D. Y. Lee

Structural and optical properties of InAs quantum dots regrown on atomic hydrogen-cleaned GaAs surface

We report the structural and optical properties of InAs quantum dots (QDs) directly regrown on air-exposed and subsequent atomic hydrogen-cleaned (AHC) GaAs (001) surface. The average size of InAs QDs on the AHC GaAs surface is 29nm, which is larger than 22nm for the conventionally grown InAs QDs on GaAs. The integrated photoluminescence intensity of the InAs QDs on the AHC GaAs measured at room temperature is larger than that of the reference sample by two orders, even though the cleaned GaAs surface directly faced the base of the InAs QDs. The decrease in the interface states between the wetting layer and AHC GaAs was confirmed by Franz–Keldysh oscillations of the photoreflectance spectra.

Confocal scanning electroluminescence spectro-microscope for multidimensional light-emitting property analysis

We report new type of micro-EL instrument and its applications for light emitting devices. Our new micro-EL, so-called confocal scanning electroluminescence sprctro-microscope (CSESM) has not only fast image acquisition time but also high image resolution. The newly developed CSESM is combined with confocal laser scanning photoluminescence micsoscope, i.e. micro-PL. Therefore, micro-EL distribution can be directly matched with micro-PL and mechanical chip structure of LED. It is fruitful for providing a fast and non-destructive method to analyze the homogeneity of LEDs in its completely proceeded form. Using this apparatus, we study local intensity and wavelength distribution of electroluminescence for InGaN/GaN blue LED chip. Our results represent that local fluctuations of electroluminescence intensity and wavelength position are closely connected with the fluctuation of local current density, i.e. current spreading features on LED chips.

Photoluminescence Studies of Mg-Doped AlxGa1-xAs Epitaxial Layers Grown by Molecular Beam Epitaxy

Magnesium (Mg)-doped AlxGa1-xAs epitaxial layers were grown by molecular beam epitaxy (MBE) at various substrate temperatures, Al mole fractions, and As/Ga beam equivalent pressure (BEP) ratios. The optical properties were investigated by photoluminescence (PL) spectroscopy. The PL emission peak intensity was increased and the emission peak was slightly blue shifted from 700.7 to 697.3 nm when the substrate temperature was increased. The full width at half maximum (FWHM) was decreased from 29.6 to 19 meV when the substrate temperature was increased. The other growth parameters, such as the As/Ga BEP ratio and the Al mole fraction, also affected the optical properties of the Mg-doped AlxGa1-xAs epilayers. The anomalous behavior of the blue- and red-shift of the PL peak energy was observed also to be temperature dependent.

Convergence of optical spectroscopic system for characterization of InGaN/GaN multi-quantum well light-emitting diodes

We present a converged spectroscopic system design for performing photoreflectance (PR), electroreflectance (ER), electroluminescence (EL), photoluminescence (PL) and photovoltage (PV) measurements of semiconductors. The design of the experimental setup is described in detail. To test the performance of the system, measurements of a series of InxGa1-xN/GaN light emitting semiconductor with different indium composition of InGaN layer are carried out by use of this system. The experimental reflection and luminescence spectra are analyzed and discussed. The experimental results demonstrate the performance of this system. Optical and electrical properties of In0.15Ga0.85N/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) with different quantum well (QW) thicknesses were investigated by electric-field dependent ER spectroscopy. From the ER measurements, we have observed the well-resolved transition peaks related to InGaN QW. Furthermore, the transitions related to yellow luminescence (YL) from Si-doped GaN and blue luminescence (BL) from Mg-doped GaN were observed in the ER spectra of In0.15Ga0.85N/GaN MQW LEDs. With increasing QW thickness, the additional transitions related to InGaN QW can be attributed to the recombination of excitons localized at the shallow potential states in InGaN QW, originating from the In-poor InGaN regions caused by indium phase separation in InGaN QW. By applying a reverse bias voltage, the ER features related to InGaN QW were shifted to higher energy, resulting from the reduction of quantum confined Stark effect in InGaN QW with increasing reverse bias voltage. On the other hand, the ER features from YL and BL band related to the deep and the shallow impurity state exhibit redshift and broaden with reverse bias voltage. These results can be attributed to the reduction of Coulomb interaction between donor and acceptor caused by the increase of depletion regions with increasing reverse bias voltage.

Focused ion beam nano patterning for fabrications of III-nitride light emitting diodes

Photonic device structures often require nano scale lithography techniques for their device fabrication. The techniques are electron beam lithography and FIB(focused ion beam) pattering. Focused ion beam etching has been used as a nanolithography tool for the creation of these nanostructures without mask. We obtain nano scale mesa patterns on InGaN/GaN LED(light emitting diodes) wafer using focused ion beam and characterized. The InGaN/GaN LED wafer was grown by molecular organic vapor deposition (MOCVD). To reduce the surface damage during FIB patterning, we used a dielectric mask layer and wet etching to eliminate re-deposition of sputtering materials and Ga+ ion implantations and ion damage layer during FIB patterning, and finally, removed SiO2 with wet etching. A metal thin layer was deposited by an ion beam sputter to avoid charging effects during FIB patterning. We obtain a 2-Dimensional patterning for the fabrication of the high brightness LEDs. This FIB pattering technique can be applied to nanofabrication optoelectronic devices.