JPH0420274B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor detector, and particularly to a light receiving element suitable for realizing high sensitivity, low dark current, and high speed performance.

Conventionally, a light receiving element having a mesa type or planar type structure as shown in FIGS. 1a and 1b has been proposed. A first conductivity type semiconductor layer 02 and a second conductivity type semiconductor layer 03 are formed on a semiconductor substrate 01, and electrodes 08 and 09 are further provided. In a mesa-type structure as shown in FIG. 1a, a high electric field is exposed at the junction end face, and the device characteristics are influenced by the properties of the surface protective film, which is not practically desirable. On the other hand, the planar structure shown in FIG. 1B (Japanese Unexamined Patent Publication No. 132079/1986) is expected to provide more stable operation than the mesa structure.
In the structure shown in Fig. 1b, an n ⁺ type
An InP layer 2, an InGaAsP layer 3, and an n-type InP layer 4 are formed. 6 is, for example, a Cd diffusion layer, and a Pn junction is formed at the end face of this diffusion layer. 7 is an insulating layer, and 8 and 9 are electrodes. However, there are drawbacks as described below.

When an InP crystal is used as a material with a large forbidden band width, it is conceivable that P, which has a high vapor pressure, dissociates during the heat treatment step of the device fabrication process after crystal growth, and the surface layer changes in quality. Therefore, the interfacial properties after the surface protective film is formed become unstable, causing an increase in dark current.

In general, in semiconductor materials, the smaller the effective mass and forbidden band width, and the higher the impurity concentration, the more
Since the electric field strength that causes breakdown decreases due to the tunnel effect, the distance l between the Pn junction surface formed in a region with a large bandgap width (e.g. InP) and a region with a small bandgap width (e.g. InGaAsP) increases. If the bandgap is small, the electric field of the material with a small bandgap reaches an electric field sufficient to cause a tunnel effect before the Pn junction formed in the region with a large bandgap causes avalanche multiplication, and the tunneling occurs. It causes surrender.

The purpose of the present invention is to increase the avalanche multiplication effect by setting the distance between the pn junction and the layer with a small forbidden band width (light absorption region), the impurity concentration of each layer, and the thickness of each layer so that avalanche multiplication effect occurs effectively. The object of the present invention is to provide a light receiving element having high sensitivity, low dark current, and high speed. Generally, tunnel current can be expressed by the following equation.

J = (m ^* ) ^1/2 q ² ε・E ³ / _n /4√2π ² h ^-2 NE ^1/2 exp (
−π (m ^* ) ^1/2 E ^3/2 /2√2qhE _n ) (1) m ^* : effective mass, q: elementary charge of electrons, h=h/2π: h is Planck's constant, N: impurity concentration E _g =: forbidden band width, ε: dielectric constant, E _n field strength Assuming that the junction is of a stepped type, the relationship between the electric field strength and the operating voltage (tunnel breakdown voltage V _T ) is given by the following equation.

V _T =ε/2qNE ² _n (2) On the other hand, the avalanche breakdown voltage V _A is given by the following equation.

V _A =60 (E _g /1.1) ^3/2・(N/10 ¹⁶ ) ^-3/4 (3) Now, taking a pn junction made of lnGaAsP as an example.
The impurity concentration, V _T =V _A , is determined as follows.

