Intersubband light emitting element

[0002] 1. Field of the Invention

[0003] The present invention relates to an intersubband light emitting element such as a light emitting diode and a laser diode. Such an element emits light of which a wavelength ranges from a near infrared region to a terahertz region.

[0004] 2. Description of the Related Art

[0005] The intersubband light emitting element is attractive as a light source for emitting light having a wavelength that ranges from the near infrared region to the terahertz (THz) region, because it has a relatively high oscillator strength and is capable of controlling a transition wavelength of the light widely utilizing the structure of the element. A conventional intersubband light emitting element has a heterojunction structure referred as a type-I structure, such as GaAs/AlGaAs and GaInAs/AlInAs.

[0006] However, the element of the type-I structure has a disadvantage that the amount of carrier leakage is high and thus the injection efficiency of the carriers is low when the carriers are injected into a quantum well of the element. This is one of the reasons why, under existing circumstances, the threshold current density of the element increases during the laser oscillation, and makes it difficult to oscillate the element continuously under a room temperature condition.

[0007] It is an object of the present invention to provide an intersubband light emitting element with a minimized leakage current and a decreased threshold current.

[0008] It is another object of the present invention to provide an intersubband light emitting element with a minimized leakage current and a decreased threshold current, that is capable of emitting light having a wavelength of the THz region.

[0009] It is another object of the present invention to provide an intersubband light emitting element with a minimized leakage current and a decreased threshold current, that is capable of emitting light having a plurality of predetermined wavelengths such as blue light and red light at the same time.

[0010] According to one aspect of the present invention, there is provided an intersubband light emitting element comprising:

[0011] a semiconducting substrate;

[0012] a first layer composed of a first semiconducting material;

[0013] a second layer composed of a second semiconducting material and making a heterojunction with the first layer, the top of a valence band of the second semiconducting material being higher in energy than the bottom of a conduction band of the first semiconducting material; and

[0014] a third layer making a heterojunction with one of the first and second layer and having a superlattice structure,

[0015] wherein one of the first and second layer is provided on the semiconducting substrate directly or through at least one semiconducting layer.

[0016] Intersubband light emission occurs when carriers injected into the second subband within a quantum well of the first layer are relaxed to the first subband within the same quantum well. In order that the intersubband light emission takes place effectively, it is necessary to prevent leakage of the injected carrier and thereby utilize the intersubband transitions of the entire carriers.

[0017] With the above-mentioned element according to the invention, the first layer and the second layer form a broken gap structure, and this structure prevents leakage of the carriers or electrons, with the result that the threshold current of the element is decreased.

[0018] The third layer having the superlattice structure forms a new energy gap. Consequently, the leakage current is further reduced and the threshold current is also further decreased.

[0019] The element according to the present invention may be constituted such that the functions of the first and the second layers are reversed to each other. In this instance, the intersubband light emission occurs when carriers injected into the second subband within a quantum well of the second layer are relaxed to the first subband within the same quantum well. The carriers are then pulled from the first subband rapidly due to an interband tunneling effect of the heterojunction structure formed with the first layer and the second layer. Similarly, the broken gap structure prevents the leakage of the carriers or holes, and the leakage current is further reduced because the third layer having the superlattice structure makes a heterojunction with the first layer.

[0020] Preferably, the element comprises a plurality of sets of the first and second layers, the sets each having a quantum well width of one of the first and second layers, the quantum well widths being different from each other among the sets. The wavelength of the light emitted from the element or a light emitting portion formed with the quantum well becomes longer as the well width becomes wider, and it is thus possible to emit the light having a plurality of predetermined wavelengths such as blue light and red light at the same time.

[0021] By way of a concrete example, the first semiconducting material is one of InAs and InAs with an impurity, the second semiconducting material is one of GaSb and GaSb with an impurity, and the superlattice structure is one of an InAs/AlSb superlattice structure, an InAs/AlSb superlattice structure with an impurity, a GaSb/AlGaSb superlattice structure, a GaSb/AlGaSb superlattice structure with an impurity, a GaSb/AlSb superlattice structure, and a GaSb/AlSb superlattice structure with an impurity. The impurity of the InAs is one of Al and Ga instead of one of In and Si, one of Sb and P instead of As and so on. The impurity of GaSb is one of In and Al instead of one of Ga, Si and Be, one of As and P instead of Sb.

