Abstract:
A device includes a semiconductor structure with at least one III-P light emitting layer disposed between an n-type region and a p-type region. The semiconductor structure further includes a GaAs x P 1-x  p-contact layer, wherein x&lt;0.45. A first metal contact is in direct contact with the GaAs x P 1-x  p-contact layer. A second metal contact is electrically connected to the n-type region. The first and second metal contacts are formed on a same side of the semiconductor structure.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a III-P light emitting device, and in particular to a contact layer for a flip chip III-P light emitting device. 
       BACKGROUND 
       [0002]    Group III-P semiconductor devices such as (Al x Ga 1-x ) 1-y In y P light emitting diodes (LEDs) are used to produce visible wavelengths from red to amber. AlInGaP LEDs are typically formed by growing epitaxial layers, including p-type and n-type layers sandwiching a light-emitting active layer, on a GaAs growth substrate. High quality ternary and quaternary substrates are very difficult to fabricate, so GaAs substrates are commonly used. To produce low-defect LED layers, the lattice constant of the (Al x Ga 1-x ) 1-y In y P epitaxial layers must match the lattice constant of the GaAs. To match the GaAs lattice constant, y=0.48. The x value is adjusted to achieve the desired emission wavelength. 
         [0003]    A flip chip III-P LED described in U.S. Pat. No. 7,244,630 is illustrated in  FIG. 1 . A lower confining layer  22  of n-type AlInP is grown on a growth substrate (not shown). The AlInP confining layer  22  has a band gap that is higher than the band gap of the active layer. An active layer  24  of (Al x Ga 1-x ) 0.47 In 0.53 P, which may comprise a plurality of layers, is grown over the confining layer  22 . A p-type upper confining layer  26  of AlInP is grown over the active layer  24 . A highly doped p-type AlInGaP contact layer  71  may be provided over layer  26 . Layers  24 ,  26 , and  71  are etched to expose the n-AlInP confining layer  22  for electrical contact. A metal n-electrode  83  is then formed to electrically contact the n-AlInP confining layer  22 , and a p-electrode  84  is formed to contact the p+ AlInGaP layer  71 . 
         [0004]    The p and n-electrodes are bonded to metal pads on the package element  87 . The substrate may be removed after bonding the electrodes to the package element  87 . Vias electrically couple the metal pads on the top of package element  87  to p- and n-electrodes  90 ,  91  on the bottom of package element  87 . Electrodes  90 ,  91  may be soldered to pads on a circuit board or to pads on another package. 
         [0005]    The top surface of the LED (the n-AlInP layer  22  in the example) is further processed to have light extraction features  92 . Such features may include roughening or other techniques, such as ordered texturing or a photonic crystal structure, to increase the light output. 
       SUMMARY 
       [0006]    It is an object of the invention to form a device with a GaAs x P 1-x  p-contact layer and a metal contact in direct contact with the GaAs x P 1-x  p-contact layer. Embodiments of the invention may have lower contact resistance than conventional III-P devices. 
         [0007]    In accordance with embodiments of the invention, a device includes a semiconductor structure with at least one III-P light emitting layer disposed between an n-type region and a p-type region. The semiconductor structure further includes a GaAs x P 1-x  p-contact layer, wherein x&lt;0.45. A first metal contact is in direct contact with the GaAs x P 1-x  p-contact layer. A second metal contact is electrically connected to the n-type region. The first and second metal contacts are formed on a same side of the semiconductor structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates a prior art flip chip III-P LED. 
           [0009]      FIG. 2  illustrates a III-P device structure grown on a growth substrate. 
           [0010]      FIG. 3  illustrates the structure of  FIG. 2  after forming a p-contact, etching a mesa, and forming an n-contact. 
           [0011]      FIG. 4  illustrates a III-P device mounted on a mount. 
           [0012]      FIG. 5  illustrates an energy band diagram for a portion of a device with a conventional contact layer. 
