4.6 Some optical properties of thin Cd3As2 films

The transmission of vacuum deposited Cd₃As₂ films has been measured in the spectral range 0.6–15 ωm. The absorption coefficient α has been calculated from the data for crystalline films obtained at substrate temperature T_s = 440 K and also for amorphous films obtained on substrates at T_s = 300?393 K. The position of the low-energy absorption edge in the spectral range 0.1–0.3 eV is discussed and the value Δ_g^opt = 0.06 eV for indirect transion Γ → Γ₁₅ is obtained for crystalline films. For amorphous films, a shift of the minimum absorption edge to $\text{Δ}\text{E}{}_{\mathrm{g}}{}^{\mathrm{opt}}\text{? 0.4?0.5}\text{eV}$ was observed. For higher energies two direct transitions have been established: the first at E₁ = 0.9 eV in the photon energy range 0.9–1.2 eV.and the second at E₂ = 1.2?1.3 eV in the photon energy range 1.3?1.6 eV. The last value is in good agreement with the value calculated by Lin Chung and with reflectivity data; it corresponds to the transition Γ₁₅ → X₁ in the hypothetical band model of Lin Chung. Differences in the reflective index values for crystalline and amorphous Cd₃ As₂ are also discussed.     

Anomalous photovoltaic effect in vacuum-deposited CdTe films

CdTe films of thicknesses varying from 125 to 1250 nm were vapour deposited onto glass substrates at oblique incidence in vacuum at substrate temperatures varying from room temperature to 250 °C. The samples are irradiated with an He-Ne laser from the direction of the deposited layer and through the substrate. It was found that the film thickness and substrate temperature play an important role in changing the magnitude and polarity of the photovoltages generated in CdTe films. A reversal in the polarity of the photovoltage is obtained in certain films depending on the deposition parameters. An attempt is made to interpret the results.     

Nb-doped TiO2 thin films for photovoltaic applications

Polycrystalline titania and Nb:TiO₂ thin films were deposited by RF magnetron sputtering. The influence of post-deposition annealing in vacuum and hydrogen atmosphere on the structure, morphology, oxidation states and optical properties was studied by X-ray diffraction, atomic force microscopy, XPS and UV–VIS spectroscopy. The heat treatment of titanium dioxide thin films in vacuum and H₂ atmosphere induces structural and morphological changes. The band gap narrowing was observed for the transparent as-deposited Nb:TiO₂ films, while annealing at 420 °C in H₂ atmosphere resulted in an enhancement of the electrical conductivity. Further on, TiO₂/p-CdTe photovoltaic devices with efficiency of 1.8% were fabricated and their characteristic ‘enhancement’ is discussed.     

Ellipsometry and optical measurements on amorphous thin films of As2Se3

Thin films of amorphous AsSe_3/2 have been prepared by thermal evaporation of the material under a vacuum of 1.33×10^?3 Pa. Reflectivity, transmission and ellipsometric measurements of the films have been carried out. The optical energy gap and the absorption coefficient as a function of wavelength were obtained. Two absorption bands were observed and interpreted in terms of defects in the AsSe_3/2 system (homopolar bonds). Analysis of reflection and transmission spectra shows that the electron density at band tails of both conduction and valency bands follows N(E)?E^1/2 (Taue plots). No considerable variations were observed on changing the film thickness.     

CuGaSe2 thin films for photovoltaic applications

Of the I-III-VI₂ group chalcopyrites, CuInSe₂ has already proved its suitability for thin film solar cells owing to its excellent optical and transport properties. CuGaSe₂ is expected to exhibit comparable properties from this point of view. With its band gap of 1.7 eV it is a candidate for use in photovoltaic tandem systems.

The preparation of CuGaSe₂ thin films by means of the vacuum evaporation of the constituent elements (four-temperature method) is described. The structural, electrical and optical properties of these films were investigated. Secondary electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction examination and measurements of the optical transmission, resistivity and thermoelectric power were used to determine the film properties relative to the preparation parameters and stoichiometry. The growth conditions were optimized for solar cell applications. Heterojunctions were prepared by the in situ evaporation of Zn_xCd_1−xS onto the CuGaSe₂ films. The characteristic data of the cells are a short-circuit current of 6 mA and an open-circuit voltage of 620 mV at an illumination at air mass 1.5 on an area of 1 cm².     

Anomalous electrical resistivity of thin gadolinium films

Electrical resistivity measurements at room temperature on thin gadolinium films in the thickness range 200–1000 Å are described. The resistivity-thickness curve in this case is anomalous in the sense that it exhibits a decrease in resistivity at lower thicknesses (less than about 500 Å). This is in marked contrast with other work and seems to represent the first observation of a resistivity decrease with decreasing thickness. It is suggested that this curious resistivity characteristic in Gd films results from the thickness dependences of the structural phase and of the spin-disorder resistivity.     

