SiGe Quantum Wells Research Articles

SiGe quantum wells implementation in Si based nanowires for solar cells applications

This study focuses on modelling and optimizing a new Si nanowire solar cell containing a SiGe/Si quantum well. Quantum efficiency measurements show that the proposed structure has a higher energy absorption advantage and stronger than that of a solar cell based on a standard Si p-i-n nanowire. As a result, the insertion of 14 layers of SiGe/Si quantum well improved the short circuit current density and the efficiency by a factor of about 1.24 and 1.37, respectively. The best concentration and radius values obtained are x = 0.05 and r = 0.190 µm, respectively, with a strain of less than 1%.

Electron energy relaxation in SiGe quantum wells

In this article, we theoretically calculated the hot electron energy relaxation induced by longitudinal acoustic (LA) phonons, longitudinal optical (LO) phonons and importantly two-transverse acoustic (2TA) phonons in two-dimensional SiGe quantum wells, by incorporating dynamic screening effect in LA phonon scattering and hot-phonon effect in LO phonon scattering mechanisms. At the low-temperature regime, the ELR is found to be dominated by LA phonons and at higher temperature ELR is dominated by LO phonons. In the intermediate temperature range, 2TA phonon scattering mechanism is very important and we incorporated in our calculations. The unscreened longitudinal acoustic (LA) phonon due to deformation potential (DP) coupling is dominant over the screened acoustic piezoelectric phonon contribution. At higher temperatures, there is a crossover from ELR due to LA phonons to ELR due to longitudinal optical (LO) phonons with the cross-over temperature being about Te∼40K. The LA phonon screening effect and hot phonon effect is demonstrated to reduce ELR significantly. In the intermediate temperature range (30 K

Density-dependent two-dimensional optimal mobility in ultra-high-quality semiconductor quantum wells

We calculate using the Boltzmann transport theory the density-dependent mobility of two-dimensional (2D) electrons in GaAs, SiGe, and AlAs quantum wells as well as of 2D holes in GaAs quantum wells. The goal is to precisely understand the recently reported breakthrough in achieving a record 2D mobility for electrons confined in a GaAs quantum well. Comparing our theory with the experimentally reported electron mobility in GaAs quantum wells, we conclude that the mobility is limited by unintentional background random charged impurities at an unprecedented low concentration of $\ensuremath{\sim}{10}^{13}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. We find that this same low level of background disorder should lead to 2D GaAs hole and 2D AlAs electron mobilities of $\ensuremath{\sim}{10}^{7}$ and $\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}/\text{V}\phantom{\rule{0.16em}{0ex}}\text{s}$, respectively, which are much higher theoretical limits than the currently achieved experimental values in these systems. We therefore conclude that the current GaAs hole and AlAs electron systems are much dirtier than the state-of-the-art 2D GaAs electron systems. We present theoretical results for 2D mobility as a function of density, effective mass, quantum-well width, and valley degeneracy, comparing with experimental data.

Metal–insulator transition and low-density phases in a strongly-interacting two-dimensional electron system

We review recent experimental results on the metal–insulator transition and low-density phases in strongly-interacting, low-disordered silicon-based two-dimensional electron systems. Special attention is given to the metallic state in ultra-clean SiGe quantum wells and to the evidence for a flat band at the Fermi level and a quantum electron solid.

Crossover between ballistic and diffusive regime in 2D SiGe quantum well

In this manuscript, we report a theoretical investigation of the magneto-conductivity (σ(B,T)) in the metallic side (where dσ/dt<0) of a low densities 2D hole system in SiGe at very low temperature (below 2K). We have reanalyzed the experimental data already prepared and published by Drichko et al. in [23]. The purpose of this study is to determine the dominant electronic transport regime by confronting the experience with current theories. Based on the Fermi-liquid approach and the ZNA theory, we have able to determine the régimes of electronic transport in 2D p-SiGe quantum well. Indeed, the careful analysis of the results shows the coexistence of the two electronic transport regimes: ballistic and diffusive and, above all, a transition from one regime to the other. At low magnetic field (less than 6T), the Fermi liquid parameter values are found negative and positive for the highest values of the parallel magnetic field. The results are consistent with recent works on graphene sample and other similar samples.

