Abstract:
A method of producing a semiconductor device can include receiving a Group III-N wafer as a substrate, initiating a first inversion domain boundary layer to form a thin etch stop layer, terminating the etch stop layer with a second inversion domain boundary layer, and subsequently continuing the active region growth.

Description:
GOVERNMENT FUNDING 
       [0001]    This invention was made with U.S. Government support through the Defense Advanced Research Projects Agency (DARPA) under Army Research Laboratory (ARL) Cooperative Agreement # W911NF-10-02-0102. The Government has certain rights in this invention. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosed technology relates to semiconductors and, more particularly, to the design and manufacturing of certain semiconductor devices such as light emitting devices (LEDs), for example. 
       BACKGROUND 
       [0003]    In many light emitting device (LED) designs, it is necessary to extract light from the backside of the device because layers above the active light-emitting layers of the device are usually, at least to some extent, optically-absorbing. In the case of laser diodes, for example, an optically-absorbing substrate generally contributes to waveguide materials loss, thus increasing the amount of gain required to achieve lasing. Aluminum nitride (AlN) substrates are electrically insulating, so leaving such substrates on the device may preclude vertical injection laser diode architectures having backside contacts. In addition, some AlN substrates may contain a high concentration of impurities. These impurities can give rise to a high level of optical absorption. Such absorption can reduce the efficiency of light extraction from the backside of the LED. In such designs, it is usually useful to be able to remove this AlN substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a schematic figure showing the relation between crystallographic orientation and chemical structure of wurtzite aluminum nitride (AlN) in accordance with certain embodiments of the disclosed technology. 
           [0005]      FIG. 2  illustrates an etch stop layer defined by two c-plane inversion domain boundaries in accordance with certain embodiments of the disclosed technology. 
           [0006]      FIG. 3  illustrates an example of a first method of producing a semiconductor device, e.g., an LED, in accordance with certain embodiments of the disclosed technology. 
           [0007]      FIG. 4  illustrates an example of a second method of producing a semiconductor device, e.g., an LED, in accordance with certain embodiments of the disclosed technology. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    Embodiments of the disclosed technology generally include one or more processes in which etch stop layers may be created by inverting the polarity of the crystal and leveraging certain properties of the material, e.g., that etching may occur on only one surface of the material. In certain embodiments, bulk aluminum nitride (AlN) material, which generally has a natural polarity with one surface of the crystal being a nitrogen-terminated face or nitrogen-face (N-face) and the opposing surface being an aluminum-terminated face or aluminum-face (Al-face), may be used as a substrate. In certain alternative embodiments, aluminum gallium nitride (AlGaN) may be used as a substrate. 
         [0009]      FIG. 1  is a schematic figure showing the relation between crystallographic orientation and chemical structure of wurtzite AlN in accordance with certain embodiments of the disclosed technology. In the example, the N-face and the Al-face are both defined. At the Al-face, the [0001] direction generally points out of the material and into the upward space outside the material. At the N-face, the [000-1] direction generally points out of the material and into the downward space outside the material. 
         [0010]    In the example, the nitrogen-terminated face may be readily etched while the aluminum-terminated face is generally not attacked by the etchant. Epitaxial growth of device structures may be initiated on the aluminum-terminated face because growth on this face generally produces better quality crystals. The exposed backside of the substrate, therefore, has the nitrogen-terminated face. The AlN substrate may thus be removed by etching the nitrogen-terminated face. 
         [0011]    The polarity of the AlN substrate can usually be reversed during molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD) growth by controlling the relative flux of the Al and N during exposure of the growth surface to magnesium (Mg). This typically results in a Mg-induced reversal of polarity at the interface and thus creates an effective etch stop layer by presenting an Al-face to the etch front as it reaches the inversion layer. 
         [0012]      FIG. 2  illustrates an etch stop layer defined by two c-plane inversion domain boundaries (IDBs) in accordance with certain embodiments of the disclosed technology. In the example, Region A can be attacked and removed by the etchant, whereas Region B, the etch stop layer, cannot be etched. In the etch stop layer, the etching rate is so low as to result in a negligible loss of material during the time the etchant is in contact with the surface. Region B may be made thin by placing two Mg-induced IDBs (IDB1 and IDB2) a short distance L apart. The active region may be grown within region C. In general, IDB1 and IDB2 may be structurally inequivalent. For example, IDB1 may invert the polarity so that the growth surface is changed from an Al-face to a N-face, and IDB2 may invert the polarity so that the growth surface is changed from the N-face back to the Al-face. 
         [0013]    The etch stop layer, e.g., Region B, may be relatively thin. Subsequent to the initiation of this layer, another layer, e.g., Region C, having the original polarity may be initiated and grown by a similar manipulation of the Mg, Al, and N flows. This layer may then present an Al growth surface for epitaxial growth of the device structure. 
         [0014]      FIG. 3  illustrates a first example of a method  300  of producing a semiconductor device, e.g., an LED, in accordance with certain embodiments of the disclosed technology. At  302 , a substrate, e.g., an AlN substrate, AlGaN substrate or other suitable substrate, having (0001) and (000-1) surfaces is received or otherwise provided. 
         [0015]    At  304 , a layer of material is grown on the substrate by directing a flux of Al atoms, or Al-containing molecules such as tri-methyl-aluminum, and a flux of N atoms, or N-containing molecules such as N 2  or NH 3 , onto the (0001) surface of the substrate. In certain situations, where the grown material is Al x Ga 1-x N, a flux of Ga atoms or Ga-containing molecules would generally be present. Here, the growth surface orientation (GSO) is (0001). The material present following completion of the steps at  302  and  304  may constitute region A as shown in  FIG. 2 . 
