Jim-Yong Chi

GaN metal-semiconductor-metal ultraviolet photodetectors with transparent indium-tin-oxide Schottky contacts

Indium-tin-oxide (ITO) layers were deposited onto n-GaN films and/or glass substrates by electron-beam evaporation. With proper annealing, we found that we could improve the optical properties of the ITO layers and achieve a maximum transmittance of 98% at 360 nm. GaN-based metal-semiconductor-metal (MSM) photodetectors with ITO transparent contacts were also fabricated. A maximum 0.12-A photocurrent with a photocurrent to dark current contrast higher than five orders of magnitude during ultraviolet irradiation were obtained for a photodetector annealed at 600/spl deg/C. We also found that the maximum photo responsivity at 345 nm is 7.2 and 0.9 A/W when the detector is biased at 5 and 0.5 V, respectively.

1.3-/spl mu/m InAs-InGaAs quantum-dot vertical-cavity surface-emitting laser with fully doped DBRs grown by MBE

We report InAs-InGaAs quantum-dot vertical-cavity surface-emitting lasers (VCSELs) grown by molecular beam epitaxy with fully doped n- and p-doped AlGaAs distributed Bragg reflectors and including an AlAs layer to form a current and waveguiding aperture. The metal contacts are deposited on a topmost p/sup +/-GaAs contact layer and on the bottom surface of the n/sup +/-GaAs substrate. This conventional selectively oxidized top-emitting device configuration avoids the added complexity of fabricating intracavity or coplanar ohmic contacts. The VCSELs operate continuous-wave at room temperature with peak output powers of 0.33 mW and differential slope efficiencies up to 0.23 W/A. The peak lasing wavelengths are near 1.275 /spl mu/m, with a sidemode suppression ratio of 28 dB.

Polarized light emission from photonic crystal light-emitting diodes

We have experimentally studied polarization characteristics of the two-dimensional photonic crystal (PhC) light-emitting diodes (LEDs) using an annular structure with square lattice and observed a strong polarization dependence of the lattice constant and orientation of the PhC. The extracted light from the GaN PhC LEDs has P∕S ratios of 5.5 (∼85% polarization light) for light propagating in the ΓX direction and 2.1 (∼68% polarization light) for the ΓM direction, respectively. Based on the couple mode theory, the dependence of polarization behaviors on different lattice constant and orientation was found to be in good agreement with theoretical discussion.

Anisotropy of light extraction from GaN two-dimensional photonic crystals

Angular distribution of light diffracted out of the plane of two-dimensional photonic crystals (PhC) has been studied in the azimuthal direction with a specially designed waveguide structure. The optical images of the light extraction patterns from the guided photoluminescence light are obtained with laser excitation in the center of the annual structure made on GaN multilayer. For increasing lattice constant, symmetric patterns with varying number of petals according to the symmetry of the PhC are observed. The appearance and disappearance of the petals can be explained using the Ewald construction in the reciprocal space. The dependence of light extraction on the lattice constant and orientation, and incident angle are investigated.

Influence of doping density on the normal incident absorption of quantum-dot infrared photodetectors

The influences of doping densities at the quantum-dot (QD) region for 30-period InAs∕GaAs quantum-dot infrared photodetectors (QDIPs) are investigated. The InAs∕GaAs QDIPs with a lower doping density can operate at high responsivity and high background-limited-performance temperature. Also observed is the decreasing photocurrent ratio of s∕p-polarized lights for the QDIPs with increasing QD doping density. Compared to the similar photocurrent ratio of s∕p-polarized lights for the GaAs∕AlGaAs quantum-well infrared photodetectors at different applied voltages, the observed voltage-dependent response ratio for QDIPs is attributed to the strong scattering characteristics of QDs occupied with photoexcited electrons for electron transport through the QD region.

