Abstract:
The present invention provides a method for preparing a novel low-resistance p-type SrTiO 3  capable of opening the way for oxide electronics in combination with an already developed low-resistance n-type SrTiO 3 . The method is characterized in that an acceptor and a donor are co-doped into a perovskite-type transition-metal oxide SrTiO 3  during crystal growth.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a method for reducing the resistance of a perovskite-type transition-metal oxide SrTiO 3 .  
         BACKGROUND ART  
         [0002]    A perovskite-type transition-metal oxide SrTiO 3  has a wide bandgap energy of 3.3 eV. Therefore, SrTiO 3  gets attention as a base material of other perovskite-type transition-metal oxides, a nonlinear resistive element, a ferroelectric material or the like, and a SrTiO 3  film having a high dielectric constant are actively developing.  
           [0003]    A high dielectric film consisting of SrTiO 3  is typically prepared through a spattering method using a SrTiO 3  sintered body as a target (e.g. Japanese Patent Laid-Open Publication Nos. 6-140385 and 9-153598).  
           [0004]    On the other hand, it has been difficult to prepare a p-type SrTiO 3  having a desirably lowered resistance even by adopting any conventional technologies. Specifically, a conventional method of doping a p-type dopant N (nitrogen) by itself has been able to increase carrier concentration only up to about 10 14  cm −3  due to the deep impurity level and resulting low activation rate of N acceptor. Further, the doping of N as an acceptor causes a compensation effect, which inevitably leads to oxygen deficiency serving as a donor. Such negative factors obstruct to achieve a desired low-resistance p-type SrTiO 3 .  
         DISCLOSURE OF INVENTION  
         [0005]    The inventors have found a new technical concept of simultaneously doping or co-doping an acceptor with a donor having a high chemical bonding strength with the acceptor intentionally to provide a shallowed accepter level and suppressed compensation effect, and have verified that the above problem could be solved by the developed technique.  
           [0006]    According to a first aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO 3 , wherein an acceptor and a donor are co-doped into a perovskite-type transition-metal oxide SrTiO 3  during crystal growth.  
           [0007]    According to a second aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO 3 , wherein an acceptor (A) and a donor (D) are co-doped into a perovskite-type transition-metal oxide SrTiO 3  during crystal growth to form an impurity complex (A-D-A) of the acceptor and the donor through a crystal growth method in a thermal nonequilibrium state, whereby an impurity formation energy is reduced by less than that in a method of doping the acceptor by itself, so as to provide an increased concentration of the acceptor.  
           [0008]    According to a third aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO 3 , wherein an acceptor (A) and a donor (D) are co-doped into a perovskite-type transition-metal oxide SrTiO 3  during crystal growth to form an impurity complex (A-D-A) of the acceptor and the donor through a crystal growth method in a thermal nonequilibrium state, whereby a Coulomb scattering mechanism having a long range force is converted into a dipole or multipole scattering mechanism having a short range force, so as to provide an increased mobility of the impurities.  
           [0009]    According to a fourth aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO 3 , wherein an acceptor (A) and a donor (D) are co-doped into a perovskite-type transition-metal oxide SrTiO 3  during crystal growth to form an impurity complex (A-D-A) of the acceptor and the donor through a crystal growth method in a thermal nonequilibrium state, whereby the acceptor has a shallower impurity level than that in a method of doping N by itself, so as to provide an increased activation rate of the acceptor.  
           [0010]    In the present invention, in order to prevent oxygen deficiency, the donor may be Nb oxide, and the acceptor may be N oxide.  
           [0011]    Further, the donor may be an n-type dopant of Nb (niobium), and the acceptor may be a p-type dopant of N (nitrogen). In this case, the n-type dopant of Nb and the p-type dopant of N are co-doped at a rate of 1:(1+x) where 0&lt;x&lt;100, so as to provide p-type carriers doped in a high concentration of 10 19  cm −3  to 10 21  cm −3 . In the ratio of Nb (niobium) to N (nitrogen), the amount of N should be greater than that of Nb to obtain a p-type SrTiO 3 , and a significant co-doping effect can be observed at the ratio of up to about 1:100.  
           [0012]    In the method of the present invention, the SrTiO 3  crystal can be grown on a substrate such as Si, SrTiO 3  or βAl 2 O 3  by employing an MBE (Molecular Beam Epitaxy) method, laser-MBE method, CVD (Chemical Vapor Deposition) method or spattering method.  
           [0013]    The CVD method promotes crystal growth under hydrogen atmosphere, and a hydrogen donor will remain in the resulting crystal because hydrogen acts as a donor. However, the remaining donor can be removed by subjecting the resulting crystal to annealing (at about 500° C.), and thereby an acceptor acts to provide a desirable low-resistance p-type crystal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1( a ) is a schematic diagram showing an impurity complex of an acceptor and a donor, which is formed in SrTiO 3  crystal-grown by a method of the present invention.  
         [0015]    [0015]FIG. 1( b ) is a schematic diagram showing the relationship between a repulsive force and an attracting force of the impurity complex.  
