JP2017017172A - Surface light emitting laser, laser array, solid-state laser, information acquisition device, image forming apparatus, and manufacturing method of laser array - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a VCSEL that reduces an increase in laser threshold and stabilizes output.SOLUTION: A surface light emitting laser comprises: a p-type first semiconductor layer 105 that is arranged on an active layer 104; a second semiconductor layer 106 that is arranged on the first semiconductor layer 105 and includes an opening 130; and a p-type third semiconductor layer 107 that is arranged on the second semiconductor layer 106 and the first semiconductor layer 105 located in the opening 130 to fill a difference in step formed by the opening 130. The first semiconductor layer 105 includes hydrogen; the first semiconductor layer 105 has a hydrogen density distribution in which the density of hydrogen of a portion corresponding to the opening 130 is lower than the density of hydrogen of a portion outside the portion corresponding to the opening 130.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a surface emitting laser, a laser array, a solid-state laser, an information acquisition device, an image forming apparatus, and a method for manufacturing a laser array.

  A material using a nitride semiconductor such as GaN is known as an edge emitting laser material. In the ridge waveguide structure often used in the edge emitting laser, lateral light confinement is performed by forming a ridge in the p-type layer on the surface. However, the current spreads in the lateral direction around the active layer and the threshold value increases. Therefore, as in Patent Document 1, a pnp-type current confinement structure in which an n-type layer having an opening is formed in a p-type layer is used in order to strengthen current confinement in the lateral direction. In a specific manufacturing method of this current confinement structure, after forming a p-AlGaN layer, an n-GaN layer is formed as a layer that blocks current. Thereafter, the n-GaN layer is etched in a stripe shape by reactive ion etching to form openings, and finally the p-GaN layer is grown, thereby realizing a pnp-type current confinement structure.

  Patent Document 1 shows a doping profile of impurities in a current confinement portion. It is described that Mg diffuses from the p-type layer formed above and below the n-type layer, and in particular, the diffusion from the p-GaN below the n-type layer is large. When Mg in the p-type layer diffuses into the n-type layer, electrons in the n-type layer are compensated, and the n-type layer becomes p-type. When the n-type layer is thin, the entire n-type layer becomes p-type, so that no band barrier is formed by the n-type layer and current cannot be blocked. Therefore, this n-type layer is formed with a thickness of 1.5 μm.

  On the other hand, a vertical cavity surface emitting laser (vertical cavity surface emitting laser, hereinafter referred to as VCSEL) also uses a current confinement structure. In the VCSEL, the electrode on the light emission side is arranged avoiding the light emission region so as not to shield light. This light emission region corresponds to the light emission region in the active layer. That is, the light emitting region of the active layer is at a position different from the region where the electrode on the light emission side is disposed. For this reason, a current confinement structure is used for flowing a current path to a portion to be a light emitting region.

JP 2004006934 A

  However, as the current confinement structure of the VCSEL, the pnp type current confinement structure as in Patent Document 1 has the following problems.

  In the pnp-type current confinement structure, a p-type layer is grown on an n-type layer in which an opening is formed. Since there is a step due to the opening, the p-type layer around the opening is directed toward the center of the opening. Grow laterally. When the thickness of the p-type layer is thinner than the thickness of the n-type layer, the step due to the opening remains without being flat. In the case of a VCSEL, the laser threshold is increased by scattering light due to the effect of the step.

  Further, when the p-type layer is grown thicker than the thickness of the n-type layer, the step can be flattened, but the n-type layer is formed thick in order to reduce the influence of Mg diffusion as described above. Therefore, the p-type layer also becomes thick. For this reason, the resonator length of the VCSEL increases, and oscillation occurs in a plurality of longitudinal modes, and the output becomes unstable.

  Therefore, an object of the present invention is to provide a VCSEL that reduces an increase in laser threshold and stabilizes output.

  One aspect of the present invention includes a first reflecting mirror, an active layer disposed on the first reflecting mirror, a p-type first semiconductor layer disposed on the active layer, A second semiconductor layer disposed on the first semiconductor layer and having an opening; and the opening on the second semiconductor layer and on the first semiconductor layer located in the opening. A p-type third semiconductor layer disposed so as to fill a step due to the portion, a second reflecting mirror disposed on the third semiconductor and at least in a region corresponding to the opening, An electrode disposed on the third semiconductor layer and in a region corresponding to the outside of the opening, wherein the first semiconductor layer includes hydrogen, and the first semiconductor The layer has a hydrogen concentration in which the hydrogen concentration in the portion corresponding to the opening is lower than the hydrogen concentration in the portion outside the portion corresponding to the opening. And having a distribution.

  According to another aspect of the present invention, there is provided a first reflector, an active layer disposed on the first reflector, and a p-type first semiconductor layer disposed on the active layer. And a second semiconductor layer disposed on the first semiconductor layer and having an opening, and on the first semiconductor layer located on the second semiconductor layer and the opening, A p-type third semiconductor layer disposed so as to fill the step due to the opening, and a second reflecting mirror disposed on the third semiconductor at least in a region corresponding to the opening And an electrode disposed on the third semiconductor layer and in a region corresponding to the outside of the opening, and the first semiconductor layer has hydrogen, and the first semiconductor layer has hydrogen. This semiconductor layer has an electrical resistance in which the electrical resistance of the portion corresponding to the opening is smaller than the electrical resistance of the portion corresponding to the outside of the opening. And having a distribution.

  According to another aspect of the present invention, there is provided a first reflector, an active layer disposed on the first reflector, and a p-type first semiconductor layer disposed on the active layer. And a second semiconductor layer disposed on the first semiconductor layer and having an opening, and on the first semiconductor layer located on the second semiconductor layer and the opening, A p-type third semiconductor layer disposed so as to fill the step due to the opening, and a second reflecting mirror disposed on the third semiconductor at least in a region corresponding to the opening And an electrode disposed on the third semiconductor layer and in a region corresponding to the outside of the opening, and the first semiconductor layer has hydrogen, and the first semiconductor layer has hydrogen. In the semiconductor layer, the carrier concentration in the portion corresponding to the opening is higher than the carrier concentration in the portion corresponding to the outside of the opening. It characterized by having a Yaria concentration distribution.

