Hisashi Sakai

Optoelectronic properties of thin film CdS formed by ultraviolet and infrared pulsed-laser deposition

Abstract Thin CdS films on glass are formed by ultraviolet (UV) and infrared (IR) pulsed-laser deposition (PLD) at 355 and 1064 nm with a repetition rate of 10 Hz and a pulse width of 10 ns and 150–180 μs, respectively. The applied laser fluence is kept in the range of 2–4 J cm −2 . By UV-PLD, the orientation of the c -axis with respect to the glass surface is adjustable via laser fluence, resulting in perpendicular and parallel oriented films at 2 and 4 J cm −2 , respectively. The orientation of IR-PLD samples is maintained perpendicularly, independent of the fluence. The optoelectronic properties of the films are studied by photocurrent (PC) and photoluminescence (PL) spectroscopy at 300 K. The PC of the UV-PLD samples reflects the turn of the c -axis, i.e. the absorption anisotropy, of CdS. The IR-PLD films, however, do not show PC because of high dark conductivity. The UV-PLD samples show PL in the range (2.27–2.45 eV). The emission below 2.45 eV is caused by formation of recombination centers. Notably, the film formed at 3 J cm −2 emits the spectral sum of the films formed at 2 and 4 J cm −2 . The IR-PLD samples show green emission (2.493 eV) clearly above the CdS bandgap due to band filling. The results pave the way for the creation of smart photonic gratings, which exhibit locally tunable optoelectronic properties.

Absorption and photocurrent properties of thin ZnS films formed by pulsed-laser deposition on quartz

The absorption and photocurrent properties of thin film ZnS on quartz glass formed by pulsed-laser deposition have been studied experimentally and theoretically at room temperature. Using the Lorentzian function to describe the exciton density of states, we show that the absorption is strongly influenced by excitonic formation. The theory for the absorption, however, does not describe the PC spectra of the film since the exciton remains electrically neutral up to fields of 1 kV/cm due to the high binding energy of 36 meV. Therefore, the fundamental absorption according to density of states and Urbach rule determines the shape of the photocurrent spectrum.

High-electric-field photocurrent in thin-film ZnS formed by pulsed-laser deposition

Photocurrent spectra of thin-film ZnS on glass fabricated by pulsed-laser deposition have been studied employing electric fields up to 20 kV/cm. The spectra show a shift towards lower energy at and beyond 10 kV/cm. By modeling the film absorption with the density of states and the Urbach rule, it is shown that, without the involvement of the Franz–Keldysh effect and excitonic transitions, the slope of the Urbach tail depends on the electric field, owing to impurity ionization.

Photoelectric dichroism of oriented thin film CdS fabricated by pulsed-laser deposition

By means of alternating (AC) and direct (DC) photocurrent measurements, the photoelectric dichroism of thin CdS films on glass is demonstrated at 300 K. The samples are formed by pulsed-laser deposition, and defined orientations, i.e. the c-axis perpendicular (CdS⊥) or parallel (CdS∥) to the glass substrate, are achieved by variation of the laser fluence. The AC peaks of the photocurrent spectra of CdS⊥ and CdS∥ are separated by 55 meV. The absorbance spectra prove that the peak shift is caused by absorption dichroism, which is comparable with that of single crystal CdS. The separation of the DC spectra (140 meV) clearly exceeds the value of the absorption dichroism due to long-term trapping of the photocarriers at the surface of the films.

Microcavity lasing of optically excited cadmium sulfide thin films at room temperature

We report what is believed to be the first observation of lasing of an optically pumped thin CdS film formed by laser ablation on glass. Laser action is observed at room temperature, and the emission peak is at 501 nm. X-ray diffraction shows that the polycrystalline films are of wurtzite structure and have (002) preferred orientation. Fabry-Perot laser modes are spaced 16 nm apart, indicating a cavity length of 2.9mum . The cavity is formed by consistently self-formed microcavities within the hexagonal lattice.

