Semiconductor optical apparatus

Abstract: A semiconductor optical apparatus of the invention has a quantum well super-lattice structure consisting of only a direct transition semiconductor or both of a direct transition semiconductor and an indirect transition semiconductor. In the semicondcutor optical apparatus of the invention, by using an absorp­ tion saturation phenomenon of excitons in the direct transition semiconductor constructing the quantum well super-lattice structure, a non-volatile information recording apparatus which can record, reproduce, and erase informatin at a high speed even at the room temperature or even by irradiating a light of a low intensity is realized.

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical apparatus using an absorption saturation phenomenon of an exciton and, more particularly, to a semiconductor optical apparatus. Various kinds of apparatuses using the absorption saturation phenomenon of an exciton can be considered. Among them, in recent years, an attention has been paid to a point that an excellent performance is provided as an optical recording apparatus. By realizing the absorption saturation phenomenon of an exciton by using a semiconductor super lattice structure, a rewritable non-volatile information recording apparatus of a high speed and a high density can be realized at room temperatures. As a method of increasing a recording density by using the absorption saturation phenomenon, an optical data recording system using wavelength dimensions has been disclosed in U.S.P. No. 4,101,976. According to this conventional system, a bulk material which causes the saturation of a uniform absorption line and shows a heterogeneous absorption line extent is included. Porphyrin, deuterium substitution porphyrin, phthalocyanine tetrazine, and tetraphenyl porphyrin can be mentioned as optical recording materials. Data bits are recorded by using the selective absorption saturation by a narrow band high power laser beam. That is, the above recording uses a phenomenon called an photochemical hole burning (PHB) and relates to a method of causing a narrow width absorption in a special wavelength portion in a wide heterogeneous absorption line. This absorption saturation phenomenon is accomplished in a manner such that ground state molecules of the same kind put under slightly different environments are converted into exciting state molecules in a phase and energy manner by irradiating a laser beam of a special wavelength, thereby finally causing a bleaching of the ground state molecules. Therefore, in principle, the above phenomenon is operable only at super low temperatures of about 20°K or less and can be caused only by irradiating a laser beam of a high intensity. Therefore, when recording bit information, it has been known that since a high power laser beam of about, e.g., 10 W/µm² is irradiated for a period of time of at least 1 msec or longer and the recorded information is rewritten by heating, it takes a time of 5 to 10 minutes to completely erase the recorded information. As mentioned above, in the optical information recording using the PHB, the phenomenon at super low temperatures of 20°K or less, preferably, the absolute zero is essentially used, a high laser power is required to write information, and the information is erased or written by heating, so that there is a problem such that the apparatus enlarges in size and becomes complicated. On the other hand, a recording density of an existing non-volatile memory is relatively so small to be 1 x 10⁶ to 4 x 10⁶ bits/cm². Even in the present large scale computers as well as super large computers which will be able to be used in future in the AI (artificial intelligence) or the like, the memory having a satisfactory memory capacity does not exist. It is demanded to realize a high performance non-volatile memory. The document Optical Engineering, vol.24 (1985) no.1, pages 68-73 discloses a quantum well device having optical bistability. It is an object of the present invention to solve the problems of the foregoing conventional techniques and to provide an optical recording apparatus to realize the super high density non-volatile information recording which can perform high speed recording, reproduction, and erasure even at temperatures near the room temperature and even by the light irradiation of a low intensity. A device according to the present invention is defined in claims 1-10 and 13 to 14. Claims 11 and 12 define methods of operating a device according to claim 3. SUMMARY OF THE INVENTION The present invention intends to realize a high speed and high density non-volatile information recording apparatus which is rewritable at the room temperature by using the exciton absorption saturation phenomenon in a semiconductor optical apparatus having a quantum well super-lattice structure. In the semiconductor optical apparatus of the present invention, the quantum well super-lattice structure is formed by only a direct transition semiconductor or by both of a direct transition semiconductor and an indirect transition semiconductor and the light which resonates with the absorption wavelength of the exciton absorption line of the direct transition semiconductor is irradiated to the quantum well super