  According to another aspect of the present invention, there is provided a method of manufacturing a laser array having a plurality of surface emitting lasers, the step of forming a first reflecting mirror, and the formation of an active layer on the first reflecting mirror. A step of forming a p-type first semiconductor layer on the active layer, a step of forming a second semiconductor layer having an opening on the first semiconductor layer, Forming a p-type third semiconductor layer on the second semiconductor layer and on the first semiconductor layer located in the opening so as to fill a step due to the opening; and Forming a second reflecting mirror at least in a region corresponding to the opening, and a region corresponding to the outside of the opening on the third semiconductor layer. Forming an electrode, the first semiconductor layer, the second semiconductor layer, and the third semiconductor. A mesa structure for separating each surface emitting laser is formed after the step of heat-treating the layer in an atmosphere not containing hydrogen atoms, the step of forming the second reflecting mirror, and the step of forming the electrode. And the first semiconductor layer has hydrogen, and the first semiconductor layer has a hydrogen concentration in a portion corresponding to the opening and a hydrogen concentration in a portion corresponding to the outside of the opening. A laser array manufacturing method having a hydrogen concentration distribution lower than the concentration.

  The present invention can provide a VCSEL that reduces the increase in laser threshold and stabilizes the output.

FIG. 3 is a schematic cross-sectional view illustrating an example of a VCSEL according to the first embodiment. FIG. 3 is a schematic diagram illustrating a current confinement structure according to Embodiment 1. FIG. 3 is a schematic cross-sectional view showing an example of a manufacturing method of the VCSEL array according to the first embodiment. FIG. 3 is a schematic cross-sectional view showing an example of a manufacturing method of the VCSEL array according to the first embodiment. FIG. 6 is a schematic diagram illustrating an example of a solid-state laser according to a second embodiment. FIG. 9 is a schematic diagram illustrating an example of a subject information acquisition apparatus according to a third embodiment. FIG. 6 is a schematic cross-sectional view illustrating an example of an image forming apparatus according to a fourth embodiment.

  Nitride semiconductors that use Mg as a p-type impurity incorporate hydrogen atoms derived from Group III and Group V materials into the crystal during crystal growth, and combine the hydrogen atoms with Mg to deactivate Mg, resulting in high resistance. become. The vertical cavity surface emitting laser (VCSEL) of the present invention employs the following current confinement structure. That is, a semiconductor layer serving as a mask for blocking hydrogen atoms is formed on the p-type semiconductor layer, and a part of the surface is selectively activated to form a hydrogen atom concentration distribution in the p-type semiconductor layer. . As a result, a current confinement structure having an electric resistance distribution is formed in the p-type semiconductor layer.

  The VCSEL of the present invention will be described below. First, terms used in this specification will be defined. In this specification, when referring to the vertical direction of the VCSEL structure, the substrate side is defined as the lower side, and the side opposite to the substrate is defined as the upper side.

(Embodiment 1)
FIG. 1A is a schematic cross-sectional view showing an example of the VCSEL of this embodiment, and FIG. 1B is a schematic top view of an example of the VCSEL of this embodiment. FIG. 1A corresponds to a cross-sectional view taken along a line AB in FIG. The VCSEL has an n-type lower reflecting mirror 102, a spacer layer 103, an active layer 104, a p-type first semiconductor layer 105, a second semiconductor layer 106, and a p-type third semiconductor layer on a substrate 101. 107 and a contact layer 108 are sequentially provided. Further, the VCSEL includes an upper reflecting mirror 109 and a first electrode 110 on the contact layer 108, and a second electrode 111 below the substrate 101.

  The first semiconductor layer 105 has an electrical resistance distribution in the in-plane direction of the substrate 101, and includes a region 105a having a high electrical resistance and a region 105b having a low electrical resistance. More specifically, the first semiconductor layer 105 has a hydrogen concentration distribution in the in-plane direction, and the hydrogen concentration in the region 105a is higher than the hydrogen concentration in the region 105b. The hydrogen concentration in the region 105b is preferably less than 50% of the Mg concentration in the first semiconductor layer 105, more preferably less than 30%, and even more preferably less than 10%. The hydrogen concentration in the region 105a is preferably 50% or more of the Mg concentration in the first semiconductor 105, more preferably 70% or more, and still more preferably 90% or more.

  An opening 130 is formed in the second semiconductor layer 106. The third semiconductor layer 107 is disposed on the second semiconductor layer 106 so as to cover and close the opening 130. Specifically, the third semiconductor layer 107 is disposed on the second semiconductor layer 106 and on the first semiconductor layer 105 located in the opening 130. Further, an upper reflecting mirror 109 is disposed on the contact layer 108 in a region corresponding to the opening 130. The first electrode 110 is disposed outside the region corresponding to the opening 130.

  The current confinement structure of this embodiment is constituted by a first semiconductor layer 105, a second semiconductor layer 106, and a third semiconductor layer 107. That is, the hole injected from the first electrode 110 passes through the contact layer 108, the third semiconductor layer 107 formed in the opening 130, and the low-resistance region 105 b of the first semiconductor layer 105, and then the active layer 104. To be supplied. On the other hand, electrons are injected from the second electrode 111 into the active layer 104 through the substrate 101, the lower reflecting mirror 102, and the spacer layer 103. Then, light is emitted by recombination of holes and electrons in the active layer 104.

  Part of the emitted light resonates between the lower reflecting mirror 102 and the upper reflecting mirror 109, and laser oscillation occurs. Light emission in the active layer 104 substantially corresponds to the opening 130, and the opening 130 corresponds to the light emission region. In this embodiment, the light emission surface of the VCSEL is on the upper reflecting mirror 109 side, and light is emitted from the side opposite to the substrate 101. However, the light may be emitted from the substrate 101 side. .

  Next, the current confinement structure of this embodiment will be described in detail. First, the thicknesses of the second semiconductor layer 106 and the third semiconductor layer 107 are described. The third semiconductor layer 107 is required to eliminate the influence of the step due to the opening 130. Therefore, the thickness of the third semiconductor layer 107 depends on a step due to the opening 130, that is, the thickness of the second semiconductor layer 106. For example, when the diameter of the opening 130 is about 10 μm or less, the thickness of the third semiconductor layer 107 is about the same as that of the second semiconductor layer 106 in order to eliminate the influence of the step due to the opening 130. Is required.

