JPH06125135A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To control finely the wavelength of a laser oscillation by a method wherein the widths of a quantum well in a conduction band and a quantum well in a valence band can be respectively changed independently. CONSTITUTION:An N-type AlAs0.57Sb0.43 layer 2, an I-type AlAs0.57Sb0.43 layer 3, an I-type InP layer 4, an I-type GaAs0.22Sb0.78 layer 5, an I-type InP layer 6, an I-type AlAs0.57Sb0.43 layer 7, a P-type AlAs0.57Sb0.43 layer 8 and a P<+> InP layer are continuously made to perform an epitaxial growth on the surface of an N-type InP substrate 1. A strain quantum well is formed of the layer 5, of which quantum wells are respectively formed in a conduction band and a valence band. Electrodes 10 and 11 are deposited and thereafter, when the laminated material is made to perform a mesa epitaxial growth to form into an element of a stripe structure and both end surfaces of the element are cleaved to form one pair of reflectors, a Fabry-Perot laser can be formed. The quantum well in the conduction band has a width about twice wider than that of the quantum well in the valence band. By changing the strength of the strain of the GaAsSb layer in the center of a well layer, the luminous wavelength of the laser can be adjusted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser having an active layer containing a quantum well structure.

Semiconductor lasers are becoming increasingly important as light sources for optical communication and optical information processing systems. Along with the expansion of application fields, fine characteristics and performance improvement that meet each purpose are required.

[0003]

2. Description of the Related Art In recent years, a device using a quantum well structure as an active layer has been developed as a technique that greatly contributes to the performance improvement of a semiconductor laser.

In the quantum well structure, the degree of freedom of electron movement is 2 due to the quantum confinement effect of electrons in the film thickness direction. As a result, the density of states becomes stepwise with respect to energy, and the energy levels take discrete values in the quantum wells of the conduction band and the valence band, and the electrons are localized at each quantum level.

Since the electronic transition occurs between the quantum levels in the conduction band and the valence band, the threshold current density is reduced and the radiation monochromaticity is enhanced. The quantum well structure is obtained by alternately stacking superlattices of different semiconductor materials (thin layers of tens to hundreds of Å whose levels are quantized).

FIG. 2 is a schematic energy band diagram of a single well structure for the sake of simplicity. In the barrier layer made of the material A having a large bandgap energy Eg, Eg
The well layers made of the small material B are sandwiched.

The depth of the potential well of the well layer is ΔEc on the conduction band side and ΔEv on the valence band side. Since both the conduction band potential well and the valence band potential well are made of the semiconductor material B, their widths are the same on the conduction band side and the valence band side. That is, L _WC = L _WV =
L _W

The energy Ej (j = 1, 2, ...) Of the quantized electrons in each well is

[0009]

[Equation 1] … (1)

However, h is Planck's constant, and m ^*
Can be expressed as the effective mass. It can be seen from the equation (1) that the quantum level energy Ej is a function of the effective mass and the well width.

In the III-V group compound semiconductor, m
^{Since *} _e << m ^* _h , the quantum level spacing, for example,
| E ₁ −E ₂ | is overwhelmingly wider on the conduction band side. The recombination transition occurs between levels having the same quantum number, but the radiative recombination is practically sufficient considering the ground level of j = 1.

[0012]

In a semiconductor laser having a quantum well structure in the active layer, the semiconductor material of the quantum well layer is changed to change m ^* or the well width L _W to change Ej, that is, the emission wavelength. It is possible to shift.

As the application field of semiconductor lasers expands, it is necessary to finely adjust the oscillation wavelength. In the conventional quantum well laser, when the semiconductor material is changed, m ^* _e and m ^*
_{Since the h} changes at the same time and the effect of the lattice strain generated in the heterojunction between the barrier layer and the well layer is added to this, the wavelength adjustment is delicate. Further, when the well width is changed, its effect appears in both the conduction band and the valence band (L _WC = L _WV ), so that highly accurate wavelength adjustment is not always easy.

It is an object of the present invention to provide a quantum well laser capable of independently adjusting the energy of quantum levels in the conduction band and the valence band and controlling the oscillation wavelength more finely.

[0015]

A semiconductor laser of the present invention has an active layer including a quantum well structure having a quantum well width of a conduction band and a quantum well width of a valence band different from each other.

In order to form a quantum well having a conduction band quantum well width L _WC and a valence band quantum well width L _WV different from each other, the quantum well is constituted by both carrier bands (conduction band and valence band). A layer of semiconductor material that forms a part of the quantum well in one carrier band and a part of the quantum barrier in the other carrier band is connected to the layer of material .

[0017]

The basic concept of the quantum well of L _WC ≠ L _WV is shown in FIG. FIG. 1 (A) shows the case where L _WC <L _WV , and FIG. 1 (B)
Indicates the case of L _WC > L _WV .

In either case, the conventional quantum well structure has a laminated structure in which a thin layer of the semiconductor C is inserted between the semiconductor A which is a barrier layer and the semiconductor B which is a well layer. That is, the quantum well structure of the related art is A / B / A, whereas the quantum well structure of the present invention is A / C / B / C / A. The semiconductor C forms a quantum well in one carrier band and a potential barrier in the other carrier band.

