RU2521156C2 - Semiconductor infrared photodiode - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: invention relates to semiconductor optoelectronics, specifically to infrared detectors, and can find application in spectrometers, detection and monitoring systems, security, fire-protection and communication systems. The infrared photodiode (1) has p and n regions (2, 3, 7) with current-conducting opaque contacts (4, 5) and an active region which is electrically connected to the p-n junction (6), wherein one or more contacts on the surface of the region which receives photons from the investigated object have a common perimeter, the value of which is selected from a range of values associated with the current spreading length. Contacts on the surface of the region receiving photons from the investigated object have elements with a repeating geometric shape, e.g. in form of a spiral or cellular structure. The active region of the photodiode is made of INAsSb, InAs, InGaAsSb, and the layer on the illuminated side is made of INAs_1-x-y Sb_xP_y (o<x<0.2, y=(2-2.2)·x) and has contacts from a series of metal layers Cr-Au_1-w-Zn_w-Ni-Au, wherein the Cr layer adjoins the surface of the p region, and w=0.01-0.2.

EFFECT: photodiode according to the invention provides high photosensitivity to radiation in the middle infrared region of the spectrum.

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Description

The invention relates to the field of semiconductor optoelectronics, specifically to infrared (IR) radiation receivers, and may find application in spectrometers, detection, tracking systems, security and fire protection systems and communications.

In the spectral region (3-5 μm), photodetectors from CdHgTe and PdSe (PbS) occupy a dominant position in terms of volume of use. This is partly due to the well-developed technology for the post-growth processing of these materials and the exceptionally large range of band gap changes in CdHgTe, which makes it possible to produce photodiodes in both the first and second atmospheric transparency windows [1, 2]. The methods for producing PbSe (PbS) are quite simple, and studies of photodetectors based on them were started in the middle of the last century. At the same time, both materials are characterized by the presence of 1 / f noise; in addition to this, high detection values are required, as a rule, thermoelectric cooling.

An alternative to these materials are A ³ B ⁵ semiconductors, which, unlike the above, have metallurgical stability and are immune to moisture. The main studies aimed at creating photodiodes from the semiconductor most suitable for the range (3-5 microns) - indium arsenide - were carried out in the 70s of the last century, and for several decades there have been industrial photodiodes manufactured by Hamamatsu, AG&G with a red border at 3.6 microns based on homo pn structures. At the same time, photodiodes based on homo pn junctions have low resistance values at the zero bias. So, for example, in [3] the values of R ₀ = 30-55 Ohm (R ₀ A ~ 0.1 Ohm · cm ² ) are given for structures obtained on the p-InAs substrate by the MOCVD method, which is not enough for many applications. Therefore, photodiodes in which heterostructures are used to achieve high values of R ₀ A are relevant. Thus, for example, by using a wide-gap “window” on the InAs surface that extends the spectral curve to the region of short wavelengths, it was possible to reduce surface recombination [4] and avoid problems associated with the formation of an inverse layer on the surface of p-type indium arsenide [5] with a simultaneous increase in the value of R ₀ A.

The creation of photodetectors with a photosensitivity boundary at a wavelength of 4-5 μm was mainly associated with the use of InAsSb as the material of the active region. InAsSb layers illuminated from the non-epitaxial side were obtained on n-InAs substrates (more precisely, on n-InAsSb buffer layers previously grown on InAs) [6], on GaAs [7], or GaSb [8] substrates. In the latter case, an InAsSb _0.1 solid solution was used, which has the same lattice period with GaSb at the epitaxy temperature.

A photodiode is known for the mid-infrared range of the spectrum, including p- and n-regions with current-conducting contacts separated by a pn junction, an active region electrically connected to the pn junction, at least one of the contacts on the surface of the region receiving photons from the studied object made transparent in the working region of wavelengths [9]. In the known solution, a continuous current-conducting contact of CdO, or ZnO, or RuSiO4 was deposited on the surface of a GaSb / In (Al) GaAsSb structure by sputtering in vacuum. These contact materials had a transparency> 80%, which made it possible to minimize contact shadowing of the active region and increase photosensitivity. However, the materials used have a low refractive index (see table 1), therefore, the presence of contacts from these materials on the light-receiving surface creates difficulties for the manufacture of effective immersion photodiodes. The dependence of the detection ability of immersion PDs on the refractive index of the layers located between the PD and the lens was considered in [10].

