Reversible optical storage on a low-doped Te-basedchalcogenide film with a capping layer
P. C. Clemens
A reversible optical storage medium with more than 4 X 104 write/erase cycles has been realized using a low-doped Te film of composition Te96.8As3.oGeO.2 encapsulated by a thick capping layer. A number of proper-ties relevant to its storage application were also determined experimentally: the sensitivity range for writ-ing (0.3-4 nJ) and erasing (34-40 nJ), the contrast of the readout signal (2:1), the limit for the erasing time(-10 sec), the bit permanence (3 weeks), and the readout signal behavior during writing and erasing.
1. IntroductionTellurium films doped with As and Ge can be
switched reversibly by means of laser light between theiramorphous and crystalline phases. Since these phasesexhibit different transparencies, such films were, aboutten years ago, subject to intensive investigations witha view to determining their usability as optical storagemedia.lA However, these investigations showed themto have a poor reversibility performance; only 10-20write/erase cycles were obtained. Apart from this, thelight energy required to attain a definite degree ofcrystallization changed as the number of cycles in-creased.4 5 This poor reversibility was due to the irre-versible formation of a phase mixture comprising tel-luric microcrystals and amorphous chalcogenide glass6
as well as to an irreversible destruction of the film byhole formation. To avoid formation of this phasemixture, Bell6 proposed that the Ge and As dopinglevels should be reduced. (Doping cannot be com-pletely obviated since it guarantees the stability of theamorphous phase at room temperature.) Hole forma-tion, on the other hand, can be restrained by encapsu-lation with a capping layer.7 8 Pure tellurium films havebeen switched repeatably between the amorphous andcrystalline phases without any hole formation or im-pairment of the optical signal after 50 cycles.8
The purpose of this paper is to show the capability ofTe films with low doping levels and a capping layer tocope with high cycle numbers. The following data,
The author is with Siemens AG Research Laboratories, D-8000Munich 83, Federal Republic of Germany.
which are important for a reversible optical memory, arealso investigated: the sensitivity range for the write anderase energies, the signal contrast, the attainable writingand erasing rates as well as the readout behavior mea-sured directly during writing and erasing.
II. Film Structure and Sample PreparationThe film structure is shown in Fig. 1. It consisted of
a 48-nm thick chalcogenide film and a 650-nm thickcapping layer of silicon oxides on a quartz substrate.
Both the chalcogenide film and the capping layerwere produced by means of thermal evaporation. Thestarting material for the chalcogenide had the compo-sition TesoAs5.5Ge14.5, that for the capping layer SiO.An x-ray analysis showed the composition of the evap-orated memory layer to be Te96.8As3.0Ge0.2; the com-position of the capping layer can be assumed to be SiO1.5on empirical grounds.9 The layer thicknesses weremonitored indirectly during the evaporation process bytaking reflection and transmission measurements witha He-Ne laser beam.
After evaporation, the deposited layer was in thecrystalline phase. It was thus unnecessary to temperthe specimen in preparation for writing in reversemode3 4 (i.e., writing by transforming the crystalline intothe amorphous phase).
Ill. Experimental SetupThe experimental setup is shown in Fig. 2. An
argon-ion laser equipped with a cavity dumper (X = 514nm) supplied the write and erase pulses. Their dura-tion and power were measured by the pulse detector.
The exposure beam was incident on the memorymedium with a focal diameter of 1 m. The transmis-sion of the memory location was measured by the signaldetector. For this purpose, the leakage beam con-stantly emitted from the Ar+ laser was used. This beamwas focused onto the memory location with such a low
Fig. 2. Setup for the write, read, and erase experiments.
power density (0.1 ,uW//um2) that it could not cause anyphase transformation in the chalcogenide.
The transmission behavior during the write and eraseprocesses was traced by a photomultiplier (Sec. IV.C).A He-Ne laser was inserted into the beam path of theAr+ laser for this purpose. Separation of the two lightwavelengths then took place behind the memory me-dium by means of a dichroic mirror. An additional redfilter ensured that the photomultiplier could detect onlythe light of the He-Ne laser. The transmission of thememory location could thus be measured without anydisturbance from the write or erase pulses emitted bythe Ar+ laser.
