HIGH EFFICIENCY LIGHT EMITTERS WITH REDUCED POLARIZATION-INDUCED CHARGES

Abstract: Naturally occurring polarization-induced electric fields in a semiconductor light emitter with crystal layers (2-7) grown along a polar direction are reduced, canceled or reversed to improve the emitter's operating efficiency and carrier confinement. This is accomplished by reducing differences in the material compositions of adjacent crystal layers (2-7), grading one or more layers to generate space charges and quasi-fields that oppose polarization-induced charges, incorporating various impurities into the semiconductor that ionize into a charge state opposite to the polarization induced charges, inverting the sequence of charged atomic layers, inverting the growth sequence of n- and p-type layers in the device, employing a multilayer emission system instead of a uniform active region and/or changing the in-plane lattice constant of the material.

Description: HIGH EFFICIENCY LIGHT EMITTERS WITH REDUCED POLARIZATION-INDUCED CHARGES RELATED APPLICATION This application claims the benefit of provisional patent application Serial No. 60/168,495, filed December 2, 1999. BACKGROUND OF THE INVENTION Field of the Invention This invention relates to light emitting compound semiconductor crystals grown on a polar surface and, more particularly, to reduction or cancellation of their naturally occurring polarization-induced charges to improve emission efficiency. Description of the Related Art Most semiconductor light emitters have a double heterostructure structure that includes an active or light- generating layer grown between two cladding layers. The various layers of the double heterostructure are fabricated from more than one material. One cladding layer is n-type, which means it contains excess free electrons, and one cladding layer is p-type, which means it contains excess holes. In general, the cladding layers have larger band- gaps than the active layer. This causes injected electrons and holes to be confined within the active layer, encourag- ing efficient recombination of free carriers through spatial localization within the active layer to produce light. In addition, laser diode (LD) emitters also have separate lignt confining layers, typically comprised of a material with an even wider bandgap, surrounding a double heterostructure. Double heterostructure semiconductor devices are αescribed in numerous publications, including O'Shea ec al, Introduction to Lasers and Their Applications, Addison- Wesiey Publishing Company, December 1978, pages 166-167. In such structures, polarization-induced charges occur when tne material composition varies in a polar direction of ts basic crystal structure. A polar direction is de- fmeα as any crystal direction not orthogonal to the polarization vector, P , of the crystal. This is especially true for materials wnose crystal bonds are naturally directional and even slightly ionic, such as in III-V or 11 -VI semiconductors. These charges can be strain-related (pie- zoelectπc) m the case of lattice mismatched materials, composition related (spontaneous) due to differences in the ionic strengths of bonds in different materials, or a combination of the two. The induced charges cause electric fields or potential gradients that have the same effect on free carriers as external fields. The phenomenon is discussed in a number of publications, including Bernardini et al, "Spontaneous polarization and piezoelectric constants of III-V nitrides," American Physical Society Journal, Physics Review B, Vol. 56, No. 16, 1997, pages RIO 024 to RIO 027, and Takeuchi et al, "Quantum-Confined Stark Effect due to Piezoelectric Fields in GalnN Strained Quantum Wells," Japanese Journal of Applied Physics, Vol. 36, Part 2, No. 4, 1997, pages L382 - L385. The magnitudes of such fields have been estimated to be as high as 2.5xl0 6 V/cm for nitride heterostructures grown on a polar surface of a crystal, Bykhovski et al., "Elastic strain relaxation and piezo-effect in GaN-AIN, Gan-AlGaN and GaN-InGaN superlat- tices", Journal of Replied Physics, Vol. 81, No. 9, 1997, pages 6332-6338. charges should be taken into account wnen considering the electrical characteristics of heterostructures grown on crystal polar surfaces. Crystal layers grown along the 0001 orientation in the case of wurtzite GaN crystal, or along the 111 orientation in the case of zincblende GaAs crystals, are two examples of crystal polar surfaces. The Bravais lattice of the wurtzite structure is hexagona_, with the axis perpendicular to the hexagons usually labeled as the c-axis or the 0001 orientation. Along this axis the structure can be thought of as a sequence of atomic layers of the same element (e.g. all Gallium or all Nitrogen) built up from regular hexagons. Due to this uniformity, each layer (or surface) is polarized ana possesses either a positive or a negative charge, generating a dipole across the atomic layers. The charge state of each layer depends upon its constituent atoms. Other examples of crystal planes with various growth direc- tions may be found in Streetman, Solid State Electronic Devices, 2nd ed., Prentice-Hall, Inc., 1980, pages 1-24, and Shuji Nakamura et a_, "The Blue Laser Diode, GaN Based Light Emitters and Lasers," Springer, 1997, pages 21-24. Until recently, internal polarization fields associ- ated with the active and cladding regions of a light emitting heterostructure have not posed significant problems. This was because light emitting diodes (LEDs) based on the more established Al-Ga-In-As-P material system have typically been grown on a non-polar crystal surface (in par- ticular the 001 zmcolende surface) . Recently, however, there has been considerable work in light emitters based on the Al-Ga-In-N ("nitride") materials system, mostly grown along the 0001 orientation of wurtzite crystal, which is a nighly polar surface. Nevertheless, nitride double heterostructures have followed conventional non-polar designs. Figure 1A is a sectional view schematically illustrating a typical conventional nitride double heterostructure semiconductor grown in a polar direction. The illustrated suDstrate layer 1 may be any material suitable for growing nitride semiconductors, including spinel (MgAl 2 0 4 ) , sapphire (A1 2 0 3 ) , SiC (including 6H, 4H, and 3C) , ZnS, ZnO, GaAs, A1N and GaN. The substrate thickness typically ranges from lOOμm to 1mm. A buffer layer 2 on the substrate 1 can be formed of A1N, GaN, AlGaN, InGaN or the like. The buffer layer facilitates possible lattice mismatches between the substrate 1 and an overlying conductive contact layer 3. However, the buffer layer 2 may be omitted if the substrate has a lattice constant approximately equal to that of the