FIELD OF THE INVENTION

The invention relates to a laser diode having type II heterojunctions, and more particularly to a II-VI compound laser diode suitable for emission of bule-green light.

KNOWN ART

It has been known that II-VI semiconductors have wide band gaps and are suitable for fabrication of blue-green laser diodes. In fact a successful emission was demonstrated at 77K. See Applied Physics Letters, Vol. 59, (1991)1272. A light emitting element utilizing II -VI semiconductor is also disclosed in Japanese Patent Early Publications 2-125477 and 2-196485.

However, laser diode having a double-heterojunction structure of II-VI compounds lattice matched with a semiconductor substrate can not effectively confine both electrons and holes simultaneously. In order to confine both electrons and holes, laser diodes must be fabricated from a combination of different materials having different lattice constants. However, the laser will suffer from numerous dislocation caused by misfit of different lattice constants of two different materials, which prevent continuous wave (cw) operation of bule-green light at room temperature.

It is therefore an object of the invention to provide a blue -green laser diode capable of cw operation at a room temperature.

It is another object of the invention to provide a II-VI blue -green laser diode capable of cw operation at a room temperature.

SUMMARY OF THE INVENTION

In one aspect of the invention, there is provided a laser, diode comprising a semiconductor substrate and epitaxial layers lattice-matched with said substrate in the sequence of:

   a p-cladding layer;

   a hole barrier layer with p-type conduction forming an interface with said p-cladding layer;

   a hole confinement layer forming an interface with said hole barrier layer, for confining therein holes;

   an electron confinement layer forming an interface with said hole confinment layer, for confining therein electrons;

   an electron barrier layer with n-type conduction forming an interface with said electron confinement layer; and

   an n-cladding layer forming an interface with said electron barrier layer,

   said electron confinement layer and said hole confinement layer forming a type II heterojuction,

   said semiconductor laser having a band line-up such that

   potential for holes (hole potential) is low in said hole confinement layer and potential for electron (electron potential) is low in said electron confinement layer;

   said hole potential being higher in said hole barrier layer than in said hole confinement layer;

   said electron potential being lower in said electron barrier layer than in said electron confinement layer;

   said p-cladding layer and said n-cladding layer having refractive indices smaller than at least one of the refractive indices of said hole confinement layer and said electron confinement layer.

In another aspect of the invention, there is provided a laser diode, comprising a semiconductor substrate and epitaxial layers lattice matched with said substrate in the sequence of:

   an n-cladding layer;

   an electron barrier layer with n-type conduction forming an interface with said electron barrier layer;

   an electron confinement layer forming an interface with said electron barrier layer, for confining electrons therein;

   a quantum well layer forming an interface with said electron confinement layer;

   a hole confinement layer forming an interface with said quantam well layer, for confining holes therein;

   a hole barrier layer with p-type conduction forming an interface with said hole donfinement layer; and

   a p-cladding layer forming an interface with said hole barrier layer,

wherein

   said sequence of layers having a type II band line-up such that

   potential for holes (hole potential) is low in said hole confinement layer and potential for electrons (electron potential) is low in said electron confinement layer;

   said hole potential and said electron potential are lower in said quantum well layer than in said hole confinement layer and said electron confinement layer;

   said hole potential being higher in said hole barrier layer than in said hole confinement layer;

   said electron potential being lower in said electron barrier layer than in said electron confinement layer;

   said p-cladding layer and said n-cladding layer having refractive indices smaller than at least one of the refractive indices of said hole confinement layer and said electron confinement layer.

Briefly stated, the invention employes so-called type II band structures in the hole confinement layer and the electron confinement layer, in which the potential for holes is low in the hole confinement layer and the potential for electrons is low in the electron confinement layer, thereby facilitating effective confinement of holes and electrons in the respective layers.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates a cross section of a laser diode defined by claim 1 along with the band line-up of the laser.

Fig. 2 illustrates a cross section of another laser diode defined by claim 10 along with the band line-up of the laser.

Fig. 3 illustrates a cross section of another laser diode defined by claim 4 along with the band line-up of the laser.

Fig. 4 illustrates a cross section of another laser diode defined by claim 5 along with the band line-up of the laser.

Fig. 5 illustrates a cross section of another laser diode defined by claim 7 along with the band line-up of the laser.

Fig. 6 illustrates a cross section of another laser diode defined by claim 9 along with the band line-up of the laser.

Fig. 7 illustrates a cross section of another laser diode defined by claim 11 along with the band line-up of the laser.

Fig. 8 illustrates a cross section of another laser diode defined by claim 12 along with the band line-up of the laser.

