Linköping Studies in Science and Technology Thesis No. 1492
Physical simulation, fabrication and characterization of Wide bandgap semiconductor devices
Sadia Muniza Faraz
LIU-TEK-LIC-2011:31
Semiconductor Materials Devision
Department of Physics, Chemistry and Biology
Linköpings universitet, SE-581 83 Linköping, Sweden
Linköping 2011 LIU-TEK-LIC-2011: 31 ISBN: 978-91-7393-148-9
ISSN:0280-7971
Printed by Liu-Tryck, Linköping, Sweden, 2011
To my Parents, family and all those who pray for my success
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Abstract
Wide band gap semiconductors, Zinc Oxide (ZnO), Gallium Nitride (GaN) and Silicon Carbide (SiC) have been emerged to be the most promising semiconductors for future applications in electronic, optoelectronic and power devices. They offer incredible advantages in terms of their optical properties, DC and microwave frequencies power handling capability, piezoelectric properties in building electromechanical coupled sensors and transducers, biosensors and bright light emission. For producing high quality devices, thermal treatment always plays an important role in improving material structural quality which results in improved electrical and optical properties. Similarly good quality of metal–semiconductor interface, sensitive to the semiconductor surface, is always required.
In this thesis we report the study of the interface states density for Pd/Ti/Au Schottky contacts on the free-standing GaN and post fabrication annealing effects on the electrical and optical properties of ZnO/Si hetero-junction diodes. The determination of interface states density (NSS) distribution within the band gap would help in understanding the processes dominating the electrical behavior of the metal–semiconductor contacts. The study of annealing effects on photoluminescence, rectification and ideality factor of ZnO/Si hetero-junction diodes are helpful for optimization and realization to build up the confidence to commercialize devices for lightening. A comparison of device performance between the physical simulations and measured device characteristics has also been carried out for pd/ZnO Schottky diode to understand the behavior of the devices.
This research work not only teaches the effective way of device fabrication, but also obtains some beneficial results in aspects of their optical and electrical properties, which builds theoretical and experimental foundation for much better and broader applications of wide band gap semiconductor devices.
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iii
Preface
This thesis is presented as partial fulfillment of the requirements for the degree of Licentiate of Philosophy, of Linkoping University.
The work described in the thesis has been carried out at Department of physics, chemistry and biology (IFM), department of Science and technology (ITN) at Linkopings University, Linkoping, Sweden and at the department of Electronic Engineering at NED University of Engineering & Technology, Karachi, Pakistan between April, 2008 and April, 2011.
List of appended publications
[1] S. M. Faraz, H. Ashraf, M. Imran Arshad, P. R. Hageman, M. Asghar, Q. Wahab, Interface state density of free-standing GaN Schottky diodes, Semicond. Sci. Technol.
25, 095008 (2010).
[2] S. M. Faraz, N. H. Alvi, A. Henry, O. Nur, M. Willander, Q. Wahab, “Post fabrication annealing effects on electrical and optical characteristics of n-ZnO nanorods/p-Si heterojunction diodes”, submitted.
[3] S. M. Faraz, Hadia Noor, M. Asghar, M. Willander, Q. Wahab, Modeling and simulations of Pd/n-ZnO Schottky diode and its comparison with measurements, Advanced Materials Research Vols. 79-82, 1317, (2009).
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Related papers not included in the thesis
[1]
Hadia Noor, P. Klason, S. M. Faraz, O. Nur, Q. Wahab, M. Willander, M. Asghar,“Influence of background concentration induced field on the emission rate signatures of an electron trap in zinc oxide Schottky devices”, J. Appl. Phys., 107, 103717 (2010)
[2]
H. Ashraf, M. Imran Arshad, S. M. Faraz, Q. Wahab, P. R. Hageman and M. Asghar,“Study of electric field enhanced emission rates of an electron trap in n-type GaN grown by hydride vapor phase epitaxy”, J. Appl. Phys., 108, 103708 (2010)
[3]
S. M. Faraz, N. H. Alvi, A. Henry, O. Nur, M. Willander and Q. Wahab, “Annealing effects on electrical and optical properties of n-ZnO/p-Si heterojunction diodes”, submitted.[4]
F. Iqbal, S. M. Faraz, A. Ali, Q. Wahab, Mikael Syrviai and M. Asghar, “Non radiative emission of Aluminum related deep level defect in p-type 6H-SiC Epilayers”, submitted.[5]
F. Iqbal, S. M. Faraz, A. Ali, Q. Wahab, Mikael Syrviai and M. Asghar, “Deep level transient spectroscopy of bulk- and N-doped 6H-SiC epilayers”, submitted.[6]
S. M. Faraz, V. Khranovskyy, R. Yakimova, A. Ulyashin and Q. Wahab, Temperature dependent current transport in Schottky diodes of nano structured ZnO grown on Si by magnetron sputtering, submitted.v
Acknowledgements
All praise is due to ALLAH (God) who enabled me to do this research work. I would like to express my sincere gratitude to my supervisor Associate Prof. Qamar ul Wahab, for his guidance, patience, discussions with valuable suggestions and encouragement during this research work. He introduced me to the most experienced and pronounced researchers of professional world, like Prof. Magnus Willander and Associate Prof. Omer Nour, department of science and technology (ITN) Linkopings University. I am thankful to them for their cooperation, useful recommendations and guidance.