Process Effects on the Noise Performance of SiGe/Si Multi Quantum Well Thermistor

In this paper, the process effects of SiGe/Si multi quantum well (MQW) stack is presented in terms of its 1/f noise performance in an intrinsic thermal detector device. In particular, boron (B) doping in SiGe quantum wells and carbon (C) delta layer at SiGe/Si interfaces are analyzed by presenting the 1/f noise voltage power spectral densities. The results indicate that 1/f noise performance can be further improved by carbon delta layers while doping in SiGe quantum wells has no significant effect. Effects of the bias voltage and the pixel size of the thermistor are obtained very much similar to the expected trend. The overall achievements proves that the desired TCR and resistance values for high performance SiGe/Si MQW thermistor material can be achieved with a desired 1/f noise performance by the introduction of carbon delta layer at SiGe/Si interfaces and in-situ doping of the SiGe layers in the MQW stack.

A Band-Engineered One-Transistor DRAM With Improved Data Retention and Power Efficiency

In this letter, a one-transistor (1T) dynamic random-access memory (DRAM) with SiGe quantum well (QW) is proposed, and its performance is validated through the technology computer-aided design (TCAD) simulation. At the write operation, band-to-band tunneling is used and 1 V or lower programming voltage is realized by inserting the SiGe QW beside the drain. This QW also functions as the storage node, which enhances not only the current sensing margin but also the retention time ( $\tau _{{\text {ret}}}$ ) compared with those of all-Si device. At an extremely scaled cell size and sub-10-ns write/erase operations, the proposed device shows 0.2-s-long $\tau _{{\text {ret}}}$ and current ratio > 104. It has been verified that a single cycle of 1T DRAM operations consumes only 93.8 fJ.

(Invited) Si(1-x)Gex/Si Mqw Based Uncooled Microbolometer Development and Integration into 130 nm BiCMOS Technology

The process of epitaxial SiGe layers are getting attractive as they offer a variety of advanced microelectronics devices compatible with silicon processing technology [1-3]. The large research effort of the last decade has focused on the growth of high quality strained SiGe layers. In particular, SiGe superlattice containing high Ge concentration is demanded in order to improve performances of the devices. Recently, IHP has achieved a significant breakthrough in SiGe/Si MQW processing technology [4]. The developed high quality of superlattice of SiGe/Si MQW structure containing 50% Ge has been successfully processed (Fig. 1) and presented as a potential thermistor material with its high performance for uncooled microbolometer applications [5]. The prototype of thermistor device with 50% Ge in SiGe wells exhibited an outstanding TCR of -5.5 %/K and a K1/f value of 3.4 × 10-15 for 200 µm× 200 µm pixel size, respectively, showing a concurrent achievement of a very high TCR and a low 1/f noise performance. One of the main challenge of SiGe/Si MQW structures is the high resistance values including high Ge concentration makes it difficult to integrate as a detector material in a complete microbolometer device. Large pixel resistances cause not only high noises and hence lower sensitivity but also a challenge to design and maximize the performance of the ROICs [6, 7]. A method to optimize and tune the resistance of the detector material in microbolometers is in-situ doping of quantum well layers in order to enhance the combined detector and ROIC performance. In [8], Boron (B) doping level in SiGe quantum wells are optimized in order to keep the pixel resistance of the thermistor in acceptable values for an ROIC design. B doping of SiGe MQWs is used for the resistance tuning of the detector. The main challenge of the method is simultaneous achievement of enough doping in the SiGe region of the MQWs while keeping the barrier intrinsic silicons non-doped. The initial pixel resistance of 3 period of SiGe/Si MQWs with 50% Ge concentration of 21 MΩ is reduced 210 kΩ for 25 × 25 µm2 pixel size by the optimization of in-situ B doping level in SiGe layers of the MQW stack [8]. The optimized B doping density of ~1 × 1018 cm-3 in SiGe wells did not cause any significant change in the TCR value whereas the 1/f noise performance is even enhanced due to the in-situ doping process. All the aforementioned results shows the successful demonstration of intrinsic thermistor based on SiGe/Si MQWs including 50% Ge with an acceptable pixel resistances. Although the successful demonstration of SiGe MQWs has been performed for high performance microbolometer applications, a full integration of the high performance SiGe/Si MQW based detector into a standard CMOS process is still challenging. SiGe/Si MQW process requires higher temperatures than the temperature budget of the post processing of CMOS process. Therefore, the transfer of the optimized detector structure to a CMOS/BiCMOS wafer needs a special layer transfer process (i.e. wafer bonding). More critically, due to the vertical working principle mechanism of the detector device, there is a challenge for full integration of the optimized detector structure into standard BiCMOS process because the suspended detector device needs to be contacted by both sides In this work, the recent progress on SiGe/Si type high performance detector structures will be presented. The process optimization of the detector by means of high TCR, low 1/f noise and appropriate resistance will be detailed. The method of integrating the developed SiGe/Si MQW structures into a 130 nm BiCMOS process will be provided. The integration method is based on the oxide-oxide fusion bonding of ROIC and detector wafer followed by post processing of the bonded wafer stack. The details of thermal conductance optimization of the microbolometer together with the mechanical stress optimization, required for the suspended detector device, will also be given. Finally, the low-pressure hermetic packaging of the microbolometer structure will also be proposed. Figure 1

