Light emitting device

Abstract: A semiconductor laser capable of emitting blue or green light is disclosed. The semiconductor laser comprises an n-type ZnMgSSe cladding layer (2), an active layer (3), a p-type ZnMgSSe cladding layer (4), a p-type ZnSe contact layer (5) and a p-type ZnTe contact layer (6) which are stacked in this sequence on an n-type GaAs substrate (1). A p-side electrode (7) is provided on the p-type ZnTe contact layer (6). An n-side electrode (8) is provided on the back surface of the n-type GaAs substrate (1). A maltiquantum well layer (9) comprising quantum wells made of p-type ZnTe and barriers made of p-type ZnSe is provided in the depletion layer produced in the p-type ZnSe contact layer (5) along the junction interface between the p-type ZnSe contact layer (5) and the p-type ZnTe contact layer (6). Holes injected from the p-side electrode (7) pass through the junction by the resonant tunneling effect through quantum levels formed in the quantum wells of the multiquantum well layer (9).

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a semiconductor device and, in particular, to a semiconductor device such as a light emitting device or the like using II-VI compound semiconductors. Description of the Prior Art Recently, in order to improve the recording density of optical discs and the resolution of laser printers, semiconductor lasers capable of emitting light having short wavelength are increasingly desired, and there are active researches to realize them. The present Applicant made vigorous researches to meet the demand and proposed a semiconductor laser capable of emitting blue or green light which uses a sort of II-VI compound semiconductors, namely, ZnMgSSe compound semiconductor, as the material for its cladding layers. In this semiconductor laser, a laser structure including an n-type ZnMgSSe cladding layer, an active layer and a p-type ZnMgSSe cladding layer (not shown) is provided on an n-type GaAs substrate, and a p-type ZnSe contact layer is provided on the p-type ZnMgSSe cladding layer. On the p-type ZnSe contact layer is provided a p-side electrode, and on the back surface of the n-type GaAs substrate is provided an n-side electrode. With this structure, the p-side electrode has a high contact resistance with respect to the p-type ZnSe contact layer, and it is difficult to obtain a good ohmic contact. Major reasons thereof are that the carrier concentration obtained by doping p-type impurities into ZnSe is as low as the order of∼10 17 cm -3 at maximum and that it is difficult to obtain a good ohmic contact with p-type ZnSe. In addition, there is a band discontinuity in valence band as large as 0.5eV at the junction interface between p-type ZnSe (energy gap: 2.8eV) and p-type ZnTe (energy gap: 2.3eV). The valence band of p-type ZnSe is curved downward toward p-type ZnTe (see Fig. 4), and the changes in valence band bulging downward behave as a potential barrier against holes injected from the p-side electrode to the p-type ZnSe/p-type ZnTe junction. OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a semiconductor device that can effectively remove a potential barrier caused by a large band discontinuity of valence bands or conduction bands at a semiconductor junction interface and therefore has a good voltage-current characteristic. A further object of the invention is to provide a semiconductor device in which a multiquantum well layer is provided in a depletion layer produced along a semiconductor junction interface and a potential barrier caused by a band discontinuity at the interface can be effectively removed by a resonant tunneling effect. A still further object of the invention is to provide a semiconductor device in which, in the presence of a large band discontinuity in valence bands or conduction bands at a semiconductor junction interface, the transmission probability of carriers through the junction when the bias voltage applied to the junction is near zero can be increased, to thereby make it easy for a current to flow through the junction. According to an aspect of the. invention, there is provided a semiconductor device, comprising the features of appended claim 1. According to another aspect of the invention, there is provided a semiconductor device, comprising the features of appended Claim 4. According to still another aspect of the invention, there is provided a semiconductor laser, comprising the features of appended claim 8. The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross sectional view of a semiconductor laser taken as a first embodiment of the invention; Fig. 2 is an energy band diagram of a portion extending from a