Si Multiple Quantum Research Articles

Photoluminescence from ultrathin Ge-rich multiple quantum wells observed up to room temperature: Experiments and modeling

Employing a low-temperature growth mode, we fabricated ultrathin $\mathrm{S}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{G}{\mathrm{e}}_{x}$/Si multiple quantum well structures with a well thickness of less than 1.5 nm and a Ge concentration above 60% directly on a Si substrate. We identified an unusual temperature-dependent blueshift of the photoluminescence (PL) and exceptionally low thermal quenching. We find that this behavior is related to the relative intensities of the no-phonon (NP) peak and a phonon-assisted replica that are the main contributors to the total PL signal. To investigate these aspects in more detail, we developed a strategy to calculate the PL spectrum employing a self-consistent multivalley effective mass model, in combination with second-order perturbation theory. Through our investigation, we find that while the phonon-assisted feature decreases with temperature, the NP feature shows a strong increase in the recombination rate. Besides leading to the observed robustness against thermal quenching, this causes the observed blueshift of the total PL signal.

Si and InGaAs Spatial Wavefunction-Switched (SWS) FETs with II–VI Gate Insulators: An Approach to the Design and Integration of Two-Bit SRAMs and Binary CMOS Logic

Electron wavefunctions are switched spatially from one quantum well to another by varying the gate voltage Vg in spatial wavefunction-switched (SWS) field-effect transistors (FETs), which comprise two or more coupled quantum wells serving as the transport channel. This is shown for Si/SiGe and InGaAs/AlInAs quantum well systems. The presence of charge in a particular well or channel is used to encode four states 00, 01, 10, 11. This unique property is used for two-bit processing, resulting in compact two-bit static random-access memory devices. Experimental data including capacitance–voltage peaks in Si and InGaAs multiple quantum well SWS-FETs has verified the SWS phenomenon. Replacing quantum wells by an array of cladded quantum dots, forming a quantum dot superlattice (QDSL) layer, enhances the contrast and noise margin in SWS-FETs. This paper reports I–V and C–V characteristics for a fabricated twin-drain SWS-quantum dot channel (QDC) FET comprising four layers of self-assembled SiOx-Si quantum dots. SWS-QDC-FETs are shown to be scalable to ∼9 nm, and comprise four layers of cladded quantum dots with an array of 3 × 3 forming the channel.

Inhomogeneities of InGaN/GaN MOVPE multi quantum wells grown with a two temperatures process studied by transmission electron microscopy

AbstractThe structural investigation of InGaN/GaN:Si multiple quantum well (MQW) samples grown by low‐pressure metal‐organic vapor phase epitaxy (LP‐MOVPE) in a two temperatures (2T) process on high‐pressure GaN mono‐crystalline substrates is performed by transmission electron microscopy (TEM). A sample in which barriers and wells were grown at 780 °C is compared with another in which the barriers were deposited at 900 °C and the wells at 730 °C. For both samples the indium composition in the QWs reaches the level of about 20 at.%. The local indium composition was measured through strain measurements by digital processing from the lattice fringes images taken by TEM. Cross‐sectional investigations are performed in two zone axes – [$1\bar {1}00$] and [$11\bar {2}0$] – with the use of axial and off‐axis illumination. During TEM investigations the formation of “false indium clusters ” of the size of 2–4 nm was observed for both samples just after 2 min of LaB6 electron beam illumination at 200 kV. Large lateral fluctuations of indium content in the QWs of the length of 30–130 nm were detected in 2T sample. The plan view analysis was carried out to characterize the anisotropy of the indium fluctuation inside the QWs.

Selectively enhanced emission and suppression in Si0.5Ge0.5/Si multiple quantum wells by photonic crystals

Selective enhancement and suppression of the photoluminescence arising from Si0.5Ge0.5/Si multiple quantum wells by photonic crystals (PCs) have been demonstrated. The formation of the stop band in PCs is designed to be a filter as well as a reflector. It is found that the self-assembled PCs are able to selectively enhance the luminescence of the type-II transitions at the interface between Si and Si0.5Ge0.5/Si layers and suppress the emission from Si. Our working principle shown here can be extended to many other material systems and should be very useful for creating high power solid-state emitters.

