Matching Items (33)

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Direct gap Ge1-ySny alloys: Fabrication and design of mid-IR photodiodes

Description

Chemical vapor deposition methods were developed, using stoichiometric reactions of specialty Ge[subscript 3]H[subscript 8] and SnD[subscript 4] hydrides, to fabricate Ge[subscript 1-y]Sn[subscript y] photodiodes with very high Sn concentrations in

Chemical vapor deposition methods were developed, using stoichiometric reactions of specialty Ge[subscript 3]H[subscript 8] and SnD[subscript 4] hydrides, to fabricate Ge[subscript 1-y]Sn[subscript y] photodiodes with very high Sn concentrations in the 12%–16% range. A unique aspect of this approach is the compatible reactivity of the compounds at ultra-low temperatures, allowing efficient control and systematic tuning of the alloy composition beyond the direct gap threshold. This crucial property allows the formation of thick supersaturated layers with device-quality material properties. Diodes with composition up to 14% Sn were initially produced on Ge-buffered Si(100) featuring previously optimized n-Ge/i-Ge[subscript 1-y]Sn[subscript y]/p-Ge[subscript 1-z]Sn[subscript z] type structures with a single defected interface. The devices exhibited sizable electroluminescence and good rectifying behavior as evidenced by the low dark currents in the I-V measurements. The formation of working diodes with higher Sn content up to 16% Sn was implemented by using more advanced n-Ge[subscript 1-x]Sn[subscript x]/i-Ge[subscript 1-y]Sn[subscript y]/p-Ge[subscript 1-z]Sn[subscript z] architectures incorporating Ge[subscript 1-x]Sn[subscript x] intermediate layers (x ∼ 12% Sn) that served to mitigate the lattice mismatch with the Ge platform. This yielded fully coherent diode interfaces devoid of strain relaxation defects. The electrical measurements in this case revealed a sharp increase in reverse-bias dark currents by almost two orders of magnitude, in spite of the comparable crystallinity of the active layers. This observation is attributed to the enhancement of band-to-band tunneling when all the diode layers consist of direct gap materials and thus has implications for the design of light emitting diodes and lasers operating at desirable mid-IR wavelengths. Possible ways to engineer these diode characteristics and improve carrier confinement involve the incorporation of new barrier materials, in particular, ternary Ge[subscript 1-x-y]Si[subscript x]Sn[subscript y] alloys. The possibility of achieving type-I structures using binary and ternary alloy combinations is discussed in detail, taking into account the latest experimental and theoretical work on band offsets involving such materials.

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  • 2016-07-13

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Electroluminescence from GeSn heterostructure pin diodes at the indirect to direct transition

Description

The emission properties of GeSn heterostructure pin diodes have been investigated. The devices contain thick (400–600 nm) Ge [subscript 1− y] Sn [subscript y] i-layers spanning a broad compositional range below

The emission properties of GeSn heterostructure pin diodes have been investigated. The devices contain thick (400–600 nm) Ge [subscript 1− y] Sn [subscript y] i-layers spanning a broad compositional range below and above the crossover Sn concentration y [subscript c] where the Ge [subscript 1− y] Sn [subscript y] alloy becomes a direct-gap material. These results are made possible by an optimized device architecture containing a single defected interface thereby mitigating the deleterious effects of mismatch-induced defects. The observed emission intensities as a function of composition show the contributions from two separate trends: an increase in direct gap emission as the Sn concentration is increased, as expected from the reduction and eventual reversal of the separation between the direct and indirect edges, and a parallel increase in non-radiative recombination when the mismatch strains between the structure components is partially relaxed by the generation of misfit dislocations. An estimation of recombination times based on the observed electroluminescence intensities is found to be strongly correlated with the reverse-bias dark current measured in the same devices.

