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Self-assembled semiconductor nanostructures formedbylattice-mismatched heteroepitaxy are defect free islands with high optical quality and suitable confinement properties for the development of optoelectronic devices. Recently, they have
been considered also as promising candidates for information storage and quantumcomputing applications. Many authors have reported experimental andtheoretical detailed work o n the growth mechanics, morphology and optical properties of these
nanostructures, though less work has been reported on their t ransport characteristics. The electron and hole energy levels and confining potentials dependstrongly on the shape, composition and surrounding material of the nanostructures, which makes
theoretical predictions complex. On the other hand, optical characterization gives information on the recombination of excitons between the valance bandandtheconduction band. S everal authors [1, 2, 3, 4] have reportedmeasurements on the
transport characteristics of InAs on GaAs nanostructures, and have emphasized the importance of a precise knowledge of the electron and hole systemindependently.
In this work we study the electron and hole systems of self-assembled InGaAs on GaAs (001) quantumrings (QR) by combining photoluminescence and capacitance-voltage measurements (CV) which provides characterization of the QR
electrons (and holes) energy levels position. The samples are grown b y molecular beam epitaxy (MBE), details on the growth of the samples and the QRcan be obtained elsewhere. To study the transport properties of carriers a metal-insulatorsemiconductor
schottky diode (MISSD) is developed. A highly doped (2x10 cm ) GaAs:Si buffer to study the conduction band properties (or a G aAs:Be f or the valence band), acts as back contact t o inject carriers. The QR are embedded between a
GaAs tunnel barrier 25 nm thick (t ); and a GaAs layer followed by an AlAs/GaAs short period super lattice (SPSL) which acts a s blocking barrier to forbid current, the thickness of this barrier is 150nm(t ). 5nmof GaAs on the top prevents surface
oxidation. A schottky contact gate is deposited on top of the structure. When anACsignal imposed upon a large negative DCvoltage is applied between the gate and the back contact at low temperature,the QR energy levels lie above the fermi energy
level and are unoccupied. For more positive DCvoltages i t is possible to align the fermi energy level with the QRlowest energy level. When this occurs, electrons tunnel in and out of the QR. This tunnel can be monitorized by measuring the
capacitance between back contact and gate with a lock-in technique. By applying more positive voltages, it is possible to monitor the energy levels of the QR. |
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