Showing papers on "Quantum well published in 2009"
TL;DR: In this article, the basic principles of x-ray diffraction of thin films and areas of special current interest, such as analysis of non-polar, semipolar and cubic III-nitrides, are reviewed, along with the basic principle of X-ray diffusion of thin thin films, and some useful values needed in calculations, including elastic constants and lattice parameters.
Abstract: The III-nitrides include the semiconductors AlN, GaN and InN, which have band gaps spanning the entire UV and visible ranges. Thin films of III-nitrides are used to make UV, violet, blue and green light-emitting diodes and lasers, as well as solar cells, high-electron mobility transistors (HEMTs) and other devices. However, the film growth process gives rise to unusually high strain and high defect densities, which can affect the device performance. X-ray diffraction is a popular, non-destructive technique used to characterize films and device structures, allowing improvements in device efficiencies to be made. It provides information on crystalline lattice parameters (from which strain and composition are determined), misorientation (from which defect types and densities may be deduced), crystallite size and microstrain, wafer bowing, residual stress, alloy ordering, phase separation (if present) along with film thicknesses and superlattice (quantum well) thicknesses, compositions and non-uniformities. These topics are reviewed, along with the basic principles of x-ray diffraction of thin films and areas of special current interest, such as analysis of non-polar, semipolar and cubic III-nitrides. A summary of useful values needed in calculations, including elastic constants and lattice parameters, is also given. Such topics are also likely to be relevant to other highly lattice-mismatched wurtzite-structure materials such as heteroepitaxial ZnO and ZnSe.
925 citations
TL;DR: The data confirm that the quantum transport through the (helical) edge channels is dissipationless and that the contacts lead to equilibration between the counterpropagating spin states at the edge, which agree quantitatively with the theory of the quantum spin Hall effect.
Abstract: Nonlocal transport through edge channels holds great promise for low-power information processing. However, edge channels have so far only been demonstrated to occur in the quantum Hall regime, at high magnetic fields. We found that mercury telluride quantum wells in the quantum spin Hall regime exhibit nonlocal edge channel transport at zero external magnetic field. The data confirm that the quantum transport through the (helical) edge channels is dissipationless and that the contacts lead to equilibration between the counterpropagating spin states at the edge. The experimental data agree quantitatively with the theory of the quantum spin Hall effect. The edge channel transport paves the way for a new generation of spintronic devices for low-power information processing.
688 citations
TL;DR: Direct observation of bound exciton pairs (biexcitons) that provide incisive signatures of four-body correlations among electrons and holes in gallium arsenide (GaAs) quantum wells are reported.
Abstract: The motions of electrons in solids may be highly correlated by strong, long-range Coulomb interactions. Correlated electron-hole pairs (excitons) are accessed spectroscopically through their allowed single-quantum transitions, but higher-order correlations that may strongly influence electronic and optical properties have been far more elusive to study. Here we report direct observation of bound exciton pairs (biexcitons) that provide incisive signatures of four-body correlations among electrons and holes in gallium arsenide (GaAs) quantum wells. Four distinct, mutually coherent, ultrashort optical pulses were used to create coherent exciton states, transform these successively into coherent biexciton states and then new radiative exciton states, and finally to read out the radiated signals, yielding biexciton binding energies through a technique closely analogous to multiple-quantum two-dimensional Fourier transform (2D FT) nuclear magnetic resonance spectroscopy. A measured variation of the biexciton dephasing rate indicated still higher-order correlations.
269 citations
TL;DR: In this paper, room-temperature photoluminescence (PL) measurements are performed on GaInN/GaN multiple-quantum-well heterostructures grown on GaN-on-sapphire templates with different threading-dislocation densities.
Abstract: Room-temperature photoluminescence (PL) measurements are performed on GaInN/GaN multiple-quantum-well heterostructures grown on GaN-on-sapphire templates with different threading-dislocation densities. The selective optical excitation of quantum wells and the dependence of integrated PL intensity on excitation power allow us to determine the internal quantum efficiency (IQE) as a function of carrier concentration. The measured IQE of the sample with the lowest dislocation density (5.3×108 cm−2) is as high as 64%. The measured nonradiative coefficient A varies from 6×107 to 2×108 s−1 as the dislocation density increases from 5.3×108 to 5.7×109 cm−2, respectively.
