Piezoelectric Polarization Fields Research Articles

Intense laser field effects on the linear and nonlinear intersubband optical properties in a strained [formula omitted] quantum well

In this work, the linear, the third-order nonlinear and the total optical absorption coefficients and refractive index changes of a strained InGaN/GaN quantum well are investigated numerically. In the effective-mass approximation, the electronic energy levels and the corresponding wave functions are calculated by taking into account the effects of spontaneous and piezoelectric polarization fields on the conduction band edge. Effects of intense laser field, In composition and the well width on the optical properties of the strained quantum well are studied. Results indicate that the laser field as well as the strain induced piezoelectric field considerably affects the confining potential of the quantum well. Results also show that the resonant peaks experience a red-shift (blue-shift) with the increase in the laser field intensity and well width (In composition).

Strain- and surface-induced modification of photoluminescence from self-assembled GaN/Al0.5Ga0.5N quantum dots: strong effect of capping layer and atmospheric condition

We report on the influence of a capping layer on the photoluminescence properties of self-assembled GaN quantum dots grown on an Al0.5Ga0.5N template. Self-assembled GaN quantum dots show a large quantum confined Stark shift and long carrier recombination time due to strong built-in spontaneous and piezoelectric polarization fields. Nevertheless, owing to strong carrier localization and suppressed nonradiative processes, these quantum dots have a high-quantum efficiency even at room temperature. Here, we show that the capping thickness has an important role on the optical properties of the GaN quantum dots. The radiative and nonradiative recombination processes of quantum dots are strongly affected by adjusting the capping thickness, and the GaN quantum dots with 12 monolayers-thick Al0.5Ga0.5N capping layer show a remarkably high internal quantum efficiency of more than 80% at room temperature. We also studied photoluminescence quenching and enhancement for surface (uncapped) quantum dots caused by photoadsorption and photodesorption of oxygen.

Performance improvement of AlGaN-based deep ultraviolet light-emitting diodes by using staggered quantum wells

The optical and physical properties of AlGaN-based deep ultraviolet light-emitting diodes (UV LEDs) with various specific designs of staggered quantum wells (QWs) are numerically investigated. Detailed analysis has been carried out on the light output power, energy band, overlap of electron and hole wavefunctions, carrier concentration, radiative recombination rate, spontaneous radiative spectrum and internal quantum efficiency. The simulated results reveal that the deep UV LEDs with the staggered quantum wells exhibit better performance than their conventional counterpart due to the diminished piezoelectric polarization fields in QWs which can increase the density of electron and hole and the overlap of the electron and hole wavefunctions, and thus enhance the radiative recombination rate.

Effects of spontaneous and piezoelectric polarization fields on the electronic and optical properties in GaN/AlN quantum dots: multimillion-atomsp3d5s*tight-binding simulations

GaN quantum dots QDs grown on AlN substrates are strong candidates for UV and near-infrared applications. The wurtzite crystal symmetry in these dots induces internal fields arising from the following: i crystal atomicity;ii strained active region; iii piezoelectricity; and iv spontaneous polarization pyroelectricity. Accurate modeling of electronic and optical properties of these QDs must capture the interplay of these atomistic and long-range fields and the size quantization on an equal footing. In this work, single-particle electronic structure and interband optical transition rates of a GaN/AlN QD grown along the c-axis are studied using a coupled molecular mechanics-atomistic 20-band sp3d5s* tight-binding VFF-TB framework. To calculate piezoelectricity, a recently reported model that takes into account both the linear and the nonlinear dependence of polarization on the strain tensors has been employed. The simulated GaN/AlN dot is realistically sized containing ~3 million atoms and of hexagonal disk shape having height and base length of 4.1 and 17nm, respectively. It is found that, in contrast to the well-studied InN/GaN systems, the pyroelectric potential in GaN/AlN dot is larger than the piezoelectric counterpart, and the effects of piezoelectric and pyroelectric fields add up. The internal fields result in a large redshift in the electronic states near the Brillouin zone center known as quantum confined Stark effect, pronounced non-degeneracy in the excited states, strongly suppressed optical transition, and anisotropic emission spectra. Copyright © 2014 John Wiley & Sons, Ltd.

