JPH07112090B2 - Semiconductor light emitting device - Google Patents

Description

Description: (a) Technical Field of the Invention The present invention relates to a semiconductor light emitting device, and more particularly to a semiconductor laser in which lights of a plurality of wavelengths are simultaneously emitted from the same optical resonator.

(B) Background of technology In optical communication and systems in which light is used as an information signal medium in various industries or consumer fields, a semiconductor light emitting device is the most important component, and a required wavelength band is realized and stable. Single fundamental zeroth order transverse mode oscillation, single longitudinal mode oscillation, reduction of light beam divergence angle, reduction of threshold current, improvement of linearity of current-light output characteristics, increase of output and temperature dependence of these characteristics A lot of efforts have been made to improve various characteristics such as reduction of properties, but it is particularly important to achieve stability of characteristics and long life.

(C) Prior Art and Problems Many semiconductor light emitting devices, particularly lasers, have been proposed to date for the above purpose, and one of them is a semiconductor laser having a quantum well structure.

The quantum well (Quantum Well) semiconductor lasers, which was not more than double heterostructure thickness of the active layer dough Buroi wavelength lambda of the carrier _d of _(GaAs in lambda _d ≒ 30 [nm]), the active layer is a quantum It functions as a mechanical well potential and becomes a two-dimensional electronic state in which the motion of carriers in the thickness direction is quantized. The quantum well laser includes a single quantum well laser composed of one well layer as an active layer and a Mu layer in which well layers and barrier layers are alternately laminated.
There is an lti Quantum Well laser.

An example of a known multiple quantum well laser is shown in FIG. 1 by a cross-sectional view. In the figure, 1 is n-type gallium
Arsenic (GaAs) substrate, 2 n-type GaAs buffer layer, 3 n-type aluminum gallium arsenide (AlxGa ₁ -xAs) clad layer, 4 multiple quantum well structure, well layer is GaAs,
Barrier layer is formed by AlxGa ₁ -xAs. Further, 5 is a p-type AlxGa ₁ -xAs clad layer, 6 is a p-type GaAs cap layer, 7 is a protective film, 8 is a p-side electrode, and 9 is an n-side electrode.

If the example of the composition of each semiconductor layer is illustrated in the above example,
This is as shown in FIG. However, corresponding parts are indicated by the same reference numerals as in FIG. In the conventional multiple quantum well laser, the composition of the barrier layer is not necessarily the same as the composition of the cladding layer, but the composition is the same between the barrier layers. The thickness of the barrier layer is also the same. Further, the composition and thickness of the well layers are the same between the well layers. Figure 3 shows the energy diagram of a GaAs quantum well sandwiched by AlxGa ₁ -xAs. In the figure, Lz represents the width of the quantum well, that is, the thickness of the GaAs well layer, and this Lz is equal to or less than the Dow-Bloey wavelength λ _d of the electron.

As shown in the figure, the conduction band barrier is given by the difference ΔEc in electron affinity between AlxGa ₁ −xAs and GaAs, and the valence band barrier is calculated from the difference in the forbidden band width Eg between AlxGa ₁ −xAs and GaAs. It is given by ΔEv minus ΔEc.

In the GaAs well layer, electrons and holes are confined by the barriers ΔEc and ΔEv, respectively, and their energy E is When the barrier height of the AlxGa ₁ −xAs barrier layer is infinite, Given in. However, Is Planck's constant, m ^* is effective mass, kx and ky are x- and y-direction components of the wave vector.

En is shown schematically in Fig. 3, Ehhn is a heavy hole Elhn
Corresponds to a light hole.

The densities of states of carriers having such energy are stepwise, and the densities of states at the band edges are significantly higher than those of three-dimensional free carriers. The selection rule for the radiative transition of carriers quantized as described above is Δn = 0. For example, the electron of E1 is recombined with the hole of Ehh ₁ or Elh ₁ . In the quantum well structure, electrons and holes are concentrated near the subband edge because the density of states at the subband edge is large as described above, and electron-hole recombination occurs between the subband edges. .

