JPH0382184A - High resistance buried semiconductor laser and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain a semiconductor laser which is free of a leakage current, high in efficiency at a threshold, small in parasitic capacity, and able to execute a high speed modulation by a method wherein semiconductor buried layers which include a semi-insulating semiconductor laser possessed of a level to trap holes and a P-type semiconductor are formed on both the sides of a mesa, a second conductivity type semiconductor is provided onto the mesa and the semiconductor buried layers, and a P-type semiconductor layer is formed between an N-type semiconductor clad layer and the semi-insulating semiconductor layer on the side face of the mesa. CONSTITUTION:A current block region is formed in a P-SI-P-N structure, and a leakage current is lessened by this region. A P-type semiconductor layer 6 is interposed between the side face of a mesa and a semi-insulating semiconductor layer to modify the side face of the mesa of P-SI-N structure into a P-SI-P-N structure so as to stop a leakage current, and the P-type semiconductor layer 6 is formed in a P<-> layer of low concentration to be high in resistance, whereby a leakage current, which flows from a P-type semiconductor clad layer 2 of the mesa to an N-type semiconductor layer 4 passing through a P<-> semiconductor layer 11 of the side face of the mesa, can be made very small. By this setup, a leakage current can be eliminated, so that a semiconductor laser of this design can be made low in oscillation threshold current and high in light emission efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a high-resistance buried semiconductor laser used as a light source for optical communications and a method for manufacturing the same.

(Conventional technology) Optical fiber communication technology has made remarkable progress in recent years, and
High-capacity optical communication systems such as 0 Mb/s and 1.6 Gb/s have been introduced not only within Japan but also with foreign countries via submarine cables. A key device indispensable to this optical communication technology is a semiconductor laser, which serves as a light source for optical signals. As systems become longer-distance and larger-capacity, semiconductor lasers are also required to have even higher output and faster speed.

Semiconductor lasers commonly used at present have a thyristor structure in which the current blocking region is pnpn. However, with this structure, when the current increases, the thyristor may turn on, or when the gain of the transistor constituting the thyristor is large, a non-negligible amount of current flows, and the optical output may become saturated. Furthermore, since the thyristor has parasitic capacitance, the RC time constant is large, making it difficult to achieve high-speed modulation.

On the other hand, semiconductor lasers that use a high-resistance semiconductor layer in the current blocking region have been gaining popularity recently because they have low leakage current, allowing low threshold operation, and have low parasitic capacitance, making high-speed modulation possible. Research and development is underway.

A conventional example of such a high-resistance buried semiconductor laser is shown in FIG. In FIG. 3(a), a mesa-shaped double heterostructure consisting of a p-type InP cladding layer 2, an InGsAsP active layer 3, and an n-type InP cladding layer 4 is formed on a p-type InP substrate 1, and this is a semi-insulating (SI) structure. It is buried with an InP layer 5, on which an n-type InP layer 7 is formed over the entire surface, and p-type electrodes 2 are formed on both sides.
1 and an n-type electrode 22 are formed. Further, FIG. 3(b) shows the case where an n-type InP substrate 8 is used, and the conductivity type of each InP layer is different from that in FIG. 3(a).
In order to make ohmic contact on the p side, p+ type In
A GaAsP contact layer 10 is formed in contact with the p-side electrode 21.

(Problems to be Solved by the Invention) The semi-insulating semiconductor layer used in the current block structure is not an insulator through which no current flows. For example, in the case of InP doped with iron (Fe), which is the most common, the Fe that forms a deep level in the forbidden band of InP functions as an electron trap. That is, when an AC voltage is applied to Fe-doped InP sandwiched between p-type and n-type InP, electrons injected from the n-type InP to the Fe-doped InP are captured by Fe traps. p-type In
Holes injected from P into Fe-doped InP recombine with electrons already captured by Fe acceptors, resulting in a recombination current. Therefore, in the structure shown in FIG. 3, the p-type InP layer 1,2 (FIG. 3(a)) or 2.9(
As shown in Fig. 3(b), there is a leakage current flowing through the semi-insulating InP layer 5, and a sufficient current blocking effect cannot be obtained, causing an increase in threshold current, a decrease in efficiency, and saturation of optical output. .

