Patent Publication Number: US-7719018-B2

Title: Light emitting devices with compact active regions

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation of U.S. application Ser. No. 10/632,720, filed Jul. 31, 2003 now U.S. Pat. No. 6,900,474, which claims the benefit of U.S. Provisional Application 60/435,838, filed Dec. 20, 2002. Both 10/632,720 and 60/435,838 are incorporated herein by this reference. 

   BACKGROUND 
   1. Field of Invention 
   The invention is related to light emitting devices with reflective contacts. 
   2. Description of Related Art 
   Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs 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; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus, also referred to as III-phosphide materials. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide by metal organic chemical vapor deposition (MOCVD) molecular beam epitaxy (MBE) or other epitaxial techniques. Often, an n-type layer (or layers) is deposited on the substrate, then an active region is deposited on the n-type layers, then a p-type layer (or layers) is deposited on the active region. The order of the layers may be reversed such that the p-type layers are adjacent to the substrate. Needed in the art are LED structures that increase the amount of light extracted from the device. 
   SUMMARY 
   In accordance with embodiments of the invention, a light emitting device includes a region of first conductivity type, a region of second conductivity type, an active region, and an electrode. The active region is disposed between the region of first conductivity type and the region of second conductivity type and the region of second conductivity type is disposed between the active region and the electrode. The active region has a total thickness less than or equal to about 0.25λ n  and has a portion located between about 0.6λ n  and 0.75λ n  from the electrode, λ n =λ vacuum /n, where n is the index of refraction in the region of second conductivity type. 
   In some embodiments, the active region includes a plurality of clusters, with a portion of a first cluster located between about 0.6λ n  and 0.75λ n  from the electrode and a portion of a second cluster located between about 1.2λ n  and 1.35λ n  from the electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a light emitting device according to embodiments of the present invention. 
       FIG. 2  illustrates a portion of the light emitting device of  FIG. 1  covered with an encapsulating gel. 
       FIG. 3  illustrates the angular distribution of flux of light emitted from III-nitride single quantum well silver/nickel contact light emitting devices having different amounts of material separating the active region from the reflective contact. The devices are fabricated on sapphire substrates and are packaged with a silicone encapsulating gel. The amount of material separating the active region from the reflective contact is expressed in terms of wavelength, λ n . 
       FIG. 4  illustrates extraction efficiency of top light as a function of separation between the active region and reflective contact for several devices. 
       FIG. 5  illustrates a method of determining the separation between the active region and reflective contact of a light emitting device. 
       FIG. 6A  illustrates one embodiment of active region  1  of  FIG. 1 .  FIG. 6B  illustrates the location of the device layers illustrated in  FIG. 6A . 
       FIG. 7A  illustrates an alternative embodiment of active region  1  of  FIG. 1 .  FIG. 7B  illustrates the location of the device layers illustrated in  FIG. 7A . 
       FIG. 8  illustrates an alternative embodiment of a light emitting device. 
       FIG. 9  is an exploded view of a packaged light emitting device. 
       FIG. 10  illustrates a thin film electroluminescent device according to embodiments of the present invention. 
   