256 citations
TL;DR: In this article, the authors reviewed current technological developments in polarization engineering and the control of the quantum-confined Stark effect (QCSE) for InxGa1-xN-based quantum-well active regions, which are generally employed in visible LEDs for solid-state lighting applications.
Abstract: This paper reviews current technological developments in polarization engineering and the control of the quantum-confined Stark effect (QCSE) for InxGa1- xN-based quantum-well active regions, which are generally employed in visible LEDs for solid-state lighting applications. First, the origin of the QCSE in III-N wurtzite semiconductors is introduced, and polarization-induced internal fields are discussed in order to provide contextual background. Next, the optical and electrical properties of InxGa1- xN-based quantum wells that are affected by the QCSE are described. Finally, several methods for controlling the QCSE of InxGa1- xN-based quantum wells are discussed in the context of performance metrics of visible light emitters, considering both pros and cons. These strategies include doping control, strain/polarization field/electronic band structure control, growth direction control, and crystalline structure control.
246 citations
TL;DR: In this article, a strain-compensated InGaN-AlGaN quantum well (QW) structure consisting of thin tensile-strained AlGaN barriers surrounding the QW was investigated as improved active regions for lasers and light emitting diodes.
Abstract: Strain-compensated InGaN-AlGaN quantum wells (QW) are investigated as improved active regions for lasers and light emitting diodes. The strain-compensated QW structure consists of thin tensile-strained AlGaN barriers surrounding the InGaN QW. The band structure was calculated by using a self-consistent 6-band kmiddotp formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The spontaneous emission and gain properties were analyzed for strain-compensated InGaN-AlGaN QW structures with indium contents of 28%, 22%, and 15% for lasers (light-emitting diodes) emitting at 480 (500), 440 (450), and 405 nm (415 nm) spectral regimes, respectively. The spontaneous emission spectra show significant improvement of the radiative emission for strain-compensated QW for all three structures compared to the corresponding conventional InGaN QW, which indicates the enhanced radiative efficiency for light emitting diodes. Our studies show the improvement of the optical gain and reduction of the threshold current density from the use of strain-compensated InGaN-AlGaN QW as active regions for diode lasers.
221 citations
TL;DR: In this article, the advantages of blue InGaN light-emitting diodes (LEDs) with inGaN barriers are studied. And the simulation results suggest that the efficiency droop was markedly improved when the traditional GaN barriers were replaced by InGaNs barriers.
Abstract: The advantages of blue InGaN light-emitting diodes (LEDs) with InGaN barriers are studied. The L-I curves, carrier concentrations in the quantum wells, energy band diagrams, and internal quantum efficiency are investigated. The simulation results show that the InGaN/InGaN LED has better performance over its conventional InGaN/GaN counterpart due to the enhancement of electron confinement, the reduced polarization effect between the barrier and well, and the lower potential barrier height for the holes to transport in the active region. The simulation results also suggest that the efficiency droop is markedly improved when the traditional GaN barriers are replaced by InGaN barriers.
212 citations
TL;DR: The edge conductance of a QSH insulator as a function of temperature in the presence of a magnetic impurity is calculated using linear response and renormalization group methods.
Abstract: Following the recent observation of the quantum spin Hall (QSH) effect in HgTe quantum wells, an important issue is to understand the effect of impurities on transport in the QSH regime. Using linear response and renormalization group methods, we calculate the edge conductance of a QSH insulator as a function of temperature in the presence of a magnetic impurity. At high temperatures, Kondo and/or two-particle scattering give rise to a logarithmic temperature dependence. At low temperatures, for weak Coulomb interactions in the edge liquid, the conductance is restored to unitarity with unusual power laws characteristic of a "local helical liquid," while for strong interactions, transport proceeds by weak tunneling through the impurity where only half an electron charge is transferred in each tunneling event.
191 citations
TL;DR: This study reports the unequivocal demonstration of midinfrared mode-locked pulses from quantum cascade lasers generated by actively modulating the current and hence the gain of an edge-emitting quantum cascade laser (QCL).
Abstract: In this study, we report the unequivocal demonstration of midinfrared mode-locked pulses from quantum cascade lasers. The train of short pulses was generated by actively modulating the current and hence the gain of an edge-emitting quantum cascade laser (QCL). Pulses with duration of about 3 ps at full-width-at-half-maxima and energy of 0.5 pJ were characterized using a second-order interferometric autocorrelation technique based on a nonlinear quantum well infrared photodetector. The mode-locking dynamics in the QCLs was modeled based on the Maxwell-Bloch equations in an open two-level system. Our model reproduces the overall shape of the measured autocorrelation traces and predicts that the short pulses are accompanied by substantial wings as a result of strong spatial hole burning. The range of parameters where short mode-locked pulses can be formed is found.
