Epitaxial growth and characterization of non-polar a-plane AlGaN films with MgN/AlGaN insertion layers

The MgN/AlGaN insertion layers were applied for the first time in the growth of non-polar a-plane AlGaN epi-layers by metal organic chemical vapor deposition technology. The full-width-at-half-maximum value of X-ray rocking curve for the a-plane AlGaN epi-layer was decreased by approximately 50.6% and the root-mean-square value of the surface was reduced by 74% by inserting the MgN/AlGaN insertion layers with optimized number of insertion pairs, which revealed that the compressive strain within the a-plane AlGaN epi-layers was effectively reduced, leading to significant improvements in crystalline quality and surface morphology, which is very helpful to fabricate high quality AlGaN-based ultraviolet light-emitting-diodes.


Introduction
AlGaN-based ultraviolet light emitting diodes (UV-LEDs) with emission wavelength between 210 and 365 nm are in demand for a wide range of applications, such as sterilization, water/air purification, and medical treatment. 1,2) Currently, most of the AlGaN-based UV-LEDs have been fabricated based on polar (0001)-oriented c-plane AlGaN epi-layers because there is less difficulty to grow relatively high quality AlGaN epi-layers on polar c-plane sapphire substrate than on other orientation sapphire substrates. However, it is well known that the light emission efficiency of the polar c-plane AlGaN-based UV-LEDs can be strongly reduced by the quantum confined Stark effect (QCSE) induced by the spontaneous and piezoelectric polarization fields along [0001] c-direction. 3) In contrast, there is no spontaneous polarization-induced built-in electric field in a non-polar AlGaN epi-layer and the QCSE is completely suppressed in non-polar AlGaN-based multiple quantum wells (MQWs). 4) Hence, high light emission efficiency can be expected for the non-polar AlGaN-based UV-LED. Unfortunately, the hetero-epitaxial growth of non-polar 112 0 -oriented a-plane AlGaN films on semi-polar 11 02 -oriented r-plane sapphire substrates has been proven to be a very challengeable task. Because of the large mismatch both in lattice constant and thermal expansion coefficient between the r-plane sapphire substrate and the non-polar AlGaN epi-layer, high density of the misfit dislocations, undulating surface morphology, and strong crystallographic anisotropy are usually generated for the non-polar AlGaN epi-layers during the epitaxial growth process. Therefore, the epitaxial growth of high quality non-polar AlGaN films plays a crucial role in the fabrication of AlGaN-based UV-LEDs.
In order to improve the crystalline quality of the non-polar GaN epi-layers, various kinds of intermediate layers [5][6][7][8] and patterned sapphire substrate (PSS) with different patterns [9][10][11][12] were applied for the epitaxial growth of the non-polar GaN films. However, up to date there were only few reports on the approaches to improve the crystalline quality and the surface morphology for the non-polar AlGaN epi-layers. Even though the first attempt to grow the non-polar a-plane AlGaN films under the two-dimensional growth mode was made in 2003, 13) it is still indispensable to further improve the crystalline quality and the surface morphology of non-polar a-plane AlGaN epi-layers, especially those with high Al composition (> 0.3) for meeting the needs to fabricate deep UV-LEDs with wavelength less than 320 nm.
In this study, the MgN/AlGaN insertion layers were applied for the first time in the epitaxial growth of non-polar a-plane AlGaN films on semi-polar r-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD) technology. X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, photoluminescence (PL) spectroscopy, and atomic force microscopy (AFM) were employed to characterize the structural and optical properties of the grown non-polar a-plane AlGaN epi-layers. The characterization results demonstrate that high crystalline quality non-polar a-plane AlGaN epi-layers with low dislocations density, smooth surface morphology, low compressive strain, and strong PL intensity have been achieved with the optimized MgN/AlGaN insertion layers. Trimethyl-aluminum (TMAl), trimethyl-gallium (TMGa), bis(cyclopentadienyl)-magnesium (CP2Mg), and ammonia (NH3) were used as the precursors for Al, Ga, Mg, and N, respectively, and hydrogen (H2) was used as the carrier gas. Similar to the epitaxial growth process described in detail in our recently published paper, 14) for the epitaxial growth of the non-polar a-plane AlGaN film without MgN/AlGaN insertion layer which was named as The crystal orientation and the Al composition for the grown non-polar a-plane AlGaN epi-layers were determined by means of high resolution (HR)-XRD 2θ-ω scans. In specific, the X-ray rocking curves (XRCs) were measured in two directions that are perpendicular to each other for evaluating the crystalline quality for the non-polar a-plane AlGaN epi-layers.

Experimental
The blocking of the threading dislocations (TDs) by the MgN/AlGaN insertion layers was confirmed with TEM measurement. Moreover, the surface morphology for the non-polar a-plane AlGaN epi-layers was characterized by AFM measurement. The structural and optical properties for the grown non-polar a-plane AlGaN epi-layers were further investigated with Raman and PL spectroscopies conducted at room temperature (RT). The HR-XRD 2θ-ω scanning curve for sample A was shown in Fig. 2(a). It was demonstrated unambiguously that the peaks located at 2θ=52.56°, 58.40° and 59.35° were the XRD diffraction peaks from the semi-polar 22 04 -oriented r-plane sapphire substrate, the non-polar 112 0 -oriented a-plane AlGaN and AlN epi-layers, respectively. 15,16) Extremely similar results were observed for samples B, C, D, and E although they are not shown in Fig. 2(a). Moreover, the Al compositions were determined to be 0.40 for all the five direction and 50.6% less along the 11 00 direction, respectively than those for sample A without any MgN/AlGaN insertion layers. However, as shown clearly in Fig. 2(b), the FWHM value of the XRC decreased slowly and tended to saturate when the number of pairs of the MgN/AlGaN insertion layers increased further from three for sample D to four for sample E.    25) As mentioned above, the reduction in the dislocation density could be ascribed to the MgN/AlGaN insertion layers-induced enhancement in the opportunity for the dislocations to annihilate, resulting in the improvement in crystalline quality. 26) In particular, as shown in Fig. 4(a), the GaN-like E2 (high) peak for sample E was located at 579.8 cm -1 which was consistent with the critical point value of 579.8 cm -1 . This fact implies that the compressive strain within sample E is nearly completely compensated, or sample E has the lowest dislocation density among the five non-polar a-plane AlGaN epi-layer samples A-E. In addition, it was found that both the phonon frequency and the FWHM value of the GaN-like E2 (high) mode decreased slowly and tended to saturate as the number of pairs of the MgN/AlGaN insertion layers increased further from three for sample D to four for sample E. of MgN/AlGaN insertion layers was much larger than that of sample D, which could be explained in terms of such a theory that the growth mode tends to transform from two-dimensional (2D) to 3D growth mode when the thickness of the insertion layers is increased too much, resulting in a degradation in surface morphology. 30) On the other hand, the surface morphology for the AlGaN epi-layers could also be degraded with the increase in Mg incorporation into the non-polar a-plane AlGaN epi-layers. 31)

Conclusions
The MgN/AlGaN insertion layers were applied for the first time in the epitaxial growth of the non-polar a-plane AlGaN films on semi-polar r-plane sapphire substrates by MOCVD.
The FWHM value of the XRC for the non-polar a-plane AlGaN epi-layer sample with three pairs of the MgN/AlGaN insertion layers was reduced to approximately a half of that for the sample without any MgN/AlGaN insertion layers. The smallest RMS value obtained from the AFM measurement was also achieved with the non-polar a-plane AlGaN epi-layer sample with three pairs of the MgN/AlGaN insertion layers, which was found to be 74% less than