Investigation of the Surface Reconstruction and Gas Phase Composition during Growth by MOVPE

Metal organic vapor phase epitaxy (MOVPE) plays an important role in the fabrication of optoelectronic devices based on III-V semiconductor materials such as telecommunications lasers, light emitting diodes, highly efficient solar cells or high frequency devices on an industrial scale. Even though the technique has been largely established since its invention in the 1960s, there are still many remaining questions to understand the physics of this deposition technique. This is caused by the complexity of the underlying thermodynamics, kinetics and hydrodynamics, resulting in a more phenomenological understanding of the epitaxial growth process. Nevertheless, the MOVPE technique is applicable for the realization of novel metastable materials paving the way for inventions in terms of new devices and device optimization. One promising approach is the use of ‘dilute nitrides’ and ‘dilute bismides’ based on the gallium arsenide (GaAs) host material for an improvement of the practicability and the energy efficiency of the currently used devices to make a small contribution to solve environmental problems such as global warming. ‘Dilute bismides’ show the capability to decrease the energy consumption of laser diodes used for the telecommunication across the internet by light emission at 1.3 µm and 1.55 µm through glass fibers. The higher predicted efficiency of ‘dilute bismides’ is based on the prevention of internal loss mechanisms in the active region of the laser devices which are limiting the efficiency of the currently used (Ga,In)(As,P) laser diodes. Likewise, the ‘dilute nitride’ material family is discussed as potential candidate for highly efficient telecommunication lasers. Here material combinations such as (Ga,In)(N,As) are discussed, due to a potentially higher efficiency, a better thermal stability and the possibility to fabricate these laser structures on the well-established GaAs substrates. Besides this, the ‘dilute nitrides’ are discussed for application as highly efficient solar cells that are promising in terms of renewable energy. Here material combinations such as (Ga,In)(N,As), Ga(N,As,Sb) or Ga(N,As,Bi) are discussed as a layer in a multi-junction solar cell concept to improve the conversion efficiency of the junction responsible for absorption of sun light with a wavelength between 1.1 µm and 1.4 µm. An increase to conversion efficiencies over 50 % could make the multi-junction solar cells more competitive with respect to Si-based solar cells in terms of cost efficiency. However, the fabrication of these devices by MOVPE with respect to the desired material composition, structural quality and purity is a current challenge. In this work a small contribution towards the possible realization of improved devices and towards a more detailed physical understanding of the related processes during growth is presented. This covers in the first part the investigation of surface structure of ‘dilute nitrides’ and ‘dilute bismides’ on an atomic scale by reflection anisotropy spectroscopy (RAS). The RAS technique is used for in-situ analysis of the arising surface reconstructions of Ga(As,Bi) and Ga(N,As) layers during the growth by MOVPE. For the Ga(N,As) material system, this study includes the investigation of the already established nitrogen precursors 1,1 di methyl hydrazine (UDMHy) and the newly synthesized di tert butyl amino arsane (DTBAA) precursor. The initial point of these investigations is focused on the analysis of the GaAs host material. For a simple nitridation of the GaAs (001), surface both precursors show a different influence on the surface reconstruction. The supply of UDMHy to the GaAs (001) surface produced a strong change of the surface structure from the As rich c(4×4)β surface reconstruction to the more Ga- or N rich (2×6)/(6×6) surface reconstruction. Compared to this, the supply of DTBAA showed no strong modulation of the As rich c(4×4)β surface. In contrast to UDMHy, the inbuilt additional As supply by DTBAA caused a stabilization of the given c(4×4)β surface. During the growth of Ga(N,As), by additional supply of tert butyl arsane (TBAs) and tri ethyl gallium (TEGa), the surface reconstruction changes to a more Ga or N rich (2×6)/(6×6) surface structure, independent from the choice of the N precursor and the underlying surface reconstruction. The latter was investigated by the growth of a Ga(N,As) layer on a prepared (2×4) reconstructed GaAs (001) surface. Based on the Ga(N,As) results, atomically abrupt Ga(N,As) layers with high N contents of presumably up to 16 % were realized by using different gas switching sequences in the growth process. These studies are of fundamental interest concerning the modification of the type-II transitions in W-type laser structures based on the Ga(N,As) and Ga(As,Sb) material systems. In case of the Ga(As,Bi) material system, similar experiments were carried out starting with the influence of the tri methyl bismuth (TMBi) precursor on the GaAs (001) surface structure. As expected from the behavior of Bi to float on the growth surface without being incorporated, the supply of TMBi caused the As rich c(4×4)β surface reconstruction to change presumably to a (4×3) surface reconstruction. This change of the surface reconstruction was directly related to the Bi surface coverage, which is an essential step for the realization of homogenous Ga(As,Bi) layers. The created Bi terminated surface was extensively studied in terms of the thermal stability, stability towards changes of the environmental conditions and its formation time. The addition of TBAs and TEGa for the growth of Ga(As,Bi) caused the surface reconstruction to further change to a Bi containing c(4×4)β surface reconstruction. Furthermore, the surface reconstruction was found to be very sensitive to variations of the TBAs/TEGa gas phase ratio, underlying former findings of a small growth window for the growth of Ga(As,Bi) on