Abstract:
A p-n tunnel junction between a p-type semiconductor layer and a n-type semiconductor layer provides current injection for an edge-emitting nitride based semiconductor laser structure. The amount of p-type material in the nitride based semiconductor laser structure can be minimized, with attendant advantages in electrical, thermal, and optical performance, and in fabrication.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application contains subject matter that is related to subject matter of U.S. Patent Application Serial. No. (Attorney Docket D/A1350Q) entitled “NITRIDE-BASED VCSEL OR LIGHT EMITTING DIODE WITH P-N TUNNEL JUNCTION CURRENT INJECTION”, filed concurrently with this application, commonly assigned to the same assignee herein and herein incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to an indium gallium aluminum nitride-based edge-emitting laser diode structure and, more particularly, to a p-n tunnel junction for current injection for the indium gallium aluminum nitride edge-emitting nitride based laser diode structure.  
           [0003]    Solid state lasers, also referred to as semiconductor lasers or laser diodes, are well known in the art. These devices generally consist of a planar multi-layered semiconductor structure having one or more active semiconductor layers bounded at their side ends by cleaved surfaces that act as mirrors. The semiconductor layers on one side of the active layer in the structure are doped with impurities so as to have an excess of mobile electrons. These layers with excess electrons are said to be n-type, i.e. negative. The semiconductor layers on the other side of the active layer in the structure are doped with impurities so as to have a deficiency of mobile electrons, therefore creating an excess of positively charged carriers called holes. These layers with excess holes are said to be p-type, i.e. positive.  
           [0004]    An electrical potential is applied through electrodes between the p-side and the n-side of the layered structure, thereby driving either holes or electrons or both in a direction perpendicular to the planar layers across the p-n junction so as to “inject” them into the active layers, where electrons recombine with holes to produce light. Optical feedback is provided by the cleaved mirrors and a standing wave is formed between the mirrors in the laser resonator with a wave front parallel to the mirrors. If the optical gain produced in the active layers exceeds the optical loss in the laser structure amplified stimulated emission is produced and coherent laser light is emitted through the mirrored edges of the semiconductor laser structure.  
           [0005]    Nitride based semiconductors, also known as group III nitride semiconductors or Group III-V nitride semiconductors, comprise elements selected from group II, such as Al, Ga and In, and the group V element N of the periodic table. The nitride based semiconductors can be binary compounds such as gallium nitride (GaN), as well as ternary alloys of aluminum gallium nitride (AlGaN) or indium aluminum nitride (InGaN), and quarternary alloys such as indium gallium aluminum nitride (InGaAlN). These materials are deposited on substrates to produce layered semiconductor structures usable as light emitters for optoelectronic device applications. Nitride based semiconductors have the wide bandgap necessary for short-wavelength visible light emission in the green to blue to violet to the ultraviolet spectrum.  
           [0006]    These materials are particularly suited for use in short-wavelength light emitting devices for several important reasons. Specifically, the InGaAlN system has a large bandgap covering the entire visible spectrum. III-V nitrides also provide the important advantage of having a strong chemical bond which makes these materials highly stable and resistant to degradation under the high electric current and the intense light illumination conditions that are present at active regions of the devices. These materials are also resistant to dislocation formation once grown.  
           [0007]    Semiconductor laser structures comprising nitride semiconductor layers grown on a sapphire substrate will emit light in the ultra-violet to visible spectrum within a range including 280 nm to 650 nm.  
           [0008]    The shorter wavelength violet of nitride based semiconductor laser diodes provides a smaller spot size and a better depth of focus than the longer wavelength of red and infrared (IR) laser diodes for high-resolution or high-speed laser printing operations and high density optical storage. In addition, blue lasers can potentially be combined with existing red and green lasers to create projection displays and color film printers. The emission wavelength of GaN-based lasers and LEDs with an AlGaN or AlInGaN active region can be tuned into the UV range of the spectrum. Emission wavelength around 340 nm and 280 nm are particularly interesting for the optical excitation of biomolecules in bacteria, spores and viruses, which can be applied e.g. in bioagent detection systems.  
