JPH0341791A - Semiconductor optical element - Google Patents

Abstract

PURPOSE:To realize a laser oscillation in a low threshold current and moreover, a high quantum effect in a quantum well type laser by a method wherein the side of a conduction band has an equivalent band end discontinuous energy value, which is formed of the lowest energy state of a miniband to be formed in a barrier layer by a superlattice structure and a well layer and is larger than that on the side of a valency band. CONSTITUTION:Minibands are formed in respect to electrons and holes in a superlattice structure. Energies of the minibands in the barrier of this superlattice structure are equivalent to the depths of wells to a quantum well layer, in short, become DELTAEc and DELTAEv. The energies DELTAE of the minibands are inversely proportional to the effective masses of the electrons and the holes. In an InGaAs(P) system, the effective mass of electrons is about 1/10 of that of holes. Accordingly, a DELTAEe of the miniband of electrons on the side of a conduction band, that is, the DELTAEc becomes about 10 times as large as a DELTAEh of the miniband of holes on the side of a valency band, that is, the DELTAEv. In such a way, a superlattice type quantum barrier is used in a system of DELTAEv>DELTAEc as well. Thereby, the relation of DELTAEc>DELTAEv can be made.

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD 1 The present invention relates to a semiconductor optical device, and particularly to a semiconductor optical device suitable for use in optical fiber communication, optical information processing, and optical measurement. [Conventional technology 1] A quantum well (QW) semiconductor laser has an increased relaxation oscillation frequency reflecting its quantum size effect. Because the spectral linewidth is reduced, it is expected to be used as a semiconductor laser for next-generation optical communications. Conventional InGa for optical communication
The value of the band edge discontinuity energy of the quantum well layer and the barrier layer of the AsP-based QW laser is larger on the valence band side than on the conduction band side. In other words, in a QW4I structure using InGaAs (P) as a quantum well layer and InP or InGaAsP as a barrier layer, the depth ΔEc of the well on the conduction band side is smaller than the depth ΔEv of the well on the valence band side (i.e. ΔEc<ΔE,). Therefore, the depth of the well felt by electrons is shallower than the depth of the well felt by holes. This ratio, that is, the value of the band edge discontinuity energy, especially the value of the conduction band discontinuity, that is, ΔEC/(ΔEc+ΔEv), is approximately 0.3 to 0 in this system.
．． It is 4. -Example: Applied, Physics, Letters 51. P, 24 (1987) (Appl.
Phys, Lett, 51, P, 24 (19
87)). In this conventional example, it is 0.38. [Problems to be Solved by the Invention] In the QW structure in which the relationship ΔEv>ΔEc as in the prior art described above, there is a problem that holes are difficult to be injected into each well layer. This will be explained using the band diagram shown in FIG. (aOb), in this example, TnGaA is used as the quantum well layer.
(a) is InP, (
b) The one using InGaAsP is shown. first,
In (a), the difference in the forbidden band width between the quantum well layer and the barrier layer is large (approximately 610 rneV), and ΔEc is approximately 232 meV.
, ΔEv is 378 meV. In this case, electrons and holes are sufficiently quantized, but the depth of the well on the hole side, which has a larger effective mass, that is, ΔEv, is too large, and holes are injected into the first well layer. It becomes difficult to inject into the second and third well layers. This is because the depth ΔEv of the well on the valence band side is so large that holes cannot overcome it. This in turn results in a significant increase in threshold current. Therefore, in the past, the barrier layer was made of InGaAsP and ΔEv
Efforts have been made to lower the value ((b) in the same figure). In this case, holes are injected into each well as shown in (b), but on the other hand, the depth ΔEc of the wells on the conduction band side also becomes smaller. In this case, the distribution of electrons with a light effective mass may exceed the energy of the well layer, as shown by the diagonal line in FIG. 2(b). There are electrons that are not confined in the well layer. As a result, the degree of quantization of electrons is reduced, and the quantum size effect of the quantum well cannot be sufficiently exploited. In this case, the threshold current is reduced, but the quantum size effect is reduced. As described above, conventional InGaAs/TnP. In the InGaAs/InGaAsP quantum well system, there is a trade-off relationship between the degree of quantum size effect and the ease with which holes can be injected into each well. This is because the depth ΔEv of the well on the hole side, which essentially has a heavier effective mass, is greater than ΔEc. On the other hand, in the GaAQAs/GaAs system, Δ
Ec is larger than ΔEv (ΔEC/(ΔEc+ΔEv
)! 0.7), there is no such trade-off. In other words, the quantum size effect can be fully exploited, and the carrier (
