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1.
Sci Rep ; 9(1): 259, 2019 Jan 22.
Artigo em Inglês | MEDLINE | ID: mdl-30670785

RESUMO

In recent years much effort has been made to increase the Sn content in GeSn alloys in order to increase direct bandgap charge carrier recombination and, therefore, to reach room temperature lasing. While being successful for the former, the increase of Sn content is detrimental, leading to increased defect concentrations and a lower thermal budget regarding processing. In this work we demonstrate strong photoluminescence enhancement in low Sn content Ge0.94Sn0.06 layers by implementing tensile strain. Fitting of the calculated photoluminescence spectra to reproduce our experimental results indicates a strain of ~1.45%, induced via an SiNx stressor layer, which is strong enough to transform the investigated layer into a direct bandgap semiconductor. Moreover, theoretical calculations, using the 8-band k·p model, show the advantages of using low Sn content tensile strained GeSn layers in respect to gain and lasing temperature. We show that low Sn content GeSn alloys have a strong potential to enable efficient room temperature lasers on electronic-photonic integrated circuits.

2.
Sci Rep ; 8(1): 15557, 2018 Oct 22.
Artigo em Inglês | MEDLINE | ID: mdl-30348982

RESUMO

Since the first demonstration of lasing in direct bandgap GeSn semiconductors, the research efforts for the realization of electrically pumped group IV lasers monolithically integrated on Si have significantly intensified. This led to epitaxial studies of GeSn/SiGeSn hetero- and nanostructures, where charge carrier confinement strongly improves the radiative emission properties. Based on recent experimental literature data, in this report we discuss the advantages of GeSn/SiGeSn multi quantum well and quantum dot structures, aiming to propose a roadmap for group IV epitaxy. Calculations based on 8-band k∙p and effective mass method have been performed to determine band discontinuities, the energy difference between Γ- and L-valley conduction band edges, and optical properties such as material gain and optical cross section. The effects of these parameters are systematically analyzed for an experimentally achievable range of Sn (10 to 20 at.%) and Si (1 to 10 at.%) contents, as well as strain values (-1 to 1%). We show that charge carriers can be efficiently confined in the active region of optical devices for experimentally acceptable Sn contents in both multi quantum well and quantum dot configurations.

3.
Adv Sci (Weinh) ; 5(6): 1700955, 2018 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-29938172

RESUMO

Growth and characterization of advanced group IV semiconductor materials with CMOS-compatible applications are demonstrated, both in photonics. The investigated GeSn/SiGeSn heterostructures combine direct bandgap GeSn active layers with indirect gap ternary SiGeSn claddings, a design proven its worth already decades ago in the III-V material system. Different types of double heterostructures and multi-quantum wells (MQWs) are epitaxially grown with varying well thicknesses and barriers. The retaining high material quality of those complex structures is probed by advanced characterization methods, such as atom probe tomography and dark-field electron holography to extract composition parameters and strain, used further for band structure calculations. Special emphasis is put on the impact of carrier confinement and quantization effects, evaluated by photoluminescence and validated by theoretical calculations. As shown, particularly MQW heterostructures promise the highest potential for efficient next generation complementary metal-oxide-semiconductor (CMOS)-compatible group IV lasers.

4.
Small ; 13(16)2017 04.
Artigo em Inglês | MEDLINE | ID: mdl-28160408

RESUMO

SiGeSn ternaries are grown on Ge-buffered Si wafers incorporating Si or Sn contents of up to 15 at%. The ternaries exhibit layer thicknesses up to 600 nm, while maintaining a high crystalline quality. Tuning of stoichiometry and strain, as shown by means of absorption measurements, allows bandgap engineering in the short-wave infrared range of up to about 2.6 µm. Temperature-dependent photoluminescence experiments indicate ternaries near the indirect-to-direct bandgap transition, proving their potential for ternary-based light emitters in the aforementioned optical range. The ternaries' layer relaxation is also monitored to explore their use as strain-relaxed buffers, since they are of interest not only for light emitting diodes investigated in this paper but also for many other optoelectronic and electronic applications. In particular, the authors have epitaxially grown a GeSn/SiGeSn multiquantum well heterostructure, which employs SiGeSn as barrier material to efficiently confine carriers in GeSn wells. Strong room temperature light emission from fabricated light emitting diodes proves the high potential of this heterostructure approach.

