Amorphous semiconductors, being intrinsically metastable in nature, exhibit a wide variety of changes in their physical properties, particularly when photoinduced using bandgap illumination. This article reviews the photoinduced phenomena exhibited by amorphous semiconductors such as amorphous hydrogenated silicon (and other tetrahedrally coordinated materials) and chalcogenide glasses. Features exhibited in common by all types of amorphous semiconductors, whether in the experimentally observed photoinduced metastability or the theoretical models used to account for such behaviour, are stressed.

Photoinduced effects and metastability in amorphous semiconductors and insulators

A programmable metallization cell ("PMC") comprises a fast ion conductor such as a chalcogenide-metal ion and a plurality of electrodes (e.g., an anode and a cathode) disposed at the surface of the fast ion conductor and spaced a set distance apart from each other. Preferably, the fast ion conductor comprises a chalcogenide with Group IB or Group IIB metals, the anode comprises silver, and the cathode comprises aluminum or other conductor. When a voltage is applied to the anode and the cathode, a non-volatile metal dendrite grows from the cathode along the surface of the fast ion conductor towards the anode. The growth rate of the dendrite is a function of the applied voltage and time. The growth of the dendrite may be stopped by removing the voltage and the dendrite may be retracted by reversing the voltage polarity at the anode and cathode. Changes in the length of the dendrite affect the resistance and capacitance of the PMC. The PMC may be incorporated into a variety of technologies such as memory devices, programmable resistor/capacitor devices, optical devices, sensors, and the like. Electrodes additional to the cathode and anode can be provided to serve as outputs or additional outputs of the devices in sensing electrical characteristics which are dependent upon the extent of the dendrite.

Programmable metallization cell structure and method of making same

Methods of forming metal-doped chalcogenide layers and devices containing such doped chalcogenide layers include using a plasma to induce diffusion of metal into a chalcogenide layer concurrently with metal deposition. The plasma contains at least one noble gas of low atomic weight, such as neon or helium. The plasma has a sputter yield sufficient to sputter a metal target and a UV component of its emitted spectrum sufficient to induce diffusion of the sputtered metal into the chalcogenide layer. Using such methods, a conductive layer can be formed on the doped chalcogenide layer in situ. In integrated circuit devices, such as non-volatile chalcogenide memory devices, doping of the chalcogenide layer concurrently with metal deposition and formation of a conductive layer in situ with the doping of the chalcogenide layer reduces contamination concerns and physical damage resulting from moving the device substrate from tool to tool, thus facilitating improved device reliability.

Integrated circuit device and fabrication using metal-doped chalcogenide materials

Ag-doped chalcogenide glasses and amorphous thin films, their preparation, properties, photodoping, photoinduced surface deposition and applications, are reviewed, expanding on the results obtained recently. The progress obtained is not only connected with better understanding of their structure, chemical bonding and properties but also in application of Ag-containing glasses and films in solid-state batteries, electrochemical sensors and optoelectronics (gratings, microlenses, waveguides, optical memories, non-linear effects).

Ag doped chalcogenide glasses and their applications

A method and apparatus is disclosed for sensing the resistance state of a Programmable Conductor Random Access Memory (PCRAM) element using complementary PCRAM elements, one holding the resistance state being sensed and the other holding a complementary resistance state. A sense amplifier detects voltages discharging through the high and low resistance elements to determine the resistance state of an element being read.

Complementary bit PCRAM sense amplifier and method of operation

To understand photosurface deposition (PSD) phenomenon, the deposition mechanism of Ag particles on Ag-rich AgAsS glasses under illumination has been investigated from a view point of electrical effects. The photo-deposited Ag particles are found to be negatively charged under illumination. PSD can effectively be suppressed by coating over the surface with semi-transparent metallic films of work function greater than 4.3 eV, which are positively charged under illumination. The Ag deposition with similar morphology as that observed in PSD can be induced by application of d.c. voltage or by irradiation of an electron beam. These observations suggest that the growth of Ag particles in PSD can be explained by a purely electrical model.

Mechanism of photosurface deposition

Non-crystalline solids and liquids with optical gaps of 1–9 eV possess Urbach edges with a universal, minimal steepness parameter of ~ 50 meV. We suggest that this value arises from thermo-dynamical density fluctuation inherent to nano-scale, medium-range, glassy structures. Other kinds of disorders appear to reduce the steepness of the optical absorption edges.

Minimal Urbach energy in non-crystalline materials

Photostructural change in amorphous selenium has been studied in situ by extended X-ray absorption fine structure (EXAFS) at 30 K. Under illumination, we observed a light-induced increase of average coordination number (by ∼5%) and in disorder while the bond length remained unchange. This change is ascribed to local formation of three-fold coordinated sites which simultaneously increases structural disorder. The light-induced change of the coordination is transient: initial coordination is restored after switching off the light, while the light-induced structural disorder remains.

Photostructural changes in amorphous selenium: an in situ EXAFS study at low temperature

Amorphous Ag–As–S films become birefringent and their surfaces are modified when exposed to linearly polarized light. The photoinduced birefringence is positive with a magnitude of +0.01, which is in contrast with that (−0.001) of As 2 S 3 . The refractive index change appears with an anisotropic surface deformation, which consists of two kinds of patterns. One is fringes in an illuminated spot and the other is streaks as cat's whiskers radiating from the spot. Compositional measurements show that the fringe accompanies an Ag-content modification of ∼5 at.%. The photoinduced anisotropy in Ag–As–S films is assumed to be caused by photo-electro–ionic interaction.

K. Tanaka

Papers

Mechanism of photosurface deposition

Minimal Urbach energy in non-crystalline materials

Photostructural changes in amorphous selenium: an in situ EXAFS study at low temperature

Photoinduced anisotropy in the ion-conducting amorphous semiconductor Ag–As–S

Photo-induced bond switching in amorphous chalcogenides