I am thankful to NED University of Engineering & Technology for financial support and providing me an excellent learning opportunity. I am thankful to Prof. Shoaib Hassan Zaidi and Prof. Abdul Qadir for guidance at NED University.
I acknowledge Mr. Ijaz Hussain and Sergej Liske for helping me in growth and device fabrication. Thanks to Franziska bayer for helping me with measurements. I appreciate the cooperation of Prof. M. Asghar Hashmi, Dr. Hina Ashraf and Hadia Noor in co-authoring the articles. Everyone at IFM, ITN (Linkopings University) and Electronics Dept. (NED University) is acknowledged for being part of a warm and friendly working environment.
My deep gratitude is due, to my parents for their devotion, sacrifices, continuous guidance, encouragement, support, and prayers for my whole life. Thanks are due, to my mother in law Mrs. Waseem Tahir for her co-operation, prayers and for always being positive. I also want to thank all my immediate and extended family members and my friends for all their love and support.
I wish to thank my ever-supporting husband Faraz Tahir, for his patience, encouragement, help and believing in my abilities. Thanks to my daughters Syeda Yusra Faraz, Syeda Javeria Faraz and my son Syed Abdullah Faraz for giving me happiness and making my life so beautiful. My love is always for you.
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Table of Contents
Abstract ... i
Preface ... iii
Acknowledgement ... v
CHAPTER-1 Introduction ... 1
1.1 Background ...1
1.2 Objective ...2
1.3 Outline ...2
CHAPTER-2 Crystal Structure and Basic Parameters ... 5
2.1 Basic crystal structure of ZnO, GaN and SiC ...5
2.2 Basic Parameters ...7
CHAPTER-3 Growth and Device Fabrication ... 9
3.1 n-ZnO/p-Si Heterojunction diodes ...9
3.1.1 Growth of ZnO nanorods ...9
3.1.2 Annealing ...10
3.1.3 Ohmic contacts to ZnO ...10
3.2 GaN Schottky diode ...10
3.2.1 Growth of free-standing GaN thick layer ...10
3.2.2 Contacts to GaN ...11
3.2.2.1 Ohmic Contacts ...11
3.2.2.2 Schottky Contacts ...12
CHAPTER-4 Characterization ... 13
4.1 Structural Characterization- Scanning Electron Microscopy (SEM) ...13
4.2 Optical Characterization- Photoluminescence (PL) ...14
4.3 Electrical Characterization ...16
4.3.1 Current-Voltage (I-V) Characteristics ...16
4.3.1.1 I-V characteristics of n-ZnO/p-Si heterojunction ...17
4.3.1.2 I-V characteristics of GaN Schottky diode ...18
4.3.2 Capacitance-Voltage Characteristics ...19
4.3.2.1 C-V characteristics of n-ZnO/p-Si heterojunction ...19
4.3.2.2 C-V characteristics of GaN Schottky diode ...20
4.3.3 Density of Interface states(Nss)...20
CHAPTER-5 Conclusion ... 23
References ... 25
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1
Chapter - 1
Introduction
1.1 Background
Recent advances in device development have allowed Si semiconductor technology to approach the theoretical limits of the material. Despite of being preferred semiconductor is still silicon; industry is now toolings up for wide bandgap semiconductors Gallium-Nitride, Silicon- Carbide and Zinc-Oxide (GaN, SiC and ZnO). Besides wide bandgap they also possess properties in optical and electrical characteristics. Only their direct bandgap allows easier integration with other optical devices. As conductivity control has been established, so the pn- junction devices such as LEDs and LDs are commercially available.
GaN is the material of choice for solid-state light emitters covering from UV, blue, and green parts of the spectrum and with the help of phosphors white light can be produced. The devices have demonstrated high performance not only for blue and ultraviolet LEDs but also for RF high-power and high-temperature applications. GaN HEMTs are the next generation of RF power transistor technology that offers the unique combination of higher power, higher efficiency and wider bandwidth than competing GaAs and Si based technologies.
ZnO has several important properties that make it a promising semiconductor for various type of electronic and photonic device applications. It has a large exciton binding energy (60 meV), compared with GaN (26meV), which makes it suitable for the fabrication of such devices that would possess bright coherent emission/detection capabilities at room and elevated temperatures.
ZnO has a high breakdown electric field, around 2 × 106 V/cm (more than double of GaAs breakdown field) making it suitable for use in high power and high gain devices. Its saturation velocity is 3.2 × 107 cm/sec at room temperature is also higher than GaN, SiC, or GaAs [1-3].
