We discuss quantum well cells (QWCs) in the quaternary system In1-xGaxAsyP1-y lattice-matched to the InP substrate (x approximate to 0.47y) for the use in solar and thermophotovoltaic (TPV) applications. The deep lattice-matched wells of up to y = 1 (In0.53Ga0.47As) can be incorporated without any strain. The effective band-gap for absorption in these quaternary QWCs can be tailored (up to similar to 1.7 mu m) to produce the ideal band-gap for a given blackbody or selective-emitter spectrum while retaining a comparatively high efficiency for the solar spectrum. This has a great potential for higher-efficiency cells, especially for hybrid solar-TPV applications. We present the results of a new model for the external quantum efficiency of InGaAsP QWCs. The model calculates the spectral response of multi-layer InGaAsP photovoltaic cells with and without quantum wells. It is in very good agreement with the experimental spectral response of InGaAsP QWCs with a variety of designs and therefore is an important tool for the optimisation of these cells. Besides improvements in quantum well geometry, processing and material quality the high efficiencies we obtain can be even further increased by additional features such as the use of back surface reflectors. We also investigate how these cells perform under several different illuminations such as solar and blackbody spectra as well as narrow-band selective-emitter spectra from erbia and ytterbia comparing them to lattice-matched InGaAs monolithic interconnected modules. We conclude that the InGaAsP QWC system is a very promising candidate for high-efficiency photovoltaic cells for hybrid solar-TPV applications.

We discuss the advantages of quantum well solar cells (QWSCs) for thermophotovoltaic (TPV) applications and illustrate them with InP/InGaAs and GaInAsP/InGaAs QWSCs which were designed for other applications and have not been optimised for TPV. It is shown that an InP p-i-n solar cell with 15 lattice matched InGaAs quantum wells (QWs) in the i region has an increase in open circuit voltage ( V oc ) of (1.7 ± 0.1) times that of a control cell of InP with InGaAs in the i-region under an illuminating spectrum close to that expected from an ideal ytterbia emitter. Also, using an InGaAsP quaternary cell of band gap wavelength of 1.1 Am with 60 InGaAs QWs under the same illuminating spectrum the current density is increased by a factor of (2.4 ± 0.1) over that of the InP QWSC. The quaternary cell also absorbs longer wavelengths without any significant loss in V OC . Better temperature coefficients for the former quantum well solar cell than the control cell are observed in a spectrum approximating a black body at 3000 K. Further advantages of QWs for narrow band and broad band illuminating spectra are discussed.

Infrared absorption measurements show that high-resistivity InP grown by metal-organic vapor phase epitaxy (MOVPE) at 500 degrees C and doped using CCl4 contains comparably high concentrations (similar to 3 x 10(18) cm(-3)) of isolated C-p accepters and H-C-p pairs. These centers give rise to localized vibrational modes at 546.9 cm(-1) (C-p) and at 2703.3, 521.1, and 413.5 cm(-1) (H-C-p). A sharp weak line, also at 546.9 cm(-1), may be due to a much lower concentration of C-In donors. It is concluded that the doping introduces C-p accepters that overcompensate grown-in donors and complete compensation results from the formation of H-C-p pairs. [S0163-1829(98)01648-8].

We discuss the advantages of quantum well solar cells (QWSCs) for thermophotovoltaic (TPV) applications and illustrate them with a test cell grown in GaInAsP/InGaAs lattice-matched to InP but not optimised for TPV. It is shown that a GaInAsP quaternary cell with a bandgap of 1.1 mu m with 60 InGaAs QWs has an open circuit voltage of (1.7 +/- 0.1) times that of a homogeneous InGaAs cell under a narrow band ytterbia-like illuminating spectrum. Similar enhancements are observed under an erbia-like illumination and in a broad band illumination approximating a black body at 3000K. The quaternary cell absorbs a similar range of wavelengths close to the InGaAs cell. Also, better temperature coefficients for the QWSC than the control cell are observed in a spectrum approximating a black body at 3000K. A comparison is made between the QWSC and two other cells, one InGaAs and one Si, using published spectral response and illuminated current/voltage data. It is shown that whilst InGaAs has a higher power output in black body spectra and Si a higher power output in an ideal ytterbia spectra the QWSC has the highest output in a combination of 2000K black body and ytterbia spectra where 56% of the power output is in the ytterbia emission band. Possible improvements to this non-optimised QWSC are discussed.

