X.Q. Shen

Arsenic Pressure Dependence of Surface Diffusion of Ga on Nonplanar GaAs Substrates

The arsenic pressure dependence of Ga adatom surface diffusion in molecular beam epitaxy (MBE) on nonplanar substrates was investigated. By using in situ scanning microprobe reflection high-energy electron diffraction (µ-RHEED), the distribution of the growth rate of GaAs on the (001) surface near the edge of the (111)A or (111)B sidewall was measured under various arsenic pressures. The surface diffusion length of Ga adatom incorporation on the (001) surface derived from the distribution is on the order of micrometers and it shows a strong dependence on arsenic pressure. A simple model based on one-dimensional surface diffusion was proposed. With this theory, the lifetime of Ga adatom incorporation on other surfaces is obtained.

Surface diffusion length of cation incorporation studied by microprobe-RHEED/SEM MBE

Abstract The surface diffusion of group III atom incorporation in the MBE of GaAs and InAs is reviewed. First the diffusion length of incorporation on the (001) top surface with the (111)A or (411)A side surfaces of V grooves is discussed. It is shown that the diffusion length takes the same value for both cases and is inversely proportional to the arsenic pressure. The same relationship is also obtained for the diffusion of In in InAs MBE. However, the diffusion length of Ga on (111)B shows an inverse parabolic dependence of the arsenic pressure. It is suggested that on the (001) surface two As 4 molecules meet to give active As atoms for the growth. On the other hand, the behavior of the As 4 molecule on the (111)B surface is still not clear. The ratio of the surface diffusion coefficient on (111)B and that on (001) is calculated. It is found that the ratio takes a value of around 140. With this ratio, the incorporation life times τ inc on (111)B and (001) surfaces are calculated as functions of arsenic pressure. It is found that the lines of the incorporation life time intersect at the arsenic pressure where flow inversion occurs.

Resharpening effect of AlAs and fabrication of quantum-wires on V-grooved substrates by molecular beam epitaxy

Abstract The resharpening effect of AlAs grown on V-grooved substrates by MBE was first observed. The surface diffusions in GaAs/AlAs and InGaAs/AlAs heterostructures were also studied by high-resolution scanning electron microscope (SEM). With the measurement of PL at 77 K and the observation of high-resolution SEM, it was found that multiple GaAs/AlAs quantum-wires with a size of about 140–160 A × 400–600 A at the bottom of the V-grooves were formed. From the results, a new way of fabricating GaAs/AlAs and InGaAs/AlAs multiple quantum-wires at the bottom of V-grooves by MBE was proposed.

Molecular beam epitaxial growth of GaAs, AlAs and Al0.45Ga0.55As on (111) A-(001) V-grooved substrates

Abstract Molecular beam epitaxial (MBE) growth of GaAs, AlAs, and AlGaAs on V-grooved GaAs substrates are studied. Observing the morphology of the heterostructures grown on the various part of V-grooves with scanning electron microscopy (SEM), it is shown that GaAs, AlAs and AlGaAs have different elemental growth parameters such as surface diffusion and atom incorporation, leading to the difference in growth behavior. Under appropriate growth conditions, in particular, the growth of AlAs results in very smooth surface on the (111)A sidewalls and drastic sharpening at the bottom of the V-grooves, whereas the growth of AlGaAs and GaAs do not show such phenomena. MBE growth mechanisms, both on the V-grooved substrates and on the (111)A flat surface, are discussed based on the phenomena observed in our experiments. Furthermore, we investigate the growth temperature dependence of the morphology and the surface diffusion of atoms during the growth of GaAs/AlAs heterostructures on the V-grooves, and demonstrate the successful fabrication of GaAs multiple quantum-wire structures at the bottom of the V-grooves by high-resolution SEM observations and cathodoluminescence measurements at 16K.

