JP2624588B2 - Compound semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a p-type InP (hereinafter referred to as p-type InP).
Notated as -InP. Other This also applies to sign) single crystal substrate, relates the long wavelength semiconductor laser formed by the epitaxial growth method, in _{particular, In 1-x Ga x As} y P 1-y layer or InP laminated immediately above the active layer By providing a thin layer in which an n-type impurity is added at a high concentration from the hetero interface with the active layer, it is intended to provide a long wavelength compound semiconductor laser having excellent characteristics at a high yield.

[0002]

2. Description of the Related Art Conventionally, long-wavelength laser devices have been known as n-InP.
Double hetero (D) on single crystal substrate by various crystal growth methods
H) It is usually manufactured using a substrate on which a structure is epitaxially grown. On the other hand, when the p-InP single crystal substrate is used, the electric conduction type of the InP cladding layer becomes n-type, and the following advantages are obtained. First, n
Since the specific resistance of the -InP layer is smaller than that of the p-InP layer by one digit or more, the electrode forming process becomes extremely easy.
Furthermore, since the n-InP layer has a higher thermal conductivity, so-called junction down bonding, in which the epitaxially grown InP clad layer is bonded to a heat sink, can more efficiently radiate heat generated during operation. It has the feature of being able to. This is, in particular,
A long-wavelength laser device generally has a large operating current value and poor temperature characteristics, and thus has a great advantage when used at a high temperature.

[0003]

However, when a p-type InP single crystal substrate is used, diffusion or auto-doping of Zn, which is an impurity source of the substrate, is inevitable, so that the threshold current density increases and the luminous efficiency deteriorates. In addition, there arises a problem such as deterioration of device production yield, and it is generally difficult to produce a laser device having excellent characteristics with good reproducibility.

Therefore, when growing a DH substrate for a long wavelength LD using a p-InP single crystal substrate, it is practical to prevent Zn as an impurity source from diffusing from the substrate into the epitaxial growth layer. This is an essential condition for obtaining an LD element having characteristic characteristics. When _{In 1-x Ga x As y} P 1-y or In _1-x Ga _x As Zn at a high concentration in the active layer made of mixed crystal will be taken, or increase the light absorption loss of the active layer, Alternatively, the characteristics of the LD element are significantly deteriorated due to pile-up at the hetero interface between the InP layer and the active layer. For this reason, conventionally, the p-InP buffer layer is grown as thick as 1 to 3 μm directly above the substrate, and the active layer is kept as far away from the substrate as possible to prevent the deterioration of the characteristics of the LD element due to the diffusion of Zn. However, as is well known, the resistivity of the p-InP layer is one or two orders of magnitude higher than that of the n-InP layer, and increasing the thickness of the p-InP buffer layer increases the resistance of the LD element to n-InP. Inevitably, the problem is increased compared to the LD element on the InP substrate, and the amount of heat generated during operation of the element and high-speed operation characteristics are degraded. In addition, growing a thick layer leads to an increase in growth time and an increase in the consumption of growth raw materials, and as a result, there is a disadvantage in that the cost of an LD device manufactured is increased.

As a laser device using a p-InP single crystal substrate, it is possible to use a substrate epitaxially grown by a liquid phase growth method, which has relatively few problems of Zn diffusion and auto doping. However, if the epitaxial structure according to the present invention is applied, the same effect as the present invention can be expected.

An object of the present invention is to solve these problems when a double heterostructure for a laser device is grown on a p-InP single crystal substrate, and to provide a compound semiconductor epitaxial substrate having excellent characteristics at a high yield. It is in.

[0007]

According to the present invention, p-
And the InP layer having a thickness by epitaxial growth of 1.0μm or less directly on the InP single crystal _{substrate, In 1-x Ga x As} y P 1-y quaternary mixed crystal of a single layer formed on the InP layer In a double-heterostructure long-wavelength laser in which an active layer made of an active layer and an n-InP layer doped with an n-type impurity is further stacked on the active layer, n- An n ⁺ -InP thin layer to which an n-type impurity is added at a higher concentration than the impurity concentration of the InP layer is provided.

Further, an InP layer having a thickness of 1.0 μm or less epitaxially grown immediately above a p-InP single crystal substrate, and a multi-layered In _1-x Ga layer formed on the InP layer.
_x As _y P _1-y and the active layer made of quaternary mixed crystal or In _1-x Ga _x As ternary mixed crystal, further, n on the active layer
In a double-heterostructure long-wavelength laser in which an n-InP layer doped with a p-type impurity is stacked, the active layer has a band gap larger than that of the light emitting layer immediately above the light emitting layer, and the I
n is substantially lattice-matched to nP and is doped with an n-type impurity.
_{-In 1-x Ga x As y} P 1-y guide layer provided with, this n
_{-In 1-x Ga x As y} P 1-y guide layer on at least 10nm higher concentration n-type impurity over the hetero interface between the light emitting layer is added ^{_{n + -In 1-x Ga x}} As y P
_{A 1-y} guide layer is provided.

