CN109935685B - Method for regulating and controlling vacancy defects in material - Google Patents
Method for regulating and controlling vacancy defects in material Download PDFInfo
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- CN109935685B CN109935685B CN201910095442.1A CN201910095442A CN109935685B CN 109935685 B CN109935685 B CN 109935685B CN 201910095442 A CN201910095442 A CN 201910095442A CN 109935685 B CN109935685 B CN 109935685B
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Abstract
The invention discloses a method for regulating and controlling vacancy defects in a material, which comprises the steps of inserting a stress applying material into a phase-change functional material to regulate and control the concentration of the vacancy defects in the phase-change functional material, thereby obtaining a composite structure generated by the embedding growth of the phase-change functional material and the stress applying material; the phase change characteristic of the composite structure is mainly determined by the phase change functional material, the phase change characteristic of the composite structure is changed based on the phase change characteristic of a phase change functional material crystal obtained from the pure phase change functional material through the regulation and control of the stress applying material, and the stress applying material is specifically a material capable of forming a crystalline state. Particularly, by inserting a stress applying material with a slightly larger constant into a phase change functional material with a slightly smaller lattice constant, the concentration of vacancy defects in the phase change storage material can be effectively reduced, so that the threshold value of the crystallization process is reduced, and the crystallization speed of the phase change storage material is effectively improved.
Description
Technical Field
The invention belongs to the field of microelectronic materials, and particularly relates to a method for regulating and controlling vacancy defects in a material.
Background
Phase change memory materials have attracted considerable attention because they can rapidly switch between a low resistance state and a high resistance state by applying an electric or optical pulse, the process of switching from high resistance to low resistance being referred to as the SET process and the reverse process being referred to as the RESET process. Phase change material based memory technology is considered to be one of the strong competitors to the next generation memory technology. The phase change memory device has the advantages of high erasing speed, large difference of two states, compatibility with the existing Complementary Metal Oxide Semiconductor (CMOS) process and the like.
Vacancies, an important defect in phase change memory materials, have been the subject of much attention. At present, research methods thereof are divided into two categories of first-principle simulation calculation and experiments. The experimental methods include a high-angle annular dark field-scanning transmission electron microscope (HAADF-STEM), an extended X-ray absorption fine structure (EXAFS), a positron annihilation spectrum (PILS), an electron spin response spectrum (EPR), and the like. However, the conventional vacancy characterization means belong to passive research methods, i.e., research is based on effective experimental means for actively regulating and controlling vacancies after the vacancies are formed, which is still deficient at present.
Related researchIt was found that the presence or absence of vacancies greatly affects the properties of the phase change material. On one hand, the existence of the vacancy effectively reduces the energy barrier of atom migration, and may play an important role in reducing the power consumption of the phase change material (Deringer V L, Lumeij M, Stoffel R P, et al]Chemistry of Materials,2013,25(11): 2220-. On the other hand, the existence of voids can form voids during repeated erasing and writing of the phase-change memory material, which can cause the performance failure of the phase-change memory material, and this may be an important factor limiting the cyclic erasing and writing capability of the phase-change material (Njoroge W K,
H W,WuttigM.Density changes upon crystallization of Ge2Sb2.04Te4.74films[J].Journal ofVacuum Science&Technology A:Vacuum,Surfaces,and Films,2002,20(1):230-233.)。
therefore, a realization method for regulating the vacancy concentration is developed as soon as possible, the generation of the vacancies is actively promoted or inhibited according to the differentiated requirements on the performance of the phase-change storage material, and the modification of the phase-change storage material is further realized, so that the improvement on the performance of the phase-change storage and the industrialization of the phase-change storage are of great significance.
Disclosure of Invention
In view of the above defects or improvement needs of the prior art, an object of the present invention is to provide a method for adjusting and controlling vacancy defects in a material, which specifically utilizes the degree of lattice mismatch to achieve different lattice stresses, thereby adjusting and controlling the concentration of vacancy defects in the material, specifically utilizes the mismatch between lattice constants of two materials to generate tensile stress or compressive stress in the material, thereby correspondingly reducing or increasing the atomic stacking density of the material, and further inhibiting or promoting the generation of vacancy defects in the material. In addition, the invention particularly inserts the stress applying material with slightly larger lattice constant into the phase change functional material with slightly smaller lattice constant to increase the energy required by the formation of the vacancy defect, thereby reducing the concentration of the vacancy defect in the phase change storage material, further reducing the threshold value of the crystallization process and effectively improving the crystallization speed of the phase change storage material.
