Abstract
The high-quality InN epifilms and InN microdisks have been grown with InGaN buffer layers at low temperatures by plasma-assisted molecular beam epitaxy. The samples were analyzed using X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and photoluminescence. The characteristics of the InN epifilms and InN microdisks were studied, and the role of InGaN buffer was evaluated.
Keywords
- InN microdisk
- InGaN buffer
- Molecular beam epitaxy
1. Introduction
III-Nitride semiconductor compounds have been extensively studied for applications in optoelectronic devices, such as solar cells and light emitting diodes (LEDs) [1–5]. The wide direct band-gap gallium nitride (GaN) and aluminum nitride (AlN) compounds, with energy gaps covering the ultraviolet spectrum, are the dominant materials for solid-state lighting devices and have been well studied to date. The molecular beam epitaxy (MBE) technique can be used to grow a thin epifilm in an ultrahigh vacuum (~10−10 torr) and low temperature condition [6]. Under such conditions, materials in the effusion cells of the MBE system are heated and they move toward the substrate to form epitaxial high purity films. The low-temperature condition is crucial to grow the compounds with a low volatilized temperature (such as In atom, 650°C). Because of the improvement of InN films grown by MBE, the direct band-gap of the indium nitride (InN) compound was demonstrated with the value of 0.64 eV rather than 1.9 eV [7, 8]. This important finding indicated that one can tune the band-gap energy to achieve the full-color spectrum (red, green, and blue) devices by changing the III-group alloy ratio without any phosphor. Besides, InN is a high potential material in optoelectronic applications due to its outstanding material properties, such as the smallest effective mass, the highest peak and saturation electron drift velocity, and the largest mobility among the nitride semiconductors [9, 10].
On the other hand, the development of the full-color spectrum micron LED is very important for the high-resolution display. The general method to fabricate micron LED is etching process to reach micro scale. However, it is not easy to downsize to 1–10 μm by etching process. In order to fabricate micron LED, a suitable micron growth base is the top priority. In recent years, the growth and characteristics of InN nanowire on Si (1 0 0) by the vapor-liquid-solid mechanism and on Si (1 1 1) by plasma-assisted molecular beam epitaxy (PA-MBE) were reported [11–13]. The wire diameter was less than 100 nm. In our previous work, we have grown the high-quality self-assembled
2. High-quality InN epifilms
2.1. InGaN buffer layer
When engineering the band structure of III-nitrides, it is difficult to grow high-quality InN thin film due to the low decomposition temperature of InN (<600°C) and the large lattice mismatch between InN and common substrates (e.g., sapphire or silicon) [15]. Therefore, determining an appropriate substrate for the growth of high-quality InN film is one of the main issues in the fabrication of full-color optoelectronic devices. The lattice mismatch between InN (
The initial methods prior to InN growth, including substrate nitridation and buffer layer deposition, have very important effects on the growth of high-quality InN films with a flat surface on a sapphire substrate. Xiao et al. grew InN films with 20 min nitridation and a low-temperature InN (LT-InN) buffer layer. By X-ray diffraction (XRD) and room temperature photoluminescence (PL) analyses, it was found that these InN films grown with LT-InN buffer layer have better quality than those without LT-InN buffer layer [16].
Meanwhile, Saito et al. reported the growth of InN films on sapphire with 1 hour nitridation and low-temperature intermediate InN buffer layers, and they found that the growth of thicker InN with a uniform surface was very difficult without intermediate layers and the electron mobility was improved by improvement of surface flatness [17]. Besides, Lu et al. studied the effect of an AlN buffer layer on the epitaxial growth of InN on the sapphire substrate by MBE and found that by using the AlN buffer layer, the structural and electrical properties of InN could be greatly improved. It was also found that a thicker AlN buffer layer was preferred when growing the InN epilayer, which could lead to better electrical properties and surface morphology [18]. From these studies, we can find that it is helpful to improve the quality of InN thin films by introducing an appropriate buffer layer. In general, a thick GaN film (>4 μm) can be grown on sapphire substrate (0 0 0 1) to form a GaN template. In this chapter, we will show firstly the high-quality epitaxial growth of InN epifilms on GaN template with an appropriate InGaN buffer layer by PA-MBE system. We designed a series of samples to study the effect of InGaN buffer layer with growth-temperature dependence.
