Study on surface defects for InSb mid-IR FPAs

Abstract

Indium-antimonide (InSb) is the most narrow band-gap material among III-V compound semiconductors. It has a high potential for device applications involving infrared detectors, high frequency electronics, and magnetic field sensors. In order to fabricate InSb devices with high performance, reducing dark current is the most important issue. The reduction of dark current has a great importance because it has strong relation with the detectivity of InSb photodiodes. Especially, InSb has a very weak atomic binding between In and Sb atom, surface defects can be easily generated by plasma energy and heat. These process-induced surface defects can act as surface trap sites in the energy bandgap and it is one of the main causes increasing the dark current. In this sense, to develop the InSb FPAs with minimized surface defects, it has to be explained what kind of surface defect can be generated by plasma or thermal energy at the each step during the fabrication process and what kind of factors can affect the generation. The objective of this thesis is to suggest the answers to the questions about both What kind of defect can be generated by plasma and heat and How to reduce or prevent it. To achieve this objective, the research has been focused on how to measure the surface defect for investigating what kind of defect are existing and on how to reduce and prevent the defect generation during fabrication process. First, by applying Raman method to plasm-induced defect analysis, it was proved that the enhancement of TO scattering was originated from plasma-induce defects. As the applied RF power in Ar-ion etching raising from 50 to 200 W, the integrated area ratio of the TO mode to that of the LO mode (ITO/ILO) increased from 0.05 to 0.23 and the intensity of the TO phonon mode was fully restores back after annealing process under Sb ambient in the spectra. It clearly indicated that the origin of unintended enhancement of TO scattering in Raman spectra is plasma-induced defect due to the preferential loss of Sb atoms near the surface, not rough surface as reported by F. Frost et al. Second, with increasing the environmental temperature from 25 to 500oC, phase change of indium-antimonide near the surface during annealing process were investigated by in-situ Raman spectroscopy. When the external temperature was above 450oC, elemental antimony phase began to form near the interface between thin native oxide and InSb substrate. Furthermore collecting spectral data with spatial resolution and encoding it in a 2D plot generates images with information complementary to optical imaging. As a result, images of Raman map measured at 450, 475 and 500oC for Sb phonon (Eg) mode represented clearly the visible growth step of Sb region on the surface. In a series of fabrication process, InSb surface is exposed to the applied plasma and heat frequently. We have developed the multi-step plasma etching to reduce the plasma-induced defects. As gradually increasing the amount of N2 gas flow during the etching process, the smooth surface was obtained. Furthermore, Raman analysis of the InSb surface after the plasma etching indicated clearly that the multi-step etching process was an effective approach in reducing the plamsa-induced defects on the surface. Moreover, we have clarified the degradation mechanism of passivation properties and suggested the critical temperature to avoid it. The shape of C-V characteristics was dramatically changed when the deposition temperature was higher than 300oC. Raman spectra represented that elemental Sb accumulation resulted from the chemical reaction of Sb oxide with InSb substrate was responsible for the failure in the C-V characteristics of MIS structure. Thus, it was proved that the temperature have to be kept below 250oC during passivation process. In conclusion, all of these works were aimed to enhance the device properties of InSb devices. Surface defects induced by plasma and thermal energy can cause the degradation of device performance. In this thesis, it was proven that Raman spectroscopy can analyze the change of surface structure very effectively. Furthermore, based on these results, we could reduce the dark current and enhance the R0A effectively.