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Ligand Exchange Reactions in Ingaas Metalorganic Vapor-Phase Epitaxy

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Ligand Exchange Reactions in Ingaas Metalorganic Vapor-Phase Epitaxy Journal of Crystal Growth 191 (1998) 332Ð340 Ligand exchange reactions in InGaAs metalorganic vapor-phase epitaxy M.J. Kappers, M.L. Warddrip, R.F. Hicks* Department of Chemical Engineering, University of California at Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095-1592, USA Received 16 July 1997; accepted 23 January 1998 Abstract Metalorganic vapor-phase epitaxy of InyGa1~yAs from triethylgallium, trimethylindium and tertiarybutylarsine was studied using on-line infrared spectroscopy to monitor the organometallic compounds in the feed and e§uent gases. The film composition was measured by X-ray di¤raction. Ligand exchange reactions between the group III sources were found to occur in the feed lines. The new species produced were trimethylgallium, dimethylethylgallium, methyldiethyl- gallium, dimethylethylindium and methyldiethylindium. The thermal stability of these species varied over a wide temperature range. For example, the ethylindium compounds started to decompose at 250¡C, while trimethylgallium began to react at 500¡C. In the square-duct reactor used in this study, the wide variation in the reactivity of the precursors resulted in films that were indium rich near the reactor inlet and gallium rich near the reactor outlet. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 82.30.Hk; 81.05.Ea; 81.15.Gh; 82.80.Ch Keywords: Ligand exchange; Metalorganic vapor-phase epitaxy; Infrared spectroscopy; InGaAs 1. Introduction highly uniform in composition, thickness and cry- stallinity, since slight variations in these properties The fabrication of In0.53Ga0.47As/InP hetero- will seriously degrade the quality of the InGaAs junction bipolar transistors (HBTs) requires accu- epilayer [1]. rate control of the indium content to within 0.1% Low-temperature growth of InGaAs is of interest in order to lattice match the InGaAs layer to the for manufacturing high-speed HBTs. Heavily p- InP substrate and to obtain the highest electron type doped InGaAs base layers are fabricated by mobilities. It is essential that the InGaAs layer is incorporating carbon into the film using CCl4 at growth temperatures between 425¡C and 500¡C [2Ð4]. Higher temperatures are not e¤ective at * Corresponding author. Tel.: #1 310 206 6865; fax: #1 310 achieving high doping levels. Trimethylindium 206 4107; e-mail; [email protected]. (TMIn) and trimethylgallium (TMGa) are typically 0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S0022-0248(98)00174-2 M.J. Kappers et al. / Journal of Crystal Growth 191 (1998) 332±340 333 used as sources for high-temperature InGaAs were used as a measure of concentration. The peak growth [5Ð7]. However, for low-temperature heights were related to the species concentration deposition, triethylgallium (TEGa) must be sub- through the following calibration procedure. The stituted for TMGa. concentrations of the sources were determined us- Ligand exchange reactions between trimethyl- ing mass spectrometry by measuring the hydrocar- amine alane and TMGa precursors were observed bons produced during complete pyrolysis of each during AlGaAs MOVPE, leading to the growth of organometallic compound at 650¡C. The mass non-uniform layers [8]. Recently, we found that spectrometer signals were calibrated using hydro- exchange reactions occur between diethylzinc and carbon gas standards. The hydrocarbon concentra- dimethylcadmium in the feed lines on the way to tions were summed and converted to a molar the reactor [9]. It can be expected that similar concentration of the precursor using the ideal gas reactions occur during the growth of InGaAs using law. Then, a graph of the peak height of a particular TEGa and TMIn, which may lead to compositional infrared band versus the precursor concentration non-uniformities in the film. was prepared. From the slope of the resulting In this study, on-line infrared spectroscopy was straight line, a calibration factor relating the inten- used to monitor the gas composition of the reactor sity of the infrared band to the precursor concen- feed and e§uent during MOVPE of InGaAs below tration was obtained. Using this calibration factor, 600¡C. Ex situ X-ray di¤raction was used to deter- the organometallic concentrations were calculated mine the InGaAs composition over the length of from the measured infrared peak heights. the film. It is shown that exchange reactions indeed In the following section, the results are discussed take place between TEGa and TMIn, resulting in terms of the relative pressure of an organometal- in the segregation of indium and gallium in the lic compound in the gas e§uent of the MOVPE InGaAs layers. reactor. The relative pressure is defined as xi, where P 2. Experimental methods x " i i 0 # 0 . (1) PTMI/ PTEG! The InGaAs films were deposited on the glass Here, P is the partial pressure of the ith or- walls of a low-pressure tube reactor at 99 Torr. i