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Properties of III-V/Si heterojunction fabricated by HVPE

Prakhar Bhargava

KTH ROYAL INSTITUTE OF TECHNOLOGY

SCHOOL OF ENGINEERING SCIENCES

DEGREE PROJECT IN SCHOOL OF ENGINEERING SCIENCES, STOCKHOLM, SWEDEN 2020

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Properties of III-V/Si heterojunction fabricated by HVPE

Prakhar Bhargava Master’s Thesis

Examiner

Prof. Sebastian Lourdudoss

Supervisors’

Dr.Yanting Sun & Axel Str

Ö

mberg

SEPTEMBER 2020

KTH Royal Institute of Technology School of Engineering Sciences Department of Applied Physics SE-100 44 Stockholm, Sweden

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Abstract

Silicon is a promising material and is used for a wide range of applications in the electronics industry because of the high quality surface passivation given by the native oxide layer SiO2e.

However, Si is not an ideal candidate for optoelectronic applications due to its indirect bandgap of 1.1eV, which limits the use of silicon for light emitting devices.

III-V semiconductor materials have relatively higher electron mobility when compared to Si, they also have a direct bandgap which makes them more suitable for the fabrication of devices for electronic and optical applications. There are different III-V semiconductor materials such as GaN, InP, GaAs, GaP which can be used in the fabrication of optoelectronic devices. The limitation is that these compound semiconductors are costly because of their scarce availability in nature hence, it is economically expensive to fabricate devices using III-V compound semiconductors. This issue of economic feasibility can be resolved by integrating III-V and Si to fabricate devices with better electronic and optical properties and reasonable cost.

Although it’s a reasonable argument to fabricate devices using III-V and Si. There is also a trade-off between enhanced electronic properties and the defects that are induced at the interface due to lattice mismatch and when the density of these defects is higher at the interface then the device performance degrades significantly. III-V compound semiconductor materials like GaAs and InP have lattice mismatch of 4% and 8% respectively with silicon, which would not be ideal as this will induce a lot of defects at the interface. GaP, on the other hand, has a lattice mismatch of 0.4% with silicon which will result in less defects.

In this project, the research is focussed on studying the properties of three different III-V/Si heterojunctions i.e. GaAs/Si, GaP/Si, and GaAsP/Si. GaAs has direct bandgap and higher electron mobility with respect to Si, GaP on the other hand has a low lattice mismatch with Si which will induce less defects. GaAsP is a ternary compound with tunnable properties that are useful to fabricate III-V/Si heterojunction with desired requirements. A low-temperature epitaxial buffer layer growth is used for the fabrication of these heterojunctions, which was performed by using cost effective hydride vapour phase epitaxy (HVPE) technique. The focus was also on studying the optical properties of Selective Area Growth (SAG) of III-V/Si structures grown on (100) and (111) Si substrates by HVPE. Various characterization tools were used for analysing the morphology and optical properties of these heterojunctions. The morphological study was performed by using the scanning electron microscope (SEM) and Atomic force microscope (AFM). The optical properties of the sample were analysed by using photoluminescence (PL) and Raman spectroscopy.

The dependence of morphology of III-V/Si heterostructure on Si substrate orientation was observed in both planar growth and SAG. The planar GaAs/Si(100) sample reveals meandering lines with step heights of 4-8nm, these could be associated with threading dislocations.

However, planar GaAs/Si(111) sample revealed a comparatively smoother surface with high density of pits. In case of SAG the growth along the (100) plane was found to be more lateral than vertical, whereas the growth along the (111) plane was both lateral and vertical with pillar- like structures. Optical analysis suggests that the crystalline quality of SAG GaAs/Si(111) was better than the SAG GaAs/Si(100) sample. The morphology was also dependent on the growth temperatures as at low temperature the islands coalesce rapidly due to the decrease in critical

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radius for nucleation of III-V compound semiconductors. A further optimization in the growth process might be required as there might be a possibility that growth in SAG GaAs/Si(100) and SAG GaP/Si(100) samples have occurred through (111) facets. These findings can be used in future for device applications, such as solar cell, multi-junction PV, etc.

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Acknowledgment

My first and foremost gratitude is towards God Almighty for keeping me and people around me in good health throughout my Masters at KTH as well as for the successful completion of this project during this unprecedented epidemic.

I would like to thank Prof. Sebastian Lourdudoss for giving me this opportunity to work in the Photonics department alongside talented researchers. I am also grateful for his constant encouragement and guidance during this work. I would also like to thank Dr.Yanting Sun who has been my immediate supervisor for all the insightful discussions and guidance. I would also like to thank Axel StrÖmberg who was always around to help me in every way possible. I would also like to thank Lakshman Srinivasan for being a wonderful colleague. I feel blessed to have been part of such an amazing group where I always felt welcomed and everyone was very kind and helpful.

I would like to thank other researchers and Ph.D. students in the Electrum Laboratory for equipment training and all other timely assistance in the lab, they are Carl Reuterskiöld- Hedlund, Reza Nikpars, Cecilia Aronsson, and Corrado Capriata.

At last, I would like to thank my parents and my brother who have been a source of inspiration and strength during my Masters at KTH. Also, I would like to thank my family away from home Hillsong Church, Sweden, for being very kind and loving.

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Table of Contents

Abstract ... 2

Acknowledgment ... 4

Table of Contents ... 5

List of Abbreviations ... 6

1. Introduction ... 7

1.1 Introduction to semiconductors and III-V/Si ... 7

1.2 Project Outline ... 7

1.3 Defects in III-V/Si heterostructures ... 9

1.3.1 Misfit dislocation defects ... 9

1.3.2 Anti-phase domains (APDs) ... 10

2. Processing Techniques ... 11

2.1 PECVD ... 11

2.2 Photolithography ... 11

2.3 RIE ... 12

2.4 Hydride vapour phase epitaxy (HVPE) ... 12

2.5 Sample Preparation ... 14

3. Characterization Techniques ... 17

3.1 SEM and EDS ... 17

3.2 Atomic Force Microscopy (AFM) ... 18

3.3 Photoluminescence ... 19

3.4 Raman Spectroscopy... 20

4. Results and Discussion... 21

4.1 GaAs ... 22

4.1.1 GaAs/GaAs/Si (Growth run – 3738) ... 22

4.1.2 GaAs/GaAs/Si (Growth run – 3750) ... 24

4.2 GaAsP/GaP/Si (Growth run– 3800) ... 34

4.2.1 Optical characterization of the samples ... 35

4.2.2 Raman analysis of GaAs and GaAsP... 43

4.3 GaP... 45

4.3.1 GaP buffer layer growth on Si – 3805 ... 45

4.3.2 GaP overlayer growth on planar and patterned Si substrates- 3770 ... 46

4.4 Summary ... 54

4.5 Future Work ... 55

5. Conclusion ... 59

References ... 60

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List of Abbreviations

AFM: Atomic Force Microscope APD: Anti-Phase Domain BSE: Back Scattered Electrons