Spin qubit manipulation of acceptor bound states in group IV quantum wells

The large spin–orbit coupling in the valence band of group IV semiconductors provides an electric field knob for spin-qubit manipulation. This fact can be exploited with acceptor based qubits. Spin manipulation of holes bound to acceptors in engineered SiGe quantum wells depends very strongly on the electric field applied and on the heterostructure parameters. The g-factor is enhanced by the Ge content and can be tuned by shifting the hole wave-function between the heterostructure constituent layers. The lack of inversion symmetry induced both by the quantum well and the electric fields together with the g-factor tunability allows the possibility of different qubit manipulation methods such as electron spin resonance, electric dipole spin resonance and g-tensor modulation resonance. Rabi frequencies up to hundreds of MHz can be achieved by electric field manipulation of heavy-hole qubits, and of the order of GHz with light-hole qubits.

On the radiative recombination and tunneling of charge carriers in SiGe/Si heterostructures with double quantum wells

For SiGe/Si(001) epitaxial structures with two nonequivalent SiGe quantum wells separated by a thin Si barrier, the spectral and time characteristics of interband photoluminescence corresponding to the radiative recombination of excitons in quantum wells are studied. For a series of structures with two SiGe quantum wells different in width, the characteristic time of tunneling of charge carriers (holes) from the narrow quantum well, distinguished by a higher exciton recombination energy, to the wide quantum well is determined as a function of the Si barrier thickness. It is shown that the time of tunneling of holes between the Si0.85Ge0.15 layers with thicknesses of 3 and 9 nm steadily decreases from ~500 to <5 ns, as the Si barrier thickness is reduced from 16 to 8 nm. At intermediate Si barrier thicknesses, an increase in the photoluminescence signal from the wide quantum well is observed, with a characteristic time of the same order of magnitude as the luminescence decay time of the narrow quantum well. This supports the observation of the effect of the tunneling of holes from the narrow to the wide quantum well. A strong dependence of the tunneling time of holes on the Ge content in the SiGe layers at the same thickness of the Si barrier between quantum wells is observed, which is attributed to an increase in the effective Si barrier height.

