TW595023B - A light emitting diode with a current blocking structure and method for making the same - Google Patents

Abstract

The present invention discloses a light emitting diode with a current blocking structure and method for making the same. The light emitting diode comprises a substrate, an epitaxy structure positioned on the substrate, an ohmic contact positioned on the epitaxy structure and a current blocking structure positioned in the epitaxy structure. The epitaxy structure comprises a bottom cladding layer, an upper cladding layer, a light emitting layer positioned between the bottom and the upper cladding layer, a window layer positioned on the upper cladding layer and a contact layer positioned between the window layer and the ohmic contact. The current blocking structure can extend from the bottom surface of the ohmic contact to the light emitting layer. According to the present invention, an implanting process is performed to implant proton into the epitaxy structure to form the current blocking structure.

Description

595023 (1) Description of the invention: 1. Field of the invention The present invention relates to a light-emitting diode and a method for preparing the same, and particularly relates to a light-emitting diode having a current blocking structure and a method for preparing the same by using a planting technique. 2. Previous technology During the past 40 years, countries around the world have been developing new light-emitting diode material systems and improving their internal quantum efficiency (Internal Quantum Efficiency). However, there is still a huge gap between the external quantum efficiency (External Quantum Efficiency) and the internal quantum efficiency of light emitting diodes. The internal quantum efficiency of a light-emitting diode with a double heterostructure (Double Heterojunction, DH) can be as high as 99%, but the external quantum efficiency is as low as a few percentage points. The main reasons are: (1) due to the light-emitting diode The current distribution relationship between the p-type coating layer, the P-type window layer, and the P-type contact layer on the light-emitting layer causes the generated large photons to be shielded by the P-type ohmic contact electrode and reflected back, and absorbed by the substrate. Reduced the probability of photons radiating out of the diode; (2) It is very difficult for photons to pass from the semiconductor material with high refractive index (Refractive Index, η) to the air with low refractive index (η = 1), which also makes the Most photons are totally reflected back and absorbed by the substrate. Currently commercialized high-brightness light-emitting diodes, including red, yellow, green, and blue, all use semiconductor materials such as aluminum gallium phosphide / gallium substrate and aluminum nitride steel / monitoring stone early-crystal substrates. , And the use of organometallic chemical vapor deposition (Metal-Organic Chemical Vapor Deposition,

H: \ HU \ HYG \ Nuclear Energy Institute \ 87172 \ 87172.DOC 595023 MOCVD) technology to produce the film structure of light-emitting diodes. FIG. 1 is a sectional view of a conventional light emitting diode 10. As shown in FIG. 1, the light emitting diode 10 includes a gallium arsenide substrate 12, a lower coating layer 14 composed of n-type aluminum gallium indium phosphide (n_AlGaInP), and an unimplanted aluminum gallium indium phosphide ( An undoped AlGalnP) light emitting layer 16 and an overcoat layer 18 made of p-type aluminum gallium indium phosphide (P-AlGaInP). In addition, the light emitting diode 10 also includes a p-type ohmic contact electrode 22 disposed on the surface of the upper cladding layer 18 and an n-type ohmic contact electrode 20 disposed on the lower surface of the gallium substrate 12. The p-type ohmic contact electrode 22 is a metal film with a diameter of about 150 micrometers, which is used as a pad for external circuits. The current applied to the light emitting diode 1Q is from top to bottom through the p-type ohmic contact electrode 22, flows through the upper coating layer 18, and is then injected into the light emitting layer 16 to generate photons. Because the p-type aluminum gallium phosphide which constitutes the upper coating layer 18 has a high resistance and is too thin, it is not easy to spread the electrode laterally uniformly, causing most of the current to be concentrated directly under the p-type ohmic contact electrode 22 . However, when the photons generated by the light-emitting layer 16 directly below the P-type ohmic contact electrode 22 are to be emitted from the light-emitting diode 10, they will be covered by the p-type ohmic contact electrode 22 above and reflected back. A portion of the photons will be absorbed by the gallium arsenide substrate 12 with a smaller energy gap, thereby limiting the external quantum efficiency of the light emitting diode 10. Is the light emitting diode 10 series directly? The overcoat layer _ 18 composed of indium aluminum gallium phosphide is used as a current spreading layer, and it is found that it faces three problems: (丨 Type-Carrier Mobility of aluminum gallium phosphide is very low, Only, the spoon is about 10cm / V-sec; (2) p-type aluminum gallium phosphide is not easy to fabricate