EDS: Energy Dispersive X-ray Spectroscopy FWHM: Full Width at Half Maxima

GaAs: Gallium Arsenide

GaAsP: Gallium Arsenide Phosphide GaN: Gallium Nitride

GaP: Gallium Phosphide

HVPE: Hydride Vapor Phase Epitaxy

MOVPE: Metal Organic Vapor Phase Epitaxy MBE: Molecular Beam Epitaxy

LEDs: Light Emitting Diodes LO: Longitudinal Optical Mode

PECVD: Plasma Enhanced Chemical Vapor Deposition PL: Photoluminescence

RIE: Reactive Ion Etching RMS: Root Mean Square SAG: Selective Area Growth SE: Secondary Electrons

SEM: Scanning Electron Microscope SI: Semi-Insulating

Si: Silicon

SiO2: Silicon dioxide Si3N4: Silicon Nitride

TO: Transverse Optical Mode

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1. Introduction

1.1 Introduction to semiconductors and III-V/Si

Semiconductors were discovered when Michael Faraday in 1833 [1] observed that there is a decrease in resistance upon heating material and this behavior of material was particularly opposite from known properties of metals. Although this observation was made in the early 19th Century, the interest in semiconductors was developed only in the 1920s after the development of photovoltaic cells. This interest led to the invention of transistors in 1947 at Bell Labs[2], later this invention of transistors revolutionized the electronics industry as today we are all surrounded by millions and billions of transistors that are used in electronics devices such as the transistor radio, video games, computer, and smartphones.

As the semiconductor industry started to grow, Germanium gained a lot of attention as it showed better performance and higher electron mobility when compared to the limited mobility of electrons in silicon. In the 1950s Carl Frosch and Lincoln Derick at Bell Labs discovered the formation of SiO2 which could be used as a mask on Si and act as a passivation layer[2].

After this, the electronics industry developed with a vast interest in Si and then almost followed Moore’s law[3] given in 1970 for the next 4 decades which suggested that the number of transistors on a chip will double every two years and hence reach a theoretical limit.

At present, the focus of research in the semiconductors is to find solutions to replace Si with other semiconductor materials so that highly efficient devices can be fabricated and the electronics industry can move past this limitation.

Silicon has also played a vital role in the field of photovoltaic as today the commercialized solar cells are made of Si, however, its theoretical limit in terms of efficiency is only 33%

whereas the multi-junction solar cell fabricated using GaInP/GaAs/GaInAsP/GaInAs has achieved a record efficiency of 46% in 2015[4]. Si has an advantage in terms of cost as it is abundantly present on earth whereas other semiconductor compounds such as GaAs, GaP, and GaAsP which could produce better efficiency but the cost of production is high as these materials are not abundant. This is why III-V integration with Si has attracted attention as properties from both materials could be used for diverse applications.

1.2 Project Outline

This project aims at studying properties of III-V/Si heterojunctions fabricated by using HVPE.

Integration of III-V on silicon can be useful in many ways as III-V semiconductor compounds have a higher bandgap which is desired in a photovoltaic device, as well as these structures, can be grown at low cost. These III-V/Si heterojunctions can be grown by using various methods such as MOVPE, MBE, HVPE, etc. In this work, the growths were performed by using HVPE. HVPE works at near-equilibrium conditions which means that both kinetics and diffusion of gaseous precursors are fast. This property results in a high growth rate, HVPE is also suitable for highly selective growth [5] as it operates at near-equilibrium. HVPE uses a cheaper precursor i.e. metal chlorides when compared to MOCVD which uses metal-organic precursor which is expensive. These qualities of growing III-V on Si with high selectivity,

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higher growth rate, and cheaper precursors make HVPE an ideal choice for the fabrication of devices[6].

The integration of these III-V compound semiconductors on Si has some challenges as there are defects that are induced due to the difference in their lattice constants and thermal expansion coefficients. These mismatch in lattice constants can generate APDs, threading dislocations as well as strain in the grown layer which results in reduced electronic and optical properties[7].

These problems with defects can be avoided by using 4-6o offcut Si substrates[8]. The two step growth method where nucleation is done at low temperature followed by overlayer growth at high temperature has shown improved structural properties with reduced threading dislocations[8].

In this project, the growth of the III-V layer was performed in two steps by using HVPE. The first step was to grow a thin buffer layer at low temperature and then over-layer growth was performed at a high temperature. Further morphology and optical characterization are carried out to study crystallinity, composition variation as well as strain in the sample.

In this work various III-V/Si heterojunctions were studied and their properties were analyzed by using different characterization techniques, which will be discussed in chapter 2. The project aims to perform an extensive analysis of both planar III-V/Si samples as well as SAG III-V/Si with substrates oriented in (100) and (111) directions. To my knowledge such a study on SAG of III-V/Si structures where growth was performed by HVPE has not been reported earlier hence it would be interesting to study these structures for development in III-V/Si heterojunction based photonic devices.

The different III-V structures that were studied are:

1. GaAs:

Gallium Arsenide (GaAs) is a type of III-V semiconductor compound which has a direct bandgap of 1.424 eV and a lattice constant of 5.653 Å[9] whereas, Si has a lattice constant of 5.431 Å and this results in a lattice mismatch of 4.08%, the lattice mismatch gives rise to misfit dislocations and strain in the GaAs layer. In this project, various GaAs samples that were grown on silicon substrates with orientation along (100) and (111) planes were analyzed. Also, the study was performed on selective area growth samples (SAG) of GaAs that were grown on Si substrates. These samples were grown using two step method so that we can avoid the propagation of defects on the surface. In the first step, a thin GaAs layer was grown at low temperature (440-460oC) and then over-layer growth was performed at high temperature (660oC).

2. GaP:

Gallium Phosphide (GaP) is another III-V compound semiconductor that has an indirect bandgap of 2.26 eV and a lattice constant of 5.450 Å while Si has a lattice constant of 5.431 Å, resulting in the lattice mismatch of 0.35% , the lattice mismatch is significantly low when compared with the mismatch between GaAs and Si. This small mismatch will significantly reduce the formation of defects in the GaP layer grown on Si substrates when compared to GaAs growth on Si. In this project, optical and morphology studies were done on GaP/Si heterojunctions grown using HVPE in two step method which includes low temperature GaP

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buffer layer growth on Si and then over-layer GaP growth at high temperature. The study was performed on both planar samples as well as SAG GaP/Si samples.

