Title Page

Abstract

Contents

Chapter I. Introduction 18

References 20

Chapter II. Overview of Silicon Quantum Dots 21

2.1. Quantum Dots 21

2.1.1. Electronic Structure of Quantum Dots 22

2.1.2. Electron Addition and Optical Gap Energies 28

2.2. Silicon Nanocrystal Quantum Dots 30

2.2.1. Light Emission from Silicon Nanocrystals 30

2.2.2. Charge Storage in Silicon Nanocrystals 32

2.2.3. Single Electron Effect in Silicon Nanocrystals 33

References 35

Chapter III. Growth and Basic Properties of Silicon Nanocrystals Embedded in Silicon Nitride Films 46

3.1. Plasma Enhanced Chemical Vapor Deposition (PECVD) System for Growth of Silicon Nanocrystals 46

3.2. Growth and Basic Properties of Silicon Nanocrystals Embedded in Silicon Nitride Films Grown by PECVD 47

References 50

Chapter IV. Effect of Hydrogen Passivation on Charge Storage in Silicon Quantum Dots Embedded in Silicon Nitride Films 55

4.1. Introduction 55

4.2. Experimental Procedure 56

4.3. Results and Discussion 56

4.4. Conclusion 60

References 62

Chapter V. Room-temperature Coulomb Blockade Effect in Silicon Quantum Dots in Silicon Nitride Films 70

5.1. Introduction 70

5.2. Experimental Procedure 71

5.3. Results and Discussion 72

5.4. Conclusion 75

References 76

Chapter VI. Strong Size Dependence of Carrier Injection Dynamics in Quantum-confined Silicon Nanocrystals 81

6.1. Introduction 81

6.2. Experimental Procedure 82

6.3. Results and Discussion 83

6.4. Conclusion 86

References 87

Chapter VII. Identification of Electronic Structure of Silicon Nanocrystals in Insulating Films 95

7.1. Introduction 95

7.2. Experimental Procedure 96

7.3. Results and Discussion 98

7.4. Conclusion 104

References 105

Chapter VIII. Conclusions 115

감사의 글 118

Curriculum vitae 119

Table 2.1. Roots of the Bessel function χnl(이미지참조) 38

Figure 2.1. Electronic density of states in 3D crystals (bulk), 2D (quantum wells), ID (quantum wires), and 0D (quantum dots) of semiconductors. 37

Figure 2.2. The six lowest energy levels of a particle in a 3D box with an infinite cubic (left part) and spherical (right Part) potential barrier. The energies can be compared directly if the edge length L of the cube is equal to the diameter d of... 39

Figure 2.3. Illustration of wave functions in a quantum dot. The wave function is composed of a Bloch function multiplied by an envelope function. 40

Figure 2.4. Quantum dot energy levels with n ＝ 1, 2, or 3 envelope and k ＝ 0 Bloch function. The expectation value of k does not change for different envelope wave functions; the kinetic energy increases. The wavenumber on the x-axis... 41

Figure 2.5. Experimental data on the size dependence of the optical gap of silicon nanocrystals that are synthesized by various methods. 42

Figure 2.6. Calculated conduction and valence energy levels for H-passivated silicon nanocrystals and calculated energy levels associated with a trapped electron and a trapped hole at a Si＝O bond on the surface. As the size decreases... 43

Figure 2.7. (a) Schematic cross-section of a silicon nanocrystal based nonvolatile memory and its conduction band diagram for (b) writing, (c) storing, (d) erasing. 44

Figure 2.8. Application areas of single electron devices. 45

Figure 3.1. Oxford Plasmalab system 100 and a schematic diagram of PECVD. 51

Figure 3.2. High-resolution TEM image of crystalline silicon nanocrystals. The inset shows the ring patterns for the transmission electron diffraction from crystalline silicon quantum dots. 52

Figure 3.3. Room-temperature PL spectra for various-sized silicon nanocrystals in silicon nitride films. 53