p-type ZnSe contact layer to a p-type ZnTe contact layer in the semiconductor laser taken as the first embodiment; Fig. 3 is a graph illustrating changes in first quantum level E 1 of quantum wells made of p-type ZnTe with width L W thereof; Fig. 4 is an energy band diagram of the p-type Znse/p-type ZnTe junction; Fig. 5 is a graph illustrating the relation between the effective carrier concentration in ZnSe and the N doping concentration into ZnSe; Fig. 6 is a graph illustrating a profile of the N doping concentration [N] of the portion extending from a p-type ZnSe contact layer to a p-type ZnTe contact layer in a semiconductor laser taken as a second embodiment of the invention; Fig. 7 is an energy band diagram of the portion extending from the p-type ZnSe contact layer to the p-type ZnTe contact layer in the semiconductor laser taken as the second embodiment; Fig. 8 is an energy band diagram of the portion extending from a p-type ZnSe contact layer to a p-type ZnTe contact layer in a semiconductor laser taken as a third embodiment of the invention; Fig. 9 is a graph illustrating a result of calculation showing how the transmission probability of holes through the p-type ZnSe/p-type ZnTe depends on energy, in the semiconductor laser taken as the third embodiment; Fig. 10 is an energy band diagram of the portion extending from a p-type ZnSe contact layer to a p-type ZnTe contact layer in a semiconductor laser taken as a fourth embodiment of the invention; and Fig. 11 is a graph showing a result of calculation showing how the transmission probability of holes through the p-type ZnSe/p-type ZnTe depends on energy, in the semiconductor laser taken as the fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the invention are explained below with reference to the drawings. Now refer to Fig. 1 illustrating a semiconductor laser taken as a first embodiment of the invention. An n-type GaAs substrate 1 has a {100}-oriented surface and is doped with Si which is an n-type impurity. On the n-type GaAs substrate 1 are stacked, in sequence, an n-type ZnMgSSe cladding layer 2 doped with Cl as an n-type impurity, an active layer 3 made of, for example, ZnCdSe or ZnSe, a p-type ZnMgSSe cladding layer 4 doped with N as a p-type impurity, a p-type ZnSe contact layer 5 doped with N as a p-type impurity, and a p-type ZnTe contact layer 6 doped with N as a p-type impurity. On the p-type ZnTe contact layer 6 is provided an p-side electrode 7 made of Au or Au/Pd. On the back surface of the n-type GaAs substrate 1 is provided an n-side electrode 8. Reference numeral 9 refers to a p-type ZnSe/ZnTe multiquantum well (MQW) layer provided in a depletion layer produced in the p-type ZnSe contact layer 5 along the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6. In other words, the p-type ZnSe/ZnTe multiquantum well (MQW) layer 9 is made by interposing quantum wells of p-type ZnTe in the depletion layer produced in the p-type ZnSe contact layer 5 along the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6. As shown in Fig. 2, although barriers made of p-type ZnSe have the same thickness L B , quantum wells made of p-type ZnTe have different thicknesses L W which increase in steps toward the p-type ZnTe contact layer 6. Specifically, the thickness L W of each quantum well made of p-type ZnTe is determined as explained below. First, Fig. 3 is a diagram showing a result of quantum mechanical calculation of a finite barrier well-type potential to know how a first quantum level E 1 of a quantum well made of p-type ZnTe changes with the thickness L W of the quantum well in a single quantum well structure arranged by sandwiching the quantum well layer of p-type ZnTe by barriers made of p-type ZnSe from opposite sides thereof. In the calculation, by taking the effective mass m h of a hole in p-type ZnSe and p-type ZnTe as the mass of an electron in the quantum well and the barriers, 0.6m 0 (m 0 is the rest mass of the electron) is used, and the depth of the well is 0.5eV. It is known from Fig. 3 that the first quantum level E 1 formed in the quantum well made of p-type ZnTe can be lowered by decreasing the thickness L w of the quantum well. This is used in the first embodiment to vary the thickness L w of the quantum well layer made of p-type ZnTe. In addition, the band bending produced in the side of p-type ZnSe from the interface between p-type ZnSe and p-type ZnTe over the width W is given by a quadratic function of the distance x from the interface between p-type ZnSe and p-type ZnTe (see Fig. 4) as follows: (x) =  T {1 - (x/W) 2 } Therefore, the p-type ZnSe/p-type ZnTe multiquantum well layer 9 can be designed on the basis of