RU2569041C1 - Heterostructure for translucent photocathode - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: claimed heterostructure comprises substrate of GaAs, first ply of GaAs of p-conductivity, second ply of AlGaAs of p-conductivity. Note here that first ply of AlGaAs is a locking composition of Al_xGa_1-xAs of p-conductivity with concentration P₁ of acceptor dopant. Active ply of GaAs features concentration P₂ of acceptor dopant. Second ply of AlGaAs is a buffer composition Al_yGa_1-yAs with concentration P₃ of acceptor dopant. Transition ply is arranged between active and buffer plies of p-conductivity and composition varying form GaAs to Al_yGa_1-yAs. Note here that aluminium content variation starting from the boundary with active ply to that with buffer ply is a monotone increasing and continuous function F₁ of transition ply depth. Acceptor dopant concentration is a monotone decreasing continuous function F₃ of transition ply depth starting from concentration P₂ nearby the boundary with active ply to concentration P₃ nearby the boundary with buffer ply.

EFFECT: higher quantum efficiency and integral response of photocathode.

2 cl, 3 dwg

Description

Technical field

The invention relates to the field of electrovacuum technology, in particular to semiconductor optoelectronic devices - photocathodes, and in particular to a heterostructure for a translucent photocathode with an active layer of gallium arsenide, photosensitive in the visible and near infrared range.

State of the art

Photocathodes with negative electron affinity based on AlGaAs / GaAs / AlGaAs heteroepitaxial structures are widely used as a photosensitive element of semiconductor optoelectronic devices: electron-optical converters, photomultipliers used in radiation detectors.

A known heterostructure design for a translucent photocathode based on gallium arsenide [I. Sakhno, A.V. Dolgikh, V.G. Chubarev, I.I. Marakhovka, Yu.G. Galitsyn, V.G. Mansurov, A.S. Suranov. Gallium arsenide photocathode based on the AlGaAs / P ⁺ -GaAs / AlaAs heteroepitaxial structure grown by molecular beam epitaxy, ZhTP, 1996, Volume 22, Issue 23] including a GaAs substrate, p-Al _0.6 Ga ₀ stop layer _{, 4} As 1.0 μm thick with an acceptor impurity concentration of 5 · 10 ¹⁷ cm ^-3 , p-GaAs active layer 1.5 ... 2.0 μm thick with an acceptor impurity concentration of 0.7 ... 9 · 10 ¹⁹ cm ^-3 , a buffer layer of p-Al _0.5 Ga _0.5 As 1.0 μm thick with an acceptor impurity concentration of 5 · 10 ¹⁷ cm ^-3 with an integrated sensitivity of more than 1200 μA / Lm.

A known photocathode design based on a GaAs heteroepitaxial structure with an AlGaAs buffer layer with a thickness of 100 ... 120 Angstroms, an active GaAs layer with a thickness of 0.35 ... 0.45 μm with an integrated sensitivity of more than 1000 μA / Lm [Utility model, patent RU 94057, H01J 31 / fifty; H01J 43/00, 05/10/2010].

A known heterostructure for a translucent photocathode proposed in [L.G. Zabelina, A.S. Petrov. Heteroepitaxial structures based on gallium arsenide for photocathodes up to 1.1 μm, Applied Physics, No. 3, 1999], including p-GaAs sequentially arranged on the substrate: Al _0.5 Ga _0.5 As retainer layer, GaAs active layer and buffer layer Al _0.5 Ga _0.5 As. They were used to fabricate translucent OES photocathodes on glass with a photoemissive sensitivity in the range of 0.5-0.9 μm, 500-1500 μA / Lm.

The closest heterostructure for a translucent photocathode was proposed in [I.V. Pinchuk. Development of semiconductor materials for night observation devices and industrial technology for their production, the Dissertation for the degree of candidate of technical sciences, Moscow. 2001]. A p-Al _x Ga _1-x As layer, a p-Al _y Ga _1-y As layer, an active p-GaAs layer doped with an acceptor impurity with a concentration of 0.8 ... 1.0 · 10 ¹⁹ cm are sequentially on the GaAs substrate ^{- 3} and a p-Al _z Ga _1-z As layer, where x = 0.15, y = 0.5, z = 0.6 with “sharp” heterojunctions. Based on these heterostructures with a diameter of 20 mm, translucent OES photocathodes were fabricated, which are the input window of the image intensifier tube and PMT with a maximum integrated “transparency” sensitivity of 1850 μA / Lm.

