Patent Description:
The two-dimensional photonic-crystal laser includes an active layer and a two-dimensional photonic-crystal layer. The active layer generates light within a specific wavelength band when supplied with carriers (carrier holes or electrons). The two-dimensional photonic-crystal layer has a configuration including a plate-shaped base body in which modified refractive index areas whose refractive index differs from that of the base body are periodically arranged two-dimensionally. The modified refractive index areas include holes (air) formed in the base body or a member different from the material of the base body. In the two-dimensional photonic-crystal laser, only a specific wavelength of light corresponding to the lattice constant of the arrangement of the modified refractive index areas in the light generated in the active layer is amplified and causes a laser oscillation, to be eventually emitted as a laser beam in a direction perpendicular to the photonic-crystal layer.

The two-dimensional photonic-crystal laser generally includes layers having various functions in addition to the active layer and the two-dimensional photonic-crystal layer described above. For example, the two-dimensional photonic-crystal laser described in Patent Literature <NUM> has a structure in which a first cladding layer, an active layer, a carrier block layer, a two-dimensional photonic-crystal layer, a second cladding layer, and a contact layer are stacked in the mentioned order on a substrate. The lower face of the substrate and the upper face of the contact layer are each provided with an electrode. Each layer other than the electrode is stacked by epitaxially growing on the substrate. As a material of the substrate, an n-type semiconductor, which is less expensive than a p-type semiconductor, is used. For the first cladding layer, a semiconductor having the same polarity as the substrate, n-type, is used, and for the carrier block layer, the base body of the two-dimensional photonic-crystal layer, the second cladding layer, and the contact layer, p-type semiconductors are used. In this type of two-dimensional photonic-crystal laser, carrier holes are supplied into the active layer from the upper electrode through the contact layer, the second cladding layer, and the base body of the two-dimensional photonic-crystal layer, and electrons are supplied into the active layer from the lower electrode through the substrate and the first cladding layer.

The first cladding layer and the second cladding layer are provided in order to enhance the confinement of light between the first cladding layer and the second cladding layer, so that the light emission efficiency in the active layer and the light amplification efficiency in the photonic-crystal layer are improved. The carrier block layer is provided in order to prevent carrier electrons from entering the photonic-crystal layer. The contact layer is provided in order to facilitate supply of carrier holes from the upper electrode.

The two-dimensional photonic-crystal laser described in Patent Literature <NUM> uses, as described above, a p-type semiconductor material as a base body of the two-dimensional photonic-crystal layer. In general, since carrier holes are lower in mobility than electrons, the material of a base body of the two-dimensional photonic-crystal layer is selected from p-type semiconductors in which the band gap, the concentration of impurities, and the like are designed so that the density of carriers (holes) in the two-dimensional photonic-crystal layer is higher than the density of carriers (electrons) in the substrate using an n-type semiconductor and the first and second cladding layers using an n-type semiconductor. However, when light generated in the active layer is amplified in the two-dimensional photonic-crystal layer, carrier holes which are free carriers absorb a portion of light, and thus, there arises a problem that when the density of carrier holes in the two-dimensional photonic-crystal layer increases, the efficiency of laser oscillation decreases.

Patent Literature <NUM> describes a two-dimensional photonic-crystal laser having a structure similar to that of Patent Literature <NUM>, in which it describes a substrate and a first cladding layer including a p-type semiconductor may be used, and a second cladding layer and a contact layer including an n-type semiconductor may be used. In this configuration, since the n-type semiconductor is used as the base body of the two-dimensional photonic-crystal layer, the carrier (electron) density can be made lower than that in the case of using the p-type semiconductor. Therefore, light absorption can be suppressed. However, since the substrate needs to be thicker than other layers, if a p-type semiconductor, which is more expensive than an n-type semiconductor, is used as the substrate, the material cost of the two-dimensional photonic-crystal laser rises.

Patent Literature <NUM> describes a photonic-crystal laser having a configuration in which a cladding layer made of an n-type semiconductor, a two-dimensional photonic-crystal layer having a base body made of an n-type semiconductor, an active layer, a carrier block layer made of a p-type semiconductor, a cladding layer made of a p-type semiconductor, and a contact layer made of a p-type semiconductor are stacked in the mentioned order on a substrate made of an n-type semiconductor. Also in this configuration, since the base body of the two-dimensional photonic-crystal layer is an n-type semiconductor, absorption of light by free carriers can be suppressed as compared with the case of using a p-type semiconductor. However, in a case where the modified refractive index areas of the two-dimensional photonic-crystal layer include holes, irregularities are formed on the surface opposite to the substrate. Also in a case where the modified refractive index areas are made of members having a material different from that of the base body, it is difficult to prepare the modified refractive index areas without forming irregularities on the surface of the two-dimensional photonic-crystal layer. Even when another layer is formed on the face having such irregularities, the irregularities remain on the upper face of the layer. In the semiconductor light-emitting element of Patent Literature <NUM>, it is necessary to stack an active layer on such a face having irregularities, but since the active layer is generally made by stacking a plurality of semiconductor layers thinner than other layers in a two-dimensional photonic-crystal laser, it is difficult to stack an active layer having desired characteristics on such a surface having irregularities.

Patent Literature <NUM> describes a device that includes a metal(III)-polar III-nitride substrate having a first surface opposite a second surface, a tunnel junction formed on one of the first surface or a buffer layer disposed on the first surface, a p-type III-nitride layer formed directly on the tunnel junction, and a number of material layers;, a first material layer formed on the p-type III-nitride layer, each subsequent layer disposed on a preceding layer, where one layer from the number of material layers is patterned into a structure, that one layer being a III-nitride layer.

