WO2020123451A1 - Enhancement of photon extraction with nanoparticles on semiconducting nanostructures - Google Patents

Abstract

Nanoparticles deposited on a semiconducting structure allow control of the surface roughness through etching, control of the refractive index by choosing a filling fraction of nanoparticles to air, and control of effective roughness by acting as scattering centers for photons.

Description

ENHANCEMENT OF PHOTON EXTRACTION WITH NANOPARTICLES ON

SEMICONDUCTING NANOSTRUCTURES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Provisional Patent Application No. 62/777,597 filed on December 10, 2018 and U.S. Provisional Patent Application No. 62/884,115 filed on August 7, 2019, the disclosures of both of which being incorporated herein by reference in their entirety.

TECHNICAL FIELD

[0002] The present disclosure relates to nanoparticles. More particularly, it relates to enhancement of photon extraction with nanoparticles on semiconducting nanostructures.

BRIEF DESCRIPTION OF DRAWINGS

[0003] The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present disclosure and, together with the description of example embodiments, serve to explain the principles and implementations of the disclosure.

[0004] Fig. 1 illustrates a schematic of nanoparticle attachment for light extraction enhancement of semiconductor nanostructures.

[0005] Fig. 2 illustrates a uniform two-dimensional pillar with a fixed geometry [0006] Fig. 3 illustrates simulated field intensity for a GaAs pillar.

[0007] Fig. 4 illustrates simulated field intensity for a GaAs pillar with 100-nm TiC nanoparticles. [0008] Figs. 5-6 illustrate a comparison of a GaAs pillar with and without nanoparticles.

[0009] Figs. 7-8 illustrate data for the addition of 15 nm etch pits between nanoparticles.

[0010] Fig. 9 illustrates data of light extraction efficiency based on different nanoparticle sizes. [0011] Fig. 10 illustrates data of directionality or angular emission based on different nanoparticle sizes.

[0012] Fig. 11 shows the effect of nanoparticle size and composition on the enhancement of light.

[0013] Fig. 12 shows a pyramidal semiconductor light emitter geometry decorated with nanoparticles.

[0014] Fig. 13 shows a light extraction enhancement effect provided by the light emitter shown in Fig. 12.

[0015] Fig. 14 shows the effect of nanoparticle size and composition on the light extractioOn enhancement provided by the emitter shown in Fig. 12.

DETAILED DESCRIPTION

[0016] The present disclosure describes the application of nanoparticles, for example dielectric nanoparticles, on semiconductor structures to modify the extraction efficiency of photons. The semiconductor structures can emit photons, and the nanoparticles enhance the efficiency of photon emission. Specifically, conformal nanoparticles can be attached to semiconducting nanostructures in three ways: (i) as etching masks to generate a controlled surface texture, (ii) as subwavelength texturing creating an effective index of refraction to reduce reflections, and (iii) as local scattering sites which randomize reflections, creating an effectively rough medium. For all three photon extraction methods, nanoparticles with lateral dimensions between kp_cak/ l 0 and kp_cak/2, where kp_cak refers to the peak photon emission wavelength of the semiconducting structure, can be used. In this context, a semiconducting structure refers to any non-planar semiconducting structure which has dimensions <10 pm. In this context, semiconductor refers to any material which may be used to emit light, either through electrical or optical excitation. Examples include but are not limited to GaAs, InP, InAs, InGaAs, GaN, InGaN, InGaP, and InAsP. In this context, lateral dimensions refers to the radius of the nanoparticles.

