We report on experimental and theoretical studies of deep-level defects in BGaAs alloys, with a particular focus on the effects of rapid thermal annealing (RTA) on their electrical and optical properties. The n- and p-type BxGa1−xAs layers, with varying boron concentration ranging from 0% to 2.0% (dilute borides), were grown by molecular beam epitaxy on GaAs at a substrate temperature of 400 °C and subsequently ex-situ annealed at 700 °C. Deep level transient spectroscopy (DLTS) was employed to investigate the properties of deep-level defects and their evolution upon annealing. In addition, density functional theory (DFT) was used to calculate defect formation energies and charge transition levels, enabling the identification of native point defects and defect complexes observed in DLTS experiments. Based on the obtained DFT results, we discuss the origin of the observed trap levels, with particular emphasis on B-related defects. We also found that RTA significantly affected both the carrier concentration and the total trap concentration in n- and p-type BGaAs as the boron content increased. Furthermore, annealing impacts the emission properties and overall optical quality of these dilute borides.

Models of GeSn and GeCSn quantum well (QW) lasers were compared to predict net gain and threshold for computing applications. GeSn showed weak confinement of electrons in both k-space (directness) and real space, as well as a weak optical confinement factor. Using material parameters from ab-initio calculations, adding 1-2% carbon to Ge or GeSn could provide all three confinements simultaneously, with up to 350 meV of electron confinement by Ge QW barriers and a direct bandgap that is 50-220 meV below the indirect gap. A 2-4x increase in electron effective mass preserves strong confinement even in narrow, 5 nm GeCSn/Ge quantum wells. Simply keeping electrons out of non-lasing, higher energy states doubles the differential gain compared with GeSn lasers and reduces free carrier absorption, while deeper QWs further enhance gain. GeCSn laser thresholds as low as 160 A/cm(2) are predicted for operation at temperatures of 100 degrees C, two orders of magnitude lower than comparable GeSn lasers.

We investigate the use of steady-state thermoreflectance to characterize the thermal conductivity of BGaInAs photodiode epitaxies and correlate these results to alloy concentration. The thermal conductivity decreases with alloy diversity, which narrows the bandgap and extends the absorption cutoff. We demonstrate the effectiveness of this thermal approach for epitaxial characterization of pre-fabrication devices.

Highly mismatched B-III-V alloys present a promising avenue for near-infrared (NIR), direct bandgap optoelectronics that can be integrated on GaAs or Si, owing to their ability to vary bandgap and lattice constant independently. Here, we report the epitaxial growth, fabrication, and characterization of nearly strain-free, all-BGaInAs, p-i-n photodiodes on GaAs substrates grown by molecular beam epitaxy. Incorporating boron effectively reduced the strain in InGaAs, yielding nearly lattice-matched BGaInAs layers with high surface quality, as confirmed by high-resolution x-ray diffraction and atomic force microscopy measurements. Strong photoluminescence (PL) intensity was observed for BGaInAs with up to 3.2% B (7% In). The operating wavelength consistently extended with increasing boron and indium incorporation as evidenced by PL and external quantum efficiency measurements, which agree with density functional theory predictions. Altogether, these results highlight precise control of the operating wavelength and suggest highly substitutional boron incorporation. BGaInAs photodiodes with low boron and indium concentrations demonstrated high sensitivity with low dark current. We also report the effective thermal conductivity of the BGaInAs p-i-n structures and find that the thermal conductivity is largely insensitive to boron content with dominant effects from the indium content. Altogether, these findings underscore the potential for strain-free BGaInAs photodiodes as promising candidates for high-performance NIR optoelectronics.

AlInSbBi is an interesting yet underexplored material system with potential applications in devices operating in the infrared range. Thus far, its development has been hindered by the challenges associated with growing Bi-containing III-V alloys. In this study, we demonstrate the first molecular beam epitaxial growth of AlInSbBi and identify a kinetically limited growth regime for the quaternary. At low Al concentrations, Bi incorporation was limited to approximately 0.4% under a wide array of growth conditions, with excess Bi precipitating out into droplets on the surface of the film. However, increasing the Al concentration in the host matrix allowed for higher Bi incorporation, possibly due to decreased local strain or the reactivity of Al. Rutherford backscattering spectrometry measurements confirmed that AlInSbBi films containing 55% Al enabled record high Bi incorporation up to 7.8%. Atomic force microscopy scans revealed the presence of Bi droplets, suggesting saturation under these growth conditions. Altogether, these findings highlight the underexplored potential of AlInSbBi, which may contribute significantly to future optoelectronic devices as a wide bandgap barrier material with tunable lattice constants and band alignments.