Large saturation velocity indicates that ZnO-based devices would be better for high frequency applications than other wideband gap semiconductors. It is resistant to radiation damage by high energy radiation therefore it is better suited for space operation. It is about 100 times more resistant than GaN against damage by high-energy radiation from electrons or protons. It is attracting the attention of the scientific community for its applications as piezoelectric films (or coatings) for surface acoustic wave devices (SAW), for IR and visible light emitting devices and UV sensing and biomedical applications where it can be used without coating.
SiC technology has achieved a broad spectrum of applications; the most beneficial advantages that SiC-based electronics offer are in the areas of high-temperature, high-power, high- frequency, radiation insensitivity, low noise capability and optoelectronic operation. The principal optoelectronic applications for SiC are low-intensity blue LEDs and substrates for GaN based high-intensity blue LEDs and blue laser diodes. Heterojunction of ZnO and SiC also give efficient light emission [4, 5]. SiC based power devices fulfill the requirements of higher blocking voltages, switching frequencies, efficiency and reliability. Its devices include rectifiers, power switches, RF and microwave power devices (p-i-n and Schottky barrier diodes, MOSFETs, JFETs and MESFETs) [6, 7]. SiC also exhibits a wide bandgap, high breakdown
2 field, high saturation velocity, high thermal conductivity, high chemical inertness and extremely high radiation resistance.
Fig 1.1 Semiconductor material power and frequency regions
Although impressive results have already been demonstrated for these wideband gap semiconductor devices but improvements are still needed in the basic crystalline quality of materials through the fabrication of complete devices with enhanced performance and reliability.
Further research work is required to better understand and improve material growth and optimize devices structures for improved electrical and optical properties.
1.2 Objectives
The aim of this thesis is to understand the behavior of metal-semiconductor contacts and to investigate the possible reasons of non idealities focusing on the effects of interface states on the other hand post fabrication annealing effects are studied on electrical and optical properties of their pn-diodes and based on the observations some conclusions are drawn. For these studies growth, fabrication and characterization of Schottky diodes and p-n diodes of wide band gap semiconductor has been studied using different techniques.
1.3 Outline
This thesis comprises of two sections. The first section contains introduction and importance, fabrication and characterization of wide band gap semiconductors Schottky and
3 heterojunction pn diodes. The introductory part provides an overview and scope of applications of GaN, SiC and ZnO. Chapter 2 highlights the structural, optical and electrical properties of wideband gap materials in a comparative manner taking into account their basic material parameters. Chapter 3 gives experimental details including growth / preparation methods along with structural, optical and electrical characterization. Results are discussed in chapter 4 and conclusions are drawn in chapter 5.
The second section presents results compiled in three publications. In paper I, Modeling of Pd/ZnO Schottky diode is done together with a set of simulations to investigate its electrical behavior. Interface states density is extracted for free standing GaN Schottky diodes in Paper II.
In paper III annealing effects on the optical and electrical properties of ZnO/Si heterojunction in different annealing ambient and temperatures are studied.
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5
Chapter-2
Crystal structures and Basic Parameters of Wide band gap semiconductors
2.1 Basic Crystal Structures of GaN, ZnO and SiC
Zinc oxide crystallizes in three forms i.e. hexagonal wurtzite, cubic zincblende, and cubic rock salt. The wurtzite structure is most stable at ambient conditions and thus most common. The zincblende form can be stabilized by growing ZnO on substrates with cubic lattice structure. In both cases, the zinc and oxide centers are tetrahedral. The rock salt (NaCl-type) structure is rarely observed and it is only observed at relatively high pressures of about 10 Gpa [8]. This basic structure is shown in Fig 2.1.
Despite of these three basic forms Zincoxide can be induced to form a very large variety of crystalline shapes using specialized growth methods. The precise shape of the crystals depends on the method of formation. In regular zinc oxide these shapes vary between acicular needles and plate shaped crystals to nano-pipes, nano-flowers, nano-combs etc.
(a) (b) (c)
Fig. 2.1 Basic crystal structures of ZnO, (a) Wurtzite structure (b) Zinc cite structure and (c) ZnO rock salt crystal structure
GaN is grown in Wurtzite and Zincblende crystal structure Fig.2.2. The reason for mostly wurtzite structure is that most of III–V nitrides are grown on sapphire substrates which generally transfer its hexagonal symmetry to the nitride film. The bandgap (Eg) is also effected by crystal structure. As Wurtzite crystal structure is a member of the hexagonal crystal system and is closely related to the structure of hexagonal diamond, therefore its energy gap is higher (3.4 eV) than Zincblende crystal with energy gap of 3.2 eV.
