Structural Inhomogeneities and Impurity Incorporation in Growth of

Article pubs.acs.org/crystal

Structural Inhomogeneities and Impurity Incorporation in Growth of High-Quality Ammonothermal GaN Substrates N. A. Mahadik,* S. B. Qadri, and J. A. Freitas, Jr. U.S. Naval Research Laboratory, Washington, D.C. 20375, United States

ABSTRACT: Ammonothermal Gallium Nitride (GaN) substrates are the most promising substrates for homoepitaxial growth of GaN films having low dislocation density. Growth-induced structural inhomogeneities in these substrates were investigated by high-resolution X-ray topography (HR-XRT) and high-resolution X-ray diffraction (HR-XRD). A one-to-one correlation of defects was observed in photoluminescence imaging. From the HR-XRD intrinsic rocking curve widths were found to be 16 arcsecs indicating superior crystalline quality. The lattice constants were measured from symmetric and asymmetric reflections to be similar to bulk values indicting low sample strian. The true dislocation density, from the HR-XRT images, was observed to be of the order of 102 cm−2 in the samples, and the radius of curvature was greater than 600 m. Growth striations were observed in the a and m-pane samples and are attributed to inhomogeneity in impurity incorporation during growth. Photoluminescence imaging showed deep level luminescent centers along the growth striations.

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INTRODUCTION Gallium Nitride (GaN) is a very important semiconductor because of its potential for the fabrication of various optoelectronic, high frequency, and high power devices.1−4 Due to the absence of naturally occurring GaN substrates, several methods have been used to grow GaN films heteroepitaxially on foreign substrates. Over the past two decades, there have been several reports on the structural properties of these GaN films grown on foreign substrates5−12 such as sapphire or silicon carbide, or quasi-bulk thick freestanding GaN substrates,13−16 that are lifted off from sacrificial substrates. However, during GaN heteroepitaxial growth using conventional deposition techniques, significant strain builds up during growth, nucleating a high concentration of dislocations, typically 107 cm−2 or higher.17,14 This is due to the lattice and thermal mismatch between the substrate and the GaN films. As a result, GaN devices fabricated on these substrates operate well below their theoretical limits.18 Significant reduction in dislocation density can be achieved if bulk samples are grown from equilibrium phase using the recently developed ammonothermal process without the use of any foreign substrates.19 Subsequent GaN films grown on these native substrates using any of the conventional film growth techniques will yield very high quality films with very low dislocation densities. In this work, we report on the structural This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society

properties of semi-insulating c, a, and m-plane wafers from GaN crystal boules grown by the ammonothermal method. Critical issues for device fabrication such as sample warpage, true dislocation density, and other extended defects are shown. In addition, optical investigation of these structural defects show a one-to-one correlation with the results from X-ray measurements.

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EXPERIMENTAL PROCEDURE

The bulk ammonothermally grown, semi-insulating GaN samples were obtained from AMMONO SA. The typical growth pressure and temperatures were 0.2−0.5 GPa and 500−600 °C, respectively. Mineralizers were used to enhance the solubility of GaN in ammonia.19 High-resolution-X-ray diffraction (HR-XRD) and rocking curves measurements were performed on a Rigaku ATX-E diffractometer, equipped with an 18 kW Cu rotating anode, a collimating mirror and four bounce Ge (220) channel cut monochromators, which provides a low divergence, pure Cu Kα1 incident beam. The sample was precisely aligned using the computer controlled X, Y, Z, and χ, ϕ stage. Multiple symmetric and asymmetric GaN reflections were measured to precisely obtain the a and c lattice parameters. Rocking curve full width at half-maximum (FHWM) was Received: September 9, 2014 Revised: November 5, 2014

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dx.doi.org/10.1021/cg5013523 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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measured to analyze the crystalline quality of the wafers. To investigate the true crystal structure, high-resolution X-ray topography (HR-XRT) was used to obtain precise dislocation density, sample warpage, and other extended defects. HR-XRT was performed using a 12 kW Cu rotating anode, in double crystal geometry with an asymmetrically cut Si monochromator, which expands the incident beam horizontally 26 times to image the entire sample. Using this geometry, we obtain a resolution of ∼1 μm to image the extended defects. The HR-XRT was performed using grazing incidence angles (213), (204), and (203) asymmetric reflections for the c, a, and m-plane samples, respectively. High-resolution photographic film with submicron resolution was used to collect the image. Photoluminescence (PL) imaging was performed by illuminating the entire sample with the 325 nm line of a HeCd laser and collecting the luminesnce images with a CCD camera. Standard bandpass filters were used to acquire images in the blue, green, and red color regions and real color images.

