JP2003163362A - Flame sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a flame sensor capable of making a stable detection of flame under high temperature and high humidity environments. <P>SOLUTION: The flame sensor has a semiconductor multilayer structure which is such that a plurality of semiconductor layers are stacked on a substrate. Part of the semiconductor multilayer structure is removed to a prescribed depth to form a step. On one of the two surfaces of the semiconductor multilayer structure with the step in between, a first electrode is formed, and a second electrode is formed on the other surface. A high-resistance structure is formed at least on a part of the surface of the step between the first and second electrodes. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flame sensor with reduced leakage current between electrodes.

[0002]

2. Description of the Related Art Conventionally, it has been proposed to use a light receiving element manufactured by using a semiconductor mesa structure in which a step is provided in a semiconductor laminated structure as a flame sensor. When the light receiving element is used as a flame sensor, it is required to satisfy the conditions peculiar to the flame sensor, but particularly, it is required to detect light of a weak flame with high sensitivity. Specifically, it is necessary to clearly distinguish the photocurrent generated by absorbing the flame light from the noise. Here, dark current exists as noise in the light receiving element, and a large dark current is observed particularly in a Schottky diode type light receiving element.

[0003]

The causes of the dark current are the tunnel current (junction leakage current) passing through the depletion layer of the junction between the semiconductor layers in the light receiving element structure, and the cause of the dark current flowing on the side surface of the light receiving element. Some are due to creeping current (leakage current around the mesa). The tunnel current depends on the defect density and the type of defects contained in the semiconductor layer, and the tunnel current cannot be completely eliminated when a bias voltage is applied between the electrodes. The creeping current depends on the state of the surface of the light receiving element, and particularly greatly depends on the fluctuation of humidity.

Conventionally, in order to reduce the creeping current, it has been practiced to implant impurities such as hydrogen and nitrogen into the surface of the light receiving element to increase the resistance. However, by implanting the impurities, the internal characteristics of the semiconductor are improved. There is a problem that the tunnel current changes and the tunnel current increases as described above.

Further, the flame sensor is often used in a hot and humid environment due to its special application, and it is required to operate stably even in such an environment.
However, it is natural that a large dark current is observed due to thermionic emission at a high temperature, and further, as described above, the dark current is increased due to an increase in the creeping current flowing on the flame sensor surface under high humidity. There is a problem of large observations.

The present invention has been made in view of the above problems, and an object thereof is to perform stable flame detection even in a hot and humid environment by reducing a leak current between electrodes. The point is to provide a flame sensor that can be used.

[0007]

A first characteristic configuration of a flame sensor according to the present invention for solving the above-mentioned problems is to provide a plurality of substrates on a substrate as described in claim 1 of the claims section. A first electrode is provided on one of two surfaces of the semiconductor laminated structure, which has a semiconductor laminated structure in which semiconductor layers are laminated, and which sandwiches a step formed by removing a part of the semiconductor laminated structure to a predetermined depth. And a second electrode is formed on the other side, and a high resistance structure is formed on at least a part of the surface of the step between the first electrode and the second electrode.

A second characteristic configuration of the flame sensor according to the present invention for solving the above-mentioned problems is, in addition to the first characteristic configuration described above, in addition to the first characteristic configuration described in claim 2 of the scope of claims. The high resistance structure is that it is an insulating film formed on at least a part of the step between the first electrode and the second electrode.

A third characteristic constitution of the flame sensor according to the present invention for solving the above-mentioned problems is, in addition to the above-mentioned first or second characteristic constitution, as described in claim 3 of the scope of claims. Then, the step between the first electrode and the second electrode is formed in a step-like shape including a plurality of small steps, and the high resistance structure is formed between the small steps formed in the high resistance semiconductor layer. It is at the boundary.