         [0016]    [0016]FIG. 2 is a schematic diagram showing an acceptor level and a donor level in an A-D-A complex of SrTiO 3  crystal-grown by a method of the present invention.  
         [0017]    [0017]FIG. 3 is a schematic vertical sectional view of a laser-MBE crystal-growth apparatus used in a first example of a method of the present invention.  
         [0018]    [0018]FIG. 4( a ) is an energy spectrum diagram showing a state density when N is doped by itself into SrTiO 3  during crystal growth.  
         [0019]    [0019]FIG. 4( b ) is an energy spectrum diagram showing a state density when N and Nb are co-doped at a ratio of 2:1 into SrTiO 3  during crystal growth.  
         [0020]    [0020]FIG. 5 is a schematic vertical sectional view of a CVD crystal-growth apparatus used in a second example of a method of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    [0021]FIG. 1( a ) is a schematic diagram showing an impurity complex of an acceptor and a donor, which is formed when SrTiO 3  is crystal-grown according to a method of the present invention. FIG. 1( b ) is a schematic diagram showing the relationship between a repulsive force and an attracting force of the impurity complex.  
         [0022]    When an acceptor (A) and a donor (D) are simultaneously doped or co-doped into SrTiO 3  through a crystal growth method in a thermal nonequilibrium state, a (A-D-A) complex as shown in FIG. 1( a ), which is an impurity complex of the acceptors  and the donors ⊚, is formed during the crystal growth.  
         [0023]    As shown in FIG. 1( b ), a repulsive force acts between the respective acceptors and an attracting force acts between each of the acceptors and the donor. Thus, the impurity complex is formed by the acceptor/donor ratio of 2:1. FIG. 2 is a schematic diagram showing an acceptor level and a donor level in an A-D-A complex of SrTiO 3  crystal-grown by a method of the present invention.  
         [0024]    When an impurity complex of acceptors  and donors ⊚ is formed by co-doping an n-type dopant of Nb (niobium) and a p-type dopant of N (nitrogen) at a ratio of 1:(1+x) where 0&lt;x&lt;100, respective wave functions of the acceptors and the donors are hybridized as shown in FIG. 2. Thus, the co-doping allows the acceptors to have a shallower impurity level than the deep impurity level provided by the conventional method of doping N by itself (hereinafter referred to as “independent doping method”), so that an activation rate of the acceptors is increased.  
         [0025]    Further, an impurity formation energy can be reduced by less than that in the independent doping, to provide an increased concentration (solid solubility limit) of the acceptors. In addtion, a Coulomb scattering mechanism having a long range force can be converted into a dipole or multipole scattering mechanism having a short range force to provide an about double-digit-increased mobility of the impurities.  
         [0026]    In the conventional method of doping N by itself into SrTiO 3 , the number of doped carriers is only about 10 14  cm −3 . By contrast, the co-doping method of the present invention can dope carriers in a high concentration of about 10 19  cm −3  to 10 21  cm −3 .  
       EXAMPLE  
     First Example  
       [0027]    Crystal Growth Through MBE Method  
         [0028]    [0028]FIG. 3 is a schematic vertical sectional view of a laser-MBE crystal-growth apparatus used in this example. A substrate 29 made of Si was housed in a chamber 21 with placing on a substrate heater 28 heated by a heater 22. A target 25 formed by sintering Sr oxide/Ti oxide in the form of a pellet was fixedly disposed on a target table 23 fixed to the end of a rotating shaft 26 to be rotated by a rotational drive unit outside the chamber 21. A N 2 O 24 as a dopant was fixedly disposed at a N 2 O gas source position of the target table 23. A excimer laser was irradiated from the outside of the chamber 21 onto the target 25 and the N 2 O 24 to introduce abrasion in them, and an n-type dopant of metal Nb (Nb oxide was used as the target) and a p-type dopant of N was co-doped at a ration of 1:2 while crystal-growing SrTiO 3  on the substrate 29 under oxygen/N 2 O atmosphere.  
         [0029]    In order to check up on the effect of the co-doping during the laser MBE crystal grow of SrTiO 3 , respective energy spectrums of state densities in the independent doping method and the co-doping method were measured to compare them with one another. The result is shown in FIGS.  4 ( a ) and  4 ( b ). FIG. 4( a ) shows the state density in the independent doping method, together with a partial state density in the 2p region of N. FIG. 4( b ) shows the state density in the method of co-doping N and Nb, together with a partial state density in the 2p region of N and a quadrupled partial state density in the 4d region of Nb. As can be seen from these energy spectrums, the conventional method of doping the acceptor N by itself provides a deep acceptor level and thereby the activation rate of a p-type carrier is low. By contrast, in the method of co-doping the acceptor N and the donor Nb at the rate of 2:1, the resulting energy spectrum of the electronic density of states shows that the acceptor/donor impurity complex comprised of N-Nb-N is formed through the co-doping, and the wave function of the acceptor N is strongly hybridized with the wave function of donor Nb to shift the acceptor level of N toward the lower energy side or the shallower side.  
