CHAPTER 2 EXPERIMENTAL APPARATUS
2.9 X- RAY D IFFRACTION (XRD)
X-rays are electromagnetic radiation with typical photon energies in the range of 100 eV~100 keV. For diffraction applications, only short wavelength x-rays in the range of a few Å to 0.1 Å (1 keV~120 keV) are used. Because the wavelength of x-rays is comparable to the size of atoms, they are ideally suited for probing the structural arrangement of atoms and molecules in a wide range of materials. The energetic x-rays can penetrate deep into the materials and provide information about the bulk structure.
X-rays are produced generally by either x-ray tubes or synchrotron radiation. In an X-ray tube, which is the primary X-ray source used in laboratory X-ray instruments, x-rays are generated when a focused electron beam accelerated across a high voltage field bombards a stationary or rotating solid target. As electrons collide with atoms in the target and slow down, a continuous spectrum of X-rays are emitted, which are termed Bremsstrahlung radiation. The high energy electrons also eject inner shell electrons in atoms through the ionization process. When a free electron fills the shell, a x-ray photon with energy characteristic of the target material is emitted.
Common targets used in x-ray tubes include Cu and Mo, which emits 8 keV and 14 keV X-rays with corresponding wavelengths of 1.54 Å and 0.8 Å, respectively.
X-rays primarily interact with electrons in atoms. When x-ray photons collide with electrons, some photons from the incident beam will be deflected away from the direction where they originally travel, much like billiard balls bouncing off one another. If the wavelength of these scattered x-rays did not change, the process is called elastic scattering (Thompson Scattering) in that only momentum has been
transferred in the scattering process. These are the x-rays that measured in diffraction experiments, as the scattered x-rays carry information about the electron distribution in materials. On the other hand, in the inelastic scattering process (Compton Scattering), x-rays transfer some of their energy to the electrons and the scattered x-rays will have different wavelength than the incident x-rays.
Diffracted waves from different atoms can interfere with each other and the resultant intensity distribution is strongly modulated by this interaction. If the atoms are arranged in a periodic fashion, as in crystals, the diffracted waves will consist of sharp interference maxima with the same symmetry as in the distribution of atoms.
Measuring the diffraction pattern therefore allows us to deduce the distribution of atoms in a material.
The peaks in X-ray diffraction pattern are directly related to the atomic distances. Consider an incident x-ray beam interacting with the atoms arranged in a periodic manner. For a given set of lattice planes with an inter-plane distance of d, the condition for a diffraction (peak) to occur can be simply written as 2d．sinθ = nλ which is known as the Bragg's law. The Bragg's Law is one of most important laws used for interpreting x-ray diffraction data.
It is important to point out that although we have used atoms as scattering points in this example, Bragg's Law applies to scattering centers consisting of any periodic distribution of electron density. In other words, the law holds true if the atoms are replaced by molecules or collections of molecules, such as colloids, polymers, proteins and virus particles.
X-ray crystallography is a standard technique for solving crystal structures. Its
basic theory was developed soon after X-rays were first discovered more than a century ago. However, over the years it has gone through continual development in data collection instrumentation and data reduction methods. In recent years, the advent of synchrotron radiation sources, area detector based data collection instruments, and high speed computers has dramatically enhanced the efficiency of crystallographic structural determination. Today X-ray crystallography is widely used in materials and biological research. Structures of very large biological machinery have been solved using this method.
2.10 References
[2.01]Shvid’ko Yu.V., Lucht M., Gerdau E. et al. J. Synchr. Rad. 9, 2002, 17 [2.02]Lucht M., Lerche M., Wille H.C. et al. J. Appl. Cryst. 36, 2003, 1075 [2.03]Gumilevskiy A.A. Watches and Watch Mechanisms. 1 (131), 1962, 26 [2.04]Wiederhorn S.M. J. Am. Ceram. Soc. 59, 1969, 485
[2.05]Hartman P., Proceedings of Mineralogical Society. Leningrad: USSR Academy of Sciences. 91 (6), 1962, 672
[2.06]C. Sartel, S. Gautier, S. Ould Saad Hamady, N. Maloufi, J. Martin, A.
Sirenko, and A. Ougazzaden, Superlattices Microstruct., 40, 2006, 476.
[2.07]Hitoshi Sato, Hirohazu Takahashi, Atsushi Watanabe and Hiroyuki Ota, Appl. Phys. Lett., 68, 1996, 3617.