A field theory of piezoelectric media containing dislocations

A field theory is proposed to extend the standard piezoelectric framework for linear elastic solids by accounting for the presence and motion of dislocation fields and assessing their impact on the piezoelectric properties. The proposed theory describes the incompatible lattice distortion and residual piezoelectric polarization fields induced by dislocation ensembles, as well as the dynamic evolution of these fields through dislocation motion driven by coupled electro-mechanical loading. It is suggested that (i) dislocation mobility may be enhanced or inhibited by the electric field, depending on the polarity of the latter, (ii) plasticity mediated by dislocation motion allows capturing long-term time-dependent properties of piezoelectric polarization. Due to the continuity of the proposed electro-mechanical framework, the stress/strain and polarization fields are smooth even in the dislocation core regions. The theory is applied to gallium nitride layers for validation. The piezoelectric polarization fields associated with bulk screw/edge dislocations are retrieved and surface potential modulations are predicted. The results are extended to dislocation loops.

Single Nanowire Light-Emitting Diodes Using Uniaxial and Coaxial InGaN/GaN Multiple Quantum Wells Synthesized by Metalorganic Chemical Vapor Deposition

We report the controlled synthesis of InGaN/GaN multiple quantum well (MQW) uniaxial (c-plane) and coaxial (m-plane) nanowire (NW) heterostructures by metalorganic chemical vapor deposition. Two kinds of heterostructure NW light-emitting diodes (LEDs) have been fabricated: (1) 10 pairs of InGaN/GaN MQW layers in the c-plane on the top of n-GaN NWs where Mg-doped p-GaN NW is axially grown (2) p-GaN/10 pairs of InGaN/GaN shell structure were surrounded by n-GaN core. Here, we discuss a comparative analysis based on the m-plane and the c-plane oriented InGaN/GaN MQW NW arrays. High-resolution transmission electron microscopy studies revealed that the barrier and the well structures of MQW were observed to be substantially clear with regular intervals while the interface regions were extremely sharp. The c-plane and m-plane oriented MQW single NW was utilized for the parallel assembly fabrication of the LEDs via a focused ion beam. The polarization induced effects on the c-plane and m-plane oriented MQW NWs were precisely compared via power dependence electroluminescence. The electrical properties of m-plane NWs exhibited superior characteristics than that of c-plane NWs owing to the absence of piezoelectric polarization fields. According to this study, high-quality m-plane coaxial NWs can be utilized for the realization of high-brightness LEDs.

Influence of AlGaN Electron Blocking Layer on Modulation Bandwidth of GaN-Based Light Emitting Diodes

GaN-based LEDs is applied to illumination and visible-light communication. In this study, we research the impact of electron blocking layer (EBL) with different aluminum (Al) contents of 20% and 15% on the modulation bandwidth of high-power LEDs. LEDs with 15% Al in EBL shows higher radiative recombination rate and hole injection efficiency. Moreover, LED with 15% Al in EBL exhibits the modulation bandwidth of 25.5 MHz versus 23.5 MHz for LED with 20% Al at 300 mA. This finding demonstrates the influence of piezoelectric polarization field and low hole injection efficiency caused by EBL on modulation bandwidth of high-power LEDs.

