GaN as a potential candidate for high speed devices

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Material characterization of InGaN/GaN layers

Introduction

In this chapter, we provide a detailed study of material quality for several types of structures InGaN/GaN. Our study concerns different compositions of indium for InGaN intrinsic layer in the PIN structure. We will analyze the InGaN material as function of indium percentage. The growth of studied samples was provided from numerous collaborators. Commercials and academic entities have established highly regarded collaboration with our group in order to have a significant feedback of their materials in terms of quality and device performance subsequently. As we work on a photodetector based on InGaN/GaN material, it is important to adjust the indium content in the InGaN layer (absorbent layer) in order to identify the optimal wavelength of excitation in the absorption phase. It is noted that the availability in the market of laser source which is expected to be the light source of excitation is a key factor. It should be defined according to requested samples in order to improve the photodetection of fabricated devices. In the Figure 2.1, we made a synthesis of reported cutoff frequency measured by optical absorption for different indium composition of InGaN layer [1]–[11].
Figure 2.20: Reported cutoff wavelengths for different values of indium composition for InGaN layer measured by optical absorption.
The objective of this synthesis is to identify the laser source used later for the characterization phase. The fact is that the quality of material decreases with the incorporation of indium due to increased lattice mismatch with the substrate and the disorder introduced by the indium atoms, causing the formation of many defects. For that, it has been decided to proceed with low concentrations of indium (x = 10%) of InxGa1-xN layer since the quality of material had better improvement and good crystalline quality. It is worth noting that the evaluation of the photodiode in high percentage of indium content permits to work around the red wavelength 600 nm. This will enable more availability in terms of laser sources. For this purpose, increasing the indium composition of InGaN toward 20%, 30% and 50% will be the succeeding objectives once we have optimized the fabrication process. According to the Figure 2.1, the laser source of excitation should be in the range of 400-700 nm (visible spectrum) in order to evaluate the photodiode response in different percentage of indium composition in InGaN layer. The first phase of fabrication and characterization will be performed for 10% of indium content of InGaN samples. For this reason, a laser source of 400 nm is expected to be the excited source of photodiode and could also excite the other composition of InGaN (20%, 30% and 50%). However this 400 nm laser source is not the optimal wavelength for the higher indium composition samples, but it allows estimating the photodiode response.
During this project it has been conducted to study the optical waveguiding properties into porous GaN structures. Samples have been grown and structured by Korea Advanced Institute of Science and Technology (KAIST) in South Korea. Films have been prepared on sapphire by MOCVD and the microstructure has been characterized in IEMN. The dispersion of refractive index for porous GaN has been studied and compared to the GaN bulk material. The control of the refractive index into GaN is therefore fundamental for the design of active and passive optical devices. The objective of this study is to control the light emission by an increased coupling of the active material with optical modes, as well as the enhancement of the light collection efficiency.
In this chapter, the configuration of InGaN/GaN for the PIN structure is discussed. As the growth of materiel is carried out by external collaborators, a number of growth options will be also mentioned. In order to evaluate the crystalline quality, density of dislocations, surface morphology, layer thickness and indium composition, structural and microstructural characterization are detailed. X-Ray Diffraction measurement (XRD) will be presented. Besides, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy TEM, Energy-dispersive X-ray spectroscopy (EDX), and Atomic Force Microscopy (AFM) are also illustrated. We will also determine the optical properties such as refractive index, photoluminescence spectra and cut-off wavelength of absorption/transmission. For this, Prism coupling, Ellipsometry, Micro-Photoluminescence (µPL) and Spectrophotometer will be employed. Samples with differnet indium composition of InGaN 10%, 20%, 30% and 50% will be compared in terms of surface morphology and dislocation density. As we acquire two different growth techniques MBE and MOCVD, the grown samples will be also evaluated. Finally, the porous GaN structure is optically characterized.

