Band-gap Renormalization Effects Research Articles

The Effect of Thickness of Delta-Strained Layers in the Design of Polarization-Insensitive Semiconductor Optical Amplifiers

We have analyzed the effect of thickness of delta (/spl delta/)-strained layer in the design of polarization insensitive semiconductor optical amplifiers. We considered a system consisting of a single quantum well (QW) with several different locations and varying thicknesses of /spl delta/ layer. For its analysis, we used the self-consistent approach of solving the Poisson, Schro/spl uml/dinger, and 4 /spl times/ 4 Lu/spl uml/ttinger-Kohn equations. The many-body effects of bandgap renormalization, Coulombic scattering interactions, and a non-Markovian distribution were all included in the model. We have found that up to eight monolayers produce very good polarization insensitivity (<1 dB) provided that the /spl delta/ layer is properly positioned inside QW. For a given thickness of /spl delta/ layer, we also provide the location inside QW which gives the best polarization insensitivity.

On the differential gain in coupled quantum wells

We have analyzed the combined self-consistent and well coupling effects on differential gain in quantum wells within the self-consistent solution of the Poisson, Schrödinger, and 4×4 Luttinger–Kohn (LK) equations. The many-body effects of band-gap renormalization, Coulombic scattering interactions, and a non-Markovian distribution are also included. The analysis has been performed for a 1.55 μm InGaAsP/InP lattice-matched system grown in the [001] direction. The analysis shows that self-consistent effects significantly affect the differential gain.

Photoluminescence of one‐dimensional electron gases in cleaved‐edge overgrowth quantum wires

AbstractWe carried out micro‐photoluminescence studies of doped multiple quantum wire structures grown by the cleaved‐edge overgrowth technique in the GaAs/AlGaAs material system. The wires are defined in a quantum well whose potential is modulated by adjacent alternating negatively and positively charged layers. We observed strong band gap renormalization effects in the quantum wires indicating that they are densely populated. The measured recombination energies are in good agreement with a simple quantum mechanical calculation. Nevertheless, the design of the sample appears to be not suitable for light scattering studies of the one dimensional (1D) electron gas, for the 1D confinement strongly depends on photoexcitation. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Nonlinear optical absorption of semiconductor quantum wires: Photoexcitation dynamical effects

We investigate the optical absorption spectra of semiconductor quantum wires and its dependence on the optical pumping power. The absorption coefficient is obtained solving the semiconductor Bloch equations in the real-space and time domains, including corrections due to band-gap renormalization, local field, and screening. We find that the energy shifts in the spectra due to increasing photoexcitation power have different behavior when treating dynamically the carrier creation rather than using stationary carrier distribution at thermal equilibrium. Dealing with the nonequilibrium distribution dynamically, we are able to describe the observed constancy of the peak position of the fundamental transition energy with the optical pumping power. The competing effects of the dynamical band-gap renormalization and the local-field correction leads to an almost cancellation of the red/blue shift energy.

Photoluminescence studies of Si-doped AlN epilayers

Si-doped AlN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates. Deep ultraviolet picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in the grown epilayers. The donor bound exciton (or I2) transition was found to be the dominant recombination line in Si-doped AlN epilayers at 10 K and its emission intensity decreases with increasing Si dopant concentration. Doping-induced PL emission linewidth broadening and band-gap renormalization effects have also been observed. Time-resolved PL studies revealed a linear decrease of PL decay lifetime with increasing Si dopant concentration, which was believed to be a direct consequence of the doping-enhanced nonradiative recombination rates.