   DETAILED DESCRIPTION 
   The examples described below are directed to semiconductor light emitting devices. Embodiments of the invention may be applicable to organic light emitting devices, or any other suitable flip chip device. 
     FIG. 1  illustrates a light emitting device according to embodiments of the present invention. A group of semiconductor layers including an n-type region  3 , a light emitting active region  1 , and a p-type region  5  are formed over a substrate  2 . The semiconductor layers may be, for example, III-nitride layers, III-phosphide layers, II-VI layers, or any other suitable material. Each of n-type region  3 , active region  1 , and p-type region  5  may include multiple layers of the same or different composition, thickness, and dopant concentration. A portion of p-type region  5  and active region  1  are removed to expose a portion of n-type region  3 . An n-electrode  10  is deposited on n-type region  3  and a p-electrode  4  is deposited on p-type region  5 . At least one of the p- and n-electrodes are highly reflective of light emitted by active region  1 . The device is physically mounted on and electrically connected to a submount  9  by interconnects  8 . 
     FIG. 8  illustrates an alternative embodiment of a semiconductor light emitting device. In the device of  FIG. 8 , p-type region  5  is separated from p-electrode  4  by a reflective surface  50  such as a distributed Bragg reflector (DBR). 
     FIG. 10  illustrates a thin film luminescent device such as an organic light emitting diode, according to embodiments of the present invention. A phosphor light emitting layer  304  is sandwiched between two insulators  303  and  305 . Contact is made to the device through metal layer  306  and transparent electrode  302 . Light is extracted through a glass superstrate  301 . Organic light emitting diodes are described in more detail in Kristiaan Neyts, “Microcavities for Electroluminescent Devices,” Chapter 4, Electroluminescence II, ed. Gerd Mueller, Semiconductors and Semimetals, Vol. 65. 
   Light extraction efficiency may be improved by controlling the placement of the light emitting layers relative to reflective layers in the device. In the device of  FIG. 1 , the placement of active region  1  is controlled relative to the highly reflective p-electrode  4 . In the device of  FIG. 8 , the placement of active region  1  is controlled relative to DBR  50 . In the device of  FIG. 10 , the placement of phosphor layer  304  is controlled relative to metal layer  306 . 
   Returning to the device of  FIG. 1 , assuming the p-electrode is a perfectly conducting metal, when the center of the active region is brought within approximately an odd multiple of quarter-wavelengths of light within the material ((2i+1)λ n /4, where i=0, 1, 2, . . . ) from the reflective p-electrode, constructive interference of the downward and upward traveling light results in a radiation pattern that emits power preferentially into the escape cone (ƒ˜0° as shown in  FIG. 1 ). This enhancement is in a direction close to normal to the semiconductor layers/substrate and is not susceptible to total internal reflection back into the semiconductor layers. Alternatively, slight detuning of the resonance condition, by moving the active region slightly closer to (or farther from) the p-electrode reflector, may be preferred to optimize the emission of light into the escape cone, and thus the total top surface extraction from the chip. For maximum efficiency in most applications, the distance between the active region and a perfectly conducting metal p-electrode should be approximately one quarter-wavelength. 
   Further retuning of the resonance condition for maximum extraction in a device with a nonideal metal contact depends on the phase shift of light reflecting off the metal. Methods for determining the phase shift of an actual reflective contact, then determining the optimal placement of an active region relative to that contact based on the phase shift are described below. Though the below description often uses the example of a III-nitride device formed on a sapphire substrate, it will be clear to a person of skill in the art that the methods described are readily applicable to other materials systems, other contact metals, and other growth substrates. 
   The total amount of light emitted from the LED (i.e., the total integrated flux) is the integrated flux emitted from the topside (towards the substrate) of the device added to the integrated flux emitted from the sides of the device. Side-emitted light is typically guided to the sides of the device by a waveguide created by reflective surfaces and various device layers having different indices of refraction. Waveguided light typically undergoes several reflections on its path to the side of the device, losing intensity with each reflection. In addition, light passing through the active region may be absorbed. Thus it is advantageous to extract as much light as possible from the topside of the device in the first pass, tending thereby to reduce internal losses and increase the total integrated flux. 
   Flipchip LEDs have a “top escape cone” near the active region such that light impinging on the topside from within the LED and lying within the escape cone exit directly from the topside of the device. For economy of language, we refer to the top escape cone merely as the “escape cone,” understanding thereby that maximum topside light emission is a significant LED performance goal. The escape cone is determined by several device parameters including the indices of refraction of the various layers within the device, according to Snell&#39;s law. Light beams impinging on the topside outside the escape cone undergo total internal reflection. Such internally reflected light typically exits from the side of the device or undergoes further internal reflections and loss of intensity within the device. Thus, one approach to increasing the intensity emerging from the topside of the LED is to increase the flux impinging on the topside that lies within the escape cone. 
