JP2002368342A - Multiplex quantum well semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multiplex quantum well semiconductor element in which uneven hole and electron implantation to each of quantum well is suppressed. SOLUTION: A substance having n-type conductivity is used as an impurity for the quantum well layer of this multiplex quantum well semiconductor element, and only the quantum well layer is doped, or it is mainly doped with the substance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser (hereinafter, referred to as an LD (Laser Diode)) as a light source in fields such as optical communication and optical measurement, and a super luminescent diode (hereinafter, SLD).
)) And light-emitting diodes (hereinafter referred to as LED
Emitting Diode)), and a semiconductor direct optical amplifier (hereinafter SOA (Semiconductor Optica
An active layer where electrons and holes recombine, such as an MQW (Multiple Quantum Well)
Quantum Well)).

[0002]

2. Description of the Related Art In an MQW structure, a quantum well layer composed of a semiconductor having a narrow band gap and barrier layers composed of a semiconductor having a band gap wider than the quantum well layer are alternately laminated. The thickness is set to about the de Broglie wavelength of electrons in the semiconductor, while the barrier layer is set to a thickness where the tunneling probability of electrons can be ignored. For example, MQW-
To briefly describe the LD structure, the MQW structure is formed by a cladding layer made of a semiconductor having p-type and n-type conductivity through a separate confinement layer and the like from the opposite side along the stacking direction axis. Electrodes are formed in these cladding layers so that carriers corresponding to their conductivity can be injected.
With such a layer structure, a quantum confinement effect of electrons and holes in the quantum well layer appears, and excellent device characteristics that cannot be obtained with a bulk-type layer structure in which the quantum effect does not work are obtained.

The movement of carriers, that is, electrons and holes in the MQW of such an MQW-LD will be described as follows. Electrons and holes injected into the MQW from the n-type and p-type conductive cladding layers, respectively, have three-dimensional degrees of freedom.
Transport near the band edge of the barrier layer in the MQW is mainly carried out by the diffusion process. These three-dimensional carriers are trapped in the quantum well during the transport process, become two-dimensional carriers that allow freedom of movement only in the two-dimensional plane within the quantum well, and then undergo stimulated emission through radiative recombination to generate laser light. Is obtained.

[0004] InP used in LD of 1480 nm light wavelength band
Taking the example of a quantum well formed by epitaxially growing a multilayer structure of InP and GaInAsP mixed crystals on a substrate,
Heterogeneous barrier on the valence band side is higher than that on the conduction band side
1.3 to 1.6 times higher. Of the careers mentioned earlier,
There are "light holes" and "heavy holes"
Heavy holes occupy the majority of holes due to their high density of states, and heavy holes with strong degeneracy due to large effective mass have a higher heterobarrier on the valence band side. After the heavy hole is once captured by the quantum well and turned into a two-dimensional heavy hole,
It becomes difficult to emit heavy holes in the three-dimensional state emitted from the quantum well.

That is, most of the heavy holes injected from the p-type conductive cladding layer into the MQW are trapped in the quantum well closest to the p-type conductive cladding layer in the MQW, and the n-type conductive cladding is The number of heavy holes reaching the quantum well near the layer is extremely small, and the quantum well in the MQW has a nonuniform heavy hole density distribution. Additionally, the mobility of the heavy holes in the valence band edge near the barrier layer ^{(70~80 cm 2 V - 1 s} - about ^1), for example electrons compared to (3500 cm ² V ^- about ^{1 - 1} s) Transport speed is as low as about 10 ³ m / s because of the extremely small size, and especially in a state where the LD is operated at a high light output, the consumption due to stimulated emission of carriers in the quantum well becomes remarkable. Even in the region of several tens of nanometers (nm), the change in the distribution of heavy hole density is large, and as shown in FIG.
It is higher on the p-type conductive cladding layer side, and the unevenness is further increased.

Further, since the carriers are arranged in the MQW so as to satisfy the positive condition of the charge, the electrons are also distributed so as to correspond to the distribution of the heavy holes, so that the carriers trapped in the respective quantum wells in the MQW. The closer the density is to the p-type conductive cladding layer, the higher the density becomes, resulting in non-uniform carrier injection into the quantum well. Such a phenomenon in MQW,
There has been a problem in that the number of quantum wells in operation is reduced, and the optical gain coefficient and differential gain coefficient of each quantum well are not uniform, so that the potential of the LD cannot be exploited.