ganometallic compound, and the superscript 0 Electronic-grade TMIn, TEGa and TBAs were refers to the partial pressures of the sources (TEGa contained in stainless steel bubblers immersed in and TMIn) initially introduced into the gas thermal regulating baths at 25¡C, 20¡C and 4¡C, manifold. respectively. The precursors were vaporized into Each InGaAs film was analyzed over the length 3Ð280 cm3/min of hydrogen, then diluted with hy- of the glass reactor tube using a h/2h scan on drogen to a total flow between 100 and 1200 cm3/ a Crystal Logic powder di¤ractometer. The tube min (NTP). The hydrogen was purified by di¤usion was sectioned into 1 cm pieces, which were split through a palladium membrane. The tubular glass axially to reveal the film on the inner wall. Di¤rac- reactor and heater design have been described tion patterns were obtained by scanning for the previously [10,11]. (1 1 1) and (3 1 1) peaks at 2h ranges of 24¡Ð29¡ and The organometallic compounds in the reactor 48¡Ð55¡, respectively, in steps of 0.01¡. Using feed and e§uent were identified by infrared and Vegard’s law and the measured peak position, the mass spectroscopies (Leybold Inficon 300 amu In Ga As film composition was calculated from mass spectrometer). Infrared spectra of the gas y 1~y streams were collected on a BioRad FTS-7 infrared a!a y" G!A4 , (2) spectrometer by passing them through a small flow a !a cell, 17.7 cm long by 3.0 cm in diameter, that was I/A4 G!A4 sealed with KBr windows. Peak heights of the char- where a is the lattice constant found by Bragg’s law, _ acteristic metalÐcarbon stretches of each species aG!A4 is the lattice constant of GaAs (5.6532 A), and 334 M.J. Kappers et al. / Journal of Crystal Growth 191 (1998) 332±340 _ aI/A4 is the lattice constant of InAs (6.0584 A). The peak position was taken to be 2h corresponding to the maximum intensity value after the di¤raction pattern was smoothed to reduce the noise. The value of y is calculated to within 5% error. 3. Results 3.1. Ligand exchange reactions Fig. 1 shows the infrared spectra of a mixture of TEGa and TMIn as a function of the relative pres- 0 sure of TMIn initially fed to the reactor, xTMI/. The absorption bands appearing between 850 and ~1 Fig. 2. Infrared spectra of the individual organometallic com- 450 cm are characteristic of the individual or- pounds in the feed after mixing TMIn and TEGa, (a) the or- ganometallic molecules in the gas. The bands oc- ganogallium species, and (b) the organoindium species. curring in this frequency range are due to the CÐH bending modes of the alkyl ligands (850Ð550 cm~1) and the stretching modes of the metalÐcarbon tain several new absorption bands near 770 cm~1, bonds (750Ð450 cm~1) [12,13]. A comparison of between 590 and 550 cm~1, and between 500 and 0 ~1 the infrared spectra of the mixtures, xTMI/ equal to 475 cm . 0.3 and 0.7, to those of the pure compounds, The new bands show similarities with those of 0 xTMI/ equal to 0.0 and 1.0, reveals that they are not the TEGa and TMIn sources initially fed, sugges- a simple weighted summation of the infrared ting the formation of new organometallic com- spectra of the latter compounds. Instead, they con- pounds by ligand exchange reactions [9]. Five new organometallic compounds are identified from the reaction between TEGa and TMIn. The infrared spectra of each organometallic compound in the mixture are shown in Fig. 2. The band positions in the spectral region between 850 and 450 cm~1 and the proposed assignments for all the modes are summarized in Table 1. The infrared spectra of the individual compounds are obtained from the mix- ture by spectral subtraction, as described elsewhere [9]. One of the infrared spectra is identical to the known vibrational spectrum of TMGa [14]. On the other hand, the characteristic InÐC stretch at 470 cm~1 for triethylindium (TEIn) is not observed in the mixture, but is included in Table 1 for refer- ence. The measured peak positions for TMIn, TMGa and TEGa agree well with those previously reported in the literature [14Ð16]. The remaining four species are identified as methyldiethylgallium (MDEGa), dimethylethylgallium (DMEGa), methyl- Fig. 1. Infrared spectra of the organometallic compounds in the diethylindium (MDEIn), and dimethylethylindium reactor feed during InGaAs MOVPE at varying ratios of TEGa (DMEIn), which are the obvious products of ligand to TMIn. exchange between TEGa and TMIn. M.J. Kappers et al. / Journal of Crystal Growth 191 (1998) 332±340 335 Table 1 Infrared band assignments of the organometallic compounds for the spectral region between 900 and 450 cm~1 Organometallic compounds Mode! TMGa DMEGa MDEGa TEGa TMIn DMEIn MDEIn TEIn" l(MC)!P 583 578 569 544 497 494 486 470 l(MC)!R 570 565 560 509 o @ (CH3)(e ) 771 769 739 738 729 728 700 Ð o A (CH3)(a2) 723 694 661 652 693 660 640 Ð !l(MC) refers to the asymmetrical metalÐcarbon stretching mode, which shows a P- and R-branch in the gas-phase (no Q-branch is o @ A observed), (CH3) refers to the CH3 rocking mode with either the e symmetry or the a2 symmetry.
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