We study experimentally and theoretically the influence of interface roughness on the mobility of two-dimensional electrons in modulation-doped AlAs/GaAs quantum wells. It is shown that interface roughness scattering is the dominant scattering mechanism in thin quantum wells with a well thickness Lw<60 Å, where electron mobilities are proportional to L6w, reaching 2×103 cm2/V s at Lw∼55 Å. From detailed comparison between theory and experiment, it is determined that the ‘‘GaAs-on-AlAs’’ interface grown by molecular beam epitaxy has a roughness with the height of 3–5 Å and a lateral size of 50–70 Å.

The electronic structures of clean InAs(lOO) surfaces have been investigated by in situ high-resolution electron-energy-loss spectroscopy. Intrinsic electron accumulation layers with carrier densities strongly depending on the surface reconstruction are formed on both As-stabilized and In-stabilized surfaces. The correlation between the surface electron densities and the surface reconstructions suggests that electrons in the accumulation layers are induced by the donorlike intrinsic surface states of InAs whose energy spectrum is determined by the surface reconstructions.It is well known that an electron accumulation layer is easily formed on InAs surfaces. Recently, this surface accumulation layer has attracted much attention because the high density of electrons on the surface has great technological importance, such as the formation of nonalloyed Ohmic contacts 1 and the realization of the three-terminal Josephson devices. 2 It is, however, not clear whether an intrinsic electron accumulation layer is present even on clean InAs (100) surfaces and how it is related with surface atomic configurations.In this work, we studied the electronic structure of both As-stabilized and In-stabilized clean InAs(100) surfaces with in situ high-resolution electron-energy-loss spectroscopy (HREELS). By analyzing the HREELS spectra, it is found for the first time that electron accumulation layers are formed on both As-stabilized and In-stabilized surfaces and, furthermore, that the electron density in the accumulation layer changes reversibly with surface reconstructions. The origin of such electron accumulation layers is discussed.HREELS is a very powerful tool to investigate semiconductor surfaces because it gives us rich information on the surface vibrational excitations which extend into semiconductors by several tens of nanometers. To pursue HREELS measurements, however, it is essential to prepare clean and undamaged semiconductor surfaces. Such techniques as cleaving, 3,4 ion bombardment and subsequent annealing, 5,6 and arsenic deposition 7,8 were used in the previous works. With these methods, however, it is difficult to obtain clean and undamaged InAs(lOO) surfaces with high reproducibility. To overcome this difficulty, the HREELS system is connected with a molecular-beam-epitaxy (MBE) chamber under an ultrahigh-vacuum condition (<3xlO~8 Pa). This configuration keeps the surface contamination negligibly small during measurements [<0.4 L (1 L = 10~6 Torrs) for 2 h] and allows us to investigate clean surfaces.The As-stabilized undoped InAs(100) surfaces were prepared as follows: 0.3-0.5-^um-thick undoped «-type InAs layers were grown on undoped InAs (100) sub-strates by MBE. The bulk electron density in the MBEgrown InAs layer is less than 2xl0 16 cm" 3 . The substrate temperature during the growth was set at 450-490 °C. The As-stabilized surfaces were obtained by cooling the arsenic-stabilized (2x4) reconstructed surfaces down to room temperature in an AS4 flux of -10 15 cm~2s _1 . During the cooling process, the reflec...

Titanium dioxide (TiO2) is a potential photosensitizer for photodynamic therapy. In this study, the mechanism of DNA damage catalyzed by photo-irradiated TiO2 was examined using [32P]-5'-end-labeled DNA fragments obtained from human genes. Photo-irradiated TiO2 (anatase and rutile) caused DNA cleavage frequently at the guanine residue in the presence of Cu(II) after E. coli formamidopyrimidine-DNA glycosylase treatment, and the thymine residue was also cleaved after piperidine treatment. Catalase, SOD and bathocuproine, a chelator of Cu(I), inhibited the DNA damage, suggesting the involvement of hydrogen peroxide, superoxide and Cu(I). The photocatalytic generation of Cu(I) from Cu(II) was decreased by the addition of SOD. These findings suggest that the inhibitory effect of SOD on DNA damage is due to the inhibition of the reduction of Cu(II) by superoxide. We also measured the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine, an indicator of oxidative DNA damage, and showed that anatase is more active than rutile. On the other hand, high concentration of anatase caused DNA damage in the absence of Cu(II). Typical free hydroxyl radical scavengers, such as ethanol, mannnitol, sodium formate and DMSO, inhibited the copper-independent DNA photodamage by anatase. In conclusion, photo-irradiated TiO2 particles catalyze the copper-mediated site-specific DNA damage via the formation of hydrogen peroxide rather than that of a free hydroxyl radical. This DNA-damaging mechanism may participate in the phototoxicity of TiO2.