RU2240631C1 - Photodetector - Google Patents

Abstract

FIELD: semiconductor electronics.

SUBSTANCE: ratio of areas of pn junction structure components and gap between pn junction and contact to substrate is chosen in avalanche-multiplication silicon lateral photodetector. Spatial charge area of pn junction fully covers gap between pn ⁺ junction and pn ⁺ contact to substrate. Avalanche multiplication of charge carriers is ensured by applying operating voltage due to concentration of electric field about small-size n ⁺ and p ⁺ areas.

EFFECT: reduced capacitance, enhanced speed and photosensitivity.

3 cl, 7 dwg

Description

The present invention relates to semiconductor electronics, semiconductor devices having sensitivity to radiation.

Semiconductor sensors of the intensity and spectrum of radiation are becoming more widespread in integrated microsystems due to the possibility of combining them with other components of microsystems using microelectronics methods and the creation of microminiature devices for monitoring and control in automated complexes for various purposes.

Among other types of semiconductor sensors of intensity and emission spectrum, the photodiode is of particular interest in connection with the possibility of obtaining high speed and photosensitivity. Details of this device are described in / 1 /. The photosensitivity of the photodiode is caused by the generation of light quanta of electron-hole pairs of current carriers that penetrate into the space charge of the pn junction, which, when a voltage is applied, create conductivity in the pn junction space charge layer. To increase the photosensitivity of the diode, device structures are developed in which the sizes, composition and absorption spectrum of all layers of the photodiode are selected, which provide the most complete conversion of light quanta into current carriers and avalanche multiplication in the electric field of the space charge layer is used, which allows to increase the photoelectric effect.

Many types of photodetectors have a three-dimensional structure, in which electrodes in contact with the areas of the cathode and anode of the diode are located on opposite sides of the semiconductor wafer.

A semiconductor ultraviolet radiation sensor with a given detection range / 2 / has a rectifying contact with a Schottky barrier formed on an epitaxial n-layer grown on an n + silicon carbide substrate. The wave range is provided by the choice of thickness and material of the metallization of the Schottky contact.

The patent / 3 / proposes a semiconductor ionizing radiation detector with a circular shape of an additional electrode located around the first electrode at a distance at least equal to the double size of the space charge region, which allows to reduce leakage currents and thereby increase the sensitivity.

The patent / 4 / proposes a widely used silicon semiconductor n + -p-p + -p-p + avalanche photodiode with a volumetric structure - n + (layer) -p (multiplication region) -p + (concentrator) -p (drift region) -p + (contact with the substrate from the bottom) - with an increased concentration of impurities at a given depth in the lightly doped p-region of the diode to separate the areas of avalanche multiplication and drift of photogenerated current carriers.

The patent / 5 / proposes the construction of a semiconductor photodiode with a pinch resistor region at the periphery of the pn junction to ensure the photodiode is operable at high voltages.

The patent / 6 / proposes a silicon low-noise avalanche photodiode with the sequence of arrangement of the nnn ++ - nnn ++ regions with the formation of a Schottky contact with region n.

The patent / 7 / proposes a soft X-ray detector with a structure including a layer for converting X-ray quanta into quanta of the visible wavelength range. Silicon photodetectors are shielded by the conversion layer and therefore only transformed quanta are perceived.

The patent / 8 / proposes the construction of a semiconductor avalanche photodiode with p + -n-p-n + mesa structure and protective peripheral p-diffusion to eliminate side effects. This design is essentially tetrode, but with floating base areas.

It is also known that a phototransistor [1] can serve as a photodetector with a large current gain, in particular, when turned on with a torn base, i.e. with two contacts, like a photodiode.

In the first phototransistors [9–12], the collector – base photocurrent arising from illumination of the pn junction was amplified by the injection current from the emitter.

In the patent [9], the voltage regulator had a photodiode connected in parallel with the collector-base junction of the bipolar transistor.

In the patent [10], the photodiode was connected parallel to the collector-base junction directly in the structure of the phototransistor, which increased the collector-base junction area and, accordingly, the conversion coefficient.