InP (corresponding to E _g λ1.35μm): ~3×10 ¹⁷ cm ^-3 In _0.79 Ga _0.21 As _0.47 P _0.53 (corresponding to E _g λ1.2μm): ~2×10 ¹⁶ cm ^-3 In _0.75 Ga _0.25 As _0.56 P _0.44 (λ1.3μm corresponding to E _g ) ~7×10 ¹⁵ cm ^-3 In _0.61 Ga _0.39 As _0.83 P _0.17 (λ1.55μm corresponding to E _g ) 8×10 ¹⁴ cm ^-3 Because avalanche multiplication effect occurs effectively for,
Since it is necessary that V _T >V _A , the impurity concentration needs to be lower than the above value. Therefore, if the structure shown in Fig. 1b is adopted, depending on the impurity concentration of the material with a large bandgap width and 1, the restrictions on the impurity concentration of the material with a small bandgap width will be somewhat relaxed, but the avalanche increase will increase. In order to expect a double effect, for a substance with a large forbidden band width, V _T 〉V _A
The following relationship is established, and the maximum electric field E _ns in the region with a small forbidden band width becomes smaller than the electric field strength E _T at the time of tunnel breakdown (Depending on the impurity concentration, the electric field strength E _A at the time of avalanche breakdown). It is important to design the device accordingly. The distance l and the impurity concentration of substances with large and small bandgap widths are respectively
If N _L and N _s are used, in order to satisfy the above-mentioned conditions, it is necessary to satisfy at least the following relationship between them. Hereinafter, this relationship to be satisfied will be explained in detail using FIG. 4. The vicinity of the pn junction of the avalanche photodiode, as schematically shown in the cross ^- sectional view in FIG. a second region 21 exhibiting the first conductivity type and having a wider forbidden band width than the first region;
A region 22 is selectively provided in the second region and exhibits a second conductivity type (for example, P ⁺ ) for forming a pn junction. The x-axis is taken in the direction of the first region with the pn junction surface 23 as the plane containing the origin, and Pn
Let the distance between the joint surface and the first region be l. Figure 4b
4 schematically shows the electric field strength in the section A-A' shown in FIG. 4a. Normally, a Pn junction is formed by diffusion, resulting in a stepped junction, and if we consider the example of P ⁺ diffusion to n ^- , the expansion of the depletion layer to the P ⁺ side can be ignored. Let E _nL be the maximum electric field strength of a material with a wide forbidden band width, and ε _L and ε _S be the dielectric constants of a material with a large band gap and a material with a small band gap width. In FIG. 4b, the maximum electric field in the second region is the electric field E ₁ of the Pn junction at point B, and E ₁ =E _nL . Further, the maximum electric field in the first region is the electric field E ₂ at the boundary with the second region at point C, and this corresponds to E _ns .

In order for the avalanche multiplication effect to occur effectively, E _ns needs to be smaller than the electric field strength E _T at the time of tunnel breakdown, as described above, and E _ns ≦ _ET .
From FIG. 4b, the electric field E ₂ becomes E ₂ =E ₁ −qlN _L /ε _L E ₂ =E _nL −qlN _L /ε _L. Here, in the case of ε _L ≠ ε _s , if the difference between ε _L and ε _s is small, it can be treated approximately as E ₂ = E _ns in practice, but strictly speaking, from the basics of electromagnetism, the value of the electric field E at the boundary is itself is discontinuous, E ₂ = E _ns does not hold, and the electric flux, D = ε _E , becomes continuous. Therefore, if the above equation is expressed in E _ns , ε _s E _ns = ε _L {E _nL −qlN _L /ε _L }, and if expressed using the condition of E _ns ≦ _ET , ε _L {E _nL − qlN _L /ε _L }≦ε _s Q _T By transforming this equation, l≧ε _L /qN _L {E _nL −ε _s /ε _L E _T } (4) is obtained.

From Equation 4, when N _L is small, l needs to be large. Furthermore, although the impurity concentration N _s is not directly expressed in equation (4), since E _T is a function of N _s , equation (4) indirectly includes N _s . The larger N _s is, the smaller E _T becomes, and therefore l needs to be larger.

In practical terms, the relationship between E _T and N _s is as follows:
It can be obtained from equation (1). When setting a practical dark current for an avalanche photodiode (for example, lnA), if the tunnel current component of this dark current is 1/10 to 1/100, the effect of tunnel breakdown is small, so it is not practical for practical use. You can ignore this. The current density J ₁ is determined from this current value and the junction area of the avalanche photodiode. Substituting J ₁ for J (current density) on the left side of equation (1), N→N _s ,
E _g →E _gs (Forbidden band width in area with small forbidden band width)ε→
By replacing ε _s , the electric field strength E _n can be calculated by numerical calculation, and the calculated E _n can be used for practical purposes.
It will give E _T. For example, N _L = 1×10 ¹⁶ cm
^-3 , N _s = 2 × 10 ¹⁶ cm ^-3 , then λ = 1.55 μm
For InGaAsP, l is approximately 1.5 μm or more. In this example, considering the above relationship, l is 1 μm
In the following element design, the wavelength corresponding to the forbidden band width corresponds to 1.3 μm or 1.5 μm.
In a device configuration using InGaAsP as a material with a narrow bandgap, the tunnel effect has a significant effect, making it difficult to create an effective avalanche photodiode. More specifically, as explained above for the photodiode having the structure shown in FIG.

