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We investigated the effects of doping on the photovoltaic efficiency in a GaAs reference cell, and in undoped, n-doped, and p-doped InAs/GaAs quantum-dot (QD) solar cells. We found that the photovoltaic efficiency of the undoped QD solar cell is almost the same as that of the reference cell. However, the efficiency improves monotonically with increasing inter-dot ndoping, while p-doping deteriorates the photovoltaic conversion. We observed a 50 % increase in photovoltaic efficiency in the device n-doped to provide approximately six electrons per dot as compared with the undoped QD cell. In this QD solar cell, the short circuit current density increases to 24.30 mA/cm2compared with 15.07 mA/cm2in the undoped QD solar cell without deterioration of the open circuit voltage. To identify the physical mechanisms that provide this improvement, we investigated the spectral characteristics of the photovoltaic response and photoluminescence of our QD solar cells. We found that the electron capture into QDs is substantially faster than the hole capture, which leads to an accumulation of electrons in QDs. The electrons trapped in dots enhance IR transitions. The built-in-dot electron charge together with charged dopants outside the dots creates potential barriers, which suppress the fast electron capture processes and at the larger scale form a potential profile which precludes degradation of the open circuit voltage. All of these factors lead to the enhanced harvesting of IR energy and a radical improvement of the QD solar cell efficiency. Higher efficiencies are anticipated with further increase of doping level and at higher radiation intensity. This makes the QD solar cells promising candidates for use with concentrators of solar radiation.

We report the substantial increase in power efficiency in InAs/GaAs quantum dot (QD) solar cells due to n-doping of the inter-dot space in p+--n+structures and investigate the physical mechanisms that provide this significant improvement. We have compared the GaAs reference cell to undoped, n-doped and p-doped QD solar cell structures and found that the short circuit current, JSC, of the undoped QD solar cell is comparable to that of the GaAs reference cell. On the other hand, while p-doping deteriorates the device performance, n-doping significantly increases JSCwithout degradation of the open circuit voltage, VOC. The photovoltaic device, n-doped to provide approximately six electrons per dot, demonstrates 60% increase in JSC, from 15.07 mA/cm2to 24.30 mA/cm2. Strong increase in the photoresponse and JSCof the IR portion of the solar spectrum has been observed for the n-doped structures. From the photoluminescence data, the electron capture noticeably dominates over hole capture leading to an accumulation of electrons in the dots. We have observed that QDs with built-in charge (Q-BIC) enhances harvesting of IR energy, suppresses the fast electron capture process, and stabilizes the open circuit voltage. All of these factors lead to a significant improvement of the cell efficiency.

We present results on design, fabrication, and characterization of hot-electron bolometers based on low-mobility two-dimensional electron gas (2DEG) in AlInN/GaN and AlGaN/GaN heterostructures. Electrical and optical characterization of our Hot Electron Bolometers (HEBs) show that these sensors combine (i) high coupling to incident THz radiation due to Drude absorption, (ii) significant electron heating by the THz radiation due to small value of the electron heat capacity, (iii) substantial sensitivity of the device resistance to the heating effect. A low contact resistance (below 0.5 ·mm) achieved in our devices ensures that the THz voltage primarily drops across the active region. Due to a small electron momentum relaxation time, the inductive part of the impedance in our devices is large, so these sensors can be combined with standard antennas or waveguides. In the capacity of the THz local oscillator (LO) for heterodyne THz sensing, we fabricated AlGaAs/GaAs quantum cascade lasers (QCLs) with a stable continuous-wave single-mode operation in the range of 2.5-3 THz. Spectral properties of the QCLs have been studied by means of Fourier transform spectroscopy. It has been demonstrated that the spectral purity of the QCL emission line doesn't exceed the spectrometer resolution limit at the level of 0.1 cm-1(3 GHz). Discrete spectral tuning can be achieved using selective devices; fine tuning can be done by thermally changing the refractive index of the material and by applied voltage. Compatibility of the low-mobility 2DEG microbolometers with QCLs in terms of LO power requirements, spectral coverage, and cooling requirements makes this technology especially attractive for THz heterodyne sensing.

We present the results of design, fabrication, and characterization of the room-temperature, low electron heat capacity hot-electron THz microbolometers based on two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. The 2DEG sensor is integrated with a broadband THz antenna and a coplanar waveguide. Devices with various patterning of 2DEG have been fabricated and tested. Optimizing the material properties, geometrical parameters of the 2DEG, and antenna design, we match the impedances of the sensor and antenna to reach strong coupling of THz radiation to 2DEG via the Drude absorption. Testing the detectors, we found that the THz-induced photocurrent, I, is proportional to the bias current, I, and the temperature derivative of the resistance and inversely proportional to the area of 2DEG sensor, S. The analysis allowed us to identify the mechanism of the 2DEG response to THz radiation as electron heating. The responsivity of our sensors, normalized to the bias current and to unit area of 2DEG, R*= I•S/ (I•P), is ~ 103W-1m2. So, for our typical sensor with an area of 1000 m2and bias currents of ~ 10 mA, the responsivity is ~ 0.01 A/W. The measurements of mixing at sub-terahertz frequencies showed that the mixing bandwidth is above 2 GHz, which corresponds to a characteristic electron relaxation time to be shorter than 0.7 ps. Further decrease of the size of 2DEG sensors will increase the responsivity as well as allows for decreasing the local oscillator power in heterodyne applications.

We report on the effect of population inversion associated with the electron and hole injection in graphene pi- n structures at the temperatures 200K-300K. It is assumed that the recombination and energy relaxation of electrons and holes is associated primarily with the interband and intraband processes assisted by optical phonons. The dependences of the electron-hole and optical phonon effective temperatures on the applied voltage, the current-voltage characteristics, and the frequency-dependent dynamic conductivity are obtained. In particular, at low and moderate voltages the injection can lead to a pronounced cooling of the electron-hole plasma in the device i-section to the temperatures below the lattice temperature. At higher voltages, the current and electronhole and phonon temperature dependences on voltage exhibit the S-shape. At a certain values of the applied voltage the frequency-dependent dynamic conductivity can be negative in the terahertz range of frequencies. The electron-hole plasma cooling substantially reinforces the effect of negative dynamic conductivity and promotes the realization of terahertz lasing. It is demonstrated that the heating of optical phonon system hinders the realization of negative dynamic conductivity and terahertz lasing at the room temperatures.