CZ307941B6 - Multilayer semiconductor structure for photoelectric devices - Google Patents

Abstract

The invention is based on using a self-ordered triple KT combination of two semiconductor materials InAs and GaAsSb for efficient light absorption, generating electron-hole pairs and their rapid separation, preventing reverse recombination of the generated charge carriers and thus the efficiency of photocurrent generation. The trio KT consists of one InAs KT for electron localization and above it a pair of GaAsSb KT for localizing holes. InAs KT and GaAsSb KT are separated from each other by a thin layer of GaAs or GaAsSb with a low Sb content. The formation of GaAsSb KT is enhanced when an aprox. 1-3 nm InGaAs undercoat is deposited under the InAs layer. This layer also increases the spatial separation of the electrons and holes, increases the surface density of InAs KT and helps the extraction of electrons from InAs KT. The structure can be incorporated into photodiodes, solar cells and tandem solar cells for light absorption in the 1,100-1,500 nm spectral range. The structure increases the efficiency of photocurrent generation by effectively suppressing the recombination of generated charge carriers.

Description

Multilayer semiconductor structure for photoelectric devices

Technical field:

The invention relates to multilayer semiconductor structures for photoelectric devices, in particular for solar cells converting light into electricity, comprising quantum dots of InAs (indium arsenide) and GaAsSb alloy semiconductor (arsenide and gallium antimonide).

BACKGROUND OF THE INVENTION:

The most widespread material for solar cells is currently silicon (Si) with a simple p-n transition, especially for ease of availability and economical manufacturing. However, for some special applications requiring high efficiency of conversion of light into electricity, more complex layered semiconductor structures, so-called tandem cells, are used, in which different regions of the solar spectrum are absorbed in different parts of the tandem heterostructure. In recent years, the possibility of using quantum dots for solar tandem cells InAs (arsenide india) has been discovered for the conversion of solar radiation in the near infrared region. Quantum dots (often referred to as KT or also from Quantum Dots QD) are lenticular formations of InAs, In (Ga) As semiconductor (Thium Indium Arsenide and Gallium Arsenide Semiconductor) or Ga (As) Sb (Thium Gallium Arsenide and Gallium Antimonide Alloy) ) in a GaAs (gallium arsenide) matrix. Their base has a diameter of several tens of nm and their height ranges from 2 to 10 nm. It is a system with non-lattice materials with high stress in the structure. CC in this system is formed by self-organization in the so-called Stránský-Krastanow growth mode. This occurs when a combination of certain types of semiconductors (eg, when InAs grows to GaAs) with different lattice constants under suitable growth conditions: In order to release the mechanical stress in the structure, a thin so-called wetting layer forms during epitaxy. layers are self-arranging small islands of InAs as so-called quantum dots.

From A. Luque et al. , Journal of Applied Physics 96, 903 (2004), General Equivalent Circuit for Intermediate Band Devices: Potentials, currents and electroluminescence, it is known that quantum dots can form a so-called mid-band in GaAs semiconductors that allow multi-photon absorption of light (concept) Intermediate band solar cells (IBSC). However, these structures have two significant disadvantages. First, the quantum dots in the structure have a small volume, which reduces the proportion of light absorbed in the quantum dots. In addition, the light-generated charge carriers (electrons and holes) are bound in the InAs dots and recombine very quickly so that only a small portion of these charge carriers contribute to the desired photocurrent. The first disadvantage is solved by a structure containing a large number of planes with quantum dots. The second disadvantage can be eliminated by creating so-called different heterostructures II. Type KT are either KT formed from GaSb semiconductor in GaAs matrix or KT InAs are covered by GaAsSb layer with sufficiently high Sb content. In heterostructure II. The two types of charge carriers have a minimum potential energy in different materials and are therefore spatially separated in the basic quantum state. (A comparison of the energy course of the conductive and valence band edges in type I and type II heterostructure is shown in Fig. 1 (a), (b)). Electrons in heterostructure II. type are found in quantum dots InAs, holes are located in GaAsSb layer just above quantum dots. This arrangement reduces the recombination of the generated charge carriers, but similarly reduces the efficiency of the generation of the electron-hole pair in light absorption.

