First-principles Study Of Amorphous HgCdTe Material

Along with the infrared detection technology development, infrared detectorshave developed to the third generation. And its typical characteristics are the largearea, multi-colored, long wave and highly integrated. The main scientific problemsof the third-generation infrared detectors development appear in the optoelectronicmaterials and the suitable substrates and the processes. These two problems aremostly related to the quality of the crystal. On the other hand, the amorphous mate-rials have the similar properties to the crystals. And it has advantages of low cost,simple process and no restrictions on the substrates. Thus, for the barricades of theinfrared detector developments, the amorphous material has great prospect in appli-cation. In this thesis, starting with Mercury Cadmium Telluride (HgCdTe) crystalstructures, we investigated the structural and properties variations. Further more,based on the concept of Continuous Random Network, we established the amor-phous models of HgCdTe semiconductors. In addition, we studied the Density ofStates and the localization of our models to find out the relationship between thenetwork disorder and the electronic peroperties. The main conclusions obtained inthis thesis are as listed follows:1. Due to the special dependency between material parameters and its electronicstructures, we firstly studied the HgTe properties with various deformations.According to the previous researches, we can know that both sp3and p-d cou-pling play an important role in the II-VI narrow-gap semiconductors. There-fore, remaining the local symmetry, only change the bond length can effec-tively modified the band gap of CdTe, while for HgTe, the impact is muchsmaller. Furthermore, through the analysis of HgTe in different structures andunder uniaxial, we found that when the bond angles changes, there is a posi-tive band gap opened in HgTe.2. Based on continuous random network concept, we successfully establishedamorphous models of HgTe. For our a-HgTe models, we found that for short order, the structures tend to remain the crystal characteristic-tetrahedralstructure. So the average coordination number is around4. However, the coor-dination number statistics show that there are some dangling bonds and float-ing bonding in the systems. And for all the models, the wrong bond ratios arearound10%. So our models present topological disorder and chemical disor-der.3. Different XC function obtained similar electronic states distributions. Highlocalized states concentrate in the core states of anions and cations. There aretail-states appearing for all bands and these states have extremely high inverseparticipation ratio values. Besides that, there are localized tail states presentedin CBM and VBM. More interesting, both of our a-HgTe models have a bandgap of about0.4eV, which is much different from the crystal property. Thesestates mainly come from Te atoms. The distortion of Te network and the ex-istence of defect states open a band in a-HgTe systems.4. The ternary amorphous models of Hg1-xCdxTe constructed by theory are rea-sonable and reliable. The structures are consistent with the experimental data.Similar to the binary structures, the a-Hg1-xCdxTe models remain the structuralcharacteristic of crystal–the tetrahedral arrangement, and the average coordi-nation number is around4. But in spite of the composition, it has a smallamount of dangling bonds and floating bonds in the systems. And the wrongbond ratios are around10%. Thus, our a-Hg1-xCdxTe models are also con-sistent with the amorphous structure characteristics.5. It is found that the different models with different components have similarelectronic states properties. The deep states, which are mainly from the corestates of atoms, naturally have highlocalization. And the tail states induced bynetwork disorder have extremely high IRP. For the bonding states, there arerelatively high localized tail states appearing in CBM and VBM. Similarly,they are related to the the network distortion of Te.6. Under the influence of the disorder, a-Hg1-xCdxTe has more complicated elec-tronic properties. By investigation the bonding mechanism, due to the sym- metry missing, the coupling between d states of cations and p states of anionsis weaken, which make the d states complete shrink below-4eV.7. We also find that, comparing with the crystal material, composition variationhas little effect on properties of Hg1-xCdxTe in amorphous structures. The dif-ferent models with different composition which we build have similar struc-tural parametes. And the density of states analysis shows that all the modelshave band gap around0.3eV. So we think the similar Te atom network is thereason of similar band gap. And the amorphous disorder weakens the impactof compositon variation on the electronic properties.