SiGe Nanomembrane Quantum-Well Infrared Photodetectors

SiGe quantum wells are promising candidates for the development of intersubband light emitters and photodetectors operating at mid- and far-infrared wavelengths. By virtue of their inherent compatibility with the Si microelectronics platform, these devices may be integrated seamlessly within complex optoelectronic systems for sensing and imaging applications. However, the development of high-quality SiGe intersubband active layers is complicated by the large lattice mismatch between Si and Ge, which limits the number of quantum wells that can be grown on bulk Si substrates before the onset of structural degradation due to inelastic strain relaxation. To address this issue, we investigate the use of lattice matched growth templates consisting of quantum-well nanomembrane stacks that were at one point free-standing, allowing for the internal stress to be relaxed via elastic strain sharing rather than defect formation. SiGe quantum-well infrared photodetectors (QWIPs) based on this approach are developed and characterized. Efficient current extraction from these ultrathin devices is obtained by bonding the nanomembranes directly on a doped Si substrate. Pronounced photocurrent peaks at mid-infrared wavelengths are measured, with improved responsivity compared to otherwise identical devices grown simultaneously on the supporting Si substrate.

Ion-sensitive field-effect transistor with sSi/Si0.5Ge0.5/sSOI quantum-well for high voltage sensitivity

An ion-sensitive field-effect transistor (ISFET) with improved sensing current and voltage sensitivity is realized by using a sSi/Si0.5Ge0.5/sSOI quantum-well (QW) heterostructure and an Al2O3 dielectric layer as sensing membrane coupling with the back-gate. The voltage sensitivity is much higher than the reference SOI ISFET, typically at low drain current due to the high hole mobility confined in the SiGe QW. A high voltage sensitivity 360mV/pH with a linearity of 99.89% was achieved for QW ISFET. The results of the planar QW ISFET show great potential for low cost, real-time monitoring of bio-chemicals due to its simplified process.

Quantum-Confined Stark Effect Analysis of GeSn/SiGeSn Quantum Wells for Mid-Infrared Si-Based Electroabsorption Devices Based on Many-Body Theory

Quantum-confined Stark effect (QCSE) of group IV Ge(Sn)/SiGe(Sn) quantum wells (QWs) on Si substrate is analyzed by microscopic many-body theory for mid-infrared (mid-IR) Si-based electroabsorption devices. To show the validity of the theory, QCSE of Ge/SiGe QW is investigated and very good agreement between theory and reported measured results is obtained. Next, the QCSE of GeSn/SiGeSn QWs is analyzed and the QW design for electroabsorption modulators to obtain large extinction ratio in mid-IR region is presented. It is shown that compressive and tensile strained well and barrier layers is preferable to obtain large extinction ratio due to its large conduction band offset.

Intersubband linear and nonlinear optical response of the delta-doped SiGe quantum well

The degree of ionization, controlled by external fields, of delta-doped layers inside the quantum wells can affect their energy structure, therefore delta-doped QWs can be used to engineer different kinds of tunable THz optical devices on intersubband transitions. Here it is calculated and analyzed the linear and nonlinear (Kerr-type) optical response, including absorption coefficient and refractive index change of 20nm-wide Si0.8Ge0.2/Si/Si0.8Ge0.2 QW structures n-delta-doped either at the center or at the edge of the well under different temperatures. The conduction subband energy structure was found self-consistently, including the calculation of the impurity binding energy. Our results show that the degree of ionization of the impurity layer as well as the heterostructure symmetry has a strong influence on optical properties of the structures in THz region.

High performance strained Si0.5Ge0.5 quantum-well p-MOSFETs fabricated using a high-κ/metal-gate last process

A high-κ/metal-gate (HKMG) last process for fabricating strained Si0.5Ge0.5 quantum-well (QW) p-channel metal oxide semiconductor field effect transistors (p-MOSFETs) is presented. Because the HKMG was formed after the ion-implant doping activation process, the thermal budget issue could be mitigated. The transistor features good transfer and output characteristics with Ion/Ioff ratio up to 106 and threshold voltage (VT) down to 0.1V. The effective hole mobility of the SiGe QW transistor reaches 215cm2/Vs for strong inversion conditions, which is 2.74 times the SOI device’s hole mobility.

