Surface Contamination with Si and O Impurities for GaN Single-Crystalline Layers Grown by Ultra-High-Vacuum Sputter Epitaxy
Authors: Mizuno Ai , Shinoda Hiroyuki , Mutsukura Nobuki 
Published: February 2020

Introduction

GaN and related alloys have been extensively investigated for their potential applications in optoelectronic and high power electronic devices. The study on surface contamination with Si and O impurities is quite important for device fabrication, especially for Si, because Si is n-type impurity for GaN. In this work, we demonstrate the GaN surface contamination and cleaning technique focused on O and Si based compounds contaminated in air atmosphere.

Experiment

GaN single-crystalline layers were grown on sapphire (0001) substrates by ultra-high-vacuum radio-frequency sputter epitaxy technique. The thermal desorption spectroscopy (TDS) measurement was carried out for as-grown GaN layers at pressure of 10–
4 Pa, and the chemical etching was also carried out using buffered hydrogen fluoride (BHF) solution. The surface state of the GaN layer was analyzed by X-ray photoelectron spectroscopy (XPS) measurement.

Results

In the XPS measurements, O1s and Si2p XPS signals were detected from as-grown GaN layer surface. The thermal treatment was then carried out in order to remove these impurities, and the results on XPS measurements indicated the significant decrease of Ga–O bonds on the GaN layer surface. Moreover, from the TDS measurements, the signal of m/z = 70 corresponding to Ga (Figure 1(a)) and the signal of m/z = 32 corresponding to O2 (Figure 1(b)) indicated rapid increase at around 500 °C, which suggests that isolated Ga and O atoms were easily released from the GaN layer surface. On the other hand, the removal of Si compounds by the thermal treatment was quite difficult, because the thermally stable Si–N bonds which would be formed by nitrogen dangling bonds, were terminated with Si atoms during such thermal treatment. The chemical etching was very effective in order to remove the Si contamination, and after the BHF etching treatment the decrease of Si compounds on GaN layer surface was confirmed by the great decrease of Si2p/Ga3d XPS signal intensity ratio. After the chemical etching treatment, the GaN layer was then exposed in air atmosphere. Two kinds of methods were utilized for the air exposure; sample was put in open air (case 1), and also was put at the inside of closed narrow space (case 2) limiting the concentrations of impurities reaching onto the sample surface. Figure 2 shows the Si2p/Ga3d XPS signal intensity ratio depending on an elapsed time in air exposure. At the case 1, the rapid increase of the intensity ratio was observed within initial 120 minutes, and then it indicated the gradual increase. On the other hand, at the case 2, the intensity ratios were always lower than those at the case 1. Therefore, the storage of the sample in air atmosphere is the important factor for Si contamination. It seems that the grade of O contamination depends greatly on temperature and humidity in air atmosphere. The results in GaN layer grown by metal-organic chemical vapor deposition method will be also presented.

Acknowledgement

A part of this work was conducted at Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by "Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Figure 1