Thin films of CoSi2 on Si1−yCy substrate layers

The reaction of Co with epitaxial Si_1−yC_y(001) films is investigated with regard to dependence on annealing temperature and C concentration y. Resistance measurements and RBS analysis reveal a small increase in the disilicide formation temperature. The electrical properties are very similar for thin CoSi₂ films grown at 650°C on Si_0.999C_0.001 and on Si. Whereas the CoSi₂ is fully polycrystalline on Si(001), partially oriented CoSi₂ has been observed on C-containing substrate layers. An increase of the number of epitaxially grown CoSi₂ crystallites has been observed with increasing C concentration.     

Investigation of polycrystalline silicon layers by electron microscopy and x-ray diffraction

The structure of polycrystalline Si layers deposited by pyrolysis of silane in a hydrogen ambient has been investigated by replica electron microscopy and X-ray diffraction. When the gas flow ratio (SiH₄/H₂) is 3·64 × 10⁻⁴ the temperature region below 900°C is a surface reaction control region and the activation energy of the chemical surface reaction rate is 1·6 eV. The temperature range above 900°C is a mass transfer region and the deposition rate is about 500 Å/min. The grains become larger and the texture of the surface of the poly Si layers becomes coarser with the deposition temperature. Some phase change was found to occur around 900°C by replica electron microscopy. The X-ray diffraction experiments show that there exist three preferred orientations of (220), (111) and (311) in the poly Si layers. The decrease of the relative intensity of the (311) orientation might have a relation to the phase change around 900°C.     

High temperature ohmic contacts to 3C–silicon carbide films

Currently, large-area 3C–SiC films are available from a number of sources and it is imperative that stable high temperature contacts be developed for high power devices on these films. By comparing the existing data in the literature, we demonstrate that the contact behavior on each of the different polytypes of SiC will vary significantly. In particular, we demonstrate this for 6H–SiC and 3C–SiC. The interface slope parameter, S, which is a measure of the Fermi-level pinning in each system varies between 0.4–0.5 on 6H–SiC, while it is 0.6 on 3C–SiC. This implies that the barrier heights of contacts to 3C–SiC will vary more significantly with the choice of metal than for 6H–SiC. Aluminum, nickel and tungsten were deposited on 3C–SiC films and their specific contact resistance measured using the circular TLM method. High temperature measurements (up to 400°C) were performed to determine the behavior of these contacts at operational temperatures. Aluminum was used primarily as a baseline for comparison since it melts at 660°C and cannot be used for very high temperature contacts. The specific contact resistance (ρ_c) for nickel at room temperature was 5×10⁻⁴ Ω cm², but increased with temperature to a value of 1.5×10⁻³ Ω cm² at 400°C. Tungsten had a higher room temperature ρ_c of 2×10⁻³ Ω cm², which remained relatively constant with increasing temperature up to 400°C. This is related to the fact that there is hardly any reaction between tungsten and silicon carbide even up to 900°C, whereas nickel almost completely reacts with SiC by that temperature. Contact resistance measurements were also performed on samples that were annealed at 500°C.     

Refractory metal silicon device technology

Films 2000–5000 Å thick of Mo or W deposited over thin films of thermally grown SiO₂ are shown to be effective high temperature diffusion masks against both phosphorous and boron. These metal films may be precisely patterned and their diffusion masking properties can be used to define the source and drain regions of MOSFETs. In this manner, self-registered MOSFETs can be fabricated with a portion of the diffusion masking metal film acting as the gate electrode. Using P or B doped deposited glasses as diffusion sources, n or p channel enhancement mode MOSFETs were made by diffusion through the exposed thin SiO₂ film into p and n type Si to form source and drain junctions. Contact was subsequently made by etching holes through the oxide layers to the source and drain regions and to the refractory metal gate electrode buried within the oxide layers. These devices exhibit channel mobilities between 200 and 300 cm²/V-sec at gate voltages about 10 V above threshold. The stability of MOS structures processed in a similar manner has been measured. After being stressed at ±6 × 10⁵ V/cm and 250°C for 15 hr, these devices exhibited shifts in their C---V characteristics less than 200 mV.     