lattice-structure of the semiconductor optical apparatus according to the invention, thereby enabling information to be recorded and reproduced at a high speed. Since the semiconductor optical apparatus of the invention has the quantum well super-lattice structure, by controlling a film thickness and compositions of the super-lattice, the exciton absorption phenomenon which occurs only at low temperatures in the case of the bulk can be caused even at the room temperature. Information can be recorded and reproduced even at the room temperature and even by the light of a low intensity. On the other hand, in the semiconductor optical apparatus of the invention, by applying a reverse bias, the recorded information can be easily erased at a high speed. In addition, a recording density of the semiconductor optical apparatus of the invention is about 10⁸ to 10¹⁰ bits/cm². As mentioned above, according to the semiconductor optical apparatus of the invention, a memory apparatus which can record, reproduce, and erase information at a high speed and a high density at the room temperature is realized. In accordance with another aspect of the present invention there is provided a semiconductor optical device comprising a semiconductor stacked structure having a first semiconductor layer, a second semiconductor layer and a third semiconductor layer stacked in an above-mentioned order, wherein the semiconductor layers form a quantum well structure therein and an energy band structure thereof has a step-like band structure to electrons in a conduction band and a well band structure at the second semiconductor layer to holes in a valence band. The above-mentioned semiconductor optical device has a plurality of the semiconductor stacked structures therein and the plurality of the semiconductor stacked structures form a multiple quantum well structure having different well widths so as to absorb light having a plurality of wavelengths in accordance with energies thereof. In accordance with a further aspect of the present invention there is provided a semiconductor optical device having a semiconductor structure including multiple quantum wells therein, the multiple quantum wells having a plurality of energy states of direct transition and indirect transition of electrons, wherein energy of incident light is absorbed so that electrons are excited through the direct transition, and at least part of the energy is held by the electrons which are in the energy state of the indirect transition. The part of energy is released by shortening a relaxation time of the indirect transition. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B are diagrams showing examples of a quantum well structure and an exciton absorption line; Fig. 2 is a conceptional diagram showing an embodiment of an optical information recording method according to the invention; Fig. 3 is a conceptional diagram showing the optical information recording system according to the invention; Figs. 4A to 4C are diagrams showing recording data bits in the embodiment; Figs. 5A and 5B are conceptional diagrams showing an embodiment of the optical information recording method according to the invention; Fig. 6A is a conceptional diagram regarding a device structure showing an embodiment of the invention; Fig. 6B is a conceptional diagram regarding a device structure showing another embodiment of the invention; and Fig. 7 is a conceptional diagram showing another embodiment of an optical information recording method according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described. When explaining the embodiment of the invention, the principle will be first described and practical devices will be subsequently described as Examples 1, 2, and 3. The invention uses the light absorption due to the generation of excitons of a direct transition semiconductor. In the generation of excitons, a steep absorption peak exists at the absorption edge. This is because in the direct transition semiconductor, the bottom of the exciton band is located at a point of the zero momentum and the excitons of the zero momentum are produced by the light irradiation. As such a semiconductor, there are known compound semiconductors of the II-VI group and III-V group such as ZnSe, ZnS, ZnTe, CdSe, CdS, CdTe, ZnSe x S 1-x (0 < x < 1), Cdse x S 1-x (0 < x < 1), GaAs, Inp, InGaAsP, etc. The absorption peak position by the excitons can be set to an arbitrary wavelength by forming a quantum well super-lattice structure. For instance, in the case of forming a super-lattice structure in which a ZnS layer of 10 nm as shown in Fig. 1 is used as a barrier layer 2 and a quantum well layer of an arbitrary film thickness is used as a light absorption layer 3, as shown in Figs. 1A and 1B, an energy shift of hundreds of meV occurs between exciton absorption lines 1 when film thicknesses (L W ) of quantum well layers are set to 2 nm and 10 nm. Similarly, in the case of forming a super-lattice structure in which Ga 0.3 Al 0.7 As of a thickness of 30 nm is used as a barrier layer and a GaAs layer of an arbitrary film thickness is used as a light absorption layer, the exciton absorption line is shifted to the side of a high energy with a decrease in GaAs film thickness. Such a control of the absorption