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.     

Elimination of dislocations in heteroepitaxial MBE and RTCVD Ge x Si1-x grown on patterned Si substrates

We have grown Ge _x Si_1-x (0 <x < 0.20,1000–3000Å thick) on small growth areas etched in the Si substrate. Layers were grown using both molecular beam epitaxy (MBE) at 550° C and rapid thermal chemical vapor deposition (RTCVD) at 900° C. Electron beam induced current images (EBIC) (as well as defect etches and transmission electron microscopy) show that 2800Å-thick, MBE Ge_0.19Si_0.81 on 70-μm-wide mesas have zerothreading and nearly zero misfit dislocations. The Ge_0.19Si{0.81} grown on unpatterned, large areas is heavily dislocated. It is also evident from the images that heterogeneous nucleation of misfit dislocations is dominant in this composition range. 1000Å-thick, RTCVD Ge_0.14Si_0.86 films deposited on 70 μm-wide mesas are also nearly dislocation-free as shown by EBIC, whereas unpatterned areas are more heavily dislocated. Thus, despite the high growth temperatures, only heterogeneous nucleation of misfit dislocations occurs and patterning is still effective. Photoluminescence spectra from arrays of GeSi on Si mesas show that even when the interface dislocation density on the mesas is high, growth on small areas results in a lower dislocation density than growth on large areas.     

Current-voltage characteristics and X- ray diffraction study of Pd/Si1- xGex schottky contacts

The electrical characteristics of Pd/p-Si_1-xGe_x Schottky contacts have been investigated. The Schottky contacts were formed by depositing Pd metal on substrates at room temperature (RT = 300K) and at low temperature (LT = 77K). Post annealing was performed in nitrogen atmosphere at 450 and 550°C, respectively, to study the effect of silicide formation on contact characteristics. The current-voltage measurements showed that the barrier height, ϕ_B, decreased with the increase of the gemanium composition. The contact postannealed at 550°C showed a current transport mechanism obviously different from the as-deposited Schottky contacts. Nearly identical characteristics were observed for the low temperature deposited contacts and the room temperature deposited contacts with 550°C post-annealing. They both showed thermionic emission dominated transport mechanism. X-ray diffraction technique was used to characterize the effect of different temperature treatments on the crystal structure. The full width at half maximum of Si_1-xGe_x(400) phase decreased at low temperature deposited sample, while it increased at room temperature deposition.     

Ultra-shallow raised p+−n junctions formed by diffusion from selectively depositedIn-situ doped Si0.7Ge0.3

In this paper, a novel raised p⁺−n junction formation technique is presented. The technique makes use ofin- situ doped, selectively deposited Si_0.7Ge_0.3 as a solid diffusion source. In this study, the films were deposited in a tungsten halogen lamp heated cold-walled rapid thermal processor using SiCl₂H₂, GeH₄, and B₂H₆. The microstructure of the Si_0.7Ge_0.3 layer resembles that of a heavily defected epitaxial layer with a high density of misfit dislocations, micro-twins, and stacking faults. Conventional furnace annealing or rapid thermal annealing were used to drive the boron from thein- situ doped Si_0.7Ge_0.3 source into silicon to form ultra-shallow p⁺−n junctions. Segregation at the Si_0.7Ge_0.3/Si interface was observed resulting in an approximately 3:1 boron concentration discontinuity at the interface. Junction profiles as shallow as a few hundred angstroms were formed at a background concentration of 10¹⁷ cm⁻³.     

Structure and electrical properties of polycrystalline SiGe films grown by molecular beam deposition

The structural and electrical properties of polycrystalline Si_0.5Ge_0.5 films 150 nm thick grown by molecular beam deposition at temperatures of 200–550°C on silicon substrates coated with amorphous layers of silicon oxynitride were studied. It is shown that the films consist of a mixture of amorphous and polycrystalline phases. The amorphous phase fraction decreases from ～50% in films deposited at 200°C to zero in films grown at 550°C. Subsequent 1-h annealing at a temperature of 550°C results in complete solid-phase crystallization of all films. The electron transport of charge carriers in polycrystalline films occurs by the thermally activated mechanism associated with the energy barrier of ～0.2 eV at grain boundaries. Barrier lowering upon additional annealing of SiGe films correlates with an increase in the average grain size.     

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.     

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.     

