Increasing Sb Composition Research Articles

Metamorphic growth of 0.1 eV InAsSb on InAs/GaAs virtual substrate for LWIR applications

Epitaxial growth of bulk InAs1-xSbx layer on GaAs substrate could open an opportunity for cost-competitive long-wavelength infrared sensors. Achieving this requires a high-quality metamorphic InAsSb layer with a uniform strain relief buffer design. We report a comprehensive analysis of metamorphic InAsSb layers grown on InAs/GaAs virtual substrate for 0.1 eV low bandgap material with an optimized growth condition and group-V flux control. We find that InAsSb surface roughens significantly with increasing Sb composition up to 58%. Lowering the growth temperature of InAsSb layers from 450 ℃ to 425 ℃ mitigated surface roughening while allowing greater than 90% strain relaxation. Moreover, we also present a dramatically increasing threading dislocation density by more than 200 times as a consequence of high Sb composition. Finally, we demonstrate a narrow energy bandgap of 0.13 eV at 10 K in the InAs0.42Sb0.58 layer, which is close to 0.1 eV at room temperature. This InAsSb film grown on InAs/GaAs template paves the way for long-wavelength infrared optoelectronics applications.

Effect of Sb composition on the band alignment of InAs/GaAsSb quantum dots* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2019YFB1503601 and 2017YFB0405302), the National Natural Science Foundation of China (Grant Nos. 61574139 and U1738114), and the Strategic Priority Research Program on Space Science, the Chinese Academy of Sciences (Grant No. XDA15051200).

Aiming to achieve InAs quantum dots (QDs) with a long carrier lifetime, the effects of Sb component in cap layers on the band alignment of the InAs/GaAsSb QDs have been studied. InAs QDs with high density and uniformity have been grown by molecular beam epitaxy. With increasing Sb composition, the InAs/GaAsSb QDs exhibit a significant red-shift and broadening photoluminescence (PL). With a high Sb component of 22%, the longest wavelength emission of the InAs/GaAs0.78Sb0.22 QDs occurs at 1.5 μm at room temperature. The power-dependence PL measurements indicate that with a low Sb component of 14%, the InAs/GaAs0.86Sb0.14 QDs have a type-I and a type-II carrier recombination processes, respectively. With a high Sb component of 22%, the InAs/GaAs0.78Sb0.22 QDs have a pure type-II band alignment, with three type-II carrier recombination processes. Extracted from time-resolved PL decay traces, the carrier lifetime of the InAs/GaAs0.78Sb0.22 QDs reaches 16.86 ns, which is much longer than that of the InAs/GaAs0.86Sb0.14 QDs (2.07 ns). These results obtained here are meaningful to realize high conversion efficiency intermediate-band QD solar cells and other opto-electronic device.

Composition-induced influence on the electronic band structure, optical and thermoelectric coefficients of the highly mismatched GaNSb alloy over the entire range: A DFT analysis

Capable of achieving wide control over energy band gap and following optoelectronic properties; the highly mismatched alloys (HMAs) are considered to be promising materials for solar energy conversion devices. The dramatic restructuring of energy bands and density of states in HMAs caused by the replacement of anions with distinctly-mismatched isovalent constituents could further be an important course in improving their thermoelectric efficiency. In this paper, we attempt to explore and address the composition-induced modifications in the electronic band structure and the resultant effects on optical spectra and thermoelectric coefficients of GaN1−xSbx based HMAs in the framework of density functional theory. We observe, the substitution of N by Sb, considerably affects its band structure and split the conduction band minimum (CBM) into sub-bands. With increasing Sb composition, the lowest sub-band stemmed from N-s electrons has experienced drastic downward shift leading to energy gap narrowing. Interestingly, the energy gap narrowing along R- Γ is found to be faster than that of Γ- Γ point leading to an amazing direct to indirect band gap crossover. On the other hand, the composition-induced energy gap narrowing stimulates the red-shift in fundamental absorption edge in both ultraviolet and the infrared regime, making the GaNSb potentially useful material for photovoltaic applications. In addition, substantial effect on the thermoelectric coefficients of GaNSb is also observed via Sb substitution. We obtain larger Seebeck coefficient, improved power factors and figure of merit (ZT) for GaNSb at low Sb substitution and found diminishing effect with the further increase of Sb composition. With enhanced Seebeck coefficient, power factor and ZT values at modest doping levels, GaNSb alloy could be a promising candidate for near or above room temperature thermoelectric applications.