High doped MBE Si p–n and n–n heterojunction diodes on 4H-SiC

The physical and electrical properties of heavily doped silicon (5×10¹⁹ cm⁻³) deposited by molecular beam epitaxy (MBE) on 4H-SiC are investigated in this paper. Silicon layers on silicon carbide have a broad number of potential applications including device fabrication or passivation when oxidised. In particular, Si/SiC contacts present several atractive material advantages for the semiconductor industry and especially for SiC processing procedures for avoiding stages such as high temperature contact annealing or SiC etching. Si films of 100 nm thickness have been grown using a MBE system after different cleaning procedures on n-type (0 0 0 1) Si face 8° off 4H-SiC substrates. Isotype (n–n) and an-isotype (p–n) devices were fabricated at both 500 and 900 °C using antimonium (Sb) or boron (B), respectively. X-ray diffraction analysis (XRD) and scanning electronic mircorscope (SEM) have been used to investigate the crystal composition and morphology of the deposited layers. The electrical mesurements were performed to determine the rectifiying contact characteristics and band offsets.     

Size dependence of hall mobility and dislocation density in Ge heteroepitaxial layers grown by MBE on a SiO2 patterned Si template

We have investigated the size dependent properties of the Hall mobility and etch pit dislocations (EPDs) in germanium (Ge) heteroepitaxial layers. Pure Ge thin films were grown by molecular beam epitaxy (MBE) using pattern guided growth at 650 °C and compared with homoepitaxially grown films. The results show enhanced Hall mobility and lower dislocation density as the pattern size decreases. The number of EPDs was decreased by one order of magnitude and the Hall mobility measured with different sizes of van der Pauw patterns was enhanced by two times as the pattern size was decreased from 200×200 μm² to 3×3 μm². Raman spectroscopy was also employed to characterize residual strain and crystalline quality of the epitaxial films with respect to the pattern size. We conclude that nano-scale pattern guided heteroepitaxial growth of Ge may be a plausible method for monolithic integration of Ge optoelectronic or electronic devices onto a Si substrate.     

Electroless deposition of Co(W) thin films

Thin films of cobalt that contain small amounts of tungsten [Co(W)] were deposited by the electroless process. Those films do not contain either phosphorus or boron which are included in most electroless cobalt films processes. The deposition bath for Co(W) thin films include Co ions, tungstate ions as a source for tungsten, di-methyl-amine-borane (DMAB) complex as a reducing agent, ammonium hydrate as a complexing agent, acetic acid for buffering and surfactants. Co(W) layers were deposited on two types of seed layers: (a) thin sputtered cobalt or copper films on 100 nm SiO₂/Si and (b) bare silicon wafers activated by an aqueous Pd/PdCl₂ solution. The deposited layer thickness range was 40–1000 nm with deposition rate at 90 °C and pH 9 of 7 nm/s for both Pd activated Si and sputtered Co seed, and 5 nm/s for the sputtered Cu seed. Lowering the temperature to 70 °C lowered the deposition rate to 0.7 nm/s for the Pd activated Si. The deposited layers were bright coloured, uniform, and with low defect density under visual inspection. The thin films composition was found to be Cobalt with 3–4 at.% tungsten for all types of seed layers. The Co(W) thin films specific resistivity was in the range of 60–90 μΩ cm. Finally we present the thin film morphology as it was characterized using atomic force microscopy and scanning electron microscopy.     

