Chemical vapor deposition of chalcogenide materials for phase-change memories

Films of chalcogenide Ge-Sb-Te materials were grown by pulsed liquid injection chemical vapor deposition (CVD) technique. Simple thermal CVD without additional process activation and CVD with remote activation of precursor decomposition process by a hot-wire were investigated and compared. Ge(NMe₂)₄, Sb(NMe₂)₃ and Te(i-Pr)₂ precursors in a form of diluted solutions in toluene were used for depositions. Film composition was controlled by injection parameters, while the thickness was directly related with number of pulses. Hot-wire activated CVD process allows the growth of chalcogenide films of clearly better quality compared to films grown by standard thermal CVD. Uniform, smooth, crystalline Ge₂Sb₂Te₅ films were grown at substrate/wire temperature 300 °C/550 °C and pressure ? 15 Torr, using nitrogen as a carrier gas, on Si, Si/SiO₂, Si/Si₃N₄ and glass substrates. Forty to forty five nanometer thick films on Si/SiO₂ substrates showed reversible electrical and optical phase switching behavior.     

Real-time stress evolution during Si1-xGex Heteroepitaxy: Dislocations, islanding, and segregation

We have used sensitive real-time measurements of film stress during Si_1-xGe_x molecular beam epitaxy to examine strain relaxation due to coherent island formation, and to probe the kinetics of Ge surface segregation. We first describe our novel curvature-measurement technique for real-time stress determination. Measurements of the relaxation kinetics during high temperature Si₇₉Ge₂₁ growth on Si (001) are reported in which formation of highly regular arrays of [501]-faceted islands produce 20% stress relaxation. An island shape transition is also observed that reduces the effective stress by up to 50% without dislocations. Nonuniform composition profiles due to Ge surface segregation during growth of planar alloy films are determined with submonolayer thickness resolution from the real-time stress evolution. Up to two monolayers of Ge can segregate to the growth surface.     

Electronic properties of Si/Si-Ge Alloy/Si(100) heterostructures formed by ECR Ar plasma CVD without substrate heating

By using our low-energy Ar plasma enhanced chemical vapor deposition (CVD) at a substrate temperature below 100 °C during plasma exposure without substrate heating, modulation of valence band structures and infrared photoluminescence can be observed by change of strain in a Si/strained Si_0.4Ge_0.6/Si(100) heterostructure. For the strained Si_0.5Ge_0.5 film, Hall mobility at room temperature was confirmed to be as high as 660 cm² V⁻¹ s⁻¹ with a carrier concentration of 1.3×10¹⁸ cm⁻³ for n-type carrier, although the carrier origin was unclear. Moreover, good rectifying characteristics were obtained for a p⁺Si/nSi_0.5Ge_0.5 heterojunction diode. This indicates that the strained Si-Ge alloy and Si films and their heterostructures epitaxially grown by our low-energy Ar plasma enhanced CVD without substrate heating can be applicable effectively for various semiconductor devices utilizing high carrier mobility, built-in potential by doping and band engineering.     

Low temperature heteroepitaxial growth of Si1−xGexon-Si by photo-enhanced ultra high vacuum chemical vapor deposition using Si2H6 and Ge2H6

Strained-layer Si_1×Ge_x-on-Si heteroepitaxy has been achieved by photolytic decomposition of disilane (Si₂H₆) and digermane (G e₂H₆) in an ultra high vacuum (UHV) chamber at substrate temperatures as low as 275°C. An ArF excimer laser (193 nm) shining parallel to the Si substrate was used as the UV light source to avoid surface damage and substrate heating. The partial pressures of the source gases in the reactor were chosen to vary the Ge mole fraction x from 0.06 to 0.5 in the alloy. The Si₂H₆ partial pressure was kept at 10 mTorr and the Ge₂H₆ partial pressure was varied from 0.13 to 2 mTorr with the laser intensity fixed at 2.75 × 10¹⁵ photons/cm²·pulse. To fit the Si_1−xGe_x growth rate and Ge mole fraction data, the absorption cross section of Ge₂H₆ at 193 nm was set to 1 × 10⁻¹⁶ cm², which is 30 times larger than that of Si₂H₆ (3.4 × 10⁻¹⁸ cm2). For Si_1−xGe_x alloy growth, the deposition rate of Si increases with Ge mole fraction, resulting in increased Si_1−xGe_x alloy growth rates for higher Ge content. The increase of the Si growth rate was attributed to the enhanced adsorption rate of Si₂H₆ pyrolytically in the presence of Ge, rather than due to photolytic decomposition reaction. The Ge mole fraction in Si_1−xGe_x alloys can be predicted by a new model for Si and Ge pyrolytic and photolytic growth. The model describes the increased growth rate of Si_1−xGe_x alloys due to a Ge₂H₆ catalytic effect during photo-enhanced chemical vapor deposition.     

