JPH11354881A - Vertical resonator surface light emitting laser device and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To greatly reduce the series resistance of a vertical resonator surface light emitting laser device, by providing the upper layer portion of a first DBR mirror and an active layer with a first mesa that is formed on the first DBR mirror, by providing a second DBR mirror with a second mesa that is formed on the active layer, and by arranging waveguides within each mesa. SOLUTION: A resonator surface light emitting structure is formed on an n substrate 1, and an n-DBR mirror 2 is formed on a buffer layer. Then, an SCH layer 4, an active layer 5, an SCH layer 6, and a p-DBR mirror 8 are sequentially formed on the mirror 2. A quantum well is included in the layer 5 between the mirrors 2 and 8 of this AlAs/AlGaAs laminated structure. A mesa 10 is formed by etching. While exposing the resultant substrate to a high- temperature water vapor, the AlAs layer that tends to be easily oxidized is oxidized sideways from the outside toward the interior of the mesa 10, thereby forming waveguides 12 in the mesa 10. Next, p-electrodes are formed on the exposed portion of the layer 6 on the layer 5, and n-electrodes are formed on the mirror 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a vertical cavity surface emitting laser device having a novel structure and a method of manufacturing the same.
More specifically, the present invention relates to a vertical cavity surface emitting laser having a high effect of confining not only light (photons) but also current (carrier) and a method for manufacturing the same.

[0002]

2. Description of the Related Art As is well known, a vertical cavity surface emitting laser device (hereinafter abbreviated as a VCSEL device) is an important optical element expected to have many applications in optical communication. However, there are various problems in lowering the threshold value of the drive current / voltage of this SEL device and using it as a high-output, low-resistance light source to generate a very short light pulse with a high output. .

In particular, a technique for epitaxially growing a VCSEL device and its electrodes with high quality is important, as is a manufacturing technique for efficiently confining current / light. That is, it is considered that a change in the manufacturing method causes a change in characteristics. VCSEL devices have the potential to generate very short light pulses (several picoseconds) of high power (hundreds of mW) at low drive threshold current values (tens of μA) and to the front of the substrate (wafer). Forming the p-type and n-type electrodes on the substrate has the effect that a laser drive test can be performed on the substrate (on wafer). This means that a laser drive test can be performed after the chip manufacturing process is completed and before the laser device is housed in a package.

In order to make the structure of the electrode such that a current flows along a specific path of the active region, that is, to enable the current to be confined, it is necessary to devise some manufacturing process. . This current confinement is due to the circular mesa etch and around the output window (ie on the active area).
Forming anode (Cr / Au) and cathode (AuG
This is made possible by forming (e / Ni / Au) on the n-DBR mirror (ie, below the active region).

After an SEL (surface emitting laser) structure is formed by epitaxial growth, the production of electrodes must be completed before this test in order to conduct an electrical pumping test on-wafer. Light confinement in the laser cavity is provided by the circular mesa of the p-DBR mirror and the sharp drop in refractive index between the active region and its periphery (in air). If the active region has a higher refractive index than its surroundings and the layers above and below,
The propagation of the electromagnetic wave is guided in a direction parallel to the layer interface. S
In the case of EL, since the refractive index of the mesa region is much larger than that of air, the electromagnetic wave is more strongly confined along the mesa than by the layer interface. High reflectivity DB above and below the active area
Due to the formation of the R mirror, the confinement factor に お け る in the VCSEL device is defined as the ratio of the light intensity in the active region to the sum of the light intensity inside and outside the active region.

That is, the confinement factor Γ is defined as follows: Γ = 1−exp (−CΔnd) (1) Where C is a constant, Δn is the difference in refractive index,
d is the diameter of the mesa. Equation (1) indicates that Δ increases as Δn and d increase. When the injection current is low, spontaneous emission occurs in all directions. However, the gain increases as the current increases, and reaches the threshold value of laser oscillation. That is, until the condition that the light wave completely traverses inside the resonator without attenuation is satisfied,
To increase. That is, Rexp [(Γg-α) L] = 1 (2) or Γg (threshold gain) = α + (1 / L) ln [1 / R] (3). Where α is the loss per unit length (due to absorption and other scattering mechanisms) and L is the cavity length. R indicates the reflectance of the DBR mirror of the resonator.

