Abstract:
A magnesium amide for use as a magnesium donor not having any Mg--C bonds. The compound is useful for doping GaN with Mg +2 . The compound of the present invention is a high molecular weight dimer, preferably a diamide containing one or more silicon substituent groups. Alternatively, the compounds of the present invention may contain amino nitrogens weakly bonded to Mg. The compounds must have sufficient volatility to be useful in chemical vapor deposition.

Description:
This application claims priority from U.S. Provisional patent application Serial No. 60/029,606 filed Oct. 24, 1996. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to dopants for semiconducting materials and, more particularly, to magnesium compounds suitable for making p-type gallium nitride (GaN). 
     BACKGROUND OF THE INVENTION 
     The preparation of blue light-emitting materials has become a heavily researched field in recent years. This interest is caused by the significant number of potential applications for blue light emitters. The two most commonly used light emitting devices (LED&#39;s) are liquid crystal displays (LCDS) and laser diodes. The two most technologically significant applications of blue LED&#39;s are electroluminescent displays and read/write heads for optical data storage. Full color electroluminescent red, green and blue (RGB) displays cannot be constructed because they are neither pure green nor pure blue LEDS. 
     Because the storage density increases inversely to the square of the light source wavelength, a blue LED laser-based optical data storage device, such as a CD-ROM, could store on the order of five times as much data as a standard red LED laser-based CD-ROM. This translates to about 3.25 Gbytes of data for a blue laser based CD, versus 650 Mbytes for the commonly used red laser based CDs. Another less obvious, significant, use of blue (or blue-green) LED&#39;s is in traffic signals. Traditional signals utilize a white incandescent light source with an appropriately colored glass filter. The use of an incandescent source, as well as a filter, yields a very inefficient device. The efficiency and longevity of traffic signals can be enhanced through the use of blue, LED&#39;s, as well as commonly available red and yellow LEDS. The energy cost savings and the lower maintenance requirements, when multiplied by the number of traffic signals in an average city, represent vast cost reductions. This approach is currently in use in Japan, where blue traffic signals, are used. 
     GaN and zinc selenide (ZnSe) have emerged as strong candidates for blue light-emitting materials; however, ZnSe suffers from short device lifetimes, relative to GaN. Furthermore, II-VI compounds are rather fragile and are grown at comparatively low temperatures. Thus, work has recently been expanding on GaN materials. 
     The thermodynamically stable phase of GaN at room temperature and atmospheric pressure is the hexagonal wurzite phase. This material is a direct bandgap semiconductor with a band gap of 3.45 eV. Light emitting diodes require a p-n junction. The fabrication of n-type GaN is not difficult, However, the fabrication of p-type GaN has presented challenges. Zinc has been used as a p-type dopant for GaN, however, its large size and propensity for forming covalent bonds have limited is usefulness. Magnesium has emerged as the dopant of choice for a p-type GaN. The most commonly utilized magnesium sources in the CVD (or MOMBE) growth of GaN:Mg have been organometallic compounds such as bis[cyclopentadienyl]magnesium (Cp 2  Mg), bis[3(dimethyl)propyl]magnesium, and bis[3-(diethylamino)propyl]magnesium. The drawback of these sources is that they create a large carbon impurity (MgC) in the deposited GaN, which destroys the material&#39;s electronic usefulness. To compensate for the detrimental effects of the carbon, two approaches have been used. The first is to apply several folds more Mg than needed. This, however, causes lattice deterioration, is expensive, and the devices burn out quickly. The second approach is to attempt to remove the carbon. To counter the formation of MgC, large quantities of H 2  have been injected into the reactor to form CH 4 , which vaporizes. However, this in turn causes the formation of MgH 2 , which also passivates the material. 
     SUMMARY OF THE INVENTION 
     The present invention is a magnesium donor designed to avoid the above problems. The compound is a volatile magnesium precursor lacking magnesium-carbon bonds. In its broadest sense, The invention encompasses all volatile Mg compounds not containing only Mg--C bonds. Since the final material, theoretically, places magnesium on a gallium site, a source material containing only magnesium-nitrogen bonds would seem most promising. The present invention, therefore, is a class of compounds useful for doping GaN with Mg. The compounds are volatile magnesium amides, not containing any Mg--C bonds. The most commonly prepared magnesium amide is tetrakis[bis(trimethylsilyl)amido]dimagnesium. This compound is a high molecular weight dimer. A more volatile, monomeric compound would be preferable for CVD applications. Optimally, the compounds are diamides containing one or more silicon groups. The compounds may contain amino-nitrogens weakly bound to Mg. The compounds must be volatile. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. 
