Donguk Nam

Strained germanium thin film membrane on silicon substrate for optoelectronics

This work presents a novel method to introduce a sustainable biaxial tensile strain larger than 1% in a thin Ge membrane using a stressor layer integrated on a Si substrate. Raman spectroscopy confirms 1.13% strain and photoluminescence shows a direct band gap reduction of 100meV with enhanced light emission efficiency. Simulation results predict that a combination of 1.1% strain and heavy n(+) doping reduces the required injected carrier density for population inversion by over a factor of 60. We also present the first highly strained Ge photodetector, showing an excellent responsivity well beyond 1.6um.

Direct bandgap germanium-on-silicon inferred from 5.7% 〈100〉 uniaxial tensile strain [Invited]

We report uniaxial tensile strains up to 5.7% along 〈100〉 in suspended germanium (Ge) wires on a silicon substrate, measured using Raman spectroscopy. This strain is sufficient to make Ge a direct bandgap semiconductor. Theoretical calculations show that a significant fraction of electrons remain in the indirect conduction valley despite the direct bandgap due to the much larger density of states; however, recombination can nevertheless be dominated by radiative direct bandgap transitions if defects are minimized. We then calculate the theoretical efficiency of direct bandgap Ge LEDs and lasers. These strained Ge wires represent a direct bandgap Group IV semiconductor integrated directly on a silicon platform.

Roadmap to an Efficient Germanium-on-Silicon Laser: Strain vs. n-Type Doping

We provide a theoretical analysis of the relative merits of tensile strain and n-type doping as approaches to realizing an efficient low-power germanium laser. Ultimately, tensile strain offers threshold reductions of over 200x, and significant improvements in slope efficiency compared with the recently demonstrated 0.25% strained electrically pumped germanium laser. In contrast, doping offers fundamentally limited benefits, and too much doping is harmful. Moreover, we predict that tensile strain reduces the optimal doping value and that experimentally demonstrated doping has already reached its fundamental limit. We therefore theoretically show large (>; 1%) tensile strain to be the most viable path to a practical germanium-on-silicon laser.

Electroluminescence from strained germanium membranes and implications for an efficient Si-compatible laser

We demonstrate room-temperature electroluminescence (EL) from light-emitting diodes (LED) on highly strained germanium (Ge) membranes. An external stressor technique was employed to introduce a 0.76% bi-axial tensile strain in the active region of a vertical PN junction. Electrical measurements show an on-off ratio increase of one order of magnitude in membrane LEDs compared to bulk. The EL spectrum from the 0.76% strained Ge LED shows a 100nm redshift of the center wavelength because of the strain-induced direct band gap reduction. Finally, using tight-binding and FDTD simulations, we discuss the implications for highly efficient Ge lasers.

Strain-Induced Pseudoheterostructure Nanowires Confining Carriers at Room Temperature with Nanoscale-Tunable Band Profiles

Semiconductor heterostructures play a vital role in photonics and electronics. They are typically realized by growing layers of different materials, complicating fabrication and limiting the number of unique heterojunctions on a wafer. In this Letter, we present single-material nanowires which behave exactly like traditional heterostructures. These pseudoheterostructures have electronic band profiles that are custom-designed at the nanoscale by strain engineering. Since the band profile depends only on the nanowire geometry with this approach, arbitrary band profiles can be individually tailored at the nanoscale using existing nanolithography. We report the first experimental observations of spatially confined, greatly enhanced (>200×), and wavelength-shifted (>500 nm) emission from strain-induced potential wells that facilitate effective carrier collection at room temperature. This work represents a fundamentally new paradigm for creating nanoscale devices with full heterostructure behavior in photonics and electronics.

