CHAPTER 1 INTRODUCTION TO III-NITRIDE SEMICONDUCTORS

1 CHAPTER 1 INTRODUCTION TO III-NITRIDE SEMICONDUCTORS 1.1 INTRODUCTION Group III-nitrides have been promising materials system for semiconductor devices applications since 1970, especially for the development of blue and ultraviolet-light emitting diodes (UV-LEDs). Among the nitrides, aluminium nitride (AlN), gallium nitride (GaN) and indium nitride (InN), have been potential candidate materials for optoelectronic applications for a wide range of photon energies, because they form a continuous alloy system (InGaN, InAlN, and AlGaN) with direct optical bandgaps of 0.7 ev for InN and 3.4 ev for GaN to 6.2 ev for AlN (Kim et al 1997, Wu et al 2002). Other advantageous are high mechanical and thermal stability, large piezoelectric constants and the possibility of passivation by forming thin layers of Ga 2 O 3 or Al 2 O 3 with bandgaps of approximately 4.2 ev and 9 ev. III nitride materials also have higher breakdown filed, higher thermal conductivity, and better chemical stability, these properties are more suitable for high power, high voltage devices working in harsh environment compared to elemental and conventional compound semiconductors, such as silicon (Si), germanium (Ge), indium phosphide (InP) and gallium arsenide (GaAs). These superior properties of III-nitrides are greatly used in electronic devices such as high electron mobility transistors (HEMTs), heterojunction

2 bipolar transistors (HBTs), Schottky diodes and metal oxide semiconductor field effect transistors (MOSFETs). 1.2 OVERVIEW OF GaN GaN was one of the foremost III nitride compound semiconductors studied (Ambacher et al 1998, Xing et al 2001). The first GaN material was produced by passing ammonia over hot gallium, which resulted small needles and platelets (Johnson et al 1932). After three decades, hydride vapor phase epitaxy (HVPE) method was used to grow GaN on sapphire substrate by Maruska (Maruska et al 1969). Unfortunately, these samples suffered from very high background electron concentrations (~ 10 8-10 20 cm -3 ) and poor crystallinity due to growth on non-native substrate. Yoshida et al (1983) employed an AlN intermediate layer and demonstrated that the cathodoluminescence efficiency of the overlaying GaN layer was improved. Later, this idea was further developed with a thin AlN nucleation layer (Akasaki et al 1989) or GaN nucleation layer (Nakamura et al 1991) grown at low temperatures (500 750 C) prior to the growth of high temperature GaN epilayer. A low temperature nucleation layer promotes the GaN growth mode from three-dimensional (3D) islands to two-dimensional (2D) growth mode and leads to a flat and smooth GaN epilayer surface. The success in p-type doping is another breakthrough in the history of GaN growth (Vechten et al 1992, Nakamura et al 1992a). Group II elements Zn, Cd, Mg or Be were investigated as p-type dopants, with these dopant GaN layers shows highly resistive nature. Pankove et al (1972) were able to make the first nitride LED consisting of an undoped n-type region, an insulating Zn-doped layer, and an indium surface contact, but this device suffered from very low efficiency. Amano et al (1989) converted an insulating Mg-doped GaN layer to a conductive p-type layer using the low energy electron beam irradiation (LEEBI) method. Later, Nakamura et al (1992) discovered that post - growth

3 thermal annealing in hydrogen free ambient was an effective way of achieving p-type conductivity in Mg doped GaN. Since then, thermal annealing has been established as a standard method for achieving p-type conductivity in MOCVD grown Mg-doped GaN. The success of p-type GaN was a milestone of having today s GaN related devices. Nakamura et al (1994) were demonstrated high-brightness blue LEDs with a brightness of over 1 candela and ultraviolet laser diodes (UV-LDs). After the discovery of very narrow band gap InN (0.7 ev instead of previously thought 1.9 ev), in 2002 (Davydov et al 2002, Wu et al 2002, Matsuoka et al 2002) III-nitrides have promised tremendous potential for photovoltaic technology (Wu et al 2003, Peng et al 1998, Ko et al 2002), optoelectronic and electronic devices. However, difficulties in growing GaN in single crystalline form, and lack of appropriate lattice matched substrates, made progress in developing this material rather slow. Currently, commercial GaN based devices are grown on heterosubstrates: usually sapphire, Si and SiC. The problems associated with wetting of the substrate, the large lattice mismatch, and different thermal expansion coefficients, resulted structural defects in epitaxial layers. These defects are known to propagate to the surface along the growth direction. The defective epilayers introduce defect states in the forbidden energy bandgap of the GaN that can act as non-radiative or radiative recombination centers and scattering centers and thus kill the lifetime and quality of optical and electronic devices fabricated on them. Different methods were attempted in order to reduce these defects, such as low temperature deposition of a nucleation layer (Iwaya et al 1998, Amano et al 1998, Yang et al 1999) which enhances the wetting of the substrate and thereby suppresses the occurrence of growth islands, and the Epitaxial Lateral Overgrowth (ELO) method (Kapolnek et al 1997, Nam et al 1997, Marchand et al 1998, Usui et al 1997). Another alternative approach to