Characterization of reduced pressure chemical vapor deposited Si0.8Ge0.2/Si multi-layers

We have investigated the Si_0.8Ge_0.2/Si multi-layer grown directly onto the Si (001) substrates using reduced pressure chemical vapor deposition. The thicknesses of the Si_0.8Ge_0.2/Si multi-layer were determined using transmission electron microscopy. From the results of energy-dispersive X-ray spectroscopy and X-ray diffraction analyses on the Si_0.8Ge_0.2/Si multi-layer, Ge composition in the Si_1?xGe_x layers was determined as ～20% and the value of residual strain ε of the Si_0.8Ge_0.2 layer is calculated to be 0.012. Three peaks are observed in Raman spectrum, which are located at approximately 514, 404, and 303 cm^?1, corresponding to the vibration of Si–Si, Si–Ge, and Ge–Ge phonons, respectively. The photoluminescence spectrum originates from the radiative recombinations both from the Si substrate and the Si_0.8Ge_0.2/Si multi-layer. For the Si_0.8Ge_0.2/Si multi-layer, the transition peaks related to the quantum well region observed in the photocurrent spectrum were preliminarily assigned to e–hh and e–lh fundamental excitonic transitions.     

Structural and electrical properties of SiGe-on-insulator substrates fabricated by direct bonding

A new method for fabricating SiGe-on-insulator substrates, i.e., direct bonding of thermally oxidized Si wafers with Si_{1 − x} Ge _x wafers cut from Czochralski-grown crystals, is suggested. Si_{1 − x} Ge _x layers no larger than 10 μm thick in SiGe/SiO₂/Si compositions were fabricated by chemical mechanical polishing. To increase the Ge content in the Si_{1 − x} Ge _x layer, thermal oxidation was used. It was shown that the increase in the Ge content and heat treatment procedures at 1250°C are not accompanied by degradation of structural and electrical characteristics of Si_{1 − x} Ge _x layers.     

Compliant substrates: A comparative study of the relaxation mechanisms of strained films bonded to high and low viscosity oxides

Relaxation of compressively strained heteroepitaxial Si_0.7Ge_0.3 films bonded to high and low viscosity glass compliant layers was investigated. These structures were formed by transferring Si_0.7Ge_0.3 films to Si substrates covered with thermal SiO₂ and borophosphorosilicate glass (BPSG) films. Relaxation was studied through thermal annealing experiments. For the low viscosity BPSG, relaxation was observed near 800°C and was accompanied by buckling of the Si_0.7Ge_0.3 film. At this temperature, no change in the Si_0.7Ge_0.3 film bonded to thermal SiO₂ was observed, and through this comparison relaxation on BPSG is interpreted as the result of viscous flow of the glass. Finally, film buckling was successfully avoided by patterning the strained films into small areas prior to annealing, and is an indication that film expansion must be considered for elastic strain relaxation on compliant media.     

Low temperature epitaxial growth of Si0.5Ge0.5 alloy layer on Si (100) by ion beam assisted deposition

The first results were reported on low temperature epitaxial growth of Si_0.5Ge_0.5 alloy layer on Si (100) by ion beam assisted deposition. Nucleation and the growth of Si_0.5Ge_0.5 alloy layer had been investigated by atomic force microscopy and reflection high energy electron diffraction analysis. The Si_0.5Ge_0.5 alloy layer nucleated on Si (100) via Stranski-Krastanov (SK) mode. The Ar ion bombard-ment improved crystallinity and prolonged layer-by-layer stage of the SK mode. The epitaxial temperature was 200°C lower than 550-600°C in molecular beam epitaxy. In order to explain the mechanism of low temperature epitaxial growth E_Ar (energy transferred to growing film by bombarding Ar ion, eV/atom) value was experimentally calculated. In conclusion, the ion bombardment induced dissociation of three-dimensional islands and enhanced the surface diffusion. The variation of tetragonal strain and its effect on electron mobility were taken into consideration. Electron mobility increased with tetragonal strain as a result of band split.     

