Method for producing a semiconductor device

Abstract: On a GaAs substrate (11) (AlYGa1-Y)0.5In0.5P crystal layers (14) (0≦Y≦1) is formed to be lattice-matched with the substrate (11). By radiating As molecular beams on the surface of the crystal layers (14) while heating the layered substrate (11) to a temperature at which In in the crystal layers (14) evaporates, the portion near the surface of the crystal layers (14) is changed into an AlYGa1-YAs crystal layer (15) (0≦Y≦1) of a thickness of several molecules, on which layer an AlXGa1-XAs crystal layer (16) (0≦X≦1) is formed. Since the surface of the AlYGa1-YAs crystal layer (15) has been purified, the formed AlXGa1-XAs crystal layer (16) has a high crystallinity, allowing production of a light emitting diode, a semiconductor laser device and the like with high efficiency.

Description: The present invention relates to a method for producing a compound semiconductor device such as the semiconductor laser device and a light emitting diode (LED). More particularly, the present invention relates to a semiconductor laser device having an excellent temperature property allowing continuous oscillation of visible light beams at room temperature, and to a method for producing a compound semiconductor device in which a semiconductor layer of a III-V group compound is formed on a GaAs substrate with high crystallinity. In recent years, in order to attain high efficiency in optical information processing systems, a semiconductor laser device capable of oscillating light beams in the range of short wavelengths is required. Particularly, an (Al Y Ga 1-Y ) 0.5 In 0.5 P crystal (0≦Y≦1) which lattice-matches a GaAs substrate has called attention in industries as a material for a visible light beam semiconductor laser capable of radiating light beams having a wavelength of the 600 nm band. The (Al Y Ga 1-Y ) 0.5 In 0.5 P (0≦Y≦1) is hereinafter referred to as AlGaInP unless otherwise specified. The molecular beam epitaxy (MBE), as well as the metal organic chemical vapor deposition (MOCVD), have been anticipated to be important methods for epitaxial growth of the AlGaInP crystal on the GaAs substrate. It has been reported that an AlGaInP group visible light semiconductor laser device produced by MBE method continuously oscillated visible light beams at room temperature. (Hayaka et al, Journal of Crystal Growth 95(1989) pp.949) Figure 5 is a sectional view of a conventional AlGaInP group visible light semiconductor laser device produced by the MBE method. A first conductivity type GaAs buffer layer 72, a first conductivity type GaInP buffer layer 73, a first conductivity type AlGaInP clad layer 74, a GaInP active layer 75, a second conductivity type AlGaInP second clad layer 76 and a second conductivity type GaInP layer 90 are formed on a first conductivity type GaAs substrate 71 in such a manner that one layer is grown on another in this order by the MBE method. On the second conductivity type GaInP layer 90 is formed an insulation silicon nitride film 91 which has a 10 µm wide stripe groove extending to reach the second conductivity type GaInP layer 90. Electrodes 85 and 84 are formed on the insulation silicon nitride film 91 and on the back surface of the substrate 71, respectively. The semiconductor laser device shown in Figure 5 is a gain guided semiconductor laser device in which current is confined by the insulation silicon nitride film 91 having the stripe groove. This semiconductor laser device has an oscillation threshold level of 93 mA and can continuously oscillate visible light beams at room temperature. This type of the semiconductor laser device, however, can not effectively diffuse heat generated in the active layer at the time of oscillation due to the low thermal conductivity of the AlGaInP crystal. As a result, the maximum temperature for continuous oscillation is as low as 35°C. A semiconductor laser device having not only a structure of effective heat emission but also a double hetero structure composed of AIGaInP crystal layers lattice-matched with a GaAs substrate will be produced if an AlGaAs crystal layer with comparatively high thermal conductivity and effective heat emission can be formed on the AlGaInP crystal layers by the MBE method. GB-A-2178 595 discloses a method of producing a semiconductor laser, wherein an AlGaAs layer is grown on an AlGaInP layer by the MBE method. However, the AlGaAs crystal layer with high crystallinity may not be formed on the AlGaInP crystal layers lattice-matched with the GaAs substrate by the MBE method if the surface of the AlGaInP crystal layer is contaminated by impurities. When the AlGaInP crystal layers and the AlGaAs crystal layer are continuously formed by the MBE method, the molecular