Conical dielectric array InGaN quantum dot structure and preparation method thereof

Abstract

The invention discloses a conical dielectric array InGaN quantum dot structure and a preparation method. The conical dielectric array InGaN quantum dot structure comprises a substrate, a GaN template layer, a conical dielectric array layer, a first InGaN quantum dot layer, a first GaN barrier layer, a second InGaN quantum dot layer and a second GaN barrier layer, the GaN template layer grows on the substrate; the conical medium array layer grows on the GaN template layer; the first InGaN quantum dot layer is grown on the GaN template layer; the first GaN barrier layer uniformly grows and completely covers the first InGaN quantum dot layer; the second InGaN quantum dot layer grows on the first GaN barrier layer; the second GaN barrier layer uniformly grows and completely covers the second InGaN quantum dot layer; and a plurality of periodically arranged InGaN quantum dot layers and GaN barrier layers are sequentially and repeatedly grown on the second GaN barrier layer to form a multi-layer InGaN quantum dot structure. According to the method, the density, the size, the distribution and the like of the quantum dots can be accurately adjusted, and the grown InGaN quantum dots have the advantages of being simple in process and high in crystal quality.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductor materials, and in particular relates to a conical dielectric array InGaN quantum dot structure and a preparation method thereof. Background Art

[0002] In recent years, the preparation technology of GaN-based semiconductor materials has made significant progress, which has greatly promoted the rapid development of optoelectronic devices such as visible light band light-emitting diodes (LEDs) and laser diodes (LDs) in the fields of display and lighting. However, traditional GaN-based light-emitting devices usually use InGaN / GaN multiple quantum well structures grown on polar surfaces as the active region. Although the material quality of polar surface growth is high, the quantum confined Stark effect (QCSE) induced by its strong built-in electric field causes the electrons and holes in the quantum well to be spatially separated, resulting in a decrease in the radiative recombination efficiency. In addition, GaN-based materials generally have the disadvantage of high dislocation density, which further affects the improvement of the device's luminous efficiency.

[0003] In order to overcome the above problems, InGaN quantum dot materials with three-dimensional confinement structures that can effectively suppress the QCSE effect and shield dislocations have become a research hotspot in the current field of GaN-based luminescent materials. However, traditional InGaN quantum dots are prepared based on the self-assembled SK growth mode, which is highly random. The obtained quantum dots have problems such as uneven distribution and poor size consistency, which restricts the improvement of the performance of quantum dot materials. In order to improve the uniformity of InGaN quantum dots, some researchers have proposed the use of selective area growth technology to etch out neatly arranged and regularly shaped "window areas", and then perform epitaxial growth of InGaN to form uniformly distributed quantum dots. However, after the InGaN quantum dot layer is grown, this method also needs to etch away excess InGaN material outside the "window area", which complicates the overall preparation process, and may also cause damage to the InGaN quantum dots during the etching process, which is not conducive to the preparation of high-quality InGaN quantum dot materials. Summary of the invention

[0004] In order to overcome the shortcomings of the prior art, the present invention provides a conical dielectric array InGaN quantum dot structure and preparation method, including a substrate, a GaN template layer, a conical dielectric array layer, a first InGaN quantum dot layer, a first GaN barrier layer, a second InGaN quantum dot layer, and a second GaN barrier layer; the GaN template layer is grown on the substrate; the conical dielectric array layer is grown on the GaN template layer; the first InGaN quantum dot layer is grown on the GaN template layer; the first GaN barrier layer is uniformly grown and completely covered on the first InGaN quantum dot layer; the second InGaN quantum dot layer is grown on the first GaN barrier layer; the second GaN barrier layer is uniformly grown and completely covered on the second InGaN quantum dot layer; multiple periodically arranged InGaN quantum dot layers and GaN barrier layers are repeatedly grown on the second GaN barrier layer in sequence to form a multilayer InGaN quantum dot structure. The present invention can realize precise adjustment of the density, size, distribution, etc. of quantum dots, and the grown InGaN quantum dots have the advantages of simple process and high crystal quality.

