3.2.2.1 Structure of amorphous Si
Amorphous Si differs from crystalline Si by having no long range order or periodicity. It can be described as having a frozen liquid structure or as a continuous random network of Si atoms. Generally each Si atom is four fold coordinated bonded to four other Si atoms but with large average distortion of bond angles and lengths that incorporate 5 and 7 ring structures as well as the 6 ring structure for the crystalline, as shown in Figure 3.4. Additionally amorphous Si has been found to contain defects without four fold coordination, such as dangling bonds and vacancy type defects (4).
3.2.2.2 Ion implantation amorphisation
Ion implantation of Si can produce amorphous Si layers or a damaged but essentially still crystalline structure depending on implantation conditions (4). For transistor device production there are distinct advantages for using implants that amorphise the Si therefore the focus of this thesis is on amorphising implants. The reasons for amorphising the Si will be discussed in some detail throughout this thesis but as a brief summary the main reasons are the production of higher quality defect free regrown layers compared to the regrowth of highly damaged layers. The junction depth can be spatially separated from the end of range, and channelling of the ions can be suppressed. The experiments carried out in this thesis on the whole use amorphised Si or are concerned with the formation of a continuous amorphous layer.
Amorphous Si layers can also be produced by deposition, e.g. chemical vapour deposition (CVD) or using low energy ion beams. However deposited amorphous Si layers are less well characterised and with less reproducible characteristics than ion implanted amorphous Si (17).
3.2.2.3 Collision Cascades, produced by single heavy and light ions
When an ion is implanted into Si it will probably undergo a hard collision with an atom near the surface. Energy will be transferred in the collision as described in chapter 2 (18). It is likely that the energy transferred will exceed the displacement
Figure 3.4 Structure of crystalline Si (left) showing the 6 ring structure and amorphous Si (right) where the 5 and 7 ring structure is visible. From (4)
energy Ed causing the target atom to be displaced. Any energy transferred exceeding Ed
becomes the resulting kinetic energy of this so-called primary recoil atom, which moves away from the ion track into the lattice. Primary recoil atoms with sufficient energy are able to displace further atoms, producing secondary recoil atoms, which in turn can create higher order recoils. This process continues, creating a collision cascade. The ion will continue travelling through the material, creating more primary recoil atoms along its path until all its energy is dissipated.
The energy deposition density is much higher for heavy ions compared to light ones. This difference results in the damage build up proceeding in a very different fashion for heavy and light ions, and is a critical factor for amorphous zone and layer formation. In this section the collision cascades of heavy ions are first described, followed by light ions, highlighting the differences. Different models for amorphisation, i.e. heterogeneous and homogeneous amorphisation, appropriate for heavy and light ions respectively, are described in the following sections, along with a brief summary of other relevant models and damage effects.
The amount of energy transferred in a collision is impact parameter dependent, with maximum transfer for a head on collision, and decreasing with increasing impact parameter. The average energy transfer is dependent on the scattering cross section (18). Heavy ions, with a larger scattering cross section are more likely to undergo collisions, and the average energy transfer is therefore higher for heavy ions (18). The mean free path for an energetic particle between collisions λd is given by
σ
λ 1N
d = (3.1)
where N is the atomic lattice density and σ is the scattering cross section, given in equation 2.6. Since σ is proportional to Z12 it follows that the mean free path for a heavy
ion is much shorter than for a light ion. It is in fact of the order of an interatomic spacing. A heavy ion therefore generates another displacement almost immediately after the first, causing the energy deposition density to be much higher for heavy ions than with light ones. Since primary recoils are created close together along the ion track, the individual cascades tend to overlap with their neighbours and the final result would be a giant collision cascade containing all the overlapping individual cascades. As the ion energy decreases along its path, its scattering cross section increases causing collisions closer together. The rate of energy deposition increases along the ion path, until towards the end of the ion path where the ion energy is low and correspondingly the total energy transferred to recoils is also low. The resulting collision cascade is therefore ellipsoidal in shape (18), as shown schematically in Figure 3.5 a). A heavy ion, such as As, hitting
a Si atom will suffer little angular deflection in a collision and will continue into the lattice making further collisions with Si atoms along its path. For heavy ions in the energy range of interest, the cascade from a single ion will produce a small amorphous zone.
Considering the behaviour of light ions, such as B, the mean energy transfer will be lower than with a heavy ion. The primary Si recoils produced from light ions will have on average less energy and consequently fewer secondary recoils will be produced. Secondary cascades will be small and may consist of only a few displaced atoms. For a light ion, the collision cross section σ is much lower. From equations 3.1 (and 2.6) it is obvious that the mean free path between collisions is much greater with a light ion than a heavy ion. This results in a much lower energy deposition density for light ions. As the small cascades will be created much further apart they will not overlap, so amorphous zones will not be produced (18). As a light ion travels a substantially greater distance between collisions a higher proportion of its energy will be lost inelastically, than with heavy ions. However this effect is less important at the low energies of interest where electronic stopping is low. A light ion has more likelihood of being deflected through large angles than heavy ions and so it will tend to take a zigzag pathway. The overall effect is that small pockets of damage are formed at locations along the path as shown schematically in Figure 3.5b). A single implanted light ion will not produce an amorphous zone.
In summary the distribution of displacements around an ion track depends strongly on the ion mass. Heavy ions have a much higher energy deposition density resulting in small amorphous zones. A light ion has a much lower energy deposition density resulting in damage which is sparse and created in pockets along a zigzag ion path. Important models of how these damage distributions produce a continuous amorphous layer are described in the following sections. The models are described with reference to the type of ion which is most appropriate.
3.2.2.4 Heterogeneous amorphisation (overlap model)
The first model views the production of a continuous amorphous layer in ion implanted Si as being due to the overlapping of the locally amorphised zones, produced by heavy ion bombardment (19). This overlapping cascade model is called heterogeneous amorphisation because it assumes that locally amorphous zones are generated heterogeneously by the incident ions within the cascade. As ion fluence increases, these amorphous zones accumulate and overlap to produce a continuous amorphous layer (19). Observations of heavy ion implant are consistent with the heterogeneous formation of an amorphous phase along a single ion track (20). The amorphous fraction increases linearly with dose for the heavy ions, but for light ions a phase change occurs over a small variation of dose once a high dose has been implanted (4). As such this heterogeneous model is more commonly associated with the build-up of damage from heavy ions. The ion energy primarily determines the width of the layer (21), with the dose having a secondary effect. Once an amorphous layer is produced, further implantation can push the depth of the amorphous layer to greater depths.
To apply this model to light ions, the number of overlapping ion tracks necessary to form an amorphous zone would need to be considered. Light ions with small cascades would require many overlapping ion tracks to produce the amorphous zones that act as a precursor for the formation of an amorphous layer (4). In comparison heavy ion implants can result in the creation of amorphous zones with no overlap necessary (4).