Optoelectronic Properties of III-Nitride Semiconductors

在本論文中, 我們提出了氮化物半導體之光學與電學的研究,其中包含氮化銦鎵磊晶薄膜, 氮化銦鎵/氮化鎵多重量子井結構, 氮化銦鎵/氮化鎵超晶格, 氮化鋁鎵/氮化鎵超晶格以及氮化銦鎵/氮化鎵奈米針. 內容如下: 藉由光激發螢光,電子掃描顯微鏡, 電子束螢光, 穿透式電子顯微鏡等量測研究出鎂摻雜氮化鋁鎵/氮化鎵超晶格光學特性與結構的關係. 我們發現側面的藍色能帶有光學異向性. 經由電子束螢光, 穿透式電子顯微鏡等量測研究我們可知鎂摻雜所形成的三角錐缺陷是造成光學異向性的來源. 我們首次在氮化銦鎵量子點中看到光調制現象.而此現象會造成材料的側面光激發螢光藍移,折射率減少以及聲子的頻率降低.除此之外, 在不同外加雷射的強度此變溫的光激發螢光峰值可以來驗證這個機制. 我們藉由奈米針的製程讓發光二極體增加其發光效率. 藉由我們的研究,我們清楚的確認奈米針可以增加發光效率是來自藉由這個製程應力被釋. 對於許多有存在應力的半導體材料中,我們的結果將對於未來發展高效能的光電元件更有幫助. 我們記錄了氮化銦鎵/氮化鎵超晶格在沒有外加磁場, 只外加電流引發自旋異向性的研究. 除此之外, 電旋獲爾效應可被內部應力控制也已經被發現. 此結果也有理論計算來佐證. 藉由XRD, 電子掃描顯微鏡, EDS, 電子束螢光等實驗,光學與結構特性的修正提供了一個直接的證據支持銦奈米柱是主導其發光來源.我們的結果對於將來光電元件發光效率的增加可提供 一個重要的貢獻. 關鍵字: 光激發螢光, 氮化銦鎵/氮化鎵, 拉曼In this thesis, we perform several studies of electrical and optical properties of nitride-based semiconductor heterostructure, including InGaN thin film, InGaN/GaN multiple quantum wells, InGaN/GaN superlattices, AlGaN/GaN superlattices, and InGaN/GaN nanotips. These results are presented as follows. The structural and optical properties of Mg-doped AlGaN/GaN superlattices have been investigated by photoluminescence (PL), scanning electron microscopy (SEM), cathodoluminescence (CL) and transmission electron microscopy (TEM). We found that the edge blue-band emission shows a strong optical anisotropy. Through the combination of the CL and TEM images, we clearly establish that the underlying microstructure responsible for the blue luminescence in Mg-doped AlGaN/GaN arises from the pyramidal defects. We have reported an intriguing photoelastic effect in InGaN QDs for the first time. The optically modulated internal strain contributes to the blueshift in edge PL spectra, a reduction of the refractive index, and a redshift in the InGaN LO phonon mode. In addition, the change of the temperature dependence of the PL emission energy under high and low excitation density can also be explained consistently. We have demonstrated a significant improvement of the emission from InGaN/GaN nanotip arrays compared with InGaN/GaN MQWs. The nanotip arrays were formed by a simple and low cost self-masked dry etching process, which is compatible with the current semiconductor technologies. Our unique approach is able to enhance the light output power by a factor of up to 10 times. Based on our study, we clearly demonstrate that the main underlying mechanism for the enhanced luminescence arises from the strain relaxation in the nanotip through its inherent characteristic of a large surface-to-volume ratio. Lateral current-induced spin polarization in InGaN/GaN superlattices (SLs) without an applied magnetic field is reported. The fact that the sign of the nonequilibrium spin changes as the current reverses and is opposite for the two edges provides a clear signature for the spin Hall effect. In addition, it is discovered that the spin Hall effect can be strongly manipulated by the internal strains. A theoretical work has also been developed to understand the observed strain-induced spin polarization. Our result paves an alternative way for the generation of spin polarized current. The correlation between optical and structure properties obtained by XRD, SEM images, EDS, and localized CL spectra provides a direct and concrete evidence to support the fact that the formation of nanoclusters is responsible for the enhanced luminescence in InGaN thin films. Our results shown here can serve as an important clue for the enhancement of the luminescent intensity in future optoelectronic devices. Key words : PL, InGaN/GaN, Raman1 Introduction………………………………………………01 1.1 