     FIG. 1 is a thermogravimetric trace of Mg[N(TMS)CH 2  CH 2  CH 2  NMe 2  ] 2 . 
     FIG. 2 is a ball and stick depiction of the structure of Mg[N(TMS)CH 2  CH 2  CH 2  NMe 2  ] 2 . 
     FIG. 3 is a depiction of the dative p-d bonding and metal disilylamides. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is a class of compounds useful for depositing Mg 2+   onto GaN. The compounds are magnesium amides having a sufficient volatility to be useful in chemical vapor deposition (CVD). The Mg is not bound to any carbons and is preferably bound only to nitrogen. However, a compound may fall within the bounds of the invention if it has one hydrogen or alkyl-Mg bond if the compound is sufficiently volatile. In other words, compounds such as HMgNR 1  R 2  and R 3  MgNR 1  R 2 , where R 1 , R 2 , or R 3  represent alkyl, are included in the invention if they have sufficient volatility. 
     Preferably, the compounds are diamides, having two amide groups connected to the Mg. Most preferably, the compounds are four--coordinate diamides, having two Mg--N amide bonds and two Mg--N bonds from tertiary amines. Also, most preferably, at least one of R 1  or R 2  is a silicon substituent. The compounds should have a vapor pressure of about 10 -2  torr at 100° C. 
     In a preferred embodiment, the compounds have the general structure: ##STR1## wherein L 1  and L 2  are not C. More preferably, the compounds have the general structure: ##STR2## wherein R 1  -R 6  are alkyl. One compound of the general structure has been synthesized and characterized, having the structure: ##STR3## 
     Magnesium bis-[γ-dimethylaminopropyl)trimethylsilylamide], 200 FIG. 2, was prepared by the reaction of the free amine with dibutylmagnesium in hexane. It is a white solid at ambient conditions and has been shown to be a monomer in the solid state by single crystal x-ray diffraction. The compound is volatile, as demonstrated by its thermogravimetric trace 100 (FIG. 1), and can be sublimed at 80° C. and 0.01 Torr. The compound, like most other group 2 element amides, is quite air sensitive. 
     Single crystals of the compound were obtained by slow sublimation. A crystal of suitable quality for x-ray diffraction was selected and mounted in a glass capillary in an inert atmosphere glove box. The unit cell was determined from 25 centered reflections with 2θ values between 20° and 35°. Unit cell data and collection parameters are given in Table 1. The magnesium atom is tetrahedrally bonded to four nitrogen atoms; two from amide bonds and two from coordinate covalent bonds to tertiary amines (FIG. 2). The amide nitrogen atoms are planar, indicating an sp 2  hybridization, due to dative bonding from the full nitrogen p-orbital to the empty silicon d-orbital 300, as shown in FIG. 3. Interatomic distance and angle data is given in Tables 2 and 3. 
     N-(γ-dimethylaminopropyl)trimethylsilylamine 200. Under argon, in a 500 ml Schlenk flask containing a magnetic stir bar, 49.38 g (0.480 mol) of 3-dimethylaminopropylamine (99%, Aldrich) was added to 120 ml of anhydrous hexanes (distilled from Na). This mixture was stirred and cooled with a constant temperature bath at 10° C. From a dropping funnel, 25 g of TMS-Cl (0.230 mol) were added dropwise to the mixture over a period of two hours. A white precipitate formed, and later redissolved forming a separate layer at the bottom of the flask. Once the TMS-Cl addition was complete, 200 ml of anhydrous ether (distilled from Na/K) was added to the flask. The mixture then was rapidly stirred overnight causing the two layers to merge into a homogeneous solution over a white precipitate. The solution was separated from the white solid by cannulation and distilled at ambient pressure to give TMS-NH--CH 2  CH 2  CH 2  NMe 2  at 160° C. Yield: 85%. Characterization:  1  H NMR: (400 MHz, positive δ downfield referenced to Si(CH 3 ) 4  =0 ppm utilizing residual CDCl 3  =7.24 ppm in solvent CDCl 3 ) 2.72 [m, 2H, --CH 2  NMe 2  ], 2.24 [m, 2H, TMS-NH--CH 2  --], 2.20 [s, 6H, --NCH 3  ], 1.54 [m, 4H, TMS-NH--CH 2  CH 2  --], 0.02 [s, 18H, CH 3  Si--].  13  C{ 1  H} NMR: (75 MHz, positive δ downfield referenced to Si(CH 3 ) 4  =0 ppm utilizing residual C 6  D 5  H=128.0 ppm in solvent C 6  D 6 ) 57.45 [--CH 2  NMe 2  ], 45.40 [--NCH 3  ], 40.00 [TMS-N--CH 2  --], 32.66 [TMS-N--CH 2  CH 2  --], -0.20 [(CH 3 ) 3  Si--N--]. Mass Spectrum: (EI, 70 eV) 174 [TMS-NHCH 2  CH 2  CH 2  NMe 2  ] + , 159 (M--Me), 129, 114, 100, 85, 72, 58. 