Extraction of rate‐equation parameters from optical modal gain measurements for multiple‐quantum‐well semiconductor lasers

AbstractRate equations for multiple‐quantum‐well (MQW) laser structure with carrier transport effects and advanced models of optical modal gain were used to extract relevant laser's parameters used in simulations. The many‐body effects of bandgap renormalization, Coulombic scattering interactions, and a non‐Markovian distribution were included in gain calculations. The importance of carrier leakage for that particular structure is indicated. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 35: 55–56, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10514

Anomalous shift of the recombination energy in single asymmetric quantum wells

Self-consistent numerical calculation and photoluminescence (PL) measurements have been used to investigate the temperature dependence of the optical Stark effect in n-doped GaAs/AlGaAs single asymmetric quantum wells (SAQWs), grown by molecular beam epitaxy. In the low-temperature regime (5 to 40 K) a remarkable blue shift (9.8 meV) is observed in the PL peak energy, as the optical excitation intensity increases from 0.03 to 90 W/cm2. The blue shift is well explained by the reduction of the two-dimensional electron gas (2DEG) density, due to a charge-transfer mechanism. At about 80 K, however, an anomalous behavior of the PL peak energy was found, i.e. a red shift has been observed as the optical excitation intensity increases. This anomalous behavior has been explained by combining the effects of band gap renormalization, band bending, temperature dependence of the band gap, temperature dependence of the 2DEG density, and temperature dependence of the fundamental energy position.

Temperature effect of magneto-photoluminescence in InGaAs/InAlAs quantum wells: application of band theory to nonparabolic conduction subband

Interband magneto-photoluminescence (MPL) spectra in n-type InGaAs/InAlAs quantum wells (QWs) lattice-matched to InP were measured between 1.4 and 275 K. From a fan chart of MPL peak energy vs. magnetic field, assignment of Landau levels (LLs) associated with conduction subband (CS) was made consistently. Two series of LL peak energies with different Landau quantum number was subtracted to cancel the effects of complex valence subband states and bandgap renormalization, etc. The energy difference of two CS LL peak series in the MPL fan chart, between Landau quantum number 0 and 1, did not agree with the measured cyclotron resonance (CR) energy or the related theoretical CS LL energy difference. The difference was 6-10 meV at 1.4 K near 10 T. Temperature dependence of the difference of CS LL peak energies was studied and the difference tended to diminish at 175-275 K. The behavior suggested that the quantity was the difference of magneto-exciton binding energy and that the exciton states were merged into LLs at higher temperature. A band parameter, Ep, in nonparabolic band theory was discussed.

Effect of an electric field on electronic excitations in double quantum wells

We have measured photoluminescence (PL) and inelastic light scattering spectra of GaAs/AlGaAs double quantum wells (DQW). The structures contain a high-mobility two-dimensional (2D) electron gas produced by modulation doping. The electron density as well as the intersubband spacings of the DQWs are changed by applying a DC voltage between a top and a bottom contact. Band-gap renormalization effects are observed in PL spectra when an excited subband becomes populated with electrons, as the 2D density increases. Inelastic light scattering yields information on the bias dependence of collective intersubband excitations of the 2D electron gas. The applied voltage reduces the coupling between layers leading to an energy shift of the excitations in each well in different directions.

Silicon behavior in GaN grown by radiofrequency plasma molecular beam epitaxy

The behavior of Si in GaN has been investigated in the wide doping range from 1×10 17 to 9×10 19 cm −3 by photoluminescence (PL). PL studies at 77 K for doped specimens show that the intensity of the near-band-edge emission increased monotonically as the Si concentration increases. The intensity of the yellow luminescence (YL), however, remains almost constant, which indicates that Si does not strongly influence the density of deep centers in GaN. The red-shift of the near-band-edge emission due to increase the Si concentration shows that bandgap renormalization (BGR) effect is much more important than the Burstein–Moss (B-M) effect in Si-doped GaN. With Si doping as high as 9×10 19 cm −3, a new peak due to donor-to-acceptor recombination appears in the PL spectrum. This donor-to-acceptor emission peak can also be observed in the PL spectrum of Si-doped GaN after thermal annealing. We attributed the appearance of the new acceptor level to the formation of Si N. A model has been proposed to illustrate the Si behavior in GaN in a wide range of Si concentrations.

