Theoretical and Experimental Investigation of Ultrathin Oxynitrides

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257

Mat. Res. Soc. Symp. Proc. Vol. 592 © 2000 Materials Research Society

SI-0

SN

0) U

LO+LA

U) .0j

SIONif~ on

~

~

TO+LA

0

LOL+TO 2T0

LO+2To

-

.,.~

..

~

1000

800

~

.,

Br .. I

S.,. ,

1400

1200

Frequency (cm-1 ) Figure 1. ATR absorption spectra from the 36 A oxynitride film on Si and a bare Si substrate with native oxide.

30 ASION

.0

0< 3000 Asi N 600

800

1000

1200

1400

Frequency (cm 1 ) Figure 2.Comparison of the absorption spectrum of the oxynitride film obtained by subtracting the two spectra in Fig. 1 with that from a thick nitride film. Figure 1 shows the ATR spectra of the thin oxynitride film on Si and of a bare Si substrate. Since the penetration depth of the IR light is much larger than the film thickness, absorption features due to multi-phonon processes in Si also appear in both spectra. The broad structure between 1000 and 1200 cm' is originated from the Si-O bond stretching vibrations of the native oxide (about 10 A measured using SE). The admixture of lattice vibration absorption of the interfacial layer that has different Si-O bonding structures with that of the regular Si-O bonds results in this broad absorption feature. The new features manifest themselves as stretching vibration peaks of Si-N bonds and Si-O bonds in the spectrum of the oxynitride film. The Si-O peak in the oxynitride is a relatively sharp peak at 1100 cm- in contrast to the broad peak for native oxide. By subtracting the two spectra using a scale factor, the absorption peaks in the oxynitride film can be singled out and plotted in Figure 2. It can be seen that the Si-N stretching vibration peak substantially blue-shifted from 830 cm' up to 960 cmr in comparison with the spectrum of a thick Si 3N 4 film (see Fig. 2).

258

THEORY To investigate an ultra thin oxynitried layer we start with a previously generated model of the Si-Si0 2 interface with the oxide thickness of 0.8 nm [6]. The interfacial region in this model is about 0.4 rum thick. Two main questions are considered. Firstly, we need to know the nitrogen distribution in the sample; secondly, it is the effect of nitrogen on the local atomic structure and electronic properties of the interface. We use a simplified density functional quantum molecular dynamics method Fireball96 [7]. The local density and pseudopotential approximations are used. Numerous recent applications of the technique to a variety of materials problems are reviewed in ref. [8].

j

Oxynitride

sio2-s I

S"

0

I

I

I

Si N2 0

>

Quartz

I II

I-

Si3N

700 750 800 850 900 9501000

Frequency (cm-)

Figure 3. Theoretical vibrational density of states at the

r point for the ultra thin oxynitride film on Si, Si-SiO 2

interface, crystalline oxynitride Si 2 N2O, .-quartz, and P3-Si 3N4 . Four modes marked with dashed lines are

discussed inthe text.

We introduce nitrogen as NO molecules into our model structure. Here we discuss the cell with the nitrogen concentration of 6.78 x 1014 cm 2 . To find the equilibrium geometry we perform molecular dynamics simulation with a fi