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Short-circuit capability is essential for the adoption of GaN power devices in motor drives for industrial and automotive applications. In this work, we report an innovative solution for GaN power switches to achieve short-circuit withstanding time (SCWT) equal to or greater than 3 micro-seconds with limited increase in on-resistance. We discuss the technology, referred to as Short-Circuit Current Limiter (SCCL) and show the experimental results including static and dynamic Ron, 400-V short-circuit, inductive switching, off-state leakage and 1000-hour high-temperature reverse-bias stress. Thanks to extended SCWT, the SCCL technology allows the industry to adopt conventional short-circuit protection schemes, with sufficient immunity to noise and switching transients.

An N-polar III-N transistor includes a III-N buffer layer, a first III-N barrier layer, and a III-N channel layer, the III-N channel layer having a gate region and a plurality of access regions on opposite sides of the gate region. The compositional difference between the first III-N barrier layer and the III-N channel layer causes a conductive channel to be induced in the access regions of the III-N channel layer. The transistor also includes a source, a gate, a drain, and a second III-N barrier layer between the gate and the III-N channel layer. The second III-N barrier layer has an N-face proximal to the gate and a group-III face opposite the N-face, and has a larger bandgap than the III-N channel layer. The lattice constant of the first III-N barrier layer is within 0.5% of the lattice constant of the buffer layer.

We report enhanced performance of InGaN solar cells grown by metalorganic chemical vapor deposition through optimization of the annealing of the epitaxial wafer before device fabrication. We varied the annealing environment gas mixtures as well as temperatures to obtain the optimized annealing condition. It was found that the major improvement of the nitride solar cell efficiency after annealing is in the increase of the Voc. In addition, annealing at the reasonably moderate temperature of 550°C in O2 environment results in the highest-efficiency InGaN solar cell devices compared with devices annealed at different temperatures and in different gas environments.

GaN nanopillar and nanostripe arrays with embedded InGaN/GaN multi quantum wells (MQWs) were fabricated from planar wafers with different quantum well widths using holographic lithography and subsequent reactive ion etching. Although the etching process led to a reduction in the MQW related luminescence, the etch related damage was successfully healed through annealing in NH3/N2 mixtures under optimized conditions, and the annealed nanopatterned samples exhibited enhanced photoluminescence (PL) compared to the planar wafers. Angular-resolved PL measurements revealed extraction of guided modes from the nanopillar and nanostripe arrays (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

N-polar (000) and Ga-polar (0001) GaN templates were patterned with holographic lithography and etched to form nanopillars (NP) and nanostripes (NS) arrays, upon which InGaN and GaN was subsequently regrown by metal organic chemical vapor deposition (MOCVD). A wide range of growth conditions were explored to determine the structural impact of the growth conditions. The results were evaluated by scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was observed that the initial polarity of the GaN template used to form the pillar or stripe nanostructures had a significant impact upon the results of the regrowth. The Ga-polar NP exhibited (0001), {010}, and {011} facets, while the N-polar NP exhibited (000) and {010} facets. The NS samples also exhibited different facet formation depending upon the initial polarity. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

N-polar (000-1) GaN templates were patterned using holographic lithography to create nanopillar (NP) and nanostripe (NS) arrays. InGaN and GaN was subsequently regrown on the patterned wafers by metalorganic chemical vapor deposition (MOCVD). The impact of changes in V/III ratio and growth temperature were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL). Highly selective growth of InGaN and GaN on planes close to the {10-10} m-planes was observed over a wide range of growth conditions as well as different facet formation compared to regrowth on Ga-polar (0001) NP and NS arrays.