In the patent [11], a semiconductor device with a reduced collector-base junction capacitance in a phototriode due to doping of the substrate to create pn junctions is proposed.

The creation of a phototransistor with avalanche multiplication near the collector-base junction [12] made it possible to further increase the photocurrent gain.

Creating a structure of phototransistors [13] with a varying composition based on complex semiconductor compounds forming heterojunctions also allows increasing the photocurrent gain.

The disadvantage of phototransistors is the low speed, especially with a small amount of light flux, due to the large capacity of the emitter.

The considered bulk structures can turn into planar structures when creating regions in a semiconductor that connect the surface of the structure to the base, i.e. with a semiconductor substrate. In the structures of photodiodes formed on a dielectric substrate, contacts to semiconductor regions are formed mainly on the surface of the device. In some cases, it is possible to deposit electrodes on a substrate before applying a semiconductor layer and thereby obtain conclusions from the lower side of the semiconductor layer by partially etching the upper layers to expose the lower layer and create contact from the upper side.

Features of creating the most suitable for integrated performance planar photodetectors with electrodes located on one side of the substrate are reflected in the following works.

The patent / 14 / proposes a photodetector for detecting infrared radiation with high photoelectric gain at high values of quantum efficiency based on an impurity compensated semiconductor with several contacts on one side of the device located at a distance that ensures the flow between the electrodes of the current limited by the space charge (SCLC), which is controlled by a photogenerated charge in the semiconductor volume.

The patent / 15 / proposes a device for implementing internal proportional amplification in a semiconductor germanium particle and radiation detector, in which the distance between the strips (electrodes) is selected according to a formula that ensures the creation of an electric field strength near one of the electrodes with a value sufficient for avalanche multiplication of current carriers and increases due to this quantum efficiency and photoelectric gain.

The patent / 16 / proposes a silicon semiconductor infrared photodiode with the formation of narrow Schottky contacts to the photosensitive region, as well as on its periphery, to ensure protection.

The closest analogue adopted by us for the prototype is the patent / 17 /. The patent proposes a high-speed silicon lateral photodetector with recessed electrodes and with thick oxide to reduce the influence of edge fields. A photodetector compatible with silicon VLSI technology has a metal-semiconductor-metal (MPM-FD) or lateral p-i-n photodetector (LPS-FD) structure in which interdigital (comb) electrodes are placed on the surface of a silicon substrate. MPM-FD electrodes have moderate barrier heights for electrons and holes relative to silicon during the formation of the Schottky barrier and are made in such a way as to penetrate the surface of a silicon semiconductor layer using self-aligned metallization technology during selective deposition, selective reaction, or etching used in the silicide technology. Production begins with the growth of an oxide film on the surface of a silicon substrate with a low recombination rate at the silicon – silicon oxide interface. The interdigital (comb) configuration is etched using photolithography in oxide, after which the metal electrodes are selectively introduced onto the silicon surface using self-aligned metallization to create thin interdigital (comb) electrodes on the active layer, which itself can be made using silicon, germanium, or thin-film technology their alloys. The distance between the electrodes can be chosen small to ensure a short transfer time and increase sensitivity. Further, insulating layers are applied to the surface, which should be: 1) transparent and insulating; 2) with optical absorption; 3) with optical reflection, so that the photogenerated carriers do not go aside without recombining. In the latter case, the photodetector acts as a resonator, increasing the number of generated carriers and thereby increasing the sensitivity of the device.

The main disadvantage of this device is that the photosensitive regions are located mainly under the electrodes of a relatively large area. These electrodes have a large capacitance that limits the speed of the photodetector, and they shield the space charge region, absorb light, and reduce sensitivity.

The purpose of the invention is to reduce the capacity of the photodetector, increase speed and increase photosensitivity.

The essence of the invention is to change the ratio of the sizes of the structural elements — the pn junction and the gap between the pn junction and the contact to the substrate — in the lateral diode photodetector with avalanche multiplication, taking into account the thickness of the pn junction space charge layer.

