JP2577347B2 - Image sensor - Google Patents

Description

Description: TECHNICAL FIELD [0001] The present invention relates to an image sensor including a heterojunction composed of two types of semiconductor layers having different optical band gaps.

[Technical background of the invention and its problems] Amorphous silicon (hereinafter referred to as a-Si) has excellent photoconductivity, and is used for photoelectric conversion elements such as solar cells, optical sensors, photoconductors, and image sensors. It is applied to a photoconductive film stacked type solid-state imaging device.

In these photoelectric conversion elements, hydrogenated amorphous silicon carbide (hereinafter referred to as a-SiC: H) and hydrogenated amorphous silicon are provided on the incident light side for the purpose of improving the sensitivity to short wavelength light (blue sensitivity). In many cases, a heterojunction a-SiC: H / a-Si: H of silicon (hereinafter referred to as a-SiH) is provided.

However, since the covalent bond radius of carbon is 0.77Å and that of silicon is 1.17Å, a number of defect levels are generated at the heterojunction interface due to lattice mismatch. Had led to a decline.

To solve this problem, at the heterojunction interface,
A method has been devised in which a semiconductor layer intermediate between the chemical compositions of the two semiconductor layers is interposed (JP-A-60-50973). FIG. 3 shows an example of this structure.
Before laminating an n-layer 12 of i: H and an i-layer 13 of a-Si: H, and laminating a window layer 14 of p-type a-SiC: H for forming a heterojunction thereon, By introducing a hydrocarbon serving as a carbon source of a-Sie: H into the monosilane and its derivative gas as a raw material gas for a short time, the intermediate layer 15 of a-SiC: H and a-Si: H is formed.
Is formed. 16 is a transparent conductive film, and 17 is a metal current collecting electrode. By providing the intermediate layer 15, distortion is reduced due to lattice mismatch, thereby preventing leakage current and improving blue sensitivity of the photoelectric conversion element.

However, the photoelectric conversion element in which the intermediate layer 15 of the chemical composition is interposed in the hetero junction has a problem that the light response speed becomes slower than the case where the intermediate layer 15 is not provided. This is considered to be due to the fact that the intermediate layer 15 is a relatively high-resistance layer and has poor photoconductivity, so that the photoexcited carriers have poor running properties in the intermediate layer 15. Here, the traveling property means the product of mobility and lifetime. If the light response speed is low, it becomes a fatal problem in a device that requires high-speed light response performance, such as an image sensor or a solid-state image sensor. In particular, this has been one of the causes of deteriorating the afterimage characteristics of the photoconductive film-stacked solid-state imaging device using a-Si for the photoconductive layer.

Therefore, the structure of the intermediate layer that alleviates the lattice mismatch of the heterojunction, satisfies the sufficient traveling property of the photoexcited carriers, and does not slow down the optical response speed is required in the image sensor and the photoconductive film stacked solid-state imaging device. Have been.

[Object of the invention]

The present invention is an improvement over the above-described drawbacks of the conventional device, in which the blue sensitivity is improved by alleviating the lattice mismatch at the heterojunction interface and improving the mobility of photoexcited carriers at the heterojunction interface layer. It is another object of the present invention to provide an image sensor capable of increasing a light response speed.

[Summary of the Invention] The present invention relates to a heterojunction interface composed of two semiconductor layers having different optical band gaps, each having an excellent electron and hole traveling property and having a different optical band gap. The semiconductor device is configured so as to interpose an ultra-thin stacked semiconductor layer in which semiconductor layers are alternately stacked.

〔The invention's effect〕

According to the present invention, there is an effect that the lattice mismatch of the hetero junction is alleviated and the traveling property of the photoexcited carriers in the junction interface layer is improved.

(Example of the invention)

An embodiment in which the present invention is applied to a photoelectric conversion element portion of a photoconductive film stacked solid-state imaging device will be described in detail with reference to the drawings.

