Direct Conduction Band Research Articles

Ultrafast carrier recombination in highly n-doped Ge-on-Si films

We study the femtosecond carrier dynamics of n-type doped and biaxially strained Ge-on-Si films which occurs upon impulsive photoexcitation by means of broadband near-IR transient absorption spectroscopy. The modeling of the experimental data takes into account the static donor density in a modified rate equation for the description of the temporal recombination dynamics. The measurements confirm the negligible contribution at a high n-type doping concentration, in the 1019 cm−3 range, of Auger processes as compared to defect-related Shockley-Read-Hall recombination. Energy resolved dynamics reveal further insights into the doping-related band structure changes and suggest a reshaping of direct and indirect conduction band valleys to a single effective valley along with a significant spectral broadening of the optical transitions.

Engineering of Germanium Tunnel Junctions for Optical Applications

Data transfer across millimeter-scale electrical wires is limited by both data rates and power density, which creates a performance bottleneck for semiconductor microchips in modern computer systems. Hence, silicon based platforms for optical communication are diligently explored for an on-chip optical data transfer. Semiconductor optical amplifiers provide signal recovery and loss compensation in advanced photonic circuits and are, thus, indispensable components for such platforms. However, silicon photonic components have to operate at much lower voltages and energy-per-bit metrics before it is worth integrating them on-chip with a CPU. The usage of tunnel junctions to control carrier injection can provide a fast and energy-saving alternative. Here, we present experimental results on direct conduction band carrier modulation in the indirect semiconductor Ge by Zener tunnel injection. Electrons are injected by a reverse-biased p-n Zener tunnel diode and recombine radiatively with holes injected by a forward biased p-i-n diode. This injection mechanism favors tunneling of electrons into the direct conduction band valley with concomitant improvements of optoelectronic properties. Benchmarking the performance, 2.42 dB transmission change at 0.9 V bias (1660 nm) at 300 K confirm the working principle. Our device can serve as a starting point to investigate the benefits of tunnel injection for silicon photonic devices.

MBE grown germanium tunnel-junctions—burstein-moss effect and band-edge luminescence in the Ge Zener-Emitter

We investigated the Burstein-Moss (BM) shift by electroluminescence experiments in the Ge Zener-Emitter. Due to fast transit times through a tunneling barrier (few ps), electrons can be accumulated in the indirect as well as the direct conduction band by Zener band-to-band tunneling. The resulting shift can thus be assigned to the band-filling effect in the indirect conduction band (BM shift of 39 meV) as well as the direct conduction band (BM shift of 58 meV). Theoretical modelling has been carried out and shows good agreement with our experimental data from the Zener-Emitter.

(Invited) Toward GeSn Lasers: Light Amplification and Stimulated Emission in GeSn Waveguides at Room Temperature

The search for Si-based lasers has been ongoing for many years. GeSn alloys represent a possible solution to this problem due to their unique band structure. Pure Ge is an indirect band gap semiconductor, but the difference between the conduction band minima at the L- and Γ-points is only 140 meV. The diamond cubic structure of Sn, α-Sn, is a semimetal. When Sn is substituted for Ge in the Ge lattice, a hybrid band structure is formed, yielding a material with a tunable band gap. For Sn concentrations around 10% or lower, the band structure is still Ge-like with the lowest conduction band minimum at the L-point. However, the energy difference between the L- and Γ-minima reduces with increasing Sn concentration. For higher Sn concentrations, the minimum at the Γ-point becomes the lowest minimum, thereby creating a direct band gap semiconductor. For the lower Sn concentration regime, where the band gap of GeSn is still indirect, it is still possible to exploit the band structure to achieve direct band gap emission. Even in pure Ge, where the energy difference between the L- and Γ-minima is highest, there is a spillover of conduction band electrons into the Γ-valley, which then transition to the valence band through optical recombination. This effect can be further exploited through the use of n-type doping. The additional electrons from the n-type dopant serve to increase the quasi-Fermi level in the material, thereby filling the L-valley states with electrons. As the material is then injected with new electrons, using either optical or electrical pumping, the new electrons will have a much higher probability of occupying the Γ-valley, where they will undergo optical recombination and emit photons with the direct band gap energy. This effect has been used previously to create optically- and electrically-pumped lasers in tensile strained Ge-on-Si. We now extend this idea to the GeSn system. In this work, I will discuss efforts to create laser structures that operate at room temperature using GeSn layers with Sn concentrations in the range of 4.4-7.2% that have been highly n-type doped (1019-1020 cm-3) with phosphorus. Waveguides were fabricated from GeSn films grown on Ge-buffered-Si substrates using low pressure chemical vapor deposition (LPCVD) of SnD4 and Ge2H6. The GeSn films were doped in situ using the custom gaseous precursor P(GeH3)3. The waveguides were defined using standard UV photolithography, and then etched using reactive ion etching in a BCl3 plasma. The end facets of the waveguides were mechanically polished to a mirror finish, and then an Al film was sputtered on one facet to form a high-reflector. The Fresnel reflection on the other facet was used as the output coupler. The GeSn waveguides were optically pumped using a 976 nm CW laser. The waveguide emission from the output coupler was collected using an optical fiber, and the pump wavelength was filtered out using a dielectric filter. The resulting emission was focused onto a LN2-cooled extended InGaAs detector, and the output intensity was measured as a function of input intensity. At low power, the output intensity is proportional to the input intensity, as one would expect from spontaneous emission. As the input intensity is increased, there is an exponential increase in output intensity, indicative of stimulated emission. This stimulated emission was observed in waveguides with Sn concentrations over the entire range tested. As the Sn content is increased, the transition to stimulated emission becomes sharper, thereby indicating the increased efficiency of optical emission due to lowering the difference between the indirect and direct conduction band minima. This is the first observation of stimulated emission in GeSn at room temperature.