5.
Opt Express ; 24(18): 20554-70, 2016 Sep 05.
Artigo em Inglês | MEDLINE | ID: mdl-27607659

RESUMO

Optical feedback effects in lasers may be useful or problematic, depending on the type of application. When semiconductor lasers are operated using pulsed-mode excitation, their behavior under optical feedback depends on the electronic and thermal characteristics of the laser, as well as the nature of the external cavity. Predicting the behavior of a laser under both optical feedback and pulsed operation therefore requires a detailed model that includes laser-specific thermal and electronic characteristics. In this paper we introduce such a model for an exemplar bound-to-continuum terahertz frequency quantum cascade laser (QCL), illustrating its use in a selection of pulsed operation scenarios. Our results demonstrate significant interplay between electro-optical, thermal, and feedback phenomena, and that this interplay is key to understanding QCL behavior in pulsed applications. Further, our results suggest that for many types of QCL in interferometric applications, thermal modulation via low duty cycle pulsed operation would be an alternative to commonly used adiabatic modulation.

6.
Opt Express ; 24(19): 21948-56, 2016 Sep 19.
Artigo em Inglês | MEDLINE | ID: mdl-27661929

RESUMO

We explain the origin of voltage variations due to self-mixing in a terahertz (THz) frequency quantum cascade laser (QCL) using an extended density matrix (DM) approach. Our DM model allows calculation of both the current-voltage (I-V) and optical power characteristics of the QCL under optical feedback by changing the cavity loss, to which the gain of the active region is clamped. The variation of intra-cavity field strength necessary to achieve gain clamping, and the corresponding change in bias required to maintain a constant current density through the heterostructure is then calculated. Strong enhancement of the self-mixing voltage signal due to non-linearity of the (I-V) characteristics is predicted and confirmed experimentally in an exemplar 2.6 THz bound-to-continuum QCL.

7.
ACS Appl Mater Interfaces ; 7(1): 62-7, 2015 Jan 14.
Artigo em Inglês | MEDLINE | ID: mdl-25531887

RESUMO

We present the epitaxial growth of Ge and Ge0.94Sn0.06 layers with 1.4% and 0.4% tensile strain, respectively, by reduced pressure chemical vapor deposition on relaxed GeSn buffers and the formation of high-k/metal gate stacks thereon. Annealing experiments reveal that process temperatures are limited to 350 °C to avoid Sn diffusion. Particular emphasis is placed on the electrical characterization of various high-k dielectrics, as 5 nm Al2O3, 5 nm HfO2, or 1 nmAl2O3/4 nm HfO2, on strained Ge and strained Ge0.94Sn0.06. Experimental capacitance-voltage characteristics are presented and the effect of the small bandgap, like strong response of minority carriers at applied field, are discussed via simulations.

8.
J Phys Chem Lett ; 5(8): 1335-40, 2014 Apr 17.
Artigo em Inglês | MEDLINE | ID: mdl-26269977

RESUMO

We developed an accurate model accounting for electron-phonon interaction in colloidal quantum dot supercrystals that allowed us to identify the nature of charge carriers and the electrical transport regime. We find that in experimentally analyzed CdSe nanocrystal solids, the electron-phonon interaction is sufficiently strong that small polarons localized to single dots are formed. Charge-carrier transport occurs by small polaron hopping between the dots, with mobility that decreases with increasing temperature. While such a temperature dependence of mobility is usually considered as a proof of band transport, we show that the same type of dependence occurs in the system where transport is dominated by small polaron hopping.

