852 research outputs found
LEC GaAs for integrated circuit applications
Recent developments in liquid encapsulated Czochralski techniques for the growth of semiinsulating GaAs for integrated circuit applications have resulted in significant improvements in the quality and quantity of GaAs material suitable for device processing. The emergence of high performance GaAs integrated circuit technologies has accelerated the demand for high quality, large diameter semiinsulating GaAs substrates. The new device technologies, including digital integrated circuits, monolithic microwave integrated circuits and charge coupled devices have largely adopted direct ion implantation for the formation of doped layers. Ion implantation lends itself to good uniformity and reproducibility, high yield and low cost; however, this technique also places stringent demands on the quality of the semiinsulating GaAs substrates. Although significant progress was made in developing a viable planar ion implantation technology, the variability and poor quality of GaAs substrates have hindered progress in process development
Ferromagnetism in semiconductors and oxides: prospects from a ten years' perspective
Over the last decade the search for compounds combining the resources of
semiconductors and ferromagnets has evolved into an important field of
materials science. This endeavour has been fuelled by continual demonstrations
of remarkable low-temperature functionalities found for ferromagnetic
structures of (Ga,Mn)As, p-(Cd,Mn)Te, and related compounds as well as by ample
observations of ferromagnetic signatures at high temperatures in a number of
non-metallic systems. In this paper, recent experimental and theoretical
developments are reviewed emphasising that, from the one hand, they disentangle
many controversies and puzzles accumulated over the last decade and, on the
other, offer new research prospects.Comment: review, 13 pages, 8 figures, 109 reference
Optoelectronic applications of heavily doped GaAs and MoSeâ/FePSâ heterostructures
Optoelectronics is quickly becoming a fast emerging technology field. It refers to detect or emit electromagnetic radiation, and convert it into a form that can be read by an integrated measuring device. These devices can be a part of many applications like photodiodes, solar cells, light emitting diode (LED), telecommunications, medical equipment, and more. Due to their different applications, the semiconductor optoelectronic devices can be divided by their operating wavelength and working mechanisms.
In this work, I have focused on semiconductor plasmonic systems operating in the mid-infrared and on the optical detectors made of 2D materials operating in the UV-visible spectral range. Mid-infrared plasmonic devices are very attractive for chemical sensing. Our results show that ultra-doped n-type GaAs is ideal for mid-infrared plasmonics, where the plasmon wavelength is controlled by electron concentration and can be as short as 4 ÎŒm. Ultra-doped n-type GaAs is achieved using ion implantation of chalcogenides like S and Te followed by nonequillibrium thermal annealing, namely ns-range pulsed laser melting or ms-range flash lamp annealing. I have shown that the maximum electron concentration in our GaAs layer can be as high as 7Ă10Âčâč cmâ»Âł, which is a few times higher than that obtained by alternative techniques. In addition to plasmonic applications, the ultra-doped n-type GaAs shows negative magnetoresistance, making GaAs potential material for quantum devices and spintronic applications.
UV-visible optical detectors are made of 2D materials based on van der Waals heterostructures, i.e. transition metal dichalcogenides (TMDCs) e.g. MoSeâ and transition metal chalcogenophosphates (TMCPs) with a general formula MPXâ where M=Fe, Ni, Mn and X=S, Se, Te. The external quantum efficiency of a self-driven broadband photodetector made of a few layers of MoSeâ/FePSâ van der Waals heterojunctions is as high as 12 % at 532 nm. Moreover, it is shown that multilayer MoSeâ on FePSâ forms a type-II band alignment, while monolayer MoSeâ on FePSâ forms a type-I heterojunction. Due to the type-I band alignment, the PL emission from the monolayer MoSeâ is strongly enhanced
Bismuth Surfactant Effects for GaAsN and Beryllium Doping of GaAsN Grown by Molecular Beam Epitaxy
Bi was investigated as a possible surfactant for growth of GaAs1-xNx layers on (100) GaAs substrates by molecular beam epitaxy using an RF plasma nitrogen source. Bi extends the useable growth conditions producing smoother surfaces to a significantly higher N content than without Bi. The conductivity of Be-doped GaAsN decreased significantly with increasing N concentration. Temperature dependent Hall measurement suggests possible compensation and increased activation energy. SIMS and Raman measurements indicate that the N composition increased by introducing Be, and for low [N], Bi. The addition of Bi during growth of Be-doped GaAsN only produced semi-insulating layers.