In order to achieve the above object, according to one aspect of the present invention, there is provided a method for regulating the concentration of vacancy defects in a phase change material based on lattice stress, wherein the method comprises inserting a stress applying material into a phase change functional material to regulate the concentration of vacancy defects in the phase change functional material, thereby obtaining a composite structure formed by the mosaic growth of the phase change functional material and the stress applying material; the phase change characteristic of the composite structure is mainly determined by the phase change functional material, the phase change characteristic of the composite structure is changed based on the phase change characteristic of a phase change functional material crystal obtained from the pure phase change functional material through the regulation and control of the stress applying material, and the stress applying material is specifically a material capable of forming a crystalline state.
As a further preferred aspect of the present invention, when the phase change functional material and the stress applying material are crystalline at the same time, there is lattice epitaxial contact of the two materials at the interface between the phase change functional material and the stress applying material;
and for the phase change functional material and the stress applying material, the absolute value of the lattice mismatch ratio between the two types of crystals of the phase change functional material obtained from the pure phase change functional material and the stress applying material obtained from the pure stress applying material is between 0.1% and 10%;
preferably, the lattice constant of the phase change functional material crystal obtained from the pure phase change functional material is smaller than the lattice constant of the stress applying material crystal obtained from the pure stress applying material.
In a further preferred embodiment of the present invention, the insertion of the stress applying material into the phase change functional material is specifically to insert a stress applying layer made of the stress applying material into a phase change functional layer made of the phase change functional material, thereby obtaining a multilayer film structure in which the phase change functional layer and the stress applying layer are alternately grown.
As a further preference of the present invention, the multilayer film structure is a periodic multilayer film structure; any period of the multilayer film structure simultaneously comprises a phase change functional layer and a stress applying layer, wherein the thickness ratio of the phase change functional layer to the stress applying layer meets 1: 10-10: 1; the thickness of any period satisfies 2-10nm, and the total period number of the whole multilayer film structure satisfies 5-100.
In a further preferred embodiment of the present invention, the multilayer film structure is formed to have different concentrations of vacancy defects by adjusting the stress applied to the phase change functional layer by adjusting the thickness of the stress application layer, and further adjusting the formation energy of vacancy defects.
As a further preferred aspect of the present invention, the phase change functional material is a microscopic non-layered phase change material having no van der waals gap between atomic layers, and the stress application material is a microscopic layered material having van der waals gap between atomic layers;
preferably, the phase change functional material is an intrinsic or element-doped simple substance material or a compound material, wherein the simple substance material is an Sb simple substance, and the compound material is a compound formed by Ge and Te, a compound formed by Ge and Sb, a compound formed by Ge, Sb and Te, a compound formed by Ge, Bi and Te, or a compound formed by Ge, Sb, Bi and Te; the doped element is at least one of C, Cu, N, O, Si, Sc and Ti; the stress applying material is a compound formed by Sb and Te, a compound formed by Bi and Te, a compound formed by Ge and Se, a compound formed by In and Se, or a compound formed by Mo and S;
more preferably, the phase change functional material is GeTe, GeSb, Ge2Sb2Te5Or Ge1Sb2Te4(ii) a The stress applying material is Sb2Te3Or Bi2Te3。
According to another aspect of the present invention, the present invention provides a composite structure obtained by using the above method for regulating the concentration of vacancy defects in a phase change material based on lattice stress, wherein the phase change material and the stress applying material are in a mosaic distribution in the composite structure.
As a further preferred aspect of the present invention, the stress applying material is crystalline.
According to still another aspect of the present invention, there is provided a phase change memory using the above composite structure.
According to a further aspect of the invention, there is provided a method of preparing a composite structure as described above, characterised in that the preparation method is based in particular on magnetron sputtering, molecular layer deposition, molecular beam epitaxy, pulsed laser deposition, physical vapour deposition, chemical vapour deposition, thermal evaporation or electrochemical growth.
Compared with the prior art, the technical scheme provided by the invention has the advantages that the stress applying material is inserted into the phase change functional material, so that the stress of the phase change functional material is regulated and controlled, and the concentration of vacancy defects in the phase change functional material is finally regulated and controlled. And based on the method for regulating and controlling the vacancy concentration by realizing different lattice stresses, the forming energy of vacancy defects can be regulated by regulating the proportion of the phase-change functional material and the stress applying material, so that different vacancy concentrations are formed.
The lattice constant mismatch ratio between the phase change functional material and the stress applying material is not too large, and the lattice mismatch ratio is defined as:
wherein b is the in-plane lattice constant of the stress applying material, and a is the in-plane lattice constant of the phase change functional material. The absolute value of the lattice mismatch between the two materials is preferably between 0.1% and 10% to ensure that epitaxial coupling between the two lattices occurs more readily, thereby ensuring that the phase change functional material and the stress exerting material should have lattice epitaxial-type contact at the interface (i.e. the two materials are nested together), rather than being all grain boundary contact.
