Study of Arsenic ion implantation of patterned strained Si NWs

A systematic study of the impact of As⁺ ion implantation on strain relaxation and dopant activation of biaxially strained SSOI layers and uniaxially strained/unstrained NWs is presented. Three aspects are investigated: (i) the quality of the single crystalline layers and the NWs, (ii) strain relaxation of the implanted NWs and (iii) dopant activation of the layers and NWs. Optimization of the doping conditions resulted into very low contact resistivities of NiSi contacts on strained and unstrained 70 nm SOI layers and Si NWs. For NW contacts values as low as 1.2 × 10⁻⁸ Ω cm² for an As⁺ dose of 2 × 10¹⁵ cm⁻² were achieved, which is 20 times lower than for planar contacts made under the same implantation and annealing conditions.     

Thermal stability of Si/Gen/Si heterostructures by photoreflectance

The thermal stability of Si/Ge_n/Si(001) heterostructures includingn = 1, 6, 20, and 100 monolayers (ML’s) is studied in connection with their electronic structures through the measurement of photoreflectance (PR). The PR spectra are observed at 90 K over the energy range 0.85–4.0 eV. Comparing the PR signals of Si/Ge _n /Si(001) heterostructures before and after thermal annealing at 600° C, it is found that the samples with less than 6 ML Ge show no change whereas those with more than 20 ML Ge show large changes. The result suggests that Si/Ge _n /Si heterostructures with Ge layer thickness less than 6 ML’s are thermally stable. For the heterostructures with 20 and 100 ML Ge, the relaxation of strain in the Ge layer is found to occur from the PR spectra ofE ₀(Ge),E ₁(Ge) andE ₁ +Δ ₁(Ge), andE ₁(Si).     

Si(100)和Si(111)衬底上的同质分子束外延

用超高真空电子束蒸发系统进行了硅的同质分子束外延.发现采用适当的表面化学处理方法,然后在超高真空中加热,可以在较低温度下(800—814℃)获得清洁和平整的有序表面.Si(100)和Si(111)的外延分别在520℃和714℃进行,外延膜的结构和电学特性良好.     

Strain relaxation in silicon ion molecular beam epitaxy on silicon (001) substrates during aging

Silicon strained epitaxial films were grown on Si (001) substrates by low energy ion beam assisted molecular beam epitaxy. Films grown in the range of 450– 550°C with concurrent Ar⁺ ion bombardment (100 eV) were characterized using x-ray diffraction and transmission electron microscopy and found to be disloca-tion free and ununiformly strained. During aging, the strained layers stay stable until 500°C. Relaxation of most of the strain occurred at temperatures of 500-650°C. At higher aging temperatures, the strained layers relaxed by the formation of dense dislocation structures.     

Si_(1-x)Ge_x/Si多层异质外延结构的研究

对制作的 Ｓｉ１－ｘＧｅｘ／Ｓｉ多层异质外延结构进行了研究。并对其做了反射高能电子衍射（ＲＨＥＥＤ）、Ｘ射线衍射（ＸＲＤ）和扩展电阻（ＳＲ）等测量,给出了利用这种结构研制出的异质结双极晶体管（ＨＢＴ）的输出特性曲线。     

Si1-xGex/Si应变材料的生长及热稳定性研究

利用分子束外延（MBE）技术生长了Ge组份为0.1-0.46的Si1-xGex外延层。X射线衍射线测试表明，SiGe/Si异质结材料具有良好的结晶质量和陡峭界面，其它参数与可准确控制。通过X射线双晶衍射摆曲线方法，研究了经700℃、800℃和900℃退火后应变SiGe/Si异质结材料的热稳定性。结果表明，随着退火温度的提高，应变层垂直应变逐渐减小，并发生了应变弛豫，导致晶体质量退化；且Ge组分越小，Si1-xGex应变结构的热稳定性越好；室温下长时间存放的应变材料性能稳定。     

Doping and processing epitaxial GexSi1−x films on Si(100) by ion implantation for Si-based heterojunction devices applications