GaAs1-xNx layers and quantum dot-like structures were grown on (100) GaAs substrates by molecular beam epitaxy. The dependence of photoluminescence emission spectra on annealing temperature is consistent with literature at lower temperatures but after annealing at 750 ÂșC a net red-shift is consistently observed. X-ray photoelectron spectroscopy measurements indicate that for different annealing times and temperatures, the nitrogen and arsenic surface concentrations changed compared to that of as-grown samples, specifically arsenic is lost from the material. Raman measurements are consistent with the trends in photoluminescence and also suggest the loss of arsenic occurs at higher annealing temperatures in both samples capped with GaAs and uncapped samples.
The real substrate temperature preliminarily estimated by pyrometer measurement was further verified and determined by RHEED pattern transition. RHEED was also employed to observe the surface reconstruction. To optimize growth conditions, surface phase diagrams of As4/Ga BEP flux vs. the real substrate temperature under fixed As4 BEP ~4.5x10-6 Torr and fixed growth rate 0.46 ÎŒm/hr (0.45ML/s) were obtained.
Different aperture plates of RF-plasma nitrogen discharge tube were used. Only the one with 10 x Ă0.2mm holes is able to produce streaky RHEED patterns under some growth circumstances, and was eventually selected to lead nitrogen species through for all growths in this work. Ga flux, N flow rate, and RF power were all found to be critical factors affecting the resultant N concentrations
The electrical properties of 60 keV zinc ions implanted into semi-insulating gallium arsenide
The electrical behavior of zinc ions implanted into chromium-doped semiinsulating gallium arsenide was investigated by measurements of the sheet resistivity and Hall effect. Room temperature implantations were performed using fluence values from 10 to the 12th to 10 to the 15th power/sq cm at 60 keV. The samples were annealed for 30 minutes in a nitrogen atmosphere up to 800 C in steps of 200 C and the effect of this annealing on the Hall effect and sheet resistivity was studied at room temperature using the Van der Pauw technique. The temperature dependence of sheet resistivity and mobility was measured from liquid nitrogen temperature to room temperature. Finally, a measurement of the implanted profile was obtained using a layer removal technique combined with the Hall effect and sheet resistivity measurements
Electrical Properties of Ion Implanted Layers in Silicon and Gallium Arsenide
Part I
With the advent of ion implantation, it has become possible to introduce many new dopant species into silicon. The electrical behavior of implanted species displaying deep energy levels was investigated in this work. Hall effect and sheet resistivity measurements were taken as a function of temperature to determine the carrier concentration, mobility, compensation, and impurity ionization energy in the implanted layers. However, since these electrical parameters varied with depth in the samples, conventional Hall effect methods were inadequate. Special differential Hall techniques were developed to characterize the inhomogeneous samples.
The validity of this differential technique was demonstrated by investigating the doping effects of indium in silicon. Differential measurements were first made on samples shallow diffused with indium. Then the results were compared with bulk values that had been obtained in a uniformly doped sample by standard methods. In addition, studies were made on indium implanted silicon to determine the influence of radiation effects. In all three cases an indium acceptor level of 160 meV was observed. Mobility plots versus temperature were also consistent with bulk measurements. However, significant compensation effects were noticed in the implanted layers.
With the analysis technique experimentally confirmed, the electrical behavior of tellurium implanted silicon was investigated. Samples were implanted with several doses to study the electrical activity as a function of impurity concentration. Isothermal anneal cycles were performed to determine the anneal temperature necessary to attain peak electrical activity. After anneal, differential Hall measurements were made from 100° to 278°K to characterize the implanted layers. Tellurium was found to behave as a donor with an energy level of 140 meV in ion implanted silicon. For room temperature e1ectron densities above 1017 carriers/cm3, the ionization energy was observed to decrease. In conjunction with this decrease, the doping efficiency of ion implanted tellurium was also observed to decrease for concentrations in excess of 1017/cm3. Both of these effects were attributed to the influence of energy level broadening.
