Advances in remote plasma-enhanced chemical vapor deposition for low temperature In situ hydrogen plasma clean and Si and Si1-xGex epitaxy

Remote plasma-enhanced chemical vapor deposition (RPCVD) is a low temperature growth technique which has been successfully employed inin situ remote hydrogen plasma clean of Si(100) surfaces, silicon homoepitaxy and Si_{1- x}Ge_x heteroepitaxy in the temperature range of 150–450° C. The epitaxial process employs anex situ wet chemical clean, anin situ remote hydrogen plasma clean, followed by a remote argon plasma dissociation of silane and germane to generate the precursors for epitaxial growth. Boron doping concentrations as high as 10²¹ cm^?3 have been achieved in the low temperature epitaxial films by introducing B₂H₆/He during the growth. The growth rate of epitaxial Si can be varied from 0.4Å/min to 50Å/min by controlling therf power. The wide range of controllable growth rates makes RPCVD an excellent tool for applications ranging from superlattice structures to more conventional Si epitaxy. Auger electron spectroscopy analysis has been employed to confirm the efficacy of this remote hydrogen plasma clean in terms of removing surface contaminants. Reflection high energy electron diffraction and transmission electron microscopy have been utilized to investigate the surface structure in terms of crystallinity and defect generation. Epitaxial Si and Si_1-xGe_x films have been grown by RPCVD with defect densities below the detection limits of TEM (~10⁵ cm^-2 or less). The RPCVD process also exploits the hydrogen passivation effect at temperatures below 500° C to minimize the adsorption of C and 0 during growth. Epitaxial Si and Si_1-xGe_x films with low oxygen content (~3 × 10₁₈ cm^-3) have been achieved by RPCVD. Silicon and Si/Si_1-xGe_x mesa diodes with boron concentrations ranging from 10¹⁷ to 10¹⁹ cm^-3 in the epitaxial films grown by RPCVD show reasonably good current-voltage characteristics with ideality factors of 1.2-1.3. A Si/Si_1-xGe_x superlattice structure with sharp Ge transitions has been demonstrated by exploiting the low temperature capability of RPCVD.In situ plasma diagnostics using single and double Langmuir probes has been performed to reveal the nature of the RPCVD process.     

Nickel silicidation techniques for strained Si1?xGex, Si1?x?yGexCy, and Si1?yCy alloys material-device applications

A nickel silicide process for Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloy materials compatible with Si technology has been developed. Low-resistivity-phase (12–20 μΘ cm) nickel silicides have been obtained for these alloys with different low sheet-resistance temperature windows. The study shows that thin (15–18 nm) silicide layers with high crystalline quality, smooth silicide surface, and smooth interface between silicide and the underlying material are achievable. The technique could be used to combine the benefits of Ni silicide and Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloys. The technique is promising for Si or Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloy-based metal-oxide semiconductor, field-effect transistors (MOSFETs) or other device applications.     

Charge collection microscopy of annealing induced electrically active defects in Si1−xGex/Si strained layer epitaxy

Images of electrically active defects in Si_1−xGe_x strained epilayers onn-type Si(100) have been obtained using charge collection microscopy (CCM) in a scanning electron microscope. Electrically active defects were generated in the defect-free as-grown material by rapid thermal annealing treatments (3 min) over a temperature range of 550 to 850° C. Point-like contrast, attributed to threading dislocations, as well as an additional line contrast due to the formation of an interface misfit dislocation network, were observed as the metastable strained Si_1−xGr_x layers relaxed upon annealing. Cross-sectional and plan-view transmission electron microscopy were used to examine the Si_1−xGe_x epilayers in finer detail. Double crystal x-ray diffraction and Raman scattering spectra were also obtained and the annealing induced peak shifts investigated.     