equation (1), by varying L w in steps so that the first quantum levels E 1 formed in the respective quantum well layers of p-type ZnTe coincide with top energies of the valence bands of p-type ZnSe and p-type ZnTe and so that they are equal with each other. Practically, coincidence within the range of thermal energy ∼kT (k: Boltzmann's constant, T: absolute temperature) is acceptable. Fig. 2 shows an example of the design for quantum well widths L w in the p-type ZnSe/p-type ZnTe multiquantum well layer 9 in the case where the width L B of each p-type ZnSe barrier is 2nm. Note here that the acceptor concentration N A of the p-type ZnSe contact layer 5 is 5 x 10 17 cm -3 and the acceptor concentration N A of the p-type ZnTe contact layer 6 is 5 x 10 19 cm -3 . In this case, as shown in Fig. 2, the widths L w of seven quantum wells, in total, are made to vary as L w = 0.3nm, 0.4nm, 0.5nm, 0.6nm, 0.8nm, 1.1nm and 1.7nm toward the p-type ZnTe contact layer 6 so that their quantum levels E 1 coincide with the Fermi levels of the p-type ZnSe and the p-type ZnTe. When designing the widths L w of the quantum wells, since levels of respective quantum wells are coupled with each other, their interactions must be taken into consideration in a strict sense. Further, effects of strain caused by lattice mismatching between the quantum wells and the barriers must also be taken into consideration. However, it is basically possible to design the multiquantum well layer to exhibit a flat quantum level as shown in Fig. 2. In Fig. 2, since holes injected in the p-type ZnTe can flow into the p-type ZnSe due to the tunneling effect through quantum levels E 1 formed in the respective quantum wells in the p-type ZnSe/p-type ZnTe multiquantum well layer 9, the potential barrier along the p-type ZnSe/p-type ZnTe interface is effectively removed. Therefore, a good voltage-current characteristic can be realized with the semiconductor laser shown in Fig. 1. Since the current crossing the p-type ZnSe/p-type ZnTe multiquantum well layer 9 is caused by the tunneling effect, a slight resistance exists. However, the use of the p-type ZnSe/p-type ZnTe multiquantum well layer 9 is greatly effective for reducing, in particular, the forward rising voltage of a diode. For manufacturing the semiconductor laser according to the first embodiment, as shown in Fig. 1, the n-type ZnMgSSe cladding layer 2, the active layer 3, the p-type ZnMgSSe cladding layer 4, the p-type ZnSe contact layer 5, the p-type ZnSe/p-type ZnTe multiquantum well layer 9 and the p-type ZnTe contact layer 6 are epitaxially grown in this sequence on the n-type GaAs substrate 1 by the molecular beam epitaxy (MBE) method. Growth of the p-type ZnSe/p-type ZnTe multiquantum well layer 9 is readily realized only by opening and closing a molecular beam shutter of the MBE apparatus. In the epitaxial growth by MBE, there are used Zn of purity 99.9999% as a Zn source material, Mg of purity 99.9999% as a Mg source material, ZnS of purity 99.9999% as a S source material, and Se of purity 99.9999% as a Se source material. Further, doping of Cl as an n-type impurity for the n-type ZnMgSSe cladding layer 2 is done by using ZnCl 2 of purity 99.9999% as a dopant, while doping of N as a p-type impurity for the p-type ZnMgSSe cladding layer 4, the p-type ZnSe contact layer 5, the p-type ZnSe/p-type ZnTe multiquantum well layer 9 and the p-type ZnTe contact layer 6 is done by irradiating N 2 plasma generated by a plasma gun using electron cyclotron resonance (ECR). Regarding the doping of the p-type impurity into ZnSe, a result of measurement by the secondary ion mass spectrometry (SIMS) shows that a p-type impurity itself can be doped up to the order of 10 18 ∼10 19 cm -3 . However, because the impurity level determined by the doping concentration of the p-type impurity is deepened, only a part of the doped impurity is activated to behave as an acceptor for supplying effective carriers. As a result, the carrier concentration obtained exhibits a low value mentioned above. Fig. 5 shows this aspect in which, regardless of an increase in the doping concentration [N] of N used as a p-type impurity, the effective carrier concentration, that is, N A -N D (N A : acceptor concentration, N D : donor concentration) is saturated with about 4 x 10 17 cm -3 . A second embodiment of the invention will now be explained. Fig. 6 shows a profile of the N doping concentration [N] in the portion extending from the p-type ZnSe contact layer 5 to the p-type ZnTe contact layer 6 in Fig. 1. In Fig. 6, the N doping concentration [N] in the p-type ZnSe contact layer 5 is uniform up to the portion distant from the junction