JP2739999B2 - Method for manufacturing semiconductor light emitting device - Google Patents

Description

The present invention relates to a semiconductor light emitting device using an InGaAlP-based compound semiconductor material, and particularly to the manufacture of a semiconductor light emitting device using a double heterojunction epitaxial wafer. About the method.

(Prior art) In recent years, chemical vapor deposition (hereinafter referred to as MOCV)
D method), it is possible to form InGaAlP on a GaAs substrate, and a visible light semiconductor laser using this technology has been attracting attention.

FIG. 5 is a cross-sectional view showing a schematic structure and a manufacturing process of a rib waveguide laser using an InGaAlP-based material. In order to produce this laser, first, as shown in FIG.
An n-InGaAlP cladding layer 52 and an InGaP active layer 5 on a GaAs substrate 51.
3. A p-InGaAlP cladding layer 54, a p-InGaP contact layer 55, and a p-GaAs cap layer 56 are sequentially formed by MOCVD. Thereafter, as shown in FIG. 5 (b), an SiO ₂ film 57 is formed on the cap layer 56, and this is used as a mask to form the cap layer 56.
Then, the contact layer 55 is etched, and the p-cladding layer 54 is further etched to a desired depth to form a mesa stripe.

Next, as shown in FIG. 5C, an n-GaAs block layer (current blocking layer) 58 is selectively grown on the side of the mesa stripe using the SiO ₂ film 57 as a mask. Subsequently, after removing the SiO ₂ film 57 and the cap layer 56, as shown in FIG.
A GaAs ohmic layer 59 is grown. Here, Si is used for the n-type impurity and Zn is used for the p-type impurity in the substrate and each layer.

However, the semiconductor laser fabricated by this type of method has the following problems. That is, in a semiconductor laser using an InGaAlP-based material, when Zn is used as a p-type impurity, it is said that a carrier concentration obtained under a constant doping condition is lower than that in a GaAlAs-based material (for example, Journa).
l of Crystal Growth 77 (1986) 374-379). In particular, Al
As the composition ratio increases, it becomes more difficult to grow a crystal with a high carrier concentration, and the carrier concentration tends to be saturated even when the Zn supply amount is increased. On the other hand, from the viewpoint of device design, a crystal having a high Al composition ratio is desired for the p-cladding layer to provide a sufficient refractive index difference from the active layer, and high-concentration p-type doping with a high Al composition is required.

An internal current confinement type laser as shown in FIG. 6 is a laser having a gain guide structure which can be formed in two crystal growth steps including a double heterojunction equivalent to the rib waveguide type laser. To produce this laser, first, as shown in FIG. 6 (a), in the first crystal growth step,
A layer structure similar to that of the rib waveguide type laser contact layer 55 is formed, and an n-GaAs block layer 66 is continuously grown. Thereafter, as shown in FIG. 6B, a part of the block layer 66 is etched to form a striped window. Next, as shown in FIG. 6C, a p-GaAs ohmic layer 59 is regrown on the contact 55 layer and the block layer 66 exposed at the window.

In the manufacturing process of the laser shown in FIGS. 5 and 6, the double hetero junction structure (the n-cladding layer 52 to the contact layer
The growth of 55) is performed under the same conditions, and the carrier concentration of each layer after growth is examined. As a result, the two layers are equal in the p-cladding layer 52, but different in the Zn-doped p-cladding layer 54. The carrier concentration of the epitaxial wafer of Example) becomes low. Zn 5 × in the structure of FIG. 6 in the doping conditions in which the carrier concentration is saturated with respect to the supply amount of 10 ¹⁷
When cm ^-3 is obtained, in the structure of FIG. 5 2.5 × 10 ¹⁷ cm ^-3
The carrier concentration is only low.

When the Zn concentration of the p-cladding layer 54 in the epitaxial crystal was examined by SIMS (Secondary lon Mas Spectrometry) analysis, it was shown that the incorporated Zn concentration itself was equivalent. This suggests that the ratio (activation rate) of Zn incorporated into the crystal as a carrier is different between the two, and cannot be improved by increasing the amount of absorbed Zn.

In the case of forming a structure in which a layer containing Zn, P, or the like having a high vapor pressure or Al or the like which is easily oxidized is formed on the surface, a cap layer (laser of several hundreds to several thousand mm) is used to prevent deterioration of the surface layer. Layer covering surface that does not directly contribute to the structure)
Is provided, but this is not effective against the problem that the carrier concentration of the Zn-doped p-cladding layer 54 decreases as described above.