Further, the InP layer grown directly on the p-InP single crystal substrate is composed of an InP layer doped with a p-type impurity and an InP layer doped with no impurity on the active layer side.

[0010]

In the present invention, since InP layer n ⁺ -InP thin layer or _{^{n + -In 1-x Ga x}} As y P 1-y guide layer or the impurity is not added is provided,
Zn or p-In added to p-InP single crystal substrate
Even if Zn added to the P buffer layer diffuses into the active layer, the pn junction interface is not n-InP layer or n-In
_{1-x Ga x As y P} 1-y moves to the guide layer causes the can to prevent it, the characteristics of the LD element is not deteriorated.

[0011]

FIG. 1 shows a first embodiment of the present invention. Figure 1 shows a metalorganic vapor phase (MOCVD) on a p-InP single crystal substrate.
Oscillation wavelength 1.3μ formed by epitaxial growth
FIG. 2 is a sectional view of a main part of an m-band compound semiconductor laser, and FIG.
FIG. 3 shows an impurity concentration distribution in the embodiment of FIG. The horizontal axis in FIG. 2 represents the impurity concentration, and the vertical axis represents the film thickness corresponding to the sectional view in FIG. In FIG. 1, reference numeral 1 denotes a p-InP single crystal substrate, which generally uses zinc (Zn) as an impurity source, and has a concentration of approximately 5 × 10 ¹⁸ /.
cm ³ . 2 is 0.5 to 0.5% Zn as an impurity source.
This is a p-InP buffer layer that is epitaxially grown on the p-InP single crystal substrate 1 by adding about 1 × 10 ¹⁸ / cm ³ , and has a thickness of about 0.5 μm. Continued
About 0.1 μm of the InP buffer layer 21 to which no impurity is added.
_{m, In 1-x Ga x} As y P 1-y quaternary mixed crystal (x = 0.2
8, y = 0.61) after growing an active layer (light emitting layer) 3 of about 0.15 μm and then adding an n-type impurity to 1 × 10 ¹⁸ /
An DH structure is formed by sequentially laminating an InP layer added to about ³ cm ³ , that is, an n-InP clad layer 4 of 1 μm or more. Further, in the present invention, an n-type impurity is added to the hetero interface between the active layer 3 and the n-InP cladding layer 4 at a higher concentration (2 to 10 × 10 ¹⁸ / cm ³ ) than the n-InP cladding layer 4. The n ⁺ -InP thin layer 41 is inserted with a thickness of about 20 nm. By taking such a laminated structure, p
-InP buffer layer 2 and In
Although the P buffer layer 21 is grown only by about 0.6 μm, the threshold current density J _th = 1.6 to 2.0
KA / cm ² , external quantum efficiency η = 25-30% (double-sided cleavage, resonator length = 300 μm), characteristic temperature T ₀ = 50-6
An LD device having excellent characteristics of 0 ° C. was obtained with a high yield.

This is an effect of the present invention in which the InP buffer layer 21 and the n ⁺ -InP thin layer 41 doped with a high concentration are inserted above and below the active layer 3. When the thickness of the p-InP buffer layer 2 is as small as 0.6 μm at the most, the above-described excellent characteristics cannot be obtained with high yield. By inserting the InP buffer layer 21 to which the impurity Zn is not added, p-In
Zn added to the P buffer layer 2 or Zn from the substrate
Can be prevented from diffusing into the active layer 3 and deteriorating the characteristics of the laser element. Further, for some reason, p-In
When a crystal defect occurs at the hetero interface between the P buffer layer 2 and the active layer 3, Zn as an impurity source and this crystal defect form a complex, which significantly lowers the crystallinity of the double hetero crystal. By inserting the buffer layer 21 between the p-InP buffer layer 2 and the active layer 3, generation of a composite is prevented, and the yield of crystal growth is improved.