     Magnesium bis[N-(γ-dimethylaminopropyl)trimethylsilylamide]. (FIG. 2) In a 100 ml Schlenk flask equipped with a magnetic stir bar and argon purge were combined 4.46 g of N-(γ-dimethylaminopropyl)trimethylsilylamine and 10 ml of anhydrous THF (twice distilled from Na). The contents of the flask were stirred and cooled to -10° C., then 12 ml of MgBu 2  (1M solution in heptane) were added slowly by syringe over a period of two minutes. The solution was allowed to warm to room temperature and then was heated to reflux for 5 hours. The flask was allowed to cool to room temperature. The THF was removed under vacuum leaving an off-white solid. The solid was sublimed at 80° C. and 10 -4  mm Hg to yield purified Mg(N{TMS}CH 2  CH 2  CH 2  NMe 2 ) 2  as a crystalline, white solid. Yield: 52%. Characterization: Mp 106° C. TGA: (FIG. 1 ) Onset of weight loss, 141° C.; 1.2% residue at 500° C.  29  Si{ 1  H} NMR: (79.5 MHz, in C 6  D 6 , positive δ downfield referenced to Si(CH 3 ) 4  =0 ppm) -8.16 [SiMe 3  ].  1  H NMR: (400 MHz, positive δ downfield referenced to Si(CH 3 ) 4  =0 ppm utilizing residual C 6  D 5  H=7.15 ppm in solvent C 6  D 6 ) 3.41 [m, 2H, --CH 2 (a) NMe 2  ], 3.03 [m, 2H, --CH 2 (b) NMe 2  ], 2.15 [s, 6H, --NCH 3 (a) ], 2.05 [m, 4H, TMS-N--CH 2  CH 2  --], 1.71 [s, 6H, --NCH 3 (b) ], 1.57 [m, 2H, TMS-N--CH 2 (a) --], 1.28 [m, 2H, TMS-N--CH 2 (b) --], 0.49 [s, 18H, (CH 3 ) 3  Si--].  13  C{ 1  H} NMR: (100 MHz, positive δ downfield referenced to Si(CH 3 ) 4  =0 ppm utilizing residual C 6  D 5  H=128.0 ppm in solvent C 6  D 6 ) 62.91 [--CH 2  NMe 2  ], 49.11 [TMS-N--CH 2  --], 47.78 [--NCH 3 (a) ], 45.82 [--NCH 3 (b) ], 32.70 [TMS-N--CH 2  CH 2  --], 2.37 [(CH 3 ) 3  Si--N--]. Mass Spectrum: (EI, 70 eV) 370.3 [Mg(N{TMS}CH 2  CH 2  CH 2  NMe 2 ) 2  ] + , 353.2, 324.4, 306.3, 277.3, 250.3, 196.2, 174.2, 159.2, 146.1, 129.1, 114.1, 100.1, 85.1, 73.1, 58.1. Elemental Analysis: Calculated: C, 51.9%; H, 11.4%; N, 15.1%. Found: C, 52.0%; H, 12.5%; N, 15.2%. 
