APPLIED PHYSICS LETTERS 88, 253504 共2006兲

Single silicon vacancy-oxygen complex defect and variable retention time
phenomenon in dynamic random access memories
T. Umedaa兲
Graduate School of Library, Information and Media Studies, University of Tsukuba, Tsukuba 305-8550,
Japan

K. Okonogi, K. Ohyu, S. Tsukada, and K. Hamada
Elpida Memory, Inc., Sagamihara 229-1198, Japan

S. Fujieda and Y. Mochizuki
System Devices Research Laboratories, NEC Corporation, Sagamihara 229-1198, Japan

共Received 27 February 2006; accepted 4 May 2006; published online 22 June 2006兲
The variable retention time phenomenon has recently been highlighted as an important issue indynamic random access memory 共DRAM兲 technology. Based on electrically detected magnetic
resonance and simulation studies, we suggest that a single Si vacancy-oxygen complex defect is
responsible for this phenomenon, when the defect is embedded in the near surface drain-gateboundary of a DRAM cell. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2213966兴
In 1992 Restle et al. reported the two-level variable retention time 共VRT兲 phenomenon of dynamic random accessmemory 共DRAM兲, which was observed universally in
64 Kbit– 16 Mbit DRAMs of various manufacturers.1 We
also observed this phenomenon in a 256 Mbit DRAM cell, asshown in Fig. 1共a兲. The two-level VRT is characterized by
two reversible states 共deemed “good” and “bad”兲 that exhibit
two discrete data retention times 共RTs兲. It was recently demonstrated that the junction leakage current 共JLC兲 of a VRT
bit reversibly changes between low and high levels, causing
the VRT phenomenon.2 Similar two-level “telegraph” phenomena were also observed as random telegraph noise in thechannel current of a small Si field effect transistor3 and in theleakage current of an ultrathin SiO2 film.4 This noise originates from the interaction between a single defect and thecarriers.3,4 Likewise, the VRT phenomenon is believed to berelated to a single “bistable” defect 共VRT defect兲;1,2 however, its microscopic entity is still unclear. Since the VRT has
a drastic impact on a DRAM’s performance andreliability,2,5,6 this is a highly important issue.
In this letter, we suggest that Si vacancy-oxygen 共VO兲complexes are the most likely source of the VRT defect. We
detected such complexes in DRAM cells by means of electrically detected magnetic resonance 共EDMR兲. The location,
electronic levels, and bistable nature of those complexes are
discussed in conjunction with the basic features of the VRT.
First, we found that the characteristic thermal activationsof the VRT in old and current DRAMs are similar. Restle etal.1 established that VRT can be described using a bistableenergy diagram with two energy barriers, Eg and Eb 关Fig.
1共b兲兴. The average lifetime for a good 共bad兲 state is in proportion to exp关Eg共b兲 / kT兴, where k is the Boltzmann constantand T is temperature.1,2 For our VRT bits, as shown in theArrhenius plot in Fig. 1共c兲, Eg and Eb were found to beapproximately 1.16 and 1.06 共±0.06兲 eV, respectively, which
are close to Restle’s values 共Eg = 0.90– 1.04 eV, Eb= 0.84– 1.08 eV, and generally Eg ⬎ Eb兲. We also found that
their activation energies were between 0.7 and 1.4 eV; howa兲

Electronic mail: umeda@slis.tsukuba.ac.jpever, most cases exhibited a barrier of approximately 1 eVand Eg ⬎ Eb. Therefore, the nature of the VRT defect is apparently unchanged in current DRAMs. This defect must belocated in the Si depletion layer since it generates the JLC.
To investigate this defect, we performed EDMR measurements on the JLC in 10 000 DRAM cells with 64 or512 Mbit technologies 共gate length= 0.25 and 0.11 ␮m andjunction area⬇ 2500 and 300 ␮m2, respectively兲. TheEDMR setup and measurement conditions used here werebasically the same as those previously reported.7,8 Figure 2
shows typical EDMR spectra for the two samples when thereverse bias 共VR兲 was applied to their pn junctions 共then+-drain to the p-well兲. Two types of defects were observedin the 64 Mbit sample; they were assigned to large VO complexes 共V6 or larger兲 or V2O and V2O2 共V2Ox兲 based on aprevious detailed study.7 The 512 Mbit sample clearly exhibited a large VO signal, but the V2Ox signal was unclear dueto noise. Nevertheless, we deduced that both types of defects
were present in every DRAM, because they were formed byion implantation in the drain 共n+兲 region,7 which is a fundamental process for DRAM technology. The defects possess
two or more Si dangling bonds 共DBs兲, which generate electronic levels Et in the gap; for the V2Ox type 共with two DBs,electron spin S = 1兲, two 共⫹兩0兲 levels lie at EV共valence band top兲 + 0.3– 0.4 eV.9 These levels are similar to
those of the well-known Pb center 共DB at Si– SiO2interface兲.9 We thus speculate that Et exists at similar positions for the large VO defects because they also have DBs.
Their EDMR signal intensities were sensitive to the gate
voltage, indicating that they were located in the near-surface
drain region close to the gate.7 These complexes are thelikely source of the VRT defect.
The JLC due to the defect J can be calculated using thewell-known Shockley-Read-Hall formula,10
J = CnC pvthni/关Cn exp共− E/kT兲 + C p exp共E/kT兲兴,
where Cn共p兲 is the electron 共hole兲 capture cross-section, vth isthe carrier thermal velocity 关=共3kT / m*兲1/2; we used the effective mass 共m* = 0.25m0兲 for Si 共Ref. 11兲兴, ni is the intrinsic
carrier concentration of Si, and E is the difference between

