RF characterization of InP double heterojunction bipolar transistors on a flexible substrate under bending conditions

This letter presents the fabrication of InP double heterojunction bipolar transistors(DHBTs)on a 3-inch flexible substrate with various thickness values of the benzocyclobutene(BCB)adhesive bonding layer,the corresponding thermal resistance of the InP DHBT on flexible substrate is also measured and calculated.InP DHBT on a flexible substrate with 100 nm BCB obtains cut-off frequency f_(T)=358 GHz and maximum oscillation frequency f_(MAX)=530 GHz.Moreover,the frequency performance of the InP DHBT on flexible substrates at different bending radii are compared.It is shown that the bending strain has little effect on the frequency characteristics(less than 8.5%),and these bending tests prove that InP DHBT has feasible flexibility.

Probing angle-resolved reflection signatures of intralayer and interlayer excitons in monolayer and bilayer MoS_(2)

Strongly bound excitons in atomically thin transition metal dichalcogenides offer many opportunities to reveal the underlying physics of basic quasiparticles and many-body effects in the two-dimensional(2D)limit.Comprehensive reflection investigation on band-edge exciton transitions is essential to exploring wealthy light–matter interactions in the emerging 2D semiconductors,whereas angle-resolved reflection(ARR)characteristics of intralayer and interlayer excitons in 2D MoS_(2)layers remain unclear.Herein,we report ARR spectroscopic features of A,B,interlayer excitons in monolayer(ML)and bilayer(BL)MoS_(2)on three kinds of photonic substrates,involving distinct exciton–photon interactions.In a BL MoS_(2)on a protected silver mirror,the interlayer exciton with one-third amplitude of A exciton appears at 0.05 eV above the A exciton energy,exhibiting an angleinsensitive energy dispersion.When ML and BL MoS_(2)lie on a SiO_(2)-covered silicon,the broad trapped-photon mode weakly couples with the reflection bands of A and B excitons by the Fano resonance effect,causing the asymmetric lineshapes and the redshifted energies.After transferring MoS_(2)layers onto a one-dimensional photonic crystal,two high-lying branches of B-exciton polaritons are formed by the interactions between B excitons and Bragg photons,beyond the weak-coupling regime.This work provides ARR spectral benchmarks of A,B,interlayer excitons in ML and BL MoS_(2),gaining insights into the interpretation of light–matter interactions in 2D semiconductors and the design of their devices for practical photonic applications.

Edge effect during microwave plasma chemical vapor deposition diamond-film:Multiphysics simulation and experimental verification

This study focused on the investigation of the edge effect of diamond films deposited by microwave plasma chemical vapor de-position.Substrate bulge height△h is a factor that affects the edge effect,and it was used to simulate plasma and guide the diamond-film deposition experiments.Finite-element software COMSOL Multiphysics was used to construct a multiphysics(electromagnetic,plasma,and fluid heat transfer fields)coupling model based on electron collision reaction.Raman spectroscopy and scanning electron microscopy were performed to characterize the experimental growth and validate the model.The simulation results reflected the experimental trends observed.Plasma discharge at the edge of the substrate accelerated due to the increase in△h(△h=0-3 mm),and the values of electron density(n_(c)),molar concentration of H(C_(H)),and molar concentration of CH_(3)(C_(CH_(3)))doubled at the edge(for the special concave sample with△h=−1 mm,the active chemical groups exhibited a decreased molar concentration at the edge of the substrate).At=0-3 mm,a high diamond growth rate and a large diamond grain size were observed at the edge of the substrate,and their values increased with.The uniformity of film thickness decreased with.The Raman spectra of all samples revealed the first-order characteristic peak of dia-mond near 1332 cm^(−1).When△h=−1 mm,tensile stress occurred in all regions of the film.When△h=1-3 mm,all areas in the film ex-hibited compressive stress.

