Surface passivation methods in production of optoelectronic devices

Abstract

The disclosure concerns a method (200) of producing a substrate with improved photocurrent generation performance, the method comprising: on a selected (202) substrate configured to generate mobile charge carriers in response to incident photons, depositing (208) a fixed charge passivation film that creates an electric field and repels photogenerated charge carriers from the substrate surface hence suppressing recombination thereof. Prior to depositing the fixed charge film, the substrate surface is pretreated (204) to generate (206) a passivation interface having reduced interface trap density. The disclosure further concerns a passivated substrate structure obtained (212) by the method, and an optoelectronic device comprising the same.

Description

[0001] SURFACE PASSIVATION METHODS IN PRODUCTION OF OPTOELECTRONIC DEVICES

[0002] FIELD OF THE INVENTION

[0003] The present invention generally relates to improving performance of optoelectronic devices resulting from improved photocurrent collection efficiency. In particular, the invention concerns passivation of substrate materials capable of generating photocurrent using thin film deposition methods.

[0004] BACKGROUND OF THE INVENTION

[0005] Efficiency of optoelectronic devices is intrinsically linked to an ability of said devices to collect photocurrent. In a variety of optoelectronic devices, such as photodetectors, LEDs, solar cells, etc., absorption of incident photons generates electron-hole pairs which split to free charge carriers (electrons and holes carrying negative and positive charge (±q) respectively) with excess free energy, whose net flux gives rise to electric current, commonly termed the photocurrent. A fraction of photogenerated charge carriers that remains available for collection by an electrode or a metal contact in order to be used for energy conversion defines efficiency of the optoelectronic device.

[0006] However, efficiency of photocurrent collection is hindered by the fact that these free charge carriers tend to recombine with their charge pairs, there after no longer contributing to the conductivity of the material. Moreover, charge traps or states defined, in physics, as any location within a solid material that restricts the movement of electrons and holes, further facilitate recombination of electron-hole pairs. A trap consists of a chemical impurity or any defect in the crystal lattice of a substrate material Specifically, traps occurring close to substrate surfaces and referred to as interface traps, act as recombination centers and greatly deteriorate the ability of conductive substrates to carry an electrical current, hence negatively affecting operation of optoelectronic devices.

[0007] Surface passivation is a well-known method of reducing the effects of charge carrier recombination in the substrates, and generally encompasses chemical and field-assisted passivation approaches. Chemical passivation suppresses surface defects / traps and is typically achieved through annealing techniques. Field- assisted passivation includes depositing, on the substrate, a layer of material with high fixed charge of the same sign with the substrate or a topmost substrate layer. This charge creates an electric field that drives charge carriers of a predetermine charge sign away from the surface, thus reducing the rate of carrier recombination.

[0008] Atomic Layer Deposition (ALD) is a technique of deposition of thin films in vapour phase which is extensively described in the art. ALD technology, generally regarded as a subclass of Chemical Vapour Deposition (CVD) processes, has proved an efficient tool for manufacturing high-quality conformal coatings on a variety of three-dimensional substrate structures.

[0009] ALD is based on alternating self-saturate surface reactions, wherein different reactants (precursors) provided as molecular compounds or elements in a nonreactive (inert) gaseous carrier are sequentially pulsed into a reaction space accommodating a substrate. Common ALD precursors include metal oxides, elemental metals, metal nitrides, metal sulfides, metal halides, metal alkylamides and water, for example. ALD enables formation of encapsulation nanolayers on semiconductor substrates with excellent large area uniformity and extremely high precision control.

[0010] ALD-deposited alumina (AI2O3) has gained interest due to its excellent passivation performance caused partly by a high density of negative charge. Negatively charged ALD-layers have proved efficient in field-assisted surface passivation of n-type semiconductor materials (having holes as minority charge carriers), such as silicon (Si), gallium nitride (GaN), gallium arsenide (GaAs), and indium phosphide (InP) used in production of optoelectronic and various high-speed electronic devices, as well as p-type semiconductor materials (having electrons as minority charge carriers), such as crystalline silicon (c-Si), used in production of solar cells, for example.

[0011] However, further prospects of application of ALD techniques in the field of surface passivation of substrates including various combinations of n-type and p-type semiconductors used in manufacturing of optoelectronic devices should further be investigated. SUMMARY OF THE INVENTION

[0012] An objective of the present invention is to solve or to at least alleviate each of the problems arising from the limitations and disadvantages of the related art. The objective is achieved by various embodiments of a method of manufacturing a substrate structure with improved photocurrent generation performance, related substrate structure, and an optoelectronic device comprising the same. In an aspect, a method of producing a substrate structure is provided, according to what is defined in independent claim 1.