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  • 2015-03-02

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Frustrated incomplete donor ionization in ultra-low resistivity germanium films

Description

The relationship between carrier concentration and donor atomic concentration has been determined in n-type Ge films doped with P. The samples were carefully engineered to minimize non-active dopant incorporation by

The relationship between carrier concentration and donor atomic concentration has been determined in n-type Ge films doped with P. The samples were carefully engineered to minimize non-active dopant incorporation by using specially designed P(SiH[subscript 3])[subscript 3] and P(GeH[subscript 3])[subscript 3] hydride precursors. The in situ nature of the doping and the growth at low temperatures, facilitated by the Ge [subscript 3]H[subscript 8] and Ge [subscript 4]H[subscript 10] Ge sources, promote the creation of ultra-low resistivity films with flat doping profiles that help reduce the errors in the concentration measurements. The results show that Ge deviates strongly from the incomplete ionization expected when the donor atomic concentration exceeds N[subscript d]  = 10[superscript 17] cm[superscript −3], at which the energy separation between the donor and Fermi levels ceases to be much larger than the thermal energy. Instead, essentially full ionization is seen even at the highest doping levels beyond the solubility limit of P in Ge. The results can be explained using a model developed for silicon by Altermatt and coworkers, provided the relevant model parameter is properly scaled. The findings confirm that donor solubility and/or defect formation, not incomplete ionization, are the major factors limiting the achievement of very high carrier concentrations in n-type Ge. The commercially viable chemistry approach applied here enables fabrication of supersaturated and fully ionized prototypes with potential for broad applications in group-IV semiconductor technologies.

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  • 2014-12-08

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Ge1-ySny (y=0.01-0.10) alloys on Ge-buffered Si: Synthesis, microstructure, and optical properties

Description

Novel hydride chemistries are employed to deposit light-emitting Ge [subscript 1- y] Sn [subscript y] alloys with y ≤ 0.1 by Ultra-High Vacuum Chemical Vapor Deposition (UHV-CVD) on Ge-buffered Si wafers. The

Novel hydride chemistries are employed to deposit light-emitting Ge [subscript 1- y] Sn [subscript y] alloys with y ≤ 0.1 by Ultra-High Vacuum Chemical Vapor Deposition (UHV-CVD) on Ge-buffered Si wafers. The properties of the resultant materials are systematically compared with similar alloys grown directly on Si wafers. The fundamental difference between the two systems is a fivefold (and higher) decrease in lattice mismatch between film and virtual substrate, allowing direct integration of bulk-like crystals with planar surfaces and relatively low dislocation densities. For y ≤ 0.06, the CVD precursors used were digermane Ge [subscript 2]H[subscript 6] and deuterated stannane SnD[subscript 4]. For y ≥ 0.06, the Ge precursor was changed to trigermane Ge [subscript 3]H[subscript 8], whose higher reactivity enabled the fabrication of supersaturated samples with the target film parameters. In all cases, the Ge wafers were produced using tetragermane Ge [subscript 4]H[subscript 10] as the Ge source. The photoluminescence intensity from Ge [subscript 1− y] Sn [subscript y] /Ge films is expected to increase relative to Ge [subscript 1− y] Sn [subscript y] /Si due to the less defected interface with the virtual substrate. However, while Ge [subscript 1− y] Sn [subscript y] /Si films are largely relaxed, a significant amount of compressive strain may be present in the Ge [subscript 1− y] Sn [subscript y] /Ge case. This compressive strain can reduce the emission intensity by increasing the separation between the direct and indirect edges. In this context, it is shown here that the proposed CVD approach to Ge [subscript 1− y] Sn [subscript y] /Ge makes it possible to approach film thicknesses of about 1  μm, for which the strain is mostly relaxed and the photoluminescence intensity increases by one order of magnitude relative to Ge [subscript 1− y] Sn [subscript y] /Si films. The observed strain relaxation is shown to be consistent with predictions from strain-relaxation models first developed for the Si[subscript 1− x] Ge [subscript x] /Si system. The defect structure and atomic distributions in the films are studied in detail using advanced electron-microscopy techniques, including aberration corrected STEM imaging and EELS mapping of the average diamond–cubic lattice.

