JPS60247979A - Semiconductor optical element - Google Patents

Abstract

PURPOSE:To obtain a photodetector, which responds at high speed and has high sensitivity, by forming structure in which electron.hole pairs shaped through the absorption of incident beams are spatially isolated instantaneously by the internal electric field of an optical absorption layer and the recombination of electrons and holes is prevented. CONSTITUTION:A non-doped InP buffer layer 12 is laminated onto a semi-insulating InP substrate 11, and two layers of non-doped In0.52Al0.48As third semiconductor layers 15 holding a non-doped In0.53Ga0.47As optical absorption layer 16 are formed between an Si doped In0.52Al0.48As first semiconductor layer 13 and a Be doped In0.52Al0.48As second semiconductor layer 14 having a conduction type different from the conduction type of the layer 13, and a plurality of these layer structure are laminated. The forbidden band width of the first-third semiconductor layers 13-15 is made larger than that of the optical absorption layer 16. A distance between electrodes extends over approximately 10mum and electrode width approximately 50mum in metallic electrodes 17, 18. Regions 7, 8 represent high impurity concentration regions for reducing contact resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a semiconductor optical device, and particularly to a photoconductive type photodetecting device.

(Prior Art) To realize an optical communication system, a semiconductor laser that converts an electrical signal into an optical signal, an optical fiber that propagates the optical signal, and a photodetector that converts the optical signal into an electrical signal are required.

This photodetector includes a 7-balance photodiode, p
There are in-line photodiodes, photoconductive elements, etc. Among them, photoconductive elements are considered to have good structural compatibility with opto-electrical integrated circuits, and have recently attracted particular attention.

Figure 1 is a structural diagram of a conventional photoconductive element, in which 1 is a semi-insulating GaAa substrate, 2 is a light absorption layer made of undoped GaAs.
layer (thickness 2 μm) (hereinafter referred to as “light absorption layer”), 3
is undoped Ato with a wider forbidden band width than the light absorption layer 2,
a Ga(1,7As layer (thickness 71ooX) (hereinafter referred to as “
4 is Si-doped Ato, 3G
a(,,7As layer (donor concentration #10"m-3, thickness 500 mm) (hereinafter referred to as "r'si doped layer"), 5
, 6 are metal electrodes, and 7 and 8 are high impurity concentration regions for reducing contact resistance. For example, light 9 with a wavelength of 08 μm is
When light enters from the doped layer 4 side, it passes through the Si doped layer 4 and the undoped layer 3 and is absorbed by the light absorption layer 2, generating electron-hole pairs. By applying a voltage to the electrodes 5 and 6, these electrons and holes can be extracted and optically detected.

FIG. 2 is a band structure diagram for explaining the operation of the device in more detail. Among the electron-hole pairs generated by the incident light 9, electrons flow into the electron storage region 10 formed in the light absorption layer 2 in contact with the undoped layer 3, and the electron concentration increases. Therefore, a constant voltage (e.g. 2V) is applied between the electrodes 5-6.
), the current increases in response to the incidence of light, making it possible to perform photodetection. Note that EF is the Fermi level,
EC is the energy level at the conduction band edge, and Ev is the energy level at the valence band edge.

In the structure shown in FIG. 1, the light absorption layer 2. A depletion layer is formed in the undoped layer 3 and the Si-doped layer 4, but most of the depletion layer is formed on the side of the light absorption layer 2 where the impurity concentration is low. The distance d from the hetero interface between the light absorption layer 2 and the undoped layer 3 to the edge of the depletion layer in the light absorption layer 2 is the impurity concentration of the light absorption layer 2, and the hetero interface between the light absorption layer 2 and the undoped layer 3. Although it varies depending on the energy position of the conduction band edge in
3. Assuming that the diffusion potential difference VD is I, the distance d=1.2 μm can be obtained by solving Poisson's equation. Here, ε0 is the dielectric constant in vacuum, ε8 is the relative permittivity, and q is the elementary charge.

Still, the electric field F in the depletion layer is given by at a point that enters the depletion layer by a distance X from the edge of the depletion layer on the substrate 1 side.

That is, the electric field F becomes smaller as it moves toward the substrate 1, and when the distance is 1 to 2 μm or more from the Peter interface, the depletion layer no longer exists and the electric field F becomes zero.

On the other hand, the incident light 9 travels from the interface between the light absorption layer 2 and the undoped layer 3 while being attenuated, but if the absorption coefficient in the light absorption layer 2 is 10% m-', then the light absorption layer 9 has a depth of 12 μm. Only 70 chi is absorbed at 2.