nitride semiconductor. The buffer layer 2 may also be omitted with some nitride growth techniques. Depending upon the material composition, the buffer layer energy bandgap may range from 2.1eV to 6.2eV, with a thickness of about 0.5μm to lOμ . The n-type contact layer 3 is also typically formed from a nitride semiconductor, preferably GaN or InGaN with a thickness ranging from 0.5μm to 5. Oμm, and a bandgap of approximately 3.4eV for GaN and less for InGaN (depending upon the Indium concentration) . A lower n-type or undoped cladding layer 4 on the conductive layer 3 conventionally comprises GaN or AlGaN, with a bandgap of 3.4eV for GaN and greater for AlGaN (depending upon the Al concentration) . Its thickness can range from lnm to lOOnm. Nitride double heterostructures typically employ InGaN as an active region 5 over the lower cladding layer, with a thickness of lnm to lOOnm. The bandgap of this layer is typically 2.0eV, but may vary depending upon the Indium concentration. A top p-type or undoped cladding layer 6 over the active region is generally comprised of AlGaN or GaN, with a thickness and bandgap energy similar to that of the lower n-type cladding layer 4. A p-type GaN conductive contact _ayer 7 on the cladding layer 6 has an energy band- gap of about 3.4eV and a thickness of about lOnm to 500nm. In general, provided the structure is grown on a polar direction such as the 0001, a polarization-induced sheet charge occurs at the interface between layers due to dif- ferent constituent materials. Of particular concern for the operation of a light emitter are the polarization- induced cnarσe sheets adjacent to the active region 5. Witr tne compound semiconductor illustrated in Figure 1A, a negative polarization-induced charge sheet density σl, with a magnitude such as 10 13 electrons/cm 2 , is typically formed at the interface between the active region 5 and the ^ ower cladding layer 4. A positive charge sheet density σ2 of similar magnitude is formed at the interface between the active region 5 and the upper cladding layer 6. The polarities of these charges depend upon the bonds of the crystal layers, which as mentioned above are directional and slightly ionic. In general, the density of a charσe sheet will depend upon both a spontaneous factor arising from compositional differences between the two lay- ers, and a piezoelectric strain arising from the lattice mismatch between the layers. For example, σl between an In 0 2 Ga 0 B N active region 5 and a GaN cladding layer 4 is about 8.3xl0 12 electrons/cm 2 . This is due to the 20% Indium content n the In 0 2 Ga 0 sN active region (spontaneous polaπ- zation) , and the strain in that layer arising from the lattice mismatch with the underlying GaN layer (piezoelectric polarization) . Interfacial charge sheets along opposite surfaces of the active region produce a dipole across the region. This dipole corresponds to an electric field whose strength depends on the magnitude of the sheet charges σl and σ2. For the case given above, a sheet charge of 8.3xl0 12 cm "2 gives an electric field of 1.5χl0 6 V/cm. Based on its origin, we will refer to this electric field as a polarization-induced field. The magnitude of the electrostatic potential drop generated by the dipole depenαs upon the thickness of the dipole layer. The thickness of the dipole layer refers to its physical dimension in the direction of growth, which is also the distance between σl and σ2. This distance can be used to determine the magnitude of the electrostatic poten ¬ tial drop in a manner similar to the determination of a ca- pacitive potential drop from the distance between two capacitor plates. A distance of lOnm between charge densities σl and σ2 as given above would result in a polarization- induced potential drop of about 1.5V across the active region 5. The net electric field across the active region also depends on a number of parameters including the doping concentration in the surrounding cladding layers, the built in voltage across the p-n junction and free carrier screening, and is therefore not generally equal to the polarization induced field. However, due to its strength, the po- larization-mduced field plays a major role in determining the net electric field. Nitride emitters grown on a 0001 (polar) surface of a crystal have a low emission efficiency of about 1% to 10%. This can be due to the presence of significant polarization fields in or adjacent to their active regions that limit their efficiency. Figure IB illustrates the energy bands corresponding to the device structure of Figure 1A. When the device is operating, the naturally occurring polariza- t on fιe_d generated oy σi and σ2 reduces the efficiency in a numoer of ways. First, the dipole leads to a spatial separation (movement m the opposite directior ) of electrons and holes within the region. As illustrated, holes in the valence band E v are attracted to the negative charge sheet σi at one end of the active region 5, while electrons m the conduction band E c are attracted to tne positive charge sneet σ2 at its other end. This spatial separation of free carriers lowers the probability of radiative recom- bination, reducing emission efficiency. Second, the energy barriers of the conduction and valence band quantum wells are reduced by quantization effects associated with the electric field. Thus, carriers below E v and above E c escape the well through the paths indicated by dashed lines A. Third, the presence of polarization-induced fields also leads to carrier overshoots, illustrated by carrier trajectories B, from the higher E c level on the σl side of the active region to the lower E c level on the σ2 side, and from the lower E v level on the σ2 side of the active region to the higher E v level on the σl side. Anotner issue of concern for applications engineers is the stability of the emission wavelength as tne applied bias is increased. If strong polarization-incaced fields are present, the emission wavelength will blue-snift as the device bias is increased. As the device bias is increased, more free carriers accumulate in the conduction and valence band wells. Since the free carriers are spatially separated, they will themselves form a dipole that opposes, or screens, the built-in polarization induced field. As the net electric field is reduced, the quantization states of the quantum wells cnange, resulting in a blue-snift of the emission wavelength. Figure 1C illustrates the energy bands of the active layer 5 and the cladding layers 4 and 6 for a light emitter operating on a non-polar surface with no polarization- induced charges. All else being equal, its emission effi- ciency is higher since the three effects discussed above are either absent or greatly