METHOD OF PRODUCING A MESA EMBEDDED TYPE OPTICAL SEMICONDUCTOR DEVICE

Abstract: An optical semiconductor device and a manufacturing method thereof more particularly, an optical semiconductor device having a mesa structure, such as a semiconductor laser, an optical modulator, an optical filter and a light receiving element, and a manufacturing method thereof. An object of this invention is to provide a mesa buried type optical semiconductor device whose structure can suppress the exposure of a surface (111)B that causes an abnormal growth when burying the both sides of a mesa structure. The mesa buried type optical semiconductor device is characterized in that it has a semiconductor multilayer (20) of the mesa type structure in which a second surface (11B) is positioned on a first surface (11A) provided on a semiconductor substrate (11) in the transverse direction of a side wall of the semiconductor multilayer (20), and a buried layer (16) which is provided so as to bury the first surface, the second surface and the side wall of the multilayer semiconductor layers (20).

Description: FIELD OF THE INVENTION The present invention relates to a method of producing an optical semiconductor device and more specifically to a structure of an optical semiconductor device and a method of producing the same such as semiconductor laser providing a mesa structure, optical modulator, optical filter or photosensitive element. A mesa embedded type optical semiconductor device is typically known as an example of the structure of optical semiconductor device. This structure is formed by etching a multilayer structure of various semiconductor layers principally working as optical semiconductor device into the mesa type structure and then embedding both sides of such structure within a semiconductor layer for rejecting current. The mesa embedded type optical semiconductor device must be produced in the stabilized high quality because it plays a very important role as the device for realizing an optical communication system or an optical computer system. PRIOR ART Fig. 1 and Fig. 2 are diagrams indicating the mesa structure of the prior art through the sectional views of principal producing processes of a mesa embedded type InP system semiconductor laser. In these figures, the reference numeral 1 denotes InP substrate; 2, active layer consisting of InGaAsP or InGaAs; 3, InP clad layer; 4, InGaAsP contact layer; 5, etching mask consisting of silicon dioxide. The mesa type structure 10 shown in Fig. 1 is formed by etching a part of the contact layer 4, clad layer 3, active layer 2 and substrate 1 using the etchant consisting of brommethanol. Moreover, the mesa type structure 10 shown in Fig. 2 is formed by etching a semiconductor layer formed by the InP material with chloric acid or hydrogen bromide and also etching a semiconductor layer formed by InGaAsP or InGaAs material with an etchant consisting of a sulfuric acid system. In Fig. 1 and Fig. 2, the flat area of substrate has the (100) plane. In the structure shown in Fig. 1 and Fig. 2, both sides of mesa type structure 10 are embedded within the InP system semiconductor layer (current rejection layer) which defines a current path. However, in the case of prior art shown in Fig. 1 and Fig. 2, a semiconductor layer is grown by the metalorganic chemical vapor deposition (MOCVD) method for embedding both sides of mesa structure and this growth layer is sometimes exposed out from the (111) B plane during the growing process. The (111) B plane grows very slowly in the MOCVD method in comparison with that of the (100) plane forming the flat surface of the substrate and therefore when this plane is exposed, a problem that irregular growth areas or holes are generated in the grown semiconductor layer is generated. Fig. 3 shows an example where both sides of mesa type structure 10 shown in Fig. 1 are embedded in the semiconductor layer 6. Fig. 4 also shows an example where both sides of mesa type structure 10 are embedded in the semiconductor layer 6 consisting of high resistance InP. In the example where the structure of Fig. 1 is embedded in the semiconductor layer 6, an irregular growth area 6A is generated as shown in Fig. 3. Moreover, in the example where the structure of Fig. 2 is embedded in the semiconductor layer 6, holes 6B are generated as shown in Fig. 4. The structures shown in Fig. 3 and Fig. 4 show the examples where normal growth is disabled due to exposition of the (111) B plane in the course of the growth. After completion of embedded growth, gold, for example, is deposited as the electrode material. However, gold easily diffuses into semiconductor material and it is necessary to deposit the TiPt layer as a barrier layer, etc. prior to the deposition of gold. Therefore, since it is difficult to deposit the barrier