Nizami Vagidov

Strong Enhancement of Solar Cell Efficiency Due to Quantum Dots with Built-In Charge

We report a 50% increase in the power conversion efficiency of InAs/GaAs quantum dot solar cells due to n-doping of the interdot space. The n-doped device was compared with GaAs reference cell, undoped, and p-doped devices. We found that the quantum dots with built-in charge (Q-BIC) enhance electron intersubband quantum dot transitions, suppress fast electron capture processes, and preclude deterioration of the open circuit voltage in the n-doped structures. These factors lead to enhanced harvesting and efficient conversion of IR energy in the Q-BIC solar cells.

Terahertz ballistic current oscillations for carriers with negative effective mass

It is shown that the stationary distribution of ballistic current carriers moving across a thin doped base is unstable if there exists a negative effective mass (NEM) part in the carrier dispersion law. Under such a condition, a regime with a quasistationary current oscillations is established for a wide range of voltages across ballistic diode. The oscillation frequency and amplitude depend on the base length, doping concentration, and applied voltage. The current oscillations take place in a short‐circuit regime (in absence of an external resonator). We consider asymmetric double quantum wells and/or composite ΓX quantum wells as possible structures allowing for the required dispersion relation with NEM part. Carrier dynamics in these structures are described quasiclassically and the validity of such a treatment is discussed.

Effective harvesting, detection, and conversion of IR radiation due to quantum dots with built-in charge

We analyze the effect of doping on photoelectron kinetics in quantum dot [QD] structures and find two strong effects of the built-in-dot charge. First, the built-in-dot charge enhances the infrared [IR] transitions in QD structures. This effect significantly increases electron coupling to IR radiation and improves harvesting of the IR power in QD solar cells. Second, the built-in charge creates potential barriers around dots, and these barriers strongly suppress capture processes for photocarriers of the same sign as the built-in-dot charge. The second effect exponentially increases the photoelectron lifetime in unipolar devices, such as IR photodetectors. In bipolar devices, such as solar cells, the solar radiation creates the built-in-dot charge that equates the electron and hole capture rates. By providing additional charge to QDs, the appropriate doping can significantly suppress the capture and recombination processes via QDs. These improvements of IR absorption and photocarrier kinetics radically increase the responsivity of IR photodetectors and photovoltaic efficiency of QD solar cells.

Conversion of above- and below-bandgap photons via InAs quantum dot media embedded into GaAs solar cell

Quantum dots (QDs) provide photovoltaic conversion of below-bandgap photons due to multistep electron transitions. QDs also increase conversion efficiency of the above-bandgap photons due to extraction of electrons from QDs via Coulomb interaction with hot electrons excited by high-energy photons. Nanoscale potential profile (potential barriers) and nanoscale band engineering (AlGaAs atomically thin barriers) allow for suppression of photoelectron capture to QDs. To study these kinetic effects and to distinguish them from the absorption enhancement due to light scattering on QDs, we investigate long, 3-μm base GaAs devices with various InAs QD media with 20 and 40 QD layers. Quantum efficiency measurements show that, at least at low doping, the multistep processes in QD media are strongly affected by the wetting layer (WL). The QD media with WLs provide substantial conversion of below-bandgap photons and for devices with 40 QD layers the short circuit current reaches 29.2 mA/cm2. The QD media with band-en...

Quantum Dot Infrared Photodetectors: Photoresponse Enhancement Due to Potential Barriers

Potential barriers around quantum dots (QDs) play a key role in kinetics of photoelectrons. These barriers are always created, when electrons from dopants outside QDs fill the dots. Potential barriers suppress the capture processes of photoelectrons and increase the photoresponse. To directly investigate the effect of potential barriers on photoelectron kinetics, we fabricated several QD structures with different positions of dopants and various levels of doping. The potential barriers as a function of doping and dopant positions have been determined using nextnano3 software. We experimentally investigated the photoresponse to IR radiation as a function of the radiation frequency and voltage bias. We also measured the dark current in these QD structures. Our investigations show that the photoresponse increases ~30 times as the height of potential barriers changes from 30 to 130 meV.

