reference will now be made to the exemplary embodiments illustrated in the drawings , and specific language will be used herein to describe the same . it will nevertheless be understood that no limitation of the scope of the invention is thereby intended . alterations and further modifications of the inventive features illustrated herein , and additional applications of the principles of the inventions as illustrated herein , which would occur to one skilled in the relevant art and having possession of this disclosure , are to be considered within the scope of the invention . ultra - high density data storage and retrieval devices . by way of background , a prior art application for utilizing the data storage and detection device of the present invention will be discussed . specifically , fig1 shows a cross - section of an ultra - high density data storage system 100 having electron emitters 102 and 104 , a phase - change data storage medium 106 and a micromover unit 110 . electron emitter 102 and 104 are mounted above the storage medium 106 , that has a number of storage areas , such as 108 , that are impacted by electron beams from the emitters . micromover 110 is connected to the storage medium 106 and moves it relative to the emitters 102 , 104 , so that each emitter can impact a number of different data storage areas . in one specific embodiment , each storage area is used for storing one bit of information . as the micromover 110 scans the medium 106 in the x - y directions to different locations , each emitter is positioned above different storage areas . with the micromover 110 , an array of field emitters can scan over the storage medium . alternatively , a micromover 110 may be connected to the emitters to move the emitters relative to the storage medium , in order to achieve a similar scanning effect . [ 0041 ] fig2 is a top view of the cross section a - a in fig1 showing the micromover 110 fabricated to scan the medium 106 in the x and y directions . the storage medium 106 is supported by two sets of thin - walled micro - fabricated flexible beams or springs 112 and 114 which flex to allow the medium 106 to move in the x direction with respect to a supporting frame 122 . a second set of springs 116 and 118 are connected between the supporting frame 122 and the outer casing 120 and flex to allow the medium 106 and frame 122 to move in the y direction with respect to the casing 120 . the field emitters scan over the medium , or the medium scans over the field emitters in the x - y directions by electrostatic , electromagnetic or piezoelectric means known in the art . writing on the data storage medium 106 is accomplished by temporarily increasing the power density of the electron beam current to modify the surface state of the storage area . reading is carried out by providing electron beams with a decreased power density and observing the effect of the storage area on the electron beams , or the effect of the electron beams on the storage area . a storage area that has been modified can represent a bit 1 , and a storage area that has not been modified can represent a bit 0 , or the storage area may be modified to different degrees to represent more than two bits . reference is made to the gibson 596 patent for more detail regarding the ultra - high data storage device shown in fig1 and 2 , including the action of the emitters , micromovers and storage medium and the functions of writing and reading data . the present invention is concerned with diodes and diode materials used in conjunction with a phase - change layer to detect the state of the data storage cells in the phase - change layer . fig3 shows a simplified diode structure 140 comprising the phase - change layer 106 described above and a second diode layer 142 forming a diode junction 144 at the interface of the two layers . typically , phase - change layer 106 and second diode layer 142 have different electrical characteristics , to encourage the movement of carriers across the junction . for example , phase change layer 106 may be n - type and second diode layer 142 may be p - type . doping may be used to change or enhance the electrical characteristics of each layer . an external circuit 146 is connected to the diode to impress a voltage across the junction so as to create a bias , either in a reverse direction or in a forward direction , depending on the needs of the circuit . an output 148 is generated by circuit 146 and is representative of the magnitude of carrier flow across junction 144 . the term “ pn characteristics ” as used herein refers to the p - type or n - type state of a material , whether formed naturally or by doping . the presence of a built - in field is a necessary but not sufficient condition for favorable electrical properties in a diode . a built - in field occurs when two materials in contact have different work functions . the work function of a material is the energy difference between a material &# 39 ; s fermi energy and the vacuum . p - and n - doped versions of a given