[0022] According to another aspect of the present invention, there is provided an intersubband light emitting element comprising:

[0023] a semiconducting substrate;

[0024] a first layer composed of a first semiconducting material;

[0025] a second layer composed of a second semiconducting material and making a heterojunction with the first layer, the bottom of a conduction band of the second semiconducting material being higher in energy than that of the first semiconducting material or the top of a valence band of the second semiconducting material being a lower in energy than that of the first semiconducting material;

[0026] a third layer composed of a third semiconducting material which is same as the first semiconducting material and making a heterojunction with the first layer; and

[0027] a forth layer composed of a forth semiconducting material and making a heterojunction with the third layer, the top of a valence band of the forth semiconducting material being a higher in energy than the bottom of a conduction band of the second semiconducting material or the bottom of a conduction band of the forth semiconducting material being a lower in energy than the top of a valence band of the second semiconducting material;

[0028] wherein one of the first and forth layer is provided on the semiconducting substrate directly or through at least one semiconducting layer.

[0029] When widening the well width of the quantum well to prolong the wavelength of the emitting light to the THz region, the second subband within the quantum well becomes lower than the top of a valence band of the third layer. Therefore, the carriers injected into the second subband are not blocked by the barrier and thus the injection efficiency of the carriers decreases.

[0030] With the above-mentioned element according to the invention, the quantum well is formed with the first to third layers. In this case, in order to prevent the decrease of the injection efficiency due to the prolongation of the wavelength to the THz region, a projecting potential barrier is formed at the center of the quantum well by the second layer of which the bottom of a conduction band is higher in energy than that of the first layer. Such a projecting potential barrier has an effect on only the carrier distributed around the center of the quantum well. Accordingly, it is possible to increase the energy of the first subband without changing the energy of the second subband. Thereby, it is possible to decrease an energy space of the subband without affecting the injection efficiency of the carriers. As a result, it is possible to realize the element with a high efficiency even if the light having the wavelength of the THz region is emitted.

[0031] The element according to the present invention may be constituted such that functions of the first and the third layers are reversed to each other. In this instance, the intersubband light emission occurs when carriers injected into second subband within a quantum well of the second layer are relaxed to first subband within the same quantum well.

[0032] Preferably, the element comprises a plurality of sets of the first to forth layers, the sets each having a potential barrier of the second layer, heights of the potential barriers being different from each other among the sets. In this way, the element capable of emitting the light with a plurality of wavelengths can be achieved. The reason is that the injection into the second subband can be performed with the same efficiency because the energy of the second subband does not substantially change even if the height of the potential barrier changes, and no carrier always exists at the first subband because the interband tunnel effect of the heterojunction structure formed with the first and second layers is remarkable and thus reabsorption of the carriers hardly occurs.

[0033] By way of a concrete example, the first semiconducting material is one of InAs and InAs with an impurity, the second semiconducting material is one of AlInAs, AlInAs with an impurity, GaInAs, GaInAs with an impurity, InAsSb, InAsSb with an impurity, AlGaSb and AlGaSb with an impurity, the third semiconducting material is one of GaSb and GaSb with an impurity.

[0034] By further comprising a fifth layer with a superlattice structure, a new energy gap is formed. As a result, the leakage current is further reduced and the threshold current is further decreased. The superlattice structure is one of an InAs/AlSb superlattice structure, an InAs/AlSb superlattice structure with an impurity, a GaSb/AlGaSb superlattice structure, a GaSb/AlGaSb superlattice structure with an impurity, a GaSb/AlSb superlattice structure, and a GaSb/AlSb superlattice structure with an impurity.

[0035] Embodiments of the intersubband light emitting element according to the present invention will be explained below with reference to the accompanying drawings.

[0036] FIG. 1 is a schematic diagram showing a first embodiment of the element according to the present invention;

[0037] FIG. 2 is a schematic diagram showing the energy levels of an InAs layer and a GaSb layer;

[0038] FIG. 3 is a schematic diagram showing the energy levels of an InAs layer, a GaSb layer an InAs/AlSb layer;

[0039] FIG. 4 is a schematic diagram showing a second embodiment of the element according to the present invention;

[0040] FIG. 5A is a schematic diagram showing the energy levels of an InAs layer and a GaSb layer as shown in FIG. 1, and

[0041] FIG. 5B is a schematic diagram showing energy levels of an InAs layer, an AlInAs layer, an InAs layer and a GaSb layer as shown in FIG. 2;

[0042] FIG. 6 is a schematic diagram showing a third embodiment of the element according to the present invention;

[0043] FIG. 7 is a schematic diagram showing the energy levels of a GaSb layer and an InAs layer;

[0044] FIG. 8 is a schematic diagram showing the energy levels of a GaSb layer, an InAs layer and a GaSb/AlGaSb layer;

[0045] FIG. 9 is a schematic diagram showing a forth embodiment of the element according to the present invention; and

[0046] FIG. 10 is a schematic diagram showing the energy levels of a GaSb layer and an InAs layer as shown in FIG. 6.

