Abstract:
This invention provides a steam oxidation method of a matter to be oxidized with proper controllability and reproducibility. It is provided a steam oxidation method, where a semiconductor substrate (a matter to be oxidized) is housed in a steam oxidation reactor and is subjected to: a first step of supplying N 2  gas to the reactor housing the semiconductor substrate and substituting the inside of the reactor with N 2  gas; a second step of stopping supply of the N 2  gas and supplying a steam-accompanied N 2  gas, in which the N 2  gas is accompanied with steam, to the reactor; a third step of increasing a temperature of the semiconductor substrate to 450° C. (a steam oxidation temperature) while supplying the steam-accompanied N 2  gas; and a fourth step of holding the semiconductor substrate for a predetermined time at 450° C.

Description:
The present application claims priority to Japanese Patent Application JP2003-184456, filed in the Japanese Patent Office Jun. 27, 2003; the entire contents of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a steam oxidation method of subjecting a matter to be oxidized housed in a reactor to steam oxidation and more particularly to a steam oxidation method of subjecting a matter to be oxidized to steam oxidation with proper controllability and reproducibilty when carrying out steam oxidation. 
   2. Description of Related Art 
   The steam oxidation method is frequently used, for example, for forming an oxidation confining type, current-confining layer of a surface emitting laser element. 
   The surface emitting laser element is a semiconductor laser element emitting laser light in vertical direction relative to a substrate surface. As a surface emitting laser element of an 850 nm wavelength band, attention is being drawn to a surface emitting laser element formed on a semiconductor substrate such as GaAs, comprising a pair of DBRs (Diffractive Bragg Reflector) consisting of a pair of AlGaAs/AlGaAs or the like of mutually different Al composition, and an active layer of an AlGaAs type that will serve as a light-emitting region provided between that pair of DBRs. 
   In such a surface emitting laser element, to enhance a light-emitting efficiency and lower a threshold current, it is necessary to limit a cross-sectional area of a current path of a current to be injected into the active layer. Conventionally, there is employed a method such as providing higher resistance of an ion injected region through an H +  ion injection. However, in recent years, as a method of limiting this current path, there is a mainstream method of forming a current-confining structure by letting a high Al containing layer such as an AlAs layer interposed in a multi-layer film and oxidizing selectively a predetermined area of the high Al containing layer for conversion thereof to Al 2  O 3  of high electric resistance. 
   Referring to  FIG. 2 , description will be made of an example of a construction of a surface emitting laser element having a current-confining structure formed by oxidation of the AlAs layer.  FIG. 2  is a sectional view showing the construction of the surface emitting laser element. 
   A surface emitting laser element  10  is, as shown in  FIG. 2 , a multi-layered structure comprising a lower DBR  14  composed of an n-type semiconductor multi-layer film, an Al 0.3  Ga 0.7 As lower clad layer  16 , an active layer  18 , an Al 0.3 Ga 0.7 As upper clad layer  20 , an upper DBR  22  composed of a p-type semiconductor multi-layer film, and a p-type GaAs contact layer  24 , all of which are sequentially formed on an n-type GaAs substrate  12 . 
   The lower DBR  14  is constructed as a semiconductor multi-layer film with an n-type Al 0.2 Ga 0.8 As layer and an n-type Al 0.9 Ga 0.1 As layer. 
   The upper DBR  22  as a semiconductor multi-layer film with a p-type Al 0.2 Ga 0.8 As layer and a p-type Al 0.9 Ga 0.1 As layer. 
   Also, the p-type Al 0.9 Ga 0.1 As layer of a first pair in the upper DBR  22  adjacent to the upper clad layer  20  is, in lieu of the p-type Al 0.9 Ga 0.1 As layer, replaced with a p-type Al As layer  26   a , and the AlAs layer, excluding a circular area in the center, is selectively oxidized and converted to an Al oxidized layer  26   b.    