     
                       TABLE 1______________________________________Unit cell and data collection parameters.______________________________________Empirical Formula MgC.sub.16 H.sub.42 N.sub.4 Si.sub.2  Formula Weight 371.01 g/mol.  Temperature 293(2) K  Wavelength 0.71073 Å  Crystal System monoclinic  Space Group P2.sub.1 /n  Unit Cell Dimensions a = 10.188(2) Å   b = 16.401(5) Å β = 101.79(3)°   c = 15.208(7) Å  Volume 2487.6(15) Å.sup.3  Z 4  Density (calc&#39;d) 0.991 g/cc  Absorption Coefficient 0.17 mm.sup.-1  F(000) 824.94  θ Range for Data Collection 2.29° to 25°  Index Ranges -12 ≦ h ≦ 11, 0 ≦ k ≦ 19,               0 ≦ 1 ≦ 18  Reflections Collected 4771  Independent Reflections 4360  Refinement Method Full-matrix least-squares on F.sup.2  Data/Restraints/Parameters 1793/0/208  Goodness of Fit on F.sup.2 2.45  Final R Indices [I &gt; 2σ(I)] R = 0.079, R.sub.w = 0.085  R Indices (all data) R = 0.163, R.sub.w = 0.096  Largest Difference Peak and Hole 0.340 and -0.180 e/Å.sup.3______________________________________ 
    
     
                       TABLE 2______________________________________Interatomic distances.    Atoms      Distance(Å)                      Atoms   Distance(Å)______________________________________MG-N1      1.979(7)    N1-C1     1.495(12)  MG-N2 2.189(8) N2-C3 1.459(17)  MG-N3 1.969(7) N2-C4 1.482(15)  MG-N4 2.212(8) N2-C5 1.442(17)  SI1-N1 1.668(7) N3-C9 1.503(14)  SI1-C6 1.835(12) N4-C11 1.437(19)  SI1-C7 1.853(12) N4-C12 1.418(17)  SI1-C8 1.859(10) N4-c13 1.427(18)  SI2-N3 1.682(7) C1-C2 1.463(18)  SI2-C14 1.874(11) C2-C3 1.537(23)  SI2-C15 1.872(12) C9-C10 1.493(23)  SI2-C16 1.820(12) C10-C11 1.51(3)______________________________________ 
    
     
                       TABLE 3______________________________________Interatomic angles.    Atoms       Angle (°)                      Atoms    Angle (°)______________________________________N1-MG-N2    97.0(3)    MG-N2-C3   113.6(7)  N1-MG-N3 137.6(3) MG-N2-C4 106.4(6)  N1-MG-N4 105.5(3) MG-N2-C5 113.0(7)  N2-MG-N3 107.1(3) C3-N2-C4 115.1(9)  N2-MG-N4 111.8(3) C3-N2-C5 104.9(10)  N3-MG-N4 97.4(3) C4-N2-C5 103.5(10)  N1-SI1-C6 112.6(5) MG-N3-SI2 130.2(4)  N1-SI1-C7 111.3(5) MG-N3-C9 115.8(6)  N1-SI1-C8 111.1(4) SI2-N3-C9 114.0(6)  C6-SI1-C7 105.4(6) MG-N4-C11 114.5(8)  C6-SI1-C8 108.8(6) MG-N4-C12 114.8(8)  C7-SI1-C8 107.3(5) MG-N4-C13 105.8(7)  N3-SI2-C14 111.7(4) C11-N4-C12 106.2(13)  N3-SI2-C15 113.4(5) C11-N4-C13 112.6(12)  N3-SI2-C16 111.4(4) C12-N4-C13 102.5(13)  C14-SI2-C15 104.9(6) N1-C1-C2 116.5(9)  C14-SI2-C16 108.5(6) C1-C2-C3 115.8(11)  C15-SI2-C16 106.4(5) N2-C3-C2 112.6(10)  MG-N1-SI1 128.3(4) N3-C9-C10 116.8(10)  MG-N1-C1 116.9(5) C9-C10-C11 115.1(12)  SI1-N1-C1 114.8(6) N4-C11-C10 114.4(13)______________________________________ 
    
     
                       TABLE 4______________________________________Atomic coordinates.    Atom    x            y         z______________________________________MG      0.85598(23)  0.34056(14) 0.19779(15)  SI1 1.08267(25) 0.46996(16) 0.30000(18)  SI2 0.68260(29) 0.32077(17) 0.35870(18)  N1 1.03505(65) 0.39205(39) 0.23031(43)  N2 0.92555(94) 0.21749(46) 0.17523(55)  N3 0.69597(65) 0.32860(44) 0.25060(45)  N4 0.75620(88) 0.39939(58) 0.07076(50)  C1 1.14391(99) 0.35599(72) 0.18994(79)  C2 1.16103(122) 0.26754(86) 0.19764(105)  C3 1.04893(168) 0.21620(73) 0.14057(92)  C4 0.93137(158) 0.17209(69) 0.26023(84)  C5 0.83008(149) 0.17175(71) 0.11077(101)  C6 1.18284(129) 0.54536(76) 0.25351(99)  C7 1.19092(119) 0.43522(93) 0.40649(73)  C8 0.93533(100) 0.52203(58) 0.32934(67)  C9 0.56473(109) 0.32078(88) 0.18474(90)  C10 0.53485(125) 0.38379(109) 0.11251(107)  C11 0.61756(172) 0.37769(112) 0.04093(111)  C12 0.81851(191) 0.38511(122) -0.00308(90)  C13 0.77494(191) 0.48495(91) 0.08512(94)  C14 0.59022(163) 0.22617(80) 0.37928(80)  C15 0.58646(123) 0.40651(79) 0.39686(81)  C16 0.84655(119) 0.31997(71) 0.43364(66)______________________________________