peak position can be obviously accomplished by changing the compound semiconductor forming a quantum well or the compositions of the elements constituting the compound semiconductor. In such a quantum well super-lattice structure, the coupling energy of the excitons increases than that in the bulk. Therefore, the excitons which are unstable in the bulk even at the room temperature are free from the thermal dissociation and stably exist, so that the absorption line appears at a position near the band edge. In this manner, in the quantum well super-lattice structure, the intensity of vibrator of the absorption line due to the excitons can be increased by 10² to 10⁴ times. As is also well known from the sum rule of the oscillator strength, such an increase in oscillator strength functions so as to decrease the oscillator strength of the absorption transition between bands which is unpreferable for the semiconductor optical apparatus. On the other hand, in the case where the exciton binding energy is increased by setting the layer thickness L w (= L w1 + L w2 ) of the super lattice structure shown in, e.g., Fig. 2 to a value of 20 nm or less, the oscillator strength can be concentrated to the transition to the ls state of the excitons. Accordingly, by making the quantum well super-lattice structure, the exciton absorption line 1 consisting of a steep single absorption transition as shown in Fig. 1 can be made appear on the lower energy side than the absorption end based on the transition between bands. When irradiating the light of the wavelength which resonates with the absorption wavelength of the excitons which were made appear, as compared with the compound semiconductors of, e.g., ZnSe, GaAs, etc. of the bulk, the absorption coefficient decreases at the incident light intensity which is lower by one digit or more in their super-lattice structures. It is considered that the foregoing phenomenon occurs because in the super-lattice structure, the shielding of the Coulomb interaction of the excitons due to the free electrons-holes is easily caused due to what is called a confinement phenomenon. Thus, the exciton absorption line can be extinguished in a short time of a few 10⁻¹² seconds even by the light irradiation of a low intensity of 0.1 to 10 mW/µm², so that the wavelength multiplex optical information recording can be performed. However, such a recording is a volatile recording. The operation regarding the non-volatile recording will now be described hereinbelow. Fig. 2 shows an example of a band model according to the invention under the field-free condition (E = 0). An axis of ordinate indicates an energy and an axis of abscissa indicates a location. A substance A (e.g., a first semiconductor), a substance B (e.g., a second semiconductor), and a substance C (e.g., a third semiconductor) form a heterojunction. The substance A forms a potential barrier for the electrons and holes. The substance B forms a potential barrier for the electrons. The substance C forms a potential barrier for the holes. The substances A, B, and C are approximately intrinsic semiconductors. The free electrons existing in the conduction band of the substance B or C cannot move through the heterojunctions between the substances A and B and between the substances A and C due to the potential barrier formed by the substance A. At the same time, the free holes existing in the valence electron band of the substance B cannot move through the heterojunctions due to the potential barriers formed by the substances A and C. On the other hand, the electrons existing in the conduction band of the substance C cannot move to the left in the diagram than the heterojunction due to the potential barrier formed by the substance B because the lowest energy which the electrons can occupy is smaller than the lowest energy in the substance B. On the other hand, the potential barrier which limits the movement of the carriers has a level of at least KT (about 26 mV), preferably, a few KT, and more desirably, a level of about 10 KT (about 0.26 V) or more. Since the forbidden band width of the substance B is smaller than those of the substances A and C, excitons are certainly first produced in the substance B by the light of an energy near the forbidden band width of the substance B. The exciton transition between the heterojunctions from the valence electron band of the substance B to the conduction band of the substance C can be ignored because the wave functions of the holes and electrons hardly overlap. The electrons in the conduction band of the substance B are promoted by the Coulomb interaction between the electrons and holes in the excitons and are relaxed to the bottom of a deep quantum well in the substance C. A time of about 10⁻¹³ to 10⁻¹² seconds or shorter as an atomic vibrating period is required for this relaxation. Fig. 5A shows a band model when a forward bias is applied to the heterojunctions in Fig. 2. That is, the polarity of the substance A is negative for the side of the substance B. All of the substances A, B, and C are approximately intrinsic and the bending of the band (formation of a depletion layer) due to the sweeping up of the free carriers is so small to be substantially ignored. Therefore, the electric fields which are