Growth of InGaN/GaN structure with different indium composition

Two important factors have been discussed in terms of material growth, the first is the substrate and the second is the growth technique. As explained in the first chapter, it is known that the large lattice constant and thermal expansion coefficient mismatches of substrates with GaN have been the major concern of studies. Recently, the majority of III-nitride growth has been performed on c-plane sapphire substrates regardless of a lattice parameter and a thermal expansion coefficient which are different from those of GaN and cause defects in the GaN material. The lattice mismatch parameter relating to GaN epilayer is about 15% for sapphire [12]. According to our requirements, grower expertizes and cost issue, the sapphire substrate will be the best candidate.
Concerning the growth technique for InGaN/GaN structures, MOCVD and MBE have provided the highest quality GaN based materials for semiconductor heterostructures. MOCVD grower in Europe has been identified and well known as one of the best III-Nitrides supplier for electronic and photonic applications, named NOVAGAN Company. It has provided 10%, 20% and 30% of indium composition of InGaN/GaN/Sapphire wafers throughout the whole project. 50% of InGaN indium content sample has been grown by MOCVD also and provided by the Institute of Materials Research and Engineering (IMRE) in Singapore. For the MBE growth provider, we have established a long-term collaboration with King Abdullah University of Science and Technology (KAUST) in Saudi Arabia. Photonics Lab in this university has grown 10% of InGaN/GaN/sapphire wafers by MBE in order to compare it with MOCVD samples in terms of material quality and device performance. The structure of PIN photodiode has been discussed with the growers according to their expertise in order to obtain the best configuration of growth. Tow type of structures will be studied in each phase of InGaN growth, multi quantum well of InGaN layer (MQW-InGaN) and Single Layer of InGaN (SL-InGaN). The objective is to analyze two different configurations in each indium percentage of InGaN and to investigate the effect of heterostructure on the photodiode response.
The proposed PIN structure consists of a thick not-intentionally-doped (n.i.d) GaN buffer layer (called also undoped GaN) on c-plane (0001) sapphire. In order to optimize a high crystalline quality GaN and to move away from the interface GaN/Sapphire, the thickness of this layer is chosen to be 2.5 µm. It is followed by 3.5 µm thick Si-doped n-type GaN layer with a carrier concentration of 3×1018 cm-3.
The thickness of “i” layer or the absorption layer, is a critical parameter and should be optimized. It is known that the collection efficiency decreases sharply with the increase in the thickness of the absorption layer [13]. As the absorption coefficient influence directly on the device operation, it is important to adjust this factor to obtain an optimal thickness of absorption layer (InGaN layer). With the value of 105cm-1 of absorption coefficient for InGaN reported in chapter 1, we need 100 nm of thickness in order to absorb efficiently the emitted light. However, a critical thickness depending on the InGaN indium content is a major criteria and could be estimated to reported models [14]. Accordingly, 100 nm of intrinsic layer of SL-InGaN will be grown for the SL configuration. For the MQW-InGaN configuration, several possibilities exist in the literature. A synthesis of reported adjustment of MQW-InGaN/GaN layer has been carried out in the Table 1.
Consequently, 10 pairs of InGaN/GaN MQWs with 2.5 nm of InGaN and 12 nm of GaN barrier has been proposed to growers and fit well with their optimized configurations. Finally, 100 nm Mg-doped p-type GaN layer is grown with a carrier concentration of 5×1017 cm-3. It has been agreed that the PIN structure will be carried out according to the Figure 2.2.

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Structural characterization techniques

X-Ray Diffraction (XRD)

The X-ray diffraction is a widely used structural analysis just as powerful and non-destructive experimental technique. XRD analysis can provide information about structural quality such as lattice parameters of crystals, strain state, crystalline orientation, layer thickness and chemical composition of alloys. X-rays has a very small wavelength in the order of atomic range and the typical wavelength used for crystallography is 1 Å (0.1 nm). It is typically produced using cupper (Cu) or molybdenum (Mo) sources. XRD technique consists of probing a crystal through x-ray radiation owning wavelength (λ) close to the interatomic spacing, and then will be scattered by atoms. The X-ray beam is generated by bombarding a metal with electrons in incidence angle called (ω) then scattered by ions which produce diffractions on different planes of the structure with angle of 2θ then collected by a removable sensor. Figure 2.3 shows the principal of XRD with planes of atoms where indicated by dotted lines [22].

