Light emitting diode with transparent window layer

The light brightness of a semiconductor LED is increased by employing a light transmitting window comprising ZnO. In another embodiment, current crowding is reduced, efficiency increased and reliability (lifetime) increased by forming a thin semiconductor transition layer to reduce contact resistance between an overlying transparent window layer and an underlying transparent current diffusion layer formed on a double heterostructure light generation region. Optimum performance is achieved employing the transition layer with a ZnO transparent window layer.

TECHNICAL FIELD
 The present invention relates to a semiconductor light emitting diode
 (LED). The present invention is particularly applicable to a semiconductor
 LED having high efficiency and reliability and comprising a
 heterostructure light generating region and a transparent window layer.
 BACKGROUND ART
 Conventional LEDs comprise a semiconductor light generation region on a
 light absorbing substrate. Such LEDs enjoy various industrial
 applications, as in optical communication systems, optical information
 processing and as a light source due to their low power consumption,
 efficiency and reliability. Efficient operation of an LED requires uniform
 lateral spreading of current injected by a front electrical contact, so
 that the current uniformly enters the light generation region, thereby
 generating light with uniformity. However, as a result of current
 crowding, current tends to concentrate under the front electrical contact,
 thereby preventing uniform light generation. Industry efforts have focused
 upon reducing the current crowding problem as well as increasing the
 brightness of emitted light.
 A traditional semiconductor LED is schematically illustrated in FIG. 1 and
 comprises a back electrical contact 10, an n-type substrate 20, a double
 heterostructure 30 (light generation region) which includes an undoped
 active layer 3b positioned between doped confinement layers 3a and 3c, and
 a front contact 70. It is in such a structure that current crowding
 typically occurs between the light generation region 30 and front contact
 70, thereby preventing uniform light generation.
 A prior effort to alleviate the current crowding effect and maximize light
 output is disclosed by Fletcher et al. in U.S. Pat. No. 5,233,204 and
 schematically illustrated in FIG. 2, wherein elements similar to those
 depicted in FIG. 1 bear similar reference numerals and, hence, are not
 described in detail to avoid repetition. The improvement disclosed by
 Fletcher et al. comprises positioning a relatively thick transparent
 semiconductor window layer 40, e.g., about 10 microns to about 50 microns,
 between the light generation region 30 and the front metal contact 70.
 Window layer 40 is desirably selected from materials having a high
 conductivity to enable rapid current spreading from front contact 70,
 thereby minimizing the current crowding effect. In addition, window layer
 40 should have a higher bandgap than that of the light generation region
 30 so that window layer 40 is transparent to emitted light. There are,
 however, drawbacks attendant upon the semiconductor LED illustrated in
 FIG. 2. For example, semiconductor window layer 40 can not include
 material systems having lattice constants which are not compatible with
 light generation region 30, thereby limiting design flexibility. In
 addition, the growth of a thick layer is time consuming.
 Another prior approach to the current crowding problem is disclosed by Lin
 et al. in U.S. Pat. No. Re. 35,665 and schematically illustrated in FIG.
 3, wherein elements similar to those in FIGS. 1 and 2 bear similar
 reference numerals. The semiconductor LED illustrated in FIG. 3 basically
 differs from that of FIG. 2 in that the thick semiconductor window layer
 40 (FIG. 2) is replaced by transparent conductive oxide window layer 50
 and an ohmic contact layer 51 which is typically a semiconductor material
 having a relatively high impurity concentration, e.g., greater than about
 1.times.10.sup.18 atoms cm.sup.-3. Ohmic contact layer 51 is provided so
 that window layer 50 can be formed on a p-type confinement layer (3c),
 thereby expanding utility to n-type gallium-arsenide (GaAs)
 substrate-based LEDs. The transparent conductive oxide 50 comprises tin
 oxide, indium oxide, or indium-tin oxide, which are conductive materials,
 relatively inexpensive and relatively easier to grow than semiconductor
 compound transparent window materials for window layer 40 (FIG. 2).