   Namely, this layer constitutes a current-confining layer  26  where the Al oxidized layer  26   b  functions as an oxidation confining type, current-confining region of high electric resistance and where the AlAs layer  26   a  functions as a current injection region. 
   The contact layer  24  and the upper DBR  22  are subjected to etching and processed to a columnar mesa post  20  of a circular cross section. 
   The contact layer  24  on an upper surface of the mesa post  30  opens a light-emitting window  32  in a vicinity of the center and is formed in a circular ring shape. 
   In manufacturing the surface emitting laser element  10 , as shown in  FIG. 3 , a multi-layered structure is formed by depositing first on the n-type GaAs substrate  12 , in the order of the lower DBR  14 , the lower clad layer  16 , the active layer  18 , the upper clad layer  20 , the upper DBR  22  having the AlAs layer  26   a , and the contact layer  24 . 
   Next, the vicinity of the center of the contact layer  24  is removed and the light-emitting window  32  is opened. 
   Subsequently, the reactive ion beam etching method (RIBE) is used to etch the contact layer  24  and the upper DBR  22  up to the upper clad layer  20 , thus forming the columnar mesa post  30 . 
   Consequently, there is obtained a semiconductor substrate  44  consisting of a multi-layered structure having the mesa post  30  as shown in  FIG. 3 . 
   Next, the semiconductor substrate  44  is heated in a steam atmosphere to oxidize the AlAs layer  26   a  until a desired oxidation confining diameter is obtained. 
   In the AlAs layer  26   a  on the upper DBR  22 , AlAs on the periphery of the mesa post  30  is selectively oxidized, generating the Al oxidized layer  26   b , while, at the same time, a central region of the mesa post structure  30  remains as the original AlAs layer  26   a.    
   In forming a current-confining structure of the oxidation confining type into a semiconductor substrate by subjecting a high Al containing layer such as the AlAs layer  26   a  to steam oxidation, a steam oxidation apparatus described below will be used. Referring to  FIG. 4 , an example of a construction of a steam oxidation apparatus for subjecting the high Al containing layer to steam oxidation will be explained.  FIG. 4  is a schematic diagram showing the construction of the steam oxidation apparatus  40 . The steam oxidation apparatus  40  is an invention disclosed in patent application Ser. No. 2003-14260. 
   The steam oxidation apparatus  40  is an apparatus to be used when forming a current-confining structure into a surface emitting laser element by subjecting the high Al containing layer to steam oxidation. As shown in  FIG. 4 , as a reactor carrying out steam oxidation, it is equipped with a horizontal-type reactor  42  of a single-slice treatment type. 
   The reactor  42  comprises a quartz chamber  48  in a horizontal square tube type, an electric heater  50  set up around the quartz chamber  48 , and a susceptor  46  housed in the quartz chamber  48 , which supports a semiconductor substrate  44  having a multi-layered structure in which the above-mentioned mesa post  30  is formed. 
   The electric heater  50  is a lamp heater, being capable of increasing a substrate temperature of the semiconductor substrate  44  by irradiation of the lamp. 
   Further, the steam oxidation apparatus  40  comprises a steam-accompanied inert gas system supplying a steam-accompanied inert gas to the reactor  42 , an inert gas system supplying an inert gas to the reactor  42 , a reactor bypass pipe  52  subjecting the steam-accompanied inert gas system and the inert gas system to reactor bypassing, and an exhaust system venting a gas discharged from the reactor  42 . 
   The exhaust system has a water-cooled trap  54 , comprising a gas discharge port  42 B of the reactor  42  and a 4 th gas pipe  56  which leads a gas transmitted from the reactor bypass pipe  52  to the water-cooled trap  54 , and a 5th gas pipe  58  which exhausts a gas that passed through the water-cooled trap  54 . 
