Chapter 1: Introduction
1.3 Organization of dissertation
This dissertation is classified into two subjects. The first subject is concerned with ordered quantum dot lattices growth and the other is with quantum ring-like nanostructures. Before getting into the main subjects, the experimental techniques used
in this dissertation are described in chapter 2. In the following, the growth condition dependence and characterization of QDs are studied in chapter 3. As for the first subject of this dissertation, the studies with ordered quantum dot lattices growth are presented at first. A chain of ordered quantum dot array formed on the GaAs (100) substrate patterned by electron beam nanolithography. Next, a new technique was developed to position a single QD on a predesigned location to improve device applications. In the second subject, fabrication method and formation mechanism of In(Ga)As semiconductor quantum rings are discussed. Then, a new growth method to produce quantum rings is proposed and demonstrated experimentally. Finally, a conclusion and a brief description in the future works are presented.
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Chapter 2
Experimental techniques
In this chapter, the experimental techniques used in these studies are presented.
They mainly consist of two parts: sample growth and material characterization. All the samples were grown with the molecular beam epitaxy (MBE) system in our laboratory.
The material characterization techniques are stated in the last section of the chapter:
photoluminescence and atomic force microscope. The concise setups and basic operating principles of measurement instruments are described briefly here.
2.1 Molecular beam epitaxy growth [2.1-2.3]
2.1.1 Introduction to our MBE system
Basically, molecular beam epitaxy (MBE) is a vacuum evaporation technique defined as the growth of epitaxial films onto the heated substrates using atomic and molecular beams produced from Kundsen effusion cells (K-cell) under ultra-high vacuum condition. It is a physical deposition, which is carried out under condition far from thermodynamic equilibrium and is governing by the kinetics of the surface process. Traditionally, this technique was developed for high-quality epilayer and ideal heterostructure growth. Nowadays, owing to its precise growth on atomic scale, MBE has been verified as the most suitable and reliable tool to construct various nanostructures with a wide range of geometries, such as quantum wells (QWs), quantum wires and quantum dots (QDs).
Our system is a Varian Gen II solid source MBE machine with schematic shown in
Fig.2.1. The components of the system can be classified into three main groups:
vacuum, epitaxy and analysis equipments.
• Vacuum equipments
It mainly consists of three connected ultra-high vacuum chambers: the growth chamber, the buffer chamber and the entry/exit (E/E) chamber. There are two gate valves used to connect or isolate the buffer chamber and the other two chambers. In order to obtain ultra-high vacuum on request for growth, these three chambers are equipped with a variety of oil-free pumps: two cryo-pumps (one for growth chamber, and the other for E/E chamber), two ion pumps (one for growth and the other for buffer), and two titanium sublimation pumps (TSP) (one for growth and the other for buffer).
Furthermore, in order to pump down the system from atmospheric pressure, a Varian scroll pump, and a turbo-molecular pump equipped with a diaphragm vacuum pump are connected in parallel to all three chambers and isolated with three metal valves, respectively.
• Epitaxy equipments
There are eight effusion cells installed in our growth chamber. Two gallium (Ga) cells (one is a SUMO cell and the other is a conventional K-cell), one aluminum (Al) cell, and one indium (In) SUMO cell supply group III sources. The SUMO cells produced by Applied EPI have several characteristics superior to the conventional K-cells. For example, the advantages provided by the SUMO cell are low defects, good uniformity, an increased capacity and so on. About group V sources, there are two arsenic (As) cells installed in the growth chamber: one is a K-cell and the other is an As valved cracker cell. As for the As valved cracker cell, it can provide two As molecular
species (As2 and As4).The amount of As flux is controlled with a needle valve. Silicon (Si) and beryllium (Be) are used as the n-type and p-type dopant sources, respectively.
With these source elements, we can grow high quality films of GaAs, AlGaAs, AlAs, InGaAs, InAlAs, and InAlGaAs on GaAs or InP substrates under proper growth conditions.