beam radiation must be switched from P to As. The above contamination occurs at the time when the layer growth is temporarily stopped for this switching after the growth of the AlGaInP crystal layers is completed. In several seconds after stopping, impurities such as oxygen and steam in the atmosphere inside a MBE apparatus attach themselves to the surface of the crystal layer on which the growth is temporarily stopped. Moreover, in order to grow the AlGaAs layer of high quality by the MBE method, the temperature of the substrate must be raised to about 620°C or higher. At such temperatures, In and P in the AlGaInP layers actively evaporate, causing to deteriorate the surface of the AlGaInP crystal layers. It is not possible to grow the AlGaAs crystal layer with high crystallinity on the deteriorated surface of the AlGaInP crystal layers. Furthermore, the semiconductor laser device having the AlGaInP crystal layers grown by the MBE method is normally of a gain-guided type as shown in Figure 5. In the gain-guided semiconductor laser device, the horizontal transverse mode of laser beams can not be fully controlled. Therefore, the development of an index-guided semiconductor laser device which can stabilize the horizontal transverse mode of laser beams is required also for the semiconductor laser device having AlGaInP crystals. Figure 6 is a sectional view of a conventional index-guided semiconductor laser device. A first conductivity type GaAs buffer layer 72, a first conductivity type AlGaInP first clad layer 74, a GaInP active layer 75, a second conductivity type AlGaInP second clad layer 76, a second conductivity type GaAs layer 78 and a second conductivity type InGaAs layer 100 are formed on a first conductivity type GaAs substrate 71 in such a manner that one layer is grown on another in this order by the MBE method. The GaInP active layer 75, the second conductivity type AlGaInP second clad layer 76, the second conductivity type GaAs layer 78 and the second conductivity type InGaAs layer 100 are etched to form a 10 µm wide ridge. This ridged surface is covered with a silicon oxide layer 101 except the top portion thereof. Electrodes 85 and 84 are then formed over the top ridged surface and on the back surface of the substrate 71, respectively. In the semiconductor laser device of the above described structure, current flows between the electrodes 85 and 84 through the top portion of the ridge where the silicon oxide layer 101 is not formed. And, the presence of the 10 µm wide thin active layer 75 enables the oscillation at a unified horizontal transverse mode. However, the semiconductor laser device of this structure is disadvantageous in that heat generated in the active layer 75 is not efficiently emitted outside the device due to the presence of recesses on the ridged surface formed by etching, as a result preventing continuous oscillation at room temperature. The method for producing a compound semiconductor device, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises the steps of forming on a GaAs substrate an (Al Y Ga 1-Y ) 0.5 In 0.5 P crystal layer (0≦Y≦1) which lattice-matches with the GaAs substrate. radiating As molecular beams on the surface of the crystal layer by an MBE method while heating the layered substrate to a temperature at which In in the crystal layer evaporates so as to change the surface of the crystal layer to an Al Y Ga 1-Y As crystal layer (0≦Y≦1) of a thickness of several molecules, and forming an Al X Ga 1-x As crystal layer (0≦X≦1) on the Al Y Ga 1-Y As crystal layer. In a preferred embodiment, the compound semiconductor device is a light emitting diode. In another preferred embodiment, the compound semiconductor device is a semiconduccor laser. In a further preferred embodiment, the compound semiconductor device is a pin-type photodiode. In the method for producing the compound semiconductor device according to the present invention, As molecular beams are radiated on the surface of the (Al Y Ga 1-Y ) 0.5 In 0.5 P crystal layer (0≦Y≦1) lattice-matched with the GaAs substrate, while the temperature of the substrate is raised to a temperature at which In in the crystal layer evaporates, thus changing the surface of the crystal layer into an Al Y Ga 1-Y As crystal layer (0≦Y≦1) of a thickness of several molecules. In this way, the surface of the (Al Y Ga 1-Y ) 0.5 In 0.5 P crystal layer is purified, at the same time preventing In and P from vaporing from the (Al Y Ga 1-Y ) 0.5 In 0.5 P crystal layer. Also, if an Al X Ga 1-X As crystal layer (0≦X≦1) of a thickness of several molecules is formed on the Al Y Ga 1-Y As crystal layer, the surface of the Al Y Ga 1-Y As crystal layer is purified, allowing obtaining high crystallinity of the Al X Ga 