III-Nitride semiconductors……………………………01 1.2 Band structure of wurtize (WZ) III-nitrides……03 1.3 properties of alloys……………………………………07 1.3.1 AlN and GaN heterostructures…………………………07 1.3.1.1 Band gap and bowing parameter………………………07 1.3.1.2 Mg-doping of AlGaN heterostructures………………09 1.3.2 InN and GaN heterostructures………………………13 1.3.2.1 Band gap and bowing parameter………………………13 1.3.2.2 Piezoelectric field in InGaN/GaN multiple quantum wells...16 1.4 Effect of strain…………………………………………17 1.5 Overview of the thesis…………………………………20 1.6 References…………………………………………………23 2 Techniques for optoelectronic measurements………28 2.1 Photoluminescence (PL)…………………………………28 2.1.1 Definition of photoluminescence……………………28 2.1.2 Brief description of general luminescence………29 2.1.3 Impurities in semiconductors…………………………31 2.1.4 Direct transition in semiconductors………………32 2.1.5 Temperature dependence photoluminescence…………34 2.1.6 Photoluminescence enhancement methods……………36 2.1.7 Conventional PL setup…………………………………37 2.2 Scanning electron microscopy (SEM)…………………38 2.2.1 Brief description of Scanning electron microscopy.....38 2.3 Cathodoluminescence(CL)………………………………42 2.3.1 Brief description of Cathodoluminescence…………42 2.3.2 Cathodoluminescence mapping…………………………44 2.3.3 Conventional CL setup…………………………………45 2.4 Electron energy dispersive X-ray spectrum (EDX)………46 2.5 Raman scattering………………………………………48 2.5.1 Brief description of Raman scattering……………48 2.5.2 Raman scattering characterization of GaN and InN compounds…52 2.5.3 Conventional Raman scattering setup………………54 2.6 References…………………………………………………56 3 Sample preparations……………………………………58 3.1 Mg-doped AlGaN/GaN Superlattices……………………58 3.2 InGaN quantum dots grown on SiNx nano masks……58 3.3 InGaN/GaN nanotip arrays………………………………59 3.4 InGaN/GaN superlattices………………………………61 3.5 InGaN epifilms……………………………………………62 3.6 References……………………………………………..…63 4 Optical anisotropy induced by pyramidal defects in Mg-doped AlGaN/GaN Superlattices………………………64 4.1 Introduction…………………………………………………..64 4.2 Experimental setup…………………………………………..64 4.3 Results and discussion………………………………………65 4.4 Summary……………………………………………………...71 4.5 References…………………………………………………….72 5 Optically modulated internal strain in InGaN quantum dots grown on SiNx nano masks…………………………73 5.1 Introduction…………………………………………………..73 5.2 Experimental setup…………………………………………..74 5.3 Results and discussion………………………………………75 5.4 Summary……………………………………………………...82 5.5 References…………………………………………………….83 6 Strong luminescence from strain relaxed InGaN/GaN nanotips for highly efficient light emitters……………...85 6.1 Introduction…………………………………………………..85 6.2 Experimental setup…………………………………………..87 6.3 Results and discussion…………………………………….88 6.4 Summary……………………………………………………..95 6.5 References…………………………………………………....96 7 Current and strain-induced spin polarization in InGaN/GaN superlattices………………………………...98 7.1 Introduction…………………………………………………..98 7.2 Experimental setup…………………………………………..99 7.3 Results and discussion………………………………....100 7.4 Summary…………………………………………………….108 7.5 References……………………………………………………110 8 Direct evidence of nanocluster-induced luminescence in InGaN epifilms…………………………………………..112 8.1 Introduction…………………………………………………112 8.2 Experimental setup………………………………………….113 8.3 Results and discussion……………………………………113 8.4 Summary…………………………………………………….119 8.5 References…………………………………………………...120 9 Conclusion……………………………………………….122 Lists of Figures Fig. 1.1: Effect of crystal field splitting and spin-orbit coupling on the valence band (near the Γ point) of WZ GaN (Ref. 21). At k = 0, the valence band