           [0009]    A prior art nitride based semiconductor laser structure  100  of FIG. 1 has a sapphire (Al 2 O 3 ) substrate  102  on which is epitaxially deposited a succession of semiconductor layers. The sapphire substrate  102  typically has a thickness of 200 micron to 1000 micron.  
           [0010]    The prior art laser structure  100  includes an n-type III-V nitride nucleation layer  104  formed on the sapphire substrate  102 . Typically, the buffer layer  104  is undoped GaN and has typically a thickness in the range between 10 nm and 30 nm.  
           [0011]    A III-V nitride contact and current-spreading layer  106  is formed on the nucleation layer  104 . The III-V nitride layer  106  is an n-type GaN:Silayer acting as a lateral n-contact and current spreading layer. The contact and current spreading layer  106  typically has a thickness of from about 1 μm to about 20 μm.  
           [0012]    A III-V nitride cladding layer  108  is formed over the contact layer  106 . The III-V nitride layer  106  is an n-type AlGaN:Si cladding layer. The cladding layer  106  typically has a thickness of from about 0.2 μm to about 2 μm.  
           [0013]    On top of the III-V nitride cladding layer  108 , a III-V nitride waveguide layer  110  is formed followed by the III-V nitride quantum well active region  112 . The n-type GaN:Si waveguide layer  110  typically has a thickness of from about 50 nm to about 200 nm. The quantum well active region  112  is comprised of at least one InGaN quantum well. For multiple-quantum well active regions, the individual quantum wells typically have a thickness of from about 10 Å to about 100 Å and are separated by InGaN or GaN barrier layers which have typically a thickness of from about 10 Å to about 200 Å.  
           [0014]    A III-V nitride waveguide layer  114  is formed over the quantum well active region  112 . The p-type GaN:Mg layer  114  serves as a waveguide layer and has a thickness of from about 50 nm to about 200 nm.  
           [0015]    A III-V nitride cladding layer  116  is formed over the waveguide layer  114 . The p-type AlGaN:Mg layer  116  serves as a cladding and current confinement layer. The III-V nitride cladding layer  116  typically has a thickness of from about 0.2 μm to about 1 μm.  
           [0016]    A III-V nitride contact layer  118  is formed over the cladding layer  116 . The p-type GaN:Mg layer  118  forms a p-contact layer for the minimum-resistance metal electrode to contact the p-side of the laser heterostructure  100 . The III-V nitride contact layer  118  typically has a thickness of from about 10 nm to 200 nm.  
           [0017]    The laser structure  100  can be fabricated by a technique such as metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy as is well known in the art.  
           [0018]    Dry-etching using CAIBE (chemical assisted ion beam etching) or RIE (reactive ion beam etching) in an Ar/Cl 2 /BCl 3  gas mixture is used to etch the prior art laser structure  100  down to the GaN:Si current-spreading layer  106 .  
           [0019]    An n-type Ti/Al electrode  120  is formed on the etched, exposed n-current-spreading layer  106  of the laser  100 , which is functioning as a lateral contact layer. A p-type Ni/Au electrode  122  is formed on the p-contact layer  118  of the laser  100 .  
           [0020]    P-type doping of InGaAlN layers is a key problem in the realization of GaN-based devices. It is difficult to achieve a high hole concentration in AlGaN alloys since the ionization energy of Mg acceptors is relatively high (˜200 meV for Mg in GaN) and increases even further with higher Al content (˜3 meV per % Al). Therefore, p-doped waveguide and cladding layers contribute significantly to the series resistance of the nitride-based laser structure, which results in higher operating voltages. Even in today&#39;s currently best violet nitride lasers, the operating voltages are on the order of 5 to 6 V, which is 2 to 3 V above the laser emission energy. For UV laser and LEDs, which require even higher Al compositions, the series resistance is going to be even larger. For a UV laser structure emitting around  340 nm the required Al composition for the cladding layers would be around 30%. The increase in Mg acceptor activation energy in the AGaN layer would result in an almost an order of magnitude drop in hole concentration compared to a Mg-doped GaN film.  
