Microelectronic Engineering 85 (2008) 2385–2387

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Microelectronic Engineering

journal homepage: www.elsevier .com/locate /mee

Growth ambient on memory characteristics in Au nanoclustersembedded in high-k dielectric as novel non-volatile memory

K.C. Chan, P.F. Lee, J.Y. Dai *

Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 May 2008Received in revised form 5 September 2008Accepted 12 September 2008Available online 30 September 2008

Keywords:Metal nanoclustersAuFloating gate memoryFlash memory

0167-9317/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.mee.2008.09.031

* Corresponding author.E-mail address: apdaijy@inet.polyu.edu.hk (J.Y. Da

In this work, we report on the findings of the effects of different ambient on memory characteristics of afloating gate memory structure containing HfAlO control gate, self-organized Au nanoclusters (NCs), anda HfAlO tunnel layer deposited by the pulsed-laser deposition. The optimized fabrication environmenthas been found and stored charge density up to 1013 cm�2 has been achieved. As the sizes of the AuNCs are smaller than 4 nm, they may be potentially used in multilayer-structured multi-bit memory cell.

� 2008 Elsevier B.V. All rights reserved.

As the technology is advancing and the size of chips is furtherscaling down, new types of memories are emerging. Nanocrystalfloating gate memory has drawn much attention recently as it ismore scalable and can be operated under relatively lower voltagethan conventional floating gate memory [1,2]. Many noble metals,such as Au, Ru and Pd are currently under investigation as the stor-age node material for NVM [3–5].

Au is selected for several reasons, including its relatively largework function and small energy perturbation [6–9]. More impor-tantly, its chemical stability favors the formation of NCs bypulsed-laser deposition during which oxygen ambient may beused. High-k dielectrics generally allow a relatively thick film witha thinner equivalent oxide thickness (EOT) in floating gate memorystructure without sacrificing non-volatility, but with an improvedretention property. Among the reported high-k dielectric materials,such as LaAlO3, HfO2, and Al2O3 [3,5,10–12]. HfAlO is chosen as thetunnel and control layer, owing to its good thermal stability andunique band asymmetry in the programming and retention modes[12,13].

From our previous studies, the combination of HfAlO/Aunanoclusters (NCs)/HfAlO trilayer floating gate memory structureoperated by Fowler–Nordheim (FN) tunneling provides promisingmemory characteristics [14]. In addition, this structure potentiallycan be applied in multilevel charge storage devices [15]. This paperreports process of trilayer floating gate structures with Au NCs fab-ricated at different gaseous ambient and compares their memory

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i).

characteristics, in order to optimize the growth condition for futureflash memory use.

In this study, the HfAlO/Au NCs/HfAlO trilayer structures withAu NCs fabricated under different gaseous ambient have beendeposited. After wet-chemical cleaning, p-type Si wafer was dippedinto HF solution to remove the native oxide. Then, approximately10 nm-thick HfAlO layer as a tunneling oxide was deposited bypulsed-laser deposition in 550 �C under 2.0 Pa of oxygen partialpressure. The laser fluence used was 5 J/cm2. After deposition ofHfAlO, a very thin layer of Au NCs was deposited in either 2.0 Paof oxygen partial pressure (sample A), or relatively high vacuum(1 � 10�4 Pa, sample B) or in 5.9 Pa of argon partial pressure (sam-ple C). The quenching effect to the laser ablated plume and rela-tively low substrate temperature are responsible for theformation of Au NCs instead of Au thin film.

The reason to select 2.0 Pa of oxygen partial pressure is that it isthe oxygen partial pressure in depositing HfAlO dielectric thin film;while argon partial pressure has been chosen as the molecules ofAr is large and it is relatively inert which gives us extra informationof the influence to the nanocluster size by the ambient gas. Then,about 20 nm-thick HfAlO layer as control oxide was sequentiallydeposited with the same deposition condition as the tunnel layer.Post-deposition thermal annealing has been performed at 850 �Cfor 30 min in N2 ambient. Pt dot electrodes with a diameter of200 lm were subsequently deposited for electrical measurement.The charge storage and retention characteristics of the MOS capac-itors were evaluated by capacitance–voltage (C–V) measurementswith Agilent 4294A impedance analyzer at room temperature,and the structural property of the Au NCs was studied by high-res-olution transmission electron microscopy (HRTEM JEOL 2010).

Fig. 2. C–V hysteresis loops of the trilayer structure. The maximum flat-bandvoltage shift (DVFB) for sample B is about 6.3 V, corresponding to 6.6 � 1013 cm�2 ofstored charge density; while the stored charge density of sample A and C are about3.0 and 2.1 � 1013 cm�2.

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Fig. 1 shows TEM images of Au NCs embedded in the HfAlO thinfilm matrix, where the trilayer structures can be clearly seen fromthe cross-section TEM images. It is worth noting that based on ourTEM study that are not shown here, Au NCs have been formed be-fore annealing. It is also measured that the thickness differences ofthe tunnel and control layers between the three samples are lessthan 3 nm. The thicknesses of interfacial layers for sample A, Band C are 4.8, 3.2 and 3.8 nm, respectively. The interface betweenthe HfAlO tunnel layer and the silicon substrate is mainly due tothe oxygen diffusion and reaction with Si substrate during thepost-deposition annealing. As revealed by the C–V results in thesubsequent sections, this interfacial layer causes no significantdegradation of the electrical properties. From the plane-viewTEM images, one can see that the Au NCs are well self-organizedand distributed uniformly. The NCs density of samples A, B and Care about 2.0 � 1012 cm�2, 1.4 � 1012 cm�2 and 1.6 � 1012 cm�2,respectively.

