RU2554302C2 - Vertically emitting laser with bragg mirrors and intracavity metal contacts - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: inside the cavity of a vertically emitting laser with Bragg mirrors and intracavity metal contacts, between a Bragg reflector and the active region there are metal layers which are simultaneously contacts and elements of the cavity which generates the characteristic mode of the electromagnetic field, wherein the thickness of the layers of the Bragg reflector adjacent to the metal layer differs from the other layers of the Bragg reflector, which provides such a spatial structure of the characteristic mode of the electromagnetic field used for laser generation that electrical field nodes coincide with the metal layers on position, which significantly reduces light absorption by the metal layers, while providing maximum overlapping of the electric field of the characteristic mode of the laser and the active region.

EFFECT: low electrical resistance of the structure, ensuring uniform pumping electrical current and inhibiting light absorption by metal layers.

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Description

The present invention relates to vertical-emitting lasers based on heterostructures operating in the visible, infrared and ultraviolet range.

A vertical-emitting laser (VIL, VCSEL (Vertical-Cavity Surface-Emitting Laser)) is a surface-emitting semiconductor laser with a vertical cavity resonator.

The design of VILs was first proposed in the late 70s of the last century [1], however, the development of semiconductor technologies made it possible to create VILs suitable for practical use only by the end of the 1990s [2].

Modern VIL design options are based on the use of vertical optical microresonators with mirrors in the form of distributed Bragg reflectors (BO) based on alternating quarter-wave layers of semiconductor materials of various compositions. In this case, as an active (light-emitting) region, as a rule, one or several quantum wells or an array of quantum dots [3], located near the maximum distribution of the optical field, are used.

VILs are the most promising radiation sources for local high-speed fiber-optic communication systems. With their help, record data transfer rates (more than 40 Gbit / s per channel [3]) in fiber lines of short distances [4] were demonstrated. Also in recent years, VIL has been increasingly used in sensors and sensors of various types [5], high-performance computer systems.

The main advantages of VILs compared to traditional strip-type injection lasers include low angular divergence and a symmetrical radiation pattern of the output optical radiation, the ability to provide submilliampere threshold currents, increased temperature and radiation stability, group manufacturing technology, and the ability to test devices directly on the wafer. The planar VIL technology allows the formation of integrated linear arrays and two-dimensional matrices with a large number of individually addressable emitters.

As a rule, VILs are grown on crystalline semiconductor or dielectric substrates; it is also possible to grow VILs on metal substrates [6]. Some designs provide for the location of metal layers between the Bragg reflectors and the substrate to increase the reflection coefficient of the Bragg mirrors [7].

The main technological problem of the implementation of VIL is the complexity of epitaxial synthesis and post-growth processing of high-alloy low-resistance RBOs. Also, high doping of conductive mirrors leads to a significant increase in optical losses due to absorption on free carriers and a high parasitic capacitance, which may limit the frequency range of devices. In this case, the electrical resistance of the Bragg mirrors leads to a decrease in the laser efficiency, and their heating due to ohmic losses degrades the characteristics of the laser and can lead to its degradation.

To create VILs with improved characteristics, it is interesting to peel active layers from the initial substrate and deposit them onto a new one using lift-off technology, which was first demonstrated to create devices based on GaAs / AlAs heterostructures [8]. In particular, this technology makes it possible to create VILs by epitaxial growing of the active region on a substrate, removing the substrate, and growing dielectric Bragg mirrors by magnetron sputtering on an active region with high crystalline perfection [9]. The lift-off technology allows you to remove the epitaxial layer from the substrate during post-growth processing. After that, the removed layer can be attached to a new substrate [10]. There are several methods for removing the substrate: selective etching, wet longitudinal oxidation, and laser removal. The selective etching method removes the sacrificial layer with a thickness of 10-50 nm, additionally grown between the active layers and the substrate. Using this method, layers from substrates with a diameter of up to 6 inches can be removed without material degradation or performance degradation [11]. Laser removal is used to remove LED structures based on III-N materials from a sapphire substrate [12]. The radiation of an excimer Kr-F laser with a wavelength of 248 nm is directed to the heterostructure through a sapphire substrate. Gallium nitride begins to decompose, and a sharp release of gaseous nitrogen occurs. The heterostructure undergoes a strong mechanical shock and the sapphire substrate is separated. The lift-off technology has already been successfully implemented in a number of semiconductor devices, such as laser diodes [13] and VIL [14]

An important direction in the development of optoelectronics is the miniaturization of lasers. One way to reduce the size of devices is to use metal cavities of various types [15]. In such structures, the characteristic volume of the laser mode is achieved, which is substantially smaller than the cube of the radiation wavelength. In some cases, lasers with metal cavities of a certain design are distinguished into a separate class of structures, the so-called “spasers" (SPASER, Surface plasmon amplification by stimulated emission of radiation) [16, 17]. In such structures, the light-emitting active region interacts with localized surface plasmons.

In 2007, it was theoretically predicted [18], and subsequently discovered [19] a new type of state of the electromagnetic field localized at the boundary of the metal and the Bragg reflector (Tamm plasmon). In a nanostructure formed by a Bragg reflector based on GaAs / AlAs with built-in quantum wells and a layer of silver, laser pumping was demonstrated at the temperature of liquid nitrogen [20].

In lasers with metal resonators, the absorption of light in a metal significantly reduces the quality factor of the resonators. As a result, the threshold pump level rises. A laser based on a metal cavity operating at room temperature and electrically pumped was demonstrated in 2013 by a group from the USA [21]. To significantly increase the characteristics of semiconductor lasers with metal resonators, it is necessary to reduce the absorption of light in the metal elements of the laser.

