Donor passivation in pseudomorphic-high electron mobility transistors due to plasma-incorporated fluorine impurities obs
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To investigate the effect of plasma-incorporated fluorine on Si donors in pseudomorphic-high electron mobility transistors (P-HEMTs), we used x-ray photoemission spectroscopy to analyze three layers near the Si ␦-doped layer and the Si ␦-doped layer itself, in which we previously found fluorine accumulation after post-thermal annealing following fluorocarbon-based plasma exposure. For this evaluation, we developed controllable and low-speed AlGaAs wet-chemical etching using citric-acid-based wet etchant. We used it to expose one of the layers to be analyzed: one 7.5 nm above the Si ␦-doped layer, one 1.5 nm above it, the ␦-doped layer itself, and one 1.5 nm below it. We found that the accumulated fluorine localized in the ␦-doped layer and reacted with Si donors. This is apparently the main reason for the carrier passivation in the fluorocarbon-based plasma-exposed P-HEMTs.
I. INTRODUCTION
We previously investigated the damage caused to pseudomorphic-high electron mobility transistors (PHEMTs) by C2F6/CHF3 reactive ion etching (RIE). Hall effect measurements and secondary-ion mass spectrometry (SIMS) analysis revealed that fluorine incorporation into the active layers of P-HEMTs caused serious degradation in device characteristics.1–4 We also found that fluorine easily accumulated in the strained layers of the P-HEMT structure, such as the heterojunctions and the ␦-doped layer. Taking this property into account, we used strained epilayers as fluorine trapping barriers in the P-HEMT structure. The strained epilayers effectively suppressed fluorine incorporation into the active layers of the P-HEMT during C2F6/CHF3 RIE.2–4 However, the actual mechanism of the fluorine damage to the active layers of P-HEMTs has not yet been determined. While donor passivation due to plasma-incorporated fluorine has been observed in the thick Si-doped GaAs layers accessible for general surface analysis,5,6 it is difficult to observe fluorine behavior in the extremely thin ␦-doped layer of P-HEMTs because preparation of sample for surface analysis using x-ray photoemission spectroscopy (XPS), for example, needs process techniques for precisely exposing the ␦-doped layer. The generally used argon ion sputtering method is incapable of doing this.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0159 J. Mater. Res., Vol. 21, No. 5, May 2006
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In this work, we used precise and low-speed citric acid-based wet-chemical etching7–10 to prepare XPS samples and investigated the state of fluorine atoms incorporated in layers near the ␦-doped layer and in the ␦-doped layer itself. II. EXPERIMENTAL
A P-HEMT structure was grown on a semi-insulating GaAs (SI-GaAs) wafer (600 m thick) by molecular-beam epitaxy (MBE). The structure was shown in Table I. We also used a SI-GaAs substrate and a Si-doped GaAs (Si: 1 × 1018 cm−1) substrate as references to identify the XPS signal. The samples were cut to about 10 × 10 mm and exposed to C2F6/CHF3
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