The Charge Storage of Doubly Stacked Nanocrystalline-Si based Metal Insulator Semiconductor Memory Structure

doi:10.4236/mnsms.2013.31B006

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Modeling and Numerical Simulation of Material Science, 2013, 3, 20-22

Published Online January 2013 (http://www.SciRP.org/journal/mnsms)

The Charge Storage of Doubly Stacked Nanocrystalline-Si

based Metal Insulator Semiconductor Memory Structure

Xiang Wang, Chao Song, Yanqing Guo, Jie Song, Rui Huang

Department of Physics and Electronic Engineering, Hanshan Normal University, Chaozhou, People’s Republic of China

Email: [email protected]

Received 2012

ABSTRACT

Doubly stacked nano cr yst alli ne-Si (nc-Si) based metal insulator semiconductor memory structure was fabricated by

plasma enhanced chemical vapor deposition. Capacitance-Voltage (C-V) and capacitance-time (C-t) measurements

were used to investigate electron tunnel, storage and discharging characteristic. The C-V results show that the flatband

voltage increases at first, then decreases and finally increases, exhibiting a clear deep at gate voltage of 9 V. The de-

creasing of flatband voltage at moderate programming bias is attributed to the transfer of electrons from the lower nc-Si

layer to the upper nc-Si layer. The C-t measurement results show that the charges transfer in the structure strongly de-

pends on the hold time and the flatband voltage decreases markedly with increasing the hold time.

Keywords: nc-Si dots; Capaci tance-Voltage Measurement; St or age

1. Introduction

In recent years, silicon nanostructures have attracted

enormous attentions.[1 -5] Particularly, metal-i nsula tor -

semiconductor (MIS) structures based on silicon quan-

tum dots are widely studied for their new physical phe-

nomena as well as their potential applications in future

memory device. The first nanocrystals based MIS mem-

ory structure was put forward by Tiwari et al., where Si

nanocrystals were used as storage nodes.[1] Compared to

the conventional nonvolatile memories with polycrystal-

line silicon or silicon nitride floating gate as the charge

storage layer, a nanocrystal based memory has been

suggested as one of the key items for increasing flash

memory stability and decreasing the node size for higher

information density. In such memory device, the Si na-

nocrystals are sufficiently isolated from each other by

insulator. Hence, the electrons are localized in the nano-

crystals and a local leakage path would not discharge all

of the nanocrystals, which means a nanocrystal based

memory should have a better reliability than convention-

al nonvolatile memories.[2-3] However, nanocrystal

based memories have a rather low charge density which

might be a drawback of this approach. In addition, the

charge retention time in single-layer nanocrystal based

memory device is also limited by the fast direct tunneling

process through the ultrathin tunneling layer. To solve

these problems, there are have some approaches, suchas

the introduction of certain tunneling dielcctric materials

and a doubly stacked dot structure. For instance, multi-

layer nc-Si structure in SiO2 matrix was investigated by

some research groups,[6-8] where electrons were injected

into the multilayer structure via a FN tunneling process at

high programming voltages. An ultrahigh stored charge

density is demonstrated in the multilayer nc-Si structure.

A self-aligned stacked double-layer nc-Si structure in

SiNx matrix with enhanced retention time has been re-

ported.[9,10]

In this work, we have prepared the stacked nc-Si

memory structure where double layers of nc-Si dots em-

bedded within triple SiNx barriers. We investigated the

charge storage mechanism in stacked nc-Si memory

structure by using capacitance voltage (C-V) measure-

ment and capacitance-time (C-t) measurements.

2. Experimental Details

The stack nc-Si dots based memory structures were fa-

bricated on n-type crystalline Si substrates (6-8 Ω cm) in

plasma enhanced chemical vapor deposition system. The

doubly stacked layers of nc-Si with the thickness of

about 5 nm were fabricated by the layer-by-layer deposi-

tion technique[11] with silane and hydrogen mixture gas.

SiNx barrier were deposited by a gas mixture of silane

and ammonia with the ratio of 1:5. The thickness of both

lower and upper tunneling SiNx barrier was about 4 nm

and the thickness of control SiNx barrier was about 20

nm. For comparison the sample with single nc-Si dots

layer was also prepared with the same deposition process.

