Temperature Dependence of Electrical Resistance in Ge-Sb-Te Thin Films

Nowadays, the Ge-Sb-Te system is studied extensively for use in the field of both electrical and optical non-volatile memories. The key of this application is based on the changes in the physical properties (electrical conductivity or refractive index) of these films as a result of structural transformation between amorphous and crystalline states. Both states are highly stable and it is relatively easy to change between them when they are prepared as thin films. In this work, structural and electrical behaviours with the temperature of thin films with compositions Ge 13 Sb 5 Te 82 , Ge 1 Sb 2 Te 4 , Ge 2 Sb 2 Te 5 , Ge 1 Sb 4 Te 7 and Sb 70 Te 30 (atomic fraction) were studied. Films were obtained by pulsed laser deposition (PLD) using a pulsed Nd:YAG laser ( λ = 355 nm) and they were structurally characterized by X-ray diffraction. Temperature dependence of electrical resistance was studied for these films from room temperature to 520 K at a heating rate about 3 K/min. During crystallization, their electrical resistance falls several orders of magnitude in a narrow temperature range. The electrical conduction activation energies of the amorphous and crystalline states and the crystallization temperature were determined. The crystallization products were characterized by X-ray diffraction. The results were compared with those obtained by other authors.


Introduction
Phase-change materials have been used in rewriteable optical data storage for years and now they seem one of the most promising materials for non-volatile electronic memory applications. For non-volatile memories, a pronounced contrast in electrical resistivity is used. This obeys to an also pronounced contrast in structure between amorphous and crystalline states. The amorphous state has a high resistance. Applying a long low-voltage pulse, locally heats the amorphous region and leads to recrystallization. A higher-voltage in a short pulse applied to the crystalline state leads to local melting and formation of an amorphous region on rapid quenching 1 .
The main properties of a good phase-change material are: high-speed phase transition, long thermal stability of amorphous state, large optical change (for rewriteable optical storage) or large resistance change (for non-volatile electronic storage) between the two states, large cycle number of reversible transitions and high chemical stability 1 . Suitable materials for non-volatile memories have been identified in the past years [1][2][3][4][5][6] , being the Ge-Sb-Te system the most studied.
The glass forming ability of the Ge-Sb-Te system, for rapid solidification from the liquid, is restricted to a small composition range near the binary eutectic Ge 15 Te 85 (at. fraction) 7 . Ge-Te system has an eutectic point at T e ~ 648 K, formed by the co-precipitation of GeTe and Te. In a previous work, we have studied the crystallization kinetics with the addition of Sb to the eutectic point, and recognized its crystallization products for the chosen alloy at the amorphization zone (Ge 13 Sb 5 Te 82 ) 8 .
The stable ternary diagram of the Ge-Sb-Te system shows three ternary compounds on the GeTe-Sb 2 Te 3 line, which can be considered as a quasi-binary system 9-10 . These three crystalline phases are Ge 2 Sb 2 Te 5 , GeSb 2 Te 4 and GeSb 4 Te 7 and they have incongruent melting points at 902, 889 and 879 K respectively. The compounds Ge 2 Sb 2 Te 5 , GeSb 2 Te 4 and GeSb 4 Te 7 have been extensively studied and have the following characteristics: high thermal stability at room temperature, high crystallization rate and very good reversibility between amorphous and crystalline phases.
Amorphous films of the GeTe-Sb 2 Te 3 pseudobinary system were obtained by an electron beam co-evaporation method 11 . They were found to have featuring characteristics for optical memory material presenting a large optical change and enabling high-speed one-beam data rewriting. Studies of calorimetric technique and X-rays diffraction show that metastable phases appear when these films are thermally treated. Yamada et al. 11 reported that Ge 2 Sb 2 Te 5 , GeSb 2 Te 4 and GeSb 4 Te 7 compounds present two crystalline states: one is a metastable face centered cubic and the other is a stable structure. The metastable crystalline structure is a rocksaltlike, where Te atoms occupy the Cl sublattice, while the Na sublattice is randomly occupied by Ge/Sb atoms and intrinsic vacancies. The stable crystalline structures in the  [12][13][14][15] . Three zones of the Ge-Sb-Te diagram were identified as suitable materials for rewriteable storage: the first in the vicinity of the Ge 15 Te 85 eutectic, the second in the region of the pseudo-binary diagram GeTe-Sb 2 Te 3 and the third in the vicinity of the Sb 70 Te 30 composition 1 . In this work, thin films of different compositions which are representative of these three zones were obtained by pulsed laser deposition (PLD). Their structural and electrical behaviours with the temperature were studied and compared with previous works.