191 citations
TL;DR: In this paper, a short-period superlattice (SPSL) doping was used for the growth of AlGaAs/GaAs heterostructures instead of the more standard n-AlGaAs doping, allowing the use of a low AlAs mole fraction spacer which leads to a lower background of impurities as well as a better interface quality.
185 citations
TL;DR: In this article, an overview on the design, fabrication, and characterization of quantum cascade detectors is given. But the authors do not discuss the performance of the quantum cascade detector at wavelengths from the near infrared at 2 mum to THz radiation at 87 mum.
Abstract: This paper gives an overview on the design, fabrication, and characterization of quantum cascade detectors. They are tailorable infrared photodetectors based on intersubband transitions in semiconductor quantum wells that do not require an external bias voltage due to their asymmetric conduction band profile. They thus profit from favorable noise behavior, reduced thermal load, and simpler readout circuits. This was demonstrated at wavelengths from the near infrared at 2 mum to THz radiation at 87 mum using different semiconductor material systems.
TL;DR: Time-resolved photoluminescence from single InP nanowires containing both wurtzite and zincblende crystalline phases is used to measure the carrier dynamics of quantum confined excitons in a type-II homostructure and demonstrates that the dynamics are consistent with the calculated distribution of confined states for the electrons and holes.
Abstract: We use time-resolved photoluminescence from single InP nanowires containing both wurtzite (WZ) and zincblende (ZB) crystalline phases to measure the carrier dynamics of quantum confined excitons in a type-II homostructure. The observed recombination lifetime increases by nearly 2 orders of magnitude from 170 ps for excitons above the conduction and valence band barriers to more than 8400 ps for electrons and holes that are strongly confined in quantum wells defined by monolayer-scale ZB sections in a predominantly WZ nanowire. A simple computational model, guided by detailed high-resolution transmission electron microscopy measurements from a single nanowire, demonstrates that the dynamics are consistent with the calculated distribution of confined states for the electrons and holes.
TL;DR: In this paper, a terahertz quantum cascade laser design that combines a wide gain bandwidth, large photon-driven transport and good high-temperature characteristics is presented, which relies on a diagonal transition between a bound state and doublet of states tunnel coupled to the upper state of a phonon extraction stage.
Abstract: A terahertz quantum cascade laser design that combines a wide gain bandwidth, large photon-driven transport and good high-temperature characteristics is presented. It relies on a diagonal transition between a bound state and doublet of states tunnel coupled to the upper state of a phonon extraction stage. The high optical efficiency of this design enables the observation of photon-driven transport over a wide current density range. The relative tolerance of the design to small variations in the barrier thicknesses made it suitable for testing different growth techniques and materials. In particular, we compared the performances of devices grown using molecular-beam epitaxy with those achieved using organometallic chemical vapor deposition. The low-threshold current density and the high slope efficiency makes this device an attractive active region for the development of single-mode quantum cascade lasers based on third-order-distributed feedback structures. Single-mode, high power was achieved with good continuous and pulsed wave operation.
TL;DR: In this article, a three-layer staggered InGaN quantum wells (QWs) light-emitting diodes (LEDs) emitting at 520-525 nm were grown by metal-organic chemical vapor deposition by employing a graded growth-temperature profile.
Abstract: Three-layer staggered InGaN quantum wells (QWs) light-emitting diodes (LEDs) emitting at 520–525 nm were grown by metal-organic chemical vapor deposition by employing graded growth-temperature profile. The use of staggered InGaN QW, with improved electron-hole wave functions overlap design, leads to an enhancement of its radiative recombination rate. Both cathodoluminescence and electroluminescence measurements of three-layer staggered InGaN QW LED exhibited enhancements by 1.8–2.8 and 2.0–3.5 times, respectively, over those of conventional InGaN QW LED.
TL;DR: The studies pave the way for quantum-dot terahertz device development, providing the fundamental knowledge of carrier relaxation times required for optimum device design.
