Patent Publication Number: US-6706542-B1

Title: Application of InAIAs double-layer to block dopant out-diffusion in III-V device Fabrication

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This present application claims priority of Provisional Application Ser. No. 60/120,821 filed Jan. 7, 2000. The present application is also a continuation-in-part application of U.S. patent application Ser. No. 09/540,471 filed on Mar. 31, 2000 and assigned to the assignee of the present invention. The disclosures of the above captioned applications are specifically incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a multi-layer dopant diffusion barrier for use in III-V semiconductor structures. 
     BACKGROUND OF THE INVENTION 
     Optical communications systems employ a variety of optoelectronic devices. Some of these optoelectronic devices incorporate a structure in which an intrinsic (i) semiconductor layer is disposed between a p-type layer and an n-type layer. This structure is commonly referred to as a PIN structure and the devices as PIN devices. The PIN structure has properties which are useful in optoelectronic devices. For example, the intrinsic layer may have a larger index of refraction than the p and n layers. In this way, a waveguide can be formed, with the p and n layers being cladding layers and the intrinsic layer being the guiding layer of the waveguide. Furthermore, energy band discontinuities in the conduction and valence bands in the PIN structure facilitate carrier confinement within the intrinsic layer, which is useful in many devices. In short, the PIN structure is well suited for a variety of light emitting and detecting optoelectronic device applications. 
     One material that is used often in PIN devices is indium phosphide (InP). In a PIN structure employing InP, the p-type layer is often fabricated by introducing zinc as a dopant. Although a suitable dopant for forming the p-type layer, zinc can readily diffuse out of the p-type layer due to the higher temperature achieved during the growth of InP. 
     In addition, many optoelectronic devices are based on a structure commonly known as a buried mesa structure. Often a PIN structure is part of the mesa, and the mesa is disposed between current blocking layers. The current blocking layers provide transverse optical and carrier confinement. In many structures, one or more of the current blocking layers is semi-insulating, iron doped indium phosphide (InP(Fe)). Unfortunately, inter-diffusion of p-dopants from the p-type layer and iron from the semi-insulating InP(Fe) layer may occur. This inter-diffusion may have detrimental effects on device characteristics. The diffusion of the p-type dopants from the p-type layer can result in leakage current in the device; and the diffusion of the iron dopants into the p-type layer can make the p-type layer more resistive. Accordingly, it is desirable to have a barrier layer to prevent the inter-diffusion. 
     In some conventional structures, an n-InP layer can be used as a dopant barrier layer. The n-InP layer may be disposed between the mesa and the current blocking layers in a buried mesa structure. This is shown in FIG.  5 . An n-InP dopant barrier layer  501  is disposed between the mesa  502  and InP(Fe) current blocking layers  503 . However, the dopant concentration in the n-InP barrier layer  501  may be relatively high (on the order of about 10 18 -10 19  atoms/cm 3 ) to prevent diffusion of p-type dopants out of the p-type layer  504 . Additionally, to ensure appropriate blocking of the dopants, it may be necessary to make the n-InP layer  501  relatively thick, on the order of tens of nanometers. Accordingly, a parasitic pn junction may be formed between the p-type layer  504  and n-type dopant barrier layer  501 . This can cause an undesired current leakage path. Moreover, the disposition of the n-type dopant barrier layer  501  between the p-type cladding layer  504  and the current blocking layers  503  can result in an undesired parasitic capacitance component. Parasitic capacitance may be particularly problematic in devices such as electro-absorptive modulators, lasers and digital devices, in general, as it adversely impacts device speed. 
     Accordingly, what is needed is a dopant diffusion barrier, which effectively blocks diffusion of dopants, while not introducing parasitic elements, such as pn junctions and capacitance, to the device. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a multi-layer dopant barrier and its method of fabrication for use in semiconductor structures. In an illustrative embodiment, the multi-layer dopant barrier is disposed between a first doped layer and a second doped layer. The multi-layer dopant barrier further includes a first dopant blocking layer adjacent the first doped layer and a second dopant blocking layer adjacent the second doped layer. 
     In another illustrative embodiment, a technique for fabricating the multi-layer dopant barrier is disclosed. A first dopant blocking layer is formed at a first temperature, and a second dopant blocking layer is formed at a second temperature over the first barrier layer. 