H: \ HU \ HYG \ Nuclear Energy Institute \ 87 丨 72 \ 87172.DOC 595023 The highest carrier inertia is only about IO8 / Cm3. When the thickness is 2 to 5 microns, the quality of the aluminum gallium indium semiconductor material will be deteriorated. FIG. 2 is a sectional view of another conventional light emitting diode 30. FIG. Comparing the figure] < Light-emitting diode 20, there are more than 30 light-emitting diodes in FIG. 2-p-type windows provided on the upper coating layer 18 | 32. In order to overcome the above-mentioned problems, the researchers epitaxially grown on the upper coating layer 18 to form a P-type window layer 32 with a thickness of about 2 ~% microns. Currently widely used semiconductor materials for the window layer include indium aluminum phosphide (InAlP), gallium aluminum arsenide (AmaAs), nitride and converted gallium. The p-type window layer 32 not only has a low resistivity, but also is clear (ie, does not absorb) the photons emitted by the light emitting diode 20. By using this p-type window layer 32, researchers have successfully developed a high brightness red light emitting diode of aluminum gallium indium phosphide. Japan's Toshiba Company first used p-type aluminum gallium arsenide (p-AlGaAs) to prepare the window layer and the current development layer in 1991, which increased the external quantum efficiency of the light-emitting diode by about 40 times. After that, Hewlett-PaCkard Company of the United States used p-type gallium (p-GaP) with a thickness of 2 to 5 microns to prepare the window layer, which effectively improved the external quantum efficiency of the light-emitting diode. FIG. 3 is a sectional view of another conventional light emitting diode 40. FIG. As shown in FIG. 3, compared to the light-emitting diode 30, the light-emitting diode 40 uses a window layer 42 disposed on the upper coating layer 18 to laterally expand the current, wherein the window layer 42 is phosphorized. A three-layer structure of gallium / amorphous gallium / gallium phosphide (~ ρ / —ρ). Taiwan's Guolian Optoelectronics has adopted this design.