Optical properties of amorphous and crystalline Sb-doped SnO2 thin films studied with spectroscopic ellipsometry: Optical gap energy and effective mass

We investigated the optical properties of amorphous and crystalline antimony (Sb)-doped tin dioxide (SnO2) thin films grown using the co-sputtering deposition method at room temperature. We used undoped and Sb-doped (8 wt. %) SnO2 targets. Varying the relative power ratio of the two targets, we controlled the Sb-composition of the SnO2:Sb thin films up to 2.3 at. % of Sb contents. Through annealing, the as-grown amorphous SnO2:Sb thin films were transformed to crystalline thin films. Dielectric functions were obtained from the measured ellipsometry angles, Ψ and Δ, using the Drude and parametric optical constant models. We determined the absorption coefficients and optical gap energies of the SnO2:Sb thin films from the dielectric functions. We found increasing optical gap energy with increasing Sb composition. Increases in the Drude tail amplitudes, a signature of free carrier concentrations, were found in annealed, crystalline thin films with increasing Sb composition. The increase in the optical gap energy with increasing Sb composition was mainly attributed to the Burstein-Moss effect. Using Hall effect measurements, we obtained Hall carrier concentrations (NHall) and electron Hall mobilities (μHall). The carrier concentrations and mobilities increased from 2.6 × 1019 cm−3 and 1.0 cm2/(V s) to 2.0 × 1020 cm−1 and 7.2 cm2/(V s), respectively, with increasing Sb contents. This result suggests that the nominally undoped SnO2 films are unintentionally n-type doped. Assuming that the NHall and optical carrier concentrations (Nopt) were the same, we obtained the effective masses of the SnO2:Sb thin films with increasing Sb compositions. The effective masses of the SnO2:Sb thin films increased from 0.245 m0 to 0.4 m0 with increasing Sb doping contents, and the nonparabolicity of the conduction band was estimated. We discussed the relation between the optical (μopt) and Hall (μHall) mobilities as a function of Sb contents and grain sizes.

InAs/GaAsSb quantum dot solar cells.

The hybrid structure of GaAs/GaAsSb quantum well (QW)/InAs quantum dots solar cells (QDSCs) is analyzed using power-dependent and temperature-dependent photoluminescence. We demonstrate that placing the GaAsSb QW beneath the QDs forms type-II characteristics that initiate at 12% Sb composition. Current density-voltage measurements demonstrate a decrease in power efficiency with increasing Sb composition. This could be attributed to increased valence band potential in the GaAsSb QW that subsequently limits hole transportation in the QD region. To reduce the confinement energy barrier, a 2 nm GaAs wall is inserted between GaAsSb QW and InAs QDs, leading to a 23% improvement in power efficiency for QDSCs.

Improved photoresponse of InAs/GaAs quantum dot infrared photodetectors by using GaAs1−xSbx strain reducing layer

The effect of a GaAs1−xSbx strain reducing layer on the performance of InAs/GaAs quantum-dot infrared photodetectors was investigated. The results suggest that increasing Sb composition x from 0 to 0.2 leads to an enhanced peak response and a pronounced narrowing of the band width of the spectral response from 3.3 to 1.5 μm. For a photodetector with GaAs0.8Sb0.2 strain reducing layer, the best responsivity obtained is 533 mA/W, which is 380 times higher than that without strain reducing layer. In addition, the operating temperature increases from 50 to 90 K when increasing Sb composition from 0 to 0.2.

Effects of a high magnetic field on the phase equilibria of Mn–Sb system during solidification process

The effects of an 11.5 T magnetic field on phase equilibria of Mn–Sb system during solidification process were investigated with the aid of direct thermocouple measurements and quantitative metallography analysis. It was found that the magnetic field could increase the eutectic line and the area fraction of the MnSb in eutectics, and thus shift the eutectic point to the high Mn concentration side. It could also increase the Sb liquidus temperatures by a decreasing trend with an increasing Sb composition, but show little effect on the MnSb liquidus temperature. The above results could be related to the variation in the magnetic susceptibility during the phase transformation.