The disadvantage of all of the above structures is the high probability of the formation of a misfit dislocation due to the difference in the crystal lattice parameters and thermal expansion coefficients of the materials of the active and subsequent p-Al _z Ga _1-z As layer (see the last mentioned link) during subsequent heat treatment of the heterostructure, which leads to to enhance the recombination of minority charge carriers at the boundary of these layers. The recombination rate at the sharp boundary of the active and buffer layers reaches 1 · 10 ⁴ cm / s. The presence between the substrate and the p-Al _y Ga _1-y As layer of the intermediate p-Al _x Ga _1-x As layer (see [IV Pinchuk. Development of semiconductor materials for night-time monitoring devices and industrial technology for their production, Dissertation on competition for the degree of candidate of technical sciences, Moscow, 2001]), introduced to reduce the density of defects in the crystal lattice in the structure, is not enough to reduce the probability of the formation of a misfit dislocation in the entire structure with “sharp” heterojunctions. Accordingly, the sensitivity and quantum efficiency of such a translucent photocathode are insufficient.

Disclosure of invention

The technical result of the invention is to increase the integrated sensitivity and quantum efficiency of a semiconductor translucent photocathode operating in the “clear” mode based on a heterostructure with an active layer of gallium arsenide by reducing the likelihood of a misfit dislocation in the transition and buffer layers of the proposed heterostructure, removing the potential barrier for minority charge carriers at the boundary of the active and transition layers and; therefore, a decrease in the recombination rate of minority charge carriers at the interface between the active and buffer layers.

To achieve a technical result, a heterostructure is proposed for a translucent photocathode containing a GaAs substrate, then a first AlGaAs layer, an active p-type GaAs layer, and a second p-type AlGaAs layer. The difference is that the first AlGaAs layer is a p-type conduction Al _x Ga _1-x As composition with a concentration of P ₁ acceptor impurity. The active GaAs layer has a concentration of P ₂ acceptor impurity. The second AlGaAs layer is a buffer composition of Al _y Ga _1-y As with a concentration of P ₃ acceptor impurity. Between the active and buffer layers, we additionally introduced a p-type transition layer of variable conductivity from GaAs to Al _y Ga _1-y As, and the change in the aluminum content, starting from the boundary with the active layer to the boundary with the buffer layer, is a monotonously increasing and continuous function F ₁ from the thickness of the transition layer, and the concentration of the acceptor impurity is a monotonously decreasing continuous function of F ₂ from the thickness of the transition layer, starting from the concentration of P ₂ at the boundary with the active layer to the concentration of P ₃ at the border with the buffer layer.

A significant difference between the proposed heterostructure of the translucent photocathode is the introduction of a transition layer of variable composition between the active layer and the second layer of a solid solution of gallium aluminum arsenide - buffer, while ensuring a monotonic and continuous change in both the composition and the level of doping along its thickness. In such a heterostructure, the probability of generating a misfit dislocation from the active layer to the buffer layer is noticeably reduced; moreover, the electric field that appears in the transition layer stimulates the drift of electrons generated by the absorption of light in the region of the transition layer toward the active layer. However, in the case when a sharp change in the concentration of sensitivity and quantum efficiency of a semiconductor semitransparent photocathode with an active layer of gallium arsenide operating in the "transparency" mode occurs at the boundary between the active and transition layers.

The technological implementation of the heterostructure of the translucent photocathode proposed in the invention is based on well-known basic methods for the manufacture of semiconductor photocathodes, including translucent ones, which are currently well developed and widely used. The proposal meets the criterion of "industrial applicability".

Brief Description of the Drawings

The invention is illustrated in FIG. 1-3.

In FIG. Figure 1 shows a fragment of the band diagram of the active, transition, and buffer layers of the heterostructure of a semitransparent photocathode with an active GaAs layer during stepwise doping of the transition layer, the composition of which varies from the composition of the active layer to the composition of the buffer layer according to a linear law.

In FIG. Figure 2 shows a fragment of the band diagram of the active, transition, and buffer layers of the heterostructure of a semitransparent photocathode with an active GaAs layer when linearly doped with a change in the thickness of the transition layer, the composition of which varies from the composition of the active layer to the composition of the buffer layer according to the linear law.

In FIG. 3 schematically depicts the inventive heterostructure design for a semiconductor translucent photocathode of gallium arsenide with a transition layer.

Embodiments of the invention

The invention is further illustrated by specific embodiments of the proposed heterostructure for a translucent photocathode based on gallium arsenide. We further give a number of examples from the entire set of manufactured devices.