When a hole trap such as titanium (Ti) or cobalt (Co) is used as a deep impurity, it works as a hole trap, so n-type InP4.7 (Figure 3 (a)) or 4.8 (
A current flows through the semi-insulating InP layer 5 from FIG. 3(b).

Unless these problems are solved, the features of high-resistance buried semiconductor lasers, such as low threshold voltage, high efficiency, and high-speed modulation, cannot be fully utilized.

(Means for Solving the Problems) A high-resistance buried semiconductor laser of the present invention includes, on a first conductivity type semiconductor substrate, at least a semiconductor active layer having a smaller energy gap than the semiconductor substrate and a second conductivity type semiconductor cladding layer. A double heterostructure is formed in a striped mesa, and a semiconductor buried layer including a semi-insulating semiconductor layer having a level for trapping holes and a p-type semiconductor layer on both sides of the mesa is formed when the semiconductor substrate is p-type. A second conductivity type semiconductor is formed on the mesa and on the semiconductor buried layer, and an n-type semiconductor cladding on the side surface of the striped mesa is formed. A p-type semiconductor layer is provided between the semiconductor layer and the semi-insulating semiconductor layer.

The manufacturing method of the present invention includes a step of forming a semiconductor layer including at least a semiconductor active layer and a second conductivity type semiconductor cladding layer on a first conductivity type semiconductor substrate, and forming the semiconductor layer including the active layer in a stripe shape. etching using the insulating film as a mask to form a mesa, and when the substrate is p-type, a semi-insulating semiconductor layer having a hole capturing level and a semiconductor buried layer constituting a p-type semiconductor device are formed in this order. , when the substrate is n-type, an n-type semiconductor cladding layer is formed on the side surface of the striped mesa by diffusion of the p-type impurity in the p-type semiconductor layer during the buried growth at the same time as the substrate is formed by buried growth. a step in which the semi-insulating semiconductor changes to p-type at the interface with the semi-insulating semiconductor, and a second step after removing the insulating film
The method is characterized by including a step of growing a conductive semiconductor layer over the entire surface.

(Function) Holes as a high-resistance semi-insulating semiconductor (SI) layer
In the p-8I-n structure using an impurity that captures , holes injected from the p-type semiconductor into S are captured. However, the electrons injected from the n-type semiconductor into the SI layer are not captured,
It recombines with the holes already captured in the 8th picture. This becomes a leakage current, and the current blocking effect using the high resistance layer is not sufficiently exhibited.

In the high-resistance buried semiconductor laser of the present invention, the current block region has a p-8I-pn structure to eliminate leakage current there. Furthermore, on the mesa side, p-8r-n
P between the mesa side and the semi-insulating semiconductor layer to eliminate the structure.
A p-type semiconductor layer is added to eliminate leakage current as a p-8I-pn structure.
By increasing the resistance of the - layer, the leakage current flowing from the p-type semiconductor cladding layer of the mesa to the n-type semiconductor cladding layer through the p-semiconductor layer on the side surface of the mesa can be made very small. Since leakage current can be eliminated in this way, a low oscillation threshold current and high luminous efficiency can be obtained.

In the manufacturing method of the present invention, the p-type impurity from the p-type semiconductor layer is automatically diffused into the semi-insulating semiconductor layer during the buried growth process, so that the semi-insulating semiconductor layer becomes p-type at the interface with the mesa side surface.
- A good current block structure can be easily obtained with good reproducibility.

We used semi-insulating InP doped with Co as an impurity.
layer, and n-type In doped with Si to serve as a donor.
A p-type InP layer doped with Zn (carrier concentration 1×10 18 cm −3 ) was grown on the P layer, and the degree of Zn diffusion was examined by SIMS measurement. Metal organic vapor phase epitaxy (MOVPE or MOCVD) was used for crystal growth. As a result, as shown in Figure 2(a), Zn diffusion of about 0.3 pm was observed in the semi-insulating InP layer, whereas as shown in Figure 2(b), Zn diffusion was observed in the semi-insulating InP layer. Zn hardly diffused into the layer. Thus, the diffusion of Zn into a semi-insulating semiconductor is 0.3 under normal crystal growth conditions.
It is about pm. There is almost no diffusion of Zn into the nff1 semiconductor layer, and there is no inversion to the p-type. Furthermore, since the concentration of Zn diffused into the p-type InP layer formed on the side surface of the mesa is about one-tenth of the Zn concentration in the p-type semiconductor layer, the p-type InP layer formed by diffusion has a low resistance. The leakage current flowing through it is very small.