That is, these problems are described in, for example, the literature, A.
Hangleiter, A. Grabmaier, and G. Fuchs, "Damping
of the Relaxastion Resonance in Multiple-quantum-
wellLasers by Slow Interwell Transport ", Appl. Ph
ys. Lett., Vol. 62 (19), pp. 2316-2318, 1993
As shown in (1)), since the heterobarrier on the valence band side is high, the trapping lifetime of a heavy hole in the three-dimensional state is captured by the quantum well, and the hole is released from the quantum well to the heavy hole in the three-dimensional state. Significantly shorter than the release lifetime, which is the process performed, and see, for example, the literature, CH Lin, CL Chua,
ZH Zhu, and YH Lo, "On Nonuniform Pumpiung
for Multiple-quantum Well Semiconductor Lasers ",
Appl. Phys. Lett.Vol.65 (19), pp.2383-2385, 1994
As described in [Reference 2]], the transport of heavy holes is reduced due to the low mobility of heavy holes.

[0008] These problems have been studied through experiments and theories in the past.
N. Tessler and G. Eisenstein, "On Carrier Injecti
on and Dynamics in Quantum Well Lasers ", IEEE J.
Quantum Electron., Vol. 29, pp. 1586-1595, 1993 [Reference 3]], and it has been pointed out that non-uniformity of carrier injection into the quantum well in the MQW exists.

[0009]

SUMMARY OF THE INVENTION In a semiconductor optical device such as an LD having an MQW having a conventional structure, the internal differential quantum efficiency and the differential gain coefficient decrease due to uneven carrier injection into each quantum well in the MQW. Therefore, suppression of device characteristics such as a decrease in optical output and a decrease in direct modulation band has occurred. In order to avoid this problem, it is necessary to reduce the height of the valence band side hetero barrier between the quantum well and the barrier layer. By reducing the height of the valence band side hetero-barrier between the quantum well and the barrier layer in this manner, the density of heavy holes emitted from the quantum well per unit time is increased, that is, the emission lifetime is shortened. You. When the rate at which heavy holes are emitted from the quantum well increases, such heavy holes may be trapped in the original quantum well, but the proportion of those trapped in other quantum wells will increase. The distribution of heavy hole density in each quantum well in MQW can be made uniform. However, since the height of the valence band side hetero-barrier between the quantum well and the barrier layer is determined by the combination of the crystal and the mixed crystal used for the quantum well and the barrier layer, the height of the hetero-barrier must be directly controlled. Is not done. In order to improve this problem, for example, in the above-mentioned document 1), an oscillation light formed by growing an MQW structure with a quantum well of GaInAs mixed crystal and a barrier layer of InGaAlAs mixed In an LD with a wavelength of about 1500 nm,
Simulations show that the distribution in the MQW is more uniform than in the conventional structure.

This is based on the following reasons. In the conventional MQW structure in which the quantum well and the barrier layer are both made of GaInAsP on the InP substrate, the valence band side hetero barrier height is 1.3 to 1.6 times the conduction band side hetero barrier height, as described above. Is a GaInAs / InGaAlAs based MQW grown on InP substrate
In this case, the height of the valence band side hetero-barrier is less than half the height of the conduction band side hetero-barrier, so that the emission lifetime of heavy holes from the quantum well is greatly reduced. But this
GaInAs / InGaAlAs based MQWs grown on InP substrates have the following problems. For example, in an LD, the width of the light emitting region is set to 1-2 μm by chemical etching in order to stabilize the oscillation light mode and to reduce the threshold injection current.
m, and buried growth is applied to this,
Stabilization of the oscillation light mode and reduction of the threshold injection current are indispensable steps. However, in a mixed crystal system containing Al, such as GaInAs / InGaAlAs MQW, an extremely stable Al oxide is formed on the surface. Is extremely difficult.

In addition, literature, Mitsuki Yamada, et al., "GaAsSb
Long Wavelength Surface Emitting Laser ", IEICE Technical Report, LQE 99-133, (20
00-02) In [Reference 4]], a GaAsSb / GaAs surface emitting semiconductor laser has a so-called modulation-doped structure in which only the barrier layer in the MQW is doped with a p-type conductive impurity. A technique for reducing the height of the electron band side hetero-barrier has been reported.

