Eunsoon Oh

EXCITATION DENSITY DEPENDENCE OF PHOTOLUMINESCENCE IN GAN:MG

Continuous redshift of an emission from 3.23 to 2.85 eV with decreasing excitation density is observed in the photoluminescence spectra of highly doped GaN:Mg. It has been explained by the formation of minibands originating from the overlap of deep levels and shallow acceptor levels. For nominally undoped samples the intensity ratio of a donor-bound exciton line and a donor–acceptor pair line changes dramatically with excitation density due to the limited number of acceptors.

Influence of potential fluctuation on optical and electrical properties in GaN

We observed strong correlation between optical properties and transport properties in GaN. Both the intensity and the energy of near-band edge photoluminescence (PL) peak in GaN:Si vary with its mobility. Such behavior has been explained by the potential fluctuation associated with the inhomogeneous impurities or local defects, leading to the space-charge scattering of carriers and the redshift of the PL line. We also discuss the strain relaxation in GaN:Si.

Comparison of Si doping effect in optical properties of GaN epilayers and InxGa1−xN quantum wells

Micro-photoluminescence (PL) spectra of Si-doped GaN epilayers and three-period In0.1Ga0.9N/In0.02Ga0.98N:Si quantum-well (QW) structures were studied and compared with macro-PL spectra. The shift of the macro-PL peak with increasing Si concentration was found to be similar to that with increasing excitation density in both GaN:Si and InxGa1−xN QWs. Also, it was observed that the macro-PL intensity increased with increasing Si concentration in GaN:Si and InxGa1−xN QWs, but the micro-PL intensity was independent of doping concentration. These results indicate that the changes of PL spectra with Si doping are mainly due to the increase of carriers.

EFFECT OF SURFACE LAYER ON OPTICAL PROPERTIES OF GAN AND INXGA1-XN UPON THERMAL ANNEALING

We investigated the effect of rapid thermal annealing (RTA) on the optical properties of GaN and InxGa1−xN. It was found that some of the changes in the photoluminescence spectra of GaN upon annealing were associated with the properties of the surface layer. For example, a new low-temperature photoluminescence line associated with donor–acceptor pair (DAP) recombination appeared at around 3.40 eV upon annealing, but disappeared after etching off the surface layer. This indicates that the acceptors responsible for the DAP emission were created only near the surface. After the RTA process, the near-band-edge emission was blueshifted, and the relative intensity of yellow luminescence with respect to the near-band-edge emission was increased, which were also attributed to the influence of the surface layer. The thermal annealing effect of an InxGa1−xN multiple quantum well structure is also discussed.

Effects of Si-doping in the barriers on the recombination dynamics in In0.15Ga0.85N/In0.015Ga0.985N quantum wells

A systematic study of time-resolved photoluminescence spectra from In0.15Ga0.85N/In0.015Ga0.985N quantum wells (QWs) with different Si-doping concentration in the barriers has been carried out. The 10 K recombination lifetimes in the QWs depend strongly on the Si-doping in the barriers, decreasing from ∼80 to ∼20 ns as the Si-doping level increases from 2×1018 to 1×1019 cm−3. The separation between the spontaneous and stimulated emission (SE) peaks and a shift of the emission peak to longer wavelengths with delay time after a pulsed excitation decrease with increasing Si doping. The increased recombination rate, the decrease of peak shift with delay time, and the reduced separation between the spontaneous and SE peaks with increasing Si doping are attributed to the screening of piezoelectric field by carriers originated from Si-doped barriers.

Structural and optical properties of undoped and doped ZnSe/GaAs strained heterostructures

Abstract Double-crystal X-ray rocking curve (DCRC), Auger electron spectroscopy (AES), transmission electron microscopy (TEM), photoluminescence (PL), and Raman scattering measurements on undoped and doped ZnSe epitaxial films grown on GaAs (100) substrates by molecular beam epitaxy were performed to investigate the structural and optical properties of the ZnSe films. From the DCRC analyses, the grown layer was found to be a ZnSe epitaxial film with high quality. The results of TEM measurements showed that the misfit dislocations of the nitrogen-doped ZnSe thin films was reduced in comparison with those of the undoped ZnSe films. The PL spectra of the nitrogen-doped ZnSe epilayer was dominated by donor–acceptor pair recombination and by a sequence of its longitudinal optical phonon replicas. Raman spectroscopy measurements showed that there was a lattice mismatch between the ZnSe epitaxial layer and the GaAs substrate and that a plasma–phonon coupling mode existed together with its characteristic longitudinal optical phonons. These results indicate that the ZnSe epitaxial films grown on GaAs hold promise for applications as buffer layers for the growth of Zn 1− x Mg x S y Se 1− y .

OPTICAL AND STRUCTURAL PROPERTIES OF ZNCDSE/ZNSSE/ZNMGSSE SEPARATE CONFINEMENT HETEROSTRUCTURES

We have grown ZnCdSe/ZnSSe/ZnMgSSe separate confinement heterostructures by molecular‐beam epitaxy. Strain on the ZnSSe layer is calculated from x‐ray and photoluminescence data. The temperature dependence of band‐gap energy and the photoluminescence intensity in the Cl‐doped ZnCdSe active layers is compared with that of undoped ones.

Excitation Density Dependence of Photoluminescence and Barrier Doping Effect in InxGa1—xN Quantum Wells

The effect of Si-doping in barriers has been investigated in photoluminescence (PL) spectra of 10 period In0.1Ga0.9N/ In0.02Ga0.98N quantum wells with various excitation densities. For the quantum wells with well width of 40 A, the PL peak was blueshifted and the intensity was increased with Si-doping under relatively low excitation density. However, such Si-doping effect was found to be less pronounced with higher excitation density. The blueshift of PL emission with increasing excitation density was larger for larger well widths, which indicates the influence of a piezoelectric potential in our samples. For sufficiently narrow quantum wells, we observed a redshift with increasing excitation density due to the bandgap renormalization effect.

Optical properties of Zn1−xMgxSeGaAs(0 0 1) epitaxial films studied by spectroscopic ellipsometry

Abstract Optical properties of zine-blende Zn 1− x Mg x Se ( x ≤ 0.21) alloys grown epitaxially on GaAs(0 0 1) by molecular beam epitaxy have been studied using spectroscopic ellipsometry at room temperature in the 1.5–5.5 eV photon energy region. Using the measured dielectric functions and their second-derivative spectra, the energies of the interband critical-point edges, E 0 , E 1 and their spin-orbit splittings, have been determined. These edges are found to shift linearly to higher energies as the Mg composition increases. The rates of increase of the E 0 and E 1 edges of the alloys are 10 and 4.7 meV/at.% respectively and their spin-orbit splittings remain at about the same as those of ZnSe. The increase rate of the E 0 edge of the present zinc-blende alloys is found to be smaller by about 20% than that of Bridgman-grown bulk sphalerite alloys [ J. Crystal Growth , 159 , 1996, 167].

Micro-photoluminescence study of InxGa1–xN/GaN quantum wells

Abstract In micro-photoluminescence (PL) study of a green In x Ga 1– x N/GaN single quantum well at room temperature, we observed some areas in the sample where only weak blue emission was detected. One of the difficulties in obtaining high quality In x Ga 1− x N/GaN quantum wells is the large difference in optimum growth temperatures for In x Ga 1– x N and GaN. By decreasing the growth temperature of GaN barriers in violet In x Ga 1– x N/GaN quantum wells, the PL emission was observed to be weaker for lower excitation density, but stronger for higher excitation density. It is emphasized that optical characterization under high excitation density is necessary for device applications such as laser diode structures.