with the experiment. The intraband relaxation time τin obtained from fitting with experiment is 25fs. This value agrees well with the calculated value (∼24fs) at the subband edge, for which most of optical transitions occur.

Double modulation-doped AlGaAs/InGaAs heterostructure with a graded composition in the quantum well

A new double modulation‐doped AlGaAs/InGaAs heterostructure with a graded composition in the InGaAs quantum well (QW) has been designed and electrically characterized. An InGaAs QW with a rectangular‐like potential profile in the presence of the two dimensional electron gas is obtained by a two‐step grading of the In composition, which results in a broad and symmetric electron distribution profile even under various voltages. The Hall measurement shows a very high electron mobility of 7230 cm2/V s and an electron sheet density of 4.1×1012/cm2 at room temperature. To our knowledge, this is the highest mobility ever reported so far for the double modulation‐doped Al0.3Ga0.7As/In0.2Ga0.8As field‐effect transistor structure.

Hydrogen effect on 670-nm AlGaInP visible laser during high temperature operation

Characteristic changes of single-mode 670-nm strained multiquantum-well (S-MQW) AlGaInP visible lasers during accelerated aging test are investigated. After a short-term continuous operation of 3 mW at 50/spl deg/C, the threshold current and slope efficiency are improved by 8.5% and 24%, respectively. In order to find out the origin of characteristic changes, photoluminescence and secondary ion mass spectrometry (SIMS) measurement were carried out. SIMS measurement revealed that atomic hydrogen, which had been formed as a zinc-hydrogen complex in the p-type InGaInP cladding layer, were dissociated and drifted into the active region of the laser structure during high temperature operation. These result in increase of the hole concentration of the p-AlGaInP layer, which enhances the heterobarrier height between the p-cladding layer and the active region In addition, the dissociated atomic hydrogen drifted into the active region by bias voltage passivates the defects in that region, which is proved by low temperature photoluminescence measurement showing that the defect-related peaks have disappeared. >

Envelope function calculations of linear and nonlinear optical gains in a strained-layer quantum-well laser

The linear and nonlinear optical gain of strained-layer InGaAs-AlGaAs quantum well (QW) lasers are studied theoretically, with band mixing effects taken into account. Effects of the biaxial compressive strain of the InGaAs-AlGaAs QW on the band structure are investigated by solving for the Pikus-Bir Hamiltonian. The biaxial compressive strain separates the HH and the LH subbands by pulling down the HH subbands and pushing the LH subbands away from the valence band edge. Since the C-HH transition is dominated by the TE polarization, it is expected that the TE mode gain would be substantially larger than the TM mode gain. The gain and the gain-suppression coefficient are calculated from the complex optical susceptibility obtained by the density matrix formalism. Optical output power is calculated by solving the rate equations for the stationary states with nonlinear gain suppression. The calculated L-I characteristics shows reasonable agreement with the experimental data. >

Non-Markovian gain and luminescence of an InGaN-AlInGaN quantum-well with many-body effects

The optical gain and the luminescence of an InGaN quantum well with quaternary AlInGaN barriers is studied theoretically. We calculated the non-Markovian optical gain and the luminescence for the strained-layer wurtzite quantum well taking into account of many-body effects. It is predicted that both optical gain and luminescence are enhanced significantly when aluminum and indium are introduced into the quaternary barrier composition. Adding the aluminum to the barrier will increase of the confinement potentials for electrons and holes, while the indium will reduce the biaxial strain, which in turn reduces the internal field caused by spontaneous polarization and piezoelectric effects.

Surface-Emitting AlGaAs/GaAs DH LED with Buried-Window Cylindrical Lens

Surface-emitting AlGaAs/GaAs double heterostructure (DH) LEDs with buried-window cylindrical lenses have been fabricated. The selective meltback and regrowth were used for the lens and DH junctions. The far-field pattern has the full width at half maximum (FWHM) of 90°. A small protrusion with the full width of 20° was observed in the far-field pattern. The maximum power of 20 mW was obtained from the 1 µm×60 µm×300 µm active layer.

Bell-shaped light emitting diodes (BS-LED) with a 45 degree corner reflector, deep side-wall, and microlens

A new type Bell Shaped Light Emitting Diode(BS-LED) with a circular 45 degree(s) corner reflector, deep side-wall and microlens is proposed and fabricated. Because the light of in-plane radiation in the active layer of Surface Emitting LED(SE-LED) can be extracted to emission surface by a circular 45 degree(s) corner reflector, the output power saturation phenomena that occur due to the in-plane superluminescence can be considerably improved. So, the light output power and the linearity of light-current curve can be improved efficiently by the corner reflector. The deeply etched side-wall can dramatically improve the external quantum efficiency of LED by side-wall reflection and photon recycling mechanism. Microlens is formed on light emission surface to improve the beam pattern. The fabricated BS-LED shows the dramatically improved external quantum efficiency up to about 8 times than that of conventional LED. The output power improvement is simulated as device design parameters. The BS-LED is characterized using spectrum, near-field pattern and light-current measurement.