H: \ HU \ HYGVf ^ Energy Institute \ 87172 \ 87172.DOC 595023 Successfully developed aluminum gallium phosphide red high-brightness light-emitting diode. FIG. 4 is a sectional view of another conventional light emitting diode 50. FIG. The light-emitting diode 50 uses a layer of transparent electrodes 52 to laterally expand the current. The transparent electrode 52 is formed by indium tin oxide (Indium Tin Oxide, IT) on the coating layer 18 composed of p-type aluminum gallium indium phosphide, and the electricity of the light-emitting diode 50 passes through the transparent electrode 5 2 Extend laterally to the upper coating 丨 8. Taiwan's Industry. The Institute of Photoelectric Industry of the Institute of Technology Research Institute first used this design to successfully develop an aluminum gallium phosphide red high-brightness light-emitting diode. However, because the interface between rhenium tin oxide and aluminum gallium indium phosphide is not easy to form a good ohmic contact, a layer of transition layer 54 (transition layer) made of p-type gallium gallium is needed between Zhongyu ° Figure 5 is another A cross-sectional view of a conventional light emitting diode 60. The light emitting diode 60 uses a current blocking structure to force the current to expand laterally. The light emitting diode 60 includes a p-type window layer 62 provided on the upper coating layer 18 and a p-type window layer 62 provided on the upper coating layer 18. An n-type epitaxial layer 64 between the layers 62. The edge η-type epitaxial layer 64 is located directly below the ρ-type ohmic contact electrode, and is formed with the upper coating layer 18-a ρη contact surface that can prevent the current from passing through. This structure of the current blocking ▲ structure effectively illuminates the light-emitting diode 6 The current of 〇 spreads laterally around the P-type ohmic contact electrode 22, instead of being concentrated in the central area or can reduce the probability of photons generated by the light-emitting layer 16 being blocked by the p-type ohmic contact electrode 22, thereby improving the light-emitting diode. External quantum efficiency of body 60. Toshiba said that the window layer was prepared by using P-type aluminum gallium arsenide (p-AlGaAs) semiconductor Λ material, and an η-type island-shaped current blocking structure was added. If the current blocking structure of the ρη junction is used, two steps are required to perform the VD epitaxial h: \ hu \ hyg \ nuclear power plant \ 871 pass 7mD〇c -10- growth. First, tens of nanometer η-type worm crystal layers 64 must be epitaxially grown on the upper coating layer 18, and then the worm chip is taken out of the reaction chamber, and lithography and etching techniques are used to form island-shaped η-type worm chips. Epitaxial layer 64. The epitaxial wafer is then sent into the MOCVD reaction chamber, and the epitaxial growth of the P-type window layer 62 is continued. In addition, the current blocking structure of the light emitting diode 60 is disposed above the upper cladding layer 18, and the current introduced from the p-type ohmic contact 22 may still flow to the light emitting layer 16 below the n-type epitaxial layer 64, and here The photons generated there will still be unable to be emitted to the outside due to the shielding of the p-type ohmic contact electrode 22. There are many technologies available to solve the problem of current development of light emitting diodes. For example, H. Sugawara et al. Used a selected area diffusion method to make a current blocking structure (see US 5,153,889). BJ Lee et al. Formed a circular hole in the middle of a transparent electrode made of ITO or ZnO using lithography and etching technology to a depth of overcoating layer consisting of p-type aluminum gallium indium phosphide, and then coated with a metal film to form Xiao's Barrier (Schokky Barrier), the metal film and p-type aluminum gallium phosphide gallium are heated to form a natural oxide by heating. This Xiao barrier and the natural oxide form a current blocking structure (refer to US 5,717,226). Patent documents such as 5,949,093, US 6,420,732 B1, EP 1,225,670 Al, US 2001/0050530 Al, US 2003/0039288 A1, and US 6,522,676 B1 also disclose various forms of current blocking structure design. III. SUMMARY OF THE INVENTION The main purpose of the present invention is to provide a light-emitting diode with a current blocking structure and a method for preparing a light-emitting diode with a current blocking structure by using the implantation technology H: \ HU \ HYGM ^ t ^ r`` \ 87172 \ 87172. DOC -11-To achieve the above object, the present invention provides a light-emitting diode with a current blocking structure, which includes a substrate, an epitaxial structure provided on the substrate, and an ohmic contact provided on the epitaxial structure. An electrode and a current blocking structure disposed in the epitaxial structure. The epitaxial structure includes a coating layer, an upper coating layer, a light-emitting layer sandwiched between the upper coating layer and the lower coating layer, and an upper layer. The window layer on the covering layer and a contact layer disposed between the window layer and the ohmic contact electrode. The current blocking structure can extend from the lower surface of the ohmic contact electrode to the light emitting layer. The light emitting device 2 having the current blocking structure of the present invention The method for preparing a polar body first forms an epitaxial structure on a substrate, wherein the epitaxial structure includes a lower coating layer, an upper coating layer, a sandwich between the upper coating layer and the lower layer. A light-emitting layer between the cladding layers, a window layer disposed on the upper cladding layer, and a contact layer disposed between the window layer and the ohmic contact electrode. Then, a photoresist layer is formed on the epitaxial structure, wherein the The photoresist layer includes at least one opening. Then, at least one implantation process is performed to introduce a proton beam having a predetermined dose and a predetermined energy into an epitaxial structure located below the opening to form a current blocking structure in the epitaxial structure. Then, the photoresist layer is removed and an ohmic contact electrode is formed on the epitaxial structure. The implantation process can be performed before or after the ohmic contact electrode is formed. Compared to the conventional technique, the present invention uses the implantation The technology forms a current blocking structure inside the light emitting diode, which has the following advantages: 1. The epitaxial wafer only needs to enter and exit the MOCVD reaction chamber once to complete all the epitaxial growth. H: \ HU \ HYG \ Nuclear Energy Institute \ 87172 \ 87172 .DOC -12- 2 · π :: = Light, only the polar body is needed; a thin layer can reach the current, which can further shorten the time for the crystal growth.: The planting process is a very stable and reliable technology. not only Yes, the rate of W can also be relatively reduced to four. Embodiments Figures 6_ through 8 are not intended for the light emitting diode method of the first embodiment of the present invention. Spectrum 1 @ ^ ^ ^ 于-基First, a worm-like structure 80 is formed. The upper layer is the upper layer. The epitaxial structure 80 includes the lower coating layer 82, the light-emitting layer I /, 4-2 84, and the lower coating layer 82. The window layer% and P on the upper cladding layer 84 and the contact layer 88 on the clover layer 86. Generally, the window C is high, which is not conducive to the lateral expansion of the current, so the present invention borrows: ;; = length: In the process of the window layer 86, a -container is additionally added. The connection cavity has a reaction cavity of 40 coffee, so that a contact layer 88 with a higher carrier concentration is formed on the surface of the window layer%. = After, can be on the contact layer 88 domain-the thickness of the layer is about i, 嶋 = Lu film or gold film 'and then use the spin coating (Spi_) to the appropriate thickness J, after soft and hard baking, and use the alignment light The mask, after exposure to silver engraving and photoresist removal procedures, left a metal ten-strand mark on the cutting path of the insect wafer as a reference for alignment. Referring to FIG. 7, a photoresist layer 100 is formed on the contact 7'88 of the exposed crystal structure 80 ', wherein the photoresist layer includes 1 pin. Then at least: the implantation process, a proton beam with a predetermined dose and a predetermined energy is opened directly under the contact layer 88, the window layer%, the upper cover

