This application is a continuation-in-part of copending application Serial No. 08/398 419 filed March 2, 1995.

CONTRACTURAL ORIGIN OF THE INVENTION

This invention was made with Government support under grant Number: DMR-9120521 awarded by the National Science Foundation and grant Number DE-FG02-85-ER45209 awarded by the Department of Energy. The Government may have certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to metalorganic deposited, thin films of perovskite oxides, such as barium titanate, strontium titanate, and their solid solutions, doped in-situ during deposition to improve dielectric properties by control of film electrical resistivity and resistance to reduction.

BACKGROUND OF THE INVENTION

As a result of their exceptional dielectric and other properties, the perovskite oxides barium titanate (BaTi03), strontium titanate (SrTi03), and their solid solutions (Bal-xSrxTi03) are promising candidates for numerous electrical device applications, including but not limited to, dynamic random access memories (DRAM's), ferroelectric random access memories (FRAM's), and insulating layers in high TsubC superconductor multilayer devices. For use in these applications, dielectric thin films are needed possessing low defect densities and high electrical resistivities (10 ¹³ to 10 ¹⁴ Ohm-centimeter) for minimizing electrical charge leakage, dc electrical degradation, and ferroelectric fatigue which is the decrease of remanent polarization with increasing switching cycles as described by

Chen et al. in J. Appl. Phys. 76, (9), 1 November 1994).

Highly insulating thin films of BaTi03, SrTi03, and Bal-xSrxTi03 with high dielectric constants have been made in the past by fully oxygenating the films during or after deposition. These thin films, however, are prone to accidental donor doping and to reduction during subsequent processing steps. Furthermore, dc electrical degradation and ferroelectric fatigue remain serious obstacles to the implementation of practical electrical device using these films.

Copending application Serial No. 08/398 419 filed March 2, 1995, describes metalorganic chemical vapor deposition of doped barium titanate thin films using a fluorinated Ba precursor such as Ba(hfa)2tetraglyme (hfa = hexafluoroacetylacetonate) that allows a substantial decrease in the operating temperature of the metalorganic vapor pressure source and process simplification.

Because the Ba source possesses fluorine, water must be introduced to the coating chamber during deposition to assist in fluorine removal. The accidental incorporation of fluorine or hydrogen into the thin film, however, may introduce accidental donor dopants on the anion sites of the oxide film, leading to detrimental decrease in thin film resistivity and dielectric properties.

An object of the present invention is to provide a method of metalorganic chemical vapor deposition of thin perovskite oxide films such as BaTi03, SrTi03, and Bal-xSrxTi03, as well as the thin films produced thereby, in a manner to improve film dielectric properties by control of film electrical resistivity and other film properties such as, for example only, resistance to reduction, accidental donor incorporation, dc electrical degradation, and ferroelectric fatigue.

SUMMARY OF THE INVENTION

The present invention provides a method of depositing high dielectric perovskite oxide thin films, such as BaTi03, SrTi03, and Bal-xSrxTi03, on a substrate by metalorganic chemical vapor deposition using reduced vapor pressure metalorganic precursors under conditions to provide in-situ doping of the thin film with a dopant of a type and in an amount effective to improve film dielectric properties by control of film electrical resistivity and other film properties such as resistance to reduction, accidental donor incorporation, dc electrical degradation, and ferroelectric fatigue. The thin film can be doped with a net excess of an acceptor dopant, such as for example only, aluminum, or net excess of cation donor dopant, such as for example only the rare earth element lanthanum in accordance with embodiments of the invention.

The above and other objects and advantages of the present invention will become more readily apparent from the following detailed description taken with the following drawings.

DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph illustrating leakage current density, J, versus bias voltage properties of undoped SrTi03 and Al doped SrTi03 thin films deposited on Nb:SrTi03 substrates pursuant to the invention.

The temperature, T(A1), of the Al precursor source is set forth on Fig. 1 for the samples tested using a magnesium electrical contact area of 1.88xl0 ^"3 cm ² .

Figure 2 is a graph illustrating the dependence of room temnperature resistivity versus the La(dpm)3 vapor pressure during film deposition of La-doped BaTi03 thin films. The line serves only as a guide through the data shown.

Figure 3 is a schematic diagram of apparatus for metalorganic chemical vapor deposition of thin films of the invention.

DESCRIPTION OF THE INVENTION

The following detailed description of the invention is offered for purposes of illustrating the present invention in greater detail and not limiting the scope of the invention.

