Effect of processing modulation on electrical performances 

Sapphire substrate

Sapphire is the most extensively used substrate for growth of the III–nitrides despite of its large structural and thermal mismatch with GaN and InN (shown in Table 1.5). This is supported by the fact that the layers grown on sapphire have, in many cases, better quality and sapphire is easily available up to inches in diameter at relatively low cost. Sapphire substrates are transparent and stable at high temperatures. In general, the quality of the films grown directly (i.e., without the buffer layer) on any plane of sapphire is poor. Obtaining a good quality nitride epilayers necessitates the nitridation of sapphire substrates and an insertion of a low temperature intermediate buffer layer. Furthermore, the optical transparency of sapphire is beneficial in backilluminated detectors and LEDs for lack of absorption. The main drawback of sapphire as a substrate, is the low thermal conductivity, which causes heat management as an important concern for high current density devices.

Silicon carbide (SiC) substrate

Extensive work has been done on the growth of III– nitrides on SiC substrates . SiC hasseveral advantages over sapphire for InN and GaN epitaxy, which includes a smaller latticemismatch and higher thermal conductivity. Additionally, SiC has good electrical conductivitywhich eases to make electrical contacts to the backside of the substrates and thereby simplifies the device structure compared to sapphire substrates. Large good quality SiC substrates are commercially available. Low temperature GaN or AlN buffer layers are also deposited on the SiC substrates before growing the nitrides. The stress developed in the films grown on SiC is smaller than that grown on sapphire, because of less lattice mismatch. SiC is also a polar material which facilitates the growth of single polar nitrides.

Silicon (Si) substrate

Generally, nitride-based devices are grown on sapphire, silicon carbide substrates. However, a considerable work has been done on the growth of group III nitrides on Si substrate.
The major attractive points of Si as a substrate includes high quality, low cost, availability of large size, good electrical and thermal conductivity. In addition, Si substrate can accomplish the integration of III nitride devices with other Si based electronics. Although the crystal quality of GaN grown on Si is still poorer than that on sapphire and SiC, research on this is in great progress. A low temperature buffer layer of AlN is usually grown on Si before the growth of the main epilayer to avoid the formation of SixNy. The growth of a polar epilayer on nonpolar substrate is more complicated due to the formation of additional defects, such as inversion domains.

Applications of group III nitrides

The properties of group III nitride binary compounds and alloys make them exclusive for applications in the fields of electronics and optoelectronics. The quest for these applications has led to extensive research work on these materials from the last two decades. The reevaluation of InN bandgap has even more broadened the spectrum of these applications.

Optical applications

The Solid State Lighting (SSL) technology has the potential to cut the world lighting energy usage by 20% and could contribute significantly to climate change solutions . So the research aims at bringing LEDs solid-state lighting as the next generation of light sources for general illumination, from homes to commercial applications. In this field, research on group III nitride semiconductors is realizing breakthroughs in efficiency and performance in terms of successful launching of nitride LEDs and Laser diodes.

LED applications

Of group III (Al, Ga, In) N system, InN plays a major role in empowering the fabrication of high efficient light emitting diodes by widening the spectral region with the tuning of indium composition as shown in Figure 1.6. In other words the group III nitrides spans from near infraFigure 1.6. (a) Bandgap of all group III nitrides as a function of molar fraction. The solid and dashed lines are bowing curves with best-fit bowing parameters red to deep ultraviolet region. The research on nitride based LEDs was kindled by the advent of blue/green LED based on InGaN heterostructure grown on sapphire substrate . Thereafter the research in this system was endured by red LED with indium rich InGaN heterostructures and white LEDs which have been developed by coating blue GaN LED with phosphors.

Laser applications

The fabrication of high quality LEDs paves the way for the realization of lasers which can operate at light wavelengths from ultraviolet (UV) to green. The blue ray disc technology has replaced the traditional DVDs as the blue laser diodes can allow five times higher storage capacity. A major breakthrough in research has been accomplished by the infrared lasing in high quality single crystalline InN nanobelts . The possibility of making ternary and quaternary nitride systems fosters the semiconductor lasers emitting from deep UV to infrared region.