In order to facilitate the injection of electrons, holes) into each well layer, it is ideal that ΔEc on the electron side with a lighter effective mass be larger than ΔEv on the hole side with a heavier effective mass. . An object of the present invention is to artificially create the relationship ΔEC>ΔEv in a quantum well structure such as an InGaAs (P) system. (Means for solving !i problem) In a material system where ΔEC<ΔEv, ΔEC
In order to create the relationship >ΔEv, the present invention discloses that the barrier layer itself is formed with a superlattice structure. This is called a superlattice quantum barrier. A superlattice structure is a structure in which two types of semiconductors having different forbidden band widths are periodically stacked. The thickness of each film is approximately 30 Å or less. Reflecting this thin film thickness, the wave functions of electrons and holes are tunnel 1!
Due to the A-zoom, the semiconductors each having a small forbidden band width are coupled. This structure is very similar to a multiple quantum well structure, but each layer is thin and features a tunnel effect. This will be explained using the first band diagram. - As an example, the quantum well layer is made of InGaAs, and the superlattice quantum barrier, which is the key point of the present invention, has a periodic structure of an InGaAs well layer and an InPl wall layer. Typical values of these sizes are: The film thickness of the InGaAs quantum well in which electrons and face plates are localized is about 5 to 15 nm, at which the quantum size effect is sufficiently exhibited. Furthermore, the film thickness of the InGaAs well layer forming the superlattice quantum barrier is approximately 0.6
~3 nm, InPFJ! The thickness of the wall layer is approximately 0.6-3
In nanometers, the repeating period of these is about 2 to 10 periods, which is the range in which the features of the present invention are particularly noticeable. Further, it is particularly preferable to dope the entire barrier layer or the superlattice type quantum barrier layer forming the barrier layer with a high concentration of P-type impurity, since the characteristics are improved by the modulation doping effect. [Operation] The control mechanism of ΔEc and ΔEv according to the present invention will be explained below with reference to FIGS. 1 and 2. Within the superlattice structure, as shown in FIG. 1, mini-bands are formed regarding electrons and holes. This miniband is a quantum level within the superlattice structure. That is, it is an energy band formed within the superlattice structure. The energy of the miniband within this superlattice barrier becomes the depth of the well with respect to the quantum well layer, that is, ΔEC and ΔEv. This mini-band is generated by the degree of coupling of wave functions between each well in the superlattice, and corresponds to the quantization level in the superlattice. It is something. Now, this quantization level energy Δ
E can be approximately expressed as. Here, m is the effective mass and L is the well width in the superlattice. In other words, mini band energy ΔE
is inversely proportional to the effective mass. In the InGaAs (P) system, the effective mass of electrons is about 1/10 of the effective mass of holes. Therefore, the electron mini-band ΔEe, ie, ΔEc, on the conduction band side is about 10 times larger than the hole mini-band ΔEh, ie, ΔEv, on the valence band side. In this way, even in a system where ΔEv is larger than ΔEc, by using a superlattice quantum barrier, ΔE c
>ΔEv relationship can be created. This is due to the fact that electrons have a smaller effective mass than holes and are more easily quantized.The above points have been explained qualitatively, but the results of actual rigorous calculations are shown in FIG. In this calculation, we used the Kronitzg-Benny model, which is very commonly used in superlattice miniband calculations. Here, the well in the superlattice is made of InGaAs, and the barrier is made of InP, and the horizontal axis of the figure is the well thickness, and the parameter is the barrier thickness. When the horizontal axis is O, it represents the so-called ΔE c and ΔEv of the material, and it can be seen that (ΔE(:/(ΔEc+ΔEv)) is 0.38. Typical ΔE c and ΔE of this superlattice barrier. If you indicate the value, type ■ (well:
, 2 nm, barrier: 3nrn), (ΔEc/(
ΔEc+ΔEV) is o, 63, 0.59 for type ■ (well and barrier both 1 nm), and 0.59 for type ■ (well: 2.5 nm).
, Barrier: Inm). It can be seen that ΔEc is sufficiently larger at 0.69. Further, when the ΔEc of these types was measured by the CV method, the values were almost the same as the calculated results. In this way, the relationship ΔEc>ΔEv can be expressed as InGaAs(P
) system was realized for the first time. This made it possible to easily inject holes while maintaining a sufficient quantum size effect. InG with relaxation oscillation frequency fr taking these into account
FIG. 6 shows the calculated value of the aAs quantum well cap dependence. The vertical axis is a value normalized by f of a DH (double hetero) laser. In this way, according to the present invention, InGaAs (
P) system QW laser can be expected to have f more than double.

