The present disclosure relates to a compound semiconductor layer formed of gallium nitride (GaN) or a nitride of gallium and other metal, and a method for forming the compound semiconductor layer. The present disclosure also relates to a method for preparing a substrate used in manufacturing of electronic or photo-electronic devices including the compound semiconductor layer. The present disclosure pertains to a technical field for forming a high quality compound semiconductor layer on a substrate, and more particularly, to a technical field for preparing a free standing compound semiconductor substrate by separating the substrate and the compound semiconductor layer.
Semiconductor materials based on nitrides of Group III elements or Group V elements already hold important positions in electronic and photo-electronic fields, which will be important more and more. In fact, the nitride based semiconductor materials may be used in a wide range of fields from laser diodes (LD) to transistors operating at high frequency and high temperature. The nitride based semiconductor materials may also be used in ultraviolet photo-detectors, surface acoustic wave detectors and light emitting diodes.
For example, although gallium nitride is widely known for its usefulness in blue light emitting diodes and high frequency and high temperature transistors, it is also being extensively researched for use in microelectronic devices. As used herein, gallium nitride includes gallium nitride alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and aluminum indium gallium nitride (AlInGaN).
To grow a gallium nitride layer of low defect density is important in manufacturing gallium nitride microelectronic devices. A substrate on which gallium nitride is grown is known as one cause of the defect. However, it is difficult to prepare a gallium nitride substrate or a substrate for growing gallium nitride without defects. Typical methods such as Czochralski method where the crystal is grown from a melt cannot be used in producing a gallium nitride single crystal for the substrate because gallium nitride is difficult to melt. Surely, gallium nitride can be molten under ultrahigh pressure, however, this is currently unavailable for commercial use due to the low productivity.
Accordingly, in such devices, the most frequently used for growing gallium nitride layer are heterogeneous substrates such as a sapphire substrate, a silicon carbide (SiC) substrate and a silicon substrate. However, because of lattice mismatch and thermal expansion coefficient difference between such substrate materials and gallium nitride, a large number of dislocations may be produced in the gallium nitride layer grown on the substrate, causing crack and bending of the gallium nitride layer. Therefore, a variety of buffer layers are often formed on the substrate before growing the gallium nitride layer thereon, or an epitaxial lateral overgrowth (ELO) method are used to reduce the dislocation generation.
In a typical ELO method, a stripe-shaped silicon dioxide (SiO2) mask is used to reduce stress caused by the lattice mismatch and the thermal expansion coefficient difference between the substrate and the gallium nitride layer. The typical ELO method will be described below with reference to FIG. 1, which is a cross-sectional view of the substrate on which the gallium nitride layer is grown according to the typical ELO method.
In the typical ELO method, the gallium nitride layer 2 is grown on the substrate 1 in a furnace, and then the substrate 1 is taken out of the furnace. The substrate is placed in a deposition apparatus so that silicon dioxide (SiO2) layer is deposited on the gallium nitride layer, and then the substrate 1 is taken out of the deposition apparatus. The silicon dioxide layer is patterned using a photolithography technique to form a silicon dioxide mask 3 on the gallium nitride layer, and then the substrate 1 is placed again in the furnace so that an ELO gallium nitride layer 4 is grown on the gallium nitride layer 2.
A portion of the ELO gallium nitride layer 4 that is laterally grown over the silicon dioxide mask 3 has relatively high quality compared to the portion that is vertically grown. This is because defects such as dislocations cannot propagate through the laterally grown portion. Therefore, by forming a device in the portion of the ELO gallium nitride layer 4 that is laterally grown over the silicon dioxide mask 3, it is possible to obtain an excellent property.
However, the ELO method requires the above described complex process such as an additional external process for forming the silicon dioxide mask, increasing process time and process cost. In addition, recently, as a plurality of silicon dioxide masks are used to improve and enlarge the function of the ELO, the number of the processes for forming the silicon dioxide mask and growing the gallium nitride layer is also increased correspondingly. Consequently, this may result in increased process cost, process complexity, time loss and economical loss, and thus result in decreased process yield.
Korean Patent Laid-Open Publication No. 2004-0078208 discloses, instead of the ELO method, a method for preparing a gallium nitride substrate by forming grooves for reducing contact area between a sapphire substrate and a gallium nitride layer. According to the method, gallium nitride epitaxial layers are grown on an upper surface and a lower surface of the sapphire substrate, respectively. In specific, a first gallium nitride epitaxial layer is grown on the upper surface of the sapphire substrate in a furnace. The sapphire substrate is taken out of the furnace, turned upside down, and then placed in the furnace again so that a second gallium nitride epitaxial layer is grown on the other surface, i.e., the lower surface of the sapphire substrate. Next, a mask pattering is performed on the lower surface using a photolithography process, and the second gallium nitride epitaxial layer is etched to form a plurality of grooves. Thereafter, a laser beam is applied thereto to etch the portion of the first gallium nitride epitaxial layer corresponding to the portion of the plurality of grooves. As a result, the void-like grooves are formed on the upper surface of the sapphire. Using these grooves, a third gallium nitride epitaxial layer is grown.
According to the above described method, the grooves prevent contact of the sapphire substrate and the gallium nitride layer for growing the third gallium nitride epitaxial layer. As such, it is possible to reduce dislocation generations in the gallium nitride layer over the grooves, and reduce crack and bending caused by the thermal expansion coefficient difference as temperature is decreased from a growth temperature to the room temperature. However, the method requires additional processes such as growing the gallium nitride layers on the upper surface and the lower surface of the sapphire substrate, respectively, performing the mask patterning on the gallium nitride layers, and applying the laser beam to form the grooves on the upper surface of the sapphire substrate. This may result in increased process time and process cost.
In addition, when the sapphire substrate is removed after growing the gallium nitride epitaxial layer on the sapphire substrate to use the gallium nitride epitaxial layer as a free standing gallium nitride substrate, the separation of the sapphire substrate from the gallium nitride epitaxial layer needs an additional process such as a laser lift-off. This may also increase the process cost, and decrease the process yield because heat applied to separate the sapphire substrate from the gallium nitride epitaxial layer may cause defects such as crack and bending in the gallium nitride epitaxial layer. If the substrate is formed of silicon, it may be easily removed by a polishing or a chemical etching. However, the silicon substrate also has a limitation that it is difficult to form a high quality gallium nitride epitaxial layer thereon.
As described above, although the mask patterning process for manufacturing the gallium nitride substrate of good quality is effective in reducing dislocations, it may increase process time and process cost.