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  • H01L21/02664—Aftertreatments

Definitions

• the present invention relates to a method for producing a group-Ill nitride wafer.
• group-Ill nitrides The interest for group-Ill nitrides has grown rapidly during the last years. One reason is the semiconducting properties of group-Ill nitrides that may be utilized in light-emitting diodes (LED) and laser diodes (LD).
• LED light-emitting diodes
• LD laser diodes
• Semiconductors are commonly manufactured from a melt.
• the melting conditions for silicon is 1400°C at ⁇ 1 atm
• for gallium arsenide is 1250°C at 15 atm
• for gallium phosphide is 1465°C at 30 atm.
• the melting conditions for group-Ill nitrides are significantly higher.
• the melting condition for gallium nitride is 2500°C at 45 000 atm. Consequently, group-Ill nitrides cannot be obtained from the melt at reasonable temperatures and pressures and therefore it is extremely difficult and would be very expensive to produce group-Ill nitrides from a melt.
• Group-Ill nitrides may instead be grown on a substrate. Due to the lack of native substrates group-Ill nitrides are fabricated on heterogeneous substrates such as sapphire, SiC or Si. It gives rise to very high dislocation densities ( ⁇ 10 9 cm "2 ) structures which is a severe problem in LEDs and LDs since dislocations reduces the lifetime and lower the external quantum efficiency with increasing current density. Deposition of a thick group-Ill nitride layer on a substrate different from the group-Ill nitride, i.e. heteroepitaxial growth, involves several problems.
• the usually large mismatch in lattice constants between the substrate and the group-Ill nitride gives rise to tensile stress during the growth and a high dislocation density.
• the usually large difference in thermal expansion coefficients between the substrate and the group-Ill nitride causes biaxial compressive stress in the group-Ill nitride layer during cooling from growth temperature.
• the compressive stress can cause cracking of both the group-Ill nitride epilayer and the substrate. Cracking during the cooling is one of the most severe problems which make the fabrication of group-Ill nitride wafers having a large area challenging.
• One way of releasing group-Ill nitride from the substrate is by laser lift-off, which has been described by W.S. Wong, T. Sands, N.W Cheung, Appl. Phys. Lett. 72 (5), (1998) 599-601.
• laser lift-off is that in order to release the complete group-Ill nitride wafer from the substrate, the laser beam has to affect the whole contact surface between the group-Ill nitride wafer and the substrate and consequently the laser setup has to be moved to all positions over the complete area of the group-Ill nitride wafer.
• the possible material of the substrate is limited since a substrate that does not absorb laser has to be used.
• laser lift-off is performed as a separate action after production of the group-Ill nitride in the reactor. Consequently, the laser lift-off is performed after cooling and therefore does not overcome the problem of cracking of the group-Ill nitride wafer during cooling. Even though laser lift-off has been suggested to be performed within the reactor in which the group-Ill nitride is produced, the equipment of a reactor with a laser setup is technically very complicated and expensive, which reduces the practical and commercial use of such a reactor.
• a second way to remove the group-Ill nitride from the substrate is to form voids in the interface between the substrate and the group-Ill nitride. Due to that, the interface between the group-Ill nitride and the substrate is weakened and upon cooling from growth
• the group-Ill nitride film is spontaneously separated due to the difference in thermal expansion between the group-Ill nitride and the substrate.
• VAS TiN buffer layer
• Preparation of group-Ill nitride wafers have also been suggested to include removing the substrate from the group-Ill nitride wafer by polishing and etching of the complete substrate.
• the polishing or etching is also performed outside the reactor after growth of the group-Ill nitride inside the reactor.
• polishing nor etching overcomes the problem of cracking of the group-Ill nitride wafer during cooling.
• One further obvious drawback of polishing and etching is the need for complete destruction of the substrate which both is time consuming and a waste of resources.
• One object of the present invention is to overcome at least some of the problems and drawbacks mentioned above.
• One object of the present invention is to obtain high quality group-Ill nitride wafers.
• One object of the present invention is to obtain group-Ill nitride wafers free from cracks or at least having reduced occurrence of cracks.
• One object of the present invention is to avoid cracking of the group-Ill nitride wafer during cooling.
• the method of the present invention enables production of high quality group-Ill nitride wafers.
• the method of the present invention reduces the occurrence of cracks in group-Ill nitride wafers.