FIG. 1 is a schematic structural cross-sectional view of a photoelectric conversion element according to the present invention. Reference numeral 21 denotes a second metal electrode, which is electrically connected to the first metal electrode that contacts the storage diode of the charge-coupled device (CCD). On top of the above 21, i-type a-
A layer 22 of SiC: H or n-type a-SiC: H is laminated, and a layer 23 of i-type a-Si: H, which is a main photocurrent generator, is laminated thereon.

Further thereon, an i-type a-SiC: H of 24 ₁ and the i-type a-Si: H-
Ultra-thin film laminate type semiconductor layer 24 is formed which consists unit lamination films are stacked 2 or more cycles by 24 _2. Then p
A type a-SiC: H25, a transparent conductive film 26 and a terminal electrode 27 are formed.

a-Si is formed by glow discharge decomposition using monosilane and higher silane as a source gas. When a-SiC: H is formed, hydrocarbons such as methane, acetylene and ethylene may be introduced into the glow discharge. . p-type and n
For the type a-Si, doping gas such as diborane and phosphine may be added during glow discharge. Note that a-Si can be formed into a film using a decomposition process by a photoexcitation process without depending on glow discharge decomposition.

25 pa-SiC of: ia-Si of H and 23: the extent of lattice mismatch at the heterojunction interface of H is燥和is of 24 ₁ ia-SiC: H
Layer thickness d ₁ and 24 ₂ of the ia-Si: depending on the H layer thickness d _2, also depends on the cycle number m of unit multilayer films. Optimal value of d _1, d ₂ and m required to alleviate the lattice mismatch varies with the conditions for forming the 24 _first layer and 24 _{a two-layer,} approximately d ₁ and d ₂ in the range of 5A～100A, m is effective as long as it is in the range of 2 to 20, and a reduction in dark current due to suppression of interfacial recombination of photoexcited carriers and suppression of recombination current accompanying relaxation of lattice mismatch was observed. As a result, improvements in characteristics such as blue sensitivity, which were not inferior to those of the conventional structure (FIG. 3), were achieved. If d ₁ and d ₂ are too thin, lattice mismatch will not be reduced, while if too thick, 24
Heterozygous _one layer and 24 _second layer becomes strong, resulting in increasing the defect level due to strain. Similarly, when the number m of periods became too large, deterioration of characteristics due to an increase in interface defect levels was observed. Only when d ₁ , d ₂ and m are at appropriate values within the above range, the strain at the pa-SiC: H heterojunction interface is absorbed by the 24 ultra-thin stacked semiconductor layers, and the effect of relaxation is reduced. is there. Of course, if the thickness of the ultra-thin stacked semiconductor layer 24 is too large, a high resistance layer will be formed, and the characteristics will be deteriorated. Therefore, it is necessary to set the total thickness of the 24 to 4000 mm or less. is there.

 Next, the improvement of the optical response will be described.

In the conventional structure shown in FIG. 3, since the intermediate layer 15 is formed only of the a-SiC: H layer, of the electrons and holes, particularly, the traveling property of the holes in the intermediate layer 15 is deteriorated, and the light response speed is reduced. Had led to a decline.

In the present invention, ia is excellent in traveling properties of electrons and holes.
-Si: than H layer 24 ₂ and the hole has excellent electron runnability ia
-SiC: the intermediate layer 24 is formed by ultra-thin film laminate structure of the H layer 24 _1, and the features of both layers are synergistic, in the intermediate layer 24, the hole without deteriorating the electron runnability travel Performance can be improved as compared with the prior art, and the light response time can be shortened.

FIG. 2 shows an optical response time in which the number m of cycles in the photoelectric conversion element unit is used as a parameter. In this measurement, the thickness of the intermediate layer 24 was fixed at a value in the range of 50 ° to 400 °, and d
₁ and the film thickness value of d ₂ and equal. When a reverse bias voltage of -1 V is applied to the photoelectric conversion film, the green LED is irradiated in a pulsed manner, and when the light is cut off, the falling signal current reaches 10% of the signal current at the time of light irradiation. The required time was measured as the light response time.