Taking the example of alternately growing the phase change functional layer material and the stress applying layer material into a stress multilayer film structure, the structure is a multilayer film structure obtained by using at least two types of materials which have different lattice constants and alternately grow; the phase change functional layer is marked as A (the layer is simultaneously a stress receiving layer) and the stress applying layer is marked as B, and the invention can be preferably compounded to form a periodic stress multilayer film structure; also, for the periodic structure, it may be preferable that the thickness of the (A + B) layer may be controlled to be between 2-10nm, the number of repeated periods of the (A + B) layer may be controlled to be between 5-100, wherein the thickness ratio of the A layer to the B layer may be controlled to be between 1:10 and 10:1 within one (A + B) period. The stress of the B layer on the A layer can be adjusted by adjusting the thickness of the B layer, and the formation energy of vacancy defects can be adjusted, so that different vacancy concentrations can be formed.
According to the invention, a microscopic non-layered phase change material is preferably adopted as a phase change functional material (the microscopic structure of the microscopic non-layered phase change material belongs to a non-layered structure, namely, Van der Waals gaps do not exist between atomic layers), and the microscopic layered material is adopted as a stress applying material (the microscopic structure of the microscopic layered material belongs to a layered structure, namely, Van der Waals gaps exist between partial atomic layers), so that compared with the microscopic layered material, the stress borne by the microscopic non-layered phase change material is not easy to relax; and compared with the microscopic non-layered material, the microscopic layered material is easy to relax due to the van der Waals gap stress, so that the constant of the lattice constant is kept, and further, the stable stress can be provided.
The method can be applied to a phase change memory to regulate and control the vacancy concentration of a phase change functional layer so as to realize the application of modification; for example, the multilayer film structure can be applied to a phase change memory to regulate and control the vacancy concentration of a phase change functional layer, so that the modification purpose is realized. The phase change function of the composite structure is mainly determined by the phase change functional material, and the stress applying material only plays a role in regulation and control; taking the multilayer film structure as an example, preferably, the stress applying layer may be easier to change phase than the phase change functional layer when selecting the material, and it forms a crystalline phase first (for example, the phase change temperature of the stress applying material may be lower than the phase change temperature of the phase change functional layer; that is, in the composite structure obtained by the present invention, the stress applying material may be crystalline or amorphous, but it is preferentially converted into crystalline phase compared with the phase change functional material in the phase change process), so that the stress is applied to the phase change functional layer through lattice matching when the phase change functional layer changes phase, thereby changing the concentration of defects in the phase change functional layer.
In addition, taking the case that the phase change functional layer material and the stress applying layer material are alternately grown into a stress multilayer film structure, and the lattice constant of the stress applying layer material is larger than the in-plane lattice constant of the functional layer material, the phase change functional layer material is applied with tensile stress by utilizing lattice mismatch between the two materials, so that the formation energy of the vacancy is increased, the concentration of the vacancy in the phase change material is reduced, and the phase change characteristics of the material and the device are improved. First principles calculations indicate that the concentration of vacancies in the functional layer can be reduced to a fraction 1406 of the original vacancy concentration by applying a tensile stress. The test results of X-ray diffraction and transmission electron microscope prove that in practical production, the stress can be generated by depositing and then annealing the multilayer film. By applying a tensile stress, the SET speed of the phase change material GeTe can be increased from 58ns to 12 ns. Because the read-write speed of the phase change memory is mainly determined by the SET speed, the method for regulating the vacancy by stress can effectively improve the read-write speed of the phase change memory, simultaneously reduce the power consumption of the SET process of the phase change memory and has important significance on future industrialization roads.
In conclusion, the invention can obtain the following beneficial effects:
the method for generating the stress with different sizes is simple and easy to implement, mature in process and easy to be compatible with the existing micro-nano processing process. And the stress is flexibly regulated and controlled, so that the active regulation and control of the concentration of the hollow site in the material are conveniently realized. The vacancy concentration can be effectively reduced by applying tensile stress on the phase change functional layer, so that the speed of the SET process is greatly increased. Provides another solution for further increasing the erasing speed of the phase change memory.
