Si SiGe Heterostructures Research Articles

Design and performance analysis of Si-SiGe heterostructure based double gate feedback FET

The design and performance analysis of a Si-SiGe heterostructure-based double gate feedback field-effect transistor (HDG FBFET) are presented in this paper. The proposed HDG FBFET is capable of providing high on current (3 × 10−4 A/μm) with a large I ON /I OFF ratio (3 × 1011) and is scalable up to 20 nm channel length. Its exceptionally steep switching characteristics (SS < 1 mV/decade) and ability to switch ON/OFF at lower gate voltage due to the use of smaller band-gap material (Si1−x Ge x ) in channel-2 and drain regions make it suitable for use in low power applications. A significant hysteresis window of 4.99 V is also achieved by the device, which can be extremely helpful for memory applications. Moreover, a comprehensive investigation of the nature of hysteresis in relation to the different device parameters has also been carried out. The designing of the device structure and all of the electrical performance characterization have been done using the Sentaurus TCAD tool.

Si/Si0.7Ge0.3 A2RAM nanowires fabrication and characterization for 1T-DRAM applications

A2RAM devices are fabricated using an adaptation of Si-nanowire process flow, including a Si-SiGe heterostructure. We present the effects of gate length, film thickness, bridge implantation conditions and the presence of LDD impacts on A2RAM memory performance. Despite the accidentally undoped polysilicon gate, we succeed in demonstrating 1T-DRAM programming and operation.

High Drive Current NMOS With Si–SiGe Heterostructure Low Electric Field Channel

A drive-current enhancement in NMOS with a compressively strained SiGe structure, which had been a difficult challenge for CMOS integration with strained SiGe high-hole-mobility PMOS, was successfully achieved using a Si-SiGe heterostructure low electric field channel of optimum thickness. A 4-nm-thick Si low-field-channel NMOS with a 4-nm-thick Si/sub 0.8/Ge/sub 0.2/ layer improved drive current by 10% with a 20% reduction in gate leakage current compared with Si-control, while suppressing threshold-voltage rolloff characteristic degradation, and demonstrated excellent I/sub on/--I/sub off/ characteristics of I/sub on/ = 1 mA//spl mu/m for I/sub off/ = 100 nA//spl mu/m. These results are the best in ever reported NMOS with a compressively strained SiGe structure and indicate that a Si-SiGe heterostructure low-field-channel NMOS integrated with a compressively strained SiGe channel PMOS is a promising candidate for high-speed CMOS in 65-nm node logic technology.

Self-Consistent Subband Structure and Mobility of Two Dimensional Holes in Strained SiGe MOSFETs

The motivation to research strained SiGe layers on relaxed silicon is to enhance the very low hole mobility observed in conventional Si MOSFETs. This is mainly due to large hole effective masses and strong surface roughness scattering. In this work, the buried SiGe quantum well, chosen as the active channel for carrier transport, enhances mobility in two ways. First, the carriers are confined in the SiGe well and are removed from the Si-SiO2 interface, so interface roughness is not a big factor. Secondly, strained SiGe has a lower hole effective mass than unstrained Si and the mobility is expected to increase as the Ge concentration increases. We present the results of a self-consistent subband structure and low field mobility calculations for holes in a Si-SiGe heterostructure FET. Our results are in agreement with the experimental work by Garone and coworkers, indicating the validity of our model.

Peculiarities of the electron field emission from quantum-size structures

The electron field emission from semiconductor based layered structures has been investigated. Among studied structures were silicon tips coated with ultra-thin DLC layer, multilayer structures Si–SiO 2–Si ∗–SiO 2 with delta-doped Si ∗ layer, nanocomposite layers SiO x N y (Si) with Si nanocrystals embedded in SiO x N y matrix, GaN layers and Si–SiGe heterostructures. All of them have such peculiarities of electron field emission as peaks on emission current–voltage characteristics and corresponding Fowler–Nordheim plots. A physical model is proposed for explanation of experimental results. All emitters have layer, cluster wire or dot with quantum-size restriction in it. As a result, the quantum well with splitted electron levels exists or appears at electric field. Additional mechanism of electron emission-resonance tunneling is realized at definite electric fields.

Simulation of quantum effects along the channel of ultrascaled Si-based MOSFETs

Quantum transport simulations, including phase-breaking scattering, are used to observe the transition from classical to quantum transport in ultrascaled Si and SiGe heterostructure MOSFETs in order to gauge the potential effectiveness of semiclassical and pure phase-coherent quantum transport models as this transition is approached. It is shown that semiclassical models of transport along the length of the channel (as opposed to normal to the channel, where the importance of quantum mechanical effects has long been recognized) may remain reliable for channel lengths down to roughly 10 nm and perhaps beyond, and likely more reliable at this point than phase-coherent quantum transport simulations even when much of the transport is coherent/ballistic. As coherent transport effects within the channel eventually do become significant for ballistic carriers, the phase-breaking scattering rate, itself, also becomes a nonlocal function of the carrier's kinetic energy placing further demands on simulation. Simulations also reaffirm that for injection into the channel, the modeling of quantum transport effects such as tunneling, particularly in Si-SiGe heterostructure MOSFET's, will be important in much longer devices. However, even for this purpose it may not be possible to neglect the effects of inelastic scattering that can provide additional tunneling "paths.".