uniformly generated are shown in the diagram. When electrons are produced in the conduction band of the substance B, the electrons are collected to a portion having a low potential near the heterojunction of the substances C and A due to the function of the electric field. On the other hand, the holes are collected to a portion of a low potential near the heterojunction of the substances B and A. Since the movement of these free carriers results in a drift due to the electric fields, they are moved at an enough high speed. Further, since the wave functions of the electrons and holes are sufficiently separated due to the influence of the electric fields, a transition propability can be extremely suppressed to a low value. Thus, by suppressing the recombination of the electron/hole pairs, the bleaching state can be held for a long period of a few months or longer as shown in Fig. 4B. The operation regarding the erasure of recorded information will now be described. Fig. 5B shows a band model when a reverse bias is applied to the heterojunctions in Fig. 2. That is, the polarity of the substance A becomes positive for the substance B. When the polarity of the applied field is reversed from the state of Fig. 5A to the state of Fig. 5B, both of the electrons and holes are collected to a portion of a low potential near the heterojunction between the substances B and C due to the operation of the electric fields. Further, the wave functions of the electrons and holes exceed the heterojunction and enter the potential barrier due to the influence of the electric fields, so that the wave functions of the electrons and holes overlap. Therefore, since the transition probability increases, the recombining process of the electrons and holes can be promoted. Thus, information can be erased at a high speed. A method of optically recording information by the non-volatile wavelength selection which is constructed by the writing, recording/holding, and erasing operations will now be described with reference to Figs. 4 and 5. n layers of sets of the super-lattice layers each having a quantum well structure shown in Fig. 2 which makes appear exciton absorption lines having different absorption peak wavelengths and narrow half-value widths are laminated. That is, information recording layers of n bits are formed. At this time, one bit denotes binary logic information in which the presence and absence of the exciton absorption line correspond to 0 and 1. The light absorbing section consisting of the set of super-lattice layers is sandwiched by insulative films which are transparent to the recording light. Further, both sides of this assembly are sandwiched by transparent electrodes 22 and a forward bias of, typically, 0 to 10⁶ V/cm is applied between the electrodes. Thereafter, the absorption saturation of the excitons is caused in each of super-lattice layers i, j, and k having absorption bands at wavelengths υ₁, υ₂, and υ₃ by the laser beams of the wavelengths υ l , υ₂, and υ₃ which are generated from a laser beam source 17 at the room temperature, so that information is recorded. The recording holding period of time is a few months or longer. After that, a probe beam of an enough weak intensity having a wavelength width wider than Δλ = λ₂ - λ₁ in Fig. 4C is irradiated, thereby measuring the absorption spectrum. As shown in Fig. 4C, for the spectrum to be measured, the absorption saturation wavelength is made correspond to a binary logic "1" and the wavelength at which the exciton absorption line exists is made correspond to binary logic "0". In this case, for instance, the reproduction of information is accomplished. Thereafter, when a reverse bias of about 0 to 16⁶ V/cm is applied between the electrodes, the exciton absorption line is recovered and the erasure of the information is accomplished. In this manner, for instance, in the semiconductor optical apparatus, information is completely written for a time of 2 msec by an energy of 20 mW. Thereafter, the information can be read out ten thousand times or more for a few months or longer. The recording density at this time is about at most 10¹⁰ bits/cm². This recording density value is larger by three order of magnitude than the maximum recording density in the existing apparatus. The information can be read out without using the probe light. In this case, as shown in Fig. 5B, a reverse bias is applied and a light emitting line is used. The light emitting line indicates the information which corresponds to the recorded information in a one-to-one corresponding relation by the light emitting signals in accordance with the difference of the quantum level of the quantum well for the holes which is provided in the valence electron band. In this case, the number of reading operation times differs depending on the intensity of the reverse bias to be applied and the information can be read out at most ten times. The operations to record, store/hold, reproduce, and erase information in the semiconductor optical apparatus of the invention have been described above on the basis of the band diagrams of the quantum well super-lattice structure. Another embodiment of a semiconductor optical apparatus of the invention will now be described. Fig. 7 is a conceptional diagram showing