In order to stabilize the output of the VCSEL by shortening the resonator length to reduce the longitudinal mode and to suppress the mode hop, it is better to reduce the thickness of the third semiconductor layer 107. It is preferable to reduce the thickness in accordance with the thickness of the third semiconductor layer 107. Therefore, the thickness of the second semiconductor layer 106 and the third semiconductor layer 107 is preferably 200 nm or less, more preferably 100 nm or less. Furthermore, the thickness of the second semiconductor layer 106 and the third semiconductor layer 107 is preferably small from the viewpoint of suppressing increase in light absorption and electrical resistance. A light absorption coefficient of a p-type nitride semiconductor like the third semiconductor layer 107 is said to be 50 cm ^{−1 to} 300 cm ⁻¹ . If the absorption coefficient is estimated to be 50 cm ⁻¹ , the light absorption of the p-type third semiconductor layer 107 is 0.1% when the thickness is 200 nm and 0.05% when the thickness is 100 nm.

  Next, a configuration in which the first semiconductor layer 105 has an in-plane distribution of electrical resistance in the in-plane direction will be described. As for the second semiconductor layer 106, it is known that in the case of a nitride semiconductor, when the Fermi level moves away from the valence band, hydrogen atoms are less likely to be present in the crystal of the nitride semiconductor. The fact that hydrogen atoms are less likely to be present means that hydrogen atoms are less likely to pass through the nitride semiconductor.

In this embodiment, a structure in which the Fermi level is separated from the valence band is formed as the second semiconductor layer 106 on the first semiconductor layer 105, so that it can be used as a mask for blocking hydrogen atoms. The structure in which the Fermi level is separated from the valence band refers to a p ⁻ type semiconductor layer in which holes are lower than a normal doping concentration or a state in which the entire semiconductor layer is depleted.

  The second semiconductor layer 106 of the present embodiment controls the second semiconductor layer 106 by controlling the Mg concentration diffused from the first semiconductor layer 105 and the third semiconductor layer and the Si concentration doped in the second semiconductor layer 106. The entire semiconductor layer 106 is depleted. The second semiconductor layer 106 has a thickness of 50 nm or more in order to block hydrogen atoms.

  With this configuration, when the entire VCSEL is annealed in the manufacturing method described later, hydrogen can be prevented from being released from the first semiconductor layer 105 in the region where the second semiconductor layer 106 is disposed. As a result, a hydrogen concentration distribution is formed in the first semiconductor layer 105. Specifically, the first semiconductor layer 105 has a hydrogen concentration distribution in which a hydrogen concentration in a portion corresponding to the opening 130 is lower than a hydrogen concentration in a portion corresponding to the outside of the opening 130. For this reason, in the part corresponding to the opening part 130 of the 1st semiconductor layer 105, Mg is activated by hydrogen escaping. On the other hand, in the portion corresponding to the portion other than the opening 130 of the first semiconductor layer 105, since hydrogen does not escape, Mg remains inactive.

  As a result, in the first semiconductor layer 105, a portion corresponding to the opening 130 is a low resistance region 105b, and the other portion is a high resistance region 105a. As a result, holes from the first electrode 110 are supplied to the active layer 104 through the low resistance region 105b. Note that, in the first semiconductor layer 105, the carrier concentration in the portion corresponding to the opening 130 in the in-plane direction is more than the carrier concentration in the portion corresponding to the outside of the opening 130 due to the influence of activated Mg. It is high.

  As a method of detecting the hydrogen concentration, secondary ion mass spectrometry (SIMS) can be used. On the other hand, as a method for measuring the distribution of electric resistance and carrier concentration, scanning capacitance microscopy (Scanning Capacitance Microscopy) can be mentioned. In addition, a scanning microwave microscopy or a scanning spreading resistance microscope may be used.

In order to deplete the entire image of the second semiconductor layer 106, the Mg concentration of the first semiconductor layer 105 and the third semiconductor layer 107 is doped with a concentration of 1 × 10 ^{19 to} 3 × 10 ¹⁹ cm ^−3. Do. On the other hand, the Si concentration doped into the second semiconductor layer 106 is the same as the Mg concentration of the first semiconductor layer 105 and the third semiconductor layer 107, or the Mg concentration of the first semiconductor layer 105 and the third semiconductor layer 107. The concentration is higher than the concentration. In that case, the influence of Mg diffused into the second semiconductor layer 106 can be canceled and the second semiconductor layer 106 can be made to be n-type. However, the crystal quality is deteriorated due to excessive doping, pits and the like are generated in the second semiconductor layer 106, leakage occurs, and the function of the current confinement structure cannot be obtained. Therefore, the Si concentration of the second semiconductor layer 106 is set lower than the Mg concentration of the first semiconductor layer 105, preferably 1 × 10 ^{18 to} 5 × 10 ¹⁸ cm ⁻³ .

  Next, a depletion layer formed in the second semiconductor layer 106 is described. When the second semiconductor layer 106 is bonded to the first semiconductor layer 105, a depletion layer is formed at the interface between the second semiconductor layer 106 and the first semiconductor layer 105 in accordance with the doping concentration. Of the depletion layer formed at the interface between the first semiconductor layer 105 and the second semiconductor layer 106, a region formed in the second semiconductor layer 106 is defined as the width Wn1 of the depletion layer. Similarly, a depletion layer is also formed at the interface between the third semiconductor layer 107 and the second semiconductor layer 106, and a region formed in the second semiconductor layer 106 is defined as a width Wn2 of the depletion layer.

FIG. 2 shows an enlarged schematic view of the region 150 in FIG. As shown in FIG. 2, in the second semiconductor layer 106, Wn1 and Wn2 are formed so as to overlap each other in the thickness direction of the second semiconductor layer 106. Therefore, the entire second semiconductor layer 106 is a depletion layer. That is, when expressed by the equation using the thickness t of the second semiconductor layer 106 and the widths Wn1 and Wn2 of the depletion layer, the following equation is obtained.
t ≦ Wn1 + Wn2 (1)
Here, the thickness Wn1 (or Wn2) of the depletion layer is expressed by the following formula (2) and boundary condition (3) (the voltage and the electric field are continuous, the electric field is 0 at the depletion layer end, and the p-side voltage at the depletion layer end Is calculated from 0 and the n-side voltage is the diffusion potential Vd). Na (x) is the distribution of Mg in the second semiconductor layer 106, and Nd (x) is the distribution of Si in the second semiconductor layer 106. x represents the thickness direction. e is the elementary charge, ε _s is the relative permittivity, and ε ₀ is the permittivity in vacuum. Further, p _p0 is the hole density in the first semiconductor layer 105 (or the third semiconductor layer 107), and _pn0 is the hole density in the second semiconductor layer 106. n _n0 is the electron density in the second semiconductor layer 106, n _p0 is the electron density in the first semiconductor layer 105 (or the third semiconductor layer 107), and Wn is a depletion layer extending to the second semiconductor layer 106. The width Wp is the width of the depletion layer extending to the first semiconductor layer 105 (or the third semiconductor layer 107). Wn is Wn1 or Wn2. In an actual device, since Mg diffuses internally, Na (x) cannot be expressed by a simple function, and it is difficult to obtain an analytical solution by numerical calculation.