reduced. Several approaches to increase GaN-based LED efficiency have been used. U.S. Patents 5,959,307 and 5,578,839, both to Nakamura et al, discuss the addition of Aluminum to the cladding layers to increase the active region barrier heights for a more efficient confinement of free carriers. This addition, however, also changes the material composition of the cladding layers from GaN to AlGaN, which act to increase both spontaneous and piezoe- lectric polarization fields. The presence of 15% Aluminum in an Al 0 l sGao 85 N cladding layer could double the polarization field in the emission layer to about 3χl0 6 V/cιr. Such fields may reduce carrier confinement and increase the spatial separation of carriers by changing the energy bands of the light emitter, thereby lowering its radiative efficiency. SUMMARY OF THE INVENTION The present invention seeks to improve the operating efficiency of a compound semiconductor LED, with layers grown along a polar direction by: reducing or canceling the effect of the crystal's naturally occurring polarization- induced charges to improve carrier confinement, reducing their spatial separation, and reducing carrier overshoot. In one embodiment, these charges are lowered by reducing differences m the material compositions of the crystal layers adjacent to the active region. The cladding layers can also be composed of a combination of elements, each of which teno to cancel the polarization effects of the others . Ore or more layers m or around the active region can also be graded in composition or doping to generate space charges that oppose the polarization-induced charges, and quasi-fields that oppose polarization-induced fields generated by the polarization-induced charges. The grading may be continuous or discrete. The compound semiconductor crystal can also have a multilayer emission system consisting of alternating light emitting and non-emitting layers to reduce the average polarization field while improving emission efficiency. The average field in the multilayer emission system as a whole is reduced or canceled compared to a single, uniform active region of comparable thickness. Various impurities can be incorporated into the crystal that ionize, based upon their energy levels, into a charge state opposite to polarization-induced charges to reduce or cancel their effect. The impurities preferably comprise group II, IV, or VI elements. The sign of the polarization induced charges can also be inverted to encourage, rather than oppose, the efficient confinement of carriers . These charges are inverted by inverting the atomic layer sequence of the crystal layer. The direction from which the carriers are injected can also be inverted, by inverting the growth order of p and n type layers, to screen polarization induced charges. The lattice constant of the lower buffer layer, contact layer, or cladding layer can also be changed by epitaxial growth techniques to more closely match the lattice constant of the active region. This reduces the strain-induced piezoelectric effect within the active region, reducing the po- larization-mduced fields for more efficient light emission. These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A is an illustrative sectional view of a known structure for a nitride light emitter; Figure IB illustrates the energy bands corresponding to the device of Figure 1A grown on a polar surface; Figure 1C illustrates the energy bands of a known light emitter grown on a non-polar surface; Figure 2A illustrates the energy bands of the active region and cladding layers with various impurities; Figure 2B is a graph illustrating impurity profiles; Figure 3A is an illustrative sectional view of a nitride light emitter having an InGaN cladding layer in ac- cordance with the invention; Figure 3B illustrates the energy bands corresponding to tne device of Figure 3A; Figures 4A and 4B respectively illustrate the energy bands of a nitride semiconductor with ternary AlGaN and quarternary AlInGaN cladding layers; Figure 5 is a graph illustrating the relationship between the concentration of atoms as a function of time during cladding growth; Figure 6A illustrates the energy bands of an active region in a quasi-field; Figures 6B and 6C respectively illustrate the energy bands of continuously graded active regions; Figure 7A illustrates the energy bands of a multilayer light emission system; Figure 7B illustrates the energy bands of a single active region with a width equivalent to the multilayer e is- sion system of Figure 7A; Figure 8A is a sectional view illustrating the atomic layer structure corresponding to a single layer of the structure shown in Figure 1A; Figure 8B is a sectional view illustrating an inverted atomic layer structure of a single semiconductor layer; Figure 8C illustrates the energy bands of cladding and emission layers corresponding to the device structure of Figure 8B; Figure 9A is a sectional view illustrating an inverted nitride light emitter with p-type layers grown before n- type; Figure 9B illustrates the energy bands corresponding to the device of Figure 9A; Figure 10A is a sectional view illustrating a nitride light emitter with a new buffer that has a lattice constant more closely matched with the active region; and Figure 10B illustrates the energy bands corresponding to the device of figure 10A. DETAILED DESCRIPTION OF THE INVENTION The following description of various embodiments of the present invention is directed to nitride emitter systems with a double heterostructure construction whose crystal layers are grown normal to the polar direction of the crystal. The nitride emitter is presumed to have the wurtzite crystal structure with layers comprising Al x In y Gax- < - v N, where O≤x≤l and O≤y≤l . Except where noted, the top surface of the crystal is the 0001 orientation witn the peπ- odic table group III polarity. The nitride emitter illustrated m Figure 1A with the corresponding band structures m Figure IB will be used as a reference for the various embodiments . SELECTIVE DOPING This embodiment reduces or cancels the adverse effects of polarization-mduced by incorporating various dopants into the semiconductor. The dopant impurity should be of a type tnat does not diffuse away from its intended position. The dopants ionize, based upon their energy levels, into either positive or negative charge states, which are opposite to the mterfacial polarization-mduced charge state, to cancel or reduce its effect. The type of dopant used de- pends upon the mterfacial charge (positive or negative) being targeted. A positive charge would require a dopant that is ionized into a negative charge state, and the opposite for a negative mterfacial charge. Figure 2A illustrates the band