The remaining 30% was absorbed outside the depletion layer, and the electrons generated thereby had to diffuse to the depletion layer and then move to the electron storage region 10 by the internal electric field. Therefore, the response speed of the photocurrent is limited by the electron diffusion process and becomes slow. Furthermore, the response speed is also influenced by the time taken for electrons and holes generated by light irradiation to move between the electrodes 5 and 60. Therefore, the structure shown in FIG. 1 has the disadvantage that the response speed is slow because the hole movement speed is slow.

If the response speed of a photodetector is slow, the communication speed of information to be transmitted must also be slow, so a photodetector with a high-speed response has been strongly desired.

Furthermore, the electron-hole pairs generated in the depletion layer recombine due to the small internal electric field, resulting in a decrease in sensitivity.

(Objects and Features of the Invention) The present invention has been made in view of the above-mentioned drawbacks of the prior art, and an object of the present invention is to provide a photoconductive photodetector with high-speed response and high sensitivity.

(Structure and operation of the invention) The present invention is characterized in that an optical element is provided with at least one pair of electrodes for electrical connection with a light absorption layer.
The first layer has a larger forbidden band width than the light absorption layer and has a different conductivity type.
a third semiconductor having a wider forbidden band width than the light absorption layer and not doped with impurities, such that the semiconductor layer and the second semiconductor layer are arranged on one side and the other side of the light absorption layer, respectively; By providing at least one set of layer structures formed with or without intervening layers, and by determining the layer thickness of the light absorption layer to be equal to or less than the layer thickness determined by the value of the impurity concentration, An internal electric field is generated within the light absorption layer in the layer thickness direction.

The present invention will be described in detail below using the drawings.

Example 1 FIG. 3 shows an example according to the present invention, in which semi-insulating InP
An undoped nP buffer layer 12 (thickness 1
2 μm, n71 x 1015L:m-3) were stacked, S]doped Ino, 5□Ato48As first
Semiconductor layer 13 (thickness: 1000 A + n ”=
52 AL04B"L
S second semiconductor layer 14 (thickness = 1000λ+p 1018tyn-3)
52Ato, 4BAs third semiconductor layer 15 (thickness=20
0X, carrier concentration ≦l x 10"z-') 71
: Undoped In(, 53Ga(1,47As) light absorption layer 16 (thickness: 5000 mm).

Carrier concentration ≦I
A plurality of these layered structures are laminated. Note that the first to third semiconductor layers 13 to 15 have a larger forbidden band width than the light absorption layer 16. 1, 17, and 18 are metal electrodes having an inter-electrode distance of about 10 μm and an electrode width of about 50 μm, and 7.8 is a high impurity concentration region for reducing contact resistance.

As is clear from the figure, centering around the light absorption layer 16,
A third semiconductor layer 15 is formed on both sides thereof, and a first semiconductor layer 13 and a second semiconductor layer 14 are formed on one side and the other side, respectively. By changing the layer structure in this way, the regions (
An internal electric field can be generated in most of the light-receiving area (hereinafter referred to as the "light-receiving area"). As will be described later, an internal electric field is generated throughout the active region of the light receiving region. Therefore, since the electrons and holes generated by light irradiation are spatially separated by the internal electric field and concentrated in the respective accumulation regions of electrons and holes, deterioration of sensitivity due to recombination etc. is avoided. High-speed response is possible.

In this example, four sets of such layer structures were constructed to increase the efficiency of converting incident light into electrical signals, but this layer structure 1
Needless to say, it is better if you are above the group.

FIG. 4 is a band structure diagram for explaining the structure of this embodiment more clearly. For example, when incident light 9 with a wavelength of 15 μm is irradiated from below the substrate 11 or from above the first semiconductor layer 13, the incident light 9 is absorbed in each light receiving region, and electron-hole pairs are generated. These electron-hole pairs are immediately separated by an internal electric field perpendicular to each light absorption layer 16, and the electron and hole pairs are i? The electron W type region 10 (hereinafter abbreviated as "electron region") and the hole accumulation region 10a (hereinafter abbreviated as "hole region") which are /J. Here, the distribution of the internal electric field of the light absorption layer 16 is determined from equation (2) by solving the Bohanon equation, and it differs depending on the layer thickness of the light absorption layer 16 and the concentration of ionized impurities. For example, the layer thickness of the light absorption layer 16 is 5oooX, and the diffusion potential difference is VD.
If is 0.75V and the relative dielectric constant cs is 12, the entire region of the light absorption layer 16 (hereinafter referred to as "active region") excluding the electron and hole accumulation region in the light receiving region is depleted. The ionized impurity concentration Ni is approximately 4×1015 crn-3
becomes. Therefore, if the impurity concentration is below this value, an internal electric field will be generated throughout the active region in the light absorption layer in the light receiving region.