• the method of the present invention enables production of group-Ill nitride wafers free from cracks.
• the method of the present invention enables production of group-Ill nitride wafers avoiding cracking of the group-Ill nitride wafer during cooling.
• the method of the present invention enables in-situ production of high quality group-Ill nitride wafers.
• the method of the present invention enables production of high quality group-Ill nitride wafers by a reduced number of operations.
• the method of the present invention enables reuse of the substrate after polishing.
• Figures 1 (a), 2(a) and 3(a) are photographs illustrating the backside of freestanding GaN produced according to embodiments of the present invention and annealed at different temperatures.
• Figures 1 (b) and 2(b) are photographs illustrating the sapphire substrate used during production of the freestanding GaN in figure 1 (a) and 2(a), respectively.
• Figure 3(b) is a photograph illustrating the surface morphology of the freestanding GaN in figure 3(a).
• the present invention relates to a method for producing a group-Ill nitride wafer comprising the steps providing a substrate, growing a main layer of a group-Ill nitride on the substrate by means of heteroepitaxial growth at a temperature within a growth temperature range of the group-Ill nitride, annealing the substrate with the main layer at a temperature sufficiently high to initiate decomposition of the group-Ill nitride at the surface facing the substrate whereby the main layer is released from the substrate, wherein the steps growing a main layer and annealing are performed in one reactor chamber.
• a crystalline layer of group-Ill nitride is grown on a crystalline substrate of a different material where the layer is in registry with the substrate.
• the heteroepitaxial growth is performed at a temperature within a growth temperature of the group-Ill nitride. Within the growth temperature range, the group-Ill nitride grows by crystallization of group-Ill nitride.
• the crystallization ceases and the crystalline group-Ill nitride starts to decompose. The decomposition starts at the surface facing the substrate because of the strains present at this surface as explained above.
• the strains lower the decomposition temperature and therefore the decomposition starts at the surface facing the substrate.
• the decomposition of the group-Ill nitride surface facing the substrate implies that the attachment of the group-Ill nitride to the substrate is broken. Consequently, the group-Ill nitride is released from the substrate.
• the decomposition should be sufficient to release the group-Ill nitride from the substrate.
• the temperature should be sufficiently high to
• the group-Ill nitride is decomposed into group-Ill metal and gaseous nitrogen.
• metal is released forming metal droplets. Consequently, the group-Ill nitride becomes metal rich at the surface facing the substrate. Presence of metal droplets lowers the decomposition temperature of the group-Ill nitride.
• decomposition of the group-Ill nitride at the surface facing the substrate is further facilitated. Thereby, the release of the group-Ill nitride wafer from the substrate is enhanced.
• the step growing a main layer of a group-Ill nitride on the substrate and the subsequent step of annealing the substrate with the main layer are performed in one and the same reactor chamber, i.e. in-situ.
• the process is simplified and the production time reduced, since it not is necessary to move the substrate with the main layer to a second production unit or reactor chamber.
• the costs are reduced since only one reactor or reactor chamber is needed instead of two different reactors or reactor chambers.
• the annealing step is performed after the step of growing a main layer without cooling or allowing the substrate with the main layer to cool.
• the annealing step is performed after the step of growing a main layer without cooling or allowing the substrate with the main layer to cool between the step of growing a main layer and the annealing step.
• no cooling occurs, neither by performing cooling or allowing the substrate with the main layer to cool, between the step of growing a main layer and the subsequent annealing step.
• no cooling occurs before the annealing.
• both the step of growing a main layer and the annealing step are performed in the same reactor chamber and do not have to be moved to a different reactor or furnace.
• the annealing step may be performed directly after the step of growing a main layer without cooling or allowing the substrate with the main layer to cool.
• the method further comprises the step separating the main layer from the substrate.
• separating the main layer from the substrate a freestanding group-Ill nitride wafer is obtained. Since the main layer of group-Ill nitride already has been released from the substrate, the separation of the main layer from the substrate is easily achieved by only bringing the main layer of group-Ill nitride and the substrate away from each other.
• the growth temperature range of a group-Ill nitride is the temperature range where the partial pressures of the precursors can maintain a driving force for crystallization of the group-Ill nitride.
• the decomposition temperature is the temperature when the material starts to decompose depending on environmental conditions.