           [0013]      FIG. 6  illustrates an energy band diagram for a portion of a device according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    In the devices such as the device illustrated in  FIG. 1 , contact layer  71  is typically GaP. It is difficult to form an Ohmic p-contact to GaP without reducing the reflection caused by photon scattering at the metal-semiconductor interface. The p-metal-semiconductor interface is preferably as smooth and uniform as possible, to minimize photon scattering. A conventional p-metal contact formed on a GaP contact layer typically introduces metal spikes and a non-uniform interface, which causes undesirable photon scattering. 
         [0015]    In embodiments of the invention, the metal p-contact is formed on a GaAsP contact layer, rather than a GaP contact layer. 
         [0016]      FIGS. 2-4  illustrate forming a device according to embodiments of the invention. In  FIG. 2 , a device structure is grown on a growth substrate  10 , which may be GaAs or any suitable growth substrate. An n-type region  12  is grown first on growth substrate  10 . N-type region  12  may include multiple layers of different compositions and dopant concentration including, for example, preparation layers such as buffer layers or nucleation layers, which may be n-type or not intentionally doped, release layers designed to facilitate later release of the growth substrate or thinning of the semiconductor structure after substrate removal, and n- or even p-type device layers designed for particular optical or electrical properties desirable for the light emitting region to efficiently emit light. N-type region  12  may include, for example, an (Al x Ga 1-x ) 0.52 In 0.48 P n-contact layer, where x=0.4. 
         [0017]    A light emitting or active region  14  is grown over n-type  12 . Active region  14  may be a single thick or thin light emitting layer, or a multi-quantum well active region including multiple thin or thick quantum well light emitting layers separated by barrier layers. 
         [0018]    A p-type region  16  is grown over active region  14 . Like the n-type region, the p-type region may include multiple layers of different composition, thickness, and dopant concentration, including layers that are not intentionally doped, or n-type layers. P-type region  16  may include, for example, a GaP or AlInP p-cladding layer. In some embodiments, p-type region  16  includes a cladding layer adjacent to the active region and a transition region disposed between the cladding layer and the contact layer described below. For example, the cladding layer may be a 1.5 μm thick Al 0.48 In 0.52 P (or AlInGaP) layer and the transition region may be a thin (thickness 20-5000 Å) compositionally graded (Al x Ga 1-x ) 0.5 In 0.5 P layer, graded for example from AlInP to (Al 0.3 Ga 0.7 ) 0.47 In 0.53 P. 
         [0019]    In accordance with embodiments of the invention, a p-contact layer  18  is grown over p-type region  16 . P-contact layer  18  may be, for example, GaAs x P 1-x . The As composition x is less than 0.45, such that the material is in the indirect band gap region, to minimize absorption. P-contact layer  18  may have a constant composition x, or may be compositionally graded from x=0 (GaP), to GaAs x P 1-x , where x&lt;0.45. In a p-contact layer  18  with constant composition x,  0 ≦x≦0.45 in some embodiments, 0.2≦x≦0.4 in some embodiments, and x=0.3 in some embodiments. In a compositionally graded p-contact layer  18 , x is graded from 0 to 0.45 in some embodiments, from 0 to between 0.2 and 0.4 in some embodiments, and from 0 to 0.3 in some embodiments. P-contact layer  18  may be doped with, for example, Mg, Zn, or C, to a concentration between 3e18 cm −3  and 1e19 cm −3 . P-contact layer  18  may be, for example, between 20 Å and 2 μm thick in some embodiments, and 0.5 μm thick in some embodiments. 
         [0020]    In some embodiments, a GaAsP p-contact layer  18  is grown using tetrabutylarsine (TBAs) as the arsine source and tetrabutylphosphine (TBP) as the phosphine source. Replacing conventional sources such as arsine (AsH 3 ) and phosphine (PH 3 ) with TBAs and TBP may permit the p-contact layer  18  to be grown at a lower temperature, which may lead to higher quality material with better contact properties. For example, the growth temperature can be reduced by as much as 100° C. when TBAs and TBP are used as sources, which may increase Mg doping incorporation efficiency by a factor of 2-2.5. The increase in Mg doping efficiency enables a lower reactor background concentration (lower residual dopants in the background) and may produce more consistent LEDs with higher light output. 