interface between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 by a predetermined distance (substantially the same as the width of the depletion layer produced in the p-type ZnSe contact layer 5 along the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6). [N] in this portion is about 4 x 10 17 cm -3 at which the effective carrier concentration saturates (see Fig. 5). On the other hand, in the portion of the p-type ZnSe contact layer 5 within a predetermined distance from the junction interface between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6, [N] continuously increases toward the junction interface. Then [N] increases stepwise in the junction interface to exhibit a sufficiently high value, for example, 2 x 10 18 cm -3 , as compared with [N] in the p-type ZnTe contact layer 6 (see Fig. 6). The carrier concentration in the p-type ZnTe contact layer 6 does not saturate at the [N] value of this order. The [N] value in the portion of the p-type ZnSe contact layer 5 within a predetermined distance from the junction interface between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 is beyond the value at which the carrier concentration saturates, and the N impurity level is deep. However, holes trapped in the deep levels can be made to fall in the p-type ZnTe contact layer 6. Therefore, a modulation of N as shown in Fig. 6 can decrease the width of the depletion layer in the p-type ZnSe contact layer 5 and can increase [N] in the p-type ZnTe contact layer 6. As a result, an increased amount of current can flow in the semiconductor laser. As described above, the carrier concentration in the p-type ZnSe contact layer 5 can be on the order of 5 x 10 17 cm -3 , and the carrier concentration in the p-type ZnTe contact layer 6 can be on the order of 10 19 cm -3 . On the other hand, a band discontinuity of about 0.5eV exists in valance bands in the junction interface between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 (see Fig. 4). If the junction is a step junction, a band bending is produced in the valence bands in a portion of the p-type ZnSe contact layer 5 along the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 over the width: W = (2ε T /qN A )1/2 where q is the absolute value of the charge of an electron, ε is the permittivity of ZnSe,  T is the magnitude of the discontinuity in the valence bands at the junction interface between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 (about 0.5eV). When the acceptor concentration N A of a portion of the p-type ZnSe contact layer 5 excluding the highly doped portion near the junction interface is 5 x 10 17 cm -3 and the acceptor concentration N A of the highly doped portion thereof is 1 x 10 18 cm -3 in average, calculation of W using equation (2) results in W = 23nm. On the other hand, W is 32nm when no highly doped portion is provided in the portion of the p-type ZnSe contact layer 5 near the junction interface. That is, because of the use of the highly doped portion in the portion of the p-type ZnSe contact layer 5 near the junction interface, the width W of the depletion layer is smaller by about 9nm than that in a structure with no highly doped portion. Because of the decreased width W of the depletion layer, holes can more easily pass through the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 due to a tunneling effect. Fig. 7 shows an example of the design for widths L w of quantum wells in the p-type ZnSe/p-type ZnTe multiquantum well layer 9 in the case where the width L B of the p-type ZnSe barrier layer thereof is 2nm. Note here that the acceptor concentration N A in the p-type ZnSe contact layer 5 is 5 x 10 17 cm -3 , that the doping concentration [N] gradually increases toward the p-type ZnTe contact layer 6 and exhibits 1 x 10 18 cm -3 in average in the p-type ZnSe/p-type ZnTe multiquantum well layer 9, and that the acceptor concentration N A in the p-type ZnTe contact layer 6 is 1 x 10 19 cm -3 . In this case, as shown in Fig. 7, the widths L w of five quantum wells, in total, are made to vary as L w = 0.3nm, 0.4nm, 0.6nm, 0.9nm and 1.6nm toward the p-type ZnTe contact layer 6 so that their first quantum levels E 1 coincide with the Fermi levels of p-type ZnSe and p-type ZnTe. As mentioned above, because of the use of the p-type ZnSe/p-type ZnTe multiquantum well layer 9 in the depletion layer produced in the p-type ZnSe contact layer 5 along the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6, the potential barrier between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 is effectively removed by a resonant tunneling effect through first quantum levels E 1 of the respective quantum wells