Direct band gap light emission and detection at room temperature in bulk germanium diodes with HfGe/Ge/TiN structure

Direct band gap (DBG) electroluminescence (EL) and photo detection were studied at room temperature for n-type bulk germanium (Ge) diodes with a fin type lateral HfGe/Ge/TiN structure. DBG EL spectra peaked at 1.55μm were clearly observed due to small hole and electron barrier heights of HfGe/Ge and TiN/Ge contacts. DBG EL peak intensity increased with increasing doping level of Ge substrate due to increased electron population in direct conduction band. The integrated intensity of DBG EL spectrum is proportional to the area of active region, implying a good surface-uniformity of EL efficiency. Small dark current intensity was measured as 2.4×10−7A under a reverse bias voltage of −1V, corresponding to dark current densities of 5.3×10−10A/μm or 3.2×10−10A/μm2. At the wavelength of 1.55μm, a linear dependence of photo current intensity on laser power was observed with a responsivity of 0.44A/W at a reverse bias voltage of −1V.

Band-Offset Engineering for GeSn-SiGeSn Hetero Tunnel FETs and the Role of Strain

In this paper a simulation study of the effect of conduction and valence band offsets on the subthreshold swing (SS) of a double-gate tunnel field-effect transistor (TFET) with gate-overlapped source is presented. The simulations show that if the pn-junction and the hetero-junction coincide, the band offsets can significantly improve the SS by suppressing the so-called point tunneling at the pn-junction. It turns out that the performance of an n-channel TFET is determined by the direct conduction band offset whereas that of a p-channel TFET is mainly effected by the energy difference between the light hole bands of the two materials. Thus, the performance of the hetero-junction TFET can be improved by selecting material systems with high conduction or valence band offsets. The misalignment between the pn-junction and the hetero-junction is shown to degrade the SS. The above-described band-offset engineering has been applied to the GeSn/SiGeSn hetero-structure system with and without strain. Simulations of GeSn/SiGeSn hetero-TFETs with band-to-band-tunneling parameters determined from pseudopotential calculations show that compressive strain in GeSn widens the design space for TFET application while tensile strain reduces it.

Direct band gap electroluminescence from bulk germanium at room temperature using an asymmetric fin type metal/germanium/metal structure

We demonstrated direct band gap (DBG) electroluminescence (EL) at room temperature from n-type bulk germanium (Ge) using a fin type asymmetric lateral metal/Ge/metal structure with TiN/Ge and HfGe/Ge contacts, which was fabricated using a low temperature (<400 °C) process. Small electron and hole barrier heights were obtained for TiN/Ge and HfGe/Ge contacts, respectively. DBG EL spectrum peaked at 1.55 μm was clearly observed even at a small current density of 2.2 μA/μm. Superlinear increase in EL intensity was also observed with increasing current density, due to superlinear increase in population of elections in direct conduction band. The efficiency of hole injection was also clarified.

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.

Effect of diameter variation on electrical characteristics of Schottky barrier indium arsenide nanowire field-effect transistors.

The effect of diameter variation on electrical characteristics of long-channel InAs nanowire metal-oxide-semiconductor field-effect transistors is experimentally investigated. For a range of nanowire diameters, in which significant band gap changes are observed due to size quantization, the Schottky barrier heights between source/drain metal contacts and the semiconducting nanowire channel are extracted considering both thermionic emission and thermally assisted tunneling. Nanowires as small as 10 nm in diameter were used in device geometry in this context. Interestingly, while experimental and simulation data are consistent with a band gap increase for decreasing nanowire diameter, the experimentally determined Schottky barrier height is found to be around 110 meV irrespective of the nanowire diameter. These observations indicate that for nanowire devices the density of states at the direct conduction band minimum impacts the so-called branching point. Our findings are thus distinctly different from bulk-type results when metal contacts are formed on three-dimensional InAs crystals.