Part II
Ion implantation was investigated as a doping process for the fabrication of submicron n-type layers in GaAs. Tellurium implantation was performed as a function of dose (3 x 1013 - 1 x 1015 Te/cm2) and substrate temperature (23°C - 350°C). After implantation, a protective dielectric coating was sputtered on the samples to prevent the GaAs from disassociating during the anneal. The protective qualities of three dielectrics (SiO2, Si3N4, AlN) were compared. Anneal temperatures ranged from 750°C to 950°C. The residual radiation damage and defects in the implanted layers were studied by photoluminescence and Rutherford backscatteringmeasurements. The electrical characteristics were analyzed by Schottky barrier capacitance-voltage and Hall effect measurements. Sequential Hall measurements in conjunction with layer removal were used to determine the carrier concentration and mobility profiles in the implanted layers. In addition, junction capacitance-voltae and current-voltage measurements were performed to evaluate the quality of implanted diodes.
The samples implanted at room temperature and subsequently annealed with a SiO2 protective coating displayed almost no electrical activity and had intrinsic regions extending several microns into the GaAs. In contrast, high electrical activity was observed in samples implanted at elevated temperatures followed by anneal with a Si3N4 coating. A doping efficiency of 50% was achieved with a carrier density approaching the maximum attainable in tellurium doped GaAs (7 x 1018 electrons/cm3). However, the electrical activity varied over a wide range for samples with identical implant conditions. This scatter in the electrical measurements was attributed to the poor adherence of the Si3N4 layers to the GaAs surface during the anneal.
The maximum electrical activity achieved using an AlN encapsulent was comparable to the value attained using a Si3N4 coating. However, the electrical activity was consistently high for the AlN protected samples and the AlN displayed better adherence to the GaAs during anneal than Si3N4.</p
Electrical Activation Studies of Ion Implanted Gallium Nitride
A comprehensive and systematic electrical activation study of Si-implanted GaN was performed as a function of ion implantation dose, anneal temperature, and implantation temperature. Additionally, Mg-implanted GaN was also investigated. Temperature-dependent Hall effect measurements and photoluminescence (PL) spectra were used to characterize the samples. GaN wafers capped with AlN were implanted with Si ions at doses ranging from 1x1013 to 5x1015 cm-2 and annealed from 1050 to 1350 °C. The optimum anneal temperature for samples implanted with the higher Si doses is around 1350 °C, exhibiting nearly 100% electrical activation efficiency. Exceptional mobilities and carrier concentrations were obtained on all Si-implanted samples. PL spectra revealed nearly complete implantation damage recovery as well as the nature of the yellow luminescence plaguing nearly all Si-doped GaN. Additionally, GaN wafers were implanted with Mg and various coimplants and annealed from 1100 to 1350 °C. All of the Mg-implanted and most of the Mg-coimplanted GaN samples became extremely resistive, and did not show definite p-type conductivity even after annealing at 1350 °C, remaining highly resistive even at a sample temperature as high as 800 K. A dominant 2.36 eV green luminescence band observed in the PL spectra of all Mg-implanted samples is attributed to a Mg-related deep complex DAP transition. The inefficient electrical activation of Mg acceptors implanted into GaN is attributed to these Mg-related deep complexes
Potential and challenges of compound semiconductor characterization by application of non-contacting characterization techniques
Trotz der im Vergleich zu Silizium ĂŒberragenden elektronischen Eigenschaften von Verbindungshalbleitern, ist die Leistung der daraus gefertigten elektrischen Bauelemente aufgrund der vorhandenen, die elektronischen Materialeigenschaften beeinflussenden Defekte nach wie vor begrenzt. Die vorliegende Arbeit trĂ€gt dazu bei, das bestehende ökonomische Interesse an einem besseren VerstĂ€ndnis der die Bauelementeleistung limitierenden Defekte zu befriedigen, indem sie die Auswirkungen dieser Defekte auf die elektronischen und optischen Materialeigenschaften von Indiumphosphid (InP) und Siliziumkarbid (SiC) aufzeigt. Zur KlĂ€rung der Effekte finden in der Arbeit sich ergĂ€nzende elektrische und optische Charakterisierungsmethoden Anwendung, von denen die meisten kontaktlos und zerstörungsfrei arbeiten und sich daher prinzipiell auch fĂŒr Routineanalysen eignen. Die erzielten Ergebnisse bestĂ€tigen und ergĂ€nzen Literaturdaten zum Defektinventar in InP und SiC nutzbringend. So wird insbesondere das Potential der elektrischen Charakterisierung mittels MDP und MD-PICTS, welche in der Arbeit erstmals fĂŒr die Defektcharakterisierung von InP und SiC eingesetzt wurden, nachgewiesen. Die experimentellen Studien werden dabei bedarfsorientiert durch eine theoretische Betrachtung des entsprechenden Signalentstehungsmechanismuses ergĂ€nzt.:1 Motivation