The operation and effect will be described below. According to the first characteristic configuration of the flame sensor according to the present invention, since the high resistance portion is structurally formed at the step between the first electrode and the second electrode to which an electric field is applied, The creeping current generated on the surface of the semiconductor laminated structure between the first electrode and the second electrode can be reduced. In addition, since the high resistance portion is not provided by changing the internal structure of the semiconductor laminated structure forming the flame sensor, the defect density generated when the high resistance portion is provided by implanting impurities unlike the conventional case There is no problem such as increase in the pulse current and increase in the tunnel current. Furthermore, even if the flame sensor is placed in a hot and humid environment exposed to the flame, the high resistance structure formed between the electrodes significantly increases the level of the creeping current flowing on the surface of the semiconductor laminated structure of the flame sensor. Can be reduced. Therefore, it is possible to provide a flame sensor capable of detecting the photocurrent generated by receiving the flame with high sensitivity by distinguishing it from the dark current.

According to the second characteristic configuration of the flame sensor of the present invention, the insulating coating is formed on the step portion between the first electrode and the second electrode, so that the first electrode and the second electrode are formed. The resistance value on the surface path of the semiconductor laminated structure between the first and second electrodes can be increased, and the inter-electrode surface of the semiconductor laminated structure can be obtained even when an electric field is applied between the first electrode and the second electrode. It is possible to greatly reduce the level of the creeping current flowing in the.

According to the third characteristic configuration of the flame sensor of the present invention, the step between the first electrode and the second electrode is formed in a step-like shape consisting of a plurality of small steps, and between the small steps. Since the boundary portion is provided in the high resistance semiconductor layer included in the semiconductor laminated structure, the distance of the surface of the semiconductor laminated structure between the first electrode and the second electrode is selectively lengthened in the high resistance semiconductor layer. be able to. As a result, the resistance value on the path of the creeping current generated on the surface of the semiconductor laminated structure between the first electrode and the second electrode can be significantly increased, so that the level of the generated creeping current can be significantly reduced. Can be made.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The Schottky diode type flame sensor illustrated in FIG. 1A has a structure in which a base structure and a device structure are sequentially deposited on a substrate 1. The underlayer structure is formed by sequentially depositing the low temperature deposition buffer layer 2, the crystal improvement layer 3, and the low temperature deposition intermediate layer 4 on the substrate 1, and the device structure is formed on the underlayer structure by the n-type semiconductor layer 5. And the i-type semiconductor layer 6 to be the light receiving region are sequentially deposited. Further, the electrode 8 is formed on the surface of the n-type semiconductor layer 5 exposed by partially etching the device structure so as to make ohmic contact, and the i-type semiconductor layer having a step different from the n-type semiconductor layer 5 is formed. An electrode 9 is formed on 6 to form a Schottky junction. Here, although the flame sensor is irradiated with light from the electrode 9 side, the light receiving region (i
The electrode 9 is formed of a transparent conductive material or the like having a good light-transmitting property so that light of high intensity is incident on the type semiconductor layer 6).

Specifically, in the flame sensor illustrated in FIG. 1A, the material of the substrate 1 is sapphire, the material of the low temperature deposition buffer layer 2 is AlN (thickness 20 nm), and the crystal improvement layer 3 is formed. Is GaN (thickness 1 μm), the material of the low temperature deposition intermediate layer 4 is AlN (thickness 20 nm), n
The material of the type semiconductor layer 5 (thickness 1 μm) is single crystal AlGaN.
The material of the i-type semiconductor layer 6 (thickness 100 to 200 nm) is single crystal AlGaN. The electrode 8 is made of Al,
The electrode 9 is formed of a metal such as Ti, Ni or Au, and the electrode 9 is formed of a transparent conductive film (ITO, SnO ₂ or the like). still,
The above-mentioned film thickness can be changed to other values.

Further, an insulating film (high resistance structure) 10 is formed on the surface of the step between the electrodes 8 and 9, and the flame sensor is applied when an electric field is applied between the electrodes 8 and 9. It is possible to significantly increase the resistance value of the path of the creeping current flowing on the surface (including the surface of the step). Although SiO ₂ or SiN can be used as the material of the insulating coating 10, other materials can be used as long as they are heat resistant within the operating temperature range of the flame sensor. As a method for producing the insulating coating 10, a usual film forming method such as a CVD method can be used. Here, the surface of the flame sensor is a portion where the semiconductor laminated structure is exposed, and the cross-sectional portion (step portion) of the mesa structure as illustrated.
Is also included.