An object of the present invention is to provide a two-dimensional photonic-crystal laser capable of suppressing decrease in the efficiency of laser oscillation in a two-dimensional photonic-crystal layer and that can be easily manufactured without increasing material cost.

A two-dimensional photonic-crystal laser according to the present invention made to solve the above problems includes:.

It should be noted that here, the terms "upper" and "lower" are used for convenience in order to describe the positional relationship among the constituent elements, but these terms do not limit the orientation of the two-dimensional photonic-crystal laser according to the present invention.

Before the operation of the two-dimensional photonic-crystal laser according to the present invention is described, the roles of the first tunnel layer and the second tunnel layer will be described. In general, in a case where an n-type semiconductor layer made of an n-type semiconductor and a p-type semiconductor layer made of a p-type semiconductor are in contact with each other, carrier holes can pass the border when a voltage that is positive on the p-type semiconductor layer side is applied, but cannot pass when a voltage that is positive on the n-type semiconductor layer side is applied. However, by providing the first tunnel layer higher in carrier (electron) density than the n-type semiconductor layer rather close to the n-type semiconductor layer between the n-type semiconductor layer and the p-type semiconductor layer, and the second tunnel layer higher in carrier (carrier hole) density than the p-type semiconductor layer rather closer to the p-type semiconductor layer between the n-type semiconductor layer and the p-type semiconductor layer, carrier holes can pass from the n-type semiconductor layer to the p-type semiconductor layer by the tunnel effect even when a voltage that is positive on the n-type semiconductor layer side is applied. The carrier (electron or carrier hole) densities can be designed by reducing the band gap of the p-type or n-type semiconductor, increasing the impurity concentration, or the like.

An operation of the two-dimensional photonic-crystal laser according to the present invention will be described. In this two-dimensional photonic-crystal laser, by applying a voltage between the first electrode and the second electrode, with positive on the first electrode side and negative on the second electrode side, carrier holes are supplied from the first electrode, and electrons are supplied from the second electrode. Then, since the first tunnel layer is higher in carrier density than the substrate (corresponding to the n-type semiconductor layer) and the second tunnel layer is higher in carrier density than the p-type semiconductor layer, carrier holes supplied from the first electrode pass through the first tunnel layer and the second tunnel layer by the tunnel effect from the substrate, reach the p-type semiconductor layer, and are supplied into the active layer as described above. On the other hand, electrons supplied from the second electrode pass through the two-dimensional photonic crystal and are supplied into the active layer. Light is generated in the active layer when carrier holes and electrons are supplied into the active layer in this manner, and laser oscillation is caused when the light is amplified in the two-dimensional photonic-crystal layer.

According to the two-dimensional photonic-crystal laser according to the present invention, since the n-type semiconductor is used as the base body of the two-dimensional photonic-crystal layer, the carrier density when the electric current of the same magnitude flows can be reduced as compared with the case where the p-type semiconductor is used as the base body. Therefore, it is possible to suppress absorption of a portion of light by free carriers in the two-dimensional photonic-crystal layer, and prevent decrease in the efficiency of laser oscillation.

In addition, in the two-dimensional photonic-crystal laser according to the present invention, since an n-type semiconductor, which is less expensive than a p-type semiconductor, is used as a substrate, it is possible to prevent the material cost from rising.

Furthermore, in the two-dimensional photonic-crystal laser according to the present invention, since the two-dimensional photonic-crystal layer is provided on the opposite side of the substrate as viewed from the active layer, the active layer can be stacked without being affected by the irregularities of the surface of the two-dimensional photonic-crystal layer, so that the active layer having desired characteristics can be easily made.

Another layer including an n-type semiconductor may be provided between the substrate and the first tunnel layer, between the active layer and the photonic-crystal layer, and/or between the photonic-crystal layer and the second electrode. For example, a carrier block layer made of an n-type semiconductor may be provided between the active layer and the two-dimensional photonic-crystal layer. A cladding layer made of an n-type semiconductor or a contact layer made of an n-type semiconductor may be provided between the two-dimensional photonic-crystal layer and the second electrode.

In addition, another layer (for example, a reflection layer between the second tunnel layer and the p-type semiconductor layer, which will be described below) made of a p-type semiconductor may be provided between the second tunnel layer and the p-type semiconductor layer and/or between the p-type semiconductor layer and the active layer. In this case, laser light is emitted from the second electrode side to the outside of the two-dimensional photonic-crystal laser.

Since the first tunnel layer and the second tunnel layer become higher in carrier density than the substrate and the p-type semiconductor layer, larger absorption of laser light by free carriers may occur. Therefore, it is desirable that the two-dimensional photonic-crystal laser according to the present invention further include a reflection layer configured to reflect laser light generated in the two-dimensional photonic-crystal layer between the second tunnel layer and the p-type semiconductor layer. This makes it possible to prevent a portion of laser light from being absorbed in the first tunnel layer and the second tunnel layer that become higher in carrier density than the other layers. As such a reflection layer, it is possible to use, for example, a distribution Bragg reflector (DBR) in which a plurality of layers including two types of p-type semiconductors having different refractive indices are alternately stacked.

