Abstract:
An InAsP active region for a long wavelength light emitting device and a method for growing the same are disclosed. In one embodiment, the method comprises placing a substrate in an organometallic vapor phase epitaxy (OMVPE) reactor, the substrate for supporting growth of an indium arsenide phosphide (InAsP) film, forming a quantum well layer of InAsP, and forming a barrier layer adjacent the quantum well layer, where the quantum well layer and the barrier layer are formed at a temperature of less than 520 degrees C. Forming the quantum well layer and the barrier layer at a temperature of less than 520 degrees C. results in fewer dislocations by suppressing relaxation of the layers. A long wavelength active region including InAsP quantum well layers and InGaP barrier layers is also disclosed.

Description:
TECHNICAL FIELD 
     The invention relates generally to light emitting devices, and, more particularly, to an indium arsenide phosphide (InAsP) quantum well active region for a long wavelength light emitting device and a method for producing same. 
     BACKGROUND OF THE INVENTION 
     Light emitting devices are used in many applications including optical communication systems. Optical communication systems have been in existence for some time and continue to increase in use due to the large amount of bandwidth available for transporting signals. Optical communication systems provide high bandwidth and superior speed and are suitable for efficiently communicating large amounts of voice and data over long distances. Optical communication systems that operate at relatively long wavelengths on the order of 1.3 micrometers (μm) to 1.55 μm are generally preferred because optical fibers generally have their lowest attenuation in this wavelength range. These long wavelength optical communication systems include a light source capable of emitting light at a relatively long wavelength. Such a light source can be, for example, a vertical-cavity surface-emitting laser (VCSEL), an edge-emitting laser, or other types of light sources. 
     These light sources include an active region into which carriers, i.e., electrons and holes, are injected. The holes and electrons recombine in the active region and emit coherent light at a particular wavelength. One manner of forming an active region in a light emitting device is to form a quantum well layer and sandwich the quantum well layer between a pair of adjacent barrier layers. The quantum well layer and the adjacent barrier layers form what is referred to as a quantum well. The quantum well layer typically comprises a low bandgap semiconductor material, while the barrier layers typically have a bandgap higher than the bandgap of the quantum well layers. In this manner, when the laser diode is subject to forward bias, electrons and holes are injected into and trapped in the quantum well layer and recombine to emit coherent light at a particular wavelength. Generally, more than one quantum well is formed in a light emitting device. The optimum number of quantum wells is dependent upon the material system from which the quantum wells are grown and on the required optical gain 
     A light emitting device has a threshold current (I th ), which is the current at which lasing action begins. The relationship between temperature and threshold current of a light emitting device is exponential, and can be characterized by the formula I th  ∝ exp T/T 0 , where T 0  is the characteristic temperature of the light emitting device. 
     A quantum well layer for a 1.5 μm wavelength light emitting device can be formed using indium gallium arsenide phosphide (InGaAsP), which can be formed over an indium phosphide (InP) substrate. Unfortunately, for a conventional light emitting device having an InGaAsP quantum well layer, the value of T 0  is small, resulting in a rapid increase in the value of I th  when temperature rises. This occurs mainly due to Auger recombination and carrier leakage, as known to those skilled in the art. Therefore, InGaAsP quantum well layers are not particularly well suited for 1.5 μm wavelength output light emitting devices in which a low threshold current and high characteristic temperature are desired. 
     A quantum well layer for a 1.3 μm wavelength light emitting device can be formed using InAsP, which can be formed over an indium phosphide (InP) substrate, and which has a higher characteristic temperature, T 0 , than InGaAsP. Depending on the arsenic fraction of a quantum well layer formed using InAsP, the operating wavelength of a light emitting device can be extended to approximately 1.3 μm. However, it would be desirable to extend the wavelength in which an InAsP quantum well layer generates photons to approximately 1.5 μm. Obtaining such an output wavelength from an InAsP quantum well layer suggests that the arsenic fraction in the InAsP layer approach 60%. Unfortunately, when using conventional processing techniques, such an arsenic fraction results in a significant lattice mismatch when the InAsP is grown over InP. The lattice mismatch can approach 2%. Thus, the InAsP quantum well layers are highly strained. These highly strained quantum well layers may relax during, or after their formation, thereby resulting in the formation of dislocations in the InAsP layer. Dislocations are stress fractures in the epitaxial film and can degrade the optical performance of the material by destroying the material&#39;s luminescence efficiency, sometimes referred to as photoluminescence intensity, thereby making the material unacceptable for use in a light emitting device. 