3. GaAsP:

Gallium Arsenide Phosphide (GaAs1-xPx) is a ternary compound semiconductor. The ‘x’ here represents the composition of arsenic and phosphorous. The properties such as lattice constants and bandgap can be tuned by varying the composition as shown in Figure 1.1. As ‘x’ increases from 0 to 1 there is a variation on both bandgaps as well as lattice constants. When ‘x’ is below 0.45 then it has a direct bandgap whereas above 0.45 it has an indirect bandgap. These tunable properties of GaAsP make this ternary compound very interesting and it has attracted a lot of attention especially for tandem solar cell application[10]. In this project, GaAsP was grown epitaxially by using HVPE. Here two step method was used for epitaxial growth, firstly the GaP buffer layer was grown at low temperature and then the GaAs0.75P0.25 (intended) layer was grown at high temperature on the GaP buffer layer.

Fig 1.1: Variation in bandgap and lattice constants in GaAsP with increase in ‘x’[11]

1.3 Defects in III-V/Si heterostructures

1.3.1 Misfit dislocation defects

Misfit dislocation defects occur due to the mismatch in lattice constants between III-V compounds and Si. This is schematically shown in Figure 1.2. The difference in lattice constants will either result in the formation of misfit dislocation or strained layer. In the case of GaP on Si, these misfit dislocations are not observed below critical thickness i.e. 90nm. This misfit dislocations in GaP generates above critical thickness, and during the cooling down process[12]. GaAs on Si has a 4.1% lattice mismatch, also the density of these defects increases due to the difference in thermal expansion coefficients and cooling down process. Ideally, the stress free GaAs on Si can be obtained by confining the dislocation network strictly at the interface however, there is a high density of stacking faults and threading dislocations near the interface region[13].

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Fig 1.2: Heterointerface with misfit dislocation[5]

1.3.2 Anti-phase domains (APDs)

These type of defects occurs due to the growth of polar semiconductor on non-polar semiconductor[14]. III-V semiconductor compounds like GaAs, GaP are polar while Si and Ge are non-polar semiconductors. Figure 1.3 illustrates the anti-phase domain in GaP growth on Si. The diamond lattice of Si (100) surface has monoatomic steps whereas the zinc-blende lattice of GaP has diatomic steps[15]. This results in the formation of APD as the domain is formed with opposite sub-lattice as the bulk (shown in Figure 1.3). Self-annihilation is possible when two APBs (Anti-phase boundaries) cross each other. This is indicated in Figure 1.3 where APBs along (111) lattice plane self annihilates. Another approach of obtaining APD free growth of GaP film on Si (100) is reported by Mitsuru SUGO et al. where Si (100) substrate was tilted by 4o towards [110] with an accuracy of 0.5o [14]. Two step growth method has been proven useful for growing APDs free GaAs on Si by MOCVD[16].

Fig 1.3: Formation of APBs in (111) and (110) plane of GaP structure on Si (100) substrate [15].

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2. Processing Techniques

There are different processing techniques involved in the fabrication of III-V/Si heterojunctions. In this project, there are different processes done on the sample at different stages. The processes involved are:

2.1 PECVD

Plasma-enhanced chemical vapour deposition (PECVD) is used for the deposition of thin films like Si3N4, and SiO2.In this process of thin film deposition, the substrate is first heated at high temperature (300-350oC), then gases (reactants) like SiH4 (carried by N2), O2, NH3, N2O, CF4, and N2 are used for deposition. PECVD has an advantage over CVD processes as the deposition of these films occurs at low temperature. In PECVD, ion bombardment allows the diffusion of species further along the surface at sufficiently low temperature. This results in better small feature filling[17]. The gases are introduced right above the substrate which is positioned between the two electrodes. The gases are then ionized to generate plasma as shown in Figure 2.1. The generated plasma chemically reacts with the substrate which results in the deposition of a thin film. The chemical reaction involved in the formation of SiO2 and Si3N4 layers are as follows:

SiH4 + 2N2O  SiO2 + 2H2 + 2N2 (2.1) 3SiH4 + 4NH3  Si3N4 + 12H2 (2.2)

Fig 2.1: Schematic of PECVD [18]

In this project, the SAG III-V/Si samples have a SiO2 thin film which is used as a mask during the epitaxial growth. The deposition of this thin film was done at 300oC, other details can be found elsewhere [19].

2.2 Photolithography

Photolithography is a process which is used for patterning the wafer, there are different types

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of exposures used in photolithography such as optical, UV, and EUV. In this project UV light is used for exposure. There are various components involved in photolithography which are shown in Figure 2.2. This method of patterning requires a mask that consists of the pattern that needs to be imprinted on the wafer. In this work the after aligning the mask with the close proximity of 50µm to the wafer, the sample is exposed to UV light, and depending upon the type of photoresist the exposed part either gets more soluble in developer (for positive photoresist) or insoluble (for negative photoresist). This occurs because positive photoresist upon exposure degrades and is easily soluble in developer solution however, negative photoresist gets strengthened upon exposure, due to cross-linking. The remaining part where the photoresist is still present is removed by using a developer solution. After this process, the mask layer which is SiO2 in this project is etched by using RIE and then the sacrificial photoresist is stripped by using plasma treatment in Tepla.

Fig 2.2: Schematic of photolithography

2.3 RIE

Reactive Ion Etching (RIE) is a dry etching technique that is used for etching the surface. In this particular project, RIE is used to etch SiO2 grown inside the circular openings that was created by using photolithography. The instrument is similar to PECVD however, the gas flow is quite different. In this process of etching, gases like CF4, O2, and Ar are used with the flow depending upon the targeted sample. RIE is a process that includes both physical sputtering and chemical reactivity of plasma-created species for etching[20]. The energetic bombardment of ions results in directional etching, which causes anisotropic etching[21]. RIE also offers to etch with high selectivity, the etching ratio of SiO2 with respect to Si is >30[22, 23].

2.4 Hydride vapour phase epitaxy (HVPE)

The growth of III-V compound semiconductors can be carried out by using different methods such as MOVPE, MBE, and HVPE. MOVPE uses metal organic precursors such as trimethylgallium (group III source) and hydrides (group V source). It is a cold wall reactor wherein precursors react only at the heated substrate, this reduces the parasitic reactions[24].

MOVPE is a diffusion controlled process which is why growth is not affected by small temperature fluctuations[5]. The limitation in the case of MOVPE is the cost of production which is due to expensive metalorganic precursors. In the case of MBE, growth is surface

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kinetics-controlled whereas in HVPE the process is thermodynamically controlled.