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  • 2014-10-07

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Compositional dependence of the bowing parameter for the direct and indirect band gaps in Ge1-ySny alloys

Description

Photoluminescence spectroscopy has been used to determine the direct gap E [subscript 0] of Ge [subscript 1− y] Sn [subscript y] alloys over a broad compositional range from pure Ge

Photoluminescence spectroscopy has been used to determine the direct gap E [subscript 0] of Ge [subscript 1− y] Sn [subscript y] alloys over a broad compositional range from pure Ge to Sn concentrations exceeding 10%. A fit of the compositional dependence of E [subscript 0] using a standard quadratic expression is not fully satisfactory, revealing that the bowing parameter (quadratic coefficient) b [subscript 0] is compositionally dependent. Excellent agreement with the data is obtained with b [subscript 0](y) = (2.66 ± 0.09) eV − (5.4 ± 1.1)y eV. A theoretical model of the bowing is presented, which explains the strong compositional dependence of the bowing parameter and suggest a similar behavior for the indirect gap. Combining the model predictions with experimental data for samples with y ≤ 0.06, it is proposed that the bowing parameter for the indirect gap is b [subscript ind](y) = (1.11 ± 0.07) eV − (0.78 ± 0.05)y eV. The compositional dependence of the bowing parameters shifts the crossover concentration from indirect to direct gap behavior to y[subscript c]  = 0.087, significantly higher than the value predicted earlier based on strictly quadratic fits.

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  • 2014-10-06

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Compositional dependence of the direct and indirect band gaps in Ge1-ySny alloys from room temperature photoluminescence: implications for the indirect to direct gap crossover in intrinsic and n-type materials

Description

The compositional dependence of the lowest direct and indirect band gaps in Ge[subscript 1−y]Sn[subscript y] alloys has been determined from room-temperature photoluminescence measurements. This technique is particularly attractive for a

The compositional dependence of the lowest direct and indirect band gaps in Ge[subscript 1−y]Sn[subscript y] alloys has been determined from room-temperature photoluminescence measurements. This technique is particularly attractive for a comparison of the two transitions because distinct features in the spectra can be associated with the direct and indirect gaps. However, detailed modeling of these room temperature spectra is required to extract the band gap values with the high accuracy required to determine the Sn concentration y[subscript c] at which the alloy becomes a direct gap semiconductor. For the direct gap, this is accomplished using a microscopic model that allows the determination of direct gap energies with meV accuracy. For the indirect gap, it is shown that current theoretical models are inadequate to describe the emission properties of systems with close indirect and direct transitions. Accordingly, an ad hoc procedure is used to extract the indirect gap energies from the data. For y < 0.1 the resulting direct gap compositional dependence is given by ΔE[subscript 0] = −(3.57 ± 0.06)y (in eV). For the indirect gap, the corresponding expression is ΔE[subscript ind] = −(1.64 ± 0.10)y (in eV). If a quadratic function of composition is used to express the two transition energies over the entire compositional range 0 ≤ y ≤ 1, the quadratic (bowing) coefficients are found to be b[subscript 0] = 2.46 ± 0.06 eV (for E0) and b[subscript ind] = 1.03 ± 0.11 eV (for E[subscript ind]). These results imply a crossover concentration y[subscript c] = $0.073 [+0.007 over -0.006], much lower than early theoretical predictions based on the virtual crystal approximation, but in better agreement with predictions based on large atomic supercells.

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Date Created
  • 2014-11-01

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Investigation of light absorption and emission in Ge and GeSn films grown on Si substrates

Description

Ge1-ySny alloys represent a new class of photonic materials for integrated optoelectronics on Si. In this work, the electrical and optical properties of Ge1-ySny alloy films grown on Si, with