On the other hand, the upper limit of l is determined by another factor. However, the structure of the semiconductor layer (composition, thickness, impurity concentration, presence or absence of an intermediate layer, etc.), the structure to obtain the guard ring effect (measures to prevent edge breakdown), and the target set values of device characteristics (noise, response speed, quantum Due to various factors such as efficiency, multiplication factor, etc.,
Since the upper limit value of l is different, it is not uniquely determined like the lower limit value of l explained earlier.

For reference, the carrier pile-up effect at the hetero interface, which is one of the factors, will be exemplified. The first conductivity type illustrated in FIG. 4 is n ^- ,
When the second conductivity type is P ⁺ , among the electron-hole pairs generated when the incident light is absorbed in the first region (a light absorption region with a small forbidden band width), the hole is It moves toward the Pn junction. At the interface between the first region (small forbidden band width) and the second region (large forbidden band width), a discontinuous band step (hetero barrier) occurs due to the difference in forbidden band width, and the hole Movement is inhibited. A phenomenon occurs in which it becomes difficult to move over this level difference until a certain amount of holes accumulate, and this phenomenon is called the hole pile-up effect. This phenomenon affects the response speed and noise characteristics of the device. Energy is required for the hole to cross the step, and in this case the energy is given by the electric field, so the electric field strength E _H (corresponding to E ₂ in Figure 4b) at the heterointerface is greater than a certain value. This is necessary. This required value differs depending on the presence or absence of an intermediate layer at the heterointerface, the structure of the semiconductor layer (composition, number of layers, etc.), and the like. N _L and ε _L are the second
However, since E _nL changes depending on the configuration for obtaining the guard ring effect, etc., the relationship between E _H and l cannot be stated uniformly. InP− that has been explained so far
In the example of InGaAsP-based materials, to give an example of specific numbers, using the equation derived earlier E ₂ = E _nL − _ql N _L /ε _L , the aforementioned N _L = 1×10 ¹⁶ cm ^-3 When taking the example case, E _nL = 4.5×10 ⁵ V/cm and E _H = 2×
At 10 ⁵ V/cm, l is approximately 1.7 μm, E _nL = 5×
10 ⁵ V/cm, and when E _H = 1×10 ⁵ V/cm, l is approximately
It becomes 2.8μm.

, the maximum electric field during operation becomes smaller, but it is necessary to limit the impurity concentration in the region where the forbidden band width is small. When the depletion layer widens by W in the region where the forbidden band width is small, the electric field strength E _s of the material with the small forbidden band width on the side of the material with the large forbidden band width is given by the following equation.

E _s = qN _s / ε _s W (5) If the depletion layer is (l + W) and W is 1 μm, then
In order to prevent E _s from exceeding E _T , the impurity concentration N _s needs to be smaller than the value below.

N _s :≦approximately 1×10 ¹⁶ cm ⁻³ forλ=1.55 μm ≦approximately 2×10 ¹⁶ cm ⁻³ forλ=1.3 μm As described above, it is necessary to consider the relationship between E _g and N _s of the substance. In compound semiconductors, the lifetime of photoexcited carriers is extremely short compared to Si, so in order to increase the photoelectric conversion efficiency, the light absorption region must be made into a depletion layer, and in order to increase the speed,
In order to reduce the junction capacitance C, it is necessary to expand the depletion layer. Junction capacitance is approximately given by the following equation.

Cε/l+WS (6) ε: dielectric constant, S: junction area Considering the practically required quantum efficiency (≧50%) and junction capacitance (≦2pF), it is not necessary for the depletion layer W to expand by about 1 μm. Conceivable.