Figure 3.4. PL peak energy for crystalline silicon nanocrystals(nanocrytals) as a function of dot diameter. Dotted lines are fitted curves for amorphous and crystalline silicon quantum dots. The filled and open circles are data points obtained for crystalline and... 54

Figure 4.1. (a) PL spectra of sample-A and B. 64

Figure 4.1. (b) and (c) Plan-view TEM images of sample-A and sample-B, respectively. The insets In (b) and (c) show an enlarged TEM view of the single Si QD. 65

Figure 4.2. C-V hysteresis curves of two types of silicon nitride films grown by (a) N₂-diluted 5% SiH₄+ N₂ and (b) N₂-diluted 5% SiH₄+ NH₃. The voltage of the top electrode is swept from -V to +V and back to -V. The total width of the... 66

Figure 4.3. (a) Charge retention characteristics of silicon quantum dots. The charge-loss rates were monitored at 0 V after injecting electrons/holes at ±12 V for 7 s. 67

Figure 4.3. (b) Schematic band diagram of silicon nitride films containing silicon quantum dots. 68

Figure 4.4. FTIR spectra of sample-A and B. sample -B Shows higher peak intensities for all the absorption bands associated with hydrogen bonding, compared to sample-A. 69

Figure 5.1. (a) Plan-view TEM image of Si QDs embedded in a silicon nitride film grown using a N₂-diluted 5% SiH₄ and NH₃ plasma. The inset shows an enlarged TEM view of the single Si QD. (b) Corresponding size distribution of the Si QDs.... 78

Figure 5.2. (a) Tunneling current versus applied voltage characteristics measured at room temperature, showing a Coulomb staircase. (b) Differential conductance versus applied voltage characteristics. The average differential conductance peak... 79

Figure 5.3. (a) Schematic illustration of a fabricated MIM device with a 10 nm-thick silicon nitride film containing Si QDs. Considering the density of the Si QDs of 2.3 × 1018 cm-3 and the film thickness of 10 nm, the device can be simplified to a...(이미지참조) 80

Figure 6.1. (a) Schematic of the tunnelling capacitor device containing size-controlled silicon nanocrystals. (b) Energy band diagram of the devices. 89

Figure 6.2. (a) C-V hysteresis curves for the devices at VINJ of +12 V and -9 V for 1 sec to inject electrons and holes, respectively. The average diameter of silicon nanocrystals used in the devices is 4.97 nm, 3.80 nm, and 3.43 nm, respectively....(이미지참조) 90

Figure 6.2. (b) VTH, at which a significant carrier injection begins to occur, and △VFB (at VINJ ＝ +12 V and -9 V for 1 sec to inject electrons and holes, respectively), which is proportional to the number of injected carriers, as a function of the...(이미지참조) 91

Figure 6.2. (c) Schematic of the conduction band diagram under a positive gate bias, showing the available potential depth for electron injection. 92

Figure 6.3. Injection time dependence of △VFB characteristics for electron (a) and hole (b) injections. VINJ of +12 V and -9 V was applied to inject electrons and holes, respectively.(이미지참조) 93

Figure 6.3. (c) Schematic of the conduction band diagram under a positive gate bias, showing the resonant and nonresonant tunneling processes. 94

Figure 7.1. (a) Schematic of fabricated devices with silicon nanocrystals. (b) Cross-sectional TEM image of a device showing the array of silicon nanocrystals between the tunneling and the blocking silicon nitride layers. The dotted line... 107

Figure 7.2. (a) Schematic band diagram of the devices in flat-band condition. 108

Figure 7.2. (b) C-V characteristics of the devices containing silicon nanocrystals with average diameters of 4.97 nm, 3.80 nm, and 3.43 nm, respectively. 109

Figure 7.2. Measured △VFB as a function of VINJ for electron injection (c) and hole injection (d). Arrows indicate the VTH, which is the starting point for significant charging. VTH was determined from the intercept with the VINJ axis by linear...(이미지참조) 110