1-x As crystal layer. Thus, the method for producing the compound semiconductor device according to the present invention can provide a high quality compound semiconductor device having the Al X Ga 1-X As crystal layer of high crystallinity formed on the (Al Y Ga 1-Y ) 0.5 In 0.5 P crystal layer. Thus, the invention described herein makes possible the objectives of providing a method for producing a high quality compound semiconductor device in which an Al X Ga 1-X As crystal layer of high crystallinity is easily formed on a deteriorated surface of an (Al Y Ga 1-Y ) 0.5 In 0.5 P crystal layer. This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figures 1(a) to 1(c) are sectional views showing the steps of the method for producing a light emitting device of the first embodiment according to the present invention; Figures 2(a) to 2(c) are sectional views showing the steps of the method for producing a light emitting device of the second embodiment according to the present invention; Figures 3(a) to 3(c) are sectional views showing the steps of the method for producing a light emitting device of the third embodiment according to the present invention; Figure 4 is a sectional view showing a pin-type photodiode produced according to the fourth emboiment of the present invention; Figure 5 is a sectional view showing a conventional semiconductor laser; and Figure 6 is a sectional view showing another conventional semiconductor laser. Example 1 Referring to Figure 1, a producing method of a light emitting device such as a light emitting diode according to the present invention is described as follows. As shown in Figure 1(a), a GaAs buffer layer 12 and an (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P layer 14 are formed on a GaAs substrate 11 in such a manner that one layer is grown on another in this order by the MBE method. The substrate temperature at this stage is 510°C. Next, after the radiation of P molecular beams is stopped, As molecular beams are radiated on the (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P Iayer 14 as shown in Figure 1(b), white the substrate temperature is raised to 620°C. This condition is maintained for several minutes. As a result, In and P near the surface of the (Al 0.7 G 0.3 ) 0.5 In 0.5 P layer 14 are replaced with As in the As molecular beams, changing the upper portion near the surface of the layer 14 to an Al 0.7 Ga 0.3 As layer 15 of a thickness of several molecules. This Al 0.7 Ga 0.3 As layer 15 is thermally stable at a temperature below about 680°C, seldom causing evaporation of the composing elements thereof. Thus, the evaporation of In and P from the layer 14 that is normally active at around 620°C is avoided by being covered with the Al 0.7 Ga 0.3 As layer 15 of a thickness of several molecules. As a result, deterioration of the layer 14 which is caused by In and P evaporation and which is most eminent at a temperature around 580°C or higher can be prevented even at a temperature as high as 620°C. Continuously after the formation of the Al 0.7 Ga 0.3 As layer 15, an Al 0.4 Ga 0.6 As layer 16 is grown on the Al 0.7 Ga 0.3 As layer 15 as shown in Figure 1(c). The growth of the Al 0.4 Ga 0.6 As layer 16 is made by the normal MBE method in which Al and Ga molecular beams in addition to the As molecular beams are radiated on the layered substrate. At this stage, the substrate temperature is 620°C. A light emitting device is thus produced. The efficiency of the light emitting device produced by the above described method was tested by comparing the photoluminescence of the Al 0.4 Ga 0.6 As layer 16 with that of the same layer of a light emitting device produced as a comparative example by a method in which the above second step of radiating As molecular beams while raising the substrate temperature to 620°C is excluded from the above-described method. The result was that the luminescence intensity of the light emitting device of this example was several times as large as that of the light emitting device of the comparative example, indicating that the crystallinity of the Al 0.4 Ga 0.6 As layer 16 of this example are superior to that of the same layer of the comparative example. One reason why the Al 0.4 Ga 0.6 As layer 16 having high crystallinity can be obtained by the method of the present invention is that, as mentioned above, the formation of the Al 0.7 Ga 0.3 As layer 15 can protect the (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P layer 14 from deterioration caused by evaporation of In and P therein. The other reason is that during the above second step In and P near the surface of the (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P layer 14 are replaced with As in the As molecular beams, removing