Elastic strain and enhanced light emission in dry etched Si/Si1-xGex quantum dots

Quantum dots of 50 ~ 60 nm diameter fabricated from both Si/Si_1-xGe_x (x = 0.1 ~ 0.3) strained layer superlattices and a strain symmetried Si₉/Ge₆ superlattice were investigated by a combination of Raman scattering, photoluminescence, and electroluminescence spectroscopy. It was found that, in addition to an enhanced luminescence intensity of the dots by over two orders of magnitude and improved luminescence quenching temperature, all of the nanostructure dots have residual built-in elastic strains, which are of the order of ~50% of the values in corresponding pseudomorphic heterostructures. This result suggests a possible mechanism for explaining the huge enhancement of the optical efficiency in our luminescence measurements.     

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.     

Electrical properties of Si1−x−yGexCy and Ge1−yCy alloys

The effects of carbon on the structural and electrical properties of Si_1−x−yGe_xC_y and Ge_1−yC_y alloys grown by the molecular beam epitaxy have been examined by Hall effect measurements, current-voltage measurements, x-ray diffraction, and atomic force microscopy. Hall effect measurements showed that the addition of carbon increased the hole mobility in GeC compared to pure Ge, we attributed this increase to improved crystalline quality and reduced surface roughness. Current-voltage characteristics of SiGeC/Si and GeC/Si heterojunction diodes showed that with increasing carbon, the reverse leakage current decreased and the forward turn-on voltage increased, attributed to the increase in bandgap energy and reduction of intrinsic carrier concentration n_i.     

Schottky barrier heights on IV-IV compound semiconductors

The variations of Schottky barrier heights on Si_1-x-y Ge_xC_y films with composition and strain have been investigated and compared to those expected for the band gap energy. The barrier on n-type does not depend on composition and strain. This independence suggests that the Fermi level at the interface between tungsten and Si_1-x-yGe_xC_y alloys (x≠0) is pinned relative to the conduction-band. For Si_1-xGe_x the barrier on p-type follows the same trends as the band gap. For the ternary alloys, the variations of the barrier on p-type seem to be too large to be only due to a variation of the band-gap. In addition, we have investigated the influence of the deposition conditions of the sputtered-W-gate on the barrier to silicon and Si_1-xGe_x. Our results show that the barrier on n-type-Si and p-type-Si_1-xGe_x-films increases when the stress retained in the W-films changes from compressive to tensile as the deposition pressure increases. The absence of change in the barrier height of W to p-type-silicon and n-type-Sij xGex-films suggests that the Fermi level at the interface with Si is pinned relative to the valence-band while it is pinned relative to the conduction when Ge is added.     

Photoluminescence spectra of Si1-xGex/Si quantum well structures grown by three different techniques

Photoluminescence (PL) spectra of Si_1-xGe_x/Si multiple quantum wells have been measured at 4.2 K for the samples grown by three different techniques; conventional molecular beam epitaxy (MBE), gas-source MBE, and ultra high vacuum chemical vapor deposition (UHV-CVD). Only in the case of conventional MBE, strong emission bands appear about 80 meV below the band gap of Si_1-xGe_x. These strong emission bands disappear after the annealing at 800° C. From the dependence of the PL intensity on the excitation power, strong emission is considered to be due to some recombination center. On the other hand, in the case of gas-source MBE and UHV-CVD, the strong emission bands are undetectable, although the band-edge PL lines of Si_1-xGe_x are clearly observed. There is no significant change in the PL spectra after the annealing. The origin of the strong emission band is considered to be defects which are characteristic of conventional MBE.     

Influence of device structure and growth conditions on the tunneling characteristics of Si/Si1−xGex double barrier structures

We have investigated different Si/Si_1−xGe_x hole resonant tunneling structures. We demonstrate the advantages of grading the Ge concentration in the spacer layers, which allows for a smoother potential profile in the spacer layers and a higher Ge concentration in the well, and hence higher bandoffsets. This leads to an improvement of the resonances seen in the I–V characteristics of these devices. Structures grown at different temperatures emphasize the importance of obtaining abrupt Si/Si_1−xGe_x double barrier heterointerfaces in order to obtain good I–V characteristics. Short-term post annealing at ⋟500°C, well below temperatures where strain relaxation or dopant diffusion into the barrier layers occur, is shown to destroy the resonances. We believe this is due to monolayer interdiffusion at the barriers, destroying the abruptness of the interfaces.     