     Forming an InAsP layer is possible using a technique known as organometallic vapor phase epitaxy (OMVPE), sometimes referred to as metal organic chemical vapor deposition (MOCVD). OMVPE uses liquid or solid chemical precursors, through which a carrier gas is passed, to generate a chemical vapor that is passed over a heated semiconductor substrate located in a reactor. Conditions in the reactor are controlled so that the combination of vapors forms an epitaxial film as the vapors pass over the substrate. OMVPE is an economical and well established technology for growing epitaxial films. 
     Unfortunately, as mentioned above, growing high optical quality InAsP is difficult because, when using conventional growth parameters, the arsenic fraction required for light emission at 1.5 μm results in dislocations in the epitaxial material sufficient to render the material unusable for a light emitting device. 
     Therefore, it would be desirable to economically mass produce a long-wavelength light emitting device having an InAsP quantum well layer using OMVPE. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention provide several methods for using OMVPE to grow high quality, long wavelength light emitting active regions. In one embodiment, the invention is a method for producing an active region for a long wavelength light emitting device, comprising placing a substrate in an organometallic vapor phase epitaxy (OMVPE) reactor, the substrate for supporting growth of an indium arsenide phosphide (InAsP) film, forming a quantum well layer of InAsP, and forming a barrier layer adjacent the quantum well layer, where the quantum well layer and the barrier layer are formed at a temperature of less than 520 degrees C. Forming the quantum well layer and the barrier layers at a temperature of less than 520 degrees C. results in fewer dislocations by suppressing relaxation of the layers. 
     Other features and advantages in addition to or in lieu of the foregoing are provided by certain embodiments of the invention, as is apparent from the description below with reference to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, as defined in the claims, can be better understood with reference to the following drawings. The components within the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the present invention. 
         FIG. 1A  is a schematic view illustrating the relevant portions of an exemplary edge emitting laser constructed in accordance with an aspect of the invention. 
         FIG. 1B  is a schematic view illustrating the active region of the laser of  FIG. 1A . 
         FIG. 1C  is a schematic view illustrating the quantum well layer and associated barrier layers of  FIG. 1B . 
         FIG. 2A  is a schematic diagram illustrating an OMVPE reactor in which a laser including the active region of  FIG. 1B  can be grown. 
         FIG. 2B  is a detailed view of the laser shown in  FIG. 2A  partway through the fabrication process. 
         FIG. 3  is a graphical illustration depicting the room temperature photoluminescence intensity obtained from a 1.5 μm InAsP quantum well layer-InGaP barrier layer active region structure of  FIG. 1B . 
         FIG. 4  is a graphical illustration showing the x-ray diffraction analysis for an InAsP quantum well layer-InGaP barrier layer active region of  FIG. 1B . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While described below using an example of an edge emitting laser that incorporates InAsP quantum well layers, other device structures can benefit from the invention. For example, a vertical-cavity surface-emitting laser (VCSEL) including high quality InAsP quantum well epitaxial layers can be economically fabricated using the concepts of the invention. 
       FIG. 1A  is a schematic view illustrating the relevant portions of an exemplary edge emitting laser  100  constructed in accordance with an aspect of the invention. Some of the elements of the laser  100  are omitted for simplicity. The laser  100  comprises an N-type, sulfur (S) doped, indium phosphide (InP) substrate  102  over which an n-type 2 μm thick, selenium (Se) doped, InP cladding layer  104  is formed. The cladding layer  104  is formed using a high bandgap, low refractive index material. 