   The steam-accompanied inert gas system consists of an H 2 O bubbler which houses pure water, into which an inert gas is transmitted to cause bubbling, and which generates a steam-accompanied inert gas; a 1st gas pipe  64  which is connected to an inert gas source transmits an inert gas whose flow is controlled by an MFC (Mass Flow Controller)  62 A into the H 2 O bubbler; and a 2nd gas pipe  68  which transmits a steam-accompanied inert gas generated in the H 2 O bubbler through an automatic valve  66 A into a gas flow-in port  42 A of the reactor  42 . 
   The inert gas system includes a 3rd gas pipe  70  which is connected to an inert gas source and transmits an inert gas, whose flow is controlled by the MFC  62 B, through an automatic valve  66 C to a gas flow-in port of the reactor  42 . 
   The reactor bypass pipe  52  has its one end connected to the 2nd gas pipe  68  through an automatic valve  66 B, and it is connected to the 3rd gas pipe  70  through an automatic valve  66 D, its other end being connected to the 4 th gas pipe  56 , whereby the steam-accompanied inert gas and the inert gas are subjected to reactor bypassing. 
   When supplying the steam-accompanied inert gas supplied from the steam-accompanied inert gas system to the reactor  42 , the automatic valve  66 A is opened, and an automatic valve  66 B is closed. When supplying the inert gas from the inert gas system to the reactor  42 , the automatic valve  66 C is opened, and the automatic valve  66 D is closed. 
   Also, by closing the automatic valve  66 A and opening the automatic valve  66 B, it is possible to convey the steam-accompanied inert gas supplied from the steam-accompanied inert gas system to the reactor bypass pipe  52 . By closing the automatic valve  66 C and opening the automatic valve  66 D, it is possible to convey the inert gas supplied from the inert gas system to the reactor bypass pipe  52 . 
   The H 2 O bubbler  60  is housed in a constant-temperature bath  72 , and water in the H 2 O bubbler  60  is held at a predetermined temperature by the constant-temperature bath  72  and by the inert gas flow which is controlled by the MFC  62 A. 
   Related Art Example 
   Steam oxidation of a high Al containing layer such as the above-mentioned AlAs layer  26   a  has thus far been carried out as follows by using steam oxidation equipment  40 , one example of which is shown in  FIG. 4 . Referring to  FIG. 5 , a steam oxidation method for oxidizing the AlAs layer  26   a  of the semiconductor substrate  44  will be described.  FIG. 5  is a time table showing a sequence of the conventional steam oxidation method. 
   First, an operating condition of the constant-temperature bath  72  and a flow rate condition of MFC  62 A are set such that the temperature of the H 2 O bubbler  60  is held at 80° C. at all times. 
   Next, the lamp heater  50  is turned on, and at a time point (a), the semiconductor substrate  44  in the normal temperature (approx. 30° C.), that is, the multi-layered structure in which the above-mentioned mesa post  30  was formed is inserted into the reactor  24 . Then, at a time point (b) when the temperature of the semiconductor substrate  44  reaches 450° C., supply of N 2  gas is started and continued for 3 minutes. 
   Subsequently, at a time point (c) of 3 minutes after the supply of N 2  gas is started, the supply of the steam-accompanied N 2  gas in lieu of N 2  gas is started. While supplying the steam-accompanied N 2  gas, at a time point (d) after a lapse of a preset time, that is, at a time point when a predetermined region of the AlAs layer  26   a  of the semiconductor substrate  44  is oxidized in steam, the supply of the steam-accompanied N 2  gas is stopped. The semiconductor substrate  44  is cooled to the normal temperature and the semiconductor substrate  44  is taken out from the reactor  42 . 
   As mentioned above, it is possible that the AlAs layer  26   a  of the semiconductor substrate  44  is oxidized in steam, thus forming the current-confining structure of the oxidation confining type. 
   Since related art technical documents regarding the conventional steam oxidation method described above were not available, disclosure of the related art technical information is omitted. 