4 ELO was the so-called Pendeo-Epitaxy (Zheleva et al 1999, Thomson et al 1999, Linthicum et al 1999). Here, the starting material was a seed GaN substrate. The seed GaN substrate was etched in columns, after the etching, the columnar posts were then capped with a mask layer of dielectric material. Thereafter, GaN growth proceeds laterally from the columns and then vertically. Eventually the GaN coalesces between and over the top of the mask to create a continuous, defect-free layer. The research efforts made so far towards improving the defect densities in GaN epitaxial layers have been instrumental in building several optoelectronic devices. For example, the ELO method has produced GaN layers in which the density of dislocations has been reduced by several orders of magnitude, from 10 10 to 10 6 cm -2. Nitride based laser diodes working at room temperature with a lifetime exceeding over 10,000 h have also been demonstrated (Nakamura and Fasol 1997). Nevertheless, the availability of bulk, single crystal GaN substrates is still highly desirable, which would lead to better device quality and reliability, longer device lifetime, and better affordability. Gallium nitride and related III-V nitrides have also attracted extensive experimental and theoretical interest on fundamental studies due to their physical properties. Commercially manufactured GaN powder is typically synthesized through the reaction of Ga 2 O 3 with NH 3, resulting in a high concentration of residual oxygen (Chow et al 2008). This high impurity concentration increases the density of dislocations in GaN single crystals and inhibits continuous growth. Commercial powders made from metal Ga are extremely expensive and the quantity available is very limited since the yield is very low. Many research groups have demonstrated the growth of GaN powders through various synthetic methods. Particular interest has been currently centered on simple and inexpensive chemical synthetic routes for producing nanocrystalline GaN.

5 1.3 CRYSTAL STRUCTURE The group III nitrides GaN, AlN and InN can crystallize in the following three different crystal structures: (1) wurtzite, (2) zinc blende and (3) rock-salt. At ambient conditions, the thermodynamically stable phase is the wurtzite structure as shown in Figure 1.1. A phase transition to the rock salt structure takes place at high pressure. In contrast, the zinc blende is metastable and may be stabilized by heteroepitaxial growth on substrate reflecting structural compatibility. Major material properties of binary III-nitride compound semiconductors as well as other important semiconductors are listed in Table 1.1. In general, single crystals are required for accurate measurements of the lattice constant and related properties. Because of lack of GaN single crystals, most of the data acquired to date stems from nitride layers grown heteroepitaxially. As a result, lattice constants and associated thermal and mechanical properties were found to exhibit significant scattering in the reported values (Trampert et al 1998). The most widely accepted lattice constants of wurtzite GaN are a = 3.189 Ǻ and c = 5.186 Ǻ (Bougrov et al 2001). Ga, In or Al N Figure 1.1 Wurtzite structure of binary III-nitride materials

6 Table 1.1 The major material properties of binary III-nitride compound semiconductors and some other important semiconductors at 300 K (Zhang 2009) Material properties Structure Lattice constant (Å) GaAs Si InP GaN AlN InN Zinc blende Diamond cubic Zinc blende 5.653 5.431 5.869 Wurtzite Wurtzite Wurtzite a = 3.18 c = 5.18 a = 3.11 c = 4.97 a = 3.54 c = 5.71 Band gap (ev) 1.42 1.12 1.35 3.44 6.2 0.7-1.0 Nature of bandgap Refractive index Dielectric constant (static) Thermal conductivity (W/cm -1 K -1 ) Electron mobility (cm 2 /V-s) Hole mobility (cm 2 /V-s) Electron saturated velocity (10 7 cm/s) Direct Indirect Direct Direct Direct Direct 3.3 3.42 3.1 2.3 2.1-2.2 2.9-3.0 12.9 11.7 12.6 10.4 8.5 15.3 0.46 1.56 0.68 2.0-2.4 3.0-3.3 0.6-1.0 8000 1400 5400 2000 300 3200 400 500 200 400 14-0.7 2.3 3.9 2.5 1.9 3.4 1.4 CHEMICAL, ELECTRICAL AND OPTICAL PROPERTIES OF GaN The elements Al, Ga, and In are forming compounds with N having the composition MN, where M represents a III-metal. The chemical bond of