The n ⁺ -InP thin layer 41 is composed of the active layer 3 and the n-
Inserted at the hetero interface with the InP cladding layer 4, the impurity density is at least twice that of the n-InP cladding layer 4,
Usually, it was set to 2 to 10 × 10 ¹⁸ / cm ³ . A p-InP single crystal substrate 1 is used, and p-InP buffer layer 2 and In
The thickness of the P buffer layer 21 is 0.6 μm as in this embodiment.
In the case where the active layer 3 is as thin as possible, the active layer 3 is grown without adding impurities by diffusion of Zn as an impurity or auto-doping of Zn from the p-InP single crystal substrate 1 as shown by a dotted line in FIG. Thus, usually p = 10 ¹⁶ to 10 ¹⁷
P-type conduction having a carrier density of about / cm ³ . Furthermore, the impurity concentration of the p-InP single crystal substrate 1 is too high, or Z added as an impurity for some reason.
When the electrical activation rate of n is low, the diffusion coefficient of Zn becomes large, and the active layer 3 and the n-InP clad layer 4
Piles up at the hetero interface, and compensates for n-type impurities. Alternatively, as a result of a decrease in the activation rate of the n-type impurity due to crystal defects occurring at the hetero interface, n-
At the hetero interface of the InP cladding layer 4, a transition layer having a low carrier density over about 10 nm is formed. Originally, it is essential for the n-InP cladding layer 4 to exhibit n-type electric conduction in order to obtain good LD characteristics. If a low carrier density transition layer is formed at the hetero interface, as described above, Transformation into p-type due to Zn diffusion or auto-doping causes so-called remote junction, which significantly impairs LD characteristics. The existence of this transition layer is one of the reasons that a laser device using the p-InP single crystal substrate 1 cannot be manufactured with a high yield. In the present invention, as shown in the embodiment of FIG. -In
N with a high concentration of n-type impurity added between the P cladding layers 4
By inserting the ⁺ -InP thin layer 41, it is possible to prevent generation of a metamorphic layer at the hetero interface.

Although the addition of a high concentration of n-type impurities to the entire n-InP cladding layer 4 is effective in preventing the formation of a p-type metamorphic layer, it is well known that good crystallinity is obtained. It is difficult to grow an InP layer to which an n-type impurity is added at a high thickness while maintaining the thickness. Also,
If the carrier density of the n-InP layer is high, that is, if the specific resistance is unnecessarily low, there is also a problem that a leakage current is likely to occur when a buried LD element is formed.
Further, when the carrier density becomes 1 to 2 × 10 ¹⁸ / cm ³ or more, the light absorption loss due to free carriers increases sharply, and the problem of deteriorating the LD element characteristics occurs. Therefore, in the present invention, a high-concentration long-wavelength LD using the p-InP single crystal substrate 1 is inserted by inserting a high-concentration n ⁺ -InP thin layer 41 having a thickness of about 20 nm from the hetero interface with the active layer 3. The device can be manufactured with a high yield.

FIG. 3 shows a second embodiment of the present invention. FIG.
Is a DF having an oscillation wavelength of 1.55 μm band formed by MOVPE epitaxial growth on a p-type InP single crystal substrate 1.
FIG. 4 is a sectional view of a main part of the B compound semiconductor laser, and FIG. 4 shows an impurity concentration distribution thereof. 1, 2, and 21 are a p-InP single crystal substrate, a p-InP buffer layer, and an InP buffer layer to which no impurity is added, respectively, as in the first embodiment. Subsequently, the InP buffer layer 21 to which no impurity is added
In the nearly lattice matched to the bandgap wavelength of about 1.3μm to _{1-x Ga x As y P} 1-y buffer layer 31 of about 0.0
_{5μm, In 1-x Ga x} As y P 1-y mixed crystal (x = 0.4
2, y = 0.90) to about 0.10 μm.
After m-growth, n-type semiconductor having a bandgap wavelength of about 1.3 μm, which is almost lattice-matched to n-InP doped with n-type impurities.
0.1 the _{In 1-x Ga x As y} P 1-y guide layer 32
It grows 0.15 μm. When using a DFB laser,
A DH structure is formed by forming a grating on this guide layer and sequentially laminating an InP layer on which an n-type impurity is added at about 1 × 10 ¹⁸ / cm ³ , that is, an n-InP cladding layer 4 of 1 μm or more. . _{In 1-x Ga x As y} P
_{The 1-y} guide layer has a thickness of at least 10 nm from the hetero interface with the active layer 3 and is highly doped with n-type impurities.
A ^{_{+ -In 1-x Ga x As}} y P 1-y guide layer 33, 1 ×
N-In _1-x doped with about 10 ¹⁸ / cm ³
The Ga _x As _y P _1-y guide layer 32 is formed. High concentration of carrier concentration layer ^{_{n + -In 1-x Ga x}} As y P 1-y guide layer 33 has a dopant concentration of at least 2 × 10 ¹⁸ / cm ^3. By using such a layer configuration,
As described in the first embodiment, even if the thicknesses of the p-InP buffer layer 2 and the InP buffer layer 21 are made as thin as about 0.6 μm, an LD element having good characteristics is grown with high yield. can do.

