Patent Publication Number: US-2007114563-A1

Title: Semiconductor device and method of fabricating the same

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
      Priority is claimed to Korean Patent Application Nos. 10-2005-0110882 and 10-2006-0102046, filed on Nov. 18, 2005 and Oct. 19, 2006, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in its entirety by reference.  
     BACKGROUND OF THE DISCLOSURE  
      1. Field of the Disclosure  
      The present disclosure relates to a GaN semiconductor device, and more particularly, to a semiconductor device having improved surface morphology characteristics, and a method of fabricating the same.  
      2. Description of the Related Art  
      Conventional GaN-based devices, for example, nitride semiconductor laser diodes, are implemented on a c-plane GaN substrate. However, the c-plane of GaN crystal is well-known as a polar plane. Thus, in nitride semiconductor laser diodes, the probability of combining electrons with holes can be reduced by the effect of an internal electric field formed by polarization of the c-plane, which lowers the luminous efficiency of the nitride semiconductor laser diodes.  
      To solve this problem, technology implementing a semiconductor device on an a-plane GaN substrate having no polarization has been developed.  
       FIG. 1  is a cross-sectional view of a conventional a-plane GaN substrate, and  FIGS. 2 and 3  are a cross-section scanning electron microscopy (SEM) photo and a surface SEM photo, respectively, showing the a-plane GaN substrate illustrated in  FIG. 1 .  
      The a-plane GaN substrate can be obtained by epitaxially growing an a-plane GaN layer  6  on an r-plane sapphire substrate  2 . However, lattice mismatch between the r-plane sapphire substrate  2  and the a-plane GaN layer  6  is about 16.2%, which is very large, and thus a V-shape defect caused by stress is typically generated on the surface of the a-plane GaN layer  6  stacked on the r-plane sapphire substrate  2 . Accordingly, when a device is implemented on the surface of the a-plane GaN layer  6  on which the V-shape defect is generated, device characteristics are lowered.  
     SUMMARY OF THE DISCLOSURE  
      The present disclosure provides a semiconductor device having improved surface morphology characteristics, and a method of fabricating the same.  
      According to an aspect of the present disclosure, there is provided a semiconductor device comprising: an r-plane sapphire substrate; an Al x Ga (1-x) N(0≦×&lt;1) buffer layer epitaxially grown on the r-plane sapphire substrate to a thickness in the range of 100-20000 Å in a gas atmosphere containing nitrogen (N 2 ) and at a temperature of 900-1100° C.; and a first a-plane GaN layer formed on the buffer layer.  
      The gas atmosphere containing N 2  may be a mixed gas atmosphere of N 2  and hydrogen (H 2 ), and the ratio of N 2  of the mixed gas may be 1-99.99%.  
      A second a-plane GaN layer may be further grown on the first a-plane GaN layer. Here, the first a-plane GaN layer may be formed of an n-type semiconductor including an n-type dopant, and the second a-plane GaN layer may be formed of a p-type semiconductor including a p-type dopant.  
      According to another aspect of the present disclosure, there is provided a method of fabricating a semiconductor device comprising: epitaxially growing an Al x Ga (1-x) N(0≦×&lt;1) buffer layer on an r-plane sapphire substrate to a thickness in the range of 100-20000 Å in a gas atmosphere containing nitrogen (N 2 ) and at a temperature of 900-1100° C. to form a buffer layer; and forming a first a-plane GaN layer on the buffer layer.  
      The method may further include forming a second a-plane GaN layer on the first a-plane GaN layer. Here, the first a-plane GaN layer may be formed of an n-type semiconductor including an n-type dopant, and the second a-plane GaN layer may be formed of a p-type semiconductor including a p-type dopant. And, the first a-plane GaN layer and the second a-plane GaN layer may be formed at a temperature of 900-1200° C., and the buffer layer may be formed under a pressure of 1-200 torr.  
      According to exemplary embodiments of the present disclosure, a semiconductor device having an improved surface morphology characteristic can be obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present disclosure will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIG. 1  is a cross-sectional view of a conventional a-plane GaN semiconductor substrate;  
       FIG. 2  is a cross-section scanning electron microscopy (SEM) photo of the a-plane GaN semiconductor substrate illustrated in  FIG. 1 ;  
       FIG. 3  is a surface SEM photo of the a-plane GaN semiconductor substrate illustrated in  FIG. 1 ;  
       FIG. 4  is a cross-sectional view of a semiconductor device according to an embodiment of the present disclosure;  
       FIG. 5  is a cross-section SEM photo of the semiconductor device illustrated in  FIG. 4 ;  
       FIG. 6  is a surface SEM photo of the semiconductor device illustrated in  FIG. 4 ;  
       FIG. 7  is a cross-sectional view of a semiconductor device according to an exemplary implementation of the semiconductor device of  FIG. 4 ;  
       FIGS. 8 through 8 C are cross-sectional views illustrating a method of fabricating a semiconductor device according to an embodiment of the present disclosure;  
       FIG. 9  is a graph of thickness of a buffer layer versus crystallinity of a first a-plane GaN layer in the method of fabricating a semiconductor device illustrated in  FIGS. 8A through 8C ; and  
       FIGS. 10A through 10D  are cross-sectional views illustrating a method of fabricating a semiconductor device of  FIG. 7 . 
