Research

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9 pages, 2461 KiB

Open AccessArticle

Strain Analysis of GaN HEMTs on (111) Silicon with Two Transitional Al_xGa_1−xN Layers

by Yuefei Cai, Chenqi Zhu, Ling Jiu, Yipin Gong, Xiang Yu, Jie Bai, Volkan Esendag and Tao Wang

Materials 2018, 11(10), 1968; https://doi.org/10.3390/ma11101968 - 13 Oct 2018

Cited by 11 | Viewed by 5092

Abstract

We have designed and then grown a simple structure for high electron mobility transistors (HEMTs) on silicon, where as usual two transitional layers of Al_xGa_1−xN (x = 0.35, x = 0.17) have been used in order to engineer the [...] Read more.

We have designed and then grown a simple structure for high electron mobility transistors (HEMTs) on silicon, where as usual two transitional layers of Al_xGa_1−xN (x = 0.35, x = 0.17) have been used in order to engineer the induced strain as a result of the large lattice mismatch and large thermal expansion coefficient difference between GaN and silicon. Detailed x-ray reciprocal space mapping (RSM) measurements have been taken in order to study the strain, along with cross-section scanning electron microscope (SEM) images and x-ray diffraction (XRD) curve measurements. It has been found that it is critical to achieve a crack-free GaN HEMT epi-wafer with high crystal quality by obtaining a high quality AlN buffer, and then tuning the proper thickness and aluminium composition of the two transitional Al_xGa_1−xN layers. Finally, HEMTs with high performance that are fabricated on the epi-wafer have been demonstrated to confirm the success of our strain engineering and above analysis. Full article

(This article belongs to the Special Issue III-Nitrides Semiconductor Research in the UK and Ireland)

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11 pages, 5202 KiB

Open AccessArticle

Characterisation of InGaN by Photoconductive Atomic Force Microscopy

by Thomas F. K. Weatherley, Fabien C.-P. Massabuau, Menno J. Kappers and Rachel A. Oliver

Materials 2018, 11(10), 1794; https://doi.org/10.3390/ma11101794 - 21 Sep 2018

Cited by 5 | Viewed by 4413

Abstract

Nanoscale structure has a large effect on the optoelectronic properties of InGaN, a material vital for energy saving technologies such as light emitting diodes. Photoconductive atomic force microscopy (PC-AFM) provides a new way to investigate this effect. In this study, PC-AFM was used [...] Read more.

Nanoscale structure has a large effect on the optoelectronic properties of InGaN, a material vital for energy saving technologies such as light emitting diodes. Photoconductive atomic force microscopy (PC-AFM) provides a new way to investigate this effect. In this study, PC-AFM was used to characterise four thick (∼130 nm) In

_{x}

Ga

_{1 - x}

N films with x = 5%, 9%, 12%, and 15%. Lower photocurrent was observed on elevated ridges around defects (such as V-pits) in the films with

x \leq 12

%. Current-voltage curve analysis using the PC-AFM setup showed that this was due to a higher turn-on voltage on these ridges compared to surrounding material. To further understand this phenomenon, V-pit cross sections from the 9% and 15% films were characterised using transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. This identified a subsurface indium-deficient region surrounding the V-pit in the lower indium content film, which was not present in the 15% sample. Although this cannot directly explain the impact of ridges on turn-on voltage, it is likely to be related. Overall, the data presented here demonstrate the potential of PC-AFM in the field of III-nitride semiconductors. Full article

(This article belongs to the Special Issue III-Nitrides Semiconductor Research in the UK and Ireland)

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11 pages, 3843 KiB

Open AccessArticle

Effects of a Si-doped InGaN Underlayer on the Optical Properties of InGaN/GaN Quantum Well Structures with Different Numbers of Quantum Wells

by George Christian, Menno Kappers, Fabien Massabuau, Colin Humphreys, Rachel Oliver and Philip Dawson

Materials 2018, 11(9), 1736; https://doi.org/10.3390/ma11091736 - 15 Sep 2018

Cited by 6 | Viewed by 3043

Abstract

In this paper we report on the optical properties of a series of InGaN polar quantum well structures where the number of wells was 1, 3, 5, 7, 10 and 15 and which were grown with the inclusion of an InGaN Si-doped underlayer. [...] Read more.