another embodiment of a semiconductor optical apparatus of the invention. In the semiconductor optical apparatus in Fig. 7, the substance B is constructed by a direct transition semiconductor and the substance C is constructed by an indirect transition semiconductor. In the semiconductor optical apparatus in Fig. 7, information is recorded by the exciton transition in the substance B by the light of an energy near the forbidden band width of the substance B in a manner similar to the foregoing semiconductor optical apparatus of the invention. The electrons excited in the conduction band of the substance B are promoted by the Coulomb interaction between the electrons and the holes in the excitons and are relaxed to the bottom of the deep quantum well in the substance C. The substance C is constructed by the indirect transition semiconductor. In the indirect transition semiconductor, the recombination of the electrons and holes is suppressed and the stored information is held for a long period of time even under the field-free condition. In the semiconductor optical apparatus of Fig. 7, the stored information can be also erased by applying a reverse bias as shown in Fig. 5B. As the means for forming the super-lattice structure, a molecular beam (MBE) method, an organometallic molecular beam (MOMBE) method, an organometallic chemical vapor deposition (MOCVD) method, an atomic layer epitaxy (ALE) method, and the like have been known. A II-VI group compound and a III-V group compound have been known as the substances which form the heterojunction to cause the quantum wells for the electrons and holes at different positions. For instance, the substance A is ZnS x Se 1-x (0 ≦ x ≦ 1), the substance B is CdTe, and the substance C is CdS. Or, the substance A is Zns x Se 1-x (0 ≦ x ≦ 1), the substance B is GaAs, and the substance C is CdS, etc. The electric field intensity is set to a value enough to perform a desired function and, typically, it is set to 0 to 10⁶ V/cm. Figs. 6A and 6B show examples of a device structure. In Fig. 6A, after the transparent electrode 22 and an insulative substance 23 were laminated on a substrate 24, a heterojunction consisting of the substances A, B, and C is formed on the opposite surface of the substrate 24. Finally, a transparent electrode 22' is again formed through the insulative substance 23. The electrode 22 is made of indium-tin oxide (ITO), thin film gold, or the like to transmit a desired light. Similarly to the substance A, a II-VI group semiconductor of the wide band gap can be also used as the insulative substance. Or, the substances such as Al x Ga 1-x AS (0 < x ≦ 1), CaF₂, BaF₂, etc., or the substance such as SiO₂, Ta₂O₅, etc. can be also used. On the other hand, similarly to the substance A, the substrate can be made of a II-VI group semiconductor of the wide band gap or the substance such as CaF₂, BaF₂, etc. In the example of the device shown in Fig. 6B having a structure such that an optical information recording layer is sandwiched by the insulative layer and electrode every absorption layer which indicates the same exciton absorption line, a voltage can be independently applied every layer constituting each bit. Therefore, the recorded information can be corrected by erasing and rerecording information every bit. Examples of practical devices will now be described in detail as embodiments of the invention with reference to the drawings. Example 1: In Fig. 6A, the ZnS monocrystalline substrate (0.5 x 10 x 10 mm³) 24 was formed by evaporation depositing the transparent electrode 22 and Ta₂O₅ insulative layer 23 of a film thickness of 400 nm onto one surface. A ZnS layer of a film thickness of 10 nm, CdTe of a film thickness of L w1 , and CdS of a film thickness of L w2 were repetitively laminated onto the (111) face of the substrate 24 in accordance with this order by the MOCVD (organometallic chemical vapor deposition) method, MBE method, or the like to thereby obtain a recording layer 25. The number of data bits is set to n = 12. First, in the first layer (n = 1) of the recording layer 25, the thickness L w1 of the CdTe light absorption layer was set to 13 nm, the thickness L w2 of the CdS layer was set to 3 nm, and a combination of three layers consisting of ZnS (10 nm), CdTe (13 nm), and CdS (3 nm) was formed. Twenty sets of these combined layers are laminated to thereby form the first layer corresponding to one data bit having a total film thickness of 520 nm. The film thickness of the ZnS layer was set to a predetermined value of 10 nm. The film thickness of CdS was set to a predetermined value of 3 nm (= L w2 ), and the second to twelfth layers were formed. For the second layer, the thickness L w1 of the CdTe light absorption layer was set to 12 nm and a set of combination consisting of ZnS (10 nm), CdTe (12 nm), and CdS (3 nm) was formed. Twenty sets of these combined layers were laminated such that the film thickness of the CdTe light absorption layer is set to about 260 nm. In this manner, the second layer corresponding to the second data bit was formed. In a manner similar to the above, the CdTe layer was reduced by a thickness of 1 nm at a time and the third to twelfth layers consisting of the super-lattice structure shown in Fig. 2 were formed. The