JP2000106426A - Quantum well optical sensor and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the effect of a voltage drop in each optical sensor element as the loss of incident light due to free-carrier absorption is kept low in a quantum well optical sensor and a method of manufacturing the quantum well optical sensor. SOLUTION: A multitude of optical sensor elements 5, which are subjected to photodetection utilizing light absorption using a carrier excitation between subbands in multiple quantum well layers 6, are arranged in an array type and at the same time, a carrier concentration in a lower contact layer 2 provided under the lower parts of the elements 5 is partially made to differ. That is, as the regions, which are overlapped with the elements 5 in a projective manner, of the layer 2 are provided as low-carrier concentration regions and the peripheries of the low-carrier concentration regions are provided as high-carrier concentration regions, reduction in an element sensitivity, which is felt as the whole quantum well optical sensor, due to free-carrier absorption is never caused and the effect of a voltage drop in each optical sensor element can be reduced. As a result, the fluctuation of a bias, which is applied to each optical sensor element, and an irregularity in the bias can be lessened, whereby the quantum well optical sensor greatly contributes to the practical use of a large-area, high-integration degree and high-resolution infrared sensor array.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quantum well type optical sensor and a method for manufacturing the same, and more particularly, to a method for reducing a voltage drop in an infrared sensor array by light absorption utilizing transition between subbands generated in a multiple quantum well. The present invention relates to a quantum well type optical sensor having a characteristic structure and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, an infrared detector for detecting infrared rays in the vicinity of the 10 μm band has a Cd composition ratio of about 0.2,
For example, a pn junction diode formed on a HgCdTe layer having a Cd composition ratio of 0.22 is used as a photodiode, and the photodiode is arranged in a one-dimensional array or a two-dimensional array, and the electrical connection between the photodiode and a read circuit is made. Infrared detectors are known in which an infrared photodiode array substrate and a Si signal processing circuit substrate are bonded to each other with metal bumps of In or the like formed on both sides in order to make a proper contact.

However, in the case of such an HgCdTe-based infrared detecting device, it is difficult to obtain a large-area substrate having good crystallinity, so that it is difficult to construct a large-sized infrared detecting array composed of multiple sensor elements. There is a problem that there is
In recent years, as a solution to such a problem, a GaAs-based semiconductor which has a good crystallinity and a large-area substrate is easily available, and the intersubband absorption in a multiple quantum well is used to reduce the vicinity of the 10 μm band A quantum well-type optical sensor capable of detecting infrared rays has attracted attention.

Here, a conventional quantum well type optical sensor will be described with reference to FIG. FIG. 6 is a schematic sectional view of a conventional quantum well type optical sensor. First, an i-type GaAs is formed on a semi-insulating GaAs substrate 41.
An n-type GaAs lower contact layer 43 is provided with a buffer layer 42 interposed therebetween.
After sequentially depositing an MQW layer 45 composed of a plurality of n-type GaAs well layers and an n-type GaAs upper contact layer 46 alternately interposed between i-type AlGaAs barrier layers via an n-type InGaP layer 44, the sensor element formation region is formed. Corresponding n
A diffraction grating 47 for deflecting the vertically incident light in an oblique direction so as to be detectable by the MQW layer 45 is provided on the surface of the n-type GaAs lower contact layer 43, and then a common electrode for forming the common electrode for the n-type GaAs lower contact layer 43 is formed. A contact opening 48 is provided, and a SiON film 49 is used as a protective film.
An ohmic electrode 50 made of Au.Ge/Ni is provided in an opening portion provided in the iON film 49, and an ohmic electrode 50 is formed thereon.
A reflection film 51 is formed by providing a u film, and an element isolation groove 52 for isolating between sensor elements is formed.

As described above, in the conventional quantum well type optical sensor, the contact layers, that is, the contact layers above and below the MQW layer 45 having the multiple quantum well structure that causes light absorption by transition between subbands.
The element structure has an element structure sandwiched between an n-type GaAs lower contact layer 43 and an n-type GaAs upper contact layer 46. Each sensor element has a diffraction element that deflects incident light vertically incident from the semi-insulating GaAs substrate 41 side in an oblique direction. The optical coupling structure includes a grating 47 and a reflective electrode 51 that reflects light transmitted through the diffraction grating 47 to the MQW layer 45 side.

