Hongping Zhao

Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells

Optimization of internal quantum efficiency (IQE) for InGaN quantum wells (QWs) light-emitting diodes (LEDs) is investigated. Staggered InGaN QWs with large electron-hole wavefunction overlap and improved radiative recombination rate are investigated for nitride LEDs application. The effect of interface abruptness in staggered InGaN QWs on radiative recombination rate is studied. Studies show that the less interface abruptness between the InGaN sub-layers will not affect the performance of the staggered InGaN QWs detrimentally. The growths of linearly-shaped staggered InGaN QWs by employing graded growth temperature grading are presented. The effect of current injection efficiency on IQE of InGaN QWs LEDs and other approaches to reduce dislocation in InGaN QWs LEDs are also discussed. The optimization of both radiative efficiency and current injection efficiency in InGaN QWs LEDs are required for achieving high IQE devices emitting in the green spectral regime and longer.

Self-Consistent Analysis of Strain-Compensated InGaN–AlGaN Quantum Wells for Lasers and Light-Emitting Diodes

Strain-compensated InGaN-AlGaN quantum wells (QW) are investigated as improved active regions for lasers and light emitting diodes. The strain-compensated QW structure consists of thin tensile-strained AlGaN barriers surrounding the InGaN QW. The band structure was calculated by using a self-consistent 6-band kmiddotp formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The spontaneous emission and gain properties were analyzed for strain-compensated InGaN-AlGaN QW structures with indium contents of 28%, 22%, and 15% for lasers (light-emitting diodes) emitting at 480 (500), 440 (450), and 405 nm (415 nm) spectral regimes, respectively. The spontaneous emission spectra show significant improvement of the radiative emission for strain-compensated QW for all three structures compared to the corresponding conventional InGaN QW, which indicates the enhanced radiative efficiency for light emitting diodes. Our studies show the improvement of the optical gain and reduction of the threshold current density from the use of strain-compensated InGaN-AlGaN QW as active regions for diode lasers.

Growths of staggered InGaN quantum wells light-emitting diodes emitting at 520–525 nm employing graded growth-temperature profile

Three-layer staggered InGaN quantum wells (QWs) light-emitting diodes (LEDs) emitting at 520–525 nm were grown by metal-organic chemical vapor deposition by employing graded growth-temperature profile. The use of staggered InGaN QW, with improved electron-hole wave functions overlap design, leads to an enhancement of its radiative recombination rate. Both cathodoluminescence and electroluminescence measurements of three-layer staggered InGaN QW LED exhibited enhancements by 1.8–2.8 and 2.0–3.5 times, respectively, over those of conventional InGaN QW LED.

III-Nitride Photonics

The progress in III-Nitride photonics research in 2009 is reviewed. The III-Nitride photonics research is a very active field with many important applications in the areas of energy, biosensors, laser devices, and communications. The applications of nitride semiconductors in energy-related technologies include solid-state lighting, solar cells, thermoelectric, and power electronics. Several new research areas in III-Nitride photonics related to terahertz photonics, intersubband quantum wells, nanostructures, and other devices are discussed.

Analysis of InGaN-delta-InN quantum wells for light-emitting diodes

The design of InGaN-delta-InN quantum wells (QWs) leads to significant redshift for nitride active region with large electron-hole wave function overlap (Γe_hh) and spontaneous emission rate. The analysis was carried out by using self-consistent six-band k⋅p band formalism. The design of active region consisting of 30 A In0.25Ga0.75N QW with InN delta-layer leads to large Γe_hh of >50% with emission wavelength in the yellow and red spectral regimes, which is applicable for nitride-based light-emitting diodes.

Analysis of Internal Quantum Efficiency and Current Injection Efficiency in III-Nitride Light-Emitting Diodes

Current injection efficiency and internal quantum efficiency (IQE) in InGaN quantum well (QW) based light emitting diodes (LEDs) are investigated. The analysis is based on current continuity relation for drift and diffusion carrier transport across the QW-barrier systems. A self-consistent 6-band k ·p method is used to calculate the band structure for InGaN QW structure. Carrier-photon rate equations are utilized to describe radiative and non-radiative recombination in the QW and the barrier regions, carrier transport and capture time, and thermionic emission leading to carrier leakage out of the QW. Our model indicates that the IQE in the conventional 24-Å In0.28Ga0.72 N -GaN QW structure reaches its peak at low injection current density and reduces gradually with further increase in current due to the large thermionic carrier leakage. The efficiency droop phenomenon at high current density in III-nitride LEDs is thus consistent with the high-driving-current induced quenching in current injection efficiency predicted by our model. The effects of the monomolecular recombination coefficient, Auger recombination coefficient and GaN hole mobility on the current injection efficiency and IQE are studied. Structures combining InGaN QW with thin larger energy bandgap barriers such as AlxGa1-xN, lattice-matched AlxIn1-xN, and lattice-matched AlxInyGa1-x-y N have been analyzed to improve current injection efficiency and thus minimize droop at high current injection in III-nitride LEDs. Effect of the thickness of the larger energy bandgap barriers (AlGaN, AlInN and AlInGaN) on injection efficiency and IQE are investigated. The use of thin AlGaN barriers shows slight reduction of quenching of the injection efficiency as the current density increases. The use of thin lattice-matched AlInN or AlInGaN barriers shows significant suppression of efficiency-droop in nitride LEDs.