Fig. 9 illustrates a cross section of another laser diode defined by claim 14 along with the band line-up of the laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is now described by way of example with reference to the accompanying drawings, in which like or corresponding components are numbered the same throughout the drawings.

Referring now to Fig. 1, there is shown a first example of a laser diode in accordance with claim 1. Fig. 1(A) illustrates a multi-layer structure of the laser diode. Fig. 1(B) illustrates the band line-up of the laser of Fig. 1(A).

This laser diode is fabricated by growing on a p-type GaAs semiconductor substrate 10 the following semiconductor layers by molecular beam epitaxicy:

   a p-cladding layer 11 (p=1x1018 cm-3, 30 nm thick) made of N -doped Zn0.48Cd0.52S0.91Se0.09;

   a hole barrier layer 12 (p=1x1018 cm-3, 30 nm thick) made of N-doped Zn0.42Cd0.58S;

   a hole confinement layer 13 (p=1x1017 cm-3, 50 nm thick) made of N-doped ZnS0.09Se0.91;

   a electron confinement layer 14 (n=1x1017 cm-3, 30 nm thick) made of Cl-doped Zn0.71Cd0.29S0.55Se0.45;

   a electron barrier layer 15 (n=1x1018 cm-3,50 nm thick) made of Cl-doped ZnS0.09Se0.91;

   a n-cladding layer 16 (n=1x1018 cm-3, 1µ thick) made of Cl -doped Zn0.48Cd0.52S0.91Se0.09, and then providing an Au n -electrode 17 and an AuZn p-electrode 18 using vacuum evaporation technique, and lastly cleaving facets forming reflective mirrors for the semiconductor laser.

These layers of the laser had defect-free, high quality crystal qualities, since they were each lattice matched with the semiconductor substrate 10. A typical working conduction band edge and a valence band edge are shown in Fig. 1 (B). A ZnCdSSe compounds heterojuction of the hole confinement layer 13 and the electron confinement layer 14 is of type II, in which the potential for the holes (hereinafter referred to as hole potential) is low in the hole confinement layer 13 and the potential for electrons (hereinafter referred to as electron potential) is low in the electron confinement layer 14. In addition, the hole potential is high in the hole barrier layer 12 and electron potential is high in the electron barrier layer 15, so that holes and electrons injected into the semiconductor laser will be trapped in the layers 13 and 14, respectively, thereby resulting in the population inversion required for (continuous wave bule -green light) lasing. The recombination of the electrons and holes proceeds in the heterogeneous interface (heterointerface) between the hole confinement layer 13 and the electron confinement layer 14, emitting desired laser light. The p -cladding layer 11 and the n-cladding layer 16 have smaller refractive indices than the hole confinement layer 13 and the electron barrier layer 15, and as a result the light emitted by the recombination will be confined between the p-cladding layer 11 and the n-cladding layer 16, with its maximum intensity lying in the region of the heterointerface between the hole confinement layer 13 and the electron confinement layer 14.

Because of the effective confinement of holes and electrons in the respective confinement layers, and of successful confinement of light, cw operation of bule-green laser was easily attained at a room temperature.

Although in the first example described hereinabove and shown in Fig. 1 the p-cladding layer 11 and the hole barrier layer 12 are made of N-doped Zn0.48Cd0.52S0.91Se0.09 compound and N-doped Zn0.42Cd0.58S compound, respectively, the layers 11 and 12 may be made of the same compound such as Zn0.48Cd0.52S0.91Se0. 09 and Zn0.48Cd0.52S. The reason for this is that simultaneous confinement of both holes and light is possible in ZnCdSSe systems due to the fact that in these ZnCdSSe compounds the smaller is the refractive indices the higher is the hole potential.

Also, instead of the p-type GaAs of the first example, another type of semiconductor such as n-type GaAs, InP, GaP, InAs, ZnSe, ZnTe may be used for the semiconductor substrate, and instead of the epitaxialy grown semiconductor layers of ZnCdSSe, one of type II epitaxial materials such as ZnCdSTe-, ZnCdSeTe-, HgCdSeTe-systems may be used as well.

Further, the epitaxial layers of the first example may alternatively substituted for by superlattices.

Referring now to Fig. 2, there is shown a second laser in accordance with claim 10. Figs. 2(A) and 2(b) respectively illustrate the cross section of the laser and its band line-up.