           [0021]    In addition, the optimum growth temperatures for Mg-doped AlGaN layers is typically lower than the growth temperatures for Si-doped or un-doped AlGaN films, because of the improved Mg incorporation efficiency at lower temperatures. However, the structural quality of nitride-based semiconductor layers is reduced, when grown at a lower temperature, which deteriorates the structural and electronic properties of the upper cladding layers and upper waveguide layers in a III-V nitride laser structure.  
           [0022]    Furthermore, in conventional InGaAlN laser diodes, GaN:Mg or InGaN:Mg are used as waveguiding layers and short period AlGaN/GaN supperlattice layers or bulk AlGaN layers doped with Mg are used as upper cladding layers. These Mg-doped layers have a significant absorption loss particularly in the blue to ultraviolet spectrum that a nitride based laser will emit light. For laser diodes operating close to the band gap of GaN (&lt;400 nm), this leads to increased distributed loss and consequently to increased threshold current densities.  
           [0023]    It is an object of this invention to provide a nitride based semiconductor laser structure with a reduced number of p-type semiconductor layers.  
         SUMMARY OF THE INVENTION  
         [0024]    According to the present invention, a p-n tunnel junction between a p-type semiconductor layer and a n-type semiconductor layer provides current injection for an edge-emitting nitride based semiconductor laser structure. The p-n tunnel junction reduces the number of p-type semiconductor layers in the nitride based semiconductor laser structure which reduces the distributed loss, reduces the threshold current densities, reduces the overall series resistance and improves the structural quality of the laser by allowing higher growth temperatures.  
           [0025]    Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained and understood by referring to the following detailed description and the accompanying drawings in which like reference numerals denote like elements as between the various drawings. The drawings, briefly described below, are not to scale.  
         [0027]    [0027]FIG. 1 is a side view of the prior art nitride semiconductor laser structure.  
         [0028]    [0028]FIG. 2 is a side view of the nitride semiconductor laser structure with a p-n tunnel junction for current injection of the present invention.  
         [0029]    [0029]FIG. 3 is a graph of the strength of the piezo-electric field versus the indium content in InGaN layers in the nitride semiconductor laser structure with a p-n tunnel junction of FIG. 2. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0030]    In the following detailed description, numeric ranges are provided for various aspects of the embodiments described. These recited ranges are to be treated as examples only, and are not intended to limit the scope of the claims hereof. In addition, a number of materials are identified as suitable for various facets of the embodiments. These recited materials are to be treated as exemplary, and are not intended to limit the scope of the claims hereof.  
         [0031]    Reference is now made to FIG. 2, wherein there is illustrated the nitride based semiconductor laser structure  200  with a p-n tunnel junction for current injection in accordance with this invention.  
         [0032]    The nitride based semiconductor laser structure  200  of FIG. 2 has a C-face (0001) or A-face (11 2 0) oriented sapphire (Al 2 O 3 ) substrate  202  on which is epitaxially deposited a succession of semiconductor layers. The sapphire substrate  202  typically has a thickness of 200 micron to 1000 micron. The sapphire substrate is an illustrative example, other possible substrates for the laser structure  200  of the present invention include silicon carbide (SiC), spinel (MgAl 2 O 4 ), aluminum nitride (AlN) or gallium nitride (GaN).  
         [0033]    The laser structure  200  includes an n-type nucleation layer  204  formed on the sapphire substrate  202 . Typically, the nucleation layer  204  is a binary or ternary III-V nitride material, such as, for example, GaN, AlN, InGaN or AlGaN. The nucleation layer  204  in this illustrative example is undoped GaN and has typically a thickness in the range between 10 nm and 30 nm.  
         [0034]    A III-V nitride contact and current spreading layer  206  is formed on the nucleation layer  204 . The III-V nitride layer  206  is an n-type GaN:Si layer acting as a lateral n-contact and current spreading layer. The contact and current spreading layer  206  typically has a thickness of from about 1 μm to about 20 μm. The III-V nitride layer  206  can alternately be n-type AlGaN:Si.  