Growth using HVPE occurs at near equilibrium which provides high growth rate and selectivity. These properties of HVPE allow performing SAG with a high aspect ratio. There are different structures with high aspect ratios that can be selectively grown by using the HVPE method such as nanopillars, and nanowires. The precursors involved in growing III-V layers by using HVPE are metal chlorides (group III source) such as GaCl and hydrides (group V source) such as PH3 and AsH3. This process occurs at near equilibrium which means the rate of deposition increases with an increase in total flow rate.

HVPE is a hot wall reactor, meaning that the temperature in the reactor is always elevated to high temperature with resistive heating of the reactor wall[5]. Since HVPE is a hot wall reactor it has a demerit of having parasitic reactions before the deposition[24]. In this project custom built Aixtron low pressure HVPE (LP-HVPE) is used for the fabrication of III-V/Si heterojunctions. It is important to note that the growth of these III-V/Si heterostructures was performed in two steps where the buffer layer was initially grown at low temperature and then the overlayer growth is performed at high temperature. Low temperature nucleation results in better crystallinity as the film coalesce before misfit dislocations are introduced[25]. Then overlayer growth is performed at a high temperature.

The schematic of this reactor is shown in Figure 2.3. The reactor consists of multiple heating zones as shown in Figure 2.3. These zones are the source zone, mixing zone, deposition zone, and loading zone. The temperature of these zones increases as we move from the loading zone to the source zone.

To conduct low temperature buffer layer growth in HVPE the precursors are separately carried into the mixing zone wherein the vapors (precursors) mix and then growth begins as the substrate is moved into the mixing zone. In this process, the mixing of precursors occurs just above the substrate which facilitates low temperature growth in HVPE[26].

Fig 2.3: Schematic of LP-HVPE reactor[19]

In this project, three different III-V structures are grown on Si by using HVPE. The three structures are GaAs, GaP, and GaAsP. The chemical reactions involved for epitaxial growth of these structures are as follows:

As discussed earlier the sources of group III elements are chlorides, for GaAs it is GaCl. GaCl is synthesized inside the reactor by flowing HCl through liquid Ga (stored in the boat) as shown in Figure 2.3. The chemical reaction is shown in equation 2.3

2Ga(l) + 2HCl(g) ⇋ 2GaCl(g) + H2(g) (2.3)

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The gaseous GaCl is carried into the growth zone where it reacts with the hydrides (group V source). GaCl is an unstable compound, hence high temperature is always maintained. The chemical reaction between GaCl and hydride (AsH3) is shown in equation 2.4

AsH3 (g) + GaCl (g)

GaAs (s) + HCl (g) + H2 (g) (2.4) A similar process is followed in the epitaxial growth of GaP on Si. The difference is in group V precursor which in the case of GaP is PH3. The chemical reaction involved in GaP growth is shown in equation 2.5

PH3 (g) + GaCl (g)

GaP (s) + HCl (g) + H2 (g) (2.5) For GaAsP growth, the composition is controlled by maintaining the flow ratio in the AsH3/PH3

[27]. The composition is also affected by the growth temperature beacuse the phosphorous incorporation decreases at a lower temperature[28]. This is also explained in temperature dependent reactivities of P and As-species in GaAsP growth[29].

HVPE reactor consists of multiple heating zones, in one of the zone substrates are heated at high temperature, in other zone Ga precursor is heated and in another zone hydride gases are monitored. The growth parameters are mentioned in Table 2.1.

2.5 Sample Preparation

In this project various III-V/Si heterostructures were studied. Table 2.1 represents the description of the samples that were investigated during this project.

Table 2.1: Details and description of the samples that are investigated in this project

c

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Fig 2.4: Schematic of processes involved in patterning substrates[19]

After patterning the substrates the last step is the growth of III-V compound semiconductors.

Both, the planar substrates and the patterned substrates were grown together in a single run.

Table 2.2 shows the various parameters that were set during each growth run. The parameters mentioned in the table are V/III ratio, total flow of precursors, growth temperature, duration for growth, thickness of the sample, and overall growth rate.

Si

(a) HF cleaned substrate

Si SiO2

(b) Oxide deposition

SiOSi 2

(c) Spin coating of photoresist Photoresist

Si

PhotoresistMask SiO2

(d) Light exposure

Si

Photoresist SiO2

(e)Patterns formed on the photoresist

Si

SiO2

Photoresist

(f) Reactive-ion etching

Si (g) Photoresist SiO2

SiO2

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Run number Layers Substrates V/III

ratio Total flow (sccm)

Temperatu

re (℃) Time

(min) Thickness Overall Growth rate 3738 GaAs

(overgr owth)

SI GaAs, Epi Si, Planar Si(100)

20 895 665 15 Planar

Si(100):

1.5 μm

Planar Si(100):

8.2 μm/h GaAs

(Buffer layer)

10 825 465 2

3750 GaAs (overgr owth)

Planar Si(100), Planar Si(111), Patterned Si(100), Patterned Si(111), Planar GaAs

20 895 665 15 planar

Si(100):

1.5μm

planar Si(100):

5.3μm/h GaAs

(Buffer layer)

10 825 439 2

3770 GaP (overgr owth)

Planar Si(100), Planar Si(111), Patterned Si(100), Patterned Si(111), Planar GaP

20 895 715 15 planar

Si(100):

3.1μm

planar Si(100):

11.0μm/h GaP (Buffer

layer)

10 825 445 2

3800 GaAsP (overgr owth)

Planar Si(100), Planar Si(111), Textured Si(100), Planar GaP

20 895 661 15 textured

Si(100):

4.2μm

textured Si(100):

14.7μm/h GaP (Buffer

layer)

10 825 444 2

3805 GaP (Buffer layer)

Planar Si(100), Planar Si(111), Textured Si(100), Planar GaAs

10 825 448 2

Table 2.2: Detail of growth parameters while growing III-V layer using HVPE

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3. Characterization Techniques

3.1 SEM and EDS

Scanning electron microscope is used for studying the morphology of the sample, it can also be used to calculate various parameters of the sample such as the thickness of the film deposited on the substrate, size of the particles, and many other morphological studies can be performed by using SEM. SEM is a powerful microscope that has a high resolution up to 1-10nm which is not possible to achieve by using the optical microscopes, as the resolution is directly related to the wavelength that is used for visualizing the sample. In the case of an optical microscope, the wavelength of the light is in a visible region which is 400nm-800nm while in the case of SEM an electron beam of wavelength in picometers is used for scanning the sample which enables observer to visualize small features that are present on the surface of the sample.

The schematic of SEM is shown in Figure 3.1. SEM consists of an electron gun, series of lenses i.e. condenser lens and objective lens. There are also magnetic coils and detectors positioned above the sample to detect the electrons that are emitted from the sample.