The mechanism is briefly described as follows. Of the holes generated from impurities doped in the barrier layer, only those near the quantum well are trapped in the quantum well due to the electric field generated between the ionized acceptor and the hole, while the others are captured in the barrier layer. remaining,
The valence band side Fermi level in the barrier layer is kept high. On the other hand, since the holes trapped in the quantum well come from the vicinity of the quantum well in the barrier layer as described above, the hole density in the quantum well does not increase so much. The valence band-side Fermi level of the quantum well is relatively low due to the large density of states resulting from the large effective mass of. In the steady state, the bands are arranged so that the Fermi level between the barrier layer and the quantum well substantially coincides with each other, so that the valence band side hetero-barrier height is reduced as shown in FIG. However, this report shows that although the injection efficiency of holes into the MQW quantum well is improved, the threshold injection current density is reduced, but the optical absorption loss is also increased. As described above, when the light absorption increases, for example, in the LD, there is a problem in that an optical output decreases due to a decrease in slope efficiency, and an undesirable characteristic such as oscillation line width expansion and wavelength chirping deteriorates due to an increase in a line width increase coefficient. There is.

The present invention has been made in order to solve such a problem. The present invention has been made to reduce the effective valence band side hetero-barrier height between a quantum well and a barrier layer, and to reduce the height of each quantum well. An object of the present invention is to provide a multiple quantum well semiconductor device in which non-uniformity of hole and electron injection is suppressed.

[0014]

According to a first aspect of the present invention, there is provided a semiconductor substrate having a first conductivity on a semiconductor substrate having a first conductivity. Layer, an active layer made of a semiconductor, a semiconductor clad layer having a second conductivity and a contact layer made of a semiconductor having a second conductivity are sequentially laminated, and on the surface of the semiconductor substrate and the surface of the contact layer. The active layer in the double hetero structure in which the electrodes are formed, wherein the active layer is formed in contact with the first conductive clad layer, and has a bandgap higher than that of the first conductive clad layer. And a semiconductor having a smaller band gap than the second conductive cladding layer formed in contact with the second conductive cladding layer. Between the Li Cheng separate confinement layer,
In a multiple quantum well semiconductor device having a layer structure in which quantum well layers and barrier layers are alternately stacked, and the quantum well layers are two or more layers, a material that makes the quantum well layers n-type conductive is doped with impurities. The present invention provides a multiple quantum well semiconductor device characterized in that only the quantum well layer or mainly the quantum well layer is doped.

Further, according to the second aspect of the present invention, the quantum well layer is doped with an n-type conductive material as an impurity by doping only the quantum well layer or mainly by doping the quantum well layer. The multiple quantum well semiconductor device according to the first aspect, wherein the obtained electron density in the quantum well layer is at least 2.3 times or more the electron density of the barrier layer.

Further, according to a third aspect of the present invention, the electron density in the quantum well layer is from 1 × 10 ²⁴ m ⁻³ to 3
The multiple quantum well semiconductor device according to the second aspect, which is between × 10 ²⁴ m ⁻³ , is provided.

The multiple quantum well semiconductor device of the present invention has a III-
Taking MQW made of Group V compound semiconductor as an example, Si, S,
A substance that makes this compound semiconductor n-type conductive, such as Ge, Se, Sn, or Te, is doped as an impurity into only the quantum well or mainly the quantum well in the MQW. As described here, impurities generate electrons as carriers,
Electrons have an effective mass that is a fraction to a tenth smaller than holes, so the state density on the conductor side is low, and the Fermi level of the conductor is easier than the valence band side with the same doping amount. There is an advantage to rise.

In addition, the so-called absorption between valence bands, which occurs due to photoexcitation of electrons from the split-off band on the valence band side to the heavy hole band, occurs on the valence band side when the hole density increases, but the conductor side does not. Has an advantage in that such a light absorption mechanism is not provided since the band has a simple structure. In addition, impurities such as Si have low self-diffusion and a sharp doping profile, and, for example, n-type conductivity in GaInAsP / InP system
The GaInAsP-based mixed crystal is compared with the p-type conductive one in FIG.
As shown in the example of InP, the electrical resistivity is one or more orders of magnitude smaller, which is a very advantageous property for a semiconductor element operated by current injection.