H: \ HU \ HYGV ^ Energy \ 87172 \ 87172.DOC -13-595023 layer 82 and light emitting layer 83 to form a current blocking structure 92 in the epitaxial structure 80. The predetermined dose is between 1X1012 and 9X1016 dopants / cm 2, and the predetermined energy is between 100 and 1000 仟 electron volts. In addition to protons, nitrogen and oxygen ions can also be used to form the current blocking structure. In addition, the implantation process can also introduce a proton beam with multiple energies into the epitaxial structure. This implantation process can be performed first with high-energy proton beams, then with low-energy proton beams, or with low-energy proton beams, followed by high-energy proton beams. Referring to FIG. 8, after the photoresist layer 100 is removed, a p-type ohmic contact electrode 90 is formed on the contact layer 88 of the crystal structure and an n-type ohmic contact electrode 94 is on the other surface of the substrate 72. To complete the light emitting diode 70. The area of the opening 102 (ie, the planting area) is preferably smaller than the area of the ρ-type ohmic contact electrode 90. Taking a circular ohmic contact electrode with a diameter of 150 μm as an example, the diameter of the corresponding planting area is between 10 μm and 140 Between micrometers. The current blocking structure 92 of the light emitting diode 70 of the present invention extends from the lower surface of the p-type ohmic contact electrode 90 to the light emitting layer 83. In addition, the present invention can also implant protons in the light-emitting layer 83 directly under the opening 102 or even in the lower coating layer 82 by increasing the energy of the proton beam. That is, the current blocking structure 92 extends from the lower surface of the p-type ohmic contact electrode 90 to the lower coating layer 82. Since most of the photons generated by the light-emitting layer 83 directly below the p-type ohmic contact electrode 90 cannot radiate the light-emitting diode 70, the present invention applies a proton to the light-emitting layer 83 directly under the p-type ohmic contact electrode 90 To prevent current from flowing here and generating light H: \ HU \ HYGM ^ 能 所 \ 87172 \ 87172.DOC -14- that cannot radiate light emitting diode 70. On the contrary, the current will flow to the light-emitting layer 83 other than directly below the P-type ohmic contact electrode 90 to generate photons that can radiate the light-emitting diode 70, thereby increasing external quantum efficiency. Compared with the conventional technique (e.g., the light emitting diode 60 shown in FIG. 5), the present invention forms a continuous extension < current blocking structure 92 between the ohmic contact electrode 90 and the light emitting layer 83. When a forward current is applied to the light-emitting diode 70, the current introduced from the p-type ohmic contact electrode 90 will first expand laterally through the contact layer 88, and then flow down to the light-emitting layer 83. In addition, since the cylindrical region immediately below the p ohmic contact electrode 9G has formed a semi-insulator ❼ | to prevent the forward current from being forced, the current is forced to pass through the region other than the cylindrical region and injected into the light-emitting layer 83 outside the cylindrical region. Therefore, the probability that the generated photons are reflected back by the P-type ohmic contact electrode 90 due to the geometric relationship is greatly reduced, and the external quantum efficiency of the light emitting monopole 70 is greatly improved. Fig. 9 is a schematic diagram of a method for preparing a light emitting diode 70 according to a second embodiment of the present invention. As shown in FIG. 9, after the epitaxial structure 80 is formed on the upper surface of the substrate 72 | a P-type ohmic contact electrode 90 is formed on the epitaxial structure 80 and an n-type ohmic contact electrode 94 is formed on the lower surface of the substrate 72. After that, a photoresist layer 110 is formed, wherein the photoresist layer 110 includes an opening 112. Then, a planting process is performed to introduce the contact layer 88, the window layer%, and the upper covering material directly under the opening 112 into the contact layer 88 with the financial amount and the f-beam% of energy. Burst structure 80. Finally, the light and the resist layer 110 are removed to complete the fabrication of the light emitting diode 70. The predetermined dose is between 1X1012 and 9X1016 dopants per square centimeter, and the predetermined energy is between 100 and 1000 仟 electron volts. Since you must first