Alternatively, a reflection layer may be provided between the two-dimensional photonic-crystal layer and the second electrode. In this case, another layer (the above-described cladding layer or contact layer) including an n-type semiconductor may be present between the two-dimensional photonic-crystal layer and the reflection layer and/or between the reflection layer and the second electrode. In this case, the laser light is emitted from the first electrode side to the outside of the two-dimensional photonic-crystal laser.

The two-dimensional photonic-crystal laser according to the present invention has a configuration including:.

By providing the first electrode on the bottom face (therefore, in the substrate) of the groove having the bottom face at the position between the upper face and the lower face of the substrate, the electric resistance between the first electrode and the active layer becomes smaller than that in a case where the first electrode is provided on the lower face of the substrate, and charges can be more efficiently supplied to the active layer. In addition, since the first electrode is provided on the bottom face of the groove having a frame-like planar shape, the shape of the first electrode also becomes frame-like, and the laser light caused by laser oscillation in the two-dimensional photonic-crystal layer passes through the frame of the first electrode and is emitted from the surface of the substrate to the outside. Therefore, it is possible to suppress the first electrode from hindering emission of laser light and causing unnecessary diffraction. Furthermore, in a case where the first electrode is provided on the lower face of the substrate, the layers of the second tunnel layer, the first tunnel layer, the p-type semiconductor layer, and the like should be prepared on one face of the substrate, and the first electrode should be prepared on the other face. Therefore, it is necessary to invert the upper and lower sides of the substrate during manufacturing. On the other hand, in a case where the first electrode is provided on the bottom face of such a groove, the first electrode is made on the same side as the second tunnel layer and the like. Therefore, it is not necessary to invert the upper and lower sides of the substrate during manufacturing, and the production becomes easy.

For example, n-type GaAs or n-type AlGaAs in which GaAs or a portion of Ga in GaAs is substituted with Al can be used as the material of the substrate and the base body of the two-dimensional photonic-crystal layer, and p-type GaAs or p-type AlGaAs can be used for the p-type semiconductor layer. In this example, it is preferable that InGaAs is used for the first tunnel layer and the second tunnel layer. InGaAs has a small band gap among GaAs and AlGaAs, and can increase the carrier density. However, since InGaAs easily absorbs light, when InGaAs is used as a material of the first tunnel layer or the second tunnel layer, it is desirable to provide a reflection layer between the second tunnel layer and the p-type semiconductor layer as described above.

In the two-dimensional photonic-crystal laser according to the present invention, a non-carrier-doped semiconductor may be used in place of the n-type semiconductor for the entire or a portion of the base body. In the case of using a non-carrier-doped semiconductor for a portion of the base body (n-type semiconductor for the remaining portion), an electric current easily flows from the second electrode side to the active layer side as compared with the case where the entire base body includes a non-carrier-doped semiconductor while suppressing absorption loss of light by free carriers as compared with the case where the entire base body includes an n-type semiconductor. In addition, use of the non-carrier-doped semiconductor for the entire base body, the absorption loss of light can be further suppressed.

Owing to the two-dimensional photonic-crystal laser according to the present invention, it is possible to suppress decrease in the efficiency of laser oscillation in the two-dimensional photonic-crystal layer, and it is possible to easily manufacture the two-dimensional photonic-crystal laser without increasing the material cost.

Embodiments of the two-dimensional photonic-crystal laser according to the present invention will be described with reference to <FIG>.

A two-dimensional photonic-crystal laser <NUM> of the first example has a configuration in which a substrate <NUM>, a first tunnel layer <NUM>, a second tunnel layer <NUM>, a p-type cladding layer (p-type semiconductor layer in the present invention) <NUM>, an active layer <NUM>, a carrier block layer <NUM>, a two-dimensional photonic-crystal layer <NUM>, an n-type cladding layer <NUM>, and a contact layer <NUM> are sequentially stacked in order from the lower side of <FIG>. A lower side of the substrate <NUM> (opposite side of the first tunnel layer <NUM>) is provided with a first electrode <NUM>, and an upper side of the contact layer <NUM> (opposite side of the n-type cladding layer <NUM>) is provided with a second electrode <NUM>.

The substrate <NUM> includes an n-type semiconductor, and the first tunnel layer <NUM> includes an n-type semiconductor higher in carrier (electron) density than that of the substrate <NUM>. The p-type cladding layer <NUM> includes a p-type semiconductor, and the second tunnel layer <NUM> includes a p-type semiconductor higher in carrier (carrier hole) density than that of the p-type cladding layer <NUM>.

The active layer <NUM> generates light emission within a specific emission wavelength band upon being supplied with carrier holes and electrons. The active layer <NUM> includes, for example, a multiple-quantum well (MQW) in which a thin film including indium gallium arsenide (InGaAs) and a thin film including gallium arsenide (GaAs) are alternately stacked in large numbers.

As illustrated in <FIG>, the two-dimensional photonic-crystal layer <NUM> has a configuration in which a plurality of modified refractive index areas <NUM> are periodically arranged two-dimensionally in a plate-shaped base body <NUM> made of an n-type semiconductor. The modified refractive index areas <NUM> typically include holes (air), but members including a material other than the base body <NUM> may be used. The arrangement of the modified refractive index areas <NUM> has a square lattice shape in the example illustrated in <FIG>, but may have another shape such as a triangular lattice shape. In addition, the planar shape of the modified refractive index area <NUM> is an equilateral triangle in the example illustrated in <FIG>, but may be other shapes including a triangle such as a right triangle other than the equilateral triangle, a circle, and an ellipse. In addition, one modified refractive index area <NUM> may be configured by combining a plurality of holes or a member including a material other than the base body <NUM>.

