Weijun Fan

Effects of rapid thermal annealing on structure and luminescence of self-assembled InAs/GaAs quantum dots

Postgrowth rapid thermal annealing was used to modify the structural and optical properties of the self-assembled InAs quantum dots grown on GaAs substrates by molecular beam epitaxy. It is found that significant narrowing of the luminescence linewidth (from 78.9 to 20.5 meV) from the InAs dot layer occurs together with about 260 meV blueshift at annealing temperatures up to 850u2009°C. Observation of high-resolution transmission electron microscopy shows the existence of the dots under lower annealing temperatures but disappearance of the dots annealed at 850u2009°C. The excited-state-filling experiments for the samples show that the luminescence of the samples annealed at 850u2009°C exhibits quantum well-like behavior. Comparing with the reference quantum well, we demonstrate significant enhancement of the interdiffusion in the dot layer.

Characteristics of InGaAs quantum dot infrared photodetectors

A quantum dot infrared photodetector (QDIP) consisting of self-assembled InGaAs quantum dots has been demonstrated. Responsivity of 3.25 mA/W at 9.2 μm was obtained for nonpolarized incident light on the detector with a 45° angle facet at 60 K. The QDIPs exhibit some unique electro-optic characteristics such as a strong negative differential photoconductance effect and blueshift of the response peak wavelength.

Electronic properties of zinc‐blende GaN, AlN, and their alloys Ga1−xAlxN

The electronic properties of wide‐energy gap zinc‐blende structure GaN, AlN, and their alloys Ga1−xAlxN are investigated using the empirical pseudopotential method. Electron and hole effective mass parameters, hydrostatic and shear deformation potential constants of the valence band at Γ and those of the conduction band at Γ and X are obtained for GaN and AlN, respectively. The energies of Γ, X, L conduction valleys of Ga1−xAlxN alloy versus Al fraction x are also calculated. The information will be useful for the design of lattice mismatched heterostructure optoelectronic devices based on these materials in the blue light range application.

VALENCE HOLE SUBBANDS AND OPTICAL GAIN SPECTRA OF GAN/GA1-XALXN STRAINED QUANTUM WELLS

The valence hole subbands, TE and TM mode optical gains, transparency carrier density, and radiative current density of the zinc‐blende GaN/Ga0.85Al0.15N strained quantum well (100 A well width) have been investigated using a 6×6 Hamiltonian model including the heavy hole, light hole, and spin‐orbit split‐off bands. At the k=0 point, it is found that the light hole strongly couples with the spin‐orbit split‐off hole, resulting in the so+lh hybrid states. The heavy hole does not couple with the light hole and the spin‐orbit split‐off hole. Optical transitions between the valence subbands and the conduction subbands obey the Δn=0 selection rule. At the k≠0 points, there is strong band mixing among the heavy hole, light hole, and spin‐orbit split‐off hole. The optical transitions do not obey the Δn=0 selection rule. The compressive strain in the GaN well region increases the energy separation between the so1+lh1 energy level and the hh1 energy level. Consequently, the compressive strain enhances the TE mode ...

Analysis of optical gain and threshold current density of wurtzite InGaN/GaN/AlGaN quantum well lasers

The valence subband structures, density-of-states, and optical gain of (0001) wurtzite InxGa1−xN/GaN quantum wells (QWs) are studied using a numerical approach. We used the effective-mass parameters of GaN and InN derived using the empirical pseudopotential method. By varying the well width and mole fraction of In in the well material, the effects of quantum confinement and compressive strain are examined. A narrower well width and a higher In mole fraction in the well lead to transverse electric enhancement and transverse magnetic suppression of the optical gain. From the relationship between the optical gain and the radiative current density, we obtain the transparent current density for a single QW to be 200u2009A/cm2. The InGaN/GaN/AlGaN separate confinement heterostructure multiple QW (MQW) laser structure is then analyzed. It is shown that a suitable combination of well width and number of QWs should be selected in optimizing the threshold current density in such MQW lasers.

Widely tunable intersubband energy spacing of self-assembled InAs/GaAs quantum dots due to interface intermixing

In this article, we showed the significant reduction of the energy spacing between ground state and excited state emissions from InAs/GaAs quantum dots ~QDs! due to interface interdiffusion induced by thermal treatment. In addition, the strong narrowing of the luminescence linewidth of the ground state and excited state emissions from the InAs dot layers for the annealed samples indicates an improvement of the size distribution of the QDs. Large blueshift of the energy positions of both emissions was also observed. High resolution x-ray diffraction experiments give strong evidence of the interface atom interdiffusion in the annealed samples. This work shows the ability to tune the wavelength for applications like infrared detectors and lasers based on intrasubband transitions of self-assembled QDs.

Polarization dependence of intraband absorption in self-organized quantum dots

Photoluminescence and intraband absorption were investigated in n-doped self-organized InAs and In0.35Ga0.65As quantum dots grown on a GaAs substrate. Intraband absorption of the dots is strongly polarized along the growth axis in the mid infrared spectral range. The absorption is maximum at around 120 meV for InAs dots and at 130 meV for In0.35Ga0.65As dots. The experimental results on InAs dots are in agreement with published theoretical calculations.

X-ray diffraction and optical characterization of interdiffusion in self-assembled InAs/GaAs quantum-dot superlattices

We report on the characterization of thermally induced interdiffusion in InAs/GaAs quantum-dot superlattices with high-resolution x-ray diffraction and photoluminescence techniques. The dynamical theory is employed to simulate the measured x-ray diffraction rocking curves of the InAs/GaAs quantum-dot superlattices annealed at different temperatures. Excellent agreement between the experimental curves and the simulations is achieved when the composition, thickness, and stress variations caused by interdiffusion are taken in account. It is found that the significant In-Ga intermixing occurs even in the as-grown InAs/GaAs quantum dots. The diffusion coefficients at different temperatures are estimated

Band offsets at GaInP/AlGaInP(001) heterostructures lattice matched to GaAs

In this work, the band offsets at the Ga0.5In0.5P/AlxGa0.5−xIn0.5P heterojunction lattice matched to (001)u2009GaAs was calculated over the whole range of aluminum composition from x=0.0 to 0.5 using the first-principles pseudopotential method with virtual crystal approximation. The valence band offset, VBO, varies with x as VBO=0.433xu2009eV, while the inferred conduction band offset CBO at Γ minimum (band-gap difference minus the valence band offset) varies in x as CBOΓ=0.787xu2009eV. Our results are in good agreement with the experimental data.

First-principles calculations of band offsets

The band offsets are calculated for both the Ga-As and interface bonds within first-principles local density functional theory. It is found that the band offsets are independent of the type of interfacial bond, while the valence band offset, VBO, decreases linearly with x as and the inferred conduction band offset CBO (the band-gap difference minus the valence band offset) decreases linearly as for .