 With continued reference to FIG. 3, the utilization of a transparent
 conductive oxide layer 50 could reduce the current crowding effect, reduce
 manufacturing time, improve efficiency and expand applicability to LEDs
 with n-type GaAs substrates. Such oxides are suitable window materials for
 LEDs employing aluminum-gallium-indium-phosphorous (AlGaInP) material
 systems, i.e. for the light generation region, which emit light having
 wavelengths ranging from about 570 to about 680 nm. However, semiconductor
 LEDs based upon FIG. 3 are also problematic. For example, tin oxide,
 indium oxide and indium tin oxide exhibit poor optical transmission at
 longer wavelengths and, hence, are not particularly suitable for use in
 semiconductor LEDs at wavelengths of about 1.3 or about 1.5 .mu.m. Such
 oxides are also toxic, and do not exhibit long term chemical stability. In
 addition, semiconductor LEDs based upon FIG. 3 exhibit an undesirably high
 contact resistance between light transmission region 30 and ohmic contact
 layer 51, which unnecessarily squanders electricity and increases the
 operating temperature, e.g., above room temperature, thereby decreasing
 device reliability, i.e. longevity.
 There exists a need for a semiconductor LED which exhibits improved light
 brightness, reduced crowding effect and increased longevity. There also
 exists a need for such a semiconductor LED which can be manufactured
 efficiently and economically.
 DISCLOSURE OF THE INVENTION
 An advantage of the present invention is a semiconductor LED exhibiting
 improved light brightness and reduced current crowding.
 Another advantage of the present invention is a semiconductor LED
 exhibiting long term stability and reduced toxicity.
 A further advantage of the present invention is a semiconductor LED
 exhibiting improved light brightness, reduced current crowding, long term
 stability and reduced toxicity which can be manufactured economically and
 efficiently.
 Additional advantages and other features of the present invention will be
 set forth in part in the description which follows and in part will become
 apparent to those having ordinary skill in the art upon examination of the
 following or may be learned from the practice of the present invention.
 The advantages of the present invention may be realized and obtained as
 particularly pointed out in the appended claims.
 According to the present invention, the foregoing and other advantages are
 achieved in part by a semiconductor LED comprising: a semiconductor
 substrate having a back electrode contact; a light generation region on
 the substrate; a transparent current diffusion layer on the light
 generation region; a dual transparent layer window on the current
 diffusion layer; and a front contact on the dual layer window.
 Embodiments of the present invention include a dual layer window comprising
 a transition layer on the current diffusion layer and a transparent window
 layer on the transition layer, wherein the transition layer has a bandgap
 selected to reduce contact resistance between the current diffusion layer
 and the window layer. Embodiments of the present invention include forming
 the transition layer at a thickness of about 10 to about 100 nm and
 forming the window layer at a thickness of about 0.5 micron to about 2
 microns. Embodiments of the present invention also include forming the
 transition layer of a doped semiconductor material, and employing a highly
 doped semiconductor material as the current diffusion layer at a thickness
 of about 5 microns to about 10 microns.
 Another aspect of the present invention is a semiconductor LED comprising:
 a semiconductor substrate having a back electrical contact; a light
 generation region on substrate; a transparent window layer comprising zinc
 oxide; and a front contact on the window layer.
 Embodiments of the present invention include forming a highly doped
 transparent current diffusion layer on a double heterostructure light
 generation region and forming a doped semiconductor transition layer
 between the zinc oxide window layer and current diffusion layer, such that
 the transition layer reduces contact resistance between the current
 diffusion layer and window layer.
 Additional advantages of the present invention will become readily apparent
 to those skilled in this art from the following detailed description,
 wherein embodiments of the present invention are described, simply by way
 of illustration of the best mode contemplated for carrying out the present
 invention. As will be realized, the present invention is capable of other
 and different embodiments, and its several details are capable of
 modifications in various obvious respects, all without departing from the
 present invention. Accordingly, the drawings and description are to be
 regarded as illustrative in nature, and not as restrictive.

DESCRIPTION OF THE INVENTION
 The present invention addresses and solves the current crowding problem in
 semiconductor LEDs, thereby enhancing efficiency and increasing the
 brightness of emitted light. Embodiments of the present invention
 additionally comprise increasing the reliability or lifetime of a
 semiconductor LED.
