Study On The Fabrication Of In(Ga)N-based Nanowire Structures By Mbe And The Physical Properties

As a representative of the third generation of semiconductor materials,InGaN-based semiconductor materials are direct bandgap semiconductors with wide wavelength tunability from near infrared to visible light spectral range.They are very promising candidates for solid state optoelectronic devices,microwave power devices and power electronic devices.However,most of InGaN-based materials are grown on foreign substrates,the large lattice mismatch and thermal mismatch result in poor crystalline quality,and high density of threading dislocations and defects.The InGaN-based nanowire structure has a lower dimension and a higher quantum confinement effect,which can effectively reduce the dislocation density of the InGaN material and form a higher internal quantum efficiency within the nanowire.In addition,in the dot-in-nanowire structures,the nanowires can form effective quantum confinement on quantum dots.This kind of structures have a wide range of applications in the fields of nano-LED,nano-laser and single photon source.However,the studies are all limited to the visible light wavelength,there are relatively few studies on the1.55μm communication wavelength.The growth and characterization of nanowire materials are the foundation for the preparation of nanodevices.In this paper,for the central wavelength of 1.55μm,the fabrication of quantum dot embedded nanowire structures using top-down technology and the quantum dot embedded nanowire structures using bottom-up technology have been made by plasma-assisted molecular beam epitaxy（PA-MBE）growth.The morphological characteristics,crystal structure and optical properties of self assembled InGaN/GaN nanowire heterostructures were also studied.The main study contents and results are as follows:1.For the 1.55μm communication wavelength,a single quantum well（SQW）film with smooth surface was fabricatied by MBE.The thickness of the well was 2⁴ monolayers,the well was high In composition InGaN,and the barrier was low In composition InGaN.The PL results showed that the emitting wavelength from the QW was around 1.55μm.Firstly,the designed InN/In_0.9Ga_0.1N SQW structure and the only In_0.9Ga_0.1N were grown by MBE on GaN template substrate,respectively.Afterward,the room temperature PL results showed the SQW structure has a peak more than the In0.9Ga0.1N sample,the peak position was at 1.498μm.In order to realize the emission around 1.55μm,the design InN/In0.88Ga0.12N SQW with InN thickness two monolayers was fabricated.TEM result showed the exist of quantum well,however,the thickness of the quantum well was 2⁴ monolayers.The quantum well was not pure InN,Some Ga atoms were incorporated into the quantum well.The interface inter-diffusion between each other led to fluctuations in the thickness of the quantum well.The photoluminescence（PL）measurements of the sample at room temperature showed that the quantum well emited at 1.559μm.The sample provided a good foundation for the subsequent top-down nanowire arrays arranged periodically.2.The quantum dot embedded in InGaN nanowire arrays with short period and long period were fabricated by the top-down method.Firstly,the patterns on the short-period nanoimprint hard template are firstly transferred to the IPS soft template by nanoimprint,and then the patterns on the IPS are further transferred onto the sample,the nanowire arrays with a period of 500 nm,a diameter of 100 nm and a height of 25 nm was successfully obtained.In order to let the quantum dot achieve better quantum confinement,and to study the luminescence properties of the individual nanowire,we further reduced the diameter of the nanowires,increased the spacing between the nanowire arrays and fabricated a long period quantum dot embedded in InGaN nanowire array structure,with a period of 3μm,a diameter of 40 nm and a height of 200 nm.TheμPL of the single nanowire showed that the luminescence intensity of the quantum dot was stronger than that of the previous short-period nanorods,and the FWHM was narrower,which proves that the reduction of the diameter of single nanowire can let the crystal lattice stress release,and enhance the internal quantum efficiency.The emitting wavelength of the quantum dot in single nanowire has also been successfully relized at 1.55μm.3.The growth and related physical properties of self-assembled InGaN/GaN nanowire heterostructures were investigated.By growing the“T”like InGaN/GaN nanowire heterostructures,the crystal quality of InGaN was improved.The In composition segregation of the In composition in In_0.5Ga_0.5N nanowire was also observed.Firstly,it was found that the morphology of GaN self-assembled nanowires was better when V/III ratio was 9.After that,the morphology and physical properties of InGaN/GaN nanowire heterostructures with different In composition were investigated.It was found that with the increase of In composition,the lateral epitaxy of InGaN was serious and the crystal quality deteriorates.In order to improve the quality of InGaN,the“T”like InGaN/GaN nanowire structure was grown and the crystal quality was improved.On the basis of this,the physical properties of single In_0.5Ga_0.5N/GaN nanowire heterostructure was studied.It was found that the“T”like InGaN nanonanowire was a core-shell structure.TheμPL ans EDS measured on single nanowire at room temperature were studied.The single In_0.5Ga_0.5N nanowire has an In composition segregation,and segregated two peaks,one was around 700 nm,the other was around 852 nm.Such a single nanowire is expected to be used in the field of quantum memory in the future.4.The bottom-up method was successfully used to fabricate a nanowire structure in which a high In composition InGaN quantum dot embedded in a low In composition InGaN quantum nanowire.The quantum dot in single nanowire emited around the 1.55μm communication wavelength.First,the designed self-assembled InN quantum dot embedded in In_0.88Ga_0.12N quantum nanowire structure was grown on GaN self-assembled nanowires by MBE.The thickness of InN was designed as two monolayers.The nanowires separated from each other,and there was no polymerization phenomenon by SEM.TEM demonstrated that there was a quantum dot in the single InGaN nanowire.The EDS results showed that the core was not pure InN,but a higher In composition InGaN.For a single nanowire,theμPL at room temperature showed that the emission intensity from the quantum dot was stronger than from the barrier and the FWHM of the peak from the quantum was narrower.The emission wavelength from the quantum dot was 1.555μm.The related experimental results have laid a foundation for the functionalization of the nanowire quantum structures and devices designed to emit at a specific wavelength.