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Zikrillayev Nurilla, Saitov Elyorr Botirov Bozorbek, Nasirdinov Bakhodirw, Kurbanov Yunus, Turayev Farxodjon, Shodiyeva Nozina, Faculty of Power Engineering, Tashkent State Technical University, Tashkent, Usbekistan E-mail: gelyor.satov@gmail.com, elyor.saitov@yandex.ru
STUDY OF THE RESULTS OF DIFFUSION DOPING TECHNIQUE FOR PRODUCING HETEROSTRUCTURES (Si-Ge) USING MICROPROBE ANALYSIS
Abstract. Large-scale application of solar power in all areas of the economy is mainly determined by the efficiency, reliability, and cost factors of solar cells. Currently, solar cells based on silicon have exhausted their peak capacity, the efficiency ratio of such elements is believed to hit 20%. Silicon -germanium solid solutions are widely used in electronics and especially in nanoscale electronics technology. The unique properties of this material make it possible to create devices with parameters superior to those manufactured on the basis of AmBV compounds. It is known that solid solutions of Six xGex is mainly produced by liquid-phase epitaxy or during cultivation. However, the above techniques barely allow to obtain solid solution on the crystal surface with desired thickness and composition. Neither allows it to engineer nanoscale structures on the basis of germanium in the silicon lattice.
Keywords:
Introduction
The technique of building nanoclusters of impurity atoms in the crystal lattice of semiconductors with desired concentration and structure is a fundamentally new approach in engineering nanoscale structures.
In compare to the existing techniques of creating nanoscale structures using expensive equipment in the process of complex technological operations, the diffusion doping technique for building nanoclusters of impurity atoms is believed to manifest several advantages among them are are the ability to
create nanoscale structures across the entire bulk of the crystalline material, capacity to control their size and structure, and the ability to create magnetic and multiply charged nanoclusters.
The development of new materials for solar power absorption, namely based Si with Ge clusters, is of particular importance due to natural obstruction related to the process of photonic-induced generation of carriers and thus the width of the forbidden band. This restriction is mainly due to the inability to use for photonic-induced generation of charge carriers with energy hv < Eg (representing the wider infrared
solar spectrum \ = 1.2^3 ^m), which is almost ~40% of solar energy.
The ultraviolet (UV) and visible solar spectra (where hv > Eg) due to the effect of thermal photon-induced generation of hot charge carriers, also results in the loss of solar energy. The existing production technology of silicon photocells virtually does not allow to eliminate these losses.
Although the above problem is partially solved by using multi-stage (sandwiched) photocells based on semiconductor compounds such as Ga As, Al Ga As, nevertheless, the production technology of such photocells is not only rather complicated, but also requires the availability of expensive equipment.
Besides that, their price does not allow for large-scale application and the creation on their basis of high-power solar stations. Therefore, this problem can be successfully solved only on the basis of the development of novel class of semiconductor materials or on the basis of new physical phenomena.
Elements of AIII and AV subgroups are inserted in[to] the lattice of germanium and silicon, mainly replacing the atoms of the main substance, and at the same time behave in accordance with their valence. The atom of the AV subgroup element allocates four valence electrons to form a chemical bond, and one of its electrons can be transferred to the conduction band.
The atom of AIII subgroup element gives three valence electrons to form a chemical bond and can attract one electron, which will lead to the formation of a hole in the valence band. Thus, the elements of the AV subgroup behave as simple donors do, and the elements of the AIII subgroup do as simple acceptors do, forming shallow energy levels in the forbidden band. The ionization energy levels of impurities of the elements of the AV and AIII subgroups, and of lithium in lightly doped Si and Ge determined via the "conductivity as a function of temperature" curve are given in (Table 1).
Table 1. - Thermal ionization energy of impurities of the elements AV, AIII of the subgroups and Li in Si and Ge
Silicon Germanium
Donors Acce Jtors Donors Acce )tors
Atom Ei, eV Atom Ei, eV Atom Ei, eV Atom Ei, eV
Li 0.033 B 0.045 P 0.0120 B 0.0104
P 0.044 Al 0.057 As 0.0127 Al 0.0102
As 0.049 Ga 0.065 Sb 0.0096 Ga 0.0108
Sb 0.039 In 0.16 In 0.0112
Bi 0.069 Tl 0.26 Tl 0.01
From (table 1) it is clear that the ionization energy of A111 and AV subgroups' impurities in germanium differ from each other significantly less than those in silicon. In germanium, the ionization energy of impurities from different subgroups differs negligently to the level of 0.01 eV, which is predicted by the hydrogen-like model for impurity atoms [1].