Table 1 Optical characteristics of contact materials Contact material Cdo Zno RuSiO ₄ Refractive Index λ = 2.2 μm 1.78 1.81 1.75

Known photodiode for the middle infrared range of the spectrum [11], which coincides with the claimed technical solution for the largest number of essential features and adopted as a prototype, while the photodiode prototype reflects the most common version of the design used in practice [12], [13]. The prototype photodiode includes p- and n-regions with current-carrying opaque contacts separated by a pn junction, an active region electrically connected to the pn junction, and one contact on the surface of the region receiving photons from the object under study. The epitaxial structure for the photodiode was obtained on a GaSb substrate by sequentially growing by a molecular beam epitaxy method a layer of an n-GaInAsSb solid solution doped with tellurium (Te) 2 μm thick and a p-GaInAsSb layer doped with germanium (Ge) 1 μm thick. In this case, the active region was made of an InGaAsSb solid solution having a lattice period close to the GaSb lattice period, the diameter of the mesa diode, i.e. the diameter of the active region was 300 μm, and the contact, consisting of an AuGe alloy, occupied only a small part of the structure. The diameter of the contact according to [12] was 50 μm.

A disadvantage of the known photodiode for the mid-infrared spectrum is the low photosensitivity at room temperature and unacceptably low at elevated temperatures.

The objective of the invention is the development of such a photodiode for the middle infrared range of the spectrum, which would have increased photosensitivity both at room and at elevated temperatures.

The problem is solved in that the photodiode for the middle infrared spectrum includes p- and n-regions with current-carrying opaque contacts separated by a pn junction, an active region electrically connected to the pn junction, with one or more contacts on the surface of the receiving Photons from the studied object have a common perimeter, the value of which is selected from the interval:

$$P_{p-n}\cdot (\frac{\sqrt{S_{p-n}}}{\mathrm{one}.3\cdot L_{spr}})+P_{cont}^{min}\ge P_{cont}\ge P_{p-n}\cdot (\frac{\sqrt{S_{p-n}}}{\mathrm{one}3\cdot L_{spr}})+P_{cont}^{min}$$

- периметр контакта с минимально возможной для используемых технологических процессов площадью.where P _pn , S _pn are the perimeter and area of the active part of the pn junction, respectively, P _cont is the total perimeter of all contact areas (s) involved in collecting the photocurrent, L _spr is the current spreading length, $$P_{cont}^{\min }$$

- the perimeter of the contact with the minimum possible area for the used technological processes.

Each of the contacts on the surface of the region receiving photons from the object under study may contain elements with a repeating geometric shape.

At least part of the contact (s) may be in the form of a spiral.

At least part of the contact (s) may have a cellular structure.

In some cases, the minimum distance between the edges of adjacent contact elements (s) does not exceed half the current spreading length L _spr .

The active region can be made from INAsSb.

The active region may additionally contain gallium atoms, while the period of the crystal lattice of the active region is close to the period of the crystal lattice of InAs.

The active region may further contain phosphorus atoms.

The radiation receiving region can be made of an InAs _1-xy Sb _x P _y solid solution (o <x <0.2, y = 2-2.2) x).

The radiation receiving region may further comprise gallium atoms.

Contact (s) to the p-region can be made (s) from a sequence of Cr-Au _1-w -Zn _w -Ni-Au metal layers, the Cr layer adjacent to the surface of the p-region, aw = 0.01-0.2.

, т.е. имел размер, обусловленный требованием получения минимального затенения активной области при сохранении возможности электрического соединения с проволокой, типичной для используемых технологических процессов. Увеличенный периметр контакта позволяет производить сбор фототока более эффективно, чем в фотодиодах с малым периметром контакта. В фотодиодах для среднего инфракрасного диапазона спектра фототок неравномерно распределен по поверхности: в непосредственной близости от контакта он максимален и минимален в областях, удаленных от контакта. Указанное свойство неравномерности усиливается с уменьшением ширины запрещенной зоны и/или при увеличении температуры фотодиода. Поэтому в фотодиодах с малым размером контакта (т.е. при малом его периметре) эффективность, особенно при длинах волн, более 3 мкм, или при температурах выше 40 С, низка. Напротив, фотодиоды с увеличенным периметром даже в случае затенения значительной части активной области (например, при использовании сплошного контакта круглой формы) имеют повышенную эффективность.In contrast to the prototype photodiode, in the inventive photodiode, the contact on the illuminated surface (the contact participating in the collection of the photocurrent) has an increased perimeter, in the prototype the contact perimeter was $$P_{cont}^{\min }$$