impurities such as oxides present near the surface of the layer 14 and purifying that area. That is, if the (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P layer 14 is contaminated by impurities such as oxygen and steam in the atmosphere inside the MBE apparatus during the inactive time of several seconds between the stop of radiation of P molecular beams and the start of radiation of As molecular beams, such contamination is purified by replacement of In and P in the contaminated area with As. As a result, the problem of contamination of the crystal surface at switching of the molecular beam radiation is resolved. Example 2 Referring to Figure 2, the second example of a method for producing a light emitting device according to the present invention is described as follows. As shown in Figure 2(a), a GaAs buffer layer 12 and a GaInP layer 13 are formed on a GaAs substrate 11 in such a manner that one layer is grown on another in this order by the MBE method. The substrate temperature at this stage is in the range of 450°C and 570°C. Then, the layered substrate is taken out of the MBE apparatus for observation of the surface of the grown crystal surface, and returned to the MBE apparatus. Next, As molecular beams are radiated on the GaInP layer 13 as shown in Figure 2(b), while the substrate temperature is raised to 620°C. This condition is maintained for several minutes. As a result, In and P near the surface of the GaInP layer 13 are replaced with As in the As molecular beams, changing the upper portion near the surface of the GaInP layer 13 to a GaAs layer 17 of a thickness of several molecules. This thin GaAs layer 17 is thermally stable at a temperature below about 680°C, seldom causing evaporation of the composing elements thereof. Thus, the evaporation of In and P from the GaInP layer 13 which is normally active at around 620°C is avoided by being covered by the GaAs layer 17 of a thickness of several molecules. As a result, deterioration of the GaIn layer 13 which is caused by In and P and which is most eminent at a temperature around 580°C or higher can be prevented even at a temperature as high as 620°C. Continuously after the formation of the GaAs layer 17, an Al 0.7 Ga 0.3 As layer 18 is grown on the GaAs layer 17 as shown in Figure 2(c). The growth of the Al 0.7 Ga 0.3 As layer 18 is made by the normal MBE method in which Al and Ga molecular beams in addition to the As molecular beams are radiated on the layered substrate. At this stage, the substrate temperature is 690°C. The thus formed AlGaAs layer 18 has showed the same high crystallinity as the AlGaAs layer 16 of Example 1. Example 3 Referring to Figure 3, a method for producing a semiconductor device according to the present invention is described as follows. As shown in Figure 3(a), a first conductivity type GaAs buffer layer 2, a first conductivity type GaInP buffer layer 3, a first conductivity type AlGaInP clad layer 4, a GaInP active layer 5, a second conductivity type AlGaInP second clad layer 6 and a second conductivity type GaInP layer 7 are formed on a first conductivity type GaAs substrate 1 in such a manner that one layer is grown on another in this order by the MBE method. The growth of each layer is made inside the MBE apparatus using P as the molecular beam source by the normal MBE method. The substrate temperature at this stage is 510°C, and the thickness of the second conductivity type GaInP layer 7 is 10 nm. Next, the layered substrate is taken out of the MBE apparatus and entered into another MBE apparatus using As as the molecular beam source. Then, As molecular beams are radiated on the second conductivity type GaInP layer 7, while the substrate temperature is raised to 620°C. This condition is maintained for several minutes. As a result, as shown in Figure 3(b), In and P near the surface of the GaInP layer 7 are replaced with As in the As molecular beams, changing the upper portion near the surface of the GaInP layer 7 to a GaAs layer 8 of a thickness of several molecules. After the formation of the GaAs layer 8, a second conductivity type GaAs cap layer 9 is grown on the GaAs layer 8 by the MBE method. Then, an insulation silicon nitride film 21 is grown on the cap layer 9 by the plasma-activated CVD method. By photo etching of the silicon nitride film 21 is formed a 10 µm wide stripe groove extending therethrough to reach the second conductivity type GaAs cap layer 9. Finally, electrodes 23 and 22 are formed on the top surface of the thus formed layers and on the back surface of the substrate 1, respectively, to produce a gain guided semiconductor laser device as shown in Figure 3(c). The semiconductor laser device produced am described above is provided with the second conductivity type