[0017] When used as an etch mask, the nanoparticles must exhibit sufficient etch contrast to the semiconductor when exposed to the etching gases or solutions. In this approach, the nanoparticles will first be deposited conformally, or nearly conformally, on the semiconducting nanostructure, followed by an etching step which selectively etches the semiconducting structure where the nanoparticles are not in contact with the surface. This will cause the surface of the semiconductor to exhibit a surface roughness with lateral spacing with the same or related dimension as the semiconducting nanoparticle. In other words, the etchant can etch the unexposed surface of the semiconductor, either in gaps left without nanoparticle attachment, or in gaps on the surface between deposited nanoparticles. For example, if the nanoparticles have an approximately spherical shape, even tightly packed arrangements of conformal nanoparticles can leave gaps exposing the surface of the semiconductors. It can be noted that multiple layers of nanoparticles or nanoparticles of different sizes within the same layer can offer process tunability. In addition, core-shell nanoparticles (for example a Ti02 nanoparticle with an Si02 shell grown by a wet chemical process or by atomic layer deposition) can be used to create gap spacings that are equal to or related to the outer diameter of the shell, but have a light scattering behavior that is dominated, or influenced by the core. It is also noted that the shell might be tuned to have a different etching contrast to the core particle (either higher or lower) to enable the particles to be shaped or to use a particle with low etch resistance, but desirable scattering properties.

[0018] When used as an effective refractive index matching medium, nanoparticles with a dielectric constant greater, equal to, or less than the semiconducting structure and air can be used. In some embodiments, the dielectric constant has a value between that of the semiconducting structure and that of air. The effective index will be controlled by a conformal coating or coatings of the nanoparticles on the semiconducting structures exhibiting (i) loose packing of the nanoparticles on the surface, (ii) a mixture of two or more nanoparticles with different dielectric constants, with the effective index of the nanoparticle coating controlled by the relative ratio of the different nanoparticles which are attached, or (iii) a mixture of multiple nanoparticles of different diameters, which when attached to the surface of the semiconductor create a random distribution with varying lateral and vertical heights. Through these three approaches, the nanoparticles create a layer between the semiconductor structure and the surrounding structure, which has an effective dielectric constant between the two values (those of the semiconductor and of the surrounding structure). This approach utilizes two concepts. Due to the intermediate effective dielectric constant, between that of air and of the semiconductor, the reflection at the semiconductor/air interface is reduced. This effect can be quantified by utilizing the Fresnel reflection equations. However, in addition to the reflection reduction caused by the introduction of a layer with intermediate dielectric, the effective dielectric constant of the nanoparticle layer is also a function of position in a direction orthogonal to the semiconductor surface. Specifically, there is a gradual change of the effective dielectric constant due to the change in the relative fraction of nanoparticle and air at each plane which is parallel to the semiconductor surface. This enables the nanoparticle layer to have a varying effective index, which further reduces the reflections by reducing the magnitude of the abrupt change in dielectric constant.

[0019] When used as effective scattering centers, the nanoparticles on the semiconductor surface act as scattering sites for the photons in the semiconductors, changing the specular reflection of internal photons at the smooth semiconducting surface, and acting to randomize the reflections by introducing an effective roughness through a nonuniform lateral dielectric surface on the semiconductor. This causes the surface to exhibit an effective roughness without physical texturing. The mechanism via which this approach leads to an improvement in light extraction is through the randomization of photon reflections at the semiconductor surface. Through this, a fraction of the reflected photons will be removed from the guided modes of the semiconductor structure and into an emitting mode of the semiconducting structures. By introducing an effective random roughness at the surface of the semiconductor, photons generated in the semiconductor which impinge on the surface at an angle which would otherwise cause total internal reflection, now have a small probability of immediate extraction, and if reflected, have a greater chance of being emitted during the next surface impingement event.

[0020] The mechanism by which the surface roughness enables scattering of photons from guided modes into emitting modes is through randomization of the reflection angles with respect to the surface. When a photon is emitted into a guided mode of the semiconductor structure, all interactions of that photon with surfaces will result in total internal reflection. In the case of a structure with a rough surface, even when photons are emitted into a guided mode of the original structure (with a smooth surface), every interaction with a surface has a probability that the photon will be reflected out of the structure's guided modes and into an emitting mode.

[0021] Another way of explaining the concept above is through the emission cone. Due to the large mismatch of the index of refraction between the semiconductor and the surrounding environment, the escape cone for photons generated in the semiconductor is small, typically of the order of 2% of the overall solid angle of photon emission. Thus, on average, only 2% of photons will be emitted after the first reflection. The remaining 98% of photons will undergo total internal reflection at the semiconductor interface. However, due to the specular reflection of a smooth surface, when the reflected photons arrive at the next surface, all of those photons will be reflected, as all of them are at an angle large enough to undergo total internal reflection. This process will continue, and nearly all photons, outside the initial 2% which escape through the escape cone, will be lost.