   SUMMARY OF THE INVENTION 
   However, in the related art steam oxidation method mentioned above, there was a problem of difficulty in forming the AlAs layer with proper controllability and reproducibility when oxidizing in steam a high Al containing layer such as the AlAs layer  26   a.    
   Accordingly, it is an object of the present invention to provide a steam oxidation method for oxidizing in steam a matter to be oxidized with proper controllability and reproducibility when oxidizing in steam a matter to be oxidized housed in the reactor. 
   The present inventor made a review to solve the above-mentioned problem and found out that in the conventional steam oxidation method, after inserting a semiconductor substrate into the reactor, there was an increase in temperature before supplying a steam-accompanied inert gas (steam-accompanied N 2  gas), so that due to residual water inside the reactor, the high Al containing layer was subjected to natural oxidation, thus deteriorating the controllability of forced oxidation through steam oxidation. 
   Namely, according to the time table shown in  FIG. 5 , from the time point (a) of inserting the semiconductor substrate  44  into the reactor  42 , heating of the semiconductor substrate  44  is started by the lamp heater  50 . 
   Consequently, before the inside of the reactor  42  is completely substituted with the N 2  gas at the time point (b) when the N 2  gas is supplied, natural oxidation of the AlAs layer  26   a  of the semiconductor substrate  44  takes place due to a minute amount of water remaining in the reactor  42 . 
   Moreover, as the temperature of the semiconductor substrate  44  is increased to 450° C. which is the steam oxidation temperature or a temperature close to the steam oxidation temperature, before the inside of the reactor  42  is completely substituted with the N 2  gas, oxidation reaction is thus promoted even more. 
   And, it was found out that as a result of this natural oxidation, it is difficult to properly control the forced oxidation by steam, thereby it is difficult to control or reproduce the proper shape of the Al oxidized layer  26   b.    
   The present inventors have conceived an idea that after substituting the inner atmosphere of the reactor having the semiconductor substrate with an inert gas and eliminating residual water in the reactor, supply of the steam-accompanied inert gas is started to increase the temperature of the semiconductor substrate to the steam oxidation temperature, and thereby natural oxidation due to residual water could be controlled. 
   Based on the information described above, to accomplish the above-mentioned need, it is provided a steam oxidation method according to the present invention, where a matter to be oxidized is housed in a steam oxidation reactor, which comprises: a step of supplying an inert gas to the reactor housing the matter to be oxidized and substituting the inside of the reactor with the inert gas; a step of stopping supply of the inert gas and supplying a steam-accompanied inert gas, in which the inert gas is accompanied with steam, to the reactor; a step of increasing a temperature of the matter to be oxidized to a steam oxidation temperature while supplying the steam-accompanied inert gas; and a subsequent step of holding the matter to be oxidized for a predetermined time at the steam oxidation temperature. 
   According to the method of the present invention, after substituting the inside of the reactor housing the semiconductor substrate with the inert gas and eliminating any residual water in the reactor, supply of the steam-accompanied inert gas is started and by increasing the temperature of the semiconductor substrate to the steam oxidation temperature, natural oxidation of the matter to be oxidized due to residual water in the reactor may be restrained. 
   In a manner described above, in the steam oxidation of the matter to be oxidized which is housed in the reactor, it is possible to enhance controllability of forced oxidation by means of steam oxidation and to carry out steam oxidation of the matter to be oxidized through proper controllability and reproducibility. 
   In the present invention, the matter to be oxidized is preferably a compound semiconductor multi-layered matter having a high Al containing layer for manufacturing a surface emitting laser element, wherein the high Al containing layer is oxidized in steam to form a current-confining structure of an oxidation confining type in the compound semiconductor multi-layered matter for manufacturing the surface emitting laser. 
   This enables, with proper controllability and reproducibility, the formation of the current-confining structure of the oxidation confining type in the compound semiconductor multi-layered matter for the surface emitting laser. 
