Toshimasa Suzuki

Arsenic-free GaAs substrate preparation and direct growth of GaAs/AlGaAs multiple quantum well without buffer layer

Abstract High-temperature treatment of GaAs substrate without As flux in a preparation chamber was investigated as a substrate surface cleaning method for molecular beam epitaxial (MBE) growth. Oxide gases such as CO and CO 2 were almost completely desorbed at a temperature above which Ga and As started to evaporate from the substrate. During the cleaning at a temperature as high as 575°C for 30 min, about 100 nm thick GaAs was evaporated from the substrate, but its surface maintained mirror-like smoothness and showed streak pattern with surface reconstruction pattern in the reflection high energy electron diffraction (RHEED) observation. Direct growth of GaAs/Al GaAs quantum well (QW) structures was tried on such surfaces without introducing any buffer layers. The QW structure showed photoluminescence with both intensity and full width at half maximum comparable with those for the QW grown on the substrate cleaned by the conventional method with introducing a GaAs buffer layer.

III–Nitride Epitaxy on Atomically Controlled Surface of Sapphire Substrate with Slight Misorientation

The importance of the atomically well-controlled surface of sapphire substrate with slight misorientation and ideally minimized surface roughness for III–nitride epitaxy is discussed in detail. An atomically controlled surface of sapphire substrate with slight misorientation angle is modeled and an almost ideal level of atomic surface roughness of sapphire substrate is found to be obtained by a chemical mechanical polishing (CMP) with colloidal silica. Cathodoluminescence (CL) imaging indicated the complete absence of subsurface damage induced by mechanical polishing. GaN and AlN thin films are grown on misoriented sapphire substrate with an atomically controlled surface by the CMP to investigate the misorientation angle of both sapphire and grown GaN and AlN thin films. An interface model is proposed to explain the difference in misorientation angle between sapphire and III–nitride thin films, providing strong evidence of the necessity of atomically controlled surface of sapphire substrate for III–nitride epitaxy.

Observation of dark line defects in InGaAs/GaAs strained layer superlattices by photoluminescence topography

Abstract The behavior of dark line defects (DLDs) in molecular beam epitaxy grown InGaAs/GaAs strained layer superlattices (SLSs) has been studied by photoluminescence (PL) topography. The density of DLDs parallel to the [011] was larger than that of those perpendicular to the [011] and increased with increasing number of SLS periods. These DLDs were considered to be originated from the locally deformed lattices by the misfit stress. The relaxation model of stress in MBE-grown InGaAs/GaAs SLSs was proposed from the obtained results.

Arsenic-free high-temperature surface cleaning of molecular beam epitxy (MBE)-grown AlGaAs layer with new passivation structure

To facilitate an MBE regrowth on an AlGaAs epilayer surface which is easily oxidized in the atmosphere, a passivation layer was provided on the top of the AlGaAs surface. This layer was either a GaAs single layer or a GaAs/ultra-thin AlGaAs/GaAs double-hetero (DH) structure layer grown at the final stage of the first MBE, and it was sublimated during the arsenic-free high-temperature surface cleaning which was carried out at the first stage of MBE regrowth. The cleaned AlGaAs surface was examined by Auger electron spectroscopy. It exhibited an intensity ratio of oxygen to Al, Ga and As much lower for the DH passivated AlGaAs than for the AlGaAs passivated by a GaAs layer. Large electron mobility was systematically obtained for the high electron mobility structure grown on the cleaned AlGaAs.

Behaviour of the superconductivity characteristics of the Bi system during water soaking

Samples of a bulk Bi-Pb-Sr-Ca-Cu-O system and a thick film of the Bi-Sr-Ca-Cu-O system are soaked in distilled water and the change in the superconductivity characteristics is measured. For the bulk samples, the maximum magnetization in the magnetization curve and the critical current density increase for the first 18 hours of soaking and then decrease during the subsequent soaking time. The critical temperature shows monotonic decrease throughout the soaking. A thick-film sample is almost the same as a bulk sample, i.e. the critical current density of the thick film first tends to increase for a short time due to its greater surface area then tends to decrease during the subsequent soaking. The critical temperature of the film sample also shows a slight deterioration as a result of the water soaking.