     In the illustrative embodiments of the present invention, dopant diffusion is significantly reduced. Moreover, parasitic pn junctions and parasitic capacitance of the above discussed prior art are substantially avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that in accordance with the standard practice in the semiconductor industry the various features are not necessarily drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
     FIG. 1 is a cross-sectional view of an exemplary embodiment including the multi-layer dopant diffusion barrier layer of the present invention. 
     FIG. 2 is a graphical representation showing the blocking of zinc dopants by the first dopant blocking layer of an exemplary embodiment of the present invention. 
     FIG. 3 is a graphical representation showing the blocking of iron dopants by the second dopant blocking layer of an exemplary embodiment of the present invention. 
     FIG. 4 is a cross-sectional view of an exemplary embodiment including the multi-layer dopant diffusion barrier of the present invention. 
     FIG. 5 is a cross-sectional view of a conventional dopant barrier layer used in a buried mesa PIN structure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described more fully with reference to accompany drawing figures in which exemplary embodiments of the present invention are described. Referring initially to FIG. 1, one such embodiment, including a mesa PIN structure  111 , is shown. A substrate  100  has an n-type semiconductor layer  103  disposed thereover. An active layer  102  of intrinsic semiconductor is disposed over the n-type layer  103 , and a p-type semiconductor layer  101  is disposed over the active layer  102 . The mesa PIN structure  111  is disposed between current blocking or confinement layers  107 . A multi-layer dopant diffusion barrier layer  110 , illustratively including a first dopant blocking layer  104  and a second dopant blocking layer  106 , is disposed between the mesa  111  and each of the current blocking layers  107 . 
     In the exemplary embodiment shown in FIG. 1, the first dopant blocking layer  104  is contiguous with vertical side  108  of the mesa  111  and blocks p-type dopants from diffusing out of the p-type layer  101 . Similarly, the second dopant blocking layer  106  is contiguous with, and prevents dopants from diffusing out of, the current blocking layer  107 . However, it is of interest to note that first dopant blocking layer  104  and second dopant blocking layer  106  need not be contiguous to p-type layer  101  and current blocking layer  107 , respectively. In fact, it is possible that the first dopant blocking layer  104  is adjacent to the p-type layer  101 , with one or more layers of suitable material disposed between the first dopant blocking layer  104  and the p-type layer  101 . Similarly, it is possible that the second dopant blocking layer  106  is adjacent to the current blocking layer  107 , with one or more layers of suitable material disposed between second dopant blocking layer  106  and the current blocking layer  107 . Moreover, a third dopant blocking layer  105  may be disposed between current blocking layers  107  and the mesa  111  to further aid in preventing dopants from diffusing out of the current blocking layers  107  and into the mesa. Finally, as is discussed in detail below, the first dopant blocking layer  104  and the second dopant blocking layer  106  of the present invention illustratively are like compounds, but have different properties which are exploited to provide clear advantages over the prior art These differences in properties are believed to be attributable to the temperature at which they are grown, with layer  104  being grown at a lower temperature than layer  106 . 
     In the illustrative embodiment shown in FIG. 1, the substrate  100  is illustratively n-type, for example n-type indium phosphide (n-InP). The n-type layer  103  (also referred to as the lower cladding layer) is illustratively n-doped InP. The active layer  102  is illustratively intrinsic InGaAsP. The p-type layer  101  (also referred to as the upper cladding layer) is illustratively p-doped InP. In the exemplary embodiment, zinc is used as the p-type dopant in layer  101 , although other dopants such as Cd, Mg and Be may be used in this capacity. Illustratively, the current blocking layers  107  are disposed on either side of the mesa  111 , effecting a buried ridge structure. Alternatively, it is possible that current blocking layer  107  substantially surrounds the mesa. This would be useful in a surface emitting device, for example. In either case, an exemplary material for current blocking layers  107  is semi-insulating InP(Fe). 
     First dopant blocking layer  104  and second dopant blocking layer  106  in the exemplary embodiment shown in FIG. 1 are indium aluminum arsenide (InAlAs). 
