R. J. Soukup

Formation of pyrite (FeS2) thin films by thermal sulfurization of dc magnetron sputtered iron

Iron films deposited by direct current magnetron sputtering onto glass substrates were converted into FeS2 films by thermal sulfurization. Experiments were carried out to optimize the sulfurization process, and the formation of FeS2 thin films was investigated under different annealing temperatures and times. High quality FeS2 films were fabricated using this process, and single phase pyrite films were obtained after sulfurization in a sulfur and nitrogen atmosphere at 450 °C for 1 h. Film crystallinity and phase identification were determined by using x-ray diffraction. The cubic phase pyrite films prepared were p-type, and scanning electron microscopy studies exhibited a homogeneous surface of pyrite. The authors have found that the best Ohmic contact for their pyrite thin films, using inexpensive metals, was Ni. The following were chosen for the study: Al, Mo, Fe, and Ni, and the one that led to the lowest resistance, 333 Ω, was Ni.

Investigation of the rf and dc hollow cathode plasma-jet sputtering systems for the deposition of silicon thin films

Abstract Both rf and dc hollow cathode plasma-jet sputtering systems have been investigated for thin film semiconductor deposition. These systems were studied as a modification of the well-known rf hollow cathode plasma jet system. The aim of this modification was to provide low temperature deposition of semiconductor silicon and silicon-based alloys as thin films with these plasma jet systems. As a first step, the deposition of an already well explored, hydrogenated amorphous silicon material, a-Si:H, was chosen for experimentation. Plasma erosion of single crystal silicon nozzles in an Ar and H2 working gas mixture was utilized for this purpose. A comparison of both dc and rf hollow cathode plasma jets has been made and correlated to the a-Si:H thin film properties. As a preliminary result, large differences between the properties of a-Si:H thin films deposited using dc and rf plasmas have been found. Monohydride Si:H composition was found for a-Si:H films fabricated using the dc plasma jet system under certain experimental conditions. However, predominantly di-hydride and multi-hydride structures and strong oxidization were found for the a-Si:H films deposited using rf plasma excitation. The sputtering efficiencies of both the rf and dc jet sources for silicon films have been found to be similar.

The advantages of Sun tracking for planar silicon solar cells

Experimental results comparing the power output of a Sun tracking solar cell with that of a stationary solar cell indicates that the tracking cell will produce over 30% more electrical energy in the course of a relatively clear day than will the stationary cell. A mathematical treatment of the problem agrees remarkably well with the experimental results predicting only slightly greater gain for the tracking cell than found experimentally. This difference can be explained by the lack of perfectly clear days during experimental testing. In fact, when a great deal of haziness or cloud cover occurs the output gain of the tracking cell over the stationary cell rapidly disappears.

Deposition of electronic quality amorphous silicon, a-Si:H, thin films by a hollow cathode plasma-jet reactive sputtering system

High quality hydrogenated amorphous silicon, a-Si:H, thin films were deposited by means of a dc hollow cathode plasma-jet with magnetic field confinement. Single-crystal silicon nozzles were reactively sputtered in a high density hollow cathode discharge. Only nontoxic gases, argon and hydrogen, were used for this purpose. Different configurations of the dc hollow cathode were used for the deposition process. Electronic quality a-Si:H thin films were achieved with light to dark conductivity ratios >106, with light conductivity near 10−5 S/cm and dark conductivity between 10−11 and 10−12 S/cm. This was accomplished with a specific configuration of the hollow cathode discharge in the silicon nozzle. Our best films have a Tauc band gap near 1.8 eV and an atomic hydrogen concentration of about 14%. The growth rate achieved for the electronic quality a-Si:H films was in the range of 2–3 μm/h.