[0005] The technical solution adopted by the present invention to solve the technical problem is as follows:

[0006] A conical dielectric array InGaN quantum dot structure, comprising a substrate, a GaN template layer, a conical dielectric array layer, a first InGaN quantum dot layer, a first GaN barrier layer, a second InGaN quantum dot layer, and a second GaN barrier layer;

[0007] The GaN template layer is grown on a substrate;

[0008] The conical dielectric array layer is grown on the GaN template layer;

[0009] The first InGaN quantum dot layer is grown on the first GaN template layer;

[0010] The first GaN barrier layer grows uniformly and completely covers the first InGaN quantum dot layer;

[0011] The second InGaN quantum dot layer is grown on the first GaN barrier layer;

[0012] The second GaN barrier layer grows uniformly and completely covers the second InGaN quantum dot layer;

[0013] A plurality of periodically arranged InGaN quantum dot layers and GaN barrier layers are repeatedly grown on the second GaN barrier layer to form a multi-layer InGaN quantum dot structure.

[0014] Preferably, the lateral size, density and distribution of the quantum dots of the first InGaN quantum dot layer are regulated by the conical dielectric array layer, that is, only the spacing regions between the conical dielectric array units can grow to form InGaN quantum dots.

[0015] Preferably, the material of the substrate is sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide; the conical dielectric array layer 12 is formed of insulating dielectric materials such as silicon oxide, silicon nitride, hafnium oxide and aluminum oxide.

[0016] Preferably, the array units in the tapered dielectric array layer are uniformly distributed in a nanometer-scale array formed by electron beam exposure, nanoimprinting, or self-aligned multiple exposure technology.

[0017] Preferably, the conical dielectric array layer is a conical or pyramidal structure with a sharp top and smooth inclined sidewalls formed by drilling technology of an isotropic etching process.

[0018] Preferably, the height of the dielectric array unit in the conical dielectric array layer is greater than the height of the quantum dots in the first InGaN quantum dot layer; the thickness of the first GaN barrier layer is greater than the height of the array unit in the conical dielectric layer; and the thickness of the second GaN barrier layer is greater than the height of the quantum dots in the second InGaN quantum dot layer.

[0019] Preferably, under the influence of stress coupling, the multi-layer InGaN quantum dot structure has a distribution consistent with that of quantum dots in the first InGaN quantum dot layer, forming a multi-layer self-aligned structure.

[0020] A method for preparing a tapered dielectric array InGaN quantum dot structure comprises the following steps:

[0021] Step 1: Take a substrate, the material of which is sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide; perform a high-temperature cleaning treatment on the substrate in a hydrogen atmosphere at 1000-1200°C for 5-20 minutes, and then perform a nitridation treatment;

[0022] Step 2: growing a GaN template layer on a substrate;

[0023] Step 3: Deposit an insulating dielectric material on the GaN template layer, and use electron beam exposure, nanoimprinting, self-aligned multiple exposure micro-nano processing technology, combined with the drilling technology of the isotropic etching process, to form a conical dielectric array layer with a uniform array of nanometer scale;

[0024] Step 4: epitaxially grow InGaN quantum dot material at the gap positions of the dielectric array units of the conical dielectric array layer to form a first InGaN quantum dot layer;

[0025] Step 5: growing a first GaN barrier layer to cover the first InGaN quantum dot layer and the tapered dielectric array layer;

[0026] Step 6: growing a second InGaN quantum dot layer on the first GaN barrier layer, and forming an InGaN quantum dot layer with a self-aligned structure by utilizing stress coupling;

[0027] Step 7: growing a second GaN barrier layer to cover the second InGaN quantum dot layer;

[0028] Step 8: On the second GaN barrier layer, multiple cycles of InGaN quantum dot layers and GaN barrier layers are repeatedly grown in sequence to complete the growth of self-aligned multi-layer InGaN quantum dots.

[0029] The beneficial effects of the present invention are as follows:

[0030] 1. Since InGaN quantum dots are obtained by epitaxial growth on the GaN template layer at the gap position of the conical array unit, high-quality selective epitaxial growth of InGaN quantum dots can be achieved, which is beneficial to improving the crystal quality of quantum dots.