We have studied the variation with applied bias and temperature of steady state photoluminescence (DCPL) and photoconductivity (DCPC) from a series of GaAs/AlGaAs single quantum well, p-i-n structures with different well widths. We present the DCPC and DCPL results, which when combined, allow us to assess how significant nonradiative recombination is in the samples and hence the quality of the material. We discuss the qualitative features in the light of a new theoretical approach presented here for the first time. This includes contributions from escape (of both electrons and holes) and makes it possible to extract from the experimental data two parameters, each reflecting the competition between escape and one of the recombination processes (radiative or nonradiative) in the absence of the other. We further comment qualitatively on the bias and temperature dependence of these different processes.

A series of Al/sub x/Ga/sub 0.52-x/In/sub 0.48/P/GaAs heterojunction bipolar transistors (HBT's) with x=0 to x=0.52 showed ideality factors close to unity for both base current and collector current and small variation in gain with temperature up to at least T=623 K across the whole range of x composition. Hole current injection from the base into the emitter in these devices was shown to be negli...

The radiative behavior of quantum-well (QW) devices depends upon the quasi-Fermi-level separation ΔEf induced in the quantum well. We present a method of obtaining ΔEf in absolute units from the emission spectra of optically or electrically biased QWs. Emission spectra are calibrated by comparison with measurements of the limiting photocurrent. A theoretical model is then used to separate out the effects of carrier generation rate and field-dependent QW absorption. We apply the method to the low-temperature photoluminescence spectra of a set of single QW p-i-n photodiodes at different electric fields. We show that modeled emission spectra agree closely with measured spectra in flatband conditions. We also observe a field-dependent loss in emission intensity—leading to a reduction in ΔEf of several meV—which we attribute mainly to carrier escape from the QW. The derived values for field-dependent nonradiative efficiency are consistent with independent measurements of low-temperature dc photocurrent, and with a simple model for thermally assisted carrier escape. We show how the method can be applied to electroluminescence spectra in order to investigate the dependence of ΔEf on applied bias.

Passively Q-switched poises of up to 94 pJ with a duration of 750 ps have been generated in a tapered bulk InGaAsP bow-tie diode laser. The device was driven by an improved, overshoot-free avalanche pulse generator which gave up to 6.25 A current pulses with a FWHM of 5.0 ns.

Electroabsorption optical modulators have been fabricated on GaInAs/GaInAsP multiquantum well structures whose bandgap had been increased by laser photoabsorption-induced disordering. Modulation depths of 20 dB have been obtained in material which has been bandgap blue shifted by as much as 120 nm, while samples shifted by 80 nm gave depths as high as 27 dB.<>

Oxide stripe lasers have been fabricated from GaInAs/GaInAsP multi-quantum well material which has undergone various degrees of intermixing by photoabsorption induced disordering (PAID). Blue shifts of up to 160 nm in the lasing spectra are demonstrated

The authors describe the fabrication of five-element antiguided laser arrays at 1.48 mu m using a novel material design which removes the need for overgrowth. Adding undoped passive waveguiding layers to the standard laser design significantly alters the shape of the far field emission from the lasers, showing that the array elements are pulled in phase with each other.<>

In this paper we present results on the spectral response of p-i-n GaAs/InGaP quantum well (QW) material grown on a 10° off GaAs (100) substrate. Room temperature photocurrent spectroscopy shows that although no excitonic features are resolvable at 0 V, clear excitonic transitions are observed on application of an electric field. This effect is accompanied by a blue shift and sharpening of the band edge. Such a phenomenon is consistent with the formation of a narrow, low band gap QW at the InGaP-GaAs interface caused by an indium memory effect during growth.

