Franz Keldysh Research Articles

Bias Dependence of Sub-Bandgap Light Detection for Core–Shell Silicon Nanowires

We experimentally demonstrate a vertically arrayed silicon nanowire-based device that exhibits voltage dependence of photoresponse to infrared sub-bandgap optical radiation. The device is fabricated using a proximity solid-state phosphorus diffusion method to convert the surface areas of highly boron-doped silicon nanowires into n-type, thus forming a radial core-shell p-n junction structure. Prominent photoresponse from such core-shell Si nanowires is observed under sub-bandgap illumination at 1310 nm. The strong bias dependence of the photoresponse and other device characteristics indicates that the sub-bandgap absorption is attributed to the intrinsic properties of core-shell Si nanowires rather than the surface states. The attractive characteristics are based on three physical mechanisms: the Franz-Keldysh effect, quasi-quantum confinement effect, and the impurity-state assisted photon absorption. The first two effects enhance carrier tunneling probability, rendering a stronger wave function overlap to facilitate sub-bandgap absorption. The last effect relaxes the k-selection rule by involving the localized impurity states, thus removing the limit imposed by the indirect bandgap nature of Si. The presented device uses single-crystal silicon and holds promise of fabricating nanophotonic systems in a fully complementary metal-oxide-semiconductor (CMOS) compatible process. The concept and approach can be applied to silicon and other materials to significantly extend the operable wavelength regime beyond the constraint of energy bandgap.

Dependence of Biasing Voltage and Illumination Power on the Built-in Electric Field of InGaP Solar Cells

The electroreflectance spectra of InGaP solar cells at various biasing voltages and illumination levels were studied. The Franz–Keldysh oscillations were observed and the electric field at the junction of solar cells can be extracted. The measured electric field decreases with increasing biasing voltage and illumination level. The theoretical electric fields as a function of the biasing voltage and the photocurrent are calculated on the basis of the depletion approximation in the junction theory and the photovoltaic effect, respectively.

Perturbative approach to space-charge field dynamics in photorefractive semiconductors

A general perturbative approach to solving photorefractive material equations for a two-wave mixing geometry is developed. The method allows the study of both stationary and transient states of the space-charge field. As an example, solutions were given for GaAs–AlGaAs photorefractive multiple quantum well structures working in the Franz–Keldysh geometry. Approximate explicit expressions describing the recording and erasure dynamics of photorefractive gratings have been derived. The results are compared with numerical solutions of a complete set of material equations.

Stationary high‐field domains as tools to measure field‐dependent carrier densities and mobilities in CdS and work functions of blocking contacts

AbstractIt is shown that stationary high‐field domains that occur in the range of negative differential conductivity, can be used to clearly identify field‐quenched states in CdS. These are distinguished as cathode‐ and anode‐adjacent domains and permit an unambiguous determination of electron density and mobility as function of the electric field. The anode‐adjacent domain permits additional insight into the high‐field properties of CdS in a field range that is now stabilized in the pre‐breakdown range. Here one finds direct evidence, by using the spectral distribution of the photoconductivity within the domain, of inverting the CdS to p‐type either by more complete quenching or by hole injection from the anode. Both types of stationary domains are determined by the work function of blocking contacts and thereby permit a closer analysis of the contact/CdS interface by shifting the space charge region away from the cathode to the bulk‐side end of the domain. This allows a more precise determination of the dependence of the work function on the photoconductivity of the adjacent CdS. The field‐of‐direction (phase portrait) analysis of the time‐independent transport and Poisson equations allows a simple classification of the two types of stationary high‐field domains relating to the two singular points in the decreasing branch of the current–voltage characteristic. This permits a transparent discussion of the field distribution of these domains that can be directly observed by the Franz–Keldysh effect. Herewith the transition between cathode‐ to anode‐adjacent domains as a function of the applied voltage can be directly followed.

