Semiconductor optical device

Abstract: An improvement for a semiconductor optical device having a region in which semiconductor or carriers in the semi­ conductor and light interact, for example, an active region for a semiconductor laser, an optical wave guide region of an optical modulator, etc. resides in a quantum well structure comprising a well region and a barrier region. A semiconductor optical device is remarkably improved in the degree of freedom of its design such as thickness and selection of material without deteriorating the quantum effect, by introducing a super lattice super-layer structure into the barrier region of the quantum well structure or defining the strain for the well region and the barrier region.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention concerns a semiconductor optical device and, more in particular, it relates to a semiconductor optical device suitable to the application for semiconductor laser devices and semiconductor light modulation devices used for optical fiber communication, optical information processing and optical measurement. Description of the Related Art A semiconductor optical devices having a quantum well represented by MQW (multiple quantum well) lasers have been noted as fundamental technique in the optical communication in the next generation, in which effects such as increase in the relaxation oscillation frequency, and reduction in the spectral linewidth reflecting the quantum size effect are expected. Values of the band-edge discontinuity energy in the quantum well region and the barrier region in conventional InGaAsP series QW (quantum well) lasers are greater on the side of the valence band than on the side of the conduction band. That is, in the QW structure using InGaAs(P) for the quantum well region and InP or InGaAsP for the barrier region, the well depth ΔEc on the side of the conduction band is smaller than the well depth ΔEv on the side of the valence band (that is ΔEc < ΔEv). Accordingly, the well depth sensed by electrons (effective well depth to the electrons) is shallower than the well depth sensed by holes (effective well depth to the holes). The ratio, i.e., the ratio of the value for the band-edge discontinuity energy, particularly, the value of the conduction band discontinuity: ΔEc/(ΔEc + ΔEv) is about from 0.3 to 0.4 in this system. As an example, it is described as 0.38 in Applied Physics Letters, Vol.51 (1987), pages 24 - 26. As in the prior art described above, in the Qw structure having a relationship: ΔEv > ΔEc, there is a problem that it is difficult to inject holes to each of the well regions. This will be explained referring to band diagrams shown in Fig. 3A and 3B. Both of Figs. 3A and 3B use InGaAs as the quantum well region, and InP is used in Fig. 3A and InGaAsP is used in Fig. 3B for the barrier region. Referring at first to Fig. 3A, the difference of the band gap between the quantum well region and the barrier region is large (about 610 meV) and ΔEc is about 232 meV and ΔEV is 378 meV. In this case, although electrons and holes are sufficiently quantitized, since the depth on the side of the holes with great effective, i.e., ΔEV is too great, the holes can be injected into the first well region but can not easily be injected into the second and the third well region, because the well depth on the side ΔEV of the valence band is too great and the holes can not override it. This leads to the remarkable increase of a threshold current. In view of the above, the barrier region has been made of InGaAsP for reducing ΔEV (refer to Fig. 3B). Thus, the holes can be injected to each of the wells as shown in the figure, whereas the well depth ΔEc on the side of the conduction band is also reduced. Then, the distribution of electrons with small effective mass exceeds the energy of the well region and electrons not confined in the well region are present as shown by the hatched line in Fig. 3B. Thus, the degree of electron quntitization is reduced and the quantum size effect of the quantum well can not fully be developed. In this case, although the threshold current is decreased the quantum size effect is reduced to bring about a technical problem that the purpose of introducing the quantum well structure is deteriorated. GB-A-2 131 610 discloses a semiconductor optical device (laser) having an InP/InGaAsP super-lattice structure with the features included in the first part of claim 1. The super-lattice structure is employed for a pair of light-guiding layers that sandwich the light-emitting region in order to suppress carrier leakage and reduce the threshold current. Semiconductor hetero-structures including a super-lattice structure with mini-bands are discussed in Siemens Forschungs- und Entwicklungsberichte, vol. 15 (1986) pp. 312-318. SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor optical device in which the threshold current is reduced while at the same time the quantum effect is retained. This object is accomplished by the device defined in claim 1. In accordance with one aspect of the present invention, there is provided a semiconductor optical device having a quantum well structure comprising a combination of a well region and a barrier region in which the band-edge discontinuity ΔEv of the valence band is greater than the band-edge discontinuity ΔEc of the conduction band. Each barrier region includes a super lattice thin-layer structure, comprising a plurality of layers, so that the lowest energy state of the mini-band formed to the barrier region with the super lattice-layer structure and thereby the effective band-edge discontinuity formed by the well region are greater