John Hart

Photoconductivity of germanium tin alloys grown by molecular beam epitaxy

Photocurrent spectroscopy was used to measure the infrared absorption of germanium-tin alloys grown by molecular beam epitaxy. To study dependence on Sn composition, the photocurrent was measured at 100u2009K on alloys of Ge1−xSnx with atomic percentages of Sn up to 9.8%. The optical absorption coefficient was calculated from the photocurrent, and it was found that the absorption edge and extracted bandgap energy decreased with increasing Sn content. For all Ge1−xSnx samples, a fundamental bandgap below that of bulk Ge was observed, and a bandgap energy as low as 0.624u2009eV was found for a Sn percentage of 9.8% at 100u2009K.

Temperature varying photoconductivity of GeSn alloys grown by chemical vapor deposition with Sn concentrations from 4% to 11%

Pseudomorphic GeSn layers with Sn atomic percentages between 4.5% and 11.3% were grown by chemical vapor deposition using digermane and SnCl4 precursors on Ge virtual substrates grown on Si. The layers were characterized by x-ray diffraction rocking curves and reciprocal space maps. Photoconductive devices were fabricated, and the dark current was found to increase with Sn concentration. The responsivity of the photoconductors was measured at a wavelength of 1.55u2009μm using calibrated laser illumination at room temperature and a maximum value of 2.7u2009mA/W was measured for a 4.5% Sn device. Moreover, the responsivity for higher Sn concentration was found to increase with decreasing temperature. Spectral photoconductivity was measured using Fourier transform infrared spectroscopy. The photoconductive absorption edge continually increased in wavelength with increasing tin percentage, out to approximately 2.4u2009μm for an 11.3% Sn device. The direct band gap was extracted using Tauc plots and was fit to a bandgap model accounting for layer strain and Sn concentration. This direct bandgap was attributed to absorption from the heavy-hole band to the conduction band. Higher energy absorption was also observed, which was thought to be likely from absorption in the light-hole band. The band gaps for these alloys were plotted as a function of temperature. These experiments show the promise of GeSn alloys for CMOS compatible short wave infrared detectors.

Properties of pseudomorphic and relaxed germanium1−xtinx alloys (x < 0.185) grown by MBE

Epitaxial layers of Ge1−xSnx with Sn compositions up to 18.5% were grown on Ge (100) substrates via solid-source molecular beam epitaxy. Crystallographic information was determined by high resolution x-ray diffraction, and composition was verified by Rutherford backscattering spectrometry. The surface roughness, measured via atomic force microscopy and variable angle spectroscopic ellipsometry, was found to scale with the layer thickness and the Sn concentration, but not to the extent of strain relaxation. In addition, x-ray rocking curve peak broadening was found not to trend with strain relaxation. The optical response of the Ge1−xSnx alloys was measured by spectroscopic ellipsometry. With increasing Sn content, the E1 and E1u2009+u2009Δ1 critical points shifted to lower energies, and closely matched the deformation potential theory calculations for both pseudomorphic and relaxed Ge1−xSnx layers. The dielectric functions of the high Sn and strain relaxed material were similar to bulk germanium, but with slightl...

Band gap and strain engineering of pseudomorphic Ge1−x−ySixSny alloys on Ge and GaAs for photonic applications

The authors report the compositional dependence of the direct and indirect band gaps of pseudomorphic Ge1−x−ySixSny alloys on Ge and GaAs with (001) surface orientation determined from deformation potential theory and spectroscopic ellipsometry measurements. The effects of alloying Ge with Si and Sn and the strain dependence of the band gaps at the Γ, Δ, and L conduction band minima are discussed. Deformation potential theory predicts an indirect to direct crossover in pseudomorphic Ge1−y−xSixSny alloys on Ge or GaAs only for very high Sn concentrations between 15% and 20%. No indirect to direct cross-over in pseudomorphic Ge1−ySny alloys (xu2009=u20090) on Ge or GaAs was found for practically approachable Sn compositions (yu2009<u200925%). The predictions for the compositional dependence of the E0, E1, and E1u2009+u2009Δ1 band gaps were validated for pseudomorphic Ge1−ySny alloys on Ge using spectroscopic ellipsometry. The complex pseudodielectric functions of pseudomorphic Ge1−ySny alloys grown on Ge by molecular beam epitaxy ...

The characteristics of GeSn p-n junction devices fabricated by molecular beam epitaxy

Germanium-tin is an emerging optoelectronic material, but its device properties are not yet well understood. To evaluate the feasibility of GeSn-based p-n junction diodes for device and circuit applications, layers of doped GeSn with Sn contents up to 10 % were grown by the method of solid source molecular beam epitaxy (MBE) at substrate temperatures near 150 °C, on Ge substrates as shown in Fig. 1 (left panel). Prior to growth, a high temperature desorption step at 850 °C for 15 minutes was used to remove the Ge surface oxides. During GeSn growth, phosphorus was used for n-type doping from a custom baffled GaP sublimation source. Boron was used as a p-type dopant from a high temperature effusion cell. The presence of dopants in the GeSn layers was verified by SIMS impurity profiling as shown in Fig. 1 (right panel). The electrical activity of the dopants was checked by the sign of the thermoelectric power and the electrical behavior of the diodes.

Band structure and optical properties of pseudomorphic Ge 1−x−y Si x Sn y on Ge

Ge is an indirect band gap material. The band structure of Ge is a strong function of strain and alloy composition, and a transition from an indirect to a direct band gap has been observed for y~6-10% for relaxed Ge1_ySny indicating the possibility of widespread applications of Ge-based photonic devices. The pseudomorphic nature of the Ge-based alloy layer on a substrate is important to keep dislocation densities low at the interface to improve the performance of the device. Band gap engineering of Ge by controlling strain and alloying with Si and Sn has attracted great interest since Ge1-x-ySixSny ternary alloy with two compositional degrees of freedom allows decoupling of the lattice constant and electronic structures. Hence the knowledge of the compositional and strain dependence of the Ge1-x-ySixSny band structure is critical for the design of photonic devices with the desired interband transition energies.