JPH04307770A - light sensor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a photoconductive type optical sensor with low dark current.

0002

[Prior Art] Semiconductor optical sensors include photodiodes and avalanche photodiodes, which apply a reverse bias voltage to a pn junction and measure the current flowing in response to light irradiation, and sensors that utilize the change in resistance caused by light irradiation. There is a photoconductive type. The present invention relates to the latter improvement. This is a simple device with an electrode attached to a semiconductor. Since electron-hole pairs are excited by light irradiation, carriers increase and electrical resistance decreases. This increases the photocurrent. It is also called a photoresistive element.

[0003] The oldest and most widely used is Cd.
It is an S cell. This has high sensitivity in the visible light range, so it has high utility value. Other photoconductive cells include CdSe, PbS, and PbSe. Naturally, a semiconductor with a narrow band gap is used for light with a long wavelength such as infrared light. The CdS conductive layer may be single crystal, polycrystalline, or sintered polycrystalline. This is CdS on a ceramic substrate.
Many have a sintered body. These photoconductive cells are described in, for example, "Semiconductor Devices" S. M. Written by G, Sangyo Tosho Co., Ltd. p.
293-295 “Optical Technology Utilization Handbook” Optronics, p.23
2 etc.

0004

[Problems to be Solved by the Invention] Conventional photoresistive elements and photoconductive elements have a drawback of large dark current. This is p
This can be said to be a drawback compared to a photodiode which reverse biases the n-junction. Unlike photodiodes, photoconductive elements do not generate carriers due to interband transition caused by light, but instead generate carriers by exciting doped impurity electrons to the conduction band as light absorption centers. A carrier is generated. Since carriers exist even in the dark (when no light is irradiated), the dark current is large. The dark current of the photoconductive element is

00
05]

[Math 1]

It is shown by [0006]. Here A is a constant of proportionality,
n is the carrier concentration of the conductive layer in the dark, and d is the thickness of the conductive layer. The proportionality constant A is

[0007]

[Math 2]

It is represented by: q is the elementary charge, μ is the carrier mobility, l is the distance between the electrodes, V is the applied voltage, and ω is the width of the electrode. Equations 1 and 2 also hold true during light irradiation, but since the value of n differs between irradiation and darkness, the current will differ. Since the value of n is quite large even in the dark, the dark current is large. It is an object of the present invention to provide a photoconductive type optical sensor with reduced dark current.

[0009]

[Means for Solving the Problems] In a photoconductive cell, since the conductive layer is in contact with the atmosphere or a high-resistance semiconductor substrate, a depletion layer is formed at the interface. Traditionally, the conduction layer has been much thicker than the depletion layer. However, in the present invention, the conductive layer is made extremely thin, making it thinner than the depletion layer in the dark. That is, the optical sensor of the present invention includes a high-resistance semiconductor substrate, a conductive layer formed on the high-resistance semiconductor substrate and having a deep level serving as a light absorption center, and two ohmic layers provided on the conductive layer. The thickness d of the conductive layer including the connecting electrode is larger than the thickness d0 of the depletion layer during light irradiation, and the thickness d1 of the depletion layer during darkness.
It is characterized by being smaller.

0010

[Operation] In conventional photoconductive photosensors, the conductive layer is much thicker than the depletion layer, and the depletion layer has no function. In other words, it can be said that conventional optical sensors ignore the depletion layer. The present invention attempts to actively utilize the depletion layer. A depletion layer is a region where carriers are removed and only impurities such as donors and acceptors remain because the potential differs at the boundary between two substances, so a charge equal to the potential must be generated. It is. When the potential difference is large, the depletion layer becomes thick, and when there are many impurities, the depletion layer becomes thin. Since there are no carriers, it is called a depletion layer. Since there are no carriers, no current flows in the depletion layer other than tunnel current.

