one II-VI compounds
As shown in the periodic table of elements in Fig. 1, group E elements (including Zn, CD, Hg) and group H elements (i.e. s, Se, Te, etc.) constitute group II and VI compounds with semiconductor properties, Its basic properties are shown in Fig. 1[5l. Except that CdTe and ZnSe have n-type and p-type conductive properties, other II-VI compounds can only obtain a single conductive form, such as p-type ZnTe and n-type CDs, CdSe, ZnS, etc. Therefore, there are some restrictions on the matching of materials. According to theoretical calculation, PN homojunction (Homo J function) The energy conversion efficiency of solar cell is the highest, and its energy gap is 1 About 5 EV, so CDT has naturally become the most suitable material for solar cells in h-group compounds.
II-VI compounds have stronger ionic bonds than V group. In addition, the vapor pressure of group II and VI elements is similar and their value is not low. Therefore, in the preparation of materials, it is easy to form compounds by self interactive coordination. For the growth of thin films, if the substrate temperature is set so that the same atoms (ii-ii or vi-vi) cannot form valence bonds, the elements of group H and group m only have interactive valence bonds to form group II-VI compounds; Under normal coating conditions, the temperature is not high, about 300 ℃.
Some of the crystalline structures of II-VI compounds are cubic sphalerite structures, such as ZnSe, ZnTe, CdTe, etc., while others are hexagonal structures, such as CdSe; CDs and ZnS have one or both of the above two structures according to the material preparation conditions. Different structures have different properties.
In semiconductor materials, the strength of valence bond is directly related to the energy gap. From the periodic table in Fig. 1, those with small atomic numbers of constituent elements in II-VI compounds have strong ionic bonds. For example, Zn Se is stronger than Zn te, and CD-s is also stronger than CD te; Their energy gap also reflects the strength of bidding bond. Compared with CD te, Zn te has stronger valence bond and larger energy gap.
- I-ill-vi group 2 compounds
Group I VI 2 compounds can be regarded as three element compounds derived from group II VI, that is, the elements of group H are replaced by group I (Cu, Ag) and group I (al, GA, in) and combined with group VI (CS, Se, TE). Because there are two kinds of atoms arranged alternately and orderly in the lattice position occupied by cations, the unit cell seems to be formed by stacking the unit cells of two amphibolites. However, the unit length ratio of c-axis and a-axis is different due to the valence bond strength of two different cations and anions, which is not equal to 2. Such a crystalline structure is called chalcopyrite structure, as shown in Figure 2.
As can be seen from the periodic table of Fig. 1, this series of compounds include 18 compounds such as group I elements Cu and Ag, group IV elements Al, in and GA and group M elements s, Se and te. Like other compound semiconductors, their energy gaps can form quaternary or even quinary compounds through the mutual substitution of homologous elements in a certain range. The biggest difference between it and tianv or HVI compounds is that there is no composition very close to the chemical ratio. The composition stability range of single phase can reach 3% ~ 5%, and the degree of deviation from the fixed ratio composition of 1:1:2 is considerable. The above situation can be seen from the pseudo binary phase diagram of Fig. 2. The situation of deviating from the fixed specific composition will produce intrinsic point defects in the material. There are 12 kinds of ternary compounds of III-VI group, and the distribution of the number of various point defects is closely related to the chemical composition and the formation energy of defects.