     The use of InAlAs is illustrative, and other materials may be used for the first and second dopant blocking layers  104  and  106 , respectively, to prevent dopants from diffusing out of layers  101  and  107 . The alternative materials would be chosen to suitably block dopant diffusion from layers  101  and  107 , but would not form a parasitic pn junction with layer  101 , for reasons discussed above. These include, for example, InAlGaAs, and may be formed by the illustrative techniques described below. 
     In keeping with the teaching of the above captioned parent application, a dopant barrier  112  may be used to prevent p-type dopants from diffusing out of the p-type layer  101  and into the active layer  102 . Dopant barrier  112  may be as set forth in the parent application. Alternatively, dopant barrier  112  may be aluminum spikes or aluminum containing spikes as set forth in U.S. patent application Ser. No. 09/540,474 filed Mar. 31, 2000. This application is assigned to the assignee of the present invention, and its disclosure is specifically incorporated by reference herein. 
     The present invention is applicable to a variety of electronic devices. Illustrative devices include, but are not limited to, optoelectronic devices such as semiconductor lasers, electro-absorptive modulators, and detectors. A variety of materials, to include III-V semiconductors, may be used to fabricate heterostructure devices using the present invention. For example, materials that can be used to form the heterostructure of layers  101 ,  102 ,  103 , include, but are in no way limited to, GaAs, GaP, InGaAsP and InGaAs. The active layer  102  may be a single intrinsic layer, or a single quantum layer, depending on the ultimate device sought. Moreover, it is clear that multiple heterojunction structures such as superlattice structures, or multi quantum well (MQW) structures may be used for the active layer  102 . As would be readily understood by one having ordinary skill in the art, the particular materials chosen, as well as the chosen stoichiometry may depend upon device application and desired characteristics. Such choices of materials and stoichiometry would be within the purview of one having ordinary skill in the art, and as such need not be discussed in further detail. 
     In the exemplary embodiment shown in FIG. 1, the first dopant blocking layer  104  is effective in preventing diffusion of p-type dopants out of the p-type cladding layer  101 . This illustrative layer of InAlAs is grown by standard technique such as metal organic vapor phase epitaxy (MOVPE). The growth temperature is in the range of approximately 500° C. to approximately 570° C.; illustratively 530° C. Alternatively, the first dopant blocking layer  104  of InAlAs may be grown by molecular beam epitaxy (MBE). In the illustrative embodiment using MBE, the growth temperature when growing the first dopant barrier layer  104  is in the range of approximately 400° C. to approximately 470° C.; illustratively 430° C. By either technique, growth at the illustrative lower temperatures provides advantageous material properties to layer  104 , as well as allowing a suitable thickness to be grown. To this end, growth of the illustrative InAlAs layer  104  at lower temperatures is believed to foster growth of a layer having a relatively high resistivity. In addition the lower temperature growth is believed to foster growth of a layer having sufficient thickness to aid in preventing p-type dopants from diffusing out of the p-type layer  101  and into the burying layers  107 . Finally, it is of interest to note that MOVPE and MBE are illustrative techniques for forming layer  104  at low temperatures. Clearly, other techniques known in the art may be used to fabricate layer  104  to achieve its desired characteristics described herein. 
     Applicants believe, without wishing to be bound to their theory, that the greater the kinetic energy of the growth species, the greater the probability that the diffusion along a vertical growth surface (such as vertical sidewall  108 ) will occur. This diffusion of the growth species at higher growth temperatures may retard growth along the vertical sidewall  108  resulting in a thinner layer&#39;s being grown along the vertical sidewall  108 . At the same time due to the higher temperature growth, a thicker layer may be grown along the horizontal surface  109  of the substrate. However, because of the relatively low growth temperatures used in growing layer  104  in the exemplary embodiment of FIG. 1, a layer of InAlAs having a thickness in the illustrative range of approximately 50 nm to approximately 100 nm, may be achieved along the vertical (e.g., (011) crystallographic plane) sidewall  108  of the mesa  111 . Moreover, because of the reduction in diffusion along the sidewall  108  as a result of the lower growth temperature, the growth rate of the portion of layer  104  along the vertical sidewall  108  is substantially the same as the growth rate of the portion of layer  104  along the horizontal (e.g., (100) crystallographic plane) surface  109  of the substrate. As a result, layer  104  has a substantially equal thickness along its vertical and horizontal portions. Applicants believe that this ability to grow layer  104  relatively thickly along the vertical sidewall  108  of the mesa  111  aids in current confinement in the device of the mesa PIN structure  111 . 
