Chapter 1. Introduction
1.4 Outline of this Dissertation
The contents of this dissertation surround the ripening dynamics of the self-assembled II-VI semiconductor QDs grown by MBE. Before discussing the experimental results, in Chapter 2, the experimental systems and techniques are introduced. The samples were grown using the Veeco-Applied-EPI 620 and Riber 32P MBE systems. The AFM was employed to measure the sample’s topography. In addition, the optical measurements, including photoluminescence (PL), and time-resolve PL (TR-PL) are presented. At the end of Chapter 2, the RTA technique is also presented.
The main focus of Chapter 3 is the growth dynamics of CdSe/ZnSe QDs. To have an ideal growth condition of QD, in Sec. 3.2, we study the effect of substrate temperatures on the formation of QD. The AFM images reveal that TG is an important parameter for controlling the dot size. TG must not be too low to ensure that sufficient kinetic energy is provided for the migration of the CdSe molecules. However, if TG is too high, the nucleation of the properly sized CdSe QDs becomes difficult. In summary, a TG of 260 oC is the most suitable for further morphology study. In Sec. 3.3, a complete transfer of the QD growth mode was discussed. As the coverage increases, the growth mode from the FM mode to the SK mode, followed by the ripened mode, was observed in an AFM study. The observations herein are consistent with the theoretical prediction in Ref. 9. The full understanding of growth dynamics is helpful in studying the size control of QD.
The applications of QDs in opto-electronic devices inevitably encounter the problems of the dot density, designed wavelength and size fluctuation. Thus good artificial control during the growth of QDs is an important issue to improve the fabrication of opto-electronic devices with QDs. The ripening dynamics of QDs
provides an additional method for artificial control on the dot density and size. For example, to achieve ultra-low QDs density for the device fabrication which involve with a single QD. In Chapter 4, we investigate the effect of ZnSe partial capping on ripening of CdSe QDs. The final morphology and optical properties of QDs depend on the coverage of the ZnSe partial capping. The mechanism of ripening depending on the thickness of the partial capping layer is discussed in Sec. 4.2. In Sec. 4.3, we presented a model to describe the enhancement factor of ripening, which is an exponential function of the partial capping thickness.
In Chapter 5, we investigate the effect of atomic oxygen on ripening of CdSe QDs. In Sec. 5.2, the results show that atomic oxygen on the surface of ZnSe buffer layer probably change the surface energy and play the role of the nucleation sites.
Therefore, the dot density increases and the dot size decreases when the CdSe coverage thickness is kept the same. Moreover, the surface migration of CdSe QDs can be significantly enhanced by introducing the atomic oxygen, and cause the size and the uniformity of QDs to increase during the growth interruption. The results are discussed in Sec. 5.3.
In Chapter 6, the main issue is to study how RTA affects the ripening and band bending of the type-II ZnTe/ZnSe QD structures. The characteristics of as-grown ZnTe/ZnSe QDs with 3.0 MLs are discussed in Sec. 6.2. The average dot diameter and height are 40.5 and 7.86 nm, respectively. The estimated dot density is 1.5 x 109 cm−2. The Stranski-Krastanow growth mode of the ZnTe/ZnSe QDs was verified and investigated by reflection high-energy electron diffraction (RHEED) and AFM studies.
In Sec 6.3, the PL spectra show a significant strong red-shift in the peak energy, when QDs are annealed at an annealing temperature (TA) that exceeds a critical temperature.
It can be well interpreted in terms of the formation of larger QDs by the migration of atoms from the neighboring small QDs, activated by RTA. However, the PL peak
does not exhibit any red-shift as TA is increased further. Moreover, the reduction of the band-bending effect by RTA was discussed in Sec. 6.4. Based on studies of the time-resolved and excitation power-dependent PL, the observed decreasing of the band-bending effect in the annealed QD could be related to the increase in the dot size.
Finally, we summarize the conclusions of these studies in Chapter 7.
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Free energy
0
V (volume)Ripening
VC V1 V2
Fig. 1.1: Schematic illustration of the free energy of an island as a function of island volume V. The dotted line indicates the total free energy of strain-free island nucleation.
ZnSe ZnTe ZnSe
△E
C~ 0.6 eV
1.8 eV
△E
V~ 1.0 eV
ZnSe ZnTe ZnSe
△E
C~ 0.6 eV
1.8 eV
△E
V~ 1.0 eV
Fig. 1.2: Schematic type-II band alignment for ZnTe QDs.