Abstract: Carrier relaxation is a key issue in determining the efficiency of semiconductor optoelectronic device operation. Devices incorporating semiconductor quantum dots have the potential to overcome many of the limitations of quantum-well-based devices because of the predicted long quantum-dot excited-state lifetimes. For example, the population inversion required for terahertz laser operation in quantum-well-based devices (quantum-cascade lasers) is fundamentally limited by efficient scattering between the laser levels, which form a continuum in the plane of the quantum well. In this context, semiconductor quantum dots are a highly attractive alternative for terahertz devices, because of their intrinsic discrete energy levels. Here, we present the first measurements, and theoretical description, of the intersublevel carrier relaxation in quantum dots for transition energies in the few terahertz range. Long intradot relaxation times (1.5 ns) are found for level separations of 14 meV (3.4 THz), decreasing very strongly to approximately 2 ps at 30 meV (7 THz), in very good agreement with our microscopic theory of the carrier relaxation process. Our studies pave the way for quantum-dot terahertz device development, providing the fundamental knowledge of carrier relaxation times required for optimum device design.
TL;DR: In this article, a nonperturbative approach based upon the Kramers-Henneberger translation transformation, followed by Floquet series expansions, yields, for sufficiently high frequencies, the so-called "laser-dressed" potential, which is taken for composing a time-independent Schrodinger equation whose solutions are the desired quasistationary states.
Abstract: When an electronic system is irradiated by an intense laser field, the potential “seen” by electrons is modified, which affects significantly the bound-state energy levels, a feature that has been observed in transition energy experiments. For lasers for which the dipole approximation applies, a nonperturbative approach based upon the Kramers–Henneberger translation transformation, followed by Floquet series expansions, yields, for sufficiently high frequencies, the so-called “laser-dressed” potential, which is taken for composing a time-independent Schrodinger equation whose solutions are the desired quasistationary states. This approach, developed originally for atoms, has been verified to be useful also for carriers in semiconductor nanostructures under intense laser fields. In quantum wells, analytical expressions for the dressed potential have been proposed in literature for a nonresonant, intense laser field polarized perpendicularly to the interfaces. By noting that they apply only for α0≤L/2, wher...
TL;DR: In this paper, the authors report the development of Al0.7Ga0.3N/AlN quantum wells with high internal quantum efficiency, where the III/V flux ratio was varied during growth by increasing the Ga flux.
Abstract: We report the development of Al0.7Ga0.3N/AlN quantum wells with high internal quantum efficiency. All samples had identical well and barrier thickness but the III/V flux ratio was varied during growth by increasing the Ga flux. The luminescence spectra show single peaks which vary from 220 nm (III/V∼1) to 250 nm (III/V⪢1) with internal quantum efficiency varying from 5% to 50%, respectively. To account for these results, a growth model was proposed in which at III/V∼1 the growth proceeds via vapor phase epitaxy, while at III/V⪢1 the growth proceeds via liquid phase epitaxy.
TL;DR: In this paper, the Auger recombination coefficient in In0.1Ga0.9N/GaN quantum wells, emitting at 407 nm, has been determined from large signal modulation measurements on lasers in which these quantum wells form the gain region.
Abstract: The Auger recombination coefficient in In0.1Ga0.9N/GaN quantum wells, emitting at 407 nm has been determined from large signal modulation measurements on lasers in which these quantum wells form the gain region. A value of 1.5×10−30 cm6 s−1 is determined for the Auger coefficient at room temperature, which is used to analyze the reported efficiency characteristics of 410 nm In0.1Ga0.9N/GaN quantum wells light emitting diodes. The calculated efficiencies agree remarkably well with the measured ones. It is apparent that Auger recombination is largely responsible for limiting device efficiencies at high injection currents.
TL;DR: In this paper, the design, fabrication, and evaluation of large-aperture, oxide-confined 850 nm vertical cavity surface emitting lasers (VCSELs) with high modulation bandwidth at low current densities are reported.
Abstract: We report on the design, fabrication, and evaluation of large-aperture, oxide-confined 850 nm vertical cavity surface emitting lasers (VCSELs) with high modulation bandwidth at low current densities. We also compare the use of InGaAs and GaAs quantum wells (QWs) in the active region. Both VCSELs reach an output power of 9 mW at room temperature, with a thermal resistance of 1.9deg C/mW. The use of InGaAs QWs improves the high-speed performance and enables a small-signal modulation bandwidth of 20 GHz at 25degC and 15 GHz at 85degC. At a constant bias current density of only 11 kA/cm2, we generate open eyes under large-signal modulation at bit rates up to 25 Gbit/s at 85degC and 30 Gbit/s at 55degC.