layer on the abnormally grown complicated region in Fig. 3 or in the hole region in Fig. 4, gold is deposited in direct in such regions. The region where gold is deposited in direct allows formation of a conductive path by diffusion of gold and accordingly a leakage current is easily generated and good performance cannot be obtained. Another example of a prior art optical semiconductor device based on a mesa structure is disclosed in Electronic Letters, Vol. 24, No. 24, November 1988, pages 1483-1484, by T.Tanbun-ek et al. The device is based on a non-re-entrant mesa having a narrowed actiye layer produced by a selective etch. The fabrication of the device is carried out in two MOVPE growth steps. In the second growth step, the slots adjacent to the active layer are filled in view to obtain a substantially planar device. A planar buried heterostructure laser device is also disclosed in Journal of Applied Physics, Vol. 64, No. 7, October 1988, pages 3684-3689 by T.Kawabata, et al. The authors investigate the growth of InP and InGaAsP layers forming the mesa using a low-pressure MOCVD technique. Finally, there is disclosed in EP-A-0 118 671 a semiconductor laser device composed of four semiconductor layers, with the optical confinement region formed as a mesa stripe. The layer structure is embedded in two separate semiconductor burying layers. OBJECT AND DISCLOSURE OF THE INVENTION It is a first object of the present invention to provide a method of producing a mesa embedded type optical semiconductor device providing a structure such that exposition of the (111) B plane which causes irregular growth at the time of embedding both sides of mesa type structure may be suppressed. It is a second object of the present invention to allow obtain a mesa embedded type optical semiconductor device produced by the above method, which enables suppression of exposition of the (111) B plane at the time of embedding both sides of the mesa type structure. In order to realize the first object, the present invention provides a method of producing a mesa embedded type optical semiconductor device as defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 to Fig. 4 are diagrams for explaining prior arts relative to the present invention. Fig. 5 is a diagram of a first embodiment indicating the basic structure of the present invention. Fig. 6 to Fig. 10 are diagrams for explaining a second embodiment indicating the fabrication processes of the present invention. PREFERRED EMBODIMENT OF THE INVENTION [Embodiment 1] Fig. 5 is a diagram for explaining the first embodiment of the present invention. Fig. 5 shows a sectional view of the first embodiment wherein the present invention is applied to a InP system semiconductor laser. In this figure, the reference numeral 11 denotes InP substrate; 11A, first plane on the substrate; 11B, second plane on the substrate; 12, InGaAsP active layer; 13, InP clad layer; 14, InGaAsP electrode contact layer; 16, an embedding layer consisting of InP; 17, barrier layer consisting of TiPt; 18, gold electrode layer; 19, electrode layer at the rear side; 20, mesa type structure. As will be obvious from Fig. 5, in this embodiment, the first plane 11A is provided on the substrate 11, moreover a multilayer semiconductor layer consisting of substrate 11, active layer 12, clad layer 13 and contact layer 14 is formed as the mesa type structure 20 and the second plane is provided at the higher area than the first plane 11A on such mesa type structure 20. Both first plane 11A and second plane 11B have the plane index number (100). In the case where the embedding layer 16 is formed for such structure with the metalorganic chemical vapor deposition (MOCVD) method, irregular growth or holes, which have been observed in the prior art, are not generated as is apparent from Fig. 5. This fact has been confirmed experimentally. Although details of mechanism of no generation of irregular growth and holes are not yet analyzed clearly, the plane index number to form the first and second planes is considered very important. Namely, in this embodiment, the first and second planes are formed with the (100) plane index number, but this (100) plane may be easily grown by the MOCVD method and has the property against that of the (111) B plane. Since these first and second planes are located in different heights, film is considered to be deposited first from these two kinds of planes in the initial stage of growth. In the prior arts, the (100) plane is formed only at the flat surface of substrate and the (111) B plane may be assumed to easily appear because the growth mainly occurs from such plane as the growth initiation plane. On the other hand, in the embodiment of the present invention, the (100) plane as the growth initiation plane is divided into two stages and