is split by the combined action of crystal field and spin-orbit coupling into A(Γ9), B(Γ7) and C(Γ7) states…………………………………………….05 Fig. 1.2: Calculated band structure near the Γ point of WZ GaN. The exciton binding energies are denoted as ΔEA b , ΔEB b , ΔEC b for the A, B, and C excitons, respectively. [ref 20]……………………………………………06 Fig. 1.3: Band gap of the epitaxial AlxGa1–xN layers as a function of Al molar fraction x ( x = 0 – 0.25 ) with considering the strain effect. (Ref. 28)……………09 Fig. 1.4: High-resolution TEM image of a pyramidal defect taken around the [10-10] zone-axis.(Ref. 38)………………………………………………………..12 Fig. 1.5: The peak wavelength of the PL spectra of InxGa1–xN films as a function of In mole fraction x. The figures are taken from Ref. 54………………………15 Fig.2.1: The schematic diagram of different processes that can give rise to light emission in semiconductors.(a) exciton recombination (b) band to band transition (c) free hole-neutral donor recombination (d) donor-acceptor recombination (e) free electron-neutral acceptor recombination………….30 Fig.2.2: Conventional PL setup………………………………………………………37 Fig.2.3: Scanning electron microscopy photo………………………………………..39 Fig. 2.4: Typical scanning electron microscopy setup……………………………….40 Fig. 2.5: The schematic diagram of Cathodoluminescence spectrometer.[ref. 14]…..46 Fig. 2.6: The model of Stokes shifted and anti-Stoke shifted of Raman scattering….50 Fig. 2.7: Schematic diagram of Raman scattering measurement. Scattered laser light is transmitted through the beam splitter to the triple monochromator, which gives the Raman spectra of the selected region. The microscope objective lens focuses the laser beam to a spot size of about 1 μm. Visible light from illuminator is reflected from the sample to a video camera, which gives a realtime image to the allow position of the probe beam. The X-Y stage can be moved by the automatic machine………………………………………55 Fig. 3.1: Atomic force microscope images of InGaN layers with 195 s of SiNx treatment on the underlying GaN layers…………………………………..59 Fig. 3.2: Scanning electron micrograph images of tilted top-view of InGaN/GaN nanotips……………………………………………………………………61 Fig. 4.1: (a) In-plane and edge photoluminescence spectra of Mg-doped AlGaN/GaN superlattices at 15 K with excitation power density of 4.8 W/cm2. (b) The schematic diagram of the edge PL measurement with different angle of polarizer. (c) Photoluminescence intensity (solid dots) with respect to the angle of analyzer taken at 15 K. The solid curve is a cos2θ fit……………66 Fig. 4.2: (a) Cross-section TEM image showing the defect density reduction in the surface of the Mg-doped AlGaN/GaN SLs. (b) High magnification of the region in (a) shows the detail of pyramidal defects. Some of the irregular dark spots are out of focus. In the inset, the enlarged Mg-related defect shows a pyramid structure…………………………………………………68 Fig. 4.3: (a) Cross-section cathodoluminescence spectra of Mg-doped AlGaN/GaN superlattices at room temperature. (b) In-plane cathodoluminescence spectra of Mg-doped AlGaN/GaN superlattices at room temperature. (c) The schematic diagram shows the electron beam with in-focus and out of focus used in our measurement. The difference between the two focus points is about 700 nm………………………………………………………………69 Fig. 4.4: (a) SEM image of the cross section of Mg-doped AlGaN/GaN SLs. (b) Monochromatic cathodoluminescence image of the same region monitored by 2.8 eV emission………………………………………………………..70 Fig. 5.1: Edge photoluminescence spectra of InGaN quantum dots (QDs) with different excitation density at room