, i.e. had a size due to the requirement of obtaining minimal shading of the active region while maintaining the possibility of electrical connection with the wire, typical for the used technological processes. The increased contact perimeter allows the collection of photocurrent more efficiently than in photodiodes with a small contact perimeter. In photodiodes for the mid-infrared range of the spectrum, the photocurrent is unevenly distributed over the surface: in the immediate vicinity of the contact, it is maximum and minimum in areas remote from the contact. The indicated property of non-uniformity increases with decreasing band gap and / or with increasing temperature of the photodiode. Therefore, in photodiodes with a small contact size (i.e., with a small perimeter thereof), the efficiency, especially at wavelengths greater than 3 μm, or at temperatures above 40 ° C, is low. On the contrary, photodiodes with an increased perimeter, even in the case of shading a significant part of the active region (for example, when using a continuous contact of a circular shape), have increased efficiency.

The values of the optimal total perimeter of all contact areas (s) involved in the collection of the photocurrent, P _cont , are inside the interval:

$$7.4\cdot P_{p-n}\cdot (\frac{\sqrt{S_{p-n}}}{L_{spr}})+P_{cont}^{min}\ge P_{cont}\ge P_{p-n}\cdot (\frac{\sqrt{S_{p-\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="00000011.png,00000012.png"\; state="\{\{state\}\}">n</figure-callout>}}}}{2.7L_{spr}})+P_{cont}^{min}$$

- периметр контакта с минимально возможной для используемых технологических процессов площадью.where P _pn , S _pn are the perimeter and area of the active part of the pn junction, respectively, P _cont is the total perimeter of all contact areas (s) involved in collecting the photocurrent, L _spr is the current spreading length, $$P_{cont}^{\min }$$

- the perimeter of the contact with the minimum possible area for the used technological processes.

The current spreading length L _spr , i.e. the distance at which the current value decreases (e) times from the value near the contact is determined by the ratio between the dynamic resistance of the pn junction at the bias zero (R _o ) and the resistances of the p and n regions and the ratios between the geometric characteristics of the sample (thicknesses, lateral size, location and shape). To calculate the values of L _{spr, the} formalism described in the monograph by F. Schubert [14], articles [15] is used or determined experimentally using atomic force microscopy in conjunction with the Kelvin probe method, which allows determining the potential distribution on the surface [16] , or use IR images (that is, 2D — intensity distribution) of the PD surface to which the bias is applied [17, 18].

контакт с соответствующей геометрией не позволяет собрать большую часть фототока и эффективность фотодиода низкая, несмотря на небольшую ступень затенения активной области.At $$P_{cont}\le P_{p-n}\cdot (\frac{\sqrt{S_{p-n}}}{\mathrm{one}3\cdot L_{spr}})+P_{cont}^{min}$$

contact with the corresponding geometry does not allow to collect most of the photocurrent and the efficiency of the photodiode is low, despite the small level of shadowing of the active region.

большая часть рожденных в активной области пар электрон-дырка дает вклад в фототок, однако общее количество рожденных пар невелико из-за затенения большей части активной области контактом. Вследствие этого эффективность фотодиода также невелика.At $$P_{cont}\ge P_{p-n}\cdot (\frac{\sqrt{S_{p-n}}}{\mathrm{one}.3\cdot L_{spr}})+P_{cont}^{min}$$

most of the electron – hole pairs produced in the active region contribute to the photocurrent, however, the total number of generated pairs is small due to the shadowing of most of the active region by the contact. As a result, the efficiency of the photodiode is also low.

The implementation of each of the contacts on the surface of the region receiving photons from the studied object with elements with a repeating geometric shape increases the perimeter of interaction with photoexcited electron-hole pairs, as indicated above, which means that this increases the efficiency of the photodiode.

