An optical semiconductor device

Abstract: An optical semiconductor device is disclosed that comprises a quantum-well structure as an active region and exhibits a nonlinear optical effect with regard to light of energy near the band gap between the allowed band edges in the active region. The quantum-­ well structure of this device is composed of alternate layers consisting of at least one first semiconductor layer with a thickness smaller than the de Broglie wavelength of carriers and at least two second semiconductor layers with a band gap greater than that of the first semiconductor layer, the alternate layers being formed along a crystal orientation in the zinc-­ blends structure. The second semiconductor layers mentioned above are of an indirect transition type.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention: This invention relates to an optical semiconductor device such as an optical modulator, an optical switch, and the like, which utilizes a nonlinear effect caused by excitons in the semiconductor layers thereof. 2. Description of the prior art: In recent years, optical semiconductor devices such as optical modulators, optical bistable devices, and the like have been extensively developed in order to realize superhigh speed optical communication, optical logic circuits, and the like. Of great interest as a means to achieve these objectives, there can be mentioned a device structure utilizing a quantum-well effect, in which tens to hundreds of two kinds of very thin semiconductor layers with different band gaps are alternately formed into a quantum-well structure. The term quantum-well structure used heroin refers to a thin-layer structure composed of alternate layers consisting of first semiconductor layers with a thickness smaller than the de Broglie wavelength of about 200 Å to 300 Å with respect to electrons or holes and second semiconductor layers with a band gap greater than that of the first semiconductor layer. In recent years, epitaxial growth techniques such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MO-CVD), etc., have been developed, by which such a thin-layer structure can be produced readily. Because each layer in the multiple quantum-well structure has a very small thickness, electrons and holes in the multiple quantum-well layer cannot move freely in the direction of thickness, so that they have a strong tendency to be confined two-dimensionally in the plane perpendicular to the direction of thickness. Moreover, the bound energy of an exciton into which an electron and a hole are bound together by their Coulombic attraction is increased because of the two-dimensional confinement of electrons and holes, so that excitons occur under heat energy at room temperature. There are proposed several optical semiconductor devices utilizing such excitons present at room temperature, which include an electric field effect optical modulator. Figure 5 shows a sectional view of a conventional electric field effect optical modulator, which is produced as follows: On a (100)-oriented n-GaAs substrate 60, an n-Al 0.3 Ga 0.7 As layer 61, a multiple quantum-well (MQW) layer (composed of alternate layers consisting of fifty undoped GaAs well layers 62 of a thickness of 100 Å each and forty-nine undoped Al 0.3 Ga 0.7 As barrier layers 63 of a thickness of 100 Å each), a p-Al 0.3 Ga 0.7 As layer 64, and a p-GaAs layer 65 are successively grown. Then, by photolithography and chemical etching techniques, the central portion of each of the n-GaAs substrate 60 and the p-GaAs layer 65 is removed into a circular shape with a diameter of 200 µm, resulting in circular windows 66. Next, an n-sided electrode 67 and a p-sided electrode 68 are formed on the back and upper faces of this device other than the circular windows, respectively. Figure 6 shows the band edges when an electric field is applied to the above-mentioned optical modulator, and Figure 7 shows absorption spectra obtained when the optical modulator is irradiated with light through the circular window. When an appropriate forward voltage is applied across the p-n junction of the optical modulator, the multiple quantum well becomes flat as shown in Figure 6a, and the wave functions of both electrons and holes in the conduction band and the valence band have a maximum value at the center of each of the well layers, so that the transition matrix element represented by the following formula (1) has a large value, resulting in large transition probability: 〈ψ c |P|ψ v 〉 (1) where ψ c and ψ v are wave functions of electrons and holes in the first quantum state, respectively, and P is momentum operator. In contrast, when a reversed bias voltage is applied across the p-n junction, the bond edges