The role of the underlying structure is to improve the crystal quality of the device structure including the light receiving region. The lattice spacing on the crystal growth surface of the sapphire substrate 1 and the single crystal Al _x Ga _1-x N (0
There is a large difference with the lattice constant of ≦ x ≦ 1), but the lattice mismatch is relaxed by the underlying structure, and due to the lattice mismatch added when growing the AlGaN layer for forming the device layer structure. The tensile stress can be made very small. Therefore, the crystal quality of the single crystal AlGaN layer related to the formation of the device structure can be improved.

As described above, the formation of the insulating coating 10 having heat resistance within the operating temperature range of the flame sensor on the surface of the step between the electrodes 8 and 9 causes a dark current. A creeping current between the electrodes 8 and 9 can be reduced, and as a result, a dark current (including a creeping current) is generated even if the photocurrent generated by absorbing the irradiated light is weak.
Can be detected separately. Although the case where the entire surface of the semiconductor laminated structure including the step portion between the electrode 8 and the electrode 9 is covered with the insulating coating film 10 is illustrated in the drawing, the insulating coating film 10 covers the electrode 8 and the electrode 9. It may be formed on a part of the surface of the step between the two.

A PIN type flame sensor is illustrated in FIG. 1 (b). The device structure is deposited on the same underlying structure as illustrated in FIG. 1A, and the device structure forms a PIN type light receiving element.
Therefore, the device structure is formed by sequentially depositing the n-type semiconductor layer 5, the i-type semiconductor layer 6 serving as the light receiving region, and the p-type semiconductor layer 7. Further, the electrode 8 is formed on the surface of the n-type semiconductor layer 5 exposed by partially etching the device structure so as to make ohmic contact, and the p-type semiconductor layer 7 having a step different from the n-type semiconductor layer 5 is formed. The electrode 11 is formed on the upper surface of the electrode 11 so as to be in ohmic contact. Here, the flame sensor is irradiated with light from the electrode 11 side. However, the electrode 11 has a mesh shape or a mesh shape so that light of high intensity is incident on the light receiving region (i-type semiconductor layer 6). It is formed of a transparent conductive material or the like having good light transmittance. Electrode 8
Is formed of a metal such as Ti, Ni, Al, Au, and the material of the electrode 11 is Ti, Ni, in the case of a mesh electrode.
Metals such as Al and Au are used, and materials such as ITO and SnO ₂ are used for the transparent conductive electrode. Also,
The material of the p-type semiconductor layer 7 (thickness: 80 nm) is single crystal AlG.
aN. In order to make ohmic contact between the p-type semiconductor layer 7 and the electrode 11, a p-type contact layer made of p-GaN or the like is provided between the p-type semiconductor layer 7 and the electrode 11. Good.

Further, as in the case of FIG. 1A, the surface of the step between the electrode 8 and the electrode 11 is made of SiO ₂ or SiN.
By forming the insulating coating (high resistance structure) 10 having heat resistance within the operating temperature range of the flame sensor, the creeping current between the electrode 8 and the electrode 11 which causes dark current is reduced. As a result, the photocurrent generated by absorbing the irradiated light can be detected separately from the dark current (including the creeping current).

As described above, FIG. 1 (a) and FIG.
As described with reference to (b), by forming the insulating coating film 10 on the step portion between the electrode 8 and the electrode 9 (or the electrode 11) on the surface of the flame sensor, the electrode 8 and the electrode 9 ( Alternatively, it was possible to increase the resistance value of the path of the creeping current generated between the electrode 11). In the figure, the case where the entire space between the electrodes is covered with the insulating coating film 10 is shown, but even when the space between the electrodes is partially covered with the insulating coating film 10, the resistance of the flame sensor surface between the electrodes is reduced. Can be higher, and as a result,
Similarly, the dark current can be reduced.