Chapter 2. Experiments and Techniques
In this chapter, we will introduce the experimental systems and techniques, including the molecular beam epitaxy (MBE), atomic force microscopy (AFM), photoluminescence (PL), time-resolved photoluminescence (TR-PL), and rapid thermal annealing (RTA) treatment.
2.1 Molecular Beam Epitaxy (MBE)
MBE is a technique for epitaxial growth via the interaction of one or several molecular or atomic beams that occurs on a surface of a heated crystalline substrate.
The samples, which were studied in this dissertation, were grown using the Veeco-Applied-EPI 620 or Riber 32P MBE systems. The Veeco Applied EPI 620 MBE system is shown in Fig. 2.1, which is a six-source vertical growth chamber. It includes the vacuum systems, analytical equipments, one introduction chamber, and one growth chamber.
The growth chamber contains all of the components and analytical equipment.
Currently, we have five cells, which contain materials Zinc (Zn), Selemium (Se), Cadmium (Cd), Manganese (Mn), and Tellurium (Te). An Addon RF-plasma source with independently separated pumping design which could provide very abrupt oxygen incorporation for supplying reactive atomic oxygen (O) radicals. The flux of oxygen gas is controlled by a mass flow controller system. The EPI 40 cc low temperature cells are used for the evaporation of the elemental solid source Zn, Te, Cd, and Se. For Mn Solid source, the EPI 40 cc standard temperature cell is used. Each of the cells has its own shutters to control the growth time. There is a main-shutter
between sources and substrate to protect substrate from evaporation before growth.
The common focus of the cells is at the sample plane on a substrate heating stage, which can be continuously rotated for highly uniform growth. Substrate motion is imparted to the manipulator’s magnetically coupled drive train from a servomotor.
The reflection high-energy electron diffraction (RHEED) system is also set up in the growth chamber. It is an invaluable tool to determine different aspects of the depositing layer. Morphological information of the surface may be interpreted from the spot and line patterns typical on the displayed phosphor screen during growth.
To maintain the low growth chamber pressure required for molecular beam epitaxy, the Veeco Applied EPI620 MBE system uses several kinds of pumps. Both the growth and introduction chambers are roughly pumped (the range between atmosphere and approximately 100 mtorr) using an oil-free mechanical diaphragm pump. The introduction chamber uses the Varian V-250 turbo-molecular pump to reduce the pressure to the high vacuum region (＜5×10-8 torr ) in all volumes. The growth chamber uses a single cryogenic pump, which is a closed loop liquid He cryogenic pumping system, to reach high vacuum, and ultrahigh vacuum (＜1×10-10 torr).
The another MBE systems: Riber 32P is shown in Fig. 2.2, which has eight materials sources, include Mg, Zn, Se, Te, Cd, ZnS, ZnCl2, and RF generated nitrogen.
The samples of ZnTe/ZnSe type-II QDs for studying the effect of RTA on the ripening and the band-bending effect were grown using this MBE system.
The detailed growth procedures of the sample structures were described in the first section of each corresponding chapters.
2.2 Atomic Force Microscopy (AFM)
Binnig, Quate, and Gerber invented the AFM in 1985. Their original AFM consisted of a diamond shard attached to a strip of gold foil. The diamond tip contacted the surface directly, with the interatomic van der Waals forces providing the interaction mechanism. The AFM was developed to overcome a basic drawback with STM, that it can only image the surfaces of conductor or semiconductor. The AFM, however, has the advantage of imaging almost any type of surface, including polymers, ceramics, composites, glass, and biological samples.
In this dissertation, the NT-MDT SOLVER P47 AFM was used for the morphology study. The schematic diagram of the AFM system is shown in Fig. 2.3.
The measurement was carried out using the semi-contact mode. The scan step in x, y direction are both 4.8 nm. The resolution is 0.01 nm in the z direction. The shape of silicon tip is conic. Its diameter and height are 30 and 70 nm (i.e. the aspect ratio, α=
H/D= 7/3), respectively. The aspect ratio of silicon tip is much lager than that of the CdSe QD (typical aspect ratio of CdSe QD is about 4.3×10-2) grown in current study.
Therefore, no further treatment on the AFM data is needed.
2.3 Photoluminescence (PL)
PL is a process in which a substance absorbs photons and then re-radiates photons. It can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. This is one of many forms of luminescence and is distinguished by photoexcitation. The period between absorption and emission is typically shorter than 1 ns or maybe as long as several hundred ns. PL is an important technique for measuring the purity and crystalline quality of semiconductor.
