Screening Of Piezoelectric Field Research Articles

CMOS-Controlled Color-Tunable Smart Display

We demonstrate a color-tunable smart display system based on a micropixelated light-emitting diode <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\mu\hbox{LED})$</tex></formula> array made from one InGaN epitaxial structure with high (0.4) indium mole fraction. When integrated with custom complementary metal–oxide–semiconductor (CMOS) electronics and a CMOS driving board with a field-programmable gate array (FPGA) configuration, this <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu\hbox{LED}$</tex></formula> device is computer controllable via a simple USB interface and is capable of delivering programmable dynamic images with emission colors changeable from red to green by tailoring the current densities applied to the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu\hbox{LED}$</tex></formula> pixels. The color tunability of this CMOS-controlled device is attributed to the competition between the screening of piezo-electric field and the band filling effect. Comparable brightness of the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu\hbox{LED}$</tex></formula> pixels emitting at different colors was achieved by adjusting the duty cycle. Further measurement suggests that this microdisplay system can also be used for high-speed visible light communications.

Carrier capture times in InGaN/GaN multiple quantum wells

AbstractWe have investigated the temperature and carrier density dependence of the carrier capture time of two optically pumped InGaN/GaN multiple quantum well laser structures by subpicosecond time‐resolved differential transmission spectroscopy. We find that the carrier capture time varies significantly with both the temperature and the carrier density. The carrier density dependence is consistent with screening of piezoelectric fields, in agreement with theoretical predictions. The sample with the lower threshold for stimulated emission has a faster carrier capture rate, which highlights the importance of carrier capture for the lasing process. At room temperature the capture time is less than 1.2 ps in both samples. (© 2003 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electrical properties and luminescence spectra of light-emitting diodes based on InGaN/GaN heterostructures with modulation-doped quantum wells

The distribution of charged centers N(w), quantum efficiency, and electroluminescence spectra of blue and green light-emitting diodes (LED) based on InGaN/AlGaN/GaN p-n heterostructures were investigated. Multiple InGaN/GaN quantum wells (QW) were modulation-doped with Si donors in GaN barriers. Acceptor and donor concentrations near the p-n junction were determined by the heterodyne method of dynamic capacitance to be about N A ≥ 1 × 1019 cm−3 ≫ N D ≥ 1 × 1018 cm−3. The N(w) functions exhibited maxima and minima with a period of 11–18 (±2–3 nm) nm. The energy diagram of the structures has been constructed. The shifts of spectral peaks with variation of current (J=10−6–3×10−2 A) are smaller (13–12 meV for blue and 20–50 meV for green LEDs) than the corresponding values for the diodes with undoped barriers (up to 150 meV). This effect is due to the screening of piezoelectric fields in QWs by electrons. The dependence of quantum efficiency on current correlates with the charge distribution and specific features in the current-voltage characteristics.

Spontaneous emission study of (111) InGaAs/GaAs quantum well lasers

Spontaneous emission spectra of (111) and (100) InGaAs/GaAs QW lasers with high In content are measured up to high injection levels and compared with the theoretical results of our own model. Piezoelectric effects are investigated in (111) devices, finding significant contribution of forbidden transitions and a strong blue shift due to carrier screening of piezoelectric field. Band gap renormalization (BGR) in (111) QW lasers is estimated from the measured broadening of the low energy side of the spectrum comparing with model, and corrected by a fundamental band edge. It is also theoretically calculated, including screening effects of the coulomb interaction, in order to explain the estimated results. BGR is found to be higher in (111) samples than in (100) ones, the difference being mainly due to the hole contribution.