         [0016]    At  306 , the polarity of the grown material is inverted, e.g., by temporarily adding a flux of magnesium or magnesium-containing molecules. The GSO is thus (000-1). For example, the flux of magnesium or magnesium-containing molecules may be selected such that approximately 1.5 monolayers of magnesium is incorporated into the material, which can result in the inversion of the polarity. Alternatively, the inversion of polarity may be accomplished by temporarily adding a flux of oxygen or oxygen-containing molecules, which may be selected such that approximately 1.5 monolayers of oxygen is incorporated into the material. 
         [0017]    At  308 , a growth is caused to occur on the (000-1) surface. This growth may include a layer of material, e.g., AlGaN, that is sufficiently thick to act as an etch stop layer. The thickness L of such an etch stop layer can be approximately 10 nm, for example. 
         [0018]    At  310 , the polarity of the grown material is inverted back such that growth may now take place on the (0001) surface. The GSO is thus (0001). The inversion of polarity may be accomplished by temporarily adding a flux of magnesium atoms or magnesium-containing molecules, for example. 
         [0019]    At  312 , one or more active regions, e.g., layers in which light may be emitted, of a semiconductor device, such as an LED or laser diode device, are grown on the (0001) surface. 
         [0020]    At  314 , the (000-1) surface of the substrate is exposed to an etchant, which may be used to remove the material in a certain region, e.g., region A in  FIG. 2 . Alternatively or in addition thereto, the etching may be used to remove at least substantially all of the substrate or all of the substrate and at least substantially all of the material grown at  304 . In certain embodiments, the etching may stop automatically after the etchant has removed all of the material up to the beginning of the etch stop layer. 
         [0021]      FIG. 4  illustrates a second example of a method  400  of producing a semiconductor device, e.g., an LED, in accordance with certain embodiments of the disclosed technology. At  402 , a substrate, e.g., an AlN substrate, AlGaN substrate or other suitable substrate, having (0001) and (000-1) surfaces is received or otherwise provided. 
         [0022]    At  404 , a layer of material is grown on the substrate by directing a flux of Al atoms, or Al-containing molecules such as tri-methyl-aluminum, and a flux of N atoms, or N-containing molecules such as N 2  or NH 3 , onto the (0001) surface of the substrate. In certain situations, where the grown material is Al x Ga 1-x N, a flux of Ga atoms or Ga-containing molecules would generally be present. Here, the growth surface orientation (GSO) is (0001). The material present following completion of the steps at  402  and  404  may constitute region A as shown in  FIG. 2 . 
         [0023]    At  406 , the polarity of the grown material is inverted, e.g., by temporarily adding a flux of magnesium or magnesium-containing molecules. The GSO is thus (000-1). For example, the flux of magnesium or magnesium-containing molecules may be selected such that approximately 1.5 monolayers of magnesium is incorporated into the material, which can result in the inversion of the polarity. Alternatively, the inversion of polarity may be accomplished by temporarily adding a flux of oxygen or oxygen-containing molecules, which may be selected such that approximately 1.5 monolayers of oxygen is incorporated into the material. 
         [0024]    At  408 , one or more active regions, e.g., layers in which light may be emitted, of a semiconductor device, such as an LED or laser diode device, are grown on the (000-1) surface. 
         [0025]    At  410 , the (000-1) surface of the substrate is exposed to an etchant, which may be used to remove the material in a certain region, e.g., region A in  FIG. 2 . Alternatively or in addition thereto, the etching may be used to remove at least substantially all of the substrate or all of the substrate and at least substantially all of the material grown at  404 . In certain embodiments, the etching may stop automatically after the etchant has removed all of the material up to the beginning of the etch stop layer. 
         [0026]    Certain embodiments generally include a growth strategy in which most of a semiconductor device, e.g., an LED, is grown by a first process, such as an MOCVD process, while the etch stop layer is grown by a second process, such as an MBE process. For example, steps  304  and  312  in  FIG. 3  may involve growth by an MOCVD process and steps  406 - 410  in  FIG. 4  may involve growth by an MBE process. Alternative embodiments may include using MOCVD growth for all steps or MBE growth for all steps. Certain alternative embodiments may include employing an approach in which only one IDB is employed. In such approaches, one could initially provoke growth on the Al-face, invert the growth to the N-face, grow the device structure on the N-face, and then remove the AlN substrate up to the etch stop layer. 
         [0027]    With reference to  FIG. 2 , it may be advantageous in certain situations for Region C to have the same in-plane lattice constant as the AN or Al x Ga 1-x N substrate. Thus, growth of Region C may include growth of a thick AlN or Al x Ga 1-x N layer so that, after removal of Region A, the wafer is not bowed and the in-plane lattice constant of Region C and the layers situated above are the same as the bulk AlN or Al x Ga 1-x N substrate. 
         [0028]    Lateral control of the polarity of the material may be used to laterally define the regions either during growth or subsequent etching of the substrate. This might also be used as an intrinsic back surface texturing technique based on selection of the lateral dimensions. By patterning the initial template, one may initiate N-face growth in certain areas and Al-face growth in other areas. Etching away the N-face regions on the back surface, for example, may give rise to texturing that could improve light extraction efficiency based on selection of the lateral dimensions. 
         [0029]    It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.