RU2069922C1 - Infrared-spectrum photodetector - Google Patents

Abstract

FIELD: electronic engineering; multielement and single-element extrinsic-conductivity photodetectors sensitive in infrared spectrum. SUBSTANCE: photodetector has substrate made of semiconductor compensated by deep multicharge impurity creating twice negative charged levels. Both sides of photodetector carry conducting buses built up of contact layer on substrate and deposited metal affording electric contact with substrate. Contact layer, as distinct from prototype, is formed from two regions at 300 K with different concentration of impurity enriching this layer. First region has maximum concentration so that it is twice as large or equal to diffusion length of carriers. Second region with impurity concentration is enriched by 1 or 2% higher than concentration of small impurities in substrate. Width of this region should be greater than length of carrier shielding at fine impurity compensation. Region of n⁺-n(p⁺-p) junction that possesses injecting properties appears in contact region during photodetector cooling. This junction incorporates fine impurity compensation plane possessing utmost shielding length of carriers injected from contact. Second region affords separation of electric contact to compensated semiconductor into two contacts performing different functions: nickel film and enriched substrate region contact that is resistive contact; second region and substrate contact which is injecting contact. EFFECT: improved design. 2 cl, 2 dwg, 3 tbl

Description

 The photodetector FPU relates to electronic equipment, in particular to photodetectors having sensitivity in the infrared range of the spectrum, multi-element or single-element with impurity photoconductivity.

 FPU for the infrared region of the spectrum is usually used in the pulse survey mode. The photovoltaic parameters of the FPU are determined by the properties of the electrical contact to the compensated semiconductor. When a voltage pulse is switched on to the FPU, the main current carriers are injected from the contact and some of them are trapped. During the pause between pulses, the captured carriers are released by thermal or photo-generation. The magnitude of the photocurrent is determined by the current recharging levels of the traps.

A photodetector device is known (VG Ivanov. FTP, 1979, issue 9, volume 13, p. 1838 1841), which contains a semiconductor photosensitive substrate into which a compensating impurity is introduced, the activation energy of which E _{a is} less than half the band gap substrate material. The impurity is multicharged with a double negatively charged level for the main carriers. FPU has external bus layers made of a material forming an ohmic contact with the substrate material and transparent to the projected radiation. The photodetector has several significant drawbacks: the instability of the electrical properties of the resulting contact of the outer layer with the substrate, the influence of the properties of the substrate surface and the properties of the outer layer on the photoelectric parameters of the FPU, poor reproducibility of the process in the manufacture of ohmic contact.

Known infrared detector matrix (patent GB N 2125217), containing a silicon substrate doped with boron and compensated by controlled divacancy, on both sides of which are formed conductive mutually perpendicular buses from Au, which in turn consist of a contact layer, which is an enrichment and film region metal deposited on the contact layer. Such a contact at low temperatures forms an n ⁺ -n (p ⁺ -p) type transition, which is formed close to the surface, which leads to a dependence of the properties of the transition itself on the surface state and on the properties of the metal, which leads to contact instability, and also do not occur properties of the injecting contact, the possibility of increasing photosensitivity.

 The main objective of the invention is, firstly, to increase the photosensitivity of the FPU in the pulse mode, and, secondly, to increase the uniformity of the distribution of photosensitivity across the field of the FPU and to obtain stable reproducible photoelectric parameters of the FPU.

 The positive effect of the use of the invention is the creation of a stable injecting contact to a compensated semiconductor, independent of the properties of the surface and the metal deposited on the surface and an increase in the photosensitivity of the FPU in the pulsed mode compared to the stationary mode.