The threshold current density is J _th (A / cm ² ) = J ₀ d / η + J ₀ (d / g ₀ ηΓ) × [α + (1 / L) ln (1 / R)] (4) ). Here, J ₀ is the initial current density before reaching the threshold, η is the quantum efficiency (conversion efficiency from electrons to photons), and g ₀ is the gain of the active layer.

Therefore, in order to reduce J _th , η,
It is necessary to increase Γ, L and R and decrease d and α.

[0009]

As described above, as described above,
Although there are various approaches to obtaining an ideal and practical VCSEL device, the present invention particularly focuses on VCSEL devices.
Focusing on the series resistance of the device, a VCSEL device with a low threshold voltage / current value for laser emission can be obtained by reducing this, and a new structure with a greatly reduced series resistance can be obtained. The purpose of the present invention is to obtain a VCSEL device and an efficient manufacturing method thereof.

[0010]

In order to solve the above problems, the present invention provides a semiconductor substrate, a first DBR mirror formed on the substrate, and a gain region formed on the first DBR mirror. In the vertical cavity surface emitting laser device having an active layer as described above and a second DBR mirror formed on the active layer, the upper layer and the active layer of the first DBR mirror are disposed on the first DBR mirror. One mesa, the second DBR mirror forms a second mesa on the active layer that is smaller than the first mesa, and the first and second mesas are at least partially formed by a lateral oxidation process. Has a narrow waveguide inside by being oxidized, and forms a first electrode on the first DBR mirror, and forms a second electrode around the side surface of the second mesa on the first mesa. Vertical cavity surface emitting laser device .

According to another aspect of the present invention, there is provided a semiconductor device comprising: a first DBR mirror; an active layer;
Stacking a plurality of layers and epitaxially growing a plurality of layers to form a DBR mirror in this order;
Forming a first mesa by mesa-etching the active layer and the second DBR mirror including the upper several layers of the mirror; and placing the semiconductor substrate having the formed first mesa in an oxidizing atmosphere. A step of oxidizing the first mesa from the outer periphery while leaving its center, and a step of selectively mesa-etching the second DBR mirror portion of the first mesa to form a second mesa having a smaller bottom area than the first mesa. Forming a mesa, forming a first electrode on the upper surface of the first DBR mirror exposed by the first mesa etching, and forming a second electrode on the upper surface of the active layer exposed by the second mesa etching And a method for manufacturing a vertical cavity surface emitting laser.

According to the above structure, since the driving current of the laser device does not flow through the upper and lower DBR mirrors, an effect of high current confinement (current confinement) can be expected. Also, the current does not flow through the high resistance p-DBR mirror, so that the series resistance of the device is greatly reduced. As a result, a laser device having a low threshold current / voltage value is obtained. Further, according to the manufacturing method, a laser device having the above-described structure can be manufactured with high accuracy and relatively easily.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Although dry etching is used for mesa etching, current is usually applied to a p-DBR mirror and an n-DBR for SEL driving regardless of either dry or wet etching. It flows from the top to the bottom through both mirrors. Light confinement is
This is achieved by the mesa structure as described above. However, it is desirable to prevent current from passing through the DBR mirror. This reduces the series resistance and consequently the threshold current, which reduces the heat generated by the SEL device. Therefore, the structure of the SEL device needs to be configured so that the confinement of current and light is as high as possible and the series resistance is reduced to prevent device heating.

The advantages of the SEL structure are that the size can be made compact and the surface emission is achieved, and at the same time, the structure of the output opening has flexibility. The flexibility in the design of the output aperture means that this aperture can be made square, oval or circular, and its size can be changed from 1 μm to about 100 μm according to the purpose. Means that you can do it. With these advantages, S
EL is an ideal choice in optical data communications.