0003-6951/2006/88共25兲/253504/3/$23.00
88, 253504-1© 2006 American Institute of Physics
Downloaded 23 Jun 2006 to 133.51.28.61. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

253504-2

Appl. Phys. Lett. 88, 253504 共2006兲

Umeda et al.

FIG. 1. 共Color online兲 共a兲 Two-level VRT phenomenon observed in 256 Mbit DRAM cell. 共b兲 Bistable energy diagram for VRT 共Ref. 1兲. 共c兲 Thermal
activation of reversible change between good and bad states for 256 Mbit DRAM cell.

the intrinsic Fermi level and the defect level 共=Ei − Et兲. We
simulated J vs Et for a typical defect level,12 as shown in Fig.
3共a兲 共the “normal” curve兲. The good and bad states correspond to energetically lower and higher defect levels, which
are separated by ⌬E共=Eg − Eb ⬇ 0.1 eV兲, as assumed in Fig.
1共b兲. The simulated change in J was 101.3, which is comparable to the observed change in RT 关see Fig. 1共a兲兴.
However, the simulated JLC from the bad state was
1 / 100 or less of the actual current level 共100 fA or more兲.
This inconsistency can be reasonably accounted for by thepresence of a strong electric field 共F兲 in the device. Using astandard numerical simulator, we simulated the electric field
distribution in a 256/ 512 Mbit DRAM cell operating undernormal charge retention. As shown in Fig. 3共b兲, the highest
electric field 共0.5 MV/ cm兲 was observed at the boundary
between the n+ drain and the gate, which coincides with theregion where the VO complexes were embedded. With sucha strong F, the 共⫹兩0兲 level of a DB can activate the PooleFrenkel effect 共PF兲.13 The trap assisted tunnel 共TAT兲 effectcan also work.11 Using previously reported formulas,11,13 wecalculated the enhancements to the electron emission andtrap rates from the 共⫹兩0兲 level,
EPF = 共qF/␲␧兲1/2 ,⌫n = exp共EPF/kT兲 +冕共Egap−Et兲/kT

EPF/kT⫻exp兵z − ␣z3/2关1 − 共EPF/zkT兲5/3兴其dz,⌫p = 1 +冕Et/kT

exp兵z − ␣关共Egap/kT兲3/2 − 共Egap/kT0− z兲3/2兴其dz,

␣ = 8␲关2m*共kT兲3兴1/2/3qhF,
where EPF is the potential-barrier reduction due to the PFeffect, q is the electron charge, F is the applied electric field,
␧ is the dielectric constant of Si, ⌫n expresses the enhancement for electron emission due to PF 共the first term兲 and TAT
共the second term兲, ⌫ p expresses the electron-trap enhancement by TAT, Egap is the band gap of Si, and h is Planck’sconstant. We estimated that the electron emission from thedefect level is enhanced 320 times at 0.5 MV/ cm, as shownin Fig. 3共c兲. Therefore, the previous simulation of J 共thenormal curve兲 should be revised to appear like the “withPF+ TAT” curve in Fig. 3共a兲. As a result, we expect the JLC
to be over 100 fA for the bad state, which is a reasonablelevel to account for the VRT.
The VRT defect should be bistable with activation energies 共Eg and Eb兲 of approximately 1 eV. We propose here amodel of strain-induced bistability for the small VO complexes 共V2Ox兲. They are formed as rare defects because their
EDMR signal intensity was very small. This corresponds toprevious findings that VRT bits appear with a small
probability.1,2,5 For the case of V2Ox under a strong latticestrain, one orientation becomes more stable than the other, as
was demonstrated in electron paramagnetic resonance experiments under artificial strain.14 Using convergent beamelectron diffraction, we observed strain of over 100 MPa in
the relevant devices.8 We deduced that the good and badstates correspond to two orientations, like those shown inFig. 4. The reorientation of V2Ox is similar to that of divacancy 共V2兲, with an estimated activation barrier of⬃1.4 eV.14 This barrier is compatible with that observed for
the VRT.
The above discussion implies that the impact of the V2Ox
complex on the VRT depends strongly on F and strain
共namely, ⌬E兲. This may explain 共1兲 why the VRT exhibits avery wide variation, such that RT is changed by several percent to 1–2 orders of magnitude1,5 共because the JLC varies in
the above range depending on F and ⌬E兲 and 共2兲 why theVRT is affected by the packaging and thermal processes5,6共because they change the distribution of strain in a device兲.