In the present invention, since the thickness d of the conductive layer is made thinner than the thickness d1 of the depletion layer in the dark, there are no carriers in the conductive layer in the dark. This means that n in number 1 is 0. Therefore, the dark current becomes smaller. In this way, the depletion layer is actively used, and the entire conductive layer becomes a depletion layer during darkness. This can be called an epoch-making photoconductive element.

However, on the other hand, if the conductive layer is a depletion layer, there seems to be a drawback that there are no carriers, no current flows, and no light is sensed. But that's not the case. The depletion layer is thought to have a constant thickness. This is because it depends on the impurity concentration and the potential difference between the two substances. If that is the case, the thickness of the depletion layer should be constant. However, this is not the case. In the case of photoconductive photosensors, they are always doped with impurities that absorb light and release carriers.
This forms a deep level in the forbidden band between the conduction band and the valence band. Why does it have to be deep? This is because if something creates a shallow level, it is easily excited by temperature and always exists as carriers. It must be a deep level that cannot be excited by heat but can only be excited by light irradiation.

The conductive layer is made of an n-type semiconductor, and is doped with an impurity at a light absorption center that forms a deep level of a donor type, for example. The concentrations of donor, acceptor, and light absorption center are ND and N
Let A , NT be neutral when the absorption center is not excited. The positive charge concentration in this part is (ND
-NA). On the other hand, when the absorption center is excited by light, the positive charge concentration in this part is (ND - NA + αNT), where the probability of excitation is α. α changes depending on the wavelength and intensity of light (0<α<1
). In this way, the concentration of positive charges changes due to light irradiation, and therefore the thickness of the depletion layer also changes. Fortunately, light irradiation increases positive charges, which thins the depletion layer. Therefore, in the present invention, the thickness d of the conductive layer is made larger than the thickness d0 of the depletion layer during light irradiation. (d
-d0) is not a depletion layer but has carriers. Since a current flows here, a photocurrent is generated.

As described above, the present invention focuses on the fact that the depletion layer becomes thinner when irradiated with light and becomes thicker when it is in the dark, and the thickness of the conductive layer is set to be an intermediate thickness between the two. In this way, dark current can be kept low. FIG. 2 shows a band diagram near the surface of the photoconductive element in the dark. The thickness of the conductive layer is d. Deeper than this is a high-resistance board. There are a surface depletion layer and an interface depletion layer on both sides of the conductive layer, and the thicknesses of each are indicated by ds and di. Since it is assumed to be n-type, there are many donor levels and few acceptor levels. There is a donor level ND near the conduction band and an acceptor level NA near the valence band. All of these are close to the band and emit carriers even at room temperature. In addition to these two shallow impurity levels, there is a deep absorption center level NT.

The depletion layer is a region where impurities release carriers and become charged in order to fill the difference in potential, but from the electrostatic Maxwell's equation, etc.

[
0016

[Math 3]

[0017]

[Math 4]

[0018]

[Math 5]

The following relationship holds true. Here, the depth from the surface is taken in the x-axis direction and can be considered as a one-dimensional problem. D is electric flux density, E is electric field, φ is potential, and ρ is charge density. In the depletion layer in the dark, ρ=q(N
D-NA) is constant. The surface potential is φs
Then these formulas are

[0020]

[Math 6]

[0021]

[0022]

[Math 7]

[0023] From here, the thickness of the surface depletion layer ds
but

[0024]

[Math. 8]

[0025] There is also a depletion layer at the interface, but if the interface potential is φi, then this thickness di is

[0026]

[Math. 9]

[0027] If the thickness of the intermediate layer is dc, then
Conventionally, the thickness d of the conductive layer was d=ds +dc +di. The carrier density of the intermediate layer dc is (ND
-NA). FIG. 3 is a band diagram of the conductive layer upon irradiation with light. The diagram shows that all absorption centers NT are excited by light. Since the electrons are excited to the conduction band, the electron density in the intermediate layer increases and the depletion layer becomes thinner. If the light absorption center is excited by α by light irradiation, the positive charge increases by αNT. Then, the thicknesses ds ′ and di ′ of the surface depletion layer and interface depletion layer during light irradiation are as follows.