As is known, for example, from Yingnan Guo, Appl. Phys. Lett. 111, 191105 (2017) Carrier Dynamics of InAs quantum dots with GaAsl-xSbx barrier layers, the quantum dots of InAs / GaAsixSb _x exhibit better properties than the quantum dots of InAs / GaAs. Even in these structures, however, the reduction in radiant recombination is insufficient, as has often been demonstrated by demonstrating relatively strong photoluminescence, that is, effective radiative recombination.

- 1 GB 307941 B6

A. Hospodková et al., J. Appl. Phys. 114, 174305 (2013), Combined vertically correlated InAs and GaAsSb quantum dots separated by triangular GaAsSb barrier, a combination of two types of KTs, namely KTs consisting of InAs and KTs consisting of GaAsSb in vertically correlated II heterostructure, is described. type. Also, these structures exhibit relatively strong radiant recombination. This is due to the very close localization of electrons and holes arranged separately but vertically one above the other.

From Hai-Ming Ji et al., Appl. Phys. Lett. 106, 103104 (2015), Hybrid type-I InAs / GaAs and type-II GaSb / GaAs quantum dot structure with enhanced photoluminescence are known hybrid heterostructures I. and II. type with GaSb / GaAs quantum dots in the GaAs matrix.

From the patent document CN 102509700 there is known a method of deposition of InAs quantum dots by self-arrangement on a pre-prepared thin layer of GaAsSb.

From U.S. Pat. No. 9,240,507, a super-lattice structure is known in which the barrier layer and the quantum dots layer alternately form a type I structure to form a super-lattice band of energies.

The structure of InAs KT surrounded by InGaAs or GaAsSb layers (ie in the case of GaAsSb with low Sb content, ie type I, where both types of charge carriers are located in InAs KT in the lowest energy state) is also used for laser applications in the region of 1300 nm. The CZ 303855 patent discloses a multilayer semiconductor epitaxial structure with InAs / GaAs quantum dots and a GaAsSb cover and stress reducing layer retaining a type I heterosection between the InAs quantum dots and the GaAsSb layer and allowing the emission of electroluminescence at a telecommunication wavelength of 1300 nm. Preservation of type I heterogeneity for these wavelengths is achieved by a graded composition of the GaAsx _x Sb _x layer so that the GaAsSb layer has a lower Sb concentration (x = 0.03 to 0.09) immediately at the quantum dots (thus ensuring sufficient the barrier for holes in InAs quantum dots and their localization in quantum dots) in the direction of epitaxial growth of the antimony concentration in GaAsSb increases to a maximum concentration ranging from x = 0.15 to 0.30. This achieves the necessary stress reduction within the quantum dots and the extension of the emitted wavelength. These properties may also be advantageous for use in photovoltaic structures.

SUMMARY OF THE INVENTION

SUMMARY OF THE INVENTION The present invention is based on the use of a self-assembled triple KT combined of two semiconductor materials InAs and GaAsSb for efficient light absorption, generation of an electron pair and rapid separation thereof to prevent back recombination of generated charge carriers. The principle of improving the photovoltaic properties of the present invention lies in the higher Sb content in the GaAsSb layer and in the formation of a functional triad of one InAs and two GaAsSb quantum dots each to provide more spatial separation of generated electrons and holes in the lowest energy state. The present invention solves the above-mentioned disadvantages by using a self-ordered triad KT, namely a quantum dot InAs for electron localization and a pair of quantum dots GaAsSb for hole localization always on the sides of the quantum dot InAs. This greatly reduces the recombination of charge carriers generated in the KT region, thereby significantly increasing the efficiency of converting the energy of incident infrared radiation into photocurrent.

Above the GaAs or InGaAs layer, there is a wetting InAs layer with a higher concentration of In, on which the InAs quantum dots are formed, with two GaAsSb quantum dots that spontaneously originate in the crystallographic direction alongside the InAs quantum dots. (-110) during GaAsSb deposition, and which partially overlap the respective InAs quantum dot from above on positions on opposite sides of the circuit. Indium arsenide quantum dots, referred to herein as InAs quantum dots, can of course

However, according to the present invention, gallium is not intentionally added.