A strontium titanate (SrTi03) thin film was grown or deposited pursuant to an embodiment of the invention by metaloraganic chemical vapor deposition on various substrates including (100)LaA103, (100) Nb:SrTi03 comprising 0.5 weight % Nb doped SrTi03, and oxidized (100) Si. However, the invention is not limited to these substrates and other substrates can used.

In particular, reactants ( i.e.metalorganic precursors) bearing the film components (e.g. strontium, titanuim, acceptor dopant such as aluminum, and oxygen) are provided in a reactor and reacted in proportions and under conditions of temperature and pressure controlled to deposit a strontium titanate thin film that is doped in-situ with aluminum as an acceptor dopant as the film is grown or deposited on the substrate. An acceptor dopant is defined as having a smaller positive charge than the host ion it replaces and thus represents a negative charge relative to the ideal oxide lattice. Other acceptor dopants which may be used in practicing the invention to control electrical resistivity of the thin film include Fe, Ga, Ni, Cr, Co, and Ca.

Metalorganic chemical vapor deposition apparatus described by L. A. Wills et al. in J. Crvst. Growth, 107, 712 (1991), the teachings of which are incorporated herein by reference, can be used in the practice of the present invention to deposit the doped strontium and/or barium titanate thin films on suitable substrates. Apparatus to this end is shown schematically in Figure 3 and comprises a low pressure, two-zone horizontal quartz reactor system having a reactor zone that is provided with

reactants (metalorganic precursors) in suitable proportions for reaction under the temperature and pressure conditions in the reaction zone to deposit or grow the doped strontium and/or barium titanate thin films on the substrate S.

In the practice of one embodiment of the invention to form an Al doped (net excess of acceptor impurities) strontium titanate thin film on the substrates mentioned above, the metalorganic precursors used were high purity (99.999%) and included:

Sr(hexafluoracetylacetonate)2(tetraglyme) titanium tetraisopropoxide [TPT; Ti(OC3H7)4], and Al(aceytlacetonate) .

The Sr and Al solid sources were placed in separate reactor source zones in the manner shown in Figure 3 and resistively heated. The liquid TPT was stored in a bubbler that was heated by a recirculating bath (not shown) . Argon (high purity) was used as a carrier gas to bring the metalorganic precursors into the reactor zone in proper proportions. Oxygen (high purity) bubbled through deionized water was used as the reactant gas for reacting with the metalorganic precursors. The argon and oxygen flow rates were controlled by mass flow controllers (MFC). Pressure gages P were used at appropriate locations of the apparatus. The reactor pressure was set by the total flow rate. An IR (infrared radiation) lamp was used to heat a SiC coated graphite susceptor SP on which the substrate is placed in the reaction zone to provide an appropriate substrate film growth temperature in the range of 725 to 820 degrees C. The deposition temperature was monitored by chromel-alumel thermocouple placed inside the susceptor. The films were deposited and cooled in an oxygen partial pressure 1.65 to 1.80 Torr.

The Al doped strontium titanate thin films were grown on single crystal LaA103, (100) Nb:SrTi03, and oxidized Si (100) substrates

mentioned above. The deposition conditions used to grow Al doped strontium titanate thin films on the aforementioned substrates are summarized in Table I.

Table I Typical AI-Dυpcd SrTιθ3 Thin Film Growth Conditions

Growth Temperature (°C) 725 - 820 Substrates S1O ₂ /S1, ( 100) LaAIO ₃ ,

( 100) Nb:SrTιO,

Growth Rate (μm/hr) 0.2 - • 0.6

Temperature of Sr Source (°C) 80 - 105

Temperature of Al Source (°C) 30 - 85

Temperature of Ti Source (°C) 25 - 45

Total Flow Rate (seem) 120

Oxygen Flow Rate(seem) 50

The Al doped strontium titanate thin films deposited on the (100) LaA103 and (100) Nb:SrTi03 substrates were epitaxial and single phase, while the films grown on oxidized (100) Si were non- textured meaning that they did not have a preferred orientation. The film thicknesses were in the range from 0.1 to 0.7 microns. Al dopant concentrations of the thin films were estimated to range from less than 0.10 atomic percent to approximately 1 atomic percent based upon energy dispersive X-ray analysis and film latttice parameter measurements.

The Al doped strontium titanate (Al-doped SrTi03) thin films as described above were found to exhibit significant improvements in both dc electrical resistivity and leakage current density over the corresponding undoped thin films. For example, the increase in dc resistivity is demonstrated in Tables II and III.

Table II Four probe dc electrical resistivity measured for undoped SrTiOj deposited on LaAlQ3.