In this embodiment, the oscillation wavelength is 1.55
μm, the In _1-x
By simply changing the composition ratio of the Ga _x As _y P _{1 -y} mixed crystal, it is of course possible to produce an LD device having another oscillation wavelength. Also, an In _1-x of the active layer 3 single composition Ga _x As _y
It was composed of P _1-y or In _1-x Ga _x As mixed crystal,
It goes without saying that the same effect can be obtained by using a superlattice structure instead. That is, the _{In 1-x Ga x As y} P 1-y mixed crystal bandgap wavelength is nearly lattice matched to InP of 1.0~1.35μm a barrier layer, I
The active layer 3 can be composed of MQW or SLS (strained superlattice or strained MQW) using nGaAs, InGaAsP or InGaAs1As mixed crystal as a well layer.

[0017]

As described above, according to the present invention, p-
In forming a long wavelength LD device using an InP single crystal substrate, a layer in which the carrier concentration of the n-type layer in contact with the p-type active layer is higher than the carrier density of the n-InP cladding layer is inserted into the pn junction interface. By doing, pI
Since the InP layer grown directly above the nP single crystal substrate is composed of the InP layer doped with a p-type impurity and the InP layer doped with no impurity on the active layer side, the p-InP buffer layer is made as thin as 1 μm or less. High yield and high performance L
D elements can be manufactured. Furthermore, p-InP
Since the thickness of the buffer layer can be reduced, the LD
The resistance value of the element can be reduced, which is advantageous not only for high-speed operation but also for reduced heat generation and advantageous for high-temperature operation.

[Brief description of the drawings]

FIG. 1 is a sectional view of a main part showing a first embodiment of a compound semiconductor laser of the present invention.

FIG. 2 is an impurity concentration distribution diagram in the embodiment of FIG.

FIG. 3 is a sectional view of a main part showing a second embodiment of the compound semiconductor laser of the present invention.

FIG. 4 is an impurity concentration distribution chart in the embodiment of FIG. 3;

[Explanation of symbols]

1 p-InP single crystal substrate 2 p-InP buffer layer 21 InP buffer layer 3 an active layer _{31 In 1-x Ga x As} y P 1-y buffer layer _{32 n-In 1-x Ga} x As y P 1-y guide layer _{^{33 n + -In 1-x Ga}} x As y P 1-y guide layer 4 n-InP cladding layer 41 n ⁺ -InP thin layer

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Trust Tsuzuki 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-2-33990 (JP, A) JP-A Heihei 2-126692 (JP, A) JP-A-3-77391 (JP, A) JP-A-3-227089 (JP, A) JPN. J. APPL. PHYS. P ART2 30-10A! (1991) p. L 1722-L1725

Claims (6)

(57) [Claims] 1. An epitaxially grown InP layer having a thickness of 1.0 μm or less directly above a p-InP single crystal substrate, and a single layer of In _1-x Ga formed on the InP layer.
an active layer made of _x As _y P _1-y quaternary mixed crystal, further, in the double heterostructure long wavelength laser n-InP layer doped with an n-type impurity on the active layer are laminated, the active layer A n ⁺ -InP thin layer to which an n-type impurity is added at a concentration higher than the impurity concentration of the n-InP layer over at least 10 nm from a hetero interface with the compound semiconductor laser. 2. An InP layer having a thickness of 1.0 μm or less epitaxially grown directly on a p-InP single crystal substrate, and a multilayer In _1-x Ga formed on the InP layer.
_x As _y P _1-y and the active layer made of quaternary mixed crystal or In _1-x Ga _x As ternary mixed crystal, further, n on the active layer
In the double-heterostructure long-wavelength laser in which an n-InP layer doped with a p-type impurity is laminated, the active layer has a band gap larger than that of the light emitting layer immediately above the light emitting layer, is substantially lattice-matched to InP, and comprising a _{n-in 1-x Ga x} as y P 1-y guide layer form impurity is added, hetero interface between the _{n-in 1-x Ga x} as y P 1-y guide layer to the light-emitting layer N ⁺ -In _{1 -x} Ga _x doped with a higher concentration of n-type impurity for at least 10 nm
A compound semiconductor laser comprising an As _y P _1-y guide layer. To 3. A side where the active layer is in contact with the InP buffer layer grown directly on p-InP single crystal substrate, an In ₁ to have a larger band gap than the light emitting layer, and without the addition of impurities nearly lattice-matched to InP _{-x Ga x As y P 1-} y compound semiconductor laser according to claim 1 or 2 having a buffer layer. 4. The compound semiconductor laser according to claim 2, wherein the light emitting layer has a multiple quantum well structure. 5. I grown on a p-InP single crystal substrate
0.1 to 2.0 × 1 Zn as a p-type impurity source in the nP layer
5. The compound semiconductor laser according to claim 2, wherein said compound semiconductor laser is added in a range of 0 ¹⁸ / cm ³ . 6. I grown on a p-InP single crystal substrate
5. The compound semiconductor laser according to claim 1, wherein the nP layer comprises an InP layer doped with a p-type impurity and an InP layer doped with no impurity on the active layer side. .