TL;DR: In this article, the authors identify a quantum well internal high density Auger-like loss process as the origin of the so-called "droop" of internal quantum efficiency (IQE) in InGaN based light emitters.
Abstract: We identify a quantum well internal high density Augerlike loss process as the origin of the so called ‘droop’ of internal quantum efficiency (IQE) in InGaN based light emitters. The IQE of such a device peaks at small current densities and then monotonously decreases towards higher currents. The origin of this ‘droop’ has been widely discussed recently and many possible mechanisms have been proposed for explaining the effect. We compare temperature and carrier density dependent electroluminescence and photoluminescence measurements of a green emitting single quantum-well (SQW) LED over a wide parameter range. The carrier-density as well as temperature dependence of efficiency is identical in both measurements, indicating that the decrease is due to a high density quantum-well internal loss process. The data can be accurately modeled assuming an Auger-like loss process with C = 3.5 × 10–31 cm6s–1. We suggest phonon- or defect-assisted Auger recombination as the origin of this loss-channel. The high current performance can be improved if a thick InGaN SQW or a multi quantum-well (MQW) is used. This is in very good agreement with theoretical simulations (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this paper, a high-power quantum well laser with high brightness in the spectral range between 650 nm and 1080 nm was presented, with a narrow vertical far-field divergence down to angles of 15 degrees.
Abstract: High-power quantum well lasers with high brightness in the spectral range between 650 nm and 1080 nm will be presented. Improved layer structures with a narrow vertical far-field divergence down to angles of 15deg (full-width at half-maximum) were developed. For these layer structures, optimized tapered lasers were processed to achieve laterally a nearly diffraction-limited beam quality with beam propagation factors smaller than 2. Depending on the emission wavelength, the tapered devices reach an output power up to 12 W and a brightness of 1 GWmiddotcm-2middotsr-1.
TL;DR: In this article, the transfer of electrons and holes between a quantum well and a quantum dot by exploiting the moving piezoelectric potential modulation induced by an acoustic phonon was demonstrated.
Abstract: It is now possible to acoustically control the transfer of electrons and holes between a quantum well and a quantum dot by exploiting the moving piezoelectric potential modulation induced by an acoustic phonon. The effect has been used to demonstrate a high-frequency single-photon source with tunable emission energy, by acoustically transferring carriers to selected quantum dots.
TL;DR: In this paper, the authors have made a systematic study of the electronic and optical properties of InGaN based quantum dot light emitters and found that variation in dot sizes can lead to emission spectrum that can cover the whole visible light range.
Abstract: In this paper, we have made a systematic study of the electronic and optical properties of InGaN based quantum dot light emitters. The valence force field model and 6×6k⋅p method have been applied to study the band structures in InGaN or InN quantum dot devices. Piezoelectric and spontaneous polarization effects are included. A comparison with InGaN quantum wells shows that InGaN quantum dots can provide better electron-hole overlap and reduce radiative lifetime. We also find that variation in dot sizes can lead to emission spectrum that can cover the whole visible light range. For high carrier density injection conditions, a self-consistent method for solving quantum dot devices is applied for better estimation of device performance. Consequences of variations in dot sizes, shapes, and composition have been studied in this paper. The results suggest that InGaN quantum dots would have superior performance in white light emitters.
TL;DR: In this article, a maskless V-grooved c-plane sapphire was fabricated, and a GaN lateral epitaxial overgrowth method on this substrate was developed, and consequently GaN films are obtained with low dislocation densities and an increased light-emitting efficiency.
Abstract: In the last few years the GaN-based white light-emitting diode (LED) has been remarkable as a commercially available solid-state light source. To increase the luminescence power, we studied GaN LED epitaxial materials. First, a special maskless V-grooved c-plane sapphire was fabricated, a GaN lateral epitaxial overgrowth method on this substrate was developed, and consequently GaN films are obtained with low dislocation densities and an increased light-emitting efficiency (because of the enhanced reflection from the V-grooved plane). Furthermore, anomalous tunneling-assisted carrier transfer in an asymmetrically coupled InGaN/GaN quantum well structure was studied. A new quantum well structure using this effect is designed to enhance the luminescent efficiency of the LED to similar to 72%. Finally, a single-chip phosphor-free white LED is fabricated, a stable white light is emitted for currents from 20 to 60 mA, which makes the LED chip suitable for lighting applications.