при условии L₁≥L диф. где D коэффициент диффузии основных носителей: L_диф. длина диффузии основных носителей; γ коэффициент рекомбинации носителей заряда; L₁ толщина первой области, и следующей за ней второй области с концентрацией примеси на 1 2 выше концентрации мелкой донорной примеси подложки, причем их толщины соотносятся следующим образом:

где L₂ толщина второй области; для улучшения омических свойств металла используется пленка никеля.This effect is achieved by the fact that in the proposed FPU for the IR region of the spectrum containing a substrate of a semiconductor doped with a shallow donor impurity and precisely compensated by a deep multiply charged acceptor impurity, which creates doubly charged levels, on both sides of which are conductive buses consisting of that created in the substrate a contact layer enriched with a specially introduced fine impurity of the same type of conductivity as the substrate, and a metal film deposited on it, creating an electrical tact with it, the contact layer consists of a first region contacting the metal with a maximum impurity concentration equal

subject to L ₁ ≥L differential. where D is the diffusion coefficient of the main carriers: L _diff. diffusion length of the main carriers; γ carrier recombination coefficient; L _{1 the} thickness of the first region, and the second region following it with an impurity concentration of 1 2 higher than the concentration of the fine donor impurity of the substrate, and their thicknesses are correlated as follows:

where L _{2 is the} thickness of the second region; To improve the ohmic properties of the metal, a nickel film is used.

где N_t n_o темновая концентрация носителей тока в зоне проводимости при точной компенсации примесей; ΔΦ высота потенциального барьера (n⁺-n) перехода.Thus, in the proposed FPU, the contact layer at room temperature consists of at least 2 regions: the first enrichment region is N ⁺⁺ (p ⁺⁺ ); the second enrichment area with a lower concentration of n ⁺ (p ⁺ ). Upon cooling, a third transition region of the same type n ⁺ n (p ⁺ p) appears in such a contact, the height of the potential barrier of which is equal to the difference between the Fermi levels in region 2 and in the bulk of the substrate. In the n ⁺ n (p ⁺ p) transition, there is a plane in which the impurities are precisely compensated. The injected carriers trapped on the traps are nonequilibrium and create an additional space charge that shields the injection contact. The maximum width of the shielding area must be in the plane with precise compensation and equal to

where N _t n _{o the} dark concentration of current carriers in the conduction band with accurate compensation of impurities; ΔΦ is the height of the potential barrier (n ⁺ -n) of the transition.

Концентрация инжектированных, захваченных носителей заряда при напряжении предельного заполнения ловушек равна концентрации пустых мест на акцепторном уровне в подложке. Толщина первой области должна быть больше или равна диффузионной длине основных носителей тока, которая определяется по формуле

где D коэффициент диффузии мелкой примеси в 1 области; γ -коэффициент рекомбинации основных носителей, N_макс максимальная концентрация примеси в 1 области.The width of region 2 should be greater than or equal to the maximum screening length in the n ⁺ n (p ⁺ p) junction, which is determined by formula (2) and write down the condition

The concentration of injected, trapped charge carriers at a voltage of the maximum filling of traps is equal to the concentration of empty spaces at the acceptor level in the substrate. The thickness of the first region should be greater than or equal to the diffusion length of the main current carriers, which is determined by the formula

where D is the diffusion coefficient of fine impurities in 1 region; γ is the recombination coefficient of the main carriers, N _{max is the} maximum impurity concentration in 1 region.

ε диэлектрическая постоянная; m подвижность носителей тока; V - напряжение питания; L расстояние между электродами; S площадь элемента. Величину тока в стационарном режиме через этот же элемент выражается

где Δn_ст. концентрация фотоносителей в зоне проводимости в стационарном режиме. Увеличение фоточувствительности зависит от исходной степени компенсации объема компенсированной подложки, от длительности импульса и паузы между ними, от уровня фоновой облученности. Амплитуда импульса фототока дается выражением