In the first modification, the inventive heterostructure design of a translucent photocathode with an active layer of gallium arsenide and with a transition layer (hereinafter “Heterostructure”) is illustrated in FIG. 3. The heterostructure consistently consists of a substrate 1 of GaAs with a thickness of 500 μm, n-type conductivity, a retaining layer 2 of p-Al _x Ga _1-x As, where the value of "x" is 0.7, a thickness of 0.9 μm, s the concentration of P ₁ acceptor impurities 2 · 10 ¹⁸ cm ^-3 ; the active layer 3 of p-GaAs with a thickness of 2.0 μm, with a concentration of P ₂ acceptor impurities 1 · 10 ¹⁹ cm ^-3 ; transition layer 4 of variable composition of p-type conductivity with a thickness of 0.55 μm and a buffer layer 5 of Al _y Ga _1-y As, where the value of "y" is 0.55, a thickness of 0.55 μm, with a concentration of P ₃ acceptor impurity 2 10 ¹⁸ cm ^-1 . In this case, the transition layer 4 has a composition that varies from GaAs at the boundary with the active layer 3 to Al _y Ga _1-y As, where the value of "y" is 0.55, at the boundary with buffer layer 5 according to the linear law calculated by the formula F ₁ (d) = k · d, where d is the thickness of the transition layer and k is a constant for a given thickness of the transition layer. It is determined that for a transition layer thickness of 0.55 μm, the constant k is 1.0. The concentration of the acceptor impurity in the transition layer decreases from 1 · 10 ¹⁹ cm ^-3 to 2 · 10 ¹⁸ cm ^-3 also according to the linear law, following the formula F ₂ (d) = a · d + c, where a and c are constants for a given thickness of the transition layer. For a thickness a equal to 0.55 μm, a and c are -1.455 · 10 ¹⁹ and 1 · 10 ^19, respectively.

An antireflection layer was grown on the buffer layer of the Heterostructure and translucent photocathodes were fabricated, on which the following output parameters were obtained:

- integrated sensitivity equal to 3190 μA / lm and

- quantum efficiency at a wavelength of 700 nm, equal to 53%, which exceeds the known values by 1.2 times.

In the second modification, the inventive Heterostructure (also illustrated in Fig. 3) consistently consists of a substrate 1 of GaAs 300 μm thick, n-type conductivity, a retaining layer 2 of Al _x Ga _1-x As, where the value of "x" is 0.5, a thickness of 0.3 μm, with a concentration of P ₁ ) acceptor impurities 1 · 10 ¹⁸ cm ^-3 ; active layer 3 of p-GaAs with a thickness of 1.0 μm, with a concentration of P ₂ acceptor impurity 5 · 10 ¹⁸ cm ^-3 ; transition layer 4 of variable composition of p-type conductivity with a thickness of 0.1 μm and a buffer layer 5 of Al _y Ga _1-y As, where the value of "y" is 0.4 with a thickness of 0.1 μm, with a concentration of P ₃ acceptor impurity 1 · 10 ¹⁸ cm ^-3 . In this case, the transition layer 4 has a composition that varies from GaAs at the boundary with the active layer 3 to Al _y Ga _1-y As, where the value of "y" is 0.4, at the boundary with buffer spout 5 according to the linear law calculated by the formula F ₁ (d) = k · d, where d is the thickness of the transition layer and k is a constant for a given thickness of the transition layer. It is determined that for a transition layer thickness of 0.1 μm, the constant k is 4.0. The concentration of the acceptor impurity in the transition layer decreases from 5 · 10 ¹⁸ cm ^-3 to 1 · 10 ¹⁸ cm ^-3 also according to the linear law, following the formula F ₂ (d) = а · d + с, where a and c are constants for a given thickness of the transition layer. For a thickness of 0.1 μm, a and c are equal to 4 · 10 ¹⁹ and 5 · 10 ^18, respectively.

An antireflection layer was grown on the buffer layer of the Heterostructure and translucent photocathodes were fabricated, on which the following output parameters were obtained:

- integrated sensitivity equal to 2760 μA / lm and

- quantum efficiency at a wavelength of 700 nm, equal to 46%.

In the third modification, the inventive Heterostructure (also illustrated in Fig. 3) consistently consists of a substrate 1 of GaAs with a thickness of 700 μm, n-type conductivity, a retaining layer 2 of p-Al _x Ga _1-x As, where the value of "x" is 0, 9, a thickness of 1.5 μm, with a concentration of P ₁ acceptor impurities 310 ¹⁸ cm ^-3 ; active layer 3 of p-GaAs with a thickness of 3.0 μm, with a concentration of P ₂ acceptor impurities of 1.5 10 ¹⁹ cm ^-3 ; a transition layer 4 of a variable composition of p-type conductivity with a thickness of 1.0 μm and a buffer layer 5 of Al _y Ga _1-y As, where the value of "y" is 0.7, a thickness of 1.0 μm, with a concentration of P ₃ acceptor impurity 3 10 ¹⁸ cm ^-3 . In this case, the transition layer 4 has a composition that varies from GaAs at the boundary with the active layer 3 to Al _y Ga _1-y As, where the value of "y" is 0.7, at the boundary with buffer layer 5 according to the linear law calculated by the formula F ₁ (d) = k · d, where d is the thickness of the transition layer and k is a constant for a given thickness of the transition layer. It is determined that for a transition layer thickness of 1.0 μm, the constant k is 0.7. The concentration of the acceptor impurity in the transition layer decreases from the value of 1.5 · 10 ¹⁹ cm ^-3 to the value of 3 · 10 ¹⁸ cm ^-3 , also according to the linear law, following the formula F ₂ (d) = a · d + с, where a and C - constants for a given thickness of the transition layer. For a thickness d of 1.0 μm, a and c are -1.2 · 10 ¹⁹ and 1.5 · 10 ^19, respectively.