Experimental
Thin films were prepared by pulsed laser deposition (PLD) from chalcogenide targets having the following compositions within the Ge-Sb-Te system: Ge 13 Sb 5 Te 82 , Ge 1 Sb 2 Te 4 , Ge 2 Sb 2 Te 5 , Ge 1 Sb 4 Te 7 and Sb 70 Te 30 (expressed as atomic fraction). Bulk samples with these compositions were first prepared by direct synthesis from pure elements (4N) in evacuated silica ampoules 16 . After this process, samples were sliced and polished to obtain PLD targets with parallel faces.
Thin films were deposited on static substrates (chemically cleaned microscope glass slides) held at room temperature, which were parallelly aligned to the target surface, inside a vacuum chamber. PLD was performed using a pulsed Nd:YAG laser (Spectra-Physics Quanta-Ray Lab-150) with deposition times of 30-40 minutes, operating at a 355 nm wavelength, with a 5 ns pulse duration and a 10 Hz repetition rate. The laser beam with a 45º angle of incidence was horizontally spanned by moving a mirror in order to get uniform ablation of the target surface. The energy density of the laser spot was 1.1-1.3 J/cm 2 .
Film thicknesses, shown in Table 1, were measured by atomic force microscopy (AFM).
Electrical resistance was measured on the surface of the deposited films, using a two-point probe over sputtered Pt-contacts, in coplanar configuration separated by a length L, connected to a low-current/high-resistance electrometer. Films were placed in a vacuum cell (evacuated to 10 -2 mbar with rotary pump) and heated by a resistance furnace with a heating rate of 3 K/min.
With the aim of studying the crystallization steps, measurements of electrical resistance were performed while heating until several upper limits of temperature for each film composition. As-obtained films and their crystallization steps were analyzed by X-ray diffraction at room temperature in a Θ-Θ diffractometer using monochromatized Cu(K α ) radiation.  [21][22] . e is the film thickness, L is length between electric contact, R g and R c are the resistance of the glass and the crystal at 298 K.

Results and Discussion
Thin film electrical resistance R of samples with compositions Ge 13 Sb 5 Te 82 , Ge 1 Sb 2 Te 4 , Ge 2 Sb 2 Te 5 , Ge 1 Sb 4 Te 7 and Sb 70 Te 30 evolve upon heating and successive cooling as shown in Figure 1. Each R(T) plot shows one or more sharp transitions where the value of resistance falls some orders of magnitude in a small temperature range. These transitions are associated with structural changes 17 .
Temperature dependence of electrical resistance R(T) in the Ge 2 Sb 2 Te 5 film has two sharp transitions at about 443 and 523K. As it is well known in many previous works, the first transformation is associated with a transition between amorphous and crystalline Ge 2 Sb 2 Te 5 (fcc metastable structure) and the second one with a transformation from fcc-Ge 2 Sb 2 Te 5 to hexagonal Ge 2 Sb 2 Te 5 (stable structure) 11 . Amorphous and fcc-Ge 2 Sb 2 Te 5 phases behave as semiconductors because their resistance decreases as temperature increases. On the other hand, stable hexagonal Ge 2 Sb 2 Te 5 phase has metallic behaviour as its resistance decreases during cooling.
Similar behaviours (two sharp transitions) are observed for the Ge 1 Sb 2 Te 4 and Ge 1 Sb 4 Te 7 films. However, only a single transition is observed in the Sb 70 Te 30 film while the Ge 13 Sb 5 Te 82 film has three transitions.
R(T) in the Ge 13 Sb 5 Te 82 film has its three sharp transitions at about 433, 518 and 538 K. Amorphous phase and crystalline phases appearing in the first two stages show semiconductor behaviour. The crystalline phase appearing in the last step shows a metallic behaviour though, as its resistance increases during heating and decreases when cooling.
R(T) in the Ge 1 Sb 2 Te 4 film has its two sharp transitions at about 420 and 575 K. In the Ge 1 Sb 4 Te 7 film its two sharp transitions are found at about 378 and 458 K. In both cases, amorphous phase and crystalline phases appearing in the first stage show semiconductor behaviour, but the last crystallized phases exhibit metallic behaviour.
R(T) in the Sb 70 Te 30 film has its only sharp transition at about 445 K. Amorphous phase and crystallized phases show semiconductor behaviour. Then, its resistance increases when cooling.
