Hyeongsoo Park

Formation of V-shaped pits in InGaN/GaN multiquantum wells and bulk InGaN films

InGaN/GaN multiquantum well structures and bulk InGaN films grown on (0001) sapphire substrate were investigated by cross-sectional transmission electron microscopy and double crystal x-ray diffraction. Highly strained InGaN layers with a high In mole fraction were found to contain V-shaped surface pits with (10–11) facet planes on pure or mixed screw treading dislocations. Phase separation was also found on thick InGaN/GaN superlattices and bulk InGaN layers. The mechanism of the surface pit formation was discussed in terms of strain energy and surface mobility of InGaN.

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.

Characteristic of InGaN/GaN Laser Diode Grown by a Multi-Wafer MOCVD System

InGaN/GaN multi-quantum well (MQW) laser diodes (LDs) were grown on c-plane sapphire substrates using a multi-wafer MOCVD system. The threshold current for pulsed lasing was 1.6 A for a gain-guided laser diode with a stripe of 10 × 800 μm 2 . The threshold current density was 20.3 kA cm −2 and the threshold voltage was 16.5 V. The optical power ratio of transverse electric mode to transverse magnetic mode was found to be greater than 50. The characteristic temperature measured from the plot of threshold current versus measurement temperature was between 130 and 150K.

Composition stoichiometry and growth rate control in molecular beam epitaxy of ZnSe based ternary and quaternary alloys

Abstract This paper presents an original thermodynamic description of (Mg,Zn)(S,Se) MBE growth, which is in a good quantitative agreement with experimental data. This approach provides large flexibility in choosing different growth regimes of pseudomorphic ZnSe-based heterostructures to obtain desirable alloy composition, surface stoichiometry, and growth rate. The possibility to control a nearly lattice-matched to GaAs composition of a Zn1−xMgxSySe1−y alloy from (x = 0, y = 0.09) to (x = 0.07, y = 0.13) only by variation of the Mg flux intensity has been theoretically revealed and experimentally realized in an optically pumped ZnMgSSe/ZnSSe/ZnCdSe SQW SCH laser structure with a threshold power density of 20 kW/cm2 at 300 K.

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.

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.

Non-destructive measurement of electron concentration in n-ZnSe by means of reflectance difference spectroscopy

Abstract Reflectance difference (RD) spectroscopy was employed for contactless determination of the free-carrier concentrations in n-ZnSe grown by molecular beam epitaxy (MBE). In materials systems that show the linear electro-optic (LEO) effect, the LEO-induced RD signal from the region of the E 1 and E 1 + Δ 1 transitions can be used to estimate the net doping level. In this study we have investigated n-type ZnSe for a wide range of carrier concentrations (9 × 10 16 to 5 × 10 18 cm −3 ). Our results show that the carrier concentration can be determined from the gauged RD spectra with an accuracy of approx. ± 20% for net donor concentrations of the order of 10 18 cm −3 .

Quantum-confined Franz-Keldysh effect in (ZnCdSe)(ZnSeS) quantum-well laser diodes

Abstract The room-temperature electro-absorption in wide ( ∼ 150 A ) ZnCdSe quantum wells is explained in terms of the quantum-confined Franz-Keldysh effect. The heavy hole-light hole splitting has been determined both for single quantum well (SQW) and multiple quantum well heterostructures, providing an insight into the distribution of intrinsic strain. The band gap shrinkage due to the many-body Coulomb interactions is shown to be negligible in the SQW laser diode with the wide quantum well.