T. Isu

Real‐time observation of molecular beam epitaxy growth on mesa‐etched GaAs substrates by scanning microprobe reflection high‐energy electron diffraction

Microscopic distribution of growth rates on mesa‐etched GaAs(001) wafers was measured in real time during molecular beam epitaxy growth by scanning microprobe reflection high‐energy electron diffraction. It has been observed that the growth rate on the GaAs(001) surface near the edge of (111)A surfaces becomes larger. The exponential variation of the growth rate as a function of the distance from the edge reflects surface diffusion of Ga atoms. The diffusion length on the (001) surface is estimated to be about 1 μm at 560 °C. The relatively larger diffusion length suggests that the incorporation rate of migrating Ga atoms by steps is much smaller than unity.

Surface diffusion length of Ga adatoms on (1̄1̄1̄)B surfaces during molecular beam epitaxy

The spatial variation of the growth rate on mesa‐etched GaAs (111)B substrates during molecular beam epitaxy of GaAs is measured from the period of the reflection high‐energy electron diffraction (RHEED) intensity oscillation using in situ scanning microprobe RHEED. The surface diffusion length of Ga adatoms on the (111)B surface is determined from the spatial variation of the growth rate. The surface diffusion length on the (111)B surface increases as the substrate temperature is raised or the arsenic pressure is decreased. The typical value of the diffusion length is about 10 μm at a substrate temperature of 580u2009°C and an arsenic pressure of 5.7×10−4 Pa, which is an order of magnitude larger than that on the (100) surface along the [011] direction. The activation energy of the surface diffusion length changes with the surface reconstruction. Anisotropic diffusion, as reported for the (100) surface, is not observed on the (111)B surface.

Observation of a large third-order nonlinear response of exciton polaritons in GaAs thin film

We investigated the third-order nonlinear response of excitons in GaAs thin films grown by molecular beam exitaxy. Degenerate four-wave mixing (DFWM) measurements were made using picosecond pulses of 3.2 ps auto correlation width at T=5 K. The DFWM signal of a 110-nm-thick GaAs film at exciton resonance energy is 25 times larger than that of a 1-μm-thick GaAs film. This enhancement is explained by the confinement effect of exciton polaritons, which is due to the nonlocal response of the center-of-mass quantized states of excitons in a radiation field.

Threshold height for movement of C60 molecules on Si(111)-7×7 with a scanning tunneling microscope

Scanning tunneling microscope has been employed for intentional movement of C60 molecules adsorbed on Si(111)‐7×7 surface. Threshold conditions at which a C60 molecule starts moving from its adsorption site are investigated in relation to tunnel current and tip bias voltage. We find that there exists threshold height for the movement process. The threshold height estimated from a planar electrode model is in agreement with the vertical height of the adsorbed C60 molecules. Present results evidence that direct contact or close proximity between a mechanical probe and C60 molecules is required for movement of the molecules to occur.

The condition for step flow in MBE growth on vicinal surfaces

We discuss the growth-mode transition from two-dimensional nucleation to step flow in MBE growth on vicinal surfaces, which is observed as the disappearance of RHEED intensity oscillations. With the sticking coefficient of adatoms to the step edges (pstep) and the coalescence probability of adatoms to form dimers (p11) taken into account, the critical condition for the growth-mode transition is derived for cases in which the surface diffusion coefficient D, as well as pstep and p11, is anisotropic. It is shown not only D, pstep and p11 along the direction perpendicular to the steps, but also those along the direction parallel to th affect the critical condition. In addition, we discuss the difference between the diffusion lengths obtained from the observation of the RHEED oscillation disappearance and those estimated from the distribution of the growth rate on patterned surfaces.

Ultra‐Fast Radiative Decay of Spatially Extended Non‐Dipole Type Excitonic State

By means of the non-local theory of the pulse response and the analysis of the exciton-radiation coupled modes, we elucidate a recently observed short response time of degenerate four-wave mixing (DFWM) of weakly confined excitons in a thin film. Especially, it is expected that the excitons with non-dipole type of center-of-mass wavefunctions exhibit a peculiar thickness dependence owing to a spatial structure of the radiation field. The experimental results of the three-beam DFWM of GaAs thin layers strongly support our theoretical explanation of the radiative decay that is beyond the long wavelength approximation.

Surface diffusion lenght during MOMBE and CBE growth measured by μ-RHEED

The growth rates of layers grown on a mesa-etched (001) GaAs surface were measured by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED) from the period of the RHEED intensity oscillation in real time. The diffusion lenght of the surface adatoms of column III elements was determined from the gradient of the variation of the growth rates in the cases of MBE, MOMBE using trimethylgallium (TMGa) and CBE using TMGa or triethylgallium (TEGa) and arsine (AsH3). The obtained values of the diffusion lengths were of the order of a micrometer in every case of the source-material combination. In the case of metalorganic materials as Ga source, it was found that the diffusion length was larger than that of Ga atom from metal Ga source. Since the substrate temperature of the present experiment is high enough to decompose TMGa and TEGa on the surface, Ga adatoms are considered to be responsible to the surface diffusion. Therefore, it is considered that the derivatives of the metalorganic molecules such as methyl radicals affect the diffusion of Ga adatoms.

Surface diffusion length during MBE and MOMBE measured from distribution of growth rates

Resumen Optoelectronics Technology Research Laboratory, 5-5 Tohkodai, Tsukuba, Ibaraki 300-26, Japan

Ultrafast optical response of excitons weakly confined in semiconductors

Ultrafast optical responses of excitons weakly confined in thin GaAs films are observed in a degenerate four-wave mixing experiment. The signal decay, which is attributed to the coherent response from the second quantized level of the center of mass motion, exhibits a nonmonotonic size dependence. The experimental results are explained by calculation results from a mode analysis based on the microscopic nonlocal theory. The decay time agrees well with the calculation of the radiative width of the confined excitons. These results show that strong coupling can be achieved between the radiation field and confined excitons by controlling the size of the thin semiconductor film.

Extremely smooth vertical facets of InP formed by reactive ion etching and selective chemical beam epitaxy regrowth

Abstract A new fabrication process, utilizing reactive ion etching (RIE) and selective regrowth by chemical beam epitaxy (CBE), was developed to form (011)-oriented InP facets vertical to the (100) substrate surface. The ridge structures were formed along the [011] direction by RIE from the wafers of InP and undoped InGaAs/InP double heterostructure (DH). The smoothness and the verticality of the sidewalls formed by RIE were improved during the selective regrowth by CBE. Scanning electron-microscope observation showed that the (011) facets formed by this process were as excellent as those formed by cleavage. Similar results were reproducibly obtained for the selective CBE regrowth on the sidewalls of the InGaAs/InP DH etched by RIE. This process is applicable to obtain highly reflective facet mirrors that are essential to fabricate short or micro cavity laser diodes.