JPH05211346A - Surface light emitting element - Google Patents

Abstract

PURPOSE:To provide a surface light-emitting element having a reduced element resistance, a low threshold value and a high efficiency. CONSTITUTION:A p-type GaAs current injection layer 32 is provided on the side of hole injection of a luminous region (a luminous layer) 2a and with mirrors 4b and 33 provided on both sides of this region 2a, the thickness of this layer 32 is a value of a product of 1/4 of the optical wavelength multiplied by an odd number.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of characteristics of a surface emitting diode and a surface emitting semiconductor laser which emits or oscillates laser light in a direction perpendicular to a substrate, and reduces element resistance and low threshold voltage. It is intended to provide a highly efficient device.

[0002]

2. Description of the Related Art A spontaneous emission control type surface emitting diode or a surface emitting type semiconductor laser which emits light or oscillates perpendicularly to a substrate is minute, has high directivity and high efficiency (or has a small threshold value), and is formed into a two-dimensional array. Since it is suitable for, it is expected to open a new way for optical applications. However, in order to efficiently control spontaneous light emission or obtain laser oscillation at a low threshold value, a mirror having a reflectance close to 100% is required. For this reason, a DBR mirror (distributed Bragg reflector) made of a semiconductor or dielectric multilayer film
Is used.

[0003]

FIG. 4 shows an example of a conventional light emitting device. In the figure, 1a is a substrate, 2 is a cavity, 3 and 4a are mirrors, and 11a and 12a are electrodes. In other words, G on the n-GaAs substrate 1a
DH (Do
uble Hetero) structure, SCH (Separated Comfined Het)
ero) structure or GRIN (Graded Index) -SCH
It has a cavity area 2 consisting of a structure and a mirror spacing of 1
Alternatively, it has a so-called microcavity shape with a half optical wavelength. A substrate-side mirror 3 made of an n-type semiconductor multilayer film and an air-side mirror 4a made of a p-type semiconductor multilayer film are provided outside the cavity region, and GaAs / AlAs
Alternatively, alternating semiconductor multilayer films each having a thickness of λ / 4n of AlGaAs / AlAs are used (where λ is the emission wavelength and n
Is each refractive index). Holes are injected from the p-type multilayer film and electrons are injected from the n-type multilayer film into the light emitting layer in the cavity, and light is emitted by recombination in the light emitting layer. The light emitting layer may be either n-type or p-type. The element is a mesa type element, and the light emission is the substrate 1
It is output through a. When an InGaAs strained quantum well is used for the light emitting layer, the emission wavelength is ˜1 μm, the energy is smaller than the band gap of GaAs, and the n-type substrate is transparent to this light, so that even when outputting from the substrate side It is not necessary to make holes in the substrate GaAs. The mesa structure is one method for forming a current confinement and a vertical light guide, but it may be another structure such as a buried type in which the surroundings are filled with a semiconductor. It is desirable that the height of the mesa is at least below the cavity region 2 for current confinement, and from the light guiding property, the mesa shape is formed as low as possible below the substrate-side mirror. In the following example, it is assumed that the height of the mesa is close to the cavity of the substrate-side mirror in order to confine the current and reduce the resistance on the substrate side.

The reflectivity of the semiconductor multi-layer film mirror increases as the refractive index difference Δn between the semiconductor layers constituting the semiconductor mirror increases and the number of films (the number of pairs) increases. However, in the conventional surface emitting device, the reflectivity is 100% required for oscillation. GaA is used as a semiconductor to obtain close reflectance.
Materials having a large composition difference such as s and AlAs are used.
In general, when semiconductor multilayer film mirrors having the same structure are compared, the reflectance of the substrate (refractive index to 3.6) side mirror is considerably smaller than that of the air (refractive index 1) side when viewed from the light emitting layer. Therefore, the number of pairs is GaA as a mirror composition.
Even if s / AlAs is used, 20-40 pairs on the substrate side,
Requires 10-25 pairs on the air side.

