GaN Quantum-well Structures Research Articles

Low-threshold voltage ultraviolet light-emitting diodes based on (Al,Ga)N metal–insulator–semiconductor structures

Al-rich III–nitride-based deep-ultraviolet (UV) (275–320 nm) light-emitting diodes are plagued with a low emission efficiency and high turn-on voltages. We report Al-rich (Al,Ga)N metal–insulator–semiconductor UV light-emitting Schottky diodes with low turn-on voltages of <3 V, which are about half those of typical (Al,Ga)N p–i–n diodes. Our devices use a thin AlN film as the insulator and an n-type Al0.58Ga0.42N film as the semiconductor. To improve the efficiency, we inserted a GaN quantum-well structure between the AlN insulator and the n-type AlxGa1−xN semiconductor. The benefits of the quantum-well structure include the potential to tune the emission wavelength and the capability to confine carriers for more efficient radiative recombination.

Analysis of Internal Quantum Efficiency and Current Injection Efficiency in III-Nitride Light-Emitting Diodes

Current injection efficiency and internal quantum efficiency (IQE) in InGaN quantum well (QW) based light emitting diodes (LEDs) are investigated. The analysis is based on current continuity relation for drift and diffusion carrier transport across the QW-barrier systems. A self-consistent 6-band k ·p method is used to calculate the band structure for InGaN QW structure. Carrier-photon rate equations are utilized to describe radiative and non-radiative recombination in the QW and the barrier regions, carrier transport and capture time, and thermionic emission leading to carrier leakage out of the QW. Our model indicates that the IQE in the conventional 24-A In0.28Ga0.72 N -GaN QW structure reaches its peak at low injection current density and reduces gradually with further increase in current due to the large thermionic carrier leakage. The efficiency droop phenomenon at high current density in III-nitride LEDs is thus consistent with the high-driving-current induced quenching in current injection efficiency predicted by our model. The effects of the monomolecular recombination coefficient, Auger recombination coefficient and GaN hole mobility on the current injection efficiency and IQE are studied. Structures combining InGaN QW with thin larger energy bandgap barriers such as AlxGa1-xN, lattice-matched AlxIn1-xN, and lattice-matched AlxInyGa1-x-y N have been analyzed to improve current injection efficiency and thus minimize droop at high current injection in III-nitride LEDs. Effect of the thickness of the larger energy bandgap barriers (AlGaN, AlInN and AlInGaN) on injection efficiency and IQE are investigated. The use of thin AlGaN barriers shows slight reduction of quenching of the injection efficiency as the current density increases. The use of thin lattice-matched AlInN or AlInGaN barriers shows significant suppression of efficiency-droop in nitride LEDs.

Theory of non-polar and semi-polar nitride semiconductor quantum-well structures

We investigate the electronic and optical properties of non-polar a- and m-planes and semi-polar InGaN–GaN quantum-well structures using the multiband effective-mass theory taking into account the many-body effects. These results are compared with those of (0 0 0 1) oriented c-plane wurtzite InGaN–GaN quantum wells. We derive explicitly the valence-band Hamiltonian and interband optical matrix elements with polarization dependence for an arbitrary crystal polar angle θ and an azimuthal angle φ. The average hole effective masses of the topmost valence band along the wave vector perpendicular to the crystal orientation for the non-polar and semi-polar cases are substantially lower than that of the c-plane case. It is found that the azimuthal angle φ does not change the shape of the band structure but shifts the sub-band position. Thus the transition energy will be dependent on both the polar angle θ and the azimuthal angle φ. It is expected that the optical matrix elements are strongly anisotropic and optical gain would be larger for the non-polar and semi-polar cases because of the vanishing of the internal electric fields.

Current injection efficiency induced efficiency-droop in InGaN quantum well light-emitting diodes

Current injection efficiency and its impact on efficiency-droop in InGaN single quantum well (QW) based light-emitting diodes (LEDs) are investigated. The analysis is based on current continuity relation for drift and diffusion carrier transport across the QW-barrier system. A self-consistent 6-band k · p method is used to calculate the band structure for InGaN QW. The analysis indicates that the internal quantum efficiency in the conventional 24-Å In 0.28Ga 0.72N–GaN QW structure reaches its peak at low injection current density and reduces gradually with further increase in current due to the large carrier thermionic emission. Structures combining 24-Å In 0.28Ga 0.72N QW with 15-Å Al 0.1Ga 0.9N barriers show slight reduction in quenching of the injection efficiency as current density increases. The use of 15-Å Al 0.83In 0.17N barriers shows significant reduction in efficiency-droop (10% reduction of the internal quantum efficiency at current density of 620 A/cm 2). Thus, InGaN QWs employing thin layers of larger bandgap AlInN barriers suppress the efficiency-droop phenomenon significantly.