 In accordance with embodiments of the present invention, the brightness of
 emitted light of a semiconductor LED is improved by strategically
 employing zinc oxide in forming the transparent window. It was found that
 zinc oxide results in improved efficiency in terms of power output and
 brightness.
 Zinc oxide has been mentioned as a suitable material for use in solar
 cells. See, Weller et al., "NOVEL TYPE OF ZnO STUDIED IN COMBINATION WITH
 1.5 eV A --SiGeiHPIN DIODES", IEEE, CH2953-8/91, pages 1290-1295 and
 Nakada et al., "TEXTURED ZnO:AL FILMS FOR SOLAR CELLS BY DC-MAGNETRON
 SPUTTERING IN WATER VAPOR PLASMA", IEEE, CH2953-8/91, pages 1389-1392.
 Solar cells are structurally and functionally different, of course, from
 semiconductor LEDs. Basically, semiconductor LED devices consume
 electricity and emit light. However, solar cells operate in the reverse
 manner, i.e. solar cells absorb and generate electricity from the absorbed
 light. Since solar cells operate at about room temperature, reliability is
 not an issue.
 Reliability of the semiconductor LEDs is a problem, however, particularly
 upon degradation of the material employed in a conventional window, e.g.
 tin oxide, indium oxide or indium tin oxide. The degradation of such
 material causes the semiconductor LED to run at a high temperature thereby
 shortening its lifetime.
 In accordance with an embodiment of the present invention, a window layer,
 such as window layer 40 of the FIG. 2 device, or window layer 50 of the
 FIG. 3 device, is fabricated of zinc oxide, thereby significantly
 enhancing the brightness of emitted light and further significantly
 enhancing reliability.
 A comparison of the optical transmission of zinc oxide vis-a-vis tin oxide
 is shown in FIG. 5. It is apparent that zinc oxide exhibits a higher
 optical transmission than tin oxide, (as well as indium oxide and indium
 tin oxide,) in the wavelength region of about 370 to about 1700 nm. Thus,
 zinc oxide is a strategically desirable material for use as a window layer
 for semiconductor LEDs emitting light ranging from about 370 to about 1700
 nm.
 In another embodiment of the present invention, a semiconductor LED is
 formed with a thin dual layer window comprising a transition layer between
 an overlying transparent window layer and an underlying current diffusion
 layer. The thin transition layer is strategically formed of a material to
 reduce contact resistance between the window layer and current diffusion
 layer, thereby reducing the amount of electricity consumed and, hence,
 avoiding an increase in operating temperature so that the lifetime of the
 semiconductor LED is increased. In embodiments of the present invention,
 the thin transition layer is formed of a material having a bandgap which
 is different from the bandgap of the overlying window layer and different
 from the bandgap of the underlying current diffusion layer. Embodiments of
 the present invention include forming the thin transition layer of a
 material having a bandgap which is less than the bandgap of the overlying
 transparent window layer and less than the bandgap of the underlying
 diffusion layer.
 Embodiments of the present include forming the transition layer at a
 thickness of about 10 to about 100 nm, e.g. at a thickness of about 50 to
 about 100 nm. Embodiments of the present invention also include forming
 the transparent window layer at a thickness of about 0.05 microns to about
 2 microns, e.g. about 0.1 micron to about 0.2 micron.
 Suitable materials for the transition layer include any of various
 semiconductor materials, such as indium-gallium-arsenic (InGaAs),
 gallium-arsenide (GaAs) and indium-galium-arsenic-phosphorus (InGaAsP).
 Embodiments of the present invention also include forming the thin
 transition layer of a semiconductor material having an impurity
 concentration of about 5.times.10.sup.17 atoms cm.sup.-3 to about
 4.times.10.sup.19 atoms cm.sup.-3.
 In embodiments of the present invention comprising the use of a thin
 transition layer to reduce contact resistance, the window layer can
 comprise tin oxide, indium oxide, indium-tin-oxide or, preferably, zinc
 oxide for enhanced brightness.
