Abstract:
Disclosed is a method for processing GaN based substrate material for manufacturing light-emitting diodes, lasers, and other types of devices. In various embodiments, a GaN substrate is exposed to nitrogen and hydrogen at a high temperature. This process causes the surface of the GaN substrate to anneal and become smooth. Then other processes, such as growing epitaxial layers over the surface of GaN substrate, can be performed over the smooth surface of the GaN substrate.

Description:
This application claims the benefit of U.S. Provisional Application No. 61/526,355, filed on Aug. 23, 2011, which is incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention is directed to optical devices. With the advent of semiconductor devices and applications thereof, GaN materials are being used for fabricating light emitting diodes (LEDs), lasers, and other types of devices. Typically, LEDs are formed over sapphire, silicon carbide, and on gallium nitride (GaN) substrates. 
     To form devices (e.g., LED or laser devices) over a GaN substrate, it is desirable to have a smooth surface. In the past, various processing techniques have been developed to achieve this goal. Unfortunately, conventional techniques have been inadequate. 
     SUMMARY OF THE INVENTION 
     Embodiments provided by the present disclosure are directed to optical devices. More specifically, embodiments of the invention provide a method for processing GaN based substrate material that can be used for manufacturing light-emitting diode, laser, and other types of devices. In various embodiments, a GaN substrate is exposed to gaseous species, which includes nitrogen and hydrogen material, at a high temperature for a predetermined period of time, and this process causes the surface of the GaN substrate to become smooth. Other processes, such as growing epitaxial layers over the surface of GaN substrate, are performed over the smooth surface of the GaN substrate. 
     In one embodiment, the invention provides a method for manufacturing semiconductor devices. The method includes providing a substrate. The substrate comprises gallium, and/or other Group III material, and nitrogen containing material. The substrate includes a top surface, which has scratches characterized by a depth of at least 8 nm. The method also includes providing a processing apparatus, which can be a chemical vapor deposition (CVD) apparatus. The CVD apparatus includes a chamber that has an initial temperature of between 10° C. and 60° C. The method additionally includes placing the substrate within the chamber of the CVD apparatus. The method also includes providing H 2  and NH 3  gaseous species within the chamber. Depending on the application, other types of gaseous species may be present within the chamber, such as N 2 , Ar, He, and others. Moreover, the method includes increasing the chamber temperature to a second temperature over a first time period. The second temperature is at least 900° C., but can also be greater than 1,000° C. The substrate may optionally be exposed to one or more of Group-III material during the second time period. 
     The method also includes subjecting the substrate to the second temperature for a second time period of about 5 minutes to 30 minutes. In addition, the method includes causing the top surface of the substrate to anneal at the second temperature. The plurality of scratches/trenches on the top surface of the substrate is characterized by a second scratch/trench depth of less than 2 nm as a result of the annealing. In various embodiments, at least a portion of the annealed substrate surface may be characterized by an RMS roughness of less than 0.05 nm over an area of 25 μm 2 . The terms scratches and trenches are used interchangeably herein to refer to substrate surface defects. 
     The method may include additional steps as well. For example, the method may include injecting NH 3  into the chamber at a second temperature at a flow rate of 8 slpm. The method can also include forming epitaxial layers over the top surface. The method may additionally include doping the substrate with indium material over the annealed top surface. The top surface of the substrate may be polished before loading into the CVD chamber. Alternatively, the substrate could then be transferred to a second chamber (part of the same or different apparatus) for further thermal treatment and/or epitaxial growth/deposition. 
     During the first time period, as the temperature within the chamber increases, the top surface may have a temperature of about 100° C. less than the second temperature. The first time period can be about 12 minutes to 18 minutes. The second time period can be about 10 minutes. 
     The substrate, with a nitridation process performed over its surface, can be used to manufacture various types of devices, such as LED chips, power electronic devices, and others. To form LED chips using the substrate, the method may also include forming a rectifying device structure over the annealed surface. For example, the method can include forming a transistor device structure over the annealed surface. Prior to, or after the nitridation process, the substrate may optionally be patterned using conventional wet/dry etching techniques, or using metal/dielectric masks such as SiO 2 , SiN, or others. 
