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Thin TiO2 films are demonstrated to be an excellent electron-selective contact for crystalline silicon solar cells. An efficiency of 21.6% is achieved for crystalline silicon solar cells featuring a full-area TiO2 -based electron-selective contact.

We report the fabrication and characterization of highly responsive ZnMgO-based ultraviolet (UV) photodetectors in the metal–semiconductor–metal (MSM) configuration for solar-blind/visible-blind optoelectronic application. MSM devices were fabricated from wurtzite Zn1–xMgx O/ZnO (x ∼ 0.44) thin-film heterostructures grown on sapphire (α-Al2O3) substrates and w-Zn1–xMgx O (x ∼ 0.08), grown on nearly lattice-matched lithium gallate (LiGaO2) substrates, both by radio-frequency plasma-assisted molecular beam epitaxy (PAMBE). Thin film properties were studied by AFM, XRD, and optical transmission spectra, while MSM device performance was analyzed by spectral photoresponse and current–voltage techniques. Under biased conditions, α-Al2O3 grown devices exhibit peak responsivity of ∼7.6 A/W at 280 nm while LiGaO2 grown samples demonstrate peak performance of ∼119.3 A/W, albeit in the UV-A regime (∼324 nm). High photoconductive gains (76, 525) and spectral rejection ratios (∼103, ∼104) were obtained for devices grown on α-Al2O3 and LiGaO2, respectively. Exemplary device performance was ascribed to high material quality and in the case of lattice-matched LiGaO2 films, decreased photocarrier trapping probability, presumably due to low-density of dislocation defects. To the best of our knowledge, these results represent the highest performing ZnO-based photodetectors on LiGaO2 yet fabricated, and demonstrate both the feasibility and substantial enhancement of photodetector device performance via growth on lattice-matched substrates. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Using a high throughput, in-line atmosphere chemical vapor deposition (APCVD) tool, we have synthesized amorphous aluminum oxide (AlOx) films from precursors of trimethyl-aluminum (TMA) and O2, yielding a maximum deposition 150 nm min–1 per wafer. For p-type crystalline silicon (c-Si) wafers, excellent surface passivation was achieved with the APCVD AlOx films, with a best maximum effective surface recombination velocity (Seff,max) of 8 cm/s following a standard industrial firing step. The findings could be attributed to the existence of large negative charge (Qf ≈ –3 × 1012 cm–2) and low interface defect density (Dit ≈ 4 × 1011 eV–1 cm–2) achieved by the films. This data demonstrates a high potential for APCVD AlOx to be used in high efficiency, low cost industrial solar cells. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Emitter formation for industrial crystalline silicon (c-Si) solar cells is demonstrated by the deposition of phosphorous-doped silicate glasses (PSG) on p-type monocrystalline silicon wafers via in-line atmospheric pressure chemical vapor deposition (APCVD) and subsequent thermal diffusion. Processed wafers with and without the PSG layers have been analysed by SIMS measurements to investigate the depth profiles of the resultant phosphorous emitters. Subsequently, complete solar cells were fabricated using the phosphorous emitters formed by doped silicate glasses to determine the impact of this high-throughput doping method on cell performance. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We study optically single self-assembled quantum dots embedded within the wide quantum well of a mixed type quantum structure. We compare the steady state and pulsed photoluminescence spectra of these dots to those of previously studied "regular" dots. We unambiguously identify experimentally emission from various discrete charge state of the dots. We provide means for optically tune the charge state of the dot, both negatively and positively. Our observations are used to accurately determine the asymmetry between the quantum dots' confined electron and hole envelope wavefunctions.

We report the experimental observation of photon antibunching in the photoluminescence of a chemically synthesized CdSe/ZnS quantum dot (QD) at room temperature and from a single self-assembled InAs/GaAs QD at 4 K. These results prove that a single quantum dot acts like an artificial atom, with a discrete anharmonic spectrum. Using the quantum regression theorem, we find that the normalized second-order correlation function g (2) (τ) can be described by a single exponential function in the weak-laser excitation regime. From this model we derive that the time constant of the photon antibunching signal is determined by the effective incoherent pump and recombination rates of the nonresonant optically pumped QD.