on the side of the conduction band than on the side of the valence band. Such a semiconductor optical device has a means such as electrodes for injecting electrons and holes (carriers) to the quantum well structure. In the conventional InGaAs/InP or InGaAs/InGaAsP quantum well system, there is a trade-off relationship between the degree of the quantum size effect and the easiness for the injection of the holes to each of the wells. This is attributable to that the well depth ΔEv on the side of the holes with greater effective mass is greater than ΔEc. On the other hand, in GaALAs/GaAs series, ΔEc is greater than ΔEv (ΔEc/(ΔEc + ΔEv) = 0.7), in which there is no such trade-off relationship as described above. That is, for sufficiently utilizing the quantum size effect and facilitating the injection of the carriers (electrons, holes) to each of the well regions, it is ideal to make ΔEc on the side of electrons with less effective mass greater than ΔEv on the side of holes with larger effective mass. For intentionally creating a relationship: ΔEc > ΔEv in the material system originally having a relationship ΔEc > ΔEv, a super lattice thin-layer structure is introduced into the barrier region in the present invention, which is referred to as a super lattice-type quantum barrier. The super lattice thin-structure comprises a plurality kinds of (usually two kinds of) semiconductors of different band gaps laminated with each other. A typical form is a structure in which a plurality of well layers of small band gaps and a plurality of barrier layers of larger band gaps are alternately laminated periodically. The thickness for each of the layers, particularly, the thickness for the barrier layer is about less than 3 nm. Due to the thin layer thickness, the wave function of electrons and holes are bonded by means of tunnelling between each of semiconductor layers (well layers) of small band gaps. The structure is similar to the multiple quantum well structure but the feature of the super lattice thin-layer structure is that the thickness for each of the layers is thin and the tunnelling effect is formed. This will be explained referring to the first band diagram. As an example, the quantum well region is made of InGaAs and the super lattice-type quantum barrier constituting the barrier region as a feature of the present invention comprises a periodical structure of the InGaAs well layers and InP barrier layers. As the typical value for the size of them, the thickness of the InGaAs quantum well region in which the electrons and the holes are locallized is about from 5 to 15 nm so as to sufficiently develop the quantum-size effect. Further, the thickness of the InGaAs well layer forming the super-lattice type quantum barrier is about 0.6 to 3 nm and the thickness for the InP barrier layer is about from 0.6 to 3 nm. The feature of the present invention develops particularly remarkably within a range from about 2 to 10 of repeating periods. However, only if the thickness for the well layer and the barrier layer is greater than 0.5 nm, the effect of the present invention can be attained sufficiently. In addition, when impurity (preferably p type) is doped at a high concentration (1 x 10 18 cm -3 , preferably, greater than 2 x 10 19 cm -3 ) to the entire barrier layer, or the super lattice-type quantum barrier forming the barrier layer, it synergistically functions with the improvement for the characteristics due to the modified doping effect, which is particularly preferred. The present invention, may also be employed in a semiconductor optical device having: a semiconductor substrate, a laminated semiconductor structure laminated on the semiconductor substrate and having an active region for emitting light by injected carriers and a resonant cavity for feeding back and simplifying the light emitted from the active region, and means for injecting the carriers into the active region, wherein the active region has the same multiple quantum well structure comprising a plurality of well regions and barrier regions as defined in claim 1. The resonant cavity may be formed by utilizing the two end faces of the device but use of a constitution similar to a distributed feedback (DFB) laser or a distributed Bragg reflection (DBR) laser utilizing a so-called distributed feedback structure such as of a grating disposed in the laminated semiconductor structure is particularly preferred for the reduction of the spectral linewith. Still further advantages of the present invention will become apparent to those of the ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. BRIEF DESCRIPTION OF THE DRAWINGS The invention may take from in various parts and arrangements of parts. The drawings are only for purpose of illustrating the preferred embodiments and are not to be construded as limiting the invention. Fig. 1 is a view illustrating a band structure in principle for explaining a mini-band used in the present invention. Fig. 2 is a graph illustrating a relationship between the lowerest energy of the mini-band and a well layer. Figs. 3A and 3B are band diagrams for a conventional QW structure. Figs. 4 , 5A, 5B and 8 are, respectively, views illustrating the cross sectional structure of one embodiment according to the present invention. Figs. 6 and 7 are views for explaining the effect of the present invention using a mini-band. Fig. 9 is a perspective view for explaining the first embodiment according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Description will now be made at first to the mechanism for controlling ΔEc and ΔEv in accordance with the present invention using a super lattice-type quantum barrier