Epitaxial substrate for high-intensity LED, and method of manufacturing same

Abstract: An epitaxial substrate for a high-intensity LED is disclosed which comprises a double-hetero structure epitaxial layer (1 to 6) formed by vapor phase epitaxy and an epitaxial layer (7) formed by liquid phase epitaxy on said double-hetero structure epitaxial layer (1 to 5), said double-hetero structure epitaxial layer (1 to 6) being formed by removing a compound semiconductor substrate (1) after said double-hetero structure epitaxial layer (1 to 6) has been formed thereon. The double-hetero epitaxial layer (1 to 6) is formed using an MOCVD process or an MBE process, and the epitaxial layer (7) is formed using an LPE process and so the resultant substrate (2 to 7) is formed by a process which shows the benefits of both processes, that is mixed-crystal ration and film thick­ ness controllability, excellent reproducibility and uniformity of the MOCVD process and the high rate of growth of the LPE process.

Description: This invention relates to a method of manufacturing a high-intensity LED epitaxial wafer, which is of the type having a double-hetero structure, by joint use of vapor phase epitaxy (an MOCVD process or MBE process) and liquid phase epitaxy (an LPE process). In order to epitaxially grow a substrate for, say, a red-light emitting high-intensity LED in the prior art, first a p -type layer of Al 0.75 Ga 0.25 As is formed as a p -type cladding layer to a thickness of 200 µm (microns) on a p -type GaAs substrate [(100) surface] by the LPE process. This is followed by forming a p -type Al 0.35 Ga 0.65 As layer as a p -type active layer to a thickness of 2-3 µm (microns), and then an n -type Te-doped Al 0.75 Ga 0.25 As layer as an n -type cladding layer to a thickness of 50 µm (microns). Next, a GaAs substrate-selective etchant (e.g. NH₄OH:H₂O₂ = 1.7) is used to remove the light-absorptive GaAs substrate, thereby providing a high-intensity LED chip. Though the LPE process exhibits a high growth rate and therefore is suitable for forming layers that are thick, it is difficult to control thickness and carrier concentration. Consequently, when an epitaxial substrate having a double-hetero structure is fabricated solely by the LPE process, a variance in thickness and carrier concentration within the wafer surface tends to occur when the active layer is formed. As a result, stable luminance cannot be obtained. Furthermore, attempting to grow a mechanically strong and thick (about 200 µm (microns)) mixed-crystal substrate solely by the MOCVD or MBE process is impractical since it involves a prolonged period of time and high cost. A principal object of the invention is to manufacture a high-quality and highly uniform epitaxial wafer in a highly efficient manner. Another object of the invention is to make joint use of the MOCVD process or MBE process and the LPE process to exploit the advantages of both processes. Still another object of the invention is to form a substrate, at a high rate of growth, in which the layer thickness and carrier concentration of an epitaxially grown layer are made uniform. A further object of the invention is to manufacture an epitaxial wafer which is high in quality, highly uniform and low in cost. At the present time, epitaxial growth of a high-intensity LED wafer largely relies upon AlGaAs, and in most cases the LPE process is employed. The LPE process features a rapid rate of growth and therefore is advantageous in forming thick layers. Though it is effective to adopt a double-hetero structure in order to achieve a high-intensity LED, control of layer thickness involves considerable difficulties when relying upon the LPE process. On the other hand, the MOCVD process or MBE process is advantageous for the epitaxial growth of a high-intensity LED substrate since epitaxial layer thickness can be accurately controlled for uniformity within the wafer surface while carrier concentration is also controlled. In many cases, however, a GaAs wafer is used as the substrate for epitaxial growth. Therefore, when it is attempted to also obtain higher intensity light, it is necessary