Editing Phase-change memory (section)

==Background==
In the 1960s, [[Stanford R. Ovshinsky]] of [[Energy Conversion Devices]] first explored the properties of [[chalcogenide glass]]es as a potential memory technology. In 1969, Charles Sie published a  dissertation at [[Iowa State University]] that both described and demonstrated the feasibility of a phase-change-memory device by integrating [[chalcogenide]] film with a [[diode]] array.<ref>{{cite thesis |last=Sie |first=C.H. |title=Memory cell using bistable resistivity in amorphous As-Te-Ge film |year=1969 |work=Retrospective Theses and Dissertations |id=3604 https://lib.dr.iastate.edu/rtd/3604 |type=PhD |publisher=Iowa State University}}</ref><ref>{{cite journal |last1=Pohm |first1=A. |last2=Sie |first2=C. |last3=Uttecht |first3=R. |last4=Kao |first4=V. |last5=Agrawal |first5=O. |year=1970 |title=Chalcogenide glass bistable resistivity (Ovonic) memories |url= |journal=IEEE Transactions on Magnetics |volume=6 |issue=3 |pages=592 |bibcode=1970ITM.....6..592P |doi=10.1109/TMAG.1970.1066920}}</ref> A cinematographic study in 1970 established that the phase-change-memory mechanism in chalcogenide glass involves [[Electric field|electric-field]]-induced [[Crystal growth|crystalline filament growth]].<ref>"Electric-Field Induced Filament Formation in As-Te-Ge Semiconductor" C.H. Sie, R. Uttecht, H. Stevenson, J. D. Griener and K. Raghavan , Journal of Non-Crystalline Solids, 2, 358–370,1970</ref><ref>{{cite web|url=https://www.youtube.com/watch?v=0bgVsOk17vw  |archive-url=https://ghostarchive.org/varchive/youtube/20211221/0bgVsOk17vw |archive-date=2021-12-21 |url-status=live|title=A Cinematic Study of Mechanisms of Phase Change Memory |publisher=YouTube |date=2012-06-21 |access-date=2013-09-17}}{{cbignore}}</ref> In the September 1970 issue of ''[[Electronics (magazine)|Electronics]]'', [[Gordon Moore]], co-founder of [[Intel]], published an article on the technology.<ref>{{Cite journal |last1=Moore |first1=Gordon E. |last2=Neale |first2=R.G. |last3=Nelson |first3=D.L. |date=September 28, 1970 |title=Nonvolatile and reprogramable, the read-mostly memory is here |url=https://objective-analysis.com/uploads/Electronics%201970%20Neale%20Nelson%20Moore.pdf |journal=[[Electronics (magazine)|Electronics]] |pages=56–60 |access-date=April 22, 2022 |archive-date=July 7, 2022 |archive-url=https://web.archive.org/web/20220707201607/https://objective-analysis.com/uploads/Electronics%201970%20Neale%20Nelson%20Moore.pdf |url-status=dead }}</ref> However, material quality and power consumption issues prevented commercialization of the technology. More recently, interest and research have resumed as [[Flash memory|flash]] and [[dynamic random-access memory|DRAM]] memory technologies are expected to encounter scaling difficulties as chip [[lithography]] shrinks.<ref>{{cite web|url=http://features.techworld.com/storage/3211959/is-nand-flash-memory-a-dying-technology/|title=Is NAND flash memory a dying technology?|work=Techworld |access-date= 2010-02-04}}</ref>

The [[crystal]]line and [[Amorphous solid|amorphous]] states of chalcogenide glass have dramatically different [[resistivity|electrical resistivity]] values. The amorphous, high resistance state represents a [[Binary numeral system|binary]] 0, while the crystalline, low resistance state represents a 1.{{citation needed|date=July 2012}} Chalcogenide is the same material used in re-writable [[Optical disc|optical media]] (such as [[CD-RW]] and [[DVD-RW]]). In those instances, the material's optical properties are manipulated, rather than its electrical resistivity, as chalcogenide's [[refractive index]] also changes with the state of the material.