structure of clad- dmg/active/claddmg layers incorporating approximately 10 13 Si atoms/cm 3 as a source of positive charge to reduce or cancel the negative mterfacial charge density σl . To reduce or cancel the positive charge density σ2, 10 13 atoms/cm" of Mg may be used as a source of negative charge. The dopant concentration profile will depend in part upon the dopant lonization energy and donor/acceptor levels. For example, a higher dopant concentration would be required if Zinc (Zn) were used as a source of negative charge dopant to reduce σ2. The impurity profile of the dopant does not have to exactly match/cancel the polarization-mduced charges to show a benefit. Other dopant impurities may include group II, IV, or VI elements. There are several methods for distributing (or incorporating) impurities into the semiconductor. For abrupt changes in material composition, m which polarization- induced charges occur in a plane of the interface, the lm- purity profile is preferably delta-doped at or near the plane. An abrupt change is one in which the change in mole fraction of a given constituent is greater than 1% over a monolayer, i.e. a single crystal layer of atoms. Delta doping attempts to confine the dopants to a single atomic layer, producing a dopant sheet rather than a dopant volume. For graded composition changes, either in discrete steps or continuous, the impurity profile is preferably also graced. Figure 2B illustrates various types of impurity profiles, including delta 61, graded in discrete steps 62 and continuously graded 63. INTERMEDIATE COMPOSITION BARRIERS This embodiment is directed to improving the emission efficiency of light emitters having GaN cladding layers. The material composition of one or both cladding layers is made intermediate with respect to the material composition of their adjacent layers to reduce or cancel both piezoelectric and spontaneous mterfacial polarization-mduced charges. Approximately 5% Indium (In) can, for example, be added to the GaN cladding layer 4 of the nitride system of Figure 1A to change the cladding layer's composition to Ino osGao 95 . This would make the cladding layer composition intermediate to the In 0 2 oGa 0 βo active region 5, with 20% Indium, and GaN conductive layer 3 with 0% Indium. As ll- lustrated in Figure 3A, the Indium reduces differences in the material compositions of the two adjacent layers, resulting in a 25% lower mterfacial polarization-mduced charge sheet density σlA of approximately 0.75xl0 13 elec- trons/crrr between the active and cladding layers. σlB is about 0.25χl0 ±3 electrons/cm 2 . If donor-type doping is pres ¬ ent near one or both of the interfaces that contain σlA or σlB, some of the piezo- duced charge will be screened out, as discussed in the selective doping embodiments above. Since σlA and σlB are both smaller than the combination σlA+σlB, this device structure eases the ability to selectively screen out some of the polarization-mduced charges, compared with the conventional structure, while keeping dopants away from the active region. This is done by heavily n-type doping the interface with σlB so that all of this charge is effectively screened. In this way, the electric field in the active region is reduced by as much as 25% over the conventional structure, increasing emission efficiency. Figure 3B illustrates the band diagram for this case. Further reduction is obtained if n-type doping extends towards interface containing σlA. The more Indium used in the cladding layer, the lower the field in the active region. However, if too much Indium is used, the car- πer confinement may be compromised. The same technique can be applied to the upper cladding layer, except that p- type doping should be used for the charge screening. By adding Indium to one or both of the cladding layers, the polarization-mduced electric field in the active region under device operation goes down, leading to an increase in the emission efficiency despite lower energy barriers. Improved LED efficiency m the 450nm to 470nm range was demonstrated using a low Indium content of about 5% m the lower cladding layer 4, compared to a cladding layer with no Indium. QUARTERNARY BARRIERS This embodiment -.s directed to improving the emission efficiency of light emitters having ternary AlGaN cladding layers. The addition of Aluminum Nitride (A1N) to the original GaN cladding layers to produce ternary AlGaN lay- ers is known. This addition generates cladding layer band- gaps that are larger than the bandgaps of their adjacent contact and active layers to produce higher energy barriers on the opposite sides of the active region, thereby improving carrier confinement. However, the added Aluminum also increases variations in material composition that produce mterfacial polarization-mduced charges. In fact, both spontaneous and piezoelectric polarization-mduced charges, which are determined by the change in the polarity of Ga to N bonds from one layer to the next, are actually increased at the active region/claddmg layer interfaces. The increase in charges is due to both the piezoelectric strain and spontaneous polarization differences n the two materials. As a result high electric fields exist n the active region of the device. One aspect of the invention reduces the strain and polarization differences by making their material compositions more similar. Indium Nitride (InN) is added to one or both of the ternary AlGaN cladding layers to produce a quarternary AlInGaN layer, which makes the average polarity of the bonds in both the cladding and the active layers similar. The presence of Indium in their layers counteracts the effect of Aluminum in generating mterfacial piezoelectric polarization-mduced charges at the active layer/cladding layer interfaces. That is, there is a smaller charge sheet between a quarternary AlInGaN cladding layer and an InGaN active region, than there is between a ternary AlGaN cladding layers and an InGaN active region. Most of the polarization-mduced charge in the structure is confined tc heavily doped regions within the cladding or contact layers that are removed from the active region and can be effectively screened. The amount of Indium added to these layers will generally depend upon the layer thick- ness, material composition, and growth limitations. For example, an Alo ι 2 Ino o 3 Gao 85 N cladding layer next to an In 0 05 Ga o 95 N active layer reduces the mterfacial piezoelectric charge density by approximately 30%, to 0.7xl0 13 electrons/cm 2 , compared to an Al 0 lsGao 85N layer next is the same active layer, reducing the electric field in the active region by approximately 30%. The Indium and Aluminum components of the quaternary cladding layers tend to cancel each