     FIG. 2  depicts light escaping from a portion of the device illustrated in  FIG. 1 . The reflective positive ohmic contact  4  lies at a separation d from active region  1  and has p-type region  5  lying between the active region  1  and the contact  4 . Region  5  can comprise one layer or can comprise multiple sublayers having distinct compositions, doping characteristics and refractive indices from sublayer to sublayer or a gradation of compositions, electrical properties and optical properties throughout the thickness of p-type region  5 . 
   To be clear in our descriptions, we consider in detail the case of a III-nitride device with Al x In y Ga z N layers, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1, including a GaN base layer  3  and a single p-type layer  5 , each of base layer  3  and p-type layer  5  having a substantially uniform index of refraction throughout. Generalizations to layers having non-uniform indices of refraction (such as arising from multiple layers of different materials, gradations of optical properties, and the like) is straight-forward by the use of optical distances obtained by summing or integrating (physical thickness of layer i)/(index of refraction of layer i) over various layers of base layer material. Therefore, examples presented herein for layers having a uniform index of refraction are illustrative and not limiting. 
   Light generated in active region  1  and emerging from the topside of the LED passes through n-type region  3 , substrate  2  and encapsulating gel  7  and undergoes refraction at each, as depicted by beam  8  in  FIG. 2 . GaN base layer  3  has an index of refraction n 1 =2.4. Sapphire substrate  2  has n 2 =1.8 and a typical encapsulating gel  7  has n 3 =1.5. Thus, refraction away from the normal occurs as depicted in  FIG. 2 , causing light  8  to emerge from substrate  2  into encapsulating gel  7  with an angle from the normal of θ 3 . 
   As light travels from the site of its formation into the encapsulating gel  7  through successive regions of lower index of refraction ( 3 ,  2  and  7 ), the possibility of total internal reflection arises at each interface. That is, if beam  8  strikes the 3-2 or 2-7 interface from the higher index side at too glancing an angle, no light will enter encapsulating gel  7 . 
   Applying Snell&#39;s Law to  FIG. 2  gives n 1  sin θ 1 =n 2  sin θ 2 =n 3  sin θ 3 . The escape cone is determined by θ 3 =90°, or sin θ 1 (escape)=(n 3 /n 1 ). Using the above values for the indices of refraction for GaN, sapphire and encapsulating gel yields θ 1 (escape)≈38.7°. Thus, light striking the n 1 -n 2  interface from the n 2  side will not emerge from the topside of the device if the angle of incidence exceeds θ 1 (escape), about 38.7°. 
   Light emitted from electron-hole recombination occurring in the active region  1  can be directed into the transparent substrate directly, such as beam  6   d , or following reflection from ohmic contact  4  such as beam  6   r . The coherence length for light emitted in active region  1  is typically around 3 μm in GaN. Thus, if separation d is less than about 50% of the coherence length (d≲1.5 μm in GaN), strong interference between direct ( 6   d ) and reflected ( 6   r ) beams is expected to occur. The interference pattern is influenced by the distance between active region  1  and reflective contact  4 . 
   The reflected light  6   r  suffers a loss in intensity and a phase shift as it bounces off reflective contact  4 . To increase the light output from a flip chip and to reduce intensity loss to contact  4 , contact  4  may be chosen to have high reflectivity. For example, contact  4  may have a reflectivity greater than 50%, usually has a reflectivity greater than 80%, and, in some embodiments, preferably has a reflectivity greater than 90%. The phase shift of the reflected light  6   r  depends on the n and k values of the metals or metal alloys used in reflective contact  4 , and therefore, the phase shift will change depending on the metal type. This phase shift also influences the interference pattern. For a perfect conducting metal, the phase shift will be 180°. Generally, the contacts on real devices are not perfect conducting metals. 
   Once the phase shift of light reflected from reflective contact  4  is determined, the interference pattern of light escaping the device can be calculated as a function of the distance between active region  1  and reflective contact  4 . As described above, the critical angle for light escaping from a GaN/sapphire/encapsulating gel interface is 39 degrees, thus only the portion of light in GaN directed within this 39 degrees can escape through the topside of substrate  2 . One way to increase light extraction from the light emitting device is to concentrate light intensity within the escape cone. Since the interference patterns emerging from the active region are controlled by the distance between active region  1  and reflective contact  4  for a given contact material, the light intensity within the escape cone may be maximized by appropriately selecting the distance between active region  1  and reflective contact  4 . 
   The electric field of the directly emitted light  6   d  from a single quantum well active region is given by:
 