The carrier block layer <NUM>, the n-type cladding layer <NUM>, and the contact layer <NUM> each include an n-type semiconductor.

The first electrode <NUM> is a plate-shaped conductor having a hollow portion formed inside, and has a frame portion <NUM>, which is a portion where the conductor remains, and a window portion <NUM>, which is the hollow portion of the conductor (see <FIG>). The second electrode <NUM> includes a conductor plate provided at a position opposing the window portion <NUM> of the first electrode <NUM> and having an area smaller than that of the window portion <NUM> (see <FIG>).

A specific example of the material of each constituent element in the two-dimensional photonic-crystal laser <NUM> will be described. It is possible to use n-type GaAs for the substrate <NUM>, n-type GaAs (for example, <NUM> to <NUM> times) higher in impurity concentration than that of the substrate <NUM> for the first tunnel layer <NUM>, p-type AlGaAs for the p-type cladding layer <NUM>, and GaAs (for example, <NUM> to <NUM> times) higher in impurity concentration than that of the p-type cladding layer <NUM> for the second tunnel layer <NUM>. In addition, n-type AlGaAs can be used for each of the carrier block layer <NUM>, the base body <NUM> of the two-dimensional photonic-crystal layer <NUM>, and the n-type cladding layer <NUM>, and the impurity concentrations of these three layers may be the same or different. For the contact layer <NUM>, n-type GaAs can be used. The impurity concentration in each layer other than the first tunnel layer <NUM> and the second tunnel layer <NUM> is set to be (for example, <NUM> to <NUM> times) higher in each layer made of a p-type semiconductor than in each layer made of an n-type semiconductor. It should be noted that the material of each layer described here is an example, and another p-type semiconductor can be used in each layer for which p-type GaAs or AlGaAs is exemplified, and another n-type semiconductor can be used in each layer for which n-type GaAs or AlGaAs is exemplified. These GaAs and AlGaAs can transmit light within a wavelength band of <NUM> to <NUM>.

As a material of each of these layers, a semiconductor such as InP, GaN, or AlInGaAsP other than GaAs and AlGaAs may be used.

The first tunnel layer <NUM> can be stacked by epitaxially growing on the substrate <NUM>. Similarly, each layer from the second tunnel layer <NUM> to the contact layer <NUM> can be stacked by epitaxially growing each layer of the immediate layer on the substrate <NUM> side.

The first electrode <NUM> and the second electrode <NUM> can be made using a method such as a vapor deposition method with a metal such as gold as a material.

The thickness of the substrate <NUM> is made sufficiently larger than the thickness of each layer from the first tunnel layer <NUM> to the contact layer <NUM>. This makes the distance between the second electrode <NUM> and the active layer <NUM> sufficiently smaller than the distance between the first electrode <NUM> and the active layer <NUM>. In addition, the thicknesses of the first tunnel layer <NUM> and the second tunnel layer <NUM> are made sufficiently smaller than the thicknesses of the respective layers from the substrate <NUM> and the p-type cladding layer <NUM> to the contact layer <NUM>. This makes it easier for carrier holes supplied from the first electrode <NUM> as described later to easily reach the p-type cladding layer <NUM> (further, the active layer <NUM> via the p-type cladding layer <NUM>). The thickness of each layer is, for example, equal to or greater than <NUM> for the substrate <NUM>, <NUM> to <NUM> for the first tunnel layer <NUM>, <NUM> to <NUM> for the second tunnel layer <NUM>, <NUM> to <NUM> for the p-type cladding layer <NUM>, <NUM> to <NUM> for the active layer <NUM>, <NUM> to <NUM> for the carrier block layer <NUM>, <NUM> to <NUM> for the two-dimensional photonic-crystal layer <NUM>, <NUM> to <NUM> for the n-type cladding layer <NUM>, and <NUM> to <NUM> for the contact layer <NUM>.

An operation of the two-dimensional photonic-crystal laser <NUM> of the first example will be described. When this two-dimensional photonic-crystal laser <NUM> is used, a voltage that is positive on the first electrode <NUM> side and negative on the second electrode <NUM> side is applied between these two electrodes. This causes carrier holes to be supplied from the first electrode <NUM> into the two-dimensional photonic-crystal laser <NUM>, and electrons to be supplied from the second electrode <NUM> into the two-dimensional photonic-crystal laser <NUM>.

Carrier holes supplied from the first electrode <NUM> pass through the substrate <NUM>, the first tunnel layer <NUM>, the second tunnel layer <NUM>, and the p-type cladding layer <NUM>, and are introduced into the active layer <NUM>. Here, since the substrate <NUM> and the first tunnel layer <NUM> include an n-type semiconductor, and the second tunnel layer <NUM> and the p-type cladding layer <NUM> includes a p-type semiconductor, a reverse bias voltage that is positive on the n-type semiconductor side and negative on the p-type semiconductor side is applied at a boundary between the first tunnel layer <NUM> and the second tunnel layer <NUM>. As well known in diodes, when such a reverse bias voltage is applied, an electric current that crosses the boundary between an n-type semiconductor and a p-type semiconductor usually hardly flows. However, in the present invention, since the impurity concentration of the first tunnel layer <NUM> is higher than that of the substrate <NUM> and the impurity concentration in the second tunnel layer <NUM> is higher than that of the p-type cladding layer <NUM>, it becomes possible to achieve a state in which the carrier (electron) density in the first tunnel layer <NUM> and the carrier (carrier hole) density in the second tunnel layer <NUM> are high. This allows the carrier holes supplied from the first electrode <NUM> and introduced into the first tunnel layer <NUM> from the substrate <NUM> side to move to the second tunnel layer <NUM> by the tunnel effect, and are introduced into the active layer <NUM> from the second tunnel layer <NUM> through the p-type cladding layer <NUM>.