Strain-Balanced ${\rm Ge}_{z}{\rm Sn}_{1-z}\hbox{--}{\rm Si}_{x}{\rm Ge}_{y}{\rm Sn}_{1-x-y}$ Multiple-Quantum-Well Lasers

We propose and analyze a strain-balanced Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-z</sub> -Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x-y</sub> multiple-quantum-well (MQW) laser. By incorporating a proper amount of -Sn into Ge, a direct-bandgap GeSn alloy can be realized to achieve population inversion in the direct conduction band. The introduction of compressive strain into the GeSn QW can effectively modify the valence band structure to reduce the threshold carrier density. We calculate the electronic band structure and the polarization-dependent optical gain of the strained Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-z</sub> -Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x-y</sub> MQW laser taking into account the effect of the L-conduction bands. We also present our waveguide design for index guidance and calculate the optical confinement factors of various regions. Our calculation indicates that the modal gain can reach the threshold condition and lead to lasing action.

Indirect optical absorption and origin of the emission from β-FeSi2 nanoparticles: Bound exciton (0.809 eV) and band to acceptor impurity (0.795 eV) transitions

We investigated the optical absorption of the fundamental band edge and the origin of the emission from β-FeSi2 nanoparticles synthesized by ion-beam-induced epitaxial crystallization of Fe+ implanted SiO2/Si(100) followed by thermal annealing. From micro-Raman scattering and transmission electron microscopy measurements it was possible to attest the formation of strained β-FeSi2 nanoparticles and its structural quality. The optical absorption near the fundamental gap edge of β-FeSi2 nanoparticles evaluated by spectroscopic ellipsometry showed a step structure characteristic of an indirect fundamental gap material. Photoluminescence spectroscopy measurements at each synthesis stage revealed complex emissions in the 0.7–0.9 eV spectral region, with different intensities and morphologies strongly dependent on thermal treatment temperature. Spectral deconvolution into four transition lines at 0.795, 0.809, 0.851, and 0.873 eV was performed. We concluded that the emission at 0.795 eV may be related to a radiative direct transition from the direct conduction band to an acceptor level and that the emission at 0.809 eV derives from a recombination of an indirect bound exciton to this acceptor level of β-FeSi2. Emissions 0.851 and 0.873 eV were confirmed to be typical dislocation-related photoluminescence centers in Si. From the energy balance we determined the fundamental indirect and direct band gap energies to be 0.856 and 0.867 eV, respectively. An illustrative energy band diagram derived from a proposed model to explain the possible transition processes involved is presented.

Theory for n-type doped, tensile-strained Ge–Si_xGe_ySn_1−x−y quantum-well lasers at telecom wavelength

We propose and develop a theoretical gain model for an n-doped, tensile-strained Ge-Si(x)Ge(y)Sn(1-x-y) quantum-well laser. Tensile strain and n doping in Ge active layers can help achieve population inversion in the direct conduction band and provide optical gain. We show our theoretical model for the bandgap structure, the polarization-dependent optical gain spectrum, and the free-carrier absorption of the n-type doped, tensile-strained Ge quantum-well laser. Despite the free-carrier absorption due to the n-type doping, a significant net gain can be obtained from the direct transition. We also present our waveguide design and calculate the optical confinement factors to estimate the modal gain and predict the threshold carrier density.

Enhanced photoluminescence of heavily n-doped germanium

We show that a significant enhancement of the direct band gap photoluminescence can be achieved at room temperature in bulk Ge and Ge-on-insulator heavily n-doped by gas immersion laser doping. The photoluminescence signal from bulk Ge and Ge-on-insulator increases with the donor concentration. An enhancement factor of 20 as compared to the undoped material is achieved near the 1550 nm wavelength for active dopant concentrations around 5×1019 cm−3. These results are supported by calculations of the Ge spontaneous emission spectrum taking into account the doping effect on the electron distribution in the direct and indirect conduction band valleys.

AlAs/GaAs quantum cascade lasers based on large direct conduction band discontinuity

The design and operation of quantum cascade (QC) lasers using AlAs/GaAs coupled quantum wells are reported. In this material system, the conduction band offset at the Γ point (∼1 eV) is much higher than in previously reported QC lasers. The use of high band discontinuity allows us to increase the energy separation among the subbands, thus suppressing thermally activated processes which limit device performance at high temperature. The measured thermal characteristics of these promising devices are strongly improved from previously reported GaAs-based QC lasers: The temperature dependence of the threshold current density is described by a very large T0 (320 K) and the laser slope efficiency does not vary for increasing heat sink temperatures. The maximum operating temperature is 230 K, limited by negative differential resistance effects that occur when the applied bias reaches 8 V.

