Elsevier

Micron

Volume 134, July 2020, 102864
Micron

Detection of Si doping in the AlN/GaN MQW using Super X – EDS measurements

https://doi.org/10.1016/j.micron.2020.102864Get rights and content

Highlights

  • The sensitivity of SSD detector allows the determination of doping in GaN (QW).

  • The use of the AlN as a standard in STEM/EDS allows the quantitative determination of Si doping.

  • Quantifying STEM/EDS studies of Si in GaN area give correct Si dope values.

Abstract

A multiple-quantum-well structure consisting of 40 periods of AlN/GaN:Si was investigated using a transmission electron microscope equipped with energy-dispersive X-ray spectroscopy. The thicknesses of the AlN barriers and the GaN quantum wells were 4 nm and 6 nm, respectively. The QW layers were doped with Si to a concentration of 1.3×1019cm-3 (0.012 % at). The procedure for quantifying such a doping level using AlN as a standard is presented. The EDS results (0.013 % at) are compared with secondary ion mass spectrometry measurements (0.05 % at).

Introduction

Multi-quantum-well structures (MQWs) based on wurtzite GaN/AlN are promising materials for the fabrication of ultraviolet optoelectronic devices (Hirayama, 2011; Nam et al., 2004; Li et al., 2018), such as: light emitting diodes (LEDs) and laser diodes (LDs). However, when compared to InGaN-based emitters (Lochner et al., 2013; Cho et al., 2013), GaN/AlN-based devices still have considerably lower efficiency. The electronic properties of GaN/AlN heterostructures are highly dependent on the density of structural defects (point (Saarinen et al., 1997; Freysoldt et al., 2014) or extended (Ban et al., 2011) defects), the large lattice mismatch between nitrides (Angerer et al., 1997; Bernardini et al., 1997a), the nonuniformity of the chemical composition (Chichibu et al., 1997a; Chichibu et al., 1997b), and the presence of polarization-related electric fields within the structure (Bernardini and Fiorentini, 1999; Bernardini et al., 1997b; Fiorentini et al., 1999), which can be partially screened by doping. The two types of doping are usually used in synthesis of high quality GaN, the first by Mg to obtain p-type (Amano et al., 1989; Nakamura et al., 1992) and second by Si to obtain n-type material (Eiting et al., 1998).

MQW structures were intensively studied for many years using various methods like extended X-ray absorption fine structure (EXAFS) or X-ray diffraction (XRD) (Zhuravlev et al., 2013; Chandolu et al., 2007) but none of these methods can obtain the content of the elements quantitatively. The method capable to determine the nanometer size structure and obtain simultaneous measurements of the chemical composition is TEM, but until recently the determination of the content of element with an atomic concentration below 1% at, was very challenging. Only after the development of energy-dispersive spectrometry (EDS) and the introduction of the new silicon drift detectors (SDD) with large active area, was the measurement sensitivity enhanced to enable the observation of elements at much smaller concentrations. The sensing material in SDD is high purity silicon, with a very low leakage current, and the detectors incorporate some new solutions, like a transversal field generated by a series of ring electrodes that causes charge carriers to drift to a small collection electrode. This geometry allows significantly higher count rates coupled with a very low capacitance of the detector. In a transmission electron microscope (TEM), these detectors may sometimes be more sensitive than electron energy-loss spectroscopy (EELS) (von Harrach et al., 2009). In this paper, we present an experimental analysis of chemical composition data from GaN/AlN MQWs where the GaN QWs were Si doped.

Section snippets

Experimental

The investigated GaN/AlN MQW sample was grown by plasma-assisted molecular-beam epitaxy (PA-MBE) (Kaminska et al., 2016a; Kaminska et al., 2016b). The sample consisting of 40 GaN/AlN periods was deposited on a commercial 1-μm-thick (0001)-oriented AlN-on-sapphire template. The MQWs were capped by 30 nm of AlN. The scheme of the investigated sample is presented in Fig. 1. The AlN quantum barriers (QBs) and the GaN quantum wells (QWs) were 4 nm and 6 nm thick, respectively. The GaN layer was

Results

The images demonstrating the general view of the layers are presented in Fig. 2. The cross-sectional images were taken in the STEM mode (Z-contrast) and were combined with EDS maps of two elements: Ga – QW (yellow color) and Al - QB (blue color). The interfaces between the AlN template and the thick GaN buffer layer are shown to be flat, proving good control of the growth of the investigated sample. In large-scale view STEM images, the QB/QW structure is generally flat, having parallel

Conclusions

An AlN/GaN:Si MQW structure, grown by PA-MBE, was investigated. The purpose of the work was to confirm the presence of silicon in the area of GaN QWs. The QW layer was doped with Si to a nominal concentration of 1.3*1019 cm3 (0.012 % at). We have presented a procedure for quantifying such small amounts of doping, using AlN as a standard. The presence of silicon in the GaN area was identified and its content was quantified as 0.013 ± 0.007 at. %, as compared with the Si concentration: 4.5 × 1019

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The study was carried out at the Biological and Chemical Research Centre, University of Warsaw, established within the project co-financed by European Union from the European Regional Development Fund under the Operational Programme Innovative Economy, 2007 – 2013 and Panda2 to 501-D312-56-0000002.

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