Nickel and aluminum ohmic contacts were formed on p-doped GeC and GeCSn epitaxial films with similar to 1%C. When a 40 nm p-GeC contact layer was added to p-Ge, annealed contact resistivity (R-c) dropped by 87% to 9.3 x 10(-7) Omega cm(2) for Al but increased by 32% to 2.9 x 10(-5 )Omega cm(2) for Ni. On the other hand, thick films of GeCSn, which showed lower active doping, had contact resistivities of 4.4 x 10(-6 )Omega cm(2) for Al and 1.4 x 10(-5) Omega cm(2) for Ni. In general, Al contacts were better than Ni, regardless of anneal, and were further improved by adding carbon. Annealing reduced R-c for both Ni and Al contacts to GeCSn by 4x, 2x for Al on GeC, and 5 orders of magnitude for Ni on GeC. It is speculated that C forms bonds with Ni that inhibit diffusion of Ni into the Ge, thus preventing the formation of low-resistance nickel germanide. Adding C, either as bulk GeCSn or as GeC contact layers, seems to significantly reduce the contact resistivity for Al contacts when compared to bulk Ge of comparable doping.

The electronic structures of three highly mismatched alloys (HMAs)—GeC(Sn), Ga(In)NAs, and BGa(In)As—were studied using density functional theory with HSE06 hybrid functionals, with an emphasis on the local environment near the mismatched, highly electronegative atom (B, C, and N). These alloys are known for their counterintuitive reduction in the bandgap when adding the smaller atom, due to a band anticrossing (BAC) or splitting of the conduction band. Surprisingly, the existence of band splitting was found to be completely unrelated to the local displacement of the lattice ions near the mismatched atom. Furthermore, in BGaAs, the reduction in the bandgap due to BAC was weaker than the increase due to the lattice constant, which has not been observed among other HMAs but may explain differences among experimental reports. While local distortion in GeC and GaNAs was not the cause for BAC, it was found to enhance the bandgap reduction due to BAC. This work also found that mere contrast in electronegativity between neighboring atoms does not induce BAC. In fact, surrounding the electronegative atom with elements of even smaller electronegativity than the host (e.g., Sn or In) consistently decreased or even eliminated BAC. For a fixed composition, moving Sn toward C and In toward either N or B was always energetically favorable and increased the bandgap, consistent with experimental annealing results. Such rearrangement also delocalized the conduction band wavefunctions near the mismatched atom to resemble the original host states in unperturbed Ge or GaAs, causing the BAC to progressively weaken. These collective results were consistent whether the mismatched atom was a cation (N), anion (B), or fully covalent (C), varying only with the magnitude of its electronegativity, with B having the least effect. The effects can be explained by charge screening of the mismatched atom's deep electrostatic potential. Together, these results help explain differences in the bandgap and other properties reported for HMAs from different groups and provide insight into the creation of materials with designer properties.

Asymmetric molecular bonds possess a microscopic second-order nonlinear optical polarizability p(2). Crystals built from them possess a macroscopic second-order nonlinear optical susceptibility, χ(2), if their structure lacks centrosymmetry. χ(2) can be enhanced by introducing additional asymmetry at the meta-structural level. Here, we use a dipole matrix formalism to calculate χ(2) of asymmetric GaAs/AlGaAs coupled quantum well structures at telecommunication frequencies, for which interband (rather than previously considered intersubband) optical transitions govern optical nonlinearities. Using unit cell and envelope wavefunctions and considering all possible transitions between two bound electron and two bound hole states, we predict tenfold enhancement in χ(2) in previously underexplored ranges of quantum well asymmetry and coupling barrier thickness. This work paves the way toward enhanced, tailorable second-order optical nonlinearities for semiconductor digital alloy and superlattice structures.

