Investigation Of Surface Morphology Of Gan-based LED Epitaxial Wafer During Sims Analysis

Secondary ion mass spectrometry (SIMS) is an irreplaceable technology forquantitative analysis in the microelectronics scientific fields due to its excellentsensitivity and high resolution. During SIMS analysis, the sputtered surfacemorphology and structure would be changed, which affects the measurementprecision. Especially for the LED epitaxial wafer with complex multilayer structure,the changes of surface morphology by ion sputtering is uncertain. However, therehave been seldom reserch concerned so far. In this paper the GaN epitaxial waferbased on Si substrate developed by Nanchang University have been studied by use ofa Cameca IMS-7f SIMS instrument, atomic force microscope (AFM) and scanningelectron microscope (SEM). We focused on the evolution of sputtered surfacemorphology and the influence to the SIMS depth profiles, A new method ofmeasuring V defect density in the epitaxial wafers were proposed. These studiesprovide a reliable reference for further understand as well as utilize SIMS abundantspectrogram information. Some innovative research results are as follows:1. When2keV Cs+ions of SIMS sputter on the smooth surface of GaN epitaxialwafer, many nano-dots appear on the crater bottoms. The diameter of the dotsincreases with sputtered depth, whereas the density of dots decreases. After SEMand EDS analysis, these nano-dots are considered metal Ga droplets.2. Combined with the binary phase diagram of GaN and nucleation growth theoryof crystal solidification, we consider that there are two aspects to make Gaforming droplets and then grow up. On the one hand is the surface temperaturerisen during ion sputtering, which make GaN thermal decomposition. On theother hand, Nitrogen element is easy to be preferential sputtering.3. According to the SIMS depth profile curve and the topography feature of craterbottom, it has been proposed that a new method for characterization of V defectdensity in GaN-based epitaxial wafer. When2keV Cs+ions sputter on the smoothsurface of GaN epitaxial wafer, both nano-dots and pits are observed on the crater bottom. These pits are related to the internal V defect density of GaN-basedepitaxial wafer. The internal V defect density of GaN-based epitaxial wafer canbe evaluated by the pits density of crater bottom.4. The characteristics of morphologies have been compared by different Cs+ionsbeam energy bombarding. If the initial surface of the wafer is smooth, thesputtered crater bottom remains flat without nano-dots after higher energy ionbeam bombarding. However, if there are V defects in the wafer surface,nano-dots appear on the crater bottom after higher energy ion beam bombarding.5. Statistical analysis on a large sample of experiment data indicates the followingtwo folds:(1) After being bombarded by low Cs+ion beam energy and smallbeam current, the sputtered surface becomes rougher, the sputtering rate getsslower and the secondary ion yields are lower; but the depth resolution isimproved.(2) After being bombarded by higher Cs+ion beam energy and largebeam current, the sputtered surface remains smooth, the sputtering rate is fasterand the secondary ion yields are higher; but the detection limits of elementalanalysis is reduced.