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE  
      The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. In the drawings, the thicknesses of layers and regions are exaggerated for clarity.  
       FIG. 4  is a cross-sectional view of a semiconductor device according to an embodiment of the present disclosure, and  FIGS. 5 and 6  are respectively a cross-section SEM photo and a surface SEM photo of the semiconductor device illustrated in  FIG. 4 .  
      Referring to  FIGS. 4 through 6 , the semiconductor device according to an embodiment of the present disclosure includes an Al x Ga (1-x) N(0≦×&lt;1) buffer layer  14  and a first a-plane GaN layer  16 , which are sequentially stacked on an r-plane sapphire substrate  12 . The semiconductor device illustrated in  FIG. 4  may be used as a semiconductor substrate for fabricating a GaN-based device.  
      The buffer layer  14  may be epitaxially grown to a thickness in the range of 100-20000 Å in a gas atmosphere containing nitrogen (N 2 ) and at a temperature of 900-1100° C. Obviously, as illustrated, the layers are not drawn to scale. Here, the gas atmosphere containing N 2  is an N 2  gas atmosphere or a mixed gas atmosphere of N 2  and hydrogen (H 2 ). When the buffer layer  14  is formed in the mixed gas atmosphere, the ratio of N 2  in the mixed gas may be 1-99.99%. In this case, the buffer layer  14  may be formed under a pressure of 1-200 torr, preferably, under a pressure of 100 torr.  
      The buffer layer  14  serves to offset lattice mismatch between the r-plane sapphire substrate  12  and the first a-plane GaN layer  16 . Thus, surface morphology characteristics of the a-plane GaN layer  16  that is epitaxially grown on the buffer layer  14  can be improved. Specifically, the first a-plane GaN layer  16  stacked on the buffer layer  14  does not include a V-shape defect and may have a mirror-like surface morphology. In particular, since an a-plane of GaN crystal is well-known as a non-polar plane, when a GaN-based device, for example, a nitride semiconductor laser diode, is implemented on the semiconductor device (see  FIG. 7 ), the luminous efficiency and optical power of the nitride semiconductor laser diode can be improved.  
       FIG. 7  is a cross-sectional view of a semiconductor device according to an embodiment of the present disclosure. As illustrated in  FIG. 7 , the semiconductor device may be implemented as a nitride semiconductor laser diode.  
      Referring to  FIG. 7 , the semiconductor device, that is, the nitride semiconductor laser diode, includes an Al x Ga (1-x) N(0≦×&lt;1) buffer layer  14 , a first a-plane GaN layer  20 , an active layer  22 , and a second a-plane GaN layer  24 , which are sequentially stacked on an r-plane sapphire substrate  12 . And, an n-electrode  30  and a p-electrode  40  are formed using a conductive material, such as Ag or Au, on a step portion of the first a-plane GaN layer  20  and the second a-plane GaN layer  24 . The illustrations are not drawn to scale.  
      The buffer layer  14  may be epitaxially grown to a thickness in the range of 100-20000 Å in a gas atmosphere containing nitrogen (N 2 ) and at a temperature of 900-1100° C. Here, the gas atmosphere containing N 2  is an N 2  gas atmosphere or a mixed gas atmosphere of N 2  and hydrogen (H 2 ). When the buffer layer  14  is formed in the mixed gas atmosphere, the ratio of N 2  in the mixed gas may be 1-99.99%. In this case, the buffer layer  14  may be formed under a pressure of 1-200 torr, preferably, under a pressure of 100 torr. The buffer layer  14  formed in such a process may serve to offset lattice mismatch between the r-plane sapphire substrate  12  and the first a-plane GaN layer  20 . Thus, a surface morphology characteristic of the first a-plane GaN layer  20  that is epitaxially grown on the buffer layer  14  can be improved. Specifically, the first a-plane GaN layer  20  stacked on the buffer layer  14  does not include a V-shape defect and may have a mirror-like surface morphology.  