[0031] 2. Due to the use of micro-nano processing technologies such as electron beam exposure, nanoimprinting, and self-aligned multiple exposure, the unit size and arrangement of the conical dielectric array at the nanoscale can be obtained, ensuring that the subsequent epitaxial growth can obtain nanoscale InGaN quantum dots;

[0032] 3. Since InGaN quantum dots grow in the gaps of the conical dielectric array units, the position, size and density of the subsequently grown InGaN quantum dots can be precisely controlled by controlling the size, spacing and distribution of the dielectric array units;

[0033] 4. Since the etching process is only for the insulating dielectric unit, and the InGaN quantum dots are formed by epitaxial growth on the GaN template layer, the damage to the InGaN quantum dots caused by the etching process is avoided to the greatest extent.

[0034] 5. Since the side walls of the conical dielectric array unit are inclined and smooth, the subsequently grown InGaN material is not easy to be deposited on the surface of the dielectric array unit. Therefore, it is not necessary to remove the InGaN film deposited on the surface of the dielectric layer in the subsequent process. Thus, the step of etching to remove the excess InGaN material outside the window growth area in the traditional selective growth technology is avoided. Not only does it simplify the growth and preparation process of quantum dots, but more importantly, it effectively avoids the damage that may be caused to quantum dots by additional etching processes, avoids the generation of defects on the surface of quantum dots, and helps to improve the crystal quality of quantum dots. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the epitaxial structure of the InGaN quantum dots of the present invention.

[0036] Figure 2 It is a preparation flow chart of InGaN quantum dots of the present invention. DETAILED DESCRIPTION

[0037] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0038] The present invention proposes a method for growing InGaN quantum dots that can accurately control the size, position, and density of InGaN quantum dots without causing etching damage and has a simple process. Compared with the SK growth mode with strong randomness, the present invention can achieve accurate adjustment of the density, size, and distribution of quantum dots. Compared with the traditional selective area growth technology, the InGaN quantum dots grown by the present invention have the advantages of simple process and high crystal quality.

[0039] The present invention aims to form an array arrangement of pointed cone-shaped units by using nano-scale etching technology, and to control the lateral size, density and distribution of quantum dots by using the pattern of the cone-shaped medium array layer.

[0040] A conical dielectric array InGaN quantum dot structure, comprising:

[0041] A substrate 10, wherein the material of the substrate is sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide;

[0042] A GaN template layer 11 is grown on a substrate 10;

[0043] A conical dielectric array layer 12 is grown on the GaN template layer 11;

[0044] A first InGaN quantum dot layer 13, grown on the first GaN template layer 11;

[0045] A first GaN barrier layer 14 is uniformly grown and completely covers the first InGaN quantum dot layer 13;

[0046] A second InGaN quantum dot layer 15 grown on the first GaN barrier layer 14;

[0047] A second GaN barrier layer 16 is uniformly grown and completely covers the second InGaN quantum dot layer 15;

[0048] A plurality of periodically arranged InGaN quantum dot layers and GaN barrier layers are repeatedly grown on the second GaN barrier layer to form a high-density multi-layer InGaN quantum dot structure.

[0049] In the first InGaN quantum dot layer 13, the lateral size, density and distribution of the quantum dots are regulated by the conical dielectric array layer 12, that is, only the spacing regions between the conical dielectric array units can grow to form InGaN quantum dots;

[0050] The conical dielectric array layer 12 is formed of insulating dielectric materials such as silicon oxide, silicon nitride, hafnium oxide, and aluminum oxide;

[0051] The array units in the tapered dielectric array layer 12 are uniformly distributed arrays at the nanoscale using techniques such as electron beam exposure, nanoimprinting, and self-aligned multiple exposure.

