Abstract:
A light emitting device includes a first active region, a second active region, and a tunnel junction. The tunnel junction includes a layer of first conductivity type and a layer of second conductivity type, both thinner than a layer of first conductivity type and a layer of second conductivity type surrounding the first active region. The tunnel junction permits vertical stacking of the active regions, which may increase the light generated by a device without increasing the size of the source.

Description:
BACKGROUND 
     1. Field of Invention 
     This invention relates to semiconductor light emitting devices and, in particular, to semiconductor light emitting devices incorporating tunnel junctions. 
     2. Description of Related Art 
     Semiconductor light-emitting devices including light emitting diodes (LEDs), vertical resonant cavity light emitting diodes (VRCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, a light emitting or active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. 
     Mg-doped III-nitride layers tend to be highly resistive, requiring high voltage drop in order to provide enough positive charge carriers (holes) in the active region. In addition, current tends to concentrate in the most direct paths from the p-contact to the active region. Accordingly, device designs which increase current spreading, particularly in the p-type layers, are advantageous. 
     Semiconductor light emitting devices are often incorporated into systems, such as projectors and optical scanning devices, that include optics such as lenses. The cost of such optics tends to increase with increasing size, thus device designs that increase brightness without increasing size are desirable. 
     SUMMARY 
     In accordance with an embodiment of the invention, a light emitting device includes a first active region, a second active region, and a tunnel junction. The tunnel junction includes a layer of first conductivity type and a layer of second conductivity type, both thinner than a layer of first conductivity type and a layer of second conductivity type surrounding the first active region. The tunnel junction permits vertical stacking of the active regions, which may increase the light generated by a device without increasing the size of the source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B illustrate single active region devices incorporating tunnel junctions, according to embodiments of the present invention. 
     FIG. 2 illustrates a multiple active region, two contact embodiment of the present invention. 
     FIG. 3 illustrates a multiple active region, multiple contact embodiment of the present invention. 
     FIG. 4 illustrates a multiple active region, two contact embodiment of the present invention incorporating a wavelength converting material. 
     FIG. 5 illustrates a packaged light emitting device. 
    
    
     DETAILED DESCRIPTION 
     The examples described below are III-nitride light emitting devices. The semiconductor layers of III-nitride devices have the general formula Al x In y Ga z N, where 0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1. III-nitride device layers may further contain group III elements such as boron and thallium and may have some of the nitrogen may be replaced by phosphorus, arsenic, antimony, or bismuth. Though the examples below describe III-nitride devices, embodiments of the invention may also be fabricated in other III-V materials systems including III-phosphide and III-arsenide, II-VI material systems, and any other materials systems suitable for making light emitting devices. 
     FIG. 1A illustrates a light emitting device with a single light emitting region, according to an embodiment of the invention. The device shown in FIG. 1A is a VRCLED or VCSEL incorporating a tunnel junction as a hole spreading layer. A light emitting active region  3  separates an n-type layer  2  and a p-type layer  4 . Active region  3  may be, for example, a single light emitting layer or a multiple quantum well structure of alternating quantum well layers and barrier layers, such as a separate confinement heterostructure, described in more detail in application Ser. No. 10/033,349, titled “Indium Gallium Nitride Separate Confinement Heterostructure Light Emitting Devices,” filed Nov. 2, 2001, and incorporated herein by reference. The n-type layer may be, for example, AlGaN doped with Si; the p-type layer  4  may be, for example, AlGaN doped with Mg; and the active region may be, for example, an InGaN multiple quantum well structure. A tunnel junction  100  is formed over p-type layer  4 . 