Although PRAM has not yet reached the commercialization stage for consumer electronic devices, nearly all prototype devices make use of a [[chalcogenide]] alloy of [[germanium]] (Ge), [[antimony]] (Sb) and [[tellurium]] (Te) called [[GeSbTe]] (GST). The [[stoichiometry]], or Ge:Sb:Te element ratio, is 2:2:5 in GST. When GST is heated to a high temperature (over 600&nbsp;°C), its chalcogenide crystallinity is lost. Once cooled, it is frozen into an amorphous glass-like state <ref>{{Cite journal|last1=Caravati|first1=Sebastiano|last2=Bernasconi|first2=Marco|last3=Kühne|first3=Thomas D.|last4=Krack|first4=Matthias|last5=Parrinello|first5=Michele|title=Coexistence of tetrahedral- and octahedral-like sites in amorphous phase change materials|journal=Applied Physics Letters|volume=91|issue=17|pages=171906|year=2007|doi=10.1063/1.2801626|arxiv=0708.1302|bibcode=2007ApPhL..91q1906C|s2cid=119628572}}</ref> and its [[electrical resistance]] is high. By heating the chalcogenide to a temperature above its [[crystallization|crystallization point]], but below the [[melting point]], it will transform into a crystalline state with a much lower resistance. The time to complete this phase transition is temperature-dependent. Cooler portions of the chalcogenide take longer to crystallize, and overheated portions may be remelted. A crystallization time scale on the order of 100[[Nanosecond|&nbsp;ns]] is commonly used.<ref>{{cite book |first=H. |last=Horii |display-authors=etal |chapter=A novel cell technology using N-doped GeSbTe films for phase change RAM |title=2003 Symposium on VLSI Technology. Digest of Technical Papers |year=2003 |isbn=4-89114-033-X |doi=10.1109/VLSIT.2003.1221143 |pages=177–8 |s2cid=40051862 |id=03CH37407}}</ref> This is longer than conventional volatile memory devices like modern [[DRAM]], which have a switching time on the order of two nanoseconds. However, a January 2006 [[Samsung Electronics]] patent application indicates PRAM may achieve switching times as fast as five nanoseconds.

A 2008 advance pioneered by [[Intel]] and [[ST Microelectronics]] allowed the material state to be more carefully controlled, allowing it to be transformed into one of four distinct states: the previous amorphous or crystalline states, along with two new partially crystalline ones. Each of these states has different electrical properties that can be measured during reads, allowing a single cell to represent two [[bit]]s, doubling [[Density (computer storage)|memory density]].<ref name=review>{{cite news |url=https://www.technologyreview.com/2008/02/04/35301/a-memory-breakthrough/ |title=A Memory Breakthrough |first=Kate |last=Greene |newspaper=Technology Review |date=4 February 2008}}</ref>

===Aluminum/antimony===
Phase-change memory devices based on [[germanium]], [[antimony]] and [[tellurium]] present manufacturing challenges, since etching and polishing of the material with [[chalcogen]]s can change the material's composition. Materials based on [[Aluminium|aluminum]] and antimony are more thermally stable than [[GeSbTe]]. [[Aluminium antimonide|Al<sub>50</sub>Sb<sub>50</sub>]] has three distinct resistance levels, offering the potential to store three bits of data in two cells as opposed to two (nine states possible for the pair of cells, using eight of those states yields log<sub>2</sub>&nbsp;8&nbsp;=&nbsp;3&nbsp;bits).<ref>{{cite web|url=http://www.kurzweilai.net/will-phase-change-memory-replace-flash-memory |title=Will phase-change memory replace flash memory? |publisher=KurzweilAI |access-date=2013-09-17}}</ref><ref>{{Cite journal | last1 = Zhou | first1 = X. | last2 = Wu | first2 = L. | last3 = Song | first3 = Z. | last4 = Rao | first4 = F. | last5 = Ren | first5 = K. | last6 = Peng | first6 = C. | last7 = Song | first7 = S. | last8 = Liu | first8 = B. | last9 = Xu | first9 = L. | last10 = Feng | first10 = S. | doi = 10.1063/1.4818662 | title = Phase transition characteristics of Al-Sb phase change materials for phase change memory application | journal = Applied Physics Letters | volume = 103 | issue = 7 | pages = 072114 | year = 2013 | bibcode = 2013ApPhL.103g2114Z }}</ref>

[[File:PRAM cell structure.svg|thumb|300px|right|A cross-section of two PRAM memory cells. One cell is in low resistance crystalline state, the other in high resistance amorphous state.]]