Semiconductor devices: electrical contact

Solid-state electronic devices such as diodes, transistors, and integrated circuits contain p-n junctions in which a p-type semiconductor is in contact with an n-type semiconductor (Fig. 3.47). The structure of a p-n junction allows an electric current to flow in only one direction. When the electrode attached to the p-type semiconductor has a negative charge, the holes in the p-type semiconductor are attracted to it, the electrons in the n-type semiconductor are attracted to the other (positive) electrode, and current does not flow. When the polarity is reversed, with the negative electrode attached to the n-type semiconductor, electrons flow from the n-type semiconductor through the p-type semiconductor toward the positive electrode. 

Interconnect. Three-dimensional structures require interconnections between the various levels. This is achieved by small, high aspect-ratio holes that provide electrical contact. These holes include the contact fills which connect the semiconductor silicon area of the device to the first-level metal, and the via holes which connect the first level metal to the second and subsequent metal levels (see Fig. 13.1). The interconnect presents a major fabrication challenge since these high-aspect holes, which may be as small as 0.25 im across, must be completely filled with a diffusion barrier material (such as CVD titanium nitride) and a conductor metal such as CVD tungsten. The ability to fill the interconnects is a major factor in selecting a thin-film deposition process. 

First, let us consider the problem of getting electrical signals into and out of a semiconductor device through metallic contacts. If the contact is with an n-type semiconductor, and if the work function of the metal is smaller than... 

Solid-state electronic devices such as diodes, transistors, and integrated circuits contain p-n junctions in which a p-type semiconductor is in contact with an n-type semiconductor (Fig. 3.48). Because solar radiation can excite electrons into a conduction band, semiconductors can be used to generate an electrical current by the action of sunlight. 

There are, however, several fields of current research in which a corresponding level of understanding would be of interest also for large molecular adsorbates. For example, adsorbate-substrate interactions are relevant in the general areas of biocompatibility [51] and chemical sensors [52]. The requirement of dye-sensitization of metal oxide semiconductors also makes this an important aspect of many molecular photovoltaic devices. In fact, a good interfacial contact between dye and substrate, characterized by long-term stability and intimate electric contact, is vital for the efficiency of e.g. the dye-sensitized solar cells which have been at the center of our attention for the last five years. 

Photovoltaic (PV) Cell An electronic device consisting of layers of semiconductor materials fabricated to form a junction (adjacent layers of materials with different electronic characteristics) and electrical contacts, capable of converting incident light directly into electricity (direct current). Photovoltaic technology works by harnessing the movement of electrons between the layers of a solar cell when the sun strikes the material. 

The application we have in mind for the metal-polymer interfaces discussed in this book is primarily that where the polymer serves as the electroactive material (semiconductor) in an electronic device and the metal is the electric contact to the device. Metal-semiconductor interfaces, in general, have been the subject of intensive studies since the pioneering work of Schottky, Stromer and Waibel1, who were the first to explain the mechanisms behind the rectifying behaviour in this type of asymmetric electric contact. Today, there still occur developments in the understanding of the basic physics of the barrier formation at the interface, and a complete understanding of all the factors that determine the height of the (Schottky) barrier is still ahead of us2. 

The materials (metals and conjugated polymers) that are used in LED applications were introduced in the previous chapter. The polymer is a semiconductor with a band gap of 2-3 eV. The most commonly used polymers in LEDs today are derivatives of poly(p-phenylene-vinylene) (PPV), poly(p-phenylene) (PPP), and polythiophene (PT). These polymers are soluble and therefore relatively easy to process. The most common LED device layout is a three layer component consisting of a metallic contact, typically indium tin oxide (ITO), on a glass substrate, a polymer film r- 1000 A thick), and an evaporated metal contact4. Electric contact to an external voltage supply is made with the two metallic layers on either side of the polymer. 

These characteristics make CP-AFM ideal for studying electrical transport of nanotubes, nanoparticle assemblies, micro- or nanofabricated semiconductor devices, and individual molecules. Detailed appraisal of these characterizations can be obtained by comparing CP-AFM and STM. Although CP-AFM and STM share high spatial resolution imaging capability (STM 0.1 mn CP-AFM -10 nm, due to larger tip apex) that is critical in linking nanoscale structure to transport properties, an important distinction is the position of the tip with respect to the sample. In the case of CP-AFM, a metal-coated tip is directly contacted to the sample under a controlled load. This means that the measured I V relationship is mainly affected by the electrical properties of the tip-sample contact. 

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