JP2780337B2 - High-resistance semiconductor layer embedded semiconductor laser - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a high-resistance-modulated double-heterostructure semiconductor laser capable of high-speed modulation.

(Prior Art) With the construction of a highly information-oriented society, the capacity of optical communication systems and the sophistication of communication networks are being promoted. One of the effective means for increasing the capacity of an optical communication system is to increase the modulation speed. 2. Description of the Related Art In an optical communication system in which a light source is modulated at a very high speed to achieve a high speed, a semiconductor laser excellent in a high-speed response is required.

In recent years, a high-resistance semiconductor layer using a deep level in a semiconductor has been used as a buried layer for effectively confining current only in the active region of a semiconductor laser and effectively confining light in the active region due to a difference in refractive index. Attention is being paid to research and development. It is known that a semiconductor laser using a high-resistance semiconductor layer as a buried layer does not use a pn junction current blocking layer for current confinement to an active region, and therefore has a small parasitic capacitance and can perform high-speed modulation. . (Applied physics letters
Letters, Vol. 45, No. 4, pp. 311) FIG. 4 shows a conventional example of a semiconductor laser using a high-resistance semiconductor layer as a buried layer. A buried current is to be effectively injected into the active layer on both sides of the stripe-shaped active layer by a high-resistance semiconductor layer 44 having a deep level for capturing electrons or holes. 41 and 43 are p-type or n-type cladding layers.

(Problems to be Solved by the Invention) In the above-described conventional technique, the current blocking layer 44 is formed by using a semi-insulating semiconductor layer (SI) that captures only one of electrons and holes in the current blocking layer 44. The cladding layer is sandwiched between the n-type cladding layer and the n-type cladding layer, and a p / SI / n structure is generated. In this part, double injection occurs and a leakage current flows outside the active region. If the current blocking layer is a semi-insulating semiconductor layer that captures electrons, a hole current flows from the p-layer. If the current blocking layer is a semi-insulating semiconductor layer that captures holes, an electron current flows from the n-layer. Leakage current flows outside the region, resulting in deterioration of semiconductor laser characteristics such as an increase in threshold current, a decrease in external differential quantum efficiency, and a decrease in maximum output. For this reason, in the conventional technique, it has been difficult to obtain a high-performance semiconductor laser using a high-resistance semiconductor layer as a current blocking layer.

An object of the present invention is to improve the above-mentioned disadvantages of the prior art and to provide an excellent semiconductor laser.

(Means for Solving the Problems) In a semiconductor laser according to the present invention, a stripe-shaped mesa having p-type and n-type cladding layers and an active layer and including at least the active layer is formed on a semiconductor substrate. In a high-resistance semiconductor layer-embedded double heterostructure semiconductor laser in which a side surface is buried with a high-resistance semiconductor layer having a deep impurity level for capturing electrons or holes, a current blocking layer is embedded in the high-resistance semiconductor layer. It is characterized in that the doping ratio of an impurity having a deep impurity level is changed.

(Operation) FIG. 5 (a) shows the concept of the energy band when a forward bias voltage is applied by contacting a p-type semiconductor layer, a semi-insulating semiconductor layer having a deep electron trapping level, and an n-type semiconductor layer. FIG. FIG. 5 (b) shows the concept of the energy band when a forward bias voltage is applied by contacting a p-type semiconductor layer, a semi-insulating semiconductor layer having a deep hole trapping level, and an n-type semiconductor layer. FIG.

In a conventional high-resistance semiconductor layer embedded semiconductor laser,
Since the p-type cladding layer, the high-resistance semiconductor layer and the n-type cladding layer are directly connected and a bias voltage is applied in the forward direction when the semiconductor laser is driven, the energy band shown in FIG. 5A or FIG. It becomes equivalent to a conceptual diagram.
Therefore, in the case of a semi-insulating semiconductor layer having a deep electron trap level, electrons and holes recombine near the interface between the p-type cladding layer and the semi-insulating semiconductor layer, and a recombination current flows. In the case of a semi-insulating semiconductor layer having a deep hole trapping level, electrons and holes recombine near the interface between the n-type cladding layer and the semi-insulating semiconductor layer, and a recombination current flows.