[2.08]R. T. Lee and G. B. Stringfellow, J. Electron. Mater., 28 (8).
[2.09]S. S. Liu and D. A. Stevenson, J. Electrochem. Soc., 125 (7), 1978, 1161 [2.10]Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San
Diego, 2001. ISBN 0-12-352651-5.
[2.11]J.I. Goldestein, D.E. Newbury, P. Echlin, D.C. Joy, C.E. Lyman, and E.
Lifshin, L. Sawyer and J.R. Michael: Scanning Electron Microscopy and X-Ray Microanalysis, 3rd edition, (Plenum Press, New York 2003).
[2.12]D.B. Williams and C. B. Carter: Transmission Electron Microscopy, Plenum Press, New York 1996.
[2.13]R.J. Keyse, A.J. Garratt-Reed, R.J. Goodhew, and G.W. Lorimer:
Introduction to Scanning Transmission Electron Microscopy, Springer, New York 1998.
[2.14]B.G. Yacobi and D.B. Holt: Cathodolumiescence Microscopy of Inorganic Solids (Plenum Press, New York 1990).
[2.15]B.G. Yacobi: Encyclopedia of Materials Characterization:
Cathodoluminescence, CL, edited by C.R. Brundle, C.A. Evans, S. Wilson, and L.E. Fitzpatrick (Butterworth-Heinemann, Boston 1992), 149-159.
[2.16]S.O. Kasap: Principles of Electronic Materials and Devices (McGraw-Hill, New York 2002).
[2.17]H. Morkoç: Nitride Semi-conductors and Devices, 2nd edition (Springer, Berlin Heidelberg 1999).
Table 2.01. Interplanar distances in sapphire. (Cu-radiation)
Table 2.02. Symbols of crystallographic planes in morphological and structural units
Table 2.03. Wulff-Bragg's angles for some GaN planes
Table 2.04. Crystallographic planes of sapphire lattice and their spherical coordinates
Figure 2.01. (a)Schematic of the arrangement of Al3+ (black circles) and octahedral hollows (small light circles) between two layers of O2- (large light circles) in the basal plane.
(b) Schematic of the packing of O2- ions (light circles) and Al3+ in the direction of the axis c.
Figure 2.02. Sapphire lattice parameters in the 4.5~374K temperature range (a, c) and more detailed measurements at temperatures below 100K (b, d).
Figure 2.03. Location of the symmetry elements and the plane of chipping in the crystals grown by the Verneuil method: (a) chip along the plane (11-20); (b) chip along the plane (10-10), (c) chip along the prism (hi-k0), (d) The location of the planes (11-20) and (10-11) with respect to the growth axis and to the c-axis for optimization of chipping.
Figure 2.04. Crystallographic diagram of sapphire.
Figure 2.05. (a) Location of the crystallographic planes of sapphire often met in practice. (b) Comparison of the symbols of facets with common trace.
Figure 2.06. Description of subsystems in a MOCVD apparatus.
Figure 2.07. Illustration of the precursors impinging on the wafer surface and the rest products.
Figure 2.08. Temperature dependence of the deposition rate: (a) exothermic reaction and (b) endothermic reaction
Figure 2.09. Fundamental physical operating mechanism of HDP-RIE.
Figure 2.10. Schematic of inductive coupling plasma source.
Figure 2.11. Schematic of SEM.
Figure 2.12. Schematic illustration of the origin of two sources of secondary electron generation in the sample.
Figure 2.13. Schematic illustration of the indirect collection of backscattered electrons by a positively biased E-T detector.
Figure 2.14. Through-the-lens (TTL) detector for SE used in high-performance field emission SEMs.
Figure 2.15. Typical experimental set-up for PL measurements.
Figure 2.16. Schematic of the luminescence transitions between the conduction band (EC), valence band (EV), excition (EE), donor (ED), and acceptor (EA) levels in a luminescent material.
Chapter 3
Experimental Procedures
3.1 Epitaxial Relationship of Gallium Nitride on Sapphire Heterosubstrate
Lacking of native substrate for GaN epitaxy layer, it is grown on foreign substrates, usually with huge lattice mismatch. Therefore, the tendency of internal strain minimization leads to a substrate-dependent atomic arrangement different from that of the heterosubstrate.
Sapphire, an ubiquitous substrate, on which can grow semiconductors including GaN. It remains the most frequently used substrate for group III-nitride epitaxial growth owing to low cost, the availability of 3-inches diameter crystals, transparent nature, thermal stability and a mature technology for nitride growth.