6
(a) (b)
Fig.2.2 Basic crystal structures of GaN, (a) Wurtzite structure (b) Zincblende crystal structure Silicon carbide is found in about 250 crystalline forms [9]. The polymorphism of SiC is characterized by a large family of similar crystalline structures called polytypes which are variations of the same chemical compound that are identical in two dimensions and differ in the third. Therefore they can be viewed as layers of lattice atoms stacked in a certain sequence [10].
Alpha silicon carbide (α-SiC) is the most commonly encountered polymorph (6H-SiC); it is formed at temperatures greater than 1700 °C and has a hexagonal crystal structure (similar to Wurtzite) shown in Fig.2.3. The beta modification (β-SiC), with a zincblende crystal structure (similar to diamond), is formed at temperatures 900-1250 °C [11] i.e. 3C-SiC. Until recently 3C- SiC have relatively few commercial uses, although there is now increasing interest in its use as a support for heterogeneous catalysts, owing to its higher surface area compared to the alpha form.
(a) (b)
(c)
Fig. 2.3 Crystal structures of three major SiC polytypes (a) 3C-SiC (b)4H-SiC (c) 6H-SiC
7
2.2 Basic Properties
Undoped ZnO is often n-type with good electrical conduction and intentional doping of B, Al, Ga, In, Si and F have been successfully incorporated to obtain high quality n-type ZnO [8,12].
The p-type conductivity in ZnO is still a challenging issue due to dominance of self compensating donor defects [13], low solubility, deep acceptor levels and insufficient excitation of carriers at room temperature. Reproducible p-type ZnO of high crystalline quality is important for the realization of robust pn-junctions, which are required in many electronic and photonic devices including LEDs [14]. P-type doping with N, P, Li, As, Sb and Cu has been investigated with some success [15-21] and co-doping of group III-V elements has also been effective for making pn-junctions, as the compensating intrinsic donors have low formation energies and are energetically stable in ZnO [22]. Dual doping of Li-N and N-As is another alternative for having acceptor states [23]. The background carrier concentration varies a lot according to the quality of the layers but is usually 1016 cm−3. The largest reported n-type doping is 1020 electrons cm−3 and largest reported p-type doping is 1019 holes cm−3, however such high levels of p-conductivity are questionable and have not been experimentally verified [3, 24].
Undoped GaN is invariably n-type usually with a high free electron concentration (1017 to 1018 cm-3) at room temperature. Although the dominant donor have not been unambiguously identified. There is a consensus that n-type conductivity is due to an intrinsic defect and most probably the nitrogen vacancy. P-type doping in GaN remains a critical issue for GaN-based electronic and optoelectronic devices. Most widely Zn, Cd, Mg, and Be have been investigated as a potential p-type dopant [25] .
Some basic properties of wide bandgap semiconductors in comparison to Si and GaAs are shown in Table-2.1
Table – 2.1 Basic Properties of wideband gap semiconductors
Properties Si GaAs Diamond GaN 4H-SiC ZnO
Band gap, Eg (eV) 1.12 1.43 5.45 3.45 3.26 3.37
Dielectric Constant 11.9 13.1 5.5 9 10.1 9
Breakdown Electric Field (MV/cm)
0.3 0.4 10 2.0 3 2
Electron Mobility (cm2/ V.sec)
1350 8500 2200 1250 1000 200
Hole Mobility (K) ( cm2/ V.sec)
480 400 850 850 115 180
Thermal Conductivity λ (W/cm. K)
1.5 0.46 22 1.3 4.9 1.3
Saturation Velocity Vsat x 107 cm/sec
1 1 2.7 2.2 2.7 3.2
Electron Affinity (eV) 4.05 4.07 –ve / +ve depending on surfaces
4.1
3.3 4.55
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Chapter- 3
Growth and Device Fabrication
3.1 ZnO-nanorods/p-Si heterojunction diodes 3.1.1 Growth of ZnO nanorods
ZnO can be grown by Metal-Organic Chemical-Vapor Deposition (MOCVD), Vapor Phase Transport (VPT), Magnetron Sputtering, Liquid Phase Growth i.e by Hydrothermal Method, Growth from Solution, Molecular-Beam Epitaxy (MBE), Pulsed Laser Deposition, and Plasma- Assisted MBE.
In recent years low temperature growth method have received more attention for the growth of ZnO nanostructures. There are three approaches for low temperature growths i.e. hydrothermal growth, chemical bath deposition (CBD) and electrochemical deposition.
One major advantage of using Chemical bath deposition (CBD) or aquous chemical growth (ACG) is that the growth temperature can be quite low (95 oC) [26, 27]. Due to low growth temperature cheaper substrate such as plastic, glass and paper can be used [28]. However, nanostructures grown by this method show a poor reproducibility, difficulty to control size and bad orientation, particularly on substrates (such as Si) with large lattice mismatch and different crystalline structures in comparison with ZnO. The most successful approach for the ACG growth techniques is growing ZnO nanorods on pretreated substrate with a two-step growth method [29, 30]. For pretreatment spin coating method is more easy and economical than other methods [30-32]. We selected spin coating technique to introduce seeding layer on our substrate.
