Ground state of excitons and charged excitons in a quantum well

A variational calculation of the ground state of a neutral exciton and of positively and negatively charged excitons (trions) in single quantum well is presented. We study the dependance of the correlation energy and of the binding energy on the well width and on the hole mass. Our results are are compared with previous theoretical results and with avalaible experimental data.Comment: 8 pages, 5 figures presented to OECS

Acceptor Raman scattering in GaAs-AlxGa1-xAs quantum-well structures

We report resonant Raman scattering from Be acceptors in GaAs-AlxGa1-xAs quantum well structures, grown by molecular beam epitaxy. Center- and edge-doped samples with well widths in the range 70-165 A were investigated as a function of temperature and uniaxial stress. The data show confinement-induced shifts and splittings of the lowest-lying acceptor levels in good agreement with recent calculations, and also excitations that may involve impurity states derived from higher subbands. The stress dependence of the spectra reveals coupling of the lowest acceptor transition to transverse acoustic phonons. Confinement-split lines exhibit a not fully understood intensity exchange with increasing temperature.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26808/1/0000364.pd

Reliability characteristics and conduction mechanisms in resistive switching memory devices using ZnO thin films

In this work, bipolar resistive switching characteristics were demonstrated in the Pt/ZnO/Pt structure. Reliability tests show that ac cycling endurance level above 106 can be achieved. However, significant window closure takes place after about 102 dc cycles. Data retention characteristic exhibits no observed degradation after 168 h. Read durability shows stable resistance states after 106 read times. The current transportation in ZnO films is dominated by the hopping conduction and the ohmic conduction in high-resistance and low-resistance states, respectively. Therefore, the electrical parameters of trap energy level, trap spacing, Fermi level, electron mobility, and effective density of states in conduction band in ZnO were identified

Anisotropic Confinement, Electronic Coupling and Strain Induced Effects Detected by Valence-Band Anisotropy in Self-Assembled Quantum Dots

A method to determine the effects of the geometry and lateral ordering on the electronic properties of an array of one-dimensional self-assembled quantum dots is discussed. A model that takes into account the valence-band anisotropic effective masses and strain effects must be used to describe the behavior of the photoluminescence emission, proposed as a clean tool for the characterization of dot anisotropy and/or inter-dot coupling. Under special growth conditions, such as substrate temperature and Arsenic background, 1D chains of In0.4Ga0.6 As quantum dots were grown by molecular beam epitaxy. Grazing-incidence X-ray diffraction measurements directly evidence the strong strain anisotropy due to the formation of quantum dot chains, probed by polarization-resolved low-temperature photoluminescence. The results are in fair good agreement with the proposed model

Screw dislocations in GaN grown by different methods

Thin film heteroepitaxy of polar materials such as GaN grown by MOCVD, MBE or HVPE Molecular Beam Epitaxy (MBE), and grown on SiC or Al2O3 is frequently hampered by the formation of structural defects - mostly dislocations, nanotubes and pinholes. The nanotubes and pinholes are empty areas either extending along the growth direction (tubes) or forming v-shape defects on {101?1} planes. In most cases they are associated with dislocations, but in some cases these defects are formed in dislocation-free areas. In this presentation it will be shown that the screw dislocations present in HVPE samples are decorated by pinholes arranged on top of each other ("bamboo" structure), but these defects are not found in MBE samples grown on the top of HVPE samples. These might suggest either different core structure of a screw dislocation in HVPE and MBE grown material or suggest a different purity of these two materials. By applying a direct reconstruction of the phase and amplitude of the scattered electron wave from a focal series of high resolution images, the core structures of screw dislocations in both materials have been studied in order to see if the core structure in HVPE template layer and the MBE overlayer is different. This will be discussed in detail during the presentation