 {right arrow over (E)}   0   =w   0 *exp(− i{right arrow over (k)}*{right arrow over (x)} )  (1)
 
The electric field of the reflected light  6   r  from a single quantum well active region is given by:
 
 {right arrow over (E)}   R   =w   R *exp(− i{right arrow over (k)}*{right arrow over (x)} Φ+Φ′))  (2)
 
where w 0  is the amplitude of emitted light  6   d , w r  is the amplitude of reflected light  6   r , k is the direction vector, x is the position vector, Φ is the phase shift upon reflection off reflective contact  4 , and Φ′ is the phase shift due to the optical path length differences.
 
   The intensity in the device, as a function of angle, θ, is then given by:
 
| {right arrow over (E)}   Total | 2 =( {right arrow over (E)}   0   +{right arrow over (E)}   R )*( {right arrow over (E)}   0   +{right arrow over (E)}   R )= w   0   2   +w   R   2 +2 w   0   w   R  cos(Φ+Φ′).  (3)
 
   The phase shift due to optical path length difference Φ′ depends on the distance d between the single quantum well active region  1  and reflective contact  4 , the angle θ, the wavelength λ, and n (the index of refraction of the device layers, GaN in the above example) according to equation (4), where λ n =λ/n: 
   
     
       
         
           
             
               
                 
                   Φ 
                   ′ 
                 
                 = 
                 
                   
                     2 
                     ⁢ 
                     π 
                     ⁢ 
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         x 
                       
                       
                         λ 
                         n 
                       
                     
                   
                   = 
                   
                     2 
                     ⁢ 
                     π 
                     ⁢ 
                     
                       
                         
                           2 
                           ⁢ 
                           d 
                           * 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           θ 
                         
                         
                           λ 
                           n 
                         
                       
                       . 
                     
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   The phase shift due to reflection from reflective contact  4 , Φ, can be calculated from the n and k values of the metal in reflective contact  4  if the metal is known. See, for example, Max Born &amp; Emil Wolf, Principles of Optics, p. 628-630 (1980), which is incorporated herein by reference. If the n and k values of reflective contact  4  are not known, the phase shift Φ may be determined by, for example, the following method. A detector is positioned to collect light that is emitted through the substrate normal to a device under test, in a small collection cone (θ˜6°). A series of devices having a varying distance between active region  1  and reflective contact  4  are fabricated including the material with unknown phase shift as reflective contact  4 . If the wavelength of emitted light is constant, the extraction efficiency of the series of devices will only vary as a function of the distance d between active region  1  and reflective contact  4 . 
   According to equation 3, the minimum in the extraction efficiency is located where Φ+Φ′=m*π (m is an odd integer), where Φ′ is the phase difference due to the optical path differences between directly emitting light  6   d  and reflected light  6   r . With a series of devices each having a single quantum well placed at a different distance from reflective contact  4 , the external quantum efficiency of reflective contact  4  may be measured by the detector as a function of distance between the single quantum well active region  1  and reflective contact  4 . Using a second reflective contact  4  with a known phase shift, another curve of external quantum efficiency versus distance can be measured or calculated for a second set of devices with the same active-region-to-reflective-contact distances. External quantum efficiency is the product of internal quantum efficiency and extraction efficiency, EQE=C ext *IQE, thus to eliminate the wafer to wafer differences in the internal quantum efficiency and to obtain the ratio of the extraction efficiencies, the points on the first curve can be divided by the points on the second curve, such that:
 