Delving into the orientation relationship between the GaN epilayer and underlying sapphire substrate, notations of Miller (hjkl) indices is warranted.
The orientation order of the GaN films grown on the main sapphire planes {basal, c-plane (0001), a-plane (11-20), and γ-plane (1-102)} by both MBE and MOCVD has been studied in great detail [3.01-3.03]. The epitaxial relationship between GaN and sapphire is insensitive to the method of growth. A few examples of the film/substrate epitaxial relationships are [3.04]: (0001)GaN || (0001)sapphire with [2-1-1
0]GaN || [1-100]sapphire and [1-100]GaN || [1-210]sapphire; (2-1-10)GaN || (01-12)sapphire with [0001]GaN || [0-111]sapphire and [0-110]GaN || [2-1-10]sapphire.
In the case of a-plane GaN on γ-plane sapphire, the epitaxial relationships are (11-20) a-plane GaN on (1-102) γ-plane sapphire withٛ[11-20]GaN || [1-102]sapphire, [0001]GaN || [-1101]sapphire, and [-1100]GaN || [11-20]sapphire owing to hexagonal symmetry. Table 3.01 shows the summary representation of epitaxial relationships of GaN grown on various planes of sapphire heterosubstrate.
The calculated lattice mismatch between the basal GaN and the basal sapphire plane is larger than 30%. However, the actual mismatch is only
, (3.01) because the small cell of Al atoms on the basal sapphire plane is oriented 30∘away from the larger sapphire unit cell. This smaller lattice mismatch can be calculated by adopting the model explained in figure 3.01. It is on this plane that the best gallium nitride has been grown with relatively small in- and out-of-plane misorientations. In general, gallium nitride on this plane show almost perfect wurtzite phase.
GaN grown on the (11-20) a-plane sapphire turns out to be (0001) oriented and anisotropically compressed. The in-plane relationship of GaN and sapphire is depicted in figure 3.02 where the [11-20]GaN is aligned with the [0001]sapphire. In this orientation, the bulk positions of both the substrate and the GaN cations lie along the sapphire [0001] direction. The mismatch between the substrate and the film is
, (3.02)
and for the [1-100]GaN parallel to the [1-100]sapphire by
. (3.03)
The aforementioned discussion is academic in that GaN grown on a-plane sapphire still has [0001] direction normal to the surface. However, growth on γ-plane with MOCVD leads to a-plane GaN, as in the figure3.03.
GaN films have been grown on the γ-plane (1-102) of sapphire purportedly to achieve a lattice mismatch smaller than on the c-plane sapphire. GaN grown on the γ-plane sapphire has been reported to assume an orientation similar to (2-1-10). The arrangement in the case of the (1-102)sapphire and (2-1-10)GaN is depicted in figure 3.04.
The lattice mismatch between the [-1101]sapphire and the [0001]GaN parallel to the [-1101]sapphire is equal to
. (3.04)
In the case whenٛ[1-100]GaN direction is parallel to the [01-2 0]sapphire direction, the lattice mismatch is
. (3.05)
The mismatch along the [0001]GaN parallel to the [-1101]sapphire is 1%, which is much smaller than the 16% mismatch alongٛ[1-100]GaN parallel to theٛ[11-20]sapphire. Growth on the γ-plane exhibits ridge-like features that allow relaxation of the
mismatch. It is assumed that the topmost O layer is desorbed and the Al layer of sapphire is then exposed. One thing must be stated is that the representation in figure 3.04 is very simple intended only to give a first-order glimpse as to how the a-plane GaN might be organized on the γ-plane sapphire with no consideration to energy minimization. More detail information about γ-plane sapphire can be seen in figure 3.05.
3.2 Nonpolar a-Plane GaN on m-Plane Sapphire
On considering the aforementioned epitaxial relationship between GaN and sapphire substrate, the best GaN can be grown with relatively small in- and out-of-plane misorientations from the c-plane sapphire. Therefore the epitaxial direction and initial plane we chose are along c-direction from c-plane sapphire respectively. From the basic crystalline structure of wurtzite, as in figure 3.06, it is clear that c-plane, a-plane and m-plane are perpendicular to each other. To achieve the demand of growing nonpolar a-plane GaN on m-plane sapphire substrate, appropriate substrate engineering is necessary to effectively expose the c-plane of sapphire.