[0022] When photons are emitted in a material with a rough surface, the probability of escape after the first reflection will still be 2%. However, as the photons will be randomly reflected from the surface, and not specularly reflected, every subsequent interaction with a surface also has a 2% chance of escape. In this way, a rough surface can dramatically enhance light extraction efficiency, as photons have a certain percentage (typically ~2%) chance of escape on every interaction with a surface. A smooth surface however, typically only offers a 2% chance of escape for photons on their first interaction, while all subsequent interactions will be continually totally internally reflected.

[0023] Examples of nanostructures to be covered with conformal layers of nanoparticles comprise, for example, arrays of pillars of different cross sections. Fig. 1 describes exemplary pillars or other similar nanostructures (110) which are covered with a layer of nanoparticles (105) to enhance light extraction. The substrate may also be a semiconductor (115).

[0024] In some embodiments, there does not need to be any etch contrast between the nanoparticles and the surface to be etched. For example, even if the nanoparticles are etched at exactly the same rate of the underlying material, the surface of the material would still be etched with pits or recession that have a depth of approximately the original nanoparticle diameter, once the nanoparticles have been fully etched. Thus, the choice of a numerical value for the etch contrast depends on the shape of recession to be etched. A deep narrow recession requires a high etch contrast of 3 : 1 or higher. A shallow and broad recession would benefit from a nanoparticle with an etch contrast the same or less than that of the material being etched. Therefore, the numerical range for the optimal etch contrast between the semiconductor and the nanoparticles depends on the aspect ratio of the recession to be etched.

[0025] In some embodiments, a range of filling factors for the nanoparticle/air layer can be used to control the refractive index. In some embodiments, the packing factor is between 0.2 and 1 0

[0026] In some embodiments, the nanoparticles need to be firmly attached to the surface to withstand subsequent fabrication processes, or normal weathering during use if the particles are meant to remain on the surface. For example, the nanoparticles need to be attached so that they are not washed or blown away in a subsequent fabrication step. To attach the nanoparticles, different methods may be used, for example chemical glue, hydrogen bonding, covalent bonding or ionic bonding.

[0027] In some embodiments, the regions where the nanoparticles are attached can be controlled through patterning of the substrate functional groups that bind with the nanoparticles. The patterning can happen in several different ways: 1. Choosing fluids that apply functional groups only in certain areas (fluids that do not wet in certain places); 2. Choosing bifunctional molecules that have substrate specific binding. With this degree of chemical specificity, the substrate itself can be pre-patterned (some areas have photoresist, some areas have metal, some areas have GaN, etc. etc.). The bifunctional molecule has one end that will only attach to GaN, for example, but not to the other materials. Then, the other end will attach to the nanoparticle; 3. The functional groups can be applied everywhere, but then selectively removed by etching through a mask.

[0028] Fig. 2 illustrates a uniform two-dimensional pillar with a fixed geometry. Light extraction is modeled by monitoring far-field transmission averaged over three orthogonal dipole orientations and five positions along the center axis of the pillar (15 averaged simulations per structure). Total light extraction is compared between a bare pillar structure (case 1), a pillar with dielectric nanoparticles deposited in a conformal layer (case 2), and a pillar with regions etched between nanoparticles (case 3).

[0029] Fig. 3 illustrates simulated field intensity for a GaAs pillar. For a bare uniform GaAs pillar, the average transmission was simulated over the visible spectrum. Transmission is impacted by the absorption of GaAs at these wavelengths (particularly in the blue and green). As expected, the transmission is highest toward the infrared, where the GaAs is lower, and there is an angle- dependent emission pattern peaking near 35° off-normal.

