| Infrared technology has a history of 200 years. Because there is a specific band based on different types of objects emitting infrared band; people can use this specific band of infrared light to achieve the objectives of the object detection and tracking. IR infrared detector system is the core component of the IR system and the pilot of infrared technology development.Infrared detectors will be mainly used to receive infrared radiation converter for ease of measurement or observation of electricity, heat and other forms of energy, which is an energy converter. Between 1950s and 1960s, HgCdTe becomes the main materials of infrared detector. Moreover, different wavelengths of infrared detectors have been made. However, this kind detectors made by HgCdTe have some disadvantages. First of all, the stability of this material is not very good. Second, HgCdTe is highly toxic material and can cause harm to the environment and operators. At last, detectors made of HgCdTe could only work under lower temperature. In view of these adverse factors, another new material is needed to be HgCdTe alternative materials.Superlattices are mentioned in 1970 by Esaki, which are made of two different kinds ofⅢ-Ⅴsemiconductor materials and multi-layer structure. SLS used to be divided into two different types according to different band structures, that is, type-Ⅰa nd type-Ⅱ.And GaAs/AlGaAs belongs to type-Ⅰ, while InAs/GaSb belongs to the later.InAs/GaSb strained layer superlattices have been studied in the past years and can be considered as an alternative to HgCdTe IR material system. Type-Ⅱsuperlattices are constructed from theⅢ-Ⅴmaterial system, and hence they have much better mechanical proerties and material uniformity than HgCdTe. On one hand, due to the reduced Auger recombination coefficient resulting from the type-Ⅱband alignment, photodetectors based on InAs/GaSb have an advantage in high temperature operation over conventional HgCdTe. On the other hand, the type-Ⅱs uperlattice has staggered band alignment such that the conduction band of the InAs layer is lower than the valence band of GaSb layer. It has been shown that the thickness of each layer in the SL is very critical to the interband optical transition. Furthermore, InAs/GaSb superlattices material system can have some advantages over bulk HgCdTe, including lower leakage currents and greater uniformity. At last, in the case InAs/GaSb SLS structures the absorption is strong for normal incidence of light. Consequently, the superlattices structures could achieve a strong optical response because of the wave functions overlap of electron and hole in the adjacent wells.GaSb and InAs have similar lattice constants(6.096? and 6.058?)which differ by﹤1% and as such are said to be well lattice matched, which eases the fabrication of dislocation-free heterostructures. Using combinations of materials having the type-Ⅱs taggered offset, the bulk conduction band edge in InAs is lower in energy than the valence band edge in GaSb. The characteristic band gap of these structures is the transition energy between the hole ground state of the valence well in GaSb and the electronic ground state of the conduction well in InAs. As a consequence of the two wells residing in different materials, the holes and electrons tend to be localized in different layers of the structures. As a result, the IR detectors'wavelength made by InAs/GaSb type-Ⅱsuperlattices covers infrared region from 3 to 30μm. The growth of InAs/GaSb is developed at Xi'an Institute of Optics and Precision Mechanics of Chinese Academy of Sciences (CAS) by metal organic chemical vapor deposition (MOCVD). The three important factors that impact the growth of the material are temperature, source flows and pressure of the reaction chamber. For optimized epitaxial layer growth conditions, temperature is the important factor that the surface morphology can be changed with different temperatures.The material sources are TMGa, TMSb, TMIn and AsH3 (which is diluted to 10% by H2). Sample 802# GaSb layer was grown on n-InAs substrate at the temperature of 520℃. While sample 812# was grown on Te-doped GaSb substrate and 5 periods InAs/GaSb superlattices. After the growth, the sample 802# surface is smooth and bright at the first observation; the surface of sample 812# was smooth and bright, but under magnification of 200 times, we saw a mist band on the surface.791#-798# were grown on GaSb, SLS. 791# was grown at 550℃, while the growth temperature of samples 792#-798# was 520℃. At the normal observation, we found there were some black dots on surface of all samples. But sample 798# had no back dots.Atomic force microscopy (AFM) has been used to determine the surface morphology of samples. Temperature of the reactor, pressure and flow of sources are the important factors that impact the surface of samples. 2D and 3D of 802#-03,802#-04,802#-05 and 812#-01are obtained from the observation of 802# by AFM. The surfaces of both samples are characterized by hillocks distribution of sizes. The GaSb layer surface has a maximum peak-to-valley height range of 84nm. The number for the InAs/GaSb superlattices is 20nm because of longer growth time for sample 802#. And the hillock density for the sample 812# is significantly lower than for the sample 802#. But compared with the sample 656#, the sample 802# has a lower hillock density.The scanning range for all of these AFM imaginations is 5μm×5μm. The superlattices sample 812# had widest atomic step than sample 802# and sample 656#, which is expected. From the discussions above, we come to a conclusion that the temperature of the reactor is kept at 520℃for InAs/GaSb superlattices growth in order to obtain better surface morphology. The diffraction curves of sample 793# and 796#-798# are obtained from X-ray diffraction. The diffraction peaks of samples 797# and 798# are higher and the diffraction intensity is stronger than samples 793# and 796#. Those can be explained they have better quality. The surface morphology of sample 812# is what we have expected from this article. |
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