However, if the composition difference is made large, the electrical energy barrier at the interface becomes large, and if the number of pairs is made large, the mirror thickness becomes large, and in either case, the element resistance becomes abnormally large. Particularly in the case of a p-type mirror, the problem of mirror resistance due to barrier and thickness is important because the compound semiconductor has a large hole effective mass and a low mobility. The device resistance increases with miniaturization, but at least the mesa height necessary for current confinement, that is, the air-side mirror has p
If it is formed in the shape, it is more disadvantageous than the n-type.

For example, when the p-type mirror is made of 20 pairs of GaAs / AlAs, the thickness is about 3 μm, the number of interfaces is 40, the element resistance of the mesa-type element having a diameter of about 10 μm is 100Ω or more, and the surface emission. Laser threshold voltage is 1
It was over 5V. For this reason, measures such as inserting an Al _0.5 Ga _0.5 As intermediate layer at the GaAs / AlAs interface have been taken, but this has not led to an essential solution and the thickness of the entire semiconductor mirror is thick, so up to about 4V. The threshold voltage could not be lowered. This is nearly three times the size of about 1.4 V in the case of a normal stripe laser. The greatest feature of the surface laser is that it can be finely and two-dimensionally integrated. However, due to miniaturization, if the voltage is high due to the high resistance even if the threshold value becomes small, the low power consumption is impaired, and the integration condition May not be met. In addition, high resistance is also disadvantageous for high speed operation.

In order to reduce the resistance at the mesa portion, first, in the element structure of FIG. 4, a structure in which p and n are reversed (the cavity may be n or p) may be considered, but in this case, the substrate The side is a p-type semiconductor mirror. Originally, a large number of pairs are required to obtain a high reflectance, but as mentioned above, since it is disadvantageous in terms of reflectance on the substrate side, the number of pairs, that is, the total number of pairs, is increased to improve the reflectance. The thickness and the number of interfaces are increased, resulting in an increase in resistance even if the shape is not a mesa. In general, p-type semiconductors have large absorption for light with energy below the band gap.

[Equation 1] Increasing the doping amount of the p-type semiconductor forming the mirror causes an increase in the threshold value due to absorption loss in the mirror. Similarly, it becomes impossible to obtain a light emission output through the p-type substrate.

The present invention has been proposed in order to improve the above-mentioned drawbacks, and an object thereof is to use an n-type semiconductor multilayer film mirror on the air side of a mesa portion, but not a p-type semiconductor multilayer film on the substrate side. The main part of the substrate-side mirror is n-thick by not using it at all or minimizing it and introducing a semiconductor layer for ohmic formation and current injection to form a p-side current path.
-Type semiconductor multi-layer film, and at this time, the ohmic-cum-injection semiconductor film is set to an odd multiple of λ / 4n so as not to impair the structural condition as a mirror, thereby providing a low resistance and a low threshold value. To provide a surface emitting laser and a surface emitting diode.

[0009]

To achieve the above object, the present invention provides a p-type current injection layer on the hole injection side of the light emitting region, and an n type or undoped layer on the side opposite to the light emitting region of the injection layer. Another aspect of the present invention is a planar light-emitting device having a mirror formed of the semiconductor multilayer film.

[0010]

The semiconductor layer for p-type ohmic formation and current injection is used on the substrate side, and the main part of the substrate-side mirror is n.
By forming the semiconductor film for ohmic / implantation into an odd multiple of λ / 4n,
This element is different from the conventional element in that the constitutional condition as a mirror is not impaired, which makes it possible to reduce the element resistance and obtain a light emitting element having a low threshold value and high efficiency.

[0011]

EXAMPLES Next, examples of the present invention will be described. It is needless to say that the embodiment is merely an example, and various modifications and improvements can be made without departing from the spirit of the present invention.

