Jinhyun Lee

“Blue” temperature-induced shift and band-tail emission in InGaN-based light sources

Electro- and photoluminescence spectra of high-brightness light-emitting AlGaN/InGaN/GaN single-quantum-well structures are studied over a broad range of temperatures and pumping levels. Blue shift of the spectral peak position was observed along with an increase of temperature and current. An involvement of band-tail states in the radiative recombination was considered, and a quantitative description of the blue temperature-induced shift was proposed assuming a Gaussian shape of the band tail.

MOCVD growth of InNxAs1-x on GaAs using dimethylhydrazine

InNAs/GaAs multiple-quantum-well samples were grown by MOCVD on (100) n + -GaAs substrates at 500 °C and 60 Torr using uncracked dimethylhydrazine (DMHy). Quantum well layers were grown using trimethylindium, tertiarybutylarsine, and 95-97.5% of DMHy in the vapor phase, while GaAs buffer, barrier, and cap layers were grown using trimethylgallium and arsine. The crystalline quality and solid phase composition were evaluated using high-resolution X-ray diffraction analysis. The nitrogen content in InNAs wells was determined to be 18%. Surface morphology was investigated by atomic force microscopy (AFM) and field emission microscopy (FEM). Photoluminescence measurements confirm that the bandgap energy of InNAs is significantly lower than that of InAs. The peak emission wavelength of ∼6.5 μm at 10 K is the longest reported so far for dilute nitride semiconductors.

Comprehensive studies of light emission from GaN/InGaN/AlGaN single-quantum-well structures

Results of our comprehensive studies of Nichia blue and green SQW LEDs are presented. These include measurements of the emission spectra over a wide range of currents, temperatures, and pressures. The observed anomalous blue shift with temperature and rapid current-induced blue shift at low excitation levels can be explained using a simple model of recombination between Gaussian band-tail states.

Effects of high electrical stress on GaN/InGaN/AlGaN single-quantum-well light-emitting diodes

Abstract We report on high-electrical-stress testing of Nichia GaN/InGaN/AlGaN single-quantum-well (SQW) light-emitting diodes. In contrast to our earlier experiments with double-heterostructure LEDs, the present SQW devices have been improved to the point that the encapsulating plastic fails under high electrical stress earlier than the diode itself.

Anomalous temperature behaviour and band tailing in InGaN/GaN heterostructures grown on sapphire by MOCVD

Summary form only given. InGaN is very attractive as the active region material for optoelectronic devices emitting near-UV and blue/green light. However, in spite of tremendous progress in realization of GaN/InGaN/AlGaN light-emitting diodes and diode lasers, the underlying fundamental processes of carrier recombination in InGaN are still poorly understood. For example, controversies surround the role of localized excitons in optical gain, or self-formation of quantum dots in InGaN. Light emission from single-quantum-well (SQW) blue and green light emitting diodes (LEDs) was studied extensively, and it was shown that their unusual temperature, current, and hydrostatic pressure behavior was consistent with involvement of density-of-states (DOS) band tails in the optical emission process. In the paper, we demonstrate that thicker InGaN/GaN heterostructures exhibit behavior similar to that observed in SQW LEDs. Samples under study were grown on (0001) sapphire substrates by atmospheric-pressure metal-organic chemical-vapor deposition (MOCVD).

InGaN-based quantum-well LEDs: explanation of anomalous electro-optical characteristics

Charge transport and luminescence in InGaN-based short-wavelength light-emitting diodes feature some anomalies: 1) low-temperature quenching of luminescence yield; 2) very large ideality factor of I-V curves. These observations are explained in terms of electron ballistic overflow and injection-induced conductivity on p-side.

Optical and electrical properties of interdigitated InGaN/GaN green light-emitting diodes

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 p-type epilayers pre-activated in N2 ambient for 4 min at 800 °C, and the other as-grown, without any pre-activation of Mg acceptors. Measured specific resistances of p-side contacts are 1.49x10-4 Ωcm2 for contacts on pre-activated samples annealed at 650°C for 4 min, and 1.55x10-5 Ωcm2 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 p-type conduction activation and for ohmic contact formation) or a single-step annealing process (simultaneous annealing for activation of p-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.

Effects of impurity-free intermixing on InGaAs/GaAs/AlGaAs broad-area diode laser performance

Summary form only given. In this paper, we report the fabrication and characterization of otherwise identical diode lasers with non-intermixed and intermixed active regions. Measured characteristics of these lasers are used to extract information about changes in threshold current density, internal optical loss, internal quantum efficiency, material gain, etc., introduced by the impurity-free vacancy diffusion process.

Applications of high-power laser technology to wide-bandgap nitride semiconductor processing

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 ).

Selective intermixing of InGaAs/GaAs/AlGaAs quantum wells with spin-on-glass/MgF2 grating caps

In order to achieve controlled degree of intermixing in selected areas (CISA), SiO2 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, SiO2/MgF2 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.