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Scaling Semi -polar Substrates

Growth on patterned sapphire can lead to large, low-cost semi-polar substrates By Keisuke Yamane from Yamaguchi University, Japan

One-fifth of the world's electricity is consumed by lighting. So, to try and trim carbon footprints, many companies and governments are funding efforts to increase the efficiency of the light bulb.

Much interest has been devoted to solid-state lighting, thanks to its potential to deliver efficacies of several hundred lumens-per-Watt, which is far higher than the best fluorescent sources of today. At the heart of this type of light source is an InGaN-based LED that typically emits in the blue and pumps one or more phosphors of longer wavelengths. White light results from colour mixing.

Most InGaN-based LEDs are fabricated on a polar {0001} (c-plane) GaN layer, which is grown on a c-plane sapphire substrate. This platform is cost-effective, and the growth technique for forming the LED epistructure on it is refined and widely used in high-volume fabs. In contract, high-performance nitride devices, such as laser diodes or very high efficiency LEDs, are fabricated on bulk GaN substrates. The reason for this is that deposition on a native substrate gives the best device performance, because it minimises interface effects and defect formation.

Regardless of the substrate, LEDs formed with c-plane GaN-based material have a major impediment to high performance: separation of the carriers by a piezoelectric field (see Figure 1(a)). This field pulls apart electrons and holes, reducing the likelihood for radiative recombination and thereby impairing efficiency.


 

Figure 1. Band alignment of an InGaN/GaN quantum well structure for polar and nonpolar/semi-polar planes. Conventional InGaN quantum wells are grown in the c-direction (left). The InGaN quantum well gets compressively strained due to the different lattice constants of the two materials. This leads to an internal piezoelectric field, resulting in a tilt of the energy band. Consequently, there is a spatial separation between the electrons in the conduction band and the holes in the valence band. Moreover, the effective band gap is slightly reduced. This effect is called the quantum confined Stark effect. Removing the internal electric field allows the electron and holes to spatially overlap perfectly, resulting in efficient recombination

To reach longer wavelengths, indium content is increased, but this also increases the strength of the unwanted electric field in the device "“ and is the primary reason for the low luminous efficiency of green light-emitting devices.

An attractive option for addressing these fields is to turn to a new approach for band engineering, which involves growth on different planes of GaN. Piezoelectric fields are absent on non-polar planes, while drastically cut on semi-polar planes, and in both cases this enables greater electron-hole overlap and more efficient devices. If device developers want to turn to a non-polar plane, they have to choose between two orientations; but if they want to employ a semi-polar plane, they have a vast number of variants to consider, and it is still debatable which is the best of them.

What they are looking for is a plane that combines ease of high-quality crystal growth with straightforward device processing and favourable bandgap engineering. One orientation with much merit is the {2021} plane, which was used by a team from Sumitomo to make the world's first green laser diode.

GaN substrates
Today, hydride vapour phase epitaxy (HVPE) is viewed as a highly practical approach to obtaining thick layers of GaN, which can serve as quasi-bulk substrates. GaN is first grown on a foreign substrate, such as GaAs or sapphire, before the two are separated. The biggest advantage of this technique is its ability to produce high-quality material at high growth rates, thanks to high surface migration of the halide species. For this reason, the vast majority of GaN substrates shipped today are produced with a HVPE process.

The thick polar GaN material produced by HVPE is also used to make semi-polar and non-polar GaN substrates, which are formed by cutting along appropriate planes (see Figure 2). However, the size of nonpolar and semi-polar GaN substrates is typically restricted to just a rectangular area of a few square millimetres, due to the dimensions of the polar GaN

 


Figure 2. The conventional fabrication process for semi-polar GaN substrates: A GaN layer more than a few millimetres thick is grown, by HVPE, on c-plane GaN template and sliced in intended semi-polar directions

Due to the small size of semi-polar substrates produced by HVPE, there has been much interest in developing alternative approaches that could yield large-sized, semi-polar and non-polar GaN platforms. Our team at Yamaguchi University has been one of the groups attempting to pioneer new technologies to produce such orientations. Since 2004, we have been using MOCVD to develop epitaxial lateral overgrowth via selective area growth on a patterned sapphire substrate.

Our approach begins by forming trenches in sapphire with a width and depth of a few micrometres (see Figure 3). The orientation of these grooves is carefully chosen to ensure that one sidewall of the trench acts as a nucleation plane for c-plane GaN growth. Under optimised conditions, the epitaxial process starts on this sidewall and continues laterally over the terraces to yield a continuous semi-polar GaN layer. With this approach, we have formed a variety of thin films of 2-inch semi-polar material, based on orientations such as {1011}, {1122}, and {2021}.