Table of contents :

General introduction
Chapter I. Introduction to III-Nitride based high speed devices
1.1. Overview of III-N Nitride materials and their applications
1.2. GaN as a potential candidate for high speed devices
1.3. GaN: Figure of merit
1.4. Photodiode as a key element
1.4.1. PIN photodiode
1.5. Fundamental properties
1.5.1. Crystal Structure
1.5.2. Spontaneous and Piezoelectric Polarization
1.5.3. Mechanical Properties
1.5.4. Crystal growth techniques
1.5.5. Substrates for III-Nitride materials
1.5.6. Defects in nitride materials
1.5.7. Doping
1.5.8. Electronic band structure and electrical properties
1.5.9. InGaN alloy
1.5.10. Carrier transport: low-field mobility for electron and hole
1.5.11. High-Field carrier Velocity
1.5.12. Optical properties
1.6. State of the art of high speed photodiode
1.7. GaN-based material for photodetectors: state of the art
References
Chapter II. Material characterization of InGaN/GaN layers
II.1. Introduction
II.2. Growth of InGaN/GaN structure with different indium composition
II.3. Characterization techniques
II.3.1. Structural characterization techniques
II.3.1.1. X-Ray Diffraction (XRD)
II.3.1.2. Scanning Electron Microscopy (SEM)
II.3.1.3. Transmission Electron Microscopy (TEM)
II.3.1.4. Atomic Force Microscopy (AFM)
II.3.2. Optical characterization techniques
II.3.2.1. Photoluminescence spectroscopy
II.3.2.2. Spectrophotometer
II.3.2.3. Prism coupling
II.3.2.4. Ellipsometry
II.4. PIN structure with 10 % of SL and MQW InGaN
II.4.1. Structural characterization
II.4.1.1. Samples grown by MOCVD
II.4.1.2. Samples grown by MBE
II.4.2. Optical characterization for samples grown by MOCVD and MBE
II.4.2.1. Photoluminescence
II.4.2.2. Spectrophotometer measurement
II.5. PIN structure with 20 % of SL and MQW InGaN
II.5.1. Structural characterization
II.5.2. Optical characterization
II.6. PIN structure with 30 % of SL and MQW InGaN
II.6.1. Structural characterization
II.6.2. Optical characterization
II.7. PIN structure with 50 % of SL InGaN
II.7.1. Structural characterization
II.7.2. Optical characterization
II.8. Characterization of nanostructured GaN
References
Chapter III. Fabrication and characterization of PIN photodiodes
III.1. Introduction
III.2. Electrical characterizations techniques
III.2.1. External Quantum Efficiency measurements
III.2.2. Photocurrent measurement using laser source
III.3. Set-up of noise measurement
III.4. Study of GaN deep etching
III.5. Fabrication of large-scale photodiode: Design 1
III.5.1. Comb and circle structures: fabrication process
III.5.2. Study of annealing effect
III.5.3. Current-voltage characterizations
III.5.4. Influence of the MQW configuration on the device performance
III.5.5. Influence of the growth technique on the device performance
III.5.6. External Quantum Efficiency measurements
III.5.7. Photocurrent measurement using laser source
III.5.8. Capacitance-Voltage characterizations
III.6. Fabrication of planar micro-photodiode: Design 2
III.6.1. Flowchart and fabrication process
III.6.2. The influence of passivation on photodiode response
III.6.3. Photocurrent measurement using laser source
III.6.4. Capacitance-Voltage characterizations
III.7. Extraction of cut-off frequency using noise measurement
III.8. Fabrication of vertical micro-photodiode
General conclusions and perspectives
References

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