growth occurs individually from the respective planes. Therefore, such main growth is considered difficult and appearance of the (111) B plane is considered to be suppressed. When the layer 16 is embedded as required as indicated in the embdodiment, the barrier layer to be formed thereafter can be uniformly deposited to the entire surface of plane and the gold electrode to be formed next will not be deposited in direct on the multilayer semiconductor layer and thereby a problem of leak current occurring in the prior art may be solved. In an alternative, a plurality of similar planes may be provided in place of the first and second planes. Moreover, these planes are provided on the substrate 11 forming a part of the mesa type structure 20 in this embodiment, but it is also enabled to provide such planes on the other layers forming the mesa type structure 20 such as active layer 12, clad layer 13 and electrode contact layer 14. [Embodiment 2] Fig. 6 to Fig. 10 are diagrams for explaining the second embodiment of the present invention. The elements similar to those in Fig. 1 are designated by the like reference numerals. Referring to Fig. 6: For example, using the MOCVD method, the active layer 12, clad layer 13 and electrode contact layer 14 are sequentially grown on the n-type InP substrate 11 having the plane index number of (100). The principal conditions of each growth layer is for example as follow. 1 For substrate 11: Impurity: S or Sn Impurity concentration: 5 x 10¹⁸ (cm⁻³) 2 For active layer 12: Material: non-doped InGaAsP Luminescence peak wavelength: 1.3 (µm) Thickness: 0.15 (µm) 3 For clad layer 13: Material: p-type InP Impurity: Cd or Zn Impurity concentration: 7 x 10¹⁷ (cm⁻³) Thickness: 1.5 (µm) 4 For electrode contact layer 14: Material: P-type InGaAsP Impurity: Cd or Zn Impurity concentration: 1 x 10¹⁹ (cm⁻³) Thickness: 0.3 (µm) Thereafter, using the chemical vapor deposition method, a silicon dioxide layer 15 is grown in the thickness of about 100 nm (1000Å). Moreover, silicon dioxide may be replaced with silicon nitride or aluminum oxide. Next, the patterning of silicon dioxide film 15 is executed by applying the resist process utilizing ordinary photolithography and the reactive ion etching (RIE) method where trichloromethane (CHF₃) is used as the etching gas. This patterning is executed so that the silicon dioxide film 15 extends in the direction of <011> of the substrate 11 and is striped in the width of about 3 4 (µm). Referring to Fig. 7: The patterning of electrode contact layer 14 may be executed using the silicon dioxide film 15 used as the mask by applying the wet etching method utilizing the etchant of sulfuric acid. Next, the etching is carried out utilizing the etchant of hydrochloric acid system and the clad layer 13 is selectively etched with the patterned electrode contact layer 14 used as the mask. Next, the etching is executed utilizing the etchant of sulfuric acid and the active layer 14 is selectively etched with the patterned clad layer 13 used as the mask. In this timing, quantity of side etching of active layer 14 is controlled by adjusting the etching time and thereby the width of active layer 14 can be controlled. Referring to Fig. 8: A stepped portion is formed on the substrate 11 by the etching utilizing the etchant of HCℓ + HNO₃ + H₂O₂. In this process, the first plane 11A is formed on the semiconductor substrate 11, the clad layer 13 is also side-etched in the lateral direction at the time of this etching and the side surface draws back to the surface continued to the electrode contact layer 14 and active layer 12, exposing the second plane 11B. With the process mentioned above, the mesa type structure 20 consisting of multilayer structure of the substrate 11, active layer 12, clad layer 13 and electrode contact layer 14 is formed. Referring to Fig. 9: Next, an embedding layer 16 is formed by utilizing the MOCVD method. The embedding layer 16 does not grow on the region where the silicon dioxide film 15 is formed. The embedding layer 16 has for example the following conditions. Material: InP Impurity: Fe Impurity concentration: 7 x 10¹⁶ (cm⁻³) The embedding layer 16 contains Fe which will become impurity to form a deep level (deep acceptor) and works as a high resistance layer. Therefore, current is limited to be injected only to the mesa structure region. The embedding layer 16 starts growth from the first plane 11A and second plane 11B and is suppressed to allow appearance of the (111) B plane during the growth. Accordingly, irregular growth or holes are never generated. The embedding layer may not only be the layer forming the PN reverse bias junction (for example, laminated structure of the N type layer and P type layer), in addition to the high resistance layer explained above, but also a combination of