Analysis using a two-layer model of the transport properties of InGaN epilayers grown on GaN template substrate

Abstract

In this paper, a two-layer model was used to investigate the electrical properties of InGaN epilayer grown by MOCVD on a semiconducting GaN template substrate. The elaboration process was optimized to obtain high-quality InGaN epilayers with respect to the indium composition. The details of structural and optical properties as well as the surface morphology evolution were studied using X-ray diffraction, photoluminescence spectroscopy, and atomic force microscopy, respectively. The electrical transport properties were measured using the van der Pauw/Hall effect method combined with a two layer model for the first time for InGaN. The obtained experimental results combined with the developed model allowed to precisely extract the electrical properties of the InGaN epilayers and correlate them to the structural properties. The developed model procedure and code are made available and can be easily adapted to other III-N and III-V compound semiconductors grown on a semiconducting substrate.

Introduction

Indium gallium nitride (InGaN) ternary alloy is one of the most promising and outstanding materials for modern photonic and electronic devices. With the recent advancement of InGaN as active materials in solid-state lighting technologies, particularly in ultraviolet/blue/green light-emitting diodes to continuous wave laser diodes [1], [2], [3], a focus has been drawn into elaborating and developing the material into a high-efficiency photovoltaic application owing to its fascinating intrinsic properties [4], [5]. The fundamental bandgap of InGaN alloy can be engineered from 0.67 eV (InN) to 3.4 eV (GaN), which is the only material among the III-V semiconductors (including III-Nitride) that provides an excellent spectral match with the solar spectrum. In addition, the absorption coefficient of InGaN is much higher than any other III-V material, thus making it possible to develop into thin-film solar cells. InGaN also possesses experimental carrier mobilities higher than $300{\mathrm{cm}}^2/\mathrm{V\; s}$ [6], high resistance to extreme conditions with an operating temperature as high as $400°\mathrm{C}$ [7], [8], a high ionizing radiation tolerance with the irradiation-induced degradation in the photoluminescence intensity lower than $0.1\mathrm{decade}/10^{12}\mathrm{MeV/g}$ [9] and a predicted lifetime better than 30 years under solar storm proton irradiation [7], and very high chemical stability [10], allowing the fabricated photovoltaic devices to be operated in a harsh environment.

The work in this area has been focused on addressing the main technological challenges, primarily on the InGaN epitaxial growth, such as the difficulty in growing high-quality In-rich monocrystalline InGaN epilayers with optoelectronic properties suitable for applications such as solar cells [11], [12], [13], [14], [15]. This is because almost all epitaxially grown In-rich InGaN films tend to possess phase separation and composition fluctuation within the grown structure due to the solid phase miscibility gap. The existence of the solid phase miscibility gap in InGaN alloy is related to the mismatch between the lattice parameters of the binary GaN and InN. Thus, it is essential to precisely control the InGaN epitaxial growth parameters such as growth temperature/pressure and precursor flow rate to realize high-quality In-rich InGaN films. Indeed, the impurities and other strain-related defects such as point, and V-defect can be introduced into the epitaxial films during the growth process due to the mismatch. These defects can contribute as a recombination site for the photoexcited carriers, lowering the overall photovoltaic properties of the fabricated devices, such as lower short circuit current density and shorter carrier lifetime. Nonetheless, the created defects induced by the mismatch can be partially mitigated by using the GaN template. This method has been demonstrated in LED technology, where thin InGaN multi-quantum wells with low indium composition can be grown relatively in strain when heteroepitaxially grown on a thick GaN buffer layer [16]. However, through this transverse structure configuration, it is known that the outcome of the crystal quality, as well as the electrical and optical properties of the InGaN epilayer are heavily dependent on the buffer layer/substrate condition.

For instance, the electronic properties of an active layer grown on a semiconducting substrate are particularly challenging to investigate using a typical electrical characterization method such as the Hall effect with van der Pauw configuration since the underlying thick semiconducting GaN template can strongly influence the outcome of the measured electrical transport properties such as resistivity, carrier concentration, and mobility. The necessity to grow InGaN on a semiconducting substrate in our case is justified by two reasons. First, the final objective of optimizing InGaN/GaN is the development of InGaN-based solar cells in the transverse configuration where the bottom layer should be highly doped for the deposition of ohmic contact. Second, the optoelectronic properties of III-Nitride materials depend on the substrate and a study on a particular substrate cannot be extrapolated to a different one. To overcome this limitation, we proposed for the first time for InGaN/GaN heterostructures a two-layer methodology using the theoretical formalism in [17] to investigate the transport properties of the InGaN epilayer grown in such structure configuration. This model extends the classical two-layer approach developed for layers with different types and/or for non-isolated layers either with top or lateral contacts [18], [19]. This classical procedure is inherently not adapted to InGaN/GaN heterostructure where the conduction between the InGaN epilayer and the semiconducting GaN substrate under the contact region as well as the conduction in the substrate itself dominates the measured electrical characteristics. The main advantage of the method used in this work is the fact that it takes into account the contribution of the GaN substrate and the conduction between it and the InGaN epilayers, permitting to achieve reliable and high precision van der Pauw and Hall effect measurements for the same InGaN/GaN heterostructure that will be used in the actual device. The procedure was applied to perform a study of the transport properties of InGaN epilayer with respect to indium composition ranging from about 4% up to 11%. Using this approach, the transport properties of the InGaN epilayers are discussed in conjunction with a detailed study of the structural properties using X-ray diffraction, atomic force microscopy, and photoluminescence spectroscopy. The developed code of the two-layer model analysis for the van der Pauw and Hall effect characterization method is made available and can be easily adapted to other materials where the active layer is grown on a semiconducting substrate.

The study of the transport properties, using this two-layer method, of InGaN/GaN heterostructure elaborated using MOCVD and characterized using X-ray diffraction, photoluminescence and AFM technique, is presented with, first, the experimental procedure described in Section 2. Then, Section 3 presents the structural and morphological properties of the elaborated layers. Section 4 develops in detail the two-layer method and its application to InGaN/GaN and discusses the correlation between the extracted electrical properties and the structural properties.