other's effect on the energy barriers associated with the active region. Althougn the addition of Indium to the AlGaN cladding layer lowers the active region's energy barriers, the confinement efficiency actually increases due to reductions in polarization-mduced charges. The cladding layers need not have identical composi- tions to improve the emitter efficiency. For example, improved LED efficiency of about 25% at an emission wavelength of 380nm was demonstrated using only an upper quaternary Alo i 5 ln 0 o 3 Ga 0 82 N cladding with energy barriers of 0.26eV Ec and O.OδeV E v . The composition of the lower • ciad- ding layer was maintained as Al 0 ιsGa 0 85 . Schematic diagrams under forward bias conditions close to turn-on of an LED (i.e. close to the threshold for light emission) , for cladding layers with ternary AlGaN and quarternary AlInGaN compositions, are illustrated in Figures 4A and 4B, respectively. In both structures the cladding layers 4 and 6 contain the same concentration of Aluminum, but with no Indium m the ternary structure. In both figures, the slope of the line 30 in the active region 5 represents the polarization-mduced field. Its strength depends upon the biasing between the contacts 3 and 7, doping in the cladding layers 4 and 6, and tne magnitude of the mterfacial polarization charge sheet densities. The biasing used for both structures is about 2.6V, with an n-type (Si) dopant concentration of approximately lxl0 18 /cm 3 m the cladding layer 4, and a p-type (Mg) dopant concentration of approximately Ixl0 19 /cm3 in the cladding layer 6. The magnitude of the polarization induced electric field generated across the active region 5 with the ternary AlGaN cladding layers is approximately 8.8xl0 3 V/cm. As illustrated m Figure 4A, this field tends to spatially separate the electrons and the holes injected from GaN contact layers 3 and 7 by confining the carriers near interfaces 31 that are furthest away from their respective injecting contacts. This phenomenon reduces the radiative recombination probability of the carriers. The quarternary AlInGaN cladding layers 4 and 6 illustrated in Figure 4B exhibit a much lower polarization induced field in the ac- tive region 5. For an Al 0 osIn 0 02 sGa 0 925N lower cladding layer 4 and an Al 0 ,,oIn 0 ιoGa 0 70N upper cladding layer 6, the field is approximately 4.6xl0 5 V/cm. These layers also exhibit lower quantization energies 31, reducing the spatial separation of free carriers to improve radiative recombination. There is a trade-off between the reduction of polarization- mduced charges and the maintenance of high energy barriers for maximum carrier confinement, which can be determined empirically. CLADDING LAYERS WITH GRADED COMPOSITION In this embodiment, the composition of one or both of the cladding layers is graded to generate a space charge that opposes mterfacial polarization effects at the ter- faces cetween the cladding layers and the active region. The grading varies the layer composition in the polar di ¬ rection to generate piezoelectric charges. The grading may oe either continuous or discrete. The distributed charge polarity should be opposite to that of the targeted mterfacial polarization charges. The polarity is determined by t e composition of the graced layer and its two adjacent layers. For example, the lower GaN contact layer 3 and the lower AlGaN cladding layer 4 of Figure 1A generate a posi- tive mterfacial charge sheet density. This charge may be distributed over the volume of the cladding layer 4 by gradually varying the Aluminum content of this layer. In particular, the lower cladding layer 4 may be graded from a GaN composition with 0% Aluminum near the GaN conductive layer 3, to Alo . ioGao g 0 N with 10% Aluminum near the InGaN active region 5. The positive space charge generated would partially counter the 0.75xl0 lj electrons/cm" negative charge sheet density σl at tne Alo . ioGao 9 oN/In 0.05 Gao 95 N interface . The negative mterfacial charge density that exists between the upper GaN contact layer 7 and the upper AlGaN claαdmg layer 6 may be distributed over the volume of layer 6 by grading its Aluminum content. The negative space charge generated would counter the positive charge density σ2 to reduce or cancel its effects upon the active region. Tne magnitude of the space charge depends upon the distance ever which the grade occurs. Figure 5 graphically illustrates the grading of the AlGaN cladding layer by the addition of Aluminum atoms (continuously 41 or discrete steps 42) to the crystal surface per unit of time, starting with 0% Aluminum. The added concentration of Gallium and Nitrogen atoms 40 is kept uniform throughout the growth period. Tne process is stopped when an appropriate cladding layer thickness and grading is achieved. A varying period super- lattice of two materials can also be used to make a compositional grade. There is a traαe-off between the optimal device per- formance due to availability of free carriers near the ac ¬ tive region, and the adverse effects of dopant diffusion into it. The space cnarge which is generated by grading the cladding layer allows the dopant impurities to be placed outside the graded layer (e.g. in the conductive layer only), while attracting their free carriers. The space charge associated with the graded composition naturally attracts the free dopant carriers into the cladding layer from the adjacent conductive layer. Therefore, doping impurities need not be near the active region to provide free carriers to this region, and may be eliminated or reduced in the adjacent cladding layers. ACTIVE REGION WITH GRADED OR MIXED COMPOSITION The effect of compositional changes in a semiconductor device on free carriers can be similar to that of an electric field, and are referred to as quasi-flelds . See Herbert Kroemer, "Band Offsets and Chemical Bonding: The Basis for Heterostructure Applications", The Journal of Physica Scπpta, Vol. T68, pages 10-16, 1996. This embodiment of the invention uses a quasi-field to counter the effects of polarization-mduced electric fields on free carriers. The establishment of a quasi-field in the active region is possible without the presence of any charges within the system. Figure 6A illustrates quasi-fields that are generated by gradients 51 n the energy bands, rather than from true electric charges. These gradients can be produced by grading the composition of the active region 5. In a quasi- field, electrons m the conduction band move to a lower en- ergy level towards the positive cnarge, and the holes in the valence oand move to a higher energy towards the negative cnarge. Note that the active region band structures in this field are not parallel. By contrast, the energy bands