Characteristics of MOCVD- and MBE-grown InGa(N)As VCSELs

We report our results on InGaNAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) in the 1.3 ?m range. The epitaxial structures were grown on (1?0?0) GaAs substrates by metalorganic chemical vapour deposition (MOCVD) or molecular beam epitaxy (MBE). The nitrogen composition of the InGa(N)As/GaAs quantum-well (QW) active region is 0?0.02. The long-wavelength (up to 1.3 ?m) room-temperature continuous-wave (RT CW) lasing operation was achieved for MBE- and MOCVD-grown VCSELs. For MOCVD-grown devices with n- and p-doped distributed Bragg reflectors (DBRs), a maximum optical output power of 0.74 mW was measured for In0.36Ga0.64N0.006As0.994/GaAs VCSELs. A very low Jth of 2.55 kA cm?2 was obtained for the InGaNAs/GaAs VCSELs. The MBE-grown devices were made with an intracavity structure. Top-emitting multi-mode 1.3 ?m In0.35Ga0.65N0.02As0.98/GaAs VCSELs with 1 mW output power have been achieved under RT CW operation. A Jth of 1.52 kA cm?2 has been obtained for the MBE-grown In0.35Ga0.65N0.02As0.98/GaAs VCSELs, which is the lowest threshold current density reported. The emission characteristics of the InGaNAs/GaAs VCSELs were measured and analysed.

Far-field of GaN film-transferred green light-emitting diodes with two-dimensional photonic crystals.

Far-field distributions of GaN-based photonic crystal (PhC) film-transferred light-emitting diodes (FT-LEDs) were investigated. The thickness of the device is about 840 nm. The emission wavelength is around 520 nm. The PhC region is a square lattice with a/lambda around 0.5. Angular-resolved measurements in the Gamma-X and Gamma-M directions were made in the polarized-resolved manner. Guided mode extraction behavior in agreement with the two-dimensional free-photon band calculation was observed. In addition, the pseudo-TM behavior for the non-collinearly coupled modes was observed. The azimuthal-mapping of the angular-resolved spectra revealed the evolution of the collinearly and the non-collinearly coupled modes. Furthermore, the light enhancement of approximately 2.7x and the collimation angle about 102.3 degrees were achieved.

Quantum-dot infrared photodetectors with p-type-doped GaAs barrier Layers

In this letter, we investigate the effects of p-type-doping concentration in the GaAs barrier layer on the multistack InAs-GaAs quantum-dot infrared photodetectors (QDIPs). The dark current decreases with the p-type-doping density in the GaAs barrier layer. The QDIP with five stacks and a p-type-doping concentration of 1/spl times/10/sup 16/ cm/sup -3/ in the GaAs barrier layer and show an observable spectral response with a peak responsivity of 0.5 A/W at 6 /spl mu/m. With increasing the p-type-doping density up to 1/spl times/10/sup 17/ cm/sup -3/, the QDIP structure shows a reduced photocurrent and a nondetectable response. The QDIPs with p-type-doped GaAs barrier layers and different stack numbers exhibit a saturation photocurrent, where the saturation voltage is proportional to the stack number.

Relaxation-induced lattice misfits and their effects on the emission properties of InAs quantum dots

Strain relaxation in InAs/InGaAs quantum dots (QDs) is shown to introduce misfits in the QD and neighboring GaAs bottom layer. A capacitance?voltage profiling shows an electron accumulation peak at the QD with a long emission time, followed by additional carrier depletion caused by the misfits in the GaAs bottom layer. The emission-time increase is explained by the suppression of tunneling for the QD excited states due to the additional carrier depletion. As a result, electrons are thermally activated from the QD states to the GaAs conduction band, consistent with observed emission energies of 0.160 and 0.068?eV which are comparable to the confinement energies of the QD electron ground and first-excited states, respectively, relative to the GaAs conduction band. This is in contrast to non-relaxed samples in which emission energy of 60?meV is observed, corresponding to the emission from the QD ground state to the first-excited state.

Temperature dependence of carrier dynamics for InAs∕GaAs quantum dot infrared photodetectors

Temperature-dependent micro-photoluminescence (μ-PL) spectra and the spectral response for the 30-period undoped InAs∕GaAs quantum-dot infrared photodetectors (QDIPs) are investigated. The competition of different transition levels within the spectra is observed. Also observed is the influence of dot size and density on the μ-PL characteristics. Consistently, the mid-infrared spectral response for the fabricated QDIPs exhibit the same energy position as the shifted PL spectra relative to the energy of wetting layer, which indicates the multi-transition mechanisms responsible for the QDIP spectral response.