Seoung-Hwan Park

Crystal orientation effects on electronic properties of wurtzite InGaN/GaN quantum wells

Crystal orientation effects on electronic properties of wurtzite InGaN/GaN quantum wells (QWs) with piezoelectric (PZ) and spontaneous (SP) polarization are investigated using the multiband effective-mass theory. With increasing crystal angle, the internal field of the InGaN/GaN QW structure changes its sign near the crystal angle of 55° while that of the GaN/AlGaN QW structure gradually decreases without changing its sign. The interband transition energy is redshifted for crystal angles near θ=0° and gradually increases with the crystal angle due to the reduced SP and PZ polarization effects. The y′-polarized optical matrix element largely increases with increasing crystal angle. This is mainly due to the fact that the states constituting the topmost valence subband near the band edge are predominantly |Y′〉-like for QW structures with larger crystal angle. Also, it is observed that the average hole effective mass is largely reduced with increasing crystal angle. In particular, the average hole effective ...

Comparison of zinc-blende and wurtzite GaN semiconductors with spontaneous polarization and piezoelectric field effects

The electronic properties of (001)-, (111)-oriented zinc-blende and (0001)-oriented wurtzite crystals are investigated theoretically, where the spontaneous polarization and piezoelectric (PZ) field effects are taken into account. We show that the Luttinger–Kohn 6×6 Hamiltonians for the valence bands of the zinc-blende crystals written in the wurtzite bases for (001) and (111) crystal orientations and the Hamiltonian for the (0001)-orientation of wurtzite crystals can all be block diagonalized to two 3×3 Hamiltonians, which have analytical solutions for eigenvalues and eigenvectors. We then derive analytical expressions for the strain dependent band-edge effective masses and interband optical matrix elements of zinc-blende and wurtzite GaN crystals and compare their numerical results as well as valence band structures. Although the compressively strained zinc-blende quantum wells in materials such as GaAs- and ZnSe-based systems show reduced threshold carrier densities due to the lower in-plane effective m...

Piezoelectric effects on electrical and optical properties of wurtzite GaN/AlGaN quantum well lasers

The piezoelectric effects on the optical gain of wurtzite GaN/AlGaN QW lasers taking into account the many-body effects are presented. The self-consistent model with piezoelectric field effect shows that band structures and optical gain are significantly affected by the piezoelectric field at relatively low carrier densities. The peak gain is redshifted and smaller when compared to the flat-band model without piezoelectric field effect. Only gain peaks corresponding to C1-HH1 and C1-LH1 transitions are observed in the investigated range and transitions for C1-HH2 and C1-LH2 are negligible due to the large subband energy spacing at low carrier densities and small matrix elements at high carrier densities. At high carrier densities, the self-consistent model shows band structures and optical properties similar to the flat-band model due to the screening effects.

Spontaneous polarization effects in wurtzite GaN/AlGaN quantum wells and comparison with experiment

Electronic and optical properties of wurtzite GaN/AlGaN quantum well (QW) structures with the spontaneous (SP) and piezoelectric (PZ) polarizations are investigated. Although the PZ field in the well is zero where there is no strain if the QW structures are grown on a thick GaN layer, there may still exist a strong field in the well due to the difference between the SP polarizations in the well and barrier regions. It is shown that the transition energies have significant dependence on both the well and the barrier widths and the many-body optical gain is reduced largely due to the SP polarization. In particular, in the case of a QW structure with a large well width, the reduction of the optical gain is dominant due to larger spatial separation between the electron and hole wave functions. These results suggest that a QW structure with a thin well width below 30 A is desirable for QW lasers. We show that the theoretical transition energies agree very well with the experimental results for several Al compo...

Spontaneous and piezoelectric polarization effects in wurtzite ZnO∕MgZnO quantum well lasers

Spontaneous and piezoelectric polarization effects on electronic and optical properties of ZnO∕MgZnO quantum well (QW) structures are investigated by using the non-Markovian gain model with many-body effects. The spontaneous polarization constant for MgO determined from a comparison with the experiment is about −0.070C∕m2, which is larger than the value (−0.050C∕m2) for ZnO. The negligible internal field effect observed in the case of ZnO∕MgZnO QW structures with relatively low Mg composition (x<0.2) and thin well width (Lw<46A) can be explained by the cancelation of the sum of piezoelectric and spontaneous polarizations between the well and the barrier. The ZnO∕MgZnO QW laser has much larger optical gain than the GaN∕AlGaN QW laser. This is attributed to the fact that the ZnO∕MgZnO QW structure has a larger optical matrix element due to the relatively small internal field, compared to the GaN∕AlGaN QW structure.