The effect of reducing the effective valence band side hetero-barrier height between the quantum well and the barrier layer according to the present invention and having an effect on the uniformity of the carrier density in the MQW will be described. In the case of a GaInAsP / InP MQW-LD as an example, usually doping is not performed in the MQW region to avoid an increase in light absorption loss. However, this remains in the conduction band hetero barrier height (V _c) relative to the valence band hetero barrier height (V _hh)
Is increased from 1.3 to 1.6 times, for example,
), N. Tessler and G. Eisenstein, "On Carrier In
jection and Dynamics in Quantum Well Lasers ", IEE
E J. Quantum Electron., Vol. 29, pp. 1586-1595, 1993
According to the above, in the range of the combination of the band gap of the quantum well and the barrier layer used conventionally, the emission lifetime of the heavy hole from the quantum well becomes several tens to 1000 times longer than the capture lifetime.

On the other hand, in the present invention, the effective valence band side hetero-barrier height (V _hh-eff ) between the quantum well and the barrier layer is reduced by the mechanism described below. FIG.
(A) is a band lineup between a quantum well and a barrier layer in an undoped MQW used for a normal LD or the like. On the other hand, in the present invention, the impurity having MQW as the n-type conductor is doped only into the quantum well or mainly into the quantum well. _F-QW ) is much higher than the barrier layer level ( _EF-Barri. ). In this state, the bands are arranged so that the Fermi levels of the quantum well and the barrier layer coincide with each other, and a quasi-equilibrium state is established. At this time, as shown in FIG. 1B, the effective valence band side hetero-barrier height (V
_hh-eff ) can be made lower than the valence band side hetero-barrier height (V _hh ) in the case of an undoped quantum well. In addition, the electrons in the quantum well diffuse into the barrier layer, so that the band is distorted at the interface between the quantum well and the barrier layer, and a spike is formed at the valence band edge of the barrier layer. Spikes are narrow due to the electric field between the ionized donor and the electrons diffused into the barrier layer, allowing holes to be easily captured and released from the quantum well by tunneling Becomes

In the present invention, the electron density of the quantum well and the barrier layer and the effective valence band side hetero-barrier height (V
_hh-eff ) is shown in Fig. 2 for the 1480 nm optical wavelength band strain MQW-LD, but it is effective as the electron density in the quantum well ( _NQW ) increases regardless of the barrier layer electron density ( _NBarr. ) _. The valence band side hetero-barrier height decreases with almost the same slope. As is clear from FIG. 2, the lower the electron density of the barrier layer is, the lower the hetero barrier height on the valence band side is, for example, in the process of fabricating a multiple quantum well in a buried semiconductor laser, diffusion of impurities, etc. By 10 ²¹
From m ^-3 to 10 ²² m ^-3 carrier density, the electron density (N _Barr. ) of the barrier layer should be set to 1
It is realistic to set to 1 × 10 ²³ m ^{-3 which} is about an order of magnitude higher. Further, N _Barr as ^{_{1 × 10 24 m -..}} 3 or more and made the valence band side hetero barrier height reduction effect becomes weaker it 2 Karawakari, N _Barr 1 × 10 ²³ as a range of m ^{- Three}
From 1 × 10 ²⁴ m ^-3 is appropriate.

From the above, the condition that the valence band side hetero-barrier height can be reduced by a small N _{Barr. Should} be the central one of the three curves in FIG.
The electron density in the quantum well at which the hetero-barrier height on the valence band side indicated by this curve is lower than the barrier height (V _hh ) of the conventional structure is 1 × 10 ²⁴ m ⁻³ . On the other hand, doping such that the electron density in the quantum well exceeds 3 × 10 ²⁴ m ⁻³ impairs the crystallinity of the quantum well, so that the electron density of the quantum well in the actually manufactured multiple quantum well is 1 × 10 ²⁴ m ^-3
It is appropriate to be between ^{3 and 1024} m ^-3 .

