Showing papers by "John Kouvetakis published in 2015"

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

Abstract: The emission properties of GeSn heterostructure pin diodes have been investigated The devices contain thick (400–600 nm) Ge1−ySny i-layers spanning a broad compositional range below and above the crossover Sn concentration yc where the Ge1−ySny 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 intensi

Non-radiative recombination in Ge1−ySny light emitting diodes: The role of strain relaxation in tuned heterostructure designs

Abstract: This paper describes the properties of Ge1−ySny light emitting diodes with a broad range of Sn concentrations (y = 0.0–0.11). The devices are grown upon Si(100) platforms using ultra-low temperature deposition of highly reactive Ge and Sn hydrides. The device fabrication adopts two new photodiode designs which lead to optimized performance and enables a systematic study of the effects of strain relaxation on emission efficiency. In contrast with n-Ge/i-Ge1−ySny/p-Ge analogs, which in most cases contain two defected interfaces, our designs include a p-layer with composition Ge1−zSnz chosen to be z < y to facilitate light extraction, but with z close enough to y to guarantee no strain relaxation at the i/p interface. In addition, a Ge1−xSnx alloy is also used for the n layer, with compositions in the 0 ≤ x ≤ y range, so that defected and non-defected n/i interfaces can be studied. The electroluminescence spectra vs the Sn content y in the intrinsic layer of the diodes exhibit a monotonic shift in the emissi...

Ge1−x−ySixSny light emitting diodes on silicon for mid-infrared photonic applications

Abstract: This paper reports initial the demonstration of prototype Ge1−x−ySixSny light emitting diodes with distinct direct and indirect edges and high quality I-V characteristics. The devices are fabricated on Si (100) wafers in heterostructure pin geometry [n-Ge/i-Ge1−x−ySixSny/p-Ge(Sn/Si)] using ultra low-temperature (T < 300 °C) depositions of the highly reactive chemical sources Si4H10, Ge4H10, Ge3H8, and SnD4. The Sn content in the i-Ge1−x−ySixSny layer was varied from ∼3.5% to 11%, while the Si content was kept constant near 3%. The Si/Sn amounts in the p-layer were selected to mitigate the lattice mismatch so that the top interface grows defect-free, thereby reducing the deleterious effects of mismatch-induced dislocations on the optical/electrical properties. The spectral responsivity plots of the devices reveal sharp and well-defined absorption edges that systematically red-shift in the mid-IR from 1750 to 2100 nm with increasing Sn content from 3.5% to 11%. The electroluminescence spectra reveal strong ...

Compositional dependence of optical interband transition energies in GeSn and GeSiSn alloys

Abstract: The dielectric functions of GeSn and GeSiSn alloys were measured in the 1–6 eV energy range using spectroscopic ellipsometry. The contributions from the E 1 , E 1 + Δ 1 , E 0 ′, E 2 , and E 1 ′ critical points in the joint density of electronic states were enhanced by computing numerical second derivatives of the measured dielectric function, and the resulting lineshapes were fitted with model expressions from which the critical point energies, amplitudes, broadenings, and phases were determined. A detailed analysis of the compositional dependence of the different transition energies is presented. By describing this dependence in terms of quadratic polynomials, the bowing parameter (quadratic coefficient) for each transition is determined. It is shown that the bowing parameters in the ternary alloy follow a distinct chemical trend, in which the ternary is well described in terms of bowing parameters for the underlying binary alloys, and these bowing parameters increase as a function of the size and electronegativity mismatch of the alloy constituents.

Non-conventional routes to SiGe:P/Si(100) materials and devices based on -SiH3 and -GeH3 derivatives of phosphorus: synthesis, electrical performance and optical behavior

Abstract: Ge–Si based n-type films are synthesized using specially designed hydrides P(SiH3)3, Ge3H8 and Ge4H10 for potential applications in next-generation CMOS technologies. The films are grown on Ge buffered Si(100) at 340 °C using two complementary methods. The first employs a gas-source molecular epitaxy approach using Ge4H10 to produce materials with P doping densities varying from 4 × 1018 to a 3.5 × 1019 cm−3 threshold. These materials are co-doped with Si concentrations ranging from 3 × 1019 cm−3 to 3.5%, roughly in proportion with the amount of P(SiH3)3 used in the reactions. The second approach applies an alternative ultra-high vacuum chemical vapor deposition (UHV–CVD) technique and Ge3H8 in place of Ge4H10 to achieve ultra-high carrier concentrations up to ~6 × 1019 cm−3. The Si content in this case is minimal—in the 2–6 × 1019 cm−3 range—indicating that the growth mechanism allows only 'impurity' levels of Si to be incorporated. The active carrier densities in both cases closely reflect the absolute P content, indicating that the P atoms are mostly substitutional. The electron mobilities are significantly higher compared to state-of-the-art prototypes, probably due to superior microstructure and dearth of inactive donors in the lattice. P–I–N diodes fabricated using the P(SiH3)3 compound show I–V characteristics comparable to state-of-the-art results for Ge-on-Si devices and are virtually undistinguishable from similar diodes doped with the P(GeH3)3 precursor. These results confirm P(SiH3)3 as a viable CVD doping source that is practical from a process standpoint and therefore attractive for industrial scale-up.

Electroluminescence from Ge1−ySny diodes with degenerate pn junctions

Abstract: The light emission properties of GeSn pn diodes were investigated as a function of alloy composition and doping levels. Very sharp interfaces between contiguous ultra-highly doped p- and n-layers were obtained using in situ doping with B2H6 and P(SiH3)3 in a chemical vapor deposition environment, yielding nearly ideal model systems for systematic studies. Changes in the doping levels and layer Sn concentrations are shown to greatly affect the electroluminescence spectra. This sensitivity should make it possible to optimize the emission efficiency for these structures in the interesting quasi-direct regime, for which direct gap luminescence is observed due to the proximity of the conduction band quasi-Fermi level to the minimum of the conduction band at the center of the Brillouin zone. Such structures represent the basic building block of Ge-based electrically pumped lasers.