Due to the small shape (particularly at diameters of 5 to 20 μm), the driving threshold value is reduced, and the SEL can be driven with a low power of several milliamps. A laser having a low threshold has an effect that modulation is easy. Further, since the shape is small, the capacity of the SEL is reduced, and as a result, high-speed driving is possible. However, the production of such a SEL is not easy and has to wait for much research. Many challenges have been made to put SEL to practical use. For example, by allowing a vertical current to flow through a small opening, the drive voltage of the multi-layer semiconductor DBR can be reduced, and the epitaxial growth technique can be improved to meet the special requirements of the SEL. . Whether a satisfactory result is obtained for such a challenge depends strongly on the desired drive wavelength. The driving wavelength depends on what material is used for the manufacture of the SEL. For example, if the wavelength is 7
To obtain a window of 80-980 nm, the aluminum-indium-gallium-arsenic system is suitable.

The structure and manufacturing method of the VCSEL device proposed in the present invention are very effective for confining current as well as light. The major feature of the present invention is that a current is prevented from flowing particularly through the p-DBR mirror in order to reduce the series resistance of the VCSEL device. p-D
BR mirrors have a lower doping concentration than n-DBR mirrors, and thus are the main series resistance sources in VCSEL devices. Therefore, by preventing the drive current from flowing through the p-DBR mirror, the series resistance of the entire device is greatly reduced, and as a result, the threshold current value of the laser device is also greatly reduced.

In one embodiment of the present invention, the dry etching for the mesa is performed by a high frequency inductively coupled plasma (HF-I
CP). This plasma method enables very precise etching control and has reproducibility. Hereinafter, a VCSEL device and a method for manufacturing the same according to an embodiment of the present invention will be described with reference to the drawings.

First, in a first stage, as shown in FIG. 1, a SEL structure is formed on an n-GaAs substrate 1 by an epitaxial growth method. An n-GaAs buffer layer having a thickness of 500 μm is formed on the n-GaAs substrate 1, and this layer is included in the substrate 1 in FIG. An n-DBR mirror 2 is formed on the buffer layer. The mirror 2 includes an n-AlAs layer 21 and n-Al _x Ga
_{The (1-x)} As layer 22 is formed by alternately forming, for example, 20 pairs.

On the mirror 2, n-Al_xGa_(1-x)A
s spacer layer 3, non-doped Al_xGa_(1-x)As
SCH (Separate Confinem)
ent Heterostructure) layer 4 is formed
And three pairs of GaAs-Al_xGa_(1-x)From As
An active layer (gain region) 5 is formed. On the active layer 5
Is a SCH layer 6 having the same structure as the layer 4 and p-Al_xGa
_(1-x)Through a spacer layer 7 composed of As,
A p-DBR mirror 8 is formed. This mirror 8 is an example
For example, 20 pairs of p-AlAs layers and p-A
l _xGa_(1-x)It consists of an As layer. P-G on mirror 8
An aAs cap layer 9 is formed. In the table below,
The mixed crystal ratio and film thickness of each layer are shown.

SEL epitaxial growth film material mixed crystal specific film thickness (μm) p-GaAs (9) 60 p-Al _x Ga _(1-x) As (8) x = 0.1 59.8 p-AlAs (8 ) 71.2 p-Al x Ga ( 1-x) As (7) x = 0.45 Al x Ga (1-x) As (6) x = 0.3 150 ~0.45 GaAs (5) 9 _{Al x Ga (1-x)} As (5) x = 0.3 7 GaAs (5) 9 Al x Ga (1-x) As (4) x = 0.45 150 ~0.3 n-Al x Ga _{(1-x) As (3} ) x = 0.45 n-AlAs (2) 71.2 n-Al x Ga (1-x) As (2) x = 0.1 59.8 n-AlAs (2 71.2 n-GaAs (Buffer Layer) 500 As described above, the laser device according to the present embodiment is, for example, an n-D having a 20-layer AlAs / AlGaAs stacked structure. The active layer 5 is a gain medium sandwiched between a BR mirror and a p-DBR mirror, and has a structure including three quantum wells. The active region is GaAs
s / Al _x Ga _(1-x) As
It emits light with a wavelength of 50 nm.

In the second stage, the SEL structure shown in FIG. 1 is dry-etched by HF-ICP using a photoresist as a mask to form a mesa 10 (see FIG. 2).
The shape of the mesa 10 is rectangular. The dry etching is performed including the upper several layers in the n-DBR mirror 2 under the active layer 5. As described above, HF-
Since the ICP method enables precise control of etching, mesa etching including the upper several layers of the n-DBR mirror 2 is possible.

