III/V Semiconductor Precursor Decomposition in a Closed Gas Cell In Situ TEM Holder

III/V semiconductor devices are used for many technological applications, e.g. LEDs, lasers and solar cells. A widely used fabrication process for these materials is metal organic vapor phase epitaxy (MOVPE), where metal organic precursor gases are supplied to a heated substrate. The precursor gases decompose thermally and the growth material adsorbs onto the crystalline substrate resulting in a layer by layer crystal growth. Mass spectrometry analysis of the gas phase in the MOVPE reactor is able to give detailed insights into the decomposition of the precursor gases and, thus, into the growth process [1]. <i>In-situ</i> studies of the growth process promise a performance improvement of the fabricated materials, however direct observations of the crystal growth in conventional MOVPE reactors are challenging. <i>In-situ</i> (scanning) transmission electron microscopy ((S)TEM) allows the investigation of dynamic processes, which occur during growth of III/V semiconductors. Gas environmental cell and heating holders enable the supply of gases while heating the sample so that conditions comparable to those during the MOVPE process can be realized in any TEM [2] and semiconductor growth can be performed. The comparability of such a micro reactor with a conventional one, however, still needs to be proven.<br/>To this end, a commercially available Protochips <i>in-situ</i> system, equipped with a quadrupole mass spectrometer working with 70 eV electron ionization, has been modified to investigate the processes occurring during semiconductor growth. In order to allow the usage of toxic and pyrophoric gases, like the precursor gases used in MOVPE growth, a gas mixing system, an appropriate gas monitoring system as well as a gas scrubbing system have been added to the setup [3]. The <i>in-situ</i> TEM holder is capable of sustaining pressures and temperatures up to 1 bar and 1000 °C, respectively. Investigated precursor gases were tertiarybutylphosphine (TBP) and trimethylgallium (TMGa), at partial pressures of 1 hPa and a V/III ratio of 1. Additionally, N₂ was used as carrier gas at pressures in the range of 100 hPa. Temperatures at which decomposition takes place were expected to be above the pyrolysis temperature of the precursors of around 450 °C [4]. The thermal decomposition of the precursor gases in the <i>in-situ</i> TEM holder tip was observed by mass spectrometry in dependence on temperature.<br/>It was found that TBP decomposes in a gas mixture of 1 hPa TBP and 99 hPa N₂ at a heating membrane temperature of 650 °C. Starting from this temperature several decomposition products, which appear due to different decomposition pathways, can be observed. Thus, distinct molecule species can be related to individual decomposition pathways. The appearance of isobutane for example indicates TBP decomposition via homolytic fission or intramolecular coupling, whereas the detection of isobutene implies a β-hydrogen elimination. Taking all detected products formed into account, then leads to the preferred decomposition pathways at a specific temperature. Our findings suggest that at lower temperatures starting from 650 °C, TBP decomposes preferentially via homolytic fission. With increasing temperatures β-hydrogen elimination becomes more favorable, dominating from temperatures above 750 °C and homolytic fission occurs less. The detected decomposition curves of the distinct species, leading to this conclusion, show similar trends compared to results obtained in an MOVPE reactor [5]. Furthermore, the decomposition of TMGa and TMGa together with TBP was observed.<br/><b>References</b><br/>[1] P. W. Lee, <i>et al., Journal of Crystal Growth</i> <b>85.1-2</b> (1987), pp. 165-174.<br/>[2] L. F. Allard, <i>et al.</i>, <i>Microscopy and Microanalysis</i> <b>18.4</b> (2012), pp. 656-666.<br/>[3] R. Straubinger, <i>et al.</i>, <i>Microscopy and Microanalysis</i> <b>23.4</b> (2017), pp. 751-757.<br/>[4] Li, S. H., <i>et al., Journal of electronic materials</i> <b>18.3</b> (1989), pp. 457-464.<br/>[5] O. Maßmeyer, et al. <i>ACS Omega</i> <b>6</b> (2021), pp. 28229-28241.