In all the phases showing a semiconductor behaviour, temperature dependence of resistance can be written as the Arrhenius-type equation in eq (1), where E a is the apparent activation energy, k is the Boltzmann's constant and R 0 is a pre-exponential factor. (1) E a values are obtained linearizing eq (1). Figure 2 shows ln(R) vs 1/T plots for each composition with linear fit where possible, corresponding to semiconductor behaviour of existing phases. Arrhenius plots on temperature dependence of resistance show different regions with the characteristic activation energies for the amorphous and crystalline states.
Thermal and electrical parameters in the glass to crystal transitions for films with compositions Ge 13 Sb 5 Te 82 , Ge 1 Sb 2 Te 4 , Ge 2 Sb 2 Te 5 , Ge 1 Sb 4 Te 7 and Sb 70 Te 30 are shown in Table 1. Onset temperature T on1 for the glass-crystal transition in Ge 1 Sb 2 Te 4 , Ge 2 Sb 2 Te 5 , Ge 1 Sb 4 Te 7 thin films are in agreement with previous works 1,11 observing that values are slightly superior in about 15-20 K. T on1 of Ge 1 Sb 2 Te 4 , Ge 2 Sb 2 Te 5 , Ge 1 Sb 4 Te 7 thin films increases with the Ge content in agreement with the bibliography 11 . The value of ratio R g /R c (ratio of amorphous state to crystalline state resistances) for the Ge 2 Sb 2 Te 5 alloy is almost an order of magnitude higher than previous results 3 .
Electrical resistance at room temperature of thin films exhibits a remarkable contrast (~MΩ -~Ω) when measured before and after the thermal treatments due to glass -crystal transition; as we mentioned, this result is an excellent property for phase-change materials. The electrical resistance at room temperature of the amorphous and crystalline phases is shown in Table 1. The changes in the resistance observed in our samples for the amorphous phase are in good accordance with previously known electrical properties of amorphous semiconductors.
X-ray diffractograms of as-deposited films and their crystallization steps at different temperatures are shown in Fig 3. In Figure 1, each red star shows the maximum temperature reached by a sample before an X-ray diffraction experiment was held at room temperature.
X-ray diffractograms of the Ge 13 Sb 5 Te 82 film are shown in Fig 3 (a). X-ray patterns of the PLD as-obtained film are characteristic of a mainly amorphous phase with traces of a crystalline phase. Crystal peaks of small intensity can be associated with either the fcc-Ge 1 Sb 2 Te 4 metastable phase or the fcc-Ge 2 Sb 2 Te 5 metastable phase 17 , as both phases are similar. When that film is heated up to T = 533 K (after the second transition), X-ray patterns show the appearance of two crystal peaks at 2Θ = 25.5° and 28.7° associated with the hcp-Ge 2 Sb 2 Te 5 stable phase 17 although it can also be associated with the hcp-Ge 1 Sb 2 Te 4 stable phase 15 . When it is heated up to T = 593 K (after the third transition), X-ray patterns show the appearance of two crystal peaks at 2Θ = 12.9° and 19.4° associated with the hcp-Ge 1 Sb 2 Te 4 stable phase 15 . It is worth mentioning that the hcp-Te crystalline stable phase is not detected in this film, despite the fact that the hcp-Te phase is observed in as-cast sample and in the crystallization of amorphous samples obtained by rapid cooling from the liquid 8 . The main peaks of hcp-Te phase associated with relative intensity of 16, 100, 36 and 25 located in the angles 2Θ = 23.06°, 27.58°, 38.29° and 40.48° respectively (JCPDS 36-1452) are not observed in the experimental X-ray patterns shown in Figure 3 (a).
X-ray diffractograms of the Ge 1 Sb 2 Te 4 film are shown in Fig 3 (b). X-ray patterns of the PLD as-obtained film are characteristic of a mainly amorphous phase with traces of the fcc-Ge 1 Sb 2 Te 4 metastable crystalline phase 15 . When that film is Figure 1. Temperature dependence of electrical resistance for the Ge 13 Sb 5 Te 82 , Ge 1 Sb 2 Te 4 , Ge 2 Sb 2 Te 5 , Ge 1 Sb 4 Te 7 and Sb 70 Te 30 thin films. Each red star shows the maximum temperature reached by a sample before an X-ray diffraction experiment was held at room temperature. heated up to T = 507 K (after the first transition, T on1 = 420 K), X-ray patterns show the appearance of the hcp-Ge 1 Sb 2 Te 4 stable phase 15 . When it is heated up to T = 644 K, X-ray patterns do not show significant structural changes: only the hcp-Ge 1 Sb 2 Te 4 phase is observed.