   The present invention has preferably an Al composition of over 80% in the high Al containing layer. This makes it possible to obtain proper effect as mentioned above. 
   In a preferred embodiment of the present invention, the temperature of the matter to be oxidized at the step of supplying the steam-accompanied inert gas to the reactor is more than 20° C. and less than 250° C. 
   The above lower limit temperature is the normal temperature, and if the above temperature exceeds 250° C., natural oxidation due to residual water in the reactor will proceed. 
   In a preferred embodiment of the present invention, the steam oxidation temperature is more than 350° C. and less than 500° C. 
   If the above temperature is less than 350° C., the oxidation rate of the matter to be oxidized is too slow to obtain a desired oxidized layer, and, if the above temperature is more than 500° C., the oxidation rate of the matter to be oxidized is too fast to obtain sufficient controllability of the shape of the oxidized layer. 
   The inert gas for the present invention is preferably N 2  gas. This makes it possible to obtain proper effect as mentioned above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a time table showing a steam oxidation method of an embodiment of the present invention; 
       FIG. 2  is a sectional view showing a configuration of a surface emitting laser element; 
       FIG. 3  is a sectional view showing a semiconductor substrate having a multi-layered structure in which a mesa post is formed; 
       FIG. 4  is a schematic illustration of an example of a configuration of a steam oxidation apparatus; 
       FIG. 5  is a time table showing a steam oxidation method of a conventional example; 
       FIG. 6A  and  FIG. 6B  are respectively infrared microscopic photographs of the semiconductor substrate  44  and another semiconductor substrate, the AlAs layers of which were steam oxidized according to the steam oxidation method of the example of an embodiment; 
       FIG. 7A  is a sectional view showing a configuration of a surface emitting semiconductor laser element of the sample  1  of the conventional example, and  FIG. 7B  is a sectional view showing a configuration of a semiconductor substrate of the sample  1  of the conventional example; and 
       FIG. 8A  and  FIG. 8B  are respectively infrared microscopic photographs of the semiconductor substrate and another semiconductor substrate, the AlAs layers of which were steam oxidized according to the steam oxidation method of the conventional method. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to attached drawings, preferred embodiments of the present invention will be described concretely and in detail, by citing examples of the embodiments. Incidentally, the embodiment described hereinbelow is illustrative. The invention is not limited thereto. 
   EXAMPLE OF AN EMBODIMENT 
   This is an example of an embodiment of the steam oxidation method according to the present invention, and  FIG. 1  is a time table showing a sequence of the steam oxidation method of the present example of an embodiment. In the present example of an embodiment, the matter to be oxidized is a semiconductor substrate  44  constituted by a multi-layered structure in which the mesa post  30  shown in  FIG. 3  is formed. Steam oxidation of the AlAs layer  26   a  of the semiconductor substrate  44  is to be carried out by using the steam oxidation apparatus  40  shown in  FIG. 4 . 
   According to a steam oxidation method of the present example of an embodiment, before the lamp heater  50  is turned on, at the time point (a), the semiconductor substrate  44  is inserted into the reactor  42 , being held on the susceptor  46 , and N 2  gas is supplied. N 2  gas supply is conducted at least longer than time required for substituting the inside of the reactor  42  with the N 2  gas. In the present example of an embodiment, it is conducted for about 3 minutes. 
   Next, at the time point (b), N 2  gas supply is stopped and supplying steam-accompanied N 2  gas to the reactor  42  is started and continued for about 1 minute. 
   Subsequently, while supplying the steam-accompanied N 2  gas, at the time point (c), a current is run into the lamp heater  50  and this is continued for about 10 minutes up to the time point (c). As a result of heating by the lamp heater  50 , the temperature of the semiconductor substrate  44  reaches 450° C. (steam oxidation temperature) in about 4 minutes and 450° C. is continued to be held up to the time point (d). This enables the AlAs layer  26   a  of the semiconductor substrate  44  to be steam oxidized from a side face of the mesa post  30  thus enabling the Al oxidized layer  26   b  to be formed. 
