Semiconductor device and method for producing the same

Details Semiconductor device and method for producing the same Semiconductor device and method for producing the same

2001-10-18

2002-10-01

Lee, Eddie (Department: 2815)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S057000, C438S059000, C438S071000, C438S072000, C438S098000

Reexamination Certificate

active

06458620

ABSTRACT:

1. Field of the Invention
The present invention relates to a photo-detecting device. In particular, the present invention relates to a photo-detecting device which provides fast photo-sensitive response, with a reduced photocurrent component (tail current) which has very slow response as compared to that of the majority of the photocurrent; and a method for producing the same.
2. Description of the Related Art
One class of photo-detecting devices having fast photo-sensitive response which are currently in wide use are so-called “pin photodiodes”. Pin photodiodes can be further classified depending on the type of material used as semiconductor material, i.e., “silicon pin photodiodes ”, which are based on silicon, and “compound semiconductor pin photodiodes”, which are based on compound semiconductor materials.
In general, a pin photodiode can be produced in the following manner.
First, a low concentration n-type semiconductor layer is allowed to grow its crystal on a high concentration n-type semiconductor substrate. Next, in predetermined regions which are to become island-like diffusion regions, a p-type impurity is diffused to some depth from the surface of the low concentration n-type semiconductor growth layer, thereby forming the island-like diffusion regions. Thereafter, a negative electrode is formed on the upper face of some of the islands of p-type diffusion regions, and a positive electrode is formed on the back face of the high concentration n-type semiconductor substrate. Thus, a pin photodiode is produced.
In the case of producing a compound semiconductor pin photodiode, e.g., InGaAs/InP, in particular, the low concentration n-type semiconductor growth layer may be formed in two layers. These two growth layers may include a light absorption layer which is adjacent to the semiconductor substrate, and a window layer formed on the light absorption layer, such that the window layer has a larger energy band gap than that of the light absorption layer. The size of the energy band gap can be adjusted by selecting the compound semiconductor material and appropriately changing the component ratios thereof. Next, a p-type impurity is diffused in the window layer to form island-like diffusion regions, whereby a compound semiconductor pin photodiode is produced. It should be noted that it is impossible to form such a window layer in a silicon pin photodiode structure because its energy band gap cannot be changed.
In a compound semiconductor pin photodiode having the above-described structure, regions of the light absorption layer which lie under the p-type diffusion regions function as photo-detecting portions. In the photo-detecting portions, a photocurrent is generated responsive to incident light which enters through the growth surface of the window layer.
Specifically, electron-hole pairs are generated through photoexcitation occurring in regions (photo-detecting portion) of the light absorption layer located under the p-type diffusion regions. The generated electron-hole pairs are dissociated by a potential barrier (electric field) at the p-n junction, so that the electrons migrate to the high concentration n-type semiconductor substrate and the holes migrate to the p-type diffusion regions. A photocurrent results from the migration of the electrons and the holes.
Compound semiconductor pin photodiodes which incorporate a window layer above a light absorption layer as mentioned above can provide an improved quantum efficiency because the window layer has a greater energy band gap than that of the light absorption layer so that the window layer becomes transparent with respect to the incident light, thereby preventing surface recombination of electron-hole pairs at the surface of the light absorption layer.
A photocurrent in a pin photodiode is primarily generated in the above-described manner. However, a photocurrent may also be generated in the case where light enters the window layer in regions other than the photo-detecting portions. Such a photocurrent may be generated due to the diffusion of holes, and has a response which is much slower than the photocurrent that is generated in the photo-detecting portions. This photocurrent having a very slow response is commonly referred to as a “tail current”, which may present a significant problem in certain applications of the photo-detecting device. The mechanism which generates a tail current will be described below.
The light entering regions of the window layer other than those corresponding to the photo-detecting portions generate electron-hole pairs in the underlying light absorption layer. However, since no potential barrier (electric field) that is associated with a p-n junction exists in these regions, the generated electrons and holes migrate due to diffusion, rather than due to an electric field. That is, the generated electrons and holes diffuse in accordance with their respective density gradients so as to permeate the surrounding low concentration regions. Since the electrons are the majority carriers in the n
−
layer (i.e., light absorption layer), it is presumable that the electrons immediately create a photocurrent before even reaching the n-substrate. On the other hand, only those of the holes which have reached the p-type diffusion regions through diffusion create a photocurrent, whereas the other holes will recombine with the electrons over a long period of time. Since the holes have a long lifetime within the light absorption layer, some holes may reach the p-type diffusion layer after having diffused through the light absorption layer over a long period of time. A tail current is defined as a component of the photocurrent that is attributable to the diffusive migration of such holes.
As described above, the cause for a tail current is the electron-hole pairs generated in regions other than the photo-detecting portions. Therefore, in order to reduce the tail current, it has been proposed to construct a photo-detecting device in which regions other than photo-detecting portions are covered by a light-shielding film such as a thin metal film. This technique for reducing the tail current is generally employed in the field of silicon pin photodiodes.
However, the aforementioned technique is difficult to apply to compound semiconductor pin photodiodes due to the nature of the actual production processes. Specifically, the production of a compound semiconductor pin photodiode requires highly precise micro-processing techniques because a depletion layer for a compound semiconductor material is much narrower than a depletion layer for silicon, as described below in more detail.
In a photo-detecting device, regions other than photo-detecting portions are usually not entirely covered by a light-shielding film such as a thin metal film because such a light-shielding film (e.g., a thin metal film) would cause short-circuiting if they contact an annular electrode, wiring and/or a pad composed of a conductive material, which are formed on the surface of a photo-detecting device on which photo-detecting regions are formed. Rather, such a light-shielding film is provided so as to have a minimum interspace with each conductive element on the surface of the photo-detecting device. The interspaces, which cannot shield incident light, should be minimized in order to minimize the tail current. Specifically, such a light-shielding film is only required to be large enough so that its inner end (i.e., the end adjoining the interspace with a conductive element on the device surface) is in an overlapping relation with the outer periphery of an underlying depleted intrinsic semiconductor layer (i.e., a depletion layer), when viewed from above the light entering surface (i.e., the upper face of the substrate). In accordance with this configuration, even if light enters the depletion layer through the interspace, a very rapid response can be obtained because of the electric field applied to the depletion layer, so that no tail current is generated. Another advantage associated with the photo-detecting device structure in whi

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