     Tunnel junction  100  includes a highly doped p-type layer  5 , also referred to as a p++ layer, and a highly doped n-type layer  6 , also referred to as an n++ layer. P++ layer  5  may be, for example, InGaN or GaN, doped with an acceptor such as Mg or Zn to a concentration of about 10 18  cm −3  to about 5×10 20  cm −3 . In some embodiments, p++ layer  5  is doped to a concentration of about 2×10 20  cm −3  to about 4×10 20  cm −3 . N++ layer  6  may be, for example, InGaN or GaN, doped with an acceptor such as Si or Ge to a concentration of about 10 18  cm −3  to about 5×10 20  cm −3 . In some embodiments, n++ layer  6  is doped to a concentration of about 7×10 19  cm −3  to about 9×10 19  cm −3 . Tunnel junction  100  is usually very thin, for example tunnel junction  100  may have a total thickness ranging from about 2 nm to about 100 nm, and each of p++ layer  5  and n++ layer  6  may have a thickness ranging from about 1 nm to about 50 nm. In some embodiments, each of p++ layer  5  and n++ layer  6  may have a thickness ranging from about 25 nm to about 35 nm. P++ layer  5  and n++ layer  6  may not necessarily be the same thickness. In one embodiment, p++ layer  5  is 15 nm of Mg-doped InGaN and n++ layer  6  is 30 nm of Si-doped GaN. P++ layer  5  and n++ layer  6  may have a graded dopant concentration. For example, a portion of p++ layer  5  adjacent to the underlying p-layer  4  may have a dopant concentration that is graded from the dopant concentration of the underlying p-type layer to the desired dopant concentration in p++ layer  5 . Similarly, n++ layer  6  may have a dopant concentration that is graded from a maximum adjacent to p++ layer  5  to a minimum adjacent to n-type layer  7 . Tunnel junction  100  is fabricated to be thin enough and doped enough such that tunnel junction  100  displays low series voltage drop when conducting current in reverse-biased mode. In some embodiments, the voltage drop across tunnel junction  100  is about 0.1V to about 1V. 
     A second n-type layer  7  is formed over tunnel junction  100 . A positive electrical contact  9  is attached to n-type layer  7  and a negative electrical contact  10  is attached to n-type layer  2 . The device shown in FIG. 1A is a VRCLED or a VCSEL, thus it also includes a bottom distributed Bragg reflector (DBR)  1  under n-type layer  2 . DBR  1  may be a dielectric or it may be formed in-situ of III-nitride materials. A top DBR  8  is formed on n-type layer  7 . As is clear to a person of skill in the art, the device shown in FIG. 1A may be fabricated as a light emitting diode, without DBRs  1  and  8 . 
     Tunnel junction  100  is fabricated such that when a voltage drop is applied across contacts  9  and  10  such that the p-n junction between active region  3  and p-type layer  4  is forward biased, tunnel junction  100  quickly breaks down and conducts in the reverse-bias direction with a minimal voltage drop. The layers in tunnel junction  100  are thin enough and doped enough that charge carriers can tunnel through them. Each of the layers in tunnel junction  100  need not have the same composition, thickness, or dopant composition. Tunnel junction  100  may also include an additional layer between p++ layer  5  and n++ layer  6  that contains both p- and n-type dopants. 
     Tunnel junction  100  acts as a hole spreading layer to distribute positive charge carriers in p-type layer  4 . Carriers in n-type III-nitride material have a much longer diffusion length than carriers in p-type III-nitride material, thus current can spread more readily in an n-type layer than a p-type layer. Since current spreading on the p-side of the p-n junction occurs in n-type layer  7 , the device illustrated in FIG. 1B may have better p-side current spreading than a device lacking a tunnel junction. 
     A single light emitting region device incorporating a tunnel junction may also be simpler to fabricate than a similar device without the tunnel junction. Since both contacts  9  and  10  are formed on n-type layers  2  and  7 , contacts  9  and  10  may be the same material, potentially eliminating from the fabrication process some deposition and etching steps required when different contact materials must be applied to the p- and n-layers of the device. 
     FIG. 1B illustrates an alternative embodiment of a single light emitting region device. Tunnel junction  100  is located beneath the active region, rather than above the active region as in the embodiment shown in FIG.  1 A. Tunnel junction  100  is located between n-type layer  2  and p-type layer  4 . Thus, the polarity of the device in FIG. 1B is the opposite of the polarity of the device in FIG.  1 A. Tunnel junction  100  of FIG. 1B also serves as a hole spreading layer by distributing positive charge carriers in p-type layer  4 . 