[0013] In an embodiment, the method comprises:

[0014] (a) obtaining a substrate configured to generate mobile charge carriers in response to incident photons, hence giving rise to photocurrent,

[0015] (b) pretreating at least one surface of the substrate to reduce a number of surface defects acting as charge carrier traps, and

[0016] (c) on a pretreated surface or surfaces of the substrate depositing a fixed charge passivation film configured to create an electric field that repels photogenerated charge carriers from the substrate surface and suppresses recombination thereof, wherein during pretreatment (b), a passivation interface with a reduced interface trap density (Dit) is formed between the substrate and the fixed charge film.

[0017] In an embodiment, the pretreatment (step b) comprises exposing the substrate surface(s) to an oxidizing gaseous species selected from the group consisting of: hydrogen peroxide, ozone, water vapour, a combination of hydrogen peroxide and water vapour, and a combination of ozone and water vapour. In another embodiment, the pretreatment step comprises exposing the substrate surface(s) to a reducing gaseous species selected from the group consisting of hydrogen, hydrogen plasma, ammonia, and ammonia plasma.

[0018] In an embodiment, the pretreatment step comprises formation of an interfacial layer on the substrate surface(s).

[0019] In an embodiment, the pretreatment step is performed by Atomic Layer Deposition (ALD). In an embodiment, during the pretreatment preferably performed by ALD, an interfacial layer is formed by exposing the substrate surface(s) to alternately repeated surface reactions of at least two precursors including a first precursor for a metal and a second precursor for oxygen. In an embodiment, the precursor for oxygen is any one of hydrogen peroxide, ozone, water vapour, a combination of hydrogen peroxide and water vapour, and a combination of ozone and water vapour.

[0020] In an embodiment, the pretreatment (step b) starts with exposing the substrate surface(s) to any one of hydrogen peroxide, ozone, water vapour, or a combination thereof.

[0021] The pretreatment (step b) can be alternatively performed by Chemical Vapour Deposition (CVD) or Physical Vapour Deposition (PVD).

[0022] In an embodiment, the fixed charge film is configured to repel photogenerated minority charge carriers from the substrate surface.

[0023] In embodiments, the fixed charge film comprises or consists of a fixed charge material composed of any compound selected from the group consisting of: aluminium(III) oxide (AI2O3), tantalum(V) oxide (Ta20s), titanium(IV) oxide (TiCh), hafnium(IV) oxide (HfCh), silicon dioxide (S1O2), and a combination thereof.

[0024] In an embodiment, deposition (step c) of the fixed charge film is performed by ALD. Alternatively, deposition of the fixed charge film can be performed by CVD or PVD.

[0025] In an embodiment, the method further comprises, at step (d), subjecting the substrate deposited with the fixed charge film to a post-deposition treatment. In preferred embodiment, the post-deposition treatment is thermal treatment (annealing).

[0026] In an embodiment, the substrate comprises or consists of a semiconductor material. In embodiments, the substrate comprises or consist of a material selected from the group consisting of: silicon (Si), including amorphous silicon, polycrystalline silicon (poly-Si), silicon carbide (SiC), gallium arsenide (GaAs), aluminium gallium arsenide (AlGaAs), gallium nitride (GaN), aluminium gallium nitride (AlGaN), aluminium nitride (AIN), indium phosphide (InP), and any combination thereof. Different crystalline structures (if applicable) and / or presence / absence of dopants in the above mentioned materials is / are not excluded.

[0027] In another aspect, a passivated substrate structure for use in optoelectronic devices, is provided, according to what is defined in the independent claim 16.

[0028] In a further aspect, an optoelectronic comprising a passivated substrate structure as defined in some previous aspect and embodiments and / or deposited with a method defined in some previous aspect and embodiments, is provided, according to what is defined in independent claim 19.

[0029] In still a further aspect, a method of reducing interface trap density and suppressing recombination and trapping of photogenerated charge carriers in photocurrent generation substrates is provided, according to what is defined in the independent claim 21.

[0030] The utility of the present invention arises from a variety of reasons depending on each particular embodiment thereof.

[0031] Overall, the invention offers a surface passivation method that combines suppressing traps (lowering trap density, Dit) and, by virtue of depositing a coating layer having a high density of fixed charge (Qf), creating an electric field preventing free charge carriers from reaching the traps. Fixed charge density (Qf) is defined as the amount of fixed charge per unit area.

[0032] By reducing the number of surface defects / traps, recombination rate of charge carriers at the substrate surface can be reduced and the lifetime of minority carriers increased, respectively. The invention hence enables manufacturing of optoelectronic devices with improved photocurrent collection performance.

[0033] Deposition layers deposited by ALD methods are pinhole-free and fully conformal, therefore, the ALD technology has a high potential in manufacturing of high- quality coatings utilized in optoelectronic devices. ALD allows for obtaining nanometer thick layers with good stoichiometric control and excellent repeatability. Due to relatively low temperatures utilized during ALD depositions (25 - 150 °C), the coatings can be conformally deposited also on sensitive substrate materials, such as those used in production of micro- and nanoelectronic devices. Since ALD coatings are typically provided in a nanometer scale, they cause minimal dimensional change to the substrate.

[0034] In the present disclosure, materials with a layer thickness below 1 micrometer (pm) are referred to as “thin films”.