Ultra-Thin Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Dislocation Blocking Layers for Ge/Strained Si CMOS Devices

We demonstrate ultra-thin (<150 nm) Si_1−x Ge _x dislocation blocking layers on Si substrates used for the fabrication of tensile-strained Si N channel metal oxide semiconductor (NMOS) and Ge P channel metal oxide semiconductor (PMOS) devices. These layers were grown using ultra high vacuum chemical vapor deposition (UHVCVD). The Ge mole fraction was varied in rapid, but distinct steps during the epitaxial layer growth. This results in several Si_1−x Ge _x interfaces in the epitaxially grown material with significant strain fields at these interfaces. The strain fields enable a dislocation blocking mechanism at the Si_1−x Ge _x interfaces on which we were able to deposit very smooth, atomically flat, tensile-strained Si and relaxed Ge layers for the fabrication of high mobility N and P channel metal oxide semiconductor (MOS) devices, respectively. Both N and P channel metal oxide semiconductor field effect transister (MOSFETs) were successfully fabricated using high-k dielectric and metal gates on these layers, demonstrating that this technique of using ultra-thin dislocation blocking layers might be ideal for incorporating high mobility channel materials in a conventional CMOS process.     

H2对UHV/CVD低温选择性外延生长Si1-xGex的影响

利用超高真空化学气相沉积(UHV/CVD)成功实现了Si_1-xGe_x的低温选择性外延生长,并研究了H2对选择性外延生长的影响及其作用机理. 以SiH4和GeH4为反应气源,在开有6mm×6mm窗口氧化硅片上进行Si_1-xGe_x外延层的生长.首先分别以不含H2(纯GeH4)和含H2(90% H2稀释的GeH4)的两种Ge源进行选择性外延生长.通过SEM观察两种情况下氧化硅片表面,发现H2的存在对选择性外延生长有至关重要的作用.接着以90% H2稀释的GeH4为Ge源,变化Si源和Ge源的流量比改变H2分压,以获得SiH4和GeH4 (90% H2）的最佳流量比，使外延生长的选择性达到最好. 利用SEM观察在不同流量比时，经40min外延生长后各样品的表面形貌，并对其进行比较，分析了H2分压在Si_1-xGe_x选择性外延生长中的作用机理.     

Role of a Si0.95Ge0.05 epilayer cap on boron diffusion in silicon under inert and dry oxidizing ambient annealing

Silicon (Si) and Si with a 60 nm Si_0.95Ge_0.05 epilayer cap (Si_0.95Ge_0.05/Si) were implanted with 60 keV, 1×10¹³ cm⁻² boron (B) followed by annealing in nitrogen (N₂) or dry oxygen (O₂) in two different anneal conditions. B⁺implantation energy and dose were set such that the B peak is placed inside Si in Si_0.95Ge_0.05/Si samples and concentration independent B diffusion is achieved upon annealing. For samples annealed above 1075 °C, Ge diffusing from the Si_0.95Ge_0.05 epilayer cap in Si_0.95Ge_0.05/Si samples reached the B layer inside Si and resulted in retarded B diffusion compared to the Si samples. For annealing done at lower temperatures, diffusion of Ge from Si_0.95Ge_0.05 epilayer cap does not reach the B layer inside Si. Thus B diffusion profiles in the Si and Si_0.95Ge_0.05/Si samples appear to be similar. B diffusion in dry oxidizing ambient annealing of Si_0.95Ge_0.05/Si samples further depends on the nature of Si_0.95Ge_0.05 oxidation which is set by the duration and the thermal budget of the oxidizing anneal.     

Growth and electrical properties of the (Si/Ge)-on-insulator structures formed by ion implantation and subsequent hydrogen-assisted transfer

Systematic features of endotaxial growth of intermediate germanium layers at the bonding interface in the silicon-on-insulator structure consisting of buried SiO₂ layer implanted with Ge⁺ ions are studied in relation to the annealing temperature. On the basis of the results for high-resolution electron microscopy and thermodynamic analysis of the Si/Ge/SiO₂ system it is assumed that the endotaxial growth of the Ge layer occurs via formation of a melt due to enhanced segregation and accumulation of Ge at the Si/SiO₂ interface. Effect of germanium at the bonding interface on the Hall mobility of holes in silicon layers with nanometer-scale thickness is studied. It is found that the structures including the top silicon layer with the thickness 3–20 nm and incorporating germanium feature the hole mobility that exceeds by a factor of 2–3 the hole mobility in corresponding Ge-free silicon-on-insulator structures.     

