Abstract:
A process for growing a semiconductor crystal, comprising growing a group III-V compound semiconductor containing P as a group V element by an organometal vapor phase epitaxy by using tertiary butyl phosphine (TBP) as a source of P constituting a grown layer and doping the semiconductor with a dopant gas during a growth of the semiconductor. In this process, the source gas and dopant gas are fed under a condition satisfying a requirement represented by the following formula, to conduct a growth of the crystal: ##EQU1## wherein n is a carrier concentration of the growing semiconductor crystal, T is a temperature of the substrate and an atmosphere in the vicinity of the substrate, P TBP   0  is an equilibrium vapor pressure of TBP or a decomposition product thereof, P D   0  is an equilibrium vapor pressure of a dopant or a decomposition product thereof, P 0  is a total pressure, f 0  is a total flow rate, and f TBP  is a flow rate of TBP.

Description:
This application is a continuation of application Ser. No. 08/097,092, filed Jul. 18, 1993, now abandoned, and is a continuation of U.S. patent application Ser. No. 07/863,783, filed Apr. 6, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for growing a semiconductor crystal. More particularly, the present invention relates to a doping method for use in the crystal growth of a compound semiconductor containing P, by metal organic vapor phase epitaxy (MOVPE). 
     2. Description of the Related Art 
     The practical use of MOVPE has been promoted as a process for growing a next generation crystal, because MOVPE has an excellent controllability and homogeneity of the film thickness and composition, etc., of the grown crystal, and enables the growth to be conducted on a substrate having a large area and a plurality of sheets of a substrate. Nevertheless, the arsine (AsH 3 ) and phosphine (PH 3 ) commonly used as a group V element source in MOVPE are extremely dangerous, as they are high-pressure gases having a strong toxicity, and this has inhibited the practical use of MOVPE. 
     The present inventors have conducted the growth of InP, InGaAs and InGaAsP crystals through the use of tertiary butyl arsine (TBA) and tertiary butyl phosphine (TBP), which are liquids having a low toxicity, as the group V element source to thereby improve the safety factor, and have succeeded in forming a crystal having a high purity. In this case, it is necessary to establish p-type and n-type doping techniques as a growing technique for preparing a semiconductor device. 
     The n-type doping of GaAs with disilane (Si 2  H 6 ) is known as an example of doping during a growth in which TBA is used as the group V element source, and although it is reported that a good doping can be conducted by this method, there is no report on the use of TBP as the group V element source. Accordingly, an attempt to grow an InP crystal has been made through the use of TBP as the group V element source and monosilane (SiH 4 , which is a dopant usually employed in the growth wherein use is made of PH 3  as the group V element material) as an n-type dopant. In this case, the growth of the InP crystal was conducted under the same conditions as usually employed for a growth under a reduced pressure through the use of PH 3  (growth temperature: 600° C., growth pressure: 76 Torr, flow rate of carrier gas: 6 liters/min, growth rate: 1.2 μm/hr, V/III ratio: 200, SiH 4  flow rate: 0.005 cc/min). Nevertheless, the intended single crystal was not grown; the grown crystal was polycrystalline. When the growth was conducted under the same conditions as used above, through the use of PH 3 , a single crystal having a surface free from defects was obtained with a carrier concentration of 2.4×10 18  cm -3 . A comparison of both cases is shown in FIG. 4. Further, in a growth in which use was made of TBP, although a single crystal was obtained by reducing the flow rate of SiH 4  to 0.0002 cc/min, it was found that the single crystal had a surface having a number of pits (small holes), i.e., the surface state was poor, as shown in FIG. 5. The carrier concentration in this case was 6.8×10 17  cm -3 . 
     Although TBP is a very safe source material, the reason for the above-described drawback is believed to be that, since the reactivity of TBP (or a deposition product thereof) is larger than that of PH 3 , a vapor phase reaction occurs between the dopant and the group V element source (TBP). This problem is characteristic of TBP, and has not hitherto been pointed out in the art. 
     SUMMARY OF THE INVENTION 
     The growth of a single crystal is indispensable to the production of an electronic device, and a surface defect, such as a pit, has a remarkably adverse effect on the properties of the device. An object of the present invention is to provide a process for growing a single crystal free from surface defects through the prevention of a vapor phase reaction between a highly reactive TBP and a dopant in the doping of a P-containing compound semiconductor (InP, InGaAsP or the like) by using TBP as the group V element source. 