FIG. 1 shows an embodiment of the light emitting device of the present invention. In the figure, 1b is an n-type or semi-insulating GaAs substrate, 2 is a DH, SCH or GRIN-SCH type cavity having a GaAs layer or an InGaAs strained quantum well as a light emitting layer, 2a is a light emitting layer, and 31 is a substrate side mirror. P-type semiconductor multilayer film, 32 is p-type GaAs (contact and injection layer), 33 is an n-type semiconductor multilayer film forming a substrate-side mirror, 4b is an air-side n-type semiconductor multilayer film mirror, and 11 is n.
Reference numeral 12 denotes a p-type electrode (ring shape). Next, each unit will be described in detail. n-type or semi-insulating GaAs
An n-type or undoped semiconductor alternating multilayer film 33 is formed on the substrate 1b as a part of the substrate-side mirror. For example,
If the composition is AlAs and GaAs, then 1/4 of that
The optical wavelengths are 80.3 nm and 66.9 nm, respectively (emission wavelength is 980 nm, AlAs and GaAs have a refractive index of 3.0, respectively).
5 and 3.66). 20.5 pairs of this material (the top layer is Al
When As) is formed, the total thickness is 3.0 μm, and the maximum reflectance when viewed from the light emitting layer side to the substrate side is 99.8%.

Next, one layer of p-GaAs is formed as the substrate-side contact / injection layer 32. The thickness of this layer is basically λ / 4n optical thickness (λ is the emission wavelength, n is the refractive index), but in order to reduce the device resistance or to facilitate the device fabrication, this layer is used. Thicker is preferable. For example, the λ / 4n thickness of GaAs is about 0.07 μm, which is extremely thin. If holes are injected through this thin layer, the device resistance is large, and it is difficult to inject them uniformly into the entire mesa. However, as will be described later, if it is an odd multiple of 5 times, 0.33μ
m becomes 9 times and becomes 0.61 μm, which is a sufficient thin film for implantation. However, since this layer is integrated with the n-type or semi-insulating semiconductor multilayer film 33 and the p-type semiconductor layer 31 described later to form one mirror, there is a condition for its thickness. Basically, it is an integral multiple of λ / 4n, but an odd multiple is allowed depending on the number of pairs of the p-type semiconductor layers 31 and the air-side mirror 4b. In other cases, the thickness may be slightly increased or decreased to adjust the phase. Details of these will be described later. The p-type carrier concentration of this layer is
For ohmic contact and current injection, it is desirable to use as much as possible, but since the threshold value increases due to absorption loss, the optimum value is selected in consideration of these factors. In some cases, p-GaAs is divided into two, and the inside is 4 × 10.
¹⁸ cm ^-3 or less, ultra-thin surface (about 100 Å)
Only the concentration may be increased to 1 × 10 ¹⁹ cm ⁻³ .

Next, as the p-type multilayer film 31, Al _{x is} generally used.
Several pairs of multilayer films of λ / 4n optical thickness of Ga _1-x As (x ≦ 1) / GaAs are formed. However, this multilayer film is not always necessary, and the height of the mesa (that is, the guide property)
And the phase adjustment as a part of the substrate side mirror. For example, in the case where the cavity 2 has one wavelength, in order to align the antinode of the standing wave with the center of the cavity (light emitting layer position) to improve the light emission gain, the refractive index of the semiconductor layer in contact with the cavity is higher than that of the semiconductor for the cavity. Must be small. For example, in this embodiment, the light emitting layer 2a is I
GaAs / InGaAs / GaA as nGaAs strained layer
Al mainly having s-SCH structure (each about 100 Å)
_{Using 0.3} Ga _0.7 As as the λ-cavity, a λ / 4n thick film of Al _0.7 Ga _0.3 As is further in contact as 31, and several pairs of p-type multilayer films are further added. Air side mirror 4b
Also, the lowermost layer in contact with the cavity is made of Al _0.7 Ga _0.3 As having a smaller refractive index than the cavity. Further, in the case where the cavity has λ / 2 with the same composition, GaAs having a larger refractive index is brought into contact with the cavity. The same applies to the air side mirror. carrier concentration of p-type multilayer film,
The Al composition and the number of pairs are selected mainly considering the element resistance and the threshold current rather than the reflectance.