The implementation of at least part of the contact (s) in the form of a spiral increases the efficiency of the photodiode, since this form of contacts provides minimal shadowing of the active region at the maximum perimeter of the contact in the case when the width of the spiral part of the contact is maximally reduced in relation to the used technological processes.

The implementation of part of the contact (s) in the form of a cellular structure is important to increase the stability (strength) of the connection of the contact with the semiconductor, i.e. to increase the efficiency of the photodiode during prolonged hours of operation or under adverse conditions, for example during thermal cycling.

Making contacts with a minimum distance between the edges of adjacent contact elements (s) exceeding half the current spreading length L _spr allows you to get a photodiode with maximum efficiency, because this ensures efficient collection of photocurrent with minimal shadowing of the active region. In photodiodes in which the distance between the edges of adjacent contact elements (s) is more than half L _spr , the photocurrent is efficiently collected, which means they are inefficient.

The execution of the active region from InAsSb provides the creation of effective photodiodes that are durable at elevated temperatures, because, unlike other materials for the mid-IR range (for example, CdHgTe), the InAsSd solid solution has metallurgical stability.

The implementation of the active region of InAsSb with an additional content of gallium atoms and a crystal lattice period close to the period of the InAs crystal lattice provides an additional increase in durability at elevated temperatures due to the "hardening" of the InGaAsSb solid solution. The proximity of the lattice periods of the InGaAsSb and InAs layer provides, on the one hand, the necessary wavelength range in the mid-IR range, and on the other hand, provides the possibility of obtaining crystalline perfect (and, therefore, effective) photodiodes.

The introduction of the fifth component, phosphorus, makes it possible to coordinate not only the lattice periods, but also the thermal expansion coefficients of the substrate and layer materials, which increases the efficiency during long-term operating time under changing temperature conditions ..

The execution of the radiation receiving region from InAs _1-xy SbxPy solid solution (o <x <0.2, y = (2-2.2) · x) provides an increase in efficiency due to an increase in the total barrier height at the hetero pn junction, provided that in defects are not created in the material (misfit dislocations), i.e. for y = (2-2.2) x. For x> 0.2, the resulting InAs _1-xy SbxPy compositions are within the immiscibility region, i.e. in areas with unstable properties, where obtaining high-quality layers is difficult.

The introduction of the fifth component, gallium, into the region of InAs _1-xy Sb _x P _y solid solution provides the possibility of matching not only the lattice periods, but also the thermal expansion coefficients of the substrate and layer materials, which increases the efficiency during prolonged use under varying temperatures.

The implementation of the contact (s) to the p-region from the sequence of metal layers Cr-Au _1-w- Zn _w -Ni-Au, in which the layer of Cr is adjacent to the surface of the p-region, aw = 0.01-0.2, increases the efficiency during long-term operating time at elevated temperatures, since it is the most reliable and long-lived of all known to the author.

The inventive device is illustrated by drawings, where

figure 1 schematically shows a first embodiment of the inventive photodiode for the middle infrared spectrum,

figure 2 schematically shows a second variant of the photodiode for the middle infrared spectrum,

figure 3 schematically shows a third embodiment of the inventive photodiode for the middle infrared spectrum,

The inventive photodiode for the middle infrared range of spectrum 1 (see Fig. 1) includes p-2 and n-regions 3 with current-conducting opaque contacts 4, 5, separated by a pn junction 6, an active region 7, electrically connected with a pn junction 6 while one or more contacts on the surface of the region receiving photons from the studied object 8 have a common perimeter, the value of which is selected from the range of values obtained by calculation.

The second embodiment of the photodiode for the mid-infrared range of spectrum 1 (see FIG. 2) differs from the first embodiment in that the contact contains elements with a repeating geometric shape, creating a cellular shape. In addition, the second contact is formed not on the back of the photodiode, as in the first embodiment, but on the side facing the object under study.

The third embodiment of the photodiode for the mid-infrared range of spectrum 1 (see FIG. 3) differs from the first embodiment in that at least part of the contact 4 is made in the form of a spiral. It also differs from the second option in that the second contact 5 is made of a limited area, i.e. "Point".