Figure 7.3. (a) Calculated VSINC versus VINJ. Dashed lines indicate VSiNC corresponding to VTH.(이미지참조) 111

Figure 7.3. (b) Measured lowest conduction and highest valence levels as a function of diameter of silicon nanocrystal. The solid lines are the least-squares fit to the quantum confinement energies according to the effective mass approximation... 112

Figure 7.3. (c) Raman spectra for the silicon nanocrystals with average diameter of 4.97 urn and the bulk crystalline silicon. 113

Figure 7.4. Comparison of the size-dependent band-gap energies determined by capacitance and PL spectroscopies. The inset shows the PL spectra for the silicon nanocrystals. The solid line indicates the least-squares fit to the band-... 114

Semiconductor nanocrystals offer great promise for developing novel devices because quantum confinement in nanocrystals leads to a variety of interesting optical and electronic properties, including tunable discrete electronic energy levels, enhanced light emission efficiency, exciton multiplication, single-photon emission, and single-electron/hole charging. In particular, silicon nanocrystals have been of great interest because silicon is the most important material for current integrated circuit industry. Unlike bulk silicon, silicon nanocrystals, which have dimensions of just a few nanometers, have tunable electronic properties by varying the size, and give rise to novel properties that will be useful for future applications, such as silicon-based light sources, solar cells, nonvolatile memories, and single-electron devices, However, in order to realize such applications, the fundamental properties of the silicon nanocrystal quantum dot needs to be understood clearly.

In this thesis, the electronic properties such as long-term charge storage, Coulomb blockade effect, size-dependent carrier injection dynamics, and absolute values of electronic energy levels are studied on the silicon nanocrystal quantum dots embedded in silicon nitride films grown by plasma-enhanced chemical vapor deposition.

The effect of in-situ hydrogen passivation on the charge storage characteristics was investigated for applications to nonvolatile silicon nanocrystal memories. To study the effect of in-situ hydrogen passivation, two types of silicon quantum dots in silicon nitride films were grown by SiH₄+ N₂and SiH₄+ NH₃plasma and metal-insulator-semiconductor(MIS) devices containing the silicon quantum dots were fabricated. The MIS devices with a silicon nitride grown by SiH₄and NH₃plasma exhibited a very low charge loss rate due to the reduction in charge loss paths through the hydrogen passivation of defects in the silicon nitride matrix and the interface between the silicon quantum dot layer and the substrate, compared to those with a silicon nitride grown using SiH₄and N₂plasma.

A room-temperature Coulomb blockade effect was observed in silicon quantum dots(Si QDs) spontaneously grown in a silicon nitride film. The metal-insulator-metal device containing the Si QDs showed a clear Coulomb staircase and differential conductance peaks at room temperature. The size distribution of the Si QDs determined by high-resolution transmission electron microscopy suggests that the measured single electron addition energy of 67 meV can be attributed to the charging energy of 63 meV of the Si QDs with the largest diameter of 4.7 nm among the various-sized Si QDs.

We report the strong size dependence of carrier injection dynamics in quantum-confined silicon nanocrystals that is investigated by using tunneling capacitor devices containing size-controlled silicon nanocrystals. As the diameter of silicon nanocrystals increases, the threshold voltage for carrier injection decreases and the number of injected carriers increases, resulting from the quantum size effect. The tunneling time for the carrier injection is shortened by about two orders of magnitude with increasing the diameter of silicon nanocrystals from 3.43 ㎚ to 4.97 ㎚, and this can be attributed to the enhanced resonant tunneling in larger silicon nanocrystals.

Finally, we report that the electronic structure of silicon nanocrystals embedded in an insulating matrix can be identified by using a capacitance spectroscopy. The absolute position of the lowest conduction and the highest valence levels of silicon nanocrystals is revealed and the significantly enhanced quantum confinement over the theoretical prediction is observed in the conduction levels of the tensile-strained silicon nanocrystals in silicon nitride films. The size-dependent band-gap energies estimated by capacitance spectroscopy agree well with those measured by photoluminescence spectroscopy.