referring to Figs. 1 and 2. As shown in Fig. 1, a mini-band is formed for electrons and holes in the super lattice super-layer structure. The mini-band is a quantum energy level in the super lattice thin-layer structure, i.e., an energy band formed in the super lattice thin-layer structure. The energy of the mini-band in the super lattice-type quantum barrier forms the depth of well to the well region, that is, ΔEc and ΔEv. The mini-band is formed by the coupling degree of the wave function between each of the well layers in the super lattice thin-layer structure and corresponds to the quantitized energy level in the super lattice thin-layer structure. Now, the quantitizing energy level ΔE can be represented approximately as: E ∝ 1 mL 2 in which m represents an effective mass, L represents a well layer thickness in the super lattice thin-layer structure. That is, the mini-band energy ΔE is in a reverse proportion to the effective mass. In InGaAs(P) series, the effective mass of electrons is about 1/10 of the effective mass of holes. Accordingly, the mini-band ΔEe for the electrons on the side of the conduction band, i.e., ΔEc is greater by about ten times of the mini-band ΔEh for the holes on the side of the valence band, that is, ΔEv. As described above, also in the system in which ΔEv is greater than ΔEc, a relationship ΔEc > ΔEv can be prepared by using the super lattice-type quantum barrier. This is attributable to that electrons have an effective mass lighter than that of holes and can be quantitized more easily. Explanations have been made qualitatively as above and Fig. 2 shows a result of an actual and exact calculation. In this calculation, a Kronig-Penny model used most generally in the calculation for the mini-band of the super lattice structure is used. In this case, InGaAs is used for the well and InP is used for the barrier in the super lattice structure, and the abscissa represents the thickness of the well and the parameter used is the thickness of the barrier. The point 0 on the abscissa represents so-called ΔEc and ΔEv of the material and it can be seen that ( ΔEc/( ΔEc + ΔEv) is 0.38. Referring to typical value for ΔEc and ΔEv of the super lattice-type quantum barrier, it can be seen that ΔEc is sufficiently larger as: ( ΔEc /( ΔEc + ΔEv) is 0.63 in the type I (well: 2 nm, barrier: 3 nm), the valve is 0.59 in the type II (both well and barrier: 1 nm) and 0.69 in the type III (well: 2.5 nm, barrier: 1 nm). Further, when ΔEc of these types are measured by a C-V method, values substantially equal to the results of the calculation were obtained. In this way, a relationship: ΔEc > ΔEv can be attained for the first time in InGaAs(P) series. This can keep the quantum size effect sufficiently and facilitate the injection of the holes. Fig. 6 shows the dependency of the relaxation oscillation frequency fr on the InGaAs quantum well width. In the figure, the ordinate represents a standerized value for fr of a DH (double hetelo) laser. In this way, fr higher than twice can be expected in an InGaAs(P) series QW laser in accordance with the present invention. The present invention will be described in details referring to preferred embodiments Example 1 In Fig. 4, a multiple quantum well active layer 44, a p-InP clad layer 45 and an n-InP cap layer 46 are formed by crystal growth on an n-InP substrate 43 by means of an MOCVD process. In this example, the multiple quantum active layer 44 has a periodical structure comprising InGaAs quantum well layers as the well region and a super lattice-type barrier constituting a barrier region each having a layer thickness of 5 to 15 nm (period:-1-20). The super lattice-type quantum barrier is constituted with well layers comprising InGaAs or InGaAsP (λ g > 1.5 µm) with a thickness of 0.6 to 3 nm and a periodical structure of barrier layers comprising InP or InGaAsP (λ g < 1.15 µm) with a thickness of 0.6 to 3 nm (period: structure 2-20). Subsequently, a SiO 2 layer 47 is formed, partially removed and then diffused with Zn to form an impurity (Zn) diffusion layer 48, thereby preparing a stripe region (2 - 10 µm width). Then, a p-electrode 49 and an n-electrode 40 as a carrier injection means are formed. A plurality of such laminate structures are formed on a single wafer as a substrate and cleaved on every devices to obtain a plurality of device chips. In the case of a semiconductor laser, the cleaved surfaces of the device constitute two reflection surfaces of a resonant cavity for feeding back and amplifying light emitted from the active layers. The reflection surface may sometime be coated with a reflection membrane, a reflection-preventive membrane and surface protection layer. The thus manufactured device has an extremely low threshold current of 10 to 20 mA due to the reduced ΔEv and has an extremely high relaxation oscillation frequency fr of about 30 GHz at 5 mW power due to the sufficient quantum size effect. Example 2 Figs. 5A and 5B show an example in which the present invention is applied to a DEB laser device. In the DEB laser, the resonant cavity for feeding back and amplifying light emitted in the active layer has a distributed feed back structure (grating). Fig. 5B is a cross sectional view of a device taken along line A - A′ in Fig. 5A. An n-InGaAsP having a thickness of 0.05 to 0.25 µm (λ g = 1.1 - 1.3 µm) optical guide layer 52, a multiple quantum active layer 54 like that in Example 1 and a p-InP clad layer 55 are formed by means of crystal growth on an n-InP substrate 58 formed with a refractive grating, by using a gas source MBE process. Subsequently, a ridged stripe is formed passing through each of the grown layers