   When heating the semiconductor substrate  44 , if the temperature gradient is too large, a wafer may possibly be damaged. Also, if the temperature gradient is too small in comparison with the overall steam oxidation time (from the time point (c) to the time point (d)), control of the shape of the Al oxidized layer is made difficult. Consequently, it is desirable for the time of temperature increase to be about half of the overall steam oxidation time. 
   Next, at the time point (d), N 2  gas for cooling is supplied in lieu of the steam-accompanied N 2  gas. At the time point (e) when the temperature of the semiconductor substrate  44  decreases to 250° C., the semiconductor substrate  44  is taken out, and the semiconductor substrate  44  is cooled down to the normal temperature. 
   By the above-mentioned procedures, it is possible to form the current-confining layer  26  of the oxidation confining type in the semiconductor substrate  44 . 
   EXAMPLE OF AN EXPERIMENT 
   Regarding the semiconductor substrate  44  having a film thickness of the AlAs layer  26   a  as 40 nm, steam oxidation was carried out according to the steam oxidation method of the present example of an embodiment, and this was treated as sample  1  of the embodiment example. 
   Also, in lieu of formation of the mesa post  30 , regarding another semiconductor substrate (not illustrated) which is a multi-layered structure having the same multi-layered structure as the semiconductor substrate  44  subjected to cleavage into a flat plate, its steam oxidation was carried out according to the steam oxidation method of the present example of an embodiment, and this was treated as sample  2  of the embodiment example. 
   Infrared microscopic photographs taken from above the semiconductor substrate regarding the sample  1  of the embodiment example and the sample  2  of the embodiment example are respectively shown in  FIG. 6A  and  FIG. 6B . 
   An arrow A in  FIGS. 6A and 6B  indicates a boundary between the AlAs layer  26   a  and the Al oxidized layer  26   b.    
   Also, in  FIG. 6A , a dark circle on the outside is the mesa post  30 , and a dark circle on the inside is an outline of the inside of a contact layer  24  of a circular ring. Further, a dark horizontal line in  FIG. 6B  is a cleavage surface. 
   In the sample  1  of the example of an embodiment, as shown in  FIG. 6A , the Al oxidized layer  26   b  is formed, reflecting a surface orientation of the AlAs layer  26   a , in a manner of surrounding the AlAs layer  26   a  in the shape of a rhomb. Also, in the sample  2  of the example of an embodiment, as shown in  FIG. 6B , oxidation of the AlAs layer  26   a  advances uniformly, so that a tip of the Al oxidized layer  26   b  is formed linear. 
   In this way, as clear from  FIG. 6A  and  FIG. 6B , the Al oxidized layer  26   b  formed according to the steam oxidation method of the examples of the present embodiment is such that its shape is properly controlled. 
   COMPARISON EXAMPLE 
   To make comparison with the samples  1  and  2  of the example of an embodiment, the samples  1  and  2  of a conventional example were experimentally made according to the steam oxidation method of the conventional example shown in  FIG. 5 . 
   In experimentally manufacturing the sample  1  of the conventional example, in lieu of the semiconductor substrate  44  of  FIG. 3 , a semiconductor substrate of a comparison example shown in  FIG. 7B  was used to carry out steam oxidation, and in lieu of the surface emitting semiconductor laser element  10  of  FIG. 2 , a surface emitting semiconductor laser element of the comparison example shown in  FIG. 7A  was manufactured. 
     FIG. 7A  is a sectional view showing a configuration of the surface emitting semiconductor laser element of the comparison example, and  FIG. 7B  is a sectional view showing a configuration of the semiconductor substrate of the comparison example. In these drawings, parts showing like configurations of the surface emitting semiconductor laser element shown in  FIG. 2  and the semiconductor substrate shown in  FIG. 3  are given like reference characters. 