In this paper we report on the optical properties of a series of InGaN polar quantum well structures where the number of wells was 1, 3, 5, 7, 10 and 15 and which were grown with the inclusion of an InGaN Si-doped underlayer. When the number of quantum wells is low then the room temperature internal quantum efficiency can be dominated by thermionic emission from the wells. This can occur because the radiative recombination rate in InGaN polar quantum wells can be low due to the built-in electric field across the quantum well which allows the thermionic emission process to compete effectively at room temperature limiting the internal quantum efficiency. In the structures that we discuss here, the radiative recombination rate is increased due to the effects of the Si-doped underlayer which reduces the electric field across the quantum wells. This results in the effect of thermionic emission being largely eliminated to such an extent that the internal quantum efficiency at room temperature is independent of the number of quantum wells. Full article

(This article belongs to the Special Issue III-Nitrides Semiconductor Research in the UK and Ireland)

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14 pages, 2678 KiB

Open AccessArticle

Temporal Encoding to Reject Background Signals in a Low Complexity, Photon Counting Communication Link

by Alexander D. Griffiths, Johannes Herrnsdorf, Christopher Lowe, Malcolm Macdonald, Robert Henderson, Michael J. Strain and Martin D. Dawson

Materials 2018, 11(9), 1671; https://doi.org/10.3390/ma11091671 - 09 Sep 2018

Cited by 4 | Viewed by 4024

Abstract

Communicating information at the few photon level typically requires some complexity in the transmitter or receiver in order to operate in the presence of noise. This in turn incurs expense in the necessary spatial volume and power consumption of the system. In this [...] Read more.

Communicating information at the few photon level typically requires some complexity in the transmitter or receiver in order to operate in the presence of noise. This in turn incurs expense in the necessary spatial volume and power consumption of the system. In this work, we present a self-synchronised free-space optical communications system based on simple, compact and low power consumption semiconductor devices. A temporal encoding method, implemented using a gallium nitride micro-LED source and a silicon single photon avalanche photo-detector (SPAD), demonstrates data transmission at rates up to 100 kb/s for 8.25 pW received power, corresponding to 27 photons per bit. Furthermore, the signals can be decoded in the presence of both constant and modulated background noise at levels significantly exceeding the signal power. The system’s low power consumption and modest electronics requirements are demonstrated by employing it as a communications channel between two nano-satellite simulator systems. Full article

(This article belongs to the Special Issue III-Nitrides Semiconductor Research in the UK and Ireland)

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8 pages, 4082 KiB

Open AccessArticle

Porous AlGaN-Based Ultraviolet Distributed Bragg Reflectors

by Peter Griffin, Tongtong Zhu and Rachel Oliver

Materials 2018, 11(9), 1487; https://doi.org/10.3390/ma11091487 - 21 Aug 2018

Cited by 15 | Viewed by 4526

Abstract

Utilising dislocation-related vertical etching channels in gallium nitride, we have previously demonstrated a simple electrochemical etching (ECE) process that can create layered porous GaN structures to form distributed Bragg reflectors for visible light at wafer scale. Here, we apply the same ECE process [...] Read more.

Utilising dislocation-related vertical etching channels in gallium nitride, we have previously demonstrated a simple electrochemical etching (ECE) process that can create layered porous GaN structures to form distributed Bragg reflectors for visible light at wafer scale. Here, we apply the same ECE process to realise AlGaN-based ultraviolet distributed Bragg reflectors (DBRs). These are of interest because they could provide a pathway to non-absorbing UV reflectors to enhance the performance of UV LEDs, which currently have extremely low efficiency. We have demonstrated porous AlGaN-based UV DBRs with a peak reflectance of 89% at 324 nm. The uniformity of these devices is currently low, as the as-grown material has a high density of V-pits and these alter the etching process. However, our results indicate that if the material growth is optimised, the ECE process will be useful for the fabrication of UV reflectors. Full article

(This article belongs to the Special Issue III-Nitrides Semiconductor Research in the UK and Ireland)

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12 pages, 7420 KiB

Open AccessArticle

Hybrid Top-Down/Bottom-Up Fabrication of a Highly Uniform and Organized Faceted AlN Nanorod Scaffold

by Pierre-Marie Coulon, Gunnar Kusch, Philip Fletcher, Pierre Chausse, Robert W. Martin and Philip A. Shields

Materials 2018, 11(7), 1140; https://doi.org/10.3390/ma11071140 - 05 Jul 2018

Cited by 11 | Viewed by 4277

Abstract

As a route to the formation of regular arrays of AlN nanorods, in contrast to other III-V materials, the use of selective area growth via metal organic vapor phase epitaxy (MOVPE) has so far not been successful. Therefore, in this work we report [...] Read more.