(6)
где τ_захв время захвата носителей тока на примесный уровень;
t_имп. длительность импульса; τ_паузы длительность паузы; С - параметр, зависящий от свойств инжектирующего контакта.In order to satisfy the condition L ₁ ≥L _differential necessary to introduce one impurity at a concentration
N _max ≥ D / L $\genfrac{}{}{0ex}{}{\text{2}}{\text{differential}}$ γ
In the contact of the metal and region 1, a region of space charge is formed with the height of the potential barrier Φ _o and the maximum electric field strength E _max. at a depth equal to Φ _o / E _max . Region 1 should be greater than the penetration depth of the electric field of the metal-substrate contact. Each region of such a contact fulfills its own functions: the first region is the function of the ohmic (injecting) contact, the metal is enriched with N ⁺⁺ layer, the second region at low temperatures of the injection contact of the N ⁺ region and the FPU substrate, i.e., it forms the directly injecting n ⁺ -n junction ( p ⁺ p). The distribution of the concentration of charge carriers in such a contact is shown in FIG. 1. Next, it will be shown how sensitivity increases in the proposed FPU. The concentration of current carriers injected from the contact is determined by the properties of the injecting contact, the voltage and dimensions of the FPU elements. In the pulsed interrogation mode of the FPU, a significant increase in photosensitivity is observed compared to the stationary mode. At the first moment, when a voltage pulse is applied, an injection trap current flows whose peak value

ε dielectric constant; m mobility of current carriers; V is the supply voltage; L distance between the electrodes; S is the area of the element. The value of the current in stationary mode through the same element is expressed

where Δn _Art. concentration of photocarriers in the conduction band in the stationary mode. An increase in photosensitivity depends on the initial degree of compensation of the volume of the compensated substrate, on the pulse duration and the pause between them, on the level of background irradiation. The amplitude of the photocurrent pulse is given by

(6)
where τ _capture time capture of current carriers to the impurity level;
t _imp. pulse duration; τ _{pauses pause} duration; C is a parameter depending on the properties of the injecting contact.

В ФПУ прототипе поверхности подложки наносится пленка золота. Золото в германии создает акцепторные уровни.An increase in photosensitivity appears as a ratio of currents

In the FPU prototype substrate surface, a gold film is deposited. Gold in Germany creates acceptor levels.

However, it is not possible to obtain a technologically reproducible contact, since the gold during deposition and subsequent annealing diffused into germanium to a depth greater than the width of the enriched region 1; therefore, a strongly overcompensated thin layer of Ge (Au, Sb) was created, which becomes more highly resistive at low temperatures. than the n ⁺ n transition and all the voltage drops on this layer, leading to a loss of the injecting properties of the contact and, accordingly, photosensitivity.

In the proposed FPU, when nickel is deposited, its diffusion also occurs, however, region 2 is much larger than the length at which nickel diffuses, and therefore the nn transition is preserved, located at a depth of ≈1.5 μm from the surface, much larger than L diff Ni
Thus, the injecting properties of the n ⁺ n (p ⁺ p) junction and the advantages of such a contact are preserved.

In FIG. 1 shows the distribution of the concentration of current carriers in the FPU; in FIG. 2 FPU based on the matrix type structure: 1 substrate; 2 region of enrichment N ⁺⁺ = 10 ¹⁹ cm ^-3 ; 3 region of enrichment N ⁺ = 10 ¹⁵ cm ^-3 ; 4 metal film; 5 transition n ⁺ - n (p ⁺ p).

An example of a specific implementation. FPU can be single-element or multi-element matrix or linear types. The proposed FPU is depicted in FIG. 2, FPU consists of a germanium substrate doped with antimony and compensated by silver (1), an enrichment region (2) in which the maximum concentration of enrichment impurity is ≈10 ¹⁹ cm ^-3 , enrichment region (3) with a concentration of ≈10 ¹⁵ cm ^-3 , region (5), which manifests itself at low temperatures and forms the n ⁺ n (p ⁺ p) transition, and a metal film (4) deposited on the substrate. The concentration in region (5) varies from ≈10 ¹⁵ cm ^–3 to the concentration of charge carriers in the substrate n _o .

