Photoconductivity of GeSn thin films with up to 15% Sn content

We have used temperature-dependent photoconductivity (PC) with different excitation wavelengths and intensities to study the photoexcited charge-carrier transport within GeSn/Ge/Si heterostructures. The evolution of the PC spectra with temperature was analyzed between 10 and 200 K. These strained GeSn films were grown with high enough Sn content such that the band gaps were direct. As such, the relationships between the band gaps and the temperature were determined using photoconductivity spectroscopy. As a result, an anomalous, linear blueshift of the PC spectral edge was found with increasing temperature. This was attributed to the variation of the GeSn band gap within the film, due to variations in strain and the increased contribution to PC from the region of highest Sn content. The change in photocurrent with excitation intensity and temperature demonstrates that conduction occurs predominantly through the GeSn and Ge layers under low optical pumping and through the Si substrate under high optical pumping. A phenomenological model of photoconductivity in GeSn as it depends on strain was proposed. This detailed understanding of the transport of photoexcited carriers along the GeSn layers is critical for developing GeSn/Ge-based optoelectronic devices.

Figure 1

The Raman spectra for samples of different thicknesses.

Figure 2

The maps for Sn content for the samples of different thicknesses derived from the measured Raman spectra using Eqs. (1) and (2).

Figure 3

(a–d) AFM images of the surface of GeSn layers of different thicknesses, (e) the surface rms roughness, and ${\varepsilon }_x$ histogram peak width on the layer thickness.

Figure 4

Reciprocal space maps (RSMs) measured around asymmetrical Ge(–2–24) reflection.

Figure 5

The lateral PC spectra of the GeSn/Ge/Si heterostructures at 10 K (a) and their near-edge region in the Tauc coordinates ${\left(I_{\mathrm{PC}}hv\right)}^2$ vs $hv$ (b).

Figure 6

The PC spectral dependencies for S45 (a) and S200 (b) samples measured at various temperatures in the range of 10–200 K.

Figure 7

The edges of the PС spectra of Fig. 6 in the Tauc coordinates (a) and the resulting temperature dependence of direct-band-gap for samples S45 and S200 in the temperature range 10–150 K (b).

Figure 8

The PL spectral dependencies for S45 (a) and S200 (b) samples measured at various temperatures in the range of 10–200 K.

Figure 9

The temperature dependence of PC for samples S45 (left) and S200 (right) at various excitation energies.

Figure 10

PC vs excitation intensity at different excitations corresponding to edge absorption of the respective layers in sample S45 at 80 K.

Figure 11

Calculated band gap maps for samples of various thicknesses.