Recent progress in δ-doping of III–V semiconductors grown by metal organic vapour phase epitaxy

Abstract In this article, δ-doping of III–V semiconductors (III–Vs) grown by metal organic vapour phase epitaxy (MOVPE) is reviewed in respect to the parametric dependencies of δ-doping concentrations, the spatial confinement of carriers and dopants in δ-doped layers, and applications of MOVPE-grown δ-doped structures to electronic and opto-electronic devices. The use of molecular doping precursors determines that δ-doping in MOVPE is more complicated than that in molecular beam epitaxy (MBE). A number of processes, such as the gas phase reactions of the doping precursors, adsorption and desorption of the doping species on the non-growing surface, surface reactions and reconstruction during a δ-doping step, have to be taken into account in parametric studies of the δ-doping concentration. Si δ-doping is dominated by the adsorption of the Si doping species, leading to a linear dependence of the electron density on δ-doping time. Si δ-doping concentration can be increased by increasing the growth temperature, the partial pressure of the Si doping precursor and/or decreasing the gas flow velocity. Using the optimised growth conditions, the electron density can be altered over the range 5 × 10 11 –10 13 cm −2 even at a growth temperature of 630°C. Zn δ-doping is dominated by the desorption of monatomic Zn with respect to the non-growing surface. Using the Zn δ-doping sequence (which does not have a post-δ-doping purge step), a very wide range of hole densities (10 12 −2 × 10 14 cm −2 ) can be achieved in (Al,Ga)As. The temperature and the partial pressure of the Zn doping precursor are the most effective parameters controlling Zn δ-doping concentration. Carbon δ-doping was achieved using different doping precursors, such as CCl 4 , trimethylgallium (TMGa) and trimethylaluminium (TMAl). The potential damage of CCl 4 to the environment and the common availability of TMGa and TMAl in the MOVPE reactors make the metal organic precursors more attractive to C δ-doping. The results show that TMAl is a more efficient C δ-doping precursor than TMGa. Free hole densities over the range 5 × 10 11 −3 × 10 13 cm −2 were reported using metal organic sources as the doping precursor. Well spatially-confined δ-doped layers were obtained in III–Vs grown by MOVPE. In the MOVPE-grown δ-doped III–Vs, thermal diffusion controls the spread of the dopants. Recent progress in δ-doping of III–Vs has led to fabrication of δ-doped electronic and opto-electronic devices. The capability of MOVPE to grow those novel devices is discussed with respect to the effects of the dopant profile width and concentration on the device performance. It is concluded that MOVPE is able to grow high quality n -type and p -type δ-doped layers in III–Vs though further research is required to design and fabricate device structures with strategic δ-doping for optimum performance.