On the other hand, electrons supplied from the second electrode <NUM> are introduced into the active layer <NUM> through the contact layer <NUM>, the two-dimensional photonic-crystal layer <NUM>, and the carrier block layer <NUM>. It should be noted that the carrier block layer <NUM> prevents carrier holes from moving from the active layer <NUM> to the two-dimensional photonic-crystal layer <NUM>, and can cause electrons supplied from the two-dimensional photonic-crystal layer <NUM> side to move to the active layer <NUM>.

When carrier holes and electrons are introduced into the active layer <NUM> in this manner, light emission within a specific emission wavelength band is generated in the active layer <NUM>. At this time, since the area of the first electrode <NUM> is larger than the area of the second electrode <NUM> and the distance between the first electrode <NUM> and the active layer <NUM> is sufficiently smaller than the distance between the second electrode <NUM> and the active layer <NUM>, the area of a charge supply area <NUM> (see <FIG>) into which carrier holes and electrons are supplied of the active layer <NUM> is close to the area of the second electrode <NUM> and becomes sufficiently smaller than the area of the first electrode <NUM>. By intensively supplying charges into the charge supply area <NUM> having such a small area, it is possible to increase the output per unit area of light generated in the active layer <NUM>. It should be noted that although the ratio of the thicknesses of each layer is not accurately illustrated in <FIG> and <FIG> for convenience of description, <FIG> in which the substrate <NUM> is illustrated to be sufficiently thicker than the other layers is closer to the actual ratio of the thickness of the substrate <NUM> and the other layers.

Regarding the light generated in the active layer <NUM>, in the two-dimensional photonic-crystal layer <NUM>, only light having a predetermined wavelength corresponding to the lattice constant of the arrangement of the modified refractive index areas <NUM> is amplified and laser oscillation occurs. Here, in the two-dimensional photonic-crystal laser <NUM> of the first example not falling into the scope of the claims, the base body <NUM> of the two-dimensional photonic-crystal layer <NUM> includes an n-type semiconductor, and electrons have higher mobility than carrier holes. Therefore, it is possible to reduce the carrier density when the same magnitude of electric current is passed as compared with a case where the base body <NUM> includes a p-type semiconductor. Therefore, it is possible to suppress absorption of a portion of light by free carriers (electrons) in the base body <NUM>, and it is possible to suppress decrease in the efficiency of laser oscillation.

For example, in a case where the material of the base body <NUM> includes n-type GaAs, the mobility of electrons is higher than the mobility of carrier holes when compared with p-type GaAs, and thus the carrier density required for flowing the same magnitude of electric current can be made about <NUM>/<NUM>. In addition to it, rather than the p-type GaAs, n-type GaAs can suppress the absorption coefficient of light in a case where the carrier density is the same to about <NUM>% (in a case where the carrier density is <NUM> × <NUM><NUM> cm-<NUM>). In consideration of these together, use of n-type GaAs for the material of the base body <NUM> makes it possible to suppress absorption of light to about <NUM>/<NUM> to <NUM>/<NUM> as compared with the case of use of p-type GaAs. It should be noted that although n-type GaAs is used as the material of the base body <NUM> as an example here, also when another n-type semiconductor such as AlGaAs is used as the material of the base body <NUM> (although numerical values are different), the carrier density and the absorption coefficient of light can be suppressed, and thus the same effects are achieved.

Together with it, by increasing the output per unit area of the light generated in the active layer <NUM> as described above, it is possible to easily cause laser oscillation in the two-dimensional photonic-crystal layer <NUM>.

The laser light thus generated is emitted from the window portion <NUM> of the first electrode <NUM> to the outside.

According to the two-dimensional photonic-crystal layer <NUM> according to the present embodiment, use of the n-type semiconductor as the material of the base body <NUM> of the two-dimensional photonic-crystal layer <NUM> makes it possible to suppress absorption of a portion of light by free carriers (electrons), and makes it possible to suppress decrease in the efficiency of laser oscillation.

In addition, since the base body <NUM> made of an n-type semiconductor is used, it is not necessary to use a substrate made of a p-type semiconductor, and it is possible to suppress the material cost by using the substrate <NUM> made of a less expensive n-type semiconductor.

Furthermore, by providing the first tunnel layer <NUM> and the second tunnel layer <NUM> between the substrate <NUM> and the p-type cladding layer <NUM>, it is possible to provide the base body <NUM> on the opposite side of the substrate <NUM> as viewed from the active layer <NUM> while using an n-type semiconductor for both the base body <NUM> and the substrate <NUM>. Since this eliminates the need for preparing the active layer <NUM> on the two-dimensional photonic-crystal layer <NUM>, the active layer <NUM> having desired characteristics can be easily prepared without being affected by irregularities generated on the surface of the two-dimensional photonic-crystal layer <NUM>.