Arsenic Pressure Dependence of the Surface Diffusion in Molecular Beam Epitaxy on (111)B-(001) Mesa-Etched GaAs Substrates Studied by In Situ Scanning Microprobe Reflection High-Energy Electron Diffraction

The arsenic pressure dependence of surface diffusion of Ga adatoms in molecular beam epitaxy (MBE) on (111)B-(001) mesa-etched substrates was investigated by means of in situ scanning microprobe reflection high-energy electron diffraction (µ-RHEED). It was observed for the first time that the direction of Ga adatom migration from or to the (111)B sidewall is changed depending on the arsenic pressure. Furthermore, the diffusion length of Ga adatoms on the (001) surface along the [10] direction was found to vary with arsenic pressures exponentially; however, it was independent of the direction of lateral migration. The diffusion length of Ga adatoms on the (001) surface along the [10] direction varied from about 0.25 µm to 1.2 µm at 600°C within our arsenic pressure range. This suggests that the lifetime of Ga adatoms before incorporation into the crystal on each surface depends strongly on arsenic pressure.

Inter-surface diffusion of In on (111)A-(001) InAs nonplanar substrates in molecular beam epitaxy

Abstract The inter-surface diffusion of In adatoms between a (111)A and a (001) surface on the InAs (111)A-(001) nonplanar substrate was investigated for the first time using microprobe reflection high-energy electron diffraction (μ-RHEED). The surface diffusion of In adatoms is found to depend strongly on the growth temperature and the arsenic pressure, while it is independent of the In flux. Furthermore, it was observed that the migration direction of the In lateral flux between the (111)A and the (001) InAs surface changes depending on the growth conditions.

In situ observation of macrostep formation on misoriented GaAs(111)B by molecular beam epitaxy

Abstract This paper reports, for the first time, the formation of straight macrosteps during the growth of GaAs on misoriented GaAs(111)B substrates inclined toward the [110] direction, and their in situ observation in a microprobe-reflection high-energy-electron-diffraction/scanning-electron-microscope molecular-beam-epitaxy system. It was found that step bunchaing usually occurs at a lower As 4 flux under a √19 × √19 surface reconstruction, while smooth surfaces are obtained at higher As 4 flux. In situ observation showed that macrosteps are formed gradually during the growth and finally develop into giant macrosteps consisting of flat terraces and rough risers with straight step edges along the [110] direction. Atomicforce-microscopy studies revealed that these macrosteps are composed of near-(111)B terraces and near-(110) risers.

Initial growth behaviors of disk-shaped mesas in GaAs molecular beam epitaxy on GaAs(111)B substrates

Abstract The real-time observations of initial growth behaviors were carried out in molecular beam epitaxy (MBE) of GaAs on disk-shaped mesa (111)B substrates by micro-probe reflection high-energy diffraction/scanning electron microscope (μ-RHEED/SEM) MBE system. It was found that the initial growth behavior depends strongly on the growth temperature. In the experiments, the initial growth process under different surface reconstructions (Ga stabilized (√19 × √19) and Ga-rich (1 × 1) HT surface reconstructions) was extensively studied. During the growth, it was found that {221} facets surrounding the foot of the mesas are formed at 580 and 610°C, but the process and time for facet appearance are quite different. At 620°C, the macrostep and associated black and white stripes composed of (√19 × √19) and (1 × 1) HT reconstructions were observed to propagate outward on the foot of the mesa, which resulted in the formation of gentle slopes with macrosteps. In the whole range of present growth temperature, it was found that Ga atoms diffuse from sidewalls to the bottom of the mesa especially in the beginning of the growth.

Arsenic pressure dependence of pure two-face inter-surface diffusion between (0 0 1) and (1 1 1)B in molecular beam epitaxy of GaAs

Abstract Inter-surface diffusion between (0 0 1) and (1 1 1)B surfaces was studied. From the measurements of the growth rates of top, side and bottom surfaces, it turned out that if the length of the sidewall is short, three-face inter-surface diffusion takes place on mesa-structures. To prevent such complexity, the (1 1 1)B long sidewall was prepared by mask LPE and pure two-face inter-surface diffusion was studied using microprobe-RHEED/SEM MBE. It was confirmed that the direction of pure two-face inter-surface diffusion between the (0 0 1) top surface and the (1 1 1)B sidewall is reversed by varying the arsenic pressure, although the critical value of the arsenic pressure was different from the previously reported value on the short-sidewall mesas.