ESD Characterization of SOI SiGe QW Gated Diode

Higher mobility channel materials, e.g., SiGe or Ge, are possibly introduced into the FinFET device architerture as the next technology option. The ESD characterization of SOI poly-defined lateral diode featuring a SiGe Quantum Well(QW) as channal was conducted by technology computer-aided design(TCAD) simulations. Meanwhile, the selective epitaxy growth of Si cap/SiGe bilayer on Si substrate was experimentally checked.

Strain-dependent intersubband absorption in the valence band of SiGe quantum wells

We present intersubband absorption measurements performed on p-type Si1 − xGex/Si(0 0 1) quantum wells (x = 0.35) in the presence of an external strain. We find that the in-plane polarized absorption band is rather insensitive to the strain. For the incident light polarized along the growth direction, the absorption strength increases under the tensile strain. The latter observation is well accounted for by a six-band k · p model and is explained by redistribution of hole population between the heavy- and the light-hole subbands caused by the strain-induced energy shifts of the bands.

Low-temperature electrical characterization of the Ti–Si(100) interface at the p-Si/SiGe/Si–Ti structure using Hall measurement analysis

Electrical characterization of a Ti–Si Schottky contact has been investigated using low-temperature Hall experiment analysis in the p-Si/SiGe/Si–Ti structure. In this approach, the density of a two-dimensional hole gas (2DHG) formed in the SiGe quantum well was a measure of charged metal-induced gap states. The Schottky barrier height (SBH) and the density of interface charges nint were evaluated via simulation of 2DHG density nh versus bias voltage applied to a Schottky gate. It was found that the density of interface charges depends on the position of the SiGe quantum well with respect to the Si–Ti interface irrespective of the bias voltage. Finally, the density of interface states was assessed via cap layer dependence of SBH and nint according to the mid-gap pinning model.

(Invited) Fast and Slow Light-Emitting Silicon-Germanium Nanostructures

Epitaxially-grown three-dimensional Si/SiGe nanostructures (NSs) produce photoluminescence (PL) and electroluminescence in the desired spectral range of 1.3-1.6 μm. We show that by controlling and modifying such Ge-rich SiGe nanoclusters during growth it is possible to fabricate very fast (PL lifetime <20 ns) and hence more efficient SiGe light-emitting devices. The results presented here demonstrate that in such Si/SiGe 3D NSs with a nominal Ge concentration approaching ~35% the PL peaked near 0.78 eV strongly depends on the Si/SiGe heterointerface abruptness. In other Si/SiGe NS/quantum-well samples with a Ge concentration approaching ~40%, we find two PL bands peaked at ~0.8 eV and ~0.9 eV at low temperatures. The PL peaked at 0.8 eV rises and decays slowly, and it quickly saturates as the excitation intensity increases. In contrast, the PL peaked at 0.9 eV shows a much shorter lifetime and exhibits a linear dependence versus excitation intensity. The slow/delayed PL at 0.8 eV is attributed to carrier recombination at the SiGe NS/Si transition layer while the faster and more efficient PL at 0.9 eV is associated with SiGe quantum wells. More complicated and similarly fast (~10-7 s) decays are observed at very high excitation intensities due to electron-hole droplet formation. The physics of carrier recombination in these Si/SiGe NSs is discussed.

Intraband optical transitions of holes in strained SiGe quantum wells

Optical absorption spectra of polarized infrared radiation in p-type SiGe quantum-well structures have been investigated. Three types of structures with different magnitudes and signs of biaxial elastic strains have been studied. The light absorption factor has been found to double in the region of interband transitions under the in-plane tensile strain of the quantum-well heterojunction. The results are attributed to a change in the order of light- and heavy-hole bands caused by the tensile strain and to the formation of the ground state of charge carriers in the light-hole band.