     According to another embodiment, the invention provides a method for manufacturing semiconductor devices. The method includes providing a substrate. The substrate comprises gallium and nitrogen containing material. The substrate has a top surface, which comprises a plurality of scratches characterized by a first scratch depth of at least 8 nm. The method also includes providing a chemical vapor deposition (CVD) apparatus. The CVD apparatus has a chamber, which has an initial temperature of between 10° C. and 60° C. The method also includes placing the substrate within the chamber of the CVD apparatus. Additionally, the method includes increasing the chamber temperature to a second temperature for a first period of time. The second temperature is at least 900° C. The method additionally includes subjecting the substrate to the second temperature for a second period of time about 5 minutes to 30 minutes and filling NH 3  gaseous species into the chamber at a flow rate of at least 5 slpm. The method also includes causing the top surface of the substrate to anneal at the second temperature. The plurality of trenches on the top surface of the substrate is characterized by a second trench depth of less than 2 nm as a result of the annealing. The method also includes forming one or more epitaxial layers over the annealed surface. 
     In certain embodiments, method for manufacturing semiconductor devices are disclosed, the methods comprising: providing a substrate, the substrate comprising gallium and nitrogen containing material, the substrate having a top surface, the top surface comprising a plurality of scratches characterized by a first depth of at least 8 nm; providing a processing apparatus, the processing apparatus having a chamber, the chamber having an initial temperature of between 10 to 60° C., the apparatus being configured to cause the chamber to reach a temperature of at least 900° C. and change gas ambient within the chamber; placing the substrate within the chamber of the processing apparatus; providing H 2  and NH 3  gaseous species within the chamber; increasing the chamber temperature to a second temperature over a first time period, the second temperature being at least 900° C.; subjecting the substrate to the second temperature for a second time period of about 5 minutes to 30 minutes; and causing the top surface of the substrate to anneal at the second temperature, the plurality of scratches on the top surface of the substrate being characterized by a second depth of less than 2 nm as a result of the annealing, to provide an annealed substrate surface. 
     In certain embodiments, methods for manufacturing semiconductor devices are provided, the method comprising: providing a substrate, the substrate comprising gallium and nitrogen containing material, the substrate having a top surface, the top surface comprising a plurality of scratches characterized by a first depth of at least 8 nm; providing a processing apparatus, the processing apparatus having a chamber, the chamber having an initial temperature of between 10 to 60° C.; placing the substrate within the chamber of the processing apparatus; increasing the chamber temperature to a second temperature over a first time period, the second temperature being at least 1,300° C.; subjecting the substrate to the second temperature for a second time period of about 5 minutes to about 30 minutes and filling NH 3  gaseous species into the chamber at a flow rate of at least 5 slpm; causing the top surface of the substrate to anneal at the second temperature, the plurality of scratches on the top surface of the substrate being characterized by a second depth of less than 2 nm as a result of the annealing to provide an annealed substrate surface; and forming one or more epitaxial layers over the anneal surface. 
     It is to be appreciated that embodiments of the invention provide numerous advantages compared to conventional techniques. Among other things, embodiments of the invention provide a process that smooth substrate surfaces, and/or reduces subsurface damage and extended defect density, thereby making the processed substrate better suited for forming various types of devices, such as LED chips, power electronics, etc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present disclosure and, together with the description, serve to explain the principles of the disclosed embodiments. Those skilled in the art will understand that the drawings, described herein, are for illustration purposes only. The drawings are not intended to limit the scope of the present disclosure. 
         FIG. 1  is a diagram showing the optical microscope image of the surface morphology of as-received bulk GaN substrate. 
         FIG. 2  shows an atomic force microscope (AFM) image of the as-received surface. The deep scratches are clearly visible on the surface. 
         FIG. 3  is a diagram of a section analysis illustrating scratch depth, which can range from few nanometers to up to 14.3 nm as shown, along the section denoted in  FIG. 2 . 
         FIG. 4  is a diagram showing the process flow of the nitridation step prior to the growth of the epitaxial layers. 
         FIG. 5  is an AFM micrograph of the substrate surface after the nitridation process. 
         FIGS. 6A-6E  illustrate the surface texture observed after nitridation at various temperatures. The surfaces shown in  FIG. 6A  and  FIG. 6D  were annealed at 1,000° C.; the surfaces shown in  FIG. 6B  and  FIG. 6E  were annealed at 1,050° C.; and the surfaces shown in  FIG. 6C  and  FIG. 6F  were annealed at 1,100° C. The surface area shown in each of  FIGS. 6A, 6B, and 6C  is 10×10 μm 2 . The surface area shown in each of  FIGS. 6D, 6E, and 6F  is 3×3 μm 2 . 