Ge-on-Si for Si-based integrated materials and photonic devices

This paper reviews the recent progress in photonic devices application of Ge-on-Si. Ge-on-Si materials and optical devices are suitable candidates for Si-based optoelectronic integration because of the mature epitaxial technique and the compatibility with Si complementary metal-oxide-semiconductor (CMOS) technology. Recently, the realities of electric-pump Ge light emitting diode (LED) and optical-pump pulse Ge laser, Ge quantum well modulator based on quantum Stark confined effect, waveguide Ge modulator based on Franz-Keldysh (FK) effect, and high performance near-infrared Ge detector, rendered the Si-based optoelectronic integration using Ge photonic devices. Ge-on-Si material is also an important platform to grow other materials on it for Sibased optoelectronic integration. InGaAs and GeSn have been grown on the Ge-on-Si. InGaAs LED and GeSn photodetector have been successfully fabricated as well.

Integrated Optoelectronic Devices on Silicon

ABSTRACTSilicon (Si) has been the dominating material platform of microelectronics over half century. Continuous technological advances in circuit design and manufacturing enable complementary metal-oxide semiconductor (CMOS) chips with increasingly high integration complexity to be fabricated in an unprecedently scale and economical manner. Conventional Si-based planar lightwave circuits (PLCs) has benefited from advanced CMOS technology but only demonstrate passive functionalities in most circumstances due to poor light emission efficiency and weak major electro-optic effects (e.g., Pockels effect, the Kerr effect and the Franz–Keldysh effect) in Si. Recently, a new hybrid III-V-on-Si integration platform has been developed, aiming to bridge the gap between Si and III-V direct-bandgap materials for active Si photonic integrated circuit applications. Since then high-performance lasers, amplifiers, photodetectors and modulators, etc. have been demonstrated. Here we review the most recent progress on hybrid Si lasers and high-speed hybrid Si modulators. The former include distributed feedback (DFB) lasers showing over 10 mW output power and up to 85 oC continuous-wave (cw) operation, compact hybrid microring lasers with cw threshold less than 4 mA and over 3 mW output power, and 4-channel hybrid Si AWG lasers with channel space of 360 GHz. Recently fabricated traveling-wave electro-absorption modulators (EAMs) and Mach-Zehnder interferometer modulators (MZM) on this platform support 50 Gb/s and 40 Gb/s data transmission with over 10 dB extinction ratio, respectively.

Molecular beam epitaxy grown Ge/Si pin layer sequence for photonic devices

A key challenge to obtain a convergence of classical Si-based microelectronics and optoelectronics is the manufacturing of photonic integrated circuits integrable into classical Si-based integrated circuits. This integration would be greatly enhanced if similar facilities and technologies could be used. Therefore one approach is the development of optoelectronic components and devices made from group-IV-based materials such as SiGe, Ge or Ge:Sn. In this paper the optoelectronic performances of a pin diode made from a Ge/Si heterostructure pin layer sequence grown by molecular beam epitaxy are discussed. After a detailed description of the layer sequence growth and the device manufacturing process it will be shown that – depending on the chosen operating point and device design – the diode serves as a broadband high speed photo detector, Franz–Keldysh effect modulator or light emitting diode.

Ge-on-Si optoelectronics

Electronic–photonic synergy has become an increasingly clear solution to enhance the bandwidth and improve the energy efficiency of information systems. Monolithic integration of optoelectronic devices is the ideal solution for large-scale electronic–photonic synergy. Due to its pseudo-direct gap behavior in optoelectronic properties and compatibility with Si electronics, epitaxial Ge-on-Si has become an attractive solution for monolithic optoelectronics. In this paper we will review recent progress in Ge-on-Si optoelectronics, including photodetectors, electroabsorption modulators, and lasers. The performance of these devices has been enhanced by band-engineering such as tensile strain and n-type doping, which transforms Ge towards a direct gap material. Selective growth reduces defect density and facilitates monolithic integration at the same time. Ge-on-Si photodetectors have approached or exceeded the performance of their III–V counterparts, with bandwidth-efficiency product >30GHz for p-i-n photodiodes and bandwidth-gain product >340GHz for avalanche photodiodes. Enhanced Franz–Keldysh effect in tensile-strained Ge offers ultralow energy photonic modulation with <30fJ/bit energy consumption and >100GHz intrinsic bandwidth. Room temperature optically-pumped lasing as well as electroluminescence has also been achieved from the direct gap transition of band-engineered Ge-on-Si waveguides. These results indicate that band-engineered Ge-on-Si is promising to achieve monolithic active optoelectronic devices on a Si platform.