As a result, in the quantum well widened by the insertion of the semiconductor C, from the equation (1),

[0020]

[Equation 2]

Since m ^* _h >> m ^* _e , Ej
It can be seen that the shift effect is larger in the case of FIG. 1B, that is, when the quantum well width of the conduction band is expanded. Hereinafter, the present invention will be described in more detail based on examples.

[0022]

EXAMPLE FIG. 3 shows an active layer quantum well structure and a laser device structure of a quantum well laser according to an example.

As shown in FIG. 3B, in the laser of this embodiment, a heteroepitaxial layer is continuously deposited on the n-type InP substrate 1 by using, for example, a metal organic molecular beam epitaxy (MOMBE) method. Can be formed with.

On the (100) plane of the n-type InP substrate 1, a 0.5 μm-thick Si-doped substrate having a carrier concentration of 5 × 10 ¹⁷ c
m ⁻³ n-type AlAs _0.57 Sb _0.43 layer 2, undoped i-type AlAs _0.57 Sb _0.43 layer 3 with a thickness of 30 nm, thickness 1 nm
I-type InP layer 4, 2 nm-thick i-type GaAs _0.22 Sb
_0.78 layer 5, i-type InP layer 6 with a thickness of 1 nm, thickness 100 n
m i-type AlAs _0.57 Sb _0.43 layer 7, Zn-doped, carrier concentration 5 × 10 ¹⁷ cm ⁻³ , p-type Al with a thickness of 1.5 μm
As _0.57 Sb _0.43 layer 8, Zn-doped carrier concentration 2 × 1
A 0 ¹⁸ cm ⁻³ p ⁺ type InP layer 9 is continuously epitaxially grown in this order.

The AlAs _0.57 Sb _0.43, which is lattice matched to InP, GaAs _0.22 Sb _{0. 78} has a larger lattice constant. Therefore, the i-type GaAs _0.22 Sb _0.78 layer 5 receives a compressive strain of about 2% from the layers on both sides. That is, i-type G that forms a quantum well in the conduction band and the valence band
The aAs _0.22 Sb _0.78 layer 5 forms a strained quantum well.

Due to compressive strain, m ^* _e is less affected, but m ^* _h is reduced. As a result, from equation (1), E
_1V is larger than that without distortion. That is, the shift is made downward in the figure. Although AlAs _0.57 Sb _0.43 is an indirect transition type semiconductor, GaAs _0.22 Sb in the radiative recombination region is used.
_0.78 is a direct transition semiconductor.

AuGe / Au electrode 10 as n-side electrode
And a Ti / Pt / Au electrode 11 as a p-side electrode, and then mesa-etched to form a stripe structure element.
When both ends are cleaved to form a pair of reflecting mirrors, a Fabry-Perot type laser is obtained as shown in FIG.

When this laser is forward biased, near-infrared light of about 1.5 μm oscillates. In the lasers described above, the conduction band quantum wells have, for example, approximately twice the width of the valence band quantum wells. i-type GaAs _0.22 Sb
_The i-type InP layers 4 and 6 arranged on both sides of the _0.78 layer 5 have an energy level difference of 0.1 eV with respect to the valence band of the i-type AlAs _0.57 Sb _0.43 layers 3 and 7 on both sides of the _0.78 layer 5. There is no hindrance.

Since the central GaAs _0.22 Sb _0.78 layer 5 has a built-in strain, its thickness is set to a critical thickness or less. Usually 2
A thickness in the range of 0Å to 100Å is selected. To obtain a larger change in transition energy by changing the width of the quantum well in the conduction band, the width G of the quantum well in the valence band is determined.
It is preferable that the aAs _0.22 Sb _0.78 layer 5 is thin.

Further, as described above, Ga at the center of the well layer is
It is possible to adjust the transition interval of the recombination region, that is, the emission wavelength by changing the composition of AsSb, that is, the magnitude of strain. If the compressive strain is increased, the emission wavelength shifts to the short wavelength side.

I on both sides of the i-type GaAs _0.22 Sb _0.78 layer 5
The thickness of the type InP layers 4 and 6 can be changed within the range of several Å to several hundred Å. At this time, E _1C is the i-type InP layer 4,
It can be reduced by about 0.01 to 0.2 eV as compared with the case where 6 is not present. That is, the emission wavelength is 0.01 to
It can be shifted to the 0.2 eV long wavelength side.

Also, the InP layers 4, 6 have a narrower bandgap than the i-type AlAs _0.57 Sb _0.43 layers 3, 7 on both sides thereof, and thus have a higher refractive index. Therefore, it also has a light confinement effect. The effect of reducing the threshold current density may occur due to this optical confinement effect.

L_WC> L_WVI explained that the case is
Width L of the quantum well of the child band_WVIs the width L of the conduction band quantum well_WCYo
May be wider. FIG. 4 illustrates another embodiment of the present invention.
1 shows a quantum well laser. FIG. 4A shows L _WV> L_WCAnd
FIG. 4 shows an energy band structure of a single quantum well structure shown in FIG.
4B is an element structure for realizing the energy band structure of FIG.
Shows the structure.