The question of whether one can effectively dope or process epitaxial Si(100)/GeSi heterostructures by ion implantation for the fabrication of Si-based heterojunction devices is experimentally investigated. Results that cover several differention species (B, C, Si, P, Ge, As, BF₂, and Sb), doses (10¹³ to 10¹⁶/cm²), implantation temperatures (room temperature to 150°C), as well as annealing techniques (steady-state and rapid thermal annealing) are included in this minireview, and the data are compared with those available in the literature whenever possible. Implantation-induced damage and strain and their annealing behavior for both strained and relaxed GeSi are measured and contrasted with those in Si and Ge. The damage and strain generated in pseudomorphic GeSi by room-temperature implantation are considerably higher than the values interpolated from those of Si and Ge. Implantation at slightly elevated substrate temperatures (e.g., 100°C) can very effectively suppress the implantation-induced damage and strain in GeSi. The fractions of electrically active dopants in both Si and GeSi are measured and compared for several doses and under various annealing conditions. Solid-phase epitaxial regrowth of GeSi amorphized by implantation has also been studied and compared with regrowth in Si and Ge. For the case of metastable epi-GeSi amorphized by implantation, the pseudomorphic strain in the regrown GeSi is always lost and the layer contains a high density of defects, which is very different from the clean regrowth of Si(100). Solid-phase epitaxy, however, facilitates the activation of dopants in both GeSi and Si, irrespective of the annealing techniques used. For metastable GeSi films that are not amorphized by implantation, rapid thermal annealing is shown to outperform steady-state annealing for the preservation of pseudomorphic strain and the activation of dopants. In general, defects generated by ion implantation can enhance the strain relaxation process of strained GeSi during post-implantation annealing. The processing window that is optimized for ion-implanted Si, therefore, has to be modified considerably for ion-implanted GeSi. However, with these modifications, the mature ion implantation technology can be used to effectively dope and process Si/GeSi heterostructures for device applications. Possible impacts of implantation-induced damage on the reliability of Si/GeSi heterojunction devices are briefly discussed.     

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.     

Ion-assisted molecular beam epitaxy of GaAs on Si(100)

GaAs was grown by molecular beam epitaxy (MBE) and ion-assisted MBE on Si(100) substrates. Three-dimensional (3D) island nucleation, observed during MBE growth, was eliminated during ion-assisted MBE when the ion energyE was >25 eV and the product ofE and the current densityJ was ≈6-12 eV mA/cm². IncreasingEJ to ≈15 eV mA/ cm² resulted in excessive ion damage. Decreasing the substrate temperature from 280 to 580° C during ion-assisted MBE yielded a slight decrease in surface roughness, and flatter surfaces were obtained for lower As⁴/Ga flux ratios. The suppression of 3D island nucleation led to an improvement in the crystalline perfection of thicker GaAs films. For example, the x-ray diffraction rocking-curve full-width-at-half-maximum values for 0.5 μm thick films grown at 380° C decreased from 1700 arcsec to 1350 arcsec when ion irradiation was used during nucleation. IAMBE allowed nucleation of thin, relatively flat-surfaced GaAs films even at 580° C, resulting in FWHM values of 1850 arcsec for 0.14 /μm thick films.     

Suppression of twin formation in CdTe(111)B epilayers grown by molecular beam epitaxy on misoriented Si(001)

CdTe(lll)B layers have been grown on misoriented Si(001). Twin formation inside CdTe(lll)B layer is very sensitive to the substrate tilt direction. When Si(001) is tilted toward [110] or [100], a fully twinned layer is obtained. When Si(001) is tilted toward a direction significantly away from [110], a twin-free layer is obtained. Microtwins inside the CdTe(111)B layers are overwhelmingly dominated by the lamellar twins. CdTe(111)B layers always start with heavily lamellar twinning. For twin-free layers, the lamellar twins are gradually suppressed and give way to twin-free CdTe(111)B layer. The major driving forces for suppressing the lamellar twinning are the preferential orientation of CdTe[11-2] along Si[1-10] and lattice relaxation. Such preferential orientation is found to exist for the CdTe(111)B layers grown on Si(001) tilted toward a direction between [110] and [100].     