The inventive photodiode for the middle infrared spectrum works as follows. An external energy source, for example, a detected object, with a temperature higher than the background, creates a stream of photons that enter the active region 7 through the surface receiving photons from the studied object 3, where they are absorbed, producing electron-hole pairs separated by a potential barrier at pn- transition 6. Separated pairs create an electric field that prevents the further movement of excess carriers to the pn junction 6. The voltage change at the pn junction using contacts 4, 5 is recorded in the external circuit in the form e useful signal. When contacts 4, 5 are closed, a current flows in the circuit, which is a useful signal - a photocurrent.

Example 1. An example of a photodiode was carried out at IoffeLED LLC using standard processes for the production of InAsSbP gradient structures on an InAs substrate by the HPE method. The samples were similar to those described previously [19] and had a smooth change in composition over the thickness of the InAsSbP gradient layers. After photolithography and removal of the substrate by chemical etching, the photodiode included a p-region from InAsSb (2), an n-region from InAsSb (7), InAsSbP (5), limited by an etching mesa with a diameter of 300 μm with current-carrying opaque contacts (4, 5) separated by the p-InAsSbP / n-InAs pn junction (6), the active region from InAsSb (7) electrically coupled to the pn junction, with the contact on the surface of the p-InAsSb region receiving photons from the object under study (4, the anode ), formed by sputtering the metal composition Cr-Au _1-w- Zn _w -Ni-Au (w = 0.05) in vacuum, had a diameter of 50 μm and was located lagged in the center of Mesa; the back opaque metal contact to n-InAsSbP (5, cathode), formed by sputtering the Cr-AuGe-Ni-Au metal composition in vacuum, was continuous, occupying the entire surface of n-InAs. Prior to photosensitivity measurements, PDs were mounted in a standard TO-18 package, while the contact with the smallest possible area for the used technological processes had a diameter of 50 μm, and the current spreading length (L _spr ) was 280 μm. In this case, the upper contact to the metal disk with a diameter of 50 μm was carried out by welding or soldering indium gold wire with a diameter of 30 μm. In the second case, it was possible to change the contact area by re-soldering the contact with a different mass of solder. A Globar with a temperature of 300 ° C served as a radiation source.

,Figure 4 presents the data of photosensitivity measurements (photocurrent at a fixed incident flux density) in a series of identical PDs described above and differing only in diameter D _cont (perimeter P _cont ) of an opaque anode on the _photon- receiving surface. As can be seen from the data in Figure 4, the sensitivity of the photodiodes, in which the perimeter of the anode satisfied the condition $$P_{p-n}\cdot (\frac{\sqrt{S_{p-n}}}{\mathrm{one}.3\cdot L_{spr}})+P_{cont}^{min}\ge P_{cont}\ge P_{p-n}\cdot (\frac{\sqrt{S_{p-n}}}{13\cdot L_{spr}})+P_{cont}^{min}$$

,

the corresponding range of values indicated by dashed lines was at least two times higher than in the known PD with point contact D _cont = 50 μm. Outside the interval, on the side of large contact diameters, the photosensitivity decreased due to the shadowing effect of the contact and there were no advantages over the known photodiode.

Example 2. In the second example of the photodiode, the latter was made from a heterostructure similar to that described in the first example, however, the mesa (active region of the pn junction) had the shape of a square with a side of 450 μm. The contact on the photon-receiving surface consisted of repeating rectangular elements connected together, forming a “comb” of 3 strips 20 μm wide, parallel to each other and electrically connected together by a 300 × 100 μm rectangular element. The extreme elements of the “comb” contact, resembling the letter “Ш” of the Russian alphabet, were located at a distance of 75 μm from the edge of the Mesa, and the contact itself had a perimeter P _cont = 1800 μm, the value of which was inside the optimal interval from our point of view 275 ....... 2275 microns obtained by calculation using the proposed ratios. For comparison, a PD was made with a point contact (D _cont = 50 μm) located in the center of the square mesa. Both PDs were mounted similarly to those described above, while the photosensitivity of PDs with point contact was 5 times lower than that of the claimed PDs with a "Ш" -shaped contact.