Compared with the existing vacancy regulation and research method, the method has two obvious advantages: (1) the method for regulating the vacancy concentration can realize active regulation of the vacancy concentration. (2) The stress applying layer (abbreviated as B layer) introduced by regulation is positioned outside the phase change functional layer (abbreviated as A layer), plays a role through a crystal boundary without entering the A layer, and the B layer only plays a role in regulating and controlling the phase change process, and the phase change process is still mainly determined by the material of the A layer (namely, the phase change characteristic of the obtained composite structure is based on the phase change characteristic of a phase change functional material crystal obtained from a pure phase change functional material, and the phase change characteristic of the composite structure generates slight change through the regulation and control of the stress applying material), so that the research process is greatly simplified. The invention enables the research on the vacancy in the phase-change material to be more convenient and faster and the result to be easier to analyze.
The lattice constant of the material is increased by utilizing the tensile stress, so that the vacancy concentration is reduced, the threshold value of the SET process of the phase change material is further reduced, and the SET operation speed of the phase change memory is increased finally. The stress regulation and control method provides a new idea for improving the erasing speed of the phase change memory.
Drawings
Fig. 1 is a schematic side view of a phase change multilayer film structure according to the present invention.
Fig. 2 is a result of an X-ray diffraction (XRD) test of the phase-change multilayer film described in the present invention.
Fig. 3 (a) is a result of a spherical Aberration Correction Transmission Electron Microscope (ACTEM) test of the phase-change multilayer film described in the present invention, and (b) is a result of an HAADF-TEM test at the interface of the phase-change multilayer film.
Fig. 4 (a) shows a model of a lattice structure of a multilayer film in the present invention in which the thickness of the a layer/the thickness of the B layer are 1:0, 1:1, 1:2, and 1:3, respectively, and (B) shows the results of simulation calculation of stress and vacancy concentration corresponding to different thickness ratios.
FIG. 5 is a schematic diagram of a process for fabricating a "T" type structure of a phase change memory cell.
Fig. 6 shows the SET speed test results after a multi-layer film of different vacancy concentrations is fabricated into a phase change memory cell.
The meanings of the reference symbols in the figures are as follows: 1 is Si substrate, 2a is thermally grown SiO2The layer 2b is a physical vapor deposition insulating layer, 3 is a stress applying layer, 4 is a phase change functional layer, 5a is a lower electrode, and 5b is an upper electrode.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
The invention provides a method for regulating the concentration of vacancy defects in a material and realizing different lattice stresses by using lattice stress, aiming at generating tensile stress or compressive stress in the material by using the mismatch between lattice constants of two materials, so that the atom stacking density of the material is correspondingly reduced or increased, and the generation of the vacancy defects in the material is further inhibited or promoted. The method can be realized based on the existing magnetron sputtering method, molecular layer deposition method, molecular beam epitaxy method, pulse laser deposition method, physical vapor deposition method, chemical vapor deposition method, thermal evaporation method or electrochemical growth method.
For example, the material structure is a multilayer film structure which has different lattice constants and is alternately grown. One of the two different films is a phase change functional layer A and is a stress receiving layer at the same time. The other layer is a stress applying layer B which only has a regulating and controlling effect on the phase change process, and the phase change characteristic of the multilayer film structure is mainly determined by the phase change functional material in the multilayer film structure.
The thickness of the (A + B) layer should be between 2-10nm and the periodicity of the (A + B) layer should be between 5-100. Preferably, the A, B two materials should have a lattice epitaxy type contact at the interface, rather than all grain boundary contacts. Therefore, the lattice constant mismatch between the a and B layer materials should not be too large to ensure that coupling between the two lattices is easier to occur. Preferably, the lattice mismatch between the two materials should be between 0.1% and 10%.
The invention provides a method for generating different stress magnitudes, namely, the stress of different magnitudes is generated on a functional layer A by regulating and controlling the thickness of a stress applying layer B. Preferably, the thickness ratio of the a layer to the B layer should be between 1:10 and 10:1 within one period.
The invention provides a preparation method of the multilayer film with stress, which is a method of magnetron sputtering, molecular layer deposition, molecular beam epitaxy, pulsed laser deposition, physical vapor deposition, chemical vapor deposition, thermal evaporation, electrochemical growth and the like. Taking a pulse laser deposition method as an example, the pulse laser deposition method can be specifically a method of alternately rotating a target material used by a phase change functional material and a stress applying material to form a multilayer film structure in which a phase change functional layer and the stress applying layer alternately grow on a substrate; the thickness of each phase change functional layer and each stress applying layer can be controlled by controlling the number of pulse lasers striking the corresponding target material, and the periodicity of the multilayer film is controlled by the periodicity of the alternate rotation of the two target materials.