The experimental setup for PL measurements is shown in Fig. 2.4. The samples were loaded on the cold finger of a closed-cycle refrigerator. The 325 nm line of a He-Cd laser was employed to excite the PL spectra. A Spex-1403 double-grating spectrometer, equipped with a thermal electric cooled photomultiplier tube, was used to analyze the PL spectra. The slit widths were set to 100 m to yield a spectral resolution of better than 0.1 meV. High spectrum resolution which is better than 0.1 meV can be obtained.
2.4 Time-resolved Photoluminescence (TR-PL)
The recombination dynamics of the samples were studied by TR-PL. A pulsed diode laser (375 nm) with a repetition rate of 2.5 MHz is employed as the excited source for TR-PL measurements. The decay traces are recorded using the time-correlated single photon counting technique with an overall time resolution of
~120 ps.
2.5 Rapid Thermal Annealing (RTA)
RTA is an alternative to conventional furnace annealing. Its advantages include short annealing time and precise control of the annealing profile. Thermally induced intermixing in such QD systems has attracted considerable attention as a way of tuning or controlling the emission properties of the QD assemblies [1-6]. Typically, a blue-shift of the peak energy is observed concurrently with a narrowing of the peak linewidth after RTA [1-3].
In this dissertation, the ZnTe/ZnSe type-II QDs were treated with RTA in pure N2
environment. RTA process was performed using tungsten-halogen lamps at temperatures from 300 to 480 °C. The rising and cooling rate of temperature is about 30 °C/sec, and the annealing time is fixed at 30 sec.
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Fig. 2.1: Veeco Applied EPI 620 MBE system
Fig. 2.2: Riber 32P MBE system
Fig. 2.3: NT-MDT SOLVER P47 AFM system
Fig. 2.4: Experimental setup for the PL measurement Argon - ion
Laser
L1
L2
L3
TRIAX550 Singal-Grating Spectrometer
Computer
PMT Closed Refrigerator
1.71.81.92.02.12.22.32.42.5
15K
PL Intensity (a.u.)
Photon Energy (eV) ZnTe
Chapter 3. Growth Dynamics of CdSe Self-Assembled Quantum Dots
In this chapter, the growth dynamics and optical properties of CdSe/ZnSe quantum dots (QDs) will be discussed. The self-assembled CdSe/ZnSe QDs were grown at various growth temperatures on GaAs (001) by molecular beam epitaxy.
Experimental results indicated an optimum growth temperature was found to be 260
oC. The Stranski–Krastanow (SK) growth mode was confirmed clearly by atomic force microscopy images. The thickness of the wetting layer of the CdSe QDs is about 2.5 mono-layers (MLs). Two types of QDs were found with the CdSe coverage of 3.0 MLs. As the coverage increases, a complete transfer of the QD growth mode from the Frank van der Merve (FM) mode to the SK mode, followed by the ripened mode, was observed in an AFM study. A schematic diagram of the growth mechanism of self-assembled CdSe QDs is presented. Moreover, the photoluminescence spectra of samples with various thicknesses were investigated. A dramatic change of optical properties confirmed that the QD structure formed with thickness above 2.5 MLs [1].
3.1 Growth of CdSe/ZnSe Quantum Dots
Self-assembled CdSe QDs were grown on a ZnSe buffer layer using Veeco Applied EPI 620 MBE system. The ZnSe buffer layer was grown on the epi-ready GaAs substrate. Before growth, the GaAs surface was cleaned by chemical etching with H2O : NH4OH : H2O2 in the ratio 50 : 5 : 5 for 2 minutes, followed by rinsing with D.I. water and final drying with nitrogen gas. The time between chemical etching and the transfer of the substrate into the vacuum chamber is less than 5 minutes. The
substrate temperature (TG) was then raised to 650 oC to desorb the residual oxide on the GaAs surface. Desorption process was monitored by a reflection high energy electron diffraction (RHEED) pattern. After desorption, TG was decreased to 350 oC to grow the ZnSe buffer layer. The ZnSe buffer layer includes several MLs grown by migration enhance epitaxy (MEE) [2], and a thickness of 20 nm grown by the conventional MBE growth mode. The average roughness of the ZnSe buffer layer is about 0.28 nm. After the flat ZnSe buffer layer was deposited, the growth of the self-assembled CdSe QDs began. Table 3.1 presents the uncapping sample parameters in the AFM study. Samples 1, 2 and 3 have the same average coverage of CdSe (3.0 MLs), but the values of TG were 240, 260 and 280 oC, respectively. The TG values of samples 4 to 7 are fixed at 260 oC. The average coverage of CdSe for samples 5 to 7 are between 2.0 and 3.0 MLs. Sample 4 was grown without CdSe coverage to investigate the roughness of ZnSe buffer. The average coverage of CdSe was determined by the growth duration and the growth rate, which was calibrated by a thick CdSe epilayer whose growth conditions were the same as those of QDs. The growth rate was determined by measuring the growth duration and the thickness of the thick CdSe epilayer. The thickness was estimated from the energy spacing of the interference peaks of the reflectivity spectrum. The thickness of the CdSe layer was further verified by optical microscopy with a resolution of 0.1μm. For a CdSe epilayer with a thickness of 1μm and accuracy of 0.1μm, the uncertainty of the growth rate is about 10%. It implies the accuracy of the CdSe coverage is about 10%, i.e. the uncertainty is less than several tenths MLs for the CdSe coverage of 2 to 4 MLs. Determining the thickness of the wetting layer is crucial. The thickness of the ZnSe buffer layers is 20 nm. Above the ZnSe buffer layer, CdSe was grown with an average coverage fixed at 3.0 MLs. Buried dots were also prepared for optical studies.