The applied substrate engineering here in this experiment was artificial surface patterning. The trench-patterned nonpolar m-plane sapphire substrate was fabricated by e-gun evaporated nickel mask and Cl2-based inductively coupled plasma reactive ion etching (ICP-RIE). Via accurate lithography aligning, symmetric crystallographic c-plane facets of sapphire with terrace/trench width 4μm/2μm and 3μm etching depth were illustrated in Figure 3.07. To effectively expose the c-plane sapphire sidewalls, terrace/trench stripes orientation were chosen parallel to [11-20]sapphire.
The growth of GaN on the trench-patterned m-plane sapphire was performed by Metal Organic Chemical Vapor Deposition (MOCVD) system. After heating up and reaching the appropriate temperature, a 30 nm GaN buffer layer was grown at 550°C, followed by main GaN growth at 1050°C with a pressure of 100 mtorr. The different growth rate and growth mode of nonpolar a-plane GaN on trench-patterned m-plane sapphire was achieved via tuning the V/III ratio of GaN growth duration from 72 to 350, 1800 and up to 9000. In order to know the dependency of growth rate of specific plane of GaN on different V/III ratio, all the growth period are fixed to the same time.
Additionally, an optimized substrate with lower terrace/trench width ratio (i.e.
2um/4um) is used to attempt to suppress the growth of GaN from the (10-10)sapphire
terraces under the V/III ratio of 72. The condition of growth temperature versus growth time was summarized in figure 3.08 where the critical timing of V/III ratio tuning was depicted also.
3.3 Nonpolar a-plane GaN on γ-plane Sapphire
It has been demonstrated that wavy, stripe-like growth feature of nonpolar a-plane GaN growing epitaxially on γ-plane sapphire leads to a large amount of threading dislocations and stacking faults [3.06, 3.07]. These defects form nonradiative centers and bring difficulty and great challenge to the growth of high quality a-plane GaN inevitably. The acquirement of crystalline quality of a-plane GaN for further optoelectronics devices application is urgent and necessary.
Recently, the technique of epitaxially lateral overgrowth (ELOG) provided a
power solution to grow a-plane GaN on γ-plane sapphire [3.08]. ELOG improves the crystalline quality by reducing the density of threading dislocations and alleviates the strain-related surface roughening and faceting. Despite the ability of improving morphology and crystalline quality, the coalescence thickness of ELOG is often more than 20μm, which is too thick to effectively control the growth uniformity. Therefore, in this experiment, a modified technique by using epitaxially lateral overgrowth on trench-patterned a-plane GaN buffer layer (TELOG, where “T” represents trench-patterned) is performed. Via applying this technique to growth procedure, a-plane GaN with relatively low dislocation density is expected. It would show the potential of simple fabrication process, low cost, and, most important of all, thinner coalescence thickness in comparison with the previous ELOG.
Figure 3.09 shows the flow chart of TELOG a-plane GaN growth. Initially, the a-plane GaN template with 1.5μm thickness was grown by MOCVD on γ-plane sapphire substrate using conventional two-step growth technique. Then a 3μm-seed/
3μm-trench was applied parallel to the direction to realize vertical c-plane sidewalls followed by etching of SiO2 using inductively coupled plasma reactive ion etching through the windows to the GaN epitaxial template film. These well-organized GaN stripes were etched down to the γ-plane sapphire substrate by reactive ion etching.
The SiO2 etching-mask was removed by hydrofluoric acid solution to avoid the complexity of growth mode. The deposition procedure of TELOG a-plane GaN film was performed by using a two-step growth technique. At initial, the sapphire substrate was heated to 700°C and then a nitridation process was revealed under gradually increasing temperature. A short period of ramping growth followed until the substrate was heated to the suitable growth temperature, i.e. 970°C. TELOG a-plane GaN was grown mainly at 970°C for a specific long period to achieve the fully coalescence of
the surface. From nitridation step to the main TELOG step, the growth condition of V/III of MOCVD was fixed at the level of 50~100. Under this specific condition, the growth direction can be selective to +c-direction of sapphire and the growth rate is enhanced. To apply a pre-strain superlattice prior to the multiple quantum wells, the temperature was slightly down to 855°C. After the supperlatice growth, the quantum wells and quantum barriers were grown periodically at the temperature of 760°C and 855°C respectively. The growth temperature of wells and barriers can be changed to achieve different lighting optical wave length from UV to green light. And one thing to note is that the samples we took to perform the further experiments were in blue lighting region. From supperlattice growth step to the MQWs growth step, the growth condition of V/III of MOCVD was fixed at the level of 5000~10000. The specific condition was chosen to be similar to the general growth condition of c-plane GaN on c-plane sapphire substrate. It would be helpful in controlling the growth rate, growth mode and achieving the smooth morphology of each layer. After all, the aforementioned growth conditions are summarized in figure 3.10.