Regarding the quantum well laser including the quantum well structure having the characteristics described above, (a) the threshold current is low. (B)
The characteristic temperature To of the threshold current is larger than that of an ordinary double hetero laser, and the stability of the threshold current with respect to temperature rise is excellent. (C) It is easy to obtain single-mode oscillation. (D) Good linearity of current-light output characteristics.
(E) High differential quantum efficiency. (F) The oscillation wavelength can be designed by selecting the well width and barrier height. However, by using the quantum well structure, there is a possibility that a function which is impossible or extremely difficult in a conventional semiconductor laser can be realized.

(D) Object of the Invention It is an object of the present invention to provide a semiconductor light emitting device, especially a semiconductor laser, with a structure in which light of a plurality of wavelengths is emitted from one optical resonator in the same manner.

(E) Structure of the Invention The object of the present invention is to alternately stack well layers having a thickness equal to or less than the Dow-Bloey wavelength of an electron wave and barrier layers having a band gap larger than the well layers. A multiple quantum well structure, wherein the well layers include layers having at least one of thicknesses and compositions different from each other, and there are singular or at least approximately matching coincident energy levels. This is achieved by a semiconductor light emitting device in which there are a plurality of groups and the allowable energy levels are different between the groups.

That is, in the present invention, by configuring a quantum well structure that selectively combines the energy levels of carriers that determine the emission wavelength, for example, light having a wavelength in the infrared region and light having a wavelength in the visible band Can be simultaneously emitted by the same optical resonator.

The energy level of carriers is approximated by the above-mentioned equation (2). For example, the thickness Lz of the well layer made of GaAs sandwiched by the barrier layer made of AlxGa ₁ -xAs and the energy gap Eg are shown in FIG. It has a correlation as shown. However, the solid line in the figure shows the energy gap formed between electrons and heavy holes, and the broken line shows the energy gap formed between electrons and light holes. Actually, it is sufficient to consider the ground state of n = 1. Therefore, the energy gap Eg can be largely changed by changing the thickness Lz of the well layer.

In the AlxGa ₁ -xAs / GaAs-based quantum well structure of the above example, the well layer is made of Alx'Ga ₁ -x'As as well as GaAs.
The energy gap Eg can also be changed by selecting the composition of the semiconductor material forming the well layer, such as selecting the composition ratio x.

Therefore, the energy gap Eg corresponding to the target wavelength is
Can be easily realized by selecting one or both of the thickness and composition of the well layer, and the present invention can be realized by forming well layer groups having different energy gaps Eg in the same multiple quantum well structure. The purpose of is achieved.

(F) Embodiments of the Invention Hereinafter, the present invention will be specifically described with reference to the drawings by embodiments.

The present invention has various embodiments depending on the combination of conditions such as one or multiple well layers for each wavelength, or whether the current injection direction is vertical or horizontal depending on the pn junction. The cross-sectional view shown in FIG. 5 (a) shows a wavelength λ ₁ = 870 [n
m] and a wavelength λ ₂ = 780 nm, two well layers are provided for each of the two wavelengths, and a pn junction is formed in the longitudinal direction. An example is shown in FIGS. 5 (b) and 5 (c). The Al composition ratio of each semiconductor layer of the example is shown.

Specific examples of the semiconductor substrate and the semiconductor layer shown in FIG. 5A are as follows, and each semiconductor layer is sequentially grown on the semiconductor substrate by a molecular beam epitaxial growth method, a metal organic thermal decomposition vapor phase growth method, or the like. .

n ⁺ type GaAs substrate 11; thickness 100 [μm], impurity concentration 1 × 10 ¹⁸ [cm ^-13 ] n ⁺ type GaAs buffer layer 12; thickness 3.5 [μm], impurity concentration 1 × 10 ¹⁸ [cm ^-3 ] N-type Al _0.5 Ga _0.5 As clad layer 13; thickness 1 to 1.5 [μm], impurity concentration 3 to 5 × 10 ¹⁷ [c
m ^-3 ] non-doped first well layer 14a ₁ ; emission wavelength λ ₁ about 870 [nm] composition GaAs, thickness Lz ₁ 12 [nm] non-doped barrier layer 14b; composition Al0.3Ga0.7As, thickness L _B 4 [ nm] non-doped second well layer 14a ₂ ; emission wavelength λ ₂ about 780 [nm] composition GaAs thickness Lz ₂ 5 [nm] p-type Al _0.5 Ga _0.5 As clad layer 15; thickness 1 to 1.5 [μm], impurities Concentration 3 × 10 ¹⁷ [cm ^-3 ] p ⁺ type GaAs cap layer 16; thickness 0.5 [μm], impurity concentration 1 × 10 ¹⁹ [cm ^-3 ] or more However, the above values are typical values. FIG. 5B shows the Al composition ratio of each semiconductor layer of this example.