TL;DR: In this article, the basic physics of electron-phonon coupling and how information can be obtained from angle-resolved photoemission experiments and first principles calculations are discussed, and several recent results for clean and adsorbate-covered surfaces, quantum wells and free-standing monolayers are also discussed.
Abstract: Over the recent years, electronic surface states have been used for a detailed spectroscopic study of the electron–phonon (e–ph) interaction, both experimentally and theoretically. This review discusses the basic physics of e–ph coupling and how information can be obtained from angle-resolved photoemission experiments and first principles calculations. Several recent results for clean and adsorbate-covered surfaces, quantum wells and free-standing monolayers are also discussed.
TL;DR: In this article, the strong coupling regime occurring between a Tamm plasmon (TP) mode and an exciton from inorganic quantum wells (QWs) was observed.
Abstract: We report on the observation of the strong coupling regime occurring between a Tamm plasmon (TP) mode and an exciton from inorganic quantum wells (QWs). The sample is formed by a silver thin film deposited onto an AlAs/GaAlAs Bragg reflector containing InGaAs QWs located in the high refractive index layers. Angular resolved reflectometry experiments evidence a clear anticrossing in the dispersion relations, a signature of the strong coupling regime. The Rabi splitting energy is 11.5 meV. The experimental data are in very good agreement with simple transfer matrix calculations. The emission from low and high energy TP/exciton polaritons is also demonstrated.
TL;DR: In this paper, the linear and third-order nonlinear optical absorptions in the asymmetric double triangular quantum wells (DTQWs) were investigated theoretically and the dependence of the optical absorption on the right-well width of the DTQWs was studied, and the influence of the applied electric field on the Optical absorption was also taken into account.
TL;DR: In this article, the authors discuss the specific features of ISB active region design using GaN/AlGaN materials, and show that the ISB wavelength can be tailored in a wide spectral range from near-to long infrared wavelengths by engineering the internal electric field and layer thicknesses.
Abstract: This paper reviews recent progress toward intersubband (ISB) devices based on III-nitride quantum wells (QWs). First, we discuss the specific features of ISB active region design using GaN/AlGaN materials, and show that the ISB wavelength can be tailored in a wide spectral range from near- to long infrared wavelengths by engineering the internal electric field and layer thicknesses. We then describe recent results for electro-optical waveguide modulator devices exhibiting a modulation depth as large as 14 dB at telecommunication wavelengths. Finally, we address a new concept of III-nitride QW detectors based on the quantum cascade scheme, and show that these photodetectors offer the prospect of high-speed devices at telecommunication wavelengths.
TL;DR: In this paper, the authors proposed a method to generate bright and dark optical solitons based on bound-to-bound intersubband transitions in an asymmetric three-coupled quantum well structure.
Abstract: We show the possibility to generate bright and dark optical solitons based on bound-to-bound intersubband transitions in an asymmetric three-coupled quantum well structure. By presenting the concept of detuning management, we show that the bright optical soliton can evolve into a dark one by adiabatically changing the corresponding one- and two-photon detunings. With adjustable carrier frequencies within the terahertz regime, we also demonstrate numerically shape-recovered collisions of two solitons in our proposed quantum wells. The present investigation may provide research opportunities in nonlinear optical experiments and may have impact on the technology for the design of semiconductor devices.
TL;DR: In this article, the authors present a study of an exciton system where electrons and holes are confined in double quantum well structures, and they show that the tail of this interaction leads to a strong correlation between excitons and substantially affects the behavior of the system.
Abstract: In this paper we present a study of an exciton system where electrons and holes are confined in double quantum well structures. The dominating interaction between excitons in such systems is a dipole-dipole repulsion. We show that the tail of this interaction leads to a strong correlation between excitons and substantially affects the behavior of the system. Making use of qualitative arguments and estimates we develop a picture of the exciton-exciton correlations in the whole region of temperature and concentration where excitons exist. It appears that at low concentration degeneracy of the excitons is accompanied with strong multiparticle correlation so that the system cannot be considered as a gas. At high concentration the dipole-dipole repulsion suppresses the quantum degeneracy. As a result there exists a temperature region where the system behaves a classical liquid; such a region does not exist in case of contact interaction. We calculate the blue shift of the exciton luminescence line which is a sensitive tool to observe the exciton-exciton correlations.