Fully strained Si0.75Ge0.25 epitaxial films with HfSiO gate dielectrics for high mobility channel metal-oxide semiconductor devices

The authors report on fully strained Si_0.75Ge_0.25 metal-oxide-semiconductor capacitors with HfSiO₂ high-k gate dielectric and TaN metal gate fabricated on Si substrates. Fully strained Si_0.75Ge_0.25 films are directly grown on Si substrates below the critical thickness. HfSiO₂ high-k gate dielectrics exhibit an equivalent oxide thickness of 13-18 Å with a permittivity of 17.7 and gate leakage current density lower than SiO₂ gate oxides by >100×. Interfacial oxide of the HfSiO₂/Si_0.75Ge_0.25 stack consists primarily of SiO₂ with a small amount of Ge and Hf. High performance SiGe field effect transistors are highly manufacturable with excellent electrical characteristics afforded by the fully strained HfSiO₂/SiGe gate stack.     

A GeSi-buffer structure for growth of high-quality GaAs epitaxial layers on a Si substrate

A SiGe-buffer structure for growth of high-quality GaAs layers on a Si (100) substrate is proposed. For the growth of this SiGe-buffer structure, a 0.8-μm Si_0.1 Ge_0.9 layer was first grown. Because of the large mismatch between this layer and the Si substrate, many dislocations formed near the interface and in the low part of the Si_0.1Ge_0.9 layer. A 0.8-μm Si_0.05Ge_0.95 layer and a 1-μm top Ge layer were subsequently grown. The strained Si_0.05Ge_0.95/Si_0.1Ge_0.9 and Ge/Si_0.05Ge_0.95 interfaces formed can bend and terminate the upward-propagated dislocations very effectively. An in-situ annealing process is also performed for each individual layer. Finally, a 1–3-μm GaAs film was grown by metal-organic chemical vapor deposition (MOCVD) at 600°C. The experimental results show that the dislocation density in the top Ge and GaAs layers can be greatly reduced, and the surface was kept very smooth after growth, while the total thickness of the structure was only 5.1 μm (2.6-μm SiGe-buffer structure +2.5-μm GaAs layer).     

Epitaxial Ge layers on Si via GexSi1-xO2 reduction: The roles of the hydrogen partial pressure and the Ge content

The epitaxial growth of an epi-Ge layer via Ge_xSi_1-xO₂ reduction in hydrogen annealing is reported. Ge_xSi+_1-x alloys with x = 0.52 and 0.82 were first grown epitaxially on Si substrates. They were then oxidized in a wet ambient and subsequently annealed in 5% or 100% H₂. The reduction of Ge from its oxide state is observed in both samples with both ambients. However, an epitaxial Ge growth is only observed in the sample with x = 0.82 after the 5% H₂ annealing. The other three cases result in the formation of polycrystalline Ge. The roles of the hydrogen partial pressure and the Ge content are discussed and conditions under which this novel mode of solid-phase epitaxy can occur are explained.     

Strain relaxation and dopant distribution in the rapid thermal annealing of Co with Si/Si1−xGex/Si heterostructure

The reaction of cobalt with the Si-sacrificial cap in the strained Si/Si_1−xGe_x/Si MBE grown heterostructure was studied. The Si-cap is added to prevent the relaxation of the SiGe and to guarantee uniform and reliable silicidation reaction. The Si_1−xGe_x epilayer, with Ge content between 18 and 28 at%, was highly B doped, while the Si-cap was undoped or B doped either during growth or by ion implantation. Cobalt evaporation was followed by rapid thermal annealing at 450–700°C for 30 sec in N₂ or Ar+10%H₂. When the silicide penetrated the Si-cap/Si_1−xGe_x interface, noticeable out-diffusion of Ge and B to the surface was observed. In spite of the presence of the Si-cap significant strain relaxation was observed in three cases: (1) in the implanted samples, although the implantation was confined to the Si-cap, (2) when the Co layer was too thick, such that the silicide penetrated the SiGe layer and (3) when the Ge content in the SiGe layer was relatively high (27.5%).     