[0030] Fig. 4 illustrates simulated field intensity for a GaAs pillar with 100-nm TiCk nanoparticles. With the addition of a monolayer of TiCk nanoparticles, the peak field intensity is improved for specific wavelengths, compared to Fig. 3. In the case of 100-nm-diameter nanoparticles, the far- field enhancement is created near the 695 nm wavelength. The nanoparticle layer acts as an antireflective coating, and the wavelengths of peak enhancement can be tuned with the nanoparticle's size.

[0031] Figs. 5-6 illustrate a comparison of a GaAs pillar with and without nanoparticles. Fig. 5 illustrates the ratio of far-field transmission for a pillar with nanoparticles to that of a bare pillar. Fig. 6 illustrates the far-field intensity at 695 nm. The ratio of the integrated far-field power transmission for the pillar covered with nanoparticles to that of the bare pillar shows the wavelength dependent enhancement - a total power enhancement of 32% is achieved at the 695 nm wavelength. The enhancement also shows angle dependence, but there is some enhancement at all angles at 695 nm for this structure.

[0032] Figs. 7-8 illustrate data for the addition of 15 nm etch pits between nanoparticles. Fig. 7 illustrates the ratio of far-field transmission for a pillar with nanoparticles and etching to that of a bare pillar. Fig. 8 illustrates the far-field intensity at 695 nm. The addition of 15 nm etch pits or recession between the TiCk nanoparticles further enhances the far-field transmission, with a peak integrated power transmission increase of 36%. The angular distribution of the far field is largely unaffected by the etch pits

[0033] In some embodiments, tuning the nanoparticle size enables modification of the light extraction efficiency across a range of wavelengths. In Fig. 9, an example of GaN pillars with 1 micron diameter and 5 micron heights decorated with SiCk nanoparticles of 50 nm, 100 nm, and 150 nm is given. It is shown that the size of the nanoparticles enables tuning of the magnitude of the light extraction enhancement (i.e., far-field transmission) across a range of wavelengths.

[0034] In some embodiments, tuning the nanoparticle size enables modification of the wavelength and bandwidth of the light extraction efficiency peak. In Fig. 9, an example of GaN pillars with 1 micron diameter and 5 micron heights decorated with SiC nanoparticles of 50 nm, 100 nm, and 150 nm is given. It is shown that the size of the nanoparticles enables tuning of the magnitude and wavelength of the peak enhancement across a range of wavelengths.

[0035] Fig. 10 shows the modification to the directionality or angular emission of the light for GaN pillars with 1 micron diameter and 5 micron heights decorated with SiCk nanoparticles of 50 nm, 100 nm, and 150 nm. It is shown that for light with a gaussian intensity distribution peaked at 460 nm with a full width half maximum of 25 nm, the light enhancement can be focused in a narrow cone around the normal direction to the pillar substrate. This enhancement is a function of the nanoparticle size and can be tuned by varying nanoparticle size. The particle size influences the directionality of the extracted light

[0036] Fig. 11 shows the effect of nanoparticle size and composition on the enhancement of light with a gaussian intensity distribution peaked at 460 nm with a full width half maximum of 25 nm emitted within +/-15 degrees of normal. As shown, both the nanoparticle composition and size can be used to tune the directionality of the emitted light.

[0037] Fig. 12 shows a pyramidal semiconductor light emitter geometry decorated with nanoparticles and Fig. 13 shows the light extraction enhancement effect of nanoparticles. Fig. 14 shows how tuning the composition of the nanoparticles can change the total enhancement for light with a gaussian intensity distribution peaked at 460 nm with a full width half maximum of 25 nm.

[0038] In some embodiments the nanoparticles act as local resonators, interacting with optical modes in the internal nanostructure (of the semiconductor) through evanescent coupling, and then re-radiating the photons into free space. The modification of the nanoparticle size and composition tunes both the interaction cross section with the nanostructure optical modes, and the subsequent radiation of photons into free space.

[0039] As described above, using a conformal nanoparticle coating on a semiconductor structure, three ways of improving light extraction efficiency can be used, either alone or in combination with one another. [0040] The examples set forth above are provided to those of ordinary skill in the art as a complete disclosure and description of how to make and use the embodiments of the disclosure, and are not intended to limit the scope of what the inventor/inventors regard as their disclosure.

