Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15<x<0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6×1012 to 2×1013 cm−2 in the GaN channel with increasing Al-concentration from x=0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by ...

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Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures

Recent research results pertaining to InN, GaN and AlN are reviewed, focusing on the different growth techniques of Group III-nitride crystals and epitaxial films, heterostructures and devices. The chemical and thermal stability of epitaxial nitride films is discussed in relation to the problems of deposition processes and the advantages for applications in high-power and high-temperature devices. The development of growth methods like metalorganic chemical vapour deposition and plasma-induced molecular beam epitaxy has resulted in remarkable improvements in the structural, optical and electrical properties. New developments in precursor chemistry, plasma-based nitrogen sources, substrates, the growth of nucleation layers and selective growth are covered. Deposition conditions and methods used to grow alloys for optical bandgap and lattice engineering are introduced. The review is concluded with a description of recent Group III-nitride semiconductor devices such as bright blue and white light-emitting diodes, the first blue-emitting laser, high-power transistors, and a discussion of further applications in surface acoustic wave devices and sensors.

https://iopscience.iop.org/article/10.1088/0022-3727/31/20/001/pdf

Growth and applications of Group III-nitrides

This paper presents growth orientation dependence of the piezoelectric polarization of InxGa1−xN and AlyGa1−yN layers lattice matched to GaN. This topic has become relevant with the advent of growing nitride based devices on semipolar planes [A. Chakraborty et al., Jpn. J. Appl. Phys., Part 2 44, L945 (2005)]. The calculations demonstrate that for strained InxGa1−xN and AlyGa1−yN layers lattice matched to GaN, the piezoelectric polarization becomes zero for nonpolar orientations and also at another point ≈45° tilted from the c plane. The zero crossover has only a very small dependence on the In or Al content of the ternary alloy layer. With the addition of spontaneous polarization, the angle at which the total polarization equals zero increases slightly for InxGa1−xN, but the exact value depends on the In content. For AlyGa1−yN mismatched layers the effect of spontaneous polarization becomes important by increasing the crossover point to ∼70° from c-axis oriented films. These calculations were performed u...

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Strain-induced polarization in wurtzite III-nitride semipolar layers

Highly efficient light-emitting diodes (LEDs) emitting ultraviolet (UV), blue, green, amber and red light have been obtained through the use of InGaN active layers instead of GaN active layers. Red LEDs with an emission wavelength of 675 nm, whose emission energy was almost equal to the band-gap energy of InN, were fabricated. The dependence of the emission wavelength of the red LED on the current (blue shift) is dominated by both the band-filling effect of the localized energy states and the screening effect of the piezoelectric field. In the red LEDs, a phase separation of the InGaN layer was clearly observed in the emission spectra, in which blue and red emission peaks appeared. In terms of the temperature dependence of the LEDs, InGaN LEDs are superior to the conventional red and amber LEDs due to a large band offset between the active and cladding layers. The localized energy states caused by In composition fluctuation in the InGaN active layer contribute to the high efficiency of the InGaN-based emitting devices, in spite of the large number of threading dislocations and a large effect of the piezoelectric field. The blue and green InGaN-based LEDs had the highest external quantum efficiencies of 18% and 20% at low currents of 0.6 mA and 0.1 mA, respectively.

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Characteristics of InGaN-Based UV/Blue/Green/Amber/Red Light-Emitting Diodes

We calculated the crystal orientation dependence of piezoelectric fields in wurtzite strained Ga0.9In0.1N/GaN heterostructures. The highest longitudinal piezoelectric field of 0.7 MV/cm can be generated in (0001)-oriented biaxial-strained Ga0.9In0.1N layer coherently grown on GaN. On the contrary, no longitudinal piezoelectric field is induced in strained layers grown along orientations at an off angle of 39° or 90° from (0001). The high symmetry planes with these angles are, for instance, (1124) and (1012) for 39°, and (1120) and (1010) for 90°. We also calculated the crystal orientation dependence of the transition probability in a 3-nm strained Ga0.9In0.1N/GaN quantum well, which indicated that the transition probability with these non-(0001) orientations becomes 2.3 times larger than that with the (0001) orientation. We conclude that high-performance strained nitride-based optical devices can be obtained by control of the crystal orientation.

