Hogarth, C A

Islam, M H

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NOTEIndian J. Phys.  69A (2), 273 -  211  (1995)I J P  A— an international journalElectron emission and memory effects in M -Si0/Sn02-M  devicesM H Islam and C A Hogarth*Depuimenl of Applied Physics & F.lecnnines, Rajshahi Univeisity. Bangladesh * Department of Physics, Diunel Ihnveisily, Oxbridge. Middlesex t'DK iPII OK.Received 2 1 July 1994, <k  < epted !4yDei ember 1994Abstract : Thin film sandwich stiucluies of M-SiO/SnOn-M weic studied with Cu anil Al as metal electiodes The complex dielectric materials SiO/SnO-, were prepared by co-evapoiulion technique The sandwich weic clccliotoimed al between 1 S and S V niespei. live ol the composition of S1O/S11O3 films hlectiofoi med M Si()/SnO_> M devices show a dilleiential negative resistance, election emission in a vacuum, voltage memory and piessurc memory effects The results are interpreted using a filamcntaiy model The effect of composition and thickness ol Si()/Snt): films on the maximum cunent ( /nu,i and maximum powei dissipation t P nulA) weic also investigatedKeywords : Voltage memoiy, pressuic voltage memory, elec lion emission PACS Nos. : 73.61 At. 85 9 0 +hR ecently, considerable atlenlion has been drawn to M ctal-lnsulator-M cial ( MIM)  devices  which show  a number o f  interesting properties such as differential negative resistance, electron em ission in a vacuum, memory and sw itching effects. Much early work was carried out using A U O 3 [ l |  and StO [21, where electrdform ing and V oltage Controlled Negative R esistance ( VCNR)  were observed. Voltage-m em ory effect on a formed AI-SiO-AI devices has been observed by Sim m ons and Vcrderber |2] Such effect has also been observed by Hogarth and Ilyas [3] in Cu-SiO /TiO -C u devices. The pressure-voltage m em ory effect has been observed by Em m er [4] on AliO^.  Abidi and Hogarth [51 and Hogarth and Ilyas [31 have observed the sw itching from a low resistance state to a high resistance state when the pressure was increased to '/60 torr from 10 s torr in BaO -SiO  and SiO -TiO  system s. The M - SiO /SnO i-M  devices had not been previously studied alter electroform ing the device. In this paper, w c report the characteristics o f M-SiO/SnCE-M devices after eleclrotorming.69A(2)-11& 1995 1ACS274 M H Islam and C A HogarthT he com plex dielectric oxide film s S iO /Sn ( ) 2 w ere prepared on C om ing g lass by using  co-evap oration  technique d eve lop ed  by Hogarth and W right [6 ]. T he a c tiv e  area o f  each  dev ice  wits 0 .0 8  cm 2. Tantalum  and Tungsten boats were used for the evaporation o f  S iO  and Sn ( ) 2 respectively . The evaporation rales w ere m onitored using quartz crystal m onitor. Cu mid A1 electrodes were evaporated from m olybdenum  boat and tungsten spiral respectively. The form ing experim ents were perform ed in a vacuum  o f  about 1 0 5 lorr. A  copper plate  placed  vertically  at 1.5 cm  above  the substrate and b iased  at + 1 0 0  V w ith  resp ect to base  electrode w as used to m easure the em issio n  current using a K eith ley  e lectrom eter  (m odelbigurc 1. ( urrent-voltage characteristics of a Figure 2. Current-voltage characteristics of an(VQ° mul%SiO / IOmol%Sn() r Cu device (insulator eJeclroformed Cu-95moI%SiO /  5mol%Sn02-Cuthickness 250 inn), before and alter eleclrofurming. (insulator thickness 250 nm) sample showing voltage -memory effect, (a) initial formed characteristics, (b) memory stale induced at 7V.A typical current-voltage ( 1 - V h) ch aracter istic s  o f  a  C u -9 0 m o l%  S iO /lQ m o l%  SnC V C u (insulator thickness 2 5 0  nm) d ev ice  before and after form in g  is sh o w n  in F igure 1. T he ( l c- Vh) characteristics in itia lly  sh o w  o h m ic  character b e lo w  0 .6  V  and n o n -o h m ic  character ab ove  0 .6 V  but at an app lied  vo ltage  o f  1.5 V  the current starts to fluctuate  and in crea ses  rap id ly  w h ich  in d ica tes d ia l e lec tro fo rm in g  has started . A  very sta b le  and reproducible (/, ™ V h) characteristics with differential negative resistance in the ( / C- V fc) curve is obtained  as sh ow n in 'formed' part o f  Figure 1 . It is  ev id en t from  this figure that the currentElectron emission and memory effects in M-SiO/SnO r M devices 275increases by m ore than tw o orders o i m agnitude alter eleclroform ing o l the dev ice . The e llc c i  o f  com position  and thickness o f  S iO /Sn ( ) 2 film  on the m axim um  current ( / nilx) and m axim um  pow er dissipation (/>rTlux)are  given in Table 1. N o  system atic change ol / ITiax with com position  w as ioun d . but it m ay be observed  that, in general, thicker lilm s  d isp la y ed  low  pow er  dissipation at 7niax and the results were con sisten t w idi the result reported earlier by Hogarth  and Taheri [7],Tuble 1 . Some parameters ol SiO/SnOi ihin films after eleclroforming of the device with Cu electrodeComposition Film l;or ruing(inol%) thickness voltageSiO Sn02nm V95 ,S 