and embedded with, a p-InP layer 56 and an n-InP layer 57. Then, a p-electrode 59 and an n-electrode 50 are formed. The active layer width is about 0.5 to 3 µm. The thus manufactured device oscillates at a threshold current of 5 to 15 mA and shows a single longitudinal mode with a side mode suppression ratio of 50 dB. Further, fr at 5 mW is as high as about 30 GHz. Fig. 7 shows the experimental values collectively for the threshold current and fr (at 5 nm) in various kind of QW lasers. At first, in the case of an InP barrier region, although the Qw effect is large and fr is high (about 25 GHZ), since the injection of holes into each of the quantum wells is insufficient, the threshold current is as high as about 100 mA. On the other hand, in a QW laser using an InGaAsP barrier region with λ g = 1.3 µm, since ΔEv can be reduced, the holes can be injected easily into each of the quantum wells and the threshold current is low. However, ΔEc is low as well, electrons are distributed over the barrier height energy to reduce the QW effect and fr is as low as 10 GHz. Further, a QW laser having an InGaAsP barrier region with λ g = = 1.15 µm situates between them. That is, it has been impossible in the conventional system to simultaneously satisfy high fr and low threshold current. On the contrary, in the QW laser according to the present invention using the super lattice-type quantum barrier for the barrier region, high fr and low threshold voltage can be attained simultaneously for the first time. In addition, chirping under high speed modulation is as low as about 30% of the conventional QW laser. Example 3 In Fig. 8, the same structure as that in Examples 1 and 2 is formed on an n-InP substrate 83. In this example, Zn is doped at 2 x 10 18 - 1 x 12 19 cm -3 only to the super lattice-type quantum barrier, and a p-type modulation doped multiple quantum well active layer and a p-InP clad layer are formed by crystal growth by using an MOCVD process. Subsequently, a ridged stripe (0.5 - 3 µm as the active layer width) is formed and then is embedded with a semi-insulating Fe-doped InP layer and, further, p-electrode 89 and an n-electrode 80 are formed. The capacitance of the thus manufactured device due to the introduction of semi-insulating InP and the resonant frequency fr can be 50 GHz due to the modulation doping effect and the QW effect. Example 4 Description will now be made to an embodiment in which the present invention is applied to a semiconductor optical phase modulator referring to Fig. 9. A multiple quantum well active layer 94 and a p-InP clad layer 95 like that Example 1 and 2 are successively grown on an n-Inp substrate 93 and then a ridged stripe as shown in the figure is formed to a typical width of 2 to 10 µm by means of etching. Subsequently, after forming a p-electrode 99 and n-electrode 90, it is cleaved and isolated into individual devices. A laser beam at a wavelength of 1.3 µm is injected on one end face of the thus manufactured optical phase modulator and the phase of light emitted from the optical phase modulator is controlled by varying the amount of the current injected to the p-electrode 99, that is, the amount of carriers injected to the multiple quantum well active layer 94. Since the variation of the refractive index of the optical phase modulator of this embodiment is as great as 5 x 10 -2 due to high QW effect, the length of the modulator for changing the phase by π can be reduced about as short as to 30 µm, which can not be attained in the prior art. Further, it is of course possible to use the optical phase modulator in this example as a basic device such as for Mach-Zehnder type modulator. The invention has been described with reference to the preferred embodiments. Obviously, modifications and alternations will occur to those of ordinary skill in the art upon reading and understanding the present invention. It is intended that the invention be construded as including all such alterations and modifications in so far as they come with the scope of the appended claims or the equivalent thereof.

Claims: A semiconductor optical device made of materials in which the valence band-edge discontinuity ΔEv is larger than the conduction band-edge discontinuity ΔEc, with a multiple quantum well structure including a plurality of well regions for carrier recombination and light emission and barrier regions for isolating the well regions, characterised in that each well region is disposed between two barrier regions and each barrier region is constituted by a super-lattice thin-layer structure comprising a plurality of semiconductor layers, so that the lowest energy state of a mini-band formed in the super-lattice thin-layer structure is greater on the side of the conduction band than on the side of the valence band. The device of claim 1, wherein the effective band edge discontinuity between said barrier regions formed with said mini-bands and said well regions is smaller on the side of the valence band than on the side of the conduction band. The device of claim 1 or 2, wherein the layers constituting said super-lattice thin-layer structure each have a thickness of 0.5 to 3 nm. The device of any preceding claim, wherein said barrier region includes a region doped with a conduction-type impurity at a density greater than 2 × 10 18 cm -3 and lower than 1 × 10 19 cm -3 .

Inventors: CHINONE NAOKI, OHTOSHI TSUKURU, SASAKI SHINJI, TSUCHIYA TOMONOBU, UOMI KAZUHISA, CHINONE, NAOKI, OHTOSHI, TSUKURU, SASAKI, SHINJI, TSUCHIYA, TOMONOBU, UOMI, KAZUHISA
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1990
Main classification: H01L29
IPC codes: H01S5, H01L33, G02F1, H01L31