On optimum designs of a RCE Si/SiGe/Si MQW photodetector for long wavelength applications

In the present paper, performance analysis of a resonant-cavity-enhanced Si/Si 1−y Ge y /Si multiple quantum well photodetector has been carried out. The effects of material parameters of active Si 1−y Ge y layers and carrier trapping by the potential barriers at the heterointerfaces of Si/Si 1−y Ge y /Si quantum wells on the bandwidth (BW) and responsivity of the detector have been investigated using numerical simulation. Results on possible optimum designs of the photodetector have been suggested to maximize responsivity-BW product.

Simulation of p–i–n heterojunctions built on strain-compensated Si/Si 0.40Ge 0.60/Si multiple quantum wells for photodetection near 1.55 µm

Self-consistent computations of the potential profile in complex semiconductor heterostructures can be successfully applied for comprehensive simulation of many device characteristics. Such computations have been used for the study of SiGe/Si multiple quantum wells (MQWs) based infrared photodetectors operating in the two low absorption windows of silica fibers, e.g. around λ = 1.3 and λ = 1.55 µm. In this paper, a versatile model is proposed for the design optimization of SiGe/Si MQWs based photodetectors. It is based on the coupled Schrödinger–Poisson equations that allow the determination of the energy quantization levels and the wave functions of charge carriers. The optimum parameters such as the layers thickness, compositions and doping of the relative MQWs based p–i–n photodetectors under an external applied electric field are determined for the aimed range of wavelength.

Electroluminescence enhancement of SiGe/Si multiple quantum wells through nanowallstructures

The enhancement of light extraction fromSi0.5Ge0.5/Si multiple quantum wells (MQWs) with nanowall structures fabricated by electroncyclotron resonance (ECR) plasma etching is presented. It is shown that the ECRplasma treatment does not damage the crystalline quality. At a driving current of5.5 × 106 A m−2, the light output intensity of the MQWs with nanowall structures shows an enhancementof about 50% compared with that of the original MQWs. In addition to the enhanced lightextraction, the improved optoelectronic properties are also attributed to the strainrelaxation in nanowall structures.

Photogalvanic effects for interband transition in p-Si0.5Ge0.5∕Si multiple quantum wells

Circular photogalvanic effect (CPGE) and linear photogalvanic effect for interband transition have been observed simultaneously in Si0.5Ge0.5∕Si multiple quantum wells. The signature of the CPGE is evidenced by the change of its sign upon reversing the radiation helicity. It is found that the observed CPGE photocurrent is an order of magnitude greater than that obtained for intersubband transition. The dependences of the CPGE on the angle of incidence and the excitation intensities can be well interpreted based on its characteristics. The large signal of spin generation observed here at room temperature should be very useful for the realization of practical application of spintronics.

Experimental evidence for index modulation by carrier depletionin SiGe∕Si multiple quantum well structures

Experimental results for the refractive index variation obtained by hole depletion in SiGe∕Si multiple quantum wells inserted in a reverse-biased p-i-n junction are reported. The electronic contribution to the index variation is unambiguously separated from the thermal one. Measured refractive index changes around 4.2×10−5V−1 are in quite good agreement with modeling.

A high-performance SiGe-Si multiple-quantum-well heterojunction phototransistor

A novel phototransistor is fabricated by placing Si/sub 0.5/Ge/sub 0.5//Si multiple quantum wells (MQWs) between the base and the collector of Si-SiGe heterojunction bipolar transistors (HPT). The SiGe-Si MQWs are used as a light absorption layer. The cutoff frequency (f/sub T/) and maximum oscillation frequency (f/sub MAX/) of the phototransistor are found to be 25 GHz, which is suitable for gigabit integrated circuit. The responsivity of 1.3 A/W (external quantum efficiency = 194%) and the pulsewidth of 184 ps at a wavelength of 850 nm are observed. The excellent electrical and optical performance of the Si-SiGe MQW phototransistor makes it attractive for future Si-based optoelectronic integrated circuit applications.