         [0035]    In the case when substrates like GaN or AlN are used, a nucleation layer is not required and the contact and current spreading layer then acts as a transition layer between the GaN or AlN substrate and the cladding layer. The transition layer can also be a graded Al x Ga 1−x N layer with its Al composition x graded between the substrate and the cladding layer.  
         [0036]    A III-V nitride cladding layer  208  is formed over the contact layer  206 . The III-V nitride layer  208  is an n-type AlGaN:Si cladding layer with an Al content larger than the contact layer  206 . The cladding layer  208  typically has a thickness of from about 0.2 μm to about 2 μm.  
         [0037]    On top of the cladding layer  208 , a III-V nitride waveguide layer  210  is formed followed by the III-V nitride quantum well active region  212 . The n-type GaN:Si waveguide layer  210  typically has a thickness of from about 50 nm to about 200 nm. The waveguide layer  210  can also be GaN:un, AlGaN:un, AlGaN:Si, InGaN:un or InGaN:Si with a band gap energy larger than the InGaAlN quantum well in the active region  212 . The quantum well active region  212  is comprised of at least one In x Al y Ga 1−x−y N quantum well (1&gt;=x&gt;=0, 1&gt;=y&gt;=0, 1&gt;=x+y&gt;=0). For multiple-quantum well active regions, the individual quantum wells typically have a thickness of from about 10 Å to about 100 Å and are separated by InGaN, AlGaN or GaN barrier layers which have typically a thickness of from about 10 Å to about 200 Å. The InGaAlN quantum wells and the InGaN, AlGaN or GaN barrier layers are typically undoped or can be Si-doped.  
         [0038]    A III-V nitride electron blocking layer  214  is formed over the quantum well active region  212 . The p-type AlGaN:Mg electron blocking layer  214  serves as a carrier confinement layer to keep electrons from leaking out of the active region  212 . The acceptor concentration of the AlGaN electron blocking layer  116  is in the range between 1*10 18  cm −3  and 2*10 20  cm −3  and typically around 1*10 20  cm −3 .  
         [0039]    The first part of the tunnel junction layer  216  is formed over the electron blocking layer  214 . The highly p-type doped GaN:Mg layer  216  serves also as part of the waveguide layer and has a thickness between 10 nm and 100 nm and typically around 20 nm. The acceptor concentration of the GaN layer  216  is in the range between 1*10 19  cm −3  and 2*10 20  cm −3  and typically around approximately 1*10 20  cm −3 . Alternatively the first part of the tunnel junction can be formed from highly p-type doped InGaN:Mg or InGaAlN:Mg. The tunnel probability increases exponentially with decreasing tunnel junction bandgap [˜exp(−E gap   1.5 )] and therefore the lower bandgap E gap  of the InGaN layer would reduce the operating voltage. In addition, the large polarization fields present in pseudomorphicaly strained InGaN films would add to the built-in field of the pn-junction and consequently increase the total electric field across the tunnel junction. As the tunnel probability increases exponentially with increasing tunnel junction field F [˜exp(−1/F)], the larger total electric field across the tunnel junction would reduce the operating voltage. A calculated curve showing the strength of the piezoelectric field in InGaN films vs. the In composition is shown in FIG. 3. As can be seen, even at moderate In content of 10% the piezoelectric field in the layer is in the order of 2 MV/cm.  
         [0040]    The second part of the tunnel junction layer  218  is formed over the first part of the tunnel junction layer  216 . The highly n-type doped GaN:Si layer  218  serves also as part of the waveguide layer and has a thickness between 10 nm and 200 nm and typically around 20 nm. The n-doping level of the GaN layer  218  is in the range between 5*10 18  cm −3  and 1*10 20  cm −3  and typically around 5*10 19  cm 1 . Alternatively the second part of the tunnel junction can be formed from highly n-type doped InGaN:Si or InGaAlN:Si. The motivation is similar as in the case of the first part of the tunnel junction layer. The tunnel probability increases exponentially with decreasing tunnel junction bandgap and increasing tunnel junction field F and therefore the lower bandgap E gap  and the polarization field of the InGaN layer would reduce the operating voltage.  
