Dae Ho Lim

Selective etching of AlGaAs/GaAs structures using the solutions of citric acid/H2O2 and de-ionized H2O/buffered oxide etch

Etching results using the solution system of citric acid/H2O2 and de-ionized H2O/buffered oxide etch are shown to provide good selective wet etching of AlGaAs/GaAs structures. For AlxGa1−xAs (x 0.7) quickly decreases with decreasing Al composition in de-ionized H2O/buffered oxide etch solution, providing a high degree of etching selectivity. These simple selective etching processes have been applied to define AlAs/GaAs distributed Bragg reflector mesas in a vertical-cavity surface-emitting laser structure.

Lateral wet oxidation of AlxGa1−xAs‐GaAs depending on its structures

Data are presented demonstrating that the lateral wet oxidation of Al(Ga)As layer is strongly influenced by its thicknesses and heterointerface structures as well as Al compositions. The oxidation length decreases rapidly with decreasing AlAs thickness in the range of <80 nm and oxidation nearly stops at a thickness of ∼11 nm. Also, the oxidation rate of AlxGa1−xAs decreases quickly with decreasing Al composition, providing a high degree of oxidation selectivity. AlGaAs layers on both sides of AlAs layer reduce the lateral oxidation rate which is enhanced by the stress induced by oxidized AlAs.

Quality-enhanced GaAs layers grown on GeSi substrates by metalorganic chemical vapor deposition

High-quality GaAs layers were grown on Si substrates by low-pressure metalorganic chemical vapor deposition. Double crystal X-ray diffraction measurement shows that the structural properties of GaAs layers are so much improved by 5 μm thick Ge buffer layer and InGaAsGaAs strained-layer superlattice. The X-ray full-width at half-maximum of the (4 0 0) reflection obtained from 2 μm thick GaAs is 98 arcsec, one of the smallest value ever reported. The surface dislocation density of the GaAs layer estimated by molten KOH study is as low as 4 × 105cm−2. Photoluminescence and Raman measurements confirm the good optical quality of the heteroepitaxial GaAs epilayer.

Sealing of AlAs against wet oxidation and its use in the fabrication of vertical-cavity surface-emitting lasers

We have studied the process of sealing an exposed AlAs to prevent further wet oxidation. A critical step in this sealing process consists of the first wet oxidation for a short time at 408 °C in a steam environment of a previously room-ambient exposed AlAs surface. During this brief wet oxidation, a dense oxide surface barrier with a thickness of 1.1 μm is formed, which further blocks diffusing oxygen species during the second wet oxidation. The effectiveness of the sealing is demonstrated through its use as a mask against wet oxidation in the fabrication of oxide-confined vertical-cavity surface-emitting lasers.

Selective Oxidation of AlGaAs/GaAs Structure and Its Application to Vertical Cavity Lasers

We report on vertical-cavity surface-emitting lasers (VCSELs) fabricated using selective oxidation to form a current aperture and an oxide surface barrier to seal buried AlAs layers in distributed Bragg reflectors. The lateral selective oxidation of the AlAs layer is strongly influenced by its thickness and heterointerface structures. The oxidation rate decreases rapidly with decreasing AlAs thickness in the range of < 80 nm, and the presence of AlGaAs layers on both sides of the AlAs layer reduces the oxidation rate. In addition, an oxide surface barrier with a thickness of ~1 µm is formed by a brief wet oxidation which blocks diffusing oxygen species during the second wet oxidation. The effectiveness of the seal is demonstrated by using the oxide barrier as a mask against wet oxidation in the fabrication of oxide-confined VCSELs. The 8×8 µm2 devices with three quantum wells exhibit a threshold current of 400 µA, and the 4×4 µm2 single-quantum devices possess a threshold current as low as 85 µA.

Temperature and emission angle dependence of photoluminescence from an InGaAs/GaAs quantum well in a microcavity structure

The spontaneous emission from an InGaAs/GaAs quantum well embedded in a semiconductor vertical-cavity structure is studied. The spontaneous emission enhancement at the cavity mode is clearly identified from the photoluminescence measurement, depending on the wavelength separation between the excitonic emission and the cavity resonance mode which is tuned by changing the measurement temperature and the emission angle. The emission intensity of the cavity resonance mode is increased as the excitonic emission approaches the cavity mode, thereby obtaining evidence of coupling between the exciton and the cavity mode. The emission wavelength of the cavity mode shifts to shorter wavelengths as the emission angle increases, which is clarified with theoretical calculations performed with the transfer-matrix method.

Self-organized growth of InGaAs/GaAs/AlGaAs quantum dot heterostructures by metalorganic chemical vapor deposition

Self-organized In0.6Ga0.4As/GaAs/AlGaAs quantum dot (QD) heterostructures were grown by metalorganic chemical vapor deposition using the Stranski–Krastanow growth mode. The atomic force microscopy image shows that the quantum dot structures are formed. The room-temperature photoluminescence (PL) measurement was employed to establish the optimized growth condition for QD structures. A strong PL emission was obtained at room temperature. Blueshifts in the PL emission from InGaAs QDs are observed as a results of postgrowth annealing and also when raising the upper cladding layer growth temperatures. Also, the strong electroluminescence at room temperature shows the possibility of QD optical device. Using the optimized growth condition, a full laser structure was fabricated by selective oxidation.

Effect of Al‐rich surface on Se δ‐doped GaAs grown by low‐pressure metalorganic chemical vapor deposition

We study Se δ‐doped GaAs grown by low‐pressure metalorganic chemical vapor deposition using hydrogen selenide as a doping precursor. The results of capacitance–voltage measurements show that the very sharp doping profile can be obtained by Se δ‐doping on an Al‐rich surface and Se segregation is also reduced by making an Al‐rich surface after δ‐doping. It is essential to utilize the δ‐doping sequence which does not have a post‐δ‐doping purge step to minimize the Se evaporation.