semiconductor necessarily have different work functions since the fermi level will be close to the valence band for a p - type semiconductor and close to the conduction band for an n - type semiconductor . alternately , the carrier type of both layers of a diode junction may be the same , but the layers differ from each other in work function to the extent that the junction between the layers has a built - in field . when two materials that are both p - type ( or both n - type ) have sufficiently different work functions and these materials are placed in contact , their vacuum levels align and charge carriers diffuse from one material to the other . this motion of charge carriers leads to a built - in electrical field that causes the materials &# 39 ; energy bands to bend . as explained above , the altered state of each storage area 108 may be detected in various ways . one approach is to store bits at data storage areas in the phase - change material by locally altering the surface of the storage medium 106 in such a way that the collection efficiency for minority carriers generated near the altered data storage area 108 is different from that of an unaltered data storage area 109 . the term “ collection efficiency for minority carriers ” is defined as the fraction of minority carriers generated by incident electrons that cross the diode junction 144 when it is reverse biased by external circuit 146 . external circuit 146 is only exemplary of the concept explained above . the actual external circuit used may be different and more complex , but still provides a bias across the diode junction and measures the current across the junction . the reading or detection operation is done by the field emitter 104 directing a narrow beam of electrons 105 or electromagnetic radiation , such as light , onto the surface of phase change layer 106 at data storage area 108 . the incident beams excite electron - hole pairs 107 near the surface of the storage area 108 . the diode 140 is reversed - biased by external circuit 146 so that the minority carriers that are generated by the incident electrons are swept toward the diode junction 144 . electrons that reach the junction 144 will be swept across the junction . in other words , minority carriers that do not recombine with majority carriers before reaching the junction are swept across the junction , causing a current to flow in the external biasing circuit 146 . writing onto diode 140 is accomplished by increasing the power density of the electron or optical beam 105 enough to locally alter some property of the diode at storage area 108 , such as collection efficiency of minority carriers . for example , the recombination rate in a written area 108 could be increased relative to an unwritten area 109 so that the minority carriers 107 generated in the written area 108 have an increased probability of recombining with majority carriers before they have a chance to reach and cross the junction 144 . hence , a smaller current flows in the external circuit 146 when the read beam 105 is incident upon a written area 108 than when it is incident upon the unwritten area 109 . conversely , one may also start with a diode structure having a high recombination rate , and then write bits by locally reducing the recombination rate . the magnitude of the current resulting from the minority carriers depends on the state of the storage area , and the current magnitude is represented by the output signal 148 to indicate the nature of the sensed data bit . ultra - high data storage and retrieval media . with the foregoing background , the present invention is concerned with suitable media or materials for fabricating diode memory cells having phase - change media diode layer for storing and detecting data . the media may include many different types of compounds and materials that have suitable phase - change characteristics for the data layer and that have suitable structural and electrical characteristics to form a suitable diode for detecting the data states of the memory cells . a class of materials called cis materials has been determined to have useful characteristics for one or more layers in ultra - high data storage memory cell diodes . as discussed above , this class of materials is based on compounds of various types of copper indium selenium ( cu — in — se ). the compounds are referred to as “ cis ,” “ cis compound ” or “ cis material ” or “ cigs ” if doped with gallium . cis compounds have a number of advantages . they may be produced as polycrystalline films , which are less expensive and faster to fabricate than slow and expensive epitaxy methods to form single crystalline layers . polycrystalline films may be produced utilizing sputtering , evaporation or electrodeposition during manufacture . further , at least some cis films have electrical properties that are relatively insensitive to defects and may be produced with higher yield over that of some epitaxial films . cis materials have gained widespread use as absorber layers in high - efficiency