         [0041]    Alternatively, the second part of the tunnel junction  218  can be formed from highly n-type doped oxides such as ZnO, CdO, MgO, SnO 2  or In 2 O 3 . N-doping in these films is achieved with Al, Ga, In, F, Sn or Si with doping levels ranging between 1*10 19  cm −3  and 1*10 21  cm −3  and typically around 1*10 20  cm −1 . ZnO, CdO, MgO, SnO 2  or In 2 O 3  can be deposited by RF magnetron sputtering, pulsed laser deposition or MOCVD. Compounds of these oxides could also be used, e.g. Cd 2 SnO 4 . The advantage of using an oxide such as ZnO lies in the fact that the band offsets between ZnO and GaN are large, with both the valence band and conduction band of ZnO lying significantly lower in energy than the corresponding bands in GaN. This band alignment facilitates tunneling from the oxide conduction band into the nitride valence band. In addition, ZnO or other oxides with band gap exceeding that of the InAlGaN active layer are transparent to the light emitted from the active layer, hence optical absorption losses are small.  
         [0042]    The tunnel junction  220  of the present invention is the interface between the p-(In)GaN:Mg layer  216  and the n-(In)GaN:Si layer  218  (or n-type ZnO, CdO, MgO, SnO 2 , In 2 O 3  layer). In order to avoid Mg p-dopant memory effects and to obtain a sharp p-n interface at the tunnel junction  220 , a growth stop of typically 60 seconds to 600 seconds is introduced between deposition of the highly p- and n-doped layers  216  and  218 . Alternatively, the sample can be taken out of the reactor and the surface can be etched (e.g. dry-etching) in order to remove any possible Mg accumulating at the surface.  
         [0043]    Depending on the total thickness of the combined tunnel junction layers a III-V nitride layer  221  can be formed in order to complete the waveguide layer. The third part of the waveguide can be formed from GaN, InGaN, AlGaN or InGaAlN. The layer can be Si-doped or undoped layer and has a thickness between 0 nm and 200 nm and typically around 60 nm.  
         [0044]    A III-V nitride or oxide cladding layer  222  is formed over the n-type waveguide layer  221  (or  218  if  221  is not formed). The n-type AlGaN:Si layer  222  serves as a cladding and current confinement layer. The cladding layer  222  typically has a thickness of from about 0.2 μm to about 1 μm. The cladding layer can also be formed from n-type ZnO, CdO, MgO, SnO 2 , In 2 O 3  layer, since most of these oxide layers have a refractive index around 2, which is smaller than that of InGaN or GaN. ZnO, CdO, MgO, SnO 2  or In 2 O 3  can be deposited by RF magnetron sputtering, pulsed laser deposition or MOCVD. Compounds of these oxides could also be used, e.g. Cd 2 SnO 4 .  
         [0045]    A III-V nitride or oxide contact layer  224  is formed over the cladding layer  222 . The n-type GaN:Si layer  224  forms a contact layer for the minimum-resistance metal electrode to contact the p-side of the laser heterostructure  100 . The contact layer  224  typically has a thickness of from about 10 nm to 200 nm. The contact layer can also be formed from n-type ZnO, CdO, MgO, SnO 2 , In 2 O 3  layer. Compounds of these oxides could also be used, e.g. Cd 2 SnO 4 .  
         [0046]    The laser structure  200  can be fabricated by a technique such as metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy as is well known in the art.  
         [0047]    Dry-etching using CAIBE (chemical assisted ion beam etching) or RIE (reactive ion beam etching) in an Ar/Cl 2 /BCl 3  gas mixture is used to etch a portion of the laser structure  200  down to the GaN:Si contact and current-spreading layer  206 .  