Thereafter, the substrate is exposed to water vapor at 350 ° C. for 1 minute, so that the AlAs layer which is easily oxidized becomes
It is oxidized laterally from the outside to the inside of the mesa,
The structure shown in FIG. In FIG. 2, the dark hatched portion indicates the oxidized portion 11 of the AlAs layer. The AlAs layer of the oxidized portion 11 is replaced with the AlAs layer of the non-oxidized portion.
Since the refractive index of light is lower than that of the layer, the waveguide 12 shown in FIG. 2 is formed in the mesa 10 as a result. At the same time AlA
Since the s layer is oxidized to become an insulator, carriers (current) are effectively confined in the waveguide 12. In this way, light and current are confined. Since the oxidation of the AlAs layer extends to several layers above the n-DBR mirror, light and current are confined in the n-DBR mirror as well as in the p-DBR mirror.

In the third stage, the p-DBR mirror 9 is again mesa-etched into a cylindrical shape by dry etching. This mesa etching is performed at 40 μm, 20 μm, and 10 μm.
Performed using a circular pattern of photoresist (eg, 1400-27) having a diameter of μm,
A cylindrical mesa 13 having a diameter of μm is obtained. In dry etching, BCl ₃ (20 sccm) and Cl ₂ (40
(sccm) by the above-mentioned HF-ICP method using gas. The height of this mesa etching is p
Reaching to most layers of the DBR mirror 9;

Damage due to dry etching is wet
Removed to the order of sub-μm by etching
You. In this wet etching, H_ThreePO_Four: H_TwoO
_Two: H _TwoO = 3: 1: 50 etchant is used
You. Thus, the mesa region 13 (see FIG. 3) is cleaned.
After etching, the entire surface of the substrate is
iO_TwoCover with layer 14. SiO_TwoFor the formation of the layer 14,
A sputtering method is used.

Next, the SiO ₂ layer 14 is HF (40%):
Selective etching is performed with an etching solution of ammonium fluoride (40%) = 1: 7 to expose a part 15 of the SCH layer 6 on the active region 5 to obtain a structure shown in FIG. In the fourth stage,
A p-electrode 16 is formed on the exposed portion 15 as shown in FIG. The p-electrode 16 is a Cr / Au layer formed by a sputtering and lift-off process.

Finally, AuG is deposited by electron beam evaporation.
e: Ni: Au = 50: 50: 250 μm thick n-electrode 17 is formed on the n-DBR mirror 2 to obtain the structure shown in FIG. After forming the electrodes, the substrate is annealed at 410 ° C. for about 30 seconds to make ohmic contacts for each electrode. As described above, the VCSEL device before being housed in the package, which can perform the characteristic test on the wafer, is completed.

In the vertical cavity surface emitting laser device having the structure described above, the p-electrode 16 is provided not on the output window of the p-DBR mirror 8 but on the active layer.
The current for driving this laser device does not flow through the p-DBR mirror 8. Since the p-DBR mirror 8 has a low concentration of p-type impurities, it serves as a main series resistance source of the entire laser device. Therefore, when a driving current does not flow through the mirror 8, the series resistance value of the laser device is reduced. Dramatically reduced. As a result, the threshold voltage of the device decreases.

The upper several layers of the n-DBR mirror 2 below the active layer are mesa-etched, and a part thereof is removed, and the n-electrode 17 is formed on the removed part. Therefore, the drive current does not flow through most of the n-DBR mirror, and the series resistance further decreases. At the same time, the mesa-etched n-DBR mirror 2
The entire p-DBR mirror including the upper several layers is subjected to the lateral oxidation treatment, so that the waveguide and the current path are narrowed. As a result, the effect of confining light and current is improved.

Further, as described above, the series resistance value is reduced, and the current is forced to flow laterally through the n-DBR mirror. As a result, the threshold current value is reduced. This means that the driving current of the laser device is reduced, and as a result, high-speed driving is possible. When the drive current is reduced, the amount of heat generated during drive is also reduced, and as a result, the life of the device is improved. At the same time, a decrease in the calorific value prevents a change in the refractive index of the p-DBR mirror,
High light confinement is maintained.