A two-dimensional photonic-crystal laser <NUM> of the second example has a configuration in which the substrate <NUM>, the first tunnel layer <NUM>, the second tunnel layer <NUM>, a reflection layer <NUM>, the p-type cladding layer (p-type semiconductor layer in the present invention) <NUM>, the active layer <NUM>, the carrier block layer <NUM>, the two-dimensional photonic-crystal layer <NUM>, the n-type cladding layer <NUM>, and the contact layer <NUM> are sequentially stacked in order from the lower side of <FIG>. A lower side of the substrate <NUM> (opposite side of the first tunnel layer <NUM>) is provided with a first electrode <NUM>, and an upper side of the contact layer <NUM> (opposite side of the n-type cladding layer <NUM>) is provided with a second electrode <NUM>. Among these constituent elements, the substrate <NUM>, the p-type cladding layer <NUM>, the active layer <NUM>, the carrier block layer <NUM>, the two-dimensional photonic-crystal layer <NUM>, the n-type cladding layer <NUM>, and the contact layer <NUM> are similar to the respective constituent elements of the two-dimensional photonic-crystal laser <NUM> of the first example and thus description thereof is omitted.

The reflection layer <NUM> includes a DBR. The DBR used in the present example has a plurality of alternately stacked layers including two types of p-type semiconductors having different refractive indices. For example, two types of layers of p-type AlGaAs having different Al contents are alternately stacked and can be used as the reflection layer <NUM>.

It is similar to the two-dimensional photonic-crystal laser <NUM> of the first example in that the first tunnel layer <NUM> includes an n-type semiconductor higher in carrier density than that of the substrate <NUM>, and the second tunnel layer <NUM> includes a p-type semiconductor higher in carrier density than that of the p-type cladding layer <NUM>. In the present example both the first tunnel layer <NUM> and the second tunnel layer <NUM> use an n-type semiconductor (first tunnel layer <NUM>) and a p-type semiconductor (second tunnel layer <NUM>) that are higher in carrier density than those in the first example When using GaAs or AlGaAs for each of the substrate <NUM> and the p-type cladding layer <NUM> (for example, n-type GaAs for the substrate <NUM>, and p-type AlGaAs for the p-type cladding layer <NUM>), it is possible to suitably use n-type InGaAs for the first tunnel layer <NUM>, and p-type InGaAs for the second tunnel layer <NUM>. InGaAs is a semiconductor that has a band gap smaller than that of GaAs or AlGaAs, and can thereby increase the carrier density.

The first electrode <NUM> is provided on the lower face of the substrate <NUM>, and the second electrode <NUM> is provided on the upper face of the contact layer <NUM>. The first electrode <NUM> is larger in area than the second electrode <NUM>. For example, it is preferable that the first electrode <NUM> is provided on the entire lower face of the substrate <NUM>, and the second electrode <NUM> is provided only near the center of the upper face of the contact layer <NUM>. As the material of the second electrode <NUM>, a material transparent with respect to laser light oscillated in the two-dimensional photonic-crystal layer <NUM> is used. On the other hand, it does not matter whether the material of the first electrode <NUM> is transparent or opaque with respect to laser light. For example, a metal material such as gold can be used as the material of the first electrode <NUM>, and indium tin oxide (ITO) can be used as the material of the second electrode <NUM>.

An operation of the two-dimensional photonic-crystal laser <NUM> of the second example will be described. Similarly to that of the first example when a voltage that is positive on the first electrode <NUM> side and negative on the second electrode <NUM> side is applied, carrier holes are supplied from the first electrode <NUM> and electrons are supplied from the second electrode <NUM>, and light emission in a specific emission wavelength band is generated in the active layer <NUM>. At this time, since the area of the first electrode <NUM> is larger than the area of the second electrode <NUM> and the distance between the first electrode <NUM> and the active layer <NUM> is sufficiently smaller than the distance between the second electrode <NUM> and the active layer <NUM>, charges are intensively supplied into an area smaller than that of the first electrode <NUM> in the active layer <NUM>, and the output per unit area of light generated in the active layer <NUM> can be increased. Regarding the light generated in the active layer <NUM>, in the two-dimensional photonic-crystal layer <NUM>, only light having a predetermined wavelength corresponding to the lattice constant of the arrangement of the modified refractive index areas <NUM> is amplified and laser oscillation occurs.

The laser light thus generated is emitted from each of the upper and lower faces of the two-dimensional photonic-crystal layer <NUM>, and the laser light emitted to the first electrode <NUM> side among them is reflected by the reflection layer <NUM> and is directed toward the second electrode <NUM> side without entering the second tunnel layer <NUM> and the first tunnel layer <NUM>. Therefore, the laser light emitted from any of the upper and lower faces of the two-dimensional photonic-crystal layer <NUM> is emitted to the outside directly from the upper face of the contact layer <NUM> or through the second electrode <NUM>.

According to the two-dimensional photonic-crystal laser <NUM> of the second example, similarly to the two-dimensional photonic-crystal laser <NUM> of the first example, use of the n-type semiconductor as the material of the base body <NUM> of the two-dimensional photonic-crystal layer <NUM> makes it possible to suppress absorption of a portion of light by free carriers (electrons), and makes it possible to suppress decrease in the efficiency of laser oscillation. In addition, it is not necessary to use a substrate made of a p-type semiconductor, and it is possible to suppress the material cost by using the substrate <NUM> made of a less expensive n-type semiconductor. Furthermore, since by providing the first tunnel layer <NUM> and the second tunnel layer <NUM> between the substrate <NUM> and the p-type cladding layer <NUM>, it is possible to provide the base body <NUM> on the opposite side of the substrate <NUM> as viewed from the active layer <NUM> while using an n-type semiconductor for both the base body <NUM> and the substrate <NUM>, it is possible to easily prepare the active layer <NUM> having desired characteristics without being affected by irregularities generated on the surface of the two-dimensional photonic-crystal layer <NUM>.