Next, another method for increasing the resistance value between the electrodes will be described with reference to FIG. 2 shows the case of a PIN type flame sensor, which is shown in FIG.
Similar to the example illustrated in (a), it can be applied to various flame sensors such as a Schottky diode type or a phototransistor type.

The respective semiconductor layers constituting the flame sensor illustrated in FIGS. 2A and 2B are the same as those illustrated in FIG. 1B, and the low temperature deposition buffer layer 2 is formed on the substrate 1. When,
Crystal improvement layer 3, low-temperature deposited intermediate layer 4, and n-type semiconductor layer 5
And an i-type semiconductor layer 6 serving as a light receiving region and a p-type semiconductor layer 7
And are sequentially deposited to be formed. The electrodes 8 and 11 are formed on the two surfaces that sandwich the step. However, unlike the flame sensor illustrated in FIGS. 1A and 1B, the flame sensor described here is not provided with an insulating coating.

First, in the flame sensor illustrated in FIG. 2A, the step portion between the surface on which the electrode 8 is formed and the surface on which the electrode 11 is formed is formed by two small step-like steps. Is forming. As a result, a boundary portion 12 parallel to the substrate is formed between the two small steps. The boundary portion 12 is provided in the high-resistance i-type semiconductor layer 6, and the electrode 8 and the electrode 1 are provided.
The length of the creeping current path generated between the electrode 8 and the electrode 1 is increased in the high resistance portion, and as a result, the resistance value between the electrode 8 and the electrode 11 is significantly increased. That is, the boundary portion 12 acts as a high resistance structure.

The step-like small step shown in FIG. 2A can be formed by partially and stepwise etching the device structure. For example, after the device structure (n-type semiconductor layer 5, i-type semiconductor layer 6, p-type semiconductor layer 7) is deposited on the underlying structure, the p-type semiconductor layer 7 and the i-type semiconductor layer 6 are partially etched by dry etching. Partially removed from above. The etching depth is adjusted by changing the etching time. Then, the exposed parts of the i-type semiconductor layer 6 and the n-type semiconductor layer 5 are partially removed from above. As a result, step-like small steps are formed as shown in the drawing, and the boundary portion 12 is formed between the small steps. As a result, the electrode 8 and the electrode 11 are provided so as to sandwich the two-step small step and the boundary portion 12. By using the same etching method, it is possible to provide more steps in steps.

Further, in the flame sensor illustrated in FIG. 2B, the boundary 12 between the two small steps has the i-type semiconductor layer 6 and p.
By being provided at the interface with the i-type semiconductor layer 7, the exposed area of the i-type semiconductor layer 6 (that is, the area of the boundary portion 12) increases, and the i-type semiconductor layer 6 on the path of the creeping current generated between the electrode 8 and the electrode 11 increases. The length of the high resistance part becomes longer. As a result, the creeping current between the electrodes 8 and 11 can be reduced, and the dark current in the flame sensor can be reduced. Figure 2
The step-like small step shown in (b) can be formed by etching as in the case of FIG. 2 (a). In addition to dry etching, wet etching can be used. For example, it is possible to use a selective etching method capable of removing only a semiconductor layer having a specific polarity by applying a current while irradiating ultraviolet rays in an acid.

Generally, a high resistance i in a semiconductor laminated structure is used.
Since the type semiconductor layer (light receiving layer) is formed very thin, i
When an electric field is applied between a pair of electrodes sandwiching the i-type semiconductor layer, the length of the i-type semiconductor layer included in the path of the creeping current generated between the electrodes on the surface of the semiconductor laminated structure (i-type semiconductor layer Corresponding to the thickness of) becomes very short. On the other hand, the above i
Since the type semiconductor layer (light receiving layer) is formed in a large area for use as a light receiving element, the boundary portion 12 parallel to the substrate surface is much larger than the cross-sectional area (thickness) of the i type semiconductor layer. It can be set large. Therefore, as described above, the boundary portion 12 is provided in the high-resistance i-type semiconductor 6,
Alternatively, it can be said that the effect provided on the i-type semiconductor layer 6 is very high.