Direct bandgap group IV materials could provide intimate integration of lasers, amplifiers, and compact modulators within complementary metal–oxide–semiconductor for smaller, active silicon photonics. Dilute germanium carbides (GeC) with ∼1 at. % C offer a direct bandgap and strong optical emission, but energetic carbon sources such as plasmas and e-beam evaporation produce defective materials. In this work, we used CBr4 as a low-damage source of carbon in molecular beam epitaxy of tin-free GeC, with smooth surfaces and narrow x-ray diffraction peaks. Raman spectroscopy showed substitutional incorporation of C and no detectable sp2 bonding from amorphous or graphitic carbon, even without surfactants. Photoluminescence shows strong emission compared with Ge.

GeSnC alloys offer a route to direct bandgap semiconductors for CMOS-compatible lasers, but the use of CBr4 as a carbon source was shown to reduce Sn incorporation by 83%–92%. We report on the role of thermally cracked H in increasing Sn incorporation by 6x–9.5x, restoring up to 71% of the lost Sn, and attribute this increase to removal of Br from the growth surface as HBr prior to formation of volatile groups such as SnBr4. Furthermore, as the H flux is increased, Rutherford backscattering spectroscopy reveals a monotonic increase in both Sn and carbon incorporation. X-ray diffraction reveals tensile-strained films that are pseudomorphic with the substrate. Raman spectroscopy suggests substitutional C incorporation; both x-ray photoelectron spectroscopy and Raman suggest a lack of graphitic carbon or its other phases. For the lowest growth temperatures, scanning transmission electron microscopy reveals nanovoids that may account for the low Sn substitutional fraction in those layers. Conversely, the sample grown at high temperatures displayed abrupt interfaces, notably devoid of any voids, tin, or carbon-rich clusters. Finally, the surface roughness decreases with increasing growth temperature. These results show that atomic hydrogen provides a highly promising route to increase both Sn and C to achieve a strongly direct bandgap for optical gain and active silicon photonics.

Device models show GeSn lasers are limited by weak electron and photon confinement. Adding carbon offers strong conduction band offsets, freeing SiGeSn layers for separate confinement heterostructures, reducing thresholds. Photoluminescence from recent growths of GeC and GeSnC quantum wells will be presented.

Tensile-strained pseudomorphic Ge1–x–ySnxCy was grown on GaAs substrates by molecular beam epitaxy using carbon tetrabromide (CBr4) at low temperatures (171–258 °C). High resolution x-ray diffraction reveals good crystallinity in all samples. Atomic force microscopy showed atomically smooth surfaces with a maximum roughness of 1.9 nm. The presence of the 530.5 cm−1 local vibrational mode of carbon in the Raman spectrum verifies substitutional C incorporation in Ge1–x–ySnxCy samples. X-ray photoelectron spectroscopy confirms carbon bonding with Sn and Ge without evidence of sp2 or sp3 carbon formation. The commonly observed Raman features corresponding to alternative carbon phases were not detected. Furthermore, no Sn droplets were visible in scanning electron microscopy, illustrating the synergy in C and Sn incorporation and the potential of Ge1–x–ySnxCy active regions for silicon-based lasers.

Highly mismatched semiconductor alloys (HMAs) offer unusual combinations of bandgap and lattice constant, which are attractive for myriad applications. Dilute borides, such as BGa(In)As, are typically assumed to be HMAs. BGa(In)As can be grown in higher alloy compositions than Ga(In)NAs with comparable bandgaps, potentially enabling routes to lattice-matched telecom lasers on Si or GaAs. However, BGa(In)As remains relatively unexplored, especially with large fractions of indium. Density functional theory with HSE06 hybrid functionals was employed to study BGaInAs with 4%–44% In and 0%–11% B, including atomic rearrangement effects. All compositions showed a direct bandgap, and the character of the lowest conduction band was nearly unperturbed with the addition of B. Surprisingly, although the bandgap remained almost constant and the lattice constant followed Vegard's law with the addition of boron, the electron effective mass increased. The increase in electron effective mass was higher than in conventional alloys, though smaller than those characteristics of HMAs. This illustrates a particularly striking finding, specifically that the compositional space of BGa(In)As appears to span conventional alloy and HMA behavior, so it is not well-described by either limit. For example, adding B to GaAs introduces additional states within the conduction band, but further addition of In removes them, regardless of the atomic arrangement.

We used ab-initio and finite element strain modeling to explain high threshold current densities in reported tensile Ge lasers. Anisotropic strains improve carrier confinement but reduces optical confinement, and intervalence band absorption still dominates. Strain anisotropy increases Jth compared with simple slab models.