FIG. 2. 共Color online兲 Typical EDMR spectra measured for JLCs of 64 and512 Mbit samples 共80 nA at VR = 9.6 V and 55 nA at VR = 5.9 V, respectively兲 at room temperature with the magnetic field parallel to 关011兴 axis.
Downloaded 23 Jun 2006 to 133.51.28.61. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

253504-3

Appl. Phys. Lett. 88, 253504 共2006兲

Umeda et al.

FIG. 3. 共Color online兲 共a兲 Simulated junction leakage current J from defect level Et, assuming that electron and hole capture cross-sections 共Cn and C p,
respectively, in cm2兲 are Cn = 1 ⫻ 10−14, C p = 1 ⫻ 10−15 for “normal” curve, Cn = 320⫻ 10−14, and C p = 1 ⫻ 10−15 for “with PF+ TAT” curve. Good 共bad兲 state is
set to EV + 0.3 共0.4兲 eV. 共b兲 Simulated two-dimensional electric field distribution in 256/ 512 Mbit DRAM cell under normal charge retention operation. 共c兲Enhancement factors for electron emission and trap rates due to Poole-Frenkel effect 共PF兲 and trap assisted tunneling effect 共TAT兲.

The role of the large VO complexes on the VRT phenomenon is not clear at present. However, their DBs caninteract with hydrogen atoms 共H兲 and potentially cause
H-related phenomena. Recent experiments suggested a correlation between H and the RT degradation observed at arelatively low T共艋250 ° C兲.6 They also suggested that VRT
is correlated with H.5 The VO complexes may be related tothese H-related phenomena.
In summary, using EDMR, we observed that the VO
complexes were present in all the DRAM cells we investigated. Numerical simulation showed that a single bistable
VO defect located in the near surface gate-drain boundary
can behave as a VRT defect.

FIG. 4. 共Color online兲 Model for bistable V2Ox defect under strain: 共a兲
Atomic view and 共b兲 energy-level view.

One of the authors 共T.U.兲 thanks Professor Junichi Isoya
共University of Tsukuba兲 for his valuable help in conducting
the experiments.
1

P. J. Restle, J. W. Park, and B. F. Lloyd, Tech. Dig. - Int. Electron Devices
Meet. 1992, 807.
2
Y. Mori, K. Ohyu, K. Okonogi, and E. Yamada, Tech. Dig. - Int. ElectronDevices Meet. 2005, 41.
3
M. J. Kirton and M. J. Uren, Appl. Phys. Lett. 48, 1270 共1986兲; P. Restle
and A. Gnudi, IBM J. Res. Dev. 34, 227 共1990兲.
4
M.-J. Chen and M.-P. Lu, Appl. Phys. Lett. 81, 3488 共2002兲.
5Y. I. Kim, K. H. Yang, and W. S. Lee, Reliability Physics Symposium
Proceedings, 2004, 42nd Annual 共IEEE International, 2004兲, pp. 667–668.
6
M. Chang, J. Lin, C.-S. Lai, R.-D. Chang, S. N. Shih, M.-Y. Wang, andP.-I. Lee, IEEE Trans. Electron Devices 52, 484 共2005兲.
7T. Umeda, Y. Mochizuki, K. Okonogi, and K. Hamada, J. Appl. Phys. 94,7105 共2003兲.
8T. Umeda, A. Toda, and Y. Mochizuki, Eur. Phys. J.: Appl. Phys. 27, 13共2004兲.
9S. Coffa and F. Priolo, Properties of Crystalline Silicon 共INSPEC, London,
1999兲, edited by R. Hull, p. 748.
10S. M. Sze, Physics of Semiconductor Devices 共Wiley, New York, 1981兲,Chap. 2.
11G. A. M. Hurkx, D. B. M. Klaassen, and M. P. G. Knuvers, IEEE Trans.
Electron Devices 39, 331 共1992兲.
12B. G. Svensson, Properties of Crystalline Silicon 共INSPEC, London,1999兲, edited by R. Hull, p. 764.
13G. Vincent, A. Chantre, and D. Bois, J. Appl. Phys. 50, 5484 共1979兲.
14G. D. Watkins and J. W. Corbett, Phys. Rev. 138, A543 共1965兲.

Downloaded 23 Jun 2006 to 133.51.28.61. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp