Mansour Mortazavi

Systematic study of GeSn heterostructure-based light-emitting diodes towards mid-infrared applications

Temperature-dependent characteristics of GeSn light-emitting diodes with Sn composition up to 9.2% have been systematically studied. Such diodes were based on Ge/GeSn/Ge double heterostructures (DHS) that were grown directly on a Si substrate via a chemical vapor deposition system. Both photoluminescence and electroluminescence spectra have been characterized at temperatures from 300 to 77 K. Based on our theoretical calculation, all GeSn alloys in this study are indirect bandgap materials. However, due to the small energy separation between direct and indirect bandgap, and the fact that radiative recombination rate greater than non-radiative, the emissions are mainly from the direct Γ-valley to valence band transitions. The electroluminescence emissions under current injection levels from 102 to 357 A/cm2 were investigated at 300 K. The monotonic increase of the integrated electroluminescence intensity was observed for each sample. Moreover, the electronic band structures of the DHS were discussed. Despite the indirect GeSn bandgap owing to the compressive strain, type-I band alignment was achieved with the barrier heights ranging from 11 to 47 meV.

Optical Characterization of Si-Based Ge1−x Sn x Alloys with Sn Compositions up to 12%

Optical properties of germanium tin (Ge1−xSnx) alloys have been comprehensively studied with Sn compositions from 0 (Ge) to 12%. Raman spectra of the GeSn samples with various Sn compositions were measured. The room temperature photoluminescence (PL) spectra show a gradual shift of emission peaks towards longer wavelength as Sn composition increases. Temperature dependent PL shows the PL intensity variation along with the temperature change, which reveals the indirectness or directness of the bandgap of the material. As temperature decreases, the PL intensity decreases with Sn composition less than 8%, indicating the indirect bandgap Ge1−xSnx; while the PL intensity increases with Sn composition higher than 10%, implying the direct bandgap Ge1−xSnx. Moreover, the PL study of n-doped samples shows bandgap narrowing compared to the unintentionally (Boron) doped thin film with similar Sn compositions due to the doping.

Study of a SiGeSn/GeSn/SiGeSn structure toward direct bandgap type-I quantum well for all group-IV optoelectronics

A SiGeSn/GeSn/SiGeSn single quantum well structure was grown using an industry standard chemical vapor deposition reactor with low-cost commercially available precursors. The material characterization revealed the precisely controlled material growth process. Temperature-dependent photoluminescence spectra were correlated with band structure calculation for a structure accurately determined by high-resolution x-ray diffraction and transmission electron microscopy. Based on the result, a systematic study of SiGeSn and GeSn bandgap energy separation and barrier heights versus material compositions and strain was conducted, leading to a practical design of a type-I direct bandgap quantum well.

Investigation of GeSn Strain Relaxation and Spontaneous Composition Gradient for Low-Defect and High-Sn Alloy Growth

Recent development of group-IV alloy GeSn indicates its bright future for the application of mid-infrared Si photonics. Relaxed GeSn with high material quality and high Sn composition is highly desirable to cover mid-infrared wavelength. However, its crystal growth remains a great challenge. In this work, a systematic study of GeSn strain relaxation mechanism and its effects on Sn incorporation during the material growth via chemical vapor deposition was conducted. It was discovered that Sn incorporation into Ge lattice sites is limited by high compressive strain rather than historically acknowledged chemical reaction dynamics, which was also confirmed by Gibbs free energy calculation. In-depth material characterizations revealed that: (i) the generation of dislocations at Ge/GeSn interface eases the compressive strain, which offers a favorably increased Sn incorporation; (ii) the formation of dislocation loop near Ge/GeSn interface effectively localizes defects, leading to the subsequent low-defect grown GeSn. Following the discovered growth mechanism, a world-record Sn content of 22.3% was achieved. The experiment result shows that even higher Sn content could be obtained if further continuous growth with the same recipe is conducted. This report offers an essential guidance for the growth of high quality high Sn composition GeSn for future GeSn based optoelectronics.

Investigation of optical transitions in a SiGeSn/GeSn/SiGeSn single quantum well structure

A SiGeSn/GeSn/SiGeSn single quantum well structure featuring type-I band alignment was comprehensively characterized. Three pump lasers with different penetration depths and photon energies were used to pinpoint the optical transition characteristics of the sample. The carrier generation, redistribution, and recombination under each pumping condition were analyzed in detail. By comparing the temperature-dependent photoluminescence spectra of the GeSn quantum well with that of SiGeSn and GeSn thin film samples possessing similar Si and Sn compositions, the optical transition mechanism was clearly identified.

Comparison study of the low temperature growth of dilute GeSn and Ge

Dilute GeSn films have been grown at the temperatures below 400 °C in a cold-walled ultrahigh vacuum chemical-vapor-deposition chamber. Diluted tin-tetrachloride (SnCl4) with a flow rate of 0.02 sccm was used as Sn precursor while the flow rate of Ge precursor germane was 10 sccm. For comparison, the Ge films were grown under the same conditions except only the precursor germane was used. Material growth study revealed the linear growth rates for both films and increased nucleation times at lower temperatures. Material and optical characterizations showed that the GeSn films featured longer nucleation times, higher growth rates, and higher crystal quality compared to those of Ge films grown at the same conditions. The growth mechanism investigation suggested that GeSn growth using SnCl4 is an exothermic chemical reaction which could lead to the improved material quality.

Band filling effects on temperature performance of intermediate band quantum wire solar cells

Detailed studies of solar cell efficiency as a function of temperature were performed for quantum wire intermediate band solar cells grown on the (311)A plane. A remotely doped one-dimensional intermediate band made of self-assembled In0.4Ga0.6As quantum wires was compared to an undoped intermediate band and a reference p-i-n GaAs sample. These studies indicate that the efficiencies of these solar cells depend on the population of the one-dimensional band by equilibrium free carriers. A change in this population by free electrons under various temperatures affects absorption and carrier transport of non-equilibrium carriers generated by incident light. This results in different efficiencies for both the doped and undoped intermediate band solar cells in comparison with the reference GaAs p-i-n solar cell device.

Study of direct bandgap type-I GeSn/GeSn double quantum well with improved carrier confinement

The GeSn-based quantum wells (QWs) have been investigated recently for the development of efficient GeSn emitters. Although our previous study indicated that the direct bandgap well with type-I band alignment was achieved, the demonstrated QW still has insufficient carrier confinement. In this work, we report the systematic study of light emission from the Ge0.91Sn0.09/Ge0.85Sn0.15/Ge0.91Sn0.09 double QW structure. Two double QW samples, with the thicknesses of Ge0.85Sn0.15 well of 6 and 19 nm, were investigated. Band structure calculations revealed that both samples feature type-I band alignment. Compared with our previous study, by increasing the Sn composition in GeSn barrier and well, the QW layer featured increased energy separation between the indirect and direct bandgaps towards a better direct gap semiconductor. Moreover, the thicker well sample exhibited improved carrier confinement compared to the thinner well sample due to lowered first quantized energy level in the Γ valley. To identify the optical transition characteristics, photoluminescence (PL) study using three pump lasers with different penetration depths and photon energies was performed. The PL spectra confirmed the direct bandgap well feature and the improved carrier confinement, as significantly enhanced QW emission from the thicker well sample was observed.

High performance Ge0.89Sn0.11 photodiodes for low-cost shortwave infrared imaging

Low-cost shortwave infrared detectors have great potential for emerging civilian night-vision applications. This paper reports the characteristics of Ge0.89Sn0.11 photodiodes monolithically grown on a Si substrate that holds great promise for those applications. At room temperature, the 500 μm diameter active area device demonstrated a longwave cutoff of 2.65 μm and a responsivity of 0.32 A/W at 2 μm, which corresponds to an external quantum efficiency of 20% without any contribution from the Ge buffer layer. The measured peak specific detectivity at 300 K and 77 K is 1.7 × 109 Jones and 4.3 × 109 Jones, respectively. The specific detectivity at 77 K is only one-order-of-magnitude lower than that of the market dominating extended-InGaAs photodiode. The detailed device analysis indicated that the 700-nm thick fully relaxed high-quality GeSn absorbing layer and the modified depletion region lead to the above-mentioned device performance.Low-cost shortwave infrared detectors have great potential for emerging civilian night-vision applications. This paper reports the characteristics of Ge0.89Sn0.11 photodiodes monolithically grown on a Si substrate that holds great promise for those applications. At room temperature, the 500 μm diameter active area device demonstrated a longwave cutoff of 2.65 μm and a responsivity of 0.32 A/W at 2 μm, which corresponds to an external quantum efficiency of 20% without any contribution from the Ge buffer layer. The measured peak specific detectivity at 300 K and 77 K is 1.7 × 109 Jones and 4.3 × 109 Jones, respectively. The specific detectivity at 77 K is only one-order-of-magnitude lower than that of the market dominating extended-InGaAs photodiode. The detailed device analysis indicated that the 700-nm thick fully relaxed high-quality GeSn absorbing layer and the modified depletion region lead to the above-mentioned device performance.

Direct bandgap type-I GeSn/GeSn quantum well on a GeSn- and Ge- buffered Si substrate

This paper reports the comprehensive characterization of a Ge0.92Sn0.08/Ge0.86Sn0.14/Ge0.92Sn0.08 single quantum well. By using a strain relaxed Ge0.92Sn0.08 buffer, the direct bandgap Ge0.86Sn0.14 QW was achieved, which is unattainable by using only a Ge buffer. Band structure calculations and optical transition analysis revealed that the quantum well features type-I band alignment. The photoluminescence spectra showed dramatically increased quantum well peak intensity at lower temperature, confirming that the Ge0.86Sn0.14 quantum well is a direct bandgap material.