Λ tn an electric field exhibit a parallel relationship, as illustrated in Figure IB. The active region can be com- positionally graded to generate a desired gradient in band structure, and thereby a desired quasi-field effect. The resulting quasi-field, however, should oppose the true e ectπc field's effect on at least one carrier type. The active region material composition may be graded (continuously or discretely) by varying its Indium content. Depending upon the region's width and the desired emitter properties, it may incorporate a graded composition from a low to a high Indium content or vice versa. Properties of concern typically include the emission wavelength and the operating current . Figures 6B and 6C illustrate the net effect of quasi-fields on the energy bands of the active region 5. In Figure 6B the active region has a continuously graced Indium concentration from a low of 5% to a high of 10%, with a gradient of approximately 1%/nm. The Indium concentration is lowest at the interface between the cladding layer 4 and the active region 5, and increases to the highest cσncen- tration on the opposite side of the active region. In this case the quasi-field reduces the polarization-mduced electric field in the valence band. A grading in the opposite direction will offset the electric field in the conduction band. In Figure 6C, the active region has an Indium gradi- ent and concentrations again from 5% to 10% and an average gradient of 1%/nm, but in the opposite direction. Since one of the carrier types, elections or holes is now spread out, tnere is better spatial overlap of tne carriers, increasing emission efficiency. MULTILAYER EMISSION WITH FIELD-COMPENSATION BARRIERS Figure 7A illustrates the energy bands for a multilayer emission system 90 with field-compensation energy barriers 93. In this embodiment, the emissions system 90 consists of multiple layers with alternating active regions 91 (i.e. light emitting) and cladding layers 92 (non- emitting) . The energy barriers 93 of the claddings 92 have the dual function of confining injected carriers in the active region 91, and opposing its polarization-mduced fields. The generated field and total thickness of the multilayer emission system 90 will in general depend upon the number, thickness, and composition of the individual active regions 91 and the cladding layers 92. The multilayer system 90, with four In 0 ιGa 0 9 active regions 91 each 2nm thick and three Al 0 osGa 0 95 N cladding layers 92 each 5nm thick, would have a total thickness of 23nm with an ap- proximate average polarization-mduced field strength of 4.5xl0 5 V/cm. The band structure of a single active region 100 with comparable volume, illustrated in Figure 7B, could exhioit a polarization field strength of 9xl0 5 V/cm. The use of a multilayer emission system increases the total volume of the active material while ensuring that the average field due to the polarization-mduced charges in the active region as a whole is reduced compared to a structure with a single active region of comparable volume. INVERTED POLARIZATION In this embodiment, the naturally occurring polarization-mduced charges of a compound semiconductor are inverted to improve carrier confinement. The interatomic bonds in GaN are naturally _onιc because the Gallium atom -_.s slightly positive and the Nitrogen atom negative, generating a dipole across t^e bond. Figure 8A illustrates individual Gallium and Nitrogen atomic layers grown along the polar direction of the crystal surface. With the order illustrated, successive atomic layers alternate oetween Ga and N, with the bottom surface 70 of each layer a single atomic layer of Gallium atoms and the top surface 71 Nitrogen. The individual dipoles n 2 across each GaN atomic pair add up to generate an average polanzation-mducea field 73 across the layer in the direction indicated. Its effect on tne energy bands of the active region was descnoed aoove m connection with Figure IB. The direction of this naturally occurring polaπza- tion-mduced field 73 can be inverted by inverting the direction of the individual atomic dipoles 72. This is accomplished by reversing the growth order of the Gallium and Nitrogen atomic layers. In Figure 8B, the inverted growth order of atomic layers starts with Nitrogen, ano than al- ternates between Gallium and Nitrogen atomic layers until the top surface Ga layer 74 is reached. The growth order of atomic layers can be changed in several ways. First, if one starts with a N-termmated GaN or AlGaN substrate, one can αrow the N-faced polarity without difficulty. However, most growth is done on sapphire or S C and the growth naturally desires to be of the Ga-polaπty. There are techniques to change the polarity . One technique involves using MBE growth under N-rich conditions to change tne polarity. A second technique uses deposition of about 1 mono- layer of Magnesium on the surface, which results m N- polaπty growth for subsequent layers. A third .ay is to use atomic layer epitaxy by MOCVD or MBE to try to force the nucleation to be m the correct polarity. The individual dipoles 75 generate a polarization- mduced field 76 in a reverse direction to field 73 of Figure 8A. Its effect on the energy bands of the active region is illustrated m Figure 8C. Although this growth order continues to generate a polarization-mduced electrical field in the active region, the field's dipole is reversed. This enables the injected carriers to screen (neutralize) the polarization-mduced charge densities σl and σ2 before the device turn-on, i.e. before free carriers start to re- combine. The electrons injected towards the active region from the cladding layer 4 above E build up near σ2 at the claddmg/active region interface. The accumulated charges of these free carriers neutralize σ2. Similarly, the holes injected from the cladding layer 6 below E v neutralize σl near the active region/claddmg interface. Before device turn-on, this process "flattens" the active region energy bands, similar to those illustrated in Figure 1C. Consequently, the efficiency of the device is not reduced by the polarization-mduced charges. A further benefit of this device structure is that the carrier overshoots denoted as paths A are dramatically reduced compared to the conventional structure. Also, the carrier confinement of both electrons and holes is increased. INVERTED STRUCTURE In a conventional LED, the n-type layers are grown before the p-type. This embodiment inverts this growth sequence. Figure 9A is a sectional view schematically illustrating an LED formed with the new growth sequence, with the p-type contact and cladding layers 83, 84 grown before the n-type cladding and contact layers 86 and 87. The thickness and material