Evidence of blocking effect on carrier trapping process by necking region in very narrow AlGaAs/GaAs V-grooved quantum wire structure

Transient band-gap renormalization (BGR) effects are investigated in AlGaAs/GaAs V-grooved quantum structures. The temperature-dependent transient BGR effects in the sidewall quantum well (SQWL) provide direct evidence of the existence of the blocking effect by the necking region barrier on the carrier trapping process. These effects provide a useful method to show the existence of the necking region, particularly for very thin SQWL structures. The temperature-dependent lifetimes of the SQWL and quantum wire (QWR) provide further proof of the carrier trapping process from the SQWL to the QWR.

Correlation between Defect Concentration and Carrier Lifetime of GaAs Grown by Molecular Beam Epitaxy at Different Temperatures

A pump-probe study of GaAs grown by molecular beam epitaxy at low temperatures (LT-GaAs) is performed. Ultrashort carrier lifetimes of the as-grown LT-GaAs increase from < 0.13 ps (measurement limitation) to 1.8±0.2 ps as the growth temperature is increased from 200°C to 320°C. The carrier lifetime was found to be approximately inversely proportional to the antisite defect concentration. This trend is found to be in reasonably good agreement with the Schokley–Read–Hall model. The decreasing trend in the amplitudes of continuous-wave and transient reflectivities (ΔR/R) as a function of the growth temperature for the LT-GaAs is explained as an induced absorption caused by dense arsenic antisite defects. The sign of the transient ΔR/R reversed for LT-GaAs grown at 200°C. This is tentatively attributed to the band gap renormalization effect.

Combined effects of screening and band gap renormalization on the energy of optical transitions in ZnO and GaN

The energy positions of the optical transitions in both GaN and ZnO were investigated when the samples were excited simultaneously with a HeCd laser and an Ar+ ion laser. The increased number of free electrons excited by the Ar+ ion laser will effectively screen both the free exciton and bound exciton transitions, resulting in a blueshift. The increased number of free electrons also produces many-body effects, which lead to a reduction of the band gap energy and thus a redshift. The resultant of screening and renormalization results in a redshift of the optical transitions in ZnO but a nearly vanishing shift in GaN.

Excitation Density Dependence of Photoluminescence and Barrier Doping Effect in InxGa1—xN Quantum Wells

The effect of Si-doping in barriers has been investigated in photoluminescence (PL) spectra of 10 period In0.1Ga0.9N/ In0.02Ga0.98N quantum wells with various excitation densities. For the quantum wells with well width of 40 A, the PL peak was blueshifted and the intensity was increased with Si-doping under relatively low excitation density. However, such Si-doping effect was found to be less pronounced with higher excitation density. The blueshift of PL emission with increasing excitation density was larger for larger well widths, which indicates the influence of a piezoelectric potential in our samples. For sufficiently narrow quantum wells, we observed a redshift with increasing excitation density due to the bandgap renormalization effect.

Band-gap renormalization and band filling in Si-doped GaN films studied by photoluminescence spectroscopy

We have studied band-gap renormalization and band filling in Si-doped GaN films with free-electron concentrations up to 1.7×1019 cm−3, using temperature-dependent photoluminescence (PL) spectroscopy. The low-temperature (2 K) PL spectra showed a line-shape characteristic for momentum nonconserving band-to-band recombination. The energy downshift of the low-energy edge of the PL line with increasing electron concentration n, which is attributed to band-gap renormalization (BGR) effects, could be fitted by a n1/3 power law with a BGR coefficient of −4.7×10−8 eV cm. The peak energy of the room-temperature band-to-band photoluminescence spectrum was found to decrease as the carrier concentration increases up to about 7×1018 cm−3, followed by a high-energy shift upon further increasing carrier concentration, due to the interplay between the BGR effects and band filling. The room-temperature PL linewidth showed a monotonic increase with carrier concentration, which could be described by a n2/3 power-law dependence.

