Yao Group

Atomic Level Analysis and Control of II-VI Semiconductor Surface
Takafumi Yao

■Object
　The bonding nature of II-VI wide gap semiconductors is characterized by a mixture of ionic bonds and covalent bond with almost the same contribution to the bonding. The crystal structure is based upon the four-fold coordination. The materials have direct bandgaps which range from visible to UV region. The purposes of this research are as follows: (1) Atomistic investigation of surface processes during molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE); (2) Control of the surface processes on an atomic level; (3) Tailoring of semiconductor quantum structures which exhibit novel properties.

■Results in Fiscal Year
　　1.Control and characterization of the surface and interface processes of II-VI/III-V heterointerfaces using three different diagnosis

1.1 Reflectance difference spectroscopy (RDS)

We have developed an analytical procedure that separates the surface- and interface-origined components in the reflectance-difference signal of a heterostructure. This procedure enables in-situ and simultaneous diagnostics of the growing surface and the buried interface.

Taking advantage of this new characterization technique, we have demonstrated that the structure of the ZnSe/GaAs interface can be controlled at atomic level by controlling the structure of the initial GaAs surface.

1.2 Scanning tunneling microscopy (STM)

STM observation of Zn-/Se-treatment and ZnSe growth on stabilized GaAs(001) surfaces of As:(2×4), As:c(4×4), Ga:(4×6) was performed. Island growth on Se-treated 2×1 and two-dimensional growth on Zn-treated 2×4 were revealed.

1.3 Transmission electron microscopy (TEM)

Zn-As and Ga-Se interfacial layers were suggested by TEM in Zn treated and Se treated or reacted ZnSe/GaAs interface, respectively.

High density of extrinsic Shockley partials were introduced in ZnSe/GaAs samples with Zn treatment on c(4×4) As-rich GaAs surface and Se-reacted ZnSe/GaAs.

Zn- and Se- terminated extrinsic Shockley partials were generated in samples with Zn- and Se- rich flux ratios, respectively.

2. Control of nitrogen doping

2.1 Chemical species in N2 plasma for p-type dopant of ZnSe and related compounds were measured using optical emission spectroscopy (OES) and laser induced fluorescence (LIF). We found that ion species form additional compensation centers in N-doped p-ZnSe.

2.2 We have developed a novel type nitrogen plasma source which completely decomposes nitrogen molecules into atoms using induction-heating.

Reflectance-difference spectra of the ZnSe/GaAs interfaces formed on three different GaAs surfaces.

Net acceptor concentration of nitrogen doped ZnSe layers as a function of the RF power. ● shows net acceptor concentration with D.C. bias (100 V).