For example, when the electron density of the barrier layer is N_Barr.: 0.
1 × 10^{twenty four} m^-3And 1 × 10^{twenty four} m ^-3If it
In each case, the electron density of the quantum well is N_QW : About 0.7 × 10^{twenty four} m
^-3And about 2.3 × 10^{twenty four} m^-3Above, effective
The valence band side heterobarrier height is shown by the oblique lines in FIG.
Valence band side heterobarrier height of undoped quantum wells in the doped region
Sa (V_hh) Can be reduced. This thing
The electron density in the quantum well is at least
Above 2.3 times the effective valence band heterogeneity
The barrier height can be reduced.

From the ease of controlling the doping amount of the barrier layer, N _{Barr .} : about 0.1 × 10 ²⁴ m ⁻³ is suitable as an MQW manufacturing condition. Therefore, the height of the hetero-barrier on the conduction band side under this doping condition (V FIG. 4 shows the relationship between _c-eff ), the valence band side hetero-barrier height, and the electron density in the quantum well. By making MQW under such conditions, N _QW : about 0.8
At 7 × 10 ²⁴ m ^-3 (arrow A in the figure), V _hh-eff
Lower than V _c-eff and N _QW : from about 1.5
At 2 × 10 ²⁴ m ^-3 , V _hh-eff is _equal to V _c-eff
From 1/2 to 1/3, V _hh-eff is significantly reduced from 60 meV to about 90 meV compared to V _hh . Here, the above-mentioned electron density can be easily achieved because the ionization rate of the donor impurity is extremely high.

Next, the length of the resonator consisting of four quantum wells is
 Optical output of 1480 nm optical wavelength band LD with 1 mm (P_OUT)
Is 400 mW, the height of the valence band side heterobarrier is reduced.
Quantity (ΔV_hh) Is MQW when 0 meV, 40meV, 80meV
Density of heavy holes in three-dimensional state (P_3D) And M
The relationship of the position (x) in QW is as shown in FIG. _hh 
Is larger, the emission lifetime of holes from the quantum well is shorter.
To become P_3D Will be higher.

Table 1 is a table showing the effective reduction of the valence band side hetero-barrier height according to the present invention and the relaxation of the bias in the MQW distribution of heavy holes having three-dimensional degrees of freedom. MQW
X: 0 nm and x: 80 nm at the ends of the region
In the density of heavy holes in the three-dimensional state at
The difference between _{3D (0)} and P _{3D (80 nm )} at x: 80 nm is almost the same at any ΔV _hh . P
_{3D (0)} and the difference between P _{3D (80 nm)} and P _{3D (0)} P
_{The 3D (80 nm)} arithmetic mean divided by the MQW area
Defining the non-uniformity of the heavy hole density in the three-dimensional state, ΔV
_{As hh} increases, this non-uniformity decreases as shown in Table 1.

[0027]

[Table 1]

FIG. 6 shows the relationship between ΔV _hh and the injection current into the four individual quantum wells at an optical output of 400 mW of this LD. In the conventional structure, the quantum well (N _QWW) closest to the p-side cladding layer is obtained. : 1) and the farthest quantum well (N _QW : 4) the difference in injection current is about 40 mA,
When V _hh take 80 meV example, this difference can be suppressed to about 6.4 mA becomes 1/6.

Thus, by adopting the present invention, the number of operating quantum wells can be reduced in the MQW,
The problem of non-uniformity of the optical gain coefficient and differential gain coefficient of each quantum well and the inability to extract the potential of LD
The problem can be solved without increasing the light absorption and lowering the buried crystal growth quality.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIGS. 7 and 8 through a manufacturing procedure of a semiconductor laser device to which the present invention is applied. Here, the material used is InP, which allows good buried growth.
aInAsP-based mixed crystal is used.

First, an n-type conductive InP buffer layer 2 is grown on an n-type conductive InP substrate 1 by a metal organic chemical vapor deposition method or the like. Next, after growing a separation confinement layer made of n-type conductive GaInAsP mixed crystal and a spacer layer made of undoped GaInAsP mixed crystal to a thickness of about 2 μm, 15 nm and 10 nm, respectively, several nm to several tens nm The MQW is formed by alternately growing quantum wells and barrier layers having a layer thickness of, and during this growth, the quantum wells and barrier layers, such as Si, S, Ge, Se, Sn, and Te, have n-type conductivity. Is doped as an impurity into only the quantum well or mainly into the quantum well.

