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УДК 541.124.16+662.612
Solid-state Synthesis of Cobalt Germanides in Epitaxial Ge/a-Co(001) and Ge/в-Co(110) Nanofilms
Liudmila E. Bykova* Victor G. Myagkov Igor A. Turpanov
Kirensky Institute of Physics, SB RAS, Akademgorodok 50, Krasnoyarsk, 660036,
Russia
Risa B. Abylkalykova
D. Serikbayev East Kazakhstan State Technical University, Protazanova 69, Ust-Kamenogorsk, 070004,
Kazakhstan
Galina N. Bondarenko
Institute of Chemistry and Chemical Technology SB RAS, K.Marksa 42, Krasnoyarsk, 660049,
Russia
Liudmila A. Lee Alexander V. Kobyakov
Siberian Federal University, Krasnoyarsk, Svobodny 79, Krasnoyarsk, 660041,
Russia
Received 10.12.2009, received in revised form 15.01.2010, accepted 10.02.2010 The experimental results of a study of solid-state synthesis of cobalt germanides in epitaxial Ge/a-Co(001) and Ge//3-Co(110) nanofilms are presented. For both polymorphic modifications of cobalt, it is demonstrated that the Co5 Ge7 phase occurs at ~ 275 ° C. When the annealing temperature increases to ~ 300 ° C, the CoGe2 phase forms, which sharply reduces the electric resistance and magnetic characteristics of the samples. The order of the formation of phases and the temperatures at which the phases are formed are not changed based on the polymorphic modification of cobalt.
Keywords: epitaxial growth, nanofilms, solid-state synthesis, Co-Ge system, cobalt germanides.
Studying the chemical interactions between the metals with various semiconductors has shown that the interface acquires new structural and magnetic properties. Chemical reactions on the interface of the films often cause solid-state reactions, which are in the focus of intensive research. The main efforts of this research are focused on studying the formation of silicides on the interface of metallic films with silicon [1]. To a lesser extent, there has been some research of solid-state reactions of metals with germanium. Most studies [2-4] show that as the annealing temperature increases, the Co5Ge7 phase on the Co/Ge interface occurs first at a temperature
* lebyk@ iph.krasn. ru © Siberian Federal University. All rights reserved
of ~ 300 °C, and then changes into the CoGe2 phase at a temperature of ~ 425°C. In some works though [5], it is shown that the CoGe phase forms first on the Co/Ge interface, and then as the annealing temperature increases the phases follow the following order: Co/Ge ^ CoGe ^ Co5Ge7 ^ CoGe2. A structural analysis of thin films of Co growing on Ge(111) and Ge(001) using photoelectric X-ray diffraction and low-energy electron diffraction, has shown a mixing of Co and Ge at a low temperature (~ 100 °C) and a possible formation of the first CoGe2 phase [6]. The results of photoelectric investigations show a mixing of the layers on the interface between Co and Ge(100) at a very low (~ 170 K) temperature [7]. The CoGe2 and CoGe phases have very low symmetry, and only Co5Ge7 has a tetragonal lattice and grows epitaxially between other phases on Ge(111) and Ge(100) surfaces in an ultrahigh vacuum [5, 6]. The chemical reactions and epitaxial growth of Ge on various fl-Co and a-Co surfaces have not been investigated.
It is well known that bulk samples of the hexagonal a-Co phase are stable at temperatures below the allotropic a ^ fl transformation. But small samples and thin films of the a-Co phase are often stable at room temperature. This paper reports the findings on solid-phase reactions on the interface between metastable cubic fl-Co(001) and hexagonal a-Co(110) films with a polycrystalline Ge layer.
Initial Ge/fl-Co(001) and Ge/a-Co(110) film structures were made using thermal evaporation on a monocrystalline Mg0(001) substrate in a vacuum of 10-5 torr. Samples with an atomic ration close to 3Ge:2Co were used in these experiments. The thickness of the films used was not more than 300 nm. In order to prevent a solid-state reaction between Ge and Co, the Ge film was precipitated at room temperature. The resulting samples were annealed in a vacuum of 10-5 torr over 25 °C for 20 minutes at temperatures between 100 °C and 350 °C. X-ray investigations using a DR0N-4-07 diffractometer (Cu Ka — radiation) were used to identify the formed phases. Fluorescent X-ray was used to identify the chemical composition and thickness of the films. Measurements of the magnetic crystallographic anisotropy and the saturation of magnetization were made using the method of torsional moment in a maximum magnetic field of 18 kOe. All measurements were made at room temperature.