• the substrate with the main layer is annealed at a temperature equal to or exceeding a decomposition temperature of the group-Ill nitride. By annealing at or above the decomposition temperature of the group-Ill nitride, the decomposition of the surface of the group-Ill nitride and in particular the surface of the group-Ill nitride facing the substrate is further enhanced.
• the substrate with the main layer may be heated to a decomposition temperature of the group-Ill nitride.
• the annealing is performed by heating the substrate with the main layer to a temperature above the growth temperature range of the group-Ill nitride.
• the substrate with the main layer is heated to a temperature at least 10°C above the decomposition temperature.
• the substrate with the main layer is heated to a temperature at least 20°C above the decomposition temperature.
• the substrate with the main layer is heated to a temperature at least 30°C above the decomposition temperature. Increased temperature increases the efficiency of the decomposition and thereby the production time is reduced.
• the main layer is grown to a thickness of at least 200 ⁇ .
• the present invention enables production of thick group-Ill nitride wafers.
• the main layer may be grown to a thickness of at least 300 ⁇ .
• the main layer may be grown to a thickness of at least 500 ⁇ .
• the main layer may be grown to a thickness of at least 1 mm.
• the annealing of the substrate is performed during a period sufficiently long to achieve decomposition of group-Ill nitride at the surface facing the substrate.
• the duration of the period is dependent on the temperature at which annealing is performed, i.e. how much above (how many degrees Celsius above) the temperature at which decomposition of the group-Ill nitride is initiated.
• the annealing of the substrate with the main layer is performed during a period of at least 2 minutes. This period of time enables sufficient decomposition of group-Ill nitride at the surface facing the substrate and thereby release of the main layer of group-Ill nitride from the substrate.
• the substrate with the main layer may be annealed during a period of at least 5 minutes, such as at least 10 minutes.
• the substrate with the main layer may be annealed during a period of maximum 30 minutes, such as 20 minutes.
• a group III nitride is a nitride of group-Ill elements, i.e. elements belonging to the group of boron (B), aluminum (Al), gallium (Ga), indium (In), thallium (Tl) and ununtrium (Uut).
• the group-Ill element of the group-Ill nitride of the main layer is gallium, aluminum, indium or a mixture thereof.
• the main group-Ill nitride may be a single group-Ill nitride, such as gallium nitride, aluminum nitride or indium nitride.
• the main group-Ill nitride may be a nitride comprising a mixture of metals, such as a combination of gallium and aluminum or gallium and indium or gallium, aluminum and indium. Such a mixture may be obtained from an alloy of different metals, such as an alloy of gallium and aluminum.
• the main layer may be made of gallium nitride, aluminum nitride, indium nitride, aluminum gallium nitride, indium gallium nitride or indium aluminum gallium nitride.
• GaN gallium nitride
• HVPE halide vapor phase epitaxy
• MOCVD metal-organic chemical vapor deposition
• MBE molecular beam epitaxy
• decomposition activation energy is about 380 kJ/mol.
• AIN aluminum nitride
• MOCVD MOCVD
• 900-1 150 °C using MBE the decomposition activation energy is about 414 kJ/mol.
• InN indium nitride
• the growth temperature range is about 500-600 °C using HVPE and
• MOCVD techniques and about 400 °C using MBE and the decomposition activation energy is about 336 kJ/mol. [O Ambacher, J. Phys. D: Appl. Phys. 31 (1998) 2653-2710].
• the growth temperature range and the decomposition activation energy is somewhere inbetween the growth temperature range and the
• decomposition activation energy respectively, the corresponding separate group-Ill nitrides, depending on the content of present metals.
• the growth temperature range and the decomposition activation energy is somewhere inbetween the growth temperature range and the decomposition activation energy, respectively, for gallium nitride and aluminum nitride, depending on the content of gallium and aluminum.
• the group-Ill element of the group-Ill nitride of the main layer is gallium or a mixture of metals including gallium.
• the main group-Ill nitride may be gallium nitride or a nitride comprising a mixture of metals of which one is gallium, such as aluminum gallium nitride, indium gallium nitride or indium aluminum gallium nitride.
• the group-Ill element of the group-Ill nitride of the main layer may be gallium or a mixture consisting of gallium and at least one of aluminum and indium.
• the metals of the group-Ill nitride of the main layer may comprise at least 1 % gallium, such as at least 5% gallium, such as at least 25% gallium.