     To solve the above-described problems, the present invention provides a process for growing a semiconductor crystal, comprising growing a group III-V compound semiconductor containing P as a group V element by a metal organic vapor phase epitaxy by using tertiary butyl phosphine (TBP) as a source of P constituting a grown layer, and doping the semiconductor with a dopant gas during the growth of the semiconductor. This process is characterized in that the source gas and the dopant gas are fed under a condition satisfying a requirement represented by the following formula, to conduct the growth of the crystal: ##EQU2## wherein n is a carrier concentration of the growing semiconductor, T is a temperature of the substrate and an atmosphere in the vicinity of the substrate, P TBP   0  is an equilibrium vapor pressure of TBP or a decomposition product thereof, P D   0  is an equilibrium vapor pressure of a dopant or a decomposition product thereof, P 0  is a total pressure, f 0  is a total flow rate, and f TBP  is a flow rate of TBP. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing the relationship between the carrier concentration n capable of conducting doping and the total pressure P 0  ; 
     FIG. 2 is a graph showing the relationship between the flow rate of SiH 4  and the carrier concentration; 
     FIG. 3 is a photograph of the surface of a single crystal prepared in the Example; 
     FIGS. 4a and 4b are photographs of the surface of a single crystal prepared by the conventional method; 
     FIG. 5 is a photograph of the surface of a single crystal corresponding to a comparative example; 
     FIG. 6 is a graph showing the relationship between the TBP flow rate and the carrier concentration in the Example; 
     FIG. 7 is a graph showing the relationship between the growth pressure and the carrier concentration in the Example; and, 
     FIG. 8 is a graph showing the relationship between the growth temperature and the carrier concentration. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Examples of useful dopants include monosilane, disilane, hydrogen sulfide, diethyl telluride, diisopropyl telluride, diethyl selenide, diisopropyl selenide, diethyl sulfide, tetraethyl tin, and tetramethyl tin. 
     The causes of the growth of a polycrystal and the formation of a pit are considered to be that TBP (or a decomposition product thereof) reacts with a dopant (or a decomposition product thereof) in a vapor phase to form a solid phase nucleus which is deposited on a substrate to inhibit a normal growth of a single crystal. It is considered that the formation of a solid phase nucleus occurs only when the growth condition exceeds a certain critical condition, and no solid phase nucleus forms when the growth condition deviates from the critical condition. For example, when the growth pressure is taken as an example, there exists a critical pressure for the formation of a solid phase nucleus determined depending upon the flow rate of TBP, the flow rate of dopant gas, the growth temperature, the flow rate of carrier gas, and the configuration of the reaction tube. When the growth pressure is below the critical pressure, the formation of the solid phase nucleus is completely prevented. Since the surface state of the crystal rapidly changes, with the critical pressure as a boundary, whether or not the solid phase nucleus has been formed can be determined from the surface state of the grown crystal. Examples of the factors to be considered as critical conditions include a growth pressure, a growth temperature, a TBP flow rate, a carrier gas flow rate, and molar fractions of the TBP (or a decomposition product thereof) and the dopant (or a decomposition product thereof). 
     A regulation of the growth pressure, the growth temperature, the TBP flow rate, the carrier gas flow rate, and the molar fractions of the TBP (or the decomposition product thereof) and the dopant (or the decomposition product thereof) is considered to effectively prevent the formation of the solid phase nucleus. 
     Further, although it is possible that the adoption of such a means will have an adverse effect on the growth of the InP crystal per se, which is a vital point even though the occurrence of the vapor phase nucleus is prevented, such an adverse effect can be avoided by exciting the crystal surface alone, by using an electron beam, X-ray or the like. 
     The features of the present invention will now be described in more detail. 
     For example, in a reaction of TBP (a gas) with a dopant D (a gas) to produce a solid, the following relationship is established between the free energy change ΔG and the respective equilibrium vapor pressures, P TBP   0  and P D   0 , of TBP and the dopant D: ##EQU3## wherein R is a gas constant and 1.987 and T is a temperature of the substrate and in the vicinity of the substrate. Since the free energy change ΔG can be expressed by the following formula 
     