In order to get epitaxial fl-Co(001) layers, cobalt was precipitated at a temperature of ~ 250°C. A strong and singular diffractional reflection of (002) fl-Co confirms the formation of an epitaxial fl-Co(001) layer (Fig. 1a). Each sample had a biaxial magnetic anisotropy with a constant of K2= -(6.0 — 7.0) • 105 erg/cm3. The light axis of magnetization of fl-Co film coincides with the direction of [110] and [1-10] of the substrate Mg0(001), which indicates the presence of orientational correlation [100](001)fl-Co ||[100](001)Mg0 during epitaxial growth of cubic cobalt on the surface of Mg0(001). These two factors indicate a crystalline perfection of the initial fl-Co(001) layers, acquired in the given technological conditions.
The epitaxy of Co on the surface of Mg0(001) radically changes when precipitation occurs at temperatures of ~ (370-400) °C . The diffraction patterns of the samples show that a-Co crystalline particles (110) grow on (001) the surface of Mg0 (Fig. 2a). The analysis performed in the work [8] shows that a-Co(110) crystalline particles grow on Mg0(001) following two epitaxial ratios: a-Co(110)[001] || Mg0(001)[110] and a-Co(110)[100] || Mg0(001)[1-10]. The constant Kef f of effective biaxial magnetic anisotropy of a-Co(110)/Mg0(001) films is Kef f = (1.1-1.2)• 106) erg/cm3. The energy of magnetic anisotropy EK of a hexagonal crystal (without taking into account the anisotropy in the plane of the film) is Ek = KiSin2p + K2Sin4p + ... for a-Co, where K1 = 4.3-106 erg/cm3, K2 = 1.2-106 erg/cm3 and p is the angle between the axis c and the direction of magnetization MS [19]. Assuming that the crystalline particles a-Co(110), growing along the axis c in the directions [110] and [1-10] Mg0, are interchangeable and all have
ЗО 40 50 60 70 80
2в (deg.)
Fig. 1. Diffraction patterns of Ge/e-Co(001) films after annealing: (a) at 20 “C, (b) at 275 “C, (c) at 300 “C, (d) at 350“C
the same volume, the constant Keff = K2 [9]. The fact that the experimental values of Keff and K2 are the same confirms the epitaxial growth of a-Co(110) crystalline particles on the surface of Mg0(001).
The graphs of the constant of biaxial magnetic anisotropy K2, the saturation of magnetization Ms and the electric resistance R as a function of the annealing temperature Ts for Ge/в-Co(001) and Ge/a-Co(110) nanofilms all have the same form taking into account experimental uncertainties. Fig. 3 shows graphs of the constant of biaxial magnetic anisotropy K2, the saturation of magnetization Ms and the electric resistance R as a function of the annealing temperature Ts for the given samples. Up until a temperature of 250 0C the values of K2 and Ms are not related to Ts, which indicates that no mixing or formation of connections has occurred on the interface between germanium and cobalt. At temperatures around 275 “C the values of K2 and Ms for the monocrystalline cobalt layer decreased for all samples and at a temperature of 300 “C all samples became completely nonmagnetic. At temperatures Ts > 300 “C the values of K2 and Ms become zero. This suggests a full mixing of the Co and Ge layers and the synthesis of nonferromagnetic cobalt germanides.
The diffraction patterns change based on the relationships of K2(Ts) and Ms(Ts). Fig. 1 shows the X-ray spectrum for Ge/e-Co(001) nanofilms at their initial temperatures and after annealing at temperatures of 275 “C, 300 “C and 3500C. After annealing at a temperature of 275 0C, the diffractional reflection for (002)e-Co decreased and new weaker peaks formed, which indicates a formation of polycrystalline phases as a product of the reactions (Fig. 1b). The diffractional reflections for many of the phases of the Co-Ge films are the same, but the reflection when 20 = 45.80 can only come from a peak of the (222)Co5Ge7 phase. This suggests that the Co5Ge7 phase forms first on the Ge/e-Co(001) interface at a temperature of 275“C. At a
30 40 50 60 70 80
2© (deg.)