HiVHUXHYGM1 \ 87172 \ 87 丨 72.DOC -15-595023 penetrates the P-type ohmic contact electrode 90, so the energy of implanting protons is more directional than that of the first embodiment W. When aligning the photomask, a P-type ohmic contact electrode can be used as an alignment reference, so it is not necessary to make the alignment mark 'in the first embodiment, that is, only two photomasks are needed, which are used to define the placement position and Ohmic contact electrode 90. The invention can also control the dose and energy of the cloth-valued proton beam 96, so that the current blocking structure 92 of the light-emitting diode 70 is generated from any position below the p-type ohmic contact electrode 90 (that is, without contacting its lower surface). .

Compared with the conventional technique, the present invention uses the implantation technology to form a current blocking structure inside the light emitting diode, and has the following advantages: The conventional technique uses the lithography and etching technology to make the current blocking structure, so the light emitting diode The body needs to undergo a second epitaxial process to complete all the epitaxial layers. In contrast, since the implantation process for forming a current blocking structure according to the present invention can be performed after all epitaxial layers are completed, the epitaxial wafer only needs to enter and exit the MOCVD reaction chamber once to complete all the epitaxial growth.

2. The light-emitting diode of the present invention only needs a thin window layer to achieve electricity; the purpose of unfolding. Because the applied current is mainly caused by the current blocking structure ^ the contact layer is expanded laterally, so the thickness of the window layer is reduced to shorten the growth time. 3. The implantation process is a very stable and reliable technology, which can not only improve the yield of light emitting diode manufacturing, but also reduce the production cost relatively. The technical content and technical features of the present invention are disclosed as above, but those who contaminate the technology may still be based on the teaching and disclosure of the invention, substitutions and modifications that depart from the spirit of the present invention. Therefore, the present invention

RVHLAHYGVK Energy Center \ 87 丨 72 \ 87I72.DOC -16- 595023 should not be limited to those disclosed in the examples, but should include various substitutions and modifications that do not depart from the present invention, and are covered by the following patent application parks. V. Brief Description of the Drawings Figure 1 is a sectional view of a conventional red light emitting diode; Figure 2 is a sectional view of another conventional light emitting diode; Figure 3 is another conventional light emitting diode A sectional view;

Preparation of Diodes

4 is a cross-sectional view of another conventional light-emitting diode. FIG. 5 is a cross-sectional view of another conventional light-emitting diode. FIGS. 6 to 8 are schematic views of a light-emitting method according to the first embodiment of the present invention; Description of element symbols 10 Light emitting diode 14 Lower covering layer 18 Upper covering layer 22 p-type ohmic contact electrode 32 window layer 42 window layer 52 transparent electrode 60 light emitting diode 64 n-type epitaxial layer 7 0 light emitting diode 80 Epitaxial structure 12 Substrate 16 Light-emitting layer 20n-type ohmic contact electrode 30 Light-emitting diode 40 Light-emitting diode 5 0 Light-emitting diode 54 Gradient layer 62 Window layer 72 Substrate 82 Covering layer

H: VHLAHYG \ Nuclear Energy Institute \ 87172 \ 87172.DOC 17- 595023 83 Luminous layer 84 Overcoat layer 86 Window layer 88 Contact layer 90 ρ-type ohmic contact electrode 92 Current blocking structure 94 η-type ohmic contact electrode 96 Proton beam 100 Light Resistive layer 102 opening 110 Photoresistive layer 112 opening HAHUXHYGM1 Hai Energy Institute \ 87172 \ 87172.DOC-18-

Claims (1)