The n-type impurities are tin (Sn), tellurium (Te) or silicon (Si), the p-type impurities are beryllium (Be) in the molecular beam epitaxial growth method, and zinc (Zn) in the organometallic pyrolysis vapor phase growth method Cadmium (Cd) is suitable.

In the above embodiment, the first well layer 14a ₁ and the second well layer 14a ₂ are made of GaAs having the same composition, but the wavelength can be selected by selecting the composition, that is, the forbidden band width as described above. can do. For example, the non-doped second well layer 14a _{2 of} the above embodiment is modified to include the non-doped first well layer 14a ₁ ; the same non-doped barrier layer 14b as the above example; composition AlxGa ₁ -xAs, x = 0.3 to 0.5 thickness LB 4 [nm ] (Same as the above example) Non-doped second well layer Emission wavelength λ ₂ Approximately 780 [nm] (Same as above example) Composition Al0.15Ga0.85As Thickness Lz ₂ 12 [nm] Various choices are possible. is there. FIG. 5C shows the Al composition ratio of each semiconductor layer in the above embodiment.

In FIG. 5 (a), 17 is a protective film, 18 is a p-side electrode, and 19 is an n-side electrode.

Next, as a second embodiment of the present invention, the wavelength λ ₁ = 870 [n
m] and a wavelength λ ₂ = 780 nm, a plurality of well layers are provided for each of the two wavelengths, and a pn heterojunction is formed in the lateral direction. The Al composition ratio of is shown in FIGS. 6 (b) and 6 (c). The same parts as those in FIG. 5 (a) are designated by the same reference numerals.

In this embodiment, each semiconductor layer shown in the XX 'section in FIG. 6 (a) is epitaxially grown. The composition ratio of Al in each semiconductor layer on the XX 'section is as shown in FIG. 6 (b). The n ⁺ type GaAs substrate 11 and the n ⁺ type GaAs are shown in FIG.
The buffer layer 12 and the n-type Al0.5Ga0.5As clad layer 13 are the same as those in the above embodiment, and the Al0.5Ga0.5As clad layer 15 and
The GaAs cap layer 16 is n-type in this embodiment. However, the impurity concentration of each semiconductor layer is 1
It may be about × 10 ¹⁸ [cm ^-3 ].

In this embodiment, the quantum well structure is composed of the following two groups.

The first group MQW-1 Each of the plurality of well layers 14a ₁ and the barrier layer 14b ₁ below are laminated alternately, the wavelength lambda _{1 =} 870 [nm]
Emits light.

Well layer 14a ₁ ; Composition: GaAs, thickness Lz ₁ ; 12 [nm] Barrier layer 14b ₁ ; Composition: Al0.5Ga0.5, thickness LB ₁ ; 4 [nm] Second group MQW-2 Well layer 14a ₂ and barrier layer 1
4b ₂ and generates light of wavelength λ ₂ = 780 [nm].

Well layer 14a ₂ ; composition; GaAs, thickness Lz ₂ ; 5 [nm] or composition; Al0.15Ga0.85As, thickness Lz ₂ ; 12 [nm] barrier layer 14b ₂ ; composition; Al0.5Ga0.5, Thickness LB ₂ ; 4 [nm] The barrier layer 14b between MQW-1 and MQW-2 may have the same composition and thickness as the barrier layers 14b ₁ and 14b ₂ , but pn
In the present embodiment in which the junction is formed in the lateral direction, it is not necessary for carriers to be injected into the barrier layer by tunnel effect, and it may be thick enough. In this embodiment, the barrier layer 14b has a thickness of 0.1 [μm. ] It is about.