     A lower separate confinement heterostructure (SCH) layer  106  is formed over the cladding layer  104 . The lower SCH layer  106  is preferably formed of a 120 nanometer (nm) thick layer of indium gallium arsenide phosphide (InGaAsP) having a bandgap wavelength of approximately 1.15 μm. The combination of the cladding layer  104  and the lower SCH layer  106  acts as a waveguide and provides optical confinement for the light generated in the active region to be described below. The composition of the lower SCH layer is preferably In 0.85 Ga 0.15 As 0.35 P 0.65 . The composition of the InGaAsP in the lower SCH layer is chosen to provide the desired optical confinement and carrier confinement properties. 
     In accordance with an aspect of the invention, an active region  150  comprising alternating indium arsenide phosphide (InAsP) quantum well layers and indium gallium phosphide (InGaP), or indium gallium arsenide phosphide (InGaAsP), barrier layers is then formed over the lower SCH layer  106 . The InAsP quantum well layers and the InGaP barrier layers form a multiple quantum well (MQW) structure. As will be described in detail below, the InAsP quantum well layers and the InGaP barrier layers are grown in an OMVPE reactor at a low growth temperature. The low growth temperature minimizes the formation of dislocations in the quantum well layers and allows the operating wavelength of the laser  100  to be extended to the 1.5 μm range. 
     A 120 nm thick upper SCH layer  108  is formed over the active region  150 . The upper SCH layer  108  is constructed similarly to the lower SCH layer  106 . A p-type, 0.1 μm thick spacer layer  112  of zinc (Zn) doped InP is formed over the upper SCH layer  108 . The spacer layer  112  defines the distance between the upper SCH layer  108  and an etch stop layer  114 . 
     The etch stop layer  114  is formed using InGaAsP and is formed over the spacer layer  112 . Preferably the etch stop layer  114  is approximately 20 nm thick. The etch stop layer  114  is used to stop an appropriate etchant at that layer if the formation of a ridge waveguide is desired. 
     A p-type 2 μm thick, Zn doped, InP cladding layer  116  is formed over the etch stop layer  114 . The combination of the cladding layer  116  and the upper SCH layer  108  acts as a waveguide and provides optical confinement for the light generated in the active region  150 . The cladding layers  104  and  116  are high bandgap, low refractive index material layers that confine carriers that are injected into the active region  150  and help to confine the light generated in the active region  150  to the upper and lower SCH layers  106  and  108 . 
     A cap layer  118  comprising a 0.1 μm thick layer of Zn doped InGaAs is formed over the cladding layer  116  and provides good ohm contact between the cladding layer  116  and metal contacts (not shown). 
     In operation, when the laser  100  is forward biased, holes from the p-type material above the active region  150  and electrons from the n-type material below the active region  150  are launched into the active region, where they settle in the low bandgap InAsP quantum well layers (to be described below). In this manner, light is generated in the active region  150 . The light is confined by the waveguide, which is formed by the MQW structure in the active region  150 , and the upper and lower SCH layers  106  and  108 , which are surrounded by the cladding layers  104  and  116 , respectively. 
     In accordance with an embodiment of the invention, InAsP quantum well layers and InGaP barrier layers that are formed as part of the active region  150  are grown in an OMVPE reactor at a reduced temperature, as will be described below with particular reference to  FIGS. 2A and 2B . 
       FIG. 1B  is a schematic view illustrating the active region  150  of the laser  100  of  FIG. 1A . As shown in  FIG. 1B , the active region  150  includes a number of quantum wells, an exemplary one of which is illustrated using reference numeral  152 . 
     The quantum well  152  includes a quantum well layer  155  sandwiched between two barrier layers  154  and  156 . In a preferred embodiment, the quantum well layer  155  is InAs 0.6 P 0.4  and is grown in a thickness of approximately 7.5 nm at a growth temperature of less than about 520 degrees C. The barrier layers  154  and  156  are each InGa 0.15 P 0.85  and are grown in a thickness of approximately 12.5 nm at a growth temperature of less than about 520 degrees C. By growing the barrier layers  154  and  156  and the quantum well layer  155  at less than about 520 degrees C., the quantum well  152  can sustain a high level of strain without forming dislocations when the material layers relax. 