 

Figure 3. Growth process of a {2021} GaN layer on patterned sapphire. (a) Formation of c-plane sapphire sidewall by dry etching, which is 74.6° inclined from the {2243} plane of the sapphire. (b) Nucleation of GaN stripes from the c-plane sapphire side-walls by MOVPE. (c) Formation of the {2021} GaN film by the coalescence of neighbouring GaN stripes. (d) Cross-sectional scanning electron microscopy of a {2021} GaN layer

Although producing these thin films is important, for practical applications bulk substrates are needed, and that means the deposition of thick, crack-free layers of GaN that are free from anomalous growth. We have done just that by turning to new sample structures for the fabrication of a semi-polar {2021} GaN substrate using SiO2 stripe-masked templates (see Figure 4). Optimised SiO2 striped masks were prepared in the direction perpendicular to the a-axis on a 2-inch {2021} GaN templates, prior to the of growth of a 1.4 mm-thick GaN layer at a deposition rate of 350 mm/h. The growth tool employed is a vertical-flow-type HVPE apparatus equipped with liquid gallium source, hydrogen chloride, ammonia, nitrogen and a hydrogen gas cylinder.

 Figure 4. Experimental set-up. (a) A 1.4 mm-thick GaN layer was grown on a SiO2 masked template. (b) HVPE apparatus can accommodate a 6-inch substrate. Four, 2-inch wafers were loaded in this work The SiO2 stripes play a crucial role in this substrate formation process. Without the striped mask, GaN forms a rough and cracked surface. But when it's there, the surface is far smoother, with roughening originating from unintentional anomalous growth regions on the template "“ note that these regions were completely embedded during selective area growth by HVPE. What's more, the SiO2 mask is effective on other planes, such as {1011} or {1122}, when growing GaN on patterned sapphire.

 

Figure 5. As grown surface of various orientations of GaN. Normarski microscope images of a {2021} GaN layer are also shown. The red arrows show the cracks Another attractive feature of our approach is the effective self-separation of the patterned sapphire and the GaN film. In comparison, typical methods employed for separating a GaN layer grown on a foreign substrate are more involved, such as mechanical polishing, laser or chemical lift-off, or self-separation via the growth of an intentional interlayer. With these more common methods, an additional process is required before or after HVPE growth of GaN.

 

Figure 6. A GaN substrate formed by chemical mechanical polishing. (a) Photograph of 2 inch c-plane, {1122} and {2021} GaN wafer. (b) Surface morphology of the chemical-mechanical polished {2021} GaN substrate measured by a scanning white light interferometer

When a layer of GaN is grown on sapphire, as the wafer cools thermal stress is induced in both materials, due to a difference in the thermal expansion coefficients, and this leads to a maximum shear stress at the heterointerface. We take advantage of that with our approach: GaN layers are formed by selective area growth from the narrow sidewall of the patterned sapphire, so there is weak adhesion at the hetero-interface. Thus, mechanical failure can easily occur at the hetero-interface to yield free-standing GaN without additional processing.

Chemical mechanical polishing of our thick layer of GaN, which has been separated from sapphire, enables the formation of a 2-inch semi-polar substrate. According to measurements acquired with a scanning white light interferometer, the typical root mean square roughness of this free-standing GaN is just 0.2 nm, implying that the surface is atomically flat. X-ray diffraction indicates that the radius of lattice curvature of the substrate is approximately 4 m. The dislocation density depends on the orientation of GaN. It is low 108 cm-2 for {2021}, low 107 cm-2 for {1011}, and mid 106 cm-2 for {1122}. One of our goals is to reduce this density by optimizing growth conditions and SiO2 mask geometry.

Our substrates have formed the foundation for the growth, by MOCVD, of an LED epiwafer structure featuring InGaN/GaN multiple quantum wells. X-ray diffraction measurements of this epistructure reveal satellite peaks, indicative of the abrupt heterointerface between the InGaN well and GaN barrier layers (see
Figure 7). Meanwhile, transmission electron microscopy reveals the high quality of the active region.
 


Figure 7. X-ray diffraction and current-voltage curves of {2021}GaN LED. (a) Measured and simulated X-ray diffraction profiles of InGaN/GaN multiple quantum wells. The inset shows (a) transmission electron microscopy image of InGaN/GaN multiple quantum wells. (b) Typical current-voltage curves of {2021} GaN LED

We have also measured the current-voltage characteristics of these LED structures, with results showing that these devices can be fabricated on the {2021} GaN substrate. If the dimensions of this platform can improve, it could play a major role in increasing the performance of commercial InGaN LEDs.

  • The development of thick layers of semi-polar and non-polar GaN was carried out within the projects Regional Innovation Cluster Program (Global Type) and Low-Carbon Research Network in the MEXT Japan.


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