Considering the above effects in the above equation (4), the lateral confinement of the upper layers of the p-DBR mirror and the n-DBR mirror improves the light confinement effect. The variable α (loss per unit length due to absorption or scattering) in (4) decreases. At the same time, the current confinement effect is also improved, so that the quantum efficiency η (electron-photon conversion efficiency) is also improved, and as a result, the threshold current value J _th is considered to decrease.

[0031]

As described above, according to the vertical cavity surface emitting laser device and the method of manufacturing the same according to the embodiment, the laser drive current does not flow through the upper and lower DBR mirrors, so that the DC resistance is reduced. The value is greatly reduced, and the threshold voltage / current value is greatly reduced. In addition, high current confinement is expected together with high light confinement effect. As a result, a very short light pulse can be obtained at high speed and high output, and the life of the laser device is also improved, so that its practical effect is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing a layer structure by epitaxial growth of a vertical cavity surface emitting laser device of the present invention.

FIG. 2 is a diagram showing a state where the layer structure shown in FIG. 1 is subjected to mesa etching and oxidation treatment.

FIG. 3 shows the structure shown in FIG.
-The figure which shows the state which performed the exposure process for electrode formation.

FIG. 4 is a diagram showing a state where a p-electrode is formed in the structure shown in FIG. 3;

FIG. 5 is a diagram showing a state in which an n-electrode is further formed in the structure shown in FIG. 4;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... n-GaAs substrate 2 ... n-DBR mirror 3, 7 ... spacer layer 4, 6 ... SCH layer 5 ... active layer 8 ... p-DBR mirror 9 ... cap layer 10 ... mesa structure 11 ... oxide part 12 ... waveguide 13: Mesa on cylinder 14: SiO ₂ layer 16: p-electrode 17: n-electrode

Claims (7)

[Claims] 1. A semiconductor substrate and a first substrate formed on the semiconductor substrate.
A vertical cavity surface emitting laser device having a DBR mirror, an active layer serving as a gain region formed on the first DBR mirror, and a second DBR mirror formed on the active layer. The upper layer portion of the DBR mirror and the active layer form a first mesa on the first DBR mirror, and the second DBR mirror has a second mesa on the active layer that is smaller than the first mesa. Forming a mesa, wherein the first and second mesas have a narrow waveguide therein by being oxidized at least in part by a lateral oxidation treatment;
A first electrode is formed on the DBR mirror, and a second electrode is formed around a side surface of the second mesa on the first mesa. . 2. The vertical cavity surface emitting laser device according to claim 1, wherein the second DBR mirror has a relatively low resistance value with a low impurity concentration. 3. The first and second DBR mirrors,
3. The vertical cavity surface emitting laser device according to claim 1, wherein the vertical cavity surface emitting laser device is formed by alternately repeating first and second layers having different compositions a plurality of times and epitaxially growing the layers. 4. A step of laminating and epitaxially growing a plurality of layers to form a first DBR mirror, an active layer, and a second DBR mirror on a semiconductor substrate in this order; Forming a first mesa by mesa-etching the active layer and the second DBR mirror including upper several layers; and placing the semiconductor substrate having the formed first mesa in an oxidizing atmosphere. Oxidizing the first mesa from the outer periphery while leaving the center thereof, and selectively mesa-etching the second DBR mirror portion of the first mesa so that the first mesa has a bottom area smaller than that of the first mesa. Forming a small second mesa; forming a first electrode on the upper surface of the first DBR mirror exposed by the first mesa etching; and exposing the second electrode by the second mesa etching Forming a second electrode on the serial active layer top surface, a vertical cavity surface emitting laser manufacturing method, including capital. 5. The vertical cavity surface emitting laser according to claim 4, wherein the first mesa etching is performed by a high-frequency inductively coupled plasma method capable of precisely controlling the etching. Production method. 6. The vertical resonance according to claim 4, wherein the step of oxidizing the first mesa is performed by exposing the substrate to a water vapor atmosphere at about 350 ° C. for about 1 minute. Manufacturing method of surface emitting laser. 7. The method according to claim 4, wherein the second mesa etching is performed by a high-frequency inductively coupled plasma method capable of precisely controlling the etching. A method for manufacturing a vertical cavity surface emitting laser.