• Analysis equipments
In addition to the vacuum and epitaxy equipments mentioned above, two analysis instruments are installed, one is the residue gas analyzer (RGA Model 100, Stanford Research Systems), and the other is the reflection high-energy electron diffraction (RHEED) assembly.
The RGA is a mass spectrometer, which is used to analyze the residue gas in the chamber. It can serve as a leaky detector by detecting He or N2 inside the chamber.
The RHEED is the most powerful tool to in-situ analyze or study the dynamics of MBE growth. By using high-energy electron beam with a very small incident angle (about 2o), the surface construction and kinetics can be analyzed with the diffraction patterns. The common applications in the MBE growth are monitoring the thermal decomposition (or desorption) of native oxide layer, and growth rate calibration.
Besides, it provides a useful and general way of characterizing QDs formation by RHEED pattern transformation.
2.1.2 Brief description of MBE mechanism
A detailed model to describe the deposition of GaAs with MBE has been proposed by Foxon and Joyce in 1970s. It basically consists of a series of surface processes in the epitaxial deposition: (i) absorption of the constituent atoms, (ii) surface migration and
then dissociation of the absorbed atoms, (iii) incorporation of the constituent atoms into the crystal lattice, and (iv) desorption of the atoms not incorporated into the crystal lattice.
2.1.3 Other aspects about MBE growth
Before regular MBE growth duties, several procedures need to be performed and checked, such as growth rate calibration, wafer bake-out, growth chamber cooling, oxide layer desorption and so on.
• Growth rate calibration
The calibration of growth rate can be done with RHEED oscillation, which is directly related to the epitaxial deposition in the 2D layer-by-layer growth mode. The origin of oscillations is given as follows: Initially, when the substrate surface is fresh with no adatoms, the intensity at the specular spot reaches its maximum value, because of the constructive interference of the diffracted electron from the crystalline substrate surface. At the moment the group III source shutter is opened, the atoms begin to adhere to the surface. Then the RHEED intensity decreased due to the destructive interference. It reaches a minimum value when half of a layer is deposited. Afterwards, the intensity gradually recovers to a peak value when a full layer is nearly completed.
Because each effusion cell gives a stable beam flux at a fixed cell temperature, the calibration is to find out the relation between beam-flux equivalent pressure (BEP) of the source elements and growth rate from the RHEED oscillations.
A GaAs (100) substrate is used to check the growth rates of both Ga and Al cells, due to the almost lattice match between AlAs and GaAs. On the other hand, an InAs (100) substrate is used for the In growth rate calibration, because of a large lattice misfit
of ~7 % between InAs and GaAs.
For our growth rate check, the oscillations are captured and recorded by a Video RHEED system, which is a powerful tool to analyze the RHEED images. It can track RHEED intensity changes and measure the rate of oscillations directly. Besides, the FFT analysis of the oscillations can accurately determine the rates from noisy signals.
One of RHEED images and growth rate for In cell are shown in Fig.2.2.and Fig. 2.3
•Wafer cleaning and growth chamber cooling down
A standard treatment procedure for wafers before growth is described as follows:
The epi-ready GaAs wafers are mounted on Mo block with In or In-free springs, and then put into E/E chamber for bake-out to remove the water absorbed during the loading procedure. After bake-out and pumping down, the pressure of the E/E chamber can be lower to about 10-9 torr. Then, we transfer the wafers into the buffer chamber. In the buffer chamber, each wafer is heated up to 400oC to bake out the residual water and organic species further. After the chamber pressure is lower than 3x10-9 torr, the wafer could be transferred into the growth chamber for epitaxial deposition.
Before the cells warmed up for sample growth, the cryoshroud surrounding the interior of the growth chamber is cooled with a flow of liquid nitrogen. The utility of cryoshroud cooling process is to trap the impurities and absorb the source elements that miss the substrate, preventing the unwanted materials from being incorporated in the epitaxial layer. The pressure of the chamber would be lower to less than about 4x10-10 torr after cooling down.