 EQE   unknown   /EQE   known   =C   ext,unknown   /C   ext,known .  (5)
 
   Equation (3) may be substituted for C ext,unknown  and C ext,known , the measured values of external quantum efficiency for both devices may be substituted for EQE unknown  and EQE known , then Equation (5) may be solved for Φ unknown metal . Since d, θ, λ, m, Φ known metal , and reflectivity are known, Φ unknown metal  can be calculated for any unknown reflective contact  4 . Other methods may be used to determine Φ. See, for example, P. Maaskant et al.,  Fabrication of GaN - based Resonant Cavity LEDs , P HYS . S TAT . S OL . (submitted Feb. 19, 2002). 
   Once the phase shift due to reflection Φ is known, the intensity can be calculated as a function of distance d and angle θ using equation 3 above.  FIG. 3  depicts computer-generated examples of the top-side far-field emitted light intensity (or flux) as a function of the direction of emission into silicone with respect to the normal to the LED, θ 3 , defined in  FIG. 2 . The curves in  FIG. 3  are based on a single quantum well III-nitride device fabricated on a sapphire substrate with silicone as encapsulating gel  7 . Various values of d are depicted, from curve a of  FIG. 3  having d=0.5λ n , to curve i of  FIG. 3  having d=1.3λ n , where λ n  is the wavelength of the light in the semiconductor material separating the active region and the reflective contact. The units of flux are arbitrary as only the variations of flux with angle are of concern. The radiation patterns depend upon the distance, d, the wavelength of emitted light, and the effective indices of refraction of the materials through which the light passes in exiting from the LED, among other factors. The radiation patterns clearly change as d changes, changing the flux lying within the escape cone of 38.7°. 
   As illustrated in  FIG. 3 , a maximum in total emitted flux may occur for a radiation pattern not peaked about the central perpendicular axis of the light emitting region. That is, spacing the reflective plane from the light emitting region such that flux intensity is directed primarily normal to the surface (0 deg. in  FIG. 3  or “on-axis”) does not necessarily lead to maximum total emitted flux. Curve “f” in  FIG. 3  provides marked on-axis peaking of emitted radiation, but at a considerable sacrifice in the total emitted flux. Thus, spacing the light emitting region from the reflector so as to maximize on-axis light emission intensity may be suboptimal for obtaining maximal LED total flux. 
   The curves in  FIG. 3  may be integrated and added to light emitted from the side of the device to generate curve a of  FIG. 4 .  FIG. 4  illustrates, for four devices, computer generated plots of extraction efficiency of total flux of a 1 mm×1 mm flip chip as a function of distance between active region  1  and reflective contact  4  divided by λ n . Curve a in  FIG. 4  represents the results for a III-nitride single quantum well device with a sapphire substrate, a silicone encapsulating gel, and a reflective contact having silver/nickel contact.  FIG. 4  illustrates the second and third maxima. The second maximum in extraction efficiency occurs between about 0.6λ n  and about 0.75λ n , and the third maximum in extraction efficiency occurs between about 1.2λ n  and about 1.35λ n . Thus, for maximum extraction efficiency in a III-nitride device with a (thin Ni)/Ag contact, the center of a single quantum well active region  1  should be placed between about 0.5λ n  and about 0.8λ n , or between about 1.1λ n  and about 1.4λ n  from reflective contact  4 . 
   The location of the maxima in a curve of extraction efficiency vs. distance may shift as the wavelength of the light changes. The wavelength of the light affects the phase shift due to reflection from the metal, Φ. If Φ is calculated from known n and k values, the calculation takes into account the wavelength. If Φ is measured as described above, Φ must be measured for a particular wavelength to ensure proper selection of the distance between the active region and the reflective contact. The table below lists spacing ranges d between a single III-nitride quantum well and a Ni/Ag contact corresponding to the second, third, and fourth maxima in a plot of extraction efficiency vs. d for three different wavelength devices. 
                                           450 nm device   505 nm device   530 nm device       n = 2.4   d/λ n     d/λ n     d/λ n                    Optimal   0.5-0.8   0.53-0.83   0.55-0.85       Preferred   0.6-0.7   0.63-0.73   0.65-0.75       Optimal   1.05-1.35   1.08-1.38   1.1-1.4       Preferred   1.15-1.25   1.18-1.28   1.2-1.3       Optimal   1.6-1.9   1.63-1.93   1.65-1.95       Preferred   1.65-1.75   1.68-1.78   1.7-1.8                    
A first maximum generally lies between about 0.1λ n  and about 0.3λ n .
 
