DOPANT MODIFICATION OF SEMICONDUCTORS WITH CONTINUOUS AND PULSED ION BEAMS
R.M. BAYAZITOV
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Continuous ion implantation is one of the widely used methods for introducing impurities in semiconductors for microelectronic devices. Ion beam formation of submicron highly doped layers on semiconductors is an important problem for creation of contact regions and active layers in microwave devices, solar cells, opto-electronic devices etc. However, after implantation consequent thermal annealing or pulse irradiation treatment is a necessary condition for removing radiation defects and for electrical activation of impurities.
In this work the behavior of impurities in the implanted semiconductors after subsequent treatment by powerful ion beams is analyzed. The main techniques for creating highly doped layers are:
-Continuous ion implantation. In experiments, industrial Si and GaAs wafers implanted by various dopant ions with continuous beams were used (up to 100 keV; P+,B+ : Si ; Te+, Si+ : GaAs). This implantation creates amorphous layers with thickness of up to 0.2 pm.
-Subsequent pulsed ion beam treatment (PIBT) (C+, IT, E= 300 keV, t=50 ns, W<1.5 J/cm2 ). The total implantation dose per one pulse is considerably lower than that at the continuous implantation. For comparison, we also used pulsed laser annealing by the nanosecond ruby laser (X = 0 .6 9 pm, t=50 ns, W<1.5 J/cm2 ).
In fig.l one can see pulsed ion beam accelerator used for PIBT. This accelerator we usually use for treatment of various construction materials, for example, for hardening tools , sputtering of different materials for surface coating. Recently it is used for treatment of semiconductors too.
After laser or pulsed ion treatment with the energy density of about 1 J/cm2 the melting depth reaches the crystalline substrate and the subsequent epitaxial crystallization takes place. This layer is characterized by the absence of growth defects and by the very high concentration of impurity in the substitution position in the crystal lattice. At high energy the disruption of the surface takes place.
PIBT has some advantages if compared to laser annealing. It has greater productivity. Also it has peculiarities in energy losses in material.
Fig .1. Pulsed ion beam beam contents vacuum Ion energy, E pulse duration, t current beam, I current density, J pulse density, W pulse frequency productivity accelerator - C+(80%), H+(20%) - 10-5 Torr - 300 keV - 50 ns - up to 10 kA - up to 200 A/cm2 - up to 3 J/cm2 - up to 0.2 c-1 - up to 1 m2/h
In fig.2 depth profiles of energy losses in amorphous Si for ion and laser irradiation are presented.
The characteristics of PIBT in comparison to laser beams is a more uniform energy depth distribution in material which causes deeper melting and diffusion of impurities without surface disruption.
Fig-3. Carrier concentration and mobility profiles for Si implanted with P+ at dose of 21016 cm-2 after pulsed laser treatment (1,2) and after PIBT (3,4). LSS - calculated phosphorus distribution after implantation.
Uniform energy depth distribution enables us to form highly doped layers in silicon (fig.3). At the optimal energy density the thickness of the doped layer is about 0.5 |im, which considerably exceeds the depth achievable by laser annealing. The concentration of carriers exceeds by about 5 times the equilibrium phosphorus solubility in silicon.
It is very difficult to produce high levels of the carrier concentration in n-GaAs by conventional methods including ion implantation and subsequent thermal or laser annealing. Thermal annealing of implanted GaAs does not cause considerable increase in the concentration of donor atoms in substitutional positions and in the carrier concentration. Nanosecond laser annealing enables one to increase concentration of atoms in substitutional positions but concentration of carriers is low due to decomposition of GaAs. Using continuous ion implantation and subsequent powerful ion beam treatment heavily doped (81019cm-3) layers may be formed in the subsurface region (0.1-0.5 ^m) (fig.4).
Both for PIBT and laser annealing we observe a significant redistribution of atoms in GaAs. Results of computer simulations of liquid phase diffusion show good agreement with experiment. The high values of the carrier concentration were obtained: up to 1020cm-3 at pulsed ion treatment and 1...31019cm-3 at pulsed laser treatment with the carrier mobility higher than
DEPTH, nm 0 150 300 450 O F 8 T,0C
Fig.4 .Si-concentration profiles obtained from AES spectra for Si+ implanted GaAs (80 keV, 1016cm-2) before, after laser treatment and after PIBT. Dashed curves-calculated distributions of Si after treatments (D=5 -10-4 cm2/c). Dotted curve- electrical activity of Si after ion pulse treatment.
Fig.5 .Sheet electron densities in Te+(60 keV) implanted GaAs after PIBT (1,3) and laser treatment (2) as a function of subsequent thermal annealing (5 min). 1,2- D=1016 cm-2; 3- D=1015 cm-2. Arrow points to a maximal density obtained after only thermal annealing of Te+ implanted GaAs in As-H2 ambient atmosphere.
Near the surface the concentration of Si atoms essentially exceeds the carrier concentration. This fact can be attributed to decomposition of GaAs and formation of neutral pairs or impurity- vacancy complexes.
Carrier concentration in n-GaAs layers more than by the order of magnitude exceeds the solubility limit in the crystal. These supersaturated GaAs solutions are unstable with respect to subsequent thermal treatments. Fig.5 shows sheet electron densities in implanted GaAs after ion (1,3) and laser (2) treatment as a function of subsequent thermal annealing (5 min). 1,2- Dose=1016 cm-2; 3- Dose=1015 cm-2. Arrow points to a maximal density obtained after only thermal annealing of Te+ implanted GaAs in As-H2 ambient atmosphere. After this stabilisation the concentration level is about one order of magnitude higher than that obtained after conventional annealing.
Calculations predict that growth velocities of several meters per seconds are achieved at PIBT. Under this conditions the great part of implanted impurities is trapped by the crystal. However, in the case of low solubility the greater part of atoms may by pushed to the surface. This segregation effect is observed for indium atoms in silicon (fig.6). At the high concentration the interface may be unstable and the impurity is pushed to the surface too. This effect leads to the cell structure formation. Structure of the layer consist of the column single- crystalline silicon.
Walls of these column consist of metallic indium. The column size depends on the pulse energy density.
Fig.6. Cellular structure formed by PIBT (W = 1.0 J cm-2 ) of In+ implanted Si, D = 21016cm-2
Fig.7. Optical micrographs of Si surface after C+ implantation (40 keV, 51017 cm-2) and subsequent PIBT (W = 1.0 J/ cm2 ).
High-dose ion implantation followed by PIBT may be also used for synthesis of submicron layers of compounds. In case of pulse treatment of the carbon implantation in silicon with subsequent PIBT, single-crystal or large-grain polycrystal layers of silicon carbide are formed. However, due to large difference of melting points of silicon and carbide, crystallization from highly undercooled melt takes place. In this case dendrite morphology of the surface is observed (fig.7). Implantation of transition metals in silicon with PIBT could give silicide formation.
Thus, due to high productivity and peculiarity of energy losses, PIBT is an effective method for semiconductor modification.