By arranging such optical sensor elements in a one-dimensional array or a two-dimensional array, an infrared detecting array is formed. A common bias for each optical sensor element is provided via an n-type GaAs lower contact layer 43. Therefore, the n-type GaAs lower contact layer 43 is usually formed of a relatively high carrier concentration layer.

In such a quantum well type optical sensor, the dark current and the sensitivity have bias dependency, and in an optical sensor array in which a large number of optical sensor elements are arranged, the MQW layer 45 of each optical sensor element is provided. The applied bias is determined by applying the n-type GaAs lower contact layer 43 and the n-type Ga
This is a value obtained by subtracting the voltage drop due to the As upper contact layer 46.

[0008]

However, in the conventional quantum well optical sensor, when the number of optical sensor elements increases, the distance from the n-type GaAs lower contact layer 43 to the optical sensor element increases, so that the voltage drop occurs. And the bias applied to the MQW layer 45 of each optical sensor element fluctuates.

[0009] This variation in the bias causes variations in the performance of each optical sensor element within the optical sensor array, for example, variations in sensitivity. To avoid such variations, the n-type GaAs lower contact layer is required. It is necessary to reduce the contact layer resistance by increasing the carrier concentration of 43.

However, if the carrier concentration is excessively increased, absorption loss of incident light occurs due to free carrier absorption in the n-type GaAs lower contact layer 43, which causes a problem that the sensitivity of the entire optical sensor array decreases.
That is, when the carrier concentration of the n-type GaAs lower contact layer 43 is low, the absorption loss of the incident light due to free carrier absorption is effectively negligible. It cannot be ignored.

Accordingly, it is an object of the present invention to reduce the effect of voltage drop in each optical sensor element while keeping the incident light loss due to free carrier absorption low.

[0012]

FIG. 1 is an explanatory view of the principle configuration of the present invention. Referring to FIG. 1, means for solving the problems in the present invention will be described. Note that reference numerals 1, 7, and 9 in FIG. 1 represent a semiconductor substrate, an upper contact layer, and a sensor electrode, respectively. See FIG. 1. (1) The present invention provides a quantum well optical sensor that performs optical detection by utilizing light absorption due to carrier excitation between subbands in a multiple quantum well layer 6, in which a large number of sensor elements 5 are arranged in an array. In addition, the carrier concentration of the lower contact layer 2 provided below the sensor element 5 is partially varied.

As described above, by partially varying the carrier concentration of the lower contact layer 2 provided below the sensor element 5, the variation of the voltage drop in each sensor element 5 can be reduced. Variations in the characteristics of each sensor element 5 and a decrease in the sensitivity of the entire sensor array can be prevented. In addition, the array arrangement is
A one-dimensional array or a two-dimensional array may be used.

(2) According to the present invention, in the above (1), a region immediately below the sensor element 5 in the lower contact layer 2 is defined as a low carrier concentration region 4, and the lower contact layer 2 is provided between adjacent sensor elements 5. Is a high carrier concentration region 3.

As described above, by setting the region of the lower contact layer 2 immediately below the sensor element 5 as the low carrier concentration region 4, the effect of the loss due to the free carrier absorption of the incident light incident on each sensor element 5 can be effectively reduced. Can be eliminated.

(3) The present invention is characterized in that, in the above (2), the high carrier concentration regions 3 are in contact with each other and are continuously connected to the lower contact electrode 8.

As described above, by bringing the high carrier concentration regions 3 into contact with each other, that is, forming them in a mesh shape and connecting them continuously to the lower contact electrode 8, when the number of the sensor elements 5 increases, Also, the voltage drop in each sensor element 5 can be effectively reduced.

(4) According to the present invention, in any one of the above (1) to (3), the carrier concentration of the low carrier concentration region 4 is lower than the carrier concentration of the well layer forming the multiple quantum well layer 6. It is characterized by the following.

In general, the carrier concentration of the well layer constituting the multiple quantum well layer 6 requires a predetermined concentration for absorbing the incident light.
By making the carrier concentration lower than that of the well layers constituting the multiple quantum well layer 6, the free carrier absorption in the lower contact layer 2 can be reduced more effectively.

(5) The present invention is characterized in that in any one of the above (1) to (4), the area of the high carrier concentration region 3 is smaller than the area of the low carrier concentration region 4.

As described above, by making the area of the high carrier concentration region 3 smaller than the area of the low carrier concentration region 4, the area occupied by the sensor element 5 can be increased. It is possible to increase the density and the number of pixels.

