Chung-Ching Yang

Structural and electronic properties of amorphous and polycrystalline In2Se3 films

Structural and electronic properties of amorphous and single-phase polycrystalline films of γ- and κ-In2Se3 have been measured. The effect of deposition conditions on the film phase was studied extensively. The stable γ phase nucleates homogeneously in the film bulk and has a high resistivity, while the metastable κ phase nucleates at the film surface and has a moderate resistivity. The microstructures of polycrystalline hot-deposited and postannealed, cold-deposited γ films are quite different but their electronic properties are similar. The increase in the resistivity of amorphous In2Se3 films upon annealing is interpreted in terms of the replacement of In–In bonds with In–Se bonds during crystallization. Great care must be taken in the preparation of In2Se3 films for electrical measurements as the presence of excess chalcogen or surface oxidation may greatly affect the film properties.

Electron-beam detection of bits reversibly recorded on epitaxial InSe/GaSe/Si phase-change diodes

We demonstrate a data read-back scheme based on electron-beam induced current in a data storage device that utilizes thermal recording onto a phase-change medium. The phase-change medium is part of a heterojunction diode whose local charge-collection efficiency depends on the crystalline or amorphous state of a bit. Current gains up to 65 at 2 keV electron beam energy have been demonstrated using InSe/GaSe/Si epitaxial diodes. Fifteen write–erase cycles are obtained without loss of signal contrast by using a protective cap layer and short write pulses. 100 write–erase cycles have been achieved with some loss of contrast. Erasure times for the bits are longer than in similar polycrystalline In–Se media films. Possible reasons for the long erasure times are discussed in terms of a nucleation- or growth-dominated recrystallization. Prospects for extension to smaller bit sizes using electron-beam writing are considered.

Enhancement of photoluminescence efficiency in binary quantum dot arrays in hybrid organic/inorganic materials

The photoluminescence (PL) quantum efficiency of dense semiconductor colloidal quantum dot (QD) arrays is significantly reduced by the quenching of the optical excitons via defect-induced nonradiative decay channels. This luminescence quenching is facilitated by the rapid migration of excitons between neighboring QDs via resonant Forster transfer processes. We propose to mitigate this quenching by using nonradiative “spacer” quantum dots (SQDs) to separate radiative primary quantum dots (PQDs). We have identified the maximum and minimum values of the PL quantum efficiency (PLQE) for different compositions of spacer-primary QD arrays. Using the kinetic Monte Carlo technique, we have found that for a given composition, the PLQE is highest for randomly distributed spacer and primary QDs, decreasing dramatically when clusters of SQDs and PQDs are formed. We have modeled cluster formation of QDs in binary QD arrays and determined the resultant PL properties. By comparing our simulation results with those we ob...

Light Emission from Polymer‐Semiconductor Nanocrystal (PFO‐CdSe/ZnS) Structures

Electroluminescence was achieved from hybrid conjugated polymer (polyfluorene) — II‐VI semiconductor nanocrystal structures. The energy of the carriers injected into the polymer was transferred into the nanocrystals causing light emission with a photon energy corresponding to the nanocrystals’ bandgap. Undesirable polymer‐nanocrystal phase segregation and presence of defective nanocrystals was responsible for the relatively low emission efficiency from the hybrid devices.

Heteroepitaxy of InSe/GaSe on Si(111) Substrates

High quality growth of InSe on Si(111) was achieved by insertion of GaSe buffer layer. Rhombohedral polytypes were formed in both the InSe and GaSe layers. Twinning and stacking disorder was often detected in these materials due to their layered structure. Moreover, in samples with a thin GaSe layer, strong interdiffusion of indium into the GaSe layer was detected that resulted in the formation of an In x Ga y Se phase. The dominant threading defects present in these InSe/GaSe heterostructures were screw dislocations, which may act as nonradiative recombination centers.