Voltage‐Current Characteristics for Electrical Conduction Through Thin MgO Films

The voltage‐current characteristics of Au–MgO–Au thin‐film emission diodes formed by electron beam evaporation of Au and MgO are consistent with an expression derived for Schottky field emission into the conduction band of the oxide. These results however indicate a potential barrier height of about 0.72 eV when determined from the zero‐field intercept of the straight line logI vs V1/2 plot and about 0.33 eV when obtained from an Arrhenius plot. These apparent barrier heights were different for devices fabricated by depositing the MgO film in an oxygen atmosphere. The apparent barrier height determined from the zero‐field intercept went up for increasing oxygen pressure and the barrier height determined from the Arrhenius plot went down. Emission of electrons into vacuum was observed from each device at very low sample biases indicating that preferential emission through pinholes in the Au overlayer was occurring.

The lensed high‐voltage vertical multijunction solar cell

The vertical multijunction solar cell with covering lens is a photovoltaic device which promises high‐voltage, high‐efficiency outputs. The structure described here is a modification of a structure previously reported. It is shown that the original high‐voltage vertical multijunction solar cell is capable of high voltages but not high efficiencies. The solar cell described here is capable of higher efficiencies, but a more detailed analysis than presented here is necessary before a comparison between this cell and a conventional solar cell can be made.

High‐voltage vertical multijunction solar cell

The vertical multijunction solar cell with covering lens is a photovoltaic device which promises efficiencies greater than that predicted under ideal conditions for any other structure. The mathematical analysis presented here illustrates this statement. In addition the structure described here is capable of a high‐voltage output for small solar cell dimensions, a feature which makes this device attractive for many applications where other designs are impractical. The analysis predicts the output short‐circuit current, open‐circuit voltage, maximum power, and an efficiency of 21% for a silicon homojunction solar cell.

Electron‐beam‐induced currents collected by a p‐n junction of finite junction depth

The hole‐electron pair generation region in a semiconductor bombarded by a scanning electron beam can be modeled as a circular cylinder tangent to the semiconductor surface. A cross‐sectional plane normal to the cylinder is then examined. Under conditions of high semiconductor surface recombination velocity, a conformal transformation of this plane can be made to yield a geometry in which the method of images can be used to match boundary conditions at the junction and at the semiconductor surface. Thus, a solution for the electron‐beam‐induced current (EBIC) problem can be found in this manner when the generation region is external to the diffused or implanted region of a finite junction depth. An infinite series of images can be used to solve the problem when the generation region is within the diffused or implanted region under conditions of low, as well as high, surface recombination velocity. Thus, expected EBIC results are presented as a function of junction depth.

Comparative calculations for thin‐film and bulk single‐crystal Schottky‐barrier solar cells

A mathematical analysis of the expected short‐circuit current density in Schottky‐barrier solar cells is presented. For a solar cell with the Schottky barrier on the bottom, back illuminated, the active semiconductor material, GaAs for this example, must be a thin film for maximum efficiency. A comparison between this cell and a single‐crystal solar cell with the Schottky barrier on the top, front illuminated is made. This comparison shows that solar cells made from polycrystalline films could deliver the same short‐circuit current as a single‐crystal solar cell provided that the minority‐carrier diffusion length in the polycrystalline films can be kept to within one order of magnitude lower than that for the single‐crystal material. The reason for this is that solar‐reflection losses for the back‐illuminated thin‐film cell can be minimized, while for the front‐illuminated single‐crystal cell the losses must always be high.

Chemical bath deposition (CBD) of iron sulfide thin films for photovoltaic applications, crystallographic and optical properties

A low temperature chemical deposition method has been developed to deposit iron/sulfur thin films onto soda lime glass substrates. The chemical bath deposition (CBD) consists of aqueous solution ferrous sulphate, disodium salt of ethylenediaminetetra-acetic acid (Na2EDTA), sodium thiosulphate and organic solutions of ethylenediamine and methanol. The experiments were performed at room temperature and under two different conditions. The films were uniform and adhered well to the soda lime glass substrates. The deposited films were additionally processed in a sulfur and nitrogen atmosphere at a variety of different temperatures to form the pyrite phase of FeS2. The as-deposited and annealed thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption, auger electron spectroscopy (AES), and resistivity. The optimization of the FeS2 pyrite growth parameters was determined using XRD. Although both methods appeared to form FeS2 the second method is the preferable one where additional sulfurization at 450 °C for one hour yielded the films with the maximum crystalline order and stoichiometry.