https://iopscience.iop.org/article/10.1143/JJAP.39.413/pdf

Theoretical Study of Orientation Dependence of Piezoelectric Effects in Wurtzite Strained GaInN/GaN Heterostructures and Quantum Wells

We demonstrate a dramatic reduction of the oscillator strength in ${\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ quantum wells due to piezoelectric fields. Our study using time-resolved photoluminescence spectroscopy reveals a strong increase of the luminescence decay time of the dominating transition with increasing well width by several orders of magnitude in parallel to a redshift of the emission peaks. The experimental results are consistently explained by a quantitative model based on the piezoelectric fields in strained wurtzite quantum wells. We estimate the piezoelectric constant of GaN to ${d}_{31}=\ensuremath{-}0.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm/V.

Reduction of oscillator strength due to piezoelectric fields in G a N / A l x Ga 1 − x N quantum wells

In this contribution, we focus on the consequences of the piezoelectric field, which is an inherent consequence of the commonly used wurtzite phase of GaN, on the optical properties of strained GaN-based quantum well structures. We demonstrate that both in GaN/AlGaN and in GaInN/GaN single quantum well structures, the piezoelectric field leads to a Stark-shift of the fundamental optical transitions, which can lead to luminescence emission far below the bulk bandgap. Due to the spatial separation of the electron and hole wavefunctions in such structures, the oscillator strength of these transitions may become extremely small, many orders of magnitude lower than in the field-free case. From specially designed structures, we can even determine the sign of the piezoelectric field and relate it to the polarity of the layers. Under high-excitation conditions, as found in a laser diode, the piezoelectric field is almost completely screened by the injected carriers. As a consequence, the stimulated emission is significantly blue-shifted compared to the photoluminescence, which has sometimes been confused with localization effects.

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The role of piezoelectric fields in GaN-based quantum wells

We have studied optical transitions in GaInN/GaN single and multiple quantum wells using time-resolved photoluminescence spectroscopy. Our results show that the energy positions of the dominant emission lines strongly depend both on the well width and on the number of wells. In the case of multiple quantum wells, time-resolved measurements clearly distinguish multiple emission lines. These observations are consistently explained by considering the large built-in piezoelectric field in strained GaInN quantum wells. The multiple emission lines are attributed to intra- and interwell transitions between nearest and next-nearest neighbors.

Intra- and interwell transitions in GaInN/GaN multiple quantum wells with built-in piezoelectric fields

The effects of piezoelectric fields on the static and dynamic optical properties of GaInN/GaN and GaN/AIGaN double heterostructures and single quantum wells are studied by time-resolved photoluminescence. We find a strong increase of the luminescence decay time of the dominating transition with well thickness by several orders of magnitude. For well thicknesses larger than about 5 nm, two emission lines with strongly differing decay times are observed, which are attributed to spatially direct and indirect transitions. Our experimental findings are consistently explained by a quantitative model based on the piezoelectric fields in strained wurtzite quantum wells.

Effects of Piezoelectric Fields in GaInN/GaN and GaN/AlGaN Heterostructures and Quantum Wells

We present time-resolved measurements on GaInN/GaN quantum wells (QWs) with varying well widths and GaInN/AlGaN/GaN QWs with asymmetric barriers Our study in GaInN/GaN QWs shows a strong decrease of oscillator strength with increasing well widths in parallel to a red shift of emission peaks, which can be well explained by the piezoelectric field The measurement in the asymmetric structure reveals enhanced oscillator strength with the AlGaN barrier on top of the GaInN QW indicating the better carrier confinement in such structure These results allow us to determine unambiguously the sign of the piezoelectric field, which points towards the substrate in a compressively strained QW

H. Kollmer

Papers

Reduction of oscillator strength due to piezoelectric fields in G a N / A l x Ga 1 − x N quantum wells

The role of piezoelectric fields in GaN-based quantum wells

Intra- and interwell transitions in GaInN/GaN multiple quantum wells with built-in piezoelectric fields

Effects of Piezoelectric Fields in GaInN/GaN and GaN/AlGaN Heterostructures and Quantum Wells

Carrier confinement in GaInN/AlGaN/GaN quantum wells with asymmetric barriers: direction of the piezoelectric field