The flame sensor shown in FIG. 3 is a combination of the flame sensors illustrated in FIGS. 1 (b) and 2 (a), and the boundary portion 12 between the step-like small steps in the i-type semiconductor layer 6 is formed.
And an insulating coating (high resistance structure) 10 is formed on a step portion (including a boundary portion between small steps) between the electrode 8 and the electrode 11. As a result, the value of the surface resistance on the path of the creeping current generated between the electrode 8 and the electrode 11 can be made extremely high, and the length of the high resistance portion on the path of the creeping current can be lengthened. . As a result, the creeping current between the electrodes 8 and 11 can be greatly reduced, and the dark current in the flame sensor can be made extremely small.

The band gap energy of the device structure when used as a flame sensor will be described below. Single crystal Al _x Ga _{1 -x} N constituting the device structure (n-type semiconductor layer 5, i-type semiconductor layer 6, p-type semiconductor layer 7)
The bandgap energy of (0 ≦ x ≦ 1) is adjusted by changing the aluminum composition ratio, and the aluminum composition ratio x and the bandgap energy are represented by the relationship shown in FIG. As can be seen from FIG. 4, by changing the aluminum composition ratio x, the band gap energy of Al _x Ga _{1 -x} N can be adjusted within the range of about 3.42 eV to about 6.2 eV. Therefore, the cutoff wavelength of the light receiving region can be adjusted between about 200 nm and about 363 nm. Further, in the case of detecting flame light in the ultraviolet light receiving element, it is sufficient to form a light receiving region having band gap energy for absorbing the light emission of the flame as shown in the emission spectrum of FIG. The emission spectrum of the flame shown in FIG. 5 is the spectrum of the flame generated when gas (hydrocarbon) is burned. Further, the spectrum of sunlight and the spectrum of room light due to light from various lighting devices are also shown at the same time.

Below, AlGa relating to the formation of the light receiving region
The band gap energy of N will be described. In order to provide the ultraviolet light receiving element with wavelength selectivity, the composition ratio of Al in the light receiving region (Al _x Ga _{1 -x} N) is adjusted,
The band gap energy is set to a desired value. For example, when it is desired to manufacture a flame sensor capable of selectively receiving the light of the flame that appears with a relatively large intensity in the wavelength range of about 344 nm or less, the bandgap energy of the light receiving region is 3.6 eV or more. Thus, the aluminum composition ratio x may be 0.05 or more. Alternatively, when it is desired to manufacture a flame sensor that receives light of flame in the detection target wavelength range without receiving light (indoor light) from various lighting devices included in the wavelength range of about 300 nm or more, The aluminum composition ratio x may be set to 0.25 or more so that the bandgap energy of the light receiving region is 4.1 eV or more. Alternatively, when it is desired to manufacture a flame sensor that receives only the light of the flame within the detection target wavelength range without receiving the light from sunlight included in the wavelength range of about 280 nm or more, the light receiving region Of aluminum composition ratio x =
It may be 0.35 or more.

Alternatively, if ambient light such as sunlight may be absorbed in the light receiving region as long as the light intensity is low, the band gap energy of the light receiving region is 4.3 eV or more (wavelength of about 290 nm or less). Thus, the aluminum composition ratio x should be 0.31 or higher. At a wavelength of about 290 nm or less, the light intensity of the ambient light becomes very small as shown in FIG. 5, and the light of the flame is large on the other hand, and as a result, the presence of the light of the flame can be detected.

Further, when the ultraviolet light receiving element is installed in a closed space such as the inside of the engine and it is desired to detect the light emission of the fuel burned therein, the above-mentioned room light and sunlight do not exist, so those are excluded. It is not necessary to set a very large bandgap energy. Therefore, among the light of the flame in the detection target wavelength range, a light emission peak (wavelength of approximately 310 nm (wavelength of about 310 nm ( 310 nm ± 10 nm): 4.0
eV) (light of a flame having a wavelength of 310 nm or more and 344 nm or less) is manufactured, the bandgap energy of the light receiving region is 3.6 eV or more and 4.0 eV or less. As described above, the aluminum composition ratio x may be 0.05 or more and 0.23 or less.