[0035] The term “optoelectronic device” is used in the context of the present disclosure to refer to any electronic device that interacts with light, which interaction is based on quantum mechanical effects of light on electronic materials, such as semiconductors, sometimes in presence of electric fields. In optoelectronic devices, optical radiation is emitted, modified or converted to other types of energy (e.g. optical-to-electrical).

[0036] The term “light” is used, in the context of present disclosure, to designate any one of visible light (VIS) and invisible forms of electromagnetic radiation, such as gamma rays, X-rays, ultraviolet (UV) and infrared (IR). Visible light is defined by a portion of the electromagnetic spectrum that a human eye can perceive and spanning wavelengths from about 380 nm to about 780 nm, more typically, from about 400 nm to about 700 nm. A UV-ray is a segment of the electromagnetic spectrum, with a wavelength ranging between about 100 nm and about 400 nm; while IR-radiation spans through wavelengths of about 780 nm and 1 mm. X-rays span between about 1-10 nm, and gamma rays have wavelengths less than 1 nm.

[0037] The term “stack” is used in the present disclosure to indicate a layered structure, which comprises two or more layers arranged on top of one another. The stack may comprise layers made of two or more different materials.

[0038] The expressions “reactive fluid” and “precursor fluids” are indicative in the present disclosure of a fluidic flow comprising at least one chemical compound (a precursor compound), hereafter, a precursor, in an inert carrier. In some instances, precursor fluid contains a precursor compound co-dosed with another precursor compound in inert gas carrier. The expression “set of precursors” relates to two or more precursor compounds used to deposit an individual material (deposition) layer.

[0039] The expression “a number of’ refers herein to any positive integer starting from one (1), e.g. to one, two, or three; whereas the expression “a plurality of’ refers herein to any positive integer starting from two (2), e.g. to two, three, or four.

[0040] The terms "first" and "second" are not intended to denote any order, quantity, or importance, but rather are used to merely distinguish one element from another, unless explicitly stated otherwise.

[0041] BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Fig. 1 is a block diagram that illustrate a method of manufacturing a substrate, according to the embodiments.

[0043] Figs. 2A and 2B schematically illustrate substrate structures 100A, 100B, according to the embodiments.

[0044] Fig. 3 is a schematic illustration of a principle underlying the improved photocurrent generation performance in a substrate produced in accordance with some embodiments of a method disclosed herewith.

[0045] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] Fig. 1 illustrates a method 200 of producing a substrate structure 100 with improved photocurrent generation performance, according to the embodiments. Enhanced substrate structures 100, 100A, 100B produced by the method 200 are shown on Figs. 2A and 2B, respectively.

[0047] Overall, the method 200 (Fig. 1) comprises pretreating, at 204 a substrate to form, on said substrate, a passivation interface 11 with reduced interface trap density, followed with depositing, at 206, a fixed charge passivation film 20 on pretreated substrate surface(s), thereafter the enhanced substrate structure 100, 100A, 100B is obtained (212). In some embodiments, a post-deposition treatment 210 is performed after depositing the fixed charge film 20.

[0048] The following description provides more detailed insights into various embodiments of the method 200.

[0049] The method starts at 202 with obtaining a substrate 10 configured to generate electric current in response to incident photons. In embodiments, the substrate comprises or consists of a semiconductor material, provided as any one of intrinsic (i-type) semiconductor (e.g. undoped silicon), p- type semiconductor (formed by doping an intrinsic semiconductor with trivalent impurity atoms, such as boron (B), indium (In), or gallium (Ga)), n-type semiconductor (formed by doping an intrinsic semiconductor with pentavalent impurity atoms, such as phosphorus (P), arsenic (As), or antimony (Sb)), a p-n junction semiconductor, and any combination thereof. In an n-type semiconductor, the electrons are the majority carriers, whereas the holes are the minority carriers. In the p-type semiconductor material, the holes are the majority carriers, whereas the electrons are the minority carriers.

[0050] In semiconductors, if the density of charge carriers in a material is much larger than the density of recombination centers, then minority carriers represent a limiting factor to conductivity. Conversely, if the density of recombination centers is larger, nearly all minority charges will be recombined and the majority carrier will dominate the photoconductivity signal, as is often the case for insulators.

[0051] Important parameters that quantitatively describe charge carrier recombination within a semiconductor substrate are minority carrier lifetime and surface recombination rate (surface recombination velocity).

[0052] The efficiency of an optoelectronic device is linked to the minority carrier lifetime (T). Lifetime parameter is a measure of how long the minority charge carrier, an electron or hole, exists within a substrate material and it is defined as an average time it takes for a minority carrier to recombine with its charge pair. This parameter determines the likelihood that a charge carrier will be collected by a metal contact or electrode and therefore used for energy conversion. A longer lifetime in the substrate material typically means a greater likelihood of charge collection in order to generate a clear image or optical signal, hence correlating to a greater efficiency of the optoelectronic device. Lifetime of photogenerated carriers can be prolonged by decreasing their recombination rate by reducing or inactivating the recombination centers that contribute to interface charge trapping.