N N⁺⁺ В прототипе
N=N⁺ в нашем случае
Ширина перехода n⁺ n определяется по формуле

При включении импульса напряжения носителя инжектируются из контакта, продиффундировав область 1 и 2, носители рекомбинируют. Инжектированные носители частично захватываются в области перехода и в подложке, создавая неравновесный связанный заряд. Этот заряд экранирует контакт, длина экранирования носителей заряда определяется по формуле при точной степени компенсации

Изменение фоточувствительности ФПУ определяется нестационарными процессами, проходящими в переходе, поэтому создание области 2 позволяет сформировать переход, ширина которого определяется длиной экранирования носителей тока на глубине ≈Lэкр. В прототипе переход образуется близко от поверхности подложки и свойства его зависят от состояния поверхности.FPU of matrix type, on one bus of which voltage pulses are supplied, an amplifier is connected to other buses. In the stationary state, the substrate at room temperature, the low-resistance and near-contact layer consists of 2 regions: the enriched region and the region of the n ⁺ n transition, which is not clearly manifested. Upon cooling to T≈40 K, the substrate becomes high-resistance and region 2 transforms into two regions, where one of them is low-resistance with a concentration higher by (1 2) than the concentration of small donor impurities in the substrate, and the second transition region is n ± n (p ⁺ p) . The height of the potential transition barrier is determined by the formula

NN ⁺⁺ In the prototype
N = N ⁺ in our case
The width of the transition n ⁺ n is determined by the formula

When a pulse is turned on, the carrier voltages are injected from the contact, diffusing region 1 and 2, the carriers recombine. Injected carriers are partially captured in the transition region and in the substrate, creating a nonequilibrium bound charge. This charge screens the contact, the length of the screening of the charge carriers is determined by the formula with the exact degree of compensation

The change in the photosensitivity of the FPU is determined by non-stationary processes taking place in the transition, therefore, the creation of region 2 allows the formation of a transition whose width is determined by the length of the screening of current carriers at a depth of ≈Lecr. In the prototype, the transition is formed close to the surface of the substrate and its properties depend on the state of the surface.

Концентрация фотоносителей определяется по формуле

где N_Sb концентрация сурьмы в подложке; N_cм3 - эффективная плотность состояний, приведенная к верхнему уровню серебра; 3η=N_Ag/N_Sb степень компенсации примесей в подложке, g_ф сечение фотоионизации верхнего уровня серебра; g коэффициент рекомбинации; I интенсивность облучения, m_o концентрация электронов на верхнем уровне серебра.The dark concentration of charge carriers in the substrate (n _o ) depends on the degree of compensation of impurities. Silver in Germany has three acceptor levels with ionization energies E _v +0.14 eV, E _c -0.28 eV, E _c -0.09 eV. If two levels are completely filled, and the upper is partially filled, then n _o is determined by the formula

The concentration of photocarriers is determined by the formula

where N _{Sb is the} concentration of antimony in the substrate; N _cm3 — effective density of states reduced to the upper level of silver; 3η = N _Ag / N _{Sb the} degree of compensation of impurities in the substrate, g _f photoionization cross section of the upper level of silver; g recombination coefficient; I radiation intensity, m _o electron concentration at the upper level of silver.

 Table 1 shows the parameters of the FPU regions.

где γ 10^-12 см^-3 • с^-1; g_ф=10^-17 см²; I 5•10¹² кван/см²с; V 1B; e 16 • 8,85 • 10^-14 ф•см^-1; L 3•10^-2 см.The theoretically estimated increase in sensitivity in the pulsed mode compared with the stationary mode, for different degrees of substrate compensation, which can be obtained, is given in table. 2. In practice, the prototype has ≈10 yield, the proposed solution is ≈30 The calculation is carried out according to the formula

where γ 10 ^-12 cm ^-3 • s ^-1 ; g _f = 10 ^-17 cm ² ; I 5 • 10 ¹² quantum / cm ² s; V 1B; e 16 • 8.85 • 10 ^-14 f • cm ^-1 ; L 3 • 10 ^-2 cm.