Crystalline (Al1-xBx)PSi3 and (Al1-xBx)AsSi3 tetrahedral phases via reactions of Al(BH4)3 and M(SiH3)3 (M = P, As)

Abstract: Crystalline Al1–xBxPSi3 alloys (x = 0.04–0.06) are grown lattice-matched on Si(100) substrates by reactions of P(SiH3)3 and Al(BH4)3 using low pressure CVD. The materials have been characterized fo...

In situ low temperature As-doping of Ge films using As(SiH3)3 and As(GeH3)3: Fundamental properties and device prototypes

Abstract: We report the development of a new method to systematically and controllably achieve very high carrier concentrations in As-doped germanium using ultra-low temperature, high efficiency routes based on the structurally and chemically compatible inorganic hydrides As(SiH3)3 and As(GeH3)3. The Ge n-layers are grown on Ge-buffered Si(100) using in situ depositions of the compounds with Ge3H8 at 330 °C. The as-grown films are found to exhibit excellent crystallinity, defect-free interfaces, atomically smooth surfaces and flat doping profiles with abrupt edges. The active carrier densities are measured to be in the range of 1 × 1019–8.4 × 1019 cm−3 irrespective of the precursor type. These carrier densities are in close agreement with atomic As concentrations measured by secondary ion mass spectrometry, indicating that the growth mechanism promotes the nearly complete substitutional incorporation of dopant atoms while suppressing the formation of non-active clusters and defects. In spite of the lower solubility of As in Ge relative to that of P, the maximum carrier concentrations obtained with As(SiH3)3 and As(GeH3)3 are roughly 30% higher than those found with the analogous P(SiH3)3 and P(GeH3)3. This result, along with the close similarity in band gap narrowing observed for the two methods, suggests that the As-doping route may be advantageous for optical devices that require the highest possible carrier concentrations to populate the conduction band valley associated with direct gap emission. On the other hand—due to the inherently shorter carrier relaxation times in As-doped Ge—the lowest observed resistivity of 5 × 10−4 Ω cm is slightly higher than the lowest resistivity from P-doped analogs. Finally, optical responsivity, electroluminescence and I–V properties of photodiodes fabricated using As(SiH3)3 and As(GeH3)3 are found to be on par with those observed from Ge-on-Si reference analogs, indicating that the chemistry approach described here represents a viable and straightforward route to doping and activation of device-quality materials.

Atomic Scale Structure and Bonding Configurations in Monocrystalline Ah-xBxPSb Alloys Grown Lattice Matched on Si(001) Platforms

Abstract: Atomic-resolution imaging and element-selective mapping of composition and structure in crystalline solids using aberration corrected microscopy with probe sizes comparable to atomic spacing is a powerful technique which has to date been successfully used to study known systems with classical structures such as hexagonal BN [1] and perovskites [2]. In this study we have used this method to directly investigate atomic scale structure, and bonding configurations in newly synthesized Al1-x BxPSi3/Si(001) alloys comprised of earth abundant elements. These materials are grown on Si buffered Si(001) via reaction of Al(BH4)3 and P(SiH3)3 using low pressure gas source molecular beam epitaxy. The reactions deliver tetrahedral AlPSi3 and BPSi3 building blocks, which are expected to incorporate intact into the crystal, creating a Si-like framework containing isolated Al-P and B-P donor-acceptor pairs. The addition of B into AlPSi3 facilitates lattice matching of the material with the Si template yielding fully relaxed structures devoid of mismatch induced defects as required for application in photovoltaic devices.

Influence of Device Microstructure on The Optical Properties of Ge1-ySny (y=00.11) LEDs Produced by Next Generation Deposition Methods

Abstract: Ge1-ySny alloys represent a promising new class of IR semiconductors with tunable direct band gaps beyond that of Ge, allowing the optoelectronic capabilities of the material to be significantly extended into the mid IR. These alloys become direct gap semiconductors for y = 0.09 [1], making them an attractive alternative to Ge for application in purely group-IV interband lasers that are integrated onto Si-platforms. Significant progress has been made in recent years with the demonstration of high efficiency photodetectors and light emitting diodes containing up to 8% Sn [2,3]. However systematic optical studies of devices with concentrations near the indirect-to-direct transition are still lacking. A major problem with the synthesis of highly saturated materials with y > 0.08 is the strong dependence of the lattice parameter on the Sn content. This produces a significant lattice mismatch with Si and Ge platforms, making it difficult to integrate materials with sufficiently low defect densities as required for viable device performance. Furthermore, the associated compressive strains become an issue, rendering the material more indirect and adversely affecting light emission. For the fabrication of practical devices, relaxed films with large thickness are desirable but such films contain deleterious misfit dislocations that increase the non-radiative recombination rate. In this paper, we report the development of optimized growth protocols that enable fabrication of a new class of photodiodes featuring thick, bulk-like components with purposely designed microstructures. The device configurations comprise nGe/Ge1-ySny/p-Ge1-ySny stacks grown directly upon Si(100). The constituent layers feature defectengineered interfaces, allowing the fabrication of devices exhibiting strong direct-gap electroluminescence over a wide concentration range 0 ≤ y ≤ 0.11 for the first time. The intrinsic active regions are grown largely relaxed on Ge-buffered Si following low-temperature routes based on specialty Ge3H8 and SnD4 chemical sources. The devices contain only one defected interface between the intrinsic layer and the Ge buffer, while all other junctions are selected to be lattice-matched and thus devoid of dislocations.