Ionization Of Quantum Dots Research Articles

A quantum-dot heterostructure transistor with enhanced maximum drift velocity of electrons

A new type of heterotransistor based on an AlGaAs/GaAs/InAs/GaAs/InAs structure with a layer of InAs quantum dots embedded directly into the GaAs channel is fabricated. High values of the maximum saturation current (up to 35 A/cm) and transconductance (up to 1300 mS/mm) are attained. The specific features of the current-voltage characteristics of the new device are explained in the context of a model that takes into account the ionization of quantum dots in high electric fields and tenfold enhancement of the electron drift velocity in a structure with an InAs quantum-dot layer in the vicinity of an AlGaAs/GaAs heterojunction.

Transport Phenomena in Two-Dimensional Structures with Quantum Dots

A model that explains the unusual characteristics of the AlGaAs/GaAs modulation-doped field-effect transistor (MODFET) with InAs quantum dots incorporated in the GaAs channel is presented. It is shown that the negative charge of electrons confined in quantum dots decreases the threshold gate–drain voltage at which the channel is fully depleted. This provides an impact ionization of quantum dots at a low drain voltage. Because of the quantum dot ionization, the quantum dot MODFET transconductance becomes large and negative. The increased transconductance, due to the additional doping of the GaAs and InAs channels by impurities, exceeds 10 mS/mm. It is shown that the insertion of InAs quantum well with quantum dots into the GaAs quantum well increases the electron maximum drift velocity up to 10 cm/s, and consequently, quantum dot MODFET current gain cut-off frequency up to few hundred gigahertz.

Electron transport in unipolar heterostructure transistors with quantum dots in strong electric fields

A model for the explaining specific features of the electron transport in strong electric fields in the quantum-dot unipolar heterostructure transistor (AlGaAs/GaAs/InAs/GaAs/InAs) is presented. It is shown that the two-step shape of the output current-voltage characteristic I D (V D ) and the anomalous dependence of the drain current I D on the gate voltage V G are caused by the ionization of quantum dots in the strong electric field at the drain gate edge. The ionization of quantum dots sets in at the drain voltage V D that exceeds the VD1 value, at which the I D (V D ) dependence is saturated (the first step of the I-V characteristic). With the subsequent increase in V D , i.e., for V D >VD1, the I D (V D ) dependence has a second abrupt rise due to the ionization of quantum dots, and then, for V D =VD2>VD1, the current I D is saturated for the second time (the second step in the current-voltage characteristic). It is suggested to use this phenomenon for the determining the population of quantum dots with electrons. The model presented also describes the twice-repeated variation in the sign of transconductance g m =dI D /dV G as a function of V G .

Persistent Spectral-Hole-Burning in Semiconductor Quantum Dots and its Application to Spectroscopy

Persistent spectral-hole-burning (PSHB) phenomena have been observed in many kinds of semiconductor quantum dots (QDs). Luminescence hole burning and charged excitons have been observed in CuCl QDs in NaCl crystals. The observed PSHB phenomena are explained by the ionization of QDs. The energy of photoionized QDs is changed from the original energy and their new energies depend on the spatial arrangement of trapped carriers. Quantum confinement of carriers and the resulting strong Coulomb interaction between confined carriers and trapped carriers cause the energy change. PSHB has been widely applied to site-selective spectroscopy of QDs. Size-dependent excited exciton quantum states in CuCl quantum cubes, size-dependent excited electron-hole quantum states in CdTe QDs and renormalization of longitudinal optical phonons confined in CuCl QDs have been revealed by studying sideband holes. This also allowed the measurement of accumulated photon echo, which clarified the very sharp homogeneous spectral linewidth of QDs under weak excitation conditions.