   Though the above example is directed to a III-nitride device with a nickel/silver contact, a silicone encapsulating gel, and a sapphire substrate, it will be apparent to a person of skill in the art that the method of choosing the spacing between the active region and reflective contact can be applied to other materials systems including but not limited to III-phosphide, III-arsenide, and II-VI, to other contact materials, to other encapsulating materials, and to other substrates. For example, a 450 nm III-nitride device with a pure silver contact has a second maximum in extraction efficiency between about 0.65λ n  and about 0.75λ n , a third maximum in extraction efficiency between about 1.15λ n  and about 1.38λ n , and a fourth maximum in extraction efficiency between about 1.73λ n , and about 1.85λ n . A 625 nm III-phosphide device (n=3.4) with a gold contact has a first maximum in extraction efficiency between about 0.1λ n  and about 0.3λ n , a second maximum in extraction efficiency between about 0.6λ n , and about 0.75λ n , a third maximum in extraction efficiency between about 1.1λ n  and about 1.25λ n , a fourth maximum in extraction efficiency between about 1.6λ n  and about 1.8λ n , and a fifth maximum in extraction efficiency between about 2.18λ n  and about 2.28λ n . 
     FIG. 5  illustrates a method for determining the spacing between the active region and reflective contact. First, in stage  202 , the escape cone is calculated using the index of refraction of the semiconductor material, the substrate, and the encapsulation material or air, if no encapsulation material is used. In stage  204 , the phase shift due to optical path length and phase shift due to reflection are then calculated or measured, as described above. The phase shift due to optical path length and phase shift due to reflection depend on the material used in the reflective contact and the wavelength of the light. In stage  206 , the radiation pattern is calculated for various spacings between the active region and reflective contact using the phase shifts determined in stage  204 , yielding a graph such as  FIG. 3 . The radiation pattern depends on the wavelength of light. In stage  208 , the extraction efficiency is calculated as a function of the distance d between the active region and the reflective contact. The extraction efficiency depends on the radiation pattern calculated in stage  206  and the escape cone calculated in stage  202 . An example of extraction efficiency as a function of d is illustrated in  FIG. 4 . The maxima in the plot of extraction efficiency as a function of d are identified in stage  210 , which determine the spacing between the active region and the reflective contact which outputs the most light. The method illustrated in  FIG. 5  is described in more detail in U.S. application Ser. No. 10/158,360, titled “Selective Placement Of Quantum Wells In Flipchip Light Emitting Diodes For Improved Light Extraction,” filed May 29, 2002, and incorporated herein by reference. 
   The method described in  FIG. 5  may be used to determine the separation between the reflective contact and active region of a III-phosphide device, where the device layers are Al x In y Ga z P, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1. III-phosphide devices often have an AlInP layer adjacent to the active region, and a GaInP contact layer adjacent to the reflective contact. The reflective contact is selected for ohmic contact with GaInP. Examples of suitable reflective contact materials are Al, Ag, and Au. The reflective contact metal may be alloyed to the GaInP, which may reduce the reflectivity of the contact. To alleviate a trade-off between ohmic properties and reflective properties, a two-part contact may be used. To form a two part contact, first a layer of suitable ohmic metal is deposited and alloyed to the GaInP. Then, the metal is etched into a pattern of fine lines whereby most of the surface area of the GaInP is revealed, the fine lines serving to conduct current into the device. The exposed areas of the GaInP are then chemically etched away revealing the AlInP cladding layer beneath, thereby eliminating a large part of the adsorbing GaInP from the optical path. Finally, a suitable reflector, chosen for its optical properties without regard to the resistivity of the contact it makes to the AlInP, is deposited over both the fine lines of the first metal and the exposed AlInP. In this manner, current is conducted into the device by the first metal, and a highly reflective second metal serves as a mirror. 
