Suk Ki Min

Selective formation of one- and two-dimensional arrayed InGaAs quantum dots using Ga2O3 thin film as a mask material

We report on the selective formation of InGaAs quantum dots (QDs) by molecular beam epitaxy. Nanoscale patterned Ga2O3 thin film deposited on the GaAs (100) substrate was employed as a mask material. Due to the enhanced migration effect of the group-III adatoms, such as Ga and In on Ga2O3 mask layer, the InGaAs QDs formed on the patterned substrate results in coalesced islands unlike those formed on the nonpatterned substrate. The estimation of the relative volume of the islands per unit area revealed that the desorption process as well as the migration of the Ga and In adatoms might occur on the Ga2O3 layer during the growth process, providing a good selective growth of self-assembled QDs.

Deposition and Properties of Reactively Sputtered Ruthenium Dioxide Thin Films as an Electrode for Ferroelectric Capacitors

Ruthenium dioxide (RuO2) films are studied for use as a bottom electrode of the (Ba, Sr)TiO3 thin-film capacitor. RuO2 films have been deposited by reactive DC magnetron sputtering of ruthenium at a relatively low sputtering power. Stoichiometric RuO2 films are obtained at oxygen partial pressures as low as 0.6 mTorr. The properties of the films have been investigated using techniques such as Rutherford backscattering spectrometry, Auger electron spectrometry, X-ray diffraction, and scanning electron microscopy. The oxygen composition in as-deposited RuOx films increases from 2.0 to 2.4 with the increase of initial O2 partial pressure from 1.2 to 5.6 mTorr at a sputtering power of 200 W. The films deposited under low oxygen partial pressures followed by annealing show preferential crystal growth in the [110] direction, whereas those deposited in high oxygen partial pressures show growth in the [101] direction. A resistivity of 65 µΩcm is obtained after annealing at 800°C. Even after high-temperature deposition and subsequent annealing processes, clear interfaces between (Ba, Sr)TiO3 and RuO2 films are obtained.

Selenium and silicon delta‐doping properties of GaAs by atmospheric pressure metalorganic chemical vapor deposition

We investigate selenium and silicon delta‐doping characteristics for GaAs layers grown by atmospheric pressure metalorganic chemical vapor deposition. Broadened capacitance‐voltage (C‐V) profiles and low doping activities are found in case of Se delta doping. In contrast, excellent delta‐doping characteristics result for silicon. The obtained C‐V full width at half maximum are in the range of 44–49 A for samples even with substrate temperatures of 700–750 °C, which are relatively high compared with the molecular‐beam epitaxy (MBE) substrate temperature. Hall mobility measurements show the mobility enhancement due to a screening of impurity charges. Also, Shubnikov–de Haas oscillations demonstrate the two‐dimensional nature of electrons confined in the delta‐doped layers. Therefore, our delta‐doped layers show no substantial differences in quality with MBE‐grown delta‐doped layers. In addition, preliminary delta field‐effect transistors having a gate length of 1.5 μm and a gate width of 300 μm are fabricated.

DIRECT FORMATION OF NANOCRYSTALLINE SILICON BY ELECTRON CYCLOTRON RESONANCE CHEMICAL VAPOR DEPOSITION

We have confirmed the direct formation of nanosized crystalline silicon during the deposition of amorphous silicon layers by electron cyclotron resonance chemical vapor deposition (ECRCVD) on silicon and silicon–dioxide substrates. Two photoluminescence (PL) peaks at 680 and 838 nm were observed at room temperature from the samples. From cross-sectional high-resolution transmission electron microscopy (HRTEM) measurements, it was confirmed that nanosize silicon crystallites of 3–5 nm in diameter were randomly distributed throughout the amorphous silicon layer. Theoretical calculations using quantum size effects gave an average crystalline size of 4 nm which was consistent with the PL peak energy at 680 nm obtained from the sample. Also, the size of the crystallites could be controlled by the change of the substrate temperature during the deposition process.

Improvement of carrier capture efficiency of short‐period GaAs/AlGaAs quantum wire array by a new lithography method

Short‐period GaAs/AlGaAs quantum wire array (QWA) was fabricated by metalorganic chemical vapor deposition on the GaAs substrate with submicron gratings. A strong photoluminescence signal derived from highly dense QWA was detected in the as‐grown sample. To identify the signal more clearly, all the layers except QWR regions should be removed. However, the small dimension of the sample made it difficult to do that with conventional lithography techniques. We have developed a novel lithography technique which can be applied to nonplanar structures. We could completely remove the (100) and the (111)A quantum well (QW) layers using the technique. As a result, we could clearly observe the optical properties of short‐period QWA by improving the carrier capture efficiency of QWR regions.