a high resistance layer and a PN reverse bias junction (for example, the structure where the embedded layer is formed by high resistance layer and N type layer and the P type layer is laminated thereon). Referring to Fig. 10: Next, after removing the silicon dioxide film 15 utilizing the etchant of HF system, a barrier layer 17, gold electrode 18 are formed and a rear surface electrode layer 19 is also formed to the rear surface of the substrate 11 to complete a semiconductor laser. Each electrode has for example the following conditions. For barrier layer 17: Material: laminated material of Ti and Pt Thickness: Ti = 100 nm (1000Å), Pt = 300 nm (3000Å) For gold electrode 18: Thickness: 3 µm For rear surface electrode layer 19: Material: laminated material of Au and Au/Ge alloy Thickness: Au = 250 nm (2500Å), Au/Ge = 50 nm (500Å) The mesa type embedded semiconductor laser formed by the processes explained above is controlled in generation of irregular growth and holes. Therefore, such a fear as generating lead current due to diffusion of gold electrode 18 into semiconductor layer can be dissolved. In above embodiment, the present invention has been applied to a structure of the semiconductor layer, but the present invention allows, as multiplicity of alternatives, application to a variety of optical semiconductor devices by modifying organization of a multilayer semiconductor layer of the mesa structure. For instance, an optical modulator may be realized by forming the active layer 12 of the multilayer semiconductor layer forming the mesa type structure with InGaAsP which absorbs the light of 1.3 µm where λ is 1.55. Moreover, an optical filter can also be realized by forming the active layer 12 with a semiconductor layer which absorbs the particular light in a structure similar to the optical modulator. In addition, a photosensitive element can also be realized by forming the active layer 12, for example, with the InGaAs light absorbing layer. The alternatives explained above are only examples and any type of optical semiconductor device employing the mesa type embedded structure allows application of the present invention.

Claims: A method of producing a mesa embedded type optical semiconductor device comprising the steps of: forming on a substrate (11) a multilayer semiconductor layer structure comprised of an active layer (12) and an upper layer (13) formed on said active layer, to form said optical semiconductor device; selectively etching said multilayer semiconductor layer structure to shape said mesa structure on a first plane (11A), such that the mesa structure has a first portion having a first width and a side wall, a second portion located on said first portion and comprising said active layer and said upper layer, said second portion having a planarized side wall defined by said active layer and said upper layer, and having a second width narrower than said first width, and a second plane (11B) located between said first portion and said second portion; and forming, by vapor growth deposition, an embedded layer (16) at said first and second planes (11A, 11B) and said planarized side wall of said multilayer semiconductor layer of the mesa type structure. The method of producing a mesa embedded type optical semiconductor device according to claim 1, wherein said first plane (11A) is exposed by simultaneous etching the substrate surface on the occasion of selectively etching said multilayer semiconductor layer structure (12, 13). The method of producing a mesa embedded type otpical semiconductor device according to claim 2, wherein said second plane (11B) is etched from the substrate surface exposed as said multilayer semiconductor layer structure (12, 13) draws back when side etching is executed after selectively etching said multilayer semiconductor layer. The method of producing a mesa embedded type optical semiconductor device according to any one of claims 1 to 3, wherein said embedded layer (16) is formed by the metalorganic chemical vapor deposition (MOCVD) method. The method of producing a mesa embedded type optical semiconductor device according to any one of claims 1 to 4, wherein said first and second planes (11A, 11B) have the surface index number (100). The method of producing a mesa embedded type optical semiconductor device according to any one of claims 1 to 5, wherein said embedded layer (16) is a current contracting layer for injecting a current only to said multilayer semiconductor layer structure (12, 13) of mesa type structure. The method of producing a mesa embedded type optical semiconductor device according to claim 6, wherein said embedded layer (16) is a high resistance semiconductor layer.

Inventors: OKAZAKI NIROU, OKAZAKI, NIROU
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: H01L33
IPC codes: H01L33, H01S5