     In addition to the desirable thickness of the portion of the layer  104  along the vertical sidewall  108 , the first dopant blocking layer  104  of the exemplary embodiment has an increased resistivity compared to a like material grown at higher temperatures. For example, in the illustrative embodiment, the InAlAs layer  104  is semi-insulating, and has a resistivity in the range of approximately 10 6  Ohm-cm to approximately 10 9  Ohm-cm. As discussed herein, InAlAs grown by the same technique at a higher temperature has a lower resistivity. This greater resistivity in layer  104  is believed to aid in current confinement in buried structure devices. Accordingly, leakage current may be reduced in devices incorporating the present invention. This can improve device performance and reliability. Again, without wishing to be bound, applicants believe that due to the comparatively low temperature at which layer  104  is grown, ambient oxygen does not evaporate and may bond to the aluminum forming deep level traps. Also, carbon may be introduced into the material during the low temperature growth sequence. The bonding of the oxygen to the aluminum and the carbon compensation at low temperature may both serve to increase the resistivity. 
     The diffusion blocking characteristics of the first dopant blocking layer  104  can be seen most readily from a review of FIG.  2 . To this end, the concentration of the illustrative dopant, Zn, shown at  201 , is relatively high in the layer of type InP (on the order of about ×10 18 /cm 3 ). However, the concentration of Zn drops off significantly in the illustrative dopant blocking layer InAlAs layer  202 , grown at low temperature as discussed above. The illustrative layer of InAlAs shown in FIG. 2 is about 800 Å thick, and as can be seen, the concentration of Zn drops off to minimal levels. 
     While the first dopant blocking layer  104  of InAlAs grown at low temperature is a suitable material to prevent the diffusion of p-type dopants from layer  101 , this material may not be suitable to prevent the diffusion of dopants out of layer  107 . To this end, in the illustrative embodiment in FIG. 1, iron doped indium phosphide is used for the current blocking layer/confinement layer  107 . As is known, iron doped indium phosphide is a suitable material to provide both current and optical confinement in buried mesa structure devices. However, the diffusion of iron dopants into the mesa structure  104  and in particular into layer  101  can have deleterious effects on the performance characteristics of a device. Unfortunately, the low-temperature InAlAs first dopant blocking layer  104  may not be suitable for blocking iron dopants from current blocking layer  107 . Applicants have found, however, that when InAlAs is grown at a higher temperature, it can effectively prevent the diffusion of iron out of layer  107 , and into the mesa  111 . To this end, in the illustrative embodiment of the present invention, layer  106  is grown by MOVPE at a temperature on the order of approximately 600° C. to approximately 650° C.; illustratively 630° C. Alternatively, layer  106  may be grown by MBE at an illustrative growth temperature of approximately 500° C. to approximately 550° C.; illustratively 530° C. 
     By either MOVPE or MBE, when grown at higher growth temperatures, the InAlAs growth rate is slower along the vertical portion  113  of the layer  106  than along the horizontal portion  114  of the layer. Again, this follows from the kinetic energy of the growth species as discussed above. Accordingly, the vertical portion  113  of layer  106  tends to be 3-10 times thinner than the horizontal portion  114 . Applicants have found that the thickness grown along vertical portion  113  of layer  106  is sufficient to effectively block the diffusion of iron dopants out of the current confinement layer  107 . To this end, layer  106  has an illustrative thickness on the order of approximately 30 nm to approximately 100 nm along its vertical portion  113 . Finally it is of interest to note that MOVPE and MBE are illustrative techniques for forming layer  106  at high temperatures. Clearly other techniques know in the art may be used to fabricate layer  106  at to achieve its desired characteristics described herein. 
     In addition to its ability to suitably block iron dopants from diffusing out of the blocking layers  107 , the second dopant blocking layer  106  is not very resistive, having a resistivity in the range approximately 10 4  Ohm-cm to 10 5  Ohm-cm. However, the second dopant barrier layer, while having a relatively low resistivity, does not adversely impact the current blocking ability of layer  107 , since it is substantially isolated electrically from the mesa  111  and from the n-InP substrate  100 . 
