Extensive literature and publications on intermediate band solar cells (IBSCs) are reviewed. A detailed discussion is given on the thermodynamics of solar energy conversion in IBSCs, the device physics, and the carrier dynamics processes with a particular emphasis on the two-step inter-subband absorption/recombination processes that are of paramount importance in a successful implementation high-efficiency IBSC. The experimental solar cell performance is further discussed, which has been recently demonstrated by using highly mismatched alloys and high-density quantum dot arrays and superlattice. IBSCs having widely different structures, materials, and spectral responses are also covered, as is the optimization of device parameters to achieve maximum performance.

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Intermediate band solar cells: Recent progress and future directions

We report the electroabsorption property of InAsP/InP strained multiple quantum wells (MQWs), grown by gas‐source molecular beam epitaxy, for 1.3 μm modulator applications. Very sharp excitonic absorption at room temperature was observed. Electroabsorption measurements performed for a ring‐shaped p‐i‐n diode, consisting of 10‐period InAs0.41P0.59(100 A)/InP(150 A) strained MQWs, reveal a significant red shift of the absorption peak with increasing reverse biases due to the quantum‐confined Stark effect. This large energy shift (e.g., ∼18 meV at an external field of 57 kV/cm) is well accounted for in the ‘‘effective well‐width’’ model. The change of the absorption coefficient at a 22 meV detuning is as large as 3510 cm−1 with a small residual absorption, which can be very useful for 1.3 μm waveguide modulators.

Electroabsorption of InAsP/InP strained multiple quantum wells for 1.3 μm waveguide modulators

We report the operation of strained‐layer InAsyP1−y/InP multiple quantum well optical modulators at wavelengths compatible with solid‐state lasers such as neodymium‐doped yttrium aluminum garnet. A structure having 50 periods of 100 A InAsyP1−y quantum wells with 100 A InP barriers is described that has an exciton peak at 1.05 μm and a single pass transmission contrast ratio of 1.4. Favorable comparison is made to similar InxGa1−xAs/GaAs structures.

InAsyP1−y/InP multiple quantum well optical modulators for solid‐state lasers

Previous strain‐relief p‐i‐n InGaAs‐GaAs quantum well (qw) modulators have incurred surface striations upon growth due to defect formation, resulting in an optically rough surface. We present here a qw modulator on a GaAs substrate with InGaAs wells and GaAsP barriers which balance the strain in the wells so that the lattice does not relax, leading to much fewer defects, and an optically smooth surface. We obtain a transmission change from 60% to 80% at 1014 nm for a sample with a 1 μm thick intrinsic region.

Pseudomorphic InGaAs‐GaAsP quantum well modulators on GaAs

Theoretical and experimental work shows that electroabsorption improves and the quantum‐confined Stark effect becomes stronger in In0.2Ga0.8As/AlxGa1−xAs quantum wells as the aluminum concentration (x) is increased in the barriers. This improved electroabsorption was used in a reflection modulator that exhibited the largest reported reflectivity change of 77%.

Electroabsorptive modulators in InGaAs/AlGaAs

We report the operation of strained‐layer InxGa1−x As/GaAs 50‐ and 100‐period multiple quantum well optical modulators at wavelengths ranging from 1.02 to 1.07 μm. Structures were grown on GaAs substrates, as well as on strain relief InxGa1−xAs buffer layers. Devices show favorable electrical characteristics and absorption contrasts up to 57% at the exciton peak. Optical modulation of a Nd:YAG laser is demonstrated, via operation of self‐electro‐optic effect devices at 1.064 μm.

InxGa1−xAs/GaAs multiple quantum well optical modulators for the 1.02–1.07 μm wavelength range

We compare InP‐based materials systems for multiple quantum well modulator application in the 1.06 μm wavelength range. Quantum well/barrier systems studied are the lattice‐matched system InxGa1−xAsyP1−y/InP, the strained system InAsyP1−y/InP, and the strain‐balanced system InAsyP1−y/InxGa1−xP. 50 period samples were grown on InP substrates by chemical beam epitaxy. We find the ternary systems to be better than the quaternary in terms of exciton peak sharpness. The InAsyP1−y/InxGa1−xP system was best overall, with our results suggesting that it is coherently strained to the InP substrate.

Multiple quantum well light modulators for the 1.06 μm range on InP substrates: InxGa1−xAsyP1−y/InP, InAsyP1−y/InP, and coherently strained InAsyP1−y/InxGa1−xP

A high‐power lamp‐pumped Nd:YAG laser has been harmonically mode locked using a specially fabricated GaP mode locker at repetition rates of 0.25, 0.5, and 1 GHz. At these ultrahigh mode‐locking frequencies the pulse duration is reduced as well. Stable 31 ps pulses have been produced with 4 W of average output power.

Mode locking of high-power neodymium:yttrium aluminum garnet lasers at ultrahigh repetition rates

The last sentence in the paragraph at the top of the second column of the second page of the article should read: ‘‘Since previously reported samples like B have been described with sharper excitonic features (HWHM of 6 meV at 300 K and 2 meV at 10 K) it is reasonable to expect that sample C could be further improved.4’’

Erratum: ‘‘Multiple quantum well light modulators for the 1.06 μm range on InP substrates: InxGa1−xAsyP1−y/InP, InAsyP1−y/InP, and coherently strained InAsyP1−y/InxGa1−xP’’ [Appl. Phys. Lett. 60, 2846 (1992)]

Theodore Sizer

Papers

InxGa1−xAs/GaAs multiple quantum well optical modulators for the 1.02–1.07 μm wavelength range

InAsyP1−y/InP multiple quantum well optical modulators for solid‐state lasers

Multiple quantum well light modulators for the 1.06 μm range on InP substrates: InxGa1−xAsyP1−y/InP, InAsyP1−y/InP, and coherently strained InAsyP1−y/InxGa1−xP

Mode locking of high-power neodymium:yttrium aluminum garnet lasers at ultrahigh repetition rates

Erratum: ‘‘Multiple quantum well light modulators for the 1.06 μm range on InP substrates: InxGa1−xAsyP1−y/InP, InAsyP1−y/InP, and coherently strained InAsyP1−y/InxGa1−xP’’ [Appl. Phys. Lett. 60, 2846 (1992)]