That is, when the doping results in the electron density distribution shown in FIG. 8A, the band gap profile of the MQW of the conventional structure shown in FIG. 8B is deformed as shown in FIG. The valence band side hetero barrier height is reduced. The barrier layer and the quantum well have no distortion,
Any mixed crystal that applies compression strain and extension strain may be used. On this, a spacer layer made of undoped GaInAsP mixed crystal and a separation confinement layer made of p-type conductive GaInAsP mixed crystal are grown to form an active layer 3 consisting of an MQW structure and a spacer layer and a separation confinement layer.

Subsequently, a p-type conductive InP clad layer 4 is grown to produce a multi-layer semiconductor substrate 5 having an active layer as shown in FIG. Next, after forming a stripe-shaped etching resistant mask 6 made of a dielectric film or the like having a width of about several μm on the multilayer structure semiconductor substrate 5,
Multi-layer semiconductor substrate using bromine-based etchant
5 is etched to a position deeper than the active layer 3 to remove the multi-layered semiconductor other than the portion protected by the etching resistant mask 6 described above. As shown in FIG. The substrate 7 on which the stripe is formed is manufactured. A p-type conductive InP first buried layer is formed on the mesa-formed stripe substrate 7 by metalorganic vapor phase epitaxy or liquid phase epitaxy.
8 and n-type conductive InP second buried layer 9 are sequentially buried and grown, then the etching resistant mask 6 is removed, and a contact layer 10 of p-type conductive GaInAsP mixed crystal is formed thereon.
Is grown to complete a buried crystal growth substrate 11 as shown in FIG.

The buried crystal growth substrate 11 has a thickness of 100 μm.
After polishing the n-type conductive InP substrate 1 side until
This polished n-type conductivity InP substrate surface and p-type conductivity
On the crystal growth surface on the GaInAsP contact layer 10 side,
u-Ge and Au-Zn are deposited by a vacuum evaporation method, and heat treatment is performed to form an n-side electrode 12 and a p-side electrode 13.
The ohmic electrode forming substrate 14 shown in d) is completed. Subsequently, the ohmic electrode forming substrate 14 was cleaved and cut at intervals of several hundred μm to several mm in order to make the resonator length in the vertical direction of the mesa stripe, thereby forming a semiconductor laser bar in which a plurality of embedded mesa stripes were arranged in parallel. rear,
This bar has a width of several hundred μm around the mesa stripe.
The semiconductor laser chip is completed by cutting at intervals.

The GaInAsP / InP-based semiconductor laser device in which a GaInAsP mixed crystal and an InP crystal layer are grown on an InP crystal substrate has been described above.
Not limited to this crystal and mixed crystal system, for example, InGaAlAs / In
The present invention uses not only III-V group compound semiconductors such as P, GaInAsP / GaAs, AlGaAs / GaAs, and AlGaInP / GaAs series but also, for example, compounds containing VII atoms such as Cl as dopants, thereby enabling the present invention to use II-VI group compounds. It is clear that the present invention can be applied to an MQW structure made of a compound semiconductor.

Since the present invention relates to the MQW structure, all other parts except the MQW structure in the first embodiment are the same, and other embodiments will be described. FIG. 9 shows a second embodiment. In this embodiment, the height of the valence band side hetero-barrier is higher than that of the conduction band side hetero-barrier, such as GaInAsP-based mixed crystal formed on an InP substrate or AlGaInP-based mixed crystal formed on a GaAs substrate. The present invention is applied to tunable LDs, SLDs, and SOAs made of higher mixed crystal materials, and the optical wavelength spectrum width and gain amplification band are particularly expanded compared to these conventional devices. I do.

The wavelength tunable LDs, SLDs and SOAs are shown in FIG. 9 (b) in order to expand the optical wavelength band in which these elements operate.
The device structure using MQW as the active layer, which is composed of quantum wells having different band gap energies as in (3), has been tried. However, when the device is actually manufactured and operated,
The expansion of the optical spectrum width and the gain amplification band has a problem that there is only a small amount centering on the optical gain spectrum of the quantum well having the smallest band gap energy. The reason for this is that in the MQW with the lowest bandgap energy in the MQW having this structure, the emission lifetime of holes from the quantum well is much higher than the capture lifetime compared to the quantum well with the larger bandgap. This is because the holes are long and the injected holes are concentrated in the quantum well having the smallest band gap. To alleviate this problem, the valence band side hetero-barrier height may be the same between quantum wells having different band gap energies in the MQW.