     The dopant diffusion blocking capability is shown graphically in FIG.  3 . In this illustrative embodiment, the Fe concentration  301  is on the order of  10   16  cm −3  to 10 17  cm −3  in the InP(Fe) blocking layer. However, as can be seen, the iron concentration drops significantly in the InAlAs layer grown at the exemplary high temperatures of the present invention. 
     In the illustrative embodiment shown in FIG. 1, a third dopant blocking layer  105  may be used. The third dopant blocking layer  105  is illustratively a layer of InP based material grown between layers  104  and  106 . In the exemplary embodiment, third dopant blocking layer  105  is undoped InP. Alternatively, layer  105  may be undoped InGaP, InGaAs or InGaAsP. Third dopant blocking layer  105  is grown by conventional technique, such as MOVPE or MBE, and adds certain advantages. Applicants theorize, without wishing to be bound to their theory, that in addition to the intrinsic properties of the materials used for diffusion blocking layers, interfaces between layers are believed to assist in blocking dopant diffusion. Accordingly, the third dopant blocking layer  105  of undoped InP (or InP-based material) disposed between layers  104  and  106  adds two hetero-interfaces to the multi-layer dopant diffusion barrier layer  110 . In the illustrative embodiment of the present invention, this layer has a thickness on the order of approximately 10 nm to approximately 100 nm. 
     The third dopant blocking layer  105  may be slightly n-doped (n-type). Illustratively, layer  105  is an n-InP layer having a thickness in the range of approximately 10 nm to approximately 100 nm; having a doping concentration in the range of approximately 1-5×10 17  cm −3 . Accordingly, it may further aid in the blocking of zinc and iron diffusion in the illustrative embodiment. In contrast to prior art n-InP dopant blocking layers, if layer  105  of the present invention is n-type, it will not form a parasitic pn junction, since it is not in contact with a p-type layer, such as upper cladding layer  101 . Moreover, because it is electrically isolated from the n-type substrate  100 , it will not provide a leakage current path. Finally, because layer  105  is relatively thin and only slightly doped n-type, it does not add significantly any parasitic capacitance component to a device. 
     Turning to FIG. 4, another embodiment of the present invention is shown. In this structure, a first doped layer  400  (for example a substrate) may be a semi-insulating layer, such as InP(Fe), and a second doped layer  404  may be p-type such as p-InP. In such a structure, a multi-layer dopant barrier  405 , having a first dopant blocking layer  403  and a second dopant blocking layer  401 , is disposed between layers  400  and  404 . Consistent with the discussion surrounding the embodiment of FIG. 1, second dopant blocking layer  401  in the embodiment of FIG. 4, is illustratively a layer of InAlAs grown at high temperature and having a suitable thickness to block dopants, such as Fe from diffusing out of the substrate  400 . Also consistent with the teaching surrounding the exemplary embodiment of FIG. 1, first dopant blocking layer  403  is illustratively a layer of InAlAs grown at low temperature and thickness to block p-type dopants (such as Zn) from diffusing out of layer  404 . Third dopant blocking layer  402  is illustratively a layer of undoped or slightly n-type InP and may be used optionally, as discussed above. The materials and techniques used to fabricate the structure of FIG. 4 are consistent with the disclosure surrounding the multi-layer dopant barrier of the illustrative embodiment of FIG.  1  and are omitted in this portion of the disclosure of the invention in the interest of brevity. 
     Finally, it is of interest to note that there are conceivably structures in which the substrate  400  is p-type (such as p-InP) and layer  404  is a semi-insulating material (such as InP(Fe)). As such, the multi-layer dopant blocking layer  405  may be used, except that layer  403  illustratively would be a dopant blocking layer suitable for blocking dopant diffusion out of the semi-insulating layer (such as InAlAs grown at high temperature). Layer  401  would be a layer dopant blocking layer suitable for blocking p-type dopants (such as InAlAs grown at low temperature). 
     The invention having been described in detail, those skilled in the art should understand that they can make various changes, substitutions and modifications thereto without departing from the theme and spirit of the invention in its broadest form. To the extent that such changes, substitutions and modifications result in a multi-layer dopant barrier in accordance with the present disclosure, such are deemed within the scope of the appended claims.