0
1 2 3 4 nJ
writing energy
Fig. 3. Detector signal (transmission of the memory location) vswriting energy for an exposure time of 20 nsec.
The red read light was incident on the memory me-dium with an intensity of 20 uW/pLm 2. Although theamorphous phase is converted to the crystalline phasein -5 sec at this intensity, it is still low enough to allowinvestigation of the interesting brief-time effects.
IV. Experimental Results
A. Write Energy and Contrast of the Readout SignalDuring the writing process, the crystalline starting
material is heated to above its melting point and thusbecomes amorphous. Since heat is conducted into thematerial, the required write energy depends on the ex-posure time.
An exposure time of 20 nsec was chosen, which cor-responds to the exposure time which would be requiredto attain a data rate of 10 Mbit/sec (compare Ref. 10),and the transmission of the film was measured as afunction of the write energy. The result is shown in Fig.3: above a threshold energy of 0.3 nJ, the detectorsignal increases with increasing write energy. Above-4 nJ, the recording becomes irreversible.
The attainable signal contrast of 2:1 is worthy of note;it leads us to expect a high signal-to-noise ratio. Thesensitivity of -1 nJ is comparable with that for irre-versible hole formation in Te films.1 The large expo-sure range between 0.3 and 4 nJ ensures independencefrom fluctuations in write energy.
B. Erase Time and Erase EnergyErasing heats the amorphous material to between its
glass-transition temperature and its melting point sothat it recrystallizes. The higher the temperature, themore rapidlly does the material crystallize. Since themelting point must not be exceeded, however, thecrystallization rate has an upper limit, i.e., the eraseprocess demands a minimum exposure time of a fewmicroseconds.12
In determining the required erase energy, amorphousmaterial was exposed for 10 ,usec and its transmissionwas measured as a function of the exposure energy.The result is shown in Fig. 4: above a threshold energyof 26 nJ, the detector signal decreases with increasingexposure energy and reaches, between 34 and 39 nJ, thevalue of the crystalline starting material. When theexposure energy is increased still further, the detectorsignal is reversed, probably due to the melting point
Fig. 4. Detector signal (transmission of the memory location) vserasing energy for an exposure time of 10,4sec. At minimum signal
level (reversal point at 39 nJ) complete crystallization is reached.
-
c 0_ E
. C
amorphous 2crystalline 1
0
Fig. 6. Direct readout during erasing. Erasing energy 39 nJ.
i 20nsJ lwrite pulse
f amorphous 2 -
i crystalline 1
U 0.5 1 5 1 0 [is
erasing time
Fig. 5. Reversal points of the detector signal, i.e., maximum at-tainable degree of crystallization (see Fig. 4), as a function of erasing
time.
being exceeded. Accordingly, the erase energy must bemaintained around 10% of the 37-nJ mark.
Erase attempts at exposure times below 10 /usecshowed a reversal of the detector signal even beforecomplete crystallization had been achieved. The re-versal points for a number of exposure times are plottedin Fig. 5. Their position is a measure of the maximumdegree of crystallization attainable in these cases. Fromthis we conclude that erase times significantly below 10psec cannot be realized.
C. Direct Readout During Writing and ErasingIt is important for memory applications that the
signal can be read immediately after the writing orerasing process, so that any errors which may have oc-curred can be corrected. To clarify this point, thetransmission was measured during the actual expo-sure.
The result for the erasing process is shown in Fig. 6.During the period of the erase pulse (10 sec) thetransmission drops to the value of the crystallinestarting material.