Electrical Characterization of Nonvolatile Phase-Change Memory Devices Using Sb-Rich Ge–Sb–Te Alloy Films

We modified the composition of Ge–Sb–Te alloys for phase-change-type nonvolatile memory devices to realize more reliable memory operations. It is expected that the stability of the low-resistance crystalline phase of the Ge–Sb–Te alloy can be improved when an appropriate amount of excess Sb is added to conventional stoichiometric Ge2Sb2Te5 (GST), because a phase transition directly occurs from the amorphous phase to the more conductive hcp phase without the formation of the less conductive metastable fcc phase. Phase-change memory devices using Sb-rich Ge–Sb–Te alloys were fabricated, where the Sb atomic ratio was controlled to be from 31 to 47%. The effect of Sb addition to GST on the phase-change switching characteristics was investigated in terms of the electrical behaviors of the fabricated memory devices. For the set operations, the threshold voltage (Vth) for electronic switching and the required current for set were observed to decrease with increasing Sb composition. The data endurance of memory devices under repetitive operations was measured to be in the range from 1×105 to 2×106 cycles. From the investigations using energy dispersive X-ray spectroscopy (EDS), it was clearly shown that there was no phase separation and/or marked compositional change of the device experiencing 2×106 successive operations, when the Sb atomic ratio in the Ge–Sb–Te alloy was 39%.

Emission Properties of InGaAsSbN Quantum Well Laser Diodes in 2 µm Wavelength Region Grown on InP Substrates

InGaAsSbN quantum well laser diodes operating in the 2 µm wavelength region were grown by molecular beam epitaxy (MBE) on InP substrates and their emission properties were studied. Two types of laser diodes with different Sb composition were compared. It was found that an increase in the Sb composition induces a marked redshift of the peak wavelength and a reduction of the spectral line width of the electroluminescence (EL) spectrum. A blueshift of the EL peak energy and a reduction of the EL spectral line width as well as a marked enhancement of the EL intensity were obtained by postgrowth rapid thermal annealing. The Sb composition dependence of the lasing characteristics was also studied. A clear redshift of the lasing wavelength was observed by increasing Sb composition.

Change in band configuration of [formula omitted] quantum wells from type-II to type-I by increasing Sb composition x

InGaAs/AlAsSb systems lattice matched to InP have two distinguishable features: a high conduction band offset and type-II band configuration. Although, the former results in a large intersubband transition (ISBT) at conduction band, the latter makes it difficult to use the effective interband transition (IBT). To overcome this latter problem, the effect of the Sb was investigated because Sb would act as a band modulator from type-II to type-I. In 0.57 Ga 0.43 As 1 - x Sb x / AlAs 0.48 Sb 0.52 single quantum well (SQW) samples with various Sb compositions x, were grown on GaAs substrates via AlAs 0.48 Sb 0.52 buffer layer. Their photoluminescence (PL) properties were examined to identify their band configurations. When the excitation laser power was increased, the PL property of In 0.57 Ga 0.43 As SQW sample, showed a larger blue shift than that of In 0.57 Ga 0.43 As 1 - x Sb x ( x ⩾ 0.13 ) ones. This indicates that the band configuration modulates from type-II to type-I when the antimonide composition is larger than 0.13. These findings indicate that new functional devices can be fabricated using a combination of IBT and ISBT.

Long-wavelength light emission and lasing from InAs∕GaAs quantum dots covered by a GaAsSb strain-reducing layer

The effects of a thin GaAsSb strain-reducing layer on the optical properties of InAs∕GaAs quantum dots (QDs) are investigated. With increasing Sb composition, the room-temperature emission wavelength of the InAs QDs increases to ∼1.43μm. For Sb compositions above 14%, the system becomes Type II, with a decrease of the photoluminescence (PL) efficiency. At a composition of 14%, the room-temperature PL efficiency is maximized, and is also significantly enhanced when compared to that of conventional InGaAs-capped InAs QDs grown under the same conditions. Room-temperature ground-state lasing at 1.292μm is demonstrated for an InAs∕GaAsSb∕GaAs structure.

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In0.53Ga0.47As/GaAs0.5Sb0.5 type II quantum well structures lattice-matched to InP substrates are a very attractive material system for mid-infrared lasers in the 2μm wavelength region. It is expected to get longer wavelength laser operation with increasing Sb composition (y) in the GaAs1-ySby layer. This letter reports the Sb composition dependence and the GaAsSb thickness dependence of the photoluminescence properties of the InGaAs/GaAsSb type II MQW structures.

Midinfrared photoluminescence of InAsSb quantum dots grown by liquid phase epitaxy

Photoluminescence in the 2–5 μm spectral region is reported from InAs1−xSbx quantum dots grown from the liquid phase at 580 °C on an InAs (100) substrate. Atomic force microscopy shows that coalesced quantum dots and then isolated quantum dots are formed with increasing Sb composition (x=0.2–0.3) and strain. The 4 K photoluminescence of the isolated and coalesced quantum dots was observed to peak in the midinfrared at 289 and 316 meV, (4.29 and 3.92 μm), respectively. These peaks are due to type II transitions and begin to quench at temperatures above 100 K as holes become thermally activated out of the quantum dot confinement potential.