Ultrafast intersubband scattering of holes in p-type modulation-doped Si 1− xGe x/Si multiple quantum wells

We report on an experimental and theoretical study of intersubband relaxation of holes in p-type modulation-doped Si 1− x Ge x /Si multiple quantum wells. The ultrafast hole dynamics is studied in pump–probe experiments with 150 fs mid-infrared pulses. For 4.4 nm wide wells ( x=0.5) the lifetime of holes in the second heavy hole subband is only 250 fs due to rapid emission of optical phonons via the deformation potential interaction. Model calculations of hole–phonon scattering give an almost identical value and point to the crucial role of cascaded scattering via an intermediate subband with light-hole-split-off symmetry.

Femtosecond intersubband scattering of holes in Si 1− xGe x/Si quantum wells

The intersubband relaxation of holes in p-type modulation-doped Si 1− x Ge x /Si multiple quantum wells is studied both experimentally and theoretically. Pump–probe experiments with 150-fs midinfrared pulses yield a lifetime of holes in the second heavy-hole subband of only 250 fs for 4.4-nm wide wells with x=0.5. This short lifetime is caused by interaction with phonons via the optical deformation potential. Calculations of hole–phonon scattering agree very well with the experimental results. The calculations show that longer heavy-hole lifetimes are possible by increasing the Ge content and the well widths.

Incorporation and optical activation of erbium in strained silicon–germanium structures

Erbium has been incorporated in strained Si/Si 0.87Ge 0.13/Si multiple quantum well structures with a density of 10 18 cm −3. The process of ion implantation was used. Samples were amorphized by silicon implantation at liquid nitrogen temperature following the erbium implant. Recrystallization and optical activation of erbium atoms were achieved simultaneously by low temperature annealings in the range of 550–650°C. Detailed study on this low temperature window for erbium activation has been done. Reduction or complete elimination of erbium luminescence was observed for recrystallized samples having an additional step of rapid thermal annealing. It was shown that the uncontrolled sharp quenching of temperature that follows the rapid thermal annealing process deteriorates the sample structure and the erbium luminescence.

Ultrafast dynamics of intersubband excitations in a quasi-two-dimensional hole gas.

We present the first study of ultrafast hole dynamics after resonant intersubband excitation in a quasi-two-dimensional semiconductor. p-type Si0.5Ge 0.5/Si multiple quantum wells are studied in pump-probe experiments with 150 fs midinfrared pulses. Intersubband scattering from the second heavy-hole back to the first heavy-hole subband occurs with a time constant of 250 fs, followed by intrasubband carrier heating within 1 ps. Such processes give rise to a strong reshaping of the intersubband absorption line, which is accounted for by calculations of the subband structure, optical spectra, and hole-phonon scattering rates.

High-Pressure Photoluminescence Studies of Pseudomorphic Si1-yCy/Si MQW Structures

We have performed photoluminescence (PL) investigations of pseudomorphic Si 1-y C y /Si (y = 0.45, 1.05, and 1.62%) multiple quantum well (MQW) structures under hydrostatic pressure (0 to 8 GPa) and at low temperatures (10 to 70 K). The main MQW-related emission, at energies below the Si band gap, consists of bound and free exciton no-phonon lines and related Si transverse-optic phonon replicas. All MQW-related PL peaks shift to lower energy with increasing pressure at a rate characteristic for Γ-X indirect transitions in tetrahedral semiconductors. The total band offset and the activation energies for decay of the free and bound exciton emission increase slightly with pressure as a result of the larger negative band gap pressure coefficient of the strained pseudomorphic Si 1 -yCy layers compared to pure silicon. A separation of biaxial strain effects on the conduction and valence band near-gap states in the pseudomorphic Si 1-y C y layers (y < 0.02) on Si indicates a decrease of the intrinsic Si 1-y C y band gap which corresponds to that of pure silicon compressed to the lattice constant of the alloy. From this a type-I band alignment with electrons and light holes localized in the SiC layers is inferred. This assignment is consistent with the dependence of PL-peak intensities and energies on excitation power, temperature and pressure.