On the other hand, FIG. 6 (a) shows a conceptual diagram of the energy band of the current blocking layer of the present invention. The figure shows the case of a semi-insulating semiconductor layer having a deep electron trapping level. By increasing the electron trapping level density near the n-type semiconductor layer, more electrons injected from the n-type layer are trapped, and p By lowering the electron trap level density in the vicinity of the p-type semiconductor layer, recombination between holes injected from the p-type semiconductor layer and electrons captured at a deep level can be suppressed. Next, FIG. 6 (b) shows a conceptual diagram of the energy band of the current blocking layer of the present invention.
The figure shows the case of a semi-insulating semiconductor layer having a deep hole trapping level. By increasing the hole trapping level density near the p-type semiconductor layer, more holes are injected from the p-type semiconductor layer. By capturing and lowering the hole trap level density in the vicinity of the n-type semiconductor layer, recombination between electrons injected from the n-type semiconductor layer and holes trapped at a deep level can be suppressed. it is conceivable that.

As described above, in the high-resistance layer embedded semiconductor laser according to the present invention, the leakage current is suppressed, and the injection current is effectively converted into light in the active layer. Efficiency and high light output can be expected.

(Example) Next, the present invention will be described with reference to the drawings. First
The figure is a diagram of the first embodiment of the present invention. In the embodiment, an example of indium phosphide (InP) -based material which is a long-wavelength-based material will be described.

First, sulfur (S) -doped n-type InP with (100) plane
A silicon (Si) -doped n-type InP layer 18 [n = 1 × 10 ¹⁸ cm] is formed on the substrate 11 by using a metal organic chemical vapor deposition (MOVPE).
^-3 ] with indium-gallium-arsenic, phosphorus (InGaAsP) having a band gap of 1 μm in thickness and 1.55 μm in emission wavelength
The active layer 19 has a thickness of 0.15 μm and is doped with zinc (Zn) p-type In.
The P layer 20 [p = 1 × 10 ¹⁸ cm ⁻³ ] has a thickness of 1.5 μm, and the Zn ^doping p-type InGaAsP contact layer 17 [p = 1 × 10 ¹⁹ cm ⁻³ ] has a thickness of 0.5 μm. I do. Next, by photolithography, <011>
In the direction, SiO ₂ stripe-shaped masks having a thickness of about 2000 ° and a width of 2 μm are formed at intervals of 300 μm. Next, the p-type InGaAsP contact layer 17, the p-type InP layer 20, and the InGaAsP
The active layer 19 and the n-type InP layer 18 have a mesa stripe height of 3.5 μm.
Etching so that Further, while the SiO ₂ stripe-shaped mask is left, a 3.5 μm-thick iron (Fe) -doped high-resistance InP layer 12 is formed on the concave portion of the mesa stripe by MOVPE.
Selective epitaxial growth so that the whole becomes flat. The right side of Fig. 1 shows the high resistance of Fe doping at this time.
4 shows the distribution of Fe doping in an InP layer. During growth, the flow rate of the dopant material is changed so that the concentration of the untrapped trap changes from 1 × 10 ¹⁶ cm ⁻³ to 1 × 10 ¹⁵ cm ⁻³ from the substrate side toward the surface side. Next, after removing the SiO ₂ stripe-shaped mask with hydrofluoric acid, it is polished until the entire thickness becomes about 120 μm, and the electrodes 10 on the p-type semiconductor side and the n-type semiconductor substrate side are formed by vacuum evaporation. After the annealing, the semiconductor laser is cleaved and separated into individual semiconductor lasers, and the entire processing is completed. Thus, a semiconductor laser whose cross section is shown in FIG. 1 is completed.