The present invention can also achieve such an object. As shown in FIG. 9A, a substance having n-type conductivity in the quantum well and the barrier layer is provided in the MQW. Among the plurality of quantum wells, the smaller the quantum well of the band gap, the higher the impurity doping amount is set, and the higher the electron density is. By doing so, as shown in FIG. 9 (c), the valence band side hetero-barrier height can be the same between the quantum wells having different band gap energies in the MQW. The carrier injection into the quantum well becomes more constant, and the emission wavelength spectrum width and gain amplification band of LD, SLD and SOA are expanded.

On the other hand, it has been described earlier that a spike occurs in the band lineup near the heterojunction interface between the barrier layer and the quantum well layer. Since the width of the spike is narrow, a heavy hole relatively easily passes through the spike due to the tunnel effect, but it is clear that the absence of the spike has a greater effect of promoting the transport of holes. FIG. 10 shows an example of a structure for suppressing spikes at the valence band edge in the present invention.

FIG. 10A does not have the spike suppressing structure of the present invention, and spikes are generated near the hetero interface. On the other hand, as shown in FIG. 10 (b), a spike is formed by forming a transition barrier layer having a band gap larger than the quantum well and smaller than the barrier layer between the barrier layer and the quantum well. Can be suppressed. The transition barrier layer only needs to have a band gap larger than the quantum well and smaller than the barrier layer, so that not only crystals and mixed crystals lattice-matched to the substrate but also strain can be applied to change the band gap. It is clear that what is done also holds.

FIG. 10 (c) shows an example in which a superlattice transition barrier layer is formed between the barrier layer and the quantum well to generate a mini band of holes at a portion where a spike is formed, thereby promoting hole transport. It is. FIG. 10D shows an example of a structure in which a spike at the valence band edge is suppressed by forming a low-doped transition barrier layer that is lower doped than the barrier layer between the barrier layer and the quantum well. . In this structure, a potential convex portion is generated on the conduction band end side, but electrons do not hinder electron transport because the effective mass of an electron is about one digit smaller than that of a hole. By adopting such a structure, the operation of the present invention is further promoted, and the optical gain coefficient and the differential gain coefficient of each quantum well in the MQW are realized to achieve the LD, SLD and SOA.
Etc. can be fully exploited.

[0042]

According to the multiple quantum well semiconductor device of the present invention, only the quantum well layer or mainly the quantum well layer is doped with a substance having an n-type conductivity for the quantum well layer as an impurity. To provide a multiple quantum well semiconductor device in which the effective valence band side hetero-barrier height between the quantum well and the barrier layer is reduced, and the nonuniformity of hole and electron injection into each quantum well is suppressed. Can be.

With respect to the multiple quantum well semiconductor optical device, LD, SLD, LED and S
As compared with semiconductor optical devices such as OA, it is possible to provide a semiconductor optical device with improved optical output, expanded direct modulation frequency band, and improved optical amplification gain. When the present invention is applied to MQW-LD, the carrier density injected into the quantum well in the MQW becomes uniform, so that the ratio of the increase of the threshold injection current to the increase of the injected carrier density decreases, In particular, the linearity of the injection current-optical output characteristics in the high output operation is improved. Further, since the light absorption loss is reduced as compared with the p-type modulation doped structure, the light output can be improved as compared with this structure. In addition, in the MQW structure with a large number of quantum well layers, operation can be performed with a high differential gain when compared with the same injection current. Higher performance can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of reducing the effective valence band side hetero-barrier height according to the present invention.

FIG. 2 is a diagram showing a relationship between an effective hetero barrier height on an effective valence band side and an electron density in a quantum well depending on an electron density of a barrier layer.

FIG. 3 is a diagram showing a range having a remarkable effect in a relationship between an electron density in a barrier layer and an electron density in a quantum well.

FIG. 4 is a diagram showing the relationship between the electron density in a quantum well and the effective hetero-barrier heights on the conduction band side and the valence band side.