Example 1 The present invention will be described in detail below. Example 1 In FIG. 4, a multi-quantum well active [94,p-InP cladding M5]n-InP cap N6 is grown on an n-InP substrate 3 by MOCVD. Here, the multi-quantum well active M4 is made of InGaAs with a film thickness of 5 to 15 nm.
Periodic structure of quantum well layer and superlattice type quantum 'R wall (period: 1
~20). Moreover, this superlattice quantum barrier has a film thickness of 0.6 to 3 nm.
nGaAs or InGaAsP (1g>1.5μm
) Well layer and InP or InG with a film thickness of 0.6 to 3 nm
Formed with a periodic structure (period = 2 to 20) of aAsP (1g>1.15μrn) barrier layer. Thereafter, the 5in2 film 7 is shaped and partially removed, and Zn diffusion 8 is performed to create a stripe region (11g: 2 to 10 μm). After this, a p-electrode 9 and an n-electrode 1o, which serve as carrier injection means, are formed. The prototype device has an extremely low threshold current of 10 to 20 mA, reflecting the reduction in ΔEv, and a relaxation oscillation frequency f, reflecting the sufficient quantum size effect. is extremely high at approximately 30 GHz at 5 mW output. Embodiment 2 FIG. 5 shows the present invention applied to a distributed feedback (DFB) laser. A film of n-InGaA with a thickness of 0.05 to 0.25 μm is formed on an n-InP substrate 3 on which a diffraction grating is formed.
sP (λg' = 1, 1 to 1, 3 μm) light guide layer 12, multiple quantum well activity N4 similar to the example construction,
Then, a p-InP cladding layer is crystal-grown by selective gas source MBE. After that, after forming a convex stripe penetrating the growth layer, a p-InPn soil layer, an n-
Embed with InPn soil layer. After this, p electrode 9, n electrode 1
form 0. Here, the active layer width is approximately 0.5 to 3 μm. The prototype device oscillates at a threshold current of 5 to 15 mA, and reflects the DFB structure to become a longitudinal single mode with a sub-mode of 50 dB. Furthermore, f at 5 mW is extremely high at approximately 30 GHz. FIG. 7 shows a summary of the experimental values of the threshold m current and fr (at 5 ohm) for various QW lasers. First I
In the case of an nP barrier, the QW effect is large and f is high (about 25 GI-Iz), but the injection of holes into each quantum well is insufficient, so the threshold current is extremely high at about 100 mA. On the other hand, Q using an InGaAsP barrier with λg = 1.3 μm
With a W laser, ΔEv can be made small, so holes can be easily injected into each quantum well, and the threshold current is low. However, since ΔEc is also low, electrons leak into the barrier layer, resulting in a low QW effect.
fr becomes as low as 10 GHz. Also, λ,=1.
A 15 μm InGaAsP barrier QW laser is in between. In other words, with the conventional method, it was impossible to simultaneously satisfy high f and low threshold current. Compared to these, QW lasers using superlattice quantum barriers have high f
, and low threshold current can be achieved for the first time. Additionally, chirping during high-speed modulation is approximately 30% that of conventional QW lasers.
becomes extremely small. Example 3゜ In Fig. 8, the structure is similar to Example 1゜2 on an n-InP substrate, but Zn is added only to the superlattice barrier layer.
X 10"~I X 10"cm-3 doping,
P-type modulation doped structure multi-quantum well active layer 15 and p-I
An nP cladding layer is grown by MOCVD. After that, a convex stripe (0.5 to 3 μm as active layer width)
After forming, it is filled with Fe-doped high-resistance InpH6, and further a p-side electrode 9 and an n-side W1 electrode 10 are formed. The parasitic capacitance of the prototype device is low due to the introduction of high resistance InP, and fr is 5 due to the modulation doping effect and QW effect.
0GHz can be achieved. Example 4 A ninth example in which the present invention is applied to a semiconductor optical phase modulator is described below.
This will be explained using figures. Example 1 on the n-InP substrate 3,
After sequentially growing a multi-quantum well structure similar to 2 and a p-InP cladding layer 5, ridge-type stripes with a typical width of 2 to 10 .mu.m are formed by etching as shown in FIG. After that, a p-side electrode 9 and an n-side electrode 10 are formed, and then separated into elements. A laser beam with a wavelength of 1.3 μm is incident on one end surface of the prototype optical phase modulator, and by changing the amount of current injected into the p-side electrode, that is, the amount of carriers injected into the multi-quantum well layer 4, the optical phase can be changed. Controls the phase of the output light from the modulator. Since the refractive index change of this optical phase modulator is as large as 5X10-'' reflecting the high QW effect, the length of the modulator for changing the phase by π is approximately 30 μm, which is unprecedentedly short. Furthermore, it goes without saying that the optical phase modulator of the present invention can be used as a basic element of a Matsuha Cender type transformer, etc. >ΔEv can be created.This can be applied, for example, to the fabrication of InGaAsP-based qw4, and with this, holes can be smoothly injected into each well layer while maintaining a high quantum effect. As a result, it is possible to realize a low threshold current and a high quantum effect (high fr, low chirping) in a QW laser of this system.