[0052] The conical dielectric array layer 12 is a conical or pyramidal structure with a sharp top and a smooth inclined side wall formed by drilling technology of an isotropic etching process;

[0053] Since the conical dielectric array layer 12 is an insulating dielectric material with a sharp top and a smooth inclined side wall, the subsequently grown InGaN material is not easy to adhere to the conical dielectric array unit, so that an InGaN film will not be formed on the surface of the dielectric array unit;

[0054] The height of the dielectric array unit in the conical dielectric array layer 12 should be greater than the height of the quantum dots in the first InGaN quantum dot layer 13;

[0055] The thickness of the first GaN barrier layer 14 should be greater than the height of the array unit in the tapered dielectric layer 12;

[0056] Due to the stress coupling effect of the first InGaN quantum dot layer 13 at the bottom, the quantum dots in the second InGaN quantum dot layer 15 are distributed in the same manner as the quantum dots in the first InGaN quantum dot layer 13;

[0057] The thickness of the second GaN barrier layer 16 should be greater than the height of the quantum dots in the second InGaN quantum dot layer 15;

[0058] Under the influence of stress coupling, the subsequently grown multi-layer InGaN quantum dots are distributed in the same manner as the quantum dots in the first InGaN quantum dot layer 13, forming a multi-layer self-aligned structure.

[0059] A method for preparing a tapered dielectric array InGaN quantum dot comprises the following steps:

[0060] Step 1: Take a substrate 10, the material of the substrate is sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide;

[0061] Step 2: growing a GaN template layer 11 on the substrate 10;

[0062] Step 3: depositing insulating dielectric material on the GaN template layer 11, and using nano-scale processing technologies such as electron beam exposure, nano-imprinting, self-aligned multiple exposure, combined with drilling technology of isotropic etching process, to form a conical dielectric array layer 12 with a uniform array of nano-scale;

[0063] Step 4: epitaxially growing InGaN quantum dot material at the gap positions of the dielectric array units of the conical dielectric array layer 12 to form a first InGaN quantum dot layer 13;

[0064] Step 5: growing a first GaN barrier layer 14 to cover the aforementioned first InGaN quantum dot layer 13 and the conical dielectric array layer 12;

[0065] Step 6: growing a second InGaN quantum dot layer 15 on the first GaN barrier layer 14, and forming an InGaN quantum dot layer with a self-aligned structure by utilizing stress coupling;

[0066] Step 7: growing a second GaN barrier layer 16 to cover the second InGaN quantum dot layer 15;

[0067] Step 8: On the second GaN barrier layer, multiple periods of InGaN quantum dot layers and GaN barrier layers are repeatedly grown in sequence to complete the growth of self-aligned multi-layer InGaN quantum dots.

[0068] Example:

[0069] See also Figure 1 The present invention provides an InGaN quantum dot structure of a conical dielectric array, comprising:

[0070] A substrate 10, wherein the material of the substrate is sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide;

[0071] The GaN template layer 11 is grown on the substrate 10; the GaN template layer is grown by a two-step growth method, that is, firstly a low-temperature GaN buffer layer with a thickness of less than 50nm is grown, and then a high-temperature GaN layer with a thickness of 1-3μm is grown. The GaN epitaxial layer can be doped according to actual needs, generally N-type Si doping, with a doping concentration greater than 1018cm -3 .

[0072] The conical dielectric array layer 12 is grown on the GaN template layer 11. The array units in the conical dielectric array layer 12 are uniformly distributed arrays at the nanoscale using technologies such as electron beam exposure, nanoimprinting, and self-aligned multiple exposure. The structure of the conical dielectric array unit is a cone or pyramid with a sharp top and a smooth inclined side wall formed by a drilling technique of an isotropic etching process.

[0073] The first InGaN quantum dot layer 13 is grown on the first GaN template layer 11; wherein, since the conical dielectric array layer 12 is an insulating dielectric material with a sharp top and a smooth inclined side wall, the subsequently grown InGaN material is not easy to adhere to the conical dielectric array unit, so that an InGaN film will not be formed on the surface of the dielectric array unit, thereby ensuring that the first InGaN quantum dot layer is completely epitaxially grown in the gap position of the conical dielectric array unit.

[0074] A second InGaN quantum dot layer 15 is grown on the first GaN barrier layer 14, and a stress coupling effect is used to form an InGaN quantum dot layer with a self-aligned structure.

[0075] Growing a second GaN barrier layer 16 to cover the second InGaN quantum dot layer 15;

[0076] On the second GaN barrier layer 16, multiple periods of InGaN quantum dot layers and GaN barrier layers are repeatedly grown in sequence to complete the growth of self-aligned multi-layer InGaN quantum dots. The growth conditions are consistent with the above.