      In the semiconductor device illustrated in  FIG. 7 , the first a-plane GaN layer  20  may be formed of an n-type semiconductor including an n-type dopant, and the second a-plane GaN layer  24  may be formed of a p-type semiconductor including a p-type dopant. Specifically, the first a-plane GaN layer  20  is an n-GaN-based III-V-group nitride-based compound semiconductor layer and in particular, may be an n-GaN layer. However, the first a-plane GaN layer  20  is not limited to this and may be another compound semiconductor layer of III-V-group in which laser oscillation (lasing) can be performed. In addition, the second a-plane GaN layer  24  is a p-GaN-based III-V-group nitride-based compound semiconductor layer and in particular, may be a p-GaN layer. However, the second a-plane GaN layer  24  is not limited to this and may be another compound semiconductor layer of III-V-group in which laser oscillation (lasing) can be performed.  
      A material layer in which lasing can be performed can be used as the active layer  22 . A material layer in which laser light having small threshold current and stable latitudinal mode characteristic can be oscillated may be used as the active layer  22 . A GaN-based III-V group nitride-based compound semiconductor layer, In x Al y Ga 1-x-y N (0≦×&lt;1, 0≦y≦1 and x+y&lt;1), in which Al is contained at a predetermined ratio may be used as the active layer  22 . The active layer  22  may have one of a multiple quantum well structure and a single quantum well structure, and the structure of the active layer  22  does not restrict the technical scope of the present disclosure.  
      As described above, the semiconductor device illustrated in  FIG. 7  has an excellent surface morphology characteristic and has the upper plane formed of a non-polar plane. Thus, the surface characteristics of thin films stacked on the first a-plane GaN layer  20 , that is, the active layer  22  and the second a-plane GaN layer  24  can be improved. As such, the internal quantum efficiency and optical extraction efficiency of the nitride semiconductor laser diode can be improved, thereby improving the luminous efficiency and optical power.  
       FIGS. 8A through 8C  illustrate a method of fabricating a semiconductor device according to an embodiment of the present disclosure. Each layer may be formed using chemical vapor deposition (CVD). CVD includes atomic layer deposition (ALD), metal organic CVD (MOCVD), and other well-known vapor deposition.  
      Referring to  FIGS. 8A through 8C , an r-plane sapphire substrate  12  is prepared and then, Al x Ga (1-x) N (0≦×&lt;1) is epitaxially grown on the r-plane sapphire substrate  12  to a thickness in the range of 100-20000 Å in a gas atmosphere containing nitrogen (N 2 ) and at a temperature of  900 - 1   100 ° C., thereby forming a buffer layer  14 . Here, the gas atmosphere containing N 2  is an N 2  gas atmosphere or a mixed gas atmosphere of N 2  and hydrogen (H 2 ). When the buffer layer  14  is formed in the mixed gas atmosphere, the ratio of N 2  in the mixed gas may be 1-99.99%. In this case, the buffer layer  14  may be formed under a pressure of 1-200 torr, preferably, under a pressure of 100 torr.  
      After the buffer layer  14  is grown, a first a-plane GaN layer  16  is formed on the buffer layer  14 . The first a-plane GaN layer  16  may be formed at a temperature of 900-1200° C. The first a-plane GaN layer  16  may be formed of an n-type semiconductor including an n-type dopant.  
      The buffer layer  14  may be interposed between the r-plane sapphire substrate  12  and the first a-plane GaN layer  16  and may serve to offset lattice mismatch between the r-plane sapphire substrate  12  and the first a-plane GaN layer  16 . As such, a surface morphology characteristic of the first a-plane GaN layer  16  that is stacked on the the buffer layer  14  is improved, does not include a V-shape defect and may have a mirror-like surface morphology. In particular, since an a-plane of GaN crystal is well-known as a non-polar plane, when a GaN-based device, for example, a nitride semiconductor laser diode, is implemented on the semiconductor (see  FIG. 10 ), the luminous efficiency and optical power of the nitride semiconductor laser diode can be improved. In particular, in the fabrication process according to the present disclosure, when the buffer layer  14  is epitaxially grown, the thickness of the buffer layer  14  is controlled such that crystallinity of the first a-plane GaN layer  16  stacked thereon is controlled. This will be described with reference to  FIG. 9 .  
       FIG. 9  is a graph of thickness of a buffer layer versus crystallinity of a first a-plane GaN layer in the method of fabricating a semiconductor device illustrated in  FIGS. 8A through 8C . Referring to  FIG. 9 , crystallinity of the first a-plane GaN layer  16  depends on the thickness of the buffer layer  14 . Specifically, as the thickness of the buffer layer  14  increases, the crystallinity of the first a-plane GaN layer  16  is improved.  