[0053] In some configurations, the substrate includes, but is not limited to: silicon (Si), including amorphous silicon, polycrystalline silicon (poly-Si), silicon carbide (SiC), gallium arsenide (GaAs), aluminium gallium arsenide (AlGaAs), gallium nitride (GaN), aluminium gallium nitride (AlGaN), aluminium nitride (AIN), indium phosphide (InP), and any combination thereof. The above mentioned substrate materials can have different crystal lattices (if a crystalline structure is applicable) and / or include any suitable dopants. Any other appropriate substrate material can be utilized.

[0054] At 204, the substrate 10 undergoes pretreatment of at least one of its surfaces, thereupon the surface(s) is / are prepared for subsequent deposition, at 208, of a passivation / encapsulation film 20. Film 20 is preferably deposited on pretreated surface or surfaces. Passivation film has a fixed charge and hence it is referred to, in the context of a substrate structure 100A, 100B, as a fixed charge (passivation) film. After depositing the fixed charge passivation film 20, a substrate structure 100 (100A, 100B) can be obtained (step 212).

[0055] In some configurations, the substrate deposited with the passivation film 20 further undergoes a post-deposition treatment step 210 (Fig. 1). Preferred post-deposition treatment methods include thermal treatment (annealing). Annealing is typically performed at temperatures within a range of 200-800 °C, dependent on a composition of the coated substrate, and the treatment duration is within a range of about 1 second to about 2 hours. By subjecting the coated substrate to postannealing (i.e. post-deposition treatment by annealing), the structure and properties of the passivation film 20 can be modified such as to increase its’ fixed charge. That is, for passivation films composed of compounds with negative fixed charge, such as for example of aluminium oxides (A1OX), post-annealing can further increase negativity of the film. Moreover, annealing contributes to further reduction of the interface trap density (Dit) at the interface between the substrate 10 and the fixed charge film 20.

[0056] The fixed charge film 20 is preferably configured to perform field-assisted passivation of the substrate 10. In field-assisted passivation, fixed charge within a passivation film material is a result of interactions with a substrate bulk. High fixed charge in the passivation film deposited on the substrate having same charge sign creates a local electric field and drive away minority carriers, reducing their concentration and improving (reducing) recombination rate. Fixed charge film carrying a significant fixed charge (having fixed charge density in the order of at least 1012cm-2) is configured to generate electric field and to introduce thus generated electric field at the substrate surface, thereby facilitating separation of electron-hole pairs and promoting formation of free charge carriers, respectively. Electric field generated by the fixed charge film further contributes to the reduction of recombination rate at a substrate surface.

[0057] Composition of the fixed charge film and the sign of its electrical charge respectively is thus selected based on the substrate. By way of example, alumina (AI2O3) deposited on silicon (Si) substrate has a high fixed negative charge density (can be for example 4x 1012cm'2), which makes it suitable for deposition on p-type silicon substrates. Due to its substantial amount of fixed negative charge, alumina passivation layer repels (minority carrier) electrons from the p-type substrate surface, hence reducing surface recombination thereof. Similar results can be achieved with passivation films 20 composed of titanium(IV) oxide (TiCh) or tantalum(V) oxide (Ta20s) both having a certain (fixed) charge.

[0058] In embodiments, the fixed charge passivation layer 20 can be deposited by Atomic Layer Deposition (ALD). The fixed charge passivation layer 20 can be further deposited by any one of Chemical Vapour Deposition (CVD) and Physical Vapour Deposition.

[0059] The basics of an ALD growth mechanism are known to a skilled person. ALD is a chemical gas phase thin film deposition method based on alternate supply of at least two reactive precursor species (in gaseous phase) to a substrate, which forces the film growth reactions to proceed only on the surface in a highly controlled manner. It is to be understood, however, that one of these reactive precursors can be substituted by energy when using, for example, plasma-enhanced ALD (PE- ALD), or photon-enhanced ALD, leading to single precursor ALD processes. Thin films grown by ALD are dense, pinhole free and have uniform thickness.

[0060] In conventional (thermal) ALD, the substrate is typically exposed to temporally separated precursor pulses in a reaction vessel to deposit material on substrate surface or surfaces by sequential self-saturating surface reactions. In the context of present disclosure, the term ALD comprises all applicable ALD-based techniques, as well as, equivalent or closely related technologies, including, but not limited the following ALD sub-types: plasma-enhanced ALD (PEALD) also referred to plasma-assisted ALD, and photon- enhanced ALD, also referred to as photo- ALD or flash enhanced ALD.

[0061] Deposition setup may be the one based on an ALD installation described in the U.S. patent no. 8211235 (Lindfors), for example, or on any installation, such as Picosun R-200, P-300 or P-1000 ALD system, available from Picosun Oy, Finland. Nevertheless, the features underlying a concept of the present invention can be incorporated into any other chemical deposition reactor generally embodied as a reactor for ALD, PEALD and / or MLD, or as Chemical Vapour Deposition (CVD) device, for example.