       FIG. 1C  is a schematic view illustrating the quantum well layer  155  and associated barrier layers  154  and  156  of  FIG. 1B . The quantum well layer  155  is in a condition referred to as “compressive” strain with respect to the InP substrate  102  ( FIG. 1A ). The bulk lattice constant of the quantum well layer  155  is larger than the lattice parameter of the InP substrate  102  ( FIG. 1A ), so that when grown over the InP substrate  102 , the quantum well layer  155  is compressively strained. The barrier layers  154  and  156  are in a condition referred to as “tensile” strain with respect to the InP substrate  102  ( FIG. 1A ). The bulk lattice constant of the barrier layers  154  and  156  is smaller than the lattice parameter of the InP substrate  102  ( FIG. 1A ), so that when grown over the InP substrate  102 , the barrier layers  154  and  156  are in tensile strain. Accordingly, the tensile strain of the barrier layers  154  and  156  “balance” the compressive strain of the quantum well layer  155 , resulting in what is referred to as a “strain balanced” condition. When a strain balanced quantum well  152  is grown at a lower than conventional temperature, the luminescence efficiency and the spectral linewidth (a measure of the optical quality of the laser  100 ) are excellent. Because it is desirable to have slightly less tensile strain in the barrier layers than compressive strain in the quantum well layers, the barrier layers  154  and  156  may be grown thicker than the quantum well layer  155 . 
     The structure of the quantum well  152  allows very high energy band offset, which helps to reduce leakage current in the active region  150 . For example, the bandgap energy (E g ) of the InAsP quantum well layer  155  is 0.73 electron volts (eV) (λ=1.7 μm) and the bandgap energy (E g ) of the InGaP barrier layer  154  is 1.45 eV (λ=0.85 μm). This results in a bandgap energy difference between the quantum well layer and the barrier layer of more than 600 millielectron Volts (meV), which is significantly higher than in conventional structures. The quantum well  155  exhibits a very high electron confinement barrier, which helps to eliminate carrier leakage in the laser  100  and leads to higher operating efficiency and a lower threshold current, due to a higher characteristic temperature, T 0  than an InGaAsP quantum well 
       FIG. 2A  is a schematic diagram  200  illustrating an OMVPE reactor  210  in which a laser including the active region  150  can be grown. Many of the details of an OMVPE reactor are omitted for clarity, as they are known in the art. A reactor controller  215  is coupled to the reactor  210  via connection  217 . The reactor controller can control various operating aspects and parameters of the reactor  210 . As will be described in greater detail below, the reactor controller  215  can be used to control, among other parameters, the temperature in the reactor  210  during epitaxial growth. 
     To facilitate OMVPE epitaxial growth, a carrier gas is bubbled through the constituent precursor compounds so that a saturated vaporous precursor is created for each compound. After the carrier gas is bubbled through the constituent precursor compounds, the saturated vaporous precursors are then diluted with other gasses as is known in the art. The vaporous precursors are transported into the reactor by the carrier gas. The vaporous precursors are pyrolized inside the reactor when they pass over a heated substrate wafer, yielding the constituent atomic elements. These elements are deposited on the heated substrate wafer, where they bond to the underlying crystal structure of the substrate wafer, thereby forming an epitaxial layer. 
     In the example shown in  FIG. 2A , and to facilitate the growth of an InAsP quantum well layer and an InGaP barrier layer, the vaporous precursors  214  may include arsine (AsH 3 ), the arsenic precursor, phosphine (PH 3 ), the phosphorus precursor, trimethylgallium (TMGa), the gallium precursor, trimethylindium (TMIn), the indium precursor, and a carrier gas. Trimethylgallium is also known in the art as an alkyl-gallium, which has the chemical formula (CH 3 ) 3 Ga, and trimethylindium is also known in the art as an alkyl-indium, which has the chemical formula (CH 3 ) 3 In. 
     Other vaporous precursors can also be used depending on the desired composition of the epitaxial layers. The carrier gas can be, for example, hydrogen (H 2 ) or nitrogen (N 2 ). The carrier gas is bubbled through these chemical precursors. These flows are subsequently combined into a vaporous mixture of the appropriate concentrations, and carried into the OMVPE reactor  210 . 