in the p-type ZnSe/p-type ZnTe multiquantum well layer 9, and holes can readily move through the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6. In order to manufacture the semiconductor laser according to the second embodiment, the foregoing method for manufacturing the semiconductor laser according to the first embodiment is used, and the power supplied to the plasma gun is varied in the form along the profile of [N] as shown by a broken line in Fig. 6, in order to obtain the doping profile shown in Fig. 6, when doping of N is done for the p-type ZnSe contact layer 5, the p-type ZnSe/p-type ZnTe multiquantum well layer 9 and the p-type ZnTe contact layer 6. The doping of the p-type impurity using the plasma gun permits relatively easy control of the doping amount and the deep level near the junction interface between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6. In the semiconductor laser according to the second embodiment, since the portion of the p-type ZnSe contact layer 5 near the junction interface between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 has a higher N doping concentration [N] than the other portion, the width of the depletion layer produced in the p-type ZnSe contact layer 5 in the junction can be decreased, and holes can easily pass through the junction. In addition, since the p-type ZnSe/p-type ZnTe multiquantum well layer 9 is provided in the junction, the potential barrier is effectively removed from the junction. A third embodiment of the invention will now be described. A semiconductor laser according to the third embodiment has the structure shown in Fig. 1. Fig. 8 shows an energy band diagram of a portion of the p-type ZnSe/p-type p-type ZnTe multiquantum well layer 9 extending from the ZnSe contact layer 5 to the p-type ZnTe contact layer 6. Fig. 8 also shows the number of atomic layers forming each of quantum wells and barriers constituting the p-type ZnSe/p-type ZnTe multiquantum well layer 9. As shown in Fig. 8, respective quantum wells vary in thickness L w toward the p-type ZnTe contact layer 6 to read: one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), two atomic layers (about 0.6nm), two atomic layers (about 0.6nm), three atomic layers (about 0.9nm), four atomic layers (about 1.2nm), four atomic layers (about 1.2nm), six atomic layers (about 1.8nm), and six atomic layers (about 1.8nm). In contrast, L B varies toward the p-type ZnTe contact layer 6 to read: five atomic layer (about 1.5nm), four atomic layer (about 1.2nm), three atomic layer (about 0.9nm), three atomic layer (about 0.9nm), three atomic layers (about 0.9nm), three atomic layers (about 0.9nm), three atomic layers (about 0.9nm), two atomic layers (about 0.6nm), two atomic layers (about 0.6nm) and one atomic layer (about 0.3nm). Quantum levels of respective quantum wells are shown by horizontal solid lines in the quantum wells in Fig. 8. Note that, in Fig. 8, the depletion layer produced in the p-type ZnSe contact layer 5 along the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 has a width (see W in Fig. 4) of about 30nm. In correspondence therewith, the acceptor concentration in the p-type ZnSe contact layer 5 is about 5.7 x 10 17 cm -3 , and the acceptor concentration in the p-type ZnTe contact layer 6 is 1 x 10 19 cm -3 . The p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 are assumed to form a step junction. Fig. 9 shows a result of calculation to show how the transmission probability of holes through the p-type ZnSe/p-type ZnTe junction depends on energy in the structure in which the p-type ZnSe/p-type ZnTe multiquantum well layer 9 is provided in the depletion layer produced in the p-type ZnSe contact layer 5 (see Fig. 1) in the junction shown in Fig. 8. The abscissa of Fig. 9 shows the energy E of holes normalized by the discontinuity ΔE v in valence bands at the junction interface between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6, that is, E/ΔE v , where the energy E is measured from the origin placed at the top of the valence band of p-type ZnTe. As shown in Fig. 9, near the portion where E/ΔE v is zero, that is, where the bias voltage applied to the junction is zero, transmission probability of holes through the junction is very small. This is because, with respect to a portion (A in Fig. 8) where the curvature of the valence band in the depletion layer produced in the p-type ZnSe contact layer 5 along the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 is small, the third embodiment makes it very difficult to determine quantum levels that permit transmission of holes by a resonant tunneling effect between quantum levels of quantum