Matthew Coppinger

Lattice constant and substitutional composition of GeSn alloys grown by molecular beam epitaxy

Single crystal epitaxial Ge1−xSnx alloys with atomic fractions of tin up to x = 0.145 were grown by solid source molecular beam epitaxy on Ge (001) substrates. The Ge1−xSnx alloys formed high quality, coherent, strained layers at growth temperatures below 250 °C, as shown by high resolution X-ray diffraction. The amount of Sn that was on lattice sites, as determined by Rutherford backscattering spectrometry channeling, was found to be above 90% substitutional in all alloys. The degree of strain and the dependence of the effective unstrained bulk lattice constant of Ge1−xSnx alloys versus the composition of Sn have been determined.

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 100 K 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 0.624 eV was found for a Sn percentage of 9.8% at 100 K.

Sensitivity of a vanadium oxide uncooled microbolometer array for terahertz imaging

The broadband imaging capabilities of a vanadium oxide microbolometer camera were investigated in the far-infrared for applications in real-time terahertz imaging and analysis. To accomplish this, we used an optical configuration consisting of a broadband terahertz source, terahertz filtering optics, and a modified commercial broadband microbolometer camera. A blackbody radiator was employed as the broadband terahertz source to illuminate the microbolometer array with all components in a nitrogen purged enclosure. Data was taken using several different levels of radiant flux intensity. Optical filtering were necessary to isolate incident radiation frequencies into a band from 1.5 to 7.5 THz. Fourier transform infrared spectroscopy was used to characterize the transmission properties of each optical component. The noise equivalent differential temperature (NEDT) and the noise equivalent power (NEP) were recorded over a range of blackbody intensities. We discuss the relative utility of these two figures of merit for terahertz imaging. For example, at a blackbody temperature of 925°C the NEDT was recorded below 800 mK, and the NEP was calculated to be 136 pW/√Hz. This study provides a complete analysis of a microbolometer as the detector component of a terahertz imaging system in a broadband imaging configuration.

Effects of boron and phosphorus doping on the photoluminescence of self-assembled germanium quantum dots

The doping of Ge quantum dots grown on Si (100) was investigated using atomic force microscopy and photoluminescence spectroscopy. The dots produced photoluminescence in the energy range from 0.75 to 0.95 eV. Compared to the undoped dots, the boron and phosphorus doped dots demonstrated a shift in peak emission toward lower energy by 60–80 meV, which is attributed to the decrease in density of the smaller sized dots. Increased photoluminescence intensity with temperature indicated higher activation energy with doping, suggesting a higher probability of radiative transitions at elevated temperatures, potentially important for various applications.

Current–Voltage Characteristics of GeSn/Ge Heterojunction Diodes Grown by Molecular Beam Epitaxy

Heterojunction diodes of p-GeSn/n-Ge were fabricated by solid-source molecular beam epitaxy on Ge substrates to investigate their electrical properties. Measurements of the current-voltage characteristics and their temperature and composition dependence were performed to extract the diode parameters of reverse saturation current, ideality factor, series resistance, and shunt resistance. The diodes showed good rectifying behavior with low turn-ON voltages in forward bias. The reverse saturation current increased with increasing Sn content and increasing temperature, and the magnitude of the breakdown voltage decreased with increasing temperature. These results suggest that Ge-Sn diodes may be useful for Ge-based circuits and optoelectronics.

Infrared photoresponse of GeSn/n-Ge heterojunctions grown by molecular beam epitaxy

Heterojunction devices of Ge(1-x)Sn(x) / n-Ge were grown by solid source molecular beam epitaxy (MBE), and the mid-infrared (IR) photocurrent response was measured. With increasing Sn composition from 4% to 12%, the photocurrent spectra became red-shifted, suggesting that the bandgap of Ge(1-x)Sn(x) alloys was lowered compared to pure Ge. At a temperature of 100 K, the wavelengths of peak photocurrent were shifted from 1.42 µm for pure Ge (0% Sn) to 2.0 µm for 12% Sn. The bias dependence of the device response showed that the optimum reverse bias was > 0.5 volts for saturated photocurrent. The responsivity of the Ge(1-x)Sn(x) devices was estimated to be 0.17 A/W for 4% Sn. These results suggest that Ge(1-x)Sn(x) photodetectors may have practical applications in the near/mid IR wavelength regime.

Structural and optoelectronic properties of germanium-rich islands grown on silicon using molecular beam epitaxy

We report on the structural and optoelectronic properties of self-assembled germanium-rich islands grown on silicon using molecular beam epitaxy. Raman, photocurrent, photoluminescence, and transient optical spectroscopy measurements suggest significant built-in strains and a well-defined interface with little intermixing between the islands and the silicon. The shape of these islands depends on the growth conditions and includes pyramid, dome, barn-shaped, and superdome islands. Most importantly, we demonstrate that these germanium-rich islands provide efficient light emission at telecommunication wavelengths on a complementary metal-oxide semiconductor-compatible platform.

Increasing the operating temperature of boron doped silicon terahertz electroluminescence devices

High power electroluminescence near 8THz was observed from boron doped silicon devices operating at heat sink temperatures up to 118K. This represents the highest emission temperature yet observed for silicon dopant-based terahertz devices, and is a significant increase from previous reports. This letter compares the temperature dependence of the emission mechanism to the dopant occupation function and describes an empirical model that fits the variation of output power with temperature, and that can guide the design of future terahertz devices.

Xenon Difluoride Dry Etching of Si, SiGe Alloy and Ge

Xenon diflouride (XeF2) vapor has been known to be able to spontaneously etch Si isotropically at high rates up to 10 mum/min. This dry etching process does not require plasmas or catalysts, and thus causes little damage to the electronic properties. It is useful for releasing free standing structures by etching away Si sacrificial layers or for gate oxide failure analysis by etching away the backside Si. In this work, the etching of Si, SiGe alloys and Ge was studied and results were discussed. Both SiGe and Ge were found to be etched by XeF2 vapor, and at faster rates than Si

Structural and light-emission properties of bulk Germanium islands grown on Silicon using Molecular Beam Epitaxy

We report on the optoelectronic properties of bulk Germanium islands formed on Silicon by Molecular Beam Epitaxy. More specifically, we will discuss the role of strains and doping in favoring efficient light-emission at telecommunication wavelengths.