The thickness of nc-Si layer, tunneling layer and the bar-

X. WANG ET AL.

Figure 1. The Capacitance-voltage characteristics of the doubly

stacked nc-Si structure.

rier layer was 5nm, 4nm and 20nm, respectively. The

samples were annealed in nitrogen ambient at 900 oC for

30 minute to improve the quality of the nc-Si and to re-

duce the density of interface states and defects in the

structure. The C-V and C-t measurements were per-

formed using Agilent 4284A LCR meter. All measure-

ments were performed at air ambient and at room tem-

perature inside a shield box.

3. Results and discussions

Figure 1 shows the C-V characteristics of the doubly

stacked nc-Si memory structure. The bias was sweeping

from the accumulation to the inversion region for the

MIS device based on the n-type substrates. In order to

obtain saturation of charge storage, before each sweep

the structure was held at a positive gate bias for 120 s.

After each sweep, electrons trapped in the structure were

erased by negative bias of -4 V for 120 s to bring the

structure to the neutral stage. As can seen in figure 1, at

low programming bias, a higher programming bias ap-

plied at the gate results in a bigger shift of the flatband

voltage, which is caused by more and more electrons

trapping in the structure. At moderate programming bias

(6 V to 9 V), the flatband voltage decreases with in-

creasing the bias. At high programming bias (10 V to 13

V), the flatband voltage increases with the bias. Under

fixed erasing bias and varied programming bias, the rela-

tionship between programming bias and flatband voltage

is of special interest.

Figure 2 shows the programming bias dependence of

the flatband voltage. With increasing the gate voltage,

the flatband voltage of the sample with double nc-Si

layers is found to first increase, then decrease, and fi-

nally increase, exhibiting a clear deep at bias of 9 V.

When the programming bias increases from 2 V to 6 V,

the flatband voltage is shift from 0.34 V to 4.47 V. With

Figure 2. The dependence of the flatband voltage on programming

bias.

further increasing the programming bias from 6 V to 9 V,

the flatband voltage decrease from 4.47 V to 3.44 V. As

the bias changes from 9 V to 13 V, the flatband voltage

increases to 4.92 V. The flatband voltage of the sample

with single nc-Si layer increases from 0.73V to 3.35V

when the bias changes from 3V to 5V. Then the flat-

band voltage keeps invariably with the increasing the

bias. we thi nk that the decreasing of flatband voltage at

moderate programming bias is attributed to the transfer

of electrons from the lower nc-Si layer to the upper nc-Si

layer .

Figure 3 shows the relationship of programming bias

and the flatband voltage under different hold time.

Each sweep measurement was performed after a posi-

tive gate bias was held at the gate for different hold

time of 120 s, 50 s and 20 s, respectively. From the

figure 4, we can see that the flatband voltage strongly

depends on the hold time. When the structure was held

at a positive gate bias for 120 s, we can see clear at

moderate programming bias (9 V). With varying the

hold time to 50 s, the flatband-voltage curve also exhi-

bits a deep, but the deep is smaller than that of the hold

time of 120 s. It demonstrates that the charge transfer is

not sufficient for the hold time of 50 s, resulting that

the amount of transfer charges is less than that of the

hold time of 120 s. Hence, the decrease of flatband

voltage and the deep is smaller than that of the hold

time of 120 s. In the case of the hold time of 20 s, we

can notice that the flatband voltage keeps unchanged

nearly, which demonstrates that few charges exist can

transfer from the lower nc-Si layer to the upper nc-Si

layer. Based on the above experimental result, we con-

clude that the charges transfer in the structure strongly

depends on the hold time and the flatband voltage de-

creases markedly with increasing the hold time.

X. WANG ET AL.

4. Conclusions

In summary, doubly stacked nc-Si based MIS memory

structure was fabricated by plasma enhanced chemical

vapor deposition. C-V and C-t measurement was used to

Figure 3. The programming bias dependence of flatband voltage

under different hold times.

investigate electron storage and discharging in the struc-

ture. The C-V experiment results show that the flatband

voltage first increases, then decreases and finally in-

creases, exhibiting a claer deep at gate voltage of 9 V.

The decreasing of flatband voltage at moderate pro-

gramming bias is attributed to the transfer of electrons

from the lower nc-Si layer to the upper nc-Si layer. The

C-t measurement results show that the charges transfer

in the structure strongly depends on the hold time.

5. Acknowledgemen t s

The authors would like to acknowledge the financial

support of Natural Science Foundation of Guangdong

Province (S2011010001853) and Foundation for Distin-

guished Young Talents in Higher Education of Guang-

dong (LYM09101, LYM11090, LYM10099).

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