In this semiconductor laser, there is almost no reactive current flowing through portions other than the active layer, and a VSB type (V-grooved Substrate Buried Heterostructure Lass) using a pn bond as a block layer.
ers) and DC-PBH type (Double Channel Planar Buried Het
A threshold current of about 10 mA, comparable to that of erostructure lasers, and a single-sided external differential quantum efficiency of about 30% are obtained. Further, since a high resistance semiconductor layer having a thickness of 2 to 3 μm is used for the current blocking layer, the parasitic capacitance is 4 to 5 pF, which is practically sufficient as a light source for an optical communication system of several gigabits per second (Gb / sec) class. Can be used.

Next, FIG. 2 is a diagram of a second embodiment of the present invention. First, the MO is placed on the S-doped n-type InP substrate 11 with the (100) plane.
Using VPE, the Si-doped n-type InP layer 18 [n = 1 × 10 ¹⁸
cm ⁻³ ] is 1 μm in thickness, the InGaAsP active layer 19 having a band gap of an emission wavelength of 1.55 μm is 0.15 μm in thickness, and the Zn-doped p-type InP layer 20 [p = 1 × 10 ¹⁸ cm ⁻³ ] is 1.5 in thickness. μm,
Zn doping p-type InGaAsP contact layer 17 [p = 1 × 10
¹⁸ cm ⁻³ ] is epitaxially grown continuously with a thickness of 0.5 μm. Next, SiO ₂ stripe-shaped masks having a thickness of about 2000 mm and a width of 2 μm are formed at 300 μm intervals in the <011> direction by a photolithography technique. Next, the p-type InGaAsP contact layer 17 and the p-type InP layer are
20, the InGaAsP active layer 19 and the n-type InP layer 18 are etched so that the height of the mesa stripe is 3.5 μm. In addition, SiO
_{2 While} leaving the stripe-shaped mask, a titanium (Ti) -doped high-resistance InP layer 15
Selective epitaxial growth of 3.5 μm is performed by MOVPE so that the whole becomes flat. The right side of FIG. 2 shows the Ti doping distribution of the Ti-doped high-resistance InP layer 15 at this time. During growth, the flow rate of the dopant material is changed so that the concentration of the untrapped trap changes from 1 × 10 ¹⁵ cm ⁻³ to 1 × 10 ¹⁶ cm ⁻³ from the substrate side to the surface side. Next, after removing the SiO ₂ stripe-shaped mask with hydrofluoric acid, it is polished until the entire thickness becomes about 120 μm, and the electrodes 10 on the p-type semiconductor side and the n-type semiconductor substrate side are formed by vacuum evaporation. ,
After annealing, the semiconductor laser is cleaved and separated into individual semiconductor lasers, and the entire processing is completed. Thus, a semiconductor laser whose cross section is shown in FIG. 2 is completed. In this semiconductor laser, a threshold current of about 10 mA and a single-sided external differential quantum efficiency of about 30% can be obtained. Further, since a high resistance semiconductor layer having a thickness of 2 to 3 μm is used for the current blocking layer, the parasitic capacitance is 4 to 5 pF,
It can be practically used as a light source for optical communication systems of several gigabits per second (Gb / sec) class.

Next, FIG. 3 is a diagram of a third embodiment of the present invention. In the embodiment shown in FIG. 1, the doping distribution of Fe changes abruptly in a portion in contact with the n-type semiconductor 18 and in a portion in contact with the p-type semiconductor 20, as shown on the right side of FIG. .
This semiconductor laser has a threshold current of around 10mA and 30%
One-sided external differential quantum efficiency before and after is obtained. Furthermore, thickness 2
Since the high-resistance semiconductor layer of about 3 μm is used for the current blocking layer, the parasitic capacitance is 4 to 5 pF, and the disease is several gigabits every (Gb / se).
c) It can be practically used as a light source for optical communication systems of the class.

When Ti is used as a dopant, the manner of changing the doping concentration is opposite to that when Fe is used.

FIGS. 1 and 2 show the case where the substrate is p-type.
The doping distribution of the current blocking layer shown in FIG. 3 has exactly the opposite shape.