However, when setting the light absorption layer thickness d, the light absorption layer 1
There is no practical problem as long as the forbidden band width of 6 is set to Eg + impurity concentration to Ni, relative dielectric constant to ε.

Note that the conduction band edge or valence band edge of the electron and hole accumulation region in the light receiving region is located on the lower energy side than the Fermi level EF, so even if no energy (light irradiation) is applied from the outside, some electrons can be accumulated. and holes gather, so depletion does not occur.

That is, since an electric field is applied to the entirety of each active region, the time taken for electrons and holes to move to the electron region and the hole region becomes extremely faster than in the conventional structure. However, even when electron holes move from each accumulation region toward the electrode, the structure is such that there are no doping impurities in or on either side of each accumulation region, making them less susceptible to scattering. ing.

Furthermore, electrons and holes behave as two-dimensional conductive particles within each accumulation region. Therefore, the response speed of the photocurrent becomes extremely fast compared to the conventional structure.

On the other hand, as in this embodiment, since electrons and holes are immediately separated into an electron region and a hole region by the internal electric field of the depletion layer region, there is no deterioration in sensitivity due to recombination, resulting in high sensitivity. Furthermore, by increasing the number of light absorption layers 16 or increasing the thickness of the light absorption layers 16 as in this embodiment, the quantum efficiency is increased and a photodetector with even higher sensitivity can be obtained. However, if an anti-reflection film is applied to eliminate reflection on the incident surface, most of the incident light can be converted into electrons and holes, resulting in high sensitivity.

Note that the method of light incidence in this embodiment is that the electrode 17
, the upper surface of the 18 side, the substrate 11 side, or the side.

Embodiment 2 FIG. 5 shows another embodiment of the present invention, in which 15a is the thickness of the third semiconductor layer (thickness: 200 layers) shown in FIG.
19 is undoped In. , 5□A1o
, 481s buffer layer (thickness: 1 μm, carrier concentration <
IX10"crn-").

FIG. 6 shows the band structure diagram. As is clear from FIG. 6, in this embodiment, the first semiconductor layer 13 (or second semiconductor layer 14) - third semiconductor layer 15 - light absorption layer 16 -
Third semiconductor layer 15-second semiconductor layer 14 (first
When stacking a plurality of layer structures such as semiconductor layers 13),
Rather than simply repeating and stacking multiple layers as in Example 1, multiple layers are stacked via a third semiconductor layer 15a which has a wider forbidden band width than the light absorption layer 16 and is not doped with impurities. . By adopting such a structure, the direction of the internal electric field in the active region can be unified, and as in the first embodiment, high-sensitivity and fast-response photodetection is possible.

Further, in this embodiment, two sets of layer structures are laminated, but three or more sets may be laminated to further improve the quantum efficiency.

Embodiment 3 FIGS. 7(a) and 7(b) show another embodiment according to the present invention, in which the layer 15 is removed from the above-described embodiments 1 and 2 of the present invention.

In Examples 1 and 2, undoped semiconductor layers 15 were formed on both sides of the light absorption layer 16. Layer 15 is provided to reduce scattering due to ionized impurities in layers 13 and 14 when electrons or holes travel parallel to the layer within each accumulation region.

Here, the effect of scattering due to this ionized impurity is low temperature (
For example, it becomes noticeable at temperatures of 77 K), and the influence of lattice scattering is large at room temperature. That is, when the semiconductor optical device according to the present invention is used at room temperature, the presence or absence of the layer 15 does not significantly affect the response speed. From this point of view, a structure in which the layer 15 is omitted as shown in FIGS. 7(a) and 7(b) does not pose any practical problem.

Embodiment 4 FIGS. 8(a), (b), (c) and (d) are other embodiments according to the present invention, in which gates for potential adjustment are added to Embodiments 1, 2 and 3 of the present invention described above. Electrode 2
0 is set.

By applying a voltage to this gate electrode 20, the dark current between the electrode 17 and the electrode 187 can be reduced.
It becomes possible to increase the sensitivity of light detection.

Note that in this embodiment, a 7-way junction is formed between the electrode 20 and the first semiconductor layer 13, so there is no need for an insulating film between the electrode 20 and the first semiconductor layer 13. ,/
If a Yottoky junction is not formed, it is necessary to insert an insulating film.

In addition, when the light is incident from the upper surface of the electrode side, the electrode 20
It is necessary to make the electrode transparent, but of course this is not necessary if the light is incident from another surface.