Further, the present inventors have studied and found that the p-GaAs cap layer 56 shown in FIG. 5A was replaced with the n-Ga cap layer 56 shown in FIG.
Even when the thickness is the same as that of the As block layer 66 and when the heat treatment at the same temperature and at the same time as the growth of the block layer in FIG. The carrier concentration of the p-cladding layer 54 in the laser was not improved.

In this type of semiconductor laser, the carrier concentration of the p-cladding layer is one of the important parameters that determine the device characteristics. If a sufficiently high carrier concentration cannot be obtained, the oscillation threshold current will increase and the temperature characteristics will decrease. It may cause a decrease.
Further, the same problem can be applied not only to a semiconductor laser but also to a light emitting diode and the like.

(Problems to be Solved by the Invention) As described above, in the production of a semiconductor laser using an InGaAlP-based material, the growth of a double heterojunction (n-cladding layer, active layer, p-cladding layer) is performed in one crystal growth step. Despite the same conditions, the carrier concentration of the Zn-doped p-cladding layer differs depending on the presence or absence or composition of the subsequently grown layer. In the case of forming, there is a problem that a sufficiently high carrier concentration cannot be obtained.

The present invention has been made in view of the above circumstances, and an object thereof is to increase the carrier concentration of a p-clad layer in a rib waveguide type, and to reduce the temperature characteristics due to the low carrier concentration. It is an object of the present invention to provide a method for manufacturing a semiconductor light emitting device that can suppress and contribute to improvement of element characteristics.

[Structure of the Invention] (Means for Solving the Problems) The gist of the present invention is that when a semiconductor light emitting device is manufactured using a double heterojunction structure made of InGaAlP, it is formed on the uppermost layer after the growth of the double heterojunction structure. The object is to increase the carrier concentration of the p-cladding layer by forming the n-GaAs layer.

That is, the present invention provides a method for manufacturing a rib waveguide type semiconductor light emitting device, wherein a double heterojunction structure portion made of In _1-Y (Ga _1-X Al _X ) _Y P is grown on an n-type GaAs substrate, and After growing an n-type GaAs cap layer on the double heterojunction structure, the cap layer is removed, a mesa stripe is formed on the double heterojunction structure, and then an n-type GaAs current is formed on the side of the mesa stripe. In this method, a blocking layer is grown, and then a p-type GaAs ohmic layer is grown on the double heterojunction structure and the current blocking layer.

(Operation) According to the present invention, after the formation of the double hetero junction structure,
By growing the n-GaAs layer as the uppermost layer, a heat treatment effect equivalent to that of the laser shown in FIG. 6 is added to the p-cladding layer. As a result, Zn incorporated in the p-cladding layer
In the first crystal growth step of forming the double heterojunction structure part, the activation rate of the semiconductor is increased to obtain a high carrier concentration equivalent to that of a laser formed by growing up to the n-GaAs block layer shown in FIG. Can be.

(Examples) Hereinafter, details of the present invention will be described with reference to the illustrated examples.

FIG. 1 is a sectional view showing a manufacturing process of a semiconductor laser according to one embodiment of the present invention. First, as shown in FIG. 1 (a), an MO is placed on an n-GaAs substrate (Si-doped: 3 × 10 ¹⁸ cm ⁻³ ) 11.
A 0.8 μm thick n-In _0.5 (Ga _0.3 Al _0.7 ) _0.5 P cladding layer (Si-doped 5 × 1
0 ¹⁷ cm ^-3) 12, a thickness of 0.06μm undoped In0.5Ga0.5P active layer 13, a thickness of _{0.8μm p-In 0.5 (Ga 0.3} Al 0.7) 0.5 P cladding layer (Zn-doped: 5 × 1
0 ¹⁷ cm ^-3 ) 14, p-In _0.5 Ga _0.5 P contact layer (Zn doped: 7 × 10 ¹⁷ cm ^-3 ) 15 having a thickness of 0.05 μm and n having a thickness of 0.7 μm
-A GaAs cap layer (Si doped: 8 × 10 ¹⁸ cm ⁻³ ) 16 is continuously grown.

Then, as shown in Fig. 1 (b), the n-GaAs cap layer _{16 (H 2 SO 4 + H} 2 O 2 + H 2 O) was removed using an etchant, followed by forming the SiO ₂ film 17. Using this SiO ₂ film 17 as a mask, as shown in FIG.
Is removed using a (Br ₂ + HBr + H ₂ O) etchant, and the p-cladding layer 14 is further etched to a desired depth with hot phosphoric acid to form a mesa stripe.