       FIGS. 10A through 10D  illustrate a method of fabricating an exemplary semiconductor device as shown in  FIG. 7 . A nitride semiconductor laser diode is implemented as a semiconductor device, as illustrated in  FIGS. 10A through 10D . Here, the method of fabricating a semiconductor device illustrated in  FIGS. 10A through 10D  may include the same processes as those of the method fabricating a semiconductor device illustrated in  FIGS. 8A through 8C . Thus,  FIGS. 8A through 8C  and a description thereof will be referred to for repeated or similar processes.  
      Referring to  FIGS. 10A and 10B , an r-plane sapphire substrate  12  is prepared and then, Al x Ga (1-x) N (0≦×&lt;1) is epitaxially grown on the r-plane sapphire substrate  12  to a thickness in the range of 100-20000 Å in a gas atmosphere containing nitrogen (N 2 ) and at a temperature of 900-1100° C., thereby forming a buffer layer  14 . After that, a first a-plane GaN layer  20 , an active layer  22 , and a second a-plane GaN layer  24  are sequentially formed on the buffer layer  14 .  
      In particular, in the fabrication process according to the present disclosure, when the buffer layer  14  is epitaxially grown, the thickness of the buffer layer  14  is controlled such that crystallinity of the first a-plane GaN layer  20  stacked thereon is controlled, as described previously.  
      The first a-plane GaN layer  20  may be formed of an n-type semiconductor including an n-type dopant, and the second a-plane GaN layer  24  may be formed of a p-type semiconductor including a p-type dopant. Each of the first a-plane GaN layer  20  and the second a-plane GaN layer  24  may be formed at a temperature of 900-1200° C.  
      Specifically, the first a-plane GaN layer  20  may be an n-GaN-based III-V-group nitride-based compound semiconductor layer, and in particular, may be an n-GaN layer. However, the first a-plane GaN layer  20  is not limited to this and may be another compound semiconductor layer of Ill-V group in which laser oscillation (lasing) can be performed. In addition, the second a-plane GaN layer  24  may be a p-GaN-based III-V-group nitride-based compound semiconductor layer, and in particular, may be a p-GaN layer. However, the second a-plane GaN layer  24  is not limited to this and may be another compound semiconductor layer of Ill-V group in which laser oscillation (lasing) can be performed.  
      A material layer in which lasing can be performed can be used as the active layer  22 . A material layer in which laser light of which threshold current value is small and latitudinal mode characteristic is stable can be oscillated may be used as the active layer  22 . A GaN-based Ill-V group nitride-based compound semiconductor layer, In x Al y Ga 1-x-y N (0≦×&lt;1, 0≦y≦1 and x+y&lt;1), in which Al is contained at a predetermined ratio may be used as the active layer  22 . The active layer  22  may have one of a multiple quantum well structure and a single quantum well structure as examples, and the structure of the active layer  22  does not restrict the technical scope of the present disclosure.  
      Referring to  FIGS. 10C and 10D , a predetermined region is selected on the second a-plane GaN layer  24  which is the uppermost layer, and is etched/removed to a predetermined depth of the first a-plane GaN layer  20 , thereby forming a step portion on the first a-plane GaN layer  20 . After that, the n-electrode  30  and the p-electrode  40  are formed using a conductive material, such as Ag or Au, on the step portion of the first a-plane GaN layer  20  and the second a-plane GaN layer  24 .  
      In the semiconductor device illustrated in  FIG. 7 , since the first a-plane GaN layer  20  has excellent surface morphology characteristics and has the uppermost plane formed of a non-polar plane, the surface characteristics of thin films stacked on the first a-plane GaN layer  20 , that is, the active layer  22  and the second a-plane GaN layer  24 , can be improved. As such, the internal quantum efficiency and optical extraction efficiency of the nitride semiconductor laser diode can be improved, thereby improving the luminous efficiency and optical power.  
      According to embodiments of the present disclosure, the nitride-based semiconductor device having an improved surface morphology characteristic can be obtained. The semiconductor device according to embodiments of the present disclosure can avoid V-shape defects and has a mirror-like surface morphology. In particular, since the a-plane of GaN crystal is well-known as a non-polar plane, when the GaN-based device, for example, a nitride semiconductor laser diode, is implemented on the first a-plane GaN layer  20  formed on the buffer layer  14  in the semiconductor device according to the present disclosure, luminous efficiency and optical power of the nitride semiconductor laser diode can be improved.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.