incline as shown in Figure 6b, and the ψ c and the ψ v are biased in the opposite directions to each other, so that special overlaps between these wave functions become small. Therefore, the matrix element of the formula (1) has a small value, resulting in a reduction of transition probability. At the some time, the quantum states Ec and Ev of electrons and holes shift to the lower energy side. The absorption spectra shown in Figure 7 reflect such an effect. The absorption spectrum ℓa shown in Figure 7(a) is obtained in the case where bonds are flat as shown in Figure 6a, and there is a sharp peak E H at the absorption edge, corresponding to the exciton transition of electrons and heavy holes. On the other hand, when a reversed bias voltage is being applied, the absorption peak E H shifts to the lower energy side and the height thereof decreases. The second absorption peak E L that appears in each spectrum corresponds to an exciton transition of electrons and light holes. The above-mentioned optical modulator with the multiple quantum well is irradiated with light of a wavelength corresponding to energy hν shown in Figure 7 through one of the circular windows, and the intensity of light emitted from the other circular window can be modulated by means of an applied voltage. In the situation of Figures 6a and 7(a), incident light is almost absorbed into the absorption peak E H at the band edge, so that the intensity of omitted light becomes small. On the other hand, in the situation of Figures 6b and 7(b), the absorption edge peak E H shifts to the lower energy side and the height thereof decreases, so that the absorbance with respect to the incident light of energy hν is remarkably decreased, resulting in an increase in the emitted light intensity. In such an optical modulator, the modulation index of emitted light is determined by the height of the absorption curve on the higher energy side of the peak E H shown in Figure 7. There is another absorption peak E L , which corresponds to an exciton transition of electrons and light holes, on the higher energy side of the peak E H so that when a voltage is applied to the optical modulator, incident light of energy hν is absorbed into the peak E L . There has been proposed a semiconductor device using the quantum effect of one dimension in which such an influence of the peak E L is reduced (T. Hayakawa et al., U.S. Patent Appln. Serial No. 159,797 corresponding to EP-A-0 281 310 published after the priority date of the present application). As an example of this semiconductor device, an optical modulator produced on a (111)-oriented GaAs substrate is disclosed therein, whereas a conventional optical modulator is produced on a (100)-oriented GaAs substrate. Figure 8 compares photoluminescene excitation spectra of multiple quantum wells above the (100)-oriented and the (111)-oriented substrates at 77K. As seen from this figure, when the (111)-oriented multiple quantum well is used, the energy separation between the peaks E H and E L becomes large and the height of the peak E H is greater than that of the peak E L . This is due to the anisotropy of the heavy-holes bond in the [100] and [111] directions. That is, the effective mass of heavy holes in the [111] direction is greater than In the [100] direction and the energy levels of heavy holes rise only slightly from the bottom of the quantum well, so that the peak E H shifts to the lower energy side, resulting in an increase in the energy separation between the peaks E H and E L . Moreover, this is because the effective mass of heavy holes in the (111) plane is greater than in the (100) plane, so that the state density of heavy holes within the quantum well becomes large, resulting in an increase in transition probability. The use of such an effect makes it possible to increase the height of the absorption curve on the higher energy side of the peak E H , so that the modulation amplitude of omitted light can be increased. As a typical example of other conventional optical semiconductor devices, there can be mentioned on optical bistable device utilizing exciton peaks such as a self-electrooptic effect device (SEED) proposed by Miller et al., which is described in detail in the following articles D.A.B. Miller, D.S. Chemla, T.C. Damen, T.H. Wood, C.A. Bvrrus, Tr, A.C. Gossard, and W. Wigmann, "The quantum well self-electrooptic effect device, optoelectronic bistability and oscillation, and self-linearized modulation," IEEE, J. Quantum Electron., vol.Qe-21, pp. 1462(1985). The operating principle of this optical switch will hereinafter be explained briefly. Figure 3 shows the optical switch in which the multiple quantum-well device shown in Figure 5 is connected in