X-ray diffraction study on an InGaN∕GaN quantum-well structure of prestrained growth

We compare the x-ray diffraction (XRD) results of two InGaN∕GaN quantum-well (QW) structures to observe the effects of prestrained growth by depositing a low-indium QW before the growth of five high-indium QWs. From the results of reciprocal space mapping, we observe the fully strained condition in the QWs of the control sample. However, in the sample of prestrained growth, the average strain is partially relaxed. By using an XRD fitting algorithm for calibrating QW parameters, we obtain reasonable values for the compositions and thicknesses of the QWs in both samples. In particular, by assuming a nonuniform strain relaxation distribution among the five high-indium QWs in the prestrained sample, we obtain reasonable composition variations among the QWs. The high-indium QW closest to the low-indium one is most strain-relaxed and has the highest indium incorporation, leading to the longest-wavelength emission. The observed red shift with increasing electron penetration depth in the cathodo-luminescence spectra of the prestrained sample is consistent with the distributions of calibrated strain relaxation and indium composition. The results of high-resolution transmission electron microscopy and effective band gap calculation also agree with the above conclusions.

Depolarization effects in (112¯2)-oriented InGaN∕GaN quantum well structures

Depolarization effects on electrical and optical properties of (112¯2)-oriented wurtzite (WU) InGaN∕GaN quantum well (QW) were investigated using the multiband effective-mass theory. These results are compared with those of (0001)- and (101¯0)-oriented WU InGaN∕GaN QW structures. The internal field is shown to become zero for (112¯2) crystal orientation near the crystal angle of 56°, irrespective of the In composition in the well. This is because the sum of the piezoelectric and spontaneous polarizations in the barrier is equal to that in the well. The optical gain of the (112¯2)-oriented QW is significantly larger than that of the (0001)-oriented QW. This is caused mainly by the increase of the optical matrix element due to the disappearance of the internal field. Also, the (112¯2)-oriented QW is found to have the optical gain comparable to that of the (101¯0)-oriented QW.

Prestrained effect on the emission properties of InGaN∕GaN quantum-well structures

The authors demonstrate the spectral redshift of the quantum wells (QWs) designated for green emission into the orange range in a light-emitting diode by adding a violet-emitting QW at the bottom in metal-organic chemical vapor deposition. An electroluminescence redshift of 53nm was obtained. The cathodoluminescence spectra indicated that the long-wavelength QWs close to the violet one were strongly influenced by this added QW and mainly emitted the orange photons. Those near the top were less affected. This influence is supposed to originate from the prestrained effect in the barrier layer right above the violet QW. Such a prestrained effect is expected to be more effective when the underlying QW is well shaped and the heterojunction strain is strong, like the case of the violet QW. This effect is weak between the high-indium QWs, in which the formation of indium-rich clusters releases the strain.

Structure and formation mechanism of V defects in multiple InGaN∕GaN quantum well layers

A variety of different transmission electron microscopy techniques, and particularly high-angle annular dark-field scanning transmission electron microscopy, has been used to reveal that V defects or inverted hexagonal pyramid defects in multiple InGaN∕GaN quantum well (QW) layers nucleate on threading dislocations that cross the InGaN QW. The defects have thin walls lying parallel to {101¯1} with the InGaN∕GaN QW structure. A formation mechanism for the V defects is proposed taking into account the growth kinetics of GaN and the segregation of In atoms in the strain field around the cores of the threading dislocations.