Fig. 2. Diffraction patterns of Ge/a-Co(110) films after annealing: (a) at 20 °C, (b) at 300 °C, (c) at 350 ° C
temperature of 300°C the reflection from (002)^-Co disappears, but the peak from (222)Co5Ge7 grows, which indicates a further increase in volume for this phase (Fig. 1c). The reflection from the CoGe2 orthorhombic phase also appears at this temperature. At a temperature of 350°C the peak from (222)Co5Ge7 decreases and the Co5Ge7 phase turns into the CoGe2 phase, which becomes the dominant product in the reactions (Fig. 1d).
As shown by the decrease of the constant of biaxial magnetic anisotropy K2(Ts) and by the decrease of the saturation of magnetization MS(Ts), the solid-state reaction in Ge/a-Co(110) nanofilms, just like in the Ge/^-Co(001) nanofilms, starts at a temperature of 275 °C. But there are no new reflections on the diffraction patterns, which might indicate the formation of a new phase that has a disordered finely-dispersed structure. The diffraction patterns after annealing at a temperature of 300 °C show reflections that belong to the CoGe2 and Co5Ge7 phases (Fig. 2b). The weak peak from (222)Co5Ge7 disappears from the diffraction pattern after annealing at a temperature of 350 °C, which suggests a decrease of the Co5Ge7 phase in the products of the reaction (Fig. 2c). Only the reflections from the CoGe2 phase remain after annealing at a temperature of 350 °C, so the transformation of the (222)Co5Ge7 peak is the same for both the Ge/a-Co(110) and the Ge/^-Co(001) nanofilms. This suggests that the disordered finely-dispersed Co5Ge7 phase in the Ge/a-Co(110) nanofilms also forms at a temperature of 275 °C and precedes the formation of the CoGe2 phase. An analysis of the above mentioned facts suggest that the order of the formation of phases in the Ge/a-Co(110) and Ge/^-Co(001) nanofilms is the same.
The average size of the Co5Ge7 and CoGe2 crystalline particles was determined from the diffraction pattern peaks using Sherrer’s formula. For the Ge/^-Co(001) and Ge/a-Co(110) samples, the average size of the crystalline particles was 17-30 nm.
One of the stages of a solid-state reaction is the breaking of chemical bonds in the reactants. The energy of the bonds in a-Co is almost the same as the energy of the bonds in ,3-Co, since the enthalpy of the AH@^a = -220 cal/mole transition ,3-Co ^ a-Co is small. This shows
□
re
*
□
(6
os
Fig. 3. Saturation of magnetization Ms, magnetic anisotropy constant K2 , electric resistance R of the epitaxial Ge/P-Co(001) and Ge/a-Co(110) nanofilms as a function of the annealing temperature Ts
that small differences in the energies of the polymorphous reactants does not have an effect on the behavior of solid-state synthesis. In most cases, the enthalpy of the transition from the amorphous phase to the crystalline phase is ~ 1000 cal/mole. Because of this, regardless of whether amorphous, polycrystalline or monocrystalline reactants are used, the order of formation of phases and the temperatures at which each phase forms are the same.
This study shows that annealing polycrystalline Ge nanofilms, precipitated on epitaxial p-Co(001) and a-Co(110) surfaces, leads to the formation of finely-dispersed polycrystalline Co5Ge7 and CoGe2 phases at temperatures of Tq1 ~ 275 °C and Tq ~ 300 °C, respectively. Small differences in the energies of the polymorphous p-Co and a-Co modifications of cobalt do not have an effect on the order of formation of phases and the temperatures at which each phase forms.
This study was supported by the Russian Foundation for Basic Research, project no. 07-0300190.
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Ts (°C)
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Твердофазный синтез германидов кобальта в эпитаксиальных Ge/a-Co(001) и Ge/в-Co(110) нанопленках
Людмила Е. Быкова Виктор Г. Мягков Игорь А. Турпанов Риза Б. Абылкалыкова Галина Н. Бондаренко Людмила А. Ли Александр В. Кобяков
Представлены экспериментальные результаты исследования твёрдофазного синтеза германидов кобальта в эпитаксиальных Ge/a-Co(001) и Ge//3-Co(110) нанопленках. Показано, что для обеих полиморфных модификаций кобальта фаза Co5 Ge7 формируется первой при температуре ~ 275 ° C. С увеличением температуры отжига при температуре ~ 300 ° C образуется фаза CoGe2, которая резко уменьшает электрическое сопротивление и намагниченность образцов. Различные полиморфные модификации кобальта не изменяют последовательность формирования фаз и их температур инициирования.
Ключевые слова: эпитаксиальный рост, нанопленки, твердофазный синтез, Co-Ge система, гер-маниды кобальта.