temperature. A guiding line is used to show the blueshift with increasing the pumping power…………………..75 Fig. 5.2: Refractive index as a function of the optical excitation density. The refractive index was calculated from the interference pattern shown in Fig. 5.2 by using the Fabry- Pérot equation listed in the text…………………………77 Fig. 5.3: Temperature dependence of the photoluminescence peak position of InGaN QDs under high and low excitation intensity……………………………..78 Fig. 5.4: Room-temperature μ-Raman scattering spectra of InGaN QDs under different excitation density………………………………………………..80 Fig. 5.5: (a) The relationship of A1(LO) and E2H phonon modes versus optical excitation density. (b) The calculated strain as a function of optical excitation density based on Eq. (5.1)……………………………………..81 Fig. 6.1: Photoluminescence spectra of InGaN/GaN nanotips (solid line) and multiple quantum wells (dot line)……………………………………………………….88 Fig. 6.2: (a) Photoluminescence (PL) spectra of as-grown InGaN/GaN multiple Quantum wells excited with optical excitation density ranging from 2.5x103 to 2x105 W/cm2. (b) The PL spectra of InGaN/GaN nanotips excited with optical excitation density ranging from 2.5x103 to 2x105 W/cm2…………89 Fig. 6.3: (a) Room-temperature Raman scattering spectra of InGaN/GaN nanotips and as-grown multiple quantum wells. (b) and (c) Room-temperature Raman scattering spectra of as-grown multiple quantum wells and nanotips under different excitation densities………………………………………..93 Fig. 6.4: Calculated strain of InGaN/GaN nanotips and as-grown multiple quantum wells as a function of optical excitation density based on Eq. (6.1)…..…..94 Fig. 7.1: The schematic diagram of the edge photoluminescence measurement with different angle of polarizer……………………………………………….100 Fig. 7.2: (a) Cross sectional photoluminescence spectra with σ+ and σ− polarizations of the InGaN/GaN superlattices in absence of a current. (b) shows the intensity variation of the cross sectional PL spectra with different orientation of polarization when the current flow ( I = +/- 80 mA ) is turned on at 13 K………………………………………………………………...101 Fig. 7.3: Differential spectra of polarized photoluminescence. Base line is taken with the current turned off……………………………………………………..102 Fig. 7.4: The degree of circular polarization as a function of excitation density and corresponding strain. The strain is calculated by the Raman spectra in Fig. 5.The in-plane electric field is applied in -y direction…………………...105 Fig. 7.5: Raman shift as a function of excitation density. In the inset, the calculated strain as a function of excitation density. It shows that the internal strain in InGaN/GaN superlattices can be manipulated by external excitation……105 Fig. 7.6: (a) The theoretical effective Rashba coupling αe = λ+α (eVÅ) (open circle) and BIA coupling β (eVÅ3) (solid circle) vs. the in-plane strain. (b) the theoretical spin Hall conductivity (σsxy) (solid square) and the degree of circular polarization (CP) (solid triangle) vs. the in-plane strain. The in-plane electric field is applied in -y direction…………………………..108 Fig. 8.1: Θ-2Θ X-ray diffraction patterns for the In0.186Ga0.814N thin film……….114 Fig. 8.2: (a) SEM image of the In0.186Ga0.814N thin film. (b) Typical cathodoluminescnce image of of the In0.186Ga0.814N thin film…………...116 Fig. 8.3: Indium and Gallium composition of the bright spot and dark region determined by energy dispersion data……………………………………116 Fig. 8.4: Local cathodoluminescence (CL) spectrum of the In0.186Ga0.814N thin film taken in the bright and dark regions……………………………………...11