total film thickness of the CdTe light absorption layer in each layer is set to about 260 nm. Therefore, for instance, the twelfth layer becomes the super-lattice film of a film thickness of 1.95 µm in which 130 layers consisting of ZnS (10 nm), CdTe (2 nm), and CdS (3 nm) were laminated. After the twelfth layer was formed, the ZnS layer of a film thickness of 100 nm is laminated. Then, the Ta₂O₅ insulative layer 23' of a film thickness of 400 nm was laminated. The transparent electrode 22' was formed on the insulative layer 23'. Both sides of the electrode were connected to a power source 4 which can apply an arbitrary electric field of 0 to 10⁶ V/cm to the optical information recording layer 25 and has a switching circuit to switch both of forward and reverse biases. The material manufactured in this manner is used as an optical recording device 5. In Fig. 3, reference numeral 11 denotes a recording laser beam source which is controlled by a laser control system 10. A laser beam emitted from the laser beam source 11 is controlled by a laser beam control system 12 and is irradiated onto the optical recording device 5. A size of irradiation light spot is set to about 1 µm. A plurality of GaAs MQW semiconductor lasers are used as the laser beam source 11. Three kinds of laser beams in which half-value widths of the semiconductor lasers are 0.5 meV or less, outputs are 50 mW, and wavelengths are 808.21 nm (1.533 eV, υ₁), 806.12 nm (1.537 eV, υ₂), and 802.96 nm (1.543 eV, υ₃) were simultaneously irradiated for a period of time of 2 msec, thereby recording information. Fig. 4B shows an absorption spectrum of the optical recording device after the information was recorded. Next, the information is read out through an optical path control system 14 by a recording reading light 13 of an output of 10 µW consisting of a wide spectrum of wavelengths in a range from 770 nm (λ₁) to 920 nm (λ₂) and is output by a data reading system 15. The result is shown in Fig. 4C. The recording density of the recording device was 7.2 x 10⁹ bits/cm². After data was recorded, the recorded data can be held even after the elapse of two years. This recording device can also operate even under the field-free condition. The recording holding time in this case is at least a few weeks. Example 2: In Fig. 6A, the ZnSe monocrystalline substrate (0.5 x 10 x 10 mm³) 24 was formed by laminating the transparent electrode 22 and Ta₂O₅ insulative layer 23 of a film thickness of 400 nm onto one surface. The recording layer 25 was formed by repetitively laminating the ZnSe layer of a film thickness of 10 nm, GaAs of a film thickness of L w1 , and CdS of a film thickness of L w2 onto the (111) face of the substrate 24 in accordance with this order by the well-known method such as MOCVD method or MBE method. The number of data bits is set to n = 12. In the first layer (n = 1) of the recording layer 25, a thickness of the GaAs light absorption layer was set to L w1 = 13 nm, a thickness of CdS layer was set to L w2 = 3 nm, and a set of combination consisting of three layers of ZnS (10 nm), GaAs (13 nm), and CdS (3 nm) was formed. Five sets of these combined layers were laminated to thereby form the first layer corresponding to one data bit having a total film thickness of 130 nm. In a manner similar to the above, a film thickness of the ZnS layer was set to a predetermined value of 10 nm and a film thickness of the CdS layer was set to a predetermined value of 3 nm (L w2 ), and the second to twelfth layers were formed. For the second layer, a thickness of the GaAs light absorption layer was set to L w1 = 12 nm and a set of combination consisting of ZnS (10 nm), GaAs (12 nm), and CdS (3 nm) was formed. Six sets of these combined layers were laminated to thereby form the second layer to record the second data bit so that a thickness of the GaAs layer is set to about 65 nm. In a manner similar to the above, the GaAs layer was reduced by 1 nm at a time and the third to twelfth layers consisting of the super-lattice structure shown in Fig. 2 were formed. For instance, the twelfth layer becomes a film consisting of the super lattice structure of a film thickness of about 560 nm in which 33 layers consisting of ZnS (10 nm), GaAs (2 nm), and CdS (3 nm) were laminated. After the twelfth layer was formed, the ZnS layer of a thickness of 100 nm is laminated. The Ta₂O₅ insulative layer 23' of a film thickness of 400 nm was then laminated on the ZnS layer. The transparent electrode 22' was formed on the insulative layer 23'. Both electrodes 22 22' were connected to the power source 4. Thus, arbitrary electric fields of both of the forward and reverse biases of 0 to 10⁶ V/cm can be applied to the information recording layer 25. The material manufactured in this manner is used as the optical recording device 5. Next, a plurality of GaAs MQW semiconductor lasers for emitting the lights of different oscilalting wavelengths were used as the recording light source. Two kinds of laser beams in which half-value widths of the lasers are 0.5 meV or less, an output is 10 mW, and wavelengths are 837.16 nm (1.480 eV, υ₁) and 831.54 nm (1.490 eV, υ₂) were simultaneously irradiated for a period of time of 1 msec, thereby recording information. Next, the recorded information