この結果、第１の半導体層１０５内に電気抵抗の分布が生じ、第２の半導体層１０６中の開口部１３０に対応した低抵抗な領域１０５ｂを介して活性層１０４にキャリアが注入される電流狭窄構造が形成される。そして、開口部１３０に対応する領域が発光領域となる。また、本実施形態の構成では、第２の半導体層１０６の厚さが小さいため、開口部１３０による段差は小さくなる。そのため、その段差を埋める第３の半導体層１０７の厚さも小さくなり、共振器長が大きくなることを低減でき、レーザ出力を安定化することができる。また、段差を埋めることができるため、光の散乱を抑制でき、レーザ閾値を小さくすることができる。

As a result, a distribution of electrical resistance is generated in the first semiconductor layer 105, and a current in which carriers are injected into the active layer 104 through the low resistance region 105 b corresponding to the opening 130 in the second semiconductor layer 106. A constriction structure is formed. A region corresponding to the opening 130 is a light emitting region. Further, in the configuration of this embodiment, since the thickness of the second semiconductor layer 106 is small, the step due to the opening 130 is small. Therefore, the thickness of the third semiconductor layer 107 filling the step is also reduced, the increase in the resonator length can be reduced, and the laser output can be stabilized. In addition, since the step can be filled, light scattering can be suppressed and the laser threshold can be reduced.

  Next, a method for manufacturing the VCSEL will be described. In particular, a method for manufacturing a VCSEL array will be described with reference to FIGS. A VCSEL element can be formed by a similar manufacturing method.

  First, as shown in FIG. 3A, an n-type lower reflecting mirror 102, a spacer layer 103, and an active layer 104 are sequentially formed on a substrate 101. The substrate 101 only needs to be able to form a nitride semiconductor, and sapphire, Si, GaAs, SiC, and GaN can be used. The lower reflecting mirror 102 may be a distributed Bragg reflector (hereinafter referred to as DBR) made of a laminate of AlGaN and InGaN. The active layer 104 uses a quantum well structure or the like in order to emit light efficiently. For example, as the active layer 104, InGaN / GaN or the like can be used.

  Further, as shown in FIG. 3A, a first semiconductor layer 105 doped with Mg as a p-type impurity is formed on the substrate 101 grown to the active layer 104. During this process, in the first semiconductor layer 105, hydrogen atoms generated by the decomposition of the group III raw material and the group V raw material are combined with Mg and taken into the first semiconductor layer 105. Almost no activation is made, and the first semiconductor layer 105 has a high resistance.

  Next, as shown in FIG. 3B, a second mask including Si and having an opening 130 is used as a mask for selectively activating hydrogen in the first semiconductor layer 105 in a later step. A semiconductor layer 106 is formed. The thickness of the second semiconductor layer 106 is 200 nm or less as described above. Further, the Si concentration of the second semiconductor layer 106 is set lower than the Mg concentration of the first semiconductor layer 105. With this configuration, a depletion layer is formed over the entire second semiconductor layer 106, and permeation of hydrogen can be suppressed.

  The opening 130 is provided corresponding to a portion in the surface of the first semiconductor layer 105 where electrical resistance is desired to be lowered. Examples of the method for forming the opening 130 include the following methods. That is, a method for selectively growing the second semiconductor layer 106 by forming a growth-inhibiting mask, photolithography, dry etching, or the like. In addition, the second semiconductor layer 106 is formed between the VCSEL elements without providing the opening 130.

  Next, as shown in FIG. 3C, the third semiconductor layer 107 and the contact layer 108 are straddled over the second semiconductor layer 106 and the first semiconductor layer 105 in the opening 130. Form. At this time, similarly to the first semiconductor layer 105, the third semiconductor layer 107 also takes in hydrogen atoms, and the third semiconductor layer 107 also has high resistance. The thickness of the third semiconductor layer 107 may be a thickness that fills the step due to the opening 130, but is preferably a thickness that does not increase light absorption or resistance.

  Next, as shown in FIG. 4A, annealing (heat treatment) is performed in an atmosphere not containing hydrogen atoms. Then, since the third semiconductor layer 107 has no surface that prevents diffusion of hydrogen atoms, hydrogen atoms in the third semiconductor layer 107 are desorbed from the surface, and Mg in the third semiconductor layer 107 is activated. And the whole becomes p-type and has low resistance. On the other hand, in the first semiconductor layer 105, hydrogen atoms can be released to the surface through the third semiconductor layer 107 in the region of the opening 130. However, in a region where there is no opening 130, hydrogen atoms are blocked by the second semiconductor layer 106 and are difficult to desorb to the surface.

  As a result, a hydrogen concentration distribution is generated in the first semiconductor layer 105 as shown in FIG. Specifically, the hydrogen concentration in the portion corresponding to the opening 130 is lower than the hydrogen concentration in the portion corresponding to the outside of the opening 130. For this reason, Mg is selectively activated in the portion of the opening 130, and the first semiconductor layer 105 immediately below the opening 130 becomes a region 105b with low electrical resistance. On the other hand, the first semiconductor layer 105 immediately below the second semiconductor layer 106 becomes a region 105a with high electrical resistance.

  It should be noted that the order of the heat treatment steps needs attention. When element isolation is performed by forming a mesa on the surface, hydrogen atoms are desorbed from the side wall surface of the first semiconductor layer 105. Therefore, it is desirable to perform this heat treatment before element isolation. Note that in the case where the heat treatment step is performed after element isolation, a similar structure can be obtained by increasing the distance from the opening 130 to the side wall surface of the mesa structure and performing the heat treatment in a short time. The distance from the opening 130 to the side wall surface of the mesa structure may be larger than the longest diameter of the opening 130.

