A Chemical Perspective of GaN Polarity: The use of Hydrogen Plasma Dry Etching Versus NaOH Wet Etching to Determine Pola
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A Chemical Perspective of GaN Polarity: The use of Hydrogen Plasma Dry Etching Versus NaOH Wet Etching to Determine Polarity Maria Losurdo, MariaMichela Giangregorio, Pio Capezzuto, Giovanni Bruno, Gon Namkoong1, W. Alan Doolittle1, and April S. Brown1 Institute of Inorganic Metodologies and Plasmas -CNR, sect. of Bari via Orabona, 4- 70126 Bari, Italy 1 Georgia Institute of Technology, School of Electrical and Computer Engineering, Microelectronic Research Center, 791 Atlantic Dr., Atlanta, GA 30332-0269, USA ABSTRACT The use of dry hydrogen plasma etching is evaluated for determination of GaN polarity and critically compared to wet etching in NaOH. It is shown that hydrogen plasma etching is effective in revealing inversion domains (IDs) and some types of dislocations. This is because the surface morphology is unchanged by the hydrogen treatment, and, hence, the surface reactivity is not masked. INTRODUCTION In the last decade, GaN has received much attention for its’ use in optoelectronic, and high power/temperature electronic devices. GaN is suitable for applications such as UV detectors, UV-blue LEDs and laser diodes. Moreover, GaN’s favorable electron velocity and thermal conductivity make it suitable for high power/temperature FETs. Nevertheless, being non-centro-symmetric due to its wurtzitic structure, GaN films exhibit polarity, i.e., GaN can be Ga-polar ([0001] direction) or N-polar ([000-1] direction). Spontaneous polarization and piezoelectric polarization depend on the film polarity, which needs to be known and controlled particularly in FETs. In fact, it is well known that Ga- and N-polarity yield GaN films with completely different chemical stability, morphological, photoluminescence and electrical properties [1]. Different methods have been proposed to determine the polarity of GaN films. First, the formation of a 2DEG at an epitaxial AlGaN barrier is a reliable but indirect method [2]. Second, the assessment of surface morphology is sometimes misleading, since it depends on the growth procedures; and, finally wet etching methods, such as by NaOH, KOH or H3PO4 [3,4], are destructive and do not allow differentiate among samples with a different density of inversion domains (IDs) or with different defect density. In this contribution, we present a process based on the interaction of GaN with H-atoms from a remote plasma source to determine GaN film polarity. The efficacy of this process results from the different surface reactivity of the N- and Ga-atoms towards H-atoms and on the different stability of the etching products (NHx or GaHx). This process can be considered nondestructive since only few tens of nm are involved in this reactive process, without degradation of the surface morphology. The high selectivity of this process combined with measurements of surface potential by Kelvin probe force microscopy (KPFM) allows differentiation between films with the same overall polarity but characterized by a different density of inversion domains (IDs). X-ray photoelectron spectroscopy (XPS) and spectros
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