     To achieve optimum layer thickness, composition uniformity and interface abruptness, additional carrier gas may be introduced to increase the flow velocity. A heated susceptor  212  comprises a heated surface (typically graphite, silicon carbide, or molybdenum) on which a crystalline substrate  102  resides. The cladding layer  110 , lower SCH layer  106 , active region  150 , including barrier layers alternating with quantum well layers, the upper SCH layer  108 , the spacer layer  112 , the etch stop layer  114 , the cladding layer  116  and the cap layer  118  are grown over the crystalline substrate  102  and form the laser  100  ( FIG. 1A ). 
     In accordance with the operation of an OMVPE reactor  210 , the vaporous precursors travel into the OMVPE reactor, as indicated using arrow  216 , and eventually pass over the heated substrate  102 . As the vaporous precursors pass over the heated substrate  220 , they are decomposed by pyrolysis and/or surface reactions, thereby releasing the constituent species on the substrate surface. These species settle on the heated surface of the substrate  102 , where they bond to the underlying crystal structure. In this manner, epitaxial growth occurs in the OMVPE reactor  210 . 
       FIG. 2B  is a detailed view of the laser  100  shown in  FIG. 2A  partway through the fabrication process. The epitaxial layers that form the laser  100  are deposited using OMVPE. In accordance with an embodiment of the invention, and to grow an active region including InAsP quantum wells, the temperature in the OMVPE reactor  210  is maintained at or below approximately 520 degrees C. during the growth of the InGaP barrier layers and the InAsP quantum well layers, while the reactor  210  is maintained at a temperature of approximately 610 to 640 degrees C. when the other layers are grown. This results in fewer dislocations by suppressing relaxation of the layers. 
     Lowering the temperature in the OMVPE reactor  210  ensures that the InAsP material in the quantum well layers and the InGaP material in the barrier layers are of high optical quality and emit light in the 1.5 nm wavelength range. 
     As shown in  FIG. 2B , a first InGaP barrier layer  154  is grown over the lower SCH layer  106 . Alternatively, the barrier layer  154  can be formed using InGaAsP, which may provide improved control of the bandgap and strain in the quantum well  152 . 
     Although the lower SCH layer  106  is formed using InGaAsP, this layer would not function as a first barrier layer. In a preferred embodiment, the lower SCH layer would be formed using In 0.85 Ga 0.15 As 0.35 P 0.65  while the first barrier layer would be formed using In 0.8 Ga 0.2 As 0.1 P 0.9  if InGaAsP were used. These different material compositions are used because of the different strain and bandgap requirements of the lower SCH layer  106  and the barrier layer  154 . 
       FIG. 3  is a graphical illustration  300  depicting the room temperature photoluminescence obtained from an InAsP quantum well layer-InGaP barrier layer active region  150  of  FIG. 1B  structured to generate light at approximately 1.5 μm. The vertical axis  302  represents photoluminescence intensity and the horizontal axis  304  represents wavelength (λ) in nanometers (nm). The curve  310  represents significant photoluminescence centered at a wavelength of 1,486 nm, with a full width half maximum (FWHM) value of 61 nm (34 meV). From the graphical illustration  300  it is clear that, at room temperature, a high quality 1.5 μm multiple quantum well structure can be grown as described above with respect to  FIGS. 2A and 2B . 
       FIG. 4  is a graphical illustration  400  showing the x-ray diffraction analysis for an InAsP quantum well layer-InGaP barrier layer active region  150  of  FIG. 1B . The vertical axis  402  represents diffracted intensity and the horizontal axis  404  represents relative rock angle, also known as diffraction angle. As shown, the curve  410  illustrates a superior multiple quantum well structure grown as described above in  FIGS. 2A and 2B . 
     It will be apparent to those skilled in the art that many modifications and variations may be made to the preferred embodiments of the present invention, as set forth above, without departing substantially from the principles of the present invention. For example, many light emitting devices can benefit from the economical growth of an InAsP quantum well active region. The InAsP active region, including InAsP quantum well layers can be used in edge-emitting as well as surface-emitting lasers. All such modifications and variations are intended to be included herein within the scope of the present invention, as defined in the claims that follow.