     FIG. 2 illustrates an embodiment of a light emitting device with multiple light emitting regions separated by tunnel junctions. The device shown in FIG. 2 includes a bottom contact  11 , a bottom DBR  12 , a first active region  14  sandwiched between a first n-type layer  13  and a first p-type layer  15 , a second active region  19  sandwiched between a second n-type layer  18  and a second p-type layer  20 , a top DBR  21 , and a top contact  22 . A tunnel junction  100 , including a p++ layer  16  and an n++ layer  17  separates the two active regions  14  and  19 . P++ layer  16  is adjacent to first p-type layer  15  and n++ layer  17  is adjacent to second n-type layer  18 . Tunnel junction  100  may have the same characteristics as the tunnel junctions described above in reference to FIGS. 1A and 1B. The device shown in FIG. 2 may be formed on a conducting growth substrate such as SiC, such that the substrate separates contact  11  and DBR  12 . Alternatively, layers  13 - 21  may be formed on an insulating substrate which is later removed. DBR  12  and contact  11  may be attached to layer  13  by, for example, wafer bonding. The device shown in FIG. 2 is a VRCLED or VCLED. Like the embodiments illustrated in FIGS. 1A and 1B, the embodiment shown in FIG. 2 can easily be fabricated as a light emitting diode by omitting DBRs  12  and  21 . In order to form both contacts on the same side of the device, a portion of the semiconductor layers may be etched away to expose a ledge on n-type layer  13 , on which contact  11  may be formed. 
     Though two active regions are illustrated in FIG. 2, any number of active regions may be included between contacts  11  and  22 , provided the p-type region adjacent each active region is separated from the n-type region adjacent the next active region by a tunnel junction. Since the device of FIG. 2 has only two contacts, both active regions  14  and  19  emit light at the same time and cannot be individually and separately activated. In one embodiment, a device may have enough junctions such that the device can operate at 110 volts. 
     Active regions  14  and  19  of FIG. 2 may be fabricated with the same composition, such that they emit the same color light, or with different compositions, such that they emit different colors (i.e. different peak wavelengths) of light. For example, a three active region device with two contacts may be fabricated such that the first active region emits red light, the second active region emits blue light, and the third active region emits green light. When activated, the device may produce white light. Since the active regions are stacked such that they appear to emit light from the same area, such devices may avoid problems with color mixing present in a device that combines red, blue, and green light from adjacent, rather than stacked, active regions. In a device with active regions emitting different wavelengths of light, the active region that generates light of the shortest wavelength may be located closest to the surface from which light is extracted, generally the sapphire, SiC, or GaN growth substrate in an LED. Placement of the shortest wavelength active region near the output surface may minimize loss due to absorption in the quantum wells of the other active regions and may reduce the thermal impact on more sensitive longer wavelength quantum wells by locating the longer wavelength active regions closer to the heat sink formed by the contacts. The quantum well layers may also be made sufficiently thin that absorption of light in the quantum well layers is low. The color of the mixed light emitted from the device may be controlled by selecting the number of active regions that emit light of each color. For example, the human eye is very sensitive to green photons and not as sensitive to red photons and blue photons. In order to create balanced white light, a stacked active region device may have a single green active region and multiple blue and red active regions. 
     Devices with multiple stacked active regions, such as the device shown in FIG. 2, may offer several advantages. First, when driven at the same current density as a similar device with a single active region, a multiple stacked active region device may operate at a higher voltage than a single active region device of the same area, due to the voltage drop required by the tunnel junctions separating the active regions. Operating at higher voltage may simplify the design and reduce the cost of power supplies and drivers. 
     Second, when operated at the same current density as a single active region device, a multiple stacked active region device may emit more light than a single active region device, while not requiring additional area. Increasing the brightness per area may reduce the cost of fabricating the device, because less substrate area is required for each device, thus more devices may be fabricated on a single substrate wafer. Also, higher brightness for a given area may reduce simplify the design and reduce the cost of secondary optics because the cost of optics tends to increase as the area of the optic increases. 
     Third, the presence of tunnel junctions separating the active regions may enhance current spreading in the device, which distributes carriers more evenly in the layers adjacent to each active region and thus permits more radiative recombinations of carriers in the active region. 