The growth condition of the samples for the optical study was the same as that of
samples 4 to 7, before a 20 nm capping layer of ZnSe was deposited.
3.2 Optimum Growth Temperatures
Figures 3.1(a)/3.1(b), Figs. 3.1(c)/3.1(d) and Figs. 3.1(e)/3.1(f) show the plane view/3D view of AFM for sample 1 (TG = 240 oC), sample 2 (TG = 260 oC) and sample 3 (TG = 280 oC), respectively. The vertical axis labeled with black and white contrast is drawn at the right-hand side of the plane views to represent the height of a QD. In Figs. 3.1(a) and 3.1(b), large islands are observed. The average diameter and height of the large islands are about 291 and 29.81 nm, respectively. The dots are large and their sizes are not uniform. As TG is increased slightly for sample 2, self-assembled CdSe QDs form, as depicted in Figs. 3.1(c) and 3.1(d). The QDs are divided into a group of large dots (H > 4 nm) and one of small dots (H < 4 nm). The average diameter of the larger (smaller) dots is approximately 86 (66) nm and the mean height is 5.35 (2.21) nm. The dot density of both groups is approximately 20.0×108 cm-2. The dot density increased abruptly as TG is increased further for sample 3, as shown in Figs. 3.1(e) and 3.1(f). The dots are very close to each other. As a result, some dots were connected. Random dot shapes are observed. Also, the size distribution is very broad. This sample is less useful for further study. The above result reveals that TG is an important parameter for controling the dot size. TG must not be too low to ensure that sufficient kinetic energy is provided for the migration of the CdSe molecules. However, if TG is too high, the nucleation of the properly sized CdSe QDs becomes difficult. In summary, a TG of 260 oC of sample 2 is the most
Figures 3.1(a)/3.1(b), Figs. 3.1(c)/3.1(d) and Figs. 3.1(e)/3.1(f) show the plane view/3D view of AFM for sample 1 (TG = 240 oC), sample 2 (TG = 260 oC) and sample 3 (TG = 280 oC), respectively. The vertical axis labeled with black and white contrast is drawn at the right-hand side of the plane views to represent the height of a QD. In Figs. 3.1(a) and 3.1(b), large islands are observed. The average diameter and height of the large islands are about 291 and 29.81 nm, respectively. The dots are large and their sizes are not uniform. As TG is increased slightly for sample 2, self-assembled CdSe QDs form, as depicted in Figs. 3.1(c) and 3.1(d). The QDs are divided into a group of large dots (H > 4 nm) and one of small dots (H < 4 nm). The average diameter of the larger (smaller) dots is approximately 86 (66) nm and the mean height is 5.35 (2.21) nm. The dot density of both groups is approximately 20.0×108 cm-2. The dot density increased abruptly as TG is increased further for sample 3, as shown in Figs. 3.1(e) and 3.1(f). The dots are very close to each other. As a result, some dots were connected. Random dot shapes are observed. Also, the size distribution is very broad. This sample is less useful for further study. The above result reveals that TG is an important parameter for controling the dot size. TG must not be too low to ensure that sufficient kinetic energy is provided for the migration of the CdSe molecules. However, if TG is too high, the nucleation of the properly sized CdSe QDs becomes difficult. In summary, a TG of 260 oC of sample 2 is the most