Characterization of germanium implanted Si1−xGex layer

Characterization of a Si_1−xGe_x layer formed by high-dose germanium implantation and subsequent solid phase epitaxy is reported. Properties of this layer are obtained from electrical measurements on diodes and transistors fabricated in this layer. Results are compared with those of the silicon control devices. It was observed that the germanium implantation created considerable defects that are difficult to eliminate with annealing. These defects result in boron deactivation in the p-type regions of the devices, giving rise to larger resistance. Optimization of the device structure and fabrication process is discussed.     

Fabrication of high-Ge-fraction strained Si1−xGex/Si hole resonant tunneling diode using low-temperature Si2H6 reaction for nanometer-order ultrathin Si barriers

Low-temperature Si barrier growth with atomically flat heterointerfaces was investigated in order to improve negative differential conductance (NDC) characteristics of high-Ge-fraction strained Si_1−xGe_x/Si hole resonant tunneling diode with nanometer-order thick strained Si_1−xGe_x and unstrained Si layers. Especially to suppress the roughness generation at heterointerfaces for higher Ge fraction, Si barriers were deposited using Si₂H₆ reaction at a lower temperature of 400 °C instead of SiH₄ reaction at 500 °C after the Si_0.42Ge_0.58 growth. NDC characteristics show that difference between peak and valley currents is effectively enhanced at 11-295 K by using Si₂H₆ at 400 °C, compared with that using SiH₄ at 500 °C. Non-thermal leakage current at lower temperatures below 100 K tends to increase with decrease of Si barrier thickness. Additionally, thermionic-emission dominant characteristics at higher temperatures above 100 K suggests a possibility that introduction of larger barrier height (i.e. larger band discontinuity) enhances the NDC at room temperature by suppression of thermionic-emission current.     

Control of implant-damage-enhanced boron diffusion in epitaxially grown n-Si/p-Si1-xGex/n-Si heterojunction bipolar transistors

Boron out-diffusion in epitaxially grown n-Si/p⁺-Si_1-xGe_x/n-Si heterojunction bipolar transistors is significantly enhanced during 850°C, 10 s rapid thermal annealing following arsenic emitter contact implantation. In this paper, we introduce three techniques which dramatically reduce boron out-diffusion during implant activation. Limiting the post-implant processing to 600°C for 2 min results in minimal diffusion giving acceptable device performance. A second technique involves pulsed laser annealing of the As implant, which removes residual defects and eliminates enhanced diffusion during subsequent thermal processing. Finally, we show that high bulk concentrations of oxygen in the Si_1-xGe_x (∼10²⁰ cm^-3) dramatically reduce the implant-damage-enhanced boron diffusion. In addition to the depth profiles, electrical measurements performed on heterojunction bipolar transistors, incorporating these fabrication techniques, show ideal collector current characteristics and confirm the absence of deleterious boron out-diffusion effects.     

Photoluminescence spectroscopy of localized excitons in Si1−xGex

We have recently found that high quantum efficiency can be achieved in strained Si_1−xGe_x alloy layers through the elimination of nonradiative channels. We observed a photoluminescence process in SiGe grown on 〈100232A; silicon by rapid thermal chemical vapor deposition, which was attributed to free excitons localized by random fluctuations in alloy composition. The external quantum efficiency of this process was measured directly for a single Si_0.75Ge_0.25 quantum well and found to be extraordinarily high, about 11.5 ± 2%. In this paper, we present additional data on the localized exciton photoluminescence, including temperature dependence, time decay curves, and effects of sample annealing.     

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.     