In this work we demonstrate a light emitting diode (LED) with m -plane quantum wells fabricated on a (000) template. N-polar, n-type GaN was grown by MOCVD on vicinal sapphire substrates. Stripes, measuring 500 nm wide, 500 nm tall and spaced 2 μm apart, were etched parallel to the 〈110〉 direction leading to sidewalls that are approximately {100}. Sputtered AlN was used as a regrowth mask on the c -plane surfaces. An active region consisting of 5 InGaN quantum wells and GaN barriers followed by p-type was grown. The regrowth occurred mostly on the exposed m -plane sidewalls, leading to lateral growth in the 〈100〉 direction. The LED was processed using conventional methods. A thick metal contact was used to connect the p-regions together. Current vs. voltage measurements showed good rectifying behavior with a turn on of about 6 volts. On-wafer electroluminescence measurements revealed a peak wavelength of 422 nm. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature-dependent carrier transport is investigated using ultrafast spectroscopy in a p-GaN/i-InGaN/n-GaN solar cell with heavily-doped layers to compensate for polarization charges at the hetero-interface. We observe a flip in the transport direction at 110 K.

Efficiencies exceeding 40% have already been achieved with GaAs-based multijunction (MJ) solar cells. In this talk, we will discuss the unique advantages and challenges of fabricating hybrid InGaN-GaAs MJ cells for ultrahigh efficiency device designs.

We report here enhanced solar energy harvesting using a hybrid solar cell with silicon solar cells (visible-infrared light) on bottom and an InGaN solar cell (UV light) on top. The InGaN solar cell with 30 QW periods has peak external quantum efficiency (EQE) of 40 % at 380 nm, an open circuit voltage (V oc ) of 2.0 V, a short circuit current (I sc ) of 0.8 mA/cm2, and fill factor of 55%. We have demonstrated that the application of an InGaN “active window” to a silicon solar cell counterbalances the encapsulation power loss typically suffered during production of a solar panel

III-nitrides have recently been demonstrated as potential photovoltaic device material particularly in the high-energy portion of the solar spectrum [1–2]. The large lattice mismatch between InN and GaN however, makes it difficult to grow good quality high In-composition InGaN films for low bandgap subcells. The integration of III-N based solar cells, which have currently been demonstrated to work well above 2.0 eV, with mature IV and III–V based solar cell technologies, which work well at bandgaps ≤ 2.0 eV, has the potential to improve the efficiency of current multi-junction solar cells. In this paper, we present the first on-wafer integration of InGaN/GaN solar cells with non-III-nitride (GaAs) solar cells.

In this work we report on the optimization of the p-GaN window layer for InGaN/GaN solar cells. We studied the effect of p-GaN thickness and growth temperature on the electrical performance. By optimizing the window thickness of In x Ga 1−x N solar cells with X In ≈0.04 we maximized short wavelength response and produced solar cells with 82% FF and Voc of 2 V and enhancement of Jsc of 80% over un-optimized devices. We also studied the effect of growth temperature of the window layer, and found that the electrical performance was greatly improved with higher growth temperatures. By increasing the p-GaN growth temperature from 890 °C to 1040 °C, reverse bias leakage was reduced by three orders of magnitude, Voc increased from 0.85 to 1.65 V and peak output power increased by nearly 100% for devices with X In ≈0.08. Surface pit density was also significantly decreased by increasing growth temperature and seems to be an important mechanism for leakage in these devices.

In this work we investigate the optical and thermal properties of In 12 Ga 88 N/GaN photovoltaic devices. These devices show very good solar cell characteristics with peak external quantum efficiency of 63%, open circuit voltage of 1.7 V, and a fill factor of 76%. Thermal measurements show no reduction in output power with increasing temperature up to 87 °C.

Planar (In,Ga)N layers were grown on nanostripe arrays composed of InGaN/GaN multi quantum wells (MQWs) by metal–organic chemical vapor deposition. The MQW nanostripe arrays with height to width aspect ratios of about 0.5 and 1 were fabricated from planar coherently strained InGaN/GaN MQW samples. Independent of their aspect ratio, the nanostripes exhibited elastic relaxation perpendicular to the stripe direction after pattern fabrication, resulting in an a⊥ lattice constant perpendicular to the stripe direction larger than that of the GaN base layer. In a subsequent step, (In,Ga)N layers were grown on top of the nanostripe arrays, leading to the formation of planar films with a similar a⊥ lattice constant as the MQW stripes beneath. Bright luminescence was recorded from the planar, partially relaxed re-grown (In,Ga)N layers grown on the stripe arrays with an aspect ratio of 1. Plastic relaxation of the MQW stripes was observed after (In,Ga)N regrowth for samples with a stripe aspect ratio of 0.5, leading to luminescence quenching.