  Next, as shown in FIG. 4B, the first electrode 110 is formed on the contact layer 108, and the second electrode is formed on the back surface of the substrate 101. Further, the upper reflecting mirror 109 is formed on the contact layer 108. Note that the first electrode 110 is formed for each element. Further, the first electrode 110 is formed in a region other than the region corresponding to the opening 130 in order to avoid the VCSEL light emission region. The upper reflecting mirror 109 is formed in a region corresponding to the opening 130.

  Finally, as shown in FIG. 4C, a mesa structure is formed so as to separate the VCSEL elements. Although not shown in the drawing, this step is performed after a protective film is provided in a region not to be etched. FIG. 4B shows an example in which the mesa structure is formed so as to separate up to the spacer layer 103, but a part or all of the lower reflecting mirror 102 may be separated by etching. Further, the spacer layer 103 may be connected by an element by separating up to the active layer 104. In the case where the spacer layer 103 is connected by an element, the second electrode 111 may be provided on the spacer layer 103. In this case, the substrate 101 and the lower reflecting mirror are not conductive. Also good.

(substrate)
As described above, sapphire, Si, GaAs, SiC, or GaN can be used for the substrate 101. In addition, a substrate in which a buffer layer such as GaN is provided on sapphire can be used as a substrate.

(Lower reflector and upper reflector)
The lower reflecting mirror 102 and the upper reflecting mirror 109 are composed of, for example, a DBR in which high refractive index layers and low refractive index layers are alternately stacked with an optical thickness of ¼ wavelength. As the lower reflecting mirror 102 and the upper reflecting mirror 109, either a semiconductor DBR made of a semiconductor or a dielectric DBR made of a dielectric can be used. In general, a DBR made of a dielectric is easier to increase the refractive index difference between a high refractive index layer and a low refractive index layer than a DBR made of a semiconductor, so that a high reflectance is achieved with a small number of layers. realizable. On the other hand, a DBR made of a semiconductor has a large number of pairs. However, it has a process advantage that a film can be formed simultaneously during crystal growth and a current can be supplied by doping.

  Silicon oxide can be used as the low refractive index layer of the dielectric DBR, and titanium oxide or tantalum pentoxide can be used as the high refractive index layer. AlGaN or GaN can be used as the low refractive index layer of the semiconductor DBR. As the high refractive index layer of the semiconductor DBR, GaN or InGaN can be used when AlGaN is used as the low refractive index layer, and InGaN can be used when GaN is used as the low refractive index layer. Further, AlGaN or InGaN may be used for both the high refractive index layer and the low refractive index layer of the semiconductor DBR, and the Al composition or In composition of each layer may be changed.

  In order to collectively form the layers by epitaxial growth, a semiconductor DBR can be used for the lower reflecting mirror 102 and a semiconductor DBR can also be used for the upper reflecting mirror 109. However, the dielectric DBR may be used for the upper reflecting mirror 109 in order to obtain a high reflectance in a wider band.

(Spacer layer)
The spacer layer 103 can be made of a nitride semiconductor such as GaN, InGaN, or AlGaN. Since the spacer layer 103 is of an n-type, it is doped with a dopant such as Si or C. The spacer layer 103 may be composed of a plurality of layers. Although not shown, a separate spacer layer may be disposed between the active layer 104 and the first semiconductor layer 105.

(Active layer)
The active layer 104 is not particularly limited as long as it is a material that generates light by injecting current. For example, a quantum well structure made of GaN and InGaN can be used as the active layer 104. The quantum well structure may be a single quantum well or a multiple quantum well. Examples of the multiple quantum well structure include a quantum well structure capable of emitting light at at least two different energy levels, a so-called asymmetric quantum well structure. In order to configure the asymmetric quantum well structure, for example, a configuration in which the thickness and composition of one quantum well layer is different from those of other quantum well layers.

  The active layer 104 may be composed of a plurality of active layers. For example, it is preferable that each of the plurality of active layers is disposed apart from each other so as to be disposed on the antinode of the electric field strength distribution in the VCSEL. The material and structure of the active layer 104 can be appropriately selected according to the wavelength desired to be oscillated.

(First semiconductor layer)
As described above, in the first semiconductor layer 105, the electric resistance of the portion corresponding to the opening 130 of the second semiconductor layer 106 is the electric resistance of the portion corresponding to the outside of the opening 130 of the second semiconductor layer 106. The structure is smaller than the resistance. Further, in the first semiconductor layer 105, the hydrogen concentration in the portion corresponding to the opening 130 of the second semiconductor layer 106 is higher than the hydrogen concentration in the portion corresponding to the outside of the opening 130 of the second semiconductor layer 106. It has a low hydrogen concentration distribution. In other words, in the first semiconductor layer 105, the carrier concentration in the portion corresponding to the opening 130 in the second semiconductor layer 106 is greater than the carrier concentration in the portion corresponding to outside the opening 130 in the second semiconductor layer 106. Has a high carrier concentration distribution.

The first semiconductor layer 105 is doped with Mg to form a p-type, and the concentration thereof is 1 × 10 ^{18 to} 5 × 10 ¹⁹ cm ⁻³ , more preferably 5 × 10 ^{18 to} It is 4 × 10 ¹⁹ cm ⁻³ , more preferably 1 × 10 ^{19 to} 3 × 10 ¹⁹ cm ⁻³ . The first semiconductor layer 105 is made of a nitride semiconductor such as GaN, InGaN, or AlGaN.

(Second semiconductor layer)
The second semiconductor layer 106 has a function of suppressing release of hydrogen from part of the first semiconductor layer 105. For this reason, the entire second semiconductor layer 106 is depleted. In addition, the second semiconductor layer 106 has an opening 130 in order to form a distribution of electrical resistance in the first semiconductor layer 105. The opening 130 does not have to be circular as shown in FIG. 1B, and may be oval or rectangular.

The second semiconductor layer 106 is doped with Si to form a depletion layer, and the concentration thereof is lower than the Mg concentration of the first semiconductor layer 105, and is 1 × 10 ^{18 to} 5 × 10 ¹⁸ cm ^{−. 3} is preferred. The second semiconductor layer 106 is formed of a nitride semiconductor such as GaN, InGaN, or AlGaN. The thickness of the second semiconductor layer 106 is preferably greater than or equal to 50 nm and less than or equal to 200 nm.