The remaining part of the air-side mirror 4b is an n-type semiconductor alternating multilayer film, for example, AlGaAs and G having a high Al concentration.
aAs is used, and the uppermost layer is GaAs, which allows easy ohmic contact, and the thickness is adjusted in consideration of the phase shift of reflection due to the metal for the electrode. FIG. 5 shows a reflection spectrum of the entire mirror (a reflection spectrum when the configuration including both mirrors and the cavity is viewed from the air side of the upper surface of the epi layer without the upper electrode). The horizontal axis represents λ (nanometer) and the vertical axis represents reflectance. The cavity region 2 has a thickness of 1λ, the composition is Al _0.3 Ga _0.7 As, and the p-type semiconductor multilayer film 31 is Al _0.6 Ga _0.4 As / GaAs / Al.
_0.6 Ga _0.4 As (ie 1.5 pairs), contact layer 3
2 shows (a) when GaAs has a thickness of 3λ / 4n {generally (2m + 1) λ / 4}, and (b) also has a thickness of 2λ / 4n (generally 2mλ / 4). In this case, the underlying n-type multilayer film is 20.5 pairs of AlAs / GaAs, and the air side is 10 pairs of n-type AlAs / GaAs.

The semiconductor film other than the contact layer is λ / 4n
The thickness is (λ = 980 nm), but when the contact layer has a thickness of an odd multiple of λ / 4n as shown in FIG.
There is a reflection dip (cavity mode) at a wavelength of 980 nm, and light emission or oscillation light is extracted at this wavelength. In the case where the thickness is an even multiple as shown in (b), the cavity wavelength deviates from the desired 980 nm, and side modes are generated at two wavelength positions that are substantially symmetrically separated. In this case, it is impossible to fabricate a desired element if the emission wavelength of the light emitting layer is kept at 980 nm. Therefore, if you want to obtain oscillation at a strictly specified wavelength, the thickness of the contact layer is
It is desirable to make it an odd multiple of a quarter optical wavelength.

However, for other purposes, the thickness of the contact layer can be deviated from an odd multiple. This allows several different oscillation wavelengths. This is the case of so-called wavelength-multiplexed light emitting elements and lasers. Normally, the emission wavelength of the light emitting layer has a certain width, but if the cavity wavelength is changed within that width, it is possible to emit light or oscillate at different wavelengths.

FIG. 6 shows the reflection spectrum when the thickness of the contact layer is 7.5 times and 6.5 times the optical wavelength of 1/4, for example, and the thickness of this layer is 1/4. The cavity wavelength, that is, the emission or oscillation wavelength can be shifted by shifting from an odd multiple of the optical wavelength.

FIG. 7 shows the thickness dependence of the oscillation wavelength centered on an odd number times (for example, 7 times) thickness. The emission wavelength width increases with the excitation current density regardless of whether the light emitting layer is a normal thin layer or a quantum well.
It is 300 to 400 nm or more. Therefore, close to FIG.
If the thickness of the contact layer is changed and an element in which the cavity wavelength is set within the emission wavelength width of the light emitting layer is produced, the emission or oscillation wavelength can be controlled. In addition, a wavelength-multiplexed light source can be provided by producing an array of a plurality of elements having different cavity wavelengths in the same wafer. When the emission width is extremely wide, it is possible to oscillate two wavelengths with one device.

FIG. 2 shows a second embodiment of the present invention, in which the light emission output is on the upper surface side opposite to the substrate in the above-mentioned embodiment, which is higher than that of the embodiment of FIG. Only the electrode structure is different. In FIG. 2, the simplest transparent electrode 11a of top emission type is used. In addition to this, the partial electrodes may be used or the dielectric mirror may be used in combination. In the case of top emission, the substrate 1b may be n-type or p-type having large light absorption. Alternatively, it may be semi-insulating. Other reference numerals are the same as those in FIG.

