Patent Publication Number: US-5891243-A

Title: Production of heavy doped ZnSe crystal

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of producing a p-type ZnSe crystal having low resistivity useful as a material for a blue laser, a blue light emitting device, etc.. ZnSe has been anticipated as a material for a blue laser, a blue light emitting device and so on, since its band gap corresponds to blue luminescence. A ZnSe crystal has been produced so far by a molecular beam epitaxy (MBE) process or a metal organic vapor deposition (MOCVD) process. 
     According to the MBE process, a metal element Zn and a non-metallic element Se are vaporized in an atmosphere at an ultra-high degree of vacuum so as to obtain an intensity of a molecular beam corresponding to vapor pressure of each element, and a crystal is grown up while controlling an atomic layer. 
     On the other hand, thermal decomposition of an organometallic compound is used for production of a ZnSe crystal in the MOCVD process. 
     In order to obtain a p-type ZnSe crystal, it is necessary to dope ZnSe with N during crystal growth. For N doping, gaseous N 2  molecules for instance are dissociated with electromagnetic wave RF, and radical N is introduced to ZnSe epitaxially growing on a substrate. 
     Since ZnSe is doped with sole N during crystal growth in a conventional method, acceptor concentration of N is not increased more than 10 18  cm -3 . Even if further addition of N to ZnSe is tried, it is impossible to increase concentration of the carrier due to the compensation mechanism; a N atom located at a substitutional position of Se transfers to an interstitial position when acceptor concentration of N exceeds 10 18  cm -3  and turns to a donor (n-type), so that N atoms acting as p-type acceptors are diseffected by N atoms acting as p-type donors. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to offer a ZnSe single crystal having low resistivity heavily doped with N at acceptor concentration of 10 19  -10 21  cm -3 . 
     According to the present invention, atomic N +   gas prepared by dissociating N 2  gas with electromagnetic irradiation and a gaseous complementary element X acting as a n-type dopant are prepared at an atomic ratio of N:X being 2:1. The atomic gases are fed to a region where a ZnSe crystal is epitaxially grown on a substrate by MBE or MOCVD process. 
     The atomic gases having the ratio of N:X being 2:1 may be prepared by decomposing XN with electromagnetic irradiation at a high temperature and adding N 2  gas to the decomposed product. 
     The substitutional element X may be In, Ga, Al etc.. In the hereunder explanation, the substitutional element X is represented by In. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a model of a ZnSe crystal simultaneously doped with In and N. 
     FIG. 2 is a schematic view illustrating an MBE apparatus for growth of a ZnSe crystal. 
     FIG. 3 is a schematic view illustrating an MOCVD apparatus for growth of a ZnSe crystal. 
     FIGS. 4(a) and 4(b) are graphs of electronic state density which shows that N substitutionally occupying a position of Se serves as a p-type acceptor, while In substitutionally occupying a position of Zn serves as a n-type donor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     When ZnSe is simultaneously doped with In acting as a n-type dopant and N acting as a p-type dopant at a ratio of In:N being 1:2, a donor In -   Zn  derived from In substitutionally occupying a position of Zn makes a couple with an acceptor N +   Se  derived from N substitutionally occupying a position of Se, and the replacement is stabilized due to an electrostatic energy generated by coupling +e with -e. The electrostatic energy stabilizes N which is further added to ZnSe. As a result, ZnSe is heavily doped with the p-type carrier (a hole) at acceptor concentration of 10 19  -10 21  cm -3 , although the acceptor concentration does not exceeds 10 18  cm -3  or so by addition of sole N. 
     The reason why simultaneous In and N doping effectively increases the concentration of the p-type carrier is supposed as follows: When ZnSe is simultaneously doped with In and N at a ratio of 1:2, In substitutionally occupies a position of Zn, and N substitutionally occupies a position of Se, as illustrated in a model shown in FIG. 1. The n-type dopant In makes an atomic 1:1 couple with the p-type dopant N, and another one N atom coordinates near the atomic couple and serves as an acceptor. Consequently, the acceptor maintains its activity up to a higher concentration level so as to enable heavy doping with the p-type dopant N at high concentration. 
     According to MBE process for growth of a ZnSe crystal, a substrate 2 is located in a vacuum chamber 1, and a Zn source 3 and a Se source 4 are individually faced to the substrate 2, as shown in FIG. 3. Zn and Se vapors are generated by heating the Zn source 3 and the Se source 4 with corresponding heaters 5, fed onto the substrate 2 and epitaxially grown as a ZnSe crystal 6 on the substrate 2. 
     During growth of the ZnSe crystal, N 2  gas is irradiated with electromagnetic wave generated by a radio frequency (RF) coil 7 and dissociated to N + . The resulting ionized N +   gas is fed to the crystal growth region. At the same time, vapor In generated by heating an In source 8 with a heater 5 is also fed to the crystal growth region. The vapor In may be also supplied from a vapor source. 
     When a flow rate of ionized N +   gas is controlled in relation with vapor In so as to adjust the atomic ratio of ionized N +   gas to vapor In at 2:1, the ZnSe crystal 6 growing on the substrate 2 becomes a p-type crystal heavily doped with N. 
     According to MOCVD process, an organic Zn gas g 1 , an organic In gas g 2 , N 2  gas g 3  and an organic Se gas g 4  are individually decomposed and dissociated to atomic Zn, In, N +   and Se with electromagnetic irradiation, and fed to a substrate 2. In this case, a flow rate of ionized N +   gas is also controlled in relation with vapor In so as to adjust an atomic ratio of ionized N +   gas to vapor In at 2:1. Due to the flow rate control, a ZnSe crystal 6 growing on the substrate 2 becomes a p-type crystal heavily doped with N. 
     The source of ionized N +   and In vapor may be a compound of InN in any above-mentioned method of epitaxial crystal growth. In this case, InN is decomposed at a high temperature by irradiation with electromagnetic wave and supplied to a region for epitaxial crystal growth. Additional N 2  gas may be fed to the region for epitaxial crystal growth to adjust the atomic ratio of ionized N +   gas to In vapor at 2:1. 
     N incorporated in the ZnSe crystal substitutionally occupies a position of Se and serves as a p-type acceptor N +   Se , while In substitutionally occupies a position of Zn and serves as a n-type donor In -   Zn . When the ZnSe crystal is doped with N and In at a ratio of 1:1, an acceptor charged with +e makes a couple with a donor charged with -e, so as to stabilize an electrostatic energy. When another one N is added to the ZnSe crystal stabilized in this way, a position of Se is stably substituted by N. Consequently, ZnSe can be heavily doped with N. 
     An upper column (a) of FIG. 4 shows a density of state of the N-doped ZnSe crystal wherein N atom substituted for Se. It is noted that N served as a p-type acceptor. On the other hand, it is noted that In served as a n-type donor from the lower column (b) of FIG. 4 showing a density of the In-doped ZnSe crystal wherein In atom substituted for Zn. From these data on the density of state, it is understood that an electrostatic energy can be stabilized by simultaneous N and In doping. 
     EXAMPLE 
     A ZnSe single crystal was used as a substrate 2 for growth of a ZnSe crystal thereon and located in a vacuum chamber 1 held at 1.3×10 -8  Pa. A ZnSe crystal was epitaxially grown at 250°-400° C. by supplying vapor Zn at a pressure of 1.3×10 -5  Pa., vapor Se at a pressure of 1.3×10 -5  Pa., ionized N +   at a pressure of 1.3×10 -7  Pa. and vapor In at a pressure of 6.5×10 -7  Pa., respectively. Said ionized N +   was prepared by irradiating N 2  gas with electromagnetic wave in a microwave range. 
     The obtained ZnSe crystal had an acceptor concentration which varied in response to a crystal growth temperature, as shown in Table 1. The ZnSe crystal was doped with N at a higher concentration at any crystal growth temperature, compared with a ZnSe crystal which was doped with sole N without feeding vapor In. 
     