3.4 KOH Etching Procedures
The KOH etching processes are performed under standard environments with normal pressure and uniform agitation of the KOH-ethylene glycol solution. These were the controlled independent variables, i.e. remained fixed through the whole experimental duration of experiments. To know the complete etching behavior of TELOG a-plane GaN with related MQWs in KOH-ethylene glycol solution, variables including etching time, concentration of KOH-ethylene glycol solution and etching
temperature were considered.
What responsible for the determination of the duration of etching is not specific but epitaxial-crystalline-quality-dependent. Here in this experiment, the etching time performed from 10 minutes to 80 minutes. An extreme long etching duration was tested to ensure the optimized window of etching.
The concentration of KOH-ethylene glycol solution is undoubtedly one of the most important factors. For the nature of chemical etching, the influence of solution concentration on etching rate of general materials is self-evident. Crystalline gallium nitride, however, shows more unique etching characteristics in chemical solutions. To effectively clarify those characteristics, different concentration with 5 wt.%, 10 wt.%
and 20 wt.%, of KOH was prepared in ethylene glycol. The high boiling point and thermal stability of ethylene glycol ensured solvent against evaporation and the concentration of KOH-ethylene glycol solution, therefore, was maintained.
Temperature, which hastens etching reactions if raised, since higher temperature increases the energy of the etchants molecules, creates more collisions between etchants and materials per unit time. In general, higher temperature leads to faster reaction. The temperature effect is usually discussed in terms of activation energy, i.e.
the chemical potential barrier. Here in this experiment, the etching temperature was controlled in the range of 100°C ~120°C. The heat source was supplied by commercial hot plate. To ensure the uniformity of temperature inside the solution, a gentle agitation was performed by magnetic stirring.
Briefly, the three factors, i.e. time, concentration and temperature will show its own dependency to the etching results. These specifically-arranged conditions can
help in clarifying the etching windows of nonpolar gallium nitride in KOH-ethylene glycol solution. The integration of the variables is organized on the basis of the coordinate systems constructed by time, concentration and temperature. The integrated variables and experimental direction are schematically shown in figure 3.11.
3.5 Measurement and Characterization
The crystalline quality of grown samples, a-plane GaN on patterned m-plane sapphire and TELOG a-plane GaN on γ-plane sapphire were characterized by X-ray diffraction (XRD). Additionally, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to observe the surface morphology, growth mode, and defect distribution of the epitaxial film.
The scanning electron microscopy also provided the ability of observing the cross-sectional image of these epitaxial samples. Specially, the etching effects of TELOG a-plane GaN in KOH-ethylene glycol solution under different conditions are clear and unequivocal in scanning electron microscopy. This offered the important information of clarifying the etching characteristics including etching direction, etching rate, and most of all, etching mechanism.
The capable of imaging at a significantly higher resolution of Transmission electron microscopy can help in determining the defects distribution and arrangement.
In accompany with the electron diffraction patterns, the types of defects in a-plane GaN on m-plane sapphire and a-plane GaN on γ-plane sapphire are clarified.
Optical properties of these a-plane GaN samples were investigated by uncontact and nondestructive photoluminescence spectroscopy and cathodoluminescence spectroscopy. The luminescence results can be categorized to show the lighting behavior and the crystalline quality respectively. Photoluminescence spectroscopy, in addition, provided the information of light extraction ability of KOH-ethylene glycol solution etched TELOG a-plane GaN MQWs. Cathodoluminescence spectroscopy mapping of the cross-sectional image of a-plane GaN on m-plane sapphire and
Optical properties of these a-plane GaN samples were investigated by uncontact and nondestructive photoluminescence spectroscopy and cathodoluminescence spectroscopy. The luminescence results can be categorized to show the lighting behavior and the crystalline quality respectively. Photoluminescence spectroscopy, in addition, provided the information of light extraction ability of KOH-ethylene glycol solution etched TELOG a-plane GaN MQWs. Cathodoluminescence spectroscopy mapping of the cross-sectional image of a-plane GaN on m-plane sapphire and