Optimization of Al/a-SiC:H optical sensor device by means of thermal annealing

The optimization of optoelectronic properties of Al/a-SiC:H Schottky diodes grown as Al/a-SiC:H/c-Si(n) structures is studied by means of thermal annealing of a-SiC:H thin films. According to the spectral response of the Schottky diodes the measured quantum efficiency, η_measured, increases with increasing annealing temperature (400–600 °C), whereas η_measured decreases for T_a>600 °C. For T_a=600 °C, optimum material quality of a-SiC:H films is achieved and the spectral response of the Al/a-SiC:H/c-S(n) structures present very high and almost constant values (η_measured80%) for the whole range of wavelengths from 500 up to 850 nm. These results show that our Al/a-SiC:H/c-S(n) structures can be very attractive as optical sensors. Diffusion length calculations as well as the mobility by lifetime product (μτ)_p of the minority carriers (holes) of a-SiC:H films present a dependence on T_a similar to that of the measured quantum efficiency. Finally, the quantum efficiency of films processed with T_a=675 °C is found to increase when the Al/a-SiC:H/c-S(n) structures are exposed to hydrogen, a result that could be promising for the construction of a hydrogen detection sensor.     

Epitaxial growth of β-SiC on Si by RTCVD with C3H8 and SiH4

The authors established that β-SiC thin films can be grown epitaxially on (100) Si substrates by rapid thermal chemical vapor deposition (RTCVD) employing carbonization with C₃H₈ , as well as post-carbonization growth using SiH₄ and C ₃H₈ in H₂. They determined the optimum carbonization conditions with respect to reaction temperature and ramp rate, gas flow rates, etc. A possible mechanism, based on nucleation density and Si surface diffusion, has been proposed for the effect of C ₃H₈ flow rate in film thickness, morphology, and void formation. Void-free SiC films have been grown on Si at high C₃H₈ flow rates. The authors determined the optimum conditions for subsequent SiC growth with respect to temperature and Si/C ratio in the gas phase. They established that the resulting SiC thin films are monocrystalline by X-ray and electron diffraction. The SiC-Si interface was investigated by cross-section transmission electron microscopy and found to be sharp and intimate where no voids are present     

Growth of InxGa(1−x)N compound semiconductors and high-power InGaN/AlGaN double heterostructure violet-light-emitting diodes

In_xGa(_1−x)N films were grown on GaN films with an indium mole fraction x up to X = 0.33 at temperatures between 720°C and 850°C. The growth rate of InGaN films had to be decreased sharply to obtain high-quality InGan films when the growth temperature was decreased. Band-gap energies between 2.67 eV and 3.40 eV obtained by room-temperature photoluminescence measurements fit quite well to parabolic forms previously obtained by Osamura et al. on the indium mole fraction x assuming that the band-gap energies for GaN and InN are 3.40 and 1.95 eV, respectively. High-power InGaN/AlGaN double-heterostructure violet-light-emitting diodes were fabricated. The typical output power was 1000 μW and the external quantum efficiency was as high as 1.5% at a forward current of 20 mA at room temperature. The peak wavelength and the full width at half-maximum of the electroluminescence were 380 nm and 17 nm, respectively.     

Study of Ta–Si–N thin films for use as barrier layer in copper metallizations

This work focuses on the deposition process, microanalytical characterization and barrier behaviour of 10–100-nm thick sputtered Ta–Si and Ta–Si–N films. Pure Ta–Si films were found to be already nanocrystalline. The addition of N₂ leads to a further grain fining resulting in amorphous films with excellent thermal stability. According to microanalytical investigations, Ta–Si barriers between Cu and Si with a thickness of only 10 nm are not stable at 600°C. Copper silicides are formed due to intensive Cu diffusion throughout the barrier. In contrast, 10-nm thick nitrogen-rich Ta–Si–N barriers remain thermally stable during annealing at 600°C and protect the Si wafer from Cu indiffusion.     

Schottky barrier height of a new ohmic contact NiSi2 to n-type 6H-SiC

We present a new ohmic contact material NiSi₂ to n-type 6H-SiC with a low specific contact resistance. NiSi₂ films are prepared by annealing the Ni and Si films separately deposited on (0 0 0 1)-oriented 6H-SiC substrates with carrier concentrations (n) ranging from 5.8×10¹⁶ to 2.5×10¹⁹ cm⁻³. The deposited films are annealed at 900 °C for 10 min in a flow of Ar gas containing 5 vol.% H₂ gas. The specific contact resistance of NiSi₂ contact exponentially decreases with increasing carrier concentrations of substrates. NiSi₂ contacts formed on the substrates with n=2.5×10¹⁹ cm⁻³ show a relatively low specific contact resistance with 3.6×10⁻⁶ Ω cm². Schottky barrier height of NiSi₂ to n-type 6H-SiC is estimated to be 0.40±0.02 eV using a theoretical relationship for the carrier concentration dependence of the specific contact resistance.     