As revealed by the plane-view TEM images, the density of AuNCs fabricated in 1 � 10�4 Pa is the lowest, compared to the others.This can be explained by the fact that as there are fewer particlescollided with the Au atoms during transportation, the plasmaplume is less concentrated, therefore, there are relatively feweradatoms reach the substrate surface. However, the density differ-ence of the Au NCs fabricated at three different ambient conditionsis not obvious. This shows that the gaseous ambient during thegrowth of the Au NCs does not have a great influential effect onthe density. Rather, from the plane-view TEM, the mean sizes ofthe Au NCs of samples A, B and C are 2.6, 2.7 and 3.8 nm,respectively.

From the high frequency (1 MHz) C–V results as shown in Fig. 2,significant hysteresis has been achieved in all trilayer floating gatememory structures, indicating successful charge storage in the AuNCs [16]. The control sample shows a typical C–V curve without anobvious loop, hence the influences of oxide trap charges or mobile

Fig. 1. Cross-sectional and plane-view HRTEM images of the trilayer floating gate mempartial pressure (sample A), or relatively high vacuum (1 � 10�4 Pa, sample B) or in 5.9

ions in the HfAlO dielectric thin layers are neglected in the follow-ing discussion. The flat-band voltage shift (DVFB) of the controlsample is negative, which can be attributed to the formation of po-sitive fixed charges due to the oxygen vacancies resulted from therelatively low oxygen pressure growth of the HfAlO layers. Asshown in the figure, the maximum flat-band voltage shift (DVFB)for sample B is about 6.3 V, corresponding to 6.6 � 1013 cm�2 ofstored charge density calculated by the equation given by Kim[17] and Tiwari et al. [1]; while the stored charge density of sam-ples A and C are about 3.0 and 2.1 � 1013 cm�2, respectively. Bycomparing the three samples, one can see that the sample B exhib-its relatively larger memory window over the other two. As shownin the plane-view TEM image in Fig. 1, the density of the three

ory structures sample A, B and C, where Au NCs are fabricated at 2.0 Pa of oxygenPa of argon partial pressure (sample C).

Fig. 3. Normalized capacitance decay characteristics (capacitance–time, C–t) forthree samples.

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samples are almost the same but the size of the Au NCs in sample Bis relatively smaller.

Fig. 3 shows the retention measurement (capacitance–time,C–t) performed at room temperature which is used to evaluatethe lifetime of the stored charges in the floating gate memory. Afterinjecting electrons at 10 V for 20 s, gate voltage was changed to theflat-band bias voltages and the duration of retention time is ex-pected to be long enough to withstand the loss of charges storedin the memory nodes.

When electric field is removed, electrons tunnel out via directtunneling. In general, smaller tunneling oxide thickness favors di-rect tunneling, leading to poor retention. For the comparison ofsamples A and B, the retention property of the former is more out-standing. As the oxygen partial pressure affects the film growth ofHfAlO, it plays an important role in the threshold voltage shift. Onepossible mechanism that can be drawn is that, at higher oxygenpressure, the HfAlO film possesses lower oxygen deficiency in-duced leakage current and thus a better charge storage retentionproperty. Other possible reasons for this difference in retentionare due to the lateral channel leakage mechanism, proposed byKim et al. [17] and the Coulomb repulsion in nanoclusters, pro-posed by Winkler et al. [18]. From the plane-view TEM images,we can see that the mean distance in the NCs of sample B is short-er. The lateral channel formed by Au NCs, which favored the trans-verse motion of electrons, generally led to larger leakage. Inaddition, the smaller Au NC size in this sample results in relativelystronger quantum confinement and Coulomb effects. The Coulombblockade effect raises the electrostatic potential of the Au NCs, andthe quantum confinement effect shifts the conduction band energy

upward and reduces the conduction band offset between the NCsand HfAlO. These two effects enhance the back tunneling of thestorage charges to the Si substrate and deteriorate the retentionproperty. Hence, though sample B possess better C–V performance,as the leakage induced is also larger, sample A or C seem to bemore suitable for application. And the result shows that thetrade-off between write/erase and retention remains a great chal-lenge for non-volatile memory.

In summary, the effects of growth condition of metal nanoclus-ter on memory properties of HfAlO/Au NCs/HfAlO floating gatememory structure have been studied. The trilayer floating gatememory structure with Au NCs fabricated at oxygen ambientexhibits relatively better memory property. The memory windowis up to 6.3 V, corresponding to 6.6 � 1013 cm�2 of stored chargedensity. The sizes of the Au NCs fabricated are smaller than 4 nmin diameter, suggesting that they may also be used in multi-bitmemory cell.

Acknowledgements

This work was supported by the Hong Kong Research GrantCouncil (No. PolyU 5157/06E). P.F. Lee is grateful to financial sup-port of PolyU Postdoctoral Fellow Grant (No. G-YX83).

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