[0041] Modifications of the above-described modes for carrying out the methods and systems herein disclosed that are obvious to persons of skill in the art are intended to be within the scope of the following claims. All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the disclosure pertains. All references cited in this disclosure are incorporated by reference to the same extent as if each reference had been incorporated by reference in its entirety individually.

[0042] It is to be understood that the disclosure is not limited to particular methods or systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the content clearly dictates otherwise. The term“plurality” includes two or more referents unless the content clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains.

Claims

CLAIMS What is claimed is:

1. A method comprising:

depositing a conformal layer of nanoparticles on a semiconductor nanostructure, the nanoparticles having an etching contrast with respect to the semiconductor nanostructure; and etching the semiconductor nanostructure to generate surface roughness;

wherein the nanoparticles have lateral dimensions between p_eak/10 and p_eak/2, where p_eak is a peak photon emission wavelength of the semiconducting nanostructure.

2. A method comprising:

depositing at least one layer of nanoparticles on a semiconductor nanostructure, the nanoparticles having a dielectric constant between that of the semiconducting nanostructure and that of air; and

controlling a refractive index of a surface layer comprising the nanoparticles and air, by controlling a filling fraction of nanoparticles for the surface layer;

wherein the nanoparticles have lateral dimensions between kp_cak/ l 0 and kp_cak/2, where kp_cak is a peak photon emission wavelength of the semiconducting nanostructure.

3. A method comprising:

depositing a layer or layers of nanoparticles on a semiconductor nanostructure, the nanoparticles having a dielectric constant greater than that of the semiconducting nanostructure and that of air; and

controlling a refractive index of a surface layer comprising the nanoparticles and air, by controlling a filling fraction of nanoparticles for the surface layer;

wherein the nanoparticles have lateral dimensions between kp_cak/ l 0 and kp_cak/2, where kp_eak is a peak photon emission wavelength of the semiconducting nanostructure.

4. A method comprising:

depositing a layer or layers of nanoparticles on a semiconductor nanostructure, the nanoparticles having a dielectric constant less or equal to that of the semiconducting nanostructure or to that of air; and

controlling a refractive index of a surface layer comprising the nanoparticles and air, by controlling a filling fraction of nanoparticles for the surface layer;

wherein the nanoparticles have lateral dimensions between p_eak/10 and p_eak/2, where p_eak is a peak photon emission wavelength of the semiconducting nanostructure.

5. A method comprising:

depositing at least one layer of nanoparticles on a semiconductor nanostructure, wherein the nanoparticles have lateral dimensions between kp_cak/ l 0 and kp_cak/2, where kp_cak is a peak photon emission wavelength of the semiconducting nanostructure.

6. A method comprising

tuning of an angular emission of photons through at least one layer of nanoparticles deposited on a semiconductor nanostructure, wherein the nanoparticles have lateral dimensions between kp_cak/ l 0 and kp_cak/2, where kp_cak is a peak photon emission wavelength of the semiconducting nanostructure.

7. A method comprising

tuning of a peak wavelength of light extraction efficiency enhancement through at least one layer of nanoparticles deposited on a semiconductor nanostructure, wherein the nanoparticles have lateral dimensions between kp_cak/ l 0 and kp_cak/2, where kp_cak is a peak photon emission wavelength of the semiconducting nanostructure.

8. A method comprising:

depositing at least one layer of nanoparticles on a semiconductor nanostructure, thereby obtaining a coated semiconductor nanostructure;

exciting the coated semiconductor nanostructure;

based on the exciting, obtaining an interaction based on evanescent coupling between the nanoparticles and optical modes of the semiconductor nanostructure; and

based on the obtaining, radiating photons into free space,

wherein the nanoparticles have lateral dimensions between kp_cak/ l 0 and kp_cak/2, where kp_eak is a peak photon emission wavelength of the semiconducting nanostructure.

9. The method according to claim 8, further comprising:

tuning a cross section of the interaction by changing a dimension or a composition of the nanoparticles.