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C h a p te r 7. Sum m ary In this work, Si, Zn and C 5-doped layers in (Al)GaAs have been grown using MOVPE and characterised using a number o f techniques. The major achievements can be summarised as follows.
It has been established that the electron confinement o f Si 5-doped (Al)GaAs grown at normal MOVPE growth temperatures is determined by thermal diffusion o f the Si in the absence o f segregation. The diffusion coefficients of the Si (of the order o f 10'16 - 10‘15 cm2/s at ^ w t h temperatures o f 650°C to 725°C) are derived using electron profiles of
as-grown Si 8-doped (Al)GaAs. It is shown that the electron density o f Si 5-doped
(Al)GaAs is limited by the partial pressure o f the Si doping species in the gas phase, whereby the Si doping species is mainly generated by means o f homogeneous reactions including gas phase decomposition o f SiH4. The major gas phase reactions are not in equilibrium and the Si 5-doping concentration significantly increases with reducing gas flow velocity, increasing temperature, and/or rising SiH4 partial pressure. In addition, the V/III mole ratio for the growth o f undoped (Al)GaAs layers, substrate orientation and purge parameters are also found to weakly affect Si doping concentration. Si 5-doped GaAs (A l^ G a ^ A s ) layers with a peak electron density o f 8 .5 x l0 18 (6 .8 x l0 ls) cm '3 for a electron profile width o f 58 (60) A have been achieved in the present work.
The existence o f 2DEGs in Si 5-doped (Al)GaAs grown by MOVPE at a relatively high temperature of 700°C has been confirmed. In this case, the subband electronic structure is comparable to theoretical calculation, in which the Si dopants are assumed to be spatially confined to a thickness less than 10 nm and also similar to the previously reported results obtained for Si 5-doped GaAs grown by MBE at relatively low temperatures o f 605°C or below. The V-shaped potential well in Si 8-doped GaAs is not ideally symmetric due to the effect of surface states and a partially bulk-doped cap layer instead of an undoped GaAs cap layer is shown to provide well symmetry. Si 5-doped GaAs is shown to possess a weak and moderately persistent photoconductivity. At 1.5K, the illumination-generated electrons occupy an additional subband. This occupancy persists once the illumination has been turned off. DX centres, similar to those previously observed in Si bulk-doped AlxG a,.xAs (x > 0.22), are not occupied by electrons in Si 8- doped AlxGa,.xAs when x is < 0.15 but are occupied for x > 0.25.
A new Zn 8-doping sequence has been developed to overcome the problem with high vapour pressure o f Zn at normal MOVPE growth temperatures. The best Zn 8-doped GaAs with the highest peak hole density of l .lx lO 19 cm'3 for the narrowest hole profile
first time using MOVPE. The hole confinement of Zn 5-doped (Al)GaAs has been shown to be determined by thermal diffusion in the absence of segregation and the diffusion coefficients at 650°C increase with increasing A1 mole fraction over the range of 10'16 to 10'15 cm2/s. Indeed, the equilibrium between the Zn adsorption and desorption can be established very rapidly and the Zn 8-doping concentration is predominantly determined by the Zn desorption. The desorption activation energy has been determined as 2.04 eV for Zn 8-doped GaAs and 1.64 eV for Zn 8-doped Al0 35Ga0 65As. The maximum hole density obtainable in Zn 8-doped GaAs is ~ 1 .5 x l0 14 cm'2. A reduced temperature and/or an increased DMZ partial pressure have been found to result in a significant increase in Zn 8-doping concentration. Due to different desorption activation energies of the Zn in 8-doped GaAs and Al0^ G a ^ A s , the Zn 8-doping concentration is found to decrease with increasing A1 mole fraction in AlGaAs. Based on these experimental result, a model is developed to quantitatively describe the Zn 8-doping concentration as a function of 8- doping parameters.
C 8-doped (Al)GaAs with very high hole density has been successfully grown by MOVPE for the first time using TMA1 as a C 8-doping precursor. Compared to TMGa, TMA1 possesses very high C 8-doping efficiency, independent of A1 mole fraction. Environmental consideration and its commercial availability in high purity form suggest that TMA1 is a promising C 8-doping precursor as an alternative to CC14. A detailed parametric study has also shown that the C 8-doping concentration and electrical activation are strongly dependent on growth conditions, such as temperature and the amount of TMGa or TMA1 totally introduced during a 8-doping step. It is found that lower than 100% electrical activation in C 8-doped (Al)GaAs is caused by self compensation of the C atoms and not by H passivation. However, using the optimised growth conditions, it is also shown that self-compensation can be eliminated. C 8-doped
pipi doping superlattices in (Al)GaAs have also been grown at 580°C using the C 8- doping technique developed in this work. The resultant bulk-doped-like hole profiles are comparable to those obtained in (Al)GaAs grown at the same temperature using conventional C bulk-doping approaches. It is suggested that C 8-doping pipi doping superlattice constitutes a promising approach to fabricate heavily C bulk-doped layers in (Al)GaAs.