Michael Iza

High quantum efficiency InGaN/GaN solar cells with 2.95 eV band gap

We report on III-nitride photovoltaic cells with external quantum efficiency as high as 63%. InxGa1−xN/GaN p-i-n double heterojunction solar cells are grown by metal-organic chemical vapor deposition on (0001) sapphire substrates with xIn=12%. A reciprocal space map of the epitaxial structure showed that the InGaN was coherently strained to the GaN buffer. The solar cells have a fill factor of 75%, short circuit current density of 4.2 mA/cm2, and open circuit voltage of 1.81 V under concentrated AM0 illumination. It was observed that the external quantum efficiency can be improved by optimizing the top contact grid.

High internal and external quantum efficiency InGaN/GaN solar cells

High internal and external quantum efficiency GaN/InGaN solar cells are demonstrated. The internal quantum efficiency was assessed through the combination of absorption and external quantum efficiency measurements. The measured internal quantum efficiency, as high as 97%, revealed an efficient conversion of absorbed photons into electrons and holes and an efficient transport of these carriers outside the device. Improved light incoupling into the solar cells was achieved by texturing the surface. A peak external quantum efficiency of 72%, a fill factor of 79%, a short-circuit current density of 1.06 mA/cm2, and an open circuit voltage of 1.89 V were achieved under 1 sun air-mass 1.5 global spectrum illumination conditions.

Measurement of electron overflow in 450 nm InGaN light-emitting diode structures

Test structures were developed to experimentally measure the presence of electron overflow in light-emitting diodes (LEDs) under typical bias conditions. These test structures are comprised of a standard LED structure with an extra Mg-doped quantum well inserted on the p-type side of the electron blocking layer. Electrons escaping the active region recombine in the extra quantum well and the corresponding photon emission is observed. No electron overflow was observed at low current densities. At intermediate current densities where efficiency droop occurs, overflow was observed and increased with increasing current density. The onset of electron overflow occurred at slightly lower current densities than the onset of efficiency droop. Auger-assisted overflow, a by-product of the Auger process, is considered in addition to traditional overflow mechanisms.

High brightness violet InGaN/GaN light emitting diodes on semipolar (1011) bulk GaN substrates

We report the fabrication of violet InGaN/GaN light-emitting diodes (LEDs) on semipolar (1011) GaN bulk substrates. The LEDs have a dimension of 300 ?300 ?m2 and are packaged in an epoxy resin. The output power and external quantum efficiency (EQE) at a driving current of 20 mA were 20.58 mW and 33.91% respectively, with peak electroluminescence (EL) emission wavelength at 411 nm. The LEDs show minimal shift in peak EL wavelength with increasing drive current along with a high EQE.

Effect of doping and polarization on carrier collection in InGaN quantum well solar cells

The effect of doping and polarization on carrier collection is investigated for InGaN quantum well solar cells. Energy band diagram simulations of actual devices indicate that spontaneous and piezoelectric polarization sheet charges can inhibit carrier collection unless these charges are screened by sufficient doping. By increasing the doping on both sides of the active region, the polarization-induced barriers to carrier collection were eliminated and the short circuit current density was increased from 0.1 to 1.32 mA/cm2 under 1.5 sun AM1.5G equivalent illumination, leading to devices with an open circuit voltage of 1.9 V and a fill factor of 71%.

Light-polarization characteristics of electroluminescence from InGaN∕GaN light-emitting diodes prepared on (112¯2)-plane GaN

Light polarization and emission spectra from InGaN∕GaN quantum-well light-emitting diodes (LEDs) were investigated. The LEDs were prepared on the (112¯2) plane of wurtzite GaN. Polarization and spectrum measurement was performed at different observation angles with respect to the LED surface. Partially polarized electroluminescence was confirmed at any angle of observation, where the emission intensity tended to be greater when a polarizer was aligned along the c axis of the InGaN∕GaN LED structure. The results clearly indicated the inclination of the c axis relative to the LED surface. As a result, two light polarizations were identified and they were assigned to two different electronic transitions in relation to emission peak energies. Possible alteration of the valence-band structure was suggested due to the induced strain.

2.5λ microcavity InGaN light-emitting diodes fabricated by a selective dry-etch thinning process

The authors report on InGaN microcavity light-emitting diodes with an effective thickness of ∼450nm at the emission wavelength of ∼415nm. The starting material for the flip-chip laser lift-off device is a structure with an active region embedding six InGaN∕GaN quantum wells, ∼60-nm-thick AlGaN, and a GaN template grown on a c-plane sapphire substrate. High-precision control of the final microcavity thickness was facilitated by SF6-based selective inductively coupled plasma etching on the flipped material with an etch rate of ⩾5:1 for GaN:AlxGa1−xN, where x⩾0.15. Pronounced microcavity effects were observed by angular measurements, in agreement with the theoretical cavity-mode dispersion characteristics.

First-moment analysis of polarized light emission from InGaN/GaN light-emitting diodes prepared on semipolar planes

A first-moment analysis has been applied to electroluminescent spectra of polarized light from InGaN/GaN light-emitting diodes prepared on (1011) and (1013) planes. The normalized first moment (the first moment divided by the zeroth moment) of emission spectra clearly fluctuated due to polarizer rotational angle, even though the spectral peak shift was not obvious. A δ-function model of emission spectra, where two δ-functions represented two polarization components of the electroluminescence, was proposed to explain the observed fluctuation. This was reproduced very well by the model, and fitting parameters appeared to be reasonable numerical values.

Electroluminescent measurement of the internal quantum efficiency of light emitting diodes

An experimental method is demonstrated for the determination of internal quantum efficiency (IQE) in III-nitride-based light-emitting diodes (LEDs). LED devices surrounded with an optically absorbing material have been fabricated to limit collected light to photons emitted directly from the quantum wells across a known fraction of the recombination area. The emission pattern for this device configuration was modeled to estimate the extraction efficiency. IQE can then be calculated from the measured input current and output power. This method was applied to c-plane InxGa1−xN-based LEDs emitting at 445 nm. Initial measurements estimate an IQE of 43%±1% at a current density of 7.9 A/cm2.

Photoelectrochemical Properties of Nonpolar and Semipolar GaN

The photoelectrochemical and electrical properties of nonpolar (1120)-oriented and semipolar (1122)-oriented GaN were compared with those of (0001)-oriented GaN. Flatband potentials were obtained in the order of (1120)<(0001)<(1122). The highest photocurrent at a zero bias had been expected for the (1120) sample considering the flatband potential, but the photocurrent of the (1120) sample was the lowest among the three. This could have been due to the electric properties of the (1120) sample used. The surface morphology changes indeed by the photoelectrochemical reactions are also discussed.