9.
Opt Express ; 21(19): 22194-205, 2013 Sep 23.
Artigo em Inglês | MEDLINE | ID: mdl-24104111

RESUMO

The terahertz (THz) frequency quantum cascade laser (QCL) is a compact source of high-power radiation with a narrow intrinsic linewidth. As such, THz QCLs are extremely promising sources for applications including high-resolution spectroscopy, heterodyne detection, and coherent imaging. We exploit the remarkable phase-stability of THz QCLs to create a coherent swept-frequency delayed self-homodyning method for both imaging and materials analysis, using laser feedback interferometry. Using our scheme we obtain amplitude-like and phase-like images with minimal signal processing. We determine the physical relationship between the operating parameters of the laser under feedback and the complex refractive index of the target and demonstrate that this coherent detection method enables extraction of complex refractive indices with high accuracy. This establishes an ultimately compact and easy-to-implement THz imaging and materials analysis system, in which the local oscillator, mixer, and detector are all combined into a single laser.

10.
Opt Express ; 21(1): 867-76, 2013 Jan 14.
Artigo em Inglês | MEDLINE | ID: mdl-23388980

RESUMO

We demonstrate electroabsorption contrast greater than 5 dB over the entire telecommunication S- and C-bands with only 1V drive using a new Ge/SiGe QW epitaxy design approach; further, this is demonstrated with the thinnest Ge/SiGe epitaxy to date, using a virtual substrate only 320-nm-thick. We use an eigenmode expansion method to model the optical coupling between SOI waveguides and both vertically and butt-coupled Ge/SiGe devices, and show that this reduction in thickness is expected to lead to a significant improvement in the insertion loss of waveguide-integrated devices.

11.
Opt Express ; 20(19): 21520-31, 2012 Sep 10.
Artigo em Inglês | MEDLINE | ID: mdl-23037272

RESUMO

Plasmonics is a field in which the light matter interaction can be controlled at the nanoscale by patterning the material surface to achieve enhanced optical effects. Realisation of micron sized silicon based plasmonic devices will require efficient coupling of light from an optical fibre grating coupler into silicon compatible plasmonic waveguides. In this paper we have investigated a silicon based plasmonic coupler with a very short taper length, which confines and focuses light from a broad input fibre opening into a plasmonic waveguide at the apex of the structure. A simple transfer matrix model was also developed to analyse the transmission performance of the coupler with respect to its key physical parameters. The proposed plasmonic coupler was optimised with respect to its different structural parameters using finite element simulations. A maximum coupling efficiency of 72% for light coupling from a 6.2 µm wide input opening into a 20 nm slit width was predicted. The simulated result also predicted an insertion loss of ≈ 2.0 dB for light coupling into a 300 nm single mode SOI waveguide from a plasmonic structure with a 10.4 µm input opening width and a taper length of only 3.15 µm. Furthermore, the application of the optimised plasmonic coupler as a splitter was investigated, in which the structure simultaneously splits and couples light with a predicted coupling efficiency of ≈ 37 % (or a total coupling efficiency of 73%) from a 6.22 µm input opening into two 50 nm wide plasmonic waveguides.

12.
Opt Lett ; 36(13): 2587-9, 2011 Jul 01.
Artigo em Inglês | MEDLINE | ID: mdl-21725488

RESUMO

We demonstrate terahertz (THz) frequency imaging using a single quantum cascade laser (QCL) device for both generation and sensing of THz radiation. Detection is achieved by utilizing the effect of self-mixing in the THz QCL, and, specifically, by monitoring perturbations to the voltage across the QCL, induced by light reflected from an external object back into the laser cavity. Self-mixing imaging offers high sensitivity, a potentially fast response, and a simple, compact optical design, and we show that it can be used to obtain high-resolution reflection images of exemplar structures.

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