The sample is placed on the stage as shown in the Figure 3.1, the electron beam is generated by using electric field/thermionic emission, this ealectron beam is then accelerated at a significantly high voltage 1-15 kV by an electron gun, this accelerated electron beam is focused on the sample which placed on the stage. To focus these electron beams series of condenser lenses are used. The magnetic coils are used for scanning the sample by moving the electron beam.

There are two major detections which takes place by collecting backscattered electrons (BSE) and secondary electrons (SE)respectively[30]. Backscattered electrons are generated when the electrons undergo elastic collisions with the sample and are reflected back, while secondary electrons are the outcome of inelastic collisions occur between the primary electron beam and the sample, these secondary electrons have the lower energy than backscattered electrons.

These secondary electrons which are emitted from the sample are then collected by the detector and the signals are amplified to obtain the morphology of the sample.

SEM can also be used to analyze the elements present in the sample, for this purpose an ESB detector is used here which detects characteristic x-rays which are emitted by the elements present in the sample, this technique is known as Energy dispersive x-ray spectroscopy (EDS or EDX). This is generally performed at high voltage (10 KeV).

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Fig 3.1: Schematic of SEM [31]

3.2 Atomic Force Microscopy (AFM)

Atomic force microscopy is a powerful tool with resolution up to the atomic scale, this is basically used for studying surface morphology of the sample. There are various parameters that can be calculated by using AFM such as surface roughness, feature size, and there are many other parameters which can be analyzed by using Gwyddion software.

The AFM instrument used in this project for the measurements is shown in Figure 3.2 (a). As highlighted there are different components in the instrument that should be calibirated before starting the measurements. Firstly the sample is loaded on the stage and placed such that the scanning region is underneath the tip. There are holes on the stage that creates a vacuum to hold the sample during the measurement, therefore turn on the vacuum switch after positioning the sample. The laser adjustment knobs don’t require constant calibration. The photodetector adjustment knobs help in the precise adjustment of laser on the cantilever, these adjustments can be seen on the screen. The camera adjustment knob is to monitor the movement of the tip.

The measurements can be started once each of these parameters is adjusted.

AFM set-up consists of various components such as cantilever, laser source, photodetector, and a sharp tip usually made of Si or Si3N4 is attached to the free end of the cantilever as shown in Figure 3.2 (b). To obtain topographic information of the sample, the laser is focused on top of the cantilever to which the tip is attached, and as the tip is brought closer to the sample there is deflection in the reflected laser beam due to the change in the oscillations of the cantilever and this deflection is then detected in photodetector which is then generated into an electrical signal to provide us a topographic image. A 3D profile of the surface is also obtained using AFM[32].

AFM can operate in three different modes i.e. contact mode, non-contact mode, and tapping mode. In contact mode, the tip is in direct contact with the sample and this is useful for soft samples as there is a risk for the tip to break if we use contact mode for hard surfaces such as silicon. In non-contact mode, the tip is at a distance of few nanometres where it can still sense the van der walls attractive forces and electrostatic repulsive forces.

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Fig 3.2: (a)The AFM instrument (b) Schematic of AFM [33]

3.3 Photoluminescence

Photoluminescence (PL) study is used for studying the crystalline quality, bandgap, and composition of the sample. The principle behind the PL is to excite the electrons from the valance band to the conduction band, this process of excitation requires that the incident photons used for excitation must have higher energy than the bandgap of the material. Now since the excited electron would tend to come to its equilibrium position i.e. valance band where it will recombine with the generated hole and in that process, the energy is released in the form of a new photon, the energy of the photon generated through recombination corresponds to the bandgap or composition of the material. This is how the bandgap is detected through PL. Also, this causes the limitation of using PL as indirect bandgap materials like GaP can’t be studied due to the involvement of phonons in the process of recombination. The process is shown in Figure 3.3.

Fig 3.3: Photoluminescence Process

The schematic of PL is shown in Figure 3.4. Typical PL setup consists of a laser source, cryostat, sample stage, monochromator, filters, and detector. In this project, laser line that was used is Argon laser or green line (514nm) which is powered at 10mW and measurements are

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performed at room temperature. Typically laser beam is focused on the sample and resulted photons are then filtered and then go through a monochromator so that background wavelength is avoided in measurements. Finally, this signal is then detected and translated into a spectrum.

Fig 3.4: Schematic of Photoluminescence [34]

3.4 Raman Spectroscopy

Raman spectroscopy is used for studying the crystallinity, composition, and strain in the sample. In this technique, the scattered photons after going under collisions with the molecules are detected. Based on the change in energy of these photons and shifts in Raman spectra this Raman scattering is classified. If the photon is excited from zero energy level and falls back to the same energy level due to elastic interaction then this scattering is known as Rayleigh scattering. In the case of inelastic collisions, there can be two alternatives where these photons either loses energy and fall back to other energy levels. If the photon gains energy and falls back to other energy levels as shown in Figure 3.5. This kind of inelastic scattering is respectively known as stokes and anti-stokes Raman scattering. This is diagrammatically represented in Figure 3.5.

Fig 3.5: Different Raman scattering processes [35]

Raman spectrometer consists of various components such as a laser source (in this project Argon laser with wavelength 514nm was used), a set of lenses are used for focusing the laser on the sample, filters (for filtering reflected light and other background light from scattered light) and a sensitive detector to detect weak signals as the intensity of scattered light is weak.

Ar - Laser

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4. Results and Discussion

There are various characterizations done on various III-V/Si samples shown in Table 4.1. The morphology of the samples was studied by using AFM, while optical properties were studied using PL and Raman spectroscopy. It is important to note that these samples were not fabricated during this project and in-depth details regarding the fabrication procedure in each step can be found elsewhere [19]

Table 4.1: Characterization work done in this project

Run No Layers Substrate AFM PL Raman SEM

(TV)

3738 GaAs + GaAs Buffer Layer Planar Si(100)  

3750 GaAs + GaAs Buffer Layer Planar Si(100)

Planar Si(111)

Patterned

Si(100)  

Patterned

Si(111)  

3770 GaP + GaP Buffer Layer Planar Si(100)

Planar Si(111)

Patterned

Si(100)

Patterned

Si(111)

3800 GaAsP + GaP Buffer Layer Planar Si(100)  

Planar Si(111)  

Textured Si(100)  

3805 GaP Buffer layer Planar Si(100)

Planar Si(111)

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22 4.1 GaAs

4.1.1 GaAs/GaAs/Si (Growth run – 3738)

In this growth run, GaAs was grown on three different substrates SI GaAs, Epi Si, and planar Si(100). The focus of this project is to study the morphology of the GaAs layer grown on planar Si(100) sample by using AFM. The growth was done in two steps, firstly GaAs buffer layer was grown at low temperature, and in the second step over-layer growth of GaAs is done at a higher temperature. Figures 4.1(a) and (b) show cross view and top-view respectively of SEM image of GaAs layer grown on planar Si(100) substrate. The cross-section view shows that the surface is quite rough as well as some grains can be observed in top view image, a slightly higher magnification images could resolve more features.