• the main group-Ill nitride layer is grown during a period sufficiently long to obtain a group-Ill nitride layer having the desired thickness. In one embodiment, the main group-Ill nitride layer is grown during a period of from 30 minutes (creating a group-Ill nitride layer having a thickness of about 200 ⁇ ) up to 12 hours (creating a group-Ill nitride layer having a thickness of about 4 mm).
• the method further comprises the step growing a buffer layer on the substrate on which buffer layer the main layer is grown, which step is performed before growth of the main layer.
• the buffer layer reduces the stresses and dislocations in the main layer.
• the buffer layer is used as a sacrificial layer which is removed and not included in the produced group-Ill nitride wafer. The presence of the buffer layer increases the quality of the main layer and consequently the group-Ill nitride wafer.
• the buffer layer may consist of a single layer.
• the step growing a buffer layer on the substrate, the step growing a main layer of a group-Ill nitride on the substrate and the step of annealing the substrate with the main layer may be performed in one reactor chamber, i.e. in-situ. Since all these three steps are performed in one and the same reactor chamber, the production is simplified, the production time is reduced and the costs are reduced.
• the buffer layer is made of group-Ill nitride.
• the buffer layer further reduces the stresses and dislocations in the main layer due to that the largest differences in lattice constants and thermal expansion coefficients between the substrate and the main layer are related to the passage from the substrate to the buffer layer.
• the differences in lattice constants and thermal expansion coefficients are larger between the substrate and the buffer layer than between the buffer layer and the main layer. Consequently, the stresses and dislocations are concentrated to the surface of the buffer layer facing the substrate.
• the buffer layer may be a buffer layer of group-Ill nitride.
• the group-Ill element of the group-Ill nitride of the buffer layer is gallium, aluminum, indium or a mixture thereof.
• the buffer group-Ill nitride may be a single group-Ill nitride, such as gallium nitride, aluminum nitride or indium nitride.
• the buffer group-Ill nitride may be a nitride comprising a mixture of group-Ill elements, such as a combination of gallium and aluminum or gallium and indium or gallium, aluminum and indium or aluminum and indium. Such a mixture may be obtained from an alloy of different group-Ill elements, such as an alloy of gallium and indium.
• the buffer layer may be made of gallium nitride, aluminum nitride, indium nitride, aluminum gallium nitride, indium gallium nitride or indium aluminum gallium nitride.
• the group-Ill element of the group- Ill nitride of the buffer layer is preferably gallium, indium or a mixture thereof, more preferably gallium.
• the main group-Ill nitride layer and the buffer group-Ill nitride layer may be made of the same or different group-Ill nitride.
• the stresses and dislocations in the main layer are further reduced due to the reasons explained above.
• the buffer layer is grown during a period of 1.5-20 minutes. This implies formation of a buffer layer reducing the stresses and dislocations in the main group-Ill nitride layer.
• the buffer layer may be grown during a period of at least 5 minutes.
• the buffer layer may be grown during a period of maximum 15 minutes.
• the buffer layer is typically grown during a period of about 10 minutes.
• the buffer layer is grown to a thickness between 20 nm and 1 ⁇ .
• the buffer may be grown to a thickness of at least 50 nm, such as at least 100 nm.
• the buffer may be grown to a thickness of maximum 500 nm, such as maximum 300 nm, such as maximum 200 nm.
• the substrate is lattice-mismatched to the group-Ill nitride.
• the substrate is made of sapphire, silicon carbide, silicon, lithium gallate, zinc oxide, diamond, magnesium oxide, gallium arsenide and/or aluminum nitride. These substrates enable growth of group-Ill nitride thereon.
• the substrate is made of sapphire and/or silicon carbide. Sapphire and silicon carbide are thoroughly tested with success as substrates for growth of group-Ill nitride. Sapphire is a crystalline form of aluminum oxide. The substrate may be sapphire. Sapphire has been shown to be suitable as substrate for growth of group-Ill nitride.
• the heteroepitaxial growth is achieved by vapor phase epitaxy (VPE), molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD).
• VPE vapor phase epitaxy
• MBE molecular beam epitaxy
• MOCVD metal-organic chemical vapor deposition
• MOCVD metal-organic chemical vapor deposition
• the heteroepitaxial growth is achieved by halide vapor phase epitaxy (HVPE).
• HVPE halide vapor phase epitaxy
• An advantage of HVPE is a high growth rate (200-300 ⁇ per hour or higher).