A typical example of the present invention is shown in FIG. The first feature is that a material layer 14 with a large bandgap width, which serves as an optical window layer, is formed on a material layer 13 with a small bandgap width, which serves as an active region, and furthermore, a material layer 14 with a large bandgap width, which serves as an optical window layer, is formed. A light-receiving element structure that reduces dark current and stabilizes the interface by forming an atomic material layer 15 and protecting it with a surface protection film 17 to stabilize the relationship between the semiconductor and the interface. It is characterized by

Furthermore, as explained above, the second feature is the impurity concentration of the material in region 14 and the material in region 13.
The light-receiving element structure is characterized in that there is a relationship between N ₁₄ and N ₁₃ such that N ₁₄ ≧N ₁₃ , and N ₁₃ is set to be 2×10 ₁₆ cm ^-3 _{or less} .

One embodiment according to the present invention is shown in FIG. 2, and its structure will be described below.

n ⁺ type InP substrate with high impurity concentration of approximately ¹⁰ cm ^-3 or more,
11, an n-type film with an impurity concentration of 9×10 ¹⁵ cm ^-3 and a thickness of 1.5 μm was grown using a known liquid phase epitaxial growth method.
An InP layer, 12, is formed, followed by an impurity concentration of 7×
10 ¹⁵ cm ^-3 , 1.3μm thick n-type In _0.61 Ga _0.39 AS _0.83 P _0.17
A layer 13 is formed. In particular, in order to receive light with a wavelength of 1.3 μm or more with sufficient sensitivity, the composition of this In _1-x Ga _x As _y P _1-y is preferably 0.47≧x≧0.25. Note that the As content is generally determined in conjunction with the Ga content.

There is a relational expression: y=x/0.48043+0.00327x. Subsequently, the impurity concentration is 9×
10 ¹⁵ cm ^-3 and a thickness of 1.8 μm n-type InP layer 14 was formed,
Finally, an n-type In _0.9 Ga _0.1 As _0.2 P _0.8 layer 15 having an impurity concentration of 7×10 ¹⁵ cm ^-3 and a thickness of 0.2 μm is continuously formed. After forming Al ₂ O ₃ and SiO ₂ films by a known gas phase chemical reaction method, removing the Al ₂ O ₃ and SiO ₂ films at inconsequential parts by a known selective photoetching method, further areas are formed. 15 is removed, and using the above insulator as a diffusion mask, Zn or
introducing Cd impurities into the regions 14 and 15;
A P ⁺ type diffusion region, 16, with a diffusion depth of 0.7 μm is formed. By the diffusion layer, 16, and the InP layer, 14.
A pn junction is formed. p-n junction surface and region, 13,
The distance between the two is 1.1 μm. Next, after removing the insulating film used as a diffusion mask, the
Form a SiO ₂ film. After removing unnecessary portions of SiO ₂ by a known selective photoetching method, a surface protective film 17 is obtained. In addition, anti-reflection film 1
7', apply a surface protective film or apply SiO ₂ or Si ₃ N ₄ of a thickness suitable as an anti-reflection film.
was re-formed and applied. After this surface electrode, 1
8, and a back electrode 19 were formed. The device was mounted on an appropriate stem and was found to work as a device.

The configuration and operation of this embodiment will be explained below. In this embodiment, since the narrow forbidden band width region 13 is surrounded by the wide forbidden band width region, the incident light is absorbed in the region 13. Also,
The surface layer is formed of an InGaAsP layer with a wide forbidden band width.
Since an insulating film for surface protection is formed thereon, the characteristics at the interface become stable, which is suitable for reducing dark current. This surface layer semiconductor layer can (1) have lattice matching with the underlying second semiconductor layer, (2) have a fixed crystal system, and (3) remain stable even when exposed to high temperatures. It has properties such as being more stable than or,
This surface layer is selected to have a composition that has a larger band gap than the semiconductor material of the active region. Also,
The pn junction is formed at a distance from the region 13 based on the above-mentioned idea, and the impurity concentration distribution is also taken into consideration, so that it is possible to maintain hard junction characteristics and efficiently collect optically excited carriers to the junction. Are suitable. Furthermore, since the spread of the depletion layer is set in consideration of the electric field distribution, the junction capacitance is reduced and it is suitable for high speed.