X-ray diffractograms of the Ge 2 Sb 2 Te 5 film are shown in Fig 3 (c). X-ray patterns of the PLD as-obtained film are characteristic of a mainly amorphous phase with traces of the fcc-Ge 2 Sb 2 Te 5 metastable crystalline phase 17 . When that film is heated up to T = 454 K (after the first transition, T on1 = 443 K), X-ray patterns show growing of the fcc-Ge 2 Sb 2 Te 5 phase 15,20 and incipient appearance of the hcp-Ge 2 Sb 2 Te 5 stable phase with a peak at 2Θ = 28.65°. When it is heated up to T = 623 K (after the second transition, T on2 = 523 K), X-ray patterns only show one crystalline phase: stable hcp-Ge 2 Sb 2 Te 5 .
X-ray diffractograms of the Ge 1 Sb 4 Te 7 film are shown in Fig 3 (d). X-ray patterns of the PLD as-obtained film are characteristic of a mainly amorphous phase with traces of a phase similar to metastable fcc-Ge 2 Sb 2 Te 5 17 . Despite metastable phases have not been found in previous structural works for the composition Ge 1 Sb 4 Te 7 , Yamada et al reported 17 that the first transformation corresponds to a metastable phase. When that film is heated up to T = 392 K (slightly higher temperature to the first transition, T on1 =378 K) X-ray patterns do not show significant structural changes. When it is heated up to T = 443 K, X-ray patterns show incipient crystallization of the hcp-Ge 1 Sb 4 Te 7 stable phase 19 in coexistence with the metastable phase. X-ray patterns only show one crystalline phase when the film reaches either T = 534 K or T = 588 K (both after the second transition, T on2 = 458 K): the stable hcp-Ge 1 Sb 4 Te 7 .
X-ray diffractograms of the Sb 70 Te 30 film are shown in Fig 3 (e). X-ray patterns of the PLD as-obtained film are characteristic of a mainly amorphous phase with traces of the Sb 72 Te 28 crystalline phase 18 . X-ray patterns only show the Sb 72 Te 28 crystalline phase when the film reaches either T = 525 K or T = 531 K (both above observed transition) 18 .

Conclusions
The activation energies reported in Table 1 for conduction in the amorphous state are the expected values for the chalcogenide amorphous materials 23 , observing that the higher values (close to 0.5 eV) are given for the films of compositions Ge 1 Sb 2 Te 4 , Ge 2 Sb 2 Te 5 and Ge 1 Sb 4 Te 7 . A higher value of activation energy implies a greater variation of the electrical resistance in the amorphous state, that is, a greater decrease in the resistance with temperature. It is also noted that the activation energy for conduction in the crystalline phase is significantly increased except in the Sb 70 Te 30 alloy, where it decreases.
The electrical measurements were made in the configuration of two points on the same side of the film. Taking into account the experimental form that was used, the product of the electrical resistance R multiplied by the thickness e of the film gives an order of magnitude of the electrical resistivity of each state. Table 1 reports the values of the product R.e for the amorphous and crystalline states, observing values between 0.6 and 32 Ωm for the amorphous state and of the order 10 -5 -10 -6 Ωm for the crystalline state.
In view of its application for non-volatile memories, it is interesting to analyze the dimensionless quotients of the crystallization and melting temperatures T on1 /T m , which determines the operating temperature range of the cell, as well as the ratio R g /R c (ratio of amorphous state to crystalline state resistances) at 298 K, which determines the resolution range in the electrical resistance between these states. It is desirable that these dimensionless parameters were the largest possible. In the first, it is intended with a larger value, to achieve greater thermal stability of the amorphous phase, while in the second it is desired to have the greatest range of values of resistance that allows to differentiate both states. Taking this into account, it is observed in Table 1, that the Ge 13 Sb 5 Te 82 alloy has the highest T on1 /T m , followed by the Sb 70 Te 30 alloy. These two alloys also have the lowest activation energy for electrical conduction in the amorphous state. With respect to the dimensionless resistance parameter R g /R c , the alloys that present the best results are those of compositions Ge 13 Sb 5 Te 82 and Ge 2 Sb 2 Te 5 with values about 3·10 6 .