Thus, the KT trio consists of one InAs quantum dot for electron localization and above it a pair of GaAsSb quantum dots for hole localization, see the self-ordered structure diagram as shown in Figure 2. This pair of GaAsSb KT arises spontaneously under suitable technological conditions on the sides of InAs KT the system has sufficient time at or after GaAsSb layer epitaxy to find thermodynamic equilibrium (typically when the layer growth time along with growth interruption after GaAsSb deposition is greater than 50 s). In this arrangement, the electrons and holes in the lowest energy state are spatially separated from each other, but the excited states in the two types of quantum dots overlap sufficiently to efficiently absorb light and generate an electron-hole pair as shown in Figure 3. Both charge carriers relax very quickly from excited to the basic quantum state (relaxation time of the order of 10 ¹¹ s). This results in their spatial separation and prevents their recombination. This is followed by the extraction of charge carriers from the triad KT region by the present built-in electric field in the PIN structure, thereby contributing to the photocurrent generated by this component when illuminated by the infrared part of the solar spectrum. In order to have a sufficiently strong spatial separation of electrons in the InAs quantum dot and holes in the GaAsSb quantum dots (to reduce the overlap of wave functions in the lowest energy state), a 1 to 3 nm thin layer of GaAs or GaAsSb s should be inserted between InAs and GaAsSb. low content of Sb. The lowest Sb content in GaAsi_ _x Sb _x in the region adjacent to KT can be selected in the range x = 0 to 0.12. This composition is selected to maintain a sufficient barrier for the baseline holes located in the GaAsSb region of quantum dots, thereby separating the holes sufficiently spatially from the electrons located in the InAs quantum dot, while keeping the barrier lower than the excited hole energy state, thereby ensuring sufficient overlapping of the electron waveforms and holes in the excited state, and thereby generation of the electron leaky pair. The concentration of antimony in GaAsi_ _x Sb _x quantum dot is in the range x = 0.2 to 0.9 so as to achieve II heterostructure. type, ie the dominant potential hole for holes in the area of GaAsSb quantum dots (see Fig. 1 b), whose barriers are formed on one side by a low Sb concentration GaAsSb layer and on the other side by a covering GaAs layer. With higher Sb content, self-organizing GaAsSb quantum dots come closer together. Suitable thicknesses of the GaAsi- _x Sb _x layer from which spontaneous KT spontaneously develops during epitaxy are in the range of 2 to 10 nm. A side effect of this GaAsSb layer is to prevent the "dissolution" of the InAs quantum dot during growth of the cover layer. It has been found that the formation of GaAsSb quantum dots is enhanced when about 0.5 to 3 nm thin InGaAs undercoat is deposited under the InAs layer. This layer has other advantageous functions: it increases the spatial separation of electrons and holes, increases the surface density of the InAs quantum dots, and facilitates the extraction of electrons from the InAs quantum dots.

The multilayer semiconductor structure according to the invention preferably comprises, from the substrate, the following layers, respectively:

The n-type GaAs layer an unalloyed GaAs layer optionally may comprise an InGaAs backing layer

InAs wetting layer (average thickness of 1.8 to 2.5 monoatomic layer) for electron localization and discrete InAs quantum dots on it optionally can contain GaAs or GaAsSb low GaSb component separation layer

GaAsSb layer of quantum dots for hole localization (thickness is 2 to 10 nm depending on GaAsSb composition) unalloyed GaAs layer p-type GaAs layer

The multilayer semiconductor structure according to the invention may advantageously comprise, from the substrate, the following layers, respectively:

n-type GaAs layer with a thickness of for example 200 nm,

An unalloyed GaAs layer having a thickness of, for example, 200 nm,

In _y Ga- _y As layer with a thickness of 0.5 to 3 nm with an In content corresponding to the above formula, where y <0.3, preferably y = 0.2,

InAs wetting layer having a thickness of 1.8 to 2.5 monoatomové layers, preferably two layers monoatomových and located thereon InAs quantum dots (11), a thin GaAsi_ _x Sb _x layer wherein x = 0 to 0.12, preferably x = 0,08, of a thickness of 1 to 3 nm,

GaAsi. _x Sb _x , where x = 0.2 to 0.9, with a thickness of 2 to 10 nm, preferably 5 nm, with quantum dots (10), an unalloyed GaAs layer with a thickness of, for example, 200 nm, and a p-type GaAs layer of a thickness of, for example, 200 nm.

