# Presentation: 356698

## Amorphous Semiconductors for Optical Memories P. Craig Taylor, Colorado School of Mines, DMR-0307594

- Semiconductors that can be switched between their crystalline and amorphous phases by a pulse of light are the foundation for one of the most important optical data storage technologies.  Technologically, the most important composition is Ge2Sb2Te5, which is the active component of re-writable DVD’s. Until recently, even something as basic as the structure of the amorphous phase was unknown.  We have performed extended x-ray absorption fine structure experiments to determine that the Ge, Sb, and Te atoms have four, three, and two-and-a-half nearest neighbors, respectively, and that some Ge-Ge bonds exist.  Simple models based on covalent bonding are consistent with this result.  This structure is *very different* than that of the crystalline phase in which all three elements have six nearest neighbors.

- Two simplistic structures of amorphous Ge2Sb2Te5.  The structure determined experimentally is midway between these two extremes, i.e., fewer 3-fold coordinated Te than in (a) and fewer Ge-Ge bonds than in (b).

**3-fold coordinated Te (80%)**

**Te**

**Te**

**Ge**

**Sb**

**Sb**

**Half as many Ge-Ge bonds as Ge-Te bonds**

**(a)**

**(b)**

**Te**

**Sb**

**Ge**

**Notes:**
A semiconductor is a solid in which the resistance to the flow of electrical current can be varied dramatically.  This property makes semiconductors the basis for all modern electronic devices, such as computers.  An amorphous semiconductor is a solid in which the atoms have a well defined number of nearest neighbor atoms as dictated by chemistry but in which there is no long range order as exists in a crystalline solid.  The picture shows part of an amorphous structure for the particular chemical composition, Ge2Sb2Te5.  The green, blue, and red balls represent tellurium, germanium, and antimony atoms, respectively, and the sticks represent the chemical bonds between the atoms. The two structures shown are the simplest predictions of the local coordination around these three elements based on simple covalent bonding arguments.  In the simplest form, these arguments [which are sometimes called the “8-N” rule, where N is the column number (valence) in the periodic table] suggest that Ge Sb and Te will have four, three, and two nearest neighbors, respectively.  This is case (b) in the figure, where a continuous network also requires that there exist like atom bonds (Ge-Ge bonds are the most likely).  If the amorphous network strongly prefers only Ge-Te and and Sb-Te bonds, then the picture in (a) holds given the 8-N rule with this “chemical” restriction.  These are the two simplest predictions, and as is often the case, the experimental evidence suggests something in between these two extremes.  Regardless of this detail, the local structural order in the amorphous phase is determined primarily by the covalent nature of the bonding.  This situation does not hold in the crystalline phases where all three elements have six nearest neighbors (octahedral).  

Nugget based on:  Baker et al PRL 96 (25) 255501 (2006)

## Amorphous Semiconductors for Optical MemoriesP. Craig Taylor, Colorado School of Mines, DMR-0307594

- Several undergraduates, including Robin Plachy, Jonathan Grant, Cody Holdaway, Ben Martin, and Michael Richardson, five graduate students, Chris Nelson, Jared Olson, Janica Whitaker, Tong Ju, Tom Herring and Heng Li, and one postdoctoral research associate, Tining Su, contributed to this work.  Plachy received a Kennicott Scholarship and a Goldwater Scholarship and is now in graduate school. Nelson and Whitaker have received their Ph.D. degrees and are currently employed as an assistant professor and a corporate research scientist, respectively.

**Societal Impact:**
- Understanding of the phase-change mechanism will lead to improved DVD’s and to improved electrically switched memories using Ge2Sb2Te5.

**Amorphous Semiconductors** **for Optical Memories****P. Craig Taylor, Colorado School of Mines, ****DMR-****0307594**

- Graduate students, David Bobela (left) and Tong Ju (right) and Fullbright Scholar, Dr. Eldar Mammadov (center) preparing a sample for a complementary measurement of the structure of amorphous Ge2Sb2Te5.