wells remoter from the p-type ZnTe contact layer 6 and those of quantum wells nearer to the p-type ZnTe contact layer 6. A fourth embodiment of the invention will now be explained. A semiconductor laser according to the fourth embodiment of the invention has the structure shown in Fig. 1. Fig. 10 shows an energy band diagram of a portion of the p-type ZnSe/p-type ZnTe multiquantum well layer 9 extending from the p-type ZnSe contact layer 5 to the p-type ZnTe contact layer 6. In Fig. 10, the p-type ZnSe/p-type ZnTe multiquantum well layer 9 has twenty-one quantum wells. Fig. 10 shows the thickness of each quantum well or barrier in terms of the number of atomic layers forming thereof. As shown in Fig. 10, thicknesses L w of respective quantum wells vary toward the p-type ZnTe contact layer 6 to read: one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3 nm), two atomic layers (about 0.6nm), three atomic layers (about 0.9nm), four atomic layer (about 1.2nm), five atomic layers (about 1.5nm), and seven atomic layers (about 2.1nm). Thicknesses L B of barriers vary toward the p-type contact layer 6 to read: four atomic layers (about 1.2nm), four atomic layers (about 1.2nm), four atomic layers (about 1.2nm), four atomic layers (about 1.2nm), three atomic layers (about 0.9nm), three atomic layers (about 0.9nm), three atomic layers (about 0.9nm), three atomic layers (about 0.9nm), two atomic layers (about 0.6nm), two atomic layers (about 0.6nm), two atomic layers (about 0.6nm), two atomic layers (0.6nm), one atomic layer (about 0.3nm), one atomic layer (about 0.3nm), one atomic layer (0.3nm), three atomic layers (about 0.9nm), four atomic layers (about 1.2nm), four atomic layers (about 1.2nm), two atomic layers and two atomic layers (about 0.6nm). In this case, in the portion of the p-type ZnSe/p-type ZnTe multiquantum well layer 9 nearer to the p-type ZnTe contact layer 6 (see B in Fig. 10), where the sum of the thickness L W of each quantum well and the thickness L B of an adjacent barrier L w + L B is relatively large, quantum levels of respective quantum wells are discretized, and these quantum levels substantially coincide with the tops of valence bands of the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6. Quantum levels of respective quantum wells in portion B are shown by horizontal solid lines in Fig. 10. On the other hand, in the portion of the p-type ZnSe/p-type ZnTe multiquantum well layer 9 remoter from the p-type ZnTe contact layer 6 (see C in Fig. 10), where the sum of the thickness L W of each quantum well and the thickness L B of an adjacent barrier L W + L B is small enough to reinforce the coupling between adjacent quantum wells, sub-bands SB 1 , SB 2 , SB 3 , and SB 4 are produced. Widths of these sub-bands SB 1 , SB 2 , SB 3 , and SB 4 increase gradually from the sub-band SB 1 produced in the portion where L W + L B is largest to the sub-band SB 4 produced in the portion where L W + L B is smallest. In addition, these sub-bands SB 1 , SB 2 , SB 3 , and SB 4 include the top energies of valence bands of the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6. Note, in Fig. 10, that the width of the depletion layer (see W in Fig. 4) produced in the p-type ZnSe contact layer 5 along the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 is about 30nm. In correspondence therewith, the acceptor concentration in the p-type ZnSe contact layer 5 is about 5.7 x 10 17 cm -3 , and the acceptor concentration in the p-type ZnTe contact layer 6 is about 1 x 10 19 cm -3 . The p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 are assumed to form a step junction. Fig. 11 shows a result of calculation to show how the transmission probability of holes through the p-type ZnSe/p-type ZnTe junction depends on energy in the structure including the p-type ZnSe/p-type ZnTe multiquantum well layer 9 in the depletion layer produced in the p-type ZnSe contact layer 5 in the junction shown in Fig. 10. The abscissa of Fig. 11 shows the energy E of holes normalized by the discontinuity ΔE v in valence bands at the junction interface between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6, that is, E/ΔE v , where the energy E is measured from the origin placed at the top of the valence band of p-type ZnTe contact layer 6. As shown in Fig. 11, when E/ΔE v is near zero, that is, when the bias voltage applied to the junction is near zero, transmission probability of holes through the junction is significantly large as compared with that in case of Fig. 9, and the energy width providing such a high transmission probability is also significantly large. This results from the following reason. In the portion