In our experiments the synthesis process of ZnO nanorods contained pre treatment of the substrate and ACG growth. Vertically aligned ZnO nanorods were grown on p-Si substrate to form pn-heterojunction. Si substrate of (100) orientation and p-type doping of 1016 cm-3 and 1.38
-cm conductivity have been used. The substrate was ultrasonically cleaned for 15 min with acetone, methanol and then rinsed in de-ionized water. Analytical reagent grade chemicals have been used without further purification and all the aqueous solutions were prepared in distilled water.
Initially a ZnO seed layer was produced by diluting Zinc acetate dihydrate (Zn (OOCCH3)2·2H2O) in ethanol. This solution was coated onto Si (100) substrates by a spin coater (Laurell WS-400-8TFW-Full) at the rate of 3000 rpm for 30s. The thickness of the zinc acetate layer can be controlled by numbers of the spin coating. For our study substrates were spin coated for three times to ensure uniform ZnO seed layer and were dried in room temperature. The substrates were annealed in air at 250 °C for 20min. This annealing temperature is a little above the decomposition temperature of zinc acetate particles. In the following, all substrates were pretreated twice for the above processes before final growth of ZnO nanorods.
10 For ACG growth an equimolar concentration of hexamethylenetetramine (HMT) (C6H12N4, 99.9% purity) and zinc nitrate hexahydrate [Zn (NO3)2.6H2O, 99.9% purity] 0.022–0.075 mM is used. The substrate was placed in the solution and heated at 96 °C for 4 hours. The pre-treated Si substrates were immersed into the aqueous solution and tilted against the wall of beaker. The angel between substrate and beaker bottom is θ. Then the beaker was put into the oven and kept in it for different time under 93°C. After growth, the substrate was removed from the solution, rinsed with de-ionized water and then dried at room temperature.
3.1.2 Annealing
Annealing helps to improve the material crystalline quality. For the study of annealing effects all the seven samples were treated independently. Six samples were annealed at 400 and 600 oC each in Air, Nitrogen and Oxygen for 15 minutes. The seventh as grown sample was kept as reference.
3.1.3 Ohmic Contacts to ZnO
Prior to ohmic contacts evaporation an insulating PPMA layer was deposited between the nanorods. Then oxygen plasma cleaning was performed to remove excess PPMA from the top surface of the nanorods. About 150 nm thick ohmic contact of Aluminum (Al) have been evaporated in vacuum chamber on p-Si substrate. Al/Pt non-alloyed circular contacts were then evaporated to form ohmic contacts to n-ZnO. The diameter of the top contact was 0.58 mm with specific contact resistance of 1.2 x 10-5 Ω-cm-2 [33]
3.2 GaN Schottky diodes
3.2.1 Growth of free standing GaN film
Growth methods for GaN single crystal are Hydride Vapour phase epitaxy(HVPE), sodium Flux method and Ammonothermal Growth method described in Table.3.1. Out of these methods HVPE growth method is now on mass production stage whereas Na flux and Ammonothermal methods are also promising and have chances to be followed in the near future.
Free-standing GaN was grown in a hydride vapor phase reactor of horizontal configuration [14].
Gallium (Ga) melt precursor has been used for the growth process which has been flushed with HCl flow at 1000 oC to form gallium chloride (GaCl) species. These GaCl species then reacted with gaseous ammonia (NH3) over the sapphire substrate and formed GaN. To grow directly on the sapphire substrate, it is necessary to first deposit a GaN nucleation layer at a pressure of 200 mbar and at a temperature of 600 ◦C with a V/III ratio of 100, preceding the epitaxial growth step, performed at an elevated temperature of 1070 ◦C and pressure of 970 mbar with an average growth rate of 250 μm h−1. GaN layers around 500 μm thick were grown in successive series of two runs. During the cooling down of the samples after the second growth run, the GaN layers were spontaneously separated into pieces from the sapphire substrate due to built-in high thermal
11 Table 3.1 Growth methods for GaN single crystal
strain [34]. The separation took place into the GaN layer as shown by the optical studies [35].
Often as-grown, free-standing HVPE GaN layers exhibit background doping of n-type. This is because the growth under extreme conditions results in an incorporation of impurities such as Si and O from the corroding quartz reactor tube [36]. However, a few GaN layers were found to have an unintentional p-type doping effect. The unintentional p-type doping would have been introduced due to problems in automated switching of the process parameters during the growth process. The non-uniformities resulted in the generation of defects responsible for the p-type GaN layers.