Sample p(300K) ^a ( ^A ) Thickness FWHM

(Ω-cm) (A) (200)

SRG094 0.97 3.9026 3700 0.92

SRG 120 135 3.9095 6578 0.60

Table III Four probe dc electrical resistivity measured for Al doped SrTiθ deposited on LaAlQ3.

Sample T(AI) °C (300K) a (A) d (A) FWHM

(Ω-cin) ( 200)

SRG 13 1 50 1.8x l0 ⁶ 3905 0.43

SRG 130 60 2.2X 10 ³ 3.9035 3220 0.72

SRG 132 60 ~1( 0.36

SRG 134 70 -102 2227 0.44

SRG 138 70 > 10 ⁹ 3.8919 0.86

SRG 143 80 >2x l() ⁹ 3.8830 2024 2.7 1

where p is room temperature resistivity, a is lattice constant, thickness is film thickness, FWHM is full width at half maximum of the (200) film reflection (diffractometer data), and d is film thickness.

Films deposited with Al precursor temperatures on the order of 70 degrees C to 80 degrees C possessed resistivities of at least 10 ⁹ ohm-centimeter, while the undoped strontium titanate thin films exhibited much lower resistivities in the range of 10 ^"1 to 10 ³ ohm-centimeter. Futhermore, Figure 1 demonstrates that Al doping of the SrTi03 thin films as described above led to a decrease in the leakage current density at an applied bias voltage of +2 volts from approximately 10 ^" " Ampere/centimeters squared for undoped SrTi03 thin films to about 10 ^"β Ampere/centimeters squared for Al-doped SrTi03 thin films, which represents a 10 ⁴ improvement in leakage current density for the Al doped thin films. These results indicate that Al acceptor doping during metalorganic chemcial vapor deposition significantly improves the dielectric properties of SrTi03 thin films deposited using the relatively low vapor pressure fluorinated strontium precursor described above. High resistivity acceptor doped ferroelectric thin films such as doped SrTi03 and BaTi03 as well as their solid solutions can be produced by metalorganic chemical vapor deposition pursuant to the invention.

In the practice of another embodiment of the invention, the thin film is doped with a net excess of cation donor impurities. A donor dopant is defined as having a higher positive charge than the host cation it replaces and thus represents a postive charge relative to the ideal oxide lattice. Other donor dopants which may be used in practicing the invention to control electrical resistivity of the thin film include Nb, Ce, Nd, Sm, Eu, Gd, and Ho.

In the practice of the invention to form a rare earth doped (net excess of cation donor impurities) barium titanate thin film on the LaA103 substrate mentioned above, the metalorganic precursors used were high purity (99.999%) and included:

Ba(hexafluoracetylacetonate)2(tetraglyme) titanium tetraisopropoxide [TPT; Ti(OC3H7)4], and solid La(dpm)3 (dpm = dipivaloylmethanate) .

The Ba and La solid sources were placed in separate reactor source zones similar to the manner shown in Figure 3 for the strontium and aluminum sources and resistively heated. The dopant vapor pressure was controlled by varying the La(dpm)3 temperature between 130 degrees C and 160 degees C and was calculated using data from R.E. Sievers et al. , Science 201, 217 (1978). The liquid TPT was stored in a bubbler that was heated by a recirculating bath (not shown). Argon (high purity) was used as a carrier gas to bring the metalorganic precursors into the reactor zone in proper proportions. Oxygen (high purity) bubbled through deionized water was used as the reactant gas for reacting with the metalorganic precursors. The argon and oxygen flow rates were controlled by mass flow controllers (MFC) . Pressure gages P were used at appropriate locations of the apparatus. In practice of the invention, reactor pressure was set by the total flow rate. An IR (infrared radiation) lamp was used to heat a SiC coated susceptor SP on which the substrate is placed in the reaction zone to provide an appropriate substrate film growth temperature in the range of 800 degrees C for a (100) LaA103

single crystal substrate. The deposition temperature was monitored by chromel-alumel thermocouple placed inside the susceptor. The films were deposited and cooled in an oxygen partial pressure of 1.65 to 1.80 Torr.

The deposition conditions used to grow La-doped barium titanate thin films on the aforementioned substrates are summarized in Table V.