Design, fabrication, and analysis of crystalline Si-SiGe heterostructure thin-film solar cells

One possible method to improve the efficiency of crystalline silicon (Si) solar cells is by alloying with germanium (Ge). Although the improved absorption of the alloy leads to a gain in the current, the reduction of the alloy bandgap causes a loss in voltage, which overrides the increased current of the SiGe-alloy solar cell. There has been a number of theoretical studies to circumvent this behavior. However, to date there has been no detailed study, which discusses the technological implementation of these concepts in solar cells. In this paper, the design issues of crystalline Si-SiGe heterostructure will be dealt with in an attempt to reduce the effect of the increased dark current of the alloyed cells, while at the same time sustaining the enhancement in the current. The enhanced back surface field at the back p/sup +/-Si/p-SiGe interface reduces the base component of the recombination current of the heterostructure cell if recombination caused by dislocations is neglected. A higher infrared (IR) response which results in a higher short-circuit current (2 mA/cm/sup 2/ higher than a reference Si cell) has been recorded for the Si-Si/sub 0.9/Ge/sub 0.1/-thin-film structure of 15 /spl mu/m thickness. The reduction in dark saturation current which has been predicted based on the theoretical calculations could not be realized in the heterostructure SiGe/Si cell due to the degradation effect of the misfit dislocations that decreases the bulk lifetime, and increases the interface recombination velocity. In a structure which contains a p/sup +/-SiGe buffer layer, an efficiency of 12.5% is achieved for a SiGe cell with 15 /spl mu/m thickness without texturing or optical confinement, which is about the same as the Si reference cell with equal active thickness, but with a higher short-circuit current. These results, for the first time, experimentally prove that alloying with Ge offers a higher current and might have a room for improving the efficiency of the multijunction solar cells or dual bandgap cells when SiGe is used to convert the IR-part of the spectrum.

Effect of illumination on transport properties of the high mobility 2D hole gas in Si–SiGe heterostructures

The effect of illumination on transport properties of the two-dimensional hole gas (2DHG) in Si–SiGe heterostructures is found to irreversibly alter its 2D transport properties, analogous to the persistent photoconductivity effect in GaAs-based devices. The relatively small change of the 2D hole concentration from 3.75×10 11 cm −2 before illumination to 4.23×10 11 cm −2 after illumination is accompanied by a significant increase in the in-plane effective mass from (0.23–0.25) m e to (0.32–0.33) m e, and an even larger increase in the quantum lifetime. To evaluate the g-factor of this highly spin degenerate 2DHG we use highly sensitive magneto photoconductivity measurements to obtain g*=9.1±0.1 after illumination, compared to an estimate for dark state g*=8.5±0.5.

Electrical characterization of [formula omitted] quantum well wires fabricated by low damage CF4 reactive ion etching

Nearly damage free etching of a high mobility SiSiGe heterostructure is obtained by using very low power reactive ion etching and precise end-point detection. Conductance versus wire width plots of 0.08 μm to 1 μm wide SiSiGe quantum well wires show the combined nonconducting width at the edges to be 0.13 μm ± 0.01 μm, in agreement with weak localization studies. Mobility vs. sheet concentration measurements indicate little or no degradation in electron mobility after processing. Furthermore, our study demonstrates very little difference between wires with and without post-RIE (reactive-ion etching) annealing and passivation treatment. Application of this process for fabricating point contacts and other quantum effect devices is demonstrated.

Quantized conductance in a [formula omitted] split-gate device and impurity-related magnetotransport phenomena

We have defined a quantum point-contact by the split-gate technique in a Si SiGe heterostructure containing a two-dimensional electron gas with an elastic mean free path of about 1.3 μm. The conductance of this device shows typical steps very close to multiples of 4 e 2 h −1. Upon application of a perpendicular magnetic field the spin and valley degeneracies are lifted and magnetic depopulation of the one-dimensional subbands can be observed. The appearance of Aharonov-Bohm oscillations for B ≥ 2T and of resonant tunneling peaks close to “pinch-off” indicates the presence of impurities close to the constriction.

Low-temperature transport and photoconductivity response of high mobility Si-SiGe heterostructures in strong magnetic fields

Low-temperature (mK) magneto-transport and photoconductivity measurements of 2D electron and hole systems (2DES, 2DHS) in high quality n- and p-type modulation-doped Si-SiGe heterostructures (respectively) have been extended to high magnetic fields. For the high mobility 2DES in n-Si, a two valley system, signatures of the fractional quantum Hall effect (FQHE) in the region v < 1 (one valley occupied) usually observed in GaAs are replicated out to v = 2 5 ( B ∼ 40 T ) . For 1 < v < 2 however (two valleys occupied) prominent FQHE states are absent. For the 2DHS in p-SiGe, in addition to the QHE two low temperature insulating phases (IP) are identified at v = 1.5 and v < ½( B ∼ 30 T). The IP at v = 1.5 has the characteristics of a Hall insulator and our measurements indicate that the important physics is related to an unusual degeneracy of adjacent Landau levels (LL) of opposite spin. Illuminating the p-SiGe samples results in a lifting of the LL degeneracy together with a quenching of the IP at v = 1.5. Magneto-photoconductivity measurements of these samples closely resemble d ρ xx/d B and provide an additional probe of this indirect bandgap system. The incorporation of a dilution refrigerator in pulsed magnets with CuAg conductor for extended use at 60 T is described.