(6) The present invention also provides a multiple quantum well layer 6
In the method of manufacturing a quantum well optical sensor for performing light detection by utilizing light absorption due to carrier excitation between subbands in the method described above, the lower contact layer 2 having a low carrier concentration is epitaxially grown, and then a portion of the lower contact layer 2 is formed. After the high carrier concentration region 3 is formed by selectively introducing impurities, the multiple quantum well layer 6 is epitaxially grown.

As described above, the high carrier concentration region 3 for reducing the voltage drop is selectively formed in a part of the lower contact layer 2 by the selective ion implantation method or the selective diffusion method immediately after the lower contact layer 2 is formed. May be formed by introducing impurities into the substrate.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Here, a manufacturing process of a quantum well type optical sensor according to an embodiment of the present invention will be described with reference to FIGS. 2 (a) and 2 (b). FIG. 2 (a) is a sectional view taken along the line AA 'in FIG. 2 (b).
FIG. 3 is a cross-sectional view taken along a dashed-dotted line connecting the i-type G layer having a thickness of, for example, 100 nm on the semi-insulating GaAs substrate 11 by MOVPE (metal-organic chemical vapor deposition).
The thickness to be the aAs buffer layer 12 and the common contact layer is 0.5 to 3.0 μm, for example, 2.0 μm.
Then, the n-type GaAs lower contact layer 13 having a carrier concentration of 1 × 10 ¹⁶ to 1 × 10 ¹⁷ cm ⁻³ , for example, 5 × 10 ¹⁶ cm ⁻³ is sequentially epitaxially grown.

Next, after providing an SiON film, an SiON mask 14 having a predetermined opening is formed by a normal photolithography process.
4 is used as an ion implantation mask to selectively implant Si ions, so that a high carrier concentration region 1 having a carrier concentration of 1 × 10 ¹⁹ cm ⁻³ or more, for example, 1 × 10 ¹⁹ cm ^−3.
6, 17 are formed. In this ion implantation step, an alignment mark is formed around the surface of the n-type GaAs lower contact layer 13 so that the alignment with the high carrier concentration regions 16 and 17 can be performed in a later step. Is desirable.

Next, after the SiON mask 14 is removed, M
By the OVPE method, n-type GaAs having a thickness of, for example, 50 nm and a carrier concentration of, for example, 5 × 10 ¹⁶ cm ^−3.
The thickness of the s layer 18 serving as an etching stopper layer is, for example, 30 nm, and the carrier concentration is, for example, 2 × 10 ^17.
cm- ^3, an i-type InGaP layer 19, an MQW layer, a thickness of, for example, 1 μm, and a carrier concentration of, for example, 2 ×
An n-type GaAs upper contact layer 21 of 10 ¹⁷ cm ^-3 is sequentially epitaxially grown.

The MQW layer serving as the light absorbing layer in this case
20 is, for example, an i-type Al having a thickness of 50 nm._0.32Ga
_0.68As barrier layer, 5 nm thick and 2 carrier concentration
× 10 ¹⁷cm^-3And the n-type GaAs well layer
The As well layers are alternately deposited so as to have 50 layers, and
I-type Al in this case._0.32Ga_0.68A
The s barrier layer is located between adjacent n-type GaAs well layers.
And make it thick enough so that electrons do not tunnel.
Good.

Next, after a photoresist is applied to the entire surface, a diffraction grating 23 is formed in a portion corresponding to the region surrounded by the high carrier concentration regions 16 and 17 by using the interference exposure method. Is formed on the surface of the n-type GaAs upper contact layer 21 by performing a reactive ion etching (RIE) using the resist mask 22 as a mask. .

Referring to FIG. 4E, after the resist mask 22 is removed, S
An iON film is provided, and an SiO film having an opening for forming a common electrode is formed by a normal photolithography process.
An N mask 24 is formed, and a contact opening 25 reaching the n-type InGaP layer 19 is formed by performing wet etching using the SiON mask 24 as a mask.

In this case, HF + H _{2 is used} as an etchant.
By using O ₂ + H ₂ O, the n-type InGaP layer 19 serves as an etching stopper layer so that GaAs and Al
GaAs is selectively etched, and n-type InGaP
Etching automatically stops at the surface of layer 19.

