S. K. Noh

Visible photoluminescence from self-assembled InAs quantum dots embedded in AlAs cladding layers

Photoluminescence (PL) from InAs self-assembled quantum dots (QD) embedded in the AlAs matrix was strong and clean around 700 nm. PL efficiency remained quite high at room temperature compared to other QD systems embedded in GaAs cladding layers. Transmission electron microscope pictures from the structure showed a clear formation of relatively small and coherently strained InAs QD. The observed blueshift with accompanying broadening of PL spectra with the increase of excitation power is interpreted in terms of local carrier tunneling in a dense QD system. The PL peak redshift with the increase of temperature was very large, as much as 228 meV. The anomalous shift is interpreted as due to activation-energy differences between dots of different sizes.

Performance improvement of InAs/GaSb strained layer superlattice detectors by reducing surface leakage currents with SU-8 passivation

We report on SU-8 passivation for performance improvement of type-II InAs/GaSb strained layer superlattice detectors (λcut-off∼4.6u2002μm). Optical and electrical behavior of SU-8 passivated and unpassivated devices was compared. The dark current density was improved by four orders of magnitude for passivated single diodes at 77 K. The zero bias responsivity and detectivity at 77 K was equal to 0.9 A/W and 3.5×1012 Jones for SU-8 passivated single pixel diodes. FPA size diodes (24×24u2002μm2) were also fabricated and they showed responsivity and detectivity of 1.3 A/W and 3.5×1012 Jones, respectively at 77 K.

Multi-stack InAs/InGaAs sub-monolayer quantum dots infrared photodetectors

We report on the design and performance of multi-stack InAs/InGaAs sub-monolayer (SML) quantum dots (QD) based infrared photodetectors (SML-QDIP). SML-QDIPs are grown with the number of stacks varied from 2 to 6. From detailed radiometric characterization, it is determined that the sample with 4 SML stacks has the best performance. The s-to-p (s/p) polarized spectral response ratio of this device is measured to be 21.7%, which is significantly higher than conventional Stranski-Krastanov quantum dots (∼13%) and quantum wells (∼2.8%). This result makes the SML-QDIP an attractive candidate in applications that require normal incidence.

Energy level control for self-assembled InAs quantum dots utilizing a thin AlAs layer

Ground-state energy of InAs quantum dots (QDs) in the GaAs matrix can be changed significantly by introducing a thin AlAs layer (1 nm). The photoluminescence (PL) peak position of the QDs grown directly on the thin AlAs layer is blueshifted by 171 meV from that of the QDs grown without the AlAs layer. QDs grown on an additional GaAs thin layer on top of the AlAs layer have PL peaks systematically redshifted to lower energy as the GaAs layer becomes thicker. Time-resolved PL shows that the QDs have similar lifetimes, attesting to the fact that all the QDs grown in this way are of high quality, although the energy level change is large and a thin AlAs layer is introduced.

Formation and optical properties of InAs/GaAs quantum dots for applications as infrared photodetectors operating at room temperature

The formation and optical properties of lateral quantum-dot infrared photodetector (QDIP) structures utilizing uniform-sized InAs QDs obtained by controlling the intensity of the reflection high-energy diffraction pattern and the As/In flux ratio were investigated. The thickness of the pseudomorphic, strained InAs wetting layer increased slightly with decreasing As/In flux ratio. The characteristic peaks corresponding to the interband and the intersubband transitions were observed, respectively, in the photoluminescence and Fourier transform infrared spectra. The absorption spectrum showed an absorption peak corresponding to the intersubband transition at around 10 μm. These results indicate that InAs/GaAs QDs obtained by controlling growth conditions hold promise for potential applications in QDIP devices operating at room temperature.