Tunable infrared photoemission sensor on silicon using [formula omitted] epitaxial heterostructures

The tunable infrared photoemission sensor (TIPS) consists of two different conducting materials (metal or p-type degenerately doped SiGe) separated by a thin undoped Si layer. The two conducting materials are chosen with different barrier heights on silicon (either Schottky barrier heights or formed by valence band discontinuities) such that the depleted Si forms an asymmetrical potential barrier to the carriers (both holes and electrons) photocreated in each conducting film. Under sub-band gap illumination, the photocurrent flowing between the two conducting films is therefore strongly dependent on the shape and height of the Si potential barrier that can be varied by a small bias (less than 1 V) applied between the two electrodes. In this paper, the TIPS principle of operation, the photoresponse and cut-off wavelength tunability are demonstrated using the Cr/Si/p + SiGe system in which the Si SiGe heterostructure is epitaxially grown on silicon and the top metal layer (Cr) is high vacuum evaporated. The quantum efficiency together with the I– V characteristics in the dark will also be presented.

Electron mobility enhancement in a strained Si channel

High-quality n-type modulation doped Si SiGe heterostructures are fabricated by rapid thermal chemical vapor deposition and electrically characterized by Hall effect measurements from temperature (RT) to 2 K. A new technique for the direct measurement of both the RT mobility and RT density of confined electrons in the Si channel is presented. It consists in chemically etching the doped SiGe overlayer, thus reducing its contribution to the transport properties of the structures. The consistent interpretation of the Hall data is made using the two-carrier-type Hall model and shows that a RT electron mobility of 2150 cm 2/V · s is systematically obtained in the strained Si channel. Furthermore, the conduction band discontinuity between the strained Si channel and the relaxed Si 0.67Ge 0.33 overlayer is experimentally determined ( ΔE c = 320 ± 30 me V). Finally, this study suggests that the RT mobility of Si SiGe heterostructures is only dependent on the Ge concentration, i.e. on the stress in the Si channel.

Two-dimensional hole gas in SiGe heterostructures: electrical properties and field effect applications

SiSiGe heterostructures offer realistic prospects for field effect transistors of enhanced performance and are being given serious consideration by the electronics industry. The author discusses our knowledge of the parameters influencing the mobility and drift velocity of holes in these materials which have already been demonstrated to exceed those in the silicon pMOS device.

A silicon molecular beam epitaxy system dedicated to device-oriented material research

Design, performance test, doping capability and grown material quality of a Balzers UMS 630 Si MBE system are reported, particularly concerning measures to obtain good quality of grown films. Good stability, reproducibility and uniformity of deposition rates (Si and Ge) and doping concentrations (Sb and B) have been obtained for growth on a 4 inch Si wafer with sample rotation using a mass-spectrometry controlled e-beam evaporation system, and home-made doping sources, respectively. The quality of grown undoped and modulation doped Si and SiGe layered structures were evaluated using high-resolution XRD, XTEM, SIMS, Hall, and PL measurements. Intense and sharp excitonic PL transitions and high carrier mobility obtained from the grown Si SiGe heterostructures and quantum wells grown at a wide substrate temperature range (320–650°C) indicate high crystalline quality of grown films. Finally, test HBT structures with a thin SiGe base have been made. Good dc characteristics and frequency performance were obtained.

Microscopic origin of optical nonlinearities associated with excitations between valence minibands in p-type GaAs-AlAs and Si-Si1-xGex heterostructures.

We have carried out full-scale semiempirical relativistic pseduopotential calculations of the frequency dependence---spanning the midinfrared to far infrared range of frequencies---of ${\mathrm{\ensuremath{\chi}}}^{(2)}$(-2\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega}) associated with excitations between valence minibands in p-type GaAs-AlAs and Si-SiGe heterostructures. We find that the spectrum is dominated by excitations lying further from the center of the Brillouin zone and depends on states lying above the semiclassical barrier.

Effect of ordering, interface imperfections and clusters, and external electric fields on optical spectra of Si–SiGe heterostructures

A microscopic theory of optical transitions in Si–SiGe heterostructures which offers a direct comparison of the strength of the optical transitions in ordered and disordered structures is presented herein, and suggests a new enhancement mechanism via localization at interface imperfections and Ge islands, and shows that even ultrashort period Si–Ge superlattices exhibit a strong red Stark shift in place of the expected blueshift familiar from analogous GaAs–AlAs systems.