is read out by irradiating the reading light 13 of an output of 10 µW consisting of a broad spectrum of wavelengths within a range from 830 nm (λ₁) to 890 nm (λ₂). The data recording density was about 10⁹ bits/cm². The data can be also read out even after the elapse of two years. Even if the recording device is operated under the field-free condition, the good recording and reproducing characteristics are obtained. The recording holding time in this case was two weeks. Example 3: In Fig. 6A, the ZnS monocrystalline substrate (0.5 x 10 x 10 mm³) 24 was formed by evaporation depositing the transparent electrode 22 and Ta₂O₅ insulative layer 23 of a film thickness of 400 nm onto one surface. The recording layer 25 was formed by repetitively laminating the ZnS layer of a film thickness of 10 nm, GaAs of a film thickness of L w1 , and GaP of a film thickness of L w2 onto the (111) face of the substrate 24 in accordance with this order by the MOCVD (organometallic chemical vapor deposition) method, MBE method, or the like. The number of data bits is set to n = 12. First, in the first layer (n = 1) of the recording layer 25, a thickness of the GaAs light absorption layer was set to L w1 = 13 nm, a thickness of the CdS layer was set to L w2 = 3 nm, and a set of combination consisting of three layers of ZnS (10 nm), GaAs (13 nm), and GaP (3 nm) was formed. Twenty sets of these combined layers were laminated, thereby forming the first layer corresponding to one data bit having a total film thickness of 520 nm. In a manner similar to the above, a film thickness of the ZnS layer was set to a predetermined value of 10 nm and a film thickness of the Gap was set to a predetermined value of 3 nm (= L w2 ), and the second to twelfth layers were formed. For the second layer, a thickness of the GaAs light absorption layer was set to L w1 = 12 nm and a set of combination consisting of ZnS (10 nm), GaAs (12 nm), and GaP (3 nm) was formed. 22 sets of these combined layers were laminated to thereby form the second layer corresponding to the second data bit such that a film thickness of the CdTe light absorption layer is set to about 260 nm. In a manner similar to the above, the GaAs layer was reduced by a thickness of 1 nm at a time and the third to twelfth layers consisting of the super-lattice structure shown in Fig. 2 were formed. A total film thickness of the CdTe light absorption layer in each layer was set to about 260 nm. Therefore, for example, the twelfth layer becomes the super-lattice film of a film thickness of 1.95 µm in which 130 layers consisting of ZnS (10 nm), GaAs (2 nm), and GaP (3 nm) were laminated. After the twelfth layer was formed, the ZnS layer of a film thickness of 100 nm is laminated. The Ta₂O₅ insulative layer 23'of a film thickness of 400 nm was then laminated onto the ZnS layer. The transparent electrode 22' was formed on the insulative layer 23'. Both sides of the electrode are connected to the power source 4 which can apply an arbitrary electric field of 0 to 10⁶ V/cm to the optical information recording layer 25 and has a switching circuit to switch both of forward and reverse biases. The material manufactured in this manner is used as the optical recording device 5. In Fig. 3, reference numeral 11 denotes the recording laser beam source which is controlled by the laser control system 10. A laser beam emitted from the laser beam source 11 is controlled by the laser beam control system 12 and is irradiated onto the optical recording device 5. A size of irradiation light spot is set to about 1 µm. A plurality of GaAs MQW semiconductor lasers are used as the laser beam source 11. Three kinds of laser beams in which half-value widths of the semiconductor lasers are 0.5 meV or less, outputs are 50 mW, and wavelengths are 866.36 nm (1.430 eV, υ₁), 863.95 nm (1.434 eV, υ₂), and 861.54 nm (1.438 eV, υ₃) were simultaneously irradiated for a period of time of 2 msec, thereby recording information. Fig. 4B shows an absorption spectrum of the optical recording device after the information was recorded. Next, the recorded information is read out through the optical path control system 14 by the recording reading light 13 of an output of 10 µW consisting of a wide spectrum of wavelengths within a range from 800 nm (λ₁) to 940 nm (λ₂) and is output by the data reading system 15. The result is shown in Fig. 4C. The recording density of the recording device was 7.2 x 10⁹ bits/cm². The recording holding time under the field-free condition was a few months and the performance of 10⁴ V/cm was held for one year or longer under the field applying condition. According to the semiconductor optical apparatus having the quantum well structure of the invention, even at temperatures near the room temperature or even when a laser beam of a low intensity is irradiated, the high density and non-volatile information recording which can record, reproduce, and erase information at a high speed can be performed. The recording density of about 10⁸ to 10¹⁰ bits/cm² can be realized. Although the semiconductor optical apparatus of the invention has the excellent performance as an information recording apparatus as mentioned above, it can be also used as a wavelength converting device or an input/output switching device.