• Native oxide desorption and growth conditions
Before epitaxial deposition, the thin native oxide needs to be removed by heating
the substrate to certain temperature (about 610oC for GaAs, 510oC for InP or InAs wafers) under enough arsenic flux. Then, about 2500 Å buffer layer is deposited to recover the substrate surface immediately.
Finally, the standard growth conditions for 2-D layer-by-layer growth mode are given as follows: The growth rate is typically around 1 µm/hr; the III-V BEP ratio (group V versus group III) and growth temperature must be kept within suitable range for different grown materials for high-quality epi-layers. For example, the III-V BEP ratio keeps around 10 (20) for As2 (As4); the growth temperature for GaAs, AlAs, AlxGa1-xAs and InxGa1-xAs materials on GaAs substrates are near 570oC and 510oC, respectively. Besides, for self-assembled growth, various growth conditions are used to obtain a diversity of nanostructures, such as QDs and QRs. We will give the detail procedures and growth conditions in the following chapters.
2.2 Material characterization [2.3-2.4]
2.2.1 Photoluminescence
Photoluminescence (PL) is a nondestructive characterization technique to identify the optical quality of semiconductors. It is a measurement defined as the creation of electron-hole (e-h) pairs in the semiconductor by optical radiation and subsequent radiative recombination photon emission. Briefly, there are three basic physical processes involved in the PL: e-h excitation, carriers’ thermalization and diffusion, and e-h recombination.
The PL system in our laboratory (as shown in Fig. 2.4) consists of an argon-ion pumping laser, an optical chopper, a lock-in amplifier, a closed-cycle cryostat, a 0.85m double grating monochromator, two photodetectors (Si-PMT and TE-cooled InGaAs)
and a set of focused/collected lens. The samples to be measured are kept in the cryostat, which provides for the varied-temperature measurements (about 20K ~ room temperature). The pumping laser is focused to excite the samples, and then the resulting luminescence is collected into the input slit of the monochromator. The grating used in the system is 600 /mm with blazing wavelength at 1000nm. The dispersed light is imaged on the output slit of monochromator and detected by the photodetector. The whole PL system is controlled by a personal computer.
2.2.2 Atomic force microscopy
• Introduction
The invention of the scanning tunneling microscope (STM) has revolutionized the field of microscopy: scanning probe microscopy (SPM). It relies on a feedback loop to control a fine tip only a few nanometers away from the sample surface while the tip is raster scanned in X and Y to record an image. Since a tunneling current is employed in STM, the application is limited to conductive surfaces. In order to analyze the features of insulting surface, a new kind of SPM, which is called atomic force microscope (AFM), was developed immediately. At first, the contact mode AFM was developed, which relied on the repulsive forces experienced by the tip measured by recording the cantilever deflection. Afterwards, a new modulation technique in AFM (tapping mode) was invented to overcome the limitations of contact mode. For consideration of sample damage and fast wear-out of tip operated in contact mode, we characterize the samples in the tapping mode. Moreover, we operate the AFM in the constant-force (or constant-interaction) mode. In the constant-force mode, in which the feedback mechanism is activated, one detects the variations of the local z-height of the tip with respect to the sample surface at the fixed force strength.
Our system (as shown in Fig.2.5) is a Digital Instruments MultiMode SPM, which basically consists of the optical head and the base. The scanner is installed within the base, and the measured sample is mounted on the top of the scanner. The optical head consists of sample space, a laser diode, mirrors, and a four-quadrant positional photodetector. The tip on the cantilever is vibrated by bimorph at its resonance. As it approaching to sample surface, the tip-sample interaction causes a change in the amplitude, the phase, and the resonance frequency of the vibrating cantilever. Therefore, during operating in tapping mode, the feedback loop keeps the cantilever to vibrate at constant amplitude (constant-force mode) by extending or retracting the scanner as it is simultaneously raster-scans in X and Y directions. Finally, the “history” of scanner movement in Z across the sample surface is converted into a 3D image of the height data.