7 these compounds is predominantly covalent. Because of the large differences in electronegativity of the two constituents, there is a significant ionic contribution to the bond, which determines the stability of these compounds. Hence, GaN is a highly stable compound even at elevated temperatures. It has bond energy of 9.12 ev/atom pair (Izabella et al 2001). In fact, this chemical stability combined with its hardness makes GaN is an attractive material for protective coating. Electrical properties of unintentionally doped GaN were reviewed extensively by Strite and Morkoc (1992). They pointed out that widely varying results reflect the different crystal quality and purity of material. The room temperature electron mobility values in bulk GaN grown with HVPE to a thickness of 60 µm were reported to be 950 cm 2 /Vs (Look et al 1997) and that reported for MOCVD grown layers was also in excess of 900 cm 2 /Vs (Nakamura et al 1992). MBE grown films, however, produce much lower mobility values in the range of 100-300 cm 2 /Vs (Ng et al 1998); this has been attributed to both high dislocation density and point defects. Other important electrical properties were reviewed by Amano et al (1989). Maruska et al (1969) were the first to accurately measure the direct band gap energy of GaN to be 3.39 ev. Pioneering work in this area was performed by Pankove et al (1972) who reported on the low temperature photoluminescence (PL) spectrum of wurtzite GaN. Since the primary interest in GaN is for its potential as a blue and UV light emitter, a lot of effort has been devoted to determine its optical properties. By tuning the composition of group III elements, the bandgap (E g ) of III-nitride materials can cover the region from 0.7 ev (InN) to 6.2 ev (AlN) with the corresponding spectral region from the infrared (1.77 μ m) to the deep ultraviolet (200 nm). The direct and tunable bandgap property makes the III-nitride materials excellent choice for optoelectronic devices such as visible and UV-LEDs, LDs, UV

8 photodiodes and avalanche photodiodes (APDs). The available data were reviewed by Akasaki et al (1994), and by Strite and Morkoc (1992). 1.5 DIFFICULTIES IN GROWTH OF BULK GaN Bulk GaN crystal growth remains a technological challenge, but the desire to have large single crystals of GaN continues for homoepitaxial growth of GaN. Growth of GaN on foreign substrates often introduces high defect densities, as mentioned in the previous section. Significant progress has been made towards reduction of such defects, and has culminated with demonstrations of real device applications using GaN and its alloy systems. Due to the extreme melting conditions of GaN as compared to other common semiconductors, which is given in Table 1.2, it cannot be grown from its stoichiometric melt by Bridgman, Czochralski and liquid encapsulated Czochralski methods commonly used for typical bulk semiconductors growth. Table 1.2 Melting temperatures and equilibrium pressure at the melting point of various semiconductors Crystal T M ( C) P M (atm) Si 1400 < 1 GaP 1465 30 GaAs 1250 15 GaN 2500 4500 GaN is a strongly bonded compound (with bonding energy of 9.12 ev/atom pair) (Izabella et al 2001) compared to other typical III-V semiconductors such as GaAs (bonding energy of 6.5 ev/atom pair). N 2 is also a very stable gas with bonding energy of 4.9 ev/atom pair. Consequently, the Gibbs free energy of Ga + N 2 is low. Thus, the free energy of GaN

9 constituents at their normal states of Ga and N 2 is not far from that of the GaN crystal (Porowski 1996). This situation makes growth of GaN difficult task. The group of III-nitrides does not exist in nature and thus must be synthesized. Crystal growth of this group of compounds using standard methods (Czochralski, Bridgman) is extremely difficult because of the following factors 1. The high melting temperature (~ 2800 K for GaN) 2. The relatively low sublimation 3. The very high equilibrium nitrogen vapor pressure (~40 k bar at melting temperature) and 4. Low solubility in acids, bases, and most other inorganic elements and compounds. 1.6 ETHYLENEDIAMINETETRAACETIC ACID (EDTA) EDTA is a widely used initialism for the organic compound Ethylenediaminetetraacetic acid. The conjugate base is named ethylenediaminetetraacetate. EDTA is a polyamino carboxylic acid and a colorless, water-soluble solid. Its usefulness arises because of its role as a chelating agent, i.e. its ability to "sequester" metal ions such as Ca 2+ and Fe 3+. After being bound by EDTA, metal ions remain in solution but exhibit diminished reactivity. EDTA is produced as several salts, notably disodium EDTA and calcium disodium EDTA. Since ethylenediaminetetraacetic acid (EDTA : C 10 H 16 N 2 O 8 ) forms stable and water-soluble metal complexes, homogeneous mixture of several metal EDTA complexes can be obtained from solution and be transferred into solid powder using a spray-dry technique, which was reported to fabricate Ba, Sr, Ti, Y and Cu-EDTA complexes and to further synthesize metal oxide powders such as (Ba, Sr)