Electrical Properties and Luminescence Spectra of Light-Emitting Diodes with Modulated Doped InGaN/GaN Quantum Wells

AbstractCharge distributions N(z) and electroluminescence spectra of blue and green light-emitting diodes (LEDs) based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells. MQWs were modulated doped by Si donors in GaN barriers, electrons from donors being in InGaN wells. N(z) were determined using dynamical capacitance (C-V) method. Acceptor and donor concentrations near the p-n- junction were approximately NA ≥1.1019 cm-3 &gt;&gt; ND ≥ 1.1018 cm-3. Functions N(z) have periodic maxima and minima; their number was 4 and a period of 10 %15 nm, according to the details of growth. The extrema reflect charge distributions in MQWs on the n-side of the junctions with accuracy in z of the order of the Debye length (2-3 nm). An energy diagram of the structures is calculated according these measurements. Shifts of spectral maxima with current (J = 10-6 – 3.10-2 A) for these LEDs are comparatively low (3-12 meV for blue LEDs and 20-50 meV for green ones), much less than for previously studied green LEDs (up to 150 meV). This behavior is explained by screening of piezoelectric fields by electrons in the wells. Quantum efficiency versus current is correlated with N(z) distributions and currentvoltage characteristics of the LEDs.

Piezoelectric-field effect on electronic and optical properties of [111] InxGa1−xAs/GaAs superlattices

We theoretically analyze effects of the piezoelectric field and its screening on the electronic and optical properties including optical matrix element and spontaneous emission for InxGa1−xAs/GaAs superlattices (SLs). For the investigation we take [111] In0.15Ga0.85As/GaAs SL with three types of structures according to well/barrier widths of 40 Å/40 Å, 100 Å/100 Å, and 160 Å/160 Å. The carrier densities are assumed to be equivalent to 5×1016, 1×1017, 5×1017, 1×1018, and 5×1018 cm−3 over 40 Å width in the wells. In the numerical computations, we use the computation model presented recently by [B. W. Kim, J. H. Yoo, and S. H. Kim, ETRI Journal 21, 65 (1999)]. The model self-consistently solves 8×8 (conduction, heavy, light, and spin split-off valence bands) effective-mass Schrödinger equation and the Hartree and exchange-correlation potential equations through the variational procedure. The results show that piezoelectric field causes significant changes in the electronic and optical properties; however, the screening of piezoelectric field by the carriers in the wells seems not very effective for device application.

Time-Resolved Photoluminescence Measurements of In0.15Ga0.85N/In0.015Ga0.985N Quantum Wells with Si-doped Barriers

ABSTRACTA systematic study of photoluminescence (PL) and time-resolved PL spectra from In0.15Ga0.85N/In0.015Ga0.985N quantum wells (QWs) with different Si doping concentration in the barriers has been carried out. As the Si doping concentration increases, the PL emission intensity was increased and the PL peak energy was blueshifted. The energy separation between the spontaneous emission (SPE) and stimulated emission (SE) peaks decrease with increasing Si doping concentration. We also observed that the slow decay time τ2 in the QWs decreases with increasing Si doping concentration, from ∼ 130 ns for [Si] = 2 × 1018 cm−3 to ∼ 30 ns for [Si] = 1 × 1019 cm−3. The PL emission peak shifts to lower energies with delay time after a pulsed excitation and this shift decreases with increasing Si doping concentration. The increased recombination rate, the decrease of peak shift with delay time, and the reduced separation between the SPE and SE peaks with increasing Si doping concentration are attributed to the screening of piezoelectric field by carriers originated from Si doped barriers.

Emission Enhancement of GaN/AlGaN Single-Quantum-Wells Due to Screening of Piezoelectric Field

AbstractPhotoluminescence (PL) enhancement due to the screening of piezoelectric field induced by Si-doping is systematically studied in GaN/AlGaN quantum wells (QWs) fabricated by metal organic vapor-phase-epitaxy (MOVPE). The PL enhancement ratio of QWs for Si-doped directly into the wells was much larger than that for doped only into the barrier layers. This result shows that the crystal quality of the quantum well is not so damaged by heavy Si-doping, which is different from the cases of GaAs or InP material systems. The PL intensity enhancement ratio was especially large for thick wells. The typical value of the enhancement ratio was 30 times for a 5 nm-thick single QW. The optimum Si-doping concentration was approximately 4×1018 cm-3. From the well width dependence of the PL enhancement ratio and PL peak shift under high excitation conditions, we determined that the dominant effect inducing the PL enhancement is screening of piezoelectric field in the QWs. These results indicate that Si-doping is very effective for the application of GaN/AlGaN QWs to optical devices.