This manufacturing method does not require any special diffusion process, and utilizes diffusion from a p-type semiconductor to a semi-insulating semiconductor during crystal growth. Therefore, semiconductor lasers can be manufactured with good controllability and reproducibility at a high yield.

(Example) The results of actually prototyping the high-resistance buried semiconductor laser of the present invention will be described below. FIG. 1 is a cross-sectional view showing the structure of a prototype semiconductor laser, in which (a) shows a case where a p-type semiconductor substrate is used, the first conductivity type is p-type, and the second conductivity type is n-type.
(b) is a case where an n-type semiconductor substrate is used and the conductivity type is opposite to (a), but here, the case (a) will be explained in detail.

All crystal growth was done by MOVPE. First, p-type I
A Zn-doped p-type InP cladding layer 2 is formed on the nP substrate 1.
(carrier concentration 5X1017cm-3, layer thickness 1pm),
Non-doped InGaAsP active layer 3 (layer thickness 0.2 pm)
, Si-doped n-type InP cladding layer 4 (carrier concentration 4
A double heterostructure crystal consisting of a layer thickness of 0.4 pm and a thickness of 0.4 pm was grown. Next, a surface 5i02 film was deposited and processed into stripes with a width of 1.5 pm by photolithography, and mesa etching was performed using a hydrochloric acid-based and bromine-methanol-based etching solution until the p-type InP substrate 1 was reached. Then, using the 5i02 stripe as a mask, a high resistance CO-doped semi-semi-insulating In surface layer 5 with an uncaptured trap concentration of 2 x 1016 cm-3 and a layer thickness of 2.3
pm), Zn-doped p-type InP layer 6 (carrier concentration 1×
1011018C, layer thickness 0.5 pm) was buried and grown. The interface between the semi-insulating InP layer 5 and the p-type InP layer 6 was arranged to be approximately on the same plane as the interface between the n-type InP cladding layer 4 and the 5i02 stripe. Furthermore, after removing the 5i02 stripes, the Si-doped n-type InP layer 7 (
carrier concentration 2 x 1018 cm-3, layer thickness 1.3 pm)
has grown. A portion of the wafer was set aside for cross-section observation using a scanning electron microscope (SEM), and the remaining wafer was
After depositing AuGeNi as the side electrode 22, 430°
The p-type InP substrate 1 is heat-treated with C for 5 minutes, polished to a total thickness of about 1100p, and the p-side electrode 21
After sputtering Ti/Pt/Au as
was heat-treated for 5 minutes. The wafers thus processed were separated so that the resonator length was 300 pm, mounted on a Si heat sink and assembled, and the device characteristics were evaluated.

As a result of SEM observation, Zn is present in the semi-insulating InP layer 5 at a concentration of approximately 0.
A p-type InP layer 11 is formed by diffusion of 3 pm, and an n-type InP layer 11 is formed.
It was found that the semi-insulating InP layer 5 changed to p-type at the interface of the P cladding layer 4 and became a p'''' type InP layer 11. Due to diffusion, the layer thickness of the semi-insulating InP layer 5 is approximately 1.711
decreased to m. The active layer width was 1.5 pm.

When we measured the current-light output characteristics of the cut out element, we found that
Threshold current is average 10mA (minimum 7mA), efficiency is average 0.23W/A, maximum light output is average 30mW (maximum 4
Good results were obtained (2 mW). This is because the leakage current in the buried portion is reduced by high-resistance embedding, and in particular, the p-type InP layer 6 and the p-type InP layer 6 formed during crystal growth.
The presence of the - type InP layer 11 makes the n-type InP layer (4
, 7) to the semi-insulating InP layer 5 is no longer present. In addition, the element capacitance is small at approximately 4 pF, and when we measured the small signal frequency response characteristics, we obtained characteristics with little response drop (roll-off) in the low frequency range.
The modulation frequency band at 5 mW optical output was 12 GHz. These characteristics are superior to conventional high-resistance buried semiconductor lasers in terms of threshold current and maximum optical output, and they are superior to conventional high-resistance buried semiconductor lasers in terms of threshold current and maximum optical output.
It is superior to lasers with a pn block structure in terms of threshold current and modulation band. In addition, it is easy to manufacture and about 2 times the conventional
The yield was twice as high.

