Quantum chemical calculation for CVD process ofGeSbTe
Akitomo Tachibana∗1 , Kazuhide Ichikawa1 and Toshimichi Shintani2
1
Department of Micro Engineering, Kyoto University, Kyoto 606-8501, Japan
2
National Institute of Advanced Industrial Science and Technology West 7A, Onogawa
16-1, Tsukuba, Ibaraki 305-8569 Japan
E-mail: akitomo@scl.kyoto-u.ac.jp

1

Introduction

It has been long known that the ab initio calculation of electronic structure of atomic andmolecular system based on quantum chemistry can be a powerful tool to analyze details ofchemical reactions. In this paper, we apply the quantum chemical calculation to chemicalvapor deposition (CVD) process of GeSbTe (GST) alloys which is used for phase changememory (PCM).
PCM is one of the most promising candidates for the next-generation memory device
and GST is widely used as the PCM material. So far, GST thin ﬁls are deposited byphysical vapor deposition such as sputtering and pulsed laser deposition. However, CVD
has many advantages and considered to be necessary for future PCM applications. The
CVD process for GST is relatively new ﬁeld of research and there remains much things
which are not known. Therefore, to establish more eﬃcient CVD process, it would be
useful to investigate the details of the process by using quantum chemical calculation
together with the experiment.
In this paper, we assume GeH3 (tBu), Sb(iPr)3 and Te(iPr)2 as precursors [1] and study
gas phase reactions among them (and carrier gas H2 ) by quantum chemical calculation.

2Calculation Method and Results

All the calculations are performed by the DFT (density functional theory) method withthe B3LYP functional [3]. The Gaussian 09 package [2] is used for the calculations. TheD95(d,p) basis set [4] is used for the H atoms, the 6-31G(d) basis set is used for the Catoms and the LanL2DZ eﬀective core potential [5] is used for the Ge, Sb and Te atoms.
First of all, we show the optimized structures of the precursors in Fig. 1. We theninvestigate how these structures decompose by itself or by reacting with the carriergas, the H2 molecules. As for the Ge precursor, we can imagine the following reaction
paths: (i-1) GeH3 (tBu) → GeH3 + C4 H9 , (i-2) GeH3 (tBu) + H2 → GeH4 + C4 H10 and (i3) GeH3 (tBu) → GeH2 + C4 H10 . The reaction (i-1) proceeds without energy barrier andis endothermic by 2.456 eV. The reaction (i-2) is exothermic by 0.561 eV with the energy
barrier of 3.534 eV. The reaction (i-3) is endothermic by 1.121 eV with the energy barrier of
2.392 eV. These paths are summarized in Fig. 2. As for the Sb precursor, we ﬁrst consider
the reactions that the isopropyl groups are removed by reacting with H2 , namely (ii2) Sb(iPr)3 + H2 → SbH(iPr)2 + C3 H8 and (ii-2) SbH(iPr)2 + H2 → SbH2 (iPr) + C3 H8 .
They are endothermic by 0.643 eV and 0.577 eV with the barriers of 1.858 eV and 1.968 eV
respectively. Then, the SbH2 (iPr) molecule may decompose by the following reactions:(ii-3) SbH2 (iPr) → SbH2 + C3 H7 and (ii-4) SbH2 (iPr) → SbH + C3 H8 . The reaction (ii-3)

