T. Martin

Structural and electrical characterization of AuTiAlTi/AlGaN/GaN ohmic contacts

AuTiAlTi/AlGaN/GaN ohmic contact structures rapid thermal annealed at 650, 750, 850, and 950u200a°C have been analyzed using complementary transmission electron microscopy and electrical characterization techniques. The relationship between annealing temperature, interfacial microstructure, and contact resistance is examined. Annealing temperatures of 750u200a°C or higher are required to produce an ohmic contact. Contacts annealed at 750 and 850u200a°C show a planar interface between contact and the AlGaN layer, with minimal consumption of the AlGaN and the formation of a thin TiN interfacial layer. Annealing at 950u200a°C produces the lowest contact resistance, with a structure showing inclusions through the AlGaN/GaN layer. These inclusions are also shown to be a Ti-nitride, having an Al/Au-rich metallurgical barrier layer surrounding them. However, this metallurgical layer does not produce an electrical barrier.

Growth mechanism studies in CBE/MOMBE

Abstract The ultra-high vacuum environment used for chemical beam epitaxy (CBE) and metalorganic molecular beam epitaxy (MOMBE) provides the important capability to perform detailed in-situ studies of the reaction processes involved. Despite the relative immaturity of both the CBE and MOMBE growth techniques, significant advances in this understanding are therefore already being made. The present paper reviews the current knowledge of CBE/MOMBE reaction processes and discusses implications of the relevant experimental observations in relation to the early theoretical models of CBE growth. The authors then propose specific topics which require further investigation if the full potential of CBE and MOMBE is to be exploited.

Structural and electrical characterization of AuPdAlTi ohmic contacts to AlGaN∕GaN with varying Ti content

AuPdAlTi∕AlGaN∕GaN ohmic contact structures with varying Ti:Al ratios have been investigated. The relationship between Ti:Al ratio, interfacial microstructure, and contact resistance is examined. Rapid thermal annealing temperatures of 850°C or higher are required to produce an ohmic contact with annealing at 950°C producing the lowest contact resistance in the majority of samples. Samples annealed at 950°C have been analyzed using complementary transmission electron microscopy and electrical characterization techniques. A thin Ti-nitride region is found to form at the contact/semiconductor interface in all samples.u2003Ti-nitride inclusions through the AlGaN∕GaN layer are also observed, surrounded by an Al∕Au rich metallurgical barrier layer, with the size of the inclusions increasing with Ti content. The size of these inclusions does not have any clear effect on the electrical characteristics of the contacts at room temperature, but samples with fewer inclusions show superior electrical characteristics at h...

Efficiency measurement of GaN-based quantum well and light-emitting diode structures grown on silicon substrates

The optical efficiency of GaN-based multiple quantum well (MQW) and light emitting diode (LED) structures grown on Si(111) substrates by metal-organic vapor phase epitaxy was measured and compared with equivalent structures on sapphire. The crystalline quality of the LED structures was comprehensively characterized using x-ray diffraction, atomic force microscopy, and plan-view transmission electron microscopy. A room temperature photoluminescence (PL) internal quantum efficiency (IQE) as high as 58% has been achieved in an InGaN/GaN MQW on Si, emitting at 460 nm. This is the highest reported PL-IQE of a c-plane GaN-based MQW on Si, and the radiative efficiency of this sample compares well with similar structures grown on sapphire. Processed LED devices on Si also show good electroluminescence (EL) performance, including a forward bias voltage of ∼3.5u2002V at 20 mA and a light output power of 1 mW at 45 mA from a 500×500u2002μm2 planar device without the use of any additional techniques to enhance the output cou...

Modelling of high temperature optical constants and surface roughness evolution during MOVPE growth of GaN using in-situ spectral reflectometry

Abstract An investigation of in-situ optical monitoring during metalorganic vapour phase epitaxy (MOVPE) growth of GaN is reported. We describe the modelling of experimental optical data to determine the important properties of the GaN films such as the high temperature optical constants, the evolving surface roughness during coalescence and the film growth rate. Real time spectral reflectometry has been implemented as a routine tool for quantitative and qualitative monitoring during GaN MOVPE growth across the wavelength range 300–750xa0nm. The technique allows rapid material optimisation for device performance and tracking of run to run variation. Two multilayer optical modelling techniques have been investigated to simulate the time-dependent reflectance signatures. A virtual interface method where the simulation must begin after a smooth surface has been achieved, and an enhanced modelling routine, developed in-house, to include the evolving surface roughness during coalescence. It is shown that the high temperature optical constants are in close agreement with the limited published literature values, and the growth rate is consistent with post-growth thickness measurements. Using atomic force microscopy (AFM) and spectroscopic ellipsometry to study the surfaces of interrupted GaN growth runs, we show for the first time that the surface topography of a GaN film during coalescence contains components of both macroscopic and microscopic roughness with respect to the optical monitoring wavelengths. Additionally, we obtain very good agreement between values of RMS roughness determined by in-situ spectral reflectometry and spectroscopic ellipsometry compared with AFM.