 In accordance with embodiments of the present invention, a current
 diffusion layer is provided on the light generation region and the
 transition layer is formed on the current diffusion layer. The current
 diffusion layer is typically formed at a thickness of about 5 microns to
 about 10 microns and typically comprises a semiconductor material, such as
 AlGaAs, gallium-arsenic-phosphorus (GaAsP), and/or gallium-phosphorus
 (GaP), with a high impurity concentration, such as about
 5.0.times.10.sup.17 atoms cm.sup.-3 to about 4.0.times.10.sup.18 atoms
 cm.sup.-3.
 An embodiment of the present invention is illustrated in FIG. 4 wherein
 elements similar to those in FIGS. 1-3 bear similar reference numerals.
 The semiconductor LED illustrated in FIG. 4 comprises back electrical
 contact 10 and semiconductor substrate 20, such as n-type or p-type InP or
 GaAs. A light generation region 30 is provided on substrate 20. Light
 generation region 30 typically comprises a conventional double
 heterostructure of an AlGaInP, AlGaAs or InGaAsP material system forming a
 PN junction. For example, layer 3a can comprise an n-type semiconductor
 material, layer 3b an undoped semiconductor material and layer 3c a
 semiconductor material containing a p-type dopant. Suitable n-type dopants
 include silicon and selenium. Suitable p-type dopants include magnesium,
 zinc and carbon.
 In the embodiment depicted in FIG. 4, a relatively thick current diffusion
 layer 80 is formed on light generation region 30. Current diffusion layer
 80 serves to reduce current crowding and is typically formed of a
 transparent semiconductor material which is heavily doped. Current
 diffusion layer 80 can be formed at a thickness of about 5 microns to
 about 10 microns of AlGaAs, InP or GaP and typically has an impurity
 concentration of about 5.times.10.sup.17 to about 4.times.10.sup.18 atoms
 cm.sup.-3. For example, current diffusion layer 80 can comprise a highly
 doped Al.sub.x Ga.sub.1-x As(x=0.6-0.8) with an impurity concentration of
 at least about 1.times.10.sup.18 cm.sup.-3.
 The transparent dual layer window 60 is then formed on current diffusion
 layer 80. Transparent dual layer window 60 comprises a thin transition
 layer 6a and a relatively thick conductive oxide window layer 6b,
 preferably zinc oxide. Thin transition layer 6a is typically formed of a
 material having a narrower bandgap than that of current diffusion layer 80
 and capable of being heavily doped with both n-type and p-type impurities.
 For example, transition layer 6a can be formed at a thickness of about 500
 .ANG. of In.sub.x Ga.sub.1-x As (x=0.5) with an impurity concentration of
 about 5.times.10.sup.17 to about 4.times.10.sup.19 atoms cm.sup.-3. Front
 contact 70 is then formed on transparent window layer 6b. All layers can
 be formed or grown by conventional techniques.
 Semiconductor LEDs in accordance with embodiments of the present invention
 can be employed for emitting light at various wavelengths. An (Al.sub.x
 Ga.sub.1-x).sub.0.5 In.sub.0.5 P system can be employed for light
 generation region 30 in manufacturing semiconductor LEDs emitting light at
 wavelengths ranging from 565 to 620 nm. For example, when x=0.4, the
 semiconductor LED emits light at 565 nm. An n-type GaAs material with an
 impurity concentration of about 2.times.10.sup.17 atoms cm.sup.-3 to
 4.times.10.sup.18 atoms cm.sup.-3 can be employed as substrate 20. Light
 generation region 30 can comprise a 1 micron thick n-type (Al.sub.x
 Ga.sub.1-x).sub.0.5 In.sub.0.5 P layer 3a formed on the GaAs substrate. A
 0.5 micron-thick undoped (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P layer
 3b is grown on layer 3a and a 1 micron-thick p-type (Al.sub.x
 Ga.sub.1-x).sub.0.5 In.sub.0.5 P layer 3c formed on layer 3b. A 5-10
 micron-thick highly doped GaP current diffusion layer 80 is then grown on
 light generation region 30. A 50-100 nm-thick In.sub.x Ga.sub.1-x
 As(x=0.5) transition layer 6a is then grown on current diffusion layer 80
 to reduce contact resistance. It was found that such a heavily doped
 p-type narrow bandgap thin In.sub.x Ga.sub.1-x As layer, with a carrier
 concentration of about 5.times.10.sup.17 to 4.times.10.sup.19 atoms
 cm.sup.-3 advantageously reduces the current crowding effect without
 adversely affecting light output.