The layers of the structure carrying the quantum dots can be repeated multiple times and combined with sufficiently large GaAs layers, the total thickness of such a structure may have a thickness of 200 nm to several µm. In addition, in a multi-layered multi-dot structure, the recombination of charge carriers migrating through the structure is reduced due to orthogonal non-crossing electron and hole trajectories in the combined In (Ga) As and GaAsSb quantum dot structure. For the structure to be functional, it must be inserted into the intrinsic area of the GaAs PIN diodes with a low built-in electric field.

The multilayer semiconductor structure can be prepared by the standard method of gaseous epitaxy of organometallic compounds (MOVPE) or by molecular beam epitaxy (MBE), or by other epitaxial technologies enabling the preparation of layers with a thickness of several nanometers (CBE, MOMBE).

Using the present invention, with the combined quantum dots described, up to 72% of the integral photocurrent generated by the absorption of light at a wavelength of 900 to 1600 nm was increased compared to single InAs quantum dots. Another significant increase in photocurrent (approximately 200x) occurred when the thin InGaAs underlayer was incorporated under the combined quantum dots.

The structure of the present invention increases the efficiency of photocurrent generation by effectively suppressing the recombination of generated charge carriers.

Clarification of drawings

Giant. 1 is a schematic representation of the energy waveform of a conductive and valence band in the direction of epitaxial growth (a) in a type I heterostructure with only InAs quantum dots, with both electrons and holes located in InAs quantum dots, (b) in a heterostructure II. of the type of the invention with combined InAs and GaAsSb quantum dots, wherein the electrons are located in InAs quantum dots and the ground quantum state holes in GaAsSb quantum dots.

Giant. 2 (a) is a schematic representation of a self-ordered triad of KTs of the invention in cross-section across the InAs quantum dot for electron localization and an adjacent pair of GaAsSb quantum dots for localizing holes in the sides of the InAs quantum dot. The individual layers are shown in the direction of epitaxial growth from the substrate upwards. Giant. Fig. 2 (b) is a photomicrograph of the surface of an exemplary structure of the invention; and Fig. 2 (c) shows an exemplary surface profile of a structure of the invention.

Giant. 3 represents the energy waveforms of the conductive and valence bands and the localization of electrons and holes in the heterostructure of the present invention in (a) an excited state that is used for light absorption and (b) a ground quantum state that is used for rapid spatial separation of electrons; and preventing back recombination.

-4GB 307941 B6

Giant. 4 is a comparison of the generated photocurrent without applied voltage depending on the wavelength of incident light for a structure with only one layer of InAs quantum dots, with five layers of InAs quantum dots and for a KT structure, according to one embodiment of the invention, with combined InAs and GaAsSb.

Giant. 5 is a comparison of the generated photocurrent without applied voltage versus the wavelength of incident light for a structure with combined InAs and GaAsSb quantum dots with a GaAs backing layer and for a structure according to the second embodiment of the invention with an embedded InGaAs backing layer of 2 nm.

Giant. 6 is a comparison of the generated photocurrent without applied voltage versus the wavelength of incident light for a single layer structure of combined InAs and GaAsSb quantum dots according to and for a five layer structure of combined KTs according to a third embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Example 1- Photovoltaic structure with combined CH

The multilayer semiconductor structure was built into the PIN diode. The structure was prepared using the standard method of low pressure gaseous epitaxy of organometallic compounds (LP MOVPE).

The structure contains the following layers in order:

n-type GaAs substrate 1 of n-type GaAs epitaxial layer 2 having a concentration of holes of 10 ¹⁷ cm ^-3, thickness 200 nm, the preparation temperature of 650 ° C unalloyed GaAs layer 3, the thickness of 200 nm, the preparation temperature of 650 ° C

InAs wetting layer 5 with 2 atomic layer thicknesses for electron localization, preparation temperature 510 ° C, layer growth time 24 seconds, interrupting growth for 15 s to create quantum dots, InAs wetting layer 5 are discrete InAs quantum dots InAs GaAsi. _x Sb _x layer 6 with a composition corresponding to the above formula, where x = 0.03 with a thickness of 2 nm, preparation temperature 510 ° C

GaAsi-xSbx layer 7 with quantum dots 10, with composition x = 0.20 and 6 nm thickness, preparation temperature 510 ° C, growth time 50 seconds, with 10 sec growth to create quantum dots 10 unalloyed GaAs layer 8, thickness 200 nm, preparation temperature 650 ° C p-type GaAs epitaxial layer 9 with a hole concentration 10 of ¹⁷ cm ³ , thickness 200 nm, preparation temperature 650 ° C

After epitaxy, PdGeAu contacts were steamed onto the cold structure. Alternatively, the contacts may be made of ITO material. The dependence of photocurrent on energy of incident radiation of prepared photodiode and comparison of photocurrent for different types of photodiodes with KT is presented in Fig.