[0028]

[Math. 10]

[0029]

[Math. 11]

The electron density of the intermediate layer is (ND −NA +α
NT). As shown in FIGS. 2 and 3, conventional photoconductive elements have sufficiently thick conductive layers, and depletion layers on surfaces and interfaces do not pose a problem. However, in the present invention, the conductive layer is made sufficiently thin so that no intermediate layer exists in the dark. This state is shown in FIG. In other words, in the present invention

0031
]

[Math. 12]

[0032] Carrier concentration (ND
-NA), so no current flows in the dark. The dark current is only the current flowing through the substrate. However, if d is too thin, no current will flow even during light irradiation, so the intermediate layer dc' is made positive during light irradiation. In other words, in the present invention,

[0033]

[Math. 13]

[0034] A band diagram at this time is shown in FIG. d1 and d0 are numbers 8 to 11
According to

[0035]

[Math. 14]

[0036]

[Math. 15]

[0037] Let us consider the ratio of photocurrent/dark current in the present invention and the conventional structure. In the conventional structure, the conduction layer is thick and the depletion layer is not a problem. The carrier concentration during irradiation is n′,
In the conventional structure, assuming the carrier concentration in the dark as n,

0
038]

[Math. 16]

[0039] In the present invention, since the dark current is only the current Ansub dsub flowing through the substrate,

[0040]

[Math. 17]

According to the present invention, an extremely large ratio can be obtained. For example, ND = 1 x 1016 cm-3, NA = 1 x 1
015 cm-3, NT = 5 x 1016 cm-3, α=
1, the current ratio in the conventional example is n'/n=5.9×1016/0.9×1016=6.
It becomes 6. At most, this is the only ratio that can be taken. However, in the case of the present invention, the above ND, NA,
For GaAs, d0 = ds for the value of NT and α
'+di'=0.12μm+0.13μm=0.25
It becomes μm. d1=ds+di=0.3μm+0
．． 34 μm=0.64 μm. High resistance GaAs substrate nsub is 1×107 cm-3, thickness dsub
is 400 μm, the thickness d of the conductive layer is 0.64 μm.
Then, the ratio of the current during irradiation and during darkness is n'(d-d0)/nsub dsub =5.8×
It becomes 106. It can be seen that the dark current is smaller in the present invention.

FIG. 1 is a graph showing the relationship between the conductive layer thickness d, dark current, and photocurrent for intuitively explaining the effect of the present invention. The horizontal axis is the thickness d of the conductive layer, and the vertical axis is the current. d0 on the horizontal axis is the depletion layer thickness at the time of light irradiation. The carrier concentration during light irradiation is n' (=ND −NA +αNT
), the magnitude of the photocurrent is An'(d-d0
). d1 is the depletion layer thickness in the dark. The carrier concentration at this time is n (=ND - NA)
Therefore, the magnitude of the dark current is An(d−d1). Since the conventional conductive layer thickness d is much larger than d1, the ratio of bright and dark currents is proportional to n'/n. In the present invention, d0 <d≦d1, so if the substrate current is ignored, the ratio of bright and dark currents is infinite. Above is n
Although the description has been made regarding a p-type conductive layer, the same thing is possible with a p-type conductive layer.

[0043]

[Example] An AlGaAs film of approximately 2 μm was deposited on a GaAs substrate.
The film was epitaxially grown by OMVPE to a thickness of m. The Al composition is 0.2. Oxygen is added as a light absorption center. The electrical conductivity was p-type and the carrier concentration was approximately 1.times.10.sup.15 cm.sup.-3 in the dark (this carrier concentration was confirmed by measurements of thicker AlGaAs layers). In order to have the element structure as shown in Figure 4,
Au/Zn/Au electrode thickness 100 Å/300 Å/15
The film was deposited to a thickness of 00 Å. The width ω of the electrodes is 4 mm, and the interval l is 1 mm. An ohmic connection electrode was formed by alloying at 400° C. for 1 minute. This optical sensor showed good sensitivity to light with a wavelength of about 1 μm. Dark current was 50 nA. During light irradiation (λ = 1 μm, intensity 10
The photocurrent (mW/cm) was 0.5 mA. The ratio of bright and dark currents in this case reaches 104.