3.2.2 Contacts to GaN
3.2.2.1 Ohmic contacts to GaN
As-grown free-standing GaN samples were cleaned chemically by the standard TL1 and TL2 processes, which included boiling in NH4OH:H2O2:H2O and HCL:H2O2:H2O: for 5 min each at 80 ◦C followed by dipping in a HF:H2O = 1:50 solution and then rinsing in de-ionized water. The
12 samples were then loaded into the metal (Al–Ti) deposition chamber for the deposition of the ohmic contacts. The ohmic contacts were deposited on one edge side of the Ga polar (0001) sample surfaces. The ohmic contact was subsequently annealed at 600 ◦C for 3 min.
3.2.2.2 Schottky Contacts to GaN film
Schottky contacts of 1 mm diameter were deposited using a shadow mask by sequentially evaporating palladium (Pd), titanium (Ti) and finally gold (Au) layers of thicknesses 40, 20 and 160 nm, respectively. Here Pd and Ti are used to provide good adhesion between Au and GaN.
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Chapter-4
Characterization
4.1 Structural characterization - SEM
The scanning electron microscope (SEM) images the sample surface by scanning it with a high- energy beam of electrons. The interaction of electrons with the atoms produces signals which are detected by some specialized detectors. These signals contain information about the topography of sample surface, composition, electrical conductivity etc. SEM images allow us to examine the diameter, length, shape and density of the ZnO nanostructures.
The morphology and the size distribution of the as grown ZnO nanorods have been studied by JEOL JSM-6301F scanning electron microscope. The chamber pressure was about 10-6 mbar.
Hexagonal –shaped vertical nanorods with a mean diameter of 160-200 nm and approximate height of 1.2 μm have been revealed as shown in Fig.4.1. Although the nanorods were not perfectly aligned on the substrate, they showed a tendency to grow perpendicular to the surface and distributed almost uniformly.
Fig.4.1. SEM images of as-grown ZnO nanorods on Si substrate
SEM picture of free standing GaN film showed smooth surface with hexagonal pits with average dislocation density (DD) in the order of 5 × 106 cm−2 as shown in Fig. 4.2. Often the DD lies from high 105 to mid 106 cm−2 ranges. The majorities of the dislocations were of edge and mixed types. The density of screw-type dislocation was suppressed by growing on c-plane sapphire substrates with a miscut of 0.3◦ towards the a-plane [37] resulting in a high structural quality of the material.
14 Fig.4.2 SEM image of free standing GaN layer
4.2 Optical Characterization - Photoluminiscence (PL)
Photoluminescence (PL) spectroscopy is the spontaneous emission of light from a material under optical excitation. When high energy light is directed on a sample, photons are absorbed and cause photo- excitations. These excitations relax and emit photons. The intensity of the spectral contents of photoluminescence is a measure of various important material parameters. The PL spectra also reveal transition energies and its intensity gives a measure of the relative rates of radiative and non-radiative recombination.
Room temperature PL spectra of ZnO usually consists of a narrow UV emission band and a broad deep level emission band. The UV emission band is dominated by the free exciton emission and is related to near band-edge transition i.e. the recombination of the free excitons.
For ZnO nanorods grown by ACG method the chemical component of the ZnO nanorods is nonstoichiometric and usually consists of excess Zn atoms and oxygen vacancies. Therefore many lattice defects and surface defects are contained in the as grown ZnO nanorods. It is observed that after annealing treatment the optical properties are improved due to decrease in surface defects [38]. Room temperature PL emission spectra of as grown and annealed ZnO nanorods are shown in Fig. 4.3. The spectra reveal that annealing ambient influences the emission characteristics of ZnO nanorods. High UV to visible emission ratio with peaks of near band edge emission (NBE) centered from 375-380 nm confirms a good optical quality with few structural
15 (a)
(b)
Fig.4.3 PL spectra of as-grown with (a) 400 oC annealed (b) both with 400 and 600oC annealed ZnO nanorods
16 defects. As the excitonic binding energy is quite high in ZnO therefore excitonic recombination is efficient than band to band transition (radiative recombination). At RT excitons are thermally stable and usually ZnO gives a strong NBE peak centered at 375nm. Strong enhancement in UV emission intensity is observed for samples annealed at 400 oC than the samples annealed at 600
oC in air, oxygen and nitrogen. Besides UV emission peaks there are weak broad deep-level emissions (DLE) centered from 510-580 nm. A broad DLE emission band from ZnO nanorods is attributed to defects such as oxygen vacancy (Vo), Zinc vacancy (Vzn), oxygen interstitials (Oi), zinc interstitials (Zni)[ 39-46], extrinsic impurities[38] and the superposition of different defect bands emitting in various wavelengths. Annealed ZnO nanorods contain majority of the defects related to oxygen vacancies generated by the evaporation of oxygen. Therefore low DLE emission intensity for N2 annealed sample at 600 oC is probably due to the suppression of re- evaporation of oxygen during annealing. Also during annealing process, excess zinc reacts with oxygen from the ambient and forms new ZnO structures different from the as grown and effecting the UV light emission. The reduced UV light emission from O2 annealed samples at 600 oC can be somehow related to structural defects [47]. Secondly, under high temperature annealing in oxygen-rich ambient, sometimes oxygen diffuses into the lattice to fill the oxygen vacancies, as a result DLE emission is quenched due to effective reduction in oxygen vacancies [48].