Emitters and detectors

The wide band gap AlN and GaN binaries posses the potential for fabrication of UV emitters and detectors. UV emitters can be used in various applications such as material identification, forensic location, disinfection and in material processing. These III-nitride based UV detectors finds outmost usage in UV sensing applications such as automobile engine combustion sensing, high temperature flame sensing, environmental monitoring, solar blind detectors, missile plume detection for military use etc. The other most significant application of III nitride semiconductors is in the fabrication of quantum infrared detectors. Photoconductors are the most common type of quantum infrared detectors which can be realized by nitride semiconductors. The narrow band gap of InN and its alloying with GaN makes it perfect for photovoltaic applications. The InGaN ternary system can be tuned to absorb the entire visible range of solar radiation and this could result in high efficiency solar cells.

Electronic applications

The unique properties of InN such as small effective mass, high electron mobilities, and high peak electron velocities make InN promising for electronic devices. InN is of great interest for realization of high speed, high performance, and high frequency devices due to its inherent unique properties. InAlN can be a good candidate for high power, high temperature microwave applications because of its higher breakdown voltages. InN is also ideal for terahertz applications.

Low frequency noise

The study of fluctuations in physical quantities can yield an insight into the physical phenomena, associated with fluctuations. The spontaneous fluctuations of physical quantities in the domain of electron devices are termed as noise and both terms; fluctuations and noise, are used interchangeably. For the outmost usage of the physical and chemical properties of a material, it is essential to predict the limitations of the device performances. To this end, electrical noise claims to be an accurate indicator of the quality of the materials and devices since it arises from various relaxation processes of the charge carriers, defects, or group of defects . Noise measurement is a diagnostic tool to explore the microscopic and/or mesoscopic properties of materials under study, as noise is sensitive to transport processes; and also noise limits the smallest signal level that can be measured.
In this work, the noise measurements were of considerable interest because of primarily two reasons. Firstly, the noise as a sensitive indicator of material quality, and therefore can be used as a feedback to growers. Secondly, it provides, by comparison with theoretical models, a way to determine the dominant conduction mechanisms.

Noise definition

In the broadest sense, noise is any unwanted signal that comes along with the desired signal. The sources of noise can be classified into mainly two categories. The first is extrinsic noise sources which comes from interactions between the investigated system or device and the external environment which may result from electrostatic or electromagnetic coupling between the circuit and the A.C power lines or fluorescent lights, cross talk between adjacent circuits, humming from D.C. power suppliers or microphonics caused by mechanical vibration of components. Most of these disturbances can often be eliminated or minimized by adequate shielding, filtering or the layout of circuit components . The intrinsic noise is the spontaneous fluctuations which result from the physics of devices and the materials that make up the electrical system. This noise is measured in terms of random fluctuations either in the voltage across the terminals of the device or current flowing through the device and is relative to static values. The noise is randomly distributed in value and sign fluctuations are small compared to static values. From signal point of view, it can be represented as a function, b(t) which is expressed in volt for voltage fluctuations and in ampere for current fluctuations. One typical view of a noise signal is shown in Figure 1.7. The mathematical properties of this function are the following: considering at an instant t=t 0, it can be written as.

White noise sources

There are two distinct types of white noise: thermal noise, shot noise.
Thermal noise: It is often termed as Johnson noise or white noise and is caused by the random motion of charge carriers in thermal equilibrium. In every conductor, above the absolute zero temperature, charge carriers are in random motion and this vibration is dependent on temperature.Since the motion is random, at any given time there might be a surge of charge on one side or the other leading to a voltage across the material. For a semiconductor of electrical resistance R at a temperature T, the spectral voltage noise density (S v) is given as in equation 1.8, which shows that S v is independent of frequency as can be seen in Figure 1. 8.

Growth Techniques

In chapter 1, we have already mentioned about the different substrates and buffer layers which can be employed in group III nitrides epitaxy. Here we will briefly describe the growth techniques for the epitaxial layers investigated in this dissertation.