Although each semiconductor layer forming the quantum well structure described above may be non-doped, it is desirable to be an n-type having an impurity concentration of about 1 × 10 ¹⁸ [cm ⁻³ ].

A protective film 17 made of, for example, silicon nitride (Si ₃ N ₄ ) or silicon dioxide (SiO ₂ ) is selectively formed on the cap layer 16 of the semiconductor substrate having the above-described structure, and this is used as a mask to acceptor impurities such as zinc. (Zn) is diffused from the surface of the cap layer 16 to a depth reaching the cladding layer 13.

This impurity diffusion causes interdiffusion of composition atoms between the semiconductor layers that form the clad layer 15 and the quantum well structure, and Al atoms are distributed almost uniformly in the semiconductor region where the quantum well structure was formed. Becomes 20 indicates a p-type region formed in this way. In addition, FIG. 6 (c)
Indicates the composition ratio of Al in the YY ′ cross section including the p-type region 20.

Well layer 14a ₂ of the well layer 14a ₁ and MQW-2 of the quantum well structure MQW-1 is an AlGaAs small GaAs or Al composition ratio, p-type region 20 and pn of these well layers 14a ₁ or 14a ₂
The junction interface is a heterojunction interface. By forming the heterojunction in this way, the injection emission efficiency is improved.

In the second embodiment described above, the quantum well structures MQW-1 and MQW-2 for each wavelength are provided between the cladding layers, but the well layers are not sorted for each wavelength and The well layers may be alternately formed.

In any structure, the light generated in the well layer spreads between the clad layers, and the light of each wavelength is confined in the same space.

Although the above description is an example of the case of two wavelengths, it is possible to form a laser of three wavelengths or more in the same manner.

Although the above description has taken the GaAs / AlGaAs quantum well laser as an example, the present invention is not limited to other semiconductor materials such as InP / InGaAaP.
The same effect can be obtained by applying to a system or the like.

(G) Effects of the Invention As described above, according to the present invention, it is possible to emit light of a plurality of wavelengths from one optical resonator at the same time, and the application of laser light is greatly expanded. Further, according to the present invention, for example, by emitting visible wavelength light (wavelength 780 {nm] and the like shown in the embodiment) together with long wavelength light used for information transmission from the same optical resonator, it becomes possible to visually check laser light. This is effective for the assembly adjustment of optical application devices. In addition, it can be similarly applied to prevention of accidents caused by laser light.

[Brief description of drawings]

FIG. 1 is a sectional view showing a conventional example of a quantum well laser, FIGS. 2 (a) and 2 (b) are tables showing composition examples of respective semiconductor layers of the conventional example, FIG. 3 is an energy diagram of a quantum well,
FIG. 4 is a table showing the correlation between the thickness of the well layer and the energy gap, FIG. 5 (a) is a sectional view showing the first embodiment of the present invention, and FIGS. 5 (b) and 5 (c) are 6 is a table showing a composition example of each semiconductor layer of the embodiment, FIG. 6 (a) is a sectional view showing a second embodiment of the present invention, and FIGS. 6 (b) and 6 (c) are each of the embodiments. It is a chart showing a composition example in a section. In the figure, 11 is an n ⁺ type GaAs substrate, 12 is an n ⁺ type GaAs buffer layer, 13 is an n type AlGaAs cladding layer, 14a ₁ , 14a ₂ etc. are well layers, 14b, 14b ₁ , 14b ₂ etc. are barrier layers, 15 is p-type or n-type Al
GaAs cladding layer, 16 is p ⁺ type or n type GaAs cap layer, 17
Is a protective film, 18 is a p-side electrode, and 19 is an n-side electrode.

Claims (1)

[Claims] 1. A multi-quantum well structure in which a well layer having a thickness equal to or less than the Dow Bloey wavelength of an electron wave and a barrier layer having a forbidden band width larger than the well layer are alternately laminated, The well layers include well layers having at least one of thicknesses and compositions different from each other, and there are a plurality of groups each including a plurality of the well layers having singular or at least approximately matching allowable energy levels. A semiconductor light emitting device, wherein the allowable energy levels are different between the groups.