Richard T. Beeler

Direct-gap photoluminescence with tunable emission wavelength in Ge1−ySny alloys on silicon

Direct-gap photoluminescence has been observed at room temperature in Ge1−ySny alloys grown on (001) Si substrates. The emission wavelength is tunable over a 90 meV (200 nm) range by increasing the Sn concentration from y=0 to y=0.03. A weaker feature at lower energy is assigned to the indirect gap transitions, and the separation between the direct and indirect emission peaks is found to decrease as a function of y, as expected for these alloys. These results suggest that Ge1−ySny alloys represent an attractive alternative to Ge for the fabrication of laser devices on Si.

Direct gap electroluminescence from Si/Ge1−ySny p-i-n heterostructure diodes

Electroluminescence spectra from Si/Ge1−ySny heterostructure diodes are reported. The observed emission is dominated by direct gap optical transitions and displays the expected compositional dependence of the peak energy. Weaker indirect gap emission is also observed, and their energies are consistent with a closing of the indirect-direct separation as the Sn concentration is increased. The intensity of the EL spectra shows a superlinear dependence on the injection current, which is modeled using a Van Roosbroeck–Shockley expression for the emission intensity. The model assumes quasiequilibrium conditions for the electrons populating the different valleys in the conduction band of Ge.

Next generation of Ge1−ySny (y = 0.01-0.09) alloys grown on Si(100) via Ge3H8 and SnD4: Reaction kinetics and tunable emission

Film growth and reaction kinetics studies have shown that trigermane (Ge3H8) is a superior Ge source for the epitaxial synthesis of Ge1−ySny/Si(100) alloys using ultra-high vacuum chemical vapor deposition. The Ge3H8/SnD4 combination yields 3-4 times higher growth rates than the traditional Ge2H6/SnD4 approach, with film Sn/Ge ratios reflecting the corresponding gas-phase stoichiometries much more closely. These advances have led to optical quality Ge1−ySny layers with Sn concentrations up to at least 9% and thicknesses approaching 1 μm. These thick films are found to be crucial for the observation of a strong, tunable photoluminescence signal near the threshold of the predicted direct-indirect bandgap crossover.

High-Performance Near-IR Photodiodes: A Novel Chemistry-Based Approach to Ge and Ge–Sn Devices Integrated on Silicon

Ge/Si heterostructure diodes based on n++Si(100)/i-Ge/p-Ge and p++Si(100)/i-Ge/n-Ge stacks and intrinsic region thickness of ~350 and ~900 nm, respectively, were fabricated using a specially developed synthesis protocol that allows unprecedented control of film microstructure, morphology, and purity at complementary metal-oxide-semiconductor compatible conditions. From a growth and doping perspective, a main advantage of our inherently low-temperature (390°C) soft-chemistry approach is that all high-energy processing steps are circumvented. Current-voltage measurements of circular mesas (60-250 μm in diameter) show dark current densities as low as 6 ×10-3 A/cm2 at -1 V bias, which is clearly improved over devices fabricated under low thermal budgets using traditional Ge deposition techniques. Spectral photocurrent measurements indicate external quantum efficiencies between 30 and 60% of the maximum theoretical value at zero bias, and approaching full collection efficiency at high reverse biases. The above Ge devices are compared to analogous low-temperature-grown (350°C) Ge0.98Sn0.02 diodes. The latter display much higher dark currents but also higher collection efficiencies close to 70% at zero bias. Moreover, the quantum efficiency of these Ge0.98Sn0.02 diodes remains strong at wavelengths longer than 1550 nm out to 1750 nm due to the reduced band gap of the alloy relative to Ge.

Temperature-dependent photoluminescence of Ge/Si and Ge1-ySny/Si, indicating possible indirect-to-direct bandgap transition at lower Sn content

Temperature (T)-dependent photoluminescence (PL) has been investigated for both p-Ge and n-Ge1-ySny films grown on Si substrates. For the p-Ge, strong direct bandgap (ED) along with weak indirect bandgap related (EID) PL at low temperatures (LTs) and strong ED PL at room temperature (RT) were observed. In contrast, for the n-Ge1-ySny, very strong dominant EID PL at LT and strong ED PL were observed at RT. This T-dependent PL study indicates that the indirect-to-direct bandgap transitions of Ge1-ySny might take place at much lower Sn contents than the theory predicts, suggesting that these Ge1-ySny could become very promising direct bandgap semiconductors.