Sub-monolayer quantum dots in confinement enhanced dots-in-a-well heterostructure

We have investigated optical properties and device performance of sub-monolayer quantum dots infrared photodetector with confinement enhancing (CE) barrier and compared with conventional Stranski-Krastanov quantum dots with a similar design. This quantum dots-in-a-well structure with CE barrier enables higher quantum confinement and increased absorption efficiency due to stronger overlap of wavefunctions between the ground state and the excited state. Normal incidence photoresponse peak is obtained at 7.5u2009μm with a detectivity of 1.2u2009×u20091011u2009cm Hz1/2 W−1 and responsivity of 0.5u2009A/W (77u2009K, 0.4u2009V, f/2 optics). Using photoluminescence and spectral response measurements, the bandstructure of the samples were deduced semi-empirically.

Fabrication Of Self-Assembled GaAs/AIGaAs Quantum Dots By Low-Temperature Droplet Epitaxy

The fabrication of nanometer-scale GaAs dots on AlGaAs layer by molecular beam epitaxy was demonstrated. Unlike the stress-driven transition of the three-dimensional growth mode in the lattice-mismatched system, the limited migration of Ga droplets on the AlGaAs layer grown at low substrate temperature was exploited to give rise to the formation of three-dimensional GaAs islands. The resulting GaAs dots show crater-like features having {111} facets. In micro-photoluminescence measurements of the buried structures, the emission spectra were clearly observed, and the sharp lines of the spectra might be considered as the exciton emissions from individual dots with various sizes.

Characteristics of Mg-doped GaN epilayers grown with the variation of Mg incorporation

Abstract We have studied the growth characteristics of Mg-doped GaN epilayers grown by MOCVD with the variation of Cp 2 Mg flow rate. To optimize the p-type conductivity. We investigated the dependence of acceptor concentration on the dopant source (Cp 2 Mg) flow rate. The van der Pauw technique, double-crystal X-ray diffractometry (DCXRD) and photoluminescence (PL) were used to characterize their crystallographic, electrical and optical properties. As the incorporation of Mg in GaN epitaxy increases, the surface morphology and crystallinity of the layers become rough and worse because of the increase of lattice distortion due to the large difference of the atomic size between Ga and Mg. With the increase of Mg incorporation, the resistivity of the epilayers increases abruptly without discontinuity because of the increase of much Mg–H complex not cracked. So, it is possible to know that only the partial amount of Mg–H complex in the layers are unbound by annealing at a certain condition. In spite of the continuous increase of Mg incorporation, the hole concentration of the epilayers first increases and then decreases from a certain Mg flow rate. As well as the results of hole concentration, the blue emission intensity of the layers in PL spectra at room temperature first increases and then decreases from a certain Mg flow rate. Therefore, it can be concluded that there is a limitation in Mg activation and the hole concentration decreases from this limit though the incorporation of Mg increases.

Thermally stable, low‐resistance PdGe‐based ohmic contacts to high–low doped n‐GaAs

Thermally stable, low‐resistance PdGe‐based ohmic contacts to high–low doped n‐GaAs have been developed. The lowest contact resistance obtained is two times lower than that of previously reported PdGe ohmic contacts. The contacts are thermally stable even after isothermal annealing for 5 h at 400u2009°C under atmosphere ambient. X‐ray diffraction results and Auger depth profiles show that the good PdGe‐based ohmic contact is due to the formation of both AuGa and TiO compounds. The AuGa compound enhances the creation of more Ga vacancies, followed by the incorporation of Ge into Ga vacancies, and the TiO compound suppresses As outdiffusion from the GaAs substrate, respectively.

Thickness dependent properties of ZnSe on (100) GaAs grown by atomic layer epitaxy

Abstract ZnSe epilayers were grown on semi-insulating GaAs (100) substrates by ALE (atomic layer epitaxy) modified from CVD (chemical vapor deposition). The optical properties of ZnSe films depending on thickness were studied through micro-Raman and PL (photoluminescence) spectroscopy. The critical thickness was determined to be about 0.1 μm by analyzing the change in the peak shift of LO-phonon modes of ZnSe films. This is confirmed from the increase of the intensities of the deep center band in the PL spectra when the thickness exceeds 0.1 μm.