Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
1998-05-13
2001-03-13
Saadat, Mahshid (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C136S258000, C136S262000, C257S053000, C438S097000
Reexamination Certificate
active
06201261
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the fabrication of semiconductor devices and more particularly to a method of fabricating highly efficient, thin film semiconductor devices using a low quality substrate and low temperature, optical processing.
2. Description of the Prior Art
Thin film semiconductor devices, used extensively in a myriad of microelectronic and opto-electronic devices, have generated considerable academic and commercial interest in recent years. In particular, these “thin” film devices (i.e., solar cells), wherein the film thickness is less than the optical absorption depth, can exhibit high solar energy to electrical energy conversion efficiencies as compared to their “thick” film counterparts.
Optimal conversion efficiency for thin film silicon solar cells is generally thought to require a grain size of several hundred microns, preferably about ten times the film thickness, and a minority carrier diffusion length of about 50 to 100 &mgr;m, or approximately twice the film thickness. In addition, to realize optimal efficiency, thin film solar cells must exhibit a variety of desirable optical and electronic properties. Such additional properties include low electrical resistivity, high optical reflectance, high optical confinement at the contact-semiconductor interface, and minimal absorption loss at the metal contact layer. Assuming the presence of all of these attributes, the theoretical efficiency limit for a single-junction, thin film multi-crystalline silicon solar cell is about 16% to 18%.
As interest in these thin film semiconductor devices intensifies, so too does the need for more efficient and economical designs. Unfortunately, simultaneous improvement in these areas, performance and cost, has been difficult due to a number of structural and functional limitations in semiconductor device fabrication. For example, most conventional semiconductor devices include costly, crystalline substrates which function as both the substrate for subsequent depositions as well as the semiconductor material itself. To make less expensive semiconductor devices requires the use of less costly, non-crystalline substrates. Unfortunately, most low-quality substrates (e.g., amorphous glass) cannot withstand the high processing temperatures required to produce a high efficiency film using conventional techniques. One conventional process for making silicon solar cells, for example, requires a thermal furnace with temperatures in excess of 850° C. (the melting point of silicon is 1430° C.). Such high processing temperatures place severe limitations on the choice of substrate (glass, for example, softens at about 500° C.). In addition to their thermal lability, low-quality substrates typically include impurities which diffuse into the semiconductor material at high processing temperatures, thereby reducing the performance of the semiconductor or even rendering the device inoperative.
One way around the temperature limitation described above is to add a metal dopant, such as tin or copper, to the semiconductor material to depress the melting point, thereby facilitating deposition of the semiconductor at lower temperatures. This technique can work reasonably well for epitaxial growth on conventional crystalline substrates, such as single or multi-crystalline silicon. However, epitaxial growth of semiconductor materials directly on crystalline substrates (without a barrier layer) results in optical continuity (i.e., negligible index of refraction change) at the film-substrate interface. Unfortunately, optical continuity at the film-substrate interface reduces photovoltaic efficiency by enabling incident electromagnetic radiation to pass through the device, rather than being absorbed by the semiconductor material to produce electricity. Deposition of a metal-doped semiconductor on a non-crystalline or lattice mismatched substrate, on the other hand, can cause a number of problems including the formation of a very fine-grain semiconductor film, incorporation of residual metal at grain boundaries, and diffusion of metal at the interfaces. It is important that the semiconductor film have a relatively large grain structure to minimize the number of grain boundaries, or minority carrier recombination sites, which function as shunts to reduce flow of generated carriers between the semiconductor device and the external electronic circuit. Thus, although certain metals can effectively lower the melting point of the semiconductor material, which in turn reduces the deposition temperature and expands the choice of acceptable substrates, such techniques produce inefficient semiconductor devices, either because of optical continuity at the film-substrate interface or large carrier recombination losses at grain boundaries.
As previously mentioned, a grain size of several hundred microns is required for optimal conversion efficiency of thin film silicon solar cells. Unfortunately, conventional techniques can produce only small-grain silicon films at low processing temperatures. To effectively enlarge the grain size of this material, and thus reduce the number of grain boundaries or recombination sites, the film must be remelted and directionally solidified (i.e., re-solidified to position the grains in a columnar orientation) or annealed for very long durations. However, this grain-enhancement process requires extensive thermal heating which, as discussed above, prevents the use of most low-quality substrates.
Other electronic and optical properties affecting the performance of a photovoltaic semiconductor device include the device's optical confinement capacity, resistance due to electrical contacts, and optical reflectance. Forming thin, conductive metal layers on semiconductor materials is an essential step in the manufacture of microelectronic and opto-electronic devices to provide electrical contacts or current carrying paths to and from the semiconductor substance. During manufacture, such thin metal layers, or contacts, are applied to the semiconductor substance by any one of several well-known deposition techniques such as vapor deposition, sputtering, or electrolytic precipitation.
It is desirable to create an interface between the semiconductor material and the metal contact layer that has both low resistivity and high optical reflectance. Low resistivity is a primary requirement of any contact on a semiconductor device to reduce the barrier to carrier flow between the semiconductor device and the external electronic circuit. It is important, therefore, that the electrical contact be ohmic, even at very high current densities. Newer high efficiency solar cell designs utilize optical confinement techniques to capture and manage incident electromagnetic radiation so that more of it is absorbed in the semiconductor to produce electricity instead of escaping or being absorbed at the contact-substrate interface and dissipated as heat. Optical confinement is facilitated by highly reflecting interfaces where the contact joins the semiconductor material to prevent escape or absorption of the electromagnetic radiation at that interface and reflecting it instead back into the semiconductor for production of electricity. High optical reflectivity, therefore, is important to increase the amount of electromagnetic radiation energy absorbed by the semiconductor material, thereby improving the operation of the semiconductor device by increasing the number of photogenerated electrons available for collection. Unfortunately, it is difficult to provide both of these conditions simultaneously, low resistivity and high optical reflectance, at the semiconductor-metal interface using conventional methods. High optical reflectance requires a clean, abrupt semiconductor-metal interface, which is difficult to achieve. When a metal layer is deposited on a semiconductor, the contact is not electrically intimate, i.e., there is generally a layer of oxide which must be broken down by sintering or alloying. Conventional thermal processing produces
Midwest Research Institute
Saadat Mahshid
White Paul J.
Wilson Allan R.
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