series with an external resistor R and a constant reversed bias voltage is applied between both sides of the multiple quantum-well device. When the multiple quantum-well device is irradiated through one of the circular windows with light of photoenergy near the band gap between the bend edges at the time when no voltage is applied, an absorption coefficient for incident light at the time when a voltage is applied becomes small because of the Stark effect of the quantum well as shown in Figures 6 and 7. Raising the intensity of the incident light increases a photocurrent that arises from the absorption of the incident light, so that while a voltage drop with respect to the external resistor R is increased, a voltage applied to the multiple quantum well is lowered. Therefore, the absorption spectrum of the multiple quantum well at the time when a voltage is applied approaches the absorption spectrum ℓa shown in Figure 7(a). When resonance occurs between the energy of incident light and the exciton transition energy of electrons and heavy holes, the absorbance of the multiple quantum well increases, and the amount of emitted light rapidly decreases. Even if the amount of the incident light is lowered under such a condition, optical output power is maintained at a low level because of a large photocurrent that arises from the exciton absorption, resulting in a hysteresis as shown in Figure 9. In this kind of optical switch, the ON/OFF ratio of bistable output power is determined by the depth of the absorption curve on the higher energy side of the peak E H shown in Figure 7. Therefore, a large ON/OFF ratio of the bistable output power can be obtained by use of a (111)-oriented quantum well. EP-A-0 311 445, which was published after the filing date of this application, discloses a semiconductor laser device which has a multiple quantum well active layer. The active layer consists of GaAs quantum well layers and Ga 1-x Al x As (x = 0. 5) barrier layers. The use of a [111] oriented substrate is not disclosed in EP-A-0 311 445. EP-A-0 281 310, which was published after the priority date of this application and which designates neither of the contracting states designated in this application, discloses an electric field effect light modulator grown on the [111]B plane of a substrate. The modulator has an multiple quantum well layer composed of GaAs quantum well layers and Ga 1-x Al x As (x = 0.7) barrier layers. SUMMARY OF THE INVENTION The optical semiconductor device of this invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises a quantum well structure as an active region and exhibits a nonlinear optical effect with regard to light of energy near the band gap between the allowed energy bands in the active region, wherein the quantum well structure is composed of alternate layers consisting of at least one first semiconductor layer with a thickness smaller than the de Broglie wavelength of electrons or holes confined in the quantum well and at least two second semiconductor layers with a band gap greater than that of the first semiconductor layer, the alternate layers being formed along a crystal orientation in the zinc-blende structure; said optical semiconductor device being characterised in that the second semiconductor layers are of an indirect transition type; and in that the crystal orientation in the [111] direction. In a preferred embodiment, the second semiconductor layers have the composition of Al x Ga 1-x As (where 0.45 < x ≦ 1). Thus, the invention described herein makes possible the objective of providing an optical semiconductor device with excellent characteristics that utilizes the effect of the exciton transition of electrons and heavy holes, such as in optical modulitor with a high modulation index and an optical switch with stable switching characteristics at low power, in which multiple quantum-well barrier layers formed along a crystal orientation in the zinc-blende structure are of an indirect transition type, so that the influence of absorption peaks for the exciton transition of electrons and light holes is reduced, resulting in excellent device characteristics. BRIEF DESCRIPTION OF THE DRAWINGS 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: Figure 1 is a sectional view of an absorption type electric field effect optical modulator of this invention, including a sectional view of the multiple quantum well of the self-electrooptic effect device of this invention. Figure 2 is a sectional view of a laser type electric field effect optical modulator of this invention. Figure 3 is a schematic view showing the constitution of a self-electrooptic effect device. Figure 4 is a schematic view showing the total reflection type optical switch