S. F. Gong

Al induced crystallization of a‐Si

The crystallization of amorphous Si induced by Al during heat treatment has been investigated by cross section and plan view transmission electron microscopy. The lowest temperature of Al induced crystallization of amorphous Si was found to be 440 K. The crystallization temperature, however, depends on the thickness of Al layers in layered structures and on the concentration of Al in co‐deposited layers below 1‐nm‐layer thickness and 15 at.% of Al concentration, respectively. Al‐induced crystallization in layered structures starts at the Al/amorphous Si interfaces and is located close to them. The amount of crystallized Si depends on the quantity of Al and on the temperature and increases with them. The mechanism of crystallization involves intermixing of Al with Si and the formation of an alloy of high metal concentration in the amorphous/crystalline interface. When the formation of this alloy is not assured due to low Al concentration, then crystallization does not start or the process of crystallization stops. In Al induced crystallization the nucleation of polycrystalline Si grains rather than their crystal growth is affected.

Al‐doped and Sb‐doped polycrystalline silicon obtained by means of metal‐induced crystallization

Thin‐film multilayer structures of a‐Si/Al/a‐Si and a‐Si/Sb/a‐Si were deposited by electron‐beam evaporation. The microstructure and the electrical properties of as‐deposited and annealed (T<1370 K) thin films were determined. A p‐n junction was formed between polycrystalline silicon (poly‐Si) doped with Sb and a p‐type Si substrate. Al and Sb were found to induce crystallization of a‐Si at 600 and 700 K, respectively. After annealing to 1370 K for 60 min, the resistivities 7.0×10−3 Ω cm for the Al‐Si sample and 1.4×10−2 Ω cm for the Sb‐Si sample were obtained. Passivation of poly‐Si grain boundaries by Sb is proposed.

Initial solid‐state reactions between crystalline Sb and amorphous Si thin films

Bilayers of Sb and Si thin films were deposited at room temperature on a thin (20–30 nm) Si3N4 film using electron‐beam evaporation. The solid‐state reactions in the bilayers were investigated using transmission electron microscope (TEM) during in situ annealing and Auger electron spectroscopy (AES). The reactions resulted in either an amorphous Sb‐Si (a‐Sb‐Si) alloy or caused crystallization of amorphous silicon (a‐Si) at low temperatures, depending on the film thickness of an a‐Si layer as well as the heating rate. As predicted from the phase diagram, no compounds between Sb and Si were observed. The initial intermixing of Sb and a‐Si was found to be anomalously fast.

Effects of Sb on phase transformations of amorphous TiSi2 thin films

Co‐deposited amorphous TiSi2 thin films with various Sb concentrations were prepared in order to study the effects of Sb on TiSi2 phase transformations. The crystallization behavior of the films was investigated by in situ annealing in a transmission electron microscope. The phase transformation from C49 TiSi2 to C54 TiSi2 in the films was examined with x‐ray diffraction. It was observed that incorporation of Sb resulted in a higher crystallization temperature of the amorphous TiSi2 film and a lower crystal growth rate. The activation energies for the crystal growth during the crystallization were determined to be 1.37, 1.62, 1.63, and 1.87 eV (±0.07 eV) for the films with 0, 0.3, 1.3, and 2.5 at. % Sb, respectively. For the C49→C54 transformation, it was, however, observed that the activation energy decreased when the Sb content increased. These indicate that the Sb incorporation in TiSi2 retards the crystallization of the amorphous TiSi2 film, but enhances the transformation from C49 TiSi2 to C54 TiSi2.

Thermodynamic investigations of solid-state Si-metal interactions. I, experimental and analytical studies of the Si-Ti binary system

An experimental study has been made on reactions in codeposited and multilayer films consisting of silicon and titanium deposited by electron beam evaporation. Transmission electron microscopy and Auger electron spectroscopy were used to determine structures and compositions. The experimental results from the codeposited films in the whole composition range (0%–100%) were compared with the predictions from the calculated free‐energy diagram of the Si‐Ti system. It is revealed that the phenomena of metal‐induced crystallization of amorphous silicon and formation of amorphous alloys appear in two different composition ranges in the binary system. Metal‐induced crystallization of amorphous silicon is attributed to lowering of bonding energy of SiSi bonds by titanium atoms in the Si‐rich composition range and the formation of an amorphous Si‐Ti alloy is attributed to the dominant SiTi bonding due to the largely negative heat of mixing between the two elements in a medium composition range.An experimental study has been made on reactions in codeposited and multilayer films consisting of silicon and titanium deposited by electron beam evaporation. Transmission electron microscopy and Auger electron spectroscopy were used to determine structures and compositions. The experimental results from the codeposited films in the whole composition range (0%–100%) were compared with the predictions from the calculated free‐energy diagram of the Si‐Ti system. It is revealed that the phenomena of metal‐induced crystallization of amorphous silicon and formation of amorphous alloys appear in two different composition ranges in the binary system. Metal‐induced crystallization of amorphous silicon is attributed to lowering of bonding energy of SiSi bonds by titanium atoms in the Si‐rich composition range and the formation of an amorphous Si‐Ti alloy is attributed to the dominant SiTi bonding due to the largely negative heat of mixing between the two elements in a medium composition range.

