New Substrate for Ga-N Based LEDs

Gallium Nitride Epi Wafers on Sapphire Substrates
Gallium Nitride Epi Wafers on SiC Substrates

Aluminum Nitride Epi Wafers on Sapphire Substrates
Aluminum Nitride Epi Wafers on SiC Substrates

Silicon Carbide Epi wafers - General Description
Silicon Carbide wafers with Reduced Micropipe Density

Novel Substrate for Blue LED Production

GaN Epitaxial Wafers are the ideal substrates for GaN homoepitaxial growth and device manufacturing. 

Substrate for III-V Nitrides Epitaxy.

Applications New type of substrate for GaN-based LEDs. For more details, please click here. GaN Epitaxial Wafers may be used as substrates for III-V nitride epitaxial growth by MBE, MOCVD and CVD. No buffer layer is required.

Depth profile for concentration Nd-Na in GaN epitaxial layer

Photoluminescence spectrum of GaN epitaxial layer

Gallium Nitride epi wafers on SiC substrates

Applications GaN Epitaxial Wafers may be used as substrates for III-V nitride epitaxial growth by MBE, MOCVD and CVD. No buffer layer is required.  GaN Epitaxial Wafers are the ideal substrates for GaN homoepitaxial growth and device manufacturing.

Technology
Gallium Nitride Epitaxial Wafer consists of a thin undoped GaN epitaxial layer grown by hydride vapor phase epitaxy (HVPE) directly on (0001)Si face on-axis 6H-SiC or 4H-SiC substrate.

Additional Information

• GaN layers and SiC substrates are electrically conducting.  Silicon carbide insures excellent heat removal from nitride device structure, which is important for high-power devices. GaN/SiC wafers may be cleaved providing mirror-like facets for nitride laser diodes. 
• GaN layers could be grown on SiC substrates supplied by customers.

General Description

Micropipes in SiC - Device Killers
Closing of Micropipes During SiC LPE Growth
Filling the Micropipe Channels in Standard Commercial SiC wafers
Properties of SiC epitaxial layers with reduced micropipe density

Micropipes in SiC - Device Killers

SiC is wide-band-gap material with well-recognized potential for high-power, high-temperature and high-frequency electronics. Fundamental parameters of SiC material are very attractive for the fabrication of semiconductor devices with superior characteristics for military and industrial needs in the aircraft and space electronics, nuclear power, automotive, and power utility industries. Silicon carbide devices will meet commercial and military need for high-current and high-voltage devices and integrated circuits for power transmission and distribution systems, hybrid- and all-electric vehicles, and other types of advanced electrical equipment and machinery.

Also, silicon carbide has found an application as a substrate material for III-nitrides device structure epitaxy, due to its good lattice match with III-nitrides and high thermal conductivity.

The factor limiting the SiC substrate application is high density of defects existing in commercially available SiC substrates. The defect is known to destroy any device if present in the device active area; this is the so-called micropipe defect.

Most views on micropipes are based around Frank’s theory of a micropipe being the hollow core of a screw dislocation with a large Burgers vector. According to the theory, the diameter of the micropipe has a direct relationship with the magnitude of the Burgers vector. Generally speaking, a micropipe is a tube (with a diameter ranging from fractions of a micron to tens of microns) that propagates through SiC crystal in the direction parallel or close to the [0001] crystallographic axis.

Today, the density of micropipe defects in standard SiC commercial wafers, which are being used as substrates for SiC device fabrication, exceeds 100 cm^-2. These micropipes, originated from SiC substrates, penetrate in device structures during epitaxial growth and cause the device failure (Figure 1).

 

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Closing of micropipes during SiC LPE growth

It has been shown that micropipes in SiC wafers may be closed during liquid phase epitaxial growth. A few independent researchers have proved this fact. The general observation was that the channel of the micropipe, penetrating from SiC substrate in to epitaxial layer, became smaller and smaller during Liquid Phase Epitaxy (LPE). Finally, the micropipe closes, forming a growth hillock on the epitaxial layer surface. In order to close micropipes during liquid phase epitaxial growth, a thick (up to 100 microns) SiC epitaxial layer should be grown on the initial SiC wafer. Due to the requirement of thick SiC layer formation, the growth temperature and supersaturation in a melt must be reasonably high and the solubility of SiC in a melt must also be high. During thick layer growth, the formation of other defects, such as foreign polytype inclusions in the grown layer, is possible. The height of growth steps forming due to step bunching on the SiC surface is larger for the thicker layers. If a thick layer is required to close micropipes, the amplitude of surface relief may reach a few microns, causing problems in growing a SiC device structure on such a surface. Experiments also show that it is difficult to close micropipes by this approach if the SiC wafer has a large misorientation angle.

 

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Filling the Micropipe Channels in Standard Commercial SiC

A new technical approach to solve the micropipe problem is proposed by TDI, Inc. The main idea of this approach is to fill the micropipes inside the micropipe channel first, and then grow an epitaxial SiC layer on the top of the wafer with filled micropipes (Figure 2). Resulting wafers consist of a SiC commercial wafer (initially having about 100 micropipes per square cm) with filled micropipes, and a SiC epitaxial layer with reduced micropipe density grown on this wafer. This approach allows us to avoid the LPE growth of thick SiC layers in order to close micropipes. The filling process takes place inside the micropipe channel and, at the same time, epitaxial growth on a flat surface is negligible. It is necessary to emphasize that for this process wafer size does not effect the micropipe filling process.

Currently TDI is manufacturing 50 mm diameter SiC epitaxial wafers with micropipe density less than 10 cm^-2.

 

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Properties of SiC epitaxial layers with reduced micropipe density

Step bunching occurs during epitaxial growth and surface morphology depends strongly on substrate orientation. The flattest surfaces are obtained on well-oriented substrates. Step height increased with the increase of the tilt angle of the substrate. The SEM image of 4H-SiC layer grown on Si face of 4H-SiC substrate with tilt angle of 8 degrees is shown in the Figure 3. Closed micropipe and growth steps are clearly seen. The height of the single step measured by Atomic Force Microscopy (AFM) does not exceed 20 nanometers. The surface between steps is atomically flat. The high-energy electron diffraction showed that the surface is of single-crystal structure; no amorphous or polycrystalline areas were observed. Although the surface quality is sufficiently high, polishing technique may be employed for its further improvement.

We etched SiC wafer with reduced micropipe density in molten KOH to count number of micropipes remained in the wafer after micropipe filling process. This method is time-consuming, but it provides reliable results on micropipe density measurements. If the micropipe is not closed during the epitaxial growth, it is clearly seen on the surface of the epitaxial layer. When the closing process is completed, a growth hillock can usually be seen on the layer surface at the point corresponding to the micropipe in the substrate. There is no difference in the micropipe closing process of 4H and 6H polytypes. However, the misorientation angle plays an important role in the micropipe closing process. It is more difficult to fill micropipe if the tilt angle is large. After epitaxial growth on on-axis wafers, some areas, about 1 cm², having no micropipes were observed on the epitaxial layers. Micropipes were not opened after short etching (about 1 minute) at 500°C. Very important to note that micropipe filling process did not depend on size of SiC wafer.

For more information on our substrates for GaN-Based LEDs, please contact Marubeni Sunnyvale and we will send you a full brochure.