[0062] An exemplary ALD reactor comprises a reaction chamber that establishes reaction space (deposition space), in which production of (nano)laminate coatings and / or coated items described in the present disclosure takes place. The reactor further comprises a number of appliances configured to mediate fluidic flow (inert fluids and reactive fluids containing precursor compounds) into the reaction chamber. These appliances are provided as a number of intake lines / feedlines and associated switching and / or regulating devices, such as valves, for example.

[0063] Substrates to be coated are typically loaded on a substrate holder, which, in turn, is inserted inside the reaction chamber.

[0064] A basic ALD deposition cycle consists of four sequential steps: (1) pulse A, (2) purge A, (3) pulse B, and (4) purge B. Reactive fluid entering the reaction chamber during pulses A and B is preferably a gaseous substance comprising a predetermined precursor chemical carried by an inert carrier (gas). Delivery of the precursor chemicals into the reaction space and film growth on the substrate is / are regulated by means of the abovesaid regulating appliances, such as e.g. three-way ALD valves, mass-flow controllers or any other device suitable for this purpose.

[0065] Overall, each deposition cycle (Pulse A - Purge A - Pulse B - Purge B) results in formation of 0.01-0.2 nm thick “sub-layer”. This sequence is repeated until the desired material thickness is achieved. By way of example, a 20 nm coating film may thus be deposited in 200 deposition cycles with exemplary growth rate 0.1 nm per deposition cycle, respectively.

[0066] By way of example, AI2O3 layer may be deposited by repeatedly exposing the substrate to pulses of TMA (trimethylaluminium, A1(CH3)3) to deposit one-atom aluminium sublayers, followed with pulses of any one of: ozone, water vapor, and a combination of ozone and water vapor as oxidants. Reaction space is purged with inert gas (e.g. argon or nitrogen) between precursor pulses in order to remove unreacted precursor and reaction by-products. For example, while thermal ALD processes utilize water vapour as an oxidant, in plasma-assisted processes, water is replaced by plasma of e.g. O2, which generates oxygen radicals.

[0067] It is preferred that ALD-deposition is performed, in the method 200, at temperatures not exceeding 400 °C, preferably, at temperatures, not exceeding 200 °C.

[0068] Overall, ALD-deposited passivation film 20 may consist of one or more ALD- coating layers, composed of same or different materials. ALD-films 20 having thickness within a range of about 5 nm to about 25 nm, preferably, within a range of about 10 nm to about 20 nm can be deposited. In some configurations, the passivation film 20 is homogenous, i.e. consists of a single material. In some other configurations, the passivation film 20 is implemented as a laminate structure or a stack comprising at least two deposition layers formed with different materials. Creating a stack of passivation layers may be beneficial in controlling the fixed charge density (Of). Stacks composed of AI2O3 and TiO2 may be created.

[0069] In some instances, the fixed charge passivation film 20 may be composed of fixed positive charge material including, but not limited to any one of: hafnium(IV) oxide (HfO2), silicon dioxide (SiO2), and a combination thereof. In similar manner, heterogenous stacks combining negative- and positive fixed charge materials and layers thereof may be created for the fixed charge passivation film 20.

[0070] A plurality of ALD-deposition cycles resulting in formation of a thin film from a timed deposition sequence, which controlled by a logic unit or a microprocessor of the related deposition setup. More simple or more complex deposition cycles / deposition sequences can be implemented. For example, deposition cycles can include three or more reactant vapor pulses separated by purging steps, or, alternatively, certain purge steps can be omitted. On the other hand, for example photon-enhanced ALD has a variety of options, such as using only one active precursor in a corresponding precursor set, with various options for purging. Reference is made back to the method step 204 (pretreatment) shown on Fig. 1.

[0071] During pretreatment step 204, a number of surface defects at an interface between the substrate 10 and the fixed charge passivation layer 20 is reduced. These defects act as charge carrier traps and are referred to as interface traps.

[0072] In embodiments, pretreatment 204 comprises exposing the substrate’s 10 surface(s) to gasses, such as reactive gasses, and / or plasma. In embodiments, pretreatment 204 comprises exposing the substrate’s 10 surface(s) to oxidizing or reducing gaseous species. Oxidizing gaseous species include, but are not limited to any one of: hydrogen peroxide (H2O2), ozone (O3), water vapour (H2O), combination of hydrogen peroxide and water vapour, and a combination of ozone and water vapour; whereas reducing gaseous species include, but are not limited to any one of: hydrogen, hydrogen plasma, ammonia, and ammonia plasma. Substrate can be exposed subsequently to oxidizing and reducing gaseous species or vice versa.

