104a of the second semiconductor layer 104, the third region 103a of the diffusion barrier layer 103, and the fifth region 102a of the light-absorbing layer 102 can have the same surface area. In other words, the first region 104a, the third region 103a, and the fifth region 102a can be completely overlapped in a vertical direction. In a cross-sectional view of the photo-detecting device 10, as shown in the lower part of FIG. 1, the thickness of the first region 104a is equal to that of the second region 104b, the thickness of the third region 103a is equal to that of the fourth region 103b, and the thickness of the fifth region 102a is smaller than that of the sixth region 102b. The thickness of the fifth region 102a of the light-absorbing layer 102 may be greater than or equal to 0.5 μm, and smaller than or equal to 3 μm, for example, 1 μm, 1.5 μm, 2 μm, or 2.5 μm. By the presence of the fifth region 102a of the light-absorbing layer 102, the signal-to-noise ratio (S/N) of the photo-detecting device 10 can be improved.

The third dopant has a first doping concentration in the semiconductor contact layer 106, a second doping concentration in the first region 104a of the second semiconductor 104, a third doping concentration in the third region 103a of the diffusion layer 103, and a fourth doping concentration in the fifth region 102a of the light-absorbing layer 102. In an embodiment, the first, second, and third doping concentrations can be roughly the same, e.g. in a range of 10¹⁸to 10¹⁹cm⁻³. The fourth doping concentration can be lower than any one of the first, second, and third doping concentrations, e.g. in a range of 10¹⁶to 10¹⁹cm⁻³.

FIG. 2 shows a photo-detecting device in accordance with an embodiment of the present disclosure wherein the upper part of FIG. 2 shows a top view of a photo-detecting device 11 and the lower part of FIG. 2 shows a cross-sectional view taken along cross-section line B-B′. The photo-detecting device 11 can include similar elements in the photo-detecting device 10. In a photo-detecting device manufacturing process, the diffusion process is performed after the epitaxy process so the semiconductor structure grown in the epitaxy process affect the dopant which is required to penetrate through multiple semiconductor layers, for example, contact layer and cap layer, and reach the light light-absorbing layer in the diffusion process. In this embodiment, the thickness of the semiconductor contact layer 106 is greater than or equal to 50 Å and is smaller than or equal to 1000 Å, such as 100 Å, 200 Å, 300 Å, 400 Å, 500 Å, 600 Å, 700 Å, 800 Å, or 900 Å. As the semiconductor contact layer 106 has a thickness smaller than or equal to 1000 Å, the controllability for the third dopant diffusion process may be enhanced, and the dark current contributed by the defect of the photo-detecting device 11 is decreased. Furthermore, the fabrication yield of photo-detecting devices 11 is increased.

As shown in the upper part of FIG. 2, after projecting the photo-detecting device 11 to a horizontal plane, the first electrode structure 108 can form an annulus, such as circle annulus or square annulus. In another embodiment, in the vertical direction, the perimeter 104a1 of the first region 104a can be aligned with the outer sidewall 108c of the first electrode structure 108. The second semiconductor layer 104 of the photo-detecting device 11 can include a seventh region 104c. The seventh region 104c includes the second dopant and the third dopant. The third dopant diffuses from the first region 104a and crosses beyond the outer sidewall 108c of the first electrode structure 108 to form the seventh region 104c in the horizontal direction. As shown in the lower part of FIG. 2, the seventh region 104c is not covered by the semiconductor contact layer 106 and the first electrode structure 108. Instead, the seventh region 104c is covered by the insulating layer 107. In an embodiment, there is a first distance L1 between the second side surface 406 of the semiconductor contact layer 106 and the seventh region 104c of the second semiconductor layer 104 in the horizontal direction. The first distance L1 is greater than or equal to 1 μm and is smaller than or equal to 20 μm, such as 5 μm, 10 μm, or 15 μm. In an embodiment, the doping concentration of the third dopant can be higher than that of the second dopant in the seventh region 104c, such that the seventh region 104c of the second semiconductor layer 104 has the second conductivity-type.

In the top view of the photo-detecting device 11, as shown in the upper part of FIG. 2. The seventh region 104c surrounds the first region 104a, and the second region 104b surrounds the seventh region 104c. The seventh region 104c is between the first region 104a and the second region 104b in the horizontal direction. Between the first region 104a and the seventh region 104c, there is no interface can be observed by Atomic Force Microscope (AFM) because the first region 104a and the seventh region 104c both have the second dopant and the third dopant. On the contrary, an interface between the second region 104b and the seventh region 104c can be observed by AFM because the second region 104b does not have the third dopant.

As shown in the upper part of FIG. 2, the light-absorbing layer 102 of the photo-detecting device 11 includes an eighth region 102c. The eighth region 102c includes the third dopant. Thus, the eighth region 102c of the light-absorbing layer 102 has the second conductivity-type. The eighth region 102c surrounds the fifth region 102a, the sixth region 102b surrounds the eighth region 102c, and the eighth region 102c is between the fifth region 102a and the sixth region 102b in the horizontal direction.

As shown in the lower part of FIG. 2, in a vertical direction, the fifth region 102a of the light-absorbing layer 102 is enclosed by the projection of the outer sidewall 108c of the first electrode structure 108 and the perimeter 102a1 of the fifth region 102a is aligned with the outer sidewall 108c of the first electrode structure 108. The third dopant diffuses from the fifth region 102a and crosses beyond the outer sidewall 108c of the first electrode structure 108 to form the eighth region 102c of the light-absorbing layer 102 in the horizontal direction. Thus, the eighth region 102c is not covered by the first electrode structure 108 and the semiconductor contact layer 106. Instead, the eighth region 102c is covered by the insulating layer 107.