GaAs. Additional experiments on prepared (2×4) GaAs (001) surfaces showed that the Ga(As,Bi) surface structure is adopting to the underlying surface reconstruction during growth. For both material systems, further analysis of the surface morphology and material composition was done by post growth investigation with atomic force microscopy (AFM) and high-resolution X ray diffraction (HR-XRD). The compositional analysis gives evidence that the N incorporation is favored on more Ga rich surfaces like the (2×4) surface reconstruction and that Bi incorporation is enhanced on the more As rich c(4×4)β surface reconstruction compared to the (2×4) surface reconstruction. The second part of this thesis covers the real time analysis of the gas phase composition during the deposition of III-V semiconductor with focus on decomposition studies of the novel nitrogen precursors DTBAA, di tert butyl amino phosphane (DTBAP) and di tert butyl arsenyl di methyl hydrazine (DTBADMHy). As a general overview the decomposition temperature and decomposition reactions of the investigated precursors were studied independently to analyze the unimolecular decomposition reactions in the used horizontal AIXTRON AIX 200 MOVPE reactor. Besides the investigation of the novel N precursors, this includes the analysis of the unimolecular decomposition reactions of the group III precursors tri methyl gallium (TMGa), TEGa, tri tert butyl gallium (TTBGa) and the group V precursors TMBi, TBAs, tert butyl phosphane (TBP) and UDMHy. The decomposition reactions of these precursors show comprehensively a good agreement to formerly published decomposition data and proof the reliability of the measurements done with this ion trap setup. Very beneficial was the investigation of all these precursors under comparable experimental conditions in the same MOVPE reactor system, which resulted in a good data base for future investigations with this setup. The decomposition studies of the novel N precursors are of even more relevance, since growth studies with these precursors showed the simultaneous incorporation of N and As from the precursors and highlighted promising N incorporation characteristics for growth of ‘dilute nitrides’ at temperatures below 500 °C. The DTBAA, DTBAP and DTBADMHy precursor all exhibit a direct N-As or N-P bond, which was found to be dissociated in the first step of the decomposition reaction under formation of aminyl radicals (NH2•). These radicals are believed to be responsible for N incorporation and lead to a limitation of the N incorporation at higher temperature due to the formation of the thermally very stable ammonia (NH3). The As or P incorporation is connected to the decomposition of larger As or P compounds formed in the first bond homolysis or heterolysis step. These larger compounds were identified as di tert butyl arsane (DTBAs•) or di tert butyl phosphane (DTBP•). Based on the unimolecular decomposition experiments the investigation of the gas phase during the growth of GaAs, GaP and GaN was investigated. The occurring bimolecular reactions were most extensively studied for the precursor combination of TBAs and the Ga precursors TMGa, TEGa and TTBGa during the growth of GaAs. This study showed a strong influence of the decomposed Ga precursor on the TBAs decomposition. Here the decomposition temperature of TBAs was reduced down to the decomposition temperature of the respective Ga precursor. This is believed to occur due to a catalytic effect of the decomposed Ga precursor on the decomposition of the TBAs leading to a reduction of the decomposition temperature of TBAs from 350 °C down to 160 °C in combination with TTBGa. This catalyzed decomposition of TBAs is especially interesting for low temperature growth of GaAs based materials. The further studies of the bimolecular decomposition of TBAs with TEGa were carried out for different gas phase ratios of TBAs/TEGa between 0.5 to 10 and by a more detailed analysis of the decomposition products, utilizing the selective removal of the trapped ions with stored wave inverse Fourier transformation (SWIFT) from the ion trap. These results show evidence for an alkyl exchange reaction to be included in the catalyzed bimolecular decomposition. The very same experiments were carried out for the bimolecular reactions between TBP and the Ga precursors during growth of GaP. Similarly, a strong reduction of the decomposition temperature of TBP by the addition of the Ga precursors was shown. However, the decomposition temperature is determined about 50 °C higher compared to the decomposition of the Ga sources. This shows that the C-P as well as the alkyl groups of the Ga precursor have to be involved in the decomposition reaction, supporting the proposed alkyl exchange reactions. The last experiments were carried out to show the investigation of adduct formation during epitaxial growth. For the precursor combinations of UDMHy with TMGa and TEGa, adduct formation is predicted, but only indirectly proven in literature. The analysis of these bimolecular reactions with the novel ion trap setup proves the formation of larger adducts such as (CH3)2NN[(CH3)2Ga]2NN(CH3)2, which already form at room temperature for the combination of UDMHy and TMGa. Analogue formation of (CH3)2NN[(C2H5)2Ga]2NN(CH3)2 is supposed for UDMHy and TEGa. Altogether, the analysis of the surface structure and the gas phase composition during growth by MOVPE has led to new insights in the deposition of III-V semiconductors with focus on novel material systems such as ‘dilute bismides’ and ‘dilute nitrides’. Both analysis techniques, the surface analysis by RAS and the gas phase analysis by mass spectrometry turned out to be very powerful for a direct feedback during the MOVPE process. The application and understanding of these techniques should be expanded to arising novel material systems. Especially, the analysis of ternary or quaternary compound semiconductors or of novel 2D materials would be desired, as these will presumably drive new applications in this field.