Pool–Frenkel thermoelectric modulation of exciton photoluminescence in GaSe crystals

Effect of external field on the exciton photoluminescence of GaSe crystals has been investigated and it has been observed that the PL is quenched with the applied field. The changes observed in the PL spectra have been analyzed with impact exciton, Franz–Keldysh and Pool–Frenkel effects. From the analyses of the experimental data, it has been found that the intensity of direct free, indirect free and bound exciton peaks decreased exponentially with the square root of applied field as I∼exp– β√ E. The energy positions of emission peaks were found to shift to longer wavelength with the applied field as Δ E∼ β√ E. From these findings, the Pool–Frenkel thermoelectric field effect is seen to be the dominant mechanism in the variation of exciton PL with the applied field even though the impact exciton and Franz–Keldysh effects contribute.

Oscillatory Structure in the Electroabsorption Spectrum of π-Conjugated Polymer Thin Films: How to Identify the Franz–Keldysh Oscillation

The nature of the oscillatory structure observed in the electroabsorption spectrum of highly ordered thin films of poly(9,9-dioctylfluorene) has been investigated and compared with that of the theoretically expected one-dimensional Franz–Keldysh (FK) oscillation. Although the observed oscillatory structure is not an FK oscillation, there are some similarities. From this comparison, we have discovered some features that can be used to distinguish whether the oscillatory structure is an FK oscillation, which include the field dependence of the oscillatory structure measured at four times the frequency of the applied electric field.

Contactless electroreflectance of AlGaN/GaN heterostructures deposited on c-, a-, m-, and (20.1)-plane GaN bulk substrates grown by ammonothermal method

Room temperature contactless electroreflectance (CER) has been applied to study optical transitions and the distribution of the built-in electric field in AlGaN/GaN heterostructures grown on c-, a-, m-, and (20.1)-plane GaN substrates obtained by the ammonothermal method. It has been clearly shown that polarization effects in the AlGaN/GaN heterostructures grown on the c-plane lead to a strong built-in electric field in the AlGaN layer. The aforementioned field was determined to be ∼0.43 MV/cm from the period of Franz–Keldysh oscillations (FKOs). In addition, polarization effects lead to the formation of a two dimensional electron gas at the AlGaN/GaN interface, which screens the band bending modulation in the GaN buffer layer, and, therefore, GaN-related excitonic transitions are not observed for this heterostructure. Such features/effects are also not observed in the AlGaN/GaN heterostructures grown on nonpolar and semipolar GaN substrates because any strong polarization effects are not expected in this case. For these heterostructures, very strong and sharp GaN excitonic resonances are clearly visible in CER spectra. The resonances are very similar to the excitonic transitions observed for the GaN epilayers deposited on nonpolar and semipolar substrates. Moreover, there is a very weak AlGaN-resonance without FKO for nonpolar and semipolar heterostructures instead of the strong AlGaN-related FKO, which is typical of polar AlGaN/GaN heterostructures.