The method for generating the stress with different sizes is simple and easy to implement, mature in process and easy to be compatible with the existing micro-nano processing process. And the stress is flexibly regulated and controlled, so that the active regulation and control of the concentration of the hollow site in the material are conveniently realized. The vacancy concentration can be effectively reduced by applying tensile stress on the phase change functional layer, so that the speed of the SET process is greatly increased. Provides another solution for further increasing the erasing speed of the phase change memory.
The following are examples:
example 1
The multilayer phase change stress film prepared in this example has a structure of [ A ]mBn]lWherein m and n respectively represent the thickness of A, B, the unit is considered as nm (nanometer), and l is the period number of the multilayer film alternate growth. 1/10<m/n<10/1, and 2<m+n<10，5<l<100, m and n are real numbers, and l is an integer. In this embodiment, A is GeTe and B is Sb2Te3M/n are 4/0, 2/2, 1.3/2.7 and 1/3 respectively, and l is 12. GeTe is a microscopic, non-layered material without van der waals gaps. Sb2Te3The phase change material is a microscopic layered material containing Van der Waals gaps, the crystallization temperature of the microscopic layered material is lower than that of GeTe, and the microscopic layered material can crystallize before GeTe in the phase change process. Therefore, the crystallization properties (such as SET speed) of the multilayer film are mainly determined by the slower crystallization speed of GeTe. Sb2Te3Although the crystallization of (A) is not critical to the crystallization process of the multilayer thin film, it can be applied to the crystallization process of GeTeThe superlattice matching applies stress to the GeTe to change the microstructure of the GeTe layer, which is specifically shown as changing the concentration of vacancy defects in the GeTe layer. The multilayer phase change stress film is prepared by a laser pulse deposition method and then is annealed by an annealing furnace to be promoted to be converted into a crystalline state. XRD and TEM tests were then used to explore its internal stresses. Finally, the difference of vacancy concentration caused by different vacancy forming energy caused by different stresses is calculated by utilizing the first principle simulation. The specific implementation method comprises the following steps:
as shown in FIG. 1, a 500 μm thick, (100) oriented Si wafer (layer 1) is selected, the surface of which has been thermally grown to form 1 μm thick SiO2Film (layer 2 a). So that it is selected to have SiO2The Si sheet of the layer is due to SiO2The silicon wafer is cut into the size of 1cm × 1cm and then put into a beaker, an appropriate amount of acetone is injected, the silicon wafer is ultrasonically cleaned for 10 minutes, the silicon wafer is cleaned for 10 minutes by absolute ethyl alcohol after the cleaning is finished, finally the silicon wafer is cleaned for ten minutes by deionized water, and the silicon wafer is blow-dried by a nitrogen gun for standby.
(2) Adhering the cleaned silicon wafer to a heater of a laser pulse deposition system by using double-sided adhesive tape, and adhering GeTe and Sb with the purity of 99.99 percent2Te3The target material is put into the cavity. The laser mode of the laser pulse deposition system is EGY NGR, and the energy is 250 mJ. The stress applying layer 3 (Sb in the present embodiment) is rapidly applied by bombarding the target material with laser2Te3) Alternating with the phase change functional layer 4 (GeTe in this embodiment) is deposited on the silicon wafer. The thickness of each material is controlled by the number of laser pulses striking the target. For GeTe and Sb in the present example2Te3Target, the deposition rate parameters of which are as follows:
GeTe, laser energy: 250mJ, laser frequency: 5Hz, target base distance: 78mm, rate 283 pulse/nm.
②Sb2Te3Laser energy: 250mJ, laser frequency: 5Hz, target base distance: 78mm, rate 167 pulse/nm.
The laser scanning areas of the two targets are set as follows: 15mm × 1mm, laser spot size: 2.1mm, 1mm, initial offset of laser spot position: 3mm, 2mm, scan rate: 1 mm/s.
(3) And (3) putting the deposited films with different components into an annealing furnace for annealing treatment, wherein the annealing temperature is 250 ℃, the heat preservation time is 1 hour, and the heating and cooling rates are both 10 ℃/min. The purpose of annealing is to promote the multi-layer phase-change film to be transformed into the crystalline state, and stress is generated due to lattice mismatch after the multi-layer film is transformed into the crystalline state.
(4) Mixing multiple crystalline GeTe/Sb2Te3The film is placed into an X-ray diffractometer for testing, and the angle scanning range is as follows: 5-60 degrees and the scanning speed is 5/min. The XRD test results are shown in fig. 2.