Further, not only the bandgap energy of the light receiving region (i-type semiconductor layer 6) but also the bandgap energy of the semiconductor layer provided around the i-type semiconductor layer 6 may be adjusted. . For example, in FIG.
In the PIN-type flame sensor shown in (b), the p-type semiconductor layer 7 arranged on the light incident surface side with respect to the i-type semiconductor layer 6
When the bandgap energy of is adjusted to be larger than the bandgap energy of the i-type semiconductor layer 6, the p-type semiconductor layer 7 acts as a light transmitting layer, and the i-type semiconductor layer 6 which is the light receiving region has a large intensity. Light is incident. Further, when the bandgap energy of the n-type semiconductor layer 5 arranged on the substrate 1 side of the i-type semiconductor layer 6 is set to be larger than the bandgap energy of the i-type semiconductor layer, it is absorbed in the light receiving region. A flame sensor that absorbs light on the lower energy side (light on the longer wavelength side) than light and does not have the problem of being sensitive to light outside the detection target wavelength range (in this case, sunlight) can do.

The above-mentioned relationship between the aluminum composition ratio x and the band gap energy is explained based on the theoretical value, and even if the film formation is performed so that the aluminum composition ratio x becomes the same, it is actually obtained. The bandgap energies of the AlGaN layers formed may be different. For example, in the case of AlGaN which is a ternary mixed crystal compound, GaN which is a binary compound is easily generated, and as a result, the band gap energy tends to shift to the low energy side (long wavelength side). Therefore, when it is desired to obtain the bandgap energy according to the theoretical value, the aluminum composition ratio may be set to a large value before the film formation.

The flame sensor according to the present invention has been described by taking as an example the flame sensor having the Schottky diode type and PIN type device structures as described above, but the element structure of the flame sensor is limited to the above. However, various other semiconductor laminated structures can be adopted. For example, the same applies when the device structure is a PN type and the light receiving region is formed in the depletion layer region of the pn junction.

[Brief description of drawings]

FIG. 1A is a configuration diagram of a Schottky diode type flame sensor, and FIG. 1B is a configuration diagram of a PIN type flame sensor.

FIG. 2A and FIG. 2B are other configuration diagrams of a PIN type flame sensor.

FIG. 3 is another configuration diagram of a PIN type flame sensor.

FIG. 4 is a graph showing the band gap energy of AlGaN.

FIG. 5 is a graph showing spectra of flame light, sunlight, and room light.

[Explanation of symbols]

1 substrate 2 Low temperature deposition buffer layer 3 Crystal improvement layer 4 Low temperature sedimentary intermediate layer 5 n-type semiconductor layer 6 i-type semiconductor layer (light receiving area) 7 p-type semiconductor layer 8 electrodes 9 electrodes 10 Insulating film (high resistance structure) 11 electrodes 12 Boundary (high resistance structure)

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2G065 AA04 BA09 CA23 CA25 DA06                 4M118 AA05 AB00 BA01 CA05 CA06                       CA19 CA32 CB01 CB14 HA25                 5F049 MA04 MA05 MB07 NA05 NA07                       NB07 PA14 QA02 QA18 QA20                       SS01 WA05

Claims (3)

[Claims] 1. A semiconductor laminated structure comprising: a semiconductor laminated structure in which a plurality of semiconductor layers are laminated on a substrate; and a step formed by removing a part of the semiconductor laminated structure to a predetermined depth is sandwiched. A first electrode is formed on one of the two surfaces and a second electrode is formed on the other surface, and a high resistance structure is formed on at least a part of the surface of the step between the first electrode and the second electrode. Flame sensor that is formed. 2. The flame sensor according to claim 1, wherein the high resistance structure is an insulating coating formed on at least a part of the step between the first electrode and the second electrode. 3. The small step formed between the first electrode and the second electrode is formed in a stepwise shape including a plurality of small steps, and the high resistance structure is formed in a high resistance semiconductor layer. The flame sensor according to claim 1 or 2, which is a boundary portion between steps.