         FIGS. 7A-7E  illustrate the surface texture observed after nitridation for various times. The figures show surfaces anneal at 1,100° C. for 0 minutes ( FIG. 7A  and  FIG. 7D ), for 10 minutes ( FIG. 7B  and  FIG. 7E ), and for 20 minutes ( FIG. 7C  and  FIG. 7F ). The surface area shown in each of  FIGS. 7A, 7B, and 7C  is 10×10 μm 2 . The surface area shown in each of  FIGS. 7D, 7E, and 7F  is 3×3 μm 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As mentioned above, GaN substrates with a smooth surface are desirable. Often, achieving a smooth surface prior to the growth of an active region is a requirement for achieving high performance devices, such as LEDs and laser diodes. A smooth growth surface can provide many benefits, such as: 
     1. Sharp interfaces between layers; 
     2. Minimize roughness-induced scattering, thereby resulting in higher carrier mobility; 
     3. Reduced point defect densities; 
     4. Uniform alloy, dopant composition indium incorporation across wafer; and/or 
     5. Prevents 3-D growth mode. 
     Achieving a smooth surface morphology is often difficult, especially for bulk GaN substrates. An in-situ surface treatment is often required to generate the step-structure, required for 2-D step-flow growth mode. The surface treatment is usually carried out in-situ and consists of ramping the temperature in a growth chamber in ammonia and hydrogen ambient, followed by nitridation of the GaN surface at an elevated temperature for a fixed amount of time. The optimum nitridation time, temperature, NH 3  flow rate, H 2  flow rate, etc. are determined empirically. 
       FIG. 1  is an optical microscope image of the surface morphology of as-received bulk GaN substrate. As shown, the optical image shows a very high density of scratches, which are most likely made during the polishing process. These scratches render the GaN substrate surface uneven, which is undesirable for the reasons explained above.  FIG. 2  shows an atomic force microscope (AFM) image of the as-received GaN surface. The deep scratches are clearly visible on the surface. The area of the AFM image is 20×20 nm 2  and the depth of the scratches as characterized by Z-range is 25 nm.  FIG. 3  shows a section analysis illustrating scratch depth, which can range from a few nanometers up to 14.3 nm as shown. 
       FIG. 4  is a simplified diagram showing the process flow of the nitridation step prior to the growth of the epitaxial layers according to an embodiment of the invention. First, a substrate is provided. For example, the substrate consists substantially of gallium and nitrogen containing materials. As explained above, the substrate may have an uneven surface with many scratches. Depending on the application, the received substrate may be polished. Active regions are to be defined and formed over the substrate. The substrate, as received, is loaded in an MOCVD chamber. For example, the initial temperature at the MOCVD temperature is at a relatively temperature of about 20° C. to about 50° C. Next, with NH 3  and H 2  gas species filled within the MOCVD chamber, the temperature is increased to at least a predetermined temperature of about 1,050° C. s over a time period of about 15 minutes. 
     It is to be appreciated that depending on the application, the temperature and the temperature ramp time may be changed. For example, the MOCVD chamber can have a thermal couple temperature of greater than 1,000° C. In an embodiment, during the temperature ramp stage r, the NH 3  flow is about 8 slpm and total H 2  flow is about 31 slpm. Among other things, the NH 3  gas preserves the surface of the substrate. Once the temperature at the MOCVD reaches the predetermined temperature, the substrate stays in the MOCVD chamber for a period of time (e.g., about 10 minutes) with the NH 3  flow at about 8 slpm and total H 2  flow at about 31 slpm, which substantially anneals the surface and enhances the smoothness and uniformity of the substrate surface. The amount of time for annealing the substrate surface varies, which can be from 5 minutes to 20 minutes. For example, the annealing process with the NH 3  gaseous species can be referred as a nitridation process. 
     After the substrate bas been subject to both high temperature and NH 3  and H 2  within the MOCVD chamber, the surface of the substrate is smoothed and thus the substrate can be used for forming various types of devices. In an embodiment, an epitaxial film is formed over the smooth surface of the substrate. For example, the epitaxial film growth can be performed after the annealing process or after another ramp up in chamber temperature with NH 3  material, as epitaxial film can be grown in high or low temperature. 
       FIG. 5  is an AFM micrograph illustrating substrate surface after nitridation process is performed according to embodiments of the invention. As shown in  FIG. 5 , a smooth surface morphology can be observed after nitridation. 