The photodetector consists of a lightly doped p-type silicon substrate, n + diffusion regions creating a pn junction with the substrate, p + heavily doped contact regions to the substrate, metal contacts to the n + region and p + contacts to the substrate. The sizes of the regions n + and p + are chosen so small that, near each of them, when the voltage is connected, an electric field concentration and an avalanche multiplication of charge carriers occur. The relatively small size and low doping level of the region in the substrate in the gap between the n + region and the p + contact to the substrate allow the space charge of the n + p junction to reach the p + contact to the substrate to propagate, and the gap between the electrodes is significantly larger than the area of the electrodes themselves. By reducing the area n + of the region with respect to the size of the photosensitive surface, the photodetector capacitance decreases and the speed increases. By creating a grid structure of electrodes with illumination from the surface of the space charge regions in the grid cells from n + pp + transitions, the concentration of the electric field in regions of small size n + regions and p + contact to the substrate, and avalanche multiplication in strong electric fields, the photosensitivity increases. Such a structure and mode of operation with regions of high electric field concentration near the n + region and p + junction create separate regions of charge carrier photogeneration in the space charge region of the n + p junction, starting from the surface of the device and avalanche multiplication near the n + region and p + junction to the substrate .

Figure 1 shows the cross section of a photodetector. The topology of the planar structure of the photodetector element is presented in figure 2. The distribution of the shock ionization rate in the structure of the device in the operating mode is given in Fig.3. A conditional image of the photogeneration of charge carriers and their multiplication in the volume of the photodetector is given in Fig.4. The dependence of the magnitude of the photocurrent on the applied voltage is shown in Fig.5. Figure 6 shows a cross section of a photodetector with a Schottky barrier at the contact to the n + region. 7 shows a cross section of a photodetector having a lightly doped region n of the type of conductivity in the gap between the n + region and the p + contact to the substrate.

Figure 1 shows a cross section of a lateral photodetector, where the device consists of a single-crystal silicon substrate of the first p-type conductivity (1) with the main surface (2) and the reverse side (3); n + region (4), located on the side of the main surface of the substrate and having a high concentration of impurities, creating a second n-type conductivity, metal contacts to n + regions (5) and to regions of p + contacts to the substrate (6), which are heavily doped with impurity, creating the first p type of conductivity, the space charge regions of the pn junction (7) in the gap between the n + region (4) and the p + contacts to the substrate (6) and in the substrate volume deeper than the n + region (4).

Figure 2 shows the topology of the planar structure of one photodetector element, which can be combined into a matrix, where the photodetector element consists of a lightly doped p-type silicon substrate (1); from n + regions (2), regions of p + contacts to the substrate (3), with a distance between n + region (2) and regions of p + contacts (3) less than the thickness of the space charge layer (4) in the n + p junction at an operating voltage .

Figure 3 shows in the form of isolines the distribution of the impact ionization rate (1 / sec / cm ³ ) in the structure of the device cell when the operating voltage is turned on, for example, 50 V to the contacts to n + regions (1) relative to p + contacts to the substrate (2), at which the n + p transition shifts in the opposite direction. The space charge region of the n + p junction propagates in the substrate and reaches the p + junction. When the device (3) is illuminated from the main surface in the space charge region (4), photo-generation of charge carriers occurs. A photocurrent flows between n + and p + regions. Near the n + and p + regions, the concentration of electric fields occurs, and in the avalanche multiplication regions (5), an increase in the concentration of charge carriers and an increase in current occur.

Figure 4 shows a cross section of the lateral photodetector with a conditional image of how in the region of the space charge (1) the incident light quanta (2) generate holes (3) and electrons (4), which move, respectively, to p + and n + regions and near these regions there is an avalanche multiplication of charge carriers (5).

Figure 5 shows the dependence of the relative magnitude of the photocurrent on the voltage at the photodetector at a certain light intensity.