Molecular Beam Epitaxy (MBE)

MBE was developed in late 1960s by A.Y. Cho, since then it has evolved into one of the most widely used techniques for producing high purity epitaxial layers. MBE can provide good uniform and atomically sharp interfaces even at substantially low growth temperatures (for instance InN the growth temperature from 420-620 °C depending on the In or N polar 2 ). As MBE is operated at high vacuum it provides accurate in-situ monitoring capabilities. Hence, MBE is very suitable for precisely controlling the growth parameters.
The principle of MBE growth essentially consists of atoms or clusters of atoms which are produced by heating up a solid or liquid source. They are then led to impinge on a hot rotating substrate, where they can diffuse and eventually form the desired film. The process takes place in anUltra High Vacuum (UHV) environment (pressure ~ 10 −9 mbar). The precursor sources can be shutoff and turned on rapidly using shutters, enabling MBE to make abrupt composition changes within a monolayer.
Nowadays, to take the advantage of liquid and gaseous sources, variations of MBE are used for Group III nitrides as well as for various dopants. There are two variations in MBE which are specific to Group III nitrides: Ammonia MBE and Plasma Assisted (PA) MBE. Ammonia MBE uses ammonia (NH 3 ) as the nitrogen source and solid sources of In, Ga and Al metals for group III components. However dissociating ammonia to make atomic Nitrogen introduces Hydrogen (H), an impurity for nitride growth. On the other hand, PAMBE uses Nitrogen as the group V source. This necessitates Radio-Frequency (RF) plasma to crack the N 2 and create highly reactive atomic N since the N 2 molecule (unlike NH3 ) has a very high thermal stability.
Metal Organic Vapor Phase Epitaxy (MOVPE) MOVPE is an efficient technique for the growth of Group III nitride heterostructures, quantum wells and superlattices. With this technique one can produce almost atomically sharp interfaces. Its high growth rate, good uniformity, large area and multiple wafer growth has attracted the nitride industry for mass production of devices.
The typical organic precursors for III nitrides are trimethylindium (TMIn) for In, trimethylgallium (TMGa) for Ga, trimethylaluminium (TMAl) for Al and ammonia (NH 3 ) as the nitrogen source. It is to be noted that, triethyls (In/Ga/Al) can also be used instead of trimethyls (In/Ga/Al). For the growth, the organic precursors are driven on to over hot substrate, with the help of carrier gases like hydrogen and nitrogen where the organic species decompose and react with the atomic nitrogen. In this process molecules of required semiconductor material are produced, which then adsorbs on the substrate surface to produce an epitaxial layer. MOVPE requires high growth temperature, as it must satisfy the conditions for NH 3 pyrolysis. However, for the growth of InN, this is an inherent disadvantage as it dissociates already below 600 °C.

Epitaxial layers used in this dissertation

The materials mentioned in this dissertation were grown by MBE and MOVPE. The major part of this thesis contains MBE grown samples. The InN samples were grown by plasma assisted Molecular beam epitaxy (MBE) at Instituto de Sistemas Optoelectronicos y Microtecnologia (ISOM), Universidad Politecnica de Madrid (UPM) in Spain. The InGaN/GaN samples, which consist of quantum well structures, were grown by MBE as well as MOVPE techniques at École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland and other samples from Centre de Recherche sur l’Hétéro-Epitaxie et ses Applications (CRHEA),Valbonne, France.

Samples characterization

The surface of the films was inspected by Scanning Electron Microscopy (SEM). The quantitative information about the surface morphology in terms of roughness (rrms) were investigated by Atomic Force Microscopy (AFM) in the tapping mode. One of the essential parts of this thesis is the micro structural characterization of the thin films, in which the interest was about all kinds of defects in the film itself and in the quality of interfaces between the various heteroepitaxial laye rs.
For such a characterization, Transmission Electron Microscope (TEM) was used. The optical properties were studied by photoluminescence spectroscopy. The electrical transport properties like the electrical resistivity and the low frequency noise were carried out using a semiconductor parameter analyzer (HP 4156B) and a dedicated low frequency noise measurement set-up, including a low temperature four probe equipment (Lakeshore TTP4).

Microscopy techniques

Atomic Force Microscopy (AFM)