Solar cell with built-in charge: Experimental studies of diode model parameters

Quantum dots acquire built-in charge due to selective n-doping of the interdot space. The quantum dots with built-in charge (Q-BIC) increase electron coupling to IR radiation and suppress photoelectron capture, which in turn decrease the recombination via quantum dots. To investigate effects of the built-in-dot charge on recombination processes and device performance, the light and dark I–V characteristics and their temperature dependences of Q-BIC solar cells are measured. Employing the diode model, the data are analyzed in terms of the ideality factor, shunt resistance, and reverse saturation current. The authors compare the n-doped Q-BIC solar cells with the GaAs p-i-n reference cell, undoped, and p-doped devices. The analysis provides a qualitative description of the effect of doping on carrier kinetics and transport. The authors show that n-doping substantially reduces the recombination via quantum dots.

Ballistic and quasiballistic tunnel transit time oscillators for the terahertz range: Linear admittance

We have considered interactions between ballistic (or quasiballistic) electrons accelerated by a dc electric field in an undoped transit space (T space) and a small ultrahigh frequency ac electric field and have calculated the linear admittance of the T space. Electrons in the T space have a conventional, nonparabolic dispersion relation. After consideration of the simplest specific case when the current is limited by the space charge of the emitted electrons, we turned to an actual case when the current is limited by a heterostructural tunnel barrier (B barrier) separating the heavily doped cathode contact and the T space. We assumed that the B barrier is much thinner than the T space and both dc and ac voltages drop mainly across the T space. The emission tunnel current through the B barrier is determined by the electric field E(0) in the T space at the boundary B barrier/T space. The more substantial is, the tunnel current limitation the higher the electric field E(0) becomes. We have shown that for a ...

Two mechanisms of the negative-effective-mass instability in p-type quantum well-based ballistic p+pp+-diodes: Simulations with a load

There exist two regimes of the negative-effective-mass (NEM) instability in ballistic p+pp+-diodes with two-dimensional hole gas in the p-base: the instability of homogeneous NEM-hole distribution in a quasineutral plasma region, and the instability of a thin accumulation layer, which forms inside a depletion region and contains NEM holes. Both instabilities lead to the development of terahertz oscillatory regimes. The regimes’ simulation in the inductance-loaded diodes with base lengths 0.05–0.15 μm demonstrates that such loads substantially enlarge the voltage range of the second regime and give rise to oscillatory regimes, which do not appear in unloaded diodes at all. Efficiencies of different oscillatory regimes are estimated.

HOT-ELECTRON TRANSPORT IN QUANTUM-DOT PHOTODETECTORS

Employing Monte-Carlo simulations we investigate effects of an electric field on electron kinetics and transport in quantum-dot structures with potential barriers created around dots via intentional or unintentional doping. Results of our simulations demonstrate that the photoelectron capture is substantially enhanced in strong electric fields and this process has an exponential character. Detailed analysis shows that effects of the electric field on electron capture in the structures with barriers are not sensitive to the redistribution of electrons between valleys and these effects are not related to an increase of drift velocity. Most data find adequate explanation in the model of hot-electron transport in the potential relief of quantum dots. Electron kinetics controllable by potential barriers and an electric field may provide significant improvements in the photoconductive gain, detectivity, and responsivity of photodetectors.

Gated negative-effective-mass ballistic terahertz generators

We consider gate control of terahertz generation in planar ballistic diodes with a negative-effective-mass section in a dispersion relation of current carriers in a current-conducting channel. Such a generation in ballistic p+pp+ or n+nn+ diodes occurs as a result of plasma instability development and self-organization of a regular oscillation regime. Conditions of existence and oscillation frequencies are calculated. The gate can also serve as an oscillation-collecting electrode. We consider double-gate designs, side by side with conventional single-gate designs. The double-gate devices allow us to separate circuits for direct and high-frequency currents.