compositions of all the layers in this embodiment are similar to those for the nitride emit- ter αescπoeo in connection with Figure 1A. Despite an inverted layer structure, mterfacial polarization induced charge sheet densities σl and σ2 are still forward due to variations in material composition between layers. Their effect on the energy bands of the active region is illustrated m Figure 9B, which is essentially similar to Figure IB. However, inverting the growth order of the p- and n- type layers has also changed the direction from which electrons and holes are injected into the active region. In this way the electrons, whicn are always injected from the n-type contact layer, are injected into the active region 85 from the contact layer 87 aoove E c , shown to the left of the active region 85. The holes are injected from the p- type contact layer 83 below E, from the right. This inverted layer sequence enables the carriers to screen the charge densities before the device turn-on in a manner similar to the previous embodiment, in which the field direction was inverted instead of the layer structure. This structure also reduces carrier overshoot and increases carrier con- finement in a manner similar to the previous embodiment. CHANGING THE LATTICE CONSTANT Figures 10A and 10B illustrate another method of engineering the polarization fields in the structure. ' By changing the m-plane lattice constant of the structure beneath the active region 113 so that the active layer is under less strain, or the strain direction is reversed, the piezoelectric polanzation-mcuced fields can be reduced, eliminated, or reversed in the active region as shown in the band diagram of figure 10B. The underlying lattice constant can be changed in several ways. First, the buffer layer 110 can be grown with a different material composition, such as InAlGaN, so that the m-plane lattice con- stant of the buffer layer is close to the m-plane lattice constant of the InGaN active layer. The bandgap of the InAlGaN puffer layer will also be larger than that of the InGaN active layer so that there will be no light absorp- tion. A second method for changing the m-plane lattice constant is to use the conventional buffer layer, but grow at least part of the n-contact layer 111 with a different material composition, such as AlInGaN, than the buffer layer so that the m-plane lattice constant is changed from that of the buffer layer to a value closer to that of the active region. Thus, the same benefits as described above are realized. A third method changes the lattice constant within the lower cladding layer 112 in the same way as described for the n-contact layer. In a fourth method, the active region is grown thick enough to cause strain- relaxation, thus eliminating the piezoelectric-mduced electric field in the active layer. The method chosen will generally be the one providing the least amount of material dislocations, preserving the materials quality while pro- vidmg the device structure advantages described above. There are a number of ways to further reduce or cancel most of the residual polarization-mduced charges. The various embodiments discussed above could be mixed-and- matched and applied to any one or all appropriate layers, emitting or non-emittmg. For any given application, the particular embodiment or combination of embodiments would depend upon the nature of the application, and can be determined empirically. For example, one possibility would be to combine AlGaN cladding layers 92 and InGaN active re- gions 91 with the counteracting effects of Aluminum and Indium mentioned in connection with quarternary barriers. Another would be to grade the Aluminum content of at least one cladding layer 92 and/or to add impurities that ionize into a charged state opposing the mterfacial poiaπzation- duced charges. While several illustrative emPodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled m the art. For example, altnough the description was directed to nitride emitters, any or all of the embodiments may be used to address the problems of polarization-mduced charges in light emitters comprised of other materials. Such varia- tions and alternate emϋodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims: WE CLAIM 1. An optical emitter, comprising: a high efficiency double heterostructure with n- type and p-type contact layers (3,7), and cladding layers (4,6) sandwiching an active light emitting region (5) and grown along a crystal polar direction, said high efficiency heterostructure having a structural difference from a base heterostructure which comprises an active region (5) with atomic dipoles which generate a polarization-induced electric field across the active region (5), and binary or ternary cladding layers (4,6) with said n-type contact layer (3) preceding said p- type contact layer (7) in growth order, said structural difference at least partially alleviating the efficiency reducing effects of a polariza- tion-induced electric field across the active region (5) of said high efficiency heterostructure compared to the polarization-induced electric field of said base heterostructure. 2. The optical emitter of claim 1, said active region (5) comprising InGaN and said structural difference comprising the addition of In to at least one of said cladding layers (4,6) such that at least one of said cladding layers (4,6) comprises Al x In y Gaι- x - y N, where O≤x≤l and O≤y≤l . 3. The optical emitter of claim 1, wherein said structural difference comprises a gradient in the composition of at least one of said cladding layers (4,6) to oppose mterfacial polarization effects at the interfaces be- tween at least one of said cladding layers (4,6) and the active region (5) . 4. The optical emitter of claim 1, wherein said structural difference comprises a gradient in the composition of said active region (5) to oppose said polarization- mduced field. 5. The optical emitter of claim 1, said structural difference comprising the addition of dopant impurities m at least one of said cladding layers (4,6) which reduce said polarization-mduced field. 6. The optical emitter of claim 1, said structural difference comprising an inversion of said active region atomic dipoles (72). 7. The optical emitter of claim 1, said structural difference comprising said p-type contact layer (83) preceding said n-type contact layer (87) in growth order. 8. The optical emitter of claim 1, said structural difference comprising multiple active regions (91) alternating with cladding layers (92) . 9. The optical emitter of claim 1, said structural difference comprising an at least partially stram-relaxed lower cladding layer (112) having an m-plane lattice constant close in value to the m-plane lattice constant of unstrained active region material (113) . 