Pt-based metallization of PMOS devices for a compatible monolithic integration of semiconducting/Yba2Cu3O7−δ superconducting devices on silicon

Mo, Pt, Pt/Mo and Pt/Ti thin films have been deposited onto Si and SiO₂ substrates by RF sputtering and annealed in the YBa₂Cu₃O_7−δ (YBCO) growth conditions. The effect of annealing on the sheet resistance of unpatterned layers was measured. A Pt-based multilayered metallization for the PMOS devices was proposed and tested for a compatible monolithic integration of semiconducting devices and YBCO sensors on the same silicon substrate. The best results were obtained with a Pt/Ti/Mo-silicide structure showing 0.472 Ω_□ interconnect sheet resistivity and 2×10⁻⁴ Ω cm² specific contact resistivity after annealing for 60 min at 700 °C in 0.5 mbar O₂ pressure.     

Formation kinetics and structure of Pd2Si films on Si

Backscattering and X-ray techniques have been used to study properties of palladium silicide (Pd₂Si) formed by evaporating thin Pd layers on Si followed by heat treatment. The rate of formation of Pd₂Si in the temperature range of 200–275°C has been measured by 2-MeV ⁴He⁺ ion backscattering. The Pd₂Si layer is found to grow at a rate proportional to the square root of time for thicknesses ranging from approximately 200–4000 Å. The rate of growth is found to be independent of Si substrate orientation or doping type and the rate constant is found to fit a single activation energy of E_a = 1·5±0·1 eV over the temperature range measured. X-ray diffraction indicates the structure to be Pd₂Si with the basal plane roughly parallel to the substrate surface for films formed on 111, 110, 100 and evaporated (amorphous) silicon substrates. The degree of preferred orientation is markedly stronger on [111] Si. Ion channeling measurements confirm that in this case the c-direction of the Pd₂Si is parallel with the [111] direction in the underlying Si.     

Buffer layers for high-quality epitaxial YBCO films on Si

Efforts aimed at producing device-quality YBa₂Cu₃ O_7-δ (YBCO) films on Si, which have resulted in films with properties comparable to what can be achieved with conventional oxide substrates such as SrTiO₃, are described. It is reported how epitaxial YBCO films were grown on Si(100) using an intermediate buffer layer of yttria-stabilized zirconia (YSZ). Both layers are grown with an entirely in situ process by pulsed laser deposition (PLD). Ion channeling revealed a high degree of crystalline perfection with a channeling minimum yield for Ba as low as 12%. The normal state resistivity was 250-300 μΩ-cm at 300 K; the critical temperature, Tc (R=0), was 86-88 K, with a transition width of 1 K. Critical current densities of 2×10⁷ at 4.2 K and 2.2×10⁶ at 77 K have been achieved. Noise measurements indicate that these films are suitable for use in highly sensitive far-infrared bolometers. Applications of this technology to produce in situ reaction patterned microstrip lines are discussed     

Analysis of Ti–Si–N diffusion barrier films obtained by r.f. magnetron sputtering

Thin films of Ti–Si–N are deposited by r.f. magnetron sputtering in a Ar/N₂ gas mixture. The magnetron discharge is operated at 10 mTorr with 5 and 10% N₂ in the gas mixture and r.f. powers ranging from 100 to 200 W. The composition and electrical resistivity of the thin films were determined by energy dispersive X-ray spectroscopy and the four-point probe method, respectively. The structure of the films was determined by high-resolution transmission electron microscopy. The Ti–Si–N films were either amorphous or contained cubic TiN nanosized grains in an amorphous phase. The diffusion barrier properties of 10-nm thick film between Cu and Si were studied from 500 to 700°C. The highest failure temperature of 650°C was obtained for Ti_37.5 Si₂₇ N_35.5 which. contains 4-nm TiN crystallites in an amorphous phase.     