(Effects of the Invention) As described above, the present invention uses a semi-insulating semiconductor layer having a deep level for capturing electrons or holes in a current blocking layer, and by changing this doping distribution,
This has the effect of realizing a low threshold current, a high external differential quantum efficiency, and an ultra-high-speed modulation characteristic of a high-resistance semiconductor embedded semiconductor laser.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of a high-resistance semiconductor embedded semiconductor laser according to the present invention. FIG. 2 is a view showing a second embodiment of the high-resistance semiconductor embedded semiconductor laser according to the present invention. FIG. 3 is a view showing a third embodiment of a high-resistance semiconductor embedded semiconductor laser according to the present invention. FIG. 4 is a sectional view showing the structure of a conventional semiconductor laser having a high-resistance current blocking layer. FIG. 5 (a) is a conceptual diagram showing a band structure when an n-type semiconductor layer, a semi-insulating semiconductor layer having a deep electron trap level, and a p-type semiconductor layer are in contact with each other and forward biased. is there. FIG. 5B shows an n-type semiconductor layer, a semi-insulating semiconductor layer having a deep hole trap level, and a p-type semiconductor layer,
FIG. 3 is a conceptual diagram showing a band structure when a forward bias is applied to this. FIG. 6A is a conceptual diagram showing a band structure when an n-type semiconductor layer, a semi-insulating semiconductor layer having a deep electron trap level, and a p-type semiconductor layer are in contact with each other. However, doping near the n-type semiconductor is increased and doping near the p-type semiconductor is reduced. FIG. 6 (b)
FIG. 3 is a conceptual diagram showing a band structure when an n-type semiconductor layer, a semi-insulating semiconductor layer having a deep hole trap level, and a p-type semiconductor layer are in contact with each other. However, doping near the n-type semiconductor is reduced and doping near the p-type semiconductor is increased. 10; electrode; 11; n-type InP substrate; 12; Fe-doped high-resistance InP
Layer, 15; Ti-doped high-resistance InP layer, 17; p-type InGaAsP contact layer, 18; n-type InP layer, 19; InGaAsP active layer, 20; p-type InP
Layer, 40; semiconductor substrate, 41; first cladding layer, 42; active layer, 43; second cladding layer, 44; high-resistance semiconductor layer, 45; contact layer, 46; insulating film, 47; electrode, 48 ;electrode.

Claims (3)

(57) [Claims] 1. A semiconductor substrate having a p-type and n-type cladding layer and an active layer, and a mesa in a stripe shape including at least the active layer is formed. In a high-resistance semiconductor layer-embedded double heterostructure semiconductor laser that is buried with a high-resistance semiconductor layer having an impurity level to serve as a current blocking layer,
The high-resistance semiconductor layer is characterized in that the concentration of doping of an impurity having a deep impurity level in the portion in contact with the p-type cladding layer and the portion in contact with the n-type cladding layer are different from each other. Semiconductor laser with embedded semiconductor layer. 2. A semiconductor substrate having p-type and n-type cladding layers and an active layer, and a stripe-shaped mesa including at least the active layer is formed. A doping concentration of an impurity having an impurity level for trapping electrons in the high-resistance semiconductor layer in a high-resistance semiconductor layer-embedded double heterostructure semiconductor laser having a current blocking layer by being buried in the high-resistance semiconductor layer. Is high in a portion in contact with the n-type cladding layer and low in a portion in contact with the p-type cladding layer. 3. A stripe-shaped mesa having a p-type and n-type cladding layer and an active layer on a semiconductor substrate and including at least the active layer, and an impurity level trapping holes on the side surface of the mesa. Doping of an impurity having an impurity level for trapping holes in the high-resistance semiconductor layer in the high-resistance semiconductor layer-embedded double heterostructure semiconductor laser which is buried with a high-resistance semiconductor layer having A high-resistance semiconductor layer-embedded semiconductor laser, wherein the concentration of the semiconductor layer is high in a portion in contact with the p-type cladding layer and low in a portion in contact with the n-type cladding layer.