First principles calculations for growth condition for
CVD process of GeSbTe
Akitomo Tachibana1∗ , Masato Senami1 , 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
Introduction
In this paper, the condition of steady growth of GeSbTe (GST) crystal by chemical
vapor deposition (CVD) process is investigated by ﬁrst principles calculations.
The CVD process for GST is relatively new ﬁeld of research and there remains muchthings which are not known. Therefore, to establish more eﬃcient CVD process, it would
be useful to investigate the details of the process by computational approach together
with the experiment. Many researches have reported that the deposited ﬁlm compositionand structure are signiﬁcantly dependent on deposition temperature, pressure, precursors,substrates, and so on [1-3]. Accrdingly, gas-phase and/or surface reactions are expected
to play important role for CVD process.
In our previous report, with assuming GeH3 (tBu), Sb(iPr)3 , and Te(iPr)2 as precursors [3], gas-phase reactions between precursors and between a precursor and H2 have been
investigated in detail. Following the work, we consider two subject, the choice of substrate
and reactions on GST surface. In the latter, we study surface reactions between GST surface and molecules produced by gas-phase reactions. We investigate the adsorption ofthese molecules on a pristine GST surface (particularly Te surface) of Ge2 Sb2 Te5 . As thestudy of substrate, we carry out the geometric optimization calculation of Ge2 Sb2 Te5 onsubstrate and show the lattice mismatch, stabilization energy, and structures.
Calculation Method and Results
All the calculations are performed by using the VASP program package with thedensity functional theory method. In this program package, periodic boundary conditions
are adopted and wave functions are expanded in plane wave functions.
First, we study the choice of substrate, where we consider W, TiN, Si, and SiO2
as a substrate material. Geometric optimization calculations are carried out for thesesubstrate and Ge2 Sb2 Te5 . As Ge2 Sb2 Te5 structure, we consider the structures reportedin Ref. [4] and all their periodic patterns. Our results are summarized in Table 1, andtheir optimized structures are shown in Fig. 1. The stabilization energy (∆E) for SiO2
is not shown, since these GST models have unacceptable structure for this supercell. For
substrates with good lattice mismatch, W, TiN, and Si, the optimized structures are notdisturbed around interface and surface. Small disorder of the structure for TiN with
Ti surface is not serious. On the other hand, for substrate with relatively large latticemismatch, SiO2 , the structure is signiﬁcantly disturbed around surface or Te-Te interface.
These results are somewhat consistent with the knowledge that the GST on SiO2 hasworse quality than that on TiN.
Next, we investigate surface reactions between a GST surface and molecules in gasphase. For this study, we use the GST surface model shown in Fig. 2. This model has Te
pristine surface, since Te surface model is most stable, and six layers, which are a part ofGST unit cell. As reactant, we consider what molecules exist in gas-phase. First of all, we∗E-mail:akitomo@scl.kyoto-u.ac.jp

assume GeH3 (tBu), Sb(iPr)3 , and Te(iPr)2 as precursors [3]. In addition, we consider thatthese precursors react with carrier gas H2 . These reactions are studied in our report of thelast year. As a result, we have shown that GeH4 , SbH(iPr)2 , SbH2 (iPr), SbH3 , TeH(iPr),and TeH2 are more stabilized after reactions. These molecules may be produced by gasphase reactions with hydrogen. We consider these molecules and precursors as reactantin this paper.
In Fig. 3, we show the optimized structures of Ge compounds on the GST surface
model. The stabilization energies of GeH3 (tBu) and GeH4 are -0.020 and 0.041 eV,respectively. Hence, our result implies that GeH3 (tBu) is not attached on this Te surface.
On the other hand, GeH4 is seen to be attached, and however the distance from thenearest Te atom is much longer (about twice ) than the Ge-Te distance in the GST
crystal. This is because hydrogen atoms prevent the Ge atom from bonding surface Teatoms directly. The optimized structures of Sb and Te compounds on the GST surface
model are shown in Figs. 4 and 5. The stabilization energies of Sb(iPr)3 , SbH(iPr)2 ,SbH2 (iPr), and SbH3 , are 1.489, 1.390, 1.425, and 1.447 eV, respectively. Surface of all
structures of Sb compounds are disturbed signiﬁcantly. The large stabilization energy iscontributed dominantly from the surface reconstruction. The distance from the nearest
Te atom is much longer than the Te-Sb distance in the GST crystal for all structures. Thestabilization energies of Te(iPr)2 , TeH(iPr), and TeH2 , are 0.013, 0.019, and 0.054 eV,respectively. The distance from the nearest Te atom is longer than the Te-Te distancein the GST crystal for TeH(iPr), while the distances for Te(iPr)2 and TeH2 are almost
the same length as the Te-Te distance in the GST crystal. Our results imply that theadsorption of Ge compounds on Te surface is more diﬃcult than Te compounds. Hence,we consider that Ge ﬂow rate is a key ingredient of CVD process. The adsorption of Sb
compounds is seen to disturb Te surface. For further understanding of CVD process, weshould consider other phenomena, the adsorption of dimer and decomposed molecules,hydrogen covered GST surface, hydrogen elimination reaction on surface, and so on.
Conclusion
We have investigated the CVD condition of steady growth of GST crystal by ﬁrst
principles calculations. The choice of substrates has been studied in terms of the optimized
structure and lattice mismatch. Our results have shown that SiO2 substrate are worse
than other substrates. We have also studied surface reactions between molecules in gasphase and Te atoms on GST surface. Ge compounds have less activity due to their stericeﬀect, while Te compounds have good reactivity for Te surfaces.
Acknowledgment: This research is granted by the Japan Society for the Promotionof Science (JSPS) through the “Funding Program for World-Leading Innovative R & D
on Science and Technology (FIRST Program),” initiated by the Council for Science andTechnology Policy (CSTP).
References[1] R. Y. Kim, H. G. Kim, and S. G. Yoon, Appl. Phys. Lett. 89, 102107 (2006).
[2] B. J. Choi et al., Chem. Mater. 21, 2386 (2009).
[3] H. Machida et al., Jpn. J. Appl. Phys 49, 05FF06 (2010).
[4] I. I. Petrov, et al, Sov. Phys. Crystallogr. 13, 339 (1968); B. J. Kooi and J. Th. M. De
Hosson, J. Appl. Phys. 92, 3584 (2002).

Table 1: Comparison of substrates. ∗ TiN is matched for TiN:Ge2 Sb2 Te5 =3:2.
Substrate Lattice mismatch ∆E (eV) Atom (Interface) Atom (Surface)W-0.04-11.31GeTe∗
TiN (Ti)-0.05-16.85
TeSbTiN (N)-0.05∗-6.53
GeTeSi0.11-4.78
SbTe
SiO2 (Si)
-0.14
TeTe
SiO2 (O)-0.14
GeTeFigure 1: The optimized structures of GST on substrate. From left to right, results for W,TiN(Ti), TiN(N), Si, SiO2 (Si), and SiO2 (O) are shown.

Figure 2: The GST surface model

(a)

(b)

Figure 3: The optimized structures of (a) GeH3 (tBu) and (b) GeH4 on the GST surface model.
(a)

(b)

(c)

(d)

Figure 4: The optimized structure of Sb compoites on the GST surface model. Compounds are(a) Sb(iPr)3 , (b) SbH(iPr)2 , (c) SbH2 (iPr), and (d) SbH3 .

(a)

(b)

(c)

Figure 5: The optimized structure of Te compoites on the GST surface model. Compounds are(a) Te(iPr)2 , (b) TeH(iPr), and (c) TeH2 .