Example 3. The samples were similar to the previous ones described in examples 1 and 2, and had a continuous lower contact (to n-InAs). The upper contact (to p-InAsSb (P)) had four modifications: 1) a circle with a diameter of 80 μm located in the center of the mesa, 2) a contact according to claim 1, to which strips 10 μm wide were added, making up the pattern in the form of a “double-leaf window frames without window ”; the central strip of contact was connected to the circle, 3) the contact of claim 2, to which two more strips were added, forming unshaded sections in the form of elongated rectangles, 4) the contact of claim 3 with the addition of four more strips, two of which are “new” stripes connected (intersected) with the central circle. The geometry of the contacts is shown in the inset to the right of Fig. 5. Photosensitivity was measured using a blackbody model heated to 573 K; it was assumed that the Johnson (thermal) noise dominates in the photodiode and that the shading by the contact was not taken into account when calculating D *, then the total power of the stream, which is incident throughout the mesa, was taken into account in the calculations; When measuring the spectra of the photoresponse, Globar was used.

Figure 5 shows the current-voltage characteristics (IV) for the four types of samples described above. The photosensitivity spectrum of the obtained diodes had a maximum at ~ 5.3 μm (300 K) (λ _cut-off. = 5.8 μm) and was pulled into the short-wavelength region due to the diffusion of photogenerated carriers to the pn junction.

As can be seen from Figure 5, three of the four samples (No. 2-4) with a developed contact structure had approximately the same values of the dark saturation current (I _sat ~ 40 mA); in the range of average voltages, an increase in current followed an increase in the area of the anode. A diode with a point contact (No. 1) was characterized by a factor of ~ 2 smaller in comparison with the above PD currents and the absence of a pronounced saturation in the reverse branch of the IV characteristic. In our case, by increasing the contact perimeter (it was possible to increase the photo signal at least threefold; Fig. 6 shows the dependences of sensitivity (S _I ), detection ability (D _λreak ), dynamic resistance at zero bias (R _o ) on the anode perimeter, confirming the above.

It should be remembered that with an increase in the contact perimeter, there is not only an increase in photosensitivity, but also a decrease in the dynamic resistance of the PD due to an increase in the area involved in the current passage, i.e. photocurrent collection areas. Therefore, when working with such PDs, it is necessary to correlate the increase in the photocurrent with a possible increase in the noise of the photodetector, consisting of their PD and signal amplifier. An approach for such an analysis is described in [20].

The author thanks Zakheim A.L., Ilyinsky N.D., Karandasheva S.A., Mzhelsky I.V., Polovinkina V.T. Remennoy M.A., Rybalchenko A.Yu., Stusya N.M. and Chernyakova A.E. for helping with the experiments.

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Claims (8)

1. A photodiode for the middle infrared range of the spectrum, including p- and n-regions with current-carrying opaque contacts separated by a pn junction, an active region electrically connected to the pn junction, with one or more contacts on the surface of the region receiving photons from the studied objects that have a common perimeter, the value of which is selected from the interval:

where P _pn , S _pn are the perimeter and area of the active part of the pn junction, respectively, P _cont is the total perimeter of all contact areas (s) involved in collecting the photocurrent, L _spr is the current spreading length,

- the perimeter of the contact with the minimum possible area for the used technological processes. 2. The photodiode according to claim 1, characterized in that each of the contacts on the surface of the region receiving photons from the object under study contains elements with a similar geometric shape. 3. The photodiode according to claim 1, characterized in that at least part of the contact (s) is made in the form of a spiral. 4. The photodiode according to claim 1, characterized in that at least part of the contact (s) has a cellular structure. 5. The photodiode according to claim 1, characterized in that the minimum distance between the edges of adjacent contact elements (s) does not exceed half the current spreading length L _spr . 6. A photodiode according to any one of claims 1 to 5, characterized in that the active region is made of INAsSb, or InGaAsSb, or INGaAsPSb. 7. A photodiode according to any one of claims 1 to 5, characterized in that the region receiving radiation is made of InAs _1-xy Sb _x P _y solid solution (o <x <0.2, y = (2-2.2) · x) . 8. The photodiode according to claim 7, characterized in that the contact (s) to the p-region is made (s) from a sequence of metal layers Cr-Au _1-w Zn _w -Ni-Au, and the layer of Cr is adjacent to the surface p- area, aw = 0.01-0.2.

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