Finally, it is necessary to note that we should check the tips used in the measurement carefully because the results of AFM images are strongly dependent on the shape and radius of the tip. Figure 2.6 shows that the surface morphology of QDs in the same sample with different tips. The measured result is approximately the sum of the nanostructures’ real size and the tip’s diameter. Therefore, a ‘very uniform’ size distribution of the nanostructures would be obtained and is nearly equal to the tip’s diameter when the tip’ size is much lager than nanostructures’.
• Brief description of operation principle
The basic principle of operation in the tapping mode AFM can be simply and phenomenally described with a forced oscillation with damping. The equation of motion and its solution have the form
m t
where F, ω, ωo , and β are driving force, driving frequency, cantilever’s resonant frequency and damping constant respectively. The solution consists of homogeneous (which is omitted above) and particular parts. The homogeneous solution comprises an exponent term derived from the damping, and would decay rapidly. The particular term is a steady-state solution.
From the formula above, the amplitude and phase of the oscillation changes with damping β. The relations between the amplitude (phase) and oscillation frequency of the cantilever are shown in Fig.2.7. Figure 2.8 gives the shift in amplitude (phase) at the fixed frequency due to the tip-sample interaction. There are at least three types of data recorded in the tapping mode AFM measurement: height, amplitude, and phase image.
As mentioned above, the height image records the ‘traveling’ of scanner needed to keep the amplitude of the cantilever constant. On the other hand, the amplitude image is the change in amplitude of cantilever due to tip-sample interaction, and phase imaging is the mapping of the phase lag between the periodic signal that drives the cantilever and the oscillations of the cantilever.
References
[2.1] E. H. C. Parker, The technology and physics of molecular beam epitaxy, London, England (1985).
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[2.3] V. Swaminathan and A.T. Macrander, Materials aspects of GaAs and InP based structures, Prentice Hall, Inc. (1991)
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Cryo-pump
Fig.2.1 A schematic of our Varian Gen II MBE system.
Fig.2.2 One of measured RHEED oscillations in our system.
In820 Growth Rate
Fig.2.3 One of calibrated plots for growth rate and beam flux equivalent pressure.
Argon Laser
Lock-in Amp. PC
Si-PMT
PD
Monochromator (SPEX-1404) M1
L2 L1 Cryo-Stat M2
Sample Chopper
Fig.2.4 A schematic of the PL measurement system.
Fig.2.5 MultiMode SPM (upper image) and optical head (lower image).
Figure 2.6 AFM images of QDs in the same sample taken with different tips
Fig. 2.7 The relations between the amplitude (phase) and oscillation frequency of the cantilever
Fig. 2.8 The amplitude (phase) shift at the fixed frequency due to the tip-sample
Chapter 3
InAs/GaAs quantum dots growth and characterization
In this chapter, the studies of self-assemble InAs quantum dots (QDs) growth on (100) GaAs substrate are presented. The investigation of QDs growth and characterization were carried out, which were based on the previous results in our laboratory. First of all, the fabrication methods of QDs and the mechanism of the self-assembled growth (Stranski-Krastanow mode) are introduced. Then, the previous results of growth conditions dependence of the QDs are reviewed briefly. At last, the studies of growth parameters dependence of the QDs are demonstrated.
3.1 Introduction to QDs fabrication
3.1.1 Fabrication methods of QDs
There has been a great amount of effort expended in the QD fabrication methods during the last two decades. The most conventional method is to lateral pattern the quantum well structure by electron beam lithography and wet or dry etching. However, lithography- and etching-based technologies will damage the structure and lead to defect problem. In order to overcome the surface state problem, two new techniques were introduced in early 1990s. In 1993, a novel fabrication technique, which was so-called thermal etching, was developed to construct high-quality and surface-state-free QDs [3.1]. This technique utilizes the property of different thermal desorption rates between different materials to result in nonuniform thermal evaporation in a thin epilay to create QDs. Figure 3.1 illustrates the growth procedure