Raman scattering investigation of twisted WS_(2)/MoS_(2) heterostructures: interlayer mechanical coupling versus charge transfer

Twisted van der Waals homo-and hetero-structures have aroused great attentions due to their unique physical properties,providing a new platform to explore the novel two-dimensional(2D)condensed matter physics.The robust dependence of phonon vibrations and electronic band structures on the twist angle has been intensively observed in transition metal dichalcogenide(TMD)homo-structures.However,the effects of twist angle on the lattice vibrational properties in the TMD heterostructures have not caused enough attention.Here,we report the distinct evolutions of Raman scattering and the underlying interlayer interactions in the twisted WS_(2)/MoS_(2) heterostructures.The shifts and linewidths of E_(2g)(Γ)and A_(1g)(Γ)phonon modes are found to be twist angle dependent.In particular,analogous to that of the twisted TMD homostructures,the frequency separations between E_(2g)(Γ)and A_(1g)(Γ)modes of MoS_(2) and WS_(2) in the twisted heterostructures varying with twist angle correlate with the interlayer mechanical coupling,essentially originating from the spacing-related repulsion between sulfur atoms.Moreover,the opposite shift behaviors and broadening of A_(1g)(Γ)modes caused by charge transfer are also observed in the twisted heterostructures.The calculated interlayer distances and band alignment of twisted WS_(2)/MoS_(2) through density functional theory further evidence our interpretations on the roles of the interlayer mechanical coupling and charge transfer in variations of Raman features.Such understanding and controlling of interlayer interaction through the stacking orientation are significant for future optoelectronic device design based on the newly emerged 2D heterostructures.

Monolayer tungsten disulfide in photonic environment:Angleresolved weak and strong light-matter coupling

Light-matter interactions in two-dimensional transition metal dichalcogenides(TMDs)are sensitive to the surrounding dielectric environment.Depending on the interacting strength,weak and strong exciton–photon coupling effects can occur when the exciton energy is resonant with the one of photon.Here we report angle-resolved spectroscopic signatures of monolayer tungsten disulfide(1L-WS2)in weak and strong exciton–photon coupling environments.Inherent optical response of 1L-WS_(2)in the momentum space is uncovered by employing a dielectric mirror as substrate,where the energy dispersion is angleindependent while the amplitudes increase at high detection angles.When 1L-WS_(2)sits on top of a dielectric layer on silicon,the resonant trapped photon weakly couples with the exciton,in which the minimum of reflection dip shifts at both sides of the crossing angle while the emitted exciton energy remains unchanged.The unusual shift of reflection dip is attributed to the presence of Fano resonance under white-light illumination.By embedding 1L-WS_(2)into a dielectric microcavity,strong exciton–photon coupling results in the formation of lower and upper polariton branches with an appreciable Rabi splitting of 34 meV at room temperature,where the observed blueshift of the lower polariton branch is indicative of the enhanced polaritonpolariton scattering.Our findings highlight the effect of dielectric environment on angle-resolved optical response of exciton–photon interactions in a two-dimensional semiconductor,which is helpful to develop practical TMD-based architectures for photonic and polaritonic applications.

Observation of Bragg polaritons in monolayer tungsten disulphide

Strong light-matter interactions involved with photons and quasiparticles are fundamentally interesting to access the wealthy many-body physics in quantum mechanics.The emerging two-dimensional(2D)semiconductors with large exciton binding energies and strong quantum confinement allow to investigate exciton-photon coupling at elevated temperatures.Here we report room-temperature formation of Bragg polaritons in monolayer semiconductor on a dielectric mirror through the exciton-Bragg photon coupling.With the negative detuning energy of -30 meV,angle-resolved reflection signals reveal anti-crossing behaviors of lower and upper polariton branches at±18°together with the Rabi splitting of 10 meV.Meanwhile,the strengthened photoluminescence appears in the lower polariton branch right below the anti-crossing angles,indicating the presence of the characteristic bottleneck effect caused by the slowing exciton-polariton energy relaxation towards the band minimum.The extracted coupling strength is between the ones of weak and distinct strong coupling regimes,where the eigenenergy splitting induced by the moderate coupling is resolvable but not large enough to fully separate two polaritonic components.Our work develops a simplified strategy to generate exciton-polaritons in 2D semiconductors and can be further extended to probe the intriguing bosonic characteristics of these quasiparticles,such as Bose-Einstein condensation,polariton lasing and superfluidity,directly at the material surfaces.