2 Theses
3 Compound semiconductors: structure and benefits
4 Growth of compound semiconductors
5 Structural defects in compound semiconductors
6 Defects and their impact on electronic material properties
7 Effect of annealing treatments on the properties of InP
8 Experimental details
9 Experimental results
10 Summary of the thesis
11 Conclusion and impact
12 Prospect of future work
13 Appendix - Theory of signal development
14 List of tables
15 List of figures
16 List of abbreviations and symbols
17 Eidesstattliche ErklÀrung - Declaration of academic honesty
18 Danksagung - Acknowledgment
19 Veröffetnlichungen - Publications
20 ReferencesAlthough the electronic properties of compound semiconductors exceed those of Silicon, the performance of respective electronic devices still is limited. This is due to the presence of various growth-induced defects in compound semiconductors. In order to satisfy the economic demand of an improved insight into limiting defects this thesis contributes to a better understanding of material inherent defects in commonly used Indium Phosphide (InP) and Silicon Carbide (SiC) by revealing their effects on electronic and optical material properties. On that account various complementary electrical and optical characterization techniques have been applied to both materials. Most of these techniques are non-contacting and non-destructive. So, in principle they are qualified for routine application. Characterization results that are obtained with these techniques are shown to either confirm published results concerning defects in InP and SiC or beneficially complement them. Thus, in particular the potential of electrical characterization by MDP and MD-PICTS measurements is proofed. Both techniques have been applied for the first time for defect characterization of InP and SiC during these studies. The respective experiments are complemented by a theoretical consideration of the corresponding signal development mechanism in order to develop an explanation approach for occasionally occurring experimental imperfection also arising during silicon characterization from time to time.:1 Motivation
2 Theses
3 Compound semiconductors: structure and benefits
4 Growth of compound semiconductors
5 Structural defects in compound semiconductors
6 Defects and their impact on electronic material properties
7 Effect of annealing treatments on the properties of InP
8 Experimental details
9 Experimental results
10 Summary of the thesis
11 Conclusion and impact
12 Prospect of future work
13 Appendix - Theory of signal development
14 List of tables
15 List of figures
16 List of abbreviations and symbols
17 Eidesstattliche ErklÀrung - Declaration of academic honesty
18 Danksagung - Acknowledgment
19 Veröffetnlichungen - Publications
20 Reference
InGaAs/GaAs Quantum Dot Solar Cells by Metal Organic Chemical Vapour Deposition
Along with the ongoing research and industry development to reduce the cost of conventional PV devices such as Si-based solar cells, significant research efforts have been focused on exploring new concepts and approaches for high efficiency III-V compound semiconductor solar cells, especially through the fast emerging nanotechnology to exploit the unique properties of nanostructures such as self-assembled quantum dots (QDs).
By incorporating self-assembled QDs into the intrinsic region of a standard p-i-n solar cell structure during the epitaxial growth, photons in the solar spectrum with energy lower than the energy gap of the host material can be absorbed by the QD layers, leading to an extended photoresponse to longer wavelengths and hence larger photocurrent. In addition, the size and composition of the QDs can be varied and thereby allowing the bandgap to be tuned for absorption in different regime of the solar spectrum. However, due to the small QD absorption cross section, the increase of photocurrent in QDSCs is not significant and always accompanied with some reduction in other device characteristics such as the open circuit voltage and fill factor.