4.3 Electrical Characterization 4.3.1 Current-Voltage (I-V) Characteristics
Electrical characteristics of devices are characterized by current- voltage (I-V) and capacitance- voltage (C-V) measurements. For GaN Schottky diodes the current and capacitance (I–V and C–
V) measurements were performed usingKiethley-237 source measure unit and Agilent 4284- LCR meter by placing the samples on a probe station. ZnO/Si heterojunction diodes were characterized by using Keithley 4200 semiconductor characterization system.
For Schottky diodes the barrier height (φB) and ideality factors (n) are extracted by using equations (1) and (2).
BkT
e q
T AA
Io * 2 / (1)
/ ) 1
Io e
qV nkTI
(2)Where A is the cross-sectional area, A* is Richardson constant and Io is the saturation current.
17 4.3.1.1 I-V Characteristics of n-ZnO/p-Si Heterojunction diode
The comparison of I-V characteristics of as-grown and annealed heterojunction diodes indicates that after annealing the properties of diodes are improved. The reverse leakage current is decreased while forward current is slightly changed. Current–voltage characteristics show a typical non linear and rectifying behavior as shown in Fig. 4.4. Annealing in air and oxygen are resulted a decrease in reverse current through the hetero-barrier repairing leakage which may be present due to the inhomogenieties at the interface.
Fig.4.4 I-V characteristics of n-ZnO/p-Si heterojunction diodes annealed at 400 and 600 oC in air, oxygen and nitrogen.
After annealing at 400 oC reverse current of as-grown diode decreased from 1.68 x 10-4 A to 1.39 x 10-4 , 4.89 x 10-5 and 1.53 x 10-5 Amps for nitrogen, oxygen and air annealed samples respectively at -10V. Air and oxygen annealed diodes exhibited more stable rectification characteristics with high ratio of IF/IR. The rectification factor 46.3 of as-grown diode at ±4V is raised to 1890 and 271 after annealing at 400 oC in air and oxygen. No significant improvements were observed in I-V behavior of diodes annealed in N2 at 400 and 600 oC. It seems that leakage current paths are generated after annealing in N2, which make tunneling current more probable and increase the reverse current after annealing at 600 oC. The ideality factors for all the heterojunction diodes are in the range of 3 to 8. Higher values of ideality factors may be related to the presence of defects in the ZnO nanorods. Usually I-V characteristics of ZnO/Si heterojunction are controlled by the interface properties and presence of oxide layer results in high ideality factors [49-51]. Improved I-V behavior for annealed diodes is most likely due to
18 improvement in the crystalline quality of ZnO nanorods [52]. Better electrical properties are observed for diodes from 400 °C annealed ZnO nanorods.
4.3.1.2 I-V Characteristics of GaN Schottky diodes
The measured forward and reverse I–V characteristics at room temperature are shown in Fig.4.5.
For n- and p-type GaN samples, respectively. Both diodes showed a reverse breakdown of around 12 and 10 V. Excellent rectification characteristics are obtained for the n-GaN Schottky diode with a rectification ratio of 767 at±1.6 V, while for p-GaN Schottky diode this ratio is 14, indicating that it is not of as good quality diode as n-type. The barrier height (
φ
B) and ideality factor (n) have been calculated using Eq. (1) and (2)(a) (b)
Fig.4.5 Current-Voltage characteristics at room temperature for (a) n-GaN (b) p-GaN Schottky diode
The forward I–V characteristics indicate that n-GaN has a double barrier. Because of double barrier, high- and low-barrier phases give different saturation currents, barrier heights, ideality factors and series resistances [53]. The barrier heights of 0.9 and 0.6 eV have been obtained from two different current slopes and corresponding ideality factors are 1.8 and 7.2. Such a behavior is attributed to thin oxide layer at the surface [54, 55] of GaN which remains active due to metal electrode deposition at comparatively low temperatures. This effect can be greatly reduced by thermal annealing at 500 oC [56]. For p-GaN Schottky diode, the barrier height is 0.6 eV and the ideality factor is 4.16.
19 4.3.2 Capacitance-Voltage (C-V) Characteristics
From the capacitance voltage measurements, by plotting the inverse squared of the junction capacitance against the applied reverse voltage, built in potential ( Vbi), barrier height (φB ) and doping concentration (Nd) can be extracted using Eq. (3) and (4).
1 22 C d q N dV
o S d
(3)
d C bi
B N
N q V kTln
(4) The distribution of interface states can be extracted for a Schottky diode from the current–
voltage (I–V) and capacitance–voltage (C–V) measured values.