The Cd3As2-NiAs pseudobinary eutectic

Criteria to be followed in predicting new eutectic systems of particular types are discussed briefly, and demonstrated in the discovery of the Cd₃As₂-NiAs eutectic. The eutectic region of the Cd₃As₂-NiAs pseudobinary phase diagram has been determined by thermal and microscopic analysis. Cd₃As₂-NiAs eutectic ingots have been directionally grown by a new process, the liquid encapsulated Stockbarger technique, and shown to exhibit a typical rod-type morphology.     

Anomalous photovoltaic effect and disorder in ZnS crystals

Some crystals of ZnS are known to produce an anomalously high photovoltage, up to several hundred volts per cm, when illuminated by uv light in the absorption edge region. This has been attributed to the presence of alternate regions of hexagonal and cubic packing with charged dislocations at the interfaces producing built-in electric fields. Differential absorption of the incident light in the hexagonal and cubic regions is believed to create the necessary asymmetry in the built-in fields, causing an addition of tiny photovoltages at a series of interfaces which finally results in the abnormally high photovoltages observed. This paper investigates the possible mechanism by which disordered ZnS crystals containing alternating regions of cubic and hexagonal packing can result. X-ray diffraction studies show that such a disordered configuration results during the 2H to 3C phase transformation in ZnS. It is suggested that the transformation occurs by the non-random nucleation of deformation faults wherein the probability (α) of random nucleation of the faults is much less than the probability (β) for the faults to occur at two-layer separations.     

Anomalous variation of resistance in bismuth thin films and the effect of substrate temperature

Bismuth thin films of thickness 720 Å have been vacuum deposited at various substrate temperatures on glass substrates and their resistances are recorded as a function of temperature after annealing the films at 200°C. The resistance (R) versus temperature (t) curves show anomalous behaviour, in that the resistance first decreases as the temperature increases and then increases as the temperature increases, thus showing a minimum in resistance, R_min at a temperature t_min. The position of R_min (i.e. t_min) and the magnitude of R_min vary with substrate temperature. However, no such anomalous behaviour is observed for the film deposited at room temperature and its resistance monotonically decreases with increase in temperature. These observations have been explained by considering that the bismuth films behave as narrow band gap semiconductors because of the quantum size effect and by the facts that in polycrystalline films the electronic mean free path will be limited by the grain size and also that the average grain size of the film increases as the substrate temperature during deposition increases.     

Antimony sulfide thin films in chemically deposited thin film photovoltaic cells

Antimony sulfide thin films of thickness ≈ 500 nm have been deposited on glass slides from chemical baths constituted with SbCl₃ and sodium thiosulfate. Smooth specularly reflective thin films are obtained at deposition temperatures from − 3 to 10 °C. The differences in the film thickness and improvement in the crystallinity and photoconductivity upon annealing the film in nitrogen are presented. These films can be partially converted into a solid solution of the type Sb₂S_xSe_3 − x, detected in X-ray diffraction, through heating them in contact with a chemically deposited selenium thin film. This would decrease the optical band gap of the film from ≈ 1.7 eV (Sb₂S₃) to ≈ 1.3 eV for the films heated at 300 °C. Similarly, heating at 300 °C of sequentially deposited thin film layers of Sb₂S₃-Ag₂Se, the latter also from a chemical bath at 10 °C results in the formation of AgSb(S/Se)₂ with an optical gap of ≈ 1.2 eV. All these thin films have been integrated into photovoltaic structures using a CdS window layer deposited on 3 mm glass sheets with a SnO₂:F coating (TEC-15, Pilkington). Characteristics obtained in these cells under an illumination of 850 W/m² (tungsten halogen) are as follows: SnO₂:F-CdS-Sb₂S₃-Ag(paint) with open circuit voltage (V_oc) 470 mV and short circuit current density (J_sc) 0.02 mA/cm²; SnO₂:F-CdS-Sb₂S₃-CuS-Ag(paint), V_oc ≈ 460 mV and J_sc ≈ 0.4 mA/cm²; SnO₂:F-CdS-Sb₂S_xSe_3 − x-Ag(paint), V_oc ≈ 670 mV and J_sc ≈ 0.05 mA/cm²; SnO₂:F-CdS-Sb₂S₃-AgSb(S/Se)₂-Ag(paint), V_oc ≈ 450 mV and J_sc ≈ 1.4 mA/cm². We consider that the materials and the deposition techniques reported here are promising toward developing ‘all-chemically deposited solar cell technologies.’     

Phase Relations in the Zn3As2–ZnAs2–CdAs2–Cd3As2 System

Phase relations along the Cd₃As₂–ZnAs₂ and Zn₃As₂–CdAs₂ joins are studied by differential thermal analysis, x-ray diffraction, and microstructural analysis. The results, in conjunction with earlier data on the CdAs₂–ZnAs₂, Zn₃As₂–Cd₃As₂, Cd₃As₂–CdAs₂, and ZnAs₂–Zn₃As₂ binaries, are used to map out the phase diagram of the liquidus surface in the composition region Zn₃As₂–ZnAs₂–CdAs₂–Cd₃As₂ of the ternary system Cd–As–Zn. The ternary eutectic revealed in this region has an approximate composition of 26 at. % Cd + 65 at. % As + 9 at. % Zn and melts at 863 K.     