         [0030]    Further, the acceptor/donor impurity complex of N-Nb-N formed through the co-doping method provides a higher solid solubility limit of the acceptor than that in the independent doping method. The shallowed acceptor level and the heightened solid solubility limit of the acceptor support the realization of a p-type SrTiO 3  having a desirably lowered resistance.  
         [0031]    The respective effects of the conventional method of doping the acceptor N by itself and the method of co-doping the acceptor N and the donor Nb at the ratio of 2:1 were compared under the condition that the substrate temperature was changed in the range of 500° C. and 800° C. during crystal growth. The result is shown in the following Table 1. The co-doping method could provide a significantly increased number, or four to six-digit-increased number, of carriers as compared to the independent doping method.  
                                 TABLE 1                           Substrate    Acceptor Concentration (number/cm 3 )            Temperature (° C.)   Co-Doping of N and Nb   Independent Doping of N               500   4.2 × 10 17     1.1 × 10 14         600   8.1 × 10 18     6.8 × 10 14         700   9.5 × 10 19     9.6 × 10 14         800   9.3 × 10 20     8.3 × 10 15                    
 
       Second Example  
       [0032]    Crystal Growth Through CVD Method  
         [0033]    [0033]FIG. 5 is a schematic vertical sectional view of a CVD crystal-growth apparatus used in this example. Based on the CVD method, organic metal complexes of Sr, Ti and Nb, i.e. Sr (DPM) 2 , Ti (DPM) 3  and Nb (DPM) 4  (DPM: dipivaloymethanato), were reserved in vessels  11 ,  12  and  13 , respectively, and used as starting materials.  
         [0034]    Ammonia was used as a starting material of N. Each of the starting materials was discharged from the corresponding vessel by introducing rare gas from a gas inlet 3 into each of the vessels. The starting materials were heated up to a high temperature at a vaporizing chamber 2, and forcibly vaporized. The vapor of the starting materials was introduced in a chamber heated by a heater 7, and directed toward a substrate 9 through a gas nozzle 6 to grow a SrTiO 3  crystal 10 on the substrate 9 heated by a substrate heater 8. During this process, oxygen gas was introduced from a gas inlet 4 into the chamber 1 to prevent oxygen deficiency. Effluent gas in the chamber 1 was discharged from a gas outlet 5 by sucking with a pump.  
         [0035]    The respective effects of the conventional method of doping the acceptor N by itself and the method of co-doping the acceptor N and the donor Nb at the ratio of 2:1 were compared under the condition that the substrate temperature was changed in the range of 200° C. and 300° C. during crystal growth. The result is shown in the following Table 2. The co-doping method could provide a significantly increased number, or four to six-digit-increased number, of p-type carriers as compared to the independent doping method. Hydrogen gas immixed during crystal growth can be removed by subjecting a SrTiO 3  thin-film after the crystal growth to annealing under oxygen atmosphere, and a further increase number of p-type carriers was observed in the annealed SrTiO 3  thin-film.  
                                 TABLE 2                           Substrate    Acceptor Concentration (number/cm 3 )            Temperature (° C.)   Co-Doping of N and Nb   Independent Doping of N               200   2.9 × 10 17     2.1 × 10 14         250   8.8 × 10 18     6.2 × 10 14         280   9.6 × 10 19     7.1 × 10 14         300   8.2 × 10 20     7.4 × 10 15                    
 
       INDUSTRIAL APPLICABILITY  
       [0036]    A low-resistance p-type SrTiO 3  will open the way for oxide electronics in combination with an already developed low-resistance n-type SrTiO 3 .  
         [0037]    Further, the p-type doping can provide adequately controlled properties of a ferroelectric material. This makes it possible to assure the capacity of a highly integrated and miniaturized memory cell, and to provide a new memory device with a high withstand voltage, usable in a DRAM (Dynamic Random Access Memories).  
         [0038]    Thus, the low-resistance p-type SrTiO 3  can be used to achieve a nonvolatile memory excellent in high-speeding, low power consumption, high-level integration and rewrite resistance properties, and thereby applied to a system LSI or an IC chip card.  
                                                   Fig. 1 (a)           acceptor/donor impurity complex           acceptor  donor           Fig. 1 (b)           repulsive force           attracting force                       formation of acceptor/donor complex           energy             conduction band           lone acceptor level  lone doner level           bandgap           valence band                       25: target             26: rotating shaft           excimer laser           O 2 /N 20 atmosphere             21: chamber           23: target table           24: N 2 O           20: SrTiO 3  crystal           29: substrate           28: substrate heater           22: heater                       Fermi level           state density (state/Ry)           energy (Ry)                       3: gas inlet           rare gas  liquid MFC                     Ti source                liquid MFC                Nb sourse                liquid MFC                Sr source           2: vaporizing chamber           4: gas inlet           oxygen gas           7: heater           6: gas nozzle           1: chamber           5: gas outlet           pump           8: substrate heater           9: substrate           10: crystal (SrTiO 3 )