[0047] FIG. 1 is a schematic diagram showing a first embodiment of the element according to the present invention. The element comprises an InAs substrate 1, an InAs/AlSb layer 2 provided on the InAs substrate 1 and having a superlattice structure, and a plurality of layered portions 3a, 3bprovided on the InAs/AlSb layer 2 successively.

[0048] The layered portion 3aor 3bhas an AlSb layer 4aor 4b, an InAs layers 5aor 5bas the first layer, a GaSb layer 6aor 6bas the second layer making a heterojunction with the InAs layer 5aor 5b, and an InAs/AlSb layer 7aor 7bas the third layer having the superlattice structure. In the embodiment, quantum well width of the InAs layer 5ais different from that of the InAs layer 5b. Thereby, it is possible to emit the light having a plurality of wavelengths. The well width can be changed by changing the thickness of the InAs layer 5aor 5b, for example.

[0049] The operation of the embodiment will be explained. As the top of a valence band of GaSb is higher in energy than the bottom of a conduction band of InAs, InAs layers 5a, 5band GaSb layers 6a, 6bhave a broken gap structure as shown in FIG. 2. As carriers or electrons injected into second subband within a quantum well of the InAs layers 5a, 5bare blocked by a bandgap of the respective GaSb layers 6a, 6bas represented by arrow, it is possible to prevent leakage of the carriers. As a result of this, the leakage current of the element is reduced and the injection of the carriers with high efficiency is possible.

[0050] In the embodiment, as shown in FIG. 3, the leakage current is further reduced and the efficiency of the injection is improved because there is the InAs/AlSb layer 7a, 7bwith the superlattice which comprises a forbidden band 8.

[0051] FIG. 4 is a schematic diagram showing a second embodiment of the element according to the present invention. The element comprises an InAs substrate 11, an InAs/AlSb layer 12 provided on the InAs substrate 11 and having a superlattice structure, and a plurality of layered portions 13a, 13bprovided on the InAs/AlSb layer 2 successively.

[0052] The layered portion 13aor 13bhas an AlSb layer 14aor 14b, an InAs layers 15aor 15bas the first layer, an AlInAs layer 16aor 16bas the second layer making a heterojunction with the InAs layer 15aor 15b, an InAs layer 17aor 17bas the third layer making a heterojunction with the AlInAs layer 16aor 16b, a GaSb layer 18aor 18bas the forth layer making a heterojunction with the InAs layer 17aor 17b, and an InAs/AlSb layer 19aor 19bas the fifth layer having the superlattice structure.

[0053] The InAs layers 15a, 15b, the AlInAs layers 16a, 16band the InAs layers 17a, 17bform quantum wells (light emitting portion), respectively. A height of projecting potential barriers formed with the AlInAs layer 16aare different from that formed with the AlInAs layer 16band thus it is possible to emit light having a plurality of wavelengths. The heights of the potential barriers can be changed by changing the composition of material of the layer forming the potential barrier, for example. In this case, the heights can be changed by changing the Al composition of the AlInAs.

[0054] The operation of the embodiment will be explained. The intersubband light emission occurs when the carriers injected into second subband E₂within a quantum well are relaxed to first subband E₁. If a well width of a quantum well of the element having a InAs/GaSb/AlSb heterojunction structure as shown in FIG. 1 is widened to prolong the wavelength to the THz region, the second subband is lower in energy than the top of a valence band of the GaSb as shown in FIG. 5A. Therefore, the carrier injected into the second subband are not blocked by the barrier and thus the efficiency of the carrier injection is decreased.

[0055] According to the embodiment, in order to prevent the decrease of the injection efficiency due to the prolongation of the wavelength to the THz region, a projecting potential barrier is formed at the center of the quantum well by the AlInAs having the top of a conduction band higher in energy than that of the InAs and lower in energy than the top of a valence band of the GaSb as shown in FIG. 5B. Such a projecting potential barrier has an effect on only the carrier distributed at the center of the quantum well. Accordingly, it is possible to increase the energy of the first subband without changing the energy of the second subband. Thereby, it is possible to decrease the energy space of the subband without affecting the injection efficiency of the carriers. As a result, it is possible to realize the element with a high efficiency even if the light having the wavelength of the THz region is emitted.

[0056] FIG. 6 is a schematic diagram showing a third embodiment of the element according to the present invention. The element performs an intersubband light emission at a valence band of itself and comprises a GaSb substrate 21, a GaSb/AlGaSb or GaSb/AlSb layer 22 provided on the GaSb substrate 21 and having a superlattice structure, and a plurality of layered portions 23a, 23bprovided on the GaSb/AlGaSb or GaSb/AlSb layer 22 successively.