[Brief explanation of drawings]

Figures 1, 2, 6 and 7 are for explaining the principle structure and effects of the present invention. Figures 3 (a) and (b) are
) is a conventional QW tank structure band diagram, FIGS. 4, 5(a, ), and 8 are cross-sectional views of the embodiment according to the present invention, and FIG. 5(b) is a band diagram of the conventional QW tank structure. A' line sectional view, No. 9
The figure is a bird's eye diagram of an embodiment according to the present invention. 1...Superlattice quantum barrier. 2...well layer, 4...multi-quantum well active layer. Number (α) (b) Figure 3 Figure 3 S Figure A' (Sink) (b+ Figure 5 1015 J, I&aA, si7 company/' width (9 star) 3rd group rod S Figure A' (sink) (b+ Figure 5 1015 i (Labor J) Figure q

Claims (1)

[Scope of Claims] 1. A multiple quantum composed of a well layer in which the band edge discontinuity energy value on the valence band side is larger than the band edge discontinuity energy value on the conduction band side and a barrier layer having a superlattice structure. The well structure is such that the equivalent band edge discontinuity energy value formed between the lowest energy state of the mini-band formed in the barrier layer by the superlattice structure and the well layer is on the conduction band side and on the valence band side. What is claimed is: 1. A semiconductor optical device characterized by having a diameter larger than . 2. The semiconductor optical device according to claim 1, further comprising means for injecting carriers into the multiple quantum well structure. 3. The semiconductor optical device according to claim 1, wherein the superlattice structure is formed of a plurality of semiconductor layers having a layer thickness of 5 to 30 Å. 4. The semiconductor optical device according to claim 1, wherein the barrier layer has a region doped with a conductivity type impurity at a density of 2×10^1^8 cm^-^2 or more. 5. A barrier layer of a multi-quantum well structure consisting of a plurality of well layers and a barrier layer is formed by a superlattice structure consisting of a plurality of semiconductor layers, and mini bands formed within this superlattice structure allow carriers to be distributed between the well layers. A semiconductor optical device characterized by increased mobility. 6. In the semiconductor optical device according to claim 6, the energy band structure formed by the well layer and the barrier layer has a band edge discontinuous energy value on the valence band side and a band edge discontinuous energy value on the conduction band side. A semiconductor optical device characterized by having a multiple quantum well structure larger than . 7. It has a multi-quantum well structure consisting of a plurality of well layers for carrier recombination light emission and a barrier layer separating these well layers, and the barrier layer is a superlattice structure consisting of a plurality of semiconductor layers. A semiconductor optical device characterized in that carriers in the multiple quantum well structure move between the plurality of well layers via mini-bands formed inside the superlattice structure. 8. In the semiconductor optical device according to claim 7, by making the equivalent band edge discontinuity energy value formed by the mini band and the well layer smaller on the valence band side than on the conduction band side, A semiconductor optical device characterized in that holes in the valence band are easily injected into the well layer.