photovoltaic power generation devices . these solar cells are made by combining cis materials with cdte or cds counterelectrodes . the excellent electrical properties of polycrystalline cis materials make them promising candidates for use in ultra - high density memory cells for several reasons . the growth of cis materials is well studied , and their electrical properties are suitable and well - known . cis materials may be grown as n - or p - type and may be doped to enhance their electrical conductivity . they also have forgiving defects in polycrystalline form . further , the low - symmetry , defected crystal structures of the cu 2 se — in 2 se 3 pseudobinaries suggest that some of the cis materials may have a high enough glass transition temperature to quench into the amorphous state and serve as good phase - change media . most of the phases in the cis system have tetragonal crystal structures , and several have large unit cells . the cuinse 2 compound has excellent electrical properties , but also has the more symmetric chalcopyrite crystal structure and a high melting temperature . see j . folmer et al ., j . electrochem . soc . 132 , 1319 ( 1985 ). in ultra - high density data storage devices , cis compounds can be useful for forming both the top layer and the bottom layer of a memory cell diode . certain chalcogenide compounds form high collection - efficiency diodes with cis materials , such as inse , in 2 se 3 and gase , in some cases as a phase - change layer and in other cases as a bottom diode layer . u . s . pat . no . 5 , 385 , 806 ( ohno et al .) considered several ternary compounds for the optical recording of data , including the chalocopyrite cuinse 2 . however the recording method envisioned by ohno was all - optical , and did not include heterojunction detection of carriers , nor were cu 2 se — in 2 se 3 films explicitly considered . as a bottom layer , cis compounds having the chalocopyrite structure , such as cuinse2 or cuingase2 , may be used . such structures are relatively stable and are not affected by typical melting temperatures needed to change the state or phase of the top layer . see a copending application , [ hp 10020 - 1669 ] entitled “ ultra - high density data storage device using phase change diode memory cells and methods of fabrication thereof ” which describes in greater detail the use of cis compounds as the bottom layer in a memory cell diode of the type discussed here . alternately , a gase layer may be used , as previously discussed . for both top and bottom layers , more exotic or complex cis structures are also desirable for forming one or both layers in memory cell diodes . as mentioned above , these complex cis compounds may also be referred to as an “ ordered - defect compound ,” “ ordered - defect material ” or “ odc .” these terms refer to cis materials having defects throughout their structure wherein the defects occur in an ordered fashion and are typically off - stoichiometric , not having a simple “ cis - 112 ” ratio like cuinse 2 or a simple “ cigs - 1112 ” ratio like cu ( inga ) se 2 . referring to fig4 a phase diagram is provided with a line 180 showing regions of stability for the cuin 3 se 5 , cuin 5 se 8 and cu 3 in 7 se 12 pseudobinary compounds . ( the region of stability for cu 3 in 5 se 9 is not shown .) according to zhang et al (“ defect physics of the cuinse 2 chalcopyrite semiconductor ,” phys . rev . vol . 57 , 9642 ( 1998 )), these compounds are stable because they contain ordered arrays of defects consisting of two cu vacancies plus one in antisite defect . these defect triplets are charge - neutral and do not contribute a deep level recombination center . this self - passivation of defects is a partial explanation of why polycrystalline cis materials can be made into relatively efficient photovoltaic devices . [ 0059 ] fig5 is a ternary phase diagram showing that odc compounds tend to be found along a pseudo - binary line 190 . based on their complex crystal structures , these phases are expected to have high enough glass transition temperatures to enable melted regions to be quenched into amorphous bits . based on the analogy with gete — sb 2 te 3 pseudobinaries , the cis materials found on the cu 2 se — in 2 se 3 tie line are expected to have shorter crystallization times than other ternary compounds in the cu — in — se system , like cuin 7 se 12 or cu 4 in 9 se 16 . in pseudo - binary systems there are several phases that are ordered - defect compounds , notably cuin 3 se 5 , cuin 5 se 8 , cu 2 in 4 se 7 and cu 3 in 5 se 9 . as mentioned above , the cis compounds may also include quaternary compounds cu — in — ga — se ( cigs ). as used herein , the term “ selected odc group ” includes the following cis compounds : ( 1 ) naturally n - type — cuin 3 se 5 (“ cis - 135 ”) and cuin 5 se 8 (“ cis - 158 ”), and ( 2 ) p - type — cu 2 in 4 se 7 (“ cis - 247 ”) and cu 3 in 5 se 9 (“ cis - 359 ”). the selected odc group also includes the foregoing compounds with gallium - doping , as well as compounds having the