Damage and strain in pseudomorphic vs relaxed GexSi1−x layers on Si(100) generated by Si ion irradiation

We compare both the strain and damage that 100 keV Si irradiation at room temperature introduces in pseudomorphic and relaxed Ge_xSi_1−x films grown on Si(100) substrates. The ion range is such that the Si/Ge_xSi_1−x interface is not significantly damaged. The amount of damage produced in pseudomorphic and relaxed Ge_xSi_1−x layers of similar x for irradiation doses up to 2.5 × 10¹⁴ Si/cm² is the same, which proves that a pre-existing uniform strain does not noticeably affect the irradiation-induced damage. However, the irradiation-induced strain does depend on the pre-existing strain of the samples. Possible interpretations are discussed. On leave from Inst. voor Kern en Stralingsfysika, Catholic University of Leuven, Belgium.     

(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。     

High-resolution transmission electron microscopy of silicide formation and morphology development of Ni/Si and Ni/Si<Subscript>1−x</Subscript>Ge<Subscript>x</Subscript>

Nickel-silicide phase formation in the Ni/Si and Ni/Si_1−xGe_x (x=0.20) systems and its correlation with variations in sheet resistance have been studied using high-resolution transmission electron microscopy (HRTEM) and related techniques. Following a 500°C anneal, uniform and low-resistivity NiSi and NiSi_1−xGe_x (x<0.20) crystalline films were formed in the respective systems. Annealed at 900°C, NiSi₂, in the form of pyramidal or trapezoidal islands, is found to replace the NiSi in the Ni/Si system. After a 700°C anneal, threading dislocations were observed for the first time in the Ni/Si_1−xGe_x system to serve as heterogeneous nucleation sites for rapid lateral NiSi_1−xGe_x growth.     

Silicon molecular beam epitaxy: Highlights of recent work

This paper describes highlights of recent work in Si MBE directed at device application of the technique. Topics include studies of doping by solid sources and co-ion-implantation, reduction of particle and metal induced defect levels, gas source MBE and recent results on high speed Ge _x Si_1-x heterojunction bipolar transistors.     

Interfacial reactions and electrical properties of Hf/p-Si0.85Ge0.15

Interfacial reactions and electrical properties of Hf/p-Si_0.85Ge_0.15 as a function of the annealing temperature were studied. Hf₃(Si_1−xGe)₂ and Hf(Si_1−xGe)₂ were initially formed at 500°C and 600°C, respectively. At temperatures above 400°C, Ge segregation out of the reacted layers associated with strain relaxation of the unreacted Si_0.85Ge_0.15 films appeared. At 780°C, agglomeration occurred in the Hf(Si_1−xGe_x)₂ films. All the as-deposited and annealed Hf/p-Si_0.85Ge_0.15 samples showed the formation of an ohmic contact. The lowest specific contact resistance around 10⁻⁵ ω cm² could be obtained for the Hf₃ (Si_1−xGe_x)₂ contacts to p-Si_0.85Ge_0.15 formed at 500°C. Below 500°C, the decrease of specific contact resistance with the annealing temperature is mainly caused by the formation of Hf₃(Si_1−xGe_x)₂ and an interfacial Ge-rich layer between the Hf₃(Si_1−xGe_x)₂ and unreacted Si_0.85Ge_0.15 films, while above 600°C, the increase of specific contact resistance may be due to the formation of Hf(Si_1−xGe_x)₂ and SiC as well as the roughness of the Hf(Si_1−xGe_x)₂ films.     

Si/Si1−x Gex epitaxial layers and superlattices. Growth and structural characteristics

Si, Ge, and Si_1−x Ge_x epitaxial layers and Si/Si_1−x Ge_x superlattices have been obtained on (100) and (111) silicon substrates by molecular-beam epitaxy. The growth processes and the structural characteristics and chemical composition of the structures were studied by x-ray diffraction and Auger spectroscopy. It is shown that under the experimental conditions for obtaining Si/Si_1−x Ge_x superlattices structurally perfect, strained superlattices with satellites up to ±5 orders can be obtained. Fiz. Tekh. Poluprovodn. 31, 922–925 (August 1997)