Co-electrodeposition of Cu3BiS3 thin films in weakly alkaline aqueous solutions for photovoltaic application

Journal of Materials Science: Materials in Electronics - The semiconductor copper bismuth sulphide (Cu3BiS3) has been perceived as a promising photovoltaic (PV) absorber material thanks to its...     

Electrical breakdown in amorphous As2S3 films

We investigated the electrical breakdown under d.c. electric fields of amorphous As₂S₃ films about 100–1000 Å thick sandwiched between two aluminium electrodes. It was found that the breakdown field decreased with increases in either the temperature or the thickness of the sample. Attempts were made to fit the observed electrical breakdown voltages with several previously proposed theories of thermal and electrical breakdown. It was found that a satisfactory fit could only be obtained by a modification of Klein's theory provided that the current was observed to depend on the voltage in an exponential manner in accordance with the Poole mechanism in which the carriers are released from high density localized states within the forbidden energy region. According to this modification, the breakdown field should depend inversely on the temperature and this agrees well with the experiment. The formation of localized conducting channels within the sample because of breakdown was verified by taking a scanning electron micrograph of the junction area of the broken film.     

Characterization of Zinc-phthalocyanine-CdS composite thin films for photovoltaic applications

The optical properties of ZnPc-CdS composite thin films have been measured. The composite layers were prepared by vacuum evaporation. The electrical conduction mechanism prevailing in these junctions was of Poole-Frenkel type and the activation energy was found to have a linear dependence with applied field. The maximum photoconductivity occurred at an energy gap around 1.5 to 1.7 eV. The photosensitivity is found to increase with increase in applied voltage.     

Anomalous behavior in ZnMgO thin films deposited by sol-gel method

The magnesium doped zinc oxide is a promising optical material to enhance the luminescence for possible application in solid state lighting. Magnesium doped zinc oxide thin films (Zn_0.85Mg_0.15O) were deposited by sol-gel route on p-type silicon and annealed at different temperatures in oxygen environment for an hour. The doping of magnesium in zinc oxide was confirmed by X-Ray diffraction and the samples were found to have wurtzite crystal structure with (002) preferred orientation. The films were characterized by Hall-effect, atomic force microscopy, UV-VIS spectroscopy, photoluminescence (PL) and work function measurements. The different studies exhibited an anomalous behavior for the film annealed at 900 °C. The Hall effect, work function measurements and UV-VIS spectroscopy indicated that the resistivity, work function and optical band gap increased as a function of annealing temperature (from 300 °C to 700 °C) however these parameters were found to decrease for the films annealed above 700 °C. The particle size increased with the annealing but for the samples annealed at 900 °C, the shape of the grains changed and became elongated like fibers as observed by the atomic force microscopy. The PL measurements displayed the existence of oxygen vacancies defects for the samples annealed at and above 600 °C. The possible mechanism for this anomaly has been discussed in this work.     

Dirac Semimetal Heterostructures: 3D Cd3As2 on 2D Graphene

Dirac semimetal is an emerging class of quantum matters, ranging from 2D category, such as, graphene and surface states of topological insulator to 3D category, for instance, Cd₃As₂ and Na₃Bi. As 3D Dirac semimetals typically possess Fermi‐arc surface states, the 2D–3D Dirac van der Waals heterostructures should be promising for future electronics. Here, graphene–Cd₃As₂ heterostructures are fabricated through direct layer‐by‐layer stacking. The electronic coupling results in a notable interlayer charge transfer, which enables us to modulate the Fermi level of graphene through Cd₃As₂. A planar graphene p–n–p junction is achieved by selective modification, which demonstrates quantized conductance plateaus. Moreover, compared with the bare graphene device, the graphene–Cd₃As₂ hybrid device presents large nonlocal signals near the Dirac point due to the charge transfer from the spin‐polarized surface states in the adjacent Cd₃As₂. The results enrich the family of van der Waals heterostructure and should inspire more studies on the application of Dirac/Weyl semimetals in spintronics.     

Modular process control system for photovoltaic thin films manufacturing

A method of temperature control for selenization process conducted in vacuum quartz tube furnace with radial energy transfer and a movable sample holder has been described. Selenization of metallic precursors to obtain CuInSe₂ absorber layer of a photovoltaic cell requires a special approach to the temperature regulation, where two-stage heating process is necessary. The modular process control system was developed in which the first module playing a supervising role was built in LabVIEW environment and the second one, called a predictor, was built in MATLAB. Relatively quick movement of the holder from the cold to the hot regions of the furnace allows for a rapid change of sample temperature, typical of rapid thermal processing processes.