10 TiO 3 and YBa 2 Cu 3 O 7 with them. It has been proposed to prepare GaN nanocrystals by a simple method. This process uses the solution of metal ethylenediaminetetraacetic acid (EDTA) as a starting material. Ga-EDTA.NH 4 complex was prepared from the mixture of GaCl 3, and EDTA.NH 4 in aqueous solution at a ph of 9. In this process, the particles of the mixture of metal-edta complexes were obtained by a drying method from solution consisting of M-EDTA complex mixture. Figure1.2 shows the chemical structure of EDTA. EDTA has been used for the preparation of GaN nanocrystals in the present investigation. Figure 1.2 Chemical structure of Ethylenediaminetetraacetic acid 1.7 SEMICONDUCTOR NANOSTRUCTURES Nano-scale materials have stimulated great interest due to their importance in basic scientific research and potential technological applications. These materials exhibit unique chemical and physical properties, differing substantially from those of the corresponding bulk solids because of their small size and extremely large surface-to-volume ratio. Nano-scale materials can be defined as systems in which at least one dimension is less than 100 nm, i.e. reducing 1, 2, or 3 dimensions of a bulk material to the nanometre scale produces nanometre thick two-dimensional (2D) layers,

11 one-dimensional (1D) nanowires or zero dimensional (0D) nanoclusters (nanoparticles/nanocrystals). The synthesis of nano-scale materials is critical and important work directed towards understanding the fundamental properties of small structures, creating nanostructured materials, and developing nanotechnologies. Miniaturisation in electronics through improvements in established top-down (cutting, etching and lithography techniques) fabrication techniques is approaching the point where fundamental issues are expected to limit the dramatic increases in computing, seen over the past several decades. Bottom-up (CVD, PVD and chemical synthesis etc.,) approaches to nanoelectronics where the functional electronic structures are assembled from well defined nanoscale building blocks such as nanocrystals, nanowires/nanotubes and molecules have the potential to go far beyond the limits of top-down fabrication techniques. Nanostructures of semiconductor materials have been the subject of intense research in the last decade owing to the novel electronic, catalytic and optical properties. A crystalline semiconductor material, which is size restricted in three dimensions such that the electron wave functions are confined within its volume, is called a semiconductor quantum dots (QDs). Semiconductor QDs refer to nanometer-sized, giant (10 3-10 5 atoms) molecules made from ordinary inorganic semiconductor materials such as Si, GaAs, CdSe, GaN, AlN and InN etc. The unusual properties of these quantum dots can be attributed to two main factors: the large surface to volume ratio of atoms and the confinement of charge carriers in a quantum mechanical box and this effect, now called quantum size effect. The optical properties of nanostructures are strongly dependent on the size of the particle. During the past two decades, chemists and physicists have extensively studied the size quantization effects in semiconductor QDs. Nanocrystalline semiconductor QDs synthesized as powders or colloids have generated great interest, since

12 this quantum, confined structures can be synthesized with a high degree of reproducibility and high quality. 1.8 INTRODUCTION TO DILUTE MAGNETIC SEMICONDUCTORS (DMS) There is great current interest in the emerging field of semiconductor spin transfer electronics (spintronics), which seeks to exploit the spin of charge carriers in semiconductors. It is widely expected that new functionalities for electronics and photonics can be derived if the injection, transfer and detection of carrier spin can be controlled above room temperature. Among this new class of devices are magnetic devices with gain, spin transistors operating at very low powers for mobile applications that rely on batteries, optical emitters with encoded information through their polarized light output, fast non-volatile semiconductor memory and integrated magnetic/electronic/photonic devices (electromagnetism-on-a-chip). Since the magnetic properties of ferromagnetic semiconductors are a function of carrier concentration in the material in many cases, then it will be possible to have electrically or optically controlled magnetism through field-gating of transistor structures or optical excitation to alter the carrier density. A number of recent reviews have covered the topics of spin injection, coherence length and magnetic properties of materials systems such as in the general areas of spin injection from metals into semiconductors and applications of the spintronic phenomena (Dietl 2002, Ohno et al 2000, Wolf et al 2001, Kikkawa et al 2000). The elements commonly used as magnetic dopants for the synthesis of DMS belong to the family of transition metals (TM) (Sc, Ti, V, Cr, Mn, Fe, Co, Ni and Cu) and rare earths (RE) (Sm, Eu, Gd, Tb, Dy, Er). The magnetic behaviour of these elements is due to the partially filled d/f states, respectively, containing unpaired spins.