[0073] During pretreatment step 204, an interface 11 with reduced interface trap density (Dit) is thus formed between the substrate 10 and the fixed charge film 20. Interface 11 is further referred to as a passivation interface and its formation is designated on Fig. 1 with a block 206. In some embodiments, pretreatment 204 comprises formation of an additional interfacial layer on the substrate surface(s) (between the substrate 10 and the fixed charge film 20). This interfacial layer has different composition and / or thickness than the fixed charge film 20.

[0074] Passivated substrate structures 100 embodied at 100A and 100B are shown on Figs. 2A and 2B, respectively. Fig. 2A shows a substrate structure 100, 100A with the passivation interface 11 A having reduced interface trap density (Dit) being formed directly in the substrate, thereby (pre)treated substrate surface itself constitutes the passivation interface. Fig. 2B shows a substrate structure 100, 100B, in which the passivation interface is formed with an (additional) interfacial layer 1 IB.

[0075] In embodiments, the interfacial layer 11B (Fig. 2B) is formed by ALD. ALD procedure generally follows the one described hereinabove. In ALD process performed during the pretreatment 204, 206, the interfacial layer 11B can be formed by exposing the substrate surface(s) to alternately repeated surface reactions of at least two precursors. In embodiments, a first precursor includes a precursor for metal and a second precursor includes a precursor for oxygen. Precursor for oxygen may be one of hydrogen peroxide, ozone, water vapour, a combination of hydrogen peroxide and water vapour, and a combination of ozone and water vapour. Precursor for metal may be represented by any compound suitable for depositing a metal element sublayer. Byway of example, the interfacial later 11B composed of AI2O3 can be ALD-deposited in a sequence of alternating pulses of TMA (as a precursor for metal) and water (as a precursor for oxygen).

[0076] In embodiments, the pretreatment process 204 performed by ALD and aiming at formation of the interfacial layer 11B starts with exposing the substrate surface(s) to an oxidant, such as any one of hydrogen peroxide, ozone or water vapour, a combination of hydrogen peroxide and water vapour, a combination of ozone and water vapor, and / or any other suitable compound suitable for priming the substrate surface. Priming of the substrate surface with oxidant is performed before stating a basic ALD cycle, i.e. before pulsing a first (metal) precursor into the reaction space. After being initially exposed to the oxidant the substrate surface may be purged with inert gas.

[0077] In some instances, the interfacial layer 11B (Fig. 2B) may be formed by any one of PEALD, photo-ALD, CVD or PVD processes.

[0078] Overall, it is preferred that the total thickness of all deposited layers 20, 11 (in particular, 11B) does not exceed about 20-25 nm.

[0079] The method according to the invention is further illustrated by the following nonlimiting examples.

[0080] Example 1. Formation of a passivated substrate structure 100A (Fig. 2A).

[0081] Substrate structure 100A was formed by obtaining a substrate made of material capable to generate photocurrent (silicon (Si), polycrystalline silicon (poly-Si), gallium arsenide (GaAs), gallium nitride (GaN), etc.). The substrate was pretreated with plasma, such as hydrogen plasma or ammonia plasma, in order to passivate interface traps and create a passivation interface 11 A with reduced interface trap density Dit. Fixed charge passivation layer 20 having the fixed charge density (Qf) sufficient to repel minority carriers from the substrate surface, as described hereinabove, was further deposited preferably using ALD. In some configurations, the fixed charge passivation layer 20 composed of AI2O3 was ALD-deposited in a sequence of alternating pulses of TMA and ozone.

[0082] In an additional example, the process of Example 1 was modified by performing the pretreatment (204, Fig. 1) with exposing the substrate surface(s) to an oxidant (short pulses thereof), such as water vapour or ozone, using the ALD-setup as described hereinabove, followed with the ALD cycle.

[0083] Example 2. Formation of a passivated substrate structure 100B (Fig. 2B).

[0084] Substrate structure 100A was formed by obtaining a substrate made of material capable to generate photocurrent (silicon (Si), polycrystalline silicon (poly-Si), gallium arsenide (GaAs), gallium nitride (GaN), etc.). The substrate was pretreated by depositing an interfacial layer 1 IB using ALD and combining ozone and water vapour as an oxidant. Interfacial layer formed a passivation interface 11B with reduced interface trap density Dit. ALD process was started with a metal precursor pulse (e.g. TMA) followed with an oxidant precursor pulse (this order can be reversed). A combination of ozone and water vapour was used as oxidant. Fixed charge passivation layer 20 having the fixed charge density (Qf) sufficient to repel minority carriers from the substrate surface, as described hereinabove, was further deposited preferably using ALD. Deposition of the fixed charge layer 20 composed of AI2O3 was the same as described in Example 1.

[0085] In an additional example, the process of Example 2 was modified by starting the pretreatment (204, Fig. 1) with exposing the surface of the substrate to an oxidant, such as water vapour or ozone, followed with the ALD cycle.

[0086] In another additional example, the process of Example 2 was modified by depositing the interfacial layer 1 IB for interfacial trap reducing preferably by ALD process utilizing sequential pulses of TMA and water vapour.

