The pyroelectric response of polycrystalline, Si-doped HfO2 layers in a thickness range of 10 nm to 50 nm is investigated employing the temperature oscillation method. The largest value of the pyroelectric coefficient is obtained for the 20 nm layer with p = 84 μC m−2 K−1, which is similar to that of lithium niobate. Furthermore, the pyroelectric coefficient is analyzed with respect to field cycling and is found to increase proportionally with the remanent polarization during wake-up, providing further evidence that the hysteresis of the material is truly ferroelectric. However, for different material thicknesses, the switchable polarization and pyroelectric coefficient are not proportional, indicating that only part of the domains is pyroelectrically active, which suggests potential for further improvement of the pyroelectric response. Due to its CMOS compatibility and conformal deposition using atomic layer deposition (ALD), Si-doped HfO2 is a promising candidate for future energy harvesting and sensor applications.

Lanthanum has been identified as a promising dopant to achieve ferroelectricity in HfO2 thin films in recent theoretical and experimental studies. In this work, the pyroelectric properties of 10 nm thick polycrystalline La-doped HfO2 layers manufactured by thermal atomic layer deposition are assessed employing a sinusoidal temperature profile. Compared to Si doping, La offers a broader dopant range in which ferroelectric behavior is obtained, making the material interesting for large-scale integration and deposition on area-enhanced substrates. Pyroelectric coefficients of up to −80 μC/m2 K are obtained using an optimized stoichiometry, which is comparable to earlier measurements with Si-doped HfO2 samples. Phase-sensitive evaluation of the measured current confirms the pyroelectric origin with minimal spurious contributions. The results are discussed with respect to the ferroelectric switching behavior, which is analyzed employing first-order reversal curve measurements. It is found that there is no simple linear relationship between the remanent polarization and the pyroelectric coefficient. Experimental evidence indicates that the pyroelectric response in polycrystalline thin films is modulated by internal bias fields, which can arise from charged defects. This illustrates the need for careful tuning of the manufacturing conditions and the film phase composition in future applications such as pyroelectric sensors, energy harvesting, or solid-state cooling.

We investigate the time-resolved photoluminescence (PL) dynamics of an undoped GaAs/AlGaAs multiple quantum well under mid-infrared (MIR) irradiation. A time-delayed MIR laser pulse from a free-electron laser, tuned to the intersubband transition energy of the quantum well, induces temporal quenching of the PL intensity with subsequent recovery. The experimental data can be accurately described by a simple rate-equation model, which accounts for the cooling of the non-radiative states to radiative states. By performing polarization sensitive measurements, we are able to discriminate the contributions of free-carrier absorption from that of intersubband absorption, where the latter is about 50 times more efficient

Time and wavelength resolved spectroscopy requires optical sources emitting very short pulses and a fast detection mechanism capable of measuring the evolution of the output spectrum as a function of time. We use table-top Ti:sapphire lasers and a free-electron laser (FEL) emitting ps pulses as excitation sources and a streak camera coupled to a spectrometer for detection. One of the major aspects of this setup is the synchronization of pulses from the two lasers which we describe in detail. Optical properties of the FEL pulses are studied by autocorrelation and electro-optic sampling measurements. We discuss the advantages of using this setup to perform photoluminescence quenching in semiconductor quantum wells and quantum dots. Carrier redistribution due to pulsed excitation in these heterostructures can be investigated directly. Sideband generation in quantum wells is also studied where the intense FEL pulses facilitate the detection of the otherwise weak nonlinear effect.

The pyroelectric response of polycrystalline, Si-doped HfO2 layers with a thickness of 20 nm is investigated in a frequency range of 2 Hz to 20 kHz. Local Joule heating of the pyroelectric material by a deposited nickel strip is used to achieve fast thermal cycles. Over the whole frequency range, a distinct pyroelectric response is registered. A pyroelectric coefficient of −72 μC/m2K is obtained at a frequency of 10 Hz, which is in good agreement with earlier low-frequency measurements. The pyroelectric current is evaluated with respect to electric field cycling, where a successive increase is observed during wake-up. By comparing measurement results in the low- and high-frequency limit, primary and secondary pyroelectric coefficients of −53 μC/m2K and −19 μC/m2K are estimated, respectively. Si-doped HfO2 is a promising material for future energy harvesting and IR sensor applications due to environmental friendliness and CMOS compatible manufacturing.