Conduction and valence band states for the highly mismatched alloy (HMA) Ge:C are projected onto Ge crystal states, Ge vacancy states, and Ge/C atomic orbitals, revealing that substitutional carbon not only creates a direct bandgap, but the new conduction band is optically active. Overlap integrals of the new Ge:C conduction band with bands of pure Ge shows the new band has almost no Ge band character. C sites structurally mimic uncharged vacancies in the Ge lattice, similar to Hjalmarson's model for other HMAs. C perturbs the entire Ge band structure even at the deepest crystal core energy levels. Projection onto atomic sites shows relatively weak localization compared with other HMAs, but does show a strong anisotropy in probability distribution. L-valley conduction band states in Ge are ruled out as major contributors to the carbon state in Ge:C, both by weak inner products between these states and by a negligible effect on optical transition strength when adding C.

BGaAs layers with the boron concentration up to 17.7% have been grown by molecular beam epitaxy on GaAs and GaP substrates and studied by photoreflectance (PR) spectroscopy. The direct optical transitions between the valence band (heavy hole, light hole, and the spin–orbit split-off band) and the conduction band have been observed in PR spectra. The bowing parameter for the band gap and the spin–orbit splitting has been determined from PR studies to be 3.6 ± 0.2 and 0.2 ± 0.1 eV, respectively. These values are very close to those determined using state-of-the-art first principle density functional methods (bdir = 3.5 ± 0.1 eV and bSO = 0.06 ± 0.02 eV). In addition, the indirect band gap has been examined theoretically and the bowing parameter has been determined to be 2.3 ± 0. eV, with the crossing between the direct and indirect gap in BGaAs at ∼12% B.

We present electronic band structure calculations of BxGa1−xAs alloys over the full composition range using HSE06 hybrid functionals. We find that at low boron percentages, the direct bandgap decreases slightly and then increases toward the large minimum direct gap of BAs as more boron is added. Our results show that the effect of isolated boron atoms on the bandgap is small (<5%) at concentrations below 13%. We estimate that BGaAs transitions from the direct to indirect bandgap at around 18% boron content. We calculate the electron effective masses in the direct bandgap region and investigate the effect of B-B pairs in nearest-neighbor group III sites on the bandgap, conduction band dispersion, and total free energy. We find that the lattice constant of BGaAs follows Vegard's law and estimate that the boron concentration required to lattice match BGaAs to silicon is outside the direct gap regime.

Tensile-strained Ge is a possible laser material for Si integrated circuits, but reports of lasers using tensile Ge show high threshold current densities and short lifetimes. To study the origins of these shortcomings, Ge ridge waveguides with tensile strain in three dimensions were fabricated using compressive silicon nitride (SiNx) films with up to 2 GPa stress as stress liners. A Raman peak shift of up to 11 cm−1 was observed, corresponding to 3.6% hydrostatic tensile strain for waveguides with a triangular cross-section. Real time degradation in tensile-strained Ge was observed and studied under transmission electron microscopy (TEM). A network of defects, resembling dark line defects, was observed to form and propagate with a speed and density strongly correlated with the local strain extracted from both modeled and measured strain profiles. This degradation suggests highly tensile-strained Ge lasers are likely to have significantly shorter lifetime than similar GaAs or InGaAs quantum well lasers.

Ge and its alloys are attractive candidates for a laser compatible with silicon integrated circuits. Dilute germanium carbide (Ge1−xCx) offers a particularly interesting prospect. By using a precursor gas with a Ge4C core, C can be preferentially incorporated in substitutional sites, suppressing interstitial and C cluster defects. We present a method of reproducible and upscalable gas synthesis of tetrakis(germyl)methane, or (H3Ge)4C, followed by the design of a hybrid gas/solid-source molecular beam epitaxy system and subsequent growth of defect-free Ge1−xCx by molecular beam epitaxy (MBE). Secondary ion mass spectroscopy, transmission electron microscopy and contactless electroreflectance confirm the presence of carbon with very high crystal quality resulting in a decrease in the direct bandgap energy. This technique has broad applicability to growth of highly mismatched alloys by MBE.

Ab-initio simulations of dilute germanium carbides (Ge:C) using hybrid functionals predict a direct bandgap with < 1% C. Growth of dilute Ge:C shows reduced direct gap consistent with the model, with no structural defects detected. Ge: C may enable lasers and compact modulators on Si.