Metal-organic vapor-phase epitaxy growth and characterization of thick (100) CdTe layers on (100) GaAs and (100) GaAs/Si substrates

The growth characteristics and crystalline quality of thick (100) CdTe-epitaxial layers grown on (100) GaAs and (100) GaAs/Si substrates in a metal-organic vapor-phase epitaxy (MOVPE) system for possible applications in x-ray imaging detectors were investigated. High-crystalline-quality epitaxial layers of thickness greater than 100 μm could be readily obtained on both types of substrates. The full width at half maximum (FWHM) values of the x-ray double-crystal rocking curve (DCRC) decreased rapidly with increasing layer thickness, and remained around 50–70 arcsec for layers thicker than 30 μm on both types of substrates. Photoluminescence (PL) measurement showed high-intensity excitonic emission with very small defect-related peaks from both types of epilayers. Stress analysis carried out by performing PL as a function of layer thickness showed the layers were strained and a small amount of residual stress, compressive in CdTe/GaAs and tensile in CdTe/GaAs/Si, remained even in the thick layers. Furthermore, the resistivity of the layers on the GaAs substrate was found to be lower than that of layers on GaAs/Si possibly because of the difference of the activation of incorporated impurity from the substrates because of the different kinds of stress existing on them. A heterojunction diode was then fabricated by growing a CdTe epilayer on an n⁺-GaAs substrate, which exhibited a good rectification property with a low value of reverse-bias leakage current even at high applied biases.     

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.     

Epitaxial growth of CdTe on (211) silicon mesas formed by deep reactive ion etching

By patterning a (211) Si substrate wafer into mesas and depositing CdTe onto this substrate by molecular beam epitaxy (MBE), we achieved the removal of nearly all threading dislocations from the epilayer. Faceting of mesa surfaces is observed and characterized. Deposition of CdTe on mesa sidewalls nucleates stacking faults along the (111) planes, which result in nonradiative carrier recombination. The density of these stacking faults can be reduced if care is taken to align the molecular beams from the effusion cells with particular crystallographic directions of the substrate.     

Characterization of PbSnSe/CdTe/Si (211) Epilayers Grown by Molecular Beam Epitaxy

Narrow-bandgap PbSnSe has received much attention as a promising alternative material for mid- and long-wavelength high performance of infrared detection at relatively high operating temperatures owing to the weak composition dependence of its bandgap, which can intrinsically result in better uniformity. Additionally, it possesses a high dielectric constant that is anticipated to be much more tolerant to defects. In addition, its growth by molecular beam epitaxy (MBE) can be easily accomplished in comparison with HgCdTe and many III–V quantum well and superlattice materials. However, overcoming the high lattice and thermal mismatches between PbSnSe and CdTe/Si substrates and improving the crystalline quality of PbSnSe grown on CdTe/Si substrates are challenges that require further study. Additionally, interdiffusion between CdTe and PbSnSe can take place and lead to nonuniform distributions of elements in PbSnSe. Epitaxial crystal PbSnSe alloy films were grown by MBE and were investigated by scanning and high-resolution transmission electron microscopy (STEM/HRTEM). Etch pit density (EPD) measurements were done to determine the density of threading defects in the films. EPD measurements on PbSnSe surfaces gave values in the mid-10⁶ cm⁻² range. The dislocations exposed as etch pits were found to accumulate and form small-angle grain boundaries lined up along the () direction, which is the intersection line between (100) and (211) growth planes.     

Epitaxial growth of zinc-blende AIN on Si(100) substrates by plasma source molecular beam epitaxy

Epitaxial zinc-blende AlN films were deposited on Si(100) substrates by plasma source molecular beam epitaxy (PSMBE). The lattice parameter of the zinc-blende AlN was determined to be 4.373Å. The epitaxial relationship between film and substrate was (100)AlN‖(100)Si and [011]AlN‖[011]Si. The zinc-blende AlN films were formed using a hollow cathode source with a pulse d.c. power supply in the PSMBE system. The high energy and large density of the Al⁺ and N⁺ species emerging from the hollow cathode and the presence of a substrate surface with cubic symmetry are probably the main factors for the formation of the metastable zinc-blende phase of AlN.     

Thermal stability of tantalum nitride diffusion barriers for Cu metallization formed using plasma immersion ion implantation

Plasma immersion ion implantation (PIII) technique was employed to form Tantalum nitride diffusion barrier films for copper metallization on silicon. Tantalum coated silicon wafers were implanted with nitrogen at two different doses. A copper layer was deposited on the samples to produce Cu/Ta(N)/Si structure. Samples were heated at various temperatures in nitrogen ambient. Effect of nitrogen dose on the properties of the barrier metal was investigated by sheet resistance, X-ray diffraction and scanning electron microscopy measurements. High dose nitrogen implanted tantalum layer was found to inhibit the diffusion of copper up to 700 °C.     

