Study of the optical and structural properties of multi quantum well structures grown on high-index surfaces
The MOVPE overgrowth of high [01@?1]-oriented ridges confined at sides by facets related to {n11} crystallographic planes is reported. We studied the influence of the side tilt on the thickness of the AlGaAs and GaAs epitaxial layers grown under the condition of the kinetic growth mode. The multi quantum well (MQW) structures were prepared on the sides of ridges tilted at 54.7^o, 45^o and 30^o to (100). The sidewall surface morphologies before and after epitaxial growth were evaluated and compared. We observed no tendency towards planarization towards a neighbouring high-index crystallographic plane, such as (211) and (311). We also showed that the quantum wells of the MQW structure make a smooth transition over the edge between the top surface and the facet as both AlGaAs and GaAs grew at similar rates on the surfaces.
Infrared absorption and current-voltage characteristic of GaAs/ AlGaAs multiple quantum wells on GaAs (111)A substrate grown by solid source molecular beam epitaxy
Abstract Infrared absorption and current–voltage characteristic of Si-doped GaAs/AlGaAs multiple quantum wells (MQW) grown on GaAs (1 1 1)A substrate by solid source molecular beam epitaxy has been presented in this paper. Strong polarization dependence was found and the normal incidence absorption is dominant in the (1 1 1)A-oriented MQW structure with the peak absorption wavelength around 7 μm. The final excited state is quite higher than the top of the barrier and the polarization property is complicated by the coupling between the valence heavy hole and light hole states in the continuum subband. The symmetric dark current characteristic to the zero voltage under positive and negative bias conditions attributes to the bias symmetric structures for (1 1 1)A-oriented MQW and the negligible outdiffusion of Si during the growth. The activation energies extracted from I – V characteristic curve decrease with increasing bias voltage in an exponential relation. The overall agreement between the activation energy and the calculated result has also been obtained.
Experimental investigation of the geotechnical properties and microstructure of lime-stabilized saline soils under freeze-thaw cycling
Abstract The Song-Nen Plain in northeastern China holds one of the three largest expanses of soda saline-alkali soil in the world. Due to its disadvantageous geotechnical properties, the saline soil in this area is unable to meet the needs of construction projects. This study evaluates the stabilizing effect of adding lime to such soils. Specifically, it investigates how the physicochemical and mechanical properties and microstructure of soda saline-alkali soil are altered by lime and how these changes are affected by both freeze-thaw cycling and the duration of curing. The results reveal that adding lime to saline soil transforms clay particles into sand and silt particles and causes the pore size distribution (PSD) to become bimodal, with small-pore and macropore populations. An increase in lime content leads to an increase in the unconfined compressive strength (UCS) but a gradual decrease in both the cation exchange capacity (CEC) and plastic index. Freeze-thaw cycles break up coarse particles and aggregate finer particles, increasing the diameter of macropores and decreasing the USC; however, the effect on specimens with >9% lime is negligible. It is further shown that the geotechnical properties of the soil change immediately after the addition of lime, indicating that the cation exchange reaction takes place very quickly. Furthermore, the pozzolanic reaction leads to an increase in the percentage of sand-sized particles and a decrease in porosity after 28 days of curing. Overall, these results indicate that it is feasible to use lime to improve the engineering properties of soda saline-alkali soil in this area.