Direct versus indirect optical recombination in Ge films grown on Si substrates

The optical emission spectra from Ge films on Si are markedly different from their bulk Ge counterparts. Whereas bulk Ge emission is dominated by the materials indirect gap, the photoluminescence signal from Ge films is mainly associated with its direct band gap. Using a new class of Ge-on-Si films grown by a recently introduced chemical vapor deposition approach, we study the direct and indirect photoluminescence from intrinsic and doped samples and we conclude that the origin of the discrepancy is the lack of self-absorption in thin Ge films combined with a deviation from quasi-equilibrium conditions in the conduction band of undoped films. The latter is confirmed by a simple model suggesting that the deviation from quasi-equilibrium is caused by the much shorter recombination lifetime in the films relative to bulk Ge.

Complementary metal-oxide semiconductor-compatible detector materials with enhanced 1550 nm responsivity via Sn-doping of Ge/Si(100)

Previously developed methods used to grow Ge1−ySny alloys on Si are extended to Sn concentrations in the 1019−1020 cm−3 range. These concentrations are shown to be sufficient to engineer large increases in the responsivity of detectors operating at 1550 nm. The dopant levels of Sn are incorporated at temperatures in the 370–390 °C range, yielding atomically smooth layers devoid of threading defects at high growth rates of 15–30 nm/min. These conditions are far more compatible with complementary metal-oxide semiconductor processing than the high growth and processing temperatures required to achieve the same responsivity via tensile strain in pure Ge on Si. A detailed study of a detector based on a Sn-doped Ge layer with 0.25% (1.1 × 1020 cm−3) Sn range demonstrates the responsivity enhancement and shows much better I-V characteristics than previously fabricated detectors based on Ge1−ySny alloys with y = 0.02.Previously developed methods used to grow Ge1−ySny alloys on Si are extended to Sn concentrations in the 1019−1020 cm−3 range. These concentrations are shown to be sufficient to engineer large increases in the responsivity of detectors operating at 1550 nm. The dopant levels of Sn are incorporated at temperatures in the 370–390 °C range, yielding atomically smooth layers devoid of threading defects at high growth rates of 15–30 nm/min. These conditions are far more compatible with complementary metal-oxide semiconductor processing than the high growth and processing temperatures required to achieve the same responsivity via tensile strain in pure Ge on Si. A detailed study of a detector based on a Sn-doped Ge layer with 0.25% (1.1 × 1020 cm−3) Sn range demonstrates the responsivity enhancement and shows much better I-V characteristics than previously fabricated detectors based on Ge1−ySny alloys with y = 0.02.

Compositional dependence of the absorption edge and dark currents in Ge1−x−ySixSny/Ge(100) photodetectors grown via ultra-low-temperature epitaxy of Ge4H10, Si4H10, and SnD4

Lattice-matched Ge1−x−ySixSny (x ≤ 0.2, y ≤ 0.05) alloys were deposited defect-free on Ge(001) substrates via low-temperature (330–290 °C) reactions of Ge4H10, Si4H10 and SnD4 hydrides, and used to fabricate pin photodetectors. The growth is carried out under gas-source molecular beam epitaxy conditions in a specially designed single-wafer reactor. Optical responsivity measurements reveal absorption edges between 0.88 eV and 0.98 eV, which are used to determine the compositional dependence of the direct band gap. A study of the I-V characteristics of the diodes shows that the dark current is very weakly correlated with the number of Si-Sn bonds in the alloy.

Photoluminescence from heavily doped GeSn:P materials grown on Si(100)

Photoluminescence has been observed at room temperature in phosphorus-dopedGe1−ySny/Si(100) alloys containing carrier densities in the 1-6 × 1019 cm−3 range. The emission intensity is one order of magnitude stronger than observed in similar undoped films, and the enhancement is consistent with theoretical predictions for doped-Ge like materials. The ratio Idir/Iind of direct over indirect gap emission is found to increase for high-Sn concentrations as a result of the reduced Γ-L valley separation in Ge1−ySny alloys. These results confirm that alloying with Sn is a viable alternative to tensile strain as a tool to enhance direct-gap emission in Ge-like semiconductors.

New strategies for Ge-on-Si materials and devices using non-conventional hydride chemistries: the tetragermane case

We introduce a practical chemical vapor deposition strategy for next-generation Ge-on-Si epitaxy utilizing recently introduced Ge4H10 hydride sources that confer unprecedented deposition efficiencies at very low-temperatures (<400 °C). The corresponding high growth rates produce thick bulk-like Ge films with structural and electrical properties significantly improved relative to state-of-the-art results obtained using conventional approaches. The use of a pure, single-source compound facilitates the control of residual doping, and enables p-i-n devices whose dark currents are not entirely determined by defects and whose zero-bias optical collection efficiencies are higher than obtained from samples fabricated using alternative Ge-on-Si approaches. The reaction pathways leading to the high-yield synthesis of Ge4H10 are identified on the basis of quantum thermochemistry simulations. The results suggest a simple approach to routine synthesis of tetragermane as the main product in quantities sufficient to be deployed as a commercial source.