   Curve a of  FIG. 4A  presents data for a single quantum well active region. However, the methods described herein are not limited to single quantum well devices and can also be used in connection with multiple quantum well (MQW) active regions. For example, the center of brightness and/or physical center of MQW active regions may be placed at a separation corresponding to a maximum on the appropriate plot of extraction efficiency of top-side flux as a function of distance between the active region and the reflective contact. Such an embodiment is illustrated in  FIGS. 6A and 6B . The active region illustrated in  FIG. 6A  has two quantum well layers  15  separated by a barrier layer  17 . As illustrated in  FIG. 6B , the center of barrier layer  17 , which is the center of active region  1 , is located at the first peak of curve b of  FIG. 4 , the curve corresponding to a two quantum well active region. The first peak of curve b corresponds to the second maximum in a plot of extraction efficiency vs. distance between the center of the active region and the p-electrode, and is located at a distance of about 0.67λ n . 
   The extraction efficiency of a device can be improved by designing the active region and the layers between the active region and the reflective electrode such that each of the quantum wells are located as close as possible to a maximum on a plot of extraction efficiency vs. distance from the reflective electrode.  FIGS. 6A ,  6 B,  7 A, and  7 B illustrate examples of devices so designed. 
   The device illustrated in  FIGS. 6A and 6B  may be, for example, a III-nitride device emitting light at a wavelength ranging from UV through green. Each of quantum wells  15  may have a thickness ranging from about 10 angstroms to about 60 angstroms, usually has a thickness between about 15 angstroms to about 40 angstroms, and preferably has a thickness of about 30 angstroms. The composition of quantum wells  15  depends on the color of light to be emitted by the device. Each of quantum wells  15  need not have the same thickness and composition. Barrier layer  17  may have a thickness ranging from about 50 angstroms to about 200 angstroms, and usually has a thickness of about 85 angstroms. A barrier layer in a shorter wavelength device may be thinner than a barrier layer in a longer wavelength device. P-type region may contain, listed in order from the active region to the p-electrode, an AlGaN confining layer  5   a , a first GaN layer  5   b , and a second GaN contact layer  5   c . In some embodiments, confining layer  5   a  may have a thickness between 100 angstroms and 1000 angstroms, and usually has a thickness between about 100 angstroms to about 400 angstroms; first GaN layer  5   b  may have a thickness between about 100 angstroms and about 1000 angstroms, and usually has a thickness between about 400 angstroms and about 900 angstroms; and second GaN contact layer  5   c  may have a thickness between about 50 angstroms and about 500 angstroms, and usually has a thickness between about 50 angstroms and about 250 angstroms. The reflective p-electrode in the device illustrated in  FIGS. 6A and 6B  may be, for example, a multi-layer electrode with a thin layer of Ni sandwiched between GaN contact layer  5   c  and a thick layer of Ag. 
   In the embodiment illustrated in  FIGS. 6A and 6B , the entire active region is located surrounding a peak on a curve of extraction efficiency vs. distance between the active region and the p-electrode. Though the active region is placed at a distance corresponding to the second maximum on the curve of extraction efficiency vs. distance in  FIG. 6B , the placement of the active region may correspond to the third or a higher local maximum. Usually the active region is not placed near the first maximum, since placement of the active region at the first maximum may result in a p-type region that is too thin. Usually the fourth and higher maxima are not used as such placement may result in a p-type region that is too thick, and a decrease in extraction efficiency. The p-type region is generally fabricated at a higher temperature than the quantum wells, thus the fabrication of a thick p-type region may result in fabrication conditions that damage the quantum wells. In addition, the extraction efficiency beyond the fourth local maximum is significantly lower than for the second, third, and fourth local maxima. 