Electrical and structural properties of GaN films and GaN/InGaN light-emitting diodes grown on porous GaN templates fabricated by combined electrochemical and photoelectrochemical etching

Porous GaN templates were prepared by combined electrochemical etching (ECE) and back-side photoelectrochemical etching (PECE), followed by the overgrowth of GaN films and InGaN/GaN multiple quantum well (MQW) light-emitting diode (LED) structures. Structural, luminescent, and electrical properties of the GaN and LED structures were studied and compared with the properties of structures grown under the same conditions on templates not subjected to ECE–PECE treatment. Overgrowth of LED structures on the ECE–PECE templates reduced strain, cracking, and micropits, leading to increased internal quantum efficiency and light extraction efficiency. This luminescence enhancement was observed in overgrown GaN films, but was more pronounced for InGaN/GaN LED structures due to suppression of piezoelectric polarization field in QWs.

Suppressed piezoelectric polarization in single InGaN/GaN heterostructure nanowires

The optical properties of single InGaN/GaN heterostructure nanowires (NWs) with a mean diameter down to 18 nm are investigated. Sharp emission lines originating from the recombination of localized excitons in the InGaN disk layer can be resolved. Excitation-dependent energy shifts, together with spectral diffusions of these emission lines, indicate the presence of a weak quantum confined Stark effect (QCSE) caused by nearby charge fluctuations, rather than the screening of piezoelectric polarizations at the InGaN/GaN interface. The absence of a piezoelectric polarization field is further confirmed by time-resolved photoluminescence measurements. Numerical simulations reveal that the elastic strain relaxation via the NW geometry is marginal and occurs only near the NW sidewalls. Carrier localization preferentially near the periphery of the InGaN disk and alloy intermixing are suggested as possible reasons for the absence of a polarization-induced QCSE in thin InGaN/GaN heterostructure NWs.

Measurement and effects of polarization fields on one-monolayer-thick InN/GaN multiple quantum wells

Polarization fields associated with one-monolayer-thick InN/GaN multiple quantum wells (MQWs) cause shifts of the photoluminescence peak that depend on the GaN barrier layer thickness. Diffraction contrast and aberration-corrected scanning transmission electron microscopy show that the InN QWs are well defined and coherently strained. Mapping of electrostatic potential using off-axis electron holography shows that the electric fields inside the GaN barriers decrease from ∼0.7 to ∼0.2 MV/cm as the barrier layer thickness increases from 5 to 20 nm. Atomistic tight-binding calculations agree closely with experiment, and confirm that changes in optical emission of these III-nitride quantum wells result from changes in the spontaneous and piezoelectric polarization fields in the InN quantum wells and the GaN barrier layers. Overall, this QW system provides the basis for InN-based light-emitting devices operating across a useful band of wavelengths at room temperature.

Enhancing the performance of green GaN-based light-emitting diodes with graded superlattice AlGaN/GaN inserting layer

Green InGaN/GaN multiple quantum wells light-emitting diodes with graded superlattice (GSL) AlGaN/GaN inserting layer are investigated numerically and experimentally. Our simulation results indicate that GSL inserting layer can decrease the effective barrier height of holes by 57 meV, which makes holes more easily inject into the quantum wells. The piezoelectric polarization field near the last barrier is suppressed effectively by introducing of the GSL inserting layer. As a result, the efficiency droop radio is improved from 35.8% to 19.4% at current density of 100 A/cm−2.

Electron mobility limited by surface and interface roughness scattering in AlxGa1−xN/GaN quantum wells

The electron mobility limited by the interface and surface roughness scatterings of the two-dimensional electron gas in AlxGa1−xN/GaN quantum wells is studied. The newly proposed surface roughness scattering in the AlGaN/GaN quantum wells becomes effective when an electric field exists in the AlxGa1−xN barrier. For the AlGaN/GaN potential well, the ground subband energy is governed by the spontaneous and the piezoelectric polarization fields which are determined by the barrier and the well thicknesses. The thickness fluctuation of the AlGaN barrier and the GaN well due to the roughnesses cause the local fluctuation of the ground subband energy, which will reduce the 2DEG mobility.