FIG. 3 shows another embodiment of the present invention shown in FIG.
The difference from the structure of FIG. 1 is that the injection layer 32 is arranged on the upper side, that is, on the air side in the drawing. 1b is a substrate, 4 is a mirror, 2 is a cavity, 31 is a p-type semiconductor multilayer film, 32 is
Is a p-type GaAs injection layer, 33 is an n-type semiconductor multilayer film, 11
Reference numerals 12b and 12b respectively indicate electrodes.

In any of the embodiments, since the substrate is semi-insulating, or the substrate-side mirror includes a pn junction, element isolation may be performed by etching up to the substrate or below the pn junction. It is possible.

[0023]

As described above, according to the present invention, a p-type current injection layer is provided on the hole injection side of the light emitting region, and an n-type or undoped semiconductor multilayer film is provided on the side opposite to the light emitting region of the injection layer. By including the mirror made of, it is possible to reduce the element resistance and obtain a light emitting element having a low threshold value and high efficiency. Further, in the above embodiment, the light emitting layer is the InGaAs strained quantum well and the multi-layer film mirror is the AlGaAs type, but the basic concept of the present invention is that the light emitting layer is GaAs or AlG.
In the case of quantum well and bulk thin film of aAs, further, InGaAsP / InP long-wave light emitting device, AlGaI
It can also be applied to nP-based visible light elements and the like. Especially in the case of long-wave system, the influence of light absorption by p-type semiconductor is larger,
The advantage of not using the p-type semiconductor multilayer film mirror is great in terms of the optical effect as well as the electrical effect. Also for the current confinement structure, it is effective for a structure such as a buried type or a heterobarrier type, which can be applied to a surface emitting type by a method usually used for a stripe type semiconductor laser. The quantum structure of the light emitting layer is not limited to the above-mentioned one-dimensional well structure, but also for two-dimensional and three-dimensional quantum wells, that is, quantum wires and quantum boxes, by setting the mirror characteristics according to the emission spectrum width and the like, The effects of the invention can be applied. Regarding the element form, not only surface emitting lasers and surface emitting diodes, but also surface bistable lasers, surface amplifiers and switches, pnp
It is also applicable to n optical thyristors and optical logic devices using them.

[Brief description of drawings]

FIG. 1 shows a sectional structure of a first embodiment of a planar light emitting device of the present invention.

FIG. 2 shows a sectional structure of another embodiment of the present invention.

FIG. 3 shows a sectional structure of another embodiment of the present invention.

FIG. 4 shows a cross-sectional structure of a conventional planar light emitting device.

FIG. 5 is a comparison of reflection spectrum characteristics of mirrors. (A) is the case where the thickness of GaAs is 3λ / 4n,
(B) is a case of 2λ / 4n.

FIG. 6 shows a contact layer thickness of a quarter optical wavelength of 7.
The reflectance is shown for 5 times and 6.5 times.

FIG. 7 shows the relationship between the thickness of the contact layer and the cavity wavelength.

[Explanation of symbols]

1b GaAs substrate 2 GaAs layer or InGaAs strain quantum well as a light emitting layer DH, SCH or GRIN-SCH type cavity 2a Light emitting layer 3 n type semiconductor multilayer film mirror 31 p type semiconductor multilayer film 32 constituting a mirror n-type or undoped semiconductor multilayer film constituting a p-type GaAs 33 mirror 4a p-type semiconductor multilayer film mirror 4b n-type semiconductor multilayer film mirror 11,11a n-type electrode 12 p-type electrode 12b ring electrode

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takashi Tadokoro 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (72) Inventor Yoshiaki Kadota 1-1-6 Uchiyuki-cho, Chiyoda-ku, Tokyo No. Japan Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. A surface characterized in that a p-type current injection layer is provided on the hole injection side of the light emitting region, and a mirror made of an n-type or undoped semiconductor multilayer film is provided on the opposite side of the injection layer from the light emitting region. Shape light emitting element. 2. The planar light emitting device according to claim 1, wherein the thickness of the current injection layer is an odd multiple of a quarter optical wavelength. 3. The planar light emitting device according to claim 1, wherein the emission or oscillation wavelength is controlled by the thickness of the current injection layer.