In FIG. 4B, the n-type InP substrate 21
On the (100) plane of, the n-type InP layer 22 having a thickness of 0.5 μm, a carrier concentration of 5 × 10 ¹⁷ cm ⁻³ , the i-type InP layer 23 having a thickness of 30 nm, and the i layer having a thickness of 3.5 nm. Type A
l _0.24 Ga _0.24 In _0.52 As layer 24, i-type Ga _0.47 In _0.53 As layer 25 with a thickness of 7 nm, i-type A with a thickness of 3.5 nm
A _0.24 Ga _0.24 In _0.52 As layer 26 is continuously heteroepitaxially grown in this order.

The three mixed crystal layers 24, 25 and 26 form a valence band quantum well, and the central mixed crystal layer 25 forms a conduction band quantum well. i-type Al _0.24 Ga _0.24 In
_{On the 0.52} As layer 26, an i-type InP layer 27 having a thickness of 100 nm, Zn doping having a thickness of 1.5 μm, and a carrier concentration of 5 ×
10 ¹⁷ cm ^-3 p-type InP layer 28, Z having a thickness of 0.5 μm
n-doped, p-type In with carrier concentration 2 × 10 ¹⁸ cm ⁻³
_{A 0.75} Ga _0.25 As _0.5 P _0.5 layer 29 is laminated.

The above heteroepitaxy is, for example, MO
It can be performed in a series of steps using the VPE method. A
l _0.24 Ga _0.24 In _0.52 As, Ga _0.47 In _0.53 As
Are almost lattice-matched to InP.

The AuGe / Au electrode 30 as the n-side electrode and the p-type InP layer 2 are formed on the back surface of the n-type InP substrate 21.
A Ti / Pt / Au electrode 31 is formed on p. 8 as a p-side electrode. The element structure is mesa-etched to form a stripe structure. Both end faces are cleaved to form a Fabry-Perot resonator.

[0038] In this embodiment, since the effective mass of electrons is hole effective mass is less than the ^{_{(m * e ≒ 0.1m * h}} ),
The change of the energy level corresponding to the change of the well width of E ₁ based on the equation (1) is about 1/10 as compared with the case of FIG.

However, when precise wavelength control is required, finer control becomes possible. In particular, in the case of this embodiment,
Since it is not necessary to consider the shift of the emission wavelength due to the distortion effect, the wavelength control can be performed more easily.

It is also possible to dope an impurity into a region that forms a barrier in both the valence band and the conduction band to form a modulation doping structure. Although the present invention has been described above with reference to the embodiments, the present invention is not limited thereto. For example, it will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

For example, the present invention can be applied to II-VI group compound semiconductors.

[0042]

Since the quantum well widths of the conduction band and the valence band can be changed independently, the laser oscillation wavelength can be finely controlled.

[Brief description of drawings]

FIG. 1 is a diagram showing the basic concept of the present invention.

FIG. 2 is an explanatory diagram of a quantum well laser according to a conventional example.

FIG. 3 is a diagram showing a quantum well laser according to an example.

FIG. 4 is a diagram showing a quantum well laser according to another embodiment.

[Explanation of symbols]

1, 21 n-type InP substrate 2 n-type AlAs _0.57 Sb _0.43 layer 3, 7 i-type AlAs _0.57 Sb _0.43 layer 4, 6, 23 i-type InP layer 5 i-type GaAs _0.22 Sb _0.78 layer 8 p-type AlAs _0.57 Sb _0.43 layer 9 p ⁺ type InP layer 10, 30 AuGe / Au electrode 11, 31 Ti / Pt / Au electrode 22 n type InP layer 24, 26 i type Al _0.24 Ga _0.24 In _0.52 As layer 25 i type Ga _0.47 In _0.53 As layer 27 i-type InP layer 28 p-type InP layer 29 p-type In _0.75 Ga _0.25 As _0.5 P _0.5 layer

Claims (5)

[Claims] 1. A semiconductor laser having an active layer including a quantum well structure in which a quantum well width (L _WC ) of a conduction band and a quantum well width (L _WV ) of a valence band in one quantum well have different values. 2. The semiconductor laser according to claim 1, wherein the quantum well structure has a strain therein. 3. A modulation-doped active layer in which impurities are doped only in a region of the quantum well structure that commonly acts as a potential barrier in both the conduction band and the valence band.
Alternatively, the semiconductor laser described in 2. 4. A GaInAs mixed crystal and an AlGaInAs mixed crystal are used as the quantum well material and InP is used as the potential barrier material, and the quantum well width in the valence band is made wider than the quantum well width in the conduction band (L _WC < L _WV ), The semiconductor laser according to claim 1. 5. InP and GaAs as the quantum well material
The Sb, using AlAsSb the material of the potential barrier, broadly the (L _WC> L _WV) than the quantum well width of the valence band of the quantum well width of the conduction band, a semiconductor according to claim 1 laser.