(Third semiconductor layer)
The third semiconductor layer 107 is configured to have a thickness that fills the step due to the opening 130 of the second semiconductor layer 106. Specifically, the thickness may be equal to or greater than that of the second semiconductor layer 106. The third semiconductor layer 107 is doped with Mg in order to form a p-type, and the concentration is preferably 1 × 10 ^{19 to} 3 × 10 ¹⁹ cm ⁻³ . The third semiconductor layer 107 is formed of a nitride semiconductor such as GaN, InGaN, or AlGaN.

(Contact layer)
There is no particular limitation on the contact layer 108 as long as it can be electrically connected to the first electrode 110, but the contact layer 108 is formed of a p-type. The Mg concentration in the contact layer 108 is higher than that of the first semiconductor layer 105 and the third semiconductor layer 107 and is preferably 3 × 10 ^{19 to} 5 × 10 ¹⁹ cm ⁻³ . The contact layer 108 is formed of a nitride semiconductor such as GaN, InGaN, or AlGaN.

(First electrode and second electrode)
The material of the first electrode 110 and the second electrode 111 is not limited as long as carriers are injected. A single metal or alloy such as titanium or gold, or a laminate of metal films can be used. For example, Ti / Au or AuGe / Ni / Au can be used as the electrode material.

  Further, as shown in FIG. 1B, the first electrode 110 is located outside the opening 130 and is disposed so as not to overlap the light emitting region (light emission region). With such a structure, the first electrode 110 may not have a donut shape. The first electrode 110 may be divided into a plurality of parts. The first electrode 110 is provided for each VCSEL element.

  Although the 2nd electrode 111 is arrange | positioned under the board | substrate 101, it is not limited to this. For example, it may be formed on the lower reflecting mirror 102 or the spacer layer 103. The second electrode 111 may also be divided.

(Embodiment 2)
Next, a configuration example of a solid-state laser including a surface-emitting laser array in which the VCSELs described in the first embodiment are integrated and arranged in an array as an excitation light source will be described with reference to FIG. FIG. 5 is a schematic diagram of a solid-state laser according to this embodiment.

  A solid-state laser 1100 according to this embodiment includes a surface-emitting laser array 1110, a solid-state laser medium 1130, and a pair of reflecting mirrors 1150a and 1150b.

  The surface emitting laser array 1110 irradiates the solid-state laser medium 1130 with excitation light 1120 having a wavelength λ. The solid-state laser medium 1130 absorbs the excitation light 1120 and emits light 1140 with laser transition. The light 1140 generated from the solid-state laser medium is repeatedly reflected by the two reflecting mirrors 1150a and 1150b, so that the solid-state laser enters an oscillation state. Then, a solid-state laser beam 1160 that has passed through the reflecting mirror 1150b is emitted from the solid-state laser 1100 in the oscillation state.

  Here, it is preferable to determine the wavelength λ of the excitation light 1120 emitted from the surface emitting laser array 1110 in accordance with the absorption spectrum of the solid-state laser medium 1130. That is, it is preferable to design the peak wavelength of the reflectance of the reflecting mirror used in the surface emitting laser array 1110 according to the absorption spectrum of the solid-state laser medium 1130. Furthermore, it is more preferable to design the reflecting mirror so that the wavelength near the peak of the absorption spectrum of the solid-state laser medium 1130 becomes the peak of reflectance. For example, when an alexandrite crystal is used as the solid-state laser medium 1130, the solid-state laser can be efficiently oscillated by setting the peak wavelength λ of the reflectance of the reflector to 400 nm near the peak of the absorption spectrum of the alexandrite crystal. Note that any solid-state laser medium may be employed for the solid-state laser according to the present embodiment.

(Embodiment 3)
Next, a subject information acquisition apparatus using the solid-state laser 1100 described in the second embodiment will be described with reference to FIG. The subject information acquisition apparatus according to this embodiment includes a solid-state laser 1100, an optical system 1200, a probe (detection element) 1400, a signal processing unit (acquisition unit) 1500, and a display unit 1600.

  First, light generated from the solid-state laser 1100 is irradiated to the subject 1000 as pulsed light 1210 through the optical system 1200. Then, a photoacoustic wave 1020 is generated in the light absorber 1010 in the subject 1000 due to the photoacoustic effect. Subsequently, the probe 1400 detects the photoacoustic wave 1020 that has propagated through the subject 1000 and acquires a time-series electrical signal. Subsequently, the signal processing unit 1500 acquires information inside the subject based on the time-series electrical signal, and causes the display unit 1600 to display information inside the subject.

  In the present embodiment, the wavelength of light that can be emitted by the solid-state laser 1100 is desirably a wavelength at which light propagates to the inside of the subject 1000. Specifically, when the subject 1000 is a living body, a suitable wavelength is 500 nm or more and 1200 nm or less. However, when obtaining the optical characteristic value distribution of the living tissue relatively near the surface of the living body, it is also possible to use a wavelength region having a wider range than the above wavelength region, for example, a wavelength region of 400 nm to 1600 nm.

  The object information according to the present embodiment includes an initial sound pressure of a photoacoustic wave, a light energy absorption density, an absorption coefficient, and a concentration of a substance constituting the object. Here, the concentration of the substance includes oxygen saturation, oxyhemoglobin concentration, deoxyhemoglobin concentration, total hemoglobin concentration, and the like. The total hemoglobin concentration is the sum of the oxyhemoglobin concentration and the deoxyhemoglobin concentration. In the present embodiment, the subject information may not be numerical data but may be distribution information of each position in the subject. That is, distribution information such as an absorption coefficient distribution and an oxygen saturation distribution may be used as the subject information.

  The solid-state laser 1100 may be configured to switch wavelengths. A plurality of different solid-state lasers 1100 can also be used.

(Embodiment 4)
Next, a configuration example of an image forming apparatus including a surface emitting laser array in which the VCSELs described in the first embodiment are arranged in an array as a light source will be described with reference to FIG. FIG. 7A is a plan view of the image forming apparatus according to this embodiment, and FIG. 7B is a side view of the apparatus.

  The rotating polygon mirror 2010 is driven to rotate by a motor 2012 shown in FIG. The surface emitting laser array 2014 serves as a recording light source, and is configured to be turned on or off according to an image signal by a laser driver (not shown).

