Electronic properties of hybrid organic–inorganic semiconductors

Hybrid semiconductors comprising networks of inorganic II–VI semiconductor segments bonded to organic solvent chains show promise as a versatile material system that offers the capability of synthesizing many new electronic structures that are desirable for photonic applications. Polarized optical absorption and reflection are measured in single crystal semiconductor nanostructures composed of arrays of monolayers or linear chains. This shows very sharp spectral signatures of excitonic transitions, providing evidence of an electronic bandstructure that is strongly modified from the bulk inorganic semiconductor. Together with x-ray diffraction structural analysis, it gives a clear image of a monolayer multiple quantum well structure in which a chemical growth process has achieved a quantum confinement larger than other known heterostructure types, together with near-zero layer width fluctuations, a very large oscillator strength and large exciton binding energy.

Figure 1

(Color) 3D sample structure determined by x-ray diffraction (see data from Ref. 5). (a) Multiple ZnTe layers bonded by ethylenediamine chains attached at Zn atoms. Hydrogen atoms are omitted for clarity and the unit cell is shown, (b) a single ZnTe layer projected along $b$ axis. Samples grow in thin platelets parallel to this figure, with edges oriented along the $a$- and $c$-axes.

Figure 2

(Color) 1D sample structure from x-ray diffraction, discussed in Ref. 6. (a) Four linear chains of ZnTe are shown coordinated by molecules of propanediamine; (b) top view of a single chain.

Figure 3

(Color) $1.6\mathrm{K}$ optical reflectivity and absorption spectra. (a) $E_{\Vert }a$ reflectivity of a freestanding, as-grown 3D sample with a thickness of $0.9\mu \mathrm{m}$. Four resonances are identified at A–D; (b) absorption for the same as-grown sample and a second 3D sample that has been etched to a thickness of $\sim 0.2\mu \mathrm{m}$. Etched samples were mounted on glass and typically blueshifted $10–20\mathrm{meV}$ by strain; data have been shifted by comparison with the freestanding sample. Right of axis break: unpolarized absorption of a freestanding 1D (wire) sample with a thickness of several microns. High absorption regions are limited by the experimental dynamic range; close examination of the 3D sample with a scanning double spectrometer found the peak absorption is still unresolved at greater than $1.8\times {10}^5{\mathrm{cm}}^{-1}$

Figure 4

(Color) Temperature evolution of the unpolarized reflectivity. A freestanding 3D sample with thickness $\approx 10\mu \mathrm{m}$ shows strong $E_{\Vert }c$ resonances superimposed on those from Fig. 3a.

Figure 5

(Color) Refractive index data in the below-gap region of the 3D freestanding sample. Solid and open points are $E_{\Vert }c$ and $E_{\Vert }a$ at $1.6\mathrm{K}$. Broadband transmission spectrum for $E_{\Vert }c$ is shown in the inset.