Photonic THz laser digitizing with thin-film GaAs on glass is demonstrated. In fact, the GaAs/glass interface can be used for an effective all-optical digitizer (35%) of transmitted and reflected laser beams. The films have been formed with pulsed-laser deposition. The switching principle is extremely straightforward: two (or more) laser rays were crossed at the same spot on the interface. Most of the experiments have been carried out with red (read) and green (write) laser rays. The information of the write beam is transferred to the read beam by means of alteration of the electronic state of the interface. Pump-probe experiments revealed that the absorption change, i.e., the alteration of the electronic state, takes place within a few picoseconds. Therefore, logic operations in the THz range become feasible. In addition, NOR and NAND gate realizations with laser crossing are possible. Besides the formation of smart all-optical cross-links, all-optical computing is further application of laser crossing. Based on the unmatched simplicity of the switch realization, switching speed, and the fairly low material quality demands, laser crossing in thin-film GaAs has the potential to be used in future optical networks. This work further stresses the hybrid mode with laser crossing in thin-film GaAs/glass interfaces.
The absorption edge of thin-film GaAs on glass has been investigated with the standard constant photocurrent method (s-CPM) method and photocurrent analysis. The films have been formed by pulsed-laser deposition (PLD) employing the 532 nm emission of a YAG:Nd laser (6 ns, 10 Hz). Notably, the films have been deposited without heating the substrate. Fitting the measured absorption data with the crystalline density of states and the Urbach tail a very good agreement has been found. X-ray analysis showed that the films are predominately oriented towards the (111) plane. The function used to fit the absorption data describes the photocurrent data at different biases as well. Annealing of the samples up to 400 K did not cause notable changes in the absorption edge and overall photocurrent spectra. The presented results reveal that "cold" PLD, i.e., without substrate heating, forms high-quality oriented photosensitive thin-film GaAs on glass, which hardly alters its optoelectronic features under thermal treatment. Under this prospect and due to the relative ease to form the films, PLD GaAs might be of interest for applications in optoelectronics and photovoltaics.
We investigated a novel possibility to attain all-optical logical gates. The host of the device was a thin-film semiconductor (CdS, GaAs, InP) on glass produced by various methods (pulsed-laser deposition and metal organic chemical vapor deposition). In the thin-film two visible laser beams, the primary and secondary ray, were crossed in the same spot. In this way, the secondary beam caused a transmission decrease in the primary beam. Laser crossing is an extremely undemanding concept based on electronic absorption alterations. Apparently, every semiconductor can be used for laser crossed all-optical logics and, in contrary to other semiconductor based concepts, laser crossing does not demand specific materials, material qualities or nonlinear features. The unmatched overall simplicity and possible THz operations recommend laser crossing for the realization of all-optical digital devices.
Hetero-pairing of thin-film GaAs on Si is of considerable interest for novel applications in optoelectronics. However, the formation of high-quality GaAs is difficult and requires expensive top technologies such as molecular beam epitaxy (MBE) and related methods. In general, MBE forms high-quality epitaxial layers but is not capable of the straightforward formation of GaAs on Si because of the 4.1% lattice mismatch between both materials. We have developed and explored the possibilities of pulsed-laser deposition (PLD) for the formation of GaAs films on (100) n-type Si substrates. The films have been produced in vacuum (10-6 torr) employing the fundamental (1064 nm), second (532 nm), and third (355 nm) harmonic emission of a Nd:YAG laser with a repetition rate of 10 Hz and a pulse duration of 6 ns. The laser was focused on (100) p-type (1019 cm-3) GaAs wafers with an energy fluence of 0.79-0.84 J/cm2. During the deposition, the substrate was not heated. The current-voltage characteristic of the samples showed rectification, i.e., the doping of the GaAs target was successfully maintained in the PLD film and a diode was formed in conjunction with the oppositely doped Si substrate. The observation of photocurrent without bias is an additional proof that an operating junction was achieved. The crystallographic quality of the films was checked by x-ray analysis and revealed that the films show [111]-oriented crystalline parts. The realization of GaAs/Si photodiodes reveals the potential of PLD to be used for the monolithic integration of GaAs photonic devices with Si circuits.
We formed p-GaAs/n-Si and n-GaAs/p-Si heterostructures by depositing thin-film GaAs on Si wafers with pulsed-laser deposition (PLD). For the GaAs ablation, the 532 nm emission of a Nd:YAG laser (10 Hz, 6 ns) with a fluence of 0.79-0.84 J/cm2 was used. The thicknesses of the films were approximately 0.5 μm. During the deposition, the substrate was not heated and the ambient pressure was kept at 10-6 torr. X-ray analysis showed that the films contain crystallites and by means of an atomic force microscope (AFM), it is demonstrated that the film surfaces are fairly smooth. Using a monochromatic light source and by means of electrical contacts on the top and the rear of the sample, we measured the photocurrent through the junction using lock-in technique. These measurements showed that the photocurrent spectra of the p-GaAs/n-Si diode crucially depend on the applied bias. At -0.7 V (reverse bias) the photocurrent maximum is at 930 nm, while at +0.5 V, the photocurrent maximum lies at 1056 nm. These maxima are in the vicinity to the bandgap of GaAs and Si, respectively. In other words, it is possible to switch between the spectral sensitivity of GaAs and Si via an applied electric field. The device can be either used as a photo-detector for which the sensitive wavelength range can be easily chosen by the applied bias or as hybrid multiplexer to convert two optical inputs into one electrical output.
One of the most straightforward methods possible is presented and investigated to form thin film GaAs. The film was deposited on unheated glass in vacuum (10-6 Torr) by the ablation from a GaAs wafer with the emission of a pulsed Nd:YAG laser (532 nm, 6 ns, 10 Hz). The photoluminescence, photocurrent, transmission and micro-Raman measurements of the films demonstrate that films with promising optoelectronic properties have been formed. Most importantly, from the viewpoint of light emitting and optoelectronic device production, the films show photoluminescence of comparable intensity with the bulk material without emissions owing to impurities, although the films show a rather flat absorption edge which indicates tail states. The observed photocurrent was in the μA/W range driven by rather moderate electric fields on the order of 100 V/cm. Concerning the material quality, the films have an extremely smooth surface as demonstrated with scanning electron microscopy. Grown GaAs films on glass substrates were amorphous evidenced by X-ray diffraction measurements, however, micro-Raman measurements showed crystalline phonon modes, suggesting that localized crystalline structure might co-exist in amorphous GaAs films.
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