• the step growing a buffer layer is achieved by the same technique as the heteroepitaxial growth of the main layer.
• both the step growing a buffer layer and the step growing a main layer are achieved by vapor phase epitaxy (VPE), such as halide vapor phase epitaxy (HVPE).
• VPE vapor phase epitaxy
• HVPE halide vapor phase epitaxy
• a gallium nitride wafer (having a thickness of 200 ⁇ or thicker) may be produced by means of HVPE by the following procedure:
• a low temperature buffer layer of GaN with a thickness of 100-200 nm is deposited on a foreign substrate of sapphire at atmospheric pressure at a growth temperature of 550-650°C using N 2 as a carrier gas.
• the temperature is increased to 1000-1050°C in N 2 and ammonia (NH 3 ) atmosphere and is kept unchanged until the GaN layer is recrystallized (10 min).
• the ambient temperature is set to a growth temperature of 950-1050°C in a mixture of N 2 and ammonia. When the temperature has been stabilized a mixture of N 2 and GaCI is added into the reaction chamber.
• a thick GaN layer (200 ⁇ or thicker) is grown at atmospheric pressure at 950-1050°C with GaCI and NH 3 as precursor and a mixture of N 2 and H 2 as carrier gas.
• the precursor gases (GaCI and NH 3 ) are switched off and the temperature of the sample is increased to 1050-1200°C (annealing temperature). The temperature is kept constant during 5-10 minutes of annealing.
• the thick GaN layer is self-separated from the sapphire substrate.
• the temperature can be immediately decreased to room temperature without cracking of the thick GaN layer.
• the experiments have been done in a vertical growth reactor at atmospheric pressure where the carrier gases and precursors are delivered from the bottom.
• the reactor consists of two zones, one lower where the Ga-boat is situated and the growth zone where the substrate holder is situated.
• the gallium boat is heated resistively while the growth zone is heated by RF induction.
• the temperature of the substrate holder and the Ga-boat is measured with a thermocouple and controlled by a temperature regulation system.
• the growing surface can be kept at the same distance from the gas inlet by substrate pulling and the substrate can be rotated at various speeds in order to improve the thickness uniformity.
• the precursors GaCI and the NH 3 were separated by a sheet flow of N 2 .
• the GaCI was synthesized within the reactor in the Ga-boat by reacting HCI with Ga at 750- 850°C. The reaction is described by:
• the GaCI reacts with NH 3 to form GaN according to the reaction:
• GaCI, NH 3 and N 2 or a mixture of N 2 and H 2 was used as gallium source, nitrogen source and carrier gas, respectively, in the experiments.
• the NH 3 flow was kept in the range 2-8 l/min, while the HCI flow rate was typically 5-80 ml/min.
• the low temperature GaN buffer layers were grown at 600 °C using a V/lll ratio in the range of 17-70 at the substrate surface.
• the growth time of the low temperature GaN buffer was typically between 90 sec and 10 min which correspond to a thickness variation about 70 nm to 350 nm, respectively. Before the growth of the main GaN layer commenced, the buffer layer was in most cases recrystallizated by annealing at 1050° C for 10 min.
• the sample was annealed at 1080° C for 10 min after the growth of a 1 .5 mm thick main GaN layer.
• the GaN layer and the sapphire were cooled down to room temperature.
• the GaN layer and the sapphire substrate were released (separated from each other).
• the backside of the GaN layer was deformed and the sapphire substrate was cracked, as illustrated in figure 1 (a) and 1 (b), respectively. It shows that the substrate and the GaN layer were under compressive stress during cooling since they were not completely separated during the annealing.
• Figure 1 (a) illustrates the backside of the 1 .5 mm thick GaN layer annealed at 1080°C during 10 minutes
• figure 1 (b) illustrates the sapphire substrate after removal of the GaN layer.
• the annealing was done at 1 130° C during 10 minutes after the growth of a 1.5 mm thick GaN layer. After cooling down to room temperature the GaN layer and the sapphire substrate were released (separated from each other). There were no deformation at the backside of GaN layer and the sapphire substrate was not cracked. See figures 2(a) and 2(b). Thus, the GaN layer was cooled down under no stress or possibly low compressive stress conditions.
• Figure 2(a) illustrates the backside of the 1 .5 mm thick GaN layer annealed at 1 130°C during 10 minutes and figure 2(b) illustrates the sapphire substrate after removal of the GaN layer.

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