(a) (b) Fig 4.1: SEM images of GaAs growth (3738) on planar Si100 (a) Cross-section (b) Top view

4.1.1.1 Morphology study of planar Si(100) sample

AFM analysis was performed by using Gwyddion software and while analyzing some operations were also used to remove unwanted artifacts such as levelling of the data, alignment of rows as well as correction of horizontal strokes. The RMS roughness of the sample is 45 nm.

There are some cavities observed in the AFM image shown in Figure 4.2 (d), these pits are nearly 150nm deep as shown by using the line profile in Figure 4.2(e), these pits are not deep enough to reach the interface as the thickness of GaAs film is 1.4 µm.

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Fig4.2: AFM images of GaAs growth (3738) on planar Si100 at the center of the sample (a) 2D image of 20x20 µm scan (b) 3D image of 20x20 µm scan (c) 2D image of 5x5 µm scan (d) 3D image of 5x5 µm scan (e) 2D image of 5x5 µm line scan measurement (f) 3D image of 2x2 µm scan

(a) (b)

(c) (d)

(e) (f)

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4.1.2 GaAs/GaAs/Si (Growth run – 3750)

In this growth run, the GaAs was grown on planar Si substrates as well as SAG of GaAs was also realised on patterned Si substrates. The difference in this run when compared to the 3738 run was the temperature at which the buffer layer was grown. The buffer layer growth temperature during the 3738 run was 25oC higher when compared to the 3750 run. This has resulted in different morphology, which can be observed by comparing the SEM top view image of planar Si100 samples from both runs (fig 4.1(b) and 4.3(a)). In this project, morphology of planar samples were studied using AFM while optical characterization was done for SAG samples using PL and Raman spectroscopy.

(a) (b) Fig4.3: SEM top-view images of GaAs growth (3750) on planar (a) Si100 and (b) Si111 [19]

4.1.2.1 Morphology Study of Planar Si(100) and planar Si(111) samples

The study of morphology was done on these planar samples similarly using AFM as mentioned in the 3738 run. The purpose of this study was to resolve the meandering lines that are observed in the SEM image of the planar Si(100) sample (fig 4.3(a)) as well as to study the spots that are observed in the SEM image of the planar Si(111)(fig 4.3(b)) sample. Also, there is a possibility of observing APDs due to the polarity mismatch between GaAs and Si.

Planar Si(100)

Figure 4.4(a) depict a 20x20 µm AFM image that reveals similar features as observed in the SEM image (Figure 4.3(a)). There is a presence of multiple steps with an approximate step size of 5nm. These step sizes are observed more clearly in higher resolution 2x2 µm AFM image (Figure 4.4(f)). These steps are also observed in SEM image similar to those meandering lines.

These steps most likely are associated with threading dislocation in the GaAs layer due to a 4%

lattice mismatch. The RMS roughness of this sample is 98nm, which is more than twice when compared to the 3738 run.

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Planar Si(111)

A similar morphology study was also performed for the planar Si(111) sample. AFM analysis was performed similarly as described earlier. The RMS roughness of the GaAs layer grown on planar Si(111) sample is 42nm. The 20x20 µm AFM image show the resemblance to the SEM image shown in Figure 4.3(b), as those spots can be observed as pits. At high resolution scan of 5x5 µm which is shown in Figure 4.5(e), these pits were analyzed to be 100nm deep meaning

Fig.4.4: AFM images of GaAs growth (3750) on planar Si(100) at the center of the sample (a) 2D image of 20x20 µm scan (b) 3D image of 20x20 µm scan (c) 2D image of 5x5 µm scan (d) 3D image of 5x5 µm scan (e) 3D image of 2x2 µm scan (f) 2D image of 2x2 µm line scan

(a) (b)

(c) (d)

(e) (f)

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that they are not in direct contact with the interface as the thickness of the GaAs film is 740nm.

These AFM images also reveal that the presence of threading dislocations is not observed, unlike the planar Si(100) sample.

Fig 4.5: AFM images of GaAs growth (3750) on planar Si111 at the center of the sample (a) 2D image of 20x20 µm scan (b) 3D image of 20x20 µm scan (c) 2D image of 5x5 µm scan (d) 3D image of 5x5 µm scan (e) Line scan across 5x5 µm scan

(a) (b)

(c) (d)

(e)

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4.1.2.2 Optical Characterization of SAG GaAs on Patterned Si(100) and Patterned Si(111) samples:

Photoluminescence and Raman spectroscopy were used to study the optical and crystalline properties of the samples. This study was done on SAG GaAs samples wherein we observe two different kinds of morphology based on the orientation of Si wafers as shown in Figure 4.6. In the case of GaAs SAG on patterned Si(100) there is more lateral growth when compared to vertical growth. However, one can observe pillars that represent there is higher vertical growth in SAG GaAs on patterned Si(111).

(a) (b)

Fig 4.6: SEM top-view images of SAG GaAs growth (3750) on patterned (a) Si(100) and (b) Si(111) [19]

Patterned Si(100)

PL mapping was done across single SAG GaAs. There is no isolated SAG in this sample. There are different regions where these SAGs have coalesced or there is only polycrystalline growth.

The figure below shows the SAGs with the least residual growth around it.

(a) (b)

The parameters used for the PL mapping across the SAG are as follows:

Fig4.7: Microscopic top-view images of SAG GaAs growth (3750) on patterned Si(100) with laser spot (a) Position of the laser spot for mapping (b) SAG pillar for mapping

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PL mapping plots of wavelength variation at maximum intensity (Figure 4.8(a)) reveals that the SAG GaAs at the bottom which is due to the stage movement and the variation in wavelength is from 874 - 878nm, which is slightly deviated from standard value i.e. 870nm. This deviation could be due to strain or surface roughness. FWHM variation plot (Figure 4.8(b)) shows that the crystalline quality of SAG is uniform. However, the wide spectra are shown in Figure 4.8(d) represents the low crystalline quality of GaAs. Figure 4.8(d) shows the PL spectra at various points on the SAG as shown in Figure 4.8(a), more details regarding these points are described in Table 4.2. A shoulder is observed at 825nm in Figure 4.8(d), indicating the presence of defects free GaAs in a wurtzite structure[36]. When III-V compounds are grown in the form of nanowire (SAG) they tend to adopt a wurtzite structure. Although, the thermodynamically stable crystal structure is zincblende [36]. SAG of GaAs is considered to be defect free because dislocation defects that propagate through the buffer layer are blocked by the mask layer and hence, the laterally grown layer should be defect free[37]. GaAs signals are also observed from outside SAG, which is due to the residual growth of GaAs on the mask and also due to the laser spot size which is quite large, and the detector detects the GaAs signals even though the laser spot is away from SAG.