where the curvature of the valence band in the depletion layer produced in the p-type ZnSe along the junction between the p-type ZnSe and the p-type ZnTe is small, that is, in the portion shown at C in Fig. 10, there are made sub-bands SB 1 , SB 2 , SB 3 and SB 4 having widths in which the top energies of valence bands of the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 are involved. Therefore, due to a resonant tunneling effect through the sub-bands SB 1 , SB 2 , SB 3 and SB 4 without discrete quantum levels formed in the respective quantum wells in the portion nearer to the p-type ZnTe contact layer 6 than the portion shown at C, that is, in the portion shown at B, holes can readily transmit through the junction. According to the semiconductor laser taken as the fourth embodiment, the p-type ZnSe/p-type ZnTe multiquantum well layer 9 is formed in the depletion layer produced in the p-type ZnSe contact layer 5 along the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6, discrete quantum levels substantially equal to the top energies of valence bands of the p-type ZnSe and the p-type ZnTe are provided in the respective quantum wells in the portion of the p-type ZnSe/p-type ZnTe multiquantum well layer 9 remoter from the p-type ZnTe contact layer 6, and the sub-bands SB 1 , SB 2 , SB 3 and SB 4 having widths including the top energies of valence bands of the p-type ZnSe and the p-type ZnTe are formed in the quantum wells in the portion of the p-type ZnSe/p-type ZnTe multiquantum well layer 9 nearer to the p-type ZnTe contact layer 6. As a result, transmission probability of holes through the junction between the p-type ZnSe contact layer 5 and the p-type ZnTe contact layer 6 can be increased when the bias voltage applied to the junction is near zero. As a result, an increased amount of current can flow in the junction. In addition, since this alleviates requirements for operating the semiconductor laser, it is possible to stabilize the operation, to elongate the life time and to improve the reliability of the semiconductor laser. As described above, the invention realizes a semiconductor laser having a good current-voltage characteristic and capable of emitting blue or green light. Having described specific preferred embodiments of the present invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of the invention as defined in the appended claims. For example, although the embodiments have been described as being applied to a semiconductor laser, the invention can be applied to a light emitting diode. In a more general aspect, the invention can be applied to various semiconductor devices having a p-p junction or an n-n junction including a band discontinuity in valence band or in conduction band along its junction interface. The invention can also be applied to semiconductor devices including a junction between a metal and a semiconductor having a large difference in work function.

Claims: A semiconductor device comprising: a first semiconductor layer (5) made of a first p-type II-VI compound semiconductor material; a second semiconductor layer (6) made of a second p-type II-VI compound semiconductor material; whereby the two II-VI materials of the first and second layer are such that a depletion layer with width W would be formed in the first layer if both layers would be brought in direct contact and in which depletion layer the top energy of valence band of the first layer is bent so that a barrier against holes injected from the second layer (6) is generated; characterised by a multiquantum well layer (9) arranged between the first semiconductor layer (5) and the second semiconductor layer (6) which multiquantum well layer (9) has a thickness which is less than said width W and comprises barriers made of said first semiconductor material and quantum wells made of said second semiconductor material, whereby the barriers and the quantumwells of said multiquantum well layer have such thicknesses that the energy of the first quantum level in each quantum well coincides within its thermal band width with the top energy of valence band of the first semiconductor layer (5) remote from the multiquantum well layer and the top energy of valence band of the second semiconductor layer (6); and holes can tunnel through said barriers from said second semiconductor layer via said multiquantum well layer (9) to said first semiconductor layer (5). The semiconductor device according to claim 1, wherein respective said quantum wells and barriers of said multiquantum well layer have thicknesses to exhibit discrete quantum levels substantially equal within their thermal bandwidth to the top energies of