     Fourth, multiple junctions may be activated by a single pair of contacts, thus mitigating electromigration problems in the contacts and the amount of space consumed by the contacts. When III-nitride devices operate at high current, metal from the contacts can migrate into the semiconductor layers and cause reliability problems in the device. In a device with multiple lateral active regions, each time an active region is added, a set of contacts must be added, thus each time an active region is added the possibility of electromigration problems increases. In contrast, in a multiple stacked active region device, the addition of an active region does not require the addition of more contacts, thus the extent of the electromigration problem for a multiple stacked active region device is the same as a single active region device with the same contact area. 
     FIG. 3 illustrates an embodiment of the invention with multiple active regions and more than two contacts. Besides contacts  11  and  22  on the ends of the device, a contact  23  is formed on a ledge etched down to p-type layer  15 . Alternatively, contact  23  may be connected to one of layers  16  and  17  in tunnel junction  100 , or to n-type layer  18 . Contact  23  makes active regions  19  and  14  independently addressable. When current is applied between contacts  11  and  22 , both active regions emit light. When current is applied between contacts  23  and  22 , only active region  19  emits light. When current is applied between contacts  23  and  11 , only active region  14  emits light. In addition, contact  23  allows for modulation of the relative current flow through the different active regions. The ability to increase the current flow in some layers and decrease the current flow in other layers can be used to tune the color of the aggregate spectrum. In some embodiments, all contacts are formed on the same side of the device. In such embodiments, the contacts are formed on multiple ledges, each connecting to a different layer. 
     In some embodiments, part of the light emitted from one or more of the active regions in a multiple stacked active region device may be converted to a different wavelength by a wavelength converting material such as, for example, a phosphor. FIG. 4 illustrates a two active region device with a phosphor layer  35 . The device shown in FIG. 4 is an LED formed on an insulating substrate  30 . Interconnects  32 , which may be, for example, solder, wires, or any other suitable connection, connect contacts  22  and  23  to a submount  34 . The device shown in FIG. 4 is a flip chip, meaning that the device is “flipped” when mounted on the submount such that light is extracted through growth substrate  30 . Phosphor layer  35  may be any suitable phosphor. Phosphor layer  35  may be formed for example over just the top of substrate  30 , over the top and sides of substrate  30 , or over the top and sides of substrate  30  and all or a portion of the sides of the semiconductor layers (shown in FIG.  4 ). Phosphor layer  35  may be thin enough such that only a portion of light emitted from active regions  14  and  19  are converted by phosphor  35 . Phosphor layer  35  may contain scattering particles in addition to a fluorescent material. One or more phosphor layers may be used on a multiple stacked active region device to form white light. For example, active region  14  of FIG. 4 may produce blue light, active region  19  may produce cyan light, and phosphor  35  may be a doped Yttrium Aluminum Garnet (YAG) or SrS phosphor that converts some of the blue light to light having a wavelength from amber through red. The blue, cyan, and amber/red light combine to produce white light. 
     In another embodiment, the wavelength converting material is located between semiconductor layers, rather than applied to the surfaces of the substrate and semiconductor layers. In such an embodiment, some of the semiconductor layers are formed on a first substrate. The last layer applied over the semiconductor layers is a phosphor layer. The remaining semiconductor layers are formed on a second substrate. The second substrate is removed and the remaining semiconductor layers are wafer bonded to the phosphor layer. In still another embodiment, growth substrate  30  is a wavelength converting material, such as single crystal YAG. 
     In some embodiments, more than one multiple stacked active region device may be fabricated monolithically on a single substrate. Monolithic light emitting device arrays are described in more detail in application Ser. No. 09/823,824, filed Mar. 29, 2001, titled “Monolithic Series/Parallel LED Arrays Formed On Highly Resistive Substrates,” and incorporated herein by reference. 
     FIG. 5 is an exploded view of a packaged light emitting device. A heat-sinking slug  100  is placed into an insert-molded leadframe  106 . The insert-molded leadframe  106  is, for example, a filled plastic material molded around a metal frame that provides an electrical path. Slug  100  may include an optional reflector cup  102 . The light emitting device die  104 , which may be any of the devices described above, is mounted directly or indirectly via a thermally conducting submount  103  to slug  100 . An optical lens  108  may be added. 
     Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concept described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.