The results described above were obtained using the p-type InP substrate 1 shown in FIG. 1(a), but when using the n-type InP substrate 1 shown in FIG.
The same applies when the P substrate 8 is used. p-type InP layer6.
9 and p-type InP cladding layer 2 as well as semi-insulating InP layer 5
Zn may be diffused into the p-type InP layer 11 to be formed between the semi-insulating InP layer 5 and the n-type InP cladding layer 4. In this example, a p+ type InGaAsP contact layer 10 is used to establish a p-side ohmic contact.

In the embodiment, Co was used as a dopant for forming the high resistance layer, but the gist of the present invention does not change at all even if an element having similar physical properties such as Ti is used. Also, I
AlGaAs/GaAs instead of nGaAsP/InP
, AIGaInP/GaInP and other compound mixed crystal double heterostructures can have similar effects.

(Effects of the Invention) According to the present invention, a semiconductor laser with no leakage current, low threshold value, high efficiency characteristics, small parasitic capacitance, and high-speed modulation can be obtained, and the manufacturing method of the present invention provides a high-performance semiconductor laser. A semiconductor laser with high yield and good reproducibility can be obtained.

[Brief explanation of drawings]

FIG. 1 (aXb) is a cross-sectional view showing the structure of a high-resistance buried semiconductor laser of the present invention, using a p-type semiconductor substrate and an n-type semiconductor substrate, respectively. Figure 2 (aXb
) is a semiconductor multilayer structure SIM, which provides a detailed explanation of the present invention.
FIG. 3 is a diagram of a concentration profile obtained by S measurement. Also, the third
Figures (aXb) are cross-sectional views showing the structure of a conventional high-resistance buried semiconductor laser. In the figure, 1... p-type semiconductor substrate, 2... p-type semiconductor cladding layer, 3... semiconductor active layer, 4... n-type semiconductor cladding layer, 5... semi-insulating semiconductor layer, 6... ... p-type semiconductor layer, 7... n-type semiconductor layer, 8... n-type semiconductor substrate, 9... p-type semiconductor layer, 10... p-type semiconductor contact layer, 11... p-type semiconductor layer, 21...p
side electrode, 22... n-side electrode.

Claims (2)

[Claims] (1) On a first conductivity type semiconductor substrate, a double heterostructure consisting of a semiconductor active layer and a second conductivity type semiconductor cladding layer having at least a smaller energy gap than the semiconductor substrate is formed in a striped mesa, and the mesa is A semiconductor buried layer including a semi-insulating semiconductor layer and a p-type semiconductor layer having a level for trapping holes on both sides is arranged so that the semiconductor substrate is p-type.
A second layer is formed in this order when the type is formed, and in the reverse order when the type is formed.
A high-resistance buried semiconductor laser characterized in that a conductive type semiconductor is formed and a p-type semiconductor layer is provided between an n-type semiconductor cladding layer on the side surface of the striped mesa and the semi-insulating semiconductor layer. (2) Forming a semiconductor layer consisting of at least a semiconductor active layer and a second conductivity type semiconductor cladding layer on a first conductivity type semiconductor semiconductor substrate, and forming a striped insulating film on the semiconductor layer including the active layer. The substrate is etched using a mask to form a mesa, and when the substrate is a p-type, a semi-insulating semiconductor layer having a hole capturing level and a semiconductor buried layer consisting of a p-type semiconductor are formed in this order. In the case of n-type, in the reverse order, it is formed by buried growth, and at the same time, during the buried growth, the p-type impurity in the p-type semiconductor layer is diffused, so that the interface with the n-type semiconductor cladding layer on the side surface of the striped mesa is formed. A method for manufacturing a high-resistance buried semiconductor laser, comprising the steps of changing the semi-insulating semiconductor to p-type, and growing a second conductivity type semiconductor layer over the entire surface after removing the insulating film.