An antireflection layer was grown on the buffer layer of the Heterostructure and translucent photocathodes were fabricated, on which the following output parameters were obtained:

- integral sensitivity equal to 3030 μA / lm and

- quantum efficiency at a wavelength of 700 nm, equal to 50%.

In the next modification, the heterostructure has a design in accordance with the first modification with a difference in the choice of substrate 1. In this case, changes in the output parameters are noted at the level of measurement error.

In the next modification, the heterostructure has a design in accordance with the first modification with a difference in the choice of p-type substrate 1. In this case, changes in the output parameters are noted at the level of measurement error.

In the following modifications, the difference from the first modification consists in another mathematical law of changes in the functions F ₁ (d) and F ₂ (d), i.e. the composition of the transition layer 4 and the concentration of the dopant. For exponential and logarithmic changes in the functions F ₁ and F ₂ , the corresponding Heterostructures were calculated. All other parameters did not differ from the heterostructure of the first modification.

For the exponential case, the composition of the transition layer varies according to the law F ₁ (d) = q · exp (d) + w, and the concentration of the acceptor impurity of the transition layer changes according to the law F ₂ (d) = t · exp (p · a), where d - the thickness of the transition layer 4, μm, q, w, t and p are constants, determined equal to 0.75, -0.75, 1 · 10 ¹⁹ , -2.926, respectively.

For the logarithmic case, the composition of the transition layer varies according to the law F ₁ (d) = g · ln (h + d), and the concentration of the acceptor impurity of the transition layer 4 changes according to the law F ₂ (d) = j · ln (e + m * d) where d is the thickness of the transition layer 4 μm, g, h, j and m are constants, are defined as 1.255, 1, 1 · 10 ¹⁹ , -2.722, respectively, and e is the base of the natural logarithm (mathematical constant, approximately equal to 2.718). Changes in the output parameters for these two cases are noted at the level of measurement error.

In the following modifications, the heterostructures are constructed in accordance with the first modification with the difference that an intermediate layer of undoped GaAs with different thicknesses of 0.125 μm, 0.05 μm, and 0.2 μm is introduced between the substrate and the stopper layer. Changes in the output parameters for the last three modifications are noted at the level of measurement error.

When using the claimed design, the characteristics of translucent photocathodes made of gallium arsenide were improved: the integrated sensitivity was increased above 2500 μA / lm and the quantum efficiency at a wavelength of 700 nm was above 45%.

Industrial applicability

The proposed heterostructure of a translucent photocathode can be used in the manufacture of a photosensitive element of optoelectronic devices: electron-optical converters, photomultipliers used in radiation detectors.

Claims (2)

1. A heterostructure for a translucent photocathode, containing a GaAs substrate, then a first AlGaAs layer, an p-type active GaAs layer, a second p-type AlGaAs layer, characterized in that the first AlGaAs layer is a retainer composition Al _x Ga _1-x As p-type conductivity with a concentration of P ₁ acceptor impurity, the active GaAs layer has a concentration of P ₂ acceptor impurity, the second AlGaAs layer is a buffer composition of Al _y Ga _1-y As with a concentration of P ₃ acceptor impurity, there is a transition layer p between the active and buffer layers -type conductivity Nogo composition of GaAs to Al _y Ga _1-y As, wherein the change in aluminum content, starting from the boundary with the active layer to the boundary with the buffer layer, is monotonously increasing and continuous function F ₁ of the thickness of the transition layer, and the acceptor impurity concentration is monotonously decreasing a continuous function of F ₃ on the thickness of the transition layer, starting from the concentration of P ₂ at the border with the active layer to the concentration of P ₃ at the border with the buffer layer. 2. The heterostructure according to claim 1, characterized in that between the substrate and the retaining layer there is an intermediate GaAs layer with a thickness of 0.05 μm to 0.2 μm.

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