Thermal Escape Of Electrons Research Articles

Spatially indirect interfacial excitons in n+-ZnO/p-GaN heterostructures

Electroluminescence properties of epitaxially grown n+-ZnO/p-GaN pn+-heterojunctions are investigated as functions of applied bias and temperature. This study reveals the existence of indirect interfacial excitons at sufficiently low temperatures. Electroluminescence feature associated with these excitons redshifts with increasing forward bias. It has been found that the binding energy of these entities can be controlled through applied forward bias and can even be made higher than that of the excitons in ZnO bulk (60 meV). However, the formation of these excitons becomes unsustainable when either the applied bias or the temperature crosses a threshold. This has been explained in terms of leakage and thermal escape of electrons (holes) into the GaN (ZnO) side. Calculations for the band diagram and the binding energy of these spatially indirect electron–hole coulomb-coupled entities are carried out. Theoretical results are found to explain the experimental findings quite well.

Mobility and activation energy of lateral photocurrent of InAs quantum dot layers with ultrafast carrier relaxation

We determined the lateral mobility of photocarriers and activation energy of the photocurrent (PC) in stacked InAs quantum dot (QD) layers with ultrafast carrier relaxation times and 1.5-μm band absorption. Lateral mobility is one of the important parameters in the application of QD layers to photoconductive antennas operating with 1.5-μm gating light for terahertz (THz) wave detection. The activation energy obtained from temperature dependence of PC spectroscopy is important for elucidating the mechanism, and it is an indicator of the temperature range for antenna operation. The dependence of the mobility on the photocarrier density was evaluated from the dependence on the pump power. The mobility sharply increased from 42 cm 2 /V･s in the dark to 416 cm 2 /V･s upon laser illumination and then decreased with the increase in the number of generated photocarriers. The broad PC peak from InAs QD layers at around 1400 nm was continuously monitored in the temperature range of 4–292 K. The activation energy of the PC from the InAs QD layers was evaluated in the temperature range of 159–240 K where the PC significantly increased. In the broad PC peak in the wavelength range of 1200–1600 nm, the activation energy increased from 48 to 62 meV with the wavelength. The PC observed at lower temperatures was not derived considering the thermal escape of electrons, but it was determined through the Auger process. The InAs QD layer with ultrafast carrier relaxation has considerable potential in photoconductive antenna applications for THz wave detection. • Lateral mobility increased: 42 cm 2 /V･s in darkness to 416 cm 2 /V･s with illumination. • Thermally stable and broad peak of InAs QD layers occurred during PC spectroscopy. • From 159 to 240 K, PC increased and activation energy of the PC depended on QD sizes. • At cryogenic temperatures, PC was derived using the Auger process.

Low temperature characterization of the photocurrent produced by two-photon transitions in a quantum dot intermediate band solar cell

Conceived to exceed the conversion efficiency of conventional photovoltaic devices, the intermediate band solar cell bases its operation on exploiting, besides the usual band-to-band optical transitions, the absorption of two sub-bandgap photons. For the present, the only technology used to implement an intermediate band in real devices has been the growth of an InAs/GaAs quantum dot superlattice. In practice, the obtained material shows two limitations: the narrow energy gap between conduction and intermediate band and the appearance of growth defects due to the lattice stress. The consequences are the presence of non-radiative recombination mechanisms and the thermal escape of electrons from the intermediate to the conduction band, hindering the splitting of the quasi-Fermi levels associated with the intermediate and conduction bands and the observation of photocurrent associated with the two-photon absorption. By reducing the temperature at which the devices are characterised we have suppressed the parasitic thermal mechanisms and have succeeded in measuring the photocurrent caused by the absorption of two below bandgap photons. In this work, the characterization of this photocurrent at low temperature is presented and discussed.

LATERAL INTERSUBBAND PHOTOCURRENT STUDY ON InAs/InAlAs/InP SELF-ASSEMBLED NANOSTRUCTURES

We present lateral intersubband photocurrent (PC) study on self-assembled InAs/InAlAs/InP (001) nanostructures in normal incidence. With the help of interband excitation, a broad PC signal has been observed in the photon energy range of 150–630 meV arising from the bound-to-continuum intersubband absorption in the InAs nanostructures. The large linewidth of the intersubband PC signal is due to the size inhomogeneity of the nanostructures. With the increase of the interband excitation the intersubband PC signal firstly increases with a redshift of PC peak and reaches its maximum, then decreases with no peak shift. The increase and redshift of the PC signal at low excitation level can be explained by the state filling effect. However, the decrease of PC signal at high excitation level may be due to the change of the mobility and lifetime of the electrons. The intersubband PC signal decreases when the temperature is increased, which can be explained by the decrease of the mobility and lifetime of the electrons and the thermal escape of electrons.

Properties of photoluminescence in type-II GaAsSb/GaAs multiple quantum wells

The optical properties of type-II GaAsSb/GaAs multiple quantum wells were investigated by photoluminescence. It was found that the peak position of photoluminescence (PL) spectra shows a giant blueshift under a moderate optical excitation level. As the pumping intensity increases further, the saturation of the peak position was observed. The giant blueshift can be interpreted in terms of the band-bending effect due to the spatially photoexcited carriers in a type-II alignment. The saturation effect was attributed to the creation of an energy barrier induced by photocarriers, which prevents the charge transfer. The change of integrated photoluminescence intensity with increasing excitation intensity is also consistent with the band-bending model. In addition, the temperature dependence of the PL spectra reveals that the thermal escape of electrons from the GaAs well into the GaAsSb barrier is responsible for the PL quenching.

Photocurrent and capacitance spectroscopy of Schottky barrier structures incorporating InAs/GaAs quantum dots

We present photocurrent, capacitance, and photoluminescence studies of GaAs-based Schottky barrier structures incorporating InAs self-assembled quantum dots. We show that the photocurrent is mainly controlled by thermal escape of electrons out of the dots and is suppressed at low temperatures, below 100 K. At higher temperatures (g185 K), we are able to control the magnitude of the photon absorption, and hence the photocurrent, by varying the bias voltage applied to the device.