Density Filter Grating Size Aperture Integration

Time(s) Step Size Microscope Objective

D0.3 300 lines/mm 1160µm 1 0.5µm 50x

Fig 4.8: PL mapping of SAG GaAs growth (3750) on patterned Si(100) substrate (a) Plot of wavelength variation at maximum intensity (b) FWHM variation (c) Maximum intensity plot at 875nm (d) PL scans at various spots shown in fig 4.8(a)

X (µm) (a)

Y m)

X (µm) (b)

Y m)

Y m)

X (µm)

(c) (d)

PL map of wavelength variation at maximum intensity PL map of FWHM variaiton

PL map of Maximum Intensity at 875nm

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Raman analysis was performed on a different SAG present in the same region. Raman analysis is useful to confirm that if the observed deviation in PL is due to the strain in the GaAs layer or sample roughness. The mapping was done across a single SAG.

(a) (b)

The parameters for Raman mapping are shown in the table below:

Raman scans observed in Figure 4.10 (f) shows the presence of both TO and LO peak, however TO peak is forbidden in (100) plane but this strong TO peak indicates that the growth has orientation along (111) plane as the growth might have occurred through facets with orientation along (111) plane. Raman shift and maximum intensity maps were plotted separately for TO and LO peaks, intensity plots for TO and LO (Figures 4.10(a), and 4.10(c)) reveals SAG region.

Also, the Raman shift map shows consistency for both TO and LO peaks which are more clearly observed in Figure 4.10 (f). The frequency resolution of the setup is 0.6cm-1due to this slight variation in that range is observed. There are no significant shifts hence the deviation in the PL spectrum is due to surface roughness and not strain.

Density Filter Grating Size Aperture Integration

Time(s) Step Size Microscope Objective

D0.3 1800 lines/mm 300µm 5 1µm 100x

Table 4.2 Details of the points mentioned in Fig 4.8(a)

Fig 4.9: Microscopic top-view images of SAG GaAs growth (3750) on patterned Si(100) with laser spot (a) Position of the laser spot for mapping (b) SAG pillar for mapping

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Fig4.10: (a) Maximum intensity plot of TO peak (b) Raman shift plot of TO peak (c) Maximum intensity plot of LO peak (d) Raman shift plot of LO peak (e) Horizontal line scan from where points A to E were chosen to plot the raman spectra (f) Raman spectra of points A to E as indicated in (e)

240 260 280 300 320

GaAs LO

Normalised Intensity

Raman Shift (cm-1)

E D C B A GaAs TO

(a) (b)

(c) (d)

(e) (f)

Maximum Intensity plot of TO peak TO peak shift

LO peak shift Maximum Intensity plot of LO peak

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Patterned Si(111)

The morphology of SAG GaAs on patterned Si(111) substrate was slightly different as discussed earlier. Also, it is observed from Figure 4.11(a) that there is no residual growth of GaAs on the mask in the region where measurements are done. PL mapping was done across a single SAG structure.

(a) (b)

The PL mapping was done following the parameters shown in the table below.

From the PL mapping results, it is observed that the variation in wavelength is between 866- 867nm which corresponds to GaAs. The SAG region is more closely observed using the FWHM plot as the mask region does not show any peak and only the noise is observed. GaAs signal is also observed from regions outside the mask as the laser spot is quite large, resultantly the detector still detects those signal as GaAs signals. However, there is another SAG very close to top right corner as seen in Figure 4.11(a), resulting in signals observed from that particular region. PL scans were plotted (Figure 4.12(d)) at various points that are mentioned in Figure 4.12(a), the detailed information can be observed in Table 4.3. There are shoulders observed around 825nm, which represents that GaAs exist in defects free wurtzite structure.

PL scan at C and E only represents the noise outside the SAG region.

Density Filter Grating Size Aperture Integration

Time(s) Step Size Microscope Objective

D3 300 lines/mm 1160µm 2 0.5µm 100x

Fig 4.11: Microscopic top-view images of SAG GaAs growth (3750) on patterned Si(111) with laser spot (a) Position of the laser spot for mapping (b) SAG pillar for mapping

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Fig 4.12: PL mapping of SAG GaAs growth (3750) on patterned Si(111) substrate (a) Plot of wavelength variation at maximum intensity (b) FWHM variation (c) Maximum intensity plot at 867nm (d) PL scans at various spots shown in

Table 4.3 Details of the points mentioned in fig 4.12(a)

800 820 840 860 880 900

0 200 400 600 800 1000 1200 1400 1600

Intensity (A.U.)

Wavelength(nm) A B

C D E F G H

(a) (b)

(c) (d)

PL map of wavelength variation at maximum intensity PL map of FWHM variaiton

PL map of Maximum Intensity at 867nm

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Raman study was performed on the same SAG where PL mapping was done. A similar profile is observed as seen in PL mapping. The intensity signal from the top right region is due to another SAG that is very close. There was a slight shift of the sample due to the movement of stage during the mapping has resulted in the shifting of SAG in the plots below. The SAG pattern is easily identified using the intensity plot as there is no residual growth in the surrounding of SAG. Raman shift and intensity maps are plotted separately for TO and LO peak. There was a consistency in both TO and LO Raman shifts plot: Figure 4.13(b) and 4.13(d). The TO peak is at 268.8 cm-1 and the LO peak is at 292.7 cm-1 . There is also a shift under 0.6cm-1 but that is just the frequency resolution of the setup. Hence, there are no significant shifts that can correspond to the presence of strain. However, there is a shift in PL scans which could be due to high surface roughness as there is no strain observed on the SAG.

X (µm) (a)

Y m)

X (µm) (b)

Y m)

X (µm) (c)

Y m)

X (µm) (d)

Y m)

Maximum Intensity plot of TO peak

LO peak shift TO peak shift

Maximum Intensity plot of LO peak

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There is a difference in crystalline quality between the SAG of GaAs on planar Si(100) and the planar Si(111) samples. The crystalline quality of SAG of GaAs on planar Si(111) was better than planar Si(100) sample. There were no Raman shifts observed in both samples, which shows that there is no strain developed in the GaAs layer. Also, the slight variation in PL peaks is probably due to the high roughness of the sample.

