B. Ullrich

Bistable Optical Thin CdS Film Devices: All-Optical and Optoelectronic Features

We have performed experiments on thin (6 µm) spray-deposited CdS films. All-optical and optoelectronic (hybrid) bistabilities were observed at 210 K. We have been successful for the first time in demonstrating that it is possible to reach, in thin CdS films, the same contrasted bistable loops in both transmission and photocurrent. The origin of the bistability is explained by the measured dependence of transmission on temperature. A generalized model based mainly on Urbachs rule is presented for calculating the temperature dependence of transmission. A good agreement between theory and measurement is obtained.

Comparison of the photocurrent of ZnSe/InSe/Si and ZnSe/Si heterojunctions

Abstract A thin (≈200 nm) ZnSe film was grown by molecular beam epitaxy on p-type Si with an InSe buffer layer in between. The InSe buffer (≈25 A) was used to bypass the huge lattice mismatch of 4.4% between ZnSe and Si in order to ensure the growth of stress-free ZnSe. The ZnSe/InSe/Si heterojunction exhibited rectifying features and intrinsic photoconductivity. It happens, however, that the shape of the photocurrent spectrum differs considerably from those of ZnSe/Si heterojunctions. The differences are explained by enhanced transport and diffusion features of excited electrons in the Si substrate due to the relaxation at the interface in comparison with ZnSe/Si samples.

Luminescence properties pf p-type thin CdS films prepared by laser ablation

Abstract Investigations of the luminescence of p-type CdS:Cu thin (less than or equal to 2 μm) films on glass substrate prepared by laser ablation were performed for the first time. The dependences of the luminescence on the Cu content in the thin films were studied at 300 K with argon laser lines at 457.9 nm, 488.0 nm and 514.5 nm. It is demonstrated that the luminescence excited with the 514.5 nm line corresponds to the donor-acceptor transition. Furthermore, it is shown that the intensity of the red emission of CdS:Cu films can be efficiently bleached by Cu doping.

The effect of the substrate on the emission features of thin CdS films formed by laser ablation

Abstract The luminescence of thin (2 μm) CdS films formed by laser ablation was investigated in the range 480–520 nm at 300 K. It turned out that the luminescence properties depend crucially on the substrate. As a main result, CdS films on glass exhibited an emission peak at the gap (around 505 nm) of thin CdS films in contrast to films on quartz. This observation is explained by the disturbed stoichiometry of thin CdS films on quartz. Furthermore, we have demonstrated that both Fabry–Perot interferences and position of the fundamental absorption edge of the thin CdS films do not depend on the substrate.

Optical and hybrid properties of the ZnSe/InSe/Si heterojunction

Abstract By molecular beam epitaxy, a thin (≈200 nm) ZnSe film was grown on a p -type Si substrate covered with an InSe buffer layer. The InSe buffer is used to bypass the huge lattice mismatch of 4.4% between ZnSe and Si in order to ensure the growth of a stress-free optically smooth ZnSe film. Reflection, photoluminescence and photocurrent properties at 300 K demonstrated that the thin ZnSe film grown represents a high quality etalon with the bandgap of cubic bulk ZnSe. Furthermore, it is shown that the ZnSe/InSe/Si heterojunction exhibits rectification and a nearly constant intrinsic photoconductivity between 500 and 900 nm. This appreciable behavior makes the ZnSe/InSe/Si heterojunction very attractive for applications to sophisticated light detectors.

The influence of the laser fluence on the transmission features of thin CdS films formed by laser ablation

Abstract Transmission threshold and steepness of the transmission edge of thin CdS films formed by laser ablation with different laser fluences (2, 4 and 5 J cm −2 ) were studied at 300 K. The threshold close to 514.5 nm is shifted to shorter wavelengths and the edge becomes steeper with increasing laser fluence. We show that the modification of the transmission features underlies the turn of the c -axis of the CdS films from a perpendicular to the surface oriented direction at 2 J cm −2 to a parallel orientation at 5 J cm −2 .

The influence of hydrogen passivation of silicon on the photocurrent of CdS/Si heterodiodes

A CdS/p-Si:H heterojunction is formed by the evaporation of CdS on a hydrogen passivated Si wafer. It is found that the substrate passivation procedure has a strong influence on the photocurrent properties of the CdS/p-Si interface. In fact, in addition to the photoresponse in the red and infrared spectral ranges, the CdS/p-Si:H heterodiode also reveals photocurrent in the blue and green. This is in contrast to CdS/p-Si and CdS/p-InP devices, which do not exhibit photocurrent in the absorption region of CdS. The positive influence of the passivation on the optoelectronic properties of the CdS/p-Si:H heterodiode is explained by the prevention of interface reactions during the formation of the device. The dependence of the photocurrent of the CdS/p-Si:H heterodiode on an applied bias gives evidence of a homojunction-like behavior.

Temperature dependence of reflectance and transmittance at 514.5 nm of CdS films formed by laser ablation

The temperature dependence of reflectance and transmittance of a thin (2 μm) CdS film prepared by laser ablation was investigated. The measurements were performed with the 514.5 nm line of an argon laser in the range 180–350 K. It occurred that due to locally enhanced absorption at the film/substrate interface, the transmitted light was considerably weaker than would be theoretically expected for a homogeneous layer. The reflectance, however, followed the theory based on Urbach’s rule. The interfacial enhancement of the absorption was confirmed by luminescence investigations.

Demonstration of the independence of absorptive all‐optical bistabilities in reflection and luminescence

The dependence on the source location of photothermal bistabilities of a thin CdS:Cu film is demonstrated by the investigation of bistability in luminescence in reflection geometry. Particularly, it is shown that the infrared portion in the reflected beam clearly exhibits contrasting bistable loops in contrast to the reflection itself. The unexpected effect is explained by the fact that bistability in luminescence is a bulk feature which is not affected by the surface region.

Angular dependence of the reflectance and transmittance of CdS films formed by laser ablation

The dependence of reflectance and transmittance on the angle of incidence and the polarization of visible light was measured for thin (≈2 μm) CdS films formed by laser ablation. The experiments were carried out at 300 K with argon and He–Ne lasers. In the blue and green spectral ranges, the experiments were described straightforwardly by the theoretical approach for highly absorptive materials, omitting the film thickness. For red light, the sample represents dielectric material and multiple reflections must be considered to fit the experiments. Furthermore, it is shown that the angular-dependent features of red light are an accurate tool to determine the film thickness.