A small difference in the ionization energies of different impurities in this case shows that for impurities of the A111 and AV subgroups in germanium, a hydrogen-like model gives a a rather satisfactory approxi-
mation. In silicon, the situation however is different: there is a large difference in the ionization energies of impurities, even from the same subgroup than in germanium for impurities from different subgroups.
The stark contrast in the values of ionization energies of impurities from the Am and AV subgroups in Si and Ge from the values predicted by hydrogen-like model is due to the fact that at interatomic distances the potential created by the impurity ion differs significantly from the potential of the point charge and depends on the chemical nature of the impurity.
This short-acting portion of the impurity potential creates an additional potential to the magnitude of the ionization energy predicted by the hydrogenlike model, the so called the displacement of the impurity level, otherwise known as chemical shift. Due to the chemical shift, the impurity levels of different impurities differ from each other.
The practical significance of impurities that give shallow energy levels in the band gap of a semicon-
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ductor is that they have low ionization energies (for example, in germanium at temperatures of10 K, almost all atoms of these elements are believed to be completely ionized) and dissolve well in the doped material, have high coefficients of solubility, and low diffusion coefficients, therefore, by embedding them into semiconductors, one can widely vary the concentration of electrons and holes in them (from the intrinsic up to 1021 cm-3).
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Figure 1. Energy levels of impurities in Si. The symbols "+" and "-" denote donor and acceptor levels, respectively
The behavior of impurities in silicon and germanium becomes more complicated as they are placed further in the periodic system from the elements of the AIV subgroup. Most of the impurities from groups I, II, VI and transition metals in lightly doped silicon and germanium are substitutional impurities, although some of them can be placed
in the lattice of the basic material and (or) in the interstices. For example, cuprum in germanium. Cu atoms enter both nodes and interstices in the Ge lattice. Magnesium, calcium, strontium and barium in silicon and germanium can also be placed both in the nodes and in the interstices of the crystal lattice [2].
Figure 2. Energy levels of impurities in Ge. The symbols "+" and "-" denote donor and acceptor levels, respectively
Impurities of I, II, VI groups and transition metals form several alternative deep local energy levels in the forbidden band of silicon and germanium. Deep states, as a rule, occur when the main contribution to the binding energy is not of electric attraction nature weakened by the dielectric constant (hydrogen-like approximation), but of a short-acting potential, which is determined by the chemical nature of the impurity.
Shallow donor states can be seen as split off from the conduction band, and shallow acceptor states from the valence band. Deep states equally belong to both bands and can be both of donor and acceptor nature. The position of the experimentally determined energy levels of some impurities from these groups in the silicon and germanium bandgap are shown in (Figures 1 and 2), respectively.
Figure 3. Diagram of the distribution of germanium atoms in the near-surface region of siliconIt can be assumed that the behavior of substitution impurities that form deep levels in Si and Ge corresponds to their valences
On the outermost orbital of atoms from this group, there are two electrons ns2, whereas all valence electrons participate in the formation of tetrahedral bonds, and in the case of elements of group VI, four valence electrons participate in the formation of these bonds and two valence electrons can be transferred to the conduction band. In the semiconductor
lattice in the ionized state, these impurities are present in the form of multiply charged ions.
It should be noted that the behavior of transition metals from groups VII, VIII and with an unfilled 5d- shell in Si and Ge is studied in less detail than of impurities of other groups (see Fig. 1 and Fig. 2). Most often they form several deep levels in the
forbidden band. As noted above, if these impurities are placed in the nodes of the Si or Ge crystal lattice, then their behavior, as a rule, corresponds to their valence, i.e. they are doubly charged acceptors [3].
Amphoteric impurities in silicon and germanium can be atoms with an unfilled 5d- - orbital, can be donors or acceptors in one of the crystalline positions, or those that can be placed both in the nodes and interstices of the crystal lattice, showing the donor and acceptor properties depending on their location [4].