Silicon heterojunction bipolar power transistor with an amorphous Si:B alloy emitter

A pnp Si heterojunction bipolar power transistor has been fabricated using an amorphous Si0.7B0.3 alloy as a wide band‐gap emitter. The amorphous alloy is formed by co‐deposition of B and Si: it has a low resistivity of 2.5×10−3 Ω cm at room temperature after annealing at 1000 °C for 30 min and a band gap of 1.70 eV when annealed at 1100 °C for 20 min. In order to make a direct comparison, a conventional transistor with a diffused emitter and a polycrystalline silicon (poly‐Si) emitter transistor have also been fabricated. It is shown that an amorphous Si0.7B0.3 alloy emitter transistor can have an electrical current gain 2–5 times higher than a poly‐Si emitter transistor and 20 times higher than a conventional transistor.

Structural and optical band‐gap properties of the amorphous semiconducting Si1−xBx alloy

Thin amorphous Si1−xBx films, with x0 ranging from 0 to 0.5, were coevaporated onto preoxidized (100) Si wafers and quartz substrates, by using a dual‐electron‐gun high‐vacuum system. In order to study how the structural and optical properties depended on concentration and annealing temperature, heat treatments of the films were carried out at temperatures from 400 up to 1000 °C. The films were characterized by means of transmission electron microscopy, Auger electron spectroscopy, and spectrophotometry. It is shown that: (i) An amorphous Si1−xBx alloy can exist up to very high temperatures (≥1000 °C), when x is larger than ∼0.4; (ii) at 400 °C the optical band gap of an amorphous Si1−xBx alloy has the maximum value (1.59 eV) for x0=0.02 and then decreases to lower values for higher concentrations (e.g., 1.18 eV for x0=0.3); (iii) the measured optical band gap of amorphous Si1−xBx increases gradually with increasing annealing temperature up to 700–900 °C, and then increases rapidly when annealed at a high...

Influence of B concentration on recrystallization of polycrystalline Si

B‐doped polycrystalline Si films were prepared by electron beam evaporation and heat treatment in order to study the influence of B concentration and annealing temperature on recrystallization of polycrystalline Si. By using cross‐sectional transmission electron microscopy and Auger electron spectroscopy it is shown that: (1) at a B concentration between 1 and 10 at. %, post‐annealing at 1100 °C results in an enhanced solid phase epitaxial growth; (2) at a B concentration higher than 10 at. %, post‐annealing results in a retarded recrystallization of polycrystalline Si; (3) a relatively stable amorphous alloy can form at a B concentration of ∼35 at. % up to 1100 °C. The mechanisms of the enhancement and the retardation are discussed.

Thermal stability and crystallization products of amorphous Si:P alloy thin films made by coevaporation of Si and P

Thermal stability and crystallization of amorphous Si:P alloy thin films consisting of 20–44 at. % P have been studied in this work. The results show that the alloys have crystallization temperatures ranging from 850 to 1150 °C, which are all higher than that of pure amorphous Si, and that the variation of resistivity of the alloys during the 120 h aging at 300 °C is small (0.6%). These results indicate that the alloys have a high thermal stability, which is in agreement with the thermodynamic prediction we have made. It has also been observed that the crystallization products for these alloys are different. A new silicon phosphide phase has been observed in the 30 at. % alloy sample and suggested to be possibly a hexagonal Si7P3 phase which has lattice parameters a=5.32 A and c=13.3 A. The alloy films were deposited onto quartz substrates and Si wafers by coevaporation of Si and P. X‐ray diffractometry and transmission electron microscopy were utilized to investigate the crystallization temperature and p...

Optical and electrical investigation of semiconducting amorphous Si:P alloy thin films

Measurements are reported on the infrared (IR) absorption, the optical band gap, and the dark conductivity of amorphous silicon‐phosphorus alloy thin films (a‐Si:P) with 20–44 at. % P prepared by coevaporation of Si and P. The results show that the optical band gap can be tailored in a range of 1.5–2.15 eV by varying the P concentration and the annealing temperature. The band gap for the sample with 20 at % P is the widest (1.70–1.82 eV) when annealed at temperatures ≤600 °C. From the IR‐absorption study, a stretching mode associated with Si—P bonds at 465–474 cm−1 has been found for all alloy films. An evolution (shift or sharpening) of the Si—P absorption band with annealing temperature and P concentration have also been observed. Based on the IR‐absorption study the change of bonding structures in the alloy films is discussed. Conductivity measurements show that two electron conduction processes mainly exist in the investigated temperature range: extended‐state conduction in the conduction band at high...