2.2. Growth of InN epifilms
Four samples were grown on 2 inch
2.3. Analysis of InN epifilms
The
The crystal structure of all samples was characterized by XRD measurements. Figure 1 shows the XRD results of all samples and indicates that
Figure 3(a–d) shows the surface morphology of
Figure 5 shows that PL spectra of sample 1 for different temperatures. The PL measurements were carried out by Ti:sapphire laser (Traix-320) with a light source from 808-nm laser and 208 mW power from 300 to 14 K. When the temperature was changed from 300 to 14 K, the position of major peak shifted from 0.698 to 0.703 eV, in good agreement with the recent data (~0.7 eV) [7, 8]. The intensity of major peak also increased. The major peaks measured at different temperatures were confirmed by a multipeak Gaussian-function curve fitting with the software Origin (Pro. 8.0). The result of the multipeak Gaussian-function curve fitting showed that the major peak was composed by three peaks and all the peak centers shifted to higher energy when the temperature was changed from 300 to 14 K. Among three fitting peaks, only one peak can be described by Varshni’s equation [21]:
In the inset of Figure 5, the theoretical fitting to Varshni’s equation is obtained with
The cross-sectional TEM specimen of sample 1 was prepared by a dual-beam focus ion beam (Seiko SII-3050), with the cleavage plane along the
2.4. Characteristics of InN epifilms
From the crystal structural analyses by XRD and TEM, we found that the crystal quality was significantly improved by decreasing the growth temperature of InGaN buffer layer. From the SEM images and AFM analyses, we also found that the surface of InN epifilm became smoother by decreasing the growth temperature of the InGaN buffer layer. From the PL measurements, we showed that the energy of 0.681 eV emitted from the InN epifilm of sample 1 (the growth temperature of InGaN buffer layer is 500°C) by the fitting to Varshni’s equation. Finally, it is suggestive that one can grow high-quality and flat InN epifilms by decreasing the growth temperature of the InGaN buffer layer. Therefore, the influence of InGaN buffer layer is very effective to grow high-quality InN epifilms and InN microstructures as well. We therefore grow InN hexagonal microdisks on the LAO substrate with the InGaN buffer layer.
3. InN hexagonal microdisks
3.1. Growth of InN microdisks
The two-orientation growth of GaN nanopillars on the LAO substrate has been reported in our previous papers [23, 24]. In this paper, we applied the two-orientation growth to grow the 2D
3.2. Analysis of InN microdisks
The crystal structure of the microdisk sample is characterized by the high-resolution X-ray diffraction (XRD; Bede D1) measurement and is shown in Figure 7. From the peak of X-ray diffraction pattern at 2
The surface morphology of the sample was evaluated by the field emission scanning electron microscope (FE-SEM, SII-3050). Figure 7(a) showed the top-view SEM image of the sample. The morphology of the sample exhibited that 3D
The microstructure of the sample was analyzed by a field emission transmission electron microscope (FE-TEM; Phillips Tecnai F-20) at an electron voltage of 200 kV. The cross-sectional TEM specimen was prepared by a dual-beam focus ion beam (FIB; Seiko SII-3050), on the cleavage plane along [
3.3. Characteristics of InN microdisks
We have grown InN hexagonal thin microdisks on the LAO substrate with the InGaN buffer layer by PA-MBE. From the SEM images and TEM analyses, we found that
4. Conclusion
In this paper, we have reported the growth and characteristics of 2D
Acknowledgments
The project was supported by the Ministry of Science and Technology of Taiwan and the Core Facilities Laboratory for Nanoscience and Nanotechnology in Kaohsiung and Pintung Area.
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