4.

Example 2 - Photovoltaic structure with combined KT and InGaAs underlay

A multilayer semiconductor structure was prepared according to Example 1 with the difference that a 2 nm InGaAs layer is sandwiched between the unalloyed GaAs layer 3 and the InAs wetting layer 5.

The structure contains the following layers:

n-type GaAs pad 1 n-type GaAs epitaxial layer 2 with a hole concentration 10 ¹⁷ cm ^-3 , thickness 200 nm, preparation temperature 650 ° C

Unalloyed GaAs layer 3, thickness 200 nm, preparation temperature 650 ° C unalloyed In _y Gai _y As layer 4, wherein the content of In corresponds to the above formula, where y = 0.2, thickness 2 nm, preparation temperature 510 Noc: 2 ° C

InAs wetting layer 5 with 2 atomic layer thickness, preparation temperature 510 ° C, layer growth time of 24 seconds, with interruption of growth for 15 s to create quantum dots, InAs wetting layer 5 are discrete InAs quantum dots InAs

GaAsi. _x Sb _x layer 6 with composition x = 0.03 thickness 2 nm, preparation temperature 510 ° C

GaAsi-xSbx layer 7 with quantum dots 10, with composition x = 0.20 and 6 nm thickness, preparation temperature 510 ° C, growth time 50 seconds, with 10 sec growth to create quantum dots 10 unalloyed GaAs layer 8, thickness 200 nm, preparation temperature 650 ° C p-type GaAs epitaxial layer 9 with a hole concentration 10 of ¹⁷ cm ³ , thickness 200 nm, preparation temperature 650 ° C

Photovoltaic energy dependence of photodiode prepared according to Example 2 with In _y Gai As _y layer 4 located between the unalloyed GaAs layer 3 and the InAs wetting layer 5. Comparison to the photocurrent for the structure of Example 1, wherein the InAs wetting layer 5 is immediately above the unalloyed GaAs Layer 3 is presented in Fig. 5.

Example 3 - Five-fold structure for use in the active region of a photodiode

5x structure comprising quantum dots overlapped structure GaAsSb graded layer of n-type GaAs substrate 1 of n-type GaAs epitaxial layer 2 having a concentration of holes of 10 ¹⁷ cm ^-3, thickness 200 nm, the preparation temperature of 650 ° C unalloyed GaAs layer 3, the thickness of 200 nm , preparation temperature 650 ° C

InAs wetting layer 5 with 2 atomic layer thickness, preparation temperature 510 ° C, layer growth time of 24 seconds, with interruption of growth for 15 s to create quantum dots, InAs wetting layer 5 are discrete InAs quantum dots InAs

GaAsi-xSbx layer 6 with composition x = 0.03, thickness 2 nm, preparation temperature 510 ° C

GaAsi-xSbx layer 7 with quantum dots 10, composition x = 0.20 and thickness 6 nm, preparation temperature 510 ° C, growth time 50 s, with growth interruption for 10 s to create quantum dots 10 unalloyed GaAs layer 8, thickness 200 nm, preparation temperature 650 ° C p-type GaAs epitaxial layer 9 with hole concentration 10 ¹⁷ cm ³ , thickness 200 nm, preparation temperature 650 ° C

Layers 3-8 are repeated 5 *.

Fig. 6 shows an increase in photocurrent compared to a structure comprising a single layer of quantum dots.