10. The optical emitter of claim 1, said structural difference comprising an at least partially stram-relaxed n-type contact layer (111) having an m-plane lattice con- stant close in value to the m-plane lattice constant of unstrained active region material (113). 11. The optical emitter of claim 1, grown on a buffer layer (110), said structural difference comprising said buffer layer (110) being at least partially strain-relaxed and having an m-plane lattice constant close in value to the m-plane lattice constant of unstrained active region material (113) . 12. A compound semiconductor device grown normal to a crystal polar direction, comprising: at least three crystal layers (2-7) whose material compositions vary in said polar direction; at least one of said layers (2-7) having an intermediate material composition relative to the material compositions of two adjacent layers. 13. The device of claim 12, wherein said at least one layer (2-7) is an Al x In y Gaι_ x - y N cladding layer, where O≤x≤l and O≤y≤l. 14. A compound semiconductor device grown normal to a crystal polar direction, comprising: a plurality of crystal layers (2-7) whose material compositions vary in said polar direction to generate at least one polarization-mduced charge concentration in at least one of said layers (2-7); at least one of said layers (2-7) comprising a cladding layer (4,6) whose composition is graded to generate a space charge over a volume of said grade that opposes said charge concentration. 15. The device of claim 14, wherein said graded cladding layer (4,6) comprises Al x In y Gaι- x - y N, where O≤x≤l and O≤y≤l . 16. A compound semiconductor device grown along a crystal polar direction, comprising: a plurality of crystal layers (2-7) whose material compositions vary in said polar direction; at least one of said layers (4,6) having its composition graded to generate a space charge over a volume of said grade that allows dopant impurities to be set back from an adjacent layer. 17. The device of claim 16, wherein said graded layer comprises Al x In y Gaι- x _ y N, where O≤x≤l and O≤y≤l. 18. A compound semiconductor device grown along a crystal polar direction, comprising: a plurality of crystal layers (2-7) whose material compositions vary in said polar direction to generate a polarization-induced electric field in at least one of said layers; one of said layers comprising an active region (5) whose composition is graded to generate a quasi-field that opposes said polarization-induced electric field. 19. The device of claim 18, wherein said grading generates gradients in the energy bands of said active region (5) to produce said quasi-field. 20. The device of claim 18, wherein said active region (5) is comprised of In y Al x Gaι- x - y N, where O≤y≤l and O≤x≤l. 21. A compound semiconductor device grown along a crystal polar direction, comprising: a plurality of crystal layers (2-7) whose material compositions vary in said polar direction to generate a polarization-mduced charge concentration in at least one of said layers; at least one of said layers (2-7) incorporating dopant impurities that ionize into a charged state opposite to at least one of said charge concentrations. 22. A compound semiconductor device, comprising: a crystal having a polar direction; and a plurality of crystal layers (2-7) whose material compositions vary in said crystal polar direction to generate a polarization-mduced electric field in at least one of said layers, at least one of said layers (2-7) having a polarity alternating atomic layer sequence that reverses the direction of said polarization-mduced electric field m at least one of said layers. 23. A compound semiconductor device, comprising: a crystal having a polar direction; and at least one each of p-type (83,84) and n-type crystal layers (86,87) whose material compositions vary in said polar direction to generate a polarization-mduced charge concentration in at least one of said layers; said p-type layers (83,84) preceding said n- type layers (86,87) in growth sequence to reverse the effect of said charge concentration on said device's perform- ance. 2 . A compound semiconductor device grown normal to a crystal polar direction, comprising: a plurality of crystal layers whose material compositions vary in said polar direction to generate a po- larization-induced electric fields in at least one of said layers; one of said layers comprising a multilayer active region that comprises alternating optical emitting and non- emitting layers (91,92), with said non-emitting layers (92) confining injected carriers in said emitting layers (91) and opposing said electric fields. 25. The device of claim 24, wherein the material composition of at least one of said non-emitting layers (92) is graded to create a quasi-field opposing said electric fields . 26. The device of claim 24, wherein the material composition of at least one of said emitting layers (91) is graded to create a quasi-field opposing said electric fields . 27. The device of claim 26, wherein the material composition of at least one of said non-emitting layers (92) is graded to create a quasi-field opposing said electric fields . 28. The device of claim 24, wherein at least one of said non-emitting layers (92) includes dopant impurities to provide space charge that will oppose said electric fields. 29. A compound semiconductor device, comprising: a crystal having a polar direction; and a plurality of crystal layers (2-7) whose material compositions vary in said polar direction to generate polarization-induced electric fields in at least one of said layers; at least one of said layers (2-7) being an active region (5) ; one of said layers (2-4), below said active re- gion (5), being at least partially strain-relaxed such that the in-plane lattice constant of said layer below the active region (5) is close to the unstrained in-plane lattice constant of said active region (5), thus reducing, eliminating, or reversing said electric field in said active re- gion (5) . 30. A compound semiconductor device, comprising: a crystal having a polar direction; and a plurality of crystal layers (2-7) whose material compositions vary in said polar direction to generate polarization-induced electric fields in at least one of said layers; at least one of said layers (2-7) being an active region (5) ; said active region (5) grown so that material in said device is at least partially strain-relaxed, thereby reducing said electric field in at least said active region (5) .

Inventors: IBBETSON JAMES, THIBEAULT BRIAN, IBBETSON, JAMES, THIBEAULT, BRIAN
Applicants: CREE LIGHTING COMPANY
Application year: 2000
Main classification: H01L33
IPC codes: H01L33