Inelastic Light Scattering by Elementary Excitations of the 2D Electron Gas at High Densities

We have investigated the dependence on carrier density of elementary excitations of the 2D electron gas formed in a modulation-doped single quantum well structure. The 2D electron density n, as obtained from lineshape fits to the photoluminescence spectra, is continuously increased by applying dc voltages ranging from 5 to 100 V between the electron gas and a back contact. A substantial population of the second electron subband is revealed at high voltages by the strong redshift of the corresponding emission line due to bandgap renormalization effects. In this high-density regime, we observe a large reduction of the exchange-correlation term of the Coulomb interaction.

Size-dependent optical nonlinearities in GaN quantum dots

We studied experimentally the influence of the size of GaN quantum dots on the exciton photoluminescence at 10K under high optical excitation for which the spatial confinement was found to be a critical parameter for optical nonlinearities such as band-gap renormalization (BGR) and state-filling effects. As the dot size decreases, the BGR characteristic of bulk like dots becomes dominated by the state-filling nonlinearity. In addition, three-dimensional confinement is demonstrated by a comparison with a quantum well.

Spatially-Localized Band-Filling Effects and Band-Gap Renormalization in Growth-Interrupted Quantum Wells

We have observed a large excitonic linewidth broadening at low temperatures in growth-interrupted asymmetric-coupled quantum wells as irradiance increases. We attribute this broadening to the decrease of the exciton binding energy due to spatially-localized band-gap renormalization. We have also observed the stepwise saturation of the photoluminescence emission peaks as irradiance increases in growth-interrupted asymmetric-coupled quantum wells with various structures and doping profiles. We attribute this saturation to the result of spatially-localized band-filling effects. Based on the time-resolved photoluminescence measurements, we have determined the nature of the recombination processes and the exciton densities. In both undoped and modulation-doped samples, the small interface island area as a result of growth-interruption allows us to generate large carrier density in the islands; both band-gap renormalization and band-filling effects become stronger even at low irradiances. In compensation-doped asymmetric-coupled quantum wells, we observed anomalously large blue shift of donor-acceptor pair transition energy as the laser intensity increases.

Band-gap renormalization and strain effects in semiconductor quantum wells

The band-gap renormalization (BGR) due to exchange-correlation effects for the electron-hole plasma (EHP) of In x Ga 1− x As-InGaAsP quantum well system with strain is investigated in terms of the sheet carrier densities and strain amounts. We calculated a 4 × 4 Luttinger-Kohn Hamiltonian for the nonparabolic valence band structure of the strained-quantum well and obtained two-component EHP self-energies within the full random-phase approximation. We noted an increasing BGR with an increase of the biaxial compressive strain. The calculated results of the band-gap wavelength for biaxial compressive strain of 0.47–1.02% show the wavelength red shift of about 4.2% at n s = 1 × 10 12 cm −2 when the BGR is included, as compared to those when the BGR is not included. The band-gap wavelength red shifts of 0.2% also have been observed for every x = 0.03 increase of the indium mole fraction.

Spatially localized band-gap renormalization and band-filling effects in three growth-interrupted multiple asymmetric coupled narrow quantum wells

For the first time to our knowledge, we have observed a large excitonic linewidth broadening at low temperatures in three growth-interrupted asymmetric coupled GaAs/Al0.3Ga0.7As narrow quantum wells as the irradiance increases. We attribute this broadening to the decrease of the exciton binding energy that results from spatially localized band-gap renormalization. We also observed the stepwise saturation of the photoluminescence emission peaks as irradiance increases. We attribute this saturation to the result of spatially localized band filling. Based on time-resolved photoluminescence measurements, we have determined the nature of the recombination processes. We have also determined the exciton densities. In both undoped and modulation-doped samples, the small interface island area that results from growth interruption allows us to generate a large carrier density in the islands; both band-gap renormalization and band-filling effects become stronger even at low irradiances. When the temperature is higher than the transition temperature, the free-carrier recombination dominates the photoluminescence spectrum. The band-gap renormalization then results in a red shift of the photoluminescence emissions.