(5) XRD tests can only indirectly reflect the microscopic phase of the material, so that the microstructure of the multilayer film is further directly observed by using TEM. In TEM observation, this example is represented by [ (GeTe)4(Sb2Te3)4]12The phase change multilayer film structure is exemplified. First, the component pair [ (GeTe)4(Sb2Te3)4]12The phase-change multilayer thin film of (1) is sampled by using a focused ion beam, and after the sampling is successfully performed, the ACTEM is used for preliminarily observing crystal grains in the multilayer thin film, as shown in (a) of fig. 3. In order to further observe the lattice coupling relationship between the functional layer and the stress applying layer, the interface between the two material lattices was observed by HAADF-TEM, and the result is shown in fig. 3 (b).
(6) First principles of computation
GeTe/Sb of different components by using Materials Studio software2Te3The multilayer film structure was modeled, and the lattice model was as shown in fig. 4 (a). The structure is optimized by using vasop software to obtain important parameters such as vacancy forming energy and concentration thereof, and the correlation calculation result is shown in (b) of fig. 4.
Using first principle to pair GeTe and Sb2Te3After respective structural optimization, GeTe has an in-plane lattice constant of
And Sb2Te3Has an in-plane lattice constant of
The lattice mismatch of the two is 2.36% (i.e., 100% × (4.33-4.23)/4.23.) GeTe is under tensile stress when the lattices of the two are epitaxially grown together, while Sb is2Te3Will be subjected to compressive stress. In the test result of XRD, the diffraction peaks are substantially all Sb2Te3The (00z) direction of (b) indicates that the crystal grains of the multilayer film produced have better spatial orientation. Sb2Te3The two-dimensional material has a lattice structure formed by stacking five layers in an out-of-plane direction, and van der Waals gaps are formed between the five layers. If Sb is present2Te3If it is subjected to compressive stress in the in-plane direction, it must extend in the out-of-plane direction. This is precisely confirmed by XRD test results. The principle of XRD testing is bragg diffraction, i.e.:
2dsinθ＝nλ (2)
wherein d is the interplanar spacing, theta is the diffraction angle, n is an integer, and lambda is the X-ray wavelength.
And Sb2Te3The number of atomic layers M in each five-layer structure is obtained by the following formula:
in the formula, d (00z) is the interplanar spacing between different crystal planes, and can be obtained from the Bragg diffraction formula, and (00z) is the corresponding crystal plane.
From the results of XRD test, it can be seen that M gradually increases with the increase of GeTe composition, indicating that Sb2Te3The stress in the surface of the GeTe material is increased gradually with the increase of the GeTe composition. I.e. GeTe with Sb2Te3The stress on the alloy is gradually reduced along with Sb2Te3The increase in thickness places an increase in tensile stress.
In the TEM test, GeTe/Sb is known from (a) in FIG. 32Te3The size of crystal grains in the multilayer film is 10nm-11About nm. The grain size is far beyond that of single-layer GeTe or Sb2Te3Thickness of GeTe and Sb2Te3The presence of stress is evidenced in another aspect by the fact that the grains are collectively formed (i.e., a lattice epitaxy-like contact is formed) after epitaxial growth. The enlarged area in the white rectangular frame in fig. 3 (a) results in fig. 3 (b), and the test result of atomic resolution in fig. 3 (b) further indicates the existence of the stress due to lattice mismatch.
Within the elastic limit of a material, the stress versus strain relationship is defined by the following equation:
σ＝E (4)
where σ is stress, E is Young's modulus, and strain. Since stress is proportional to strain, and only GeTe/Sb of different compositions are of interest in this example2Te3The relative value of the stress between the films, so the magnitude of the strain of the GeTe lattice can be used to represent the magnitude of the stress in this embodiment in a simplified manner.
The first principle calculation shows that when GeTe/Sb2Te3The stress application layer Sb having a thickness ratio of 4/0, 2/2, 1.3/2.7 and 1/3, respectively2Te3The lattice tensile strain caused to the functional layer GeTe was 0%, 0.84%, 1.3%, and 1.51%, respectively. Tensile straintIs defined by the following equation:
in formula (II), a'GeTeIn-plane lattice constant of GeTe subjected to tensile stress after formation of multilayer film structure, aGeTeIs the in-plane lattice constant of GeTe without tensile stress.
Then, under the action of different tensile stresses, the formation energy of Ge vacancies in GeTe is calculated. Ge vacancy formation can be calculated from the following equation:
in the formula, EfFor vacancy forming energy, EtotFor total system energy, VGeTe is GeTe with vacant sites, niμiRespectively representing the number of atoms lost by the vacancy-containing GeTe and the chemical potential of the corresponding atom.