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 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.     

In situ B-doped Si epitaxial films grown at 450‡ C by remote plasma-enhanced chemical vapor deposition: Physical and electrical characterization

In situ boron doping of Si epitaxial films grown at 450‡ C by remote plasma-enhanced chemical vapor deposition (RPCVD) has been studied using secondary ion mass spectroscopy (SIMS), Hall effect measurements, defect etching in conjunction with Nomarski microscopy, cross-sectional transmission electron microscopy (XTEM), and current-voltage measurements. Boron incorporation is shown to be controllable and electrically active from 7 × 10¹⁷ to over 10²⁰ cm^-3, with no dependence on process parameters (temperature, rf power, and substrate bias) in the ranges studied, other than the B₂H₆/SiH₄ gas-phase ratio. No change in deposition rate upon introduction of B₂H₆ dopant gas is seen, contrary to what has been observed in several higher-temperature CVD processes. No defects such as stacking faults are seen under Nomarski microscopy, but a visible haze covers some areas ofin situ B-doped wafers. This haze appears to consist of amorphous cone-shaped structures with their apexes at the substrate-epilayer interface. The origin of the conical defects is believed to be related to some phenomenon at the initiation of growth. In order to evaluate the electrical quality ofin situ B-doped epilayers,P ⁺/N mesa diodes have been fabricated using both homoepitaxial and heteroepitaxial (Ge_xSi_1-x)p-type epitaxial films. The electrical junction in these diodes coincides with the (epi-substrate)—interface in the grown films. To avoid interdiffusion or annealing effects during diode fabrication, all processing temperatures were kept at or below 450‡ C. Ideality factors are 1.2-1.3 for all diodes, indicating diffusion-limited transport rather than recombination in the depletion region.     

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.     

The influence of defects on device performance of MBE-grown Si homojunction and strained Si1-xGex/Si heterostructures

Different Si homojunction and strained Si_1-xGe_x/Si heterojunction diodes and bipolar transistors have been fabricated by Si-MBE. The effect of annealing on Si homojunction diodes and transistors are studied. It is found that annealing generally improves the Si device performance, such as the ideality factor and breakdown characteristics. The influence of⁶⁰Co γ irradiation on the Si_1-xGe_x/Si diode performances are investigated by studying the temperature dependence of their electrical characteristics, and the results are correlated with the quality of the MBE-films. γ irradiation causes a drop in material conductivity due to the generation of atom-displacement defects in the whole volume of the wafers and increases the defect density at hetero-interfaces. The forward I-V curves of Si_1-xGe_x/Si devices may shift towards lower or higher voltages, depending on the film quality and the irradiation dose. The increase of defect density in strained Si_1-xGe_x/Si films appears to occur easier for the films with lower quality. Electrical measurements and calculations show that the defect-associated tunneling process is important in current transport for these MBE grown Si homojunction and strained Si_1-xGe_x/Si heterojunction devices, which have initially medium film quality or have been treated by irradiation.     

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.     

Room temperature oxidation of Cu3Ge and Cu3(Si1−xGex) on Si1−xGex

Room temperature oxidation of Cu₃(Si_1−xGe_x) and Cu₃Ge films grown on Si_1−xGe_x at a temperature of 200–400°C was studied using transmission electron microscopy (TEM) in conjunction with energy dispersive spectrometry (EDS). For Cu₃(Si_1−xGe_x) and Cu₃Ge films grown at 200°C and then exposed to air, room temperature oxidation occurred. The Cu₃Ge film was superior to the Cu₃(Si_1−xGe_x) film in reducing the oxidation rate because of its higher Ge concentration. Annealing at higher temperatures such as 400°C resulted in Ge segregation out of the Cu₃(Si_1−xGe_x) film or Si diffusion from the Si_1−xGe_x substrate into the Cu₃Ge overlayer, and hence enhanced the oxidation rate of Cu₃(Si_1−xGe_x) and Cu₃Ge films. The present study shows that upon exposure to air even the Cu₃Ge film grown on Si_1−xGe_x is subject to room-temperature oxidation, revealing that the use of Cu₃Ge contacts on Si_1−xGe_x may be limited by some strict conditions.     