[0057] The layered portion 23aor 23bhas an AlSb layer 24aor 24b, a GaSb layers 25aor 25bas the first layer, an InAs layer 26aor 26bas the second layer making a heterojunction with the GaSb layer 25aor 25b, and a GaSb/AlGaSb layer 27aor 27bas the third layer having the superlattice structure. In the embodiment, a quantum well widths of the GaSb layer 25ais different from that of GaSb layer 25b. Thereby, it is possible to emit the light having a plurality of wavelengths. The well width can be changed by changing the thickness of the GaSb layer 25aor 25b, for example.

[0058] The operation of the embodiment will be explained. As the top of a valence band of GaSb is higher in energy than the bottom of a conduction band of InAs, GaSb layers 25a, 25band InAs layers 26a, 26bhave a broken gap structure as shown in FIG. 7. As carriers or electrons injected into second subband within a quantum well of the GaSb layers 25a, 25bare blocked by a bandgap of the respective InAs layers 26a, 26bas represented by arrow, it is possible to prevent the leakage of the carriers. As a result of this, the leakage current of the element is reduced and the injection of the carriers with high efficiency is possible.

[0059] In the embodiment, as shown in FIG. 8, the leakage current is further reduced and the efficiency of the injection is improved because there is the GaSb/AlGaSb layer 27a, 27bwith the superlattice which comprises a forbidden band 28.

[0060] FIG. 9 is a schematic diagram showing a forth embodiment of the element according to the present invention. The element performs an intersubband light emission at a valence band of itself and comprises a GaSb substrate 31, a GaSb/AlGaSb layer 32 provided on the GaSb substrate 31 and having a superlattice structure, and a plurality of layered portions 33a, 33bprovided on the GaSb/AlGaSb layer 32 successively.

[0061] The layered portion 33aor 33bhas an AlSb layer 34aor 34b, an GaSb layers 35aor 35bas the first layer, an AlGaSb layer 36aor 36bas the second layer making a heterojunction with the GaSb layer 35aor 35b, a GaSb layer 37aor 37bas the third layer making a heterojunction with the AlGaSb layer 36aor 36b, an InAs layer 38aor 38bas the forth layer making a heterojunction with the GaSb layer 37aor 37b, and a GaSb/AlGaSb layer 39aor 39bas the fifth layer having the superlattice structure.

[0062] The GaSb layers 35a, 35b, the AlGaSb layers 36a, 36band the GaSb layers 37a, 37bform quantum wells (light emitting portion), respectively. A height of projecting potential barriers formed with the AlGaSb layer 36ais different from that formed with the AlGaSb layer 36band thus it is possible to emit light having a plurality of wavelengths. The heights of the potential barriers can be changed by changing the composition of material of the layer forming the potential barrier, for example. In this case, the heights can be changed by changing the Al composition of the AlGaSb.

[0063] The operation of the embodiment will be explained. The intersubband light emission occurs when the carriers injected into second subband E₄within a quantum well are relaxed to first subband E₃. If a well width of a quantum well of the element having a InAs/GaSb/AlSb heterojunction structure as shown in FIG. 1 is widened to prolong the wavelength to the THz region, the second subband is higher in energy than the bottom of a conduction band of the InAs as shown in FIG. 10A. Therefore, the carriers injected into the second subband are not blocked by the barrier and thus the efficiency of the carrier injection is decreased.

[0064] According to the embodiment, in order to prevent the decrease of the injection efficiency due to the prolongation of the wavelength of the light to the THz region, a projecting potential barrier is formed at the center of the quantum well by the AlGaSb having the top of a valence band higher in energy than the bottom of a conduction band of the InAs and lower in energy than the top valence band of the GaSb as shown in FIG. 10B. Such a projecting potential barrier has an effect on only the carrier distributed at the center of the quantum well. Accordingly, it is possible to increase the energy of the first subband without changing the energy of the second subband. Thereby, it is possible to decrease the energy space of the subband without affecting the injection efficiency of the carriers. As a result, it is possible to realize the element with a high efficiency even if the light having the wavelength of the THz region is emitted.

[0065] While the present invention has been described above with reference to certain preferred embodiments, it should be noted that it were presented by way of examples only and various changes and/or modifications may be made without departing from the scope of the invention. For example, it is possible to construct the element according to the present invention in combination the layered portions as shown in FIG. 1 and those as shown in FIG. 4. One of the layered portions as shown in one of FIGS. 1, 4, 6 and 9 can be provided on the substrate directly. A single layered portion as shown in FIGS. 1, 4, 6 and 9 can be used. The substrate can be composed of any other semiconducting material than the InAs and the GaSb. The layer having the superlattice structure can be composed of semiconducting material substantially lattice-matched to the substrate instead of InAs/AlSb. A layer of mixed crystal based on the InAs can be used instead of the InAs layer, and a layer of mixed crystal based on the GaSb can be used instead of the GaSb.