The open circuit voltage V/sub oc/ and reference voltage V/sub ref/ defined as a measure of the dark current quality, have been studied for a large number of quantum well (QW) solar cells and homogenous control cells. Samples were grown in the Al/sub x/Ga/sub 1-x/As/GaAs and GaAs/In/sub y/Ga/sub 1-y/As material systems. For both combinations, QW solar cells show a better voltage performance in V/sub oc/ and V/sub ref/ than one would expect from a single bandgap solar cell with the same effective absorption bandgap E/sub a/. For the AlGaAs/GaAs cells, V/sub oc/ is related to structural parameters of the QW cells such as the well width L/sub W/ and the Al fraction x. For the strained GaAs/InGaAs cells a relationship is found between V/sub ref/ and the barrier width L/sub B/, which is a dominant parameter in determining strain relaxation and defect formation at a fixed In fraction.

The open circuit voltage Voc and reference voltage V ref defined as a measure of the dark current quality, have been studied for a large number of quantum well (QW) solar cells and homogenous control cells. Samples were grown in the AlxGa1-xAs/GaAs and GaAs/InyGa1-y As material systems. For both combinations, QW solar cells show a better voltage performance in Voc and Vref than one would expect from a single bandgap solar cell with the same effective absorption bandgap Ea. For the AlGaAs/GaAs cells, V oc is related to structural parameters of the QW cells such as the well width LW and the Al fraction x. For the strained GaAs/InGaAs cells a relationship is found between Vref and the barrier width LB, which is a dominant parameter in determining strain relaxation and defect formation at a fixed In fraction

The authors have studied the variation in DC photocurrent with bias and temperature from GaAs-Al/sub x/Ga/sub 1-x/As single quantum wells embedded in p-i-n diodes. They found that the observed temperature response shows Arrhenius behaviour with a field-dependent activation energy close to the hole well depth. This can be accounted for using a model based on the competition between photocarrier escape and recombination. Using reasonable values for the diode's built-in voltage and the quantum-well recombination lifetime, good quantitative agreement between theory and experiment is achieved if it is assumed that the recombination rate is governed by the fastest escaping carriers, which are light holes in the present devices.< >

Quantum well intermixing using dielectric caps has been studied using photoluminescence at 77 K. The structures which have been investigated, including shallow depth single quantum wells and multiquantum well waveguiding material, are highly sensitive to the presence of surface defects during annealing. Samples capped with either silicon nitride or silica have shown considerable energy shifts after processing in a rapid thermal annealer, and large energy shifts have also been found in uncapped material. Samples capped with strontium fluoride have shown negligible intermixing of the quantum wells.

In this paper we shall look at a technique, known as impurity free vacancy diffusion (IFVD) for selectively altering the optoelectronic response of quantum well material after growth with a view to monolithic device integration. We will discuss the mechanism, practical considerations and some possible applications.

Nonlinear refractive index measurements have been made at 1.06 μm using a time-division interferometer, and n2 measured to be about −3×10−17 m2 W−1 for a TM pump and TE probe. The refractive index change was also seen to contain a positive component at small pump-probe delay times, corresponding to an intensity-dependent nonlinear refractive index of n2≊2×10−17 m2 W−1.

Time-resolved nonlinear absorption modulation due to two-photon absorption (TPA) and free carrier absorption (FCA) is demonstrated in GaAs/AlGaAs waveguides at 1.06-mu-m. For a peak input intensity of 4.5 X 10(11) Wm-2, there was a maximum absorption modulation of 35%. This was caused by TPA, so that the recovery time was limited by the laser pulsewidth alone. At a peak input intensity of 1.5 X 10(12) Wm-2, a 65% absorption modulation was observed. This consisted of two components: an instantaneous effect due to TPA and also a slower component, attributed to FCA, whose recovery time was limited by the carrier lifetime. A theoretical model was also derived which provided a good fit to the experimental data.

Micro-resonator modulators exploiting the quantum confined Stark shift and incorporating the asymmetric Fabry-Perot have been fabricated which exhibit high contrast (15 - 20 dB), and large reflectivity changes (> 50%) at low voltages (3V - 4V). This paper will review progress made in the development of these devices and will consider how the performance can be modified, post growth, by use of intermixing techniques to modify the shape of the quantum wells. The asymmetric Fabry Perot structure is also a convenient configuration to investigate the intermixing process itself. Measurement of the lateral extent of the intermixing process promoted by vacancies in GaAs/AlGaAs structures obtained using this structure are reported.