JPS614291A - Surface light-emission laser - Google Patents

Abstract

PURPOSE:To provide a surface light-emission laser, which can be implemented even at a low threshold current value, by forming a light resonator structure with an active layer, which comprises a semiconductor heterogeneous multilayer film and has an active function and a distribution feedback type light resonator function; providing clad layer for confining minority carriers; and arranging said structure and layer vertically with respect to a substrate side by side. CONSTITUTION:A light resonator structure 11 with an active layer comprises a semiconductor heterogeneous multilayer film and it is set in a P type. Therefore, minority carriers, i.e., electrons in this case, can be confined by energy- gap differences between a P type clad layer 12 and a P type active layer in the structur 11. A part, which is located at the right side of the light resonator structure 11 with active layer, is a carrier supplying layer 16 (electrons in this case), which is set in an N type by doping suitable atoms in the semiconductor heterogeneous multilayer film. An electrode 15 on the side of the N layer is formed on the upper surface of the carrier supplying layer 16. An electrode 14 on the side of the P type clad layer is formed on the surface of the etched- out part on the left.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to lasers, and more particularly to improvements in distributed feedback surface emitting lasers.

A surface emitting laser that can obtain laser output light perpendicular to the substrate does not require the 9J opening process. Due to its advantages such as ease of increasing the area, applications to laser arrays and high-output lasers are being considered.

・In particular, surface-emitting lasers using semiconductor heteroepitaxial multilayer films can monolithically form an optical resonator with wavelength selectivity and an active layer in -times of epitaxial growth, so there is no need for substrate etching to form a reflective layer. It has the added advantage of having a stable oscillation wavelength, and is expected to be advantageous for integration.

However, since the laser length of this type of surface emitting laser is generally short and sufficient gain cannot be obtained, an element with a practical threshold current has not yet been obtained.

Generally, the oscillation condition of a semiconductor laser is satisfied when the product of the laser gain and the laser length exceeds the loss of the optical resonator.

Among these, the laser gain is controlled by the minority carrier confinement efficiency and the coupling efficiency between the carrier recombination region and the optical resonator.

Further, the laser length corresponds to the length in the optical path direction between the optical resonators of the active layer, and is limited by the minority carrier diffusion length or the thickness of the epitaxial layer in the case of a surface emitting laser. .

On the other hand, the loss of the optical resonator is expressed as In(1/R), where R is the equivalent reflectance of the optical resonator nose end surface. For example, if the reflectance is increased to 85%, the loss itself is approximately It can be reduced to 1/20.

A conventional surface emitting laser basically has a pn junction consisting of an active layer 4 formed in a layered manner and cladding layers 3 and 5 above and below the active layer 4, as shown in FIG. A vertical optical resonator is constructed by providing a metal mirror 1 that also functions as a current injection electrode for this laser, and the output laser beam is directed to the mirror 1.1 as shown by the large arrow.
It is emitted in the opposite direction.

In addition, as is already known, in order to cause the desired current confinement effect, a portion to be constricted is left under the upper metal layer L.
7 is provided with an insulating layer 2 that covers the other parts, and the metal layer l is provided with an upper cladding layer 3 in the part where this insulating layer 2 is not provided.
Similarly, the substrate 6 is also formed in the narrowed area, and the lower metal layer 1 is also in contact with the raft layer 5 on the side of the substrate 5 in this formed area.

However, in the case of such a structure, the direction of current injection and the direction of optical resonance by the optical resonator coincide, so if the thickness L of the active layer 4 is reduced in order to increase the injection density of minority carriers, this will have the opposite effect. The result is that the laser length of the light emitting laser is shortened. Therefore, in this type of laser element, it has been impossible to increase the laser length beyond about one leg. Further, in terms of structure, since etching of the substrate is required to form the above-mentioned production-limited portions, there are also disadvantages in that integration is difficult and mechanical strength is weakened.

On the other hand, as shown in FIG.
There is also a conventional structure that uses semiconductor hetero multilayer films 8 and 9 as an oscillator reflector, but although it can avoid mechanical strength problems and the need for substrate processing, it does not change the situation that it is difficult to increase the laser length. Ta. The same reference numerals refer to both components in the conventional example shown in FIG. It is sandwiched between a multilayer S mirror 9, and the metal layers 7, 7 on both the upper and lower end surfaces of the element function only as electrodes.

In any case, in order to increase the laser gain of such a vertical oscillation type surface emitting laser, it is necessary to remove the contradictory elements shown in the above two sides and do not impair the minority carrier confinement efficiency. It is necessary to obtain sufficient laser length.