of this invention. Figure 5 is a sectional view of the conventional absorption type electric field effect optical modulator. Figures 6a and 6b are schematic diagrams showing the band edges in the multiple quantum well of the optical modulator shown in Figure 5 when an electric field is applied to the optical modulator. Figure 7 shows absorption spectra for the multiple quantum well of the optical modulator shown in Figure 5 when an electric field is applied to the optical modulator. Figure 8 shows excitation spectra for the (111)-oriented and the (100)-oriented quantum wells. Figure 9 is of characteristic curves showing the optical bistability of the self-electrooptic affect device. Figure 10 is of a characteristic curve showing the relationship between the Al mole fraction (i.e., x) in Al x Ga 1-x As barrier layers in the (111)-oriented quantum well and the peak intensity ratio, in the excitation spectra of the (111)-oriented quantum well wherein the peak E H corresponds to the transition of electrons and heavy holes, and the peak E L corresponds to the transition of electrons end light holes. Figure 11 is a sectional view of the intersecting region of the total reflection type optical switch shown in Figure 4. DESCRIPTION OF THE PREFERRED EMBODIMENTS In any device utilizing the exciton transition oil electrons and heavy holes, the energy separation between the peaks E H and E L as shown in Figure 7 can be enlarged by use of a (111)-oriented quantum well. In order to reduce the influence of the peak E L further, it is desired to raise the intensity ratio between the peaks E H and E L . Figure 10 shows the relationship between the peak intensity ratio and the Al mole fraction in the barrier layers, which relationship is obtained from photoluminescene excitation spectra for the (111)-oriented multiple quantum wells with various Al mole fractions. As seen from this figure, when the Al mole fraction is 0.45 or more, the ratio of the intensity oil the peak E H to that of the peak E L is considerably increased. If the Al mole fraction in the barrier layers is selected in this region (i.e., 0.45 ≦ x ≦ 1), there can be obtained optical semiconductor devices with extremely excellent characteristics. Example 1 Figure 1 shows an absorption type electric field effect optical modulator of this invention, which is produced as follows: On the plane of an n-GaAs substrate 10 that is inclined from the (111) plane at an angle of 0.5 degrees, an n-Al 0.3 Ga 0.7 As layer 11, a multiple quantum-well layer (composed of alternate layers consisting of forty-nine undoped GaAs well layers 12 of a thickness of 100 Å each and fifty undoped AlAs barrier layers 13 of a thickness of 100 Å each), a p-Al 0.3 Ga 0.7 As layer 14, and a p-GaAs layer 15 are successively grown by molecular beam epitaxy or the like. Then, by photolithography and chemical etching techniques, the central portion of each of the n-GaAs substrate 10 and the p-GaAs layer 15 is removed into a circular shape with a diameter of 200 µm, and a p-sided electrode 16 and an n-sided electrode 17 are disposed on the upper face oil the p-GaAs layer 15 and the back face of the n-GaAs substrate 10, respectively. The resulting optical modulator in irradiated with monochromatic light, which in emitted from another laser light source and has a wavelength of 848 nm, through one of the circular windows, and the intensity of emitted light through the other circular window is modulated by the application of a reversed bias voltage to the multiple quantum well layer. The wavelength of the incident light corresponds to an energy gap between the band edges of the multiple quantum-well layer with no voltage. As a reference standard, an absorption type electric field affect optical modulator with a multiple quantum-well layer composed oil alternate layers consisting of forty-nine undoped GaAs well layers and fifty undoped Al 0.3 Ga 0.7 As barrier layers was produced on the (100) plane oil an n-GaAs substrate as shown in Figure 5, and another absorption type electric field effect optical modulator with the some structure as that of Figure 5 was produced on the (111) plane of an n-GaAs substrate. As a result, the modulation amplitude of the optical modulator of this example was four times as large as that of the optical modulator with the multiple quantum-well layer comprising the Al 0.3 Ga 0.7 As barrier layer above the (100)-oriented substrate and was two times as large as that of the optical modulator with the multiple quantum-well layer comprising the undoped Al 0.3 Ga 0.7 As barrier layer above the (111)-oriented substrate. Example 2 Figure 2 shows a laser type electric field effect optical modulator of this invention, which was produced