As shown in FIG. 2, it was confirmed that the conventional structure (FIG. 3) m can shorten the optical response time according to the present embodiment. Actually, the afterimage characteristics of the photoconductive film-stacked solid-state imaging device using this embodiment are greatly improved as compared with the conventional structure.

In the intermediate layer 24 of ultra-thin film laminate type semiconductor layer, one of the reasons that the travel of the photo-excited carriers is improved, the local electric field is generated at the bonding interface between the optical band gap is different from 24 _one layer and 24 _second layer, Another point is that the photoexcited carriers are accelerated by the electric field, and the average carrier speed is increased.

The optical band gap in the intermediate layer 24 of this embodiment is ia−
SiC: Although H layer 24 ₁ is formed so as to be 1.9EV～2.8EV (referred to as Eg _1), from the means of increasing the carrier average velocity of the, it is necessary to differentiate the Eg ₁ and Eg _2.

The combination of Eg ₁ and Eg ₂ different 24 _one layer and 24 _two layers constituting the ultra-thin film laminate type semiconductor layer 24, another embodiment, the first
The combinations shown in the table are also effective for improving the characteristics.

In Table 1, 24 _1-layer and 24 ₂ layers a-SiC: when formed by H is to regulate the amount of hydrocarbon introduced into the blow discharge, optical band gap Eg ₁ and Eg _{The two} can be different.

It should be noted that a unit laminated film of the ultra-thin laminated semiconductor layer 24
The film thicknesses d ₁ and d ₂ and the optical bandgap Eg ₁ and Eg ₂ of the _one layer and the _two layers need not be the same in all periods, and may have different values in each period. Needless to say.

Further, according to the present invention, the impurity concentration of the dopant is 1 × 10
If it is ¹⁸ (cm ⁻³ ) or less, it can be regarded as an i-type layer.

In the above, the case of the a-SiC: H / a-Si: H heterojunction has been described. However, another heterojunction, that is, a heterojunction using a nitride film, such as a-SiN: H / a-Si: H or silicon. Heterojunction using germanium alloy film, for example, a
Similar effects can be obtained by applying the present invention to -SiC: / a-Si: H. a-SiN: H / a- Si: in the case of H heterojunctions one layer at least 24 _one layer and 24 ₂ a-SiN: formed by H, a-SiC:
H / a-Si: Ge: in the case of H heterojunctions 24 _one layer and 24 of the _two layers at least one layer a-SiC: may be formed with H.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of the structure of a photoelectric conversion element according to the present invention, FIG. 2 is a view of light response time with the number of periods m of a unit laminated film as a parameter, and FIG. FIG. 25,23: Heterojunction forming layer 24: Ultra-thin stacked semiconductor layer

 ────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-62-159475 (JP, A) JP-A-61-144833 (JP, A) JP-A-60-100485 (JP, A)

Claims (5)

(57) [Claims] 1. A heterojunction comprising an i-type hydrogenated amorphous silicon and a semiconductor layer having a different optical band gap from the i-type hydrogenated amorphous silicon. An image sensor comprising an ultra-thin laminated semiconductor layer formed by alternately laminating semiconductor layers having different optical band gaps from each other between semiconductor layers of different types. 2. The image sensor according to claim 1, wherein said ultra-thin stacked semiconductor layer is made of a semiconductor having amorphous silicon as a base. 3. The method according to claim 1, wherein the heterojunction is made of i-type hydrogenated amorphous silicon and p-type hydrogenated amorphous silicon carbide or i-type hydrogenated amorphous silicon.
3. The image sensor according to claim 1, wherein the image sensor is made of hydrogenated amorphous silicon carbide or n-type hydrogenated amorphous silicon carbide. 4. The semiconductor device according to claim 1, wherein said ultra-thin stacked semiconductor layer comprises hydrogenated amorphous silicon and hydrogenated amorphous silicon carbide. Alternatively, the image sensor according to claim 3. 5. The image sensor according to claim 1, wherein the image sensor is a photoconductive film-stacked solid-state imaging device.