In Fig.6. shows a cross section of the lateral photodetector with a Schottky contact to the n + region, where the device consists of a single-crystal substrate of the first type of conductivity (1) with the main surface (2) and the reverse side (3); n + region (4), located on the side of the main surface of the substrate and having a high concentration of impurities, creating a second type of conductivity; metal contacts to the n + region (5) and to the regions of p + contacts to the substrate (6), which are heavily doped with an impurity that creates the first type of conductivity, the space charge regions of the pn junction (7) in the gap between the n + region (4) and p + contacts to the substrate (6) and deeper than the n + region (5), the space charge region (8), formed due to the contact potential difference of the metal electrode and the semiconductor n + region and which blocks the capacitance of the n + p junction, reducing the total capacitance of the photodetector.

7 shows a cross section of the lateral photodetector, where the device consists of a single-crystal substrate of the first type of conductivity (1) with the main surface (2) and the reverse side (3); n + region (4), located on the side of the main surface of the substrate and having a high concentration of impurities, creating a second type of conductivity; metal contacts to the n + region (5) and to the regions of p + contacts to the substrate (6), which are heavily doped with an impurity that creates the first type of conductivity, the space charge regions of the pn junction (7) in the gap between the n + region (4) and p + contacts to the substrate (6) and deeper than the n + region (5), the lightly doped n region (8) in the gap between the n + and p + regions, which at the operating voltage is completely blocked by the space charge of the pn junction and which enhances the electric field concentration near the p + region .

The cross section in FIG. 1 and the topology in FIG. 2 show the structure of the lateral photodetector with dimensions in the main plane of the semiconductor substrate smaller than the thickness of the space charge layer of the pn junction between the lightly doped semiconductor substrate of the same type of conductivity, for example, p-type, and the heavily doped diffusion region the second type of conductivity, for example, n + type. The concentration of impurities in the substrate may be 10 ¹³ -10 ¹⁶ cm ^-3 . The impurity concentration in the n + region can be 10 ¹⁶ -10 ²¹ cm ^-3 at least three orders of magnitude greater than in the substrate. The thickness of the space charge layer for a sharp pn junction at the same voltage, for example 10 V, at the indicated impurity concentrations in the silicon substrate is from 35 to 1 microns. When the impurity concentration in the substrate is 10 ¹⁴ cm ^–3 and the voltage at the transition is 50 V, the thickness of the space charge layer is 20 μm. The sizes of the diffusion regions n + and p + regions in the main plane of the semiconductor substrate are chosen to be significantly smaller than the thickness of the space charge region, for example, less than 5 μm. This size ratio ensures the efficient use of the substrate surface, where electrons and holes are generated during illumination and a photocurrent occurs in the space charge region, while diffusion n + and p + regions do not screen the light flux. The small sizes of the n + p junction, respectively, have a small value of the junction capacitance, which is important for obtaining high-speed photodetector. An increase in surface utilization efficiency and a decrease in capacity are also achieved due to the ring geometry of the structural element, in which the diffusion n + region is located in the center of the ring, and the diffusion p + region by the ring, for example, in a hexagonal shape, covers the n + region. The topology of the planar structure of one element of the device allows, if necessary, to connect the elements into a matrix of a large area, combining adjacent diffusion p + regions and connecting the elements by metallization, as is customary in the manufacture of integrated circuits.

Figure 3 shows the calculated distribution of the rate of generation of charge carriers characterizing the feature of this photodetector, which shows that when choosing small sizes n + and p + of regions of the order of one micron and a gap between these regions of the order of 5 microns in the operating mode, the electric field concentration areas and there is a multiplication of charge carriers. When the device is illuminated from the main surface in the space charge region, photo-generation of charge carriers occurs, conventionally shown in Fig. 4. Between the n + and p + regions and, respectively, between the metal electrodes, a photocurrent flows to these regions. The concentration of the electric field near the n + and p + regions creates an avalanche multiplication region in which an increase in the concentration of charge carriers and an increase in current occur. Characteristic of this photodetector is also the separation of the photocarrier generation region and two regions of charge carrier multiplication, as well as the reduction of the screening of the generation region by diffusion n + and p + regions.