The effect of intentional p-GaN surface roughening on the performance of c-plane InGaN/GaN solar cells was investigated. Surface roughness was introduced by growing the p-GaN at a relatively high rate and low temperature which resulted in a faceted surface with a high density of V-defects. Increasing the surface roughness led to a 69.4% increase in short circuit current density. Similar surface roughening techniques should also be applicable for increasing the extraction efficiency of InGaN/GaN light-emitting diodes.

A two-step GaN barrier growth methodology was developed for InxGa1−xN/GaN multiple quantum well solar cells in which a lower temperature GaN cap layer was grown on top of the quantum wells (QWs) and then followed by a higher temperature GaN barrier layer. The performance of the solar cells improved markedly by increasing the low temperature GaN cap layer thickness from 1.5 to 3.0 nm. High-angle annular dark field scanning transmission electron microscopy and atom probe tomography measurements showed that increasing the GaN cap layer thickness improved the uniformity and increased the average indium content of the QWs.

We report on the unique thermal properties of In0.28Ga0.72N/GaN multiple quantum well solar cells. The devices exhibited an external quantum efficiency of 69% (26%) at 390 nm (460 nm), an open circuit voltage of 2.04 V, a fill factor of 63%, a short circuit current density of 2 mA/cm2, and a peak output power of 2.63 mW/cm2 at room temperature under 1-sun AM1.5G illumination. Thermal measurements showed that the peak output power increased with temperature up to 2.73 mW/cm2 at 70 °C, signifying the potential of III-nitride solar cells for concentrator photovoltaic applications.

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%.

We demonstrate high quantum efficiency InGaN/GaN multiple quantum well (QW) solar cells with spectral response extending out to 520 nm. Increasing the number of QWs in the active region did not reduce the carrier collection efficiency for devices with 10, 20, and 30 QWs. Solar cells with 30 QWs and an intentionally roughened p-GaN surface exhibited a peak external quantum efficiency (EQE) of 70.9% at 390 nm, an EQE of 39.0% at 450 nm, an open circuit voltage of 1.93 V, and a short circuit current density of 2.53 mA/cm2 under 1.2 suns AM1.5G equivalent illumination.

High external quantum efficiency (EQE) p-i-n heterojunction solar cells grown by NH3-based molecular beam epitaxy are presented. EQE values including optical losses are greater than 50% with fill-factors over 72% when illuminated with a 1 sun AM0 spectrum. Optical absorption measurements in conjunction with EQE measurements indicate an internal quantum efficiency greater than 90% for the InGaN absorbing layer. By adjusting the thickness of the top p-type GaN window contact layer, it is shown that the short-wavelength (

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.

GaN nanopillar and nanostripe arrays with embedded InGaN∕GaN multi-quantum wells (MQWs) were fabricated by holographic lithography and subsequent reactive ion etching. Etch related damage of the nanostructures was successfully healed through annealing in NH3∕N2 mixtures under optimized conditions. The nanopatterned samples exhibited enhanced luminescence in comparison to the planar wafers. X-ray reciprocal space maps recorded around the asymmetric (101¯5) reflection revealed that the MQWs in both nanopillars and nanostripes relaxed after nanopatterning and adopted a larger in-plane lattice constant than the underlying GaN layer. The pillar relaxation process had no measurable effect on the Stokes shift typically observed in MQWs on c-plane GaN, as evaluated by excitation power dependent photoluminescence (PL) measurements. Angular-resolved PL measurements revealed the extraction of guided modes from the nanopillar arrays.

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/cm(2), and an open circuit voltage of 1.89 V were achieved under 1 sun air-mass 1.5 global spectrum illumination conditions. (C) 2011 American Institute of Physics. [doi:10.1063/1.3540501]