Imaging of thickness and compositional fluctuations in InGaN∕GaN quantum wells by scanning capacitance microscopy

We have used scanning capacitance microscopy (SCM) and atomic force microscopy (AFM) to characterize structural and electronic properties of InxGa1−xN∕GaN quantum-well structures at the nanoscale. Macroscopic capacitance-voltage measurements combined with numerical simulations indicate that either electron or hole accumulation in the quantum-well layer can be induced by application of forward or reverse bias, respectively. Under reverse bias conditions (hole accumulation), features corresponding to monolayer fluctuations in InxGa1−xN quantum-well thickness are clearly evident. Under forward bias conditions (electron accumulation), samples exhibiting high luminescence efficiencies are found to contain regions of increased carrier accumulation within the quantum well, which on the basis of spatially resolved spectroscopy, bias-dependent imaging, and numerical simulations are attributed to nanoscale In-rich clusters in the quantum well. No such features are observed in samples exhibiting lower luminescence efficiencies. Together, these studies demonstrate the ability to image, and distinguish, nanoscale variations in subsurface electronic properties arising from either monolayer thickness fluctuations or compositional inhomogeneities in InxGa1−xN∕GaN quantum-well structures.

Effects of interfacial layers in InGaN∕GaN quantum-well structures on their optical and nanostructural properties

We compared the optical properties and material nanostructures between several InGaN∕GaN multiple quantum-well (QW) samples of different interfacial layers. In some of the samples, InN interfacial layers were inserted between the wells and barriers to improve the QW quality and hence the light-emission efficiency. Compared with a widely used barrier-doped QW structure, the insertions of the InN interfacial layers (silicon doped or undoped) do enhance the photon emission efficiencies. Of the two samples with InN interfacial layers, the one with intrinsic InN interfacial layers had the higher photoluminescence (PL) and electroluminescence (EL) efficiencies. Cluster structures are clearly observed in this sample, resulting in strong carrier localization. In this sample, we also observed a temperature-dependent S-shape variation in the PL spectral peak, a strong photoluminescence excitation (PLE) intensity, and a steep PL decay time variation beyond its peak as a function of temperature. On the other hand, both carrier localization and quantum-confined Stark effect (QCSE) were relatively weaker in another sample, which includes silicon-doped InN interfacial layers. The broadening of the InGaN well layers, in one sample, by inserting silicon-doped InGaN interfacial layers led to the sharpest cluster structures and the strongest carrier localization among the four samples. Therefore, in this sample we observed quite high PL and EL efficiencies, increasing EL spectral peak energy with temperature, a strong PLE intensity, and a steep PL decay time variation beyond its peak in temperature dependence. Compared with the aforementioned samples, the widely used QW structure (the reference sample) shows the lowest PL and EL emission efficiencies, the smallest PL and EL emission photon energies, and the generally longest PL decay times. This suggests that the QCSE is the strongest in this sample.

Observation of In concentration variations in InGaN∕GaN quantum-well heterostructures by scanning capacitance microscopy

Scanning capacitance microscopy and spectroscopy have been used to analyze nanoscale variations in electronic properties in In0.15Ga0.85N∕GaN quantum-well structures grown by metalorganic chemical vapor deposition. Scanning capacitance imaging reveals that localized regions within the In0.15Ga0.85N quantum well, up to ∼25nm in radius and present at densities in the range of 109–1010cm−2, exhibit markedly increased electron accumulation relative to surrounding areas. Spatially resolved scanning capacitance spectroscopy combined with numerical simulations indicates that these regions of enhanced electron accumulation are characterized by locally increased In concentration in the quantum well. The presence of these localized In-rich regions is correlated with reported observations of increased luminescence efficiency, presumably due to carrier localization and consequently enhanced radiative recombination, in very similarly grown samples. In addition, these results demonstrate the ability, using a surface characterization technique, to image variations in composition in a subsurface quantum well with nanoscale spatial resolution.

Optical characteristics of hexagonal GaN self-assembled quantum dots: Strong influence of built-in electric field and carrier localization

Optical characteristics of hexagonal GaN self-assembled quantum dots (QDs) were systematically studied by photoluminescence (PL), PL excitation (PLE), time-resolved PL, and cathodoluminescence (CL). We observed a Stokes-like shift between PLE absorption edge and PL emission from the GaN QDs as well as from the Al(Ga)N base layer. With decreasing emission energy, the measured lifetime of the hexagonal GaN QDs emission increased, while that of the cubic GaN QDs kept almost constant. The optical emission from the GaN QDs was measured as a function of temperature from 10 to 300 K, and their properties were compared with GaN quantum-well structures. With increasing temperature, the PL intensity of Al(Ga)N base layer or GaN quantum wells was dramatically decreased, while that of GaN QDs was not changed much. We observed CL images showing strong carrier localization in GaN QDs. Therefore, we conclude that the GaN QD emissions are strongly influenced by built-in electric field as well as by carrier localization in the QDs.