As can be seen from table 2, an increase in the photocurrent can be obtained on a substrate with overcompensated impurities, with accurate compensation of impurities, an increase in photosensitivity is not observed compared with the stationary mode. The spread of degree of substrate compensation mainly given antimony distribution in the radial distribution of germanium at 2 (3η ≠ 1,0 1,02), which leads to a change in the photosensitivity in the stationary regime at 40KΔn ₁ / Δn ₃ 4,1 • 10 ^12/8 • ¹⁰ July 5 • 10 ^4, in the pulsed mode, the ratio is 4.1 • 10 ^12/8 10 ⁷¹²⁵ • = • 4 February ¹⁰ times, t. e., the photocurrent increases 5 • 10 ^4/4 • February ¹⁰
10 ² times.

With a distribution of 0.5%, Δn ₁ / Δn ₂ = 4.1 • 10 ^12/3 • 10 ⁸ 10 ³ , in the pulsed mode 4.1 • 10 ^12/3 • 10 ⁸ • 31 = 30, i.e. photocurrent increases by 30 times.

Since the increase in the photocurrent in the overcompensated regions of the substrate is greater than in the regions with exact compensation, the pulsed interrogation mode reduces the inhomogeneity of the photosensitivity distribution over the working field of the FPU. With degrees of compensation 3η = 1,0; 3η = 1.02 the spread of the photocurrent is ≈5 • 10 ⁴ times in the stationary mode, in the pulsed mode ≈4 • 10 ² times.

происходит разделение функций инжектирующего контакта к компенсированной подложке на две: первую функцию омического контакта выполняет контакт металла и поверхности обогащенной области N⁺⁺, вторую функцию именно инжектирующего контакта выполняет переход от области 2 к объему подложки. Ширина области 2 должна быть ≥Lэкр, n⁺-n перехода при точной компенсации примесей. Это позволяет получить технологически воспроизводимый контакт, уменьшение неоднородности распределения фоточувствительности, увеличения фоточувствительности в импульсном режиме опроса при низких температурах, исключается влияние поверхности на инжектирующий контакт.Thus, when creating the transition region 2 with a thickness

there is a separation of the functions of the injecting contact to the compensated substrate into two: the first function of the ohmic contact is performed by the contact of the metal and the surface of the enriched N ⁺⁺ region, the second function of the injecting contact is the transition from region 2 to the volume of the substrate. The width of region 2 should be ≥Lecr, n ⁺ -n junction with accurate compensation of impurities. This makes it possible to obtain a technologically reproducible contact, a decrease in the heterogeneity of the photosensitivity distribution, an increase in photosensitivity in a pulsed interrogation mode at low temperatures, the influence of the surface on the injection contact is excluded.

 2. Compared with gold deposited on a substrate, a more stable ohmic contact was obtained on compensated germanium.

 In the table. Figure 3 shows FPUs with different metal films.

Claims (2)

1. A photodetector for the infrared region of the spectrum, containing a substrate of a semiconductor doped with a shallow donor impurity and precisely compensated by a deep multiply charged acceptor impurity, creating doubly charged levels, on both sides of which conductive buses are created, consisting of a contact layer created in the substrate enriched a specially introduced fine impurity of the same type of conductivity as the substrate, and a metal film deposited on it, which creates electrical contact with it, characterized the fact that the contact layer consists of the first region in contact with the metal, with a maximum concentration of impurities, determined by the formula

provided L ₁ ≥L _{differential,}
where D is the diffusion coefficient of the main carriers;
L _diff diffusion length;
γ recombination coefficient of current carriers;
L _{1 the} thickness of the first region and the second region following it with an impurity concentration of 1 2% higher than the concentration of the fine donor impurity of the substrate, and their thicknesses are correlated as follows:

where L _{2 is the} thickness of the second region;
ε dielectric constant;
Dv is the height of the potential barrier of the n ⁺ n- or p ⁺ p- junction;
e is the charge of an electron;
N _{t the} concentration of traps at the levels of acceptor impurities with accurate compensation of impurities, equal to the dark concentration of current carriers;
E _{max the} maximum field of the space charge region of the contact metal substrate;
v _{o the} height of the potential contact barrier of the metal substrate.  2. The device according to claim 1, characterized in that nickel is used as a metal film.

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