Analytical modeling and analysis of Al m Ga1−m N/GaN HEMTs employing both field-plate and high-k dielectric stack for high-voltage operation

In this paper, a new high-voltage Al m Ga1Â?m N/GaN HEMT (High Electron Mobility Transistors) with Field-Plate and high-k dielectric stack, Graded two-dimensional electron gas (2DEG) Channel Field-Plate Stack dielectric (GCFPS) HEMTs structure has been reported. The proposed structure has shown enhancements of the performances of the GaN-based HEMTs taking into account the effects of spontaneous and piezoelectric polarization fields. In order to analyze this structure, a 2D analytical model has been developed where the expressions for 2D channel potential and electric field distribution have been derived. It was shown that the GCFPS design exhibits significantly reduction of the electric field peaks along the 2DEG channel. Therefore, the breakdown voltage (BV) is greatly improved in comparison with the standard AlGaN/GaN FP-HEMTs. The developed model is validated by the good agreement with the 2D simulated data.

Investigation of the electroluminescence spectrum shift of InGaN/GaN multiple quantum well light-emitting diodes under direct and pulsed currents

Electroluminescence (EL) spectra of InGaN/GaN multiple quantum well light emitting diodes with different piezoelectric polarization fields were investigated under pulsed and direct currents. We find a positive correlation between the piezoelectric polarization field and the thermally induced red-shift of the EL spectra at high direct currents above 25 A/cm2. Under pulsed current, when thermal effects are negligible, a non-uniform EL spectrum blue-shift rate as a function of injection level is observed and compared with numerical results obtained by both self-consistent and non-self-consistent K·P methods. We conclude that the screening effect is positively related to the piezoelectric polarization field, but the band filling-induced blue-shift is almost independent from the piezoelectric field. The electrostatic fields induced by free carriers in the quantum wells increase rapidly with current but tend to saturate at higher injection where the band filling effect becomes the dominant mechanism for the blue-shift. Finally, at high injection above 30 A/cm2, an increase in Auger recombination and carrier leakage holds the spectral peaks almost constant in position.

Analysis of boundary conditions for the envelope functions of GaN/InGaN(0001) heterostructures

Within the framework of the method of envelope functions, various matching conditions on the GaN/InGaN(0001) heteroboundaries are considered. The conditions of matching the envelope functions at which functions are joined with functions and derivatives are joined with derivatives for states in the center of the flat Brillouin zone are conventionally used. In this case, the envelope functions are symmetric (antisymmetric) about the quantum well center. This should not be the case for the anisotropic nitride heterostructures. The matching conditions based on pseudopotential calculations are suggested. These conditions qualitatively differ from those used conventionally and lead to discontinuities of the envelope functions on the heteroboundaries and removal of the above-mentioned symmetry even disregarding embedded spontaneous and piezoelectric polarization fields. The electron envelope functions appear shifted from the quantum well center in the direction of the [0001] hexagonal axis, and the holes envelope functions are shifted in the opposite direction. Different matching conditions considered in the present work lead to the energy levels in the well that differ from each other by several millielectronvolts. The electron and hole envelope functions important for the optical characteristics differ insignificantly when different matching conditions are employed; however, some of them become nonzero due to removal of the artificial symmetry.

Static and dynamic properties of multi-section InGaN-based laser diodes

We have studied multi-section InGaN multiple-quantum-well (MQW) laser diodes grown on c-plane freestanding GaN substrate consisting of an absorber section (AS) and an amplifier gain section. As a result of the interplay between external bias applied to the AS and the internal piezoelectric and spontaneous polarization fields inherent to c-plane InGaN MQWs, the devices exhibit non-linear non-monotonic variations of the threshold current due to the quantum-confined Stark effect that takes place in the AS MQWs. We report on how this effect tailors the lasing characteristics and lasing dynamics, leading from a steady-state cw lasing regime for an unbiased AS to self-pulsation and Q-switching regimes at high negative absorber bias.