Figure 5 shows the calculated dependence of the relative magnitude of the photocurrent on the applied voltage to the photodetector. A positive voltage relative to the p + contact region to the substrate of the order of 50 V is applied to the contact to the n + region.

For definiteness, we consider that the substrate is silicon p-type conductivity with an acceptor concentration of 10 ¹⁴ cm ^-3 . The manufacture of a photodetector begins with the formation of windows in the oxide covering the substrate. Ion doping of the n + and p + regions is carried out through these windows using photomasks. To activate impurities, the plates are annealed. At the final stage of manufacturing photodetectors, metal electrodes, for example, aluminum, are formed through the windows in the oxide using photolithography.

To further reduce the capacitance of the photodetector in series with the n + pn junction, a potential barrier can be formed at the contact of the n + region with the metal, as shown in the cross section of the lateral photodetector with the Schottky contact (Fig. 6). The layer of the space charge region formed due to the contact potential difference of the metal electrode and the semiconductor n + region creates an additional capacitance, which, due to series connection, blocks the capacitance of the n + p junction, reducing the total capacitance of the photodetector.

To enhance the avalanche multiplication on the p + contact region to the substrate in the gap, it is possible to form a lightly doped n region in the gap between the n + and p + regions, which at the operating voltage is completely blocked by the space charge of the pn junction and which enhances the electric field concentration near the p + region due to the formation of the p + n junction, as shown in the cross section of the lateral photodetector (Fig.7).

When voltage is applied to the photodetector, a dark leakage current flows through the device, and when illuminated from the side of the main surface of the plate, a photocurrent generated by light and obtained as a result of avalanche multiplication in regions of concentration of the electric field of charge carriers is added. The concentration of the electric field on elements of small size occurs both in the p + region and in the n + region; therefore, two regions of avalanche multiplication exist and contribute to the increase in photosensitivity. With the introduction of a lightly doped n- region between p + and n + regions, which completely overlaps with the space charge region at the operating voltage, it allows increasing the concentration of the electric field near the p + region and, accordingly, avalanche multiplication.

The relative change in current determines the sensitivity of the photodetector to lighting. The change in photodetector current is measured by an external device. The rate of change of the readings of the measuring device depends on the value of the capacitance of the photodetector, which is determined by the capacitance of the pn junction. The small dimensions of the pn junction and the high applied voltage create an extremely small photodetector capacitance. The series connection with the junction of the capacitance of the Schottky diode at the contact of the metal contact - n + region further reduces the capacitance of the photodetector.

The functioning of the photodetector is largely determined by the edge regions of the diode; therefore, the name “craidiode” can be used for this type of photodetector.

Sources of information

1. Semiconductor photodetectors. / Edited by V. I. Stafeev. // Moscow, Radio and communications, 1984.

2. RF patent 2178601.

3. RF patent 2061282.

4. US patent 4142200.

5. US patent 5223919.

6. US patent 4060820.

7. US patent 5352897.

8. US patent 3886579.

9. US patent 3244949.

10. US patent 3714526.

11. U.S. Patent 3,532,945.

12. US patent 5602413.

13. US patent 6137123.

14. RF patent 1816166.

15. RF patent 2141703.

16. U.S. Patent 4,531,055.

17. US patent 5525828 - prototype.

Claims (3)

1. A photodetector comprising lateral recessed electrodes on the surface of a silicon substrate with metal contacts to n + and p + regions, characterized in that the distance between the electrodes at an operating voltage is less than the thickness of the edge regions of the space charge of the pn junction, and the sizes of the electrode regions are less than the thickness area of space charge. 2. The photodetector according to claim 1, characterized in that in the space between the electrodes there is a lightly doped region with a conductivity type opposite to that of the substrate, which, when the operating voltage is completely depleted in the main charge carriers. 3. The photodetector according to claim 1, characterized in that the metal contact to the n + region has a Schottky barrier.

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