In contrast, during writing the transmission starts toincrease after an initial delay (Fig. 7). The process isnot completed until the elapse of 80-160 nsec (de-pending on the write energy) after the write pulse. Aconceivable explanation for this is that the moltenmaterial is as opaque as the crystalline material and-100 nsec is required for it to cool down to the trans-parent solid-state amorphous phase. A corre-
°E 75 4'V 0
amorphous 2-
crystalline 1-
O-
Fig. 7. Direct readout during writing. Writing energy 2.0 nJ.
spondingly strong temperature dependence of theohmic resistance of amorphous TesoAs5Gel5 was mea-sured by Haberland.13
D. Cycle NumberBy cycle number we mean the number of write/erase
cycles possible for a given memory location. Our in-vestigtions were carried out, in line with the results re-ported in Secs. IV.A and IV.B, using a write energy of1.2 nJ, a write time of 20 nsec, an erase energy of 39 nJ,and an erase time of 10 /isec. Every new writing orerasing process had to be preceded by the cooling of thememory location to its starting temperature, which re-quired a total of -2 sec/cycle. About 5 X 103 cycleswere attained.
Both the write and the erase energy could be keptconstant for all the cycles. This is not possible in thecase of the familiar highly doped chalcogenide films. 4
Figure 8 shows the detector signal for the first 50 cy-cles [Fig. 8(a)] and the unreduced signal contrast after5300 cycles [Fig. 8(b)]. We assume that the irregular-ities in the signal are due to mechanical oscillations ofour experimental setup.
In a verification experiment using a test specimen ofthe same material but without a capping layer, only10-20 cycles were attained. This confirms the decisiveeffect of the capping layer on reversibility. Anotherexperiment was therefore carried out in which thecapping layer was made even thicker, 1100 nm insteadof 650 nm. This allowed over 4 X 104 cycles to be real-ized. Figure 9 shows the detector signal for cycles from40300 to 40400.
It would, however, be too early yet to exclude thepossibility that a Te alloy may be found with a dopinglevel low enough to prevent decomposition but highenough to ensure the desired long-term stability of theamorphous phase.
The relatively long erasing time is clearly not affectedsignificantly by doping. It always lies within the 1-10-,usec region, for materials as diverse as Te81-Ge15Sb2 S2,2 Te93Ge2As5," and TeN6 .8Ge0 2As3.. Thisis an important disadvantage of chalcogenides com-pared with magnetooptical materials whose write anderase times are <50 nsec.14
The 1-nJ/bit sensitivity of the memory materialpresented here is comparable with that toward irre-versible hole formation in Te films. The use of asemiconductor laser will, therefore, lead to data ratesof 10 Mbit/sec as have already been attained in the caseof Te films. 1 In addition, high storage densities anda high signal-to-noise ratio may be expected formemories made from this material, but negative factorsare low erase rates (0.1 Mbit/sec) and a stored-infor-mation stability limited to a few weeks.
t
20s
Fig. 9. Cycles 40300-40400 for a material with a particularly thick(1100-nm) capping layer.
E. Stability of the Amorphous PhaseThe lifetime of the amorphous phase was determined
by daily observation of the recorded amorphous bitsunder the microscope: after -3 weeks, the materialreturned to the crystalline phase.
In an experiment with material of lower doping level,Te98Asj.Ge0o. instead of TeN6.8As3Geo.2, the lifetime ofthe amorphous phase proved to be only 3 h; with anundoped Te film it was as low as 7 sec. These resultsshow that the lifetime of the amorphous phase dependsvery sensitively on the degree of doping (see Table I)and that it increases rapidly with increasing dopinglevel. Long-term stability is obtained with the morestrongly doped Te88As 5Ge7 films known from the lit-erature,3 but whose reversibility is limited to -20 cyclesas a result of decomposition.
V. DiscussionBy using low-doped Te films (Te96.sAs3.OGeo. 2) with
thick capping layers, a reversible optical storage mate-rial with over 4 X 104 possible write/erase cycles wasrealized. The low doping levels meant, however, thatthe long-term stability of the amorphous phase exhib-ited by Te films with higher Ge-As doping levels3 waslost.
The author would like to thank E. Storck and U.Wolff for their helpful discussions and critical readingof the manuscript and 0. Eberspdcher for his x-rayanalysis of the samples.
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