In this thesis, self-assembled In0.5Ga0.5As/GaAs QDSCs have been designed, fabricated, characterized and investigated in comparison with conventional GaAs p-i-n solar cells. The properties and fundamental mechanisms behind their complicated photoelectrical behaviours were analysed and understood. Several approaches were proposed and carried out to improve the device performance of QDSCs, either during the epitaxial growth process or after the growth and fabrication of the solar cells.
Stacking more QD layers is supposed to enhance the total volume of QD material and hence the light absorption. We carried out experiments to grow QDSC structures with increased number of QD layers. However, much reduced photocurrent and conversion efficiency for 15 and 20-layer samples were observed, which could be due to low carrier extraction efficiency and strain-induced defects. In order to improve the carrier extraction efficiency and consequently more enhanced photocurrent, modulation doping has been introduced into QDs layers to partially populate the confined states with electrons. The modulation doping has been found to be effective to improve carrier transport and collection efficiency, leading to an enhancement of the external quantum efficiency over the whole solar cell response range and thus the conversion efficiency.
We have also taken two different post-growth approaches to improve the QDSC efficiency, namely the rapid thermal annealing and surface plasmonic light trapping. Firstly, QDSCs with different layers were annealed at various temperatures between 700 and 850 °C with the device annealed at the highest temperature of 850 °C displayed the highest efficiency increase of 41.42 % from 10.26 % to 14.51 %, compared to the as-grown sample. Secondly, it was found that a combination of 120 nm diameter hemispherical Ag nanoparticle and a 5 nm thick TiO2 dielectric film pre-deposited on the back of the GaAs substrate was the optimum light trapping configuration for our QDSC. The QDSC spectral response was improved by 35.7% over the 900 nm- 1200 nm wavelength range, leading to enhancements in both Jsc and Voc and an overall efficiency enhancement of 7.6 % compared to the reference QD solar cell
Compensation and Characterization of Gallium Arsenide
The properties of transition metals in gallium arsenide have been previously investigated extensively with respect to activation energies, but little effort has been made to correlate processing parameters with electronic characteristics. Diffusion of copper in gallium arsenide is of technological importance due to the development of GaAs:Cu bistable photoconductive devices. Several techniques are demonstrated in this work to develop and characterize compensated gallium arsenide wafers. The material is created by the thermal diffusion of copper into silicon-doped GaAs. Transition metals generally form deep and shallow acceptors in GaAs, and therefore compensation is possible by material processing such that the shallow silicon donors are compensated by deep acceptors. Copper is an example of a transition metal that forms deep acceptors in GaAs, and therefore this work will focus on the compensation and characterization of GaAs:Si:Cu.
The compensation of the material has shown that the lower diffusion temperatures (500-600°C) form primarily the well-known CuB centers whereas the higher temperature anneals (\u3e$750°C) result in the formation of CuA. Using compensation curves, the copper density is found by comparing the compensation temperature with copper solubility curves given by others. These curves also show that the formation of CuB, EL2, and CuA can be manipulated by varying processing parameters such as annealing temperature and arsenic pressure. The compensation results are confirmed using Temperature-Dependent Hall (TDH) measurements to detect the copper levels. Also, the photoconductive properties of the material under illumination from 1.06 and 2.1 Όm wavelength laser pulses have been used to demonstrate the effects of the different processing procedures. The persistent photoconductivity inherent to these devices under illumination from the 1.06 Όm laser pulse is used to predict the concentration of the CuB level, and the fast hole capture times of the various acceptors are found through the response to a 140 ps (FWHM), 2.1 Όm laser pulse. Finally, the physical distribution of the copper atoms in the gallium arsenide wafer is examined using Glow Discharge and Secondary Ion Mass Spectroscopy (GDMS and SIMS). These techniques have been used to show that the copper diffusion in gallium arsenide is non-uniform with respect to depth and surface position
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