Enhanced cation-substituted p-type doping in GaP from dual surfactant effects

We report first principles calculations demonstrating a dual-surfactant effect of Sb and H on enhanced Zn, Mg, Be and Cd incorporation in organometallic vapor phase epitaxially grown GaP films. The combined effects of Sb and H lower significantly the film doping energy during the epitaxial growth of all the p-type dopants studied, while neither Sb nor H can work alone as effectively. The role of H is to redistribution. We also predict that due to the low electronegativity of Mg, Sb and H will enhance Mg doping the least among these dopants because Mg as an electron reservoir itself may negate the electron reservoir effect of Sb. Our findings provide an important general physical understanding for p-type doping in III—V thin films. Published by Elsevier B.V. In epitaxial growth, surfactants have been proved to be effective to control the thin film microstructure, composition and morphology and hence to improve the thin film properties and device performance. Copel et al. in 1989 first used As as a surfactant in the growth of Si/Ge/Si(0 0 1) to suppress island formation [1]. Surfactant effects may affect crystal growth in various ways: (1) they can change the growth mode. In addition to Copel’s work [1], the growth mode of Ag on Ag(111) is also changed when Sb is used as a surfactant [2,3]. (2) Surfactants can reduce interface roughness. For example, Bi as a surfactant reduces the surface roughness of InGaAs grown on GaAs substrates [4]. (3) Interface alloy intermixing can be suppressed by surfactants. For example, H can suppress the interface intermixing of Ge(0 01) covered Si [5]. (4) They can be used to change the surface reconstruction and, hence, induce the formation of various new ordered phases. For example, Sb is known to suppress Cu–Pt ordering in GaInP [6]. At higher concentrations, it can change the surface reconstruction from ð2 4Þ to ð2 3Þlike inducing a new ordered phase in InGaP [6]. (5) Surfactants can also strongly affect the incorporation of dopants in semiconductors [7,8]. This last effect will be the focus of this paper. B.V. The surfactant effects listed above may be attributed to several physical mechanisms. Surfactants can change the growth thermodynamics, by altering the surface energy. For example, surface As is known to lower the surface energy of the Si/Ge/Si system to suppress island formation [1]. In addition to changing the thermodynamics, surfactants can also change the growth kinetics, such as surface diffusion [2] and the size of step-edge barriers [3]. For example, Sb as a surfactant has been shown to reduce the mobility of Ag adatoms. This results in a higher island density leading to a change of growth mode. Sb as a surfactant on Ag (111) or GaAs can also reduce the step edge barrier and promote smoother growth morphologies [3,9]. Obtaining high doping levels in high bandgap materials has been a difficult problem for decades. This hinders high-level p-type doping in III–V materials such as phosphide and nitride semiconductors. This may be caused by several factors, including the limited solubility of acceptors, H passivation of acceptors, and high acceptor-hole binding energies [10,11]. An effective approach to achieving high p-type doping levels in GaInP, GaP, and GaAs employs the use of surfactants during organometallic vaporphase epitaxy (OMVPE) growth [6–8]. For example, a recent study showed that Sb can be used to enhance the incorporation of dopants, such as Zn [7,8], and reduce unintentional impurities, such as C, S, and Si [8]. In addition to Sb, surface H was postulated to also play a role in the doping process [7,8]. The enhanced Zn