Three-dimensional confinement in the conduction band structure of InP

Strong quantum confinement in InP is observed to significantly reduce the separation between the direct and indirect conduction band states. The effects of three-dimensional confinement are investigated by tailoring the initial separation between conduction band states using quantum dots (QDs) of different sizes and hydrostatic pressure. Analyses of the QD emission spectra show that the X(1c) states are lowest in energy at pressures of approximately 6 GPa, much lower than in the bulk. The transition to the X(1c) states can be explained by either a sequence of gamma-L and L-X crossings, or by the crossover between strongly coupled gamma and X states.

Superlattice Calculation in an Empirical spds* Tight-Binding Model

ABSTRACTWe propose an empirical tight-binding method for tetrahedrally coordinated cubic materials and apply it to group IV and III-V semiconductors, extending existing calculations by the inclusion of all five d-orbitals per atom in the basis set. The symmetry character of the conduction states at the surface of the Brillouin zone is considerably improved compared to calculations in smaller bases, and the corresponding band positions can be obtained within the experimental uncertainties. Because the distance dependence of the tight-binding parameters is derived from deformation potentials, the model is particularly suited for an investigation of strained superlattices where the states at direct or pseudo-direct conduction band minima are composed of wavefunctions of all the minima at Γ, X, and L of the constituents. Investigations of GaAs/AlAs and short-period superlattices indicate a strong mixing between the conduction band valleys in the miniband structure, and the results are in better agreement with experiments than state-of-the-art empirical pseudopotential calculations.

Near-Band-Edge Photoluminescence of Sulfur-Doped GaAs Prepared by Liquid Phase Epitaxy

Photoluminescence measurement is used to characterize the sulfur-doped GaAs epitaxial layers grown on GaAs(100) substrates by liquid phase epitaxy. The dependences of spectral line shape on the doping levels ranged from 5×1017 to 1.8×1018 cm-3 as functions of excitation power and temperature have been investigated. Three main competing near-band-edge radiative transitions are identified as: direct conduction band to valence band transition, conduction band filling levels to valence band tail transition, and donor-related states to valence band tail or to acceptor transition. In order to explain the spectral shifting and broadening observed in heavily sulfur-doped GaAs, the luminescence peak emitted from the conduction band filling to valence band tail transition is also compared with theoretical calculations where the carrier concentration dependence of peak energy can be well described based on the heavy doping induced band-tail effect.

Tunneling calculations for GaAs-Al/sub x/Ga/sub (1-x)/As graded band-gap sawtooth superlattices

The transmission resonance spectra and tunneling current-voltage characteristics for direct conduction band electrons in sawtooth GaAs-Al/sub x/Ga/sub (1-x)/As superlattices are computed. The authors use a transfer matrix method, working within the framework of the effective mass approximation; band nonparabolicity and band-bending at heterojunctions are neglected. Only direct-gap interfaces are considered. It is found that sawtooth superlattices exhibit resonant tunneling similar to that in step superlattices, manifested by correlation of peaks and regions of negative differential resistance in the current-voltage curves with transmission resonances. The step-barrier superlattice always presents some barrier to tunneling, no matter how high the electric field strength.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Auger recombination in direct-gap semiconductors: band-structure effects

Auger recombination in direct-gap semiconductors is reinvestigated taking into account a realistic band structure instead of using the usual parabolic approximation. It is found that the direct conduction band process is negligible in large-gap semiconductors (Eg>or=0.6 eV), contrary to what has been found using the parabolic approximation. The new values for the probabilities of the valence band process and of the phonon-assisted Auger processes are also found to be smaller, but not by so much. Altogether, smaller values for the total Auger recombination are obtained and these are in agreement with experimental results. In addition, the effect of carrier degeneracy is shown to be weak, apart from where carrier concentrations are extremely high (n>or=1020 cm-3). Finally, the uncertainties involved in the quantitative values of the Auger coefficients are discussed.

Threshold-temperature characteristics in (GaAl)As visible laser diodes emitting below 750 nm

The temperature dependence of the threshold current Ith has been measured for (GaAl)As v-channeled substrate inner stripe lasers emitting in the wavelength range of 781–697 nm. Ith (350 K)/Ith (300 K) is found to increase below 750 nm with decreasing wavelength in spite of the fairly large heterojunction step height Δx of 0.4. This is well explained by the model of carrier leakage due to unconfined carriers in the active layer. The calculation indicates that the threshold-temperature sensitivity below 750 nm is almost determined by the carrier leakage in the cladding layer when Δx is smaller than ∼0.4, and it is determined by the temperature dependence of the fraction of the total electrons in the direct conduction band of the active layer when Δx is larger than ∼0.6.