Closest to the proposed invention and adopted as a prototype is a vertical-emitting laser. The disadvantages of such lasers are optical losses on free carriers due to the high doping of the conductive mirrors and a sufficiently high electrical resistance of the Bragg mirrors, which leads to a decrease in the laser efficiency and degradation of its characteristics. In this regard, there is a need to increase the efficiency and improve the parameters of laser radiation of the laser.

The problem solved by the present invention is the creation of vertically emitting lasers with homogeneous layered metal contacts located inside the resonator, with an increased efficiency and improved parameters of laser radiation.

The technical result that allows us to complete the task is to reduce the electrical resistance of the structure, ensuring uniformity of the pump electric current, and also suppressing the absorption of light by metal layers.

The result is achieved due to the fact that inside the resonator of a vertically emitting laser with Bragg mirrors and intracavity metal contacts, metal layers are introduced between the Bragg reflector and the active region, which are both contacts and resonator elements forming the eigenmode of the electromagnetic field, the layer thickness of the Bragg reflector being adjacent to the metal layer, differs from the other layers of the Bragg reflector, which provides such spaces nnuyu structure eigenmode of the electromagnetic field used for lasing, the electric field components that are the same in position with the metal layers, which significantly reduces the absorption of light by the metal layers, while ensuring the maximum overlap of the electric field eigenmode of the laser and the active region.

The design of a vertical-emitting laser with Bragg mirrors and intracavity metal contacts includes:

- lower Bragg mirror (1, figure 1);

- phase-matching layer of the Bragg reflector of a modified thickness (2, figure 1);

- metal layer (3, figure 1);

- light emitting active region (4, Fig. 1);

- metal layer (5, figure 1);

- phase matching layer of the Bragg reflector of a modified thickness (6, Fig. 1);

- Bragg reflector (7, figure 1).

Figure 1 presents the basic diagram of the cross section of a nanoheterostructure selected as a prototype of the claimed invention. The following successive layers and their composition are indicated. Bragg reflectors are multilayer periodic structures.

Figure 2 presents a detailed cross-sectional diagram of a nanoheterostructure of a special original design, demonstrating the essence of the claimed invention. The following successive layers and their composition are indicated. The profile of the electric field of the eigenmode is shown.

The structure of the VIL with Bragg mirrors and intracavity metal contacts with the semiconductor active region is made as follows:

1) a light-emitting active region with quantum wells or quantum dots is grown on a crystalline substrate by the epitaxy method (4, Fig. 1), and near the edges of the active region, the doping level is chosen sufficient to create ohmic contacts, and a “sacrificial” layer is located between the active region and the substrate ,

2) a metal contact layer is sprayed onto the active region (3, Fig. 1),

3) a phase matching layer of the Bragg reflector is grown on a metal layer (2, Fig. 1),

4) a Bragg reflector is grown on a phase-matching layer by the method of magnetron sputtering (1, Fig. 1),

5) the grown structure is separated from the substrate, the “sacrificial” layer is removed,

6) on the opened side of the active region a metal layer is sprayed (5, Fig. 1),

7) a phase matching layer of the Bragg reflector is grown on a metal layer (6, Fig. 1),

8) a Bragg reflector is grown on a phase-matching layer by the method of magnetron sputtering (7, Fig. 1).

The structure of the VIL with Bragg mirrors and intracavity metal contacts with the active region of organic material is made as follows:

1) a Bragg reflector (1, Fig. 1) is grown on an amorphous or crystalline substrate, on which a phase matching (2, Fig. 1) and a metal layer (3, Fig. 1) are grown,

2) an organic active region is applied to the metal layer by spin-coating or chemical deposition (4, Fig. 1),

3) a metal layer is applied to the active region (5, FIG. 1), on which a phase matching layer is grown (FIGS. 1, 6). and a Bragg reflector (7, FIG. 1).

The characteristic thicknesses of the VIL layers with Bragg mirrors and intracavity metal contacts:

- light-emitting active region with quantum wells or quantum dots 100-400 nm (2, figure 2)

- intracavity metal contact layers 20-50 nm (3, figure 2)

- phase matching layers - 40-100 nm (4, figure 2)

- layers of the Bragg reflector 50-150 nm (5, figure 2).

The thicknesses of the layers are selected in such a way as to ensure the location of the nodes of the electric field of the eigenmodes of the resonator, which coincides with the metal layers and the maximum overlap of the eigenmodes of the electric field with the active region (figure 2).

The creation, in accordance with the declared features and design, of an IDF with Bragg mirrors and intracavity metal contacts makes it possible to manufacture lasers by vertical emission of radiation having improved characteristics provided by a uniform pump current density in the active region and increased efficiency due to the supply of the pump current directly from the metal contacts to the active area.

BIBLIOGRAPHY

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Claims (1)

A vertically emitting laser with Bragg mirrors and intracavity metal contacts, characterized in that metal layers are introduced into the cavity between the Bragg reflector and the active region, which are simultaneously contacts and resonator elements forming an eigenmode of the electromagnetic field, the layer thickness of the Bragg reflector adjacent to the metal layer, differs from other layers of the Bragg reflector, which provides such a spatial structure with of the natural mode of the electromagnetic field used for laser generation, that the nodes of the electric field coincide in position with the metal layers, which significantly reduces the absorption of light by metal layers, while ensuring the maximum overlap of the electric field of the eigenmode of the laser and the active region.

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