as follows; On a (111)-oriented n-GaAs substrate 20, an n-Al 0.75 Ga 0.25 As layer 21, a multiple quantum-well layer (composed of alternate layers consisting of ten undoped Al 0.2 Ga 0.8 As well layers 22 oil a thickness of 100 Å each and nine undoped Al 0.5 Ga 0.5 As barrier layers 23 of a thickness of 40 Å each), a p-Al 0.75 Ga 0.25 As layer 24, and a p-GaAs layer 25 were successively grown by molecular beam epitaxy. Then, by a reactive ion-beam etching technique, the semiconductor growth layers on the n-GaAs substrate were selectively etched as shown in Figure 2, so that a laser oscillation pert and a modulation part were formed separately from each other. The laser oscillation part and the modulation part had a width of 30 µm and a length of 200 µm. Next, an n-sided electrode 27 and p-sided electrodes 28 were formed on the back face of the n-GaAs substrate and an the upper face of the p-GaAs layers 25 of the laser oscillation part and the modulation part, respectively. The laser light emitted from the laser oscillation part by current flowing in the forward direction is modulated, while being guided in the modulation part, by the application of a reversed bias voltage thereto, and is emitted from the modulation part. As a reference standard, a laser type electric field effect optical modulator with a different structure from that of Figure 2 was produced as follows; On a (111)-oriented n-GaAs substrate, an n-Al 0.55 Ga 0.45 As layer, a multiple quantum layer (composed of alternate layers consisting of ten undoped GaAs well layers of a thickness of 100 Å each and nine undoped Al 0.3 Ga 0.7 As barrier layers of a thickness of 40 Å each), a p-Al 0.55 Ga 0.45 As layer, and a p-GaAs layer were successively grown by molecular beam epitaxy. Thereafter, the optical modulator was finished in the same way as that of the above-mentioned example of Figure 2. The modulation amplitude of the optical modulator of this example was compared with that of the reference standard optical modulator, and it was found that the former was two times as large as the latter. This is because the Al mole fraction (i.e., x=0.5) in the multiple quantum-well barrier layer of the optical modulator of this invention is higher than that of the reference standard optical modulator, so that the exciton absorption effect by electrons and light holes is reduced. Example 3 The self-electrooptic effect device of this invention will hereinafter be explained with reference to Figure: 1 and 3. As shown in Figure 3, a pin device with the same multiple quantum-well structure as that of Example 1 shown in Figure 1 is connected to an external resistor R with a resistance of 1 MΩ, and is also connected to a power source with a constant voltage of 20 volts so that a reversed bias voltage is applied to the multiple quantum-well layer oil this pin device. When the device is irradiated with light having a wavelength of 855 nm through one of the circular windows, the light emitted from the other circular window shows bistable characteristics as shown in Figure 9. As a reference standard, a self-electrooptic effect device using the optical modulator with the multiple quantum-well comprising the undoped Al 0.3 Ga 0.7 As barrier layers as shown in Figure 5 was produced. The ON/OFF ratio oil output power in the bistable state of the self-electrooptic effect device of this example was two times as large as that of the reference standard device. This is because the Al mole fraction (i.e., x=0.5) in the multiple quantum-well barrier layer of the self-electrooptic effect device of this invention is higher than that of the reference standard device, resulting in a weak absorption at the peak E L of the spectra shown in Figure 7. Example 4 Figure 4 shows a total reflection type optical switch in which by changing the refractive index at an intersecting region x between two optical waveguides, incident light L₁ is transmitted or reflected in the desired direction L₂ or L₃. The basic principle of such an optical switch is described in detail in the following article: C. S. Tsai, B. Kim, F. R. El-Akkari, "Optical channel waveguide switch and coupler using total internal reflection," IEEE, J. Quantum Electron, vol. QE-14, pp. 513 (1978). As a method for changing the refractive index at an intersecting region x of such an optical switch, there is a well known method in which a multiple quantum-well layer is formed at the intersecting region and the refractive index of the layer is controlled by means of a voltage applied thereto. Such a method is described in detail in the following article; N. Nagai, Y. Kan, M. Yamanishi, and I. Suemune, "Electroreflectance spectra and field induced variation In refractive index of a GaAs/AlAs