Ingrid Moerman

Indium segregation in InGaN quantum-well structures

Direct evidence for In-segregation in InGaN/GaN quantum-well structures is given via highly spatially resolved energy dispersive x-ray analysis performed in a dedicated scanning transmission electron microscope. The In fluctuations become increasingly pronounced in the vicinity of dislocations. The latter assist In diffusion and cause severe Ga/In intermixing.

Improved AlGaN/GaN high electron mobility transistor using AlN interlayers

This work reports on the effects of AlN interlayers embedded into the GaN semi-insulating buffer of AlGaN/GaN high electron mobility transistors, in comparison with standard heterostructures without AlN interlayers. Detailed optical and structural characterization data are presented, along with computer simulation results. The AlN interlayers generate a compressive strain in the GaN topmost layer, which slightly reduces the total polarization field, but most important, it prevents the AlGaN barrier from plastic relaxation. The final result is an enhanced polarization field with respect to standard heterostructures, providing an increased channel carrier density and pinch-off voltage. Electrical characterization confirms the advantages of using AlN interlayers, reaching maximum drain current density and extrinsic transconductance as high as 1.4 A/mm and 266 mS/mm, respectively, for 0.2-μm gate length.

Material optimisation for AlGaN/GaN HFET applications

An optimisation of some growth parameters for the epitaxy of AlGaN-GaN based heterostructure field effect transistors (HFET) at low pressure in a new 3 * 2 MOVPE reactor is presented. Some possible processes for the growth of semi-insulating buffers have been identified and are described. TEM analysis shows that the insulating character is not due to a high density of dislocations, whereas SIMS analysis shows that classical impurity (Si, O and C) concentrations are in the same range as in conductive undoped layers. Further studies are needed to identify the traps responsible for the compensation of the GaN layers. The properties of the two-dimensional electron gas (2DEG) located at the AlGaN-GaN interface can be tuned by modifying the characteristics of the AlGaN layer and of the insulating buffer. The best mobility (1500 cm 2 V -1 s -1 for n∼6 × 10 12 cm 2 ) is obtained when using a thick buffer layer, whereas the sheet carrier density is found to increase with the Al content in the undoped supply layer and reaches 1.1 x 10 13 cm 2 for a composition of 24%.

Mobility collapse in undoped and Si-doped GaN grown by LP-MOVPE

The room temperature carrier mobility in bulk GaN layers is found to improve drastically by up to a factor of 20 once the incorporated silicon is higher than a critical value. A theoretical model taking into account several scattering mechanisms has been developed to account for the temperature dependence of conduction band mobility m(T) and carrier density n(T) deduced from Van der Pauw Hall measurements. For significantly Si-doped layers, both m(T) and n(T) can be quite accurately reproduced. In lightly doped and undoped samples, m(T) cannot be explained just using this model. TEM observation shows the presence of a more diffuse distribution of threading dislocations in the Si-doped material compared to the undoped one. The grain boundaries in this latter case are likely to correspond to high energetic barriers that carriers can overcome only by some tunneling process resulting in very low mobility.

Relationship between classical and quantum lifetimes in AlGaN/GaN heterostructures

Carrier transport in a set of AlGaN/GaN heterostructures from different sources with a range of carrier densities and mobilities has been investigated at low temperature and high magnetic fields. The Shubnikov-de Haas oscillations have been analysed to extract the quantum scattering lifetime, τq, and this is compared with the classical transport lifetime, τt, derived from the low-field mobility. The relationship between these parameters has been observed to depend systematically on the low-field mobility of the samples studied, and indicates that higher-mobility samples suffer less scattering from centres close to the two-dimensional conducting channel.

2DEG Mobility in AlGaN–GaN Structures Grown by LP‐MOVPE

Two-dimensional electron gas (2DEGs) could be tailored through the growth by LP-MOVPE of intentionally undoped AlGaN-GaN heterostructures with 6% 1.4 x 10 13 cm -2 . A two-subband model for 2DEG transport, taking into account phonons and impurity scattering mechanisms is proposed to explain semi-quantitatively the soft decay regime.

Quantifying the smoothing of GaN epilayer growth by in situ laser interferometry

Abstract We present a metric for quantifying the roughening observed during the in situ laser reflectance monitoring of GaN metal organic vapor-phase epitaxy. A roughness factor, F rs , is derived by comparing the absolute reflectance at the turning points (peaks and troughs) in the interferogram, to that which would be expected from consideration of the equations of thin film interference for 2-D epitaxy. This factor is seen to be sensitive to the recrystallisation period prior to epilayer deposition, and ammonia flow during epilayer growth. This is the first report in the literature of a quantitative analysis of the roughening observed during the early stages of gallium nitride growth.

Interpretation of the Temperature‐Dependent Transport Properties of GaN/Sapphire Films Grown by MBE and MOCVD

Electron transport in Si-doped and unintentionally-doped GaN films grown on sapphire by MOCVD and MBE has been analysed assuming that a parallel conducting channel, via an impurity band, is present. No dependence on growth method or dopant type was observed, but other trends were apparent: a) the activation energy for the impurity band fell with increased doping; b) the temperature of the minimum in the Hall carrier density versus temperature curves increased with doping, but did not depend strongly on the absolute value of mobility; c) the ratio of the mobility in the GaN conduction band to that in the impurity band also showed systematic behaviour, possibly arising from structure-related scattering processes. An STM study of the surface characteristics of some of these samples suggests that potential variations associated with particular structural features may be important in influencing the electrical properties.

Multiple parallel conduction paths observed in depth-profiled n-GaN epilayers

We have combined plasma etching with the Hall effect and resistivity measurements between 10 and 300 K to study the depth distribution of conduction in silicon (Si)-doped GaN epitaxial layers grown on sapphire substrates by two different metalorganic chemical vapor deposition processes. Reduction of the epitaxial layer thickness produces a linear decrease of the sheet carrier density with depth in the doped region, whilst in one sample, in the region less than ∼0.3 μm from the interface, the sheet carrier density tends to flatten out to a value of ∼3×1013u2009cm−2. The former is indicative of a uniform dopant distribution in the epitaxial material, and the latter reveals the existence of mobile charge near the interface. These experiments allow the properties of the doped material to be deconvoluted from those of the interface region, and the temperature dependence of these properties indicates the presence of two parallel conduction paths in the doped material: the conduction band and an impurity band. Thus ...

EFFECTS OF CARRIER GAS ON THE PROPERTIES OF INGAN/GAN QUANTUM WELL STRUCTURES GROWN BY MOCVD

In this paper we report on the influence of the carrier gas used during growth on the optical and microstructural properties of InxGa1—xN/GaN multi-quantum well structures (L = 50 A, x = 0.15) grown by MOCVD. The sample grown with hydrogen carrier gas resulted in a luminescence spectrum that consisted of a single line (FWHM = 57 meV) whereas growth of the barrier material using a nitrogen carrier gas results in luminescence spectra with FWHM in the range of 150 to 200 meV. The change in the optical properties correlates with changes in the microstructural properties. The inferior quality of the samples grown using nitrogen carrier gas is thought to be due to the increased density of growth nucleation sites formed during the growth process resulting in increased interface roughness and a progressive increase in the well thickness throughout the structures.