[0044]

According to the present invention, since the thickness d of the conductive layer is made smaller than the thickness d1 of the depletion layer in the dark, it is possible to provide a photoconductive type optical sensor with extremely low dark current. In addition, by changing the type of element that creates the deep level that is the center of light absorption, the depth of the energy level ΔET can be changed.
The wavelength of light that provides optimal sensitivity can be varied.

[Brief explanation of drawings]

[Fig. 1] In a photoconductive type optical sensor, the conductive layer thickness and
A graph showing the relationship between dark current and photocurrent.

FIG. 2 is a band diagram of a conductive layer in the dark in a conventional optical sensor.

FIG. 3 is a band diagram of a conductive layer during light irradiation in a conventional optical sensor.

FIG. 4 is a schematic diagram of the current measurement system of the optical sensor of the present invention.

FIG. 5 is a band diagram of the conductive layer of the optical sensor of the present invention in the dark.

FIG. 6 is a band diagram of the conductive layer of the optical sensor of the present invention when irradiated with light.

Claims (4)

[Claims] Claim 1: A high-resistance semiconductor substrate, a conduction layer formed on the high-resistance semiconductor substrate and having a deep level serving as a light absorption center, and two ohmic connection electrodes provided on the conduction layer. An optical sensor comprising: a conductive layer having a thickness d greater than a depletion layer thickness d0 during light irradiation and smaller than a depletion layer thickness d1 during darkness. Claim 2: A high resistance GaAs substrate and a high resistance GaAs substrate.
An oxygen-doped AlGaAs conductive layer formed on the S substrate and an Au/Z conductive layer provided on the AlGaAs conductive layer.
including two ohmic connection electrodes made of n/Au,
An optical sensor characterized in that the thickness d of the AlGaAs conductive layer is larger than the thickness d0 of the depletion layer during light irradiation and smaller than the thickness d1 of the depletion layer during darkness. 3. A high-resistance semiconductor substrate, an n-type conductive layer formed on the high-resistance semiconductor substrate and having a deep donor level serving as a light absorption center, and two optical conductive layers provided on the conductive layer.
The surface potential of the conductive layer is φs, the interface potential of the conductive layer is φi, the dielectric constant of the conductive layer is ε, the n-type impurity concentration is ND, the p-type impurity concentration is NA, and the light absorption center is The concentration of donor impurity is NT
When the thickness d of the n-type conductive layer is d1 = (2ε)1/2 {q(ND −
NA)}-1(φs1/2+φi1/2)
d0 = (2ε)1/2 {q(ND −NA +N
An optical sensor characterized in that d0 < d < d1 with respect to d1 and d0 defined by T )}-1 (φs1/2+φi1/2). 4. A high-resistance semiconductor substrate, an n-type conductive layer formed on the high-resistance semiconductor substrate and having a deep acceptor level serving as a light absorption center, and two ohmic conductive layers provided on the conductive layer. The surface potential of the conductive layer is φs, the interfacial potential of the conductive layer is φi, the dielectric constant of the conductive layer is ε, the n-type impurity concentration is ND, the p-type impurity concentration is NA, and the acceptor is the center of light absorption. When the impurity concentration is NT, the thickness d of the n-type conductive layer is d1 = (2ε)1/2 {q(ND −
NA −NT )}-1(φs1/2+φi1/2)
d0 = (2ε)1/2 {q(ND −N
An optical sensor characterized in that d0 < d < d1 with respect to d1 and d0 defined by A)}-1 (φs1/2+φi1/2).