valence bands of said first p-type II-VI compound semiconductor and said second p-type II-VI compound semiconductor in a portion of said multiquantum well layer nearer to said second p-type II-VI compound semiconductor and to produce sub-bands having widths involving the top energies of valence bands of said first p-type II-VI compound semiconductor and said second p-type II-VI compound semiconductor in a portion of said multiquantum well layer remoter from said second p-type II-VI compound semiconductor. A semiconductor device according to one of the preceding claims, wherein said first semiconductor layer (5) is made of ZnSe and said second semiconductor layer (6) is made of ZnTe. A semiconductor device comprising: a first semiconductor layer (5) made of a first n-type II-VI compound semiconductor material; a second semiconductor layer (6) made of a second n-type II-VI compound semiconductor material; whereby the two II-VI materials of the first and second layer are such that a depletion layer with width W would be formed in the first layer if both layers would be brought in direct contact and in which depletion layer the bottom energy of conduction band of the first layer is bent so that a barrier against electrons injected from the second layer (6) is generated; characterised by a multiquantum well layer (9) arranged between the first semiconductor layer (5) and the second semiconductor layer (6) which multiquantum well layer (9) has a thickness which is less than said width W and comprises barriers made of said first semiconductor material and quantum wells made of said second semiconductor material, whereby the barriers and the quantumwells of said multiquantum well layer have such thicknesses that the energy of the first quantum level in each quantum well coincides within its thermal band width with the bottom energy of conduction band of the first semiconductor layer (5) remote from the multiquantum well layer and the bottom energy of conduction band of the second semiconductor layer (6); and electrons can tunnel through said barriers from said second semiconductor layer via said multiquantum well layer (9) to said first semiconductor layer (5). The semiconductor device according to claim 5, wherein respective said quantum wells and barriers of said multiquantum well layer have thicknesses to exhibit discrete quantum levels substantially equal within their thermal bandwidth to the bottom energies of conduction bands of said first n-type II-VI compound semiconductor and said second n-type II-VI compound semiconductor in a portion of said multiquantum well layer nearer to said second n-type II-VI compounds semiconductor and to produce sub-bands having widths involving the bottom energies of conduction bands of said first n-type II-VI compound semiconductor and said second n-type II-VI compound semiconductor in a portion of said multiquantum well layer remoter from said second n-type II-VI compound semiconductor. A semiconductor device according to claim 1 or claim 4, wherein the impurity concentration within said first semiconductor layer (5) is higher in a portion nearer to the interface between said multiquantum well layer (9) and said second semiconductor layer (6) than in a portion farer from said interface. A semiconductor device according to claim 1 or claim 4, wherein the impurity concentration within said first semiconductor layer (5) gradually increases toward the interface between said multiquantum well layer (9) and said second semiconductor layer (6). A semiconductor laser, comprising: a first electrode (8); a semiconductor substrate (1) on said first electrode (8); a first cladding layer (2) on said semiconductor substrate; an active layer (3) on said first cladding layer (2); a second cladding layer (4) on said active layer (3); a second electrode (7); characterized by a semiconductor device (5, 6, 9) according to any of the preceding claims disposed between said second cladding layer (4) and said second electrode (7). The semiconductor laser of claim 8, wherein said first cladding layer and said second cladding layer are made of Zn 1-X-Y Cd X Mg Y S 1-a-b Se a Te b (0≤X< 1, 0 < Y < 1, 0< a ≤ 1, 0 ≤ b < 1). The semiconductor laser of claim 8, or 9, wherein said active layer (3) is made of ZnCdSe or ZnSe.

Inventors: AKIMOTO KATSUHIRO, HIEI FUTOSHI, IKEDA MASAO, IOCHI YOSHINO, ISHIBASHI AKIRA, ITO SATOSHI, MIYAJIMA TAKAO, OZAWA MASAFUMI, AKIMOTO, KATSUHIRO, HIEI, FUTOSHI, IKEDA, MASAO, IOCHI, YOSHINO, ISHIBASHI, AKIRA, ITO, SATOSHI, MIYAJIMA, TAKAO, OZAWA, MASAFUMI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1993
Main classification: H01S3
IPC codes: H01L33, H01S5, H01L21, H01L29