Next, after the SiON mask 24 is removed, a new SiON film 26 is provided on the entire surface, and a common electrode forming portion and a sensor electrode forming portion are formed using a photoresist pattern (not shown). After selectively removing the SiON film 26, Au.
After partially providing an ohmic electrode 27 made of Ge / Ni, a reflective electrode 28 made of Au is provided on the surface thereof,
A common electrode and a sensor electrode are selectively formed by a lift-off method using a photoresist pattern.

In this electrode forming step, the ohmic electrode 27 provided on the diffraction grating 23 is
3 and is formed only partially on the top of the convex portion and the bottom of the concave portion of the diffraction grating 23, so that the reflectivity of the reflective electrode 28 is impaired in most of the diffraction grating 23. Nothing.

Next, after removing the photoresist pattern (not shown), a new photoresist is applied, and a resist mask (not shown) having an opening corresponding to the element isolation region is formed, as shown in FIG.
Then, dry etching is performed using this resist mask as a mask to remove a part of the SiON film 26. Subsequently, the n-type GaAs upper contact layer 21 and the MQW layer 20 are removed to form an element isolation groove 29.

Next, after removing the resist mask, a new SiON film 30 serving as a protective insulating film is deposited, and the reflection electrodes 28 of the common electrode portion and the sensor electrode portion are formed by a normal photolithography process. A part is exposed, and an In bump 32 is formed on the exposed portion via a Ti / Au film 31 serving as a barrier layer and an adhesion improving layer by an evaporation method.

As described above, in the embodiment of the present invention, the high carrier concentration region 17 is formed so as to surround each sensor element, and this high carrier concentration region 17 is formed via the high carrier concentration region 16. Since it is connected to the common electrode formed in the contact opening 25, even if the distance from the common electrode to the sensor element is long, the distance from the common electrode to the sensor element is 1 × 10 ¹⁹ cm ⁻³ or more. Since the voltage is applied through the carrier concentration regions 16 and 17, the voltage drop is sufficiently small.

Therefore, the variation in the applied voltage applied to each sensor element is reduced, so that it is possible to reduce the variation in performance between the sensor elements constituting the optical sensor array, for example, the variation in sensitivity and dark current value. it can.

Signal light is incident on each sensor element through a low carrier concentration region surrounded by a high carrier concentration region 17, and the area of the high carrier concentration region 17 is surrounded by the high carrier concentration region 17. Since the area of the low carrier concentration region is smaller than that of the low carrier concentration region, the influence of the absorption loss of the incident light due to the free carrier absorption in the high carrier concentration region 17 can be reduced. can do.

In particular, n-type GaAs lower contact layer 13
The carrier concentration in the low carrier concentration region surrounded by the high carrier concentration region 17 is 5 × 10 ¹⁶ cm ⁻³ ,
Since the carrier concentration of the n-type GaAs well layer forming the W layer 20 is lower than 2 × 10 ¹⁷ cm ⁻³ , the influence of the absorption loss of incident light due to free carrier absorption can be reduced more reliably.

Although most of the signal light incident from the semi-insulating GaAs substrate 11 is transmitted without being absorbed by the MQW layer 20, the diffraction grating 23 provided on the surface of the n-type GaAs upper contact layer 21 causes the signal light to pass through. The light is reflected and deflected in an oblique direction, and is again incident on the MQW layer 20 in an oblique direction.
It is effectively absorbed in the W layer 20.

The signal light to be transmitted through the diffraction grating 23 is reflected by the reflection electrode 28, and is again transmitted to the MQW.
Since the light is reflected on the layer 20 side and is absorbed again in the MQW layer 20, the light detection efficiency can be further improved.

The embodiments of the present invention have been described above. However, the present invention is not limited to the configurations and conditions described in the embodiments, and various modifications are possible.
The high carrier concentration region may be formed by using a selective diffusion method instead of the ion implantation method.

In the description of the above embodiment, the n-type InGaP layer 19 is used as an etching stopper layer in order to enhance the controllability of the etching step when forming the contact opening 25 and the like. Another semiconductor layer may be used as the etching stopper layer, and further, the etching stopper layer is not necessarily provided.

Further, in the above description of the embodiment, the MQW layer 20 is composed of an Al _0.32 Ga _0.68 As / GaAs junction, but the composition of the barrier layer is Al _0.32 Ga
It is not limited to _0.68 As, but Al _x
Ga _1-x As may be used, and the thickness of the barrier layer, the composition ratio, and the thickness of the n-type GaAs well layer are appropriately set so that a quantum level and a sub-band corresponding to the wavelength to be detected are formed. Just choose.