Optical gain in InGaN∕GaN quantum well structures with embedded AlGaN δ layer

Optical gain characteristics of InGaN∕GaN double quantum well (QW) structures with embedded AlGaN δ layer are investigated using the multiband effective mass theory. These results are compared with those of single QW structure without a δ layer. The theoretical energies show very good agreement with the experimental results for both single and double QW structures. The inclusion effect of a δ layer is found to be dominant at a relatively low carrier density. A double QW structure has larger optical gain than the single QW structure, in particular, at higher carrier density.

High-efficiency staggered 530 nm InGaN/InGaN/GaN quantum-well light-emitting diodes

Optical properties of staggered 530 nm InGaN/InGaN/GaN quantum-well (QW) light-emitting-diodes are investigated using the multiband effective mass theory. These results are compared with those of conventional 530 nm InGaN/GaN QW structures. A staggered InGaN/InGaN/GaN QW structure is shown to have much larger spontaneous emission than a conventional InGaN/GaN QW structure. This can be explained by the fact that a staggered QW structure has much larger matrix element than a conventional QW structure because a spatial separation between electron and hole wave functions is substantially reduced with the inclusion of a staggered InGaN layer. A staggered QW structure shows that the peak position at a high carrier density (530 nm) is similar to that at a noninjection level.

Optical gain of strained GaAsSb/GaAs quantum-well lasers: A self-consistent approach

We present a self-consistent model for the band structure and optical gain spectrum of GaAs1−xSbx/GaAs quantum-well (QW) lasers with carrier population. Experimental data indicate that this material system has a type-II QW configuration. By fitting the experimental photoluminescence data from various groups using our proposed empirical model, which assumes that 90% of the band-gap bowing parameter (1.2 eV) appears in the valence band, we find that the unstrained valence band edge discontinuity ratio Qv0 is close to 0.9 for an arbitrary Sb mole composition x of GaAs1−xSbx/GaAs QWs and the QW structure becomes type-II when the compressive strain induced band-edge shifts are taken into account. We show that for type-II QWs the self-consistent solution, which solves the Schrodinger equation and Poisson equation simultaneously, is necessary. Due to the free-carrier screening effect in the self-consistent potential, the electrons are attracted toward the barrier–well interfaces by the holes in the wells, and, t...

Dip-shaped InGaN/GaN quantum-well light-emitting diodes with high efficiency

Optical properties of dip-shaped InGaN/GaN quantum well (QW) light-emitting diodes are investigated using the multiband effective-mass theory. These results are compared with those of conventional and staggered InGaN/GaN QW light-emitting diodes. In the case of a dip-shaped QW structure, the carrier density dependence of the transition wavelength is reduced due to a relatively small internal field effect. Also, we observe that the heavy-hole effective mass around the topmost valence band is greatly reduced with the inclusion of the dip-shaped layer. The spontaneous emission peak of a dip-shaped QW structure is shown to be larger than that of a staggered QW structure or a conventional QW structure. This is mainly due to the fact that a dip-shaped QW structure has larger optical matrix elements produced by Kane’s parameter.

Depolarization effects in (112¯2)-oriented InGaN∕GaN quantum well structures

Depolarization effects on electrical and optical properties of (112¯2)-oriented wurtzite (WU) InGaN∕GaN quantum well (QW) were investigated using the multiband effective-mass theory. These results are compared with those of (0001)- and (101¯0)-oriented WU InGaN∕GaN QW structures. The internal field is shown to become zero for (112¯2) crystal orientation near the crystal angle of 56°, irrespective of the In composition in the well. This is because the sum of the piezoelectric and spontaneous polarizations in the barrier is equal to that in the well. The optical gain of the (112¯2)-oriented QW is significantly larger than that of the (0001)-oriented QW. This is caused mainly by the increase of the optical matrix element due to the disappearance of the internal field. Also, the (112¯2)-oriented QW is found to have the optical gain comparable to that of the (101¯0)-oriented QW.