However, it is not just a matter of increasing the length of the laser; making it too long may cause manufacturing problems.6 For example, as shown in Figure 1 (G), the conventional horizontal DH (Double Peter junction) Let's assume a vertical oscillation type device by simply laying the laser on its side. The numerals and symbols in the figure correspond to those in FIG. The fold-opening surfaces 9.9 located at the upper and lower ends of the active layer 4 in orthogonal directions serve as mirrors for forming the west resonator.・In such a case, the laser length is the distance indicated by the symbol in the diagram, and it can be quite long, but conversely, in order to ensure the same characteristics as the original horizontal type in the structure, it is necessary to completely remove the laser length from the existing horizontal type. Similarly, the actual length L must be 150 to 200 lengths. This is way too long.

This is because, in order to actually make a laser with the configuration shown in Figure 1(C), it is not possible to form each layer vertically one by one and then lay them horizontally and assemble them, as in the original horizontal type. Especially when considering integration, it must be fabricated from the beginning on an appropriate substrate (not shown), lying horizontally as shown in the figure. Must do. However, as mentioned above, the laser length, which ranges from 1503 nm to 200 legs, far exceeds the thickness limit that can be produced at once using existing epitaxial growth techniques.

In addition, structurally, such a hypothetical element also loses the inherent advantage of a surface-emitting laser, which is that it does not require an arm-opening surface.

The present invention has been made in view of these conventional circumstances, and improves the current injection and minority carrier confinement structure of a distributed feedback surface emitting laser having an active layer of a semiconductor heteromultilayer film. It is an object of the present invention to provide a practical surface-emitting laser that eliminates the drawbacks of the previous example and has a low threshold current and is easily manufactured.

In order to achieve these objects, the present invention provides an active layer non-optical resonator structure having both an active function and a feedback type optical resonant function constituted by a semiconductor heteromultilayer; We propose that the cladding layers for confining minority carriers are formed perpendicularly to the substrate and in parallel with each other.

In this way, sufficient laser gain can be obtained even in a film thickness range that can be manufactured using ordinary epitaxial growth techniques, etc., and a practical vertical oscillation type surface emitting laser with a low threshold current can be provided.

The semiconductor heteromultilayer film itself used in the active layer non-optical resonator structure of the present invention has already been disclosed by the present applicant, and in JP-A-5
9-313988%/,' [4: 1ゎwork...

It may be a known one such as.

For example, in the one described in the above-mentioned publication, the basic period of change in refractive index in the thickness direction of the semiconductor hetero multilayer film is 1/2 of the tube wavelength of the laser beam, and the refractive index is 1/2 of this 1/2 wavelength. The fundamental period is an integral multiple of 1/4 pipe wavelength (including l)
A high refractive index medium layer having a thickness of
Alternatively, it is obtained by continuously changing the composition in the thickness direction with a period of 1/2 tube wavelength.

Examples of the basic structure of the present invention using such a multilayer film include the structures shown in FIGS. 2(A) and 2(B).

In the embodiment shown in FIG. 2(A), a semiconductor hetero multilayer film with a 1/2 wavelength period, which is disclosed in the above-mentioned Japanese Patent Laid-Open No. 59-38988, is formed vertically or vertically on a substrate IO. A p-type rad layer 12 for confining minority carriers is formed on one side in the lateral direction, and a p-type rad layer 12 is formed on the other side in the lateral direction. In each case, an n-type cladding layer 13 is similarly formed in a direction perpendicular to the substrate 10. Accordingly, current injection electrodes 14.15 are also formed along the outer sides of both cladding layers 12.13, perpendicular to the substrate IO.

With such a structure, the laser length can be shortened to about 1/20 of the hypothetical device structure shown in FIG. 1(C) without increasing the threshold current. That is, the first
In the case shown in Figure (C), the laser length is required to be 15011m to 200JIm to realize this hypothetical structure.However, in the structure of the embodiment of the present invention shown in Figure 2(^), the laser length is 7 to 10J1m is sufficient.

With a thickness of this order, the active layer/optical resonator structure 11 and the cladding layers 12, 13 on both sides thereof can be easily formed using existing epitaxial growth techniques.

However, this thickness or laser riL is sufficiently long compared to the realized conventional device shown in FIGS. 1(^) and 1(B).

In other words, the conventional frugal element f shown in FIGS. 1(A) and (B)
- Compared to the structure of this embodiment, the current injection direction and the direction of the optical resonator are perpendicular to each other. I.J.
It can be made sufficiently short to about 1 m, and the minority carrier confinement efficiency can be greatly increased.