       FIGS. 6A-6F  illustrates surface texture observed after nitridation is performed at different temperatures according to embodiments of the invention. As shown in  FIG. 6 , smooth surface can be achieved by performing nitridation at 1,050° C. At different temperature levels, a nitridation process can have a different effect on surface smoothness. For example, nitridation at 1,000° C. or 1,100° C. can result in a different level of surface smoothness. For example, the nitridation temperature may vary from 800° C. to 1,200° C. As shown in  FIG. 6A , steps on the substrate surface appear to curve around the scratches. As shown in  FIG. 6C , after nitridation, the annealed substrate surface is characterized by defined terraces and ridges. 
       FIG. 7A-7F  illustrates surface texture observed after nitridation is performed for different durations according to embodiments of the invention. As shown, many scratches are present on the surface of the substrate if a (i.e., 0 minute) nitridation process is performed. At 10 minutes or 20 minutes of nitridation, the surface of the substrate can be much smoother. For example, during the nitridation process can vary from 3 minutes to over 20 minutes. For example, various nitridation conditions may lead to dissimilar step structure on substrates with different characteristics, such as miscut, variation, surface finish, polish, thickness, doping, and others. In certain embodiments, optimal nitridation conditions are to be varied according to the substrate surface. As shown in  FIG. 7D , steps appear to curve around scratches on the unannealed substrate surface. 
     The nitridation process illustrated in  FIG. 4  and described above provides several advantages. Among other things, the mechanism that results in smooth surface morphology can be attributed to surface-desorption and mass-transport effects. 
     According to an embodiment, the invention provides a method for manufacturing semiconductor devices. The method includes providing a substrate. The substrate comprises gallium and nitrogen containing material. The substrate includes a top surface, which comprises a plurality of scratches characterized by a first scratch depth of at least 8 nm. The method also includes providing a chemical vapor deposition (CVD) apparatus. The CVD apparatus includes a chamber that has an initial temperature of between 10° C. and 60° C. The method additionally includes placing the substrate within the chamber of the CVD apparatus. The method also includes providing H 2  and NH 3  gaseous species within the chamber. Moreover, the method includes increasing the chamber temperature to a second temperature over a first time period. The second temperature is at least 900° C., but can also be greater than 1,000° C. The substrate is exposed to one or more of Group-III material during the second time period. 
     The method also includes subjecting the substrate to the second temperature for a second time period of about 5 minutes to 30 minutes. In addition, the method includes causing the top surface of the substrate to anneal at the second temperature. The plurality of trenches on the top surface of the substrate is characterized by a second trench depth of less than 2 nm as a result of the annealing. In various embodiments, at least a portion of the annealed surface may be characterized by an RMS roughness of less than 0.05 nm over an area of 25 μm 2 . 
     The method above may include additional steps as well. For example, the method includes injecting NH 3  into the chamber at the second temperature at a flow rate of 8 slpm. The method can also include forming one or more epitaxial layers over the top surface of the smooth substrate. The method may additionally include doping the smooth substrate with indium material over the annealed top surface. The top surface of the substrate may be polished before loading into the CVD chamber. 
     During the first time period as the temperature within the chamber increases, the top surface of the substrate may have a temperature of about 100° C. less than the second temperature. The first time period can be about 12 minutes to about 18 minutes. The second time period can be about 10 minutes. 
     The substrate, with the nitridation process performed over its surface, can be used to manufacture various types of devices, such as LED chips, power electronic devices, and others. To form LED chips using the smooth substrate, the method may also include forming a rectifying device structure over the annealed surface. For example, the method can include forming a transistor device structure over the annealed surface. 
     According to another embodiment, the invention provides a method for manufacturing semiconductor devices. The method includes providing a substrate. The substrate comprises gallium and nitrogen containing material. The substrate has a top surface, which comprises a plurality of scratches characterized by a first scratch depth of at least 8 nm. The method also includes providing a chemical vapor deposition (CVD) apparatus. The CVD apparatus has a chamber, which has an initial temperature of between 10° C. and 60° C. The method also includes placing the substrate within the chamber of the CVD apparatus. Additionally, the method includes increasing the chamber temperature to a second temperature over a first time period. The second temperature is at least 900° C. The method additionally includes subjecting the substrate to the second temperature for a second time period of about 5 minutes to 30 minutes and filling NH 3  gaseous species into the chamber at a flow rate of at least 5 slpm. The method also includes causing the top surface of the substrate to anneal at the second temperature. The plurality of trenches on the top surface of the substrate is characterized by a second trench depth of less than 2 nm as a result of the annealing. The method also includes forming one or more epitaxial layers over the annealed surface. 
     While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. Therefore, the above description and illustrations should not be taken as limiting the scope of the invention which is defined by the appended claims.