Design Analysis of Staggered InGaN Quantum Wells Light-Emitting Diodes at 500–540 nm

Staggered InGaN quantum wells (QWs) are analyzed as improved active region for light-emitting diodes (LEDs) emitting at 500 nm and 540 nm, respectively. The calculation of band structure is based on a self-consistent 6-band <i>k</i>middot<i>p</i> formalism taking into account the valence band mixing, strain effect, and spontaneous and piezoelectric polarizations as well as the carrier screening effect. Both two-layer staggered In<i>x</i>Ga_1- <i>x</i>N/In<i>y</i>Ga_1- <i>y</i>N QW and three-layer staggered In<i>y</i>Ga_1- <i>y</i>N/In<i>x</i>Ga_1- <i>x</i>N/In<i>y</i>Ga_1- <i>y</i>N QW structures are investigated as active region to enhance the spontaneous emission radiative recombination rate (<i>R</i> _sp) for LEDs emitting at 500 nm and 540 nm. Analysis of the spontaneous emission radiative recombination rate (<i>R</i> _sp) shows significant enhancement for both two-layer staggered InGaN QW and three-layer staggered InGaN QW, in comparison to that of the conventional In<i>z</i>Ga_1- <i>z</i>N QW. The studies of the carrier lifetime indicate a significant reduction of the carrier lifetime for staggered InGaN QWs, which contribute to the enhancement of the radiative efficiency for both two-layer staggered InGaN QW and three-layer staggered InGaN QW LEDs emitting at 500 nm and 540 nm.

Effect of crystal-field split-off hole and heavy-hole bands crossover on gain characteristics of high Al-content AlGaN quantum well lasers

The optical gain characteristics of high Al-content AlGaN quantum wells (QWs) are analyzed for deep UV lasers. The effect of crystal-field split-off hole (CH) and heavy-hole (HH) bands crossover on the gain characteristics of AlGaN QW with AlN barriers is analyzed. Attributing to the strong transition between conduction–CH bands, the TM spontaneous emission recombination rate is enhanced significantly for high Al-content AlGaN QWs. Large TM-polarized material gain is shown as achievable for high Al-content AlGaN QWs, which indicates the feasibility of TM lasing for lasers emitting at ∼220–230 nm.

Spontaneous Emission and Characteristics of Staggered InGaN Quantum-Well Light-Emitting Diodes

A novel gain media based on staggered InGaN quantum wells (QWs) grown by metal-organic chemical vapor deposition was demonstrated as improved active region for visible light emitters. Fermis golden rule indicates that InGaN QW with step-function like In content in the well leads to significantly improved radiative recombination rate and optical gain due to increased electron-hole wavefunction overlap, in comparison to that of conventional InGaN QW. Spontaneous emission spectra of both conventional and staggered InGaN QW were calculated based on energy dispersion and transition matrix element obtained by 6-band k middotp formalism for wurtzite semiconductor, taking into account valence-band-states mixing, strain effects, and polarization-induced electric fields. The calculated spectra for the staggered InGaN QW showed enhancement of radiative recombination rate, which is in good agreement with photoluminescence and cathodoluminescence measurements at emission wavelength regime of 425 and 500 nm. Experimental results of light-emitting diode (LED) structures utilizing staggered InGaN QW also show significant improvement in output power. Staggered InGaN QW allows polarization engineering leading to improved luminescence intensity and LED output power as a result of enhanced radiative recombination rate.

Surface plasmon dispersion engineering via double-metallic Au/Ag layers for III-nitride based light-emitting diodes

Double-metallic Au/Ag layers deposited on top of InGaN/GaN quantum wells (QWs) are used to tune the Purcell peak enhancement of the radiative recombination rate for nitride light-emitting diodes. By modifying the Au/Ag thicknesses, the Purcell factor can be widely tuned between the surface plasmon frequencies of Au/GaN and Ag/GaN. Photoluminescence studies demonstrated the concept of the Purcell factor tuning by using the double-metallic Au/Ag layers.