4.2 GaAsP/GaP/Si (Growth run– 3800)

In this run, GaAsP was grown on four different substrates i.e. planar GaP, planar Si(100), planar Si(111), and textured Si(100). The focus of the study in this project is only on III-V/Si.

Therefore, the optical characterization is done for three samples only. In this run the GaAsP was grown in two step method, firstly the GaP buffer layer was grown at low temperature, and then in the second step overlayer GaAsP growth is done at high temperature. Figure 4.14 shows the top view SEM images of the samples. The morphology in these images suggests a clear presence of grains and high surface roughness.

(a) (b) (c) Fig4.14: SEM top-view images of GaAsP growth (3800) on (a) planar Si(100) and (b) planar Si(111) (c) Textured Si(100)

[19]

Fig4.13: (a) Maximum intensity plot of TO peak (b) Raman shift plot of TO peak (c) Maximum intensity plot of LO peak (d) Raman shift plot of LO peak (e) Horizontal line scan from where points A to E were chosen to plot the raman spectra (f) Raman spectra of points A to E as indicated in (e)

(e) (f)

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4.2.1 Optical characterization of the samples GaAsP/GaP/Planar Si(100)

PL mapping was done on the large area 150x150 µm2. Figure 4.15(a) shows the laser spot position at the center of the mapping. The parameters for the mapping were listed below:

PL mapping was done at the center of the sample and over a large area. The plot of wavelength variation at maximum intensity suggests that the composition of GaAsP was uniform. The composition was calculated using the following equation:

0.19X2 – 1.55X +2.78 = Eg, where X = composition of Arsenic and Eg is the bandgap (4.1) PL spectra from various coordinates are plotted, these coordinates are shown in Figure 4.15(b) and detailed analysis of these coordinates is mentioned in Table 4.4. The uniform arsenic composition was calculated to be 0.83 and hence, the composition of the grown layer is GaAs0.83P0.17. Also, there are two particular spots of interest in Figure 4.15(b), these spots are B and E. As observed, the composition of As at B is the least and the intensity plot shows maximum intensity which indicated the defect density is least at this point. However, FWHM is very high which means that the crystalline quality is low. At point E there is a slightly higher concentration of As i.e. 0.84 however the crystalline quality is not varied much.

Density Filter Grating Size Aperture Integration

Time(s) Step Size Microscope Objective

D3(Max.Intensity) 300 lines/mm 1160µm 1 15µm 50x

X (µm)

Y m)

(a) (b)

PL map of wavelength variation at maximum intensity

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Raman mapping was also done on the same area with the following parameters:

The FWHM plot of the GaAs TO and LO peak (Figures 4.16(a) and (c)) shows uniform crystalline quality, as observed in the PL intensity plot. TO peak is forbidden in (100) plane however, here both TO and LO high intensity peaks are observed. This suggests that the growth might have occurred through facets with orientation along (111) plane. Raman shift plot of the GaAs TO peak in Figure 4.16(b) shows a consistent peak at 268.9 cm-1 and the red spots are under the margin of error with a peak at 269.5 cm-1. A shift towards lower frequency is observed in the GaAs LO peak (Figure 4.16(d)) this is due to the presence of phosphorous[38]. In ternary alloys, the Raman peak (TO or LO) of a compound gets broadened asymmetrically upon the incorporation of other elements[39]. The FWHM calculation of GaAs TO peak suggests asymmetrical curve broadening when compared to GaAs samples from 3750 run. The broadening occurs due to phosphorous incorporation and it suggests the decrease in crystalline quality of GaAs. Figure 4.16(f) shows the scans from the horizontal points at the center of the sample shown in Figure 4.16 (e). A very low intensity GaP TO peak is observed at 367.5 cm-1.

Density Filter Grating Size Aperture Integration

Time(s) Step Size Microscope Objective

D3(Max.Intensity) 1800 lines/mm 1160µm 10 15µm 50x

Fig4.15: PL mapping of GaAsP growth (3800) on planar Si(100) substrate (a) Laser spot position at the center before the mapping (b) Plot of wavelength variation at maximum intensity (c) Maximum intensity plot (d) PL scans at various spots

Table 4.4 Details of the points mentioned in Fig 4.15(b) X (µm)

(c)

Y

(d)

PL map of Maximum Intensity

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Fig4.16: Plots of GaAs TO and LO peaks (a) FWHM plot of TO peak (b) Raman shift plot of TO peak (c) FWHM plot of LO peak (d) Raman shift plot of LO peak (e) Horizontal line scan from where points A to E were chosen to plot the raman spectra (f) Raman spectra of points A to E as indicated in (e)

-60 -40 -20 0 20 40 60

60 40 20 0 -20 -40 -60

B(Y)

A(X)

268.5 268.6 268.8 268.9 269.1 269.2 269.3 269.5

X (µm) (b)

Y (µm)

-60 -40 -20 0 20 40 60

60 40 20 0 -20 -40 -60

B(Y)

A(X)

286.7 287.2 287.6 288.0 288.4 288.8 289.2 289.6 290.0

X (µm) (d)

Y m)

-60 -40 -20 0 20 40 60

60 40 20 0 -20 -40 -60

Y(µm)

X(µm)

6.140 6.460 6.780 7.100 7.420 7.740 8.060 8.380

(a)

(c)

(e) (f)

FWHM plot of TO peak

FWHM plot of LO peak

TO peak shift

LO peak shift

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GaAsP/GaP/Planar Si(111)

PL mapping was done over a large area of 150x150 µm2 at the center of the sample. The parameters set for the mapping are as follows:

Figure 4.17(a) shows the position of the laser spot at the center of the sample. PL wavelength variation plot at maximum intensity shows the uniformity of the GaAsP layer with As composition of 0.82. The composition of As was calculated using equation 4.1. Figure 4.17(d) shows the PL spectra at various coordinates mentioned in Table 4.5 and shown in Figure 4.17(b). Significant composition variation is not observed in spite of mapping on the large area, this indicates a good uniformity in terms of the composition of GaAsP layer.

Density Filter Grating Size Aperture Integration

Time(s) Step Size Microscope Objective

D0.3 300 lines/mm 1160µm 1 20µm 100x

Fig4.17: PL mapping of GaAsP growth (3800) on planar Si(111) substrate (a) Laser spot position at the center before the mapping (b) Plot of wavelength variation at maximum intensity (c) Maximum intensity plot (d) PL scans at various spots

Y m)

X (µm)

Y m)

X (µm) (a)

(b)

(c) (d)

PL map of wavelength variation at maximum intensity

PL map of Maximum Intensity

References

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