When the device is biased in the reverse direction, the depletion layer spreads to regions 14 and 13 just below the junction. Therefore, light wavelengths up to the long wavelength end corresponding to the forbidden band width of the region 13 are efficiently absorbed, and the generated holes are collected in the pn junction by the drift electric field. Main prototype pin
The main characteristics of the photodiode are the wavelength sensitivity range 1.0
~1.55μm, quantum efficiency 65% (1.3μm) junction capacitance
0.8pF, dark current less than 0.1nA (10V).

The effects of this embodiment will be explained below.

(a) Dark current can be reduced by using the interface characteristics between the InGaAsP layer and the insulating film.

(b) By forming the layer structure as described above, it is possible to prevent an increase in dark current due to the tunnel effect.

(c) By forming the layer structure as described above, optically excited carriers can be efficiently collected at the junction, resulting in high sensitivity.

(d) By forming the layer structure as described above, the optically excited carriers can be collected at the junction at a drift speed, and the speed can be increased.

(e) By forming the layer structure as described above, the width of the depletion layer can be increased, so that the junction capacitance can be reduced, which is effective in increasing the speed of the device.

As another example, there is a case where the incident direction of light is set to the InP substrate side. FIG. 3 is a sectional view of the device showing this example. The same symbols as in FIG. 2 indicate the same parts. Layer 15 is an InGaAsP layer for surface protection. The difference from the embodiment in FIG. 2 is that the electrode 1
9 removes the metal in the region located below the diffusion region 16, and the electrode 18 is provided over the entire surface since no incident light is required. It is.

Moreover, although the example of the InP-InGaAsP-based material has been described as an example above, the material system is not limited to this.

For example, the 14th layer is GaAlSb, the 13th layer is aSb, the 12th layer is GaSb, and the 15th layer is GaSb.
An optical semiconductor device having the same purpose can be realized using a material system mainly composed of GaSb using GaAlAsSb.

According to the present invention, the distance between the pn junction and the light absorption region, the impurity concentration of each layer constituting the device, and the thickness of each layer are taken into consideration so that avalanche multiplication effect occurs effectively, resulting in high sensitivity. A light receiving element with low dark current and high speed can be obtained.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing an example of a conventional element structure, in which (a) shows a mesa type structure and (b) shows a planar type structure. FIGS. 2 and 3 show cross-sectional views of an element structure according to an embodiment of the present invention. FIG. 4 is a diagram schematically showing the vicinity of a pn junction of an avalanche type photodiode, in which (a) is a cross-sectional view, and (b) is a diagram showing the electric field strength in the A-A' cross section shown in (a). Explanation of symbols, 01...n ⁺ type InP substrate, 02...n type
InGaAs, 03...P type InGaAs, 08,09...electrode, 1,11...n ⁺ type InP substrate, 2...n ⁺ type InP layer,
12... n-type InP layer, 3, 13... n-type InGaAsP layer,
4, 14... n-type InP layer, 6, 16... p-type InGaAsP
Diffusion layer, 7, 17, 17'...insulating film, 15...n type
InGaAsP layer, 8, 18...P-type electrode, 9, 19...
n-type electrode.

Claims (1)

[Scope of Claims] 1. A semiconductor multilayer structure comprising a substrate, a semiconductor multilayer structure provided on the substrate, and a set of electrodes electrically connected to the semiconductor multilayer structure, wherein the semiconductor multilayer structure has a a first region with a small forbidden band width indicating conductivity type 1, and a first region with a wider forbidden band width than this first region.
a second region exhibiting a conductivity type; and a region exhibiting a second conductivity type selectively provided within the second region for forming a Pn junction within the second region; The impurity concentration and dielectric constant of the first region and the second region are N _s , ε _s , and N _L , ε _L , and the first region depends on the impurity concentration N _s of the first region. When the electric field strength at the time of tunnel breakdown is E _T , the maximum electric field strength in the second region is E _nL , and the elementary charge is q, the distance l between the Pn junction and the first region is l≧ε _L An optical semiconductor device characterized by satisfying the relationship: /qN _L {E _nL −ε _s /ε _L E _T }. 2. In the optical semiconductor device according to claim 1, the impurity concentration of the first region is 2×
10 ¹⁶ cm ^-3 or less.