Growth temperature dependence of substitutional carbon incorporation in Si 1− yC y alloys and Si 1− yC y/Si multiple quantum wells

The carbon incorporation into lattice sites in Si 1− y C y epilayers and Si 1− y C y /Si multiple quantum wells grown by molecular beam epitaxy was investigated as a function of growth temperature in the range of 350–650°C using Raman spectroscopy, double crystal X-ray diffraction (DCXD), and atomic force microscopy (AFM). The substitutional carbon concentration deduced from the simulation of DCXD results decreases with increasing growth temperature due to the strain relaxation process which occurred by the change of the growth mode from two-dimensional to three-dimensional island growth.

Growth of SiGe/Si multiple quantum wells by ultra-high-vacuum electron cyclotron resonance chemical vapor deposition

Pseudomorphic Si 1- x Ge x single layers and Si 1- x Ge x /Si multiple quantum wells (MQWs) were successfully grown on Si(100) wafers at low temperatures (440–510°C) by an ultra-high-vacuum electron cyclotron resonance chemical vapor deposition (UHV-ECRCVD) technique. Growth kinetics of UHV-ECRCVD was reported, and the unique influence of hydrogen plasma on the growth process was discussed. Transmission electron microscopy (TEM) was used to investigate the structural quality of Si 1- x Ge x single layers and MQWs. It was shown that the Si 1- x Ge x /Si MQWs were free of extended defects, fully strained, and maintained good uniformity of thickness. Double axis rocking curves and reciprocal space maps by triple axis X-ray diffraction were employed to extract structural parameters from the samples. Interfaces were flat and coherent, and no appreciable tilts or mosaic spreading were observed. It was demonstrated that UHV-ECRCVD is suitable for low-temperature growth of quantum structures with high structural quality.

Optimization of growth conditions for strained Si/Si 1− yC y structures

Preparation of Si 1− y C y /Si(001) and Si/Si 1− x Ge x /Si 1− y C y /Si(001) heterostructures using molecular beam epitaxy (MBE) with C obtained from sublimation of SiC in a high-temperature cell is reported. Accumulation of surface roughness is found to occur during growth of C-rich layers eventually leading to a reduction of the C-induced strain. The roughness can be suppressed at increased growth rates and/or decreased substrate temperature during growth of Si 1−yC y layers. Modulation of the growth temperature has been found advantageous for optimizing both the morphology and photoluminescence properties. Near band-edge luminescence from Si 1− y C y /Si multiple quantum well (MQW) structures has been observed and a conduction band edge shift equal to 63 meV/% C has been deduced from an effective mass calculation.

Ultrathin pseudomorphic Sn/Si and SnxSi1−x/Si heterostructures

Ultrathin, coherently strained Sn/Si and SnxSi1−x/Si alloy quantum well structures with substitutional Sn incorporation far in excess of the equilibrium solubility limit have been fabricated via substrate temperature and growth flux modulations in molecular beam epitaxy. Sn/Si single and multiple quantum wells with Sn coverage up to 1.3 ML, Sn0.05Si0.95/Si multiple quantum wells of up to 2.0 nm, and Sn0.16Si0.84/Si multiple quantum wells of up to 1.1 nm are determined to be pseudomorphic, and coverage-dependent Sn segregation dynamics are observed.

Growth of strained [formula omitted] structures by MBE

Carbon doping during Si molecular beam epitaxy (MBE) has been studied with C obtained by sublimation of SiC in a high-temperature cell. Accumulation of surface roughness has been observed, which can lead to reduction of C-induced strain. This surface roughening is reduced at high growth rates and/or low substrate temperatures. Modulation of the growth temperature during the growth sequence has been used to prepare Si 1 − yC y Si multiple quantum well (MQW) structures with optimized photoluminescence and X-ray diffraction (XRD) properties. Preparation of Si 1 − yC y Si 1 − xGe x MQW structures is reported and during growth at 430 °C formation of Ge-rich precipitates occurs, while growth at 370 °C leads to very abrupt interfaces.