         [0048]    Dry-etching using CAIBE (chemical assisted ion beam etching) or RIE (reactive ion beam etching) in an Ar/Cl 2 /BCl 3  gas mixture is used to etch a narrower portion of the laser structure  200  down to the electron blocking layer  214  in order to form a ridge-waveguide structure. The exposed etched surfaces can be covered with a insulating dielectric  225  like SiO 2 , silicon-oxy-nitride or Si 3 N 4 , which can be deposited by e-beam evaporation, sputtering or CVD.  
         [0049]    An n-type Ti/Al electrode  226  is formed on the etched, exposed n-current-spreading layer  206  of the laser  200 , which is functioning as a lateral contact layer. A n-type Ti/Al electrode  228  used as the p-side electrode is formed on the p-side contact layer  224  of the laser  200 .  
         [0050]    The III-V nitride layers can be doped p-type or n-type by conventional processes. Examples of p-type dopants include, but are not limited to, Mg, Ca, C and Be. Examples of n-type dopants include, but are not limited to, Si, O, Se, and Te.  
         [0051]    Current flowing between the electrodes  228  and  226  causes the nitride based semiconductor laser structure  200  to emit a light beam  230  from the active region  212  through the side  232  of the laser structure  200 . The p-n tunnel junction  220  provides current injection from the p-side of the laser structure. The very high doping levels of p and n conductivity types on opposite sides of the tunnel junction  220  between the waveguide layers  216  and  218  (and in the case of a InGaN tunnel junction piezoelectric fields) cause a sufficient amount of carriers to tunnel from the conduction band of layer  120  into the valence band of layer  118 . The voltage applied between the electrodes  228  and  226  reverse biases the tunnel junction  220  and the current flows across the junction by quantum mechanical tunneling with a relative small voltage drop.  
         [0052]    The voltage drop across the tunnel junction  220  depends on the doping levels in the layers  216  and  218  bounding the junction, on the band gap of the materials of these layers, and on the band offsets between these layers in the case of a heterojunction. Using as high a doping level as possible in the layers  216  and  218  minimizes the voltage drop across the tunnel junction. The voltage drop across the tunnel junction is further reduced by piezoelectric fields if InGaN layers are use in the tunnel junction.  
         [0053]    This invention proposes a nitride based semiconductor laser structure  200  with a p-n tunnel junction  220  for current injection from the p-side of the structure.  
         [0054]    The absorption or distributed optical loss in the nitride based semiconductor laser structure  200  and therefore the threshold current densities for the nitride based semiconductor laser structure  200  are reduced by eliminating almost all p-doped layers from the device structure (i.e. the upper p-GaN:Mg waveguide layer, the upper p-AlGaN:Mg cladding layer and the upper p-GaN:Mg contact layer). The reduction in optical loss by using a p-n tunnel junction  220  for current injection is particularly useful for nitride based semiconductor lasers which emit light in the near UV and UV spectrum. The reduction in threshold current densities by using a p-n tunnel junction  220  for current injection is particularly useful for nitride based semiconductor lasers which have high current densities relative to laser structures fabricated from arsenides and other semiconductor materials.  
         [0055]    Replacing the p-doped semiconductor layers with more conductive n-doped semiconductor layers in the nitride based semiconductor laser structure  200  with a p-n tunnel junction  220  for current injection reduces the overall series resistance of the laser structure, since most of the excess voltage drop is across the p-layers and p-contact. Although the additional voltage required for current injection through the p-n tunnel junction will offset this benefit somewhat, the net operating voltage of the nitride based semiconductor laser structure should not increase if the tunnel junction is properly designed with thin layers and high doping concentrations.  
         [0056]    In addition, the large voltage drop across the p-layers reduces the efficiency of the AlGaN electron blocking layer in a conventional laser structure. The resistance in a tunnel junction laser structure depends only on the tunnel current and not on the conductivity of the p-layers (as in the conventional laser structure) and therefore the voltage drop across the electron blocking layer should be smaller.  
         [0057]    Furthermore the structural quality of the upper cladding and waveguide layers could be improved, because eliminating p-doping makes higher growth temperatures and better growth control possible, which is preferable for high quality GaN layers.  
         [0058]    While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.