these types of samples needs to be optimized to remove the residual damage. Another significant observation is that this image was taken in a single exposure, which indicates that the wafer curvature is less than 1.6 × 10−3 m−1 or radius of curvature is greater than 600 m from the size of the sample. Typical nonbulk GaN has a radius of curvature of 0.1−2 m.15 A single defect bounded by dislocations was observed on the HRXRT image of this sample, as shown in Figure 2a, which is a typical signature of stacking faults. This defect is a singular case as the other samples did not show this feature. Figure 2b shows the PL image of this sample acquired with the red filter. Small punctual defects are observed, and they luminesce in the green and the red colors. The origin of these defects is not related to the threading dislocations and is further discussed below. Figure 3a shows the HR-XRT image of the a-plane sample. The threading dislocation density is on the order of 10 cm−2. Misfit dislocations are also observed emanating from the edge of the sample, tilted toward the surface. These are also on the order of 10 cm−2. These could have been generated by the, yet nonoptimized, CMP process. Growth striations are observed in this sample along the a- and m-planes and are detected by the HR-XRT image (Figure 4a). These are due to inhomogeneity in impurity incorporation, nonstochiometry, or some strain build up. The growth striations are probably created on the growth plane, and we assume that the crystals were grown along the c axis and later cut along the a- and m-planes. The GaN boules were likely grown in multiple growth steps, with unintentional differences in impurity incorporation at the initiation of each growth step. This is observed in the PL images, as luminescence intensity varies across the sample, as shown in Figure 3b. From the periods of the growth striations, we determine the growth steps to be between 2 and 3 mm intervals. The PL image of the a-plane sample, shown in Figure 3b, was taken with the standard red filter. The growth striations have a one-to-one correlation with the corresponding HR-XRT image. Note that these samples are semi-insulating, and are part of preliminary growth efforts by AMMONO SA to grow semiinsulating GaN boules. The lack of reproducible impurity incorporation and distribution in sequential growth are being addressed, and recently grown boules appear to have more uniform impurity distribution. In addition to the growth striations, we also observe luminescent dots along the striations. PL images taken using a variety of filters show that these luminescent dots have spectral emission extending in the green and red region. They are not observed on luminescence imaging acquired with the blue/violet filters, which indicates that these luminescent centers are deep levels in GaN. Similar broad emission bands have been previously observed in GaN (e.g. the yellow and green bands), and they have been attributed to point defects.23 However, additional experiments must be carried out to accurately identify the origin of these defects. Figure 4a shows the HR-XRT image of the m-plane sample. The average dislocation density is similar to the other ammonothermally grown GaN samples on the order of ∼20 cm−2. Damage from the CMP is also seen, which may be the cause of misfit dislocations. Growth striations, similar to the aplane sample, are observed, which are attributed to the inhomogeneity in impurty distribution. Figure 4b shows the PL image of this sample taken with the red filter, which shows the growth striations and the luminescent dots along their boundaries, similar to the features observed in the a-plane sample.

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RESULTS AND DISCUSSION Multiple symmetric and asymmetric HR-XRD scans were used to obtain the a and c-lattice parameters using least-squares method for all the samples as listed in Table 1. These values are Table 1a and c-Lattice Parameters Using Least-Squares Method Obtained from Multiple Symmetric and Asymmetric HR-XRD Scans sample

c (Å)

a (Å)

c-plane a-plane m-plane

5.1856(1) 5.1851(1) 5.186(1)

3.1890(5) 3.1892(4) 3.1891(2)

the same as those reported for bulk GaN lattice constants,20 indicating almost no strain in the samples. Sharp rocking curves were obtained for the symmetric reflections from all the samples as shown in Figure 1, with their fwhm values ∼16

Figure 1. High-resolution rocking curves of the ammonothermally grown, semi-insulating c, a, and m plane GaN samples.

arcsecs. From this value, the upper limit of the average dislocation densities in the samples can be calculated to be below 105 cm−2.21,22 The true dislocation density was observed using HR-XRT imaging. Figure 2a shows the HR-XRT image of the (213) reflection from the c-plane GaN sample. In this image of the 1 cm2 sample, we observe 16 threading dislocations, which indicates a dislocation density of the order of 20 cm−2. This is a remarkable result in comparison to previously investigated freestanding GaN, where typical densities are of the order of 107 cm−2.15 We also observe some chemical mechanical polishing (CMP) damage in the semi-insulating GaN c-plane sample. The CMP process for B



Article

Figure 2. (a) High-resolution XRT image of the c plane of GaN sample. Grazing incidence angle of ∼0.7° (b) Corresponding PL emission image in the red color of the GaN sample.

Figure 3. (a) High-resolution XRT image of the a plane GaN sample. Grazing incidence angle ∼1°. (b) Corresponding PL emission image in the red color of the GaN sample showing growth striations and luminescent dots.

Figure 4. (a) High-resolution XRT image of the m plane GaN sample. Grazing incidence angle ∼7°. (b) Corresponding PL emission image in the red color of the GaN sample showing growth striations and luminescent dots.

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CONCLUSIONS

growth. Photoluminescence imaging confirmed the existence of these growth striations and verify the presence of luminescent

The structural properties of ammonothermally grown c-, a-, and m-plane GaN substrates were investigated using high-resolution XRD and topography. The GaN substrates have average dislocation densities less than 102 dislocations cm−2. The sample radius of curvature was remarkably greater than 600 m for all the samples. CMP damage was also observed in the samples, which possibily generates surface strain to cause misfit dislocations in these semi-insulating samples. Growth striations were observed in the a- and m-plane samples, which are due to inhomogeneity in impurity incorporation during sequential

centers along the growth striations, which are characterized by a broad emission extending in the green and the red spectral region. Broad emission bands have been previously observed in GaN and have been attributed to point defects. However, additional experiments must be performed to verify the characteristics of these recombination centers and their role in the material properties. C



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Article

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Structural Inhomogeneities and Impurity Incorporation in Growth of

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