As a route to the formation of regular arrays of AlN nanorods, in contrast to other III-V materials, the use of selective area growth via metal organic vapor phase epitaxy (MOVPE) has so far not been successful. Therefore, in this work we report the fabrication of a highly uniform and ordered AlN nanorod scaffold using an alternative hybrid top-down etching and bottom-up regrowth approach. The nanorods are created across a full 2-inch AlN template by combining Displacement Talbot Lithography and lift-off to create a Ni nanodot mask, followed by chlorine-based dry etching. Additional KOH-based wet etching is used to tune the morphology and the diameter of the nanorods. The resulting smooth and straight morphology of the nanorods after the two-step dry-wet etching process is used as a template to recover the AlN facets of the nanorods via MOVPE regrowth. The facet recovery is performed for various growth times to investigate the growth mechanism and the change in morphology of the AlN nanorods. Structural characterization highlights, first, an efficient dislocation filtering resulting from the ~130 nm diameter nanorods achieved after the two-step dry-wet etching process, and second, a dislocation bending induced by the AlN facet regrowth. A strong AlN near band edge emission is observed from the nanorods both before and after regrowth. The achievement of a highly uniform and organized faceted AlN nanorod scaffold having smooth and straight non-polar facets and improved structural and optical quality is a major stepping stone toward the fabrication of deep UV core-shell-based AlN or Al_xGa_1-xN templates. Full article

(This article belongs to the Special Issue III-Nitrides Semiconductor Research in the UK and Ireland)

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9 pages, 1490 KiB

Open AccessArticle

High-Temperature Molecular Beam Epitaxy of Hexagonal Boron Nitride with High Active Nitrogen Fluxes

by Tin S. Cheng, Alex Summerfield, Christopher J. Mellor, Andrei N. Khlobystov, Laurence Eaves, C. Thomas Foxon, Peter H. Beton and Sergei V. Novikov

Materials 2018, 11(7), 1119; https://doi.org/10.3390/ma11071119 - 30 Jun 2018

Cited by 18 | Viewed by 5097

Abstract

Hexagonal boron nitride (hBN) has attracted a great deal of attention as a key component in van der Waals (vdW) heterostructures, and as a wide band gap material for deep-ultraviolet devices. We have recently demonstrated plasma-assisted molecular beam epitaxy (PA-MBE) of hBN layers [...] Read more.

Hexagonal boron nitride (hBN) has attracted a great deal of attention as a key component in van der Waals (vdW) heterostructures, and as a wide band gap material for deep-ultraviolet devices. We have recently demonstrated plasma-assisted molecular beam epitaxy (PA-MBE) of hBN layers on substrates of highly oriented pyrolytic graphite at high substrate temperatures of ~1400 °C. The current paper will present data on the high-temperature PA-MBE growth of hBN layers using a high-efficiency radio-frequency (RF) nitrogen plasma source. Despite more than a three-fold increase in nitrogen flux with this new source, we saw no significant increase in the growth rates of the hBN layers, indicating that the growth rate of hBN layers is controlled by the boron arrival rate. The hBN thickness increases to 90 nm with decrease in the growth temperature to 1080 °C. However, the decrease in the MBE temperature led to a deterioration in the optical properties of the hBN. The optical absorption data indicates that an increase in the active nitrogen flux during the PA-MBE process improves the optical properties of hBN and suppresses defect related optical absorption in the energy range 5.0–5.5 eV. Full article

(This article belongs to the Special Issue III-Nitrides Semiconductor Research in the UK and Ireland)

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Review

Jump to: Research

9 pages, 3547 KiB

Open AccessReview

Hysteretic Photochromic Switching (HPS) in Doubly Doped GaN(Mg):Eu—A Summary of Recent Results

by Paul R. Edwards, Kevin P. O’Donnell, Akhilesh K. Singh, Douglas Cameron, Katharina Lorenz, Mitsuo Yamaga, Jacob H. Leach, Menno J. Kappers and Michal Boćkowski

Materials 2018, 11(10), 1800; https://doi.org/10.3390/ma11101800 - 22 Sep 2018

Cited by 5 | Viewed by 3351

Abstract

Europium is the most-studied and least-well-understood rare earth ion (REI) dopant in GaN. While attempting to increase the efficiency of red GaN light-emitting diodes (LEDs) by implanting Eu⁺ into p-type GaN templates, the Strathclyde University group, in collaboration with IST Lisbon and [...] Read more.

Europium is the most-studied and least-well-understood rare earth ion (REI) dopant in GaN. While attempting to increase the efficiency of red GaN light-emitting diodes (LEDs) by implanting Eu⁺ into p-type GaN templates, the Strathclyde University group, in collaboration with IST Lisbon and Unipress Warsaw, discovered hysteretic photochromic switching (HPS) in the photoluminescence spectrum of doubly doped GaN(Mg):Eu. Our recent work, summarised in this contribution, has used time-, temperature- and light-induced changes in the Eu intra-4f shell emission spectrum to deduce the microscopic nature of the Mg-Eu defects that form in this material. As well as shedding light on the Mg acceptor in GaN, we propose a possible role for these emission centres in quantum information and computing. Full article

(This article belongs to the Special Issue III-Nitrides Semiconductor Research in the UK and Ireland)

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