  As shown in FIGS. 7A and 7B, the laser light thus optically modulated is irradiated from the surface emitting laser array 2014 toward the rotary polygon mirror 2010 via the collimator lens 2018. The rotating polygon mirror 2010 rotates in the direction of the arrow, and the laser light output from the surface emitting laser array 2014 is reflected as a deflected beam that continuously changes the emission angle on the reflecting surface as the rotating polygon mirror 2010 rotates. Is done.

  The reflected light is subjected to correction of distortion and the like by the f-θ lens 2020, irradiated to the photosensitive drum (photosensitive member) 2000 through the reflecting mirror 2016, and scanned on the photosensitive drum 2000 in the main scanning direction. At this time, the image of a plurality of lines corresponding to the surface emitting laser array 2014 is formed in the main scanning direction of the photosensitive drum 2000 by the reflection of the beam light through one surface of the rotating polygon mirror 2010.

  The photosensitive drum 2000 is charged in advance by a charger 2002 and sequentially exposed by scanning with a laser beam to form an electrostatic latent image. Further, the photosensitive drum 2000 is rotated in the direction of the arrow, and the formed electrostatic latent image is developed by the developing device 2004, and the developed visible image is transferred to the transfer paper by the transfer charger 2006. The transfer paper on which the visible image is transferred is conveyed to a fixing device 2008, and after fixing, the transfer paper is discharged out of the apparatus.

Example 1
An example corresponding to Embodiment 1 of the present invention will be described with reference to FIGS.

  First, a GaN substrate is used as the substrate 101 as shown in FIG. A GaN substrate is set in a MOCVD apparatus and heated. A carrier gas and a source gas are supplied to grow a semiconductor layer on the GaN substrate. First, the lower reflecting mirror 102 having n-type conductivity is grown. The lower reflecting mirror 102 is a distributed Bragg reflecting mirror (DBR) in which low refractive index layers and high refractive index layers are alternately stacked. The low refractive index layer is made of AlN or AlGaN, and the high refractive index layer is made of InGaN or GaN. Next, an n-type spacer layer 103 for adjusting the position of the standing wave in the resonator is grown, and an active layer 104 having an InGaN / GaN 5-period multiple quantum well structure is formed thereon as an active layer 104. Grow.

Next, p-AlGaN (not shown) is grown as an electron blocking layer for blocking current, and on that, as shown in FIG. 3A, the Mg concentration of the first semiconductor layer 105 is 1 ×. 10 ¹⁹ cm ⁻³ of p-GaN is grown. p-GaN has high resistance due to incorporation of hydrogen atoms contained in NH ₃ , trimethyl gallium and the like used as raw materials during growth.

Next, as shown in FIG. 3B, AlGaN having a thickness of 100 nm and a Si concentration of 2 × 10 ¹⁸ cm ⁻³ is formed as the second semiconductor layer 106 having the opening 130. The reason for using AlGaN is that deformation of the opening 130 due to heat when p-GaN is grown on AlGaN can be suppressed.

The procedure for forming the opening 130 is as follows. The substrate 101 is taken out from the MOCVD apparatus, and a selective growth mask such as SiO ₂ or SiN _x is formed at a location corresponding to the opening 130 by using photolithography and dry etching (RIE or ICP). Next, the substrate 101 is set in an MOCVD apparatus, and AlGaN is grown while supplying SiH ₄ . After the growth of the AlGaN 305, the substrate 101 is taken out of the MOCVD apparatus and the selective growth mask is removed, whereby the second semiconductor layer 106 having the opening 130 is formed.

In this step, AlGaN, which is the second semiconductor layer 106, grows while Mg is diffused from the p-GaN constituting the first semiconductor layer 105. The distribution of Mg that diffuses is experientially distributed with an exponential function in which the concentration decreases by one digit at a thickness of 100 nm. In this example, Mg has an exponential distribution with about 1 × 10 ¹⁹ cm ⁻³ on the AlGaN substrate 101 side and about 1 × 10 ¹⁸ cm ⁻³ on the AlGaN surface side.

Next, as shown in FIG. 3C, p-GaN having an Mg concentration of 1 × 10 ¹⁹ cm ⁻³ is grown as the third semiconductor layer 107. Again, Mg diffuses from the p-GaN of the third semiconductor layer 107 to the AlGaN of the second semiconductor layer 106. This Mg is more difficult to diffuse than Mg contained in the first semiconductor layer, and diffuses about several tens to 50 nm.

Then, p ⁺ -GaN highly doped with Mg is grown as the contact layer 108. The third semiconductor layer 107 and the contact layer 108 also have high resistance due to incorporation of hydrogen atoms contained in the growing raw material.

  Next, as shown in FIG. 4A, the substrate 101 is taken out of the MOCVD apparatus and heat-treated at 900 ° C. in a nitrogen atmosphere. As a result, hydrogen atoms are released from part of the first semiconductor layer 105, the third semiconductor layer 107, and the contact layer 108. In the first semiconductor layer 105, the second semiconductor layer 106 is formed in addition to the opening 130, so that hydrogen atoms are desorbed only in the portion of the opening 130, and almost all hydrogen atoms remain in the other portions. . As a result, as shown in FIG. 4B, the region 105a in the opening portion 130 has a low resistance, and the region 105b remains at a high resistance.

  Next, as shown in FIG. 4B, the dielectric DBR and the first electrode 110 are formed on the contact layer 108 as an upper reflecting mirror, and the second electrode 111 is formed on the back surface of the substrate 101.

  Finally, as shown in FIG. 4C, a protective film (not shown) is provided in a region not to be etched, and a mesa for separating the VCSEL for each element is formed by dry etching. A mesa structure is formed so as to separate up to the spacer layer 103.

  Through the above steps, a nitride semiconductor surface emitting laser (VCSEL) having a resonator composed of a lower reflecting mirror provided on the substrate side of the active layer and an upper reflecting mirror provided on the surface side of the active layer is completed.

  The VCSEL thus formed has a hydrogen concentration distribution in the first semiconductor layer 105 in which the hydrogen concentration in the portion corresponding to the opening 130 is lower than the hydrogen concentration in the portion corresponding to the outside of the opening 130. Have. That is, the first semiconductor layer 105 has an electrical resistance distribution in which the electrical resistance of the portion corresponding to the opening 130 is smaller than the electrical resistance of the portion corresponding to the outside of the opening 130. As a result, it has a current confinement structure that facilitates the flow of holes selectively in the region 105b with low electrical resistance.