   In the surface emitting semiconductor laser element of the comparison example  74 , as shown in  FIG. 7A , a contact layer  24 , an upper DBR  22 , an upper clad layer  20 , an active layer  18 , a lower clad layer  16 , and a part of a lower DBR  14  are etched to be processed into a mesa post  20  of a columnar shape having a circular section. 
   Also, in the n-type Al 0.9 Ga 0.1 As layer of the first pair in the lower DBR  14  adjacent to the lower clad layer  16 , in lieu of the n-type Al 0.9 Ga 0.1 As layer, the n-type AlAs layer  28   a  is formed, and excluding a circular region in the center, the AlAs layer in the vicinity thereof is selectively oxidized and converted to the Al oxidized layer  28   b . Namely, this layer constitutes a second current-confining layer  28  in which the Al oxidized layer  28   b  functions as a current-confining region of a high electric resistance of oxidation confining type and the AlAs layer  28   a  functions as a current injection region. 
   The surface emitting semiconductor laser element  74  of the comparison example has a like configuration of the surface emitting semiconductor laser element  10  shown in  FIG. 2  excluding the above. In the surface emitting semiconductor laser element  74  of the comparison example, the current-confining layer  26  formed in the upper DBR  22  is treated as a first current-confining layer  26 . 
   The semiconductor substrate  76  may be obtained, as shown in  FIG. 7B , in a process of forming a columnar mesa post  30 , except for etching the portion of, the contact layer  24 , the upper DBR  22 , the upper clad layer  20 , the active layer  18 , the lower clad layer  16 , and also the portion of the lower DBR  14 , up to reaching part of the lower DBR  14 , by the same manufacturing method as of the semiconductor substrate  44  shown in  FIG. 3 . 
   In the present comparison example, regarding a semiconductor sbustrate  76 , thickness of whose first AlAs layer  26   a  and second AlAs layer  28   a  are respectively 70 nm, steam oxidation was carried out according to the steam oxidation method of the conventional example, and this was treated as the sample  1  of the conventional example. 
   Also, in lieu of formation of the mesa post  30 , regarding another semiconductor substrate (not illustrated) which is a multi-layered structure having the same multi-layered structure as the semiconductor substrate  76  subjected to cleavage into a flat plate, its steam oxidation was carried out according to the conventional steam oxidation method, and this was treated as the sample  2  of the conventional example. 
   Infrared microscopic photographs of the sample  1  of the conventional example and the sample  2  of the conventional example taken from above the semiconductor substrate are respectively shown in  FIG. 8A  and  FIG. 8B . 
   In  FIG. 8A  and  FIG. 8B , an arrow A indicates a boundary between the first AlAs layer  26   a  and the first Al oxidized layer  26   b , and arrow B is a boundary between the second AlAs layer  28   a  and the second Al oxidized layer  28   b.    
   Also, in  FIG. 8A , a dark circle on the outside is the mesa post  30 , and a dark circle on the inside is an outline of the inside of a contact layer  24  of a circular ring. Further, a dark horizontal line in  FIG. 8B  is a cleavage surface. 
   In the sample  1  of the conventional example, as shown in  FIG. 8A , the Al oxidized layers  26   b  and  28   b  both have a scattering in the oxidized length from the mesa post  30 , their shapes are not isotropic, and controllability is poor. Also, in the sample  2  of the conventional example, as shown in  FIG. 8B , the Al oxidized layers  26   b  and  28   b  both have an oxidized length from the cleavage surface which is not constant, and their shapes are not properly controlled. In this manner, semiconductor substrates subjected to steam oxidation according to the steam oxidation method of the conventional example have the Al oxidized layers which are not formed with good controllability. 
   By comparing the samples  1  and  2  of the example of the present invention to the samples  1  and  2  of the conventional example, it may be stated that the steam oxidation method of the example of an embodiment has proper controllability regarding the shape of the oxidized layers.