Thermal stability of Ni silicide films on heavily doped n and p Si substrates

Electrical and structural properties of Ni silicide films formed at various temperatures ranged from 200 °C to 950 °C on both heavily doped n⁺ and p⁺ Si substrates were studied. It was found that surface morphology as well as the sheet resistance properties of the Ni silicide films formed on n⁺ and p⁺ Si substrates at the temperatures higher than 600 °C were very different. Agglomerations of Ni silicide films on n⁺ Si substrates begin to occur at around 600 °C while there is no agglomeration observed in Ni silicide films on p⁺ Si substrates up to a forming temperature of 700 °C. It was also found that the phase transition temperature from NiSi phase to NiSi₂ phase depend on substrate types; 900 °C for NiSi film on n⁺ Si substrate and 750 °C for NiSi film on p⁺ Si substrate, respectively. Our results show that the agglomeration is, especially, important factor in the process temperature dependency of the sheet resistance of Ni silicides formed on n⁺ Si substrates.     

Photoluminescence study of GaAs films on Si(100) grown by atomic hydrogen-assisted molecular beam epitaxy

Preliminary experimental results and analysis of photoluminescence (PL) measurements performed on GaAs heteroepitaxial films, which have been grown on Si(100) substrates by atomic hydrogen-assisted low-temperature molecular beam epitaxy technique have been presented and discussed. The results have also been compared with those obtained for GaAs homoepitaxial films. Furthermore, minority carrier lifetimes in n-GaAs on Si have been characterized by the PL decay method and an average lifetime of as high as 8.0 ns has been successfully obtained, which is the highest value ever reported to date.     

HgCdTe heteroepitaxy on three-inch (112) CdZnTe/Si: Ellipsometric control of substrate temperature

The technique of spectroscopic ellipsometry (SE) has been utilized to monitor in real-time and precisely control the surface temperature of Hg_1−xCd_xTe during molecular beam epitaxy. Due to the temperature dependence of the Hg sticking coefficient under Hg-deficient growth conditions, the near-surface composition of an epilayer is extremely sensitive to surface temperature. SE data were acquired in real time and modeled using a previously established library of dielectric functions of Hg_1−xCd_xTe as a function of composition. Utilizing SE-generated compositional profiles as a guide, substrate heating power was adjusted in such a way as to minimize composition transients. To demonstrate the effectiveness of the technique, we have used SE to control the temperature of HgCdTe epilayer surfaces during deposition on three-inch (211)CdZnTe/ZnTe/Si composite substrates mounted on indium free holders.     

Galvanomagnetic properties of two-dimensional electron gases in strain relaxed InxGa1−xAs(x<0.40) heterostructures grown by molecular beam epitaxy on GaAs substrates

We have investigated, as a function of indium content x, the galvanomagnetic and Shubnikov de Haas (SdH) properties of two-dimensional electron gases (2DEG) formed at lattice matched, strain relaxed InAlAs/InGaAs heterojunctions. These were grown by molecular beam epitaxy on GaAs misoriented substrates with a two degree offcut toward the nearest (110) plane. Variable temperature resistivity and Hall measurements indicate an increase in the electron sheet density n_s from 0.78×10¹²cm⁻² for x=0.15 to 1.80×10¹² cm⁻² for x=0.40 at 300K, and from 0.75×10¹²cm⁻² to 1.67×10¹²cm⁻² at T=1.6K. The room temperature electron mobility, measured along the in plane [110], direction is independent of indium content and equals approximately 9500 cm²/Vs. For T<50K, the mobility is independent of temperature decreasing with increasing x from 82000 cm²/Vs for x=0.15 to 33000 cm²/Vs for x=0.40. The ratios (τ_t/τ_q) at 1.6K between the electron relaxation time τ_t and the single particle relaxation time τq, for the strain relaxed specimens, as well as for pseudomorphically strained Al_0.35Ga_0.65As/In_0.15Ga_0.85As structures grown on GaAs substrates, and In_0.52Al_0.48As/In_0.53Ga_0.47As heterostructures grown lattice matched on InP substrates. Such a study indicates the presence of inhomogeneities in the 2DEGs of the strain relaxed specimens which appear to be related to the process of strain relaxation. Such inhomogeneities, however, have little effect on the electron relaxation time τ_t which, at low temperatures, is limited principally by alloy scattering.