101 substrate 104 active layer 105 first semiconductor layer 106 second semiconductor layer 107 third semiconductor layer 110 first electrode 111 second electrode 130 opening

Claims (17)

A first reflector;
An active layer disposed on the first reflector;
A p-type first semiconductor layer disposed on the active layer;
A second semiconductor layer disposed on the first semiconductor layer and having an opening;
A p-type third semiconductor layer disposed on the second semiconductor layer and on the first semiconductor layer located in the opening so as to fill a step due to the opening;
A second reflecting mirror disposed on the third semiconductor and at least in a region corresponding to the opening;
An electrode disposed on a region corresponding to the outside of the opening on the third semiconductor layer,
The first semiconductor layer comprises hydrogen;
The surface emitting laser according to claim 1, wherein the first semiconductor layer has a hydrogen concentration distribution in which a hydrogen concentration in a portion corresponding to the opening is lower than a hydrogen concentration in a portion corresponding to the outside of the opening. A first reflector;
An active layer disposed on the first reflector;
A p-type first semiconductor layer disposed on the active layer;
A second semiconductor layer disposed on the first semiconductor layer and having an opening;
A p-type third semiconductor layer disposed on the second semiconductor layer and on the first semiconductor layer located in the opening so as to fill a step due to the opening;
A second reflecting mirror disposed on the third semiconductor and at least in a region corresponding to the opening;
An electrode disposed on a region corresponding to the outside of the opening on the third semiconductor layer,
The first semiconductor layer comprises hydrogen;
The surface emitting laser according to claim 1, wherein the first semiconductor layer has an electric resistance distribution in which a portion corresponding to the opening has a smaller electric resistance than a portion corresponding to the outside of the opening. A first reflector;
An active layer disposed on the first reflector;
A p-type first semiconductor layer disposed on the active layer;
A second semiconductor layer disposed on the first semiconductor layer and having an opening;
A p-type third semiconductor layer disposed on the second semiconductor layer and on the first semiconductor layer located in the opening so as to fill a step due to the opening;
A second reflecting mirror disposed on the third semiconductor and at least in a region corresponding to the opening;
An electrode disposed on a region corresponding to the outside of the opening on the third semiconductor layer,
The first semiconductor layer comprises hydrogen;
The surface emitting laser according to claim 1, wherein the first semiconductor layer has a carrier concentration distribution in which a carrier concentration in a portion corresponding to the opening is higher than a carrier concentration in a portion corresponding to the outside of the opening.   The surface emitting laser according to claim 1, wherein the second semiconductor layer is a depletion layer as a whole. Of the depletion layers generated by the junction of the first semiconductor layer and the second semiconductor layer, the depletion layer on the second semiconductor layer side is Wn1, and the third semiconductor layer and the second semiconductor layer are Wn2 is a depletion layer on the second semiconductor layer side of the depletion layer generated by the junction of
The thickness t of the second semiconductor layer is:
t ≦ Wn1 + Wn2
The surface emitting laser according to claim 1, wherein: The first semiconductor layer includes Mg;
The surface emitting laser according to claim 1, wherein the second semiconductor layer contains Si.   The surface emitting laser according to claim 6, wherein the concentration of Si contained in the second semiconductor layer is lower than the concentration of Mg contained in the first semiconductor layer.   8. The surface emitting device according to claim 1, wherein the first semiconductor layer, the second semiconductor layer, and the third semiconductor layer are made of a nitride semiconductor. laser. A laser array having a plurality of surface emitting lasers,
9. The laser array according to claim 1, wherein at least one of the plurality of surface emitting lasers is the surface emitting laser according to claim 1. A pair of reflectors;
A solid-state laser comprising: a laser medium disposed between the pair of reflecting mirrors; and a light source for exciting the laser medium,
A solid-state laser, wherein the light source has the laser array according to claim 9. A solid-state laser according to claim 10;
A detection element that detects an acoustic wave generated by irradiating the subject with light emitted from the solid-state laser and outputs an electrical signal;
An information acquisition apparatus comprising: an acquisition unit that acquires information inside the subject based on an electrical signal output from the detection element. A laser array according to claim 9;
And a photoreceptor on which a latent image is exposed by light emitted from the surface emitting laser. A method of manufacturing a laser array having a plurality of surface emitting lasers,
Forming a first reflecting mirror;
Forming an active layer on the first reflecting mirror;
Forming a p-type first semiconductor layer on the active layer;
Forming a second semiconductor layer having an opening on the first semiconductor layer;
Forming a p-type third semiconductor layer on the second semiconductor layer and on the first semiconductor layer located in the opening so as to fill a step due to the opening;
Forming a second reflecting mirror on the third semiconductor at least in a region corresponding to the opening;
Forming an electrode on the third semiconductor layer in a region corresponding to the outside of the opening;
Heat-treating the first semiconductor layer, the second semiconductor layer, and the third semiconductor layer in an atmosphere containing no hydrogen atoms;
A step of forming a mesa structure for separating each surface emitting laser after the step of forming the second reflecting mirror and the step of forming the electrode;
The first semiconductor layer comprises hydrogen;
The method of manufacturing a laser array, wherein the first semiconductor layer has a hydrogen concentration distribution in which a hydrogen concentration in a portion corresponding to the opening is lower than a hydrogen concentration in a portion corresponding to outside the opening. .   14. The method of manufacturing a laser array according to claim 13, wherein the second semiconductor layer is a depletion layer as a whole. Of the depletion layers generated by the junction of the first semiconductor layer and the second semiconductor layer, the depletion layer on the second semiconductor layer side is Wn1, and the third semiconductor layer and the second semiconductor layer are Wn2 is a depletion layer on the second semiconductor layer side of the depletion layer generated by the junction of
The thickness t of the second semiconductor layer is:
t ≦ Wn1 + Wn2
The method for manufacturing a laser array according to claim 13 or 14, wherein: The first semiconductor layer includes Mg;
16. The method of manufacturing a laser array according to claim 13, wherein the second semiconductor layer contains Si.   The method for manufacturing a laser array according to claim 16, wherein the concentration of Si contained in the second semiconductor layer is lower than the concentration of Mg contained in the first semiconductor layer.

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