Amorphous semiconductor superlattice is attractive for developing new electronic devices. The superlattice structure with a-Si: H and a-AIO has a deep quantum well of a-Si: H, since a-AIO has a larger bandgap and a smaller electron affinity as compared with those of a-Si: H. Photo-CVD with ArF excimer laser (λ=193nm) makes use of photolysis of the source gases and is suitable for the superlattice formation because of little interface damage. We form a-Si: H/a-AIO superlattice and investigate its electrical and optical properties. Deposition of a-Si: H layer and a-AlO was performed with Si2H6 gas, and TMA (trimethylaluminium) and oxygen gas on quartz or silicon substrates heated at 200°C and 300°C, respectively. Si, O and Al signals increase and decrease periodically on the depth profiles by Auger electron spectroscopy analysis, showing the periodical change of the composite elements. Optical bandgap was detemined from wave length dependence of absorption coefficient (Tauc plot). The optical bandgap increases with the decrease of the well-layer thickness of the superlattice, which is due to the quantum level formed in the quantum well. Photo- and dark-conductivity of superlattice paralell to the layers increase as compared with that of monolayer a-Si: H. This result is an inherent characteristics of superlattice.
In a double quantum well structure which has two quantum wells with different well width, we have quantized levels in each well at different energies. If the potential barrier thickness between the wells is thin enough, the transfer of electrons from one well to the other is due to the tunneling. In this system, because of the energy and momentum conservation law, the tunneling must be assisted by emmision or absorption of phonons. Using the femtosecond population mixing methods in the photoluminescence intensity measurements, we studied the tunneling time at 77 K in GaAs/Al0.3Ga0.7As double quantum well structures grown by MBE. The tunneling time was a strong function of the difference energy as well as of the potential barrier thickness. If the difference energy is larger than LO phonon energy, the tunneling can be due to emission of LO phonons. If it is smaller than LO phonon energy, on the other hand, the tunneling is due to the acoustic phonon process. Thus the tunneling time depends strongly on the difference energy. Simple analysis shows that the tunneling time could be at subpicosecond for LO phonon process if the potential barrier thickness is less than 3nm. The potentiality of the structure for the optical devices is discussed. The switching of the absorption coefficient due to the bleaching effect is expected to occur within several picoseconds by the tunneling transfer of electrons from one quantum well to the other.
The ZnSe-ZnS and Cd0.3Zn0.7S-ZnS strained-layer superlattices (SLSs) were grown by low-pressure metalorganic chemical vapor deposition on (100) GaAs and ZnS substrates. Excitonic properties of the multiple-quantum well SLSs are reviewed on the basis of our recent experimental results which were revealed by photoluminescence, absorption and reflectance studies. In ZnSe-ZnS SLS, the heavy-hole and light-hole exciton states whith are drived from the strain-induced valence band splittings, were observed in the absorption spectra. Strong quantum confinement effect on the excitonic emissions in Cd0.3Zn0.7S-ZnS SLS can be observed through the temperature- and well-dependence of the emission spectra. Intense exciton emission was found around 3.28eV at room temperature. The effects of external electric-field on the exciton-emission intensity and its peak position are explained by a quantum confined Stark effect.
We have measured the transfer efficiency of electrons from one Quantum Point Contact (QPC) to another QPC as a function of magnetic field. We obtained a double-peak profile for the point contact quantized to two wave modes. We suggested that this profile reflects the angular distribution of electrons emitted from the QPC using Fraunhofer diffraction approximation through the QPC by an electron propagating according to Green's function. However we used classical Lorentz force for analysis to account for the effect of the magnetic field. The analysis seems to be an unnatural mixture of quantum mechanics and classical mechanics. This paper describes quantum mechanical Fraunhofer diffraction analysis including the magnetic field to resolve this unnaturalness. This analytic result agreed with the above analytic and experimental results, confirming that we have measured angular distribution of electron wave injected through a QPC. It should be noted that our double-peak profile is explained by Fraunhofer diffraction of electron wave. This paper also discusses the interference between the 1st and 2nd mode elctron waves.