Using molecular beam epitaxial technique, the second laser shown in Fig. 2 was fabricated by first growing, on a semiconductor substrate, the following layers in the sequence listed:

   a n-cladding layer 22 (n=1x1018 cm-3, 1µ thick) made of Cl -doped ZnTe0.46Se0.54;

   a electron barrier layer 23 (n=1x1017 cm-3,50 nm thick) made of Cl-doped Zn0.52Cd0.48Se,

   an electron confinement layer 24 (30 nm thick) made of Zn0. 76Cd0.24Te0.23Se0.77,

   a quantum well layer 25 (7 nm thick) made of Zn0.3Cd0.7Se,

   a hole confinement layer 26 (50 nm thick) made of Zn0.52Cd0. 48Se,

   a hole barrier layer 27 (p=1x1017 cm-3, 30 nm thick) made of N-doped Zn0.76Cd0.24Te0.23Se0.77, and

   a p-cladding layer 28 (p=1x1018 cm-3, 1µ thick) made of N -doped Zn0.48Cd0.52S0.91Se0.09, and then forming an Au p -electrode 29 and an AuGeNi n-electrode 30, and finally cleaving facets to form reflective mirrors.

The second laser diode has conduction band edges and valence band edges as indicated by a band line-up shown in Fig. 2(B). It should be noted that the lattice constant (lattice length) of the quantum well layer 25 is longer than that of the semiconductor substrate 21, causing the quantum well to be strained. Other epitaxial layers are lattice-matched with the substrate 21. The quantum well layer 25 has a thickness less than critical thickness and has scarce dislocation, exhibiting excellent emission efficiency.

Heterojunctions of ZnCdTeSe systems have so-called type II band line-ups in which a region of low hole potential is spaced apart from a region of low electron potential. As a result, these systems will not form a double heterojunctions which are commonly known to exist in III-V semiconductors.

In the second laser diode described above, electron potential is low in both the electron confinement layer 24 and quantum well layer 25, and high in electron barrier layer 23 and the hole confinement layer 26, so that electrons confined in the electron confinement layer 24 may be (easily) injected into the quantum well layer 25. The hole potential is low in both the quantum well layer 25 and the hole confinement layer 26 but is high in the electron confinement layer 24 and in the hole barrier layer 27, so that holes confined in the 26 may be (also easily) injected into the quantum well layer 25. The stress of the quantum well 25 changes the band structure and causes both of the hole and electron potentials in the well to be lowered, creating so-called type I structure. Consequently the electrons and holes injected into the well may effectively recombine to emit (blue-green) light.

Since the refractive indices of the electron barrier 23 and the hole confinement layer 26 are larger than the refractive indices of the n-cladding layer 22 and the p-cladding layer, emitted light is confined in a region between the n-cladding layer 22 and the p-cladding layer 28, with its maximum intensity being in the quantum well 25. Thus, cw lasing even at a room temperature was successfully observed.

It should be understood that, although the quantum well 25 of the second example is a single planar layer and has a strained structure, any one of alternative quantum wells such as lattice -matched type I quantum wells, multiple quantum wells, quantum wires, and quantum boxes may be used.

Also, it should be understood that, instead of the hole barrier layer 27 and p -cladding layer 28 having different compositions, these layers may be given the same composition if they are made of an ZnCdTeSe system, provided that the system has a smaller refractive index for a higher hole potential.

In the second example the semiconductor substrate is an n -type InP and the epitaxial layers are selected from ZnCdTeSe systems. However, an alternative semiconductor substrate such as p-type InP, GaAs, GaP, InAs, ZnSe, and ZnTe may be used equally well. Epitaxial layers may be selected from other type II materials such as ZnCdSTe, ZnCdSSe, and HgCdSeTe systems.

The epitaxial semiconductors in the second example may be substituted for by superlattices.

Referring to Fig. 3, there is shown a band line-up of a third example of the semiconductor laser in accordance with claim 4. The structure of this laser is basically the same as the first laser shown in Fig. 1 except that the hole confinement layer 13 of the first laser is replaced by a type II superlattice which has 10 periods of alternate ZnS0.09Se0.91 (4 nm thick)- Zn0. 42Cd0.58S (1 nm thick) layers, and the electron confinement layer 14 is replaced by a type II super lattice which has 6 periods of alternate ZnS0.09Se0.91 (1 nm thick)-Zn0.42Cd0.58S (4 nm thick) layers. This semiconductor laser was also observed to lase continuously at room temperatures, resulting in emission of blue light.

In this laser, the hole potential is sufficiently low in the superlattice hole confinement layer 13, and the electron potential is sufficiently low in the superlattice electron confinement layer 14. It should be noted that the hole confinement layer 13 and the hole confinement layer 14 have lattices well matched with the substrate, and that superlattices reduce dislocation and defects. Thus, this type of laser diode exhibited a better over-all crystal qualities than the first laser. It should be also noted that a great degree freedom is obtained in designing laser diodes by simply controlling the thickness of the superlattices to thereby modify the hole and electron potentials in the confinement layers.