[0077] See also Figure 2 , and combined with Figure 1 As shown, the present invention provides a method for preparing an InGaN quantum dot structure of a conical dielectric array, comprising the following steps:

[0078] Step 1: Take a substrate 10, the material of which is sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide; perform a high-temperature cleaning treatment on the substrate in a hydrogen atmosphere at 1000-1200° C. for 5-20 minutes, and then perform a nitridation treatment.

[0079] Step 2: grow a GaN template layer 11 on the substrate 10. The GaN template layer is grown by a two-step growth method, that is, first grow a low-temperature GaN buffer layer with a thickness of less than 50nm, and then grow a high-temperature GaN layer with a thickness of 1 to 3μm. The GaN epitaxial layer can be doped according to actual needs, generally N-type Si doping, with a doping concentration greater than 10 18 cm -3 . More specifically, the growth method of the low-temperature GaN buffer layer is: controlling the flow rate of trimethylgallium to 8-20sccm, the flow rate of NH3 to 500-1200sccm, the reaction pressure to 133-400mbar, the growth time to 80-500s, and the growth temperature to 450-600°C. The growth method of the high-temperature GaN layer is: controlling the flow rate of trimethylgallium to 50-150sccm, the flow rate of NH3 to 3000-8000sccm, the reaction pressure to 133-400mbar, the growth time to 500-3000s, and the growth temperature to 1100-1300°C.

[0080] Step 3: Deposit insulating dielectric material on the GaN template layer 11, and use nano-scale processing technologies such as electron beam exposure, nano-imprinting, self-aligned multiple exposure, combined with the drilling technology of isotropic etching process, to form a conical dielectric array layer 12 with a uniform array of nano-scale. Among them, the structure of the conical dielectric array unit is a cone or pyramid with a sharp top and smooth sidewalls formed by the drilling technology of isotropic etching process. Isotropic drilling technology uses plasma or wet etching process to evenly etch materials in all directions to form a structure with a sharp top and smooth sidewalls. The etching gas uses CF4 / O2 or SF / N2, the etching rate is 10-20nm / min, the etching temperature is 20-40℃, and the etching time is 10-30s.

[0081] Step 4: epitaxially grow InGaN quantum dot material in the gap position of the dielectric array unit of the conical dielectric array layer 13 to form the first InGaN quantum dot layer 12. Among them, since the conical dielectric array layer 12 is an insulating dielectric material with a sharp top and a smooth inclined side wall, the subsequently grown InGaN material is not easy to adhere to the conical dielectric array unit, so that the InGaN film will not be formed on the surface of the dielectric array unit, ensuring that the first InGaN quantum dot layer is all epitaxially grown in the gap position of the conical dielectric array unit. Specifically, the growth method of the InGaN quantum dot layer is: control the flow rate of trimethyl indium to 200-400sccm, the flow rate of triethyl gallium to 20-100sccm, the flow rate of NH3 to 5000-10000sccm, the growth temperature to 800-900℃, and the growth time to 50-150s.

[0082] Step 5: growing a first GaN barrier layer 14 to cover the aforementioned first InGaN quantum dot layer 13 and the conical dielectric array layer 12 .

[0083] Specifically, the thickness of the GaN barrier layer is 10 to 20 nm, and at least all InGaN quantum dots are covered. Adjusting its thickness can change the coupling strength between quantum dots in each layer. The growth method is: controlling the flow rate of NH3 to 5000 to 10000 sccm, the flow rate of triethylgallium to 20 to 100 sccm, the growth temperature to 800 to 900°C, and the growth time to 100 to 300 s.

[0084] Step 6: Grow a second InGaN quantum dot layer 15 on the first GaN barrier layer 14, and use stress coupling to form an InGaN quantum dot layer with a self-aligned structure. The self-alignment principle makes the second InGaN quantum dot layer 15 correspond to the first InGaN quantum dot layer 13 in the upper and lower positions, ensuring that the quantum dots grown subsequently are also uniformly distributed with high density.

[0085] Step 7: growing a second GaN barrier layer 16 to cover the second InGaN quantum dot layer 15;

[0086] Step 8: On the second GaN barrier layer, multiple cycles of InGaN quantum dot layer and GaN barrier layer are repeatedly grown in sequence to complete the growth of self-aligned multi-layer InGaN quantum dots. The growth conditions are the same as above.