generalized formula cu ( in 1 - x ga x ) se 2 . data storage and retrieval devices having a plurality of memory cells may be are fabricated using heterojunction and homojunction diodes based on the odc media mentioned above . for example odc heterojunction diodes may be formed from two or more of the following materials in various combinations : inse , gase , cuinse 2 , cis - 135 , cis - 158 , cis - 247 , cis - 359 and ga - doped versions of the preceding . the following odc diode combinations have been found to be effective and are referred to herein as the “ selected odc diode group ”: where 135 is cuin 3 se 5 , 158 is cuin 5 se 8 , 247 is cu 2 in 4 se 7 , and 359 is cu 3 in 5 se 9 . other odc compound combinations may also be effective as diodes besides those described in the selected odc group and are meant to be included within the scope of the present invention , including a memory cell diode with a structure selected from a group consisting of the following configurations : wherein the odc film is an ordered - defect material and where the order of listing indicates the deposition order of the two layers . [ 0074 ] fig6 shows a schematic side view of a diode configuration 200 in accordance with one embodiment of the present invention is shown . a phase - change layer 202 is composed of a suitable phase - change media that can be reversibly changed from an insulating ( amorphous ) to a semiconducting ( crystalline ) state by heating and cooling it at suitable rates . as discussed above , useful materials for the phase - change media include inse , gase , in 2 se 3 and odc pseudo - binaries having ratios cis - 135 , cis - 158 , cis - 247 , and cis - 359 . melting temperatures , in degrees , for these compounds were found to be in the vicinity of the following : inse — 660 c , in 2 se 3 — 900 c , gase — 950 c , cuinse 2 — 1002 c , and odc pseudobinaries — about 900 c . when the phase - change layer 202 is changed from an amorphous state to a crystalline state at a storage area 204 , its electrical properties may significantly change . accordingly , the number of carriers swept across the diode junction may be significantly different between crystalline and amorphous states . a second diode layer 206 is disposed below the phase - change layer 202 to form a diode junction 207 . as discussed above , useful materials for the second diode layer 206 include gase , in 2 se 3 , cuinse 2 , cu ( in 1 - x ga x ) se 2 and odcs having ratios cis - 247 and cis - 359 . referring to fig7 an ultra - high density memory storage diode 210 is shown in which a phase - change layer 212 is separate from a diode 214 , composed of diode layers 216 and 218 . a diode junction 217 lies between the diode layers and is not in contact with the phase - change layer 212 . in this embodiment , the selection of the phase - change layer 212 can be focused entirely on media that have excellent phase - change characteristics for local memory cell regions and that have significantly different carrier generation or recombination characteristics in different states or phases . the two diode layers 216 and 218 below the phase - change layer 212 are not limited to phase - change requirements but can be focused on various characteristics that are important for suitable functioning of the diode , such as pn characteristics and doping . looking at fig8 in one embodiment of the present invention , an ultra - high density memory storage diode 220 may be fabricated by depositing one or more layers of odc materials on a substrate where the odc materials serve as the diode materials . for example , a given substrate 222 is selected , typically silicon or silicon dioxide , upon which a buffer layer 224 is fabricated . on top of the buffer layer 224 , is formed a counter electrode semiconductor layer 226 , such as gase or cuinse 2 . next a phase - change data layer 228 , including an odc , is formed on the counter electrode semiconductor layer 226 , on which to form data storage regions 229 . after the data layer 228 is fabricated , a cap layer 230 may , optionally , be formed thereon . within this structure , certain layers may include a dopant introduced into a selected layer . use of cis pseudo - binaries in an ars media heterojunction device provides greater design flexibility in fabrication . for example , an odc / cuinse 2 heterojunction can be produced by sputtering , evaporation or chemical vapor deposition and has useful electrical properties in polycrystalline form . odc compounds may also be used in conjunction with gase or inse in diode - formed heterojunctions . in addition , gase , grown epitaxially on si ( 111 ), may be utilized as a buffer layer with heterojunction diodes . the interfaces between these two chemically related and structurally similar compounds provide good electrical quality . it is to be understood that the above - referenced arrangements are illustrative of the application for the principles of the present invention . it will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth in the claims .