 A 0.2 micron-thick zinc oxide layer is then formed as the transparent
 conducting oxide layer 6b on top of layer 6b. The zinc oxide is
 advantageous in various respects. For example, zinc oxide is non toxic,
 chemically stable and can be deposited at a relatively low cost. In
 addition, zinc oxide exhibits higher optical transmission than tin oxide,
 indium oxide and indium-tin oxide, in the wavelength region of about 370
 to 1700 nm.
 Embodiments of the present invention also encompass semiconductor LEDs
 emitting light at about 650 nm employing an Al.sub.x Ga.sub.1-x As
 material system for the light generation region 30. For example, adverting
 to FIG. 4, light generation region 30 can comprise a 1 micron-thick n-type
 Al.sub.0.6 Ga.sub.0.4 As layer 3a on an n-type GaAs substrate. A 0.5
 micron-thick undoped AlGaAs layer 3b is formed on 3a, and a 1 micron-thick
 p-type Al.sub.0.6 Ga.sub.0.4 As layer 3c is formed on 3b. A 5-10
 micron-thick highly doped Al.sub.x Ga.sub.1-x As (x=0.6-0.8) current
 diffusion layer 80 is formed on layer 3c. A 50-100 nm-thick highly doped
 p-type In.sub.x Ga.sub.1-x As (x=0.5) transition layer 6a is grown on top
 of current diffusion layer 80. A 2 micron-thick zinc oxide window layer 6b
 is then formed on transition layer 6a. By selecting "x" as 0.6, the
 resulting semiconductor LED would emit light at a wavelength of about 620
 nm.
 Embodiments of the present invention also include semiconductor LEDs
 emitting light at relatively long wavelengths, such as 1.3 or 1.5 micron,
 employing an InGaAsP-InP material system for light generation region 30.
 For example, adverting to FIG. 4, substrate 20 comprises n-type InP with
 an impurity concentration of about 5.times.10.sup.17 to about
 3.times.10.sup.18 atoms Cm.sup.-3. The light generation region 30 formed
 on substrate 20 includes a 1 micron-thick n-type InP layer 3a, a 0.2
 micron-thick InGaAsP layer 3b as the active layer on 3a, and a 1
 micron-thick p-type InP layer 3c on active layer 3b. A 5-10 micron-thick
 highly doped p-type InP layer is grown as the current diffusion layer 80
 on layer 3c. Subsequently, a 50-100 nm-thick In.sub.0.5 Ga.sub.0.5 As
 transition layer 6a is formed on current diffusion layer 80. A 0.2
 micron-thick zinc oxide layer is then grown on the transition layer 6a,
 and contact layer 70 is formed thereon.
 Thus, the present invention advantageously enables the formation of
 semiconductor LEDs which exhibit significantly reduced current crowding
 and high efficiency together with significantly enhanced brightness and
 longevity. Semiconductor LEDs in accordance with embodiments of the
 present invention enjoy applicability in various technological industries,
 including optical communication, optical information processing, and as a
 light source. Semiconductor LEDs in accordance with embodiments of the
 present invention can be fabricated employing conventional equipment and
 techniques, to manipulate various wavelengths depending upon the
 particular semiconductor material system selected.
 In the previous description, numerous specific details are set forth, such
 as specific materials, structures, thicknesses, dopant concentrations,
 etc., to provide a better understanding of the present invention. However,
 the present invention can be practiced without resulting to the details
 specifically set forth. In other instances, well known processing
 materials have not been described in detail in order not to unnecessarily
 obscure the present invention.
 Only the preferred embodiment of the present invention and but a few
 examples of its versatility are shown and described in the present
 disclosure. It is to be understood that the present invention is capable
 of use in various other combinations and environments, and is capable of
 changes and modifications within the scope of the inventive concept as
 expressed herein.