Tri‐isopropyl gallium: A very promising precursor for chemical beam epitaxy

The first reported use of tri‐isopropyl gallium (TiPGa) in chemical beam epitaxy (CBE) is described. Hall measurements performed on the resulting undoped GaAs epitaxial layers indicate an order of magnitude reduction in unintentional carbon impurity levels compared to structures grown under comparable conditions using the standard CBE precursor, triethyl gallium. 2 K photoluminescence spectra match those recorded elsewhere from state‐of‐the‐art high purity GaAs material grown by molecular beam epitaxy, and 77 K Hall measurements on intentionally n‐type doped GaAs layers confirm residual acceptor levels in the low 1014 cm−3 range. The early data obtained already provide a clear indication of the important potential of TiPGa as an improved precursor for the CBE growth of Ga‐containing III–V materials.

Molecular‐beam epitaxy of (100) InSb for CdTe/InSb device applications

Interest in InSb and InSb‐based heterostructures has recently been renewed in view of their important potential applications in infrared, logic, and novel quantum‐well devices. Our work to date has concentrated on the growth of CdTe/InSb multilayer structures in which the properties of the InSb constituent layers have a very significant influence on subsequent device performance. The present paper describes data obtained during a systematic investigation of the growth, using molecular‐beam epitaxy (MBE), of (100) InSb homoepitaxial layers, specifically for CdTe/InSb device applications. Modulated‐beam mass spectrometry experiments have shown that polycrystalline InSb can be used as an MBE source of antimony, and the properties of InSb epilayers grown using either elemental antimony or polycrystalline InSb as the group‐V source are compared. Cross‐sectional transmission electron microscope analysis indicates that very high structural quality layers can be produced and has also identified the mechanisms whi...

Optical investigation of exciton localization in AlxGa1−xN

The optical properties of AlxGa1−xN epilayers with x ranging from 0.08 to 0.52 have been studied by photoluminescence (PL). The temperature dependent PL of the AlxGa1−xN epilayers shows a classical “S-shape” behavior. This behavior is attributed to exciton localization due to compositional fluctuations in the AlxGa1−xN layers. The localization parameter σ extracted from temperature dependent PL, which gives an estimate of degree of localization, is found to increase with Al composition, up to a value of 52meV at the highest Al composition studied. Several phonon replicas are observed at the lower energy side of the main excitonic emission peak in these epilayers at low temperature. In all cases, the Huang-Rhys parameter has been estimated. The Huang-Rhys parameter is found to increase with x indicating that the degree of localization again increases with x. In addition, the Huang-Rhys parameter is found to increase with higher order phonon replicas.

Analysis of thin AlN carrier exclusion layers in AlGaN/GaN microwave heterojunction field-effect transistors

We present a study of the effect of the growth of a thin AlN exclusion layer between the AlGaN barrier layer and GaN buffer layer in microwave heterojunction field-effect transistor structures. A dramatic improvement in carrier drift mobility is observed and we present evidence from electronic structure calculations and capacitance–voltage experiments that this improvement is associated with reduced alloy scattering. However, no significant benefit is seen at low carrier concentrations. Reduced electron trapping in the AlGaN is an additional benefit.

New metalorganic gallium precursors for the growth of GaAs and AlGaAs by CBE

Abstract The use of triethylgallium (TEGa) for the CBE growth of GaAs and AlGaAs leads to very significant reductions in unintentional carbon incorporation compared to corresponding layers grown using trimethylgallium (TMGa). However, in a continuing effort to generate even further reductions in impurity levels, the present paper provides the first reported comparison of the tri-isopropylgallium (TIPGa) and tri-tertiarybutylgallium (TTBGa) precursors for CBE growth applications. The use of TTBGa is found to lead to unacceptably low GaAs growth rates, an effect which is attributed to a steric influence on the chemisorption process. In contrast, the TIPGa-grown GaAs layers exhibit very important improvements in electrical and optical properties compared to corresponding TEGa-grown material. Results of initial AlGaAs growth experiments performed using TIPGa are also presented.