Atomic Force Microscopy is a basic technique to determine the surface morphology at atomic resolution and as well as the quantitative surface roughness of thin films. The AFM consists of a microscale cantilever with a sharp tip (probe) mounted at the end of the cantilever and used to scan across the surface of the specimen. The AFM tips are typically made from silicon nitride or silicon with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke’s law. In general, the force acting between the cantilever and the sample is a sum of Vander Waals, electrostatic, magnetic, electrodynamic, chemical bonding and capillary forces, which are compensated by elasticity forces resulting from the cantilever bending and the sample deformation. Typically, the deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes. A feedback mechanism is employed to adjust the tip-to-sample distance to maintain a constant force between the tip and the sample. The sample can move in the z direction for maintaining a constant force, and the x and y directions for imaging the surface in Å scale by three piezo crystal sensor which allows driving very precise sample movements. A schematic diagram of AFM is shown in Figure 2.1. There are three scanning modes associated with AFM, namely; contact mode, non-contact mode and tapping mode. In the contact mode, the tip is static and in contact with the sample and the image is obtained by repulsive forces between the tip and the sample. This technique can often damage either the sample surface or the tip. In non-contact mode, the tip oscillates above the surface, and the image is obtained from the attractive forces between the tip and the sample. In tapping mode, the image is obtained by the tip, which just taps the surface for small periods of time. This method lessens the damage done to the surface and the tip compared to the case of contact mode. An important parameter which characterizes the surface of thin films is the roughness. The rms (root mean square) roughness is the standard deviation of the z values in a given area. It is calculated by the AFM software which processes the acquired images.

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Table des matières
Chapter 1 
Introduction to III-nitride semiconductors and low frequency noise
1.1 Brief History of Nitrides
1.2 Properties of III nitride semiconductors
1.2.1 Crystalline structure
1.2.2 Crystal polarity
1.2.3 Dislocations
1.2.4 Spontaneous and piezoelectric polarization
1.3 Group III nitrides epitaxy
1.3.1 Substrates and buffer layers for group III nitrides Sapphire substrate Silicon carbide (SiC) substrate Silicon substrate
1.4 Applications of group III nitrides
1.4.1 Optical applications
1.4.2 Electronic applications
1.5 Low frequency noise
1.5.1 Noise definition
1.5.2 Noise spectral density
1.5.3 White noise sources
1.5.4 Lorentzian noise sources
1.5.5 1/f noise sources
1.5.6 1 /f noise models
1.6 References
Chapter 2 
Experimental techniques
2.1. Growth techniques
2.1.1Molecular Beam Epitaxy (MBE)
2.1.2Metal Organic Vapor Phase Epitaxy (MOVPE)
2.1.3Epitaxial layers used in this dissertation
2.2 Samples characterization
2.2.1 Microscopy techniques Atomic Force Microscopy (AFM) Electron microscopy Scanning Electron Microscopy (SEM) Transmission Electron Microscopy (TEM) Scanning Transmission Electron Microscopy (STEM) TEM sample preparation
2.2.2 Optical characterization Photoluminescence spectroscopy (PL)
2.2.3 Electrical characterization Probe Stations Peripheral apparatus Current-Voltage (I – V) and resistance measurement Instrumentation for DC measurements Instrumentation for DC measurements
2.3 References
Chapter 3 
Plasma Assisted Molecular Beam Epitaxial InN layers electrical conduction
3.1 Introduction
3.1.1 PAMBE growth regimes of InN
3.1.2. Transport properties of indium nitride
3.2 Objectives of this research
3.3 Description of samples
3.3.1 Samples schematic
3.3.2 Samples geometries and fabrication process
3.4 Effect of processing modulation on electrical performances
3.4.1 Characterization Surface morphology by AFM and SEM Photoluminescence characteristics Electrical properties studies at room temperature
3.5 In and N rich InN layers
3.5.1Surface morphology
3.5.2 Electrical properties studies at room temperature
3.5.3 Electrical properties studies with temperature
3.6 Conclusions
3.7 References
Chapter 4 
InGaN Quantum wells: Transmission Electron Microscopy and Photoluminescence studies
4.1 Introduction and Motivation
4.1.1 Effect of polarization fields in InGaN/GaN QWs
4.1.2 Origins of high efficiency emission in InGaN Quantum Wells
4.2 Samples
4.3 Microstructure studies of InGaN QWs for V-pits and its association with PL studies
4.3.1 Microstructure
4.3.2 Optical properties of InGaN/GaN QWs
4.4 Microstructure analysis with CTEM, HRTEM and STEM
4.4.1 Microstructure and chemical composition studies with HRTEM
4.4.2 HAADF Investigations
4.4.3 Comparison of extracted In composition with literature
4.5 Conclusions
4.6 References
Chapter 5 
Conclusions and perspectives
5.1 InN layers
5.1.1 Conclusions
5.1.2 Open questions and future work
5.2 InGaN/ GaN QWs
5.2.1 Conclusions
5.2.2 Open questions and future work
5.3 References
Annex I 
Annex II

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