to remove the GaAs substrate owing to its visible light-absorbing property. Accordingly, attempting to grow a thick mixed-crystal substrate (e.g., a substrate having a thickness of about 200 µm (microns)) solely by the MOCVD process or MBE process in order to furnish mechanical strength involves considerable time and high cost. This makes the effort impractical. EP-A-0031808, page 2, lines 9-24, describes that, after an epitaxial protective layer of a group III-V compound is grown on a first layer containing Al, a second layer is grown by liquid phase epitaxy. Thus, a protective layer is formed to prevent oxidation when epitaxial growth is performed in two stages. According to the present invention we provide a method for manufacturing an epitaxial wafer for a high intensity LED, comprising an n-type cladding layer, a p-type active layer and a p-type cladding layer, characterized in that said method comprises: a first step of forming an n⁺-type contact, said n-type cladding layer, said p-type active layer, said p-type cladding layer and an anti-oxidation layer by vapor phase epitaxy on a compound semiconductor substrate to form a double-hetero structure; a second step of forming an epitaxial layer by liquid phase epitaxy on said anti-oxidation layer whereby the anti-oxidation layer is removed; and a third step of removing said substrate. The present invention combines the merit of the MOCVD or MBE process, namely the controllability of the layer thickness and carrier concentration of the epitaxially grown layer, with the merit of the LPE process, namely the high rate of growth. As shown in (a) of Fig. 1, an n +-type contact layer 2, an n -type cladding layer 3, a p -type active layer 4, a p -type cladding layer 5 and an anti-oxidation layer 6 are formed on a compound semiconductor substrate 1 by vapor phase epitaxy, thereby forming a double-hetero structure. Next, as shown in (b) of Fig. 1, a thick epitaxial layer 7 is formed on the oxidation-preventing layer (removed by melt-back) by liquid phase epitaxy, after which the compound semiconductor substrate, which is highly absorptive of the wavelength of emitted light, is removed. This makes it possible to reduce cost greatly. Other features and advantages of the present invention will be apparent from the following description taken in conjunction with accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. In the accompanying drawings: Fig. 1 illustrates the structure of an epitaxially grown layer according to the present invention; Fig. 2 is a view illustrating the structure of a grown layer in an embodiment of the invention; and Fig. 3 is a view in which luminance according to the invention is compared with that of the prior art. An embodiment of the present invention will now be described with reference to the drawings. Fig. 2 is a view illustrating an embodiment of the present invention. The structure shown in Fig. 2 includes a GaAs substrate 1, an n +-type Al 0.75 Ga 0.25 As layer 2, an n -type Al 0.75 Ga 0.25 As cladding layer 3, a p -type Al 0.35 Ga 0.65 As active layer 4, a p -type Al 0.6 Ga 0.4 As cladding layer 5, and a p -type GaAs layer 6 as an oxide film-preventing layer. In a first step according to the present invention, an n +-type Al 0.75 Ga 0.25 As layer having a carrier concentration of 3 x 10¹⁹ cm⁻³ and a thickness of 5 microns, an n -type Al 0.75 Ga 0.25 As cladding layer having a carrier concentration of 2 x 10¹⁷ cm⁻³ and a thickness of 5 microns, a p -type Al 0.35 Ga 0.65 As active layer having a carrier concentration of 5 x 10¹⁶ cm⁻³ and a thickness of 1 µm, a p -type Al 0.75 Ga 0.25 As cladding layer having a carrier concentration of 1 x 10¹⁸ cm⁻³ and a thickness of 10 microns, and a p-type GaAs layer having a carrier concentration of 1 x 10¹⁸ cm⁻³ and a thickness of 1 µm are formed, in the order mentioned, on a 300 micron-thick GaAs substrate which is 2° off the (100) surface. These layers are formed by vapor phase epitaxy (an MOCVD process, MBE process, etc.) at a substrate temperature of 750°C using trimethyl gallium [Ga(CH₃)₃], trimethyl aluminum [Al(CH₃)₃], hydrogen selenide (H₂Se) and diethyl