Example 4 - Thirty-fold structure for use in the active region of a photodiode (solar cell)

Structure comprising a quantum dot structure 30x superimposed GaAsSb graded layer of n-type GaAs substrate 1 of n-type GaAs epitaxial layer 2 having a concentration of holes of 10 ¹⁷ cm ^-3, thickness 200 nm, the preparation temperature of 650 ° C unalloyed GaAs layer 3, the thickness of 200 nm , preparation temperature 650 ° C

InAs wetting layer 5 with 2 atomic layer thickness, preparation temperature 510 ° C, layer growth time of 24 seconds, with interruption of growth for 15 s to create quantum dots, InAs wetting layer 5 are discrete InAs quantum dots InAs

GaAsi-xSbx layer 6 with composition x = 0.03, thickness 2 nm, preparation temperature 510 ° C

-6GB 307941 B6

GaAsi. _x Sb _x layer 7 with quantum dots 10, composition x = 0.20 and thickness 6 nm, preparation temperature 510 ° C, growth time 50 s, interrupt growth for 10 s to create quantum dots 10 unalloyed GaAs layer 8, thickness 200 nm, preparation temperature 650 ° C p-type GaAs epitaxial layer 9 with hole concentration 10 ¹⁷ cm ³ , thickness 200 nm, preparation temperature 650 ° C

Layers 3-8 are repeated 30 times.

Industrial applicability

The invented multilayer semiconductor structure is suitable for incorporation into photodiodes, solar cells or tandem solar cells for light absorption in the spectral range 1100 to 1500 nm.

PATENT CLAIMS

Claims (8)

Multilayer semiconductor epitaxial structure for photoelectric devices, which comprises quantum dots of indium InAs arsenide and GaAsSb gallium arsenide and antimide alloy semiconductor, characterized in that two GaAsSb quantum dots (10) are arranged in triplicate with one InAs quantum dot ( 11), wherein two GaAsSb quantum dots (10) are located next to the InAs quantum dots (11) at a level above the base of the InAs quantum dots (11) on opposite sides at positions determined by the crystallographic direction (-110) during GaAsSb deposition, the two The GaAsSb quantum dots (10) from above partially overlap the respective InAs quantum dots (11). Multilayer semiconductor structure according to claim 1, characterized in that it comprises an n-type GaAs layer (2), an unalloyed GaAs layer (3), The InAs wetting layer (5) and the InAs quantum dots (11) thereon, GaAsSb layer (7) with quantum dots (10), unalloyed GaAs layer (8), and p-type GaAs layer (9). Multilayer semiconductor structure according to claim 2, characterized in that the GaAsSb layer (7) with quantum dots (10) is separated from the InAs of quantum dots (11) by a thin GaAs or GaAsSb layer (6) of GaAsi composition. _x Sb _x , where x = 0 to 0.12. Multilayer semiconductor structure according to claim 2 or 3, characterized in that an InGaAs layer (4) of the composition In _y Gai _{y y} As is located below the InAs layer (5), where y <0.3. Multilayer semiconductor structure according to one of the preceding claims, characterized in that the GaAsSb quantum dots (10) have a composition GaAsi- _x Sb _x , where x = 0.2 to 0.9. ^{5 * * * * * * *} Multilayer semiconductor structure according to one of the preceding claims, characterized in that it comprises an n-type GaAs layer (2), an unalloyed GaAs layer (3), In _y Gai- _y As layer (4), where y <0.3, with a thickness of 0.5 to 3 nm, An InAs wetting layer (5) having a thickness of 1.8 to 2.5 of the monoatomic layer and the InAs quantum dot (11) located thereon, GaAsi-xSbx layer (6), where x = 0 to 0.12, with a thickness of 1 to 3 nm, - 7 CZ 307941 B6 GaAsi. _x Sb _x layer (7), where x = 0.2 to 0.9, with a thickness of 2 to 10 nm with quantum dots (10), an unalloyed GaAs layer (8) and a p-type GaAs layer (9). Multilayer semiconductor structure according to any one of the preceding claims, characterized in that it comprises a layer of unalloyed GaAs (3) of two to thirty times repeated an unalloyed GaAs layer (3). In _y Gaiy _y As layer (4), The InAs wetting layer (5) and the InAs quantum dot (11) located thereon, the GaAsi-xSbx layer (6), GaAsi-xSbx layer (7) with quantum dots (10), unalloyed GaAs layer (8). Use of a multilayer semiconductor structure according to any one of the preceding claims for the production of solar cells or tandem solar cells.