Next, as shown in FIG. 1D, an n-GaAs block layer (Si-doped: 8 × 10 ¹⁸ cm ⁻³ ) 18 acting as a current blocking layer is formed on the side of the mesa stripe using the SiO ₂ film 17 as a mask. To
Selectively grow to a thickness of 1.5 μm. Next, FIG. 1 (e)
After removing the SiO ₂ film 17 as shown in FIG. 3 (a), a p-GaAs ohmic layer (Zn-doped, 1 × 10 ¹⁹ cm ⁻³ ) 19 is grown to 3 μm as shown in FIG. The laser is completed.

Here, the conditions of the first crystal growth step for forming the structure of FIG. 1A are as follows: growth temperature T _G = 750 ° C., pressure P = 25T
orr, the total flow rate V _S = 1 / min, the molar ratio of group III and group V elements (V / III) = 500, and the second and third layers for forming the n-GaAs block layer 18 and the p-GaAs ohmic layer 19 The growth conditions for the crystal growth process of Example 1 are T _G = 650 ° C., pressure P = 50 Torr, V _S = 1 / mi.
n, V / III = 250.

The carrier concentration of the p-cladding layer of the wafer prepared by the method of this embodiment was measured by the CV method.
¹⁷ cm ^-3 was obtained, which is about twice as large as that of the wafer prepared by the conventional method (FIG. 5). The semiconductor laser fabricated from this wafer has reduced thermal resistance due to the increased carrier concentration of the p-cladding layer 14, and has a maximum oscillation temperature of 50 ° C or less.
Improvements in temperature characteristics, such as an increase to 70 ° C, were observed, and stable operation was observed even at high temperatures. Incidentally, the cap layer 16 formed on the uppermost layer of the double hetero junction structure portion is n-Ga
As described above, the above-described effect of improving the carrier concentration can be obtained, but improvement of the carrier concentration cannot be obtained as p-GaAs.

Thus, according to the present embodiment, it was formed on the GaAs substrate 11.
In the production of a rib waveguide semiconductor laser using a double heterojunction structure made of InGaAlP, a p-InGaAlP cladding layer 14 is formed by forming an n-GaAs layer 16 as an uppermost layer after forming a double heterojunction structure. The carrier concentration can be sufficiently increased, and the device characteristics of the semiconductor laser can be improved.

 Next, another embodiment of the present invention will be described.

InGaAlP-based material has the largest energy gap among III-V group semiconductor compounds except nitride, and is 0.5-0.6 μm
It is attracting attention as a light emitting element material for a band. In a semiconductor light emitting device made of InGaAlP, its emission wavelength is determined by the band gap energy of InGaAlP which is a light emitting region. It is known that InGaAlP has a different atomic arrangement and a different band gap energy in the crystal depending on the crystal growth method and growth conditions, even though the mole fraction of the elements in the crystal, that is, the composition is the same. (For example,
J. Grystsl Growth 93 (1988) 406-411). Also, it has been reported that the band gap of p-type InGaP changes depending on the doping amount (Jpn. J. Appl. Phys. 27).
(1988) L1549-L1552).

The present inventors have found that a series of experiments, the band gap by doping Zn into quaternary mixed crystal of _{In 1-XY Ga X Al Y} P greatly changes. As shown in FIG. 2, the point at which the band gap changes occurs when the hole concentration is about 1 × 10 ¹⁸ cm ⁻³ when y = 0, but is lower when y = 0. ¹⁷ cm ^-3
The band gap changes near. Furthermore, I = 0.2 for y = 0.2
In _{n 1-XY Ga X Al Y} P, we found that variation of the band gap is larger than 2 times compared with the case of y = 0.

Generally, a double hetero structure is used to effectively confine carriers in a semiconductor light emitting device. In that case, it is necessary to increase the Al composition in InGaAlP in order to increase the band gap of the p-type semiconductor layer. But Z
When n is used as a p-type dopant, the Al composition y is 0.25
Above this, it is difficult to form a low-resistance p-type layer because the rate of Zn incorporation and the rate of electrical activation of Zn in the crystal rapidly decrease. In fact, p-type In with high Al composition
A prototype LED with a double heterostructure was fabricated using GaAlP. However, the series resistance was very high, and the heat generation of the device was large, resulting in poor life characteristics.

As described above, it has been difficult to achieve high luminous efficiency and obtain sufficient luminous intensity in a semiconductor light emitting device having a single hetero structure, a homo structure, or a double hetero structure having a light emitting layer made of InGaAlP. In this embodiment, this point is improved to realize a semiconductor light emitting device having high luminous efficiency.