         ΔG=ΔH-ΔST 
    
     wherein ΔH is an enthalpy change and ΔS is an entropy change, the product of the equilibrium vapor pressures P TBP   0  ·P D   0  can be expressed by the following formula: ##EQU4## Nevertheless, the reaction of the gaseous TBP with the gaseous D actually occurs when the respective partial pressure, P TBP  and P D , of the gaseous TBP and the gaseous D exceeds the product of the equilibrium vapor pressures P TBP   0  ·P D   0  (i.e., P TBP  ·P D  &gt;P TBP   0  ·P D   0 ). 
     Whether or not the reaction actually occurred can be determined by an observation of the surface of the grown crystal. Therefore, when the product of partial pressures at a boundary where the change of the crystal surface occurs is determined by varying the product of the partial pressure P TBP  ·P D  while the temperature is fixed to T 1 , this gives the product of the equilibrium vapor pressures (T=T 1 ) at the temperature T 1 . Further, at a different temperature T 2 , P TBP   0  ·P D   0  (T=T 2 ) is determined. When the product of the equilibrium vapor pressures P TBP   0  ·P D   0  is determined at two different temperatures, ΔH and ΔS can be determined according to the above-described formula, and thus the product of equilibrium vapor pressures can be obtained at an arbitrary temperature. 
     This will now be described in more detail by taking as an example the use of SiH 4  as a dopant. The following results were obtained at a growth temperature of 600° C. 
     
         ______________________________________GrowthPressure  P.sub.TBP P.sub.SiH4                      P.sub.TBP · P.sub.SiH4                               Reaction______________________________________76     8.3 × 10.sup.-4            3.3 × 10.sup.-9                      2.7 × 10.sup.-12                               occurred20     2.2 × 10.sup.-4            4.4 × 10.sup.-9                      9.7 × 10.sup.-13                               did not occur______________________________________ 
    
     From the above results, it is apparent that the P TBP   0  ·P SiH4   0  value is 1×10 -12  to 2×10 -12 . 
     Further, the same experiment as described above was conducted at 580° C. and as a result, it was found that the P TBP   0  ·P SiH4   0  value was 0.8×10 -12 . 
     From the results of the above-described two experiments, ΔH and ΔS can be determined according to the above-described formula (4) and are found to be the following respective values. 
     
         ΔH=-4.65×10.sup.3 [cal/mol] 
    
     
         ΔS=0.8272 [cal/mol·deg] 
    
     Therefore, the P TBP   0  ·P SiH4   0  value is given by the following formula: ##EQU5## 
     As described above, to prevent the formation of a solid phase nucleus, i.e., a reaction of the gaseous TBP with the gaseous D, it is necessary to satisfy a requirement represented by the following formula: 
     
         P.sub.TBP ·P.sub.D ≦P.sub.TBP.sup.0 ·P.sub.D.sup.0(6) 
    
     Specific examples of the means for satisfying the above-described requirement include a change in the P TBP   0  and P D   0  values through a variation in the growth temperature and a change in the P TBP  and P D  values through a variation in the growth pressure, the flow rate of carrier gas and the molar fractions of the gaseous TBP and D. Note the carrier concentration must of course be a desired value. 
     Further, the respective partial pressures, P TBP  and P D , of the TBP and the dopant can be expressed by the following formulae: ##EQU6## wherein P 0  is a total pressure, f 0  is a total flow rate, f TBP  is a flow rate of TBP, and f D  is a flow rate of the dopant. When the f TBP  value is assumed to be constant, the condition under which the doping can be conducted can be expressed by the following formula, through the substitution of the formulae (2) and (3) for the formula (1). ##EQU7## Further, since the carrier concentration n is proportional to the α power of the flow rate of the dopant and to the β power of the total pressure P 0 , it can be expressed by the following formula: 
     
         n=η(T)·f.sub.D.sup.α ·P.sub.0.sup.β(10) 
    