In addition to the effects similar to those of the first example, according to the two-dimensional photonic-crystal laser <NUM> of the second example, since the reflection layer <NUM> is provided between the second tunnel layer <NUM> and the p-type cladding layer <NUM>, laser light does not enter the first tunnel layer <NUM> and the second tunnel layer <NUM> higher in carrier density than other layers, and a portion of laser light can be prevented from being absorbed in the first tunnel layer <NUM> and the second tunnel layer <NUM>. In addition, since the absorption of the laser light is prevented in this manner, a material higher in carrier density than the material used in the first example such as InGaAs can be used as the material of the first tunnel layer <NUM> and the second tunnel layer <NUM>, and thus, the carrier density supplied into the active layer <NUM> can be further increased and the intensity of the laser light can be further increased.

<FIG> presents results of simulation of the device having the structure of Patent Literature <NUM> (A case where a base body made of a p-type semiconductor is used. Hereinafter, it is referred to as "comparative example". ) and the device having the structure of the second example (A case where a base body made of a n-type semiconductor is used, Hereinafter, it is referred to as "examples". As described above, since the use of the n-type semiconductor (n-type GaAs) makes it possible to suppress absorption of light to about <NUM>/<NUM> to <NUM>/<NUM> as compared with the case of use of the p-type semiconductor (p-type GaAs), simulation was performed with the absorption loss of <NUM>-<NUM> in the comparative example and with the absorption loss of <NUM>-<NUM>, which is <NUM>/<NUM> of that in the comparative example, in the example. At this time, the in-plane loss (loss radiated to the outside of the device in the direction parallel to the plane) is calculated as <NUM>-<NUM> in both the comparative example and the example, and the radiation coefficient is calculated as <NUM>-<NUM> in the comparative example and <NUM>-<NUM> in the example. Due to the reduction of the loss, the threshold electric current density is reduced from <NUM> kA·cm-<NUM> (comparative example) to <NUM> kA·cm-<NUM> (example), and the oscillation threshold electric current value has decreased. The slope efficiency is <NUM> W/A in the comparative example, whereas it is <NUM> W/A in the example. As described above, in the example, due to the reduction of the loss, oscillation with a low threshold and operation with high slope efficiency become possible.

<FIG> illustrate a schematic configuration of a two-dimensional photonic-crystal laser <NUM> of this embodiment. This two-dimensional photonic-crystal laser <NUM> is different from the two-dimensional photonic-crystal lasers <NUM> and <NUM> of the first and second examples in that a groove <NUM> is included, and a first electrode <NUM> is provided on a bottom face of the groove <NUM>, and a reflection layer <NUM> is provided between the n-type cladding layer <NUM> and the contact layer <NUM>. Hereinafter, similar configurations to those of the two-dimensional photonic-crystal lasers <NUM> and <NUM> of the first and second examples will not be described, and only the above-described differences will be described.

The groove <NUM> penetrates the reflection layer <NUM>, the n-type cladding layer <NUM>, the two-dimensional photonic-crystal layer <NUM>, the carrier block layer <NUM>, the active layer <NUM>, the p-type cladding layer <NUM>, the second tunnel layer <NUM>, and the first tunnel layer <NUM> from the surface of the contact layer <NUM>, and is dug down to a position between the upper face and the lower face of the substrate <NUM>. The shape (planar shape) of the groove <NUM> in a cross section parallel to the two-dimensional photonic-crystal layer <NUM> (the same applies to other layers such as the contact layer <NUM>) is a frame shape. The shape of the first electrode <NUM> provided on the bottom face of the groove <NUM> is a frame shape similar to the planar shape of the groove <NUM>, and is similar to the shape of the frame portion <NUM> of the first electrode <NUM> in the first example. Since the first electrode <NUM> is provided on the bottom face of the groove <NUM> in this manner, the position of the first electrode <NUM> in the vertical direction is a position between the upper face and the lower face of the substrate <NUM>.

The reflection layer <NUM> is provided between the n-type cladding layer <NUM> and the contact layer <NUM> as described above. As the reflection layer <NUM>, similarly to the reflection layer <NUM> in the second example, one including DBR can be used.

According to the two-dimensional photonic-crystal laser <NUM> of an embodiment, by providing the first electrode <NUM> on the bottom face of the groove <NUM> having the bottom face at the position between the upper face and the lower face of the substrate <NUM>, the electric resistance between the first electrode <NUM> and the active layer <NUM> is reduced as compared with the case of the first example in which the first electrode <NUM> is provided on the lower face of the substrate <NUM>. This makes it possible to supply charges to the active layer <NUM> more efficiently.

In addition, according to the two-dimensional photonic-crystal laser <NUM> of an embodiment, since the planar shape of the first electrode <NUM> is a frame shape, the laser light passes through inside the frame of the first electrode <NUM> and is emitted from the surface of the substrate <NUM> to the outside. Therefore, it is possible to suppress the first electrode <NUM> from hindering emission of laser light and causing unnecessary diffraction.