Furthermore, in a separate comparison with the conventional element shown in FIG. 1B, in which a semiconductor hetero multilayer film is similarly used only for the reflector, in this element, the active layer 4 is formed by the upper and lower multilayer films 8, 91. In contrast, in the device of the present invention, in the embodiment shown in FIG. 2(A), as well as in the embodiment shown in FIG. 2(CB) and FIG. Since all of the low energy band gap material films in film 11 can be used as active layers, the laser length can be increased without the limitation of minority carrier diffusion length. Furthermore, since the injection of minority carriers is performed along the barrier of the engineering band gap of the semiconductor heteromultilayer film, there is also the advantage that parasitic resistance is reduced.

The actual manufacturing procedure of the example device shown in FIG.
After forming a layer to an appropriate thickness, one side of the layer is removed by step etching, leaving a portion of the required width W in the figure, and then a p-type cladding layer 12 is formed on this removed portion. Then, the opposite side is removed again by step etching, leaving a portion of the required width W in the figure, and the n-type cladding layer 13 is grown in a liquid phase on this removed portion, or alternatively, selective MBE, which has been developed in recent years, is used. It is conceivable to selectively form both the cladding layers 12 and 13 using a method and selective ion plantation.

In the second embodiment shown in FIG. 2(B), the atomic mixing effect due to Zn diffusion is used to form the p-type cladding layer 12, and the active layer/optical resonator structure II is set to be p-type. By doing so, minority carriers, that is, electrons in this case, can be confined due to the energy gap difference between the p-type cladding layer 12 and the p-type active layer in the structure 11.

A further difference from the first embodiment is that instead of the n-type cladding layer 13, the portion located on the right side of the active layer/optical resonator structure 11 is doped with appropriate atoms into the semiconductor hetero multilayer film. Therefore, the carrier (electron in this case) supply layer 1B is set to be n-type. The n-layer electrode 15 is formed on the upper surface of the carrier supply layer 16.

Figure 3 (A) shows a more specific example in which the basic structure shown in Figure 2 (B) is realized as an actual element using an AlGaAs system, and Figure 3 (B) shows a scanning electron microscope. (
SEM) The cross-sectional structure is shown in a photograph.

As a starting wafer for producing a surface-emitting laser device with this structure, an undoped GaAs substrate 10 is used as a starting wafer.
As a semiconductor hetero multilayer film used to form each layer of the p-type cladding layer 12, the active layer/optical resonator structure 11, and the electron supply layer 18, 50 pairs of AIn, 3Gao, 7As (58 nm) and GaAs (83 nm) formed by the MBE method were used.

In the cross-sectional photograph in FIG. 3(B), among the horizontal stripes, white stripes are AIo, 3Gao, and yAs thin films, and black stripes are GaAs thin films. The total thickness of the epitaxial layer on this substrate 10 is six legs including the GaAS buffer layer.

In the active layer/optical resonator structure 11 of the present invention, the vertical distributed feedback optical resonator is made of AlGa
It is formed by the unevenness of the refractive index of the As/GaAs multilayer film.

Si is added to the region located on the right side of the active layer/optical resonator structure 11 in the AlGaAs-based GaAs multilayer film during MBE growth, and an n-type electron supply region Jl of the Si dovetail is added.
&1G, and the carrier concentration 1m corresponding to the GaAs layer is 2×1018c+a-3.

It shows a P-type region diffused outside the left-hand side portion dotted in FIG. The dotted portion 12 becomes the p-type cladding layer 12.

In FIG. 3(A), by diffusion of Zn from the etching step shown by the imaginary line Es to the right side in the lateral direction, a substantially perpendicular pn junction boundary is obtained as shown in FIG. 3(B). .

In the actual production of this device, a SiN film (CVD) is used as a mask for step etching and Zn diffusion.
1000 people) were used. Also, step.

Etching conditions are 30% H2O2: 30% NH4
The film was left at room temperature for 8.5 minutes at OH:H2O=5:40:10 (VIllll), and a mask was formed in the (110) direction corresponding to the reverse mesa.

The conditions for Zn diffusion were 1100'C! , 14 hours.

Under these diffusion conditions, in the case of a GaAs substrate, about three legs of the p-type layer 12 with a carrier concentration of 7.times.10.sup.19 cm.sup.-3 are formed.

Next, the diffused wafer was heat-treated at 1000° C. for 1 minute in a quartz ampoule while facing the dummy GaAs substrate. This heat treatment improved the luminous efficiency of the device by about -0.