13 1.9 POTENTIAL SEMICONDUCTOR MATERIALS FOR SPINTRONICS There are two major criteria for selecting the materials for semiconductor spintronics. First, the ferromagnetism should be retained to practical temperatures (i.e. > 300 K) (Choi et al 2002, Medvedkin et al 2000). Second, it would be a major advantage if there were already an existing technology base for the material in other applications. Most of the work in the past has focused on (Ga,Mn)As and (In,Mn)As. There are indeed major markets for their host materials in infrared light emitting diodes and lasers and high speed digital electronics (GaAs) and magnetic sensors (InAs). In single phase samples carefully grown by molecular beam epitaxy (MBE), the highest Curie temperatures reported are ~ 110 K for (Ga, Mn)As and ~35 K for (In,Mn)As (Nazmul et al 2003, Schallenberg et al 2006). One of the most effective methods for investigating spin-polarized transport is by monitoring the polarized electroluminescence output from a quantum-well light-emitting diode into which the spin current is injected. Quantum selection rules relating the initial carrier spin polarization and the subsequent polarized optical output can provide a quantitative measure of the injection efficiency. Other materials for which room temperature ferromagnetism has been reported include (Cd, Mn)GeP 2 (Medvedkin et al 2000), (Zn,Mn)GeP 2 (Medvedkin et al 2002), ZnSnAs 2 (Choi et al 2002), and (Zn,Co)O (Ueda et al 2001). Some of these chalcopyrite s and wide bandgap oxides have interesting optical properties, but they lack a technology and experience base as large as that of most semiconductors. The key break through that focused attention on wide bandgap semiconductors as being the most promising for achieving practical ordering temperatures was the theoretical work of Dietl et al (2002). They predicted that cubic GaN doped with ~5 at.% of Mn and containing a high

14 concentration of holes (3.5 10 20 cm -3 ) should exhibit a Curie temperature exceeding room temperature. There has been tremendous progress on both the realization of high quality (Ga, Mn)N epitaxial layers and on the theory of ferromagnetism in these so-called dilute magnetic semiconductors (DMS). The term DMS refers to the fact that some fraction of the atoms in a nonmagnetic semiconductor like GaN is replaced by magnetic ions. A key, unanswered question is whether the resulting material is indeed an alloy of (Ga, Mn)N or whether it remains as GaN with clusters, precipitates or second phases that are responsible for the observed magnetic properties (Schilfgaarde and Mryasov 2001). Figure 1.3 shows the predicted Curie temperatures of various semiconductor materials as a function of bandgap. From the Figure 1.3 it is clear that GaN shows larger Curie temperature compared to other semiconductor materials. Figure 1.3 Predicted Curie temperatures as a function of bandgap, along with some experimental results (Dietl et al 2000)

15 1.10 SCOPE OF THE THESIS The detailed study had been made on the synthesis and characterization of pure and cobalt doped GaN nanocrystals by simple and inexpensive chemical method. Ga-EDTA complex and NH 3 was used as a source for the synthesis of pure GaN nanocrystals. The pure GaN nanocrystals were synthesized at various temperatures from 600 to 900 C in NH 3 atmosphere. The synthesized GaN nanocrystals were characterized by various techniques like X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray analysis (EDAX), photoluminescence (PL), Raman and Fourier transform infrared spectroscopy (FTIR). The size of the GaN nanocrystals achieved by this method was around 20 nm. The cobalt (Co) doped Ga-EDTA complex was used a source material for the synthesis of Co-doped GaN nanocrystals. Co-doped GaN nanocrystals were synthesized at 900 C in NH 3 ambient. The Concentrations of Co was varied as 5 and 8 mol % in GaN nanocrystals. The changes in structural, morphological, optical and magnetic properties were observed for the variations in concentration of cobalt doping and discussed. GaN nanotips were grown on sapphire substrate at 1000 C by chemical vapour deposition method (CVD). Pure Ga and GaN nanocrystals were used as a source material for the GaN growth. Self assembled single zone furnace was used for the growth. The growth was carried out for a period of 1 h. The grown GaN nanotips were characterized by XRD, SEM, atomic force spectroscopy (AFM), PL and Field emission measurements. The results show that the GaN nanotips are suitable for the field emission device applications.