     wherein η(T) is a function of the temperature T and the flow rate f III  of the group III element compound and can be expressed by the following formula: ##EQU8## In the formula, A, B, α and β are each a constant determined by the kind of dopant. For example, in the case of monosilane (SiH 4 ), A=2.400×10 21 , B=-1.855×10 24 , α=1.2 and β=1.6; in the case of disilane (Si 2  H 6 ), A=9.60×10 20 , B=-5.50×10 23 , α=1.2 and β=1.6; and in the case of hydrogen sulfide (H 2  S), A=8.39×10 19 , B=-7.75×10 22 , α=1 and β=1.4. 
     From the formulae (4) and (5), however, the dependency of the carrier concentration capable of conducting doping upon the temperature T and the total pressure P 0 , is finally determined as shown in the formula (1). In the formula (1), the terms having a temperature dependency are η(T) and (P TBP   0  ·P D   0 ). These dependencies are expressed by the formulae (11) and (4). 
     The use of SiH 4  as a dopant will now be described in more detail. When the temperature is constant, the relationship between the carrier concentration n capable of conducting the doping and the total pressure P 0  is schematically shown in FIG. 1. The portion defined by an oblique line is a carrier concentration range wherein the doping can be conducted. It is apparent that the maximum carrier concentration is lowered with an increase in the pressure. Further, as the temperature is increased, both the η(T) and (P TBP   0  ·P D   0 ) values in the formula (1) are increased (see formulae (11) and (4)), and therefore, the oblique line in FIG. 1 shifts to the dotted line. This means that the resultant maximum carrier concentration is increased with an increase in the temperature. 
     In the actual growth of a crystal, when the pressure is very low or the temperature is very high, a sufficient group V element compound pressure can not be applied, and thus the quality of the crystal is deteriorated. Therefore, the growth is preferably conducted under the conditions of a temperature of from 580° to 650° C. and a pressure of from 10 to 50 Torr, from the practical viewpoint. 
     A method of lowering the growth pressure through the use of SiH 4  as a dopant in the growth of an InP crystal will now be described as an example. 
     The growth was conducted under the conditions of a growth temperature of 600° C., a growth pressure of 20 Torr (76 Torr in the prior art), a carrier gas (hydrogen) flow rate of 6 liters/min, a TBP flow rate of 50 cc/min, and a trimethyl indium (TMI as a source of In) flow rate of 0.25 cc/min. The flow rate of SiH 4  was varied from 3.4×10 -4  cc/min to 4×10 -3  cc/min. When the temperature of the substrate was raised, a flow of the carrier gas and TBP was allowed, to thereby prevent a dephosphorization of the substrate. When the temperature of the substrate reached a predetermined growth temperature, a flow of TMI and SiH 4  was allowed, to thus initiate the growth. After a growth for a desired time, the supply of TMI and SiH 4  was stopped to lower the temperature. The dependency of the n-type carrier concentration upon the flow rate of SiH 4  is shown in FIG. 2. A single crystal free from a surface defect was also obtained in a sample subjected to doping in the highest carrier concentration, i.e., 3×10 18  cm -3 . A photograph of the surface of the single crystal is shown in FIG. 3. Compared with the case wherein a pit is formed by doping in the growth in a lower carrier concentration, i.e., 6.8×10 17  cm -3 , under a pressure of 76 Torr (see FIG. 5), it is apparent that a lowering in the growth pressure effectively eliminates this problem. 
     As described above, when an n-type doped InP crystal is grown by using trimethyl indium (TMI) and tert-butylphosphine (TBP) as sources and silane (SiH 4 ) as a dopant, five factors, i.e., TMI flow rate, TBP flow rate, SiH 4  flow rate, growth pressure (Pg) and growth temperature (Tg), are considered to be the growth conditions necessary for determining the carrier concentration. 
     The change of the carrier concentration where four factors among the five growth conditions are fixed, with only one factor being varied, will now be described. 
     [Dependency upon TMI Flow Rate] 
     The TMI flow rate is inversely proportional to the carrier concentration. Specifically, when the TMI flow rate is doubled the growth rate is doubled, and thus the carrier concentration is reduced by half. 
     [Dependency upon TBP] 
     As the TBP flow rate is increased, the carrier concentration is decreased. This is shown in FIG. 6. 
     [Dependency upon SiH 4  ] 
     When the flow rate of SiH 4  is increased, the carrier concentration is increased substantially in proportion to the increase of the flow rate (see FIG. 2). 
     [Dependency upon Growth Pressure] 
     When the growth temperature (Pg) is increased, the carrier concentration is increased in proportion to the second power of the Pg value. This is shown in FIG. 7. 
     [Dependency upon Growth Temperature] 
     The carrier concentration is increased with an increase in the growth temperature (Tg). This is shown in FIG. 8. 
     Nevertheless, it is preferable to obtain a desired carrier concentration through the regulation of the flow rate of SiH 4 , while the growth conditions other than the flow rate of SiH 4  are fixed. 
     As described above, according to the present invention, a good doping becomes possible in a growth wherein use is made of TBP, which is a safe group V element source.