zinc [Zn(C₂H₅)₂] as the gases. Next, a Zn-doped, p -type Al 0.6 Ga 0.4 As layer having a carrier concentration of 1 x 10¹⁸ cm⁻³ and a thickness of 120 µm is formed on the substrate by an LPE process. The epitaxial conditions at this time are a temperature of 875°C, with the melt being GaAs: 32.0 mg, Zn: 1.7 mg, Al: 6.3 mg in 1 g of Ga. At this time the p -type GaAs layer 6 (1 µm) formed by MOCVD is completely removed by melt-back, and a p -type Al 0.75 Ga 0.25 As layer is formed anew. Though the above embodiment is described in connection with an example in which a p -type Al 0.6 Ga 0.4 As layer is formed by the LPE process, it is not necessary to limit the invention to this material. The epitaxial layer grown by the LPE process can be formed by any material that facilitates ohmic contact and that has a band gap less than that of the cladding layer of the double-hetero epitaxial layer and greater than that of the double-hetero active later so as not to absorb the emitted light. [Example for Purpose of Comparison] When the luminance of the LED chip fabricated in accordance with the present invention and the luminance of a conventional LED chip are compared in the wafer surface, the result is as shown in Fig. 3. Fig. 3(b) illustrates luminance (in units which are arbitrary) in the direction of the chip shown in Fig. 3(a). In the present invention, it will be understood that uniformity in the surface (especially at the wafer periphery) is improved by forming the active layer uniformly, and that the luminance level is also improved. Thus, in accordance with the invention as described above, uniformity of the double-hetero structure portion is improved by using the MOCVD process or MBE process, and a variance in luminance in the wafer surface can be reduced. In addition, by using the LPE process, the epitaxial substrate can be formed in a short period of time. Since the LPE-grown layer is only a single layer, the melt is of one type and it is possible to charge a number of sheets, as a result of which total cost can be reduced, in comparison with the conventional LPE process. Furthermore, since the GaAs substrate is removed and the n-type layer becomes the top layer, the requirement for mesa etching can be reduced to a thin 11 microns and yield can be raised when forming chips. Further, the GaAs substrate surface is polished and exhibits a mirror surface on which vapor phase epitaxy is performed. Therefore, the n ⁺-type AlGaAs surface is obtained as a mirror surface when the GaAs is removed, and handling as at the time of pattern formation is comparatively simple. Flattening as by polishing is unnecessary.

Claims: A method for manufacturing an epitaxial wafer for a high intensity LED, comprising an n-type cladding layer (3), a p-type active layer (4) and a p-type cladding layer (5), characterized in that said method comprises: a first step of forming an n⁺-type contact (2), said n-type cladding layer (3), said p-type active layer (4), said p-type cladding layer (5) and an anti-oxidation layer (6) by vapor phase epitaxy on a compound semiconductor substrate (1) to form a double-hetero structure; a second step of forming a layer (7) by liquid phase epitaxy on said layer (6) whereby the anti-oxidation layer (6) is removed; and a third step of removing said substrate (1). A method according to Claim 1, wherein the epitaxial layer (7) formed by liquid phase epitaxy comprises a material having a band gap less than a band gap of a cladding layer of the double-hetero epitaxial layer and greater than a band gap of the double-hetero active layer. A method according to Claim 1 or 2, wherein the anti-oxidation layer (6) is a GaAs layer.

Inventors: GOTOH HIDEKI C O MITSUBISHI MO, NOGUCHI MASAHIRO MITSUBISHI MO, SHIMOYAMA KENJI C O MITSUBISHI, GOTOH, HIDEKI C/O MITSUBISHI MONSANTO CHEMICAL CO., NOGUCHI, MASAHIRO MITSUBISHI MONSANTO CHEMICAL CO., SHIMOYAMA, KENJI C/O MITSUBISHI MONSANTO CHEM. CO.
Applicants: MITSUBISHI CHEM IND, MITSUBISHI KASEI POLYTEC CO, MITSUBISHI KASEI CORPORATION, MITSUBISHI KASEI POLYTEC COMPANY
Application year: 1989
Main classification: H01L33
IPC codes: H01L21, H01L33