Full Length ArticleAngular dependent XPS study of surface band bending on Ga-polar n-GaN
Introduction
Large polarization in III-nitride materials results in bound polarization charges on surfaces [1], which give rise to a distribution of compensating electron states to satisfy the charge neutrality. These compensating states, including internal ionized states and external surface states, affect the internal electric field near the surface and play a crucial role in electric properties of these materials [2]. The external surface states [3] can be surface vacancies, dangling bonds, structural defects, surface oxides, or absorbates, which intimately depend on the specific deposition and processing conditions. Identifying these states is of key importance to understand basic device behavior and further to guide device fabrication. Surface pretreatments on GaN before metal deposition, such as reactive ion etching (RIE) [4] and inductively coupled plasma (ICP) etching [5], [6], have been found to be able to reduce the resistivity of metal/n-GaN contacts effectively. Surface N-vacancies on n-GaN are usually considered to be benefit to achieving low contact resistivity [5], [7]. Other surface states, such as oxides and absorbates, may have comparable effects on the contact resistivity too, but have been seldom investigated so far.
Surface band bending is a measure of internal compensation states in space charge region and thus the surface states (the denser the internal compensation states are, the less the compensation from surface states will be) [8]. Angular dependent X-ray photoelectron spectroscopy (ADXPS) can detect photoelectrons emitted from different depth of sample, and so far has been widely applied to characterize surface band bending [8], [9], [10]. However, surface band bending has been underestimated in most cases because XPS sampling depth usually is much smaller than the total depletion width [9]. To improve the measurement accuracy, a modified method was proposed by considering the difference in binding energy (BE) separation between core level and Fermi level measured from sample surface and bulk [8].
In this work, we focused on surface band bending in Ga-polar n-GaN after different surface treatments by using the improved ADXPS technique. It is known that a core level peak, obtained by ADXPS, is an integration of photoelectrons that from certain probing depth depending on emission angle [11]. So, core level on surface was determined by collecting photoelectrons emitted within the detection depth of first several monolayers, and core level in bulk was estimated to the best by collecting photoelectrons with the deepest emission depth. We have not only observed the surface band bending of GaN that was induced with different surface treatments, but also figured out the origins of the surface band bending by taking surface states into the analysis. A close correlation between oxygen-containing absorbates (besides N-vacancies) and surface band bending has been established.
Section snippets
Material and surface treatments
Ga-polar n-GaN samples were grown on a 2-inch sapphire (0001) substrate in a metal-organic chemical-vapor deposition (MOCVD) system with a 2 μm Si-doped n-GaN epitaxial layer on top of a 2 μm undoped GaN buffer layer. The carrier concentration of n-GaN was about 1.0 × 1018 cm−3 from a room temperature (RT) Hall Effect measurement.
In order to investigate the effects of surface treatments on surface band bending, three types of sample were prepared, as shown in Fig. 1. Except as-grown GaN
Surface band bending and metal contact properties
The surface Ga 3d core levels (CLGa3d−EF)surf of n-GaN samples with different surface treatments are extracted and shown in Fig. 3, where the Ga 3d peak was deconvoluted into four peaks, Ga-N, Ga-O, Ga-Ga metal, and N 2s. The deconvolution rules are presented in figure caption. The characteristic peak of Ga 3d (Ga-N) of the as-grown sample is positioned at 19.87 ± 0.10 eV. After alkaline cleaning, Ga 3d (Ga-N) peak shifts to lower binding energy, 19.47 ± 0.10 eV, which is quite in agreement
Conclusions
Ga-polar n-GaN with different surface treatments were measured via angular dependent XPS to analyze the surface band bending and its effect on contact barrier. Samples subjected to alkaline cleaning and ICP treatment showed the largest and smallest upward surface band bending, respectively. The small upward surface band bending on n-GaN etched by ICP was attributed to the existence of more surface oxygen-containing absorbates besides of oxide layers and N-vacancies. Density of positive surface
Acknowledgements
This work was funded by the National Natural Science Foundation of China (Grant numbers: 61674165, 61604167, 61574160, 61704183, 61404159, 11604366), the Natural Science Foundation of Jiangsu Province (Grant numbers: BK20170432, BK20160397, BK20140394); the National Key R&D Program of China (grant number 2016YFB0401803); and the Strategic Priority Research Program of the Chinese Academy of Science (grant number XDA09020401). F. S. L acknowledges support from the Youth Innovation Promotion
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