Reduction of boron penetration through thin silicon oxide with a nitrogen doped silicon layer

This paper deals with the development of the disilane Si₂H₆ gaseous source for gate technology and more precisely, reports on the use of nitrogen doped silicon (NIDOS) deposited from disilane and ammonia for the realisation of polycrystalline gate. Boron diffusivity into the NIDOS films is studied thanks to SIMS experiments, and results are extended to the fabrication of P⁺-poly-Si/NIDOS/SiO₂/Si capacitive structures. Electrical characterisations evidenced finally the influence of boron and nitrogen atoms on the electrical properties of PMOS devices.     

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.     

Effects of oxygen doping on properties of microcrystalline silicon film grown using rapid thermal chemical vapor deposition

An oxygen doped microcrystalline silicon (μc-Si) deposition process is developed by mixing small amounts of nitrous oxide (N₂O) with silane (SiH₄) in a rapid thermal chemical vapor deposition (RTCVD) reactor. The effects of oxygen doping on the properties of RTCVD μc-Si films are studied. Experimental results show that the RTCVD process provides high deposition rates for μc-Si and polycrystalline silicon (polySi) films at elevated deposition temperatures and pressures. The surface roughness of the RTCVD μc-Si films can be significantly reduced compared to that of conventional LPCVD polySi films. Steep side walls can be realized due to the small grain size of the μc-Si films. The sheet resistance of BF₂ doped μc-Si films is slightly higher than that of BF₂ doped polySi films, whereas sheet resistances of P and As doped μc-Si films are much higher than those of the corresponding P and As doped polySi films. Measurements of the catastrophic breakdown strength of metal-oxide-semiconductor (MOS) capacitors indicate that the quality of gate electrodes fabricated using μc-Si is improved relative to that of MOS capacitors fabricated using polySi gate electrodes.     

Segregation of phosphorus and germanium to grain boundaries in chemical vapor deposited silicon-germanium films determined by scanning transmission electron microscopy

The segregation of phosphorus and germanium to grain boundaries in P implanted, Si_0.87Ge_0.13 films deposited by chemical vapor deposition (CVD), was investigated using energy dispersive x-ray (EDX) micro-analysis. A quantitative analysis of the x-ray spectra obtained at grain boundaries showed that the excess amount of P varied with the crystallography of the boundary but that the segregation always followed an equilibrium process with an activation energy of 0.28 eV. On the other hand, Ge did not segregate to grain boundaries in either P implanted Si_0.87Ge_0.13 films nor in intrinsic Si_1−xGe_x films, containing 2, 13 and 31 at.% Ge. Possible reasons for the absence of Ge segregation are discussed.     

Polycrystalline silicon resistors for integrated circuits

The suitability of thin films of doped polycrystalline silicon on SiO₂ substrates for the production of high value resistors for monolithic integrated circuits is considered. Resistors fabricated from this material posses the advantages of high sheet resistivity and dielectric isolation while still preserving an all silicon technology compatible with conventional production techniques.Relevant structural and electrical properties of doped polycrystalline films produced by both vacuum evaporation onto hot substrates with gas-doping and by diffusion-annealing of amorphous films have been investigated. Sheet resistivities and TCR values measured on 2500 Å polycrystalline films have proved superior to those encountered with conventional diffused resistors. Typically films with sheet resistivities of 1 kΩ/□ had TCR's of ?1000 ppm/°C while conventional diffused resistors are generally made from material of 200 Ω/□ and +2000 ppm/°C TCR. Etched resistor line widths of 0·25 mil. have been obtained in the polycrystalline material employing conventional photolithographic techniques. The temperature stability and linearity of doped polycrystalline resistors have been investigated.     

Ozone/organic-source APCVD for conformai doped oxide films

Borophosphosilicate glass (BPSG) films were deposited with a new chemical vapor deposition method using ozone and organic sources under atmospheric pressure condition at 400° C called ozone/TEOS APCVD. Trimethylborate (TMB:B(OCH₃)₃) and trimethylphosphate (TMP:PO(OCH₃)₃) were utilized to dope phosphorus and boron into deposited films. Boron and phosphorus concentration in the films are easily controlled at up to 15 mole%. Conformality and reflow characteristics for the BPSG film are superior to those for BPSG film deposited by the SiH₄ + PH₃ + B₂H₆ + O₂ CVD method. Planarization of underlying patterns, which have half-micron spaces with high aspect ratio, was successfully accomplished.