FIG. 3 is a sectional view showing a schematic configuration of a light emitting diode (LED) in the present embodiment. In the figure, 31 is a p-GaAs substrate,
32 is a p-In _0.5 Ga _0.3 Al _0.2 P semiconductor layer, and 33 is a p-In _0.5 Ga _0.3 Al
_{A 0.2} P light emitting layer, 34 is an n-In _0.5 Al _0.5 P semiconductor layer, 35 is an n-side ohmic electrode, and 36 is a p-side ohmic electrode. The p-type semiconductor layer 32 has a hole concentration of 5 × 10 ¹⁷ cm ⁻³ and a thickness of 2.5 μm.
m, the hole concentration of the p-type light emitting layer 33 is 1 × 10 ¹⁷ cm ⁻³ ,
The thickness is 0.5 μm, the donor concentration of the n-type semiconductor layer 34 is 1 × 10 ¹⁸ cm ⁻³ , and the thickness is 3 μm.

FIG. 4 shows the relationship between the current and the light emission intensity of the LED shown in FIG. As shown by the solid line in FIG. 4, the light emission intensity of the LED of this example increased in proportion to the current, and a light emission intensity sufficient for practical use was obtained. The broken line in FIG. 4 shows the conventional example.
According to the ED, a light emission intensity five times or more can be obtained.

Such high luminous efficiency can be obtained for the following reasons. FIG. 2 shows a change in the hole concentration of InGaAlP and a change in band gap energy obtained from PL measurement at room temperature. As the hole concentration increases, the band gap energy changes to a higher energy side. In other words, the hole concentration of the p-type semiconductor layer is 7 × 10 ¹⁷ cm ⁻³ and the energy gap is 1.17 eV, but the hole concentration of the p-type light emitting layer is 1 × 10 ^{17
Since the} energy gap is 2.12 eV at ¹⁷ cm ^-3 , the two layers have an energy gap difference of 50 meV despite having the same composition. Due to this energy gap difference, carriers can be effectively confined in the light emitting layer, and the high luminous efficiency as described above can be realized.

As can be seen from FIG. 2, the change amount of the energy gap is larger at y = 0.2 than at y = 0. That is, y
InGaAlP that has a light-emitting layer near = 0.2 and emits green light
This embodiment has a great effect in the semiconductor light emitting device. In n-type InGaAlP, the band gap energy did not change depending on the donor concentration. In the case of n-type, the band gap energy is 1.88 eV at y = 0 and 2.1 at y = 0.2.
2 eV, which was the same as the band gap energy in the low doping region of p-type InGaAlP. That is, even if the light emitting layer is n-type, if the doping amount of the p-type InGaAlP semiconductor layer is increased and the energy gap is increased,
The same effect as described above can be obtained. Although the LED is described in the above example, it goes without saying that the active layer and the p-type cladding layer can be applied to the semiconductor laser shown in FIG. 1 and other semiconductor lasers.

[Effects of the Invention] As described in detail above, according to the present invention, when a semiconductor laser is manufactured using a double heterojunction structure made of InGaAlP, n-GaAs is formed on the uppermost layer after the growth of the double heterojunction.
By forming the layer, the carrier concentration of the p-cladding layer can be increased, a decrease in temperature characteristics due to a low carrier concentration can be suppressed, and a semiconductor laser that can contribute to an improvement in device characteristics can be manufactured.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a manufacturing process of a semiconductor laser according to one embodiment of the present invention. FIGS. 2 to 4 are views for explaining another embodiment of the present invention, and FIG. Characteristic diagram showing the relationship between concentration and band gap energy,
FIG. 3 is a cross-sectional view showing an LED structure, FIG. 4 is a characteristic diagram showing a relationship between current and light emission intensity, and FIGS. 5 and 6 are cross-sectional views for explaining a conventional problem. . 11 ... n-GaAs substrate 12 ... n-InGaAlP cladding layer 13 ... InGaP active layer 14 ... p-InGaAlP cladding layer 15 ... p-InGaP contact layer 16 ... n-GaAs cap layer 17 ... SiO ₂ film (mask) 18 ... n -GaAs block layer (current blocking layer) 19 ... p-GaAs contact layer

Claims (2)

(57) [Claims] 1. A double heterojunction structure comprising In _{1 -Y} (Ga ₁ -xAl _x ) _YP is grown on an n-type GaAs substrate, and an n-type Ga is formed on the double heterojunction structure.
Growing an As cap layer, removing the cap layer, then forming a mesa stripe in the double heterojunction structure, and growing an n-type GaAs current blocking layer on the side of the mesa stripe. And a step of growing a p-type GaAs ohmic layer on the double heterojunction structure and the current blocking layer. 2. The Al composition of the p-type cladding layer of the double hetero junction structure is equal to or greater than the Al composition of the active layer, and the p-type impurity of the p-type cladding layer is n-type InGaAl having the same composition.
2. The method for manufacturing a semiconductor light emitting device according to claim 1, wherein the doping is performed in a range where the energy gap is larger than P.