Reflection properties of oriented thin CdS films formed by laser ablation

Oriented thin (≈2 μm) films, CdS, prepared by laser ablation were characterized by the dependence of external and internal reflection on both the angle of incidence and the polarization of laser light. The samples exhibit perpendicular and parallel orientation of the crystallographic axis with respect to the surface of the glass substrate. The experiments were performed at 300 K using low intensity (<1 W/cm2) cw emissions at 476.5, 514.5 and 632.8 nm of argon and He–Ne lasers respectively. For blue and green light, the results are very well described by the theoretical models based on Fresnel reflection. In contrast to the external features, the internal reflectance exhibits dichroism and birefringence of the samples at 514.5 nm, revealing the sensitivity of the internal reflection technique to the optical anisotropy of the films. Considering multiple-beam interference, the model of Fresnel also describes satisfactorily the results for red light. However, a rather sensitive dependence on the incoming He–Ne laser intensity was observed. In fact, by increasing the intensity of 64 mW/cm2 by about one order of magnitude, only the external reflectance shows good agreement with the theory, whereas the internal reflection properties are obviously influenced by additional effects, such as non-linear change of the optical constants, which are not included in Fresnel reflection considerations.

Measurement of Density Distribution of Metastable He Atom in a Plane-Parallel Hollow Cathode He Plasma

The density distribution of the singlet metastable He atom in a plane-parallel hollow cathode plasma was measured by the laser-induced fluorescence (LIF) method along with the absorption method, where the alignment of the atom excited by the linearly polarized laser light, the self-absorption of the fluorescence light by the 21S atom in the plasma and the effect of the atomic collisions as well as the electronic ones were considered. The alignment is demonstrated to play important roles in the determination of distribution of the metastable He atom density in a glow plasma with the electron density of (2.0±0.5)×1011 cm-3, temperature of 1.3 eV and gas pressure of 0.7 Torr. Applicability of a simplified model used for the analysis of the present experiment was also discussed.

Contribution of High-Velocity Particles to Stress of Thin Film Produced by Laser Ablation.

The origin of stress in thin CuInSe2 films produced by laser ablation is investigated by the observation of the laser plume using the multichannel spectrometer and the laser transmittance method. As the laser energy density on the target increases above 1 J/cm2, the fraction of fast particles (atoms and ions) decreases and that of slow particles (droplets) increases. The stress in the thin film is the greatest at 1 J/cm2, and decreases below and above 1 J/cm2. From these results it is clear that the compressive stress of the thin film is caused by the peening effect due to the incidence of fast atoms and ions, and the slow, heavy species contribute to the relaxation of stress.

Laser-induced fluorescence measurements of electric field distribution in the plasma edge

Electric field distribution is measured in a plane-parallel hollow cathode discharge by using the Stark effect and the quadrupole moment transition with the aid of the laser-induced fluorescence technique. The He 2 1 S atoms are excited through the forbidden or allowed transitions produced by laser wavelength scanning. The intensity ratio of two fluorescences is proportional to the forbidden absorption coefficient due to the Stark effect. Moreover, in order to observe the absolute field strength without the intensity calibration, the fluorescence intensity ratio due to the Stark effect is compared with that due to the quadrupole moment transition. The observed potential differences over the dark space are in good agreement with results of probe measurements.

Influence of the c-axis orientation on the optical properties of thin CdS thin films formed by laser ablation

Thin (approximately equals 1.5 micrometer) CdS films were prepared on glass by laser ablation using fluences of 2 - 5 Jcm-2. We demonstrate that such an increase of the laser fluence turns the orientation of the c-axis of the films from perpendicular to parallel with respect to the substrate surface. The influence of this orientation variation on the optical properties of the films is studied by photocurrent, transmission and z-scan measurements. All experiments were carried out at 300 K using monochromatic light or the cw emission of argon and He-Ne lasers at 514.5 and 632.8 nm, respectively. The transmission threshold and the photocurrent maxima are shifted to shorter wavelengths and the transmission edge becomes steeper with increasing the laser fluence. The nonlinear absorption and refraction indices were evaluated for 514.5 nm and 632.8 nm by z-scan technique. It occurred that at 514.5 nm the photo-thermal heating due to effective absorption dominates and, therefore, refractive nonlinearities are not provable. At 632.8 nm, however, the samples are transmissive and refractive nonlinearities are clearly observed. Higher nonlinear coefficients of absorption and refraction were found for samples with parallel c-axis. As far as we are aware, this work represents the first study of the influence of the crystal direction on the photocurrent and z-scan features of oriented thin CdS films.