Properties of the quantum wires grown on V‐grooved Al0.3Ga0.7As/GaAs substrate by atmospheric pressure metalorganic chemical vapor deposition

We report the successful growth of quantum wire (QWR) structures of AlxGa1−xAs/ GaAs /AlxGa1−xAs on a V‐grooved Al0.3Ga0.7As/GaAs substrate. The samples are studied by photoluminescence spectra. The structures are grown by atmospheric pressure metalorganic chemical vapor deposition (MOCVD). The GaAs QWRs are fabricated on a V‐grooved Al0.3Ga0.7As/GaAs substrate instead of GaAs substrate. Due to the effect of the Al0.3Ga0.7As layer, a necking area is formed in the side quantum wells (QWLs) near the bottom of the V groove. This results in a reduction of the lateral widths of the QWRs. Also, the luminescence of the QWRs is greatly enhanced in spite of low packing density. It is the first structure made in attempts to produce the quantum‐size effects on a V‐grooved substrate with non‐(111) facets.

Effective carrier confinement of a short-period GaAs/AlGaAs quantum wire array

Quantum wire (QWR) arrays grown on a GaAs substrate with V-grooved submicrometre gratings were investigated by high-resolution scanning electron microscopy (SEM) and temperature-dependent photoluminescence (PL) spectra. All samples were grown by atmospheric pressure metalorganic chemical vapour deposition (MOCVD). Elliptical QWR arrays having a central thickness of 20 nm and an effective width of 40 nm were produced at the bottom of the V-grooved gratings. An intense and sharp PL peak with a full width at half maximum (FWHM) of 6 meV was observed from the QWR array at 21 K, which shows a sufficient mode coupling among the neighbouring QWRs and an effective carrier confinement at the quantum wire subbands. The distinct temperature dependence of the PL spectra also provides evidence of a well-fabricated QWR array. The small variation of the FWHM and peak energy from the temperature-dependent PL spectra implies that the carrier confinement at the QWR subbands is retained up to elevated temperatures.

Performance of GaAs-AlGaAs V-grooved inner stripe quantum-well wire lasers with different current blocking configurations

GaAs-AlGaAs V-grooved inner stripe (VIS) quantum-well wire (QWW) lasers grown by metalorganic chemical vapor deposition with different current blocking configurations, n-blocking on p-substrate (VIPS), p-n-p-n blocking on n-substrate (VI(PN)/sub n/S) and p-blocking on n-substrate (VINS) have been fabricated and characterized. The VIPS QWW lasers show the most stable characteristics with effective current confinement: one of the lasers shows fundamental transverse mode, lasing up to 5 mW/facet, typical threshold current of 39.9 mA at 818.5 mm, an external differential quantum efficiency of 24%/facet, and characteristic temperature of 92 K. The current tuning rate was almost linear at 0.031 mm/mA, and the temperature tuning rate was measured to be 0.14 nm//spl deg/C. Comparison of the light output versus current characteristics of the lasers with different current blocking configurations is presented here.

A short-period GaAs-AlGaAs quantum-wire array laser with a submicrometer current blocking layer

The structure and device characteristics of a 700-nm-pitch GaAs-AlGaAs quantum-wire array laser (QWAL) with a dielectric defined current blocking layer are reported. The high wire density of the QWAL has been expected to yield more efficient carrier capture, but large spacing between the quantum wires was found to deteriorate the laser characteristics. In this work, we have improved electrical confinement into the active regions by incorporating a SiO/sub 2/ film onto the large spacing. Room-temperature pulsed operation with an output power of 9 mW at 191-mA injection current was achieved for a 200/spl times/500 /spl mu/m laser with uncoated facet. The threshold current density was 0.14 kA/cm/sup 2/. The dependence of the threshold current and the maximum power on the cavity length and width was also studied.

Fabrication of V-grooved inner stripe GaAs-AlGaAs quantum-wire lasers

V-grooved inner stripe (VIS) GaAs-AlGaAs quantum-wire (QWR) lasers were successfully fabricated by, combining two-step metalorganic chemical vapor deposition (MOCVD) growth with a wet-etching technique. In order to achieve low threshold current density and high reliability, a conductive stripe width (W), a thickness (t/sub p-CBL/), and a doping concentration (n/sub p-CBL/) of the p-GaAs current-blocking layer (CBL) were determined to be W=1.2 /spl mu/m, t/sub p-CBL/=2 /spl mu/m, and n/sub p-CBL/=1/spl times/10/sup 18/ cm/sup -3/. The leakage currents passing through the CBL were also estimated using a modified P-SPICE. Thus far, a threshold current of 45 mA and an output power of 4 mW at 51 mA have been achieved under room-temperature pulsed operation for some devices with uncoated facets.