CoSi2 ohmic contacts to n-type 6H---SiC

Cobalt disilicide (CoSi₂) ohmic contacts possessing low specific contact resistivity (_c < 3.0 ± 0.4 × 10⁻⁵ ωcm²) to n-type 6H---SiC are reported. The contacts were fabricated via sequential electron-beam evaporation of Co and Si layers followed by a two-step vacuum anealing process at 500 and 900°C. Stochiometry of the contact so formed was confirmed by Rutherford backscattering spectrometry and X-ray diffraction. Specific contact resistivities were obtained via current-voltage (I-V) analysis at temperatures ranging from 25 to 500°C. _c is compared as a function of carrier concentration, current density, temperature and time at elevated temperature.     

Characterization of phosphorus implanted low pressure chemical vapor deposited polycrystalline silicon

Thin (3000–5000Å) low pressure chemically vapor deposited (LPCVD) films of polycrystalline silicon suitable for microelectronics applications have been deposited from silane at 600°C and at a pressure of 0.25 Torr. The films were phosphorus implanted at 150 KeV and electrically characterized with the annealing conditions and film thickness as parameters, over a resistivity range of four orders of magnitude (10³–10⁷Ω/□). Annealing during silox deposition was found to result in a lower film resistivity than annealing done in nitrogen atmosphere. Resistivity measurements as a function of temperature indicate that the electrical activation energy is a linear function of 1/N(N is the doping concentration), changing from 0.056 eV for a doping concentration of 8.9 × 10¹⁸ cm⁻³ to 0.310 eV for doping concentration of 3.3 × 10¹⁸ cm⁻³. The grain boundary trap density was found to have a logarithmically decreasing dependence on the polysilicon thickness, decreasing from 1.3 × 10¹³ cm⁻² for 2850Å polysilicon film to 8.3 × 10¹² cm⁻² for 4500Å polysilicon film.     

Conduction type and defect levels of β-FeSi2 films grown by MBE with different Si/Fe ratios

[1 0 0]-oriented β-FeSi₂ films were grown on Si(0 0 1) substrates by molecular beam epitaxy (MBE) with a deposited Si to Fe atomic ratio (Si/Fe ratio) varied from 1.6 to 2.8. It was found that the conduction type of the β-FeSi₂ films changed from p- to n-type between the deposited Si/Fe=2.4 and 2.8. Rutherford Back Scattering (RBS) measurements revealed that the real Si/Fe ratio of β-FeSi₂ is 2.0–2.1 for all the samples after 900°C annealing for 14 h, showing that stoichiometry of the grown films is almost satisfied even though the deposited Si/Fe ratio was away from stoichiometry.     

SiC/Si heterojunction diodes fabricated by self-selective and byblanket rapid thermal chemical vapor deposition

SiC/Si heterojunction diodes have been fabricated by two different rapid thermal chemical vapor deposition (RTCVD) processes: a localized self-selective growth and blanket growth. The self-selective growth of crystalline cubic (β) SiC was obtained by propane carbonization of the Si substrate in regions unprotected by an SiO₂ layer, producing planar diodes. Mesa diodes were fabricated using the blanket growth of polycrystalline β-SiC produced by the decomposition of methylsilane (CH₃SiH₃). The SiC/Si heterojunction diodes show good rectifying properties for both device structures. Reverse breakdown voltage of 50 V was obtained with the self-selective SiC/Si diode. The mesa diodes exhibited even higher breakdown voltages (V_br) of 150 V and excellent ideality factors of 1.06 at 25°C. The high V_br and good forward rectifying characteristics indicate that the SiC/Si heterojunction diode represents a promising approach for the fabrication of wide-gap emitter SiC/Si heterojunction bipolar transistors