GaAs cap layer 9 including GaAs which has higher thermal conductivity than GaInP , allowing heat generated in the active layer 5 to be efficiently diffused outside the semiconductor laser device. As a result, the semiconductor laser device of this example can show an excellent temperature property, compared with the one shown in Figure 5, enabling continuous oscillation of 670 nm wavelength light beams at room temperature. In a comparative example of a semiconductor laser device produced by the same method described above except excluding the step of radiating As molecular beams while raising the substrate temperature to 620°C, continuous oscillation at room temperature was not possible because the threshold current increased. The reason is that the surface of a second conductivity type GaInP layer of the comparative example was not sufficiently purified inside the MBE apparatus after being contaminated in the atmosphere and had a crystal layer formed thereon. As a result, the formed crystal layer could not have high crystallinity and therefore the radiative efficiency of laser beams was low compared with that of the semiconductor laser device of Example 3. Example 4 Referring to Figure 4, a method for producing a pin-type photodiode according to the present invention is described as follows. As shown in Figure 4, a first conductivity type GaInP buffer layer 3, a GaInP active layer 5 and a second conductivity type GaInP layer 7 are formed on a first conductivity type GaAs substrate 1 in such a manner that one layer is grown on another in this order by the MBE method. The growth of each layer is made inside the MBE apparatus using P as the molecular beam source by the normal MBE method. Next, the layered substrate is taken out of the MBE apparatus and entered into another MBE apparatus using As as the molecular beam source. Then, As molecular beams are radiated on the second conductivity type GaInP layer 7, while the substrate temperature is raised to 620°C. This condition is maintained for several minutes. As a result, In and P near the surface of the GaInP layer 7 are replaced with As in the As molecular beams, changing the upper portion near the surface of the GaInP layer 7 to a GaAs layer 8 of a thickness of several molecules. After the formation of the GaAs layer 8, a second conductivity type GaAs layer 9 is grown on the GaAs layer 8. Then, electrodes 23 and 22 are formed on the top surface of the second conductivity type GaAs layer 9 and on the back surface of the substrate 1, respectively. Then, a predetermined portion of the electrode 23 and the second conductivity type GaAs layer 9 are etched to reach the surface of the GaAs layer 8 so as to form a light-receiving portion of the photodiode. Further, predetermined portions of the electrode 23, the second conductivity type GaAs layer 9, the GaAs layer 8, the GaInP layer 7, the GaInP active layer 5 and the first conductivity type GaInP buffer layer 3 are etched to produce the pin-type photodiode as shown in Figure 4. The sensitivity of the thus produced pin-type photodiode proved to be superior to that of a pin-type photodiode produced in the same method except excluding the step of radiating As molecular beams on the layer 7 and raising the substrate temperature to 620°C. This is because, according to the method of Example 4, formation of the surface state is reduced even on the surface where the layer growth is temporarily stopped. It in understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention.

Claims: A method for producing a compound semiconductor device comprising the steps of: forming on a GaAs substrate (11) an (Al y Ga 1-y ) 0.5 In 0.5 P crystal layer (0≦y≦1) (13,14) which lattice-matches with the GaAs substrate; radiatinq As molecular beams on the surface of the crystal layer (13,14) by an MBE method while heating the layered substrate to a temperature at which In in said crystal layer evaporates so as to change the surface of the crystal layer to an Al y Ga 1-y As crystal layer (0≦ Y≦1) (15,17) of a thickness of several molecules; and forming an Al x Ga 1-x As crystal layer (0≦x≦1) (16,18) on the Al y Ga 1-y As crystal layer (15,17). A method according to claim 1, wherein the compound semiconductor device is a light emitting diode. A method according to claim 1, wherein the compound semiconductor device is a semiconductor laser. A method according to claim 1, wherein the compound semiconductor device is a pin-type photodiode.

Inventors: HOSODA MASAHIRO, MATSUI SADAYOSHI, SUYAMA TAKAHIRO, TAKAHASHI KOSEI, TSUNODA ATSUO, HOSODA, MASAHIRO, MATSUI, SADAYOSHI, SUYAMA, TAKAHIRO, TAKAHASHI, KOSEI, TSUNODA, ATSUO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: H01S3
IPC codes: H01L33, H01S5, H01L31, H01L21