Benjamin R. Conley

Direct-bandgap GeSn grown on silicon with 2230 nm photoluminescence

Material and optical characterizations have been conducted for epitaxially grown Ge1−xSnx thin films on Si with Sn composition up to 10%. A direct bandgap Ge0.9Sn0.1 alloy has been identified by temperature-dependent photoluminescence (PL) study based on the single peak spectrum and the narrow line-width. Room temperature PL emission as long as 2230 nm has also been observed from the same sample.

Room-temperature electroluminescence from Ge/Ge1-xSnx/Ge diodes on Si substrates

Double heterostructure Ge/Ge1-xSnx/Ge light-emitting diodes (LEDs) with 6% and 8% Sn were grown on Si substrates using chemical vapor deposition. The electroluminescence emission spectra from the fabricated LEDs were investigated at room-temperature under different injection levels. The observed emission peaks at 0.645 eV and 0.601 eV are attributed to the direct bandgap transition of the Ge0.94Sn0.06 and Ge0.92Sn0.08 layers, respectively. Moreover, the integrated emission intensity increases as the Sn composition increases under the same injection condition.

Temperature dependent spectral response and detectivity of GeSn photoconductors on silicon for short wave infrared detection.

The GeSn direct gap material system, with Si complementary-metal-oxide semiconductor (CMOS) compatibility, presents a promising solution for direct incorporation of focal plane arrays with short wave infrared detection on Si. A temperature dependence study of GeSn photoconductors with 0.9, 3.2, and 7.0% Sn was conducted using both electrical and optical characterizations from 300 to 77 K. The GeSn layers were grown on Si substrates using a commercially available chemical vapor deposition reactor in a Si CMOS compatible process. Carrier activation energies due to ionization and trap states are extracted from the temperature dependent dark I-V characteristics. The temperature dependent spectral response of each photoconductor was measured, and a maximum long wavelength response to 2.1 μm was observed for the 7.0% Sn sample. The DC responsivity measured at 1.55 μm showed around two orders of magnitude improvement at reduced temperatures for all samples compared to room temperature measurements. The noise current and temperature dependent specific detectivity (D*) were also measured for each sample at 1.55 μm, and a maximum D* value of 1 × 10(9) cm·√Hz/W was observed at 77 K.

Competition of optical transitions between direct and indirect bandgaps in Ge1−xSnx

Temperature-dependent photoluminescence (PL) study has been conducted in Ge1−xSnx films with Sn compositions of 0.9%, 3.2%, and 6.0% grown on Si. The competing between the direct and indirect bandgap transitions was clearly observed. The relative peak intensity of direct transition with respect to the indirect transition increases with an increase in temperature, indicating the direct transition dominates the PL at high temperature. Furthermore, as Sn composition increases, a progressive enhancement of direct transition was observed due to the reduction of direct-indirect valley separation, which experimentally confirms that the Ge1−xSnx could become the group IV-based direct bandgap material grown on Si by increasing the Sn content.

Si based GeSn photoconductors with a 1.63 A/W peak responsivity and a 2.4 μm long-wavelength cutoff

Thin-film Ge0.9Sn0.1 structures were grown by reduced-pressure chemical vapor deposition and were fabricated into photoconductors on Si substrates using a CMOS-compatible process. The temperature-dependent responsivity and specific detectivity (D*) were measured from 300 K down to 77 K. The peak responsivity of 1.63 A/W measured at 1.55 μm and 77 K indicates an enhanced responsivity due to photoconductive gain. The measured spectral response of these devices extends to 2.4 μm at 300 K, and to 2.2 μm at 77 K. From analysis of the carrier drift and photoconductive gain measurements, we have estimated the carrier lifetime of this Ge0.9Sn0.1 thin film. The longest measured effective carrier lifetime of 1.0 × 10−6 s was observed at 77 K.

Shortwave-infrared photoluminescence from Ge1−xSnx thin films on silicon

Ge1−xSnx thin films with Sn composition up to 7% were epitaxially grown by chemical vapor deposition on silicon. Temperature-dependent photoluminescence was investigated and the peaks corresponding to the direct and indirect transitions were observed in a wavelength range from 1.6 to 2.2 μm. The exact peak positions obtained from Gaussian fitting were fitted with an empirical temperature dependent band-gap equation (Varshni relationship). The separation between direct and indirect peaks was equal to 0.012 eV for GeSn thin film with 7% Sn content at room temperature. This observation indicates that the indirect-to-direct crossover would take place at slightly higher Sn compositions.

High efficiency MJ solar cells and TPV using SiGeSn materials

We report on the design and efficiency of using the new material system GeSn and SiGeSn for thermophotovoltaic (TPV) and multi-junction (MJ) solar cells. With the addition of SiGeSn alloy to the current MJ design proposed by EMCORE, bandgap energy tuned layers can be monolithically deposited creating a Group IV based, fourjunction solar cell capable of a maximum high efficiency of 47% under AM0 conditions. We also propose the use of GeSn as a compatible relaxed film and active layer for a SiGeSn based TPV.

CVD growth of Ge 1−x Sn x using large scale Si process for higher efficient multi-junction solar cells

Multi-junction solar cell efficiency gains due to GeSn and SiGeSn have already shown that a need exists for significant advancement in growing this material in a commercially available CVD chamber. Ge1-xSnx films have been grown via an Epsilon RPCVD single wafer CVD deposition tool on Si using a relaxed Ge buffer layer. The material and optical properties of these films have been characterized for various compositions. We present the characterization of strained Ge1-xSnx with x = 0.9 % to 7 % and photoluminescence of Ge1-xSnx grown via a commercial CVD reactor. This commercial growth accessibility shows that this ternary material should allow for further advancements in multi-junction photovoltaics using Si CMOS compatible processes.

Temperature Dependent Spectral Response and Responsivity of GeSn Photoconductor on Si

The spectral response and responsivity of a GeSn photoconductor were measured from 300 to 77 K. The maximum responsivity of 0.06 A/W was measured at 1550 nm for 10 volts bias at 140 K.

Strain Relaxation and Material Quality Improvement of Compressively Strained GeSn Epitaxial Films through a Cyclic Rapid Thermal Annealing Process

GeSn films were annealed in cycles of 30 s at 450 and 500 C. The annealing temperature and number of cycles for material quality enhancement and relaxation depends upon Sn mole fraction and film thickness.