In FIG. 3B, the contrast of the horizontal stripes due to the multilayer film has completely disappeared in a range approximately two rows to the right from the etching step Es. This is due to the diffusion of Zn.
The Ga, As/GaAs multilayer film has a uniform A1 composition.
This shows that it has been transformed into an lGaAs region 12. Since this p-type AlGaAs layer 12 acts as a diffusion barrier for electrons injected from the n-type electron supply layer IO side, an electron confinement effect can be expected.

The pfJ region due to Zn diffusion is an etching stela.

A region 11 is formed which extends further by about IJ1 m from the right-hand side boundary of the uniform AlGaAs layer near the top.

This region is shown in FIG. 3(B) as a multilayer film region 11 with strong horizontal striped conolasts, and this region 1
1 is an active layer/optical resonator structure 11 of the main-emitting laser.

However, as mentioned earlier, in order to achieve low threshold current properties, it is desirable to concentrate the injected current in as small a volume as possible. Therefore, in the actual implementation of this device, the width of the pn junction formed by Zn diffusion was limited to about 3JIIn.

Moreover, the loss of the optical resonator is ``the second part and the bottom reflection 4
7, □. Ah□. E1.4.20.9 As an engineering consideration, it is desirable to inject the current into the geometrical center of the optical resonator.

In order to satisfy this condition, the electrode 1 on the n-type electron supply layer 18 side
No. 5 was formed by digging one layer into the substrate 10 side, and by making 10 pairs of multilayer films from the bottom near the substrate undoped, undesirable current injection into the surface and substrate side was restricted.

The electrode 14 on the P-type rad layer 12 side is formed on the surface of the etched portion on the left side in FIG. 3(A).

As shown in FIG. 3(B), in this embodiment, the thickness of the GaAs layer 21 near the center is set to be twice that of the other layers. This is to match the phase conditions of the light that is multiple-reflected on the reflective layer consisting of the upper 20 pairs and the lower 30 pairs, and serves as a kind of phase shifter 21. In this way, since the laser oscillation wavelength and the Bragg period of the multilayer film match, high wavelength selectivity and reflectance can be maintained under short cavity conditions.

FIG. 4 shows the oscillation spectrum of the laser shown in the third figure. According to this figure, the threshold current of the created laser is 15
It can be seen that the current is 120mA at 0K and 80mA (pulse) at 54K, which is sufficiently low.

Furthermore, the width of the two oscillation spectra is sufficiently narrow, within five layers, as shown in J2 on the left in the figure.

The temperature characteristic of the oscillation spectrum is 0.48 A/2 between 54 K and 125 K. This value is the same as the conventional horizontal DFB.
(distributed feedback) lasers. The dependence of the oscillation wavelength on the excitation current can also be ignored up to about twice the threshold current. Therefore, the stability of the oscillation frequency due to the distributed feedback optical resonator of the third illustrated surface emitting laser constructed according to the idea of the present invention can be confirmed.

In the embodiment shown in the third figure, the semiconductor heteromultilayer film used is an AlGaAs layer and a GnAs system, but other systems may be used as long as continuous heteroepitaxy is possible and lattice matching can be achieved. For example, in the case of TI-IV group compound semiconductors, zinc-chalcogenide based,
In the m-v group, Ga1nAsP system, C1aA, 1InP
Examples include GaAlSb type, GaAlSb type, and the like.

In any case, as described above, the minority carrier confinement structure and lateral current injection structure according to the present invention using an active layer/optical resonator structure made of a semiconductor heteromultilayer film can stabilize the wavelength of a distributed feedback surface emitting laser. This has an extremely large effect in that it can guarantee high performance and low threshold current characteristics, and can enable continuous oscillation at room temperature for the first time.

[Brief explanation of drawings]

Figure 1 is a schematic configuration diagram of a surface emitting laser that can be estimated from conventional existing and horizontal lasers, Figure 2 is a schematic configuration diagram of a basic embodiment of the present invention, and Figure 3 is similar to Figure 2 (B). FIG. 4 is a perspective view showing a schematic configuration of a surface-emitting laser according to a more specific embodiment created in accordance with the basic structure shown, and FIG. 4 is a laser oscillation spectrum of the third illustrated embodiment. In the figure, 10 is a substrate, 11 is an active layer/chip structure made of a semiconductor hetero multilayer film, and 12 is a P-type re-rad (see Fig. 3).
A)!

Claims (1)

[Scope of Claims] An active layer/optical resonator structure having an active function and a distributed feedback optical resonance function constructed using a semiconductor hetero multilayer film; A distributed feedback surface emitting laser comprising: a cladding layer; the active layer/optical resonator structure and the cladding layer are each formed perpendicularly to the substrate and in parallel with each other.