[0087] In both Examples presented above, ALD deposition was performed at 200 °C and below; and the total thickness of a coating film (layers 11 and 20) deposited on the substrate 10 did not exceed 20 nm.

[0088] In Examples 1 and 2 (including their modifications), the ALD deposition of the fixed charge film 20 may be replaced by any one of CVD or PVD techniques.

[0089] Reference is made to Fig. 3, schematically illustrating a principle underlying the improved photocurrent generation performance in a passivated substrate structure 100 (such as 100A, 100B) produced in accordance with embodiments of a method described hereinabove. Passivated substrate structure 100 comprises a substrate body 10 deposited with a fixed charge passivation film 20 having a high fixed charge density (Qf), preferably that of negative fixed charge. Passivation interface 11 having low interface defect density (Du) is formed between the substrate body 10 and the fixed charge film 20. Two-stage passivation treatment of the substrate body 10 (formation of the passivation interface 11 during pretreatment and formation of the fixed charge passivation film 20) thus enables formation of a substrate structure 100, in which: 1) charge carrier recombination rate (R) is reduced (by electric filed created by fixed charge film 20) hence increasing the lifetime (T) of minority carriers in the substrate, and 2) interface trap density is reduced (by passivating interface traps during formation of the passivation interface 11). Charge carriers (electrons and holes) are denoted on Fig. 3 with encircled minus and plus signs, respectively.

[0090] Hence, when incident photons (arrows denoted with symbol y standing for Greek letter gamma) of some wavelengths are absorbed into the substrate structure 100A, 100B (substrate body 10 thereof), thus generated charge carriers are less prone to recombination due to significant fixed charge of the passivation film 20 and reduced amount of interface traps at an interface between the pretreated substrate surface and the fixed charge film (due to provision of the passivation interface 11). Recombination rate of charge carriers is thus lowered, and their lifetime is increased, respectively, which altogether contributes to improved photocurrent collection efficiency.

[0091] In a further aspect, a passivated substrate structure 100, 100A, 100B for use in optoelectronic devices is provided. In an embodiment, the passivated substrate structure comprises: a substrate body 10 configured to generate mobile (i.e. free) charge carriers in response to incident photons, hence giving rise to photocurrent, and a fixed charge passivation film 20 formed on at least one surface of the substrate body and configured to generate electric field and to introduce thus generated electric field at the substrate body surface thereby promoting formation of charge carriers in the substrate body and reducing their recombination rate. A passivation interface 11 having a reduced interface trap density (Dit) is formed, in said substrate structure 100, between the substrate body and the fixed charge layer.

[0092] The passivation interface 11 can be provided as a passivated surface (11 A) of the substrate body (i.e. without having an additional layer of material deposited on the substrate). Alternatively, the passivated interface can be provided as interfacial layer 1 IB formed between the substrate and the fixed charge layer.

[0093] In an embodiment, in said passivated substrate the fixed charge film 20 is a negative charge film deposited on the substrate body made of p-type semiconductor material.

[0094] The substrate structure 100 can be manufactured using the method 200 according to some previous embodiments.

[0095] In a further aspect, an optoelectronic device is provided, comprising the passivated substrate structure 100 (100 A, 100B) described hereinabove. A variety of optoelectronic devices may be conceived including, but not limited to: photoconductive cells, photodetectors, photodiodes, phototransistors, photovoltaic cells (solar cells), light emitting diodes (LEDs), optical couplers, such as optical fibers, lasers, laser diodes, and any combinations thereof.

[0096] Due to reduced losses in charge carrier transport efficiency through the substrate material and hence enhanced photocurrent collection efficiency, performance of the optoelectronic devices comprising the passivated substrate structure 100 described hereinabove and / or produced by the method 200 can be improved, respectively.

[0097] In still further aspect, a method of reducing interface trap density and suppressing recombination and trapping of photogenerated charge carriers in photocurrent generation substrates is provided, the method comprising:

[0098] (a) obtaining a substrate configured to generate mobile charge carriers in response to incident photons, hence giving rise to photocurrent,

[0099] (b) pretreating at least one surface of the substrate to reduce a number of surface defects thereon, and

[0100] (c) on a pretreated surface or surfaces of the substrate, depositing a fixed charge passivation film carrying a substantial fixed charge and hence configured to create an electric field that repels photogenerated charge carriers from the substrate surface and suppresses recombination thereof, wherein during pretreatment (b), a passivation interface with a reduced interface charge density (Dit) is formed between the substrate and the fixed charge film.

[0101] In an embodiment, the method further comprises, at step (d), subjecting the substrate deposited with the fixed charge film to a post-deposition treatment, wherein the post-deposition treatment is preferably thermal treatment (annealing).

[0102] It shall be appreciated by those skilled in the art that with the advancement of technology the basic ideas of the present invention may be implemented and combined in various ways. The invention and its embodiments are thus not limited to the examples described hereinabove, instead they may generally vary within the scope of the claims.