         [0021]    In  FIG. 3 , contact metals are formed and a mesa is etched in the device. A p-contact may be formed first, for example by depositing an ohmic contact metal in discrete regions  21 . Ohmic contact metal  21  may be, for example, AuZn or Al, formed in dots, then annealed. A mirror  20 , which may be, for example, silver, is formed over ohmic contact metal regions  21 . Ohmic regions  21  are between 1 and 5 μm in diameter in some embodiments and 3 μm in diameter in some embodiments, and are spaced between 5 and 15 μm apart in some embodiments and 10 μm apart in some embodiments. 
         [0022]    Portions of the p-contact, p-type layers 16 and 18, and active region  14  may be removed to expose a portion  27  of n-type region  12 . An n-contact  25 , which may be, for example, AuGe, is formed on the exposed portion  27  of n-type region  12 . The n- and p-contacts  25  and  20 / 21  may be electrically isolated by a trench which may be filled with an insulating material  23 . The portion of n-type region  27  exposed by etching for forming a contact may be distributed across the device. 
         [0023]    In  FIG. 4 , the device is attached to a mount and the growth substrate is removed. N- and p-contacts  25  and  20  are electrically and physically connected to mount  87  by n- and p-interconnects  35  and  33 . The interconnects may be, for example, solder, gold, or any other suitable material. The device may be attached to mount  87  by, for example, reflowing solder interconnects or ultrasonic bonding of gold interconnects. Vias (not shown in  FIG. 4 ) electrically couple metal pads on the top of mount  87  to p- and n-electrodes  90 ,  91  on the bottom of mount  87 . Electrodes  90 ,  91  may be soldered to pads on a circuit board or to pads on another package. 
         [0024]    After mounting the device on mount  87 , growth substrate  10  may be removed, for example by etching. The semiconductor structure  30  remaining after removing the growth substrate may be thinned. The top surface may be textured, for example by roughening or etching to form a pattern such as a photonic crystal, to improve light extraction. 
         [0025]      FIG. 5  illustrates the band diagram of a portion of a device including a conventional GaP contact layer.  FIG. 6  illustrates the band diagram of a portion of a device including a GaAsP contact layer and a graded (Al x Ga 1-x ) 0.5 P transition layer according to embodiments of the invention. The notch in the valence band between p-type region  16  and p-GaP contact layer  40  in  FIG. 5  can trap holes. As illustrated in  FIG. 6 , the valence band of a GaAsP contact layer  18  aligns more favorably with that of the transition layer in p-type region  16 , and GaAsP has a smaller band gap, which may increase the active hole concentration and reduce the contact resistance, which in turn may reduce the turn-on voltage and increase wall-plug efficiency. 
         [0026]    In addition, GaAsP p-contact layer  18  is highly transparent to wavelengths between 580 and 620 nm, a wavelength range commonly emitted by the active region  14  described above. The transparency of GaAsP p-contact layer  18  may reduce internal absorption and increase extraction of light from the device. 
         [0027]    Further, forming a conventional p-metal contact on a GaP contact layer often results in metal spikes and a non-uniform interface, which causes undesirable photon scattering. Metal spikes may be formed during the alloying process (which may be, for example, a high-temperature anneal) after p-metal deposition. During the alloying step, metals diffuse into the p-contact semiconductor layer at a non-uniform rate. As a result, some areas of the contact layer have a larger metal penetration, while other areas have less. The non-uniform penetration may cause photons to be scattered or absorbed at the interface, for example when the diffused metal forms an alloy with the p-contact layer that absorbs photons. GaAs x P 1-x  contact layers according to embodiments of the invention, as described above, have a more favorable band lineup with the p-contact metal, such that alloying may not be necessary, or the alloying temperature may be reduced, resulting in a more uniform interface and fewer metal spikes. 
         [0028]    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.