Gregory Sun

Design of an electrically pumped SiGeSn/GeSn/SiGeSn double-heterostructure midinfrared laser

This paper presents the conception, modeling, and simulation of a silicon-based group-IV semiconductor injection laser diode in which the GeSn-alloy active region has a direct band gap wavelength in the 1.8 to 3.0 μm midwave infrared for 6%–12% α-Sn. The strain-free monolithic P-type semiconductor/Intrinsic semiconductor/N-type semiconductor (PIN) bulk heterostructure, grown lattice matched upon a relaxed GeSn-buffer on silicon-on-insulator, is believed to be manufacturable in a complementary metal-oxide semiconductor fab. Detailed modeling is given for the type-I band offsets, carrier lifetimes, infrared gain profile and laser threshold current density Jth in a Fabry–Perot cavity having 20–100 cm−1 loss. The laser’s temperature of operation is determined by a combination of the radiative lifetime and the nonradiative lifetime due to unwanted Auger electron-hole recombination. If we keep Jth below 10 kA/cm2, then we find that this laser requires cooling in the 100–200 K range, whereas Jth at 300 K appears...

Design of a Si-based lattice-matched room-temperature GeSn/GeSiSn multi-quantum-well mid-infrared laser diode.

This paper presents modeling and simulation of a silicon-based group IV semiconductor injection laser diode in which the active region has a multiple quantum well structure formed with Ge(0.9)Sn(0.1) quantum wells separated by Ge(0.75)Si(0.1)Sn(0.15) barriers. These alloy compositions were chosen to satisfy three conditions simultaneously: a direct band gap for Ge(0.9)Sn(0.1), type-I band alignment between Ge(0.9)Sn(0.1) and Ge(0.75)Si(0.1)Sn(0.15,) and a lattice match between wells and barriers. This match ensures that the entire structure can be grown strain free upon a relaxed Ge(0.75)Si(0.1)Sn(0.15) buffer on a silicon substrate - a CMOS compatible process. Detailed analysis is performed for the type I band offsets, carrier lifetime, optical confinement, and modal gain. The carrier lifetime is found to be dominated by the spontaneous radiative process rather than the Auger process. The modal gain has a rather sensitive dependence on the number of quantum wells in the active region. The proposed laser is predicted to operate at 2.3 μm in the mid infrared at room temperature.

Mid-infrared electroluminescence from a Ge/Ge0.922Sn0.078/Ge double heterostructure p-i-n diode on a Si substrate

We report the observation of mid-infrared room-temperature electroluminescence from a p-i-n Ge/Ge0.922Sn0.078/Ge double heterostructure diode. The device structure is grown using low-temperature molecular beam epitaxy. Emission spectra under various injection current densities in the range of 318 A/cm2–490 A/cm2 show two distinct profiles peaked at 0.545 eV (2.275 μm) and 0.573 eV (2.164 μm), corresponding to indirect and direct bandgaps of the Ge0.922Sn0.078 active layer, respectively. This work represents a step forward towards the goal of an efficient direct-bandgap GeSn light-emitting device on a Si substrate by incorporating higher Sn content of 7.8% in a diode structure that operates at lower current densities.

GeSn-based p-i-n photodiodes with strained active layer on a Si wafer

We report an investigation of GeSn-based p-i-n photodiodes with an active GeSn layer that is almost fully strained. The results show that (a) the response of the Ge/GeSn/Ge heterojunction photodiodes is stronger than that of the reference Ge-based photodiodes at photon energies above the 0.8 eV direct bandgap of bulk Ge (<1.55 μm), and (b) the optical response extends to lower energy regions (1.55–1.80 μm wavelengths) as characterized by the strained GeSn bandgap. A cusp-like spectral characteristic is observed for samples with high Sn contents, which is attributed to the significant strain-induced energy splitting of heavy and light hole bands. This work represents a step forward in developing GeSn-based infrared photodetectors.