Tunable excitonic emission of monolayer WS2 for the optical detection of DNA nucleobases

Two-dimensional transition metal dichalcogenides （2D TMDs） possess a tunable excitonic light emission that is sensitive to external conditions such as electric field, strain, and chemical doping. In this work, we reveal the interactions between DNA nucleobases, i.e., adenine （A）, guanine （G）, cytosine （C）, and thymine （T） and monolayer WS2 by investigating the changes in the photoluminescence （PL） emissions of the monolayer WS2 after coating with nucleobase solutions. We found that adenine and guanine exert a clear effect on the PL profile of the monolayer WS2 and cause different PL evolution trends. In contrast, cytosine and thymine have little effect on the PL behavior. To obtain information on the interactions between the DNA bases and WS2, a series of measurements were conducted on adenine-coated WS2 monolayers, as a demonstration. The p-type doping of the WS2 monolayers on the introduction of adenine is clearly shown by both the evolution of the PL spectra and the electrical transport response. Our findings open the door for the development of label-free optical sensing approaches in which the detection signals arise from the tunable excitonic emission of the TMD itself rather than the fluorescence signals of label molecules. This dopant-selective optical response to the DNA nucleobases fills the gaps in previously reported optical biosensing methods and indicates a potential new strategy for DNA sequencing.

Spatial variations of valley splitting in monolayer transition metal dichalcogenide

In monolayer group-VI transition metal dichalcogenides(TMDs),valley splitting features have received a lot of attention since it can be potentially utilized for information storing and processing.Among the known two-dimensional(2D)TMDs,monolayer WSe2 or MoSe2 has been mostly selected for excitonic and valleytronic physics studies because of its sharp and well-resolved excitonic spectral features.Meanwhile,their high optical quality leads to a tremendous desire for developing promising WSe2-and MoSe2-based valleytronic devices.Toward this goal,exploring the uniformity of valley features crossing an entire piece of monolayer becomes necessary and critical.Here,we performed the systematic magnetophotoluminescence mapping measurements on mechanically exfoliated monolayer WSe2 and observed unconventional spatial variations of valley splitting.The observed nonuniformity is attributed to the modulated doping,which is probably due to the different distributions of unintentional absorbates across the sample.Such an unexpected doping effect shows the nonnegligible influence on the valley Zeeman splitting of the trion emission(XT)while affecting that of the neutral exciton emission(X0)trivially,evidencing for the large valleytronic sensitivity of the charged exciton.This work not only enriches the understanding of the doping effect on valley splitting but also is meaningful for developing 2D valleytronics.

An investigation of the DC and RF performance of InP DHBTs transferred to RF CMOS wafer substrate

This paper investigated the DC and RF performance of the In P double heterojunction bipolar transistors（DHBTs）transferred to RF CMOS wafer substrate.The measurement results show that the maximum values of the DC current gain of a substrate transferred device had one emitter finger,of 0.8μm in width and 5μm in length,are changed unobviously,while the cut-off frequency and the maximum oscillation frequency are decreased from 220to 171 GHz and from 204 to 154 GHz,respectively.In order to have a detailed insight on the degradation of the RF performance,small-signal models for the In P DHBT before and after substrate transferred are presented and comparably extracted.The extracted results show that the degradation of the RF performance of the device transferred to RF CMOS wafer substrate are mainly caused by the additional introduced substrate parasitics and the increase of the capacitive parasitics induced by the substrate transfer process itself.