Enhancement of light-emission efficiency of ultraviolet InGaN/GaN multiple quantum well light emitting diode with InGaN underlying layer

Two ultraviolet InGaN/GaN light emitting diodes (LEDs) with and without InGaN underlying layer beneath the multiple quantum wells (MQWs) were grown by metal-organic vapor phase epitaxy. Based on the photoluminescence excitation measurements, it was found that the Stokes shift of the sample with a 10-nm-thick In0.1Ga0.9N underlying layer was about 64 meV, which was smaller than that of the reference sample without InGaN underlying layer, indicating a reduced quantum-confined Stark effect (QCSE) due to the decrease of the piezoelectric polarization field in the MQWs. In addition, by fitting the photon energy dependence of carrier lifetime values, the radiative recombination lifetime of the sample with and without InGaN underlying layer were obtained about 1.22 and 1.58 ns at 10 K, respectively. The shorter carrier lifetime also confirmed that the QCSE in the MQWs was weakened after inserting the InGaN underlying layer. In addition, although the depth of carrier localization in the sample with InGaN underlying layer became smaller, the nonradiative recombination centers (NRCs) inside it decreased, and thus suppressed the nonradiative recombination process significantly according to the electroluminescence measurement results. Compared to the reference sample, the efficiency droop behavior was delayed in the sample with InGaN underlying layer and the droop effect was also effectively alleviated. Therefore, the enhanced light-emission efficiency of ultraviolet InGaN/GaN MQW LEDs could be attributed to the decrease of QCSE and NRCs.

Differential phase contrast 2.0—Opening new “fields” for an established technique

Differential phase contrast microscopy has become known as a high resolution imaging technique for magnetic micro-structures in the past. The method senses the local induction by measuring the deflection of the probe beam after it passes through a specimen area carrying a magnetic field. Little attention has been paid, however, to the fact that this technique is also capable of measuring electric fields. An application of the technique to measure piezoelectric polarization fields inside multi-layered structures such as quantum wells is demonstrated. For this purpose, piezoelectric fields within non-centrosymmetric crystal structures, based on GaN/InGaN/GaN quantum wells, are investigated. It can be shown that the technique is sensitive to these fields and yields detailed information about the field distribution. The specific information and experimental limitations as well as artefacts of the technique will be discussed in detail and first measurements are shown. The main advantages turn out to be high sensitivity for electric fields, combined with a very high resolution, which is limited only by the STEM probe size. Another advantage is the large achievable field of view.

Two-dimensional electron gas mobility limited by barrier and quantum well thickness fluctuations scattering in AlxGa1−xN/GaN multi-quantum wells

We calculate the electron mobility limited by the AlxGa1−xN barrier and the GaN well thickness fluctuations scattering of the two-dimensional electron gas (2DEG) at AlxGa1−xN/GaN multi-quantum wells (MQWs) with a triangle potential well. For this potential well, the ground subband energy is governed by the spontaneous and piezoelectric polarization fields and the fields are determined by the barrier and well thicknesses in undoped AlxGa1−xN/GaN MQWs. Thus, the thickness fluctuations of AlxGa1−xN barrier and GaN well will cause a local fluctuation of the ground subband energy, which will reduce the 2DEG mobility.

Natural band alignments of InN/GaN/AlN nanorod heterojunctions

Valence band alignments of wurtzite III-nitride semiconductor heterojunctions are investigated using cross-sectional scanning photoelectron microscopy and spectroscopy on the nonpolar side-facet of a vertically −c-axis-aligned heterostructure nanorod array. The nonpolar measurement geometry and near fully relaxed lattice structure allow for the determination of “natural” band alignments without the influence of spontaneous and piezoelectric polarization fields. The valence band offsets of InN/GaN, GaN/AlN, and InN/AlN are measured to be 0.8 ± 0.1, 0.6 ± 0.1, and 1.4 ± 0.1 eV, respectively. These results are in good agreement with previous data for heteroepitaxial films and obey the expected transitivity rule.