quantum well structure at room temperature," Jpn. J. Appl. Phys., vol. 125, pp. L640 (1986). Figure 11 shows the structure of the multiple quantum-well layer of the optical switch of this example, which is produced as follows: On the plane of an n-GaAs substrate 130 that is inclined from the (111) plane at an angle of 0.5 degree, an n-Al 0.6 Ga 0.4 As layer 131, a multiple quantum-well layer (composed of alternate layers consisting of fifty undoped GaAs well layers 132 of a thickness oil 100 Å each and forty-nine undoped Al 0.6 Ga 0.4 As barrier layers 133 oil a thickness of 200 Å each), a p-Al 0.6 Ga 0.4 As layer 134 and a p-GaAs layer 135 are successively grown by molecular beam epitaxy or the like. Then, an n-sided electrode 136 and a p-sided electrode 137 are formed on the back face of the n-GaAs substrate 130 and the upper face of the p-GaAs layer 135, respectively. When no voltage is applied between the electrodes 136 and 137, incident light L₁ is transmitted in the direction L₂ as shown in Figure 4. When a reversed bias voltage is applied across the multiple quantum-well layer, the refractive index at the intersecting region between two optical waveguides is decreased, so that the incident light L₁ is totally reflected in the direction L₃ as shown in Figure 4. As a reference standard, a total reflection type optical switch with undoped Al 0.4 Ga 0.6 As layers used as the multiple quantum-well barrier layers were produced. The optical switch of this example provided a higher reflective index at a lower voltage applied thereto then that of the reference standard optical switch. This is because the influence of peaks E L in the absorption spectrum shown in Figure 7 is reduced. Although the above-mentioned examples disclose only optical semiconductor devices in which Al x Ga 1-x As layers (where 0.45 < x ≦ 1, that is, these layers have the composition of an indirect transition type) are used as multiple quantum-well barrier layers, the composition of barrier layers is not limited thereto, but any barrier layers of an indirect transition type can be used in the multiple quantum-well layer, which is composed of III-V group semiconductor layers grown in the [111] direction, to attain the same quantum effect. For example, the multiple quantum-well layer can be composed of (Al x Ga 1-x ) 0.51 In 0.49 P layers (where 0 ≦ x ≦ 0.67) as well layers end (Al x' Ga 1-x' ) 0.51 In 0.49 P layers (where 0.67 ≦ x ≦ 1) as barrier layers, both of which are alternately grown on a (111)-oriented GaAs substrate. Moreover, when the thickness of each layer is small as in the multiple quantum-well layer, even if there is a mismatch between the lattice constants, semiconductor crystal layers with high quality can be obtained. Therefore, the multiple quantum-well layer composed of alternate layers consisting of (Al x Ga 1-x ) 0.47 In 0.53 As layers as well layers and (Al x' Ga 1-x' ) y' In 1-y' As layers of an indirect transition type (e.g., Al y' In 1-y' As where 0.68 ≦ y' ≦ 1) as barrier layers, the alternate layers being grown on a (111)-oriented InP substrate, and the multiple quantum-well layer composed of alternate layers consisting of Al x Ga 1-x Sb layers (where x ≦ 0.2) as well layers and Al x' Ga 1-x' Sb layers (where 0.2 ≦ x' ≦ 1) as barrier layers, the alternate layers being grown on a (111)-oriented GaSb substrate, can be used to attain the same quantum effect. It is 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: An optical semiconductor device comprising a quantum-well structure as an active region and exhibiting a nonlinear optical effect with regard to light of energy near the band gap between the allowed energy bands in said active region, wherein said quantum well structure is composed of alternate layers consisting of at least one first semiconductor layer (12) with a thickness smaller than the de Broglie wavelength of electrons or holes confined in the quantum well and at least two second semiconductor layers (13) with a band gap greater than that of said first semiconductor layer (12), said alternate layers being formed along a crystal orientation in the zinc-blende structure; said optical semiconductor device being characterised in that said second semiconductor layers (13) are of an indirect transition type; and in that the crystal orientation is the [111] direction. An optical semiconductor device according to claim 1, wherein said second semiconductor layers have the composition of Al x Ga 1-x As, where 0.45 ≦ x ≦ 1.

Inventors: HAYAKAWA TOSHIRO, KONDO MASAFUMI, SUYAMA TAKAHIRO, TAKAHASHI KOSEI, HAYAKAWA, TOSHIRO, KONDO, MASAFUMI, SUYAMA, TAKAHIRO, TAKAHASHI, KOSEI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G02F1
IPC codes: G02F3, G02F1