In the third example described above, the superlattices have a rectangular configuration in cross section, which may be changed to any other superlattice configuration of type II having, for example, a triangular-wave cross section.

Referring to Fig. 4, there is shown a band line-up diagram of a laser diode in accordance with claim 5, which is a modification of the first laser defined by claim 1.

This arrangement is obtained by modifying the composition of the hole confinement layer 13 of the first example to Zn1-0.58xCd0. 58xS0.09+0.91xSe0.91 -0.91x, and the composition of the electron confinement layer 14 to Zn1-0.58yCd0.58yS0.09+0.91ySe0.91-0.91y, where x and y are fractional concentrations of relevant elements in the corresponding compounds. The value of x gradually decreases from 1 at the interface with the hole barrier layer 12 to 0 at the interface with the electron confinement layer 14. The thickness of the layer 13 is 50 nm. The value of y gradually increases across 30 nm thick layer 14 from 0 at the interface with the electron barrier layer 15 to 0.5 at the interface with the hole confinement layer 13. Holes injected from the hole barrier layer 12 into the hole confinement layer 13 will accumulate near the interface with the electron confinement layer 14 due to the potential gradient in the hole confinement layer 13. Electrons injected from the electron barrier layer 15 into the 14 will also accumlate near the interface with the hole confinement layer 13 due to the potential gradient in the electron confinement layer 14. Consequently, the holes and the electron that has collected near the interface will undergo recombination with a high recombination rate. A laser diode fabricated in this manner exhibited an improved quantum efficiency higher than the first laser by 10%.

Instead of using ZnCdSSe systems for the hole confinement layer and the electron confinement layer in the fourth example described above, CHIRP (coherent heterointerfaces for reflection and penetration) superlattices and other alternative material systems may be used equally well.

Fig. 5 illustrates a band line-up diagram of a fifth laser diode in accordance with claim 7, in which the arrangement of the multi -layers is basically the same as in the first example. However, a hole barrier layer 12 has composition N-doped Zn1-0. 58xCd0.58xS0.09+0.91xSe0.91-0.91x and a electron barrier layer 15 has composition Cl-doped Zn1-0.58yCd0.58yS0.09+0.91ySe0.91-0.91y, where x and y are fractional concentrations of the relevant components. The value of x gradually increases in the hole barrier layer 12 from 0.9 at the interface with the p-cladding layer 11 to 1 at the interface with the hole confinement layer 13. The layer 12 has a thickness of 30 nm and N doping concentration of 1x1018 cm-3. The value of y gradually increases in the electron barrier layer 15 from 0 at the interface with the electron confinement layer 14 to 0.9 at the interface with the n -cladding layer 16. The electron barrier layer 15 has a thickness of 50 nm and Cl doping concentration of 1x1018 cm-3.

This type of laser diode has no heterobarrier for holes between the p-cladding layer 11 and the hole barrier layer 12, so that holes may easily cross the interface between the layers so that hole current may be easily injected into the hole confinement layer. Also, no heterobarrier for electrons exists between the n -cladding layer 16 and the electron barrier layer 15, so that electrons may be also injected easily. Consequently, a laser diode fabricated in this manner lased at a lower lasing voltage than the first laser by 0.1 Volt, and consumes less power.

In stead of providing both the hole barrier layer 12 and the electron barrier layer 15 with linear gradients of composition, the compound layers may be provided with step-wise compositions.

Fig. 6 illustrates a band line-up diagram of a laser diode in accordance with claim 9. This laser also has basically the same arrangement as the first laser except for varied compositions of the hole confinement layer 13 and the electron confinement layer 14. The composition of the hole confinement layer 13 is now ZnS0.09Se 0.91, while the composition of the electron confinement layer 14 is Zn0.71Cd0.29S0.55Se0.45. The layers 13 and 14 are characterized by radiative recombination centers of Te doped in the regions near the interface of the hole confinement layer 13 and the electron confinement layer 14 to an extent of 1x1020 cm-3. As shown in Fig. 6, the potential levels associated with the Te recombination centers (dotted lines) are lower than the band edges (solid lines) for both holes and electrons, so that the recombination of the holes and electron trapped near the centers may effectively proceed. In fact, experiment showed that the laser has a smaller threshold current for lasing than the first laser.

Te used in this laser may be substituted for by any other suitable element commonly known to serve as recombination centers.