Experimental and Discussion
The diffusion process was carried out in stages by gradually increasing the temperature, starting from room and rising it to the diffusion temperature. The diffusion temperature would increase from room temperature and in 3 hours would reach 1200 °C, and then the samples were kept at this temperature for 2-4 hours. This experiment was repeated several times and each time five samples were used.
The concentration profile of germanium atoms and their distribution across the depth was studied using X-ray microprobe analysis of Joel super probe JXA-8800 R/RL. (Figure 3) shows the distribution of germanium atoms (as well as the distribution of silicon atoms) in the near-surface region of sili-
Figure 4. Pictures of the microstructure of the butt side of sample received on Joel super probe JXA-880 R / RL with clusters of impurity atoms of Ge atoms at T = 300K
con (the diffusion was carried out at temperature T=1180 °C).
As can be seen from the figure, to depth of d ~ 0.6 ^m on the silicon surface, the concentration of germanium atoms is greater than that of silicon, i.e. Si1 xGex solid solution is being formed at x > 0.5, then the concentration of germanium atoms decreases sharply, and at d > 3 ^m it decreases to a substantial degree, that due to the limited sensitivity of the device, their concentration is even difficult to determine.
Thus, in this case, one can obtain a continuous solid solution Si1 xGex, where x varies in the range of 0 4 0.5. Figure 4 shows the microstructure of this sample from the front and surface sides. The figure clearly shows the boundary between Ge and Si, as well as the thickness of the doped Ge layer, which is quite accurately consistent with the results shown in (Figure 3).
An analysis of the above results reveals the following important phenomena. The maximum penetration depth of germanium atoms under these diffusion conditions should be roughly d~ 0.5-0.6 ^m.
This value is almost 6-8 times less than in the experimental data obtained (Figure 3), i.e. This means that according to the new doping technology, the diffusion process of germanium atoms is significantly accelerated, and, accordingly, the diffusion coefficient of germanium increases by 6 4 8 times.
Figure 5. Pictures of the microstructure of the surface of sample received on Joel super probe JXA-880 R / RL with clusters of impurity Ge atoms at T = 300K
To identify such significant discrepancies, the authors carried out diffusion of germanium into silicon at T = 1200 °C using the conventional diffusion technology. As shown by the results of research using the X-ray microprobe microanalysis method, in such samples the depth of penetration of germanium is almost consistent with the reference diffusion coefficients.
The data in (Fig. 3) allow us to make an important conclusion that the diffusion process of germanium atoms in silicon can be significantly accelerated using the new technology.
By controlling the heating rate between the stages and choosing the parameters of the stages in the diffusion process, it is possible to obtain a Si1-xGex solid solution with the required thickness and composition. It is possible to obtain in a sufficiently thin near-surface region of silicon a continuous solid solution type Si1-xGex with a value ofx = 0 ^ 1[5].
It has been revealed that the proposed low-temperature multi-stage technique of doping semiconductors with impurities is a practically new technical solution of the diffusion process. This technique has the following advantages over high-temperature diffusion:
1) Eliminates the erosion of the crystal surface during the diffusion of impurities (Mn, Ni, Se, Te, ...), which always takes place during high-temperature diffusion;
2) Eliminates the formation of various types of alloys, silicides and other compounds Si + impurity, which also occurs during normal diffusion;
3) It significantly stimulates the diffusion process, which provides not only time and energy savings, but also more uniform doping of samples.
4) The new multi-stage low-temperature doping method developed by the authors ensures the formation of clusters of impurity atoms in the bulk of the crystal. By varying doping conditions, one can control the parameters of clusters of impurity atoms.
Conclusion
Thus, it can be argued that the silicon with het-erostructure-nature clusters is the material of future energy, since it will be possible to create sandwich-shape photocells on a single crystal on the basis of such materials and the need for complex technological operations and expensive AIIIBV and AIIBVI semiconductor materials is substantially eliminated. Of great practical interest is the information on the use of clusters such as micro and nanoscale hetero-junctions to create new classes of microelectronic devices, as well as photocells.
The team of authors is grateful to Academic Bakhadyrkhanov M. K. for his valuable advice and discussion of the experimental results.
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