Elsevier

Journal of Crystal Growth

Volumes 496–497, August–September 2018, Pages 74-79
Journal of Crystal Growth

Mesa orientation dependence of lateral growth of GaN microchannel epitaxy by electric liquid-phase epitaxy using a mesa-shaped substrate

https://doi.org/10.1016/j.jcrysgro.2018.04.011Get rights and content

Highlights

  • Growth of GaN microchannel epitaxy by electric liquid-phase epitaxy was optimized.

  • The (−1 2 −1 2) and (1 −2 1 2) facets on the sides strongly suppressed the lateral growth.

  • The (−1 2 −1 2) and (1 −2 1 2) planes are stable under Ga-rich condition of electric liquid-phase epitaxy.

  • The mesa orientation of [1 1 −2 0] resulted in a wide lateral growth without formation of any facets.

Abstract

Growth of (0 0 0 1) GaN microchannel epitaxy by electric liquid phase epitaxy using a mesa-shaped substrate was optimized to enhance lateral growth by systematically changing the mesa direction. It was found that the formation of the (−1 2 −1 2) and (1 −2 1 2) facets on the sides strongly suppressed lateral growth. The area of the facets increased as the offset angle of the mesa direction from the [1 1 −2 0] axis increased. At an offset angle of 30°, lateral growth was fully suppressed by the formation of the (−1 2 −1 2) and (1 −2 1 2) facets on the whole sides. The (−1 2 −1 2) and (1 −2 1 2) planes are thought to be stable in electric liquid-phase epitaxy under our extremely Ga-rich experimental conditions. These facets formed readily on the sides when the growth front directed to [1 −2 1 0]. On the other hand, the [1 1 −2 0] mesa orientation resulted in wide lateral growth. This is because the (1 −1 0 0) facets are less stable than the (0 0 0 1) and (0 0 0 −1) planes, and do not hinder lateral growth.

Introduction

Recently, gallium nitride has attracted much attention because of its wide range of applications, such as blue LEDs, ultraviolet LDs, and power transistors [1], [2], [3]. GaN is normally grown by metal organic chemical vapor deposition (MOCVD), which involves expensive equipment and source materials. In order to increase the applications of GaN, it is desirable to cut down the production costs. Liquid-phase epitaxy (LPE) [4], [5], [6], [7] is a promising candidate for growing GaN cost-effectively [8], [9], [10], [11]. It is also capable of enhancing lateral growth. Wide lateral growth also serves to increase the area without dislocations in microchannel epitaxy (MCE) [12], [13], [14], [15], [16], [17], [18], [19], [20]. In MCE, a narrow opening called a “microchannel” is cut into a silicon dioxide mask covering the substrate, and a layer is grown laterally from the microchannel. Consequently, the dislocation density decreases dramatically in the laterally grown area. This is because the mask stops the propagation of dislocations into the layer. In LPE, wide lateral growth is realized by the large difference between the lateral and vertical growth rates. A low supersaturation of LPE is useful for suppressing 2D nucleation and thus terminating vertical growth by the formation of a low-index plane on the top.

However, GaN LPE was previously performed under extremely high pressures and high temperatures [21], [22], [23], [24], and the addition of Na was required to increase the solubility of nitrogen in the solution [25], [26], [27]. Several years ago, Hussy et al. reported low-pressure solution growth (LPSG) of GaN at atmospheric pressure without the addition of Na to the solution [28]. The resulting GaN layer exhibited good characteristics, but the growth rate was as low as about 0.1 μm/h. Therefore, the growth rate is enhanced by flowing an electric current through the solution. The electromigration associated with this current flow is expected to increase the growth rate. This technique is similar to liquid-phase electroepitaxy (LPEE), but the Peltier effect does not occur because the current does not flow through the substrate. Thus, we refer to the new technique as “electric liquid-phase epitaxy (e-LPE)” to distinguish it from LPEE.

GaN MCE by e-LPE was performed using a mesa-shaped substrate, and dislocations were successfully reduced in the laterally grown regions [10], [29]. As such, the (0 0 0 1) and (0 0 0 −1) planes were formed on the top and bottom of the grown layer, which successfully terminated the vertical growth. However, the growth condition for GaN MCE by e-LPE has not been fully optimized. The microchannel direction is an important parameter that controls the width of the grown layer through the formation of side facets. In this study, therefore, the mesa orientation is changed systematically to optimize lateral growth for obtaining a wide GaN MCE layer by e-LPE.

Section snippets

Experimental

The (0 0 0 1) GaN template used as substrate consisted of a 4.4-μm-thick MOCVD-grown GaN layer on a sapphire substrate. GaN mesa structures with a period of 40 μm were etched from the template using a H4P2O7 solution by a mask fabricated by conventional photolithography. The mesa orientation was changed systematically from 0 to 30° with respect to the [1 1 −2 0] axis. Fig. 1 shows Nomarski differential interference contrast microscopy (NDICM) images of the mesa structures cut in different

Results and discussion

The surface NDICM images of the GaN MCE layers are shown in Fig. 3. They reveal that selective growth proceeded perfectly, without the formation of any polycrystals on the sapphire substrates, and a flat (0 0 0 1) plane grew on the top of the layers. Straight sides were obtained for the layer grown from the mesa with an offset angle of 0° from [1 1 −2 0], while zigzag sides were obtained for layers grown from mesas with nonzero offset angles. The width of the layers decreased with increasing

Conclusion

The mesa orientation dependence of the lateral growth of GaN MCE by e-LPE was investigated to optimize the growth conditions for obtaining wide lateral growth. It was found that the width of the grown layer decreased as the offset angle of the mesa from the [1 1 −2 0] axis was increased. This is because coverage of the side facets increased with the offset angle. The formation of (−1 2 −1 2) and (1 −2 1 2) facets on the sides terminated the lateral growth. At an offset angle of 30°, almost all

Acknowledgement

This work was partly supported by Grants-in-Aid for Priority Area (B) (No. 15H03558), Challenging Exploratory Research (No. 2660008), Specially Promoted Research (No. 25000011), and Scientific Research on Innovative Areas (No. 26105002) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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