Fig. 7 illustrates another band line-up for a seventh laser diode in accordance with claim 12, which is also essentially the same in structure as the first laser except that this laser has an electron confinement layer 24 of type II superlattice consisting of 7 periods of alternate ZnTe0.46Se0.54 (3 nm thick) -Zn0.52Cd0.48Se (1 nm thick) layers, and a hole confinement layer 26 of type II superlattice consisting of 12 periods of alternate ZnTe0.46Se0.54 (1 nm thick) -Zn0.52Cd0.48Se (3 nm thick) layers.

The superlattice electron confinement layer 24 has an appropriately low potential for electrons for confining electrons therein, while the superlattice hole confinement layer 26 has an appropriately low potential for holes for confining holes therein.

Use of the superlattices may improve the over-all crystal qualities of the laser, thereby improving the quantum efficiency (defined by the ratio of the output power to the input power) of the laser.

The superlattices used in the seventh example described above are rectangular in cross section, which may be replaced by any of other type II superlattice having different cross sections, for example one having a triangular cross section.

Fig. 8 illustrates still another band line-up diagram of a laser diode in accordance with claim 13, which has a multi-layer arrangement similar to the second example (defined by claim 10) but has different compositions for electron confinement layer 24 and hole confinement layer 26. The composition of the electron confinement layer 24 is Zn0.52+0.48xCd0.48-0.48xSe1-0.46xTe0.46x and the composition of the hole confinement layer 26 is Zn0.52+0. 48yCd0.48-0.48ySe1-0.48yTe0.48y, where x and y are fractional concentration of relevant elements in the respective compounds. The value of x increases in a monotonic fashion in the 30 nm thick electron confinement layer 24 from 0 at the interface with the electron barrier layer 23 to 0.5 at the interface with the quantum well layer 25. The value of y increases in a monotonic fashion in the 50 nm thick hole confinement layer 26 from 0 at the interface with a quantum well layer 25 to 0.5 at the interface with a hole barrier layer 27.

Because the potential for electrons decreases in the electron confinement layer 24 towards the quantum well layer 25, electrons may effectively drift from the electron confinement layer 24 to the quantum well layer 25. On the other hand, the potential for holes decreases in the hole confinement layer 26 towards the quantum well layer 25, so that holes may be effectively drift from the hole confinement layer 26 to the quantum well layer 25. Consequently, electrons and holes may recombine effectively in the quantum well layer 25, allowing for a high quantum efficiency.

Although the relative concentrations x and y varies continuously in the eighth example, x and y may vary equally well in steps. It should be understood that the above electron confinement layer 24 and the hole confinement layer 26 may be fabricated using CHIRP superlattices instead of ZnCdSeTe systems.

Fig. 9 illustrates a band line-up diagram of a laser diode according to claim 15, which is obtained by partly modifying the compositions of the electron barrier layer 23. The electron barrier layer 23 has now composition Zn0.52+0.48xCd0.48-0.48xSe1-0. 48xTe0.48x, where x decreases in a monotonic fashion in the electron barrier layer 23 from 1 at the interface with an n -cladding layer 22 to 0 at the interface with a electron confinement layer 24. The electron barrier layer 23 has a thickness of 50 nm and Cl doping concentration of 1x1018 cm-3. Since there is no barrier for electrons at the interface between the n-claddig 22 and the electron barrier layer 23, electrons may be easily injected from the n-cladding layer 22 into the electron barrier layer 23 under low resistivity in laser operation. Lattice matching between the n -cladding layer 22 and the semiconductor substrate 21 also helps increase lasing efficiency.

Although the composition of the compounds layer 23 of Fig.9 is varied continuously, it may be varied in steps. Further, the layer 23 may be fabricated using semiconductor materials other than ZnCdSeTe systems.

As described above in detail, the invention utilizes II-VI compound semiconductors forming type II heterojuctions, which makes the hole potential low in the hole confinement layer and high in the hole barrier layer as well as in electron confinement layer, and makes the electron potential low in the electron confinement layer and high in the electron barrier layer as well as in the hole confinement layer, thereby facilitating effective confinement of holes and electrons in the respective confinement layers. Accumulation of each injected holes and electrons in the respective layers may result in effective recombination of the holes and the electrons in the heterojunction or the quantum well between the two confinement layers, emitting blue-green light, with a high quantum efficiency.

In addition to the recombination in the heterojuction, the quantum well and recombination centers having low potential for both holes and electrons may enhance such lasing, as described above.

The intensity of the light may be maximized in the region of the hole confinement layer and the electron confinement layer by appropriate choice of p- and n-cladding layers having sufficiently low refractive indices and by controlling the thicknesses of the hole barrier layer, the hole confinement layer, the electron confinement layer, and the electron barrier layer.