proceeds without the barrier and is endothermic by 1.787 eV. The reaction (ii-4) is endothermic by 1.544 eV with the barrier of 1.817 eV. We also investigate the decomposition
into radicals: (ii-5) Sb(iPr)3 → Sb(iPr)2 + C3 H7 and (ii-6) SbH(iPr)2 → SbH(iPr) + C3 H7 .
The energy needed for these reactions are 1.797 eV and 1.821 eV respectively which aresomewhat smaller than the barriers for the reactions (ii-1) and (ii-2). As for the Te precursor, we show the results for the reactions as regards TeH(iPr): (iii-1) TeH(iPr) + H2 →TeH2 + C3 H8 , (iii-2) TeH(iPr) → TeH + C3 H7 and (iii-3) TeH(iPr) → Te + C3 H8 . The reaction (iii-1) is exothermic by 0.592 eV with the energy barrier of 4.025 eV. The energy
needed for the decomposition processes (iii-2) and (iii-3) are much lower with 1.931 eV
and 1.966 eV respectively.
We next examine the optimized structures and stabilization energy of possible dimers
formed from the molecules which are derived from the precursors through the reactions
such as described above. The structures of dimers which are derived from pairs of Geprecursors are shown in Fig. 3. Those from Sb pairs are shown in Fig. 4 and Te pairs inFig. 5. The dimers which stem from diﬀerent kinds of precursors are shown in Figs. 6,
7 and 8, respectively for combinations of Ge-Sb, Ge-Te and Sb-Te. Let us focus on theGe-Ge dimers and discuss their stabilization energy. The stabilization energy for the reactions 2GeH2 (tBu) → (GeH2 (tBu))2 is 2.593 eV, GeH3 + GeH2 (tBu) → GeH3 GeH2 (tBu) is2.596 eV and 2GeH3 → (GeH3 )2 is 2.584 eV. Remind that the energy needed for the reaction (i-1) is 2.456 eV which is smaller than the stabilization energy of the dimer formation.
This implies that GeH3 tends to form the dimers rather than goes back to the original
precursor.

3Conclusion

We have applied quantum chemical calculation to the possible reactions of the precursors
for the CVD process of GST alloys. Some likely reaction pathways are suggested regarding
decomposition of the precursors and dimer formations from them. The whole picture ofthe process would be revealed by more computational investigation and comparison of
the results with experimental data.

Acknowledgment
This research work is supported by the national project “Green Nanoelectronics”.

References[1] H. Machida et al., JJAP 49 05FF06 (2010).
[2] Gaussian 09, Revision A.1, M. J. Frisch et al., Gaussian, Inc., Wallingford CT, 2009.
[3] C. Lee, W. Yang, and R. G. Parr, Phys. Rev. B 37, 785 (1988); A. D. Becke, J.
Chem. Phys, 98, 5648 (1993).
[4] T. H. Dunning Jr., P. J. Hay, in: H. F. Schaefer III (Ed.), Modern TheoreticalChemistry, Plenum, New York (1976).
[5] P. J. Hay and W. R. Wadt, J. Chem. Phys. 82, 299 (1985).

(b)

(a)

GeH3(tBu)(c)

Sb(iPr)3

Te(iPr)2

Figure 1: The optimized structures of (a) Ge precursor: GeH3 (tBu), (b) Sb precursor:Sb(iPr)3 and (c) Te precursor: Te(iPr)2 .

Figure 2: The possible reaction pathways as regards Ge precursor GeH3 (tBu). The energy
is expressed in units of eV.

(b)

(a)

(GeH2(tBu))2(c)

GeH3GeH2(tBu)(GeH3)2

Figure 3: The optimized structures for Ge-Ge dimers.

(b)

(a)

(Sb(iPr)2)2(c)

(e)

(d)

(SbH(iPr))2

Sb(iPr)2SbH(iPr)

Sb(iPr)2SbH2(f)(SbH2)2

SbH(iPr)SbH2

Figure 4: The optimized structures for Sb-Sb dimers.

(b)

(a)

(Te(iPr))2(c)Te(iPr)TeH(TeH)2

Figure 5: The optimized structures for Te-Te dimers.

(b)

(a)

GeH3SbH2

(c)

(e)

(d)

GeH2(tBu)SbH2

GeH3SbH(iPr)

GeH3Sb(iPr)2(f)GeH2(tBu)Sb(iPr)2

GeH2(tBu)SbH(iPr)

Figure 6: The optimized structures for Ge-Sb dimers.

(a)

(b)

GeH3TeH(c)

GeH2(tBu)TeH(d)

GeH3Te(iPr)

GeH2(tBu)Te(iPr)

Figure 7: The optimized structures for Ge-Te dimers.

(b)

(a)

SbH2TeH

(c)

(e)

(d)

SbH2Te(iPr)

SbH(iP)T H
SbH(iPr)TeH

Sb(iPr)2TeH(f)

Sb(iPr)2Te(iPr)

SbH(iPr)Te(iPr)

Figure 8: The optimized structures for Sb-Te dimers.