Photoreflectance study of InAs ultrathin layer embedded in Si-delta-doped GaAs/AlGaAs quantum wells

Photoreflectance and photoluminescence studies were performed to characterize InAs ultrathin layer embedded in Si-delta-doped GaAs/AlGaAs high electron mobility transistors. These structures were grown by Molecular Beam Epitaxy on (100) oriented GaAs substrates with different silicon-delta-doped layer densities. Interband energy transitions in the InAs ultrathin layer quantum well were observed below the GaAs band gap in the photoreflectance spectra, and assigned to electron–heavy-hole (Ee–hh) and electron–light-hole (Ee–lh) fundamental transitions. These transitions were shifted to lower energy with increasing silicon-δ-doping density. This effect is in good agreement with our theoretical results based on a self-consistent solution of the coupled Schrödinger and Poisson equations and was explained by increased escape of photogenerated carriers and enhanced Quantum Confined Stark Effect in the Si-delta-doped InAs/GaAs QW. In the photoreflectance spectra, not only the channel well interband energy transitions were observed, but also features associated with the GaAs and AlGaAs bulk layers located at about 1.427 and 1.8eV, respectively. By analyzing the Franz–Keldysh Oscillations observed in the spectral characteristics of Si-δ-doped samples, we have determined the internal electric field introduced by ionized Si-δ-doped centers. We have observed an increase in the electric field in the InAs ultrathin layer with increasing silicon content. The results are explained in terms of doping dependent ionized impurities densities and surface charges.

Optical characterization of Cu 2ZnSnSe 4 grown by thermal co-evaporation

Cu 2ZnSnSe 4 thin films with different substrate temperature and Cu flux were grown by thermal co-evaporation. Raman scattering, photoluminescence, and contactless electroreflectance (ER) measurements were performed. The Raman spectra of Cu 2ZnSnSe 4 show two main peaks at 170 and 192 cm − 1 . The photoluminescence spectrum shows a peak below 1.0 eV. Franz–Keldysh oscillations (FKOs) were observed in the ER spectra. From the analysis of the FKOs, the bandgap energy of Cu 2ZnSnSe 4 thin films is estimated to be 1.07 eV at 90 K and 0.99 eV at room temperature. We conclude that the bandgap energy of Cu 2ZnSnSe 4 thin films is around 1.0 eV.

Photoreflectance study of GaMnAs layers grown by MBE

GaMnAs layers were grown by MBE on GaAs (001) substrates. The structural properties of the epilayers were studied by atomic force microscopy, secondary ion mass spectroscopy, high-resolution X-ray diffraction, and Raman spectroscopy. Photoreflectance spectra at room temperature exhibited Franz–Keldysh oscillations; from an analysis of these oscillations we obtained the built-in internal electric field and the bandgap energy of the GaMnAs epilayers. We studied the variation of these parameters as a function of Mn in the epilayers. In addition the holes concentration in the samples was extracted from the bandgap narrowing value and compared with the carrier density obtained by the Hall measurements.

Dynamical Franz–Keldysh effect in GaAs/AlGaAs multiple quantum wells induced by single-cycle terahertz pulses

We studied absorption spectra near the band gap of GaAs/AlGaAs multiple quantum wells subjected to strong single-cycle terahertz pulses. With an increasing terahertz electric field, the heavy-hole 1s exciton energy near the band gap redshifts, reaches a maximum, and turns to blueshift. The blueshift increases proportionally to the ponderomotive energy, i.e., the terahertz electric field squared, which differs from the response expected from the Frantz–Keldysh effect (FKE) in a dc electric field. This fact suggests that the observed ponderomotive energy dependence of the blueshift can be ascribed to the dynamical FKE.

Photoreflectance analysis of a GaInP/GaInAs/Ge multijunction solar cell

We have analyzed the photoreflectance spectra of a GaInP/GaInAs/Ge triple junction solar cell. The spectra reveal signatures from the window layer and middle and top subcells included in the stack. Additional contributions from the multilayer buffer introduced between the mismatched bottom and middle cells have been detected. Franz–Keldysh oscillations (FKOs) dominate the spectra above the fundamental bandgaps of the GaInP and GaInAs absorbers. From the FKO analysis, we have estimated the dominant electric fields within each subcell. In light of these results, photoreflectance is proposed as a useful diagnostic tool for quality assessment of multijunction structures prior to completion of the device or at earlier stages during its processing.