Plasmonic light-emission enhancement with isolated metal nanoparticles and their coupled arrays

We present a systematic study of the enhancement of radiative efficiency of light-emitting matter achieved by proximity to metal nanoparticles. Our goal is to ascertain the limits of the attainable enhancement. Two separate arrangements of metal nanoparticles are studied, namely isolated particles and an array of particles. The method of analysis is based on the effective mode volume theory. Using the example of an InGaN∕GaN quantum-well active region positioned in close proximity to Ag nanospheres, we obtain optimal parameters for the nanoparticles for maximum attainable enhancement. Our results show that while the enhancement due to isolated metal nanoparticles is significant, only modest enhancement can be achieved with an ordered array. We further conclude that a random assembly of isolated particles holds an advantage over the ordered arrays for light-emitting devices of finite area.

Enhancement of optical properties of nanoscaled objects by metal nanoparticles

We provide a simple analytical model for the modification of optical properties of active molecules and other objects when they are placed in the vicinity of metal nanoparticles of subwavelength dimensions. Specifically, we study the enhancement of optical radiation, electroluminescence, and photoluminescence absorbed or emitted by these objects. The theory takes into account the radiative decay of the surface plasmon mode supported by the metal nanospheres--a basic phenomenon that has been ignored in electrostatic treatment. Using the example of Ag nanospheres embedded in a GaN dielectric, we show that enhancement for each case depends strongly on the nanoparticle size-enabling optimization for each combination of absorption cross section, original radiative efficiency, and separation between the object and metal sphere. The enhancement effect is most significant for relatively weak and diluted absorbers and rather inefficient emitters that are placed in close proximity to the metal nanoparticles.

High efficiency thin-film crystalline Si/Ge tandem solar cell

We propose and simulate a photovoltaic solar cell comprised of Si and Ge pn junctions in tandem. With an anti-reflection film at the front surface, we have shown that optimal solar cells favor a thin Si layer and a thick Ge layer with a thin tunnel hetero-diode placed in between. We predict efficiency ranging from 19% to 28% for AM1.5G solar irradiance concentrated from 1 approximately 1000 Suns for a cell with a total thickness approximately 100 microm.

Plasmon Enhancement of Luminescence by Metal Nanoparticles

We present a simple analytical yet rigorous model that adequately describes the luminescence enhancement of optical emitters that are placed in the vicinity of metal nanoparticles of subwavelength dimensions. The theory takes into account the radiative decay of the surface plasmon mode supported by the metal nanospheres-a basic phenomenon that has been ignored in electrostatic treatment. Using the example of Au nanospheres embedded in the GaN dielectric, we show that enhancement for each case depends strongly on the original radiative efficiency of the emitter, the nanoparticle size, and the separation between the emitter and metal nanosphere. We demonstrate that strong enhancement favors the closely spaced emitters and metal nanospheres, but putting them too close to each other does not always produce additional enhancement. Thus, our model provides analytical treatment of the luminescence quenching and can be used to optimize both nanoparticle size and its separation from the emitter to yield maximum enhancement factor.

Terahertz gain in a SiGe/Si quantum staircase utilizing the heavy-hole inverted effective mass

Modeling and design studies show that a strain-balanced Si1−xGex/Si superlattice on Si1−yGey-buffered Si can be engineered to give an inverted effective mass HH2 subband adjacent to HH1, thereby enabling a 77 K edge-emitting electrically pumped p–i–p quantum staircase laser for THz emission at energies below the 37 meV Ge–Ge optical phonon energy. Analysis of hole-phonon scattering, lifetimes, matrix elements, and hole populations indicates that a gain of 450 cm−1 will be feasible at f=7.3 THz during 1.7 kA/cm2 current injection.

Silicon-based interminiband infrared laser

Simulations of a room-temperature p-i-p coherently strained Si0.5Ge0.5/Si superlattice quantum-parallel laser diode have been made. Calculations have been made of the local-in-k-space population inversion between the nonparabolic heavy-hole valence minibands, HH2 and HH1. Lasing is at 5.4 μm and the optical dipole matrix element is 3.7 A. Analysis of radiative-and-phonon scattering between the “mixed” bands indicates a lifetime difference between the upper and lower states of 2.4 ps. At an injected current density of 5000 A/cm2, a laser gain of 134 cm−1 is calculated.