595023 Patent application scope: 1. A light emitting diode with a current blocking structure, comprising: a substrate; an epitaxial structure disposed on the substrate, the epitaxial structure including a coating layer, an upper coating layer, A light-emitting layer sandwiched between the upper coating layer and the lower coating layer, and a window layer disposed on the upper coating layer; an ohmic contact electrode is disposed on the crystal structure, and a current blocking structure is located in the lei Within the crystalline structure and extending from below the ohmic contact electrode to at least the light emitting layer. 2. For example, the light-emitting diode with a current blocking structure in the scope of patent application No. 1 further includes a contact layer disposed between the window layer and the ohmic contact electrode for laterally expanding the current. 3. For example, the light-emitting diode with a current blocking structure in the first patent application scope, wherein the current blocking structure further extends to the lower coating layer. 4. For the light-emitting diode with a current blocking structure as described in item 1 of the patent application scope, wherein the area of the current blocking structure is smaller than the area of the ohmic contact electrode. 5. For example, the light-emitting diode with a current blocking structure in the first patent application scope, wherein the current blocking structure extends from the lower surface of the ohmic contact electrode. 6. A method for preparing a light-emitting diode with a current blocking structure, including the following steps: forming an epitaxial structure on a substrate, wherein the epitaxial structure includes a lower coating layer, an upper coating layer, and a sandwich between the A light-emitting layer between the upper coating layer and the lower coating layer, and a window layer disposed on the upper coating layer; the energy store \ 87172 \ 87172.DOC forms a photoresist layer on the epitaxial structure, and the photoresist layer includes At least one opening; performing at least one implantation process to form a current blocking structure in the epitaxial structure; removing the photoresist layer; and forming a contact electrode on the crystal structure. 7. The method for preparing a light-emitting diode with a current blocking structure according to item 6 of the patent application, wherein the implantation process introduces a proton beam having a predetermined dose and a predetermined energy into the epitaxial structure. 8. The method for preparing a light-emitting diode with a current blocking structure according to item 7 of the patent application, wherein the predetermined dose is between 1 X 1012 and 9 X 1016 dopants / cm 2. 9. The method for preparing a light-emitting diode with a current blocking structure according to item 7 of the scope of the patent application, wherein the predetermined energy is between 100 and 1000 shi electron volts. I. The method for preparing a light-emitting diode with a current blocking structure according to item 6 of the application, wherein the implantation process introduces a plurality of energy beams into the epitaxial structure. II. The method for preparing a light-emitting diode with a current blocking structure according to item 6 of the patent application ', wherein the dopant used in the implantation process is selected from the group consisting of protons, gas ions, and oxygen ions. 12 · —A method for preparing a light-emitting diode with a current blocking structure, including the following steps: forming an epitaxial structure on a substrate, wherein the epitaxial structure includes a coating layer, an upper coating layer, and a sandwiching layer; Between the upper coating layer and the lower coating layer H: \ HU \ HYG \ Nuclear Energy Institute \ 87172 \ 87172.DOC 595023 and a window layer disposed on the upper coating layer; forming an ohmic contact electrode on the epitaxial structure Forming a photoresist layer on the epitaxial structure, and the photoresist layer includes at least one opening; and performing at least one implantation process to form a current blocking structure in the epitaxial structure. 13. The method for preparing a light-emitting diode with a current blocking structure according to item 12 of the application, wherein the implantation process introduces a proton beam having a predetermined dose and a predetermined energy into the epitaxial structure. 14. The method for preparing a light-emitting diode with a current blocking structure according to item 13 of the application, wherein the predetermined dose is between 1 × 1012 and 9 × 10 × 6 dopant / cm 2. 15. The method for preparing a light-emitting diode with a current blocking structure according to item 13 of the scope of the patent application, wherein the predetermined energy is between 100 and 1000 仟 electron volts. 16. The method for preparing a light-emitting diode with a current blocking structure according to item 12 of the application, wherein the implantation process introduces a shellfish beam having a plurality of energies into the crystal structure. 17β The method for preparing a light-emitting diode with a current blocking structure according to item 12 of the scope of patent application, wherein the dopant used in the implantation process is selected from the group consisting of protons, nitrogen ions, and oxygen ions. H: \ HU \ HYG \ Nuclear Energy Institute \ 87 丨 72 \ 87 丨 72.DOC