Optical properties of fresh dislocations in GaN

Optical properties of fresh dislocations, (a/3)[112¯0]-type edge dislocations on the (11¯00) prismatic plane, introduced into GaN by plastic deformation at elevated temperatures were investigated by photoluminescence and optical absorption measurements. Plastic deformation acts as an effective passivation, leading to remarkable reduction of near-band-edge photoluminescence intensity centered at 3.48eV and noticeable red-shift of the optical absorption edge. In a model of the Franz–Keldysh effect, the induced edge dislocations posses nonradiative trap sites around 3e/c along their core, resulting in the reduction of free-carrier concentration. Also, the induced dislocations give rise to some luminescence peaks in the energy range 1.7–2.4eV, differing from the yellow luminescence, which implies the formation of radiative recombination centers by the dislocations.

A review on quantum well structures in photonic devices for enhanced speed and span of the transmission network

Quantum well devices feature heterostructures of very thin epitaxial layers of group III-V and II-VI semiconductor materials. Quantum well devices are integrated monolithically with various optoelectronics devices to provide photonic integrated circuits. The representative structure could be realized with GaAs wells with GaAlAs barriers for wavelengths around 0.9 μm and InGaAsP are used for longer wavelengths. Together with quantum well, superlattice structure is another popular design for InGaAs Avalanche Photo Diode (APD). Quantum well structures find their applications in improved lasers, superlattice for photodiodes, modulators and switches. Consequences of quantum well theory are available today in terms of quantum wires and quantum dots. Upon the application of the normal electric field to quantum well structures, exciton pairs becomes more and more confined and the sharp exciton absorption peaks are observed. The effect is termed as “Quantum Confined Stark Effect”. The electro-absorption effect is approximately 50 times larger in multiple quantum well structures than it is in bulk semiconductors. Another electro-absorption effect known as “Franz Keldysh Effect” has been employed in monolithic waveguide detector. These effects lead to electro-absorption lasers (EAL) as well as electro-absorption laser modulators (EML).

Nonlinear Franz–Keldysh effect: two-photon absorption in a semiconducting quantum well

Two-photon absorption (TPA) in a semiconducting quantum well (QW) is investigated theoretically in the presence of the electric field. Numerical results show that the TPA depends strongly on the relative orientations of the electric field, the direction of optical polarization, and the direction of carrier confinement in the QW. The TPA is finite at photon energy below the threshold energy where the TPA would vanish in the absence of the electric field for any case. The TPA is large when the electric field is along the direction of carrier confinement in the QW. Moreover, when the electric field is in the plane of the QW, the difference Δα between the TPAs in a finite electric field and in zero field at the threshold energy increases almost linearly with increasing the electric field. In addition, it is also found that, in all cases, the TPA is less in the QW than that in the bulk.

Optical investigation of an AlGaN/GaN interface with the presence of a two-dimensional electron gas

In this paper we presented the study of an AlGaN/GaN heterostructure with the presence of a two-dimensional electron gas (2DEG) by using X-ray reciprocal space mapping (RSM), reflectance (R), photoluminescence (PL), electroreflectance (ER) and photoconductivity (PC) measurements. Some important properties such as alloy composition, strain status and layer thickness of Al x Ga 1− x N layer and carrier concentration of 2DEG are determined. The structure strain was analyzed by asymmetric RSM. We observed that the AlGaN layer grown on GaN layer was fully strained. The band gap energies of GaN and Al x Ga 1− x N are identified by PL and R spectrum, from which the Al composition x is determined to be 0.13. Using a semitransparent gate, an external dc bias was applied to control the carrier concentration of 2DEG underneath the Al x Ga 1− x N layer. We have also performed the ER and PC measurements at various external dc biases. In the ER spectra, not only the energy gap transitions of GaN and Al x Ga 1− x N but also the 2DEG feature and the Franz–Keldysh oscillations (FKOs) were observed. Through the analysis of FKOs, the strength of surface electric field and the 2DEG concentration can be determined. The optical absorption phenomenon in the AlGaN/GaN interface observed by the bias-dependent PC spectroscopy are presented and discussed.