480 395 5 380 >95 5 250 > 290 10 250 1 585 15 250 1 570 30 500 >Voltage ior maximum aim: illMaximumcurrentAMaximum power rliss mW.5 1 1 x 10  1 s 51 4 0 x 10 : 1843 X.U x 10 2409 0 x 10 “ 2703 (» 8 x 10 J 2040 3 8 x 10 - 228F igu re 2 sh o w s a vo ltage  m em ory e ffec t o f  a C u -95m ol%  S i O / 5 m ol%  S n O r Cu d ev ice . C urve b in Figure 2 sh ow s the high resistance state ('OFF' slate) T his state is stable  and reproducible provided that the threshold voltage ( Vth ~ 1 5 V) is not exceeded .H ie system  m ay be transform ed fiom  the high resistance 'o ff  state into the original lo w  resistance 'on' state using a voltage supply Vh > Vlh. T he ratio o f  resistance o l these  suites is estim ated as 1.5 x  1 0 2 (from  Figure 2).F igure 3 sh o w s a Ir~Vb  characteristic alter form ing o f  an A I-90 m ol%  S i O/ l O  mol'/r Sn ()2-A l (insulator th ickness -  2 5 0  nm ) d ev ice  at different pressures (from  10 s torr to 7 0 0  to n ) . It m ay be seen  lliat at a tm ospheric  pressure die current fo l lo w s  the sam e padi as recorded at 10 2 to n  upto a certain critical voltage (V 7 ). Increasing die volUige above V r  the current decreases and the develop m en t o f  a h igh  resistance siate-occurs. D uring die reverse  n m  (reducing voltage form  8  V to 0  V ) the system  conductiv ity  is sign ifican tly  reduced and the Ic- V b  characteristic becom e m onotonic (curve C  in Figure 3). T he high resistance m em ory  state rem ains unchanged during further voltage cy c les , as long as die sam ple is kept at h igh  pressu re  (7 6 0  torr). W h en  d ie cham ber is  ev acu ated  again  to 10 s torr and 1 , - V h characteristics recorded from 0  V , the cureeni sh ow s the sam e value as recorded at 7 6 0  toir but on ly  upto the threshold vo ltage  ( V th ~4  V ). B eyond  V !h ( - 4  V) the /^ ^ c h a r a c te r is t ic s  again  sh arp ly  returns to  'form' v a lu es o l the cu rves* lor  the vacuum  reg im e ol1 0 _s ton .T he variation o f  em issio n  current I e and circulating current I( w ith applied  vo ltage is sh ow n  in F igure 4  for C u -90  m ol%  S iO /lO  m ol%  S n t ) 2 -C u d ev ice . The em issio n  current is increased 3 orders o f  m agnitude (from  10~ 9  to 10 * A ) w hen die app lied  vo ltage is increased276 M H Islam and C A Hogarthfrom 7 V lo 13 V (Figure 4). The em ission current density at room temperature is found to hr approximately 50 A cm 2 for a Cu-95 mol% SiO /5 inol% SnCVCu device (insulator thickness 380 nm) with a maximum bias voltage o f 13 V.Figure 3. Effect* of varying ambient air pressure on tiie l c Vh ciiaraclcnstics of a formed AI-90 m o l%- Si () / 10 mol^SoO t-AI (insulator tluckness 250 nm)Figure 4. Variation of circulating current ( /() and emission current ( le) as a function of applied bias lor a C u -95 m ol%SiO /5m ol%Sn()2~Cu (insulaior thickness -  380 nm) device.fh e  observation o f eiecirofonning. voltage-controlled negative resistance and electron em ission can all be interpreted on the basis o f  filamentary conduction m odel as originally suggested by D eam aley et a l  |8). During the forming process, a large number o f  m etallic lilaincnts develop  from the anode through the insulator to the cathode and bridge the intcrelectrode space. The filamcms crealed are not perfectly uniform but contain weak points and they becom e heated due to the How o f electrons. At higher voltages significant power will be dissipated resulting in an easy  rupture o f filam ents at their weak points due to Joule heating. A s a result die conductivity is reduced and a differential negative registancc region developed in the 1 - V h curve. During reverse voltage cycle  (m axim um  to m inim um ) die cunent incicascs again due to re-growth of filam ents by som e kind o f  ion ic  migration.I herefore reproducible l c- V h characteristics were obtained with increasing and decreasing voltages.Deam aley et a l  [8J suggested that election  em ission  m ay be due to the generation ol hot electrons at the weak points o f  the filam ents near the anode by die application o f high electric field. I hese hot electrons then emerge from the top electrode (anode) into a vacuum.E lectron emission an d  memory effects in M-SiO/SnOr A / devices 277References[1] T W Hickrnotl J. Appl. Phys 33 2669 (1962)[2] J G Simmons and R R Verderbcr Proc Roy. Soc A301 77 (1967)[3] C A Hogarth and M Eyas Thin Solid Films 103 267 (1983)[4] 1 Emmer Phys. Status Sol. 3 K. 191 (1970)[5] A A Abjdi and C A Hogarth Thin Solid Films 2 203 (1974)[6] C A Hogarth and L A Wnghl Proi lnt. Con] on the Phxsii'f oj Semiconductors, Mostmv, U ningiadp 1274 (1968)[7] C A Hogarth and E H Z Tahen lnt. J Electron 37 143 (1974)181 G Dearnalcy, A M Stoneham ami 0 V Morgan Rep Prog Phys. 33 1129 (1970)

Electron Emission and Memory Effects in M-SiO/SnO2-M Devices

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Electron Emission and Memory Effects in M-SiO/SnO2-M Devices

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