   Though two quantum wells are illustrated in  FIG. 6A , more or fewer quantum wells may be used provided the portion of the active region furthest from the peak is reasonably close to the peak. In a device with three quantum wells separated by two barrier layers, the center of the middle quantum well is optimally located at a distance from the p-electrode corresponding to a peak on a curve of extraction efficiency vs. distance. 
   In the embodiment illustrated in  FIGS. 6A and 6B , the total thickness of the active region is limited such that all of the active region is close to the peak on the curve of extraction efficiency vs. distance between the active region and the p-electrode. For example, the total thickness of the active region may be selected such that the active region may not be larger than 0.35λ n  and is usually not larger than 0.15λ n , and usually does not extend beyond 0.05λ n , on either side of the peak. The four curves in  FIG. 4  demonstrate that as the active region becomes more compact, the total extraction efficiency improves.  FIG. 4  illustrates the extraction efficiency of four devices, a single quantum well device (curve a), a two quantum well device with a thin barrier (curve b), a two quantum well device with a thick barrier (curve c), and a four quantum well device. At the first peak shown in  FIG. 4 , the device with the most compact active region (the single quantum well device) has the highest extraction efficiency, while the device with the thickest active region (the four quantum well device) has the lowest extraction efficiency, assuming uniform filling of carriers in the quantum wells. 
   In embodiments with few (e.g. 1-3) quantum wells, the center of brightness of the active region is expected to be the physical center of the active region. In active regions with quantum wells of different composition or thickness, or with more than three quantum wells, the center of brightness may not be the physical center of the active region. In such devices, the center of brightness of the active region may be located at a peak on a curve of extraction efficiency vs. distance. 
   In some embodiments, the quantum wells in the active region may be clustered around peaks on a curve of extraction efficiency vs. distance, with thin barrier layers separating the quantum wells in each cluster and thick barrier layers separating the clusters. The center of brightness of each cluster may be placed at a separation corresponding to a maximum on the appropriate plot of extraction efficiency of top-side flux as a function of distance between the active region and the reflective contact.  FIGS. 7A and 7B  illustrate such a device. The device illustrated in  FIG. 7A  has two clusters of quantum wells, each with two quantum wells. The first cluster includes two quantum wells  15   a  separated by a barrier layer  17   a , the center of which is located at the first peak shown in  FIG. 4  (the second local maximum). The second cluster includes two quantum wells  15   b  separated by a barrier layer  17   b , the center of which is located at the second peak shown in  FIG. 4  (the third local maximum). The two clusters are separated by a thick barrier layer  17   c . The clusters may have more or fewer than two quantum wells, and need not have the same number of quantum wells. 
     FIG. 9  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 . Alternatively, slug  100  may provide a pedestal without a reflector cup. 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. For example, the invention is not limited to the contact materials and semiconductor materials described in the examples. Specifically, though many of the examples are directed to III-nitride flip chip devices with silver contacts, the invention may also be applicable to other reflective contacts and to other materials systems, such as III-phosphide, III-arsenide, and II-VI materials. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.