In the description of the above embodiment, the MQW layer 20 is constituted by an AlGaAs / GaAs multiple quantum well structure.
A multi-quantum well structure using another III-V compound semiconductor such as an aAs-based compound semiconductor may be used, and any material may be used as long as the material is lattice-matched to a large-area substrate to be used.

Further, in the description of the above embodiment, as is apparent from FIG.
Although described as a dimensional array, a one-dimensional array is also targeted.

[0046]

According to the present invention, the region which is projectedly overlapped with the sensor element of the lower contact layer is a low carrier concentration region and the surrounding region is a high carrier concentration region. Since the influence of the voltage effect can be reduced without causing a decrease in element sensitivity as described above, fluctuations and variations in bias applied to each sensor element can be reduced,
This greatly contributes to the practical use of a large-area, high-integration and high-resolution infrared sensor array.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a basic configuration of the present invention.

FIG. 2 is an explanatory diagram of a manufacturing process of the quantum well type optical sensor according to the embodiment of the present invention halfway;

FIG. 3 is an explanatory diagram of a manufacturing process of the quantum well type optical sensor according to the embodiment of the present invention in the middle of FIG. 2 and thereafter;

FIG. 4 is an explanatory diagram of a manufacturing process of the quantum well type optical sensor according to the embodiment of the present invention in the middle of FIG. 3 and thereafter;

FIG. 5 is an explanatory diagram of a manufacturing process of the quantum well optical sensor according to the embodiment of the present invention after FIG. 4;

FIG. 6 is a schematic sectional view of a conventional quantum well optical sensor.

[Explanation of symbols]

 Reference Signs List 1 semiconductor substrate 2 lower contact layer 3 high carrier concentration region 4 low carrier concentration region 5 sensor element 6 multiple quantum well layer 7 upper contact layer 8 lower contact electrode 9 sensor electrode 11 semi-insulating GaAs substrate 12 i-type GaAs buffer layer 13 n-type GaAs lower contact layer 14 SiON mask 15 Si ions 16 high carrier concentration region 17 high carrier concentration region 18 n-type GaAs layer 19 n-type InGaP layer 20 MQW layer 21 n-type GaAs upper contact layer 22 resist mask 23 diffraction grating 24 SiON Mask 25 Contact opening 26 SiON film 27 Ohmic electrode 28 Reflection electrode 29 Element isolation groove 30 SiON film 31 Ti / Au film 32 In bump 41 Semi-insulating GaAs substrate 42 i-type GaAs buffer layer 43 n-type Ga As lower contact layer 44 n-type InGaP layer 45 MQW layer 46 n-type GaAs upper contact layer 47 diffraction grating 48 contact opening 49 SiON film 50 ohmic electrode 51 reflective electrode 52 element isolation groove

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4M118 AA06 AB01 BA05 CA15 CA32 CA34 CB02 CB14 EA04 EA20 FB08 FB09 FB18 FB19 FB20 FB24 GA09 5F049 MA02 MB07 MB11 NA14 NA20 NB05 PA03 PA10 PA14 QA16 QA18 RA02 SE05 SE16 SS

Claims (6)

[Claims] In a quantum well type optical sensor for detecting light by utilizing light absorption due to carrier excitation between subbands in a multiple quantum well layer, a large number of sensor elements are arranged in an array, and A quantum well optical sensor characterized in that the carrier concentration of a lower contact layer provided below is partially varied. 2. A region of the lower contact layer immediately below a sensor element is a low carrier concentration region, and a region between adjacent sensor elements of the lower contact layer is a high carrier concentration region. 2. The quantum well type optical sensor according to 1. 3. The quantum well type optical sensor according to claim 2, wherein said high carrier concentration regions are in contact with each other and are continuously connected to a lower contact electrode. 4. The quantum well according to claim 1, wherein a carrier concentration of the low carrier concentration region is lower than a carrier concentration of a well layer forming the multiple quantum well layer. Type optical sensor. 5. The quantum well type optical sensor according to claim 1, wherein the area of the high carrier concentration region is smaller than the area of the low carrier concentration region. 6. A method for manufacturing a quantum well photosensor for performing photodetection utilizing light absorption due to carrier excitation between subbands in a multiple quantum well layer, wherein a lower contact layer having a low carrier concentration is epitaxially grown. Forming a high carrier concentration region by selectively introducing impurities into a part of the lower contact layer; and epitaxially growing the multiple quantum well layer.