Table V

Growth Tcmpcruluic (°C) 800 ° C

Substrates ( IOO) LnΛIO.j

Growth Rate (μm/ i) 0 . 3-0 . 8 Temperature of BaSourec (°C) 115 ° C Temperature of LΛ.Sourcc (°C) 120-160 ^ c Temperature of Ti Source (°C) _{45 c} Total Row Rale (seem) ₁₂ o Oxygen Row Rale (seem) 50

The thin La-doped barium titanate films deposited on the (100) LaA103 substrates were epitaxial and single phase having a (hOO) orientation, indicating that the lattice a-axis was perpendicular to the substrate surface. In-plane epitaxy of the thin films was verified using high resolution transmission electron microscopy (HRTEM). The film-substrate interface was found to be nearly atomically abrupt and the lattice mismatch was accommodated by a regular array of misfit dislocations. Occasionally, small islands of amorphous secondary phase, identified as the Ba2Ti04 phase, were observed at the film-substrate interface. These amorphous regions, however, did not appear to hinder the subsequent epitaxial film growth. The film thicknesses were in the range from 0.30 to 0.80 micron.

Cross-sectional electron energy loss spectroscopy (EELS) and convergent beam electron diffraction anaylsis of the optical and structural properties of the films indicated that the films assumed the bulk characteristics of tetragonal BaTi03 at distances greater than 40 nanometers away from the film-substrate

interface. At lower magnification, cross-sectional transmission electron microscopy revealed misoriented columnar BaTi03 grains that were 0.2 micron in diameter extending through the thickness of the film. Based on X-ray diffraction results indicating highly oriented films both in the a-axis direction perpendicular to the substrate surface and in the plane of the film, the grain boundaries were low angle in nature with no secondary phases observed at the grain boundaries.

The La-doped BaTi03 thin films were specular in appearance and transparent. Atomic force microscopy measurements indicated a root mean square surface roughness on the order of 10 to 20 nanometers. The films were found to be uniform over a large area and to possess a fine granular morphology presumably resulting from the formation of columnar sub-grains. The average surface feature size for La-doped and undoped BaTi03 thin films was 0.2 to 0.3 microns in diameter.

The room temperature dc electrical resistivity of the undoped BaTi03 thin films grown using the high purity Ba precursor described above ranged between 10 ⁵ to 5 X 10 ⁷ Ohm-centimeter, while that of undoped stochiometric material is approximately 10 ¹⁰ Ohm-centimeter and higher. For La(dpm)3 vapor pressures between 5 X 10 ^"4 Torr and 2.5 X 10 ^"3 Torr, (source temperatures of 130 degrees C to 150 degrees C), the room temperature resistivity decreases to a minimum of 55 Ohm-centimeter, Figure 2. At low concentrations of La dopant in the film, the low resistivity of the films is attributed to substitution of trivalent La ^*3 donors for the divalent Ba ⁺² cation, creating LaBa ⁺ donor centers, although Applicants do not wish to be bound by this explanation.

Estimates of the La concentration in the low resistivity La-doped BaTi03 thin films were made by measuring film lattice paramters. The lattice parameters measured ranged between 3.992 Angstroms to 3.995 Angstroms, compared to an average of 3.994 plus or minus 0.002 Angstroms for the undoped BaTi03 films (bulk BaTi03; a = 3.994 Angstroms). The difference in lattice parameters between

doped and undoped films therefore falls within plus or minus 0.002 Angstrom standard deviation. This indicated that the La concentration was less than approximately 0.6 atomic % in the semiconducting films based on lattice parameter data for LaxBal-xTi03 from Eylem et al. , Chem. Mater. 4, 1038 (1992).

As shown in Figure 2, when the La doping is increased by raising the La(dpm)3 vapor pressure beyond 2.5 X 10 ^"3 Torr (source temperature greater than 150 degrees C), the BaTi03 resistivity increases sharply to approximately 2 X 10 ⁶ Ohm-centimeter, equivalent to that measured for undoped BaTi03 thin films. In bulk ceramics processed in an oxidizing atmosphere, electrons no longer provide charge compensation beyond a critical La concentation of approximatley 0.1 to 0.6 atomic percent La. Concentrations of La less than approximately 0.6 atomic % are below the resolution of the lattice parmeter data. Beyond this critical minimum La concentration, the LaBa ^* centers are presumably compensated, leading to the increase in resistivity observed in Figure 2, although Applicants do not wish to be bound by this explanation.

From Figure 2, it is apparent that control of electrical resistivity of the La doped BaTi03 thin films can be achieved through control of the in-situ La doping during metalorganic chemical vapor deposition of the film.

Although the embodiment of the present invention involving doping with a net excess of cation donors has been described hereabove with respect to the rare earth La dopant, the invention it is not so limited and can be practiced using other rare earth dopants including but not limited to Ce, Nd, Sm, Eu, Gd, and Ho alone or in combination as well as non-rare earth dopants such as Nb.

Although certain specific embodiments and features of the invention have been described hereabove, it is to be understood that modifications and changes may be made therein without departing from the spirit and scope of the invention as defined

in the appended claims.