Note that although the semi-insulating substrate 11 is used in this embodiment, it is needless to say that a conductive substrate may be used instead and the semiconductor layer may be formed via a semi-insulating buffer layer.

In the above explanation, InP is used as the substrate, and InO, 53Gao, and 4
Although 7As and inO, 52A4-48As were used, the fc combination is not limited to this. For example, when using an InP substrate, I
nk- and □GaxAtyAs with different compositions may be used as the light absorption layer and the layers on both sides thereof. Also In
4-. -In4-xGax instead of GaxA4As
Two types of compositions of AsyP may be used. Furthermore, GaAs
When using as a substrate, a combination of two types of compositions in Ate- and GaxAs may be used. The following description only shows some examples, and a wide range of compound semiconductors in general can be used, and the substrate may be an insulator such as At203.

In the production of such device structures, crystal growth methods such as the molecular beam lobster/yal method, organometallic vapor phase deposition, vapor phase lobster/kill method, and liquid phase lobster/kill method can be applied. Also, electrode formation and the like can be manufactured using conventional techniques.

(Effects of the Invention) The present invention immediately spatially separates electron-hole pairs created by absorbing incident light by the internal electric field of the light absorption layer 16.
Since recombination of electrons and holes can be prevented, it is a photodetecting element with high speed response and high sensitivity, and can be applied to elements for optical fiber communication or optoelectronic integrated circuits, and its effects are extremely large.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the structure of a conventional photoconductive element, FIG. 2 is a band structure diagram of the conventional example shown in FIG. 1, FIG. 3 is a perspective view showing the structure of a photoconductive element according to the present invention, and FIG. The figures are a band structure diagram of the embodiment shown in FIG. 3, FIG. 5 is a perspective view showing another embodiment according to the present invention, FIG. 6 is a band structure diagram of the embodiment shown in FIG. 5, and FIGS. The figure is a perspective view showing another embodiment according to the present invention. DESCRIPTION OF SYMBOLS 1... Substrate, 2... Light absorption layer, 3... Undoped layer, 4... S1 doped layer, 5, 6... Metal electrode, 7.
8... High impurity concentration region, 9... Incident light, 10...
・Electron accumulation region, 10a...Hole accumulation value range, EF...
・Fermi level, EC... energy level at the conduction band edge,
Ev... Energy level at the edge of the valence band, 11... Substrate, 12... Buffer layer, 13... First semiconductor layer,
14. second semiconductor layer, 15, 15a... third semiconductor layer, 16. light absorption layer, 17, 18... metal electrode, 2o. gate electrode. Patent applicant International Telegraph and Telephone Co., Ltd. Agent Inuzuka 1 off-campus person rate 1 figure d rate 20 0 7″3 ゝ30th 7th

Claims (3)

[Claims] (1) In a semiconductor optical device comprising at least one pair of electrodes electrically connected to a light absorption layer, a first semiconductor layer having a larger forbidden band width and a different conductivity type than the light absorption layer; It includes at least one set of layer structures in which second semiconductor layers are respectively disposed on one side and the other side of the light absorption layer, and the layer thickness of the light absorption layer is determined by the value of the impurity concentration. 1. A semiconductor optical device characterized in that an internal electric field is generated in a layer thickness direction in a depletion layer region in the light absorbing layer in a light receiving region by determining the layer thickness to be equal to or smaller than the layer thickness. (2) The impurity concentration of the light absorption layer is set to N1. The relative permittivity is ε
3. The forbidden band width is Eg, the elementary charge is q, and the dielectric constant in vacuum is ε. 2. The semiconductor optical device according to claim 1, wherein the layer thickness d of the light absorption layer is determined so as to satisfy the following conditions when closed. (3) In a semiconductor optical device comprising at least one pair of electrodes electrically connected to a light absorption layer, a first semiconductor having a wider band gap (and having a different conductivity type) than the light absorption layer; via a third semiconductor layer which has a wider forbidden band width than the light absorption layer and is not doped with impurities so that the second semiconductor layer and the third semiconductor layer are arranged on one side and the other side of the light absorption layer, respectively. A depletion layer region in the light absorption layer in the light receiving region is formed by setting a layer structure formed by a small number of layer structures (one set in each), and determining the layer thickness of the light absorption layer to be equal to or less than the layer thickness determined by the value of the impurity concentration. A semiconductor optical device characterized in that it is configured such that an internal electric field is generated in the layer thickness direction. Further, the thickness d of the light absorption layer is determined to satisfy the condition defined by q, the elementary quantity of electricity, and ε, the dielectric constant in vacuum. A semiconductor optical device according to scope 3.