The calculation results are shown in the following table:
TABLE 1 tensile stress (measured as strain) versus Ge vacancy forming energy
Further, the concentration of Ge vacancies which can be caused by different Ge vacancy forming energies is calculated according to the following formula
Where N is the total number of Ge sites, N is the number of Ge vacancies, EvIs the energy required to move a Ge atom from an internal lattice point to a surface lattice point of the lattice. k is a radical ofBBoltzmann constant. T is the temperature. Likewise, only GeTe/Sb of different compositions are of interest in this example2Te3Relative value of the concentration of vacancies in the film. So for comparison purposes, the minimum value of the vacancy concentration (GeTe/Sb) can be used here2Te3Vacancy concentration value at 1nm/3 nm) was normalized as a reference value. The results of the different calculations are shown in the following table:
table 2 corresponding tensile stress (measured as strain) to Ge vacancy concentration
Example 2
In this embodiment, a multi-layer stress film structure is prepared as a phase change unit and subjected to a SET speed test. Multilayer film [ A ]mBn]lWherein A is still GeTe and B is still Sb2Te3GeTe and Sb2Te3The ratio of the thicknesses of (a) to (b) is still: 4/0, 2/2, 1.3/2.7, 1/3, the number of cycles is still fixed at 12. The preparation process of the multilayer film structure phase change unit is as follows:
(1) as shown in (a) of fig. 5, a layer 5a of the lower electrode is first deposited on the substrate composed of the layer 1 and the layer 2a by magnetron sputtering, and the layer 5a is made of inert electrode material such as Pt, TiW, Ta, etc.
(2) Depositing an insulating layer 2b on the 5a layer by physical vapor deposition (PECVD), wherein the 2b layer is SiO2、SiC、Al2O3Etc. insulating material.
(3) A photoresist mask with a circular aperture of 250nm diameter was formed on the insulating layer 2b using an electron beam exposure system (EBL).
(4) The insulating layer 2b is etched using a plasma etching technique (ICP), the insulating layer 2b at the position of the pinhole is exposed and thus etched away, and the other portion of the insulating layer 2b is masked by the photoresist and thus not etched. Etching is performed until the 2b layer is etched through to expose the lower electrode 5 a. After the etching is completed, the photoresist is removed by using the photoresist remover, and finally the final effect shown in (a) of fig. 5 is obtained.
(5) And (3) utilizing an ultraviolet lithography system to etch a square hole structure with the size of 100 microns multiplied by 100 microns on the small hole. The square hole is aligned with the center of the circular small hole etched by the ICP.
(6) Depositing 4 GeTe/Sb with different stresses in the square hole by using a laser pulse deposition system2Te3The specific procedure of the multilayer film was the same as in (2) in example 1. After the deposition of the multilayer film structure is finished, depositing an upper electrode layer 5b on the multilayer film by using a magnetron sputtering system, wherein the upper electrode layer is made of inert electrode materials such as Pt, TiW, Ta and the like.
(7) The photoresist of the ultraviolet photoetching is removed by utilizing a stripping process, so that GeTe/Sb among different phase change units is removed2Te3Multiple layers of films and upper electrode layer 5b to make different phase change unitsThey are separated from each other, and the final effect diagram is shown in fig. 5 (b).
The B1500a semiconductor characteristic tester was used to test 4 kinds of multilayer film phase change cells with different stresses, the main test content is the SET speed, and the test result is shown in fig. 6.
Due to Sb2Te3Acting only on the phase-change process (Sb)2Te3Compared with GeTe, the SET speed is higher; and when the two materials form a multilayer film composite structure, the SET process of the multilayer film needs the two materials to reach a crystallization state at the same time to be completed, so the SET speed of the multilayer film is mainly determined by GeTe with a slower SET speed), and only tensile stress is provided, so the SET speed of different phase change units is different mainly due to different Ge vacancy concentrations of the GeTe after the GeTe is stressed. As can be seen from fig. 6, the SET speed is faster and faster as the tensile stress of GeTe increases, i.e., Ge vacancies in GeTe decrease. The specific results are shown in the following table:
TABLE 3 tensile stress (measured as strain) versus SET velocity for phase change cells
Since the read-write speed of the phase change memory is mainly determined by the SET speed, the embodiment has important significance in improving the read-write speed of the phase change memory and reducing the power consumption.
In the above embodiments, the planar stacking structure of the multilayer film structure is taken as an example, and the stress applying material may be inserted into the phase change functional material in other shapes, for example, in a core-shell structure or a randomly distributed insertion manner.
Besides the specific materials used in the above embodiments, the phase change functional material in the present invention may also be a simple substance of Sb, a compound formed by Ge and Te, a compound formed by Ge and Sb, a compound formed by Ge, Sb and Te, a compound formed by Ge, Bi and Te, or a compound formed by Ge, Sb, Bi and Te; the above compounds can also be freely combined with doping elements of C, Cu, N, O, Si, Sc and Ti. The stress applying material may be a compound of Sb and Te, a compound of Bi and Te, a compound of Ge and Se, a compound of In and Se, or a compound of Mo and S.