Furthermore, the two-dimensional photonic-crystal laser <NUM> of an embodiment can be easily prepared in the following points. In the two-dimensional photonic-crystal lasers <NUM> and <NUM> of the first and second examples, since each layer such as the second tunnel layer <NUM> is prepared on the upper face of the substrate <NUM> and the first electrodes <NUM> and <NUM> are prepared on the lower face, it is necessary to invert the upper and lower sides of the substrate <NUM> between the preparation of each layer and the preparation of the first electrodes <NUM> and <NUM>. On the other hand, in the two-dimensional photonic-crystal laser <NUM> of an embodiment, since both of each layer such as the second tunnel layer <NUM> and the first electrode <NUM> are prepared on the upper face of the substrate <NUM>, it is not necessary to invert the upper and lower sides of the substrate <NUM>, and manufacturing becomes easy.

While embodiments according to the present invention have been described above, the present invention is not limited to these embodiments, and various modifications can be made within the scope of the frame of the present invention as defined by independent claim <NUM>.

For example, in the two-dimensional photonic-crystal laser <NUM> of each of the embodiments, an n-type semiconductor is used as the material of the base body <NUM>, but in order to further suppress the absorption loss of light due to free carriers, as illustrated in <FIG>, a non-carrier-doped semiconductor (described as "i" in the figure) may be used as the material of a base body 161A of a two-dimensional photonic-crystal layer 16A. Alternatively, in order to facilitate the flow of electric current from the second electrode <NUM> side to the active layer <NUM> side, as illustrated in <FIG>, an n-type semiconductor may be used for a portion <NUM> (in one example, <NUM>% in area ratio) of a base body 161B of a two-dimensional photonic-crystal layer 16B, and a non-carrier-doped semiconductor may be used for a remaining portion <NUM> (in the one example, <NUM>% in area ratio).

While in the two-dimensional photonic-crystal laser <NUM> of the second example, the second electrode <NUM> smaller in area than the first electrode <NUM> is provided near the center of the upper face of the contact layer <NUM>, but instead, a window-shaped electrode having a frame portion and a window portion similar to those of the first electrode <NUM> in the first example may be provided on the upper face of the contact layer <NUM> as the second electrode. In general, in a case where one electrode is provided with an electrode covering the entire substrate as in the first electrode <NUM> of the second example, providing a window-shaped electrode to the other electrode makes it difficult to supply charges near the center of the two-dimensional photonic-crystal layer. However, as in the second example, use of an n-type semiconductor for the cladding layer (n-type cladding layer <NUM>) provided between the second electrode <NUM> and the two-dimensional photonic-crystal layer <NUM> and the contact layer <NUM> makes it possible to increase the mobility of charges (electrons). Therefore, even if a window-shaped electrode is used for the second electrode <NUM>, it is possible to supply charges near the center of the two-dimensional photonic-crystal layer <NUM>.

While in the two-dimensional photonic-crystal laser <NUM> of an embodiment, the first electrode <NUM> is provided on the bottom face of the groove <NUM>, and the reflection layer <NUM> is provided between the n-type cladding layer <NUM> and the contact layer <NUM>. However, after the first electrode <NUM> is provided to the bottom face of the groove <NUM>, the reflection layer <NUM> may be provided between the second tunnel layer <NUM> and the p-type cladding layer <NUM> in place of the reflection layer <NUM>. Alternatively, after the reflection layer <NUM> is provided between the n-type cladding layer <NUM> and the contact layer <NUM>, the first electrode <NUM> may be provided on the lower face of the substrate <NUM> without providing the groove <NUM> and the first electrode <NUM> on the bottom face of the groove <NUM>.

Claim 1:
A two-dimensional photonic-crystal laser (<NUM>; <NUM>; <NUM>) comprising:
a) a substrate (<NUM>) made of an n-type semiconductor;
b) a p-type semiconductor layer (<NUM>) provided on an upper side of the substrate (<NUM>) and made of a p-type semiconductor;
c) an active layer (<NUM>) provided on an upper side of the p-type semiconductor layer (<NUM>);
d) a two-dimensional photonic-crystal layer (<NUM>; 16A; 16B) provided on an upper side of the active layer (<NUM>) and including a plate-shaped base body (<NUM>; 161A; 161B) made of an n-type semiconductor in which modified refractive index areas (<NUM>) whose refractive index differs from the base body (<NUM>; 161A; 161B) are periodically arranged;
e) a first tunnel layer (<NUM>) provided between the substrate (<NUM>) and the p-type semiconductor layer (<NUM>) and made of an n-type semiconductor having a carrier density higher than a carrier density of the substrate (<NUM>);
f) a second tunnel layer (<NUM>) provided in contact with the first tunnel layer (<NUM>) between the first tunnel layer (<NUM>) and the p-type semiconductor layer (<NUM>), and made of a p-type semiconductor having a carrier density higher than a carrier density of the p-type semiconductor layer (<NUM>);
g) a first electrode (<NUM>; <NUM>; <NUM>) provided on a lower side of the substrate (<NUM>) or in the substrate (<NUM>);
h) a second electrode (<NUM>; <NUM>) provided on an upper side of the two-dimensional photonic-crystal layer (<NUM>; 16A; 16B); and
i) a groove (<NUM>) provided from a surface on an upper side of the two-dimensional photonic-crystal laser (<NUM>; <NUM>; <NUM>), having a bottom face at a position between an upper face and a lower face of the substrate (<NUM>), and having a frame-like shape in a cross section parallel to the two-dimensional photonic-crystal layer (<NUM>, 16A, 16B),
wherein the first electrode (<NUM><NUM>; <NUM>) is provided on the bottom face of the groove (<NUM>).