Fabrication of Strained-Si/Strained-Ge Heterostructures on Insulator

Ultrathin strained-Si/strained-Ge heterostructures on insulator have been fabricated using a bond and etch-back technique. The substrate consists of a trilayer of 9 nm strained-Si/4 nm strained-Ge/3 nm strained-Si on a 400-nm-thick buried oxide. The epitaxial trilayer structure was originally grown pseudomorphic to a relaxed Si_0.5Ge_0.5 layer on a donor substrate. Raman analysis of the as-grown and final transferred layer structures indicates that there is little change in the strain in the Si and Ge layers after layer transfer. These ultrathin Si and Ge films have very high levels of strain (∼1.8% biaxial tension and 1.4% compression, respectively), and are suitable for enhanced-mobility field-effect transistor applications.     

Effect of composition on deep levels in heteroepitaxial GexSi1−x layers and evidence for dominant intrinsic recombination-Generation in relaxed ge layers on Si

Electron traps, hole traps, and the dominant recombination-generation (R-G) centers have been investigated with deep level transient spectroscopy and current-voltage/temperature measurements in heteroepitaxial Ge_xSi_1-x alloys with x ranging from 0.15 to 1, grown on graded Ge_y.Si_1−y/Si substrates. For all samples with compositions x < 0.85, which retain the Si-like conduction band structure, we detect a dominant electron trap and R-G center whose activation energy is ΔE = 0.5 eV, independent of composition. This energy agrees with that of electron traps previously reported for plastically deformed (PD) Si, suggesting a connection to the Si-like band structure. This 0.5 eV level dominates the reverse leakage current over a wide range of growth and annealing conditions for the 30% Ge samples, indicating that the electronic state at ΔE = 0.5 eV is a very efficient R-G center, as would be expected from its midgap position. Alternatively, for strain relaxed, pure Ge (< 1), we detect electron traps at E_c − 0.42 eV and E_c − 0.28 eV, in agreement with the literature on PD Ge and Ge bicrystals. These energies are significantly different from those observed for x < 0.85, and we conclude that these changes in activation energy are due to changes in the conduction band structure for high Ge content. Moreover, in contrast with the Si-like samples (x < 0.85), the reverse leakage current in the relaxed Ge cap layer is not controlled by deep levels, but is rather dictated by intrinsic, band-to-band generation due to the reduced bandgap of Ge as compared to Si-like alloys. Only for reverse bias magnitudes which incorporate a significant portion of the graded buffer within the depletion region do R-G centers dominate the reverse leakage current. These results confirm the high quality of the strain-relaxed, pure Ge cap region which was grown on a Ge_ySi_1−y/Si step graded heterostructure (where y was increased from 0 to 1) by ultra high vacuum chemical vapor deposition. Finally, we report for the first time, what is apparently the dislocation kink site state at E_c − 0.37 eV, in a Ge_xSi_1−x alloy.     

Studies of interdiffusion in GemSin strained layer superlattices

We present the results on the characterization and interdiffusion behavior of Ge _m Si _n strained layer superlattices (SLS’s) composed of alternating monolayers of pure Ge and pure Si. Such Ge _m Si _n SLS’s were grown on top of thick relaxed Ge _y Si_1-y buffer layers so as to symmetrize the strain distribution and to maintain the pseudomorphic growth of the superlattices. Samples with different superlattice periodicities (i.e. d = d_Ge + d_Si and different layer thickness ratios (i.e. d_Ge:d_Si were prepared for comparison. Raman scattering spectroscopy and x-ray diffraction were used to characterize these samples. Initial results on thermal stability of these Ge _m Si _n SLS’s are also reported     

(001)、(101)、(111) 双轴应变Si、应变SiGe空穴散射机制

本文基于费米黄金法则和波尔兹曼碰撞项近似理论,对Si基应变材料各空穴散射机率与应力强度、晶向的关系进行了深入的研究。结果表明：1）在应力的作用下,Si基应变材料总散射几率明显降低;2）当Ge组分为0.2时,总散射几率量化排序为应变Si/(111)Si1-xGex>应变Si/(101)Si1-xGex>应变Si1-xGex/(111)Si>应变Si1-xGex/(101)Si>应变Si/(001)Si1-xGex>应变Si1-xGex/(001)Si;3）应力作用下空穴声学声子散射几率的降低是引起Si基应变材料总散射几率降低的主要原因。本文量化结论可为Si基应变及其他应变材料的相关研究提供重要理论参考。