FIG. 5 is a diagram showing the effective reduction of the valence band side hetero-barrier height and the mitigation of the bias of the distribution of heavy holes having three-dimensional degrees of freedom in the MQW according to the present invention.

FIG. 6 is a diagram showing the effective reduction of the valence band side hetero-barrier height according to the present invention and the current injected into the individual quantum well in the MQW.

FIGS. 7A and 7B are diagrams showing a manufacturing process of a semiconductor laser device to which the present invention is applied, wherein FIG.
(B) shows a substrate on which a mesa stripe is formed;
FIG. 3A is a diagram showing a buried crystal growth substrate, and FIG. 4D is a diagram showing an ohmic electrode formation substrate.

8A and 8B are diagrams for explaining the first embodiment of the present invention, in which FIG. 8A shows an electron density distribution in an MQW, FIG. 8B shows a band gap profile of an MQW having a conventional structure, and FIG. )
FIG. 3 is a diagram showing a band gap profile of MQW in the present invention.

9A and 9B are diagrams for explaining a second embodiment of the present invention, in which FIG. 9A shows an electron density distribution in an MQW, FIG. 9B shows a band gap profile of an MQW having a conventional structure, and FIG. )
FIG. 3 is a diagram showing a band gap profile of MQW in the present invention.

10A and 10B are diagrams for explaining an example of a structure for suppressing spikes at the valence band edge in the present invention, wherein FIG. 10A shows a band gap profile without a spike suppression structure, and FIG. (C) shows the bandgap profile when the barrier layer is provided to form the spike suppressing structure, (c) shows the bandgap profile when the superlattice transition barrier layer is provided to form the spike suppressing structure, and (d) shows the lightly doped transition barrier layer. FIG. 9 is a diagram illustrating band gap profiles when a spike suppressing structure is provided by providing the same.

FIG. 11 is a diagram illustrating a relationship among conductivity, carrier density, and resistivity of InP.

FIG. 12 is a diagram showing a bias of a heavy hole density distribution having three-dimensional degrees of freedom in a conventional MQW.

FIG. 13 is a diagram showing the principle that the effective valence band side hetero-barrier height is reduced by p-type modulation doping in the prior art.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 n-type conductive InP substrate 2 n-type conductive InP buffer layer 3 active layer 4 p-type conductive InP clad layer 5 multi-layered semiconductor substrate 6 striped etching resistant mask 7 mesa-formed striped substrate 8 p-type conductive InP first buried layer 9 n-type conductive InP second buried layer 10 contact layer 11 buried crystal growth substrate 12 n-side electrode deposited with Au-Ge 13 p-side electrode deposited with Au-Zn 14 ohmic Electrode forming substrate

Claims (3)

[Claims] A first conductive substrate having a first conductivity;
A semiconductor clad layer having a conductive property, an active layer made of a semiconductor, a semiconductor clad layer having a second conductive property, and a contact layer made of a semiconductor having a second conductive property, and The first conductive layer, wherein the active layer is formed in contact with the first conductive clad layer in a double hetero structure in which electrodes are formed on a substrate surface and the contact layer surface, respectively. A separation confinement layer made of a semiconductor having a smaller band gap than the cladding layer having conductivity, and a band gap formed between the cladding layer having the second conductivity formed in contact with the cladding layer having the second conductivity. A quantum well layer and a barrier layer are alternately stacked between a separation confinement layer made of a semiconductor having a small size and a quantum well layer having a layer configuration in which the quantum well layer is two or more layers. In well semiconductor device, wherein a quantum well layer and the quantum well layer material as an impurity to n Katashirube Den resistance only, or multiple quantum well semiconductor device characterized by mainly doping the quantum well layer. 2. An electron density in the quantum well layer obtained by doping only the quantum well layer or mainly by doping the quantum well layer with a substance that makes the quantum well layer have n-type conductivity. 2. The multiple quantum well semiconductor device according to claim 1, wherein is equal to or more than 2.3 times the electron density of the barrier layer. 3. The electron density in said quantum well layer is 1 × 10 ^24
3. The multiple quantum well semiconductor device according to claim 2, wherein the value is between m- ³ and 3 * 10 < ²⁴ > m < ^-3 >.