                       TABLE 1
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EFFECT OF SIMULTANEOUS N AND In DOPING ON
ACCEPTOR CONCENTRATION
          acceptor concentration (cm.sup.-3)
temperature of
            simultaneous doping
substrate (°C.)
            with N and In
                         doping with sole N
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250         1 × 10.sup.18
                         1 × 10.sup.17
300         2 × 10.sup.19
                         1 × 10.sup.17
350         2 × 10.sup.20
                         3 × 10.sup.17
400         2 × 10.sup.21
                         1 × 10.sup.18
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     The similar simultaneous In and N doping was adopted in MOCVD process. In this case, a ZnSe crystal was also heavily doped with a p-type carrier at an acceptor concentration of 10 19  -10 21  cm -3 . 
     According to the present invention as aforementioned, ZnSe is simultaneously doped with a n-type dopant X and a p-type dopant N. Due to the simultaneous doping, a ZnSe crystal can be heavily doped with the p-type carrier at an acceptor concentration of 10 19  -10 21  cm -3 , compared that the acceptor n of the p-type carrier has been not more than 10 18  cm -3  or so in a conventional method. 
     The p-type ZnSe crystal obtained in this way exhibits excellent electric properties such as great p-type electronic conductivity and consequent low resistivity, since substantially all the N atoms incorporated in the crystal serve as active acceptors. Consequently, the doped ZnSe crystal is used as a material for an intensified blue laser, a blue light emitting device or the like in a high-density optical memory device, a full-color display, etc..