In addition, in practical application, in the method for regulating and controlling the concentration of vacancy defects in the phase change material based on lattice stress, a composite structure in which the stress applying material is crystalline can be formed by in-situ heating, and of course, a composite structure in which the stress applying material is amorphous can also be formed without in-situ heating. In the amorphous composite structure, the stress application layer B is preferably lower in crystallization temperature than the phase change functional layer a, so that the stress application layer B is preferentially converted into a crystalline state in the process of applying a temperature field or an electric field, thereby generating a stress regulation effect in the subsequent phase change process of the phase change functional layer a.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (9)
1. A method for regulating and controlling the concentration of vacancy defects in a phase-change material based on lattice stress is characterized in that a stress applying material is inserted into a phase-change functional material, so that the concentration of the vacancy defects in the phase-change functional material is regulated and controlled, and a composite structure generated by the embedding growth of the phase-change functional material and the stress applying material is obtained; the phase change characteristic of the composite structure is mainly determined by the phase change functional material, the phase change characteristic of the composite structure is changed based on the phase change characteristic of a phase change functional material crystal obtained from the pure phase change functional material through the regulation and control of the stress applying material, and the stress applying material is specifically a material capable of forming a crystalline state.
2. The method for modulating the concentration of vacancy defects in a phase change material based on lattice stress of claim 1, wherein when the phase change functional material and the stress applying material are crystalline at the same time, there is lattice epitaxial contact of the phase change functional material and the stress applying material at the interface where the two materials interface;
and, for the phase change functional material and the stress applying material, an absolute value of lattice mismatch between a phase change functional material crystal obtained from the pure phase change functional material and a stress applying material crystal obtained from the pure stress applying material is between 0.1% and 10%.
3. The method for modulating the concentration of vacancy defects in a phase change material based on lattice stress of claim 2, wherein the crystal lattice constant of the phase change functional material obtained from the phase change functional material is smaller than the crystal lattice constant of the stress applying material obtained from the stress applying material.
4. The method for regulating and controlling the concentration of vacancy defects in phase change materials based on lattice stress as claimed in claim 1, wherein the step of inserting stress applying materials into the phase change functional materials is to insert stress applying layers formed by the stress applying materials into the phase change functional layers formed by the phase change functional materials, so as to obtain a multilayer film structure formed by the phase change functional layers and the stress applying layers through alternate growth.
5. The method for modulating the concentration of vacancy defects in a phase change material based on lattice stress of claim 4, wherein the multilayer film structure is a periodic multilayer film structure; any period of the multilayer film structure simultaneously comprises a phase change functional layer and a stress applying layer, wherein the thickness ratio of the phase change functional layer to the stress applying layer meets 1: 10-10: 1; the thickness of any period satisfies 2-10nm, and the total period number of the whole multilayer film structure satisfies 5-100.
6. The method for regulating and controlling the concentration of the vacancy defects in the phase change material based on the lattice stress as claimed in claim 5, wherein for the multilayer film structure, the stress on the phase change functional layer is regulated by regulating the thickness of the stress applying layer, and further the formation energy of the vacancy defects is regulated, so that different concentrations of the vacancy defects are formed.
7. The method for modulating the concentration of vacancy defects in a phase change material based on lattice stress as claimed in claim 1, wherein the phase change functional material is a micro non-layered phase change material having no van der waals gap between atomic layers, and the stress applying material is a micro layered material having van der waals gap between atomic layers.
8. The method for regulating and controlling the concentration of vacancy defects in phase change material based on lattice stress as claimed in claim 7, wherein the phase change functional material is an intrinsic or element-doped elemental material or a compound material, wherein the elemental material is a simple substance of Sb, the compound material is a compound formed by Ge and Te, a compound formed by Ge and Sb, a compound formed by Ge, Sb and Te, a compound formed by Ge, Bi and Te, or a compound formed by Ge, Sb, Bi and Te; the doped element is at least one of C, Cu, N, O, Si, Sc and Ti; the stress applying material is a compound formed by Sb and Te, a compound formed by Bi and Te, a compound formed by Ge and Se, a compound formed by In and Se, or a compound formed by Mo and S.
9. The method for regulating and controlling the concentration of vacancy defects in phase change material based on lattice stress as claimed in claim 8, wherein the phase change functional material is GeTe, GeSb, Ge2Sb2Te5Or Ge1Sb2Te4(ii) a The stress applying material is Sb2Te3Or Bi2Te3。
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