Theoretical Analysis of GeSn Alloys as a Gain Medium for a Si-Compatible Laser

In this paper, a theoretical analysis of unstrained GeSn alloys as a laser gain medium was performed. Using the empirical pseudopotential method, the band structure of GeSn alloys was simulated and verified against experimental data. This model shows that GeSn becomes direct bandgap with 6.55% Sn concentration. The optical gain of GeSn alloys with 0-10% Sn concentration was calculated with different n-type doping concentrations and injection levels. It is shown theoretically that adding Sn greatly increases the differential gain owing to the reduction of energy between the direct and indirect conduction bands. For a double-heterostructure laser, the model shows that at a cavity loss of 50 cm-1, the minimum threshold current density drops 60 times from Ge to Ge0.9Sn0.1, and the corresponding optimum n-doping concentration of the active layer drops by almost two orders of magnitude. These results indicate that GeSn alloys are good candidates for a Si-compatible laser.

Fluorine passivation of vacancy defects in bulk germanium for Ge metal-oxide-semiconductor field-effect transistor application

Vacancy defects in germanium (Ge) adversely impact the electrical performance of Ge based metal-oxide-semiconductor field-effect transistor (MOSFET) in several ways. They behave as an acceptor site, thereby deactivating n-type dopants in the source/drain region. They can also increase substrate leakage currents and impact carrier lifetime in the channel region. In this paper, we characterize and verify the electrical behavior of vacancy defects in Ge using spreading resistance profiling (SRP). Effect of thermal annealing on the vacancy concentration is studied. Finally, passivation of these defects using fluorine (F) ion-implant is shown to demonstrate the feasibility of performance enhancement in Ge-MOSFETs.

Direct Bandgap Light Emission from Strained Germanium Nanowires Coupled with High-Q Nanophotonic Cavities

A silicon-compatible light source is the final missing piece for completing high-speed, low-power on-chip optical interconnects. In this paper, we present a germanium nanowire light emitter that encompasses all the aspects of potential low-threshold lasers: highly strained germanium gain medium, strain-induced pseudoheterostructure, and high-Q nanophotonic cavity. Our nanowire structure presents greatly enhanced photoluminescence into cavity modes with measured quality factors of up to 2000. By varying the dimensions of the germanium nanowire, we tune the emission wavelength over more than 400 nm with a single lithography step. We find reduced optical loss in optical cavities formed with germanium under high (>2.3%) tensile strain. Our compact, high-strain cavities open up new possibilities for low-threshold germanium-based lasers for on-chip optical interconnects.

Study of Carrier Statistics in Uniaxially Strained Ge for a Low-Threshold Ge Laser

In this paper, we present a comprehensive study of carrier statistics in germanium with high uniaxial strain along the [100] direction. Several types of PL experiments were conducted to investigate polarization-, temperature- and excitation-dependent carrier statistics in germanium under various amounts of uniaxial strain. With the ability to clearly resolve multiple photoluminescence peaks originating from strain-induced valence band splitting, we experimentally observed strongly polarized light emission from direct band gap transitions. Our experiments also confirm that uniaxial strain increases the hole population in the highest valence band as well as the electron population in the direct conduction band. Based upon our experimental results, we present theoretical modeling showing that the lasing threshold of a germanium laser can be reduced by >100× with 2.5% strain.

Observation of improved minority carrier lifetimes in high-quality Ge-on-insulator using time-resolved photoluminescence.

We report improved minority carrier lifetimes in n-type-doped and tensile-strained germanium by measuring direct bandgap photoluminescence from germanium-on-insulator substrates with various levels of defect density. We first describe a method to fabricate a high-quality germanium-on-insulator substrate by employing direct wafer bonding and chemical-mechanical polishing. Raman spectroscopy measurement was performed to assess the purity of the transferred layer on an insulator. Using time-resolved photoluminescence decay measurement, we observe that minority carrier lifetimes can be improved by over a factor of 3 as the defective top interface of our material stack is removed. Our high-quality germanium-on-insulator should be an ideal platform for high-performance, germanium-based photonic devices for on-chip optical interconnects.