SIMS Quantification of Matrix and Impurity Species in III-Nitride Alloys
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Date
2006-02-09
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Abstract
New applications in optoelectronic devices and high power electronic devices continue to be developed using III-Nitride. In the case of Al[subscript x]Ga[subscript 1-x]N, the quantification of matrix and impurity species is essential for matrix composition analysis, dopant control, and impurity control. Dynamic SIMS quantification in Al[subscript x]Ga[subscript 1-x]N is challenging because of matrix and charging effects. The secondary ion yield of matrix and impurity species varies in Al[subscript x]Ga[subscript 1-x]N with different AlN mole fraction (x). Al[subscript x]Ga[subscript 1-x]N also shows charging effects when the material becomes more insulating with increasing AlN mole fraction.
In this work, a SIMS quantification method is developed for the Al[subscript x]Ga[subscript 1-x] N system over the range of x = 0 to 1. A set of Al[subscript x]Ga[subscript 1-x]N films prepared on SiC or sapphire substrates with AlN mole fraction ranging from 0 to 0.58 were implanted with ¹⁶O, ²⁴Mg and ²⁹Si. Very high Al concentration Al[subscript x]Ga[subscript 1-x]N samples were created using high dose ion implantation of Ga into AlN. With these samples, calibration curves of matrix ion intensity ratio for quantification of Ga and Al matrix constituents, Relative Sensitivity Factors (RSF) for impurity species, and sputter rate as a function of AlN mole fraction were obtained. Using these calibration curves, the matrix and impurity concentrations of an unknown Al[subscript x]Ga[subscript 1-x]N sample can be determined, and the elemental composition of multi-layer Al[subscript x]Ga[subscript 1-x]N samples can be measured. Electron beam charge neutralization methods for high Al content
Al[subscript x]Ga[subscript 1-x]N are shown. The calibration curves in Al[subscript x]Ga[subscript 1-x]N using O₂⁺ bombardment with positive secondary ion detection, using Cs⁺ bombardment with negative secondary ion detection and MCs⁺ detection are developed. The ionization mechanisms under these conditions are rationalized.
Using the sputtering conditions stated above, the sputter yield decreases with AlN mole fraction in Al[subscript x]Ga[subscript 1-x]N and the rate of decrease in the sputter rate versus x declines as x increases. In the range of x=0 to 0.58, the matrix ion intensity ratios of Al-containing ions over Ga-containing ions appear to increase linearly with the corresponding matrix mole fraction ratio or AlN mole fraction. For higher x, the inverse plots of the ratio of Ga-containing ions over Al-containing ions as a function of GaN mole fraction or mole fraction ratio appear to increase linearly in the range of x=0.39 to 1. The RSFs for Si and Mg normalized to the appropriate Ga-containing matrix ions decrease exponentially with x in the range of x=0 to 0.58; those normalized to the N-containing matrix ions have smaller variation with x in the range of x=0 to 0.58. The exponential correlation of RSFs with x is consistent with that of ion yield with the surface work function.
Based on the calibration curves developed in this work at multiple analysis conditions, the matrix elements in Al[subscript x]Ga[subscript 1-x]N can be quantified in the range of x=0 to 1, and the impurity species can be quantified over x=0 to 0.58. The technique can be employed for impurity control, composition and growth rate determination, as well as structural analysis of the finished optoelectronic and electronic devices.
The ionization yields of both positive and negative ions are studied when x is changed in Al[subscript x]Ga[subscript 1-x]N. The yield variation is mainly caused by the increase of surface concentration of primary species due to the sputter yield reduction as x is increased.
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Keywords
Secondary Ion Mass Spectrometry, III-nitride, AlGaN, Quantification, SIMS
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Degree
PhD
Discipline
Materials Science and Engineering