Micro-Raman study of the residual stress in molecular-beam-epitaxy-grown AlxGa1−xAs/GaAs multilayer structures

Abstract Extensive Raman-scattering investigations in the cleaved cross-sections of molecular-beam-epitaxy-grown Al x Ga 1− x As / GaAs multilayer structures on GaAs or silicon substrates have been carried out by micro-Raman spectroscopy. The structures of the epitaxial layers are Al 0.3 Ga 0.7 As/Al 0.2 Ga 0.8 As/Al 0.1 Ga 0.9 As/GaAs (type A) and Al 0.1 Ga 0.9 As/Al 0.2 Ga 0.8 As/Al 0.3 Ga 0.7 As/GaAs (type B). The largest residual stress was observed in the Al 0.3 Ga 0.7 As layer of the type B structure on GaAs substrate and the value was estimated to be about 1.5 × 10 10 dyn cm −2 in the compressive mode. It was also found that the mode of the residual stress in the substrates differed according to the epitaxial structures grown on them. A model of residual stress in Al x Ga 1− x As / GaAs multilayers grown on GaAs or silicon substrates is proposed: namely, if a substrate exists on the Al 0.1 Ga 0.9 As side, compressive stress acts on the substrate, and on the Al 0.3 Ga 0.7 As side, tensile stress acts.

Characterization of MBE grown InGaAsGaAs strained-layer superlattices and single quantum wells by photoluminescence topography

A photoluminescence (PL) topography which allows the imaging of crystal defects of semiconductors nondestructively has been applied to characterize MBE grown In0. 1Ga0. 9AsGaAs strained-layer superlattices (SLSs) and single quantum wells (SQWs). In SQWs dark line defects (DLDs) originate from misfit dislocations have been observed when well width is more than 40 monolayers. In SLSs cross stripes which are parallel and perpendicular to (011) have been observed. A propagation model of misfit dislocations is proposed by considering PL topograph patterns and surface morphology.

Influence of Mis-Orientation of C-plane Sapphire Substrate on the Early Stages of MOCVD Growth of GaN Thin Films

We have studied the early stages of GaN growth to realize the growth mechanism of GaN thin films on mis-oriented sapphire substrates which affects the surface and crystal quality of GaN thin films. As the result, it was found that the larger mis-orientation angle helps the growth of the larger grain of GaN and leads to the earlier shift of growth mode from 3D to 2D. The AFM observation of closed-coalesced GaN thin films revealed the difference in the micro-step structures by the mis-orientation angle of sapphire substrate. The result of x-ray rocking curve as a function of mis-orientation angle well matched with the microstructure of GaN surface, indicating that the larger mis-orientation angle helps the column ordering of GaN crystals.

High-temperature surface cleaning of AlGaAs without As flux for MBE regrowth

Abstract AlGaAs surfaces were cleaned by high-temperature heat treatment without As flux in a preparation chamber for regrowth of active layers by molecular beam epitaxy (MBE). Reflection high-energy electron diffraction from the cleaned AlGaAs surface showed a spotty pattern, but surface roughness observed by an atomic force microscope was smaller than the surface of cleaned GaAs. Auger electron spectroscopy showed that a cleaned surface was covered by Al, As and O 2 . Surface sublimation measured by quadrupole mass spectroscopy during the heat treatment decreased drastically with increasing alloy composition x , suggesting that the cleaned surface consisted of AlAs or its oxide. Quantum well (QW) structures were grown on the cleaned surfaces and photoluminescence (PL) spectra were measured. Well-defined PL peaks corresponding to stacked single QWs (SQWs) with 500 nm thick buffer layer were obtained at 300 K, which were in contrast to the SQWs grown on the surface cleaned by the conventional method. The PL intensity was influenced strongly by the thickness of the buffer layer when it was thinner than 300 nm. GaAs passivation of the AlGaAs surface was effective for obtaining intense PL spectrum from the regrown SQWs.

Raman study of the stress in MBE-grown AlxGa1-xAs on Si

Abstract Cleaved cross sections of two types of AlxGa1-xAs/GaAs multi-heterostructures grown on Si and GaAs substrates have been measured by micro-Raman spectroscopy to characterize the variation of the residual stress. Sample structures are Al0.3Ga0.7As/ Al0.2Ga0.8As/Al0.1Ga0.9As/GaAs/sub. (type A) and Al0.1Ga0.9As/Al0.2Ga0.8As/Al0.3Ga0.7As/GaAs/sub. (type B). Compressive stress has been found to exist in all AlxGa1-xAs layers grown on GaAs substrates. The value of the compressive stress in the type B Al0.3Ga0.7As layer, which was put between Al0.2Ga0.8As and GaAs layers whose lattice constants are both slightly smaller than that of the Al0.3Ga0.7As layer, has been estimated to be about 1.5×1010 dyn/cm2 from the shift of the TO phonon peak frequency. Compressive stresses of 1.0×109 and 1.2×1010 dyn/cm2 have been found in Al0.3Ga0.7As layers of type A and type B samples grown on Si substrates respectively. A tensile stress of (2.5–12)×109 dyn/cm2 has been found to exist in the other four AlxGa1-xAs layers and GaAs buffer layers grown on Si substrates. The origin of the residual stress has been discussed from the difference of the thermal expansion coefficients and the lattice mismatch.