Claims

Claims1. A method of producing a substrate structure with improved photocurrent generation performance, the method comprising:(a) obtaining a substrate configured to generate mobile charge carriers in response to incident photons, hence giving rise to photocurrent,(b) pretreating at least one surface of the substrate to reduce a number of surface defects acting as charge carrier traps, and(c) on a pretreated surface or surfaces of the substrate depositing a fixed charge passivation film configured to create an electric field that repels photogenerated charge carriers from the substrate surface and suppresses recombination thereof, wherein during pretreatment (b), a passivation interface with a reduced interface trap density (Dit) is formed between the substrate and the fixed charge passivation film.

2. The method of claim 1, wherein pretreatment (b) comprises exposing the substrate surface(s) to an oxidizing gaseous species selected from the group consisting of: hydrogen peroxide, ozone, water vapour, a combination of hydrogen peroxide and water vapour, and a combination of ozone and water vapour.

3. The method of claim 1, wherein pretreatment (b) comprises exposing the substrate surface(s) to a reducing gaseous species selected from the group consisting of hydrogen, hydrogen plasma, ammonia, and ammonia plasma.

4. The method of any preceding claim, wherein pretreatment (b) comprises formation of an interfacial layer on the substrate surface(s).

5. The method of claim 4, wherein pretreatment (b) is performed by Atomic Layer Deposition (ALD).

6. The method of claim 5, wherein, during the pretreatment (b), the interfacial layer is formed by exposing the substrate surface(s) to alternately repeated surface reactions of at least two precursors including a first precursor for a metal and a second precursor for oxygen.

7. The method of claim 6, wherein the precursor for oxygen is any one of hydrogen peroxide, ozone, water vapour, a combination of hydrogen peroxide and water vapour, and a combination of ozone and water vapour.

8. The method of any one of claims 5-7, wherein the pretreatment (b) starts with exposing the substrate surface(s) to any one of hydrogen peroxide, ozone or water vapour.

9. The method of claim 4, wherein pretreatment (b) is performed by any one of Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD).

10. The method of any preceding claim, wherein the fixed charge passivation film is configured to repel photogenerated minority charge carriers from the substrate surface.

11. The method of any preceding claim, wherein the fixed charge passivation film comprises or consists of a fixed charge material composed of any compound selected from the group consisting of: aluminium(III) oxide (AI2O3), tantalum(V) oxide (Ta20s), titanium(IV) oxide (TiCh), hafnium(IV) oxide (HfCh), silicon dioxide (SiCh), and any combination thereof.

12. The method of any preceding claim, wherein deposition (c) of the fixed charge passivation film is performed by any one of Atomic Layer Deposition (ALD), Chemical Vapour Deposition (CVD), and Physical Vapour Deposition.

13. The method of any preceding claim, further comprising: (d) subjecting the substrate deposited with the fixed charge passivation film to a postdeposition treatment, wherein the post-deposition treatment is preferably annealing.

14. The method of any preceding claim, wherein the substrate comprises or consists of a semiconductor material.

15. The method of claim 14, wherein the substrate comprises or consist of amaterial selected from the group consisting of: silicon (Si), poly crystalline silicon (poly-Si), silicon carbide (SiC), gallium arsenide (GaAs), aluminium gallium arsenide (AlGaAs), gallium nitride (GaN), aluminium gallium nitride (AlGaN), aluminium nitride (AIN), indium phosphide (InP), and any combination thereof.

16. A substrate structure for use in optoelectronic devices, comprising:- a substrate body configured to generate mobile charge carriers in response to incident photons, hence giving rise to photocurrent, and- a fixed charge passivation film formed on at least one surface of the substrate body, wherein a passivation interface having a reduced interface trap density (Dit) is formed between the substrate body and the fixed charge passivation film.

17. The substrate structure of claim 16, wherein the passivation interface is provided as an interfacial layer formed between the substrate and the fixed charge passivation film.

18. The substrate structure of any one of claims 16 or 17, wherein the fixed charge passivation film is a fixed negative charge film deposited on the substrate body made of p-type semiconductor material.

19. An optoelectronic device comprising a passivated substrate structure as defined in any one of claims 16-18.

20. The optoelectronic device of claim 19, configured as a device selected from the group consisting of: a photoconductive cell, a photodetector, a photodiode, a phototransistor, solar cell, a light emitting diode (LED), an optical coupler, such as an optical fiber, a laser, a laser diode, and any combination thereof.

21. A method of reducing interface trap density and suppressing recombination and trapping of photogenerated charge carriers in photocurrent generation substrates, the method comprising:(a) obtaining a substrate configured to generate mobile charge carriers in response to incident photons, hence giving rise to photocurrent,(b) pretreating at least one surface of the substrate to reduce a number of surface defects thereon, and(c) on a pretreated surface or surfaces of the substrate, depositing a fixed charge passivation film configured to create an electric field that repels photogenerated charge carriers from the substrate surface and suppresses recombination thereof, wherein during pretreatment (b), a passivation interface with a reduced interface charge density (Dit) is formed between the substrate and the fixed charge passivation film.

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