The lower part of FIG. 2 shows a cross-sectional view of the photo-detecting device 11. The thickness of the seventh region 104c is the same with the first region 104a and/or the second region 104b thereof. The thickness of the eighth region 102c is the same with the fifth region 102a and is smaller than the thickness of the sixth region 102b. The thickness T1 of the light-absorbing layer 102 is greater than or equal to 1 μm, and is smaller than or equal to 5 μm. The thickness of the eighth region 102c is greater than or equal to 0.1 μm, and is smaller than or equal to 1 μm, such as 0.2 μm, 0.4 μm, 0.6 μm, or 0.8 μm. The thickness of the sixth region 102b may be as large as the thickness T1 of the light-absorbing layer 102.

In an embodiment, the photo-detecting device 11 can optionally include the diffusion barrier layer (not shown), which can be referred to FIG. 1, located between the second semiconductor layer 104 and the light-absorbing layer 102 in the vertical direction. The diffusion barrier layer can include a ninth region (not shown) surrounding the third region 103a referred to the FIG. 1 in the horizontal direction. The ninth region includes the second dopant and the third dopant. The doping concentration of the third dopant can be higher than that of the second dopant in the ninth region such that the ninth region of the diffusion barrier layer 103 has the second conductivity-type.

In the top view of the photo-detecting device 11, as shown in the upper part of FIG. 2, the fourth region (not shown), which can be referred to the fourth region 103b shown in FIG. 1, surrounds the ninth region. The ninth region is between the third region and the fourth region in the horizontal direction. In the vertical direction, the perimeter of the third region (not shown) may be aligned with the outer sidewall 108c of the first electrode structure 108. The third dopant diffuses from the third region and cross beyond the outer sidewall 108c of the first electrode structure 108 to form the ninth region of the diffusion barrier layer in the horizontal direction. Thus, the ninth region is not covered by the first electrode structure 108 and the semiconductor contact layer 106. Instead, the ninth region is covered by the insulating layer 107.

In an embodiment, the seventh region 104c (or the first region 104a) and the eighth region 102c (or the fifth region 102a) stack sequentially along the vertical direction. The stacked seventh region 104c (or the first region 104a) and eighth region 102c (or the fifth region 102a) have a depth D1, which is the diffusion depth of the third dopant. In an embodiment including the diffusion barrier layer, the depth D1 is constituted by the stack including the seventh region 104c (or the first region 104a), eighth region 102c (or the fifth region 102a), and the ninth region (or the third region). The depth D1 is greater than or equal to 2 μm and is smaller than or equal to 4 μm, such as 2.5 μm, 3 μm, or 3.5 μm. The seventh region 104c and the eighth region 102c form an outer ring having a width W21. The width W21 is greater than or equal to 2 μm and is smaller than or equal to 3 μm, such as 2.5 μm. In an embodiment, the ratio of the depth D1 to the width W21 is greater than 0.67 and smaller than 2, such as 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or 1.9.

In an embodiment, the photo-detecting device 11 includes an interlayer 105 between the light-absorbing layer 102 and the first semiconductor layer 101. The interlayer 105 includes undoped or unintentionally-doped semiconductor with the energy band gap Eg₁greater than the energy band gap Eg of the light-absorbing layer 102. The interlayer 105 has a width which is same with the first width W1 of the substrate 100, the first semiconductor layer 101, the light-absorbing layer 102, and the second semiconductor layer 104. The interlayer 105 has a thickness greater than or equal to 0.5 μm and is smaller than or equal to 3 μm, such as 1 μm, 1.5 μm, 2 μm, or 2.5 μm. With the interlayer 105, the dark current and the capacitance of the photo-detecting device 11 can be reduced.

FIG. 3 shows Secondary Ion Mass Spectroscopy (SIMS) spectrum of an embodiment of the photo-detecting device 11 without diffusion barrier layer 103 along the C-C′ line in FIG. 2. In FIG. 3, from the left side to the right side, it can be separated into three regions representing the third dopant concentration of the second semiconductor layer 104, the light-absorbing layer 102, and interlayer 105 respectively. When the thickness of the semiconductor contact layer 106 is greater than or equal to 50 Å, and is smaller than or equal to 1000 Å, the third dopant can properly diffuse into the light-absorbing layer 102 from the second semiconductor layer 104. The third dopant can diffuse underneath the interface of the second semiconductor layer 104 and the light-absorbing layer 102 at a depth, which is greater than or equal to 0.1 μm, and smaller than or equal to 1 μm, such as 0.2 μm, 0.4 μm, 0.6 μm, or 0.8 μm. In an embodiment, the element showed in FIG. 3 is the third dopant. In FIG. 3, along the direction from the second semiconductor layer 104 to the interlayer 105, the third dopant concentration in the second semiconductor layer 104 decreases from an order of 10⁹cm⁻³to an order of 10¹⁸cm⁻³, and there is an order of 10¹⁹cm⁻³cm at the interface of the second semiconductor layer 104 and the light-absorbing layer 102. The decreased gradient rate of the third dopant in the second semiconductor layer 104 is around 10¹⁶˜10¹⁷cm⁻³·nm⁻¹. Along the direction from the light-absorbing layer 102 to the interlayer 105, the concentration of the third dopant decreases from the order of 10¹⁹cm⁻³in the light-absorbing layer 102 to the order of 10¹⁷cm⁻³steeply, and the decreased gradient rate of the third dopant is 10¹⁶˜10¹⁹cm⁻³·nm⁻¹. In an embodiment, the decreased gradient rate of the third dopant in the light-absorbing layer 102 is greater than the decreased gradient rate of the third dopant in the second semiconductor layer 104. In other words, the diffusion coefficient of the third dopant in the second semiconductor layer