Ge1-ySny alloys represent a new class of photonic materials for integrated optoelectronics on Si. In this work, the electrical and optical properties of Ge1-ySny alloy films grown on Si, with concentrations in the range 0 ≤ y ≤ 0.04, are studied via a variety of methods. The first microelectronic devices from GeSn films were fabricated using newly developed CMOS-compatible protocols, and the devices were characterized with respect to their electrical properties and optical response. The detectors were found to have a detection range that extends into the near-IR, and the detection edge is found to shift to longer wavelengths with increasing Sn content, mainly due to the compositional dependence of the direct band gap E0. With only 2 % Sn, all of the telecommunication bands are covered by a single detector. Room temperature photoluminescence was observed from GeSn films with Sn content up to 4 %. The peak wavelength of the emission was found to shift to lower energies with increasing Sn content, corresponding to the decrease in the direct band gap E0 of the material. An additional peak in the spectrum was assigned to the indirect band gap. The separation between the direct and indirect peaks was found to decrease with increasing Sn concentration, as expected. Electroluminescence was also observed from Ge/Si and Ge0.98Sn0.02 photodiodes under forward bias, and the luminescence spectra were found to match well with the observed photoluminescence spectra. A theoretical expression was developed for the luminescence due to the direct band gap and fit to the data.

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  • 2011

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A study of two high efficiency energy conversion processes: semiconductor photovoltaics and semiconductor luminescence refrigeration

Description

As the world energy demand increases, semiconductor devices with high energy conversion efficiency become more and more desirable. The energy conversion consists of two distinct processes, namely energy generation

As the world energy demand increases, semiconductor devices with high energy conversion efficiency become more and more desirable. The energy conversion consists of two distinct processes, namely energy generation and usage. In this dissertation, novel multi-junction solar cells and light emitting diodes (LEDs) are proposed and studied for high energy conversion efficiency in both processes, respectively. The first half of this dissertation discusses the practically achievable energy conversion efficiency limit of solar cells. Since the demonstration of the Si solar cell in 1954, the performance of solar cells has been improved tremendously and recently reached 41.6% energy conversion efficiency. However, it seems rather challenging to further increase the solar cell efficiency. The state-of-the-art triple junction solar cells are analyzed to help understand the limiting factors. To address these issues, the monolithically integrated II-VI and III-V material system is proposed for solar cell applications. This material system covers the entire solar spectrum with a continuous selection of energy bandgaps and can be grown lattice matched on a GaSb substrate. Moreover, six four-junction solar cells are designed for AM0 and AM1.5D solar spectra based on this material system, and new design rules are proposed. The achievable conversion efficiencies for these designs are calculated using the commercial software package Silvaco with real material parameters. The second half of this dissertation studies the semiconductor luminescence refrigeration, which corresponds to over 100% energy usage efficiency. Although cooling has been realized in rare-earth doped glass by laser pumping, semiconductor based cooling is yet to be realized. In this work, a device structure that monolithically integrates a GaAs hemisphere with an InGaAs/GaAs quantum-well thin slab LED is proposed to realize cooling in semiconductor. The device electrical and optical performance is calculated. The proposed device then is fabricated using nine times photolithography and eight masks. The critical process steps, such as photoresist reflow and dry etch, are simulated to insure successful processing. Optical testing is done with the devices at various laser injection levels and the internal quantum efficiency, external quantum efficiency and extraction efficiency are measured.

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Date Created
  • 2010

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The optical and electronic properties of Ge₁-ySny and Ge₁-x-ySixSny materials and devices for silicon integrated optoelectronics

Description

Group-IV semiconductor alloys are of interest for Si-integrated optoelectronic applications due to the band gap tunability and enhanced optical capabilities that can be achieved through compositional tuning. This work advances

Group-IV semiconductor alloys are of interest for Si-integrated optoelectronic applications due to the band gap tunability and enhanced optical capabilities that can be achieved through compositional tuning. This work advances the field by presenting a systematic study of the optical and electronic properties of Ge1-ySny and analogous Ge1-x-ySixSny alloys.

The fundamental direct and indirect band gaps of Ge1-ySny materials are measured by room temperature photoluminescence in samples containing 0 ≤ y ≤ 0.11 and a transition to direct gap materials is found to occur at yc = 0.087. This result is enabled by the development of sample growth and processing protocols that produce high-quality materials epitaxially on Ge-buffered Si(100) substrates. Strategies to optimize the optical performance are explored by varying the film thickness, thermal and surface treatments, and n-type doping. The electrical and optical properties of diodes based on these materials are characterized by current-voltage, optical responsivity, and electroluminescence measurements. These show improved optical performance near yc with tunable emission out to 2500 nm. Measuring the carrier lifetimes in devices with strain relaxed and fully strained interfaces show significantly longer lifetimes in the fully strained case.