In this work, the root cause of variation in surface recombination for Si wafers after different cleaning processes and for different passivation layers is investigated using a combination of calibrated photoluminescence (PL) imaging and transmission electron microscopy (TEM). The use of a HF-last or oxide-last cleaning and/or conditioning process is shown to have a strong impact on surface recombination for SiNx passivated surfaces, but little impact for Al2O3/SiNx stacks. For a SiNx passivation layer, cross-sectional TEM imaging revealed the formation of a â1-2nm SiOx interlayer resulting from a controlled oxidation during the last cleaning/conditioning step. The presence of the SiOx layer reduces the interface defect density (Dit,midgap) by an order of magnitude and dramatically increases the effective carrier lifetime. However, for Al2O3/SiNx passivated surfaces, TEM studies revealed that a SiOx layer is formed at the interface between the c-Si and AlOx even for c leaning processes ending with HF-last treatment due to which the cleaning sequence has minimal impact on the effective carrier lifetime.

We report on the fabrication and characterization of solar blind photodetectors (SBPs) based on undoped β-Ga2O3 and Zn doped (∼5 × 1020 cm−3) β-Ga2O3 (ZnGaO) epitaxial films with cutoff wavelength of ∼260 nm. The epilayers were grown on c-sapphire by the metal organic chemical vapor deposition technique and their structural, electrical and optical properties were characterized using various methods. As grown films have a large number of defects, resulting in detectors with enhanced internal gain, hence, high spectral responsivity >103 A/W. Post growth annealing in oxygen improved the quality of the epilayers, leading to detectors with reduced dark current (∼nA to ∼pA) and increased out of band rejection ratio. At 20 V bias, a ZnGaO detector showed a peak responsivity of 210 A/W (at 232 nm) and an out of band rejection ratio (i.e., R232 nm/R320 nm) of 5 × 104. Alternatively, for a β-Ga2O3 detector these parameters were found to be five times and three times lower, respectively, suggesting that ZnGaO detectors have superior performance characteristics. These results provide a roadmap toward achieving high responsivity SBPs based on epitaxial ZnGaO films, laying a solid foundation for future applications.

We report experimental observation of optically-pumped continuous-wave lasing from self-assembled quantum dots (QDs) embedded in high-quality factor (Q) microdisk laser structures. For large diameter (4.5 μm) microdisks containing InAs QDs, the emission spectra show simultaneous lasing on one to five well separated modes (Q = 10000-17000) in the wavelength range between 920 and 990 nm. For small diameter (2 μm) microdisks containing (In,Ga)As QDs, lasing occurs even when the estimated average number of QDs inside the cavity-mode volume is unity.

We resolve spatially, spectroscopically and temporally the photoluminescence emission from single self-assembled In(Ga)As/GaAs quantum dots, The temporal evolution of the emission spectrum after pulsed excitation is measured for various excitation intensities at various ambient temperatures. The evolution of the spectrum with the increase in both steady state and pulse excitation intensities is measured as well. A multi-exciton model is used for calculating the temporal and excitation intensity dependence of the measured spectra. The quantitative agreement between the measured and calculated spectra provides an unambiguous determination of the radiative lifetime of a single quantum dot exciton. This lifetime is 4-6 ns long and is temperature independent. The reduced spatial coherence between the confined exciton and the radiation electromagnetic field quantitatively explains this long radiative time.

We explore electron transfer in double quantum well structures induced by femtosecond mid-infrared intersubband excitation. Spatial transfer of electrons from one quantum well to its hole filled neighbor is detected by recombination luminescence. The process results in upconversion of the mid-infrared exciting light to near-infrared luminescence. Two mid-infrared pulses with variable time delay allow us to display the field and intensity autocorrelation function for the upconverted signal and measure the electron transfer dynamics. Electron transfer between two GaAs quantum wells separated by 300 nm can be saturated and the intensity autocorrelation function exhibits a slow 18 ps recovery. Transfer between wells separated by only 25 nm is coherently controlled by the phase of the two collinear infrared pulses.