Claims: A semiconductor optical device for receiving light and recording information comprising a multiple quantum well super-lattice structure having a first semiconductor layer (A), a second semiconductor layer (B) and a third semiconductor layer (C) repetitively stacked in the above-mentioned order wherein a band gap of said second semiconductor layer (B) is smaller than the band gaps of the semiconductor of the first and third semiconductor layers (A, C), said multiple quantum well super-lattice structure having a conduction band structure wherein the conduction band for electrons is step-like and sequentially decreases from said first layer (A) to said third layer (C) whereby a quantum well for electrons is formed with minimum energy for electrons in said third layer (C) and having a valence band structure wherein a quantum well for holes is formed with minimum energy for holes in said second layer (B) whereby light is absorbed in an exciton absorption state of said second layer (B) and then the generated electrons are stored in an energy state of said third layer (C). A device according to claim 1, wherein said second semiconductor layer (B) is formed of direct transition semiconductor. A device according to claim 1, wherein the multiple quantum well super-lattice structure has a plurality of exciton absorption states so that light having a plurality of wavelengths is received in the exciton absorption states in accordance with energies thereof. A device according to claim 2, wherein said third semiconductor layer (C) is formed of an indirect transition semiconductor. A device according to claim 1, wherein said first, second and third semiconductor layers (A, B, C) are laminated in accordance with this order, potential barriers for electrons existing in said third semiconductor layer (C) are formed by said first semiconductor layer (A) and said second semiconductor layer (B), potential barriers for holes existing in said second semiconductor layer (B) are formed by said first semiconductor layer (A) and said third semiconductor layer (C), and all said potential barriers are equal to or larger than 1 KT. A device according to claim 5, wherein all said potential barriers are equal to or larger than 5 KT. A device according to claim 5, wherein all said potential barriers are equal to or larger than 10 KT. A device according to claim 1, wherein said first semiconductor (A) is ZnS, said second semiconductor (B) is CdTe, and said third semiconductor (C) is CdS. A device according to claim 1, wherein said first semiconductor (A) is ZnS, said second semiconductor (B) is GaAs, and said third semiconductor (C) is CdS. A device according to claim 1, wherein said first semiconductor (A) is ZnS, said second semiconductor (B) is GaAs, and said third semiconductor (C) is GaP. A method of operating the device according to claim 3, wherein information is written by an exciton absorption phenomenon of said second semiconductor layer (B), and by applying an electric field in such a direction that the first semiconductor layer (A) is set to be negative and the third semiconductor layer (C) is set to be positive, the stored information is held. A method of operating the device according to claim 3, wherein information is written by an exciton absorption phenomenon of the second semiconductor layer (B), and by applying an electric field in such a direction that the first semiconductor layer (A) is set to be positive and the third semiconductor layer (C) is set to be negative, the stored information is erased. A device according to any of claims 1 to 10, that comprises a plurality of said multiple quantum well super-lattice structures. A device according to claim 13, wherein said plurality of multiple quantum well super-lattice structures are disposed in a two-dimensional arrangement.

Inventors: KANEHISA OSAM, MIGITA MASAHITO, SHIIKI MASATOSHI, UDA TSUYOSHI, KANEHISA, OSAM, MIGITA, MASAHITO, SHIIKI, MASATOSHI, UDA, TSUYOSHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: H01L31
IPC codes: G11C7, H01L31