Claims

1. A tapered dielectric array InGaN quantum dot structure, characterized in that: It includes a substrate, a GaN template layer, a conical dielectric array layer, a first InGaN quantum dot layer, a first GaN barrier layer, a second InGaN quantum dot layer, and a second GaN barrier layer; The GaN template layer is grown on a substrate; The conical dielectric array layer is grown on the GaN template layer; The first InGaN quantum dot layer is grown on the first GaN template layer; The first GaN barrier layer grows uniformly and completely covers the first InGaN quantum dot layer; The second InGaN quantum dot layer is grown on the first GaN barrier layer; The second GaN barrier layer grows uniformly and completely covers the second InGaN quantum dot layer; A plurality of periodically arranged InGaN quantum dot layers and GaN barrier layers are repeatedly grown on the second GaN barrier layer to form a multi-layer InGaN quantum dot structure.

2. The tapered dielectric array InGaN quantum dot structure according to claim 1, characterized in that: The lateral size, density and distribution of the quantum dots of the first InGaN quantum dot layer are regulated by the conical dielectric array layer, that is, only the spacing regions between the conical dielectric array units can grow to form InGaN quantum dots.

3. The tapered dielectric array InGaN quantum dot structure according to claim 1, characterized in that: The material of the substrate is sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide; the conical dielectric array layer 12 is formed of insulating dielectric materials such as silicon oxide, silicon nitride, hafnium oxide and aluminum oxide.

4. The tapered dielectric array InGaN quantum dot structure according to claim 1, characterized in that: The array units in the tapered medium array layer are uniformly distributed in a nanometer scale by using electron beam exposure, nano-imprinting, and self-aligned multiple exposure technology.

5. The tapered dielectric array InGaN quantum dot structure according to claim 1, characterized in that: The conical dielectric array layer is a conical or pyramidal structure with a sharp top and a smooth inclined side wall formed by a drilling technique of an isotropic etching process.

6. The tapered dielectric array InGaN quantum dot structure according to claim 1, characterized in that: The height of the dielectric array unit in the conical dielectric array layer is greater than the height of the quantum dots in the first InGaN quantum dot layer; the thickness of the first GaN barrier layer is greater than the height of the array unit in the conical dielectric layer; and the thickness of the second GaN barrier layer is greater than the height of the quantum dots in the second InGaN quantum dot layer.

7. The tapered dielectric array InGaN quantum dot structure according to claim 1, characterized in that: Under the influence of stress coupling, the multi-layer InGaN quantum dot structure is consistent with the distribution of quantum dots in the first InGaN quantum dot layer, forming a multi-layer self-aligned structure.

8. A method for preparing the tapered dielectric array InGaN quantum dot structure as claimed in claim 1, characterized in that: The following steps are involved: Step 1: Take a substrate, the material of which is sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide; perform a high-temperature cleaning treatment on the substrate in a hydrogen atmosphere at 1000-1200°C for 5-20 minutes, and then perform a nitridation treatment; Step 2: growing a GaN template layer on a substrate; Step 3: Deposit an insulating dielectric material on the GaN template layer, and use electron beam exposure, nanoimprinting, self-aligned multiple exposure micro-nano processing technology, combined with the drilling technology of the isotropic etching process, to form a conical dielectric array layer with a uniform array of nanometer scale; Step 4: epitaxially grow InGaN quantum dot material at the gap positions of the dielectric array units of the conical dielectric array layer to form a first InGaN quantum dot layer; Step 5: growing a first GaN barrier layer to cover the first InGaN quantum dot layer and the tapered dielectric array layer; Step 6: growing a second InGaN quantum dot layer on the first GaN barrier layer, and forming an InGaN quantum dot layer with a self-aligned structure by utilizing stress coupling; Step 7: growing a second GaN barrier layer to cover the second InGaN quantum dot layer; Step 8: On the second GaN barrier layer, multiple cycles of InGaN quantum dot layers and GaN barrier layers are repeatedly grown in sequence to complete the growth of self-aligned multi-layer InGaN quantum dots.

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