The direct and indirect band gaps of Sn-rich (y > x) Ge1-x-ySixSny materials are measured by room temperature photoluminescence on optimized samples. These data confirm a transition to direct gap materials occurs for the ternary alloy as well. Devices based on compositions 0.02 ≤ x ≤ 0.10 and 0.03 ≤ y ≤ 0.11 are characterized by current-voltage, optical responsivity, and electroluminescence measurements and show competitive performance with analogous devices based on Ge1-ySny materials. A detailed study of the direct gap in Ge1-xSix alloys gives parameters crucial en route to a global description of the Ge1-x-ySixSny fundamental band gaps.

Archetypal laser device designs on Si are explored by fabricating degenerate pn junction diodes and highly doped waveguide devices based on high-quality Ge1-ySny materials. The diodes showed tunnel-like current-voltage characteristics and tailored electroluminescence based on the doping profile. The waveguides demonstrate emission under optical stimulation.

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Date Created
  • 2015

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Synthesis of hybrid (III-V)y(IV)5-2y semiconductors: a new approach to extending the optoelectronic capabilities of Si and Ge technologies

Description

Modern semiconductor technologies have been dominated by group-IV materials and III-V analogues. The development of hybrid derivatives combining appropriate members of these systems has been of interest for the purpose

Modern semiconductor technologies have been dominated by group-IV materials and III-V analogues. The development of hybrid derivatives combining appropriate members of these systems has been of interest for the purpose of extending the optoelectronic capabilities of the state-of-the-art. Early work on pseudo-binary (III-V)-IV alloys, described with the general formula (III-V)1-x(IV2)x,

showed limited progress due to phase segregation, auto-doping and compositional inhomogeneities. Recently, new techniques were introduced for synthesizing new classes of (III-V)-IV hybrid materials using reactions of V(IVH3)3 molecules [V = N, P, As and IV = Si, Ge] with group-III elements (B, Al, Ga, In). The reactions produce (III-V)-IV3 building blocks that interlink to form diamond-like

frameworks in which the III-V pairs incorporate as isolated units within the group-IV lattice. This approach not only precludes phase segregation, but also provides access to structures and compositions unattainable by conventional means. Entire new families of crystalline (III-V)-IV3 and (III-V)y(IV)5-y alloys with tunable IV-rich compositions, different from conventional (III-V)1-x(IV2)x systems, have been grown on Si(100) and GaP(100) wafers as well as Si1-xGex and Ge buffer layers which, in most cases, provide lattice matched templates for Si integration.

In this work, materials in the In-P-Ge, Ga-As-Ge and Ga-P-Si systems that would exhibit direct-gap behavior were targeted. A series of (InP)yGe5-2y alloys with tunable Ge contents above 60% were synthesized by reactions of P(GeH3)3 and indium atoms and were studied for bonding, structure, and optical response. (GaAs)yGe5-2y analogues were also grown and exhibited strong photoluminescence for applications in mid-IR photonics. The GaPSi3 alloy and Si-rich derivatives were produced via reactions of P(SiH3)3 and [H2GaNMe2]2 and exhibit enhanced absorption in the visible range. Quaternary analogues in the Al1-xBxPSi3 system were grown on Si via reactions of Al(BH4)3 and P(SiH3)3 leading to the formation crystalline materials with extended absorption relative to Si. This makes them imminently suitable for applications in Si-based photovoltaics. The work emphasized use of quantum-chemical simulations to elucidate structural, thermodynamic, and mechanical properties of the synthesized systems. The theory also included simulations of new synthetic targets such as BNC3, BNSi3, BPC3, and BPSi3 with interesting mechanical properties and strong covalent bonding.

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Date Created
  • 2017