4.3.2.1 C-V Characteristics of n-ZnO/p-Si heterojunction
C-V measurements have been performed at frequencies from 10 KHz to 1MHz and a linear relationship is revealed in 1/C2 plot. The voltage intercepts of the extrapolated straight lines along the voltage axis are 0.65 and 0.7V corresponding to the devices annealed at 600 oC in nitrogen and oxygen. Fig 4.6.
Fig.4.6 1/C2 vs. voltage plot obtained at 1MHz.
20 4.3.2.1 C-V Characteristics of GaN Schottky diodes
The capacitance of the n-GaN Schottky diode has been measured at 100 KHz and 1 MHz. The barrier height obtained from the C-V measurements is 1.03 eV which is in close agreement with the theoretical barrier height of 1.1 eV obtained from the Eq.5
B
m q
(5)Here,
φ
mis the work function of metal andχ
is the electron affinity of GaN. Generally the barrier heights obtained from the C–V measurements are comparatively larger than the obtained values from I–V. The barrier heights obtained from the I–V measurements are more meaningful for assessing the diode performance as the interface traps neither respond to applied ac signals nor contribute to capacitance at higher frequencies [57]. In such cases large barrier heights obtained from C–V are attributed to the interface states due to the intervening insulating layer, contaminations in the interfaces, fixed surface polarization charges, deep impurity levels, edge leakage current and nitride-related other defects [58].4.3.3 Density of Interface states (Nss)
Surfaces and interfaces are a strong perturbation of the periodicity of a crystal lattice. Atoms at the surfaces/interfaces cannot have the same bonding structure as bulk atoms due to the lack of neighboring atoms. Since the periodicity ends at a surface/interface it results in addition of electronic states within the forbidden gap of the semiconductor. These partially filled electron orbital, or dangling bonds, are electronic states located in the forbidden gap of the semiconductor where they act as recombination centers. Depending on the charge these can be acceptor-like or donor-like states. In a Schottky diodes also, a thin insulating interfacial layer is always present due to incomplete covalent bonds, chemical reaction and sharp discontinuity between semiconductor crystal and metal [59,60], which results in a new dielectric phase. The mere existence of an electric charge in the thin interfacial layer, which is neither semiconductor nor metal, introduces a high density of interface states [59, 61]. If the desity of interface states is high enough then the carriers can recombine rapidly.
By using the (C–V) and forward (I–V) measured values, the distribution of the interface states (NSS) with respect to the band gap energies (EC−ESS) can be calculated using Eq. (6) [62]
D S i
SS nV W
N q
1
1
(6) Here εi is the permittivity of the interfacial layer, WD is the width of the space charge region extracted from the experimental C–V data [63] and δ is the thickness of the interfacial layer from Eq.(7) and (8). The voltage-dependent ideality factor n (V) has been extracted from equation (9)
21
2
1
ma ma ma
i C
C G
C (7)
i o
i
A C
(8)
Io
kT I V qV n
ln
(9) The capacitance of the interfacial layer is calculated from the high-frequency C–V measurements at 1 MHz using Eq.(7)
S
bo
e V IR
V n
1
1
V
(10) qE
Ec SS e (11)
Moreover taking into account the series resistance RS, the bias-dependent effective barrier height and the energy of the interface states with respect to the conduction band edge (EC−ESS) are obtained by equations (10) and (11).
22
23
Chapter-5
Conclusion
The studies of heterojunction and Schottky diodes of wide bandgap semiconductors are presented. Their structural, optical and electrical properties are studied.
Both n- and p-GaN Schottky diodes have been grown by HVPE and showed typical rectifying behavior with double barrier height for n-GaN diode. The current transport mechanisms have also been confirmed by temperature dependent I-V measurements. The density of interface states, extracted for the n-GaN Schottky diodes have been found in the range of 4.2 × 1012 to 3.9
× 1011 eV−1 cm−2 in the band gap below conduction band from Ec-0.9 to Ec-0.99 eV. The results yield that the interface states play a very important role in the current flow mechanism in electronic devices and they must be kept as low as possible in order to reduce the surface recombination and tunneling.
To study the annealing effects seven samples have been grown by ACG technique and annealed at 400 and 600 oC each and one has been kept as a reference. PL spectra exhibited higher ultraviolet (UV) to visible emission ratio with a strong peak of near band edge emission (NBE) centered from 375-380 nm and very weak broad deep-level emissions (DLE) centered from 510- 580 nm. Improvements in rectifying behavior along with a decrease in reverse current have been observed. This work demonstrates that post fabrication annealing of ZnO nanorods improves optical and electrical properties of devices.
A comparison between physical simulation and measured device characteristics have also been carried out in this study. Pd/ZnO Schottky diodes have been modeled and a number of simulations have been run to study current- voltage (I-V) characteristics. The simulation results have been compared with the experimental results and other reported results in the literature. The discrepancy in simulated and experimental results is might be due to the theoretically established parameters of ZnO used for simulation. A high surface density of defects and interface states can also be responsible for the deviation.
24
25
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